Conjugated Oligomers as Fluorescence Marker for the Determination

Article Cite This: Chem. Mater. XXXX, XXX, XXX−XXX

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Conjugated Oligomers as Fluorescence Marker for the Determination of the Self-Healing Efficiency in Mussel-Inspired Polymers Johannes Ahner,†,‡ David Pretzel,†,‡ Marcel Enke,†,‡ Robert Geitner,§ Stefan Zechel,†,‡ Jürgen Popp,§,⊥ Ulrich S. Schubert,*,†,‡ and Martin D. Hager*,†,‡ †

Laboratory of Organic and Macromolecular Chemistry (IOMC), Friedrich Schiller University Jena, Humboldtstraße 10, Jena D-07743, Germany ‡ Jena Center for Soft Matter (JCSM), Friedrich Schiller University Jena, Philosophenweg 7, Jena D−07743, Germany § Institute of Physical Chemistry, Friedrich Schiller University Jena, Helmholtzweg 4, Jena D−07743, Germany ⊥ Leibniz Institute of Photonic Technology (IPHT) e.V., Albert-Einstein-Straße 9, Jena D−07745, Germany S Supporting Information *

ABSTRACT: Within the current study, a novel approach for the detailed determination of the scratch healing efficiency in mussel-inspired polymer films is presented. For this purpose, a sensor molecule was incorporated into a self-healing polymer network based on reversible zinc−histidine interactions. The fluorescence of the sensor molecule was monitored enabling a detailed depth- and time-resolved determination of the healing efficiency by means of confocal laser scanning microscopy (CLSM). Finally, this concept represents an efficient and detailed approach for the determination of the scratch selfhealing efficiency in polymer films and can also be applied for other scratch self-healing systems, which are based on reversible dynamic bonds.

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interactions.15,16 In this context, one highly interesting nature-inspired class are metallopolymers based on the reversible zinc(II) ion−histidine interactions,17−19 which are also responsible for the healing ability of mussel byssal threads.20−22 In general, the healing process of all these systems was mainly tested either using scratch or bulk healing experiments.23 The utilization of the right characterization technique strongly depends on the intended potential application of the self-healing material. For instance, scratch healing experiments are more suitable for materials intended to be utilized as coatings. Whereas bulk healing experiments are commonly used to determine the healing efficiencies by the comparison of the mechanical properties (e.g., fracture toughness of the healed polymer compared with the original fracture toughness). Particularly, scratch healing tests based on the visual inspection of the polymer films (e.g., by an optical microscope) are often limited to the “superficial” observation of the healing event itself. Often the classification of the extent of the damage is very limited, i.e. scratch is present up to being completely healed without any intersteps. Consequently, the determination of a scratch healing efficiency is not often reported in literature so far. Nevertheless, first attempts toward this direction were already performed by analyzing the scratch

INTRODUCTION Self-healing polymers represent a promising class of materials in the context of sustainability and long-term use, which are able to (partially) heal damage and restore their mechanical properties.1 Generally, self-healing polymers can be divided into intrinsic and extrinsic self-healing polymers.2 Whereas the healing of intrinsic ones is based on dynamic interactions within the polymer (network) structure,3 extrinsic ones are based on the embedding of microcapsules or nano/microtubes filled with a healing agent.4 This encapsulated healing agent is typically a polymerizable monomer, which is released by the occurrence of a damage, e.g., a crack. Subsequently, the monomer polymerizes, triggered by an embedded catalyst within the matrix, resulting in crack closure. The major disadvantage of extrinsic systems based on microcapsules is that the healing ability at each spot of the material is only nonrecurring due to the consumption of the (encapsulated) healing agent during the self-healing process. In contrast, for intrinsic self-healing polymers, often a stimulus is required to induce the healing process. However, intrinsic self-healing systems are able to heal mechanical damage more often and feature an enormous potential because different triggers can be applied.3 The dynamic interactions within intrinsic self-healing systems can be either based on dynamic covalent interactions, e.g., reversible thiol−ene5 and Diels−Alder reactions,6−8 or focus on reversible supramolecular interactions such as metal− ligand,9−11 hydrogen and halogen bonds,12−14 or π−π© XXXX American Chemical Society

