Solid-state poly(methyl methacrylate) (PMMA) nanofoams. Part I: Low-temperature CO2 sorption, diffusion, and the depression in PMMA glass transition

Abstract

In this paper, we report on the solubility and diffusivity of CO2 in PMMA in the range −30 °C to 100 °C, and on the depression in the glass transition temperature when the polymer acquires an equilibrium mass% of CO2 in this temperature range. Solubility of CO2 in PMMA increases ten-fold from 3.9% at 100 °C to 39.3% at −30 °C. We observe a drop in ΔHs, the heat of sorption, at the transition of CO2 from vapor phase to liquid phase, leading to a weaker dependence of solubility on temperature in the liquid CO2 regime. Four distinct diffusivity regions are identified, where diffusivity may increase or decrease with the decreasing temperature, characterized by the different activation energies. Besides the two transitions (normal vitrification and retrograde vitrification) between glassy state and rubbery state observed previously in PMMA-CO2, we also find a new transition in the vicinity of CO2 vaporization temperature. The dissolution of CO2 in PMMA lowers its glass transition temperature in a non-linear fashion and the incorporation of 39.3% CO2 in PMMA decreases its Tg from 103 °C to −12.5 °C. In part II of this paper, we describe a process, based on low-temperature CO2 saturation, that creates PMMA nanofoams with 30–40 nm cells.