479099 Understanding the Thermal Transition in Polyelectrolyte Multilayers

Monday, November 14, 2016
Grand Ballroom B (Hilton San Francisco Union Square)
Adam Bachmann1, Yanpu Zhang2, Melissa Santos2, Yossef A. Elabd2 and Jodie L. Lutkenhaus2, (1)Department of Chemical Engineering, University of Florida, Gainesville, FL, (2)Artie McFerrin Department of Chemical Engineering, Texas A&M University, College Station, TX

Polyelectrolyte multilayer (PEM) assemblies are functional materials created by combining positively and negatively charged polymers, polyelectrolytes, in a controlled manner. The properties of PEMs can be finely tuned by a wide variety of internal and external parameters, such as ionic strength and the identity of the polyelectrolytes. The tunability of PEMs allow them to be used for purposes as varied as drug delivery, gas separations and self-healing materials. Their physical features including thermal properties are crucial for the aforementioned potential applications. It has been observed that PEMs of poly(diallyldimethylammonium chloride) /poly(styrene sulfonate) (PDAC/PSS) undergo a thermal transition when hydrated. This transition was classified as a glass-melt transition because calorimetric studies suggested it was a second order phase transition as seen by the discontinuity of the heat capacity. Recent studies have indicated that water and excess counter-ions are needed for the transition to be observed and molecular dynamics simulations have shown this transition is perhaps a dehydration-driven process as the tightly-bound water in the PEMs moves farther away from the PSS site as temperature increases while counter-ions play a plasticizing role.

To shed light on this significant but elusive question, here, we directly observe the water effects in the thermal properties of PEMs by performing attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) of PEMs made of PDAC/PSS. Specifically, the water and polymer absorption spectrum with varying temperatures were recorded using ATR-FTIR, which were further decomposed to yield information about the different types of water sorbed in the PEM and how it interacts with the sulfonate group.

The spectral results reveal that the way water associates with the PEM shifts as temperature increases, from a tightly bound state to free water. This event happens between 45°C and 50°C for PDAC/PSS assembled with 0.5 M NaCl, indicating the same thermal transition temperature detected from previous work and other techniques. This finding supports the mechanism proposed by the prior simulational work in that the thermal transition is the result of dehydration from the sulfonate group in PEMs. Understanding the nature of the thermal transition in PEMs from strong polyelectrolytes allows for more informed design of these functional materials.

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