Pressure, Temperature, and Thickness Dependence of CO₂-Induced Devitrification of Polymer Films

The glass transition temperature is known to increase with decreasing film thickness h for sufficiently thin poly(methyl methacrylate) films supported by silicon oxide substrates. We show that this system undergoes a CO₂ pressure-induced devitrification transition, P_g, which is film thickness dependent, P_g(h)=ΔP_g+P_g^bulk. P_g^bulk is the bulk glass transition and ΔP_g can be positive or negative depending on T and P. The phenomenon of retrograde vitrification, wherein the polymer exhibits a rubbery-to-glassy-to-rubbery transition upon changing temperature isobarically, is also shown to occur in this system and it is film thickness dependent.