Sorbitol As a Chain Extender of Polyurethane Prepolymers to Prepare Self-Healable and Robust Polyhydroxyurethane Elastomers

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Article Sorbitol as a Chain Extender of Polyurethane Prepolymers to Prepare Self-Healable and Robust Polyhydroxyurethane Elastomers

Sang Hyub Lee, Se-Ra Shin and Dai-Soo Lee *

Department of Semiconductor and Chemical Engineering, Chonbuk National University, 567 Baekje-daero, Deokjini-gu, Jeonju 54896, Korea; [email protected] (S.H.L.); [email protected] (S.-R.S.) * Correspondence: [email protected]; Tel.: +82-63-270-2431

Received: 13 September 2018; Accepted: 29 September 2018; Published: 30 September 2018

Abstract: A self-healable polyhydroxyurethane (S-PU) was synthesized from sorbitol, a biomass of polyhydric alcohol, by a simple process that is suitable for practical applications. In the synthesis, only two primary hydroxyl groups of sorbitol were considered for the chain extension of the polyurethane (PU) prepolymers to introduce free hydroxyl groups in PU. As a control, conventional PU was synthesized by hexane diol mediated chain extension. Relative to the control, S-PU showed excellent intrinsic self-healing property via exchange reaction, which was facilitated by the nucleophilic addition of the secondary hydroxyl groups without any catalytic assistance and improved tensile strength due to the enhanced hydrogen bonding. We also investigated the effect of the exchange reaction on the topological, mechanical, and rheological properties of S-PU. The suggested synthetic framework for S-PU is a promising alternative to the conventional poly hydroxyurethane, in which cyclic carbonates are frequently reacted with amines. As such, it is a facile and environmentally friendly material for use in coatings, adhesives, and elastomers.

Keywords: sorbitol; self-healing; polyhydroxyurethane; exchange reaction

1. Introduction Polyurethane (PU) is one of the most versatile polymers. It is widely used in foams, coatings, adhesives, and elastomers due to its excellent mechanical properties and chemical resistance. Moreover, its properties are readily adjusted by varying either the process or the compositions of the isocyanates and polyols constituting its backbone via polymerization [1–4]. Its versatility has sparked great interest in self-healable PUs with higher durability when compared to the standard PUs. Moreover, self-healable materials are eco-friendly and cost-effective [5–32]. The development of self-healable PUs has signiﬁcantly advanced in recent decades. The self-healability and reprocessability of PUs has been achieved by several strategies based on reversible dynamic covalent chemistry, such as the Diels–Alder (DA) and retro-DA reactions [11–14], disulﬁde metathesis [15–23], alkoxyamine chemistry [24], the amine–urea exchange reaction [25], and the reversible acetal chemistry [26]. However, dynamic chemistry brings into play the functional compounds into the PU system, thereby increasing the cost of the raw materials and the process complexity. Furthermore, the introduction of dynamic chemistry into the urethane system frequently not only aggravates the mechanical properties but the practical applicability of PUs synthesized by dynamic chemistry would also be restricted. The transcarbamoylation reaction (TCR) is a dynamic chemistry based on exchange reactions among the carbamate groups. The TCR has recently attracted more attention because it achieves excellent self-healability and reprocessability of the PU network system based on the dynamic

Molecules 2018, 23, 2515; doi:10.3390/molecules23102515 www.mdpi.com/journal/molecules Molecules 2018, 23, 2515 2 of 13 chemistry [27–31]. Unfortunately, the TCR of PUs generally requires high temperature (>200 ◦C) without catalysts, and decomposes or dissociates the carbamate linkages, resulting in adverse side reactions. Dichtel et al. synthesized a polyhydroxyurethane (PHU) vitrimer from a six-membered cyclic carbonate. They reported that the nucleophilic addition of a free hydroxyl group to the carbamate group enables the exchange reaction among the carbamate groups under relatively mild conditions, unlike the harsh conditions of previous reports [32]. However, the secondary reaction and low reactivity between the cyclic carbonate and amine derivatives were reported to lower the average molecular weight of cyclic carbonate-based PHUs [33–36], and the side reactions at high temperatures lowered the mechanical properties of PHUs [30,32]. Furthermore, manufacturing PHUs from cyclic carbonates is a slow, complex process than processing conventional PUs. Thus, it is difﬁcult to upscale cyclic carbonate-based PHUs to industrial applications. Practical applicability of PHU preparation requires a simpler manufacturing process. Sorbitol is a sugar alcohol obtained from the reduction of glucose [37–39]. This renewable resource is a common ingredient in food, cosmetics, medical applications, and polymer syntheses [40–44]. As the sorbitol molecule contains two primary hydroxyl groups and four secondary hydroxyl groups, it may be considered a chain extender of N=C=O-terminated PU prepolymers. Most primary –OH groups are expected to react with the N=C=O group of the prepolymer, thereby allowing the chain extension reaction, while most secondary –OH groups remain as free –OH units due to the large difference of reactivity toward the isocyanate groups [45–47]. Free –OH groups in PU may accelerate the activation of the TCR under mild conditions through nucleophilic addition to carbamate groups [32]. Inspired by these revelations, we simply prepared self-healable PHUs using sorbitol as the chain extender of conventional PU prepolymers. The prepared sorbitol-extended PU (S-PU) exhibited stronger mechanical properties and higher self-healing efﬁciency than the control PU chain extended with a short diol, 1,6-hexane diol. The short-term heating without any catalyst increased the excellent self-healing property of S-PU through the exchange reactions of the carbamate groups via nucleophilic addition of the hydroxyl groups. To the best of our knowledge, there was no report to use sorbitol as a chain extender of PU prepolymers, thereby generating PHU elastomeric systems with self-healing ability and enhanced mechanical properties.

