J. Phys. Chem. 1993,97, 67536759
6753
Diffuse Reflectance Laser Photolysis and Luminescence Study on Poly(ethylene terephthalate) Powder Norimasa Fukazawa, Kazunori Yoshioka,t Hiroshi Fukumura, and Hiroshi Masuhara' Department of Applied Physics, Osaka University, Suita, Osaka 565. Japan, and Department of Polymer Science and Engineering, Kyoto Institute of Technologv, Matsugasaki, Kyoto 606, Japan Received: January 5. 1993; In Final Form: March 31, 1993
Photochemical and photophysical primary processesof solid poly(ethy1eneterephthalate) powder were investigated with nanosecond diffuse reflectance laser photolysis and conventional steady-state luminescence techniques. The existence of four transient species was confirmed. To assign these transients, dependencies of the transient absorption spectra and decay kinetics on foreign gas, dopant aromatic compounds which act as a photosensitizer or quencher, and crystallinity were investigated. In addition, the temperature effect was examined. As a result, the transient species, observed only under an aerated condition, was assigned to the precursor species leading to the photooxidation or photodegradation reaction. Other transient species were assigned to three different triplet states, which correspond to three different local structures: monomer site, dimer site, and a site which forms an excimer in the excited singlet state. The generation of three different triplet states is ascribed to an inhomogeneity of the aggregation and relative orientation of the monomer unit. It is discussed that crystalline and amorphous regions, and their boundary region, have a key role in photochemical and photophysical processes in polymer powder.
Introdllction For elucidating a photochemical reaction mechanism, it is very important and indispensable to measure directly the dynamic behavior of excited molecules and chemical intermediatesby laser photolysis. This techniquehas made great contributions to studies on physical, chemical, and biological systems.' Rather simple systems such as molecules in dilute solutions and in the gas phase have been frequently examined by the technique, while it is also powerful for photochemical studies of molecular aggregates, molecular organizates, and polymers. A typical example of the application of laser photolysis to polymer systems is investigation of photochemical reactions such as a- (Norrish type 1) and 8-scissions (Norrish type 11)of carbonyl-containingcompounds.2 Another interesting topic is the photoinduced charge separation process of polymers, since this process is closely related to photoconductivity and solar energy conversion. Interchromophoric interactions and configurational and conformational structures in the excited singlet, triplet, cationic, and anionic states were elucidated on the basis of absorption spectral data. Since the excitation intensity in laser photolysisis relatively high, nonlinear photochemical behavior characteristic of polymers is easily induced and has received much attention as a new topic. Mutual interactions between excited states in one polymer chain, electron transfer from the higher excited states to the neighboring chromophores, and transient polyelectrolyte formation are such special photochemical beha~ior.~ Most of these investigations were carried out in solution, and measuring transient absorption spectra of polymers in the solid state has been an important subject for a long time. However, simple applicatioin of the conventional transmittance laser photolysis method to polymer systems is often impossible, since optical conditions are quite different. Thus, various efforts have been put forth, particularly time-resolvedreflection spectroscopy, which is considered to be very useful for such solids. The first such effort is diffuse reflectance laser photolysis, which was originally proposed by Wilkinson,4 and its time resolution was extended to the picosecond time r e g i ~ n . ~ This ,~ is a very powerful technique for studying the photochemical primary processes of solid materials, bccause it gives absorption t
Kyoto Institute of Technology.
0022-3654/93/2097-6753$04.00/0
spectra of transient species in highly opaque and optically scattering materials. The technique is widely used, and photophysical and photochemical processes in various heterogeneous systems are studied in detail: energy photoionization,9J0charge separation and injection,"-l4 and heat generation in the system.15 The second effort is time-resolved attenuated total reflection UV-visible spectroscopy, where an evanescent wave from an internal reflection element to the organic solid is used as a monitoring light.16 Only the surface as well as interface layers with a submicrometer thickness can be examined. The third is transient absorption spectroscopyunder multiple reflection,which makes it possible to measure transient absorption spectra of transparent ultrathin films. Because solid-state polymers have characteristic structure, morphology, and function which are not realized in solution systems, their laser photolysis, energy transfer, and fluorescence quenching studies will provide new viewpoints. Aggregation and mutual penetration of polymer chains18a,band/or crystallinity lead to characteristic properties.18cvd Furthermore, all the structures and properties are inhomogeneousand have a gradient from the surface/interface to the bulk.19 Thus, photochemical and photophysical behavior should be elucidated in relation to morphologies. However, such a study by transient absorption spectroscopy has never been reported as far as we know. In the present work, poly(ethy1ene terephthalate) (abbreviated hereafter as PET) is studied, because it is extensively used in its solid state, for example, fiber, packaging films, and substrate for microelectronics. Consequently, many investigations of this polymer have been reported: photodegradation and photooxidation reactions,2O its photoconductive property,21 and laser ablation.22 Concerning fundamental primary photoprocesses,an excitation wavelength dependence of the fluorescencespectra of PET film was recently reported, and a new photophysical model was proposed.23 Although many researchers have tried to clarify the photophysical processes of PET, luminescence spectroscopy is not enough to reveal relationships between the aggregation/ orientation of the monomer unit and photophysicalpr0perties.*&3~ We applied nanosecond diffuse reflectance laser photolysis technique to PET powder and measured two transient absorption bands.34 Multicrystalline powders of the monomeric reference 0 1993 American Chemical Society
Fukazawa et al.