Received: February 9, 2018 Revised: March 21, 2018

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DOI: 10.1021/acs.chemmater.8b00623 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

Figure 1. Principle of self-healing procedure: (a) virgin cross-linked polymer film of copolymer P4 with Zn(OAc)2 × 2H2O and incorporated fluorescence marker 10 (defined depth level-dependent emission intensity), (b) damaged cross-linked polymer film resulting in decreased emission intensity within the scratch, (c) induction of mobile phase by thermal treatment, (d) healed cross-linked polymer film with virgin emission intensity, and (e) schematic representation of utilized materials for the design of the self-healing coating.

depth, area, and volume using the surface of the material.24−26 However, the main drawback of this method still exists and is based on the missing depth-dependent information, in particular, after the self-healing process due to the monitoring from the surface/outside. Thus, this contribution addresses this challenge by means of using confocal laser scanning microscopy (CLSM) in combination with an oligomeric fluorophore incorporated into a cross-linked mussel-inspired self-healing network as a fluorescent marker for the healing of the scratch during thermally triggered self-healing. CLSM was utilized due to the possibility of the depth-dependent detection of the fluorescence intensity. Consequently, this method allows the determination of the scratch healing efficiency on a macroscopic scale based on depth-dependent fluorescence intensity scanning. In particular, after the scratch closure, the self-healing efficiency and the homogeneity can be detected in detail in different depth levels. Moreover, it has to be noticed that cavities or impurities, which can be incorporated during damage and healing procedure, would be detectable with the usage of this new approach in different depth levels after the refilling of the scratch. This CLSM approach enables “X-ray vision” of the material by simple fluorescence.

histidine units of the copolymer), and the desired amount of Zn(OAc)2 × 2H2O based on the total amount of histidine units were temperature treated at 60 °C until the fluorescence intensity was constant in each depth level, representing the homogenization of the network during annealing. These prepared self-healing copolymer films exhibit a specific fluorescence intensity from 406 to 510 nm at an excitation wavelength of 405 nm (Figure 1a). Damaging of the copolymer films, e.g., by scratching, should result in a decreased fluorescence intensity within the scratch (Figure 1b). Induction of the mobile phase, based on the thermally triggered exchange reaction between metal-histidine complexes, should result in a refilling of the scratch and an increasing fluorescence intensity within the healed scratch (Figure 1c). After the self-healing procedure, the restoration of the fluorescence intensity in different depth levels within the healed scratch could be utilized for the determination of the self-healing efficiency (Figure 1d). Synthesis of the Fluorescence Marker and the SelfHealing Polymer. First, the π-conjugated fluorescence marker 10 is prepared, which is required for the monitoring of the healing process. The fluorescence behavior is based on the wellknown phenylene−ethynylene moiety.27,28 Phenylene−ethynylene based oligomers exhibit high extinction coefficients, high fluorescence quantum yields (>95%), and thermal stabilities up to 400 °C.29,30 Consequently, a phenylene−ethynylene based π-conjugated oligomer represent a highly promising candidate as fluorescence marker for the determination of the self-healing efficiency in thermally triggered self-healing systems. A detailed synthetic procedure of the utilized fluorescence marker 10 is described in the Supporting Information (Scheme S1 and

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RESULTS AND DISCUSSION General Principle of the Study. The general principle for the determination of the self-healing efficiency using CLSM is schematically depicted in Figure 1. First, cross-linked virgin copolymer films of the utilized metallopolymers consisting of copolymer P4, fluorescence marker 10 (2 mol% based on the B


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Figure 2. Schematic representation of the utilized fluorescence marker 10, the model compounds 11 and 13, and the self-healing polymer P4.

