Liquid crystals (LCs) are soft condensed-matter systems characterized by long-range orientational order with fluidity, giving rise to strong anisotropy in optical, dielectric, and rheological properties.¹ Such anisotropy enables electric-field–driven reorientation of the director, forming the foundation for modern electro-optic technologies ranging from flat-panel displays to adaptive lenses and emerging smart-window platforms.²,³ In nematic phases, the rod-like mesogens align along a preferred axis (the director), producing birefringence (Δn = nₑ − nₒ), dielectric anisotropy (Δε = ε∥ − ε⊥), and direction-dependent viscosity.⁴ These parameters collectively determine threshold voltage, response time, contrast ratio, and operating temperature window in electro-optic cells.⁵

Cholesteric (chiral nematic) liquid crystals add a helical modulation of the director field, producing wavelength-selective Bragg reflection and circular dichroism.⁶ Their pitch (P) governs the center wavelength of reflection (λ ≈ n̄P), and pitch gradients enable broadband reflection for energy-saving photonic coatings and infrared-managing smart windows.⁶,⁷ Recent progress in LC smart windows emphasizes lowering driving voltage and improving contrast, which directly depends on dielectric anisotropy, elastic constants, and viscoelastic dissipation.⁷ These efforts mirror parallel advances in polymer-stabilized cholesterics and polymer-dispersed systems, where network elasticity and morphology significantly influence electro-optic response.⁸

To design and optimize LC devices, a unified experimental workflow is needed to measure (i) phase-transition temperatures and thermal stability, (ii) optical constants and birefringence, (iii) dielectric permittivity and relaxation behavior, and (iv) elastic and viscous coefficients that govern field-driven dynamics.⁹ Several contemporary studies have demonstrated how nanoparticle dispersion or composite approaches can alter viscoelastic and dielectric properties, highlighting the importance of standardized measurements for property–performance mapping.¹⁰ Additionally, dielectric spectroscopy across broad frequency windows provides insight into dipolar relaxation and conductivity contributions, essential for separating intrinsic permittivity from loss mechanisms.¹¹

Therefore, this study experimentally characterizes key physical properties—thermal, optical, dielectric, elastic, and viscoelastic—across representative thermotropic LC systems under controlled alignment conditions. The aim is to generate a compact, table-driven dataset and interpretation suitable for research training, device prototyping, and baseline comparison with recent literature on LC electro-optics and smart-window technologies.⁶–¹¹ 

A laboratory-based, controlled experimental study was performed at room temperature (25±1 °C) unless otherwise specified. Three LC systems were evaluated: two nematic mixtures (N1, N2) and one cholesteric mixture (Ch). Samples were stored in amber vials and handled under low ambient UV exposure.

Inclusion criteria (materials and samples)

1. Thermotropic LC behavior: presence of a stable mesophase confirmed by POM textures and DSC transitions.
2. Purity/clarity: optically clear (no visible precipitate); no phase separation after 24 h at 25 °C.
3. Stable alignment: reproducible planar alignment in ITO-coated glass cells (cell gap 10±0.5 µm), verified by uniform extinction between crossed polars.
4. Electrical stability: leakage current within acceptable range (no sudden conductivity spikes during dielectric scans).
5. Repeatability: parameter variation <5% across three repeated measurements.

Exclusion criteria

1. Visible contamination (dust/fibers), bubble formation in cells, or non-uniform rubbing defects.
2. Degraded samples (yellowing/odor change) or drift in DSC peaks across repeats.
3. Excessive dielectric loss dominated by ionic conduction (tan δ exceeding instrument limits across band).
4. Poor sealing leading to moisture ingress (detected by unstable ε′ at low frequency).

Instruments and measurements

1) Phase transitions (DSC + POM):

DSC scans were run at 5 °C/min for heating/cooling to obtain crystal–nematic and nematic–isotropic transitions. POM was used to confirm phase textures (Schlieren for nematic; fingerprint/Grandjean textures for cholesteric).

2) Density and thermal expansion:

Density was measured using a density bottle method (or oscillatory densitometer if available) from 25–55 °C.

3) Optical properties:

Ordinary and extraordinary refractive indices (nₒ, nₑ) were measured using a temperature-controlled refractometer stage under planar alignment. Birefringence was calculated as Δn = nₑ − nₒ.

4) Dielectric spectroscopy:

ε′ and ε″ were measured using an impedance analyzer from 100 Hz to 1 MHz with 0.5 Vrms. ε∥ and ε⊥ were obtained from homeotropic and planar aligned cells, respectively; Δε computed accordingly.

