Valsecchi Michele, Ramadani Jona, Williams Daryl, Galindo Amparo, Jackson George
Department of Materials, Imperial College London, South Kensington Campus, LondonSW7 2AZ, U.K.
Department of Chemical Engineering, Imperial College London, South Kensington Campus, LondonSW7 2AZ, U.K.
J Phys Chem B. 2022 Nov 10;126(44):9059-9088. doi: 10.1021/acs.jpcb.2c04600. Epub 2022 Nov 1.
Predicting the absorption of gases and liquids in semicrystalline polymers is of critical importance for numerous applications; the mechanical and transport properties of these materials are highly dependent on the amount of solutes dissolved in their bulk. For most semicrystalline polymers which are in contact with an external fluid, the observed uptake of the solute is found to be lower than that predicted by treating the amorphous domains of the polymer as subcooled polymer melts at the same thermodynamic state. This observation has recently led to the hypothesis that the amorphous domains effectively behave as polymer liquids subject to an additional "constraint pressure" which reduces the equilibrium solubility in the domains. We present a new statistical mechanical model of semicrystalline polymers. The constraint pressure emerges naturally from our treatment, as a property of the interlamellar amorphous domains caused by the stretching and localization in space of the tie-molecules (polymer chains linking different lamellae). By assuming that the interlamellar domains exchange monomers reversibly with the lamellae, the model allows one to simultaneously predict the increase of constraint pressure at low temperatures and the variation of the lamellar thickness as a function of temperature─a phenomenon known as premelting. The sorption isotherms of a range of fluids in different polyethylene and polypropylene samples are determined experimentally and the data is compared with calculations of the new model using the SAFT-VR Mie EoS. In order to accurately predict the absorption close to the vapor pressure of the penetrant, we find that it is essential to include the "free", unconstrained amorphous domains in the description, resulting in a multiscale model with two adjustable parameters (the fractions of tie-molecules and free amorphous domains) that characterize the morphology of a given semicrystalline polymer sample. The trends observed for the adjusted parameters qualitatively match other estimates reported in the literature.
预测半结晶聚合物中气体和液体的吸收对于众多应用至关重要；这些材料的机械和传输性能高度依赖于溶解在其本体中的溶质数量。对于大多数与外部流体接触的半结晶聚合物，发现溶质的观察吸收量低于将聚合物的非晶域视为处于相同热力学状态的过冷聚合物熔体时所预测的吸收量。这一观察结果最近导致了一种假设，即非晶域实际上表现为受到额外“约束压力”的聚合物液体，该压力降低了域内的平衡溶解度。我们提出了一种新的半结晶聚合物统计力学模型。约束压力自然地从我们的处理中出现，作为由连接不同片晶的连接分子（聚合物链）在空间中的拉伸和定位引起的片晶间非晶域的一种性质。通过假设片晶间域与片晶可逆地交换单体，该模型允许人们同时预测低温下约束压力的增加以及片晶厚度随温度的变化——一种称为预熔化的现象。实验测定了不同聚乙烯和聚丙烯样品中一系列流体的吸附等温线，并将数据与使用SAFT-VR Mie状态方程的新模型计算结果进行了比较。为了准确预测接近渗透剂蒸气压时的吸收，我们发现有必要在描述中包括“自由”的、无约束的非晶域，从而得到一个具有两个可调参数（连接分子分数和自由非晶域分数）的多尺度模型，这两个参数表征了给定半结晶聚合物样品的形态。调整参数观察到的趋势在定性上与文献中报道的其他估计相符。
