SynopsisExtruded, injection-molded, unoriented crystallized specimens and capillary rheometer efflux strands of commercially stabilized polypropylene without nucleating agents were examined by optical microscopy and x-ray diffraction to determine the conditions for ,%form crystallization as a function of the distance from the surface and of the shear rate at commercial processing conditions. Results demonstrate that a t all "cooling conditions" AT = T,,, -Tb (defined as the melt temperature T , minus the bath temperature Tb) effects of strain flow initiate nucleation of &form crystals. The shear rate is demonstrated to be important for &form crystallization. A critical average threshold value for the shear rate of approximately 3 X lo2 sec-l has to be exceeded. The 0 modification is mostly connected with type-I11 spherulites and partly to row structures, and it is observed at processing conditions in oriented structures only.

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Crystallinity measurements were carried out with the help of X-ray diffraction, differential scanning calorimetry (DSC) and density measurements. The results of this study provide a better understanding of the morphology and complex thermal behaviour of polypropylene fibres. Crystallinity studies show that the crystallinity of PP fibres when measured by X-ray and DSC methods as a function of draw-ratios increases with increasing draw-ratios while the crystallinity of the same samples as derived by density measurements shows an inverse trend. These discrepancies are accounted for by the formarion of microvoids within the polypropylene fibres due to drawing. In the DSC scan two endothermic peaks were found to occur in some of the samples. This is explained by the presence of two morphological forms i. e. lamellar and fibrillar morphology in the same sample. The crystallinity values obtained from different methods are correlated with mechanical properties, such as tenacity and elongations.

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