Langmuir 1989, 5. 903-910 adsorbate-induced peak positions with those of unsaturated C4 hydrocarbons (Le., cis- and trans-butene), which are also shown in Figure 4,18 suggests that the surface species that arises by warming the surface to 200 K corresponds to trans-2-butene. It should be emphasized that the adsorbate-induced peak positions do not yield such good correspondence to any other Cz, Cs, or C4 hydrocarbons and that the gas-phase spectra have been rigidly shifted to obtain the agreement shown in Figure 4. Since thermal desorption spectroscopy indicates that 200 K is below the temperature a t which surface species dehydrogenate, it is proposed that the change in the spectrum is due to a thermal transformation of the surface species rather than self-hydrogenation of the butadiene. Finally, the inelastic cutoff in the spectrum in Figure 4 is a t 16.8 eV, corresponding to a work function increase of -0.1 eV compared to the clean surface. In this case, the surface work function differs only slightly from that of the clean metal, indicating a lower extent of electron transfer from the substrate to the adsorbate than in the case of molecular butadiene. The transformation of chemisorbed butadiene to the trans-butene-like species is thermally induced and therefore slightly activated. The observation of such a thermal transformation is in agreement with the calculations of Baetz01d.l~ These suggest that there should be a considerable strengthening of the middle C-C bond with a concomitant weakening of the terminal C-C bonds of butadiene on chemisorption, especially for chemisorption on metals toward the center of the periodic table. This prediction is entirely consistent with the experimental observations since, in contrast to butadiene, in butenes the bond order of the middle carbon
903
is larger than for the terminal carbons. 5. Conclusions The results of our spectroscopic investigation of the low-temperature chemisorption of butadiene on Mo( 100) are substantially in agreement with the results of a tight-binding calculation by Baetz01d.l~ That is, the occupied orbitals of butadiene are only slightly perturbed when it is chemisorbed on a metal toward the center of the transition series (characterized by Nd = 5.0). Bonding to the surface is by electron donation from the metal to the adsorbed molecule since chemisorption is accompanied by a work function increase. The molecular plane is tilted a t -40' to the surface following chemisorption of butadiene on Mo(100) at 120 K. Warming a butadiene-covered surface to 200 K results in the thermal transformation of butadiene to a different species having a photoelectron spectrum that corresponds well to that of gas-phase trans-2-butene. This observation is in line with theoretical predictions which suggest that the middle C-C bond of butadiene should be strengthened relative to terminal C-C bonds on chemisorption.
Acknowledgment. We thank the donors of the Petroleum Research Fund, administered by the American Chemical Society, and the University of Wisconsin-Milwaukee Graduate School for their support. We are grateful for an Atlantic Richfield Foundation Grant of the Research Corporation. We are also grateful to the personnel of NSLS, particularly Dr. M. Sagurton, for their assistance during these experiments. Registry No. Mo, 7439-98-7; butadiene, 106-99-0.
Dispersion Polymerization of Styrene in Polar Solvents. Characterization of Stabilizer in Ordinary and Precipitated Particles by Fluorescence Quenching Frangoise M. Winnik* and Anthony J. Paine Xerox Research Centre of Canada, Mississauga, Ontario, Canada L5K 2L1 Received October 25, 1988. I n Final Form: February 6, 1989 Fluorescence-quenching experiments were performed on methanolic and aqueous suspensions of dispersion-polymerized polystyrene (PS) particles stabilized with pyrene-labeled (hydroxypropy1)cellulose (HPC). The results were compared to those obtained from novel precipitated particles, prepared by dissolving the original dispersion-polymerized particles in dioxane and precipitating with controlled addition of methanol. For particles dispersed in methanol, Stern-Volmer plots of I o / I vs [quencher] were nonlinear for quenchers ethylpyridinium bromide (EPB) and N,N-dimethylaniline (DMA),saturating at a value of I o / I consistent with an inaccessible fraction of fluorophores. With a modified Stern-Volmer model, the accessible fraction was calculated to be 0.5 for EPB quenching and 0.65-0.75 for DMA quenching. Remarkably similar results were obtained from the original and precipitated particles. These results support a morphological model with most of the steric stabilizer (Le., the accessible fraction of fluorophores) located in a thin layer of grafted HPC-PS on the particle surface in both kinds of particles. For particles dispersed in water, quenching by DMA was shown to involve significant swelling of the particles by DMA, with solution-based quenching being dominant. Introduction During dispersion polymerization, an initially homogeneous reaction mixture containing monomer, initiator, solvent, and steric stabilizer reacts to form polymer particles 0.1-15 pm in diameter, often of excellent monodispersity. The dispersion polymerization of styrene in polar solvents has been the object of a detailed mechanistic investigation in this laboratory.'V2 One aspect of this work 0743-7463/89/2405-0903$01.50/0
addressed issues concerning the role of the stabilizer in dispersion polymerization.' Focussing on one steric stabilizer, (hYdroxsropy1)ceUulose(HPC), demonstrated that (1) Ober, C. K.; Lok, K. P.; Hair, M. L. Poly. Sci., Polym. Lett. Ed.
