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European Polymer Journal

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Arm-length-dependent phase transformation and dual dynamic healing behavior of supramolecular networks consisting of ureidopyrimidinone-end- functionalized semi-crystalline star polymers Woojin Leea, Seung-Yeop Kwaka,⁎, Jae Woo Chungb,c,⁎

a Department of Materials Science and Engineering, and Research Institute of Advanced Materials (RIAM), Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea b Department of Organic Materials and Fiber Engineering, Soongsil University, 369 Sangdo-ro, Dongjak-gu, Seoul 06978, Republic of Korea c Department of Information Communication, Materials, and Chemistry Convergence Technology, Soongsil University, 369 Sangdo-ro, Dongjak-gu, Seoul 06978, Republic of Korea

A R T I C L E I N F O

Keywords: Crystalline physical bond Dual dynamic supramolecular network Phase transformation Self-healing Ureidopyrimidinone

A B S T R A C T

A rigid crystalline structure suppresses chain mobility and diffusion of self-healing supramolecular networks, lowering their healing capabilities. Herein it is presented that the incorporation of crystals in self-healing su-pramolecular networks as an effective way to overcome the inherent lack of mechanical strength of self-healing supramolecular networks, instead of a healing barrier. We prepare dual dynamic supramolecular networks composed of two types of dynamic physical bonds, i.e., quadruple hydrogen bonds and crystalline physical bonds, using the ureidopyrimidinone (UPy)-end-functionalized semi-crystalline star-shaped poly(ε-caprolactone) s (USPs). With increasing arm-length of the USPs, the phase of the network changes from UPy-stacked crystals to an amorphous phase and further to chain-folding polymeric crystals. The healing capabilities are also enhanced with increasing the arm-length of the USPs. Such changes in phase and healing capability are strongly associated with the crosslinking density of the network. In addition, the appropriate arm-length balance of USPs can provide a mechanically rigid semi-crystalline supramolecular network with a highly efficient healing property due to their reversible dual dynamic features, associated with the re-association of UPy quadruple hydrogen bonds and restoration of crystalline physical bonds during healing.

1. Introduction

Since the pioneering works of White and Leibler [1,2], the study of self-healing materials has been of great scientific interest owing to their unique properties and wide range of applicability in energy harvesting [3], healthcare [4,5], construction [6], automotive and aerospace in-dustries [7–9]. Additionally, self-healing materials find use in electro-nics and biotechnology, e.g., in electrodes [10], transistors [11–14], and e-skin [15,16]. Such a self-healing technology is expected to im-prove the reliability, sustainability and prolong the service lives of di-verse products [17].

In contrast to a micro-encapsulated extrinsic healing system, which allows healing of materials only a limited number of times [18–21], a self-healing supramolecular network, composed of reversible dynamic bonds, such as hydrogen bonds [22], ionic interactions [23], metal-

ligand coordination [24], and reversible covalent bonds [25,26], can be repeatedly healed by the reversible association of such dynamic bonds [27–30]. In addition, the latter has the advantages of simple production [31] and enables initial suppression of damage propagation at a mo-lecular level [32]. Among the various dynamic bonds, hydrogen bonding is commonly used for self-healing supramolecular network formation because of the facile adjustment of bonding strength and directionality [33–35]. Particularly, ureidopyrimidinone (UPy) is one of the most promising hydrogen-bonding candidates for self-healing su-pramolecular materials because it forms a strong dimer via quadruple hydrogen bonds with reversible association/dissociation behavior. Fa-ghihnejad et al. prepared a supramolecular network composed of amorphous poly(butyl acrylate) main chains with low glass transition temperature (Tg) and UPy pendent groups, and showed that the net-works were ~80% healed at room temperature within 3 h [36]. Wie

https://doi.org/10.1016/j.eurpolymj.2020.109976 Received 18 May 2020; Received in revised form 20 July 2020; Accepted 19 August 2020

⁎ Corresponding authors at: Department of Organic Materials and Fiber Engineering, Soongsil University, 369 Sangdo-ro, Dongjak-gu, Seoul 06978, Republic of Korea (J.W. Chung). Department of Materials Science and Engineering, and Research Institute of Advanced Materials (RIAM), Seoul National University, 1 Gwanak- ro, Gwanak-gu, Seoul 08826, Republic of Korea (S.-Y. Kwak).

E-mail addresses: sykwak@snu.ac.kr (S.-Y. Kwak), jwchung@ssu.ac.kr (J.W. Chung).

European Polymer Journal 138 (2020) 109976

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et al. reported that a UPy-functionalized supramolecular network composed of 2 types of polymers with low Tg showed high self-healing efficiency; the sample was 88% healed within 48 h at 40 °C [37]. Folmer et al. also reported that UPy-modified amorphous poly(ethy-lene-co-butylene) with low Tg displayed excellent restoration of its mechanical properties after healing [38].

As mentioned above, most UPy-functionalized self-healing supra-molecular networks with great healing efficiencies have amorphous structures with low Tg, at least below room temperature, as well as bond association/dissociation ability of UPy. This is because the low-Tg

amorphous phase increases the probability of re-association of UPy hydrogen bonds dissociated by a mechanical crack, i.e., healing cap-ability, due to their high chain mobility and diffusability [39–41]. However, such networks are extremely soft and most exhibit very low mechanical properties below the level of a few MPa [42–44], thereby restricting their applicability in various industries and environments. To overcome this issue, the incorporation of a rigid crystalline phase in the supramolecular networks to reinforce its mechanical strength could be an effective strategy. Nevertheless, studies on self-healing semi- crystalline supramolecular networks have not attracted much attention thus far, because the rigid crystalline phase in supramolecular networks can act as a healing barrier [45,46]. In addition, a detailed under-standing of the relationship between the semi-crystalline supramole-cular network structure and its healing property is lacking. Hence, a novel self-healing semi-crystalline supramolecular network with ex-cellent mechanical properties and facile self-healing performance must be developed and the relationship between the supramolecular semi- crystalline structure and its self-healing behavior must be elucidated to facilitate the design of an advanced self-healing material.

