Rheological technique for determining the order–disorder transition of block copolymers

Abstract

AbstractA rheological technique is proposed for determining the thermally induced order–disorder transition of block copolymers. In the present investigation, a cone‐and‐plate rheometer was used to measure dynamic storage and loss moduli, G′(ω) and G″(ω), as a function of angular frequency ω of a commercial grade polystyrene‐block‐polyisoprene‐block‐polystyrene (SIS) tri‐block copolymer (KRATON D‐1107, Shell Development Company) in the temperature range from 140 to 240°C. For comparison purposes, dynamic viscoelastic properties of a commercial grade low‐density polyethylene (LDPE) were also determined in the temperature range from 160 to 238°C. We have found that log G′ versus log G″ plots for the LDPE show no temperature dependence, whereas log G′ versus log G″ plots for the SIS block copolymer do show systematic temperature dependence in the temperature range 140–230°C. This observation leads us to conclude that the order–disorder transition of the SIS block copolymer takes place gradually as the temperature is raised from 140 to 230°C. This conclusion is in good agreement with that drawn from the study of Roe (Ref. 33), who employed the same block copolymer using small‐angle x‐ray scattering. It is not possible to reach such a conclusion using log G′(ω) versus log ω, log G″(ω) versus log ω, or log η′(ω) versus log ω plots in which η′ is the dynamic viscosity. We have demonstrated further that the use of frequency‐temperature superposition is inappropriate for investigating the rheological behavior of block copolymer in the temperature range over which a thermally induced transition from an ordered structure to a disordered homogeneous phase occurs. We therefore suggest that when using information on dynamic viscoelastic properties, log G′ versus log G″ plots be used for determining the thermally induced order–disorder transition of block copolymers.