Light-Healable Epoxy Polymer Networks via Anthracene Dimer

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Applications of Polymer, Composite, and Coating Materials

Light-Healable Epoxy Polymer Networks via Anthracene Dimer Scission of Diamine Crosslinker Timothy Hughes, George P. Simon, and Kei Saito ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b02521 • Publication Date (Web): 07 May 2019 Downloaded from http://pubs.acs.org on May 7, 2019

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ACS Applied Materials & Interfaces

Light-Healable Epoxy Polymer Networks via Anthracene Dimer Scission of Diamine Crosslinker Timothy Hughesa, George P. Simonb* and Kei Saitoa* a

School of Chemistry, Monash University, Clayton, VIC 3800, Australia, e-mail:

[email protected]; Fax: +6139905851; Tel: +61399054600. b

Department of Materials Science & Engineering, Monash University, Clayton, VIC 3800,

Australia. E-mail: [email protected]; Fax: +61 399054934; Tel: +61 399054936. Keywords: epoxy, self-healing, coatings, anthracene, cycloaddition, diamine, crosslink, light.

Abstract: Two anthracene-based diamine crosslinkers were used to cure a range of commercially-available monomers to produce four highly photoreversible crosslinked epoxy polymers. Through the careful selection of the epoxy monomers used, the properties of the resultant polymer networks were varied to create a coating material that possessed room temperature light stimulated healing. Of the four coatings created, the best healing performance was exhibited by the two most flexible systems, both of these also exhibited the thermal and mechanical performance necessary for coatings. By using anthracene, the utilisation of a wide range of wavelengths in the healing process is possible, which in applications such as industrial coatings would be of significant benefit.

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Introduction Through the use of self-healing polymers the life-span of a plastic component in application can be increased, and by extension the waste and consumption of finite resources needed to produce the polymers can be lessened. The self-healing of a thermoset occurs in two-stages: material must bridge the gap between the damage surfaces and then solidify to result in closer of the gap. The approaches to these two stages can be broadly categorised into extrinsic or intrinsic.1-3 Extrinsic systems contain a flowable material embedded in the crosslinked structure that can flow to fill the gap resulting from the damage, and upon hardening cause healing. The benefit of this approach is that the healing begins as soon as the damage occurs, which is ideal when the polymer is used for protection of a substrate from the environment such as anti-corrosion applications. However as the polymers bridging the gap do not fully match the crosslinked structure, issues can arise such as poor adhesion between the two polymers, differences in mechanical performance and lack of healing of subsequent damage in the same area.4-10 Intrinsic systems were developed to address these issues.11-13 Intrinsic healing involves the polymer transitioning between a crosslinked structure to a flowable material that can move into the damage site and reharden. Intrinsic healing often requires a stimulus with commonly used stimuli including heat, light or chemical-triggers such as redox or pH-changes.14-22 Recently, the use of light-healable systems has increased as the undisruptive healing conditions are seen as a green approach and allow the use of intrinsic self-healing polymers on a wide range of sensitive substrates. Herein, we present self-healing epoxy crosslinked polymers that utilise UV light to trigger the transition from a rigid crosslinked structure to the flowable state necessary to heal damage and


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then reharden back to a crosslinked structure. The transition is produced by a photoreversible unit in the crosslinker that cleaves and reforms in response to UV light hence breaking and reforming the crosslinks similar to previous work conducted by our group.23,24 The moiety utilised in this case is the anthracene dimer which is positioned in the centre of a diamine structure which can be used as a crosslinker for epoxy monomers. Through the use of epoxy the mechanical strength of the resulting polymer networks will be high, as well as the crosslink density, and thus a great change of properties upon decrosslinking is possible and hence a hard, rigid crosslinked network can be decrosslinked to a soft and flowable material for room temperature photo-healing abilities. The crosslinker approach, which has been successfully utilised by our group,25-27 allows the formation of a wide range of polymer networks with vastly different self-healing, thermal and mechanical performance by the simply thermal curing of commercially-available monomers, in this case epoxies, without the need for complex synthesis often seen when photoresponsive moieties

are

introduced

through

monomer

synthesis.28-32

The

anthracene

[4+4]

photocycloaddition was utilised as the dynamic chemistry due to the high photoreversibility of the process inside polymer networks, needed to ensure regain of the original polymer network properties after healing. Photocycloaddition has been utilised for light-responsive polymers with great success.33-50 However, the utilisation of anthracene is less prevalent, surprisingly so due to its exceptional performance previously demonstrated by works such as Froimowicz et al.51 In contrast to Froimowicz et al.,51 the concentration of anthracene groups in this work is much higher which as alluded to earlier will allow a greater change in polymer properties when cleaving the anthracene dimer allowing for a mechanically robust crosslinked network to transition to a flowable material that ideally can heal scratches.


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In this work, two anthracene-based crosslinkers were synthesised. The first, 9(aminomethyl)anthracene dimer (AMAD), was found to have limited reactivity only exhibiting polymerisation with one of the chosen monomer. Hence a second crosslinker, 9(aminopropoxy)anthracene dimer (APAD), was synthesised to encourage reaction with more of the monomers of interest as well as having the added benefit of increased flexibility. From the two crosslinkers, four polymer networks resulted that exhibited the desired self-healing in response to UV light, whilst also exhibiting the mechanical performance needed for use as coating materials.

