Co-reporter:Jingcheng Fu, Rachel L. Abbett, Hadi M. Fares, and Joseph B. Schlenoff
ACS Macro Letters October 17, 2017 Volume 6(Issue 10) pp:1114-1114
Publication Date(Web):September 22, 2017
DOI:10.1021/acsmacrolett.7b00668
Hydrated polyelectrolyte complexes, H-PECs, have recently started attracting renewed interest as a class of highly solvated/plasticized blends. H-PECs are observed to undergo a transition in mechanical properties close to room temperature. Whether this is a true glass transition has been questioned recently: the material has an unusually low modulus in the “glassy” state and molecular dynamics simulations have suggested temperature-induced dehydration and water structure changes are responsible for the transition. Using in situ infrared spectroscopic methods on thin films of a widely studied H-PEC we find no definitive evidence for changes in the hydration state of functional groups, the water content, or water structure on passing through Tg for stoichiometric and nonstoichiometric H-PECs. These complexes represent a promising platform for fundamental studies of the glass transition, since the coupling between chains can be modified by “doping” the material with salt, which breaks ion pairing cross-links. The Fox equation was used to estimate Tgs for paired and unpaired oppositely charged repeat units.
Co-reporter:Joseph B. Schlenoff;Carlos J. Arias;Richard L. Surmaitis
Langmuir May 31, 2016 Volume 32(Issue 21) pp:5412-5421
Publication Date(Web):2017-2-22
DOI:10.1021/acs.langmuir.6b00784
The adhesion of living eukaryotic cells to a substrate, one of the most complex problems in surface science, requires adsorption of extracellular proteins such as fibronectin. Thin films of polyelectrolyte complex made layer-by-layer (polyelectrolyte multilayers or PEMUs) offer a high degree of control of surface charge and composition—interconnected and essential variables for protein adhesion. Fibroblasts grown on multilayers of poly(styrenesulfonate), PSS, and poly(diallyldimethylammonium), PDADMA, with increasing thickness exhibit good adhesion until the 12th layer of polyelectrolyte has been added, whereupon there is a sudden transition to nonadhesive behavior. This sharp change is due to the migration of excess positive charge to the surface—a previously unrecognized property of PEMUs. Precise radiotracer assays of adsorbed 125I-albumin show how protein adsorption is related to multilayer surface charge. With more negative surface charge density from the sulfonates of PSS, more albumin adsorbs to the surface. However, a loosely held or “soft corona” exchanges with serum protein under the Vroman effect, which is correlated with poor cell adhesion. A comprehensive view of cell adhesion highlights the central role of robust protein adhesion, which is required before any secondary effects of matrix stiffness on cell fate can come into play.
Co-reporter:Hadi M. Fares and Joseph B. Schlenoff
Macromolecules May 23, 2017 Volume 50(Issue 10) pp:3968-3968
Publication Date(Web):May 9, 2017
DOI:10.1021/acs.macromol.7b00665
Association between positive, Pol+, and negative, Pol–, units on polyelectrolytes drives spontaneous formation of a range of morphologies, some with “fuzzy” structure but most essentially amorphous. An excess of one type of charge over the other, known as overcompensation or overcharging, is essential for certain types of processing, such as the formation of polyelectrolyte “multilayers” on substrates or “polyplex” nanoparticles in solution. In this work, uniform, stoichiometric, smooth thin films of polyelectrolyte complex, PEC, from poly(diallyldimethylammonium), PDADMA, and poly(styrenesulfonate), PSS, were prepared starting from rough, nonstoichiometric multilayers of these materials. A narrow concentration range of added salt was found which promoted steady-state bulk overcompensation of PEC films in the presence of a large excess of polycation or polyanion without loss of PEC to solution. The extent of overcompensation, about 35% for PDADMA in 1.0 M NaCl and about 40% for PSS in 1.4 M NaCl, was independent of solution polymer concentration and only weakly dependent on salt concentration. A virtual dependence of overcompensation on molecular weight was also determined. Erosion/instability of films for [NaCl] > 1.4 M was observed, with more prominent or faster erosion for higher molecular weight PSS. The mechanism for overcompensation in this entropically driven system was attributed to the formation of a Donnan ion equilibrium between the PEC and solution phases.
Co-reporter:Yara E. Ghoussoub
Langmuir May 12, 2015 Volume 31(Issue 18) pp:5078-5085
Publication Date(Web):2017-2-22
DOI:10.1021/acs.langmuir.5b00975
Little is known concerning the interface between a polyelectrolyte multilayer, PEMU, and its substrate. Recent models suggest that excess polymer charge, compensated by counterions, remains buried within the PEMU, especially for thicker films having a nonlinear component to their growth. We report a novel approach for making free-standing multilayers of poly(diallyldimethylammonium) (PDADMA) and poly(styrenesulfonate) (PSS): after assembly on aluminum substrates, films were released by brief immersion in aqueous alkali. The multilayers were then flipped, allowing access to the initially buried substrate/PEMU interface. Experiments were performed to show that this method of release, one of many established for PEMUs, perturbed the surface and bulk of the film minimally. Film/solution and film/substrate interfaces were compared using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). AFM was used to record topography and perform nanoindentation, while XPS provided surface elemental composition. All three methods revealed data consistent with an excess of PDADMA at the buried interface. This excess PDADMA was then complexed with additional PSS to yield “nanosandwiches” of nonstoichiometric PEMU between layers of stoichiometric PEMU.
