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Langmuir 1998, 14, 6956-6968
Polymer-Dispersed Liquid Crystal Monolayers T. E. Herod and R. S. Duran* George and Josephine Butler Polymer Research Laboratories, Department of Chemistry, University of Florida, Gainesville, Florida 32611 Received February 10, 1998. In Final Form: August 14, 1998 The first examples of two-dimensional polymer-dispersed liquid crystals (PDLCs) have been prepared via the Langmuir-Blodgett technique and analyzed using tapping mode atomic force microscopy (TMAFM). Characterization of the thermodynamics, polymerization kinetics, and topology of mixtures of 2-pentadecylaniline (2PDA) and a ferroelectric liquid crystal termed 10PPB2 was investigated. Systematic pressure versus area isotherms indicate that the mixed monomer/liquid crystal exists as a phase-separated monolayer after the evaporation of the spreading solvent, typical of a solution-induced phase separation (SIPS). Interfacial polymerization of the monomer in the presence of the liquid crystal has been accomplished. Langmuir-Blodgett-Kuhn (LBK) films obtained by transferring the polymer/liquid crystal system on freshly cleaved mica surfaces revealed a polymer-dispersed liquid crystalline system with circular liquid crystalline domains trapped within a poly(alkylaniline) matrix. Statistical characterization of domain parameters (i.e. domain density, mean diameter, and polydispersity) was performed, and the effect of monolayer and polymerization conditions on these parameters was investigated. From these studies it was found that domain morphology could be controlled by the manipulation of monolayer spreading and compression rates. The high anisotropy and controlled symmetry of these monolayer liquid crystal domains make them attractive models of bulk PDLCs.
Introduction The controlled preparation of micro- and nanometersize features in materials, by physical or chemical methods, has become an important branch of advanced materials in recent years. One example is the class of composite materials termed polymer-dispersed liquid crystals (PDLCs). PDLCs are composites that incorporate microdroplets of a nematic liquid crystal dispersed into a polymer matrix. These materials show promise in designing a wide variety of new liquid crystalline-based technologies, such as large-area displays, smart windows, light valves, and filters.1-4 PDLCs are usually formed by either emulsion- or phase separation-type processes.5 In the former, the liquid crystal is emulsified in an aqueous medium, and once the composite is formed, water is removed. Composite materials formed in this manner are typically termed nematic curvilinear aligned phases. PDLCs are formed by the induced phase separation of liquid crystal after the formation of the film. This technique is widely used with thermoplastic polymers such as polystyrene, PMMA, and PVP. Phase separation is usually induced by changes in temperature, polymerization, or cross-linking of the binder, or solvent evaporation. In temperature-induced phase separation (TIPS) the liquid crystal and polymer start out as a single-phase mixture at an elevated temperature, while in polymerization-induced phase separation (PIPS), phase separation is induced by reduced solubility of the liquid crystal with growing polymer chains or the forming polymer network. Solvent-induced phase separation (SIPS) is another technique used with the fully formed polymer. The polymer and liquid crystal are * To whom correspondence should be addressed. (1) Graighead, H. G.; Cheng, J.; Hackwood, S. Appl. Phys. Lett. 1982, 40, 22. (2) Ferason, L. Int. Symp. Digital Technol. Pap. 1985, 16, 68. (3) Drzaic, P. S.; J. Appl. Phys. 1986, 60 (6), 2141. (4) Doane, J. W.; Vaz, N. A. P.; Chidichimo, G. Appl. Phys Lett. 1986, 40, 269. (5) Bouteiller, L.; Le Barny, P. Liq. Cryst. 1996, 21 (2), 157.
dissolved in a solvent, and the solution is used to cast the film. The films are coated onto a substrate, and the solvent is evaporated. During the evaporation process, the liquid crystal and polymer phase separate, forming the composite. While the polymer and liquid crystal must be soluble in the solvent, they cannot be miscible with each other; otherwise, no phase separation will occur upon evaporation of the solvent. The technique can be used with a wide range of polymer/liquid crystal systems; however, it is difficult to control the evaporation rate. Because the evaporation rate defines the morphology of the composite, it is thus difficult to control the reproducibility of the systems, which is necessary for large-scale industrial applications.4,5 The Langmuir-Blodgett technique has been used to study a wide range of interfacial properties.6-8 Confinement of surfactant molecules to a two-dimensional, planar air/liquid interface allows for the study of complex processes that may otherwise be difficult to observe in three dimensions. For example, our group has published several papers on the interfacial polymerization of long chain alkyl derivatives of aniline and pyrrole9-11 as well as the mixing and phase behavior of polymer and copolymer liquid crystalline blends.12,13 Imaging of Langmuir films directly at the air/water interface is usually accomplished using either fluores(6) MacRitchie, F. Chemistry at Interfaces; Academic Press: New York, 1990. (7) Birdi, K. Lipid and Biopolymer Monolayers at Liquid Interfaces; Plenum Press: New York, 1989. (8) Roberts, G. Langmuir-Blodgett Films; Plenum Press: New York, 1990. (9) Servay, T. S.; Tindall, D.; Herod, T. E. Polymer Prepr. 1995, 36 (1), 582. (10) Weerasekera, G.; Servay, T. S.; Herod, T. E.; Tindall, D. Manuscript in preparation. (11) Zhou, H.; Stern, R.; Batich, C.; Duran, R. Makromol. Chem. Rapid Commun. 1990, 11, 409. (12) Thibodeaux, A. F.; Ra¨dler, U.; Shasidhar, R.; Duran, R. S. Macromolecules 1994, 27, 784. (13) Thibodeaux, A. F. Blends in Two Dimensions: Mixtures of Liquid Crystalline Polymers at the Air-Water Interface. Ph.D. Thesis, University of Florida, Gainesville, FL, 1993.
