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Poly(N-isopropylacrylamide) microgel synthesised by emulsion polymerization
Abstract
Smart polymers have been one of the most popularly studied materials owing to their capability to alter physio-chemical behaviour upon exposure to specific external stimuli. The biocompatible thermally responsive poly(N-isopropylacrylamide), PNIPAm shows reversible transition between hydrophilic-hydrophobic characteristics at the vicinity of human physiological temperature has great potential to propel the development of smart tissue engineering scaffold and drug delivery. However, the limited availability and its high cost have dampened the extent of research on this polymer. To address these challenges, the current work demonstrates an economical lab-scale polymerization of crosslinked PNIPAm and the optimised parameters to produce mono-dispersed polymer hydrogel particles were investigated. Characterisation of the synthesized PNIPAm polymer revealed particle size polydispersity index of 0.215, indicative of distribution within the mono-dispersed range, with average hydrodynamic diameter of 346.3 nm. Zeta-potential of the synthesized PNIPAm was found to be-20.6 mV, suggesting an incipient instability in terms of colloidal coagulation. Viscosity of the synthesized PNIPAm (4 wt% concentration in methanol) was 28.6 cP. Thermal gravimetric analysis (TGA) indicated the thermal degradation of main chain PNIPAm fell in the range of 340 to 480C.
Smart thermosensitive polymer such as poly (N-isopropyl acrylamide) (PNIPAM) and dominant fibrous protein of connective tissue such as collagen (CLG) possess great potential in biomedical and tissue engineering applications. The objectives of current work aim to explore potential of PNIPAM and collagen by (i) establish a stable procedure to extract collagen from fresh water Tilapia fish scale (TFS) and (ii) fabricate PNIPAM and hybrid PNIPAM-CLG nanofibrous scaffolds through electrospinning technique and investigate their material-process-structure behaviour. Type I collagen was derived through acid hydrolysis of TFS. Electrospinning of PNIPAM was carried out with 16, 18 and 20 wt% PNIPAM concentration in methanol (MeOH) while PNIPAM-CLG was prepared through blending measured quantity of PNIPAM dissolved in water with collagen dissolved in acetic acid. Material properties, viscosity, morphology and thermo-physical behaviors of the derived collagen, electrospun PNIPAM and PNIPAM-CLG scaffolds were characterized. Results from SDS-PAGE and FTIR confirmed that the isolated TFS collagen is of type I. EDX revealed that demineralization eliminated the aluminium, magnesium, silicon and phosphorus while significantly reduced the sulfur elements from raw TFS. SEM observation of the collagen morphology shown a fluffy and fibrillary lamellae structure. Electrospun scaffolds were successfully fabricated with 16 and 18 wt% PNIPAM in MeOH. Both homogeneity and average fibre diameter (D avg ) were greater in the 18 wt% PNIPAM scaffold, in which the D avg for 16 and 18 wt% were ~110 and ~131.7 nm respectively. However, PNIPAM at 20 wt% failed to be electrospun owing to its excessively high viscosity. On the other hand, SEM observation revealed that the electrospun hybrid PNIPAM-CLG scaffold has D avg of ~105.5 nm amid the presence of numerous elongated beads.
Owing to their highly desirable properties that combine the properties of both hydrogels and nanomaterials, smart nanogels own great potentials as active nanocarriers in medical applications. In this paper, thermo-responsive nanogels with uniform sizes less than 50 nm in diameter were synthesized using potassium persulfate (KPS)/N,N,N′,N′-tetramethylethylenediamine (TMEDA) as a initiator system via a facile surfactant-free precipitation radical polymerization of N-isopropylacrylamide (NIPAM) at room temperature. Both transmission electron microscopy and dynamic light scattering were used to characterize the morphologies and diameters of the PNIPAM nanogels. All nanogels with spherical shape exhibited a narrow size distribution, and the finest nanogels were 43 nm in diameter on average. The very fine and highly pure nanogels would be more promising for drug delivery carriers than the bulk gels or microgels.
The production of new multi-responsive poly(N-isopropylacrylamide-allyl acetic acid) [P(NIPAM-AAA)] copolymer microgel by free radical emulsion polymerization is reported. Inside this copolymer microgel CuO nanoparticles were generated by in situ reduction of copper nitrate followed by the air oxidation process, their fabrication is confirmed by powdered X-ray diffraction analysis and the average size of CuO NPs was found to be 24 nm having monoclinic shapes. By using dynamic laser light scattering the swelling & de-swelling behaviour of the pure microgel was examined at different temperature and pH values. The copolymer microgel becomes unstable at low pH as well as at high temperature values respectively.UV-visible spectra show a red shift in surface plasmon resonance λSPR of CuO nanoparticles. The catalytic property of hybrid microgel was inspected by observing the reduction of 4-nitrophenol into 4-aminophenol in the presence of excess NaBH4 with various concentrations of catalyst at room temperature. The hybrid microgels also have good anti-bacterial activity against both Gram-positive (C.Albicans) and Gram-negative (E. coli) bacteria.
