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Rates of Decay of Phosphorescence from Triphenylene in Acrylic Polymers
by F. C. Unterleitner and E. I. Hormats Quantum Physics Laboratory, General Dynumics/Electronics, Rochester, New York (Received December 28, 1964)
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The rates of decay of phosphorescence from triphenylene molecules dissolved in polymethyl methacrylate and polybutyl acrylate have been studied between - 120 and 100”. In both materials the decay is found to be nonexponential and independent of pump intensity, implying a distribution of sites within the polymer. The knee in the temperature dependence of integrated phosphorescence intensity from polybutyl acrylate coincides with the glass-rubber phase transition. Evidence for the existence of glassy regions in the rubber phase is presented.
Introduction The luminescent properties of aromatic organic materials dissolved in transparent plastics have been used to study energy transfer,l lifetimes of triplet states,2 and molecular polarization3 of aromatic compounds. This luminescence may also be used to study properties of the plast,ic matrix itself; for example, the rate of diffusion of oxygen in polymethyl methacrylate has been measured by use of oxygen quenching a t this l a b ~ r a t o r yand , ~ the internal structure of polymer solutions has been studied by fluorescence polarization.6 This paper indicates the possibility of utilizing the integrated intensity of phosphorescence and the time dependence of phosphorescence decay rate after short flash excitation to obtain information on the temperature dependence of polymer microstructure. The intensity and decay time of phosphorescence decrease markedly as the matrix turns from a glass to a viscous liquid.6 Since this change from glass to viscous liquid is somewhat analogous to the change in a polymer from a hard form to a rubbery one, it was thought that the change in intensity might be used as a measure for the glass transition temperature, Tg. Many temperature-dependent properties of polymers undergo abrupt changes and discontinuities a t Tg. Most of these are macroscopic in nature, such as the density, elasticity, or gas permeability, and show only a gradual change in the vicinity of Tg. Thus it was hoped that a property. depending on the internal structure, such as the phosphorescence quenching, might give a more serviceable and accurate measurement of the glass transition temperature of polymers. The Journal of Physical Chemistry
With this in mind, an investigation into the decay time and intensity of phosphorescent organic compounds above and below the glass transition was undertaken. Triphenylene was chosen as the indicator since it has a relatively long decay time, has quite intense phosphorescence, and is soluble in monomeric materials. Experimental measurements have been made using twoacrylic polymers that are similar chemicallybut have quite different physical properties. One is polymethyl methacrylate (PMMA) which has been used extensively. The second is polybutyl acrylate (PBA) which was chosen because it has a low glass transition temperature of -56”
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Experimental Materials. The methyl methacrylate and butyl acrylate monomers (Rohm and Haas) were inhibited by MEHQ and contained other trace impurities which produced strong phosphorescence in the polymer, especially a t low temperatures. The monomers were therefore pursed by successive washings with 5% sodium nitrite, 5% sodium bisulfite, 5% sodium hydroxide, and three with distilled water. The mono(1) K.B. Eisenthal, J. Chem. Phya., 39, 2108 (1963). (2) W.H.Melhuiah and R. Hardwick, Tram. Furaduy SOC.,58, 1908 (1962). (3) M. A. El-Sayed, J . Opt. SOC.Am., 53,797 (1963). (4)E.Hormata and F. Unterleitner, to be published. (5) Y. Nishijims, “Lumineacence of Organic and Inorganic Material~,’’John WiIey and Sons, Ino., New York, N. y., 1962,p. 236. (6) G. Porter and L. J. Stiet, Nature, 195, 991 (1962). (7) F. Bueche, “Physical Properties of Polymers,” Interscience Publishers, Inc., New York, N. Y., 1962,p. 110,Table 4.