10.1021/la980167u CCC: $15.00 © 1998 American Chemical Society Published on Web 10/31/1998
Polymer-Dispersed Liquid Crystal Monolayers
Figure 1. (A) 2-Pentadecylaniline monomer (2PDA). (B) Liquid crystal (10PPB2).
cence14 or Brewster angle microscopies.15 The disadvantages of fluorescence microscopy are the unknown effect of the dye on the monolayer phase or mixing properties, and for BAM a disadvantage is the limiting resolution. In contrast, the various modes of atomic force microscopy16,17 are valuable tools for the surface scientist. These techniques involve the direct imaging of surface interactions between the scanning tip and the sample surface. In this study, tapping mode AFM, a technique developed by Digital Instruments to minimize the damage to the sample surface caused by the scanning tip, was employed. In addition, by monitoring the lag in the phase of the tip oscillations caused by materials of differing elastic properties (phase contrast), the imaging of phase-separated blends can be realized. We report here results on amphiphilic polymerizable amphiphile/nonpolymerizable liquid crystal mixtures and show that microphase-separated mixtures of the polymer and liquid crystal can be successfully transferred to a solid substrate (mica) and imaged using TMAFM. This technique allows us to directly resolve single domains in this system, with diameters ranging between about 0.1 and 10 µm. Understanding the domain formation in such model 2D systems may help in better control of the interface formation in 3D polymer/liquid crystal systems, which is among the primary processes in SIPS, TIPS, or emulsion-type processes. Experimental Procedure Materials. The monomer 2-pentadecylaniline (2PDA)18 (Figure 1) was purified via column chromatography (200 mesh silica gel) for these experiments. The ferroelectric liquid crystal (R)4′-(ethoxycarbonyl-1-ethoxy)phenyl 4-[4-(9-decenyloxy)phenyl]benzoate (10PPB2) (Figure 1) was obtained from R. Shashidhar of the Naval Research Laboratories; its bulk thermal transitions are cryst 67 SC* 95.4 SA 125.4 iso (temperatures in °C).19 For monolayer characterization and thermodynamic studies, a subphase of 0.1 M H2SO4 prepared from 18 MΩ Millipore water (Milli-Q) and concentrated sulfuric acid obtained from Fisher Scientific was used. Polymerization studies used the same subphase with the addition of 0.03 M (NH3)2S2O8 (APS) as an oxidizing agent. The APS was obtained from Fisher Scientific and used without further purification. Freshly cleaved mica, obtained from Carr-McMasters, was used as the LangmuirBlodgett-Kuhn (LBK) substrate. Solutions of 2PDA, 10PPB2, and 2PDA/10PPB2 mixtures were prepared in chloroform by dissolving 0.500-1.200 mg of surfactant per 1 mL of solvent. Mixtures were prepared at 0.2, 0.4, 0.6, and 0.8 mole fractions of 2PDA. All glassware was cleaned via the following procedure: (14) Ulman, A. An Introduction to Ultrathin Organic Films: from Langmuir-Blodgett to Self-Assembly; Academic Press: Boston, 1991. Mo¨hwald, H.; Lo¨sche, M. Rev. Sci. Instrum. 1984, 55 (12), 1968. (15) He´non, S.; Meunier, J. Rev. Sci. Instrum. 1991, 62 (4), 936. Ho¨nig, D.; Mo¨bius, D. J. Phys. Chem. 1991, 95, 4590. (16) Sarid, D. Scanning Force Microscopy with Applications to Electric, Magnetic, and Atomic Force; Oxford University Press: New York, 1991. (17) DiNardo, J. N. Nanoscale Characterization of Surfaces and Interfaces; VCH: New York, 1994. (18) Bodalia, R.; Duran, R. J. Polym. Sci., Part A: Polym. Chem. 1993, 31, 2123. (19) Naciri, J.; Pfeiffer, S.; Shashidhar, S. Liq. Cryst. 1991, 10, 585.
Langmuir, Vol. 14, No. 24, 1998 6957 it was (1) filled with absolute ethanol and sonicated at 40 °C for 30 min, (2) flushed 10 times with Milli-Q water, (3) filled with a solution of 5 vol % Microclean and 95% Milli-Q and sonicated at 40 °C for 30 min, (4) flushed with Milli-Q water 10 times, (5) filled and sonicated as in step 1, and (6) dried by flushing with N2. Pressure/Area Isotherms. Monolayers were characterized using a KSV 5000 LBK Teflon trough system isolated on a vibration-free table and contained in a dust-free hood. The temperature of the trough and subphase was kept constant at 25.0 ( 0.2 °C using a constant-temperature recirculating bath. Between 40 and 100 µL of the surfactant solution was spread at the subphase/air interface, and the solvent was allowed to evaporate for approximately 10 min. The surface pressure (π) was measured as a function of the mean molecular area (A) using the Wilhelmy method. Two Teflon barriers were used to decrease the mean molecular area at the constant compression rate 10.0 ( 0.5 Å2/molecule‚min. Four isotherms were collected for each mixture in order to obtain a mean isotherm. Typical reproducibility was better than 0.5 Å2/molecule and 0.5 mN/m in surface pressure. In addition, isobaric stability measurements were performed on each pure component separately. Monolayer Polymerization. Polymerization of 2PDA is described in detail by previous papers.20,21 The amphiphilic aniline monomer was chemically polymerized at the air/liquid interface by placing a strong oxidant (0.03 M (NH4)2S2O8) in the acidic subphase (0.1 M H2SO4) and compressing the film by decreasing A at the constant rate 10.0 ( 0.5 Å2/molecule‚min until the desired surface pressure is reached. The reaction was monitored by observing the mean molecular area change (∆A), at isobaric conditions, with time. The monotonic decrease in area during polymerization is due to the replacement of van der Waals radii by covalent bonds between monomer molecules and changes in their conformation. Two identical sets of monolayer polymerization experiments were performed for molecular weight and TMAFM analysis. Polymer reserved for molecular weight analysis was collected after the polymerization was finished and washed with Millipore water and extracted with HPLC grade THF. Solvent was removed, and the dark blue material was redissolved into THF and analyzed using GPC relative to monodisperse poly(styrene) standards. Transfer and Atomic Force Microscopy. Monolayers designated for AFM analysis were prepared in the same way as detailed in the preceding section. Before the surfactant solution was spread, a freshly cleaved mica substrate was placed into the mechanical dipping arm of the trough system and lowered into the subphase. The solution was spread onto the subphase surface, and the polymerization was monitored as previously described. Once the polymerization was complete, a separate program was started which measures the ratio of transferred material to theoretically transferred material or the transfer ratio (RT) at a constant surface pressure. Transfer was performed by the vertical deposition method with the dipping speed 10 mm/min. All measured values of RT were equal to 1.0 ( 0.1, indicating the monolayer was successfully transferred to the substrate. Once transfer was complete, samples were dried in a desiccator for approximately 4 h. Samples were then mounted onto stainless steel sample pucks and placed onto a Digital Instruments Nanoscope III TMAFM multimode head equipped with a 100 µm piezoelectric scanner. Details of the instrument as well as scanning tip and standard operating conditions are reported elsewhere.22 The scanner was calibrated before the experiments with a 5 µm gold calibration grid, and scans were obtained at 0° and 90° scanning directions to ensure reproducibility; z calibration was performed by the factory during the study. Isotherm and AFM data were plotted and analyzed with the Origin software package.
