Order and dynamics inside H-PDLC nanodroplets: an ESR spin probe study

Area of Science:

• Materials Science
• Condensed Matter Physics
• Polymer Science

Background:

• Holographic polymer-dispersed liquid crystals (H-PDLCs) utilize alternating LC and polymer layers to form diffraction gratings.
• Understanding the internal structure of LC nanodroplets is crucial for optimizing H-PDLC device performance.

Purpose of the Study:

• To investigate the order and dynamics of BL038 liquid crystal (LC) within nanosized droplets in reflection-mode H-PDLC devices.
• To determine the local director configuration and model the nanodroplet organization.
• To correlate molecular organization with H-PDLC device properties.

Main Methods:

• Electron Spin Resonance (ESR) spin probe technique across a range of temperatures (nematic to isotropic LC phases).
• Scanning Electron Microscopy (SEM) to analyze nanodroplet size and shape distribution.
• Modeling of director configuration based on experimental data.

Main Results:

• LC nanodroplets exhibit a bidimensional distribution, elongated and aligned parallel to the layers on average.
• Internal director configuration is a uniaxial quasi-monodomain aligned along the nanodroplet's long axis.
• Molecular organization is primarily governed by confinement, maintaining alignment even under external magnetic fields.

Conclusions:

• Confinement-induced molecular ordering in H-PDLC nanodroplets influences device electro-optical properties.
• This molecular organization may explain the higher switching voltages and faster turn-off times observed in H-PDLCs compared to standard PDLCs.