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Wormlike Polystyrene Brushes in Thin Films S. S. Sheiko,*,† M. Gerle,‡ K. Fischer,‡ M. Schmidt,‡ and M. Mo¨ller† Organische Chemie III/Makromolekulare Chemie, Universita¨ t Ulm, D-89069 Ulm, Germany, and Johannes-Gutenberg-Universita¨ t, Institut fu¨ r Physikalische Chemie, Jakob-Welder-Weg 11, D-55099 Mainz, Germany Received February 10, 1997. In Final Form: May 12, 1997X Mono- and multilayer films were prepared on mica by solution casting of a high molecular weight polymacromonomer, i.e., a polymethacrylate of about 1000 repeating units each of which was substituted by a polystyrene chain with a molecular weight of about 5000 Da. The films were studied by tapping scanning force microscopy. The material showed a remarkable preference for forming well-defined monolayers of a thickness of 6.5 ( 0.2 nm consistent with the hard core diameter of the collapsed cylindrical brush molecules. When the films were probed with high normal force, the single molecules were observed to organize in a dense nematic-like packing as expected for inherently stiff molecules. In order to minimize the surface area, the polymacromonomers form hairpins, i.e., rather tight folds parallel to the surface. The orientational order was shown to be highest in a monolayer, S ) 0.74, and to decrease for layers at a larger distance from the flat substrate to a value S ) 0.65.
Introduction Only recently it was shown that macromonomers, i.e., linear macromolecules with a polymerizable end group, can be linked to a high degree of polymerization.1,2 A maximum substituted comb polymer or polymer brush is formed, whose backbone significantly exceeds the length of the side chains. At the same time, the coil radius of the side chains greatly exceeds their spacing of 2.5 Å along the backbone. The conformation of these covalently linked cylindrical brushes is controlled by the coiling and the excluded volume of the side chains irrespective of the “unperturbed” flexibility of the backbone segments. Macromolecular brushes have been described theoretically based on scaling concepts3 and using mean-field analytical approaches.4 It has been shown that depending on the length of the side chains, the configuration of comb polymers changes from that of a statistical coil toward a wormlike chain, like the one schematically shown in Figure 1, with a persistence length approaching the overall chain contour length.5 High molecular weight polymacromonomers have been synthesized from polystyrene chains with a methacrylate end group. In dilute solutions these polymers were shown to exhibit a persistence length which exceeds the Kuhn length of poly(methyl methacrylate) (PMMA) by a factor of 20 and more depending on the length of the side chains. The high rigidity imparted by the steric interaction of the side chains explains the spontaneous orientational ordering observed in semidilute solutions.6 Apparently, intermolecular ordering of the uncharged brushes is controlled by the repulsion of the side chains.7 Chain flexibility and intermolecular interaction might be dif* To whom correspondence should be addressed: e-mail,
[email protected]. † Universita ¨ t Ulm. ‡ Johannes-Gutenberg-Universita ¨ t. X Abstract published in Advance ACS Abstracts, July 1, 1997. (1) Tsukahara, Y.; Tsutsumi, K.; Yamashita, Y.; Shimada, S. Macromolecules 1990, 23, 5201. (2) Wintermantel, M.; Schmidt, M.; Tsukahara, Y.; Kajiwara, K.; Kohjiya, S. Macromol. Chem., Rapid Commun. 1994, 15, 279. (3) de Gennes, P.-G. Macromolecules 1980, 13, 1068. (4) Zhulina, E. B.; Borisov, O. V.; Pryamitsyn, V. A.; Birshtein, T. M. Macromolecules 1991, 24, 140. (5) Fredrickson, G. H. Macromolecules 1993, 26, 2825. (6) Wintermantel, M.; Fischer, K.; Gerle, M.; Ries, R.; Schmidt, M.; Kajiwara, K.; Urakawa, H.; Wataoka, I. Angew. Chem. 1995, 107, 1606. (7) Zhulina, E. B.; Borisov, O. V.; Pryamitsyn, V. A. J. Colloid Interface Sci. 1990, 137, 495.
