This grant provides funding for exploring the feasibility of manipulating a polymer-clay composite in which the particle layer interspacing is controlled via a pH- and ionic concentration-responsive polymer. This exploratory research will investigate the extent to which interlayer spacing of polymer-clay composites synthesized with a responsive polymer can be directly manipulated with pH- or ionic concentration-adjusted pore fluids. Clay-polymer composites will be synthesized with Montmorillonite clay and polyacrylamide using a solution intercalation technique. Characterization of the resultant materials and measurement of the interlayer spacings will be performed using X-ray diffraction, ellipsometry, and transmission electron microscopy at the particle level. At the meso-scale, swell potential measurements will be performed.
If successful, the results of this work will contribute to the establishment of a novel field of research: engineered soils using functional polymers. Developing this field of research will lead to a broader approach to geotechnical engineering practices. The main contribution expected from this work is to show that the interlayer spacing of the synthesized polymer-clay composites can be manipulated for the purpose of controlling, and even reversing, a meso-scale soil material property, swell potential. The results of this work will provide a greater understanding of responsive polymer-clay composite systems. Advanced knowledge of polymer-clay composites will lead to the development of tunable engineered soil materials to improve the performance of barrier systems, clay liners, filters, and contaminant removal systems.
Advanced knowledge of clay-polymer composites will lead to the development of tunable engineered soil materials to improve the performance of barrier systems, clay liners, filters, and contaminant removal systems. This work focused on the development of a modifiable clay-polymer material, the properties of which are "tunable" by changes in the surrounding fluid chemistry. In other words, the structure - expansion or contraction - of these clay-polymer composites changes with some external trigger. For this study, the clay-polmer composites consisted of montmorillonite, a swelling clay, and polyacrylamide, a responsive polymer. The external trigger used was fluid pH and ionic strength. Polyacrylamide expands under high pH or low ionic strength conditions and contracts under low pH or high ionic strength conditions. The results of this work will contribute to the establishment of a novel field of research: engineered soils using functional polymers. This research represents the next generation of engineered soils that can be altered both at the space and time scales. These engineered soils can be designed for specific applications and improve performance of grouting materials, filters, impervious barriers (water or gas), and contaminant barriers. The most important findings from this work are: 1. Clay-polymer composites formed with a functional, or responsive, polymer were successfully produced using an optimized technique. The synthesis technique, called solution intercalation, simply relies on mixing the clay material with the polymer in solution. The main factor influencing the amount of composite successfully produced is the clay-to-polymer volume ratio. Drying temperature, mixing temperature, and solution pH had very little influence on the amount of composite produced. 2. Real time spectroscopic ellipsometry (SE) was used to investigate the interaction between the polymer molecules and the clay mineral surface. A simulated surface (silica wafer) was used instead of clay particles due to the limitations of the SE device. However, the silica wafer closely models the surface of the crystaliine structure of the clay particles. The results of the SE showed that the polymer responded according to the predicted response, i.e. the polymer attached to the mineral surface increased in thickness at high pH (pH ~ 11.5) and decreased in thickness at low pH (pH ~ 3). The polymer thickness also decreased with increasing ionic strength. These results show that the polymer responds to environmental conditions even when attached to a mineral surface. This result increases the likelihood that the composite will respond in a way that is analogous to the polymer response. 3. Composite swell measurements were performed to link the microscale behavior observed with SE to mesoscale composite behavior. The purpose of this task was to show that the clay-polymer composites respond to changes in the surrounding fluid chemistry just as the polymer molecules do (as observed in SE). These swelling tests consisted of measuring the swelling ratio - ratio between the mass of the completely swollen composite material and the dry composite material mass - at varios pH conditions (low to high pH). Results showed that the swelling of the composited increased with increasing pH (pH ~3 to pH ~ 11.5). These results are consistent with the results observed using spectroscopic ellipsometry. Furthermore, the composite was shown to swell less or more than clay alone, depending on pH. This demonstrates the controllable nature of this material.
