438
Langmuir 1988, 4, 438-445
where c is the velocity of light, and &/dQ is the average of the first derivative of the polarizability a of methane with respect to the normal coordinate Q of the vibration. If we adopt the literature valuez2for (da/dQ, Le., 2.27 X 10-ls cm2.g-1/2.p/2(Ris Avogadro’s number), values of E summarized in Table I1 are obtained. It must be pointed out that these E values are of a reasonable order of magnitude compared with the values (1.6-2.6 X lo5 esu) reported for Na+ in the NaA zeolite.12 It is also important to recognize that the field strength E decreases according to the sequence Li+ > Na+ > K+ > Rb+ > Cs+. This proves the consistency of the present discussion about q,:, vl, and A. Finally, we discuss the chemical potential difference Ap20. As already mentioned, the slope and the intercept at the ordinate of the straight line shown in Figure 7 give the value of KlK2. It must be pointed out that K1(4.105 X Pa-’ at 226 K) obtained from the adsorption data for silic&te* can approximately be regarded to be common
for every adsorption systems studied. Thus we can readily evaluate K 2 and the corresponding Ap20 value for each adsorption system. Table I1 also summarizes the values of K 2 and ApzO. The chemical potential difference Ap20 is expected to have an intimate relation to the heat of adsorption qs:. Since the enthalpy of adsorption of methane onto the cationic sites from the silicalite-like site can be approximated by AHo = -(q,O - 20.92 kJ/mol, we expected that information about the corresponding entropy of adsorption ASo would be obtainable by comparing Ap20 with q,:. Unfortunately, however, as we can see in Figure 8, the difference between Aw20 and qs: - 20.92 is too small to be discussed; i.e., a theoretical entropy analysis appears meaningless. Thus we can obtain little information about the difference between the motional state of methane over the silicalite-like site and that over the cationic site. It is only suggested that the motional-state difference mentioned above would not be large.
(22) Kahn,R.; Cohen de Lara, E.; MBller, D. J.Chem. Phys. 1986,83, 2653.
Registry No. CHI, 74-82-8; Li, 7439-93-2; Na, 7440-23-5;K, 7440-09-7; Rb, 7440-17-7.
Oxygen Quenching Studies of Nonaqueous Dispersions of Poly(viny1 acetate) Labeled with Phenanthrene Groups’ Luke S. Egan and Mitchell A. Winnik* Department of Chemistry and Erindale College, University of Toronto, Toronto, Ontario, Canada M5S 1 A l
Melvin D. Croucher Xerox Research Center of Canada, 2660 Speakman Dr., Mississauga, Ontario, Canada L5K 2Ll Received January 20, 1987. I n Final Form: August 7, 1987 This paper describes experiments on nonaqueous dispersions of poly(viny1acetate) (PVAc) particles sterically stabilized with poly(Zethylhexy1methacrylate) (PEHMA). Particles were prepared with trace amounts of phenanthrene (Phe) groups covalently bound to either the PVAc core phase or the PEHMA stabilizer. Phe fluorescence was very similar in degasaed samples of the two materials, but strong differences were noted in the presence of oxygen. Detailed oxygen fluorescencequenching studies provide information about particle morphology, about swelling of the stabilizer phase by the dispersion medium, particularly for PEHMA trapped inside the particle, and, most importantly, about the PEHMA-PVAc interface. In the presence of the dispersion medium (here cyclohexane),the interface is very diffuse: the presence of 7 w t % of PEHMA in the particle transforms nearly half the PVAc component into a phase swollen with cyclohexane and extremely permeable to oxygen.
Introduction In a previous paper2 we described the synthesis and characterization of nonaqueous dispersions (NADs) of poly(viny1acetate) (PVAc) particles sterically stabilized with poly(2-ethylhexyl methacrylate) (PEHMA) in which fluorescent groups were covalently attached to either the PVAc or PEHMA molecules. In these materials, PVAc is the major component, representing 90+ % by weight of the ca. 300-nm-diameter particles. I t is insoluble in the hydrocarbon media in which the particles are colloidally dispersed and is referred to as the “core polymer”. The stabilizer, PEHMA, comprises the rest of the material. It (1) Luminescence Studies of Polymer Colloids 17. For the previous paper in this series, see: Winnik, M. A.; Egan, L. S.; Tencer, M.; Croucher, M. D. Polymer 1987,28, 1553-1560. (2) Egan, L. S.; Winnik, M. A.; Croucher,M. D. J . Polym. Sci., Polym. Chem. Ed. 1986,24, 1895-1913.
is very soluble in hydrocarbons. A surface shell of PEHMA, grafted to the PVAc, confers colloidal stability to the particles. As our studies indicate, the remainder of the PEHMA is buried in the particle interior. Our objective is to use fluorescence quenching experiments to elucidate the morphology of the particles, which we take to be a prototype typical of other NAD systems of industrial importance. LPhe
phe
SLP
0743-746318812404-0438$01.50/0 0 1988 American Chemical Society
CLP
Langmuir, Vol. 4 , No. 2, 1988 439
Oxygen Quenching Studies of Poly(uiny1 acetate)
Table I. Reagent Quantities (g/mL in Isooctane) Used in the Synthesis of Core- and Stabilizer-Labeled PVAc Particles and a PVAc-PheMMA CopolymerD stabilizer lEHMAl lAIBNl lPheMMAl St-UL 0.250 0.0038 O.ooO0 St-Phe 0.254 0.0038 0.0014 colloidb [stabilizer] [BPI [VAI 0.0027 0.497 ULC 0.133 0.456 0.133 0.0024 SLP-Phe CLP-Phec 0.154 0.0032 0.538 I
F i g u r e 1. ‘Microdomain” model for the morphology of PVAc particles sterically stabilized with PEHMA.
