Characterization of Poly(silane) Thin Films Oriented by the Langmuir-Blodgett Technique

Abstract

Characteristic properties of order parameters and layer structures of highly oriented poly [(m-hydroxyphenyl)butylsilane] (PPBS), poly[[(m-potassiooxy)phenyl]butylsilane] (PPBS-K), and poly-[(6-hydroxyhexyl)hexylsilane] (PHHS) were studied by polarized fluorescence measurement and small-angle X-ray diffraction. Samples were prepared by the Langmuir-Blodgett (LB) technique. The temperature dependence of the order parameter (P 2 , P 4 ) in PPBS and PHHS can be determined by the fluorescence depolarization ratios (r 1 and r 2 ). The order-disorder conformational transitions of PPBS and PHHS occur at T c = 130.0 and 100.0 °C, respectively. P 2 's and P 4 's of PPBS are higher than those of PHHS. P 2 's of PPBS and PHHS depend clearly on the temperature, while the P 4 's show steady negative values which increase slightly near T c . X-ray diffraction patterns taken at room temperature show that the steroregularities of PPBS, PPBS-K, and PHHS thin films are completely different. PPBS shows a long and a short period peak. This long period peak is observed for the first time in oriented thin films for poly(silane)s. That is, the oriented PPBS film is constituted as a bilayer structure with one hydrogen bond and one hydrophobic bond in view of facts that the temperature dependence of intensity, peak shift, and full width at half maximum in two X-ray diffraction peaks are clearly different. One layer spacing and amorphous states are found in PPBS-K, and PHHS does not have a regular layer but indicates a broad amorphous feature. The amorphous states of PPBS-K and PHHS are considered to be of different origin because the amorphous X-ray diffraction feature of PPBS-K differs clearly from that of PHHS. The sharp peak of PPBS-K cannot originate from the same layer spacing of PPBS since hydrogen bonding is stereochemically hindered by the potassium atoms of PPBS-K. The disappearance of two layer spacings in PPBS is in accordance with the Lindeman's criterion of melting, when the temperature approaches T + ∼1.0 where T + means a reduced temperature. It can be also found that deviations of the second-order parameters around their equilibrium values in PPBS and PHHS approach ∼1/5, as the temperature is raised near T* = 1.0 where T*(= T/T c ) is a reduced temperature.