Void formation and growth in amorphous thermoplastic polymer materials

The detrimental effects of voids on the mechanical properties of polymeric structures have been documented by several researchers. The present research focussed on the identification of the chief mechanisms of void formation during the processing of amorphous thermoplastic polymeric materials and the development of a predictive model to enable the suppression of void formation phenomena. The cause of void formation in amorphous thermoplastics, such as PEI, was investigated on the basis of material characterization and processing experiments and identified, via mass spectrometry, as moisture absorbed from the ambient. A mechanism for moisture-induced void formation during the processing of amorphous thermoplasic materials was postulated. Based on the proposed mechanism, a comprehensive numerical model for void nucleation and growth in amorphous thermoplastic materials was formulated, incorporating the effect of processing parameters, such as the heating rate, on the density and spatial distribution of voids. The numerical model was used to conduct parametric studies in order to elucidate the effect of material and process parameters on void formation phenomena. Experimental investigations of the effect of material and process parameters on void formation and growth in amorphous thermoplastics were conducted, using PEI as a representative amorphous thermoplastic polymer system. The effect of absorbed moisture on material properties, such as the glass transition and degradation temperatures of the polymer were experimentally characterized. Material parameters, such as the temperature variation of moisture diffusivity in PEI, were experimentally determined or were estimated based on semi-empirical considerations. Predictions from the numerical model on the effect of processing parameters and material properties on void nucleation and growth were verified by correlating with experimental data obtained for the PEI-moisture system. The model was successfully applied to define drying protocols and the suppression of void formation.