Scheme S2). The final marker 10 exhibits several nitrogenbased binding sites, e.g., amine, amide, and imidazole, for later complexation with zinc(II) ions (Figure 2). Furthermore, model structure 11, which mimics the functional part of the polymer, was synthesized. The synthetic procedure was performed according to literature, including minor adoptions.17 Model structure 11 exhibits nearly the same nitrogen-based ligand moieties as the fluorescence marker 10 for complexation with zinc(II) ions and is required to study the complexation behavior. Additionally, fluorescence model marker 13 was prepared for detailed structure investigations with zinc(II) ions and for an investigation of the influence of the histidine moiety on the complexation behavior. A detailed description of the synthetic procedure is located in the Supporting Information (Scheme S3). In contrast to fluorescence marker 10, model compound 13 features a phenyl unit instead of the tritylprotected imidazole moiety of the histidine resulting in minor nitrogen-based complexation options for zinc(II) ions. The polymer design in general was inspired by the natural mussel byssal threads, which utilize zinc(II)−histidine interactions for a self-healing process.18,21,31 For this purpose, a histidine functionality was introduced for a subsequent cross-linking with zinc(II) ions. A detailed description of the synthetic procedure is located in the Supporting Information (Scheme S4, Figure S1, and Figure S2). Lauryl methacrylate (LMA) was utilized as comonomer due to beneficial properties of poly(lauryl methacrylate) (PLMA) for the self-healing properties, e.g., flexibility, solubility and low glass transition temperature (Tg).9 The obtained molar mass and dispersity values (Đ) are summarized in Table S1. The final copolymer P4 exhibits several nitrogen-based binding sites, e.g., amine, amide, and imidazole, for later coordination of zinc(II) ions (Figure 2). Investigation of the Complexation Behavior. To understand the self-healing mechanism in detail and utilize the fluorescence marker 10 for the determination of the healing efficiency, the investigation of the binding affinities on the molecular level represents a crucial prerequisite. First, the complexation of zinc(II) ions with the model compound 11 was studied to reveal the complexation behavior of the histidine moieties, which are incorporated into the polymer structure of copolymer P4. For this purpose, isothermal titration calorimetry (ITC) was performed to determine the thermodynamic parameters of the metal−ligand interaction (e.g., stoichiometry and the binding affinity of zinc(II) acetate and compound 11)

(Figure S3).17 The obtained results are summarized in Table 1. Finally, a ratio of compound 11:Zn(OAc)2 × 2H2O of Table 1. Summarized ITC Measurement Results of Compounds 10, 11, and 13 Titrated with Zinc(II) Acetate

compound

compound:Zn(OAc)2 × 2H2O mol % ratio

complex formation constant Ka (M−1)

enthalpy ΔH (kJ/mol)

entropy ΔS (kJ/ mol·K)

