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The dynamic moduli, E′ and E″, and tan δ for nylon–CR and PET–CR composites with unidirectional short fibers were studied as a function of temperature by using a Rheovibron. The temperature dependence of tan δ showed two dispersion peaks for nylon–CR composite. The peak at −28°C corresponded to the main dispersion of CR and the peak at 100°C to the α-dispersion of nylon 6. For a PET-CR composite, in addition to the individual dispersion of CR and PET, a small and broad peak was observed at about 90°C. The angular dependence of E′ indicated that the short fibers assumed good orientation. The storage modulus for the composites was given by the parallel model as E′ = Vf′Ef + VmE′m., where E′c, E′f and E′m were the storage modulus for the composite, fiber, and matrix and Vf and Vm were the volume fraction of fiber and matrix, respectively. In the transverse direction of fiber, the peak values of tan δ at −28°C were given by the following equation; tan δc = tan δm − δVf, where tan δc and tan δm are the loss tangent for the composite and matrix, respectively, and α is coefficient depending on fiber type. The results indicated that a region with strong interaction was formed between fibers and CR matrix.

The dynamic moduli, E′ and E″, 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 as , where E and E are the storage moduli for fiber and matrix and Vf and Vm are the volume fraction of fiber and matrix, respectively. In the transverse direction of fibers, the composite modulus E was expressed by the logarithmic law of mixing as follows: . 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)m 1 − β · Vf, where (tan δ⊥max)c and (tan δmax)m are 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 composite is the largest one among those of the composites.

Short fiber–elastomer composites with 10 vol % fiber, nylon 6–CR and PET–CR composites, absorbed water either in the moisture atmosphere or in water. The effect of absorbed water on the viscoelastic properties for these composites was investigated. The temperature dependence of tan σ for the nylon–CR composite showed that the α-dispersion peak of nylon shifted to lower temperatures with increasing absorbed water content and that after displacement of the α-dispersion peak the additional small hump appeared at about 90°C. For the PET composite, the α-dispersion peak of PET shifted slightly to lower temperatures and the small shoulder at 90°C diminished with increasing absorbed water. The additional dispersion probably was caused by the interface between fiber and CR matrix and was independent of fiber orientation. The results suggested that nylon fiber absorbed a larger amount of water than CR matrix, while the water absorption for PET fiber was considerably less than for nylon fiber. The absorbed water in nylon fiber bonded stronger than that in CR matrix and was only slightly diminished by heat treatment under 100°C.