Co-reporter:Jose D. Delgado and Joseph B. Schlenoff
Macromolecules June 13, 2017 Volume 50(Issue 11) pp:4454-4454
Publication Date(Web):May 25, 2017
DOI:10.1021/acs.macromol.7b00525
Polymers made from zwitterionic repeat units (bearing no net charge) have intriguing solution properties, especially in contrast to polyelectrolytes, such as an apparent indifference to salt concentration. These polyzwitterions (PZs) have come under renewed scrutiny because of their use in high performance antifouling coatings. Here, an amidosulfobetaine polymer was used to shed light on the complex and poorly understood response of PZ solution conformation to ionic strength. A Hofmeister anion series NaX, where X = SO42–, Cl–, Br–, NO3–, ClO4–, and SCN–, provided a systematic way to tune PZ/ion interactions. A consistent picture of PZ conformation emerged, where the role and location of counterions (how they pair with the polymer chain) depend on their position in the Hofmeister series. At least four regimes of PZ conformation/interaction as a function of ionic strength were observed, the last showing no change in coil size (hydrodynamic radius) as a function of ionic strength for all monovalent salts in the concentration range 0.6–4 M. Hydrophobic (less hydrated) anions ClO4– and SCN– yielded a clear minimum in coil size at lower [NaX], whereas PZ in solutions of hydrophilic ions SO42– and Cl– showed only a hint of the much-discussed “anti-polyelectrolyte” expansion of PZ with increasing [NaX]. Static light scattering results, when analyzed using Stockmeyer’s theory of scattering from multicomponent systems, revealed that NaX is associated with PZ with a corresponding increase in apparent molecular weight. Static light scattering measurements at low [NaX] show solution ions are excluded from PZ coils dressed with hydrophobic NaX. Dynamic light scattering in salt-free solutions at elevated temperatures revealed substantial chain stiffening of PZ, thought to be caused by nearest-neighbor interactions between zwitterion groups. DLS yielded a fast mode in these salt-free solutions, ascribed to soliton-like transport of waves of associated zwitterionic groups along the PZ backbone.
Co-reporter:Jingcheng Fu, Hadi M. Fares, and Joseph B. Schlenoff
Macromolecules 2017 Volume 50(Issue 3) pp:
Publication Date(Web):January 26, 2017
DOI:10.1021/acs.macromol.6b02445
Polyelectrolyte complexes, PECs, are spontaneously formed blends of polyelectrolytes bearing positive, Pol+, and negative, Pol–, repeat units. Many interesting PEC morphologies have been observed, ranging from dense precipitates to liquidlike coacervates to quasi-stable nanoparticles, depending on the identity of the polymers and the preparation conditions. While the number of polyelectrolytes available to synthesize these materials is large and increasing, the corresponding number of Pol+/Pol– combinations is vast. This work quantitatively compares the binding strengths between a selection of positive and negative polyelectrolytes by evaluating the extent to which ion pairs between them are broken by a common salt, KBr. Comparison of association constants or Gibbs free energies between different classes of ionic functionality reveals that more “hydrophilic” PECs are more weakly associated, small primary amines bind strongly, carboxylates bind weakly, and aromatic sulfonates interact more strongly than aliphatic ones. The use of “charge density” to predict binding strength is shown not to be justified. Ion diffusion coefficients through PECs also approximately follow water content and are inversely related to interaction strength.
Co-reporter:Nandita Abhyankar, Yara E. Ghoussoub, Qifeng Wang, Naresh S. Dalal, and Joseph B. Schlenoff
The Journal of Physical Chemistry B 2016 Volume 120(Issue 27) pp:6771-6777
Publication Date(Web):July 1, 2016
DOI:10.1021/acs.jpcb.6b02697
Amorphous hydrated complexes of the polyelectrolytes poly(styrene sulfonate) (PSS) and poly(diallyldimethylammonium) were doped with the spin-5/2 ion Mn2+. X-band electron paramagnetic resonance (EPR) measurements of the Mn2+ spins within these stoichiometric polyelectrolyte complexes (PECs) revealed an octahedral coordination environment, similar to that observed in aqueous solutions of Mn2+. This octahedral symmetry of the [Mn(H2O)6]2+ complexes, observed in fully hydrated PECs, is somewhat distorted because of the wide range of ion pairs possible with the sulfonate group on PSS. As the Mn2+ concentration was increased, the linewidths broadened, indicating the dominance of dipolar broadening over exchange narrowing in determining the linewidths; that is, any exchange narrowing was masked by the large dipolar broadening. The calculated linewidths were used to estimate the strengths of the dipolar interactions, and hence the distances between the Mn2+ spins, on the basis of a simple model of regularly spaced spins. The distances calculated by this method were roughly comparable to the geometric average distances calculated on the basis of the Mn2+ concentrations and densities of the doped PEC samples. From a comparison of their EPR spectra, the ion environments in the doped, fully hydrated PECs were found to be similar to those in hydrated classical ion exchange resins. EPR spectra before and after drying of the PECs indicate the replacement of octahedrally coordinated water by oxide anions from the polyanion chain and the corresponding loss of the symmetric environment of Mn2+ ions.
