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Enhanced Adhesion of Dopamine Methacrylamide Elastomers via Viscoelasticity Tuning Hoyong Chung,† Paul Glass,‡ Jewel M. Pothen,† Metin Sitti,‡,§ and Newell R. Washburn*,†,‡ Departments of Chemistry, Biomedical Engineering, and Mechanical Engineering, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213, United States Received September 10, 2010; Revised Manuscript Received December 3, 2010
We present a study on the effects of cross-linking on the adhesive properties of bio-inspired 3,4-dihydroxyphenylalanine (DOPA). DOPA has a unique catechol moiety found in adhesive proteins in marine organisms, such as mussels and polychaete, which results in strong adhesion in aquatic conditions. Incorporation of this functional group in synthetic polymers provides the basis for pressure-sensitive adhesives for use in a broad range of environments. A series of cross-linked DOPA-containing polymers were prepared by adding divinyl cross-linking agent ethylene glycol dimethacrylate (EGDMA) to monomer mixtures of dopamine methacrylamide (DMA) and 2-methoxyethyl acrylate (MEA). Samples were prepared using a solvent-free microwave-assisted polymerization reaction and compared to a similar series of cross-linked MEA materials. Cross-linking with EGDMA tunes the viscoelastic properties of the adhesive material and has the advantage of not reacting with the catechol group that is responsible for the excellent adhesive performance of this material. Adhesion strength was measured by uniaxial indentation tests, which indicated that 0.001 mol % of EGDMA-cross-linked copolymer showed the highest work of adhesion in dry conditions, but non-cross-linked DMA was the highest in wet conditions. The results suggest that there is an optimal cross-linking degree that displays the highest adhesion by balancing viscous and elastic behaviors of the polymer but this appears to depend on the conditions. This concentration of cross-linker is well below the theoretical percolation threshold, and we propose that subtle changes in polymer viscoelastic properties can result in significant improvements in adhesion of DOPA-based materials. The properties of lightly crosslinked poly(DMA-co-MEA) were investigated by measurement of the frequency dependence of the storage modulus (G′) and loss modulus (G′′). The frequency-dependence of G′ and magnitude of G′′ showed gradual decreases with the fraction of EGDMA. Loosely cross-linked DMA copolymers, containing 0% and 0.001 mol % of EGDMAcross-linked copolymers, displayed rheological behavior appropriate for pressure-sensitive adhesives characterized by a higher G′ at high frequencies and lower G′ at low frequencies. Our results indicate that dimethacrylate cross-linking of DMA copolymers can be used to enhance the adhesive properties of this unique material.
Introduction The modified amino acid 3,4-dihydroxyphenylalanine (DOPA) is found in many marine organisms’ adhesive proteins, such as those in mussels1-4 and polychaete.5,6 Proteins incorporating the DOPA functionality contribute to strong adhesive structures, allowing marine organisms to fix their bodies to various types of surfaces, such as polymers, ceramics, and metals, even in aqueous conditions. Because of these properties, DOPAcontaining synthetic polymers have been synthesized and used in adhesive applications, such as microstructured fibrillar adhesives.7,8 These linear copolymers are based on dopamine methacrylamide (DMA) and methoxyethyl acrylate (MEA), and compositions containing 11.3 mol % DMA appear to show the highest adhesion,7,8 so they were the focus in this research. The strength of adhesion can be determined by measuring bond failure energy.9,10 The strength of bond failure energy depends on two terms: viscoelastic deformation and interfacial bonding.11,12 The interfacial bonding mechanisms of DOPAcontaining polymers have been extensively studied. Messersmith et al. demonstrated that a covalent bond was formed through * To whom correspondence should be addressed. E-mail: washburn@ andrew.cmu.edu. † Department of Chemistry. ‡ Department of Biomedical Engineering. § Department of Mechanical Engineering.
