Tuning morphology and thermal transport of asymmetric smart polymer blends by macromolecular engineering

Abstract

A grand challenge in designing polymeric materials is to tune their properties by macromolecular engineering. In this context, one of the drawbacks that often limits broader applications under high temperature conditions is their poor thermal conductivity $\kappa$. Using molecular dynamics simulations, we establish a structure-property relationship in hydrogen bonded polymer blends for possible improvement of $\kappa$. For this purpose, we investigate two experimentally relevant hydrogen bonded systems -- one system consists of short poly({N}-acryloyl piperidine) (PAP) blended with longer chains of poly(acrylic acid) (PAA) and the second system is a mixture of PAA and short poly(acrylamide) (PAM) chains. Simulation results show that PAA-PAP blends are at the onset of phase separation over the full range of PAP monomer mole fraction $\phi_{PAP}$, which intensifies even more for $\phi_{PAP} > 0.3$. While PAA and PAP interact with preferential hydrogen bonding, phase separation is triggered by the dominant van der Waals attraction between the hydrophobic side groups of PAP. However, if PAP is replaced with PAM, which has a similar chemical structure as PAP without the hydrophobic side group, PAA-PAM blends show much improved solubility. Better solubility is due to the preferential hydrogen bonding between PAA and PAM. As a result, PAM oligomers act as cross-linking bridges between PAA chains resulting in a three dimensional highly cross-linked network. While $\kappa$ for PAA-PAP blends remain almost invariant with $\phi_{PAP}$, PAA-PAM systems show improved $\kappa$ with increasing PAM concentration and also with respect to PAA-PAP blends. Consistent with the theoretical prediction for the thermal transport of amorphous polymers, we show that $\kappa$ is proportional to the materials stiffness, i.e., the bulk modulus K and sound velocity v of PAA-PAM blends.