Thermal transitions in hydrated layer-by-layer assemblies observed using electrochemical impedance spectroscopy.

Layer-by-layer (LbL) assemblies have been of great interest due to their versatile functionality and ease of fabrication, but their response to temperature is not completely understood. It has been recently shown that hydrated LbL assemblies of poly(diallyldimethylammonium chloride) (PDAC) and poly(styrene sulfonate) (PSS) under go a thermal transition much like a "glass-melt" transition. This thermal transition is of great interest because many LbL applications are found in water. Here, we report upon the nature of this thermal transition as probed using electrochemical impedance spectroscopy (EIS) as a function of assembly salt concentration, film thickness, and outermost layer. EIS reveals that the transition is signified by a structural rearrangement of virtual pores, resulting in increased conductivity and decreased surface coverage of the electrode. Two separate thermal transitions are obtained from changes in the film resistance (Ttr,Rf) and the charge transfer resistance (Ttr,Rct). Only Ttr,Rct is strongly dependent on film thickness, salt concentration, and outermost layer, for which values ranging from 50 to 64 °C were observed. As the assembly salt concentration increases from 0.5 M to 1.0 M NaCl, Ttr,Rct increases by about 10 °C. Below 20 layers, deviations of Ttr,Rct with respect to outermost layer appear, in which PSS-capped LbL films tend to show elevated Ttr,Rct values. These results suggest that extrinsic charge compensation plays a large role in the value of Ttr,Rct in which a large degree of extrinsic charge compensation drives Ttr,Rct towards higher values. On the other hand, Ttr,Rf is largely unaffected by assembly parameters, and closer in value to prior reports via calorimetry and quartz crystal microbalance with dissipation.

A comparison of thermal transitions in dip- and spray-assisted layer-by-layer assemblies.

Spray-assisted layer-by-layer (LbL) assembly is far more rapid than conventional dip-assisted assembly methods and has gained widespread interest recently. Even so, it has remained unclear as to how the structure and properties of the resulting LbL film vary with processing method. Here, we compared the thermal properties of poly(ethylene oxide) (PEO)/poly(acrylic acid) (PAA) and PEO/poly(methacrylic acid) (PMAA) hydrogen-bonded LbL assemblies prepared using both dip-assisted and spray-assisted deposition methods. While the surface morphologies of PEO/PAA LbL assemblies were similar, those of PEO/PMAA LbL assemblies were greatly influenced by deposition method. In both PEO/PAA and PEO/PMAA LbL assemblies, glass transition temperatures were not influenced by deposition method, but the transition's breadth was consistently larger for the spray-assisted LbL films. These results indicate that the internal structure of spray-assisted LbL films is slightly more heterogeneous, possibly arising from the shorter time scale of deposition.

Effect of thickness on the thermal properties of hydrogen-bonded LbL assemblies.

Layer-by-layer (LbL) assemblies have attracted much attention for their functional versatility and ease of fabrication. However, characterizing their thermal properties in relation to the film thickness has remained a challenging topic. We have investigated the role of film thickness on the glass transition temperature (T(g)) and coeffecient of thermal expansion for poly(ethylene oxide)/poly(acrylic acid) (PEO/PAA) and PEO/poly(methacrylic acid) (PEO/PMAA) hydrogen-bonded LbL assemblies in both bulk and ultrathin films using modulated differential scanning calorimetry (modulated DSC) and temperature-controlled ellipsometry. In PEO/PAA LbL films, a single, well-defined T(g) was observed regardless of film thickness. The T(g) increased by 9 °C relative to the bulk T(g) as film thickness decreased to 30 nm because of interactions between the film and its substrate. In contrast, PEO/PMAA LbL films show a single glass transition only after a thermal cross-linking step, which results in anhydride bonds between PMAA groups. The T(g), within error, was unaffected by film thickness, but PEO/PMAA LbL films of thicknesses below ~2.7 µm exhibited a small amount of PEO crystallization and phase separation for the thermally cross-linked films. The coefficients of thermal expansion of both types of film increased with decreasing film thickness.