Experimental Materials The following chemicals were used as supplied by their respective suppliers. 9-anthraldehyde oxime (Sigma-Aldrich), lithium aliminium hydride (Sigma-Aldrich), anthrone (Sigma-Aldrich), 3-bromopropylamine hydrobromide (Sigma-Aldrich), di-tert-butyl dicarbonate (Sigma-Aldrich), bisphenol A diglycidyl ether 332 (DER332) (Sigma-Aldrich), 1,4-butanediol diglycidyl ether (BDE) (Sigma-Aldrich), poly(ethylene glycol) (600) diglycidyl ether (P6DE) (Polysciences) and poly(ethylene glycol) (1000) diglycidyl ether (P10DE) (Polysciences). Equipment/Characterisation A UVP UV crosslinker (CL-1000) and a custom-built UV reactor equipped with 8W 254 nm or 365 nm lamps (5 or 16, respectively) were used to perform the photoreactions under an air environment. NMR, Mass and IR spectra were recorded on Bruker DRX 400, Agilent 6120 Quadrupole LC/MS and Agilent Cary 630 FTIR spectrometers, respectively. UV-vis spectrometry was performed using a Shimadzu UV-1800 spectrometer. A Tosoh High


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Performance EcoSEC HLC-8320 GPC System, equipped with a guard column (TSKgel Alpha) and two analysis columns (TSKgel Alpha-4000 (400 Å pore size) and TSKgel Alpha-2500 (125 Å pore size)) in series, was used to collect size exclusion chromatograms and the calculations based on polystyrene standard calibration. A Struers Duramin A300 durometer was used for hardness measurements and an Olympus GX51 was used for optical microscope images. Differential scanning calorimetry (DSC) analysis and thermogravimetric analysis (TGA) were performed using a PerkinElmer DSC8000 and Mettler TGA/DSC1 STAR system, respectively, at 10 ᵒC/min unless otherwise stated. Synthesis of AMAD crosslinked polymer Synthesis of anthracen-9-ylmethanamine hydrochloride (AMA.HCl) 52 9-anthraldehyde oxime (2.55 g, 11.53 mmol) was dissolved in dry THF (40 mL) under argon, then the resultant solution added dropwise to a suspension of LiAlH4 (1.25 g, 32.9 mmol) in dry THF (50 mL). The mixture was refluxed under argon for 1.5 hours and then Na2SO4.10H2O was added, whilst cooling in a water bath, until gas no longer evolved. The mixture was filtered, the filtrate dried, the residue dissolved in CHCl3 (80 mL) and 1 M HCl (50 mL) was added. The resultant precipitate was collected and dried to afford a yellow solid (0.81 g, 3.34 mmol, 29%); 1

H NMR (400 MHz, d6-DMSO) δ 5.06 (s, 2H), 7.60 (t, 2H), 7.68 (t, 2H), 8.17 (d, 2H), 8.45 (d,

2H), 8.70 (br s, NH), 8.76 (s, 1H) ppm;

13

C NMR (100 MHz, d6-DMSO) δ 34.61, 124.61,

125.63, 125.95, 127.41, 129.54, 129.63, 130.69, 131.39 ppm; IR (ATR) 3412, 3047, 3025, 2870, 1621, 1595, 1580, 1560, 1496, 1448, 1123, 884, 839, 726 cm-1; MS (ESI) m/z (%) 208 ([M – Cl]+, 15), 191 ([M – HCl + Na]+, 100).


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Synthesis of 5,12:6,11-bis([1,2]benzeno)dibenzo[a,e][8]annulene-5,11(6H,12H)diyldimethanamine dihydrochloride (AMAD.2HCl) 52 AMA.HCl (1.51 g, 6.24 mmol) was dissolved in methanol (24 mL) and the resultant solution irradiated for 20 hours in a UV reactor. The solution was evaporated to dryness and then stirred together with isopropanol (50 mL) for an hour. The solids were collected and dried to afford a white powder (0.92 g, 1.90 mmol, 61%); 1H NMR (400 MHz, CD3OD) δ 4.08 (s, 2H), 4.36 (s, 4H), 6.95 (td, 4H), 7.00 (td, 4H) 7.05 (dd, 4H), 7.10 (dd, 4H) ppm;

13

C NMR (100 MHz,

CD3OD) δ 55.67, 59.48, 123.98, 126.44, 126.53, 128.27, 139.86, 142.66 ppm; IR (ATR) 3298, 2921, 2849, 1654, 1640, 1629, 1598, 1499, 1476, 1452, 1138, 1101, 788, 772, 748 cm-1; MS (ESI) m/z (%) 499 ([M + Na]+, 30), 261 ([M + 2Na]2+, 100). Synthesis of 5,12:6,11-bis([1,2]benzeno)dibenzo[a,e][8]annulene-5,11(6H,12H)diyldimethanamine (AMAD) AMAD.2HCl (0.91 g, 1.88 mmol) was dissolved in water and ethanol (1:1, 120 mL) and to the solution was added 1M KOH solution (4 mL, 4 mmol). The resultant precipitate was collected by filtration and dried to afford AMAD in high purity (0.67 g, 1.62 mmol, 86%). 1H NMR (400 MHz, d6-DMSO) δ 3.69 (s, 2H), 3.86 (s, 4H), 6.76 (t, 4H), 6.82 (t, 4H), 6.90 (d, 4H), 7.08 (d, 4H) ppm; IR (ATR) 3385, 3326, 3066, 3014, 2908, 2864, 1674, 1578, 1471, 1451, 1378, 1319, 1272, 1165, 1130, 1080, 1047, 935, 860, 820, 777, 751, 684, 643 cm-1; MS (ESI) m/z (%) 415.1 ([M+H]+, 100); m.p. = 226.2 ᵒC. Preparation of the AMAD crosslinked polymer (AMAD/DER332) AMAD (18.9 mg, 4.56 µmol) was mixed with DER332 (31.1 mg, 9.12 µmol) to form a paste which was cast onto a glass slide and then heated at 120 ᵒC for 16 hours to form a transparent


