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Polymer

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Understanding and controlling the self-healing behavior of 2-ureido-4[1H]-pyrimidinone-functionalized clustery and dendritic dual dynamicsupramolecular networkJoo Ho Yanga, Junhaeng Leea, Seayoung Lima, Sunghoon Junga, Soon Ho Jangb, So-hyun Janga,Seung-Yeop Kwakc, Seokhoon Ahnd, Yong Chae Jungd, Rodney D. Priestleye, Jae Woo Chunga,f,∗a Department of Information Communication, Materials and Chemistry Convergence Technology, Soongsil University, 369 Sangdo-ro, Dongjak-gu, Seoul 156-743, SouthKoreab Smart Convergence Research Team, Korea Textile Development Institute, 136 Gukchae bosang-ro, Seo-gu, Daegu, 41842, South Koreac Department of Materials Science and Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, South Koread Institute of Advanced Composite Materials, Korea Institute of Science and Technology (KIST), 92 Chudong-ro, Bongdong-eup, Wanju-gun, Jeonbuk, 55324, South Koreae Department of Chemical and Biological Engineering, Princeton University, Princeton, NJ, 08544, United Statesf Department of Organic Materials and Fiber Engineering, Soongsil University, 369 Sangdo-ro, Dongjak-gu, Seoul, 156-743, South Korea

H I G H L I G H T S

• HMP forms a dual dynamic network composed of two different dynamic crosslink modes.

• The healing efficiency of HMP decreases with increasing the HSP portion in HMP.

• The UPy dimer-stacked clustery network acts as an obstacle to the healing of HMP.

A R T I C L E I N F O

Keywords:Dual dynamic network (DDN)Hydrogen bondPoly(ε-caprolactone)Self-healingUreidopyrimidinone (UPy)

A B S T R A C T

To optimize a dynamic network structure is a key feature for a supramolecular self-healing material capable ofperfectly and repeatedly restoring its morphology and performance following a mechanically damaging event.Despite significant progress in self-healing supramolecular networks over the past few decades, many questionssurrounding the dynamic responses of these networks during healing still remain. Herein, we present the self-healing behavior of a dual dynamic supramolecular network (DDN) consisting of a weak clustery supramolecularnetwork and a strong dendritic supramolecular network simultaneously. The DDN is easily prepared by thecomplexation of linear and non-linear poly(ε-caprolactone)s end functionalized with 2-ureido-4[1H]-pyr-imidinone, which exhibits 100%-optical and 95%-mechanical healing efficiencies within 2 and 5min, respec-tively, at 90 °C. In addition, the DDN film is shown to repeatedly self-heal ten times, even when damaged at thesame position. DDN healing depends significantly on the density of the supramolecular network, and is speci-fically accomplished by the reassociation of hydrogen bonds between UPy moieties at the thermally swollenstatus of the dendritic network following the thermal disassembly of the clustery network. The DDN-coatednylon fabric exhibits excellent self-healing characteristics, which opens opportunities for creating scratch-pro-tective materials.

1. Introduction

Self-healing materials have attracted significant attention over thepast few decades due to their abilities to autonomously restore theiroriginal properties following damage [1–4] and their wide rangingapplications in the automotive [5–7], aerospace [8], and construction

industries [9,10]. In addition, self-healing materials find uses in elec-tronic applications that range from e-skin [11,12], to flexible electrodesand transistors [13–17]. In contrast to a micro-encapsulated extrinsichealing system, which can only heal once [18,19], a supramolecularnetwork formed by dynamic bonds, such as hydrogen bonds [20–27], π-π stacking interactions [28,29], host-guest interactions [30,31], metal-

https://doi.org/10.1016/j.polymer.2019.03.027Received 31 December 2018; Received in revised form 9 March 2019; Accepted 14 March 2019

∗ Corresponding author. Department of Organic Materials and Fiber Engineering, Soongsil University, 369 Sangdo-ro, Dongjak-gu, Seoul, 156-743, South Korea.E-mail address: jwchung@ssu.ac.kr (J.W. Chung).

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ligand [32–34], and reversible covalent bonds [35–38], can repetitivelyheal by reversible intermolecular associations through molecular re-cognition [24]. Hence, supramolecular networks are not only able toself-heal cracks in external areas, but also repair invisible internal da-mage in materials at the molecular level, thereby preemptively sup-pressing the growth of breaks, and resulting in durable and sustainablematerials.

Of particular interest is 2-ureido-4[1H]-pyrimidinone (UPy) becauseit endows oligomers or polymers with reversible, self-complementary,quadruple hydrogen bonding with a high association constant(Kdim= 6 × 107M−1 in chloroform at room temperature) through asimple functionalization process [39–42]. This enables oligomers orpolymers to exhibit bulk material properties similar to those of high-molecular-weight covalent polymers or cross-linked elastomeric rub-bers. In particular, because the quadruple hydrogen bonding of UPyreversibly dissociates and associates, many researchers have attemptedto apply UPy in self-healing systems. However, although the UPymoiety is endowed with a high association force that strongly rebinds acracked surface, the high binding force heavily reduces chain mobility,which is kinetically unfavorable for self-healing and reduces the self-healing efficiency [20,24,43,44]. The trade-off relationship betweensupramolecular binding strength and molecular mobility makes it dif-ficult to develop self-healing polymers with concurrent rapid healingand good mechanical properties. To solve this problem, significant ef-fort has been devoted to optimizing the UPy-functionalized supramo-lecular network structure in recent years. Bosman et al. prepared asupramolecular hydrogel composed of poly(ethylene glycol) and UPygroups and showed that the hydrogel healed at room temperature byproviding water molecules that acted as plasticizers [45,46]. Wanget al. reported that a UPy-functionalized supramolecular networkcomposed of polymers with low glass-transition temperatures exhibiteda high self-healing efficiency [47]. Yoshie et al. reported that control ofpolymer crystallization was important for the self-healing of UPy-functionalized poly(ethylene adipate) [48]. Despite this effort, UPy-functionalized supramolecular network healing is not yet fully under-stood, and the structural optimization of UPy-functionalized supramo-lecular networks with simultaneously improved mechanical propertiesand self-healing abilities remains challenging. Specifically, most self-healing studies have focused on technical issues associated with self-healing performance, such as healing temperature, healing rate, andhealing efficiency. From this perspective, understanding the relation-ship between the UPy-functionalized supramolecular structure and itshealing ability, and, in turn, elucidating the self-healing mechanism, isvital.

