POLYMERS Key-and-lockcommodity self-healing copolymersof the copolymer composition, globular confor-mations, similar to non–self-healable composi-tions (ranges I and III) are preferable,

POLYMERS

Key-and-lock commodityself-healing copolymersMarek W. Urban1,2,3*, Dmitriy Davydovich1,3, Ying Yang1,3, Tugba Demir1,3,Yunzhi Zhang2, Leah Casabianca2

Self-healing materials are notable for their ability to recover from physical or chemicaldamage.We report that commodity copolymers, such as poly(methyl methacrylate)/n-butylacrylate [p(MMA/nBA)] and their derivatives, can self-heal upon mechanical damage.This behavior occurs in a narrow compositional range for copolymer topologies that arepreferentially alternating with a random component (alternating/random) and is attributedto favorable interchain van der Waals forces forming key-and-lock interchain junctions.The use of van der Waals forces instead of supramolecular or covalent rebonding orencapsulated reactants eliminates chemical and physical alterations and enables multiplerecovery upon mechanical damage without external intervention. Unlike other self-healingapproaches, perturbation of ubiquitous van der Waals forces upon mechanical damage isenergetically unfavorable for interdigitated alternating/random copolymer motifs thatfacilitate self-healing under ambient conditions.

Advances in the last two decades inmaterialscapable of self-healing focused primarily onincorporating physical and chemicalmech-anisms into polymernetworks. Thesemech-anisms can be conveniently classified into

the following categories: embedding reactiveencapsulated fluids that burst open upon dam-age to fill and repair damaged areas (1); incor-porating covalent (2–6) or supramolecular (7–12)dynamic bonds that, upon cleavage, reform poly-mer networks; physically dispersing nanomate-rials that enable repair in response tomagnetic orelectromagnetic fields (13, 14); introducing phase-separated morphologies that facilitate damageclosure (11, 15); and incorporating living organismscapable of remending damaged structures (16).Polymers—and in particular copolymers, if de-signed properly—can encodemolecular featuresby placement of repeating units that interact witheach other (17). However, for self-repair to occur,synchronized chemical andphysical events (18, 19),potentially driven by van der Waals (vdW) inter-actions, must take place.We synthesized a series of copolymers using

atom transfer radical polymerization (ATRP),statistical free radical polymerization, and colloi-dal polymerization. The methyl methacrylate/n-butyl acrylate (MMA/nBA) molar ratios werevaried from 30/70 to 70/30, while maintainingsimilar molecular weights for all compositionsfor each synthesis method (ATPR: ~25 kD; sta-tistical: ~60 kD; colloidal: ~700 kD). Copolymersynthesis and properties are summarized in tableS1. Figure 1A illustrates selected optical imagesof p(MMA/nBA) copolymer films produced by

ATRP in the 40/60 to 55/45 compositional rangethat were damaged (0 hours) and allowed to self-heal (~14 hours). Self-repair occurs without ex-ternal intervention only within narrow 45/55 to50/50MMA/nBA compositional ranges (movie S1)Outside this range, self-repair does not take placeeven days after damage, even though the glasstransition temperature (Tg) for nBA-richer 40/60copolymers is below ambient conditions (25°C,relative humidity = 50%). For undamaged co-polymer films, when MMA/nBA molar ratios in-crease, Young’s moduli (E) also increase (Fig. 1B).However, ~14 hours after damage, only 45/55 to50/50 p(MMA/nBA) copolymer compositions re-cover 90 to 100% (±5%) of their original tensilestrains, respectively (Fig. 1B, B5 andC5). The 45/55self-healing copolymer exhibits moderate tough-nesswith tensile strain of ~550%and stress valuesof ~8.6MPa after self-healing (~600% and 10MPabefore damage). By contrast, the copolymer filmsoutside this range exhibit ~55 and 10% recovery(Fig. 1B, A5 and D5), respectively. Similar be-havior, although with longer self-healing times(~86hours), are observed for copolymersproducedby colloidal radical polymerization (fig. S1 andtable S1-B).It is reasonable to hypothesize that for co-

polymers with 45/55 to 50/50 MMA/nBA molarratios, the neighboringMMAandnBA copolymerunits and their distribution may play some rolein self-healing as these compositions are expectedto form random and/or alternating chain topol-ogies. To test this hypothesis, MMA and nBAmonomers were copolymerized to obtain num-ber averagemolecularweightMn =~20- to 30-kDapMMA-b-pnBAblock copolymerswith controlledbock sizes and the number of blocks rangingfrom two to six (tables S2 and S3). These blockcopolymers do not exhibit self-healing underthe same conditions.To experimentally assess molecular events as-

