Langmuir 1988,4,989-998
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Fluorescence Studies of Polymer Adsorption. 1. Rearrangement and Displacement of Pyrene-Terminated Poly(ethylene glycol) on Colloidal Silica Particles Kookheon Char, Alice P. Gast,* and Curtis W. Frank Department of Chemical Engineering, Stanford University, Stanford, California 94305-5025 Received February 18, 1988. I n Final Form: April 27, 1988 Adsorption of pyrene end-labeled poly(ethy1ene glycols) (Py-PEG-Py) on colloidal silica particles and their subsequent rearrangement and/or displacement by the addition of unlabeled PEG are studied in water by using photostationary fluorescence and time-resolved fluorescence decay. The tagged polymers we use in this study have molecular weights 4250 and 8650 based on the PEG backbone. The molecular weight of the displacing polymer (PEG) ranges from 1470 to 22000. Experimental conditions for the adsorption of Py-PEG-Py are chosen to ensure that a negligible amount of Py-PEG-Py exists in free solution. For the adsorption of Py-PEG-Py (8650) of 1 X lo4 M on silica particles at 0.1 w t % concentration there is an overshoot in the excimer to monomer intensity ratio, Ie/Im,relative to that of bulk solution, (Ie/Im),,, as the displacer (PEG (22000))concentration is increased. This overshoot is not detected for the adsorption of Py-PEG-Py (4250). Analysis of the supernatant solution shows that the overshoot may be attributed to the rearrangement of Py-PEG-Py (8650) on the silica surface due to the adsorption of incoming displacer prior to displacement of the tagged PEG into solution. We speculate that there is a competition between rearrangement of Py-PEG-Py on the surface and displacement into bulk solution that depends upon the absolute number of Py-PEG-Pysegments actually in contact with the surface. The effects of the molecular weight of the displacing polymer and the initial concentration of Py-PEG-Py on (Ie/Irn)/(Ie/Irn),, are also studied. The measurement of excitation spectra and time-resolved fluorescence decays for both monomer and excimer emission provides insight into the state of the Py-PEG-Py when adsorbed on a silica surface.
Introduction Polymer adsorption has long been a subject of both theoretical and experimental interest. Polymers adsorbed at the solid-liquid interface are utilized in many technologically important processes such as colloidal stabilization in organic media, flocculation, adhesion, and lubrication. The configuration of the adsorbed polymer chains as well as the amount adsorbed plays crucial roles in these processes. Many reviews1 address the important applications of polymer adsorption. Theoretical interest in polymer adsorption began with statistical mechanical models of an ideal isolated chain on the surface.2 This theory has since been extended to include the effects of excluded volume and the dependence of interactions between adsorbed chains on their configur a t i ~ n .Since ~ then, several analytical models have been applied to the study of polymer adsorption. De Gennes4 applied scaling theory to the adsorption of a flexible polymer chain on a planar surface from good solvents. A self-consistent field description of polymer configurations, similar in form to a diffusion equation, was originally developed by Edwards5 for the study of a polymer in so(1)(a) Stromberg, R. R. Treatise on Adhesion and Adhesives; Patrick, R. L., Ed.; Marcel Dekker: New York, 1967;Vol. 1, Chapter 3. (b) Lipatov, Y. S.; Sergeeva, L. M. Adsorption of Polymers; Wiley: New York, 1974. (c) Takahashi, A.; Kawaguchi, M. Adu. Polym. Sci. 1982,46, 1. (d) Fleer, G. J.; Lyklema, J. Adsorption from Solution at the Solidlliquid Interface; Parfitt, G. D., Rochester, C. H., Eds.; Academic: New York, 1983;Chapter 4. (2)(a) Frisch, H. L.; Simha, R.; Eirich, F. R. J. Chem. Phys. 1953,21, 365. (b) Silberberg, A. J.Phys. Chem. 1962,66,1872,1884.(c) DiMarzio, E. A.; McCrackin, F. L. J. Chem. Phys. 1965,43,539.(d) Roe, R.J. J. Chem. Phys. 1965,43,1591.(e) Silberberg, A. J. Chem. Phys. 1967,46, 1105. (f) Chan, D.; Mitchell, D. J.; Ninham, B. W.; White, L. R. J.Chem. SOC.,Faraday Trans. 2 1975,71,235. (3)(a) Silberberg, A. J. Chem. Phys. 1968,48,2835.(b) Hoeve, C. A. J. J. Polym. Sci., Part C 1970,30,361. (c) Hoeve, C. A. J. J. Polym. Sci., Part C 1971,34, 1. (4)(a) De Gennes, P. G. Scaling Concepts in Polymer Physics; Cornell University Press: Ithaca, NY, 1979. (b) De Gennes, P. G. Rep. Prog. Phys. 1969,32,187.(c) De Gennes, P. G. J.Physique 1976,37,1445.(d) De Gennes, P. G. Macromolecules 1980,13,1069. ( 5 ) Edwards, S. F. Proc. Phys. SOC.1966,85,613.
