Polymer Durability - American Chemical Society

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Downloaded by PENNSYLVANIA STATE UNIV on January 27, 2016 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/ba-1996-0249.ch014

Luminescence from Nile Red in Poly(n-butyl methacrylate) G. D. Mendenhall Department of Chemistry, Michigan Technological University, Houghton, MI 49931

Films of poly(n-butyl

methacrylate)

containing the fluorescent dye Nile

Red displayed both spontaneous and stimulated luminescence emission. Brief exposure of the dyed film to fluorescent light resulted in a delayed luminescence that decayed in a time scale of minutes and whose inten sity was reduced to negligible levels when the film was first cooled to 0 °C. The conventional fluorescence lifetime of the dye in the

polymer

matrix, 5 ns, differed little from the value of 4 ns in ethyl acetate so lution. The temperature-dependence

(4-35 °C) of the lifetime in both

media was small. The dyed film also showed spontaneous

luminescence

that was ascribed to reactions of benzoyl peroxide present as residual initiator and was greatly reduced by prior purification of the polymer. When Nile Red was added to unpurified played

spontaneous

luminescence

with the tendency of the solvent to

solvents, the solutions dis

intensities

that roughly

correlated

autoxidize.

HAVE DESCRIBED A DELAYED LUMINESCENCE that appears on a time

scale of minutes to days after brief exposure of a large (20 cm ) surface area of both polymeric and nonpolymeric solids to visible (1, 2), U V (2), or ionizing radiation (3). This light emission constitutes the slowest of all observable, stimulated luminescent processes in the material. In nearly every instance in which we have been able to observe this delayed luminescence, it decayed with a hyperbolic rather than exponential law, that is I(t) *** at~ , over a given decade. In this equation, I is the rate of luminescence emission, α is a constant, and η is usually around 1.0. This rate law is characteristic of a charge-recom bination processes (4), although much more complicated decay forms have been derived (5). 2

n

0065-2393/96/0249-0213$12.00/0 © 1996 American Chemical Society In Polymer Durability; Clough, Roger L., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1996.

POLYMER DURABILITY

214

Billingham et al. (6) and Mendenhall and Guo (7) described large changes in delayed luminescence from two-component epoxy systems undergoing cure. In an attempt to examine the relation between the luminescence and physical changes in a polymeric system, we have investigated the effect of temperature on the recombination luminescence from a dye, Nile Red, dispersed in a polymer with a glass transition temperature (T ) close to ambient temperature. This dye is highly fluorescent, dissolves in a variety of solvents, and displays the additional useful feature that the position and intensity of the fluorescence maximum are highly solvent-dependent (8). If a diffusive recombination of charges were involved in the delayed emission from such a system, we hoped that any changes in the rate of diffusion above and below T would be reflected in large changes in the rate of luminescence decay.


g

g

Experimental Poly(n-butyl methacrylate) was obtained from DuPont as Elvacite grade 2044 and had a stated weight-average molecular weight of 337K, a number-average molec ular weight of 95.9K, and a P D 3.51. The polymer, which smelled strongly of nbutanol, was purified by precipitation from reagent acetone (5.6 mL/g polymer) into reagent methanol (26 mL/g polymer). The spongy mass of polymer was col lected and pressed between paper towels to remove as much liquid as possible. The precipitation was repeated twice more, and the final odorless product was dried overnight on a vacuum fine at 1.3 Pa. The precipitations would presumably deplete lower molecular weight fractions of the polymer in addition to small mol ecule impurities, but this point was not examined. The Nile Red (1; Polysciences, Inc.) was a crystalhne material that was used as received. Both purified and unpurified polymer were made into films by dissolving 0.12 g of polymer in 4.0 m L of 7.5 Χ 10~ M ethyl acetate in Nile Red in a glass scintillation vial with diameter 2.5 cm, followed by slow evaporation while covered with a beaker at room temperature in a hood. The film from the purified polymer was then covered with a 0.5-cm layer of mercury to act as a thermal ballast and to increase the efficiency of the stimulation and detection. The film was calculated to have an average thickness of 0.25 mm and a dye concentration of 0.002 m. Steady-state luminescence from the films was measured with a thermostatted Turner Designs, Inc., Luminometer (20-s delay, 60-s measurement). Decay curves were measured in the Luminometer after stimulating the films for 10 s by resting them on a light box consisting of white fluorescent lamps and a plastic diffusion 5

1

In Polymer Durability; Clough, Roger L., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1996.

14.

