This ultrahigh molecular weight (UHMW) adhesive polymer derives its enhanced cohesive strength, creep resistance, and thermal stability from the tangles that naturally form among its long molecular chains. Polymers such as poly(methyl acrylate) consist of exceptionally long strands of repeating units. The more repeating units in a single spaghetti-like strand, the higher the molecular weight of the polymer, with strands weighing in at several million times the weight of a single hydrogen classified as UHMW polymers. Although tangles feature in many polymers, in non-UHMW polymers, entanglement produces only weak physical linkages between strands. This results in a material that is not cohesive enough for applications as a pressure-sensitive adhesive, as found in products such as stickers and tape that are applied simply by pressing down. Chemists typically compensate for weak physical linkages by inducing the different strands to form chemical bonds (cross-links) with each other, linking the various strands more tightly. However, this solution hinders the polymer’s ability to stick to other surfaces (adhesion) even as it boosts its ability to stick to itself (cohesion). A pressure-sensitive adhesive with strong cohesion without cross-links should have better mechanical properties, including a closer balance between cohesion and adhesion. Moreover, by relying on transient entanglements between chains instead of permanent cross-links, these adhesives can be readily debonded to promote the recyclability of electronic wastes.
Researchers at the University of Florida have developed adhesives from UHMW polymers whose hefty molecular weights make them harder to pull apart when entangled, enhancing their cohesion without resorting to cross-links. They also observed that increasing the molecular weights of these materials promoted many desirable mechanical properties, such as creep resistance and toughness.
Pressure-sensitive adhesive with enhanced mechanical and thermal properties without requiring cross-linkers
Commercial adhesive polymers such as those in Sticky Notes and labels can form bonds when pressed down but also should be judged according to their ability to avoid sliding (shear resistance), ability to retain shape over long times (creep resistance), and ability to stay intact at high temperatures (thermal stability). Finally, they should not leave behind a residue when peeled away (cohesive failure), necessitating strong cohesion within the molecules making up the adhesive. The structure of the polymer, including physical entanglement of the chains and chemical cross-links between them, underpins cohesion and other desirable properties. This technology bolsters the contribution of the entangled chains to cohesion by increasing the molecular weight of the chains up to the ultra-high molecular weight (UHMW) regime. To access UHMW materials, light-mediated reversible deactivation is employed to synthesize the polymers. This method not only simplifies production by minimizing steps but also enhances cohesive strength and thermal stability by doing away with the need for cross-linkers.