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deformation sensors for biodegradable poly(1,4-butylene succinate) films


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www.rsc.org/materials | Journal of Materials Chemistry

Bis(benzoxazolyl)stilbene excimers as temperature and deformation sensors for biodegradable poly(1,4-butylene succinate) films{
Andrea Pucci,*ab Flavia Di Cuia,c Francesca Signoriab and Giacomo Ruggeriad
Received 21st August 2006, Accepted 9th November 2006 First published as an Advance Article on the web 24th November 2006 DOI: 10.1039/b612033d In this work, new and innovative polymeric film sensors based on excimer luminescence and responsive to both mechanical and temperature stress were obtained through the dispersion of moderate amounts (0.02–0.2 wt%) of the food-grade dye bis(benzoxazolyl)stilbene (BBS) into a thermoplastic aliphatic biodegradable polyester [poly(1,4-butylene succinate), PBS]. Emission from BBS excimers emerged with dye concentrations higher than 0.05 wt% conferring to the film a green luminescence (lexc. = 366 nm). On applying mechanical stress at rt, the PBS reorganization efficiently breaks the BBS arrangements leading to the prevalence of blue emission from the excited isolated chromophores. Moreover, the optical behaviour of PBS–BBS quenched blends was thermally affected in the range 50–80 uC, providing composite films characterized by a very sensitive temperature dependent luminescence response. The easy modulation of the luminescent properties of polyester films by varying the supramolecular organization of BBS dispersed molecules by thermal or mechanical perturbations suggests various applications in the field of smart and intelligent films from thermoplastic materials.

Introduction
In the past years, a growing interest has been devoted to the luminescence behaviour of macromolecules due to the sensitive response of the photophysical techniques for the study of the dynamic physical properties of macromolecules (i.e. energy transfer, polarization and trapping phenomena). Interactions between the excited state of an aromatic molecule and the ground state of the same molecule actually give rise to excimer (excited dimer) formation, which represents a powerful diagnostic tool of interacting chromophores.1–3 Comparison between the signal intensities of monomeric fluorescence (defined as the contribution of an isolated chromophore covalently attached to the polymer chain or dispersed therein) and the excimer contribution are efficiently used to obtain accurate information on polymer structure and conformation,4–9 as well as on the mixing at the molecular level in polymer blends.9–16 Excimer formation in polymer solutions is widely considered to be a diffusion-controlled process and is influenced by different factors such as solvent, microstructure and macromolecule conformation.2,17,18 Differently, in polymer films the formation of excimers (static excimers) arises from the structural constraints of the polymer chains. In this case the
a b

Dipartimento di Chimica e Chimica Industriale, Universita di Pisa, Via ` Risorgimento 35, 56126 Pisa, Italy. E-mail: apucci@dcci.unipi.it PolyLab-CNR, c/o Dipartimento di Chimica e Chimica Industriale, Universita di Pisa, via Risorgimento 35, I-56126 Pisa, Italy ` c PolyLab-CNR e Dipartimento di Fisica, Universita di Pisa, largo ` Pontecorvo 3, I-56127 Pisa, Italy d INSTM, Unita di Ricerca di Pisa, Via Risorgimento 35, 56126, Pisa, ` Italy { Electronic supplementary information (ESI) available: Optical microscopy image of PBS–BBS film and first heating DSC scan of PBS–BBS quenched films. See DOI: 10.1039/b612033d