6754 The Journal of Physical Chemistry, Vol. 97, No. 25, 1993 compound and oligomerswere also examined in an effort to assign the transients. To reveal the photophysical and photochemical processes in detail, a systematicstudy is presented here. Excitation wavelength dependence of transient absorption spectra, its decay kinetics, and quenching effect by foreign gas were investigated, since multiple excited species were involved. We have also examined the effect of doped triplet sensitizer or quencher in the PET powder and the temperature effect, in an effort to identify the excited species. On the basis of these results, we discuss the photodynamics of PET powder in view of the heterogeneous structure of PET powder.
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Experimental Section PET was kindly supplied in pellet form by Kaneka Co. Ltd. The partly crystallized sample was ground into powder by a vibratory ball-mill (Tokyo Kagaku Sangyo, ROKT-5) at 77 K. The obtained PET powder was used without further purification, because its transient absorption spectra were not significantly changed by further purification, namely, reprecipitation from l,l, 1,3,3,3-hexafluoro-2-propanol-methanol solution. Dimethyl terephthalate (Tokyo Kasei, abbreviated hereafter as DMTP) was recrystallized twice from ethanol. Benzophenone (Tokyo Kasei) was recrystallizedtwice fromethanol. pTerphenyl (Dotite scintillationgrade) was zone refined (100 passes). These organic molecules were used as a monomeric reference for the polymer, photosensitizer, and quencher of the triplet state, respectively. Powder samples with dopant organic molecules were prepared in the following way. PET powder was swollen in a benzene solution of sensitizer or quencher, and the solution was stirred for 2-3 h. After leaving the solution for 1 day, we removed the solvent by a freeze-drying method. Heat treatment of PET powder was carried out at 2 W 2 3 0 OC for 10 h under a stream of nitrogen gas (1-2 L/min). A change in the crystallinity of PET powder was monitored by measuring IR spectra. A FT-IR spectrometer (FUJI ELECTRIC, FIRIS100) with a diffuse reflectance attachement was used. All the samples were contained in a rectangular Suprasil cell with 2-mm or 1-cm thickness and deaerated for at least 10 h. The samples for steady-state phosphorescence measurement at 77 K were contained in a round Suprasil cell with 2-mm or 1-cm diameter. Fluorescence and phosphorescence spectra of powder samples were measured from the front surface on a fluorescence spectrometer (Hitachi, MPF-4). To eliminate the effect of stray light on emission spectra, an interference band-pass filter (289 nm) was used for excitation below 320 nm. The phosphorescence measurement was carried out at 77 K with a chopper. Its observation time (0,)and dead time (4) were as follows: condition I: 0,= 10 ms, 0% = 20 ms condition 11: 0, = 1.5 ms, D, = 2.5 ms The spectral sensitivity of the instrument was not corrected. The detailsof our microcomputer-controllednanosecond diffuse reflectance laser photolysis were given elsewhere.35 Excimerlasers (Lumonics EX-510,308 nm, 8-12 ns, and Lambda Physik EMG 101 MSG,35 1 nm, 18 ns) were used for an excitation light pulse. A 150-W or 300-W dc Xenon lamp (Wacom KXL 150 or KXL 300), which was additionally pulsed for ca. 200 j ~ sin fwhm synchronouslywith the laser pulse, was employed as the analyzing lamp. The transient absorption intensity was displayed as percentage absorption (% abs) defined as % ' abs(A,t) = @,(A,?)
- R(X,r)+ E(h,t))/R,(X,t)
where R(X,t) and Ro(X,t)represent the intensity of the diffuse reflected analyzing light at wavelength X with and without
400
450 500 550 Wavelength (nm)
Figure 1. Transient absorption spectra of PET powder by 308-nm excitation at room temperature (a): the gate time is 0.1-0.8 ps ( 0 )and 25-28 ps (A). Spectra by 351-nm excitation at room temperature (b): the gate time is the same as in (a). Spectra by 308-nm excitation at 77 K (c): the gate time is 1-3 ps ( 0 )and 25-28 @ (A).
excitation, respectively. E(X,t) is emission intensity induced by laser excitation and is subtracted for correction.
