Glass transitions of freely suspended polymer films

We present a modified version of our 'sliding model', where chain arcs, bet ween two contacts at the surface, may move if all the barriers along the ar e are weaker than a certain threshold. An important advance of the revised model is that the high limiting chain lengths N* observed in the experiment s (N* similar to 10(4)) are naturally accounted for. In this model, a film of thickness h < 2R(0) (where R-0 is the coil size) c an show either a 'sandwich' structure with two mobile sublayers (at low tem peratures T < T(h)), or a single mobile layer at T > T(h). But the accident occurring at T = T(h), does not necessarily coincide with the apparent gla ss transition T-a(h), determined by the intersection of two tangents in a p lot of thickness versus temperature. (C) 2000 Academie des sciences/Edition s scientifiques et medicales Elsevier SAS.