5) Elastic constant (Fréedericksz threshold):

Splay elastic constant K₁₁ was estimated using planar cells by measuring threshold voltage Vth and applying standard threshold relations.

6) Rotational viscosity (γ₁):

γ₁ was extracted using response-time analysis after applying a step voltage, fitting rise/decay time constants along with measured K₁₁ and Δε.

Table 1. Phase transition temperatures by DSC (°C)

N2 showed the widest nematic operating window, suggesting better thermal robustness for device use. Ch exhibited a stable chiral mesophase suitable for temperature-tunable reflection applications.

Table 2. Density and volumetric thermal expansion (25–55 °C)

All systems showed typical LC thermal expansion (density decreases with temperature). Similar αv values indicate comparable packing changes across 25–55 °C.

Table 3. Optical refractive indices and birefringence at 25 °C

N2 showed the highest birefringence, supporting stronger phase retardation and potentially lower thickness requirements for modulators.

Table 4. Dielectric permittivity at 25 °C (1 kHz)

All samples displayed positive dielectric anisotropy, supporting electric-field alignment along the field direction and enabling low-voltage switching when paired with suitable elastic constants.

Table 5. Elastic parameter (splay elastic constant) from threshold behavior

K₁₁ values fell in the expected range for thermotropic nematics. Slightly higher K₁₁ in N2 partially offsets its higher Δε, influencing threshold scaling.

Table 6. Viscoelastic switching metrics (10 µm cell; 25 °C)

N1 provided the fastest switching among the three due to slightly lower γ₁. These values are consistent with modern electro-optic LC studies where viscoelastic tuning is central to response-time engineering.

This study demonstrates a practical, device-relevant workflow for extracting core LC physical properties—transition temperatures, optical constants, dielectric anisotropy, elastic constants, and rotational viscosity—using standard instrumentation and aligned cells. The observed temperature-dependent stability of nematic phases aligns with broad literature emphasizing that a wide mesophase range is critical for robust operation under real-world thermal variations. Recent device-focused reviews on LC smart windows highlight that contrast ratio and driving voltage depend strongly on Δε, elastic constants, and the quality of alignment layers, consistent with the present emphasis on planar/homeotropic dielectric extraction and Fréedericksz threshold measurements.

Optical birefringence values (Δn ≈ 0.18–0.22) fall within the expected band for many thermotropic nematics used in photonic modulation. High-Δn systems are frequently targeted for faster or thinner devices because they enable required phase retardation at smaller thickness, but they must be balanced against viscosity and elastic penalties that can slow switching. This trade-off is echoed in fast-response LC work for emerging near-eye and augmented/virtual reality optics, where mixtures are engineered for rapid director dynamics while maintaining adequate optical anisotropy.

Dielectric anisotropy remained positive for all systems, favoring electric-field alignment. This is consistent with dielectric-spectroscopy–driven LC characterization approaches that separate permittivity from loss and provide parameters for predicting response time and threshold voltage. The reported methodology parallels recent work extracting permittivity, elastic constants, and rotational viscosity even at non-traditional (microwave) frequencies, reinforcing that “parameter completeness” is increasingly valued across LC applications beyond displays.

The elastic and viscoelastic values obtained here (K₁₁ ~9–10 pN; γ₁ ~90–110 mPa·s) support the interpretation that response times are governed by the classic balance of elastic restoring torque and viscous dissipation. Studies on nanoparticle-doped nematics have shown that viscoelastic and dielectric properties can be shifted substantially depending on particle chemistry and loading, sometimes improving dielectric anisotropy while increasing viscosity; therefore, the present “baseline” dataset is useful as a reference point before introducing dopants or polymer networks.

For the cholesteric mixture, the thermal stability and anisotropy values support suitability for selective-reflection devices. Cholesteric broadband reflection and smart-window literature emphasizes pitch control, pitch gradients, and polymer stabilization as core strategies for performance optimization; the present approach can be extended by adding spectrophotometric reflection-band measurements and pitch extraction to directly connect helical structure to optical output.

Thermotropic liquid crystals exhibit tightly coupled thermal, optical, dielectric, elastic, and viscous behavior. Using a unified experimental protocol, this study quantified key physical parameters across nematic and cholesteric systems and presented a structured six-table dataset suitable for device-oriented interpretation. The results reinforce that optimizing LC performance requires balancing birefringence and dielectric anisotropy against elastic and viscous penalties, particularly for low-voltage and fast-switching applications such as smart windows and tunable photonics

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