Langmuir 1995,11, 1680-1687
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Adsorption Behavior of Functionalized Polystyrene-block-Polybutadiene with Randomly Attached Adsorbing Sites D. F. Siqueira,’ U. Breiner,t R. Stadler,*and M. Stamm*$; Max Planck Institut fur Polymerforschung Ackermannweg 10, 55021 Mainz, Germany, and Johannes-Gutenberg Uniuersitat, Becherweg 18, 55099 Mainz, Germany Received June 29, 1994. I n Final Form: February 2, 1995@ Null ellipsometry is used to obtain the adsorption isotherms for functionalized polystyrene-blockpolybutadiene diblock copolymers,in which polar urazole groups, 443,5-dioxo-1,2,4-triazolidin-4-yl)benzoic acid, are attached randomly to a short polybutadiene block. The block copolymer molecular weight and the number of functional groups are varied. Adsorption investigations are performed from very dilute solution of these copolymers in toluene, using silicon wafers as the substrate. Toluene is a good solvent for both blocks and a nonsolvent for the functional groups. The system largely behaves like a conventional block copolymer with selective adsorption of one block. The conformation of the adsorbed chains has “brush”character. The surface density scales with the size of the dangling, nonadsorbing blocks and is essentially independent of the number of functional groups of the adsorbing block. A model for this behavior is proposed based on largely independent adsorbing urazole groups connected via flexible nonadsorbing polybutadiene sequences. A “zipper”mechanism for desorption is envisaged. In a still better solvent, tetrahydrofuran, no adsorption takes place.
Introduction The formation of polymer layers on solid surfaces through adsorption has gained much attention in the technological and academic fie1ds.l One major problem in the application of dispersions is the tendency of particles to aggregate owing to attractive van der Waals forces. The adsorption of a polymer can prevent this aggregation process through a strong repulsion between the polymeric layers on the particle surface. On the other hand, if the polymeric chains contain for instance two functional ends, they can anchor on two different particles, leading to a flocculationprocess. In the biomedical field the adsorption of proteins on polymeric substrates plays a n important role in the development of biocompatible materials and chromatographic technique^.^,^ Many experimental studies about the adsorption of block copolymers from dilute solutions on a solid surface by means of different techniques have been reported in the literature (see refs 4-7). Common theoretical treatments of copolymer adsorption predict that the adsorbed amount and the geometric parameters of the adsorbed layer are mainly controlled by the quality of the solvent and the molecular characteristics of the chains.s-12 In the case of
* Author to whom correspondence should be addressed.
’Max Planck Institut fur Polymerforschung.
Johannes-Gutenberg Universitat. Abstract published in Advance A C S Abstracts, April 1, 1995. (1)Napper, D. H. Polymeric Stabilization of Colloidal Dispersions; Academic Press: London, 1983. (2)Andrade, D. J., Ed. Surface and Interfacial Aspects ofBiomedica1 Polymers; Plenum Press: New York, 1985; Vol. 2. (3)Kennedy, J. P. Trends Polym. Sci. 1993, 1, 381. (4)Dorgan, J. R.; Stamm, M.; Toprakcioglu, C.; Jerome, R.; Fetters, L. J. Macromolecules 1993, 26, 5321. (5) Guzonas, D. A., Boils, D., Tripp, C. P.;Hair, M. L.Macromolecules 1992,25,2434. (6) Tsai, W. H.; Boerio, F. J.;Clarson, S. J.; Parsonage, E. E.; Tirrell, M. Macromolecules 1991,24, 2538. (7) Bosse, F.; Schreiber, M.; Eisenberg, A. Macromolecules 1993,26, 6447. ( 8 ) Marques, C. M.; Joanny, J. F. Macromolecules 1989, 22, 1454. (9)Marques, C. M.; Joanny, J. F.; Leibler, L. Macromolecules 1988, 21, 1051. (10)Marques, C. M.; Joanny, J. F. Macromolecules 1990, 23, 268. (llIAlexander, S. J. Phys. 1977, 38, 983. (12) de Gennes, P. G. Macromolecules 1980, 13, 1069. @
block copolymer adsorption from a nonselective solvent,8 where one block adsorbs while a t high surface coverage the other block dangles in solution in a “brushlike” conformation, the adsorbing energy or binding energy is expected to increase as the adsorbing block becomes longer. In this work, the adsorption of functionalized polystyrene-block-polybutadiene,in which the butadiene segments carry some functional stickers, is investigated by means of null ellipsometry. The polar stickers are 4-(3,5dioxo-1,2,4-triazolidin-4-yl)benzoicacid units. The adsorption studies are performed in toluene, which is a good solvent for both blocks, but a nonsolvent for the functional groups. In order to detect aggregation in solutions prepared with toluene, dynamic light scattering measurements are also performed. The substrate, a silicon wafer, is selective just for the functional groups, since the nonfunctionalized blocks do not adsorb on the silicon oxide surface from toluene. In order to verify the theoretical predictions, the copolymer samples are synthesized with different molecular characteristics and different number of functional groups. The original aim was to vary the adsorption characteristics by changing the number of functional groups leaving the block copolymer otherwise constant. This could provide a test for scaling theories. Adsorption isotherms as well as some geometrical parameters of the adsorbed layer are presented, as far as they can be obtained with ellipsometry. The influence of solvent quality is also investigated. Thus, for comparison, adsorption measurements of the functionalized diblock copolymers have been made from tetrahydrofuran (THF), which it is a good solvent for both blocks a s well as for the functional groups. This paper has the following form: first the Experimental Section describes the materials and methods used. Second the Results and Discussion section contains (i) the dynamic light scattering measurements; (ii) the adsorption isotherms; (iii) the influence of the number of functional groups on the adsorbed amount, geometric parameters, and scaling laws, and (iv) the adsorption behavior in THF. Experimental findings are discussed and an adsorption model is presented.
