Langmuir 1992,8, 2070-2071
2070
Notes Evidence for the Preferential Solvent Uptake by Adsorbed Block Copolymer Daniele Boils, Carl P. Tripp, David Guzonas, and Michael L. Hair'
Xerox Research Centre of Canada, 2660 Speakman Drive, Mississauga, Ontario, Canada L5K 2Ll Received April 7, 1992.In Final Form:June 1, 1992
Recently1t2 we have published experimental results, using the surface force apparatus (SFA), to study the adsorption of PS-block-PEOdiblock copolymers onto mica from a nonselective solvent, toluene. When one attempts to fit experimental data to current theoretical models,3 the most important parameter is the adsorbed amount of polymer on the surface as this reflects the surface density of the polymer on the surface, Le., the area of surface occupied by one adsorbed molecule. In scaling models this parameter controls the adsorption process, and it is therefore important to know it with accuracy. However, obtaining this quantity is often difficult. In surface force measurements, obtaining a value for the adsorbed amount is nontrivial because it involves a knowledge of the refractive index of the adsorbed layer. This latter parameter is difficult to measure because there is, for the present system, a small difference between the refractive indices of the polymer and the solvent. Some measurements of refractive index have been made by Taunton et aL4 for end-adsorbed polystyrene molecules, but the experimental data show wide scatter. Marra and Hair5 overcame this difficulty by collapsing their polymer layers in a nonsolvent mixture of heptane and toluene, and we have used the same procedure in a recent publication.2By using a mixed solvent, in which the two solvents have very different refractive indices, the contrast between the refractive indices of the polymer and the solvent is increased. In both of the previous a p p r o a c h e ~ ~there ,~,~ was obtained a reasonable fit with the scaling behavior (eq 1)proposed by Alexander.6 This is not surprising as
any systematic error between the two methods would appear in the prefactor, where L is the length of the brush, a is the monomer size, Npsis the number of segments of polystyrene, and u is the surface density. We were, however, wary of attaching a great significance to the absolute numbers obtained for adsorbed quantities using a mixed solvent to collapse the polymer layers because of the possibility that one of the components of the solvent mixture could be preferentially adsorbed in the polymer layer. (1) Guzonas, D. A,; Boils, D.; Hair, M. L. Macromolecules 1991,24, 3383. ( 2 ) Guzonas,D. A.; Boils, D.;Tripp, C. P.; Hair, M. L.Macromolecules 1992, 25, 2434.
(3) Marquee, C . M.; Joanny, J. F. Macromolecules 1989,22, 1454. (4) Taunton, H. J.; Toprakcioglu, C.; Fetters, L. J.; Klein. J. Macromolecules 1990,23, 571 (5) Marra, J.; Hair, M. L. Colloids Surf. 1988, 34, 215. (6) Alexander, S. J. J. Phys. (Paris) 1988, 38,983.
Recently, we have developed an infrared spectroscopic technique7v8which has sufficient sensitivity to enable the quantitative determination of monolayer coverages of adsorbed polymer layers on mica surfaces. The accuracy of this measurement is about 5 % . The method is carried out on a dried film and does not make any assumptions about the partitioning of the solvent. We applied this technique to measure the adsorbed amounts of a series of PS-block-PEO block copolymers, and we now compare these values with those obtained in the surface force apparatus (SFA). The results of both experiments are listed in Table I, for seven block copolymers. We note that with the exception of the material labeled 87/29, the value reported by the SFA is about 50% of the value determined by the FTIR technique. The adsorbed amount in the SFA was calculated from the refractive index of a collapsed polymer layer between the two mica surfaces. The polymer was first adsorbed onto the mica from toluene solution and was then collapsed onto the surface by the addition of a poor solvent for the polystyrene block. A heptane/toluene mixture in a ratio of 67 7% to 33% in volume was usually chosen. By either measuring the refractive index of the material in the SFA or by comparison with a polystyrene under the same conditions, a volume fraction, t#Jsf, of the adsorbed polymer layer can be calculated according to the following equation: 4sf
