Isomerism. 16.† Hexagonal Columnar Phase (ɸ_h) in a Nondiscotic Copolyether Based on 1,2-Bis(4-hydroxyphenyl)ethane, 1,8-Dibromooctane, and 1,12-Dibromododecane and the Novel 2-Dimensional-3-Dimensional ɸ-S_B Transition

The copolyethers based on 1,2-bis(4-hydroxyphenyl)ethane (BPE) and a 1/1 molar ratio of 1,8-dibromooctane and 1,12-dibromododecane [BPE-8/12(l/l)] was characterized by a combination of techniques consisting of differential scanning calorimetry (DSC), simultaneous X-ray diffraction/DSC (XDDSC), and thermal optical polarized microscopy. On cooling from the isotropic phase, BPE-8/12(1/1) exhibits a hexagonal columnar (ɸ_h) followed by a smectic-B (S_B) phase. Both mesophases are enantiotropic. To our knowledge this represents the first example of a flexible nondiscotic liquid-crystalline polymer that displays a hexagonal columnar mesophase. The new ɸ_h-S_B transition represents an almost ideal case of a 2-dimensional-3-dimensional transition where longitudinal long-range order is established without significantly affecting the in-plane order. The ɸ_h phase of BPE-8/12(1/1) [which can be viewed as a copolymer containing structural units derived from polyethylene and poly(p-xylylene)] bridges the gap between the columnar phases of polyethylene and of poly(p-xylylene). The present new addition to the list of columnar phases in flexible nondiscotic polymers is particularly significant as the phase occurs above the temperature range of a smectic phase. A further proof is thus provided that this class of “hexagonal” phases is liquid crystalline in nature.