1985,23,103. Lek, K. P.; Ober, C. K. Can. J. Chem. 1985,63,209. Ober, C. K.; Lok, K. P. Macromolecules 1987, 20, 268. (2) (a) Deslandes, Y.; Gerroir, P.; Harris, J.; Henrissat, B.; Paine, A.
J. Manuscript in preparation. Paine, A. J. Manuscript in preparation.
0 1989 American Chemical Society
Winnik and Paine
904 Langmuir, Vol. 5 , No. 4, 1989
particles consisting of a copolymer of p-styrenesulfonate and dodecyl methacrylate.' Alternatively, the fluorescent tag may be attached to the stabilizer, as described in the case of poly(viny1 acetate) particles stabilized with phenanthrene-labeled poly(2-ethylhexyl methacrylate)? This paper presents fluorescence data on polystyrene particles prepared in the presence of a labeled stabilizer. The specific latices studied here are (first) ordinary particles prepared in the presence of pyrene-labeled (hydroxypropy1)cellulose (HPC-Py) and (second) novel particles prepared by dissolving the original dispersionpolymerized particles in dioxane and precipitating by controlled addition of methanol. Steady-state and timeresolved fluorescence data are reported for the two types of particles. The accessibility of the pyrenes of the labeled particles to quenchers of fluorescence was examined. The results of quenching experiments corroborate and refine the particle morphological characteristics that were inferred from the direct observation of particles by electron microscopy.
Experimental Section Reagents. (Hydroxypmpyl)ceUdcme (HPC, Klucel L, nominal
H
0.5 microns
Figure 1. Transmission electron micrograph of a crose section of particles (LpPy) embedded in Araldite 6026 resin after s t a i n i with uranyl acetate. the true steric stabilizer is graft HPC-styrene copolymer synthesized in situ during dispersion polymerization. Thii confirmed an earlier suggesti0n.l This stabilizer was detected on the surface of the particles by examination of electron micrographs of microtomed particles stained with HPCspecific reagents (Figure 1).2" It was not possible, however, to determine if this surface layer accounted for all the stabilizer incorporated in the particles or if part of the stabilizer was buried in the core of the particles. In an attempt to confirm the transmission electron microscopy results and to investigate the nature of the stabilizer anchoring, we have prepared fluorescently labeled particles and examined their fluorescence spectroscopy in different environments. Fluorescence-labeling experiments require that a dye be introduced in one component of a system. In selecting a labeling strategy, one has to take into consideration the chemistry of the material investigated and the ultimate goal of the study, as discussed in detail in recent reviews?,' In the case of colloidal polymeric particles, a fluorescent label may be attached either to the main polymer or to the stabilizer. Specific examples of particles carrying the label on the main polymer include nonaqueous dispersions of poly(methy1 methacrylate) particles and poly(viny1 acetate) particles5" and aqueous dispersions of negatively charged (3) Winnik, M. A. In Photophysical and Photochemical Toob in Polymer Science; Winnik, M.A,, Ed.; D. Reidel: Dordreeht. Holland, 1986, pp 611-627. 14) Winnik, M.A. In Polymer Surfoeos ond Inlerfoees; Feast. W. J.. Munro. H.S.. Eds.; Wiley: Chiehester. England. 1987, pp 1-31. ( 5 ) &an. L. S.; Winnik. M. A,; Cmueher. M. D.Longmuir 1988,4.438. (6) Winnik. M. A,: Eean. L. S.:Owens. S. M.:Ottewill. R. H.Anal. Chim. Acto 1986. l89,8?I
mol wt. 1OOOOO) was purchased from Aldrich. Ethanol and methoxyethanol were reagent grade solvents. For spectroscopic measurements. spectral grade solvents were used. Styrene and benzoyl peroxide (Aldrich) were used without purification after it was established that purification had no impact on the particle size or molecular weight. Water was deionized with a Millipore MiUiQ water purification system. NJV-Dimethylaniline(DMA, Fisher, w e n t ) was purified hy distillation. Sodium iodide (Fisher Scientific) and ethylpyridinium bromide (EPB, Eastman Kodak Chemicals)were used without purification. Pyrene-labeled HPC was available at two pyrene contents: a higher level loading with one pyrene molecule per 56 glucose units (designated as HPCPy/56)and a lower level loading with one pyrene per 216 glucose units (HPC-Py/216); they are characterized as described elsehere.^,^
Instrumentation. Molecular weights were measured with a Hewlett Packard 1090 GPC using THF as solvent (1.0 mL min-') and HP PL gel columna The instrument was calibrated by using moncdiiperse polystyrene standards from h u r e Chemical HP 1037A refractive index and HP E451 diode array UV-vis detectors were both employed and separately calibrated. Particle size and size distributions were measured by using a 256-channel Coulter Multisizer with 3@pm aperture tube. UV-vis absorption spectra were recorded with a Hewlett Packard 8450A diode array spectrometer. Fluorescence spectra were run on a SPEX Fluorolog 212 spectrometer equipped with a DM3000F data system. Fluorescence decay mesurements were performed with a homebuilt instrument in the laboratory of Professor M. A. Winnik (University of Toronto, Toronto). This instrument has been described in detail elsewhere.'" Ordinary Labeled Particles. A 2 3 3 " round-bottom flask equipped with a mechanical stirrer, a thermometer, and a nitrogen inlet was placed in a thermostated bath kept at 71.5 'C. It was charged with HPC (1.03 g). HPC-Py/% (0.482 g), ethanol (55.0 mL), and methoxyethanol (30.0 mL). The mixture was stirred for 1.5 h to ensure complete dissolution of HPC. A freshly prepared solution of BPO (0.315 g) in styrene (15. 