Chiral Liquid Crystalline Side-Chain Polymers. Design, Synthesis and Properties
The potential use of chiral liquid crystalline materials, low molar mass as well as polymers, in technical applications has been recognized for some decades now. A number of interesting phenomena based on the spontaneous ordering of the liquid crystal forming molecules are strong reasons for the research effort in this area. The utilization of chiral liquid crystalline materials and the tailoring of specific properties demand greater knowledge of structure - properties relationships.
In the present work, the effect of structural changes in the mesogen and the effect of variations in polymer chain length have been investigated with respect to phase behaviour and electro-optical properties. Lateral substituents of different polarity (nitro, chloro and methyl) were introduced at different positions in the mesogenic core. The polymer chain length was increased through fractionation and through reaction with a difunctional chain extender.
The lateral substituents change the preferred packing of the side-chain precursors, leading to lowered transition temperatures and destabilization of the more ordered phases. The more polar substituents increase the attractive forces between the mesogens through dipolar interactions. The transition temperatures are raised and the more ordered phases are stabilized, thus counteracting the steric effect. The influence of the lateral substituents on the phase behaviour of the polymers is less pronounced. The phase separation of the polymer chains from the mesogens maintains the smectic phases even when the corresponding side-chain precursor barely shows liquid crystallinity at all. The electro-optical properties are affected in a direct way by the polarity of the substituent and indirectly through changes in the phase behaviour. One side-chain precursor shows very large spontaneous polarization, >700 nC/cm2. The same material also exhibits a field controllable SHG effect.
The changes in molar mass and molar mass distribution of the chiral liquid crystalline polymers affect the ferroelectric and electroclinic switching as well as the mechanical strength of the material. A doubling in average polymer chain length by removal of low molar mass material increases the response times markedly, while the mechanical strength is almost unchanged. A further twofold increase in average polymer chain length does not result in slower switching but increases the mechanical strength considerably. By combining the lubricating effect of the short polymer chains with the mechanical strength provided by long chains, it should be possible to achieve switchable free-standing films.
Abbreviations: SHG second harmonic generation, a non linear optical effect