In this study, we investigated the self-healing behavior of UPy-functionalized supramolecular networks prepared by the complexationof linear and non-linear poly(ε-caprolactone)s functionalized with 2-ureido-4[1H]-pyrimidinone; these had dual dynamic network (DDN)structures that were simultaneously composed of clustery networksformed by the stacking of UPy dimers and dendritic networks formed byquadruple hydrogen bonding between UPy moieties located at thechain ends of linear and non-linear poly(ε-caprolactone)s.

2. Experimental section

Materials. 2-Amino-4-hydroxy-6-methylpyrimidine (125.13 gmol−1),hexamethylene diisocyanate (HDI, 168.2 gmol−1), dibutyltin dilaurate(DBTDL), tin(II) 2-ethylhexanoate, and 1,1,1-tris(hydroxymethyl)propane(TMP, 134.17 gmol−1) were purchased from Sigma Aldrich, Ltd., Korea.ε-Caprolactone (CL, 114.14 gmol−1) and diethylene glycol (DEG,106.12 gmol−1) were purchased from the Sejin Chemical Industry Co.,Korea. TMP was dried under vacuum at 60 °C for 24 h prior to use toremove residual water; all other chemicals were used without furtherpurification.

Synthesis of 2-(6-isocyanatohexylamino)-6-methyl-4[1H]-pyr-imidinone (UPy-NCO). 2-(6-Isocyanatohexylamino)-6-methyl-4[1H]-

pyrimidinone (UPy-NCO) was prepared according to the method re-ported by Meijer et al. [49] Briefly, 2-amino-4-hydroxy-6-methylpyr-imidine (10 g, 79.9 mmol) was added to a 500mL round-bottomedflask, and HDI (100mL, 623mmol) and pyridine (7mL) were added.The mixture was stirred at 100 °C for 16 h under dry nitrogen. Heptane(125mL) was poured into the mixture, and the white precipitate wascollected by filtration. The powder was washed three times withacetone (125mL) to remove unreacted HDI and then dried overnightunder high vacuum at 60 °C.

Synthesis of hydrogen-bonded linear and non-linear star-shaped poly(ε-caprolactone)s (HLP and HSP). The calculated quan-tities of DEG and TMP, as core materials, were separately added toreaction flasks containing CL (20 g, 175mmol) and Sn(Oct)2 (0.0675 g,0.166mmol), to give linear oligo(ε-caprolactone) (LP) and the non-linear star-shaped oligo(ε-caprolactone) (SP), respectively. After reac-tion at 110 °C for 16 h under a flow of nitrogen, the reaction mixturewas cooled to 60 °C and dissolved in 300mL of chloroform. Then, 1.5equivalent ratio of UPy-NCO and 5 drops of DBTDL were added to thesolution. After it was stirred at 60 °C for 16 h under dry nitrogen, 3 g ofsilica and 2 drops of DBTDL were added, and the mixture was furtherreacted for 4 h to eliminate unreacted UPy-NCO moieties. The silica wasfiltered and the filtrate was condensed on a rotary evaporator. Thecondensed filtrate was precipitated in excess cold heptane, to give awhite powder. The precipitate was washed several times with heptaneand dried at room temperature for 24 h in vacuo (yield: 90.4%). TheUPy-functionalized LP and SP are referred to as “hydrogen-bondedlinear poly(ε-caprolactone)” (HLP) and “hydrogen-bonded star-shapedpoly(ε-caprolactone)” (HSP), respectively.

Preparation of the hydrogen-bonded supramolecular networks.Mixtures of HLP and HSP with various weight ratios (10:0, 9:1, 7:3, 5:5,3:7, and 0:10) were used to prepare the hydrogen-bonded supramole-cular networks by solvent casting using chloroform. After solvent eva-poration at room temperature, the films were finely cut into pieces,followed by pressing at 7000 psi for 30min at 95 °C to remove residualsolvent and produce films with uniform thicknesses (Fig. S1, SupportingInformation). The samples are referred to as“ HMPx-y”, where “HMP”indicates an HLP/HSP mixture and “x” and “y” indicate the weightproportions of HLP and HSP, respectively.

Fabrication of self-healing polymer coated nylon fabric. Acommercial PU-coated fabric was prepared, and then HMP7-3 wascoated on the PU-coated fabric using a hot-press under 3500 psi at100 °C for 15 s, resulting in HMP-coated fabric.

Characterization. 1H NMR spectra were recorded at 400MHz inCDCl3 on a Bruker Avance 400 spectrometer. Functional groups werecharacterized by Fourier-transform infrared (FT-IR) spectroscopy usinga Bruker VERTEX 70 spectrometer with a spectral resolution of 2 cm−1

over the 4500–600 cm−1 range using the ATR (attenuated total re-flectance) method. Molecular weights and polydispersity indexes (PDI)of the synthesized materials were determined by gel permeation chro-matography (GPC; Waters 1515–2414) equipped with a refractive index(RI) detector and matrix-assisted laser-desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS; Applied BiosystemsVoyager-DE STR). THF was used as the GPC eluent at a flow rate of1.0 mmmin−1 at 40 °C. A GPC calibration curve was constructed usingmonodispersed polystyrene standards. Dihydrobenzoic acid (DHB)dissolved in tetrahydrofuran (THF) was used as the MALDI-TOF-MSmatrix.

