High intrinsic thermally conductivity side-chain liquid crystalline polysiloxane films grafted with pendent difunctional mesogenic groups

Abstract

A series of polymers with high intrinsic thermal conductivity, named side-chain liquid crystal polysiloxanes (SCLCPs), is prepared via introducing a mesogenic structure using the monomers 4,4′-bis(5-hexenyloxy)biphenyl (S-CL) and 4-cyano-4′-hex-5-en-1-oxy-biphenyl (M). The effects of the flexible main chain and mesogenic units on the thermal conductivity properties are investigated in depth. S-CL is a crosslinking monomer and presents smectic liquid crystal texture, and it can be used as a difunctional monomer to avoid the introduction of non-mesogenic properties. Owing to the microscopic ordered structure obtained from S-CL and M, the thermal conductivity (λ) values of SCLCP5 in the through-plane (λ_⊥) and in-plane (λ_‖) directions are 0.354 W m⁻¹ K⁻¹ and 1.585 W m⁻¹ K⁻¹, respectively, achieving a significant increase of 190.2% compared to SCLCP1. This is mainly due to the formation of a crosslinked structure and the orderly liquid crystal orientation, reducing the dissipation of phonons due to the random entanglement of molecular chains. In addition, the SCLCPs exhibit clear nematic liquid crystal structures, and as the amount of S-CL is increased the crystallite size calculated from XRD analysis gradually decreases, from 18 nm to 9 nm, and the crystal phase becomes more integrated and regular. Furthermore, SCLCPs present a wide service temperature range (22.75–323.3 °C) and good thermal stability (T_{Heat resistance index} (T_HRI): 194.7 °C). With the introduction of flexible segments, SCLCPs show better flexibility, further extending their possible applications in the field of thermoelectric materials.