Authors: Joshua T. O’Neal
Source: OakTrust (Texas A&M University Libraries) (article, 2018-05-04)
Field: Materials Science — Surfaces, Coatings and Films

Layer-by-Layer Assembly of Responsive Polyelectrolyte Thin Films and Nanocomposites This thesis presents three investigations into the responsive physical and thermal properties of polyelectrolyte layer-by-layer assemblies. The first study focuses on hydrogen bonded, layer-by-layer assembled films and nanocomposites Polyelectrolyte multilayers and layer-by-layer assemblies are susceptible to structural changes in response to changes in the post-assembly environment. The study demonstrates the first reported use of spray-assisted layer-by-layer assembly to produce hydrogen-bonding polymer nanocomposites of poly(ethylene oxide) (PEO) and poly(methacrylic acid) (PMAA) containing discrete regions of gold nanoparticles (AuNPs) vertically positioned throughout the film structure with no substantial aggregation. Elevating the environmental pH disrupts the hydrogen bonding network, resulting in release of the AuNPs, and the rate of release is shown to depend on the pH of the environment. \nThe second study deals with the swelling response of electrostatic layer-by-layer thin films to changes in ionic environment. The effects of a wide range of KBr concentrations (0 to 1.6 M) on the swelling and deswelling of LbL assemblies formed from poly(diallyldimethylammonium) polycation (PDADMA) and poly(styrenesulfonate) polyanion (PSS) in 0.5 M NaCl were investigated using quartz-crystal microbalance with dissipation (QCM-D) monitoring. The swelling behavior is compared with responses to KCl, NaBr, and NaCl at various concentrations. The results demonstrate anion type dominates swelling response, and that Br- ions have a much greater effect on the structure of as-prepared thin films than Cl- at ionic strengths above assembly conditions, likely due to the chaotropic nature of Br^-. 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. \nThe third study focuses on the thermal response of electrostatic layer-by-layer assemblies of PDADMA and PSS to changes in hydration level and counterion type. Modulated differential scanning calorimetry (MDSC) is employed to quantify the influence of changing hydration level, ionic strength and type, as well as assembly conditions on the glass transition temperature (Tg) of free-standing PDADMA/PSS LbL films assembled in 0.5 M NaCl. The results indicate that assembly conditions and post-assembly hydration level are the primary factor in determining the Tg of PDADMA/PSS LbL films when hydration is limited to less than 35 wt%. Hydrating with KBr solutions resulted in no apparent effect on the Tg of the LbL films. Conversely, films assembled in NaCl and KBr at identical ionic strengths show a nearly 20°C difference in Tg, indicating a different internal structure of extrinsic and intrinsic ion pairing. Finally, PDADMA/PSS LbL films show remarkably similar Tg behavior to PDADMA/PSS complexes when hydrated to similar levels. Polyelectrolyte ; Layer by layer ; Layer (electronics) ; Nanocomposite ; Materials science ; Nanotechnology ; Thin film ; Polymer ; Polymer science ; Composite material Polymer Surface Interaction Studies ; Advanced Polymer Synthesis and Characterization ; Hydrogels: synthesis, properties, applications