Constitutive modeling for mechanical behavior of polymer-clay nanocomposite foams

Abstract

Constitutive equations for characterizing the nonlinear mechanical behavior of polymer/clay nanocomposite foams were studied. To consider the influence of the crystallinity on the mechanical properties of the foams, both amorphous and semi-crystalline polymers were used. Several viscoelastic models were accounted for the modeling of tensile behavior. The constitutive equation for nonlinear tensile behavior of microcellular Poly(methyl-methacrylate) (PMMA) foams was expressed in terms of material properties and physical characteristics of foams such as strain, strain rate, elastic modulus, relative density of foam, and relaxation time constant. In the constitutive model for tensile behavior of PMMA/clay nanocomposite foams, the reinforcing effect by intercalation of the clays and the detrimental effect by clay agglomeration were considered. The constitutive equation was expressed in terms of clay morphology and material properties. The aspect ratio of clays and the expansion of clay layer spacing in the intercalated clay clusters were proved to play a vital role in the reinforcing mechanism. For the modeling of semi-crystalline polymer/clay nanocomposite foams, the effects of nanoclay loadings, crystallinity, strain rate, and foaming conditions on the foam morphology and mechanical properties of microcellular high density polyethylene (HDPE)/clay nanocomposites were studied. Intercalated clay structures were investigated in the nanocomposites, and microcellular foams were manufactured by a batch foaming process. Tensile mechanical tests indicated that Young's modulus and the tensile strength were reinforced on the whole with crystallinity and nanoclay contents. Also, a constitutive model for tensile behavior of HDPE/clay nanocomposite foams was proposed. The elastic modulus of the foams was developed using micromechanics theory and representative volume element (RVE) concept. The constitutive model for HDPE/clay nanocomposite foams was expressed in terms of microstructural properties of polymer, and physical properties of the foams. Employing the proper constitutive laws, the macroscopic material behaviors of polymer/clay nanocomposite foams could be represented. The developed constitutive models were demonstrated by experiment.