Molecular dynamics simulation of the structure of C50H102 n-paraffin crystal and ethylene-propylene random copolymers

A C-50 n-paraffin crystal was simulated by methods of molecular dynamics an d the crystal structure variations were studied in the temperature range fr om 200 to 450 K. Subject to 3D periodic boundary conditions at 200 K, the s ample consisting of 48 n-C50H102 chains represented a virtually perfect ort horhombic crystal with the unit cell parameters a = 7.416 Angstrom, b = 4.7 23 Angstrom, c = 2.625 Angstrom. At T = 450 K, the crystal transforms into a melt. Prior to melting (at T similar to 400 K), the orthorhombic crystal exhibits the formation of a hexagonal mesophase characterized by a high mol ecular mobility. The thermal expansion of the crystal, variation of the mea n density, and evolution of the chain conformational composition were follo wed in the course of heating. The results of model calculations are in good agreement with experimental data available in the literature. Statistical ethylene-propylene copolymers were modeled by randomly substituting CH3 gro ups for hydrogen atoms in the orthorhombic C-50 n-paraffin crystal. The CH3 group concentration varied from zero up to a level corresponding to 40% of the total carbon content in the system. The development of distortions in the C-50 crystal in the course of the methyl substitution for hydrogen was traced. When the content of propylene units exceeds 15%, the entire system exhibits amorphization at 200 K caused by breakage of the long-range order as a result of incorporation of the CH3 groups acting as structural defects . The amorphization process is anisotropic, being first developed in the b axis and then in the a axis of the C-50 crystal. The variation of the con f ormational composition of chains in the course of introduction of CH3 group s is analyzed.