Results and Discussion Monolayer Characterization. Isotherms of 2PDA/ 10PPB2 mixtures at 25.0 °C can be seen in Figure 2. (20) Kloeppner, L. J.; Duran, R. S. Polym. Prepr. 1997, 38 (2), 684. (21) Kloeppner, L. J.; Duran, R. S. Langmuir, in press. (22) Kajiyama, T.; Tanaka, K.; Ge, S.-R.; Takahara, A. Prog. Surf. Sci. 1996, 52 (1), 1.
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Figure 2. Isotherms of 2PDA/10PPB2 mixtures at 25.0 °C and a 9.5 Å2/(molecule‚min) compression rate. Each isotherm is a mean isotherm of five experiments.
Monolayer characterization of 2PDA and 10PPB2 was performed and is in agreement with previous publications.23,24 A notable feature of the 10PPB2 isotherm is the appearance of four discernible transitions. The first transition observed is the onset at A0 ) 38.5 Å2/molecule. Phase transitions are seen at A1 ) 33.8 Å2/molecule and A2 ) 28.5 Å2/molecule, and collapse of the monolayer is observed at Ac ) 24.5 Å2/molecule; the acidity of the subphase does not seem to significantly influence the monolayer isotherm. The surface pressures of the above transitions are π1 ) 8 mN/m and π2 ) 15 mN/m, respectively. The collapse of the film is seen at πc ) 38 mN/m. The pure 2PDA isotherm is relatively featureless, indicating a fluid monolayer with a collapse pressure πc at approximately 40 mN/m. Equilibrium spreading pressures (ESPs) of the pure components were determined by spreading an excess of surfactant solution onto the subphase at constant A. After a certain amount of the surfactant is deposited, the surface pressure rises until the ESP is reached, and any additional amount of surfactant is present in a 3D form at or near the surface and results in a constant or steadily decreasing surface pressure. ESP values of the 2PDA monomer and 10PPB2 liquid crystal have not been previously reported and were found to be 35.0 and 14.6 ( 0.5 mN/m, respectively. 2PDA collapses relatively close to its ESP while 10PPB2 can be compressed to considerably higher pressures. Large differences between ESP and collapse pressure have been observed for materials that are crystalline in the bulk. For example, the ESP of stearic acid is 5 mN/m, but rapid compression results in a collapse pressure of 40 mN/m or more.25 The bulk 10PPB2 liquid crystal forms small platelike crystals at room temperature, and its first mesophasic transition temperature is 94 °C. The ESP of 10PPB2 approximately corresponds to the second transition (A2) shown in Figure 2. Previous Brewster angle microscopy studies26 have reported that the 10PPB2 monolayer rigidifies and then apparently forms 2D crystals upon isobaric annealing. It is likely that the ESP experiments represent the pressure of the most condensed fluid monolayer which can be formed, as an excess of bulk 3D liquid crystal is disordered by the (23) Bodalia, R.; Duran, R. S. J. Am. Chem. Soc. 1993, 115, 11467. (24) Rettig, W.; Naciri, J.; Shashidhar, R.; Duran, R. S. Macromolecules 1991, 24, 6539. (25) Gaines, G. L. Insolulable Monolayers at the Liquid-Gas Interface; Interscience Publishing: New York, 1966. (26) Adams, J.; Rettig, W.; Duran, R. S. Macromolecules 1993, 26, 2871.
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Figure 3. Transition surface pressure (π) dependence of 2PDA/ 10PPB2 monolayers. Collapse pressure of 2PDA (2), collapse pressure of 10PPB2 (O), and first transition pressure of 10PPB2 (9). Dotted lines are not best fit lines but intended for an aid to the eye.
surface and comes to equilibrium with the resulting 2D monolayer. The transition observed by compressing low concentrations of liquid crystal bound to the monolayer may result in significant nonequilibrium character and more condensed metastable phases because nucleation to the 3D crystal is suppressed or hindered. Overall, these considerations allow some indication of the experimental conditions needed to form true PDLC monolayers. Polymerization above the ESP of the liquid crystal results in thicker collapsed solid phases of the liquid crystal that are no longer as highly anisotropic. Transition pressures determined from the monolayer isotherms of the pure and mixed components can be used to extract thermodynamic relationships of monolayer mixing. One argument, developed by Crisp et al.,27 is analogous to Gibb’s thermodynamic relationships dealing with ideal mixing behavior in 3D. The argument states that, for ideally miscible monolayers, the ESP of the monolayer will vary as a function of composition and that, for ideally immiscible monolayers, the ESP of the least stable component will be independent of the monolayer composition. Figure 3 shows the transition pressures plotted as a function of the mole fraction of 2PDA. This plot shows the first transition pressure of 10PPB2 (π10PPB2(1)), the 10PPB2 collapse pressure (π10PPB2(C)), and the 2PDA collapse pressure (π2PDA) for the pure monolayers and 10PPB2/2PDA mixtures. The 10PPB2 transition pressure values were independent of composition with the exception of the collapse pressure of pure 10PPB2, which suggests monolayer phase separation. The decrease of π10PPB2(C) is possibly due to the destabilization of 10PPB2 in the presence of the 2PDA monomer. The collapse pressure of 2PDA is constant until a 2PDA mole fraction of 0.4 is reached. This decrease in 2PDA collapse pressure at low mole fractions is also possible due to destabilization of the low-2PDA-concentration monolayer or solvation of residual 10PPB2 by 2PDA at high surface pressure. Additional thermodynamic information can be extracted from isotherm data.25 For ideally immiscible (or ideally miscible) monolayers, many fundamental properties are additive. In addition, the excess free energy of mixing (∆Gxs mix) is routinely calculated using the Goodrich method.28,29 However, calculations of ∆Gxs mix assume that the monolayer is stable and the compression isotherm is (27) Crisp, D. J. Surface Chemistry Supplemental Research; Butterworth: London, 1949. (28) Goodrich, F. C. Proc. Int. Congr. Surf. Act., 2nd 1957, 1, 85.
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a
b Figure 5. Excess free energy of mixing for the 2PDA monomer/ 10PPB2 mixed monolayer system at 25.0 °C.
