Langmuir 1994,10, 3156-3160
3156
Effects of Adsorbing-Block Molecular Weight on the Thickness of Adsorbed Diblock Copolymers Richard M. Webbert and John L. Anderson* Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213 Received August 2, 1993. I n Final Form: June 13, 1994@ Adsorbed layers of a diblock copolymer were characterized by measurements of hydrodynamicthickness. (PVPPS),was adsorbed to porous mica membranes from The polymer, poly(2-~inylpyridine)/polystyrene toluene. A series of the diblock copolymer having a constant PS block (Nps !=z 430 monomers) was used to study the effects of the size of the anchoring group (PVP)on the thickness and coverage of the layer. ~ ) in n-heptane ( L H after ~ ) removal of the The hydrodynamic thickness was measured in toluene ( L H and toluene by drying. The coverage (a,chains per area on the pore wall) was estimated fiomLHm by assuming both blocks collapsed into their bulk densities after being dried and after contact with heptane. Though there is a large scatter of our estimates of u,they are in reasonable agreement with the more accurate ~ at first with the size determinations of Parsonage et al. (Macromolecules1991,24, 1987). L Hincreases of the PVP block and then decreases when NpVpINps > 0.1. The maximum in L Hoccurs ~ at NpVpINps !=z 0.05-0.10; our data are not extensive enough to precisely determine the position of the maximum. Using the data for u from Parsonage et al., we conclude that our results for the dependence of LHon Npvp are not consistent with the scaling relation LH NPSall3.
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Introduction The adsorption and assembly of diblock copolymers a t interfaces play a central role in the effective use of these polymers in a number of applications, for example, the stabilization of colloidal paint particles, the compatibilization of polymer blends, and the enhancement of adhesion in polymerlmatrix comp~sites.l-~ Two distinct mechanisms can drive the adsorption of the diblock to a solifliquid interface. One is based on a specific affinitiy between one of the blocks (A)and the surface. Adsorption occurs even though the solvent is adequate for solvating both blocks. Models for this case, adsorption from “nonselective solvents”, are a ~ a i l a b l e . ~The , ~ second mechanism involves a lyophobic effect on block A, forcing it to an interface. In this case the solvent and the surface both can play a role in the adsorption process;6-8however, in many cases the lyophobic effect dominates and the chemistry ofthe surface is of secondary i m p o r t a n ~ e . In ~J~ this paper we are concerned with adsorbed layers whose formation was driven by the second mechanism, “selective solvation” of the diblock copolymer. A schematic of a diblock layer formed by a selectivesolvent (lyophobic)effect is shown in Figure 1. One block (A with NA monomer units) adsorbs to the surface while the solvated (nonadsorbed) block (B with N B monomer t Present address: Lubrizol Corp., 29400 Lakeland Blvd., Wickliffe, OH 44092. Abstract published inAdvanceACSAbstracts, August 15,1994. (1)Jakubauskas, H.J. Coat. Technol. 1986,58,71. (2) Bates, F.S.Science 1991,251,898. (3) Ploehn,H.J.;Russel,W. B. Interactions between Colloidal Particles and Soluble Polymers; Advances in ChemicalEngineering 15;Academic Press: New York, 1990. (4) Evers, 0. A.; Scheutjens, J. M. H. M.; Fleer, G. J. J . Chem. Soc., Faraday Trans. 1990,86, 1333. ( 5 ) Marques, C.; Joanny, J. F. Macromolecules 1989,22,1454. (6) Marques, C.;Joanny, J. F.; Leibler, L. Macromolecules 1988,21, 1051. (7) Munch, M. R.; Gast, A. P. Macromolecules 1988,21,1366. (8) van Lent, B.; Scheutjens, J. M. H. M. Macromolecules 1989,22, 1931. (9) Munch, M. R.; Gast, A. P. Macromolecules 1990,23,2313. (10) Parsonage, E.;Tirrell, M.; Watanabe, H.; Nuzzo, R. G. Macromolecules 1991,24,1987. @
PS (tail 01
PVP t1 (head or
Figure 1. Expected conformation of an adsorbedlayer of poly(2-vinylpyridine)/polystyrene ~WPIPS) at the tohenelmuscovite mica surface. The PVP block is nonsolvated by toluene and anchors the copolymer. units) expands away from the interface to form a more diffuse layer. The assembly of diblocks a t an interface, as depicted in Figure 1, is driven by the dissimilar chemical nature of the monomers comprising the two homopolymer chains of the copolymer. The important question is how the amount adsorbed and the thickness of the layer are related to the block sizes NA and NB. There have been relatively few studies of diblock layers a t solidAiquid interfaces in which the effect of block size was studied in a systematic manner using polymers of narrow molecular weight distribution. Guzonas e t a1.”J2 determined both the equivalent thickness (Dd21, as measured by the surface force apparatus, and the coverage (a = chaindarea) for a series of poly(ethy1ene oxide)/ polystyrene diblocks, PEOPS, adsorbed to mica surfaces from toluene. They considered the adsorption to be from a nonselective solvent; that is, the adsorption was driven by a n affinity between the surface and the PEO block. From the results for u determined by infrared spectroscopy, the data of Guzonas et a1.12support the scaling theory of Alexander13 and deGennes:14 (11)Guzonas, D.;Boils, D.; Hair, M. L. Macromolecules 1991,24, 3383
(12) Guzonas, D.; Boils, D.; Tripp, C. P.; Hair, M. L. Macromolecules 1992,25,2434. (13)Alexander, S . J . Phys. (Les Ulis, Fr.) 1977,38,983. (14) de Gennes, P. G. Macromolecules 1980,13,1069.