In this report we examine the fluorescence behavior of particles labeled with phenanthrene (Phe) groups, particularly the sensitivity of Phe fluorescence to quenching by oxygen. The samples in which Phe is linked to the PEHMA are called “stabilizer-labeled” particles and are denoted SLP. Those in which Phe is bound to PVAc are called “core-labeled”particles and are denoted CLP. Phe serves as a fluorescent sensor of events occurring selectively within the PEHMA and PVAc phases of the NAD. In order to facilitate data interpretation, these properties are compared to those of five separate model systems: phenanthrylmethyl pivalate and Phe-labeled free stabilizer (St-Phe) in cyclohexane solution; dried powders of SLP and CLP, and a film of PVAc containing covalently bound Phe groups. Phenanthrene was chosen as the sensor because of its reluctance to form excimers, a factor which promotes simple (exponential) fluorescence decay behavior in polymeric system^.^ This feature is particularly useful in fluorescence quenching studies, where the differences in the ability of certain solutes to reach different portions of a structure induce nonexponential components in the fluorescence decay curve. Analysis of these fluorescence decays (I@))in conjunction with steady-state data provides very powerful insights into the morphology and dynamics within the labeled regions within the particle^.^ The fluorescence quenching experiments show very pronounced differences between the homogeneous solutions of St-Phe and phenanthrylmethyl pivalate on one hand and of SLP particles on the other. They demonstrate that a significant portion of the stabilizer chains is trapped inside the particle core where the Phe groups are less accessible to quenching by oxygen. This result is a consequence of the detailed nature of the anchoring mechanism for the stabilizer chains. Quenching data for dispersions and powdered samples of CLP particles were compared as well. We find that a large fraction of the Phe groups (ca. 50%) in CLP particles is readily quenched by oxygen when the particles are dispersed in cyclohexane; the remaining Phe fraction is protected from oxygen. The addition to the system of alcohols that swell the PVAc phase of the particles allows the Phe groups in the protected fraction to be quenched. We propose that the accessible population of Phe labels is located in a solvent swollen “interphase” layer surrounding dense PVAc regions which contain buried Phe groups. These results are inconsistent with a simple core-shell model of particle stru~ture.~ An~ interpenetrating network model with (3) (a) Holden, D. A,; Ng, D.; Guillet, J. E. Br. Polym. J. 1982, 14, 159-162. (b) Birks, J. B.; Georghiou, S. J.Phys. B 1968, 958-965. (4) (a) Pekcan, 0.; Winnik, M. A.; Egan, L.; Croucher, M. D. Macromolecules 1983, 16, 699-702. (b) Winnik, M. A. Polym. Eng. Sci. 1984, 24, 87-97. (c) Egan, L. S.; Winnik, M. A.; Croucher, M. D. Polym. Eng. Sci. 1986, 26, 15-27.
“Copolymer [PVAc-Phe] was prepared by using 0.0107 g of PheMMA, 9.32 g of vinyl acetate, and 0.047 g of AIBN. Unlabeled (ULC) and core-labeled particles (CLP-Phe) were prepared by using unlabeled stabilizer St-UL. Stabilizer-labeled colloids (SLP-Phe) were prepared with labeled stabilizer St-Phe. The quantity of stabilizer in each synthesis corresponds to 22.6 w t % relative to the total weight of reagents added. ‘PheMMA was added to a level of 0.0002 g/mL of isooctane in the synthesis of CLP-Phe.
phase-separated domains within the particle, Figure 1, can accommodate these observations.
Experimental Section Materials. The preparation and purification of stabilizer and core-labeled nonaqueous dispersions6have been described in detail elsewhere.2 Here we present only a brief outline of the two-step synthetic method employed. SLPs were prepared as follows: the first step involved preparation of the labeled stabilizer chain. Phenanthrene groups were introduced by means of the PheMMA comonomer (1) during the free radical polymerization of EHMA. We refer to the copolymer poly[(2-ethylhexyl methacrylate)co-(9-phenanthrylmethyl methacrylate)] sample as St-Phe. By UV spectroscopy St-Phe was calculated to contain 0.41 mol % Phe groups. In the second step, vinyl acetate was polymerized in isooctane at 80 OC in the presence of St-Phe plus a free radical initiator to yield a stable dispersion of particles approximately 0.3 Mm in diameter. T o prepare CLPs, VAc was copolymerized (step 2) with 1 by using unlabeled PEHMA as stabilizer. The reagent quantities used in the preparation of CLC and SLC are listed in Table I. 0
CH3
II I
CH2-OC-C=CH2 I
1
0
CHI
II I CH20C-C-CH3 I
I
2
9-Phenanthrylmethyl methacrylate (PheMMA, 1) and 9phenanthrylmethyl pivalate (PheMP, 2) were prepared and purified according to the methods described by Ng and Guillete6 A copolymer of vinyl acetate and PheMMA (PVAc-Phe) was prepared by bulk copolymerization a t 55 OC using AIBN as initiator and the reagent quantities in Table I. The resulting copolymer (0.03 mol % PheMMA units) was dissolved in ethyl acetate, purified by precipitation (2X) into spectrograde cyclohexane, and dried in vacuo. Methods. Steady-state fluorescence spectra were run on a SPEX Fluorolog I1 spectrometer. Fluorescence intensities were determined from the area under the curves. Fluorescence decay profiles (Aex 295nm) were collected at 23 OC by using the timecorrelated single-photon counting technique (TCSPC)’ and were ( 5 ) Dispersion Polymerization in Organic Media; Barrett, K. E. J., Ed.; Wiley-Interscience: New York, 1975. (6) Ng, D.; Guillet, J. E. Macromolecules 1982, 15, 724-727. (7) OConnor, D. V.; Phillips, D. Time-Correlated Single Photon Counting; Academic: London, 1984.
