Article

Swelling and Exchange Behavior of Poly(sulfobetaine)-Based Block Copolymer Thin Films

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Abstract

The humidity-induced swelling and exchange behavior of a block copolymer thin film, which consists of a zwitterionic poly(sulfobetaine) [poly(N,N-dimethyl-N-(3-(methacrylamido)propyl)ammoniopropanesulfonate) (PSPP)] block and a nonionic poly(N-isopropylacrylamide) (PNIPAM) block, are investigated by time-of-flight neutron reflectometry (TOF-NR). We monitor in situ the swelling in the H 2 O atmosphere, followed by an exchange with D 2 O. In the reverse experiment, swelling in the D 2 O atmosphere and the subsequent exchange with H 2 O are studied. Both, static and kinetic TOF-NR measurements indicate significant differences in the interactions between the PSPP 80 -b-PNIPAM 130 thin film and H 2 O or D 2 O, which we attribute to the different H-and D-bonds between water and the polymer. Changes in the chain conformation and hydrogen bonding are probed with Fourier transform infrared spectroscopy during the kinetics of the swelling and exchange processes, which reveals the key roles of the ionic SO 3⁻ group in the PSPP block and of the polar amide groups of both blocks during water uptake and exchange.

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... As it is an aprotic solvent, an H/D exchange between the water and cosolvent species is very slow and can be excluded, which allowed circumnavigating a mixture-dependent scattering length density (SLD) of the individual solvents. Supplementary SR measurements using solvent vapors with alternate isotopic compositions are carried out to evaluate the impact of isotopic effects [60,61] on the film swelling behavior (see Section 3.2.2. and Figure S4, Supporting Information). ...
... We attribute this difference to the presence of residual H 2 O (SLD H2O ¼ À0.561 Â 10 À6 Å À2 ), which the film took up in ambient conditions and which is not removable by drying the film in a The obtained SLD profile of the diblock copolymer film in the D 2 O-hydrated state supports this explanation: Usually, solvent enrichment layers are observed toward the interfaces in the hydrated equilibrium state of responsive thin films. [60][61][62] In the present study, a D 2 O enrichment is found at the polymerÀair interface, indicated by an increased SLD, as was expected. [60][61][62] However, at the polymerÀsilicon substrate interface, the opposite occurs with the SLD value being lower than that of the average D 2 O-containing polymer film. ...
... [60][61][62] In the present study, a D 2 O enrichment is found at the polymerÀair interface, indicated by an increased SLD, as was expected. [60][61][62] However, at the polymerÀsilicon substrate interface, the opposite occurs with the SLD value being lower than that of the average D 2 O-containing polymer film. Its exact value arises from the presence of H 2 O, which accumulated in proximity to the native hydrophilic SiO 2 layer of the substrate, and is exchanged at a slower rate compared with the water species in proximity of the constant D 2 O vapor flow. ...
Article
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To systematically add functionality to nanoscale polymer switches, an understanding of their responsive behavior is crucial. Herein, solvent vapor stimuli are applied to thin films of a diblock copolymer consisting of a short poly(methyl methacrylate) (PMMA) block and a long poly(N‐isopropylmethacrylamide) (PNIPMAM) block for realizing ternary nanoswitches. Three significantly distinct film states are successfully implemented by the combination of amphiphilicity and co‐nonsolvency effect. The exposure of the thin films to nitrogen, pure water vapor, and mixed water/acetone (90 vol%/10 vol%) vapor switches the films from a dried to a hydrated (solvated and swollen) and a water/acetone‐exchanged (solvated and contracted) equilibrium state. These three states have distinctly different film thicknesses and solvent contents, which act as switch positions “off,” “on,” and “standby.” For understanding the switching process, time‐of‐flight neutron reflectometry (ToF‐NR) and spectral reflectance (SR) studies of the swelling and dehydration process are complemented by information on the local solvation of functional groups probed with Fourier‐transform infrared (FTIR) spectroscopy. An accelerated responsive behavior beyond a minimum hydration/solvation level is attributed to the fast build‐up and depletion of the hydration shell of PNIPMAM, caused by its hydrophobic moieties promoting a cooperative hydration character. Ternary nanoswitches are realized using PMMA‐b‐PNIPMAM diblock copolymer thin films. Three distinct film states (off, on, and standby) are successfully prepared by the combination of amphiphilicity and co‐nonsolvency effect. The exposure to N2, pure water vapor, and mixed 90 vol% water/10 vol% acetone vapor switches the films between dried, hydrated (solvated, swollen) and water/acetone exchanged (solvated, contracted) states.
... 20,21 Remarkably, their film thicknesses and incorporated water contents varied in different isotope water vapor atmospheres (H 2 O and D 2 O). This is attributed to the marked H-D isotope sensitivity of the poly(sulfobetaine)s, 15,16,22 whereas PNIPMAM exhibits a rather minor H/D isotope sensitivity. 18 For its part, PNIPMAM has the particularity to show a distinct so-called co-nonsolvency effect when adding ethanol to PNIPMAM−water mixtures. ...
... 39,40 Experiments addressing a cononsolvency effect of polymers in the substance-rich regime, e.g., in thin film geometry, have, to the best of our knowledge, not been reported so far. As mentioned above, polymer thin films are known to behave differently from the corresponding aqueous solution, 20,22 and the increased polymer concentration is expected to affect solvent−cosolvent interactions as well. 56 To be able to extend the knowledge about the earlier introduced doubly thermoresponsive DBC thin films, consisting of a poly(sulfobetaine) and a PNIPMAM block, we found it necessary to improve the understanding of the behavior of the individual blocks. ...
... The sample preparation for SR and FTIR experiments was already described in detail previously. 20,22 Briefly, Si (2 × 2 cm 2 for SR and 1 × 1 cm 2 for FTIR measurements) was cleaned in an acid bath (54 mL of H 2 O, 84 mL of H 2 O 2 , and 198 mL of H 2 SO 4 at 80°C for 15 min) and rinsed thoroughly with deionized water afterward. For the SR measurements, the substrates were treated with O 2 plasma (200 W for 10 min) before thin films were spin-coated (2500 rpm, 900 s) out of TFE solution with a polymer concentration of 25 g L −1 . ...
... Besides the fundamental understanding of the absorption and release mechanisms of the respective gaseous or liquid molecules into and out of thin films, which is still a rare topic in the literature, 35,40,64,65 a reproducible swelling and collapse process is of equally high importance. In particular, swelling with water molecules is interesting for thermoresponsive polymers. ...
... To enable in situ measurements, a custom-made sample environment was used, as was already described in our previous work. 35,64,65 Thermal control was assured using a JULABO F12 MC thermal bath. The RH and temperature within the sample environment were tracked with a SHT31 humidity and temperature sensor. ...
... The thin film on a Si substrate was placed horizontally on a sample stage in a custom-made sample environment for RH and temperature control, as was already described in our previous work. 35,64,65 The sample stage was surrounded by a reservoir. Injection of D 2 O in this reservoir increased the RH in the sample chamber. ...
Article
The cyclic swelling and collapse behavior of a doubly thermoresponsive diblock copolymer thin film, consisting of a zwitterionic poly(sulfobetaine), poly(N,N-dimethyl-N-(3-methacrylamidopropyl)-ammoniopropane sulfonate) (PSPP), and a nonionic poly(N-isopropylmethacrylamide) (PNIPMAM) block, is investigated in situ at three characteristic temperatures with time-of-flight neutron reflectometry. With increasing temperature, the thin film becomes less hydrophilic, which leads to a decreased but faster water uptake. This response of the block copolymers in the thin-film geometry differs greatly from their known aqueous solution behavior. In the cyclic experiments at constant temperature, the behavior is reproducible in terms of mesoscopic parameters such as swelling ratio and water content, even though Fourier transform infrared spectroscopy reveals altered swelling mechanisms, which are attributed to a complex interplay between different water species. Thus, the overall reduced hydrophilicity affects the overall swelling behavior of the thin film but not the hydration of particular functional groups of the diblock copolymer PSPP-b-PNIPMAM.
... 34,36,46,64−67 In thinfilm morphology, yet, only the kinetics of swelling and exchange processes at constant temperature have been reported in one of our previous works. 58 In the present work, we investigate the swelling behavior in D 2 O vapor of a DBC thin film of the block copolymer poly(N,N-dimethyl-N-(3-methacrylamidopropyl)-ammoniopropane sulfonate)-block-poly(N-isopropylmethacrylamide) (PSPP 500 -b-PNIPMAM 150 ). Its chemical structure is displayed in Figure 2. ...
... The resulting χ eff amounts to 0.71 ± 0.05, while τ is 31 ± 1 min. This indicates a slightly higher hydrophilicity of the PSPP 500 -b-PNIPMAM 145 toward D 2 O and a faster swelling process as compared to observations for a similar DBC (PSPP 80 -b-PNIPAM 130 ) thin film in our previous work, which 58 From eq S6, the maximum swelling ratio d max /d ini is calculated, which depends on the nonconstant relative humidity and χ eff . Hence, it expresses the theoretical limit to which the DBC thin film can swell at a given ambient relative humidity. ...
... This is in agreement with findings from our previous work. 58 The steeper increase and the establishment of an equilibrium after shorter times of the S−O peak shift compared to the evolution of the D 2 O content highlight the assumption that, despite the lower affinity, D 2 O hydrates the SO 3 − group first, before the rest of the polymer chain. Figure 4e shows the deuteration of the amide group. ...
