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Progress in Organic Coatings 72 (2011) 21– 25

Contents lists available at ScienceDirect

Progress in Organic Coatings

jou rn al h om epage: www.elsev ier .com/ locate /porgcoat

olour responsive smart polymers and biopolymers films throughanodispersion of organic chromophores and metal particles

ndrea Puccia,b,c, Giacomo Ruggeria,c,d, Simona Broncod,c, Francesca Signorid,c,ilippo Donatia, Marco Bernabòa, Francesco Ciardelli a,d,∗

Department of Chemistry and Industrial Chemistry, University of Pisa, via Risorgimento 35, I-56126 Pisa, ItalyCNR NANO Institute Nanoscience-CNR, piazza San Silvestro 12, I-56127 Pisa, ItalyINSTM, c/o Department of Chemistry, and Industrial Chemistry, University of Pisa, via Risorgimento 35, I-56126 Pisa, ItalyCNR-IPCF Istituto per i Processi Chimico Fisici, via Moruzzi 1, I-56124 Pisa, Italy

r t i c l e i n f o

rticle history:eceived 22 November 2010ccepted 17 December 2010

eywords:olymersilms

a b s t r a c t

Molecular optical brighteners incorporated into filmable polymers to a very low extent from 0.05 to0.5 wt.% by melt-processing, provide materials which show a well-defined excimer emission by increas-ing dye concentration flanked by a characteristic change of the luminescent properties of the films. Then,the original colour of the film, provided by the radiative transitions of the isolated dye molecules, may berestored after polymer stretching that promotes the aggregates break-up and the alignment of the singlechromophore molecules along the stretching direction thus providing an easy detectable colour change.

ptical responsemart materialstimulated light emission

The approach can be extended to polymer blends allowing to obtain a wide range of films with differ-ent structure and performances in colour changes determined by external stimuli. Also, nanostructuredpolymer films with unusual and anisotropic optical properties can be obtained through the dispersionof gold and silver nanoparticles. The metal nanoparticles can be produced directly inside the polymermatrix by a photo-reduction process. Both organic chromophores and metal particles nanostructuredsystems can be used to prepare “smart” coating materials with a broad type of structural characteristics

stor

and capable to detect and

. Introduction

Thermoplastic polymers for their excellent thermomechanicroperties, chemical stability and low environmental impact arexpanding everyday their application area, including film coatingnd powder coating. The above properties are largely determinedy the covalent saturated structure which is not suitable for obtain-

ng aesthetic properties, colour and response to external stimuli.hese features became more and more attractive properties of coat-ng materials and can be provided by the dispersion of conjugatedrganic molecules and metal derivatives thanks to their availableelocalized electron system [1].

Recently, the possibility to apply the formation of excimersnside polymer matrices for the preparation of polymers with

built-in” temperature and deformation sensors has been effec-ively demonstrated [2–4]. Small amounts of excimer formingligo(p-phenylene vinylene) synthetic chromophores dispersednto a ductile host polymer matrix (i.e., low density polyethylene)

∗ Corresponding author at: Department of Chemistry and Industrial Chemistry,niversity of Pisa, via Risorgimento 35, I-56126 Pisa, Italy.

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

300-9440/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.porgcoat.2010.12.015

es external stimuli of different origin and under different conditions.© 2011 Elsevier B.V. All rights reserved.

as very small (nano-) aggregates of dyes may be produced by guest-diffusion or by rapid quenching of the homogeneous melt blends asevidenced by the formation of excimers. On applying a mechanicaldeformation to the film, the shear-induced mixing between the twophases promotes the break up of the dye supramolecular structureleading to a change of the material emission properties. Anothersensing mechanism may be produced on the contrary by mixing thedyes with a macromolecular system characterized by a glass tem-perature above the operating temperature. In this case the sensordyes are kinetically trapped inside the glassy amorphous phase ofthe host polymers and the formation of thermodynamically stableexcimers occurs just after an annealing above the glass transi-tion temperature. We have efficiently applied the photophysics ofthe low cost, commercial bis(benzoxazolyl)stilbene (BBS) for thedetection of tensile deformation in thermoplastic films [5]. BBSchromophores belong to a well known class of stilbene derivativesgenerally employed as optical brighteners in many polymer objectsand textiles. It is a very good additive for thermoplastic materials,

but other dyes can also be used to expand application possibili-ties. Moreover, the combination of the optical properties of metalclusters with the mechanical ones of thermoplastic host materialshas recently received remarkable attention due to the very attrac-tive optical features of polymer nanocomposites [6]. Nanoparticles

