Langmuir 1994,10, 4475-4482
4475
Surface Properties of Pluronic-Coated Polymeric Colloids Jenq-Thun Li,? Karin D. Caldwell,*>?and Natalya Rapoport* Center for Biopolymers at Interfaces, Department of Chemical Engineering, Department of Materials Science and Engineering, University of Utah, Salt Lake City, Utah 84112 Received April 21, 1994. In Final Form: September 7,1994@ With the help of field-flowfractionation, photon correlation spectroscopy,and electron spin resonance techniques, as well as more conventional labeling approaches, we have accomplished the analytical characterization of the adsorption complexesformed between, on the one hand, a series of selected triblock polymeric surfactants with comparable lengths of their central poly(propy1ene oxide) (PPO)block but with varying poly(ethy1eneoxide) (PEO)block lengths, and on the other a series of differently sized polystyrene colloids. For a given surfactant, it was found that both surface concentrations and adlayer thicknesses are strongly related to the particle size, such that smaller particles take up fewer polymer molecules per unit area than the larger ones. The reduced crowding around each PEO chain results in thinner adlayers and higher chain mobilities. In adsorption complexes involving 69-nm polystyrene particles, the adlayer thickness is close to the diameter calculated for the free PEO chain in aqueous solution. In addition, for particles of a given size, it is the size of the surfactant’s hydrophobic center block (PPO), rather than its flanking tails (PEO),that determines the surface concentration. Thus, triblocks of similar PPO size showed comparable surface concentration, while the longer PEO chains were associated with thicker adlayers as well as greater chain dynamics.
Introduction Characterizations of adsorbed polymer films (adlayers) have led a number of s ~ i e n t i s t s l -to~ propose that the configuration of a polymer in the adsorbed state may differ significantly from its configuration in bulk solution. Experimentally, studies of adsorption phenomena generally require quantification of the adlayer thickness and the degree of surface coverage. In these studies, layer thickness is usually determined by the size difference of the adsorbate before and after polymer adsorption. As a n example, by investigating the surface properties of poly(ethylene oxide), poly(ethy1ene oxide)-containing copolymers and poly(viny1 alcohol) (PVA) on precipitated silica and polystyrene (PSI latex particles, Killmann et a1.2 showed the adlayer thickness measured by photon correlation spectroscopy (PCS) to be more molecular weight sensitive on latex particles than on precipitated silica. Characterization of surface coverage frequently involves the mixing of a polymer solution with a given amount of adsorbents, e.g., small particles, and removing the equilibrated adsorption complexesfrom the incubating solution. After this separation, the surface coverage is calculated from the measured amount of adsorbed polymer on each particle, determined either indirectly from the degree of solute depletion or by direct quantification of the adsorbed polymer. As an example of the latter, Tadros and Vincent used a colorimetric assay for PVA to determine the effect of electrolyte on the adsorption of this polymer to PS latex parti~les.~ The conformation of diblock copolymers attached to a n interface has been the subject of many ~ t u d i e s . ~Due - ~ to differences in solubility, it is believed that the soluble
* Corresponding author.
Department of Chemical Engineering.
* Department of Materials Science and Engineering.
* Abstract published in Advance ACS Abstracts, November 1, 1994. (1)Alexander, S. J. Phys. (Paris) 1977,38,983. (2) Killmann, E.,Maier, H.; Baker, J. A. Colloids Surf 1988,31,51. (3)Tadros, TH. F.;Vincent, B. J.ColloidZnterface Sci. 1979,72,505. (4)Milner, S.T.; Witten, T. A.; Cates, M. E. Macromolecules 1991, 24, 1987. ( 5 ) Parsonage, E.; Tirrell, M.; Watanabe, H.; Nuzzo, R. G. Macromolecules 1991,24,1987. (6) Marques, C . ;Joanny, J. F.; Leibler, L. Macromolecules 1988,21, 1051.
blocks in these situations extend out into the liquid phase to form a “brush”layer while the insoluble blocks anchor the molecule on the solid surface. The surface density of such adsorbed diblocks can be predicted by a function called the “asymmetry ratio”, which is related to the ratio of the number of monomers in each block. In the case of poly(2-vinylpyridine)-poly(styrene) adsorbed on flat mica surfaces from toluene solution, Parsonage et al.5 found surface densities in good agreement with the model developedby Marques et al.6for asymmetry ratios ranging from 10to 100. For triblock copolymersof either the AAIA or BIAIB type, in which the Ablocks are strongly attracted to the surface while the B blocks are not, Balazs et al.’ have simulated the effect of molecular architecture on block copolymer adsorption to flat surfaces. For this situation their model suggests that the BIAA type gives rise to lower surface coverage and thicker adlayers than the AIBIA arrangement. Since adsorbed polymers may contain more or less flexible chains, dynamic information is needed to obtain a full understanding of their surface behavior. The dynamic properties of a polymer depend on its chemical composition and its level of solvation, and studies of polymer dynamics can therefore aid in the formulation of models regarding their structure in the free and adsorbed state. The conformational changes of polymers induced either by temperature shiftss or by their adsorption a t the solid/liquid interfaceg can be revealed by measurements of polymer chain mobility. In ref 9, it was shown that the fraction of adsorbed segments of various polymers can be distinguished by the ratio of fast mobility (far away from the surface) to slow mobility (close proximity to the surface). These authors suggested that adsorbed poly(methyl methacrylate) on silica surfaces remained in a flat configuration of low mobility throughout a range of surface concentrations, whereas the conformatiodmobility of poly(vinylpyro1idone)on carbon surfaces was found to vary with surface coverage. Similarly, Thambo and Miller,lo in studies of polymer adsorption on mica, found the polystyrene conformation to depend on surface cover(7)Balazs, A.C.;Gempe, M.; Lantman, C. W. Macromolecules 1991, 24,168. (8)Tenhu, H.; Sundholm, F. Br. Polym. J . 1990,23,129. (9)Robb, I. D.;Smith, R. Polymer 1977,18, 500. (10)Thambo, G.;Miller, W. G. Macromolecules 1990,23,4397.
