CARBON-DIOXIDE FOAMING OF GLASSY-POLYMERS

The mechanism of foaming a glassy polymer using sorbed carbon dioxide is studied in detail. A glassy polymer supersaturated with nitrogen fo rms a microcellular foam, if the polymer is quickly heated above its g lass transition temperature. A glassy polymer super-saturated with CO2 forms this foam-like structure at much lower temperatures which indic ates the T(g)-depressing effect of CO2. Having this interpretation in mind, the overall sample morphology, i.e., a porous foam enclosed by d ense outer skins, can be completely explained. The dense skins, howeve r, are not homogeneous but show a nodular structure when analyzed by S EM and AFM. Foaming experiments with samples having a different therma l history suggest that the nucleation mechanism underlying the foaming process is heterogeneous in nature. (C) 1994 John Wiley & Sons, Inc.