... Interfacial water enrichment layers toward the polymer−substrate and polymer−vapor interfaces are in agreement with previous studies addressing the swelling behavior of spin-coated responsive thin polymer films on Si substrates in saturated water vapor. 41,42,55 In the exchanged equilibrium states, for both sequences A and B, toward both interfaces, layers of lower SLD than the bulk are found, which, in contrast to solvent enrichment toward the interfaces in the hydrated state, indicate a lower average material density. This 144.9 ± 0.6 0.50 ± 0.02 water/methanol exchanged (V) ...
... These deviations for the exchange processes in sequences A and B can be attributed to the higher strength of H 2 O hydrogen bonds compared to D 2 O hydrogen bonds, as reported in our recent studies. 41,42 In addition, due to the different vapor pressures of deuterated and nondeuterated solvent species, the molar ratios between water and cosolvent in the vapor phase at identical gas flow rates are slightly different between the two experimental sequences, which also in part can be the reason for the observed differences. ...
Article
We demonstrate that the cononsolvency effect strongly affects the responsive behavior of PMMA-b-PNIPAM films in the polymer-rich part of the phase diagram. Thin films equilibrated in water vapor show a swelling and contraction at the exchange of the surrounding atmosphere to mixed vapor of water and methanol. The film kinetics are investigated in-situ by spectral reflectance and time-of-flight neutron reflectometry. The water and methanol contents are calculated using scattering contrast variation (D2O/CH3OH and H2O/CD3OH vapor sequences). The hydration and co-solvent exchange processes at the PNIPAM amide functional group are resolved with Fourier-transform infrared spectroscopy. Results on both mesoscopic and molecular length scales indicate the film response is governed by a methanol absorption phase leading to additional swelling, followed by contraction with little change in the water/methanol ratio.
... The PBS measurements were performed to investigate the characteristics of the HF polymer self-assemblies in an environment with a similar ionic strength to the biological environment, since the ionic strength can have an impact on the thermoresponsive behaviour and the lower critical solution temperature (LCST). The measurements in D 2 O were performed to compare the results collected by SANS, which required measurement in D 2 O, while the results with H 2 O were used in lieu of D 2 O measurements to ensure compatibility with the NMR measurements [48][49][50][51]. ...
... The thermoresponsive behaviour of our copolymer shows a moderate variation between the solvents used. Obtaining this data is therefore important to conduct a reliable comparison of results from experimental methods carried out in different environments [48][49][50][51]. ...
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Fluorine-19 MRI is a promising noninvasive diagnostic method. However, the absence of a nontoxic fluorine-19 MRI tracer that does not suffer from poor biodistribution as a result of its strong fluorophilicity is a constant hurdle in the widespread applicability of this otherwise versatile diagnostic technique. The poly[N-(2-hydroxypropyl)methacrylamide]-block-poly[N-(2,2-difluoroethyl)acrylamide] thermoresponsive copolymer was proposed as an alternative fluorine-19 MRI tracer capable of overcoming such shortcomings. In this paper, the internal structure of self-assembled particles of this copolymer was investigated by various methods including 1D and 2D NMR, dynamic light scattering (DLS), small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS). The elucidated structure appears to be that of a nanogel with greatly swollen hydrophilic chains and tightly packed thermoresponsive chains forming a network within the nanogel particles, which become more hydrophobic with increasing temperature. Its capacity to provide a measurable fluorine-19 NMR signal in its aggregated state at human body temperature was also investigated and confirmed. This capacity stems from the different fluorine-19 nuclei relaxation properties compared to those of hydrogen-1 nuclei.
... System specific parameters are derived on the basis of a model approach to quantify the temporal evolution and the polymer−solvent interaction during the hydration and the solvent exchange process with respect to the involved materials. [43][44][45]48,49 ■ EXPERIMENTAL SECTION Materials, Microgel Synthesis, and Sample Preparation. Materials. ...
... However, as the polymer film shrinks, a simultaneous rearrangement of the microgel occurs, enabling a reincorporation of D 2 O into the polymer bulk. 48 Nevertheless, the overall increase in thickness when the atmosphere is exchanged from D 2 O vapor to H 2 O vapor, as well as the overall decrease of the vice versa exchange, are again an indication for the higher affinity toward H 2 O. Regarding the changes in water content shown in Figure 6d, the overall changes are in relation to what is expected from the swelling ratio values. This is in accordance with the assumption of the free volume inside the polymer film that was filled during the hydration process. ...
Article
The investigation of the response kinetics of smart colloidal microgel films is crucial for their optimization to enable advanced applications. We study the classical thermo-responsive microgel model system N-isopropylacrylamide (NIPAM) cross-linked with N,N’-methylenebisacrylamide (BIS). Without the typically used polyelectrolyte coating of the substrate, thin microgel films are prepared in a single spin-coating step. Atomic force microscopy (AFM) measurements reveal an extremely dense packing, resulting in a homogeneous compact thin film of microgel particles. The hydration kinetics of these films in H2O and D2O atmospheres as well as the kinetics of the solvent exchange between both water species is investigated with in situ time-of-flight neutron reflectometry (TOF-NR) and in situ Fourier-transform infrared spectroscopy (FTIR). With accounting for a non-constant humid atmosphere, the intrinsic diffusion dynamics of water molecules into the thin microgel film are modeled and the specific time constant τ and the effective Flory-Huggins interaction parameter χeff are determined. Comparing the results in H2O and D2O atmosphere, TOF-NR and FTIR spectroscopy results show an increased affinity of the microgel films towards H2O as compared to D2O. From the FTIR spectroscopy data we further identify different kinetics of intermolecular processes and order them according to their temporal evolution.
... The role of the specific SVA pathway on the swelling ratio, morphology, orientation and grain size were elucidated by Jung et al. [24] by following different processing protocols while annealing the poly(styrene)-block-poly(dimethylsiloxane) (PS-b-PDMS) films in a mixture of solvents. Kreuzer et al. [58] investigated the swelling and the exchange kinetics by exposing poly(sulfobetaine)-based BCP thin films to H 2 O (water) and D 2 O (deuterated) vapours, and showed the effect of the polymer-solvent interactions on the swelling behaviour of the films. The influence of the substrate on the swelling kinetics, steady-state solvent uptake and chain dynamics was reported by Ogieglo et al. [5] and Stenbock et al. [59]. ...
Article
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We studied the kinetics of swelling in high-χ lamellar-forming poly(styrene)-block- poly(lactic acid) (PS-b-PLA) block copolymer (BCP) by varying the heating rate and monitoring the solvent vapour pressure and the substrate temperature in situ during solvo-thermal vapour annealing (STVA) in an oven, and analysing the resulting morphology. Our results demonstrate that there is not only a solvent vapour pressure threshold (120 kPa), but also that the rate of reaching this pressure threshold has a significant effect on the microphase separation and the resulting morphologies. To study the heating rate effect, identical films were annealed in a tetrahydrofuran (THF) vapour environment under three different ramp regimes, low (rT<1 °C/min), medium (2<rT<3 °C/min) and high (rT>4 °C/min), for 60, 90 and 120 min, respectively, while the solvent vapour pressure and the substrate temperature were measured in real time. The translational order improved significantly with increasing the heating rate. The solvent mass uptake calculated for the different ramp regimes during annealing is linearly proportional to time, indicating that the swelling kinetics followed Case II diffusion. Two stages of the swelling behaviour were observed: (i) diffusion at the initial stages of swelling and (ii) stress relaxation, controlled at later stages. Films with a faster rate of increase in vapour pressure (rP>2 kPa/min) reached the pressure threshold value at an early stage of the swelling and attained a good phase separation. According to our results, highly ordered patterns are only obtained when the volume fraction of the solvent exceeds the polymer volume fraction, i.e., (φs≥φp), during the swelling process, and below this threshold value (φs=0.5), the films did not obtain a good structural order, even at longer annealing times.
... With the global economy and technological advancements, smart textiles [3][4][5] are one of the research hotspots in the field of textiles and garments. Intelligent textiles are a sort of smart fabric or material which can be responsive to the external environment or an outside stimulus in behavior, including electrical, chemical, biological agents or physical temperature [6,7]. Currently, in order to introduce some kind of advanced function or special performance, smart textile materials are often modified by direct coating technology such as roll coating, the spraying method or deposition and surface treatment approaches, such as plasma treatment technology, ultraviolet irradiation [8], etc. ...
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There is a significant interest in developing environmentally responsive or stimuli-responsive smart materials. The purpose of this study was to investigate multi-function responsive cotton fabrics with surface modification on the nanoscale. Three technologies including electrospinning technology, interpenetrating polymer network technology, and cross-linking technology were applied to prepare the multi-function sericin/poly(N-isopropylacrylamide)/Poly(ethylene oxide) nanofibers, which were then grafted onto the surfaces of cotton textiles to endow the cotton textiles with outstanding stimuli-responsive functionalities. The multi-function responsive properties were evaluated via SEM, DSC, the pH-responsive swelling behavior test and contact angle measurements. The results demonstrate that with this method, multi-function responsive, including thermo- and pH-responsiveness, cotton fabrics were fast formed, and the stimuli-responsiveness of the materials was well controlled. In addition, the antimicrobial testing reveals efficient activity of cotton fabrics with the sericin/PNIPAM/PEO nanofiber treatments against Gram-positive bacteria and Gram-negative bacteria such as Staphylococcus aureus and Escherichia coli. The research shows that the presented strategy demonstrated the great potential of multi-function responsive cotton fabrics fabricated using our method.
