Influence of Phosphonium Alkyl Substituents on the Rheological and Thermal Properties of Phosphonium-PAA-Based Supramolecular Polymeric Assemblies

Abstract

A noncovalent synthetic strategy to supramolecular polymeric assemblies, including network structures, is described by the complexation of various phosphonium monocations and dications with the multianion, poly(acrylic acid). The alkyl chains surrounding the phosphonium cation were systematically varied from butyl, hexyl, to octyl in order to probe the effect of sterics and ion pairing on the resulting macroscopic properties of the assemblies. The supramolecular assemblies were characterized by TGA, DSC, oscillatory rheometry, steady-state flow rheometry, and SAXS. The rheological and thermal properties, as well as the flow activation energies, are highly dependent on the alkyl chain length. All of the supramolecular assemblies have glass transition temperatures lower than room temperature and range from 8 °C to below −40 °C. Di-ButC10PAA has the shortest alkyl chain length and affords the highest glass transition temperature. Correspondingly, it shows the largest viscosity and storage and loss moduli. For example, its viscosity is 3 orders of magnitude greater than di-OctC10PAA. In creep-recovery experiments, di-ButC10PAA shows the highest percent of strain recovery after the stress is removed, followed by di-HexC10PAA and di-OctC10PAA. The rheological and thermal properties of monoIL-PAA assemblies show similar alkyl chain length dependence, but the magnitude is significantly less because of the lack of cross-linking. A reversibility test of the supramolecular networks demonstrates that the ionic network material can fully reassemble within a short time period after disruption of the network due to heat or shear without sacrificing the mechanical properties.