This study applies a novel technique, multichannel spectroscopic ellipsometry (SE), to characterize adsorbed responsive polymer molecules on simulated clay mineral surfaces representing "tunable" clay-polymer nanocomposites (CPN). SE is a non-destructive, non-invasive, highly accurate technique that has commonly been used to characterize layered materials systems. Predicting conformational behavior of the polymer molecules in CPNs is critical, but not fully understood due to many factors simultaneously affecting the behavior of the clay-polymer system. Since the conformation of responsive polymer molecules varies with external environmental conditions, SE results can be used to predict clay fabric changes due to micro-scale conformational changes of adsorbed polymer molecules. The purpose of this study is to investigate pH- and time-dependent conformational behavior of a responsive polymer on a simulated clay mineral surface and to link the micro-scale conformational variations to the meso-scale swelling potential of tunable CPNs. Using in-situ real-time spectroscopic ellipsometry (RTSE), conformational behavior of the responsive polymer is characterized as functions of pH and time. Low swelling potential measured at pH 3 is likely due to contracted coil conformation of the polymer. On the other hand, high swelling potential measured at pH 11.5 is likely due to extended conformation, and the conformation may be further extended over time.
The conformational behavior of polymers in clay-polymer nanocomposites (CPN) is not fully understood because of the many factors involved. The purpose of the present study was to investigate the conformational behavior of a polymer at the micro- and meso-scales in order to predict the behavior of tunable CPN. The study used a pH-responsive polymer, polyacrylamide, which has time-dependent hydrolysis response properties, to examine micro-scale conformational behavior of the polymer adsorbed on representative clay-mineral surfaces, SiO2 and Al2O3. A nanocomposite and a microcomposite were used to link meso-scale CPN behavior to micro-scale polymer conformation. The conformational behavior was characterized using in situ, real-time spectroscopic ellipsometry. The contracted coil conformation of polyacrylamide was observed at pH = 3, while extended conformation was observed at pH = 11.5 on both SiO2 and Al2O3 surfaces. At pH = 11.5, the polymer conformation changed from expanded coil to extended conformation over time. The polymer conformation changed more rapidly with the Al2O3 surface due to mineral dissolution at pH = 3 and 11.5. Swelling tests were conducted as functions of pH and time to link the micro-scale phenomena to meso-scale CPN behavior. The results indicated that the swelling potential of CPN corresponded to the conformation of adsorbed polyacrylamide, which varied with pH and time. The swelling potential of CPN was maximized at pH = 11.5 and decreased with decreasing pH, corresponding to the observed micro-scale conformational behavior.