Polymer Journal, Vol. 17, No.3, pp 517-524 (1985)

Photochemistry in Polymer Solids V. Decay of Benzophenone Phosphorescence in Polystyrene and in Polycarbonate

Kazuyuki HORIE, Masako TSUKAMOTO, Keiko MORISHITA, and ltaru MITA

Institute of Interdisciplinary Research, Faculty of Engineering, University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153, Japan (Received June 15, 1984)

ABSTRACT: Decay curves and lifetimes of benzophenone phosphorescence in polystyrene and in bisphenol A polycarbonate (BPA-PC) at 80----433 K significantly reflect change in the molecular motion of matrix polymers such as glass transition, /3-transition, and }'-transition. The non-single• exponential decay. curves were observed in both polymers at temperatures between T, and T., and analyzed using the diffusion-controlled rate coefficient with a time dependent transient term for the dynamic quenching process of benzophenone triplet by phenyl or phenylene groups in the matrix polymers. The diffusion coefficients, D, of the reacting functional groups in polystyrene and BPA-PC for the wide temperature range below T. are much larger than that in PMMA in the same temperature range, showing a higher quenching ability of these polymers for the benzophenone triplet. KEY WORDS Benzophenone I Phosphorescence I Polystyrene I Poly- carbonate I Non-Exponential Decay I Molecular Motion I Glass Transition Temperature I /3-Transition Temperature I y-Transition Temperature I

Measurements of and phos• ponential decay profile ofbenzophenone phos• phorescence decays provide valuable infor• phorescence in PMMA was ascertained,4 in• mation on molecular motion, excited energy dicating the absence of biphotonic triplet• transfer and migration, and microstructure in triplet annihilation processes under the present polymer solids. In our previous papers, 1•2 the experimental conditions. The decay study of decay curves of benzophenone phosphores• benzophenone phosphorescence has been ex• cence in poly(methyl methacrylate) tended to cases in polystyrene and bisphenol A (PMMA) and other acrylic polymers were polycarbonate in the present paper. observed to deviate markedly from the single• The temperature,6 •7 concentration,8 and exponential type in the temperature range molecular weight9 •10 dependences of fluores• between Tp (onset of ester side group rotation cence intensity and lifetime as well as tem• of the matrix acrylic polymers) and Tg (glass perature dependence11 or polarization12·13 of transition temperature), while the phosphores• phosphorescence, have been studied for vari• cence decays exponentially for temperqtures ous chromophores in polystyrene. The effects below Tp and above Tg for acrylic polymers. of molecular motion6 ·7 •11 •12 and free volume10 The deviation was attributed to the diffusion• of the matrix polymer as well as energy trans• controlled dynamic quenching (endothermic fer to the matrix polymer14 have been dis• energy transfer) of the benzophenone triplet by cussed in some cases, but in other cases poly• side chain ester groups in these polymers. 3 The styrene is regarded as an inert rigid matrix for intensity independence of the non-single-ex- photophysical processes of the excited chro-

517 K. HORIE et a/.

mophores. The of solid poly• film containing 1.5% benzophenone (80 11m ( oxycarbonyloxy-1 ,4-phenyleneisopropy• thickness) was prepared by solvent casting on lidene-1,4-phenylene) (BPA-PC) has been the a quartz plate from BPA-PC and benzo• subject of several investigations15 - 17 because phenone solution in dichloromethane and was of the occurrence of photo-Fries rearrange• evacuated at room temperature for two days ment by deep UV irradiation inspite of its and at 1ooac for 4 h before the phospho• practical importance from the stand point of rescence measurements in vacuum. A solvent• excellent transparency and high impact cast film of polystyrene (Mn=9.6 x 104 , strength. The effect of polycarbonate matrix MwfMn = 1.1) containing 4.0% benzophe• on the photophysical or photochemical proc• none was also prepared in a similar manner ess of the molecularly dispersed chromo• from tetrahydrofuran solution. phores has not been reported so far to our knowledge. Measurements of Phosphorescence Decay The present paper is concerned with the A pulsed nitrogen (A vco C950B) with mechanism and kinetics of the non-exponen• a pulse width of 10 ns as exciting light at tial decay of benzophenone phosphorescence 337 nm, a cryostat (Oxford DN704), a mono• in polystyrene and BPA-PC at 80-433 K, chromator (Jasco CTlO), a photomultiplier on the application of a concept of a (HTV R374), a.transient time converter (Riken diffusion-controlled reaction with a time• Denshi TCG8000), and a desk-top computer dependent rate coefficient to dynamic quench• (YHP 9825T) were used to measure benzo• ing in the solid state. phenone phosphorescence decay at 80--433 K. The details of the measurements are given EXPERIMENTAL elsewhere. 2

