Polymer Composition and Substrate Influences on the Adhesive Bonding of a Biomimetic, Cross-Linking Polymer

Article pubs.acs.org/JACS Polymer Composition and Substrate Influences on the Adhesive Bonding of a Biomimetic, Cross-Linking Polymer Cristina R. Matos-Pérez,† James D. White,† and Jonathan J. Wilker*,†,‡ † Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907-2084, United States School of Materials Engineering, Purdue University, Neil Armstrong Hall of Engineering, 701 West Stadium Avenue, West Lafayette, Indiana 47907-2045, United States ‡ ABSTRACT: Hierarchical biological materials such as bone, sea shells, and marine bioadhesives are providing inspiration for the assembly of synthetic molecules into complex structures. The adhesive system of marine mussels has been the focus of much attention in recent years. Several catecholcontaining polymers are being developed to mimic the crosslinking of proteins containing 3,4-dihydroxyphenylalanine (DOPA) used by shellfish for sticking to rocks. Many of these biomimetic polymer systems have been shown to form surface coatings or hydrogels; however, bulk adhesion is demonstrated less often. Developing adhesives requires addressing design issues including finding a good balance between cohesive and adhesive bonding interactions. Despite the growing number of mussel-mimicking polymers, there has been little effort to generate structure−property relations and gain insights on what chemical traits give rise to the best glues. In this report, we examine the simplest of these biomimetic polymers, poly[(3,4dihydroxystyrene)-co-styrene]. Pendant catechol groups (i.e., 3,4-dihydroxystyrene) are distributed throughout a polystyrene backbone. Several polymer derivatives were prepared, each with a different 3,4-dihyroxystyrene content. Bulk adhesion testing showed where the optimal middle ground of cohesive and adhesive bonding resides. Adhesive performance was benchmarked against commercial glues as well as the genuine material produced by live mussels. In the best case, bonding was similar to that obtained with cyanoacrylate “Krazy Glue”. Performance was also examined using low- (e.g., plastics) and high-energy (e.g., metals, wood) surfaces. The adhesive bonding of poly[(3,4-dihydroxystyrene)-co-styrene] may be the strongest of reported mussel protein mimics. These insights should help us to design future biomimetic systems, thereby bringing us closer to development of bone cements, dental composites, and surgical glues. ■ to high-energy surfaces via metal chelation,14−18 individual metal−ligand bonds,16,19 nonspecific adsorption,18 or hydrogen-bonding.18,20 Oxidative21,22 and enzymatic21−23 crosslinking may also be involved. Incorporating DOPA and analogous reactive groups such as catechol (i.e., 1,2-dihydroxybenzene) into polymers is being pursued for a variety of applications. This field is expanding rapidly, especially in the past 5 years, with many laboratories contributing.24 Mussel mimetic polymers are being generated from polypeptides,25−27 polyamides,28 polyacrylates,17,29−35 polyethylene glycols,36−52 polystyrenes,53−59 and polyurethanes.60 These polymers are enabling the development of imaging agents,48 nanoparticle shells,44,48,61 elastomers,30,33,59 resins,58,62 coacervates,31 hydrogels,36−38,42,43 surface treatments,27,40,49,52 antibacterial coverings,51,63 and antifouling coatings.34,35,45−47,50,51 A subset have shown the ability to bond two substrates together.25,26,29−33,36−42,53,54,60 Whereas a coating requires only adhesive bonding to the surface of interest, bulk glues also need the presence of cohesive forces. These cohesive interactions are required to form the majority of the material and reach between substrates to yield a INTRODUCTION Adhesives play a prominent role in everyday life, being used in many industries including aerospace, automobile manufacturing, housing construction, wood products, packaging, and labeling.1,2 Worldwide revenue generated by adhesives topped $40 billion in 2010.3 New roles for specialty adhesives will be found once we can develop the materials in demand for applications such as surgical adhesives, orthopedic cements, and dental glues. Marine biology can provide inspiration for the design of such materials. The natural adhesive system of marine mussels is receiving growing interest in the context of biomimetics. These shellfish affix themselves to wet rocks by assembling