10 11 13

1.26:1.00 2.12:1.00 1.39:1.00

4.24 × 104 2.97 × 104 1.22 × 103

−56.95 −32.51 −61.21

−99.24 −21.66 −142.80

2.12:1.00 was achieved, which is the expected value. Additionally, a complex formation constant of 2.97 × 104 M−1 was determined by the ITC measurements, which is in the typical range of different zinc imidazole complexes.17 Moreover, the complexation behavior of compound 11:Zn(OAc)2 × 2H2O was investigated in detail using NMR and FT-Raman spectroscopy (Figures S4 and S5), resulting in the elucidated structure depicted in Figure 3b (M11). The second part of the molecular structure analysis focused on the investigation of the fluorescence marker 10, in particular the complexation with Zn(OAc)2 × 2H2O. For the later purpose of incorporation into the polymeric structure, it is required to prove a similar complexation behavior of marker 10 in combination with Zn(OAc)2 × 2H2O when comparing to the binding behavior/ strength of the copolymer to Zn(OAc)2 × H2O. As a consequence, ITC studies of the fluorescence marker 10 with Zn(OAc)2 × 2H2O were also performed (Figure S6). The obtained complex formation constant of 4.24 × 104 M−1 for fluorescence marker 10 is in the same dimension as for polymer model compound 11 (Table 1). Furthermore, the ratio between fluorescence marker 10 and Zn(OAc)2 × 2H2O was determined to be 1.26:1.00 instead of expected 1.00:1.00 This deviation can be attributed to the oligomerization during the titration process.32 Thus, the concentration and the complex constant determines the degree of polymerization (oligomerization), resulting in an incomplete polymerization process and free ligand moieties. The degree of oligomerization is approximately 4 under the applied measurement conditions. Further investigations on fluorescence marker 10 were performed using 1H NMR spectroscopy in deuterated dichloromethane (Figure S7) and FT-Raman measurements C


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Figure 3. Schematic representation of the obtained structures in solution and solid state of (a) fluorescence marker 10 with Zn(OAc)2 × 2H2O (M10) and (b) model compound 11 with Zn(OAc)2 × 2H2O (M11).

Figure 4. Optical properties of copolymer P4 and 2 mol % of fluorescence marker 10 in solution (CHCl3, c (marker) = 10−6 mol/L) with and without Zn(OAc)2 × 2H2O: (a) absorption spectra, (b) emission spectra (λex = 405 nm).

(Figure 4a). The absorption maximum at 315 nm corresponds to the absorption of copolymer P4 and marker 10, whereas the absorption maximum at 370 nm was nearly completely assigned to the fluorescence marker 10. Consequently, an excitation at 405 nm of the mixture of P4 and fluorescence marker 10 causes an exclusive excitation of the sensor, resulting in the emission spectrum of the chromophore. The emission spectrum is characterized by the vibrational fine structure of the chromophore with 410 nm as the maximum and 430 nm as a bathochromic shoulder (Figure 4b). Cross-linking of the materials by addition of Zn(OAc)2 × 2H2O resulted in no pronounced changes in the absorption or emission spectrum. This phenomenon is explainable by the fact that zinc represents a d10 element, which does not alter the spectroscopic properties of π-conjugated oligomers after complexation.33 Self-Healing Studies. After the detailed investigation of the complexation behavior of the fluorescent cross-linker and of the histidine moiety, which was incorporated into the polymer side chain (see above), this molecular information can be utilized for the synthesis and design of a self-healing metallopolymer. For this purpose, a mixture of copolymer P4, 2 mol% fluorescence marker 10, and the corresponding amount of Zn(OAc)2 × 2H2O were drop-casted out of a chloroform:methanol (5:1) mixture. Afterward, the polymer

in solid state (Figure S8). However, the complexation of the imidazole nitrogen atom to zinc(II) ions could not be studied due to the aromatic signals of the fluorescent core, which overlap with those of the imidazole moiety when using 1H NMR and in FT-Raman spectroscopy. However, the entire complexation behavior of fluorescence marker 10 with Zn(OAc)2 × 2H2O (Figure 3a, M10) was investigated using fluorescence model compound 13. The detailed study is described in the Supporting Information (Scheme S3, Figures S9−S12). In conclusion, the complexation behavior of marker 10 and polymer model 11 with Zn(OAc)2 × 2H2O was investigated, mimicking the behavior in later self-healing polymer networks. It was determined that marker 10 and polymer model 11 showed an identical complexation behavior with Zn(OAc)2 × 2H2O, which represents an crucial prerequisite for the subsequent self-healing studies. Optical Properties. The optical properties of the fluorescence marker 10 were investigated for later application as marker for the determination of the self-healing efficiency (Figure 4). For this purpose, a mixture of copolymer P4 and 2 mol % fluorescence marker 10 were investigated using UV/vis spectroscopy in chloroform (c (marker) = 10−6 mol/L). The absorption spectrum showed maxima at 315 and 370 nm D


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Figure 5. Optical microscope images of thermally triggered self-healing of drop-casted cross-linked polymer films of copolymer P4 with incorporated fluorescence marker 10 (2 mol %) and the corresponding amount of Zn(OAc)2 × 2H2O at different temperatures (a marker made with a pencil was set in the left upper corner in all images). Self-healing polymer films before thermal treatment at (a) 25, (b) 40, and (c) 60 °C and after thermal treatment for 24 h at (d) 25, (e) 40, and (f) 60 °C.

