Supramolecular Structure of Liquid-Crystalline Polyesters in Triclinic Cell

We have analyzed the phase behavior of a series of liquid-crystalline polyesters bearing the n-alkoxy pendant group. For n ) 1-4, this class of polymers shows a crystalline phase, called phase I, with a layered molecular packing and two chains per unit cell. For n > 4, a new molecular packing is observed. The new phase, called phase II, is characterized by the supramolecular assembly of 10-12 chains, each structurally independent of the others. This assembly is located in a large triclinic cell: in the case of P(5), the cell parameters are a ) 18.67 Å, b ) 17.54 Å, c ) 21.23 Å, R ) 111.1°, â ) 81.7°, and ç ) 105.5°, with 10 chains per unit cell (dcalcd ) 1.23 g cm-3). The molecular packing of phase II is well described in terms of a “fracture” of the layered molecular structure of phase I. This fracture is restored when samples are dipped into chlorinate solvents. The reason the packing fracture of phase I occurs is not clear: we believe that a complex balance between “packing incompatibility” of rigid chains and flexible side groups might be responsible for this phenomenon