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Structural Characterization of Thin Hydroxypropylcellulose Films. X-ray Reflectivity Studies G. Evmenenko,* C.-J. Yu, S. Kewalramani, and P. Dutta Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60208-3112 Received June 24, 2003. In Final Form: December 12, 2003 Thin solid films of hydroxypropylcellulose (HPC) have been investigated using synchrotron X-ray reflectivity. Evidence of preferential alignment of HPC molecules at the substrate surface is obtained. In the surface region the liquid crystalline domains of HPC are preferentially oriented parallel to the substrate, whereas in the bulk they are mostly distributed randomly. Incorporation of colloidal particles in the film-substrate region destroys the preferential alignment. It is also found that in conditions of restricted geometry (very thin films), a minimum film thickness is required to produce lateral packing order perpendicular to the substrate surface.
Introduction The structure of liquid crystalline (LC) systems near surfaces is of fundamental scientific and technological importance. It is generally accepted that the breaking of the continuous translational symmetry of a liquid by a flat solid surface induces stratifications and oscillations in the density and changes the molecular packing in the interface regions. However, the complex surface behavior in confined systems (e.g., thin films) and factors governing different interactions at solid-liquid interfaces are still poorly understood. This is due to the interplay of many different factors such as (i) confinement and related molecular mobility, (ii) chain “stiffness”, and (iii) interfacial interactions. These issues yield interesting results in liquid crystals, as the surface-induced order can propagate into the bulk liquid crystal. For instance, significant orientational order can be surface induced in the bulk isotropic phase of nematic liquid crystals over mesoscopic or even macroscopic distances.1,2 Besides low molecular liquid crystals, during the past 2 decades, there is an increasing interest in liquid crystalline polymers from both the scientific and commercial points of view.3,4 This interest is associated with the possibility of combining the properties of low molecular weight liquid crystals with the polymeric properties and the ability of such systems to form high modulus materials with unique properties. It is important to understand the mechanism responsible for the surface-induced liquid crystalline alignment, including the possibility of its control. The reflection of X-rays or neutrons is a useful tool to get information about the structure of thin films in the direction perpendicular to the surface. The first use of X-ray reflectivity measurements as a probe of the bulk liquid/solid interface in an isotropic phase of a liquid crystalline system was made * To whom correspondence should be addressed: G. Evmenenko, Department of Physics and Astronomy, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208-3112 USA. Phone: 847-491-3477. Fax: 847-491-9982. E-mail: g-evmenenko@ northwestern.edu. (1) Sheng, P. Phys. Rev. Lett. 1976, 37, 1059. (2) Miyano, K. Phys. Rev. Lett. 1979, 43, 51. (3) Liquid-Crystalline Polymer Systems: Technological Advances; Isayev, A. I., Kyu, T., Cheng, S. Z. D., Eds.; ACS Symposium Series 632; American Chemical Society: Washington, DC, 1996. (4) Wang, X.-J.; Zhou, Q.-F. Liquid Crystalline Polymers; World Scientific: Singapore, 2004 (in press).
by Ocko on cyanobiphenyl compounds.5 Ordering effects of a side group liquid crystalline polymer PMA64 were investigated as a function of temperature and film thickness using the X-ray reflectivity technique.6 The smectic layers were oriented parallel to the surface, and the range of interface influence on smectic layer order was found to be approximately 600 Å.6 Different orientations of the liquid crystalline polymer chains depending on the film thickness (perpendicular to the substrate for films thicker than 10 nm, and parallel to the substrate in thinner films) were found for the smectic C* phase by X-ray reflectivity and scanning tunneling microscopy.7 Thin films of a polyacrylate with phenyl benzoate mesogenic side groups in the nematic phase show dewetting as was obtained using a combination of X-ray reflectivity (XRR) and atomic force microscopy (AFM).8 The authors found that the smectic lamellae are better developed at the film-air and film-substrate interfaces than in the middle of the film and the density modulation decays with increasing distance from the interfaces. Surface-induced homeotropic ordering at the interface between a nematic liquid crystal in the isotropic phase and the silanated glass substrate was observed for 4-cyano4′-n-octylbiphenyl samples by AFM and Brewster angle ellipsometry.9 More studies on surface and thin film structures of liquid crystals and polymeric crystalline systems by using X-ray surface scattering can be found in reviews.10 However, all these results are mostly related to the smectic liquid crystal phases. In the case of cholesteric (or chiral nematic) liquid crystals or crystalline polymer films with a zigzag conformation of molecules, studies of surface-induced structural changes are rather rare. A study using grazing incidence X-ray scattering shows crystalline-like ordering near the air surface in films of an aromatic polyimide more than 400 Å thick, while no such ordering was found (5) Ocko, B. M. Phys. Rev. Lett. 1990, 64, 2160. (6) Mensinger, H.; Stamm, M.; Boeffel, C. J. Chem. Phys. 1992, 96, 3183. (7) Henn, G.; Stamm, M.; Poths, H.; Ru¨cker, M.; Rabe, J. P. Physica B 1996, 221, 174. (8) Ostrovskii, B. I.; Sentenac, D.; Samoilenko, I. I.; de Jeu, W. H. Eur. Phys. J. E 2001, 6, 287. (9) Kocˇevar, K.; Musˇevicˇ, I. Phys. Rev. E 2002, 65, 021703. (10) Sinha, S. K. In Liquid Crystals: Experimental Study of Physical Properties and Phase Transitions; Kumar, S., Ed.; Cambridge University Press: London, 2001; pp 333-392.
