ARTICLE pubs.acs.org/Macromolecules
Fluorescence Behavior of Dyes in Thin Films of Various Polymers Atsuomi Shundo,† Yohei Okada,† Fuyuki Ito,‡ and Keiji Tanaka*,† † ‡
Department of Applied Chemistry, Kyushu University, Fukuoka 819-0395, Japan Department of Chemistry, Shinshu University, Nagano 380-8544, Japan ABSTRACT: We have studied the fluorescence behavior of a dye, 6-(N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)hexanoic acid (NBD), in thin films of polymers with various polarities, such as poly(methyl methacrylate) (PMMA), Arton, poly(styrene) (PS), hydrogenated polystyrene (H-PS), and Zeonex. In the case of well-dispersed systems, the fluorescence behavior of NBD could be explained in terms of the mobility of the polymer matrix. This was the case for PMMA, Arton, and PS. On the other hand, when H-PS or Zeonex was used, NBD aggregated in the films, leading to unusual fluorescence behavior: the fluorescence lifetime and maximum wavelength increased with a decrease in the film thickness. Time- and space-resolved fluorescence spectroscopy using an evanescent wave excitation revealed that the aggregation states of NBD near the interface varied with the film thickness. While NBD molecules near the interface aggregated in a thick film, such was not the case in a thin film. Angular-dependent X-ray photoelectron spectroscopy (ADXPS) structurally confirmed the above observations; that is, the film thickness greatly influenced the depth profile of the NBD composition in the polymer films.
As mentioned above, fluorescence spectroscopy enables us to gain direct access to local dynamics in polymer films. However, fluorescence properties are dependent on the place where the dye molecules exist. For instance, in general, the fluorescence lifetime, which determines the intensity, at the interface is not the same as that in the bulk. Thus, as long as the spectroscopy is applied to determine the local Tg, the absolute value of the fluorescence lifetime, as well as the emission wavelength, is not so important. This is because the Tg can be defined as a temperature at which the decrement of fluorescence lifetime, or intensity, changes with increasing temperature. In this case, although the extent to which the fluorescence lifetime changes with temperature is important, the absolute lifetime of the fluorescence is not necessary. However, to apply fluorescence spectroscopy further in order to study the physical properties of polymers, especially in confined spaces, we require a better understanding of the factors that control the fluorescence behavior of dyes in a confined space. The knowledge can be also useful for the fabrication of the aforementioned functional devices. This study revealed the fluorescence behavior of dyes in a polymer film as the film thickness decreases. We propose a possible explanation for the peculiar thickness dependence seen in some polymers on the basis of space-resolved fluorescence spectroscopy using evanescent wave excitation, in conjunction with X-ray photoelectron spectroscopy to provide structural information.
1. INTRODUCTION The idea of incorporating dyes into a polymer matrix has attracted a great deal of interest as a simple fabrication scheme for functional devices. For example, thin polymer films containing fluorescence dyes have been extensively studied for sensors,13 image patterning,46 optical storage media,79 and solar cells.1012 In these applications, one of the crucial points is to control the dispersion state of dyes in the polymer films. This is simply because aggregation formation of the dyes often alters the original fluorescence properties, such as the emission band position and lifetime.1316 In general, the dispersion state of dye molecules in a polymer is closely related to their miscibility. Hence, an effect of polymer polarity on the fluorescence behavior of the dye, especially in thin films, should be studied as the first benchmark for developing the functional materials mentioned above. It has been widely accepted that the fluorescence properties of a dye that is molecularly dispersed in a polymer matrix are influenced by the mobility of the surrounding polymer chains.1719 This feature makes it possible to determine the glass transition temperature (Tg) of the matrix polymer by measuring the temperature-induced change in the fluorescence emission.2025 Recently, this notion was applied to study the spatial distribution of Tg in thin polymer films. Torkelson and co-workers presented clear evidence for a mobility gradient in thin polymer films, in which a dye labeled-layer was inserted at an arbitrary position.21,22,24 As a result, the segmental dynamics at the surface were much faster than that in the corresponding bulk. We also investigated the polymer dynamics at the interface with inorganic solids using evanescent wave excitation, showing that the interfacial segmental dynamics was much slower than that in the bulk.23,25 r 2011 American Chemical Society
Received: August 20, 2011 Revised: October 29, 2011 Published: December 05, 2011 329
dx.doi.org/10.1021/ma201901x | Macromolecules 2012, 45, 329–335
Macromolecules
ARTICLE
Table 1. Characteristics of Polymers Used in This Studya
a Chemical structures, number-average molecular weights (Mn), molecular weight distribution (Mw/Mn), glass-transition temperatures (Tg), dielectric constants at 1 MHz (ε), refractive indices at 430 nm (n), and critical angles against a SLAH-79 substrate with a refractive index of 2.05 (θc).
