Small Angle X-Ray Diffraction Study for the Determination of Structural Parameters of Two Mesomorphic Compounds Having Smectic C Phase

Abstract

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 S_AN in PPOB and at S_C -N transitions indicate that the transitions are of the first order.

Key Words:

• X-ray diffraction
• tilt angle
• smectic C
• orientational order parameters