Microgels have a wide range of possible applications and are therefore studied with increasing interest. Nonetheless, the microgel synthesis process and some of the resulting properties of the microgels, such as the crosslinker distribution within the microgels, are not yet fully understood. An in depth understanding of the synthesis process is crucial for designing tailored microgels with requested properties. In this work, rate constants and reaction enthalpies of chain propagation reactions in aqueous N-isopropylacrylamide/N,N’-methylenebisacrylamide and aqueous N-vinylcaprolactam/N,N’-methylenebisacrylamide systems are calculated to identify possible sources of an inhomogeneous crosslinker distribution in the resulting microgels. Gas phase reaction rate constants are calculated from B2PLYPD3/aug-cc-pVTZ energies and B3LYPD3/tzvp geometries and frequencies. Then, solvation effects based on COSMO-RS are incorporated into the rate constants to obtain the desired liquid phase reaction rate constants. The rate constants agree with experiments within a factor of 2-10, while the reaction enthalpies deviate less than 5 kJ/mol. Further, the influence of rate constants on the microgel growth process is analyzed and it is shown that differences in the magnitude of the reaction rate constants are one source of an inhomogeneous crosslinker distribution within the resulting microgel.
Poly(N-isopropylacrylamide), PNIPAM, is a thermoresponsive polymer widely known for its lower critical solution temperature (LCST) phenomenon at 32°C in aqueous solutions. Precise tuning of the LCST of PNIPAM to a broader temperature range offers a larger window of applications especially in the field of biotechnology and nanotechnology. A series of free radical random copolymerizations between N-isopropylacrylamide (NIPAM) and various imidazolium based ionic liquids (ILs) were conducted. IL structures were varied in terms of their alkyl chain length at the N-3 (or N-1) position of the imidazolium ring and counter anion. The LCST behavior of the aqueous solutions of the copolymers was investigated through UV-VIS transmission measurements. The results confirm that introduction of IL into the PNIPAM offers a wide range of LCST behaviors with a synergism between the hydrophobic part of the ionic liquid and the basic strength of the counter anion, probably by varying the hydrogen bonding abilities of the copolymer.
This contribution is focused on the determination of polymerization and crosslinking mechanism of poly (N-isopropyl-acralymide) hydrogel and its swelling properties. Hydrogels were synthesized by environmental friendly UV polymerization method, from monomer N-isopropylacrylamide (NIPAM) and crosslinker N, N´ethylenebisacrylamide (BIS) of different concentrations. UV polymerization was performed in an UV chamber using UVA light with the wave length 350 nm. Surface morphology and pore structure analysis was carried out using SEM microscopy. The polymerization and crosslinking mechanism was determined by Fourier Transform Infrared spectroscopy (FT-IR). It was confirmed that crosslinker concentration influences the hydrogel structure and swelling properties. By increasing the crosllinker concentration the hydrogel structure changes from homogen to heterogen and the equilibrium degree of swelling decreases. Keywords: poly (N-isopropylacrylamide) hydrogel, UV polymerization, FTIR, polymerization mechanism, swelling properties, SEM V prispevku je predstavljen mehanizem polimerizacije inzamrèenja poli(N-isopropilakrilamidnega) hidrogela in njegova sposobnost nabrekanja. Hidrogele smo sintetizirali z okolju prijazno metodo UV polimerizacije iz monomera N-isopropilakrilamida (NIPAM) inzamrèevalca N, N´etilenebisakrilamida (BIS) razli~nih koncentracij. UV polimerizacija je potekala v UV komori z UVA svetlobo valovnedoìine 350 nm. Povr{insko morfologijo in strukturo por smo spremljali s SEM mikroskopijo. Mehanizem polimerizacije inzamrèenja smo dolo~ili na podlagi posnetih spektrov s Fourier Transform Infrarde~o spektroskopijo (FT-IR). Ugotovili smo, da koncentracijazamrèevalca vpliva na strukturo hidrogela in njegovo sposobnost nabrekanja. Z ve~anjem koncentracijezamrèevalca pride do spremembe v strukturi hidrogela iz homogene v heterogeno, kar vpliva na zmanj{anje sposobnosti nabrekanja. Klju~ne besede: poli (N-isopropilakrilamide) hidrogel, UV polimerizacija, FTIR, mehanizem polimerizacije, nabrekanje, SEM
