Thermo-responsive Behaviors Of PNIPAM/Cyclodextrin

Poly(N-isopropylacrylamide) (PNIPAM) is a typical temperature-sensitive polymer. These studies have been popularly researched for decades including the phase transition mechanism and applications of PNIPAM. Through a lot of researches, some models of the phase transition have been built and many factors affecting the LCST of PNIPAM have been identified. Hydrogel is one of application directions of PNIPAM. Recently, the preparation of PNIPAM hydrogel with fast response speed has been focused on. In this paper, we studied about the phase transition mechanism of PNIPAM aqueous solution and preparation of PNIPAM hydrogel with fast response speed. The work is concluded as follows:1. PNIPAM with low molecular weight and terminal alkynyl group was used to investigate the influence of cyclodextrins on the phase transition of PNIPAM in aqueous solution with the combination of different techniques, including NOESY NMR spectroscopy, high sensitivity micro-DSC, isothermal titration calorimetery and FTIR. Because of low molecular weight, the alkynyl end-groups play an important role on the phase transition of PNIPAM, leading to the decrease of phase transition temperature with heating-cooling cycle of micro-DSC measurement. In the presence of cyclodextrin, the phase transition behavior of PNIPAM differs from the type and concentration of cyclodextrin, in the viewpoints of phase transition temperature and enthalpy change upon heating and cooling. Based on the inclusion complexation and hydrogen-bonding complexation between PNIPAM units and cyclodextrin molecules, the influence of cyclodextrin on PNIPAM phase transition was discussed.2. Graft-sliding comb-type PNIPAM hydrogel was prepared via common radical crosslinking polymerization of PNIPAM and the inclusion complex of poly(ethylene glycol)/a-CD, following with grafting PNIPAM chains onto a-CD. Structure and interior morphology of the hydrogel were characterized by FTIR and SEM. After studying the temperature responsive performance of this PNIPAM hydrogel, the data of swelling ratio and swelling/deswell kinetics show that the increase of the a-CD and the grafted PNIPAM chains reduce swelling rate and swelling speed. It is because that the interaction of the a-CD with PNIPAM chains restricts the conformation change of PNIPAM chains, resulting in a lower swelling speed. Moreover, the sliding grafted PNIPAM chains reduce the porous ratio of the hydrogel network, resulting in a lower degree of swelling.3. Graft-sliding and graft-fixed comb-type P(DMAEMA-g-NIPAM) hydrogels were separately prepared through radical copolymerization, ATRP, RAFT and "click" reaction to investigate the influence of grafting chain mobility on stimuli-response behavior. Both of the hydrogels composed by PDMAEMA as hydrogel network, poly(propylene glycol) (PPG) as cross-linking agent and PNIPAM as grafting chains. The hydrogels have dual pH and thermal responsive ability benefiting from the virtues of PDMAEMA and PNIPAM. Structure and interior morphology of the hydrogels were characterized by FTIR and SEM. At the same time, stimuli-responsive performance of the hydrogels was studied in different temperature and pH values using a gravimetric method. We found that the swelling rates of the two hydrogels were low and the difference between the stimuli-responsive performances of the two hydrogels was not obvious. It is because that PPG have the poor solubility in water and the content of PNIPAM was small in the two hydrogels.