Structure–Property Relations of Polyampholyte Hydrogels, Graphene Composites, and Biodegradable Elastomers and Their Applications in Energy Generation and Storage Devices

• Author / Creator

  Xinda Li

• A polyampholyte is a polyelectrolyte which is composed of macromolecules that contains both cationic and anionic ionisable groups. Understanding the structure of polymer chains in polyampholyte hydrogels and its effect on the phase behavior of the contained water is a challenging problem that has a broad impact on diverse applications, such as gel electrolytes, lubrication layers, and anti-biofouling coatings. In this dissertation, we have investigated the structure of a charge-balanced polyampholyte, poly(4-vinylbenzenesulfonate-co-[3-(methacryloylamino) propyl] trimethylammonium chloride). The polymer structure was probed by variable-temperature small-angle X-ray scattering (SAXS); highly hydrated globules with a radius of gyration of 2 ~ 2.5 nm formed a network structure in the charge-balanced polyampholyte hydrogels, whereas the size and the clustering are dependent on synthesis parameters. These highly hydrated network globules formed percolated polymer-rich domains while sub-micron, slush-like ice crystals formed from water-rich domains, resulting in ion-conducting channels that are rich in amorphous water molecules at low temperatures. Solid-state nuclear magnetic resonance (NMR) spectroscopy confirmed the mobility of these amorphous water molecules at temperatures as low as –54 C. We also visualized the globular structure of polyampholyte hydrogel with scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM) for the first time. Differential scanning calorimetry (DSC) was used to estimate water states in the polyampholyte hydrogel.
  Based on the understanding of temperature-dependent structure evolution of polyampholyte hydrogel, we fabricated two devices using polyampholyte hydrogel. First, we found that the upper critical solution temperature (UCST) for the phase separation between water-rich and polymer-rich phases in the polyampholyte hydrogel can be finely raised (~ +40 °C) by substituting a small number of cationic monomers (~ 0.5 wt%) to slightly more hydrophobic ones during the random copolymerization process. By harnessing this scientific observation, we developed a smart coating that becomes opaque to both visible and mid-infrared radiation at room temperature to achieve privacy and heat retention in cold night weather. Second, a flexible and self-healing supercapacitor with high energy density in low-temperature operation was fabricated using the polyampholyte hydrogel as a gel electrolyte. The resulting supercapacitor device showed a high energy density of 30 Wh/kg, and a capacitance retention of ~90% after 5000 charge–discharge cycles. At low temperature (−30 °C), the supercapacitor had an energy density of 10.5 Wh/kg at a power density of 500 W/kg.
  Inspired by the new graphene chemistry we developed for the aforementioned supercapacitor electrodes, we also devised a self-reinforcing conductive coating strategy where graphene nanoflakes (GNF) were wrapped by self-assembling reduced graphene oxide (RGO) for electrical conductivity and mechanical integrity. The conductivity of the GNF-RGO coating reached 4.47 × 104 S/m. The coating was then applied on 3D printed porous elastomers, resulting in flexible radio frequency (RF) antennas and strain sensors with arbitrary shapes for internet-of-things (IoTs) applications. The same conductive coating strategy also converted a commercial polyurethane (PU) sponge into electricity generators, where zinc oxide (ZnO) nanowires were hydrothermally grown on top of the three-dimensional graphene networks coated on the inner wall of the PU sponge. The nanogenerator yielded an open circuit voltage of ∼0.5 V and short circuit current density of ∼2 μA/cm2, while the output was found to be consistent after ∼3000 cycles.
  Finally, we studied the synthesis of poly (glycerol sebacate) (PGS), a synthetic biocompatible elastomer developed as a template for cardiac cells, to establish criteria for quick and consistent synthesis for tailored mechanical properties. Here, we suggested that the degree of esterification (DE) could be used to predict precisely the physical status, the mechanical properties, and the degradation of PGS. Young’s modulus was shown to linearly increase with DE, which was in agreement with an entropic spring theory of rubbers. To provide a processing guideline for researchers, we also provided a physical status map as a function of curing temperature and time. The amount of glycerol loss, obtainable by monitoring the evolution of the total mass loss and the DE during synthesis, was shown to make the predictions even more precise. Quick synthesis could be achieved by employing a microwave oven instead of convection heating for prepolymerization, but microwave heating led to severe mismatch between monomeric units by uncontrolled evaporation of glycerol, leading to an inconsistency in mechanical properties.

• Subjects / Keywords

  • Biodegradable Elastomers
  • Polyampholyte Hydrogels
  • Energy Storage Devices
  • Graphene Composites

• Graduation date

  Fall 2018

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3SX64S11

• License

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