Dynamic light scattering in the polarized geometry has been used to investigate the dynamics of composition fluctuations in solutions of two poly(styrene-i>Zocfe-l,4-isoprene) diblock copolymers in a nonselective, good solvent both far from and near to the ordering transition. Four different relaxation mechanisms were identified. One relaxation relates to the cooperative diffusion of copolymer chains, and it behaves similarly to that in semidilute homopolymer solutions. As concentration increases, two additional relaxations gain in amplitude: the wavevector-independent internal or breathing mode, predicted by theory, with amplitude increasing with scattering angle, and the recently established for copolymer melts diffusive relaxation; the relaxation times of both processes increase with copolymer concentration. In addition, another very slow process is evident at low scattering angles and is related to the long-range density fluctuations. A theoretical analysis is presented that predicts the existence and the behavior of the first three relaxations. The new diffusive relaxation is attributed to the composition polydispersity of the diblocks; its intensity is proportional to the degree of polydispersity; its rate is governed by the self-diffusion of the copolymer chains; both its intensity and its relaxation time increase with the copolymer concentration, in contrast to the behavior of the cooperative diffusion.

Dielectric spectroscopy (DS) and dynamic light scattering (DLS) are employed to study the dynamics of poly(«-lauryl methacrylate) (PnLMA) (Mw = 1.1 x 105) at temperatures below and above the glass transition temperature Tg (Tg 225 K). The DS and DLS data show no evidence for the splitting between the primary (a-) and secondary (/?-) relaxations within the experimental frequency range. The main process affecting DS and DLS is the mixed a/3-relaxation which bears similarities to a single a-process with regard to the T-dependence of the relaxation times. Excellent agreement between the two sets of experimental relaxation times was found. The distribution of relaxation times exhibits a pronounced temperature dependence: from a Kohlrausch-Williams-Watts (KWW) exponent of /3kww 0.25 at T Tg to /?kww ss 1 at T « Tg + 100 K and at a frequency of about 1 MHz. PnLMA is therefore, to our knowledge, the only polymer with a Debye (fi = 1) distribution of relaxation times at megahertz frequencies so far. The extremely broad distribution near Tg and the weak normalized temperature dependence of the relaxation time apparently contradict the phenomenologically established correlation between the two in many glass formers. This contradiction might arise from concentration fluctuations of the long alkyl chain which can broaden the relaxation spectrum near Tg. In addition to the main /3-process, a "fast" /8-relaxation can be resolved in both experiments and originates from the polyethylene-like alkyl chain.

Photon correlation spectroscopy and pulsed-field-gradient nuclear magnetic resonance have been utilized in order to investigate the characteristic features of the recently established (both experimentally and theoretically) diffusive "polydispersity" relaxation process for concentration fluctuations in homogeneous diblock copolymer melts and solutions. This is accomplished using semidilute solutions of a symmetric mixture of two diblock copolymers with similar molecular weights and almost mirror compositions in a common good solvent. Mixing of the two asymmetric diblocks leads to a system with almost symmetric composition and with narrow molecular weight distribution, but with large effective composition polydispersity. Above the order to disorder transition (ODT), the theoretical expressions for the amplitude and the relaxation rate of the polydispersity mode can quantitatively describe the observed diffusive relaxation. Similarly to the situation in homopolymer blends, the thermodynamic forces can significantly retard the diffusion coefficient and lead to an increase in the dynamic intensity with increasing copolymer concentration. The intervention of the ODT alters this concentration dependence at copolymer concentrations #J near but below #JODT.

Dielectric relaxation spectroscopy has been used to investigate the normal-mode relaxation in disordered diblock copolymer melts far from the order-to-disorder transition (ODT). The dielectric spectra are analyzed in order to quantitatively obtain the distribution of relaxation times in the disordered diblocks. The width of the relaxation function shows significant broadening relative to the respective homopolymer distributions when the temperature is decreased and/or the molecular weight is increased. The broadening is attributed to composition fluctuation effects on the normal-mode relaxation. These effects are theoretically accounted for by considering both the short-range fluctuations due to chain connectivity and the long-range concentration fluctuations in diblock copolymers due to the proximity to the ODT and their coupling to the individual block segmental dynamics. Theory not only captures the relative features of the distributions but also quantitatively compares very well with the experimental dielectric spectra.

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