Shear‐induced β‐form crystallization in isotactic polypropylene

Abstract

Abstract Extruded, 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 β‐form crystallization as a function of the distance from the surface and of the shear rate at commercial processing conditions. Results demonstrate that at all “cooling conditions” Δ T = T m − T b (defined as the melt temperature T m minus the bath temperature T b ) effects of strain flow initiate nucleation of β‐form crystals. The shear rate is demonstrated to be important for β‐form crystallization. A critical average threshold value for the shear rate of approximately 3 × 10 2 sec −1 has to be exceeded. The β modification is mostly connected with type‐III spherulites and partly to row structures, and it is observed at processing conditions in oriented structures only.