Co-reporter:Yara E. Ghoussoub and Joseph B. Schlenoff
Langmuir 2016 Volume 32(Issue 15) pp:3623-3629
Publication Date(Web):April 7, 2016
DOI:10.1021/acs.langmuir.6b00672
To make a two-dimensional Janus object, the perfluorinated anionic polyelectrolyte Nafion was adsorbed to the surface of ultrathin films of polyelectrolyte complex. Nafion changed the wetting characteristics of the polyelectrolyte multilayer (PEMU) of poly(diallyldimethylammonium) and poly(styrenesulfonate) from hydrophilic to hydrophobic. PEMUs assembled on aluminum substrates and terminated with Nafion could be released by exposure to alkali solution, producing free-floating films in the 100 nm thickness regime. Water contact angle measurements showed a strong difference in hydrophilicity between the two sides of this Janus film, which was further characterized using atomic force microscopy and X-ray photoelectron spectroscopy (XPS). XPS revealed different fluorine contents on both sides of the PEMU, which could be translated to a Nafion gradient through the film. Fourier transform infrared spectroscopy showed the Nafion-containing films were much more resistant to decomposition by high salt concentration.
Co-reporter:Qifeng Wang
Advanced Materials 2015 Volume 27( Issue 12) pp:2077-2082
Publication Date(Web):
DOI:10.1002/adma.201405376
Co-reporter:Jingcheng Fu, Qifeng Wang, and Joseph B. Schlenoff
ACS Applied Materials & Interfaces 2015 Volume 7(Issue 1) pp:895
Publication Date(Web):December 19, 2014
DOI:10.1021/am5074694
Iron oxide nanoparticles of diameter ca. 12 nm were dispersed into polyelectrolyte complexes made from poly(styrenesulfonate) and poly(diallyldimethylammonium). These nanocomposites were plasticized with salt water and extruded into dense, tough fibers. Magnetometry of these composites showed they retained the superparamagnetic properties of their constituent nanoparticles with saturation magnetization that scaled with the loading of nanoparticles. Their superparamagnetic response allowed the composites to be heated remotely by radiofrequency fields. While the modulus of fibers was unaffected by the presence of nanoparticles the toughness and tensile strength increased significantly.Keywords: magnetic nanoparticle; PDADMA; polyelectrolyte complex; PSS; radiofrequency heating; SPION
Co-reporter:Kristopher D. Kelly and Joseph B. Schlenoff
ACS Applied Materials & Interfaces 2015 Volume 7(Issue 25) pp:13980
Publication Date(Web):June 16, 2015
DOI:10.1021/acsami.5b02988
Thin films of complexes made from oppositely charged polyelectrolytes have applications as supported membranes for separations, cell growth substrates, anticorrosion coatings, biocompatible coatings, and drug release media, among others. The relatively recent technique of layer-by-layer assembly reliably yields conformal coatings on substrates but is impractically slow for films with thickness greater than about 1 μm, even when accelerated many fold by spraying and/or spin assembly. In the present work, thin, uniform, smooth films of a polyelectrolyte complex (PEC) are rapidly made by spin-coating a polyelectrolyte coacervate, a strongly hydrated viscoelastic liquidlike form of PEC, on a substrate. While the apparatus used to deposit the PEC film is conventional, the behavior of the coacervate, especially the response to salt concentration, is highly nontraditional. After glassification by immersion in water, spun-on films may be released from their substrates to yield free-standing membranes of thickness in the micrometer range.Keywords: layer by layer; LbL; membrane; multilayers; separations; viscosity;
Co-reporter:Hadi M. Fares, Yara E. Ghoussoub, Richard L. Surmaitis, and Joseph B. Schlenoff
Langmuir 2015 Volume 31(Issue 21) pp:5787-5795
Publication Date(Web):February 22, 2015
DOI:10.1021/la504910y
Polyelectrolyte multilayers (PEMUs) are made from various combinations of polyanions and polycations. It is now understood that these ultrathin films of polyelectrolyte complex may also incorporate counterions derived from the solutions from which the PEMU was deposited or exchanged into the film postassembly. If these ions are required to compensate nonstoichiometric ratios of polycation and polyanion they cannot leave the film and exert considerable influence on film properties, such as modulus and permeability. These “extrinsic” charges also complicate fundamental studies on PEMUs. We report a method to remove almost all ionic content from a PEMU made of poly(diallyldimethylammonium chloride), PDADMAC, and poly(styrenesulfonate), PSS. In this method, a high salt concentration plasticizes the multilayer past its glass transition, dispersing all the buried excess PDADMA throughout the film. Exposure to a solution of PSS in a lower salt concentration consumes excess PDADMA near the surface without overcompensating with PSS. The process is repeated in a cyclic fashion, removing >95% of the ions charge present in the as-made PEMU.
Co-reporter:Qifeng Wang and Joseph B. Schlenoff
RSC Advances 2014 vol. 4(Issue 87) pp:46675-46679
Publication Date(Web):26 Sep 2014
DOI:10.1039/C4RA08733J
Polyelectrolyte complexes, PECs, are formed spontaneously by the interaction of oppositely charged polyelectrolytes. When hydrated, PECs are tough, elastic, biocompatible materials, but when dry they are hard and brittle. In either form, PECs have long been considered “unprocessable”. Here, we show that PECs are transformed from brittle to tough (when dry) by extrusion into highly strained fibers. Partial molecular strain in dry fibers may be relaxed by exposure to solutions of salt or hot water under a salt/temperature equivalence. Efficient shape memory of a complex strained in hot water is possible by cooling and recovering the original shape later in hot water.
