Dielectric relaxations in ultrathin isotactic PMMA films and PS-PMMA-PS trilayer films
Delft University of Technology, Department of Polymer Materials and Engineering, Julianalaan 136, 2628BL Delft, The Netherlands
2 Department of Physics, University of Guelph, Guelph, Ontario, N1G 2W1, Canada
Published online: 5 November 2003
The local and cooperative dynamics of supported ultrathin films (L = 6.4 - 120 nm) of isotactic poly(methyl methacrylate) (i-PMMA, g/mol) was studied using dielectric relaxation spectroscopy for a wide range of frequencies (0.1 Hz to 106 Hz) and temperatures (250 - 423 K). To assess the influence of the PMMA film surfaces on the glass transition dynamics, two different sample geometries were employed: a single layer PMMA film with the film surfaces in direct contact with aluminum films which act as attractive, hard boundaries; and a stacked polystyrene-PMMA-polystyrene trilayer film which contains diffuse PMMA-PS interfaces. For single layer films of i-PMMA, a decrease of the glass transition temperature T g by up to 10 K was observed for a film thickness L < 25 nm (comparable to R EE ), indicated by a decrease of the peak temperature T α in the loss ε ″(T) at low and high frequencies and by a decrease in the temperature corresponding to the maximum in the apparent activation energy E a (T) of the α-process. In contrast, measurements of i-PMMA sandwiched between PS-layers revealed a slight (up to 5 K) increase in T g for PMMA film thickness values less than 30 nm. The slowing down of the glass transition dynamics for the thinnest PMMA films is consistent with an increased contribution from the less mobile PMMA-PS interdiffusion regions.
PACS: codes. – 64.70.Pf - Glass transition / codes. – 77.22.Gm - Dielectric loss and relaxation
© EDP Sciences, Società Italiana di Fisica, Springer-Verlag, 2003