It is all about water: thermal transitions in polyelectrolyte assemblies occur via a dehydration mechanism
Polyelectrolyte multilayers, formed from the assembly of oppositely charged species from aqueous solutions, have long been known to possess an anomalous thermal transition signified by a dramatic decrease in modulus and increase in diffusion. This transition can be used in, e.g., designing smart, responsive coatings for energy or bioengineering applications such as drug transport. In the absence of a better word, the transition has been called a glass transition or a glass-melt transition. Researchers at Aalto University Department of Chemistry have for the first time deduced the microscopic nature of this transition based on molecular simulations and supporting experimental evidence.
Figure 1: Cartoon of the role of water in the thermal transition of polyelectrolytes. The research work revealed the hydrogen bond life time and their number experiences a sudden decrease at the transition temperature. This indicates the polyelectrolytes are experiencing dehydration. Figure by Maria Sammalkorpi.
The research team led by Dr. Maria Sammalkorpi, Aalto University, in collaboration with Dr. Jodie Lutkenhaus, Texas A&M University, has shown the transition is actually driven by dehydration. This finding is significant because it reverses a prior assumption that the thermal transition in polyelectrolyte systems is related to disruption of polycation-polyanion bonds. Furthermore, the observed mechanism bears close resemblance to lower critical solution temperature (LCST)-type mechanism and connects thus polyelectrolyte materials with a broad range of synthetic and biological materials experiencing dehydration-driven thermal transitions. In total, the findings show water-polyelectrolyte interactions should be a focus in future design of thermoresponsive polyelectrolyte materials.
The findings have been published recently in ACS Macro Letters. The scope of the journal includes high-impact research of broad interest in all areas of polymer science and engineering. The work is part of an NSF Materials World Network collaboration project funded by Academy of Finland and NSF, USA.
Research group leader, Academy of Finland Research Fellow Maria Sammalkorpi, Department of Chemistry, Aalto University School of Chemical Technology
Erol Yildirim, Yanpu Zhang, Jodie L. Lutkenhaus, and Maria Sammalkorpi, “Thermal Transitions in Polyelectrolyte Assemblies Occur via a Dehydration Mechanism“ ACS Macro Letters, 2015, 4, pp 1017–1021.