Viscosities were measured using a capillary viscometer (Micro-Ubbelohde; type no. 537 20, SI Analytics) using a Mainz AVS 460 in-strument (solvent: chloroform) at 25 °C in order to verify the formationof supramolecular networks. Differential scanning calorimetry (DSC;Discovery DSC 25, TA Instruments) was performed under a flow ofnitrogen. After preheating to 100 °C, the samples were cooled and thenreheated from −90 to 150 °C at 10 °C min−1. Small-angle X-ray scat-tering (SAXS) experiments were carried out using a Bruker D8 Discoverdiffractometer with a VANTEC-500 detector, using CuKα radiation

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(1.5418 Å). Wide-angle X-ray diffraction (WXRD) data were recordedon a Bruker D8 Advance diffractometer with DAVINCI design, withCuKα radiation.

Tensile testing was carried out with a universal testing machine(UTM; DR UTM 100, Dr-Tech) with a 100 kgf load cell at a strain rate of10mmmin−1 at room temperature according to the ASTM D 638-V 100specification, and the specimens were tested at least five times to ensurereproducibility. Dynamic mechanical analysis (DMA; DMA 8000,Perkin-Elmer) was conducted in tension mode on 6×20×0.8mm3

rectangular strips at a frequency of 1 Hz at temperatures between −80and 100 °C, and at a heating rate of 3 °C min−1 with an oscillatory strainof 5 μm under a nitrogen atmosphere. Activation energies were de-termined from the loss-modulus (E″) peak in multi-frequency mode(frequencies: 0.5, 1, 2, 5, 10, 12, and 15 Hz) while heated from 0 to90 °C at a rate of 1 °C min−1. The following Arrhenius equation de-scribes the relationship between frequency and activation energy

=f A ER T

log log2.303

1a(1)

where f is the testing frequency, R is the universal gas constant, T is thetemperature (K), and A is the frequency factor. The activation energy,Ea, which is a quantitative indicator of the self-healing threshold, wasdetermined from the slope of the linearly fitted relationship betweenlog f and T−1. The rheological behavior of the supramolecular networkswas examined using a TA Instruments Discovery HR-3 rheometer withan environmental test chamber (ETC) under a nitrogen atmosphere.Dynamic isothermal frequency sweeps were performed using 25-mm-diameter parallel geometry plates at temperatures that ranged from 70to 170 °C, in 20 °C intervals, at an angular frequency (ω) that rangedfrom 0.1 to 100 rad s−1. Dynamic temperature sweeps were performedat a frequency of 0.65 rad s−1 while heated at 5 °C min−1 from 70 to200 °C. The oscillation strain regime was determined from strain-am-plitude experiments at selected temperatures.

The self-healing properties were analyzed by scratch recovery ex-periments. The tested films (LWH = 1 cm ×1 cm ×0.8mm), with20–70-μm-wide diagonal cross-shaped notches (crack depth: 0.5 mm),were placed on the heating stage at the desired temperature, and thenotch width was monitored by optical microscopy (OM; OlympusBX51). The width was reported as the average value of five experi-ments. The optically observed self-healing efficiency was determined bycalculating the average crack width from five experiments before andafter healing treatment as follows:

= ×Optically observed healing effeciency W WW

( ) 100optinitial heal

initial (2)

where Winitial is the initial crack width and Wheal is the crack width afterself-healing treatment.

To determine the mechanically tested self-healing efficiency, dog-bone-shaped samples were prepared and notched to a depths of 0.4 mm(2/3 of the dog-bone thickness) with a razor blade. The damagedsamples were then heated at 90 °C for 10min on a hot plate. Tensiletesting was performed on the above-mentioned UTM at 10mmmin−1 atroom temperature, and the mechanically tested self-healing efficiencies,ηtough, were calculated as the proportion of restored toughness relativeto the original toughness by the following efficiency equation

= ×Mechanically tested healing effeciency TT

( ) 100toughheal

initial (3)

where Tinitial and Theal are the toughnesses of the original and notch-healed samples, respectively.

3. Results and discussion

To prepare the hydrogen-bonded linear poly(ε-caprolactone) andthe non-linear star-shaped poly(ε-caprolactone) (i.e., HLP and HSP),linear oligo(ε-caprolactone) (LP) and non-linear star-shaped oligo(ε-

caprolactone) (SP) were first synthesized by the conventional ring-opening polymerization of the CL monomer. The molecular weights ofLP and SP were controlled by manipulating the molar ratio of the CLmonomer to the hydroxyl functionalities of the core materials (Figs.S2–S4, and Table S1 in the Supporting Information). The terminal hy-droxyl groups of LP and SP were then functionalized with UPy moietiesthrough the formation of urethane linkages, resulting in the formationof HLP and HSP, respectively. HLP and HSP exhibited FT-IR absorptionbands at 1663, 1584, and 1523 cm−1 that correspond to the quadruplehydrogen bonds of UPy; they also exhibited carbonyl stretching bandsat 1699 cm−1 that correspond to the newly formed urethane (Fig. S5 inthe Supporting Information). Moreover, the isocyanate-stretching bandof UPy-NCO, observed at 2280 cm−1, was completely absent in thespectra of HLP and HSP. These results confirm the successful in-troduction of the UPy moiety onto LP and SP, and the absence of UPy-NCO. 1H NMR spectroscopy revealed that HLP and HSP have Mn valuesof about 4,000 and 3,500 gmol−1, respectively, and that the ends of theHLP and HSP chains are fully functionalized with hydrogen-bondingUPy moieties (Figs. S6 and S7 in the Supporting Information). In ad-dition, the appearance of three sharp peaks at 10.14, 11.85, and13.12 ppm confirm that self-complementary UPy dimers were success-fully formed through DDAA-aligned (D=donor, A= acceptor) quad-ruple hydrogen bonding in both HLP and HSP, which is in goodagreement with the FT-IR results. The synthetic scheme is presented inFig. 1.

