Effect of pressure on thermal conductivity and pressure collapse of ice in a polymer-hydrogel and kinetic unfreezing at 1 GPa

Abstract

We report a study of aqueous solutions of poly(vinylalcohol) and its hydrogel by thermal conductivity, κ, and specific heat measurements. In particular, we investigate (i) the changes in the solution and the hydrogel at 0.1 MPa observed in the 350-90 K range and of the frozen hydrogel at 130 K observed in the range from 0.1 MPa to 1.3 GPa, and (ii) the nature of the pressure collapse of ice in the frozen hydrogel and kinetic unfreezing on heating of its high density water at 1 GPa. The water component of the polymer solution on cooling either first phase separates and then freezes to hexagonal ice or freezes without phase separation and the dispersed polymer chains freeze-concentrate in nanoscopic and microscopic regions of the grain boundaries and grain junctions of the ice crystals in the frozen state of water in the hydrogel. The change in κ with temperature at 1 bar is reversible with some hysteresis, but not reversible with pressure after compression to 0.8 GPa at 130 K. At high pressures the crystallized state collapses showing features of κ and specific heat characteristic of formation of high density amorphous solid water. The pressure of structural collapse is 0.08 GPa higher than that of ice at 130 K. The slowly formed collapsed state shows kinetic unfreezing or glass-liquid transition temperature at 140 K for a time scale of 1 s. Comparison with the change in the properties observed for ice shows that κ decreases when the polymer is added.

Thermal properties of microscale inorganic light-emitting diodes in a pulsed operation

“…The thermal conductivity of 0.52 W/(m K) and thermal diffusivity of 3.2 Â 10 À7 m 2 /s of the PI layer 18,19 are close to those of hydrogel [thermal conductivity 0.6 W/(m K) and thermal diffusivity 2.3 Â 10 À7 m 2 /s] 20,21 such that the PI layer and hydrogel substrate are modeled as a single hydrogel layer. Furthermore, the hydrogel substrate has a thickness ($2 mm) much larger than that of SU8 encapsulation (7 lm) and the thickness (6.54 lm) and in-plane size (100 lm) of l-LED, and is therefore modeled as a semi-infinite substrate.…”

“…(18) and the accurate 3D FEA for the pulsed peak power Q 0 ¼ 30 mW with the duty cycle D ¼ 50% and the period t 0 ¼ 1:0 ms. The thermal conductivity, heat capacity, mass density, and layer thickness were 0.20 W/(m K), 1200 J/(kg K), 1190 kg/m 3 , and 7 lm for SU8; 3,23 160 W/(m K), 700 J/(kg K), 2329 kg/m 3 , and 6.54 lm for l-ILED; 22 0.52 W/(m K), 1150 J/(kg K), 1430 kg/m 3 , and 2 lm for PI; 18,19 and 0.6 W/(m K), 2375 J/(kg K), 1112 kg/m 3 , and 2 mm for hydrogel, 20,21 respectively. The top surface of SU8 had natural convection with the coefficient of heat convection 25 W/(m 2 K) while the other surfaces were at a constant ambient temperature T 1 ¼ 30 C. The continuum element DC3D8 in the ABAQUS software was used in FEA.…”

“…The l-ILED has much larger thermal conductivity [160 W/(m K)] 22 20 Therefore, the temperature increase in l-ILED is relatively uniform. 3,6,7 In addition, the uniform volumetric heat source of l-ILED also helps its temperature to be uniform.…”

Microgels and Nanoparticles: Where Micro and Nano Go Hand in Hand

“…Middle red lines correspond to npNI-PAM = 1.40 and = 0.55 W/m K and bottom green lines to npNIPAM = 1.41 and = 0.50 W/m K. These two cases represent the tendency of the pNIPAM refractive index (n) to increase with temperature [65,66] and that of the thermal conductivity of the Au NP environment (C) to decrease. The latter effect is expected because the thermal conductivity of the polymer is lower than that of water [67][68][69][70]. Since water molecules are released from the polymer network upon heating, the average conductivity of the Au NP environment will be closer to that of the polymer for increasing temperature values.…”

Multifunctional e-spun colloidal nanofiber structures from various dispersed blends of PVA/ODA-MMT with PVP/ODA-MMT, poly(VP-alt-MA) and AgNPs incorporated polymer complexes as electro-active platforms

“…As resistance had a tendency to decrease with increasing temperature (Figure 10e), the dependence on the pressure tended to decrease (Figure 10 f). Several researchers reported the effect of pressure on thermal conductivity of various polymer melts, as well as poly(vinyl alcohol) gels [58,59]. The thermal conductivity of both amorphous and semi-crystalline polymers was found to increase with increasing applied pressure and generally with increasing temperature.…”