RESUMO
Polyelectrolyte multilayers (PEMs) are thin films formed by the alternating deposition of oppositely charged polyelectrolytes. Water plays an important role in influencing the physical properties of PEMs, as it can act both as a plasticizer and swelling agent. However, the way in which water molecules distribute around and hydrate ion pairs has not been fully quantified with respect to both temperature and ionic strength. Here, we examine the effects of temperature and ionic strength on the hydration microenvironments of fully immersed poly(diallyldimethylammonium)/polystyrene sulfonate (PDADMA/PSS) PEMs. This is accomplished by tracking the OD stretch peak using attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy at 0.25-1.5 M NaCl and 35-70 °C. The OD stretch peak is deconvoluted into three peaks: (1) high frequency water, which represents a tightly bound microenvironment, (2) low frequency water, which represents a loosely bound microenvironment, and (3) bulk water. In general, the majority of water absorbed into the PEM exists in a bound state, with little-to-no bulk water observed. Increasing temperature slightly reduces the amount of absorbed water, while addition of salt increases the amount of absorbed water. Finally, a van't Hoff analysis is applied to estimate the enthalpy (11-22 kJ mol-1) and entropy (48-79 kJ mol-1 K-1) of water exchanging from low to high frequency states.
RESUMO
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.
RESUMO
Self-healing is the ability of a material to repair mechanical damage. The lifetime of a coating or film might be lengthened with this capacity. Water enabled self-healing of polyelectrolyte multilayers has been reported, using systems that grow via the interdiffusion of polyelectrolyte chains. Due to high mobility of the polyelectrolyte chains within the assembly, it is possible for lateral diffusion to heal over scratches. The influence of metal ions and nanoparticles on this property has, however, not been previously studied. Here we demonstrate that the incorporation of silver nanoparticles reduced in situ within the branched poly(ethyleneimine)-poly(acrylic acid) polyelectrolyte multilayer structure speeds the ability of the multilayer assembly to self-heal. This enhancement of property seems to not be due to changes in mechanical properties but rather in enhanced affinity to water and plasticization that enables the film to better swell.