0743-7463/88/2404-0989$01.50/0
lution. This theory was adapted to investigate the configurations of polymers adsorbed to surfaces.6 Recently, this method was generalized by Ploehn et al.,' resulting in a new surface boundary condition describing adsorbed chain behavior. Scheutjens and Fleer819 developed a quasi-crystalline lattice model allowing the calculation of the distribution of train, loop, and tail configurations as well as the adsorption isotherm and segment density distribution. Other techniques such as Monte Carlo simu1ationslO and renormalization methods" have also been applied. In addition to theoretical advances, a variety of experimental techniques have been developed for the investigation of polymer adsorption,'2 allowing one to measure important parameters such as the bound fraction, layer thickness, and density profile. The amount of polymer adsorbed and the layer thickness can be measured by el1ip~ometry.l~Infrared and pulsed NMR measurements yield information on the fraction of segments bound to the surface.14J5 The hydrodynamic thickness of polymer layers can be obtained by either photon correlation spec(6)Jones, I. s.;Richmond, P. J. Chem. SOC.,Faraday Trans. 2 1977, 73,1062. (7)Ploehn, H. J.; Russel, W. B.; Hall, C. K. Macromolecules 1988,21, 1075. (8)Scheutjens, J. M. H. M.; Fleer, G. J. J.Phys. Chem. 1979,83,1619. (9)Scheutjens, J. M. H. M.; Fleer, G. J. J.Phys. Chem. 1980,84,178. (10)(a) McCrackin, F. L. J. Chem. Phys. 1967,47,1980. (b) Clark, A. T.; Lal, M.; Turpin, M. A.; Richardson, K. A. J. Chem. Soc., Faraday Discuss. 1975,59, 189. (c) Eisenriegler, E.;Kremer, K.; Binder, K. J. Chem. Phys. 1982,77,6296. (11)Wang, Z.G.;Nemirovsky, A. M.; Freed, K. F. J. Chem. Phys. 1987,86,4266. (12)Cohen Stuart, M. A.; Cosgrove, T.; Vincent, B. Adu. Colloid Interface Sci. 1986,24, 143. (13)(a) Takahashi, A.; Kawaguchi, M.; Kato, T. Adhesion and Adsorption of Polymers; Plenum: New York, 1980. (b) Kawaguchi, M.; Hattori, S.; Takahashi, A. Macromolecules 1987,20,178. (14)(a) Vander Linden, C.; Van Leemput, R. J. Colloid Interface Sci. 1978,67,48. (b) Kawaguchi, M.;Maeda, K.; Kato, T.; Takahashi, A. Macromolecules 1984,17, 1666. (15) (a) Cosgrove, T.; Vincent, B.; Cohen Stuart, M. A.; Barnett, K. G.; Sissons, D. S. Macromolecules 1981, 14, 1018. (b) Cosgrove, T.; Barnett, K. G. J. Magn. Reson. 1981,43,15.
0 1988 American Chemical Society
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990 Langmuir, Vol. 4, No. 4, 1988
troscopy or capillary flow mea~urement,l~*'~ while smallangle neutron scattering yields the density profile of the polymer layer away from the surface.18 Several investigators have observed that adsorbed polymers do not desorb upon dilution of the supernatant but will exchange with polymers in ~olution.'~Furusawa et a1.20studied the displacement of polymers using gel permeation chromatography (GPC) and showed that the exchange or displacement efficiency improves with increasing molecular weight of the displacing polymer. Even though they noted a preference in polymer adsorption depending not only on the molecular weight of the adsorbate but also on the displacing polymer concentration, they did not discuss in detail the effect of displacer concentration on the exchange. Pefferkorn et a1.21used radioactively labeled high molecular weight polyacrylamide to study the dynamic exchange of polymers at the interface with polymers in solution. They concluded that the exchange takes place at the interface at a very slow rate, and the rate of desorption increases proportionally with the number of molecules in the solution. Luckham and Kleinzz recently observed the displacement of nonionic surfactant initially on the surface by longer poly(ethy1ene oxide) chains in their direct force measurement between surfaces bearing adsorbed polymer layers. Cohen Stuart et al.23,24 studied the displacement of polymers by small molecules with different surface affinities both theoretically and experimentally. From their experiments, they were able to determine the segmental adsorption energy of the polymers. Fluorescence techniques have long been employed to investigate the dynamics of chemical species at the molecular level. There is a large amount of literature dealing with photophysical studies of the dynamics of either synthetic or natural molecules.26 Among the various fluorescence methods, excimer formation has frequently been used to study polymer dynamics. In this method, an aromatic ring in an electronically excited state must encounter an identical chromophore in the ground state during the lifetime of the excited state. If the two chromophores are in the proper geometry, they can form an excited-state dimer giving a broad emission at a lower energy than that of the individually excited aromatic ring. Examples of the application of excimer formation include detection of association complex formation between two different water-soluble polymers26and use as probes of the end-to-end cyclization of polymer chains.27 Among the (16)(a) Cohen Stuart, M. A.; Waajen, F. A. W. H.; Cosgrove, T.; Vincent, B.; Crowley, T. L. Macromolecules 1984,17,1825.(b) K h a n n , E.;Maier, H.; Kaniut, P.; Gutling, N. Colloids Surf. 1985,15,261. (17)(a) Varoqui, R.; Dejardin, P. J . Chem. Phys. 1977,66,4395.(b) Gramain, Ph.; Myard, Ph. Macromolecules 1981,14, 180. (18)Barnett, K. G.; Cosgrove, T.; Crowley, T. L. F.; Tadros, Th. F.; Vincent, B. The Effect of Polymers on Dispersion Stability; Tadros, Th. F., Ed.; Academic: New York, 1982. (19)Grant, W. H.; Smith, L. E.; Stromberg, R. R. J. Chem. Soc., Faraday Discuss. 1975,59,209. (20)Furusawa, K.; Yamashita, K.; Konno, K. J. Colloid Interface Sci. 19112 Rfi RF; ----I --7
(21)Pefferkom, E.; Carry, A.; Varoqui, R. J. Polym. Sci., Polym. Phys. Ed. 1985,23,1997. (22)Luckham, P. F.;Klein, J. J . Colloid Interface Sci. 1987,117,149. (23)Cohen Stuart, M. A.; Fleer, G. J.; Scheutiens, J. M. H. M. J. Colloid Interface Sci. 1984,97,515. (24)Cohen Stuart, M.A.; Fleer, G. J.; Scheutjens, J. M. H. M. J. Colloid Interface Sci. 1984,97,526. (25)Lakowicz. J. R. PrinciDles - of. Fluorescence SDectroscoDv: . _Plenum: New York, 1983. (26)(a) Bednar, B.; Li, Z.; Huang, Y.; Chang, L. C. P.; Morawetz, H. Macromolecules 1985.18.1829. (b) Turro. N. J.: Arora. K. S. Polymer 1986,27,783. (c) Oyama; H. T.; Tang, W.'T.; Fr'ank, C.' W. Macromolecules 1987,20, 474. (d) Oyama, H. T.; Tang, W. T.; Frank, C. W. Macromolecules 1987,20, 1839.
many possible fluorescent chromophores, pyrene and its derivatives have been used extensively to study excimer formation due to their long excited-state lifetime and their spectral sensitivity to the surrounding mediurnaz8 There are two major advantages of a fluorescence technique for the study of polymer adsorption. The first relates to the ease with which the labeled polymer concentration in the supernatant may be determined after centrifugation. The traditional measurements of turbidityz9and refractive index30 are less accurate than use of the absorption of ultraviolet light from the pyrene labels attached to the PEG polymer. Therefore, with fluorescently tagged chains it is possible to study the adsorption of polymer from very dilute solution. The second advantage is due to the highly localized information provided by the technique of excimer fluorescence. The excimer complex is only formed when the aromatic rings approach each other within about 4-5 A. Thus, it is possible to draw reasonably precise conclusionsabout the spatial distribution of the chromophores and, hence, the portion of the polymer chain to which they are attached. In this paper we use terminal pyrene tags to probe both adsorption and configurational changes of poly(ethy1ene glycol) (PEG) on small silica particles. Photostationary and transient excimer fluorescence are used to study polymer adsorption and the subsequent displacement or rearrangement by the adsorption of additional chains. We discuss the effect of the molecular weight and concentration of the displacing polymer chain on the displacement and/or rearrangement of the preadsorbed labeled polymers.
Experimental Section Materials. Pyrene end-labeled poly(ethy1ene glycols) (Py-
PEG-Py) were prepared by the direct esterification between hydroxy groups located at both ends of the PEG backbone and the carboxy group of 1-pyrenebutyric acid following the method developed by Cuniberti and Peri~o.~'Details of the synthesis and purification were described previ~usly.~~ We denote the pyrene tagged samples as Py-PEG-Py (4250)and Py-PEG-Py (8650), where the numbers in parentheses represent the weight average molecular weight of the original untagged PEG samples as reported by Polysciences Inc. The structure of Py-PEG-Py is shown below:
Silica particles (LudoxAM) were obtained from Du Pont. Some of the surface silicon atoms have been replaced with aluminum atoms in order to create a fixed negative charge independent of (27)(a) Winnik, M. A. Acc. Chem. Res. 1985,18,73.(b) Winnik, M. A. Photophysical and Photochemical Tools in Polymer Science; Winnik, M. A., Ed.; NATO AS1 Series C: Mathematical and Physical Sciences; Reidel: Dordrecht, 1986;Vol. 182. (c) Winnik, M. A.; Redpath, A. E. C.; Paton, K.; Danhelka, J. Polymer 1984,25,91. (d) Redpath, A. E. C.; Winnik, M. A. Ann. N.Y. Acad. Sci. 1981,366,75. (28)Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. SOC.1977,99, 2039. (29)Rubio, J.; Kitchener, J. A. J.Colloid Interface Sci. 1976,57,132. (30)Joppien, G. R. J. Chem. Phys. 1978,82,2210. (31)Cuniberti, C.; Perico, A. Eur. Polym. J. 1977,13,369. (32)Char, K.;Frank, C. W.; Gast, A. P.; Tang, W. T. Macromolecules 1987,20,1833. (33)(a) Howard, G. J.; McConnell, P. J . Phys. Chem. 1967,71,2974. (b) Hommel, H.;Legrand, A. P.; Tougne, P.; Balard, H.; Papirer, E. Macromolecules 1984,17, 1578. (c) Killmann, E.: Eisenlauer. J.: Korn. M. J. Polym. Sci., Polym. Symp. 1977,61, 413.
Langmuir, Vol. 4, No. 4, 1988 991
Fluorescence Studies of Polymer Adsorption pH that leads to a suspension that is stable in the neutral pH range. The silica particles were supplied as a suspension with solid content around 30%. The average particle diameter in this suspension is 15 f 2 nm by dynamic light scattering. Glassdistilled deionized water was the solvent. Sample Preparation. In order to make final 0.1 wt % Ludox AM + 1 X lo4 M Py-PEG-Py solutions, 0.2 wt % Ludox AM solutions and 2 x lo4 M Py-PEG-Pysolutions were f i t prepared separately. The silica suspension was slowly added to the 2 X lo4 M Py-PEG-Pysolution during vigorous stirring. This mixture was then agitated by an arm shaker overnight to ensure that it reached equilibrium. For the displacement experiments, the sample containing silica particles and Py-PEG-Pywas later mixed with a solution containing untagged PEG chains, and the mixture was again shaken for 8 h before fluorescence measurement. Equilibrium was usually attained well within the first hour, often within approximately 5 min. The final colloidal suspension was transparent due to the small size of the silica particles; there was no sign of flocculation. The pH of the solution was maintained around pH 7 for all samples. The adsorption isotherm was obtained by first ultracentrifuging a 20-cm3sample of the solution containing silica particles and polymer chains for 4 h at 85000g. About 6 cm3of the supernatant solution was then carefully withdrawn from the top of the centrifuge tube, and the UV absorption was measured. The concentration of pyrene labels in the supernatant was determined from the pyrene extinction coefficient of about 35 500 M-’ cm-’ at 343 nm. Fluorescence Measurement. Photostationary-statespectra were measured on a SPEX Fluorolog 212 spectrofluorometer as emission from the front face of the cell at an angle of 22.5O from the incident light. The excitation wavelength was 343 nm, corresponding to the ‘La band of the pyrene ring, and the spectra were corrected for instrumental response from 360 to 600 nm. Two spectral parameters are of particular interest: the excimer to monomer intensity ratio, &/Im,calculated from envelope intensity measurements around 475-485 nm for the excimer (I,)and at 376 nm for the monomer (Im),and Ill&, the ratio of intensities of the first (376 nm) and third (386 nm) monomer fluorescence bands. Excitation spectra were measured by scanning the excitation wavelength from 300 to 360 nm at two fixed emission wavelengths: 376 nm for monomer emission and 480 nm for excimer emission. We measured the fluorescence lifetimes on a single photon counting PRA (System 3000) instrument. The excitation wavelength was 343 nm, monomer emission was monitored at 376 nm, and excimer emission was monitored at 480 nm. We determined the lifetimes and preexponential factors from the best fits to a multiexponential decay using a nonlinear least-squares deconvolution method.
Results Adsorption Isotherm. Measurement of intramolecular excimer formation provides information about the spatial distribution of the chromophores attached to the chain ends. In order to study the change in excimer formation due to changes in the spatial distribution of pyrenes upon adsorption onto the silica surface, we need an adsorption condition where we are able to monitor the fluorescence due to adsorbed chains alone. Thus, we first studied the adsorption isotherms for Py-PEG-Py to determine the conditions where virtually no tagged chains remain in solution. We used two different silica particle concentrations to obtain isotherms for two different molecular weights, i.e., 0.05 wt % silica for the adsorption of Py-PEG-Py (8650) and 0.1 wt % silica for Py-PEG-Py (4250). It was not possible to carry the adsorption to a high enough polymer concentration to reach a plateau for the isotherm. This limitation arose from difficulties with the UV spectrophotometry at high polymer concentrations, where the absorbance is not linear in concentration due to a decrease in the extinction coefficient. Despite these limitations, we were able to define the regimes where virtually no tagged chains exist in free solution: below a concentration of 2
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Wave1 e n g t h Inml Figure 1. Photostationary emission spectra of 3 X lo4 M PyPEG-Py (4250) with particle concentrations shown in figure. X lo4 M for the Py-PEG-Py (8650) in 0.05 wt % silica and below 6 X 10” M for Py-PEG-Py (4250) in 0.1 wt % silica. All of the experiments presented below were carried out within these regimes.
Photostationary Fluorescence Measurement. A. Adsorption of Py-PEG-Py. In our previous paper32 we examined the fluorescence, spectra of Py-PEG-Py in water as well as in methanol/water mixtures and concluded that there is a hydrophobic attraction between pyrene moieties in water. Moreover, this hydrophobic interaction increases as the molecular weight of the backbone chain is decreased. With the solution properties of these molecules characterized, we are able, in this study, to monitor the dramatic decrease in excimer formation upon adsorption as an experimental probe of chain configuration. The enhanced excimer formation in dilute solution due to hydrophobic attraction between chromophores magnifies the effect of the restricted pyrene mobility when the chains are adsorbed to a surface. Typical fluorescence emission spectra of pyrene-tagged PEG in water are shown in Figure 1 for various particle concentrations with the spectra normalized to the highest monomer band. The spectra generally consist of two parts: monomer emission bands and an excimer emission peak. The five monomer emission bands are characteristic of the pyrene vibronic structure: three distinct peaks (376, 396, 418 nm) and two appearing as shoulders (381, 386 nm). Emission from the excimer gives a very broad structureless band around 480 nm. As shown in Figure 1, the excimer to monomer intensity ratio (Ie/I,) decreases dramatically as the particle concentration is increased. These results are quantified in Figure 2, where the change in Ie/I,,, relative to that of the solution without particles, (Ie/I,J0, is plotted against the silica particle concentration. Analysis of the supernatant solution confirmed that no Py-PEG-Py remained in the bulk solution, implying that the fluorescence signals were solely due to Py-PEG-Py adsorbed onto the colloidal silica particles. The silica concentration at which there is, on average, one chain per particle is noted by the arrow. The dashed line in Figure 2 is simply an asymptotic extension of the apa t high silica content. parent plateau in (Ie/Im)/(Ie/Im),, This plateau represents the situation for which there would be no more than one labeled chain per particle, with many particles having no adsorbed chains. With strictly only one chain per particle the contribution to Ie/Immust, of course, be intramolecular. We interpret the increase in (Ie/Im)/(Ie/Im)o above the dashed line at the average single chain/particle point as an indication that some particles contain more than one labeled chain, leading to intermo-
992 Langmuir, Vol. 4, No. 4, 1988
Char et al. 1
1%
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1