Effects of the operating policies—the initial initiator amount; the initial emulsifier amount; the monomer addition mode: batch or semibatch; and the monomer addition rate under “monomer-starved conditions” for the control of particle size distribution (PSD)—were studied through a model that simulates batch and semibatch reactor operations in conventional emulsion polymerization. The population balance model incorporates both the nucleation stage and the growth stage. The full PSDs were reported, which have normally been omitted in earlier studies. It was shown through simulations that the broadness of the distributions, both initial (obtained after the end of nucleation) and final (after complete conversion of monomer), can be controlled by the initial initiator amount and the emulsifier amount. The higher initiator amounts and the lower emulsifier amounts favor narrower initial and final distributions. The shape of the initial PSDs and the trends in the average size and range were preserved with subsequent addition of monomer in the batch or in the semibatch mode, although the final PSD was always considerably narrower than that of the initial PSD. The addition of monomer in the semibatch mode gave narrower distribution compared to that of the batch mode, and also, lower monomer addition rates gave narrower distributions (larger average sizes), which was a new result. It was further shown through simulations that, under monomer-starved conditions, the reaction rate closely matched the monomer feed rate. These conclusions are explained (1) qualitatively—the shorter the length of the nucleation stage and the larger the length of the growth stage (provided the number of particles remains the same), the narrower is the distribution; and (2) mathematically—in terms of the “self-sharpening” effect. Experimental evidence in favor of the self-sharpening effect was given by analyzing the experimental particle size distributions in detail. The practical significance of this work was proposed. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 2884–2902, 2004
Poly(alkylacrylamides), most notably poly(N-isopropylacrylamide) (PNIPAM), are widely studied thermoresponsive polymers. Linear PNIPAM, poly(N-n-propylacrylamide) (PNnPAM) and poly(N-cyclopropylacrylamide) (PNCPAM) were synthesized via RAFT polymerization to obtain polymers with low dispersity and varying terminal groups. The polymers were studied with a combination of small-angle neutron scattering (SANS), turbidimetry measurements, and dissipative particle dynamics (DPD) simulations to determine the influence of terminal group and monomer structure on assembly and thermoresponsive behavior in solution. Large scale, fractal assemblies (larger than 1 μm) are observed below the critical temperature for polymers terminated with short hydrophobic groups, but for long hydrophobic terminal groups, polymers formed small micelles (ca. 15 nm). The two cases give rise to two distinct thermoresponsive behaviors: the fractal assemblies display the traditional coil-to-globule behavior, whereas the corona of the micelles collapsed well below the critical temperature due to n-clustering, while the micelles remained in solution. The results indicate that, despite making up only a small fraction of the polymer, the terminal groups play a large role in both the conformation and assembly of PNIPAM and its analogs below the critical temperature, and hints at the possibility of new design strategies for thermoresponsive materials.
Poly(N-isopropylacrylamide. -co-Methacrylic acid) (PNIPAM-MAA) microgels with different spatial distribution of functional carboxylic (COOH) groups were synthesized by a combination of semi-batch and temperature-programmed surfactant-free precipitation polymerization. This synthetic approach enabled functional COOH groups and crosslinks to be controlled in a better way so that they were either predominately concentrated at the periphery, leading to a functional dense-shell (DS), or mainly located inside the microgel, resulting in a functional dense-core (DC) particle. Because of the specially designed internal structures of these two microgels, we were able to investigate the effect of the charge group distribution on the corresponding Pickering emulsions. By examining the emulsion stability at different pH values and particle concentration using these two microgel samples as particular emulsifiers, our results showed that the soft, negatively charged microgel with a pure PNIPAM shell is the best emulsion stabilizer since the microgel is very hydrophilic and also retains its interfacial activity at the same time. On the other hand, the functional DS microgel lost most of its interfacial activity due to the dramatic increase in hydrophilicity, particularly in alkaline condition. Nevertheless, it is still capable of stabilizing oil/water emulsion by its high volume fraction in the continuous phase. Although the particles are not strongly adsorbed to the interface, the movement of the particles is significantly restrained. Therefore, they are still separating the oil droplets from coalescence.
