Introduction of Reversible Urethane Bonds Based on Vanillyl Alcohol for Efficient Self-Healing of Polyurethane Elastomers

molecules

Article Introduction of Reversible Urethane Bonds Based on Vanillyl Alcohol for Eﬃcient Self-Healing of Polyurethane Elastomers

Dae-Woo Lee †, Han-Na Kim † and Dai-Soo Lee * Division of Semiconductor and Chemical Engineering, Chonbuk National University, Baekjedaero 567, Jeonju 54896, Korea; [email protected] (D.-W.L.); [email protected] (H.-N.K.) * Correspondence: [email protected]; Tel.: +82-63-270-2310 Contributed equally to this work. † Academic Editors: Elisabete C.B.A. Alegria and Maximilian N. Kopylovich Received: 4 April 2019; Accepted: 9 June 2019; Published: 12 June 2019

Abstract: Urethane groups formed by reacting phenolic hydroxyl groups with isocyanates are known to be reversible at high temperatures. To investigate the intrinsic self-healing of polyurethane via a reversible urethane group, we synthesized vanillyl alcohol (VA)-based polyurethanes. The phenolic hydroxyl group of vanillyl alcohol allows the introduction of a reversible urethane group into the polyurethane backbone. Particularly, we investigated the effects of varying the concentration of reversible urethane groups on the self-healing of the polyurethane, and we proposed a method that improved the mobility of the molecules contributing to the self-healing process. The concentration of reversible urethane groups in the polyurethanes was controlled by varying the vanillyl alcohol content. Increasing the concentration of the reversible urethane group worsened the self-healing property by increasing hydrogen bonding and microphase separation, which consequently decreased the molecular mobility. On the other hand, after formulating a modified chain extender (m-CE), hydrogen bonding and microphase separation decreased, and the mobility (and hence the self-healing efficiency) of the molecules improved. In VA40-10 (40% VA; 10% m-CE) heated to 140 ◦C, the self-healing efficiency reached 96.5% after 30 min, a 139% improvement over the control polyurethane elastomer (PU). We conclude that the self-healing and mechanical properties of polyurethanes might be tailored for applications by adjusting the vanillyl alcohol content and modifying the chain extender.

Keywords: vanillyl alcohol; self-healing eﬃciency; mechanical property; reversible urethane bond

1. Introduction Polyurethane elastomer (PU) is a versatile polymer with many industrial applications. PUs is generally synthesized from polyols, diisocyanate, and a chain extender by the prepolymer method. The physical properties of PU can be adjusted by varying the types and amounts of polyols, isocyanates, and chain extenders in the precursor mixture. PU is a segmented block copolymer composed of a soft domain (polyols) and a hard domain (isocyanate and the chain extender). The polyol in the soft domain gives the PU its elastic quality, while the hard domain (depending on its nature) confers hardness and rigidity. Thus, microphase separations and the contents of the soft and hard domains significantly affect the mechanical properties of PU. Thermal stability of PU has also been reported. Chain scission occurs in the hard segment when PU is heated above 200 ◦C[1–3]. PU is widely used as thermal insulation foams and as adhesives, fibers, coatings, shock-absorbers, and elastomers in industrial fields [4–7]. Recently, many studies have focused on the self-healing and shape memory properties of PU. Self-healing PUs are materials capable of repairing damage via release of healing agents from embedded

Molecules 2019, 24, 2201; doi:10.3390/molecules24122201 www.mdpi.com/journal/molecules Molecules 2019, 24, 2201 2 of 13 microcapsules or a temporary increase in mobility leading to re-flow of the material in the damaged area. Compared to other materials, there are many choices to impart self-healing properties to Pus, as the physico-chemical properties of PU can be adjusted with the types and amounts of polyols, isocyanates, and chain extenders. Many researchers are improving the durability and applicability of polymer products by enhancing their self-healing properties. Various fabrication methods for self-healing polymers have been suggested. In early research, self-healing was extrinsically enhanced by dispersing microcapsules in the polymer [8]. When cracks grew in the polymer, microcapsules were broken and released a healing agent that filled the cracks and healed the polymer. However, owing to the low storage stability of the healing agent stored in the microcapsule, extrinsic self-healing cannot be sustained. While enclosed in the microcapsules, the healing agent exists in the uncured state. However, when released during the first healing, the healing agent is irreversibly cured, preventing repetitive self-healing. To overcome these problems, later researchers developed intrinsic self-healing methods. These methods can be categorized into two types. Associative self-healing methods include hydrogen bonding, host–guest interactions, or exchange reactions involving transesterification, disulfide metathesis, transalkylation, transcarbamoylation, or azomethine metathesis [9–13]. Dissociative self-healing methods are based on the Diels–Alder reaction and a reversible urethane or hindered urea group [14–18]. Especially, Cao et al. studied thermal self-healing of thermosetting PU based on the reversible features of phenolic urethanes [18]. However, the mechanical and self-healing properties usually exhibit an inverse relationship. Polymers with aromatic disulfide groups can self-heal even at room temperatures, but their mechanical properties are low. In this aspect, hydrogels and supramolecular polymers have been extensively studied [18–21]. However, polymers that self-heal via reversible urethane groups must be heated to above the dissociation temperature of reversible urethane groups (i.e., to above 200 ◦C). Hindering the urea groups may lower the dissociation temperature of the polymer by the bulk structure around the urea group, enabling self-healing at room temperature. Unfortunately, the mechanical properties of polymers that self-heal via the reversible urethane or urea groups are relatively poor [17,18]. Light-triggered self-healing polymers are also studied. In this case, inorganic fillers such as carbon nanotubes (CNTs), gold nanoparticles (AuNPs), and graphene, heating is assisted by light. Polymers heated by light can undergo self-healing via the dissociation of hydrogen bonds or melting of crystals [22–24]. Polymer self-healing is also affected by various factors that influence the diffusion properties of polymers, such as hydrogen bonding, phase mixing, and crystallization. The hardness of polymers that self-heal by disulfide metathesis is very low [21]. Recently, polymers showing high self-healing efficiency even at low temperatures have been reported. Scratched PU film comprising poly(tetramethylene ether glycol), isophorone diisocyanate, and aromatic disulfide was completely healed after only 30 min at 40 ◦C[25]. Yanagisawa and co-workers reported complete healing of urethane elastomer at 36 ◦C after 24 h via hydrogen bonding of thiourea [26]. Leibler and co-workers reported that the self-healing properties of polymers were directly relatable to their stress relaxation times [27–29]. However, applicability of the above-mentioned healable polymers with reversible urethane bonds are limited by various conditions. For instance, polymers that self-heal only at high temperatures under inert gas conditions are unsuitable for low-temperature applications. In the present study, we investigate whether the concentration of the dynamic covalent bonds influences the self-healing property of PU. Typical urethane bonds constitute aliphatic hydroxyl and isocyanate groups, which are stable and indecomposable below 200 ◦C. The phenolic hydroxyl group can also react with the isocyanate group, blocking the isocyanate and improving the storage stability of the prepolymer. Urethane groups constructed from phenolic hydroxyl and isocyanate can re-dissociate into phenol and isocyanate via a reversible reaction. Inspired by these facts, we chose vanillyl alcohol (VA) as the diol and mixed it with a polyol to synthesize a VA-based PU with two hydroxyl groups (a phenolic hydroxyl group and an aliphatic hydroxyl group) that self-heals under heating. VA enables the introduction of a reversible urethane Molecules 2019, 24, x FOR PEER REVIEW 3 of 13

hydroxyl group) that self-heals under heating. VA enables the introduction of a reversible urethane group composed of a phenolic hydroxyl with an isocyanate group into PU. The mechanical and self-healing properties of PUs with different concentrations of the dynamic chemical bonds are investigated. In addition, the self-healing properties of PU are improved by a modified chain extender (m-CE) that improves the mobility of the molecules.

