Mesophases were identified, and the phase transition temperatures were determined using a Zeiss-Axioscope polarised optical microscope (POM) (Mshot, Guangzhou, China) equipped with a Linkam-THMS-600 variable-temperature stage at a scan rate of 2 °C/min. The samples were heated from 25 to 160 °C and cooled from 160 to 25 °C.