Helical assembly, liquid crystalline behavior and crystallization of a nonracemic chiral main-chain liquid crystalline polyester and its monomers

In this thesis, A non-racemic chiral main-chain liquid crystalline (LC) polyester was synthesized from (R)-(−)-4′-{lcub}ω-[2-( p-hydroxy-o-nitrophenyloxy)-1-propyloxy]-1-undecyloxy{rcub}-4-biphenyl carboxylic acid via an A-B type condensation polymerization. The polymer was abbreviated as PET(R*-11) where eleven is the number of methylene units in the chain backbone.; PET(R*-11) exhibited a LC chiral smectic C (S_C*), a chiral smectic A (S_A*) and a twist grain boundary Smectic A (TGBA*) phase with increasing temperatures as identified by differential scanning calorimetry, polarized optical microscopy, and wide angle X-ray diffraction (WAXD).; Flat-elongated lamellae were observed in transmission electron microscopy (TEM) for thin-film melt crystallized samples. Its crystal structure was determined to be a monoclinic basic unit cell with a = 1.03 nm, b = 0.47 nm, c = 6.43 nm and γ = 83° via selected area electron diffraction (SAED), and these measurements were confirmed by two-dimensional WAXD fiber patterns. Furthermore, SAED results showed that two kinds of crystal twins existed in this polymer: “micro-twinning” within one single lamellar crystal and “rotation-twinning” between two lamellae. This rotation-twinning was possibly a result of a mechanism of a soft epitaxy between two contacting (001) folded surfaces along the (3¯10) planes. Right-handed helical lamellar crystals were also found under the same crystallization conditions, and their pitch lengths were on the micrometer scale as observed by TEM and atomic force microscopy. The helical crystal structure was found to be identical to that of the flat-elongated counterparts. Dendritic crystals with fractal geometry were also obtained as the result of diffusion-controlled crystal growth.; The phase structures of two monomers of PET(R*-11) were studied. One of them has carboxylic acid end group (named as K11-acid), while another has ester end group (named as Kl 1-ester). Kl 1-ester does not show any LC behavior, while cholesteric phase and Sc* phases can be obtained in K11-acid. The hydrogen-bonding between two carboxylic acid groups of K11-acid plays an important role in its liquid crystal phase formation. No helical crystals were observed in the monomers. These results might suggest that the enhanced chirality strength in the polymer chain of PET(R*-11) stabilize the helical structure in highly ordered crystals.