Saloplastic Macroporous Polyelectrolyte Complexes: Cartilage Mimics

Complexes of sodium poly(4-styrenesulfonate) (NaPSS) and poly(diallyldimethylammonium chloride) (PDADMAC) were formed on mixing equimolar solutions in high salt concentration. Under ultracentrifugal fields, the complex precipitates were transformed into compact polyelectrolyte complexes (CoPECs), which showed extensive porosity. The mechanical properties of CoPECS make them attractive for bioimplants and tissue engineering applications. Free NaPSS chains in the closed pores of CoPECs create excess osmotic pressure, which controls the pore size and contributes to the mechanical resistance of the material. The mechanical properties of CoPECs, modulated by the ionic strength of the doping medium, were studied by uniaxial tensile testing and the stress−strain data were fit to a three-element Maxwell model which revealed at least two regimes of stress relaxation

Mechanical Properties of Osmotically Stressed Polyelectrolyte Complexes and Multilayers: Water as a Plasticizer

Compacted, macroporous complexes of poly­(styrenesulfonate) and poly­(diallyldimethylammonium) were dehydrated under defined osmotic stress using poly­(ethylene glycol), PEG. A strong mechanical response to dehydration was observed. At the lowest osmotic stress applied, macropores within the complex were compacted, and the material became transparent. With additional osmotic stress, the decrease in water content with increasing stress slowed considerably, but the complex became much stiffer, the equilibrium modulus reaching several hundred MPa. Concurrently, the complexes became more brittle. Multilayers of the same polyelectrolytes reached equilibrium hydration levels much faster and also increased significantly in modulus. Using an empirical fit, the plasticizing efficiency of water was shown to be exceptionally strong

Homogeneity, Modulus, and Viscoelasticity of Polyelectrolyte Multilayers by Nanoindentation: Refining the Buildup Mechanism

Atomic force microscopy, AFM, and nanoindentation of polyelectrolyte multilayers, PEMUs, made from poly­(diallyldimethylammonium), PDADMA, and poly­(styrene sulfonate), PSS, provided new insight into their surface morphology and growth mechanism. A strong odd/even alternation of surface modulus revealed greater extrinsic (counterion-balanced) charge compensation for fully hydrated multilayers ending in the polycation, PDADMA. These swings in modulus indicate a much more asymmetric layer-by-layer growth mechanism than previously proposed. Viscoelastic properties of the PEMU, which may contribute to cell response, were highlighted by variable indentation rates and minimized by extrapolating to zero indentation rate, at which point the surface and bulk equilibrium moduli were comparable. Variations in surface composition were probed at high resolution using force mapping, and the surface was found to be uniform, with no evidence of phase separation. AFM comparison of wet and dry films terminated with PSS and PDADMA revealed much greater swelling of the PDADMA-terminated PEMU by water, with collapse of surface roughness features in dry conditions. Dynamic and static contact angle measurements suggested less rearrangement for the glassy PSS surface

Thermal Transformations in Extruded Saloplastic Polyelectrolyte Complexes

Extruded, salt-plasticized complexes of hydrated poly­(styrenesulfonate), PSS, and poly­(diallyldimethylammonium), PDADMA, were analyzed by differential scanning calorimetry and dynamic mechanical thermal analysis. Whereas the enthalpic signatures were weak, the latter technique revealed a strong transition in modulus, identified as a glass transition. The temperature of this transition, <i>T</i>_g, varied with deformation rate as expected from time/temperature superposition. <i>T</i>_g also decreased with increasing salt doping, which breaks ion pairing in the complexes, confirming the plasticizing effect of doping. Time, temperature, and salt concentration data were superposed to demonstrate the trends/equivalence of these three variables, and an empirical equation was used to connect them. Measurement time regimes were discussed with reference to the average lifetime of an ion pair

A Small-Angle Neutron Scattering Study of the Equilibrium Conformation of Polyelectrolytes in Stoichiometric Saloplastic Polyelectrolyte Complexes

Stoichiometric polyelectrolyte complexes, PECs, from fully sulfonated poly­(styrenesulfonate), PSS, as polyanion and poly­(diallyldimethylammonium chloride), PDADMA, as polycation, were prepared by mixing them at optimized polyelectrolyte and NaCl concentrations. The complexes were compacted by ultracentrifugtion and then annealed in NaCl solutions at elevated temperatures to allow the polymers to fully intermix and relax. Small-angle neutron scattering, SANS, with contrast matching, was used to study single polyelectrolyte chain dimensions in PECs made from a mixture of deuterated and protonated PSS chains. Two PSS molecular weights in PECs were investigated at various ionic strengths. SANS curves, form factor fits, and corresponding Kratky plots indicate the Gaussian nature of the polyelectrolyte chains in the complexes regardless of molecular weight. PSS coils were slightly larger than the unperturbed dimension, more so for the higher molecular weight material, which was attributed to an effective stiffening of the chain due to ladderlike interactions between polyelectrolytes