Page 235 - Book Hosokawa Nanoparticle Technology Handbook

4.4 NANOCOMPOSITE STRUCTURE FUNDAMENTALS
composites due to the following reasons. First, the Needless to say, the dispersion of clay has the most
minerals are natural products and are easily obtain- profound influence on the properties and functions of
able at low cost. Second, they have nanoscale layered the composites in general and the mechanical proper-
structures by nature, which are ideally suited for fab- ties in particular. Thus, it is quite reasonable to give the
ricating nanostructured composites. Third, the clay first priority to investigations, the objects of which are
minerals are inorganic substances with high chemical to clarify the effect of filler dispersion on the mechan-
and heat resistance. For synthesis of the composites ical properties of nanocomposites. Note however that
one often uses organic dispersing agents and surface orientation is a factor which cannot be ignored when
modifiers to increase dispersibility of the clay miner- the anisotropic properties of composites are studied.
als in matrix polymer. Sometimes, one employs Here a brief explanation is given on the mechanical
organic swelling agents to enlarge the interlayer spac- blending of clays and polymers. In the blending, one
ing of clays. Even if these additives are used, not frequently employs rotating screw-type equipment to
much chemical change occurs for these additives and allow clay minerals to disperse in polymers under the
the clays. This indicates that the clay minerals are influence of high shear generated by the rotating
rather non-reactive with organic chemicals. Fourth, as action of screw. The equipment yields both uniformly
Okamoto [4] reported, the clay minerals showed self- dispersed and well-exfoliated clays in matrix polymer.
assembling abilities when a proper environment was In certain cases the equipment is also useful to allow
provided. Moreover, the minerals were able to assist fillers to orient toward a desired direction in polymer.
crystallization of matrix polymer, which was helpful As shown in Fig. 4.4.14 three different kinds of basic
to improve the mechanical properties of the resulting structures are formed by this method depending upon
composites [5]. In general, both self-assembling and the physical and chemical properties of clays and poly-
crystallization of polymer are processes conducive to mers, physicochemical characteristics of clay surfaces,
improving mechanical properties of polymer-clay and operating conditions of the rotating screw
composites. blenders. In Fig. 4.4.14, we see two significantly dif-
In the following part, discussions will be presented ferent structures for polymer–clay composites. One is
regarding the dispersion and orientation of clay miner- a structure in which matrix polymer is intercalated into
als in polymer matrix. For the fabrications of polymer- the nanoscale layers of clay. The other is an exfoliated
clay mineral composites two methods are usually structure in which the clay layers are well-dispersed in
employed. One is to blend a clay mineral with a molten polymer matrix [6]. Results of many studies have indi-
polymer using extruders or injection molding cated that the latter gives more favorable mechanical
machines. The other is in-situ polymerization of properties as compared to the former especially in
monomer in the presence of clay. For these two terms of tensile strength and impact strength.
processes efforts are mainly placed on finding effi- Masenelli-Variot and his coworkers [7] investigated
cient techniques to prepare polymers with uniformly the mechanical properties of a polyamide–clay
dispersed fillers. At present, not much consideration is composite and found that the tensile properties were
given to the orientation of clay in polymer matrix. not much different for both an exfoliated and an

Figure 4.4.14
Structure of polymer-clay nanocomposite.

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