Stiffening and softening of freshly prepared and aged CTA, PTMSP, and PIM-1 films exposed to volatile compounds

Glassy polymers stiffen or soften when exposed to volatile compounds, depending on the specific combination of polymer-compound and the specimen history. Relevant to the long-term applicability of the separation membranes, three common membrane glassy polymers are studied in this work. Freshly prepared and 2-years aged films from cellulose triacetate (CTA), poly[1 (trimethylsilyl) 1 propyne] (PTMSP), and the archetypal polymer of intrinsic microporosity (PIM 1) were tested using isothermal Dynamic Mechanical Analysis (DMA) at varied vapor activity. Vapors of organic compounds, in which the polymers do and do not dissolve in the liquid phase (solvents and nonsolvents), were studied at 40 °C, namely: dichloromethane (DCM, solvent), p xylene (solvent for PTMSP and PIM 1), and methanol (nonsolvent). Functional groups of the mer units sensitive to the dissolution were identified using Raman spectroscopy. All aged films were stiffer than the freshly prepared ones. Stiffening prevailed for most freshly prepared film-vapor pairs at low vapor saturations (activity < ≈0.4), except CTA and PIM 1 in nonsolvent methanol vapors. Softening prevailed for the aged films and higher vapor saturations (activity > ≈0.6). Vapors of the solvents and nonsolvents did not show the expectable prevalence to softening and stiffening, respectively. Physical aging influenced the stiffening and softening of polymer glasses expectably.