2. EXPERIMENTAL SECTION 2.1. Sample and Film Preparation. Poly(methyl methacrylate) (PMMA), Arton, polystyrene (PS), hydrogenated polystyrene (H-PS), and Zeonex were used as matrix polymers. PMMA and PS were purchased from Polymer Source Inc. and were used as received. Arton was kindly supplied from JSR Co. H-PS and Zeonex were also kindly supplied from Zeon Co. Arton, H-PS, and Zeonex were purified by reprecipitation. Table 1 summarizes the chemical structure, the numberaverage molecular weight (Mn), and molecular weight distribution (Mw/Mn), where Mw is the weight-average molecular weight, and the glass transition temperature (Tg), together with the dielectric constant (ε) at 1 MHz, the refractive index (n) at a wavelength of 430 nm, and the critical angle calculated against a glass substrate (SLAH-79) with a refractive index of 2.05. Mn and Mw/Mn were determined by gel permeation chromatography (Tosoh HLC-8220) with Shodex KF-804L and KF-805L columns against PS standards. The bulk Tg was measured by differential scanning calorimetry (Extra6000 DSC6220, SII Co., Ltd.). As a fluorescent probe, 6-(N-(7-nitrobenz-2-oxa-1,3-diazol-4yl)amino)hexanoic acid (NBD) was purchased from Sigma-Aldrich Co. and used as received. The chemical structure of NBD is shown in Figure 1a. Thin polymer films containing NBD were prepared on a SLAH-79 substrate by a spin-coating method. The molar ratio of NBD to a repeating unit of polymer was fixed to be ∼0.1%. All films were dried under vacuum at room temperature for 24 h. The thickness of the films was determined by ellipsometry (M-150, Jasco Co., Ltd.). 2.2. Characterizations. Ultraviolet (UV)visible (vis) and fluorescence spectra for a tetrahydrofuran (THF) solution of NBD were recorded by a U-4100 spectrophotometer (Hitachi High-Technologies Co.) and a F-4500 fluorescence spectrophotometer (Hitachi High-Technologies Co.), respectively. For the time-resolved fluorescence spectroscopy, NBD dyes were excited with the second-harmonic generation of a modelocked titanium:sapphire laser (Spectra Physics, Tsunami; full width at half-maximum (fwhm) = 1.5 ps; wavelength = 430 nm) equipped with a pulse selector and a harmonic generator. A streak scope (Hamamatsu Photonics, C4334-01) was used to detect the time-resolved fluorescence from excited NBD molecules. An excitation pulse was irradiated on the film from the substrate side via a prism at a certain angle. When the incident angle (θ) is larger than the critical angle (θc), the excitation pulse is totally reflected at the interface between the polymer and the substrate. In this case, an evanescent wave, where the electric field exponentially decays along the direction normal to the interface, is generated at the polymer interface. Information near the substrate interface was selectively extracted on the basis of this evanescent wave excitation. The θc values for the matrix polymers are summarized in Table 1. In the case of θ < θc, the excitation pulse went through the internal bulk phase of the film,
Figure 1. (a) Chemical structure of 6-(N-(7-nitrobenz-2-oxa-1,3-diazol-4yl)amino)hexanoic acid, NBD, (b) UVvis and fluorescence spectra for a THF solution of NBD with a concentration of 10 μM, and (c) fluorescence decay curve for the solution.
