Supercritical CO₂-Induced Stereocomplex Formation of Highly Stereoregular Isotactic Poly(methyl methacrylate) and Syndiotactic Poly(methyl methacrylate) Blends

Abstract

Crystalline stereocomplexes of highly stereoregular isotactic poly(methyl methacrylate) (it-PMMA; mm = 97.2%) and syndiotactic poly(methyl methacrylate) (st-PMMA; rr > 86.1%) blends formed by treatment with supercritical CO2 at pressures from 5 to 25 MPa and at temperatures in the 30−170 °C range were investigated by means of differential scanning calorimetry (DSC) measurements as a function of treatment temperature, pressure, and it-PMMA/st-PMMA mixing ratio. The ΔH of the stereocomplex had a maximum at 33 wt % it-PMMA content, in agreement with stereocomplexes formed in some solvents and in bulk thermal annealing methods. For the case of over 50% it-PMMA content, two endothermic peaks, Tm1 and Tm3, were detected. Examination of the effect of heating rate from 5 to 40 °C/min on the DSC results showed that the melting temperatures, Tm1 and Tm3, corresponded to the fringed micellar structure and the lamellar crystallites of the complexed sections, respectively. For 33% or less it-PMMA content, the crystalline stereocomplex formed by supercritical CO2 had only one endothermic peak that occurred over a narrow temperature range, which was comparable with that obtained by thermal annealing or solvent treatment.