MENDENHALL

ChemiluminescencefromNile Red

215

plate. The luminescence up to 4 min after the end of the stimulation period was monitored periodically (5-s delay, 10-s measurement), although in most cases only the initial data points were sufficiently intense. The data were fitted to the em pirical equation


ln(Z - bkg) = b - n\n(t) where the background (bkg) is the luminescence from the film before it was irradiated, and t is in minutes from the end of the irradiation period to the mid point of the luminescence measurement. Values of chemiluminescence in the ta bles are unfiltered readings direcdy from the digital display of the instrument. Fluorescence emission spectra were determined from ethyl acetate solutions, 1.3 X 10~ M in Nile Red, with a Spex Fluorolog, and fluorescence lifetimes were measured with the same solutions in an apparatus described elsewhere (9). The fluorescence measurements of the polymer containing Nile Red were conducted with a portion of the same film (from purified polymer) from which the delayed emission was measured. The dynamic probe measurements were made on a PerMn-Elmer Series 7 Thermal Analysis System. 6

Results and Discussion Delayed Luminescence from P o l y m e r - D y e Mixtures. In qualitative experiments we observed no luminescence from solid, unpurified poly(n-butyl methacrylate), but a relatively strong spontaneous luminescence was observed when the polymer was dissolved in ethyl acetate containing Nile Red. The luminescence from the film obtained from the Nile Red solution upon evaporation of the ethyl acetate was much stronger than from the orig inal solution and was further increased by a factor of 4 after exposure of the film to fluorescent fight. However, the intensity of the spontaneous lumines cence in the film declined within a few days at room temperature, and the color of the film changed from red to yellow during this time. When the experiments were repeated with purified polymer, the back ground luminescence of a film containing Nile Red was very small, and stim ulation of the film with fluorescent light gave very weak decay curves whose initial values were fitted individually to the empirical equation ln(I — bkg) = A + nln[t(mm)] and r = —0.985 to —0.999. Plots of η showed considerable scatter and re vealed no significant change over the temperature range of the study (Figure 1). O n the other hand, the calculated intensities at 1.0 min (= A) and the background emission were temperature-dependent, and Figure 1 reveals a slight discontinuity at 24 ± 2 °C. Physical Property Measurements. The discontinuity in the value of A (Figure 1) with temperature seemed to correspond to a small inflection


216

POLYMER DURABILITY


2

Φ ** Φ Ε m

CL

Figure 1. Values of η (Μ), Α (φ), and l„ (•) obtained from the initial portion of delayed emission from 0.002 m Nile Red in poly(n-butyl methacrylate). Values correspond to the equation l(t) — At~" + l . ss

in the differential scanning calorimetry curve at 23 ± 2 °C that we observed in the same film. However, measurement of the loss tangent with a dynamic stress technique gave a more clearly defined T of 32 ± 2 °C (Figure 2). Values of T are recognized to be dependent on the method of measurement (10); therefore, the differing numerical values are not necessarily inconsistent. g

g

Photophysical Measurements. The fluorescence characteristics of Nile Red in ethyl acetate solution and dispersed in the solid polymer film were determined by conventional measurements with steady-state and pulsed methods. The summed fluorescence decays were fitted to single-exponential decay over several half-lives. The results are summarized in Table I. The fluorescence lifetime was about 20% longer and the emission maxi mum about 10-nm shorter in the polymer than in ethyl acetate. There ap peared to be a slight shift in the emission to longer wavelengths in ethyl acetate as the temperature was lowered. However, no distinct differences are associated with temperatures above and below T of the polymeric medium. Therefore, the much stronger temperature dependence of the delayed emisg


MENDENHALL

217

Chemiluminescence from Nile Red


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POLYMER DURABILITY

218

Table I. Fluorescence of Nile Red Medium EtAc


Polymer

Temperature (°C) 0 4 6.5 20 24 0 4 10 20 40.2

Lifetime (ns)

NOTE: — is not determined. Fluorescence decay measurements with X Steady-state experiments.

a

a

4.4

—

13

1.8 1.9

(nmf

—

1.8

594 594 586, 589 590

— —

1.0 1.0 1.1

582 582 580

—

4.0 5.3 5.0 4.8 exc

Relative Intensity

—

—

= 400 or 480 nm. Estimated error is ±0.1 ns.