meaning that the data so obtained reflects bulk information. For the bulk measurements, a θ value of 45° was used for all polymers. The chemical composition at the film surface of the polymers containing NBD was examined by X-ray photoelectron spectroscopy (XPS, PHI 5800 ESCA system, Physical Electonics, Co., Ltd.) with a monochromatized Al Kα source operated at 14 kV and 24 mA. The C1s peak was calibrated to a binding energy of 285.0 eV for the neutral carbons to correct the charging energy shifts. The analytical depth (dXPS) of XPS from the outermost surface is defined by 3λe sin θe, where λe and θe are the inelastic mean-free path of photoelectrons in the solid and the emission angle of photoelectrons, respectively. The λe for C1s photoelectrons was taken to be 3.1 nm, calculated by Ashley’s equation.26 330
dx.doi.org/10.1021/ma201901x |Macromolecules 2012, 45, 329–335
Macromolecules
ARTICLE
Figure 2. NBD fraction dependence of the fluorescence lifetime and maximum wavelength for NBD dispersed in a PMMA film with a thickness of 2 μm. Solid lines are guides to the eye. The depth profile of NBD composition in the vicinity of the film surface was analyzed by angular-dependent XPS (ADXPS)27 over a range of θe from 15° to 90°. Also, the depth profile of NBD composition in the films near the substrate interface was also obtained. To do so, the films were floated off on a water surface and then picked up by attaching the substrate from the air side.28 Figure 3. Film thickness dependence of (a) fluorescence lifetime and (b) normalized lifetime for NBD dispersed into various polymers: PMMA, Arton, PS, H-PS, and Zeonex. The molar ratio of NBD to a monomer unit in polymer is 1.0 103. Solid lines are guides to the eye.
3. RESULTS AND DISCUSSION 3.1. Fundamental Fluorescence Behavior of NBD. Figure 1b shows the UVvis and fluorescence spectra for NBD in THF with a concentration of 10 μM. The maximum wavelengths in absorption (λmax) and fluorescence emission (λem) were 457 and 517 nm, respectively. Since the emission band slightly overlapped with the absorption at 457 nm, a wavelength of 430 nm was selected to excite NBD for the fluorescence study. Figure 1c shows the time (t) dependence of fluorescence intensity (I) for NBD excited at 430 nm. The decay curve was fitted by the following double-exponential equation with two time constants, τfast and τslow:
I ¼ I0 fð1 xÞ expðt=τfast Þ þ x expðt=τslow Þg
the τ value was longer in the order of PMMA, Arton, PS, H-PS, and Zeonex. This order agreed with that for the dielectric constant (ε) collected in Table 1, implying that the τ for NBD is a function of the polarity of the matrix polymer. This is because the dipole moment of NBD in the excited state is quite different from that in the ground state.30 When the film became thinner than ∼100 nm, the τ value was dependent on the thickness. To make a comparison among polymers easier, τ was normalized by the average value for the thick films, as shown in panel b of Figure 3. When PMMA was used as a matrix polymer, τ first decreased with decreasing thickness. However, in an ultrathin region thinner than 10 nm, the trend reversed and τ started to increase. Similar behavior could be seen for the Arton and PS films. In the case of H-PS, the trend was also similar to that for PMMA, Arton, and PS. However, the minimum lifetime was observed at a thickness of ca. 40 nm, which was thicker than for the others. The curve for H-PS was just like that for one of the others but shifted toward a thicker region. On the other hand, τ simply increased with decreasing thickness for the Zeonex film. To discuss the possibility of aggregation formation of NBD in the thinner films, fluorescence spectra were collected. Figure 4 shows the spectra for NBD in five different polymer films with thicknesses of approximately 200 and 10 nm. The fluorescence spectra for NBD in PMMA and Arton films were insensitive to the thickness. Also, the spectrum for NBD in a 10 nm thick PS film was slightly shifted to a higher wavelength side from the original, but the extent was actually trivial. On the other hand, a thinning-induced red shift of the spectrum was observed for the H-PS and Zeonex films. Thus, although it can be hardly claimed solely from Figure 4 that NBD was homogeneously distributed in thin PMMA and Arton films, it is apparent that the distribution of NBD was inhomogeneous, that is, aggregation of NBD occurred, in the H-PS and Zeonex films. Next, we revisit the thickness dependence of the fluorescence lifetime for NBD shown in Figure 3. The fluorescence lifetime of NBD decreased with decreasing film thickness except in Zeonex.
