Crystallization in miscible and immiscible polymer blends

Abstract

Crystalline morphology influences greatly mechanical properties. Firstly the influence of crystallization temperature on elastic behavior of miscible polypropylene (PP)/hydrogenated polybutadiene (hBR) blends is discussed in terms of residual strain values after pre-defined stretching. The structure was directly observed in nano-scale by transmission electron microscopy (TEM) and quantitatively evaluated by differential scanning calorimetry (DSC). PP/hBR blends serve as a model for explanation of the elastic behaviour of polyolefinic TPE containing PP. There is a big difference in structure and properties for neat PP and PP/hBR blend. Regardless on crystallization temperature, PP shows always yielding point, i.e. plastic deformation after stretching even to small deformation. In neat PP, the crosshatch lamellar crystals were always formed. The situation for PP/hBR blend was completely different. The elastic properties are highly affected by crystallization temperature. The optimum in temperature of cooling medium was found to be around room temperature. In this case the formation of long crosshatch lamellae was prevented and the small fragmented crystal lamellae were formed. The fragmented crystallites play a role of tie points for amorphous chains to render good elastomeric behaviour. There is no yielding point in stress-strain curves for those blends.