Langmuir 1994,10,3500-3506
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Terminally Attached Polystyrene Chains on Modified Silicas T. Cosgrove,* T. G. Heath,? and K. Ryan$ School of Chemistry, University of Bristol, Bristol BS8 ITS,U.K. Received March 3,1994. I n Final Form: June 19, 1994@ Polystyrene of various molecular weights has been terminally grafted onto surface-modified silicas. Small-angleneutron scattering has been used to investigate the structure of these adsorbed layers. For grafted layers in which the polymer also physically adsorbs, there is no clear evidence of scattering from fluctuations at the highest scattering vector studied (0.7nm-'). Experimentscarried out where the particle scattering length density has been matched to the substrate give virtually the same volume fraction profiles as do those which are not matched. However, for a system where the polymer does not physically adsorb, it is not possible to give an absolute indication of a possible contribution from fluctuations at high Q because of uncertainty in the background scattering. This problem can be overcome by fitting the data sets such that any Lorentzian contribution at high Q is effectively eliminated, minimizing any anomalous scattering. The results are compared with mean-field calculations and scaling predictions for polymer adsorption. For systems where the adsorption enthalpy is unfavorable, the volume fraction profiles of the adsorbed layer show a distinct maximum, whose position depends on molecular weight and surface coverage. The root mean square thickness of the adsorbed layer in this case follows reasonably well the scaling prediction of Alexander and de Gennes. For favorable values of the adsorption enthalpy, the volume fraction profiles fall monotonically in a manner similar to that found for homopolymer adsorption.
Introduction The adsorption of polymers a t interfaces and the stabilization of colloidal suspensions by such adsorbed layers have been the subject of many experimental and theoretical' studies. One case of considerable interest is that of the polymer brush regime: in which the adsorbed polymer chains are grafted to a solid surface, via a reactive polymer end group with a sufficient surface density such that extensive overlap occurs. At a given surface density, the chains are forced to stretch away from the surface so that there is a linear dependence of thickness on chain length.2 In contrast to physically adsorbed homopolymer layers, the individual polymer segments need not have a favorable energetic interaction with the surface (relative to that of the solvent). Indeed, a repulsion of the grafted (or terminally attached) chain from the surface of colloidal particles might be expected t o impart greater steric stability to the system, particularly a t low surface coverage. From a practical point of view it is often found that higher adsorbed amounts of polymer may be obtained by a grafting reaction than can be obtained with physically adsorbed polymer chains in equilibrium with chains in solution. Several terminally grafted systems have been studied experimentally. These include both aqueous systems, e.g. poly(ethy1ene oxide)/polystyrenelatex3j4and poly(ethy1ene o~ide)/silica,~ and nonaqueous systems, e.g. poly(dimethPresent address: Chemical Studies Department, A.E.A. Technology, Hanvell, Oxon OX11 ORA, U.K. Present address: Shell Research Ltd., Thornton Research Centre, P.O. Box 1, Chester CH1 3SH, U.K. Abstract published inAdvance ACSAbstracts, August 15,1994. (1)Fleer, G. J.;Cohen Stuart, M. A.; Scheutjens,J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers ut Interfaces; Chapman and Hall: London, 1993. (2) Milner, S. T. Science 1991,251,905. (3)Cosgrove, T.;Ryan, K. Langmuir 1990,6,136. (4) Cosgrove, T. J. Chem. SOC.Faraday Tram. l990,86,1323. ( 5 ) Hommel, H.; Legrand, A. P.; Balard, H.; Papirer, E. Colloids Surf. 1988,36,262. @