Herein, we prepared a dual dynamic supramolecular network composed of two types of dynamic physical bonds, i.e., quadruple hy-drogen bonds and crystalline physical bonds, using the ureidopyr-imidinone (UPy)-end-functionalized semi-crystalline star-shaped poly (ε-caprolactone)s (USPs) and investigated the relationship between their supramolecular structure and self-healing behavior. These semi- crystalline networks showed phase-transformation with changing arm- length of USPs, and their healing performances were also considerably affected by the arm-length of USPs. In addition, an appropriate arm- length can enable the production of mechanically rigid semi-crystalline supramolecular networks with highly efficient healing capabilities.

2. Experimental section

2.1. Materials

2-Amino-4-hydroxyl-6-methyl pyrimidine (125.13 g/mol), hexam-ethylene diisocyanate, dipentaerythritol, tin(II) 2-ethylhexanoate, and dibutyltin dilaurate were purchased from Sigma-Aldrich Ltd., Korea. ε- Caprolactone was purchased from Tokyo Chemical Industry Co., Ltd., Japan. Toluene, heptane, and pyridine were sourced from Daejung Chemical & Materials Co., Ltd., Korea. Chloroform (99.5%) was sup-plied by Samchun Pure Chem. Ltd., Korea. All chemicals were used as received without further purification.

2.2. Synthesis of ureidopyrimidinone (UPy)-functionalized star polymer (USP series)

To synthesize the ureidopyrimidinone (UPy)-functionalized star poly(ε-caprolactone)s (USPs), we first synthesized the hydroxyl end- functional six-arm star poly(ε-caprolactone)s (SPs) via ring-opening bulk polymerization (ROP) of ε-caprolactone (ε-CL) and then in-troduced the UPy moiety to the hydroxyl end of SPs. The typical pro-cedure used for the synthesis of USPs was as follows: ε-CL (20 g, 175 mmol) were added to a 100 mL three-necked flask, and different calculated quantities of dipentaerythritol (DPTOL) (3.713 g, 1.485 g, 0.743 g, 0.571 g, and 0,464 g, which are 0.083, 0.033, 0.017, 0.013,

and 0.011 equiv. with respect to ε-CL) were further added to give SPs with different arm-length. The target SP arm-length were individually 2, 5, 10, 13, and 16 ε-CL monomeric units, i.e., 2, 5, 10, 13, and 16 of degree of polymerization of the arm (DParm). The mixture was pre-heated to 150 °C with vigorous stirring to form a homogeneous phase. After cooling to 110 °C, a catalytic amount of Sn(Oct)2 was added to the flask and the ROP was performed for 18 h under N2 atmosphere. The resulting bulk SP (3 g) and 6-fold excess mol of isocyanate-functional ureidopyrimidinone (UPy-NCO) that was synthesized as reported by Meijer et al. [38] were dissolved in 150 mL of chloroform and the so-lution was heated to 65 °C. After adding 5 drops of dibutyltin dilaurate (DBTDL), the reaction was continued for 16 h. To remove the unreacted UPy-NCO, silica-gel and DBTDL were added to the mixture and the reaction was carried out for an additional 4 h at 65 °C. The mixture was filtered to remove the silica-gel and the filtrate was concentrated using a rotary evaporator. The concentrated filtrate was poured into 700 mL of cold methanol, resulting in a white powder as precipitates. The precipitates were washed thrice with methanol and dried at room temperature for 24 h in vacuo.

1H NMR (600 MHz, CDCl3, δ) of UPy-NCO (Fig. S2): 13.08(s, 1H, CH3CNH), 11.83(s, 1H, CH2NH(C]O)NH), 10.16(s, 1H, CH2NH(C]O) NH), 5.78(s, 1H, CH]CCH3), 3.26 (m, 4H, NH(C]O) NHCH2+CH2NCO), 2.19(s, 3H, CH3C]CH), 1.58(m, 4H, NCH2CH2), 1.39(m, 4H, CH2CH2CH2CH2CH2).

1H NMR (600 MHz, CDCl3, δ) of representative SPs (SP16, Fig. S3): 4.02 (t, polyester main chain eCH2eCOOeCH2CH2CH2CH2CH2), 3.60 (t, eCOOeCH2CH2CH2CH2CH2eOH), 3.34 (t, DPTOL core eOCH2e), 2.31 (t, eCH2eCOOeCH2CH2CH2CH2CH2, eCOOCH2), 1.58e1.63 (m, eCH2eCOOeCH2CH2CH2CH2CH2), 1.35–1.43 (m, eCH2e

COOeCH2CH2CH2CH2CH2). 1H NMR peak analysis results of SP2, SP5, SP10, and SP13 were shown in the supporting information (Fig. S3).

1H NMR (600 MHz, CDCl3, δ) of USP2 (Fig. 1b): 13.13 (s, 1H, CH3CNH), 11.86 (s, 1H, CH2NH(C]O)NH), 10.13 (s, 1H, CH2NH(C]

O)NH), 5.85 (s, 1H, CH]CCH3), 4.75 (s, 1H, eCH2eCOOeNH-CH2e), 3.60 (t, eCOOeCH2CH2CH2CH2CH2eOH), 3.34 (t, DPTOL core eOCH2e), 3.23 and 3.12 (s, 4H, eCOONH-CH2CH2CH2CH2CH2CH2- NHCONHe), 2.31 (t, eCH2eCOOeCH2CH2CH2CH2CH2, eCOOCH2), 1.58–1.63 (m, eCH2eCOOeCH2CH2CH2CH2CH2), 1.35–1.43 (m, eCH2eCOOeCH2CH2CH2CH2CH2).

