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

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12.2 Organic Chromophores as the Dispersed Phase 385

the data (provided by optical experiments and by quantum - mechanical calcula-

tions) acquired for dispersion of the dyes into the polymer matrix, revealed that

the optical properties and responsiveness to mechanical stimuli were heavily

dependent on the compactness of the perylene aggregates provided by the different

molecular structures of the dyes.

12.2.3

Polymeric Indicators to Thermal Stress

12.2.3.1 Oligo( p - Phenylene Vinylene) as Luminescent Dyes

Another sensing mechanism may be produced, in contrast, by mixing the cyano -

OPVs dyes (or other suitable excimer - forming dyes) with a macromolecular system

characterized by a glass transition temperature ( T

) which is above the operating

temperature. Hence, molecularly mixed, glassy composites can readily be pro-

duced via melt processing and rapid quenching of the melts. In this case, the

sensor dyes were kinetically trapped inside the glassy amorphous phase of the host

polymers, namely poly(methyl methacrylate) ( PMMA ) and poly(bisphenyl A car-

bonate) ( PC ), with the formation of thermodynamically stable excimers occurring

just after annealing above T

[24] . Subjecting blends of sufﬁ ciently high dye con-

centrations to temperatures above their T

- values (130 ° C for PMMA, 150 ° C for

PC) led to permanent and signiﬁ cant changes in the materials ’ emission spectra,

Figure 12.4 Fluorescence emission spectra

( λ

exc.

= 277 nm) of 0.1 wt% poly(1,4 - butylene

succinate)/bis(benzoxazolyl)stilbene ﬁ lm

(PBSBBS - 0.1), before (0%) and after

solid - state drawing (from 30 to 70%

elongation). The spectra are normalized to

the intensity of the isolated BBS molecules

peak (430 nm). Inset: digital image of the

uniaxially oriented PBSBBS ﬁ lm containing the

0.1 wt% of BBS molecules, recorded under

excitation at 366 nm (50% elongation).

386 12 Optically Responsive Polymer Nanocomposites Containing Organic Functional Chromophores

due to phase separation of the polymer and the excimer - forming dye. This effect

appears to bear signiﬁ cant potential for technological applications, including the

use of dye/polymer blends as time - temperature indicator s ( TTI s). In fact, kinetic

experiments performed as a function of time, temperature and dye concentration,

resulted in some well - ﬁ tted, monoexponential growth functions. In particular,

plotting the rate of aggregation against the inverse of the annealing temperature

indicated the presence of a linear, Arrhenius - type behavior, which suggested that

an aggregation of the dye molecules might be predicted when dispersed into a

polymer ﬁ lm. Moreover, the derived thermodynamic parameters could easily be

determined via luminescence experiments.

Recently, additional data have been acquired by using semicrystalline

[ poly(ethylene terephthalate) ; PET ] and poly(alkyl methacrylate) - based matrices

[25, 35] . Homogeneous blends can be produced by conventional melt - processing

protocols that are concluded with a quenching step, with the materials thus pre-

pared displaying emission spectra characteristic of the dyes ’ monomer emission.

For example, annealing above the T

- value of the polyester (80 ° C) led to the forma-

tion of excimers and caused signiﬁ cant changes in the materials ’ emission char-

acteristics. The aggregation processes in the semicrystalline PET/cyano - OPVs

blends investigated appeared to be well - described by single - exponential transfor-

mation kinetics, while the ﬂ uorescence color of these materials was characteristic

of their thermal history (Figure 12.5 ).

In particular, cyano - OPV dyes with a longer, rigid conjugated core showed

slower aggregation rates due to their limited mobility within the polymer

matrix [35] .

An extreme, yet intriguing, application of the multifunctional chromogenic

cyano - OPV dyes was recently reported by Weder et al. [36] . The group reported the

preparation and characterization of a new shape memory polymer ( SMP ) with

built - in temperature - sensing capabilities, by incorporating a cyano - OPV dye into

a semi - crystalline crosslinked poly(cyclo - octene) (PC ’ O) by guest diffusion. SMPs

have the ability to memorize a permanent shape, and so can be manipulated and

ﬁ xed to a temporary shape under speciﬁ c conditions of temperature and stress,

Figure 12.5 Images of initially quenched blends ﬁ lms of

0.9 wt% poly(ethylene terephthalate)/cyano - oligo( p - phenylene

vinylene) upon annealing for the time and at the temperature

indicated ( λ

exc.

