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Progress in Organic Coatings 58 (2007) 105–116

Conferring dichroic properties and optical responsiveness to polyolefinsthrough organic chromophores and metal nanoparticles

Andrea Pucci a, Giacomo Ruggeri a,b, Simona Bronco c, Monica Bertoldo c,Chiara Cappelli c, Francesco Ciardelli a,c,∗

a Department of Chemistry and Industrial Chemistry, University of Pisa, via Risorgimento 35, I-56126 Pisa, Italyb INSTM, c/o Department of Chemistry and Industrial Chemistry, University of Pisa, via Risorgimento 35, I-56126 Pisa, Italy

c PolyLab-CNR-INFM, c/o Department of Chemistry and Industrial Chemistry, University of Pisa, via Risorgimento 35, I-56126 Pisa, Italy

Received 26 May 2006; accepted 28 August 2006

bstract

Properly designed organic chromophores based on functionalized terthiophenes by push–pull substituents and stilbene derivatives were efficientlyncorporated into polyolefins (polyethylene (PE), polypropylene (PP) and their copolymers, even with polar functionalities) at various concentrationfrom 0.05 to 2 wt.%) either by casting from solvents or by processing in the melt. The uniaxial orientation of the derived films by mechanicalrawing provided highly anisotropic films with optical dichroic behaviour in the visible both in absorption and in emission. A different opticalesponse was obtained with bis-benzoxazolyl-stilbene (BBS) optical brighteners incorporated into PE and PP films (ranging from 0.05 to 0.5 wt.%)y melt-processing showing for the last polymer matrix a well-defined excimer green emission by increasing dye concentration flanked by a clearhange of the luminescent properties of the films. The original blue colour of the film provided by the radiative transitions of the isolated BBSolecules, may be restored after polymer stretching that promotes the breakup of the aggregates and the alignment of the single chromophoreolecules along the stretching direction thus providing a very sensitive tool for determining the polymer orientation. With the same approach

anocomposites with unusual and anisotropic optical properties were obtained through the dispersion of preformed gold nanoparticles and gold-inding chromophores in a stretched PE matrix (∼4 wt.%) or produced directly inside a polymer matrix by a photo-reduction process. This kind ofanostructured materials can be applied to different fields that ranges from sensors (i.e., sensitive devices for UV-degradable substances, moleculartrain sensor for polymer matrices) to photonics (i.e., linear absorbing polarizer, displays, non-linear optical devices).

2006 Elsevier B.V. All rights reserved.

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eywords: Nanocomposites; Polyethylene; Organic chromophores; Gold nanopa

. Introduction

Polyolefins for their excellent thermomechanical properties,hemical stability and low environmental impact are expandingveryday their application area. This includes also the coatinghere they have been widely used for film coating and pow-er coating. The above properties are largely determined by theovalent saturated hydrocarbon structure, which is not suitable

or obtaining aesthetic properties, colour and response to exter-al stimuli, which are more and more attractive properties ofoating materials.

∗ Corresponding author at: Department of Chemistry and Industrial Chemistry,niversity of Pisa, via Risorgimento 35, I-56126 Pisa, Italy.el.: +39 050 2219229; fax: +39 050 2219320.

E-mail address: fciard@dcci.unipi.it (F. Ciardelli).

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300-9440/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.porgcoat.2006.08.018

s; Dichroic optical properties (absorption and emission); Optical responsiveness

Such additional properties can be provided to polyolefins byisperding into the paraffin original structure conjugated organicolecules and metal derivatives with their available delocalized

lectron system.This very sustainable approach is rich of new opportuni-

ies; indeed the optical properties of chromophores employeds dispersed additives for the preparation of polymer linearolarizers, coloured filters and molecular probes for polymereformation are strongly affected by the chromophore tendencyo anisotropically distribute along the oriented macromolecularhains during the polymer matrix mechanical deformation [1].ppropriate dichroic dyes features are: (a) dispersibility in theolymer matrix, (b) presence of a rigid-rod-like absorbing core

for orientation) and (c) high molar absorption related to a highipole moment. Depending on the structure complexity of theye molecules, the transition dipole is not necessarily parallelo the molecular axis and the dye, after polymer deformation,

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06 A. Pucci et al. / Progress in Or

ay not be necessarily aligned to the oriented polymer chains,hus resulting in a clear mismatch angle [1,2]. The resultingbsorption anisotropy can be used at the same time to study thehromophores orientation [3] and to produce linearly polarizedight. In the last case, the polarizing efficiency depends, to a firstpproximation, on the possibility to transfer the axial orienta-ion of the host to the low molecular weight guest. It has beenhown that the theoretical limiting properties of a polarizer cane approached by an appropriate combination of ultradrawableolymers (e.g., polyolefins with drawing ratios >15, in particu-ar ultra high molecular weight polyethylene (UHMWPE)) andighly anisotropic and strongly absorbing dyes [4–6]. As theonsequence a very good agreement with pseudo-affine defor-ation model predictions was often recorded.The characterization of UHWWPE-based polarizers and the

alculation of their ultimate properties have extensively beeniscussed in the recent literature [4,5]. The parameters used forhis characterization are dichroic ratio (R) and order parameterS), which are directly related to molecular orientation of theyes, and polarizing efficiency (PE) and single piece transmit-ance (Tsp), which define the polarizer performance. They areefined as follows:

= A‖A⊥

, S = (R − 1)

(R + 2), PE = (T⊥ − T‖)

(T⊥ + T‖),

sp = (T⊥ + T‖)

2

here A and T indicate respectively absorbance and transmit-ance, the subscripts ‖ and ⊥ denote respectively the directionarallel or perpendicular to the drawing axis. In the use of highlyctive dyes a general problem is that extended conjugation andigh dipole moment enhance aggregation and phase separa-ion phenomena; therefore, a limiting factor is the small guestoncentrations that can be used. Recently, linear low-densityolyethylene (LLDPE) has also been used for dye dispersionsith the advantage to decrease chromophores aggregation and

o reach high dichroic ratios at moderate drawings [7].Also considerable efforts have been devoted to the develop-

ent of new efficient linear polarizers, e.g. by recycling reflectedr scattered radiation [8], or directly generating polarized lightsing dichroic photoluminescence [7,9–11], for applications asptical components in optical instruments and liquid-crystal dis-lays. In order to overcome the widely encountered problem ofpacity derived from heterogeneous and bad dispersed com-osites, visible absorbing nanoparticles have been effectivelymbedded into polymer matrices in non-aggregated assemblies12–14]. The development of methods to control size, morphol-gy and aggregation of inorganic nanoparticles is a subject ofarticular interest, since these variables dramatically influenceheir optical properties, and therefore offer ideal means for con-rolling them [15–17]. Differently from smooth metal surfacesr metal powders, clusters of noble metals, such as gold, silver

r copper, assume a real and natural colour due to the absorptionf visible light at the surface plasmon resonance frequency, andhis, as described by the Drude–Lorentz–Sommerfeld [15,16]heory and shown by a huge number of experimental data

awed

Coatings 58 (2007) 105–116

16,18], is much affected by cluster size. In particular, theecrease in metal particle size leads to broadening of the absorp-ion band, decrease of the maximum intensity and often to

hypsochromic (blue) shift of the peak, and these effectsepend also on cluster topology and packing. For example,he anisotropical orientation of dipoles in nanoparticles by anlectric [19,20] or magnetic field [21,22], or by a templatinggent [23–25] or by a uniaxially oriented host polymer matrix14,26–31], generates two different excitation modes: with pho-ons polarized along the aggregation direction, leading to aathochromic (red) shift of the surface plasmon resonance, orrthogonally to it, resulting in a hypsochromic (blue) shift [15].nterestingly, the combination of the optical properties of metallusters with the mechanical ones of thermoplastic host materialsas recently received remarkable attention due to the very attrac-ive optical features of polymer nanocomposites [12,30–33].

hen dispersed into polymers in non-aggregated form, nanopar-icles with very small diameters (a few nm) allow the preparationf materials with much reduced light scattering properties forpplications as optical filters [14], linear polarizers [26,28,30,31]nd optical sensors [34]. The control in nanoparticle size, shapend spatial distribution may actually provide composite materialith modulated optical properties.In this general scenario, the present paper describes work

arried out in the authors laboratory aimed to provide ethy-ene and propylene polymers with new optical properties byanodispersion of organic chromophores and noble metal parti-les. Both dispersed systems are characterized by absorbing andmitting visible light thus providing the polyolefin thin filmsith unique optical response which can be modulated by exter-al stimuli directed to the dispersed nanophase or to the hostatrix.

. Polyolefin films with optical dichroic properties

.1. Terthiophene dyes

We have recently studied absorption polarizers based on guesterthiophene chromophores dispersed in oriented polyethylenePE) host matrices [3]. These chromophores were chosen dueo the linear and easy-to-functionalize structure; moreover, thelight non-coplanarity of the aromatic rings in the solid statetorsion angles of 171–174◦ between two molecules of thio-hene) [35] helps in keeping low intermolecular interactions thusaintaining the �-orbital overlapping and the conjugation [36].lso the photophysical properties of oligo- and polythiophenesave been widely employed for electronic and optoelectronicevices [37], as photodynamic sensitizers and phototoxic pesti-ide agents [38].

In order to render the dichroic chromophores: (a) compatibileith polyethylene, (b) structurally rigid—with a rodlike absorb-

ng core (for orientation), and (c) with high molar absorption,enerally correlated to a high dipole moment, thioalkyl chain

nd electron withdrawing groups, as push and pull substituentsere introduced in the C5- and C5′′-position, respectively. Lin-

ar alkyl chains were used to allow crystallization of the modifiedye as crystallization is believed to be beneficial by occurring

ganic Coatings 58 (2007) 105–116 107

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A. Pucci et al. / Progress in Or

epitaxially on oriented fibres thus enhancing the stability of theyes’ mechanical orientation [2].

