Comparison of KBr and NaCl effects on the glass transition temperature of hydrated layer-by-layer assemblies.

The influence of assembly and post-assembly conditions on the glass transition temperature of free-standing poly(diallyldimethyl ammonium) (PDADMA)/poly(4-styrene sulfonate) (PSS) layer-by-layer (LbL) films assembled in 0.5M NaCl and 0.5M KBr are explored using modulated differential scanning calorimetry. Upon completion, PDADMA/PSS LbL assemblies are hydrated using solutions containing various concentrations of KBr. The data indicate that water provides the primary driving force for changes in the glass transition temperature of completed films rather than the post-assembly salt type. However, upon changing the assembly salt conditions from NaCl to KBr, the glass transition temperature shows a decrease of nearly 20 °C. Additionally, the composition of the films upon analysis with 1H NMR spectroscopy and neutron activation analysis indicates an elevated number of extrinsic binding sites within the film structure when KBr is the assembly salt. This shows a clear link between the assembly conditions and the internal structure and, therefore, the thermal properties of PDADMA/PSS LbL assemblies.

Molecular Origin of the Glass Transition in Polyelectrolyte Assemblies.

Water plays a central role in the assembly and the dynamics of charged systems such as proteins, enzymes, DNA, and surfactants. Yet it remains a challenge to resolve how water affects relaxation at a molecular level, particularly for assemblies of oppositely charged macromolecules. Here, the molecular origin of water's influence on the glass transition is quantified for several charged macromolecular systems. It is revealed that the glass transition temperature (Tg) is controlled by the number of water molecules surrounding an oppositely charged polyelectrolyte-polyelectrolyte intrinsic ion pair as 1/Tg ∼ ln(nH2O/nintrinsic ion pair). This relationship is found to be "general", as it holds for two completely different types of charged systems (pH- and salt-sensitive) and for both polyelectrolyte complexes and polyelectrolyte multilayers, which are made by different paths. This suggests that water facilitates the relaxation of charged assemblies by reducing attractions between oppositely charged intrinsic ion pairs. This finding impacts current interpretations of relaxation dynamics in charged assemblies and points to water's important contribution at the molecular level.

QCM-D Investigation of Swelling Behavior of Layer-by-Layer Thin Films upon Exposure to Monovalent Ions.

Polyelectrolyte multilayers and layer-by-layer assemblies are susceptible to structural changes in response to ionic environment. By altering the salt type and ionic strength, structural changes can be induced by disruption of intrinsically bound ion pairs within the multilayer network via electrostatic screening. Notably, high salt concentrations have been used for the purposes of salt-annealing and self-healing of LbL assemblies with KBr, in particular, yielding a remarkably rapid response. However, to date, the structural and swelling effects of various monovalent ion species on the behavior of LbL assemblies remain unclear, including a quantitative view of ion content in the LbL assembly and thickness changes over a wide concentration window. Here, we investigate the effects of various concentrations of KBr (0 to 1.6 M) on the swelling and de-swelling of LbL assemblies formed from poly(diallyldimethylammonium) polycation (PDADMA) and poly(styrene sulfonate) polyanion (PSS) in 0.5 M NaCl using quartz-crystal microbalance with dissipation (QCM-D) monitoring as compared to KCl, NaBr, and NaCl. The ion content after salt exchange is quantified using neutron activation analysis (NAA). Our results demonstrate that Br- ions have a much greater effect on the structure of as-prepared thin films than Cl- at ionic strengths above assembly conditions, which is possibly caused by the more chaotropic nature of Br-. It is also found that the anion in general dominates the swelling response as compared to the cation because of the excess PDADMA in the multilayer. Four response regimes are identified that delineate swelling due to electrostatic repulsion, slight contraction, swelling due to doping, and film destruction as ionic strength increases. This understanding is critical if such materials are to be used in applications requiring submersion in chemically dynamic environments such as sensors, coatings on biomedical implants, and filtration membranes.

Hydrogen-bonded polymer nanocomposites containing discrete layers of gold nanoparticles.

The ability to finely control the spatial location and relative concentration of a nanofiller within a polymer nanocomposite is desirable, especially when faced with fillers that are susceptible to aggregation. In the case of gold nanoparticles (AuNPs), spatial organization is of interest as it offers a means to harness optical and electrical properties, but well-defined placement of AuNPs within a polymer matrix is generally challenging because of phase separation. Here, we demonstrate for the first time the spray-assisted layer-by-layer assembly (LbL) of hydrogen-bonding polymer nanocomposites of poly(ethylene oxide) (PEO) and poly(methacrylic acid) (PMAA) containing discrete regions of AuNPs vertically positioned throughout the film structure. Analysis of the internal structure using cross-sectional transmission electron microscopy (TEM) imaging shows that the AuNP regions are clearly separated by "empty" regions of polymer with no drift or aggregation of the Au NPs during or after fabrication. A UV-vis spectroscopy study of the stimuli-responsive properties of the spray-assisted LbL nanocomposites shows the release of AuNPs as induced by elevating the environmental pH above the critical pH at which the PEO and PMAA hydrogen bonds are disrupted. We anticipate that this work enables the spatial organization of other nanofillers in stimuli-responsive hydrogen-bonding nanocomposites by spray-assisted LbL assembly.