0743-7463/94/2410-3156$04.50/00 1994 American Chemical Society
Effects of AdsorbingBlock Molecular Weight
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L NBall3 To our knowledge, this is the first rigorous experimental test of eq 1with adsorbed diblock copolymers. Taunton et al.15 obtained data in agreement with eq 1for terminallyattached polymer chains, although a rigorous test of the equation was not made. Wu et measured the hydrodynamic thickness (LH) and surface coverage of diblocks of (dimethy1amino)ethyl methacrylate (A) and n-butyl methacrylate (B) adsorbed to silica particles from 2-propanol. This system represents a nonselective solvent system, where the adsorption was driven by the A blocks affinity for the silica surface. Wu et al. studied a set of diblocks with constant degree of polymerization (NA N B = constant) but variable block sizes. Their data show sharp maxima in both LH and u as N A is increased, in qualitative agreement with theoretical calculations of Evers et aL4 In this paper we present experimental results showing the effects of adsorbing block size on the hydrodynamic thickness of adsorbed diblock copolymers formed because of selective solvation. The copolymer poly(2-vinylpyridine)/polystyrene (PVPPS)was adsorbed to the pores of mica membranes from toluene. The PVP block adsorbed because of its incompatibility with toluene; the data of Parsonage et al.1° indicate that this is a selective-solvent system. The study reported here is complementary to a previous paper1' which gives experimental results for the effects of the size of the solvated block (PS) on the layer thickness when Npvp was constant a t 25 monomers. The data of rep7 are correlated by
+
L, = 1.48Np:.815 (inA) where NPSvaried between 150 and 778. These results will be used along with the new data reported here for LH versus NA,with N Bfixed a t about 430, to systematically look a t layer thickness versus the size of both blocks. Although we estimate the surface coverage by collapsing the PVPPS layers in n-heptane, these values of u are not sufficiently accurate to test eq 1;hence, we rely on the data of Parsonage et for the values of u.
Experiments Complete details of the copolymer synthesis and experimental procedures can be found elsewhere.18 Diblock Copolymers. Table 1contains the block molecular weights of the diblock copolymers of poly(2-viny1pyridine)l polystyrene (PWPS)used in this study. The P W P S diblock copolymers were synthesize by anionic polymerization of styrene and 2-vinylpyridine. Polymerization of purified styrene diluted in THF was initiated with n-butyllithium. A sample of the polystyrl anion was removed from the reaction flask, terminated with methyl alcohol, and reserved for characterization. After addition of 2-vinylpyridine monomer to the reaction flask containing polystyrl anion, the reaction was terminated with methyl alcohol. The number average molecular weight, M,, and polydispersity index, MwlMn,for each PS precursor were determined by gel permeation chromatography (GPC) in THF. Elemental analysis (Desert Analytics Inc., R. C. Johnson) was used to determine the relative amounts of carbon, hydrogen, and nitrogen upon pyrolysis of the diblock samples and therefore the ratio of the number of styrene monomers to vinylpyridine monomers, NpdNpvp. The values of M , for the PS precursors and the elemental analyses were used to calculate the values of (15) Taunton, H. J.; Toprakcioglu, C.; Feters, L. J.; Klein, J. Macromolecules 1990,23,571. (16)Wu, D. T.; Yokoyama, A,; Setterquist, R. L. Polym. J . 1991,23,
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7119
(17)McKenzie,P. F.;Webber,R. M.;Anderson,J. L.Langmuir, 1994, 10,1539. (18)Webber, R.M. Ph.D. Thesis, Camegie MellonUniversity, 1991.
Langmuir, Vol. 10,No. 9, 1994 3157 Table 1. Block Molecular Weights of P W P S Diblock Copolymers" PS414b 91427 501433c 571414 731444
1.04 1.04 1.14 1.04 1.03
9 50 57 73
0.020 0.117 0.138 0.165
a The diblock samples are designated by the number of P W and PS monomer units (Npvp and Npg), respectively. Nps (calculated from the PS number average molecular weight, MnPs)and Mwps/ MnPs were determined from GPC measurements on the PS precursors to the diblock synthesis. Npvp was determined from elemental analysis measurements (Desert Analytics Inc., R. C. Johnson) and MnPs.The monomer molecular weights are 105 and 104 for PVP and PS, respectively. PS414 is the polystyrene homopolymer that was the precursor to the diblock 57/414. Synthesized at the IBM Research Laboratory at Almaden, CA (G. Hadziioannou and co-workers). The molecular weight reported here for the PVP block was determined from elemental analysis and the reported value of Npg.
