A micromechanical model taking into account the contribution of α- and γ-crystalline phases in the stiffening of polyamide 6-clay nanocomposites: A closed-formulation including the crystal symmetry
Résumé
A clay-induced crystal transformation has been widely pointed out in semi-crystalline polymer-clay nanocomposites, from the α-form to the γ-form in the particular case of polyamide 6 (PA6). The proposition of a predictive model taking explicitly into account the polymer crystalline structure is still needed for a reliable prediction of the structure-property relationship and for a better understanding of the reinforcement mechanism in such systems. The aim of this paper is to present an approach issued from the continuum-based micromechanical framework using self-consistency condition to predict the overall stiffness of PA6-clay nanocomposites. Besides the effect of clay particle characteristics, the micromechanical model introduces the contribution of α- and γ-form crystals in the overall stiffness by considering the PA6 matrix as a heterogeneous medium containing distinct amorphous and crystalline phases. A closed-formulation of the micromechanical model is derived using the Walpole spectral decomposition of the stiffness tensors for the two monoclinic crystalline phases. Two possible representations of the microstructure are considered: the first one considers that all the phases are independent and the second one that the γ-crystalline phase constitutes an interphase region around clay particles. The micromechanical model using the two morphological representations is used to predict the overall stiffness of PA6 nanocomposites reinforced with montmorillonite clay (adjusted from 1 wt% up to 20 wt%) for which the polymer crystalline structure was characterized by infrared spectroscopy and calorimetry. The respective role of clay particles and of two crystalline species in the stiffening of exfoliated and intercalated PA6-clay nanocomposites is discussed with respect to the micromechanical model.