Study on the Behaviors of Different Polystyrene-block-Poly(methyl

Mar 6, 2003 - Aggregation behavior of a polystyrene–b-poly(phenylsilsesquioxane) H-type copolymer at the air/water interface. Yongsok Seo , Sang Gil...
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Langmuir 2003, 19, 3313-3322

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Study on the Behaviors of Different Polystyrene-block-Poly(methyl methacrylate) Diblock Copolymers Adsorbed at the Air/Water Interface Yongsok Seo,*,† Alan R. Esker,‡ Daewon Sohn,§ Hyong-Jun Kim,†,‡ Sangwook Park,|,⊥ and Hyuk Yu| Supercomputational Modeling and Simulation Laboratory (SMSL), Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul, Korea 130-650, Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, Department of Chemistry, Hanyang University, Haengdangdong 17, Sungdongku, Seoul, Korea 133-791, and Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706 Received September 30, 2002. In Final Form: January 10, 2003 Using surface quasi-elastic light scattering (SLS), we have studied the monolayer behavior of diblock copolymers A-B, where the A block is polystyrene (PS, surface inactive and water insoluble) and the B block is poly(methyl methacrylate) (PMMA, surface active but water insoluble), at the air/water interface (two-dimensional domain) in terms of the power spectra and the surface viscoelasticity. The polystyrene blocks of the diblock copolymers have similar molecular weights, but the molecular weights of the poly(methyl methacrylate) blocks vary. Substantial changes in viscoelastic behavior accompany changes in surface concentration and can be attributed to differences in molecular packing resulting from micelle formation and surface phase behavior, which are consistent with atomic force microscope (AFM) images of monolayer films transferred from air/water interfaces to solid substrates. The state change is similar to previous results obtained using the dynamic (stepwise compression) method. At submonolayer coverage, there is a clear, molecular mass dependent deviation from pure liquid dynamics in the raw SLS data, frequency shifts, and damping coefficients, along with corroborative AFM images that are consistent with a biphasic state consisting of gaslike and liquidlike domains, where the condensed liquidlike domains are composed of surface micelles. The ill-defined PS segment sits on top of the PMMA segment without much contact with the water. The PS segment forms the core of the micelles. As the concentration goes up, these systems show characteristic surface viscoelastic behavior. Since only the PMMA segment is surface active while PS is not, the general behavior of PS-PMMA diblock copolymer films at high surface pressure (>7 mN/m) is similar to that of a pure PMMA film whereas different behavior was observed at low surface pressure (submonolayer region). Viscoelastic parameters, that is, the dynamic dilational elasticity, d, and viscosity, κ, deduced from the SLS data indicate that the rigidity of the liquidlike domains increases smoothly showing large d and κ values, regardless of the PMMA molecular weight.

Introduction Block copolymers have interested scientists due to their many important industrial applications. Also, their interfacial behaviors have an enormous impact on the properties of polymer alloys and blends.1-4 Additionally, they have assumed importance in areas other than industrial and technical applications. There has been tremendous scientific interest in the study of block copolymers at interfaces due to the importance of understanding polymers in confined geometries.1-12 Block * To whom correspondence should be addressed. E-mail: ysseo@ kist.re.kr. † Korea Institute of Science and Technology. ‡ Virginia Polytechnic Institute and State University. § Hanyang University. | University of Wisconsin. ⊥ Current address: LG Chem Research Park, P.O. Box 61, Yusongku, Daejon, Korea. (1) Scheutjens, J. M. H. M.; Fleer, G.; Cohen-Stuart, M.; Cosgrove, T.; Vincent, B. Polymer Interfaces; Chapman and Hall: London, 1993. (2) Physics of Polymer Surfaces and Interfaces; Sanchez, I. C., Ed.; Butterworth-Heinemann: Stoneham, 1992. (3) Faraday Discuss. 1995, 98. Complete issue dedicated for polymers at interfaces. (4) De Gennes, P. G. Macromolecules 1980, 13, 1069; Macromolecules 1981, 14, 1037; J. Phys. (Paris) 1976, 37, 1445. (5) Alexander, S. J. Phys. (Paris) 1977, 38, 983. (6) Milner, S. T.; Witten, T. A.; Cates, M. Europhys. Lett. 1988, 5, 413.