ð1Þ
where I0 and x are the fluorescence intensity right after the excitation and the fraction of the slow component, respectively. The overall lifetime of the emission (τ) was defined as τ ¼ ð1 xÞτfast þ xτslow
ð2Þ
The lifetime for NBD in THF was 10.6 ( 0.06 ns. In general, the fluorescence behavior of a dye in a solid is not the same as that in a solution because the surrounding environment of the dye differs in the two states. Thus, as a typical polymer solid, τ and λem for NBD dispersed in the PMMA film with a thickness of ∼2 μm were examined. Figure 2 shows the NBD concentration dependence of the τ and λem values. In the case of a NBD concentration lower than 0.1 mol % in the 2 μm thick film, both τ and λem values were almost constant, and no signature for the aggregation of NBD was observed. When the NBD concentration went beyond 0.1 mol %, the τ and λem became shorter and longer, respectively. This might be due to an energy transfer from the monomers to the aggregate sites or to the excimers.16,29 Thus, hereafter, the NBD concentration in the polymer films was fixed to be 0.1 mol % unless otherwise stated. 3.2. Film Thickness Dependence of Fluorescence Behavior. Figure 3a shows the thickness dependence of the fluorescence lifetime for NBD dispersed in films of PMMA, Arton, PS, H-PS, and Zeonex. The τ values of NBD in the polymer films were in the range from 3 to 9 ns, which were smaller than that obtained in the THF solution. In a thick region of a few hundreds of nanometers, 331
dx.doi.org/10.1021/ma201901x |Macromolecules 2012, 45, 329–335
Macromolecules
ARTICLE
Figure 4. Fluorescence spectra for NBD dispersed into the films of (a) PMMA, (b) Arton, (c) PS, (d) H-PS, and (e) Zeonex, with film thicknesses of ca. 200 and 10 nm. The molar ratio of NBD to a monomer unit in polymer is 1.0 103.
Here, z is the depth from the interface. The analytical depth (d) is defined as the position at which Iev becomes Iev,0/e, namely, the value of Iev corresponding to dp/2. Equations 3 and 4 indicate that the d value can be regulated by changing θ. For the interfacial measurements, the d value was kept to be 35 nm, which corresponded to θs of 60.7° for PMMA and 62.0° for Zeonex. The d value of 35 nm is larger than the thickness of the thin films studied here ( θc. Figures 5 and 6 show the fluorescence decay curves and spectra for NBD in the films of PMMA and Zeonex, which represent typical polar and nonpolar cases, respectively. The data for approximately 200 and 20 nm thick films are presented. To excite NBD dyes, transmitted pulses and evanescent waves were used to obtain the information in the bulk and at the interface, respectively. In the case of PMMA, the lifetime was longer at the interface than in the bulk. On the other hand, the λem for NBD at the interface was almost the same as that in the bulk, indicating that the NBD dyes were well-dispersed even in the interfacial region. So far, many researchers have reported that the segmental motion of PMMA at the interface with hydrophilic substrates was less active than that in the bulk.25,3638 If this is the case, since the fractional amount of nonradiative pathways to the ground state for excited NBD decreases at the interface thanks to the less active molecular motion, it is reasonable that the lifetime for NBD at the interface became longer, as shown in panels a and c of Figure 5. The fluorescence behavior of NBD in the Zeonex films definitely differed from those in PMMA. In the case of the thick film, the τ and λem at the interface were respectively shorter and longer than the bulk values. The red shift of λem in panel b of Figure 6 means that aggregate formation of NBD dyes was more striking in the interfacial region than in the bulk. Thus, it is
This might be rationalized by two factors. First, the local concentration of NBD starts to fluctuate in the films with decreasing thickness. This means that the concentration of NBD increases and decreases depending on the position. At a position where the local concentration is higher, the lifetime decreases, as seen in Figure 2, indicating that the energy transfers from the monomers to the aggregate sites. Second, the molecular motion of polymer chains alters the fluorescence lifetime of NBD. In general, the polymer mobility at the surface and substrate interfaces is respectively enhanced and depressed in comparison with that in the internal region of the film.3134 Hence, the fluorescence lifetime should be shorter at the surface region than in the bulk because the fractional amount of nonradiative pathways to the ground state increases due to the enhanced mobility of the matrix polymer. Since the ratio of the surface area to the total volume increases with decreasing film thickness, the surface effect becomes more striking with decreasing thickness. That is, the fluorescence lifetime decreases with decreasing thickness. However, an increase in the lifetime was observed in the ultrathin films as well as in Zeonex films. This cannot be simply explained in terms of these two factors. Such a discrepancy motivates us to examine the fluorescence dynamics of NBD at the substrate interface. 3.3. Fluorescence Behavior at the Substrate Interface. The fluorescence spectrum and lifetime of NBD at the interface with the solid substrate can be selectively obtained by evanescent wave excitation.23,25 To compare the interfacial fluorescence behavior with that in the internal bulk, the measurements were made at two different θs being smaller and larger than the θc. The penetration depth (dp) of the evanescent wave is given by35 dp ¼ λðsin2 θ sin2 θc Þ1=2 =2πn
ð3Þ
where λ is the wavelength of the excitation. In fact, the relation between the depth and the electric field intensity (Iev) is much more important for the interfacial selectivity than the dp value.35 Iev ¼ Iev , 0 expð2z=dp Þ
ð4Þ 332
dx.doi.org/10.1021/ma201901x |Macromolecules 2012, 45, 329–335
Macromolecules
ARTICLE
Figure 7. Sin θe dependence of the ratio of peak intensity due to N1s and C1s (IN/IC) in XPS spectra for the PMMA films with thicknesses of (a, b) 207 nm and (c, d) 10.5 nm. The solid circles are the experimental data. The insets show the model depth profiles of atomic ratio of nitrogen to carbon to obtain the best fit to the data. The molar ratio of NBD to a monomer unit in PMMA is 1.0 103.
