Molecular dynamics study of water diffusion in an amphiphilic block copolymer with large difference in the blocks＇ glass transition temperatures

作     者：Yang Zhou Phillip Choi 

作者机构：Department of Chemical and Materials Engineering University of Alberta Edmonton Edmonton AB T6G 1H9 Canada 

出 版 物：《Frontiers of Chemical Science and Engineering》 (化学科学与工程前沿（英文版）)

年 卷 期：2017年第11卷第3期

页      面：440-447页

核心收录：

学科分类：0832[工学-食品科学与工程（可授工学、农学学位）] 08[工学] 0817[工学-化学工程与技术] 081701[工学-化学工程] 0703[理学-化学] 083201[工学-食品科学] 

基　　金：WestGrid and Compute/Calcul Canada Agriculture Funding Consortium 

主　　题：molecular dynamics simulation amphiphilic block copolymer free volume water diffusivity fujita model 

摘      要：Isothermal-isobaric molecular dynamics simu- lation was used to study the diffusion mechanism of water in polyurethane-block-poly（N-isopropyl acrylamide） （PU- block-PNIPAm） with a hydrophobic PU/hydrophilic PNIPAm mass ratio of 1.4 to 1 at 298 K and 450 K. Here, the experimental glass transition temperature （Tg） of PU is 243 K while that of PNIPAm is 383 K. Different amounts of water up to 15 wt-% were added to PU-block- PNIPAm. We were able to reproduce the specific volumes and glass transition temperatures （250 K and 390 K） of PU- block-PNIPAm. The computed self-diffusion coefficient of water increased exponentially with increasing water concentration at both temperatures （i.e., following the free volume model of Fujita）. It suggested that water diffusion in PU-block-PNIPAm depends only on its fractional free volume despite the free volume inhomo- geneity. It is noted that at 298 K, PU is rubbery while PNIPAm is glassy. Regardless of temperature, radial distribution functions showed that water formed clusters with sizes in the range of 0.2-0.4 nm in PU-block- PNIPAm. At low water concentrations, more clusters were found in the PU domain but at high water concentrations, more in the PNIPAm domain. It is believed that water molecules diffuse as clusters rather than as individual molecules.