2. Experiment

2.1. Materials 4,40-Methylene-bis(phenyl isocyanate) (MDI) was purchased from Aldrich Chemical (Young-in) in Korea and used as received. Poly(tetramethylene ether glycol) (PTMEG) (Number average molecular weight (Mn): 2000 g/mol), sorbitol, and 1,6-hexane diol (HD), also purchased from Aldrich Chemical (Young-in) in Korea, were vacuum-dried at 60 ◦C for 1 day prior to use. Dimethylformamide (DMF) was used for solution casting of the prepared S-PU and H-PU.

2.2. Synthesis of PU Prepolymers PU prepolymers were synthesized through the one-pot reaction between MDI and PTMEG without any solvent or catalyst. First, 2 mol of MDI and 1 mol of PTMEG were placed in a ◦ round-bottomed ﬂask reactor with N2 gas purging at 50 C and stirred to form a homogeneous mixture. After homogeneous mixing, the reaction temperature increased to 60 ◦C and the mixing was continued until the NCO% of the resultant reached its theoretical value. The NCO% change was determined following the ASTM D1638-74. Once the NCO% had reached its theoretical value, the reaction was completed and the prepared PU prepolymer was dissolved in DMF (at a weight ratio of prepolymer:DMF = 2:1). Molecules 2018, 23, 2515 3 of 13

2.3. Preparation of S-PU and H-PU The prepared PU prepolymer/DMF solution was mixed with 1,6-HD or sorbitol, initiating the chain extension of the N=C=O-terminated PU prepolymer. The stoichiometric amounts of HD and sorbitol were calculated based on the NCO% determined by ASTM D1638-74, sorbitol being considered diol. The PU prepolymer/DMF solution and chain extenders were thoroughly mixed by mechanical stirringMolecules for 1 h 2018 at room, 23, x FOR temperature, PEER REVIEW and then poured into a Teﬂon mold. The mold containing the 3 of 13 homogenous mixture was placed in a 110 ◦C convection oven for 1 day to remove the DMF and to facilitatecontaining the polymerization the homogenous of the PUmixture prepolymer was placed by its in chain a 110 extenders.°C convection The oven PU ﬁlms for 1 prepared day to remove by the chain extensionDMF and of to sorbitol facilitate and the HD polymerization were named S-PUof the andPU H-PU,prepolymer respectively. by its chain The preparationextenders. The steps PU films and typicalprepared chemical by chain structures extension of S-PU of andsorbitol H-PU and are HD shown were in named Scheme 1S-PU. and H-PU, respectively. The preparation steps and typical chemical structures of S-PU and H-PU are shown in Scheme 1.

Scheme 1. Preparation of S-PU and H-PU from isocyanate-terminated PU prepolymers. Scheme 1. Preparation of S-PU and H-PU from isocyanate-terminated PU prepolymers. 2.4. Characterization 2.4. Characterization The chemical structures of the prepared samples were characterized by FT-IR spectroscopy (FT/IR 300E, JASCO,The Easton, chemical MD, structures USA). In addition, of the prepared the exchange samp reactionles were of characterized S-PU was investigated by FT-IR usingspectroscopy a thermally(FT/IR controllable 300E, JASCO, IR pellet Easton, loader. MD, The USA). potassium In addition, bromide the exchange (KBr) tablet reaction was placed of S-PU on was the FT-IRinvestigated using a thermally controllable IR pellet loader. The potassium bromide (KBr) tablet was placed on sample loader and its temperature was successfully controlled by N2 gas purging. The molecular weightsthe and FT-IR polydispersity sample loader indices and (PDIs)its temperature were determined was successfully by gel permeationcontrolled by chromatography N2 gas purging. The (GPC) (Alliancemolecular e2695, weights Waters, and Milford,polydispersity MA, USA). indices The calibration(PDIs) were curves determined for GPC wereby gel prepared permeation using polystyrenechromatography standards. (GPC) (Alliance The GPC e2695, apparatus Waters, was Mi equippedlford, MA, with USA). a refractive The calibration index curves detector, for GPC and thewere eluent prepared used was using DMF polystyrene ﬂowing atstandards. 0.4 mL/min. The GPC Differential apparatus scanning was equipped calorimetry with (DSC), a refractive index detector, and the eluent used was DMF flowing at 0.4 mL/min.◦ Differential scanning (Q20, TA Instrument, New Castle, DE, USA) was carried out at a heating rate of 10 C/min with N2 gas purging.calorimetry The dynamic (DSC), (Q20, mechanical TA instrument, properties New and stressCastle relaxation, DE, USA) behaviors was carried of the out samples at a heating were rate of investigated10 °C/min using with a dynamic N2 gas mechanicalpurging. The analyzer dynamic (DMA) mechanical (Q800, prop TA Instrument).erties and stress The dimensionsrelaxation behaviors of the DMAof the samples samples were were 13 mm investigated× 5 mm × using1 mm a (length dynamic× width mechanical× thickness), analyzer and (DMA) the heating (Q800, TA rate wasInstrument). 5 ◦C/min. The The dimensions stress relaxation of the DMA test wassample carrieds were out 13 atmm 130 × 5◦C mm under × 1 mm a strain (length of 5%.× width × The dimensionalthickness), changesand the heating of the samples rate was were 5 °C/min. characterized The stress using relaxation a thermomechanical test was carried analyzer out at 130 °C (TMA)under (Q400, a TA strain Instruments) of 5%. heatedThe dimensional at 5 ◦C/min. changes Atomic of force the microscopy samples were (AFM) characterized (Multimode-8, using a thermomechanical analyzer (TMA) (Q400, TA Instruments) heated at 5 °C/min. Atomic force microscopy (AFM) (Multimode-8, Bruker, Billerica, MA, USA) was employed to observe the micro- phase separated structures of S-PU and H-PU. The changes in the morphological features of the self- healed PU films were observed by field emission scanning electron microscopy (FE-SEM) (AIS2100, SERON, Uiwang-si, Korea). The tensile properties and self-healing efficiencies of the prepared PU samples were investigated employing a universal testing machine (UTM) (LR5K Plus, LLOYD, West Sussex, UK). All measurements in the tensile tests were carried out at 25 °C with a cross-head speed of 500 mm/min. The oscillatory shear tests were conducted employing a rheometer (TA, AR2000, New Castle, DE, USA).

