Synthesis and characterization of new systems of side-chain liquid crystalline polymers and orientation studies
The research work described in this thesis consists of four separate projects. In the first project, we have prepared a series of interpenetrating networks based on two covalently cross-linked side-chain liquid crystalline polymers (SCLCPs). We discuss the synthesis and characterization as well as the potential of this novel liquid crystalline system in the search of new functional materials. In the second project, two SCLCPs were grafted onto a styrene-butadiene-styrene (SBS) tri-block copolymer through a radical polymerization of the mesogenic monomers in a SBS solution. We show that the resulting SCLCP-based materials have the interesting feature to behave like a thermoplastic elastomer, and that a monodomain structure of the mesogens can easily be induced by a film stretching and then preserved after a two-step thermal annealing process. This special technique is applicable to a large number of SCLCPs, and represents a new approach for achieving a control of the monodomain structure. In the third project, the mechanically-induced orientation of the mesogens in two groups of ion-containing SCLCPs was investigated. We found that in the case of a SCLCP carrying biphenyl mesogenic moieties the ionic aggregates show no influence on the orientation. By contrast, in the case of a SCLCP containing phenyl benzoate units, the achievable orientation decreases as the ion content increases, indicating that the ionic aggregates reduce the order inside nematic domains. In the last project, the orientational relaxation behavior of a stretched SCLCP was studied by heating the deformed film under strain to temperatures near its phase transitions. A mesophase-transition-induced reorientation phenomenon was observed for the first time. When the stretched SCLCP undergoes a smectic-nematic transition, the pendent mesogenic groups, which are initially oriented perpendicularly with respect to the stretching direction, flip 90 [degre] to give rise to a parallel orientation. We discuss this observation and other related phenomena.
- Sciences – Mémoires