copolymers are also very intriguing because they can have a pronounced amphiphilic character, depending on the choice of the blocks. Block copolymer films on a fluid interface are easy to handle because the amount of polymer spread on the interface can be precisely controlled and because the chain densities can be continuously varied.9 In view of this, the properties of interfacial layers formed by diblock copolymers have recently received considerable attention. Many experimental studies of the adsorption behaviors of polymers at interfaces have been carried out,5-16 while other studies have been more concerned with micelles on the surface.17-19 (7) Zhulina, E. B.; Borisov, O. B.; Pryamitsin, V. A. J. Colloid Interface Sci. 1990, 137, 495. (8) Shull, K. R. J. Chem. Phys. 1991, 94, 5723 and references therein. (9) Goncalves da Silva, A. M.; Filipe, E. J. M.; d’Oliveira, J. M. R.; Martinho, J. M. G. Langmuir 1996, 12, 6547. (10) Kent, M. S.; Lee, L; Farnoux, B.; Rondelez, F. Macromolecules 1992, 25, 6240. (11) Runge, F. E.; Kent, M.; Yu, H. Langmuir 1994, 10, 1962. (12) Kumaki, J. Macromolecules 1986, 19, 2258; 1988, 21, 749. Kumaki, J.; Kishikawa, Y.; Hashimoto, T. J. Am. Chem. Soc. 1996, 118, 3321. (13) Goncalves deSilva, A. M.; Ganboa, A. L. S.; Martinho, J. M. G. Langmuir 1998, 14, 5327. (14) Prokop, R. M.; Hair, M. L.; Neumann, A. W. Macromolecules 1996, 29, 5902. (15) Sauer, B. B.; Yu, H.; Kim, M. W. Langmuir 1989, 5, 278. (16) Granick, S.; Herz, J. Macromolecules 1985, 18, 460. (17) Lin, B.; Rice, S. A.; Weitz, D. A. J. Chem. Phys. 1993, 99, 8308.

10.1021/la020819l CCC: $25.00 © 2003 American Chemical Society Published on Web 03/06/2003

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For diblock copolymers at an air/liquid interface, when one block is soluble in the liquid while the other is not, the concept of a “tethered chain” appears to appropriately describe the interfacial chain density.9 The soluble chains should form an adsorbed layer (pancake) in the low-density limit, a three-dimensional configuration (mushroom) at somewhat higher densities where the soluble blocks are stabilized but do not overlap with each other, and a stretched configuration (brush) at sufficiently high coverage.9-16 A few earlier studies have been concerned with the conformations of block copolymers on surfaces and their transitions to other conformations in a liquid. Those studies used polystyrene-b-poly(ethylene oxide) (PS-PEO)9,14,15 or polystyrene-b-poly(dimethyl siloxane) (PS-PDMS)10,15,16 because the PS block anchors the chain firmly to the surface. Kent et al. performed extensive investigations of a PS-PDMS diblock copolymer film at an air/ethyl benzoate interface.10,11 Those studies were concerned with the three-dimensional conformational changes of diblock copolymers having a soluble block in a liquid. On the other hand, it has been known for many years that block copolymers can form surface micelles.17-19 The driving force for the formation of the quasi-2D micelles is a delicate balance between hydrophobic and hydrophilic factors. The self-assembling of amphiphilic diblock copolymers generally results in aggregates of a core-shell structure. Aggregates with a spherical morphology have been identified in many block copolymer solutions, but other morphologies have also been observed, depending on the relative dimensions of the corona and the core block,18,19 and are referred to as “star” micelles or “crewcut” micelles.19 Though many studies have addressed amphiphilic diblock copolymers, little work has been done on diblock copolymers in which both blocks are hydrophobic, such as polystyrene-block-poly(methyl methacrylate) (PS-PMMA). The surface behavior of the PS-PMMA diblock copolymer is quite interesting because PS-PMMA diblock copolymers are expected to stay on the water surface.12 In our previous study, we investigated the dynamic interfacial behavior of a PS-PMMA diblock copolymer at an air/water interface (two-dimensional domain).20 The surface pressure isotherms exhibited two phase-transitions attributed to surface micelle formation at low surface pressure and to further assembly of surface micelles at high surface pressure. Atomic force microscope images were correlated with the aggregates of the chains forming the quasi-two-dimensional surface micelles at low surface pressure. Chains having PMMA blocks that are shorter than the PS blocks form micelles with many block copolymer molecules, whereas chains having PMMA blocks that are 4 times longer than the PS blocks form micelles with several molecules.20 Compression to high surface concentration caused the micelles to take on a more compact structure. Changes in the interfacial structure were observed by Brewster angle microscopy as the micelles were forced together.20 In this study, we further investigate the surface viscoelastic properties of PS-PMMA monolayers at an air/water interface by using surface quasi-elastic laser (18) Zhu, J.; Eisenberg, A.; Lennox, R. B. J. Am. Chem. Soc. 1991. Zhu, J.; Eisenberg, A.; Lennox, R. B. J. Am. Chem. Soc. 1991, 113, 5583. Zhu, J.; Lennox, R. B.; Eisenberg, A. J. Phys. Chem. 1992, 96, 4727. Zhu, J.; Eisenberg, A.; Lennox, R. B. Macromolecules 1992, 25, 6547. Zhu, J.; Eisenberg, A.; Lennox, R. B. Macromolecules 1992, 25, 6556. (19) Zhang, L.; Eisenberg, A. Macromolecules 1999, 32, 2239; Science 1995, 268, 1728; Macromolecules 1996, 29, 8805; J. Am. Chem. Soc. 1996, 118, 3168. (20) Seo, Y.; Paeng, K.; Park, S. Macromolecules 2001, 34, 8735.

