CO2-induced stereocomplex formation of stereoregular poly(methyl methacrylate) and microcellular foams

Crystalline stereocomplex formation of isotactic poly(methyl methacrylate) (it-PMMA) and syndiotactic poly(methyl methacrylate) (st-PMMA) induced by h igh-pressure CO2 at pressures in the 5-35 MPa range and at temperatures in the 30-110 degrees C range was demonstrated by means of differential scanni ng-Calorimetry (DSC) and wide-angle X-ray scattering (WAXS) investigations as a function of treatment pressure, temperature, and the it-PMMA/st-PMMA r atio. At a constant it-PMMA/st-PMMA ratio, the melting temperature of the s tereocomplexes was shown to increase with increasing treatment pressure and temperature. The amount of the stereocomplex formed by high-pressure CO2 w as found to be higher than that by thermal annealing in bulk. The heat of m elting of the stereocomplex was the highest for the 1/2-1/1 it-PMMA/st-PMMA ratio. Compared with amorphous PMMA, the crystalline stereocomplex induced by high-pressure CO2 contributes to the superior morphology of the microce llular foams obtained. For the CO2-treated polymer, the average cell size d ecreases and the average cell density increases as the amount of the stereo complex that, was formed increases.