S0743-7463(97)00132-7 CCC: $14.00
Figure 1. Schematic illustration of the polymacromonomer brushes. Table 1. Molecular Characterization of the Polystyrene Graft Poly(methyl methacrylate) Brushes1-3 parameters
values
method used for determination
Mw side chains (g/mol) Mw polymacromer (g/mol)a Mw/Mn polymacromer Lw contour length (Å)a persistence length (Å) cross-sectional radius of gyration (Å) hard core radius (Å)
4950 5.1 × 106 1.5 2500 1035 52
MALDI-TOF-MS GPC GPC degree of polymerization MALLS-GPC SAXS
32.8
density of PS, F ) 1.05 g/cm3
a Estimated molecular weight from GPC by comparison of the elution volume with GPC/LS data on the nonfractionated sample.
ferent in the solvent free state when the molecules must either collapse or strongly interpenetrate to reach the bulk density. This question will be studied by neutron scattering in the near future. In this communication we report on the structure and formation of ultrathin films of polystyrene brushes which were cast from dilute solution on a hard, atomically flat surface of mica. Molecular ordering in thin films has been studied using tapping mode scanning force microscopy. Experimental Section Synthesis and characterization of the styrene polymacromonomers have been described before.1,2 The molecular characterization of the wormlike polystyrene brushes studied here is summarized in Table 1. The samples were fractionated by preparative high-performance liquid chromatography (HPLC) in order to remove nonreacted oligostyrenes and obtain a fraction with a more narrow molecular weight distribution.
© 1997 American Chemical Society
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Figure 2. SFM micrographs of thin films prepared by casting of the polystyrene brushes on the surface of mica from toluene solutions of different concentrations: (a) 5 × 10-4 wt %; (b) 10-3 wt %; (c) 10-2 wt %; (d) 10-1 wt %. The cross-sectional profiles recorded along the indicated line (d) demonstrate a layer thickness of 6.5 ( 0.2 nm for the monolayer (c) and multilayer (d) films. Scanning force microscopy (SFM) micrographs were recorded with a Nanoscope III (Digital Instruments, St. Barbara) operated in the tapping mode at a resonance frequency of about 320 kHz. The measurements were performed at ambient conditions using Si probes with a spring constant of ∼50 N/m. Tips with an apex radius less than 10 nm were selected by means of a well-defined stepped structure of a SrTiO3 single crystal wafer constructed of alternating (103) and (101) planes.8 The molecular resolution demonstrated in Figures 4-6 was only resolved when the sample was probed by the tip with the rather high force (=50 nN). Samples for microscopy measurements were prepared by solution casting at ambient conditions. Typically, one drop (1 (8) Sheiko, S. S.; Mo¨ller, M.; Reuvekamp, E. M. C. M.; Zandbergen, H. W. Phys. Rev. B 1993, 48, 5675.
µL) of a dilute solution in toluene was cast on a 1 × 1 cm2 surface of mica. Layered crystals of the muscovite mica were freshly cleaved prior to casting. The concentrations were varied from 5 × 10-4 to 10-1 wt % to control coverage of the substrate.
Results and Discussion Figure 2 shows a series of SFM micrographs which were measured on samples prepared from solutions of different concentrations. Ultradilute solutions yielded flat monolayer islands evenly distributed over the substrate surface (Figure 2a). At higher concentrations, the isolated lamellae fused into each other forming an interconnected cellular structure (Figure 2b) which is followed by a
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Figure 3. Enlarged area of the SF micrograph in Figure 2a. Single brush molecules are depicted aside the monolayer islands.
Figure 5. High magnification of the marked area in Figure 2d. In the right bottom corner, the surface is covered partly by an additional layer of molecules. The image demonstrates how individual molecules run smoothly across the step between the layers. The cross-sectional profile recorded along the dashed line yields a layer thickness of about 6.5 nm.
The cross-sectional profiles along the lines marked in the micrographs demonstrate a uniform film thickness of 6.5 ( 0.2 nm. When the coverage of the surface was increased further, a multilayer structure was observed with a discrete layer thickness of about 6.5 nm (Figure 2d). When samples were annealed at temperatures up to 150 °C, i.e., 60 °C above the glass transition temperature, the observed structures did not change. Only a few single molecules could be detected lying aside the lamellae (Figure 3). Typically, these had adopted an extended straight conformation, which is consistent with the inherent stiffness observed in solution. The molecular structure inside the condensed layers was visualized by adjusting the scanning conditions. In tapping SFM, a cantilever with a sharp tip is oscillated by a piezo driver near its resonance frequency. As the tip eventually taps the surface, both amplitude and phase of the oscillation change, which is exploited to record an image with a topographic or a viscoelastic contrast respectively.11 Therefore, the tapping SFM is utilized to probe variations of the micromechanical material properties of the films.12-14 Figure 4. High-magnification SF images of a monolayer island depicted in Figure 2a. Phase contrast (a) allows complete resolution of the molecules discriminating the chain ends, whereas the micrograph in (b) gives the height variation or topography.
