3D crystal of n-paraffin C-50 was created by computer molecular dynamics method. The crystal consists of 48 chain molecules containing 48 -CH₂-groups and two -CH₃ end groups each. Constant ...temperature-constant pressure calculations, spatially periodic boundary conditions, integration step 0.001 ps were used. Chemical bonds were treated as absolutely rigid, bond angles bending were treated trough U(θ)=½Kθ(θ-θ₀)² function and torsion rotations trough Uφ=Cφ/21+Zφ(3cos³(φ)-2cos(φ)) function. Non-bonded interactions were represented as truncated and shifted L-D 6-12 potential function. All atoms were introduced explicitly. At 200K and 36 ps of relaxation we got computer C-50 crystal with orthorhombic symmetry (o-phase) and cell parameters a=7,416Å, b=4,723Å, c=2,625Å and density ρ=1,014 g/cm³ which are close to known from literature. Heating of the crystal up to 450K have shown thermal expansion along all crystallographic axes well coinciding with known experiment. At 400K the system posses transition to hexagonal phase (h-phase). At 450K the crystal melts, but high orientation of chain axes still exists in the system. Analysis of rotational and translation mobility of h-phase at 400K have shown that all chains undergo fast rotation around chain axes for angles about 200-360⁰ during 15 ps. Quenching of the h-phase to 200K decrease the rotation amplitude up to ±12° corresponding to vibrations. The same type vibration motions (around chain axes) exists in o-phase at 200K. Replacing of some H-atoms in chains of initial crystal by -CH₃ groups gave us the possibility to mimic Ethylene/Propylene copolymers. In this work we have created only random E/P copolymers with propylene (PP) units content up to 20%. At constant temperature (200K) increase of PP units in the copolymer produce a disordering initially perfect C-50 crystal. The process goes anisotropically: long range order becomes violated along b-direction of the initial crystal at PP=3,5% and along a-direction pronounced order still exists at PP=10%. Analysis of structural alterations which appear in the system upon introduction of chemical defects (-CH₃ groups) was done by calculations of local volumes (Vloc) of each site containing three chain carbon atoms with the closest neighbours from the surrounding chains. Increase of PP concentration in copolymer increases Vloc and broaden it up considerably. The result shows broad distribution of crystalline cell sizes in copolymers. Local volumes of cells containing -CH₃ groups might be larger or smaller than mean Vloc value. Some conclusions about conformation defects in chains appearing due to “copolymerisation” (appearing of PP-units in chains) are drawn.
