A dual functional epoxy material with autonomous damage indication and self-healing

Abstract

Autonomic indication of mechanical damage and self-healing epoxy materials was conducted using 2′,7′-dichlorofluorescein (DCF) and glycidyl methacrylate (GMA) solution. When mechanical damage occured, the released DCF reacted with the amine group in the crack plane to sense the damage colorimetrically, and the GMA rebound the cracks in the epoxy matrix by chemical or physical interaction.

---

The autonomous indication of mechanical damage for polymers is one of the most investigated and challenging research fields in intelligent polymer materials.^1–6 The detection of small crack damage in polymers is extremely important, because it allows timely discovery of the defect, thus preventing significant degradation of the integrity, functionality and mechanical performance of the polymers.^7,8 Currently, the widely used strategy for the early detection of damaged polymers involves the introduction of a “triggered-compound” into the polymer.^9,10 That is to say, when the mechanical damage (e.g., impact, stretch, compression, or abrasion) occurs, this compound is released and reacts with the active sites of the crack plane, generating the proper signal either colorimetrically or fluorimetrically.^11,12 Among all the “triggered-compound”, 2′,7′-dichlorofluorescein (DCF) is the most favourable one, because it delivers a relatively strong and stable colorimetric signal.¹³ Sottos's group¹⁰ has developed a microcapsule-based polymer coatings with in situ visual damage indication. DCF, as a damage indicator, was dissolved into ethyl phenyl acetate (EPA). When the polymeric coatings suffer from abrasion, the DCF/EPA mixture was released from microcapsules; meanwhile the EPA brings the DCF to the damaged site and reacts with amine group, triggering a strong colour change.

Comparatively, research in self-healing polymers also made remarkable progress during the last decade,^14–17 with broad applications in the area of polymer coatings and composite materials.^18–20 The mechanism of the self-healing process shortly follows: upon mechanical damage of the polymer, the healing agent inside the microcapsules or microvasculars is released and delivered to the damage site, connecting the cracks via chemical reaction or physical interaction.²¹ Concerning the healing agent for epoxy materials, glycidyl methacrylate (GMA) is regarded as the best choice, because it is a nontoxic solvent with low viscosity, and it possesses reactive groups (epoxide and C=C) that reconnect cracks with the epoxy matrix by cross-linking in the presence of a curing agent such as amine,²² Lewis acid,^23,24 mercaptan,^25,26 imidazole,^27,28 and many other derivatives of the above species. Many reports have shown that the GMA recovers fracture toughness with high efficiency.^22,29

Following our interest for these two related fields enlightened by Sottos's concept,¹⁰ the ideal intelligent polymer should combine early damage indication of the polymeric materials and self-healing. This would be recognized as the man-made skin, i.e. when a wound occurs, injured tissue is immediately reported by dark red colour of bleed preventing further injury. This then initiates healing itself by its own organism growth.³⁰ Therefore, we intend to design a dual-functional epoxy composite that autonomous indicates the mechanical damage and at the same time heals itself.

The polymeric sample is prepared as shown in Scheme 1: first, 50 g EP-4100HF (bisphenol-A based epoxy resin) and 3 g latent curing agent (EH-4360S) are blended with a 3-roll shear mixing machine. Then, 5.75 g of diethylenetriamine (DETA) is added to the mixture and degassed in vacuum to form the mixture 1 (The stoichiometric ratio of EP-4100HF and DETA is 1 : 1). The mixture 1 is postured into the pre-treated stainless steel mould with a hollow glass fiber and cured at 25 °C for 48 h, yielding the product 2. Finally, the 10⁻³ mol L⁻¹ DCF/GMA solution was filled into the hollow glass fiber under capillary pressure to form the dual functional epoxy composite 3. As shown in Scheme 1, when the damage occurs in the composite 3, the DCF releases from the hollow glass fiber and fills the cracks plane under capillary pressure. Subsequently, a dramatic colour change from light yellow to bright red is generated when DCF species come into contact with the amine-cured epoxy matrix. Meanwhile, GMA swells the epoxy matrix adjacent to the cracks and contacts the latent curing agents. Then, appropriate heating (120 °C/20 min) triggers the cross-linking that bonds the closed crack faces. The feasibility of the device design will now be demonstrated in detail.

Scheme 1 Schematic graphs of the preparation, damage indication and self-healing process.

