Investigation of the Molecular Mechanism of the Environmental Stress Cracking of Polyethylene by Molecular Simulation
Creators: Kavyani Baghbaderani, Sajjad
Publisher: name: University of Alberta Library
Year: 2024

<p>In this molecular dynamics (MD) simulation study, we investigated the impact of an environmental stress cracking (ESC) agent, nonyl ethoxylate (NE), on the packing and free volume coalescence in between polyethylene chains and the ESC agent, focusing on linear polyethylene (LPE) and branched polyethylene (BPE). Our study considered the size and spatial distribution of free volume holes at the [PE]-[NE] interfaces, analyzing the behavior of ethylene (E) and ethylene oxide (EO) segments of NE. BPE models with 10 and 82 hexyl branches/1,000 backbone carbons were employed, and NE concentrations ranged from 0.001 to 1.4 wt%. The results revealed that, regardless of NE concentration, there were no significant changes in the packings of LPE and BPE chains, as characterized by radial distribution functions (RDFs). However, molecular packings at the [PE]-[NE] interfaces varied with NE concentration, with E segments exhibiting stronger association with PE chains compared to EO segments. The Voronoi tessellation method was employed to assess free volume hole size and spatial distributions, indicating that more and larger holes formed around E segments. The Fourier mode analysis further explored the dynamics of free volume coalescence, showing that the power of free volume coalescence around NE molecules was mitigated by the presence of short-chain branches, reducing the free volume coalescence at the [PE]-[NE] interfaces.<br>
Building on this, we extended our investigation to blends of unbranched and branched polyethylene with NE at 0.5 wt%. The branched polyethylene chains had 10 and 82 hexyl branches/1,000 backbone carbons, and we introduced a four-arm small alkane (7,12 hexyl octadecane) into the blend. Previously, we showed that hexyl branches in branched polyethylene suppressed free volume coalescence, potentially slowing down cavitation. This part of the study, revealed that hexyl branches in the polyethylene chains of the blend exhibited behavior similar to that in pure branched polyethylene. However, hexyl branches in 7,12 hexyl octadecane intensified free volume coalescence, particularly around the hydrophilic EO segments of NE. Addition of 7,12 hexyl octadecane did not appear to slow down void formation, i.e. cavitation, suggesting that the effect of hexyl branches on coalescence dynamics depends on the size of the backbone to which they are attached. This study also revealed that the cavitation may start from the EO-segment of the NE molecule. <br>
Expanding our focus to ESC resistance, we examined the impact of vinyl acetate-modified branch ends on free volume coalescence around NE distributed in a branched polyethylene. Compared to methyl branch ends, vinyl acetate branch ends exhibited higher affinity for the hydrophilic EO-segment of NE. Vinyl acetate branch ends also reduced the power of free volume size fluctuations around both E- and EO-segments, with the reduction correlating with increasing vinyl acetate concentration. The results aligned with experimental observations that the addition of copolymer ethylene vinyl acetate to low-density polyethylene improves ESC resistance, suggesting that vinyl acetate is capable of impeding crack propagation in PE.<br>
In summary, our MD simulations provided insights into the intricate interplay between environmental stress cracking agents, polyethylene chains, and various structural modifications. The findings contribute to a deeper understanding of ESC phenomena and offer potential pathways regarding predicting ESC by computational platforms.</p>