Helical Twist Senses, Liquid Crystalline Behavior, Crystal Microtwins, and Rotation Twins in a Polyester Containing Main-Chain Molecular Asymmetry and Effects of the Number of Methylene Units in the Backbones on the Phase Structures and Morphologies of Its Homologues

A nonracemic chiral main-chain liquid crystalline (LC) polyester was synthesized from ( R )-(−)-4‘-{ω-[2-( p -hydroxy- o -nitrophenyloxy)-1-propyloxy]-1-undecyloxy}-4-biphenylcarboxylic acid via an A−B type condensation polymerization. The polymer was abbreviated as PET(R*-11), where 11 is the number of methylene units in the chain backbone. Other PETs(R*- n ) ( n = 7−11) in this series were also studied. 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 light 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: “microtwinning” 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 folded surfaces along the {3̄10} 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. It was particularly surprising that in this series of PETs(R*- n ) the odd−even methylene unit number effects existed on broad length scales. This included not only thermodynamic properties of the S C * and the S A * LC transitions but also the helical crystal handedness and the crystal structures.