Phase Transition between Matrix and Inclusions in Polymeric Composites
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PHASE TRANSITION BETWEEN MATRIX AND INCLUSIONS IN POLYMERIC COMPOSITES
P.S. THEOCARIS Department of Theoretical and Applied Mechanics The National Technical University of Athens 5, Heroes of Polytechnion Avenue, GR-157 73 Athens,
Greece
ABSTRACT The influence of boundary layer on the coefficient of adhesion between fibers and matrix in polymer composites was investigated. A multilayer model was introduced consisting of the two main-phases of the composite,that is the elastic inclusion and the viscoelastic matrix and a third intermediate phase consisting of a series of cylindrical layers of mechanical and physical properties varying between the limit values of the matrix and inclusion. The extent of the interlayer between main phases may be determined from the overall properties of the composite and the matrix. The model proved to be very flexible and describes conveniently the behaviour of the material. Moreover, the model allowed the definition of an adhesion coefficient, convenient to describe the quality of adhesion of the component materials and the introduction of an improved expression for the law of mixtures in polymeric composites.
INTRODUCTION The physical behaviour of a two-phase composite material is depending on the characteristic properties of the filler that is the geometry of the inclusions, their size, the size-distribution, the orientation of inclusions, the volume fraction, the relative positions between the inclusions, physical state, etc. On the other hand, the physical behaviour of the system depends equally on the matrix characteristics. Such characteristics are the physicochemical state of the matrix, the degree of polymerization (in cases where there is a polymeric matrix), the crystallinity, the degree of cross-linking, etc. Finally, beyond these parameters, there is also a parameter which is of cardinal importance for the characterization of the composite material. This parameter is the interaction between the matrix and the inclusion. By the term interaction of phases we are referring to the degree of contact and the cohesive forces, which are developed on the interface of the two main phases. All theoretical models introduced in the literature assume the surfaces of the inclusions to be perfect mathematical surfaces. However, in reality the surfaces of the inclusions are rough. The irregularities of the physical surfaces are, in many cases, of the order of microns and such a situation results in stress concentrations near the corners. These stress concentrations are not predicted from the theoretical models, which assume continuity of stresses and displacements at the interfaces. Moreover, because of the roughness of the surfaces of the inclusions, we may consider that the real contact surface between the two phases is greater than the respective contact surface predicted from the theoretical model. During the manufacturing of the composite material, the presence of the inclusion in the matrix material affects the rate of polymerization of the polymeric matrix, as well as the mobilit
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