Dynamics in Thin
Polymer Films
Glass Transition
Our measurements of the glass transition temperature Tg in freely-standing polystyrene
(PS) films have revealed very large Tg reductions (up to 80oC) for very thin films [1-3].
By characterizing in detail the dependence of Tg on film thickness h and molecular
weight Mw [3], we observed a beautiful scaling dependence of Tg on h that describes all
of the measured reduced Tg values (> 40) by only 4 parameters. These results have
attracted the attention of Nobel Laureate P.-G. de Gennes, who has proposed a
mechanism by which enhanced mobility of the segments at the free surfaces can result in
enhanced mobility to depths comparable to the overall size of the polymer molecules [4].
This has led to a collaboration that has resulted in a joint publication [5].
We have recently measured the glass transition and hole growth in freely-standing films
of linear PS and cyclic PS for films of different film thickness and two different,
relatively low Mw values [6]. We find that there are no measurable differences between
the results obtained for the two molecular architectures. More interestingly, following the
annealing of the films on mica substrates at high temperatures for an extended time, we
observe small, irreversible changes in the thickness h and index of refraction n of the film
using ellipsometry and scanning probe microscopy during the initial stages of annealing
the films in the freely-standing state. With repeated thermal cycles, the h and n values
approach constant values at a given temperature, indicating that the film evolves toward a
stable state and that the initial annealing of the PS films on mica is insufficient to relax
the polymer chains.
Very recently, Michael Wübbenhorst (TU Delft) has constructed a dielectric
spectroscopy experiment in my laboratory to measure dielectric relaxation processes in
thin polymer films. Preliminary results on poly (methyl methacrylate) (PMMA) films of
different tacticities have shown that there are shifts and broadenings to both local and
cooperative relaxation processes as the film thickness is reduced [7].
[1] J.A. Forrest, K. Dalnoki-Veress, J.R. Stevens and J.R. Dutcher, Phys. Rev. Lett. 77,
2002 (1996).
[2] J.A. Forrest, K. Dalnoki-Veress and J.R. Dutcher, Phys. Rev. E 56, 5705, (1997).
[3] K. Dalnoki-Veress, J.A. Forrest, C.A. Murray, C. Gigault and J.R. Dutcher, Phys.
Rev. E 63, 031801 (2001).
[4] P.-G. de Gennes, Eur. Phys. J. E 2, 201 (2000).
[5] K. Dalnoki-Veress, J.A. Forrest, P.-G. de Gennes and J.R. Dutcher, J. Phys. IV 10,
Pr7-221 (2000).
[6] C.A. Murray, J. Thomas, J.R. Dutcher and G.B. McKenna, Phys. Rev. E, in
preparation.
[7] M. Wübbenhorst, C.A. Murray, J.A. Forrest and J.R. Dutcher, submitted for
publication.
Chain Diffusion
At elevated temperatures, freely-standing polymer films are unstable to the formation of
holes which grow to ultimately destroy the film. Using optical microscopy, we have
measured the growth of the radius of isolated holes in very thin freely-standing PS films
as a function of film thickness [1]. In agreement with previous work on thick films of a
much less viscous polymer [2], we find that the holes grow exponentially with time, with
uniform thickening of the films. The characteristic growth time decreases with decreasing
film thickness, which can be understood quantitatively in terms of nonlinear viscoelastic
effects (shear thinning). We have also developed a new differential pressure experiment
(DPE) which allows us to obtain a sensitive measure of the onset of hole formation upon
heating of freely-standing polymer films [3]. The DPE has been used to probe hole
formation and growth in very thin PS films that have dramatically reduced values of Tg.
We find that hole formation occurs only at temperatures comparable to the bulk value of
Tg. Because holes can form only if there is sufficient chain mobility across the entire film,
the combined Tg and hole growth results provide evidence for a variation in mobility
across the thickness of the films.
[1] K. Dalnoki-Veress, B.G. Nickel, C. Roth and J.R. Dutcher, Phys. Rev. E 59, 2153
(1999).
[2] G. Debrégeas, P. Martin, F. Brochard-Wyart, Phys. Rev. Lett. 75, 3886 (1995).
[3] C. Roth, B.G. Nickel, J.R. Dutcher and K. Dalnoki-Veress, submitted for publication.