Developing sulfur polymers

RESEARCHERS at the University of Liverpool, UK, are making significant progress in developing new sulfur polymers which could provide an environmentally-friendly alternative to some traditional plastics. In two recent papers, they improved the properties of the materials via crosslinking and, for the first time, demonstrated chemically-induced repair.

The work follows on from Liverpool’s earlier discovery of a novel catalytic process for making plastics from sulfur. 

More than 70m t/y of sulfur is produced as waste product from many industrial processes. In recent years, a growing number of materials scientists have become interested in using it as alternative to oil for plastic manufacture. Additionally, sulfur offers the opportunity to develop more easily-recycled plastics.

In the first of the recent studies, the Liverpool researchers made a discovery that addresses the weakness of sulfur polymers, a factor that has limited application.

Tom Hasell, chemist and Royal Society University Research Fellow at Liverpool, explained that sulfur-sulfur bonds have the benefit of being reversible, but they are weaker than the bonds found in many conventional polymers, which are typically carbon-carbon bonds.

The researchers discovered that the mechanical properties, including strength, of sulfur polymers could be altered with the addition of urethane bonds to the polymer structure. Tensile strength could be increased by up to 135 times (to more than 20 MPa) with the addition of polyurethane bonds.

According to Hasell, the addition of urethane bonds strengthens the polymers by crosslinking the sulfur polymer strands – which are otherwise linear. The sulfur bonds remain weak, but the addition of crosslinking means that more bonds need to break to enable the polymers to come apart.

Additionally, by adjusting the crosslinking density, the researchers showed that it was possible to control the properties of the polymers. The polymers showed a tendency to vary from weak-and-soft to strong-but-hard with increasing crosslinking density. Combining high strength with high flexibility for sulfur polymers is still challenging, according to the researchers.

Furthermore, polymer networks with a suitable degree of crosslinking (anything from a 50% theoretical crosslinking degree) exhibit an excellent shape memory effect. The shape memory effect is a phenomenon in which a material recovers its original shape and size when heated above certain characteristic transformation temperature.

The crosslinked sulfur polymers with 50% and 100% crosslinking used by the researchers in their study could be temporarily reshaped by heating them above their glass transition temperature – the temperature at which 30–50 carbon chains start to move – and then cooling. The original shape could be recovered when the polymer was reheated. Permanent reshaping could be achieved by heating the polymers above solid-solid transition temperature, allowing dynamic sulfur-sulfur bond exchange.

The researchers found that the permanent shape of the crosslinked polymers could be changed multiple times, and still recover the permanent shape after a reversible shape change.

According to Tom Hasell, the shape memory aspect of sulfur polymers is enabled by the relative weakness of sulfur-sulfur bonds which mean that they can be broken apart with a little heating and then recombined to form another bond. This also the characteristic that makes sulfur polymers easy to recycle.

The easy recyclability of sulfur polymers remains despite the addition of crosslinking, and the modified polymers open up the potential for application in a wider range of areas, such as soft robotics, medicine, and self-repairing objects. Additionally, there is a wider range of crosslinking degrees and crosslinking agents to explore as a way of further tuning sulfur polymer properties to meet various practical needs.

In the second paper, Liverpool teamed up with researchers at Flinders University, Australia to demonstrate that sulfur polymers could form materials which can be repaired at room temperature through phosphine or amine-catalysed bond exchange.

According to the researchers, an important goal is to develop methods for adhesion and repair of polymers made by inverse vulcanisation – such as sulfur polymers – that do not require energy intensive heating. Inverse vulcanisation is a process that can be used to generate stable sulfur polymers through ring-opening polymerisation followed by crosslinking with an unsaturated organic molecule.

For the work, the researchers made model rubber-like sulfur terpolymers using sulfur, canola oil, and dicyclopentadiene (DCPD). When cut into two pieces, pyridine or tributylphosphine – both of which are nucleophilic – could induce reparative bond exchange at room temperature. Analysing the tensile strength at different times during repair revealed that maximum strength of the polymer was achieved after less than one hour for tributylphosphine and two hours for pyridine, showing that repair is relatively rapid at room temperature.

Repair could be achieved by just putting the pieces in contact, but in this case it was not uniform. The researchers found that uniformity could be increased by applying pressure. When 10% compression was applied to the bonding interface, the reacting surfaces had more uniform contact, achieving more thorough repair. With 10% compression, 74% tensile strength was recovered using pyridine.

The researchers also discovered that triethylamine performed well in repair and could restore the polymer back to its original strength.

Investigating the impact of polymer composition on repair, the team made polymers with differing amounts of sulfur, finding that repair worked better with higher amounts of sulfur when canola oil and DCPD were kept constant.

The rubber-like material and repair catalysts could be used with low energy consumption to produce flexible, repairable, and sustainable objects, providing a useful application of these new sulfur polymers. The study demonstrated applications in latent adhesives, additive manufacturing, polymer repair, and recycling.

According to Hasell, the researchers are currently interested in exploring the limits – such as the number of times the polymers can be recycled and effects on the properties, and whether the materials can be made cheaper or more renewable –  and then trying to improve the materials.

He said: “The low cost of sulfur, as a large scale byproduct, means these materials could be interesting industrially. We want to understand what the challenges and opportunities would be for their practical application – and that means we would love to hear from specialists in industry.”

Hasell noted that process and chemical engineers could help to address challenges involved in scaleup, adding that he’d be keen to “see some of the sulfur polymers now being reported made and tested on a bigger scale. And that means getting the materials out of the hands of chemists, and into the hands of chemical engineers”.

“Both of these papers really show the potential of polymers made from waste sulfur to be a viable replacement material for some traditional petrochemical based plastics.

“We are excited to see what ideas researchers have for using these new findings, in particular the memory shape and “re-programming” properties.”

Angewandte Chemie: http://doi.org/d6kr

Chemical Science: http://doi.org/d6ks