Results and Discussion Trrnsient Absorption Spectra of PET Powder. Transient absorption spectra of PET powder observed under deaerated condition are shown in Figure 1. The spectra by excitation with a 308-nm pulse were composed of two broad bands around 430 and 520 nm. However, when the sample was excited at 351 nm, the absorption band at 520 nm was not observed. As shown in Figure 2, the decay curves at 430 nm can be analyzed by a twoexponential function as in 1: '% abs(t) = A, exp(-k,t)
+ A, exp(-k2r)
(1) The decay rate constants, kl and k2, were estimated to be 5 X 102s-l and 5 X 10s-I, respectively. The ratio of amplitude factors, A1/A2, depends upon excitation wavelength and crystallinity. These are summarized in Table I. ForeignGasEffectonTrrrnsieatSpecies. Toassign thetransient species, we examined the effect of some gasses on decay kinetics. A quenching process of the transient absorption at 430 nm by oxygen is dynamic, and an increase of decay rate was observed with an increase of oxygen pressure. All of the dacay curves observed under oxygen could be approximately analyzed by a single-exponentialfunction. This indicates that the quenching reaction is fast enough compared to both decay processes with kl and k2, giving an apparent single-component decay. The observed pseudo-first-order decay rate constant, kobrwas plotted against oxygen pressure in Figure 3. Its slope gives the secondorder rate constant of the quenching reaction. The oxygen concentration in the PET is given by [Oz] = SP, whereS and Prepresent solubility of oxygen and oxygen pressure, respectively. As S is 0.72 X 1 V cm3/(cm3Pa),'6 the quenching rate constant kq is calculated to be 1.6 X lo6 M-l s-I. For a diffusion-controlled quenching process, the Smoluchowskiqua-
Photochemical/Photophysical Processes in Polymer Powder
The Journal of Physical Chemistry, Vol. 97, No. 25, 1993 6755
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TABLE I: Ratio of Amplitude Factors (AI/Az) in DOubleExponential Analysis of tbe Transient Absorption at 430 11111 excitation wavelength, nm 308 35 1
untreated 0.45 1.4
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tion can be applied where kdi& the rate constant of the diffusion-controlledreaction,
D is the sum of the diffusion coefficients of the reactants (chromophore and oxygen), and R is their interaction radius.37 We assume here that the quenching probability per encounter is 1 and R is 0.6 nm. The diffusion coefficient of oxygen in PET film, 1.5 X m9em2s-1,36 was used as D. The kdiffwascalculated to be 1.59 X IO6 M-l s-l, which is in good agreement with the experimentally determined value. Therefore, we conclude that
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Time (ps) Figure 4. Transient absorption spectra of PET powder which were observed when the sample was excited with 308-nm light under aerated condition(a). The gate time is 1-3 p s (0)and 25-28 @ ( 0 ) . Thedecay curves of PET powder at 520 nm under aerated (b) and deaerated (c) conditions were also shown. These two decay curves were measured under the same 308-nm light intensity at room temperature.
the transient species at 430 nm is quenched by oxygen through a diffusion-controlled process. The triplet state and anion radical are generally quenched by oxygen with a diffusion-controlled rate. To discriminate both species, we examined also the effect of NzO gas for transient absorption at 430 nm. Nz0 is a quencher widely used for assigning the anion radicals; however, no quenching behavior was observed. Thus, we conclude that the transients at 430 nm are the triplet states of PET powder. Under aerated condition, a new transient absorption was observed around 520 nm, and it decays at 8 X lo4 s-l as shown in Figure 4. Since the intensity of this band was dependent upon the oxygen pressure and was not affected by NZand NzO, we suppose this transient species is a precursor of a photooxidation or photodegradation product of PET, on which a lot of papers have been published.z0 The assignment of the transient observed at 520 nm under deaerated condition will be discussed later. Dophtg Effects of Triplet Sensitizer and Quencber. From the phosphorescence spectrum (Figure 8), the lowest triplet energy level of PET is estimated to be around 24 0oO em-'. In the present work, benzophenone (E(T1) = 24 500 em-' in PMMA3*) and pterphenyl (E(T1) = 20600 em-l in EPA39) are used as photosensitizer and quencher, respectively. Furthermore, photophysicaland photochemical primary processes of these aromatic compounds are well investigated in solid state by diffuse reflectance laser p h o t o l y ~ i s . ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ When the sample doped with benzophenone (1.1 X lezmol/ (mol of monomer unit)) was examined with a 351-nm pulse, benzophenone was mainly excited because the absorbance of benzophenone under the present concentration is much larger than that of PET at this wavelength. The transient absorption spectrum has two broad absorption bands around 430 and 520 nm as shown in Figure 5a. Although benzophenone was mainly excited, the T-T absorption of benzophenone at 540 nm (Figure
Fukazawa et al.