10.1021/la035118i CCC: $27.50 © 2004 American Chemical Society Published on Web 01/27/2004
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near the substrate interface.11 The 100-Å-thick films have a crystalline-like structure comparable to that seen near the surface of the thick films. We have chosen for our studies hydroxypropylcellulose (HPC), one of the most common lyotropic water-soluble ethers of native cellulose, whose concentrated aqueous solutions display optical properties typical of cholesteric liquid crystals.12,13 The ability to modify, regenerate, and reshape cellulose and polysaccharide derivatives with unique chemical, physical, and physiological properties has raised interest in these polymers over the past decade.14 In particular, such interest originates from new insights into the anisotropic liquid-crystalline state and possibilities of highly ordered thin film architectures of cellulose derivatives. The molecular structure of the HPC in aqueous solutions is characterized by semirigid segments with a persistence length of about 70 Å (dependent upon the solvent and other factors). The phase diagram of aqueous solutions of HPC shows that, at room temperature, solutions with concentrations higher than 65 wt % are fully anisotropic with cholesteric structures such as polygonal focal conics and planar configurations.13 The chirality of the recurring anhydroglucose units controls the formation of cholesteric mesophases. Ordered phase formation in HPC solutions and the influence of different factors, such as concentration,12-18 temperature,19-21 shear,22-28 pressure,29 etc., were reported in numerous studies that allow us to consider this mesomorphic polymer as a convenient model system for study of interfacial effects in thin liquid crystalline films. The use of anisotropic HPC films for alignment of nematic liquid crystals was recently proposed.30-32 This fact opens new perspectives for preparing specifically aligned layers for which the topography can be tuned in order to optimize the electro-optical properties of the devices in which they are applied. Our former synchrotron X-ray reflectivity studies on nanometer-scale films of molecular liquids (tetrakis(2ethylhexoxy)silane, octamethylcyclotetrasiloxane, and (11) Factor, B. J.; Russel, T. P.; Toney, M. F. Macromolecules 1993, 26, 2847. (12) Werbowyj, R. S.; Gray, D. G. Mol. Cryst. Liq. Cryst. 1976, 34, 97. (13) Werbowyj, R. S.; Gray, D. G. Macromolecules 1980, 13, 69. (14) Cellulose Derivatives: Modification, Characterization, and Nanostructures; Heinze, T. J., Glasser, W. G., Eds.; ACS Symposium Series 688; American Chemical Society: Washington, DC, 1998. (15) Guido, S. Macromolecules 1995, 28, 4530. (16) Wignall, G. D.; Annis, B. K.; Triolo, R. J. Polym. Sci., Polym. Phys. 1991, 29, 349. (17) Keates, P.; Mitchell, G. R.; Peuvrel, E. Polymer 1992, 33, 3298. (18) Shtennikova, I. N.; Lavrenko, P. N.; Korneeva, E. V.; Kolbina, G. F.; Strelina, I. A.; Shibaev, V. P. Polym. Sci., Ser. A 1995, 37, 853. (19) Immaneni, A.; Kuba, A. L.; McHugh, A. J. Macromolecules 1997, 30, 4613. (20) Tanaka, A.; Onoda, H.; Nitta, K. Polym. J. 2000, 32, 665. (21) La´rez-V, C.; Crescenzi, V.; Ciferri, A. Macromolecules 1995, 28, 5280. (22) Navard, P. J. Polym. Sci., Polym. Phys. Ed. 1986, 24, 435. (23) Hongldarom, K.; Secakusuma, V.; Burghardt, W. R. J. Rheol. 1994, 38, 1505. (24) Walker, L.; Wagner, N. J. Rheol. 1994, 38, 1525. (25) Hongladarom, K.; Ugaz, V. M.; Cinader, D. K.; Burghardt, W. R.; Quintana, J. P.; Hsiao, B. S.; Dadmun, M. D.; Hamilton, W. A.; Butler, P. D. Macromolecules 1996, 29, 5346. (26) Ugaz, V. M.; Cinader, D. K.; Burghardt, W. R. Macromolecules 1997, 30, 1527. (27) Roschinski, C.; Kulicke, W.