© Copyright 1998 American Chemical Society
APRIL 14, 1998 VOLUME 14, NUMBER 8
Letters Polymer-Dispersed Liquid Crystal Films Studied by Near-Field Scanning Optical Microscopy Erwen Mei and Daniel A. Higgins* Department of Chemistry, Kansas State University, Manhattan, Kansas 66506 Received July 24, 1997. In Final Form: February 12, 1998 Topographic and near-field scanning optical microscopy images of nematic liquid crystalline (LC) droplets dispersed in polymer films are presented. It is conclusively shown that the topographic images provide information invaluable to a complete understanding of the droplet formation process, shape, and internal LC organization. The topographic data indicate that the droplets are encapsulated in a thin, relatively solid polymer shell. Many droplets in these films are observed to take on spheroidal and discoidal shapes. A new class of toroidal droplets, possibly formed by droplet rupture during film formation, is also observed and characterized. Polarization-dependent optical images are employed to determine the LC organization in these droplets.
Introduction films1,2
are Polymer-dispersed liquid crystal (PDLC) promising new optoelectronic materials being developed for use in flat-panel displays and windows.3-6 Usually comprised of micrometer-sized droplets of nematic liquid crystal (LC) suspended in a polymer matrix, PDLC films are electrically switchable between an opaque state, in which the LC droplets strongly scatter light, and a transparent state. Light scattering efficiency and contrast ratio (between the two states) depend on the optical properties of the polymer and LC, the detailed LC organization (the director configuration), and droplet size and shape. LC organization within the droplets is * Corresponding author. (1) Drzaic, P. S. J. Appl. Phys. 1986, 60, 2142-2148. (2) Doane, J. W.; Vaz, N. A.; Wu, B.-G.; Zumer, S. Appl. Phys. Lett. 1986, 48, 269-271. (3) Doane, J. W. Polymer Dispersed Liquid Crystal Displays. In Liquid Crystals: Applications and Uses; Bahadur, B., Ed.; World Scientific: Singapore, 1990; Vol. 1; pp 361-395. (4) Kitzerow, H.-S. Liq. Cryst. 1994, 16, 1-31. (5) Coates, D. J. Mater. Chem. 1995, 5, 2063-2072. (6) Fuh, A. Y.-G.; Huang, C.-Y.; Sheu, C.-R.; Lin, G.-L.; Tsai, M.-S. Jpn. J. Appl. Phys. 1994, 33, L870-L872. (7) De Gennes, P. G.; Prost, J. The Physics of Liquid Crystals, 2nd ed.; Oxford University Press: New York, 1993. (8) Zumer, S.; Doane, J. W. Phys. Rev. A 1986, 34, 3373-3386.
governed primarily by polymer-LC interfacial interactions and LC-LC interactions.7-9 A number of theoretical studies have modeled LC organization for spheroidal and ellipsoidal droplets.9-11 Experimental studies employing a variety of “bulk” spectroscopic techniques have proven the existence of the theoretically predicted director configurations and their dependence on droplet size, shape, and interfacial interactions.8,12-14 However, these studies have only provided an “average” picture of LC droplets. For detailed studies of the morphological dependence of LC organization, characterization of the droplets on an individual basis is required. To this end, conventional visible15 and IR16 microscopic methods have been utilized to study PDLC materials. However, these methods provide limited optical resolution, with ≈250 nm resolution in visible-light images, and (9) Onozawa, T. Liq. Cryst. 1994, 17, 635-649. (10) Erdmann, J. H.; Zumer, S.; Doane, J. W. Phys. Rev. Lett. 1990, 64, 1907-1910. (11) Williams, R. D. J. Phys. A 1986, 19, 3211-3222. (12) Montgomery, G. P.; West, J. L.; Tamura-Lis, W. J. Appl. Phys. 1991, 69, 1605-1612. (13) Whitehead, J. B.; Zumer, S.; Doane, J. W. J. Appl. Phys. 1993, 73, 1057-1065. (14) Golemme, A.; Zumer, S.; Doane, J. W.; Neubert, M. E. Phys. Rev. A 1988, 37, 559-569.