6756 The Journal of Physical Chemistry, Vol. 97, No. 25, 1993 -
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450 500 550 Wavelength (nm) Figure 5. (a) Transient absorption spectra of PET powder doped with benzophenone (1.1 X 10-2 mol/(mol of monomer unit)). The gate time is 1-3 ps (0),9-1 1 ps (A), and 25-28 ps (0). (b) Transient absorption spectra of benzophenone microcrystals. The gate time is the same as given in (a). (c) Transient absorption spectra of PET powder doped with pterphenyl(l.5 X 10-3 mol/(mol of monomer unit)). The gate time is 2-6 m8 (O), 22-24 ms (A),and 70-72 ms (0).
5b) was not clearly identified. Furthermore, the absorption intensity of the bands due to PET was increased relative to those without benzophenone. On the other hand, the sample doped with p-terphenyl(l.5 X 10-3 mol/(mol of monomer unit)) showed a transient absorption spectrum with a peak at 460 nm (Figure 5c) instead of the absorption at 430 nm of PET. Whilep-terphenyl was not excited with the 351-nm pulse, this can be assigned to the triplet state of molecularly dispersedp-terphenyl accordingto the l i t e r a t ~ r e . ~ ~ The present quenching experiments can be explained in terms of the triplet-triplet energy transfer. This is further verification for the triplet absorption at 430 nm. TrmsientAbaorptionat 77K. The transientabsorptionspectra OfPETpowder at 77 K are flatter than thmeat roomtemperature, when the sample was excited with a 308-nm pulse (Figure IC). This is attributed to the fact that the decay rate of transient absorption at 520 nm becomes slow at low temperature in the examined time range. It is worth noting that the rise component of the absorption at 430 nm was observed, and it coincided to the decay of that at 520 nm, giving the same time constant (Figure 6). This implies that at least one transient species giving absorption at 430 nm is generated from the transient at 520 nm; namely, the intermediate species at 520 nm is a precursor of the transient one at 430 nm. Although the transient absorption at 430 nm is assigned to the triplet state of PET powder as described above, the transient species at 520nm is not a singlet stateof PET powder. This is because the decay time observed at 520 nm is a few microseconds at room temperature and extremely longer than the fluorescence lifetime of PET films (several nanosecond~).~~ One possible explanation is that this transient at 520 nm is also a triplet state. Since the PET powder used in the present study was semicrystallized,crystalline and amorphous regions exist in thesample polymer. It is considered that aggregation and mutual orientation of the monomer DMTP unit in PET is different
Figure 6. Transient absorption rise and dccay curvea of PET powder (a, b) and annealed PET powder (c, d) at 77 K. The observationwavelength is given in the figure. Initial spikce (-) are attributed to an artifact which could not be corrected because of an extremely intense emhion of PET powder.
Wavelength (nm) 400 3(
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20 30 W avenumb er (x1O cmFigure 7. Fluoreaccnce spectra of PET powders excited at 289 nm (a) and at 340 nm (b). The d i d and dashed linea are the spectra of original and annealed PET powders, respectively.
between crystalline and amorphous regions. Therefore, plural triplet states with different structures in different environments may be formed in the sample polymer. According to this idea, the observed rise and decay behavior in Figure 6 is explained 811 a triplet-triplet energy migration proces% between two sites with different structure. These rise and decay behaviors became fast upon annealing, as shown in Figure 6. Since crystallinity of PET is increased upon annealing, the result is probably due to the fact that the triplet-triplet energy migration efficiency increaserr with the crystallinity. -lF Spectra. The fluorescence spectrum of PET powder showed an excitation wavelength dependence au given in Figure 7. When the sample was excited with 289-nm light, a broad peak at 370 nm and a shoulder at 335 nm were observed. On the other hand, when excited with 340-nm light, a peak at 390 nm and two shoulders at 370 and 410 nm were obrved.
