Toward a unified view of stress in small-molecular and in macromolecular liquids

Abstract

Interfacial tension behavior of binary and ternary mixtures of partially miscible Lennard-Jones fluids: A molecular dynamics simulation A molecular dynamics study of macromolecules in good solvents: Comparison with dielectric spectroscopy experimentsWe consider a family of molecular liquids, each consisting of linear molecules with N covalent bonds, focusing specifically on Nϭ1 ͑diatomic liquid͒, Nϭ3 ͑four-atom molecular liquid͒, and Nϭ200 ͑macromolecular liquid͒. The bonded and nonbonded potentials, u b (r) and u nb (r), are the same for each system, with u b representing stiff linear springs and u nb corresponding to the repulsive portion of the Lennard-Jones potential. The relaxation of the stress difference , following a constant-volume elongation of the system, is determined in terms of interatomic interactions by nonequilibrium molecular dynamics simulations. It is found that the nonbonded interactions make the principal contribution to while the bonds make a negative contribution. For all systems studied it is found that, following a short induction period after the start of relaxation, ϭC͗͗P 2 ( b )͘͘, where ͗͗P 2 ( b )͘͘(t) is a measure of the individual bond orientation and the proportionality constant Cϭ3.5 within simulation accuracy, for Nу3. The principal difference between the behavior for small and large N is the rate at which ͗͗P 2 ( b )͘͘(t) decays. An explanation of the broad applicability of the relation ϭC͗͗P 2 ( b )͘͘ is presented in terms of the concepts of steric shielding, intrinsic interaction distributions, and intrinsic stresses. The failure of this relation during the short induction period is explained in terms of anisotropies in atom distributions present immediately after deformation.

Testing Rigidity Transitions in Glass and Crystal Forming Dense Liquids: Viscoelasticity and Dynamical Gaps

“…Thus, viscoelasticity will contain very valuable information concerning the rigidity of dense liquids close to a glass transition. For organic glasses, there are some early works concerning the study of flexible and rigid polymer models and how this is related with relaxation (Bartenev, 1970;Picu and Weiner, 1998;Picu et al, 1999). For inorganic glasses, this area is still open in many important aspects (Scopigno et al, 2007;Gueguen et al, 2015;Zhou et al, 2017;Zhu et al, 2018;Sen et al, 2019).…”

Estimation of Macromolecular Configurational Properties from Atomistic Simulations of Oligomers under Nonequilibrium Conditions

“…A comparison of these two approaches has been presented previously 11. The fundamental difference between our approach and that of applying stress or bias to the chain ends has also been discussed by Picu et al15 In either case, an important outcome of such oriented, nonequilibrium simulations is a direction‐dependent quantity, akin to a persistence length of the polymer, that depends on not only the stiffness of the chain but also the state of flow; it turns out to be a rapidly convergent function of the simulated oligomer size. After presenting the forward mapping, we describe a method for the potentially more useful “reverse” mapping, in which a local 〈 P 2 〉 is mapped to an observed 〈 QQ 〉, for example, from a process simulation.…”

Intrinsic Distribution and Atomic Level Stress in Polymeric Melts

“…The intrinsic stress is a cylindrical tensor in which the only nonzero components are 〈〈σ 11 〉〉 and 〈〈σ 22 〉〉 = 〈〈σ 33 〉〉. Most remarkably, it was observed that, when computed from atoms which are not at the chain ends, its components are independent of chain length , Since the intrinsic stress is directly related to the global stress t ij through eq 10 and 11, its invariance with respect to deformation and chain length suggests that it can be used to great advantage in describing stress production and relaxation.…”

“…The inability of the molecular picture of stress production to account for a number of effects such as the behavior of small molecular systems and the high-frequency response, , stimulated the development of an atomic-level description. − In this new framework, stress is computed on the atomic level by explicitly considering both bonded and nonbonded interactions. Moreover, it was shown that the contribution to stress of nonbonded interactions is more significant than that of bonded interactions.…”

“…Interestingly, it was observed that the intrinsic stress tensor is insensitive to the deformation of the melt and is therefore a system parameter for given thermodynamic conditions. Furthermore, the stress−optical coefficient representing the ratio of birefringence to global stress, can be expressed in terms of intrinsic stresses. , These remarkable properties point to the investigation of the structural origins of the intrinsic stress. The results of this investigation are discussed next.…”