= 365 nm). Reproduced with permission from

12.2 Organic Chromophores as the Dispersed Phase 387

yet subsequently relax to the original, stress - free state upon application of an

external stimulus [37] . Weder ’ s group described a new SMP/dye system in which

a built - in sensor provided additional functionality. The material was based on a

chromogenic sensor dye that provided a simple and clear signal indicating if the

set/release temperature had been reached. The dye concentration was chosen to

allow dye aggregation upon drying, which resulted in the formation of excimers.

Exposure of these phase - separated blends to temperatures above the melting point

( T

= 47 – 48 ° C) of the polymer led to dissolution of the dye molecules, yielding a

pronounced change in their optical characteristics.

Brieﬂ y, the permanent rod - shaped PCO/dye blend was heated to 80 ° C, deformed

to a temporary shape (spiral), and ﬁ xed by cooling to room temperature. The spiral

was slowly immersed into a silicon oil bath (ca. 75 ° C), allowing recovery of the

rod shape which was ﬂ anked by a color change from orange to yellow - green

(Figure 12.6 ).

12.2.3.2 Bis(Benzoxazolyl) Stilbene as Luminescent Dye

Polymeric ﬁ lm sensors that were based on excimer luminescence and responsive

to temperature stress were also obtained through the dispersion of moderate

amounts (0.02 – 0.2 wt%) of the food - grade dye BBS into the thermoplastic aliphatic

biodegradable polyester, PBS, by melt - mixing [33] . Analogous to the cyano - OPVs

polymer blends, rapid quenching at 0 ° C of the PBS - BBS mixtures from the melt

promoted the very ﬁ ne molecular dispersion of BBS dyes that were kinetically

Figure 12.6 Image of the recovery from temporary shape

(spiral, orange emission) to the permanent shape (rod, green

emission) for a crosslinked poly(cyclo - octene)/cyano - oligo( p -

phenylene vinylene) blend. The sample was immersed in

silicon oil at ∼ 75 ° C (under illumination at 365 nm) [36] .

Reproduced with permission of the Royal Society of

Chemistry.

388 12 Optically Responsive Polymer Nanocomposites Containing Organic Functional Chromophores

trapped within the PBS matrix, thus avoiding the formation of excimers. The

luminescent behavior of PBS - BBS - quenched ﬁ lms was thermally controlled, with

BBS aggregation tendencies and emission color changes proportional to the

increasing annealing temperature (from 50 to 80 ° C). The thermal stress applied

to ﬁ lms led to the generation of thermodynamically stable aggregates among BBS

dyes, promoting the color change of the material from blue (emission at 435 nm)

to green (emission at 490 nm) (Figure 12.7 ).

The molecular dispersion of BBS dyes into quenched PBS ﬁ lms should allow a

prompt optical response to thermal stimuli, that was faster with respect to the

isolated dyes incorporated into a glassy amorphous polymer matrix. In the case of

PMMA, this occurred at a lower annealing temperature due to the T

value ( − 34 ° C)

of the PBS supporting matrix.

12.2.3.3 Anthracene Triaryl Amine - Terminated Diimide as Luminescent Dye

Another example of thermochromic ﬂ exible materials based on polyethylene was

recently reported [38] in which the authors described the synthesis and incorpora-

tion into PE of a novel anthracene triaryl, amine - terminated diimide ﬂ uorescent

sensor (Scheme 12.2 , 4 ). When incorporated into the polymer (0.1 wt%, by melt -

mixing), the ﬁ lms showed an emission at approximately 620 nm which was pro-

gressively red - shifted by increasing the annealing temperature to 543 nm (green

color) at 150 ° C. Although it is likely that room - temperature emission from the

anthracene diimide - doped PE was due to the formation of molecular aggregates,

Figure 12.7 Fluorescence emission spectra ( λ

exc.

= 277 nm) of

initially quenched 0.05 wt% poly(1,4 - butylene succinate)/

bis(benzoxazolyl)stilbene ﬁ lm, and its color evolution as a

function of the annealing time at 65 ° C. Inset: The same ﬁ lms,

with images recorded under irradiation at 366 nm for speciﬁ ed

times (h).

12.3 Metal Nanostructures as the Dispersed Phase 389

this mechanism cannot fully explain the material ’ s thermochromic behavior in

these ﬁ lms. The authors suggested that heating at 150 ° C led to disruption of the

dye aggregates, while the highly viscous medium of the polymer hampered full

conjugation of the molecule, thus providing the same green emission.

