Effect of nanoparticles on the thermal transitions of hydrated layer-by-layer assemblies
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Nanoparticles can have a profound effect on a polymers glass transition temperature (T_(g)). Many layer-by-layer (LbL) assemblies contain nanoparticles for added functionality, but the resulting effect of nanoparticles on an LbL films properties is not known. Previously, we have shown that a nanoparticle-free LbL film containing strong polyelectrolytes, poly(diallyldimethylammonium chloride)/poly(styrene sulfonate) (PDAC/PSS), exhibited a thermal transition somewhat akin to a glass transition using quartz crystal microblance with dissipation (QCM-D) and modulated differential scanning calorimetry (MDSC). In the work presented here, layers of negatively charged nanoparticles of either spherical or platelet morphology have been inserted at varying locations throughout PDAC/PSS LbL films. QCM-D and MDSC were used to determine the effect that these nanoparticles have on the previously measured thermal transitions as a function of placement within the film and particle shape. Using QCM-D we observed clear, reproducible T_(g)s in all LAP film configurations and in one particular SiO_(2) configuration. All observed T_(g)s, regardless of nanoparticle morphology, were elevated with respect to those found in neat PDAC/PSS films. Additionally, there was little difference noted between the transition values for the two particular morphologies. It was discovered that the highest glass transition temperatures were observed for film configurations where the nanoparticles were added during the middle bilayer. We attributed this phenomenon to the increased available nanoparticle surface area with which nearby polymer chains could form bonds. Unfortunately the extremely weak and broad thermal transitions observed with MDSC proved to be inconclusive in either supporting or refuting these observations made via QCM-D.
