[Show abstract][Hide abstract] ABSTRACT: New thermo associating polymers were designed and synthesized by grafting amino terminated poly(ethylene oxide-co-propylene oxide) (PEPO) onto carboxymethyl guar (CMG) and carboxymethyl tamarind (CMT). The grafting was performed by coupling reaction between single bondNH2 groups of PEPO and single bondCOOH groups of CMG and CMT using water-soluble EDC/NHS as coupling agents. The grafting efficiency and the temperature of thermo-association, Tassoc in the copolymer were studied by NMR spectroscopy. The graft copolymers, CMG-g-PEPO and CMT-g-PEPO exhibited interesting thermo-associating behavior which was evidenced by the detailed rheological and fluorescence measurements. The visco-elastic properties (storage modulus, G’; loss modulus, G”) of the copolymer solutions were investigated using oscillatory shear experiments. The influence of salt and surfactant on the Tassoc was also studied by rheology, where the phenomenon of “Salting out” and “Salting in” was observed for salt and surfactant, respectively, which can give an easy access to tunable properties of these copolymers. These thermo-associating polymers with biodegradable nature of CMG and CMT can have potential applications as smart injectables in controlled release technology and as thickeners in cosmetics and pharmaceutical formulations.
[Show abstract][Hide abstract] ABSTRACT: We investigated the effect of specific interactions on the structure of interfaces between a brush and a hydrogel on the polymer chain length scale. We used a model system for which the interactions between the brush and the gel are switchable. We synthesized weak polyelectrolyte brushes of poly(acrylic acid) and hydrogels of polyacrylamide and poly(N,N-dimethylacrylamide) which interact solely when the poly(acrylic acid) is mainly in its acidic form. The monomer density profiles of the poly(acrylic acid) brush immersed in pure deuterium oxide (D2O) or in contact with a D2O-swollen gel were determined by neutron reflectivity. At pH 2 when the brush is in its neutral form, it interacts with the gel by hydrogen bonds while at pH 9 when the brush is a polyelectrolyte it is not interacting with the gel. Our results show that the presence of interactions with the gel at pH 2 increases the swelling ratio of the brush relative to that in pure D2O, meaning that the brushes exhibit conformations which are more extended from the surface than in the absence of interactions.
[Show abstract][Hide abstract] ABSTRACT: Based on specific interactions taking place between poly(N,N-dimethylacrylamide) [PDMA] and silica nanoparticles, two strategies were used to improve the viscoelastic properties of aqueous formulations. First, PDMA oligomers were grafted onto a non-adsorbing poly(acrylamide-co-sodium acrylate) backbone. With this architecture, the binding process of PDMA side-chains with silica nanoparticles was shown to proceed very similarly to free PDMA chains and to promote the formation of hybrid physical networks. The viscoelastic properties of these systems are controlled by the concentration of inorganic cross-links and the fraction of adsorbing grafts involved in the formation of bridges between particles. An optimum weight ratio between silica and grafts was found for the viscoelastic properties, in agreement with the saturation of silica beads by the PDMA precursor. While the homogeneous formation of such hybrid assemblies remains limited to mixtures involving low polymer concentrations (≤ 2 wt%), we show that homogeneous hybrid networks can be readily prepared by direct polymerization of DMA monomer (≥ 5 wt%), with or without chemical cross-linker, in a suspension of silica nanoparticles. In this case, the specific interactions taking place between the silica filler and the whole polymer network give rise to a very unusual combination of properties where elasticity, dissipation, strength and strain at failure are enhanced simultaneously. While the intrinsic structure of the network can be controlled by the level of chemical cross-linking, physical interactions dominate the dissipation process with a dramatic time dependence of the mechanical properties.