0 mL) was added all at once. The reaction mixture was stirred for 41 h. After that time, the flask was removed from the heating bath and methanol (100 mL) was added. The polymeric particles were separated from the cooled mixture by centrifugation (loo00 rpm, 15 min) and washed twice with methanol (150 mL) and once with water (125 mL). (7) Miyashita, T.: Ohsava, M.:Malsuda, M. Moemmoloeules 1988,19, 585. (8) Winnik. F. M.: Winnik, M. A,; Tazuke. S.: Ober, C. K. M o e m muieculea 1987.m.38. 191 Winnik. F. M. Macromolecules lY87.m.2745. 110) Martmhu. J.; Egan. L. S.; Winnik M. A. A w l . Chom. 1987.59. 861.
Dispersion Polymerization of Styrene in Polar Solvents
~
Table I. Physical Data for the Latex Particle Samples in This Study LP-PV LP-Pv-W LP-Pv-PPT 2.9 (1.39) 4.5 (2.20) particle diameter 2.4 (1.48) (GSD)," Irm M , (MWD) of total 65000 65 000 sample (2.87) (2.77) 6.0 X 4.9 X loT7 4.2 X [Py], mol g-' particle wt % HPC-Py/56 1.10 0.89 0.86
Langmuir, Vol. 5, No. 4, 1989 905 on
on
I
I ocn2cncnJ ocn2CncHJ t n 2 on
OCH2CHCH J
I
~~
The particle size distribution (PSD) of ordinary particles contains several overlapping populations, whereas the PSDs of precipitated particles contain only one very broad population. The GSDs [(dS4/dl6)'/*]are reported for cumulative volume distributions. Approximately half the polymeric material was suspended in water and isolated by freeze-drying (sample LP-Py), while the remaining material was subjected to additional washes with methanol (3 times) and water (3 times) before isolation by freeze-drying (sample LP-Py-W). The total amount of particles isolated was 10.7 g (79%). Size and molecular weight measurements are reported in Table I. The amount of HPC-Py incorporated in the particles was determined by UV absorption of solutions of the polymer in T H F , using 4-pyrenylbutanol" (c = 45000 L mol-' cm-' a t 344 nm in T H F ) as the standard. The concentrations of pyrene and HPC-Py in samples LP-Py and LP-Py-W are reported in Table I. Unlabeled particles (4.3 pm, GSD 1.59) were prepared under the same conditions. Precipitation of Labeled Particles. A sample of LP-Py-W (1.00 g) in dioxane (20.0 mL) was stirred overnight in a sealed 500-mL Erlenmeyer flask. Under vigorous stirring, methanol was added to the dioxane solution with a peristaltic pump a t a rate of 22 mL min-' for 16 min, to give a final volume of ca. 350 mL. The resulting latex was stirred an additional 30 min, centrifuged, washed with methanol (2 X 100 mL), filtered through a 20-fim nylon mesh, washed with water (2 X 100 mL), and freeze-dried, to give precipitated labeled particles, designated LP-Py-PPT (0.60 g; 60% yield). The size, size distribution, and pyrene contents are reported in Table I. Fluorescence Measurements. The excitation and emission band paths were set a t 1.8 nm unless otherwise indicated. The excitation wavelength was 345 nm. Spectra were recorded a t 25 "C. The samples were stirred magnetically during scanning to prevent settling of the particles. Emission spectra were not corrected. Fluorescence intensities were calculated by integration of the emission spectrum (in wavelength units) in ranges specified for each quencher. For turbid samples, a background spectrum was subtracted before integration to remove the contribution due to scattering. This spectrum was obtained by measuring under identical conditions the scattering of suspensions of nonfluorescent polystyrene particles of similar size. Aqueous solutions of HPC-Py were prepared 24 h before the measurements to allow complete dissolution of the polymer. Particles were dispersed in water or methanol by 10-min sonication to make dispersions. In most cases, the particle concentration was 1.0 g L-'. For quenching experiments, a weighed amount of a solution of quencher in methanol or water was added to aliquots of the polymeric suspensions (2.0 or 4.0 mL). The concentrations of the quencher solutions were such that the change in volume due to the addition of quencher did not exceed 4%. Quenchers were also added to similarly prepared, but unlabeled, suspensions for measurements of background spectra due to scattering. Suspensions in methanol were degassed by vigorous bubbling of methanol-saturated argon through the solution for 1 min immediately prior to the measurements. After it had been ascertained that pyrene emission intensities were unaffected by degassing in water, aqueous suspensions were not degassed. Quencher Partition Measurements. 1. In Water (DMA). T o suspensions of latex particles (0.31-3.76 g L-l) in water (5.0 mL) was added a known amount of methanolic DMA (0.1 M). The suspensions were kept in capped vials a t room temperature for the desired equilibration time. The particles were separated (11) Schnieders, C.;
1801.