Egan et al.
440 Langmuir, Vol. 4, No. 2, 1988 I
Table 11. Characteristics of Sterically Stabilized PVAc Nonaaueous Diswrsions
system ULC SLP-Phe CLP-PheC
Phe groups stabilizer in in stabilizer, particles, wt% mol % 0.00 0.41 0.00
6.5 1.9
I
I
I
I
I
I
I
I
I
molecular mean weightb ( X particle g/mol) diameter,O stabilizer/ nm colloid 295 (270) 314 (250) 247
33000/72000 40000/85000 33 000/79 000
"The distribution of' diameters is such that 90% of the particles has diameters in the range f60 nm. Values in parentheses are estimates from scanning electron micrographs. These values represent the most probable molecular weight relative to polystyrene standards, calculated from the peak maxima of differential refractometer GPC traces. The actual distributions of molecular weights for each of the colloid samples are rather broad (ca. 30002500000). "he wt % of PheMMA observed in this sample of core-labeled particles is 0.028. analyzed by employing the iterative reconvolution method.8 Phenanthrene emission was measured a t wavelengths between 345 and 395 nm by employing Corning 7-60 and Schott KV-370 filters. Due to the opacity of the colloidal samples, a component due to scattered light could not be avoided in their fluorescence decays. Nitrogen-, air-, and oxygen-saturated samples for steady-state and transient fluorescence measurements were prepared in quartz cells (1X 1cm sample chamber fused to 4 X 0.9 cm round quartz) capped with rubber septa. Cyclohexane-saturated gas was bubbled through the samples for 90 s via a long (15 cm) stainless steel syringe needle introduced into the septum caps. A second syringe needle, inserted in the septum, allowed gas to escape from the sample cell, thus maintaining the gas pressure a t ca. 1 atm. Samples having varying oxygen concentrations were prepared by adjusting the oxygen composition in the gas stream entering the cells. This was accomplished by running nitrogen and oxygen lines a t varying flow rates, in parallel, through identical gas bubblers containing equal volumes of solvent. The two gas streams were combined at a Y joint and then passed through the samples. This method was calibrated by examining the effect of oxygen concentration on the fluorescence quenching of small molecules in homogeneous solution. Powdered colloidal samples and films were prepared in 12mm-0.d. quartz tubes fitted with rubber septa. Cyclohexane dispersions were added to the tubes, and the solvent was evaporated on a rotary evaporator under a water aspirator (ca. 10 Torr) vacuum for a total of 30 min. After 10 min, a thin layer of powdered particles coated the walls of the sample tubes. The samples were placed in vacuo ( 4 . 0 mmHg) for 2 days. Prior to analysis, each of these samples was purged with air, oxygen, or argon for 30 min. A film of PVAc-Phe copolymer was prepared in the same manner as above but from a methanol solution (0.03 g/mL) of the polymer.
Results Characterizationof Composition. Alkane dispersions of stabilizer- and core-labeled PVAc particles were characterized for particle size b y using a commercial particle analyzer based upon light-scattering measurements. The particles themselves, a f t e r freeze drying, formed transparent solutions i n polar solvents such as e t h y l acetate, tetrahydrofuran, and chloroform. lH NMR studies of these materials i n CDC13 provided one measure of their composition. Chromophore content was determined b y UV absorption measurements of solutions in ethyl acetate. GPC measurements provided molecular weight estimates, (8) Demas, J. N. Excited State Lifetime Measurements; Academic: New York, 1983; Chapters 8 and 9. (9) Encyclopedia of Polymer Science and Technology;Bikales, N. M., Ed.; Wiley-Interscience: New York, 1971; Vol. 15, pp 580-584.