h

sion from Nile Red in poly(n-butyl methacrylate) are not due to unusual prop erties of the fluorescent state of the dye. Attempts to measure the activation energy of the spontaneous chemilu minescence were discontinued when it was found that when the film was cooled and then warmed to the original temperature, the chemiluminescence level measured at the original temperature was not restored. Experiments with Nile R e d and Benzoyl Peroxide. Steenken (II) and others (12, 13) reported chemiluminescent reactions between ben zoyl and related peroxides and aromatic amines. Because Nile Red has two amine functionalities, it was not too surprising that a solution 0.084 m M in Nile Red in freshly distilled ethyl acetate luminesced strongly when benzoyl peroxide was added to give an initial concentration of 3.2 m M . However, the time dependence of the luminescence was not simple (Table II). Experiments with Nile R e d and Solvents. Additional control ex periments revealed that reagent-grade ethyl acetate gave easily measured, spontaneous chemiluminescence with dissolved Nile Red, even though the solvent did not give a positive test for peroxides with KI-starch paper. How ever, solvent freshly distilled from N a B H was minimally chemiluminescent when Nile Red was dissolved in it. Because peroxidic impurities were presumably responsible for the spon taneous luminescence of the dye both in polymers and in solution, could the dye, added as a solution to organic media, serve as a sensitive peroxide indi cator? To address this question, 2-mL portions (by pipet) of a stock solution of Nile Red (0.29 mM) in benzene were treated with an equal volume of various organic liquids, and the resulting chemiluminescence was recorded for three successive 20-s readings. Except for technical-grade methyl ethyl ketone, 4


14. MENDENHALL


219

Table II. Luminescence from Nile Red and Benzoyl Peroxide


Time (h) 0 3 8 19 28 45

Relative CL 0.586 2.39 1.22 1.11 0.90 1.06

NOTE: Ambient temperature and concentrations described in text. Luminescence reading before addition of peroxide was 0.004.

Successive Readings

Figure 3. Initial chemiluminescence readings from mixtures of 2.0 mL of 0.2 mM Nile Red in benzene with 2.0 mL of various liquids at ambient temperatu



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POLYMER DURABILITY

all of the samples were reagent grade but were not purified. The results are shown in Figure 3. Except for diehloromethane, the relative values of the luminescence correspond generally to the propensity of the solvents to un dergo autoxidation. The luminescence from the solution of Nile Red and the sample of benzaldehyde, taken from an old bottle that had been stored at room temperature in air, was so intense that it tripped the safety shutter of the instrument during the third reading. Although the experiments with solvents were convenient to carry out, it may be possible to characterize the state of oxidation of polymers by addition of Nile Red dissolved in a compatible solvent. Under these conditions the intensity of spontaneous chemiluminescence may reflect peroxide content, whereas shifts in the fluorescent wavelengths (8) may reflect the accumulation of oxidation products whose polarity differs from that of the starting polymer.

Conclusion The prompt emission (fluorescence) from Nile Red in poly(n-butyl methacrylate) changes little on lowering the temperature through T . The intensity of the delayed emission from the same system, whose mechanism was not established, is temperature dependent, but the changes near T are so small that this approach for this polymer does less well than conventional methods. The spontaneous luminescence from the dye-polymer combination is in ferred to arise largely from impurities, including benzoyl peroxide present as residual polymerization initiator. The luminescence induced by Nile Red may be useful as a qualitative means to characterize the degree of oxidation of organic media. g

g

Acknowledgment The fluorescence lifetimes were measured in the laboratory of T. W . Wilson, Harvard University. I also thank George Chemparathy for carrying out the dynamic probe experiment.

References 1. Mendenhall, G. D.; Agarwal, H . K. J. Appl. Polym. Sci. 1987, 33, 1259-1274. 2. Guo, X.; Mendenhall, G. D. Chem. Phys. Lett. 1988,152, 146-150. 3. Hu, X.; Mendenhall, G. D. In Radiation Effects on Polymers; Clough, R. L.; Shalaby, S. W., Eds.; ACS Symposium Series No. 475; American Chemical Society: Washington, DC, 1990; pp 534-553. 4. Debye, P.; Edwards, J. O . J. Chem. Phys. 1952, 20, 236-239. 5. Tachiya, M . Int. J. Radiat.Appl.Instrument. C Radiât. Phys. Chem. 1987, 30, 75-81. 6. Billingham, N. C.; Burdon, J. W.; Kozielski, Κ. Α.; George, G. A.Makromol.Chem. 1989, 190, 3285.


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MENDENHALL


221

7. 8. 9. 10.

Mendenhall, G. D. Angew. Chem. Int. Ed. Eng. 1990, 29, 362-373 (Figure 8). Greenspan, P.; Mayer, E. P.; Fowler, S. D. J. Cell.Biol.1985, 100, 965-973. Wilson, T.; Frye, S. L.; Halpern, A. M. J. Am. Chem. Soc. 1984, 106, 3600-3606. Seymour, R. B.; Carraher, C. E., Jr. Polymer Chemistry, 2nd ed.; Dekker: New York, 1988; p 32. 11. Steenken, S. Photochem.Photobiol.1970, 11, 279-283. 12. Matisova-Rychla, L. Rychly, J.; Lazar, M . J. Luminesc. 1972, 5, 269-276. 13. Schuster, G. B. Acc. Chem. Res. 1979,12, 366-373.


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RECEIVED for review January 26, 1994. ACCEPTED revised manuscript January 10, 1995.


Polymer Durability - American Chemical Society

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