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Hydrogels based on N-isopropylacrylamide and sodium acrylate as ionic comonomer were synthesized by free r a d i c a l polymerization in wa t e r using N ,N ? -methylenebisacrylamide as crosslinker and ammonium persulfate as initiator. The glass transition of dried copolymers poly(N-isopropylacrylamide) (PNIPA) and poly(sodium acrylate) (SA) gels and demixing/mixing transition of PNIPA-SA hydrogels swollen with increasing amounts of water were studied using conventional differential scanning calorimetry. In the crosslinked polymers, the glass transition linearly increases, and the transition range becomes broader, with increasing crosslinker content. Increasing content of ionic comonomer also produces an increase of glass transition temperature, which moves to higher temperatures with higher sodium acrylate fraction. The influence of chemical structure of PNIPA-SA hydrogels on the lower critical solution temperature (LCST) of PNIPA-SA/water mixtures during heating and cooling was quantified as function of the content of the crosslinker and the ionic comonomer, as well as water content of the hydrogel in the range from 95 to 70 wt%. At parity of water content, the LCST occurs at higher temperatures for gels containing higher amounts of sodium acrylate. Simi larly, the int roduction of N ,N ? -methylenebisacrylamide causes an increase of the LCST, which grows with increasing of crosslinking degree of the hydrogel.

A detailed morphological analysis of the melting process of poly(butylene terephthalate) (PBT) is presented. Investigations were conducted by polarizing optical microscopy, coupling the measurement of depolarized light intensity with examination of morphology of PBT spherulites during their fusion. These analyses allowed to prove that the spherulitic superstructure of PBT developed upon isothermal crystallization from the melt does not vary with temperature until complete melting takes place, despite the large reorganization involved in the fusion process. It is thus shown that the double melting behavior commonly observed in PBT does not arise from the presence of various spherulitic morphologies with different thermal stabilities, and the structural changes caused by the reorganization processes do not involve variations in the spherulitic morphology. The recrystallization of PBT above the apparent melting point was also analyzed, and the role of memory effects of crystalline precursors remaining in the melt was discussed.