Orientation induced chromism in poly(methylhexysilane)

The orientation-induced conformational transition was studied on thin films of poly(methylhexylsilane) (PMHS). The orientation behavior of molecular c hains was analyzed by the inclination-angle-dependence of the polarized UV spectrum and the FTIR reflection-absorption spectroscopy (RAS). The changes in conformation of PMHS chains were monitored with the UV absorption spect rum. It was shown that the structure of the spin-cast film depended upon th e thickness of films. In thick film (408 nm), molecular chains are not orie nted, and favor the disordered conformation, which shows absorption band at 300 nm. In contrast, extended conformation with trans-rich segments is pro duced in addition to the disordered conformation in thin spin-cast films (4 5-100 nm). The extended conformation gives rise to a sharp absorption band at 338 nm. The molecular chains with extended conformation are oriented par allel to the substrate surface. The uniaxially oriented films were prepared by drawing the him on supporting polymeric substrates and by crystallizati on on highly oriented polymer films. As the degree of orientation is increa sed, the 338 nm band increases in intensity relative to the 300 nm band. Th e 338 nm band dominated the UV spectrum of the highly oriented PMHS him ins tead of the 300 nm band. A conformational transition to the extended confor mation from disordered state is induced by the molecular orientation, exhib iting a drastic spectral change in the UV spectrum. (C) 1999 Elsevier Scien ce Ltd. All rights reserved.