2. Results and Discussion

2.1. Characterization of Vanillyl Alcohol (VA)-Based Polyurethane Elastomers (PUs) Figure 1 shows the Fourier-transform infrared (FT-IR) spectra of the raw materials of the VA-based PUs. Hydroxyl groups and isocyanates were confirmed at 3000–3500 and 2270 cm−1, Molecules 2019, 24, 2201 3 of 13 respectively. Hydrogen-bonded and free hydroxyl groups of VA were observed at 3340 and 3160 cm−1, respectively. VA-based PUs synthesized by the pre-polymer method are illustrated in Scheme group1. The peak composed at 2270 of cm a phenolic−1 in the FT-IR hydroxyl spectrum with of an the isocyanate VA-based group PUs disappeared into PU. The after mechanical curing atand 110 self-healing°C for 24 h (see properties Figure 2). of Figure PUs with 2(A) di showsfferent the concentrations FT-IR spectra ofof therepresentative dynamic chemical PUs prepared bonds from are investigated.VA and m-CE, In and addition, Figure the 2(B) self-healing magnifies properties these spectra of PU in are the improved 2000–1500 by cm a modified−1 wavenumber chain extender region. (m-CE)The absorption that improves peaks theof the mobility hydrogen-bonded of the molecules. and free carbonyl groups in the PUs appeared at 1703 and 1733 cm−1, respectively. The magnitude of the hydrogen-bonded carbonyl peak increased with 2.increasing Results andVA Discussioncontent (see Figure S2). The I1703/I1733 ratios were 1.33, 1.77, 1.85, 1.89, and 2.21 for the control PU, VA10, VA20, VA30, and VA40, respectively, implying that increasing the VA content 2.1. Characterization of Vanillyl Alcohol (VA)-BasedPolyurethane Elastomers (PUs) increased the hydrogen bonding in the PUs. However, the hydrogen-bonded carbonyl peaks in the VA40-5Figure and1 VA40-10shows the PUs Fourier-transform were largely suppressed infrared (FT-IR) by the spectra m-CE. ofSpecifically, the raw materials the I1703 of/I1733 the ratios VA-based were 1 PUs.0.95 and Hydroxyl 0.92 for groupsVA40-5 and and isocyanates VA40-10, respectively. were confirmed This result at 3000–3500 implied and that 2270hydrogen cm− , bonding respectively. was 1 Hydrogen-bondeddecreased, so the andmolecules free hydroxyl became groups more of mobi VA werele. Table observed 1 summarizes at 3340 and the 3160 average cm− , respectively. molecular VA-basedweights of PUs the synthesized synthesized by PUs, the pre-polymerdetermined methodby gel permeation are illustrated chromatography in Scheme1. The(GPC). peak The at 1 2270decrease cm− inin average the FT-IR molecular spectrum ofweights the VA-based with increa PUssing disappeared VA content after was curing attributable at 110 ◦C forto 24the h (seelow Figurereactivity2). Figureof the2 (A)phenolic shows hydroxyl the FT-IR groups spectra of of VA. representative On the other PUs hand, prepared the fromm-CE VA increased and m-CE, the 1 andaverage Figure molecular2(B) magnifies weight these of the spectra VA-based in the PUs, 2000–1500 owing to cm the− catalyticwavenumber effect region.of the tertiary The absorption amine of 1 peaksm-CE. of the hydrogen-bonded and free carbonyl groups in the PUs appeared at 1703 and 1733 cm− , respectively.Cao et al. The [18] magnitude confirmed ofthe the reversibility hydrogen-bonded of urethane carbonyl groups peak formed increased by the with phenolic increasing hydroxyl VA contentgroup and (see isocyanate Figure S2). by The FT-IR I1703 /Ispectroscopy.1733 ratios were To 1.33, confirm 1.77, the 1.85, regenerated 1.89, and 2.21 isocyanate for the control groups, PU, a VA10,model VA20,compound VA30, was and synthesized VA40, respectively, by reacting implying two moles that increasingof 4,4’-methylene the VA diphenyl content increased diisocyanate the hydrogen(MDI) with bonding three moles in the PUs.of VA. However, The complete the hydrogen-bonded consumption of carbonyl isocyanate peaks groups in the in VA40-5 the model and VA40-10structure PUswas wereconfirmed largely by suppressed titration following by the m-CE. ASTM Specifically, D2572-91. theAdditionally, I1703/I1733 ratiosthe hydroxyl were 0.95 values and 0.92of the for model VA40-5 structure and VA40-10, were respectively.116.3 mg KOH/g. This result The impliedhydroxyl that value hydrogen was determined bonding was according decreased, to soASTM the molecules4274D. FT-IR became spectra more of the mobile. model Table compound1 summarizes could be the obtained average at molecular elevated temperatures weights of the by synthesizedheating the PUs,sample determined block and by are gel given permeation in Figure chromatography S3. In order to (GPC). prevent The the decrease reaction in of average NCO moleculargroups and weights moisture with in increasing the air, VAFT-IR content spectra was were attributable collected to thefrom low the reactivity samples of in the an phenolic N2 gas hydroxylenvironment groups inside of VA.the sample On the otherblock. hand, The characterist the m-CE increasedic peak ofthe isocyanate average groups molecular generated weight by of the VA-basedreversible PUs,properties owing of to urethane the catalytic groups effect appeared of the tertiary at 2270 amine cm−1 above of m-CE. 140 °C in FT-IR spectra.

Figure 1. Fourier-transform infrared (FT-IR) spectra of the raw constituents of the synthesized polyurethane elastomer (PU). Note: PTMEG, poly(tetramethylene ether)glycol; MDI, 4,4’-methylene diphenyl diisocyanate; BDO, 1,4-butanediol; VA, vanillyl alcohol; and m-CE, modiﬁed chain extender.