[Show abstract][Hide abstract] ABSTRACT: Adhesives are made of polymers because, unlike other materials, polymers ensure good contact between surfaces by covering asperities, and retard the fracture of adhesive joints by dissipating energy under stress. But using polymers to 'glue' together polymer gels is difficult, requiring chemical reactions, heating, pH changes, ultraviolet irradiation or an electric field. Here we show that strong, rapid adhesion between two hydrogels can be achieved at room temperature by spreading a droplet of a nanoparticle solution on one gel's surface and then bringing the other gel into contact with it. The method relies on the nanoparticles' ability to adsorb onto polymer gels and to act as connectors between polymer chains, and on the ability of polymer chains to reorganize and dissipate energy under stress when adsorbed onto nanoparticles. We demonstrate this approach by pressing together pieces of hydrogels, for approximately 30 seconds, that have the same or different chemical properties or rigidities, using various solutions of silica nanoparticles, to achieve a strong bond. Furthermore, we show that carbon nanotubes and cellulose nanocrystals that do not bond hydrogels together become adhesive when their surface chemistry is modified. To illustrate the promise of the method for biological tissues, we also glued together two cut pieces of calf's liver using a solution of silica nanoparticles. As a rapid, simple and efficient way to assemble gels or tissues, this method is desirable for many emerging technological and medical applications such as microfluidics, actuation, tissue engineering and surgery.
[Show abstract][Hide abstract] ABSTRACT: The swelling of poly(acrylic acid) brushes in aqueous solutions is studied by neutron reflectivity. The brushes are synthesized on silicon wafers using a “grafting onto” approach and their stretching is investigated at various pH, grafting densities, and chain lengths. Neutron reflectivity provides the average thickness and, more interestingly, the density profile of the brushes. The profiles obtained experimentally are in good agreement with theory. The swelling ratio of the poly(acrylic acid) brushes is compared with that of other polymer brushes formerly investigated using classic scaling laws. Owing to a new representation with the normalized thickness, it is demonstrated that there is a general behavior for all polymer brushes with a master curve for neutral polymer brushes and for polyelectrolyte brushes.
Macromolecular Chemistry and Physics 12/2013; 214(24). · 2.39 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Nanohybrid gels based on poly(acrylamide-co-N,N-dimethylacrylamide), P(AAm-co-DMA), and colloidal silica nanoparticles were synthesized by radical polymerization, and the influence of hydrogen bonding between DMA and silica nanoparticles on the dynamics of the network and the nanoparticles were studied by dynamic light scattering. As previously reported, in the case of polyacrylamide homopolymer, PAAm, which have no hydrogen bonding with silica, we observed two decay modes in the hybrid gels, a gel mode and a Brownian diffusion mode of silica nanoparticles. When we increased the DMA weight fraction (or physical cross-link density), we observed (1) a higher scattered light intensity than the silica suspension and the gel without silica, (2) an increased plateau value of the autocorrelation function, (3) a slowing down of the silica diffusion mode, and (4) no influence on the gel mode. These results, compared with those of the mixtures of the linear PDMA and silica nanoparticles in solution (showing an increase in the scattered light intensity and in the hydrodynamic radius), indicate that the P(AAm-co-DMA) hybrid gels have a more heterogeneous structure due to the adsorption of the polymer on the silica nanoparticles, and that the silica nanoparticles, trapped in the network due to the adsorption of the polymer, show a cage dynamics in the network. The local viscoelastic properties probed by the bound nanoparticles are discussed.
[Show abstract][Hide abstract] ABSTRACT: Two series of Hydrophobically Modified Polyacrylamides (HMPAMs) were prepared by free radical copolymerization of trimethylsilylacrylamide with 3-pentadecylcyclohexylacrylamide (PDCAM) or dodecylacrylamide (DDAM) followed by removal of trimethylsilyl protecting groups. The solution properties of HMPAMs were studied in dilute aqueous solution by fluorescence spectroscopy, NMR and viscometry. The properties in aqueous solution of HMPAMs clearly displayed the formation of hydrophobic interactions between alkyl stickers. While dodecyl containing HMPAMs mainly showed a progressive collapse of the copolymer coil with the increasing fraction of hydrophobic moieties (weak intra-chain associations), pentadecylcyclohexyl groups containing HMPAMs exhibited a much lower solubility in water due to strong intra- and inter-chain associations. Experimental data were analyzed using the mean field theory developed by Semenov and Rubinstein (SR) for associating polymers. Using a set of realistic parameters, taking into account the solubility of the backbone, the fraction of stickers and the strength of hydrophobic attractions, we show that the SR model offers a quantitative description of the thermodynamic properties of HMPAMs in terms of individual and collective behavior of polymers chains; i.e. swelling and phase separation, respectively.
