Geckeler K.E., Nishide H. (Eds.) Advanced Nanomaterials

12.3 Metal Nanostructures as the Dispersed Phase 395

undergo thermolysis reactions at moderately low temperatures (120 – 200 ° C), pro-

ducing polymer - embedded metal or metal sulﬁ de clusters. The rapid and revers-

ible color switch (from dark brown to light yellow) that was observed when exposing

the nanocomposite ﬁ lms at approximately 80 ° C was attributed to a change in the

surface - plasmon absorption of silver nanoparticles that follows the variation of

inter - particle distances due to polymer matrix expansion [104] .

12.3.2.2 The Use of Metal Nanorods

During the past few years, there has been an increasing interest towards the optical

properties of elongated nanostructures, as nanorods or nanowires , mainly because

their absorption spectrum is characterized by a longitudinal and a transverse

SPR, rather than the single resonance, as observed for spherical nanoparticles

[40, 106 – 109] . In other words, despite spherical particles having the greatest

symmetry, they have only one plasmon resonance (all modes are degenerate).

However, by extending a particle in one dimension, a second, lower - energy

resonance band in the longitudinal direction (longitudinal mode) becomes

apparent, while the original plasmon resonance (transversal mode) remains

unaffected. In general, the number of SPR peaks will increase as the particle

symmetry decreases.

In general, those one - dimensional ( 1 - D ) structures that range in size from a few

hundreds of nanometers to several micrometers, or which have a high aspect ratio

(the ratio between the longest and shortest structure axes), are referred to as

nanowires, and smaller more elongated structures as nanorods.

In the past, many techniques have been developed for the preparation of 1 - D

nanostructures, and in order to understand this basic growth process it will ﬁ rst

be necessary to consider the spontaneous growth process [110] . In a previous

report [44] silver nanorods were prepared via a seed - mediated process and, depend-

ing on the surfactant (capping material) used, it was possible to produce shapes

other than spheres during seed - mediated growth. Several theories [111] have cen-

tered on a preferential adsorption of the surfactant to certain crystal facets on seed

particles; in other words, the surfactant binds to the surface radially, but not axially.

Unfortunately, however, this behavior blocks crystal growth on these surfaces,

such that the particle can grow only in the axial direction.

As reported by the present authors ’ group and elsewhere [109, 112, 113] , in order

to produce nanocomposite - containing metal nanostructures which are anisotropic

and thus potentially more sensitive and responsive to any mechanical stimuli

applied to the polymer matrix, silver nanorods were mixed with an aqueous solu-

tion of PVA, such that a homogeneous ﬁ lm was obtained after solvent

evaporation.

Following uniaxial stretching, the elongated silver nanostructures (mostly nano-

rods, but mixed with other spherical assemblies generated during solution casting

as a consequence of particle coalescence) that were embedded into the polymer

matrix assumed the direction of the drawing, as indicated using transmission

electron microscopy ( TEM ) (Figure 12.11 ). These dispersed nanorods had diam-

eters between 30 and 35 nm, and a length < 180 nm.

396 12 Optically Responsive Polymer Nanocomposites Containing Organic Functional Chromophores

As expected, the oriented nanocomposite ﬁ lms showed a strong dichroism when

irradiated with a linearly polarized light (Figure 12.12 ). Changing the relative

orientation between the stretching direction of the ﬁ lm and the polarization vector

of the incident light induced a suppression of the longitudinal SPR at 640 nm in

favor of the transversal SPR at 420 nm. The two isosbestic points, placed at 327

Figure 12.11 Bright - ﬁ eld TEM image of PVA - oriented ﬁ lm

containing silver nanostructures [114] .

Figure 12.12 UV - visible spectra of oriented Ag nanorods/PVA

ﬁ lm as a function of the angle between the light polarization

and the drawing direction [114] .

