Phase behavior and dynamics of a micelle-forming triblock copolymer system

TL;DR

This study investigates the phase behavior and mechanical properties of Synperonic F-108, a triblock copolymer that forms micelles and crystalline phases, revealing its soft solid-like behavior and relaxation dynamics through rheological measurements.

Contribution

The paper provides a detailed phase diagram and rheological characterization of Synperonic F-108, highlighting its elastic and viscous properties and slow relaxation dynamics.

Findings

Micelles form spherical structures with dense PPO cores and hydrated PEO coronas.

At certain concentrations and temperatures, the material exhibits a soft solid-like phase.

The material shows frequency-independent storage modulus and slow relaxation behavior.

Abstract

Synperonic F-108 (generic name, 'pluronic') is a micelle forming triblock copolymer of type ABA, where A is polyethylene oxide (PEO) and B is polypropylene oxide (PPO). At high temperatures, the hydrophobicity of the PPO chains increase, and the pluronic molecules, when dissolved in an aqueous medium, self-associate into spherical micelles with dense PPO cores and hydrated PEO coronas. At appropriately high concentrations, these micelles arrange in a face centred cubic lattice to show inverse crystallization, with the samples exhibiting high-temperature crystalline and low-temperature fluidlike phases. By studying the evolution of the elastic and viscous moduli as temperature is increased at a fixed rate, we construct the concentration-temperature phase diagram of Synperonic F-108. For a certain range of temperatures and at appropriate sample concentrations, we observe a predominantly elastic response. Oscillatory strain amplitude sweep measurements on these samples show pronounced peaks in the loss moduli, a typical feature of soft solids. The soft solid-like nature of these materials is further demonstrated by measuring their frequency dependent mechanical moduli. The storage moduli are significantly larger than the loss moduli and are almost independent of the applied angular frequency. Finally, we perform strain rate frequency superposition (SRFS) experiments to measure the slow relaxation dynamics of this soft solid.