Exfoliation clays

As it is seen from Figurel, there is a peak characteristic at region 20 = 7,0° (d = 1,19 nm) for nalchikit, peak characteristic at 20 = 3,5° (d = 2,47 nm) for nalchikit-M. In the course of introdueing up to 5%wt of nalchikit-M to polyethyleneterephthalate polymer matrix there is no peak. It indicates about intercalation of clay leaves to separate silicate layers. Diffraction patterns exhibit full clay exfoliation. 

Synthetic procedures include (a) monomer impregnation of clay followed by polymerization, (b) intercalation of monomers/polymers in clay, and (c) clay exfoliation techniques. 

MMT and EPON-828, the DSC curve (Fig. 17) indicates that the spontaneous clay exfoliation epoxide polymerization occurred at an onset temperature of 229 °C. On the basis of the integrated peak area, the heat of reaction for the composite was 572 J/g. DSC curves for the neat epoxy resin and the pristine [H3N(CH2)n-COOH]+-MMT are shown for comparison in Figs. 18 and 19 respectively. In the absence of a catalyst, self polymerization of the neat resin occurred at a much higher onset temperature of 384 °C but the heat of reaction (611 J/g) was comparable to that observed for the corresponding nanocomposite (572 J/g), when corrected for the presence of clay (572/0.95) or 602 J/g. 

Fig. 30 A,B. Proposed model for the fracture of A a glassy B a rubbery polymer-clay exfoliated nanocomposite with increasing strain...

Mohanty, S., and Nayak, S. K., Effect of clay exfoliation and organic modification on morphological, dynamic mechanical, and thermal behaviour of melt-compounded polyamide-6 nanocomposites, Polym. Compos., 28, 153-162 (2007). 

Quite a lot of studies have dealt with the use of organoclays in emulsion polymerization. In most of these studies, the organoclay is dispersed in water and the polymerization proceeds as in conventional emulsion polymerization by monomer diffusion from the droplets to the organophilic clay surface, where propagation of polymer chains takes place. However, in a few examples, the organoclay is dispersed in the monomer phase. This monomer clay suspension is next emulsified (sometimes with the aid of ultrasound to help dispersion and promote clay exfoliation) and the resulting droplets are polymerized [262-267], The latter processes look closer to suspension or miniemulsion (depending on the nature of the initiator) than emulsion polymerization and will not be discussed further. 

Osman, M. A., Mittal, V, and Suter, U. W. 2007. Poly(propylene)-layered silicate nanocomposites Gas permeation properties and clay exfoliation. Macromolecular Chemistry and Physics 208 68-75. 

A viable process for manufacturing polyolefin-clay nanocomposifes by in situ polymerization requires adequate catalytic activity, desirable polymer microstructure, and physical properties including processibility, a high level of clay exfoliation fhaf remains stable under processing conditions and, preferably, inexpensive catalysf components. The work described in the previous two sections focused on achieving in situ polymerization with clay-supported transition metal complexes, and there was less emphasis on optimization of polymer properties and/or clay dispersion. Since 2000, many more comprehensive studies have been undertaken that attempt to characterize and optimize the entire system, from the supported catalyst to the nanocomposite material. The remainder of this chapter covers work published in the past decade on clay-polyolefin nanocomposites of ethylene and propylene homopolymers, as well as their copolymers, made by in situ polymerization. The emphasis is on the catalyst compositions and catalyst-clay interactions that determine the success of one-step methods to synthesize polyolefins with enhanced physical properties. 

The effect of stereochemistry and polydispersity on nanocomposite properties has yet to be explored fully. An intriguing result was obtained by Quijada et al. who performed melf mixing of PP-g-MA with octadecylamine-modified montmorillonite or hectorite, then blended these materials with different isotactic polypropylenes. Nanocomposites with better clay exfoliation were obtained using metallocene polypropylene compared to Ziegler-Natta polypropylene, presumably because of ifs lower polydispersity. While stereoselective metallocene catalysts have been used successfully for in situ propylene polymerization in combination with clay supports (see Section 5.2.2.2), the materials properties of these nanocomposites have thus far received insufficienf affention. 

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