Matter
ArticleTunable, reusable, and recyclable perfluoropolyether periodic dynamic polymers with high underwater adhesion strength
Progress and potential
Most adhesives lose adhesion in the presence of water. Previous attempts at underwater adhesion have used bio-inspired designs with certain functional groups to stay adhered even when immersed in water. Here, we show that polymers with dynamic bonds embedded in a hydrophobic, nanophase-separated morphology can operate as effective underwater adhesives. Moreover, the polymer adhesive can be readily recycled due to the lack of permanent covalent crosslinking. This work shows how dynamic polymers with tunable structures can be designed for high-strength recyclable underwater adhesives, which could enable applications such as reversibly attachable and detachable waterproof wearable devices.
Graphical abstract
Introduction
Adhesives are ubiquitous in everyday life, including both pressure-sensitive adhesives (PSAs) that are activated by pressing the adhesive onto a substrate (e.g., Scotch tape or sticky notes) and curable adhesives, which are applied in the liquid state and then cured by air, heat, or light into a solid (e.g., glue or epoxy).1,2,3,4 In either case, good adhesion strength is achieved by simultaneously maximizing the substrate-adhesive contact area and the cohesive strength of the bulk adhesive material.5,6,7 The former requires the adhesive to readily flow over a surface at accessible timescales, while the latter requires sufficient physical or chemical crosslinking to dissipate energy.
While conventional adhesives have been well-optimized for dry conditions, most lose adhesion in the presence of water, which is a critical concern for biomedical and structural applications.8 Water interferes with adhesives via two key mechanisms.5 First, interfacial or boundary layer water can prevent good contact and reduce the available surface area between the substrate and adhesive.9 Second, water can diffuse into the bulk adhesive material and reduce the overall cohesive strength either by interfering with physical crosslinks or as a chemically inert plasticizer.5 Adhesives developed to address these issues can be loosely classified as moisture insensitive (i.e., adhered in conditions with interfacial water or high humidity), water resistant (i.e., adhered in dry conditions and used in wet conditions), or, in the most extreme case, underwater (i.e., adhered and used while immersed in water). For example, Deng et al. have recently developed a moisture-insensitive adhesive for wound care that rapidly adheres to wet or bleeding tissues by removing boundary layer water.8 Importantly, however, the adhesive must remain dry before application, rendering the material unusable in underwater conditions.
Understandably, the design of new synthetic adhesives for underwater use is challenging and has inspired many approaches.5,10 In many cases, researchers have adopted bio-inspired designs that use supramolecular or electrostatic interactions based on the underwater adhesion mechanisms of mussels, sandcastle worms, or remoras.11,12,13,14,15,16 For example, many mussel-inspired designs incorporate catechol groups such as dihydroxyphenylalanine (DOPA) to mimic the functional groups present in mussels.17,18,19,20,21,22 Another promising alternative is the use of pre-crosslinked hydrogels, which have been show to reversibly adhere underwater to a variety of substrates through different combinations of supramolecular interactions.23,24 Critically, these mechanisms focus on achieving strong adhesion in a state where the adhesive is swollen with water.
