Relationship between microstructure changes and embrittlement during chemo-oxidative degradation of bimodal HDPE with short chain branches

Abstract

Bimodal HDPE with short chain branches (SCBs) has been the focus of recent research to provide long-term durability and improved resistance to cracking for pipe applications. This study aims to identify the structural changes of bimodal HDPE with SCBs when degraded by a chemo-oxidative environment and to interpret the ductile-brittle transition relative to microstructure including the molecular weight distribution (MWD), molecular entanglement and lamellar dimensions. Sheets of commercially available bimodal HDPE with SCBs preferentially placed in the high molecular weight (HMW) region were exposed to an oxidative environment (5 ppm chlorinated water at 65 °C) for up to 3000 h. Sample microstructure was evaluated by FT-IR, DSC, SAXS, GPC-IR, TREF and cross-fractionation chromatography (CFC), while mechanical performance was characterized by tensile tests. Data revealed an increase in carbonyl index, overall mass crystallinity (61–70 %), and crystalline lamellar thickness (98 $\dot{A}$ to 113 $\dot{A}$) with exposure time. Also observed was a decrease in weight average molecular weight (242–17 kg/mol) and amorphous lamellar thickness ($68\dot{A}to48\dot{A}$). Analysis of TREF and CFC indicated that, during chemo-oxidative degradation, chain scission primarily occurs in the high molecular weight tie chains with SCBs. After 1500 h exposure, the strain hardening modulus and strain at break are reduced to 4.57 MPa and 3.04, respectively. This onset of embrittlement was evaluated relative to changes in microstructure: the number of molecular entanglements (which was reduced from 736 to 74) and critical values for HDPE molecular weight (38 kg/mol) and amorphous lamellar thickness (53 $\dot{A}$). The data show that these critical values and reduced entanglement standards are comparable to those developed for evaluation of brittle behavior in undegraded HDPE.