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