Proceedings Article | 24 October 2000
KEYWORDS: Reflection, Polymers, Luminescence, Polarization, Absorption, FT-IR spectroscopy, Polymer thin films, Infrared radiation, Glasses, Ultraviolet radiation
Thermotropic nematic and chiral-nematic poly(p-phenylenes) were prepared into well aligned solid films. Dilute solution photoluminescence spectroscopy revealed that, with excitation at 350 nm, the conjugated polymer backbone was the predominant light emitter. Linearly polarized photoluminescence, with (lambda) ex equals 350 nm, and FTIR linear dichroism, performed on a uniaxially aligned film, yielded the orientational order parameter, S, pertaining to the conjugated polymer backbone and nematic pendants, at 0.62 and 0.73, respectively. The supramolecular structure in a uniaxially aligned film was also elucidated by these measurements; the polymer backbone and the nematic pendants were found to be collinear and lie predominantly along the buffing direction, i.e. along the director of the film. For emission outside the selective reflection region, also referred to hereafter as the resonance region, S was estimated at 0.67 for the conjugated backbone within the framework of a recent theory. At a decreasing chiral mole fraction, films prepared with chiral- nematic copolymers showed a selective reflection region in the ultraviolet to visible and infrared region. Moreover, the chiral-nematic films were found to consist of a left-handed helical stack of quasinematic layers with (S)-(-)-1- phenylethanol as the chiral moiety. For emission within the resonance region, the highest degree of circular polarization ever reported for neat conjugated polymers was observed. The left-handed component of the emission was found to be strongly suppressed within the resonance region and enhanced at its edges in close agreement with the results obtained when a rod- like luminophore was doped into a chiral-nematic host. In sharp contrast, regardless of the polarization state of incident light, propagation alone through a chiral-nematic film did not result in handedness reversal of the transmitted light. Viewing angle measurements revealed that the right- handed component of the emission follows Lambert's cosine law closely regardless of emission wavelength. The left-handed component of the emission, on the other hand, was found to be strongly dependent on both emission wavelength and viewing angle. This dependence can be explained, at least in part, to the shift of the resonance region to shorter wavelengths with increasing viewing angle.
