Computer modelling of structure and dynamics of C-50 n-paraffin crystal, hexagonal phase of C-50 crystal and ethylene/propylene statistical copolymers

3D crystal of n-paraffin C-50 was created by computer molecular dynamics me thod. The crystal consists of 48 chain molecules containing 48 -CH2-groups and two -CH3 end groups each. Constant temperature-constant pressure calcul ations, spatially periodic boundary conditions, integration step 0.001 ps w ere used. Chemical bonds were treated as absolutely rigid, bond angles bend ing were treated trough U(theta)=1/2K(theta)(theta-theta(0))(2) function an d torsion rotations trough U-phi=C-phi/2[1 + Z(phi)(3cos(3)(phi)-2cos((phi) )] function. Non-bonded interactions were represented as truncated and shif ted L-D 6-12 potential function. All atoms were introduced explicitly. At 2 00K and 36 ps of relaxation we got computer C-50 crystal with orthorhombic symmetry to-phase) and cell parameters a=7,416 Angstrom, b=4,723 Angstrom, c=2,625 Angstrom, and density rho=1,014 g/cm(3) which are close to known fr om literature. Heating of the crystal up to 450K have shown thermal expansi on along all crystallographic axes well coinciding with known experiment. A t 400K the system posses transition to hexagonal phase (h-phase). At 450K t he crystal melts, but high orientation of chain axes still exists in the sy stem. Analysis of rotational and translation mobility of h-phase at 400K ha ve shown that all chains undergo fast rotation around chain axes for angles about 200-360 degrees during 15 ps. Quenching of the h-phase to 200K decre ase the rotation amplitude up to +/-12 degrees corresponding to vibrations. The same type vibration motions (around chain axes) exists in o-phase at 2 00K. Replacing of some H-atoms in chains of initial crystal by -CH3 groups gave us the possibility to mimic Ethylene/Propylene copolymers. In this wor k we have created only random E/P copolymers with propylene (PP) units cont ent up to 20%. At constant temperature (200K) increase of PP units in the c opolymer produce a disordering initially perfect C-50 crystal. The process goes anisotropically: long range order becomes violated along b-direction o f the initial crystal at [PP]=3,5% and along a-direction pronounced order s till 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 (V-loc) of each site containing three chain carbon atoms with the closest neighbours from the surrounding chains. incre ase of [PP] concentration in copolymer increases V-loc and broaden it up co nsiderably. The result shows broad distribution of crystalline cell sizes i n copolymers. Local volumes of cells containing -CH3 groups might be larger or smaller than mean V-loc value. Some conclusions about conformation defe cts in chains appearing due to "copolymerisation'' (appearing of PP-units i n chains) are drawn.