Segmental motion in poly(styrene-b-1,4-isoprene) block copolymers in the disordered state

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Abstract
The segmental dynamics associated with two primary relaxation processes in three poly(styrene‐b‐1,4‐isoprene) diblock copolymers with different proximity to the order–disorder transition (ODT) were investigated by photon correlation spectroscopy. Utilizing the selectivity of the depolarized light scattering, the presence of two distinct orientational processes were unambiguously revealed by the same experimental technique near but above ODT. These primary α‐relaxation modes result from the composition field configuration (‘‘pattern’’) in disordered block copolymers with correlation length ξc larger than the cooperative size ξα of the α relaxation near the glass transition Tg. The existence of a slowly fluctuating composition pattern can lead to the observed excess broadening of the segmental distribution of relaxation function L(log τ) beyond that of bulk homopolymers. For the slow α process, assigned to segmental orientation within microregions rich in polystyrene, a new finding is the temperature dependence of the width of the relaxation time distribution function. The relaxational characteristics of the two processes are discussed in the framework of free volume concepts and composition fluctuations within the cooperative volume (∼ξα3).