405 WAVELENGTH
310
500
(n m )
Figure 2. (a) Steady-state fluorescence spectra of a deoxygenated
sample of powdered SLP-Phe particles collected with front-face illumination (Aex 295 nm) at 23 "C. Insert: plot of fluorescence intensity versus time for the desorption of oxygen from a powdered sample of CLP-Phe particles (argon gas is introduced).
based upon polystyrene standards. These data are presented i n Table 11. CH3
I -(CHZ-C)~ I c=o I 0 I ~
CHz CHCHzCHa
I I CH2
CH3
I I c=o
(cH2-c)~-
I
@$ 0
I
I
CH~CHZCH~
Labeled Particles. The labeled copolymer, St-Phe, which serves as stabilizer in stabilizer-labeledparticles, has the chemical structure shown above. The extent of Phe incorporation is small (0.41 mol %, Table 11). Since the reactivity ratios of the monomers are known under these reaction conditions ( E H M A , rl = 0.5; PheMMA, r1 = 4), we can assert that the Phe groups are statistically distributed along the PEHMA chain. Since this polymer has M = 40000, there is on average only one Phe group per chain. The core-labeled particles CLP-Phe were prepared with monomers with a more serious mismatch of reactivity ratios. Here by keeping the e x t e n t of labeling very low (0.028 mol %), blocks of Phe groups,could be avoided. Such blocks give a strong nonexponential component to the Phe fluorescence decay curves. Details dn the preparation and characterization of the materials studied here are given i n Tables I and 11. The glass transition temperatures Tgof both SLP-Phe and CLP-Phe powders were found to be identical with that of a sample of pure P V A c homopolymer (lit. valuelo Tg (PVAc) = 32 "C). (10) Polymer Handbook, 2nd ed.; Brandrup J., Immergut, E. H., Eds.; Wiley-Interscience: New York, 1975; p 151 (111). (11) Birks, J. B. Photophysics of Aromatic Molecules; Wiley-Interscience: New York, 1971. (12) Tazuke, S.; Ooki, H.; Sato, K. Macromolecules 1982,15,400-406. (13) Martinho, J.; Egan, L. S.; Winnik, M. A. Anal. Chem. 1987, 59, 861-864. (14) Lackowitz, J. R. Principles of Fluorescence Spectroscopy; Plenum: New York, 1983. (15) (a) Meares, P. J.Am. Chem. SOC.1954, 76, 3415-3422. (b) See ref 10,p 237(III).
Langmuir, Vol. 4, No. 2, 1988 441 Table IV. Mean Fluorescence Decay Time" ( r o / ( r )and ) Intensity (Zo/Z) Ratios from Oxygen Quenching Studies of Cyclohexane Dispersions of SLP-Phe and CLP-Phe Particles at 23 OC (L 295 nm)
4-
>
5
condition [O,],Mb nitrogen 0 air 2.08 X IO" oxygen 10.8 X
3-
W
I-
f
a 2-
TU/(.) PIX SLP-Phe CLP-Phe SLP-Phe CLP-Phe 1 1 1 1 1.87 1.10 1.77 1.16 5.26 1.22 5.05 1.48
" Mean decay times in the absence of quencher are 43.4ns (SLP-Phe) and 42.3 ns (CLP-Phe). bConcentrationof oxygen in the cyclohexane medium.
0
J
I-
00
29
87 TIME
145 263 (nsec)
i2 f I
rO/(r)
Figure 3. Transient fluorescence decay curves of (a) phenanthrylmethyl pivalate (2 X lo4 M) and (b) SLP-Phe (5.6 mg/mL) in deoxygenated cyclohexane at 23 O C . A,, 295 nm, A, 345-395 nm. Table 111. Fluorescence Decay Time' (ro/r)and Intensity (Zo/Z) Ratios from Oxygen Quenching Studies of PheMp and St-Phe in Cyclohexane Solution at 23 OC (& 295 nm) TO/ T
condition [02], Mb nitrogen 0 2.08 X air oxygen 10.8 X
PheMp 1 2.89 11.53
Table V. h;an Fluorescence Decay Time" ( r O / ( r )and ) Intensity (Zo/Z) Ratios from Oxygen Quenching Studies of Powdered Samples of SLP-Phe, CLP-Phe, and PVAc-Phe at 23 OC (&=295 nm)
PII
St-Phe PheMp 1 1 2.16 2.94 11.16 6.15
St-Phe 1 1.96 6.47
"Decay times in the absence of quencher are 46.59 ns (PheMp) and 46.65 ns (St-Phe). bConcentration of oxygen in the cyclohexane medium.
Spectroscopy. The most prominent peak in the UV spectrum of the model compound phenanthrylmethyl pivalate (PheMP) is a sharp band at 295 nm (0 band of the La transition). It is accompanied by vibrational fine structure at 283 and 275 nm. These features are preserved with some broadening in St-Phe and in solutions of SLPPhe and CLP-Phe dissolved in ethyl acetate. The fluorescence spectra of all the Phe-labeled samples are similar in shape and closely resemble that of PheMP. An example is shown in Figure 2 for a dried powder sample of CLP-Phe. There are no indications in any of these spectra of Phe excimer emission, which would under any circumstances be unexpected: Phe was chosen for its reluctance to form excimers. Shown in Figure 3 are the fluorescence decay profiles of PheMP (2 X 10"' M) and SLP-Phe (5.6 mg/mL) in deoxygenated cyclohexane at 23 "C. The decay of PheMP is clearly exponential, with a lifetime ( T ) of 46.6 ns. The particle samples show a small additional component at short times. Part of this intensity is due to scattered light from the turbid samples and part is due to a small fraction of proximate Phe groups, which undergo self-quenching and show a lifetime of ea. 4 ns. The major, exponential component has essentially the same lifetime (here 45.7 ns for SLP-Phe) as the model compound. Oxygen Quenching Studies. Oxygen quenches Phe fluorescence, and the extent of quenching increases with O2concentration. The data are presented in Table I11 for solutions of PheMP and St-Phe in cyclohexane, in Table IV for SLP-Phe and CLP-Phe dispersions in cyclohexane, and in Table V for dried powders of SLP-Phe and CLPPhe. These data are presented in the form P / I for intensities and To/. for decay times, where the superscript O refers to the value in the absence of oxygen. For samples
condition SLP-Phe CLP-Phe argon 1 1 air 1.02 1.01 oxygen 1.16 1.04
PVACPhe 1 1.00 1.01
PII SLP-Phe CLP-Phe 1 1 1.03 1.01 1.20 1.05
'Decay times in the absence of quencher are 41.6 ns (SLP-Phe), 40.2 ns (CLP-Phe), and 43.8ns (PVAC-Phe).