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brown film; IR (ATR) 3320, 2959, 2924, 2867, 1654, 1603, 1578, 1507, 1456, 1384, 1362, 1293, 1228, 1178, 1030, 1010, 787 cm-1. Synthesis of APAD crosslinked polymers Synthesis of N-Boc-3-bromopropylamine53 Di-tert-butyl dicarbonate (7.00 g, 32.0 mmol) was dissolved in MeOH:CH3CN (1:1, 34 mL), to which was added triethylamine (9.2 mL, 65.7 mmol) and the solution stirred at room temperature for 15 minutes. 3-bromopropylamine hydrobromide (3.97 g, 18.1 mmol) in MeOH:CH3CN (1:1, 17 mL) was added dropwise to the reaction flask at room temperature whilst stirring vigorously for 2 hours. The solution was then concentrated to dryness by rotary evaporation, the resultant residue dissolved in ethyl acetate (60 mL), transferred to a separation funnel and washed five times with water (20 mL each). The organic phase was collected and evaporated to dryness to afford N-Boc-3-bromopropylamine as a viscous yellow oil (3.87 g, 16.3 mmol, 90%); 1H NMR (400 MHz, CDCl3) δ 1.44 (s, 9H), 2.05 (m, 2H), 3.26 (m, 2H), 3.44 (t, 2H), 4.76 (s(amine), 1H) ppm; 13C NMR (100 MHz, CDCl3) δ 27.6, 28.6, 31.0, 32.9, 39.2, 156.2 ppm; IR (ATR) 3360, 2979, 2942, 2882, 1689, 1247, 1150, 779 cm-1; MS (ESI) m/z (%) 239.04 ([M+H]+, 100), 241.04 ([M+H]+, 80). Synthesis of tert-butyl (3-(anthracen-9-yloxy)propyl)carbamate (Boc-APA) 54, 55 Anthrone (1.75 g, 9.01 mmol) was dissolved in dry THF (14 mL) after which potassium hydroxide (1.33 g, 23.7 mmol) was added and the mixture stirred at room temperature for 15 minutes. To the mixture was added N-Boc-3-bromopropylamine (2.84 g, 12.0 mmol) and the reaction heated to reflux for 20 hours. The reaction mixture was cooled to room temperature, filtered, the filtrate dried and the residue from the filtrate dissolved in chloroform (60 mL). The solution was then washed three times with water (30 mL each) and evaporated to dryness under


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vacuum to afford a yellow oil. Methanol was added to the oil and the resultant mixture filtered and upon evaporation of the filtrate Boc-APA was afforded in high purity (3.01 g, 8.56 mmol, 95%); 1H NMR (400 MHz, CDCl3) δ 1.46 (s, 9H), 2.23 (m, 2H), 3.26 (m, 2H), 3.61 (m, 2H), 5.00 (s(amine), 1H), 7.47 (m, 4H), 8.00 (m, 2H), 8.23 (m, 2H), 8.24 (s, 1H) ppm; 13C NMR (100 MHz, DMSO) δ 28.3, 30.5, 37.0, 73.3, 121.9, 122.0, 124.1, 125.5, 125.7, 126.7, 131.9, 150.7, 155.8 ppm; IR (ATR) 3345, 2973, 2930, 2876, 1688, 1622, 1503, 1442, 1390, 1247, 1159, 1086, 737 cm-1; MS (ESI) m/z (%) 353.19 ([M+H]+, 100), 250.20 ([M-Boc]+, 60). Synthesis

of

di-tert-butyl

((5,12:6,11-bis([1,2]benzeno)dibenzo[a,e][8]annulene-

5,11(6H,12H)-diylbis(oxy))bis(propane-3,1-diyl))dicarbamate (Boc-APAD) Boc-APA (1.00 g, 2.85 mmol) was dissolved in ethyl acetate (18 mL) and irradiated within the stated custom-built UV reactor for 24 hours under 365 nm light. The resultant mixture was filtered and the residue washed with ethyl acetate to afford Boc-APAD in high purity as a white solid (0.68 g, 0.97 mmol, 68%); 1H NMR (400 MHz, CDCl3) δ 1.50 (s, 18H), 1.99 (m, 4H), 3.45 (m, 4H), 3.62 (t, 4H), 4.42 (s, 2H), 5.19 (s(amine), 2H), 6.86 (m, 8H), 6.97 (m, 4H), 7.03 (m, 4H) ppm;

13

C NMR (100 MHz, CDCl3) δ 28.5, 30.2, 63.4, 64.5, 79.2, 89.4, 125.3, 125.6, 126.0,

127.7, 140.8, 141.3, 156.1 ppm; IR (ATR) 3424, 3346, 3071, 2977, 2950, 1717, 1685, 1496, 1365, 1247, 1169, 1057, 1042, 783 cm-1; MS (ESI) m/z (%) 737.34 ([M+Cl]-, 100). Synthesis of 3,3'-(5,12:6,11-bis([1,2]benzeno)dibenzo[a,e][8]annulene-5,11(6H,12H)diylbis(oxy))bis(propan-1-amine) (APAD) Boc-APAD (58.4 mg, 83.2 µmol) was dissolved in DCM (15 mL) to which was added 1.25M HCl in methanol (0.13 mL, 0.16 mmol) whilst cooling in an ice bath. After 18 hours at room temperature the solvent was removed by vacuum and the residue was dissolved in methanol, filtered and the filtrate evaporated to dryness to produce APAD in high purity (31.5 mg, 63.0


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µmol, 75%); 1H NMR (400 MHz, D2O) δ 2.28 (m, 4H), 3.32 (t, 4H), 3.79 (t, 4H), 6.98 (m, 8H), 7.14 (m, 4H), 7.19 (m, 4H) ppm; 13C NMR (100 MHz, CD3OD) δ 28.8, 38.8, 49.9, 63.4, 91.0, 124.2, 126.5, 128.1, 135.5, 142.3 ppm; IR (ATR) 3423, 3071, 2950, 2768, 1676, 1496, 1365, 1164, 1060, 1041, 782 cm-1; MS (ESI) m/z (%) 503.27 ([M+H]+, 100); m.p. = 188.2 ᵒC. Preparation of the APAD crosslinked polymers (APAD/BDE, APAD/P6DE and APAD/P10DE) APAD (5.00 mg, 9.95 µmol) was mixed with BDE (4.00 mg, 19.9 µmol), methanol was added to aid mixing of the components. The uncured mixture was cast onto a glass slide and heated at 120 ᵒC for 16 hours to form a transparent orange film; IR (ATR) 3304, 3078, 1688, 1598, 1450, 1365, 1281, 1166, 1046, 932, 759 cm-1. APAD (5.00 mg, 9.95 µmol) was mixed with P6DE (14.0 mg, 19.9 µmol), methanol was added to aid mixing of the components. The uncured mixture was cast onto a glass slide and heated at 100 ᵒC for 16 hours to form a transparent yellow film; IR (ATR) 3415, 2871, 1674, 1452, 1350, 1282, 1087, 1037, 941, 762 cm-1. APAD (5.00 mg, 9.95 µmol) was mixed with P10DE (21.9 mg, 19.9 µmol), methanol was added to aid mixing of the components. The uncured mixture was cast onto a glass slide and heated at 100 ᵒC for 16 hours to form a transparent slightly yellow film; IR (ATR) 3442, 2870, 1672, 1454, 1350, 1283, 1089, 1038, 938, 760 cm-1. Preparation of crosslinked polymer samples For different analyses, a range of samples of differing specifications were prepared. The UVvis spectroscopy samples were created by spin-coating the uncured mixture on small quartz squares, typically resulting in film thicknesses of between 300 – 500 nm. To eliminate the substrates influence on other measurements, thicker samples were made and used for IR


