Schiff碱类向列液晶熔融与清亮过程热力学

用差示扫描量热法及热台显微镜法研究了Schiff碱类向列液晶PEBAB及PBAPB的熔融与清亮过程.给出PEBAB的Tm=378.53K,△Hm=26.48±0.20kJ/mol,△Sm=69.96±0.53J/K.mol,Tc=399.80K,△Hc=929±53J/mol,△Sc=2.32±0.13J/K.mol;对于PBAPB,Tm(II)=352.86K,Tm(I)=354.74K,Tc=384.54K,△Hc=866±25J/mol,△Sc=2.25±0.07J/K.mol.在通过二次加热法消除熔前效应后,得到△Hm(I+II)=12.36±0.17kJ/mol,它不随升温速率改变而变化.本工作测得的清亮焓,在3%的偏差范围内符合平均场理论要求;△H1/△H2=T1/T2;所得清亮熵也与大多数向列型液晶相近.在PBAPB中,在318K左右,还观察到固→固相变等情况,相变焓是13.96±0.35kJ/mol.

Thermodynamics fo the melting and clearing processes of schiff base nematic liquid crystals

The melting and clearing processes of the Schiff base nematogens, p-ethoxybenzylidene-p'-aminobenzonitrile (PEBAB) and p-pentyloxybenzylidene-p'-aminophenylbutanoate (PBAPB) were studied by DSC and hot-stage microscopy. The temps., heats and entropies of melting and transition were determine The melting enthalpy of mixture of the two compounds is 12.36 ?0.17 kJ/mol. It is independent of the heating rates which ranged from 1.25 to 20.00 K/min. According to the mean field theory, there is a relationship between the clearing enthalpies and the clearing points of the nematic materials, i.e. DH1/DH2 = T1/T2. The clearing entropies are close to those of most nematic materials. In the case of PBAPB, a solid-solid transition and others were observed in some samples heated doubly and kept at room temperature The enthalpy of solid-solid transition is equal to 13.96?.35 kJ/mol.