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) and wide angle X-ray diffraction (WAXD) in a combined main-chain/side-chain liquid crystalline (LC) polyester, PEFBP(n). This series of PEFBP(n) polymers was synthesized from the polycondensation of 2,2-bis(trifluoromethyl)-4,4-biphenyldicarbonyl chloride with 2,2-bis{-[4-(4-cyanophenyl)-phenyoxy]-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 formation on different length scales during the evolution from liquid crystal to crystalline states. Alternating bands of the films mechanically-sheared at 190 °C are formed with a spacing of 3 ± 0.5 m in PLM, and recognized to be primary bands. PLM and AFM results show that these bands are seen due to the change of optical birefringence constructed mainly by alternating film thickness (and thus, retardation). Based on polarized FT-IR results, both the backbones and side chains of the polymers are orientated parallel to the shear direction. Secondary fibrillar bands develop within the primary bands after the sample is subsequently crystallized at 105 °C. These bands show a zigzag arrangement and possess a lateral size of 250 ± 50 nm determined by AFM. High resolution AFM observations illustrate that these bands consist of aggregated edge-on crystal lamellae having a thickness of approximately 20 nm. The lamellar crystals are assembled together and lie across the film thickness direction. The mechanism for the formation of these secondary zigzag bands originates from the expansion of the lattice dimension along the chain direction on a molecular scale during the nematic to crystalline phase transition and crystallization in the partially confined LC primary bands, which form macroscopic zigzag buckling.