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spectroscopy and hardness analysis, these were formed by deposition of the uncured mixture solutions with a pipette onto a glass slide. These thicker samples had coating thicknesses in the range of 100 – 500 µm.

Results and Discussion Two anthracene-based crosslinkers, 9-(aminomethyl)anthracene dimer (AMAD) and 9(aminopropoxy)anthracene dimer (APAD) were synthesised as Figure 1 and 2. From the two crosslinkers, four polymer networks were prepared (Figure 3). One of the polymer networks was from the AMAD crosslinker with the epoxy monomer DER332, and the other three were the APAD crosslinker reacted with the epoxy monomers BDE, P6DE and P10DE giving polymer networks with a wide range of properties. The key thermal properties of the polymer networks and the confirmation of their intended behaviour under UV light were determined by several analytical techniques.

Figure 1: Synthesis scheme of the first anthracene-based diamine crosslinker, AMAD.


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Figure 2: Synthesis scheme of the second anthracene-based diamine crosslinker, APAD.

Figure 3: Generic reaction scheme for the curing of the various epoxy monomers with the respective commercially-available epoxy monomers to form the crosslinked epoxy polymers AMAD/DER332, APAD/BDE, APAD/P6DE and APAD/P10DE.


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Thermal property analysis The curing process was first investigated to ensure that the crosslinker was performing as intended. The curing was confirmed by reacting the uncured mixtures of crosslinker and monomer in a DSC. For all systems, a curing curve was observed and are displayed in Figure S1. The curing curve for APAD/P6DE and APAD/P10DE appeared less clear than the others due to the event being overlapped by an endothermic peak from the melting of the monomers. The DSC curve of these two monomers alone displayed an endothermic peak which is shown in Figure S2. The curing temperatures were 99 ᵒC, 105 ᵒC, 16 ᵒC and 38 ᵒC for AMAD/DER332, APAD/BDE, APAD/P6DE and APAD/P10DE, respectively and hence all further crosslinked polymer samples were cured at least 15 ᵒC higher than these values. The curing process was also monitored by monitoring of the IR spectra of the uncured mixtures and the potentially reacted polymer networks over time (Figure 4 & S3), which involved monitoring the complete disappearance of the signal ca. 910 cm-1 corresponding to the epoxy groups of the monomers.56 For all the systems it can be seen that the epoxy signals completely disappear, a hydroxyl signal appears and the C-O signals at around 1100 cm-1 change significantly, all of which indicate opening of epoxy rings. Monitoring of other groups, such as the amine signals, was not possible due to the overlapping of the signals. After the curing was confirmed, the polymer networks were then tested by TGA to determine their decomposition temperature. Three of the four polymer networks were seen to exhibit similar decomposition temperatures (Figure S4), with the more aromatic AMAD/DER332 having a greater thermal stability. This greater decomposition temperature, of 298 ᵒC, is likely due to the aromatic structure of the monomer resulting in the polymer network being less susceptible to oxidation or free radical degradation, with the high C-O-C bond content of the


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other epoxies causing a lower thermal stability.57 The slight differences in aromatic and C-O-C content of the other polymer networks show the same effect, where the more aromatic crosslinked polymer, APAD/BDE, has a higher decomposition temperature due to the higher crosslink density that results from the shorter epoxy chain. Although the DSC and IR monitoring demonstrated the reaction is occurring a further test is needed to ensure the curing has continued to a high degree. This determination was made by conducting a post-curing stage. This involved subjecting the cured polymer networks to a temperature 20 ᵒC higher than their original curing temperature and monitoring any changes in the IR spectrum. All the crosslinked polymers exhibited no changes to key signals from the original cured spectrum to that after the post curing stage. With the results from this test coupled with the DSC results above, the curing conditions selected above were deemed sufficient to completely cure the crosslinked polymers. Once the parameters needed for complete cure of the polymer networks were determined, samples of each crosslinked system were analysed by DSC to determine the glass transition temperature (Tg) which would provide information on the potential applicability of each polymer network in coating applications. The values from the DSC analysis are displayed in Table 1 and Figure S3, it can be seen that the highly aromatic crosslinked polymer AMAD/DER332 has the highest Tg of 155 ᵒC, which is far in excess of that required for a coating material. However the values of APAD/BDE and APAD/P6DE are very useful being close to that of PMMA (105 ᵒC). This is due to the lower aromatic content of the epoxy monomers leads to a much more flexible crosslinked structure compared to AMAD/DER332. The Tg of APAD/P10DE is much lower than the ideal value for an automotive coating but may be useful for a range of other applications.58, 59


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Table 1: Glass transition temperatures (Tg) of the four polymer networks when in the crosslinked state and the decrosslinked state, after 254 nm irradiation, as measured by DSC using 10 ᵒC/min heating rate. AMAD/DER332

APAD/BDE

APAD/P6DE

APAD/P10DE

Crosslinked

155 ᵒC

126 ᵒC

100 ᵒC

53 ᵒC

Decrosslinked

85 ᵒC

31 ᵒC

22 ᵒC

-3 ᵒC

80

80

60 40 uncured 3h 6h cured (16h)

20 950

0 1400

900

1200

1000

Transmittance (%)

100

Transmittance (%)

100

60 40 20 950

0 1400

800

100

80

80

60 40 uncured 3h 6h cured (16h)

20 950

0 1400

900

1200 1000 -1 Wavenumber (cm )

800

Transmittance (%)

100

Transmittance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 42

900


uncured 3h 6h cured (16h) 800

60 40 20 950

0 1400

900


uncured 3h 6h cured (16h) 800

Figure 4: IR spectroscopy monitoring of the curing process over time by observation of the reduction of the epoxy group signal for the 4 crosslinked polymer systems: AMAD/DER332 (top left), APAD/BDE (top right), APAD/P6DE (bottom left) and APAD/P10DE (bottom right). The polymers were cured at 120 ᵒC, 120 ᵒC, 100 ᵒC and 100 ᵒC, respectively. Decrosslinking and recrosslinking of the polymer networks To ensure the occurrence of desired opening and closing of the anthracene dimer within the epoxy polymer networks induced by different wavelengths of UV light (Figure 5), a series of studies were performed.


