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

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Photochemistry in Polymer Solids V. Decay of Benzophenone Phosphorescence in Polystyrene and in Polycarbonate 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 fluorescence 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 photochemistry 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 light 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 laser (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 Luminescence 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 oxygen 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.
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