MOLECULAR-DYNAMICS SIMULATION OF THE COLLAPSE OF POLY(1,4-TRANS-BUTADIENE) TO A GLOBULE AND TO A THIN-FILM

The formation and the structure of a polymer thin film have been studied by molecular dynamics simulations. A poly(1,4-trans-butadiene) chain with a degree of polymerization of 180 and terminated by methyl groups was initially constructed so that all dihedral angles at the skeletal bonds are in the trans state. The model film, exposed to vacuum on both sides, was formed by a transition of the fully extended chain to a coil at a temperature of 300 K, with the use of periodic boundary conditions in two dimensions. A collapse transition of the extended chain to a globule was also simulated as a comparison. Simulations have been performed for a duration of 1500 ps. The conformations of the chains in the film and in the globule were examined. The collapse process of the chains can be divided into two regions for the transition to both the coil and the globule. In the first region the chain size shows a dramatic drop, while in the second region it decreases slowly with fluctuations. The density near its center of mass is close to the bulk density of amorphous poly(1,4-trans-butadiene). The coil that is the parent chain for the film exhibits a very loose structure, and its density at the center of mass is close to zero. The radius of gyration for the coil from the simulation is very close to the estimated value from rotational isomeric state theory. The local density of the film follows a sigmoidal profile at the free surfaces. The maximum thickness of the polymer/vacuum interface is about 10 Angstrom, as judged by the density profile. The backbone bonds exhibit a tendency for parallel orientation in the vicinity of the free surfaces. The trans conformation is favored in the equilibrium distribution of the dihedral angles at the CH2-CH2 bonds within the film.