12.4 Conclusions 397

and 550 nm, were originated by the two independent optical modes (transversal

and longitudinal) that characterized the optical absorption of silver nanorods. The

large, reversible variation ( ∼ 220 nm) in wavelength of the two absorption maxima

was also ﬂ anked by an increase in a red component placed at 500 nm and coupled

with a strong decreasing of the blue contribution located at 640 nm. Accordingly,

the ﬁ lm changed its color under linearly polarized light from blue (parallel con-

ﬁ guration, 0 ° ) to brown - red (perpendicular conﬁ guration, 90 ° ), with only a moder-

ate drawing (draw ratio ≥ 2) (Figure 12.13 ).

Another recently reported and highly interesting example concerned the applica-

tion of gold nanorods to polymers [115] . This group described the generation of

gold nanorod ( GNR ) – elastin - like polypeptide ( ELP ) nanoassemblies, the optical

responses of which could be manipulated based on their exposure to near - infrared

( NIR ) light. Cysteine - containing ELPs were actually self - assembled on GNRs

mediated by gold – thiol bonds, which led to the generation of GNR – ELP nanoassem-

blies. Exposure of these assemblies to NIR light at 720 nm resulted in a heating

of the GNRs due to SPR (directed in the NIR region of the absorption spectrum

at 710 and 820 nm). Subsequent heat transfer from the GNRs led to an increase

in the temperature of the self - assembled ELP above its T

g

- value (i.e., 33 – 34 ° C),

which in turn led to a phase transition and aggregation of the GNR – ELP assem-

blies and a clear change in the optical density of the system. Based on these ﬁ nd-

ings, it was suggested that polypeptides might be interfaced with GNRs to produce

optically responsive nanoasssemblies for sensing and drug delivery applications.

12.4

Conclusions

The information provided in this chapter provides a clear demonstration of the

potential offered by controlling the dispersion of luminescent aromatic molecules

and noble metal nanoaggregates into a wide range of polymer matrices, down to

the nanometer scale. Whilst most of the polymers have been characterized by their

excellent thermomechanical properties, a lack of any particular optical response

was apparent wavelengths above 200 nm. In contrast, the addition of very small

Figure 12.13 Images of oriented Ag nanorods/PVA ﬁ lm with

polarization direction of the incident light parallel (blue, 0 ° )

and perpendicular (brown - red, 90 ° ) to the drawing

direction [114] .

398 12 Optically Responsive Polymer Nanocomposites Containing Organic Functional Chromophores

( < 4 wt%) amounts of correctly selected active dyes or noble metal (nano)assemblies

allowed the production of ﬂ exible or rigid polymeric thin ﬁ lms which demon-

strated remarkable optical responses towards external stimuli. Luminescent, exci-

mer - forming dyes have been described as a new class of optical indicators for many

thermoplastic and elastomeric polymer matrices for mechanical and thermal uses.

However, the greater the interaction and asymmetry of the metal nanostructures,

the higher is the dichroic response of the SPR following mechanical stimulation

of the nanocomposite ﬁ lm. Thus, these hybrid materials may be regarded as

extremely important for future advanced applications, including the possibility of

inducing a nanophase organization through external stresses.

Acknowledgments

These studies were supported ﬁ nancially by the FIRB Project RBNE03R78E_005

of the Italian Ministry of University and Research ( MIUR ).

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Nanocomposites Based on Phyllosilicates: From

Petrochemicals to Renewable Thermoplastic Matrices

Maria - Beatrice Coltelli , Serena Coiai , Simona Bronco , and Elisa Passaglia

403

Advanced Nanomaterials. Edited by Kurt E. Geckeler and Hiroyuki Nishide

ISBN: 978-3-527-31794-3

13

13.1

Introduction

Polymer – phyllosilicate nanocomposites, which are characterized by the presence

of ﬁ llers that are less than 100 nm in at least one dimension, have been the subject

of many reviews and reports during the past decade [1, 2] . The excellent balance

between performance and ﬁ ller content (2 – 5% by weight of phyllosilicates is suf-

ﬁ cient to provide clear improvements in properties) makes these composites espe-

cially interesting as innovative materials for a variety of applications. Such

improvements, which include high moduli, increased strength and heat resist-

ance, and decreased gas permeability and ﬂ ammability, can be ascribed to a fun-

damental feature of polymer nanocomposites, namely that the small size of the

ﬁ llers leads to a dramatic increase in interfacial area when compared to traditional

composites [3] .