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Bruker, Billerica, MA, USA) was employed to observe the micro-phase separated structures of S-PU and H-PU. The changes in the morphological features of the self-healed PU films were observed by field emission scanning electron microscopy (FE-SEM) (AIS2100, SERON, Uiwang-si, Korea). The tensile properties and self-healing efficiencies of the prepared PU samples were investigated employing a universal testing machine (UTM) (LR5K Plus, LLOYD, West Sussex, UK). All measurements in the tensileMolecules tests 2018 were, 23, x FOR carried PEER out REVIEW at 25 ◦ C with a cross-head speed of 500 mm/min. The oscillatory shear4 of 13 tests were conducted employing a rheometer (TA, AR2000, New Castle, DE, USA). TheThe samples samples were were placed placed on on the the 25 25 mm mm parallel parallel plate plate of of the the rheometer rheometer at at room room temperature temperature and and heated to 210◦ °C. The samples were retained at 210◦ °C with N2 gas purging until they had fully melted. heated to 210 C. The samples were retained at 210 C with N2 gas purging until they had fully melted. TheThe gap gap was was set set to to 1.0 1.0 mm. mm. The The frequency frequency sweep sweep (from (from 620 620 to to 0.01 0.01 rad/s) rad/s) were were then then performed performed under under 0.1%0.1% strain strain after after trimming. trimming.

3.3. Results Results and and Discussion Discussion

3.1.3.1. Synthesis Synthesis of of S-PU S-PU and and H-PU H-PU TheThe chemical chemical structuresstructures ofof S-PUS-PU and H-PU were were characterized characterized by by their their FT-IR FT-IR spectra spectra shown shown in −1 inFigure Figure 1.1 .The The peak peak at at2270 2270 cm cm−1 correspondingcorresponding to the to N=C=O the N=C=O groups groups of PU ofprepolymers PU prepolymers did vanish did vanishafter the after chain the extension chain extension reaction reaction with sorbitol with sorbitolor HD. In or the HD. spectrum In the spectrum of chain-extended of chain-extended S-PU, the −1 S-PU,peak thedue peak to –NH due tostretching –NH stretching at 3310 at cm 3310−1 is cm attributedis attributed to urethane to urethane linkage linkage formation formation [48]. [ 48The]. −1 Thehydrogen-bonded hydrogen-bonded (H-bond) (H-bond) –OH –OH stretching stretching at 3500–3400 at 3500–3400 cm−1 cm and theand free the –OH free –OHstretching stretching at 3630– at −1 3630–36253625 cm−1 cmare attributedare attributed to the free to the –OH free groups –OHgroups of sorbitol, of sorbitol, which did which not didparticipate not participate in the urethane in the urethanelinkage linkageformation. formation. Although Although the primary the primary –OH of –OH sorbitol of sorbitol is more is more reactive reactive with with N=C=O N=C=O than than the thesecondary secondary –OH –OH [45–47], [45–47 ],the the spectrum spectrum exhibits exhibits the the characteristic characteristic –C–O –C–O peaks ofof bothboth thethe secondarysecondary −1 andand the the primaryprimary hydroxyl hydroxyl groups groups at at 1111 1111 and and 1065 1065 cm cm−1, respectively, respectively (Supplementary (Supplementary material, Material, Figure FigureS1). This S1). indicates This indicates that during that during the chain the chainextensio extension,n, some some of the of secondary the secondary –OH –OH groups groups reacted reacted with withthe N=C=O the N=C=O group group of the of PU the prepolymer. PU prepolymer. On the On othe ther otherhand, hand, the H-PU the H-PU spectrum spectrum (with (withno free no –OHs) free − –OHs)exhibits exhibits only onlythe –NH the –NH stretching stretching of the of theurethane urethane unit unit (3310 (3310 cm cm−1) and1) and the the –C–O–C– –C–O–C– stretching stretching of −1 ofpolyol polyol (1107 (1107 cm cm−1) (Supplementary) (Supplementary material, Material, Figure Figure S1) S1) [49–52]. [49–52 The]. The molecular molecular weights weights (Mn (M andn and Mw) Mandw) andPDIs PDIs measured measured with with GPC GPC for forS-PU S-PU and and H-PU H-PU are are summarized summarized in inTable Table 1 1(Raw (Raw data data of of GPC GPC in inSupplementary Supplementary material, Material, Figure Figure S2 S2).). The The molecular molecular weights weights and PDIs ofof S-PUS-PU andand H-PUH-PU are are similar similar becausebecause both both polymers polymers were were prepared prepared by by the the same same isocyanate–hydroxyl isocyanate–hydroxyl group group reaction reaction between between the the PUPU prepolymer prepolymer and and the the hydroxyl hydroxyl chain chain extender extender (sorbitol (sorbitol or or HD). HD).

Figure 1. FT-IR spectra of the chain extenders (sorbitol and HD), PU prepolymer, S-PU, and H-PU. Figure 1. FT-IR spectra of the chain extenders (sorbitol and HD), PU prepolymer, S-PU, and H-PU.

Table 1. Composition and molecular weights of the prepared PUs.

Composition (Molar Ratio) Sample Mn (g/mol) Mw (g/mol) PDI MDI PTMEG Sorbitol HD S-PU 2 1 1 – 56,600 126,900 2.24 H-PU 2 1 – 1 64,600 131,900 2.04

Figure 2a shows the possible H-bonds in S-PU. In conventional polyether–polyol-based PU systems, the –NH and C=O units of urethane inter- or intra-molecularly interact with the –C–O–C

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Table 1. Composition and molecular weights of the prepared PUs.

