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.

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.

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:

https://doi.org/10.1039/D4PY00522H

Grafting polymers to graphene oxide goes better through branching

Single-layer graphene is 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. It conducts heat and electricity, has excellent mechanical strength, and is impermeable to gases except hydrogen gas. Its drawback: how to disperse it in a liquid. When you try to do this flexible sheets of graphene tend to stack as a result of attractive van der Waals interactions, making it virtually an impossible material to disperse as single sheets.

As a compromise, people came up with graphene oxide (GO), it’s oxidised analogue. The presence of oxygen atoms containing functional groups, such as hydroxy, epoxy, carboxylic acid, ketone, or aldehyde, provides it with a polarity, which makes it possible to disperse GO as single sheets in polar solvents, such as water or DMSO at low concentrations in the absence of electrolyte or other colloidal particles. The drawbacks: graphene oxide sheets have holes. Whereas graphene can be seen as a pristine, flexible bedsheet, graphene oxide is a bedsheet attacked by moths. This makes the material less suitable for barrier property reinforcement. Graphene oxide is also less electrically conducting as oxidation has partially destroyed its conjugation. Nevertheless, it still shows great potential for a range of applications, for example, those that need mechanical reinforcement, such as polymer nano-composites, but only if we can enhance its dispersibility in liquids or polymer melts.

Grafting polymer chains from the surface of GO allows it to be dispersed more easily. This can provide additional electrostatic and steric stabilization against sheet stacking at higher concentrations in less polar solvents or contain soluble polymer chains or non-interacting particles that can trigger depletion flocculation.

There are several approaches to accomplishing this, each with advantages and disadvantages. A particular challenge is linking a sufficient mass of polymer chains to the GO’s surface. In our gold open access paper recently published in the RSC journal Polymer Chemistry, we report a way to address this, employing radical polymerization reactions that lead to a branched polymer architecture. Methacrylate-based macromonomers made by catalytic chain transfer polymerization were used together as molecular weight regulators with dimethacrylates as crosslinkers to introduce branching points.

You can read the paper here:

https://doi.org/10.1039/D4PY00300D