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several micrometers resolution when IR light is employed. In addition, such methods do not provide detailed topological information and, hence, have only been used to characterize droplet size and shape in two-dimensions (parallel to the plane of the film). Such methods also provide little information on the types of morphological film defects present in PDLCs. Electron microscopy has yielded much more detail as regards these issues1,2,17 but has provided little information on LC organization. Clearly, a microscopic method which simultaneously provides both optical and morphological data will yield valuable new information. Near-field scanning optical microscopy (NSOM)18-21 is a new microscopic method which simultaneously provides high-resolution optical and topographic information and, hence, meets these needs. Since its development, NSOM has proven powerful for nanometer to micrometer scale imaging of a variety of materials,22-32 and polarizationdependent NSOM has been shown to be particularly useful for studying local molecular organization.23,31,32 The first polarization-dependent NSOM images of PDLC-based materials were recently published by Vaez-Iravani and co-workers, demonstrating its applicability to these important systems.25 Here, we report the first application of NSOM to the detailed characterization of LC droplet shape and molecular organization in PDLC films. Specifically, nematic LC droplets encapsulated in poly(vinyl alcohol) (PVA) films are studied by polarization-dependent NSOM. We observe a range of droplet shapes and LC organization. The droplets are divided into three shape categories: those of oblate spheroidal shape, those of discoidal shape, and those of toroidal shape. To our knowledge, droplets of toroidal shape, which yield a toroidal director configuration, have not been observed in these films in the past. However, droplets characterized as being of spherical shape with toroidal director configurations have been described.4,11,33 The topographic information is of great importance in the determination of three-dimensional droplet shape and the characterization of LC organization. The results are shown to be particularly important for the case of toroidal (15) Ondris-Crawford, R.; Boyko, E. P.; Wagner, B. G.; Erdmann, J. H.; Zumer, S.; Doane, J. W. J. Appl. Phys. 1991, 69, 6380-6386. (16) Challa, S. R.; Wang, S.-Q.; Koenig, J. L. Appl. Spectrosc. 1995, 49, 267-272. (17) Pierron, J.; Tournier-Lasserve, V.; Sopena, P.; Boudet, A.; Sixou, P.; Mitov, M. J. Phys. II 1995, 5, 1635-1647. (18) Du¨rig, U.; Pohl, D. W.; Rohner, F. J. Appl. Phys. 1986, 59, 33183327. (19) Harootunian, A.; Betzig, E.; Isaacson, M.; Lewis, A. Appl. Phys. Lett. 1986, 49, 674-676. (20) Betzig, E.; Trautman, J. K.; Harris, T. D.; Weiner, J. S.; Kostelak, R. L. Science 1991, 251, 1468-1470. (21) Vanden Bout, D. A.; Kerimo, J.; Higgins, D. A.; Barbara, P. F. Acc. Chem. Res. 1997, 30, 204-212. (22) Betzig, E.; Trautman, J. K. Science 1992, 257, 189-195. (23) Betzig, E.; Chichester, R. J. Science 1993, 262, 1422-1425. (24) Mcdaniel, E. B.; Hsu, J. W. P.; Goldner, L. S.; Tonucci, R. J.; Shirley, E. L.; Bryant, G. W. Phys. Rev. B 1997, 55, 10878-10882. (25) Ade, H.; Toledo-Crow, R.; Vaez-Iravani, M.; Spontak, R. J. Langmuir 1996, 12, 231-234. (26) Talley, C. E.; Cooksey, G. A.; Dunn, R. C. Appl. Phys. Lett. 1996, 69, 3809-3811. (27) Dunn, R. C.; Allen, E. V.; Joyce, S. A.; Anderson, G. A.; Xie, X. S. Ultramicroscopy 1995, 57, 113-177. (28) Buratto, S. K.; Hsu, J. W. P.; Betzig, E.; Trautman, J. K.; Bylsma, R. B.; Bahr, C. C.; Cardillo, M. J. Appl. Phys. Lett. 1994, 65, 2654-2656. (29) Moyer, P. J.; Walzer, K.; Hietschold, M. Appl. Phys. Lett. 1995, 67, 2129-2131. (30) Birnbaum, D.; Kook, S.-K.; Kopelman, R. J. Phys. Chem. 1993, 97, 3091-3094. (31) Higgins, D. A.; Vanden Bout, D. A.; Kerimo, J.; Barbara, P. F. J. Phys. Chem. 1996, 100, 13794-13803. (32) Higgins, D. A.; Kerimo, J.; Vanden Bout, D. A.; Barbara, P. F. J. Am. Chem. Soc. 1996, 118, 4049-4058. (33) Drzaic, P. S. Mol. Cryst. Liq. Cryst. 1988, 154, 289-306.
