(Physical Chemistry Chemical Physics) Exploration of the unusual two-step volume phase transition of poly(N-vinylcaprolactam-co-hydroxyethyl methacrylate) hydrogel
作者：Gehong Su, Liyang Jia, Xueqian Zhang, Yulin Zhang, Pengchi Deng and Tao Zhou*
關鍵字：1
論文來源：期刊
發表時間：2018年
It is important to investigate the phase transition mechanism of stimuli-sensitive hydrogels due to its great guiding significance for the application of stimuli-sensitive hydrogels in biomedical applications. In this work, the novel thermo-sensitive poly(N-vinylcaprolactam-co-hydroxyethyl methacrylate) (PVCL-co-HEMA) hydrogel was successfully synthesized via free radical polymerization, and then temperature-dependent FTIR spectra combining with the newly developed the scaling moving-window two-dimensional (scaling-MW2D) correlation spectroscopy and generalized two-dimensional correlation analysis were utilized to investigate its volume phase transition (VPT) mechanism upon heating. Conventional 1D FTIR spectra and Boltzmann fitting results revealed that PVCL-co-HEMA hydrogel exhibited a distinct VPT behavior from the neat PVCL hydrogel due to the incorporation of PHEMA. The essential reason is that a part of water molecules was still confined in the PVCL-co-HEMA network after phase transition at high temperature, rather than continuously being expelled out of the gel with the increase of temperature. Scaling-MW2D spectra revealed that the phase transition of PVCL-co-HEMA hydrogel could be divided into two steps (I and II), and further confirmed the transition regions of these two steps were 25.0-32.3 °C and 32.3-46.8 °C, respectively. The transition regions of both these two steps were obviously lower than that of the neat PVCL hydrogel. According to the generalized 2D correlation analysis of step I, we concluded that the disassociation of the hydrogen bonds between incorporated PHEMA moieties and water molecules is the driving force of the local hydrophobic formation process (step I) , and its occurrence at a lower temperature is the main reason for the VPTT decreasing of PVCL segments. Furthermore, we found that the disassociation of the hydrogen bonds between C=OVCL groups and water molecules is the driving force for the chain collapse (step II), and the driving effect of PVCL segments on PHEMA during phase transition was confirmed. Combining with the obtained sequential order of the steps I and II, an unusual two-step VPT mechanism for PVCL-co-HEMA hydrogel upon heating was proposed.