Many studies have been initiated by considering conventional petrochemical -

derived polymer matrices, such as polyoleﬁ ns, polyamides, and polyesters. Typi-

cally, the preparation methods underwent intense examination, the aim being to

correlate the structure and morphology of these new nanostructured materials

with their ultimate properties.

Today, the development of renewable polymeric materials with excellent proper-

ties forms the subject of active research interest worldwide [4] , with aliphatic poly-

esters being among the most promising materials for the production of

high - performance, environment - friendly, biodegradable plastics [5] . The need to

optimize their properties, in order that in time they can replace present - day com-

modities, requires the assessment of new, selective, efﬁ cient, and sustainable

preparation methods. Hence, a comparative review of the preparation methods for

both commodities and renewable polyester - based materials should allow compari-

sons to be made of these two classes of material in terms of their potential and

future opportunities.

404 13 Nanocomposites Based on Phyllosilicates

13.1.1

Structure of Phyllosilicates

Phyllosilicates [6] are an essential constituent of argillaceous sedimentary rocks

and of some low - grade metamorphic rocks. The group includes micas, clays, and

serpentine, which are soft minerals with a variable, but generally low, density.

Minerals belonging to the phyllosilicate group are characterized by their possess-

ing layers of [SiO

4

]

4 −

tetrahedra linked together to form a ﬂ at sheet. Rings of tet-

rahedra are linked by shared oxygens to other rings in a two - dimensional ( 2 - D )

plane that produces a sheet - like structure. Typically, the sheets are then connected

to each other by layers of cations, together with water or other low molecules

devoid of any electrical charge. In some substrates, the sheets are rolled into tubes

that produce ﬁ bers, as in asbestos serpentine. The silicon : oxygen ratio is generally

1 : 2.5, because only one oxygen is bonded exclusively to the silicon, while the other

three oxygens are half - shared (1.5) with other silicons.

13.1.1.1 Clays

In the case of polymeric composites [7] , the most important subcategory is that of

clays [8] , including kaolinite , montmorillonite/smectite , illite (mica) and chlorite

subgroups (Table 13.1 ). The clay minerals form part of a general, but important,

group that contain a high percentage of water trapped between the silicate sheets.

Most clays are chemically and structurally analogous to other phyllosilicates, but

contain varying amounts of water and so allow more substitution of their cations.

Clay minerals tend to form microscopic to submicroscopic crystals, with the per-

centage of water changing as a function of the humidity change. In the presence

of water, clays become plastic and can be molded and formed. The expansion of

clay is due to a progressive inclusion of water between the stacked silicate layers.

Clays are usually mixed not only with other clays but also with microscopic crystals

of carbonates, feldspars, micas, and quartz.

In the case of polymer nanocomposites, the most important ﬁ llers belong to

the group of smectites. The term “ smectite ” is used to describe a family of

expansible 2 : 1 phyllosilicate minerals. Speciﬁ c minerals included in the smectite

family [9] include pyrophyllite, talc, vermiculite, sauconite, saponite, nontronite,

montmorillonite, and several less common species. Smectites [10] consist of a

single octahedral sheet sandwiched between two tetrahedral sheets, with the

octahedral sheet sharing the apical oxygens of the tetrahedral sheet. The octahe-

dral sheet may be either dioctahedral or trioctahedral. Some minerals of the

smectite groups are neutral (e.g., kaolinite, pyrophillite, talc), but others are

characterized by negatively charged layers. The charge arises from substitutions

in either the octahedral sheet (typically from the substitution of low - charge species

such as Mg

2+

, Fe

2+

, or Mn

2+

for Al

3+

in dioctahedral species), or in the tetrahedral

sheet (where Al

3+

or occasionally Fe

3+

substitutes for Si

4+

), thus producing one

negative charge for each such substitution. The charge, which usually expressed

as a charge/unit formula, is fundamental to correlate the type of mineral with

its ﬁ nal use (Table 13.2 ).

Geckeler K.E., Nishide H. (Eds.) Advanced Nanomaterials

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