Seo et al. Table 1. Molecular Weight Data for PS-PMMA Diblock Copolymers

samplea

product name

P70 P155 P392 P656

(P722) (P224) (P309) (P419)

Mn of PS

tacticity in PMMA block Mn of PMMA PDIb syndio iso atactic

146 700 70 700 1.11 143 800 154 800 1.12 154 800 392 300 1.08 140 000 656 000 1.32

0.79 0.813 0.818 0.450

0.002 0.004 0.003 0.000

0.208 0.183 0.179 0.550

a The numbers after the P mean the molecular weights of the PMMA segment in kilograms/mole. b Polydispersity index of the diblock copolymer, PDI ) Mw/Mn, where Mn is the number average molecular weight in grams/mole and Mw is the weight average molecular weight in grams/mole.

light scattering (SLS) and theoretical models. Since the PS block is not surface active and insoluble to water and the surface activity is totally attributed to the PMMA block, the monolayer behavior is expected to follow that of PMMA. However, the PS-PMMA diblock copolymer forms a surface micelle while PMMA just forms a condensed monolayer, which suggests their behavior may be different from that of PMMA. In this study, we use the capillary wave technique to characterize the diblock copolymer films in order to probe the changes in the surface viscoelasticity and morphology at an air/water interface as a function of the PMMA segment length. Experimental Section Materials. The PS-PMMA diblock copolymers were purchased from Polymer Source, Inc. (Canada). Table 1 lists some characteristics of these block copolymers. As can be seen from columns 3 and 4 of Table 1, they have similar PS block lengths (ca. 147 000 ( 7000) but mainly differ in the molecular weights of the PMMA blocks and in their tacticities. P70 has a PMMA block (Mn ) 70 000) that is shorter than the PS block. In P155, the PMMA block (Mn ) 155 000) is almost the same length as the PS block, whereas P392 and P656 have PMMA blocks (Mn ) 392 000 and 656 000, respectively) that are longer than the PS blocks. The numbers after P mean the molecular weights of the PMMA segments in kilograms/mole. These are the same materials we used in our previous study.20 Surface Quasi-Elastic Light Scattering Measurement. Spectrograde chloroform (Aldrich) without further purification was used as the spreading agent. Distilled water was deionized by using a Millipore Q2 system and was used for the subphase. A Teflon trough with internal dimensions of 110 mm × 285 mm × 12.5 mm was filled to its brim with the subphase. After the surface was cleaned by several passages of the Teflon float, a sandblasted platinum plate (11 mm × 26 mm × 0.1 mm), previously soaked in a HNO3/H2SO4 mixture and washed thoroughly with pure water, was carefully hung on the suspending wire of an electrobalance (Cahn model 2000). The polymer monolayer was spread on the subphase surface by applying a suitable volume of polymer solution with a Hamilton microsyringe. To obtain a stable monolayer on the subphase, we allowed at least 20 min for the solvent to evaporate after each spreading. The voltage output of the electrobalance control unit was read directly by using a digital voltmeter. The final precision of the surface-pressure measurement was estimated to be (0.02 mN/ m. The surface layers were prepared by the successive addition method using a Hamilton syringe rather than the stepwise compression method in order to remove the complexity caused by the dynamic factor.11 The temperature of the subphase was controlled by circulating thermostated water through a glass coil placed in the bottom of the trough. The circulating water temperature was maintained at 23 ( 0.1 °C by using a Lauda bath (Lauda RM6). Monolayer films at an air/water interface have the ability to damp out waves, called capillary waves or ripplons, propagating on the liquid surface. These waves result from spontaneous density fluctuations of the underlying bulk liquid. Despite their small amplitudes (3∼5 Å), these waves efficiently scatter light.21

