Revolutionizing Adhesion: How Polymer Microstructure Outperforms Super Glue!

By altering the microstructure of a natural polyester, scientists in the US have crafted a compostable adhesive polymer that surpasses super glue in strength. Through precise management of the sequence and alignment of monomer units, the team successfully adjusted both the adhesive and thermomechanical characteristics, tailoring the polymer for various practical applications.

‘Adhesives stand as a critically significant element of polymer science where biodegradability provides an end-of-life benefit in the absence of suitable alternatives,’ states Michael Shaver, a polymer specialist at the University of Manchester, UK. The capacity to bond surfaces is foundational to numerous facets of everyday existence, spanning packaging, electronics, and sealants employed in construction. Nevertheless, the vast majority of available commercial products are constructed from petroleum-derived polymers, which lack recoverability and biodegradability. Thus, bio-based and biodegradable adhesive polymers present an appealing alternative, although identifying materials that embody the ideal blend of adhesive, mechanical, and chemical characteristics poses a considerable challenge.

Designed adhesion

Instead of commencing from scratch, a group led by Colorado State University’s Eugene Chen aimed to modify an existing sustainable polymer, the natural polyester poly(3-hydroxybutyrate) (P3HB), to showcase adhesive capabilities. Initially discovered in certain microorganisms, contemporary synthetic chemical and biological techniques now permit enhanced control over the construction of the polymer chain, aiding in the production of P3HB plastics with various internal configurations known as microstructures.

The monomer unit, β-butyrolactone (BBL), features a chiral carbon, and the arrangement of these stereogenic centers relative to each other – referred to as tacticity – has a significant effect on the overall physical characteristics of the polymer. ‘Regulating the tacticity and positioning of the chain will influence how the chain can pack or crystallize,’ clarifies Charles Romain, a chemist specializing in sustainable polymers at Imperial College London, UK. ‘If a material is semi-crystalline, it’s prone to being hard and rigid. If it’s amorphous, it’s likely more flexible.’

Altering the microstructure

In pursuit of the ideal equilibrium of these attributes, Chen’s team initiated the creation of a range of potential P3HB microstructures. They opted for a more complex monomer unit known as 8DL, characterized by having two chiral carbons, and formulated a set of catalytic conditions to meticulously regulate the orientation of each stereogenic center during its incorporation into the polymer chain. ‘There exist three extreme tacticities: isotactic,’ – where all R groups align on the same side of the chain – ‘syndiotactic,’ – where the R groups alternate sides – ‘and atactic,’ – a random configuration – explains Chen. ‘Yet, there also exist sub-tacticities among these extremes, for instance, iso-rich, syndio-rich, and block structures.’

Once eight distinct microstructures were established, the team examined the physical and adhesive properties of each. While the thermomechanical traits scaled predictably with the degree of crystallization, the intermediate sub-tacticities displayed an unexpected trend in adhesive strength. The three extreme structures, iso-, syndio-, and atactic, demonstrated minimal to no adhesion. However, the syndio-rich variants, containing between 42% and 66% organized syndiotactic units, exhibited remarkable adhesive strength when evaluated on aluminium substrates. The 52% blend outperformed commercial Gorilla and EVA glue. This leading candidate was then subjected to a series of practical tests. The team extruded the adhesive via a glue gun, sealing a cardboard box weighing 4.4kg of books, while a subsequent demonstration applied the glue to steel plates, successfully lifting a 9kg weight through this structure.

As per Chen, it is the combination of stereoregular crystalline units and more flexible amorphous segments that allows this specific P3HB microstructure to function as an adhesive. ‘The syndio-rich stereomicrostructure provides a moderate degree of crystallinity from the stereoregular domain, essential for thermoplastics to be adhesive, alongside a viscous quality stemming from the stereoirregular – atactic – amorphous domain, necessary for close contact between the polymer and substrate surfaces,’ he elucidates.

A commercial outlook?

The team’s meticulous strategy has garnered admiration from others within the domain; however, the expense and complexity of synthesizing the intricate 8DL monomer means there remains progress to be made before the polymer can serve as a realistic commercial alternative. ‘It serves as an excellent case of employing stereoselective catalysis to refine the tacticity and characteristics of the polymer,’ notes Romain. ‘The principal challenge will be in the monomer production at a large scale.’

‘It’s a significant contribution, and there are numerous different avenues they could pursue,’ adds Shaver. ‘Clearly, ample work is required to commercialize this, but this team has intelligently examined the economics and lifecycle to effectively plan for future research targeting genuinely sustainable production methods for these monomers,’