Figure 4. (a) Excess mean molecular area and (b) excess compressibility for the 10PPB2/2PDA mixed monolayer system at 10 and 5 m/m.
reversible. Isobaric stability measurements on the pure monolayers of 10PPB2 and 2PDA revealed each of these monolayers slowly densify over long periods of time. BAM studies of 10PPB2 suggest this area change is most probably due to ordering transitions caused by annealing of the monolayer. Kinetic studies of the 2PDA monolayer suggest very slow polymerization of the monomer.21 These systems are technically irreversible, and a quantitative calculation of ∆Gxs mix cannot be justified. However, each isotherm is reversible (below the plateau for 10PPB2) when compressed at rapid rates. Therefore, ∆Gxs mix estimations together with qualitative additive property comparisons of 10PPB2/2PDA monolayer mixtures are possible. The ∆Gxs mix value is the difference between weighted sums of pure component isotherms and those of the mixed isotherm; any substantially low value due to isobaric creep of the mixed free energy term should be compensated for by a correspondingly low value of the weighted, pure free energy terms. Figure 4 shows the additive property comparison of the excess mean molecular area (∆Axs mix) and the compressibility (C ) -(1/A)(∂A/∂π)). The plot of ∆Axs mix as a function of composition (Figure 4a) shows ideal behavior at higher surface pressures and a positive deviation from ideal behavior at lower surface pressures. This implies the two phases repel due to line tension and electrostatic effects. The magnitude of the deviation is proportional to the line energy and the total length associated with the domain edges and thus the size of the domains. For a totally phase-separated system, deviations from ideal behavior can only come from interfacial interactions between the two phases. This line of contact is minimized (29) Bacon, K. J.; Barnes, G. T. J. Colloid Interface Sci., 1978, 67 (1), 70.
by domain growth via coalescence or coarsening processes (Ostwald ripening). The larger the deviation from additive behavior, the smaller the domains should be. The excess compressibility plot (Figure 4b) supports the preceding argument. The excess free energy of mixing was calculated for the 2PDA monomer/liquid crystal system (Figure 5). Thermodynamic arguments state that ∆Gmix must be negative for the mixing process and that positive values of ∆Gxs mix suggest at least partial demixing of the components.29 Although the uncertainty in these measurements is large, a summary of the additive properties and ∆Gxs mix data indicates these films exist as phase-separated systems, trapped in a metastable state due to their monolayer thickness and the increased viscosity of the forming polymeric continuous phase. Overall, the isotherms suggest simple phase separation, though the surface orientation may introduce some emulsion-like character. The final morphology of the monolayer is driven by the surface energy retained in the system after the evaporation of the spreading solvent. Yet during formation, diffusion and hydrodynamic flow, controlled by concentration gradients, and the macroscopic network formation play an important role. As such, one might expect different processing variables such as the nature of the spreading solvent to affect the morphology observed. Brewster Angle Microscopy. The monomer/liquid crystal system was observed using Brewster angle microscopy. Upon spreading of the mixed solution, a coarse “grainy” appearance was noticed which is indicative of differing morphologies at the interface, whereas a homogeneous monolayer would show no features. The characteristic feature size was near the resolution limit of the microscope and thus is not shown. The features were seen at surface areas far larger than the onset area, and their texture did not change noticeably upon compression. Thus phase separation occurs immediately upon spreading of the film. Such 2D processes are not influenced by gravity and sedimentation and thus compliment work on PDLCs formed under microgravity conditions.30 Polymerization Kinetics. Mixed monolayers were then spread onto an oxidative subphase to determine if the 2PDA monomer would polymerize in the presence of 10PPB2. Figure 6 shows the polymerization rate of the 2PDA/10PPB2 mixture at different compositions. The isobaric creep of 10PPB2 followed first-order exponential decay and, after surface area normalization, was subtracted from each experiment. (30) Jin, J. M.; Parbhakar, K.; Dao, L. H. Langmuir 1996, 12, 2096.
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Figure 6. Polymerization rates (corrected for isobaric creep of the 10PPB2) of 2PDA/10PPB2 mixtures at 25.0 ( 0.2 °C, 10.00 ( 0.05 mN/M, and 10.0 ( 0.5 Å2/(molecule‚min) initial compression speed.
Figure 6 shows that for the 80%, 60%, and 40% mole fractions of 10PBB2 the polymerization rate is simply the corresponding fraction of the pure 2PDA polymerization rate. The polymerization at 20% 2PDA did not follow the trend; however, at such low monomer concentrations it is difficult to follow the polymerization rate. In each case, polymer was obtained and analyzed by GPC and yielded molecular weights similar to that of the pure polymer formed under the same conditions. This is expected for a phase-separated system in which the liquid crystal has no influence on the polymerization process. The overall result is thus a polymer-dispersed liquid crystal monolayer that has been formed by a solution-induced phase separation (SIPS) of the monomer and the liquid crystal. Atomic Force Microscopy. Poly(2PDA)/10PPB2 monolayers were transferred onto freshly cleaved mica substrates. Sections of the monolayer mixtures on mica were cut from the sample to be imaged with TMAFM. This allowed for the procurement of AFM data from the section without the actual destruction of the entire sample. Sections of these samples were saved under ambient conditions for future stability studies. Figure 7 is a 3D surface plot of a 60% poly(2PDA)/40% 10PPB2 mixture. This figure clearly shows the phase separation of the two components into circular microdomains with diameters ranging from 2 to 5 µm distributed within a continuous phase. As they occupy 40% of the transferred film surface, it is believed that the domains are composed of the liquid crystal, surrounded by a polymer continuous phase. The liquid crystal domains had a thickness of approximately 2 nm beyond the surface of the polymer. While the polymer thickness cannot easily be determined directly from the TMAFM measurements, calculated curve fits determined from surface plasmon data for pure 2PDA and 10PPB2 LBK films on gold indicate monolayer thicknesses of 1.4 and 2.2 nm, respectively.31 These values are reasonable in light of the fact that the maximum polymer layer thickness (assuming no tilt, and excluding possible counterions) is estimated at approximately 20-24 Å and that of the liquid crystal is approximately 31-37 Å. This suggests the polymer has a relatively high tilt angle, while the pure liquid crystal monolayer domain is in a less tilted state. Figures 7 and 8 show examples of the polymer/liquid crystal morphology that is “frozen” in a nonequilibrium (31) Johannsmann, D.; Herod, T.; Wu, J.; Forstmann, G.; Weiler, C.; Duran, R. S. Submitted to Supramol. Sci.