-M. Macromol. Chem. Phys. 2000, 201, 2031. (28) Caputo, F. E.; Burghardt, W. R. Macromolecules 2001, 34, 6684. (29) Kunugi, S.; Yoshida, D.; Kiminami, H. Colloid Polym. Sci. 2001, 279, 1139. (30) Mori, N.; Morimoto, M.; Nakamura, K. Macromolecules 1999, 32, 1488. (31) Almeida, P. L.; Godinho, M. H.; Cidade, M. T.; Figueirinhas, J. L. Mol. Cryst. Liq. Cryst. 2001, 368, 121. (32) Sebastia˜o, P. J.; Cruz, C.; Pires, D.; Ferraz, A.; Brogueira, P.; Godinho, M. H. Liq. Cryst. 2002, 29, 1491.
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Figure 1. Schematic representation of the chemical structure of HPC molecule.
tetrakis(trimethylsiloxy)silane) and siloxane oligomers (poly(dimethylsiloxanes), poly(methylhydro-dimethyl)siloxanes) deposited on solid surface have given some insight into molecular ordering in such systems and the effect of different factors.33-37 Molecular layering and/or conformation changes are induced by geometrical confinement and are determined by the nature of the solid surface (hydrophobic or hydrophilic), the shapes of the molecules of the liquid, their molecular weights, and the flexibility of the molecular chains. In this work we used XRR to continue our study of molecular ordering in interfaces to the more directly applicable system of liquid crystalline polymer films of HPC. The effect of colloidal particles incorporated in the ordered phase of HPC films was studied as well. Experimental Section HPC (Klucel EF, LOT 8622) was obtained from Hercules Inc. A nominal value for Mw is reported in the product literature as 80 000.38 Molar substitution (the average number of moles of hydroxypropyl substituent per mole of anhydroglucose residue) is 3.9 according to the manufacturer. The chemical structure of an HPC molecule is presented in Figure 1. HPC powdered samples were dried under vacuum at 70 °C for 6 h before use. Aqueous solutions with polymer concentration ranging from 0.038 to 5 wt % were prepared. Deionized, distilled water (18 MΩ cm) was used for preparing solutions. The solutions were continuously stirred until homogenization at room temperature. The substrates (3 in. × 1 in. × 0.1 in.), silicon (100) with native oxide, were purchased from Umicore USA Inc., Semiconductor Processing Division. They were cleaned in a strong oxidizer, a mixture of 70% sulfuric acid and 30% hydrogen peroxide (70:30 v/v), for 1 h at 90 °C, rinsed with copious amounts of distilled water, and stored under water before use. Prior to preparing the films, the wafers were removed from the water and blown dry under a stream of nitrogen. We spread thin films by dipping the substrates in the HPC solutions, and withdrawing them at constant speed of 1.5 mm/s to get uniform film thickness, with subsequent air-drying in horizontal position at room temperature. Solution concentration and withdrawal speed affected the final HPC film thickness. The parameters used for film deposition (33) Yu, C.-J.; Richter, A. G.; Datta, A.; Durbin, M. K.; Dutta, P. Phys. Rev. Lett. 1999, 82, 2326. (34) Yu, C.-J.; Richter, A. G.; Kmetko, J.; Dugan, S. W.; Datta, A.; Dutta, P. Phys. Rev. E 2001, 63, 021205. (35) Evmenenko, G.; Dugan, S. W.; Kmetko, J.; Dutta, P. Langmuir 2001, 17, 4021. (36) Evmenenko, G.; Yu, C.-J.; Kmetko, J.; Dutta, P. Langmuir 2002, 18, 5468. (37) Evmenenko, G.; van der Boom, M. E.; Yu, C.-J.; Kmetko, J.; Dutta, P. Polymer 2003, 44, 1051. (38) Hydroxypropylcellulose: Chemical and Physical Properties; Hercules Inc.: Wilmington, DE, 1981.
