Static and Dynamic Fluorescence of Pyrene as Probes of Site Polarity

Georgetown UniVersity, Washington, D.C. 20057-1227. ReceiVed: September 1, 1999; In Final Form: April 5, 2000. Static and dynamic fluorescence of pyre...
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J. Phys. Chem. B 2000, 104, 5905-5914

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Static and Dynamic Fluorescence of Pyrene as Probes of Site Polarity and Morphology in Ethylene-co-(Vinyl Acetate) (Eva) Films E. A. Prado,† S. B. Yamaki,† T. D. Z. Atvars,*,† O. E. Zimerman,*,‡,§ and R. G. Weiss‡ Departamento de Fı´sico-Quı´mica, Instituto de Quı´mica, UniVersidade Estadual de Campinas, Caixa Postal 6154, 13083-970, Campinas, SP, Brazil, and Department of Chemistry, Georgetown UniVersity, Washington, D.C. 20057-1227 ReceiVed: September 1, 1999; In Final Form: April 5, 2000

Static and dynamic fluorescence of pyrene has been used to study the polarity of microenvironments of ethyleneco-(vinyl acetate) copolymers of varying compositions. The polarity of these copolymers, defined by the Py-scale, has been compared with low-density polyethylene and poly(vinyl acetate). Linear correlations between vinyl acetate content and either the vibronic ratio, I/III, of the pyrene fluorescence spectra or the fluorescence decay constants, τF, have been obtained. We have also studied the photophysical properties of pyrene sorbed on the polymer surface by vapor deposition or incorporated within the polymer by swelling with a solvent. The photophysical parameters determined by both experiments are virtually the same, demonstrating that pyrene senses the same type of environment throughout the polymer film. Furthermore, in each of the copolymers, the probes experience an average polarity rather than one from domains enriched in either ethylene or vinyl acetate.

Introduction Polyethylene (PE) is a partially crystalline polymer consisting of a crystalline phase (composed of chain-folded lamellae with thicknesses in the range of 100-200 Å and widths in the micron range),1,2 an interfacial region next to the lateral faces of the crystallites),3-6 and an amorphous region (where chain segments are considered to be randomly oriented). Poly(vinyl acetate) (PVAc) is an amorphous glassy polymer (Tg ∼30 °C). Therefore, at room temperature, the movements of the macromolecular segments are nearly frozen. Nevertheless, the existence of a free volume distribution in the amorphous polymer results in a micro-heterogeneous medium from the morphological point of view.7 Ethylene-co-(vinyl acetate) (EVA) is a family of random copolymers whose properties and applications strongly depend on the relative proportions of ethylene and vinyl acetate (VAc).8-11 They differ from the properties of nonpolar polyethylene and polar poly(vinyl acetate). Morphologically, the EVA copolymers are composed of hierarchical organized structures that include a crystalline phase (comprised of polymethylenic segments), an interfacial region (with both ethylenic and VAc segments that have been excluded from the crystalline domains), and a chemically and morphologically heterogeneous amorphous phase (containing VAc and noncrystallized polymethylenic segments). The relative composition of the latter depends on the VAc content in the copolymer. The EVA melting temperature decreases with increasing VAc content, and, at g40% VAc, the material is completely amorphous. Increasing the fraction of VAc segments decreases the size of the crystalline domains and increases their size distribution. As the ethylene * To whom correspondence should be addressed. † Universidade Estadual de Campinas. ‡ Georgetown University. § Present address: Edge BioSystems, 19208 Orbit Drive, Gaithersburg, Maryland 20879, U.S.A. E-mail: [email protected].