Cao et al. [18] confirmed the reversibility of urethane groups formed by the phenolic hydroxyl group and isocyanate by FT-IR spectroscopy. To confirm the regenerated isocyanate groups, a model compound was synthesized by reacting two moles of 4,4’-methylene diphenyl diisocyanate (MDI) with three moles of VA. The complete consumption of isocyanate groups in the model structure was confirmed by titration following ASTM D2572-91. Additionally, the hydroxyl values of the model structure were 116.3 mg KOH/g. The hydroxyl value was determined according to ASTM 4274D. FT-IR spectra of the model compound could be obtained at elevated temperatures by heating the sample block and are given in Figure S3. In order to prevent the reaction of NCO groups and moisture in the air, FT-IR spectra were collected from the samples in an N2 gas environment inside the sample block. MoleculesMolecules 20192019,, 2424,, xx FORFOR PEERPEER REVIEWREVIEW 44 ofof 1313

Molecules 2019, 24, 2201 4 of 13 FigureFigure 1.1. Fourier-transformFourier-transform infraredinfrared (FT-IR)(FT-IR) spectraspectra ofof thethe rawraw constituentsconstituents ofof thethe synthesizedsynthesized polyurethanepolyurethane elastomerelastomer (PU).(PU). NoteNote:: PTMEG,PTMEG, poly(tetramethylenepoly(tetramethylene etheether)glycol;r)glycol; MDI,MDI, 4,4’-methylene4,4’-methylene The characteristicdiphenyldiphenyl diisocyanate;diisocyanate; peak of isocyanate BDO,BDO, 1,4-butanediol;1,4-butanediol; groups generated VAVA,, vanillylvanillyl by the reversiblealcohol;alcohol; anan propertiesdd m-CE,m-CE, modifiedmodified of urethane chainchain groups extender. 1 appearedextender. at 2270 cm− above 140 ◦C in FT-IR spectra.

FigureFigure 2.2. FT-IRFT-IR spectraspectra ofof PUsPUs preparedprepared from from VA: VA: ((AA)) fullfullfull spectraspectraspectra andandand ((B(BB))) magniﬁed magnifiedmagnified partialpartialpartial spectraspectraspectra ofof (A).(A).

SchemeScheme 1.1. SynthesisSynthesis of of VA-based VA-based PUs.PUs.

TableTable 1.1. MolecularMolecular weightsweights ofof thethe synthesizedsynthesized PUs.PUs. Molecular Weight Sample Code Molecular Weight Sample Code Mn Mw Mz Mw/Mn Mn Mw Mz Mw/Mn ControlControl PUPU 18,000 18,000 31,000 31,000 47,000 47,000 1.78 1.78 VA10VA10 22,00022,000 93,00093,000 495,000495,000 4.194.19 VA20VA20 28,00028,000 69,00069,000 156,000156,000 2.452.45

Molecules 2019, 24, 2201 5 of 13

Table 1. Molecular weights of the synthesized PUs.

Molecular Weight Sample Code Mn Mw Mz Mw/Mn

Molecules 2019, 24, x FOR PEERControl REVIEW PU 18,000 31,000 47,000 1.78 5 of 13 VA10 22,000 93,000 495,000 4.19 VA20VA30 28,00015,000 69,00033,000 156,00060,000 2.24 2.45 VA30VA40 15,00012,000 33,00026,000 46,000 60,000 2.10 2.24 VA40VA40-5 12,00024,000 26,00072,000 214,000 46,000 3.00 2.10 VA40-5VA40-10 24,00027,000 72,00084,000 258,000 214,000 3.09 3.00 VA40-10 27,000 84,000 258,000 3.09 2.2. Microdomain Structure of VA-based PUs 2.2. MicrodomainVA-based PUs Structure synthesized of VA-BasedPUs in this study appeared opaque, mainly because of the crystallites formedVA-based by the PUshard synthesized segments. The in thismicrophase-sep study appearedarated opaque, structures mainly of the because VA-based of the PUs crystallites strongly formeddepended by theon the hard VA/PTMEG1000 segments. The microphase-separatedratio. Figure 3 shows structuresthe small-angle of the X-ray VA-based scattering PUs strongly (SAXS) dependedprofiles of on the the control VA/PTMEG1000 PU and VA-based ratio. Figure PUs.3 showsThe SAXS the small-angle profiles confirmed X-ray scattering the scatterings (SAXS) profiles of hard ofand the soft control segments PU and domains. VA-based The PUs.interdomain The SAXS distance profiles of confirmedhard domain the in scatterings the SAXS ofprofiles hard andwas soft ~20 segmentsnm, similar domains. with the The interdomain interdomain distance distance of of the hard ha domainrd domains in the in SAXS previous profiles studies was ~20of PUs nm, [30–33]. similar withThe peak the interdomain positions of distance the VA-based of the hardPUs domainsappeared in in previous a higher studies q range of than PUs [in30 –the33 ].control The peak PU, positionsindicating of a the lower VA-based interdomain PUs appeared distance in aof higher the VA-based q range than PUs in than the control that of PU, the indicating control PU. a lower The interdomaininterdomain distance distances of were the VA-based 19, 18, 18, PUs and than 17 nm, that respectively, of the control in PU. the The VA10, interdomain VA20, VA30, distances and VA40 were 19,samples; 18, 18, and 17they nm, were respectively, 15 and 14 in nm, the respectively, VA10, VA20, VA30,in the andVA40-5 VA40 and samples; VA40-10 and samples they were (in15 which and 14phase nm, mixing respectively, was induced in the VA40-5 by the and m-CE). VA40-10 VA-bas samplesed PUs (in exhibited which phase much mixing broader was widths induced than by thethe m-CE).control VA-basedPU (Figures PUs 3 and exhibited S4), implying much broader that micro-phase widths than separation the control and PU crystallinity (Figure3 and were Figure hindered S4), implyingby VA as thatwell micro-phase as by m-CE. separation and crystallinity were hindered by VA as well as by m-CE.

Figure 3. Figure 3. Small-angle X-rayX-ray scatteringscattering (SAXS)(SAXS) proﬁlesprofiles ofof thethe VA-basedVA-based PUs.PUs. 2.3. Thermal Analyses of PUs Based on VA 2.3. Thermal Analyses of PUs Based on VA The thermal stabilities of the VA-based PUs were analyzed by a thermogravimetric analyzer (TGA)The (Figure thermal4 and stabilities Figure S5). of the VA-based VA-based PUs PUs were were less thermallyanalyzed stableby a thermogravimetric than the control PU. analyzer More (TGA) (Figures 4 and S5). VA-based PUs were less thermally stable than the control PU. More speciﬁcally, the 5% decomposition temperature of the Control PU was 311 ◦C, whereas those of the specifically, the 5% decomposition temperature of the Control PU was 311 °C, whereas those of the VA-based PUs decreased from 303 ◦C to 298 ◦C as the VA content increased from 10% to 40%. That is,VA-based the thermal PUs stability decreased decreased from 303 with °C to increasing 298 °C as VA the content. VA content According increased to Zoran from et.10% al., to increasing 40%. That theis, the hard thermal segment stability decreased decreased the thermal with increasing decomposition VA content. temperature According of segmented to Zoran et. PUs al., as increasing the hard segmentthe hard wassegment less thermallydecreased stablethe thermal than the decomposit soft segmention temperature [34]. Increasing of segmented the VA content PUs as increased the hard thesegment proportion was less of hardthermally segment stable in thethan VA-based the soft se PUs,gment thereby [34]. Increasing reducing their the VA thermal content stabilities increased in comparisonthe proportion with of thehard control segment PU. in However, the VA-based in the PU VA-baseds, thereby PUs reducing incorporating their thermal m-CE, stabilities the thermal in stabilitycomparison was with hardly the changed control byPU. increasing However, the in VAthe concentration.VA-based PUs Weightincorporating loss due m-CE, to volatile the thermal small stability was hardly changed by increasing the VA concentration. Weight loss due to volatile small molecules released by thermal degradation was not observed below 200 °C. The TGA results are listed in Table S1.