Michael addition between catechol and organic surfaces. They also suggested that a reversible coordination bond formed between catechol groups and metal surfaces.13 Other explanations of the DOPA adhesion mechanism include the high cohesive strength from a network within DOPA including protein chains.14-16 Yamamoto et al. proposed two modes of behavior of DOPA-containing proteins toward glass and polyethylene surfaces. The glass surface, a typical polar interface, has an attraction with the hydrophilic side chains, such as hydroxyl and amino group in DOPA-containing proteins. In contrast, HDPE, a typical nonpolar surface, attracts hydrophobic parts of side chains, thus, providing versatility in forming adhesive bonds.17 Previously, mechanistic characterization of interfacial binding between DOPA-modified hydrogel and Ti surface have been reported by using critical energy release rate18,19 measurement and JKR method.20,21 The viscoelastic properties of DOPA-containing adhesives are not entirely understood. Previous work demonstrated that polymer cross-linking DOPA-containing adhesives alters adhesion.22-26 But, in these reports, cross-links were formed from the catechol groups using metal fillers, for which iron worked efficiently in most cases. To the best of our knowledge, there are no reports investigating viscoelastic properties of DOPA-based polymers to find a relation between adhesion property and rheology of polymer. It has been shown that
10.1021/bm101076e 2011 American Chemical Society Published on Web 12/23/2010
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Scheme 1. Preparation of Covalently Cross-Linked Poly(DMA-co-MEA)
engineering the polymer architecture to allow for complementary groups, such as lysine residues in the case of tissues, to facilitate binding the substrate.20,21 For a pressure-sensitive adhesive, strong adhesion depends on the bond-forming and bondseparation process. Efficient bond formation requires fast transport of polymer chains toward adherent surfaces to form a robust connection. A key factor of the bonding process is the viscous character of the polymer, which contributes to fast surface wetting. However, in the bond-separation process, the adhesion is strengthened by small degrees of resistance to internal motion.27,28 More generally, the resistance to internal energy dissipation can be represented as an elastic behavior of the adhesive polymer. Accordingly, to develop a strong adhesive, the material must show an optimal balance between viscous flow and elastic behavior. Various methods have been developed to adjust the viscoelastic properties of adhesives such as usage of small molecule additives,29-32 copolymerization,33,34 chain transfer agents,35 controlling degrees of cross-linking,27,36,37 and using different amount of solvents to control swelling of crosslinked adhesives.12,38 In this article, we report on the effects of cross-linking on the adhesion and viscoelastic properties of DMA-containing elastomers. A series of covalently cross-linked poly(DMA-coMEA) was prepared with divinyl cross-linking agent, ethyleneglycol dimethacrylate (EGDMA). The effects of cross-linking on adhesion were studied by indentation adhesion test in dry and fully submerged aqueous conditions.
Experimental Section Materials. MEA and EGDMA were purchased from Aldrich Chemical Co. (Milwaukee, WI) and used as received. The photoinitiator Irgacure 819 was purchased from Ciba Specialty Chemicals and used as received. DMA was synthesized and characterized as reported by a method described elsewhere.8 Preparation of Polymer Thin Films. To prepare the copolymer, DMA (550 mg), MEA (2530 mg), and Irgacure 819 (9.17 mg) and a designated amount of EGDMA (ranging from 0 mg to 43.43 mg) were mixed. The reaction mixture was heated in a commercial microwave (Sharp Corp.) for 2 min to make the solution homogeneous and transparent. Then 100 µL of the homogeneous solution was immediately transferred to 1 × 1 cm custom-made mold for photopolymerization. The transparent polymerization mold was tight assembly of commercially available polydimethylsiloxane (Sylgard 184 silicone elastomer, Dow