content is increased, the glass transition temperature decreases and the material becomes more rubbery at room temperature. Besides these morphological modifications, surface properties are also changed (N. B., a significant improvement of adhesion) by introduction of polar VAc groups in the polymer chains.8 Static and dynamic fluorescence spectroscopies of molecules dissolved in or linked to polymer chains have been used to obtain information about the microstructure of polymeric materials.7,12-14 When the fluorescence spectrum and/or dynamics of decay of the probe excited states are sensitive to the polarity of their local environment, they can provide useful information about the polarity of occupied sites in homogeneous or micro-heterogeneous systems (such as micelles,15 LangmuirBlodgett monolayers,16 polymers17 and inorganic solids).18,19 In micro-heterogeneous media, as EVA copolymers may well be, fluorescence decay profiles are especially valuable to discern the type and number of environments in which the probe molecules reside. In this work, we investigate the micropolarities of EVA copolymers of varied compositions using pyrene as a fluorescence probe because its static and dynamic emissions are strongly dependent on the polarity of the local medium in which it resides.18-22 The Py-scale values20,21 of these copolymers are compared with those of a nonpolar, low-density polyethylene and a polar PVAc homopolymer. Pyrene has been applied to polymer film surfaces by vapor deposition and in the film bulk by swelling the polymer matrix with a solution and then removing the solvent. The probes allow surface and bulk polarities to be compared using data from static excitation and fluorescence spectra and dynamic fluorescence decays. Experimental Section Materials and Analyses. Low-density polyethylene (LDPE; blown type NA-203, additive free, from Poliolefinas, Brazil)23 was obtained in film form. Pyrene (Py) (Aldrich, 99%) was

10.1021/jp9931455 CCC: $19.00 © 2000 American Chemical Society Published on Web 05/31/2000

5906 J. Phys. Chem. B, Vol. 104, No. 25, 2000 recrystallized three times from 96% ethanol, chromatographed on an alumina column using benzene as eluent, and sublimed under vacuum to yield mp 148-149 °C (lit. mp 149-151 °C;24 > 99.5% pure by HPLC analyses). Methanol (Mallinckrodt UltimAR 99.9%) and n-heptane (Fisher, spectrophotometric grade) were used as received. Molecular weights and molecular weight distributions of the PVAc homopolymer and EVA copolymers (Aldrich) were determined by gel permeation chromatography using a Waters chromatograph, THF as solvent, and polystyrene standards. A system of three coupled columns Ultrastyragel, with particle size of 7 µm, dimension 7.8 × 300 mm, for size separation range ca. 2000 to 4 × 106 g mol-1 was employed. The polymer films were prepared by pressing melts between two Teflon plates (to avoid adhesion on the substrate surfaces) using a Carver model 2925 press and a force of 4 tons for 2 min. The molding temperatures were 50 °C for PVAc and 110 °C for all EVA samples. The thicknesses of the films were 70-80 µm. Before being used, the films were immersed overnight in n-heptane to remove antioxidants, washed with methanol, and dried in the air. The vinyl acetate contents in copolymers were determined under argon by thermogravimetric analysis (TGA) using a DuPont 2000 general model V4.1c calorimeter at a heating rate of 10 °C/min. Heats and temperatures of transitions were measured on a DuPont 2910 differential scanning calorimeter (DSC) controlled by a TA module 2000 and data analysis system. The DSC thermograms for PVAc and LDPE homopolymers and EVA copolymers were heated from room temperature to 150 °C, cooled to -100 °C, and heated to 150 °C; heating and cooling rates were 20 °C/min. Surface Deposition of Pyrene. A film maintained at room temperature was placed on top of a flask containing pyrene crystals that was heated at 40 °C for ca. 5 min. Pyrene “area” concentrations on film surfaces were too low to be measured reasonably by absorption spectroscopy. The bandwidths in the excitation spectra were similar to those of the bulk-doped samples, and no excimer emission could be detected in the emission spectra. On these bases, the area concentrations for the film surface and a slice of the bulk are similar (vide infra; ∼10-5 M Py25). Bulk Deposition of Pyrene. Film strips (ca. 2.5 × 0.5 cm) were immersed overnight in (1-3) × 10-5 or ca. 10-4 M pyrene in n-heptane solutions. The films were rinsed thoroughly with methanol to remove pyrene on the surfaces and then were dried at room temperature. The doped films were flame-sealed in VitroCom flattened glass capillaries (8 mm (id) × 4 mm (id) × 40 mm, VitroCom) after being degassed at < 10-5 Torr to remove air and residual heptane. From Beers law and assuming 336 ) 55 550 M-1 cm-1,26 the Py concentrations were calculated to be about the same as the original doping solutions.25 Static and Dynamic Emission Measurements. Both surface and bulk doped films were analyzed almost immediately. No more than 3 h or 1 day after film doping was required to complete steady-state or dynamic fluorescence measurements, respectively. All data were collected at ambient temperatures unless indicated otherwise. UV/vis absorption spectra were recorded on a Perkin-Elmer Lambda-6 spectrophotometer. An undoped clean film in a Vitrocom capillary was used as reference. Steady-state excitation and emission spectra were obtained on a Spex Fluorolog 111 spectrofluorimeter (linked to a personal computer) with a 150 W high-pressure xenon lamp using 0.50 mm and 0.25 mm slits in the excitation and emission monochromators, respectively, for emission spectra and inverted

