Origin of weak layer contraction in de Vries smectic liquid crystals

Area of Science:

• Materials Science
• Condensed Matter Physics
• Liquid Crystals

Background:

• Smectic-A (SmA) and smectic-C (SmC) phases are key liquid crystal phases with distinct molecular ordering.
• The de Vries model describes specific liquid crystal behaviors, but molecular conformation effects are complex.
• Understanding molecular shape and orientation is crucial for predicting liquid crystal phase properties.

Purpose of the Study:

• To investigate the structural and conformational changes of mesogens with trisiloxane end segments in SmA and SmC phases.
• To determine the orientational distribution function (ODF) and molecular tilt angles.
• To analyze the factors influencing SmC layer contraction and the nature of the SmA-SmC transition.

Main Methods:

• Structural investigations using X-ray diffraction or similar techniques.
• Analysis of molecular conformation (linear vs. bent) in different smectic phases.
• Determination of orientational order parameters (⟨P2⟩, ⟨P4⟩, ⟨P6⟩) and tilt angles for siloxane and hydrocarbon segments.

Main Results:

• Mesogens exhibit linear conformations in SmA and bent ('hockey stick') conformations in SmC phases.
• The ODF is sugarloaf-shaped, not volcano-shaped, with significant positive orientational order parameters.
• Reduced fluctuations and narrowed ODF increase effective molecular length, counteracting tilt and reducing SmC layer contraction.

Conclusions:

• The bent molecular conformation in SmC phases is a key structural feature.
• The observed ODF and orientational order parameters deviate from typical expectations.
• The interplay between molecular tilt, fluctuations, and ODF significantly impacts SmC layer spacing and transition behavior (tricritical vs. first-order).