Molecules 2019, 24, 2201 6 of 13

molecules released by thermal degradation was not observed below 200 ◦C. The TGA results are listed in TableMolecules S1. 2019, 24, x FOR PEER REVIEW 6 of 13

FigureFigure 4. Thermogravimetric4. Thermogravimetric analyzer analyzer (TGA) (TGA) thermograms thermograms of theof the VA-based VA-based PUs: PUs: (A )( weightA) weight loss loss profilesprofiles and and (B) 1st(B) derivatives1st derivatives of the of weightthe weight loss loss profile. profile. Dynamic mechanical properties in the PUs were investigated by a dynamic mechanical analyzer Dynamic mechanical properties in the PUs were investigated by a dynamic mechanical (DMA). The results are displayed in Figure5 and Figure S6. The soft-segment domains of the PUs analyzer (DMA). The results are displayed in Figures 5 and S6. The soft-segment domains of the underwent glass–rubber transitions below room temperature. The glass transition temperature of the PUs underwent glass–rubber transitions below room temperature. The glass transition temperature soft-segment domains (Tgs) manifested as a peak in the tan delta curve. In Figure S6, the Tgs increased of the soft-segment domains (Tgs) manifested as a peak in the tan delta curve. In Figure S6, the Tgs with increasing VA concentration in the PU, reflecting the decreased content of the polyol constituting increased with increasing VA concentration in the PU, reflecting the decreased content of the polyol the soft domain and the increased content of the urethane group capable of hydrogen bonding (per unit constituting the soft domain and the increased content of the urethane group capable of hydrogen mass). Meanwhile, the Tgs of the PUs with m-CE in Figure5 decreased because the hydrogen bonding bonding (per unit mass). Meanwhile, the Tgs of the PUs with m-CE in Figures 5 decreased because and hard segment packing were disturbed by the m-CE side-chain. In Figure S6, the flow temperature the hydrogen bonding and hard segment packing were disturbed by the m-CE side-chain. In Figure (Tflow), at which the storage modulus of the PUs decreased after the rubbery plateau region, was lower S6, the flow temperature (Tflow), at which the storage modulus of the PUs decreased after the rubbery in the VA-based PUs than in the control PU, and it increased with increasing VA content. This implied plateau region, was lower in the VA-based PUs than in the control PU, and it increased with that the VA hindered the microphase separation, and dissociation of reversible urethane groups was increasing VA content. This implied that the VA hindered the microphase separation, and lower than Tflow of the control PU. However, Tflow in the VA-based PU increased as the VA content dissociation of reversible urethane groups was lower than Tflow of the control PU. However, Tflow in increased because the hard segment concentration increased. the VA-based PU increased as the VA content increased because the hard segment concentration At low temperatures, hydrogen bonds increased the storage modulus of the VA-based PUs over increased. the control PU (as confirmed by FT-IR). In Figure5, the m-CE also decreased the storage modulus (which was below Tgs) and Tflow of the VA-based PUs, again by interfering with the hydrogen bonding and hard segment packing (confirmed by FT-IR). The stress relaxations in the PUs were investigated by employing DMA, and the test results are given in Figure6 and Figure S7. Table S2 summarizes the relaxation times at which the storage moduli reached 1/e of their initial values. At 140 ◦C the relaxation time was 539 s in the control PU and 71, 102, 120, and 162 s in the VA10, VA20, VA30,

Molecules 2019, 24, x FOR PEER REVIEW 7 of 13

Molecules 2019, 24, 2201 7 of 13 and VA40 samples, respectively. The relaxation times of the VA-based PUs were lower than in the control PU, but they increased with increasing VA content. This trend was attributable to the decreasing content of PTMEG1000 (which constituted the soft segment) and increased disturbance of the microphase separation as the VA content increased. Self-healing of polymers has been frequently investigated in stress-relaxation tests, and stress-relaxation time was closely related with self-healing properties [20,21,25,26]. Figure7 shows Arrhenius plots of the relaxation times of the VA-based PUs [29,34]. The relaxation times of the VA-based PUs indeed ﬁtted the Arrhenius law, and the activation energies of the VA-based PUs (determined from the Arrhenius slopes) were lower than that of the control PU, but they increased with increasing VA content because molecular mobility of VA-based PUs decreased as a result of the increasing hydrogen bond and hard segment. However, the activation energy of VA40-10 (with 10% m-CE) reached 108.5 kJ/mol. The activation energy of VA40-10 decreased to 63.4% compared with VA40 because mobility of molecules was improved by the introductionMoleculesFigure 2019 5., 24 of Dynamic, m-CE.x FOR PEER mechanical REVIEW analyzer (DMA) thermograms of the VA-based PUs: (A) storage7 of 13 moduli and (B) tan delta.

At low temperatures, hydrogen bonds increased the storage modulus of the VA-based PUs over the control PU (as confirmed by FT-IR). In Figure 5, the m-CE also decreased the storage modulus (which was below Tgs) and Tflow of the VA-based PUs, again by interfering with the hydrogen bonding and hard segment packing (confirmed by FT-IR). The stress relaxations in the PUs were investigated by employing DMA, and the test results are given in Figures 6 and S7. Table S2 summarizes the relaxation times at which the storage moduli reached 1/e of their initial values. At 140 °C the relaxation time was 539 s in the control PU and 71, 102, 120, and 162 s in the VA10, VA20, VA30, and VA40 samples, respectively. The relaxation times of the VA-based PUs were lower than in the control PU, but they increased with increasing VA content. This trend was attributable to the decreasing content of PTMEG1000 (which constituted the soft segment) and increased disturbance of the microphase separation as the VA content increased. Self-healing of polymers has been frequently investigated in stress-relaxation tests, and stress-relaxation time was closely related with self-healing properties [20,21,25,26]. Figure 7 shows Arrhenius plots of the relaxation times of the VA-based PUs [29,34]. The relaxation times of the VA-based PUs indeed fitted the Arrhenius law, and the activation energies of the VA-based PUs (determined from the Arrhenius slopes) were lower than that of the control PU, but they increased with increasing VA content because molecular mobility of VA-based PUs decreased as a result of the increasing hydrogen bond and hard segment. However, the activation energy of VA40-10 (with 10% m-CE) reached 108.5 kJ/mol. The activation energy of

VA40-10 decreased to 63.4% compared with VA40 because mobility of molecules was improved by FigureFigure 5.5.Dynamic Dynamic mechanical mechanical analyzer analyzer (DMA) (DMA) thermograms thermograms of the of VA-based the VA-based PUs: (A PUs:) storage (A) modulistorage the introduction of m-CE. andmoduli (B) tanand delta. (B) tan delta.