To verify the hydrogen-bonded supramolecular structures of HLP/HSP mixtures, i.e., the HMPs, their viscosities were measured as func-tions of concentration. As shown in Fig. 2, HLP, HSP, and HMP ex-hibited significantly higher specific viscosities (ηsp) than pristine LP orSP over the entire concentration range. In particular, the greater HSPcontent in HMP, the more viscosity increment of HMP at high con-centrations. This indicates the virtual chain extension through strongintermolecular dimerization involving UPy moieties located at the HLPand HSP chain ends [42,50–52]. Moreover, the non-linear chain to-pology of HSP clearly led to an HMP supramolecular gel structure inwhich the HLP and HSP moieties are dendritically networked. Such adendritic supramolecular network could be highly attractive for a stableself-healing system.

The supramolecular structures of HMPs with various ratios oflinear/non-linear components were further investigated by DSC. Asshown in Fig. 3a, the oligomeric LP exhibited two endothermic peaks at48.86 °C and 54.01 °C that correspond to the melting of crystalline PCL,Tm,PCL, whereas HMP10-0, i.e., the pure HLP, exhibited a single PCLmelting peak at 50.23 °C. This demonstrates that the HMP10-0 chainsare more linear and longer through chain extension as a result of hy-drogen-bonding-induced UPy dimerization [53]. Interestingly, in ad-dition to the PCL-crystal melting peak, HMP10-0 also showed anotherendothermic peak at 70.28 °C, which corresponds to the melting ofclustered UPy dimers, Tm,UPy, which is attributed to the thermal dis-ruption of the out-of-plane arrangements of UPy dimers formed bypolar interactions between urethane groups adjacent to UPy dimers,and the thermal disruption of the in-plane arrangements of UPy dimersformed by hydrogen bonding between the cytosine alkene protons andthe pyrimidine carboxylates of neighboring dimers [44,54]. Hence, weconclude that HMP10-0 has a clustery supramolecular network struc-ture induced by UPy dimer-stacked crystals, such as in a thermoplasticelastomer, which is contrary to our expectations that HMP10-0 shouldhave a linearly chain-extended supramolecular structure. The meltingenthalpy (ΔHm,PCL) of the crystalline PCL in HMP was observed to de-crease considerably, from 33.93 J g−1 to 1.59 J g−1, with increasingHSP content; it eventually disappeared in neat HSP, i.e., HMP0-10. Incontrast, the melting enthalpy (ΔHm,UPy) of the UPy clusters in HMPsharply increased, from 0.86 J g−1 to 10.33 J g−1, with increasing HSPcontent (Fig. 3b). Likewise, as the HSP proportion in HMP was in-creased, the Tm,PCL of HMP gradually decreased, from 50.23 °C to42.42 °C, while Tm,UPy gradually increased, from 70.28 °C to 79.63 °C. A

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similar HMP-crystallization trend was observed in the cooling curves(Fig. S8 in the Supporting Information). This change in the crystallinestructure of HMP can be explained in terms of HSP-promoted inter-ference with PCL chain arrangement due to the formation of a dendriticand clustery supramolecular network (i.e., increased physical cross-linking), and the slower crystallization kinetics of PCL chains resultingfrom increased amounts of rigid UPy dimers. Considering that actualportion of UPy moiety present in HMP10-0, HMP7-3, HMP5-5, HMP3-7,and HMP0-10 were 14.7%, 17.8%, 19.9%, 22.0%, and 25.1%, it is re-liable that the rigid UPy dimers interfered with crystallization behaviorof PCL chains in HMP.

The glass-transition temperatures (Tgs) of LP, SP, and the HMPs areshown in Fig. 3c. The Tg of HMP10-0, i.e., neat HLP, was found to be−56.04 °C, which is higher by 4 °C than that of oligomeric LP devoid of

UPy moieties, and is attributed to decreased chain mobility resultingfrom the entanglement of extended supramolecular chains and theformation of the clustered UPy crystal. However, a much lower Tg wasobserved for HMP7-3 than HMP10-0, despite its higher network densitydue to the addition of HSP. This reduction in Tg is presumably due todecreased PCL crystallinity resulting from the formation of a dendriticsupramolecular network as well as a clustery supramolecular networkthrough the addition of HSP. The Tg of HMP increased from −58.69 °Cto −49.15 °C as the HSP content exceeded 30% (i.e., HMP5-5, HMP3-7,and HMP0-10); these HMPs exhibited higher Tg values than HMP10-0,which is attributed to the formation of a highly intense clustery anddendritic supramolecular network, despite the dramatic decrease inPCL crystallinity. The above-mentioned DSC-determined thermal dataare listed in Table 1.

Fig. 1. Synthetic scheme of UPy-NCO (1), linear oligomer (LP, 2a), non-linear oligomer (SP, 2b), linear UPy end-functionalized polymer (HLP, 3a), and non-linearUPy end-functionalized polymer (HSP, 3b).

Fig. 2. Capillary specific viscosity of LP, SP, and HMPs as a function of concentration.