0743-7463/94/241Q-4475$04.5QIQ 0 1994 American Chemical Society
Li et al.
4476 Langmuir, Vol. 10, No. 12, 1994 age while poly(methy1methacrylate), being more strongly adsorbed to mica, remained in a flat surface arrangement. In studying the polymer motion on different oxide surfaces, Miller et al.ll found poly(viny1 acetate) to show a more flexible conformation on silica surfaces than on alumina or titanium dioxide surfaces. Although the conformation of polymers adsorbed on solid surfaces may be affected by the curvature of adsorbates, there are only very few published results that support this notion, and the surface properties are most commonly assumed to be constant for a given surfactanthubstrate pair.l2 Nevertheless, in a study of adsorption complexes formed between PVA of various molecular weights and PS latex particles of different diameter, Garvey et al.13 found the thickness ofthe adlayer to be larger on particles with larger radius than on their smaller counterparts. From the above observations, it is clear that the interaction between adsorbed polymers and their substrates is significantly influenced by the physicochemical nature of both which, in turn, governs the surface conformation of the polymer. A series of poly(ethy1ene oxide)-poly(propy1ene oxide) triblock copolymers with the trade names of Pluronic or Poloxamer have generated much interest both in academic research and for industrial use because oftheir exceptional versatility. Pluronic surfactants are widely used as cleaning detergents, emulsifiers, colloid stabilizers, etc. because oftheir high chemical and thermal stability. Their physical properties in aqueous solution have been examined by static and dynamic light scattering, nuclear magnetic resonance, and fluorescence spectroscopy and found to strongly depend on the weight ratio of poly(ethylene oxideYpoly(propy1ene oxide) (PEOPPO), the incubation temperature, and the presence of electrol y t e ~ . ~Pluronics ~ - ~ ~ have lately generated considerable interest in biotechnology and medical applications because of their low toxicity and lack of immunogenic activity. For example, these compounds have been applied to reduce protein adsorption and platelet adhesion on hydrophobic ~urfaces,~'J* to serve as protective agents in cell reactors,lg and to stabilize both colloidal drug targeting carriersz0 and fluorocarbon emulsions.z1 Although numerous applications of these compounds have been described lately, no systematic studies have related the chemical composition of these nonionic surfactants to their physical properties in the adsorbed state. Here, we report on an investigation of the surface arrangement and dynamic characteristics of a series of Pluronics adsorbed on PS latex particles of different curvature. These surfactants are selected to probe the effect of PEO chain length and the hydrophilidhydrophobic block length ratio on the adsorption characteristics. (11)Miller, W. G.; Rudolf, W. T.; Veksli, Z.; Coon, D. L.; Wu, C. C.; Liang,T. M. Molecular Motion in Polymers by ESR;Boyer, R. F., Keinath, S. E., Eds.; Harwood Academic: New York, 1980; p 145. (12) Baker, J. A.; Berg, J. C. Langmuir 1988,4, 1055. (13)Garvey, M. J.; Tadros, TH. F.; Vincent, B. J. Colloid Interface Sci. 1976,55,440. (14)Attwood, D.; Collett, J. H.; Tait, C. J. Int. J.Pharm. 1986,26, 25. (15)Zhou, Z.; Chu, B. Macromolecules 1988,21,2548. (16) Almgren, M.; Bahadur, P.; Jansson, M.; Li, P.; Brown, W.; Bahadur, A. J. Colloid Interface Sci. 1992,151,157. (17)Li, J.-T.; Caldwell, K. D.; Tan, J. S. Particle Size Distribution II; (Provder, T., Ed.; American Chemical Society: Washington, DC, 1991; Chapter 16. (18)Amiji, M.; Park, K. Biomaterials 1992,13,682. (19) King, A. T.; Davey, M. R.; Mellor, I. R.; Mulligan, B. J.; Lowe, K. C. Enzyme Microb. Technol. 1991,18,148. (20)Muller, R. H. Colloidal Carriers for Controlled Drug Delivery and Targeting; CRC Press: Boston, MA, 1991. (21) Nagata, Y.; Mondon, C. E.; Cooper,A. D. Metabolism 1990,39, 682.
