Article pubs.acs.org/cm
Triggered Detection and Deposition: Toward the Repair of Microcracks Vinita Yadav,‡ Ryan A. Pavlick,‡ Stephen M. Meckler,‡ and Ayusman Sen* Department of Chemistry, Pennsylvania State University, University Park, Pennsylvania 16802, United States S Supporting Information *
ABSTRACT: Developing methods to detect and repair damage in polymers is an active area of research. Many of the previously described methods suffer from the lack of long-term stability of the reagents, which are typically preincorporated into the polymer. Also, they tend to be specific to certain types of polymeric materials. In this paper, we describe a general method for detection and repair of cracks in polymers using a salt-triggered mechanism. The process consists of a polymer embedded with salt that works as a “reporter”. Upon addition of a detection or healing agent, damaged areas in the polymer leach out salt, powering flows and activating the reagents in the fluid. Detection is possible with fluorescent quantum dots, which aggregate at the crack site. Repair is shown to occur through two different strategies. The first repair strategy involves high ionic strength triggered destabilization of oil-in-water emulsions, transporting polymerization agents, resulting in polymer deposition at the damage site. The second, more biocompatible strategy, involves using an enzyme, urease, and its catalytic hydrolysis of urea to deposit solid calcium carbonate in the crack. The solution of the detection or healing agent can be added “as needed” thereby overcoming the problem of reagent instability. and repair of bone microcracks.14 Here we describe a robust reusable procedure to both detect cracks and repair them in polymeric materials. This methodology uses either fluorescent quantum dots, emulsions with repair agents, or enzymes, in combination with a polymer-embedded “reporter” to detect and heal cracks in the material. Salt driven density flows can actively transport active agents to the target site. These flows occur when the salt concentration increases sufficiently to cause a local density change. Such density driven flows are very common in nature at mineral salt deposits and estuaries. Local fluid density varies with salt concentration, causing flows dictated by the direction of gravity.15−19 To make use of this mechanism, a “reporter” salt was embedded into a polymer matrix. Calcium chloride was chosen because it is highly soluble in water (solubility, 74.5 g/ 100 mL at 20 °C20) and nontoxic. Calcium chloride crystals were embedded into a polydimethylsiloxane (PDMS) matrix, forming a layered material (3−6 mm in thickness) (Figure 1). When the surface is cracked the calcium chloride begins to dissolve into the supernatant aqueous solution, creating a change in the local density powering density driven flows.
1. INTRODUCTION The detection and repair of damaged polymeric materials is an active area of research.1−13 Early systems utilized a catalyst (Grubbs’ catalyst) and monomer (dicyclopentadiene, DCPD) embedded in the polymer to repair damage.3−6 More recent systems have utilized supramolecular interactions, photochemistry, and thermal heating to seal cracks.7−9 However, these systems have long healing times. Moreover, many of the previously reported methods suffer from the lack of long-term stability of the reagents, which are typically preincorporated into the polymer. Also, they tend to be specific to certain types of polymeric materials. Pang and Bond demonstrated that glass structures filled with a repair agent and UV dye embedded in an epoxy resin can simultaneously leach dye to visualize the crack and epoxy to repair it.10,11 This method suffers from two drawbacks: (i) it has a onetime use in a given location and (ii) the UV dye degrades over time if exposed to light. Kolmakov et al.12,13 developed a “repair and go” system based on surface interactions. This system uses oil−water microcapsules to deliver encapsulated nanoparticles to damaged areas on a polymer surface, allowing the fluorescent detection of those sites. Our approach is conceptually different from the above and focuses on density driven flows that can be engineered to bring together detection or healing agents to the crack site, allowing for both detection and repair. Moreover our scheme requires the addition of active reagents only when needed, eliminating the need for them to be pre-embedded within the polymer matrix and which can degrade over time. Previously we demonstrated use of diffusiophoretic flows to enable detection © 2014 American Chemical Society
2. EXPERIMENTAL SECTION PDMS/CaCl2 Sample Synthesis. PDMS elastomer base was mixed with the cross-linker, Sylgard 184 Silicone Elastomer kit (Dow Corning), in a 10:1 ratio. Then 3 g of this mixture was added to a 60 × Received: June 20, 2014 Published: July 19, 2014 4647
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was removed, and the sample was rinsed with DI water and left to dry in a nitrogen atmosphere. The crack was then imaged using an FEI Quanta 200 Environmental Scanning Electron Microscope. XRD. XRD patterns were collected using PANalytical Empyrean theta−theta goniometer with Cu−K-alpha radiation, and programmable divergence slit (2 mm, 1.0 degree antiscatter, specimen length 10 mm) and diffracted (2 mm, 0.02 mm nickel filter) optics in reflection geometry. Data was collected at 45 kV and 40 mA from 5 to 70 degrees 2-theta using PIXcel 3D detector in scanning mode with a PSD length of 3.35 degrees 2-theta and 255 active channels for duration of ∼0.5 h. Resulting patterns were corrected for both 2-theta and position by comparison to ICDD (calcite PDF #00-005-0586 and aragonite PDF #00-041-1475) and analyzed with Jade+9 software by MDI of Livermore, CA.