... Amphiphilic block copolymers, with hydrophilic and hydrophobic segments connected by covalent bonds, can self-assemble into micelles with various morphologies and structures in selective solvents [1][2][3][4]. Because of the unique physical and chemical properties, block copolymer micelles have been used in many technical fields, including drug delivery [5,6], templates [7][8][9], hydrogels [10,11], and membranes [12][13][14]. In past decades, the self-assembly of block copolymers in selective solvents has been extensively studied experimentally [15,16] and theoretically [17][18][19][20]. ...
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A series of linear amphiphilic pentablock terpolymer PAA x -b-PS48-b-PEO46-b-PS48-b-PAA x (A x S48O46S48A x ) with various lengths x of the PAA block (x = 15, 40, 60, and 90) were synthesized via a two-step atom transfer radical polymerization (ATRP) using Br-poly(ethylene oxide)-Br (Br-PEO46-Br) as the macroinitiator, styrene (St) as the first monomer, and tert-butyl acrylate (tBA) as the second monomer, followed with the hydrolysis of PtBA blocks. The A x S48O46S48A x pentablock terpolymers formed micelles in dilute aqueous solution, of which the morphologies were dependent on the length x of the PAA block. Cryogenic transmission electron microscopy (cryo-TEM), dynamic light scattering (DLS), and zeta potential measurement were employed to investigate the morphologies, chain structures, size, and size distribution of the obtained micelles. The morphology of A x S48O46S48A x micelles changed from spherical vesicles with ordered porous membranes to long double nanotubes, then to long nanotubes with inner modulated nanotubes or short nanotubes, and finally, to spherical micelles or large compound vesicles with spherical micelles inside when x increased from 15 to 90. The hydrophobic PS blocks formed the walls of vesicles and nanotubes as well as the core of spherical micelles. The hydrophilic PEO and PAA block chains were located on the surfaces of vesicle membranes, nanotubes, and spherical micelles. The PAA block chains were partially ionized, leading to the negative zeta potential of A x S48O46S48A x micelles in dilute aqueous solutions.
... Therefore, the exchange of H 2 O for D 2 O was used to investigate the origin of LCST in aqueous solutions of other polymers [36]. In polysulfobetaines, the effect is opposite: the UCST is considerably higher in D 2 O than in H 2 O, pointing to a different origin of the shift [30,37]. ...
Article
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Multiresponsive polymers offer a wealth of possibilities to design switchable materials which respond to more than one stimulus. We describe first the response of polymers to a single external stimulus, namely to temperature, light, pH value, redox changes, and low molecular weight species, and discuss the influence of these stimuli on the polymer chain polarity. Then, we review multiresponsive homopolymers and statistical and block co-polymers. Finally, we discuss the similarity of multiresponsive synthetic polymers to biopolymers. As a conclusion, multiresponsiveness opens up a broad area for combining different properties in one system, enabling numerous possible applications.
... 12 In addition to the above-mentioned biological properties, zwitterionic sulfobetaine polymers are generally thermosensitive in aqueous solutions and exhibit upper critical solution temperature (UCST) phase transitions. [13][14][15] The zwitterionic betaine hydrogels, formed by virtue of a strong ability of zwitterionic moieties to bind water molecules, will have various potential applications, such as so contact lenses, 16,17 wound dressing, 3,18,19 drug release [20][21][22] and non-fouling materials. 23,24 It implies that zwitterionic betaine hydrogels will hold great application potentials as biomaterials. ...
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The development of zwitterionic hydrogels possessing both excellent self-healing and mechanical properties is of great significance. Herein, a class of zwitterionic sulfobetaine nanocomposite hydrogels was prepared by UV-initiated copolymerisation of zwitterionic sulfobetaine monomer N,N-dimethyl-N-(3-methacrylamidopropyl)-N-(3-sulfopropyl) ammonium betaine (DMAPMAPS) and 2-hydroxyethyl methacrylate (HEMA) in the presence of exfoliated clay platelets uniformly dispersed in an aqueous medium. The effects of the hydrogel compositions, including the DMAPMAPS/HEMA mass ratio and the amount of clay, on the self-healing behaviors and mechanical properties of the nanocomposite hydrogels were investigated. The results indicate that the fabricated zwitterionic sulfobetaine nanocomposite hydrogels can autonomously repair incisions or cracks at ambient temperature without the need for any stimulus and possess excellent mechanical properties.
... Besides, many other resins were also introduced to CNCs to fabricate nanocomposite films which are responsive to various stimuli such as heat, solvent, or pH. 29,30 Among various polymers, poly(N-isopropylacrylamide) (PNIPAM) is considered as one of the most attractive resins due to its unique heat and humidity dual stimuli responsibility. For example, Wang et al. 31 developed the PNIPAM-based films, which exhibit a strong response under a heat stimulus over a wide range of thickness. ...
Article
It has been widely reported that cellulose nanocrystals (CNCs) demonstrate a special structural colour, which stems from chiral nematic domains. Herein, the humidity and heat dual response nanocomposite films with multi-layered helical structure were prepared by self-assembling of CNCs and hydrazone groups modified poly(N-isopropylacrylamide) (PNIPAM) copolymers. Furthermore, glutaraldehyde was involved to act as chemical linker to improve cyclic stability by forming acylhydrazone bonds. The structural colour of the films could be easily regulated by humidity, heat or the content of modified PNIPAM copolymers. The absorption of water in higher humidity led to volume expansion of the resin, resulting in a red shift for up to 145 nm. In contrast, the resin shrank under the temperature above the lower critical solution temperature (LCST) of PNIPAM, leading to a blue shift for up to 87 nm. It was notable that the change of colour can be easily captured by naked eyes. Moreover, the films exhibited excellent stability and cyclicity in response to either vapour or liquid water due to the chemical linking between CNCs and resins. The as-prepared CNCs/PNIPAM nanocomposite films with humidity or heat responsibilities are promising in stimuli-responsive sensors, printing industry and surface decorations, etc.
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The water vapor-induced swelling, as well as subsequent phase-transition kinetics, of thin films of a diblock copolymer (DBC) loaded with different amounts of the salt NaBr, is investigated in situ. In dilute aqueous solution, the DBC features an orthogonally thermoresponsive behavior. It consists of a zwitterionic poly(sulfobetaine) block, namely, poly(4-(N-(3'-methacrylamidopropyl)-N,N-dimethylammonio) butane-1-sulfonate) (PSBP), showing an upper critical solution temperature, and a nonionic block, namely, poly(N-isopropylmethacrylamide) (PNIPMAM), exhibiting a lower critical solution temperature. The swelling kinetics in D2O vapor at 15 °C and the phase transition kinetics upon heating the swollen film to 60 °C and cooling back to 15 °C are followed with simultaneous time-of-flight neutron reflectometry and spectral reflectance measurements. These are complemented by Fourier transform infrared spectroscopy. The collapse temperature of PNIPMAM and the swelling temperature of PSBP are found at lower temperatures than in aqueous solution, which is attributed to the high polymer concentration in the thin-film geometry. Upon inclusion of sub-stoichiometric amounts (relative to the monomer units) of NaBr in the films, the water incorporation is significantly increased. This increase is mainly attributed to a salting-in effect on the zwitterionic PSBP block. Whereas the addition of NaBr notably shifts the swelling temperature of PSBP to lower temperatures, the collapse temperature of PNIPMAM remains unaffected by the presence of salt in the films.
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The FlexiProb project is a joint effort of three soft matter groups at the Universities of Bielefeld, Darmstadt, and Munich with scientific support from the European Spallation Source (ESS), the small-K advanced diffractometer (SKADI) beamline development group of the Jülich Centre for Neutron Science (JCNS), and the Heinz Maier-Leibnitz Zentrum (MLZ). Within this framework, a flexible and quickly interchangeable sample carrier system for small-angle neutron scattering (SANS) at the ESS was developed. In the present contribution, the development of a sample environment for the investigation of soft matter thin films with grazing-incidence small-angle neutron scattering (GISANS) is introduced. Therefore, components were assembled on an optical breadboard for the measurement of thin film samples under controlled ambient conditions, with adjustable temperature and humidity, as well as the optional in situ recording of the film thickness via spectral reflectance. Samples were placed in a 3D-printed spherical humidity metal chamber, which enabled the accurate control of experimental conditions via water-heated channels within its walls. A separately heated gas flow stream supplied an adjustable flow of dry or saturated solvent vapor. First test experiments proved the concept of the setup and respective component functionality.