An alternative approach is to design strongly hydrophobic self-adhesive materials that can remove interfacial water and maintain bulk cohesive strength by preventing water swelling. Previous work showed that combining hydrophobic poly(N-vinyl caprolactam) (PVCL) with short-molecular weight poly(ethylene glycol) (PEG) created a PSA with high adhesion strength for low water contents, but adhesion failed in conditions when water content exceeded 30 wt %.25 Yu et al. developed a crosslinked ionogel with high reversible underwater adhesion strength that is filled with a fluorinated ionic liquid to prevent water swelling for over 10 days.26 Ahn et al. demonstrated strong underwater adhesion of silicone surfaces using host-guest interactions, but this required pre-functionalization of the surfaces with cucurbituril host and aminomethylferrocene guest moeities.27 Alternatively, adding hydrophobic aliphatic side chains to polyesters with DOPA functional groups was shown to improve underwater adhesive performance but required a UV-mediated, chemical crosslinking step.28 Finally, poly(catechol-styrene) polymers have shown exceptionally strong underwater adhesion but must be pre-dissolved in chloroform when applied to the substrate and cured for 24 h before testing.29
We hypothesized that the tunable structure of dynamic polymers could be used to design simple, solvent-free, hydrophobic PSAs with good underwater adhesion. Previous work has shown that long-chain, entangled polymers can dramatically improve adhesive strength of hydrogels by increasing bulk cohesive strength and preventing delamination or fracture at the interface.30,31 Similarly, we theorized that dynamic polymers, which possess physical crosslinking from both supramolecular interactions and topological entanglements, could exhibit high cohesive strength while also readily flowing over a surface. Moreover, our recent work has shown that dynamic polymers with evenly spaced dynamic bonds along their backbone, termed periodic dynamic polymers, can exhibit well-defined supramolecular structures, which could improve nanophase separation between the backbones and the dynamic bonds and thus limit bulk water diffusion.32,33,34 For a hydrophobic backbone, we selected perfluoropolyether (PFPE) due to its high chain flexibility, low glass transition temperature, and excellent solvent resistance.35 PFPE-based dynamic polymers have been used for many applications including antifouling coatings or electrode coatings in batteries.36,37 Supramolecular telechelic PFPE polymers with 2-ureido-4[1H]-pyrimidone (Upy) end groups, PFPE-based vitrimers, and crosslinked PFPE polyurethanes have been previously reported but only exhibit terminal flow at or above 100°C, rendering them unsuitable for use as adhesives.35,38,39
In this work, we embed periodically placed urethane bonds into a PFPE backbone to create linear periodic dynamic polymers with a nanophase-separated microstructure. We optimize the bonding interactions to tune the rheological properties of the polymers to obtain high-strength adhesives and show that the hydrophobicity of PFPE enables underwater adhesion by removing interfacial water and preventing water diffusion into the bulk material. Importantly, these dynamic polymer PSAs can be applied in underwater conditions to a variety of substrates at room temperature, without any solvent or curing steps. Due to their reversible dynamic crosslinks, they can be easily removed and reapplied without additional stimuli and can be readily recovered and recycled after use.
Section snippets
Design and synthesis of PFPE-based dynamic polymers
We synthesized the PFPE-based dynamic polymers using a solvent-free reaction between an initial PFPE-diol (Mn ≈ 1700 g/mol, Fluorolink E10-H) and various liquid diisocyanates (Figures 1 and S1–S5). This procedure allows for periodic placement of the dynamic hydrogen bonding groups along the PFPE backbone, which has been shown to influence film microstructure and bond clustering.32,33,37 In total, we synthesized five different polymers from five diisocyanates: 4,4′-methylenebis(cyclohexyl
Lead contact
Further information and requests for resources and reagents should be directed to and will be fulfilled by the lead contact, Zhenan Bao ([email protected]).
Materials availability
This study did not generate new unique reagents.
Materials
Diol-terminated PFPE oligomers (Fluorolink E10-H, Mn = 1.7 kDa) were purchased from Solvay (Brussels, Belgium). All diisocyanates were purchased from Sigma-Aldrich (Burlington, MA, USA). All reagents and solvents were commercially available and used without further purification.
Acknowledgments
The authors thank Ivan Rajkovic and Thomas Weiss for their support during SAXS beamtime. C.B.C. acknowledges support from the Department of Defense (DoD) through the National Defense Science & Engineering Graduate (NDSEG) Fellowship Program. This work is in part supported by the Army Research Office Materials Design Program (grant no. W911NF-21-1-0092). This work is in part supported by Stanford’s Precourt Pioneering Project on Plastics Innovation. Part of this work was performed at the
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