Photochemical/PhotophysicalProcesses in Polymer Powder These results on PET powder are in accordance with the fluorescence behavior of its films.23-32 Therefore, we concluded that photophysical properties of PET are common to films and powder, and our powder preparation method did not change appreciably the microstructure and inhomogeneity of PET. According to the result on the film, the broad peak at 370 nm and the shoulder at 335 nm, which is observed when excited with 289-nm light, could be assigned to the excimer fluorescence and the monomer fluorescence of PET, respectively.24.25~28 The emissionobserved when PET was excited with 340-nm light could be assigned to the fluorescencefrom the excited state of a groundstate stable dimer24*26 (hereafter referred to as dimer). Recently, Hemker et al. proposed a similar photophysical model of PET film which explains the excitation wavelength dependenceof the fluorescence spectra.23 The 289-nm light excites directly the monomer units in the main chain of the polymer (monomer excitation), and the 340-nm light directly excites a trap site which is assigned to dimer (dimer excitation). It is worth noting that three fluorescent speciesare detected in PET. They also suggested that the dimer exists in the amorphous region of PET film. Fluorescencespectrum of PET powder was confmed to change with crystallinity, as clearly shown in Figure 7. When the sample was excited with 289-nm light, the monomer emission band at 335 nm was enhanced relative to the excimer band by crystallization of sample. This means that the number of a monomer site (M site) is increased and the energy migration from the M site to an excimer-forming site (E site) is restrained with crystallization of PET powder.28 On the other hand, when the sample was excited with 340-nm light, the fluorescence spectra of PET powder do not significantly change upon annealing, because the present wavelength can directly excite the D site, which is the trap site of the excitation energy. Phosphorescence Spectra. Phosphorescence and its excitation spectra of PET powders at 77 K are shown in Figure 8. Although the spectra of multicrystalline DMTP powder, which is a monomeric reference, were not changed with excitation wavelength, the spectra of PET powder showed a considerable excitation wavelength dependence. The spectral shape of PET powder was also changed with the chopping speed of the sector, as shown in Figure 8. These results indicate that at least two phosphorescent species exist in PET powder. Moreover, the phosphorescence spectral shape was affected by crystallization of PET powder, as was confirmed for fluorescence spectra. Phosphorescence and excitation spectra of annealed samples were shown in Figure 9. The excitation spectra of PET powders are divided roughly into two regions. One (around 310 nm) corresponds to monomer absorption, and the other (around 350 nm) is due to dimer absorption, which is obtained by comparing them with the fluorescence excitation spectra. The intensity around 350 nm in the phosphorescence excitation spectra was increased with observation wavelength. This result implies that the dimer phosphorescence is more intense at longer wavelength, although both spectra are overlapped with each other. The contribution of dimer phosphorescence was increased by crystallization, while the contributionof dimer absorption in excitation spectra was relatively decreased, as clearly shown in Figures 8 and 9. We consider that this is due to the increase of efficiency for triplet energy migration from M to D sites with crystallization of PET powder. ShpcturpI Conelderation for the Transient Species. These results on fluorescence and phosphorescence spectra of PET powder are consistent with the transient absorption spectroscopy giving plural triplet states. We consider that three different triplet states are derived from the aggregation and orientation of monomer DMTP units of PET. When the sample was excited at 308 nm,monomer and excimer emissions of PET wereobserved, but the dimer fluorescencewas not. Thus, the triplet state of the M and the E sites of the polymer should be generated through
The Journal of Physical Chemistry, Vol. 97, No. 25, 1993 6757
Wavenumb er (x1O3 cm-l) Wavelength (nm)
, 25 30 35 40 Wavenumb er (x1O3 cm-l)
Figure 8. Phosphorescence spectra of PET powder excited a t 289 MI (a) and at 351 nm (b). The solid and dashed lines were measured under condition I and condition I1 (see Experimental Section), respectively. Phosphoresccnce excitation spectra of PET powder (c). The solid and dashed lines were observed at 420 and 520 nm, respactively.
the intersystem crossing from their excited singlet state. Since the phosphorescence spectrum is structured and the geometry of the triplet excimer is in general different from that of the corresponding singlet excimer?' we suspect that the E site does not form an excimer in the triplet state. On the other hand, the dimer fluorescence was exclusively observed by excitation with 351 nm, and the triplet state of the D site should be generated through its intersystem crossing process. It is well-known that the singlet excimer has a symmetrical sandwich structure or a partial overlapped structure of two adjacent aromatic groups. It is suggested also that the dimer in PET has a sandwich structure of two adjacent aromatic rings.26 From this similarity of geometrical structure, we expect that the electronic structures of the E and the D sites resemble each other in the triplet state, giving dimer phosphorescence. Furthermore, they will give a similar Tn TI absorption spectrum. The transient absorption at 430 nm can thus be attributed to the sum of Tn T1 absorption of these two sites. The long-lived transient at 430 nm whose decay constant kl is 5 X 10 s-1 is assigned to the triplet state of the E site (3E*). The reason is as follows. (i) This transient is a main species by monomer excitation of PET powder before annealing, and its contribution was decreased when the crystallinity of the sample was increased, namely, when the amount of the E site was decreased. (ii) When the sample was excited with the 351-nm laser, Le., when the D site is excited, the contribution of this transient was small. Consequently,the short-lived transient was assigned to the triplet state of the D site (3D*). According to the results of transient absorption measurement at 77 K,we found that theenergy migration rate from the transient at 520 nm to that at 430 nm increased with crystallinity. On the other hand, we also clarified from the phosphorescence measurement that the efficiency of triplet energy migration from M to D sites increased with crystallinity. Furthermore, the transient
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Fukazawa et al.
