Langmuir 1994,10, 1841-1847
1841
Adsorption of Photoactive Amphiphilic Polymers onto Hydrophobic Polymer Films: Polystyrene-block-poly(2-vinylnaphthalene)- block-poly(methacrylic acid) T. Cao, W. Yin, J. L. Armstrong, and S. E. Webber* Department of Chemistry and Biochemistry and Center for Polymer Research, The University of Texas at Austin, Austin, Texas 78712 Received March 22, 1993. I n Final Form: December 27, 199P Diblock polystyrene-block-poly(methacry1ic acid) polymers have been synthesized using anionic polymerizationtechniques incorporating an average of one naphthalene per polymer at either the beginning of the polystyrene block or at the junction between the polystyrene and poly(methacry1ic acid) blocks. These polymers have been shown in earlier work to form stable micelles in solvent mixtures from 8020 dioxane/H*Oto pure water. In the present paper we have examined the ability of these polymers to adsorb on polystyrene films and have used photophysical techniques to deduce the exposure of the naphthalene group to the aqueous phase. Scanning electron microscopy images demonstrate that the intact micelle adsorbs onto the polystyrene surface and can achieve a nearly close-packed monolayer coverage. The micelles adhere tenaciously to the polystyrene film and lower the contact angle of water from ca. 90° for untreated polystyrene to 30-40' after surface adsorption. The naphthalene groups at the polystyrenepoly(methacry1ic acid) junction are partially exposed to the aqueous phase while the naphthalene at the poly(styrene) end is totally protected.
Introduction Polymers are well known to be surface modifiers, and block polymers have been studied extensively with this motivation.' In the present paper we study an amphiphilic block copolymer, polystyrene-block-poly(2-vinylnaphthalene)-block poly(methacry1ic acid) (l), abbreviated PSN-PMA hereafter. We have studied polymer micelles formed from these materials,2but in the present paper we explore the way in which these polymers adsorb onto a polystyrene film and the extent of the exposure of the naphthalene to the aqueous phase.
onstrate that the exposure of the naphthalene to the water phase is greatly diminished upon adsorption. Scanning electron micrographs show that intact micelles are adsorbed such that the photophysical modifications must be the result of a relatively subtle change in the micelle structure.
Experimental Section Two polymerswere used, PS-N-PMAand N-PS-PMA,in which the naphthalene group is at the beginning of the polystyrene block. Thesepolymers have been used in earliermicelle studies,2 and their physical properties are summarized in Table 1. N-PSPMA was studied by way of comparison with PS-N-PMA. Electron micrographswere acquired on an Electroscan Model E-3 environmental scanning electron microscope (ESEM) at a beam energy of 20 kV in 6-9 Torr of background water vapor.' After it was established that the close-packed images discussed later were present, the samples were sputtered with ca. 150A of Au to improve the resolution. Electron micrographs at 60' tilt 1 were taken on a Philips Model SEM-515 scanning electron microscope at 25.3 kV with a beam size of l00A and a Au thickness of ca. 100 A. Samples for scanning electron microscopy (SEM) Our motivation for this effort is in developing a system studies were prepared as follows: (1) A polystyrene (PS) film that is capable of interfacial electron-transfer r e a ~ t i o n : ~ was spin cast from benzene (30mg/mL) on a polished Cu or Al disk and the film allowed to anneal in the presence of benzene vapor. (2) One drop of the micelle solution of various concentra2S+'D*(film) + A(aq) D'+(film) + A'-(aq) (1) tions (maximum of 4.0 mg/mL) in 6040 v/v dioxane/HzO was placed on the PS surface for ca. 20 min. Then the surface was In the present paper we explore the quenching of the washed extensivelywith water and spun dry (typically10times). excited state of naphthalene covalently bonded to the block (3) For coated samples half the film was scraped off and the Au polymer by a simple ion (Tl+(aq)). These results demcoating applied. This assured good electrical contact between the Au coating and the metal substrate. * To whom correspondenceshould be addressedat the Department Fluorescence studies were carried out using either a SPEX of Chemistry and Biochemistry. Fluorolog or TRACOR diode array fluorimeter for steady-state