[Show abstract][Hide abstract] ABSTRACT: The strain rate effect on large strain dissipation and behavior recovery are presented to understand the toughening effect of silica nanoparticles in nanohybrid hydrogels. Such nanohybrid gels combine a poly(N,N-dimethylacrylamide) (PDMA) covalent network and physical interactions by adsorption of polymer chains at the silica nanoparticle surface. A series of model nanohybrid gels has been designed to obtain a well-controlled architecture. First insights on the structure (SANS) demonstrated that silica nanoparticles were well-dispersed in the gel, including after cyclic mechanical loading. The characteristic times involved in the nanoparticle/polymer association were investigated by large strain mechanical cycling varying the strain rate from 3 × 10–4 s–1 to 0.6 s–1. The mechanical behavior of the hybrid hydrogel varies tremendously over a relatively small range of strain rates, ranging from almost non dissipative (at slow strain rates) to highly dissipative at high strain rates. However, upon cycling over time-scales of tens of seconds, the strong physical interactions taking place between nanosilica particles and PDMA network chains enabled the hydrogel to recover its initial mechanical properties. The main feature of this work is the remarkable role played by silica nanoparticles in the network to promote transient and recoverable connectivity by reversible adsorption/desorption processes. The strong strain rate dependence suggests that toughening mechanisms operating at standard strain rates as often reported, maybe quite different at slower or larger strain rates.
[Show abstract][Hide abstract] ABSTRACT: We investigated the surface structure of hydrogels of poly(N,N-dimethylacrylamide) (PDMA) hydrogels synthesized and cross-linked simultaneously by redox free radical polymerization. We demonstrate the existence of a less cross-linked layer at the surface of the gel at least at two different length scales characterized by shear rheology and by neutron reflectivity, suggesting the existence of a gradient in cross-linking. The composition of the layer is shown to depend on the degree of hydrophobicity of the mold surface and is weaker for more hydrophobic molds. While the macroscopic tests proved the existence of a relatively thick under-cross-linked layer, we also demonstrated by neutron reflectivity that the gel surface at the submicrometric scale (500 nm) was also affected by the surface treatment of the mold. These results should have important implications for the measurement of macroscopic surface properties of these hydrogels such as friction or adhesion.
[Show abstract][Hide abstract] ABSTRACT: We have developed a new experimental methodology to investigate the adhesive properties of hydrogels on solid surfaces under fully immersed conditions. The method, based on contact mechanics, provides time-resolved reproducible and quantitative data on the work of adhesion between a hydrogel at swelling equilibrium and a planar surface grafted with responsive brushes. We used poly(N,N-dimethylacrylamide) (PDMA) and polyacrylamide (PAM) as model gels and poly(acrylic acid) (PAA) as pH dependent polymer brush. The effect of pH, contact time and debonding velocity on adhesive interactions was specifically investigated. As expected from molecular interactions, we found that adhesion increased as the pH decreased and this was attributed to the formation of hydrogen bonds at the interface. Surprisingly, however, the buildup of adhesion increased slowly with the time of contact up to one hour and depended markedly on debonding velocity despite the very elastic nature of the hydrogels. Furthermore, the maximum pH where adhesion was observed was significantly higher for the couple PAM–PAA than for the couple PDMA–PAA, in contrast with the onset of molecular interactions in dilute solutions.
[Show abstract][Hide abstract] ABSTRACT: A guideline for the synthesis of poly(acrylic acid) brushes on planar silica surfaces by the “grafting-onto” approach is described. It is demonstrated that some thermal precautions must be taken to obtain extended brushes. It is also shown that neutron reflectivity is well suited for the characterization of each step of the synthesis, while it is (unfortunately) rarely used for that purpose. The steps are the following: first, the substrates are covered with a self-assembled monolayer of epoxy-terminated molecules; then, the poly(tert-butyl acrylate) brushes are built using preformed and end-functionalized chains; finally, the deprotection of the ester group is performed using a pyrolysis reaction to convert the poly(tert-butyl acrylate) brushes into poly(acrylic acid) brushes.