22 A. Pucci et al. / Progress in Organic Coatings 72 (2011) 21– 25

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from aggregachromic dyes dispersed into semicrystalline poly-mers with Tg below the operating temperature [7]. This was thecase of BBS/poly(1,4-butylene succinate) (PBS) blends based on athermoplastic aliphatic polyester, a polymer matrix having sim-

Fig. 1. Stretched PBS film

ith very small diameters (a few nm) dispersed into polymers inon-aggregated form, allow the preparation of materials with mucheduced light scattering properties for applications as optical filters,inear polarisers and optical sensors. The control in nanoparticleize, shape and spatial distribution may actually provide compositeaterials with modulated optical properties.In this general scenario, the present paper describes the recent

ork carried out in the authors laboratory aimed to providehermoplastic polymers with attractive optical properties by nan-dispersion of organic chromophores and noble metal particles.oth systems are characterized by absorption and/or emission ofisible light thus providing the polymer thin films with modu-ated optical response to external stimuli directed to the dispersedanophase or to the host matrix.

. Polymers and biopolymers smart films throughanodispersion of organic chromophores

Polypropylene (PP), polyethylene (PE) and polyesters (poly(1,4-utylene succinate), PBS) films which contained different con-entrations of the food-grade, luminescent dye bis(benzoxazolyl)tilbene (BBS, inset Fig. 1) were efficiently prepared by processingn the melt [5,7]. BBS has an advantage over other synthetic dyesor the low cost and higher thermal stability. Notably, it was clearhat the emission characteristics of PP and PBS composite filmsepended on the BBS concentration and the polymer deformation.

well-defined excimer band was observed with more than 0.2 wt.%f BBS, and this conferred to the film a green luminescence. Duringrawing, the polymer matrix reorganization caused a break in theBS excimer-type arrangement characterised by green emission,

eading to a prevalent blue emission of the single molecules in thetretched area of the film (Fig. 1).

The orientation of the dispersed dyes along the stretching poly-er matrix (i.e. the dichroic behaviour of the oriented composites),as shown to depend on the morphology of the nanostructured

omposite with reference to the dispersion of the stress orientedonjugated molecules. This phenomenon was particularly pro-ounced for low density PE/BBS blends that, owing to the absencef BBS aggregates due to the low polymer crystallinity, showedfter deformation excellent dichroic properties in absorption andptical performances as linear polariser close to the pseudo-affineeformation scheme [8].

Analogous thermoplastic films sensitive to mechanical stressere prepared by using a different luminescent probe based onerylene derivatives [9]. Perylene dyes containing different periph-ral alkyl chains were synthesized and efficiently dispersed at low

oadings (from 0.01 to 0.1 wt.%) into linear low-density polyethy-ene (LLDPE) by processing in the melt. Both perylene bisimides

ere found to generate supramolecular aggregates promoted by–� intermolecular interactions between the conjugated planar

tructure of the dyes as shown by spectroscopic investigations on

aining dispersed BBS dye.

heptane solutions or dispersions into the LLDPE polymer matrix.The occurrence of this phenomenon effectively changed the emis-sion of the dyes from red (interacting dyes) to yellow-green(non-interacting dyes) (Fig. 2).

In particular, the data acquired for dyes dispersions into thepolymer matrix revealed that the optical properties and respon-siveness to mechanical stimuli are strongly dependent on thecompactness of perylene aggregates provided by the differentmolecular structure of dyes.