Table 1. Physical Properties of the Selected Pluronics trade name Pluronic P105 Pluronic F68 Pluronic F88 Pluronic F108
mol w t n 6 500 8 400 11 400
14 600
PEO/PPO/PEO" 37/56/37 76130176 104/39/104 129/56/129
HLBa 12-18 224 >24 >24
density (gImLlb 1.112 1.173 1.175 1.186
Data are from BASF Co. Measured by the authors.
Polystyrene latex spheres are useful as model substrates because they have a broad range of uniform particle sizes and well characterized properties. For a given Pluronic, we are specifically interested in the surface properties of layer thickness, surface concentration, and surface dynamics and their relationship to the different curvatures presented by the core particles when various sizes of particles are employed. In the present study, adlayer thicknesses were carefully measured both by field-flow fractionation (FFF)and by photon correlation spectroscopy (PCS),whereas surface concentrations were determined by a newly developed approach based on the retention behavior in sedimentation FFF.zz In addition, chain mobilities of the adsorbed polymers were characterized by electron spin resonance (ESR) measurements after introduction of a free radical substituent into their poly(ethylene oxide) hydroxyl tails. When PS colloidal particles are used as substrates in an aqueous medium, it is believed that the hydrophobic PPO center block is responsible for anchoringthe polymer to their hydrophobic surfaces whereas the hydrophilic PEO blocks will solvate and extend into the aqueous milieu. It is the purpose of this study to seek correlations between the motion of the PEO chains, on the one hand, and adlayer thickness and surface density of surfactant, on the other, for a series of four different Pluronics adsorbed on PS colloids with a 4-fold variation in size.
Experimental Section 1. Materials. Polystyrene (PS)latex standards with nominal diameters of 69,130,212, and 272 nm (from Seradyn) were used as substrates in the adsorption experiments, while the adsorbing species were the polymeric surfactants Pluronic P105, F68, F88, and F108 from the BASF Corp. "he physicochemical properties of these triblock copolymers are summarized in Table 1. The 3carboxy-2,2,5,5-tetramethyl-l-pyrrolidi(3-carboxy-PROXYL) was purchased from Aldrich, while thionyl chloride and all organic solvents used in the spin labeling were from EM or Mallinckrodt. The adsorption of surfactant was performed by incubating the latex particles (2.5% (w/w)) with Pluronic (4.0% (w/w)) in phosphate-buffered saline (PBS, Z = 0.15 M, pH 7.4) for periods of 24 h under constant end-over-end shaking at room temperature. For the ESR experiments, the various free radical labeled Pluronics, dissolved in doubly distilled water, were used in the coating process. The coated particles were then freed from unbound and loosely bound labeled species by means of centrifugation in a table-top centrifuge (Eppendorf5414) operating at 16000g,removal of the supernatant, and resuspension in pure water. "his procedure was repeated until the washings were free from measurable free radicals. In the case of the small particles (69 nm in diameter), the washings were performed by resuspension and refiltration on an Amicon Centricon-30 filter (MW 30 000 cutoff). 2. Determination of Adsorbed-Layer Thickness. FFF was the primary method used in the precise sizing of both bare particles (before adsorption) and surfactant-coated particles (after adsorption), followed by thickness (6)calculation (22) Li, J.-T.; Caldwell, K. D. Langmuir 1991,7, 2034.
Pluronic-Coated Polymeric Colloids
Langmuir, VoE. 10,No. 12,1994 4477
The bare particles were sized using the highly size-selective sedimentation FFF (sedFFF, see below) for all particles, except the smallest one ofthe set (69 nm in diameter) which lies outside the resolution range for our sedFFF system. Since the exact density, required for sizing by sedFFF, is not available for the polymer-particle complexes, a combination of sedFFF and PCS has been used in sizing the coated particles. In this approach the sample is simply separated by sedFFF, whereby potential aggregates are removed, to leave a monomer cut of uniform size which is suitable for characterization by PCS. Specifically, fractions (generally 2 mL each) were collected at each peak maximum during the elution course, and their particle diameters were subsequently evaluated using a PCS instrument from Brookhaven Instruments, consisting of a BI 200 goniometer and a BI 2030 72 channel autocorrelator. Detailed description of instrumentation and operation procedure have been reported e1sewhere.l7,22 The size of the smallest particles of the set was measured by flowFFF, before and after Pluronic coating, using a cross-flow of 15.6 mL/h and a carrier flow of 6.5 m u . The carrier was 0.1% FL-70 for analyses ofthe bare particles and PBS for the adsorption complex. According to G i d d i n g ~the , ~ ~retention ratio R of a sample under the "normal mode" of FFF operation is expressed by
v" = R = 6 l [ ~ o t h (-~ 2A] ) Vr 2A
(2)
where v" is the void volume of the channel and V, is the observed retention volume. In flowFFF, under a certain cross-flowVused as the separation field, the hydrodynamic diameter d of a particulate sample is explicitly related to an experimentally measured value of parameter I in the following way24
kTv" =(3qAVw2)
(3)
where 77 is the carrier viscosity, w is the thickness of the separation channel, T i s the absolute temperature, and k is the Boltzmann constant, respectively. The flowFFF channel (from FFFractionation, Inc.) used in this study has channel dimensions of 27.60 x 2.00 x 0.0254 cm3, and a measured void volume of 1.17 mL. All reported size values are the means of a t least three determinations made under identical conditions. 3. Sedimentation Field-Flow Fractionation for the Quantification of Adsorbed Pluronic Surfactants. In sedFFF, samples are separated according to differences in buoyant mass.26 "his is accomplished by exposing the thin separation chamber (dimensions 94.50 x 2.00 x 0.0254cm3)to an externally applied sedimentation field which forces suspended particulates to migrate radially and establish mass/diffision controlled accumulation zones near one of the channel walls. Whether this occurs at the outer or the inner wall is determined by the relative densities of the particles and their surrounding fluid. The laminar flow of liquid through this channel leads to tangential transport of the various sample zones a t rates proportional to their extension from the wall. Due to the significant velocity gradients which develop near the walls in these thin, ribbon-like FFF channels, small differences in mass, and thus in compressed layer thickness, are transformed into rather substantial differences in elution time. A recent study from this laboratoryz2has shown that one can accurately convert a recorded retention under a centrifugal field into the mass of adsorbed polymer on colloidal particles when the densities of the core colloid, the unsolvated polymer, and the suspension medium are known. In other words, the observed retention provides a direct measure of the surface concentration of adsorbed polymer assumingthe surface area of the core particle (23)Giddings, J. C.Science 1993,260,1456. (24)Giddings, J. C.; Yang, F. J.;Myers, M. N. Science 1976,193, 1244. (25)Li, J. M.; Caldwell, K. D.; Machtel, W. J. Chromatogr. 1991, 517,361.
