Nonlinear viscoelastic properties are reported for composites of fumed silica with various surface treatments and matrices of poly(vinyl acetate) of different molecular weights as well as a copolymer matrix of vinyl acetate and vinyl alcohol. Data above the glass transition temperature are reported here. The increase in the composite storage and loss moduli measured at low strains, and their relative rates of decrease with strain, are found to depend on filler surface treatment. The nonlinear behavior of the loss factor with strain is dramatically altered by filler treatment and quite revealing as to the likely mechanism causing the nonlinearity. In addition, the relative reinforcement and the degree of nonlinearity are found to be the highest for the lowest molecular weight matrices. The effect of copolymer substitution for the homopolymer matrix is equivalent to an increase in molecular weight. The primary underlying mechanism for reinforcement and nonlinear behavior appears to be the filler−matrix inte...

Reinforcement Mechanism of Nanofilled Polymer Melts As Elucidated by Nonlinear Viscoelastic Behavior

A single fumed silica, consisting of spherical primary particles (∼10 nm diameter) which are fused into aggregates (∼200 nm in diameter after mixing) was subjected to different surface treatments, then mixed with polymethylsiloxanes (PDMS, Mw=2000–325 000) and tested under oscillatory shear. The dynamic moduli showed a strong strain dependence, while the frequency dependence changed dramatically with the silica concentration. Experimental results are explained by agglomeration of silica aggregates through bridging polymer chains. Agglomeration increased with silica concentration, amount of silanols on the silica surface, and polymer molecular weight, and the break down of agglomeration occurred at decreasing strain levels with increasing filler concentration. Suspensions made with low molecular weight PDMS showed a different dynamic behavior due to the absence of entanglements and the small probability of bridging chains.

Effect of reinforcing fillers on the rheology of polymer melts

We explore filler reinforcement (i.e., increase of elastic modulus G‘ due to incorporation of fillers) and Payne effect (i.e., decrease of G‘ at large strain amplitudes) in terms of the matrix molecular weight, filler loading, and time scales used to probe the viscoelasticity of filled melts. Use of monodisperse non-cross-linked 1,4-polybutadiene (PBD) along with a silica filler allows illustration of different mechanisms of filler reinforcement in the elastic and liquid regimes. The greater filler reinforcement for matrices of higher molecular weight indicates the filler association through chain adsorption and bridging. The role of matrix-mediated filler−filler interactions is explicitly illustrated in terms of the increased elastic modulus in the terminal region due to replacement a fraction of the matrix chains with longer chains of the same kind. The Payne effect is seen to be time-dependent and comprised of an instantly recoverable and a slowly recovering component. Measuring G‘ at both high and low...

Investigating linear and nonlinear viscoelastic behavior using model silica-particle-filled polybutadiene

The mechanical properties of polymeric materials quite generally dependon time, i.e., on whether they are deformed rapidly or slowly. The timedependence is often remarkably large. The complete description of themechanical properties of a polymeric material commonly requires thatthey be traced through 10, 15, or even 20 decades of time. The class ofpolymeric materials referred to as thermorheologically and/orpiezorheologically simple materials allows use of the superposition ofthe effects of time and temperature and/or time and pressure in suchmaterials as a convenient means for extending the experimental timescale. This paper presents a critical review of models proposed todescribe the effect of temperature and/or pressure on time-dependentthermorheologically and/or piezorheologically simple polymericmaterials. The emphasis here is on the theoretical aspects, althoughexperimental results are used as illustrations wherever appropriate.

The Effect of Temperature and Pressure on the Mechanical Properties of Thermo- and/or Piezorheologically Simple Polymeric Materials in Thermodynamic Equilibrium – A Critical Review

The elastoplastic behavior of silica–silicone compounds has been characterized by a yield stress σs and an elastic modulus G0. Scaling laws have been established for the changes in rheometrical parameters with the volume fraction Φv of silica: σs∝(Φv)3.3 and G0∝(Φv)4.2 and single master curves have been obtained whatever the silica type used. The mesoscopic structure of the compounds has been studied using x-ray and light scattering and a semidilute fractal structure has been observed, with a fractal dimension D≈1.8 and a characteristic length scale 4 μm. In this paper, a formulation of the nonfluctuating semidilute fractal concept allows us to relate the fractal dimension to the rheological scaling laws: σs∝(Φv)4/(3−D) and G0∝(Φv)5/(3−D).

Shear elasticity and yield stress of silica–silicone physical gels: Fractal approach

The viscometric, stress relaxation, and stress growth rheological properties were measured for various molecular weight PDMS fluids filled with fumed silica. The stress growth function exhibited significant overshoot, when the continuous phase molecular weight was slightly greater than the entanglement molecular weight; however, significant overshoot peaks were not observed, when the continuous phase molecular weight was less than or much greater than the molecular weight between entanglements. The experimentally observed transient rheological properties are rationalized in terms of a molecular model, where interparticle interactions occur via entanglements of the polymer adsorbed on the silica surface. When the molecular weight of the adsorbed polymer is greater than the entanglement molecular weight, the strength of the interparticle interaction will increase substantially and the particle diffusivity will substantially decrease.

The effect of molecular weight on the rheological properties of poly(dimethylsiloxane) filled with fumed silica

An additive compliance model was used to simulate thermorheologically complex behavior of a linear viscoelastic material. Creep compliance isotherms were generated using the model and timetemperature shift factors were determined. Thea

 T
 shift factors depend upon both time and temperature in a transient experiment and can be unambiguously determined for all regions of time and temperature. The dynamic response was also obtained using the additive compliance model, and the frequency and temperature dependence of the shift factors were determined for both the in-phase and out-of-phase components of the complex compliance. Thea

 T
 shift factor of the in-phase component was a function of both frequency and temperature and was a well defined quantity. The shift factor for the out-ofphase component of the dynamic compliance was not identical to that of the in-phase component; furthermore, this shift factor was undefined for certain regions of frequency and temperature.

J. M. Caruthers

Papers

The effect of molecular weight on the rheological properties of poly(dimethylsiloxane) filled with fumed silica

Consequences of thermorheological complexity in viscoelastic materials