Macromolecular Chemistry and Physics 02/2012; 213(3). · 2.39 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The formation of interpolymer complexes (IPC) between poly(acrylic acid) (PAA) and poly(acrylamide) (PAM), poly(N,N-dimethylacrylamide) (PDMA), and statistical copolymers of acrylamide (AM) and N,N-dimethylacrylamide (DMA) has been studied as a function of pH, salt concentration and temperature (0–70 °C). The cloud points of dilute solutions were measured by turbidimetry and phase diagrams were determined as a function of temperature and pH in pure water and as a function of pH and salt concentration at room temperature. For each temperature and salt concentration a critical pH (pHcrit) below which IPC are observed was defined. In the case of PAA/PAM, pHcrit continuously decreased with increasing temperature, from pH 3.5 at 0 °C to pH 1.9 at 60 °C (UCST-type). In the case of PAA/PDMA, pHcrit, increased with temperature. The LCST-type behavior of the hydrogen-bonding complex formed between PAA and PDMA was attributed to the dimethyl substitution of amide groups that puts in hydrophobic interactions at high temperature. PAA and statistical copolymers P(AM-co-DMA) showed an intermediate behavior between PAA/PAM and PAA/PDMA with a continuous shift from UCST-type to LCST-type with increasing amount of DMA. This behavior can be attributed to changes in configurational entropy due to the IPC formation and (for PDMA) to the release of water molecules initially confined in hydrophobic hydration cages around DMA units. While at low salt concentration, the stability of PAA/PAM and PAA/PDMA complexes only slightly increases with the screening of ionized acrylic units, there is a sharp increase of pHcrit at high salt concentration in relation with the weakening of the solvent quality. In this regime, the complex formation of PAA/PDMA is greatly enhanced compared to PAA/PAM due to the interference of hydrophobic interactions.
[Show abstract][Hide abstract] ABSTRACT: pH and thermoresponsive polymers have been prepared by copolymerizing N-isopropylacrylamide (NIPAM) with various amounts of ionizable comonomers, either acrylic acid (AA) or N-[3-(dimethylamino)propyl]methacrylamide (MADAP). In aqueous solution, the LCST-type phase transition of these copolymers studied by differential scanning calorimetry is strongly influenced by the comonomer ratio. Under un-ionized conditions, the phase transition temperature progressively increases with MADAP content at pH 12 while it remains unchanged or slightly decreases with AA at pH 3 due to the formation of hydrogen bonds between AA and NIPAM units. When the copolymer chains are progressively charged by tuning the pH, the phase transition of PNIPAM-AA and PNIPAM-MADAP is shifted at higher temperature and is no longer observable below 60 °C when the ionic content exceeds 10%. By comparison with these single systems, where the association properties can be finely adjusted by coupling hydrophobic attractions and electrostatic repulsions, we also investigate the possibility to couple hydrophobic interactions with electrostatic attractions by mixing oppositely charged copolymers: PNIPAM-AA and PNIPAM-MADAP. This study was carried out with the copolymer pair containing 10 mol % of ionizable groups (A10 and M10) which was the most adaptable one from the point of view of responsivity. At pH 7, when AA and MADAP units are ionized, the copolymer chains are separately soluble in water in the whole temperature range while their mixture, also soluble at room temperature, phase separates upon heating. The original feature highlighted in this work is that the phase transition proceeds through a selective mechanism between complementary chains (formation of a reversible interpolyelectrolyte complex) and that this selectivity can be switched with the pH. Indeed, starting at room temperature with the copolymer mixture (A10 and M10), which remains homogeneous at all pH, we demonstrate that the association process can be switched by increasing the temperature and that the pH can be used to specifically address these associations: from A10/A10 at low pH to A10/M10 at pH 7 up to M10/M10 at high pH. Finally, the responsive precursors A10 were grafted onto a polyacrylamide backbone, and the viscoelastic properties of graft copolymers were studied in the semidilute regime and compared with calorimetric data. In this way, we show that the same set of interactions can be readily applied to more sophisticated macromolecular assemblies with the responsive formation of physical gels under pH and temperature control.
