Critical thickness in silicone resin

Abstract

The critical thickness at which a toughness transition occurs in a polymer was determined for the Dow Corning 4-3136 silicone resin. 4-3136 is macroscopically brittle, but becomes up to ten times more ductile when it is processed in the form of a sufficiently thin film. Thickness was controlled for the thin films of amorphous polymer through solution composition variation. The films were spun on highly polished stainless steel at a constant angular velocity. The transition was observed over a range of film thicknesses at several different curing temperatures. Observation of cracking induced by bending and tensile strain was used as the gauge for the transition from brittle to ductile. The critical thickness was observed to have an unambiguous value at low curing temperatures. At higher curing temperatures, the transition occurred more gradually. The critical thickness also increased up to intermediate curing temperatures, then decreased at the highest cure temperature. Crosslink density is theorized as the cause for this cure temperature induced alteration of the critical thickness.