Prado et al. configuration for excitation spectra. Two different sample orientations were used for samples containing pyrene on a surface. Back-face excitation was performed by placing the Pydoped face away from the incident radiation beam; emission was collected at 90° with respect to the excitation. Front-face excitation was performed by placing the surface with pyrene nearer the excitation beam and collecting the emission at an angle of ca. 35° to it. Samples were excited at 330, 333, 336, and 339 nm (spanning the 1La pyrene absorption band),33 and emissions were collected from 350 to 600 nm to span the monomer and (expected) excimer emission wavelengths. Fluorescence rise and decay histograms were obtained with an Edinburgh Analytical Instruments model FL900 single photon counting system using H2 as the lamp gas. The excitation wavelength was 337 nm and the emission wavelengths for collection of counts were 374, 379, 384, 389, and 394 nm (corresponding to the pyrene fluorescence peak maximum). Samples in VitroCom capillaries were aligned at 45° to the incident radiation, and emission was detected at a right angle from the back face of the film. An “instrument response function” was determined using Ludox as scatterer. Typically, the half-width duration of the pulses was ca. 2.5 ns. In all cases, at least 104 counts were collected in the peak channel. Data were collected in 1023 channels (1.009 ns/channel) and analyses were based on 1003 channels. Deconvolution was performed by nonlinear least-squares routines that minimize χ2 using software supplied by Edinburgh. Fits were deemed acceptable when χ2 values were 20 Å deep) covered by a thin VAc-enriched surface layer (ca. 20 Å deep).11a The apparent inconsistency between the higher surface energy of PVAc (37 mN/m2 versus 25.5-36 mN/m2 for polyethylene)8 and the VAc-enriched surface may be explained by the rejection of VAc units from PE crystallites in semicrystalline EVAs. As a consequence, amorphous PE segments near a surface are constrained and immobilized by the PE crystallites flanking them. This explanation is consistent with results from X-ray photoelectron spectroscopy that amorphous EVAs (50-70% VAc) have an excess of ethylene at the surface.11b Heats of melting and melting temperatures (Figure 1b) were determined from DSC thermograms, and the percentages of crystallinity, K (Table 1), were calculated from the melting enthalpy of crystalline polyethylene (286 kJ g-1).27 A simultaneous decrease of both the temperature and enthalpy associated with melting the ethylenic chains is observed as the VAc content increases. At 40% of VAc, the polymer is almost completely

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Figure 1. (a) Thermal degradation (TGA) curves for EVA samples. Heating rate 10 °C/min. (b) DSC heating thermograms (20 °C/min) of LDPE, PVAc, and EVA-X copolymers (second heating scans).

TABLE 1: Physical Properties of LDPE and PVAc Homopolymers and EVA Copolymers sample VAca

PVAc 100% 97% 30

Xb Tg (°C) Tm (°C) Mn 19,650 Mn/Mw 3.3 Kdsc (%)c a

EVA- 40 EVA-25 EVA-18 EVA-9 40% 40% -17 22,510 2.9

25% 28% -16 70 18,301 2.7 5

18% 20% -14 84 8,918 1.4 17

9% 10% -12 104 25

LDPE

110 49,000 10.4 41

b

Nominal content. Percent of VAc from TGA measurements. See text. c Degree of crystallinity from DSC thermograms.

