Fabrication and physical properties of transparent poly (methyl-methacrylate)-layered silicate nanocomposites
Abstract
Transparent polymer nanocomposites have promising potential for protective coating applications with improved surface resistance, higher temperature performance and low gas permeability for containers and films. Extremely thin protective layers are required for improved performance of various electronic devices in aviation, aerospace and medical equipment as well as for lenses and fiber optics in optical communications. This research study developed a method for fabricating optically transparent nanocomposites of poly(methyl-methacrylate)(PMMA) and a commercial organically-modified layered silicate CloisiteRTM 6A (C6A). The nanocomposites were produced by dispersing C6A and PMMA separately in a common solvent xylene followed by mixing the two solutions by mechanical stirring and/or ultrasonic agitation and then removing the solvent by evaporation. Processing conditions such as the mixing methods and times and the rates of solvent removal were investigated in order to achieve a high degree of dispersion and exfoliation of C6A in the polymer matrix and produce a nanocomposite material with high optical transparency. Small-angle x-ray scattering (SAXS) was used to monitor the morphology of the C6A after each processing step. Thin films of PMMA/C6A nanocomposites were produced by casting and spraying. SAXS results suggest that C6A was partially exfoliated in the composite material with an average of 2 to 3 platelets per crystallite. Transmission electron microscopy (TEM) confirmed the existence of both exfoliated and intercalated C6A in PMMA. One mm thick discs were obtained by molding the sprayed films. The optical transmission of the nanocomposite films and discs was measured with an UV/VIS spectrometer. The spectroscopic results served to identify the best process for producing PMMA-C6A films of high optical transparency. Even the nanocomposite films containing up to 20 wt.% C6A prepared by this process exhibited optical transmittance in the range of 80 to 90%. Thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) were used to characterize the thermal behavior of the PMMA-C6A nanocomposites. It was found that the onset of the decomposition temperature increased by 13°C to 17°C as a function of the clay loading while the glass transition temperature increased by an average of 12°C, independent of the percent clay loading. For nanocomposites up to 12 wt.% C6A the storage modulus was maintained to over 30°C above pure PMMA. The nanocomposites of high C6A content also demonstrated up to five orders of magnitude greater resistance to nitrogen gas diffusion over that of the pure PMMA. The process sequences developed in this research study successfully produced a transparent nanocomposites. The process could be used to deposit protective coatings on various shaped substrates or could be molded into larger parts increasing the potential use of this nanocomposite material.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 2003
- Bibcode:
- 2003PhDT........30V