| Stress relaxation in uniaxial extension and dynamic shear moduli G' and G'' were studied in networks of vinyl-terminated poly(dimethyl siloxane) (PDMS) of five different molecular weights crosslinked with tetrafunctional hydrosilane and containing 10 and 15 wt % of two samples of unattached linear hydroxyl-terminated PDMS (M(,w)(' )=(' )700,000 and 950,000). After conversion to the shear relaxation modulus G(t), the two sets of measurements were combined and the contribution of the unattached species G(,1)(t) was calculated by difference. After multiplication by (1-v(,2)('2))('-1)G(,N)('0)/G(,e), where v(,2) is the volume fraction of the network, G(,N)('0) is the plateau modulus of the uncrosslinked polymer, and G(,e) is the equilibrium modulus of the network containing unattached molecules, G(,1)(t) was compared with G(t) in the uncrosslinked environment. The form of the relaxation modulus was essentially the same in both environments, but was slower in the networks by 1 to 2 orders of magnitude. The retardation increased gradually with increasing G(,e), which is a measure of topological obstacles represented by crosslinks plus trapped entanglements. It appears that in these systems where the topology of the obstacles to reptation is fixed, either reptation is severely restricted or alternative modes of configurational rearrangement which contribute substantially to relaxation in the uncrosslinked polymer are suppressed.;Further rubber networks were prepared by crosslinking styrene-butadiene random copolymer (67% styrene content) with (gamma)-radiation. Before crosslinking, the polymer was stretched in simple extension, allowed to relax at constant strain and temperature for a controlled time, and then quenched 20(DEGREES)C below the glass transition temperature (T(,g)) and maintained at this temperature during crosslinking. Upon warming above T(,g), the equilibrium length was intermediate between the stretched and unstretched lengths, and was describable as the result of a force balance between a crosslink network (with rest length of the stretched polymer) and a trapped entanglement network (with rest length of the unstretched polymer). Similarly, the equilibrium force at extensions beyond the equilibrium length was describable as the result of additive contributions from the crosslink and trapped entanglement networks. These experiments confirm the contribution to the equilibrium modulus of permanently trapped entanglements (or equivalent trapped topological restraints). |