Composition (Molar Ratio) Sample Mn (g/mol) Mw (g/mol) PDI MDI PTMEG Sorbitol HD S-PU 2 1 1 – 56,600 126,900 2.24 H-PU 2 1 – 1 64,600 131,900 2.04

Figure2a shows the possible H-bonds in S-PU. In conventional polyether–polyol-based PU systems, the –NH and C=O units of urethane inter- or intra-molecularly interact with the –C–O–C units of polyol, confirming that H-bonding of the urethane moiety is possible [53]. However, the free –OH groups of S-PU can form further H-bonds in the hard and soft domains, as illustrated in Figure2a. Molecules 2018, 23, x FOR PEER REVIEW 5 of 13 The formation of H-bonding with unreacted –OH of S-PU was investigated in the FT-IR spectra (see Figureunits S1). of The polyol, strength confirming of H-bonds that H-bonding largely of affected the urethane the thermal moiety is and possible mechanical [53]. However, properties the free of S-PU. In conventional–OH groups PUsof S-PU prepared can form from further the H-bonds PU prepolymer in the hard and and diols, soft domains, a micro-phase-separated as illustrated in Figure structure resulting2a. The from formation the partial of H-bonding solubility with of unreacted the hard and–OH softof S-PU domains was investigated is usually in seen the [FT-IR54,55 spectra]. The DSC (see Figure S1). The strength of H-bonds largely affected the thermal and mechanical properties of S- thermogram of H-PU given in Figure S3 revealed clear glass transition temperatures (Tg) of the hard and softPU. segments.In conventional Meanwhile, PUs prepared as the temperaturefrom the PU increasedprepolymer in and the diols, DMA a measurements micro-phase-separated in Figure S4, structure resulting from the partial solubility of the hard and soft domains is usually seen [54,55]. The a typical glass-rubbery transition with rubbery plateau region appeared, reflecting the well-organized DSC thermogram of H-PU given in Figure S3 revealed clear glass transition temperatures (Tg) of the micro-phasehard and separatedsoft segments. structures. Meanwhile, On as the the other temp hand,erature the increased glass transition in the DMA temperature measurements of thein soft segmentFigure was S4, nota typical seen inglass-rubbery the DSC thermogram transition with of S-PUrubbery (Figure plateau S3), region and appeared, the low associationreflecting the in the hardwell-organized domains caused micro-phase a rapid decreaseseparated instructures. the storage On the modulus other hand, in thethe glass rubbery transition plateau temperature region of the DMAof data, the soft with segment an increased was not seen loss in factor the DSC (tan thermoδ) (Figuregram S4)of S-PU [54,55 (Figure]. Figure S3),2 andb shows the low the association stress–strain curvesin ofthe S-PU hard domains and H-PU caused determined a rapid decrease in tensile in the tests. storage The mechanicalmodulus in the properties rubbery plateau (elongation region at of break (ε) andthe stressDMA valuedata, with (σ)) an were incr highereased loss in S-PUfactor than(tan δ in) (Figure H-PU. S4) Specifically, [54,55]. Figure the 2b values shows of theε andstress–σ were 658%strain and 22.9curves MPa, of S-PU respectively, and H-PU in determined S-PU, versus in tensile 434% tests. and 20.5The MPa,mechanical respectively, properties in (elongation H-PU. The at higher break (ε) and stress value (σ)) were higher in S-PU than in H-PU. Specifically, the values of ε and σ mechanical properties of S-PU are attributed to the formation of H-bonds as displayed in Figure2a. were 658% and 22.9 MPa, respectively, in S-PU, versus 434% and 20.5 MPa, respectively, in H-PU. Although the H-bonding improved the σ and ε of S-PU, the strain hardening effects were slightly The higher mechanical properties of S-PU are attributed to the formation of H-bonds as displayed in delayed,Figure occurring 2a. Although around the H-bonding 430% strain improved in S-PU, the versus σ and ε 300% of S-PU, strain the strain in H-PU. hardening The strain effects hardening were of H-PUslightly at lowerdelayed, strain occurring is attributable around 430% to the strain micro-phase in S-PU, versus separated 300% structurestrain in H-PU. of the The hard strain and soft domainshardening [56]. On of H-PU the other at lower hand, strain the relativelyis attributable less to micro-phase-separated the micro-phase separated structure structure of S-PU,of the causedhard by the H-bondand soft formation,domains [56]. weakened On the other the strainhand, the hardening relatively in less tensile micro-phase- test. Theseparated difference structure in micro-phase of S- separationPU, caused of S-PU by the and H-bond H-PU formation, could be weakened confirmed the in st AFMrain hardening images given in tensile in Figure test. The S5. difference H-PU showed in domainsmicro-phase of hard separation segments inof S-PU submicron and H-PU size could while be S-PU confirmed did not in AFM due to images the relatively given in Figure less micro-phase S5. H- separatedPU showed structures. domains It of is hard speculated segments that in submicron the hydroxy size while urethane S-PU unitdid not introduced due to the relatively for the exchange less micro-phase separated structures. It is speculated that the hydroxy urethane unit introduced for the reaction with the urethane chains lowered the micro-phase separation of S-PU over the control PU exchange reaction with the urethane chains lowered the micro-phase separation of S-PU over the (H-PU), but the hydrogen bonds formed by the hydroxyl groups conferred excellent mechanical control PU (H-PU), but the hydrogen bonds formed by the hydroxyl groups conferred excellent properties,mechanical comparable properties, to comparable those of H-PU. to those of H-PU.

FigureFigure 2. Characteristics 2. Characteristics of of S-PU S-PU investigated: investigated: ( (aa) )possible possible hydrogen hydrogen bonds bonds in S-PU; in S-PU; (b) tensile (b) tensile propertiesproperties of S-PU of S-PU and and H-PU H-PU at at 25 25◦C. °C.