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Figure 1. Near-field scanning optical microscope employed in these studies. Polarized light exiting the near-field probe and passing through the sample is collected with an oil immersion objective. An analyzer is used to select the polarization of light to be detected. The sample sits on a piezo-electrically driven stage for raster scanning in X and Y, as well as for positioning the sample within the near-field of the probe (Z motion).
droplets for which conventional optical images alone may lead to the erroneous assignment of the director configuration and/or droplet shape. Experimental Section All PDLC films studied here were prepared by the polymer encapsulation method.3 A 2% (by weight) aqueous solution of PVA (Aldrich) was employed. Nematic liquid crystal (E7, Merck) was added to this solution and emulsified in an ultrasonic bath, yielding a 0.67% (by weight) suspension of LC. Thin films were prepared from this emulsion by depositing one or two drops onto a glass microscope cover slip and allowing the solvent to evaporate. All samples were dried overnight in a 40 °C oven prior to use. The LC droplets formed were found to range from ≈20 µm in diameter down to less than 1 µm. Local film thickness ranged from ≈1 µm (in central film regions) down to ≈100 nm (at the edge). Film thickness was determined by mechanically removing small regions of the film and then topographically imaging the resulting film edge. A modified Topometrix Aurora NSOM was used to record all near-field optical and topographic images. A diagram of the instrument is shown in Figure 1. NSOM probes were produced in house from single mode optical fiber using the method originally developed by Betzig et al.20 All optical images reported were found to be the same for a variety of probes. Shear-force feedback, described elsewhere,34,35 was used to maintain a probe-sample separation of 5-10 nm. For optical imaging, the 514 nm line of an argon ion laser was used and was coupled into the cleaved end of the probe fiber. The polarization of the light from the probe was controlled by λ/2 and λ/4 plates (Special Optics) placed prior to the fiber coupler. With this configuration, linearly polarized light was obtained from each NSOM probe employed. The purity of polarization (measured in the far-field) was always better than 90% (typically ≈99%). The light exiting the probe and passing through the sample was collected by an oil immersion objective (100×, 1.25 numerical aperture). An analyzer (Karl Lambrecht, >105 extinction) was set to pass light polarized perpendicular to the polarization of the light from the probe. For detection, an image of the probe aperture was produced on a 200 µm pinhole mounted on the front of a photomultiplier tube, or on the 150 µm diameter active area of a silicon single-photon-counting avalanche diode (EG&G). (34) Betzig, E.; Finn, P. L.; Weiner, J. S. Appl. Phys. Lett. 1992, 60, 2484-2486. (35) Toledo-Crow, R.; Yang, P. C.; Chen, Y.; Vaez-Iravani, M. Appl. Phys. Lett. 1992, 60, 2957-2959.
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Figure 2. (A, C, and E) Topographic images of oblate spheroidal, discoidal, and toroidal droplets, respectively. (B, D, and F) Topographic line scans corresponding to the dark lines superimposed on the images in (A), (C), and (E), respectively. Explanations for the droplet shape classification are given in the text and require consideration of the optical data shown in Figure 3.
Results and Discussion Figure 2 shows topographic images for three different LC droplets. All three droplets appear nearly round (in the plane of the film) and protrude a significant distance above the film surface. However, they are topologically very different. The droplet shown in Figure 2A can be classified as an oblate spheroid, while those shown in parts C and E of Figure 2 have very different shapes, as clearly shown by the topographic line scans in Figure 2. It appears that a portion of the LC may have leaked from the droplets in parts C and E of Figure 2 sometime during film formation and that the polymer shell then collapsed. This process results in a range of nonspherical droplet shapes. If the upper shell surface collapses to the point of contact with the lower shell surface, while trapping LC in the outer circumferential regions, a toroidal shape is formed. Alternatively, if the droplet surface is only slightly depressed, a discoidal shape will result. Characterization of numerous droplets indicates that both cases occur frequently. The droplets shown in Figure 2 are representative of what is in fact a continuous distribution of droplet shapes (from spheroidal to toroidal) found in these films. It is difficult to classify these droplets as discoidal or toroidal using only the data presented in parts C and E of Figure 2. The images suggest that both may be toroidal in shape. The optical data presented below provide additional evidence that is used to make a conclusive assignment. The topographic images also provide valuable information on the nature of LC encapsulation. Most notably, stable, reproducible, topographic images of the E7-PVA droplets are readily obtained. Such results prove that the LC droplets are encapsulated in a relatively “solid” polymer shell. In contrast, no topographic images of LC droplets formed on the surface of a different polymeric material, poly(butyl methacrylate), could be obtained.