12.3

Metal Nanostructures as the Dispersed Phase

12.3.1

Optical Properties of Metal Nanoassemblies

Today, nanoscience represents one of the most rapidly growing research areas,

allowing the manipulation of matter at the nanoscale and allowing the controlled

fabrication of such systems and devices. Engineered nanoparticles offer potential

applications in many areas beneﬁ cial for humankind, including sensors, medical

imaging, drug delivery systems, sunscreens, cosmetics, and many others [39 – 41] .

Ever since the birth of nanotechnology, it has been clear that the optical properties

of nanostructured metal particles depend heavily not only on their dimensions but

also their shape [42, 43] . The absorption of visible light by metal nanoparticles was

attributed to the induction of a collective oscillation of the free conduction elec-

trons, promoted by their interaction with electromagnetic ﬁ eld. Indeed, when the

wavelength of an incident radiation is comparable with the mean free path of the

conduction electrons of a metal particle, the electric component of the electromag-

netic incident ﬁ eld induces a polarization of the conduction electrons, giving rise

to surface plasmon absorption [43, 44] .

In particular, noble metal nanoparticles or semi - conducting nanocrystals embed-

ded in bulk polymer matrices demonstrate enhanced optical (absorption, lumines-

cence, and nonlinearity) [42, 43, 45 – 51] and magnetic properties [52] , due to the

stabilizing effects of size and aggregation provided by the macromolecular support.

When dispersed into polymers in a nonaggregated form, however, those metal

nanoparticles with very small diameters (a few nanometers) will allow the prepara-

tion of materials with much - reduced light scattering properties for applications as

optical ﬁ lters, linear polarizers, and optical sensors [53, 54] .

For example, clusters of noble metals such as gold, silver, or copper, assume a

real and natural color due to the absorption of visible light at the surface plasmon

resonance ( SPR ) frequency. The application of composites based on polymeric

materials and containing noble metal nanoparticles depends strictly on an ability

to control and to modulate their size, shape, and extent of aggregation. As described

by the Drude – Lorentz – Sommerfeld theory [42, 43] , a decrease in metal particle

size leads to a broadening of the absorption band, a decrease in the maximum

intensity, and often also to a hypsochromic (blue) shift of the peak. These effects

may also depend on cluster topology and packing [44, 55] .

In addition, the anisotropic orientation of dipoles in nanoparticles by a

uniaxially oriented host polymer matrix leads to the generation of two different

390 12 Optically Responsive Polymer Nanocomposites Containing Organic Functional Chromophores

excitation modes: (i) with photons polarized along the aggregation direction,

leading to a bathochromic (red) shift of the SPR; or (ii) with photons polarized

orthogonally to the aggregation direction, resulting in a hypsochromic (blue) shift

(Figure 12.8 ) [53] .

The most common procedure for obtaining a dispersion of metal nanoparticle s

( MNP s) in a polymer matrix is to prepare a colloidal solution of stabilized MNPs,

to mix this with the desired polymer in a mutual solvent, and then to cast a

ﬁ lm by evaporation from the solution [53] . In contrast, few examples have been

reported demonstrating the dispersion of preformed MNPs in a polymer matrix

by melt - mixing at high temperature [56, 57] .

Usually, a water - soluble metal salt is moved into an organic solvent by

using tetra - alkylammonium bromide as phase - transfer agent, followed by

successive reduction with sodium borohydride in the presence of an alkylthiol as

surface stabilizer to prevent coalescence of the growing nanoparticles [58, 59] .

In addition to thiols, a number of different surface stabilizers have been used,

including amines [60] , poly(vinyl pyrrolidone) ( PVP ) [61] , and sodium poly(acrylate)

[62] . By using the above - described colloid chemistry technique, MNPs have

been dispersed in ultra - high molecular - weight polyethylene ( UHMWPE ) [11,

45] , high - density polyethylene ( HDPE ) [57] , poly(vinyl alcohol) ( PVA ) [53, 63,

64] , poly(dimethylsiloxane) ( PDMS ) [65, 66] , and poly(styrene - b - ethylene/

propylene) [67] .

An alternative approach for preparing nanocomposites containing metal

nanoparticles involves an in situ formation of nanoparticles directly within the

Figure 12.8 I n ﬂ uence of the polarization direction of light on

the surface plasmon resonance (SPR) band of uniaxially

oriented polymer ﬁ lm containing gold nanoparticles. The

polarization direction was either parallel (0 ° ) or perpendicular

(90 ° ) to the drawing axis.