a cross-linked matrix of proteins.4,5 Essential to the cross-linking chemistry of these proteins is the 3,4-dihydroxyphenylalanine (DOPA) residue.4,5 Several proteins have been isolated from mussel adhesive plaques, each with DOPA comprising between 3 and 30% of the total amino acid content.4,5 A mechanism we have proposed for the formation of mussel adhesive involves Fe3+ templating DOPA residues followed by redox chemistry to generate radicals.6−13 Reactivity of these radicals may bring about protein−protein coupling for cohesive bonding within the bulk material and protein− substrate linkages for surface adhesive bonding.12,13 Alternatively, or perhaps complementary, is direct binding of DOPA © 2012 American Chemical Society Received: April 8, 2012 Published: May 14, 2012 9498 dx.doi.org/10.1021/ja303369p | J. Am. Chem. Soc. 2012, 134, 9498−9505 Journal of the American Chemical Society Article dihydroxystyrene)-co-styrene] found lap shear bulk adhesion at up to 1.2 ± 0.5 MPa.53 Over 1 MPa (∼145 pounds per square inch (psi)) can be considered in the realm of high-strength bonding and, once achieved, will enable development of applications in several fields.1,2 Of course, even stronger bonding is often desired. Several factors influence the performance of an adhesive, including the substrate type, surface preparation (e.g., roughness), cure conditions (e.g., temperature, time, humidity), solvent, concentration, and viscosity.2 Beyond such formulation issues, an appealing chemical aspect to explore is that of polymer composition. By varying the ratio of 3,4-dihydroxystyrene:styrene within poly[(3,4-dihydroxystyrene)-co-styrene], we can gain access to a family of adhesive copolymers with varied degrees of cross-linking. This type of systematic study has not been carried out in detail with any other mussel mimetic polymer system. Bonding performance described below was examined on an array of low- to high-energy surfaces: poly(tetrafluoroethylene) (PTFE, common name for the DuPont product Teflon), poly(vinyl chloride) (PVC), polished aluminum, sanded steel, and wood. Polymer composition turns out to be a major factor dictating bonding performance. This study presents the synthesis, characterization, and bulk adhesion of several polymers. We are excited to report that the strongest bonding of these polymers displays adhesion on par with that of commercial products such as “Krazy Glue”, albeit with very different adhesion chemistry. functional glue. Too much cohesion, however, will result in a hardened material without significant affinity for a surface. Likewise, too much adhesive bonding will come at the expense of cohesion, and the bulk material will not exist. This balance of cohesion and adhesion can be elusive, with no way to predict where an optimal interplay may reside. Despite the growing number of synthetic systems mimicking aspects of mussel adhesive proteins, there have been few detailed and systematic studies to illustrate which aspects of the polymers give rise to the greatest bulk adhesion. In particular, performance enhancements will arise from understanding how the polymer composition dictates function. In other words: How much pendant catechol should a polymer contain in order to achieve the strongest bulk bonding? To answer this question, we embarked upon a structure−property study in which the relative contributions of cohesion and adhesion could be changed systematically by altering the polymer composition. The resulting insights will show where one might find the highest-performing biomimetic material. In an effort to gain straightforward chemical insights and also to keep future scale-up in mind, our mimics of mussel adhesive proteins are kept as simple as possible. The DOPA amino acid can be stripped down to only a catechol group pendant from a polymerizable olefin, hence the choice of 3,4-dihydroxystyrene (Figure 1). To minimize structural and thermal perturbations to the host polymer resulting from this monomer, polystyrene was chosen to represent a protein backbone (Figure 1). Styrene is commercially available and easy to polymerize on large scales. A further advantage for these studies is that polystyrene alone does not exhibit any appreciable bonding capability.53 The target copolymer is thus poly[(3,4-dihydroxystyrene)-co-styrene], shown in Figure 1. ■ EXPERIMENTAL SECTION Styrene and 3,4-dimethoxystyrene monomers were purchased and purified