We have investigated the optical properties of Vectra A910, a commercially produced liquid-crystalline copolyester consisting of 4-hydroxybenzoic acid and 6-hydroxy-2-naphthoic acid subunits. The polymer is highly fluorescent in the near UV with an emission maximum at 428 nm (2.90 eV) and an excitation maximum at 376 nm (2.66 eV). The time-resolved fluorescent decays at room temperature were found to fit stretched exponential decays. Furthermore, the fluorescence kinetics were found to be highly dependent upon the emission wavelength. These results are consistent with the disorder from the random nature of the copolymer.

State-of-the-art timing technique is used to determine the 'speed of light' in an upper level undergraduate laboratoryexperiment. In thisexperiment we use the wrrelated 51 1 keV photons from positron annihilation obtained from a IOOpCi Na-22 source. The photons were detected by two BaF2 scintillation detectors and, after careful time calibration of the entire counting system, the prompt coincidence peaks were each recorded as a function of the position of one of lhe detectors. As a result, the 'speed of tight' was determined to be 3.002(16) x 101ocms~i, which is very close to the defined value of 2.99192 x l0l0cms-'. This experiment demonstrates the utilization of various nuclear instrumentation and timing equipment.

We have investigated the temperature-dependent time-resolved fluorescence of Vectra A910, a commercially produced main-chain liquid-crystalline copolyester composed of 4-hydroxybenzoic acid (HBA) and 6-hydroxy-2-naphthoic acid (HNA). The experimental temperature range was from −180 to +400 °C, in which all four previously reported phase transitions were observed. At all observed temperatures, the time-resolved fluorescence was found to fit first-order stretched exponential decays. These decays are indicative of inherent disorder based on the random intrachain sequencing of the HBA and HNA subunits and the distribution of torsional angles between adjacent HNA chromophores causing variations of the π-orbital conjugation lengths. The kinetics of the fluorescence emission is rotationally coupled to the HNA chromophores, which can be associated with the observation of two separate temperature realms. The first realm, in which the HNA subunits are rotationally frozen, is characterized by a vibrationally coupled emission only. This realm is distinguished by static disorder within the stretched exponential envelope. In the second-higher temperature realm, the HNA torsional rotations feature both vibrationally and rotationally coupled excited-state relaxation. The rotational coupling manifests as dynamic disorder in the stretched exponential decays. The dynamic disorder is indicative by a broadening of the rotational distribution with increased temperature. The second temperature-dependent realm can be further characterized by a classical Arrhenius behavior leading to an activation energy of 1.357 kcal/mol (474.9 cm-1 or 0.05885 eV).

This work encompassed the development for a frequency-domain impedance and dielectric spectrometer using ubiquitous test and measurement equipment, i.e., signal generators and digital oscilloscopes. Various methods of amplification, noise rejection, and computations were employed to achieve the desired goals. The frequency range of 100 mHz-1 MHz was tested using air capacitors of 3.7 and 14.5 pF and an applied voltage range of 10-300 mV. The multichannel instrument produced a stable and reproducible dual-phase (real and imaginary or magnitude and phase) current sensitivity of 250 fA with an average phase stability of less than 0.5 degrees (tan delta<10(-2)) and a single-phase (magnitude only) current sensitivity of 60 fA.