12.3 Metal Nanostructures as the Dispersed Phase 391

polymer matrix [63, 68 – 80] . This process is relatively straightforward and requires

simply a reduction of the metal ions precursors by either a photochemical or

a thermally induced process. However, in contrast to nanoparticles prepared

via colloid chemistry, controlling the size distribution of these in situ - prepared

particles is often more difﬁ cult due to inﬂ uential factors such as the polymer

matrix composition, and the time and energy density of the photo - or thermo -

irradiation process.

12.3.2

Nanocomposite - Based Indicators to Mechanical Stress

12.3.2.1 The Use of Metal Nanoparticles

Nanoparticle dispersions in a polymer matrix can be rendered macroscopically

anisotropic, a feature that has allowed their use in nonlinear optical devices and

linear absorbing polarizers, for example, in display applications [11, 45, 53, 57,

81 – 83] . Highly dichroic noble metal nanoparticles are efﬁ ciently obtained after

mechanical drawing of the polymer matrix. The uniaxial orientation of the mac-

romolecular ﬁ bers promotes the anisotropic distribution of both the crystalline

and amorphous phases, which then determines the alignment of the metal parti-

cles along the direction of drawing [53, 84] .

Examples of this include PVA and HDPE ﬁ lm composites with alkyl thiol - coated

gold and silver particles which, once uniaxially oriented by stretching, present

angular dependencies of the absorption intensity and the color of the transmitted

light [53] (Figure 12.9 ).

Figure 12.9 UV - visible spectra of drawn nanocomposites

comprising high - density polyethylene and gold nanoparticles,

taken in linearly polarized light at different angles between the

polarization direction of the incident light and the drawing

direction [53] . Reproduced with permission of John Wiley &

Sons, Ltd.

392 12 Optically Responsive Polymer Nanocomposites Containing Organic Functional Chromophores

The absorption of photons is dominated by the excitation of surface plasmons

in the metal particles and their aggregates [42, 43] . For example, drawn polyethyl-

ene/silver nanocomposites exhibited a strongly polarization - dependent color. Yet,

the color of the light transmitted through the oriented nanocomposite shifted from

red to yellow when the angle between the polarization axis of an interposed polar-

izer and the drawing direction of the ﬁ lm was varied from 0 ° to 90 ° [57] .

Moreover, the optical response of metal nanoparticles can be strongly enhanced

through the introduction of photoactive organic molecules, possibly combined

with control of the nanoparticle dimensions [85] . The presence of direct electronic

interactions between metal and metal - bound chromophores is of particular inter-

est, because it could allow for a ﬁ ne modulation of the optical properties by induc-

ing an energy transfer from the excited state of the chromophore to the SPR of

the metal [86, 87] .

For example, the dispersion of gold nanoparticles and gold - binding chromo-

phores in a stretched polymer matrix of PE produces nanocomposites with unusual

and anisotropic optical properties [45] . Strongly dichroic, terthiophene - based

chromophores, which previously had been used in anisotropic PE dispersions for

the preparation of linear polarizers [88] , were modiﬁ ed with a thiol group and used

for the preparation of gold nanoparticles. These authors demonstrated that the

electronic systems of the chromophores were coupled with the gold nanoparticles,

and that the polarization of the absorbed radiation could be preserved during

energy transfer between a chromophore and a metal particle.

An interesting method for the production of dichroic nanocomposites with

a reversible optical response to mechanical stress has been recently reported

[65, 66] . Here, Caseri et al. [65] based their studies on the difference in the trans-

mitted color of gold and silver elastomeric nanocomposites in dry and in swollen

samples, based on the change in interparticle distances which resulted from the

swelling process. Caseri ’ s group showed that a given dry gold – polymer nanocom-

posite could adopt not only one but several colors, and that these colors could be

switched through swelling processes, including reversible and irreversible dich-

roic – monochroic color transitions. A lightly crosslinked rubber such as PDMS was

selected as the supporting polymer matrix, as it can take up large quantities of

solvent without dissolution, and might also afford reversible dichroic effects upon

deformation. The oriented PDMS composite ﬁ lms appeared blue – gray with light

polarized parallel to the drawing axis, and red with light in a perpendicular orienta-

tion; the dichroism was preserved upon removing the strain. Swelling (with

toluene) of the dichroic oriented ﬁ lms caused the specimens to become pink,

indicating that the individual particles in the linear assemblies had disconnected

and turned blue and nondichroic following evaporation of the toluene.