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The surface light scattering method takes advantage of this scattering and has been used to study capillary waves on a variety of polymeric and surfactant monolayers.22 Recently, this method has been frequently used to study the monolayer behaviors of block copolymers and surfactants at air/water interfaces.21-34 Because we expect the dynamic film properties to be sensitive to the structure of the floating chains, the viscoelastic behavior of the PS-PMMA diblock copolymer should be significantly different from that of other block copolymers which have a block that is soluble in the liquid. For each surface concentration, the surface light scattering measurements were conducted after the static surface tension had reached a stable value. Since the details of the surface light scattering apparatus are described elsewhere,11,26,29 we shall briefly outline the basic schemes of the apparatus here. Light from a He-Ne laser (7 mW, 632.8 nm, Milles Griot) is incident at an angle of 64.31° from the outward normal to the surface. All optical components, including a cabinet made of Plexiglas enclosing the trough and the electrobalance, are placed on an isolated (vibration-free) optical table stabilized by a set of coiled springs suspended from a steel frame superstructure. The key feature of this technique is the use of a transmission diffraction grating and a neutral density filter to provide a reference beam for heterodyne detection of the scattered light.29 We select three different scattering wave vectors, k ) 262.5, 324.3, and 384.8 cm-1, corresponding to the fourth, fifth, and sixth diffraction orders of the optical grating. The power spectra from pure surfaces and monolayer-covered surfaces are acquired on a fast Fourier transform (FFT) spectrum analyzer (Nicolet 446A) and are fitted with a Lorentzian function to obtain the frequency shift, fs, at the peak and the full width at halfheight, ∆fs. Although the actual form of the power spectrum is not strictly Lorentzian,31 this approximation is used throughout because, based on the work by Earnshaw et al.,25,28 the nonLorentzian nature of the power spectrum accounts for an error of less than 0.25% in the frequency shift, fs, and of less than 1% in the full width at half-maximum intensity, ∆fs, over the range of wave vectors used here. These errors are much smaller than the magnitude of the instrumental correction and are also smaller than the random experimental errors which are typically on the order of 0.5-1% for fs and 5-10% for ∆fs,c, the instrumental corrected full width at half-maximum intensity.32 The method of Hard et al.27 is used to correct the value of ∆fs for a Gaussian instrumental width to give ∆fs,c. Once this is done, fs and ∆fs,c can be used to solve the dispersion equation33,35

η2(k - m*)2 )

[

η (k + m*) +

][

]

gF ω*F *k2 σ*k2 + η(k + m*) + iω* iω* iω* ik

(1)

(21) Braslau, A.; Perchan, P. S.; Swislow, G.; Ocko, B. M.; Als-Nielsen, J. Phys. Rev. A 1988, 38, 2457. (22) Light Scattering by Liquid Surfaces and Complimentary Techniques; Langevin, D., Ed.; Surfactant Science Series, Vol. 41; Marcel Dekker: New York, 1992; Chapters 1-6, 10, and 11. (23) Richards, R. W.; Rochford, B. R.; Taylor, M. R. Macromolecules 1996, 29, 1980. (24) Henderson, J.A.; Richards, R. W.; Pernfold, J.; Shaklecton, C.; Thomas, R. K. Polymer 1991, 32, 3284. (25) Earnshaw, J. C.; McGivern, R. C.; Winch, P. J. J. Phys. (Paris) 1988, 49, 1271. (26) Sano, M.; Kawaguchi, M.; Chen, Y. L.; Skarlupka, R. J.; Chang, T.; Zografi, G.; Yu, H. Rev. Sci. Instrum. 1986, 57, 1158. (27) Hard, S.; Newman, R. D. J. Colloid Interface Sci. 1987, 120, 15. (28) Earnshaw, J. C.; McCoo, E. Phys. Rev. Lett. 1994, 72, 84. (29) Esker, A. R. Ph.D. Dissertation, University of Wisconsin (Madison), Madison, WI, 1996. (30) Buzza, D. M.; Jones, J. L.; McLeish, T. C. B.; Richards, R. W. J. Chem. Phys. 1998, 109, 5008. Milling, A. J.; Hutchings, L. R.; Richards, R.W. Langmuir 2001, 17, 5305. (31) Earnshaw, J. C.; McLaughlin, A. C. Proc. R. Soc. London 1993, A440, 519; 1993, A443, 663. (32) Earnshaw, J. C.; McGivern, R. C.; McLaughlin, A. C.; Winch, P. J. Langmuir 1990, 6, 649. (33) Earnshaw, J. C.; McCoo, E. Langmuir 1995, 11, 1087. (34) Kawaguchi, M.; Sauer, B. B.; Yu, H. Macromolecules 1989, 22, 1735. (35) Lucassen-Reynders, E. H.; Lucassen, J. Adv. Colloid Interface Sci. 1969, 2, 347.

Figure 1. General solution to the dispersion eq 1, ∆fs,c vs fs for water at 23 °C (ref 29). The solid lines correspond to constant dilational elasticity, d, while the dashed lines correspond to constant dilational viscosity, κ. The roman numerals I and V indicate the pure liquid limit and the limit for a surface film with an infinite lateral modulus. For a purely elastic surface film (µ, κ)0), the limits correspond to II ) maximum velocity limit, III ) maximum damping limit, and IV ) minimum velocity limit. Limit VI represents the maximum damping coefficient that can be observed on a purely viscous surface film (d ) 0). The * terms represent complex quantities which are defined as

ω* ) ω0 + iR

(2)

σ* ) σd + iω0µ

(3)

* ) d + iω0κ

(4)

iω*F η

(5)

and

(

m* ) k2 +

1/2

)