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Figure 5: Reversible photocycloaddition process of the anthracene dimer inside the crosslinker causing decrosslinking and recrosslinking of the network by 254 nm and 365 nm UV light, respectively. The first involved subjecting a sample of each polymer network to irradiation of 254 nm UV light, with the structure of the polymer network monitored by IR. From the comparison of the spectra of the two states (Figure 6) it can be seen that in all polymer networks the anthracene dimers are cleaved by the 254 nm irradiation and the anthracene structure is regained resulting in the decrosslinked structure. The most clearly identifiable signals from these spectra that indicate this transition is the loss of the signal at ca. 1675 cm-1 corresponding to the benzene groups of the anthracene dimer and the appearance of the signals at ca. 1655 cm-1 and ca. 1525 cm-1 characteristic of the anthracene aromatics. Also very evident are the signals corresponding to the ‘out-of-plane’ C-H vibrations of the anthracene unit. The anthracene dimer signal at ca. 765 cm-1 shifts to ca. 740 cm-1 and two additional signals appear at ca. 880 cm-1 and ca. 840 cm-1. For the polymer networks produced from the APAD crosslinker there is also a change to the two C-O bond signals of the crosslinker. Initially these appear around ca. 1050 cm-1 with the C-O of the epoxy chain. However after opening of the anthracene dimer these bonds become very close to an aromatic structure which results in a shift upward; the signal for C-O with the carbon now part of the aromatic structure shifts drastically to ca. 1345 cm-1, whereas the other shifts slightly up to ca. 1100 cm-1 due to the oxygen shielding the effect of the aromatic on the C-O bond on the opposite side of the oxygen.60


15


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Figure 6: IR monitoring of the decrosslinking process induced by 254 nm UV light for the 4 crosslinked polymers AMAD/DER332 (top left), APAD/BDE (top right), APAD/P6DE (bottom left) and APAD/P10DE (bottom right). UV light doses of 6.85 J/cm2, 2.74 J/cm2, 0.70 J/cm2 and 0.70 J/cm2, respectively. To monitor the entire photoreversible process, UV-vis spectroscopy was used to observe the distinct characteristic signals of anthracene between 320 – 420 nm (Figure 7).61 Samples of the polymer networks were prepared by spin coating and curing on a small quartz square which, through use of a cuvette-shaped holder, allows analysis of the crosslinked polymers. The samples were irradiated with 254 nm UV light to cause anthracene dimer scission and the changes in the UV spectra monitored. After the changes ceased, decrosslinking was said to be complete and the samples then irradiated with 365 nm UV light to reform the anthracene dimers and thus recrosslink the material, again monitoring the changes in the spectra. In the case of the AMAD/DER332 system, there were difficulties in the sample preparation due to the poor solubility of the crosslinker making the spin coating difficult. However the UV spectra can still


16

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give a good indication of the behaviour of the crosslinker, even if it is possibly not as accurate as the analysis for the APAD-based polymer networks. For all the polymer networks the characteristic peaks of the anthracene increased in response to the 254 nm irradiation, indicating the scission of the dimers and the formation of the anthracene structure. After the decrosslinking was completed, the sample were subjected to 365 nm UV light irradiation and the anthracene signals were seen to decrease, and in all cases almost completely disappear, suggesting a very high degree of recrosslinking. By comparing the absorption intensity of the anthracene peaks after decrosslinking and recrosslinking to the original value before irradiation, a recrosslinking percentage can been calculated. This value is the approximate percentage of anthracene dimers that are reformed by the 365 nm UV light after being cleaved by the 254 nm UV light. The recrosslinking percentages are 88%, 91%, 94% and 99%, respectively, for AMAD/DER332, APAD/BDE, APAD/P6DE and APAD/P10DE. This high recrosslinking percentage suggests the anthracene groups of the opened dimers are in close proximity and in the correct orientation necessary for photodimerisation. This local registration of the anthracene groups is likely due to the strong π-π stacking holding the anthracene groups in position after the anthracene dimers are opened by the 254 nm UV light, resulting in a high number of the opened dimers being able to redimerise upon 365 nm UV light irradiation. The UV-vis study was conducted for several more cycles and the percentage of the anthracene dimers cleaved is displayed in Figure S5. It can be seen from the extended study that the photoreversibility efficiency decreases slightly over more cycles, however the recrosslinking percentage does not decrease below 80% across the 7 cycles meaning the original crosslinked structure can be reformed to a high degree.


17


1.6

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Page 18 of 42

0 J/cm

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Figure 7: UV-vis spectroscopy monitoring of the key anthracene signals for the (a) decrosslinking and (b) recrosslinking processes within the polymer networks induced by 254 nm and 365 nm UV light, respectively, for the 4 crosslinked polymer systems: AMAD/DER332, APAD/BDE, APAD/P6DE and APAD/P10DE (from top to bottom respectively).


