Quantifying the Contribution of London Dispersion Interaction and Adjacent Chain Packing on the Polymer Stiffness: A DFT Study

Linear polymers often have the role of sustaining tension along their chains, and thus, the longitudinal modulus of such polymers has been of great interest. To quantify the contribution of individual chain properties and their association with crystals, we studied the deformation behavior of three major classes of linear polymers, polyethylene, poly(p-phenylene terephthalamide), henceforth abbreviated as PPTA, and cellulose, using density functional theory (DFT). Polyethylene, the simplest polymer, elongates by simply widening its bond angle and stretching its C-C bonds. The single-chain spring constant along the chain is not affected by packing, and the longitudinal Young’s modulus is proportional to the number of chains per cross-sectional area. In contrast, the torsion around chemical bonds plays an important role in the deformation of PPTA and cellulose. Finally, lateral packing contributes to the stiffening of individual chains, the effect of which can be modeled by either removing the correction for London dispersion interactions or removing adjacent chain packing. This finding highlights the different structure-property relationship between crystalline polymers with and without hydrogen-bonding networks and will provide molecular insights that may assist the molecular design of high-performance polymeric materials.