Eur. Phys. J. E 5, 531-538 (2001)
The glass transition of thin polymer films: some questions, and a possible answer
S. Herminghaus, K. Jacobs and R. SeemannDepartment of Applied Physics, University of Ulm, 89069 Ulm, Germany stephan.herminghaus@physik.uni-ulm.de
(Received 23 April 2001)
Abstract
A simple and predictive model is put forward explaining the
experimentally observed substantial shift of the glass transition
temperature, 
, of sufficiently thin polymer films. It focuses
on the limit of small molecular weight, where geometrical `finite
size' effects on the chain conformation can be ruled out. The
model is based on the idea that the polymer freezes due to memory
effects in the viscoelastic eigenmodes of the film, which are
affected by the proximity of the boundaries. The elastic modulus
of the polymer at the glass transition turns out to be the only
fitting parameter. Quantitative agreement is obtained with our
experimental results on short chain polystyrene (MW = 2 kg/mol), as well as with earlier results obtained with
larger molecules. Furthermore, the model naturally accounts for the weak
dependence of the shift of upon the molecular weight. It
explains why supported films must be thinner than free
standing ones to yield the same shift, and why the latter depends
upon the chemical properties of the substrate. Generalizations for
arbitrary experimental geometries are straightforward.
64.70.Pf - Glass transitions.
68.60.Bs - Mechanical and acoustical properties.
81.05.Kf - Glasses (including metallic glasses).
83.10.Bb - Kinematics of deformation and flow.
© EDP Sciences, Società Italiana di Fisica, Springer-Verlag 2001
