7 research outputs found
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><sub>g</sub>, varied with deformation rate as expected from time/temperature
superposition. <i>T</i><sub>g</sub> 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