Effect of draw ratio on the microstructure, thermal, tensile and dynamic rheological properties of insitu microfibrillar composites

Microfibrillar composites (MFCs) were prepared using different draw/stretch ratios [viz. 2, 5, 8 and 10] from polypropylene/polyethylene terephthalate (PP/PET) blends. Scanning electron microscopy [SEM] images revealed that PET microfibrils were highly oriented after melt blending and drawing. After the conversion of drawn (stretched) blends to MFCs the PET microfibrils were found to be randomly distributed in the PP matrix. The tensile strength and modulus of the MFCs were found to be higher for the samples drawn at stretch ratios 5 and 8 on account of the long PET microfibrils they possessed. The non isothermal crystallization behaviour of the neat blend (as extruded), stretched blend and the MFC was compared. The oriented PET fibrils in the stretched blend were found to have a greater nucleating effect for the crystallization of PP than the spherical PET particles in the neat blend and randomly oriented short PET fibrils in the MFC. Dynamic rheology studies indicated the storage modulus and loss modulus of MFCs were enhanced as draw ratio increases up to an optimized level beyond which they decrease. When the draw ratio increased up to the optimized level the MFCs tended to be more viscous, especially at low frequency, whereas further increasing the draw ratio resulted in a decrease in the complex viscosity. The microfibrils of PET in the MFC were found to perturb the relaxation of molten PP matrix.