Sorption and Diffusion of Methane and Carbon Dioxide in Amorphous Poly(alkyl acrylates): A Molecular Simulation Study

Molecular simulations were carried out to understand the structural
features and the sorption and diffusion behavior of methane and carbon
dioxide in amorphous poly­(alkyl acrylates) in the temperature range
of 300–600 K. The hybrid Monte Carlo/molecular dynamics approach
was employed to address the effects of polymer swelling and framework
flexibility on the gas sorption. Simulations show that the glass-transition
temperature decreases with the side-chain length of poly­(alkyl acrylate),
consistent with experiments. This is due to the fact that the shielding
of the polar ester groups increases with the side-chain length. The
simulated sorption isotherms for methane and carbon dioxide were in
agreement with the experimental data. The polymer swelling becomes
more pronounced, especially in the case of sorption of carbon dioxide.
A significant swelling occurs, possibly because of the greater interaction
between carbon dioxide and the polar ester groups in the polymers.
The uptake of methane and carbon dioxide by poly­(alkyl acrylates)
generally increases with the side-chain length. Our simulations confirm
the experimental findings that the diffusion coefficients of methane
and carbon dioxide in poly­(alkyl acrylates) increase with the side-chain
length. Interestingly, the activation energies of gas diffusion decrease
with the side-chain length. The diffusion coefficients of the penetrants
have an exponential relationship with the void fraction, which is
in agreement with the free volume theory.

本研究通过分子模拟手段，探究300~600 K温度区间内无定形聚丙烯酸酯（poly(alkyl acrylates)）中甲烷与二氧化碳的结构特征、吸附及扩散行为。研究采用混合蒙特卡洛（Monte Carlo）/分子动力学（Molecular Dynamics）联用方法，以考量聚合物溶胀与分子链骨架柔性对气体吸附过程的影响。模拟结果表明，聚丙烯酸酯的玻璃化转变温度随侧链长度增加而降低，该结论与实验观测一致，这一现象源于极性酯基的空间遮蔽效应随侧链长度增加而增强。模拟得到的甲烷与二氧化碳吸附等温线与实验数据吻合良好。聚合物溶胀现象愈发显著，尤以二氧化碳吸附场景下为甚，出现显著溶胀的原因可能在于二氧化碳与聚合物中极性酯基之间存在更强的相互作用。聚丙烯酸酯对甲烷与二氧化碳的吸附量总体随侧链长度增加而升高。本模拟验证了实验结论：聚丙烯酸酯中甲烷与二氧化碳的扩散系数随侧链长度增加而增大。值得注意的是，气体扩散的活化能随侧链长度增加而降低。渗透质的扩散系数与空隙率（void fraction）呈指数关系，该结果符合自由体积理论（free volume theory）。