1H NMR (600 MHz, CDCl3, δ) of USP5 (Fig. 1b): 13.12 (s, 1H, CH3CNH), 11.86 (s, 1H, CH2NH(C]O)NH), 10.12 (s, 1H, CH2NH(C]

O)NH), 5.86 (s, 1H, CH]CCH3), 4.74 (s, 1H, eCH2eCOOeNH-CH2e), 3.60 (t, eCOOeCH2CH2CH2CH2CH2eOH), 3.33 (t, DPTOL core eOCH2e), 3.22 and 3.14 (s, 4H, eCOONH-CH2CH2CH2CH2CH2CH2- NHCONHe), 2.30 (t, eCH2eCOOeCH2CH2CH2CH2CH2, eCOOCH2), 1.57–1.65 (m, eCH2eCOOeCH2CH2CH2CH2CH2), 1.32–1.43 (m, eCH2eCOOeCH2CH2CH2CH2CH2).

1H NMR (600 MHz, CDCl3, δ) of USP10 (Fig. 1b): 13.13 (s, 1H, CH3CNH), 11.85 (s, 1H, CH2NH(C]O)NH), 10.14 (s, 1H, CH2NH(C]

O)NH), 5.85 (s, 1H, CH]CCH3), 4.72 (s, 1H, eCH2eCOOeNH-CH2e), 3.58 (t, eCOOeCH2CH2CH2CH2CH2eOH), 3.32 (t, DPTOL core eOCH2e), 3.25 and 3.17 (s, 4H, eCOONH-CH2CH2CH2CH2CH2CH2- NHCONHe), 2.31 (t, eCH2eCOOeCH2CH2CH2CH2CH2, eCOOCH2), 1.56–1.62 (m, eCH2eCOOeCH2CH2CH2CH2CH2), 1.33–1.40 (m, eCH2eCOOeCH2CH2CH2CH2CH2).

1H NMR (600 MHz, CDCl3, δ) of USP13 (Fig. 1b): 13.11 (s, 1H, CH3CNH), 11.84 (s, 1H, CH2NH(C]O)NH), 10.13 (s, 1H, CH2NH(C]

O)NH), 5.83 (s, 1H, CH]CCH3), 4.74 (s, 1H, eCH2eCOOeNH-CH2e), 3.59 (t, eCOOeCH2CH2CH2CH2CH2eOH), 3.32 (t, DPTOL core eOCH2e), 3.21 and 3.13 (s, 4H, eCOONH-CH2CH2CH2CH2CH2CH2- NHCONHe), 2.31 (t, eCH2eCOOeCH2CH2CH2CH2CH2, eCOOCH2), 1.57–1.63 (m, eCH2eCOOeCH2CH2CH2CH2CH2), 1.36–1.43 (m, eCH2eCOOeCH2CH2CH2CH2CH2).

1H NMR (600 MHz, CDCl3, δ) of USP16 (Fig. 1b): 13.13 (s, 1H, CH3CNH), 11.86 (s, 1H, CH2NH(C]O)NH), 10.13 (s, 1H, CH2NH(C]

O)NH), 5.85 (s, 1H, CH]CCH3), 4.75 (s, 1H, eCH2eCOOeNH-CH2e),

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3.60 (t, eCOOeCH2CH2CH2CH2CH2eOH), 3.34 (t, DPTOL core eOCH2-), 3.25 and 3.15 (s, 4H, eCOONH-CH2CH2CH2CH2CH2CH2- NHCONHe), 2.31 (t, eCH2eCOOeCH2CH2CH2CH2CH2, eCOOCH2), 1.58–1.63 (m, eCH2eCOOeCH2CH2CH2CH2CH2), 1.35–1.43 (m, eCH2eCOOeCH2CH2CH2CH2CH2).

2.3. General characterizations

1H NMR (nuclear magnetic resonance) were recorded at 600 MHz in chloroform-d on a Bruker Avance 600 spectrometer. Size exclusion chromatography (SEC) was carried out at 35 °C in THF and DMF solvent (flow rate = 1.0 mL min−1) on an Ultimate 3000 equipped with a UV/ VIS diode array detector and three KF, KD, and GF columns. Number- average molecular weight (Mn) and molecular weight distribution (Mw/ Mn) were calculated by calibrating with linear polystyrene standards in THF for SPs and linear PMMA standards in DMF for USPs. Fourier transform infrared (FTIR) spectra were obtained for samples pelletized with KBr powder over the range from 4000 to 600 cm−1 using a

Thermo Scientific Nicolet iS10 IR spectrophotometer (Thermo-Fisher Scientific, Waltham, MA, USA). Differential scanning calorimetry (DSC; Discovery DSC 25, TA Instruments) measurements were performed under a nitrogen flow over the temperature range of −90 to 150 °C at a heating rate of 10 °C min−1. Wide-angle X-ray diffraction (WXRD; New D8 Advance, Bruker) patterns were collected at room temperature using Cu Kα radiation (λ = 1.541 Å), at a voltage of 40 kV, over the 2θ range of 5 − 40° with scan rate of 2° min−1. Small angle X-ray scattering (SAXS) was conducted using the D8 Discover (Bruker) system. The wavelength of the radiation source was 1.5418 Å and the sample-to detector distance was 34.5 cm. The patterns were collected at room temperature over the 2θ range of 0.2-9°. Dynamic mechanical analysis (DMA) was conducted on a Perkin-Elmer instrument DMA 8000 in tension mode at a frequency of 1 Hz range from −80 to 80 °C with the heating rate of 3 °C min−1 with oscillatory strain of 5 μm under a ni-trogen atmosphere. Rheological analysis was performed on a TA in-strument DHR-2 hybrid rheometer using 25 mm parallel-plate alu-minum geometry with an environmental test chamber (ETC) under nitrogen atmosphere. Frequency sweep tests were conducted with an-gular frequency (ω) ranging from 0.1 to 100 rad·s−1. The oscillation strain region of all experimental samples was selected from strain am-plitude experiments at each temperature.

