| Summary: | 碩士 === 國立成功大學 === 化學系 === 87 === Molecular dynamics simulation and transition state theory have been used to study the transport phenomena of simple gases, like O2, CH4, CO2 and N2. The amorphous polymer system investigated were polyphenylene ether films, including poly(2,6-dimethyl-1,4-phenylene oxide) (PMPO), poly(2,6-diphenyl-1,4-phenylene oxide) (PPPO), and 25/75 random copolymer of PMPO and PPPO (COPOLYMER). The X-ray powder diffraction pattern, diffusion coefficients, sorption isotherms, solubility coefficients and free volume fractions were computed and compared with experiment.
The X-ray diffraction patterns obtained from computer simulation were found to be in accord with experiment. Simulation results also showed that the substitution of different functional groups affects the free volume fractions of polyphenylene ethers and diffusion coefficients of gases. The free volume fractions of these polymers has the order：PMPO > COPOLYMER > PPPO. The diffusion rate for various gases were found to have the same trend, which agrees with experiment. Apparently, the amount of free volume plays a crucial role on the gas diffusion properties for this series of polymer. In general, gas molecules with small sizes are favored for diffusion. The amount of dissolved gases increases with pressure. Among them, CO2 has the highest solubility, while N2 is the lowest. The solubility coefficients calculated by the Henry''s law give the same trend. Regardless the types of gases, the solubility coefficients for gases has the following order：PMPO > COPOLYMER > PPPO, which is similar to diffusion results.
The solubility coefficients of gases within polymer were also computed by a virtual particle insertion method. The solubility coefficient of gases in these three polymers had the same trend: CO2 > CH4 > O2 > N2, which was in accord with experiment. The solubility coefficients of gases obtained from the transition state theory were more accurate than that from the virtual particle insertion method.
With the aid of these calculations it will be helpful to understand the relationship between the chemical structure of polymer and gas permeability.
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