is accurately determined prior to coating. Since the approach has been described in detail in ref 22, we will here only briefly summarize a few key features. In sedFFF, the experimental I value (calculated from the corresponding retention volume using eq 2)is a function of the strength of the applied field G and the buoyant mass m' of each particle26
A=-
kT m'Gw
(4)
For coated particles, m' can be explicitly expressed as
where @A, @B, and @care densities of the core particle, polymer, and liquid medium, respectively. The masses ofthe core particle and the adsorbed polymer are represented by mA and mB. By combining eqs 4 and 5 one obtains
After a slight rearrangement, eq 6 can be shown to express mB as a function of the retention parameters for bare and coated particles, I A and &&dl recorded a t a fured field strength
Since the size (mass) ofthe core particle can be precisely measured by many sizing techniques including FFF and PCS, and since the densities of each component, @A, @B, and ec, may be determined by pychnometry, the mass of adsorbed materials mB is readily obtained from dcoated according to eq 6 or 7. Thus, the value for the surface area occupied per surfactant molecule (inverse of surface concentration) is expressed as
where d~is the diameter of the core particle, MW is the molecular weight of adsorbed polymer, and NA represents Avogadro's number. The accuracy of this sedFFF based measurement for the determination of polymer adsorption has been verified in parallel experiments using radioisotope-labeled surfactants.22 Furthermore, the validity of this method in determining the amount of adsorbed protein on small polymeric particles has been proven by direct quantification using amino acid analysis.27 4. ESR Investigation of Pluronic Dynamics in Adsorption Complexes with Polystyrene Colloids. In order to measure the dynamics ofthe PEO-PPO-PEO triblock copolymer by ESR, the 3-carboxy-PROXYL probe was covalently attached to the hydroxyl ends of the flanking PEO blocks. The probe molecule is assumed not to interfere with the host polymer, but to reflect its chain motion. Free radicals containing a nitroxide moiety, such as the one selected here, are commonlyused in ESR experiments because of the following three convenient features: (1)they are remarkably stable; (2) they offer functional groups that facilitate labeling; (3) the shape of the triplet spectrum is characteristic of the motion of the probe. The spin labeling reaction applied to the selected Pluronics is outlined in Figure 1. A 0.6-mL aliquot of 0.33 M 3-carboxyPROXYL free radical dissolved in a 1:l mixture of benzene and pyridine was mixed with SOClz to give a 1:1.2 mole ratio of free radical to SOC12. After 15 min, this solution was added to 4.5 (26)Giddings, J. C.; Yang, F. J. F.; Myers, M. N. Anal. Chem. 1974, 46,1917. Dalgleish, D. G. J.Chromatogr. (27)Caldwell, K. D.;Li, J.M.; Li,J.-T.; 1992,604,63.
Li et al.
4478 Langmuir, Vol. 10, No. 12, 1994
0' FIOB-OH
I
0
?C - 0 - F l O 8 I
0
Figure 1. Schematic representation of the synthesis of spin-
labeled Pluronic F108.