2.4. Self-healing tests

The synthesized USPs (3.0 g) were dissolved in 45 mL chloroform. The solutions were poured in a glass petri dish, and dried in an oven at 25 °C for 24 h, resulting in clear USP films. The USPs films were ad-ditionally hot-pressed under 4500 psi at 100 °C to ensure that the healing test specimens had the same dimensions (5 mm × 5 mm × 0.8 mm). The specimens with 0.4-mm-long diagonal cracks were placed in a convection oven at the desired temperature and their crack-healing behaviors were monitored using an optical micro-scope (Upright Metallurgical Microscope BX53MTRF, Olympus). To quantitatively monitor the healing performances of USPs, the tensile tests of USPs were carried out using a universal testing machine (UTM; DR UTM 100, Dr-Tech) with a 100-kgf load cell at a strain rate of 2 mm min−1 at room temperature. The tensile strength test specimens were notched to a depth of 0.4 mm with a razor blade, and the damaged specimens were heated at 80 °C, 70 °C, 60 °C, and 50 °C for 20 min in a convection oven. Then, the % values were calculated in terms of the recovery of healed elongation at break relative to the virgin elongation at break using the following equation:

= ×Mechanical healing effeciency ( ) 100healed

virgin% (1)

where εvirgin and εhealed are the elongations at break of the virgin and healed samples, respectively. To confirm the repeatable healing per-formance of USP, the healed USPs were subjected to stress relaxation measurement under 0.5 mm of strain, and the applied load (N) values between the pristine and the healed USP specimens were compared as per the following equation:

= ×Repetitive self healing efficiency NN

( ) 100Rhealed

pristine,% (2)

During the repetitive healing test, the same position on the USP film was notched each time and the notch was repeatedly healed at 60 °C for 10 min.

3. Results and discussion

To prepare the USPs, SPs were first synthesized by ring-opening polymerization (ROP) of the ε-caprolactone (ε-CL) monomer, initiating from the hydroxyl moieties of the dipentaerythritol (DPTOL) core. The arm-lengths of the SPs were controlled by varying the feed molar ratios of the ε-CL monomer to the DPTOL core. The UPy moieties were then

Fig. 1. (a) FT-IR and (b) 1H NMR spectra of the synthesized USPs.

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functionalized by reacting the terminal hydroxyl groups of SPs with the isocyanate groups of UPy-NCO (Scheme 1).

Fig. 1a shows that the FT-IR stretching peak at 2280 cm−1, corre-sponding to the isocyanate stretching of UPy-NCO, disappeared com-pletely and the N-H stretching peaks appeared at ~3200 cm−1 for all the USPs. Moreover, the USPs showed stretching bands at 1667, 1546 and 1529 cm−1, corresponding to the quadruple hydrogen bonds of UPy moieties, without the hydroxyl stretching bands at 3500–3600 cm−1 observed for SPs (Fig. S1) [45]. The 1H NMR spectra (Fig. 1b) of USPs showed three sharp peaks at 10.14, 11.86, and 13.13 ppm and a broad peak at 4.75 ppm, associated with the quad-ruple hydrogen bonds of UPy moieties and the urethane linkages, re-spectively, as well as the typically observed poly(ε-caprolactone) peaks. In addition, the integral ratio between the characteristic methylene groups (peak e, 3.13 and 3.23 ppm, 2H) adjacent to the UPy moiety and the methylene group (peak f, 3.34 ppm, 2H) of the DBTDL core showed that the USPs had 100% UPy end-functionality, indicating that the hydroxyl end groups of the SPs were fully converted into UPy with quadruple hydrogen bonding groups.

The average arm-length, i.e., the degree of polymerization of arm (DParm), and the number-average molar mass (Mn,NMR) of the USPs were determined from the ratio of the proton integrals of the main chain methylene groups (peak 4 in Fig. 1b) and those of the DPTOL core methylene groups (peak f in Fig. 1b). As listed in Table 1, the measured DParm values of USP2, USP5, USP10, USP13, and USP16 were 2.3, 5.1, 9.6, 13.1, and 15.9, respectively, being in good agreement with the target DParm values. The calculated Mn,NMR values based on the DParm

values for USP2–USP16 were in the ranged from ~3,600 to 13,000 g mol−1, and the SEC analysis (Fig. S4) also revealed similar Mn

values in the range from 4,800 to 14,880 g mol−1. Furthermore, the USPs had a narrow polydispersity index (PDI) range, between 1.08 and 1.25. Thus, it was concluded that a precisely controlled synthesis of USPs with different arm-lengths was achieved.

The above-mentioned FT-IR and 1H NMR results revealed that UPy moieties positioned at the arm-end of the USPs were strongly dimerized

with one another complementarily via quadruple hydrogen bonding, forming supramolecular networks of the USPs. Hence, we investigated the supramolecular structures of USPs with different arm-lengths using differential scanning calorimetry (DSC). As shown in Fig. 2a, SP13 and 16 showed distinct single endothermal peaks at ~47 °C, corresponding to typical poly(ε-caprolactone) (PCL) crystalline melting. Although SP2, 5, and 10 showed lower melting temperatures and broader double- melting peaks than SP13 and 16, which is often the case for PCL with low molar mass [47], they also displayed an endothermic peak attri-butable to the melting of the crystalline PCL chain, indicating that all the SPs had semi-crystalline structures owing to PCL chain-folding. On the other hand, following the introduction of the UPy moiety into the chain ends of SPs, the melting peaks of the PCL crystals disappeared for all the USPs, except for USP16 that had the longest arm-length. Wide- angle X-ray diffraction (WXRD) profiles, which is widely used to de-termine the degree of crystallinity corresponding to the regularity of polymeric chains, of USP2, 5, 10, and 13 showed very broad halos, while USP16 exhibited typical PCL chain crystalline diffraction peaks at 21.3° (110) and 23.5° (200) (Fig. 2c). This implied that the crystal-lization of the PCL chain for USP was suppressed by the introduction of UPy, except for USP16 with the longest arm-length. Rather, USP2, 5,

Scheme 1. Synthesis of UPy-NCO, six-arm star-shaped poly(ε-caprolactone)s (SPs), and UPy-end functionalized six-arm star-shaped poly(ε-caprolactone)s (USPs).