(Re(m*) > 0)

where k is the wave vector, ω0 is the real frequency (≡2πfs), R is the imaginary frequency (≡π ∆fs,c) which is zero for spatially damped waves, and σ* is the transverse modulus made up of a real term, σd, the dynamic surface tension, and an imaginary part containing the transverse viscosity, µ. * is the complex lateral modulus made up of the dilational elasticity, d, and its corresponding dilational viscosity, κ. The complex wave vector m* (Re(m*) > 0) is associated with how deeply the surface wave penetrates into the underlying liquid. The SLS technique allows the use of theoretical models to deduce the surface viscoelastic properties of the films. In the general case, four separate properties affect the propagation frequency, and the temporal damping constant of the waves causes problems in the unambiguous determination of these four properties.29-34 It is impossible to interpret two observables in terms of the four physical properties without extra information or a priori assumptions. In this study, the dispersion equation is solved for the two lateral (dilational) moduli, d and κ, by assuming the static and the dynamic surface tensions to be equal, σd ) σs, and the transverse viscosity, µ, to be zero.30 By use of a nonlinear regression fitting method, these four variables can be determined simultaneously, but the analysis needs to be done carefully.23,24 Recently, Earnshaw and co-workers31-33 investigated pure liquids and layers covered with soluble surfactants, experimentally using SLS and numerically, for some cases in which the transverse viscosity was nonzero. However, unless the wave frequency was enormously high (above 106 Hz), the resulting values for the transverse viscosity were so low as to be negligible compared to the values for the surface tension.25 Other results were similar. The obtained transverse viscosity showed a rapid decrease to zero with increasing surface concentration.26 Thus, µ is negligible unless the surface concentration is very low and the frequency is enormously high. A more detailed theoretical analysis for the effect of the surfactant monolayer on the hydrodynamic modes and SLS of the fluid-fluid interface was recently provided by Richards and co-workers.30 Since their

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Figure 2. Plots of the surface pressure, Π, the capillary wave frequency, fs, and the corrected full width at half-maximum intensity, ∆fs,c, versus the average surface area per monomer (0 ) 4th order, b ) 5th order, 4 ) 6th order). Molecular weights are given in Table 1. The overall trends are quite similar to each other. The molecular weight dependent difference in the viscoelastic properties of the monolayers prior to the onset of the increase in surface pressure disappears if we plot these figures versus the surface area per PMMA monomer. analysis is applicable for the polymeric surfactant monolayer to be in the brush state, which is different from our system, it is not applied to the present study. Capillary waves are neither purely transverse nor purely longitudinal. For this reason, the observed capillary wave frequency ω* represents a combination of the two whereby the damping observed on the surface results from a resonant-like coupling between the two modes. Perhaps the best way to show the consequences of this coupling of the transverse and the longitudinal modes is by plotting the general solutions to the dispersion equation as was first done by Hard and Newman.27,29 Figure 1 shows a plot of the corrected full width at half-maximum intensity, ∆fs,c, which is related to the damping coefficient for capillary waves (R ) π∆fs,c), versus the capillary wave frequency, fs, for “isobars” of fixed elasticity and viscosity at a fixed wave vector on a pure water surface at 23 °C.29 The solid lines in the plot correspond to the dilational elasticity, d, while dashed lines correspond to the dilational viscosity, κ. The upper edges of the plot correspond to a perfectly elastic surface film, while the lower part corresponds to a perfectly viscous film. A perfectly elastic surface film has some important limiting behavior.29 Limit I is the pure liquid limit, and limit II is the maximum velocity limit where the wave actually propagates faster than on a pure liquid. Here, the resonance condition is such that the motion in the longitudinal direction is favored at the expense of the transverse mode, leading to the observed result. The third limit, III, corresponds to the case of a maximum damping coefficient, where the wavelengths and the frequencies of the transverse and the longitudinal waves at the surface are equal. Further increases in elasticity lead to two other limits corresponding closely to an incompressible surface film (infinite complex lateral modulus or infinite complex dilational elasticity). Limit IV, the first one encountered, is the minimum velocity limit for a perfectly elastic surface film. Here, as * f ∞, resonant coupling leads to a case where transverse motion is favored over longitudinal motion. Further increases in lateral modulus lead to limit V, where longitudinal motion ceases. Since the lateral modulus is a complex quantity, the infinite limit must apply not only to purely elastic surface films but also to purely viscous surface films (infinitely viscous). Analogous to limit III, limit VI corresponds to the

damping coefficient for a purely viscous surface film. More details of and further insights into the limiting behavior of the monolayer dispersion equation are presented elsewhere.29,36,37 We used this analysis scheme in this study to investigate the PS-PMMA monolayer behavior in two-dimensional space. Atomic Force Microscope (AFM) Measurements. The monolayer morphology was observed using a commercial atomic force microscope (AutoProbe CP, Park Scientific Instruments Co.) with the contact mode in air at 23 °C. The microscope was equipped with a microfabricated V-shape silicon cantilever (force constant, 0.03 mN/m) on a 5-µm scanner. The monolayer was deposited on a cleaned silicon wafer. Cleaning was done by putting it in a 1/3 solution of 30% H2O2/concd H2SO4 at room temperature for 15 min and rinsing with Millipore deionized water. The wafer was immersed in the water before the solution spreading to provide a hydrophilic surface and to transfer the single layer of surface molecules.12 The monolayer was transferred on the wafer at constant surface pressure under a constant vertical lifting speed of 1 mm/min. It is generally accepted that no readily apparent structural defects are observed in the transferred monolayers if the lifting rate is kept below 5 mm/min.18

