Neal Roger Langley

Physical structure of polysilicate particles in organic media

Abstract Physical structure was determined for colloidal particles of a hydrophobic polysilicate sample dispersed in nonaqueous media. Some silicone products are compounded with such a material which is made by capping water-soluble silicates with organosilyl groups. Solutions densities and viscosities were measured and GPC, SAXS, and TEM were performed on the sample dispersed in THF and PDMS fluid media. Results showed that particles are relatively compact with little solvation or aggregation and have a reasonably spherical shape. The approximate ranges of particle diameter and molecular weight are, respectively, 2 to 10 nm and 2000 to 200,000 with broad, bimodal distributions. The larger particles seem to have lower densities within their hydrodynamic volumes than do the smaller ones. The average “hard-core” density is 1.17 g/cm3, similar to that of many (R SiO 3 2 ) x silicone resins but much less than that of amorphous fused silica. The difference is attributed to poorly packed silicate structures in the polysilicate and silicone resins.

Effects of entanglements on elastic modulus of elastomer networks after terminal relaxation

Abstract Theoretical predictions of the equilibrium elastic modulus of crosslinked amorphous polymers at low strain are compared. Different theories relate this modulus to the network structure, but are based on the different concepts of trapped entanglements, constrained junctions and constrained chains. To distinguish preferred theories, experimental conditions should be chosen to maximize differences of the predicted moduli. This is found to depend on choosing a polymer with high plateau modulus and either ideal end-linking or random crosslinking at a rather low level of random crosslinking (2 to 5 times that at the gel point). High initial values of Mn and Mw/Mn should be used, with no scission or dilution during crosslinking. Earlier tests of the theories were conducted under conditions where a ratio of the predicted moduli was less than 4. Experiments are proposed which would increase it to >20.

Effects of Interfacial Diffusion Barriers on Thermal Stability of Ceramic Fibers

High-strength Si-N-C-O ceramic fibers have been made by pyrolysis of polymer fibers. The ceramic fibers are intended as reinforcement for high-performance composites and require good retention of mechanical properties after extreme thermal exposures. Strength retention of the fibers depends on growth of their critical flaws. This, in turn, can depend on the extent of decomposition reactions. These reactions require gas transport through the outer fiber surface and are subject to control by effective diffusion barriers at this interface. A new technique was used to study the effect on flaw growth from barrier coatings of carbon applied to a fiber fracture surface. A CVD coating of SiC applied to a strand of fibers can suppress gas diffusion through the fiber surface. The effectiveness of these barriers is reported in terms of the rate of flaw growth, the extent of chemical decomposition and gas diffusion, and the loss of fiber strength.