Thermal properties of cross-linked poly(N-isopropylacrylamide) [P(N-iPAAm)], poly(methacrylic acid) [P(MAA)], their random copolymers [P(N-iPAAm-co-MAA)], and sequential interpenetrating polymer networks (IPNs)

The thermal behavior of cross-linked poly(N-isopropylacrylamide) [P(N-iPAAm)], poly(methacrylic acid) [P(MAA)], their random copolymers and their sequential interpenetrating polymer networks (IPNs) has been investigated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The state of the water has been studied on the basis of DSC traces of swollen samples. Several types of water have been found depending on water concentration and polymer structure. The lower critical solution temperature (LCST) as studied by DSC only occurs in the high N-iPAAm containing hydrogels. For the low water concentration regime, LCST seems to depend on the T-g value. TGA analysis of the dry samples indicates that the copolymers having higher content in N-iPAAm monomer have a lower thermal stability than that of the component homopolymers. On the contrary, sequential IPN samples degrade at higher temperature than their component homopolymers. and copolymers with similar composition, indicating that interchain interactions improve thermal stability. The T-g of the same series of materials has been also measured. A T-g vs composition plot of P(N-iPAAm-co-MAA) copolymers presents a S-shaped curve indicating that structural units interact among them through strong specific interactions. For interpenetrating networks, it seems that only one T-g occurs, indicating a good compatibility and interpenetration.