Figure 5. Fluorescence decay curve and spectra for NBD dispersed in the PMMA films with a thickness of (a, b) 217 nm and (c, d) 20 nm. The data indicated by transmission and evanescent correspond to those obtained by excitation of NBD at θ < θc and θ > θc, respectively. The molar ratio of NBD to a monomer unit in PMMA is 1.0 103.
interfacial region due to a lack of NBD molecules. Hence, in the case of the thin film, the effect of the interfacial chain mobility was more dominant than the effect of aggregate formation on the lifetime of NBD. Interestingly, the λem value at the interface was shorter than that in the bulk for the thin film. This implies that in the case of the thin Zeonex film the extent of NBD aggregate formation was more marked in the bulk region than in the interfacial region. 3.4. Depth Profile of NBD in Films. We finally come to the distribution of NBD dyes in the films along the direction normal to the interface determined by XPS. Here, the films of PMMA and Zeonex were again examined as typical polar and nonpolar cases, respectively. To analyze the NBD concentration at the substrate interface, the films prepared on the substrates were floated off onto the water surface, and then they were attached to different substrates from the air side, resulting in an upside-down version of the films.28 Since nitrogen atoms exist only in NBD, the intensity ratio of XPS N1s to C1s peaks (IN/IC) reflects the NBD composition. Figure 7 shows the sin θe dependence of IN/IC for 207 and 10.5 nm thick PMMA films. As stated in the Experimental Section, sin θe corresponds to the analytical depth; a smaller θe corresponds to a shallower depth from the outermost region. While panels a and c show the data for the surface side, panels b and d show the data for the substrate interface side. The IN/IC slightly decreased with decreasing sin θe, meaning that the concentration of NBD dyes was a little lower in proximity to the surface. In general, a component with lower surface energy is enriched at the surface due to the requirement for the minimization of the total free energy of the system.3942 Hence, it is reasonable that the higher surface energy component, namely NBD, prefers to be outermost in the surface region.43 In the case of the interfacial region, on the contrary, the value of IN/IC increased with decreasing sin θe. Thus, it can be claimed that NBD dyes were enriched at the substrate interface because of an attractive interaction between NBD and the hydrophilic substrate.
Figure 6. Fluorescence decay curve and spectra for NBD dispersed in the Zeonex films with thicknesses of (a, b) 227 nm and (c, d) 18 nm. The data indicated by transmission and evanescent correspond to those obtained by excitation of NBD at θ < θc and θ > θc, respectively. The molar ratio of NBD to a monomer unit in Zeonex is 1.0 103.
