WORK OF ADHESION INFLUENCE ON THE RHEOLOGICAL PROPERTIES OF SILICA FILLED POLYMER COMPOSITES

As the work of adhesion, W-a, increases between a silica filler surface and a polymer matrix, the dynamic viscosity and the sheer modulus of the composite material increase. The logarithms of these properties decrease linearly as W-a decreases. At lower dynamic test frequencies, a change in W-a has a more dramatic impact on these properties than at higher frequencies. An ''effective silica particle size'' model can be used to explain why W-a affects the viscosity and the shear modulus of a composite. According to that model, the thickness of the interphase layer increases as the W-a increases. An increase in effective particle size decreases the ''free'' polymer volume, and the decrease free volume polymer causes both the viscosity and the shear modulus to increase. Increasing the dynamic test frequency releases some of the immobilized polymer from the filler surface which causes the effective particle size to decrease. As the effective particle size decreases because of the increased testing frequency and approaches the mean size of the original filler, the impact of the W-a value on viscosity and shear modulus should decrease. However, the friction experienced between the filler interphase and the polymer, the so called ''skin friction'', depends on the magnitude of W-a and the more general term, bond energy density (BED). The skin friction determines the viscosity of the composite, particularly at lower frequencies. Higher W-a values induce higher skin friction and thereby higher flow resistance (viscosity) as polymer chains move along the filler surface.