with nonexpontial decays, mean decay times ( T ) were calculated (after correction for scatter) from the expression
where I(t)describes the form of the fluorescence decay. The transient decay data in Tables IV and V actually represent ratios of mean decay times ( T O ) / ( 7). The insert in Figure 2 shows the time-dependent desorption of oxygen from an air-saturated sample of powdered CLP-Phe particles at 23 OC. Notice that it requires approximately 5 min to rid the system of the oxygen quencher and achieve an equilibrium value of the fluorescence intensity. Using this method, we found equilibrium in powdered SLP-Phe particles to occur within 10-15 s. For particle dispersions, equilibrium is achieved even more rapidly. We fmd that for cyclohexane samples of PheMP, St-Phe, and SLP-Phe fluorescence quenching is very efficient. On the other hand, much smaller changes are observed for the dispersion of core-labeled particles and the SLP-Phe powder. The Phe* fluorescence in the PVAePhe film and CLP-Phe powder samples is quenched only slightly. These variations are attributed to differences in oxygen permeability in the particular phase containing the phenanthrene labels. The data are rich in information. As will be discussed below, these data provide detailed insights into the Phe label location and, hence, overall particle morphology. Note the similarity of P / I and T O / . ratios for the model systems PheMP and St-Phe (Table 111) and of P / I and ( T O ) / (7)for SLP-Phe, as well as the discrepancy of these values for CLP-Phe (Table IV). The simple Stern-Volmer model (see below) predicts identical values for P/Z and T O / . at any given quencher concentration. Differences in these values point to a breakdown of the assumptions of the model. Discussion Fluorescence Quenching by Oxygen. When samples containing fluorescent groups are exposed to air or their solutions are saturated with oxygen, the fluorescence in-
Egan et al.
442 Langmuir, Vol. 4, No. 2, 1988
tensities of the samples decrease and the rates of fluorescence decay increase. These phenomena are due to oxygen quenching the singlet excited state of the group. The mechanism of quenching involves a sequence of spin-allowed internal conversion processes, which takes place within a weakly associated encounter complex. The quenching process normally transforms the group to its singlet ground state or to its excited triplet state.16 Data generated from oxygen quenching studies on small molecules in homogeneous solution are usually analyzed by using the Stern-Volmer r e l a t i ~ n ' ~ J ~ P / I = 70/7 = 1 + k , 7 0 [ 0 2 ] (2) In this equation, IP and 7,7Oare the fluorescence intensities and lifetimes in the presence and absence of oxygen, respectively, and k, is the bimolecular quenching rate constant. This equation requires that the collection of fluorescent labels decay according to a single-exponential decay law and, moreover, that quenching interactions occw with a unique rate constant k,. From the slope of a plot of P / I (or ~ O / T ) versus [Q],k , can be determined provided that is known. Diffusion coefficients related to quenching events can be calculated from the time-independent Smoluchowski-Einstein17 equation . l r N ~ ( D f l +D,)@ k, = (3)
'I
J
12
- 0
2
0
6
1,
8
1 0 1 2
[OXYGEN] ( M x lo3)
Figure 4. Plots of T o / ? and P / I vs oxygen concentration for samples of phenanthrylmethyl pivalate (a), phenanthrene-labeled stabilizer St-Phe (b), and particles labeled in the stabilizer phase with phenanthrene SLP-Phe (c) in cyclohexane at 23 "C. 0 and A represent the steady-state and transient decay data, respectively. The transient data for SLP-Phe are plotted as To/( T ) vs [O,]. I
I
I
I
I
1
1000
where Dfland D, are the diffusion coefficients of the excited fluorophore and quencher, respectively, p is the quenching probability per collision, R is the sum of the collisional radii (Rfl+ R,), and N Ais Avogadro's number. For the phenanthrene-oxygen pair, p is unity and R is 5.4 A.18
Equations 2 and 3 can also be applied to the case of quenching of polymer-bound excited states in solution so long as the fluorescence decay is exponential and k , is single-~alued.~" A simplifying factor in the interpretation of k , is the general assumption that Dfl and transient fluorescence (TO/.) quenching studies on PheMP and dissolved stabilizer precursor St-Phe using oxygen as a quencher. The solid lines through the points represent least-squares fits to the data. The rate constant for oxygen quenching of PheMP (k, = 2.02 X 1O1O M-l s-l) was found to be about twice that for quenching of phenanthrene labels attached to St-Phe (16) Birks, J. B. Organic Molecular Photophysics;Wiley-Interscience: New York, 1975; Vol. 2, Chapter 9. (17) Rice, S. A. In Comprehemiue Chemical Kinetics;Bamford, C. H., Tipper, C. F. H., Compton, R. G., Eds.; Elsevier: New York, 1985; Vol. 25, pp 7-39. (18) Whitaker, T.; Bushaw, J. J.Phys. Chem. 1981, 85, 2180-2182.
