AbstractThe dynamic moduli,E′ andE″, and tan δ for PET–CR, PET–EPDM, and PET–UR composites with unidirectional short fibers were studied as a function of temperature by using a Rheovibron. The temperature dependence of tan δ showed three peaks for PET–elastomer composites. The peaks at the low temperature corresponded to the main dispersion of the respective matrixes and the peak at about 140°C to the α‐dispersion of PET fiber. A small and broad peak observed at a temperature between 60 and 120°C may be caused by the relaxation of the interface region between fibers and matrix. The longitudinal storage modulus for the composite E ∥′was given by the parallel model asdocumentclass{article}pagestyle{empty}begin{document}$ {rm E'}_parallel = V_f cdot E'_f + V_m cdot E'_m $end{document}, whereE f′andE m′are the storage moduli for fiber and matrix andVfandVmare the volume fraction of fiber and matrix, respectively. In the transverse direction of fibers, the composite modulusE ⊥′was expressed by the logarithmic law of mixing as follows:documentclass{article}pagestyle{empty}begin{document}$ log E'_ bot = V_f cdot log E'_f + V_m cdot log E'_m $end{document}. The peak values of tan δ from the main dispersion of the respective matrixes were given by the equation, (tan δ⊥max)c/(tan δmax)m1 − β ·Vf, where (tan δ⊥max)cand (tan δmax)mare the maximum values of the loss tangent for the composite and matrix, respectively, and β is coefficient depending on matrix's type. The β value of PET–CR com
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