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Abstract
This paper reports the electro-optical properties of high resistivity nematic liquid crystals sandwiched between ferroelectric polymer films. Interactions between liquid crystals and the film result in a series of interesting optical and electro-optical features. For example, the visualization of ferroelectric domains by means of liquid crystals has been known for decades. However, here we demonstrate that liquid crystals can also reveal the fractal dimension of multi—domain poly(vinylidene fluoride)-based films. Unidirectionally rubbed films made of poly(vinylidene fluoride)-based (PVDF) materials align liquid crystals (LC) homogeneously, with the pretilt angle on the order of 1–2 degrees. This property was implemented in the design of hybrid cells composed of liquid crystals sandwiched between PVDF-based films. The designed PVDF|LC|PVDF cells exhibit tunable electro-optical performance originating from the presence of the PVDF-based films. More specifically, (i) the threshold voltage characterizing the transition of liquid crystals from a planar to a homeotropic state can be tuned by varying the film thickness, and (ii) total fall time (turn-off time) can be controlled by varying the frequency and amplitude of the driving voltage. This frequency dependence of the fall time is strongly pronounced at a relatively high voltage applied across the cell. In the low frequency regime, an increase in the turn-off time can be approximated as a linear function of the applied electric field. An electric-field induced polarization of the PVDF-based films is considered a major reason leading to the afore-mentioned amplitude and frequency dependence of the switching time.
Highlights
Studies of the behaviour of liquid crystals on ferroelectric substrates date back to the 1970s.1–3. These very rst papers reported the visualization of ferroelectric domains of triglycine sulfate (TGS) crystals by observing a thin layer of nematic liquid crystals sandwiched between the cleavage surface of the ferroelectric crystal and a cover glass.[1,2,3]
The combination of nematic liquid crystals and ferroelectric polymer lms results in new features of optical and electrooptical behaviour observed in such materials
Paper visualization of domains in ferroelectric crystals by decorating them with liquid crystals has been known since the early 1970s
Summary
Studies of the behaviour of liquid crystals on ferroelectric substrates date back to the 1970s.1–3 These very rst papers reported the visualization of ferroelectric domains of triglycine sulfate (TGS) crystals by observing a thin layer of nematic liquid crystals sandwiched between the cleavage surface of the ferroelectric crystal and a cover glass.[1,2,3] The domain visualization is explained considering the dependence of the liquid crystal alignment on the surface tension coefficients characterizing an interface between liquid crystals and substrates.[1,2,3] Follow up efforts[4,5] were directed to study the domain structure of TGS crystals along with anchoring and alignment effects in liquid crystals on the ferroelectric surface.[6,7] The visualization of ferroelectric domains by means of liquid crystals was achieved in a variety of inorganic crystals.[8,9,12,13,14] In addition to the visualization of the domain structure, the “decoration” of a ferroelectric surface by liquid crystals can reveal the switching processes (switching dynamics) in ferroelectric crystals.[10,11,12,13,14,15,16,17,18] Another interesting application of nematic liquid crystals includes the visualization of stress elds in ferroelectrics.[19]. Studies of the behaviour of liquid crystals on ferroelectric substrates date back to the 1970s.1–3. The use of ferroelectric liquid crystals can overcome shortcomings of nematics such as slow response, high uidity, and their instability against disclinations.[18]
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