RATESOF DECAY OF PHOSPHORESCENCE FROM TRIPHENYLENE IN ACRYLIC POLYMERS
mers were then dried over Drierite and vacuum distilled. These materials, when outgassed and polymerized with a,a-azodiisobutyronitrile (Azo) catalyst, had barely detectable phosphorescence even a t 77°K. For the samples reported here, 0.10% w./w. triphenylene and 0.033% w./w. Azo were dissolved in the methyl methacrylate and 0.098% w./w. triphenylene and 0.011% w./w. Azo were dissolved in the butyl acrylate. The monomer solutions were then transferred to 8-mm. Pyrex tubes, sealed to a vacuum rack, and outgassed by several cycles of freezing with liquid nitrogen and warming under vacuum. The tubes were then fiIIed with dry nitJrogen and sealed off. Experiments on oxygen diffusion in similar polymethyl methacrylate samples have been carried out in this laboratory and the results indicate that this procedure is very effective in removing .molecular oxygen from the samples, perhaps because the polymerization process assists in scavenging the residual molecular oxygen. The samples were slowly polymerized in order to ensure uniform polymerization; the temperature cycle started at 45" and final cure was 1 day at 110". The PBA was quite rubbery with a sticky surface when cooled and removed from the Pyrex tube. Upon removal from the tubes, test samples were cut to 5-em. length and immediately placed in the test cell with a dry nitrogen atmosphere. Transition Temperature. The glass transition temperature of the PBA sample used for the phosphorescent measurementsi was measured by dilatometry using denatured 95% alcohol as the working fluid. This was done to determine whether the phosphor acted as a plasticizer or if any unpolymerized material remained to plasticize the material. The glass transition temperature, as determined by the break in the volume-temperature plot, was found to be -55", in excellent agreement with Bueche.' Phosphorescence Measurements. The phosphorescent spectrum emitted by these samples was measured with a phosphoroscope having a 1-msec. delay time. It consists of a strong peak centered at 21,500 ern.-' at room temperature with about seven subsidiary peaks. A slight blue shift and considerable sharpening of the spectrum occurs as the temperature is lowered to 77°K. The spectrum correlates well with that reported for triphenylene in EPA glass at 77°K. by Clar and Zander.8 Since no measurable phosphorescent intensity was noted aside from the triphenylene peaks, decay time measurements were made without a monochromattor in order to increase the intensity range available for the measurements. Plastic samples were placed in a quartz tube sample holder (Figure 1) and were flash-pumped with a xenon flashlamp having
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Sample in Teflon Support Elliptical.Reflector
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Figure 1. Flash apparatus.
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Figure 2. Intensity of phosphorescence from triphenylene in polymethyl methacrylate as a function of time after pump flash a t three temperatures.
(8) E. Clar and M . Zander, Chem. Ber., 89, 749 (1966).
Volume 69, Number 8 August 1966
F. C. UNTERLEITNER AND E. I. HORMATS
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10. c
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Figure 3. Rate constants for triphenylene in polymethyl methacrylate a t three temperatures.
a 100-psec. flash duration. The total phosphorescence was measured by an EM1 6256B photomultiplier without intervening optical elements, aside from the quartz windows of the cooling cell and a shutter. Care was taken not to exceed amp. anode current in the photomultiplier, as recommended by the manufacturer for stable linear operation, and peak luminous flux was maintained below photocathode saturation levels. Photomultiplier current was recorded on an Offner strip chart recorder except for times less than 0.1 sec. after the flash, for which oscilloscope photographs were used. The sample was cooled with cold gaseous nitrogen obtained by evaporation of liquid nitrogen. The rate of gas flow and hence the sample temperature could be controlled by the electrical power passed through a 100-w. heater in the liquid nitrogen dewar. Tank nitrogen passed over a hot nichrome ribbon was used to heat the sample above room temperature. Again control of the gas flow and the power input to the heater controlled the sample temperature. The sample temperature was measured by a copper-constantan thermocouple, held onto the surface of the sample by a strip of Mylar tape, and a Rubicon portable potentiomet er. The Journal of Physical Chemistry
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Figure 4. Intensity of phosphorescence from triphenylene in polybutyl acrylate a8 a function of time after pump flash a t four temperatures.
Results The intensity of phosphorescence as a function of time after flash is plotted in Figure 2 for three different temperatures of PMMA containing 0.1% triphenylene by weight. Within the error of the measurements the initial intensity, extrapolated to t = 0, is the same for all temperatures up to 80", implying that efficiency of internal conversion into triplet excitation is independent of temperature over this temperature range. We define the instantaneous rate constant k ( t ) by the relation
k(t) =
d
- dt - In (I(t)jIo)
which reduces to the normal rate constant for the case of exponential decay. The rate constants for the three intensity curves shown in Figure 2 are plotted in r'1' g ure 3. The indicated k, values were obtained by plotting k(t) vs. l / t and extrapolating to zero. It is clear that even a t -120" there is still appreciable departure from exponential decay. The glass-rubber transition in PMMA takes place a t about 105",' so that all of these decay curves are representative of the glass phase of the material.
