K. Severing, E. Stibal-Fischer, A. Hasenhindl, H. Finkelmann, K. Saalwächter.
Phase Biaxiality in Nematic Liquid Crystalline Side-Chain
Polymers of Various Chemical Constitutions.
J. Phys. Chem. B. Accepted.
In a previous deuterium NMR study conducted on a liquid crystalline (LC) polymer
with laterally attached book-shaped molecules as the mesogenic moiety we have
revealed a biaxial nematic phase below the conventional uniaxial nematic phase
(Phys. Rev. Lett., 92 (2004) 125501). In order to elucidate
details of its formation, we here report on deuterium NMR experiments that have
been conducted on different types of LC side-chain polymers as well as on
mixtures with low-molar-mass mesogens. Different parameters that affect the
formation of a biaxial nematic phase, such as the geometry of the attachment,
the spacer length between the polymer backbone and the mesogenic unit, as well
as the polymer dynamics, were investigated. Surprisingly, also polymers with
terminally attached mesogens (end-on polymers) are capable of forming
biaxial nematic phases if the flexible spacer is short and thus retains a
coupling between the polymer backbone and the LC phase. Furthermore, the most
important parameter for the formation of a biaxial nematic phase is the dynamics
of the polymer backbone, as the addition of a small percentage of low molar mass
LC to the biaxial nematic polymer from the original study served to shift both
the glass transition and the appearance of detectable biaxiality in a very
similar fashion. Plotting different parameters for the investigated systems as a
function of T/Tg also reveals the crucial
role of the dynamics of the polymer backbone and hence the glass transition.