Results and Discussion Surface Light Scattering Power Spectra. The surface pressure, Π, as a function of the area per monomer is shown in Figure 2 for three block copolymers. Also, the frequency shift, fs, which is a measure of the propagation velocity of the capillary waves, and the corrected width, ∆fs,c, which is related to the wave damping, are plotted in Figure 2 as functions of the area per monomer for the wavenumbers k ) 262.5, 324.3, and 384.1 cm-1 corresponding to the fourth, the fifth, and the sixth diffraction orders of the optical grating, respectively. The trends look (36) Esker, A.; Zhang, L; Olsen, C. E.; No, K.; Yu, H. Langmuir 1999, 15, 1716. (37) Esker, A. R.; Zhang, L.; Sauer, B. B.; Lee, W.; Yu, H. Colloids Surf., A 2000, 171, 131.

PS-b-PMMA Monolayer Dynamics

similar to each other, and the effect of molecular weight differences are not immediately obvious. The lift-off area per monomer (PS + PMMA) seems to depend on the molecular weight (7, 11, and 16 Å2 for P70, P155, and P656, respectively), but the lift-off areas per PMMA monomer were not very different from each other (18, 18.9, and 19.2 Å2 for P70, P155, and P656, respectively). This indicates that an ill-defined PS segment sits on the top of the PMMA segment without much contact with water and the PS segment does not interfere with the PMMA segment, which is legitimate considering the immiscibility between PS and PMMA as well as their insolubility in the water. The π-A isotherms show a sharp increase with decreasing A, which is the same as the result for the PMMA monolayer. For the PMMA monolayer, the π-A isotherm is referred to as a condensed isotherm. The trend of the frequency shifts, fs, versus A resembles that of the pure PMMA monolayer. With decreasing A, fs decreases in a convex manner as the surface tension decreases before attaining a constant value. The characteristic profile of fs for PMMA also appears here, that is, fs discontinuously drops by about 300 Hz, while the surface pressure is still zero. For the PMMA monolayer, this is ascribed to the onset of a biphasic state.34 We infer that this is due to the biphasic state of the PS-PMMA monolayer forming surface micelles rather than patches like those formed in the PMMA monolayer. This is shown later in AFM pictures. No discernible variation in the surface concentration dependence of fs with respect to k is seen for PS-PMMA block copolymers. The common trend in corrected ∆fs,c as a function of A also follows that of PMMA, but again there exists some subtle difference. The values of ∆fs,c for PMMA show that at a concentration where the surface pressure is still almost zero, the damping coefficient undergoes a sharp increase and then gradually decreases with decreasing surface area. Kawaguchi et al.34 ascribed this again to the scattering from the condensed patches of PMMA, like the abrupt drop in fs. For the block copolymers, the damping coefficient undergoes a sharp increase and then a decrease with decreasing surface area. The maximum occurs because of the resonant coupling of the transverse and longitudinal modes.35 The difference in behavior of ∆fs,c after the maximum implies that there is a subtle difference in monolayer behavior after the resonant concentration possibly due to the structural variation. There is a gradual structural variation for block copolymers after the resonant concentration,20 whereas the PMMA monolayer shows the sudden appearance of patches.34 Though the peak positions in ∆fs,c appear differently when it is plotted versus the area per monomer, the peak position difference is again not remarkable if we plot it against the area per PMMA monomer. The same is true for the limit area, A0. Again, these points mean that the PS segment does not interfere with the PMMA segment. An alternative way to represent the frequency and the temporal damping characteristics of capillary waves propagating on a liquid surface covered with a monolayer film is to determine fs and ∆fs,c values as functions of the static surface pressure, π, and to compare their limiting behaviors. This method was recently applied to investigate the static and dynamic properties of calixarene monolayers at the air/water interface36 as well as other vinyl polymer monolayers.37 Figure 3 represents experimentally determined fs and ∆fs,c values as a function of the static surface pressure at a wavelength of k ) 324.3 cm-1. The limiting behaviors of the propagation characteristics are also indicated on the plots. Starting at low π, the block copolymers show an early departure from limit I for pure