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In neutron scattering on soft matter, an important concern is the control and stability of environmental conditions surrounding the sample. Complex sample environment setups are often expensive to fabricate or simply not achievable by conventional workshop manufacturing. We make use of state-of-the-art 3D metal-printing technology to realize a sample environment for large sample sizes, optimized for investigations on thin film samples with neutron reflectometry (NR) and grazing-incidence small-angle neutron scattering (GISANS). With the flexibility and freedom of design given by 3D metal-printing, a spherical chamber with fluidic channels inside its walls is printed from an AlSi10Mg powder via selective laser melting (SLM). The thin channels ensure a homogeneous heating of the sample environment from all directions and allow for quick temperature switches in well-equilibrated atmospheres. In order to optimize the channel layout, flow simulations were carried out and verified in temperature switching tests. The spherical, edgeless design aids the prevention of condensation inside the chamber in case of high humidity conditions. The large volume of the sample chamber allows for high flexibility in sample size and geometry. While a small-angle neutron scattering (SANS) measurement through the chamber walls reveals a strong isotropic scattering signal resulting from the evenly orientated granular structure introduced by SLM, a second SANS measurement through the windows shows no additional background originating from the chamber. Exemplary GISANS and NR measurements in time-of-flight mode are shown to prove that the chamber provides a stable, background free sample environment for the investigation of thin films.
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The impact of thermal history on the kinetic response of thin thermo-responsive di-block copolymer poly(diethylene glycol monomethyl ether methacrylate)-block-poly(poly(ethylene glycol) methyl ether methacrylate), abbreviated as PMEO2MA-b-POEGMA300, films is investigated by in-situ neutron reflectivity. The PMEO2MA and POEGMA300 blocks are both thermo-responsive polymers with a lower critical solution temperature. Their transition temperatures (TTs) are around 25 °C (TT1, PMEO2MA) and 60 °C (TT2, POEGMA300). Thus, by applying different temperature protocols (20 to 60 °C or 20 to 40 to 60 °C), the PMEO2MA-b-POEGMA300 thin films experience different thermal histories: The first protocol directly switches from a swollen to a collapsed state, whereas the second one switches first from a swollen to a semi-collapsed and finally to a collapsed state. Although the applied thermal histories differ, the response and final state of the collapsed films are very close to each other. After the thermal stimulus, both films present a complicated response, composed of an initial shrinkage followed by a rearrangement. Interestingly, a subsequent reswelling of the collapsed film is only observed in case of having applied the thermal stimulus of 20 to 40 °C. The normalized film thickness and the D2O amount of each layer in the PMEO2MA-b-POEGMA300 films are consistent at the end of the two different thermal stimuli. Hence, it can be concluded that the thermal history does not influence the final state of the PMEO2MA-b-POEGMA300 films upon heating. Based on this property, these thin films are especially suitable for the temperature switches on the nanoscale which may experience different thermal histories.
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A thermo-responsive random copolymer of poly (N-isopropylacrylamide-co-furfuryl methacrylate) (NF-10) has been fabricated and applied in Nylon fabrics for smart textile applications. The copolymer was synthesized by free radical copolymerization of N-isopropylacrylamide (NIPAm) and furfuryl methacrylate (FMA) using 2,2′-azobis (2-methylpropionitrile) (AIBN) initiator. The copolymer was characterized by ¹H nuclear magnetic resonance (NMR), fourier-transform infrared spectroscopy (FTIR) and gel permeation chromatography (GPC). The fabrics were dipped in a solution which NF-10 mixed with bismaleimide (BMI) to form a Diels-Alder bonded cross-linked fabric (D-A fabric) through Diels-Alder reaction, as a covalent network structure that around the fibers of the fabrics. Additionally, D-A fabric has been provided the excellent washing fastness of 86% and controlled the pore sizes by the temperature of phase change of NF-10, investigating via scanning electron microscope (SEM) and pore size analyzer (PSA). Finally, PNIPAm-based copolymer exhibits the excellent performances, promising for the smart textile applications and commercialization in the future.
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The poly(2-methyl-2-oxazoline)-b-poly[N-(2,2-difluoroethyl)acrylamide] self-assembled nanoparticles show great potential in biological applications as, e.g., ¹⁹F MRI tumour diagnostic agents. As such their internal structure needs to be thoroughly investigated while the results also help to understand temperature-dependent self-assembly behaviour of copolymers comprised of the hydrophilic and thermoresponsive blocks in general. In this paper, we elucidate internal structure of the nanoparticles formed by temperature-driven self-assembly of the abovementioned diblock copolymer by various methods. The majority of our polymers form micelle-like structures above the cloud point temperature (CPT). However, the presence of thermoresponsive homopolymer chains even in amounts below standard detection limits as well as strong hydrogen bonding causes the formation of larger aggregates interconnected by hydrophobic networks and hydrogen bonds, which contain large amounts of solvent. In the case of our particles, we also observe a strong influence of solvent on CPT, particle formation and overall sample behaviour. The ability of nanoparticles to provide usable ¹⁹F NMR signal was also confirmed.
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This feature article reviews our recent advancements on the synthesis, phase behavior, and micellar structures of diblock copolymers consisting of oppositely thermoresponsive blocks in aqueous environments. These copolymers combine a nonionic block, which shows lower critical solution temperature (LCST) behavior, with a zwitterionic block that exhibits an upper critical solution temperature (UCST). The transition temperature of the latter class of polymers is strongly controlled by its molar mass and by the salt concentration, in contrast to the rather invariant transition of nonionic polymers with type II LCST behavior such as poly(N-isopropylacrylamide) or poly(N-isopropyl methacrylamide). This allows for implementing the sequence of the UCST and LCST transitions of the polymers at will by adjusting either molecular or, alternatively, physical parameters. Depending on the location of the transition temperatures of both blocks, different switching scenarios are realized from micelles to inverse micelles, namely via the molecularly dissolved state, the aggregated state, or directly. In addition to studies of (semi)dilute aqueous solutions, highly concentrated systems have also been explored, namely water-swollen thin films. Concerning applications, we discuss the possible use of the diblock copolymers as "smart" nanocarriers.
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Polysulfobetaines in aqueous solution show upper critical solution temperature (UCST) behavior. We investigate here the representative of this class of materials, poly (N,N-dimethyl-N-(3-methacrylamidopropyl) ammonio propane sulfonate) (PSPP), with respect to: (i) the dynamics in aqueous solution above the cloud point as function of NaBr concentration; and (ii) the swelling behavior of thin films in water vapor as function of the initial film thickness. For PSPP solutions with a concentration of 5 wt.%, the temperature dependence of the intensity autocorrelation functions is measured with dynamic light scattering as function of molar mass and NaBr concentration (0–8 mM). We found a scaling of behavior for the scattered intensity and dynamic correlation length. The resulting spinodal temperatures showed a maximum at a certain (small) NaBr concentration, which is similar to the behavior of the cloud points measured previously by turbidimetry. The critical exponent of susceptibility depends on NaBr concentration, with a minimum value where the spinodal temperature is maximum and a trend towards the mean-field value of unity with increasing NaBr concentration. In contrast, the critical exponent of the correlation length does not depend on NaBr concentration and is lower than the value of 0.5 predicted by mean-field theory. For PSPP thin films, the swelling behavior was found to depend on film thickness. A film thickness of about 100 nm turns out to be the optimum thickness needed to obtain fast hydration with H2O.
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Thermo-responsive polymers undergo a reversible coil-to-globule transition in water after which the chains collapse and aggregate into bigger globules when passing to above its lower critical solution temperature (LCST). The hydrogen bonding with the amide groups in the side chains has to be contrasted with the hydration interaction of the hydrophobic main-chain hydrocarbons. In the present investigation we study molecular changes in the polymer poly(N-isopropyl acrylamide) (PNIPAM) and in its monomer N-isopropyl acrylamide (NIPAM) in solution across the LCST transition. Employing Fourier-transform infrared spectroscopy we probe changes in conformation and hydrogen bonding. We observe a nearly discontinuous shift of the peak frequencies and areas of vibrational bands across the LCST transition for PNIPAM whereas NIPAM exhibits a continuous linear change with temperature. This supports the crucial role of the polymer backbone with respect to hydration changes in the amide group in combination with cooperative interactions of bound water along the backbone chain.
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Untethered microtools that can be precisely navigated into deep in vivo locations are important for clinical procedures pertinent to minimally invasive surgery and targeted drug delivery. In this mini-review, untethered soft grippers are discussed, with an emphasis on a class of autonomous stimuli-responsive gripping soft tools that can be used to excise tissues and release drugs in a controlled manner. The grippers are composed of polymers and hydrogels and are thus compliant to soft tissues. They can be navigated using magnetic fields and controlled by robotic path-planning strategies to carry out tasks like pick-and-place of microspheres and biological materials either with user assistance, or in a fully autonomous manner. It is envisioned that the use of these untethered soft grippers will translate from laboratory experiments to clinical scenarios and the challenges that need to be overcome to make this transition are discussed.
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The rehydration of thermoresponsive polystyrene-block-poly(methoxy diethylene glycol acrylate)-block-polystyrene (PS-b-PMDEGA-b-PS) films forming a lamellar microphase-separated structure is investigated by in situ neutron reflectivity in a D2O vapor atmosphere. The rehydration of collapsed PS-b-PMDEGA-b-PS films is realized by a temperature change from 45 to 23 °C and comprises (1) condensation and absorption of D2O, (2) evaporation of D2O, and (3) reswelling of the film due to internal rearrangement. The hydrophobic PS layers hinder the absorption of condensed D2O, and a redistribution of embedded D2O between the hydrophobic PS layers and the hydrophilic PMDEGA layers is observed. In contrast, the rehydration of semiswollen PS-b-PMDEGA-b-PS films (temperature change from 35 to 23 °C) shows two prominent differences: A thicker D2O layer condenses on the surface, causing a more enhanced evaporation of D2O. The rehydrated films differ in film thickness and volume fraction of D2O, which is due to the different thermal protocols, although the final temperature is identical.
