Simultaneous interpetrating network based on castor oil elastomers and polystyrene. III. Morphology and glass transition behavior

Abstract Electron microscopy and dynamic mechanical spectroscopy (DMS) techniques were used to study the morphology and glass transition behavior of simultaneous interpenetrating networks (SIN's), based on three different castor oil derived elastomers, and polystyrene (PS) plastic erosslinked with 1 percent divinyl benzene. The castor oil elastomers consisted of either the sebacic acid polyester, 2,4‐tolylene diisocyanate polyurethane, or the mixed poly(ester‐urethane). Emphasis was placed on two compositions having 10 and 40 percent elastomer contents by weight of each type, the remainder being PS. In all cases, a two‐phase morphology emerged. With the 10 percent elastomer composition, the use of vigorous stirring during the early stages of reaction resulted in materials having the crosslinked polystyrene as the continuous phase and elastomer domains (ranging from 100 to 8000 nm in size) as the discontinuous phase. The elastomer domains contained a polystyrene cellular structure, with the polystyrene cell sizes ranging from 50 to 300 nm size. Several examples showed morphologies resembling high impact polystyrene. Materials having a 40 percent elastomer content always showed a continuous phase of castor oil elastomer, with the PS displaying a bimodal size cellular structure. Domain sizes ranged from 10 to 860 nm. The DMS studies showed two well‐defined glass transitions near their respective homopolymer glass transitions, but shifted inwards to greater or lesser extents indicating some molecular mixing between the two polymers. The glass transition of the pure elastomer phase occurred at −66°C for the castor oil polyester elastomer, −4°C for the castor oil polyurethane elastomer and −50°C for the castor oil poly(ester‐urethane) elastomer. Phase separation in these materials is postulated to occur by two mechanisms: (1) multiple precipitation of polystyrene chains at progressive levels of polymerization and (2) microsyneresis processes. The thermodynamics of mixing and phase separation in polymerizing SIN's is examined in some detail.