Although these films appeared identical to the E7-PVA films under a conventional light microscope, the nearfield probe was observed to simply penetrate these droplets. The differences observed between the two PDLC systems indicate that the PVA shell must be strong enough to withstand most mechanical forces imparted by the nearfield probe. While it is generally accepted that the probe is not continuously in contact with the sample surface,34,35 it is likely that minor contact frequently occurs,36 especially as the tip encounters the rapidly changing topography in the vicinity of a droplet. It should be noted that inadvertent crashes of the NSOM probe into E7-PVA droplets were observed to lead to droplet rupture. However, such probe-induced rupturing of the droplets only occurred infrequently (i.e., much less than once per hundred images, on average). The topology observed in parts C and E of Figure 2 did not result from probe crashes. The thickness of the PVA shell can be estimated from topographic images of intentionally ruptured droplets. Ruptured droplets have an appearance similar to those shown in parts C and E of Figure 2. For this measurement, the film thickness in the vicinity of a ruptured droplet was first determined. The depth of the crater resulting from droplet rupture was also measured and was subtracted from the local film thickness. From these studies, an upper limit of 20-40 nm can be placed on the thickness of the PVA shell covering most LC droplets. The topographic results also provide information on the shell formation process. It is proposed here that the shell begins to form and thicken very early during macroscopic film formation and may occur prior to incorporation of some droplets in the film. In fact this is expected since PVA is a known emulsion stabilizer, indicating that it forms at least a thin film around the LC droplets in the (36) Gregor, M. J.; Blome, P. G.; Schofer, J.; Ulbrich, R. G. Appl. Phys. Lett. 1996, 68, 307-309.
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Figure 3. (A and B) Cross-polarized optical images of spheroidal droplets recorded by NSOM with the output polarizer oriented 5° from vertical (on the image). The droplet imaged in A is topographically imaged in Figure 2A; similar topography is observed for the droplet shown in B. The optical images for both droplets indicate that the LC molecules are organized in the bipolar configuration. For (A), the average director orientation is normal to the film plane while for B it is oriented approximately parallel to the film plane. (C and D) Cross-polarized optical images of the droplet shown in Figure 2C with the output polarizer at 5° and 50° from vertical, respectively. The images indicate that the LC molecules are organized in the bipolar configuration in this discoidal droplet. The average director is oriented in the plane of the film, rotated approximately 45° from horizontal. (E and F) Crosspolarized optical images of the droplet shown in Figure 2E. The output polarizer is oriented as in C and D, respectively. The optical patterns and the topography indicate that the droplet is of toroidal shape with the LC molecules organized in a toroidal (rather than bipolar) configuration.
