Dielectric behavior was examined for nanodomain-forming styrene−isoprene (SI) diblock and SIIS triblock copolymers, the former having type-A dipoles aligned along its I block, whereas the latter being the head-tohead dimer of the former and having the dipoles inverted at the midpoint of the middle II block. The slow dielectric relaxation at low temperatures ( cylinder (Cyl) > lamella (La). This order appears to naturally reflect the domain continuity that helps large-scale motion of the I block without violating the osmotic constraint. In the Sph and/or Cyl domains, all II blocks of the SIIS copolymer have their ends fixed on the surface of the same domain and are geometrically classified as loop-type blocks. These loop-type blocks are naively expected to behave similarly, in a dynamic sense, to the tail-type I blocks of SI. Nevertheless, comparison of the dielectric data of the SIIS and SI copolymers suggested that some of the II blocks are classified as dynamically pseudobridge-like blocks behaving differently from the tail-type I blocks. This behavior of the II blocks in the Sph/Cyl domains was speculated to reflect their end-to-end distance: This distance governs the direction of II block tension that acts on the midpoint of this block thereby affecting the dielectrically detected midpoint motion. The II block in the Sph/Cyl domain appears to exhibit the pseudobridge-like behavior when it has a large end-to-end distance and the two halffragments of this block pull the midpoint in the opposite directions. This argument is crude but seems to be physically acceptable, as suggested from an approximate analysis of the distribution of the end-to-end distance in those domains.

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Atactic polystyrene (PS) has the type-B dipole perpendicular to the chain backbone so that its local, segmental motion activates the dielectric relaxation. For monodisperse oligostyrene (OS) and PS samples of various molecular weights M, details of this motion were examined at temperatures T well above T g through comparison of the complex modulus, G* = G′ + iG′′, and the complex dielectric permittivity, ε* =ε′ – iε′′, measured as functions of the angular frequency ω. For the OS samples, G*(ω) and ε*(ω) fully relaxed through the segmental dynamics thereby exhibiting respective terminal relaxation tails (low-frequency tails), G′(ω) ∝ ω2, G′′(ω) ∝ ω, Δε′(ω) ≡ ε′(0) – ε′(ω)∝ ω2, and ε′′(ω) ∝ ω, at ω below the segmental relaxation frequency ωs. For the PS samples, G*(ω) relaxed partly through the segmental dynamics and then exhibited the polymeric full relaxation characterized by the Rouse-like behavior followed by the terminal flow behavior (with/without intermediate entanglement plateau depending on M). In contrast, ε*(ω) of the PS samples still relaxed completely through the segmental dynamics. For respective samples, the G*(ω) and ε*(ω) data in the segmental relaxation zone exhibited very similar relaxation mode distribution and had the same time–temperature shift factor. Nevertheless, a ratio of the dielectrically and viscoelastically detected segmental relaxation times, r(M) = ωs,G/ωs,ε, and the dielectric relaxation intensity, Δε(M), decreased with increasing M up to M* ≅ 2 × 103 and then became insensitive to M on a further increase of M. The viscoelastic segmental relaxation reflects the cooperative torsion of the repeating units along the molecular backbone (as noted from rheo-optical data), while the dielectric segmental relaxation detects reorientational motion of those units affected by both intra- and intermolecular cooperativity (as noted from the basic dielectric expression). The observed decreases of r(M) and Δε(M) suggested that the dimension ξm of the whole OS molecule (over which the cooperative torsion occurs) is smaller than the length scale ξc for the intermolecular cooperative motion and that ξm approaches ξc on an increase of M up to M*. Consequently, the high-M PS molecules having ξm > ξc exhibited the M-insensitive r(M) and Δε(M). Thus, the M value for the crossover between these two regimes, M* ≅ 2 × 103, can be taken as the molecular weight of the cooperative sequence along the PS backbone. Furthermore, the quantitative similarity of the viscoelastic and dielectric mode distributions suggests that the cooperative torsion of the repeating units along the molecular backbone is governed by the cross-correlation of the units belonging to different molecules.