Meso-Scale Behavior Characterization of “Tunable” Clay-Polymer Composites

“Tunable” clay-polymer composites are designed to be modifiable in-situ through controlled manipulation of interparticle spacings via the surrounding fluid chemistry. However, little is known about the change in the meso-scale behavior of this composite due to alteration of interparticle and/or interlayer spacings as a function of polymer conformation (molecule size and shape). These composites are made by combining a non-ionic responsive polymer – polyacrylamide (PAM) – with one of two clay types: kaolinite (micro-composite) and montmorillonite (nano-composite). The range of ionic concentrations and pH of the test solutions were selected to promote extended, partially extended, and coiled conformations of the polymer. This study investigates the meso-scale properties dependent on the interparticle and interlayer spacings of the polymermodified clay materials.

Fourier Transform Infrared Spectroscopy results imply that the interactions between the polymer and the clay surfaces cause protonation of the amide group, which has a direct impact on PAM molecule conformation. Sedimentation and scanning electron microscopy results suggest that PAM molecules tend to promote higher order face-to-face aggregation, which leads to higher liquid limits (LL) and shear strength relative to pure clay. Edge-to-edge fabric formation of the micro-composite was observed at coiled conformation due to preferential adsorption of PAM at kaolin particle edge surfaces. Variation in LL depends on both the molecule conformation and particle arrangement. Triaxial consolidated undrained tests on the micro-composite and drained direct shear tests on the nano-composite demonstrated that the composite shear strength behavior depends on the polymer-mineral surface bonds (i.e. mechanical bonds), interparticle and/or particle-polymer and/or polymer-polymer forces, and the apparent viscosity of the polymer molecule at coiled or extended conformation. The nano-composite swelling volume was less than pure clay. Free swelling volume of the nano-composite depends on the test condition at extended conformation of PAM. However, swelling volume decreases at coiled conformation of PAM compared to swelling at partially extended conformation. The hydraulic conductivity of the nano-composite increases significantly at coiled conformation, but did not change significantly at extended conformation of PAM. Conformational changes of polymer molecules and the corresponding changes in observed meso-scale behavior demonstrate the “tunable” nature of the composite.