Materials Measurements of Spectra Benzophenone and benzoyl peroxide were The phosphorescence spectra of benzo• purified by the recrystallization from ethanol. phenone in polystyrene under vacuum was Styrene was distilled under reduced pressure measured at 80 and 293 K with a Jasco FP-500 and stored in a dark refrigerator. A standard type spectrofluorimeter and Oxford DN704 sample ofBPA-PC (Mw= 33,800, MwfMn= 2.5) type cryostat. was purchased from Scientific Polymer Product, Inc. RESULTS AND DISCUSSION

Sample Preparation Temperature Dependence of the Decay Curves The purge of is important in the The phosphorescence spectra of benzophe• study of triplet lifetimes even in solid matrices. none in polystyrene excited at 337 nm are A solution of benzophenone (8.9 X 10- 3 M) shown in Figure 1. Three peaks at 420, 450, and benzoyl peroxide ( 1 x 10-3 M) in styrene and 480 nm observed at 80 K correspond to monomer was evacuated by several freeze• the vibrational structure of benzophenone pump-thaw cycles in a high vacuum system, phosphorescence.18 The emission lifetimes at sealed in a cylindrical Pyrex cell with a diam• these wavelengths were r0 = 4.2 ms at 80 K. eter of 10 mm, then polymerized at 70oC for The minor peak at 390 nm consisted of two 120 h, and postcured at l20°C for 20 h. The components: one with a very short lifetime resulting rod sample in the sealed cell was used ( r < 10 ns) and phosphorescence with r0 = for phosphorescence measurements. In the 4.2 ms. The phosphorescence of benzophenone case of benzophenone in BPA-PC, a BPA-PC in the polystyrene rod sample could be hardly

518 Polymer J., Vol. 17, No.3, 1985 Photochemistry in Polymer Solids V.

10 -IBO'C(a)

-60'C(a) 7

550 A (nm) Figure 1. Phosphorescence spectra of benzophenone in polystyrene ([BP]=8.9 X 10- 3 M) at -193°C (I) and 20oc (2), and in PMMA ([BP]=1.7x 10- 3 M) at 20°C (3). Excitation wavelength is 337 nm.

2(1 ms(a) 0 ' 'e ' 2 j ms(b) I I I 0 0.4 l}Bms(c) I I I I 0 aeJ,Js (d) I 'e ' I 16 J(J.Js I•) 9 I I I PC Time Figure 3. Semilogarithmic decay curves of benzo• phenone phosphorescence in bisphenol A polycarbonate (BPA-PC) excited by the laser pulse at 337 nm. Temperature and symols for the time scales appear next to the curves.

observed at room temperature with the present spectrofluorimeter, indicating a predominant quenching process for the benzophenone tri• plet by the polystyrene matrix. The benzo• phenone phosphorescence was observed in PMMA rod sample even at room temperature, as shown in Figure I. Typical decay curves of benzophenone phosphorescence at 450 nm at various tem• peratures in polystyrene rod and BPA-PC film PS Time samples excited by a IO-ns nitrogen laser pulse Figure 2. Semilogarithmic decay curves of benzo• at 337 nm are shown in Figures 2 and 3, phenone phosphorescence in polystyrene excited by 10 ns nitrogen laser pulse at 337 nm. Temperature and respectively. The same decay profiles with the symbols for the time scales are given beside the curves. same lifetimes were observed for polystyrene

Polymer J., Vol. 17, No. 3, 1985 519 K. HORIE et a/.

