Nanofiller-Polymer Interactions At and Above the Glass Transition Temperature
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Nanofiller-Polymer Interactions At and Above the Glass Transition Temperature Ai-jun Zhu and Sanford S. Sternstein Materials Science and Engineering Department Rensselaer Polytechnic Institute Troy, NY 12180, USA ABSTRACT Rheological data are reported for a series of fumed silica filled PVAc samples, using fillers of different specific surface areas and surface treatments. Data at the glass transition temperature and 45 C above Tg are presented. The addition of filler systematically increases Tg, and all samples obey time-temperature superposition. However, temperature normalized and frequency normalized plots of loss modulus indicate that there is no change in the dispersion of the glass transition, with the only exception being a surface modified with covalently bonded polymer chains. Thus, contrary to expectations, an increase in filler content or change in surface treatment has no effect on the relative shape of the relaxation time spectrum at the glass transition. At 45 C above Tg, different behavior is observed. The filler concentration has a major effect on the nonlinearity of dynamic moduli vs. strain amplitude, with higher filler content reducing the strain amplitude at which nonlinear behavior is observed. Specific filler surface treatments result in major changes in the shape of the loss factor versus strain amplitude relationship. These results suggest that interfacial interactions strongly modify the far-field polymer behavior with respect to chain entanglement slippage at large strains. INTRODUCTION Addition of inorganic fillers can significantly change the mechanical properties of a polymeric material. Dynamic mechanical properties of filled polymers can show a wide variety of phenomena [1]. For example, the strain amplitude corresponding to the onset of nonlinear behavior is reduced by the addition of filler [2]. The strong amplitude dependence of dynamic moduli is similar to the Payne effect found in filled elastomers. As for the glass transition temperature (Tg), it has been reported to increase as a function of the filler content [3], but it has also been observed that the Tg seemed not affected by filler loading up to 20% by volume [4]. On the other hand, it has been documented frequently that damping (loss factor) is decreased in the glass region [3, 5, 6], and is broadened along the temperature scale by fillers [1]. In rubbery matrices, fillers tend to increase relative damping [7- 9]. Interfacial bonding strength is receiving considerable attention in filled systems [10-14]. Surface treatment of fillers can provide better dispersion in polymeric matrices, and increase mechanical properties such as modulus, strength and yield stress [12,14]. Silane coupling agents with different end groups are widely used to modify silica particle surfaces. Energy dissipation has been found to be dependent on the dimensions of the interfacial region between the matrix and the filler phase [11], and enhancement of the rubber glass transition peak has been observed when the surface treatment resulted in an increa
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