Pattern formation and evolution in thin polymer films
Abstract
Thin polymer films are important for many technologies. They are used
as coatings, adhesives, lubricants and for device technologies, such as polymer
based light-emitting diodes. Several concerns arise when processing and using
thin polymer films. Properties of thin polymer films (e.g., viscosity, diffusion,
glass transition temperature) are different from bulk properties due to finite size
effects (e.g., confinement of the chains) and to interfacial interactions (e.g.,
presence of the free surface and the substrate). Moreover, the stability of the film
on the substrate is of concern. Thin polymer films, of thickness h < 100 nm,
fabricated on a substrate may rupture under destabilizing forces, such as van der
Waals forces. Rupturing exposes the underlying substrate and the exposed
regions will grow, provided that the spreading coefficient is negative. This
process is known as dewetting. Thus far, two dewetting morphologies have been
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identified but little is understood about their formation and evolution. The first
morphology consists of circular holes throughout the film and the second
morphology is reminiscent of patterns associated with spinodal decomposition
processes. In this research, we investigated four problems. First, we examined
fundamental questions related to the formation and evolution of patterns on the
substrate. We documented the existence of different dynamic stages of evolution
associated with different driving forces for both “conventional” morphologies
(circular holes and “spinodal-like”). Second, we discovered a new morphology
that occurs in a thin random copolymer film on a silicon substrate. This
morphology results from heterogeneous interactions of the chain segments with
the substrate. Third, we examined flow processes in thin polymer films (chain
dynamics near surfaces). We show that a fingering instability develop
spontaneously at the moving liquid front when the film is below a critical
thickness that depends on the length of the chains. This behavior is an intrinsic
property of entangled polymer liquids and is nonexistent in simple liquids under
the same flow conditions. Fourth and finally, based on our understanding of
structural instabilities in thin films, we developed an alternate method to measure
viscosity as a function of film thickness in these systems.
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