Corning, Midland, MI) having rectangular-shape gap that was sealed on a slide glass. Uncured liquid solution mixture was poured into the gap forming a thin film. This assembly was placed between two strong UV light sources (365 nm, Model UVGL-25, UVP LLC, Upland, CA and 312 nm, 8000 µw/cm2, FisherBioTech UV transilluminator, Fisher Scientific, Pittsburgh, PA) to initiate radical polymerization. The photopolymerization was performed for 15 min to obtain poly(DMA-co-MEA) with degrees of cross-linking ranging from 0 to 1% by mole fraction. The chemical structures of synthesized DMA monomer and non-cross-linked poly(DMA-co-MEA) was determined
by 1H NMR (Bruker Avance 300). Deuterated dimethyl sulfoxide (DMSO-d) was used as a solvent for DMA and deuterated chloroform (CDCl3) was used to dissolve poly(DMA-co-MEA). 1H NMR spectra of DMA and poly(DMA-co-MEA) are shown in Figure S1. Molecular weight and polydispersity index of polymer samples were analyzed by Waters GPC (Polymer Standards Services (PSS) columns (guard, 105, 103, and 102 Å)) using DMF as an eluent (flow rate 1.00 mL/min, 50 °C) with differential refractive index (RI) detector. The apparent molecular weights (Mn) and polydispersities (Mw/Mn) were determined with a calibration based on linear poly(methyl methacrylate) standards using WinGPC 7.0 software from PSS. GPC trace is illustrated in Figure S2. Optical Microscopy. The cross-section of adhesive layer was examined and imaged by using a Leica DMIL LED (Leica Microsystems Inc., Bannockburn, IL). The adhesive layer was cut with a razor to expose the cross-section and then flipped to face the cross-section side top. The images were obtained by objectives of 4× and 10× magnifications. The average thickness of the polymer films was 500 ( 52 µm. Indentation Adhesion Test. In uniaxial indentation tests, the samples were fixed to the bottom of a 2 mm deep tank on a glass slide mounted to an inverted optical microscope (TE200, Nikon). A 6 mm diameter glass hemisphere was attached to the stem of a load cell (GSO-50, Transducer Techniques), which was controlled by a linear stage (MFACC, Newport Corporation). Custom software was written to control the motion of the glass hemisphere while collecting data from the load cell. For testing under aqueous conditions, the shallow tank was filled with deionized water, entirely submerging the sample. For each sample in both wet and dry conditions, the samples were indented with the glass hemisphere at 0.1 mm/s speed until a predefined preload force was reached. The glass hemisphere was then retracted at the same speed, while the adhesion force at the material interface was measured. The maximum contact area39 was calculated from the indentation depth of the hemisphere, which was computed from the distance between the position of the hemisphere when contact with the test surface was initiated and the position of the hemisphere at the moment when the maximum preload was reached. Contact areas were obtained for each individual adhesion test, and varied from 1-8 × 10-6 m2, depending on the preload force and sample properties. Two-tailed t tests were used to demonstrate statistical significance between each group of adhesion tests. Dynamic Mechanical Analysis. The viscoelastic property measurement was performed with a Gemini 200 Rheometer (Bohlin Instruments, Malvern, U.K.). Flat 2-mm-thick polymer samples were photopolymerized and cut to fit the size of 20-mm-diameter parallel plate. Frequency sweeps from 100 to 0.01 Hz were conducted to measure storage modulus (G′) and loss modulus (G′′) at 23 °C.
Results and Discussion Synthesis of Cross-Linked Poly(DMA-co-MEA). Crosslinked poly(DMA-co-MEA) was prepared by photopolymerization of DMA, MEA, and EGDMA. DMA was synthesized by previously reported methods.7,8 The synthesis scheme of cross-linked poly(DMA-co-MEA) is illustrated in Scheme 1.
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Figure 1. Optical microscope image of cross-linked poly(DMA-coMEA) with 0% EGDMA. The thickness of the polymer layers was 500 ( 50 µm, as measured with optical microscope (×4 magnification; scale bar represents 100 µm).
