Ajay Vidyasagar

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 films internal structure as a function of film depth.

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.

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 transitions 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.

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.