The Journal of Physical Chemistry, Vol. 97, No. 25, 1993 Wavelength (nm) 700 600 500 400
SCHEME I: Photochemical Primary Processes of PET Powder M TIM* (30811110 j EM
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Wavenumb er (xlO3 cm-l) Figure 9. Phosphorescence and excitation spectra of annealed PET powder. The notations are the same as those in Figure 8. at 520 nm whose decay time is less than 1 ps was observed with only monomer excitation. Therefore, the transient at 520 nm is assigned to the triplet state of the M site. Rise and decay processes shown in Figure 6 were ascribed to the triplet energy migration processes from M to D sites. As discussed above, when the M site in amorphous region was excited, the singlet energy migration from M to E sites efficiently occurs, and the triplet state of the E site is exclusively generated in amorphous region. The dynamic behavior given in Figure 6 is attributed to the triplet energy migration from the M site, which exists in crystalline region, to the D site. The D site is not formed in crystalline region, because the structure of this site cannot be formed in this region. Moreover, the dimer fluorescence was observed even for amorphous PET films, so the D site should exist in the amorphous region of PET. However, our experimental results indicate that the contribution of the D site increases with crystallinity. At the present stage of investigation, there are two possible explanations: (i) the D site exists in a boundary region between the amorphous and crystalline regions or (ii) the D site corresponds to a defect site of a crystallite of PET. If the E and D sites exist in different regions, namely, amorphous and the defect site in crystalline region, it is expected that the decay behavior at 430 nm observed under oxygen could not be accounted for by the simple exponential function because of the difference of the diffusion rate of oxygen in these regions. Therefore, explanation (i) is more plausible. Photochemical and Photophysical Processes of PET Powder. From the above considerations, we propose a scheme of photochemical primary processes of PET as summarized in Scheme I. When the sample is excited with light below 320 nm (monomer excitation), the singlet state of the M site is generated. If the M site is in the amorphous region, singlet energy migration from M to E sites occurs efficiently. Consequently, 3E* of PET, which absorbs 430 nm light, is dominantly generated in the amorphous region by monomer excitation. If the M site in the crystalline region was excited, energy migration from M to E sites did not
occur in the singlet state and the ,M*, which absorbs light around 520 nm, is generated through its intersystem crossing process. Since triplet energy migration from M to D sites efficiently occurred, the3D*whichabsorbslight around430nmisgenerated. When the sample is excited to the energy states above 320 nm, the D site was directly excited and, then, the jD* state of PET is mainly observed. Energy Level of the Triplet States in PET Powder. Recently, LeFemina and Arjavalingam reported a phosphorescence peak centered at 534 nm (18 600 cm-l), which appears in the perpendicular polarized spectrum of PET film.29 They also reported the energy of the dimer phosphorescence level as 2.3 eV (1 8 600 cm-l), which was calculated by using the CNDO/S3-CI method. We consider this value to be too low compared to our results for the photosensitization experiment. If the triplet energy level of the dimer is 2.3 eV, the triplet state ofp-terphenyl, whose energyis 2.55 eV (20 600cm-'),could not be observed. Similarly, if the energy level of 3E* was less than 2.55 eV, 3E*should be formed by accepting excitation energy from the triplet state of p-terphenyl photosensitized by other triplet states of PET. However, 3E* was not observed for the sample doped with p-terphenyl. Thus, we conclude that the energy levels of the jD* and 3E* states are in the range from 2.55 eV (20 600 cm-l) to 3.0 eV (24 500 cm-I), where the latter value is the triplet energy level of the benzophenone.
Conclusion We find the existence of four transient species in PET powder by the nanosecond diffuse reflectance laser photolysis technique: (1) the transient around 520 nm which decays less than 1 ps, (2) the transient at 520 nm which appears under oxygen, (3) the transient at 430 nm whose decay rate constant is 5 X 10 s-l at room temperature, and (4) the transient at 430 nm whose decay rate constant is 5 x IO2 s-l at room temperature. We assigned (3) and (4) to the triplet state of PET from the effect of gas and doping with benzophenone andp-terphenyl. From the discussion for theeffect of photochemical primary processeson crystallization and the dependence on excitation wavelength, we assign (3) to the triplet state of dimer site and (4) to the triplet state of the site which acts as an excimer in singlet state. We assign (1) to the monomer triplet state of PET beecause this transient was only generated with monomer excitation. Furthermore, we assign (2) to the intermediate species leading to photooxidation or photodegradation from the effect of oxygen. The inhomogeneity of aggregation, orientation of the monomer unit, and energy migration are the reasons for the generation of three different triplet states of PET.
