HIERARCHICAL STRUCTURE GRADIENTS DEVELOPED IN INJECTION-MOLDED PVDF AND PVDF-PMMA BLENDS - I - OPTICAL AND THERMAL-ANALYSIS

The structural gradients developed along and across the flow direction of injection-molded PVDF and PVDF/PMMA parts were investigated by opt ical microscopy and thermal analysis techniques. The spatial variation of crystallinity across the thickness direction was found to be insen sitive to the process variables: injection speed and mold temperature. This relatively flat crystallinity profile across the thickness of th e parts was found to decrease with the increase of PMMA concentration. The blends become noncrystallizable beyond about 40-45% PMMA concentr ation. The influence of flow history on the structural evolution acros s the thickness was observed in the peak position of the cold crystall ization region. This peak temperature showed a minimum at depths where shear effects are at their maximum. This was attributed to the increa sed levels of chain orientation frozen in the amorphous portions of th ese regions which crystallize at lower temperatures upon heating. (C) 1998 John Wiley & Sons, Inc.