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The complex interaction of polyelectrolyte multilayers (PEMs) physisorbed onto end-grafted polymer brushes with focus on the temperature-responsive behavior of the system is addressed in this work. The investigated brush/multilayer composite consists of a poly(styrene sulfonate)/poly(diallyldimethylammonium chloride) (PSS/PDADMAC) multilayer deposited onto the poly(N-isopropylacrylamide-b-dimethylaminoethyl methacrylate) P(NIPAM-b-DMAEMA) brush. Ellipsometry and neutron reflectometry were used to monitor the brush collapse with the thickness decrease as a function of temperature and the change in the monomer distribution perpendicular to the substrate at temperatures below, across and above the phase transition, respectively. It was found that the adsorption of PEMs onto polymer brushes had a hydrophobization effect on PDMAEMA, inducing the shift of its phase transition to lower temperatures, but without suppressing its temperature-responsiveness. Moreover, the diffusion of the free polyelectrolyte chains inside the charged brush was proved by comparing the neutron scattering length density profile of pure and the corresponding PEM-capped brushes, eased by the enhanced contrast between hydrogenated brushes and deuterated PSS chains. The results presented herein demonstrate the possibility of combining a temperature-responsive brush with polyelectrolyte multilayers without quenching the responsive behavior, even though significant interpolyelectrolyte interactions are present. This is of importance for the design of multicompartment coatings, where the brush can be used as a reservoir for the controlled release of substances and the multilayer on the top as a membrane to control the diffusion in/out by applying different stimuli.
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We investigate the phase behaviour of aqueous dispersions of poly-N-isopropyl acrylamide (PNiPAM) microgels above their lower critical solution temperature (LCST) and find that beyond a well-defined concentration the systems exhibit a peculiar behaviour: the microgels assemble into space-spanning gels that shrink in time while maintaining the shape of the container in which they have been formed. Over a wide range of concentrations this shrinking behaviour is independent of PNiPAM concentration, but systematically depends on temperature in a temperature range significantly exceeding the LCST. The overall shrinking characteristics are consistent with those expected for scaffolds made of materials that exhibit thermal contraction. However, for the PNiPAM assemblies contraction is irreversible and can be as large as 90%. Such characteristics disclose complex interactions between fully collapsed PNiPAM and water well beyond the LCST, the origin of which has yet to be elucidated.
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The structures and synthesis of polyzwitterions ("polybetaines") are reviewed, emphasizing the literature of the past decade. Particular attention is given to the general challenges faced, and to successful strategies to obtain polymers with a true balance of permanent cationic and anionic groups, thus resulting in an overall zero charge. Also, the progress due to applying new methodologies from general polymer synthesis, such as controlled polymerization methods or the use of "click" chemical reactions is presented. Furthermore, the emerging topic of responsive ("smart") polyzwitterions is addressed. The considerations and critical discussions are illustrated by typical examples.
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Poly(N-isopropylacrylamide) (PNIPAM) brushes integrating poly[oligo(ethylene glycol) methacrylate] (POEGMA) as the core were prepared via successive atom transfer radical polymerization (ATRP). The dynamic thermal phase transition behavior of PNIPAM brushes was studied by means of IR spectroscopy in combination with the perturbation correlation moving window (PCMW) technique and two-dimensional correlation spectroscopy (2Dcos) analysis. Compared with aqueous dispersions of PNIPAM brushes covalently bound to the surface of gold nanoparticles and hydrophobic hyperbranched polyester core, an increase in the double phase transition temperature was observed due to the existence of the POEGMA core, which was favourable to the hydrophilic condition. With PCMW analysis, the phase transition temperature (ca. 36 °C) as well as the transition temperature range (33-41 °C) during the heating process were determined. 2Dcos was employed to discern the sequence order of group motions during heating. It is concluded that the PNIPAM in the inner zone responds earlier than that located in the outer zone.
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The switching kinetics of thin thermo-responsive hydrogel films of poly(monomethoxy-diethyleneglycol-acrylate) (PMDEGA) are investigated. Homogeneous and smooth PMDEGA films with a thickness of 35.9 nm are prepared on silicon substrates by spin coating. As probed with white light interferometry, PMDEGA films with a thickness of 35.9 nm exhibit a phase transition temperature of the lower critical solution temperature (LCST) type of 40 degrees C. In situ neutron reflectivity is performed to investigate the thermo-responsive behavior of these PMDEGA hydrogel films in response to a sudden thermal stimulus in deuterated water vapor atmosphere. The collapse transition proceeds in a complex way which can be seen as three steps. The first step is the shrinkage of the initially swollen film by a release of water. In the second step the thickness remains constant with water molecules embedded in the film. In the third step, perhaps due to a conformational rearrangement of the collapsed PMDEGA chains, water is reabsorbed from the vapor atmosphere, thereby giving rise to a relaxation process. Both the shrinkage and relaxation processes can be described by a simple model of hydrogel deswelling.
Article
The effect of chain architecture on the swelling and thermal response of thin films from an amphiphilic three-arm star-shaped thermo-responsive block copolymer poly(methoxy diethylene glycol acrylate)-block-polystyrene ((PMDEGA-b-PS)3) is investigated by in situ neutron reflectivity (NR) measurements. The PMDEGA and PS blocks are micro-phase separated with randomly distributed PS nanodomains. The (PMDEGA-b-PS)3 films show a transition temperature (TT) at 33 °C in white light interferometry. The swelling capability of the (PMDEGA-b-PS)3 films in D2O vapor atmosphere is better than the one of films from linear PS-b-PMDEGA-b-PS triblock copolymers, which can be attributed to the hydrophilic end groups and limited size of the PS blocks in (PMDEGA-b-PS)3. However, the swelling kinetics of as-prepared (PMDEGA-b-PS)3 films and the response of the swollen film to a temperature change above TT are significantly slower than in PS-b-PMDEGA-b-PS films, which may be related to the conformation restriction by the star-shape. Unlike in the PS-b-PMDEGA-b-PS films, the amount of residual D2O in the collapsed (PMDEGA-b-PS)3 films depends on the final temperature. It decreases from (9.7 ± 0.3) % to (7.0 ± 0.3) % or (6.0 ± 0.3) % when the final temperatures are set to 35 °C, 45 °C and 50 °C, respectively. This temperature-dependent reduction of embedded D2O originates from the hindrance of chain conformation from the star-shaped chain architecture.
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Polymer electrodes made of poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) are used in many applications but are also sensitive to humidity. We study humidity induced changes of PEDOT:PSS electrodes as monitored with in situ time-of-flight neutron reflectivity (TOF-NR) measurements under high humidity conditions. The influence of the solvent additive Zonyl and a post-treatment of PEDOT:PSS films with ethylene glycol (EG) serving as electrodes are analyzed with respect to the swelling ratio and water uptake. Depending on the applied PEDOT:PSS treatment PEDOT and PSS enrichment layers are clearly identified with TOF-NR at the substrate-polymer and polymer-air interface, respectively. The additive Zonyl reduces the water uptake and limits film swelling. EG post-treatment further increases hydrophobicity and thereby water incorporation into the PEDOT:PSS film is stronger suppressed. The characteristic time constants and effective interaction parameters extracted from the kinetic NR data show that additive and prost-treatment reduce the sensitivity of the PEDOT:PSS electrodes to humidity.
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The area of artificial muscle is a highly interdisciplinary field of research that has evolved rapidly in the last 30 years. Recent advances in nanomaterial fabrication and characterization, specifically carbon nanotubes and nanowires, have had major contributions in the development of artificial muscles. However, what can artificial muscles really do for humans? This question is considered here by first examining nature's solutions to this design problem and then discussing the structure, actuation mechanism, applications, and limitations of recently developed artificial muscles, including highly oriented semicrystalline polymer fibers; nanocomposite actuators; twisted nanofiber yarns; thermally activated shape-memory alloys; ionic-polymer/metal composites; dielectric-elastomer actuators; conducting polymers; stimuli-responsive gels; piezoelectric, electrostrictive, magnetostrictive, and photostrictive actuators; photoexcited actuators; electrostatic actuators; and pneumatic actuators.
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The isothermal vacuum-induced dehydration of thin films made of poly(methoxy diethylene glycol acrylate) (PMDEGA), which were swollen under ambient conditions, is studied. The dehydration behavior of the homopolymer film as well as of a nanostructured film of the amphiphilic triblock copolymer polystyrene-block-poly(methoxy diethylene glycol acrylate)-block-polystyrene, abbreviated as PS-b-PMDEGA-b-PS are probed, and compared to the thermally induced dehydration behavior of such thin thermo-responsive films when they pass through their LCST-type coil-to globule collapse transition. The dehydration kinetics is followed by in-situ neutron reflectivity measurements. Contrast results from the use of deuterated water. Water content and film thickness are significantly reduced during the process, which can be explained by Schott second order kinetics theory for both films. The water content of the dehydrated equilibrium state from this model is very close to the residual water content obtained from the final static measurements, indicating that residual water still remains in the film even after prolonged exposure to the vacuum. In the PS-b-PMDEGA-b-PS film that shows micro-phase separation, the hydrophobic PS domains modify the dehydration process by hindering the water removal, and thus retarding dehydration by about 30%. Whereas residual water remains tightly bound in the PMDEGA domains, water is completely removed from the PS domains of the block copolymer film.
