Dynamic Shear Compliance of Polymer Melts and Networks

Abstract

The mechanical relaxation processes in polymer melts and networks are discussed. This is done by decomposition of master curves of the dynamic shear compliance into (i) the glass relaxation with its plateau compliance J_eN, (ii) the shearband process, the relaxation strength ΔJ_B of which is reciprocal to the total crosslink density p_c and (iii) the flow relaxation ΔJ_F and viscous flow (for uncross-linked melts only). The plateau compliance J_eN is exponentially reduced only by effective crosslinks (p_c ^*≈p_c/30). This behaviour is understood on the level of a meander superstructure, including shear-bands. The observed saturation in J_eN at higher DCUP-crosslinking — which doesn’t appear with radiation — can be explained by the lack of chemically induced effective crosslinks across the interfaces between meander cubes (because of larger shear fluctuations in these areas). Whereas crosslinking changes strongly the form and position of the master curve, a filler content reduces the compliance but keeps its lgw-dependence about constant. Quantitatively the filler effect on the zero shear compliance is described on the level of the meander model and its coarse grain structure: the paraelasticity of a file of meanders across a coarse grain of diameter D_CG is blocked up, if it contains at least one filler particle (of diameter D_F).

Therefore J(Ф_F)/J(0) = (1-Ф_F) ^D _CG ^/D _F, the probability of finding no particle in a file (Ф_F total volume fraction of filler). Analyses on 4 series of filled cis-1,4-poly(isoprenes) can be accounted for by assuming the size of the (hypothetical) coarse grains of the polymer to be some 300nm. Also the prediction (by the simple formula), that smaller filler particles should be more effective in shear compliance reduction, is most probably valid.