At low temperatures, hydrogen bonds increased the storage modulus of the VA-based PUs over the control PU (as confirmed by FT-IR). In Figure 5, the m-CE also decreased the storage modulus (which was below Tgs) and Tflow of the VA-based PUs, again by interfering with the hydrogen bonding and hard segment packing (confirmed by FT-IR). The stress relaxations in the PUs were investigated by employing DMA, and the test results are given in Figures 6 and S7. Table S2 summarizes the relaxation times at which the storage moduli reached 1/e of their initial values. At 140 °C the relaxation time was 539 s in the control PU and 71, 102, 120, and 162 s in the VA10, VA20, VA30, and VA40 samples, respectively. The relaxation times of the VA-based PUs were lower than in the control PU, but they increased with increasing VA content. This trend was attributable to the decreasing content ofFigure PTMEG1000 6. Stress-relaxation (which constituted curves of the the VA-based soft segment) PUs at and 130 ◦increasedC. disturbance of the microphase separation as the VA content increased. Self-healing of polymers has been frequently investigated in stress-relaxation tests, and stress-relaxation time was closely related with self-healing properties [20,21,25,26]. Figure 7 shows Arrhenius plots of the relaxation times of the VA-based PUs [29,34]. The relaxation times of the VA-based PUs indeed fitted the Arrhenius law, and the activation energies of the VA-based PUs (determined from the Arrhenius slopes) were lower than that of the control PU, but they increased with increasing VA content because molecular mobility of VA-based PUs decreased as a result of the increasing hydrogen bond and hard segment. However, the activation energy of VA40-10 (with 10% m-CE) reached 108.5 kJ/mol. The activation energy of VA40-10 decreased to 63.4% compared with VA40 because mobility of molecules was improved by the introduction of m-CE.

Molecules 2019, 24, x FOR PEER REVIEW 8 of 13

Molecules 2019, 24, x FOR PEER REVIEW 8 of 13 Figure 6. Stress-relaxation curves of the VA-based PUs at 130 °C. Molecules 2019, 24, 2201 8 of 13 Figure 6. Stress-relaxation curves of the VA-based PUs at 130 °C.

Figure 7. Arrhenius plots of the relaxation times τ of the VA-based PUs. Figure 7. Arrhenius plots of the relaxation times τ of the VA-basedVA-based PUs.PUs. Next, VA-based PUs were investigated by differential scanning calorimetry (DSC). Tgs increased with increasing VA content. More specifically, Tgs was −45 °C in the control PU and −43, −40, −36, and Next,Next, VA-basedVA-based PUsPUs werewere investigatedinvestigated byby didifffferentialerential scanning scanning calorimetry calorimetry (DSC). (DSC).T Tgsgs increasedincreased with−20 °C increasing in the VA10, VA content. VA20, MoreVA30, specifically, and VA40T samples,gs was 45respectively.C in the control The hard PU anddomain43, melted40, 36, at with increasing VA content. More specifically, Tgs was −45− °C◦ in the control PU and −43,− −40,− −36,− and andapproximately20 C in the 150 VA10, °C. However, VA20, VA30, when and the VA40hydrogen samples, bonds respectively. were disrupted The by hard m-CE, domain Tflow decreased. melted at −20 −°C in◦ the VA10, VA20, VA30, and VA40 samples, respectively. The hard domain melted at For instance, the Tgs and melting temperature of VA40-10 were −30 °C and 143 °C, respectively. The approximatelyapproximately 150150 ◦°C.C. However,However, whenwhen thethe hydrogenhydrogen bondsbonds werewere disrupteddisrupted by by m-CE, m-CE,T Tflowflow decreased. Forincrease instance, in Tgs the withTgs VAand content melting was temperature attributable of VA40-10to phase weremixing30 andC hydrogen and 143 C,bonding respectively. by VA The(the For instance, the Tgs and melting temperature of VA40-10 were −−30 ◦°C and 143 ◦°C, respectively. The DSC results are displayed in Figure S8 and summarized in Table S1.) increaseincrease inin TTgsgs with VA content was attributable to phasephase mixing and hydrogen bonding by VA (the(the DSCDSC resultsresults areare displayeddisplayed inin FigureFigure S8S8 andand summarizedsummarized inin TableTable S1.) S1.) 2.4. Mechanical Properties of the VA-based PUs 2.4. Mechanical Properties of the VA-basedPUs 2.4. MechanicalStress–strain Properties properties of the ofVA-based the PUs PUs were studied in tensile tests, and they are presented in FiguresStress–strainStress–strain 8 and S9. properties Interestingly,properties of of the increasing the PUs PUs were werethe studied VA studied content in tensile in improved tensile tests, andtests, the they tensile and are they strengths presented are presented and in Figure moduli 8in andFiguresof the Figure PUs, 8 and S9.but S9. Interestingly,it Interestingly,lowered the increasing elongation increasing the at the VAbreak. VA content contentFigure improved 8improved shows therepresentative the tensile tensile strengths strengths tensile and properties and moduli moduli ofof theofthe the PUs,synthesized PUs, but but it lowered it PUs.lowered The the the elongationmolecular elongation atweight break. at break. of Figure the Figure 8VA-based shows 8 shows representative PUs representative decreased tensile withtensile properties increasing properties of theVA of synthesizedthecontent. synthesized At PUs.first PUs. Theglance, molecularThe this molecular trend weight seemedweight of the incompatibleof VA-based the VA-based PUs with decreased PUs the decreasedincreasing with increasing with tensile increasing VAstrength content. VA of Atcontent.VA-based first glance, At PUs first this with glance, trend higher seemedthis VA trend content, incompatible seemed but incompatibleit with can thebe explained increasing with theas tensile follows.increasing strength When tensile of the VA-based VAstrength content PUs of withVA-basedincreased, higher PUsthe VA decreasedwith content, higher butmolecular VA it cancontent, beweight explained but and it ca mi asn crophasebe follows. explained separation When as follows. the in VA the contentWhen VA-based the increased, VA PUs content were the decreasedincreased,more than molecularthe offset decreased by weight strong molecular and hydrogen microphase weight bonding; separationand mi consequently,crophase in the VA-basedseparation the tensile PUs in the were strength VA-based more and than PUs modulus offset were by strongmoreimproved. than hydrogen offset bonding; by strong consequently, hydrogen bonding; the tensile consequently, strength and modulusthe tensile improved. strength and modulus improved.

FigureFigure 8.8. Stress–strainStress–strain curvescurves ofof PUsPUs basedbased onon VA.VA. 2.5. Self-Healing Properties ofFigure the VA-basedPUs 8. Stress–strain curves of PUs based on VA. 2.5. Self-Healing Properties of the VA-based PUs Although VA-based PUs showed earlier stress relaxation at elevated temperatures (above 120 C) 2.5. Self-HealingAlthough VA-based Properties ofPUs the showedVA-based earlier PUs stress relaxation at elevated temperatures (above ◦120 than at low temperatures, their tensile properties at room temperature were superior to those of the °C) than at low temperatures, their tensile properties at room temperature were superior to those of controlAlthough PU. Self-healing VA-based properties PUs showed of the earlier samples stress were rela evaluatedxation at on elevated dog-bone temperatures specimens. (above The center 120 the control PU. Self-healing properties of the samples were evaluated on dog-bone specimens. The of°C) each than prepared at low temperatures, specimen was their cut tensile with a properties knife, and at the room two temperature halves were were immediately superior reattached.to those of center of each prepared specimen was cut with a knife, and the two halves were immediately Thethe control self-healing PU. Self-healing process is imaged properties in Figure of the S10. samp Theles specimenwere evaluated was healed on dog-bone in a convection specimens. oven The at centerreattached. of each The prepared self-healing specimen process was is cutimaged with ina knife,Figure and S10. the The two specimen halves were was immediatelyhealed in a 140 ◦C for 30 min. Tensile properties of the VA-based specimens at elevated temperatures after cutting reattached. The self-healing process is imaged in Figure S10. The specimen was healed in a are summarized in Table S3. The healing eﬃciency decreased with increasing concentration of VA in