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gave typical values of film thickness between 20 Å and up to a few thousand angstroms depending on HPC concentration. To study the effect of colloidal dispersions in ordered phase of thin liquid crystalline films and associated topological defects, Nyacol 215 colloidal silica (Eka Chemicals Inc.) with an average particle size of about 40 Å was used. Complex solutions of HPC and colloidal particles were prepared by mixing 70 mL of 1 wt % HPC solution with different amounts (0.25, 0.50, 1.0, and 2.0 mL) of 15 wt % aqueous colloidal solution (pH ) 11) with subsequent stirring. Thin films of HPC + SiOx were made as described above. XRR studies were performed at beamline X23B of the National Synchrotron Light Source using a four-circle Huber diffractometer in the specular reflection mode (i.e., incident angle was equal to exit angle). X-rays of energy E ) 10.0 keV (λ ) 1.24 Å) were used for all measurements. The beam size was 0.35-0.40 mm vertically and 1.0-2.0 mm horizontally. The samples were kept under slight overpressure of helium during the measurements to reduce the background scattering from the ambient gas and radiation damage. The experiments were performed at room temperature. The off-specular background was measured and subtracted from the specular counts.
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Figure 2. Specular X-ray reflectivity data for thin films of HPC of different thicknesses.
Results and Discussion Morphologically, water-cast HPC films have been observed to have a randomly oriented planar structure at all levels, from the molecular to the supermolecular.16,39 The generation of molecular orientation in HPC films can be interpreted in terms of the formation of anisotropic domains.17,25,39 The loss of long-range order in such ordered systems is commonly described according to the onedimensional paracrystalline model.40,41 In the crystalline domains chain segments are arranged more or less in a parallel manner, whereas the orientation of domains varies and depends on external conditions. We know that molecular packing is strongly perturbed at the interface regions and in confined systems. Further questions are as follows: What kind of structural changes can we expect in the case of very thin HPC solid films? What happens to the structural parameters of anisotropic domains? What is the spatial orientation? To answer these questions, we used synchrotron X-ray reflectivity, which provides direct structure information. Figure 2 shows typical XRR data for thin HPC films with thicknesses of around 200 Å and thicker. XRR patterns exhibit clear minima, “Kiessig fringes”,42 that correspond to the destructive interference of the reflections from the top and bottom of the film. We can estimate the total thickness of the film from 2π/∆qz, where ∆qz is the spacing between the minima. The reflectivity pattern contains a prominent peak at about qz ) 0.60 Å-1 due to the formation of the anisotropic liquid crystal phase.16,17 Simple comparison of structural parameters characterizing HPC ordering in thin films and in bulk shows a higher level of LC alignment for the former. The breadth and position of the scattering peak for the bulk HPC indicate presence of a LC structure with spacing of ∼11.6 Å and with a correlation length of ∼30 Å.17 For thin films in the range of thicknesses about 200-5000 Å the spacing and crystallite size are a j ) 2π/qm ) 10.3-10.5 Å and L ) λ/(β cos θ) ) 50 Å, where θ is the incident angle, qm ) (4π/λ) sin θ, and β is the position and the full width at a half-maximum, respectively, of the Bragg peak.41 If we calculate the integrated intensity of the LC peak for HPC films of different thicknesses and divide it by film (39) Samuels, R. J. J. Polym. Sci. 1969, 7, 1197. (40) Hosemann, R.; Bagchi, S. N. Direct Analysis of Diffraction by Matter; North-Holland: Amsterdam, 1962. (41) Vainshtein, B. K. Diffraction of X-rays by Chain Molecules; Elsevier Publishing Company: Amsterdam-London-New York, 1966. (42) Kiessig, H. Ann. Phys. 1931, 10, 769.
