聚N，N-二乙基丙烯酰胺低聚物水溶液的分子动力学研究

MOLECULAR DYNAMICS SIMULATION OF A MODEL OLIGOMER FOR POLY (N,N-DIETHYLACRYLAMIDE) IN WATER

对50个单元构成的聚N,N-二乙基丙烯酰胺(PDEA)低聚物的水溶液体系进行了分子动力学的研究,分别模拟了300 K时的伸展链、310 K时的伸展链以及紧缩链与水构成的体系,对溶液中PDEA周围溶剂水分子的分布情况以及水分子形成氢键的情况进行了统计,结果表明在PDEA周围的水产生了比本体水更有序的结构,形成了更多的氢键,这种有序结构维持到第二水合层甚至更远.发生相分离后,PDEA与水分子形成的氢键大部分未被破坏,水合层中每个水分子形成的氢键数也没有明显变化,但水合层(形成有序结构的水分子)内水分子数目的减少使得总的氢键数目减少,从而造成体系能量增加及熵增加.同时还研究了聚合物及水分子的自扩散系数,表明PDEA影响周围水分子结构的同时,对水的动力学性质也产生了很大影响.

Poly （N, N-diethylacrylamide） （PDEA） is a temperature sensitive polymer, whose aqueous solution separates into two phases when the temperature excess the lower critical solution temperature （LCST）, accompanying with the collapse of the chains. The purpose of this work was to distinguish the different explanations of the energy change during transition and to investigate the microscopic details of the PDEA-water interaction related to single- chain conformational changes occurring across the LCST. The molecular dynamics （MD） was used to simulate the aqueous solutions of thermoresponsive PDEA molecules with 50 units at 300 K and 310 K for the compact and extended chain conformations, and the number of hydrogen bonds （HB） in the solution was analyzed in more detail. The MD simulations were performed by using the Tinker program with the force field OPLS. The simulation system consisted of 6103 TIP3P water molecules surrounding one PDEA molecule. In order to maintain simulations within the canonical ensemble （NVT）, the Berendsen thermostat was used. The integration step was set to be 1 fs, and coordinates were saved every 0.3 ps. The total simulation time including the equilibration and production run, in which statistical data were gathered, was 2000 ps, where the last 300 ps of each simulation was utilized as the production run period. After the simulation, the trajectory files can be analyzed as required. Analyses of trajectories show that the water molecules mainly contact with the noupolar groups of PDEA. The average number of HB per water molecule varies with the distance of water to polymer in the PDEA aqueous solution. The water molecules form more hydrogen bonds and are in the more ordered structure in the first shell and the second shell around the polymer than those in the bulk water. When the polymer chain transfers from the extended conformation to the compact one, the number of water molecules in the first shell, especially the number of water molecules surrounding the nonpolar groups of PDEA, reduces accompanying with the decrease of the HB number in the solution. As a result, both the enthalpy and entropy of the solution increase. It supports the assumption that the energy change mainly comes from the aheruation of the number of hydrogen bonds between water molecules in the first-shell. The self-diffusion coefficients of polymer molecules and water molecules were also been discussed. The value of self-diffusion coefficient for water molecules surrounding the PDEA is smaller than that in the bulk water, and the value of self- diffusion coefficient for PDEA increases as the polymer chain is compacted.