Damage indication and mechanisms

The colour change of the epoxy film (product 2) before and after soaking in DCF/GMA solution for 60 min was determined by UV-vis spectroscopy (Fig. 1a), and a strong transmittance absorbance peak emerged. It is known that the damage mechanism is attributed to the conversion of the DCF from the acid form (light yellow) to the basic form (bright red) in the presence of amine (Fig. S1, ESI†).¹⁰ Therefore, when the damage occurs, the DCF releases and reacts with the residual amine group in the matrix, the amine removes the proton of the two phenol group in DCF, and it produces the basic form of DCF (Fig. S2, ESI†).¹⁰ In the process the GMA not only acts as the healing agent but also as a solvent helping release of DCF in the epoxy matrix. Compared with the DCF/EPA mixture, DCF/GMA shows a better compatibility and stability (Fig. S3 and S4, ESI†), because GMA with a higher Hansen solubility parameter than EPA.³¹ In addition, the good solubility of DCF in GMA enhances the sensibility of DCF upon amines. The swelling and colour change of the epoxy film was calculated using the weighting method. The curing degree of epoxy sample was 97.38% determined by FTIR spectroscopy (Fig. S5, ESI†). As demonstrated in Fig. 1b, the mass uptake of epoxy film is much larger in DCF/GMA solution than that in DCF/EPA solution. Besides, the epoxy film soaked in DCF/GMA solution also shows a remarkable colour change and a dramatic swelling (Fig. S6, ESI†). Therefore, DCF/GMA is preferred to DCF/EPA for damage indication. In order to evaluate the crack damage indication performance of DCF in the epoxy matrix, the photographs and the optical microscopy graphs of composite 3 (Fig. 2 and S7, ESI†) are carried out before and after the damage. As shown in Fig. 2a and e, the hollow glass fiber was colourless, while the epoxy matrix was transparent and with a bright colour. Then the DCF/GMA solution was filled into the hollow glass fiber, and a bright light yellow colour was observed (Fig. 2b and f). When the epoxy matrix with glass fiber (composite 3) was damaged, the DCF/GMA solution flowed out through cracks and penetrated into the cracks plane in the epoxy matrix. Then DCF reacted with residual amine in the cracks, and the epoxy matrix showed a red colour (see Fig. 2c, d, g and h).

Fig. 1 (a) visible spectra and colour change of epoxy sample before and after soaking in GMA/DCF solution; (b) mass uptake of the 4100HF/DETA epoxy film with stoichiometric ratio of 1 : 1.

Fig. 2 Photographs (a–d) and optical microscopy graphs (e–h) of the epoxy matrix before and after mechanical damage; (a and e) epoxy matrix with hollow glass fiber; (b and f) epoxy matrix with hollow glass fiber filled with DCF/GMA solution; (c and g) colour change of the epoxy matrix immediately after mechanical damage; (d and h) colour change of the epoxy matrix after mechanical damage for 48 h.

Healing performance and mechanisms

The self-healing process by GMA happens when residual amine is present in the epoxy matrix. It has been demonstrated by Zhang's group²² that when the epoxy sample was healed at 25 °C for 72 h, the highest healing efficiency was up to 90% and even reached 100% when the healing temperature was increased to 160 °C. However, in practise most of the epoxy polymers are cured at room temperature, and it is difficult to heat to a temperature as high as 160 °C and to maintain it for 72 h. Thus, in order to accelerate the healing rate, decreasing the healing temperature and reducing the healing time are required. Therefore, a modified aliphatic polyamine latent curing agent (EH-4360S) is added here into the composite 3. This curing agent uniformly dispersed in the epoxy matrix and was stable at room temperature (Fig. 3a). However, it rapidly reacted with epoxy resin and GMA at above 80 °C. This was illustrated by in situ FTIR spectroscopy (Fig. 3b, S8 and S9, ESI†), the characteristic absorptions of GMA at 908 cm⁻¹ (epoxide group) and 1637 cm⁻¹ (C=C) changing dramatically (Fig. S9, ESI†). For the purpose of quantitative estimation of the changes, the integral area ratio A_epoxide/A_C=O (noted α) and A_C=C/A_C=O (noted β) were calculated (Fig. 3b).²⁹ The calculated results indicated that the epoxide group has been largely consumed, while the C=C bond slightly decreased after the reaction at 120 °C for 20 min. It means that the epoxide group of GMA is much more active than C=C bond during curing reaction under current conditions. The deduced curing reaction mechanisms are shown in Fig. 3c. In the presence of EH-4360S, the ring-opening reaction occurred between the GMA and the amine in EH-4360S.²² In addition, the nucleophilic addition reaction between the amine of EH-4360S and the C=C bond of GMA was also carried out (Fig. 3c), and finally the random copolymer was yielded.^22,29 That is to say, the self-healing process in our system is attributed to solvent effect and chemical reaction between GMA and EH-4360S.²² Hence, to evaluate the self-healing process of the composite, the fractured planes of cracks in the epoxy sample filled with EH-4360S before and after healing was observed by the SEM (Fig. 4 and S10, ESI†), and to accelerate the self-healing process, the damaged sample was heated to 120 °C for 20 min. As shown in Fig. 4b, the fractured planes of epoxy sample with EH-4360S before healing was smooth and with spherical void, which meant a brittle fracture happened. While after the self-healing at 120 °C for 20 min, the spherical particles disappeared and a toughness surface of the crack was observed, which indicated the happening of self-healing process of the fractured crack.

