Rapid Note
Glass transitions in thin polymer films
P.G. de Gennes
Collège de France, 11 place M. Berthelot, 75231
Paris Cedex 05, France
pierre-gilles.degennes@espci.fr
Received 7 December 1999
Abstract
Freely standing polystyrene films show an anomalous
drop of the glass temperature Tg when the molecular weight
is high and the thickness smaller than the coil size R0. We
present here a tentative explanation for these features, where
two types of motions compete: a) standard motions, controlled by
the free volume, and independent of chain length, b) collective
motions along the chain, which require a weaker free volume
(except for the end groups). For bulk systems, the standard
motion always wins because of the end group hindrance. But for
films thinner than the coil size, the dominant process may be the
collective motion of a "loop'' which does not involve the chain
ends. What matters then is not the overall polymerisation index
(N), but the length g of a typical loop starting from the
surface (which is a more fluid region) and reaching deep into the
film. With these ingredients, some surprising aspects of
Tg(h,N) may possibly be understood.
PACS
68.10.Et Interface elasticity, viscosity, and
viscoelasticity -
81.05.Kf Glasses (including metallic glasses) -
83.10.Nn Polymer dynamics
Copyright EDP Sciences, Società Italiana di Fisica, Springer-Verlag