Miillen, K.; Huber, W. Tetrahedron 1984, 40,
n
n 0CnZ:nCn3
bn
yn2
ocn2cncn3 I
ocn2cncnJ I OH
Figure 2. Idealized structure of pyrene-labeled (hydroxypropy1)cellulose (HPC-Py/56). by centrifugation (15 "C, 6000 rpm). The concentration of DMA in the supernatant, [DMAIs, was determined spectrophotochemically (c = 850 L mol-' cm-' a t 298 nm). From the experimental [DMA], value and the initial concentration of DMA in each suspension, [DMAIo, the concentration of DMA in the polystyrene phase, [DMA],, was calculated and expressed in moles of DMA per unit weight of the polymeric phase (taken as the s u m of the weights of polystyrene and DMA) according to the equation
where n is the concentration of DMA in the polymer phase (that is, DMA missing from the supernatant [DMA], - [DMAIs; in mol L-'), g is the weight of the polymeric phase in g, and M = 121 is the molecular weight of DMA. 2. In Methanol (DMA a n d EPB). The concentration of quencher in the supernatant was determined spectrophotochemically following the procedure devised for DMA partitioning in aqueous latices. Within experimental error, this concentration was identical with the initial quencher concentration.
Results Latex Particle Preparation and Characterization. The labeled steric stabilizer employed in this study is a pyrene-labeled (hydroxypropy1)cellulose ( H P C - P Y / ~ ~ ) . ~ " In this polymer, pyrene groups are attached at random to the polymer via butoxy linkages (Figure 2). The polystyrene particles were prepared by free-radical dispersion polymerization of styrene in ethanol/methoxyethanol using 11 wt % steric stabilizer comprised of a 2:l mixture of HPC and HPC-Py/56. A t the end of the polymerization, the particles were isolated by centrifugation and washed with water and methanol to remove excess stabilizer. To test the effectiveness of the washes, the sample was divided into two portions after the normal three washes, and approximately half the sample was subjected to twice as much further washing, as described in the Experimental Section. This gave rise to two samples, herein referred to as LP-Py and LP-Py-W, respectively. The level of pyrene incorporation in the particles determined by UV spectroscopy of solutions of polymer dissolved in T H F is reported in Table I. It can be noted that the most vigorous washing of the particles resulted in a detectable but small decrease of the pyrene concentration in the particles. At the same time, the particle size increased slightly, possibly due to loss of fine material in the six extra washing steps. Polystyrene particles were also prepared by precipitation of LP-Py-W from dioxane, with methanol. This procedure generated particles with very slightly lower pyrene content, higher average particle size, and wider size distribution than the original particles from which they were prepared (Table I). Fluorescence Spectra of Labeled Latex Particles. 1. Steady-State Fluorescence. Pyrene fluorescence spectra were measured from particles suspended in water and in methanol and from polymer solutions in tetra-
906 Langmuir, Vol. 5, No. 4 , 1989
Winnik and Paine Table 11. Fluorescence Decay Parameterso
latex LP-Py-w
LP-Py-PPT HPC-Py/56
solvent water methanol water methanol water methanol
ns 63 64 67 61 34 19
71.
"Intensities were fit to a sum of exponentials: I ( t ) = data. The latex concentration was 1.0 g L-l.
a1
72, ns
a2
X2
0.44 0.42 0.52 0.49 0.30 0.27
154 167 159 189 115 114
0.56 0.58 0.48 0.52 0.70 0.83
1.12 1.01 0.95 1.02 1.07 1.05
Eai exp(-t/ri)
(7),
ns
131 145 130 159 106 108
The angular brackets indicate mean lifetimes calculated from the
Table 111. Quenching Data of HPC-Py/216 in quencher solvent KSV: L mol-' ( T ) " , ns DMA water 322 f 5 106 DMA methanol 992 f 8 108 EPB methanol 868 16 108
*
Solution k,, s-l mol-' 3.0 x 109 9.2 x 109 8.0 x 109
Pyrene "monomer" emission.
