(653a) Suppression of Crystallization in Ultra-Thin and Thin Films of Cellulose Acetates and Their Blends and Its Effect on Gas Transport Characteristics

The effect of the film thickness on the physical properties of polymers has been extensively studied, particularly for thin films in the thickness range of 10 nm – 1,000 nm, which is of interest for gas separation membranes. Under such thickness confinement, the crystal orientation and the degree of crystallinity can drastically differ relative to the bulk. As such, the gas transport properties of bulk polymers may not be representative of the properties obtained from commercial membranes, which possess a very thin selective layer between 50-200 nm. This study looks at semi-crystalline cellulose acetates (CAs), which are the workhorse polymeric membranes for CO₂/CH₄ gas separation. CAs exhibit CO₂ permeability of 1.8-6.6 Barrers (1 Barrer = 10^-10 cm³ (STP) cm/cm² s cmHg) at 35 ^oC. However, commercial CAs membranes (with a thickness of 100 – 200 nm) are reported to have CO₂ permeability of 5 - 20 Barrers, which is much higher than that determined for the bulk films. This study reconciles the discrepancy in gas permeability between bulk CA films and asymmetric membranes from perspectives of thickness-confinement and crystallization suppression. Freestanding cellulose diacetate (CDA), cellulose triacetate (CTA), and blends of CDA-CTA films with thicknesses ranging from 60 nm to 20 µm were prepared, and their crystallinity was determined using Differential Scanning Calorimetry and Wide-angle X-ray Diffraction. Gas solubility and permeability are satisfactorily correlated with the crystallinity using the empirical equations available in the literature. We demonstrate convincingly that the micro- or nano-confinement and dynamics in thin-film polymers are instrumental in understanding gas transport properties of industrial asymmetric membranes.