3. RESULTS AND DISCUSSION In order to demonstrate the damage detection scheme, first the salt-polymer layered material was prepared as described in the Experimental Section. For the controls the salt incorporation step was excluded. CaCl2-impregnated PDMS was cut with a scalpel and the width of the cuts ranged from 30 to 70 μm, the length was approximately 1 cm, and the depth was about 1 mm. Upon exposure of the crack to an aqueous solution, calcium chloride begins to dissolve into the supernatant, creating a change in the local density leading to directional fluid flows (Figure S6). For detection, the scratched surface was exposed to an aqueous solution of fluorescent 20 nm CdSe/ZnS core−shell quantum dots (Qdots 655 ITK carboxyl quantum dots). As the salt leached out, the ionic strength in the immediate vicinity increases, causing the particles’ zeta potentials to drop (see the SI for details). This drop decreases the electrostatic repulsion between the particles, causing the Qdots to aggregate at the damaged site. As particles aggregate in the crack, the overall concentration of quantum dots in solution decreases. This leads to the decrease of fluorescence intensity away from the crack compared to inside the crack, allowing for detection, and this was monitored using confocal microscopy. The ratio of the fluorescence intensity at the crack to that in the surrounding area increased over time (Figure 2, Figure S1).
Figure 1. Schematic of the surface coating setup. A calcium chloridePDMS layered material is formed. Upon scratching the surface, calcium chloride is exposed to water, causing an increase in the ionic strength and local density. This causes the quantum dots (Qdots) to aggregate at the crack site due to the decreased electrostatic repulsion. 15 mm polystyrene Petri-dish and placed in a vacuum desiccator for 30 min for degassing. The dish was then placed in an oven at 60 °C for 1 h to cure the PDMS. A thin layer or line of solid CaCl2 (∼0.2 g, Sigma-Aldrich) was added on top of the PDMS base. Then an additional 3 g of a mixture of 10:1 PDMS and cross-linker was poured on top to cover the solid CaCl2. The coating was then again placed in the vacuum desiccators for 30 min followed by curing at 60 °C for 1 h. A portion of the coating was cut out of the Petri-dish and sealed onto a glass slide using a high frequency generator (model BD-10AS, ElectroTechnic Products, INC). Fluorescence Confocal Imaging. Confocal images were collected on a Leica TCS SP5 confocal instrument. Ar laser was used for the imaging of the quantum dots. Qdot 655 ITK carboxyl quantum dots were purchased from Life Technologies. These quantum dots are 20 nm crystals of a semiconductor material (CdSe), which are shelled with an additional ZnS semiconductor layer. This core−shell material is coated with a polymer layer that has −COO− surface groups. Ten μL of a 8 μM stock of 655 nm excitation carboxylated quantum dot solution was first diluted to 1000 μL. The gasket on top of the coating was filled with this solution and sealed. The setup was inverted and placed on the microscope. The fluorescence intensity was monitored at the crack with a 10× objective every 10 min after an initial time of 5 min. Emulsion Fabrication. Two 1 mL centrifuge tubes were filled with 450 μL of nanopure H2O and 20 μL of oleic acid. One tube was then filled with 40 μL of 5 wt % Grubbs Catalyst, second Generation (Figure S3), in 1,1,2-trichloroethane. To the other tube, 40 μL of liquid dicyclopentadiene, heated to 43 °C, was added. The tubes were then emulsified for 180 s each forming the emulsions (Figure S4). Equal volumes of each emulsion mixture were then added to a container. For microscopy experiments this mixture was diluted until the solution became translucent. For the deposition experiments the solution was used without dilution. Emulsion Stability. The stability of the emulsions in salt solutions was tested. First, a 1:1 mixture of dicyclopentadiene emulsions and Grubbs Catalyst, second Generation emulsions was diluted in aqueous solutions with CaCl2 concentrations between 0 and 1 M. After 1 h the solutions were imaged with an optical microscope, and the emulsion concentration was measured as the number of emulsions in a given frame. After 20 h the emulsions in the vials with CaCl2 concentrations of 0.1 M and above had visibly cleared. Polymer Deposition. The system’s capacity to deposit polydicyclopentadiene into the crack was studied. A 1:1 mixture of the monomer and catalyst emulsions was placed in a spacer above a cracked sample with and without embedded salt. After 1 h the spacer
Figure 2. Fluorescence confocal imaging of the crack at 5 and 35 min (scale bar, 325 μm). Density driven flows produce increased fluorescence at the damage site, allowing its detection.