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To obtain more detailed structural insight into the HMP supramo-lecular clustery network, we subjected HMP films to WXRD and SAXSexperiments. As shown in Fig. 3d, the characteristic diffraction peaks at2θ values of 21.3° (110) and 23.6° (200), which correspond to crys-talline PCL, were observed to dramatically decreased in intensity withincreasing HSP content in the HMP. On the other hand, diffractionpeaks at 2θ =8.7° and 17.2°, which were not observed in the spectra ofLP or SP (Fig. S9 in the Supporting Information), were observed in thespectra of all HMP samples; both peaks gradually increased in intensitywith increasing HSP content. The diffraction peak at 2θ =17.2° is at-tributed to the out-of-plane stacking of dimerized UPys, with an inter-planar distance of 0.5 nm [54], The diffraction peak at 2θ =8.7° isrelated to the in-plane zipper-like alignment of dimerized UPys throughweak hydrogen bonds between the keto oxygen and the adjacent aro-matic proton of UPy; this alignment has a repeat distance of 1 nm [55].These results clearly show that HMP has a clustery structure formed bythe out-of-plane and in-plane stacking of UPy dimers. In addition, the

HMP-generated scattering profiles exhibit single scattering maxima atq ≈0.6–0.8 nm−1 (d=∼8–10 nm) (Fig. 3e), which suggests the pre-sence of a nanoscale structure in the HMP (i.e., a UPy cluster) [54,56].The scattering intensity becomes conspicuous and the q value of thescattering maximum shifts toward larger q values (or shorter spacings)with increasing HSP content. These WXRD and SAXS results indicatethat the introduction of HSP in the HMP led to an enhanced clustery-network density, whereas the amount of crystalline PCL gradually de-creased owing to the network structure formed at higher HSP levels.From the combined viscometry, DSC, WXRD, and SAXS results, weconclude that HMP forms a dual dynamic network (DDN) structure si-multaneously composed of two different dynamic reversible cross-linking modes, that is, a quadruple-hydrogen-bonded dendritic supra-molecular network and a clustery supramolecular network induced bythe complexation of linear/non-linearsupramolecular polymers and thestacking of UPy dimers, respectively, as shown in Fig. 4. Such a DDN isexpected to greatly influence the self-healing performance and

Fig. 3. DSC thermograms of (a) LP, SP, and HLP and (b) HMPs during the heating scan. (c) Magnified plot of the region around Tg for LP, SP, and HMPs. (d) WXRDprofiles of HMPs. (e) SAXS profiles of HMPs.

Table 1Thermal properties of LP, SP, and HMPs.

Sample Subsequent heating scan Subsequent cooling scan

°T C[ ]g°T C[ ]m,PCL H [J/g]PCL °T C[ ]m,UPy H [J/g]UPy °T C[ ]c,PCL H [J/g]c,PCL °T C[ ]c,UPy H [J/g]c,UPy

LP −60.16 48.86&54.01 82.54 n.a. n.a. 23.48 77.26 n.a. n.a.SP −61.87 36.75&45.86 69.69 n.a. n.a. 11.57 68.43 n.a. n.a.HMP10-0 −56.04 50.23 33.93 70.28 0.86 17.15 32.47 51.84 3.541HMP7-3 −58.69 44.61 20.05 70.82 2.87 −23.64 13.42 53.67 6.419HMP5-5 −55.40 42.59 2.34 78.78 6.84 n.a. n.a. 60.23 9.742HMP3-7 −54.83 42.42 1.59 78.79 6.83 n.a. n.a. 61.47 8.999HMP0-10 −49.15 n.a. n.a. 79.63 10.33 n.a. n.a. 59.37 9.796UPy-NCO n.a. n.a. n.a. 85.89 2.73 n.a. n.a. 25.3 2.640

n.a.: not applicable.

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mechanical properties of the HMP.The self-healing behavior of HMP under a variety of time and

temperature conditions was examined by optical microscopy. As shownin Fig. 5a, no healing of HMP films with diagonal cross-shaped notcheswas observed at 60 °C even after 2 h of heat treatment. With the ex-ception of HMP10-0, all HMPs exhibited notched surfaces when heat-treated at 70 °C for 60min (Fig. 5b), while HMP10-0 showed a clearlyhealed surface after 5min at 70 °C. At 80 °C, HMP10-0 and HMP7-3were successfully healed after 5 and 10min of heat treatment, respec-tively (Fig. 5c), and at 90 °C, the notches on the surfaces of both sampleswere completely invisible after 1min (Figs. 5d and S10, and Movie 1 inthe Supporting Information). However, HMP5-5 required 10min forcomplete healing at 90 °C, and HMP0-10 was not sufficiently healedeven after 10min at 90 °C. These results show that the self-healing rateslows with increasing levels of HSP in the HMP, or in other words, moreenergy is required to heal HMPs with higher DDN densities.

The optically observed self-healing efficiency (ηopt) was quantita-tively determined using Equation (2). As shown in Fig. 6a, HMP10-0exhibited an ηopt of 57.73% within 1min and was fully repaired after5min of heat treatment at 70 °C. The ηopt of HMP7-3 treated at 70 °Cincreased rapidly within 10min of heat treatment, but increased moreslowly after 10min of treatment, and it failed to reach 100% even after1 h of heat treatment at 70 °C. The same trend was also observed forHMP5-5 treated at 70 °C. Notably, the ηopt of HMP0-10 was only31.82% after 10min of heat treatment, and no further increase inhealing efficiency was observed after 1 h. The ηopt of each HMP washigher at 80 °C than at 70 °C, and HMP10-0 and HMP7-3 were fullyhealed after 10min of heat treatment (Fig. 6b). However, HMP5-5 andHMP0-10 failed to fully heal, even after 20min of heat treatment. At90 °C, HMP10-0 and HMP7-3 showed almost identical healing behavior,and were 100% healed within 1min. HMP5-5 also achieved full scratchrepair after 10min, although the healing rate was slower than that ofHMP10-0 or HMP7-3. However, HMP0-10 still failed to fully self-repairafter 10min (Fig. 6c). The differences in the healing properties of theHMPs are attributable to differences in their network densities. HMP10-0 exhibited the fastest repair rate, suggesting that it should be the mostsuitable self-healing material among the HMP samples. Despite this, thesurface morphology of HMP10-0 was visually poor following heat