amorphous and rubbery. Additionally, the width of the melting transition peak increases with the increase of VAc due to the presence of smaller sized polyethylene crystallites with a larger size distribution. The presence of large amounts of random VAc segments in the polymer chains makes crystallization more difficult. The shapes of the thermograms in the glass transition region are very complex for all of the copolymers, and the Tg values could not be determined precisely. For our PVAc, Tg ) 30 °C. Location of Py Molecules after Vapor Deposition. Arguments based on theory place pyrene molecules from vapor deposition deeper than the confines of the surface layers (i.e., g40 Å from the surface).11 Taking the value of the diffusion coefficient D for Py in PE and PVAc as 1.6 × 10-13 cm2 sec-1 (the value in poly(isobutyl methacrylate) just above its Tg at 64 °C),28 the distance traveled by a pyrene molecule in 1 h (the average interval between sample preparation and the beginning

Figure 2. Fluorescence intensity of pyrene in LDPE versus temperature: (a) sorbed on surface (9) and in the bulk (b) immediately after preparation; (b) sorbed on the surface immediately after preparation (9) and after 24 h (2). λex ) 337 nm.

of measurements) is ∼0.3 µm. According to this criterion, even if D were lowered by a factor of 10, pyrene should act as though it is in the bulk, at least for dynamic measurements! However, it is known that even in homopolymers such as polyethylene, chains near the surface can be oriented and organized very differently than those in the bulk.29 As a result, calculations such as those above may not be valid. Despite these calculations, there is significant experimental evidence that Py molecules remain on the surfaces in an operational sense during the duration of the preparation/ measurement periods. Washing a Py-deposited PE surface with methanol 3 h after vapor deposition resulted in almost complete loss of fluorescence intensity; when the period between deposition and washing with methanol was 24 h, >90% of the initial fluorescence intensity was lost. Thus, both the steady-state and time-resolved measurements on samples with surface-deposited pyrene report the behavior of probes at the surface (at least as defined empirically by the penetration depth of the nonswelling solvent30 methanol). Furthermore, the decrease in fluorescence intensity as temperature is increased is more acute for pyrene sorbed on the surface than doped in the bulk (Figure 2a). The intensity profiles follow the same trend whether from samples freshly prepared or aged at room temperature for 24 h (Figure 2b).31 Also, there is no evidence for diffusion from surface to interior (bulk) sites induced by electronic excitation of the dopant molecules. Fukumura et al.,32 using intense laser pulses

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Figure 3. Excitation (a,c) and fluorescence (b,d) spectra of pyrene in EVA-9: at (1-3) × 10-5 M Py (a,b) on the surface (- - -) and in the bulk (___); in the bulk (c,d) at (1-3) × 10-5 M (__) and 10-4 M (- - -).

that created high local concentrations of excited-state molecules on a polymer surface, did observe light-induced probe molecule migration into the bulk. Our time-resolved and steady-state fluorescence measurements were performed under low fluxes of radiation. Steady-State Excitation and Emission Spectra. The absorption (not shown) and excitation (Figures 3 and 4) spectra of surface and bulk pyrene in EVA films were recorded. Both the excitation and absorption spectra are composed of three vibronic bands centered at 308, 320, and 336 nm, associated with the 1A - 1L electronic transition.33 Excitation spectra of pyrene a in the bulk or on the surface of homopolymers or copolymers are similar and are independent of emission wavelength. For reasons that will be discussed later, the half-widths of the pyrene excitation spectra in the more dilute samples (Figure 4) increase slightly as X increases. Excitation spectra of the more dilute (10-5 M) or more concentrated (ca. 10-4 M) pyrene (bulk) in EVA-9, recorded under the same instrumental conditions, showed remarkable differences (Figure 3c). The more concentrated sample displays broader excitation and absorption spectra, including an increased red-edge of the vibrational bands. Both effects can be attributed to the presence of ground-state associated species or a wider distribution of occupied sites. Since diffusion in polymer films is slow with respect to the rate constants for decay of excited singlet states of pyrene,12 any excimeric emission must arise from ground-state associated species, also. However, no excimeric emission characteristic of sandwich-like dimers33 is observed in any of our films at the Py concentrations employed. Since pyrene molecules in constraining media such as our polymers may also form ground-state dimers with very different geometries,16-19,34-38 the absence of excimeric emission is not

