Polymer Networks for Thermoresponsive Shape Stabilization of an Inorganic Phase Change Material

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

Self-Healing Phase Change Salogels with Tunable Gelation Temperature

Chemically cross-linked polymer matrices have demonstrated strong potential for shape stabilization of molten phase change materials (PCM). However, they are not designed to be fillable and removable from a heat exchange module for an easy replacement with new PCM matrices and lack self-healing capability. Here, a new category of shapeable, self-healing gels, “salogels”, is introduced. The salogels reversibly disassemble in a high-salinity environment of a fluid inorganic PCM [lithium nitrate trihydrate (LNH)], at a preprogrammed temperature. LNH was employed as a high latent heat PCM and simultaneously as a solvent, which supported the formation of a network of polyvinyl alcohol (PVA) chains via physical cross-linking through poly­(amidoamine) dendrimers of various generations. The existence of hydrogen bonding and the importance of low-hydration state of PVA for the efficient gelation were experimentally confirmed. The thermal behavior of PCM salogels was highly reversible and repeatable during multiple heating/cooling cycles. Importantly, the gel–sol transition temperature could be precisely controlled within a range of temperature above LNH’s melting point by the choice of dendrimer generation and their concentration. Shape stabilization and self-healing properties of the salogels, taken together with tunability of their temperature-induced fluidization make these materials attractive for thermal energy storage applications that require on-demand removal and replacement of used inorganic PCM salt hydrates

Properties of a Terpolymer-Treated Soil: A ¹³C NMR Study

The Young's modulus and the secant modulus of a terpolymer-treated soil as a function of the polymer's characteristics are discussed in the context of a more general inelastic property known as the toughness parameter. The soil chosen was a sample of the State of Qatar subsoil. The terpolymer, designated TPAM, was characterized by a backbone structure of acrylamide, anionic carboxylate, and cationic (3-acrylamidopropyl-trimethylammonium chloride) repeat units. The backbone unit ratio was estimated from ¹³C NMR analyses. TPAM was synthesized by straightforward NaOH hydrolyses of an acrylamide/cationic copolymer. The correlations between the NaOH molarity of the hydrolysis solution, with the corresponding ratio of the anionic and cationic units, were shown to have a significant influence on the value of the toughness parameter. It is speculated that controlling the anionic and cationic ratio of a terpolymer is a general approach to optimize the toughness parameter of treated soils. Measurements of the molecular weight of TPAM were made, and comments on the importance of this feature are given. The equivalent viscosity was also recorded. It is pointed out that the work is particularly relevant to the practical problem of subsoil pavement stabilization in which the terpolymer acts as a soil binder. Suggestions on further work are given.The authors would like to acknowledge the Qatar National Research Fund (a member of the Qatar Foundation) for their support under the NPRP award [NPRP 5-508-2-204]