Clay-Polymer Nanocomposites: Polyether and Polyimide Systems
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Polyamic acid-clay intercalates then were prepared by reaction at 25 OC of the desired organoclay with the polyamic acid solution. Allowing the polyamic acid - clay suspensions to dry on a clean glass plate afforded self-supporting films, which then were heated at 300 oC for 3.0 hr to form the cured clay-polyimide hybrid composites. X-ray powder diffraction patterns were recorded with a Rigaku Rotaflex X-ray diffractometer. TEM images were obtained using JEOL 100CX transmission electron microscope. Clay-epoxide mixtures for DSC analyses were sealed hermetically and recorded under N2 on a Du Pont 910 differential scanning calorimeter. RESULTS AND DISCUSSION Polyether System The reaction of EPON-828 resin with acidic forms of montmorillonite at temperatures in
the range 150-300 °C results in the polymerization of the epoxide and the concomitant delamination of the clay structure. At the polymerization-delamination temperature (PDT) a dramatic liquid-to-powder transformation occurs which increases the bulk volume of the reaction mixture by a factor of 4 - 6. At the onium ion clay loadings used in the present work, the fraction of epoxide functional groups capable of reacting with onium ions on the clay surface is limited by stoichiometry to the range 1.4-2.8 %. Thus, the protonated aminocarboxylic acid and primary diamine cations on the exchange sites of montmorillonite act primarily as acid catalysts rather than curing agents. Consequently, the primary polymerization reaction we are observing is the formation of a polyether.
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Figure 1. XRD powder patterns for (a) [H3N(CH2)1I COOH]+-montmorillonite air-dried at 25 °C; (b) [H3N(CH2)llCOOH]+-montmorillonite dried at 229 oc and (c) clay-polyether nanocomposite containing 5 wt% [H3N(CH2)I 1COOH]+-montmorillonite.
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The delamination of the montmorillonite clay in the polymerized epoxy resin is confirmed by X-ray powder diffraction. As shown in Figures la and lb, [H3 N(CH 2 )llCOOH]+montmorillonite remains crystalline over the temperature range 25-229 oC. However, no clay diffraction peaks are observed for a 5:95 (w/w) clay-polyether nanocomposite (Figure Ic). The absence of a 17.0-A peak for [H3N(CH2)llCOOH]+-montmorillonite suggests that the clay particles (tactoids) have been exfoliated and dispersed at the molecular level. Transmission electron microscopy provided unambiguous evidence for the delamination of the clay in the polyether matrix. TEM micrographs for a 5 wt% [H3 N(CH 2 )llCOOH]+montmorillonite-polyether nanocomposite revealed that the micron-sized clay tactoids have been expanded by the polymer into accordion-like packets in which the interlayer spacings range up to 2,000 A. The clay-bound polyether becomes phase-segregated from the extragallery resin during the polymerization process. This phase segregation explains why a powdered morphology is realized for the nanocomposite rather than a monolithic structure. The following empir
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