Adapted Creep Test for the Study of Non-Equilibrated Thin Polymer Films

In a wide variety of applications, preparation of thin polymer films involves a rapid transition from
a solution into a dry glass. The limited time of this transition is insufficient for the polymer chains
to reach an equilibrium state. Thus, polymer chains are locked in out-of-equilibrium conformations
accompanied by residual stresses in the film. These preparation-induced non-equilibrium
conformations and the corresponding residual stresses in thin polymer films have two important
consequences. First, since the non-equilibrium conformations tend to relax towards equilibrium,
the fabricated polymer film shows time dependent properties that can affect the performance,
reliability and durability of material components and devices. Second, the polymer film shows
properties deviating from the bulk equilibrium properties. However, the origin of such deviations
is not still satisfactorily unveiled. Deviations from the bulk behavior observed in thin polymer
films have been also attributed to an increased molecular mobility at the polymer-air interface,
which has a major contribution in thin films in comparison to the bulk due to the large surface-tovolume
ratio. Therefore, preparation-induced residual stresses represent a persistent unsolved
problem in polymers science and technology that has generated a lot of activities aiming at a better
understanding on how to deal with, how to tailor and how to manage such stresses in thin polymer
films.

According to Albert Einstein: “Scientists investigate that which already is; engineers create that
which has never been”. In order to “create that which has never been”, we managed to clearly
demonstrate the macroscopic consequence of residual stresses introduced into thin polymer films
during spin coating process within this thesis. To this end, we adapted the conventional creep test
to filaments resulting from automatically crumpling thin films of polystyrene as a model polymer.
We analyzed the changes in the length of filaments at temperatures above the glass transition
temperature, where both the residual stresses and the external applied stresses are expected to be
comparable to the rubbery modulus of the polymer. The first key observation was found for an ascast
film, where a surprisingly rapid and large contraction of the filament in the creep experiment
was observed. However, a filament obtained from an annealed film showed an elongation in time,
the expected typical response of equilibrated polymer melts. The contraction of the filament
obtained from an as-cast film is attributed to the recovery of equilibrium conformations of the
initially non-equilibrated polymer chains that were frozen-in during spin coating.

In order to “investigate that which already is”, specifically, residual stresses and the associated
phenomena on a molecular scale, creep experiments were performed by systematically changing
the applied stress and the temperature. From creep experiments at different applied stresses, we
calculated the amount of residual stresses introduced by spin coating. The results showed that the
degree of contraction in the creep experiments, and thus residual stresses, depended on rotation
speed during film preparation through spin coating, and increased with increasing the rotation
speed. From the creep experiments on a filament at different temperatures in the range of Tg to Tg
+ 75 °C, we found the same degree of contraction independent of temperature. However, the
filament contracted in a shorter time, indicating faster relaxation of residual stresses, as the
temperature was increased. An analysis of the temperature dependence of our relaxation times
suggests that segmental rearrangements with an activation energy of about 70 kJ/mol relax residual
stresses in non-equilibrated polymer films. Contraction of a filament can be considered as a release
of energy stored on a molecular scale in terms of changes in filament length. A simple calculation
showed that energies as large as 21 kBT was stored in each polymer chain during the spin coating
process. The stored energy decreased with decreasing the rotation speed, indicating that
preparation conditions play a crucial role in determining the molecular conformations and the
macroscopic behavior of thin polymer films.