film samples as those for polystyrene rod chain phenylene group in PBA-PC should sample. The phosphorescence intensity, cP(t), cause the deviation in the phosphorescence decreases as a single exponentially at tempera• decay from a single exponential type. It should tures below the y-transition temperature, TY, be noted that the intrinsic (chemical) rate corresponding to phenyl or phenylene group constant, k, for the quenching of ben• rotation (Tr -lOOac for polystyrene11 and zophenone triplet by polystyrene was about also Tr - lOOoC for BPA-PC19). There was a 103 times that by the ester group in the acrylic deviation from single exponential, which be• polymers2 (k = 3.9 x 103M - 1 s - 1 at 30°C). The came more pronounced with increasing tem• possibility of non-exponential decay due to perature. The regression to the single exponen• triplet-triplet annihilation can be eliminated in tial decay above Tg, clearly observed in the present cases by the intensity independence poly(methyl acrylate) and other methacrylic of the non-exponential decay profiles. polymers, 1.2 was rather ambiguous in the pre• sent cases. Kinetic Parameters for Quenching in Poly• The lifetimes of benzophenone phospho• styrene and BPA-PC rescence due to spontaneous deactivation, Kinetics for the non-exponential decay of r0= l/k0, where k 0 =kPT+kn is the rate con• benzophenone triplet eBP*) in polymer solids stant for spontaneous deactivation consisting due to diffusion-controlled dynamic quenching of phosphorescence (kpT) and non-radiative including non-equilibrium .Jt term have deactivation (kn) processes, were 4.2 ms in been presented and used to explain the decay polystyrene and PBA-PC at 80 K. A small profiles of benzophenone phosphorescence in difference in r0 from those in PMMA and PMMA and other acrylic polymers. 3 The same other acrylic polymers (r0=5.0ms at 80K/·2 kinetic scheme (eq 1 and 2) was applied to the 4.6ms at 77K20) was probably due a weak present cases. interaction of benzophenone triplet with these aromatic matrices. The kn should be negli• 3BP* BP (1) gibly small compared to kPT in PMMA at k 77 K. 20 The smaller lifetimes for the phos• 3BP*+Q BP+Q (2) phorescence of triphenylene and coronene in Here, Q denotes the phenyl group in poly• polystyrene at 77 K compared to those in styrene or phenylene group in BPA-PC, and PMMA at 77 K were also observed. 14 kq, the quenching rate coefficient of benzo• Deviation in the decay curves from the phenone triplet by the phenyl or phenylene exponential above Tr of the matrix polymers group. Since the diffusion process of a phenyl suggests a new intermolecular deactivation or phenylene group approaching triplet benzo• process for the benzophenone triplet. The phenone is controlled by the side-chain rota• quenching of this triplet by a phenyl group was tion and local segmental motion of the poly• proposed to proceed through an exciplex for• mer chains, the bimolecular rate coefficient, kq, mation between the excited of is expressed by eq 3, benzophenone and n-orbital of the phenyl 21 group. The quenching rate constant of ben• 4nRDN { R } zophenone triplet by polystyrene in solution kq 1 +4nRDN/k 1 + (1 +4nRDN/k).jillt was found to be 1.2 x 106 unit-M- 1 s- 1 in benzene22 and 4.0 x 106 unit-M- 1 s- 1 in ace• (3) tonitrile23 at 30°C. Thus, the dynamic quench• where D is the sum of the diffusion coefficient ing of benzophenone triplet by the side-chain for the carbonyl group in benzophenone and phenyl group in polystyrene or by the main- that for the phenyl or phenylene group, R, the

520 Polymer J., Vol. 17, No. 3, 1985 Photochemistry in Polymer Solids V.

reaction radius between the reacting two ... ___ _ groups, k, the intrinsic (chemical) rate con• --- stant, and N, the Avogadro number divided by 103 . When kq is controlled by the diffusion process of the two groups (k 4nRDN), eq 3 is reduced to eq 4,

kq =4nRDN(l (4) -2 with 0 1.0 2.0 3.0 I (ms) A=4nRDN, Figure 4. Curve fitting profile of experimental data and the rate coefficient kq includes a time• ( 0) with eq 7 for benzophenone phosphorescence in dependent term important at the very early BPA-PC at -20oC. The solid line is calculated for -(t/r)-C(t/r)112 using r=2.5ms and C=2.6, and the stage of the reaction where the steady-state dotted line, for - (t/r). diffusive flux of the quenching group is not yet