Photochemical/PhotophysicalProcesses in Polymer Powder
The Journal of Physical Chemistry, Vol. 97, No. 25, 1993 6759
Matsuura, T.; Elsevier: Amsterdam, 1989;p 15. (17) Itaya, A.; Yamada, T.; Masuhara, H. Chem. Phys. Lett. 1990,174, 145. (18) (a) Morishima, Y.; Tominaga, Y.; Nomura, S.;Kamachi, M.; Okada, T. J. Phys. Chem. 1992,96,1990.(b) Liana, P.; Brown, W. J. Phys. Chem. 1992,96,6439.(c) Jenkins, R. M.; Hammond, G. S.;Weiss, R. G. J. Phys. Chem. 1992,96,496.(d) Sumpter, B. G.; Getino, C.; Noid, D. W. J. Phys. Chem. 1992,96,2761. (19) Itaya, A.; Yamada, T.; Tokuda, K.; Masuhara, H. Polym. J. 1990, 22,697. (20) (a) Osborn, K. R. J. Polym. Sci. 1959,38,357.(b) Pacifici, J. 0.; References and Notes Straley, J. M. Polym. Lett. 1969, 7, 7. (c) Campbell, D.; Turner, D. T. Polym. Lett. 1967,6, 1. (d) Marcotte, F. B.; Campbell, D.; Cleaveland, J. (1) Claesson, S.Fast Reactions and Primary Processes in Chemical A,; Turner, D. T. J. Polym. Sci. A-1 1967,5, 481. (e) Blais, P.; Days, M,; Kinetics; Almqvist and Wibell: Upsala, 1967. Wiles, D. M. J. Appl. Polym. Sci. 1973,17,1895 and references therein. (f) (2) Schnabel, W. Developments in Polymer Photochemistry-J; Allen, Merrill, R. G.;Roberts, C. W. J. Appl. Polym. Sci. 1977,21,2745. N. S., Ed.; Applied Science Publishers: London, 1982;Chapter 7. (3) (a) Masuhara, H.; Ohwada, S.;Mataga, N.; Itaya, A.; Okamoto, K.; (21) (a) Comins, J. D.; Wintle, H. J. J. Polym. Sci., Polym. Phys. Ed. Kusabayashi, S.J. Phys. Chem. 1980,84,2363.(b) Masuhara, H.; Tanaka, 1972,I O , 2259. (b) Hayashi, K.; Yoshino, K.; Inuishi, Y. Jpn. J. Appl. Phys. J. A,; Mstaga, N.; Sisido, M.; Egusa, S.; Imahishi, Y. J. Phys. Chem. 1986, 1975.14,39. (c) Takai, Y.; Osawa, T.; Kao, K. C.; Mizutani, T.; Ida, M. 90,2791.(c)Masuhara,H.;Tanaka, J.A.;Matap,N.;Higuchi,Y.;Tazuke, Jpn. J. Appl. Phys. 1975,14,473. (d) Takai, Y.; Osawa, T.; Mizutani, T.; S. Chem. Phys. Lett. 1986, 125, 246. (d) Masuhara, H.; Ohwada, S.; Ieda, M. J. Polym. Sci., Polym. Phys. Ed. 1977,15, 945. Yamamoto, K.; Mataga, N.; Itaya, A,; Okamoto, K.; Kusabayashi, S . Chem. (22) (a) Srinivasan, R.; Leigh, W. J. J. Am. Chem. Soc. 1982,104,6784. Phys. Lett. 1980,70,276. (b) Lazare, S.;Srinivasan, R. J. Phys. Chem. 1986,90,2124. (c) Dyer, P. (4) (a) Kessler, R. W.; Wilkinson, F. J. Chem. Soc.,Faraday Trans. I E.; Oldershaw, G. A.; Sidhu, J. J. Phys. Chem. 1991,95,10004.(d) Lazare, 1981, 77, 309. (b) Wilkinson, F.; Kelly, G. P. Photochemistry on Solid S.;Soulignac, J. C.; Fragunaud, P. AppLPhys. Lett. 1987,50,624.(e) Novis, Surfaces; Anpo, M., Matsuura, T., Eds.; Elsevier: Amsterdam, 1989;p 30. Y.; Pireaux, J. J.; Brezini, A,; Petit, E.; Caudano, R.; Lutgen, P.; Feyder, G.; ( 5 ) (a) Wilkinson, F. J. Chem. Soc., Faraday Trans. II 1986,82,2073. Lazare, S.J . Appl. Phys. 1988,64,365. (b) Wilkinson, F.; Willsher, C. J.; Leicester, P. A.; Barr, J. R. M.; Smith, M. (23) Hemker, D. J.; Frank, C. W.; Thomas, J. W. J . Polym. 1988,29,437. J. C. J. Chem. Soc., Chem. Commun. 1986,1216. (c) Kelly, G. P.; Leicester, (24) Allen, N. S.;McKeller, J. F. Makromol. Chem. 1978,179,523. P. A.; Wilkinson, F.; Worrall, D. R.; Ferreira, L. F. V.; Chittock, R.; Toner, (25) Takai, Y.; Mizutani, T.; I d a , M. Jpn. J. Appl. Phys. 1978,17,651. W.Spectrochim.Acta 1990,46A,975.(d) Wilkinson, F.;Kelly,G. P.; Michael, (26) Hennecke, M.; Fuhnnann, J. Makromol. Chem. Symp. 1986,5,181. C. J. Photochem. Photobiol. 