w
-
Abstract published in Advance A C S Abstracts, May 15, 1994. (1)(a) For a recent monograph see: Physics ofPolymer Surfaces and Interfaces; Sanchez, I., Ed.; Manning PublicationsCo.: Greenwich, CT, 1992. (b) For an extensive review see: Halperin, A.; Tmell, M.; Lodge, T. P. Adu. Polym. Sci. 1991,100,31.(c) Milner, S.T. Science 1991,251, 905. (d) Cohen Stuart, M. A.; Cosgrove, T.; Vincent, B. Adu. Colloid Interface Sci. 1986,24,143. (2)(a) Prochbka, K.; Kiserow, D.; Ramireddy, C.; Tuzar, 2.;Munk, P.; Webber, S. E. Macromolecules 1992, 25, 454. (b) Kiaerow, D.; ProchAzka, K.; Ramireddy, C.; Tuzar, 2.; Munk, P.; Webber, S. E. Macromolecules. 1992,25,461. 0
(3)(a) See the plenary lectures of the Sixth International Conference on Photochemical Conversion and Storage of Solar Energy, Paris, 1986, inNewJ. Chem. 1987,ll.(b)Fox,M.A., C h o n , M.,Eds.Photoinduced Electron Transfer;Elsevier Science Publishers: Amsterdam, 1989. (c) Hurst, J. K. Dynamics of Charge Separation Across Vesicle Membranes. In Kinetics and Catalysis in MicroheterogeneorurS y s t e m ;Griihl, M., Kalyanasundaram, K., Ede.; Marcel Dekker, Inc.: New York; pp 183222. (4)Danilatos, G. D. Microsc. Res. Tech. 1993,25,354.
0743-7463/94/2410-1841$04.50/0 0 1994 American Chemical Society
Cao et al.
1842 Langmuir, Vol. 10, No. 6, 1994
Table 2. Contact Angles and Micelle Densities for
Table 1. Properties of Polymers.
PS-N-PMA Micelles Adsorbed onto PS PS-N-PMA(SN1A-1) N-PS-PMA(N4SA-2)
48.0 45.0
54400 48500
1.15 1.08
7Oe 80
*
Results 1. Adsorption of PS-N-PMA. These polymers have a complex solution equilibrium that has to be taken into account in adsorption studies. The solid polymer can be dissolved directly into a mixture of 20:80 water/dioxane, and on the basis of quasi-elastic light scattering (QELS) micelles are formed directly. So far as we have been able to determine from sedimentation velocity measurements, there is no measurable “unimer” present (i.e., molecularly dissolved polymer), but polymer micelles do undergo dynamic equilibration (“hybridization”) that depends on the molecular weight of each segmenL6 We have found that in order to adsorb these polymers onto polystyrene film the solution must be rich in dioxane, so we assume that the adsorption of the block copolymer has a mechanism similar to that of micelle hybridization. The dioxane also serves to soften the polystyrene film, which presumably could affect adsorption processes. It is also possible to adsorb these polymers by allowing a concentrated benzene solution of polystyrene to slowly evaporate while in contact with a pure aqueous solution of polymer micelles. However,these films are not sufficiently smooth for contact angle measurements, and this method was not used to prepare the films used in this paper. It has been proposed7 that the initial step of micelle adsorption is the adsorption of the intact micelles onto the surface. Polystyrene (Aldrich)with a molecular weight of 280 000 was extensively purified by precipitation: three times from CHzCl2 into methanol, once from T H F into methanol, and two times from benzene into methanol. A smooth polystyrene film was spin-cast onto a quartz plate from a 30 mg/mL benzene solution. A few drops of micelle solution in 6040 v/v dioxane/HzO mixed solvent was placed in contact with the film for 15-20 min. Excess solution was removed and the film allowed to dry, followed by extensive washing with water (see Experimental Section). Different concentrations of micelle solution were used, as is discussed next. The contact angle of a droplet of water is reduced from -90’ for untreated polystyrene to 36 f 3” for the region treated by the block polymer at the highest concentration used. The contact angle drops steadily with the concentration of micelle in the bulk solution, and the number of micelles per unit area increases, the later based on SEM images (see Table 2 and Figure 1). Approximately 1min after the water droplet is placed in contact with the (5) Cao, T.; Munk, P.; Ramireddy, C.; Tuzar, Z.; Webber, S. E. Macromolecules 1991, 24, 6300. (6) Tian, M.;Qin, A.; Ramireddy, C.; Webber, S. E.; Munk, P.; Tuzar, 2.; Prochbka, K. Langmuir 1993,9, 1741. (7) For a theoretical discussion of this phenomenon see: (a) Johner, A.; Joanny, J-F. Macromolecules 1990, 23, 5299. (b) Marques, C . M.; Leibler, L.; Joanny, J-F. Macromolecules 1988, 21, 1051.
