Rheologica Acta Rheol Acta 27:397-404 (1988) Dynamic-mechanical properties of interfacially modified glass sphere filled polyethylene J. D. Miller, H. Ishida, and F. H. J. Maurer Department of Macromolecular Science, Case Western Reserve University, Cleveland, Ohio (U.S.A.) and DSM Research BV, Geleen (The Netherlands) Abstract: Model composites of spherical glass particles dispersed in a matrix of high density polyethylene were prepared both with and without interfacial modification by an azidofunctional trialkoxysilane. Dynamic mechanical measurements of the com- posites in the melt state were recorded. The unmodified composites behave as theoret- ically predicted and the effect of particle-particle interaction at high volume fractions can be measured. The composites with a modified interfacial region have greater shear moduli due to the effect of a region surrounding the particle modified by the silane. The material in this region is largely bound to the glass surface and was examined by Fourier transform infrared spectroscopy after extraction of the bulk matrix. Theoret- ical calculations are shown to be useful in calculating the mechanical properties and volume fraction of the interfacial region. Key words: _Composite, silane coupling agent, ~nterface, _Fourier transform infrared spectroscopy 1. Introduction A long term goal in the study of composite materials is the understanding of the physico-chemical role of the interface and interfacial regions. An important compo- nent in composites is the coupling agent. Ideally, incorpo- ration of a coupling agent results in a chemical linkage between a reinforcement and matrix phase and promotes transfer of stress, exclusion of water from the interface, cohesive failure, and other desirable qualities. The precise role which the coupling agent plays has not been deter- mined, due in large part to the difficulty in examining this region of the composite. This paper describes an attempt to determine the effect of coupling agent chemistry on the rheological properties of model composites. The role of interracial chemistry in the properties of filled polymers has been examined by a number of authors. The influ- ence of a layer of bound polymer at the matrix-filler interface has been discussed by Maurer et al. for kaolin filled polyethylene [1] and Kendall and Sherliker for polyethylene silicate composites [2, 3]. Morphological changes in matrix polymers which reside near the particle surface have been observed by Dolakova and Hudecek [4] and Chacko [5]. The rheological effects of the incor- poration of coupling agents and surface modifiers in composites have been studied by Han [6] and Bigg [7]. They noted both large 276 and small differences in viscosity, depending on the specific com- bination of polymer, reinforcement, and surface modifier. This paper describes rheological experiments with a model composite system which behaves as theoretically predicted. Such a system is necessary in order to qualita- tively and quantitatively characterize the role of inter- facial modification. In previous studies problems have arisen due to the undefined nature of the composite system. For instance, if a coupling agent is added to a filled polymer the agent may couple polymer to the filler thus tending to increase the viscosity of the melt. At the same time the presence of the agent may increase the dispersion of the particles in the matrix tending to reduce the viscosity of the melt. These changes resulting from the interfacial modification have opposing influences on the properties of the composite and it becomes impossible to separate the individual effects. Therefore, it is necessary to work with a controlled system where the changes can be isolated and attributed to specific events. In the present investigation a composite system ap- proaching ideal behavior has been defined (glass spheres in a high density polyethylen matrix). Modification of the covalent chemical interaction between the reinforcement and matrix phases has been carried out using an azido-