Marine mussels clinging to rocks inspire the development of novel materials. Characterization of mussel adhesive plaques describes a matrix of proteins containing 3,4-dihydroxyphenylalanine (DOPA), which provides cross-linking chemistry that allows mussels to attach fi rmly. Several synthetic polymer systems have been developed based on this DOPA chemistry. High strength bonding has been achieved with poly[(3,4-dihydroxystyrene)-co -styrene], a simplifi ed mimic of mussel proteins in which 3,4-dihydroxystyrene provides the cross-linking and adhesion of DOPA. The poly(styrene) host polymer stands in for a protein backbone. Prior efforts showed that a monomer ratio of 1:2 3,4-dihydroxystyrene:styrene within the statistical copolymer poly[(3,4dihydroxystyrene)-co -styrene] yields the highest adhesion. To enhance adhesive performance of this biomimetic polymer, a systematic study is carried out in which a range of cross-linking agents, cure times, cure temperatures, polymer concentrations, and fi llers are examined. Lap shear adhesion testing revealed substantial increases in bond strength from each study. Consensus conditions are then determined and bonding performance is assessed on several substrates. Adhesion of this system turns out to be one of the strongest of all biomimetic polymers. These studies show that DOPA chemistry may be able to stand alongside of cyanoacrylate (e.g., Super Glue) and epoxy when it comes to high strength bonding.

Characterization of marine biological adhesives are teaching us how nature makes materials and providing new ideas for synthetic systems. One of the most widely studied adhering animals is the marine mussel. This mollusk bonds to wet rocks by producing an adhesive from cross-linked proteins. Several laboratories are now making synthetic mimics of mussel adhesive proteins, with 3,4-dihydroxyphenylalanine (DOPA) or similar molecules pendant from polymer chains. In select cases, appreciable bulk bonding results, with strengths as high as commercial glues. Polymer molecular weight is amongst several parameters that need to be examined in order to both understand biomimetic adhesion as well as to maximize performance. Experiments presented here explore how the bulk adhesion of a mussel mimetic polymer varies as a function of molecular weight. Systematic structure-function studies were carried out both with and without the presence of an oxidative cross-linker. Without cross-linking, higher molecular weights generally afforded higher adhesion. When a [N(C4H9)4](IO4) cross-linker was added, adhesion peaked at molecular weights of ~50,000-65,000 g/mol. These data help to illustrate how changes to the balance of cohesion versus adhesion influence bulk bonding. Mussel adhesive plaques achieve this balance by incorporating several proteins with molecular weights ranging from 6000 to 110,000 g/mol. To mimic these varied proteins we made a blend of polymers containing a range of molecular weights. Interestingly, this blend adhered more strongly than any of the individual polymers when cross-linked with [N(C4H9)4](IO4). These results are helping us to both understand the origins of biological materials as well as design high performance polymers.

Elemental sulfur (S 8 ) is produced in abundance during petroleum refinement, generating millions of tons of waste. Inverse vulcanization utilizes this waste as a feedstock to create new materials. Heating S 8 above 159 °C initiates ringopening, forming radicals that react with difunctional monomers to create polysulfides. High temperature requirements limit the types of monomers that can be incorporated by inverse vulcanization. However, cleaving the linear sulfur chains present in the polysulfides requires less energy. Here, poly(S− divinylbenzene) with varied sulfur contents has been synthesized to act as a prepolymer capable of radical formation at much lower temperatures (90 °C). Dynamic sulfur bonds initiate polymerization with a family of vinylic and allylic ethers. Terpolymers were characterized by NMR spectroscopy, gel permeation chromatography, and differential scanning calorimetry to examine the impact on structure and physical properties. This method expands on inverse vulcanized polymers to create a mild, solvent-free route to polysulfide synthesis.

Adhesives releasing carcinogenic formaldehyde are almost everywhere in our homes and offices. Most of these glues are permanent, preventing disassembly and recycling of the components. New materials are thus needed to bond and debond without releasing reactive pollutants. In order to develop the next generation of advanced adhesives we have turned to biology for inspiration. The bonding chemistry of mussel proteins was combined with preformed poly(lactic acid), a bio-based polymer, by utilizing side reactions of Sn(oct) 2 , to create catechol-containing copolymers. Structure−function studies revealed that bulk adhesion was comparable to that of several petroleum-based commercial glues. Bonds could then be degraded in a controlled fashion, separating substrates gradually using mild hydrolysis conditions. These results show that biomimetic design principles can bring about the next generation of adhesive materials. Such new copolymers may help replace permanent materials with renewable and degradable adhesives that do not create chronic exposure to toxins.

Natural materials have been used as glues throughout human history. Over the last century, society has come to rely heavily on synthetic, petroleum-based adhesives instead, consuming ∼14 million tons per year. In recent years, however, there has been a resurgence of glues formed with renewable materials. This work seeks to integrate the two to form strong adhesives. Here, elemental sulfur was combined with diallyl sulfide (DAS), diallyl disulfide (DADS), and garlic essential oil (GEO) to form adhesive polymers from recycled petroleum waste and renewable monomers. The labile sulfur bonds in DADS and GEO allowed these monomers to be homopolymerized, forming polysulfides entirely from renewable monomers. Heating these materials causes them to transition from viscous liquids to hardened solids. A family of copolymers containing different garlic components and varying sulfur-to-monomer ratios were synthesized, characterized, and tested for this study. Polymer structures were confirmed by 1 H NMR. Changes to the polysulfide material properties upon curing were examined by gel permeation chromatography and differential scanning calorimetry. Characterization data of cured polymers were used to choose the optimal cure temperature for adhesion studies. The adhesion strength of polysulfides with varying compositions was determined by single-lap shear testing. Strong bonding was obtained for all garlic-based polysulfides with strengths 3 times higher than commercial hide glue.