Thermally Induced Phase Transitions and Morphological Changes in Organoclays

Abstract

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 degrees 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 degrees C. The transition temperatures of the DSC peaks (Ttr) in the organoclays varied slightly with the surfactant content; however, they were significantly lower than the melting temperature of the free surfactant (dimethyldihydrotallowammonium chloride; Tm = 70 degrees C). FTIR results indicated that within the vicinity of Ttr, 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 Ttr, 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 Ttr, 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 Ttr 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 degrees 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 degrees 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.

Characterization of twin‐screw‐extruder‐compounded polycarbonate nanoclay composites

“…The high intensity peak of PC/I.34TCN nanocomposites, however, corresponds to about 1.45 nm interlayer spacing, suggesting that a significant proportion of the PC matrix did not intercalate into the nanoclay interlayer. In fact, the interlayer spacing appears to be confined, i.e., reduced from the initial interlayer spacing (d 001 ¼ 2.00 nm), most likely a result of the thermal degradation and/or desertion of the surfactant component of I.34TCN nanoclay during melt processing, as confirmed by other authors [18,22]. Confinement is also supported by the TGA results of the nanoclays.…”

“…Ammonium alkyl surfactants and a variety of metal ions, which exist in commercial organoclays, contribute significantly to the color change of nanocomposites. A great concern are surfactants, which may decompose far below the melt processing temperatures used for PC [8,18]. Xie et al [19] showed that organic modifiers present inside organically modified montmorillonites decompose in a temperature range of 150-5008C.…”

Nanostructure and micromechanical properties of reversibly crosslinked isotactic polypropylene/clay composites

“…In particular, polymer-clay nanocomposites have received an increasing attention. [2][3][4] It is worth mentioning the outstanding contribution of Okada and Usuki in this field, both in the synthesis and development. 5 By using appropriate methods, it is possible to obtain nanocomposites with intercalated and/ or exfoliated phases.…”

Exfoliation of a stack of platelets and intercalation of polymer chains: Effects of molecular weight, entanglement, and interaction with the polymer matrix

“…Understanding the exfoliation of clay platelets in a polymer matrix is very important in designing nano-clay composites with the desirable thermal and mechanical characteristics [1][2][3][4][5][6][7][8][9] such as reducing gas permeability and flame propagation and enhancing strength with respect to tensile response. Controlling the distribution of clay platelets in appropriate polymer matrix is crucial to achieve such thermomechanical characteristics.…”