Table 1 Molecular weights of UPy-end functionalized star-shaped poly(ε-caprolactone)s (USPs).

Sample a DParm Mn,NMR of Arm (g/mol)

Mn,NMR

(g/mol) Mn,GPC

(g/mol) Mw/Mn

USP2 2.3 262 3600 4800 1.08 USP5 5.1 584 5580 6840 1.12 USP10 9.6 1095 8580 9990 1.17 USP13 13.1 1499 11,000 12,440 1.22 USP16 15.9 1822 13,000 14,880 1.23

a USPx indicates the average number of the CL monomer units incorporated in the USPs.

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and 10 showed small new endothermic peaks at 83 °C, 73 °C, and 59 °C, respectively, corresponding to the melting of UPy crystals (Tm,UPy) formed by the stacking of UPy dimers [48–50]. The endothermic peaks of UPy crystals became more pronounced and their melting tempera-tures increased considerably with decreasing arm-length of the USPs. Small-angle X-ray scattering (SAXS) measurements, which can be a useful tool for confirming the micro-phase separation regarding su-pramolecular agglomerates, also showed the same results. USP2, 5, and 10 exhibited the primary scattering peak induced by the presence of stacked UPy crystals in USPs (Fig. 2d) [51]. The primary scattering

maxima (q*) in SAXS shifted toward higher values with decreasing arm- lengths of the USPs, suggesting that the average domain spacing (D) between the UPy crystals decreased and stacking of UPy crystals in-creased with decreasing arm-length of USPs. However, USP13 and 16 showed no discernible primary scattering peaks, indicating the absence of UPy crystals (Table 2). Unlike the endothermic peaks of the PCL and UPy crystals that were selectively observed with varying arm-lengths of USPs, broad endothermic peaks in the range 100–125 °C were found for all the USPs. It is well known that the UPy moieties can be dimerized via quadruple hydrogen bonds that typically start breaking in the

Fig. 2. DSC thermograms of the (a) SPs and (b) USPs during the heating scan. (c) WXRD patterns and (d) SAXS profiles of USPs at room temperature. (e) Tg,USP - Tg,SP

values as a function of arm-lengths of USPs. (f) DSC heating curves of quenched USP2 relative to pristine USP2.

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temperature range 110–130 °C [45,50]. Thus, the endothermic peaks in the range 100–125 °C for all the USPs probably resulted from the thermal dissociation of UPy dimers via hydrogen bond cleavage. In other words, this result revealed that all the USPs had supramolecular network structures formed via UPy dimerization regardless of their arm-lengths, when the temperature was below the thermal dissociation temperature of the UPy dimers, as mentioned above. Therefore, the combined results of 1H NMR, DSC, WXRD, and SAXS confirmed that the USPs had phase-transformable supramolecular network structures de-pending on their arm-lengths. When USPs with short arm-length, i.e., USP2, 5, and 10, the stacked-UPy crystals were induced by urea polar interactions, which acted as the thermo-reversible physical crosslinking points, and they exhibited the stacked-UPy crystal dominant Dual Dy-namic Network phase composed of two dynamic components, i.e., 1) stacked-UPy crystals and 2) UPy quadruple hydrogen bonding. With increasing the arm-length of USPs, the network structure of USPs changed to the Mono Dynamic Network phase (USP13), which were solely composed of thermo-responsive UPy quadruple hydrogen bonding. For the USP16 with the longest arm-length, they form not only UPy quadruple hydrogen bonding, but also thermo-switchable physi-cally crosslinked points composed of PCL chain-folded crystalline phase resulted from folding a sufficiently long PCL polymeric chain. Thus, they again displayed the PCL chain crystalline dominant Dual Dynamic Network phase comprised of two different dynamic components, which were 1) PCL chain-folded crystalline induced by ester polar interactions and 2) UPy quadruple hydrogen bonding. It is thought that these thermo-responsive dynamic components, i.e., UPy quadruple hydrogen bonding, stacked-UPy crystals, and PCL chain-folded crystals, pre-sumably induce the thermo-reversible association/dissociation beha-vior of USPs. The thus-determined supramolecular network structures of USPs with various arm-lengths are schematically illustrated in Fig. 3.

The difference in the Tg values of USPs and SPs (Tg,USP − Tg,SP) as a function of the arm-length of the USPs is presented in Fig. 2e. As shown in Fig. 2e, all of the (Tg,USP − Tg,SP) values were positive because the USPs had higher Tg values than the SPs regardless of the arm-length. Moreover, we found that the differences in Tg,USP and Tg,SP increased drastically with decreasing arm-length of USPs, whereas the SPs showed Tg at about −60 °C, typically observed for PCL. USP2 showed a 58.5 °C increase in Tg relative to SP2, while the Tg of USP16 was only 5.9 °C higher than that of SP16. To understand why Tg increased con-siderably with decreasing arm-length of USPs, we compared the heating thermograms of relatively slow-cooled and quenched USP2. As shown in Fig. 2f, the quenched USP2 presented no endothermic peak corre-sponding to the melting of the stacked UPy crystals, while USP2 cooled at the rate of 10 °C min−1 clearly showed a UPy crystalline melting peak. Nevertheless, the Tg values of both USP2 specimens were almost the same (~0 °C). This suggests that the stacked UPy crystals did not affect the segmental motion of the USPs. In addition, no PCL crystal melting was observed in the heating thermogram of the quenched USP2, despite the absence of stacked UPy crystals that would prohibit PCL crystallization. We did not think that the absence of PCL crystal melting in the quenched USP2 was due to the quenching effect. If it was assumed that the quenching process led to the absence of PCL crystal, the quenched USP2 should have shown a PCL cold crystallization