continuous film with holes (Figure 2c). The incomplete coverage results from dewetting of the substrate during evaporation of the solvent, thus preventing thinning of the solution layer below a certain equilibrium thickness.9,10
(9) Brochard-Wyart; F.; Daillant, J. Can. J. Phys. 1990, 68, 1084. (10) Reiter, G. Langmuir 1993, 9, 1344. (11) Zhong, Q.; Inniss, D.; Elings, V. B. Surf. Sci. 1993, 290, L688. (12) Radmacher, M.; Fritz, M.; Kacher, C. M.; Cleveland, J. P.; Hansma, P. K. Biophys. J. 1996, 70, 556. (13) Winkler, R. G.; Spatz, J. P.; Sheiko, S.; Mo¨ller, M.; Reineker, P.; Marti, O. Phys. Rev. B 1996, 54, 8908. (14) Burnham, N. A; Behrend, O. P.; Oulevey, F.; Gremand, G.; Gallo, P.-J.; Gourdon, D.; Dupas, E.; Kulik, A. J.; Pollock, H. M.; Briggs, G. A. D. Nanotechnology, in press.
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Figure 6. Fourier filtered (a, b) and unprocessed (c, d) SF micrographs of a multilayer film (a, c) and a monolayer (b, d) of the polystyrene brushes on mica. Clearly, the order is significantly higher for the monolayer film. The filtered image (b) demonstrates well-defined domain boundaries.
Figure 4 depicts tapping force micrographs of a monolayer island which have been recorded with increased tapping force (reduced set point). As the tip was forced to indent the sample at a depth of about 3 nm, the contour of the single molecules was resolved in the images. The approximate value of the peak force corresponding to the 3 nm indentation was calculated to be about 50 nN.13,15 The micrographs reflect the variation of the height as well as the shift in phase of the cantilever oscillation upon scanning the surface. Apparently, the core and the shell of the adsorbed brushes responded differently to the applied force resulting in 5° difference in the phase shift and about 5 Å larger deformation for the shell. Remarkably, the densely packed molecules were strongly bent and folded parallel to the substrate surface. Mostly, a hairpin folding of the backbone was observed. The intermolecular distance was measured to be 82 ( 2 Å, which is somewhat larger than the film thickness 65 Å (15) Sheiko, S. S.; Muzafarov, A. M.; Winkler, R. G.; Getmanova, E. V.; Eckert, G.; Reineker, P. Langmuir, in press.
and can be regarded as an indication for film thinning caused by the interaction of the molecules with the substrate. The length of the straight segments between the bending points varied between 250 and 400 Å. The attained contrast was sufficient to discern the ends of some of the molecules and to measure their contour length. Values from 900 to 2700 Å are consistent with the contour lengths Ln ) 1700 Å and Lw ) 2500 Å as estimated from the gel permeation chromatography (GPC) trace. One of the most clearly and completely resolved brush molecules is indicated by the arrow in Figure 4a. Also multilayered films displayed strong bending of the constituting molecules. Figure 5 shows the surface of a thick film consisting of three monolayers, in which one can see fragments of both the top layer and the sublayer. Following the molecules at the border line of the top layer, it becomes apparent that particular molecules can be part of different layers. The SFM images suggest that it is rather feasible for the molecules to track across the layers. This is also supported by the observation that the lateral
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δk(rij) cos2 Θij ∑ i,j
Sk(r) ) 2〈cos2Θij〉N - 1 ) 2
N
-1
δk(rij) ∑ i,j
Figure 7. Orientational order parameter as a function of the distance between the adsorbed brushes constituting the monolayer (solid line) and multilayer (dashed line) films.