0
6
29 TIME
263 in s e c )
Figure 5. Fluorescence decay curves of cyclohexane dispersions of SLP-Phe (10 mgiml) at 23 "C with oxygen concentrations of 0 M (a), 2.08 X 10- M (b), and 10.8 X M (c). A,, 295 nm, Aem 345-395 nm.
(1.15 X 1O1O M-ls-l). From eq 3, we calculate values of 4.7 X and 2.7 X cmz/s for the diffusion coefficients ( D ) , respectively. Curve c in Figure 4 shows the steady-state (P/r>and transient (TO/ (7))data from oxygen quenching studies on SLP-Phe, plotted according to eq 2. Note that the values of 7 O / (7)fall on the line defined by the Steady-stateresults. This system follows the Stern-Volmer equation. The x-axis here still refers to [O,] in the bulk phase. If we assume an identical oxygen concentration in the stabilizer phase (a partition coefficient of unity); we calculate from eq 2 a value of k , = 0.81 X 1O'O M-' s-l ( D = 1.9 X cm2/s). The fact that the k , value is less than that for St-Phe indicates that the oxygen concentration, or oxygen mobility, in the stabilizer domain of the particle is reduced over that of the solution as a whole. The fluorescence decay profiles in Figure 5 are from cyclohexane dispersions SLP-Phe in the presence and absence of oxygen. The initial short components (scattered light, etc.) are ignored here. The long component decay in the deoxygenated SLP-Phe sample is exponential (7 = 45.7 ns). Two things happen to this fluorescence decay curve when oxygen is introduced: (1)the slope of the long component is drastically reduced; (2) the decay profiles appear more steeply curved. The latter effect is very
Langmuir, Vol. 4, No. 2, 1988 443
Oxygen Quenching Studies of Poly(uiny1 acetate) I
I
I
I
I
I
1
5 1
I
0 0 0
40
80
'
TIME
(n
I20 sec)
'
I$O
'
Figure 6. Fluorescence decay profile of deoxygenated (a)and
oxygenated (b) cyclohexane dispersions of SLP-Phe particles measured at 23 OC, subsequent to particle synthesis. kx295 nm, A,,
40
I20
80 TIME
$0
I60
290
( n sec)
Figure 7. Fluorescence decay curves of cyclohexane dispersions of CLP-Phe (21 mg/mL) at 23 O C with oxygen concentrations identical with those shown in Figure 5.
345-395 nm.
different from that for oxygenated and deoxygenated samples of St-Phe, where the decay profiles remain exponential under all conditions. These results coupled with those from GPC analysis imply that, as a result of PVAc graft formation during particle synthesis, phenanthrene groups are not located in identical environments in SLPPhe particles. Rather, there is a distribution of Phe environments in the particles, each characterized by its own oxygen concentration and quenching rate constant. The fact that the deviations from exponential fluorescence behavior in the presence of oxygen are small implies that the distribution of environments in SLP-Phe particle dispersions is relatively narrow. The fluorescence quenching experiments described above were performed on well-aged (2 years) SLP-Phe particles. The following result provides evidence that the morphology of our PEHMA-stabilized PVAc particles evolves with time. Figure 6 shows the fluorescence decay profiles of deoxygenated and oxygenated cyclohexane dispersions of SLP-Phe particles collected shortly after the SLP-Phe particles were prepared. Compared with curve c in Figure 5, the decay in Figure 6 shows a much more distinct long component. Its lifetime is equal to 40 ns; characteristic of phenanthrene chromophores located within the PVAc core polymer and protected from oxygen. This observation (found in another similar system as well) is important because it indicates that a signifcant quantity (at least 6 % ) estimated from decay parameters) of stabilizer chains was trapped within the particle core, as predicted by the "microdomain model". The Tgof the PVAc core polymer is just above room temperature. Over time, it appears that physical aging leads to phase separation, making the stabilizer-bound Phe groups more susceptible to oxygen quenching. Core Phase. We anticipated no quenching of Phe groups in the core phase of the PEHMA-PVAc particles. From the known solubility of O2in PVAc16 and its diffusion coefficient, we calculate from eq 2 and 3 that there should be less than 1% quenching of Phe fluorescence in a PVAc matrix. We actually observe 1% quenching of this fluorescence when a film of the PVAc-Phe copolymer is exposed to 1 atm of oxygen gas (Table V). Since this is virtudy the limit of detectability of fluorescence intensity changes, our anticipation of negligible quenching is substantiated. Even when this film (ca. 90 pm thick) is allowed to sit for weeks in contact with cyclohexane, and saturated with 02,no increase in quenching is observed.
