Nonequilibrium molecular dynamics simulation of dendrimers and hyperbranched polymer melts undergoing planar elongational flow

Abstract

The planar elongational melt rheology and structural properties of dendrimers and hyperbranched polymers of different molecular weights (generations 1-4) and their linear counterparts have been studied using nonequilibrium molecular dynamics simulation techniques in the isothermal-isobaric ensemble. The extensional viscosity showed three distinctive regions against strain-rate, including an initial Newtonian region at low strain-rates, followed by a thickening behavior at medium strain-rates and terminated with a thinning region at very high strain-rates, in agreement with the Sarkar and Gupta model [J. Rein. Plast. Comp. 20, 1473-1484]. In addition, a structural analysis was performed to study the size, shape, and spatial distributions within globular dendrimers and hyperbranched polymer molecules under planar elongational flow (PEF). Ratios of the eigenvalues of the gyration tensor showed that contrary to shear flow, under PEF even at low strain rates, dendrimers and hyperbranched molecules have ellipsoidal conformations and change to a much more flattened prolate shape at higher strain rates. In combination with the eigenvalue ratios, the distribution of monomers from the central core of the molecules showed that the thickening region occurs due to branches being stretched, and terminal thinning behavior stems primarily from flow-induced alignment and finite extensibility effects. V

Communication: Role of short chain branching in polymer structure and dynamics

“…[1][2][3][4][5][6][7][8][9] Thus far, however, most research efforts aiming to explore the fundamental role of branches in polymer science have mainly focused on long-chain branched polymers, 2,[10][11][12][13][14][15][16] although it is equally well known that short-chain branching generally significantly affects a wide variety of physical properties such as crystallinity, melting point, modulus, and the hardness of polymeric materials. [16][17][18] From a thermodynamic viewpoint, 19 the standard approach would be to analyze the structure of polymers in solution or melt by accounting simultaneously for the energetics (polymer-polymer and polymer-solvent) and (intramolecular and intermolecular) entropy of the system, and then to determine the properties of the system based on the resulting structural information.…”

“…We conducted a precise analysis of the effect of the chain branching to accurately quantify its consequential effects on the rheological properties of polymeric systems by executing extensive atomistic nonequilibrium molecular dynamics (NEMD) and coarse-grained Brownian dynamics (BD) simulations by precisely controlling the branch length and branching frequency along the chain backbone (thereby overcoming current experimental limitations, e.g., the fact that NMR (Nuclear Magnetic Resonance) cannot distinguish between branches containing more than six atoms 15 ). Three representative polyethylene (PE) melt systems with the same molecular weight (C 178 H 358 ) were employed in this study: (a) linear, (b) H-shaped, with each molecule containing 78 and 25 carbon atoms in the backbone and each of its four branches, respectively, and (c) short-chain branched (SCB), with each molecule containing 128 and 5 carbon atoms in the backbone and each of its 10 branches, respectively (see the supplementary material 30 ).…”

A molecular dynamics investigation of the planar elongational rheology of chemically identical dendrimer-linear polymer blends

“…Hunt and Todd 11 studied the planar shear and elongational behaviour and diffusion 12 of model linear chain polymer with constrained and flexible bond chains using the same method. Subsequently, Hajizadeh et al 13 applied these techniques to investigate the melt planar elongational rheology and structural properties of pure dendrimer and hyperbranched model polymers. van den Noort and Briels 14 performed NEMD simulations on model core-shell systems to study elongational viscosity and shear banding, using the method devised by Todd and Daivis.…”

“…This architectural characteristic is the key for most of their interesting properties, such as high solubility, low viscosity, and encapsulating capability. The flow properties of dendrimers 24,25 and linear polymer melts [9][10][11][12][13]26,27 have been studied previously using nonequilibrium molecular dynamics simulation techniques. Under shear, similar to the conventional linear chain polymers, dendrimers in the melt undergo a transition from the Newtonian regime characterized by the approximately constant viscosity to the non-Newtonian shear-thinning regime.…”

“…29 Under PEF, the strain-rate dependence of the extensional viscosity for linear chain polymers shows three distinctive regions, including an initial Newtonian region at low strainrates, followed by a thickening behavior at medium strainrates, and terminating with a thinning region at very high strain-rates. 9,13,30 Dendrimers show much lower melt viscosities compared to their linear counterparts, and the elongation thickening almost vanishes for high molecular weight dendrimers, 13 due to globular structure and constrained geometry. Neelov and Adolf studied the intrinsic viscosity of dendrimer 31 and hyperbranched polymer 33 solutions under elongational flow using Brownian dynamics simulations.…”