(648b) Computing Mechanical Properties of Elastomers Under Multiaxial Deformation Using Molecular Modeling

In order to develop a better understanding of the impact of microscopic changes on macroscopic properties of elastomers, this work focuses on developing novel numerical methods for evaluating changes in elastomer chain conformations under multiaxial deformation. Random elastomer chain conformations were generated under unperturbed conditions using Flory’s Rotational Isomeric State (RIS) Approach. After the unperturbed state of chain ensembles was analyzed through size and shape studies, the deformed state of the ensembles was explored through molecular modeling techniques to quantify external stresses. Uniaxial deformation, equibiaxial deformation and shear were applied to unperturbed chain ensembles by computing changes in the probability density distribution of chain end-to-end vectors and elastic free energy. The approach for computing changes in elastic free energy involved developing a probability distribution-based numerical method that can be applied across multiple modes of deformation. In order to determine the accuracy of the numerical method, it was initially applied to generated Gaussian chains and compared against known analytical equations. Then the numerical model was extended to computing elastic free energy change, force, and stress on RIS cis- and trans-1,4-polybutadiene chains. Compression forces were much greater than tension forces. Equibiaxial and uniaxial stresses in the same direction of extension were equal in magnitude and greater in magnitude than shear stress. Forces and stresses increased with deformation and showed expected dependences on chain volume and temperature. Significant variation was observed in moduli with number of chain repeat units, and the expected linearly proportional variation was observed with temperature. Chains of fewer repeat units correspond to smaller molecular weights between cross-links, leading to a more tightly cross-linked network with larger moduli than that of chains of more repeat units. Young's and shear moduli computed from the numerical model were in good agreement with available experimental results. Implementation of the numerical model along with the RIS method allows for the ability to include effects of various surface chemistries and end groups on conformation distribution to facilitate the study of filled systems such as rubber elastomers used in tires.