mL of a 0.011 M Pluronic solution in pyridine and was shaken overnight. The product was precipitated with ethyl ether, and the precipitate was recovered by filtration and then dissolved in chloroform. The precipitation procedure was repeated three times, and the product was recovered by evaporation of the remaining solvent under vacuum overnight. For the low molecular weight PluronicP105, the reagent concentrationshould be twice as high as in the procedure described above; the labeled P105 can only be precipitated by extremely large amounts of petroleum ether (the optimum volume ratio is 1:80) and is therefore somewhat more difficult to purify than the other surfactants which all have higher PEOPPO ratios. The degree of substitution of the product was measured by comparing the intensity of doubly integrated ESR spectra of spin-labeled Plqonic with the corresponding spectra of nitroxide radical standard solutions. Chain mobility measurementswere performed at 25 "C using an ESR spectrometer (ER200,Bruker) operating at the X-band frequency of 9.15 GHz, scanning from 3445 to 3505 G. Experiments were done in quartz ESR sample tubes or in an aqueous flat sample cell (Wilmad). Sincethe rotationof the polymer chains in our case was found to be isotropic and in a fast tumbling range, the chain mobility could be characterized quantitatively ~ values by the calculated rotational correlation time Z R . ~Thus, for ZR were derived from the following relationship Z , = (6.73x
10-10)AHo[(I~I+l)'2 - 11
(9)
where A H 0 is the width of the central peak, and ZO and Z+1 represent the intensities of the central and strong field peaks, respectively.
Results and Discussion In aqueous environments a stabilization of polymeric colloids is effectively accomplished by the adsorption of amphipathic surfactants that are composed ofhydrophobic blocks, which serve to anchor the macromolecule on the colloid surface, and hydrophilic blocks, which extend into the surrounding liquid. In so far as this coating can provide efficient interparticle repulsion, the colloids remain stably suspended as distinct entities. Accordingly, a nonionic polymeric surfactant of Pluronic type, which contains two hydrophilic PEO blocks flanking one hydrophobic PPO block, is likelyto reorient from a spherically symmetrical block arrangement in solution to a new and less symmetrical conformation as it adapts to the hydrophobic PS particles. Its conformation in the adsorbed state is thus one where the PPO block adheres firmly to the (28) Kuznetsov, A. N.; Wasserman, A. M.; Volkov, A. N.; Korst, N.
N . Chem. Phys. Lett. 1971,12,103.
Figure 2. Schematic representation of Pluronic adsorbed to PS surfaces: (A) the PPO block anchors the macromolecule to
the hydrophobic surface,while the PEO chains extend into the aqueous environment; (B) a high surface concentration forces an extended PEO chain conformation("brush");(C)a low surface concentration allows "mushroom-like"behavior. particulate surface while the two flanking PEO blocks remain strongly solvated with a negligible adsorption to the surface (Figure 2A). The more hydrophobic the particle surface, the stronger are the binding forces, which are thought to be largely hydrophobic in nature.29 In the present study we set out to answer the following two general questions: (1)Is there a difference in polymer surface arrangement between particles of different diameter coated with the same polymeric compound? (2)Is there a difference in adsorption between Pluronic surfactants with different PEO block length and PEOPPO composition when adsorbed to PS particles of identical size? 1. Effect of Substrate Curvature on Pluronic Adsorption. By relating the measured parameters of adlayer thickness and polymer surface concentration to the size of the colloidal substrates to which they were adsorbed, we found for each Pluronic with a n 80% PEO content that both the thickness and the amount of adsorbed polymer per unit surface area (or the inverse of occupied surface area per polymer molecule) increase with increasing particle size (Figures 3 and 4). Although these trends are the same for the three analogues referred to as F108, F88, and F68 (see Table l), the Pluronic P105 with its 50% PEO content behaves differently, as its adsorption behavior does not appear affected by substrate curvature. The exact reason for this is not known, but ESR studies of the PEO chain dynamics in the various mother liquors have indicated that P105 forms micelles at the 4% (w/v) concentration used in the a d ~ o r p t i o n . ~ ~ Since the latex particles were all polymerized from the same monomer, their chemical nature and surface prop(29) Li, J.-T.; Carlsson, J.;Caldwell, K. D. Polym. Muter. Sci. Eng. 1993,69,62. (30)Rapoport, N.; Caldwell, K. D. Colloids Surf., in press.