Figure 3. Intensity/thickness of LC peak for HPC films as a function of film thickness. Inset: averaged off-specular (background) X-ray reflectivity (θ ) (2θ)/2 ( 0.1°) in the range of LC peak for HPC films of different thicknesses.
thickness, we can see that the relative LC peak intensity increases for thinner films (Figure 3). The background scattering (off-specular, when incident and scattered angles are kept according to θ ) (2θ)/2 ( 0.1° shows a bulk peak, whose intensity is approximately equal to 50% of the specular peak for film of 5000 Å thick, to about 6% for 2000 Å film, and this peak practically disappears for films of thickness less than 1000 Å (the inset of Figure 3). These data show the direct effect of confinement in thin HPC films. Since the structural parameters of crystallites do not change as film thicknesses decrease, the increase of relative specular peak reflectivity can only be explained by a significant rise in the fraction of LC domains with a planar orientation parallel to the substrate as the films became thinner. The question is whether the improvement in ordering is only at the interfacial region or in the whole film. Figure 4 shows the corresponding Fourier transformations (so-called one-dimensional Patterson function) of the XRR data for different thickness of HPC films. The Patterson function is sensitive to the relative positions of interfaces in the electron density distribution and the positions of peaks in P(z) corresponding to the distances between regions where the density is changing rapidly. The substrate and film-gas interfaces give large primary
Thin Hydroxypropylcellulose Films
Figure 4. Patterson functions from the XRR data obtained for ∼60-400 Å films. The curves are shifted vertically for clarity.
maximum, and its position indicates the overall thickness of the film. The secondary peaks near the 20 Å region are due to density oscillations in the films. If the interfacial roughnesses at the substrate-film and film-gas interfaces are the same, by symmetry in the definition of the Patterson function we should have also the secondary peaks at the position 20 Å less from the primary peak position. As one can see from Figure 4, with increasing of film thickness, the oscillations of P(z) become “asymmetrical” with respect to two present interfaces as the film gets thicker. The absence of strong “symmetric” oscillations close to this primary maximum means that one interface is rougher than the other and that the density fluctuations within the film are near the smoother interface.34 The substrate has a very low and practically constant roughness (σSi-film ∼ 3 Å), but film “smoothness” significantly decreases for thicker films as indicated by the smearing of Kiessig fringes (Figure 2) and becomes about 20 Å for 900 Å thick film.43 Thus, such asymmetric Patterson function oscillations can only be obtained if the preferential ordering of LC domains is located in the filmsubstrate interface region of HPC solid films, but not close to the film-gas surface. There is another way to look at surface structure of HPC film. For incidence angles θ less than the critical angle of HPC film nearly all of the radiation is totally reflected and the scattering mainly stems from the nearsurface region. The critical angle θc,HPC of HPC was calculated from electron density 0.44 e Å-3 for HPC film.44 The obtained value 0.14° (at energy of X-rays E ) 10.0 keV) is identical to that experimentally estimated from our X-ray reflectivity data. The penetration depth of the X-rays, Λ, was varied by changing θ. The calculated dependence is shown in Figure 5a.45 XRR measurements were performed as a function of scattering (detector) angle 2θ at constant values of incident angle θ. Figure 5 represents the scattering intensity for the 5000 Å thick (43) The variation in thicknesses, ∆t ) λ/[4(θn2 - θc2)1/2], will broaden the nth fringe maximum at θn (θc is the critical angle for total reflection of X-rays) so that it can no longer be observed as was first described by Wainfan, N.; Parrat, L. G. J. Appl. Phys. 1960, 31, 1331. (44) The electron density FHPC was calculated from the chemical composition of HPC according to equation: FHPC ) ∑ibidNA/M. Here bi is the scattering amplitude for the ith atom (the summation is carried out over monomer unit), NA is the Avogadro number, M is molar mass of the HPC monomer, and d is the physical density of HPC film (1.2 g/cm3). (45) Tolan, M. X-ray Scattering from soft-matter thin films: materials science and basic research; Springer Tracts in Modern Physics; Springer: Berlin, 1999; Vol. 148.