The molar ratio of DMA and MEA was constant for all compositions at 1:7.2, which was previously reported to have good adhesive properties.8 The photopolymerized poly(DMAco-MEA) with 0% EGDMA was characterized using 1H NMR, which confirmed that the composition of the copolymer reflected the monomer feed, while analysis of GPC data indicated that Mn was 51000 g/mol and the PDI was 2.4. Five different portions of EGDMA were tested: 0, 0.001, 0.01, 0.1, and 1%. All other monomer feeds and conditions of polymerization were held constant. The mixture of reagents was photopolymerized in custom-made molds with designated sizes for each different characterization. Photopolymerization was an efficient method to prepare bulk adhesives for these experiments because the reaction was completed in 15 min with less chance of film debonding from the coating substrate surface through the use of thermal initiators. Furthermore, in contrast to cross-linking through metal-based coordination bonds,22-26 the divinyl crosslinker does not interfere with the catechol group that is mainly responsible for the unusual adhesive performance of this material making this an attractive strategy for exploring the effects of cross-linking. Lightly cross-linked poly(DMA-co-MEA) was prepared with consistent thickness to avoid substrate-induced artifacts on indentation test results.40 The adhesive layer thickness across different samples was measured to be 500 ( 52 µm with a representative sample shown in Figure 1. Determination of Adhesion as a Function of Degree of Cross-Linking. The adhesion of lightly cross-linked poly(DMAco-MEA) under wet and dry conditions was evaluated using a custom-made uniaxial indentation instrument. The measurement probe was a 6-mm-diameter glass hemisphere, and adhesion tests were performed with three different preload forces ranging from 10 to 50 mN. Force-displacement curves with two different cross-linking degrees are shown in Figure 2. The indentation probe was pressed into the sample until the designated preload force was reached. Adhesion force curves were obtained as a function of displacement during the pull-off process until the probe completely debonded from the adhesive material surface. In uniaxial indentation tests, the area between the approach and retract curves represents the adhesion hysteresis of the interface. For each test, an effective work of adhesion was obtained by normalizing this value by the maximum contact area.39 The work of adhesion as a function of preload force for the different compositions is reported in Figure 2. As shown in Figure 3, work of adhesion displayed monotonic increases in as a function of preload increase, although the dependence was weaker for the more highly cross-linked samples. The work of adhesion for poly(DMA-co-MEA) samples with 0% cross-
Figure 2. Force vs displacement plots of cross-linked poly(DMA-coMEA) with 0.001% EGDMA (blue solid line) and 0% EGDMA (red dashed line) in wet condition of 50 mN preload. Arrows show the preload and pull-off processes during the indentation test. For each material, the effective work of adhesion was calculated by integrating the area between the approach and retract curves and normalizing this value by the maximum contact area.
Figure 3. Work of adhesion in wet (top) and dry (bottom) conditions as a function of preload force for covalently cross-linked poly(DMAco-MEA) with different EGDMA fractions. Probe speed was 0.1 mm/s for all measurements.
linking under dry conditions was very high, with an average value of nearly 100 J/m2. The relative magnitude of the error bars are similar to what were observed by Shull and co-workers
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in their investigation of photocurable and biodegradable block copolymer,21 suggesting that there may be some intrinsic variability in the adhesive responses of DMA-based materials. Interestingly, this degree of variability was not observed under for any of the cross-linked samples, suggesting that it may be due to the lower adhesive interactions or the lower molecular weight components of the sample, although further testing would be required to verify either hypothesis. Furthermore, in dry conditions highly cross-linked materials exhibited very low adhesion forces, consistent with the intuition that the polymer chains were not able to reorganize effectively under the applied load. Samples with 0.001% of EGDMA had an average work of adhesion of 47 J/m2, lower than the average value of the non-cross-linked samples but lacking the variability observed there, although given the magnitude of the error in the measurement of the non-cross-linked sample, it cannot be determined which, if either, had strong adhesive interactions. For the wet adhesion tests, the probe and sample were fully immersed in water. The overall adhesion force in