High Ionic Strength Promotes the Formation of Spherical Copolymer Particles

Stabile, thermoresponsive colloidal clusters: an unusual morphology of polymer dispersions

A new class of colloidal polymeric particles consisting of polystyrene spheres grown in a poly(N-isopropyl acrylamide) precursor scaffold is synthesized via redox-initiated heterophase polymerization. The morphology and thermoresponsiveness of these assemblies is proven by electron microscopy investigations and temperature-dependent measurements of the change of both the speed of sound travelling through the dispersion and the hydrodynamic particle size. Electron microscopy (EM) micrographs (transmission and scanning EM as well cryo-scanning EM) prove the existence of colloidal clusters when the freeze-dried copolymer is redispersed in pure water. The clusters have a size of several micrometers, contain about 800 polystyrene particles with diameter below 100 nm, and show a highly reproducible thermoresponsive behavior with a lower critical solution temperature corresponding to that of pure poly(N-isopropyl acrylamide).

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.