In the last decade, a variety of methods for fabrication of three-dimensional biomimetic scaffolds based on hydrogels have been developed for tissue engineering. However, many methods require the use of catalysts which compromises the biocompatibility of the scaffolds. The electrochemical polymerization (ECP) of acrylic monomers has received an increased attention in recent years due to its versatility in the production of highly biocompatible coatings for the electrodes used in medical devices. The main aim of this work was the use of ECP as scaffold fabrication technique to produce highly porous poly(N-isopropylacrylamide) (PNIPAM)/hydroxyapatite (HAp) composite for bone tissue regeneration. The prepared PNIPAM-HAp porous scaffolds were characterized by SEM, FTIR, water swelling, porosity measurements and X-ray diffraction (XRD) techniques. FTIR indicates that ECP promotes a successful conversion of NIPAM to PNIPAM. The water swelling and porosity were shown to be controlled by the HAp content in PNIPAM-HAp scaffolds. The PNIPAM-HAp scaffolds exhibited no cytotoxicity to MG63 cells, showing that ECP are potentially useful for the production of PNIPAM-HAp scaffolds. To address the osteomyelitis, a significant complication in orthopedic surgeries, PNIPAM-HAp scaffolds were loaded with the antibiotic oxacillin. The oxacillin release and the bacterial killing activity of the released oxacillin from PNIPAM-HAp against S. aureus and P. aeruginosa were demonstrated. These observations demonstrate that ECP are promising technique for the production of non-toxic, biocompatible PNIPAM-HAp scaffolds for tissue engineering.
Both water and methanol are good solvents for poly(N-isopropylacrylamide) (PNIPAM), while PNIPAM does not dissolve in their mixed solvents, this phenomenon is called cononsolvency. Cononsolvency is closely related to many phenomena in life but so far, its mechanism is still controversial. In this work, the dielectric behavior of PNIPAM methanol aqueous solution was studied in the frequency of 40Hz-40GHz. From lower frequency to higher frequency, four relaxations were found. They are, respectively, from global chain motion, local motion of backbone, motion of side chain group, and the dipole orientation of the solvent molecule. The solvent dependence of dielectric parameters for the chain motion implied that the PNIPAM chain has undergone the coil-globule-coil transition. Dielectric analysis to microwave frequency showed that the volume of the bound solvent units on PNIPAM chain increases with the increasing methanol concentration, which suggested that the structure of solvation units bound on PNIPAM side chains undergo a changing process experience from water to water-methanol cluster to the ternary methanol cluster. This work reveals the structure and dynamics of the PNIPAM chain and the solvent unit that involved in the solvation of PNIPAM, and provides some new insight into the cononsolvency phenomenon.
When methanol is added to water at room temperature and 1 atm, poly (N-isopropylacrylamide), PNIPAM, undergoes a coil-to-globule collapse transition. This intriguing phenomenon is called cononsolvency. Spectroscopic measurements have shown that application of high hydrostatic pressure destroys PNIPAM cononsolvency in water-methanol solutions. We have developed a theoretical approach that identifies the decrease in solvent-excluded volume effect as the driving force of PNIPAM collapse on increasing the temperature. The same approach indicates that cononsolvency, at room temperature and P = 1 atm, is caused by the inability of PNIPAM to make all the attractive energetic interactions that it could be engaged in, due to competition between water and methanol molecules. The present analysis suggests that high hydrostatic pressure destroys cononsolvency because the coil state becomes more compact, and the quantity measuring PNIPAM-solvent attractions increases in magnitude due to the solution density increase, and the ability of small water molecules to substitute methanol molecules on PNIPAM surface.
The influence of α-methyl substitutions on the interpolymer complexation between polyisopropylacrylamides (poly( N -isopropylacrylamide) and poly( N-isopropylmethacrylamide)) and polyacrylic acids (poly(acrylic acid) and poly(methacrylic acid)) has been studied. The influence of solvents on the complexation process and, therefore, the effect of polymer–solvent interactions on the complex formation have been analyzed. Specific interactions were analyzed by Fourier transform infrared spectroscopy, and the composition of the obtained complexes was determined by elemental analysis. Finally, the thermal behavior of the obtained interpolymer complexes has been studied by calorimetry and thermogravimetry.