*
M , for the PVP blocks that are listed in Table 1. The diblock samples are designated by the number of PVP and PS monomer units (Npw and Nps), respectively. Note that one polymer in Table 1,designated PS414,was a PS homopolymer with Nps = 414;this polymer was the PS precursor to PVPPS diblock 571 414 and therefore had the same M , and M,. Hydrodynamic Thickness. The hydrodynamic thickness of a polymer layer was measured by determining the change in hydrodynamic permeability of a porous membrane after adsorption ofthe polymer to the pores.lg The membranes used in these experiments were made from thin sheets of muscovite mica by the track-etch procedure, which produces membranes with uniform and straight pores.20 The cross section of the capillary pores in such membranes is a 60" rhombus. The permeability of a '%are" membrane, ~ H O before , adsorption of the polymer is described by the Hagen-Poiseuille equation
k,,,
PQO E
nxR;
-= 0.68 AP 81
(3)
where QOis the flow rate of polymer-free solvent resulting from the applied pressure difference AP and p is the viscosity of the solvent. Ro is the radius of the bare capillary pores, n is the number of pores, and 1 is the length of the pores. n and 1 were determined independently during the membrane fabrication procedure. The factor 0.68 corrects for the pore geometry; Ro is the radius of a circle having the same cross sectional area as the pores. The hydrodynamic thickness of the polymer layer, LH,is defined as the thickness of an impermeable layer on the pore wall that would account for the reduction in permeability of the membrane caused by the adsorbed polymer layer
(4) where k~ is the permeability measured after polymer adsorption. The factor 1.21again corrects for the geometry of the rhomboidal pore cross section.21 After determination of the bare pore radius, PVPPS diblock copolymers were adsorbed to the pores of a membrane by exposing the membrane to prefiltered toluene solutions having a polymer concentration (cads) between 30 and 80 pg/mL. The polymer solutions were contacted with the membrane for approximately 100 h; experience with this systemlg has shown that further exposure of a membrane to the polymer solution does not produce an appreciable change in either the amount of polymer adsorbed or the thickness of the layer. Note that the exposure time is at (19)Webber, R. M.; Anderson, J. L.; Jhon, M. S. Macromolecules 1990,23,1026. (20) Quinn, J. A.; Anderson, J. L.; Ho, W. S.; Petzny, W. J. Biophys. J. 1972,12,990. (21) Idol, W. K.; Anderson, J. L. J.Membr. Sci. 1986,28, 269.
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Webber a n d Anderson
Table 2. Values of Hydrodynamic Thickness in Toluene (LH~ and ) in n-Heptane After Removal of Toluene by Drying ( L H ~ ) " 4
PS414 91427 91427 501433 501433 501433 501433 571414 571414 571414 571414 731444 731444 731444 731444
30 76 48 70 70 34 30 76 80 32 30 74 78 30 30
0 217(&4) 172(f2) 378(f62) 364(&24) 294(&10) 326(+2) 173(f2) 181(&21) 231(+2) 275(&5) 127(&3) 114(&9) 174(f 1) 182(&7)
12(+2) 57(f33)
1.7 7.2
20(f3) 17(&2) 48(f2)
2.5 2.2 6.3
17(f2) 44(f3)
2.2 5.7
25(f2) 6(f4) ll(f1)
3.0 0.7 1.3
The polymer average (a)was determined from L H and ~ eq 5 with epvp = 1.17 and eps = 1.05 glcm3. The error bars included with the LHmeasurements are one standard deviation about the mean value of LH. least 3 orders of magnitude greater than the time required for a polymer molecule to diffuse the length of the membrane pores. Measurement of the hydrodynamic thickness of the polymer layer in toluene, L H ~was , performed by replacing the polymer solution with pure toluene that had been filtered through 0.2pm pores in a Nucleopore membrane. Desorption of the polymer from the pores of the mica membrane, which would presumably have been indicated by a reduction in hydrodynamic thickness of the polymer layer over time, was not observed throughout the course of an experiment, sometimes as long as several weeks. The range of pore radius among the membranes was 2300-3600 A; this is approximately 10 times the thickness of the adsorbed polymer layer, so that effects of pore size on polymer adsorption and layer conformation should have been minimal. The surface coverage for the adsorbed polymer layers was estimated by the following procedure: (1)toluene was removed from the polymerlmembrane system by heating the membrane a t 50 "C for approximately 72-96 h; (2) the membrane was then placed in contact with filtered n-heptane, a nonsolvent for both the PVP and PS blocks; (3) the hydrodynamic thickness in heptane, L H ~was ~ , measured. Collapse of the polymer layer was indicated byLHdH