The process was quantitatively analyzed by calculating the ratio of the fluorescence intensity at the crack to the intensity 100 μm away from the crack. At 5 min the ratio was 1; this increased to 13 after 35 min, over one order of magnitude increase (Figure S2). This is also apparent in optical images of the polymer surface where, with salt, the quantum dots aggregate around the crack, making the damage easy to distinguish by naked eye (Figure 3). 4648
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With an understanding of the density driven flow pattern, two different repair strategies were devised. The first repair method involved a polymerization catalyst and a monomer incorporated into two separate oil-in-water emulsions (see the Experimental Section for details). The separation of the monomer from the catalyst is itself important, for certain polymerizations occur too fast to allow adequate mixing of reactants, as is seen in the well-known Grubbs’ ring opening metathesis system.21−25 A damaged salt-embedded polymer releases ions, which are utilized to generate density driven flows. As the flows bring the emulsions over the crack, the high ionic strength at the damaged site would cause the emulsions to break open and aggregate, allowing the catalyst and reactant to mix. This mixing will cause polymer formation and deposition at the crack, repairing the damage (Figure 5).
Figure 3. Image of the crack with a quantum dot solution a) with and b) without CaCl2 embedded in the PDMS after 1 h. The quantum dots clearly concentrate at the crack when there is salt.
When the quantum dots were added over scratched PDMS containing no salt, there was no significant change in the fluorescence intensity ratio over time (Figure S1). This process is not limited by the number of damage sites and was even shown to work on multiple cracks (Figure 4). Increased
Figure 4. Fluorescence confocal imaging of multiple cracks at 5 and 35 min (scale bar, 500 μm). The fluorescence intensity remains constant at the crack, while the quantum dots outside the crack diffuse away, showing a more defined fluorescence from the damaged site.
Figure 5. Schematic of a surface healing system using a salt/PDMS film. The catalyst emulsions (red) and monomer emulsions (gray) flow over the crack due to density driven flows. At the crack the emulsion stability is lowered due to the high ionic strength, and the emulsions begin to break apart and aggregate. This process causes polymer deposition.
distance between the cracks or their size did not obstruct the detection either. Hence, this scheme should also be applicable to smaller cracks within the limits of fluorescent detection. While effective for crack detection, this method, like previously reported systems, does not repair the damage. Prior to venturing into repair strategies, we sought to better understand the flow characteristics and mechanisms. A crack was formed in a mineralized polymer as previously described. Oil-in-water emulsions were synthesized (see the Experimental Section for details) and introduced to the damage site. The emulsions were seen to flow away from the crack close to the surface and toward the crack in a plane above the surface at a speed of 3.0 ± 1 μm/s. Upon inverting the setup, the flow direction reversed such that the emulsions flowed toward the crack at the surface and away from the crack in a plane just above the polymer surface at a speed of 4.0 ± 1 μm/s (Figure S6, Video S1). The flow directions were shown to be independent of tracer charge as identical flow patterns were observed for polystyrene particles with positive (amine functionalized latex particles), negative (carboxylated latex particles), and neutral (latex particles) surface charge. This is indicative of a density driven flow, since the flow is dependent on the setup’s orientation relative to the gravitational vector and not the charge of the tracers. These flows work as a useful targeting mechanism since they continuously transport emulsions toward the crack.
To evaluate how the salt perturbs the stability of the emulsions, the emulsions were suspended in solutions ranging from 0 to 1 M CaCl2 and allowed to sit for 1 h. As the salt concentration increased, fewer emulsions were observed, and more floating polymer deposits were observed. The number of emulsions was counted in each solution, and a clear decrease in emulsion concentration was observed as a function of salt concentration (Figure 6). The instability was visually noticeable after 20 h, when the 0 and 0.1 M solution still looked turbid but the solutions from 0.1 to 1 M were clear and showed deposits floating on top (Figure 6). This perturbation occurs because, as the ionic strength increases, the zeta potential decreases, lowering the repulsive forces between emulsions. This loss in repulsive forces allows the emulsions to begin to coalesce and flocculate, causing the reactants to mix.26 After verifying the basic principles of this method, the polymer repair strategy was put to test. A mixture of catalyst and monomer emulsions (1:1) was introduced into the spacer over the PDMS/CaCl2 sample that was cut using a scalpel as with the detection setup. Two setups, one inverted and one upright, were tested since different orientations cause flows in different directions. The spacers were then removed after 1 h, and the samples were washed extensively with DI water, dried, and studied with an environmental scanning electron micro4649
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ions would be expected to combine with the leached calcium ions to deposit calcium carbonate at the crack (Figure 8).