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state due to the high viscosity of the growing polymer phase formed and the low diffusivity of the liquid crystal molecules through this phase. Height mode images, like Figure 7, allow a topological survey of the monolayer domains. Phase contrast mode AFM (PCAFM), shown in Figure 8, supports the height mode data. In PCAFM, differences in the surface properties, such as elastic modulus or frictional coefficients, are dependent on the type of material that is scanned.32,33 These properties influence the amplitude and phase of the cantilever oscillation; therefore, PCAFM should be able to image phase-separated monolayers by the influences each material has on the phase of the resonant conditions of the cantilever. Figure 8 shows a TMAFM image of 60% polymer/40% liquid crystal in height and PCAFM modes. The relative cantilever deflection is similar for both materials, but the tip interactions at the polymer/liquid crystal domain interface improve, resolving domain boundaries, and thus diameters are more accurately measured. The scanning tip interactions with the liquid crystal and polymer are expected to be significantly different due to elasticity differences between the two materials and thus allow for confirmation that two separate material phases are present. It is believed that these figures represent the first imaging of a polymer-dispersed liquid crystal monolayer (PDLCM). This system looks analogous to an emulsiontype preparation of a typical PDLC. The difference in this case is that the monomer/polymer itself is acting as the surfactant and drying of an aqueous polymer solution does not occur in the same manner. This effect is useful, as residual water left in 3D emulsion-type PDLCs tends to reduce the holding ratio and impair long-term aging.5 In addition, the matrix polymer phase is formed and studied, in situ, at the air/subphase interface. As a consequence, the study of these types of PDLCMs may lead to practical applications and the improvement of 3D PDLCs. We suppose the monomer/liquid crystal mixture exhibiting a certain phase behavior and morphology is compressed to a pressure where the subsequent polymerization further freezes in the domain structure. In addition, the liquid crystal domains are interesting in their highly anisotropic form, microns wide and nanometers thick with a defined direction of molecular orientation. As such, they may have interest as model 2D particles of controllable shape. A recent publication by Frommer and co-workers reports hexagonal ordered, disk-shaped domains of a long chain hydrocarbon dispersed within a perfluoranoic acid phase.34 In that study, the circular domains of the hydrocarbon within the fluorocarbon were imaged by frictional force microscopy, where instead of normal forces on the cantilever tip, lateral tip interactions delineated the phases.35,36 Interestingly, the domain morphology is similar in both cases, even though, in the present study, the continuous phase is polymerized after spreading of the monolayer film. The current study is also the first time the analogy to a 2D PDLCM is made with a bulk system. (32) Kajiyama, T.; Ohki, I.; Tanaka, K.; Ge, S.-R.; Yoon, J.-S.; Takahara, A. Macromolecules 1994, 27, 7932. (33) Kajiyama, T.; Ohki, I.; Tanaka, K.; Ge, J.-S.; Takahara, A. Proc. Jpn. Acad. 1995, 71B, 75. (34) Xiao, S. J.; Lu, Z. H.; Yang, X. M.; Wei, Y.; Tai, Z. H. Surf. Sci. 1994, 316, L1110. (35) Frommer, J.; Overney, R. M.; Meyer, E.; Gu¨ntherodt, H. J.; Fujihira, M.; Takano, H.; Gotoh, Y. Langmuir 1994, 10, 1281. (36) Frommer, J.; Overney, R. M.; Meyer, E.; Gu¨ntherodt, H. J.; Fujihira, M.; Takano, H.; Gotoh, Y.; Brodbeck, D.; Lu¨thi, R.; Howald, L. Nature 1992, 359 (10), 133.
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Figure 7. Tapping mode AFM surface plot of the 60% poly(2PDA)/40% 10PPB2 mixture.
Figure 8. Height (left) and force modulation mode (right) images of the 60% 2PDA/40% 10PPB2 monolayer.
Precise control and manipulation of the size, density, and shape of the liquid crystal domains would allow PDLC
manufacturers to tailor the electro-optical properties of these composites.5 The domain properties are dependent
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Table 1. Domain Sample Statistics for the 58% poly(2PDA)/42% 10PPB2 Monolayer domain parameter µm2)
domain density (2500 d h (µm) from cumulative plot σd (µm) from cumulative plot d h (µm) from Gaussian fit σd (µm) from Gaussian fit % LC content
intersample value
intersample error
intrasample value
intrasample error
140 2.98 0.83 3.19 0.75 43.1
(16 (0.16 (0.04 (0.24 (0.10 (2.0
141 2.98 0.85 3.13 0.70 42.1
(10 (0.14 (0.05 (0.15 (0.16 (1.1
on the conditions of the composite formation, so it is not unreasonable to assume that manipulation of the monolayer conditions might lead to varying domain morphologies. There are different methods and procedures that can be used to describe the droplet size distribution in PDLCs. One approach is to introduce the concept of a spread parameter (S), where D90 and D10 are defined as droplet diameters that satisfy the condition
S ) (D90 - D10)/(D90 + D10) that 10% of the total liquid crystalline volume (or area in the two-dimensional case) is contained in droplets of diameters e D10 and 10% is contained in droplets of diameters g D90. By definition, 0 e S e 1, and D10 and D90 are determined from the true area histograms.37 The droplet size is considered to be polydisperse if S ≈ 2/3. For example, studies on an epoxy/7-cyanobiphenyl system report a monodisperse PDLC (S ) 0.372) by curing the composite at 80 °C and a polydisperse PDLC (S ) 0.823) by curing at 30 °C. The spread parameter for the 40% 10PPB2/60% poly(2PDA) system was determined by sampling a 50 × 50 µm2 TMAFM image with the total liquid crystalline area trapped within the droplets (ALC) determined to be ≈800 µm2 and the critical diameters D10 ) 2.33 µm and D90 ) 4.77 µm. S was calculated to be approximately equal to 1/3, which suggests that the domain size distribution for this system is relatively monodisperse compared to that for conventional PDLCs. Particle size analysis of PDLCs often involves determining a mean diameter (d) and the standard deviation of the distribution (σd), which can describe the polydispersity of the particles. Typically, the distribution is determined, as in the previous paragraph, by volumetric (or in this case, from the domain area) histograms. The mean diameter and the standard deviation of the distribution can be defined in a cumulative plot in which the points represent the amount of particulate material that is contributed by particles below a specific size. This gives the cumulative plot a continually rising character.38,39 The mean diameter is determined from the diameter on the cumulative plot at 50%, and σd is determined from the slope. Histograms indicate that the particle size distributions also follow Gaussian statistics; thus, values of d and σd can be determined by statistical methods. Figure 9 shows a histogram and a cumulative plot of the particle size diameters for the 42% 10PPB2/58% poly(2PDA) monolayer system. Step sizes were set at 0.25 µm, which is representative of the uncertainty in diameter values. Values of d and σd will be tabulated with respect to different monolayer conditions in the following sections after a brief discussion on error analysis. (37) Vaz, N. A.; Smith, G. W.; VanSteenkiste, T. H.; Montgomery, G. P., Jr. SPIE Liquid-Cryst. Devices Mater. 1991, 1455, 110. (38) Jelı´nek, Z. Particle Size Analysis; John Wiley and Sons: New York, 1970. (39) Stockham, J. D.; Fochtman, E. G. Particle Size Analysis; Ann Arbor Science; Ann Arbor, MI, 1977.