The adsorption of poly (N-isopropyl acrylamide) (PNIPAM) onto talc from aqueous solutions has been studied using the in situ methodology of particle film attenuated total reflection (ATR) Fourier transform infrared (FTIR) spectroscopy. PNIPAM was observed to adsorb significantly onto the talc particle film at a temperature below its lower critical solution temperature (LCST). Peak shifts were seen in the adsorbed layer FTIR spectrum that match those observed when PNIPAM solution is heated above its LCST. This observation indicates that adsorption causes a conformational re-arrangement similar to that seen when PNIPAM undergoes a coil-to-globule transition, in this case presumably induced by hydrophobic interactions between PNIPAM and the talc basal plane surface. The kinetics of adsorption are seen to be complex, with potential influences of conformational rearrangement and differential adsorption kinetics for the two dominant surface regions of talc particles. The adsorbed PNIPAM was seen to be exceptionally resistant to removal, with no desorption occurring when a background electrolyte solution was flowed over the adsorbed layer. Spectra acquired of the adsorbed polymer layer heated above the LCST reveal that a further conformational rearrangement takes place for the adsorbed layer, finalizing the transition from coil-to-globule that was initiated by the interaction with the mineral surface.
Aqueous solutions of poly(N-isopropylacrylamide) (PNIPAM) exhibit a temperature responsive change in conformation. When the temperature is increased, the polymer transitions from an extended coil conformation to a collapsed structure. We performed molecular dynamics simulations of aqueous solutions of single chain, syndiotactic PNIPAM oligomers over a wide range of temperatures and varying degrees of polymerization to elucidate the effect of oligomer length on the single chain transition temperature, T1. We have reproduced recent measurements of the transition temperature increasing with decreasing oligomer chain length. Examination of the chain structure reveals that conformations above T1 bend to bring hydrophobic segments together to shield them from the water. The constraints of the dihedral dynamics require elevated temperatures for shorter chains to bend sharply enough in order to undergo the transition. This result is confirmed by calculations of the solvent accessible surface area, which shows an increase in shielding of the hydrophobic groups with increasing oligomer length above T1.
Emulsion polymerization involves the propagation reaction of free radicals with monomer molecules in a very large number of discrete polymer particles (1016–1018 dm−3) dispersed in the continuous aqueous phase. The nucleation and growth of latex particles control the colloidal and physical properties of latex products. This review article focused on the polymerization mechanisms and kinetics involved in such a heterogeneous polymerization system over the preceding 10-year period. First, an overview of the general features of emulsion polymerization was given, followed by the discussion of several techniques useful for studying the related polymerization mechanisms and kinetics. Emulsion polymerizations using different stabilizers were studied extensively in the last few years and representative publications were reviewed. The performance properties of some specialty polymerizable, degradable or polymeric surfactants and surface-active initiators were also evaluated in emulsion polymerization. At present, the particle nucleation process is still not well understood and deserves more research efforts. This article continued to discuss the origin of non-uniform latex particles from both the thermodynamic and kinetic points of view. This was followed by the discussion of various reaction parameters that had significant effects on the development of particle morphology. Recent studies on the polymerization in non-uniform polymer particles were then reviewed. Finally, the polymerization mechanisms, kinetics and colloidal stability involved in the versatile semibatch emulsion polymerization were reviewed extensively.
A lower critical solution temperature (LCST) in an aqueous environment has been observed with poly(N-isopropylacrylamide) (pNIPAM) deposited onto solid surfaces from a plasma glow discharge of NIPAM vapor. The synthesis and spectroscopic data (ESCA, FTIR) for the plasma polymerized NIPAM (ppNIPAM) shows a remarkable retention of the monomer structure. The phase transition at 29 degrees C was measured by a novel AFM method. The phase transition was surprising because of the expectation that the plasma environment would destroy the specific NIPAM structure associated with the thermal responsiveness. The phase change of ppNIPAM is also responsible for the changes in the level of the meniscus when coated capillaries are placed in warm and cold water. Plasma polymerization of NIPAM represents a one-step method to fabricate thermally responsive coatings on real-world biomaterials without the need for specially prepared substrates and functionalized polymers.
Based on a detailed study of the fundamental vibrations in the mid-infrared (MIR) region and supported by NH-proton deuteration results, the assignment of the overtone and combination bands in the near-infrared (NIR) spectrum of poly(N-isopropylacrylamide) (PNIPAM) is presented. Variable-temperature experiments and two-dimensional correlation infrared spectroscopy are used to determine the chemical mechanism and changing sequence of groups in PNIPAM; we conclude that bonded NH groups turn into free NH groups during the heating process, while the CH groups on the side-chains change prior to those on the main chains. A heterospectral dynamical correlation between the NIR and MIR regions or H-included groups in both regions was also performed. The temperature-induced dissociation of the hydrogen-bonded NH groups is found to proceed earlier than the conformational changes in the hydrocarbon chains.
- Oligomers In Water
Oligomers in Water," Macromolecules, vol. 45, no. 16, pp. 6697-6703, Aug. 2012.