Molecules 2019, 24, x FOR PEER REVIEW 9 of 13

Molecules 2019, 24, 2201 9 of 13 convection oven at 140 °C for 30 min. Tensile properties of the VA-based specimens at elevated temperatures after cutting are summarized in Table S3. The healing efficiency decreased with theincreasing PU. Figure concentration9 shows the of tensile VA in properties the PU. Figure of representative 9 shows the PUstensile before properties and after of representative healing. VA-based PUs PUsbefore healed and lessafter e ffhealing.ectively VA-based than the control PUs healed PU because less effectively of the strong than hydrogen the control bonds PU and because close packing of the ofstrong hard hydrogen segment, which bonds lowered and close the mobilitypacking ofof the ha molecules.rd segment, More which specifically, lowered self-healing the mobility effi ciencyof the wasmolecules. 69.3% inMore the controlspecifically, PU and self-healing 60.5%, 50.7%, efficiency 42.9%, was and 69.3% 32.4% in in the the control VA10, VA20, PU and VA30, 60.5%, and 50.7%, VA40 samples,42.9%, and respectively. 32.4% in However, the VA10, incorporating VA20, VA30, the m-CE and dramatically VA40 samples, improved respectively. the healing However, efficiency ofincorporating samples with the the m-CE same dramatically VA concentration. improved Owing the to healing the higher efficiency molecular of samples mobility with of molecules, the same VA the healingconcentration. efficiency Owing reached to the 88.4% higher and molecular 96.5% in VA40-5 mobility and of VA40-10. molecules, The the side healing chain efficiency of m-CE hindered reached hydrogen88.4% and bonding 96.5% in among VA40-5 the and urethane VA40-10. groups. The Results side chain of repetitive of m-CE self-healing hindered hydrogen tests are displayed bonding inamong Figure the S11. urethane After groups. the second Results healing of repetitive cycle, the self-healing healing e ffitestsciency are displayed of the control in Figure PU was S11. 43.3%After (decreasedthe second fromhealing 69.3% cycle, after the the healing first cycle),efficiency but of that the of control VA40-10 PU was was 87.6% 43.3% (down (decreased from from 96.5% 69.3% after theafter first the cycle). first cycle), This resultbut that confirmed of VA40-10 that thewas m-CE 87.6% dramatically (down from improved 96.5% after the the healing first cycle). efficiencies This ofresult VA40-10. confirmed that the m-CE dramatically improved the healing efficiencies of VA40-10.

FigureFigure 9. 9.Stress–strainStress–strain curves curves of VA-based of VA-based PUs PUsbefore before (solid (solid line) line)and after and healing after healing at 140 at °C 140 (broken◦C line).(broken line).

3. Materials and Methods 3. Materials and Methods 3.1. Materials 3.1. Materials Butyl glycidyl ether (BGE, Mw: 130.19, 95%), diethanolamine (DEA, Mw: 105.14, 99%), Butyl glycidyl ether (BGE, Mw: 130.19, 95%), diethanolamine (DEA, Mw: 105.14, 99%), poly(tetramethylene ether)glycol (Mw: 1,000, 99%) (PTMEG1000), and 1,4-butanediol (BDO; 99%) were poly(tetramethylene ether)glycol (Mw: 1,000, 99%) (PTMEG1000), and 1,4-butanediol (BDO; 99%) obtained from Sigma–Aldrich. Vanillyl alcohol (VA, Mw: 154.17, 98%) was purchased from Tokyo were obtained from Sigma–Aldrich. Vanillyl alcohol (VA, Mw: 154.17, 98%) was purchased from Chemical Industry in Korea. 4,4’-methylene diphenyl diisocyanate (MDI, 99%) was obtained from Tokyo Chemical Industry in Korea. 4,4’-methylene diphenyl diisocyanate (MDI, 99%) was obtained BASF (BASF Korea, Yeosu, Korea). from BASF (BASF Korea, Yeosu, Korea). 3.2. Preparation of Modiﬁed Chain Extender 3.2. Preparation of Modified Chain Extender The m-CE was obtained by reacting DEA (10.5 g, 0.1 mol) with BGE (13 g, 0.1 mol). The mixture was stirredThe m-CE at room was temperatureobtained by forreacting 24 h. TheDEA mixture (10.5 g, was 0.1 initially mol) with opaque BGE but (13 gradually g, 0.1 mol). became The transparentmixture was through stirred at the room exothermic temperature reaction. for Figure24 h. The S11 mixture showsthe was FT-IR initially spectra opaque of DEA, but gradually BGE, and became transparent through the1 exothermic reaction. Figure S11 shows the FT-IR 1spectra of DEA, m-CE. The epoxy peak at 915 cm− completely disappeared after the reaction. In the H -NMR spectrum −1 (FigureBGE, and S12), m-CE. the peaks The epoxy at 3.6 andpeak 3.8 at ppm 915 werecm attributablecompletely todisappeared the primary after and secondarythe reaction. hydroxyl In the 1 groups,H -NMR respectively. spectrum (Figure Complete S12), consumption the peaks at of 3.6 the and secondary 3.8 ppm amine were was attributable conﬁrmed to by the amine primary titration and followingsecondary ASTM hydroxyl D2074. groups, respectively. Complete consumption of the secondary amine was confirmed by amine titration following ASTM D2074. 3.3. Preparation of Control PU and VA-basedPUs 3.3. Preparation of Control PU and VA-based PUs VA-based PUs were synthesized by the prepolymer method. The prepolymer was synthesized by reacting 0.1 mol of PTMEG1000 and 0.2 mol of MDI at 70 ◦C for 3 h. The synthesized prepolymer was

Molecules 2019, 24, 2201 10 of 13 mixed with the desired amounts of MDI, BDO, m-CE, and VA. After degassing, the mixtures were poured into a glass mold to prepare PU films and cured at 110 ◦C for 24 h in a convection oven. PU films obtained after the cure were employed for characterizations. Table2 lists the compositions of the VA-based PUs. The middle and final digits of the sample codes VA40-5 and VA40-10 denoted the molar percentages of VAs in the polyols and m-CE in the BDO, respectively.

Table 2. Composition of the synthesized PUs.

