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

Abstract 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.