RATESOF DECAYOF PHOSPHORESCENCE F'ROM TRIPHENYLENE IN ACRYLIC POLYMERS
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Figure 5. Rate constants for triphenylene in polybutyl acrylate a t four temperatures.
The phosphorescence intensity after flash of PBA with 0.10% w./w. triphenylene is shown in Figure 4. It is evident that the intensity drops off rapidly with temperature above T , = -55". The instantaneous rate constants derived from these data are shown in Figure 5. Well below T, the curves match those for PMMA quite closely. At -57" (-T,) the rate has a plateau considerably above the rate for PMMA, and a t -45" (lo" above Tg)the plateau is still noticeable, but it is a t a much higher value of rate constant. By -14" the plateau may have shifted to rate constants too short to be observed in these experiments, and only the residual slow decay is observed. At room temperature, k ( t ) us. t is almost identical with that in PMMA a t the same temperature for 0.5 sec. < t < 5 sec., but the intensity is down by about five orders of magnitude. If the integrated phosphorescent emission is plotted as a function of temperature, Figure 6, there is a knee in the PBA curve which corresponds within experimental error to the glass transition temperature. If this can be shown to be generally valid, it could prove to be a relatively simple method for determining T,. The dependence of the rate constant on time after the pump flash proved to be quite reproducible for different samples and pump flash energies. The independence of flash energy, as clearly shown in Figure
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Figure 7. Fbte constants for triphenylene and polymethyl methacrylate at two different flash energies.
7, indicates that the effect is not due to the interaction between molecules in the excited state, which has been observed in crystalline and liquid environments, and is strongly dependent on triplet state concentration (that is, pump intensity). The most reasonable explanation appears to be that there is a distribution of sites in the polymer in which the triphenylene (or other Volum 69,Number 8 August 1966
JERRY GOODISMAN
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phosphor) molecules may be located and that these sites differ in local vibrational properties and hence in thermal quenching coefficient. Although this view does not seem to conflict with any of the experimental evidence presented, the relationship between the thermal quenching of phosphorescence and the polymer environment does not appear to be understood on a theoretical level ; therefore comparisons with theory cannot be made.
Conclusions Several conclusions can be drawn from these data without further analysis. The time dependence of decay rate in PBA in the rubbery phase is seen to have a plateau of uniform rate followed by a tail of varying rate extending to the limits of observability. The existence of a plateau in k ( t ) implies that there are a large number of sites having the same rate constant, if the statistical interpretation of the previous section is
valid. The “tail” coincides almost perfectly with that observed in PMMA at corresponding temperatures, implying that glass-like regions continue to exist in the rubbery phase. A comparison of residual intensity leads to the conclusion that perhaps part by volume of PBA maintains the glassy phase microstructure at 20”. The temperature dependence of rate constants in PMMA clearly indicates that considerable variations in microstructure exist even at - 120°,but more work needs to be done to clarify the nature of the interaction which leads to nonradiative quenching of the triplet state in such an environment before specific deductions based on these observations can be made. Acknowledgments. The authors wish to acknowledge the many helpful discussions with Dr. Ernest Brock and Dr. Jack Taylor, and the valuable assistance of Mr. Kermit Mercer with the experimental work.
Scaling in Isoelectronic Molecules
by Jerry Goodisman’ Department of Chemistry, Univeraity of Illinois, Urbana, Illinois 61803
(Received December $0, 1964)
A modification of a scaling method introduced for atoms by Ellison enables one to use expectation values calculated for one molecule in calculations of the energy of a second molecule isoelectronic to the first. In going from Hz to Hez2+,the results are only fair, but in going from LiF to BeO, the results are sufEciently good to allow prediction of equilibrium distance and several expectation values as well as energy. The dipole moment is a notable exception, which reveals one basic dissimilarity between the two molecules, the ionic character. LiF dissociates to ions, Be0 to neutral atoms, causing our method to break down a t large internuclear distance. The inverse scaling transformation, from Be0 to LiF, is also accomplished, with similar results.
In an a priori molecular calculation, physical intuition may help in choosing the form for a trial wave function for variation, but one is very rarely in the position of being able to use the wave function for one System for a calculation on another, however closely they are related physically. For atoms, one can use The Journal of Physical ChemiEtry
such fundamental theorems as the virial and HellmanFeynman theorems to elucidate relations between wave functions for various different systems.2 Ellison (1) Part of research supported by National Science Foundation. (2) For example, P.-0.~ a w d i nJ, . MOLS p d r y . , 3,46 (1969).