Figure 6. Graph displaying the number of emulsion particles observed at 50× magnification as a function of CaCl2 concentration after 1 h. There is a dramatic drop in particles as the CaCl2 concentration approaches 0.1 M. Inset shows images of the oil-in-water emulsions after 20 h. The 0 and 0.01 M salt solutions are still turbid, while the higher salt solutions are clear and show floating polymer deposits.
Figure 8. Schematic of a surface healing system using a salt/PDMS film. The urease enzymes (blue) and urea molecules (gray) move over the crack due to density driven flows. While this occurs, the urea is converted by the urease to carbonate ions (pH ∼ 10.3). The carbonate ions then react with the leaching calcium ions forming solid calcium carbonate.
scope (ESEM) (Figure 7, Figure S5). The samples showed substantial filling of the crack with polymer deposition centered at the crack. Deposition in the crack was more uniform in the inverted samples, since the flow at the surface was directed toward the crack allowing for better delivery of the repair reagents. Micro AT-IR was used to analyze the polymer deposits. The IR spectrum showed key peaks for poly-DCPD at 1665 and 956 cm−1 for the acyclic CC and the acyclic C−H stretching, respectively (Figure S7).27,28 A control with the emulsion solution over a cut in pure PDMS film showed no polymer deposition. Having established this polymeric repair strategy, we explored the possibility of a more biofriendly approach. The concept involved the addition of a premixed solution of the enzyme urease (2 μM) and its substrate urea (1 M) to a calcium ion-leaching crack. At pH 10.3, carbonate ions are formed due to enzymatic hydrolysis of urea. These carbonate
A damaged PDMS/CaCl2 system was prepared as before. When the crack in this system was exposed to a mixture of urease and urea (pH ∼ 10.3; a drop of 0.1 M ammonium hydroxide was added to raise the pH.) and left undisturbed for 1 h, a white precipitate was observed to deposit at the crack (Figure 9A). The precipitate showed aragonite and calcite-like morphology (Figure S8). XRD analysis at the crack site also confirmed the presence of calcium carbonate, in aragonite and calcite forms (Figure 9B). Supporting IR spectra were also collected at the crack site, (Figure S9) to corroborate the XRD findings. Carbonate vibration bands29 around 1460 (symmetric stretching) and 880 (out-of-plane bending) cm−1 confirmed the presence of the precipitated carbonate.
Figure 7. ESEM images of polymer deposition at the damage site. The strategy works well for both single (A-C) and multiple cracks (D-F). A, D) The image of cut polymer with no salt after 1 h exposure to emulsions. B, C, E, F) PDMS/CaCl2 in inverted setup after 1 h exposure to emulsions. 4650
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions ‡
These authors contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by a 3M Summer Research Fellowship (S.M.M.), Penn State MRSEC (NSF-DMR0820404), and by the Air Force Office of Scientific Research (FA9550-10-1-0509).
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Figure 9. A) ESEM images showing the control (left) and sample (right) where the crack was exposed to the urease-urea mixture without and with the underlying calcium chloride layer, respectively. Scale bar is 100 μm. B) XRD analysis of the crack site confirming the presence of calcite (red bars-standard) and aragonite (blue barsstandard). The amorphous halo at lower two-theta values is due to PDMS.
To conclude, the versatility of a salt gradient-powered system has been demonstrated in its ability to perform both crack detection and repair. The salt acts as a “reporter” in aqueous systems containing either detection or healing agents. The flows generated by the leaching salt direct the reagents, Qdots, emulsions, or enzymes, toward the crack without the addition of an external energy source. The high ionic strength focuses the Qdots at the site of damage for both fluorescent and visual imaging. The salt triggered polymerization system is also a useful method for directed polymer deposition. The efficacy of the system, especially in an inverted setup, suggests its practical applicability for coatings. Finally an enzyme-mediated bio- and ecofriendly repair system involves calcium carbonate deposition at the crack. Since the “reporter” salt can be impregnated into most polymer systems, this methodology is quite general Furthermore, the detection and healing solutions can be assembled “as needed,” thereby overcoming the problem of reagent instability. Finally, the method should be useful for coatings on materials that are not easy to remove and repair. A comprehensive assessment of various salt/polymer combinations and the effect of salt impregnation on polymer properties is planned. Future systems will also seek to improve the deposition process and allow for restoration of the polymer properties.
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REFERENCES
ASSOCIATED CONTENT
S Supporting Information *
Figures S1−S9 and Video S1. This material is available free of charge via the Internet at http://pubs.acs.org. 4651
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(28) Dragutan, V.; Dragutan, I.; Dimonie, M. A Selective Route for Synthesis of Linear Polydicyclopentadiene. Green Metathesis Chemistry, NATO Science for Peace and Security Series A: Chemistry and Biology; Springer: 2010; pp 369−381. (29) Mecozzi, M.; et al. Analyst 2001, 126, 144−146.
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