Search

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 [...]

Accurate prediction of solubility of polymers in solvents a priori is highly desirable in practice. To this end, the Flory-Huggins interaction parameter χ is commonly used and molecular dynamics simulation, a powerful computational tool, has been used for such a purpose. To calculate w, there exist three possible strategies using molecular dynamics simulation. One is through the calculation of Hildebrand solubility parameters of the pure components while the other two are to calculate the enthalpy of solvation and Gibbs free energy of solvation for the solution, respectively. This study evaluated these three strategies using binary solutions containing a hydrophobic or hydrophilic polymer (polyisobutylene, polystyrene, cis and trans polybutadiene, cis and trans polyisoprene, poly(ethylene oxide), and polyacrylamide) and an aliphatic solvent - cyclohexane. We found that χ determined via solubility parameters predicted the solubility trend but deviated significantly from experimental values. On the other hand, the enthalpy of solvation approach provided the most accurate χ values, compared to experiment, at a reasonable computational demand, especially for hydrocarbon polymers, while the Gibbs free energy of solvation approach, though more computationally intensive, did not significantly improve χ from the enthalpy of solvation approach. In particular, the Gibbs free energy of solvation approach overestimated χ for non-polar polymers. A conformational analysis of the solvated polymers revealed that all polymers collapsed in cyclohexane with polyethylene oxide and polyacrylamide collapsed the most as expected. For the two polar polymers used, the collapse was evidenced by abrupt changes in radius of gyration (Rg) and solvent accessible surface area (SASA) in the early stage of molecular dynamics simulation trajectories, and plateauing at much lower final values. Conversely, the hydrocarbon polymers exhibited minimal deviation from the expected Rg and barely any change in SASA with time. Our findings demonstrated that there exist differences in the accuracy and computational resources used when different molecular dynamics simulation strategies are used in the determination of χ. [ABSTRACT FROM AUTHOR]

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]

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 carried out molecular dynamics (MD) simulations on polyethylene (PE) with linear, ring, and four‐arm symmetrical star structures at various molecular weights (420 − 5700 g ⋅ mol−1) and temperatures (410−450 K). We then analyzed the MD data using the technique of proper orthogonal decomposition (POD) to obtain the time correlation functions of different eigenmodes, thereby calculating the viscosity of the aforementioned macromolecules. The time correlation functions show that the ring and star PEs relax much faster than their linear counterpart. Free volume size distributions and the mean fractional free volume of the equilibrated PE melts were determined by Voronoi tessellation (VT). The mean fractional free volume and the corresponding effective pressure were then used to calculate viscosity using a polymer free volume theory recently developed in our lab. The POD and VT approaches yielded similar viscosity values. Furthermore, they both successfully predicted the crossover in the molecular weight dependence of viscosity. It is interesting to note that the difference in the free volume parameters (ϕ+ − F) of various structures always fall within the range of 0.02−0.06. Here, F and ϕ+ signify the probability of a bead having enough free volume for the activation of diffusive motion or momentum transfer and the minimum required fraction of such beads, respectively. The temperature dependence of viscosity as obtained from the POD and VT approaches gave comparable apparent activation energy (Eaapp) values of 5.8−6.4 and 5.4−6.4 kcal ⋅ mol−1, respectively, for the three structures. [ABSTRACT FROM AUTHOR]