[Show abstract][Hide abstract] ABSTRACT: We have investigated the mechanical and fracture properties of hybrid hydrogels made of chemically crosslinked polyacrylamide (PAAm) and silicananoparticles. Unlike the case where the polymer is poly(dimethylacrylamide) (PDMA), the introduction of silicananoparticles at 6–7 vol% does not lead to any significant reinforcement in the mechanical properties (modulus, hysteresis and fracture toughness) of the hydrogels. The key difference between the two hybrid gels is that PAAm does not absorb on silica while PDMA does [L. Petit, L. Bouteiller, A. Brulet, F. Lafuma and D. Hourdet, Langmuir0, 2007, 23(1), 147–158]. These results stress the importance of polymer/filler interactions in controlling the macroscopic mechanical properties of hybrid hydrogels and suggest that a strong but breakable interaction leads to superior fracture toughness.
[Show abstract][Hide abstract] ABSTRACT: A new reversible gelation pathway is described for alginates in aqueous media. From various samples differing by their mannuronic/guluronic content (M/G), both enthalpic and viscoelastic experiments demonstrate that alginates having a high M content are able to form thermoreversible assemblies in the presence of potassium salts. The aggregation behavior is driven by the low solubility of M-blocks at low temperature and high ionic strength. In semidilute solutions, responsive assemblies induce a strong increase of the viscosity below a critical temperature. A true physical gel is obtained in the entangled regime, although the length scale of specific interactions between M-blocks decreases with increasing density of entanglements. Cold setting takes place at low temperatures, below 0 °C for potassium concentrations lower than 0.2 mol/kg, but the aggregation process can be easily shifted to higher temperatures by increasing the salt concentration. The self-assembling process of alginates in solution of potassium salts is characterized by a sharp gelation exotherm and a broad melting endotherm with a large hysteresis of 20-30 °C between the transition temperatures. The viscoelastic properties of alginate gels in potassium salts closely depend on thermal treatment (rate of cooling, time, and temperature of storage), polymer and salt concentrations, and monomer composition as well. In the case of alginates with a high G content, a similar aggregation behavior is also evidenced at higher salt concentrations, but the extent of the self-assembling process remains too weak to develop a true gelation behavior in solution.
[Show abstract][Hide abstract] ABSTRACT: Responsive polymers were prepared by copolymerizing a small amount of ionizable monomers, acrylic acid or N,N-dimethylaminopropylmethacrylamide,
with N-isopropylacrylamide (NIPA) and the solubility in aqueous solution of these PNIPA derivatives were quantitatively studied.
From differential scanning calorimetry experiments, it was shown that the pH strongly influences the phase separation of these
copolymers (temperature and enthalpy), which totally disappears when ionizable groups are fully charged. At pH 7, all PNIPA
copolymers remain soluble in water at all temperatures but their mixtures show a phase separation above a critical temperature
due to the formation of a reversible inter-polyelectrolyte complex. When the responsive stickers are grafted on a poly(acrylamide)
backbone, pH and temperature are still able to drive the association process at a local scale, giving rise to a sol/gel transition
of semi-dilute solutions. The structure and the viscoelastic properties of these macromolecular assemblies are investigated
by small angle neutron scattering and rheology and their responsivity is discussed as a function of pH and temperature.