sufficient to conclude that ground-state dimers are not present. However, the dynamic fluorescence decay data that will be presented later does effectively eliminate the presence of groundstate dimers. Fluorescence spectra of pyrene are composed of several vibronic bands associated with the 1Lb f 1A transition.33 In the usual notation, the 372-374 nm band is called peak I and the others are numbered from II to V in order of decreasing energy.20 Pyrene fluorescence spectra (λex ) 336 nm) in several of our polymers (Figure 5) show an additional low-intensity and higher energy peak at ca. 366 nm, which has been assigned as the 0-0 band of the electronic transition from dichroic absorption and fluorescence spectra;39 additionally, it exhibits an interesting temperature dependence.14 The shapes of the fluorescence bands of pyrene on a polymer surface or in its bulk (Figure 5) are very similar for all of our polymer matrices when the pyrene dopant concentration is ca. (1-3) × 10-5 M. The broad humps in the fluorescence spectra at 450-470 nm are due to an experimental artifact. A small, ca. 2 nm, red shift is observed in the fluorescence spectra of pyrene in PVAc (λI ) 374 nm) compared to LDPE (λI ) 372 nm) for both surface and bulk environments. A small dependence of the fluorescence wavelengths on solvent polarity is expected for pyrene.20,21 In addition, the steady-state fluorescence spectra of Py in the more dilute samples are almost independent of the excitation wavelength. The dependence of the excitation and emission spectra on pyrene concentration is particularly interesting. The excitation spectra are broader at the higher concentration (Figure 3c). To visualize this comparison, the excitation and fluorescence spectra of 3 × 10-5 M and 10-4 M pyrene in (bulk) EVA-9 have been normalized with respect to intensity in Figures 3c and 3d. Similar

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Figure 4. Corrected and normalized excitation spectra of pyrene on the polymer surface (- - -) and in the bulk at (1-3) × 10-5 M (___) of the polymer films. λem ) 373 nm.

behavior was observed for the other homo- and copolymers.13,14 No structureless red-shifted excimer-type emission was observed from the 10-4 M (bulk) pyrene concentration in the EVA copolymers. Excimer emission of pyrene in low viscosity isotropic solutions can be detected easily at this concentration.33 The difference between the photophysical properties of pyrene in the constraining and nonconstraining media suggests that the threshold concentration to obtain excimer emission in the polymeric media is higher than those used in the present work. In addition, we assume that the relative decrease in the intensity of vibrational band I in the more concentrated samples is due to radiative energy transfer.13,33,40 Another important characteristic of pyrene fluorescence is the intensity dependence of some vibrational bands on solvent polarity.20,21 This dependence has been attributed to different

contributions of Herzberg-Teller and Born-Oppenheimer interactions that depend on coupling of the vibrational normal mode to the electronic transition.22 The I/III vibrational band intensity ratios have been measured for pyrene sorbed on polymer surfaces and in the polymer bulk ((1-3) × 10-5 M) using λex ) 336 nm (Table 2, Figure 5). These ratios are relatively independent of excitation wavelength, and increase with the VAc content in the copolymer (from 0.61 for LDPE, a nonpolar medium, to 1.46 for PVAc). They are virtually the same at the surface and in the bulk of a film. They are also comparable to the values reported for n-alkanes (0.60) and ethyl acetate (1.37).20,21 On the basis of these results, the polarity of the polymers increases with >X, as expected. Several conclusions emerge from the steady-state emission measurements. The polarities sensed by pyrene at the surface

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Figure 5. Normalized fluorescence spectra of pyrene sorbed on the surface (- - -) and in the bulk (___) of various polymers at (1-3) × 10-5 M. λex ) 336 nm.

