Phase Behaviour of Block Copolymer Pluronic: A Rheological Perspective

We study temperature induced phase change of a block copolymer Pluronic F127 (PEO₁₀₀-PPO₆₅-PEO₁₀₀) over a concentration range of 20-35 wt%. While this temperature dependent phase change visually appears like a liquid-soft solid transition, termed as gel in the literature, there is a debate regarding precise microstructure of the soft solid state. In this work, we conduct frequency (ω) sweep at various temperatures on F127 solution. We observe that irrespective of the concentration, F127 solution shows all the rheological characteristics of sol – gel – glass transition. This suggests the transition of a liquid-like sample to a space spanning percolated network, whose rheological characteristic is increase in tan δ with ω, followed by a glass transition as characterized by a peak in tan δ. Interestingly, the maxima in tan δ shifts to lower temperatures with an increase in ω, which illustrates the rate dependence of glass transition. The temperature associated with the glass transition decreases with increase in F127 concentration. The glass transition behavior is also observed at high temperature during gel melting. Finally, we construct a phase diagram and correlate it w.r.t various phase diagrams of F127 available in the literature.