Co-reporter:Qifeng Wang and Joseph B. Schlenoff
Macromolecules 2014 Volume 47(Issue 9) pp:3108-3116
Publication Date(Web):April 28, 2014
DOI:10.1021/ma500500q
Stoichiometric polyelectrolyte complexes (PECs) of the strong polyelectrolytes poly(styrenesulfonate) (PSS) and poly(diallyldimethylammonium) (PDADMA) were dissociated and dissolved in aqueous KBr. Water was added to dilute the salt, allowing polyelectrolytes to reassociate. After appropriate equilibration, these mixtures yielded compositions spanning complexes (solid) to coacervates (elastic liquid) to dissolved solutions with increasing [KBr]. These compositions were defined by a ternary polymer/water/salt phase diagram. For coacervates, transient microphase separation could be induced by a small departure from equilibration temperature. A boundary between complex and coacervate states was defined by the crossover point between loss and storage modulus. Salt ions within the complex/coacervate were identified as either ion paired with polyelectrolytes (“doping”) or unassociated. The fraction of ion pair cross-links between polyelectrolytes as a function of KBr concentration was used to account for viscosity using a model of “sticky” reptation.
Co-reporter:Maria Ferriz-Mañas and Joseph B. Schlenoff
Langmuir 2014 Volume 30(Issue 29) pp:8776-8783
Publication Date(Web):2017-2-22
DOI:10.1021/la5015785
Zeta potentials of surfaces bearing stable mono- or multilayers of polyelectrolyte were determined using the spinning disk method recently described by Sides et al. (Langmuir 2004, 20, 11493−11498). In this technique, the streaming potential difference between two electrodes, one at the disk surface, is quantitatively related to zeta potential. Variables such as rotation speed, electrolyte concentration, and electrode distance from the disk surface were explored and used to validate the recently-described theory, which emphasizes minimal contribution to net potential from surface conductivity. Layer-by-layer oscillations in sign and magnitude of the zeta potential were observed, in accord with prior work using electrophoretic mobility of multilayer-coated particles and other streaming potential measurements. The open geometry and the excellent mass transport of the spinning disk allowed in-situ observation of surface charge switching during the addition of a layer. As with all zeta potentials, especially those recorded at soft interfaces, translating results to quantitative densities of fixed surface charge is a challenge.
Co-reporter:Ramy A. Ghostine ; Marie Z. Markarian
Journal of the American Chemical Society 2013 Volume 135(Issue 20) pp:7636-7646
Publication Date(Web):May 14, 2013
DOI:10.1021/ja401318m
Radioactive counterions were used to track the ratio of positive to negative polymer repeat units within a polyelectrolyte multilayer made from poly(diallyldimethylammonium chloride), PDADMAC, and poly(styrene sulfonate), PSS. For this widely employed pair of “linearly” assembled polyelectrolytes it was found that the accepted model of charge overcompensation for each layer is incorrect. In fact, overcompensation at the surface occurs only on the addition of the polycation, whereas PSS merely compensates the PDADMAC. After the assembly of about a dozen layers, excess positive sites begin to accrue in the multilayer. Treating the surface as a reaction–diffusion region for pairing of polymer charges, a model profile was constructed. It is shown that different reaction–diffusion ranges of positive and negative polyelectrolyte charge lead to a blanket of glassy, stoichiometric complex growing on top of a layer of rubbery, PDADMAC-rich complex. Though overcompensation and growth was highly asymmetric with respect to the layer number, entirely conventional “linear” assembly of the multilayer was observed. The impact of asymmetric growth on various properties of multilayers is discussed.
Co-reporter:Ramy A. Ghostine, Rana M. Jisr, Ali Lehaf, and Joseph B. Schlenoff
Langmuir 2013 Volume 29(Issue 37) pp:11742-11750
Publication Date(Web):September 4, 2013
DOI:10.1021/la401632x
The surface roughness of polyelectrolyte multilayers made from poly(diallyldimethylammonium chloride), PDADMAC, and poly(styrene sulfonate), PSS, was measured as a function of film deposition conditions. For dry multilayers, the significant roughness which builds up for thicker films is much more apparent for multilayers terminated with PSS. Corresponding roughness for PDADMA-capped multilayers may be seen by imaging in situ under electrolyte. Roughness may be substantially reduced, but not eliminated, by annealing in salt. Annealing does not lead to loss of polyelectrolyte from the film, even under conditions where the salt concentration is high enough to place the film properties beyond the glass transition. Roughness does not correlate with the molecular weight of the polyelectrolyte and is thus not caused by solution or film polymer chain conformations. The wavelength of the roughness features is approximately proportional to film thickness, which supports a mechanism whereby roughness is generated by anisotropic swelling due to water and polyelectrolyte addition in a manner similar to water uptake in hydrogels. Roughness is preserved by the glassy PSS layer and probably incorporated within the film as it grows.
Co-reporter:Rana M. Jisr, Thomas C. S. Keller III, and Joseph B. Schlenoff
Langmuir 2013 Volume 29(Issue 50) pp:15579-15588
Publication Date(Web):2017-2-22
DOI:10.1021/la403853z
A series of copolyelectrolytes with randomly positioned fluorinated (hydrophobic) and zwitterionic (hydrophilic) repeat units was synthesized and used to assemble multilayers. Regular layer-by-layer growth was observed for polymers with a charge density as low as 6%. The hydrophobicity of these “schizophobic” surfaces increased with increasing fluorine content. Polymer-on-polymer stamping was used to create patterned areas of low and high friction, probed by lateral force microscopy using a modified hydrophobic tip. “Contractile” A7r5 smooth muscle cells adhered to the fluorinated surfaces, but the introduction of zwitterion functionality induced a motile, less firmly attached morphology consistent with the “synthetic” motile phenotype of this cell line. In contrast with cells well adhered (on fluorinated) or completely nonadhering (on zwitterionic) films, incorporation of closely spaced repeat units with strongly contrasting hydrophobicity appears to generate intermediate cell adhesion behavior.