wet conditions was much lower than the dry adhesion test, which was attributed to weaker surface bonding interactions and weakening of cohesive interactions on organic adhesives.41-43 Interestingly, the variability in the measured work of adhesion was lower for samples in wet conditions than under dry conditions, and the samples with 0.001% EGDMA had significantly higher (p < 0.01) adhesion values than any other composition, with a maximum average value of 6.7 N/m2. In contrast, the poly(DMA-co-MEA) sample with 0% EGDMA had an average value of 3.8 N/m2. Both values were significantly larger than observed in DMA-containing hydrogels, which had a maximum value for work of adhesion of 0.4 J/m2 in aqueous environments, but the volume fraction of DMA in these samples was diluted through swelling in water. This improvement in adhesive interactions from cross-linking could be attributed to subtle molecular interactions occurring near the contacting surface area of poly(DMA-co-MEA) in water, such as changes in internal stress change of material44 and interfacial energy variation.43,45 In addition, we noted that, under dry conditions, fibril formation was common for formulations containing 0 and 0.001% EGDMA, but this was observed less frequently under wet conditions. Much of the high work of adhesion of poly(DMAco-MEA) may be traced to this phenomenon, which may be inhibited by an aqueous interface that could make fibril formation energetically unfavorable. For comparison, samples of poly(MEA) were prepared and tested under dry and wet conditions as a function of EGDMA cross-linking, and the results are shown in Figure 4. In both dry and wet conditions, the work of adhesion of poly(MEA) is significantly lower than that of the poly(DMA-co-MEA) materials. Under dry conditions, samples with 1% EGDMA had the highest work of adhesion, but under wet conditions the greatest work of adhesion was observed for samples containing 0.001% EGDMA. While the wet adhesion was nonzero, it was generally 50% that of the lightly cross-linked DMA copolymers, indicating that indeed DMA enhances adhesion in aqueous environments. Work of adhesion under dry and wet conditions at 50 mN preload are summarized in Figure 5 as a function of EGDMA concentration. As discussed, poly(DMA-co-MEA) with 0.001% EGDMA showed the highest work of adhesion in wet conditions, significantly higher (p < 0.01) than any other compositions. Poly(MEA) exhibited higher wet adhesion property for 0.01% and above cross-linking according to Figure 5; however, in low cross-linking degree, 0 and 0.001%, work of adhesion of poly(DMA-co-MEA) overwhelmed poly(MEA) substantially
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Figure 4. Work of adhesion in wet (top) and dry (bottom) conditions as a function of preload force for covalently cross-linked poly(MEA) with different EGDMA fractions. Probe speed was 0.1 mm/s for all measurements.
(p < 0.01). Poly(MEA) has slight cross-linking dependency on work of adhesion but its difference was not meaningful compared to the work of adhesion of poly(DMA-co-MEA). In Figure 5, work of adhesion in dry condition is much higher than wet condition especially in low cross-linking degree. A total of 0 and 0.001% of poly(DMA-co-MEA) revealed much higher, 20 times higher adhesion for 0% EGDMA in average, work of adhesion than poly(MEA). Although there is large variation in work of adhesion for 0% EGDMA poly(DMA-coMEA), work of adhesion of poly(MEA) shows definite (p < 0.01) weak adhesion in the same cross-linking condition. In spite of large enhancement in work of adhesion compared to poly(MEA), 0 and 0.001% EGDMA poly(DMA-co-MEA) revealed poor statistically significant difference due to high variation of work of adhesion. Overall, DMA component enhanced both wet and dry adhesion over MEA homopolymer. Typically, 0.001% EGDMA poly(DMA-co-MEA) showed better wet adhesion than any other tested materials. Investigations of other systems have shown that increasing the elastic component of viscoelastic response through crosslinking,27,36,37 increasing polymer viscosity through the use of a chain transfer agent,35 or using block copolymer architectures46 can significantly affect dry adhesion. Cross-linking DMA copolymers appeared to have had a significant effect on wet adhesion, although the variability in response of the non-crosslinked poly(DMA-co-MEA) does not rule out a weaker adhesive interaction than for lightly cross-linked samples than adhesion under dry conditions. We suggest that, under dry conditions, interfacial interactions dominated the adhesive response, but
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Figure 5. Indentation adhesion test results (work of adhesion) with a preload of 50 mN at 0.1 mm/s speed in dry and wet conditions as a function of various portions of the cross-linking agent, EGDMA.