endotherm peak during the heating process following quenching, si-milar to the case of SP2 in Fig. 2a. However, the quenched USP2 did not show such a peak. This clearly indicated that UPS2 did not originally enable PCL crystal formation irrespective of the presence or the absence of UPy crystals. Accordingly, we thought that the greatly increased Tg

and the restricted PCL crystallization in the USPs with short arm- lengths were attributed to the dense supramolecular crosslinking structure with short crosslinking length. On the other hand, increasing arm-length of the USPs increased the crosslinking length of the supra-molecular network formed by quadruple UPy association, and further enabled PCL chain crystallization in the USP supramolecular network. Therefore, based on the above-mentioned results, it is thought that the major factor affecting the phase transformation of the USPs is the arm- length. USPs having a short arm-length formed stacked UPy crystals resulted from a short crosslinking length, thereby exhibited the stacked- UPy crystals dominant dense cross-linked network phase. Meanwhile, as the arm-length of USPs increased, the resulting crosslinking length sequentially increased. Due to this, a sufficiently long PCL chain, which were derived from UPy quadruple hydrogen bonding, could be folded to form a chain-folded crystalline dominant network phase.

The effect of the arm-length of USPs on their self-healing perfor-mance was investigated under various time and temperature condi-tions. Solution-casted films of USPs were prepared; USP2, 5, 10, and 13 were found to be transparent, while USP16 was slightly translucent due to the high crystallinity of USP16 (Fig. S6), in good agreement with the results of DSC analysis. Their healing performances were visualized using an optical microscope (OM) (Fig. 4). As shown in Fig. 4, USP13 and 16 showed healed morphologies following heat treatment for 40 min and 10 min at 60 °C, respectively, while the cracks of USP2, 5, and 10 were not healed. For heat treatment at 50 °C, only USP16 with the longest arm-length showed healing behavior, and eventually all the USPs were unhealed at 40 °C even after 40 min of heat treatment. These results show that the arm-length of the USPs critically affected their healing performance: both temperature and time required for self- healing decreased with increasing arm-length of the USPs (see also Fig. S7 in supporting information). Such an arm-length-dependent healing behavior of USPs was presumably due to the change in the crosslinking length of the USP. A longer arm-length implied longer crosslinking length of the USPs, which enhanced chain mobility, and in turn, en-hanced the opportunity for re-association of UPy dimers that were broken by a crack, i.e., improved the self-healing performance of the USPs. In addition, we found that all the temperatures required for self- healing of the USPs were higher than the melting temperatures of the stacked UPy crystals or PCL chain crystals (Fig. S8). At temperatures lower than the melting temperature of the USPs, there was no sig-nificant change in the crack morphology even after prolonged heat treatment (Fig. S7). Oya et al. have also reported similar results, i.e., the inability of a crystalline polymer to heal at temperatures lower than Tm

[46]. Thus, it was thought that the UPy and PCL crystals acted as a threshold barrier for the self-healing initiation of USPs [45]. In this regard, it was noteworthy that the healing performance of USP16 with a crystalline PCL chain, which could hinder its self-healing behavior, was superior to that of USP13, which had a fully amorphous supramolecular network structure with no obstacle to self-healing. This observation

Table 2 Thermal properties, scattering vectors at primary peak positions, and average spacings between UPy crystalline domains of USPs.

Tg (°C) ΔHm,UPy (J/g) Tm,Upy (°C) ΔHm,PCL (J/g) Tm,PCL (°C) qpeak (Å−1) D (nm)

USP2 −0.92 2.9145 83.73 n.a. n.a. 0.107 5.8 ± 0.1 USP5 −30.11 1.8465 73.17 n.a. n.a. 0.078 8.1 ± 0.1 USP10 −47.04 0.8058 58.56 n.a. n.a. 0.060 10.4 ± 0.1 USP13 −51.11 n.a. n.a. n.a. n.a. n.a. n.a. USP16 −53.65 n.a. n.a. 21.081 38.96 n.a. n.a. UPy-NCO n.a. 2.7265 85.89 n.a. n.a. – –

n.a.: not applicable.

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could be explained by the low melting temperature of the PCL crystals (Tm,PCL: 38 °C) for USP16. At 50 °C, the PCL crystals in USP16 were melted and USP16 subsequently had an amorphous supramolecular network structure similar to USP13; in other words, the supramolecular structure of USP16 changed from the dual dynamic network composed of UPy quadruple hydrogen bonds and PCL crystalline physical bonds to the mono dynamic network composed of only UPy quadruple hydrogen bonds. In this case, USP16 had higher chain mobility than USP13 due to the longer crosslinking length, which led to better healing capability than USP13.