= (nsf - n w 1 J (np01-
nsolv)
(2)
where nsfis the refractive index of the collapsed polymer layer as measured by the SFA in the presence of the solvent, nmlvis the refractive index of the bulk solvent, and n,l is the refractive index of the polymer. This volume fraction, together with the thickness of the collapsed layer and the bulk density of the polymer, can be used to evaluate the adsorbed amount according to the following e q ~ a t i o n : ~ 2% = dJsftP
(3) where &is the adsorbed amount, &is the polymer volume fraction, t is the thickness of the compressed polymer layer as measured by the SFA, and p is the density of the bulk polymer. In all cases the subscript sf refers to a surface force measurement. In the derivation of 3, it is assumed that the solvent mixture is identical in the bulk and in the adsorbed polymer layer, and that the solvent does not partition differently between the two blocks of the copolymer. The former will be the focus of our discussion. The infrared measurements are done in the absence of solvent. As discussed el~ewhere,~J the block copolymers are adsorbed onto mica from toluene. They are then rinsed with toluene, and the adsorbed layer is dried in nitrogen. The quantitative measurement is taken on this drysample. We have therefore repeated a similar protocol in the SFA. After the polymers adsorbed from toluene, the polymer layers were then collapsed in the nonsolvent (heptane/ toluene mixture) and the refractive index was recorded. After the solvent was drained, the polymer layers were dried by flowing nitrogen through the box for 40 h. There was a gradual diminution in the thickness, and the value for the adsorbed amount measured in this way for the 50014 copolymer was 1.4 f 0.3 mg/m2. This value is much ~~~~~
~
~~
(7) Tripp, C. P.; Hair, M. L. Appl. Spectrosc. 1992,46,100. (8) Tripp, C. P.; Hair, M. L. Langmuir 1992,8, 241.
0743-7463/92/24Q0-2070$03.00/00 1992 American Chemical Society
Langmuir, Vol. 8, No. 8, 1992 2071
Notes Table I polymer
data from FTIR,s (malm2)
10014 151/43 214 334119 6214 50014 36311
1.5 1.0 1.2 1.4 2.3 1.3 1.0
datifrom surface force, s (me/m*) 2.8 2.5 2.7 2.6 4.0 2.4 2.15
lower than that of 2.4 f 0.3 mg/m2 calculated from the collapsed layers in a poor solvent and is within the experimental error of 1.3 f 0.1 mg/m2 obtained from IR. We therefore conclude that the difference between the adsorbed amount evaluated by IR and SFA experiments on the layers collapsed in toluene/heptane mixtures is a systematic error and suggest that this difference can be accounted for by a selective uptake of toluene in the polymer layer. Let us now consider a particular example, the polymer labeled 334/19 collapsed in a heptane/toluene (67/33) solvent mixture. The refractive index of heptane is nh = 1.388, the refractive index of toluene is nt = 1.496, and the calculated refractive index of the solvent mixture is nsolv = 1.423 (the measured value for the mixed solvent is nsolv = 1.42). Given that the PS/PEO (334/19) has a PEO content of 12%, the calculated refractive index of the
polymer is npol = 1.577. The refractive index of the medium, nsf, is equal to 1.53. By rearrangement of eq 2 the refractive index of the solvent surrounding the layer of adsorbed material is (4) nso1v = (dJsFPOl - nsfV(@Jsf - 1) Substituting C$IR (0.33) for &f (0.691, we now recalculate nsOlv.The value of ~ I has R been calculated according to R &f: eq 3 by substituting SIRfor Ssfand ~ I for = 2sIR/tp (5) The value of nsolvthus obtained is 1.5 as opposed to the value of 1.423 corresponding to the heptane/toluene solvent. In other words, the solvent in the polymer is pure toluene.
Although it is not surprising that some toluene is preferentially adsorbed into the polymer from a mixed solvent, it is not at first intuitive to expect that one pure component appears to be preferred in the collapsed brush. Johner and Marquesg have recently argued that retained solvent can induce new wetting transitions in brush layers. The concept of a mixed solvent raises interesting possibilities. Acknowledgment. We thank S. Besner (University of Montreal) for performing the measurement of the refractive index of the mixed solvent. Ragistry No. (PS)(PEO) (block copolymer), 107311-90-0; toluene, 108-88-3. ~~
(9) Johner, A.; Marques, C. M. Private communication.
