Correlation between Flow-Induced Nucleation Morphologies and Strain in Polyethylene: From Uncorrelated Oriented Point-Nuclei, Scaffold-Network, and Microshish to Shish

Abstract

Extension-induced crystallization of lightly cross-linked high density polyethylene (XL-HDPE) has been studied with a combination of extensional rheology and in situ synchrotron radiation small-angle X-ray scattering (SR-SAXS) measurements, where XL-HDPE is a dynamic asymmetric system containing both cross-linked network and free chains (23 wt % gel fraction). SR-SAXS results revealed that the nucleation morphologies can be divided into four regions in strain space, namely uncorrelated oriented point-nuclei, scaffold-network nuclei, microshish nuclei, and shish nuclei. The definition of these four regions coincides nicely with the transitions in stress–strain curves, which allows us to establish a correlation between extension-induced conformations of chains and morphologies of nuclei. Even orientation of cross-linked network and free chains leads to the formation of uncorrelated oriented point-nuclei in region I, while the emergence of dynamic asymmetric nature due to disentanglement of free chain results in scaffold-network nuclei in region II. Formation of microshish in region III requires not only orientation but also stretch of chain segments, and finally nearly full extension of chain segments corresponds to shish nuclei in region IV.