Phase transformations in a chiral main-chain liquid crystalline polyester involving double-twist helical crystals

A main-chain non-racemic chiral liquid crystalline polymer has been synthes ized from (R)-(-)-4'-{omega-[2-(p-hydroxy-o-nitrophenyloxy)-1-propyloxy]-1- nonyloxy}-4-biphenyl carboxylic acid, abbreviated PET(R*)-9. Based on diffe rential scanning calorimetry, wide angle X-ray diffraction (WAXD) and polar ized light microscopy experiments, this polymer undergoes at least three li quid crystalline (LC) transitions in addition to crystallization. The LC tr ansition sequence is I <->(3.21 kJ/mol) (1.85 degrees C) TGBA* <->(1.27kJ/mol) (1.75/kJ/mol) S-A * <->(0.35 kJ/mol) (130 degrees C) S-C* <->T-37 degrees C(g) The transition sequence is reversible since they are LC phases, which are c lose to thermodynamic equilibrium. Among these phases, the twisted grain bo undary smectic A (TGBA*) phase is, for the first time, found in a main-chai n LC polymer. The TGBA* is only stable within a temperature region of 10 de grees C between the Smectic A* (S-A*) phase and the isotropic melt. Crystal lization that takes place in the (S-A*) phase can form both hat-elongated a nd double-twist helical single lamellar crystals, as observed by transmissi on electron microscopy (TEM). Analysis of the WAXD fiber patterns of this p olymer indicates that the crystal structure of PET(R*)-9 in the bulk is ort horhombic, identical with that determined using selective area electron dif fraction in TEM [Macromolecules 32 (1999) 524; Phys Rev B 60 (1999) 12 675] . (C) 2000 Elsevier Science Ltd. All rights reserved.