attained. 0 -100 -140 ('C) The decay rate of benzophenone triplet is given by eq 5, and thus, Ia' 3 - d[ BP*]/dt = (k0 + kq[QJWBP*] (5) :§. § 10' <:(. ! so that, ± 10' [3 BP*] = [3 BP*] 0 (6) 1 2 xexp {-(k0 + A[Q])t-2B[Q]t 1 } for the concentration of benzophenone triplet,

[3 BP*), at time, t, where [3 BP*) 0 is the initial concentration of benzophenone triplet. The Figure 5. Arrhenius plots of kinetic parameter 1/r (0, e) and B (,6, .A.) for the phosphorescence decay of phosphorescence intensity, tP(t), is propor• benzophenone in polystyrene (e, .A.) and BPA-PC tional to kpT[3 BP*], so we get finally (0, ,6).

1 2 In tP(t) = - (k0 + A[Q])t-2B[Q]t 1 (7) 4. = -(t/r)-C(t/r)112 profile is shown in Figure The standard deviation was 2-3% in usual cases, but some• where, times several percent at temperatures near T . g of the matnx polymers. The temperature de- I/r=k0 +A[Q]=k0 +4nRDN[Q] (8) pendence of the parameters, 1/r and B, cal• B= Cr- 112 /(2[Q])=4R2(nD) 112 N. (9) culated using phenyl or phenylene group con• The curve fitting for the non-single-ex• centration [Q] = 10.0 M (polystyrene) and ponential phosphorescence decay with eq 7 by 9.45 M (BPA-PC) is given in Figure 5. the non-linear least-square method calculated The onset of deviation from single-ex• with a YHP 9825T desk-top computer gives ponential decay is observed in Figure 5 as the the values of r and C, and hence B from eq 9 appearance of parameter B at y-transition for each decay curve. A typical curve fitting temperature, TY, corresponding to the re• laxation of the phenyl or phenylene group

Polymer J., Vol. 17, No. 3, 1985 521 K. HoRIE et a/.

rotation in polystyrene11 and in BPA-PC. 19 rescence, but the Arrhenius plot of the lifetime The nature of the molecular motion concern• of pyrene phosphorescence in PBA-PC26 ing the y-relaxation of BPA-PC at - 1oooc is showed a marked break at 150°C correspond• related to the motion of the monomer unit as a ing to the Tg of BPA-PC.19 Other breaks whole with a rapid phenylene ring rotation, 19 observed at - 20°C for B of polystyrene and though this is still open to discussion. 24•25 The at 100 and 20oc for 1/r and B of BPA-PC in kinetic parameter, 1/r, is almost constant over Figure 5 would correspond to the {3-transition a wide range of the low temperature region by temperatures due to the local mode relaxation as much as 120-130° below each Tg, and has of the main chain. 27 The {3-relaxation in poly• almost the same value for polystyrene and carbonate has been explained by packing de• 19 BPA-PC. This suggests that 4nRDN[Q] k0 in fects in the glassy state or by local chain• eq 8, so that in this temperature range. backbone motion depending widely on anneal• 28 The lifetime, r0 = 1/ k0 , of the benzophenone ing and/or drawing conditions of the sample. triplet is 4.2 ms at 80 K for these polymers. The Thus, it should be noted that benzophenone

r0 was 3. 7 ms in polystyrene and 2.9 ms in and other phosphorescent chromophores can BPA-PC at - 20°C due to a slight increase in act as new molecular probes for detecting the the non-radiative deactivation (kiT). The onset sub-glass transitions of polycarbonate and of the increase in 1/r suggests that 4nRDN[Q] other polymers which changed considerably by

becomes larger than k0 in a higher temperature annealing and/or drawing of the samples. A range. The approximation of preliminary result in the case of benzophenone in PMMA was shown previously. 1 The cross•

