Water-driven Glass Transition Dynamics in Polyelectrolyte Complexes and Multilayers

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PART 1: NON-TECHNICAL SUMMARYCharged polymers (polyelectrolytes) impact society in areas ranging from personal care and energy to health care. These polymers bear a charged group that can associate with oppositely charged macromolecules to form a larger superstructure called a polyelectrolyte complex. The nature of how these polyelectrolytes associate is represented by the number of associations or electrostatic crosslinks, which in turn influences the physical properties of the complex. One important challenge is that these complexes should bear desired properties at real-world conditions, such as certain relative humidity values and temperatures. However, there remains a lack of understanding regarding how and why water and temperature dictate the thermal and mechanical properties of a given complex. One significant challenge is that it is not clear how these physical properties connect to the original structure of the electrostatically crosslinked polyelectrolyte complex. This project addresses this challenge by examining the dynamics and mobility of the polyelectrolyte and of water within the structure at varying environmental conditions. These findings will be compared to the structure of the complex and physical properties such as stiffness. The knowledge developed in this project could lead to a predictive relationship that may broadly describe the physical properties of any complex in response to environmental conditions. The proposed work offers several opportunities for educational enrichment of a broad spectrum of members of the general public. The educational objectives and broader impact for this work include: hands-on research opportunities for undergraduates, K-12 outreach through TAMU's Chemistry Open House through demonstrations to the public, Science Night demonstrations at local elementary schools, and online outreach through the EngineerGirl website.PART 2: TECHNICAL SUMMARYCultivating a deeper understanding of the physical properties of polyelectrolyte complexes as they connect to the complex's structure is an important challenge for advancing their wider application. However, this understanding is complicated by complex factors such as the electrostatic crosslinking density, water content, and temperature. For example, it is known that the glass transition temperature (Tg) is intimately tied to the water content and ion pairing within the complex, which is in turn influenced by salt and/or pH. However, the current understanding is somewhat empirical or qualitative, and a quantitative connection to a physical dynamic process is needed. The central goal of this project is to elucidate the physical origin of the glass transition-ion pairing-water relationship via characterization of the glass transition dynamics and the molecular structure of the complex. This will be accomplished by spectroscopic characterization, dynamic mechanical analysis, and thermal analysis. These complexes will be further explored for their stiffness at different time scales, temperatures, and humidity values. The central hypothesis is that the dynamics of the polymer chain are influenced by water mobility at the electrostatic crosslink and by the association/dissociation of the crosslink itself. Water may lubricate the electrostatic crosslink and promote temporary dissociation of the electrostatic crosslink. One significant outcome of this work is possibly a new physics-based understanding that quantitatively connects the structure of a complex and the surrounding environmental conditions to the resultant physical properties of that complex..This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.