Effects of a Bound Polymer Layer Near the Solid Substrate of Supported Polymer Films
Jack F. Douglas, Francis W. Starr, Wengang Zhang
It is widely appreciated that an attractive polymer-substrate interaction can slow relaxation in thin supported polymer films and polymer nanocomposites. Recent measurements and simulations on nancomposites have indicated that this slowing of polymer dynamics occurs more strongly near the particle surface where a \bound layer" with a much lower mobility exists, strongly influencing the thermodynamics and dynamics of the material. Here we use molecular simulations to show that a bound interfacial layer having a very similar nature arises in thin supported polymer films. This bound polymer effectively insulates the remainder of the film from the strong interfacial interactions, and the resulting thermodynamically determined Tg is relatively insensitive to the polymer-substrate interaction strength when those interactions exceed polymer-polymer interactions. The presence of this bound layer gives rise to an additional relaxation process in the self-intermediate scattering function that is not observed in the bulk material and leads to a slowing down of the average relaxation time of the lm as a whole. The overall relaxation time does not grow in proportion to the strength of the substrate attraction, due to the weak coupling of the substrate relaxation to the interior of the film. At large substrate attraction, the bound layer effectively 'cloaks' the substrate with a relatively immobile particle layer that is chemically compatible with the more mobile film interior, minimizing the effects on changes to Tg.