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

The elastic properties of α and β 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 α₂ lattice agreed with published results of structure, density, thermal expansion, and stiffness. Monoclinic α 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 α₁ lattice and the arrangement with one vacancy also display a lower stiffness than α₂, which can be attributed to a less dense crystal and decreased inter-chain cooperative forces due to periodicity disruption.

Two variations of the metastable β modification were sampled to assess the most energetically favorable configurations. Then, both were subjected to the same procedure, validated first for α. A trigonal mono-chiral system, β₂, and a stable bi-chiral one with a four-chain frustrated orthorhombic cell, β₁, were found, both presenting novel characteristics. Both β structures display a less stable conformation than α₂, 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 β₂ 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 β-kind of crystal is in between the values that we obtained from the simulations of β₂ and β₁ implies that the experimental observations consist of both of these β crystals, where β₁ might work as an interface between monochiral β₂ layers.