DYNAMIC SHEAR COMPLIANCE OF POLYMER MELTS AND NETWORKS IN DEPENDENCE ON CROSS-LINKING DENSITY

Mechanical relaxation processes in polymer melts and networks are discussed. This is performed by decomposing master curves of the dynamic shear compliance into i) glass relaxation with its plateau compliance J(eN; ii) shearband process with its relaxation strength-DELTA-J(B), which is reciprocal to the total crosslink density P(c); and iii) flow relaxation-DELTA-J(F) and viscous flow (for uncrosslinked melts only). Plateau compliance J(eN) is exponentially reduced only by effective crosslinks (p(c)* almost-equal-to p(c)/30). This behavior is understood on the level of a meander superstructure, which includes shearbands. The observed saturation in J(eN) at higher dicumylperoxide (DCUP) cross-linking - which doesn't appear with radiation - can be explained by the lack of chemically induced effective crosslinks across the interfaces among meander cubes. This lack may be a consequence of DCUP molecules concentrating at the interfaces and thereby preventing the contact and radical recombination between chains at adjacent meander faces. Crosslink densities p(c) (per monomer), determined from the reduction of shearband relaxation strength, vary linearly with the crosslinking agent and read: p(c) almost-equal-to 2.4 10(-2) Dose/MGy and p(c) almost-equal-to 0.97 . 10(-2) DCUP/phr for radiation and DCUP crosslinking, respectively. This implies, e.g., that a dose of 0.4 MGy (40 Mrad) is equivalent to 1 part DCUP phr in a crosslinking polyisoprene. From activation-curve analysis it follows that 3r/d stays constant, and epsilon-s - epsilon-so (free energy of formation of a segment-dislocation) and Q(y) - Q(yo) (activation energy for segmental jumps) vary with the square of p(c), as does the glass temperatur T(g) - T(go) from DSC measurements.