These criteria have guided the design of several chro-ophores, whose key feature is a rigid-rod-shaped central part,

ased on a high dipole moment terthiophene unit, and linked tone or two long aliphatic chains (derivatives A–F in Fig. 1).n addition similar chromophores with branched alkyl chainderivatives A2–D2 in Fig. 1) were prepared in order to limit theegregation connected to the easy crystallization of the linearlkyl derivatives.

A model study of dispersion based on different alkyl function-lized terthiophene molecules evidenced the strong influence ofhe dye crystallization degree on the phase dispersion behaviournto solution-casted UHMWPE films [39,40]. In particular, lin-

ar and long alkyl lateral chains (number of carbon atomsomposing the chain >10) appeared able to confer the guestolecule the best affinity for PE, but at the same time the high

acking tendency increased the dye crystallinity [40]. On the

Fig. 1. Typical chromophores dispersed in polyolefins matrix.

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ig. 2. UV–vis spectra of (a) oriented 1.6 wt.% D/UHMWPE film and (b) ori-nted 2.5% M/UHMWPE film with polarization respectively parallel (0◦) anderpendicular (90◦) to the drawing axis. Dr = 30 in both cases.

ther hand, the introduction of branched alkyl functionaliza-ion on the terthiophene nuclei reduced the melting temperaturend tendency to crystallization of the dye favouring its disper-ion in the PE apolar matrix [39]. Also films prepared fromelt-processable LLDPE and HDPE and terthiophene deriva-

ives showed a high degree of homogeneity [41,42]. The dyesere dispersed singularly or mixed to produce a gray colour,

n concentrations varying from 1 to 3% (w/w) in the polymer.hen mixed, the chromophores concentrations were adjusted

n the basis of their extinction coefficients, to obtain the samebsorbance for every dye and build a neutral gray polarizer.fter drawing the guest–host dispersions in the solid-state, thelms presented an extremely pronounced dichroism, for disper-ions of pure chromophores (Fig. 2a) or of mixtures of themFig. 2b).

A summary of the optical properties presented by these polar-zers is shown in Table 1. To make the comparison possible,

he data are collected at the absorption maximum of each chro-

ophore. The dichroic ratio values as high as 65 (mixture of A,, and D) place our systems at the top edge of the PE-dispersibleyes for polarizers applications.

108 A. Pucci et al. / Progress in Organic Coatings 58 (2007) 105–116

Table 1Terthiophene/UHMWPE polarizers performances as a function of the chro-mophore load and drawing ratio (Dr)

Dye Concentration(wt.%)

Dr R S PE Tsp

A 1.0 20 21 0.87 0.11 0.89A 1.0 40 18 0.85 0.14 0.86A 1.4 20 17 0.84 0.11 0.89A 1.4 40 25 0.89 0.09 0.91A 3.0 40 12 0.79 0.13 0.86B 1.0 40 28 0.90 0.13 0.88B 2.0 20 25 0.89 0.22 0.80B 2.0 40 39 0.93 0.35 0.73B 3.0 40 40 0.93 0.13 0.88C 2.0 20 4.3 0.52 0.40 0.44C 2.0 30 12 0.78 0.21 0.69C 2.0 40 12 0.78 0.19 0.69C 3.0 20 7.8 0.69 0.36 0.69C 3.0 30 20 0.86 0.10 0.75C 3.0 40 10 0.76 0.09 0.89D 1.0 20 29 0.90 0.09 0.91D 1.0 40 26 0.89 0.07 0.93D 1.6 20 24 0.88 0.18 0.83D 1.6 30 29 0.90 0.10 0.91D 1.6 40 17 0.84 0.11 0.89D 2.0 20 26 0.89 0.07 0.93D 3.0 20 26 0.89 0.15 0.86D 3.0 40 15 0.83 0.22 0.79E 2.0 20 12 0.79 0.24 0.77E 2.0 30 16 0.83 0.16 0.84E 2.0 40 19 0.85 0.05 0.94E 3.0 20 13 0.79 0.24 0.77E 3.0 30 18 0.85 0.08 0.91E 3.0 40 30 0.9 0.06 0.93F 2.6 20 14 0.81 0.35 0.69F 2.6 30 19 0.85 0.37 0.70F 2.6 40 15 0.82 0.25 0.77F 3.9 20 12 0.77 0.31 0.72F 3.9 30 14 0.81 0.28 0.75F 3.9 40 10 0.76 0.21 0.79M (A + B + D) 2.5 20 42 0.93 0.20 0.83M (A + B + D) 2.5 30 65 0.96 0.22 0.81M

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Table 2Absorption dichroic behaviour of films as a function of polymer matrix, chro-mophore load and drawing

Dye wt.% Polymer matrix Type of mixing Dr R

A 1.4 UHMWPE Solution-casting 20 17 ± 2A 1.4 UHMWPE Solution-casting 40 25 ± 1A 0.5 LLDPE Melt-processing 4 18 ± 1AA

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Despite the analogous dipolar structure, in all cases the chro-mophores functionalized by the branched alkyl lateral chaindisplay an emission dichroism much lower than that shown by

Table 3Absorption and emission dichroic behaviour of PE films as a function of chro-mophore load and drawing

