Critical thickness in silicone thermosets

Abstract

Critical thickness effects are utilized to achieve high fracture toughness in brittle polymers. The postulate of critical thickness, which is: "Macroscopically brittle polymers deform in a ductile fashion below a critical dimension" is validated for silicone thermosets and polystyrene using novel "direct" methods by measuring failure strain in thin films. A discussion on polymer intrinsic deformation mechanisms is presented. Using these intrinsic deformation mechanisms as bases, it is argued that all polymers are ductile at the molecular level. Accordingly, it is suggested that polymer properties below a certain length scale (defined as critical thickness) are dominated by intrinsic deformation characteristics. Two analytical models have been developed which predict critical thickness based on the physical properties of the polymer. The first model is based on an energy criterion according to which crack initiation does not take place if the crack driving force is less than the crack resistance. Such a condition for a brittle polymer is achieved at the critical thickness. The second model is based on a mechanics criterion according to which a minimum film thickness (critical thickness) is required for typical fracture features like crazes to exist within it. Further, using these theoretical models as a basis, the effect of network density, temperature and strain rate on critical thickness is discussed. It is also shown that fracture strain is the characteristic material property to measure film toughness. A variety of silicone thermosets are studied to demonstrate engineering applicability of critical thickness.

(cont.) The selected polymers include a commercial laminate poly-phenyl- methyl-silsesquioxane resin, an experimental high temperature poly-methyl- silsesquioxane resin and an optical polysiloxane resin. In addition to silicone thermosets, polystyrene is studied as a reference polymer. A bending technique has been developed in order to determine failure strain of thin films (on substrates) of these materials. Using this technique, the failure strain is evaluated as a function of film thickness. Further, from a plot of failure strain as a function of film thickness the critical thickness is determined. For polystyrene, a critical thickness value of approximately 0.1 um is observed. The strain to failure of polystyrene films below the critical thickness is >15%, a marked increase over bulk material fracture strain (-2%). For each of the silicone thermosets, a range of curing temperatures is investigated to determine the influence of curing temperature on critical thickness. For the poly-phenyl-methyl-silsesquioxane resin, the optimum thin film properties are observed when it is cured at 225 ⁰C. The critical thickness is observed to be -5 gm with a strain to failure of -13% (bulk strain to failure <2%). Molecular engineering of the poly-phenyl-methyl-silsesquioxane by modifying the chemical structure using functionalized PDMS is shown to increase critical thickness to 0lm. For this PDMS modified poly-phenyl-methyl-silsesquioxane, critical thickness values have been determined over a range of test temperatures from -40 ⁰C to 75 ⁰C. Results indicate that the test temperature does not influence the critical thickness. For the poly-methyl- silsesquioxane, the critical thickness is observed to be greater than 2.51um with a strain to failure of 15% when cured at 250 ⁰C.

(cont.) Two optical polysiloxane resins are studied. The first resin, commercially called the PF-1202 resin is studied for a single cure temperature and is observed to have a critical thickness of -0.2-0.3um. The properties of the second resin, named MP-101, are studied for a range of cure temperatures. The best performance, with 8% strain to failure and a 0.5um critical thickness, is observed for a 50 ⁰C cure temperature. Infrared spectroscopy measurements in reflection mode are carried out to compare orientation effects in thin (below critical thickness) vis-A-vis thick (above critical thickness) films for polystyrene, poly-phenyl-methyl-silsesquioxane and PDMS modified poly-phenyl-methyl-silsesquioxane. It is observed that both the thick as well as thin films do not exhibit any substantial orientation. A correlation between molecular orientation and fracture properties cannot be made. Infrared spectroscopy has also been used to determine the "nature" of strain (elastic or plastic) these thin films (below critical thickness value) exhibit when stretched to values higher than their bulk counterparts.