Molecular Weight and Distribution Dependent Critical Stress of Fragmentation of Highly Oriented Amorphous Network in Stretched Isotactic Polypropylene

Isotactic polypropylene (iPP) can be stretched to a large strain developing a macroscopic transparent structure with aligned fibrillar/micro-fibrillar crystals embedded into a highly oriented amorphous network at elevated temperatures but far below the melting point. Fragmentation of this highly orientated amorphous network occurs during further tensile stretching, indicated by strong stress-whitening over the whole specimen. Microscopically, stress-whitening is considered due to the cavitation processes within the sample attributed to the mechanical instability of the micro-fibrils/fibrils caused by the breakage of the load-bearing inter-fibrillar/micro-fibrillar tie chains. We investigated the molecular weight and its distribution dependencies of critical stress for initiating such cavitation-induced stress-whitening in a series of polypropylene samples. It turns out that the critical stress for triggering the cavitation at a large strain stage is determined by the weight-average molecular weight (Mw) rather than the number-average molecular weight or molecular weight distribution (MWD). This phenomenon hinted that the load-carrying elements in the fibrillar system were long chains, which contributed a large proportion to the calculation of Mw. Furthermore, two iPP samples with similar Mw but extremely different in MWD were chosen to ensure that they owned a comparable number of inter-fibrillar/micro-fibrillar tie chains but distinct numbers in chain ends. The identical critical stress for triggering large strain cavitation presented in these two samples demonstrated that the inter-fibrillar/micro-fibrillar tie chains overrode the chain ends in dominating the stability of the fibrillar network.