Dye Alkylderivatization

wt.% Drawratio

R λexc. (nm) RE

A Linear C18 1.4a 20 17 ± 2 410 13 ± 2A Linear C18 1.4a 40 25 ± 1 410 18 ± 1A2 Branched C8 0.7a 20 10 ± 1 405 4 ± 1A2 Branched C8 0.7a 40 20 ± 1 405 7 ± 1A2 Branched C8 2.0 30 8 ± 1 405 10 ± 1A2 Branched C8 2.0 40 22 ± 1 405 9 ± 2B Linear C18 2.0 20 25 450 5 ± 2B Linear C18 2.0 40 39 450 2 ± 2D Linear C18 1.6 20 24 ± 1 525 24 ± 1D Linear C18 1.6 30 29 ± 2 525 39 ± 1D2 Branched C8 0.8 10 15 ± 1 – –D2 Branched C8 0.8 20 27 ± 1 490 5 ± 1D2 Branched C8 0.8 30 60 ± 1 490 9 ± 1D2 Branched C8 1.3 20 13 ± 1 490 3 ± 2D2 Branched C8 1.3 30 17 ± 1 – –D2 Branched C8 1.3 40 44 ± 1 490 7 ± 1M Linear C 2.5 20 42 ± 1 450 37 ± 2

(A + B + D) 2.5 40 23 0.88 0.21 0.81

ilm obtained by solution-casting.

All the chromophores, except C, show a dependence of PEersus Tsp close to the ultimate properties calculated for molec-larly dispersed dyes with Dr and ε‖/ε⊥ → ∞, according to theseudo-affine deformation scheme.

Actually, the melt-processing of high density and linear lowensity polyethylenes with different dichroic dyes allowed thereparation of better dispersed polymer blends, characterizedy higher anisotropic behaviour with respect to the same filmsrepared by solution casting [7,42]. The SEM micrograph takenn the surface of the film does not show the presence of anyhromophore aggregates (Fig. 3a). In particular, the EDS spec-rum (Fig. 3b), performed on the rough regions of the film,videnced the absence of the sulphur atom (no any signal at

.31 keV present) indicating the very homogeneous distributionf the terthiophene derivative in the polymer matrix. Probably,hose rough regions could be caused by the use of aluminiumoils during the preparation of the film.

MM

0.5 LLDPE Melt-processing 5 24 ± 10.5 LLDPE Melt-processing 8 42 ± 2

Successively, the LLDPE films were uniaxially oriented at10 ◦C, close to the melting temperature of A (115 ◦C) at drawatios of 4, 5 and 8 and in spite of the reduced drawing extentichroic ratio larger than 40 are observed for the linear chro-ophore A when oriented in LLDPE polymer films and larger

han in UHMWPE films (Table 2).The push–pull terthiophene based chromophores with linear

lkyl chain dispersed in UHMWPE show after orientation anxcellent anisotropic behaviour also in emission with values ofE sometimes even higher than 30 (Table 3). A typical exam-le of the dichroism is given by the oriented UHMWPE filmontaining 1.4 wt.% of A at draw ratio of 40 (Fig. 4) [10].

Dichroism in emission (RE) values are about one order ofagnitude larger for A, D and A + B + D dyes mixture (M) (span-

ing from 20 to 40). Broader emission bands (spanning from00 to 800 nm) were obtained with mixtures (M) of A + B + Dhromophores (Fig. 5), confirming their possible application inneutral grey” polarizers, and limited self-quenching of the flu-rescence that results extremely dichroic with RE values well

18

Linear C18 2.5 30 65 ± 2 450 32 ± 2* Linear C18 2.5 20 15 ± 2 450 3 ± 3

a Films with the same mol% of chromophore.

A. Pucci et al. / Progress in Organic Coatings 58 (2007) 105–116 109

Fig. 3. SEM micrograph of 0.5 wt.% A/LLDPE film (a) and ene

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pdrawing direction of the polymer matrix leading to extremelyhigh anisotropic properties in absorption, makes the same filmsunsuitable for emission linear polarizer applications probablybecause of the increased fluorescence depolarization.

ig. 4. Emission spectra recorded with polarization respectively parallel (0◦)nd perpendicular (90◦) to the drawing direction of a 1.4 wt.% A/UHMWPElm; Dr = 40.

erthiophene derivatives bearing the long and linear lateral chainTable 3).

A comparison between the absorption and emission spectraf respectively 1.6 wt.% D/UHMWPE (Fig. 6a) and 0.8 wt.%

2/UHMWPE (Fig. 6b) is reported. Both films contain the sameolar content of chromophore and they are oriented at the same

rawing ratio (30).

ig. 5. Absorption and emission spectra (λexc. = 450 nm) of an oriented 2.5 wt.%/UHMWPE film (Dr = 20).

F(

rgy dispersive (EDS) spectrum of the polymer surface (b).

The branched D2 dispersed in UHMWPE displays a nar-ower bandwidth with respect to D/UHMWPE film. Both thebsorption and the emission spectra suggesting its better phaseispersion as the branching limits the formation of microcrys-alline aggregates of dye and the supramolecular stacking insidehem [43,44].

