How asymmetric chirality and chain density affect chain stiffness of polymer melts

Abstract

Chain stiffness plays a critical role in influencing the thermodynamic and dynamic properties of polymers. Here, all-atom molecular dynamics simulations are employed to study the effects of chain tacticity and temperature on polymer stiffness. Our results show that local stiffness is enhanced by introducing an asymmetric stereochemical unit with opposite chirality joined by all-trans series on polymer chains. The free energy landscape of adjacent dihedral pairs can explain the various chain stiffness of polymer chain with different tacticities. In literature, the temperature dependence of chain stiffness measured by small-angle neutron scattering (SANS)-based experiments contradicts with rotational isomeric state (RIS) predictions and recent single chain simulation results. Our results show that chain stiffness of isotactic PP and PB-1 melt is almost temperature independent, which is consistent with SANS-based measurements. By considering the chain density of polymer melt which is ignored by original RIS theory and single-chain simulations, we point that RIS theory and SANS-based experiments can be harmonic, but the chain density must be explicitly considered for polymer melt.