Deformation Mechanisms in Polymer-Clay Nanocomposites.

Abstract

Nanoscale control of structure in polymer nanocomposites is critical for their performance but has been difficult to investigate systematically due to lack of suitable experimental model. This thesis investigated roles of various structural parameters in layered polymer-montmorillonite (MTM) clay nanocomposites manufactured using a layer-by-layer (LBL) technique. A continuum-based constitutive model was developed to predict the stress-strain response of the nanocomposites at low strain-rates. The systematic control over the nano-structure using the LBL method allowed an investigation of role of parameters like nanoparticle volume fraction, nanoparticle layer separation, nanoparticle layer stratification and interface between the polymer and nanoparticles. A series of polyurethane (PU)-MTM nanocomposites with a wide range of volume fractions of MTM nanoparticles was manufactured by varying the MTM layer separation. The nanocomposites demonstrated an increasing yield strength and stiffness with increased MTM volume fraction. A transition from ductile to brittle behavior was observed at a high volume fraction of nanoparticles and a critical nanoparticle separation was found to exist, below which brittle behavior dominated the response of the nanocomposites. The presence of nanoparticle stratified layer was believed to provide an additional slip mechanism, resulting in increased ductility. The interface between the polymer and the nanoparticle layers was altered by incorporating polyacrylic acid (PAA) using an exponential (e)-LBL method. The presence of a stronger interface resulted in enhanced stiffness and strength in the nanocomposites. For the development of the constitutive model, the nanocomposite volume was assumed to be occupied by multi-layers of bulk polymer and effective particles consisting of MTM layers and a modified PU interphase region in proximity to MTM layers. A hyperelastic model was used to capture the response of bulk polymer. The effective particle component of the model consisted of a linear elastic spring, a viscoplastic dash-pot and a non-linear spring element to capture the initial elastic response, yield strength and strain-hardening response, respectively. The model predicted all the major features of the uniaxial stress-strain constitutive response of a family of PU-MTM nanocomposites, thus confirming the efficacy of the proposed constitutive model.