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Figure 3. fs-Π (Α) and ∆fs,c-Π (Β) for PS-PMMA monolayers on a water surface at 23 °C and k ) 324.3 cm-1 (similar trends were observed for other wave vectors): (4) P70, (O) P155, (]) P392, and (0) P656. The curves marked by the roman numerals correspond to the calculated values for I ) the pure liquid limit, III ) the values at the maximum damping coefficient for a perfectly elastic surface, and V ) the values corresponding to infinite lateral modulus dynamics (see text).

liquids (Figure 3A). This is ascribed to the fact that PSPMMA diblock copolymers form surface micelles at submonolayer concentration. As shown later, these micelles have very large viscoelastic parameter values which are in agreement with values expected for an incompressible surface film, limit V, indicated by dashed curves in Figure 3. Values of ∆fs,c versus π for diblock copolymers show a single distinct maximum arising from a resonant coupling of transverse and longitudinal modes around the π value of 0.12 mN/m, regardless of the molecular weight. PMMA did not show this maximum. This implies that the PS-PMMA monolayer is not quite as condensed as PMMA. The spread pure PMMA monolayer shows a sudden onset of the biphasic state with coexisting patches in the low surface pressure range, while the PS-PMMA diblock copolymer shows a smoother progression from the spread monolayer to surface micelles (see Figure 4 below and Figure 2 of ref 38). In the low surface pressure region ( 0 mN/m) are consistent with surface heterogeneity.

condensed monolayer in which PS cores are dispersed (Figure 4). This condensed monolayer restricts the lateral movement of molecules. Thus, the values of d decrease after the maximum appears. For all three diblock copolymers, the d curves show a similar characteristic behavior in the high π region. The static elasticity and the dynamic elasticity differ significantly for all three block copolymer monolayers. Over almost the entire range of surface mass density, we find d > s. The difference between d and s is due to the existence of a heterogeneity in the monolayer, which we believe in this case is due to the surface micelles.11,36 In the dynamic process, additional energy is consumed to reorganize the molecules and micelles to remove the empty space between them. Dynamic elasticity is much larger than s, especially around the maximum, where the resonant coupling between two modes happens. The values of the dilational loss modulus term, ωκ, are also quite large. As we have seen in Figure 3, PS-PMMA diblock copolymers exist as incompressible surface films at the fully covered monolayer state. At high surface pressure, d does not go to zero while s approaches it. Another interesting point is that the maximum peaks of d and κ occur at the same surface area per PMMA monomer, which

is a natural consequence in that the surface activity comes from the PMMA block. The ratio of ωκ to d, which is the value of the loss tangent, tan δ, of the surface dilational mode of a viscoelastic body, is quite large (>10) for P155 and P656, especially when the concentration is above the value corresponding to the maximum in d. This implies that the PS-PMMA monolayer enhances ωκ much more than d in the close-packed state, which gives rise to a far more “lossy” viscoelastic object. As we mentioned above, an alternative way to represent the results is to adopt an analysis scheme plotting the general solutions to the dispersion equation, as was first done by Hard and Newman.27,29 The dynamic results from SLS are presented in Figure 6 for each polymer monolayer. Taking into consideration the frequency independence, we average the values of fs and ∆fs,c for the three wave vectors at the same surface pressure. In this manner, the trajectory can be observed with increasing surface pressure, from right to left. Errors are estimated to be 0.5% for fs and 5% for ∆fs,c. Several facts should be noted from Figure 6. The polar profiles are totally different from that of PMMA, especially in the low surface pressure region. PS-PMMA diblock copolymers exhibit a smooth transition with a large viscosity as Π increases from 0 mN/m, whereas

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PMMA showed a jump from limit I of a bare liquid surface to limit V, the incompressible limit where lateral moduli d and κ have very large values, due to the coalescence of PMMA patches existing as a heterogeneous film prior to the monolayer state.37 The transition for PS-PMMA films occurs through the viscoelastic path as the surface pressure increases. Water present at submonolayer coverage is squeezed out of the film with the surface pressure. In the dilute region, where the surface pressure is still zero, dilational viscosity increases more rapidly than the dilational elasticity. This is more obvious for short chains (P70 and P155), which have bigger aggregation numbers than the long chains (P656).20 In the submonolayer range (less than 0.3 mN/m), a few molecules aggregate to form micelles (Figure 4). Since the water is still present, dilational viscosity appears first. It is obvious that significant dilational elasticity appears at the concentration where enough micelles form a liquidlike film (Figures 4 and 6). Since the films are not perfectly elastic, the minimum velocity limit of a perfectly elastic surface film, limit IV, was never reached. The magnitudes of the elasticity and the viscosity are much larger than those of a normal surfactant monolayer (such as a fatty acid40), as a result of micelle formation and a liquidlike monolayer. Diblock copolymers exist as an incompressible surface at the fully covered monolayer state. Also, the surface dilational viscosity does not show any unrealistic negative values at high surface pressure (>2 mN/m), while that of the PMMA showed large deviations around limit V.30,37 In this regard, PS-PMMA diblock copolymers seem to form a more stable monolayer than PMMA. Deviations from the Kelvin Limit. The propagation characteristics of spontaneously formed capillary waves reflect the hydrodynamic motions of a monolayer at the interface.40 We now compare the propagation rates of the capillary waves on film-covered surfaces relative to those waves on a pure surface at a comparable static surface tension. Using the terms of the zeroth-order solution to eq 1, with * ) 0, µ ) 0, and σd ) σs, we obtain