Co-reporter:Ramy A. Ghostine, Rabih F. Shamoun, and Joseph B. Schlenoff
Macromolecules 2013 Volume 46(Issue 10) pp:4089-4094
Publication Date(Web):May 1, 2013
DOI:10.1021/ma4004083
Doping constants and diffusion coefficients for an extruded, stoichiometric, dense polyelectrolyte complex, PEC, were determined for a Hofmeister series of anions. These thermodynamic and kinetic parameters describe the extent and speed to which a complex of poly(styrenesulfonate) and poly(diallyldimethylammonium) may be doped. Both parameters followed a Hofmeister ordering and covered a wide range of response. Differences between doping and undoping kinetics were observed, with the latter adhering well to classical diffusion from the cylindrical geometry employed. Tracer diffusion of radiolabeled Na+, compared with coupled diffusion of NaCl, revealed slightly faster diffusion of Na+ compared to Cl– ions within the PEC.
Co-reporter:Jessica E. Coughlin;Andreas Reisch;Marie Z. Markarian
Journal of Polymer Science Part A: Polymer Chemistry 2013 Volume 51( Issue 11) pp:2416-2424
Publication Date(Web):
DOI:10.1002/pola.26627
ABSTRACT
Sulfonation of narrow polydispersity polystyrene, PS, standards remains the method of choice for generating polystyrene sulfonate, PSS, samples with defined composition. Although a variety of sulfonation techniques have been described, relatively little is reported on the material obtained, which is used for so many studies on the fundamental behavior of polyelectrolytes. Here, we show that powdered polystyrene treated with concentrated sulfuric acid (96%) at 90 °C without catalyst yields fully sulfonated PSS. Extensive characterization with 1H and 13C NMR as well as size exclusion chromatography coupled with static and dynamic light scattering shows no evidence of sulfone crosslinking or chain degradation under the conditions used. Though mono-sulfonated as soon as it dissolves in the acid, the PSS contains about 6% meta substitution. Sulfonation kinetics for this heterogeneous reaction depend strongly on particle size, sulfuric acid content and temperature. For preparing perdeuterated PSS from the corresponding PS it is essential to employ D2SO4, as about half of the aromatic units undergo H/D exchange during sulfonation. The remaining ortho H/D may be exchanged with extended exposure to the concentrated sulfuric acid, but the meta site is deactivated. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 2416–2624
Co-reporter:Zaki G. Estephan, Hanaa H. Hariri, and Joseph B. Schlenoff
Langmuir 2013 Volume 29(Issue 8) pp:2572-2579
Publication Date(Web):February 8, 2013
DOI:10.1021/la304872d
Stable aqueous dispersions of superparamagnetic iron oxide nanoparticles were synthesized in one step in the presence of a zwitterionic siloxane as the stabilizing/capping/solubilizing ligand. The hydrodynamic diameter of the particles was tuned by controlling the concentration of zwitterion siloxane, which ultimately yielded monodisperse nanoparticles small enough for renal filtration (<6 nm diameter). The zwitterated nanoparticles were readily dispersed and stable in aqueous media in the pH range 6–9 but exhibited lower magnetization values than nonzwitterated materials due to amorphous content and spin canting, typical for particles of such size. Turbidimetry and light scattering studies revealed no interaction between the particles and proteins, suggesting the materials will circulate well in vivo.
Co-reporter:Rabih F. Shamoun;Andreas Reisch
Advanced Functional Materials 2012 Volume 22( Issue 9) pp:1923-1931
Publication Date(Web):
DOI:10.1002/adfm.201102787
Abstract
Tough, dense polyelectrolyte complexes (PECs) with well-defined cross-sections are prepared using a laboratory extruder and plasticizing the complexes with salt water. Stoichiometric starting materials yield stoichiometric complexes of poly(diallyldimethylammonium) (PDADMA) and poly(styrene sulfonate) (PSS). As an example of this enabling technology, macroscopic tubes of PEC are produced. Microscopy images of cross-sections of rods, tape, and tubes show a pore volume of less than 10% in the bulk of the extruded complex and fully dense material towards the surface, where the shear is greatest. Thermal gravimetric analysis reveals the expected salt content for PECs doped with NaCl, and a lack of salt for PECs rinsed in water. The fact that doped PECs are transparent suggests they are supersaturated with salt. Residual stress following extrusion is relieved by exposure to solutions of NaCl. Stress relaxation experiments show decreasing equilibrium moduli as a function of increasing salt doping, consistent with prior results on multilayers of the same polymers.
Co-reporter:Marie Z. Markarian, Haifa H. Hariri, Andreas Reisch, Volker S. Urban, and Joseph B. Schlenoff
Macromolecules 2012 Volume 45(Issue 2) pp:1016-1024
Publication Date(Web):December 20, 2011
DOI:10.1021/ma2022666
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.
Co-reporter:Haifa H. Hariri, Ali M. Lehaf, and Joseph B. Schlenoff
Macromolecules 2012 Volume 45(Issue 23) pp:9364-9372
Publication Date(Web):November 21, 2012
DOI:10.1021/ma302055m
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.