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The 2-fold thermoresponsive diblock copolymer PSPP498-b-PNIPMAM144, which consists of a zwitterionic polysulfobetaine (PSPP) block and a nonionic poly(N-isopropylmethacrylamide) (PNIPMAM) block, is prepared by consecutive RAFT polymerizations. It combines the upper and lower critical solution temperature (UCST and LCST) behaviors, respectively, of the constitutive homopolymers in aqueous solution. We investigate the temperature-dependent phase behavior and the self-assembled structures of the block copolymer in D2O by turbidimetry and by small-angle neutron scattering (SANS) in salt-free solution and in the presence of small amounts of NaCl and NaBr. For comparison, solutions of PNIPMAM homopolymer in D2O are studied as well. Turbidimetry indicates thermally induced “schizophrenic” aggregation behavior for PSPP498-b-PNIPMAM144. SANS reveals that conventional star-like core–shell micellar structures are formed above the LCST transition, whereas below the UCST-transition, structure formation is much less pronounced. This is attributed to the different types of interactions, namely hydrophobic and ionic ones, dominating in the different regimes. Despite the increased polarity contrast between the zwitterionic and the nonionic blocks, and the much wider separation of the UCST- and LCST-based cloud points, CPUCST and CPLCST, the structural features of the new PSPP498-b-PNIPMAM144 resemble the ones found previously for the also 2-fold thermoresponsive analogue PSPP432-b-PNIPAM200, for which both phase transition temperatures nearly coincide. Remarkably, the addition of small amounts of NaBr or NaCl to the solution of PSPP498-b-PNIPMAM144 causes a significant increase of CPUCST, as well as minor but notable changes in the self–assembled structures, but no gross alterations of the phase behavior.
Article
A 2-fold thermoresponsive diblock copolymer PSPP430-b-PNIPAM200 consisting of a zwitterionic polysulfobetaine (PSPP) block and a nonionic poly(N-isopropylacrylamide) (PNIPAM) block is prepared by successive RAFT polymerizations. In aqueous solution, the corresponding homopolymers PSPP and PNIPAM feature both upper and lower critical solution temperature (UCST and LCST) behavior, respectively. The diblock copolymer exhibits thermally induced “schizophrenic” aggregation behavior in aqueous solutions. Moreover, the ion sensitivity of the cloud point of the zwitterionic PSPP block to both the ionic strength and the nature of the salt offers the possibility to create switchable systems which respond sensitively to changes of the temperature and of the electrolyte type and concentration. The diblock copolymer solutions in D2O are investigated by means of turbidimetry and small-angle neutron scattering (SANS) with respect to the phase behavior and the self-assembled structures in dependence on temperature and el...
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Nonvolatile, soft photonic films that reflect UV/vis light were prepared by enthalpy-driven swelling of lamellar-forming polystyrene-b-poly(2-vinylpyridine) (PS–P2VP) block copolymer thin films with a neat protic solvent. These films are very sensitive to further swelling with the addition of a small amount of acid. Transmission electron microscopy and ultrasmall-angle X-ray scattering of the films before and after the addition of the neat protic solvent revealed selective swelling of the P2VP phase while maintaining the lamellar morphology due to the presence of the layered glassy PS domains. P2VP is swollen due to the hydrogen bonding between a hydroxy group of a protic solvent and the pyridyl group of P2VP. The interdomain distance of the neat PS–P2VP film as measured by U-SAXS increased by about 200% and the films acted as a 1D photonic crystal reflecting UV light. Moreover, by exposing the neat PS–P2VP films to a mixture of a protic solvent and a small amount of sulfonic acid that can protonate the pyridyl groups of the P2VP block, the degree of swelling, therefore the interdomain distance and the wavelength of the reflection light, became significantly larger, resulting in color variations across the visible spectrum and suggesting that such a nonvolatile material system can be a sensor of the acid concentration in the millimolar regime for anhydrous solutions.
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The self-assembly of block copolymers to generate nanopatterns is of great interest as an inexpensive approach to sub-20 nm lithography. Compared to thermal annealing, solvent vapor annealing has several intriguing advantages with respect to the annealing of thin films of block copolymers, particularly for polymers with high interaction parameters, χ, and high molecular weights. In this methods paper, we describe a controlled solvent vapor flow annealing system with integrated in-situ microscopy and laser reflectometry, as well as a feedback loop that automatically controls the solvent vapor flow rate, based upon real-time calculations of the difference between thickness set point and the observed film thickness. The feedback loop enables precise control of swelling and deswelling of the polymer thin film, the degree of swelling at the dwell period, and preprogrammed complex multi-step annealing profiles. The in-situ microscope provides critical insight into the microscopic morphological evolution of the block copolymer thin films during annealing process. This device could be a powerful tool for understanding and optimizing solvent annealing by providing multiple sources of in-situ information, at both the micro- and nanoscale.
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Block-copolymers self-assemble into diverse morphologies, where nanoscale order can be finely tuned via block architecture and processing conditions. However, the ultimate usage of these materials in real-world applications may be hampered by the extremely long thermal annealing times-hours or days-required to achieve good order. Here, we provide an overview of the fundamentals of block-copolymer self-assembly kinetics, and review the techniques that have been demonstrated to influence, and enhance, these ordering kinetics. We discuss the inherent tradeoffs between oven annealing, solvent annealing, microwave annealing, zone annealing, and other directed self-assembly methods; including an assessment of spatial and temporal characteristics. We also review both real-space and reciprocal-space analysis techniques for quantifying order in these systems.
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In 1968, Heskins and Guillet published the first systematic study of the phase diagram of poly(N-isopropylacrylamide) (PNIPAM), at the time a "young polymer" first synthesized in 1956. Since then, PNIPAM became the leading member of the growing families of thermoresponsive polymers and of stimuli-responsive, "smart" polymers in general. Its thermal response is unanimously attributed to its phase behavior. Yet, in spite of 50 years of research, a coherent quantitative picture remains elusive. In this Review we survey the reported phase diagrams, discuss the differences and comment on theoretical ideas regarding their possible origins. We aim to alert the PNIPAM community to open questions in this reputably mature domain.
Article
Polymer topology and reduced dimensions can have a significant impact on the properties and phase transitions of polymeric films with thicknesses below a few hundred nanometers. We study the impact of these effects in the case of thin films of cyclic and linear poly(N-isopropylacrylamide) (PNIPAM) exposed to water vapor. Specifically, we monitor the swelling kinetics of the thin PNIPAM films, their interfacial interactions, and their LCST-type demixing phase transition, using white light interferometry and X-ray reflectivity. As the film thickness decreases, the swelling ratio increases, presumably due to the increasingly dominant effect of polymer/substrate interactions. The time constants of the swelling process depend on both the film thickness and the PNIPAM topology. Consistent with earlier observations for PNIPAM solutions, in thin swollen films of comparable concentration, cyclic PNIPAM exhibits a broader and thus less cooperative demixing transition than the linear PNIPAM counterpart.
Article
A series of zwitterionic model polymers with defined molar masses up to 150,000 Da and defined end groups are prepared from sulfobetaine monomer N,N-dimethyl-N-(3-(methacrylamido)propyl)ammoniopropanesulfonate (SPP). Polymers are synthesized by reversible addition-fragmentation chain transfer polymerization (RAFT) using a functional chain transfer agent labeled with a fluorescent probe. Their upper critical solution temperature-type coil-to-globule phase transition in water, deuterated water, and various salt solutions is studied by turbidimetry. Cloud points increase with polyzwitterion concentration and molar mass, being considerably higher in D2O than in H2O. Moreover, cloud points are strongly affected by the amount and nature of added salts. Typically, they increase with increasing salt concentration up to a maximum value, whereas further addition of salt lowers the cloud points again, mostly down to below freezing point. The different salting-in and salting-out effects of the studied anions can be correlated with the Hofmeister series. In physiological sodium chloride solution and in phosphate buffered saline (PBS), the cloud point is suppressed even for high molar mass samples. Accordingly, SPP-polymers behave strongly hydrophilic under most conditions encountered in biomedical applications. However, the direct transfer of results from model studies in D2O, using, e.g. (1)H NMR or neutron scattering techniques, to 'normal' systems in H2O is not obvious.
Article
This Perspective provides a critical analysis of the current knowledge concerning solvent vapor annealing (SVA) of block polymer thin films. Herein, we identify key challenges that will be important to overcome for future development of SVA as a practical, reliable, and universal technique for the valorization of block polymer thin films in a wide range of technologies. The Perspective includes a brief background on thin film block polymer self-assembly, a historical account of the SVA technique, an overview of the SVA fundamentals that are necessary to develop a more comprehensive picture of the overall process, and summaries of relevant and important contributions from the recent literature. We also offer our outlook on SVA and suggest important future directions.