These results indicate that the increased chromophore dis-ersion while promotes the alignment of the dyes along the

ig. 6. Absorption and emission spectra of (a) oriented 1.6% D/UHMWPE andb) 0.8 wt.% D2/UHMWPE films (Dr = 30 in both cases).

110 A. Pucci et al. / Progress in Organic Coatings 58 (2007) 105–116

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Table 4Experimental polarizers performance of LLDPE/BBS films on varying chro-mophore load and drawing ratio

Sample Dye wt.% Dr R S PE Tsp

LLBBS-0.02 BBS 0.02 6 31 0.91 0.33 0.73LLBBS-0.02 BBS 0.02 8 48 0.94 0.21 0.82LLBBS-0.02 BBS 0.02 10 99 0.97 0.33 0.75LLBBS-0.2 BBS 0.2 6 36 0.92 0.73 0.55LLBBS-0.2 BBS 0.2 8 67 0.96 0.69 0.58LLBBS-0.2 BBS 0.2 10 81 0.96 0.72 0.57LLBBS-0.5 BBS 0.5 6 40 0.93 0.71 0.56LL

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ig. 7. UV–vis absorption spectra as a function of the polarization angle θ forriented LLBBS-0.5 (Dr = 10) and molecular structure of BBS (inset).

.2. Stilbene dyes

Also the commercial dye with stilbene chromophore struc-ure, 4,4′-bis(2-benzoxazolyl)-stilbene (BBS, Fig. 9), wasfficiently incorporated into polyolefins [45,46].

Linear low-density polyethylene (LLDPE) films containingifferent concentrations of BBS were prepared by compressionoulding of the respective LLDPE based mixtures (samplesere named by listing polymer (abbreviated as LL), guestolecule and concentration, e.g. LLBBS-0.02), which were

btained by blending the components at 180 ◦C in a Brabenderype mixer. The binary LLDPE based films were successivelyriented by uniaxial tensile drawing at different drawing ratiosDr, defined as the ratio between the final and the initial lengthf the sample) at the temperature of 90 ◦C and the anisotropicehaviour of the stretched films was evaluated by UV–vis spec-roscopy in polarized light.

The absorption spectrum in polarized light of the LLBBS-0.5lm, with a draw ratio of 10, is reported in Fig. 7 for differentalues of the angle θ between the light polarization direction andhe film orientation.

When the polarization of the incident light is parallel to thetretching direction, the film shows an absorption maximumointed at 378 nm. On the contrary, in the perpendicular con-guration the absorption band results completely suppressed,

hus clearly indicating a pronounced anisotropic behaviour. Theichroism increases with Dr from 6 to 10 and is substantiallyndependent of composition (Table 4).

The best dichroic ratios as high as 100 and order parameteralues close to the unity suggest a striking alignment (on aver-ge) of the BBS molecules along the deformation direction ofhe polymer matrix.

.3. Metal nanoparticles

Noble metal nanoparticles incorporated in polymeric matri-es, depending on particle size, shape and aggregation, mayonfer tuneable absorption and scattering characteristics to the

csg

LBBS-0.5 BBS 0.5 8 69 0.96 0.30 0.76LBBS-0.5 BBS 0.5 10 96 0.97 0.32 0.75

erived thin films [12,18]. When dispersed into polymers in non-ggregated form, nanoparticles with very small diameters (a fewm) allow the design of materials with much reduced light scat-ering properties, overcoming the widely encountered problemf opacity of heterogeneous composites for optical applications14,33].

Even more interesting is the fact that nanoparticle disper-ions in a polymer matrix can be rendered macroscopicallynisotropic, a feature that has allowed their use in non-linearptical devices and linear absorbing polarizers, e.g. for displaypplications [26–28,47].

For example, poly(vinyl alcohol) [14] and high densityolyethylene (HDPE) film composites with alkyl thiol coatedold and silver particles, once uniaxially oriented by stretching,resent angular dependencies of the absorption intensity andhe colour of the transmitted light. The absorption of photonss dominated by the excitation of surface plasmons in the metalarticles and their aggregates [15,16].

The optical response of metal nanoparticles can be modulatednd enhanced through the introduction of photoactive organicolecules, possibly combined with control of the nanoparticle

imensions [48]. The presence of direct electronic interactionsetween metal and metal-bound chromophores is of particularnterest, because it could allow for a fine modulation of the opti-al properties by inducing an energy transfer from the excitedtate of the chromophore to the surface plasmon resonance ofhe metal [49,50].

Dispersion of gold nanoparticles and gold-binding chro-ophores in a stretched polymer matrix of UHMWPE gives

anocomposites with unusual and anisotropic optical proper-ies. Two types of gold nanoparticles were prepared, followingiterature procedures [23].

Analogously, DT nanoparticles (without chromophore) wererepared by chemical reduction of hydrogen tetrachloro-urate(III) trihydrate in the presence of dodecyl mercaptane andT nanoparticles (with chromophore) by the same proceduresing a mixture of dodecyl mercaptane and of 5′′-thiooctadecyl-2,2′:5′,2′′]terthiophene-5-thiol, C18S-TT-SH, a thiol derivativef a strongly dichroic terthiophene-based chromophores.