Figure 9. Representative domain distribution histogram and cumulative plot of the 58% poly(2PDA)/42% 10PPB2 monolayer taken from TMAFM images. The line represents a Gaussian fit to the histogram data.
Domain shapes observed in this work were generally circular, which allowed descriptions of the total liquid crystalline area, domain density (domains per 2500 µm2), average diameter, and size distributions to be obtained. The statistical error limits were determined for these values, between samples and between different areas within one sample, by sampling five different TMAFM scans. Table 1 summarizes these results and establishes acceptable error limits. The data in this table show a slightly higher statistical error between samples than scans taken on different areas of the same sample, which is to be expected. The cumulative method yielded error values slightly better than those of the Gaussian method and therefore was employed. TMAFM images were obtained for each mixture composition and are shown in Figure 10. These data support the interpretation that the domains consist of the liquid crystal phase at all compositions in that the number of domains (or domain density) increases with increasing mole fraction of 10PPB2. At a composition of 80% liquid crystal, a number of large domains were observed, likely resulting from coalescence of smaller liquid crystalline domains. Coalescence of liquid crystal domains at high polymer concentrations would be a signature of late stage nucleation and growth, where larger domains are formed at the expense of the smaller ones.40-42 No domains trapped in the middle of a coalescence event, or cocontinuous morphologies, were observed though. This would imply that if coalescence occurred, it would have been during spreading or compression and before polymerization increased the viscosity of the aniline phase. The domain parameter values of the concentration study are shown in Table 2. Experiments were performed to determine the concentration dependence on the domain (40) Furukawa, H. Phys. Rev. A 1985, 31 (2), 1103. (41) Marand, H.; Snyder, C. R. Macromolecules 1996, 29, 7508. (42) Haas, C. K.; Torkelson, J. M. Phys. Rev. E 1997, 55 (3), 3191.
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Figure 10. TMAFM images of (a) 80% poly(2PDA)/20% 10PPB2, (b) 60% poly(2PDA)/40% 10PPB2, (c) 40% poly(2PDA)/20% 10PPB2, and (d) 20% poly(2PDA)/40% 10PPB2. The height reference is 3 nm. Table 2. Compositional Study of Domain Sample Statistics for 10PPB2/poly(2PDA) Determined by Cumulative Analysis of Domain Diameters Observed in TMAFM Images mole fraction domain density 2PDA (2500 µm2) 0.2 0.40 0.60 0.80
180 169 119 29
d h (µm)
σd (µm)
% LC content (TMAFM)
3.49 3.10 3.09 3.96
1.57 1.02 0.81 0.89
82.7 58.5 38.9 15.4
parameters. The two main sources of error in the percent liquid crystal content values are the uncertainty in the actual domain diameter, measured directly from the image, and the error relating to the finite number of domains captured within the image. At high liquid crystalline content, the error in the actual population of domains is small, but that due to diameter uncertainty is additive and increases with increasing population. The
uncertainty in the diameter was of order 0.25 µm, and for 180 domains, this corresponds to a 5% concentration uncertainty. For low liquid crystal concentrations, the error due to diameter uncertainty is small, but that due to the uncertainty in the population is larger, which is due to the greater freedom the domains have to diffuse over the sample area. For example, two domains of 4 µm average diameter, over a sample area of 2500 µm2, result in a concentration uncertainty of 1.6%. As expected, the domain parameters are affected by the composition of the film. It was thought that the domain density might be linear with respect to composition. However, this is not the case and the relationship was found to fit a second-order polynomial with the domain density rapidly decreasing as the amount of 10PPB2 decreases. Before a direct correlation of the domain density to composition can be made, the amount of domain coalescence must be determined. It is thought that domain
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Figure 11. Fifty and ten micrometer scans of the poly(2PDA)/10PPB2 sample polymerized at 12 °C.
coalescence occurs at all compositions, though at different rates, likely related to their collision frequency. Additional factors, such as domain diffusion and line tension effects, make the determination of these trends difficult. A clear trend in d could not readily be determined, though d is larger at the more dilute mole fractions of each component, which may be explained in the case of high liquid crystalline content. Again, domain coalescence acts to increase d at the expense of smaller domains. At low liquid crystalline content, an explanation is less obvious. One would think that d would be approximately the same at the other compositions; however, this is clearly not the case. A trend is seen in the values of σd with respect to composition. These values decrease as the 10PPB2 content is decreased, with the exception of the 20% 10PPB2 film. At this composition the number of domains may not give an accurate indication of the actual polydispersity due to the low sample population. Finally, the actual 10PPB2 percentages of the domains as determined by the histograms are within the errors determined earlier, with the exception of the 20% 10PPB2 composition. It is believed that this deviation is due to the small sample population and a sample that is on the small side of the statistical mean of the data. No other scans at this composition were studied. It has been shown that PDLCs in 3D usually exhibit upper critical solution temperature (UCST) behavior;43 therefore, variations in the temperature might lead to different morphologies. Samples were prepared at different polymerization temperatures. It has been previously established that as the temperature is increased, the polymerization rate increases. The temperature range for this analysis is somewhat limited, as the LB technique (43) Chyla, A.; Kucharski, S.; Janik, R.; Sworakowski, J.; Bienkowski, M.; Thin Solid Films 1996, 284-285, 496.
Table 3. Temperature Study of Domain Sample Statistics for the poly(2PDA)/10PPB2 Monolayer temp (°C)
domain density (2500 µm2)
d h (µm)
σd (µm)
18.0 25.0 32.0
155 140 124
2.78 2.98 2.97
0.803 0.830 0.890
is restricted to the temperature range of approximately 0-50 °C. Samples were prepared at 41, 32, 25, 18, and 12 °C. These experiments revealed no clear trends regarding the density, size, or polydispersity with temperature, as shown in Table 3. These results do indicate that processes analogous to TIPS due to local cooling from solvent evaporation are likely not important in the domain formation. However, at 12 °C the domain geometry was substantially altered, as shown in Figure 11. This image shows much smaller, irregularly shaped domains. Lower temperature allows for the cohesive forces between the liquid crystal molecules to dominate, resulting in a crystalline 10PPB2 monolayer state, which apparently fractures to form the shardlike domains. We suppose this irregular geometry is frozen in place and relaxation of the liquid crystalline domains to a lower energy geometry is hindered by the high viscosity of the polymer matrix after transfer. Polymerization of the monomer at 41 °C resulted in a domain morphology typical of the monomer/liquid crystal system, where liquid crystalline domains are not held within the matrix as rigidly and tend to relax and mechanically deform during the transfer process. This is likely due to overoxidation of the polymer, and although phase separation was observed, the domains were elliptical and irregular. It was hoped that an UCST point could be observed and the films could be monitored above this temperature; however, phase separation was still observed at high mole fractions of 2PDA and high temperatures, which indicates the critical point for this system is beyond the temperature range easily accessible.