Pluronic-Coated Polymeric Colloids
Langmuir, Vol.10,No. 12, 1994 4479 concentrations to be comparable for surfactants with similar size of the PPO blocks. This fact is illustrated in Figure 4 by the close agreement between data points for each of the F-type surfactants on any given particle size. Although differences are small, the F68 and F88 surfactants with their shorter PPO blocks are always somewhat less tightly packed than their F108 counterpart. The significant difference in surface concentration observed for the P105 surfactant, with its lower PEO content, is likely related to its tendency to form micelles a t the concentration used for a d s ~ r p t i o n Nevertheless, .~~ for the three members of the F-series studied here, the closepacking on the surface appears to be insensitive to the size of their PEO blocks. By contrast, Figure 3 gives clear evidence that the thickness ofthe adsorbed layer increases with increasing PEO block size. Figure 4 is thus a n indication that the surface concentration of selected surfactants is dominated by the PPO block. Similar observations to the effect that the adsorption of block copolymers is strongly dependent on only one of the constituent polymer blocks were also reported by Parsonage et al.6 3. A Dynamic Polymer Adlayer. The conformation of a polymer chain is reflected in its dynamics. This implies that the segmental motion in a solvated polymer is collectivelygoverned by the polymer-solvent interactions, the temperature, the chemical nature of the polymer, as well as its size and configuration. In the present study, block size and surface concentration are the two parameters subjected to variation, and their effects on the dynamics of the PEO end groups are followed by ESR. As outlined in the methods section, the PEO end groups were labeled by the introduction of a nitroxyl spin probe (3carboxy-PROXYLprobe, molecular weight 186). In order to verify that the motion of this probe reflected the motion of the polymer chain, rather than the probe itself, we compared the dynamics of the probe in free and attached form. The mobility of the free probe was first examined in the aqueous environment of interest here. The high value of 2.86 x 1O1O s-l for the inverse of its correlation time, l / z ~ , proved that this small molecule undergoes fast random motion in the selected solvent. The nitroxide radical was then conjugated to the Pluronic molecules according to the labeling chemistry outlined in the methods section. As indicated in Figure 5, the attachment of the probe to the polymeric surfactant (F108 in this case) somewhat restricted its motion. This is clear evidence that the probe reflects the motion of the macromolecule. In the adsorbed state, both the size of the substrate particles and the nature of the adhering polymer were found to affect the motion of the radical probe, as illustrated in Figures 5 and 6. The spectra presented in Figure 5 demonstrate the restriction of PEO chain motion in the adsorption complex between Pluronic F108 and PS272 as compared to the chain motion in an aqueous solution of the surfactant. Figure 6 , in turn, illustrates the substantially restricted motion of these polymer chains in the adsorption complex with large PS particles compared to their behavior on small PS particles where their mobility is affected only slightly. One can explain this by correlating chain mobility to other surface parameters mentioned previously. Thus, on small particles (69 nm in diameter), the surface density of polymer is low and the chains are correspondingly highly mobile, while on large particles (272 nm in diameter), the polymer layer is less mobile because of high surface coverage. The effect of block size on PEO mobility is visualized in Figure 7. Since the radical probe is attached to the terminal hydroxyl of the PEO chain, the ESR measure-
1
A F68 A
P105
A
0
50
loo
150
200
250
I 300
Particle Size (nm)
Figure 3. Curvature-dependent thickness of the adsorbed
Pluronic layer. Size determinations are based on a minimum of three measurements and have a standard deviation of better than 1%. JJ
FlO8
30 -
0 F8B A F68 A P105
A
A
0
50
100
150
200 Particle Size (nm)
250
300
Figure4. Curvature-dependentsurfacedensity of the adsorbed Pluroniclayer. The experimental data represent averages of at least three runs with standard deviations of less than 3%. erties could be considered nearly identical. Indeed, there appeared to be no significant difference in surface charge between these PS particles, according to observations by laser doppler a n e m ~ m e t r yand , ~ ~we therefore interpret our observed substrate curvature effects as being due to steric influences a t the interface. In the context ofPluronic adsorption, our data indicate PS colloids with sizes less than 272 nm to be curved surfaces, since above this size, the adsorption behavior becomes nearly constant.22 2. Effect of Pluronic Compositionon Adsorption. The adsorption of block copolymers is apparently controlled by the chemical composition of the different blocks, the properties of the surface, and the nature of the solvent. The latter factor is not evaluated in the present case, since the adsorption of Pluronic is being performed in the same aqueous buffer throughout, and the effect of substrate curvature can be eliminated by comparing the adsorption behavior on PS colloids of the same size. In this way, the effect of block copolymer composition on the adsorption can be isolated from other factors of significant influence. It was expected that the adsorbed amount of Pluronic would be strongly influenced by the size, or molecular weight, of both the hydrophilic PEO blocks and the hydrophobic PPO block, since such a n adsorption is believed to be a competitive process between good solvation, due to the PEO portion, and strong binding due to the PPO portion of a polymer molecule. However, under specific circumstances it may be essentially determined by only one of the blocks. In the case of Pluronic adsorption to PS particles of the same size, we found the surface (31)Caldwell, I1 D.;Gao, Y.S. Anal. Chem. IWS,65, 1764.
Li et al.
4480 Langmuir, Vol. 10,No. 12, 1994 1.8,
I
~
P105
F88
F68
F108
Pluronics Figure 7. Mobility of PEO chains in various Pluronics in adsorption complexes with particles of different size.
Figure 5. ESR spectra: (A) free 3-carboxy-PROXYLin water solution; (B) spin-labeledPluronic F108 in water solution; (C) spin-labeled Pluronic F108 in adsorption complex with PS272 (aqueous suspension). OJ
;
E
,J P105
1.8 1.6
0.5
F68
F88
F108
~
~
1.4.
1.2.
A
1.0,
I Free Radical
FIOB'
F108'-Ps69
F108'-PS272
Figure6, Mobility of nitroxyl radical in differentenvironments.
ments indicate the motion of the PEO tails. Consequently, it is reasonable to relate the polymer chain mobility to the number of PEO monomer units in each flanking block. From the figure one finds that, for particles of a given size, the mobility increases with increasing number of PEO monomer units. I t is also clear that the effect of PEO chain length on probe dynamics is much more pronounced on small particles than on large ones, likely due to the differences in close packing, as demonstrated in Figure 4. Similarobservations have been made by others studying the behavior of PEO on silica bead^.^^^^^ Here, homopolymers of different molecular weight were first chemically bound to the silica surface and then terminally labeled with a free radical. These studies showed the surface dynamics to depend on temperature, solvent, polymer molecular weight, and grafting density. There was no molecular weight dependence seen for the polymer motion in bulk solution, even though the MW of PEO spanned a much wider range (400-6000) than in the Pluronics selected here. Nevertheless, such a dependence appeared, and became significant, when the polymers were grafted onto the solid surface; in the quoted case the PEO rotational correlation times increased around 1 order of magnitude. It was clear that the end-group mobilities of the grafted chains became much more restricted for low (32) Hommel, H.; Facchini, L.; Legrand, A. P.; Lecourtier, J. Eur. Polym.J. 1978, 14,803. (33) Ouada, H. B.; Hommel, H.; Legrand, A. P.; Balard, H.; Papirer, E . J. Colloid Interface Sci.1988,22, 441.