resulting static excimers are very sensitive to chromophore aggregation and, as expected, to any variations in the spatial distribution and alignment of molecules in the local environment.19–23 Recently, the possibility to apply the formation of excimers inside polymer matrices for the preparation of polymers with ‘‘built-in’’ temperature and deformation sensors has been effectively demonstrated.24–27 Small amounts of excimerforming oligo(p-phenylene vinylene) synthetic chromophores dispersed into a ductile host polymer matrix (i.e., low density polyethylene) as very small (nano-) aggregates of dyes may be produced by guest-diffusion or by rapid quenching of the homogeneous melt blends as evidenced by the formation of excimers. On applying a mechanical deformation to the film, the shear-induced mixing between the two phases promotes the break up of the dye’s supramolecular structure leading to a change of the material’s emission properties.26,27 Another sensing mechanism may be produced on the contrary by mixing the dyes with a macromolecular system characterized by a glass temperature above the operating temperature. In this case the sensor dyes are kinetically trapped inside the glassy amorphous phase of the host polymers [i.e., poly(methyl metacrylate) and poly(bisphenyl A carbonate)] and the formation of thermodynamically stable excimers occurs just after an annealing above the glass transition temperature.24 Very recently, additional results have been also reported by using semicrystalline poly(ethylene terephthalate) based matrices.25 Analogously, we have efficiently applied the photophysics of the low cost, commercial bis(benzoxazolyl)stilbene (BBS, inset Fig. 2, discussed later) for the detection of tensile deformation in poly(propylene) (PP) films.28 BBS chromophores belong to a well known class of stilbene derivatives generally employed as optical brighteners in many polymer objects and textiles.29,30 BBS is a very good additive for thermoplastic materials due to
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Scheme 1 Molecular structure of poly(1,4-butylene succinate).

PDI = 2.2, Melt Flow Rate (MFR, g/10 min, 2.16 kg/190 uC) = 1.8 ? 0.2, supplied by Showa Highpolymer CO, LTD., Japan] was used as polymer host matrix. Samples were named by listing polymer (abbreviated as PBS), guest molecule concentration and, where performed, quenching: e.g. PBSBBS-0.02q. Apparatus and methods The thermal behaviour of pristine PBS was evaluated by DSC under a nitrogen atmosphere by using a Mettler Toledo StarE System, equipped with a DSC822c module. PBS samples were heated from 280 to 220 uC at 10 uC min21 (1st heating), cooled to 280 uC at the same scan rate (1st cooling), then heated again to 220 uC at 10 uC min21 (2nd heating). Samples were then quenched to 280 uC, kept at this temperature for 3 min, and finally heated again to 220 uC at 10 uC min21 (3rd heating). Melting enthalpies were evaluated from the integrated areas of melting peaks by using indium for calibration. PBS pellets were dried for 2 h under reduced pressure (0.01 mbar) at 60 uC before processing. PBS blends were prepared in a Brabender plastograph mixer (mod. OHG47055, 30 cm3) under nitrogen atmosphere by introducing about 20 g of the polymer and 0.02–0.2 wt% of BBS in the mixer at 200 uC with a rotor speed of 50 rpm. After 10 min, the mixing was stopped and the recovered materials were ground at room temperature by using an IKA MF10 analytical mill. The powder obtained was successively moulded between two aluminium foils under compression in a press (Campana PM20/200) at 200 uC for approximately 3 min. After removal from the press, the films were allowed to slowly reach room temperature (y5 uC min21) or immediately quenched (PBSBBS-q samples) by immersion in an ice–water bath, obtaining binary samples with thickness of about 150–200 mm. Annealing experiments were performed by placing the PBSBBS-q films in a multifunctional heating Binder oven with forced convection (Series FED) at different temperatures in the range 50–80 uC. Stress–strain experiments were performed by using a TINIUS OLSEN H10KT, equipped with a 500 N crosshead cell (argolin LTD) at elongation rate of 1 mm min21. The specimens were prepared according to the ASTM D1708 standard, i.e. 22.25 mm (0.876 inches) of working specimen length. Digital images were obtained by using a Canon PowerShot Pro1 camera exposing the films under a Camag UV-Cabinet II equipped with Sylvania 8W long-range lamps (366 nm). Atomic force microscopy was performed in tapping mode under ambient conditions using a Digital Instruments Multimode AFM, equipped with the Nanoscope IIIa controller. The tapping tips were mounted on 115–135 mm long, single beam silicon cantilevers, with resonant frequencies in the range 230–300 kHz and a spring constant of 20–80 N m21. UV-Vis absorption spectra of solutions and polymer films were recorded under isotropic conditions with a Perkin-Elmer Lambda 650. Steady-state fluorescence spectra of PBS–BBS films were acquired at room temperature under isotropic excitation with the help of a Perkin-Elmer Luminescence spectrometer LS55 controlled by FL Winlab software and equipped with the
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its excellent dispersibility in the molten polymer and its properties comply with the U. S. Food and Drug Administration (FDA) regulations, making it ideal as an additive for indirect food and consumer packaging materials.31 We demonstrated that the emission characteristics of BBS doped PP blends depend on BBS concentration and polymer deformation, providing composite films very sensitive to mechanical stress. Intrigued by the possibility to extend the sensor applications of this stilbene derivative also to other classes of thermoplastic materials, in this work we report the application of the BBS photophysics to a commercially available synthetic aliphatic polyester, i.e. poly(1,4-butylene succinate) (PBS, Scheme 1), a polymer matrix having similar thermal transitions and linearity to polyethylene (PE) but composed of more polar repeating units. Actually, PBS is characterized by a glass transition temperature (Tg) = 234 uC and a melting temperature (Tm) = 113 uC (evaluated from the second heating DSC scan), and it is as processable as PE for film blowing, extrusion coating and foaming for packaging applications [decomposition onset temperature (N2, 10 uC min21, 5% weight loss) = 357 uC]. In addition, PBS is known as one of the most promising biodegradable polyester materials, useful in reducing environmental problems caused by conventional plastic waste. Like cellulose and paper, PBS is stable in the atmosphere but degradable in compost, wet soil, fresh water or seawater and activated sludge where micro organisms are present.32 Therefore, the transfer of the photoluminescent properties reached by using conventional polyolefins (PP and PE) to a biodegradable polyester matrix should be an interesting task. Accordingly, in the present paper, melt-processed PBS–BBS films have been prepared focusing the attention on the processing procedure. In particular, after removal from the press at 200 uC, part of the prepared films were allowed to slowly reach room temperature thus producing micro-/nanophase separated thermodynamically stable blends.26,28 On the other hand, part of the removed films were immediately quenched at 0 uC, thus producing homogeneously and molecularly mixed PBS–BBS blends. All the prepared films have been studied in terms of their emission behaviour as a function of BBS concentration, drawing extent and thermal treatment and the results are discussed in terms of their use as PBS temperature and deformation sensors.