over of kq from the diffusion-controlled to an is supported by the fact that the activation activation-controlled value observed in the energies, E, for 1/r (41--46 kJ mol- 1) are case ofpoly(methyl acrylate? was not noted in almost twice as large as E for B (20-22 kJ the present cases because of the much larger mol- 1) in the temperature range of Tg- intrinsic rate constant, k, for quenching by the 120o < T < Tg. The glass transition tempera• phenyl or phenylene group than k for quench• ture, Tg, was well observed as a break at 100°C ing by the ester group. The values of Tg, Tp, in the Arrhenius plot of 1/r of polystyrene and TY for these polymers are summarized in (Figure 5) due to change in the temperature Table I. dependence of D from below to above Tg. In A combination of eq 9 and 10 gives the case of BP A-PC, measurements at several C=4R312Nlf2[QJ112 (11) points for T> Tg were difficult due to the very rapid decay of benzophenone phospho- suggesting C to be constant for T> Tg-120°.

Table I. Transition temperatures and kinetic parameters for phosphorescence decays of benzophenone in polymer matrices

D £forD T, Tp r. R ko Polymer at cm2 s- 1 kJ mol- 1 oc oc oc A -193oC at 20°C at T= r. TT•

Polystyrene -100 -20 100 5.8 2.4 X 102 4.6 x 10- 13 1.5 x 10- 11 16 40 100 BPA-PC -100 20, 100 150 5.2 2.4 X 102 1.3 x 10- 13 4.o x 10-Jo 30, 44 100 PMMA3 -40 (Tp) 40 (T•. ) 110 5.0 2.0 X 102 5.0 x 10- 14 1.5 x 10-12 29 (T < T •. ) 40 (T•. < T < T•) 131

522 Polymer J., VoL 17, No.3, 1985 Photochemistry in Polymer Solids V.

200 •c to those for PMMA at the same temperatures

10-10 inspite of the higher Tg than that of PMMA. This is another indication of a large free 10-11 volume density of polycarbonate,29 which is

consistent with its high impact strength. The 'E values of D and R for these polymers along -" 10-13 with the activation energy E for D in each Cl 10"" region are listed in Table I. It should be noted that D in the present paper is defined as the 10"5 translational diffusion coefficient for the react• lo·• ing functional groups but not for the . 2 3 5 7 The diffusion process at temperatures below Tg 1/T (10"' K"' J probably arises from rotation of the ben• Figure 6. Arrhenius plots of diffusion coefficient, D, of interacting groups calculated from 1/r (0, e) and B zophenone molecule and segmental motion (6, A) for dynamic quenching of benzophenone triplet within a few monomer units of matrix poly• by the phenyl group in polystyrene ( e, A) and by the mers, but not by the mass diffusion over long phenylene group in BPA-PC (0, 6). The value of D distance. Nevertheless it may be said that a for PMMA is shown by the dotted line. comparison of D values for various polymers will provide new clues for estimating the The average reaction radius, R, for the quen• mobility and reactivity of small or ching of benzophenone triplet by the phenyl or groups in polymer matrices. phenylene group was determined from eq 11 to In conclusion, the non-single-exponential be 5.8A for polystyrene and 5.2A for BPA• decay curves for benzophenone phosphore• PC. The values of 5-6 A for the reaction radius scence in polystyrene and BPA-PC for the between functional groups are reasonable for temperature range of TY < T < Tg were ana• the triplet quenching through exciplex for• lyzed from the effects of a time-dependent mation. transient term in the diffusion-controlled in• The diffusion coefficient, D, for the reacting termolecular rate coefficient, kq, for the carbonyl group and phenyl or phenylene quenching of the benzophenone triplet by group was calculated from eq 9 and 10 to• phenyl or phenylene group of the matrix gether with the average value of R given above, polymers. The resulting kinetic parameters and is shown against 1/T in Figure 6. The such as 1/r, B, and D well reflect molecular breaks corresponding to Tg at lOOoC and Tp at motion, glass transition, and other second• - 20°C were observed for polystyrene, and ary transitions of the matrix polymers. two breaks corresponding Tp at 100 and 20°C for BPA-PC. The temperature dependence of Acknowledgement. This work was sup• D obtained for the phosphorescence decay of ported in part by a Grant-in-Aid for Scientific benzophenone in PMMA3 is also shown in Research (No. 57550561) from the Ministry of Figure 6 by the dotted line. The ten times Education, Science, and Cultur.e of Japan. higher value of D at 20°C in polystyrene than in PMMA accounts for the disappearance of REFERENCES the phosphorescence spectra of benzophenone at room temperature in Figure 1 on changing I. K. Horie and I. Mita, Chern. Phys. Lett., 93, 61 the matrix polymer from PMMA to poly• (1982). 2. K. Horie, K. Morishita, and I. Mita, Kobunshi styrene. BPA-PC showed very high D values Ronbunshu, 40, 217 (1983). for the temperature range below Tg compared 3. K. Horie, K. Morishita, and I. Mita, Macro-

