+
+
Langmuir 1996, 12, 1425-1427
1425
Fluorescence Study of Hydrophobically Modified Polyelectrolytes in Aqueous Solution: Effect of Micellization P. Guenoun,*,†,‡ S. Lipsky,§ J. W. Mays,| and M. Tirrell† Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, Service de Physique de l’Etat Condense´ , C.E.A./Saclay, 91191 Gif sur Yvette Cedex, France, Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455, and Department of Chemistry, University of Alabama at Birmingham, Birmingham, Alabama 35294 Received December 4, 1995X Fluorescence emission techniques were applied to the study of aqueous solutions of a hydrophobically modified polyelectrolyte, a block copolymer of sodium poly(styrenesulfonate) (NaPSS) and poly(tertbutylstyrene) (PtBS). At low concentrations, the spectrum of NaPSS/PtBS is essentially identical to that obtained for NaPSS homopolymer. Above a certain concentration, which we identify as corresponding to the critical micelle concentration (cmc), substantial differences are noted in the spectra. Specifically, an increase in the intensity of the excimer peak at 330 nm is observed for the diblock solution relative to the homopolymer solution. The cmc is estimated to be in the range of 2 × 10-7 to 2 × 10-6 g/g for the diblock.
Introduction Fluorescence emission from polymer or polyelectrolyte solutions is able to provide much structural and conformational information.1-3 Recent examples include the study of polyelectrolyte conformation by observing the natural fluorescence4-6 and the description of a micellization process in block copolymers by monitoring the emission which is expected to originate at the core of the micelles.7 The first example is based upon the existence of two kinds of emission from phenyl rings along a polymeric chain (e.g., poly(styrenesulfonate), PSS), namely, the monomer and excimer emissions.8 The latter mechanism of emission is due to a cooperative process between two adjacent phenyl rings whereas the monomer emission involves only one ring. Monitoring the ratio of excimer to monomer fluorescence intensity can give information about the chain conformation when the concentration of polymer or the added salt concentration is varied. The second example is useful in determining the critical micelle concentration (cmc), whose value in polymeric systems is usually too low to be investigated by scattering techniques. In this case, one expects the fluorescence emission of an added fluorescent probe to change when crossing the cmc, thus allowing measurement of its value. The natural fluorescence can also be used to quantify this transition. A combination of the two above methods can be fruitful in the case of hydrophobically modified polyelectrolytes, † Department of Chemical Engineering and Materials Science, University of Minnesota. ‡ Service de Physique de l’Eta Condense ´ , C.E.A./Saclay. § Department of Chemistry, University of Minnesota. | Department of Chemistry, University of Alabama at Birmingham. X Abstract published in Advance ACS Abstracts, February 15, 1996.
(1) Morawetz, H. Science 1979, 203, 405. (2) Nishijima, Y. Prog. Polym. Sci. Jpn. 1973, 6, 199. (3) Pekcan, O.; Winnick, M. A.; Croucher, M. D. Phys. Rev. Lett. 1988, 61, 641. Pekcan, O.; Egan, L. S.; Winnick, M. A.; Croucher, M. D. Macromolecules 1990, 23, 2210. (4) Turro, N. J.; Okubo, T. J. Phys. Chem. 1982, 86, 159. (5) Ander, P.; Mahmoudhagh, M. K. Macromolecules 1982, 15, 213. (6) Torkelson, J. M.; Lipsky, S.; Tirrell, M.; Tirrell, D. A. Macromolecules 1983, 16, 326. Major, M.; Torkelson, J. M. Macromolecules 1986, 19, 2801. (7) Kiserow, D.; Prochazka, K.; Tuzar, Z.; Munk, P.; Webber, S. E. Macromolecules 1992, 25, 461 and references therein. (8) For a recent review, see Kalyanasundaram, K. Photochemistry in Microheterogeneous Systems; Academic Press: New York, 1987.
for example, diblocks of poly(tert-butylstyrene) (PtBS) and PSS. When the length of the PSS block is greater than the length of the PtBS moiety, scattering techniques show the existence of micelles in aqueous solutions.9 Our aim here is to locate the cmc in such a system and to characterize the chain conformation by studying the natural fluorescence of the solutions. In the following we present fluorescence spectra and compare them with spectra obtained from aqueous solutions of the homopolymer PSS in order to elucidate the peculiarities induced by the association process. Experimental Section The diblock copolymer of PtBS and NaPSS was produced by sulfonation of a precursor diblock of PtBS and polystyrene (PS). The initial diblock, having 8.9% PtBS by weight, was prepared by anionic polymerization. This product had a weight-average molecular weight (Mw) of 46 000 by size exclusion chromatography and a polydispersity of 1.04, based on a polystyrene calibration. This indicates weight-average degrees of polymerization of 26 and 403 for PtBS and PS, respectively. This material was selectively sulfonated by the method of Valint and Bock10 to yield 89% sulfonation of the PS segment (elemental analysis for sulfur). The sulfonic acid groups were neutralized using sodium methoxide to generate the final PtBS/NaPSS diblock, which is designated MT-2. The NaPSS homopolymer with Mw ) 100 000 was purchased from Pressure Chemical Co. and used as received. Optically clear solutions were obtained by dissolving the polymer in pure distilled and deionized water (Millipore purification unit). Solutions were allowed to equilibrate for a few days and were then diluted further to achieve the final desired concentrations. A sonication procedure was sometimes used to speed up solution equilibration; no significant changes have been found in the spectra except for very low concentrations (see discussion below). All fluorescence measurements were made with a Spex F212 spectrofluorometer. Air-equilibrated samples in quartz cuvettes at 22 °C were illuminated at normal incidence with a 1000-Watt xenon arc through a Spex 1680 double grating monochromator (200 nm blaze), and the emission was collected in front-face geometry (22.5° from the normal) and focused onto a second Spex 1680 double grating monochromator (400 nm blaze). Both excitation and analysis monochromators were operated at a bandpass of ca. 5 nm. The spectrally dispersed fluorescence was focused onto the front surface of a thermoelectrically cooled Hamamatsu R955 photomultiplier, and its signal amplified and (9) Guenoun, P.; Davis, H. T.; Tirrell, M.; Mays, J. W. Submitted to Macromolecules. (10) Valint, P. L.; Bock, J. Macromolecules 1988, 21, 175.
+
+
1426
Langmuir, Vol. 12, No. 6, 1996
Figure 1. Comparison of fluorescence emission spectra for NaPSS homopolymer and PtBS/NaPSS diblock at high concentrations. In all figures, spectra have been rescaled in intensity in order to evidence the differences in shapes.
Letters
Figure 2. Comparison of fluorescence emission spectra for NapSS homopolymer at low and high concentrations.
counted with the standard fluorolog DMIB electronics. Excitation was usually at 240 nm. The emission spectra are uncorrected for spectral response of the analyzing system.
Results and Discussion The comparison of emission spectra of NaPSS (Mw ) 1 × 105) and of the diblock MT-2 (Mw ) 8 × 104) is shown in Figure 1 for high concentrations. Both curves exhibit typical features of monomeric and excimeric emissions, respectively, around wavelength 290 nm and 330 nm, although this behavior is more pronounced for the diblock. Our results for PSS are in reasonable agreement with those obtained by other authors. Turro and Okubo4 were the first to observe the monomeric and excimeric contributions to the emission of PSS. Their determination of the above wavelengths corresponds to ours, although differences are apparent in the shapes of the spectra observed. Ander and Mahmoudhagh5 have studied the influence of the nature of the added salt, and Major and Torkelson6 have published the most quantitative study of the evolution of the spectra with salt concentration and molecular weight. Results of these two groups agree with ours with regard to the shape of the spectra. The excimer to monomer ratio has been shown to be insensitive to salt concentration over a very wide range,5,6 indicating that the excimer emission is not very sensitive to the overall conformation of the chain. Changes in the excimer to monomer ratio are probably explained by modifications of the counterion-polyion interaction at the local scale. Counterion condensation is probably a good candidate model11,12 in order to explain why, at small scale, phenyl sulfonated groups can adopt a regular alignment (isotactic). The increase of the excimer to monomer ratio in the diblock would then mean a more regular ordering of the phenyl groups at the local scale. We believe that this result is compatible with the existence of micelles at this concentration as evidenced by other techniques.9 These micelles have been shown to behave as roughly neutral objects implying that a majority of the counterions are trapped within a micelle either as physically bound counterions along the chain or as ions whose mobility is limited to the volume within the micelle. This should ease excimeric configurations of the phenyl groups because of an increased condensation of the counterions on the sulfonated groups or an increased screening of the groups by the counterions. When the concentration of polymer is varied from 2 × 10-3 to 2 × 10-7 g/g, the emission (11) Manning, G. S. J. Chem. Phys. 1969, 51, 924. (12) Ray, J.; Manning, G. S. Langmuir 1994, 10, 962, 2450.
Figure 3. Comparison of fluorescence emission spectra for NaPSS homopolymer and PtBS/NaPSS diblock at ca. 2 × 10-6 g/g.
Figure 4. Comparison of fluorescence emission spectra for NaPSS homopolymer and PtBS/NaPSS diblock at ca. 2 × 10-7 g/g.
spectrum of PSS varies little (Figure 2), in agreement with ref 6 where no variation of the ratio excimer to monomer has been reported in this range. A different result is observed for the diblock whose spectrum at c ) 2.3 × 10-6 g/g gets closer to the generic PSS spectrum (see Figure 3). For c ) 2.3 × 10-7 g/g, despite the noise due to the very weak emission from this sample, it is observed that the PSS and diblock spectra are essentially identical (Figure 4). All spectra at low concentrations were corrected for a weak background that was observed from a pure water sample illuminated at the same excitation wavelength of 240 nm. At such a low concentration diblock chains seem to behave like unassociated chains. Conse-
+
+
Letters
quently we can locate the cmc as occurring between 2 × 10-7 and 2 × 10-6 g/g for the diblock copolymers. For concentrations in the vicinity of the cmc, we have observed that sonication of samples in a regular ultrasound bath can alter the spectra, apparently increasing the excimer/ monomer ratio. For higher concentrations, no effect is observed. It is not clear at present if these effects can be explained by chain degradation only or by a disturbance
Langmuir, Vol. 12, No. 6, 1996 1427
of the equilibrium which takes place at the cmc. More experiments are needed to develop a more quantitative understanding of this aspect of the sonication influence. Acknowledgment. We thank Professor Turro for communication of original results and we are grateful for support of this research by NATO (CRG 930892). LA951509O
