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The dc component Δn of the electric birefringence of poly(γ‐benzyl‐L‐glutamate) in m‐cresol is measured under an ac electric field at frequencies from 0.5 Hz to 200 kHz for solutions covering the dilute and semidilute regions. The dispersion curve indicates that at low frequencies Δn decreases with increasing frequency (low‐frequency relaxation). For high‐molecular‐weight polymers at high concentration, Δn becomes negative at high frequency and its absolute value decreases with further increase in frequency (high‐frequency relaxation). A unified theory for the two relaxations is developed on the basis of a model in which, in the semidilute regime, the rodlike polymer is confined in a cage formed by neighboring polymers and the lifetime of the cage lies between relaxation times of the two relaxations. The low‐frequency relaxation is ascribed to end‐over‐end rotation of the polymer and the high‐frequency relaxation to the rotation within a limited angle in the cage. The dependences of relaxation parameters on polymer concentration and molecular weight are reasonably explained by the theory.

The rotational diffusion constant Dr of entangled rodlike polymers in semidilute solution was calculated by our refining quantitatively the Doi-Edwards tube model. The methods of probability and stochastic process theory were used to calculate the mean lifetime of tubes, while the mean step angle of jump diffusion was evaluated by computer simulation on the size distribution of tubes. Our result eliminates a large discrepancy of Dr between the Doi-Edwards theory and a variety of experiments on rodlike polymers.PACS numbers: 61.25.Hq, 05.60. + WThe system of rodlike polymers in a semidilute solution is of great interest because of its simpler mathematical structure and its more conspicuous entanglement effect compared with the nondilute system of flexible polymers. Recent theoretical treatment by Doi and Edwards^'^ provides the basis for an understanding of the dynamical behavior of a system of strongly interacting rodlike polymers whose length L is much larger than the rod diameter d. Using the tube model, they derived the relationship between Dj. and Dj.^ 09 the rotational diffusion constants of rodlike polymers in semidilute and infinitely dilute solutions, respectively, as

Photoluminescence (PL) spectra and decay dynamics were studied for the spontaneously oxidized porous Si with subsequent various thermal annealing procedures. The PL decay was highly nonexponential and well described by the stretched-exponential function. The PL lifetime was shorter for the higher PL photon energy, but at the same photon energy it decreased by an order of magnitude by the thermal annealing in N2 gas, in parallel with the large PL intensity decrease. This PL quenching upon the annealing is presumably ascribable to both the structural changes and dangling bond formations in porous Si, as suggested by ESR measurements and the annealing experiments in H2 gas.