Molecular Weight and Distribution Dependent Critical Stress of Fragmentation of Highly Oriented Amorphous Network in Stretched Isotactic Polypropylene
Ying Lu, Dong Lyu, Cenhui Han, Yongfeng Men
Abstract
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 ( M w ) 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 M w . Furthermore, two iPP samples with similar M w 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.