Langmuir 1993,9, 2243-2246
Surface Micelle Formation at the Air/Water Interface from Nonionic Diblock Copolymers S. Li, S. Hanley, I. Khan, S. K. Varshney, A. Eisenberg,' and R. B. Lennox'
Department of Chemistry, McGill University, 801 Sherbrooke Street West, Montreal, Quebec H3A 2K6,Canada Received September 18, 1992. In Final Form: May 3, 1993
Introduction I t was recently shown that a block copolymer composed of a polystyrene block (260 units) and an alkylated (decylated) poly(viny1pyridine) block (240 units) spontaneously forms highly regular aggregates at the aidwater interface.' These aggregates are believed to consist of a central core of polystyrene chains from which radiate the ionic poly(viny1pyridinium) chains. Because of their relative uniformity, and the obvious parallels to threedimensional aggregates,the term surface micelleshas been used to describe these aggregates at the aidwater interface.'" Several important features of these surface micelles have been explored, including (i) their aggregation numbers (N,,, ref 2) and the relative monodispersity of NW, (ii) the role played by the hydrophilicity of the alkyl derivatizing agents in determining the features of the compression isotherm: and (iii) the effect of varying experimental conditions such as temperature, electrolyte composition, and electrolyte concentration.6 Of particular interest is the observation of polymorphism in these surface micelles, where the balance between block sizes determines whether small circular, rodlike, or large planar aggregates form.4 This surface micelle polymorphism is very predictable and is reminescent of polymorphism in both block copolymer and small molecular weight amphiphile aggregates. These strong parallels highlight the important role 2D-aggregation phenomena will play in understanding the detailed structure and energy requirements of the self-assembly phenomena. All of the 2 D surface micelle-forming systems reported to date'" have been block polyelectrolytes. The energies associated with surface adsorption and self-assembly will clearly be large, given the incompatabilityof the two blocks and the disparate affinities of the two blocks for the water surface. In the following, we describe an extension to these block polyelectrolyte studies, where we show that both surface pressure/area isotherms of the Langmuir monolayers and their corresponding LB films are consistent with surface micelle formation in diblocks whose A block is hydrophobic (polystyrene) and whose B block is polar but nonionic (poly(n-butylmethacrylate),poly(tert-butyl methacrylate), poly(dimethylsiloxane),and poly(tert-butyl acrylate)). Systems whose nonionic blocks are chemically and structurally quite different are shown in the following to yield 2D surface micelles at the aidwater interface with many of the same features as those reported
* To whom correspondence may be addressed.
(1) mu, J.; Eisenberg,A.; Lennox, R. B. J. Am. Chem. SOC.1991,113, 5583. (2) Zhu, J.; Lennox, R. B.; Eisenberg, A. Langmuir 1991, 7, 1579. (3) Zhu, J.; Eisenberg,A.;Lennox, R. B. Makro. Chem. (Macro.Symp.) 1992, 53, 211. (4) Zhu, J.; Lennox, R. B.; Eisenberg, A. J.Phys. Chem. 1992,96,4727. (5) Zhu, J.; Eisenberg, A.; Lennox, R. B. Macromolecules 1992, 25, 6547. (6) Zhu, J.; Eisenberg, A.; Lennox, R. B. Macromolecules 1992, 25, 6556.
0743-7463/93/2409-2243$04.00/0
2243
previously'" and in some respects (i.e. monodispersity and packing regularity), with enhanced properties.
Experimental Section Materials. The block copolymers used in this study were all synthesized by the technique of sequential anionic polymerizat i ~ n The . ~ materialssynthesizedare (P(Saocrb-nBuMAd,P(SW b-tBuMA& P(SWb-D&), and P(Sm-b-tBuAzza).The polydispersity of the PS blocks is 1.05(measured by GPC, Varian). Styrene,n-butylmethacrylate(n-BuMA),tert-butylmethacrylate (t-BuMA),dimethylsiloxane (DMS),and tert-butyl acrylate (tBuA) were all obtained from Aldrich (Milwaukee,WI) and were purified by distillation prior to use. Homopolymers P(tBuA)1, and P(tBuA)llm were also synthesized by anionicpolymerization;their polydispersity is 1.05. Langmuir Film BalanceMeasurements. Surfaceprewuref area curves were obtained using a Lauda Model D film balance, equipped with a barrierfpressure transducer measuring device and a circulatingtemperature controller (Haake,Germany).The water subphase was prepared from howe-distilled water which was subsequentlypassed through a Milli-Q(Bedford,MA) water purification system equipped with an organic removal cartridge. Stock solutions of the block copolymers were prepared in freshly distilled CHCh to give a final concentration of ca. 5 mJ 10mL. In a typical experiment, 100pL of the stock solutionwas deposited on the water surface in equally spaced (over the initial 970cmzof area) 5-pL drops. This procedure ensures rapid and reproducible spreading of the polymer across the water surface. Compression(atca. 6 cmZ/min)was initiated after a 30-mindelay, so as to allow the spreading solvent to evaporate. All isotherms were obtained at 25.0 .+ 1 O C . A , is the area at which r 1.0mN/m and A1 is the area at which the first phase transition becomes apparent. Film collapse is apparent in most samplesas a sharp discontinuityin the isotherm in the high T , low area/molecule region of the isotherm. Langmuir-Blodgett Films. Langmuir-Blodgett (LB)films of these materials were prepared using previously described techniques.l-3 In brief, LB films were deposited under constant 7r conditions (Le. 2 mNfm) onto carbon-coated Formvar/Cu transmission electron microscope grids (TEM experiments) or freshly cleaved mica (AFM experiments). All LB films were deposited on the upstroke and are 1 layer thick. Transfer ratios (at 2 mN/m) are 1.0 f 0.1. Each LB film was examined by transmission electron microscopy (TEM) and atomic force microscopy (AFM) after the film had been shadowed at 15-20° with a 6040 Pt:Pd The TEMs exhibit features directly related to the topology of the film. Atomic force microscopy studies of representative LB films exhibit exactly the same features (topology and dimensions) as the metal-coated LB films using a Digital Nanoscope I1 AF'M operated in the constant force mode.