Threshold temperature indicators based on polymers in theglassy amorphous state (that is with Tg above the operating tem-perature) were recently realised starting from newly preparedcyclic olefin copolymers such as ethylene/norbornene copolymers(E-co-N) and BBS as thermochromic dye [10]. The use of ethy-lene/norbornene systems with different copolymer composition,morphology (since the prepared E-co-N copolymer was semicrys-talline, N content of 15.3 mol.%) and Tg, allowed both the use ofsmaller amounts of dye (less than 0.1 wt.%) and the preparation ofmaterials with thermochromic properties (both in absorption andin emission) in temperature regime starting from 64 ◦C. In detail,upon heating E-co-C/BBS blends which contain molecularly dis-persed and kinetically trapped BBS chromophores at a temperatureabove the Tg, the chromophore molecules started to self-assembleinto stable aggregates leading to a permanent change of the com-posite luminescent properties, i.e. from blue monomer emission togreen aggregate emission (Fig. 3).

Thermochromic polymer blends can be also realised starting

Fig. 2. Emission of a LLDPE film containing the 0.1 wt.% of perylene bis-alkyl imidebefore and after orientation (Dr = 8). Image of the oriented film (Dr = 8) under irra-diation at 366 nm.

A. Pucci et al. / Progress in Organic Coatings 72 (2011) 21– 25 23

Fig. 3. Emission spectra (�exc. = 277 nm) of an E-co-C film containing the 0.5 wt.%of BBS. As a function of the annealing time at 67 ◦C (from t = 0 min to 180 min, step3ii

ipimaintoabdbtp(

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0 min). The spectra are normalized to the emission intensities attributed to thesolated BBS molecules (465 nm) and pictures of the same film taken under therradiation at 366 nm.

lar thermal transitions (Tg = −34 ◦C, Tm = 113 ◦C) and linearity toolyethylene but composed of more polar repeating units contain-

ng 0.02–0.2 wt.% of BBS molecules dispersed by processing in theelt. Rapid quenching from the melt at 0 ◦C of the BBS/PBS mixtures

llowed to maintain the molecular dispersion of BBS dyes providedn the melt, thus avoiding the formation of excimers. The homoge-eous BBS/PBS blends appeared optically stable at room tempera-ure for more than 2 months after preparation when the 0.05 wt.%f BBS is present. Phase separation due to BBS segregation intoggregates occurred by increasing temperature (from 50 to 80 ◦C) ory raising dye concentration up to 0.1 wt.% (at which the thermo-ynamically favoured aggregation forces between dye moleculesecame predominant over polymer viscosity). In particular, thehermal stress applied to films led to the BBS self-assembly thusromoting the colour change of the composite material from blueemission at about 440 nm) to green (emission at about 500 nm).

What observed before for BBS/PBS blends could not be directlyxtended to an analogous aliphatic polyester, i.e. the poly(lacticcid) (PLA), since BBS showed within this last matrix higherolubility which prevents the generation of supramolecular dyeggregates after thermal stimuli. However, as very recently

eported, the temperature responsiveness of a PLA based materialas achieved by blending PLA with a minor amount (15 wt.%) ofolymer having the requested BBS solubility features, such as PBSFig. 4) [11,12].

Fig. 4. Materials and processing conditions for the prepa

Fig. 5. Picture of the PLA85/PBS15 BBS 0.07 film containing the 0.07 wt.% of BBStaken under irradiation at 366 nm after the superimposition of a one euro centheated at 80 ◦C.

In detail, the blends consisting 85 wt.% of PLA and 15 wt.% ofPBS (PLA85/PBS15) and containing the 0.07 wt.% of dye showedan evident change of the luminescent properties from bluemonomer emission to green excimer emission when thermallysolicitated at temperatures higher than PLA’s Tg (>60–70 ◦C). Thepeculiar behaviour of the PLA/PBS blend in terms of aggrega-tion/disaggregation of the dye with varying temperature (in therange between 60 and 100 ◦C) seemed to be related to the effectsprovided by the temperature on the thermally induced mobilityand change of polymer matrix thermal features. The transition ofthe matrix from the glassy to the viscous state promoted by ther-mal solicitations, induced the cold crystallization of PBS whichacted as crystallization nuclei for PLA (resulting in a 23% higher PLAcrystalline content in the blend), thus strongly favouring the aggre-gation of BBS chromophores dispersed in the amorphous phases ofthe blend (Fig. 5).