2 t c 1r C
I 100
1/
I I I 200 300 400 [OXYGEN] ( M - ' )
JQO
Figure 8. (Top, a) Stem-Volmer plot (p/Zv9 0,) for cyclohexane dispersion of CLP-Phe particles at 23 "C. (Bottom) modified - I) vs [OJ' for cyclohexane disStern-Volmer plots (P/(P persions of CLP-Phe (b) and SLP-Phe (c) particles.
Imagine our surprise when we observed that exposure to oxygen of CLP-Phe dispersions in cyclohexane led to immediate and pronounced decreases in the fluorescence intensity. Some aspect of particle morphology permits Phe groups bound to PVAc to reside in an environment of high oxygen permeability. Fluorescence decay curves from oxygepated, aerated, and argon-purged samples of CLP-Phe particles in cyclohexane at 23 OC are presented in Figure 7. The decay time of the long component for the deoxygenated sample is 45 ns. Notice that the slope of the long component is reduced only slightly with increasing oxygen concentration; for the oxygen equilibrated sample, the long component lifetime is 41 ns. These results imply that the Phe groups yielding the long-lived fluorescence are buried deep within the PVAc core polymer. This contrasts with the results from oxygen quenching of SLP-Phe particles, which indicate that the majority of stabilizer-bound Phe labels are located in solvent-swollen regions surrounding PVAc domains where they are readily accessible to oxygen. A Stern-Volmer plot of fluorescence intensities (Figure 8, top) is curved to the right: increasing increments of
Egan et al.
444 Langmuir, Vol. 4, No. 2, 1988 quencher concentrations are less effective at quenching fluorescence. This behavior is characteristic of polymer systems containing nonuniform distribution of fluorophores, with some of the dye molecules protected from quenching by their environment. A deeper insight into the actual distribution of Phe groups in the CLP-Phe particles can be obtained from consideration of the data in Figure 8. If we assume a model in which only a fraction f a of the Phe groups is readily quenchable by oxygen and 1 - f a are protected, the Stern-Volmer equation can be rewritten in the formlg
- P- -
1
+ -1
50% R-OH
(4)
P - 1 fakq.O[Q] fa A plot of P / ( P - I) versus [quencherl-' will yield a linear
plot with l/faas the y-intercept. Furthermore, from the slope of the plot, (fak,TO)-', the quenching rate constant for the accessible population can be calculated. The data for SLP-Phe (Figure 4)and CLP-Phe plotted according to eq 4 are compared in the bottom portion of Figure 8. For SLP-Phe particles, f a is unity and the magnitude of Iz, calculated from the slope (curve b in Figure 8) is equal to that obtained from Stern-Volmer analysis, as expected. Of importance is the fact that the oxygen quenching data for CLP-Phe particles yields a linear plot when analyzed by eq 4. This supports our assumption that for CLP-Phe particles in cyclohexane populations of buried and accessible phenanthrene groups do indeed exist. The y-intercept for curve b in Figure 8 (CLP-Phe particles) is approximately equal to 2 (f, = 0.5). In other words, in the dispersed state, half of the phenanthrene groups are accessible to oxygen and half are buried. The quenching rate constant ( k , ) for the accessible fraction in CLP-Phe particles (0.4 X 1O1O M-' s-l, presuming a partition coefficient of unity) is found to be only a factor of 2 less than that for quenching of phenanthrene groups bound to SLP particles. Therefore, the accessible PVAc-bound Phe groups in CLP-Phe colloids appear to be located in a solvent-swollen environment similar to that in which the stabilizer chains are located. Model of Particle Morphology. GPC data indicate that all stabilizer chains are covalently grafted to the PVAc core polymer. Fluorescence quenching data indicate that 50% of the phenanthrene groups in CLP-Phe particles are located in a solvent-swollen environment similar to that of the stabilizer phase. Taken together, these results imply that the particles contain a substantial solvent-swollen interphase region consisting of both PEHMA and PVAc chains. In addition, the data suggest that a significant quantity of stabilizer chains reside within the interior of the particles.4b We propose, then, that the interphase region exists within the core of the particles and provides a boundary between the pure PEHMA (solvent swollen) and PVAc phases. According to this model, the buried Phe population in CLP-Phe particles would be located within the pure PVAc domains, in accord with DSC results.2 Using the value f a = 0.5 for CLP-Phe, we estimate the interphase to comprise up to 50% of the total volume of the polymer particles. Quenching the Buried Fluorophores: Particle Swelling. Oxygen quenching experiments at 23 OC on CLP-Phe particles dispersed in mixtures of cyclohexane and simple alcohols were performed. Figure 9 shows plots of P / I vs [oxygen]for cyclohexane dispersions of CLP-Phe
OL
A
h 4 6
Ib
b
h 4 &
[Oz] ( M x 1 0 3 )
1'0
IC
Figure 9. Plots of P / I vs [O,]for samples of CLP-Phe particles at 23 "C. Curves a, b, and c are for dispersions in mixtures of
95% cyclohexane/5% methanol, ethanol, and propanol, respectively (0.86 mL of alcohol/g of particles). Curves a', b', and c' are for dispersions in mixtures of 50% cyclohexane/50% methanol, ethanol, and propanol, respectively (15.6 mL of alcohol/g of particles). Curve d is for CLP-Phe particles in the absence of added alcohol.
