DYNAMIC VISCOELASTICITIES FOR SHORT-FIBER THERMOPLASTIC ELASTOMER COMPOSITES

Dynamic moduli, E' and E'', and loss tangent tan delta were investigat ed for thermoplastic elastomers (TPEs), styrene-isoprene-styrene copol ymers (SISs), styrene-butadiene-styrene copolymer (SBS), and Hytrel an d composites reinforced by poly(ethylene terephthalate) (PET) short fi bers. The styrenic TPEs have a typical rubbery behavior and the Hytrel TPE has medial characteristics between rubber and plastic. Both E' an d E'' of the composites depended on the matrix as well as the fiber lo ading and fiber length. Based on the viewpoint of different extensibil ity between the fiber and the matrix elastomer, a triblock model was c onsidered for estimating the storage modulus of the short fiber-TPE co mposites as follows: E(c) = alphaV(f)E(f) + beta(1 - V(f))E(m), where alpha and beta are the effective deformation coefficients for the fibe r and the matrix elastomer, respectively. They can be quantitatively r epresented by modulus ratio M (= E(m)/E(f)) and fiber length L: alpha = (L(n) + k)M/(L(n)M + k), beta = (1 - alphaV(f))/(1 - V(f)), where th e constants n and k are obtained experimentally. When k = 0.022 and n = 0.45, E(c) of the TPE composites agreed well with the prediction of the proposed model. The relaxation spectrum of the composites showed a distinct main peak ascribed to the matrix elastomer, but no peak to t he PET fiber. The relative damping of main relaxation, (tan delta(max) )c/(tan delta(max))m decreased monotonously with increasing fiber load ing and fiber length and with decreasing modulus of matrix elastomer. Thus, the relative damping may be attributed not only to the volume ef fect of matrix, but also to the unevenness of the strain distribution in the matrix phase, which depends on the fiber length and the matrix' s modulus. The findings prove that the different extensibility between fiber and matrix and the uneven distribution of strain in matrix were important for the short-fiber reinforcement of the TPE composites. (C ) 1993 John Wiley & Sons, Inc.