The entropy of the rotational conformations of (poly)isoprene molecules and its relationship to rubber elasticity and temperature increase for moderate tensile or compressive strains

Abstract

Molecular networks comprised of crosslinked cis-1,4 polyisoprene, often referred to as "natural rubber," are one of the most common systems for the study of rubber elasticity. Under moderate tensile or compressive strain, network chains begin to assume straighter paths, as local molecular kinks are removed. Isoprene units along the chain backbone are mechanically forced from their equilibrium distributions of 18 possible rotational states into a smaller subset of states, restricted to more linear conformations with the greatest end-to-end distances. There are two consequences to this change: both the configurational entropy and average internal energy decrease. We find that the change in entropy, and resulting change in free energy, gives rise to an elastic force. We derive an expression for a chain extension force constant that we have incorporated in an explicit, three-dimensional meso-scale network simulation code. Using this force model, our simulations predict a macroscopic stress-strain relationship that closely matches published experimental values. We also predict a slight increase in temperature resulting from the change in average internal energy in the affected isoprene units that is consistent with experiments.

Elastocaloric effect dependence on pre-elongation in natural rubber

“…Isoprene units along the chain backbone are mechanically forced from their equilibrium distributions to a smaller subset of states, restricted to more linear conformations with the greatest end-to-end distances. 27 As a consequence, further elongation from a pre-elongation of 5 results in lower degree SIC than a no pre-elongation process, the material being closer from the saturation of SIC, and is the reason for the decrease of the b value when pre-elongations locate at SIC region.…”

“…26 In the medium extension, the rubber elasticity is caused by the rotational conformations of (poly)isoprene molecules. 27 In the high extension, the rubber elasticity is caused by the purely enthalpic chain stretching. 16 As Hanson and Martin have demonstrated, bond rupture occurs at relatively large strains for polybutadiene, which implies that purely enthalpic chain stretching must commence well before tensile failure occurs.…”

Parallel Replica Dynamics of Bead‐Spring Elastomers at Low Strain Rates

“…The majority of bead‐spring based network models for elastomers require expensive equilibration calculations to generate a network structure at thermodynamic equilibrium, even with advanced equilibration methods . Thus for this work, the network construction procedure of the EPNet model was used as it has been shown to respect the statistics of chain length distributions between crosslinks, without the use of expensive equilibration calculations. While the EPNet model itself has drawbacks, it is emphasized that the intent of this article is to demonstrate the applicability of the ParRep method to a network model of elastomers.…”

A new paradigm for the molecular basis of rubber elasticity

“…In EPnet [37], the total change in internal energy (from equilibrium) is obtained by summing over all chains in the simulation cell that are in extension regime Ib. The temperature increase, as a function of network strain, is calculated by dividing this energy by the material density ρ and specific heat c (the values for polyisoprene are 0.93 g/cm 3 and 1.95 J/g · K, respectively [50]):…”