Saturated fatty acids are an exceptionally important class of liquids, used in many consumer products and suggested as phase change materials (PCMs) for thermal energy storage, in part because they crystallize with minimal supercooling. Here we investigate fatty acid nucleation to understand why crystallization is so facile, as a step toward identifying potential mechanisms for the suppression of supercooling in other PCMs. We find that fatty acid supercooling can be induced only if the liquid is first heated above a material-dependent threshold temperature. NMR spin–lattice relaxation time studies show that the average mobility of the alkyl chains in the fatty acids increases more rapidly with temperature above the supercooling threshold temperature, and NMR T 1 hysteresis also sets in at that temperature. Measurements of the real portion of the dielectric constant as a function of temperature show that a liquid fatty acid heated far above its melting point behaves with an apparent temperature upon cooling that is higher than the actual temperature, when compared to its behavior at the same temperature upon heating. Our results suggest that molecular clusters in the liquid fatty acids break apart when the liquids are heated above their threshold temperature and do not immediately re-form on cooling. The breakup of clusters leads to an increase in the mobility of the fatty acid molecules. Because the clusters do not re-form quickly on subsequent cooling, nucleation does not occur, and substantial supercooling results.
3D printing has attracted considerable attention due to its rapid responsiveness, convenient operation, and high level of personalization. However, the continually increasing waste of printed polymers imposes a heavy burden...
Preventing ice growth on infrastructure, vehicles, and appliances remains a significant engineering challenge. Damage caused by ice growth on these installations can be expensive to repair, and their failure can be dangerous. Materials such as cross‐linked polymer networks make effective anti‐ice coatings and can prevent ice growth: reducing the cost of infrastructure repairs and limiting downtime. A link between cross‐link density and ice adhesion has been demonstrated, such that lower cross‐link density materials tend toward lower ice adhesion. Here we describe a method of lowering cross‐link density by incorporating the covalently bound comonomers methyl methacrylate, lauryl methacrylate, and styrene into UV‐cured PDMS‐based polymer networks. Cross‐link density, hardness, surface roughness, and ice adhesion on these materials are tested, showing the influence of comonomer proportions on their properties. Durability is found to increase with the addition of 5, 10, and 25 wt% comonomer, with little to no effect on ice adhesion until 25 wt%, where increases in ice adhesion are observed. Coatings show promisingly low ice adhesion of ~50 kPa, maintaining this low adhesion for up to 50 deicing cycles.
Solutions of polyvinyl alcohol (PVA) in water can form gels upon repeated freezing and thawing. These PVA cryogels have applications as biomaterials, including artificial tissue and drug delivery systems. We have studied the dielectric properties of PVA cryogels within the freeze–thaw cycles as a function of both frequency and temperature in order to understand the physical changes that take place during the thermal cycling process. Our results indicate that most of the changes in dielectric properties occur during the cooling phase of the first cycle and suggest that the solution must be cooled below a critical temperature of about 263 K for the formation of a gel that persists after thawing. The material’s dielectric spectrum shows the presence of several relaxation processes. We identify one of these with the dielectric relaxation of ice and two others with motions of the PVA polymer chains. The temperature dependence of the polymeric relaxation times suggests that they are both thermally activated, with an activation energy of roughly 300 kJ/mol.
The dielectric properties of poly(ethylene oxide)–multiwalled carbon nanotube (MWCNT) nanocomposites have been studied over a wide range of frequency (0.1–106 Hz) and temperature (180–300 K). Nanocomposites were prepared by both melt mixing and twin-screw extrusion, and the concentration of MWCNTs was varied from 0 to 5 wt. %. Both the real and imaginary parts of the complex permittivity increase with the increasing MWCNT concentration. We observe a percolation transition in the DC conductivity of the composites above a critical MWCNT concentration p c. The data from the twin-screw extruded samples give a very well-defined value of p c and a percolation exponent of 1.9 ± 0.2, in good agreement with theoretical predictions. In contrast, both the percolation threshold and the critical exponent were more poorly defined for the melt-mixed nanocomposites. This indicates that the conductive properties of these materials can strongly depend on the details of sample preparation. Our data suggest that the dc conductivity of the nanocomposites is due to the conduction along the nanotubes, coupled with thermally activated transport of electrons across thin polymer bridges, which separate the nanotubes. The frequency dependence of the dielectric spectrum was studied as a function of temperature and composition. The primary dielectric relaxation process is due to the motions of electric dipoles on the polymer backbone. At low MWCNT concentrations, the relaxation involves the entire polymer chains and is slowed substantially when a low concentration of MWCNT is added. At higher MWCNT concentrations, the relaxation becomes much faster. We attribute this to binding of the polymer chains to the nanotubes, which reduces the length of the chain segments contributing to the dielectric relaxation.
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