3. Polymer smart films through dispersion of metalnanoassemblies

Noble metal nanoparticles incorporated in polymeric matricesmay confer tuneable absorption and scattering characteristics tothe derived thin films which depend on particle size, shape andaggregation [13]. When dispersed into polymers as non-aggregated

form, nanoparticles with very small diameters (a few nm) allow thedesign of materials with much reduced light scattering properties,overcoming the widely encountered problem of opacity of hetero-geneous composites for optical applications. Even more interesting

ration of the biodegradable and responsive blend.

24 A. Pucci et al. / Progress in Organic Coatings 72 (2011) 21– 25

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nance at 640 nm in favour of the transversal one at 420 nm.Moreover, preliminary interesting results were obtained after

uniaxial orientation (Dr = 5) of the composite PVA/Ag nanowiresfilm at 110 ◦C. As reported by UV–vis absorption experiments inpolarized light as in Fig. 9, the mechanical drawing induced to

ig. 6. Silver nanoparticle embedded in a PVA matrix; generation after sun- (UV) orhermal-promoted reduction process.

s the fact that nanoparticle dispersions in a polymer matrix can beendered macroscopically anisotropic, a feature that has allowedheir use in nonlinear optical devices and linear absorbing polariz-rs, e.g. for display applications.

An interesting approach for the preparation of nanocompositelms containing metal nanoparticles involves the in situ forma-ion of the nanoparticles directly within the polymer matrix.his process is simple and just requires the reduction of theetal ions precursors by a photochemical or a thermal-induced

rocess. Recently, polymeric films based on poly(vinyl alcohol)nd poly(ethylene)-co-(vinyl alcohol) matrices and nanostructuredold have been prepared by an UV photo-reduction process [14].n this case the polymer matrix based on vinyl alcohol repeatingnits, acts as co-reducing agent, as protective agent against particlegglomeration and as macroscopic support. The very fast processrovided gold nanoparticles with average diameters ranging from

to 20 nm depending on the host polymer matrix and the irra-iation time. Interestingly, uniaxial stretching of the Au/polymeromposites promoted anisotropic packing of the embedded goldanoparticles along the drawing direction of the film, resulting in

shift of the absorption maximum of gold well above 30–40 nm83 nm max.) and thus producing a well-defined colour changerom blue to purple.

Recent developments have been focused in this direction inrder to optimize the preparation Ag/PVA nanostructured films bysing alternative “in situ” methods such as sun- (UV) or thermal-romoted reduction processes [15]. The very easy and fast methodsrovide dispersed Ag nanoparticles (less than 4 wt.%) with averageiameters ranging from 15 to 150 nm depending on the type ofreparation and efficiently stabilised by the chelating properties ofhe PVA hydroxyl groups (Fig. 6).

After uniaxial orientation, the Ag/PVA nanocomposites show aery pronounced dichroic behaviour thanks to the anisotropic dis-ribution of the silver assemblies along the stretching direction.nterestingly, oriented samples when observed through a linearolarizer show colour of the films markedly deepening on the rel-tive orientation between the polarizer and the drawing directionf the film (Fig. 7).

In the last years there has been an increasing interest towardshe optical properties of elongated nanostructures, as nanorods oranowires, because their absorption spectrum is characterised by

longitudinal and a transverse surface plasmon resonance insteadhe single one of isotropic spherical nanoparticles. Generally, one-imensional structures longer than some hundreds nanometresntil micrometres or having a high aspect ratio (ratio between

ongest and smallest structure axis) are called nanowires whilemaller elongated structures will be called nanorods [16].

Ag nanorods and nanowires were prepared by a seed-mediatedrocess: depending on surfactant (capping material), it is possi-

Fig. 7. UV–vis spectra of oriented sun-promoted Ag/PVA nanocomposite films as afunction of the angle between the polarization of light and the drawing direction ofthe film (parallel and perpendicular configuration). Inset: images of the same filmsunder polarized light.

ble to produce shapes other than spheres during seed mediatedgrowth. Several theories normally centre on the preferentialadsorption of the surfactant to certain crystal facets on seed parti-cles. In other words, the surfactant binds to the surface radially butnot axially. This behaviour blocks crystal growth on these surfacesand the particle can only grow in the axial direction. Nanocom-posite films containing metal nanostructures were prepared bymixing silver nanorods or nanowires with a water solution con-taining PVA and a homogeneous film was obtained after solventevaporation. After uniaxial stretching, the elongated silver nanos-tructures embedded into the polymer matrix assume the directionof the drawing [17].

