Influence of Hydrophobicity on Polyelectrolyte Complexation

Abstract

Polyelectrolyte complexes are a fascinating class of soft materials that can span the full spectrum of mechanical properties from low-viscosity fluids to glassy solids. This spectrum can be accessed by modulating the extent of electrostatic association in these complexes. However, to realize the full potential of polyelectrolyte complexes as functional materials, their molecular level details need to be clearly correlated with their mechanical response. The present work demonstrates that by making simple amendments to the chain architecture, it is possible to affect the salt responsiveness of polyelectrolyte complexes in a systematic manner. This is achieved by quaternizing poly­(4-vinylpyridine) (QVP) with methyl, ethyl, and propyl substituentsthereby increasing the hydrophobicity with increasing side chain lengthand complexing them with a common anionic polyelectrolyte, poly­(styrene­sulfonate). The mechanical behavior of these complexes is compared to the more hydrophilic system of poly­(styrene­sulfonate) and poly­(diallyldimethyl­ammonium) by quantifying the swelling behavior in response to salt stimuli. More hydrophobic complexes are found to be more resistant to doping by salt, yet the mechanical properties of the complex remain contingent on the overall swelling ratio of the complex itself, following near universal swelling–modulus master curves that are quantified in this work. The rheological behaviors of QVP complex coacervates are found to be approximately the same, only requiring higher salt concentrations to overcome strong hydrophobic interactions, demonstrating that hydrophobicity can be used as an important parameter for tuning the stability of polyelectrolyte complexes in general, while still preserving the ability to be processed “saloplastically”.

Recent progress in the science of complex coacervation

“…32,[41][42][43][44][45][46] Turbid mixtures correspond to phase separation. However, other methods such as UV-Vis, 34,47,48 or fluorescence spectroscopy, 27,40,49 ionic conductivity, 34,50,51 thermogravimetric analysis (TGA), 35,51 NMR spectroscopy, 50 radiolabelling, 52 and quartz crystal microbalance (QCM) 53 have been used to measure concentrations of polymers and/or salt. Surface properties such as the interfacial tension between the surfactant and coacervate phase have been measured using atomic force microscopy 38,54 and surface forces apparatus.…”

Underwater Adhesion of Multiresponsive Complex Coacervates

“…Below the CSC, the mechanical properties can be tuned by choosing the appropriate salt concentration: the higher the I , the weaker are the electrostatic interactions keeping together the complex coacervate phase, which is therefore more mechanically compliant . Consequently, by decreasing I , polyelectrolyte mixtures can result in materials spanning from viscoelastic liquids to solid complexes …”

“…[28] Consequently, by decreasing I, polyelectrolyte mixtures can result in materials spanning from viscoelastic liquids to solid complexes. [29,30] In a previous work, [31] we have reported the development of a fully synthetic complex coacervate-based adhesive with a setting mechanism activated by a change in temperature. The adhesive formulation is obtained by mixing oppositely charged polyelectrolytes bearing thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) chains at an added salt concentration just below the CSC, in order to obtain a fluid complex coacervate phase of low viscosity but still separated from water before injection.…”

Influence of the Salt Concentration on the Properties of Salt‐Free Polyelectrolyte Complex Membranes

“…It has been reported in a number of papers that water hydrating the polyionic structure in PECs can act as a plasticizer and that, consequently, the T g usually decreases as the degree of hydration increases. [6,21] Therefore, the water uptake of the saltfree films prepared here was measured in order to elucidate to what extent the [NaCl] used during processing influences the water content of the samples and how it latter impacts the T g . The PEC films were thus immersed for 24 h in water, in order to achieve equilibrium swelling.…”