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-
Results and Discussion The T-A isotherms of P(S-b-nBuMA), P(S-b-tBuMA), P(S-b-tBuA),andP(S-b-DMS)block copolymer Langmuir monolayers spread at the aidwater interface are provided in Figure 1. In each case, the isotherms exhibit relatively complex properties with one or more plateau regions apparent. Plateau regions in the isotherms are characteristic of each of the polar homopolymer block materials and are attributable to pressure-induced reorganization/ reorientation phenomena. The precise nature of these reorientations is of particular interest because they are the result of the balance between block-block and blockinterface interactions. The acrylate material is unusual compared with the other samples in that as many as three plateaus are observed in its isotherm (Figure Id). Comparison to the homopolymer, which exhibits only one plateau (at ?r = 22 mN/m) establishes that the multiple plateaus are a property of the diblock material itself. Given (7) Gauthier, S.; Eisenberg, A. Macromolecules 1987,20,760.
1993 American Chemical Society
Notes
2244 Langmuir, Vol. 9, No. 8,1993 Area ( n m '/nBMA Unit)
E
\
z
E
v
60
1
0
Area ( n m '/PtBMA Unit) 0
0.05 0.10 0.15 0.200.250.30 0.35 0.40
0.20
0.30
0.40
0.50
z
5or
::I, 10
1 i
1::I,
1
,
\
,
10
20
30
40
50
60
70
80
90
,I
30 40
0 0
0.10
,
,
10
15
,\
0
100
5
0
20
25
30
35
46
40
50
Mean Area (nm2/molecule)
Area (nm*/molecule)
Area (nm'/tBA 0
0.20
Unit) 0.40
0.60
z
E
"
0
20
40
60
80
100
120
140
160
Area (nm2/molecule)
Area ( n m '/molecule) h
30
-
25 20 15 10
PtBA (150)
0
0
40
80
120
160
200
240
280
320
360
1
400
Area (nm2/molecule) Figure 1. P A isotherms (25 "C)obtained by compression of a diblock copolymer sample at the &/water interface: (a, top left) P(Snro-b-nBuMAsor); (b,top right) P(Slw-b-tBuMA70);(c,middle left) P ( S , & J - D M S ~(d,) ;middle right) P(S,-b-tBuAm); (e, bottom) homopolymersP(tBuA)I, and P(tBuA)1lm. Area is provided in units of nma/polarresidue when possible. The latter scale is accessible by subtracting the PS core area from the total area, as determined from the transmission electron micrographs.
that the highest plateau in the diblock is a t the same ?r as the homopolymer, we conclude that the lower pressure transitions originate from n-BuA reorientations and that the plateau a t ?r = 22 mN/m probably corresponds to the n-BuA chains folding over one another. Accompanying each isotherm are TEM photomicrographs (Figure 2) and AFM topographs (Figure 3) of the corresponding LB film obtained at xr = 2 mN/m. In each case, structures characteristic of the recently documented surface micellesld are evident. These LB films immediately show that the surface micellizationphenomenon is a verygeneral one in that it occurs even when the B block is nonionic,provided of course that the B block is surface-adsorbed. Evidently the 2D self-assembly of the hydrophobic PS blocks provides enough enthalpy to overcome the sub-
stantial entropy losses attendant with the orientation of a number of B-blocks into a highly defined, close-packed geometry. The aggregation numbers (N,) of the surface micelles visualized in Figure 1 can be determined using recently establishedtechniques (Table I,ref 2). N,values,ranging from 95to 200, tend to be larger for these nonionic materials (for a similar size of block) compared to the ionic materials previouslystudied. This increase in NW is consistent with expectations based on the limitations of aggregate growth caused by the balance between electrostatic repulsions and PS-PS cohesion energies in the block polyelectrolyte case. Charge neutralization in small molecular weight amphiphiles for example can lead to aggregate growth* and even to morphology changes.9 The polymeric ma-
Langmuir, Vol. 9,No. 8,1993 2245
Notes
if
I
Figure 2. Transmission electron micrographs (TEM) of Langmuir-Blodgett (LB) films of materials examined in Figure 1: (a, top left) P(Smb-nBuMAm); (b, top right) P(Slm-b-tBuMA,o);(c, bottom left) P(S8m-b-DMSw); (d, bottom right) P(S~~-b-tBu.A222). In each case, the LB film was deposited, at 2 "am-1 on the upstroke, onto carbon-coated Formvar/Cu electron microscope grids.
