可多次使用的液晶型类玻璃高分子

A Liquid Crystalline Vitrimer with Better Actuation Repeatability

通过高温下酯交换反应的进行,含酯键的液晶型类玻璃高分子(liquid crystalline vitrimer),能够通过简单拉伸进行取向,获得随温度变化可逆伸缩的智能材料.在目前已报道的此类主链型高分子中,酯交换剧烈发生需要的临界温度(Tv),与液晶弹性体发生可逆形变的温度(Ti,即液晶相-各向同性相转变温度)相隔较近,导致材料的使用温度范围比较窄,而且多次升降温后,取向及可逆形变会消失.为解决此问题,本文在原来体系的基础上,通过共聚合另外一种液晶基元,有效地降低了Ti,从而拓宽Ti与Tv之间的距离.这不仅使材料的使用次数明显增加,还能延长此类液晶弹性体的使用期限.

Liquid crystalline elastomers （LCEs） can show reversible shape change when the materials go through isotropic-liquid crystal phase transition （Ti）. To get such actuation, LCEs have to be aligned into monodomain samples, in which the mesogens are orientated more or less in a uniform direction. Normally, LCEs are covalently cross-linked, which makes the alignment very difficult. We have previously shown that this problem can be solved by designing LCEs with exchangeable links. The material for demonstration was synthesized by reacting an epoxy-terminated biphenyl mesogen with sebacic acid in the presence of triazobicyclodecene catalyst for transesterification. The resultant material is in fact a liquid crystalline vitrimer. When the material is stretched above the topology freezing transition temperature （Tv）, the accelerated transesterification reaction allows topology adjustment. The alignment can be fixed by cooling afterwards. However, as the 73 is very close to Tv in this sample, it is easy for the alignment to disappear when the temperature increases to above 73 to trigger the actuation. This not only limits the operating temperature range, but also destroys the alignment after multiple heating-cooling cycles. Consequently, it is desirable to increase the distance between Ti and Tv by reducing 73. To this end, a second epoxy-terminated cyanostilbene mesogen （EP1 l） was added into the main chain of the liquid crystalline vitrimer. The result shows that the copolymerization of the two types of mesogens can effectively decreases Ti without affecting the Tv too much. It broadens the range between Ti and Tv. The more EP 1 1, the lower 73. When the content of EP1 1 is 20 mol%, Ti is 82 ℃, which is 31 ℃ lower than the previous liquid crystalline vitrimer. The copolymer can also be used to make into monodomain LCE by stretching at high temperature. The resultant monodomain LCE has a similar reversible elongation-contraction actuation ratio as the previous liquid crystalline vitrimer. More importantly, the large distance between 73 and Tv makes it possible to trigger the actuation by heating it to a temperature of 1 5 ℃ above Ti without destroying the alignment. Little fatigue has been identified after repeating the reversible elongation-contraction actuation for 120 times.