X-ray diffraction analysis of 10,12-pentacosadiynoic acid Langmuir-Blodgett films during polymerization

Abstract

10,12-pentacosadiynoic acid (PCDA) LB films were deposited on two different subphases of CdCl2 and TbCl3 respectively and polymerized under UV-irradiation. It is found that the different subphase influenced the molecular packing, and in turn, influenced the polymerization behavior in the LB films. The molecular arrangement structures of unpolymerized and photopolymerized LB films were investigated by low angle X-ray diffraction. The periodic spacing of molecules in the LB films and the inclination angle of molecules on the substrate were calculated from the X-ray diffraction profiles. The unpolymerized PCDA LB film had one set of diffraction peaks, however, two sets of diffraction peaks were clearly observed in partly-polymerized PCDA LB films deposited on CdCl2 subphase and reduced to one set of diffraction peaks corresponding to the polymerized LB films. On the other hand, the PCDA LB films deposited on TbCl3 subphase had only one set of diffraction peaks throughout. This is interpreted to correspond to different mechanisms of chain propagation, i.e., heterogeneous chain propagation for the PCDA LB film deposited on CdCl2 subphase and homogeneous polymerization for the LB film deposited on TbCl3 subphase. A molecular arrangement model in LB film was employed to explain the phenomenon of the “even-odd intensity oscillation” qualitatively.