Recently, nanocomposites based on PVA and poly [ethylene - co - vinyl alcohol]

polymers and nanostructured gold have been efﬁ ciently prepared using a UV

photoreduction process [89] . In this case, the polymer matrix based on vinyl

alcohol repeating units, acted both as a coreducing agent, as a protective agent

against particle agglomeration, and as a macroscopic support. The very rapid

process provided dispersed gold nanoparticles with average diameters ranging

12.3 Metal Nanostructures as the Dispersed Phase 393

from 3 to 20 nm, depending on the host polymer matrix and the irradiation time.

Uniaxial drawing of the Au/polymer composites promoted anisotropic packing of

the embedded gold nanoparticles along the stretching direction of the ﬁ lm, and

this resulted in a shift of the SPR of gold well above 30 – 40 nm (83 nm maximum),

thus producing a well - deﬁ ned polarization - dependent color change from blue to

purple [89] . The optical responsiveness to mechanical stimuli appeared similar

to that reported previously by Smith et al. for optically anisotropic polyethylene –

gold nanocomposites obtained by introducing preformed and annealed alkyl - thiol -

protected gold nanoparticles into polyethylene [81] . The UV photogeneration of

gold nanoparticles directly into a polymer ﬁ lm precursor represents an easier,

faster, and highly competitive technique for the preparation of optically responsive

nanocomposites to mechanical stimuli, however.

Among the large number of hybrid organic – inorganic systems investigated,

those nanocomposites based on PVA and silver have attracted great interest

because of their speciﬁ c optical, catalytic, electronic, magnetic, and antimicrobial

properties [64, 68, 76, 90 – 97] . Recent developments have been focused in this

direction in order to optimize the preparation Ag/PVA nanostructured ﬁ lms by

using alternative “ in situ ” methods such as sun - (UV) [76, 98, 99] or thermally

promoted reduction processes [100] .

These simple and very rapid methods provide dispersed Ag nanoparticles

( < 4 wt%) with average diameters that range from 15 to 150 nm, depending on the

type of preparation [101] . Thermal annealing and UV irradiation in fact form

the basis for these very efﬁ cient methodologies, because they take advantage of

the formation of a complex between the PVA matrix, while silver nitrate [102] : Ag

ions can easily be chelated by the hydroxyl groups of the polymer and then reduced

directly in the host matrix (Scheme 12.3 ).

Scheme 12.3 Thermally promoted formation of silver nanoparticles within the PVA matrix.

394 12 Optically Responsive Polymer Nanocomposites Containing Organic Functional Chromophores

Ag(0) formation was progressively followed using both UV - visible and Fourier

transform - infrared (FT - IR) spectroscopies, monitoring the evolution of the SPR of

silver (ca. 430 nm) and the carbonyl absorption band of PVA (ca. 1720 cm

− 1

respectively [101] .

After uniaxial orientation, the Ag/PVA nanocomposites showed a pronounced

dichroic behavior, based on the anisotropic distribution of the silver assemblies

along the stretching direction. In uniaxially oriented samples, the light absorption

was seen to depend heavily on the angle between the polarization direction of

the incident light and the orientation of the particles embedded onto the host

polymeric matrix. Simply by observing oriented samples through a linear polar-

izer, it is possible to observe that the color of the ﬁ lms depends markedly on the

relative orientation between the polarizer and the drawing direction of the ﬁ lm

(Figure 12.10 ).

The maximum shifts measured for the oriented ﬁ lms were more than 100 nm,

while the spectra showed a well - deﬁ ned isosbestic point, thus conﬁ rming the

existence of two different populations of absorbing nanoparticles [89] . Interest-

ingly, the shift in wavelength of the silver SPR band also occurred at moderate

drawings (draw ratio ≥ 2). The nanocomposite ﬁ lms thus produced were highly

optically responsive to mechanical stimuli as uniaxial deformations.

Analogously, thermochromic ﬁ lms based on silver/polystyrene nanocomposites

have been reported, these being prepared by the thermal annealing of silver

dodecylmercaptide/polystyrene blends at approximately 200 ° C [103 – 105] . It was

shown that alkanethiolates of transition metals dissolved in polymers would

Figure 12.10 UV - visible spectra of oriented sun - promoted

Ag/PVA nanocomposite ﬁ lms as a function of the angle

between the polarization of light and the drawing direction of

the ﬁ lm. The insets show images of the same ﬁ lms under

polarized light.