A new three-phase model to estimate the effective elastic properties of semi-crystalline polymers: Application to PET

In this paper, the effective elastic properties of semi-crystalline polymers are computed through a new three-phase model. The formulation of this approach is based on recent thermal analyses which revealed the existence of an intermediate phase during cooling from the melt in semi-crystalline polymers. At the microscopic level, a three-phase composite inclusion constituted by three adjacent layers (a crystalline lamella, a rigid amorphous interphase and a mobile amorphous phase) is considered to estimate, via homogenization methods, the effective elastic properties of the material. Our model is applied to poly(ethylene terephthalate) and a good agreement is obtained, for different crystallinities, between our predicted results and the experimental ones found in the literature. The model is also compared to the N-phase inclusion model of Hori and Nemat-Nasser [Hori, M., Nemat-Nasser, S., 1993. Double-inclusion and overall moduli of multi-phase composites, Mechanics of Materials 14, 189–206] by considering an extension of the double-inclusion model to a three-phase inclusion model.