Thermally Induced Phase Transitions and Morphological Changes in Organoclays

Thermal transitions and morphological changes in Cloisite organoclays were investigated by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), Fourier transform infrared (FTIR) spectroscopy, and in situ simultaneous small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) over the temperature range of 30−260 °C. On the basis of DSC and FTIR results, the surfactant component in organoclays was found to undergo a melting-like order−disorder transition between 35 and 50 °C. The transition temperatures of the DSC peaks (T_tr) in the organoclays varied slightly with the surfactant content; however, they were significantly lower than the melting temperature of the free surfactant (dimethyldihydrotallowammonium chloride; T_m = 70 °C). FTIR results indicated that within the vicinity of T_tr, the gauche content increased significantly in the conformation of surfactant molecules, while WAXD results did not show any change in three-dimensional ordering. Multiple scattering peaks were observed in SAXS profiles. In the SAXS data acquired below T_tr, the second scattering peak was found to occur at an angle lower than twice that of the first peak position (i.e., nonequidistant scattering maxima). In the data acquired above T_tr, the second peak was found to shift toward the equidistant position (the most drastic shift was seen in the system with the highest surfactant content). Using a novel SAXS modeling technique, we suggest that the appearance of nonequidistant SAXS maxima could result from a bimodal layer thickness distribution of the organic layers in organoclays. The occurrence of the equidistant scattering profile above T_tr could be explained by the conversion of the bimodal distribution to the unimodal distribution, indicating a redistribution of the surfactant that is nonbounded to the clay surface. At temperatures above 190 °C, the scattering maxima gradually broadened and became nonequidistant again but having the second peak shifted toward a scattering angle higher than twice the first peak position. The changes in SAXS patterns above 190 °C could be attributed to the collapse of organic layers due to desorption and/or degradation of surfactant component, which was supported by the TGA data.