Scheme 1. Schematic Representation of the Exchange Reaction between the Zinc−Histidine Complexes during the Thermally Triggered (60 °C) Self-Healing Procedure

films were dried at room temperature for 24 h. Subsequently, the pristine polymer films were annealed at 60 °C for about 3 days until a homogeneous surface fluorescence (λem = 406−510 nm at 405 nm excitation) of film was achieved detected by CLSM. To achieve a sufficient high level of fluorescence but without any quenching effects, only 2 mol% of marker 10 was incorporated into the copolymer network. The thermal properties of the metallopolymer network with/without incorporated fluorescence marker 10 and of the pure sensor were studied using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) to be suitable for the applied thermal window during self-healing studies (Figures S13 and S14). Subsequently, the polymer films were damaged in a controlled manner using a scalpel to evaluate the healing behavior first using optical brightfield microscopy (Figure 5). After storage of the damaged sample over 24 h at ambient temperature, the scratch kept the same shape. Subsequently, the induction of the mobile phase for potential self-healing was thermally triggered in a drying oven. First, the damaged sample was thermally treated at 40 °C for 24 h. Under these conditions, no healing was observed despite small changes of the edges of the scratch, which is related to an elastic recovery phenomenon of the material as known from literature for other healable polymers.25,26 In contrast, thermal treatment at 60 °C resulted in a complete self-healing of the scratch surface visible using brightfield optical microscopy. This phenomenon is based on the reversibility of the cross-linked copolymer network at 60 °C, resulting in an increased mobility of the whole material

(including the fluorescence marker). A detailed investigation on the anticipated self-healing mechanism was already performed, which is based on the reversible interaction between zinc− histidine (Scheme 1).17,18 Similar thermally triggered selfhealing systems were studied using conventional brightfield optical microscopy surface images for the determination of the self-healing ability.6,9,19 However, the surface images of conventional brightfield microscopy provide only very limited insights into the self-healing process and its efficiency, in particular about the healing on a molecular scale. To gain detailed information about the scratch healing process with an exclusively optical and noncontacting method, fluorescence marker 10 was incorporated into the cross-linked copolymer P4 featuring a comparable complex formation constant with Zn(OAc)2 × 2H2O (see above). As a consequence, the selfhealing efficiency can be determined in detail using confocal laser scanning microscopy (CLSM) after treatment at different temperatures. To apply this novel concept for self-healing efficiency determination by the use of the fluorescence intensity of the incorporated fluorophore 10, first, it has to be proven that the mobility of the copolymer material of P4 and the fluorescence marker 10 during thermal treatment at 60 °C is in an equal range, which represents a prerequisite for the presented selfhealing concept. For this purpose, a “marker” was set by the application of a focused high-energy illumination induced photobleaching (λex. = 405 nm) in a small and defined polymer film area (Figure 6, marked in green). Thereby, the E


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Figure 6. Time- and depth-dependent CLSM measurements in fluorescence mode (λex = 405 nm) with the fluorescence channel (λem = 406−510 nm) monitoring thermally triggered self-healing procedure, in particular the virgin damaged cross-linked copolymer film, after 1, 2, and 8 h of thermal treatment at 60 °C: (red) Homogeneous area within the scratch, (yellow) heterogeneous area covering the majority of the analyzed defect, (orange) specific area with residual removed film material, (blue) intact and undamaged reference area for each measurement, and (green) photo bleached marker area.

