Mechanistic understanding of miscibility effects on fracture strength of polymer interfaces

This research has provided a fundamental framework based on both experiment and theory to understand the fracture behavior as a function of system miscibility for nonreinforced polymer interfaces. The system chosen for study was polystyrene (PS) and the statistically random copolymer poly (styrene-r-4-bromostyrene) (PBS), where the volume fraction of bromine in the copolymer, f, and the degree of polymerization, N, control the miscibility. The phase behavior of thin film blends of PS and PBS as a function of f and N was studied using atomic force microscopy (AFM) and small angle X-ray scattering (SAXS). Simulations based on the Flory-Huggins theory and the Flory-Huggins interaction parameter, chi, measured from SAXS were used to predict phase diagrams for all the systems studied. Using the phase diagrams, a miscibility map as a function of f, N, and the symmetry of N was developed and showed good agreement with compatibility (measured using AFM). A modified double cantilever beam geometry was employed to measure the Mode I fracture energy (Gc) of PS/PBS interfaces as a function of miscibility. The fracture surfaces were analyzed using scanning electron microscopy and optical microscopy to discern the failure mechanism. The fracture experiments showed that three interdiffusion/fracture regimes exist and that maximizing interfacial strength is based on the competition between gradient-driven and miscibility-limited interdiffusion and can be controlled by optimizing the miscibility of the system. Finally, a new stochastic model, which takes into consideration system miscibility, was developed to calculate Sigmaeff for partially miscible polymer interfaces. Based on this Sigmaeff, a new equation for fracture of non-reinforced systems was postulated which correctly predicts the transition from chain pullout to crazing. This equation incorporates system miscibility via Sigmaeff, the interfacial width, and the average distance between entanglements. As a function of system miscibility, the model also accurately predicts a maximum in Gc as a result of the competition between gradient-driven and miscibility limited interdiffusion. The use of the miscibility criterion proposed for the fracture mechanism may be the first step in revealing the universal nature of the pullout to crazing transition