Phase change materials, also known as PCMs, can capture, store, and release energy when they undergo a phase transition. One class of PCMs with relatively high latent energy storage capabilities are alkane-based waxes. These store energy upon melting and will release it again when they solidify through crystallization. To make use of this so-called latent heat is helpful for many applications, and indeed can be found, for example, in building insulation and temperature regulation materials. If, however, we would like to use this concept in a temperature-regulating fluid, we need to disperse the PCMs into a liquid, such as water, that in itself has a high heat capacity and thus the ability to store energy.

Creating a dispersion of PCMs is a complex task that presents an intellectual challenge. If we were to emulsify molten wax using ordinary surfactants into droplets dispersed in water, a problem would arise upon cooling. When certain wax droplets crystallize, these will touch other droplets in their vicinity triggering crystal growth and fusion of the droplets. The emulsion is destabilized and, upon heating, will not return to its original state. This complexity underscores the urgent need for innovative solutions in the field of materials science.

The solution to this challenge is to encapsulate the wax. This involves surrounding the wax droplets with a protective shell that prevents fusion. However, a new problem arises when the capsules are large, such as in the micron length scale or upwards. In these cases, the capsules can still jam together and obstruct the flow of the liquid. To overcome this, we need to miniaturize the entire system and fabricate wax nanocapsules that remain colloidally stable throughout their application. This approach not only solves the problem but also allows us to blend nanocapsules containing different waxes, thereby tuning the energy storage characteristics.

Our paper recently published in the Royal Society of Chemistry journal Polymer Chemistry and entitled: “Phase change material nanocapsules for latent function thermal fluids with tuneable thermal energy storage profiles” does exactly that.

We prepared mini-emulsions, which are stable tiny droplets of various methacrylates in the presence of trimethylolpropane trimethacrylate (TMA) as a crosslinker, and n-hexadecane (HD), n-octadecane (OCT), and n-docosane (DOC) as PCM. One important aspect was to use an ω-unsaturated poly(n-butyl methacrylate-b-[(methacrylic acid)-co-(methyl methacrylate)]) macromonomer as a reactive macromolecular emulsifier, to secure colloidal stability of our nanocapsule systems. The performance of a thermal fluid of DOC nanocapsules was tested against water, with promising results. As a tunability concept, crosslinked poly(methyl methacrylate) nanocapsules of n-octadecane (OCT) and n-docosane (DOC) were blended as a tuneable latent function thermal fluid.

You can read the paper here:

https://doi.org/10.1039/D4PY00789A