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3.2. Exchange Reaction and Thermomechanical Properties of S-PU Exchange reactions of carbamates initiated by free –OH groups are the probable exchange reaction among the hydroxy urethane units at elevated temperatures. Exchange reactions occur via nucleophilic addition of –OHs to carbamate under thermal activation and are temperature-dependent [29,32]. To investigate the exchange reaction mechanism among the hydroxy urethane units of S-PU, the FT-IR peaks corresponding to the free hydroxyl groups of S-PU were monitored during the heating process Molecules 2018, 23, x FOR PEER REVIEW 6 of 13 (from 80 to 140 ◦C) and shown in Figure3 after the normalization using peaks of methylene units. −1 Thehydroxyl peak groups intensities were of gradually –C–O at 1065increased cm withcorresponding increase of totemperature the primary by hydroxyl the exchange groups reaction were graduallygiven in Figure increased 4. In with contrast, increase the of –C–O temperature peaks at by 1111 the exchange cm−1 corresponding reaction given to inthe Figure secondary4. In contrast, –OH of −1 thesorbitol –C–O were peaks reduced at 1111 a cm little correspondingand shifted to tolowe ther secondarywavenumbers –OH with of sorbitol increasing were temperature. reduced a little Peaks and shifteddue to tofree lower –OH wavenumbers groups at 3625 with and increasing 3630 cm− temperature.1 are attributable Peaks to duesecondary to free –OH–OH groupsand primary at 3625 –OH and −1 3630groups, cm respectively.are attributable It was to observed secondary that –OH the and peak primary intensities –OH groups,at 3630 cm respectively.−1 increased It waswhile observed peak at −1 −1 that3625 thecm− peak1 decreased intensities with at increase 3630 cm of theincreased temperature. while These peak spectroscopic at 3625 cm featuresdecreased are with attributed increase to ofthe the thermally temperature. activated These exchange spectroscopic reaction features among arethe attributed–OH and carbamate to the thermally groups activated of S-PU molecules exchange reactionas described among in Figure the –OH 4 [29,32]. and carbamate However, groups in the H-PU of S-PU spectrum, molecules increasing as described the temperature in Figure4[ altered29,32]. However,the peak intensities in the H-PU and spectrum, shifted the increasing –NH (H-bonded the temperature urethane) altered and –C–O–C the peak (ether intensities in polyol) and shifted peaks thetoward –NH lower (H-bonded wavenumbers urethane) (Supplem and –C–O–Centary (ethermaterial, in polyol)Figure S6). peaks These toward trends lower correspond wavenumbers to the (Supplementarydissociation of H-bonds Material, in Figure H-PU. S6). The These exchange trends reaction correspond occurring to the in dissociation S-PU is absent of H-bonds in H-PU inbecause H-PU. Thethe exchangehydroxyl reactiongroup enabling occurring the in uncatalyzed S-PU is absent exchan in H-PUge reaction because of the carbamate hydroxyl groupgroups enabling is lacking. the uncatalyzedAccording to exchange these results, reaction the of hydroxyl carbamate unit groups is essential is lacking. for Accordingthe efficient to exchange these results, reaction the hydroxyl among unitthe hydroxyl-urethane is essential for the units efﬁcient under exchange mild conditions reaction among(relatively the low hydroxyl-urethane temperature at unitsambient under pressure mild conditionswithout catalyst). (relatively low temperature at ambient pressure without catalyst).

FigureFigure 3.3. Temperature-dependentTemperature-dependent FT-IRFT-IR spectraspectra ofofS-PU: S-PU:( a(a)) at at low low wavenumbers wavenumbers (1140–1040 (1140–1040 cm cm−−11); ((bb)) atat highhigh wavenumberswavenumbers (3800–2800(3800–2800 cmcm−−11).).

Figure 4. Schematic of the exchange reaction among the hydroxy urethane units of S-PU. Figure 4. Schematic of the exchange reaction among the hydroxy urethane units of S-PU.

The thermally activated exchange reaction of S-PU was probed by TMA. Figure 5a shows the dimensional changes in S-PU and H-PU measured by TMA. The coefficients of thermal expansion (CTEs) are summarized in Table 2. During the first and the second cycle, H-PU shows the typical linear expansion of PU [57,58], with no significant change in the CTE. However, the TMA of S-PU yielded two maximum points, at 79 °C and 149 °C. In the initial state, S-PU exhibited similar thermal expansion to H-PU. However, the dimensions of S-PU decreased after 79 °C, possibly because the thermally activated exchange reaction at 79 °C altered the configuration of this polymer (see Figure 4). The dimensional reduction was followed by a gradual increase in thermal expansion, although