WAVELENGTH (nm)
Figure 3. Fluorescence spectra of HPC-Py/56 in water and of the latex particles LP-Py dispersed in water; the polymer concentration was 100 ppm and the latex concentration was 1.0 g L-l. A,, = 345 nm. The spectra were normalized at the wavelength of maximum emission.
hydrofuran. They were compared to those of solutions of the stabilizer (HPC-Py/56) in the same solvents (Figure 3). The spectra of the latter feature a well-resolved emission of intensity ZM due to excited isolated pyrenes ("monomer" emission) with the [O,O] band at 376 nm and a broad featureless emission of intensity ZE centered a t 490-495 nm due to pyrene excimers. The ratio of the two emission intensities (ZE/ZM) is low in T H F (
a. Ordinary Particles
[DMA]:
0
370
450
400
500
540
WAVELENGTH (nm) b. Precipitated Particles
Figure 5. Fluorescence emission spectra of the latex particles LP-Py dispersed in water (1.0 g L-l) with increasing amounts of DMA; Xext = 345 nm.
-- I
.
0
,
I
I
5
10
15
[WENCHER]
(lo3 mol
I -m-
Time0
.-o-.
2 Hours
--A--
2 Days
..-A-..
3 Days
I
L-l)
Figure 4. Ratio Io/I of fluorescence intensity in the absence and presence of quencher as a function of quencher concentration (DMA or EPB) in suspensions of the particles LP-Py (a) and LP-Py-PPT (b) in methanol (1.0 g L-l). The curves correspond to eq 7 (see text).
added in increasing concentration to a 1.0 g L-l suspension of particles in methanol. To ensure equilibration, the suspensions were allowed to stand for 2 h after addition of quencher. It was established that longer equilibration times did not affect the quenching results for these two quenchers. Changes in the ratio I o / l were measured as a function of quencher concentration for EPB and DMA. In all cases, lo/lincreased rapidly a t low quencher concentrations and only slightly for quencher concentrations higher than 5 X M (Figure 4a). Experiments performed on the latex LP-Py-PPT gave similar results (Figure 4b). A modified Stern-Volmer model which provides the curved lines through the data in Figure 4 will be presented in the Discussion. 2. Particle Suspensions in Water. When the particles were dispersed in water, their response to quencher addition was quite different. The quenching of latex fluorescence by the cationic quencher ethylpyridinium cation was inefficient with the three latices LP-Py, LPPy-W, and LP-Py-PPT. In all cases, more than 80% of the total fluorescence remained unquenched for EPB concentrations as high as 2 X M. For the NaI quenching experiments, the particles were dispersed in a M sodium chloride solution in order to maintain the ionic strength of the medium constant with increasing quencher concentration. The iodide ion was a poor quencher of fluorescence for the native particles ( I o / l = 1.2 for [I-] = M), and it did not quench the fluorescence of the particles prepared by precipitation at concentrations as high as 2 X M. On the contrary, addition of DMA to aqueous latices resulted in efficient quenching of the particle fluorescence. For initial DMA concentrations ([DMA],) higher than 8
0
5.0 [DMA],
10.0
15.0
(103mol L-')
Figure 6. Ratio &/I of fluorescence intensity in the absence and presence of DMA as a function of quencher concentration in suspensions of the particles LP-Py in water (1.0 g L-l). Measurements were performed immediately after DMA addition (curve l),then 2 h (curve 2), 2 days (curve 3), and 3 days (curve 4) after the addition of DMA to the latices. X M, quenching was accompanied by the occurrence of a new broad emission centered at 480 nm attributed to a pyrene:DMA exciplex. This emission became the major component of the weak emission from latices in which the DMA concentrations were higher than 2 X M (Figure 5). DMA quenching results were initially difficult to reproduce, until it was noticed that the quenching efficiency of DMA in water was affected significantly by the time elapsed between the addition of DMA to the suspensions and the recording of the fluorescence spectra. The effect of equilibration time was examined systematically with the latex LP-Py a t a concentration of 1 g L-l. Spectra were measured immediately after DMA addition (time 0), 2 h, 24 h, and 3 days. A curious phenomenon was noticed. At time 0, the ratio Io/I of pyrene fluorescence in the absence and presence of quencher increased approximately linearly with DMA concentration, for [DMA] < 8 X M, and it increased sharply for [DMA] > 9 X M (Figure 6). When measurements were repeated after equilibration times of increased lengths, the quenching efficiency at [DMA] > 9 X M decreased until after 3 days the ratio I o / I reached a value that fitted linearly with the data points collected at lower DMA concentrations. This behavior was observed for the latices LP-Py, LP-Py-W, and LP-Py-PPT. Since the solubility of DMA in polystyrene is higher than in water, it was expected that the DMA concentration in
Winnik and Paine
908 Langmuir, Vol. 5, No. 4 , 1989 Table IV. Concentration of DMA in Polystyrene Latices and Quenching Data latex concn, g L-1 0.31
0.47
0.72
1.91
3.76
[DMAlo, [DMAls, lo3 mol lo3 mol [DMA],, L-' L-l mol kg-' 0 0 0 3.57 2.2 0.26 5.20 4.7 0.51 6.08 9.1 1.20 1.70 6.77 13.0 0 0 0 3.15 0.25 2.7 4.48 4.9 0.25 5.56 1.10 9.1 6.11 13.0 2.0 0 0 0 0 2.20 2.4 0.41 3.50 4.8 4.76 0.99 9.1 1.75 5.39 13.0 0 0 0 1.91 0.59 5.4 2.82 0.93 9.1 3.38 2.10 13.0 3.85 3.00 16.7 0 0 0 1.12 5.3 0.50 2.31 1.10 13.0 2.51 3.10 16.7