with alumina columns for removal of polymerization inhibitors. Details are provided in our earlier report.53 Solvents were commercial anhydrous grade. A Varian Inova-300 MHz spectrometer was used to collect NMR spectra. Gel permeation chromatography (GPC) data were obtained using a Polymer Laboratories PL-GPC20 system and THF eluent. Polystyrene GPC standards (Varian, Inc.) were used for instrument calibration. Differential scanning calorimetry (DSC) data were obtained with a TA Instruments DSCQ2000 calorimeter. Synthesis of Poly[(3,4-dimethoxystyrene)-co-styrene] Copolymers. In a typical polymerization, 2.86 mL (24.9 mmol) of styrene and 3.70 mL (25.0 mmol) of 3,4-dimethoxystyrene were added to a round-bottom flask with 30 mL of anhydrous toluene. The reaction was cooled to −78 °C, and, after 10 min, 0.17 mL of nbutyllithium was added dropwise. The solution turned orange, was stirred under an argon atmosphere for 8 h at −78 °C, and then was allowed to warm to room temperature over 12 h of reaction. Polymerization was quenched by addition of ∼1 mL of methanol. Further addition of ∼100 mL of cold (−20 °C) methanol precipitated the polymer. After isolation by filtering and drying under vacuum, at least three rounds of dissolution in chloroform (∼15 mL) and precipitation with methanol (∼100 mL) were used to remove unreacted monomers. Yield of poly[(3,4-dimethoxystyrene)33-costyrene67] was 4.4 g, 33 mmol, 66%. 1H NMR (CDCl3): δ 0.6−2.3 ppm (broad, polymer backbone), 3.4−3.8 ppm (broad, methoxy peaks), 6.0−7.4 ppm (broad, aromatic). Synthesis of Poly[(3,4-dihydroxystyrene)-co-styrene]. Treatment with BBr3 and an acidic workup yielded the catechol-containing polymers according to our previous methods.53 A typical deprotection was accomplished by dissolving poly[(3,4-dimethoxystyrene)33%-costyrene67%] (4.4 g, 33 mmol) in 50.0 mL of anhydrous dichloromethane (DCM) under an argon atmosphere. The reaction was cooled to 0 °C, and, after 10 min, BBr3 (1.2 mL, 13 mmol) was added dropwise over 10 min. The solution was warmed to room temperature and stirred overnight (∼12 h). The polymer was treated with 1% HCl followed by an aqueous workup to obtain poly[(3,4-dihydroxystyrene)33%-co-styrene67%] (3.6 g, 27 mmol, 82%). Loss of the 1H NMR Figure 1. Mussel adhesive is comprised of DOPA-containing proteins. These proteins are mimicked with synthetic polymers by placing pendant catechol groups along a polymer chain. One of the simplest possible mimics is poly[(3,4-dihydroxystyrene)-co-styrene], in which polystyrene represents the protein backbone and DOPA is represented by 3,4-dihydroxystyrene. Copolymers were prepared by a two-step synthetic route developed in our laboratory previously.53 We have also made cationic versions of these cross-linking polymers.54 Polymerization of styrene and 3,4-dimethoxystyrene yielded polymers for which the ratio of monomers in the final polymers was generally a reflection of the starting feed.53 The styrene and 3,4dihydroxystrene monomers distribute throughout the copolymer statistically or randomly, thereby providing a suitable model for how DOPA residues are located within mussel adhesive proteins.53 The relatively simple synthesis allows access to large quantities of polymer, up to ∼20 g per reaction in an academic laboratory. Our initial effort with poly[(3,49499 dx.doi.org/10.1021/ja303369p | J. Am. Chem. Soc. 2012, 134, 9498−9505 Journal of the American Chemical Society Article Table 1. Characterization Data for Poly[(3,4-dimethoxystyrene)-co-styrene] Copolymers feed (%) polymer observed (%) 3,4-dimethoxystyrene styrene 3,4-dimethoxystyrene styrene Mn Mw PDI Tg (°C) 0 5 9 15 22 50 50 51 53 100 95 91 85 78 50 50 49 47 0 5 10 15 19 26 33 42 36 100 95 90 85 81 74 67 58 64 32 300 37 500 39 800 40 700 40 900 49 600 57 500 50 575 32 700 38 400 48 800 50 000 48 700 54 500 65 800 84 200 61 700 43 800 1.2 1.3 1.2 1.2 1.3 1.3 1.5 1.2 1.3 106 103 100 93 97 67 62 60 68 methoxy peaks indicated complete deprotection. 