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Figure 5. Diffuse scattering pattern in the region of LC peak for HPC film of about 5000 Å thick shown for different angles of X-ray incidence. (a) Calculated penetration depth Λ of X-rays (λ ) 1.24 Å) into HPC solid film as a function of the incident angle. (b) Intensity of LC peak divided by penetration depth Λ is shown as a function of Λ.
film in the region of LC peak for different angles of X-ray incidence. No LC peak is present as θ f 0 that corresponds to the penetration depth of about 43 Å. With increasing penetration of X-rays into HPC film, the LC peak becomes more pronounced, and the intensity of the peak divided by the corresponding penetration depth reaches a plateau at Λ ∼ 100 Å as shown in Figure 5b. No change in the peak shape and its position was found as the incident angle approaches the critical angle of HPC. The exponential behavior of Λ close to θc,HPC did not allow us to perform a more detailed analysis of differences in ordering at the surface and in the bulk. However, our data suggest that thin films of HPC are roughly uniform in structure from the film surface throughout the film depth. It appears that the preferential ordering parallel to the surface direction is concentrated close to the film-substrate interface. What happens to the LC structure when colloidal particles are dispersed in an anisotropic substance? Such systems (related to a new field, now called “liquid crystal colloids”) are known to generate nontrivial topological constraints and singularities that affect the physical properties of the “host” system.46,47 Thin solid films of HPC of about 1000 Å thickness with increasing amount of SiOx particles were made according to the same procedure as for conventional HPC films. The formed periodic structures of the particles can be locked within the film after solvent evaporation. In Figure 6 the specular XRR patterns for such complex systems are shown for comparison. As seen, the LC peak diminishes with increasing concentration of colloidal particles (in the top inset of the figure the behavior of LC maximum after subtracting the baseline is shown) and a maximum at qz ) 0.07 Å-1 appears. This maximum is attributed to the periodic spacing between colloidal particles and corresponds to a distance of about 80 Å (approximately two diameters of SiOx particles). However, the off-specular X-ray reflectivity (θ ) (2θ)/2 ( 0.1°) in the range of LC peak, which is related to bulk ordering in the films, shows only a slight increase in integrated peak intensity at initial addition of colloidal particles into the (46) Poulin, P.; Stark, H.; Lubensky, T. C.; Weitz, D. A. Science 1997, 275, 1770. (47) Zapotocky, M.; Ramos, L.; Poulin, P.; Lubensky, T. C.; Weitz, D. A. Science 1999, 283, 209.
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Figure 6. Specular X-ray reflectivity pattern for complex films of HPC (thickness of about 1000 Å) with different amounts of Nyacol 215 colloidal silica, which was added to 70 mL of original HPC solution: 9, 0.25 mL; 0, 0.5 mL; b, 1.0 mL; O, 2.0 mL. (top inset) The corresponding LC peak intensities after subtracting the reflectivity baselines are shown. (bottom inset) The corresponding averaged off-specular X-ray reflectivity (θ ) (2θ)/2 ( 0.1°) in the range of LC peak after subtracting the baseline.
HPC film (the bottom inset of Figure 6). Fixed position of colloidal interference maximum and an increase in its intensity as concentration of particles increases means that there is compensation of negative particle charge and aggregation of small colloids due to changing of pH during dissolution of small amount of initial stock solution of Nyacol 215 in a large volume of neutral HPC solution. These aggregates are accumulated at the substrate by the action of gravitation and hydrogen bonding with a silicon oxide surface during the film deposition procedure and are locked there after evaporation of water.48 XRR data show that the added colloidal particles concentrated in the film-substrate region destroy the preferential crystallite alignment in that area. The data also confirm that the HPC crystallites (or domains) have a preferential orientation parallel to the substrate in the film-substrate interface region, whereas in the bulk they are mostly distributed randomly. This is another argument in support of our conclusion about preferential ordering of LC domains in the film-substrate interface region of HPC solid films. Let’s consider the thinnest HPC films. Figure 7 shows normalized reflectivity data (R/RF) from a typical scan of an HPC film of 20.6 Å. This thickness equals approximately to two diameters of HPC chain. A detailed description of XRR data analysis was described in our previous papers.35-37 The corresponding one-dimensional Patterson function calculated from XRR data shows only the large primary maximum due to the substrate-film and film-air interfaces (see the bottom inset in Figure 7). The solid line in Figure 7 shows the best fit assuming a uniform electron density film with error-function-broadened interfaces. The electron density profile obtained from this fit is presented in the top inset. Perfect fitting implies that no electron density variations inside the film are resolved by the X-ray data, while recent Monte Carlo simulations show the increased tendency of parallel (48) Evaporation should be slow enough to allow all colloidal particles to fall down to the substrate surface and to avoid locking them into a film bulk. In the last case the structure picture is more complicated due to different topological defects introduced by colloids.