TABLE 2: Decay Constants τF (ns) and Vibrational Intensity Ratios I/III for Fluorescence of Pyrene Sorbed on the Surface (S) and in the Bulk (B) of Polymer Films. λEm ) 393 Nm, λEx ) 336 nm LDPE I/III τF (ns)

EVA-9

EVA-18

EVA-25

EVA-40

PVAc

S

B

S

B

S

B

S

B

S

B

S

B

0.61 395 ( 1

0.60 392 ( 3

0.83 373 ( 3

0.83 370 ( 2

0.96 359 ( 1

0.91 358(2

0.98 346(2

1.02 347(2

1.04 352(1

1.21 348 ( 1

1.49 311 ( 3

1.46 299(1

and in the bulk of the materials appear to be virtually the same. The half-widths of the pyrene excitation spectra increase with X content, suggesting a correlation between polymer structure and the distribution of sites where pyrene molecules are located. As noted previously,41-45 chromophores dissolved in solid matrices, in contrast to the liquid phase, exist in different types of environments, which produce an inhomogeneous broadening

of the electronic spectra. The chromophores suffer different types of interactions with their local environments, and the measured spectra are a population-weighted sum of the excitation characteristics of individual chromophores in each site type. The absorption of each molecule/polymer site is associated with one potential energy surface. Since the polymer matrices are micro-heterogeneous materials, pyrene molecules in different

Pyrene Fluorescence as Polarity Probe sites are being submitted to different energy potential surfaces, and, consequently, they will decay from and to slightly different states after excitation. Therefore, changes of the spectral halfwidths in the more dilute samples must be associated with different polymer sites producing broader inhomogeneous spectra. The spectral broadening may be attributed to several factors, including different distributions of occupied sites when pyrene concentrations are different, a variety of Py/polymer van der Waals interactions, different correlations between the time constants for solvent relaxation (τR) and Py excited-state lifetimes (τF), and the dimensions of the site where a Py molecule is located and the stiffness of its “walls.”46 Some of the variables mentioned above are not important in our experiments. For instance, at ca. 10-5 M pyrene, the probability of intermolecular energy transfer by radiative or nonradiative pathways33 is minimal. Also, since our polymeric films are additive-free, extrinsic plasticizer effects other than that produced by the presence of pyrene, are not important. Furthermore, each sample was placed under vacuum to remove molecular oxygen and residual heptane (i.e., a potential plasticizer) that was used in the bulk doping process. Thus, the most important factor for inhomogeneous broadening should be the relative values of τF and τR. In general, there are two distinct types of inhomogeneous broadening; in some cases, they may be operative simultaneously. In a very rigid medium, where τF , τR, the mechanism is “static”. In a more mobile medium where τF may be .τR, the mechanism may be “dynamic.” The two extremes can be analyzed using different strategies.41-45 If the molecular probe is a fluorophore, as in the case of pyrene, both “static” and “dynamic” processes can be studied using steady-state and time-resolved spectroscopic techniques. The lifetime of the electronically excited state can be determined by time-resolved spectroscopy and then correlated with the relaxation time(s) of the matrix. The “red-edge effect”41-45 present in the excitation spectra is consistent with a static mechanism operative either in frozen solutions or in polymer matrices at temperature below the glassy transition. In principle, irradiation at the maximum of peak absorption (where molecules in a broad distribution of sites are excited) should lead to broader emission spectra than when irradiation is at the red edge (where molecules in fewer site types are excited). The latter excitation should result in both a red-shifted and a narrower emission spectrum.43 Since the dynamic mechanism implies that the natures of the sites experienced by individual molecules during their excited-state lifetimes will change, its effect, a dependence of the shape of the emission spectrum on time during periods near τR, should be detectable only in time-resolved emission spectra.45 Steadystate spectroscopic techniques were unable provide any evidence for inhomogeneous broadening since red-edge excitation did not result in a corresponding shift in the fluorescence spectra. However, there is some evidence for inhomogeneous broadening of the pyrene spectra (produced by overlapping vibrational bands from molecules in slightly different sites). For example, vibrational band III is very well defined in LDPE and in EVA copolymers, but not in PVAc. Since PVAc is an amorphous polymer with Tg ) 30 °C, the mobility of the polymer chains should be very restricted and the relaxation time of the medium should be very slow (i.e., the static mechanism for the inhomogeneous broadening should be important). However, the proximity to Tg at which the experiments were conducted and the probable local plasticizing effects of pyrene molecules on their solubilization sites46a,47 may provide sufficient chain

