STRESS-RELAXATION AND DYNAMIC VISCOELASTIC PROPERTIES OF END-LINKED POLY(DIMETHYL SILOXANE) NETWORKS CONTAINING UNATTACHED POLY(DIMETHYL SILOXANE)

Stress relaxation in uniaxial extension and dynamic shear moduli G??? and G??? have been studied in networks of vinyl???terminated poly(dimethyl siloxane) (PDMS) of five different molecular weights (M n from 1800 to 29,200) crosslinked with cis???dichlorobis (diethyl sulfide) platinum (II) and containing 10 and 15 wt % of two samples of high???molecular???weight unattached linear hydroxyl???terminated PDMS (M w 700,000 and 950,000). The M w /M n ratio of both the network prepolymers and the unattached linear species was approximately 2. In stress relaxation the stretch ratio was 1.25 or less and the shear relaxation modulus was calculated from the neo???Hookean stress???strain relation. In the dynamic measurements, the strain amplitude was 15% or less; after conversion to the timedependent 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 urn:x-wiley:00981273:media:POL180191107:tex2gif-stack-1)???1G urn:x-wiley:00981273:media:POL180191107:tex2gif-stack-2/G e, where v 2 is the volume fraction of network, G urn:x-wiley:00981273:media:POL180191107:tex2gif-stack-3 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 11(t ), the relaxation modulus was essentially the same in both environments. The relaxation was slower in the networks than in the uncrosslinked polymer by 1 to 2 orders of magnitude, and it increased gradually with increasing G e, which is a measure of total to pological obstacles represented by crosslinks plus trapped entanglements. A similar but less striking difference between relaxation in a network and in the homologous environment of a linear polymer was previously observed in end???linked polybutadiene networks and the butadiene phase of a styrene???butadiene???styrene block copolymer. It appears that, in these systems where the topology of the obstacles is fixed, the reptation is severely restricted or else alternative modes of configurational rearrangement which contribute to relaxation in the uncrosslinked polymer are suppressed