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 α 2 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 α 1 lattice and the arrangement with one vacancy also display a lower stiffness than α 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 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, β 2 , and a stable bi-chiral one with a four-chain frustrated orthorhombic cell, β 1 , were found, both presenting novel characteristics. Both β structures display a less stable conformation than α 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 β 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 β -kind of crystal is in between the values that we obtained from the simulations of β 2 and β 1 implies that the experimental observations consist of both of these β crystals, where β 1 might work as an interface between monochiral β 2 layers. • Elastic modulus decreases more for more chain-conformational defects. • Pristine α 2 has higher stiffness than non-perfect α 1 or α 2 in presence of vacancy. • Mono-chiral β 2 and bi-chiral β 1 are less stable than pristine α 2 . • Elastic modulus in chain direction below room temperature is highest for β 2 . • Temperature dependence of elastic modulus is stronger for β than for α structures.