Competition between crystallization and dewetting fronts in thin polymer films
We investigate the competition between dewetting and crystallization fronts in thin polymer films where the velocities of these fronts are modulated by varying the temperature from a low temperature regime, where crystallization dominates, to a high temperature regime, where dewetting is more rapid and dominates the film structure. Specifically, thin films of poly(caprolactone) (PCL) on n-octyldimethylchlorosilane (ODS)-treated substrates are investigated as a model system exhibiting this general phenomenon. Structure formation at low temperatures is dominated by polycrystalline spherulitic structures that nucleate at random positions within the plane of the film, but at intermediate temperatures, where the crystallization and dewetting are strongly coupled, dewetting occurs preferentially at the grain boundaries between the spherulites. We term this phenomenon crystallization-induced dewetting (CID) and provide evidence that it arises from the high stresses at the spherulite grain boundaries that accompany local volume changes of the polymer material upon crystallization. CID is prevalent in a temperature range in which the angles between the grains are larger than a critical angle and this effect allows for considerable control over the resulting film morphology.