ordering of the molecules in multilayer films is decreased and that the molecules are more coiled compared to the monolayer (Figure 6). The observation that the molecules undergo ordering which depends on the film thickness deserves special attention. In the following we describe changes of the molecular structure as the film thickness is increased from a monolayer to several molecular layers. In addition to the original images of the monolayer and the multilayer film, Figure 6 depicts the same images after a Fourier filtering procedure was applied. The processed images allow us to visualize the ordering of the molecules more clearly and to eliminate angstrom scale fluctuations in the images. To evaluate the orientational correlation of the adsorbed brushes, their contour lines in parts a and b of Figure 6 were broken up into a sum of vectors ei with a length smaller than the intermolecular distance a ) 8.2 nm. The coordinates of the vectors were determined relative to the left-down corner of the micrographs. The orientational ordering in two dimensions (d ) 2) was analyzed in terms of the order parameter
S)
d〈cos2Θij〉 - 1 ) 2 〈cos2Θij〉 - 1 d-1
where Θij is an angle between two arbitrary vectors ei and e j. When all brushes are uniformly oriented parallel to each other, the order parameter is equal to 1 (S ) 1), whereas the value S ) 0 corresponds to a random distribution of Θij. The actual order parameter was calculated as a δ-function of the distance rij between the geometric centers of the vector ei and ej (16) Onsager, L. Ann. N. Y. Acad. Sci. 1949, 51, 627. (17) Flory, P. J. Proc. R. Soc. London, Ser. A 1956, 234, 60, 73. (18) Magonov, S. N.; Whangbo, M.-H. Surface Analysis with STM and AFM; VCH: Weinheim, 1996; p 308. (19) Spatz, J. P.; Sheiko, S.; Mo¨ller, M. Macromolecules 1996, 29, 3220. (20) Sheiko, S. S.; Eckert, G.; Ignat’eva, G.; Muzafarov, A. M.; Spickermann, J.; Ra¨der, H. J.; Mo¨ller, M. Macromol. Rapid Commun. 1996, 17, 283. (21) Sheiko, S. S.; Gauthier, M.; Mo¨ller, M. Macromolecules 1997, 30, 2343.
where δk(rij) ) 1 at ka < rij < (k + 1)a, otherwise δk(rij) ) 0 (k ) 0, 1, 2, ...), N is the total number of the vectors, and a ) 8.2 nm is the average distance between the molecules in Figure 6. Figure 7 shows the variation of the order parameter as a function of the intermolecular distance determined for the monolayer and three layer film in Figure 6. At short distances the ordering in the monolayer is characterized by S ) 0.74, which is consistent with S ) 0.8 reported for nematic ordering in 3D lyotropic systems.16,17 The orientation parameter decreases exponentially with the distance and reaches S ≈ 0 (isotropic structure) at about 65 nm. The latter provides a good estimation for the average size of the domains depicted in Figure 6a. For the multilayer film, the order parameter is smaller, decays much faster, and levels off at a distance of about 35 nm. Conclusion The data show that scanning force microscopy is a very powerful tool to depict the molecular configuration and ordering of wormlike polymer brushes. Weak molecular segregation is demonstrated by the layered structure of the films and the remarkable resolution of single molecules by probing the films with high normal forces. The side chains of adjacent molecules interpenetrate only weakly and there is little overlap of the collapsed brush molecules. The contrast, which was achieved only at about 1-3 nm indentation of the tip into the sample, indicates a decrease in hardness from the core of the cylindrical brushes to the circumference. The second principal observation of this study is the strong bending of the polystyrene brushes in the condensed films and the easy formation of monolayers in spite of the high persistence length observed in dilute solution. Dense packing and the corresponding orientational ordering of the brushes (as was observed in semidilute solutions) is preserved by the frequent formation of a hairpin-like folding. Although no changes in SFM images were observed after long term annealing of the samples above the glass transition temperatures of polystyrene (PS) and PMMA, the structures formed by solution casting may be far from equilibrium. Probably, folding is enforced by the dewetting process where adsorption and capillary forces are effective. The observed structures are rather similar to those of block and graft copolymers of PS and PMMA.18,19 However the driving forces which induce these meander type structures are different. Whereas the copolymers undergo phase separation because of chemical incompatibility of PS and PMMA, the molecular segregation and ordering of the uncharged brushes are controlled by the steric repulsion of the side chain corona as is typical for other densely branched systems.19-21 As might be expected, the order is highest in a monolayer on the flat substrate and decays for thicker films where one molecule can be part of different layers. Acknowledgment. We are indebted to Dr. Knoll, BASF AG Ludwigshafen, for performing preparative fractionation of our polymacromonomer sample. Financial support of the DFG and of the Fonds der Chemischen Industrie is gratefully acknowledged. LA970132E