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Figure 12. Image of the 2PDA/10PPB2 film at 0 min polymerization time. Table 4. Polymerization Time Study of Domain Sample Statistics for the poly(2PDA)/10PPB2 Monolayer polymerization time (min)
domain density (2500 µm2)
d h (µm)
σd (µm)
20 40 60 240
95 106 115 122
3.27 3.26 3.22 3.09
0.778 0.890 0.850 0.830
The polymerization times of the samples were varied to observe the influences on the domain behavior. For comparison, the polymerization of pure 2PDA is essentially complete after 40-50 min. Samples were prepared at the times 0, 2, 5, 20, 40, 60, 240, and 1200 min. The 0 min sample was prepared by calculating the amount of solution needed to produce a film at 10 mN/m with no barrier compression. The film was spread, solvent was allowed to evaporate for 1 min, and then the film was transferred. For samples prepared between 20 and 240 min reaction times, a clear domain parameter dependence on the reaction time was not detected (Table 4). A slightly increasing and linear dependence of the domain density on the polymerization time was observed, though this trend was within the error of the measurement. Samples prepared at 2, 5, and 1200 min looked remarkably similar to the monomer/liquid crystal system in that irregularly shaped elliptical domains were observed, and this supports the earlier conclusion that samples polymerized at high temperatures are either monomer-like, or fluid-like overoxidized polymer degradation products. Figure 12 shows the TMAFM image of the mixture transferred at the 0 min polymerization time. Deformation along the transfer direction is seen, as in the other low reaction time samples, but surprisingly domains in this film are an order of magnitude smaller than domains observed previously. The smaller domain size may be
due to the spreading processes. The monolayer of surfactant mixture was formed by spreading the solution until the set surface pressure was reached rather than spreading and subsequent compression. The spreading solution is a true dilute solution with the surfactant molecules showing little interaction between each other, not an emulsion, as the solution is extremely clear and both components are readily dissolved into the organic solvent. As the mixed surfactant solution is spread at the interface, phase separation occurs when solvent evaporates, both components orient and adsorb to the water, and domains disperse across the interface. When spreading continues to higher initial surfactant surface concentration so that the surface pressure becomes consequential, any additional drop of solution spread at the interface can result in rapid resolvation of the existing monolayer. Subsequent solvent evaporation results in a higher energy film with smaller domains. If this is the case, the size of the PDLCM domains is likely due to a process more analogous to a solution-induced phase separation process than that of an emulsion-type PDLC5 even though the molecular orientation has some emulsion-like character. It is also interesting to note that no examples of polymer domains dispersed in a continuous liquid crystalline matrix were observed, likely due to viscosity and line tension between the LC and the monomer. It would be interesting to find conditions where monodisperse conducting polymer “nanodiscs” could be formed and their quantum confinement properties systematically investigated. To further probe spreading effects on morphology, the monomer and liquid crystal were spread onto the interface from separate solutions to determine if this had an effect on the domain morphology. In this case the monomer was slowly spread onto the interface, followed by the spreading of 10PPB2. The polymerization was initiated,
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Herod and Duran Table 6. Compression Rate Study of Domain Sample Statistics for the poly(2PDA)/10PPB2 Monolayer
Figure 13. Nanometer-sized domains in a polymer/10PPB2 monolayer spread from separate monomer and liquid crystal solutions. Table 5. Surface Pressure Study of Domain Sample Statistics for the poly(2PDA)/10PPB2 Monolayer surface pressure (mN/m)
domain density (2500 µm2)
d h (µm)
σd (µm)
2 5 10 15 20 30
55 79 140 116 210 206
3.32 2.98 2.98 3.36 2.48 1.91
0.950 0.930 0.830 1.06 0.790 0.510
and the normal kinetics of the polymerization for that composition were observed. The AFM image of that sample is shown in Figure 13 and reveals a phaseseparated PDLCM, but with nanometer-sized domains. It is interesting to note that, by estimating from the mean molecular area, the number of 10PPB2 molecules in even the largest domain is on the order of 103. The size of the domains in this PDLCM allows the possibility of quantum confinement of the liquid crystal domains. At this small size, quantum effects should dominate over the macroscopic properties of the domains and could allow for the study of the quantum effects in liquid crystal domains. In addition, the scattering properties of the composite material would be substantially different than those of the larger PDLC materials. By using this type of domain morphology, it might be possible to construct electro-optic films based on the quantum confinement of these very small liquid crystalline domains. Samples prepared in this way were found to be reproducible when the monolayer and spreading conditions are kept constant. Domain parameters from experiments performed to determine the effect of polymerization surface pressure are summarized in Table 5. Polymerization of samples was limited to surface pressures of 2 mN/m and above, as experiments below this surface pressure would result in extremely long polymerization times and problems with
compression rate (Å2/(molecule‚min))
domain density (2500 µm2)
d h (µm)
σd (µm)
5 10 20 100
95 106 115 122
3.27 3.26 3.22 3.09
0.778 0.890 0.850 0.830
the signal-to-noise ratio. It must be noted that samples prepared above 20 mN/m are above the critical transition pressure and may not result in monolayer formation. Statistics performed on these samples do not show a clear pressure dependence on the average domain diameter for the samples polymerized at 15 mN/m and below; however, above this pressure the domain size decreases rapidly, likely due to the collapse of the liquid crystal phase. The domain density and polydispersity do show a dependence on the surface pressure below the collapse pressure of the liquid crystal, which is likely a reflection of the compressibility of the films. The compression rate dependence on the domain morphology was also investigated. It has been shown that the compressibility and isotherm behavior of the 2PDA monomer are independent of the compression rate under these conditions, whereas the behavior of 10PPB2 is relatively dependent on the compression rate. Samples were prepared at the compression rates 100, 20, 10, 5, and 0.5 Å2/(molecule‚min) to determine if there was any correlation between the domain morphology and compression rate. As shown in Table 6, results of these experiments indicate no clear trend regarding the domain size, density, or distribution for the compression rates 5 Å2/(molecule‚min) and above. An interesting result at the compression rate 0.5 Å2/(molecule‚min) was observed. Figure 14 shows the composite film formed at this slow compression rate. Long, twisting domains of 10PPB2 were seen dispersed within the polymer matrix. Analysis of these images suggests that the 10PPB2 domains exhibit a helical nature, as repeatable twisting is seen in each domain. The twisted structure may be due to the chiral nature of this ferroelectric liquid crystal. There are reports of other unique noncircular shapes being formed in the coexistence region due to the chiral nature of some Langmuir films.44,45 There may also be some macroscopic orientation due the compression and/or transfer processes, but this was not studied in detail. Overall, this morphology is a surprising result, in that all other studies on this mixture indicate that domains form at the advent of the solvent evaporation after the initial spreading. For a possible explanation, one must look at the processes involved in the polymerization. At very slow compression rates, the polymerization rate cannot be fit to simple pseudo-first-order kinetics. The alkylaniline polymerization rate is dependent upon the surface pressure. At high compression rates the final surface pressure is reached before the polymerization goes to significant completion. However, at very slow compression rates, the polymerization begins before the final polymerization surface pressure is obtained, and therefore the result is a constantly changing rate constant. It has been theorized that the circular domains are formed at the initial spreading of the film, but this is a nonequilibrium condition with the 10PPB2 molecules arranged in some high-energy conformation. The initially slow and constantly changing rate of the polymerization could possibly allow for a (44) McConnell, H. M.; Lee, K. Y. J. Phys. Chem. 1993, 94, 9532. (45) Ho¨nig, D.; Mo¨bius, D. J. Phys. Chem. 1991, 95, 4590.
