A new scheme for determination of the carrier mobility and main-chain conformation of an electroactive polymer chain by transient electric birefringence

A new scheme of experimental criterion is proposed for estimating the carrier mobility and main-chain conformation of an electroactive polymer chain by the technique of transient electric birefringence (TEB). The rise response Δn_r(t) and the decay response Δn_d(t) in the TEB satisfy in the Kerr regime (i) Δn_r(t) = Δn _st-Δn_d(t) for a polymer molecule of arbitrary conformation with an induced dipole moment alone arising from carriers highly mobile along the main chain, (ii) Δn_r(t) = Δn _st-(3/2)Δn_d(t/3) + (1/2)Δn_d(t) for a rodlike polymer molecule with a permanent dipole moment alone originating from carriers with low mobility, and (iii) Δn_r(t) = Δn _st-2Δn_d(t/2) + Δn_d(t) for a random-coiled polymer with a permanent dipole moment alone due to low-mobility carriers, where Δn_st = lim_t-∞ Δn _r(t). Then, comparison of the TEB rise and decay responses gives us information on the carrier mobility and main-chain conformation. This criterion is valid also for polydispersed polymer samples. By applying the criterion to the exemplifying data of the TEB responses for soluble urethane-substituted polydiacetylenes, it is found that the polydiacetylene molecules take a random-coil conformation with a permanent dipole moment in a good solvent, while in a poor solvent the carrier mobility depends crucially on the solvent condition.