I o / I H,,calcd 1.0
1.47 1.61 2.30 3.13 1.0 2.29 2.16 3.01 3.77 1.0
2.09 2.08 3.04 4.12 1.0 2.27 2.50 4.17 5.46 1.0
1.78 2.80 4.15
1 1.80 2.77 3.90 5.76 1 1.64 2.24 3.15 3.96 1 1.38 1.78 2.42 2.96 1 1.31 1.54 1.72 1.91 1 1.16 1.40 1.45
the aqueous phase ([DMA],) would be quite different from the DMA concentration inside the particle ([DMA],). The concentration of DMA in the aqueous phase was measured spectrophotometrically after a 3-day equilibration of the latices in the presence of DMA for 19 different samples (Table IV). The following general trends were observed. First, [DMAIs was much lower than [DMA],, especially at low [DMA],. Second, [DMA], was very high, especially at low latex concentrations. Values as high as 6.8 mol kg-' were obtained, which, when compared to the molality of pure DMA (8.2 mol kg-'), indicate that the particle composition was severely affected by the presence of the quencher. From the calculated amount of DMA incorporated in the polymeric phase, it is possible to evaluate a volume swelling ratio, H,, of the particles suspended in water in the presence of DMA. This value was used to estimate that the average diameter of the particles increased from 2.4 to about 4.3 km in the experiments with a latex of concentration 0.31 g L-' and from 2.4 to about 2.7 pm in the experiments with a latex of concentration of 3.76 g L-l.
Discussion Whereas fluorescence quenching of free pyrene-labeled stabilizer dissolved in methanol or water obeys SternVolmer kinetics, quenching of sterically stabilized particles does not. Quenching of both ordinary and precipitated particles dispersed in methanol displays a saturation where I o / I levels off as [Q] increases (Figure 4). Quenching by DMA of particles dispersed in water is more complex, owing to a significant quencher concentration inside the particles (Figure 6). These complicated results can be accounted for by considering the location of the steric stabilizer in the particles. Location of the Stabilizer in the Particles. Quenching of small molecule chromophores in homogeneous solution is usually analyzed by the Stern-Volmer relation (eq 1). This relation requires that the emission of the fluorescent groups decays according to a single-exponential law and that the quenching interactions occur with a unique rate constant k,.12 From the slope of a plot of I o / I versus [Q],k, can be determined provided that T~
is known. Quenching of polymer-bound chromophores in solution can also be analyzed by the Stern-Volmer relation, if the fluorescence decay is single-exponential and k, is single-valued. If the decay of chromophore emission in the absence of quencher does not obey a single-exponential decay law, as in the case of solutions of HPC-Py, the Stern-Volmer relation may still be applied if the mean decay time ( T ) is~ substituted to T ~ . Other facts have to be taken into consideration when interpreting quenching data on heterogeneous samples, such as polymeric latices containing fluorophores in the condensed phase. On one hand, they concern the quencher: its concentration on the interior and exterior of the particles may be different, its diffusion coefficient throughout the sample may not be uniform, and its sorption may be time dependent. In addition, the distribution of fluorophores in the polymeric phase may not be uniform, with some chromophores protected from quenching by their environment. This situation can be approximated by assuming the existence of two populations of fluorophores, one of which is accessible to quenchers. A modified Stern-Volmer model has been derived to describe this ~ituation.'~?~ It assumes a model in which only a fraction, fa, of the chromophores is readily quenchable while a fraction (1 - f a ) is protected against quenching. The Stern-Volmer equation is then rewritten in the form
A plot of lo/(lo - I) versus [&I-' w ill yield a linear plot with l the slope from l/fa as the y-intercept and ( f & ~ ~ ) - as which the quenching rate constant of the accessible population can be calculated. In order to accommodate all the situations encountered in this study in a single model, we have defined here the two populations of fluorophores in the following manner: one population (S) is accessible to quenchers dissolved in the continuous phase; the other (P)is buried in the interior of the particles and therefore accessible only to quenchers dissolved in the polymer phase. The total fluorescence emission in the absence of quencher, Io, results from emission of both populations. It is given by In the presence of a quencher, the fluorescence intensity for each fluorophore population is decreased according to the Stern-Volmer equation, and the total intensity becomes
where [&Is and [Qlp are the quencher concentrations in the solvent and in the polymer phase respectively, and Ks and KP are the Stern-Volmer constants for quenching of the chromophores in the solvent phase and in the polymer phase, respectively. Division of eq 3 by eq 4 yields eq 5, where fs is the fraction of the initial chromophores ac+ IoP)]: cessible to quenchers in the solvent [fs = IOS/(IOS
There are cases where the quencher is restricted to one phase. For example, when a quencher is soluble only in (14) Lehrer, S.S. Biochemistry 1971, 10, 3254.
