Segmental and chain dynamics in a diblock copolymer melt near the microphase separation transition

Abstract

The dynamics of segmental and chain orientation in poly(styrene- b-isoprene) (P(S- b-I)) block copolymer with PI fraction f PI = 0.61, degree of polymerization N = 70 and χN ⋍ 9 at 25°C (χ is the interaction parameter) were investigated by depolarized Rayleigh scattering (DRS) and dielectric spectroscopy (d.s.) over a broad temperature range (240–370 K) in the disordered phase. Two distinct primary relaxation processes with vastly different timescales were associated with segmental orientation within PS- and PI-rich regions selectively probed by DRS and d.s. techniques. The width of the distribution relaxation function for the slow segmental process was found to increase with decreasing temperature, whereas the freezing of the slow segmental fluctuations was observed for the first time within the time window of photon correlation spectroscopy. Compared with an equivalent PI homopolymer chain, the first normal mode describing the end-to-end fluctuation of the copolymer chain displayed slower time τ 1 but similar amplitude δε N f PI . The new experimental results were discussed in terms of the fluctuation picture of block copolymers in the disordered melt, the chain conformation and the friction coefficient ratio of the foreign blocks.