Phase transition from focal conic to cubic smectic blue phase in partially fluorinated cyano-phenyl alkyl benzoate ester doped with ultrahigh twisting power chiral dopant

TL;DR

This study uncovers a novel phase transition from focal conic smectic to blue phase in a fluorinated liquid crystal doped with a high-twisting chiral agent, highlighting electric and thermal control of domain structures for device applications.

Contribution

It reports a new phase transition mechanism and the influence of cooling rate on domain size in a fluorinated liquid crystal doped with ultrahigh twisting power chiral dopant.

Findings

Focal conic domains can grow several times with slower cooling rates.

Blue phase exhibits unique curved platelet textures different from conventional cholesteric blue phases.

Domain sizes are tunable via electric field and thermal conditions.

Abstract

Blue phase liquid crystal (BPLC) has important applications in adaptive lenses and phase modulators due to its polarization-independent property. During our efforts for development of the new materials, we found a novel phenomenology of phase transition, from focal conic smectic to smectic blue phase in a partially fluorinated cyanophenyl alkyl benzoate ester based nematic liquid crystal (LCM-5773) doped by ultra-high twisting power [H.T.P~160 um^-1] chiral dopant (R5011/3 wt%). Polarized optical microscopy (POM) investigations revealed focal conic and fan-shaped textures typical for columnar mesophases. These focal conic domains (FCDs) are squeezed under electric field and finally at a critical electric field they undergo a dark state. When the electric field is withdrawn, the FCDs are regrown in a one dimensional array with smaller domain size. Interestingly, we have observed the domain size of the FCDs can grow several times by decreasing the cooling rate (0.02 degrees(C)/min.) ten times without any change in the phase sequence. In blue phase (BP), we have observed curved platelet texture and grain boundaries filled by small platelets, which is completely different from conventional cholesteric BP. The blue phase platelet size (PLS) also increases significantly at low cooling rates. The thermal control of FCD and PLS size has increasing demand for the construction of devices with optimal performances.