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The thermally activated exchange reaction of S-PU was probed by TMA. Figure5a shows the dimensional changes in S-PU and H-PU measured by TMA. The coefficients of thermal expansion (CTEs) are summarized in Table2. During the first and the second cycle, H-PU shows the typical linear expansion of PU [57,58], with no significant change in the CTE. However, the TMA of S-PU yielded two maximum points, at 79 ◦C and 149 ◦C. In the initial state, S-PU exhibited similar thermal expansion to H-PU. However, the dimensions of S-PU decreased after 79 ◦C, possibly because the thermally activated exchange reaction at 79 ◦C altered the configuration of this polymer (see Figure4). The dimensional Molecules 2018, 23, x FOR PEER REVIEW 7 of 13 reduction was followed by a gradual increase in thermal expansion, although the CTE was significantly ◦ lowerthe CTE in the was high significantly temperature lower range in than the high in the temperature low temperature range range than(40~79 in the lowC), astemperature shown in Table range2. ◦ Above(40~79 149°C), C,as theshown CTE wasin Table reduced 2. Above by the 149 exchange °C, the reaction CTE was accompanying reduced by the the stress exchange relaxation reaction [59]. ◦ Onaccompanying the other hand, the stress the CTE relaxation of H-PU [59]. was On much the ot higherher hand, than the that CTE of S-PUof H-PU above was 79 muchC and higher did than not decreasethat of S-PU with above further 79heating. °C and did These not results decrease reveal with that further the exchangeheating. These reaction results of S-PU reveal molecules that the restrictsexchange the reaction thermal of expansion S-PU molecules of the polymer restricts matrix. the thermal Unlike expansion the first cycle, of the the polymer second matrix. measurement Unlike ◦ yieldedthe first nocycle, dimensional the second reduction measurement at 79 yieldedC and theno dimensional thermal expansion reduction was at still 79 °C suppressed and the thermal by the exchangeexpansion reaction was still (as suppressed evidenced byby the the reduced exchange CTE). reaction Furthermore, (as evidenced the maximum by the temperaturereduced CTE). of ◦ TMAFurthermore, thermogram the maximum in S-PU increased temperat fromure of 149 TMA to 152thermogramC, implying in S-PU that increased S-PU was from configurationally 149 to 152 °C, rearrangedimplying that by the S-PU exchange was reaction.configurationally The effect rearranged of configuration by the change exchange in S-PU reaction. was also The confirmed effect byof theconfiguration repeated heating change and in S-PU cooling was tests also in confirmed DMA. Although by the repeated the DMA heating thermograms and cooling of H-PU tests remained in DMA. constant,Although the the lowered DMA thermograms micro-phase separation of H-PU remained caused by constant, three repeated the lowered tests of micro-phase heating and quenchingseparation 0 reducedcaused by the three storage repeated modulus, testsG of, of heating S-PU (Supplementary and quenching Material,reduced Figurethe storage S7). Loweredmodulus, micro-phase G′, of S-PU 0 separation(Supplementary also slightly material, reduced Figure the S7). G Loweredin the rubbery micro-phase plateau separation region of also S-PU slightly (Figure reduced5b). However, the G′ in ◦ 0 atthe higher rubbery temperatures plateau region (above of S-PU 125 (FigureC), the G5b).of Howe S-PUver, clearly at higher improved temperatures in cycles 2(above and 3 125 (Figure °C),5 theb, inset),G′ of S-PU implying clearly that improved the exchange in cycles reaction 2 and affected3 (Figure the 5b, configuration inset), implying change that inthe S-PU, exchange conferring reaction a partialaffected gel-like the configuration property. change in S-PU, conferring a partial gel-like property.

FigureFigure 5.5. Thermal properties ofof S-PUS-PU andand H-PUH-PU duringduring thethe repeatedrepeated heatingheating cycle:cycle: ((aa)) thermalthermal expansionexpansion measured by by TMA; TMA; (b ()b storage) storage modulus modulus versus versus temperat temperatureure of S-PU of S-PU in repeated in repeated DMA DMAmeasurements. measurements. Table 2. The coefﬁcients of thermal expansion (CTEs) of S-PU and H-PU. Table 2. The coefficients of thermal expansion (CTEs) of S-PU and H-PU. CTE (1/◦C) CTE (1/°C) 1st1st Cycle Cycle 2nd Cycle 2nd Cycle a b a b S-PU S-PU 0.4840.484 a 0.1320.132 b 0.398 0.398 a 0.306 0.306 b H-PU H-PU 0.4360.436 0.459 0.459 a ◦ b ◦ a CTECTE at at 40~79 °C.C. bCTE CTE at at 100~140 100~140C. °C.

To clarify how the exchange reaction affects the rheological properties of the polymer systems, we investigated the dynamic rheological properties of S-PU and H-PU under oscillation mode at 180, 190, 200, and 210 °C. At elevated temperatures, S-PU showed good thermal stability with no dissociation of the urethane unit or by-product formation through side reactions (Supplementary material, Figure S8). Owing to its linear structure, H-PU exhibited liquid-like behavior (i.e., G′ < G″) over the whole range of available angular frequencies at 180 °C as shown in Figure 6a. S-PU exhibited similar liquid-like behavior at high frequencies (G′ < G″), but its flow resistance dramatically increased with decreasing angular frequency. After the crossover point (where G′ = G″) at 1.62 rad/s, S-PU exhibited a solid-like property (G′ > G″), which implies the formation of a cross-linked gel-like structure [60–63]. This transition of S-PU from G′ < G″ to G′ > G″ at low frequencies was attributed to the exchange reaction accompanying the formation of gels. Low frequencies provide a sufficient time

MoleculesMolecules2018 2018,,23 23,, 2515 x FOR PEER REVIEW 88 ofof 1313

scale for active exchange with abundant –OH groups. The resultant gel formation by the exchange reactionTo clarifywas also how evidenced the exchange by the reaction increased affects complex the rheologicalviscosity (η*) properties after the ofcrossover the polymer point systems,(Supplementary we investigated material, theFigure dynamic S9) [63,64]. rheological properties of S-PU and H-PU under oscillation ◦ modeFigure at 180, 6b gives 190, 200,the frequency and 210 dependenciesC. At elevated of G′ temperatures,and G″ in S-PUS-PU at different showed temperatures, good thermal and stabilityTable 3 lists with the no crossover dissociation points of the at each urethane temperatur unit ore. by-product As the temperature formation increased, through sidethe rate reactions of the (Supplementaryexchange reaction Material, increased, Figure so S8).the crossover Owing to itspoints linear were structure, found H-PU at higher exhibited frequencies liquid-like (shorter behavior time 0 ◦ (i.e.,scales). G < G G”)′ also over increased the whole with range temperature, of available implying angular that frequencies the rate of atgel 180 formationC as shown was controlled in Figure6 bya. 0 S-PUthe rate exhibited of the exchange similar liquid-like reaction. On behavior the other at highhand, frequencies in H-PU (which (G < G”),lacks butthe itshydroxy ﬂow resistance unit), the 0 dramaticallyexchange reaction increased was with not decreasingactivated and angular the G frequency.′-to-G″ crossover After the was crossover not observed point (where until 210 G =°C. G”) This at 0 1.62implies rad/s, a sluggish S-PU exhibited gel formation a solid-like by the property exchange (G > reaction G”), which among implies the thecarbamate formation groups, of a cross-linked which was 0 0 gel-likeactivated structure at high temperatures [60–63]. This by transition –OH groups of S-PU (Supplementary from G < G” material, to G > G”Figure at low S10) frequencies [27–30]. That was is, attributedthe hydroxy to theurethane exchange unit reaction of S-PU accompanying enables the exchange the formation reaction of gels. between Low frequencies –OH and carbamate provide a sufﬁcientgroups at time lower scale temperatures for active exchange than the withexchange abundant reaction –OH among groups. carbonate The resultant groups gel in formation H-PU. The by thehydroxy exchange urethane reaction unit was is promising also evidenced for realizing by the increased a self-healing complex polymer, viscosity as it ( ηinitiates*) after thethe crossoverexchange pointreaction (Supplementary at relatively low Material, temperature, Figure S9)thereby [63,64 minimizing]. damage by oxidation or side reactions.

0 FigureFigure 6.6.Rheological Rheological propertiesproperties ofof S-PUS-PU andand H-PU:H-PU: ((aa)) StorageStorage modulimoduli ((GG,′, ﬁlledfilled symbols)symbols) andand lossloss ◦ 0 modulimoduli ((GG”,″, openopen symbols)symbols) versusversus angularangular frequencyfrequencyof ofS-PU S-PU (red) (red) and and H-PU H-PU (black) (black) at at 180 180 °C;C;( b(b)G) G′ ◦ ◦ ◦ ◦ andand G”G″ versusversus angularangular frequencyfrequency ofof S-PUS-PU atat 180180 °C,C, 190190 °C,C, 200200 °CC and 210 °C.C.

0 FigureTable6b 3. gives Angular the frequencies frequency dependenciesof G′-to-G″ crossover of G inand S-PU G” and in S-PUH-PU atat different temperatures. temperatures, and Table3 lists the crossover points at each temperature. As the temperature increased, the rate of the S-PU H-PU exchange reaction increased, so the crossover points were found at higher frequencies (shorter time scales). G0 also increased180 °C with 190 temperature, °C 200 °C implying 210 °C that 180 the °C rate 190 of °C gel 200 formation °C 210 was °C controlled by the rateω of (rad/s) the exchange 1.62 reaction. 9.55 On 131.57 the other 159.47 hand, in – H-PU –(which lacks – the 0.08 hydroxy unit), the exchange reaction was not activated and the G0-to-G” crossover was not observed until 210 ◦C. This3.3. Self-Healing implies a sluggish gel formation by the exchange reaction among the carbamate groups, which was activated at high temperatures by –OH groups (Supplementary Material, Figure S10) [27–30]. That is, The self-healing properties of H-PU and S-PU were investigated in a tensile test of pristine the hydroxy urethane unit of S-PU enables the exchange reaction between –OH and carbamate groups (uncut) and healed samples after cut. For the self-healing tests, the dog-bone shaped H-PU and S-PU at lower temperatures than the exchange reaction among carbonate groups in H-PU. The hydroxy samples were cut into two pieces by a razor blade. The pieces were then contacted closely and urethane unit is promising for realizing a self-healing polymer, as it initiates the exchange reaction at incubated for 120 min in a convection oven at 130 °C (Supplementary material, Figure S11). Figure relatively low temperature, thereby minimizing damage by oxidation or side reactions. 7a,b shows the tensile test results of the uncut and self-healed samples of H-PU and S-PU, respectively. To evaluate the ability of self-healing of S-PU and H-PU, repeated cutting-healing tests Table 3. Angular frequencies of G0-to-G” crossover in S-PU and H-PU at different temperatures. with specimens for tensile tests were performed up to three times on same samples and the results of the tensile tests are summarized in TableS-PU 4. After self-healing, the values H-PU of σ, ε and E of H-PU were largely reduced when cutting-healing180 ◦C 190 test◦C was 200 repeat◦C 210ed◦ Cup 180to three◦C 190times◦C because 200 ◦C H-PU 210 ◦C self-heals by a physical mechanism based on the diffusion of molecules above Tg [65,66]. As already confirmed by ω (rad/s) 1.62 9.55 131.57 159.47 – – – 0.08 the rheological properties, the carbamate groups spontaneously exchange at high temperatures (about 210 °C) without catalysts. On the other hand, the tensile properties of S-PU maintained its integrity after repeated cutting-healing tests, because physical self-healing was combined with the