Bright-field TEM pictures of DT and TT nanoparticles fromolloidal dispersions showed for both systems an approximatelypherical shape, more regular for the former and slightly elon-ated for the latter (Fig. 8). The examination of elemental carbon

A. Pucci et al. / Progress in Organic Coatings 58 (2007) 105–116 111

f DT (left) and TT (right) nanoparticles.

dtt

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Fig. 8. Bright-field TEM pictures o

istribution maps revealed the presence of the organic phase dis-ributed around the metal particles and at the interface betweenhem.

From these nanoparticles polymeric gold nanocompositesere prepared by casting a solution of 20 mg of nanoparti-

les and 500 mg of UHMWPE in 75 mL p-xylene at 125 ◦C,nd recovering the film after solvent evaporation at room tem-erature. The aggregation size of DT nanoparticles is stronglynfluenced by the mixing process with UHMWPE; in fact, the DTarticles embedded in the polyethylene matrix showed a >100%ncrease in the size of the smaller particle population. The sameehaviour is not observed for TT nanoparticles probably due toheir higher thermal stability induced by the terthiophene basedhiol.

After uniaxial film stretching in the solid state, the goldanoparticles were oriented along the drawing direction andssumed an anisotropic distribution (Fig. 9). Nanoparticle sizend size distribution were substantially unchanged in the processf thermo-mechanical elongation, thus suggesting aggregationhenomena to be negligible.

The gold plasmon absorption (550 nm) was characterized byoor dichroism, in line with literature reports for other small-ized gold nanoparticles [14,26–28]. In contrast, high dichroism

dichroic ratios between 14 and 30 recorded at 400 nm, depend-ng on the drawing ratio) was observed for the TT nanoparticleerthiophene band; this indicates a sensitivity of the adsorbedhromophores to mechanical orientation (Fig. 10). It is impor-

c

tr

Fig. 9. TEM image of the oriented T

ig. 10. UV–vis spectra of the 4 wt.% TT/UHMWPE oriented film (Dr = 30)s a function of the angle between the polarisation of light and the drawingirection of the film.

ant to note that these terthiophene chromophores linked to goldanoparticles show a higher dichroic ratio for absorption thanhe micro-aggregates of very similar dyes dispersed in orientedolyethylene. The anisotropic orientation of these gold-bound

hromophores appears to be very efficient at the molecular level.

UHMWPE nanocomposites exhibited luminescence, even inhe absence of aromatic groups with a maximum at 435 nm,ecorded upon excitation at the gold absorption band around

T/UHMWPE film (Dr = 20).

112 A. Pucci et al. / Progress in Organic

Fig. 11. (a) Fluorescence emission of oriented 4 wt.% DT and TT UHMWPEnanocomposites, using the gold excitation wavelength (288 nm)); (b) fluores-cence emission of oriented 4 wt.% nanocomposites TT compared with that of a0td

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.5 wt.% C18S-TT-SH/UHMWPE blend without gold nanoparticles, using theerthiophene (390 nm) excitation wavelength. The chromophore fluorescenceisappears when gold nanoparticles are present. In both cases Dr = 30.

88 nm (Fig. 11a). The photoluminescence of TT nanocom-osites was strongly enhanced compared to DT nanoparticlesispersed in UHMWPE (Fig. 11a). This increased luminescenceight be due to the interaction with a terthiophene-based thiol,hich may modify the electronic levels of the gold and there-

ore the emission process involving the sp states [51]. Moreover,T nanoparticles dispersed in polyethylene were smaller (aboutnm) than for DT (about 4–5 nm). It has been reported thatery small metal particles (∅ less than 5 nm) provide high emis-ion intensities, due to a more efficient coupling of the incidentadiation to their surface plasmon [52].

On the other hand, when exciting TT nanocomposites athe chromophore excitation wavelength (390 nm), the typicalerthiophene fluorescence centred at 470 nm (and observed inolyethylene dispersions without gold) completely disappeared,ikely due to a quenching process promoted by the noble metalFig. 11b) [49,50]. This suggests that the TT nanocompositehromophores are essentially all present in a gold-bound formhat changes their emission behaviour.

Even more interestingly, the emission stimulated by irradi-tion in the gold band region (288 nm) was dichroic for bothT and TT samples. The emission dichroism of gold nanoparti-

les anisotropically distributed along the UHMWPE fibres might

Itin

Coatings 58 (2007) 105–116

e due to electromagnetic energy transport between plasmon-olariton modes of closely spaced oriented particles [53]. Thishenomenon has previously been reported for, e.g., metalanoparticle plasmon waveguides and is generally observed inystems that contain closely spaced optically excited atoms,olecules or nanocrystalline semiconductors [54,55]. TT sam-

les, however, exhibited a much higher dichroism (at a drawingatio of 30, the dichroic ratios are respectively 23 and 9 for TTnd DT) and sensitivity to the drawing extent of the polyethy-ene matrix. This extent of the dichroic enhancement in the TTystems is unlikely to be explained by the effect of particlesize alone, even if we take into account that small particleshould be more sensitive to mechanical orientation in a highlyiscous matrix. It appears that the dichroic enhancement mayather be due to an energy transfer mechanism between theell-oriented chromophores and gold that preserves the polar-

zation of the radiation. This would be in line with the previouslyeported observation that terthiophene nanoparticles have a lessntense but still dichroic [56] absorption band (ε ∼ ε370/3) in the60–300 nm spectral region. This band is likely to be respon-ible for the chromophore excitation observed in the presentxperiments.

