Molecular dynamics analysis of iPP-polymorphs; Investigating thermal expansion and elastic properties

Abstract

The elastic properties of $\alpha$ and $\beta$ crystals of isotactic polypropylene are obtained by molecular dynamics simulations of a crystalline domain resembling an infinite crystal. The helical polymer chains are modeled with an all-atom force-field. The assessment of the pristine ${\alpha }_2$ lattice agreed with published results of structure, density, thermal expansion, and stiffness. Monoclinic $\alpha$ systems with particular imperfections are sampled to assess the effect of defects on conformational stability and elastic properties. The sensitivity of elastic moduli with temperature correlates with the helicity disruption: the more chain-conformational defects, the stronger the decrease in elastic modulus. The non-perfect chiral up-down ${\alpha }_1$ lattice and the arrangement with one vacancy also display a lower stiffness than ${\alpha }_2$, which can be attributed to a less dense crystal and decreased inter-chain cooperative forces due to periodicity disruption.

Two variations of the metastable $\beta$ modification were sampled to assess the most energetically favorable configurations. Then, both were subjected to the same procedure, validated first for $\alpha$. A trigonal mono-chiral system, ${\beta }_2$, and a stable bi-chiral one with a four-chain frustrated orthorhombic cell, ${\beta }_1$, were found, both presenting novel characteristics. Both $\beta$ structures display a less stable conformation than ${\alpha }_2$, observed by a higher specific volume, a lower transition temperature, and a more significant dependence of the elastic moduli with temperature. Remarkably, the mono-chiral ${\beta }_2$ crystal showed higher elastic modulus than any other crystal below room temperature, related to a more efficient global methyl interlocking between chains. The fact that the experimental value for the density of the $\beta$-kind of crystal is in between the values that we obtained from the simulations of ${\beta }_2$ and ${\beta }_1$ implies that the experimental observations consist of both of these $\beta$ crystals, where ${\beta }_1$ might work as an interface between monochiral ${\beta }_2$ layers.