Examination of Non-liquidlike Behaviors in Molten Polymer Films
By Z. H. Yang1, Y. Wang2, Lela
Todorova1, and Ophelia K. C. Tsui1,*
1Department of Physics, Boston University, Boston,
MA02215. 2Department of Physics, Hong Kong University of
Science & Technology, Clear Water Bay, Hong Kong.
Several experiments showed that polystyrene ultrathin films
with thicknesses, h, comparable to 2RG (where RG is the gyration
radius of the polymer) exhibited non-liquidlike behaviors even in
the molten state. By measuring the surface structure of polymer
films subjected to different degrees of thermal annealing, we
show that similar non-liquidlike behaviors can be reproduced if
the annealing time is under τ(qlceq(h)), the relaxation time of
the capillary wave mode with wavevector equals to the
lower-cutoff wavevector, qlceq(h) that characterizes the surface
spectrum of the film at equilibrium. The liquidlike behaviors are
recovered upon annealing beyond τ(qlceq(h)). Because the value of
τ(qlceq(h)) often amounts to days and even years, insufficient
annealing constitutes a likely cause for the non-liquidlike
behaviors. On the other hand, non-liquidlike behaviors can also
be caused by strong adsorption of the polymer chains to the
substrate, causing the interfacial layer to be solidlike. To shed
light to the origin of the observed non-liquidlike behaviors, we
measure the surface dynamics of films with h = 2RG, 4RG and 8RG
undergoing the glass transition, and find that they are all the
same. Our results are in favor of insufficient annealing as the
origin of the non-liquidlike behaviors.