OJ
,
P105
F68
F88
F108
'
1.0
Figure 8. Mobility of PEO chains and adlayer thickness in Pluronic adsorption complexes with (A, top) PS272 and (B, bottom) PS69.
molecular weight polymers than for their high molecular weight analogues. Indeed, the motion of the shorter chains was found to be close to the lower limit of the fast tumbling range. The situation is quite different for Pluronic adsorbed on PS particles. From the patterns of the ESR spectra and the determined rotational correlation times, the adsorbed Pluronics appear to have much faster motion than the silica-grafted PEO chains. Killmann et aL2have compared the adsorption properties of the homopolymer PEO on silica beads and on polystyrene particles. According to these authors, the PEO layer thickness formed from one and the same sample is smaller on the silica surface than on the polystyrene surface. I t was explained that PEO appears to have a flat configuration on the silica surface because of the favorable potential for hydrogenbond formation between SiOH groups and PEO, while the polymer is much further extended into the aqueous environment when attached to the polystyrene surface with which it is unable to form such strong bonds. The relationship found in the present study between surface dynamics and PEO block length for Pluronic adsorption on the 272- and 69-nm particles, respectively, is further illustrated in Figure 8. As can be seen from both figures, the PEO dynamics on the surface is positively
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“mushroom” conformation (Figure 2C). Unlike the homopolymer PEO, which is believed to adsorb onto surfaces in the form of trains, loops, and tails, with all segments 8 PEO equally likely to adsorb, the unique structure of these pluroNc adsorbed block copolymers is a balanced product of the interactions of each block with solvent and surface. Through their structural adaptation to the surface, the Pluronic surfactants are more permanently binding than the highly dynamic, loosely associated homopolymer PEO. The difference in surface adhesion between the PEO homopolymerand the PEO-containing copolymer has been experimentally demonstrated in a separate study from this l a b ~ r a t o r y Part . ~ ~ of the research objectives for that 0 study was to examine the protein adsorption to PS colloids 0 20 40 60 80 100 120 140 treated with homopolymer PEO (MW 22 000)as compared PEO Monomer Units to a treatment with the Pluronic F108 (MW 14 600). Since Figure 9. Comparisonof layer thickness of adsorbed Pluronic it is believed that longer PEO chains confer a better protein and the hydrodynamic diameter (2R,) of PEO homopolymer in resistance than shorter ones, the homopolymer treatment free solution, adapted from ref 38. should have a better or at least an equal effect compared to that of a n F108 treatment if both were stably attached correlated with the length of the PEO blocks. In free to the particulate substrates. However, our data indicate solution, PEO behaves as a highly flexible polymer according to dynamic studies by electron spin r e s o n a n ~ e , ~ ~ the F108 coated PS to have a minimal protein uptake, nuclear magnetic r e l a ~ a t i o nand , ~ ~light ~ c a t t e r i n gIn . ~ ~ while the protein uptake on homopolymer PEO coated particles is indistinguishable from that found on nonthe adsorption complex, the mobility of the polymer chains treated bare particles. Proteins can therefore easily is restricted by the PPO-bound terminals, a restriction displace the homopolymer PEO layer and adsorb to the that becomes less significant the longer the PEO chain. exposed PS substrates, while they leave the F108 coated In addition, our data indicate the PEO tails to become particles virtually intact. This fact, together with the much more mobile on small particles than their counapparent leakage of PEO from adsorption complexes when terparts on the large particles. This can be explained by the coated particles were exposed to large amounts of the polymer chains having more freedom of movement on diluent, again illustrates that there exist only weak the less packed surface (small particles) than on a more attractions between PEO and the hydrophobic PS surface. densely packed surface (large particles). To further illustrate this point, the adlayer thickness is included in Conclusions Figure 8. As was pointed out in the discussion of Figure 3, the adlayer thickness is a function of the size of the The understanding of the interfacial properties of PEO chains for both particle substrates. On the small PS Pluronic surfactants on polymeric colloids is of considerlatex, the thickness of each particular Pluronic adlayer is able interest for processes such as steric stabilization and close to twice the radius R, of its PEO chains in free suppression of protein adsorption. We have performed solution, while on the large particle (PS272), the PEO a n analytical characterization of the surface properties of chains have a much more extended conformation resulting a set of selected Pluronics with similarly sized PPO center in a layer thickness of approximately 4R,. The R, values blocks, but with varying lengths of their PEO blocks. These for homopolymer PEO with different molecular weights surfactants were adsorbed t o PS latex particles of different, were taken from the work of Timasheff et al. and were well-characterized sizes. The characteristics of the adbased on measured