Co-reporter:Rabih F. Shamoun, Haifa H. Hariri, Ramy A. Ghostine, and Joseph B. Schlenoff
Macromolecules 2012 Volume 45(Issue 24) pp:9759-9767
Publication Date(Web):December 11, 2012
DOI:10.1021/ma302075p
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, Tg, varied with deformation rate as expected from time/temperature superposition. Tg 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.
Co-reporter:Ali M. Lehaf, Haifa H. Hariri, and Joseph B. Schlenoff
Langmuir 2012 Volume 28(Issue 15) pp:6348-6355
Publication Date(Web):April 5, 2012
DOI:10.1021/la300482x
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.
Co-reporter:Layal L. Rouhana ; Maroun D. Moussallem
Journal of the American Chemical Society 2011 Volume 133(Issue 40) pp:16080-16091
Publication Date(Web):August 11, 2011
DOI:10.1021/ja2041833
The adsorption of water-soluble alkane thiols and their corresponding disulfides onto gold was followed in real time using highly sensitive surface conductivity measurements. Particular attention was paid to producing clean surfaces and to the purity of the adsorbates. The rate of mass transport to the surface was constant, controlled, and measured, over the whole time course of the experiment (1–104 s), by convective diffusion. An adsorption rate equation derived for coupled steady state convective-diffusion mass transport and Langmuir kinetics shows that systems limited by mass transport must also be slowed by Langmuir kinetics. Thiols and disulfides adsorbed at the same rate, limited mainly by mass transport. The distinct slowdown in adsorption rate for longer alkanethiols, attributed to conformational transitions (lying down → standing up), was less evident for the neutral thiols/disulfides. The slower rate of charged thiol adsorption is thought to stem from steric interactions of large, hydrated tail groups, although calcium as a counterion accelerated monolayer formation. The adsorption kinetics of a charged thiol were almost the same under screened (by extra added salt) or unscreened conditions. Therefore, long-range electrostatic interactions appear to be less important than short-range steric ones in limiting adsorption rates at surfaces.
Co-reporter:Jessica S. Martinez, Thomas C. S. Keller III, and Joseph B. Schlenoff
Biomacromolecules 2011 Volume 12(Issue 11) pp:
Publication Date(Web):October 25, 2011
DOI:10.1021/bm201142x
The cytotoxicity of polyelectrolytes commonly employed for layer-by-layer deposition of polyelectrolyte multilayers (PEMUs) was assessed using rat smooth muscle A7r5 and human osteosarcoma U-2 OS cells. Cell growth, viability, and metabolic assays were used to compare the responses of both cell lines to poly(acrylic acid), PAA, and poly(allylamine hydrochloride), PAH, in solution at concentrations up to 10 mM and to varying thicknesses of (PAA/PAH) PEMUs. Cytotoxicity correlated with increasing concentration of solution polyelectrolytes for both cell types and was greater for the positively charged PAH than for the negatively charged PAA. While metabolism and proliferation of both cell types was slower on PEMUs than on tissue culture plastic, little evidence for direct toxicity on cells was observed. In fact, evidence for more extensive adhesion and cytoskeletal organization was observed with PAH-terminated PEMUs. Differences in cell activity and viability on different thickness PEMU surfaces resulted primarily from differences in attachment for these adhesion-dependent cell lines.
Co-reporter:Zaki G. Estephan, Philip S. Schlenoff, and Joseph B. Schlenoff
Langmuir 2011 Volume 27(Issue 11) pp:6794-6800
Publication Date(Web):May 2, 2011
DOI:10.1021/la200227b
A direct, head-to-head comparison of the efficacy of a zwitterionic versus a poly(ethylene glycol), PEG, coating in preventing protein adsorption to silica and aggregation of silica nanoparticles is presented. The same siloxane coupling chemistry was employed to yield surfaces with similar coverages of both types of ligand. Nanoparticle and planar surfaces were challenged with salt, serum, lysozyme, and serum albumin at 25 and 37 °C. While both types of surface modification are highly effective in preventing protein adsorption and nanoparticle aggregation, the zwitterion provided monolayer-type coverage with minimal thickness, whereas the PEG appeared to yield a more three-dimensional coating. The mechanism for adsorption resistance is thought to be based on preventing ion pairing between protein and surface charges, which releases counterions and water molecules, an entropic driving force enough to overcome a disfavored enthalpy of adsorption.
Co-reporter:Ali M. Lehaf, Maroun D. Moussallem, and Joseph B. Schlenoff
Langmuir 2011 Volume 27(Issue 8) pp:4756-4763
Publication Date(Web):March 28, 2011
DOI:10.1021/la200229h
Photo-cross-linkable polyelectrolyte multilayers were made from poly(allylamine) (PAH) and poly(acrylic acid) (PAA) modified with a photosensitive benzophenone. Nanoindentation, using atomic force microscopy (AFM) of these and unmodified PAH/PAA multilayers, was used to assess their mechanical properties in situ under an aqueous buffer. Under the conditions employed (and a 20 nm radius AFM tip), reliable nanoindentations that appeared to be decoupled from the properties of the silicon substrate were obtained for films greater than 150 nm in thickness. A strong difference in the apparent modulus was observed for films terminated with positive as compared to negative polyelectrolytes. Films terminated with PAA were more glassy, suggesting better charge matching of polyelectrolytes. Multilayers irradiated for up to 100 min showed a smooth, controlled increase in the modulus with little change in the water contact angle. The permeability to iodide ion, measured electrochemically, also decreased in a controlled fashion.
