By using optical second harmonic generation (SHG), we have investigated the molecular orientation of liquid crystal (LC) monolayers in contact with polyimide films exposed to linearly polarized ultraviolet light (LPUVL) at the wavelength of 266 nm.
For different exposures at 3, 8, 20, and 80 J/cm2, the second harmonic (SH) signal from the LC monolayer was measured as a function of the rotation angle of the sample around the surface normal.
The SH signal has no rotation angle dependence, independent of the energy density of exposure.
On the other hand, uniform parallel (homogeneous) alignment of bulk LC was observed for a LC cell made with two polyimide-coated substrates exposed to LPUVL at 8 J/cm2.
From these results we found that the in-plane anisotropy of the LC monolayer in contact with the polyimide film is very small, if any, even though the polyimide film can induce the homogeneous alignment of bulk LC.
The average tilt angle of LC molecules in the monolayers in contact with the polyimide films was also determined.
We found that the average tilt angle of LC molecules measured from the surface normal decreases with the increase of UV exposure.
The cause of separation in gel electrophoresis between highly homologous ss (single-stranded) DNAs as observed in SSCP (single-strand conformation polymorphism) was pursued. Advancing our previous explanation [J. Biochem. 99, 663-671 (1986)], the mobility difference of ssDNAs was correlated to differences in their dynamic conformation (not to differences in their most stable structure), focusing on point-substituted sites. The contribution of semistable conformation dynamics was considered to be critical. Putative factors which may influence mobility (i.e., length of ssDNAs, location of substituted sites, and types of substitutions) were experimentally examined and critically discussed. Understanding of these phenomena should yield improvements in various techniques, such as SSCP, and in evaluation of the solution structures of DNA.
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