Interrelationships of Nanometer and Subnanometer Structures in a Polynorbornene Containing Second Generation Liquid-Crystalline Monodendrons as Side Groups

Abstract

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 (SA and ) phases, and one hexatic B ( ) phase were observed. Among them, the N phase only appeared in a narrow temperature window of less than 2 °C. The phase might be monotropic and metastable with respect to the 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 SA and phases also possessed the ordered layer structures on the nanometer scale despite different layer spacings (5.35 nm vs 4.90 nm). In the phase, the layer structure was much larger than those in both the SA and the 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.