Langmuir, Vol. 5, No. 4, 1989 909
Dispersion Polymerization of Styrene in Polar Solvents
Table V. Estimated Fractions of Accessible Psrenes and Stern-Volmer Constants for Particles Disuersed in Methanol linear fit fit according to eq 5 particles" LP-Py LP-Py-w LP-Py-PPT
" Latex concentration:
quencher EPB DMA DMA EPB DMA
Ksv, L mol-' 88 f 11 131 f 8 125 f 12 82 f 8 132 f 8
r2 0.898 0.975 0.938 0.934 0.973
Ksv, L mol-' 875 f 129 355 f 66 648 f 135 514 f 120 341 f 41
uy
0.192 0.141 0.277 0.139 0.183
fs
r2 0.999 0.999 0.998 0.999 0.999
0.49 f 0.01 0.70 f 0.03 0.65 f 0.02 0.51 f 0.03 0.75 f 0.02
UY
0.039 0.067 0.093 0.054 0.061
1 g L-', 7.0
the continuous phase, then [Q], = 0 and eq 4 and 5 simplify to eq 6 and 7, respectively:
I
a. 0
6.0
(7) This analysis was applied to the quenching experiments reported here. We will limit our discussion to the situations where significant quenching occurred. NaI and EPB quenching of the fluorescence of aqueous latices was too small to give significance to the interpretation. 1. DMA and EPB Quenching of Latex Particles in Methanol. These two experiments provide examples of cases where eq 6 and 7 apply, since it was ascertained experimentally that in methanolic suspensions neither EPB nor DMA penetrate inside the particles to any measurable extent. Examples of curves resulting from fitting the experimental data to eq 7 are shown in Figure 4. The estimated values of Ksv form a linear fit to the simple Stern-Volmer equation, and the values of f s and Ks for the latex particles LP-Py, LP-Py-W, and LPPy-PPT are summarized in Table V. Two statistical parameters are reported to allow comparison of the two fits. Clearly, the quality of the fit obtained by using eq 7 improved on the fits to a simple Stern-Volmer equation. This analysis lends support to the model of fluorophore distribution in two different environments and consequently to a particle morphology where the stabilizer is present on the surface of the particle and within the particle core. From the specific fs values for different quenchers and different latex particles, a few trends become apparent. First, for a given latex the fractions of pyrene accessible to DMA and to EPB are different. Approximately 50% of the fluorophores is accessible to EPB in methanol. The fraction accessible to DMA, 65-75%, is consistently higher for all latices, compared to EPB. This may reflect increased penetration of the nonionic DMA into the steric barrier, when compared to the ionic EPB, allowing DMA to find more fluorophores closer to the surface. Second, the fraction of accessible pyrenes decreases slightly after exhaustive washing of the particles. This may indicate that the washings resulted in preferential extraction of surface stabilizer, thus increasing the fraction of stabilizer buried within the particles. Precipitation of the particles had the opposite effect. It led to a small enhancement of the fraction of pyrenes accessible to the quencher in the continuous phase, possibly suggesting liberation of trapped pyrenes during the dissolution process. 2. DMA Quenching of Latex Particles in Water. Given the partitioning of DMA between the two phases of the latex suspensions, the representation of the quenching data as a function of [DMAIo in Figure 6 is naive. The changes in the ratio Zo/Z may be expressed as a function of either [DMAIs or [DMAIp, possibly permitting us to distinguish between quenching of fluores-
AOdO
0 ' 00
A /I0
-
0
4.0
Latex Concentration
2.0
1.0 [DMA],
IO
0
%NO
A '
I
5.0
0
0
4.0 5.0
IO
0
.HO
3.0
4.0
(lo3 mol L-')
1
A 0
I
l.oL+--00
4.0
2.0
[DMA](,
6.0
8.0
(mol kg-l)
Figure 7. Ratio Io/I of fluorescence intensity in the absence and presence of DMA as a function of quencher concentration in the water phase (a) and in the polymer phase (b) for the particles LP-Py of latex concentrations ranging from 0.31 to 3.76 g L-l. Measurements were performed 3 days after the addition of DMA to the latices. Table VI. Estimated Fractions of Accessible Pyrenes and Stern-Volmer Constants for Quenching by DMA of Ordinary Particles Dispersed in Water
Ks, L KP,L model mol-' mol-' f. U" linear fit: 1587 f 96 1.00 0.61 quenching in water linear fit: 0.48 f 0.07 0 1.26 quenching in polymer fit according to eq 3 840 f 200" 4.8 f 4.6* 0.59 f 0.06 0.43 a
k, = 5.8 X lo9 mol-'
s-'.