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A solubility study was conducted as a further means of following the decrosslinking process. This involved submerging glass mounted samples in DMF and exposing the submerged samples to intervals of 254 nm irradiation doses whilst monitoring the mass lost from the samples. As the 254 nm irradiation decrosslinks the structure, the sample becomes soluble and hence enters solution. After set intervals the solution is removed, the remaining sample dried and weighed and fresh solvent added for the next irradiation interval. This was continued until the sample completely dissolves which further indicates that decrosslinking is occurring and to a high enough degree to render the polymer networks soluble. These polymer solutions are then combined, dried and tested by GPC, DSC and 1H NMR to characterise the decrosslinked state of the polymer networks. The GPC of the crosslinkers and monomers used were also run for comparison purposes (Figure S6). The graph of the mass loss from the crosslinked polymer samples with increasing irradiation dose can be seen in Figure 8. As expected the mass of soluble polymer increases with the irradiation dose, suggesting the material is becoming uncrosslinked and dissolving in solution. Another key observation is that the soluble portion of polymer before irradiation is quite low which is also as expected as a crosslinked polymer is not soluble. However as there is an inherent error in the measuring of the components for curing and due to unreacted or partially-reacted material being confined in regions where reactants are spatially separated, small oligomeric material which was not perfectly incorporated into the crosslinked structure will remain soluble. The GPC chromatograms (Figure 8) of the dissolved polymer solutions all display similar qualities: there are some distinct peaks after 16 minutes that can be attributed to small amounts of the systems monomer and crosslinker, and for times less than 16 minutes, broad irregular peaks exist that correspond to high molecular weight material are observed. The presence of this high molecular weight material confirms that the original


19


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crosslinked polymer has been decrosslinked by the 254 nm irradiation resulting in polymer fragments being liberated from the crosslinked structure and entering solution. As the broad nature of the peaks suggests, the fragmentation is random and results in a large variation of the molecular weight of the resultant fragments. The APAD/P10DE system appears to fragment to a lower molecular weight material than the others as the highest peak, and hence highest concentration of material, centres around 15 minutes retention time whereas the others peaks reside around 14 minutes. This could suggest that the decrosslinked material from the APAD polymers may possess a lower decrosslinked Tg and hence be prone to flow which is advantageous in the self-healing process. The DSC analysis of these dried polymer solution samples gives the Tg of the decrosslinked state which garners greater understanding of the decrosslinked material and the potential for room temperature photo-healing. This is because, for room temperature healing to occur, the decrosslinked material must be able to flow, the potential for which would be indicated by a Tg lower than room temperature. As shown in Table 1 and Figure S7 there is a wide range of decrosslinked Tg from the different polymers, the Tg of the very rigid polymer AMAD/DER332 has decreased substantially from the crosslinked value but remains much higher than room temperature suggesting heating will be necessary to induce the flow of this decrosslinked material for the self-healing process. APAD/BDE and APAD/P6DE have much lower values but may still be too high for healing without the aid of heat. However APAD/P10DE has a decrosslinked Tg sufficiently low that there is potential for room temperature healing as the decrosslinked material should readily flow. From these results the effect of the monomer structure is evident; the polymer with the most rigid monomer DER332 still exhibits a very high Tg when decrosslinked due to the lack of flexibility in both the monomer portion and crosslinker


20

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coupled with the high aromatic content. The value for APAD/BDE is lower than expected due to the high crosslinked Tg being within 30 ᵒC of the AMAD/DER332 system. This can be explained by the greater flexibility of the BDE and APAD relative to DER332 and AMAD, as once the material is decrosslinked the flexible and highly rotational bond content allows for a material that is much less resistant to flow which is exhibited by the greater decrease in Tg of APAD/BDE, being 95 ᵒC compared to the 70 ᵒC decrease exhibited by AMAD/DER332. These observations are also seen for the APAD/P6DE and APAD/P10DE polymers. However the decrease in Tg is less, as the long chains leads to a greater spacing between the crosslinks therefore a softer material with lower Tg, with a less drastic effect on the material when the crosslinks are broken. The NMR spectra of the GPC residue from the solubility study of AMAD/DER332 along with the NMR spectra of the epoxy monomer DER332 and AMA.HCl for comparison, is displayed in Figure 9. From the decrosslinked polymer spectra there are several signals that can be seen to correspond to a structure that is slightly shifted from the epoxy monomer, DER332, spectra. The two aromatic signals around 7 ppm and the central methyl group signal at 1.6 ppm are still present. Most notable is the loss of the signal at 4.3 ppm due to the breaking of the epoxy ring during curing causing the out-of-plane hydrogens adjacent to the epoxy ring to become one signal exhibited at 3.8 ppm. The remaining aromatic signals can be attributed to the anthracene moiety and appear at 9, 8.3, 8 and 7.5 ppm. The position and number of anthracene aromatic signals, in comparison to the AMA.HCl spectra, indicates the anthracene dimer has been cleaved as intended. Overall the NMR spectra of the decrosslinked polymer suggest that the intended mechanism is responsible for the conversion of the material from a crosslinked to a soluble polymer, as the presence of key epoxy signals and the position of the anthracene signals indicate


21


no other unwanted degradation of the polymer has occurred. For further comparison, the NMR of AMAD can be seen in Figure S9.

80

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AMAD/DER332 APAD/BDE APAD/P6DE APAD/P10DE

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Figure 8: Monitoring of the mass lost from the polymer networks during irradiation with 254 nm UV light for the solubility study of the 4 polymer systems: AMAD/DER332 (top left), APAD/BDE (top right), APAD/P6DE (middle left) and APAD/P10DE (middle right). The GPC chromatograms (bottom) of the dried solutions from this solubility study before (left) and after (right) irradiation with UV light were also collected. UV light doses of 20.6 J/cm2, 16.4 J/cm2, 12.3 J/cm2 and 12.3 J/cm2, respectively.


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Figure 9: 1H NMR spectra of the soluble polymer chains from the solubility test (middle) of AMAD/DER332, the epoxy monomer DER332 (bottom) and AMA.HCl (top), all in d6-DMSO.


