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Naturally produced by bacteria, polyhydroxyalkanoates (PHAs) such as poly(3-hydroxybutyrate) (P3HB) have been attracting attention as potentially greener alternatives to plastics like polyethylene and polypropylene. And polymer chemist Eugene Chen of Colorado State University thinks that, with some careful engineering of its microstructure, P3HB could replace even more plastics.
The new adhesives' mix of stereocontrolled (green cylinders) and random (blue squiggles) segments gives it just the right blend of strength and flexibility
“There are an astronomical number of possibilities,” Chen says. “The design space is very, very vast.”
Like polypropylene, P3HB has methyl groups poking off its polymer backbone. The form of P3HB that’s made by bacteria has all its methyl groups aligned in the same direction. This results in a strong, stiff, brittle material. But Chen says altering the stereochemistry makes it possible to unlock new properties.
His group recently teamed up with chemical engineer Ting Xu of the University of California, Berkeley, and Gregg Beckham of the National Renewable Energy Laboratory to figure out just the right stereochemical arrangement to make P3HB into a strong yet flexible adhesive (Science 2025, DOI: 10.1126/science.adr7175).
The work “does a very nice job of expanding the rather short list of sustainable adhesives,” Jonathan Wilker, who researches biobased adhesives at Purdue University and was not involved in the work, says in an email.
The researchers mixed and matched biobased dimethyl diolide and β-butyrolactone monomers with stereoselective metal catalysts to create a library of P3HBs with different arrangements of methyl groups. They then used that library to examine how changing the polymer’s microstructure influenced its physical properties.
The collaborators found that to make a good adhesive, they needed a polymer with some segments where the methyls alternate directions, locking together like the teeth of a zipper, and some where the direction is more random. The zippering gives the polymer internal strength, while the random sections impart adhesion and flexibility.
“People have this idea that [polymers] need to be absolutely molecularly defined,” Xu says. But a little bit of disorder, strategically applied, can produce very interesting things, she adds.
The researchers created several different semicrystalline P3HBs with slightly varied proportions of random and ordered segments, resulting in varied adhesion strengths.
The adhesive P3HBs that the team created stick to wood, glass, and metal surfaces as well as or better than commercial glues such as ethylene vinyl acetate (EVA). Two steel plates glued together vertically with the new adhesive can support suspended weight of up to 9.1 kg, compared with less than 6.8 kg with EVA. The researchers could apply it easily using a regular hot glue gun.
Sophie Guillaume, a polymer chemist at the Rennes Institute of Chemical Sciences, says the finding that P3HB can be engineered to have adhesive properties is “a major step forward” for sustainable adhesives and PHAs.
Beckham and his team analyzed the cost and environmental impacts associated with making the adhesive form of P3HB. They found that the cost could be competitive with EVA, though greenhouse gas emissions would be higher. Chen says part of the environmental impact could be mitigated by the fact that P3HB is both recyclable and biodegradable. His team and Beckham’s are working together on a more industrially viable way to make it.
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