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Figure 2. Effect of particle concentration on the excimer to monomer intensity ratio relative to that of the polymer solution, (Ze/Zm)/(Ze/Zm)o for 3 X lo4 M Py-PEG-Py (4250). lecular excimer concentration. At low silica particle concentration, the intermolecular contribution to I e / I mdue to interactions between adsorbed chains is extensive, causing a dramatic increase in Ie/Im. The decrease in Ie/Imupon adsorption implies that there are fewer pyrene-pyrene contacts leading to excimer formation. This could result from one of several effects. We first discuss two possibilities that we believe are of lesser importance. First, the pyrene excimer could become destabilized by some form of quenching by the silica surface. Since the surface of Ludox AM particles is hydrophilic, it is unlikely that the pyrene is preferentially adsorbed on the silica. This is verified by the observation that neither the ratio of the first to third monomer emission peak, 11/13 (a measure of the local environment around the pyrene), nor the monomer excitation spectra described below changes after adsorption. Second, a polymer chain could bridge multiple particles together. This possibility can be neglected since the polymer chain is quite short compared to the average interparticle distance and no macroscopic aggregation is observed in these suspensions. The observation that Ie/Imlevels off at higher particle concentrations also indicates that bridging is not important in these studies. We believe that the decrease in Ie/Imby the increase of particle concentration is primarily due to two other effects: (1)that the pyrene moieties are further apart and (2) that the PEG backbone is tightly bound to the surface so that intramolecular cyclization is hindered. There are a number of references%reporting that the PEG chains adsorbed on silica particles assume a flattened configuration for a variety of experimental ranges tested. Despite the relatively strong hydrophobic attraction of the terminal pyrenes of Py-PEG-Py (4250) in bulk solution, it seems that the presence of the silica surface breaks up the pyrene aggregates and consequently lowers Ie/Im. This observation suggests that the average distance between pyrene groups attached to the ends of the PEG chain is increased upon adsorption. B. Rearrangement and Displacement with PEG. One of our objectives is to determine how the average number of excimer-forming sites within the terminal pyrene moieties changes upon adsorption of the tagged chain and upon addition of untagged polymers. To examine this we chose experimental conditions for preadsorption of Py-PEG-Py such that there was approximately
~-~ -da--p-zz
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Figure 3. Effect of molecular weight and concentration of untagged displacing polymer on (Ie/Zm)/(Ze/I)o of Py-PEG-9. (8650) with an initial concentration of 1 x l 0 I M Py-PEG-Py (8650) and 0.1 wt % Ludox AM for displacing polymers: 0 ,PEG (22000); A, PEG (8650);+, PEG (4250). one polymer chain on each particle. This initial concentration of Py-PEG-Py was increased later in order to study its effect on Ie/Im. Moreover, care was taken to ensure the absence of detectable free Py-PEG-Py in the supernatant solution to eliminate free pyrene end-labeled chain contributions to Ie/Im. We also note that the preadsorption and exchange reactions (displacementwith untagged PEG) occur very rapidly within 5 min. This observation contrasts with the very slow exchange (ca. 15 h) observed by Pefferkorn et aLZ1using a radiotracer counting technique to measure the exchange between labeled and unlabeled polyacrylamide adsorbed on glass beads. The probable reason the exchange rate is so slow in their system is that the adsorbent surface is saturated with polymer, i.e., within the plateau region of the adsorption isotherm, while here we are studying polymer configurations at low surface coverage. Also, the molecular weight of the polymer used in their study is quite high (M,= 1.2 X lo6,polydispersity = 1.25) compared to those in our system such that more entanglements between chains as well as more attachments of segments along the polymer to the surface reduce their exchange rates. The effect of the displacing polymer molecular weight and concentration on I e / I mrelative to the same ratio for a bulk solution, (Ie/Im)o, is shown in Figure 3 for preadsorbed pyrene end-labeled polymers of molecular weight 8650. Since the initial concentration of Py-PEG-Py (8650) is such that there is approximately