1H NMR (CDCl3): δ 0.6−2.3 ppm (broad, polymer backbone) and 6.0−7.4 ppm (broad, aromatic). Adhesion Studies. Substrates for lap shear testing were prepared by cutting each material into rectangular pieces, 8.89 cm long × 1.25 cm wide. A centered hole of 0.64 cm diameter was drilled into each adherend 2.22 cm from one end. Aluminum was 0.318 cm thick, type 6061 T6, and mirror polished with Mibro no. 3 and Mibro no. 5 polish followed by washing with hexanes, ethanol, acetone, and then deionized water, 30 min each, and air-dried overnight. The steel adherends, 0.318 cm thick, were sanded with 50 grit sandpaper prior to testing and then washed with ethanol, acetone, and hexanes. PVC (0.318 cm thick) and PTFE (0.953 cm thick) were obtained from Ridout Plastics (San Diego, CA). Red oak was purchased at a local hardware store and, after cutting to 1.27 cm thick, had a surface roughness approximately equivalent to that of 220 grit sandpaper. The wood adherends were cut and adhesion strength was measured parallel to the wood grain, running along the 8.89 cm edge of the adherend. Water loss from these wood substrates may have occurred during the adhesive cure. Massing of several oak adherends before versus after a typical cure treatment of 1 h at room temperature, 22 h at 55 °C, and 1 h at room temperature revealed an average 4.12% decrease (e.g., from 10.1 to 9.68 g). Lap shear adhesion measurements were conducted on an Instron 5544 materials testing system equipped with a 2000 N load cell. Copolymer solutions in 1:1 acetone/DCM (0.3 g/mL, 22.5 μL) were added to each adherend. Next, 15 μL of cross-linking solution (or solvent when not adding the cross-linker) was added to deliver 0.33 equiv of cross-linker per catechol group. The adherends were overlapped at 1.25 × 1.25 cm in a lap shear configuration (Figure 2). Each assembly was allowed to cure for 1 h at room temperature, 22 h at 55 °C, and then 1 h cooling at room temperature. Figure 2 shows a representative extension versus force plot used for quantifying adhesion. The early region of the trace is flat while the crosshead moves up to begin loading the sample. Once the bond begins to be stressed, a rise is seen until the sudden drop, indicating bond breakage. Adherends were pulled apart at a rate of 2 mm/min. The maximum bonding force in Newtons was recorded. Final adhesive force in megapascals was obtained by dividing the maximum load at failure, in Newtons, by the measured area of adhesive overlap in square meters. For the polymer composition studies in Figure 3, each sample was tested a minimum of 20 times, averaged, and reported with error bars showing ±1 standard deviation. The comparisons to commercial adhesives in Tables 2 and 3 were each tested a minimum of 10 times, averaged, and reported with error bars showing ±1 standard deviation. Tensile adhesion tests were carried out in an analogous manner using aluminum rods of 1 cm diameter. dimethoxystyrene content of each final polymer was similar to that placed in the feed. Table 1 provides mole percent data for each monomer in the feed versus that found in the isolated polymers. For targeting low catechol polymers (e.g., 5.7a 0.5 ± 0.1 >5.7a 3.8 ± 0.7 0.7 0.4 0.36 1.5 0.7 0.2 0.1 0.04 0.3 0.1 polished aluminum 4 7 4 7 11 ± ± ± ± ± 1 1 1 1 2 sanded steel red oak ± ± ± ± ± 5.1 ± 0.9 10 ± 1 5±2 >10b 4±2 6 5 5.5 10 9 2 1 0.9 2 1 Substrate failed while adhesive bond remained intact. bExceeded range of the instrument. conditions, concentration, added filler, viscosity, and addition of adhesion promoters have all been examined. By contrast, poly[(3,4-dihydroxystyrene)33%-co-styrene67%] is a relative newborn and, within the scope of this academic study, already performs comparably to commercial products. Ideally, an adhesive should be tailored for a target substrate. The poly[(3,4-dihydroxystyrene)-co-styrene] with the strongest bulk adhesion on aluminum is not necessarily the best polymer for other substrates. Beyond changing polymers for each surface, a detailed series of formulation efforts may enhance performance even further. Comparisons to Other Biomimetic Adhesive Polymers. We wished to place the performance of poly[(3,4dihydroxystyrene)33%-co-styrene67%] system within the expanding scope of other polymeric mussel protein mimics. Many of these new systems are being used most often to generate coatings27,34,35,40,45−47,49−52,63 or hydrogels,36−38,42,43 among several other end goals, and some have shown adhesion.25,26,29−33,36−42,53,54,60 Direct comparisons of adhesive performance are difficult to make given how many variables are present including test methods, substrate composition, surface preparations, solvents, viscosity, cure time, cure temperature, and the presence or absence of water, among several other conditions. Some of the stronger mussel mimics reported are a polyurethane at 5.2 MPa,60 polypeptides