intrinsic v i s c ~ s i t i e s . ~ The ’ , ~ ~finding, sorbed Pluronics were parameterized by three measurable for small particles, that the adlayer thickness coincides factors: thickness of adlayer, surface concentration, and with the hydrodynamic diameter of a comparable PEO surface mobility; the analytical techniques used here to chain in free solution (Figure 9) suggests that the measure these factores include FFF, PCS, and ESR. anchoring PPO block strongly adsorbs to the surface Field-flowfractionation, specificallyin its sedimentation providing a negligible layer thickness compared to that modification, has been adapted to serve as a sensitive of the highly solvated PEO blocks. analytical balance for this purpose, while the flow modification has permitted a sizing of the adlayers formed From the above discussion, we believe the molecular during polymer adsorption to the colloidal substrates. The architecture of adsorbed Pluronic on the hydrophobic PS surface to conform to the schematic illustration in Figure layer thickness has additionally been measured by means 2, with the PEO blocks stretching out into the aqueous of PCS, a s is commonly done. For the surfactant-coated solution, while PPO is being strongly attracted to the particles, prefractionation by FFF removes any aggregates surface with a flattened configuration (Figure 2A). We which might have formed during the adsorption procedure and permits a more accurate layer-thickness determinacan rationalize the conformation of the adsorbed Pluronic tion than a direct PCS measurement. surfactants using the scaling concept of de G e n n e ~i.e., ;~~ the higher the surface concentration, the further the Until the development by usz2 and independently by polymer chains extend out from the surface in a “brush” Beckett et al.,41the sedFFF had not previously been used type arrangement (Figure 2B), while a low surface for measuring surface concentrations of adsorbed matericoverage causes the polymer chains to collapse into a als. Here, we have shown that the increases in particle retention associated with a n uptake of surfactants can be directly related to the mass adsorbed per particle, which (34) Lang, M. C.;Laupretre,F.;Noel, C.; Monnerie,L. J.Chem. SOC., Faraday Trans. 2 1979,75,349. is immediately translated into the surface concentration.
*
(35) Breen, J.;van Duijin, D.;de Bleijser, J.;Leyte, J. C. Ber. Bunsen-
Ges. Phys. Chem. 1986,90,1112.
(36) Zhou, P.;Brown, W.Macromolecules 1990,23,1131. (37) Arakawa, T.; Timasheff, S. N. Biochemistry 1986,24,6756. (38) Bhat, R.;Timasheff, S . N. Protein Sci. 1992,1 , 1133. (39) De Gennes, P.G. Adu. Colloid Interface Sci. 1987,27,189.
(40) Tan, J. S.;Butterfield, D. E.;Voycheck, C. L.;Li, J.-T.;Caldwell, K.D. Biomaterials 1993,14,823. (41) Beckett, R.;Ho,J.; Jiang, J.;Giddings, J.C. Langmuir 1991,7, 2040.
4482 Langmuir, Vol. 10, No. 12, 1994
Compared to conventional labeling methods, which generally involve delicate synthetic work and time-consuming product purification, sedFFF offers an efficient means for determining the surface distribution of adsorbed materials. The ESR technique has allowed us to monitor the motion of the spin probe attached to the surfactant polymers. From these observations it is clear that the Pluronic adsorbed on PS latices forms a highly mobile polymer layer whose thickness and mobility stem from the hydrated flexible PEO chains. The noted faster motion of the adsorbed Pluronic on smaller substrate particles gives strong support for the independent finding of a substrate curvature effect on polymer adsorption. Thus, the polymer molecules were found to undergo relatively slow motion on low curvature surfaces with high packing, while their labeled ends revealed faster tumbling on highly curved surfaces with low coverage. The existence of a strong curvature effect on the surface arrangement of the adsorbent, demonstrated by the three measurable factors discussed here, is only infrequently reported in the literature. The surface motion was also found to depend on PEO chain length: the longer these chains, the faster their motion. Since the spin labels are introduced into the hydroxyl ends of the PEO chains, whose other ends are
Li et al. attached to the PPO blocks, this increased motion with increasing length strongly suggests that the PEO blocks are only weakly, if a t all, involved in the actual attachment to the surface. For the adsorption of a given surfactant, it is found that smaller particles take up fewer polymer molecules per unit area than do the larger particles, resulting in thinner adlayers on the small latex. However, this arrangement allows the PEO chains greater freedom of motion, to the point where the measured chain mobility on a 69-nm PS colloid is close to that found for the surfactant in free solution. The third parameter investigated here, namely the surface concentration resulting from adsorption to PS latices of a given diameter, is determined by the size of the hydrophobic PPO center block, rather than by the length of the flanking PEO tails. Thus, Pluronics with PPO blocks of similar size show comparable surface concentrations, regardless of the PEO block length; however, the longer the PEO chains, the thicker is the adlayer being formed and the greater is the dynamics of their end-groups.
Acknowledgment. Support for this work by Grant No. GM 38008-05 from the National Institutes of Health is gratefully acknowledged.