As expected, the oriented nanocomposite films showed strongdichroism when irradiated with a linearly polarised light (Fig. 8).The change of the relative orientation between the stretching direc-tion of the film and the polarization vector of the incident lightinduced the suppression of the longitudinal surface plasmon reso-

Fig. 8. UV–vis spectra of a PVA/Ag nanorods oriented film (Dr = 5) as a function ofthe angle between the polarization light and the drawing direction and images ofthe same film observed through a linear polarizer.

A. Pucci et al. / Progress in Organic

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ig. 9. UV–vis absorption spectra of an oriented PVA/Ag nanowires film as a functionf the angle between the polarization of light and the drawing direction of the film:arallel (0◦) and perpendicular (90◦) configuration.

he film a pronounced dichroic behaviour. The main absorptionf the oriented film pointed at about 390 nm mostly reduced itsntensity when the polarization direction of the incident lighthifted from the perpendicular (90◦) to the parallel (0◦) configu-ation with respect to the drawing axis. Most likely, the anisotropicehaviour was attributed to the orientation of silver nanowiresnd larger metal aggregates (not completely removed by theurification step) embedded in the film towards the stretchingirection. As a matter of fact, the main absorption at 380–400 nm,ttributed to the transverse plasmon resonance of nanowires,trongly reduced its contribution when the polarization direc-ion of the incident light was parallel (0◦) to the PVA matrixrientation.

Accordingly, those absorption features conferred the drawnlms a strongly polarization-dependent colour: the colour of theransmitted light through the oriented nanocomposite film shiftedrom blue-gray to yellow-green when the angle between the polar-zation axis of an interposed polarizer and the drawing direction

as varied from 0◦ to 90◦.

. Conclusions

The dispersion, controlled down to the nanometer dimension,f active dyes and/or noble metal nanoaggregates into thermoplas-ic polymer matrices is a very sustainable and very versatile routeo obtain smart polymer film of different nature with detectableesponse to external stimuli.

Indeed, all filmable polymers with the corresponding saturatedtructure and well localised electrons are characterized by excel-ent thermomechanical properties and weatherability, but lack of

ny particular optical response being transparent at wavelengtharger than 200 nm. However, the addition of very low amounts ofroperly selected molecules with an extended system of delocal-

zed electrons as well as gold or silver nanostructures formed of fewtoms has allowed to produce oriented polymer thin films showing

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Coatings 72 (2011) 21– 25 25

remarkable dichroic properties in absorption and emission in thespectral range of visible light.

The concept demonstrated in our previous work [1] was furtherexpanded with new results concerning the change of colour aftermechanical or thermal stress and change of colour with orienta-tion/temperature.

The new data reported here demonstrate that the colourresponse of nanodisperse dyes and metal nanoassemblies isstrongly affected by the nature of polymer matrix with specificreference to interphase interactions, polymer morphology andtransition temperatures.

The approach here presented and developed has then a provedgeneral validity; indeed the preparative route and the materialproperties predict the possible use of different of light sensitivespecies in very low amount to confer a smart optical behaviour tofilms derived from different polymers and blends, including struc-tures stabilized by cross-linking. On these bases the procedure cancertainly be used for the obtainment of smart coating products[18] showing external stimuli response and information storingcapacity.

Acknowledgements

Partial financial support by MIUR-PRIN 2008 2008SXASBC iskindly acknowledged.

References

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[2] A. Pucci, G. Ruggeri, F. Ciardelli, Optically responsive polymer nanocompos-ites containing organic functional chromophores and metal nanostructures,in: K. Geckeler, H. Nishide (Eds.), Advanced Nanomaterials, vol. 1, Wiley-VCH,Weinheim, Germany, 2010, pp. 379–401.

[3] B.R. Crenshaw, M. Burnworth, D. Khariwala, A. Hiltner, P.T. Mather, R. Simha,C. Weder, Macromolecules 40 (7) (2007) 2400–2408.

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