emulsion. Although surface active species typically form monolayer films at such interfaces, the stability of the shell provides proof that it is almost always thicker. Should film thickening only occur very late in the deposition process, surface droplets with little or no polymer shell would be found since many are certainly deposited during the latter stages of film growth, when most of the polymer has already been deposited. However, exactly when the polymer shell thickens and solidifies cannot be directly determined here. The morphological differences observed between the droplets depicted in Figure 2 also provide valuable information that aids in interpretation of the optical images used to determine LC organization. Figure 3 presents crossed-polarized, transmitted-light, NSOM images of the droplets shown in Figure 2. The contrast in these images is dominated by polarization rotation due to the local birefringence of the droplets. The largest optical signals represent transmitted light intensities at least 10-20 times larger than the background measured off the droplets. In addition, the maximum transmitted intensity observed over spheroidal droplets (i.e., Figure 3A,B) is often as large as ≈50% of the maximum signal expected for 90° rotation of the polarization. Although high-resolution features are not readily observed in these images (≈250 nm wide features are present), the upper regions of the droplets are indeed within the near-field of the NSOM probe since the polymer
shell is at most 20-40 nm thick. Optical contrast from features deeper within some droplets (in the far field) most certainly is incorporated in these images and cannot be readily removed. This fact is consistent with the large optical contrast observed for spheroidal droplets. However, as has been shown in previous theoretical37,38 and experimental31 studies, the portion of the sample in the near field is expected to most strongly influence the coupling of near-field light to the far-field. While contrast due to far-field interactions may mask some highresolution near-field features, this fact does not detract from the emphasis of the present work, which is to utilize the optical and topographic information provided simultaneously by NSOM to obtain crucial new information on three-dimensional droplet morphology and LC organization. As a final technical note, it has been shown that transmitted-light NSOM images often suffer from the coupling of topographic information into the optical images.39 This near-field effect results from changes in the near-field-to-far-field coupling efficiency when a (37) Heinzelmann, H.; Huser, T.; Lacoste, T.; Gu¨ntherodt, H.-J.; Pohl, D. W.; Hecht, B.; Novotny, L.; Martin, O. J. F.; Hafner, C. V.; Baggenstos, H.; Wild, U. P.; Renn, A. Opt. Eng. 1995, 34, 2441-2454. (38) Novotny, L.; Pohl, D. W.; Regli, P. J. Opt. Soc. Am. A 1994, 11, 1768-1779. (39) Valaskovic, G. A.; Holton, M.; Morrison, G. H. J. Microsc. 1995, 179, 29-54.
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Figure 4. (A and B) The idealized bipolar organization of nematic LC molecules within spherical droplets. The director configuration is viewed parallel to the optical axis (the average director alignment) in (A) and perpendicular to the optical axis in (B). (C) The proposed toroidal organization of LC molecules within toroidal droplets. The shaded (dark) and unshaded (light) regions show the expected optical pattern for a droplet with the given alignment, when imaged between crossed polarizers.
topographic feature is encountered. In the images presented here, the vast majority of the contrast results solely from polarization rotation, as is demonstrated by the fact that the light and dark regions within the individual droplets in Figure 3 do not correlate with any observable topographic features in Figure 2. In addition, the high contrast observed is inconsistent with such a phenomenon. However, the very weak dark rings observed around the edges of the droplets in Figure 3 likely result from this effect. The transmitted light intensity changes by only ≈10% of the background level in these edge regions, demonstrating that such effects are of minimal importance here. The optical images recorded can therefore be directly used to characterize the LC organization. For the E7PVA system, the LC molecules in spherical droplets are expected to be arranged in the “bipolar configuration”.8 The director configurations for a spherical droplet with its average director (the droplet’s optical axis) oriented normal to the plane of the film, and in the plane of the film, are shown in parts A and B of Figure 4, respectively. Theoretical studies have presented calculated optical images for such droplets.4,15 The shaded regions superimposed in parts A and B of Figure 4 depict the dark/light optical patterns expected for crossed polarization conditions. The pattern shown in Figure 4A corresponds well with that observed for the oblate spheroidal droplet shown in Figure 3A, indicating that the optical axis for this droplet is oriented normal to the film surface. The optical image of a second spheroidal droplet (topography not shown) is presented in Figure 3B. This pattern corresponds to that expected for a spheroidal droplet with its optical axis in the plane of the film (Figure 4B) but rotated significantly (≈45°) from the polarization axes. As has been shown in the past,1,15,40 most spheroidal droplets yield optical patterns different from that shown in Figure 4. These patterns correspond to cases in which the optical axis is oriented at an intermediate angle. A random distribution for the optical axis orientation is expected for spheroidal droplets since there is (at best) a very shallow energy minimum for its orientation along any particular direction in this case.4,8,9 It should be noted that optical images such as those presented in parts A and B of Figure 3 do not provide conclusive proof that the droplets are indeed spheroidal in shape. Such a conclusion can only be reached after consideration of the corresponding topographic images. Droplets of distinctly nonspherical shapes are observed throughout these films. In fact, droplets similar to those shown in parts C and E of Figure 2 are observed at least as frequently as spheroidal droplets. Again, the topog(40) Jain, S. C.; Rout, D. K. J. Appl. Phys. 1991, 70, 6988-6992.