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Mechanical property analysis The hardness of the polymer networks were determined via the use of a durometer to better understand the potential applications of the polymers and to gain an insight into the changes in mechanical performance that occur during the irradiation stages. Thick samples of the crosslinked polymers were made for this purpose to ensure the measurements had no potential for influence from a substrate. The hardness of the polymers were recorded at three stages; the initial crosslinked sample before irradiation, after a dose of 254 nm irradiation to induce decrosslinking and finally after a dose of 365 nm irradiation to recrosslink the material. The irradiation doses used for decrosslinking were those which were found to be sufficient to produce complete decrosslinking, according to the IR photoreversibility studies conducted earlier, and for recrosslinking the doses were three times those found to give the maximum amount of recrosslinking according to the UV photoreversibility studies conducted earlier. As can be seen in Table 2 the AMADDER332 polymer network, as expected from the rigid structure, exhibits a very high hardness value of 49.8 HV which is well in excess of that required of a coating material as the commercial automotive coating material PMMA exhibits a hardness of between 20 – 25 HV.62, 63 APAD/BDE also is quite hard, however APAD/P6DE and APAD/P10DE have more comparable values. All of the polymers exhibit softening in response to the 254 nm irradiation further indicating decrosslinking as the AMAD/DER332 does not soften as significantly as the other polymers, which is consistent with the decrosslinked Tg measurements gathered previously. It is important to note is that even in this decrosslinked state, all the polymers retain a hardness great enough to still function as a coating which suggests that the healing process would not compromise the functionality or performance of the materials. Upon recrosslinking with 365 nm UV light irradiation, all of the polymers percentage regain a high


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percentage of the original hardness, which matches with the results from the previously performed photoreversibility study. The photoreversibility study suggested that almost all the cleaved dimers are reformed and hence the material is almost back to the original structure and thus would exhibit similar mechanical performance, in this case hardness. Table 2: Vickers hardness (HV) measurements for the different states of the polymer networks. AMAD/DER332

APAD/BDE

APAD/P6DE

APAD/P10DE

Crosslinked

49.8 ± 0.245

38.4 ± 0.408

28.2 ± 0.327

12.3 ± 0.776

Decrosslinked

42.4 ± 0.490

29.4 ± 1.06

20.3 ± 0.735

7.1 ± 0.816

Recrosslinked

49.4 ± 0.163

37.2 ± 0.245

26.8 ± 0.653

11.9 ± 0.531

Self-healing of crosslinked epoxy polymer Glass-mounted samples of the polymer networks were used for the self-healing tests. Scratches were made on the samples surfaces using a razor blade. Optical microscopy (OM) was utilised to monitor the scratch dimensions and scratch healing (Figure 10 and 11). The healing treatment was the irradiation of the samples with 254 nm UV light in fixed time intervals to determine the minimum irradiation dose necessary to heal the scratches, and where necessary simultaneous heating was used to induce flow in the decrosslinked polymers to facilitate healing. A sample of AMAD/DER332 was subjected to a 25 µm wide scratch and irradiated with 254 nm UV light to induce healing. The scratch was observed to heal after a 13.0 J/cm2 dose of 254 nm irradiation while being heated to 125 ᵒC. This high temperature was necessary to induce healing to overcome the poor mobility of the decrosslinked polymer, as suggested by the high decrosslinked Tg. Tested in the same way, APAD/BDE was observed to heal a 20 µm scratch with a 13.7 J/cm2 irradiation dose while heating to 60 ᵒC, APAD/P6DE was observed to heal a 19 µm scratch with 10.3 J/cm2 of irradiation at 40 ᵒC and APAD/P10DE healed a 16 µm scratch


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with a 8.2 J/cm2 irradiation dose without any heating necessary at room temperature. It can be seen that the temperature required to induce healing while decrosslinking correlates to the decrosslinked Tg. APAD/P10DE exhibits room temperature healing due to the decrosslinked Tg being much lower than room temperature, suggesting the decrosslinked material would readily flow at room temperature. As a result of the observations, from the three systems that require heat to heal, a relation between healing temperature required and decrosslinked Tg can be established. Due to limited data points, the relationship is assumed to be linear and is constructed for the purpose of establishing an estimate of the maximum decrosslinked Tg that a polymer could exhibit whilst possessing the ability to heal at room temperature. The graph with the line of best fit is shown in Figure 12 and by assuming the room temperature to be 30 ᵒC, due to the mild heating that would occur from irradiation, the estimated maximum decrosslinked Tg is ca. 11 ᵒC. This suggests that a polymer more rigid than APAD/P10DE could be created that could still be capable of healing at room temperature provided that the decrosslinked Tg were to remain below ca. 11 ᵒC. From the structure of the monomers used with APAD, another correlation can be derived which allows the estimation of the minimum chain length of a PDE-type monomer, that when used to form a polymer with APAD would result in the required decrosslinked Tg which was estimated to allow room temperature healing. To ensure healing was not due to the heating alone, a heat control experiment was conducted for each crosslinked polymer that required heating to heal (Figure S8). For this test a damaged sample was produced in the same fashion as above and subjected to the same temperatures for the same amount of time as required for healing, but without UV light irradiation. For AMAD/DER332, there was a slight reduction in the scratch width but did not lead to the levels


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of healing seen when in combination with UV light irradiation, for all the other samples no noticeable variation in scratch dimensions were observed confirming that the healing was due to the UV light induced decrosslinking of the polymers. As the self-healing polymer networks created herein are based on the intrinsic approach, they should be capable of performing the main benefits of this approach, repeatable healing. The APAD/P10DE sample, which was previously healed, was irradiated with 11.0 J/cm2 of 365 nm UV light to recrosslink the material and then is subjected to another scratch perpendicular to the intial scratch. The recrosslinking irradiation dose was three times the dose shown to give maximum recrosslinking during the UV photoreversibility study above. This second scratch (Figure 11) was seen to be of 28 µm width and found to heal after a 12.3 J/cm2 dose of 254 nm UV light at room temperature. This healing performance was similar to that of the initial scratch, with the higher irradiation dose being attributed to the greater width of the second scratch. This demonstrates the ability of these polymer networks to repeatedly heal damage in the same area and hence possess the inherent ability of the intrinsic design.


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Figure 10: Images of the UV light induced healing of scratches on the surface of the crosslinked polymer systems: AMAD/DER332 at 125 ᵒC (top), APAD/BDE at 60 ᵒC (middle) and APAD/P6DE at 40 ᵒC (bottom) before (left) and after (right) the healing treatment, collected by OM. UV light irradiation doses of 13.0 J/cm2, 13.7 J/cm2 and 10.3 J/cm2, respectively.