Fig. 3 (a) schematic graph (left) and photograph (right) of EH-4360S in the epoxy matrix; (b) reaction degree of GMA/EH-4360S (100 : 30 wt) at various temperatures (heating rate: 5 °C min⁻¹) and various reaction times at 120 °C; (c) curing reaction mechanisms of GMA with EH-4360S.

Fig. 4 (a) photograph of healed epoxy matrix with glass fiber filled with DCF/GMA solution; (b) fractured plane of crack in the epoxy sample filled with EH-4360S before healing; (c) and (d) fractured planes of crack in the epoxy sample filled with EH-4360S after healing at 120 °C for 20 min.

In addition, to further evaluate the healing efficiency between GMA with epoxy matrix filled with EH-4360S, the manual healing experiment was conducted. It is noting that the healing efficiency (η) is defined as the ratio of maximum tensile stress of healed materials (σ_healed), to that of virgin ones (σ_virgin):³²

As depicted in Fig. S11b,† the healing efficiency for the epoxy samples filled with EH-4360S was up to 95%. Also, the healing efficiency of the epoxy sample that is filled with EH-4360S was much higher than that of the unfilled one, which may be taken into account by the curing reaction of the GMA with the polyamine. In summary, the epoxy sample filled with EH-4360S has a high healing efficiency as previously reported,²² but with a lower temperature and a shorter healing time.

Conclusions

The epoxy polymer composite containing both autonomous indication and self-healing properties is successfully designed and prepared. On one hand, the DCF/GMA system for the autonomic indication of mechanical damage was easily detected by colorimetric method, and the indicating colour change is highly stable, which is better than in the conventional system (e.g. DCF/EPA). On the other hand, in the presence of latent curing agent (EH-4360S) the self-healing process of the epoxy matrix was quickly achieved at 120 °C within 20 minutes via cross-linking, and the healing efficiency was up to 95%. Future work will focus on the filling of indication and self-healing materials in a microcapsule instead of a hollow glass fiber.

Acknowledgements

Support from the Natural Science Foundation of China (51573172, 11405149, and 51401187), and Director's Funds of China Academy of Engineering Physics (SJZ201506) is gratefully acknowledged.

