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

The elastic properties of a and /i 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 a2 lattice agreed with published results of structure, density, thermal expansion, and stiffness. Monoclinic a 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 inelastic modulus. The non-perfect chiral up-down a1 lattice and the arrangement with one vacancy also display a lower stiffness than a 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 /i modification were sampled to assess the most energetically favorable configurations. Then, both were subjected to the same procedure, validated first for a . A trigonal mono-chiral system, /i2, and astable bi-chiral one with a four-chain frustrated orthorhombic cell, /i1, were found, both presenting novel characteristics. Both /i structures display a less stable conformation than a 2 , observed by a higher specific volume, a lower transition temperature, and amore significant dependence of the elastic moduli with temperature. Remarkably, the mono-chiral /i2 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 /i-kind of crystal is in between the values that we obtained from the simulations of /i2 and /i1 implies that the experimental observations consist of both of these /i crystals, where /i1 might work as an interface between monochiral /i2 layers.