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Figure 7. Specular X-ray reflectivity data for very thin HPC film of thickness T ) 20.6 Å). Solid line represents the best fit assuming a uniform electron density within the film. (top and bottom insets) Electron density profile and Patterson function obtained from the XRR data presented in the main panel.
Figure 8. Specular X-ray reflectivity for HPC film of 33.7 Å thickness: dash line is the best fit assuming a uniform electron density film; solid line represents the best variable-density fit that gives electron density profile shown in the top inset. (bottom inset) Patterson function for the XRR data presented in the main panel.
segmental alignment of semiflexible polymers at the interface with solids.49 This deviation from theory may be because the equilibrium arrangement of HPC molecules for such small film thicknesses results in overlapping of HPC molecules that leads to a lack of LC structure. As the film thickness increases, the structural picture is changed. XRR data for a HPC film of 33.7 Å thickness are presented in Figure 8. The corresponding Patterson function P(z) is shown in the bottom inset. The large primary maximum represents the overall thickness of the film. The existence of secondary maxima shows, without any model-dependent assumptions, that the electron density does not have a uniform but a complicated profile. This result was supported by fitting the XRR data. The dashed line in Figure 8 shows a best fit assuming a uniform electron density profile into the film. The deviation from the experimental data is easily visible especially around the region of LC peak for HPC. To improve the fitting we (49) Vacatello, M. Macromol. Theory Simul. 2002, 11, 53.
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used a sinusoidal electron density variation superimposed on an average HPC bulk density of 0.44 e Å-3.35 This model gives a good fit with a period of oscillations about 9 Å. The electron density distribution calculated from the variabledensity fit is shown in the top inset of Figure 8. It should be noted that the 30 Å film thickness is of order of a crystallite size characteristic for a LC structure of HPC.17,39 X-ray diffraction analysis shows that the crystallite is approximately 470 Å long and 31 Å in diameter.39 The molecules of HPC have a diameter of 8 Å, which means that the crystallite contains only 13 molecules and the model of the crystallite is suggested to be composed of long molecular rods packed together to form a thin microfibril. Hence, it is reasonable that the minimum value to make ordered molecule alignment in thin HPC films is equal to the microfibril diameter. The packing of the molecules in the centered rectangular unit cell gives an equatorial X-ray reflection, which corresponds to a repeat distance of a j ) 11.3 Å perpendicular to the chain axis.39 X-ray scattering data for a solid HPC bulk of the same molecular weight as used in this study are reported to give a diffuse peak that corresponds to distance of 11.6 Å.17 In our case, X-ray reflectivity data show slightly lesser values of a j ) 9-10.5 Å and a bigger crystallite size of 50 Å. Both results can be explained as due to structural reorganization of HPC molecules in the region next to the Si substrate, which leads to a change in alignment of
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neighboring chain segments in a direction of a rectangular packing. This is a surface-induced effect of formation of LC structure in conditions of confined geometry during preparation of very thin water-cast films of HPC. Conclusions Synchrotron X-ray reflectivity was applied to study of structural alignment near the interfaces and in the bulk of thin solid films of hydroxypropylcellulose. In the bulk the LC phase represents a statistically random distribution of crystallites, whereas they have a higher degree of orientational order parallel to the surface close to the filmsubstrate interface. Essentially, HPC chains show a pronounced tendency to order in the interfacial region, and the flexibility of HPC chains together with a confinement effect requires a minimum scale range for such finitesize molecules to organize in a restricted geometry. Acknowledgment. This work was supported by the U.S. National Science Foundation under Grant No. DMR0305494 and was performed at beamline X23B of the National Synchrotron Light Source, which is supported by the U.S. Department of Energy. We also thank Dr. T. Budtova (CEMEF, France) for suggesting HPC as an object for studies. LA035118I
