Abstract
The thermotropic collapse of the hydrogel of copolymer N-isopropylacrylamide with acrylic acid (NIPA−AAc) upon heating has been studied by high-sensitivity differential scanning calorimetry. Curves of the polymer partial heat capacity have been measured at different pH (from pH 2 to 11) and different concentrations of cationic ligands: CaCl2, dodecyltrimethylammonium bromide (DTAB), and cetyltrimethylammonium bromide (CTAB). Dependencies of the main thermodynamic parameters of the gel collapsetemperature, enthalpy, and width of the transition on pH and ligand concentrationhave been obtained. It has been found that the collapse is characterized by a negative value of the heat capacity increment Δtcp. Its average value determined over all variables studied is Δtcp = −0.62 ± 0.02 J/(g K). This result is in accordance with the observed correlation between the transition enthalpy and temperature of the NIPA−AAc gel. The free energy of the collapse has been calculated as a function of pH and concentration of C...
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