Influence of silica reinforcement upon the glass transition behavior of acrylic polymers

The glass transition temperatures (T_g) of poly(acrylic acid), poly(methyl acrylate), and poly(ethyl acrylate) filled with submicron particulate silicas and silicates have been measured by dynamic mechanical spectroscopy and differential scanning calorimetry. The peak temperatures of the damping factor (tan δ) and the dynamic shear loss modulus (G″) were shifted by an amount which depended upon the quantity and type of filler added to each polymer. The temperatures corresponding to the step discontinuity of specific heat also shifted, but to a lesser extent than those measured mechanically. The degree of T_g shift per unit of volumetric filler addition increased with polymer pendant group polarity for both measurement methods. Utilizing a T_g-crosslinking analogue, a model was developed that related positive T_g shifts to polymeric segmental adsorption onto filler surfaces. This model also incorporated negative contributions to the T_g shift from energy storage mechanisms arising from particle–particle interactions, as well as corrections due to effective surface area available for polymer adsorption.