Notes and references

1. N. Bruns, K. Pustelny, L. M. Bergeron, T. A. Whitehead and D. S. Clark, Angew. Chem., Int. Ed., 2009, 48, 5666.
2. S. Karthikeyan and R. P. Sijbesma, Macromolecules, 2009, 42, 5175.
3. K. M. Wiggins, J. N. Brantley and C. W. Bielawski, Chem. Soc. Rev., 2013, 42, 7130.
4. N. R. Sottos, Nat. Chem., 2014, 6, 381.
5. D. A. Davis, A. Hamilton, J. L. Yang, L. D. Cremar, D. Van Gough, S. L. Potisek, M. T. Ong, P. V. Braun, T. J. Martinez, S. R. White, J. S. Moore and N. R. Sottos, Nature, 2009, 459, 68.
6. Y. L. Chen, A. J. H. Spiering, S. Karthikeyan, G. W. M. Peters, E. W. Meijer and R. P. Sijbesma, Nat. Chem., 2012, 4, 559.
7. B. J. Blaiszik, S. L. B. Kramer, S. C. Olugebefola, J. S. Moore, N. R. Sottos and S. R. White, Annu. Rev. Mater. Res., 2010, 40, 179.
8. A. Stoddart, Nature Reviews Materials, 2016, 1, 16004.
9. K. Makyla, C. Müller, S. Lörcher, T. Winkler, M. G. Nussbaumer, M. Eder and N. Bruns, Adv. Mater., 2013, 25, 2701.
10. W. L. Li, C. C. Matthews, K. Yang, M. T. Odarczenko, S. R. White and N. R. Sottos, Adv. Mater., 2016, 28, 2189.
11. S. Lörcher, T. Winkler, K. Makyla, C. Ouellet-Plamondon, I. Burgert and N. Bruns, J. Mater. Chem. A, 2014, 2, 6231.
12. C. W. Hsieh, C. H. Chu, H. M. Lee and W. Y. Yang, Sci. Rep., 2015, 5, 10376.
13. X. F. Zhang, J. Zhang and L. Liu, J. Fluoresc., 2014, 24, 819.
14. K. D. Xander, F. E. Hillewaere and P. Du, Prog. Polym. Sci., 2015, 49, 121.
15. M. D. Hager, P. Greil, C. Leyens, S. V. D. Zwaag and U. S. Schubert, Adv. Mater., 2010, 22, 5424.
16. C. J. Kloxin and C. N. Bowman, Chem. Soc. Rev., 2013, 42, 7161.
17. Y. Yang, X. Ding and M. W. Urban, Prog. Polym. Sci., 2015, 49, 34.
18. S. R. White, N. R. Sottos, P. H. Geubelle, J. S. Moore, M. R. Kessler, S. R. Sriram, E. N. Brown and S. Viswanathan, Nature, 2001, 409, 794.
19. A. E. Hughes, I. S. Cole, T. H. Muster and R. J. Varley, NPG Asia Mater., 2010, 2, 143.
20. H. J. Jin, C. L. Mangun, A. S. Griffin, J. S. Moore, N. R. Sottos and S. R. White, Adv. Mater., 2014, 26, 282.
21. C. E. Diesendruck, N. R. Sottos, J. S. Moore and S. R. White, Angew. Chem., Int. Ed., 2015, 54, 10428.
22. L. M. Meng, Y. C. Yuan, M. Z. Rong and M. Q. Zhang, J. Mater. Chem., 2010, 20, 6030.
23. D. S. Xiao, Y. C. Yuan, M. Z. Rong and M. Q. Zhang, Adv. Funct. Mater., 2009, 19, 2289.
24. X. J. Ye, J. L. Zhang, Y. Zhu, M. Z. Rong, M. Q. Zhang, Y. X. Song and H. X. Zhang, ACS Appl. Mater. Interfaces, 2014, 6, 3661.
25. Y. C. Yuan, M. Z. Rong and M. Q. Zhang, Polymer, 2008, 49, 2531.
26. Y. C. Yuan, M. Z. Rong, M. Q. Zhang, J. Chen, G. C. Yang and X. M. Li, Macromolecules, 2008, 41, 5197.
27. T. Yin, M. Z. Rong, M. Q. Zhang and G. C. Yang, Compos. Sci. Technol., 2007, 67, 201.
28. K. R. Hart, N. R. Sottos and S. R. White, Polymer, 2015, 67, 174.
29. D. Y. Zhu, J. W. Guo, G. S. Cao, W. L. Qiu, M. Z. Rong and M. Q. Zhang, J. Mater. Chem. A, 2015, 3, 1858.
30. M. Nosonovsky and P. K. Rohatgiin, Biomimetics in Materials Science, Springer-Verlag, New York, 2012.
31. W. P. Ma, W. Zhang, Y. Zhao, H. L. Yu, S. J. Wang and Y. Wang, Mater. Lett., 2016, 163, 244.
32. E. Tsangouri, D. Aggelis and D. V. Hemelrijck, Prog. Polym. Sci., 2015, 49, 154.

---

Footnote

† Electronic supplementary information (ESI) available: Experimental details, UV-vis spectra of DCF, ¹H NMR spectra of DCF, DETA and DCF precipitates, the solubility of DCF in GMA and EPA solvent, ¹H NMR spectra of GMA, DCF, and DCF in GMA solution, FTIR spectra of epoxy polymer before and after curing at 25 °C for 48 h, swelling and color change of epoxy film with different soaking time, photographs of color change of epoxy matrix after mechanical damage, in situ FTIR spectra and curing degree of 4100HF/EH-4360S (100 : 30 wt) with temperature and GMA/EH-4360S (100 : 30 wt) with temperature and time, SEM micrographs of the healed crack surface, healing process of epoxy sample and healing efficiency. See DOI: 10.1039/c6ra13519f

---