Kinetic Effects on the Phase Behavior and Microstructural Transitions of a Thermoresponsive Polymer Solution

Abstract

Abstract The thermoresponsive behavior of Pluronic F127 (PF127) triblock copolymer solutions is fundamentally governed by temperature-dependent micellization and complex self-assembly of these micelles. However, a comprehensive understanding of the temperature-dependent kinetics and reversibility of PF127 phase transitions during cyclic heating–cooling remains limited. This study systematically 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 (DSC) 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 applied thermal ramp rates. Furthermore, we employ rheological characterization which reveals a sharp sol to soft-solid transition upon heating. Interestingly, we observe a novel multistep transition during the cooling cycle, indicating a more complex reorganization pathway with intermediate metastable states than typically assumed for reversible micellization. We also investigate the effect of thermal cycling on the reversibility and thermal stability of the polymer system. Our findings indicate that the characteristic multistep cooling transition is transient, gradually weakening with successive thermal cycles. We also present a phenomenological 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 under thermal equilibrated conditions 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.