treatment at 60 °C for after 1 and 24 h were performed using CLSM in reflection mode (λ = 405 nm) and in fluorescence mode (λex = 405 nm) including a specific fluorescence channel for the detection of the fluorophores (λem = 406−510 nm) (Figure S16). The obtained results represent a further proof for an equal mobility of the copolymer material of P4 (detected as the majority of the reflected signal) in relation to the fluorescence marker 10 during the healing process. With this knowledge in hand, CLSM was utilized as a convenient method for the depth- and time-dependent determination of the selfhealing efficiency using the fluorescent properties of the incorporated marker 10. For this purpose, the self-healing polymer film was damaged by scratching with a spatula (macroscopic damage), and CLSM measurements were performed directly and after 1, 2, 4, 8, and 24 h of thermal

fluorescence marker 10 was photodamaged, e.g., by alkoxychain cleavage,34 resulting in a loss of characteristic fluorescence intensity at the utilized excitation wavelength of 405 nm. During the thermally triggered self-healing process of the crosslinked copolymer film, the bleached fluorescence intensity of the marked area stayed nearly constant within the standard deviation (Figure S15). This observation indicates that the mobility of the fluorescence marker 10 is clearly connected and restricted to the plasticity of the polymeric material during thermal treatment, in particular during scratch refilling. Thus, the fluorescence marker shows no independent migration within the polymeric material, which would be detectable as a recovery of the fluorescence in the photobleached area over the healing time. Additionally, depth level-dependent measurements of the self-healing copolymer film after scratch damage and a thermal F


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Figure 7. Summarized depth- and time-dependent fluorescence intensity (λem = 406−510 nm) in relation to the reference of (a) homogeneous area within the scratch (marked in red), (b) large area of the scratch, including defects (marked in yellow), (c) a specific area with large defects (marked in orange) determined by CLSM, and (d) depth- and time-dependent fluorescence intensity of the reference area (marked in blue) during thermally triggered self-healing process of cross-linked fluorescent copolymer film (Figure 6, Figure S17). The green dotted lines in panels a−c represent 100% virgin fluorescence intensity.

treatment at 60 °C using the same recording modes as described above (Figure 6, Figure S17). However, to prove the concept, the fluorescent intensity of the fluorophore 10 was determined at three different depth levels, i.e., the surface, −20 μm, and −40 μm for each specific healing time. Moreover, three different areas were measured to include several visible defect qualities (Figure 6): (a) a homogeneous region without defects (marked in red), (b) a large area of the scratch (marked in yellow), and (c) a specific region with residual film material (marked in orange). For each measurement, the obtained time- and depth-dependent fluorescence intensities were compared to the corresponding fluorescence intensity of the virgin polymer film at the respective depth level (Figure 6, marked in blue). It was determined that the fluorescence intensity of the blue marked area stayed nearly constant over timea prerequisite for taking this spot as an intact film reference (Figure 7d). In case of the homogeneous area, a complete self-healing of the whole defect was accomplished after 8 h of thermal treatment where fluorescence from deep, middle, and surface related zones was uniformly above 100% (Figure 6 (marked in red), Figure 7a). Interestingly, the self-healing efficiency in the depth of −40 μm reached 100% already after 1 h of thermal treatment at 60 °C, whereas in the depth of only −20 μm, 100% restoration was achieved after an additional hour of heating, and the surface zone indeed required 8 h of thermally triggering for complete restoration. This indicates that the healing process originates in the scratch basis and progresses toward the surface in a zip-like

