Polymer–CO₂ systems exhibiting retrograde behavior and formation of nanofoams

The sorption of compressed gases in polymers causing a reduction in the glass transition temperature ( T g ) is well established. There is, however, limited information on polymer–gas systems with favorable interactions, producing a unique retrograde behavior. This paper reports on using a combination of established techniques of in situ gravimetric and stepwise heat capacity ( C p ) measurements using high‐pressure differential scanning calorimetry (DSC) to demonstrate the occurrence of this behavior in acrylonitrile–butadiene–styrene copolymer (ABS)–CO 2 and syndiotactic poly(methyl methacrylate) (sPMMA)–CO 2 systems. The solubility and diffusion coefficient of CO 2 in the range 0 to 65 °C and pressures up to 5.5 MPa were determined, which resulted in a heat of sorption of − 15.5 and − 15 kJ mol −1 , and an activation energy for diffusion of 28.3 and 32.1 kJ mol −1 in the two systems, respectively. The fundamental kinetic data and the changes in C p of the polymer–gas systems were used to determine the plasticization glass transition temperature profile, its relationship to the amount of gas dissolved in the polymer, and hence the formation of nano‐morphologies.