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Figure 14. Poly(2PDA)/10PPB2 monolayer formed at a very slow compression rate: (a) 50 µm and (b) 10 µm scans.
transition in the domains to a lower energy liquid crystalline morphology. However, a more likely explanation is the 2D crystallization of 10PPB2 during the long
polymerization of the matrix. It is believed that this is the first report of this type of 2D morphology in a LB monolayer at such lateral dimensions and, as a result,
6968 Langmuir, Vol. 14, No. 24, 1998
may be useful as a means of shape control in PDLC, or generally nanoparticle, systems. Last in the series of experiments, the pH and oxidant concentration effects on the domain behavior were studied. The acidic nature of the subphase pulls the amine group on the aniline into the subphase, forcing gauche conformations of the alkyl chains. As a result, the surface area of the ortho-substituted aniline is larger than those of the meta or para isomers. It would be reasonable to assume that by varying the subphase pH, and thus modifying the aniline conformation, different domain sizes and shapes could be formed. Although the isotherm and polymerization kinetics were indeed different, the pH and ionic strength of the subphase seemed to have no influence on the composite morphology, though the domain parameter statistics were not as rigorously determined as in the previous studies. At very low acidic concentrations, it has been documented that the monomer monolayer does not polymerize, and this is supported in the AFM images by irregular elliptical-shaped domains typical of the low-molecularweight composites. In addition, variation of the oxidant concentration did not produce an effect on the domain behavior, which supports the interpretation that the domains form during spreading. The polymerization of the monolayer freezes in the high-energy morphology and prevents diffusion, mass transfer, or rearrangement of the domains. LBK films of these monolayers were observed to be extremely stable. Samples stored at ambient conditions showed identical morphology by AFM one year after initial preparation. This demonstrates the added utility of this system, as a problem with many liquid crystal systems is their tendency to “de-wet” at the surface over time.46 It is possible that the polymer increases the stability of the liquid crystal at the interface. Instability of ultrathin films is typically caused by perturbations due to macroscopic phenomena, such as Rayleigh-Taylor and Helmholtz instabilities, that lead to excess intermolecular interactions which can locally destabilize thin domains in the film. One utility of these PDLCMs could possibly be as a surface pretreatment to give added stability to a bulk liquid crystal system. The topology of the domains and associated interfacial line tension effects that leads to added stability of the LBK film may also be used in stabilizing wetting of bulk liquid crystal overlayers. More generally, it may be possible to control the morphology near polymer surfaces in PDLC systems in useful ways by analogous notions of low-dimensional confinement of the liquid crystal domains within the viscous polymer matrix. Continuing experiments to control the polydispersity of the domains in order to obtain a monodisperse domain distribution, and to control the size of the domains, are underway. It is important to be able to control the domain size distribution in these PLDC systems. Monodisperse domains would allow transmission of only certain wavelengths, whereas a polydisperse domain size distribution would allow transmission of multiple wavelengths. Preliminary experiments seem to indicate that the closer the (46) Sharma, A. Langmuir 1993, 9, 861.
Herod and Duran
system comes to the critical point, the more monodisperse the domain size distribution becomes. It is not possible to reach this point with this system using the conventional Teflon Langmuir trough. It is interesting, however, to use this information in addressing problems related to theoretical investigations into PDLCs. In a review article published recently by Bouteiller and Le Barny summarizing the current state of PDLCs,5 the need for more theoretical models for scientists to use in designing more efficient large area displays and the need for more information about the polymer/liquid crystal interface are pointed out. The LBK technique is a useful tool that takes advantage of the low dimensionality of a system, thus simplifying complex relationships and reducing complicating variables that plague the 3D systems, such as gravity, sedimentation, and hydrodynamic flow. Conclusions Reported herein is the thermodynamic, kinetic, and phase behavior regarding the formation of a 2D PDLC monolayer system. Mixtures of the system were spread at the air/liquid interface, and observations of the isotherm and thermodynamic behavior suggest a phase-separated system. Excess free energy of mixing calculations for this system, although not quantitative, were used in conjunction with additive property analysis to extract qualitative information. Polymerization of monomer to polymer was accomplished in the presence of the liquid crystal and was characterized by GPC and polymerization kinetics. The monolayer system was successfully transferred onto mica substrates and imaged using TMAFM. The AFM images show a 2D PDLC monolayer with the surfaceactive polymer as the continuous matrix. Such studies using Langmuir monolayers result in interesting, extremely anisotropic disk-shaped liquid crystalline domains. The domain formation process showed some similarity to SIPS and emulsion-like processes used in PDLC formation. Formation of the polymer and the interfacial characteristics of the surfactants draw similarities to an emulsion-type process; however, evaporation of the solvent induces phase separation, and the sizes of the domains are dependent on the evaporation rate, as in SIPS. At very slow compression/reaction rates a unique ribbon-like morphology was observed. Generally such studies also may provide models to aid theoretical investigations into the phase dynamics and nucleation and growth processes that govern the preparation of 3D PDLCs. Acknowledgment. The authors would like to acknowledge L. Kloeppner of the University of Florida for synthesis of the 2PDA monomer and derivatives, and Drs. R. Shashidhar and J. Naciri of the Naval Research Laboratories for synthesis of the 10PPB2 and derivatives. The Office of Naval Research, the NSF NYI program (Grant DMR9357462), and the NSF ERC on Particle Science and Technology at the University of Florida are acknowledged for financial support. Micron Semiconductor, Inc., MicroCal Software, E.M. Separations, and KSV Finland are acknowledged for support. LA980167U
