Phase transitions of liquid crystal confined in electrospun polymer nanofibres

Abstract

Phase behaviour as well as phase transitions of $4'$-pentyl-4-biphenylcarbonitrile (5CB) liquid crystal (LC) confined in an amorphous polymer matrix of electrospun nanofibres were investigated. The nanofibres were fabricated from simple monoaxial electrospinning using 5CB/polymer mixtures as well as coaxial electrospinning, where the polymer solution and neat 5CB constituted the shell- and core-forming fluids, respectively. The 5CB was found to be miscible with polystyrene (PS) and poly(4-vinyl pyridine) (P4VP). This was evident from the sharp drop in the glass transition temperature ($T_{\mathrm{g}}$) of PS and P4VP in their mixtures with 5CB. Hence, the phase transitions of 5CB were completely suppressed in its mixture with PS and P4VP in electrospun nanofibres as ascertained from DSC and polarized optical microscopy measurements. However, the electrospun nanofibres composed of poly(vinyl pyrrolidone) (PVP) and 5CB showed phase-separated morphology. The phase-separated morphology was unambiguously characterized using SEM and TEM measurements. Furthermore, the phase separation resulted in 5CB exhibiting its liquid crystalline characteristics. However, the radial constraint of the nanofibres led to the formation of small-sized 5CB domains with limited spatial connectivity, which resulted in deviation from the known phase-transition characteristics of 5CB. It was also observed that the inherent orientation of the nanofibres favours the nematic to crystalline transition in the blend nanofibres. The present study gives new insight and understanding about phase behaviour of LC in electrospun polymer fibre and has technological relevance for the design of LC-based flexible fibrous components with tunable optical, thermal and dielectric properties.