manner.35 The same trend was visible after evaluation of the large heterogeneous defect area (Figure 6 (marked in yellow), Figure 7b), exclusively the surface fluorescence intensity did not reach 100% fluorescence restoration after 24 h of thermal treatment at 60 °C. The heterogeneity of the defect results in a varying defect healing efficiency becoming evident in the relatively high standard deviation within the measured fluorescence intensities; still, the association between visual microscopic observations and measured values proves the reliability of the applied method. However, in the case of both the homogeneous and heterogeneous defect area, the relative fluorescence intensities in the depth of −40 μm increased significantly up to 150% of the original fluorescence intensity after thermal treatment up to 2 h. Further heating led to a decrease of the fluorescence intensity closer to the 100% restored fluorescence intensity. Presumably, this behavior can be explained with the ongoing healing process and the measurement setup. The fluorescent intensity is detected above the surface. Consequently, the fluorescent signal in −40 μm is extenuated due to the material between the measured area and the detector. During the healing process, in particular at the beginning, there is no material present between the detector and the −40 μm area, resulting in an overestimated signal intensity and, consequently, in a fluorescent intensity of 150% in relation to the fluorescence intensity at −40 μm of the intact reference area (Figure 6 (marked in blue). This assumption is confirmed by the phenomenon of a decreasing fluorescence intensity at a depth G


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Chemistry of Materials level of −40 μm during the further ongoing self-healing process. However, after 8−24 h of thermal treatment, all fluorescence intensities were striving toward a specific steady state value. Moreover, a large defect with residual material was investigated in terms of self-healing using fluorescent properties of the inserted chromophore (Figure 7c). In this case, the fluorescent properties of the cross-linked copolymer film were restorable only up to around 80% in relation to the original ones. This might be caused by the wrapped morphology of the remaining material within the defect, which likely hinders undisturbed material migration into the defect. A further potential explanation is the presence of impurities within this scratch area, resulting in an incomplete healing process. However, the influence of morphological scratch/defect characteristics on the self-healing efficiency displays another feasible parameter to be studied in further investigations with the presented method.

ORCID

Jürgen Popp: 0000-0003-4257-593X Ulrich S. Schubert: 0000-0003-4978-4670 Martin D. Hager: 0000-0002-6373-6600 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank the Deutsche Forschungsgemeinschaft (DFG) for financial support within the framework of the priority program SPP1568 (Design and Generic Principles of Self-healing materials, PO563/25 2, HA6306/31, and SCHU1229/13-1). The LSM880 ELYRA PS.1 was funded with a grant from the DFG (USS91b). S.Z. is grateful to the Carl-Zeiss foundation for funding.

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CONCLUSION Overall, a tailor-made fluorescence marker was incorporated into a mussel-inspired scratch-healing polymer network for the detailed depth- and time-dependent determination of the selfhealing efficiency using the fluorescent properties of the sensor by means of confocal laser scanning microscopy (CLSM). Depending on the depth level and the analyzed defect-areas within the polymer film, the restoration of the fluorescence intensity was found to be 73% up to 100% in relation to the original fluorescence intensity of the intact polymer film. In contrast to the conventional self-healing efficiency determination methods using the area or the volume of the scratch and measuring only the surface of the material,24−26 this novel approach exhibits the possibility for the time- and depthdependent and more detailed determination of the self-healing efficiency, particularly after the complete scratch closure. Thus, incorporated impurities, heterogeneities, and cavities would be detectable within the polymer network after the self-healing process using the fluorescent properties of the incorporated sensor. Moreover, the complexation behavior within the polymer network was studied in solution and in the solid state on a molecular level using nuclear magnetic resonance (NMR) spectroscopy, FT-Raman spectroscopy, and isothermal titration calorimetry (ITC). These results demonstrate that confocal laser scanning fluorescence microscopy in combination with the presented sensor system represents a reliable and promising tool for the determination of scratch self-healing efficiency with a high spatial resolution (x- and y-direction on the surface and depth-level) and temporal resolution.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b00623. Experimental section, NMR measurements, ITC investigations, FT-Raman spectra, TGA measurements, DSC spectra, and additional information for the applied selfhealing studies (PDF)

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REFERENCES

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