Keywordsresponsive polymers-associating polymers-pnipa
[Show abstract][Hide abstract] ABSTRACT: The adsorption process onto a silica surface of oligomers from three different monomers, N,N-dimethylacrylamide (DMA), N-isopropylacrylamide (NIPA) and ethylene oxide (EO), was studied. From the adsorption isotherms and calorimetric experiments, it was shown that N-alkylacrylamide derivatives strongly interact with silica nanoparticles. At low coverage (Γ < 0.5 mg m−2), the polymers were adsorbed in a flat conformation with a high proportion of trains, while for higher coverages (Γ = 0.5 – 1 mg m−2), the adsorption of new chains proceeded by chain displacement, forming an increasing proportion of loops and tails. Similarly, PEO chains readily interacted with silica nanoparticles but showed a lower affinity with an adsorbed amount (Γ ≅ 0.6 mg m−2) that was approximately half of the value obtained for both PDMA and PNIPA. When these oligomers were grafted onto a non-adsorbing poly(acrylamide-co-sodium acrylate) backbone (PAMH), the binding process of these adsorbing side-chains with silica nanoparticles proceeded very similarly and gave rise to the formation of hybrid hydrogels above critical copolymer and silica concentrations. The viscoelastic properties of these networks were controlled by the concentration of inorganic cross-links and the fraction of adsorbing grafts that participated in the formation of bridges between particles, the others being involved in inelastic loops or pendant chains. For all the hybrid mixtures investigated, an optimum weight ratio between silica and grafts was found for the viscoelastic properties, which was in agreement with the saturation of silica beads by the graft precursors. Due to the temperature-dependence of the solubility of the PNIPA side-chains in aqueous solutions, the PAMH-g-PNIPA copolymer was also able to self-assemble with temperature, giving rise to a hybrid conetwork in the presence of added silica.
[Show abstract][Hide abstract] ABSTRACT: New hybrid assemblies with thermothickening properties are designed using poly-(N-isopropylacrylamide) (PNIPA) as responsive trigger. The main components of the formulations are hydrolyzed polyacrylamide grafted with PNIPA side-chains (PgPNIPA) and core–corona silica–PNIPAnanoparticles (SgPNIPA). PNIPA brushes with controlled molar mass are synthesized in a two-step procedure through a “grafting from” approach involving thiol groups chemically anchored on the silica surface. Particles are characterized by FT-IR, 1H NMR, 13C NMR, solid state CP/MAS1H–29Si NMR, acid–base and chemical titrations, and dynamic light scattering. Rheology and microcalorimetry studies are used to investigate the self-assembling behaviour of hybrid formulations. For SgPNIPA aqueous dispersions, a collapse of the corona is induced upon heating. It starts from the silica core at low temperature (29 °C) and spreads progressively toward the outer part of the corona, leading to a reversible aggregation of the particles above 34 °C. When the same particles are added into PgPNIPA solutions, original self-assembling behaviours are observed. Contrary to neat silica particles, the presence of a dense PNIPA corona grafted on the silica surface prevents specific interactions between the grafted copolymer and the surface of the silica core. When the formulation is heated, the hybrid mixture exhibits two distinct phase transitions correlated to the PNIPA corona (30–50 °C) and the PNIPA grafts (40–60 °C). A synergy between these two transitions is observed above 30 °C with a noticeable increase of viscosity, followed above 40 °C by the self-assembling behaviour of PNIPA grafts and the formation of a physical network with improved viscoelastic properties.
[Show abstract][Hide abstract] ABSTRACT: The synthesis and mechanical characterization of novel, tough poly(N,N-dimethylacrylamide) (PDMA)−silica hydrogel hybrids are presented to understand the role played by strong physical interactions between silica nanoparticles and the PDMA polymer on the properties of chemically cross-linked highly swollen PDMA networks. A detailed comparison of the hybrids with unmodified PDMA gels indicates that the incorporation of silica nanoparticles in the hydrogel increases the compression strength and the fracture toughness of notched samples up to an order of magnitude while increasing its modulus by a factor of 6 with a volume fraction of particles of the order of only 7%. The hybrid gels present a strain-dependent hysteresis but no permanent damage or residual strain upon unloading even after repeated cycling, a very unique property for such tough hydrogels. The reason for this exceptional increase in toughness is attributed mainly to the combined effect of breakable silica/polymer bonds and of a wide distribution of elastic chain lengths.
[Show abstract][Hide abstract] ABSTRACT: We discovered that the free radical polymerization of N,N-dimethylacrylamide in water can lead, above a certain concentration, to gels without any added difunctional crosslinker. These so called “self-crosslinked” hydrogels were prepared and their weak mechanical properties were improved by introducing silicananoparticles.