~
H
0.5 1.0 2.0 3.0 4.0 a
(de@ ~ O
89 86 73.5 59 49 36
0
0 All polymers are 100%hydrolyzed. The notation in parentheses refers to earlier designations of samples. See ref 6. cBased on untagged polymers of very similar composition because this particular polymer was not characterized in ref 6.
work5 and a picosec correlated single photon fluorescence lifetime system described elsewhere.2 Details of the polymer adsorption procedure will be described in the text. For fluorescence measurements, the thin film was positioned against the front face of a 4 x 1 cm cuvette and front face excitationand detection were used, For micelles standard right angle geometry was used.
micollesicm2
oncn (mg/mL)
/mica
(x 10-9)
d (nm)b
0.144 0.446 0.644 0.771 0.915
10.6 13.8 17.5 19.1 19.7
41.5 64.1 68.5 71.7 76.9
See eq 4 of the text. See eq 5 of the text. 25
y
20
0 r ~i
15
. E v)
s -! 10 al
.-
E
5
0
1
0
1
3 C( mglm L)
2
2 3 C( mgim L)
4
4
Figure 1. Comparison of (a) the number of micelles per square centimeter from SEM and (b)the HzO contact angle as a function of the concentration of the bulk solution. polystyrene the final 0 ~ is~achieved. 0 The diminished contact angle persists for as long as the film has been kept (months) and is not affected by further washing with water. The N-PS-PMA material behaves in a similar way. Thus, we conclude that these block polymers have been irreversibly adsorbed onto the polystyrene film. We interpret the diminished 0 ~ as~ the 0 result of the PMA segments extending away from the surface, rendering it more hydrophilic. Micrographs of PS/micelle films were taken using the ESEM technique4 which permits an insulating material to be imaged. The images obtained from this technique were like those presented in Figure 2, which were obtained using traditional SEM methodology with a 100-A Au coating, albeit with poorer contrast and definition. The approximate micelle diameter was 50 nm, which compares well with micelles that were deposited on an aluminum substrate. This is much smaller than the hydrodynamic diameter of ca. 120 nm at high pH and low ionic strength, as determined by QELS (see later discussion). This discrepancy is not surprising because in high pH solution the hydrocynamic diameter is dominated by the fully extended corona while under SEM conditions the corona should be collapsed. For latex microspheres used as light scattering standards (obtained from Polysciences) the
Langmuir, Vol. 10, No. 6, 1994 1843
Adsorbed Photoactive Block Polymers
In the second term fdcis the fraction of the area occupied by the micelle for the closest micelle packing. Since eps = 90°, we obtain
Using the k value derived from the N-1 vs c-l plot (data not shown, k = 2.49 mL/mg) and assuming cos 8 m j c = 1, we obtain the values for fmjc listed in Table 2. If surface roughness is taken into account, the derived fdCvalues should be divided by r, and as stated above, we would expect r to increase with N. An apparent diameter (d)for the micelle can be obtained for the micelle from fdcand M
The derived d values are given in Table 2. We note that they increase steadilywith N. Since the surface roughness should be close to unity at the lowest coverage, this variation suggests that r might be as large as ca. 4 for maximum coverage. We also note that this calculation is very sensitive to the value of the contact angle at low coverage because of the rapid variation of cos 8 near 90°. Equation 4 suggests the following:
Figure 2. SEM imagesof the polystyrene surface after adsorption of polymer micelles (Au-coated)observed at normal incidence and 60° from normal for the upper and lower images,respectively.
SEM and light scattering radii agreed as expected because these particles do not have a corona. The SEM of the Au-coated polystyrene film with the adsorbed micelles demonstrates that most adsorption involves intact micelles with a clear separation between most individual particles (although some aggregates are seen), as one would expect from steric hindrance between coronas. We may attempt to understand our contact angle measurements according to the theory of contact angles of nonuniform surfaces. According to Adamsom8 for a composite surface
,e
COS
= r COS , ,e
= fl COS
e, + f2 COS e,
(2)
where f1 and f2 correspond to the fraction of exposed area of each type. For a rough surface the experimental Clem will underestimate if the roughness factor r is greater than unity and