Composition (Molar Ratio) Sample Code PTMEG1000 VA MDIz BDO m-CE Control PU 1 - 2 1 VA10 0.9 0.1 2 1 VA20 0.8 0.2 2 1 VA30 0.7 0.3 2 1 VA40 0.6 0.4 2 1 VA40-5 0.6 0.4 2 0.95 0.05 VA40-10 0.6 0.4 2 0.90 0.10

3.4. Characterization Synthesis of the m-CH and VA-based PUs were confirmed by Fourier-transform infrared (FT-IR) spectroscopy (FT-IR-302; Jasco,). H1-NMR spectra were recorded in chloroform-d with a Bruker AM400 spectrometer (600 MHz). Molecular weights of the VA-based PUs were determined by GPC (Agilent 1200S; Agilent) employing a refractive index detector (Optilab rEX; Wyatt). Samples were dissolved in DMF/THF (1:1 wt./wt.), and polystyrene standards were used for universal calibration. The flow rate of the GPC measurement was 1.0 mL/min. Thermal properties of the VA-based PUs were measured by DSC (Q20; TA Instruments). The samples were initially cooled to 90 C and maintained − ◦ at this temperature for 1 min. In all measurements, the temperature was ramped up to 240 ◦C at 10 ◦C/min. A second scan was conducted under the same conditions. All tests were performed in an N2 atmosphere. Dynamic mechanical properties and relaxation times of the VA-based PUs were measured by DMA (Q800; TA Instruments) using a film tension clamp. The films for DMA were prepared by carefully cutting the cured PU films into 5.3 mm-wide sections. Dynamic mechanical properties were measured between 90 and 240 C inclusive at a heating rate of 5 C/min. Relaxation − ◦ ◦ tests were also performed at 110, 120, 130, 140, 145, and 150 ◦C for 20 min under an axial force of 0.01 N and a deformation of 1%. Tensile strengths and percentage elongations were measured by a universal testing machine (LR5K UTM; Lloyd) following the ASTM-D638 method. SAXS patterns (D8 DISCOVER, Bruker) were collected at room temperature using Cu–Kα radiation (λ = 1.541 Å) at a voltage of 50 kV and a current of 1000 µA. The Bragg angle (2θ) ranged from 0◦ to 9◦.

4. Conclusions This study investigated the self-healing of VA-based PUs via reversible urethane bonds formed by the reaction of phenolic hydroxyl and isocyanate groups, which can be efficiently exploited for self-healing PUs. In particular, we investigated the influence of VA and m-CE contents on the self-healing properties by measuring the thermal and dynamic mechanical properties of the prepared samples. We confirmed that the urethane bonds can be reversibly dissociated and that the isocyanate groups regenerated above 140 ◦C. As the VA content of the polyols increased from 10% to 40%, the relaxation time reduced from that of the control PU. Increasing the VA content decreases the self-healing efficiency by increasing the hydrogen bonding of hard segments and decreasing the proportion of soft segments, thereby reducing the mobility of the molecules. To overcome this problem, molecular mobility was increased by incorporating m-CE into the VA-based PUs. Hydrogen bonding and hard Molecules 2019, 24, 2201 11 of 13 domain packing, which improved the mechanical properties but hindered the molecular movements, was reduced by the m-CE. Consequently, the I1703/I1733 ratio in the FT-IR spectrum, which indicated the extent of hydrogen bonding in the PU, significantly decreased to 0.92 in VA40-10. On the other hand, the I1703/I1733 value of VA40 (without the m-CE) was 2.21, indicating strong hydrogen bonds in this sample. The interdomain distance in the SAXS profile of VA40-10 was only 14 nm, reflecting decreased microphase separation. Meanwhile, the relaxation time of the VA-based PUs at 140 ◦C increased from 71 to 162 s with increasing VA content. But after incorporating the m-CE, it significantly decreased, reaching 65 and 59 s in the VA40-5 and VA40-10 samples, respectively. This result was also closely related to microphase separation and hydrogen bonding in the PUs. Thus, disrupting the hydrogen bonds and microphase separation notably affected the self-healing properties of the PUs. Reduced hydrogen bonds and lower microphase separation improved the mobility of the molecules. Thus, reducing the molecular weight by dissociating the reversible urethane bonds efficiently contributes to the self-healing of PUs. To ensure efficient PU self-healing, the molecular structure design should increase both the molecular mobility and the concentration of dynamic covalent bonds.

Supplementary Materials: The following are available online. Table S1: Thermal properties of synthesized PUs, Table S2: Relaxation time and activation energy of synthesized PUs, Table S3: Tensile strength and healing efficiency of synthesized PUs at 140 ◦C for 30 min, Figure S1: Synthesis of m-CE, Figure S2: FT-IR spectra of VA-based PUs, Figure S3: FT-IR spectra of model structure for reversible urethane bond at elevated temperature, Figure S4: SAXS profile of VA-based PUs, Figure S5: TGA (A) and DTG (B) curves for the VA-based PUs, Figure S6: DMA thermograms of VA-based PUs, Figure S7: Stress relaxation curves of VA-based PUs at various temperatures, Figure S8: DSC thermograms of VA-based PUs, Figure S9: Stress–strain curves of VA-based PUs, Figure S10: Image of self-healing test for VA40-10, Figure S11: Tensile properties of PUs after repeated healing at 140 ◦C, Figure S12: FT-IR spectra of DEA, BGE and m-CE, Figure S13: H1-NMR spectrum of m-CE. Author Contributions: D.-W.L. and D.-S.L. contributed to the manuscript via literature search, study design, data analysis, and writing; D.-W.L. and H.-N.K. performed the experiments and analyzed the data. Funding: This work was supported by ATC project of KEIT funded by Ministry of Trade, Industry, and Energy (Project No: ATC-10048672) in Korea. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Hentschel, T.; Münstedt, H. Kinetics of the molar mass decrease in a polyurethane melt: A rheological study. Polymer 2001, 42, 3195–3203. [CrossRef] 2. Grassie, N.; Zulfiqar, M. Thermal Degradation of the Polyurethane From 1,4-Butanediol and Methylene Bis(4-Phenyl Isocyanate). J. Polym. Sci. Polym. Chem. Ed. 1978, 16, 1563–1574. [CrossRef] 3. Montaudo, G.; Puglisi, C.; Scamporrino, E.; Vitalini, D. Mechanism of thermal degradation of polyurethanes. Effect of ammonium polyphosphate. Macromolecules 1984, 17, 1605–1614. [CrossRef] 4. Zia, K.M.; Bhatti, H.N.; Ahmad Bhatti, I. Methods for polyurethane and polyurethane composites, recycling and recovery: A review. React. Funct. Polym. 2007, 67, 675–692. [CrossRef] 5. Wegener, G.; Brandt, M.; Duda, L.; Hofmann, J.; Klesczewski, B.; Koch, D.; Kumpf, R.J.; Orzesek, H.; Pirkl, H.G.; Six, C.; et al. Trends in industrial catalysis in the polyurethane industry. Appl. Catal. A Gen. 2001, 221, 303–335. [CrossRef] 6. Janik, H.; Marzec, M. A review: Fabrication of porous polyurethane scaffolds. Mater. Sci. Eng. C 2015, 48, 586–591. [CrossRef] 7. Zdrahala, R.J.; Zdrahala, I.J. Biomedical applications of polyurethanes: A review of past promises, present realities, and a vibrant future. J. Biomater. Appl. 1999, 14, 67–90. [CrossRef] 8. Samadzadeh, M.; Boura, S.H.; Peikari, M.; Kasiriha, S.M.; Ashrafi, A. A review on self-healing coatings based on micro/nanocapsules. Prog. Org. Coatings 2010, 68, 159–164. [CrossRef] 9. Li, X.; Gao, Y.; Boott, C.E.; Winnik, M.A.; Manners, I. Non-covalent synthesis of supermicelles with complex architectures using spatially confined hydrogen-bonding interactions. Nat. Commun. 2015, 6, 1–8. [CrossRef] [PubMed] 10. Syrett, J.A.; Becer, C.R.; Haddleton, D.M. Self-healing and self-mendable polymers. Polym. Chem. 2010, 1, 978–987. [CrossRef] Molecules 2019, 24, 2201 12 of 13

