Page 236 - Book Hosokawa Nanoparticle Technology Handbook

FUNDAMENTALS CH. 4 CONTROL OF NANOSTRUCTURE OF MATERIALS
intercalated composite. This result seems to be differ- material during the course of composite fabrication.
ent from those obtained by other investigations. For Simply speaking, the direction tends to become par-
kneading of polymers and clays, Dennis and his allel to the flow direction of polymer in a high-shear
coworkers [8] have made a comprehensive study on zone. In a lower-shear zone, interestingly enough, the
the relationships among the operating conditions of chain axes of polymer crystallites aligned perpendic-
hardware, the degrees of both dispersions and exfolia- ularly to the layers of montmorillonite. As an exten-
tions of fillers in the resulting composites and their sion of studies related to the orientation of fillers for
mechanical properties. The results of this study pro- polymer-based composites, Okamoto and his cowork-
vide a useful guide for predicting the mechanical ers [10] investigated the orientation of clay under an
properties of polymer–clay composites obtained by elongational flow of polypropylene–clay system and
conventional kneading equipment. Besides the dis- found that a card-house structure of clay was formed
persed and exfoliated conditions of clays in polymer, in the polymer. The formation was caused by the
there is another important factor influential on the interaction of negatively charged surfaces of clay lay-
mechanical properties of polymer–clay composites. ers and positively charged end sections of the clays.
That is an interaction between polymer chains and clay Fong and his coworkers [11] studied the orientation
minerals on the interfaces of these two components. of montmorillonite clay in Nylon 6 for the electro-
Concerning the orientation, Kojima and his col- spinning process. In their study the clay layers
leagues [9] investigated the effects of flow conditions aligned in the direction perpendicular to the axes of
of a montmorillonite-Nylon 6 composite and shear Nylon fibers. As shown above a number of investiga-
stress exerted upon molten composite on the orienta- tions have been conducted on the orientations of
tions of these two components. They prepared the fillers in polymer-based nanocomposites. Regardless
composite using in-situ polymerization for Nylon 6 of the fact that the filler orientation is an important
in the presence of the layered montmorillonite, factor for the design of advanced composite materi-
formed films by employing an extruder and exam- als, not much data are available to find quantitative
ined specimens prepared by injection molding. The relationships between the orientation of fillers and
composite specimens thus prepared had a laminated physical properties of composites. For dispersion, the
structure composed of ultrafine crystallites of Nylon degree of dispersion can be controlled by the knead-
6 and the montmorillonite layers comprised of a few ing temperature and distribution of shearing velocity.
or several monolayers of montmorillonite. In terms On the contrary, it is not easy to control the orientation
of the orientation of the montmorillonite and the of fillers during the course of composite fabrication.
Nylon 6 crystallites, each of specimens had three dis- As already stated, the anisotropy in mechanical, trans-
tinct layers in the direction of the thickness of speci- port, and optical properties are closely related to the
men. They are surface, intermediate, and middle orientations of polymer crystals, polymer chains, and
layers. The surface layer was located in the vicinity of laminated fillers. Therefore, it is urgent to establish
the specimen surface, while the middle layer was sit- technical methods by which the orientations of fillers
uated in the central section of the specimen. The can be precisely regulated.
intermediate layer was formed between these two
layers. It should be noted that the shear stress was (3) Aspect ratio ranges from 10 to10,000
very high in the surface layer. In this layer both the Fibrous fillers such as glass fibers, various whiskers
montmorillonite monolayers and the chain axes of and tube-type fillers represented by carbon nan-
Nylon 6 crystallites showed orientations parallel to otubes are considered to fall into this category.
the specimen surface and the directions of the axes Particularly single-walled carbon nanotubes
were random within the layers of the Nylon 6 crys- (SWNTs) exhibit unique characteristics in mechani-
tallites. In the intermediate layer where the shear cal, thermal, optical, electrical, electronic, and trans-
stress was reduced, the montmorillonite layers port properties. For this reason, SWNTs attract a
aligned parallel to the specimen surface and the good deal of attention as a filler for advanced com-
direction of the monolayers were random. In this posite materials and the worldwide competition is
zone the chain axes of the Nylon 6 crystallites were quite keen in development of composites containing
perpendicular to the montmorillonite layers. In the SWNTs. The followings show the recent techniques
middle layer with the lowest shear stress as compared related to how to obtain well-dispersed SWNTs in
to two other layers, the directions of the montmoril- polymer and how to align SWNTs in a desired direc-
lonite layers were perpendicular to the flow direction tion. They are equally challenging problems to over-
of the composite and the directions of monolayers come for the fabrication of the polymer–SWNT
were random. The Nylon 6 crystallites were ran- composites. Because of the highly aggregative nature
domly oriented around the flow axis of the compos- of SWNTs it is rather hard to obtain uniform and sta-
ite and the chain axes of Nylon 6 were perpendicular ble dispersion of SWNTs. To solve this problem
to the montmorillonite layers. The results indicated many methods have been proposed. Chemical modi-
that the direction of clay orientation was influenced fication of SWNT surfaces is an effective way to
significantly by the flow conditions of polymeric weaken the aggregation force of SWNTs. In addition,

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