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Water-based nanocapsule dispersions for tuneable energy storage
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.
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:
Templating Calcium Phosphate onto Graphene Oxide Sheets
Single-layer graphene oxide sheets are interesting as a flexible 2D material, with xy-dimensions variable up to a centimetre in length and a z-thickness of a single carbon atom. The presence of oxygen atoms with functional groups, such as hydroxy, epoxy, carboxylic acid, ketone, or aldehyde, provides graphene oxide (GO) with polarity. This unique property allows GO to disperse as single sheets in polar solvents like water or DMSO at low concentrations, in the absence of electrolytes or other colloidal particles.
Single-layer graphene oxide sheets are interesting as a flexible 2D material, with xy-dimensions variable up to a centimetre in length and a z-thickness of a single carbon atom. The presence of oxygen atoms with functional groups, such as hydroxy, epoxy, carboxylic acid, ketone, or aldehyde, provides graphene oxide (GO) with polarity. This unique property allows GO to disperse as single sheets in polar solvents like water or DMSO at low concentrations, in the absence of electrolytes or other colloidal particles.
Our recent findings, published in RSC Polymer Chemistry (https://doi.org/10.1039/D4PY00300D), have shown that grafting polymer chains from the surface of GO significantly enhances its dispersion. This is particularly true when the polymer chains are of a branched chain architecture. The grafted branched polymer chains provide additional electrostatic and steric stabilization against sheet stacking and crumpling, a discovery that could have significant implications in the field of materials science.
SEM images of single graphene oxide sheets that are decorated with a layer of calcium phosphate, deposited to a templated synthesis route.
The 2D architecture of single GO sheets dispersed in water is an exciting template for depositing inorganic materials, especially now that the grafted polymer chains can be engineered to withstand the ionic precursor concentrations so that the sheets remain dispersed and can additionally serve as crystal nucleation sites.
In our paper published in the Journal of Colloid and Interface Science we show that it is possible to grow Calcium Phosphate onto single dispersed sheets of GO in water. The work was led by former PhD researcher dr. Wai Hin Lee.
Prof. dr. ir. Stefan Bon says: “This paper shows that as a result of the grafted branched polymer, the use of dispersed Graphene Oxide sheets as a template for mineralization is now possible and straightforward. It opens up all sorts of possibilities for hybrid material design”
You can read the paper here: https://doi.org/10.1016/j.jcis.2024.06.221
Water-Based Acrylic Latexes are excellent Pressure Sensitive Adhesives when Branched
Water-based pressure sensitive adhesives (PSAs) are typically made by emulsion polymerization using a low glass transition temperature base monomer, such as n-butyl acrylate or 2-ethyl hexylacrylate, together with a range of functional comonomers. Typically these include a high glass transition temperature comonomer, such as styrene or methyl methacrylate and monomers that can promote wetting and undergo secondary interactions such as (meth)acrylic acid.
Water-based pressure sensitive adhesives (PSAs) are typically made by emulsion polymerization using a low glass transition temperature base monomer, such as n-butyl acrylate or 2-ethyl hexylacrylate, together with a range of functional comonomers. Typically these include a high glass transition temperature comonomer, such as styrene or methyl methacrylate and monomers that can promote wetting and undergo secondary interactions such as (meth)acrylic acid.
A golden rule for good adhesive performance is that the polymer latex particles must contain a certain fraction of gel, that is, cross-linked material. This typically is 50-70%. This gel content optimizes the balance between tack and cohesive forces within the adhesive.
Schematic of the branched polymer chain architecture approach to deliver good performance in pressure sensitive adhesives (PSAs).
In our paper, led by PhD researcher Emily Brogden and published in the RSC journal Polymer Chemistry, we challenged this view. Prof. dr. ir. Stefan Bon says: “We designed a range of polymer latexes with low gel content. To balance the adhesive forces, we introduced a branched polymer chain architecture instead. We show that excellent water-based acrylic PSAs can be made using this approach.”
Moreover, the chemical composition of our PSA was designed with sustainability in mind. The base monomer selected was the bio-based 2-octyl acrylate, the high Tg component bio-based 2-isobornyl acrylate. The other monomers and chain transfer agent show promise to be or become fully bio-based.
The paper entitled “Water-Based Polymer Colloids with a Branched Chain Architecture as Low-Gel Pressure-Sensitive Adhesives” has gold open access and can be read here: