Small angle X-ray diffraction study for the determination of structural parameters of two mesomorphic compounds having smectic C phase

X-ray diffraction study was carried out for the pentyl and octyl members of the Series 4-n-alkoxy phenyl 4 ' -n octyloxy benzoate (PPOB and OPOB in short). The first shows S-C, S-A and N phases while the second compound has only S-C and N phases. The transition temperatures were determined by studying texture under polarizing microscope. Apparent molecular length in nematics, layer line spacings in smectics, tilt angle in S-C, inter molecular distances and orientational order parameters were determined from X-ray diffraction patterns of aligned samples at different temperatures. Temperature variation of the orientational order parameters agree fairly well with Maier-Saupe theory in the nematic phase and with McMillan's theory in the smectic phase with tilt angle correction in S-C. Discontinuities Of < P2 > and < P4 >- at SAN in PPOB and at S-C-N transitions indicate that the transitions are of the first order.