Thermal analysis of PNIPAAm-PAAm mixtures via photopyroelectric technique

Abstract

Crosslinked poly (N-isopropylacrylamide) (PNIPAAm) is a temperature-sensitive hydrogel with approximately 32 0C lower critical solution temperature (LCST) in pure water. The PNIPAAm chains hydrate to form extended structures in water when the latter is below LCST. However, when heated to above LCST, they become compacted by dehydration. In this work, we have used the PNIPAAm- polyacrylamide (PAAm) mixtures with varying molar percentages of PNIPAAm and PAAm that were prepared by free radical crosslinking copolymerization. N' -N'methylenebis (acrylamide) (BIS) and ammonium persulfate (APS) were used as crosslinker and an initiator, respectively. A photopyroelectric technique was applied to test the universality of the sol- gel transition as a function of polymer concentration ratios and to determine thermal effusivity of PNIPAAm- PAAm mixtures. The effects of varying PAAm concentrations on the LCST were studied by determining the temperature dependence of the thermal effusivity, with emphasis is on the influence on the properties and formation on the LCST of PNIPAAm- PAAm mixtures of the acrylamide concentration.