聚(乙烯基对苯二甲酸二烷基酯)的非寻常相转变

以聚(乙烯基对苯二甲酸二烷基酯)(PDAVT)为对象,用热分析、X射线衍射、流变及固体核磁共振等方法考察了甲壳型液晶高分子中存在的"低温无序、高温有序"的非寻常相行为.实验证明,在升温过程中,烷基基团为丁基、己基和辛基的PDAVT(P4、P6和P8)可从无定形态发育出柱状(Col)液晶相,导致在高温区体系模量升高.降温时P6和P8可完全回到无定形态,表现出典型的"各向同性相重入"行为.PDAVT液晶相畴的生长具有一维生长的特点,是成核控制的,升高温度可加快Col相的形成.固体核磁共振实验表明,样品从无定形态转变为Col相的过程与侧链运动性的不断加强有密切关系.我们也初步探索了剪切场或拉伸场对液晶转变的影响,发现当温度不高时,剪切或拉伸既不改变液晶转变温度,也没有提高液晶态样品的液晶化程度.综合分析多方面的实验结果可以看出,PDAVT的非寻常相行为是熵主导的.侧链的加速运动会增强"甲壳效应",Col相是侧链熵最大化的结果.

Unusual Phase Transition of Poly(di(alkyl) vinylterephthalates)

An unusual phase behavior,showing the disordered phase at low temperatures and the ordered phase at high temperatures,which is known as the "isotropic phase reentry",has been observed in mesogen-jacketed liquid crystalline polymers and other side-chain jacketed polymers,for example,in some poly(di(alkyl) vinylterephthalates) (PDAVTs).In general,such a phase behavior is considered entropy dominant.However,the mechanism underneath is not fully addressed yet.Here,we study in detail the feature of phase transition of the PDAVT samples with the alkyl group of butyl,hexyl and octyl (denoted as P4,P6 and P8,respectively) by differential scanning calorimetry (DSC),X-ray diffraction (XRD),rheology,and solid-state NMR.Undetectable in DSC experiment,the columnar liquid crystalline (Col) phase formation is demonstrated by XRD upon heating the PDAVT samples from isotropic state.It is also evidenced by rheology measurement,showing that,after glass transition,the samples have their shear storage modulus increased by two orders of magnitude at high temperature.Upon cooling,P4 retains its Col phase probably due to the molecular motion frozen by glass transition,while P6 and P8,which have quite low glass transition temperature (Tg),can fully return to the isotropic state,exhibiting the typical behavior of "isotropic phase reentry".It is found that the Col phase formation is nucleation-limited,with the feature of one-dimensional growth.At higher temperature,the nucleation barrier is drastically reduced,resulting in a much faster growth rate of Col phase.Sufficient development of the Col phase at the temperature above Tg makes the samples solid-like.Solid-sate NMR experiment reveals that increasing the temperature activates the alkyl tail motion first,which in fact triggers the Col phase formation.Effects of shearing and stretching on the Col phase formation are further examined.It is found that the external fields applied cannot induce the isotropic-to-Col transition of PDAVT when the temperature is close to the transition temperature,although the PDAVT chains can be oriented to some extent.On the basis of the experimental data obtained using these techniques,we conclude that the maximization of side-chain entropy is the driving force for the isotropic-to-Col phase transition.Namely,only at sufficiently high temperature can the strong side-chain motion enhance the "side-chain jacketing" effect,making the chains more rod-like which will pack parallelly to form the Col phase.