Experimental
Materials 4,49-Bis(2-benzoxazolyl)stilbene (BBS, 97%) was purchased from Aldrich and used without further purification. Poly(1,4butylene succinate) [PBS, Bionolle 1001, Mw = 1.8 6 105,
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Front Surface Accessory: i.e., the position of the sample was adjusted in the direction of the excitation beam in such a way that the optical axes of excitation and emission crossed in the film plane. The films roughness was diminished, using ultrapure silicon oil [Poly(methylphenylsiloxane), 7101 fluid, Aldrich] to reduce surface scattering between the polymeric films and the Suprasil quartz slides, used to keep them planar. The fluorescence quantum yield (Wf) in solutions and polymer films was determined relative to quinine sulfate (Wf = 0.54 in 0.1 M H2SO4) using the following relation:33 ??  {As  2 n s 0 If (u) du 1{10 Wf ~Wf ? ? s If (u) du 1{10{A n2 s 0 where Ws is the ?quantum yield of standard and the integrals f ?? ? s 0 If (u) du and 0 If (u) du are the areas under the emission curves (If = emission intensity at frequency u) of the investigated compound and standard (s), respectively. A and As are the absorbances of the dye and standard, respectively, at the excitation wavelength (277 nm). n is the refractive index of the medium. The refractive index of PBS is 1.45.34 Lifetime measurements were performed on an inverted Leica TCS SP2 microscope by using a two-photon excitation source. A titanium–sapphire laser (Mira 900; Coherent, Palo Alto, CA) was pulsed at 800 nm with 76 MHz repetition rate and pulse width 80 fs. Fluorescence from the sample was detected by an avalanche photodiode (Hamamatsu) and registered by a time-correlated single-photon counting PC card (TCSPC 830, Becker & Hickl, Berlin) using lifetime mode. The signal was acquired at 430 and 500 nm. The emission data were analysed by means of non-linear least-square fitting procedures performed by a JANDEL (AISN software) routine and by Origin 7.5, software by Microcal Origin1.