Co-reporter:Lara A. Al-Hariri, Andreas Reisch, and Joseph B. Schlenoff
Langmuir 2011 Volume 27(Issue 7) pp:3914-3919
Publication Date(Web):February 22, 2011
DOI:10.1021/la104109f
Different positive polyelectrolytes having the same charge density, molecular weight, and molecular weight distribution were employed for polyelectrolyte multilayer (PEMU) assembly. The polycations differed only in the heteroatom on which the positive charge resided: poly(vinyl benzyl trimethyl ammonium) chloride, poly(vinyl benzyl trimethyl phosphonium) chloride, and poly(vinyl benzyl dimethyl sulfonium) chloride. While the ammonium repeat unit has been employed on numerous occasions for PEMU assembly, the phosphonium and sulfonium units are relatively neglected. The polyanions, poly(styrene sulfonate), PSS, or poly(acrylic acid), PAA, were typical pH-independent or pH-dependent polymers, respectively. All three polyoniums were quite similar in showing linear layer-by-layer buildup with PSS and exponential growth with PAA, under the conditions employed. Hydration and wettability were also similar between polyoniums.
Co-reporter:Ramy A. Ghostine and Joseph B. Schlenoff
Langmuir 2011 Volume 27(Issue 13) pp:8241-8247
Publication Date(Web):June 8, 2011
DOI:10.1021/la2015258
The diffusion coefficient is a fundamental parameter for devices exploiting the ion transport properties of polyelectrolyte multilayers (PEMUs) and complexes. Here, the transport of ferricyanide through a multilayer made from poly(diallyldimethylammonium chloride) (PDADMA) and polystyrene sulfonate (PSS) was studied as a function of temperature or salt concentration. Accurate and precise measurements of ion diffusion coefficients were obtained using steady-state electrochemistry to determine the flux and Fourier transform infrared (FTIR) spectroscopy to measure the PEMU concentration. It was found that the concentration of ferricyanide inside the film decreased with temperature. Membrane transport is strongly thermally activated with activation energy 98 kJ mol–1. A potential shift with decreasing salt concentration in cyclic voltammograms was translated into a differential flux caused by significantly higher diffusion coefficients for ferricyanide as compared to ferrocyanide.
Co-reporter:Andreas Reisch, Maroun D. Moussallem, and Joseph B. Schlenoff
Langmuir 2011 Volume 27(Issue 15) pp:9418-9424
Publication Date(Web):June 30, 2011
DOI:10.1021/la201588s
In situ nanoindentation was performed on a multilayer of poly(acrylic acid) and a high molecular weight, pendant chain polyviologen under controlled electrochemical potential. The modulus of the thin film of polyelectrolyte complex was reversibly modulated, by about an order of magnitude, upon changing the state of charge within the material using the electrochemically active and addressable viologen repeat units. The applied potential, under aqueous conditions, is believed to control the extent of cross-link formation. Simultaneous quartz crystal microbalance measurements revealed the flux of ions into or out of the multilayer during redox cycling. Apparent film modulus also depends on the identity of the last layer.
Co-reporter:Lara A. Al-Hariri, Lianqing Zheng, Wei Yang, and Joseph B. Schlenoff
Macromolecules 2011 Volume 44(Issue 17) pp:6663-6668
Publication Date(Web):August 3, 2011
DOI:10.1021/ma200930d
Coordinated experimental and orthogonal space random walk (OSRW) molecular dynamics simulation studies were performed to understand the exceptionally low temperature required for thermal conversion of a poly(phenylenevinylene) (PPV) precursor containing the macromolecular counterion poly(ethylene glycol)-4-nonylphenyl-3-sulfopropyl ether (PEGNOPS). Simulations of this solvent-free system converged for starting points where the backbone was artificially expanded or compressed. In this example of a polymer organized by a polymer, the simulations revealed the ability of PEGNOPS to partially preorder the precursor chain in a conformation that favors the E2 elimination pathway.
Co-reporter:Haifa H. Hariri and Joseph B. Schlenoff
Macromolecules 2010 Volume 43(Issue 20) pp:8656-8663
Publication Date(Web):September 22, 2010
DOI:10.1021/ma1012978
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.
Co-reporter:Zaki G. Estephan, Jad A. Jaber, and Joseph B. Schlenoff
Langmuir 2010 Volume 26(Issue 22) pp:16884-16889
Publication Date(Web):October 13, 2010
DOI:10.1021/la103095d
Using a short-chain zwitterionic organosiloxane, silica nanoparticles were stabilized against aggregation by high ionic strength and/or proteins. Turbidimetry and dynamic light scattering showed that “zwitterated” nanoparticles did not exhibit a significant increase in hydrodynamic radius. When challenged with 3 M NaCl or 50% fetal bovine serum, aggregation was inhibited for at least 24 h, longer with mild heat treatment, which produced nanoparticles with zero net surface charge. These findings suggest “zwitteration” of silica-capped nanoparticles provides excellent stability for in vivo circulation diagnostics and therapies.
