Study On Phase Transition Mechanism Of Temperature-sensitive Hydrogel By Near-infrared Spectroscopy

Temperature-sensitive hydrogels have been widely applied in drug delivery,tissue engineering and other fields.The mechanism of phase transition of temperature-sensitive hydrogels was studied by near-infrared(NIR)spectroscopy in this paper.Firstly,the temperature-sensitive PNIPAM hydrogels were synthesized by NIPAM polymerization initiated by APS,and their morphology and functional groups were characterized by SEM and IR,respectively.The results showed that the surface is porous and the cross section is stratifiedand full of folds.In addition,the band assignment of the groups in the NIR region was determined.Secondly,the NIR spectra of the PNIPAM hydrogel were measured during the temperature variation process and analyzed by Principal Component Analysis(PCA),second derivative and Multivariate Curve Resolution-Alternating Least Squares(MCR-ALS).It showed that the phase transition of hydrogels is reversible in the process of heating and cooling,and the groups begin to change before the phase transition temperature occurring.The reason of phase transition is that the hydrogen bonds between hydrophilic groups and water decrease with temperature increasing in the heating process.The phase transition is hysteretic in the cooling process.The reason is that the molecular chains are wound in the phase transition process,and the winding is not untied in time during the cooling process.Finally,the influence of different conditions on the phase transition temperature was explored.The results showed that water content in the system and the molecular weight of PNIPAM have little effect on the phase transition temperature,the addition of NaCl can reduce the phase transition temperature,and the phase transition temperature increases with the increase of AA in the copolymer P(NIPAM-co-AA).In addition,the phase transition of P(NIPAM-co-AA)hydrogel is a gradual change process while PNIPAM hydrogel is a mutation process.