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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 [...]

The relative concentrations of the chains (TCs) in six single-site linear low-density polyethylene blown films (LLDPE) were measured using a polarized Fourier transform infrared (FTIR) technique along with a newly developed sample preparation strategy. Before the FTIR measurements, the films were first subjected to a tensile strain of 20% using a homemade appliance and then annealed at a temperature of 60 °C over a time period of 24 h to relax all nonTCs in the amorphous phase. The relative TC concentrations were inferred from the FTIR measurement of the orientation order of the amorphous chains in the stretched films. It is believed that a greater concentration of TCs leads to a higher orientation order for the amorphous chains. The results showed that the quantity of TCs is essentially determined by the chains with the appropriate branch frequency (~12 branches per 1000 backbone carbons based upon the films used in this work), especially those of the high molar mass chains as formation of TCs requires suitable ethylene sequence lengths between branches. The dart impact strength of the films appeared to be positively related to the relative concentration of TCs; films with greater concentrations showed higher dart impact strength. No clear correlations were observed between the relative concentration of TCs and the tear resistance of the films. However, the TC concentration seems to be related to the magnitude of the difference between the transverse direction and machine direction tear resistance. Key words: tie chains, polarized Fourier transform infrared (FTIR), linear low-density polyethylene (LLDPE) blown films, dart impact. Faisant appel a la technique d'infrarouge a transformee de Fourier polarisee (IR-TF) et a une nouvelle strategie de preparations des echantillons, on a etudie les concentrations relatives de chames liees (CL) de six films souffles de polyethylene de faible densite, lineaire et a site unique, Avant d'effectuer les mesures IR-TF, les films ont ete soumis a une force tensile de 20 % a l'aide d'un appareil maison et ils ont ensuite ete recuits a une temperature de 60 °C pour une periode de 20 heures afin de detendre toutes les chames non liees dans la phase amorphe. Les concentrations relatives de chaines liees ont ete deduiles de la mesure IR-TF de l'ordre d'orientation des chaines amorphes dans les films etires. On croit qu'une concentration plus elevee de chaines liees conduit a un ordre d'orientalion plus eleve pour les chaines amorphes. Les resultals montrent que la quantile de chaines liees est essenliellement determinee par les chaines comportant la frequence appropriee de ramification (environ 12 branches par 1000 atonies de carbone dans le squelette si on se base sur les films utilises dans cette etude), particulierement pour les chaines de masse moleculaire elevee puisque la formation de chaines liees necessite des longueurs appropriees de sequence d'ethylene entre les branches. Le force d'impact au dard des films semble etre liee d'une facon positive a la concentration des chaines liees; les films en comportant de plus grandes concentrations presentent une force d'impact au dard plus elevee. On n'a observe aucune correlation entre la concentration relative des chaines liees et la resistance aux dechirures des films. Toutefois, la concentration des chaines liees semble etre liee a l'amplitude de la difference entre la direction transverse et la direction mecanique de la resistance aux dechirures. Mots-des : chaines liees, infrarouge a transformee de Fourier, films souffles de polyethylene de faible densite et lineaire, impact au dard. [Traduil par la Redaction], Introduction Among the structural features in polyethylene blown films, tie chains (TCs) are believed to play a significant role in determining their physical properties as TCs bridge adjacent lamellae. (1) [...]

With a simple thermodynamic analysis, we show that polymers do not satisfy the two major criteria that are required for establishing a unique relationship between cohesion energy density (CED) and internal pressure (IP). Since IP can take values independently of CED, we contend that the IP/CED ratio cannot be established a priori, as the ratio depends on the thermodynamic variables (temperature, pressure, etc.) and the system of interest. POLYM. ENG. SCI., 45:798-800, 2005. © 2005 Society of Plastics Engineers, INTRODUCTION The concept of solubility parameter has been frequently used for the prediction of miscibility of binary polymer blends, as the difference in solubility parameters of the blend components is [...]

A three-dimensional lattice Boltzmann model is used for simulating the pulsating flows in a rectangular duct due to low, moderate, and high periodic pressure gradients. The studies have shown that stress is maximum in the middle of the duct walls and decreases to zero towards the corners, which has helped in designing and optimizing medical apparatuses and understanding the medical flow phenomena.

Surface tensions (gamma) of normal alkanes and methyl methacrylate (MMA) oligomers at various molecular weights in the low molecular weight range are calculated using a new molecular dynamics simulation strategy. The MD simulations, even with the use of a generic force field, reproduced the experimentally observed molecular weight dependence of gamma for both series of oligomers.

Molecular dynamics (MD) simulation is used to study the stability and molecular origin of nanometer-sized water clusters in n-heptane, with the full coverage of adsorbed 6-methyl-2-naphthatenepropionic acid at the water/n-heptane interfaces, at two temperatures. The results show that the coalescence of two water clusters fully covered by naphthenic acid molecules is a low-probability event even at evaluated temperatures and the mobility of naphthenic acids is enhanced with more n-heptane molecules present outside a water droplet.