treatment, exhibiting severe deterioration, whereas HMP7-3 clearlyshowed a smooth healed surface while maintaining the original shapeof the film (Fig. 6d). These results suggest that HMP10-0 healed mainlythrough filling of the notched surface by the rapid and abrasive flow ofthe matrix. In contrast, HMP7-3, with a DDN structure composed ofclustery and dendritic supramolecular networks, presumably healed bythe reassociation of UPy moieties through the gentle chain diffusion ofthe HMP7-3 matrix, a consequence of a strongly tied dendritic networkcapable of acting as the structural framework. Interestingly, whenHMP7-3 was prepared using a short-chain HLP (Mw=1,500 gmol−1)and a short-chain HSP (Mw=1,600 gmol−1) (Figs. S11 and S12 in theSupporting Information), no sign of healing was observed in the cor-responding HMP7-3 (Fig. S13 in the Supporting Information). This isattributed to the shorter distances between network tie points owing tothe short chain lengths of the HLP and HSP units, which results in theformation of a DDN with considerably higher density and constrainedchain mobility, and hinders self-healing behavior. Hence, we concludethat in addition to the mixing ratio of the linear/non-linear supramo-lecular polymers, the polymer chain length also plays an important roleduring the self-healing of the supramolecular network.

Fig. 6e shows images depicting the repeatable self-healing of HMP7-3. During repeated healing tests, the same position on the film wasrepeatedly notched, and the notched sample was heated at 90 °C for5min each time. As shown in Fig. 6d, the damaged surface of HMP7-3was fully repaired at 90 °C even after 10 scratch–healing cycles. Inaddition, DSC experiments revealed that the thermal properties ofHMP7-3, such as Tg, Tm, and enthalpy, did not change significantlyduring the 10 heating cycles (Fig. S14 in the Supporting Information).Hence, HMP7-3 was shown to repeatedly heal upon heat treatment at90 °C without any significant change to its DDN structure.

The healing efficiency (ηtough) of HMP7-3 was investigated me-chanically by tensile-testing experiments, and calculated usingEquation (3) (Table 2). In order to confirm whether or not the heatapplied during the healing experiment affects the mechanical propertiesof the material, we first tensile tested a sample of HMP7-3 that had beenannealed under the same conditions used in the healing experiment.The results confirm that the mechanical properties of the annealedHMP7-3 were almost identical to those of pristine HMP7-3 (Fig. S15 in

Fig. 4. Schematic images of dendritic supramolecular network and hierarchically clustery supramolecular network in HMPs.

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the Supporting Information), which indicates that the heat treatmentitself does not significantly affect the mechanical properties of HMP7-3.As shown in Fig. 7a, the notch in the HMP7-3 tensile test specimenclearly disappeared after heat treatment at 90 °C for 10min, which is ingood agreement with the results shown in Fig. 5d. This healed HMP7-3sample exhibited a similar stress–strain curve to pristine HMP7-3, andits mechanical healing efficiency (ηtough) was approximately 96%,which is indicative of successful self-healing (Fig. 7b). However, theHMP7-3 sample healed at 80 °C for 10min had an ηtough value that wasconsiderably lower (just ∼16%). Moreover, the ηtough of HMP7-3 wasdramatically lower, at about 33%, when the healing time at 90 °C was

reduced to 1min, even though the notch on the surface of the HMP7-3film visually disappeared (Figs. 5d and 7c). These results indicate thathigher healing energy than that required for visible recovery is requiredto ensure sufficient mechanical recovery of the supramolecular net-work. In the case of HMP0-10, with high dual dynamic network density,a poor ηtough of 12% was observed when heat-treated at 90 °C for 10min(Fig. S16 in the Supporting Information). Therefore, the combined OMand UTM results reveal that the molecular structure of the supramole-cular network, such as crystallinity, molecular weight, and networkdensity, critically affect the self-healing behavior of the network.

The dynamic mechanical and rheological properties of the HMPswere investigated in order to ascertain the relationship between su-pramolecular structure and self-healing performance and, in turn, toelucidate the self-healing mechanism of the clustery and dendritic dualdynamic supramolecular network. Prior to these experiments, thethermal stability of HMP was examined by TGA, which confirmed thatHMP was thermally stable at the temperatures used during the dynamicmechanical and rheological experiments (Fig. S17 in the SupportingInformation). Fig. 8a displays dynamic mechanical analysis (DMA)plots for the various HMPs, which reveals that all of the HMPs exhibittypical polymer complex viscosity (η*) curves, featuring regions thatcorrespond to glass, glass-transition, rubbery-plateau, and terminalflow behavior. The glass transition points of the HMPs were in the−56 °C to −49 °C temperature range, and the rubbery plateauslengthened in the order: HMP10-0 < HMP7-3 < HMP5-5 < HMP0-10, indicating that the DDN density increases with increasing HSPproportion. A dramatic decrease in η* was observed after the rubberyplateau, with terminal flow behavior resulting from the melting of PCLcrystals and UPy clusters. The terminal flow onset temperature wasobserved to be ∼70 °C for HMP10-0, ∼75 °C for HMP7-3, ∼80 °C forHMP5-5, and over ∼90 °C for HMP0-10, which clearly reveals its de-pendence on DDN density. We noted that optically observed self-healing was achieved at 70 °C for HMP10-0, at 80 °C for HMP7-3, and at90 °C for HMP5-5, which strongly suggests that HMP self-healing occursafter the disappearance of the PCL crystals and the UPy clusters thatrestrict the molecular motion of HMP. Fig. 8b displays complex visc-osity data acquired by small-amplitude oscillatory shearing above thetemperature range of the DMA experiments. As can be seen, the η*values for HMP7-3, HMP5-5, and HMP0-10, decrease sharply between70 and 80 °C, which is consistent with the above-mentioned DMA re-sults. In the case of HMP10-0, η* did not decrease within this tem-perature range because its PCL crystals and UPy clusters had alreadymelted below ∼70 °C. Beyond this region, the η* of each HMP pla-teaued somewhat in the 80–120 °C temperature range. Interestingly,these HMPs exhibited 104 times higher η* plateau values compared tothe oligomeric LP or SP that showed Newtonian η* values under 10 Pa sin this temperature region. Moreover, the η* plateau values of the HMPsincreased in the order: HMP10-0 < HMP7-3 < HMP5-5 < HMP0-10, highlighting the dependence of η* on HSP content. Crucially, η* wasobserved to decrease again at temperatures above ∼120 °C for eachHMP, and eventually all HMPs exhibited a second terminal region, witha viscosity of ∼10 Pa s above about 180 °C. Previously, Long et al.reported that the melt viscosity of a UPy-functionalized polymer de-creased to almost the level of the pristine polymer at temperaturesabove 80 °C because of UPy dimer dissociation; i.e., breakage of thecomplementary quadruple hydrogen bonds of UPy [57]. However, ourresults show that: 1) the HMPs have higher viscosity plateaus than thatof the oligomeric LP or SP in the 80–120 °C range; 2) the η* plateauvalue of HMP is dependent on the HSP; and 3) a double decrease in η*with temperature is observed, which suggests that the dramatic de-crease in complex viscosity observed at 70–80 °C originates from thedisassembly of UPy dimer stacked clusters into single UPy dimers, ra-ther than the dissociation of quadruple hydrogen bonds between UPymoieties. This occurs by the thermal disruption of polar interactionsbetween urethane functionalities adjacent to UPy dimers; dissociationof the quadruple hydrogen bonds of UPy gradually occurs above 120 °C,