one polymer for each particle, the initial drop in (Ie/Im)/(Ie/Im)o at zero displacer suggests that the intramolecular cyclization of the adsorbed chain is significantly hindered by the multiple attachment of segments to the surface. Upon addition of untagged PEG chains to the mixture, the excimer intensity rises with a dependence on the initial displacer concentration increasing with the molecular weight of the displacer. One surprising observation about Figure 3 is that when PEG (22000) is added to the preadsorbed Py-PEG-Py (8650) there is a large peak in (Ie/Im)/(Ie/Im)o at a con) o to a centration of 1 X lo4 M and ( I e / I m ) / ( I e / I mdrops constant value at a concentration just above 1 x lo4 M. The Ie/Imvalue at the peak is much larger than that of the bulk solution. Also note in Figure 3 that when this system is combined with an untagged polymer of the same molecular weight the maximum value of Ie/I,,, relative to
Langmuir, Vol. 4, No. 4, 1988 993
Fluorescence Studies of Polymer Adsorption
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8
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Figure 4. Dependence of the relative absorbance of the supernatant solution and I e / I m on displacer concentration for displacement by PEG (22 000) with an initial concentration of PyPEG-Py (8650)equal to 1X lo4 M in a suspension of 0.1 w t % Ludox AM.
(Ie/Im)ois smaller and the peak is broader than that for displacement with PEG (22 000), and the displacer concentration giving the maximum (Ie/Im)/ (Ie/Im)ois higher. When the preadsorbed Py-PEG-Py (8650) is displaced with a chain of lower molecular weight, no overshoot is observed and (Ie/Im)/(Ie/IJ0 does not reach a constant final value until a significantly higher displacer Concentration. On the basis of these observations we conclude that a displacer of higher molecular weight displaces or rearranges preadsorbed Py-PEG-Py (8650) more effectively. We attribute the large overshoot of Ie/Imrelative to its bulk value to a configurational change in Py-PEG-Py (8650) on the particle surface immediately before it is displaced. In order to establish that this is the case, we analyzed the supernatant solution after centrifugation, as shown in Figure 4. The relative optical density (OD,/OD,) remains low at the displacer concentration corresponding to the maximum in (Ie/Im)/(Ie/Im)o, implying that at this concentration most of the pyrene end-labeled PEGSremain on the surface. Therefore, this maximum value must be associated with interactions among polymer chains still on the surface. We also observe that the preadsorbed pyrene end-labeled PEG displacement is apparently not complete since (Ie/Im)/(Ie/Im)oremains below unity and the optical density of the supernatant solution recovers to only 67% of that of a bulk solution. Interpretation of this observation requires an assumption that there is no effect on I e / I m when labeled and unlabeled chains are mixed in bulk solution. This assumption is verified by studies of mixtures of 1 X 10* M Py-PEG-Py (8650) and 8 X 10* M PEG (22 000) resulting in Ie/Im(=0.104) unchanged from that of the tagged chains alone. This result thus suggests that some of the tagged polymer chains remain interacting with the surface even when subjected to extreme displacer M, for PEG (22000)). concentrations (e.g., 1 X Since displacement with chains smaller than the preadsorbed tagged polymer does not reach a plateau until much higher concentration, as shown in Figure 3, it seems possible that (Ie/Im)/(Ie/&J0 may depend on the displacer segment concentration rather than the displacer concenfrom tration itself. In Figure 5 we replot (Ie/Im)/(Ie/Im)o Figure 3 against the displacer segment concentration, i.e., the displacer concentration multiplied by the degree of
ft 01 0
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31SPLACER CONCENT3A;lOh
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Figure 6. Effect of the initial concentration of Py-PEG-Py(8650) on the fluorescence intensity ratio (Ie/Im)/(Ie/Iml0 plotted against the displacer concentration for displacer PEG (22000) at the following Fy-PEG-Py (8650) concentrations: 0 , 1 X lo4 M; A, 2 X lo4 M; +, 4 X lo4 M with Ludox AM concentration equal to 0.1 wt %. Table I. Estimated End-to-End Distance for PEG Samples of Different Molecular Weights in Water at 303 K mol wt, g/mol polydispersity end-to-end distance, %, 1470 4250 8650 22000
1.05
