Abstract
A multilayer film consisting of 32 alternating layers of poly(methyl methacrylate) (PMMA) and polystyrene (PS) produced by forced assembly multiplying co-extrusion has been foamed using supercritical CO2 as a physical blowing agent. Foamability was first assessed by foaming under an induced temperature gradient. Foams were then generated under isothermal conditions at 60, 70, 80, and 90 ⁰C and 100, 200, and 300 bar using a special mold. PMMA layers were found to foam at 60, 70, 80, and 90 ⁰C while PS layers displayed porous morphology only in foams generated at 80 and 90 ⁰C. The films were observed to display delamination during foaming which became more severe in foams generated from higher temperatures with longer saturation times. Preferential foaming in the PMMA over the PS layers is discussed and is linked to the differences in the CO2 sorption and the depression of Tg in the respective polymers.
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