Effect of the surfactant Span80 on the electro-optical properties of polymer-dispersed liquid crystal gratings

Polymer/liquid crystal (LC) composites represent a class of advanced materials for optical modulation, leveraging externally controlled refractive index changes. Among these, polymer-dispersed liquid crystal (PDLC) gratings have emerged as a promising device, where the alignment of LC molecules can be electrically controlled to modulate the grating’s diffractive efficiency. However, conventional PDLC gratings face critical limitations, including low diffractive efficiency and high driving voltages. In this study, the surfactant, sorbitan monooleate (Span80)), was doped into PDLC gratings to investigate its concentration-dependent effects on electro-optical properties. Experimental results demonstrate that doping with 1.4 wt% Span80 significantly enhances the diffractive efficiency from 18.8% to 84.7%, while reducing the driving voltage required for maximum efficiency from 1.7 V to 1 V. Contact angle measurements of polymer composites doped with Span80 (0–2 wt%) using a surface tension analyser showed that the addition of surfactant altered the polymer interfacial anchoring properties. Polarising optical microscopy (POM) and scanning electron microscopy (SEM) observations reveal that the improved performance originates from enlarged LC droplets and reduced anchoring energy at the polymer–LC interface. These modifications resulted in smoother interfacial morphology and more complete phase separation. This work provides a practical strategy for optimising PDLC-based tunable photonic devices.聚合物/液晶（LC）复合材料是一类依托外场调控折射率变化的先进光学调制材料。其中，聚合物分散液晶（PDLC）光栅已成为极具应用前景的器件，其液晶分子的排列可通过电场调控，从而实现光栅衍射效率的调制。然而，传统PDLC光栅存在诸多关键局限，包括衍射效率偏低、驱动电压过高等问题。本研究将表面活性剂失水山梨醇单油酸酯（Span80）掺杂至PDLC光栅中，探究其浓度对电光性能的影响规律。实验结果表明，掺杂1.4 wt%的Span80可将衍射效率从18.8%显著提升至84.7%，同时将达到最大衍射效率所需的驱动电压从1.7 V降至1 V。通过表面张力分析仪对掺杂0~2 wt% Span80的聚合物复合材料开展接触角测量，结果显示该表面活性剂的加入改变了聚合物的界面锚定性能。偏光光学显微镜（POM）与扫描电子显微镜（SEM）观测结果显示，性能提升源于液晶微滴尺寸增大以及聚合物-液晶界面的锚定能降低。上述改性使得界面形貌更为平整，相分离更为完全。本研究为优化基于PDLC的可调谐光子器件提供了切实可行的策略。