Potential energy calculations of ethylene–butadiene copolymers

Abstract

AbstractThe copolymer ethylene–butadiene (PEBU) was investigated by van der Waals potential energy calculations as a function of four internal rotation angles. The butadiene/ethylene molar ratio was assumed to be very small, no greater than 0.25. The energy values corresponding to the fully extended chain of PEBU and polyethylene (PE) are nearly equal, so that the replacement of a H2CCH2 unit with the trans HCCH group (TDB) in PE crystals can be considered feasible, as found by x‐ray diffraction measurements. The three deepest minima of the van der Wals energy correspond to folded chains, with the double bonds lying in the fold parts. Similar calculations carried out on a PE chain point out that TDB behaves as fold‐supporting. The two energetically more favorable conformations of PEBU reproduce rather well the (110) and (200) folds in PE crystals. The path of the (110) → (200) transition does not appear to be hindered by a large potential barrier. If the HCCH group is in the cis conformation, no stable folded chain can be obtained by means of only four intramolecular degrees of freedom. On the basis of these results it is reasonable to suppose that many TDB's could be located on the surface of PEBU single crystals.