Page 234 - Book Hosokawa Nanoparticle Technology Handbook

FUNDAMENTALS CH. 4 CONTROL OF NANOSTRUCTURE OF MATERIALS
effective conductivity is 1/10 1/20 of the Ni or YSZ amount of nanofillers, both dispersion and orientation
original conductivity. could be the most dominating parameters. In this
review we intend to show the results of recent investi-
gations related to the structure of polymer-based
References nanocomposites. Special emphasis is placed on the
methods useful for obtaining satisfactory dispersion
[1] C.H. Kuo, P. Gupta: Acta. Metall. Mater., 43, 397–403
(1995). and orientation of nanofillers in polymer matrix.
In order to discuss the dispersion and orientation of
[2] M. Suzuki, T. Oshima: Powder Technol., 35, 159–166 fillers in host materials it is adequate to define the
(1983). shapes and sizes of fillers. The aspect ratios of fillers
[3] S.H. Chan, Z.T. Xia: J. Electrochem. Soc., 148, were used for the definition. Three different ranges of
A388–A394 (2001). aspect ratios were considered: (1) aspect ratio is
closed to unity, (2) aspect ratio is approximately 100
4.4.3 Structure of filler orientation in matrix (fillers with the aspect ratio are exemplified by clay
minerals) (3) aspect ratio ranges from 10 to 10,000
Recently polymer-nanofiller composites have been (the aspect ratios of many rod-like and fiber-like
attracting considerable worldwide attention as cutting fillers fall in this range).
edge materials which benefit human welfare. The
composites exhibit unique properties and functions, (1) Aspect ratio is close to unity
both of which are different from those of component For fillers with almost perfect sphericity (the aspect
materials. During the past 10 years a large number of ratio is close to unity) much effort is placed on how to
research and development projects have been con- arrange the spherical particles rather than how to ori-
ducted to investigate relationships between the struc- ent. One possible method is to prepare the one- or two-
tures and the various properties of the composites. The dimensional arrays of nanoparticle precursors on the
properties cover a broad area including mechanical, surface of polymer. After the arrangement, the precur-
electronic, electrical, optical, thermal, chemical, trans- sors are allowed to react with other chemicals to form
port, biological, medical, and so forth. Obviously, the the arrays of nanoparticles [1]. This method is
foregoing properties of polymer–nanofiller compos- expected to be used to fabricate electronic and optical
ites originate partly from the properties of materials devices. Another plausible method is to arrange vari-
used for the composites. However, the elicitation of the ous dendorimers in polymer, which possess abilities to
unique properties is mainly attributed to the following bond to metal clusters. The addition of the metal clus-
structural characteristics which nanocomposites usu- ters to the dendrimers leads to form polymeric com-
ally possess. First, both filler-matrix contact area and posites with aligned nanoclusters [2]. By selecting
filler-matrix interaction increase significantly when various combinations of metals and dendric polymers,
the nanoscale filler and matrix are intimately mixed. it is possible to synthesize nanocomposites with many
Second, many nanocomposites have an ultrafine lay- varieties of characteristics and functions. Kumacheva
ered structure that helps to improve some properties and his coworkers [3] proposed a method to form a
such as mechanical, thermal, barrier, and the like. three-dimensional configuration of nanoparticles.
There exist at least two other important factors to be First, they fabricated a three-dimensional structure of
considered especially for the improvement of mechan- nanoscale latex particles composed of a hard core and
ical strength of polymer-based nanocomposites. They a soft shell. Then, this structural material was heat-
are the degree of dispersion and the orientation of treated to melt the soft shell made usually of a poly-
fillers. As one of the short-term research projects in mer. The treatment yielded three-dimensionally
the domain of nanotechnology research, the United assembled hard spheres in a continuous medium of the
States has selected a project to develop construction polymer. It is thus possible to synthesize various func-
materials with substantially increased mechanical tionalized nanocomposites when materials with differ-
strength per unit weight as compared to existing con- ent kinds of functionalities are chosen for the
struction materials. Presumably the construction mate- hard-core components. The hard core materials
rials will be used for vehicle parts in automobiles, include chromophores, fluorescent dyes, metal com-
aircrafts, and spaceships. In short, the project aims to plexes, and the like.
invent light-weight and mechanically strong materials
to be used in these vehicles. Development of polymer- (2) Aspect ratio is approximately 100
based nanocomposites appears to be very promising Layered silicates are representative clay minerals
toward attaining this goal. Many polymer-based often used as the fillers of polymer-based nanocom-
nanocomposites contain less than 10% in weight of posites. The silicates contain silicate platelets with a
fillers. The amount is much less compared to the filler thickness of about 1 nm and a side length of about 100
contents of conventional composite materials. To aim nm. Thus, they are considered to be fillers with an
for a substantial increase of mechanical strength of aspect ratio of about 100. The clay minerals are quite
polymer-based nanocomposites with such a small attractive as fillers especially for polymer-based

210