Porous Layer-by-Layer Films Assembled Using Polyelectrolyte Blend to Control Wetting Properties.

Porous layer-by-layer (LbL) films have been employed for the implementation of superwetting surfaces, but they are limited to the LbL films consisting of only two oppositely charged polyelectrolytes. In this study, LbL films were assembled using a cationic polymer blend of branched poly(ethylene imine) (BPEI) and poly(allylamine hydrochloride) (PAH), and anionic poly(acrylic acid); they were then acid-treated at pH 1.8-2.0 to create a porous structure. The films of 100% BPEI exhibited a relatively smooth surface, whereas those of the 100% PAH exhibited porous surfaces. However, various surface morphologies were obtained when BPEI and PAH were blended. When coated with fluorinated silane, films with 50% and 100% PAH exhibited relatively higher water contact angles (WCAs). In particular, films with 50% PAH exhibited the highest WCA of 140-150° when treated at pH 1.8. These fluorinated films were further infused with lubricant oil to determine their feasibility as slippery surfaces. The water and oil sliding angles were in the range of 10-20° and 5-10°, respectively. Films prepared with the BPEI/PAH blend showed lower water slide angles than those prepared with 100% BPEI or PAH. Acid treatment of LbL films assembled using a polyelectrolyte blend can effectively control surface morphologies and can potentially be applied in superwetting.

Reversibly pH-Responsive Nanoporous Layer-by-Layer Microtubes.

Nanoporous layer-by-layer (LbL) microtubes consisting of poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA) are prepared by LbL deposition in porous templates followed by postassembly acid treatment. The formation of the nanoporous structure is studied as a function of solution pH, treatment time, and number of layers. Pore formation is most effective at pH 1.8, requiring only 5 min to achieve a complete transition, and is shown to be reversible. Whereas the inner surface of the porous microtubes is rough, the outer surface is smoother and exhibited isolated pores, suggestive of an asymmetric porous structure.

Temperature-triggered shape-transformations in layer-by-layer microtubes.

One-dimensional layer-by-layer (LbL) nano- and microtubes have been extensively studied for energy and biomedical applications. Here, we report a temperature-triggered transformation in shape for LbL microtubes consisting of poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA). PAH/PAA microtubes were assembled onto porous sacrificial templates. Released microtubes transform to closed ellipsoids upon heating in water. On the other hand, unreleased microtubes (remaining within the template) exhibit a completely different behavior in which periodic perforations appear, suggestive of Rayleigh instabilities. In both cases, the transformations occurred near and above the glass transition temperature (Tg) of the hydrated PAH/PAA LbL assembly, suggesting that the transformation in shape is tied to the thermal properties of the LbL assembly.

Thermal transitions in dry and hydrated layer-by-layer assemblies exhibiting linear and exponential growth.

Layer-by-layer (LbL) assemblies are remarkable materials, known for their tunable mechanical, optical, and surface properties in nanoscale films. However, questions related to their thermal properties still remain unclear. Here, the thermal properties of a model LbL assembly of strong polyelectrolytes, poly(diallyldimethylammonium chloride)/poly(styrene sulfonate) (PDAC/PSS), assembled from solutions of varying ionic strength (0-1.25 M NaCl) are investigated using quartz crystal microbalance with dissipation (QCM-D) and modulated differential scanning calorimetry. Hydrated exponentially growing films (assembled from 0.25 to 1.25 M NaCl) exhibited distinct thermal transitions akin to a glass transition at 49-56 °C; linearly growing films (assembled without added salt) did not exhibit a transition in the temperature range investigated and were glassy. Results support the idea that exponentially growing films have greater segmental mobility than that of linearly growing films. On the other hand, all dry LbL assemblies investigated were glassy at room temperature and did not exhibit a T(g) up to 250 °C, independent of ionic strength. For the first time, thermal transitions such as T(g) values can be measured for LbL assemblies using QCM-D by monitoring fluctuations in changes in dissipation, allowing us to probe the film's internal structure as a function of film depth.