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Rotational diffusion coefficients ( D r ) and viscosities (η) of rigid rod molecules consisting of N beads were first revisited for the situation at low density. Starting from this, the transport coefficients at high density condition were expressed in terms of different density pair correlation functions, such as intermolecular radial distribution function and structure factor. This was attained using a mode-coupling theory approach to approximate the intermolecular force–force correlation function, and thus the friction coefficient. As these density pair correlation functions are dependent on N, they determine how "entangled" the rigid rod molecules are, and therefore reproducing the crossover in the log –log plot of transport coefficients with N. It was found that there are crossovers in transport coefficients from D r ∼ N − 2.3 and η ∼ N 2.3 at low N to D r ∼ N − 6.5 and η ∼ N 6.5 at high N. The strength of N dependence is mainly determined by the competition between available free space and the compressibility of the system. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Using a many-chain system of Gaussian chains interacting with each other through the Lennard-Jones (LJ) potential, we demonstrate that there exists a connection between polymer fractional free volume (f) and the entanglement chain length (Ne). Here, f is determined by the generic van der Waals (GvdW) equation of state using the intermolecular radial distribution function [g(r)] of the Gaussian chain generated by the polymer reference interaction sites model while Ne by the polymer mode-coupling theory (PMCT) for strongly coupled macromolecules. The key concept in PMCT is that intermolecular forces experienced by a bead in a chain surrounded by other chains are strongly coupled to the site specific intramolecular radial distribution function. The calculated Gp and η of polyethylene with chain lengths from 500 to 2000 interacting with one another through the LJ potential agree well with experiment and molecular dynamics simulation. [ABSTRACT FROM AUTHOR]

A theoretical method based on the simple Vogel–Fulcher law for estimating the zero shear viscosity of two glass-forming liquids, 1,3-bis(1-nathtyl)-5–(2-naphthyl) benzene and polystyrene, over a range of temperatures from that close to their glass transition temperatures to high temperatures was reported. One parameter of this model, i.e., the critical number density, at which particles are trapped in a cage that only segmental vibration is possible was identified using the intermolecular radial distribution function derived from the Percus–Yevick integral equation and a simplified schematic mode-coupling theory. [ABSTRACT FROM AUTHOR]

We used molecular dynamics (MD) simulation to investigate whether an environmental stress cracking (ESC) agent would help the cavitation process in linear polyethylene (LPE) and branched polyethylene (BPE) by examining the free volume hole size and spatial distributions at the [PE]-[ESC agent] interfaces. The BPE models used contained 10 and 82 butyl/hexyl branches/1000 backbone carbons and the ESC agent used was nonyl ethoxylate (NE) at concentrations 0.001–1.4 w t %. The results show that there exist no significant changes in the packings of LPE and BPE chains, as characterized by their radial distribution functions (RDFs), in the presence of NE regardless its concentration. However, molecular packings at the [PE]-[NE] interfaces vary with NE concentration. In particular, ethylene (E) segments of NE, compared to the ethylene oxide (EO) segments, exhibit stronger association with LPE and BPE chains. We used the Voronoi tessellation method to determine the size and spatial distributions of free volume holes at the [PE]-[NE] interfaces and found that more and larger free volume holes formed around the E-segments than the EO-segments. And such effect was found to be more pronounced in the case of LPE chains than the BPE chains. We further investigated the free volume holes coalescence dynamics using the Fourier mode analysis. The number of frequency modes and the corresponding intensities of the E-segments were significantly higher than those of EO-segments, suggesting that E-segments exhibited a higher tendency to form more and larger free volume holes. This suggests that [PE]-[E-segments] interfaces have a higher chance to form larger voids (i.e., cavitation, a feature of brittle failure that is experimentally observed in ESC). The tendency intensified with increasing NE concentration but was reduced by the presence of short chain branches. The high mobility of terminal carbons in the short chain branches is believed to suppress the formation of large free volume holes at the [PE]-[E-segments] interfaces by absorbing the newly formed free volume by that of the terminal carbons. [Display omitted] • Free volume holes coalescence occurs at the hydrophobic part of an ESC agent. • Free volume holes coalescence likely causes cavitation. • Short chain branches suppress free volume holes coalescence. [ABSTRACT FROM AUTHOR]