Results and discussion
BBS optical properties in solution The opto-electronic properties of BBS in tetrachloroethane (TCE) solutions are reported in the literature.28,31 BBS shows an absorption band centred at 377 nm, mostly located in the near-UV region of the electromagnetic spectrum of light and attributed to a p–p* transition. The emission spectrum of the same solution displays, when excited at 277 nm, an emission feature characterized by a well-defined vibronic structure attributed to the 0–0, 0–1 and 0–2 radiative transitions, pointed at 410, 425 and 450 nm, respectively. In addition, no detectable bands attributed to aggregation phenomena of the chromophores are visible in both absorption and emission with increasing BBS concentration. In solution, the formation of BBS excimers is avoided due to the high chromophore diffusion rate with respect to the excited state lifetime.1,18 Optical properties of PBSBBS films 150–200 mm thick poly(1,4-butylene succinate) (PBS) films containing different concentrations of BBS (0.02–0.2 wt% BBS) were prepared by compression moulding of the
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respective PBS–BBS mixtures (PBSBBS-0.02, PPBBS-0.05, PBSBBS-0.1 and PBSBBS-0.2 respectively), obtained by blending the components at 200 uC for 10 min in a Brabender type mixer. The molecular dispersion of the stilbene derivative into PBS was evaluated by optical microscopy (see ESI{ for a PBSBBS-0.2 film, containing the highest BBS concentration), which evidenced the absence of macro-sized aggregates of the guest molecules, indicating a similar behaviour to that shown for BBS–PE and BBS–PP blends.28,35 In addition, atomic force microscopy (AFM) performed on the PBSBBS-0.02 film showed only the typical corrugate surface of films obtained by compression moulding [Fig. 1(a)]. In contrast, by increasing the BBS concentration from 0.02 wt% in PBSBBS-0.02 to 0.2 wt% in PBSBBS-0.2, the surface texture of PBS film changed displaying well-defined geometric structures, most of them less than 0.3–0.5 mm long [Fig. 1(b) and (c)]. Those structures embedded into the polymer continuous matrix are probably attributed to the formation of supramolecular assemblies among BBS molecules, whose dispersibility into PBS decreases with concentration. The luminescence spectrum of PBSBBS-0.02 film showed an emission feature totally similar to that reported for isolated BBS chromophores dissolved in TCE solutions, with emission peaks slightly red-shifted (y5 nm) and centred at 408, 430 and 455 nm. This behaviour indicates that at low concentration the stilbene derivatives are almost molecularly dispersed in the amorphous phase of PBS in the fashion of non-interacting chromophores (Fig. 2). On the contrary, on increasing BBS concentration from 0.02 to 0.2 wt% in PBS blends, a new emission band emerges in the green region of the electromagnetic spectrum (500 nm), the intensity of which tends to overcome the emission contribution of the isolated BBS chromophores (400–450 nm). The occurrence of this broad, unstructured emission, which is red-shifted relative to the transitions of the BBS isolated molecule, may suggest that the new luminescence contribution comes from excimer-type arrangements in the solid state.1,3,19 Actually, this phenomenon, caused by the presence of dimers with associated excited electronic states, is favoured by the p–p

Fig. 1 AFM surface plot images of PBSBBS-0.02 (a) and PBSBBS0.2 (b) films with cross-sectional analysis (c).

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Fig. 2 Fluorescence emission spectra and quantum yield values (lexc. = 277 nm) of PBSBBS films as a function of dye concentration, expressed as the wt% of BBS (inset) molecules with respect to the polymer matrix. The spectra are normalized to the intensity of the isolated BBS molecular peak (430 nm).

oriented by using a tensile drawing machine at 1 mm min21. The effect produced by the mechanical deformation is depicted in Fig. 3 exposing a PBSBBS-0.1 tape under the excitation at 366 nm: the oriented portion of the film (central part, elongation = 50%) changed its emission from green to blue, which is the typical luminescence of the isolated BBS molecules [Fig. 3(a)]. This interesting behaviour is confirmed by fluorescence spectroscopy, analysing the film before and after the tensile drawing [Fig. 3(b)]. On applying progressive uniaxial deformation on PBSBBS-0.1 film, the excimer band at 500 nm strongly decreased its contribution as far as 40–50% of film elongation [from IE/IM = 0.8 at 0% to IE/IM = 0.4 at 40%, where IE is the fluorescence intensity at 500 nm (excimer band) and IM is the intensity at 430 nm (BBS monomeric emission)], thus being completely suppressed. This behaviour suggests the collapse of the supramolecular BBS arrangement, which is responsible for the excimer

