Christopher G. Robertson

Heterogeneity of structural relaxation in a particle-suspension system

We found the mechanism of how a silica-particle–reinforced elastomer after being sheared in oscillation at a strain could produce a spectrum hole or a drop in its dissipation spectrum at that amplitude, but not at other amplitudes. We also found that sequential holding the system at two strains could produce one or two holes depending on the deformation histories. When the second holding strain was higher than the first, the effect of hole burning of only high strain was evident, whereas the effects of hole burning at both strains were revealed when the second strain was lower than the first. This unexpected discovery suggests that the structural relaxations of a particle-suspension system are heterogeneous, and shearing at a fixed strain only affects part of its relaxation spectra.

MEMORY OF PRIOR DYNAMIC STRAIN HISTORY IN FILLED RUBBERS

Abstract We recently discovered that particle-reinforced rubbers after being sheared (or aged) in oscillation at a frequency ƒa at a small strain γa (e.g., ∼1% strain) for time ta can often display a spectrum hole or drop in their dissipation spectra. The location of the hole depends on the aging strain amplitude γa. The depth of this hole is influenced by both the oscillatory aging frequency ƒa and the aging duration ta, and follows a simple power relationship of the product of ƒa and ta. Sequential shear at two strains reveals that when γa1>γa2 the resulting dynamic spectra appear to be a combination of that aged at γa1 and γa2, whereas for γa1>γa2, the resulting dynamic spectra only reflect the characteristic hole burning of the second strain after holding at γa2. This new memory effect occurs at very small strains in filled elastomers and involves material stiffening during the strain aging; both of those features are quite different from the Mullins effect. Also, this new memory is found to last for ...

Assignment of effective network chains in cured rubbers derived from chemical crosslinking, entanglements, polymer end linking to carbon black and filler interaction. VII. Tensile retraction measurements

The individual contributions to effective network chains from chemical cross-links and from trapped polymer chain entanglements were estimated for gum rubber compounds using Tensile Retraction (TR). In addition, the influences of carbon black and of polymers with functional end-groups, on the character of the polymer network in filled compounds were explored. The effective network chain contributions were established for gum vulcanizates through an entanglement model and an independent estimate of the polymer molecular weight between entanglements, M e . It was found that M e was related closely to the extrapolated y intercepts obtained from the TR of cured gums. The gum compounds were used to further estimate the effects of fillers and functional end-groups on the total number of effective network chains. Comparing an α,ω-difunctional SBR with its non-functional counterpart enabled the assessment of the effects of functional end-groups. The comparison allowed for the determination of the probability, n, that a functional end group reacts with carbon black. The π was seen to increase as the volume fraction of filler increased, and π 2 provided an estimate of the fraction of functional polymer chains that react with carbon black at both ends.