We compare the characteristics of polymer networks synthesized in the same or in different times of reaction, always after gelation. Several poly(N-vinylimidazole) hydrogel samples labeled with fluorescein (PVI) were synthesized by radical copolymerization in aqueous solution with N,N‘-methylenebis(acrylamide) (BA) as cross-linker, with different times of polymerization, and with different concentrations of VI and BA in the polymerization feeding mixture. The glass transitions of samples synthesized with larger BA concentration were broader and shifted to larger T g and with smaller heat capacity jump. Throughout postgel reactions, T g decreases from a value typical of rigid domains enriched in BA to values closer to that of linear PVI, and the average density of permanent cross-links (obtained with the DiMarzio equation) decreases. In apparent contradiction with these facts, the equilibrium swelling degree in water and methanol, gravimetrically determined, decreases significantly upon increasing the time of polymerization. The effective cross-linking density, obtained from swelling measurements, increases throughout postgel reactions, contrary to permanent cross-links. It was concluded that spatial inhomogeneities give a porous structure characterized by high swelling and large T g which during postgel reactions evolves to a less porous and less rigid material. The characteristics of the fluorescence spectra of labeled samples report on the access of water to polymer material and support that conclusion.

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This article reports the scaling laws relating the synthesis conditions with the crosslinking density (νe) and swelling degree (S) of poly(N‐vinylimidazole) hydrogels (PVI) prepared by radical crosslinking copolymerization in aqueous solution, with N,N′‐methylene bisacrylamide (BA) as crosslinker. Multiple linear regression of νe versus BA concentration ([BA]) and total comonomers concentration (CT) in double log scale render the scaling law νe ∼ C italicT0.81 × [BA]1.04 as comparable to that predicted by the model of polymer network with pendant vinyl groups (νe ∼ CT × [BA]), and showing inverse dependence on CT to that expected, following from stoichiometry, for an ideal network (νe ∼ 2[BA]/CT). S scales with νe through a solvent‐dependent exponent ranging from −0.46 to −0.54, only slightly over the value predicted by the Flory–Rehner theory (−0.6) or the blob's model by de Gennes (−0.5 to −0.8). Finally, the scaling law of S with the composition of the reacting mixture is also solvent‐dependent and it seems to result not only from the dependence of νe on CT and [BA] but also from that of v2r, the polymer volume fraction in the reference state, and χ, the polymer–solvent interaction parameter. Models used seem to overestimate the contribution of entanglements to the effective crosslinking density of PVI. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 263–269, 2007

The liquid crystal formed by surfactant AOT/water mixtures in its lamellar mesophase at a spacing between lamellae, d, equal to 8 nm, is mixed with poly(dimethylacrylamide) (PDMAA) polymers of low molecular weight in the range M h n) 2-20 × 10 3 ; the polymers are synthesized by living radical polymerization. The lowest molecular weights do not affect d significantly, indicating that the polymer coils penetrate inside the lamellar phase and dissolve in the water layers. With higher molecular weights, d decreases with added polymer, this decrease being stronger as the molecular weight of the polymer is higher, and the mixture becomes microheterogeneous. This indicates that the higher-molecular-weight polymers are segregated in a separate microphase that partially deswells the lamellae and that this segregation increases with the molecular weight of the polymer. The law, which governs the deswelling of lamella with added polymer, is deduced assuming that a fraction of each polymer can dissolve in the lamellar phase, while the rest of the polymer is segregated from it. This latter fraction is then obtained for each polymer simply by fitting to this law the experimental d as function of polymer concentration. It is proposed that the reason for this fractionation of polymer is that the lamellar structure acts as a grating which sieves the polymer coils according to their size relative to d, chains with molecular weight above a certain cutoff value, determined by d, being excluded from the interlamellar space. The fraction of chains excluded from the lamellae is calculated comparing the experimental molecular weight distributions (from SEC) of the polymers with the cutoff values determined by d. The results show that the polymer samples synthesized here cover the whole spectrum of behaviors, from almost total penetration to almost total exclusion. Both this method of cutting off the molecular weight distribution according to d and the other method of fitting d to the law for lamellar deswelling give similar results for the fraction of polymer that is segregated from the lamellae.

Swollen polymer networks exhibit multiscale pores filled with solvent. Such porosity, inherent to cross-linked polymers, determines some of their most relevant physical properties and applications. In this research, several samples of chemically crosslinked poly(N-vinylimidazole) were synthesized with the same permanent crosslinking density at two different conversions, and their inherent porosity was characterized on freeze-dried specimens by SEM, TEM and nitrogen physisorption. It was thus found that all of the samples showed pores, both on the nanometer and the micrometer scales, whose dimensions were mostly equal to or larger than the mesh size of the primary polymer network (22 nm) and whose volume and specific surface decreased with increasing conversion. Micropores have, in all cases, a very minor contribution. Samples synthesized with the largest comonomer concentrations show quasi-spherical mesopores (90 nm average diameter at any conversion) and macropores (from 5 to 10 microm with increasing conversion), whereas the mesopores of samples synthesized with the largest crosslinker ratios were channel-like (150 nm) and the macropores were interconnected contiguous voids (3 microm). Samples with intermediate compositions exhibit the lowest porosity due, mostly, to interconnected mesopores. The differences in shape were ascribed to the mechanism of phase separation, taking place during polymerization, even for samples that are transparent following polymerization. The inherent porosity is a significant source of spatial inhomogeneity, which contributes to the increase in turbidity. Light scattering decreases with increasing ionization when the degree of protonation is greater than 10%. An important consequence of the inherent porosity is that the degrees of swelling determined either gravimetrically or through size measurements are not equivalent.

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Crosslinked poly(N,N-dimethylacrylamide) hydrogel samples were synthesized with various total comonomer concentrations and crosslinker ratios in the reacting mixture, and at two different temperatures: room temperature and the boiling point of the reacting mixture, about 80 • C. During gelation of samples prepared at the higher temperature, the bubbles of the boiling system were trapped in positions homogeneously distributed, and post-gel reactions fixed them. These samples were macroporous, showing about three times the swelling capacity of conventional hydrogels synthesized at room temperature with the same composition. Both types of hydrogel swollen at equilibrium in water deswelled exponentially with time when they were immersed in acetone or dioxane. The rate of shrinking was higher for macroporous than for conventional samples and it was smaller in dioxane, the solvent with higher viscosity (η), although there was no proportionality to the solvent fluidity, η −1 . The morphology, which was in the scale of micrometres (as revealed by scanning electron microscopy), played a minor role in the shrinking rates of both types of gels. The excess swelling and the faster solvent response of macroporous gels were ascribed to the store and draining capacity of macropores at the millimetre scale.

Dimethylacrylamide and N,N′-methylenebis(acrylamide) are copolymerized in the lyotropic AOT/water medium to yield several types of macroporous hydrogels. These gels are formed in two different macroscopic phases of the reacting system. The morphology of the gels, once isolated, is widely different depending on the macroscopic phase in which they were formed. Inside each phase, the polymerization induces an additional microscopic phase separation, with the result of deswelling of the original lamellar AOT/water structure. The morphology of the isolated gels is not a copy of this original nanostructure, instead, it has features whose length scales are larger than those of the starting phases. More complex morphological patterns develop in the presence of surfactant, as a result of the phase separation process. The type of morphology appears to be correlated with the proportion of water contained in the macroscopic phases. The swelling of the isolated gels in water does not follow the increase with chain length between cross-links predicted by network theory, probably because of the contribution from the macropores.