Thermo/pH-Responsive Multiamide-Functionalized Y-Junction-Bearing Polyacrylamides with Substituent-Dependent Thermal Reversibility

Abstract

Chain architecture and chemical composition of polymers can play vital roles in regulating thermoresponsive properties. Despite tremendous progress, it remains difficult to achieve LCST/upper critical solution temperature (LCST/UCST) behavior with tunable thermal hysteresis and outstripping. This study affords a promising strategy of an integrated multiamide and specific substituents to address the challenge. The incorporation of two substituents involving isopropyl, 2-diethylaminoethyl, or carbamoylmethyl into each Y junction of multiamide-functionalized Y-junction-bearing polymers (MAYJPs) allows enhancing polymer–polymer interactions. The disruption of intrinsic balances among hydrogen bonding, electrostatic interactions, and hydrophilic-to-hydrophobic ratio renders a multitunable phase transition. The location switching of heterosubstituents can result in either an inverse phase transition or a significantly different phase transition temperature. The thermodynamic or dynamic control over the hydration status of subunits leads to the occurrence of four kinds of thermal reversibility involving slight or significant hysteresis, consecutive outstripping and hysteresis, and pronounced outstripping upon heating–cooling cycles. The solvent isotope effect results in a distinct type or temperature of phase transition, and the pH effect is reflected in the pH-induced increase, decrease, or V-shaped evolution of phase transition temperature. In addition, the phase transition can accompany morphology transformation among spheres, vesicles, nanotubes, and lamellae. These fundamental findings are beneficial for gaining insights into the multiamide-related phase transition and thermal reversibility. Owing to the diversity of substituents, MAYJPs may serve as promising thermoresponsive polymers for multipurpose applications.