conceivable that the effect of aggregation for NBD on its lifetime was dominant over the effect of the interfacial chain mobility. When the Zeonex film became thinner at ca. 20 nm, the trend reversed. That is, the interfacial τ was larger than the bulk value. Currently, the NBD concentration in the sample was fixed, meaning that the total number of NBD molecules was smaller in the thin film than in the thick film. Hence, it would be possible that the NBD concentration at the interface was smaller in the thin film than in the thick film, resulting in a lesser extent of aggregate formation at the interface of the thin film because a sufficient amount of NBD could not be partitioned to the 333
dx.doi.org/10.1021/ma201901x |Macromolecules 2012, 45, 329–335
Macromolecules
ARTICLE
In XPS, the photoelectron intensity for the j-core level at θe is expressed as27 Ij ðθÞ ¼ Fk
Z ∞ 0
nj ðzÞ expfz=ðλj sin θe Þg dz
ð5Þ
where z and nj(z) represent the depth and the composition at z, respectively. F and k are the transmission function and a factor related to the sensitivity, respectively. Hence, even though the analytical depth is z, photoelectrons are not uniformly emitted from the depth region from the surface to z. Instead, the detected amount of photoelectrons exponentially decays with increasing depth to z. This means that the dependence of surface composition on sin θe, as in Figure 7, cannot be simply regarded as the compositional depth profile. Thus, the following treatment was made to extract a plausible composition profile near the surface in the real space. The NBD composition in a polymer film f(z) was assumed to be expressed by the Gaussian function f ðzÞ ¼ a expðz2 =b2 Þ þ c
ð6Þ
where a, b, and c were constants and were the fitting parameters. Moreover, IN/IC obtained by XPS at a given θe can be expressed by IN =IC ¼
Z ∞ 0
=
Figure 8. Sin θe dependence of the ratio of peak intensity due to N1s and C1s (IN/IC) in XPS spectra for the Zeonex films with thicknesses of (a, b) 189 nm and (c, d) 11.2 nm. The solid circles are the experimental data. The insets show the model depth profiles of atomic ratio of nitrogen to carbon to obtain the best fit to the data. The molar ratio of NBD to a monomer unit in Zeonex is 1.0 103.
f ðzÞ expðz=λj sin θe Þ dz
Z ∞ 0
expðz=λj sin θe Þ dz
ð7Þ
interfacial sides, respectively. Insets denote the depth dependence of IN/IC to reproduce the experimental relation between sin θe and IN/IC. In the case of the 189 nm thick film, the NBD was preferentially segregated at the substrate interface, and the NBD concentration at the outermost region was roughly 3 times that of the bulk. Given that the NBD molecules were heterogeneously dispersed in the interfacial plane as well, aggregation can be serious, leading to an obvious reduction of the NBD lifetime via overcoming the effect of the depressed chain mobility at the interface. The concentration profile was quite unique for the 11.2 nm thick film. In this case, the NBD concentration at the outermost interface with a substrate was almost the same as that at the surface side. In addition, a depletion layer of NBD existed in the depth region from 6 to 8 nm from the air surface. This implies that the aggregation of NBD near to the interface had been suppressed. The information about the NBD composition obtained by XPS is in good accord with the discussion of the fluorescence behavior of NBD.
Accordingly, the sin θeIN/IC curve was fitted with eqs 6 and 7 using a, b, and c parameters. Solid curves in Figure 7 denote the best-fitted curves obtained on the basis of the procedure mentioned above. Insets in Figure 7 show the model depth profiles of IN/IC for the best fits. In the case of a 207 nm thick PMMA film, NBD was slightly depleted at the film surface. On the other hand, the concentration of NBD at the outermost interfacial region with the substrate was approximately twice that introduced due to the favorable interaction between NBD and the hydrophilic substrate. We have already examined the concentration dependence of the lifetime for NBD shown in Figure 2. Even though the NBD concentration doubles to 0.2 mol %, the lifetime will remain unchanged. Thus, the effect of depressed molecular motion of PMMA in the interfacial region overcomes the effect of the concentration increment on the fluorescence behavior. In the case of a 10.5 nm thick PMMA film, since the analytical depth was comparable to the film thickness, the insets of panels c and d possess bilateral symmetry. That is, the NBD concentration profile from the surface can be superimposed on that from the substrate interface after reversing one of them from right to left. This is a signature that our fitting process was reliable. The NBD concentration profile in the thin film was essentially the same as that in the thick one. Nevertheless, the lifetime of NBD in the thin film was shorter than that in the thick film. This was the case in not only in the bulk but also at the interface, as shown in Figure 5. This can be explained in terms of a surface effect. The segmental mobility of polymer at the surface is enhanced in comparison with that in the bulk. This makes the lifetime of dyes in the surface region shorter. When a film becomes thinner, the surface and interfacial ratio to the total volume increases. Eventually, the effect of the surface mobility interferes with that of the interfacial mobility. Figure 8 shows the sin θe dependence of IN/IC for 189 and 11.2 nm thick Zeonex films. Similarly to Figure 7, the upper and lower panels show the data for the surface and substrate