terials described here do present an important balancing factor however, in that the ionic chains will tend to be highly extended in pure water and will have a surface emergence "footprint" of only one to two monomers in width. If they are well spread on the water surface, the nonionic blocks will adopt a much wider footprint because
of the 2D "random coil" which would be expected to be adopted. Characterization of the chain elongation state in the nonionics is the topic of an ongoing study.1° (8) Tanford, C.The Hydrophobic Effect, 2nd ed.;Wiley New York, 1980.
Notes
2246 Langmuir, Vol. 9, No. 8, 1993
Figure 3. Atomic force topographs of LB films (2 "om-l) of (a, top) P(Sm-b-nBuMAm) and (b, bottom) P(S8m-b-DMSm) deposited onto freshly cleaved mica surfaces. Both samples have been metal shadowed as per TEM experiments. Scale in (a) is 800 nm by 800 nm; scale in (b) is 1pm by 1 pm.
Table I. Aggregation Numbers of Diblock Copolymer Surface Micelles sample P(SWb-nBuMAm) P(Sm-b-tBuMAm) a Determined
Naano ~~~
190 95
sample P(S1Wb-tBuMA70) P(Smb-DMSm)
"a
140
b
using total area method (ref 2) from TEM photo-
graphs. Cannotbedeterminedusingestablishedtechniquesbecause the surface micelles fuse to form strings of aggregates as in Figures 2c and 3b.
One can subtract the PS contribution to the total area per molecule to give estimates of the area per nonionic
block residue (Figure 1). In the n-BuMA and t-BuMA cases, the A, values are similar to that determined by models (-39 A2/residue). AI values represent a -30% reduction in the available area in each case and correspond to values possiblefor surfaceanchored residueswhich have become reoriented during the compression process. With respect to the condensed region in the isotherm, in the n-BuMA case, the area per residue is as small as 7 A2. Only the formation of a 3D structure of some type is consistent with this extensive area reduction. Brinkhuis and Schoutenll have provided evidence that only isotactic PMMA homopolymer undergoes a pressure-induced surface adsorbed +surface-absorbeddouble helix transition. Evidence for rodlike polymers forming multilayered structures in the transition region, with the rod axis being parallel to the water surface12has also been presented. It is quite unlikely that either phenomenon would occur with the samples shown in Figure 1, however, given that the plateau ends a t 13 A2 per residue, after which the film becomes quite incompressible. Moreover, the P(Sm-bnBuMAm) material studied here is predominately syndi0ta~tic.l~ It is apparent, instead, that for the materials studied here, that individual diads can become water solubilizedto some extent or that clusters of diads or triads become solubilized. This may be examined by studying a variety of P(S-b-nBuMA) diblocks whose n-BuMA block varies in tacticity. The more isotactic the material, the more condensed the possible packing, because of the increased possibility that some ternary structure will be induced. The relatively large area values prior to collapse clearly indicate that pressure-induced solubilizationdoes not occur in a wholesale fashion as it evidently does for the block polyelectrolytes (refs 1 and 2) and that a solubilization mechanism may only be operative for opportune diads, triads, etc. In this preliminary report, we have established that the self-associationof diblock copolymers into highly defined structures at the air/water interface is a highly general phenomenon. A number of earlierstudies using segregated diblocks at the air/water interface did not have access to the LB/TEM visualization technique described here and may thus benefit from a reevaluation of isotherm features in the context of interacting polymer aggregates rather than simply interacting polymer molecules. Further detailed studies of self-association of diblock copolymers in two-dimensions are currently in progress.
Acknowledgment. NSERC Canada is acknowledged for its support of this work in the form of operating grants to A.E. and R.B.L. ~~~~
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(9) Fendler, J. H. Membrane Mimetic Chemistry; Wiley: New York, 1982. (10) Li, S. Unpublished results. (11) Brinkhuis, R. H. G.; Schouten, A. J. Macromolecules 1991,24, 1487. (12)Duda, G.; Schouten, A. J.; Amdt, T.; Lieaer, G.; Schmidt, G. F.; Bubeck, C.; Wegner, G. Thin Solid Films 1988,159,221. (13) As estimated from 1% NMR measurements. We thank Dr. Z. Gao, M a i l l University, for making these measurements.