Article
In situ time-resolved spectroscopic ellipsometry is used to study the dynamics of n-hexane diffusion into, and the corresponding induced swelling of, ultra-thin polystyrene films. The experimental conditions are carefully selected to facilitate the observation of anomalous Case II diffusion in the system, thereby allowing the probing of the chain-relaxation dynamics of a sharp moving penetrant front within the films. It has been found that the two different approaches to the obtained data are in quantitative agreement and suggest the existence of a finite thickness region of 14 +/- 3 nm in the outer film interface that is instantly swollen after contact with the penetrant. The thickness of this fast swelling layer is found to be independent of swelling temperature and physical aging time. After the interface is swollen, the diffusion front velocity shows no significant spatial variations in the direction perpendicular to the substrate, but is strongly dependent on temperature and sample aging history.
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To study the thickness dependence of methanol diffusion into poly(methyl methacrylate) (PMMA) films, an experimental method based on optical reflectivity was proposed. The method can cover a thickness range from the micrometre down to the nanometre scale using a single set-up. Methanol molecules diffuse into PMMA following a Case II mechanism in which they induce segmental motion of the matrix polymer. We observed two diffusion modes, fast and slow, assignable respectively to the outermost layer and the remaining internal region including the substrate interface. When the film becomes thinner, the ratio of the interfacial area to the total volume increases. This results in slowing down of the methanol diffusion due to the contribution of the depressed segmental mobility at the substrate interface. Interestingly, the thickness of the mobility-enhanced layer at the outermost region of the film similarly decreased with decreasing total thickness owing to the effect of the substrate interface.
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In this brief review we address a range of interesting applications and prospects of responsive hydrogel thin films for the fabrication of “smart” responsive surfaces, membranes, sensors with various transduction mechanisms, micro/nanoactuators, and capsules. We show that hydrogel thin films compete with grafted polymers and demonstrate strong advantages for the fabrication of robust multifunctional and multiresponsive surfaces. This article reviews recent publications on the synthesis of responsive hydrogel thin films and hybrid films with entrapped nanoparticles and reagents by the chemical crosslinking of reactive polymers, layer-by-layer deposition, and block-copolymerself-assembly, as well as examining those publications to determine a range of applications.
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In this work, from the discussion on water structure and clusters, it can be deduced that the OH stretching vibration is closely related to local hydrogen-bonded network for a water molecule, and different OH vibrations can be assigned to OH groups engaged in various hydrogen bonding. At ambient condition, the main local hydrogen bonding for a molecule can be classified as DDAA (double donor–double acceptor), DDA (double donor–single acceptor), DAA (single donor–double acceptor) and DA (single donor–single acceptor) and free OH vibrations. As for water at 290K and 0.1MPa pressure, the OH stretching region of the Raman spectrum can be deconvoluted into five sub-bands, which are located at 3014, 3226, 3432, 3572, and 3636cm−1, and can be assigned to νDAA-OH, νDDAA-OH, νDA-OH, νDDA-OH, and free OH2 symmetric stretching vibrations, respectively.
Article
ATR FTIR and Raman spectra of polymers containing amide groups in the main chain and in the side chain and of the amide low-molecular-weight model compounds in water media were measured. The hydrophobic and hydrophilic interactions of the dissolved compounds with the neighboring water molecules are reflected in the wavenumbers of the CH3 stretching and of the Amide I and II vibrations. The possibility of the existence of β-sheet-like structures in polypeptides surrounded by water molecules is also discussed.
Article
A series of new hydrolytically stable poly(carbobetaine)s was synthesised varying the length and the position of the hydrophobic side chains, together with their cationic analogues. The synthetic strategy avoids the presence of residual salt in the polybetaines, thus making them well suited for model studies. The betaine monomers as well as their cationic analogues bearing the equivalent of an undecyl chain exhibit surfactant properties (“surfmers”). The corresponding polymers, however, which are formed by cyclopolymerisation, are insoluble in water, though hygroscopic. In contrast, the hydrophobic chains favour the solubility in alcohols and allow the formation of superstructures in bulk. The poly(carbobetaine)s form homogeneous blends with selected inorganic salts, to provide organic–inorganic hybrid materials.
Article
A series of new monomeric and polymeric carbobetaines based on acrylamides has been synthesized and characterized. Due to long hydrocarbon substituents, the compounds have chemical structures of surfactants and polysoaps, respectively. Results are compared with those of analogous poly(sulfobetaine)s. The poly(carbobetaine)s are more hygroscopic and show improved solubility. Viscometric studies in ethanol show no, or only weak, polyelectrolyte behaviour. Thermal stability is decreased, and glass transitions occur at lower temperatures. X-ray diffractograms indicate the presence of superstructures whose detailed forms depend on the polymers' geometry.
Article
White light interferometry was used to determine the swelling of poly(3-alkylthiophene)s (P3ATs), with different head-to-tail regioregularity, exposed to different volatile compounds (VCs): humidity, cyclohexanone, tetrahydrofuran and chloroform. Film expansion increases always from regioregular R-P3DDT, R-P3HT to regiorandom P3BT, P3DDT. Regular solution approach was used to determine Flory–Huggins interaction parameter between P3ATs and VCs. Solubility parameters δP∼16.9±0.8, 15.3±0.4, 13.1±0.3, and 12.5±0.2MPa1/2 evaluated for P3DDT, P3BT, R-P3HT, and R-P3DDT, respectively, were confirmed by the values extrapolated from surface tension data. Evaluated δP values should allow an easier strategy for P3AT solution processing aimed at optimized film structure of P3AT polymers and their blends.
Article
The contrast-variation technique is employed in multiple-contrast neutron/X-ray reflectometry experiments to highlight scattering from different structural components that are present at a surface or interface. The advantage of this technique is that the structural model used to describe the interfacial scattering length density profile must apply to all the contrasts measured. A new reflectivity analysis package, MOTOFIT, which runs in the IGOR Pro environment (http://www.wavemetrics.com), has been created to aid the simultaneous fitting (with the same structural model) of these multiple-contrast data, using an intuitive graphical user interface, which most co-refinement packages do not possess. MOTOFIT uses a slab-model approach with the Abeles matrix method, and extensions for surface roughness to perform non-linear least-squares regression on the experimental reflectivity curves. Other features, such as the ability to create complicated interparameter constraints or analyse reflectivity from multilayers, simulated annealing, etc., make MOTOFIT a powerful reflectometry analysis package
Article
Skin hydration plays an important role in the optimal physical properties and physiological functions of the skin. Despite the advancements in the last decade, dry skin remains the most common characteristic of human skin disorders. Thus, it is important to understand the effect of hydration on Stratum Corneum (SC) components. In this respect, our interest consists in correlating the variations of unbound and bound water content in the SC with structural and organizational changes in lipids and proteins using a non-invasive technique: Raman spectroscopy. Raman spectra were acquired on human SC at different relative humidity (RH) levels (4-75%). The content of different types of water, bound and free, was measured using the second derivative and curve fitting of the Raman bands in the range of 3100-3700 cm(-1). Changes in lipidic order were evaluated using νC-C and νC-H. To analyze the effect of RH on the protein structure, we examined in the Amide I region, the Fermi doublet of tyrosine, and the νasymCH3 vibration. The contributions of totally bound water were found not to vary with humidity, while partially bound water varied with three different rates. Unbound water increased greatly when all sites for bound water were saturated. Lipid organization as well as protein deployment was found to be optimal at intermediate RH values (around 60%), which correspond to the maximum of SC water binding capacity. This analysis highlights the relationship between bound water, the SC barrier state and the protein structure and elucidates the optimal conditions. Moreover, our results showed that increased content of unbound water in the SC induces disorder in the structures of lipids and proteins.
Article
Interest in thermoresponsive polymers has steadily grown over many decades, and a great deal of work has been dedicated to developing temperature sensitive macromolecules that can be crafted into new smart materials. However, the overwhelming majority of previously reported temperature-responsive polymers are based on poly(N-isopropylacrylamide) (PNIPAM), despite the fact that a wide range of other thermoresponsive polymers have demonstrated similar promise for the preparation of adaptive materials. Herein, we aim to highlight recent results that involve thermoresponsive systems that have not yet been as fully considered. Many of these (co)polymers represent clear opportunities for advancements in emerging biomedical and materials fields due to their increased biocompatibility and tuneable response. By highlighting recent examples of newly developed thermoresponsive polymer systems, we hope to promote the development of new generations of smart materials.
Article
The kinetics of swelling and shrinking of gels is theoretically generalized and experimentally studied. A new relation, in addition to the differential equation developed by Tanaka and Fillmore, is formulated to solve the kinetics of gels having arbitrary shape. Using our new theory, we provide explicit solutions for long cylinder and large disk gels. These solutions predict that the effective diffusion constants of long cylinder and large disk gels are 1.5 and 3 times smaller than that of a spherical gel. The relaxation times of long cylinder and large disk gels with small shear modulus μ are approximately 2.0 and 5.7 times longer than that of a sphere, where the diameters of the cylinder and the sphere and the thickness of the disk are the same. The theory also concludes that the experimentally measured kinetics of a long cylinder and disk gels along the z axis and the radial axis are the same. The results have been excellently confirmed by the experiments.
