Publications (6)4.19 Total impact
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Article: Structure of surfaces and interfaces of poly(N,N-dimethylacrylamide) hydrogels.
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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.Langmuir 07/2012; 28(33):12282-7. · 4.19 Impact Factor -
Article: Thermoresponsive Interpolyelectrolyte Complexation: Application to Macromolecular Assemblies
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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.09/2011; -
Chapter: pH- and Thermo-responsive Polymer Assemblies in Aqueous Solution
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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-pnipa09/2010: pages 19-22; -
Article: Large Strain and Fracture Properties of Poly(dimethylacrylamide)/Silica Hybrid Hydrogels
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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.02/2010; -
Article: Synthesis and Rheological Behavior of New Hydrophobically Modified Hydrogels with Tunable Properties
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ABSTRACT: New hydrophobically modified hydrogels have been designed in order to obtain a series of gels with identical elastic moduli but variable dissipative properties. The synthesis of these systems has been realized following a three-step procedure: (1) introduction of double bonds onto a poly(acrylic acid) backbone [PAA], (2) hydrophobic modification of the PAA with dodecylamine, and (3) cross-linking of double bonds using dithiol. The characterization of gel precursors shows that hydrophobically modified polymers self-assemble in semidilute solution forming physical gels with temporary hydrophobic clusters. The gelation mechanism induced by reacting pendant double bonds with dithiol was studied by DSC, specific titrations, and rheology. The gelation process was not perturbed by the presence of the hydrophobic groups, and the kinetics follows a first-order dependence on thiol. In the entangled regime, the thiol conversion reaches around 80%, but only about 10% of the thiols effectively promote the formation of chemical cross-links while the other 90% are incorporated into the gel as loops or dangling chains. Once the gels are formed, NMR and SANS clearly demonstrate that hydrophobic side chains continue to form micelles within the network and that these micelles display a much better long-range order than their un-cross-linked precursors in aqueous solutions. All gels, both hydrophilic and hydrophobically modified, display a storage modulus G‘ which only depends on total polymer concentration and can be described on the basis of the percolation theory (G‘ ε2.6) with ε, the reduced concentration defined from a fixed concentration at the gel point Cg = 2%. On the other hand, the loss modulus G‘ ‘ increases dramatically relative to the corresponding hydrophilic gel, when hydrophobic groups which formed reversible associations are introduced.10/2006; -
Article: Hydrophobically Modified Dimethylacrylamide Synthesis and Rheological Behavior
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ABSTRACT: The synthesis and rheological behavior of hydrophobically modified copolymers based upon N,N‘-dimethylacrylamide (DMAM) containing dodecyl or octadecyl groups are described. The polymers were synthesized by free radical copolymerization in homogeneous solutions of toluene. This synthesis method ensured that the hydrophobic groups were incorporated individually into the copolymer, i.e., in a nonblocky fashion. This method contrasts with the more commonly produced hydrophobically modified polyacrylamides, synthesized by a micellar polymerization technique, resulting in multiblock structures. Associative behavior of the DMAM copolymers in water was investigated by viscosity measurements. Significant enhancement in viscosity was measured in the semidilute unentangled and entangled regimes. Viscosity enhancement was attributed to the formation of intermolecular hydrophobic aggregates, which act as transitory physical cross-links. While it is well established in the literature that blocky copolymers containing hydrophobic groups can significantly enhance solution viscosity, the same effect produced by hydrophobically modified acrylamide polymers based on randomly copolymerized hydrophobic and hydrophilic monomers is less well-known or understood. The results presented in this paper demonstrate that if long alkyl chains are used as stickers, then hydrophobic aggregation between neighboring chains can promote viscoelastic properties in the semidilute regime. The rheological behavior of these statistical copolymers can be described on the basis of recent theoretical models specially developed for solutions of associating polymers.03/2005;