ωk ) 2πfs ≈ [(σsk3)/F]1/2

(7)

which was first proposed by Kelvin and later rederived by Levich.41 Thus, the ratio

(σsk3)/(F(2πfs)2) ) 1

(8)

holds for pure surfaces without any monolayer in the limit of negligible gravitational wave contribution, which is the case for all SLS measurements. The ratio is a measure of the predicted propagation rate of capillary waves based on the surface tension alone relative to the experimentally observed rate.40 Any monolayer-covered surface should definitely depart from this limit because of some dilational character. Figure 7 shows a plot of the ratio in eq 8 versus A for three polymer monolayers. The overall change in monolayer behavior expressed in terms of this ratio reflects changes in Π, s, fs, and ∆fs,c. All three monolayers show a positive deviation from the Kelvin limit, which indicates that the capillary wave propagation rate is slower than rates predicted by using the surface tension in the absence of monolayer viscoelasticity. These slowdowns are definitely related to the high dilational viscosity of the polymer monolayers. The departure from the Kelvin limit also (40) Lee, W.; Esker, A.R.; Yu, H. Colloids Surf., A 1995, 102, 191. (41) Levich, V. J. Physicochemical Hydrodynamics; Prentice-Hall: Englewood Cliffs, NJ, 1962.

Figure 7. Comparison of the rates of capillary wave propagation with the monolayers of PS-PMMA diblock copolymers on a water surface relative to Kelvin’s limit ((σsk3)/(F(2πfs)2) ) 1): (A) P70, (B) P155, and (C) P656. All data points are averages of three wave vectors k ) 324.3, 385.5, and 445.6 cm-1 (4th, 5th, and 6th order, respectively).

appears at a large surface area, where the surface pressure is still zero. Although the surface area where the departure starts appears to depend on the length of the diblock copolymer, it is nearly identical if we report the data as a function of the area per PMMA monomer. This point reiterates that the surface activity totally comes from the PMMA segment and the PS segment does not interfere with the PMMA segment. Conclusions The surface properties of a series of diblock copolymers (A-B), where the A block is polystyrene (PS, surface interactive and water insoluble) and the B block is poly(methyl methacrylate) (PMMA, surface active but water insoluble), on the air/water interface have been examined by using the surface quasi-elastic light scattering method. The PMMA block length was varied while the PS block length was nearly constant. Stable monolayers of PSPMMA diblock copolymers exhibit substantial changes in molecular packing at a low concentration because of a phase change including micelle formation followed by the clumping of the micelles.20 The frequency shift, fs, a measure of the propagation velocity of the capillary waves, shows a noticeable change at a low concentration, which, we believe, is a sign of the onset of a biphasic state (micelles) in the PS-PMMA monolayer. The frequency shift, fs, and the corrected full width at half-maximum intensity, ∆fs,c, which is related to the wave damping, show

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the progress in the monolayer state. Values of ∆fs,c versus π for diblock copolymers show a single distinct maximum arising from a resonant coupling of transverse and longitudinal modes around the Π value of 0.12 mN/m, regardless of the molecular weight. The PS-PMMA monolayers become strongly viscoelastic when the surface pressure is still very low (7 mN/m), diblock copolymer films follow the pure PMMA monolayer behavior. The capillary wave propagation rate becomes slower than the rate predicted by using the surface tension in the absence of monolayer viscoelasticity. A positive departure from the Kelvin limit is observed, and it starts from the same surface area per PMMA monomer regardless of the PMMA molecular weight. Again this corroborates the fact that the surface activity is mainly attributed to the PMMA segment with essentially no contribution from the PS segment. Acknowledgment. We appreciate Mr. Kiwook Paeng at Hanyang University (Korea) for his help with the AFM pictures. Financial support from the Korea Institute of Science and Technology (KIST) (Grant No. 2E17421) to Y. Seo, from the Thomas F. Jeffress and Kate Miller Jeffress Memorial Trust to A. Esker (J-553), from the KOSEF to D. Sohn (Grant No. R14-2002-004-01002), and from the National Science Foundation to H. Yu (NSFDMR0084301) is greatly appreciated. Note Added after ASAP Posting. This article was released ASAP on March 6, 2003, with errors in the caption of Figure 5, errors get much bigger as * f ∞, and in the Acknowledgment, the Korea Research Foundation was changed to KOSEF. The correct version was posted on March 13, 2003. LA020819L