分子模拟研究小分子在聚硅氧烷中扩散行为

Study of Diffusion of Small Molecules in the Silicone Polymers Using Molecular Simulation

采用分子动力学(MD)法,在298 K的温度下,对H2,He,O2,N2,CO2,CH4,H2O以及ethanol在聚二甲基硅氧烷(PDMS),聚丙基甲基硅氧烷(PPMS),聚辛基甲基硅氧烷(POMS)以及聚苯基甲基硅氧烷(PPhMS)4种聚硅氧烷体系中的扩散进行模拟.选用对凝聚态的结构和性质描述更为准确的COMPASS分子力场模型.体系进行能量优化后得到的密度和玻璃化温度的计算值和实验值的对比表明模拟体系接近于真实体系.随后进行分子动力学模拟得到小分子的运动轨迹,对小分子扩散的两种方式进行了讨论,探讨了侧链对扩散过程的影响.

Molecular dynamics simulation was used to estimate the diffusion coefficients of H2, He, 02, N2, CO2, CH4, H2O and ethanol at 298 K in poly （dimethylsiloxane） （PDMS）, poly （propylmethylsiloxane） （PPMS）, poly （octylmethylsiloxane） （POMS） and poly（phenylmethylsiloxane） （PPhMS）. Atomic simulation techniques have been proven to be a useful tool for the understanding of structure and dynamics of dense amorphous membrane polymer and of transport processes in these materials. The Condensed-phase Optimized Molecular Potentials for Atomistic Simulation Studies（COMPASS）force field was used to construct the silicone polymers. It is found that the COMPASS force field is good at describing the property of polymers. Molecular dynamics simulation（NPT ensemble）was used to obtain specific volume as a function of temperature. The calculated glass transition temperature and density after refinement are in good agreement with the experimental results. Diffusion coefficients were obtained from molecular dynamics（NVT ensemble）using up to lns simulation times. After molecular dynamic simulation, the trajectories of the small molecules in the polymer matrix were obtained. Then diffusion coefficients were calculated from the Einstein relation revealing a considerable agreement between simulated and calculated data. Accordingly two types of motions of the small molecules diffusion in the polymers are discussed. Finally,the effect of the side chain on the diffusion of the small molecules is also presented.