Polymer Composites Containing Phase‐Change Microcapsules Displaying Deep Undercooling Exhibit Thermal History‐Dependent Mechanical Properties

Abstract

they melt into a liquid and releasing heat as they solidify into a solid. The greater the latent heat of fusion, the greater the energy absorbed/released during these phase transitions. [5,6] As various groups have shown, by encapsulating micron to millimeter diameter droplets of a PCM, functionalities ranging from temperaturedependent permeability to enhanced solar thermal energy storage efficiency can be realized. [7,8] Once encapsulated, PCMs can be embedded in a diversity of matrices, which may enable new applications in areas ranging from thermal energy storage and conversion, electronics, [9-12] smart fibers and textiles, and thermal comfort in vehicles. [13] A conventional approach to encapsulation is through shell polymerization in oil-in-water (o/w) or water-in-oil-in-water (w/o/w) double emulsions, such as those used to prepare the self-healing microcapsules first reported by Moore et al. [14] Challenges with this approach are the generally wide size distribution of the capsules prepared, compatibility between the capsule shell polymerization chemistry and the core chemistry, and control of the synthetic parameters required to enable reproducible results. [15-17] Because the thermal transitions of PCMs are sensitive to the Microencapsulated materials are receiving broad attention for applications as diverse as energy storage and conversion, biomedicine, self-healing materials, and electronics. Here, a general microfluidic approach is presented to prepare phase-change material-infilled microcapsules with unique thermal and mechanical properties. Aqueous sodium acetate solutions are encapsulated by an acrylate-based shell via a microfluidic method. To understand and optimize microcapsule formation, flow behavior during the encapsulation is numerically simulated. When the microcapsules are embedded in an acrylate matrix (same composition as the shell wall material), the microcapsules exhibit a significant 46.6 ºC difference between the crystallization and melting temperatures as determined by differential scanning calorimetry at a rate of 10 ºC per min. Variable temperature dynamic mechanical analysis over the range of 50 to-90 ºC reveals up to a 50% change in the composite's elastic modulus at a given temperature, depending on if the sample is being cooled or heated, due to significant undercooling of the core material crystallization as shown by X-ray diffraction.

Plastic straining and concomitant microstructure recrystallization of Ni-Cu alloy in the undercooled condition

“…In conclusion, complete grain refinement at large undercooling has strong texture and annealing twins. Recrystallization is driven by deformation energy, and new deformation free grains are formed in the deformed matrix through the formation and subsequent migration of large angle grain boundaries [14]. Due to solidification of undercooled melt, the lattice distortion caused by solute capture and dendrite deformation can store deformation energy.…”

“…Grain refinement mechanism of undercooled solidification microstructure of binary alloys is a topic of debate for decades [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]. For highly undercooled single phase binary alloys, recent studies on Fe-Ni [3], Ni-Cu [4], Fe-Co [6] and Ni-C [9] alloys show that two grain refinement could be found.…”

Systematic investigation on the swelling behaviors of acrylonitrile‐butadiene rubber via solubility parameter and Flory‐Huggins interaction parameter

“…Furthermore, swelling disrupts the order of structure and increases heterogeneity of internal structures of polymer, and thus significant changes in structure and mechanical properties happen after 2 days of the elastomer immersed in various liquids. 9,10 Rubber products used in oil industry appear severe changes in mechanical properties under the conditions of continuous rotation, shear stress and extrusion stress. [11][12][13] In practice, the swelling process can be promoted by the aggressive media, extraction of small additives and chemical reactions.…”

Structure Transition Mechanism in Undercooled CuNi Alloys

“…When the initial undercooling degree of the alloy melt is greater than a certain critical value, the solidification structure changes from developed dendrites to fine equiaxed crystals, and the grain size decreases by about 2 orders of magnitude [1][2][3][4][5][6][7][8][9][10] . Kattamis et al found that CuNi alloy presents developed dendrites when undercooling is less than 175 K and fine equiaxed crystals when undercooling is greater than 175 K. Recently, some researchers [4,[11][12][13][14] found that the grain refinement phenomenon of columnar dendrite to equiaxed crystal also exists under small undercooling.…”