1990,52, 309. (27) Padhye, M. R.;Tamhane, P. S.Angew. Makromol. Chem. 1978,69, (6) (a) Ikeda, N.; Imagi, K.; Masuhara, H.; Nakashima, N.; Yoshihara, 33. K. Chem.Phys.Lert. 1987,140,281.(b) Ikeda,N.;Koshioka,M.;Masuhara, (28) Cao, T.; Magonov, S.N.; Qian, R. Polym. Commun. 1988,29,43. H.; Yoshihara, K. Chem. Phys. Lett. 1988,150,452. (29) (a) LaFemina, J. P.; Arjavalingam, G. J. Phys. Chem. 1991,95,984. (7) (a) Wilkinson, F.; Ferreira, L. F. V. J. Lumin. 1988,40/41,704.(b) (b) LaFemina, J. P.; Carter, D. R.; Bass, M. B. J. Phys. Chem. 1992,96,2767. Wilkinson, F.; Leicester, P. A,; Ferreira, L. F. V.; Freire, V. M. M. R. (30) Phillips, D. H.; Schug, J. C. J . Chem. Phys. 1969,50, 3297. Photochem. Photobiol. 1991, 54, 599. (c) Wilkinson, F.; Willsher, C. J.; Bourdelande, J. L.; Font, J.; Greuges, J. J. Photochem. 1987,38,381. (31) Cheung, P.-S. R.; Roberts, C. W.; Wagener, K. B. J. Appl. Polym. (8) (a) Turro, N. J.; Zimmt, M. B.; Gould, I. R. J. Am. Chem. Soc. 1985, Sci. 1979,24, 1809. 107,5826.(b) Drake, J. M.; Levitz, P.; Turro, N. J.; Nitsce, K. S.;Cassidy, (32) Chen, L.; Jin, X.;Du, J.; Quian, R. Makromol. Chem. 1991,192, K. F. J. Phys. Chem. 1988,92,4680. 1399. (9) Oelkrug, D.; Reich, S.;Wilkinson, F.; Leichester, A. P. J. Phys. (33) Sonnenschein, M. F.; Roland, C. M. J . Polym. 1990,31,2023. Chem. 1991,9$ 269. (34) Imagi, K.; Ikeda, N.; Masuhara, H.; Nishigaki, M.; Isogawa, M. (10) Iu, K.-K.; Thomas, J. K. J. Phys. Chem. 1991, 95,506. Polym. J. 1987,19,999. 111) Pouliauen. J.: Fichou. D.: Valat, P.: Kossanvi, J.: Wilkinson, F.: (35) Koshioka, M.; Mizuma, H.; Imagi, K.; Ikeda, N.; Fukumura, H.; Wiilsher, C. J: J. Phorochem. '1986,35,381. Masuhara, H.; Kryschi, C. Bull. Chem. SOC.Jpn. 1990,63,3495. (12) (a) Kamat, P. V.; Gopidas, K. R.; Weir, D. Chem. Phys. Lett. 1988, (36) Brandrup; Immergut Polymer Handbook, 3rd ed.; John Wiley and 149,491.(b) Gopidas, K. R.; Kamat, P. V.; George, M. V. Mol. Cryst. Liq. Sons: New York, 1989;p VI-443. The value of the PET film with 40% Cryst. 1990,183,403.(c) Gopidas, K. R.; Kamat, P. V. J. Phys. Chem. 1989, crystallinity was used, because the PET powder discussed in the present paper 93,6428. was semicrystallized. (13) Levin, P. P.:Ferreira, L. F. V.; Costa, S.M. B. Chem. Phys. Lett. (37) Noyes, R. Prog. React. Kine?. 1961,I , 129. 199'0,173,277. (38) Graves, W. E.; Hofeldt, R. H.; McGlynn, S.P. J. Chem. Phys. 1972, (14) Koshioka, M.; Ikeda, N.; Itaya, A.; Masuhara, H. Chem. Lett. 1992, 56, 1309. 1165. (39) Siebrand, W. J . Chem. Phys. 1967,47,2411. (15) Wilkinson, F.;Willsher, C. J.; Uhl, S.;Honnen, W.; Oelkrug, D. J . (40) Wilkinson, F.; Willsher, C. J. Chem. Phys. Lett. 1984,104, 272. Photochem. 1986,33,273. (41) (a) Chandra, A. K.; Lim, E. C. J . Chem. Phys. 1968,49,5066.(b) (16) (a) Ikeda, N.; Kuroda, T.; Masuhara, H. Chem. Phys. Lett. 1989, Chandross, A. E.; Dempster, C. J. J . Am. Chem. SOC.1970,92,704. 156,204. (b) Masuhara, H. Photochemistry on Solid Surfaces; Anpo, M.,
Acknowledgment. We are indebted to Prof. Akira Itaya of Kyoto Institute of Technology for his helpful discussion. We thank Dr. Ryutaro Tanaka and Dr. Kazuko Hayashi of Government Industrial Research Institute, Osaka, for use of the vibratory ball-mill. The present work is supported by a Grantin-aid from the Japanese Ministry of Education, Science and Culture (63430003).
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