Interrelationships of nanometer and subnanometer structures in a polynorbornene containing second generation liquid-crystalline monodendrons as side groups

A series of polynorbornenes containing monodendrons as side groups [DPNB(n)] with different degrees of polymerization (n) were synthesized. These dendritic side groups were second generation monodendrons based on the AB₂ mesogenic monomer, 13-hydroxy-1-(4-hydroxyphenyl)-2-(4-hydroxy-4″-p-terphenylyl)tridecane building blocks. Although the mesogens were rodlike, the polymer samples were overall conformationally flexible and displayed liquid-crystalline (LC) behavior. Above n = 20 in DPNB(n), both the transition temperatures and heats of transitions were independent of n. Therefore, a DPNB(40) was chosen as the representative polymer for this study. The polymer exhibited complicated phase transformation behaviors which were associated with two different length scales. During cooling, a nematic (N) phase, two smectic A (S_A and S_A^m) phases, and one hexatic B (S_B^h) phase were observed. Among them, the N phase only appeared in a narrow temperature window of less than 2 °C. The S_A^m phase might be monotropic and metastable with respect to the S_B^h phase. The N phase was formed via the orientation order of the LC mesogens in the monodendrons of the side groups on the subnanometer scale. In addition to the molecular orientation order, both of the S_A and S_A^m phases also possessed the ordered layer structures on the nanometer scale despite different layer spacings (5.35 nm vs 4.90 nm). In the S_B^h phase, the layer structure was much larger than those in both the S_A and the S_A^m phases (8.84 nm), and its lateral molecular packing exhibited a two-dimensional hexagonal array on the subnanometer scale (a = 0.52 nm and γ = 120°). Interrelations of these nanometer and subnamometer structural formations in these phases were discussed.