PolymerJ., Vol. 17, No.3, 1985 523 K. HORIE et a/.

molecules, 17, 1746 (1984). 18. M. W. Wolf, K. D. Legg, R. E. Brown, L.A. Singer, 4. K. Horie and I. Mita, Eur. Polym. J., 20, 1037 (1984). and J. H. Parks, J. Am. Chern. Soc., 97, 4490 (1975). 5. I. M. Fraser, J. R. MacCallum, and K. T. Moran, 19. A. F. Yee and S. A. Smith, Macromolecules, 14, 54 Eur. Polym. J., 20, 425 (1984). (1981). 6. C. W. Frank and L. A. Harrah, J. Chern. Phys., 61, 20. W. H. Melhuish, Trans. Faraday Soc., 62, 3384 1526 (1974). (1966). 7. H. ltagaki, K. Horie, and I. Mita, Eur. Polym. J., 19, 21. M. W. Wolf, R. E. Brown, and L.A. Singer, J. Am. 1201 (1983). Chern. Soc., 99, 526 (1977). 8. G. E. Johnson, Macromolecules, 13, 145, 839 (1980). 22. I. Mita, T. Takagi, K. Horie, and Y. Shindo, 9. S. N. Semerak and C. W. Frank, Macromolecules, 14, Macromolecules, 17, 2256 (1984). 443 (1981). 23. K. Horie, T. Takagi, I. Mita, Y. Shindo, H. Sato, and 10. R. 0. Loutfy, Macromolecules, 16, 678 (1983). Y. Tanaka, Polym. J., 16, 887 (1984). II. A. C. Somersall, E. Dan, and J. E. Guillet, 24. A. A. Jones, J. F. O'Gara, P. T. Inglefield, J. T. Macromolecules, 7, 233 (1974). Bendler, A. F. Yee, and K. L. Ngai, Macromolecules, 12. L. J. Miller and A.M. North, J. Chern. Soc., Faraday 16, 658 (1983). Trans. 2, 71, 1233 (1975). 25. P. Tekely and E. Turska, Polymer, 24, 667 (1983). 13. R. D. Burkhart and A. A. Abla, J. Phys. Chern., 86, 26. K. Horie, M. Tsukamoto, and I. Mita, Preprints, 1st 468 (1982). SPSJ International Polymer Conference, Kyoto, 14. A. N. Jassim, J. R. MacCallum, and T. M. Shepherd, 1984. Eur. Polym. J., 17, 125 (1981). 27. G. E. Robert and E. F. T. White, "The Physics of 15. N. Tsubakiyama, S. Sasaki, S. Hiraki, and C. Glassy Polymers," R. H. Haward, Ed., Applied Kujirai, Kobunshi Ronbunshu, 31, 629 (1974), Science Publishers, London, 1973, p 153. 16. A. Gupta, A. Rembaum, and J. Moacanin, Macro• 28. M. Kochi, T. Sasaki, and H. Kambe, Polym. J., 10, molecules, 11, 1285 (1978). 169 (1978). 17. A. Gupta, R. Liang, 1. Moacanin, R. Goldbeck, and 29. B. D. Malhotra and R. A. Pethrick, Eur. Polym. J., D. Kliger, Macromolecules, 13, 262 (1980). 19, 457 (1983).

524 Polymer J., Vol. 17, No. 3, 1985.