Kinetic effects on the phase behavior and microstructural transitions of a thermoresponsive polymer solution

TL;DR

This study explores how temperature influences the phase transitions and microstructure of Pluronic F127 solutions, revealing complex multi-step transitions, transient behaviors, and a comprehensive kinetic model through experimental and theoretical analyses.

Contribution

It introduces a detailed kinetic model and uncovers novel multi-step cooling transitions and metastable states in thermoresponsive Pluronic solutions.

Findings

Micellization temperature depends on thermal ramp rate.

A sharp sol to soft-solid transition occurs upon heating.

Cooling transitions involve multiple steps with transient metastable states.

Abstract

The thermoresponsive behavior of Pluronic F127 solutions is governed by temperature-dependent micellization and complex self-assembly of these micelles. This study investigates the effect of thermal stimuli on the kinetics of phase transition of Pluronic systems during heating and cooling cycles. We employ Differential Scanning Calorimetry measurements to investigate the dependence of the micellization temperature on thermal stimuli, revealing that both the micellization temperature and the peak intensity vary systematically with the applied thermal ramp rate. Furthermore, we employ rheological characterization which reveals a sharp sol to soft-solid transition upon heating. Interestingly, we observe a novel multi-step transition during the cooling phase, indicating a more complex reorganization pathway with intermediate metastable states than typically assumed for reversible micellization. Our findings indicate that the characteristic multi-step cooling transition is transient, gradually weakening with successive thermal cycles. We also present a comprehensive mathematical model which accurately captures the kinetics and multiple step transition in viscoelastic parameters. Significantly, the distinct peaks in Small-Angle X-ray Scattering (SAXS) measurements clearly reveal the evolution from a disordered unimers/micelles state at low temperatures to a highly ordered lattice with long-range spatial correlation at elevated temperatures. We also present a comprehensive phase diagram highlighting the critical role of thermal stimuli and pathways in defining the phase behavior of Pluronic system. This work, therefore, offers essential experimental and theoretical insights into the thermally driven self-assembly, transition kinetics, and microstructural evolution of thermoreversible Pluronic solution.