4. CONCLUSIONS We have studied the fluorescence behavior of NBD dye dispersed in thin films of polymers with various polarities, which regulate the interaction with the NBD. The fluorescence behavior of NBD in thick films of a polymer depended on which kind of a polymer was used as a matrix. When a polymer film becomes thinner, the ratio of surface and interfacial areas to the total volume increases. Thus, to understand the fluorescence behavior of a dye in a thin polymer film, the surface and interfacial effects should be taken into account. The segmental mobility of polymers at the surface and the substrate interface are respectively enhanced and depressed in comparison with that in the internal bulk phase. Since the fractional amount of nonradiative pathways to the ground state of excited NBD molecules is closely related to the polymer mobility, the fluorescence lifetime at the surface and interface should be shorter and longer than that in the bulk, 334
dx.doi.org/10.1021/ma201901x |Macromolecules 2012, 45, 329–335
Macromolecules
ARTICLE
respectively. Such an idea was applicable to explain the fluorescence behavior of NBD, which was well-dispersed in thin films of PMMA and Arton, and probably PS as well. However, this was not the case for H-PS and Zeonex. The distribution of NBD in the surface and interfacial regions is extremely inhomogeneous, leading to the aggregation of NBD molecules. The aggregation formation of dye molecules make the fluorescence lifetime shorter. Thus, in addition to the surface and interfacial effects related to polymer mobility, the dispersion state of dye molecules should be also taken into account. In other words, estimating precisely the surface and interfacial effects associated with the dispersion state of dye helps us to design and construct thin polymer films containing fluorescence dyes with desired properties.
(20) White, C. C.; Migler, K. B.; Wu, W. L. Polym. Eng. Sci. 2001, 41, 1497–1505. (21) Ellison, C. J.; Torkelson, J. M. Nature Mater. 2003, 2, 695–700. (22) Priestly, R. D.; Ellison, C. J.; Broadbelt, L. J.; Torkelson, J. M. Science 2005, 309, 456–459. (23) Tanaka, K.; Tsuchimura, Y.; Akabori, K.; Ito, F.; Nagamura, T. Appl. Phys. Lett. 2006, 89, 061916–12. (24) Roth, C. B.; McNerny, K. L.; Jager, W. F.; Torkelson, J. M. Macromolecules 2007, 40, 2568–2574. (25) Tanaka, K.; Tateishi, Y.; Okada, Y.; Nagamura, T.; Doi, M.; Morita, H. J. Phys. Chem. B 2009, 113, 4571–4577. (26) Ashley, J. C. IEEE Trans. Nucl. Sci. 1980, NS-27, 1454–1458. (27) Andrade, J. D. In Surface and Interfacial Aspects of Biomedical Polymers; Andrade, J. D., Ed.; Plenum Press: New York, 1985; Vol. 1, Chapter 6, p 105. (28) Kawaguchi, D.; Tanaka, K.; Takahara, A.; Kajiyama, T. Macromolecules 2001, 34, 6164–6166. (29) Phillips, D.; Rumbles, G. Polym. Photochem. 1984, 5, 153–170. (30) Mukherjee, S.; Chattopadhyay, A.; Samanta, A.; Soujanya, T. J. Phys. Chem. 1994, 98, 2809–2812. (31) Forrest, J. A.; Jones, R. A. L. In Polymer Surfaces, Interfaces and Thin Films; Karim, A., Kumar, S., Eds.; World Scientific: Singapore, 2000; Chapter 7, p 251. (32) Keddie, J. L.; Jones, R.; Cory, R. A. Europhys. Lett. 1994, 27, 59–64. (33) Forrest, J. A.; Dalnoki-Veress, K.; Stevens, J. R.; Dutcher, J. R. Phys. Rev. Lett. 1996, 77, 2809–2005. (34) Tanaka, K.; Hashimoto, K.; Kajiyama, T.; Takahara, A. Langmuir 2003, 19, 6573–6575. (35) Matsuoka, H. Macromol. Rapid Commun. 2001, 22, 51–67. (36) Wallace, W. E.; van Zanten, J. H.; Wu, W. L. Phys. Rev. E 1995, 52, R3329–R3332. (37) Miyazaki, T.; Nishida, K.; Kanaya, T. Phys. Rev. E 2004, 69, 061803–15. (38) Rittigstein, P.; Torkelson, J. M. J. Polym. Sci., Part B: Polym. Phys. 2006, 44, 2953–2943. (39) Bhatia, Q. S.; Pan, D. H.; Koberstein, J. T. Macromolecules 1988, 21, 2166–2175. (40) Jones, R. A. L.; Kramer, E. J.; Rafailovich, M. H.; Sokolov, J.; Schwarz, S. A. Phys. Rev. Lett. 1989, 62, 280–283. (41) Tanaka, K.; Kajiyama, T.; Takahara, A.; Tasaki, S. Macromolecules 2002, 35, 4702–4706. (42) Geoghegan, M.; Krausch, G. Prog. Polym. Sci. 2003, 28, 261–302. (43) The surface energy of NBD was estimated to be 66 mJ m2, being higher than that for PMMA of 43 mJ m2, on the basis of an atomic group contribution method by van Krevelen.
