端炔基对水溶液中聚(N-异丙基丙烯酰胺)相转变的影响

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

通过原子转移自由基聚合,制备出低分子量、窄分布、端炔基的线形聚(N-异丙基丙烯酰胺)(alkynylPNIPAM),利用高灵敏示差扫描微量热技术,在升—降温循环过程中确定端炔基对alkynyl-PNIPAM相转变行为的影响.结果显示,随循环次数增加,升温热容曲线明显变宽,降温热容曲线由双峰逐渐变为单峰;升温和降温的相转变温度有逐渐降低的趋势,但焓变的变化很小.聚合物的低的分子量使端基效应不可忽视,alkynylPNIPAM的端炔基可诱导分子链聚集,形成小的端基聚集体,由于这些聚集体中链内和链间氢键作用,链构象调整变得困难,导致PNIPAM链在升温时更易塌缩、降温时更难解离.

Alkynyl-capped PNIPAM with low molecular weight（ Mn= 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（ Cp）,phase transition temperature（ PTT） and enthalpy change（ ΔH） 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 ΔH 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 π-π 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.