Article
Temperature dependent phase behavior of poly(N-isopropylacylamide) (PNIPAM) microgels in water/methanol mixtures of different composition was studied with dynamic light scattering (DLS) and small-angle neutron scattering (SANS). Using DLS, it is possible to measure the diffusion coefficient, and thus the size of particles exactly and directly; the variation of the phase transition temperature in the different solvents is also easy to detect by this method. With SANS measurements in D2O/MeOD mixtures, some of the DLS results were confirmed. Moreover, SANS measurements give valuable information on the particle structure in different solvents. The experiments were compared with the theory of competitive hydration introduced by Tanaka et al. We found a good agreement of theory and experiment, and obtained the theoretical predictions: around the transition temperature, the composition of the bound methanol along the chains is higher than that of the outer solution, while the whole methanol composition inside the gel is lower. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym. Phys. 2013, 51, 1100–1111
Article
The application of thermal neutron scattering to the study of the structure and dynamics of condensed matter requires a knowledge of the scattering lengths and the corresponding scattering and absorption cross sections of the elements. Ln some cases, values for the individual isotopes are needed as well. This information is required to obtain an absolute normalization ofthe scatteredneutron distributions, tocalculate unit-cell structure factors in neutron crystallography, and to correct for effects such as absorption, self-shielding, extinction, multiple scattering, incoherent scattering, and detector efficiency.
Article
We investigate the effect of block copolymer (BCP) molecular weight and connectivity on the morphology and time–temperature dependent swelling of thin film hydrogels created through layer-by-layer (LbL) assembly of BCP micelles with poly(acrylic acid). BCPs of poly(N,N-dimethylaminoethyl methacrylate) (D) and poly(propylene oxide) (P), a P–D diblock, a long D–P–D triblock, and a short D–P–D triblock copolymer, were compared in terms of their temperature response in solution and within micelle–polyelectrolyte multilayers (mPEMs). The critical micellization concentration and micellization temperature of the BCPs in solution, as well as the swelling transition temperature, Tstt, of the mPEMs, decreased with increasing P block length. AFM imaging of dry mPEMs shows regular dimpled surface structures that arise from surface relaxation of micelles. When the mPEMs are cooled below Tstt in water, the thin 200 nm films can swell reversibly between 3 and 6 times their dry thicknesses within 2 min. The degree of swelling (τ = wet thickness/dry thickness) increases with undercooling (ΔT = Tstt – T) and shows time dependencies related to ΔT and the constituent BCP connectivity. While the diblock films swell uncontrollably and lose integrity within 30 min at ΔT ≥ 6 °C, the triblock copolymer multilayers are able to sustain steady τ values (in the range of 4–10) under equivalent conditions. The differences in dynamic swelling behavior originating from BCP architecture have important implications in their utility as temperature responsive surfaces.
Article
We report on the fabrication of multifunctional ratiometric probes for glucose and temperatures based on thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) microgels covalently incorporated with glucose-recognizing moieties, N-acryloyl-3-aminophenylboronic acid (APBA), fluorescence resonance energy transfer (FRET) donor dyes, 4-(2-acryloyloxyethylamino)-7-nitro-2,1,3-benzoxadiazole (NBDAE), and rhodamine B-based FRET acceptors (RhBEA). P(NIPAM-APBA-NBDAE-RhBEA) microgels containing FRET pairs and APBA were synthesized via free radical emulsion copolymerization. The spatial proximity of FRET donors and acceptors within microgels can be tuned via thermo-induced microgel collapse or glucose-induced microgel swelling at appropriate pH and temperatures, leading to the facile modulation of FRET efficiencies. APBA moieties within P(NIPAM-APBA-NBDAE-RhBEA) microgels can bind with glucose at appropriate pH to form cyclic boronate moieties, which can decrease the pKa of APBA residues and increase the volume phase transition (VPT) temperature of microgels. The gradual addition of glucose into fluorescent microgel dispersions at intermediate temperatures, i.e., between microgel VPT temperatures in the absence and presence of glucose, respectively, can lead to the reswelling of initially collapsed microgels. Thus, P(NIPAM-APBA-NBDAE-RhBEA) microgels can serve as dual ratiometric fluorescent probes for glucose and temperatures by monitoring the changes in fluorescence emission intensity ratios. Moreover, P(NIPAM-APBA-NBDAE-RhBEA) microgels at pH 8 and 37 °C can serve as a ratiometric fluorescent glucose sensor with improved detection sensitivity as compared to that at 25 °C. MTT assays further revealed that thermoresponsive microgels are almost noncytotoxic up to a concentration of 1.6 g/L. These results augur well for the application of P(NIPAM-APBA-NBDAE-RhBEA) microgels for multifunctional purposes such as sensing, imaging, and triggered-release nanocarriers under in vivo conditions.
Article
The past two decades have evidenced a tremendous growth in the field of responsive polymers, which can exhibit reversible or irreversible changes in physical properties and/or chemical structures to an external stimulus such as pH, temperature, ionic strength, light irradiation, mechanical forces, electric and magnetic fields, specific analytes, external additives (ions, bioactive molecules, etc.), or a combination of them. Responsive polymers can exist in the form of solutions, gels, self-assembled nanoparticles, (multilayer) films, and bulk solids. The field of responsive polymers has nowadays evolved well beyond the demonstration of novel and interesting properties. Currently, the exploitation of useful and advanced functions, e.g., drug or gene carriers with triggered release properties, catalysis, detection and imaging, environmentally adaptive coatings, and self-healing materials, has emerged to be a more relevant subject. In this Perspective, we focus on recent developments of responsive polymer-based chemo- and biosensors, highlighting this concept with selected literature reports. Such functional polymeric materials show prominent advantages such as tunable detection sensitivity, structural stability, aqueous dispersibility, biocompatibility, processability, and facile integration into detection devices, as compared to their small molecule analogues.
Article
Various polycarboxybetaines were synthesized using controlled radical polymerization processes followed by polymer analogous transformations. Structural variation of these exclusively aromatic or heteroaromatic systems includes distance between charges, hybridization and substitution of the ammonium moiety, and additional alkyl chains between the charges. For the first time capillary electrophoresis was used for systematic characterization of the solution properties. It was found to be a very sensitive method to detect even small structural differences and the resulting pH-dependent solution behavior. The electrophoretical mobility compared with an internal standard and the amount of its pH dependence are direct measures for the interaction of charges of polycarboxybetaines in solution. This method clearly reveals a gradual and more or less predictable content of “free” cationic charges dependent on structural and solution parameters. Results obtained coincide in a fairly good way with data from FTIR measurement and charge titration.
Article
We report the first application of pressure perturbation calorimetry (PPC) to determine the hydration properties of poly(N-isopropylacrylamide) (PNIPAM) in H2O and in D2O as the solutions undergo a temperature-induced phase transition. The technique, which measures the heat change resulting from a pressure change above a solution of PNIPAM placed in a microcalorimeter cell, yields the temperature dependence of the coefficient of thermal expansion, αp, of the polymer in solution and the change in volume of the solvation layer around the polymer chain. In the temperature ranges below and above the phase transition, αp of PNIPAM in H2O increased linearly with temperature. It underwent a sharp increase at the transition temperature, Tmax, then rapidly decreased. The phase transition was accompanied by an increase in the partial specific volume of the hydrated polymer. This increase was significantly higher for solutions of PNIPAM in D2O, compared to H2O. A study by PPC of the phase transition of hydrophobically modified PNIPAM samples that undergo micellization in water demonstrated that the hydration of the polymeric micelles varies significantly as a function of the degree of hydrophobic substitution and length of the alkyl group linked to the polymer.
Article
Thermal properties of the novel, double thermosensitive block copolymer, poly(N-isopropyl acrylamide)-block-poly(3-[N-(3-methacrylamido-propyl)-N,N-dimethyl]-ammonio propane sulfonate) (PNIPA-b-PSPP) have been studied in pure and saline (NaCl) aqueous solutions by dynamic laser light scattering. The block copolymer [Mn(PNIPA) = 10 800 g/mol and Mn(PSPP) = 9700 g/mol] exhibits both upper (UCST about 9 °C) and lower (LCST about 32 °C) critical solution temperatures in pure water. The addition of NaCl enhances the solubility of the zwitterionic block, PSPP, leading to the disappearance of the UCST. On the other hand, the solubility of PNIPA in water decreases as NaCl is added. At 20 °C, the copolymer shows a bimodal size distribution through the NaCl concentration range of 0−0.93 M above a certain limiting polymer concentration. The slow and fast components of the diffusion coefficients of the polymer have been calculated. A gradual addition of salt turns the mutual interactions from zwitterionic attractions between PSPP blocks to hydrophobic attractions between PNIPA blocks. The formation of the aggregates and the aggregate sizes at T < UCST and T > LCST are influenced by polymer and salt concentrations. Below the UCST, the aggregates in saline polymer solutions are somewhat larger than those in pure polymer solutions. Above LCST, the aggregate size is determined by the salt concentration.
Article
Solvent vapor annealing of block copolymer thin films can produce a range of morphologies different from the equilibrium bulk morphology. By systematically varying the flow rate of two different solvent vapors (toluene and n-heptane) and an inert gas, phase maps showing the morphology versus vapor pressure of the solvents were constructed for 45 kg/mol polystyrene-block-polydimethylsiloxane diblock copolymer films of different thicknesses. The final morphology was correlated with the swelling of the block copolymer and homopolymer films and the solvent vapor annealing conditions. Self-consistent field theory is used to model the effects of solvent swelling. These results provide a framework for predicting the range of morphologies available under different solvent vapor conditions, which is important in lithographic applications where precise control of morphology and critical dimensions are essential.