Fast-switching electrooptical films
Liquid-crystal-based electrooptical film (EOF) is a composite material composed of liquid crystals encapsulated in a polymer framework.1 Applying an external electric field to the device induces a change in the refractive index of the liquid crystal and subsequently results in either a match or mismatch with that of the polymer component. In a mismatch situation light is scattered at the crystal/polymer interface and the device appears opaque. The change in refractive index—and thus of the transparency—is a function of the voltage applied. This way the EOF can be switched between light-transparent and opaque states simply by applying the right electric current.2 This makes EOFs potentially attractive for use in applications such as spatial light shutters, switchable privacy windows, temperature sensors, and flexible displays. Improvements to EOF design will facilitate their practical application. In particular, a polymer-encapsulated liquid-crystal film is needed that is fast switching in response time.
The EOF's switching response depends on the morphology, size, and size distribution of the liquid-crystal capsules. To examine the effects of composition on electrooptical performance, we developed an EOF comprised of a polymer-encapsulated liquid crystal doped with carbon nanotubes (CNTs). The crystal/polymer was then sandwiched between two substrates equipped with active electrodes. Three different samples were prepared using different concentrations of CNTs. We examined their dielectric, electric-optical, and morphological properties.
In general, the electric response is dominated by the film capacitance, which depends on the dielectric constants of both the liquid-crystal and polymer components. At zero voltage this capacitance depends on the liquid-crystal alignment and varies considerably depending on film composition. We applied a 100mV field to samples with a film thickness of 15μm. The real parts of the permittivity increased with increasing CNT concentration. The imaginary parts reached a maximum value at a frequency of 80kHz. The dielectric-relaxation frequency also increases with increasing CNT concentration, indicating that the average size of the liquid-crystal droplets becomes ever smaller because of the CNT dopants.
We studied the electrooptical performance of these EOFs by measuring the transmittance and response time as a function of applied voltage (see Figure 1). The transmittance increases with increasing voltage, while the threshold voltage and the steepness of the transmittance-voltage curve for CNT-containing EOFs was smaller and steeper, respectively, than that of nondoped EOFs. The transmittance often depends on the sample's history. In the experiment, not all films returned to the original scattering state after field removal. This condition is called persistence.3 The rise time is inversely proportional to the applied voltage, while the decay time is voltage independent (see Figure 2). However, both the rise and decay times of the CNT EOFs were faster than for films without CNTs. In general, the EOFs containing CNTs can be switched from opaque to transparent within 200μs under an applied voltage of 4V/μm. This rise time was faster than for EOFs without CNTs at different gray levels. The decay time is related to the viscoelasticity of the liquid crystal. Under the same applied field the EOF with 0.01% CNTs exhibited the quickest fall time.
Improvements in both the rise and fall times for CNT-containing EOFs are attributed to increases in the anchoring energy at the liquid-crystal/polymer interface and decreases in the liquid-crystal droplet size resulting from CNT doping. Scanning-electron-microscope images of these samples show that EOFs doped with 0.01% CNT have the smallest liquid-crystal-domain sizes, which supports this rationale.
These new EOFs offer the advantages of being polarizer-free and suitable for use in flexible substrates or multiple stacked layers, and having a fast response time. EOFs can combine different types of polymers and liquid crystals and arrangements of liquid-crystal orientation. The work function of doped EOFs can be altered depending on factors such as the type of polymer matrix used (hydrophilic, hydrophoisotropic, or mesogenic), dopant composition (nanoparticle, nanotube, dye, pigment, chiral compound, thermotropic or lyotropic liquid crystal), substrate (upper or lower, transparent or opaque, black or white), and the process for display addressing (active or passive matrix, electrical or optical). A liquid-crystal display would include the display film and either one or two substrates for supporting the display film. It could be mounted onto flexible substrates. A 3D display film could consist of a stack of EOFs. We are currently working on developing azo-dye-doped polymer-dispersed liquid-crystal systems for use in photosensitive smart windows.
We thank Liou Qiu for her work on the scanning-electron-microscope study.
Lu Lu is a second-year student.
Liang-Chy Chien is a professor of the chemical physics interdisciplinary program.
Shin-Ying Lu is a fourth-year student.