ylsilo~ane)/silica,6.~ polystyrene/graphite,@and polystyrene/ s i l i ~ a . ' * ~Small-angle J~ neutron scattering, nuclear magnetic resonance (NMR),3,s,14 and electron spin resonance (ESRI5techniques have been applied to these systems. The aim of these studies was to evaluate the structure of the adsorbed layer both directly through the determination of the volume fraction profile and indirectly through the mobility of the adsorbed segments. Both the molecular weight and coverage dependence of the structure of the adsorbed layers have been discussed in detail. The main result of this work is the confirmation that these systems have highly extended volume fraction profiles especially a t high adsorbed amounts compared to physically adsorbing polymers. Detailed theoretical work on terminally attached chains has been carried out using a number of approaches, including self-consistent mean-field theory,15J6 Monte Carlo sim~lation,'~J' and scaling theory.18J9 One important prediction from the first two of these theoretical approaches is that for moderate adsorbed amounts, the shape of the volume fraction profile depends rather critically on the surface affinity of the polymer. Although this effect has been reported for a rather low molecular weight s y ~ t e mno , ~ detailed experimental study on the effects of surface affinity has been reported. Another major (6) Auroy, P.; Auvray, L.; Leger, L. J.Phys.: Condens. Mutter 1990, 2,317. (7) Edwards, J.;Lenon, S.; Toussaint, A. F.; Vincent, B. ACS S y m p . Ser. 1984,No. 240, 281. (8)Bamett, K. G.; Cohen Stuart, M.; Cosgrove, T.; Sissons, D. S.; Vincent, B. Macromolecules 1981,14, 1018. (9) Cosgrove, T. Macromol. Rep. A29 (Supp. 21, 125. (10)Auvray, L.; Auroy, P.; Mir, Y. Phys. Rev.Lett. 1992,69, 93. (11)Cosgrove, T.; Crowley, T. L.; Heath, T. G.; Ryan, K. Macromolecules 1988,20, 2879. (12) Crowley, T. L. D. Phil. Thesis, University of Oxford, 1984. (13)Auvray, L.; Cotton, J. P.Macromolecules 1987,20,202. (14)Cosgrove, T.; Griffiths, P. C. Adu. Colloid Interface Sci. 1992, 42,175. (15)Cosgrove, T.;van Lent, B.; Leermakers, F.; Heath, T. G.; Scheutjens, J. M. H. M. Macromolecules 1987,20,1692. (16) Milner, S.;Witten, T.; Cates, M. Macromolecules 1988,21,2610. (17) Chakrabarti, A.; Toral,R. Macromolecules 1990,23,2016. (18)de Gennes, P. G. J. Phys. (Paris) 1976,37, 1443. (19) Alexander, S. J. Phys. (Paris) 1977,37,977.
0743-746319412410-3500$04.50/00 1994 American Chemical Society
Polystyrene on Modified Silicas Table 1. Grafting InformatiodPolymer Characterization
code rdnm MJg mol-l MdMn solvent rGmax/mg m-2 1.23 DMF DPS24 192f 10 24000 6.5f0.1 24000 1.23 toluene 3.7 f 0.1 DPS24T 58 f 4 PS12 80 f 10 12 000" 1.3 DMF 5.5 f 0.1 PS5 80 f 10 4 70Oa 1.7 DMF 2.7 f 0.1 a Prepared by J. Edwards7 prediction from all three theories is the linear dependence ofthe layer thickness, 6, on chain length,N, at fxed surface density, g . In this paper, a system ofgrafted polystyrene (PS)chains on silica particles has been studied using photon correlation spectroscopy (PCS)' and SANS. The effects of molecular weight and surface modification have been investigated. The results of these experiments are compared with theoretical predictions based on meanfield and scaling theories.
Experimental Section Materials. Monodisperse colloidal silica was prepared by the base hydrolysis of tetraethyl orthosilicate (BDH Ltd., U.K.) in ethanol and water.20 The resulting dispersion was dialyzed against distilled water and redispersed several times in twice distilled water. The particle size distribution was determined from electronmicrographs of the dispersion,taken with a JEOL 100CX-CMelectron microscope using a TG23 size analyzer (Carl Zeiss Ltd.). Particle radii, ro, are given in Table 1. Narrow molecular weight distribution homopolymers of polystyrene were prepared by the anionic polymerization of styrene (Aldrich Ltd. U.K.)in toluene using n-butyllithium (Aldrich)as the initiator. Termination of the living polymer was carried out by addition of dichlorodimethylsilane (BDH). Subsequent addition of methanol gave a methoxysilane-terminated polymer suitable for grafting onto silica particles. The solvent was removed by evaporationunder reduced pressure and the polymer was then purified by precipitation from solution, firstly by addition of hexane to a toluene solution of the polymer and then by addition of methanol to a solution of the polymer in trichloromethane. The purified polymer was then freeze-dried frombenzene. The molecularweight distributionwas determined by gel permeation chromatography using a Waters instrument with trichloromethaneas solvent and calibratedwith polystyrene standards. Details are given in Table 1. The graftingof polystyrene onto silicawas carried out in either dimethylformamide (DMF) or toluene using the method of Edwards et a1.' In the former case the silica was centrifuged and redispersed 4 times into DMF and then added to a solution of the polymer in the same solvent. The mixture was refluxed for 5 days under nitrogen, after which the coated particles were transferred into tetrachloromethane by further