stacking interactions between the BBS molecules and probably promoted by the more planar chromophore conformation in the solid state.36 In addition, the low diffusion rate of BBS dyes into the polymer matrix facilitates the aggregation of at least two dyes (with an inter-planar distance between their con? jugated structure of about 3–4 A),37–39 and planar sandwichtype conformation provides the formation of excimers. Moreover, the quantum yields (W) calculated for PBSBBS films at different dye concentrations display an increasing behaviour spanning from 0.2 for PBSBBS-0.02 to 0.42 for PBSBBS-0.1, indicating an improvement in the quantum efficiency derived by the formation of excimers. On the contrary, the PBSBBS-0.2 film containing 0.2 wt% of BBS shows a reduced W value (0.30) probably due to the formation of micro-sized bulk dye aggregates that induced quenching phenomena.40 Due to the very high thermal transition of BBS (melting point = 360 uC), the thermal stability of the BBS supramolecular structure in PBS films was only qualitatively evaluated by placing the binary film in contact with a hot plate at the temperature of 130 uC. Exposing the 200 mm thick film at 130 uC under the excitation of a long-range UV lamp (366 nm), the green luminescence of the excimer emission progressively suppressed in 1–2 min giving rise to the typical blue luminescence of the isolated BBS chromophores. This phenomenon appeared reversible once the heating was stopped and faster by decreasing BBS concentration, i.e. on passing from 0.2 to 0.05 wt%. In this last case, the green–blue–green reversible colour changes of the composite film were instantaneous due to the lower thermal stability of BBS aggregates induced by the higher solubility of the dye into the PBS matrix at low concentration, in accordance with the results obtained recently for BBS–PP blends.28 The stability of the BBS aggregates dispersed into PBS films was also investigated by applying a mechanical stress at room temperature, thus excluding any effect produced by the heating. For this purpose, the binary films were uniaxially
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Fig. 3 (a) Digital image of the uniaxially oriented PBSBBS-0.1 film containing the 0.1 wt% of BBS molecules, taken under excitation with a long-range UV lamp (l = 366 nm, 50% elongation); (b) fluorescence emission spectra (lexc. = 277 nm) of PBSBBS-0.1 film, 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); (c) indications of shearing stress caused during the preparation of a specimen from a PBSBBS-0.1 film.

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emission as a consequence of the PBS matrix orientation achieved by tensile drawing. Actually, it is well established that during polymer deformation, the mechanical forces are able to unfold the macromolecular chains leading to microfibrils resulting in oriented crystalline and amorphous regions.30 At the same time, the dyes dispersed in the polymer as isolated molecules or aggregates, and exclusively located in the amorphous phase, are aligned along the stretching direction.41–43 On increasing dye concentration from 0.1 to 0.2 wt%, the stability of the BBS aggregates against mechanical deformation rises significantly. Actually, after 80% elongation a PBSBBS-0.2 film still showed a well-defined excimer band with an IE/IM ratio of about 0.7. On the contrary, for the same reason, PBSBBS-0.1 and PBSBBS-0.05 composites, characterized by a higher emission sensitivity to mechanical forces, clearly show portions of the film subjected to shearing stress [Fig. 3(c)]. In order to study the excimer disruption of BBS chromophores by mechanical forces from the photophysical view point, lifetime experiments were performed on PBSBBS-0.1 film before and after uniaxial tensile drawing. While the information from a steady-state scan represents the averaged behaviour of what occurs during the entire scan, fluorescence decay (occurring on the nanosecond timescale) provides many details on the environment that the luminescent molecule inhabits.3 It is well known actually that the excimer emission shows long radiative lifetime, caused by the symmetry forbidden nature of the transition.22,44 The photoluminescent (PL) decay profiles of the PBSBBS0.1 film were analysed by using a two-photon excitation source consisting of a titanium–sapphire laser, pulsed at 800 nm with a 76 MHz repetition rate. Data were collected at 430 (emission from the isolated BBS chromophores, data not reported. The PL decays of the oriented and unoriented films resulted substantially superimposed) and 500 nm (emission from the BBS excimers, Fig. 4). Fig. 4 shows the PL decay traces collected at 500 nm for a PBSBBS-0.1 film before (open squares) and after tensile