’ AUTHOR INFORMATION Corresponding Author
*Tel +81-92-802-2878, Fax +81-92-802-2880, e-mail k-tanaka@ cstf.kyushu-u.ac.jp.
’ ACKNOWLEDGMENT We thank Prof. Toshihiko Nagamura at Kyushu University for his fruitful discussions. This research was partly supported by Grant-in-Aid for Young Scientists A (No. 21685013), for Scientific Research on Innovative Areas “Molecular Soft-Interface Science” (No. 21106516), and for the Global COE Program “Science for Future Molecular Systems” from the Ministry of Education, Culture, Sports, Science and Technology, Japan. ’ REFERENCES (1) Crenshaw, B. R.; Weder, C. Adv. Mater. 2005, 17, 1471–1476. (2) Greene, N. T.; Shimizu, K. D. J. Am. Chem. Soc. 2005, 127, 5695–5700. (3) Thomas, S. W.; Joly, G. D.; Swager, T. M. Chem. Rev. 2007, 107, 1339–1386. (4) Aoki, A.; Ghosh, P.; Crooks, R. M. Langmuir 1999, 15, 7418–7421. (5) Onoda, M.; Tada, K. Curr. Appl. Phys. 2006, 6, 887–890. (6) Kim, J.-M. Macromol. Rapid Commun. 2007, 28, 1191–1212. (7) Wang, G.; Hou, L.; Gan, F. Phys. Status Solidi A 1999, 174, 269–275. (8) Na, H.-S.; Kim, J.-H.; Hong, K.-M.; Ko, B.-S.; Kim, B.-C.; Han, Y.-K. Mol. Cryst. Liq. Cryst. 2000, 349, 35–38. (9) Maeda, M.; Ishitobi, H.; Sekkat, Z.; Kawata, S. Appl. Phys. Lett. 2004, 85, 351–353. (10) Wang, P.; Zakeeruddin, S. M.; Moser, J. E.; Nazeeruddin, M. K.; Sekiguchi, T.; Gr€atzel, M. Nature Mater. 2003, 2, 402–407. (11) Nogueira, A. F.; Longo, C.; Paoli, M. A. D. Coord. Chem. Rev. 2004, 248, 1455–1468. (12) Zhang, W.; Cheng, Y. M.; Yin, X. O.; Liu, B. Macromol. Chem. Phys. 2011, 212, 15–23. (13) Birks, J. B. Photophysis of Aromatic Molecules; Wiley-Interscience: London, 1970. (14) Wang, Y. Chem. Phys. Lett. 1986, 126, 209–214. (15) Shimizu, M.; Mochida, K.; Katoh, M.; Hiyama, T. J. Phys. Chem. C 2010, 114, 10004–10014. (16) Ito, F.; Kakiuchi, T.; Sakano, T.; Nagamura, T. Phys. Chem. Chem. Phys. 2010, 12, 10923–10927. (17) Nishijima, Y. Ber. Bunsenges. Phys. Chem. 1970, 74, 778–784. (18) Bokobza, L.; Pajot, E.; Monnerie, L.; Bouas-Laurent, H.; Castellan, A. Polymer 1981, 22, 1309–1311. (19) Anwand, D.; M€uller, W.; Strehmel, B.; Schiller, K. Makromol. Chem. 1991, 192, 1981–1991. 335
dx.doi.org/10.1021/ma201901x |Macromolecules 2012, 45, 329–335
