Studies on the Effect of Terminal Alkynyl Groups on the Phase Transition of Poly (N-isopropylacrylamide) in Aqueous Solutions by Micro-DSC

Alkynyl-capped PNIPAM with low molecular weight (M-n = 9800) and narrow distribution was prepared through ATRP to investigate the effect of alkynyl groups on the phase transition of PNIPAM in aqueous solutions with high sensitivity differential scanning micro-calorimetry (micro-DSC), in the viewpoints of thermal capacity (C-p), phase transition temperature (PTT) and enthalpy change (Delta H) upon heating and cooling. Being different from PNIPAM with high molecular weight, this alkynyl-capped PNIPAM shows that with heating-cooling cycle of micro-DSC measurement, the thermal capacity diagramss upon heating become significantly wider while those upon cooling change gradually from the bimodal curves to the unimodal curves. As well, PTT upon both heating and cooling is decreased and Delta H is hardly changed with heating-cooling cylces. The results suggest that the alkynyl end-groups would play an important role in influencing the phase transition of PNIPAM. Probably due to hydrophobic or pi-pi interactions, the alkynyl end-groups might induce the assembly of some PNIPAM chains to form small aggregates. The PNIPAM chains in those small aggregates are more convenient to collapse simultaneously upon heating and more difficult to stretch upon cooling due to chain relaxation until the small aggregates and free chains of PNIPAM reach the dynamic equilibrium, resulting in the gradual decrease of PPT.