Molecules 2018, 23, x FOR PEER REVIEW 9 of 13

exchange reaction activated by the nucleophilic addition of free –OH groups to carbamates, which occurred at a lower temperature than the carbamate exchanges in H-PU. MoleculesIn 2018the ,tensile23, 2515 test, the tensile strength (σ) and Young’s modulus (E) of S-PU were found 9to of be 13 slightly increased after self-healing and post curing process (Figure S12 and Table S1). This 3.3.improvement Self-Healing may be attributed to the cross-linked gel-structure formed by the thermally activated exchange reaction, as illustrated in Figure 7c. The increased gel fraction in healed S-PU was also confirmedThe self-healing by gel fraction properties measurements of H-PU in and DMF. S-PU The were gel fraction investigated of S-PU in almost a tensile doubled test of after pristine the (uncut)thermal andtreatment healed of samples the S-PU after sample cut. for For self-healing the self-healing or post tests, cure the (Table dog-bone 4 and shapedTable S1). H-PU In other and S-PUwords, samples the exchange were cut reaction into two in piecesS-PU not by aonly razor induces blade. recombination The pieces were among then contactedthe chains closelyof the ◦ anddamaged incubated S-PU, for but 120 also min improves in a convection the mechanical oven pr atoperties 130 C (Supplementarythrough the configuration Material, change Figure from S11). Figurelinear 7toa,b gel-like. shows The the self-healing tensile test resultsefficiencies of the of uncut S-PU and H-PU self-healed given samples in Table of4 were H-PU obtained and S-PU, by respectively.the following To equation, evaluate the ability of self-healing of S-PU and H-PU, repeated cutting-healing tests with specimens for tensile tests were performed up to three times on same samples and the results of ⁄ the tensile tests are summarizedSelf– healing efficienc in Table4. Aftery σ self-healing, σ the values of 100 %σ, ε and E of H-PU were(1) largelywhere reducedσhealed and when σpost-cured cutting-healing are tensile teststrengths was repeated of self-heal up toed three and times undamaged because post H-PU cured self-heals samples, by a physicalrespectively. mechanism In Table based 4, the onself-healing the diffusion efficiencies of molecules of S-PU above were T excellentg [65,66]. while As already those of conﬁrmed H-PU were by therelatively rheological low properties,and decreased the carbamate very much groups in repeated spontaneously tests. The exchange difference at high in temperaturesself-healing (aboutability ◦ 210betweenC) without S-PU and catalysts. H-PU was On also the otherinvestigated hand, the in an tensile SEM properties analysis of of the S-PU cut maintaineddamage and its self-healed integrity aftersamples. repeated The damage cutting-healing on the S-PU tests, surface because was physical almost self-healing fully repaired was after combined heating with at 130 the °C exchange for 120 reactionmin, whereas activated the bydamage the nucleophilic on H-PU surface addition was of ha freerdly –OH recovered groups to(Supplemen carbamates,tary which material, occurred Figures at a lowerS13 and temperature S14). than the carbamate exchanges in H-PU.

Figure 7. Results of self-healing at 130 ◦°CC for 120 min: ( (aa)) tensile tensile properties properties of of H-PU H-PU after after self-healing; self-healing; ((bb)) tensiletensile propertiesproperties ofof S-PUS-PU afterafter self-healing;self-healing; ((cc)) schematicschematic ofof thethe self-healingself-healing mechanismmechanism andand conﬁgurationalconfigurational changechange ofof typicaltypical S-PUS-PU afterafter thethe exchangeexchange reaction.reaction.

Table 4. Tensile properties and gel fraction of S-PU and H-PU after repeated cutting/self-healing cycles.

S-PU H-PU Properties Self-Healed S-PU Self-Healed H-PU Uncut S-PU Uncut H-PU 1st 2nd 3rd 1st 2nd 3rd σ (MPa) 22.92 25.24 23.76 22.67 22.53 5.182 4.541 2.554 ε (%) 660 620 580 590 430 290 260 130 E (MPa) 4.447 4.893 5.012 5.221 4.655 4.353 4.117 3.832 Self-healing – 93.79 88.29 84.24 – 22.68 19.87 11.17 efﬁciency (%) Gel-fraction a (%) 25.4 52.7 53.2 53.1 35.8 36.2 36.3 35.2 a The gel fractions were calculated as (W1 − W2)/W1 × 100 (where W1 is the initial sample weight and W2 is the weight of the residual solid contents after immersing the sample in DMF for 5 h at 25 ◦C). Molecules 2018, 23, 2515 10 of 13

In the tensile test, the tensile strength (σ) and Young’s modulus (E) of S-PU were found to be slightly increased after self-healing and post curing process (Figure S12 and Table S1). This improvement may be attributed to the cross-linked gel-structure formed by the thermally activated exchange reaction, as illustrated in Figure7c. The increased gel fraction in healed S-PU was also confirmed by gel fraction measurements in DMF. The gel fraction of S-PU almost doubled after the thermal treatment of the S-PU sample for self-healing or post cure (Table4 and Table S1). In other words, the exchange reaction in S-PU not only induces recombination among the chains of the damaged S-PU, but also improves the mechanical properties through the configuration change from linear to gel-like. The self-healing efficiencies of S-PU and H-PU given in Table4 were obtained by the following equation,

Self–healing efﬁciency = (σhealed/σpost−cured) × 100 % (1) where σhealed and σpost-cured are tensile strengths of self-healed and undamaged post cured samples, respectively. In Table4, the self-healing efﬁciencies of S-PU were excellent while those of H-PU were relatively low and decreased very much in repeated tests. The difference in self-healing ability between S-PU and H-PU was also investigated in an SEM analysis of the cut damage and self-healed samples. The damage on the S-PU surface was almost fully repaired after heating at 130 ◦C for 120 min, whereas the damage on H-PU surface was hardly recovered (Supplementary Material, Figures S13 and S14).

4. Conclusions A self-healable sorbitol-based PHU (S-PU) was prepared by simply introducing sorbitol as a bifunctional chain extender of the PU prepolymers. The hydroxy urethane units in S-PU not only improved the tensile properties over conventional PU, but also conferred excellent self-healing properties. The former improvement is attributable to the formation of hydrogen bonds of hydroxyl groups; the latter is conferred by nucleophilic addition of hydroxyl groups to the carbamate units without a catalyst. Moreover, the exchange reaction caused a conﬁguration change from a linear to a cross-linked structure, leading to reversible gel formation with enhanced mechanical properties and suppressed thermal expansion during the heating process. A facile and environmentally friendly material for use in coatings, adhesives, and elastomers could be prepared by the suggested synthetic framework for PHU.

Supplementary Materials: The Supplementary Materials are available online. Author Contributions: S.H.L. and D.-S.L. contributed to the manuscript via literature search, study design, data analysis and writing; S.H.L. and S.-R.S. performed the experiments and analyzed the data. Funding: This work was supported by the project for technology development of sports industries of KISS funded by Ministry of Culture, Sports, and Tourism (Project No: S072015192015) in Korea. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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