Melt Fracture of Polymer Thin Film at Strongly Attractive Surfaces
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Melt Fracture of Polymer Thin Film at Strongly Attractive Surfaces Chunhua Li∗, Jun Jiang, Miriam H. Rafailovich, Jonathan C. Sokolov Department of Material Science and Engineering, SUNY at Stony Brook, Stony Brook, NY, 11794-2275 ABSTRACT In this study, we report an interesting phenomenon of “melt fracture” which was observed when a high viscosity film dewets from a film of lower viscosity. We propose that this phenomena is similar to the “melt fracture” or “shark skin” that is observed when extruding bulk polymer. We hypothesized that the “melt fracture” occurs as a result of shear which is imposed by the dewetting layer on the visco-elastic lower layer. The dewetting layer is adhered to the lower layer via entanglements across the polymer/polymer interface. When the other interface of the liquid film is adsorbed to an attractive substrate interface, a velocity gradient occurs in the film and therefore can result in the shear gradient. We proposed that if this shear rate exceeds the natural reptation time, melt fracture of thin film resulted. Screening the substrate interaction by first deposition a very thin layer of immiscible polymer such as poly (vinyl-pyridine) PVP reduced the degree of melt fracture. A DI 3000 Atomic Force Microscopy (AFM) was used to quantify the depth and the dynamics of the melt facture process. INTRODUCTION In the bulk, “melt fracture” or “shark skin” occurs when the polymer melt is extruded too rapidly and some of the polymer adheres to the surface of the extruder. Even though it is a well-known phenomenon which interferes with bulk melt processing, it has not been reported in thin films. In this study, we report the “melt fracture” of polymer thin film which was observed when a high viscosity film was allowed to dewet from a film of lower viscosity. The recent push for miniaturization of electromechanical devices has necessitated the development of thin film lubricant layers with nanoscale control of surface morphology. In addition, it has been proposed to use dewetting dynamics for in situ measurements of the viscosity of these thin lubricant layers [1]. This technique relies on the accuracy of theoretical models that relate dewetting velocity to viscous dissipation in the substrate during hole opening. Since “melt facture” is another form of energy dissipation, its contribution must be considered. Consequently it is important to study this phenomenon in greater detail in order to understand its origin and devise methods for minimizing it. EXPERIMENTAL DETAILS
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Commercially available monodisperse polystyrene (PS) and Poly methyl methacrylate (PMMA) of various molecular weights, listed in table 1, were used in this study. The substrates in all cases were Silicon wafers, with [1,0,0] orientation purchased from Wafer world, Inc. The silicon wafers used in the experiment were first cleaned by sonicating in toluene to remove organic residues. The surfaces were then cleaned in boiling 25% H2O2 hydrogen peroxide solution, stripping with a 15% HF solution, and f
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