J. Phys. Chem. B, Vol. 104, No. 25, 2000 5911 mobility on a molecular scale to allow the dynamic mechanism to contribute. Although pyrene excitation spectra are broadener in the EVA copolymers than in LDPE, no red-edge effects are apparent in the fluorescence spectra. On this basis, dynamic processes may dominate in all of the polymers whether Py is located at surface or bulk sites: τF . τR. Several studies have shown that probe molecules in a polymer matrix are distributed between different site types that can be differentiated on the basis of free-volume, shape, polarity, etc. In partially crystalline materials, the distributions involve division of probe molecules between the amorphous and interfacial (i.e., near the outside of crystal surfaces) regions.12,13,40,48 Relaxation times for EVA copolymers have not been reported. Nevertheless, it is reasonable to assume that their relaxation processes are more rapid than the analogous ones in LDPE. VAc groups produce, simultaneously, a relative increase of the amorphous proportion of the material and a reduction of the melting temperature of the crystalline ethylenic blocks. Since the EVA copolymers become more rubbery as the VAc content is increased, these synergistic effects appear to increase chain mobility. From 13C high-resolution, solid-state NMR studies of LDPE doped with (∼1% w/w) pyrene, the spin-lattice relaxation times of carbon nuclei within the crystalline and amorphous phases are estimated to be 190 and 0.5 s, respectively.17 The relaxation time in the amorphous phase of ethylenepropylene rubber is reduced further, to 0.12-0.2s, indicating that the amorphous region of LDPE is somewhat less mobile than that of the rubber.17 Similarly, the rotational correlation time of pyrene molecules in the amorphous part of polyethylene (i.e., the correlation time of C-H dipolar interactions) is < 50 ns.17 From these results, we assume that the rotational correlation time of pyrene in EVA is also < 50 ns. If the fluorescence lifetime is much longer than this value, the dynamic mechanism should dominate. Dynamic Fluorescence Measurements. Dynamic fluorescence rise and decay curves for pyrene on the polymer surfaces and in the polymer bulk were recorded and analyzed. Excluding a very fast decay component from reflected light and weak emission from the glass capillaries, temporal decay curves for pyrene in all samples could be fitted satisfactorily to singleexponential functions. Table 2 includes the decay constants, τF, at λem ) 393 nm. The shorter decay components from the glass represent < 8% of the total and are not reported. The decay constant for pyrene in LDPE (bulk) is in the range ca. 394-397 ns for several emission wavelengths and is consistent with the values reported by Mayer et al. (410 ( 18 ns) for pyrene in LDPE and in 3-methylpentane (375 ns).49 For pyrene in PVAc, a significant reduction of τF (≈300 ns) is found. The decay constants in the EVA copolymers are intermediate between those in LDPE and in PVAc and increase with decreasing X. Similar values were obtained for pyrene on the surface or within the bulk of one polymer type. These results are consistent with the known dependence of the excited singletstate lifetime of pyrene on the polarity of liquid media. They also follow the changes in the I/III vibronic ratios from static pyrene fluorescence measurements (Figure 7, Table 2).50 The decay constants for pyrene in the more dilute and more concentrated samples are virtually indistinguishable. For instance, the decay constants are 370 ( 2 (dilute) and 373 ( 3 ns (concentrated) in EVA-9 and 357 ( 3 (dilute) and 348 ( 1 ns (concentrated) in EVA-40. This is not surprising since the rate of diffusional self-quenching is lowered enormously by the viscous media. The absence of a shorter decay component

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Figure 6. Normalized fluorescence spectra of (1-3) × 10-5 M pyrene in the bulk at λex 330 (___), 333 (- - -), 336 (- - -), and 339 nm (...).

characteristic of excimer decay suggests that ground-state dimers of pyrene, if present, are in very low concentrations.12 Concluding Remarks For aromatic guest molecules to display very well resolved fluorescence spectra in (low temperature) Shipolskii matrices33 there must be a good correspondence between the size and shape of the solvent cage and the molecular dimensions of the guest. Aromatic hydrocarbons doped into polymers, even at very low temperatures, do not provide high-resolution spectra due to the large distribution of sizes and shapes of the guest sites. Nevertheless, the appearance of the fluorescence spectra in these media can be narrowed or broadened depending on structural factors. For instance, the low-temperature fluorescence spectrum of anthracene has better resolved vibronic structure in polyethylene than in polystyrene, poly(vinyl chloride), or poly(vinyl acetate).51 Moreover, the spectrum in high-density polyethylene