Co-reporter:Marie Z. Markarian and Joseph B. Schlenoff
The Journal of Physical Chemistry B 2010 Volume 114(Issue 32) pp:10620-10627
Publication Date(Web):July 26, 2010
DOI:10.1021/jp103213w
The isothermal hybridization of complementary oligonucleotides, 15-mer, 25-mer, 35-mer, and a molecular beacon, was investigated under varying conditions of molecular crowding and ionic strength, using hypochromicity to follow strand pairing and polyethylene glycol as a crowding agent. Thermodynamic analysis of the results revealed the addition of counterions to the oligonucleotide backbones, ΔΨ, to be dependent on the strand GC content and the molecular crowding. A decrease in ΔΨ was observed, with both increasing GC% and solution PEG content. In contrast, the number of bound water molecules depended on the activity of Na+, where two regimes were observed. At aNa+ < 0.05 and increasing molecular crowding, water molecules were released into the DNA solutions, and oligonucleotide pairing was favored with both increasing hydrophobic forces, whereas at aNa+ ≥ 0.05, water molecules were bound to the strands, and the extent of double strand formation decreased with increasing PEG wt %.
Co-reporter:Lara A. Al-Hariri, Joseph B. Schlenoff
Polymer 2010 Volume 51(Issue 14) pp:2993-2997
Publication Date(Web):24 June 2010
DOI:10.1016/j.polymer.2010.05.016
Thermal elimination of a water soluble poly(xylylidene) precursor to poly(phenylene vinylene) (PPV) was accomplished in air at 80 °C using the macro-counterion poly(ethylene glycol)-4-nonylphenyl-3-sulfopropyl ether (PEGNOPS). The high relative photoluminescence efficiency was ascribed to the low carbonyl content, with a possible contribution from PPV chain separation, which minimizes radiationless interchain exciton formation. In addition, the PEGNOPS precursor showed no carbonyl formation on long term storage, in contrast to the precursors with dodecylbenzene sulfonate and chloride counterions.
Co-reporter:Maroun D. Moussallem, Scott G. Olenych, Shannon L. Scott, Thomas C. S. Keller III and Joseph B. Schlenoff
Biomacromolecules 2009 Volume 10(Issue 11) pp:
Publication Date(Web):October 9, 2009
DOI:10.1021/bm9007309
Smooth muscle cells convert between a motile, proliferative “synthetic” phenotype and a sessile, “contractile” phenotype. The ability to manipulate the phenotype of aortic smooth muscle cells with thin biocompatible polyelectrolyte multilayers (PEMUs) with common surface chemical characteristics but varying stiffness was investigated. The stiffness of (PAH/PAA) PEMUs was varied by heating to form covalent amide bond cross-links between the layers. Atomic force microscopy (AFM) showed that cross-linked PEMUs were thinner than those that were not cross-linked. AFM nanoindentation demonstrated that the Young’s modulus ranged from 6 MPa for hydrated native PEMUs to more than 8 GPa for maximally cross-linked PEMUs. Rat aortic A7r5 smooth muscle cells cultured on native PEMUs exhibited morphology and motility of synthetic cells and expression of the synthetic phenotype markers vimentin, tropomyosin 4, and nonmuscle myosin heavy chain IIB (nmMHCIIB). In comparison, cells cultured on maximally cross-linked PEMUs exhibited the phenotype markers calponin, smooth muscle myosin heavy chain (smMHC), myocardin, transgelin, and smooth muscle α-actin (smActin) that are characteristic of the smooth muscle “contractile” phenotype. Consistent with those cells being “contractile”, A7r5 cells grown on cross-linked PEMUs produced contractile force when stimulated with a Ca2+ ionophore.
Co-reporter:Claudine H. Porcel and Joseph B. Schlenoff
Biomacromolecules 2009 Volume 10(Issue 11) pp:
Publication Date(Web):October 19, 2009
DOI:10.1021/bm900373c
Precipitates of polyelectrolyte complexes were transformed into rugged shapes suitable for bioimplants by ultracentrifugation in the presence of high salt concentration. Salt ions dope the complex, creating a softer material with viscous fluid-like properties. Complexes that were compacted under the centrifugal field (CoPECs) were made from poly(diallyldimethyl ammonium), PDADMA, as polycation, and poly(styrene sulfonate), PSS, or poly(methacrylic acid), PMAA, as polyanion. Dynamic mechanical testing revealed a rubbery plateau at lower frequencies for PSS/PDADMA with moduli that decreased with increasing salt concentration, as internal ion pair cross-links were broken. CoPECs had significantly lower modulii compared to similar polyelectrolyte complexes prepared by the “multilayering” method. The difference in mechanical properties was ascribed to higher water content (located in micropores) for the former and, more importantly, to their nonstoichiometric polymer composition. The modulus of PMAA/PDADMA CoPECs, under physiological conditions, demonstrated dynamic mechanical properties that were close to those of the nucleus pulposus in an intervertebral disk.
Co-reporter:Jingcheng Fu
Journal of the American Chemical Society () pp:
Publication Date(Web):January 15, 2016
DOI:10.1021/jacs.5b11878
Driving forces for association between oppositely charged biological or synthetic polymers in aqueous solution have long been identified as electrostatic in origin. This attraction is broken down into an entropic component, due to loss of counterions, and an enthalpic component, stemming from Coulombic attraction between opposite charges. While the balance between entropic and enthalpic contributions shifts according to the conditions, the presence of exotherms or endotherms on mixing, though small, are viewed as signatures of Coulombic interactions which support theories of polyelectrolyte association rooted in continuum electrostatics. Here, a head-to-head comparison is made between mechanisms based on electrostatics and those based on specific ion pairing, or ion exchange. Using a Hofmeister series of counterions for a common polycation, poly(diallyldimethylammonium), enthalpy changes on association with poly(styrenesulfonate) are shown to derive from changes in water perturbation, revealed by Raman scattering studies of water O–H vibrations. The free energy for complexation is almost completely entropic over all salt concentrations.
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