bonding up to 4.7 MPa,25 and a poly(ethylene glycol)/ polyacrylate at 1.2 MPa.29 Fusion proteins have been expressed and modified to contain DOPA.76,77 These representations of mussel proteins can adhere up to 4 MPa.67,78 The data in Table 3 indicate that poly[(3,4-dihydroxystyrene)33%-co-styrene67%] is the strongest bonding synthetic mimic of mussel adhesive tested to date. Maximum adhesion was at 10 ± 1 MPa for the cross-linked polymer joining wood. Polished aluminum, sanded steel, and PVC were adhered at greater than 5.7 MPa, also stronger than that reported for other biomimetic adhesives. Adhesion Strength of Synthetic Mimics Compared to Plaques from Live Mussels. Recently we developed a method for quantifying adhesive performance of the glue produced by live mussels.79 On aluminum these shellfish adhere at 0.3 ± 0.1 MPa.79 The byssal adhesive system of mussels is comprised of plaques contacting the surface and threads connecting each plaque to the animal’s soft inner body (Figure 1). Tensile measurements were required to obtain accurate adhesion data for the byssus. We were curious to see how the performance of our biomimetic polymers compared to the “real” material produced by mussels. The polymer lap shear data from above (cf. Table 3) cannot be compared directly to tensile measurements. In a lap shear test, the substrates are overlapped and force is applied parallel to the adhesive bond (Figure 2). Tensile testing is an end-toend butt joint, and the applied force is perpendicular to the glue. Consequently, we gathered tensile adhesive data for poly[(3,4-dihydroxystyrene)33%-co-styrene67%] on aluminum rods. Pairs of tensile substrates were bonded together using 13.5 mg of dissolved poly[(3,4-dihydroxystyrene)33%-co-styrene67%] and (IO4)− over the 1 cm diameter overlap area. Testing revealed that the polymeric adhesive was so strong that not all of the joined substrates could be broken within the 2000 N capacity of our materials testing system. Some bonded substrates pairs did separate and provided a lower limit of ≥9 MPa for poly[(3,4-dihydroxystyrene)33%-co-styrene67%] on aluminum in tensile mode. The biomimetic system appears to bond with significantly greater force than the natural material after which the polymer was designed. Although we may be trying to make the strongest possible glue, mussels need only adhere as strongly as their environmental conditions dictate. Indeed, if these shellfish were affixed to rocks any more strongly, detachment forces exerted by waves or predators might pull on the byssus to the point of damaging the soft, internal tissues to which the threads connect (Figure 1). ■ CONCLUSIONS With their ability to remain affixed to rocks in the turbulent intertidal zone it is no wonder that mussels have inspired so much research. Here we have presented a structure−property study on the simplest mimic of mussel adhesive proteins. Several copolymers were synthesized, characterized, and examined for adhesive properties. A systematic approach was taken in order to determine the polymer composition giving rise to the greatest bulk adhesion. With the cross-linking and surface bonding chemistries present in these copolymers, the strongest adhesive is likely to provide a balance between cohesive and adhesive bonding. Adhesion was quantified on substrates ranging from low-energy, smooth plastics to highenergy, roughened metal. Performance was benchmarked against common commercial glues as well as the native material produced by live mussels. Adhesive performance of the biomimetic polymer was comparable, and in some cases better, than commercial products and the plaques of living shellfish. Relative to other mussel mimetic polymers, poly[(3,4dihydroxystyrene)-co-styrene] appears to be the strongest bulk adhesive. These comparisons, although interesting, are difficult to make directly, given the broad variations in conditions. Overall, these results help attest to the value of using blueprints from biology when designing new materials. Such a biomimetic approach may aid development of the adhesives needed for industrial or biomedical applications including wood glues without toxic formaldehyde, surgical reattachment of soft tissues, and cements for connecting metal implants to bone. ■ AUTHOR INFORMATION Corresponding Author wilker@purdue.edu 9503 dx.doi.org/10.1021/ja303369p | J. Am. Chem. 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