Fig. 5. Optical micrographs of self-healing test using HMP10-0, HMP7-3,HMP5-5, and HMP0-10 samples at (a) 60 °C, (b) 70 °C, (c) 80 °C and (d) 90 °C atvarious time points.

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which leads to the thermal dismantling of the dendritic supramolecularnetwork formed by this bonding between UPy moieties at the ends ofthe HLP and HSP chains. Therefore, the DDN structures of these HMPsmost likely undergo two step transitions: 1) from the rigid network intothe soft gel by the disassembly of the relatively weak clustery supra-molecular network at 70–80 °C, and 2) from the soft gel into the liquidoligomer through the thermal deconstruction of the strong dendriticsupramolecular network at 120–180 °C.

Fig. 9 displays log G″ vs. log G′ plots for the HMPs at various tem-peratures, but at the same frequency; these plots are generally used toinvestigate structural changes, such as the first heat-induced transitionsfor phase-separated block copolymers and liquid crystalline polymers[58,59]. If the log G″ vs. log G′ plot is independent of temperature, i.e.,if the log G″ vs. log G′ plots acquired at different temperatures are su-perimposable, then the material exhibits no structural change in theexamined temperature range. As shown in Fig. 9, few differences wereobserved between the log G″ vs. log G′ plots for HMP10-0 in the

70–110 °C temperature range, which indicates that no abrupt structuralchange within HMP10-0 occurred at these temperatures, presumablybecause the crystalline PCL chains and UPy clusters had already meltedbelow 70 °C. However, HMP7-3, HMP5-5, and HMP0-10 exhibiteddiscrete log G″ vs. log G′ plots between 70 °C and 90 °C, suggestive ofstructural changes within these HMPs. With the DSC and DMA results inmind, these structural changes are believed to be attributable to thebreakup of the clustery supramolecular network in each system. Inaddition, a discontinuity is observed between 110 °C and 130 °C in thelog G″ vs. log G′ plot for each HMP, and each plot discretely shiftstoward lower G′ and G″ values above 120 °C as the temperature is in-creased. This clearly reveals that abrupt structural changes associatedwith the dissociation of the quadruple hydrogen bonds of the UPymoieties gradually occur in this temperature range, which could resultin the oligomerization of HLP and HSP.

On the basis of the above results, the following self-healing me-chanism for an HMP with a dual dynamic supramolecular network

Fig. 6. Plots of optically observed healing efficiencies of HMP10-0, HMP7-3, HMP5-5 and HMP0-10 at (a) 70 °C, (b) 80 °C and (c) 90 °C. (d) Photo images of HMP10-0and HMP7-3 before and after self-healing (e) 10 repeated cracking and healing cycles of HMP7-3 at 90 °C for 5 min.

Table 2Toughness and healing efficiency of HMP7-3 with temperature and time variation.

Pristine Temperature variation (at 10 min) Time variation (at 90 °C)

90 °C 80 °C 1 min 5 min 10 min

Toughnes [kJ m ]3 55.58±2.19 53.43±3.12 9.23±4.50 18.30±4.89 52.82±13.44 53.43±3.12Healing efficiency [%] – 96.13 16.60 32.93 95.03 96.13

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structure is proposed, as illustrated in Fig. 10. Firstly, below 70 °C, theHMP contains crystalline PCL moieties, as well as simultaneous clusterysupramolecular and dendritic supramolecular networks, which restrictsHMP chain mobility; hence, the HMP exhibits somewhat solid-likeproperties. Healing does not occur below 70 °C because the processrequired to reform the UPy hydrogen bonds that were dissociated bythe external shock is dynamically restricted by the lack of mobility ofthe rigid chains. Secondly, the HMP gains a considerable amount of

chain mobility in the 70–90 °C temperature range, resulting from thethermal disassembly of the clustery supramolecular network and themelting of crystalline PCL. The HMP then transforms into a thermallyswollen gel held together by the dendritic supramolecular networkconnected through strong quadruple hydrogen bonds between UPymoieties at the ends of the polymer chains; consequently, the ability forquadruple hydrogen bonds of UPy in HMP is still alive, and this ma-terial behaves like a soft rubber in the 70–120 °C temperature region.

