Phase structures, transition behavior and surface alignment in polymers containing rigid-rod backbones with flexible side chains - Part VI - Novel band structures in a combined main-chain/side-chain liquid crystalline polyester: From liquid crystal to crystalline states

Physical origins of banded structures appearing on different length scales have been investigated using polarized light and atomic force microscopies (PLM and AFM), polarized Fourier Transform infrared spectroscopy (FT-IR) an d wide angle X-ray diffraction (WAXD) in a combined main-chain/side-chain l iquid crystalline (LC) polyester, PEFBP(n). This series of PEFBP(n) polymer s was synthesized from the polycondensation of 2,2'-bis(trifluoromethyl)-4, 4'-biphenyldicarbonyl chloride with 2,2'-bis{omega-[4-(4-cyanophenyl)-pheny oxy]-n-alkoxycarbonyl]}-4,4'-biphenyl diol. In this paper, we focus on one polymer [PEFBP(n = 11)] of this series to illustrate the band structural fo rmation on different length scales during the evolution from liquid crystal to crystalline states. Alternating bands of the films mechanically-sheared at 190 degrees C are formed with a spacing of 3 +/- 0.5 mu m in PLM, and r ecognized to be primary bands. PLM and AFM results show that these bands ar e seen due to the change of optical birefringence constructed mainly by alt ernating film thickness (and thus, retardation). Based on polarized FT-IR r esults, both the backbones and side chains of the polymers are orientated p arallel to the shear direction. Secondary fibrillar bands develop within th e primary bands after the sample is subsequently crystallized at 105 degree s C. These bands show a zigzag arrangement and possess a lateral size of 25 0 +/- 50 nm determined by AFM. High resolution AFM observations illustrate that these bands consist of aggregated edge-on crystal lamellae having a th ickness of approximately 20 nm. The lamellar crystals are assembled togethe r and lie across the film thickness direction. The mechanism for the format ion of these secondary zigzag bands originates from the expansion of the la ttice dimension along the chain direction on a molecular scale during the n ematic to crystalline phase transition and crystallization in the partially confi ned LC primary bands, which form macroscopic zigzag buckling. (C) 20 00 Kluwer Academic Publishers.