Liquid Crystalline SIloxane Dimers - Advanced Materials with Enhanced Properties
Doctoral thesis, 2006
The liquid crystal state is a complex state with many kinds of different molecular attractive and repulsive forces that results in various kinds of phases. Since the pioneering works by Vorländer in the early 20th century, liquid crystal research has to a great extent been depending on careful systematic studies of the structure/property relationships in series of similar liquid crystalline materials.
The present work is focused on chiral dimers both in their own right and as models for polymeric liquid crystals. Special thought has been given to using phase segregation between different parts of the molecules to enhance the dimer properties in comparison to the corresponding monomers. Both ends of the monomers have been used to form the dimers and both alkane and siloxane chains of varying lengths have been used as linking groups. In further studies on phase segregation a fluorinated tail was added to one series of monomers and dimers.
All the dimers coupled with a trisiloxane with the chiral groups outwards show the antiferroelectric phase, which in several cases has a tilt angle greater than 40 degrees over a wide temperature range, as well as high spontaneous polarisation in the field-induced ferroelectric state. A non fluorinated dimer with a spacer length of six carbons showed a remarkable field-induced antiferro- to ferroelectric transition of second order with close to 45 degrees tilt angle. This particularly important combination of properties is very useful for fast switching and high contrast in liquid crystal displays. For the dimers with fluorinated tails the tilt angle exceeded 45 degrees for all spacer lengths, thus giving the opportunity to study the isotropic state that occurs in horizontally aligned antiferroelectric liquid crystals at 45 degrees, as well as the 90 degree flip of the optic axis that occur when passing 45 degree tilt.
One dimer was cyclized into a macrocyclic material that showed a high tilt twisted ferroelectric phase with extraordinary low switching threshold and linear field dependence of the apparent tilt of the sample optic axis, which are attractive features for display applications.
45 degree tilt angle