A Monte Carlo simulation on the effects of chain end modification on freely standing thin films of amorphous polyethylene melts

The effects of chain end modification on freely standing amorphous polyethy lene thin films with a thickness 2R(g) to 4R(g) (radius of gyration) are st udied by a Monte Carlo (MC) simulation on a high coordination lattice. The rotational isomeric state (RIS) model is incorporated into the simulation a s short-range interactions, and a Lennard-Jones (LJ) potential is used to c alculate the long-range interactions. The modification of the chain ends is introduced by changing the well-depth (epsilon) of the LJ potential as com pared to the middle beads. Chain ends with epsilon(end) less than or equal to epsilon(middle) show segregation near the free surface, but chain ends w ith epsilon(end) much greater than epsilon(middle) prefer to remain in the bulklike region. Both entropy and enthalpy are major factors in the determi nation of the distribution of the chain ends. The effects of the free surfa ce can extend farther than R-g into the interior of the thin film. However, for the case of relatively attractive chain end (epsilon(end) slightly lar ger than epsilon(middle)), a fairly large length of isotropic region is obs erved at the molecular level due to the balance of the entropic effect to p refer a free surface and the enthalpic effect to prefer bulk region. The re pulsive chain ends (epsilon(end) < epsilon(middle)) induce faster dynamics in the middle region of the film as observed by the center-of-mass displace ment and the chain shape autocorrelation function.