0743-7463/95/2411-1680$09.00/00 1995 American Chemical Society
Absorption Behavior of Functionalized Block Polymers
Langmuir, Vol. 11, No. 5, 1995 1681
Table 1. Molecular Characteristics of the Functionalized Block Copolymersu M w (g/mol) GPC PS 9.2 104 8.4 104 4.8 x 104 PB 5.2 x 104 PB109u40 9.95 x 104 1.09 x 105 P(S-b-B)318~8 3.18 105 2.9 105 P(S-b-B)31 8 ~ 6 3.18 105 2.9 105 P(S-b-B)318~4 3.18 105 2.9 105 P(S-b-B)318~2 3.18 105 2.9 105 6.3 x lo4 P(S-b-B)68~6 6.8 x lo4 6.3 x lo4 P(S-b-B)68~4 6.8 x lo4 2.89 x 104 P(S-b-B)31~6 3.1 x 104 2.89 104 P(S-b-B)31~4 3.1 104 LS
sample
MwIMn 1.07 1.2 1.1 1.1 1.1
fraction of PB in copolymer (wt %)
1.1 1.1
1.08 1.08 1.07 1.07
number of functional groups, m
40 8 6 4
2.0 2.0 2.0 2.0 2.9 2.9 5.5 5.5
2 6
4 6 4
dnldc (mug) 0.1001 0.0077 0.0985 0.0985 0.0985 0.0985 0.0975 0.0975 0.095 0.095
R, (nm)
19k 1 20 f 1 21f2 20 f 1 16 f 1 15.5 f 0.8 11.7 f 0.9 11 f 1
a LS and GPC denote light scattering and gel permeation chromatography experiments, respectively, used to determine the molecular weight. dnldc denotes the Increment of the refractive index in toluene at 20.0 "C. Rh indicates the mean value of hydrodynamic radius obtained at different scattering angles (6 = 68", 88", 118", and 148")from DLS measurements for solutions at the concentration of0.6 mg/mL of the corresponding sample in toluene at 20.0 "C.
Y
h
v c 7
m
Figure 1. Schematics of the attachment of the functional acid (urazole group 4-(3,5-dioxo-1,2,4-triazolidin-4-yl)benzoic group) to the butadiene blocks.
Experimental Section Materials. Polystyrene (PS), polybutadiene (PB), and polystyrene-block-polybutadienediblock copolymer P(S-b-B) were prepared by anionic sequential polymerization. The homopolybutadiene and the polybutadiene blocks were reacted with 4-(3,5-dioxo-1,2,4-triazolidin-4-yl)benzoic acid13 as shown in Figure 1. The resulting4-(3,5-dioxo-1,2,4-triazolidin-4-yl)benzoic acid units (4-urazoylbenzoic acid units) strongly alter the bulk properties of functionalized polybutadienes as the result of a supramolecular assembly ofthe stickers. We denote the diblock copolymers with urazole groups attached as P(S-b-B)u. P(S-bB)u with different molecular characteristics and a different number of functional groups were prepared and characterized, as shown in Table 1. The functionalized homopolymer PB and diblock copolymers are represented as PBnum and P(S-b-B)num, respectively, with n indicating the molecular weight of the block copolymer and m denoting the mean number of urazole groups attached. Molecular weights are determined by light scattering (see below) and gel permeation chromatography (polystyrene standard). The styrene to butadiene ratio is obtained by nuclear magnet resonance (200 MHz lH-NMR) in CDC13. The distribution of urazole groups is supposed to be a Poisson distribution as judged from the reaction scheme and should be relatively narrow.14-16 Reagent grade toluene and tetrahydrofuran (THF) (Riedel-de Haen) were distilled over sodium metal and filtered through a Millipore filter (0.2 pm) prior to use. Methods. Dynamic Light Scattering (DLS) Measurements. DLS measurements were performed with a commercial digital correlator (ALV 3000) equipped with an krypton ion laser (1= 647.1 nm, 400 mW) at 20.0 "C. Solutions of P(S-b-B)u were (13)Hilger, C.; Stadler, R. Makromol. Chem. 1991,192,805. Hilger, C.; Drager, M.; Stadler, R. Macromolecules 1992,25,2498. Hilger, C.; Stadler, R. Macromolecules 1992,25, 6670. (14) Stock, J. PhD. Thesis,Johannes-GutenbergUniversitat, Mainz,
1993.
(15)Butler, G. B.; Williams, A. G. J.Polym. Sci.:Polym. Chem. 1979, 17, 1117. (16) Butler, G. B. J. Mucromol. Sci. Chem. (A) 1981, 16, 757.
Figure 2. Typical autocorrelation function obtained from dynamic light scattering measurements for a solution of 0.6 mg/mL of P(S-b-B)318u6 in toluene a t 20.0 "C and a t the angles of 58', 88", 118", and 148", respectively. prepared in toluene at the concentration of 0.6 mg/mL, filtered through a Millipore filter (0.2 pm), and then measured in the range of scattering angles (6) of 58'-148'. For solutions of concentration