Figure 6. G′ and G′′ as a function of frequency ω for cross-linked poly(DMA-co-MEA) with various portion of EGDMA.
under wet conditions, subtle variations in the viscoelastic properties of the adhesive were more important. Viscoelastic Properties of Cross-Linked Poly(DMA-coMEA). Dynamic mechanical analysis was performed to investigate viscoelastic properties of the copolymers. Both elastic and viscous moduli, G′ and G′′, are shown in log-log scale as a function of oscillation frequency (ω) in Figure 6. As shown in Figure 6, all poly(DMA-co-MEA) samples exhibited higher G′ than G′′ over the entire frequency range, suggesting that even the 0% of EGDMA poly(DMA-coMEA) had predominantly elastic responses in the frequency range probed. The rheological properties of non-cross-linked poly(DMA-co-MEA) from dynamic mechanical analysis were still dominated by an elastic response at these frequencies. In Figure 6, the G′′ values of the 1% EGDMA were much lower than other degree of cross-linkings and the frequency dependence of G′ was weaker, suggesting that this composition was behaving closer to an elastomer. For the other compositions, the G′′ slopes did not show significant dependence on the degree of cross-linking. In dynamic mechanical analysis, the slope of G′ as a function of frequency showed dependence on the degree of cross-linking as discussed. To do a more precise study, the slope of each sample was determined between frequencies 0.01 and 100 s-1 and reported in Figure 7. When this slope is high, the polymer displays relatively greater solid-like character at high frequency
Figure 7. Slopes of G′ of cross-linked poly(DMA-co-MEA) vs mole % of EGDMA.
and, in contrast, stronger liquid-like character at low frequency. In Figure 6, the G′ slope was found to decrease monotonically with increasing EGDMA concentration. A high modulus at high frequency and low modulus at low frequency is an important characteristic for pressure-sensitive adhesives, and typically, a ratio of G′ at 100 s-1 and G′ at 0.01 s-1 (i.e., G′100/G′0.01, equivalent to the slope of modulus) between 5 and 300 is considered optimal for pressure-sensitive adhesive.47 In this range of slope, the adhesive polymer has a soft response during the slow bonding process onto the adherent surface so that the polymer chain mobility and wettability become high enough to rapidly maximize contact. In contrast, the debonding process is a fast process and so the adhesive polymer should be more resistant to external forces and resist debonding. Based on these guidelines, the poly(DMA-co-MEA) samples containing 0.001 and 0% EGDMA appeared to have suitable viscoelastic properties for pressure-sensitive adhesives.
Conclusions A series of lightly cross-linked poly(DMA-co-MEA) samples were prepared using divinyl group cross-linking agent, EGDMA, and a solvent-free, microwave-assisted copolymerization method. Adhesive properties of various degrees of cross-linked poly(DMA-co-MEA) were measured using uniaxial indentation tests and compared to those of poly(MEA). Work of adhesion in wet conditions demonstrated the strongest adhesion results with
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0.001% EGDMA, significantly stronger than non-cross-linked poly(DMA-co-MEA) or any of the poly(MEA) formulations. Under dry conditions, the work of adhesion was highest for poly(DMA-co-MEA) having 0% EGDMA, although the variability in the measured values was particularly large for this composition and condition. One significant factor that appeared to result in very large adhesive interactions was whether fibrils formed during probe pull-off from the polymer surface. This appeared to be inhibited by cross-linking and by testing in wet environments. The relationships of adhesion with rheological and mechanical properties were investigated through dynamic mechanical analysis. Dynamic mechanical analysis revealed that a large change in G′ with the amount of cross-linking agent because of polymer’s viscoelasticity change. The plot of G′ slope indicated that the synthesized lightly cross-linked poly(DMAco-MEA) with 0 and 0.001% were a suitable material for pressure-sensitive adhesive. Based on these results, it appears that the optimal cross-linking point is at very low fractions of EGDMA, suggesting that a subtle balance of viscous and elastic components was necessary to enhance adhesion of DMA-based materials. Acknowledgment. This work was partially supported by the National Science Foundation CMMI-0800408 program. We gratefully acknowledge Annette Jacobsen and Susana Steppan for assistance with these measurements as well as the Center for Molecular Analysis at Carnegie Mellon University (NSF CHE-9808188) and the NMR facility at Carnegie Mellon University (NMR instrumentation partially supported through NSF CHE-0130903). Supporting Information Available. 1H NMR of DMA monomer and 1H NMR and GPC characterization of poly(DMAco-MEA). This material is available free of charge via the Internet at http://pubs.acs.org.
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