drawing (open circles, 50% elongation) and fitted by biexponential functions. The unoriented film was characterized by lifetime parameters of about 981 and 10 431 ps, decay values found to be typical of substituted stilbene derivatives, respectively in isolated and rigid (as well as in excimer-type arrangements) environments.45–48 Interestingly, after film drawing the long lived component strongly reduced its contribution providing a PL trace characterized by a decay behaviour typical of the isolated BBS chromophores with a lifetime parameter (1010 ps) comparable with that calculated from the decay measured at 430 nm (951 ps). These results are perfectly in agreement with those reported for steady-state luminescent investigations on PBSBBS-0.1 film, in which the emission of the oriented film (50% elongation) is only dominated by the isolated BBS chromophore contribution at 430 nm. Optical properties of PBSBBS quenched films Recently, the morphology control at the nano-/micro-scale of luminescent dyes incorporated into polymer matrices has been extensively investigated in literature also taking into account the high sensitivity of the composite films to thermal stress.24,25,49 Analogously, BBS chromophores were dispersed into the PBS matrix even as isolated molecules at a concentration higher than 0.02 wt%. Actually, the rapid quenching at 0 uC of PBSBBS-0.05 and PBSBBS-0.1 mixtures from the melt (200 uC) in a ice–water bath, well above the Tg of the host polymer matrix (234 uC), promoted the very fine molecular dispersion of BBS dyes that were kinetically trapped within the PBS matrix, thus avoiding the formation of excimers (Fig. 5, time = 0 h, dashed line). These results were also supported by exposing the quenched films under the UV lamp excitation at 366 nm; the collected blue emission confirmed the absence of supramolecular chromophoric assemblies responsible for the green excimer emission. Moreover, the very fine molecular distribution of BBS molecules into the PBS film is probably helped by the reduced

Fig. 4 Photoluminescence decay lifetime experiments performed on PBSBBS-0.1 film before (%) and after tensile drawing (#, 50% elongation), and bi-exponential fitting functions (grey and black lines, respectively).

Fig. 5 Fluorescence emission spectra (lexc. = 277 nm) of initially quenched PBSBBS-0.05 film as a function of the annealing time at 25 uC (inset: hours). The spectra are normalized to the intensity of the isolated BBS molecular peak (430 nm).

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crystallinity of the polymer matrix occurring after quenching at 0 uC. The first heating DSC scan performed on the PBSBBS-0.05q film 24 h after the preparation (see ESI{) actually showed a cold crystallization peak at about 87–88 uC (DHc = 5.39 J g21) and a melting temperature of 112 uC (DHm = 51.31 J g21). On the contrary, the cooling and the second heating scans provided thermal transitions totally similar to those shown by the neat PBS and the respective unquenched PBSBBS-0.05 (the very low BBS concentration did not affect the thermal transition of the PBS matrix). The molecular dispersion of BBS dyes into quenched PBS films should allow a prompt optical response to thermal stimuli, that is faster with respect to isolated dyes incorporated into a glassy amorphous polymer matrix, for example poly(methyl methacrylate). However, one of the critical points for the preparation of threshold temperature indicators based on luminescent sensors is the optical response stability at room temperature. For this purpose, the emission behaviour of PBSBBS quenched films (PBSBBS-q) was investigated in detail by emission spectroscopy monitoring of the IE/IM ratio with time at 25 uC (Fig. 5). The luminescence spectra reported in Fig. 5 of PBSBBS0.05q films analyzed up to 1 month after preparation display the typical emission feature of BBS molecules embedded into PBS in a fashion of non-interacting chromophores and indicate the long-time stability of their phase dispersion as well. Only 2 months after preparation, the film showed the progressive emergence of the excimer band at 500 nm (IE/IM ratio higher than 0.5–0.6) caused by the thermodynamically favoured aggregation process among the p-conjugated BBS molecules. In contrast, PBSBBS-0.1q film showed IE/IM ratio higher than 0.7 just 1 h after the preparation, indicating the promoted aggregation between the isolated BBS molecules with increasing concentration.50,51 The sensitivity of PBSBBS-0.05 quenched mixture to thermal stress was then investigated by placing the film specimen into a constant temperature oven at 50, 65 and 80 uC respectively, and monitoring the evolution of the IE/IM ratio as a function of the annealing time. It is shown that for quenched mixtures at 25 uC the ratio excimer to monomer emission is small because excimer formation is hampered by the high viscosity of the PBS polymer matrix. On the contrary, at very high temperature, excimer emission is also suppressed due to the high dissociation rate of the excimer.52,53 As a consequence, there is an intermediate temperature at which the excimer emission intensity relative to the excited isolated BBS molecules displays a maximum. In particular, this temperature depends both on the nature of the excimer system and on the polymer matrix characteristics, such as viscosity and activation to viscous flow.54 In Fig. 6(a) the evolution of the excimer band is reported for the PBSBBS-0.05q sample as a function of the annealing time at 65 uC as well as the progressive colour change of the film from the blue (typical of the excited isolated BBS chromophores) to the green emission (typical of the excimer-type BBS arrangements). As expected, the aggregation rate increased with temperature showing a very rapid process for samples annealed at T . 65 uC [Fig. 6(b)], the temperature at which the completion
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Fig. 6 (a) Fluorescence emission spectra (lexc. = 277 nm) of initially quenched PBSBBS-0.05 film and its colour evolution as a function of the annealing time at 65 uC (inset: hours); (b) excimer emission intensity relative to the excited isolated BBS molecules (IE/IM ratio) as a function of time at different annealing temperatures [50 uC (#), 65 uC (n) and 80 uC (%)] for a PBSBBS-0.05q film and mono-exponential growth fitting functions; (c) natural logarithm of the aggregation constant rates [t, obtained from the mono-exponential fits of data reported in (b)] against the inverse of temperature.