centrifugation/ redispersioncycles. In the latter case silicawas centrifuged and redispersed in a toluene/ethanol mixture and then added to a solutionof polystyrene in toluene. The ethanol was distilled off and the mixture refluxed for 5 days under nitrogen. After this, the particles were transferred into tetrachloromethane as described above. Although a gel was formed when the silica was added to the polymer solution,the graftingreaction gave a stable dispersion which consisted of mainly single particles. An aliquot of the DMF grafted sample was subsequently treated with trimethylchlorosilane (BDH) to give silica particles with a methylated surface. The amount of polymer grafted, rGmax, was calculated from elemental analysis; the results are also shown in Table 1. Techniques. The PCS data were obtained using a Malvern K7025 multibit correlator with a Coherent Radiation mode stabilized krypton ion laser operating at a wavelength of 647 nm. The SANS data were obtained at the Institut Laue-Langevin, Grenoble, France, using the D17 camera. Data were collected at wavelengths at 1.45and 0.9 nm and a sample detector distance (20) Stober,W.; Fink, A.; Bohn,E. J . Colloid Interface Sei. lS66,26, 62.
Langmuir, Vol. 10, No. 10, 1994 3501 of 2.8 m, giving an overall Q range of 0.06-0.92 nm-l. The two data sets were combined before analysis.
Theoretical Methods and Analysis Photon Correlation Spectroscopy. The hydrodynamic thickness ofthe adsorbed layers was determined by photon correlation spectroscopy (PCS).' For discrete, monodisperse, spherical particles the measured diffusion coefficient, D,,gives the hydrodynamic radius of the particle, rH, from the Stokes-Einstein equation
where is the viscosity of the dispersion medium, T the absolute temperature, and k g the Boltzmann constant. The hydrodynamic thickness, &, of the adsorbed layer is the difference in rH between the coated and bare particles. The technique can be used to measure rH for the grafted samples where there is a sufficientlyhigh surface coverage to render them stable to flocculation. However, the bare particles are not stable in the solvent medium used (tetrachloromethane) and so a comparable value of rH cannot be obtained directly. To overcome this problem, two different approaches were used. In the first the value for rH in a polar solvent was used and in the second the size was calculated from the SANS data with the adsorbed layer at zero contrast with the solvent (see below). Small-AngleNeutron Scattering. The analysis and interpretation of small-angle scattering from dilute suspensions of small particles carrying polymer layers has been considered by both Cosgrove et al. and Crowley"J2 and by Auvray and Cotton.13 The intensity of the observed coherent scattering per particle, I(Q),may be expressed as the sum of three terms corresponding to the contributions to the scattering from the adsorbed layer or coating, Ic(Q),the particle, IJQ), and a n interference term Ipc(Q) where kP and k c are the differences in neutron scattering length densities of the particle and adsorbed layer with respect to the solvent and C is a n instrumental constant. Q is the magnitude of the scattering vector, 4n sin(8/2)lA, where A is the neutron wavelength and e is the scattering angle. In principle, the average volume fraction profile of the polymer normal to the interface, @(z),may be obtained from either the Zc(Q) or the Ip,(Q)terms. The problems associated with extracting this information in each case have been discussed in detail.l0-I3 In most of the experiments described here, the neutron scattering length densities of the particle and the solvent were matched such that AeP = 0, in order to isolate the Zc(Q) term. This was achieved by the addition of a small amount (10%) of dichloromethane to the pure solvent medium. Under such conditions, the scattering is then given byI2
(3) where Vequals the volume per particle. The contribution to the scatteringfrom any fluctuations in the mean volume fraction profile has been discussed extensively by both the Bristol and Paris g r o u p ~ . ~ OFor - ~physically ~ adsorbing polymers, where the volume fraction profile shape is similar to a n exponential, neither group has found any strong indication of fluctuations, at least within the errors of the experiment. However, for systems where a terminally attached polymer does not also physically adsorb, e.g. poly( dimethylsiloxane) (PDM5V6 and polystyrenelo terminally attached to various pretreated porous silicas,
Cosgrove et al.