. Optical responsiveness

.1. Dye assembly variations in PP matrix induced byechanical deformation

The chromophore (BBS) in tetrachloroethaneTCE) shows an absorption band centred at 377 nmε = 62,000 L mol−1 cm−1), mostly located in the near-UVegion of the electromagnetic spectrum of light. The emissionpectrum of the same solution displays, when excited at 277 nm,n emission feature characterized by a well-defined vibronictructure attributed to the 0–0, 0–1 and 0–2 radiative transitions,ointed at 410, 425 and 450 nm, respectively. In addition, noetectable bands attributed to aggregation phenomena betweenhe chromophores are visible in both absorption and emissionith increasing the BBS concentration.On the other side, polypropylene (PP) films containing 0.02,

.2 and 0.5 wt.% BBS (PP/BBS-0.02, PP/BBS-0.2 and PP/BBS-

.5, respectively) prepared by compression moulding of theespective PP/BBS mixtures, obtained by blending the compo-ents at 260 ◦C in a Brabender type mixer are characterizedy the absence of micro-sized aggregates of the luminescentolecules. The UV–vis spectra of PP/BBS films show in solu-

ion absorption bands centred at about 375 and 400 nm, with thentensity increasing with BBS concentration (Fig. 12). In par-icular, despite the high scattering contribution of the 100 �mhick PP matrix, the films do not display any absorption bandttributed to the formation of BBS aggregates. However, differ-nt from UV–vis absorption spectra, the emission characteristicsf PP/BBS films change with BBS concentration (Fig. 12).

ndeed while the PP/BBS-0.02 film shows an emission featureotally similar to that reported for isolated BBS chromophoresn TCE, with increasing in BBS concentration 0.2–0.5 wt.%, aew emission band emerges in the green region of the elec-

A. Pucci et al. / Progress in Organic Coatings 58 (2007) 105–116 113

Fig. 12. (a) UV–vis spectra of PP/BBS films as a function of dye concentration.The spectrum of the PP/BBS-0.5 film is also reported after depuration from mostof the scattering components by subtraction of the spectrum of a neat PP filmwith the same thickness; (b) fluorescence emission spectra (λexc. = 277 nm) ofPt

ttc

imdcs

dertd[f

Fig. 13. Fluorescence emission spectra (λexc. = 277 nm) of PP/BBS-0.5 filmcim

idst

teBauDudgaafTpaB0gsteEmm

3situ gold nanoparticles production

P/BBS films as a function of dye concentration. The spectra are normalized tohe intensity of the isolated BBS molecules peak (409 nm).

romagnetic spectrum (500 nm). The intensity of this new bandends to overcome the emission contribution of the isolated BBShromophores (400–450 nm) (Fig. 13).

The occurrence of this broad, unstructured emission, whichs red-shifted relative to the transitions by the BBS isolated

olecule, and the presence of an absorption behaviour indepen-ent of concentration, may suggest that the new luminescenceontribution comes from excimer-type arrangements in the solidtate [57–59].

In TCE solutions the formation of BBS excimers is avoidedue to the high chromophore diffusion rate with respect to thexcited state lifetime [59,60]. By contrast, the low diffusionate and low solubility of BBS dyes into PP matrix facilitateshe aggregation of at least two molecules (with inter-planar

istance between their conjugated structure of about 3–4 A)61,62], where planar sandwich-type conformation provides theormation of excimers. i

ontaining the 0.5 wt.% of BBS molecules, before and after solid-state draw-ng (Dr = 8). The spectra are normalized to the intensity of the isolated BBS

olecules peak (409 nm).

The uniaxially stretched (130 ◦C) PP/BBS films show annteresting luminescence behaviour. Indeed, after mechanicaleformation the oriented portion of the film changes its emis-ion from green to blue, which is the typical luminescence ofhe isolated BBS molecules.

The fluorescence of a PP/BBS-0.5 film, before and afterensile drawing (Dr = 8) shows for the oriented tape only themission bands characteristic of the radiative transitions of theBS monomers, whereas the excimer band at 500 nm resultslmost completely suppressed, due to the collapse of the molec-lar BBS arrangements, responsible for the excimer emission.uring polymer deformation, the mechanical forces are able tonfold the macromolecular chains [1] and, at the same time, theyes dispersed in the polymer as isolated molecules or aggre-ates, and exclusively located in the amorphous phase, undergon alignment along the stretching direction. Dyes aggregatesre generally destroyed during polymer deformation, whichavours the uniaxial orientation of the single molecules [3,63].his is not completely true for micro-sized aggregates, whosehase separation from the polymer matrix hampers their breakupnd limits the anisotropic arrangements of the dyes [3,7,64].BS molecules dispersed in PP at a concentration higher than.2 wt.% result arranged in well-dispersed few molecules aggre-ate where �–� interaction between the conjugated planartructures leads to the excimers generation. These small (downo SEM resolution) well-dispersed luminescent aggregates resultasily broken by mechanical stretching of the host PP matrix.mission features of the oriented films remain stable after 2onths, confirming the low diffusion rate of BBS dyes into PPatrix.