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Figure 11: Images of the healing of the first scratch (top) on the surface of polymer network APAD/P10DE and the healing of a subsequent scratch (bottom) on the same sample, in the same region, after a second healing treatment. The scratch widths were 16 µm and 28 µm, respectively, and the healing irradiation doses were 8.2 and 12.3 J/cm2, respectively.


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80 60 linear fit y = 1.2984x + 15.271 R² = 0.9911

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Figure 12: Plot of the healing temperature vs. the decrosslinked Tg (left) constructed from the observations of AMAD/DER332, APAD/BDE and APAD/P6DE and the decrosslinked Tg vs. monomer molecular weight (right) constructed from the observations of APAD/BDE, APAD/P6DE and APAD/P10DE.

Conclusions Several photo-responsive polymer networks were synthesised by the creation of two anthracene-based diamine crosslinkers. These crosslinkers were used to cure four commerciallyavailable epoxy monomers, resulting in the formation of four light-healable crosslinked epoxy polymers. The curing and coating properties of the polymer networks were tested by DSC and hardness tests and showed a wide range of potential coating applications for the different crosslinked polymers. The healing effect was due to the photoreversible cleavage of the anthracene dimer in the centre of the crosslinker, resulting in a transition from a rigid material to a soft, flowable material and then back to rigid once the damage site was adequately filled. The occurrence of this mechanism within the polymer networks was confirmed, and the healing effect observed and explained, by several analytical tests.


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Three of the polymers were found to heal with the aid of mild to intense heating and the fourth exhibited healing at room temperature. From the comparison of these systems it was found that the decrosslinked Tg was the determining factor for the temperature needed for healing and hence a correlation to healing temperature could be established, which helps inform the choice of monomer for further self-healing polymer investigation. The healing ability of these polymer networks whilst possessing high hardness and glass transition temperature values suggests that these crosslinked polymers could be used as commercial coatings that possess the ability to self-heal. Further work is being conducted to determine the extent to which this concept can be implemented to make polymer networks with potentially greater performance and with further possible applications.

Supporting information The Supporting information contains 11 figures. Curing curve of the respective unreacted crosslinker and epoxy monomer mixtures for the 4 polymers; DSC curve of the epoxy monomers displaying melting point peaks as an endothermic event; Comparison of the uncured and cured IR spectrum for the 4 crosslinked

polymers;

TGA

of

the

crosslinked

polymers;

Extended

UV-vis

photoreversibility study for the crosslinked polymers through 7 irradiation stages; GPC analysis of the synthesised crosslinkers and the commercially-available epoxy monomers; Glass transition temperatures displayed on the DSC curves of the crosslinked polymers; OM images of scratches on the surface of the three polymers subjected to heating without irradiation for the heat control;

1

H NMR spectra of the synthesised anthracene

crosslinkers; Enlarged version of the zoomed in section of the IR monitoring of the


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decrosslinking process induced by 254 nm UV light for the 4 crosslinked polymers; Images of a scratched sample of polymer from the cross-section view.

Acknowledgements The authors acknowledge the Chemicals and Plastics Manufacturing Innovation Network (C&PMIN) and training program (GRIP) curated by Monash University and supported by Chemistry Australia, the Victorian Government and 3M Australia. K. S. would like to thank the PRESTO, JST (JPMJPR1515), for financial support.

Declaration of Interests The authors declare no competing interests.

References 1. Wu, D. Y.; Meure, S.; Solomon, D. Self-Healing Polymeric Materials: A Review of Recent Developments Prog. Polym. Sci. 2008, 33, 479-522. 2. Y. Yang, Y.; Urban, M. W. Self-Healing Polymeric Materials Chem. Soc. Rev. 2013, 42, 7446-7467. 3. Guimard, N. K.; Oehlenschlaeger, K. K.; Zhou, J.; Hilf, S.; Schmidt, F. G.; Barner‐ Kowollik, C. Current Trends in the Field of Self‐Healing Materials Macromol. Chem. Phys., 2012, 213, 131-143.


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4. Dry, C. Procedures Developed for Self-Repair of Polymer Matrix Composite Materials Compos. Struct. 1996, 35, 263-269. 5. White, S. R.; Sottos, N.; Geubelle, P.; Moore, J.; Kessler, M. R.; Sriram, S.; Brown, E.; Viswanathan, S. Autonomic Healing of Polymer Composites Nature 2001, 409, 794-797. 6. Pang, J. W. C.; Bond, I. P. A Hollow Fibre Reinforced Polymer Composite Encompassing Self-Healing and Enhanced Damage Visibility Compos. Sci. Technol. 2005, 65, 1791-1799. 7. Yin, T.; Rong, M. Z.; Zhang, M. Q.; Yang, G. C. Self-Healing Epoxy Composites – Preparation and Effect of the Healant Consisting of Microencapsulated Epoxy and Latent Curing Agent Compos. Sci. Technol. 2007, 67, 201-212. 8. Yuan, Y. C.; Rong, M. Z.; Zhang, M. Q.; Chen, J.; Yang, G. C.; Li, X. M. Self-Healing Polymeric Materials using Epoxy/Mercaptan as the Healant Macromolecules 2008, 41, 5197-5202. 9. Patrick, J. F.; Hart, K. R.; Krull, B. P.; Diesendruck, C. E.; Moore, J. S.; White, S. R.; Sottos, N. R. Continuous Self-Healing Life Cycle in Vascularized Structural Composites Adv. Mater. 2014, 26, 4302-4308. 10. Zhu, D. Y.; Cao, G. S.; Qiu, W. L.; Rong, M. Z.; Zhang, M. Q. Self-Healing Polyvinyl Chloride (PVC) Based on Microencapsulated Nucleophilic Thiol-Click Chemistry Polymer 2015, 69, 1-9. 11. Fiore, G. L.; Rowan, S. J.; Weder, C. Optically Healable Polymers Chem. Soc. Rev. 2013, 42, 7278-7288.


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Table of content entry

254 nm 365 nm

scratch

254 nm

0.1 mm


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Graphical abstract 79x38mm (150 x 150 DPI)


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Light-Healable Epoxy Polymer Networks via Anthracene Dimer

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