11. Zhang, M.; Xu, D.; Yan, X.; Chen, J.; Dong, S.; Zheng, B.; Huang, F. Self-healing supramolecular gels formed by crown ether based host-guest interactions. Angew. Chemie–Int. Ed. 2012, 51, 7011–7015. [CrossRef] 12. Cook, T.R.; Zheng, Y.R.; Stang, P.J.Metal-organic frameworks and self-assembled supramolecular coordination complexes: Comparing and contrasting the design, synthesis, and functionality of metal-organic materials. Chem. Rev. 2013, 113, 734–777. [CrossRef][PubMed] 13. Chen, S.; Mahmood, N.; Beiner, M.; Binder, W.H.Self-Healing Materials from V- and H-Shaped Supramolecular Architectures. Angew. Chemie–Int. Ed. 2015, 54, 10188–10192. [CrossRef] 14. Zhu, K.; Song, Q.; Chen, H.; Hu, P. Thermally assisted self-healing polyurethane containing carboxyl groups. J. Appl. Polym. Sci. 2018, 135, 1–7. [CrossRef] 15. Karami, Z.; Zohuriaan-Mehr, M.J.; Rostami, A. Bio-based thermo-healable non-isocyanate polyurethane DA

network in comparison with its epoxy counterpart. J. CO2 Util. 2017, 18, 294–302. [CrossRef] 16. Yu, F.; Cao, X.; Du, J.; Wang, G.; Chen, X. Multifunctional Hydrogel with Good Structure Integrity, Self-Healing, and Tissue-Adhesive Property Formed by Combining Diels-Alder Click Reaction and Acylhydrazone Bond. ACS Appl. Mater. Interfaces 2015, 7, 24023–24031. [CrossRef][PubMed] 17. Ying, H.; Zhang, Y.; Cheng, J. Dynamic urea bond for the design of reversible and self-healing polymers. Nat. Commun. 2014, 5, 1–9. [CrossRef] 18. Cao, S.; Li, S.; Li, M.; Xu, L.; Ding, H.; Xia, J.; Zhang, M.; Huang, K. A thermal self-healing polyurethane thermoset based on phenolic urethane. Polym. J. 2017, 49, 775–781. [CrossRef] 19. Pérez-San Vicente, A.; Peroglio, M.; Ernst, M.; Casuso, P.; Loinaz, I.; Grande, H.J.; Alini, M.; Eglin, D.; Dupin, D. Self-Healing Dynamic Hydrogel as Injectable Shock-Absorbing Artificial Nucleus Pulposus. Biomacromolecules 2017, 18, 2360–2370. [CrossRef][PubMed] 20. Pepels, M.; Filot, I.; Klumperman, B.; Goossens, H. Self-healing systems based on disulfide-thiol exchange reactions. Polym. Chem. 2013, 4, 4955–4965. [CrossRef] 21. Gao, W.; Bie, M.; Liu, F.; Chang, P.; Quan, Y. Self-Healable and Reprocessable Polysulfide Sealants Prepared from Liquid Polysulfide Oligomer and Epoxy Resin. ACS Appl. Mater. Interfaces 2017, 9, 15798–15808. [CrossRef] 22. Li, G.; Zhang, H.; Daniel, F.; Weizheng, F.; Xia, H.; Zhao, Y. A composite material with room temperature shape processabillity and optical repair. J. Mater. Chem. C 2016, 4, 5932–5939. [CrossRef] 23. Habault, D.; Hongji Zhang, H.; Zhao, Y. Light-triggered self-healing and shape-memory polymers. Chem. Soc. Rev. 2013, 42, 7244–7256. [CrossRef][PubMed] 24. Xu, W.M.; Rong, M.Z.; Zhang, M.Q. Sunlight driven self-healing, reshaping and recycling of a robust, transparent and yellowing resistant polymer. J. Mater. Chem. A 2016, 4, 10683–10690. [CrossRef] 25. Kim, S.M.; Jeon, H.; Shin, S.H.; Park, S.A.; Jegal, J.; Hwang, S.Y.; Oh, D.X.; Park, J. Superior Toughness and Fast Self-Healing at Room Temperature Engineered by Transparent Elastomers. Adv. Mater. 2018, 30, 1–8. [CrossRef][PubMed] 26. Yanagisawa, Y.; Nan, Y.; Okuro, K.; Aida, T. Mechanically robust, readily repairable polymers via tailored noncovalent cross-linking. Science 2018, 359, 72–76. [CrossRef][PubMed] 27. Capelot, M.; Montarnal, D.; Tournilhac, F.; Leibler, L. Metal-catalyzed transesterification for healing and assembling of thermosets. J. Am. Chem. Soc. 2012, 134, 7664–7667. [CrossRef][PubMed] 28. Denissen, W.; Rivero, G.; Nicolaÿ, R.; Leibler, L.; Winne, J.M.; Du Prez, F.E. Vinylogous urethane vitrimers. Adv. Funct. Mater. 2015, 25, 2451–2457. [CrossRef] 29. Montarnal, D.; Capelot, M.; Tournilhac, F.; Leibler, L. Silica-Like Malleable Materials from. Science 2011, 334, 965–968. [CrossRef][PubMed] 30. Koberstein, J.T.; Galambos, A.F.; Leung, L.M. Compression-molded polyurethane block copolymers. 1. Microdomain morphology and thermomechanical properties. Macromolecules 1992, 25, 6195–6204. [CrossRef] 31. Saiani, A.; Rochas, C.; Eeckhaut, G.; Daunch, W.A.; Leenslag, J.-W.; Higgins, J.S. Origin of Multiple Melting Endotherms in a High Hard Block Content Polyurethane. 2. Structural Investigation. Macromolecules 2004, 37, 1411–1421. [CrossRef] 32. Leung, L.M.; Koberstein, J.T. Small-angle scattering analysis of hard-microdomain structure and microphase mixing in polyurethane elastomers. J. Polym. Sci. 1985, 23, 1883–1913. [CrossRef] Molecules 2019, 24, 2201 13 of 13

33. Kojio, K.; Matsuo, K.; Motokucho, S.; Yoshinaga, K.; Shimodaira, Y.; Kimura, K. Simultaneous small-angle X-ray scattering/wide-angle X-ray diﬀraction study of the microdomain structure of polyurethane elastomers during mechanical deformation. Polym. J. 2011, 43, 692–699. [CrossRef] 34. Zoran, S.P.; Zoltan, Z.; Joseph, H.F.; William, J.M. Thermal degradation of segmented polyurethanes. J. Appl. Polym. Sci. 1994, 51, 1087–1095. [CrossRef]

Sample Availability: Samples of the compounds are not available or not from the authors.

© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).