(709b) Diffusion and Sorption Phenomena of Organic Vapor Penetrants in Unmodified and Ethylenediamine Vapor Phase Cross-Linked Matrimid Thin Films
Sorption of organic penetrants in polymeric films and membranes can have a significant impact on diffusion and induced relaxative sorption of the penetrants in the polymer. Matrimid is a commercial polyimide with a glass transition temperature of 320 °C and is known for its high chemical and thermal stability. This work examines C1-C7 alcohol vapor sorption in unmodified and vapor phase ethylenediamine cross-linked Matrimid thin films ranging from 0.1 to 2.0 µm in thickness. A marked dependence of diffusion coefficient at infinite dilution on film thickness for unmodified films was observed spanning approximately two orders of magnitude for each respective penetrant over the 0.1 to 2.0 µm thickness range. The presence of alcohol vapor even at low activity causes a high degree of plasticization in unmodified Matrimid. Swelling allows continued sorption and consequently increases the apparent diffusivity of the alcohol penetrants over an order of magnitude at activities near unity. Ethylenediamine cross-linking is used to mitigate plasticization effects by preventing polymer chain motion. Films were cross-linked using ethylenediamine vapor and therefore were not exposed to any additional solvents prior to sorption and diffusion experiments in contrast to traditional polymer cross-linking methods. Ethylenediamine cross-linking reduced C6 and C7 alcohol sorption by 90% compared to unmodified Matrimid, while C1-C5 sorption was largely unchanged. Diffusion coefficients for C3 and higher alcohols decreased an order of magnitude in cross-linked films. Relaxation induced sorption behavior was significantly reduced for C4 and higher alcohols. This work demonstrates that ethylenediamine vapor can be used as a surface-only cross-linking technique for polyimide films and membranes and can have a significant reduction in plasticization effects for molecularly larger penetrants.