.2. Photochemical tuning of films optical properties by in

Field-responsive gold nanoparticles can be produced directlynside a polymer matrix by a photo-reduction process [65]. Dis-

114 A. Pucci et al. / Progress in Organic Coatings 58 (2007) 105–116

Fb

paawnTppmit(

ifilpUata

sbam

caih[ofdc

Fr(

fUmatrices) to photonics (i.e., linear absorbing polarizer, displays,non-linear optical devices). The presented results can be viewedas a demonstration of the general validity of this approach aimed

ig. 14. Bright-field transmission electron micrograph and particles size distri-ution of Au/EVAl27 film irradiated for 30 min.

ersions of HAuCl4 and ethylene glycol in polymer films, suchs poly(ethylene-co-vinylalcohol) (EVAl copolymers with 0.27nd 0.44 ethylene molar fraction) and of poly(vinylalcohol),ere irradiated with a strong UV source providing goldanoparticles already after a short time (∼5 min) [66–68].he resulting gold particles were efficiently stabilized by theresence of electron-donor hydroxyl groups composing theolymer matrices, which prevented agglomeration and for-ation of micro-sized phase separated metal aggregates. By

ncreasing the irradiation time to 30 min the average diame-er of gold nanoparticles was reduced from 12–2 to 3–4 nmFig. 14).

During nanoparticles preparation the solid-state photo-nduced reaction (Au3+ → Au0) was monitored by analysing thelm with different spectroscopic techniques such as FT-IR (evo-

ution of the typical carbonyl band at about 1740–1720 cm−1

roofing the oxidation of the ethylene glycol during the process),V–vis (disappearing of the Au3+ absorption band at 300 nm

nd evolution of the surface plasmon absorption band attributedo nanostructured Au0 and XRD (evolution of diffraction peaksssigned to the face-centred cubic (fcc) unit cell of gold) [69].

The irradiated samples oriented via mechanical stretching,howed a highly pronounced anisotropic absorption (∼70 nmand shift) when analyzed with polarized light according to thelignment of the gold nanoparticles along the oriented polymeratrix (Fig. 15).The anisotropically distributed and interacting gold parti-

les are known to display a bathochromically (i.e., red-) shiftedbsorption band, when the polarization vector of the photonss aligned with the stretching direction of the film, and aypsochromic (blue-) shift for the cross-polarized absorption14,16]. Such dichroic behaviour is even better evidenced by

ptical microscopy in polarized light (Fig. 16) in which the sur-ace plasmon resonance shift from ϕ = 0◦ (parallel to the drawingirection) to ϕ = 90◦ (perpendicular) is associated with a clearhange in colour from blue to purple.

F(p

ig. 15. UV–vis spectra in polarized light of an oriented film obtained by photo-eduction and bright-field transmission electron micrograph of the same filminset) (Dr = 5).

This kind of nanostructured materials can be applied to dif-erent fields that ranges from sensors (i.e., sensitive devices forV-degradable substances, molecular strain sensor for polymer

ig. 16. Optical microscopy images of oriented Au/EVAl44 oriented filmDr = 5) with polarization direction of the incident light parallel (0◦) and per-endicular (90◦) to the drawing direction (white bar = 1 mm).

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o confer stimuli sensitive properties by dispersing species withelocalized electron systems in thermoplastic polymers consist-ng of macromolecules with single covalent bonds.

. Conclusion

The results presented and discussed in the present reviewaper provide a clear demonstration about the potentialityffered by the dispersion, controlled down to the nanometerimension, of conjugated organic molecules and noble metalanoaggregates into thermoplastic polymer matrices.

Indeed monoalkene polymers, commercially known withhe broad name of polyolefins, are characterized by excellenthermomechanical properties and weatherability but lack ofny particular optical response being transparent at wavelengtharger than 200 nm. On the other side the addition of very lowless than 5 wt.%) amount of properly selected molecules withn extended system of delocalized electrons as well as prop-rly coated gold nanoparticles formed of few atoms has allowedo produce oriented polyolefin thin films showing remarkableichroic properties in absorption and emission in the spectralange of visible light. Additional peculiar optical response wasbserved connected to the change of colour after mechanicaltress and change of colour with orientation.

The key factors in determining this behaviour are the inter-ace interaction between the polymer matrix and the dispersedanophase, the method of preparation of the composite andhe order parameters and aspect ratio of the dispersed phase

olecules or particles.Considering the general validity of the approach and methods

sed and the enormous number of nanoparticles available as ofan-made and natural polymers known, unlimited possibilities

an be predicted.

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