The effect of nanosized silica particles on the properties of poly(vinyl acetate) (PVAc) was investigated for a range of silica concentrations encompassing the percolation threshold. The quantity of polymer adsorbed to the particles ("bound rubber") increased systematically with silica content and was roughly equal to the quantity shielded from shear stresses ("occluded rubber"). This bound and occluded polymer attained a level of ∼12% at a silica volume content of 28%; nevertheless, the glass transition properties of the PVAc, including the glass transition temperature, local segmental relaxation function and relaxation times, and the changes in thermal expansion coefficient and heat capacity at T g , were unaffected by the interfacial material. That is, there is no indication that the local segmental dynamics of the chains adjacent to silica particles differ from the motions of the bulk chains. Interestingly, the volume sensitivity of the segmental dynamics, as determined from the scaling exponent γ in the relation T g ∼ V g -γ in which V g is the specific volume at the glass transition, becomes stronger with increasing silica concentration. Moreover, this dependence of γ increases abruptly at the filler percolation threshold. The implication of this result and possible directions for new research are considered.

Polybutadiene (PB) has a low glass temperature Tg and exhibits rubbery behavior during mechanical perturbation. The corresponding PB-based polyurea (PU) has a higher Tg and fails in a brittle mode for high strain rates. However, unlike in glasses, this brittle failure is accompanied by large energy dissipation. Dielectric relaxation measurements demonstrate that whereas the PB segmental dynamics are faster than the strain rate during impact loading, for PU these motions are on the order of the strain rate, ∼105s−1. Consequently, impact induces a transition to the glassy state, with the accompanying response markedly different from that of a rubber.

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The introduction of silanes to improve processability and properties of silica-reinforced rubber compounds is critical to the successful commercial use of silica as a filler in tires and other applications. The use of silanes to promote polymer-filler interactions is expected to limit the development of a percolated filler network and may also affect the mobility of polymer chains near the particles. Styrene-butadiene rubber (SBR) was reinforced with silica particles at a filler volume fraction of 0.19, and various levels of filler-filler shielding agent (n-octyltriethoxysilane) and polymerfiller coupling agent (3-mercaptopropyltrimethoxysilane) were incorporated. Both types of silane inhibited the filler flocculation process during annealing the uncured rubber materials, thus reducing the magnitude of the Payne effect. In contrast to the significant reinforcement effects noted in the strain-dependent shear modulus, the bulk modulus from hydrostatic compression was largely unaltered by the silanes. Addition of polymer-filler linkages using the coupling agent yielded bound rubber values up to 71%; however, this bound rubber exhibited glass transition behavior which was similar to the bulk SBR response, as determined by calorimetry and viscoelastic testing. Modifying the polymer-filler interface had a strong effect on the nature of the filler network, but it had very little influence on the segmental dynamics of polymer chains proximate to filler particles.

The longitudinal relaxation time tau of a series of alkyl-isothiocyanato-biphenyls (nBT) liquid crystals in the smectic E phase was measured as a function of temperature T and pressure P using dielectric spectroscopy. This relaxation time was found to become essentially constant, independent of T and P, at both the clearing point and the lower temperature crystalline transition. tau(T,P) could also be superposed as a function of the product TV(gamma), where V is the specific volume and gamma is a material constant. It then follows from the invariance of the relaxation time at the transition that the exponent gamma superposing tau(T,V) can be identified with the thermodynamic ratio Gamma=- partial differential log(T(c)) partial differential log(V(c)), where the subscript c denotes the value at the phase transition. Analysis of literature data on other liquid crystals shows that they likewise exhibit a constant tau at their phase transitions. Thus, there is a surprising relationship between the thermodynamic conditions defining the stability limits of a liquid crystalline phase and the dynamic properties reflected in the magnitude of the longitudinal relaxation time.

Dielectric relaxation times were measured for 1,4-polyisoprenes (PI) of different molecular weight. From the data, the number of dynamically correlated segments, N c , was calculated using an approximation to the dynamic susceptibility. N c increases with approach to the glass transition in the usual fashion and also increases with increasing molecular weight of the PI. The latter effect is ascribed to the loss of the configurational mobility conferred by the chain ends. The correlation volume was also estimated from calorimetry and, because PI has a dielectrically active normal mode, from the intersection of the extrapolated segmental and normal mode relaxation times. The three methods yield consistent results, although the last has large uncertainty due to the ambiguous connection between dynamic correlation lengths and volumes. Using the equation of state for the polymers, the dependence of the relaxation times on the scaling variable TV γ , where V is specific volume and γ is a material constant, was calculated. For the lowest molecular weight PI, there is a small difference in γ for the segmental and chain modes. The scaling exponent is also marginally smaller for the lower molecular weight sample, suggesting, in contrast with the behavior of other polymers, that in PI the volume dependence becomes weaker with decreasing molecular weight.

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Wave propagation and retraction velocities were measured for two elastomers, a 1,4-polybutadiene and a polyurea, freely retracting from large tensile strains ͑ഛ2͒. From these data the stress-strain response was calculated. The achievable strain rate depends on the initial strain and the viscoelasticity of the material, with values exceeding 1800 s −1 attained herein. Thus, the method can be used to characterize the mechanical behavior at high strain rates, as well as high strains. A drawback is that the strain rate is not constant during the retraction. The kinetic energy of retraction reflects the unrelaxed stress, providing a straightforward determination of strain energy and its dissipation. The two elastomers represent extremes of viscoelastic behavior, as reflected in the retraction response at both low and high strain rates.