The complex viscosity data of the USPs measured using dynamic mechanical analysis (DMA) and a rheometer well supported our con-jecture why crystalline USP16 showed superior healing to amorphous USP13. As shown in Fig. 5a, all the USPs exhibited complex viscosity (η*) curves for a typical polymer, featuring regions corresponding to glass, glass transition, rubbery plateau, and terminal flow. The rubbery plateau modulus decreased in the following order: USP2 > USP16 > USP5 > USP10 > USP13, which apparently followed the trend of the arm-length effect, i.e., a higher crosslinking density of USPs, which was

derived from decreasing the crosslinking length of USPs, induced higher rubbery plateau modulus. Here, the crosslinking length of USPs, which was formed by UPy quadruple hydrogen bonding, might be estimated from the approximate end-to-end distance (< R2 >) values obtained through the freely rotating chain (FRC) model assumption, which al-lows the polymeric chain to cross itself, of the USP polymers. The cal-culated crosslinking length and the order is as follows; USP2 (9.5 Å) < USP5 (23.7 Å) < USP10 (47.4 Å) < USP13 (61.7 Å) < USP16 (75.9 Å), respectively [53]. These crosslinking length of USPs can be considered as to be inversely proportional to the crosslinking density. Thus, it is thought that the longer arm-length, i.e., the longer crosslinking length, the lower crosslinking density of USPs. Unexpectedly, however, USP16 exhibited higher rubbery plateau modulus than USP5, 10, and 13, although it had the lowest crosslinking density. Generally, a highly crystalline polymer shows a high rubbery plateau [52,53]. Hence, the high rubbery plateau of USP16 was likely attributable to its high crystallinity due to the dominant PCL chain- folding crystal in its dual dynamic supramolecular network. After the rubbery plateau, there was a dramatic decrease in the η* of USPs,

Fig. 3. Schematic of the semi-crystalline supramolecular dual dynamic network structure of USPs with different arm-lengths.

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corresponding to the terminal flow behavior. The onset temperatures of terminal flow for USP2, 5, 10, 13, and 16 were 85, 70, 65, 40, and 35 °C, respectively. A lower terminal flow temperature indicates lower thermal energy required to move molecules. Hence, it was thought that healing initiation was easiest for USP16 that exhibited the lowest onset temperature of terminal flow. In addition, when the temperature was above 50 °C, USP16 with its relatively high rubbery plateau owing to the PCL chain crystals displayed the lowest η* among the USPs. This indicates that USP16 had the fastest molecular motion when its crys-talline phase was eliminated, resulting in increased re-association probability of UPy due to a more favorable molecular flow.

The η* results of USPs observed by using a rheometer operated at 90 °C, which can reveal the original mobility nature of the network itself because there were no UPy and PCL crystals at that temperature, also showed the same results. As shown in Fig. 5b, η* of the USPs gradually reduced with increasing arm-length of the USPs, and

eventually USP16 displayed the lowest η* among all the USPs. This strongly suggests that USP16, which had the lowest crosslinking density owing to its longest arm-length, possessed the fastest molecular mobi-lity under the thermal healing condition of the USPs, resulting in high self-healing performance. Therefore, it was suggested that the healing performances of semi-crystalline dual dynamic supramolecular net-works of USPs were strongly associated with the melting behavior of the crystalline phase and the network crosslinking density.

The self-healing efficiency ( %) could be quantified by comparing the mechanical property values of virgin and healed specimens, ob-tained by tensile tests. The heat treatments for the healing tests were carried out at different temperatures for 20 min. Fig. 6a shows the tensile strength and elongation at break values of USPs. With increasing arm-length of USPs, their tensile strengths gradually decreased, while the elongation at break values gradually increased. This was attributed to the decrease in crosslinking density of the USPs with increasing arm-

Fig. 4. Optical microscopy images of the self-healing supramolecular networks heat-treated at 60 °C for 5, 10, 20, and 40 min for all the USPs. Heat treatments at 50 °C and 40 °C were performed for 20 and 40 min for USP13 and 16, respectively.

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length. However, USP16 with its longest arm-length exhibited the highest tensile strength and lowest elongation at break value. This was probably because USP16 had a rigid dual dynamic network structure with a highly enhanced PCL crystalline phase. The PCL chain-folding crystals in USP16 could act as not only a physical crosslinker but also a rigid reinforcer, enhancing the mechanical property of USP16.

Fig. 6b presents the mechanically observed healing efficiency values ( )% of USPs by comparing the elongation at break (ε) values for virgin and healed samples at different healing temperatures [54]. As shown in Fig. 6b, USP2, 5, 10, 13, and 16 exhibited mechanical self-healing ef-ficiencies of 27.7%, 32.2%, 61%, 92.3%, and 92.2% at 80 °C, respec-tively, indicating increased recovery of mechanical properties with in-creasing arm-length of the USPs. For heat treatment at 70 °C, although the % values of all the USPs were reduced compared to those upon heat treatment at 80 °C, the healing efficiencies still increased with in-creasing the arm-length of the USPs. The USPs treated at 60 °C also displayed a trend similar to the case of heat treatment at 70 °C. How-ever, USP13 exhibited a rather reduced % value at 60 °C (59%) com-pared to that at 70 °C, while the % of USP16 (78.4%) at 60 °C indicated that its healing efficiency was maintained relative to that obtained for 70 °C. USP2 and 5 exhibited poor recovery of mechanical properties (less than 22% of their % values), and USP10 also displayed a rather low healing efficiency (~36%). For heat treatment at 50 °C, the self- healing efficiency of USP13 was reduced to 26%. USP16 exhibited a relatively higher % value (~40%) than the other USPs. This indicates that the parameters of self-healing performance of the USPs, such as healing temperature, time, and efficiency, were significantly dependent on the arm-length of the USPs. Furthermore, this result showed that

USP16 has the best healing capability among all the USPs, in good agreement with the optically observed healing performances of the USPs. Although the mechanically measured healing efficiencies were slightly different from those determined from OM observations, the overall trend of the former was not significantly different from the optically observed healing performances. The slight differences may have resulted from the different measurement methods for self-healing performances. Indeed, it has been previously reported that the me-chanically measured healing efficiency is lower than the optically ob-served one [45].