particles in the absence of additives (d) and in the presence of added methanol (a), ethanol (b), and propanol (c). Fluorescence quenching is enhanced over that shown in Figure 7 (curve d in Figure 9), even when methanol is present at a level of only 5% v/v. Of equal importance is the fact that a linear Stern-Volmer plot is obtained when methanol is added. A linear Stern-Volmer plot implies a value of fa = 1. Thus, as a result of swelling of the PVAc phase by methanol, the previously "buried" phenanthrenes in CLP-Phe become accessible to quenching by oxygen. The plots (Figure 9) for CLP-Phe samples analyzed in the presence of 5% v/v ethanol (b) and propanol (c) are similar in shape and in intensity to that obtained in the absence of added alcohol. The P / I values indicate a much lower degree of particle swelling relative to samples containing methanol. Ethanol swells the particles less than methanol but to a greater extent than propanol. The fact that the Stern-Volmer plots are c w e d indicates that the degree of swelling is sufficiently low so as to prevent significant quenching of the buried Phe fraction. However, in the presence of 50% v/v ethanol and propanol, linear plots of P/I w [oxygen] (b' and c' in Figure 9) are obtained. Films and Powders. When powder samples of SLCPhe and CLC-Phe are exposed to oxygen, the amount of fluorescence quenching is much less than in the case of the dispersions (Table V).20 For example, at 1 atm 02,P / I = 5.0 for the stabilizer-labeled dispersion (80% of the fluorescence intensity is quenched) and 1.16 for the dry powder. For CLP-Phe we find P / I = 1.2 for the dispersion and 1.05 (5% quenching) for the powder. These results emphasize the role of the external medium on quenching processes that occur in the particle interior. We note also that oxygen permeability in the dried stabilizer phase is higher than in the core of dried CLP-Phe particles. Moreover, the permeability of oxygen in the powdered particles is greater than in the PVAc-Phe film sample. This latter effect would be due to the perturbation imposed by the stabilizer chains on the PVAc phase.
Summary Fluorescence quenching studies on labeled polymers represent a new method to obtain information about ~
(19) (a) Lehrer, S.S.Biochemistry 1971,10,3254-3259. (b)Miyashita, T.; Ohsawa, M.; Matsuda, M. MacromoZecuZes 1986, 19,585-588.
(20)The extent of quenching here is too small for any curvature in the Stern-Volmer plots to be detected.
Langmuir 1988,4,445-448 solvent penetration and about interface structure. The method depends upon the fact that oxygen solubilityvaries among different polymers, and among different phases in a multicomponent polymer material, and its diffusion coefficient is also very sensitive to ita environment. Oxygen quenching depends upon both factors, and such experiments are useful for studying oxygen permeability in specific phase of a complex material. Here we examine nonaqueous dispersions of PVAc particles containing 7 wt Ti PEHMA as the steric stabilizer. Much of the stabilizer is in fact trapped within the particle core. When the PEHMA chains are labeled with Phe (SLP-Phe) and cyclohexane dispersions are exposed to air or oxygen, fluorescence quenching is extensive and follows a simple Stern-Volmer model. These results indicate substantial swelling of the PEHMA phase, even that in the particle interior. While some differences in the degree of swelling are inferred from the distribution of oxygen quenching rates, this distribution is rather narrow. The dispersion medium promotes oxygen permeability in the PEHMA phase. Fluorescence from the dried particles shows much less sensitivity to oxygen. Studies of the particles labeled in the PVAc phase (CLP-Phe) provide new insights into the behavior of the PVAc-PEHMA interface in the presence or absence of the
445
dispersion medium. PEHMA promotes quenching of some of the Phe groups in the PVAc phase. For dried samples this fraction is small, and the experiments are limited by the restricted permeability of O2 in PEHMA. In the presence of cyclohexane the interface is swollen into an extensive interphase and comprises nearly half of the PVAc present. The fact that such a small amount of PEHMA can have such a profound effect on the PVAc phase is remarkable. The ultimate objective of this research is twofold: to map out in detail the morphology of these materials and to understand the mechanism of their formation. The oxygen quenching experiments reported here take us a step further along the path toward our goals. In addition, they provide a tool which shows promise for the study of other types of polymer blends.
Acknowledgment. We thank NSERC Canada and the donors of the Petroleum Research Fund, administered by the American Chemical Society, for their support of this research. Registry No. (EHMa)(VAc) (PheMMA)(copolymer), 112655-57-9; (PheMMA)(VAc)(copolymer), 112655-58-0; (EHMA)(VAc)(copolymer),30815-04-4; PheMP, 81558-09-0; (EHMA)(PheMMA)(copolymer), 105288-31-1; 02,7782-44-7.
Thermal Evolution of Acetylene Adsorbed on Pt( 11 1) Neil R.Avery CSIRO Division of Materials Science and Technology, Locked Bag 33, Clayton, Victoria 3168, Australia Received July 29, 1987. In Final Form: October 10, 1987 The adsorption and thermal reactivity of C2H2and C2Dzon a Pt(ll1) surface have been studied by high-resolution electron energy loss (EEL) and thermal desorption (TD) spectroscopies. At
