Free Surface Profile and Surface Tension in a Polymer Melt: A Monte Carlo Study

Monte Carlo (MC) lattice simulation studies have been performed for a compressible polymer melt over a wide range of cohesion energies or densities. The free surface of the melt was examd. with respect to its concn. profile, surface thickness D, and surface tension s. For various reduced intersegmental energies, e (e <0), the authors found that s is proportional to -e1/2 and that D varies linearly with (ec - e)-1/2, where ec is a crit. value of the reduced intersegmental energy; both relations are analogous to those for a polymer-polymer interface. The surface thickness for a representative system was calcd. to be in the range D ~ 1.5-4.0 nm. Relative to the values found with the equation-of-state theory for surface tension of Sanchez and Poser and with the functional integral approach of Hong and Noolandi, the authors obsd. somewhat lower densities and thicker interfaces for given cohesion energies. The obsd. surface profiles are also sym. rather than asym. as predicted by these theories. The authors' equation for the dimensionless parameter sred vs. Tred provides realistic values for the surface tension and its temp. coeff. using parameters obtained from data for the bulk polymers. The microscopic anal. of the interface from the MC simulation results revealed a surface enrichment by chain ends, a layering of coil centers beneath the surface, a continuous variation in intersegmental contacts, and a strong deformation of coils in the interface. The authors also found a slight variation of coil dimensions in the bulk for different temps. and cohesion energies. [on SciFinder (R)]