Film thickness and blending can affect crystallization and chain dynamics of semi-crystalline polymers, particularly for submicron films. Herein we apply these insights to elucidate a long-standing puzzle for industrial asymmetric membranes for CO₂ removal from natural gas, which are based on blends of cellulose diacetate (CDA with a degree of acetylation or DS of 2.45) and cellulose triacetate (CTA with a DS of 2.87). The asymmetric membranes comprise thin selective layers (~100 nm) and have been reported to exhibit gas permeability much higher than the bulk blends. We hypothesize that the film thickness and blending confine crystallization and decrease the crystallinity, increasing gas permeability while retaining high selectivity. Specifically, we systematically determined crystallinity for CTA and two blends with various thicknesses ranging from ≈500 nm to ≈20 μm. Adding 75 mass% CTA in CDA (CDA-CTA75) decreases the crystallinity from 34 vol% to 23 vol% at ≈ 20 μm (accompanied by CO₂ permeability increase from 3.9 Barrer to 7.1 Barrer), while decreasing the film thickness of CDA-CTA75 from ≈20 μm to ≈ 1.0 μm decreases the crystallinity from 23 vol% to 13 vol% and increases CO₂ permeability from 7.1 Barrer to 14 Barrer. Understanding polymer chain dynamics and crystallization in thin films can be instrumental in designing high-performance membranes for industrial gas separations.

薄膜厚度和共混会影响半结晶聚合物的结晶和链动力学，特别是对于亚微米薄膜。本文中，我们利用这些见解阐明了用于CO _2的工业不对称膜的一个长期难题。从天然气中去除，这是基于二乙酸纤维素（乙酰化度为CDA或DS为2.45）和三乙酸纤维素（CTA DS为2.87）的混合物。不对称膜包括薄的选择层（〜100 nm），据报道其透气性远高于本体混合物。我们假设膜厚度和共混物限制了结晶并降低了结晶度，增加了气体渗透性，同时保留了高选择性。具体来说，我们系统地确定了CTA和两种混合物的结晶度，厚度范围从≈500nm到≈20μm。在CDA（CDA-CTA75）中添加75质量％的CTA会使结晶度从≈20μm的34vol％降低至23vol％（伴随CO ₂渗透率从3.9 Barrer增加到7.1 Barrer），同时将CDA-CTA75的膜厚度从≈20μm减小到≈1.0μm，将结晶度从23 vol％增加到13 vol％，并将CO ₂渗透率从7.1 Barrer增加到14 Barrer。了解薄膜中的聚合物链动力学和结晶可能有助于设计用于工业气体分离的高性能膜。