(with a higher content of linear segments) is narrower than in LDPE (with a higher content of branched segments).51 Evidence for strong coupling between some pyrene vibronic modes and the LDPE matrix has also been found. It is based upon changes in the dynamic deformation of the polymer when pyrene is added as a filler.52 This unusual behavior has been attributed to the presence of pyrene at sites in the amorphous and interfacial regions of the polymer matrix.52 If, as expected, vibronic coupling creates new relaxation pathways for electronically excited pyrene molecules, emission should be from a narrow distribution of potential energy surfaces. Consequently, the fluorescence spectrum of pyrene in LDPE should be narrower than that in the EVA copolymers or in PVAc. However, the narrowing may be small because of the motional relaxation of pyrene molecules in the polymer sites being more rapid than the rate at which their excited singlet states emit; the fluorescence decay constants are larger in PE than in EVA.

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Figure 7. Relationship between X and N (VAc) (N ) (100 - X)xMn/MW(VAc)) and vibronic ratios I/III and fluorescence decay constants for (1) LDPE, (2) EVA-9, (3) EVA-18, (4) EVA-25, (5) EVA-40 and (6) PVAc for surface (9) and bulk (b) doped films.

Since fluorescence spectra and decays attributed to isolated molecules and ground-state dimers are very similar, the Py must be separated from each other in the amorphous and interfacial regions of the polymers by g10 Å (i.e., the minimum distance between two pyrene moieties to avoid significant energy transfer).33 Some sites of LDPE are known to be able to accept two pyrene molecules (by swelling the polymer and employing very large concentrations of pyrene in the swelling liquid).53 Such sites, if present in our work, must be “stressed” since the free volume of almost all of the sites in native (undoped) LDPE films is less than the 322 Å3 (ref 54) necessary to accommodate even one pyrene molecule. The absence of the structureless, red-shifted emission typical of sandwich-type excimers and a fluorescence decay component characteristic of their presence33 argue against a mechanistically significant population of groundstate dimers. The morphologies of the EVA copolymers and their dependence on VAc content are very complex. As a consequence, pyrene molecules should be distributed in different types of sites even within the families defined by the amorphous and interfacial regions of one film. Although the polarity changes within the series of EVA are discernible, the multiplicity of sites within one EVA is not! Thus, the I/III vibronic ratio from pyrene fluorescence increases regularly from polyethylene to the copolymers with increasing VAc content to PVAc. However, the decay constants for pyrene fluorescence, which should be a more stochastic measure of the different site types, are monoexponential in each of the polymer matrixes! A possible explanation for the lack of more than one decay constant in each film is that the mobility of the pyrene molecules is sufficient to lose any site anisotropy within their excited-state

lifetimes: τF . τR. Similar monoexponential decays have been observed for the fluorescence of 1-ethylpyrene in LDPE and high-density polyethylene, media where site shape anisotropy should be more prevalent and τR should be longer than in the EVA copolymers or PVAc. Both the decay constants and the I/III ratios change in a regular fashion as the (more polar) VAc content increases in the polymer films. Additionally, both the static and dynamic fluorescence measurements indicate that the polarity experienced by pyrene molecules at the surface or in the polymer bulk is the same. The unexpectedly “simple” steady-state and decay characteristics of pyrene fluorescence in this series of polymer films provide a clear (but probe-specific) picture of their microdomains. In some ways, they must be anisotropic and heterogeneous; on the time scale sampled by the excited singlet state of pyrene and within the sensitivity range of its steady-state emissions, the microdomains are isotropic and homogeneous. The results suggest that site anisotropy should be detected more readily with a probe whose excited singlet state lifetime is much shorter than that of pyrene. What the “world” looks like depends on the idiosyncrasies of the observer! Acknowledgment. T.D.Z.A. thanks FAPESP and R.G.W. thanks the U.S. National Science Foundation and the Petroleum Research Fund (administered by the American Chemical Society) for their financial support of this research. References and Notes (1) Geil, P. H. Polymer Single Crystals, Wiley-Interscience: New York, 1963.

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