CHARACTERIZATION OF STRESS-WHITENING OF TENSILE YIELDED ISOTACTIC POLYPROPYLENE

The microstructure of tensile tested isotactic polypropylene (IPP) spe cimens was studied by grey level measurement, scanning electron micros copy (SEM), differential scanning calorimetry (d.s.c.) and X-ray diffr action (XRD). SEM revealed that the necked regions of specimens in whi ch stress-whitening had occurred as determined by the grey level measu rement had craze-like structures which were parallel to the drawing di rection. D.s.c. analysis showed that the necked regions of tensile spe cimens which remained transparent after yielding had an additional low -melting temperature peak. However, no additional melting peaks were f ound in the stress-whitened specimens. Inspection of the XRD patterns indicated that, apart from its original cu-crystallites (a monoclinic structure) which were broken and reoriented after drawing, there was n o new types of crystals formed in the transparent specimens. It was fo und by quantitative XRD analysis that the crystallites were broken int o finer pieces in the whitened specimens than in the transparent ones. (C) 1997 Elsevier Science Ltd.