The repeatable healing performance of USP16 was confirmed by the stress relaxation experiments. As shown in Fig. 6c, the repeatable healing efficiency ( )R,% of USP16 was 66%, 62%, and 67% at each scratch-healing cycle. These values were consistent with the above- mentioned mechanically observed healing efficiency values, confirming the successfully repeatable self-healing behavior of USP16. Moreover, USP16 showed almost the same melting enthalpy values for PCL crystal during four sequential DSC heating scans under the same healing con-dition, indicating the reformation of the PCL crystalline physical bonds that was broken by thermal healing. Therefore, the mechanical prop-erty of USP16 was repeatedly rejuvenated via recovery of PCL chain crystalline physical bonds that act as a crosslinker and a reinforcer for the USP network.

Therefore, it was concluded from the combined optical and me-chanical measurements of healing performance that the self-healing behavior of the USP16 with the dual dynamic network structure was initiated after the melting of the PCL chain crystals. Subsequently, the damaged site of the USP16 was mended by re-association of UPy, re-sulting in preliminary formation of a mono dynamic network based on the UPy dimerization. Then the mechanical properties of the network were rejuvenated via recovery of the crystalline phase, indicating complete restoration of the dual dynamic network of USP16. Thus, USP16, with such a dual dynamic network structure, showed better healing capability and 6 times higher tensile strength than USP13 having amorphous supramolecular network structure. As shown in Fig. 6e, the freestanding USP16 film was very rigid, showing a flat and uncurved film image even when a weight of 0.5 kg was suspended from it (Fig. 6e). A survey of the mechanical strengths of other self-healing supramolecular networks with over 70% healing efficiencies also showed how high the mechanical strength of USP16 was (Fig. 6f). This was due to the temperature-responsive dynamic characteristics of the crystalline phase of USP16. The crystalline phase in USP16 can re-inforce the mechanical property of the network under normal condition such as room temperature. However, the supramolecular network structure of USP16, mechanically locked by the crystalline phase at room temperature, will be open above the melting temperature and the network will gain high mobility, resulting in enhanced healing cap-ability.

Therefore, the incorporation of the hard crystalline phase with such thermo-switchable dynamic features within UPy quadruple hydrogen bonded supramolecular networks can be achieved via adjustment of the arm-length to the extent of 16 in this work. It allows the design of advanced self-healing materials with facile healing performances (ca. 78% healing efficiency in 20 min at a temperature of 60 °C) and robust mechanical properties (mechanical strength: 12.3 MPa) at the same time, unlike typical reinforcing fillers. The healing mechanism of USP16 with dual dynamic network structure is schematically illustrated in Fig. 7.

Meanwhile, excessively long arms of USPs are probably unfavorable for healing due to the significantly increased melting temperature and enthalpy value of the PCL chain crystalline phase, which probably had a negative influence on the initiation of self-healing (Fig. S10).

4. Conclusions

Despite their excellent self-healing capabilities, the poor mechanical

Fig. 5. (a) Complex viscosity curves of USPs as a function of temperature based on dynamic mechanical analysis. (b) Complex viscosities of USPs as a function of angular frequency scan at 90 °C based on rheometric measurements.

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strengths of self-healing supramolecular networks have limited their widespread industrial applications. To reinforce their mechanical properties, the semi-crystalline supramolecular network is a promising candidate to resolve the above-mentioned issue, if the side effect of the crystalline phase acting as a healing barrier could be manipulated sufficiently. Such a semi-crystalline supramolecular network allows si-multaneous improvement of both the self-healing efficiency and me-chanical properties. In this study, we attained a self-healable supra-molecular network with enhanced healing capability and mechanical properties through the formation of a dual dynamic network structure composed of UPy quadruple hydrogen bonds and crystalline physical bonds using UPy-end-functionalized semi-crystalline star-shaped

supramolecular polymers. In particular, the control of crosslinking density for the network, via adjustment of the arm-length for the UPy- functionalized semi-crystalline star-shaped supramolecular polymers, was critical to achieving the mechanically robust semi-crystalline su-pramolecular network with highly efficient healing capability due to their arm-length-dependent phase transformation and dual dynamic healing behavior. This study opens the possibility to designing ad-vanced self-healing materials for engineering applications that require high mechanical strength, and studies are currently underway to de-velop supramolecular networks with much milder healing conditions by tuning the crystallinity using copolymerization.

Fig. 6. Mechanical self-healing tests for USPs. The tensile strength and elonga-tion at break values of pristine USPs. (b) Mechanically observed self-healing efficiencies of USPs by tensile strength tests as a function of the arm-length of the USPs at various temperatures. (c) The repeatable healing performance of USP16. During the three-times re-peated healing test, the same position on the USP16 film was notched each time and the notch was repeatedly healed at 60 °C for 10 min. (d) The recovery of crystalline melting en-thalpy for USP16 after four DSC heating scans. Each experiment was performed after the temperature was lowered to 0 °C at a rate of 10 °C/min and then heated to 60 °C (self-healing temperature), followed by an iso-thermal step of 10 min (self-healing time). (e) Photographic image of rigid USP16 freestanding film with a load of 0.5 kg and (f) Ashby plot of “tensile strength” versus “temperature for suf-ficient (higher than 70% healing effi-ciency) self-healing” of the USPs and other reported self-healable supramo-lecular polymers [2,55–79].

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CRediT authorship contribution statement

Woojin Lee: Conceptualization, Methodology, Validation, Investigation, Writing - original draft. Seung-Yeop Kwak: Conceptualization, Methodology, Supervision, Project administration, Funding acquisition, Writing - review & editing. Jae Woo Chung: Conceptualization, Methodology, Supervision, Project administration, Funding acquisition, Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influ-ence the work reported in this paper.

Acknowledgments

This research was supported by the Individual Basic Science & Engineering Research Program (NRF-2016R1D1A1B01012377) and the Basic Science Research Program (NRF-2017R1A5A1015596) of the National Research Foundation (NRF) of Korea funded by the Ministry of Science and ICT.

Appendix A. Supplementary material

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.eurpolymj.2020.109976.

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