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raphy indicates that these droplets might best be described as being of toroidal or discoidal shape. Again, these droplets likely result from droplet rupture during film formation. NSOM images of the droplet shown in Figure 2C are shown in parts C and D of Figure 3. The optical patterns are similar to those expected for the bipolar configuration. For this droplet, the optical axis is oriented in the plane of the film and rotated ≈45° (in the opposite direction from the droplet shown in Figure 3B) off horizontal on the image. When both optical and topographic data are taken into account, it can be concluded that this droplet is of discoidal, rather than toroidal, shape. It is much flatter than the spheroidal droplets, but a significant quantity of birefringent LC remains in the very center of the droplet, as evidenced by the nonzero intensity of transmitted light in this region. In contrast to the spheroidal droplets, all discoidal droplets studied were found to have their optical axes aligned approximately parallel to the film surface. The horizontal alignment of the optical axis for discoidal droplets is dictated by droplet shape and is expected.4,15 While discoidal and spheroidal droplets have been observed previously, a new class of droplets is observed in these films as well. These droplets yield topographic images such as that presented in Figure 2E, which is very similar to that observed for discoidal droplets. However, their optical images are dramatically different, as shown in parts E and F of Figure 3. The optical patterns observed are similar to those expected for small spheroidal droplets of bipolar organization (with a normally oriented optical axis, see Figure 4A).15 Alternatively, the optical images might be interpreted as resulting from a known toroidal configuration within a spheroidal droplet.4,33 However, such a configuration is not expected for the E7 system because the LC does not have the appropriate bend and splay constants.4,33 Were the topographic images not available, the toroidal configuration would therefore be rejected as a possibility. However, the topographic information indicates that the droplet itself may have a toroidal shape. The data shown in Figure 2E suggests that the depression in the center of this droplet is not as pronounced as in the case of the discoidal droplet shown in Figure 2C. However, the NSOM probe may be too large to properly reproduce the topography in the narrow depression observed for this droplet. The shape of the line scan (see Figure 2F) near the bottom of the depression lends further proof to this conclusion. Were the probe to actually reach the bottom of the depression, as is observed for the discoidal droplet, a flat region in the center of the droplet might be expected. The depression observed in parts E and F of Figure 2 is therefore likely much deeper than that indicated by the topographic image, and the droplet may therefore be of toroidal shape. The optical images provide further proof of this interpretation. A toroidal droplet shape will in fact yield a toroidal director configuration, similar to that depicted in Figure 4C. The toroidal configuration proposed here is distinctly different from that observed in the past for spherical droplets.4,33 Therefore, this class of droplet will certainly exhibit different optical properties than previously observed spheroidal droplets of either bipolar or toroidal organization. Because of the much greater surface-tovolume ratio, the electric-field switching behavior should differ as well.40 A complete understanding of macroscopic PDLC film optical properties requires that such droplets be identified and characterized. However, without both the optical and topographic information provided by NSOM, studies of these and other new droplet shapes may be difficult.
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The above discussion applies to droplets with the idealized director configurations shown in Figure 4. However, many droplets are often observed to yield optical images different from what is expected, as is demonstrated by the distortion of the optical pattern on the left side of the droplet shown in Figure 3A. Such distortions are attributable to defects in the LC organization brought about by morphological defects in the surrounding polymer shell. While the influence of such physical defects on LC organization is well-known, new information directly relating specific types of morphological defects with specific distortions in the director field can now be obtained through the simultaneously recorded optical and topographic information provided by NSOM. For this particular droplet (Figure 3A), the shell appears to be somewhat flattened on the left side and its maximum height occurs to the left of the apparent center of the droplet, potentially leading to the observed distortion in the LC organization. Conclusions In summary, NSOM has been used to optically and topographically characterize PDLC films. The topographic data were used to show that the LC droplets in
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the E7-PVA system are encapsulated in a “solid” polymer shell. The topographic images were also used to characterize film morphology and droplet shape attributes in three dimensions. Some droplets were observed to take on the well-known spheroidal shape while others took on distinctly nonspherical shapes. Toroidal droplets, which have not been characterized in the past, were observed and the LC director organization was determined. These droplets are of particular importance because they may represent a significant fraction of LC droplets in PDLC films and likely have dramatically different optical properties from others observed in the past. Such droplet shapes are difficult to identify without taking into account both optical and topographic information, as provided simultaneously by NSOM. Acknowledgment. This work was supported in part by the National Science Foundation under Grant No. EPS9550487 with matching support from the State of Kansas. The National Science Foundation CAREER Award program (CHE-9701509) and Kansas State University are also acknowledged for their support of this work. LA9708288