of the crystallization process of the PBS quenched matrix appears faster. This process involving the host PBS matrix is likely the most effective driving force of BBS excimer
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formation due to the progressive chromophore segregation in the amorphous phase of the host polymer.25 In order to gain more complete understanding of the investigated phenomenon, dielectric measurements will be performed in the future on PBSBBS quenched and annealed films.55,56 All the kinetic experiments were well-fitted by monoexponential growth functions expressed as IE/IM = A + B exp(2t/t) (1)

MIUR-FIRB 2003 D.D.2186 grant RBNE03R78E is kindly acknowledged.

References
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where A is the IE/IM ratio at equilibrium, t the annealing time, and B and t are constants related to the amplitude and rate of the aggregation process, respectively. As extensively reported in literature, the rates of most reactions increase as the temperature is raised with rate constants that follow the Arrhenius equation.57 Analogously, by plotting ln(t) against the inverse of the annealing temperature a linear Arrheniustype behaviour appears evident indicating that the aggregation process of BBS molecules might be predicted even in the solid state when dispersed into a polymer film and the derived thermodynamic parameters can be easily determined by means of luminescent experiments.

Conclusions
In summary, luminescent polymeric sensors on deformation and thermal stress were obtained by the modulation of the supramolecular architecture of BBS molecules dispersed into a thermoplastic biodegradable polyester matrix provided by the preparation procedure described. In particular, we have prepared PBS films with modulated emission properties by the very fine molecular dispersion of moderate (0.02–0.2 wt%) amounts of commercial BBS optical brighteners via melt-processing. When the concentration of BBS is increased to over 0.05 wt%, the luminescence of the film changes from blue to green. This suggests the formation of excimers, which is promoted by the p–p interaction between the conjugated planar structure of the dyes. The original blue colour of the film that is provided by the radiative transitions of the isolated BBS molecules, is restored after polymer stretching, which promotes the breakup of the aggregates responsible for the formation of excimers. Even more interestingly, the luminescence behaviour of PBSBBS quenched films can be thermally controlled with BBS aggregation tendency and emission colour changes proportional to increasing temperature (from 50 to 80 uC). In conclusion, the capability to easily modulate the luminescent properties of polyester films simply by adjusting the supramolecular architecture of BBS dispersed molecules promoted by thermal or mechanical stress suggests various applications in the field of smart and intelligent films based on thermoplastic materials, which appear suitable not only for polyolefins but also for other linear polymers including aliphatic polyesters.

Acknowledgements
The authors wish to thank Prof. Francesco Ciardelli (DCCI, Pisa) for very helpful discussion. Financial support by
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