Phase separation of PMMA modified vinyl-ester thermosets: morphology, thermodynamics and mechanical properties

Abstract

In this Chapter, the initial miscibility, the developed morphologies, and the final properties of styrene(St)/vinyl-ester(VE) thermosets modified with poly(methyl methacrylate) (PMMA) are discussed. The effect of changing the molecular weight and the polydispersity of the VE oligomer and the PMMA modifier are presented. Firstly, the miscibility of binary and ternary physical mixtures of the components involved in the different studied formulations is analyzed. The experimental liquid-liquid equilibrium curves (e.g. cloud-point curves) allow computing the binary interaction parameters, χ, in the framework of the Flory-Huggins theory for polydisperse systems. These parameters are used to model the quasiternary phase diagram that represents the initial thermodynamic state of each particular system. This miscibility behavior originates quite different morphologies in the cured materials, generated by polymerization induced phase separation (PIPS) mechanism. For instance, dispersion of thermoplastic-rich particles in a thermoset-rich matrix, cocontinuous structure, dispersion of thermoset-rich particles in a thermoplastic-rich matrix (phase-inverted structure), or typical macrophase morphology characterized by droplets-like domains with secondary phase separation inside the droplets can be observed. These morphological structures are directly related to the thermal and mechanical properties, as well as the volume shrinkage of the final systems. The evaluation of the dynamic mechanical behavior, flexural modulus, compressive yield stress, and fracture toughness shows that the addition of PMMA increases the fracture resistance without significantly compromising the thermal or mechanical properties of the vinyl-ester networks, which is inevitable when using elastomeric additives. The reason for the existence of an optimum modifier concentration is also discussed.