Fig. 7. Mechanical self-healing test for HMP. (a) Schematic of notch formation and healing (b) stress–strain curves for HMP7-3 with temperature variation (fixedfactor: time, 10min), and (c) time variation (fixed factor: temperature, 90 °C).

Fig. 8. Complex viscosity curves from (a) DMA and (b) rheometry for the HMPs during temperature scan.

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Fig. 9. Log G″ vs. log G′ plots for (a) HMP10-0, (b) HMP7-3, (c) HMP5-5, (d) HMP0-10.

Fig. 10. Schematic image of the self-healing mechanism of HMPs.

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Hence, the HMP experiences a favorable environment where UPymoieties dissociated by the external shock can approach each otherclosely, and self-healing occurs by the reformation of quadruple hy-drogen bonds between dissociated UPy units in this temperature region.Thirdly, the HMP eventually starts to lose its rubbery properties above120 °C and enters into a liquid-like state because the quadruple

hydrogen bonds between UPy moieties begin to dissociate; the supra-molecular PCL oligomerizes at these temperatures, resulting in thecomplete loss of self-healing properties.

Fig. 11a–d displays the frequency-swept G′ plots at various tem-peratures, which reveal that the G′ values of the HMPs decrease withincreasing temperature. In particular, as shown in these plots at 90 °C,

Fig. 11. Logarithmic plots of G′ vs. frequency and healing behavior of HMPs at (a) 70 °C, (b) 90 °C, (c) 130 °C and (d) 150 °C. (e) XRD profiles of HMP7-3 with variedtemperature, and (f) the magnified plot. (g) E″ values for HMP7-3; multiple DMA thermograms for various frequencies (0.5, 1, 2, 5, 10, 12, and 15Hz) are plotted. (h)Arrhenius plot of the transition temperature as a function of frequency for HMPs, yielding an apparent activation energy for self-healing for this transition.

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HMP10-0, HMP7-3, and HMP5-5 exhibit G′ values below ∼105 Pa andwere completely healed. In contrast, HMP0-10 displays G′ values over∼105 Pa and was not healed. These results reveal that a certainthreshold energy is required to initiate the self-healing of HMP. XRDpattern measured at various temperatures showed that the diffractionpeaks corresponding to PCL crystal clearly disappeared at 70 °C and thediffraction peak corresponding to UPy cluster nearly disappeared at90 °C (Fig. 11e and f). This means that UPy clustery network should beeliminated as well as PCL crystal for self-healing initiation, or in otherwords, this showed that the UPy clustery network acted as a thresholdobstacle for self-healing initiation. Indeed, several studies showed thatamorphous supramolecular networks crosslinked by UPy units, espe-cially where clustered UPy crystal was absence, could be healed atlower temperature than 90 °C because they did not have a crystallinestructure that acted as an obstacle for an efficient self-healing, even if alonger healing time may be required [48,60,61]. Hence, the thermalactivation energy (Ea) for the initiation of HMP self-healing was eval-uated using Equation (1), which is based on the frequency dependenceof the temperature of the E″ peak (Figs. 11g and S18 in the SupportingInformation). As shown in Fig. 11h, plotting of the logarithm of fre-quency vs. the reciprocal temperature (T−1) yielded a straight line in allcases, indicating Arrhenius behavior. The calculated Ea values requiredto initiate self-healing were determined to be: 131.36 kJmol−1 forHMP7-3, 140.09 kJmol−1 for HMP5-5, 139.35 kJmol−1 for HMP3-7,and 155.52 kJmol−1 for HMP0-10. These results indicate that thethreshold Ea for the initiation of self-healing increases with increasingclustery network density in HMP; consequently, more energy is re-quired to self-heal a more highly networked HMP.

To further assess the self-healing performance of HMP, we prepared

two types of nylon fabrics, one PU-coated and the other HMP-coated,and compared their self-healing behavior. As shown in Fig. 12, theHMP-coated fabric was completely healed after heat treatment at 90 °Cfor 10min in an oven, whereas the scratch on the PU-coated fabric wasevident even after heat treatment. Interestingly, the scratch on thesurface of the HMP-coated fabric was even healed with a hair dryer.Hence, we anticipate that the self-healing DDN will lead to durable andsustainable consumer products that recover their original appearancesand mechanical properties after suffering damage by external stimuli.

4. Conclusions

In this study, we prepared supramolecular networks composed oflinear and non-linear supramolecular polymers end-functionalized withUPy, and investigated their self-healing behavior. The supramolecularnetwork structure, such as network density and degree of crystallinity,were easily controlled by adjusting the proportions of the linear andnon-linear supramolecular polymers in the mixture. Each supramole-cular network displayed a dual dynamic network (DDN) structureconsisting of a weak clustery supramolecular network and a strongdendritic supramolecular network. The self-healing efficiency de-creased with increasing amounts of the non-linear supramolecularpolymer in the network, due to decreased molecular mobility resultingfrom the formation of an enhanced clustery and dendritic network. Inparticular, we found that the clustery network acted as an obstacle tothe self-healing of the UPy-functionalized supramolecular network.Hence, the UPy-functionalized dual dynamic supramolecular networkswere able to be healed by reforming the UPy quadruple hydrogen bondsonly in the thermally swollen dendritic network state following the

Fig. 12. Self-healing of two types of nylon fabrics at 90 °C for 10min in an oven.

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disassembly of the clustery network. These results provide the foun-dations for creating possible advanced self-healing supramolecularnetworks. We are currently developing a UPy-functionalized supramo-lecular network capable of self-healing at temperatures lower thanthose used in this study, by controlling the clustery network of the non-linear star-shaped supramolecular polymer.

Acknowledgements

This work was supported by the Samsung Research Funding Centerof Samsung Electronics under Project Number SRFC-TB1603-03.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.polymer.2019.03.027.

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