k, = 2.8 X lo-? mol-'
s-'.
cence originating from pyrene groups on the surface of the particles, e.g., accessible to DMA in the aqueous phase, or from pyrene groups deeper inside the particles and accessible to DMA in the polymeric phase. To test this hypothesis, the ratio I o / I for the quenching by DMA of pyrene fluorescence in aqueous latices of five concentrations ranging from 0.31 to 3.76 g L-' was plotted as a
910 Langmuir, Vol. 5, No. 4 , 1989
function of [DMA]s and [DMAIp, according to the Stern-Volmer equation I o / I = 1 Ksv [Q], where [Q] = [DMAIs, Ksv = Ks (Figure 7a), [Q] = [DMAlp, and K s ~ = KP (Figure 7b). Clearly the poorest fit to the model is obtained when I o / I is plotted against [DMAIp. The fit to the model is better when I o / I is plotted against [DMAIs ( K s = 1590 f 100 L mol-'). The same data were analyzed by using eq 5, which allows quenching in both phases (Table VI). The quality of the fit improved somewhat (uy = 0.43) compared to the model, which assumed total accessibility of fluorophores to DMA in the water phase ( a y = 0.61). The analysis gave a value of about 0.60 for the fraction of pyrenes accessible to DMA in the water phase. This value is in good agreement with the fraction of accessible pyrenes calculated in the case of latex suspensions in methanol. The quenching rate constant, k,, of pyrene fluorescence by DMA in the aqueous phase was slightly higher than for particles in methanol or for the polymer in solution (Table 111). A value of k , for quenching of pyrene by DMA in heptane where the process is diffusion controlled is 1.9 X lo9 mol-' s-l.16 For quenching by DMA in the polymeric phase, we observe that the constant Kp is smaller than Ks by more than 2 orders of magnitude. This value corresponds to a quenching rate constant of the same magnitude as those reported for pyrene in micelles ((1-3) X lo7 mol-' Assuming that the rate constant k, varies as the inverse of the viscosity, the lower value of k , in the polymer phase can be attributed to the high microviscosity sensed by the pyrene label in the polymeric p h a ~ e . ' ~ J ~ Comparison of Original and Precipitated Particles. One objective of this study was to use fluorescence techniques to compare the stabilizer morphology and distribution in original particles obtained by dispersion polymerization and in precipitated particles made therefrom. Our results can be summarized as follows: (1)There is an
+
(15) Chu, D. Y.; Thomas, J. K. Macromolecules 1987,20, 2133. (16) Katusin-Razem,B.; Wong, M.; Thomas, J. K. J. Am. Chem. SOC. 1978, 100, 1679. Atik, S. S.; Thomas, J. K. Ibid 1981, 103, 3550. (17) Olea, A. F.; Thomas, J. K. J. Am. Chem. SOC. 1988,110,4494and references therein. (18) Kalyanasundaram, K. Photochemistry in Microheterogeneous Systems; Academic Press, 1987; pp 48-75.
Winnik and Paine almost equal amount of HPC-Py / 56 in washed original particles and in precipitated particles (Table I). (2) There are no significant differences in the fluorescence decay parameters for the two kinds of particles (Table 11). (3) The fraction of pyrenes accessible to DMA in methanol is slightly higher in precipitated particles than in original particles (Table V). (4) The fraction of pyrenes accessible to cationic quencher EPB in methanol is approximately the same for both kinds of particles (Table V). In addition, DMA was found adsorbed into precipitated particles from water in much the same way as for LP-Py. The gist of these results is that precipitated particles have essentially the same stabilizer morphology as the original particles from which they were made. Apparently, the stabilizer is almost entirely reincorporated onto the particle surface during the precipitation process.
Conclusion We have reported fluorescence quenching of pyrene-labeled HPC-stabilized polystyrene particles in methanol and in water. The results are consistent with the presence of the majority of the label in an accessible HPC layer on the surface of the particles, confirming our previously reported transmission electron microscopic observations. Some fraction of between 25% and 50% of the pyrene labels is apparently inaccessible to solution-based quenchers and could be trapped inside the particle. Precipitated particles have a morphology substantially identical with the original particles from which they were made. This similarity may suggest that the inaccessible fraction of stabilizer is somewhere nearer the particle surface rather than buried deep inside. It also indicates that dispersion polymerization may involve a similar precipitation/accretion mechanism, as explored in subsequent papers.2 Acknowledgment. We thank Dr. C. Pham Van Cang for fluorescence decay measurements and Dr. Y. Deslandes for transmission electron micrographs. Helpful comments and suggestions by Professor M. A. Winnik are also gratefully acknowledged. Registry No. HPC, 9004-64-2;EPB, 18820-82-1;DMA, 12169-7; polystyrene, 9003-53-6; sodium iodide, 7681-82-5.
