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Polyethylene (PE) is susceptible to environmental stress cracking (ESC). In the presence of an amphiphilic compound and subjected to stress, ESC starts with cavitation followed by slow crack growth and ends with brittle fracture. In this study, we used molecular dynamics simulation to study how branch ends which were chemically modified with vinyl acetate groups affect free volume coalescence around an amphiphilic compound, nonyl ethoxylate (NE), dispersed in branched PE models. Branch ends modified with vinyl acetate significantly impact the free volume coalescence dynamics around NE. Compared with methyl branch ends, vinyl acetate branch ends show a much higher affinity, as quantified by the corresponding radial distribution functions, for the hydrophilic ethylene oxide-segment of NE. However, this is not the case for the hydrophobic ethylene-segment. Interestingly, vinyl acetate branch ends tend to reduce the power (amplitude) of the free volume size fluctuations around both the ethylene oxide- and ethylene-segments. Indeed, the powers of the fluctuations with different PE branching characteristics decrease with increasing vinyl acetate concentration. We believe that free volume coalescence leads to cavitation and that vinyl acetate branches could inhibit cavitation, thereby crack propagation in PE. The power results seem to be consistent with the experimental observation that the addition of copolymer ethylene vinyl acetate to low-density polyethylene improves the ESC resistance of the blend. Highlights * Vinyl acetate branches reduce free volume coalescence activities. * Vinyl acetate branches are attracted to the hydrophilic segment of NE. * Cavitation likely starts from the free volume coalescence around NE. KEYWORDS coalescence dynamics, environmental stress cracking, free volume, polyethylene, vinyl acetate branch ends, 1 | INTRODUCTION Polyethylene (PE) is a widely used polymer known for its excellent mechanical properties, chemical resistance, and durability. (1,2) However, polyethylene is susceptible to a phenomenon called environmental [...]

We used molecular dynamics simulation to study the size distribution and coalescence of free volume around an amphiphilic molecule (nonyl ethoxylate (NE)) at a concentration of 0.5 wt% in blends of linear and branched polyethylene that contained a small four-arm alkane (7,12 hexyl octadecane). The branched polyethylene chains had 10 and 82 hexyl branches/1000 backbone carbons. The coalescence dynamics (fluctuation of free volume in time) was quantified by the number of Fourier frequencies and the corresponding power (amplitude). In our previous work, we hypothesized that cavitation, the first step of environmental stress cracking observed experimentally, starts from the free volume coalescence at the interface between NE and polyethylene molecules and showed that hexyl branches in the branched polyethylene molecules suppress such coalescence, suggesting that cavitation, a much longer time scale process, could also be slowed down. The current work showed that the behavior of hexyl branches in branched polyethylene in a blend with linear polyethylene was similar to that in pure branched polyethylene, but that the hexyl branches in the 7,12 hexyl octadecane exhibited the opposite behavior. In particular, they intensified free volume coalescence, especially around the hydrophilic ethylene oxide segments of NE. The addition of 7,12 hexyl octadecane to branched polyethylene alone or blended with linear polyethylene does not seem to slow down free volume coalescence (cavitation), leading us to conclude that the effect of hexyl branches on the free volume coalescence around NE depends on the size of the backbone to which the branches are attached. Highlights * Hexyl branches reduce free volume coalescence activities. * The longer the backbone is, the stronger the hexyl branch effect. * More coalescence activities occur on the hydrophilic segment of NE. KEYWORDS cavitation, free volume coalescence dynamics, free volume size, hexyl branches, polyethylene blends, 1 | INTRODUCTION Polyethylene (PE) is commonly used in a wide range of consumer and industrial products due to its versatility, low cost, and ease of manufacturing. (1) Some of [...]