3502 Langmuir, Vol. 10, No. 10,1994 anomalous scattering a t high Q was found. This was particularly evident in a Porod plot (Q41(Q)us Q). However, this contribution to the total scattering is still rather small a t intermediate Q and to minimize these effects the following procedure has been adopted. The contribution from fluctuations is expected to follow a Lorentzian law by analogy to polymer in a uniform bulk solution6
-
+
(4) I ( Q ) 1/(1 E2Q2) where Z(Q) represents the contribution from fluctuations and 6 is the correlation length which is of the order of the grafting spacing, D. At low Q the contribution tends to unity and a t high Q to Q-2. In all of our data we have multiplied the scattering by Q2 and then subtracted any baseline. This procedure effectively removes the major contribution to fluctuations at high Q . This was also the procedure adopted for our earlier data for grafted poly(ethylene oxide) on deuterated polystyrene latex.3 The profiles, &I, have been obtained by the Hilbert transform method developed by Crowley1J2and by a leastsquares fitting of the calculated intensity from model profiles8 using eq 3 and, where appropriate, eq 4. The Hilbert method requires fitting to both low and high Q using limiting forms of eq 3. At low Q , the scattering from the adsorbed layer with the particle a t contrast with the dispersion medium can be expressed as a modified Guinier law12
where u is the second moment of the volume fraction - ( z ) ~which , can be obtained profile, given by u2 = (9) directly from the low angle fit. If the profile, &z), is known, then UPRO the second moment calculated from integrating the profile can also be Calculated. Comparison of these two values is useful in assessing the consistency of the data. At the limit of high Q, extrapolation of eq 3 leads to a law of the form I Q-" where n is often close to the Porod law value ( n = 4 ) dependent on the form of Q)(z). Fitting the data to a function of the form
-
+
I ( Q ) = A&-" B (6) where B is the incoherent background, is also required by the extrapolation to ensure a smooth decay of the experimental data prior to the numerical inversion. In all the experiments cited here the high Q data could be fitted to eq 5 with n = 4.0 f0.5, consistent with the Porod law. In the least-squares analysis, no such extrapolations are necessary. Mean Field Calculations Mean field calculations have been carried out by using the Scheutjens-Fleer model, suitably modified for terminally attached chains.15 Several approaches exist where a real space polymer chain has been scaled to a lattice polymer (e.g. ref 3); however, in this paper we take a more pragmatic view. The number of monomers per lattice site has been taken to be 6, based on the persistence length for the polystyrene chain, and a n approximate equivalence of 1mg m-2 to one theoretical monolayer has been used to scale the adsorbed amount. The Flory-Huggins x parameters have been taken from the literature and values of%*,the net solvent-surface interaction, have been chosen to reflect the known adsorption affinities for polystyrene on silica from the various solvents used and to give the best fit to the data.
Table 2. Polymer Dimensions samde
ds2Ynm2
alnm2
DPS24 PS12 PS5
66.4 27.3 11.9
6.1 3.6 2.8
Results and Discussion (i) Grafting and Adsorption Experiments. In contrast to physically adsorbed polymers, the extent of adsorption of grafted polymers is governed by the kinetics of the grafting process rather than by an adsorptiondesorption equilibrium. At short times, the grafted amount. r G , rises rapidly since there is a large area of surface accessible to the reactive polymer ends. As the surface coverage builds up, a "steric" barrier develops, which tends to hinder further grafting by excluding the penetration of the free (unreacted) polymer chains into the interfacial r e g i ~ n .The ~ final surface coverage obtained, rGmax, is dependent on the polymer solution concentration and molecular weight. The values of rGmax for the various samples studied are listed in Table 1. In the experiments where PDMS was grafted into porous silica,6 however, considerably higher adsorbed amounts were obtained. This may be due to a combinationofkinetic and steric factors and the more flexible nature of PDMS compared to polystyrene. However some entrapment of the chains, especially at high molecular weights, cannot be completely ruled out. The area per grafted polymer chain, a,was calculated for each sample. This can be compared to the area projected onto a plane by a polystyrene coil in a dilute solution of a good solvent (e.g. toluene), given by z(s2), where is the radius of gyration. These values are given in Table 2. When aln(s2)
