Polymer Networks for Thermoresponsive Shape Stabilization of an Inorganic Phase Change Material
Abstract
Temperature responsive polymeric networks have achieved for reversible shape stabilization of an inorganic salt hydrate phase change material (PCM). The unique feature of these networks is the capability to simultaneously provide shape stabilization of a liquid salt hydrate PCM at lower temperature, and to reversibly adjust its assembly strength in response to a temperature increase. Specifically, lithium nitrate tri-hydrate (LNH) as an inorganic ionic liquid (IL) and a high-latent-heat PCM was employed as a solvent for a neutral polymer, poly(vinyl alcohol), PVA. LNH solvent presents a water-salt medium with extremely high concentration of salt (~18 M).
Two distinct approaches for gelation − crosslinker-free and crosslinker-assisted − were explored. In the crosslinker-free case, addressing polymer solubility issues and understanding the nature of bonding that lead to gelation were the primary focus. In the second approach, gelation was facilitated by the addition of physical crosslinking molecules - poly (amidoamine) dendrimers. Strengthening of physical crosslinking in gels through dendrimers of various generations enhanced mechanical properties, enabled precise control of the gelation temperature, and afforded shapeable, self-healing materials. We showed that the activation energy for dissociation of dynamic crosslinks that are critical for self-healing was ≈130–140 kJ/mol, as determined by rheology and dynamic light scattering for different types of crosslinks (linear and branched). Finally, we aimed to understand correlations between gelation and solvation of polymer chains by studying dilute polymer solutions. Using Fourier transform infrared spectroscopy (FTIR), fluorescence correlation spectroscopy (FCS) and viscometry, we showed that when LNH is used as a solvent instead of water, polymer chains are more expanded, less hydrated, and more permeable to a solvent. We argue that those features, taken together with binding of hydrated Li+ ions to PVA chains revealed by 7Li NMR spectroscopy, strongly contribute to distinct solubility and gelation properties of PVA in this inorganic IL. We believe that understanding solvation and ion-binding capability can offer crucial insights in designing polymer-based shape stabilization matrices for inorganic PCMs.
Citation
Karimineghlani, Parvin (2020). Polymer Networks for Thermoresponsive Shape Stabilization of an Inorganic Phase Change Material. Doctoral dissertation, Texas A&M University. Available electronically from https : / /hdl .handle .net /1969 .1 /192560.