sociated with self-healing or lack thereof, we

used internal reflection infrared imaging (IRIRI),proton nuclear magnetic resonance (1H NMR),and electron spin resonance (ESR), along withstress-strain and dynamic mechanical analysis(DMA). The results of these experiments showthat reversible spectroscopic changes are onlyobserved for self-healable copolymer composi-tions. In IR analysis (figs. S2 and S3), they aremanifested by the intensity changes of the C=O(1728 cm−1) and C-O-C (1158 cm−1) normal vibra-tions due to conformational changes of MMAand nBA repeating units (20). In 1H NMR, thekey features are the changes in themethyl groupshielding-deshielding during the damage-repaircycle for self-healing copolymer compositions(figs. S4 to S6 and tables S4 and S5) (21, 22). Uponmechanical damage, the resonances at 0.98 partsper million (ppm) (a) and 0.96 ppm (b) increase(deshielded) at the expense of diminishing0.93-ppm (c) and 0.90-ppm (d) peaks (shielded),suggesting a closer chain packing. If strong vdWforces contribute to interchain cohesiveness,mechanical damagewill alter the distribution ofshielded and deshieldedmethyl groups along thepolymer backbone. Because mechanical damagemay also lead to the formation of free radicals,ESR analysis of damaged copolymers showedthat, regardless of the copolymer composition,the concentrations of free radicals are in the4.5 to 8 × 10−7mol/liter range (fig. S7) and appearto have no relation to self-healing. Junction den-sities (nj) due to chain entanglements or adja-cent chain interactions were obtained from themeasurements of viscoelastic length transitions(VLTs) in dynamicmechanical analysis (DMA) asa function of copolymer composition (fig. S8 andtable S6) (23). ForMMAandnBAhomopolymers,the nj values are 93.1 and 60mol/m3, respectively,but an increase up to 123.6 mol/m3 is observedfor self-healable compositions.Molecular dynamics (MD) simulations were

employed under isothermal (NVT) and isoener-getic equilibration (NVE) conditions as a functionof copolymer composition to determine copolymerconformations, end-to-enddistances (r), and cohe-sive energy densities (CEDs). These results areplotted in Fig. 2A (table S7A), and further exper-imental details along with the results of MDsimulations are provided in the supplementarymaterials. Figure 2A shows that the equilibriumcohesive energy densities (CEDeq) (curve a), aswell as the end-to-end chain distances (req)(curve a’) both reach maxima for self-healable45/55 to 50/50MMA/nBA compositions (rangeII). Copolymer interchain packing (Fig. 2B) isgreater within self-healing compositional range II,whereas non–self-healable ranges I and III exhib-it less interwinding chains. Further, representa-tive copolymer chains extracted from each rangeshown in Fig. 2C indicate that, within self-healingrange II, the chains exhibit extended helix-likeconformations with average req values of ~34 Å,whereas within ranges I and III, globular shapeswith req values in the ~25 to 29 Å range areobserved. The results of MD simulations aresummarized in Table 1 and show that the CEDvalues upon reaching equilibrium (CEDeq) increase

RESEARCH

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1Department of Materials Science and Engineering, ClemsonUniversity, Clemson, SC 29634, USA. 2Department ofChemistry, Clemson University, Clemson, SC 29634, USA.3Center for Optical Materials Science and EngineeringTechnologies (COMSET), Clemson University, Clemson, SC29634, USA.*Corresponding author. Email: [email protected]

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for self-healing compositions (range II). Withinrange II, the vdWeq density values reach 1.96 ×105 kJ/m3, thus indicating that the extended-chain helix-like conformations are energeticallypreferable. It is also useful to examine chainconformations equilibrated in the absence ofinterchain vdW interactions for all composi-tions. The results of MD simulations for singleisolated 30/70, 45/55, and 70/30 p(MMA/nBA)chains shown in Fig. 2D illustrate that, regardlessof the copolymer composition, globular confor-mations, similar to non–self-healable composi-tions (ranges I and III) are preferable, and thesingle-chain end-to-end distances (req) are withinthe 21.7 to 27.8 Å range. Thus, without interchainvdW interactions, globular chain conformationsprevail regardless of the copolymer topology.In all MD simulations, an experimental aver-

age copolymer density of 1.125 g/cm3 was used.In separate simulations conducted under thesame conditions, copolymer chainswere allowedto have excess free volume by assuming an initialdensity of 0.5 g/cm3, thus enabling chainmotionin and out of the physical cell boundaries uponreaching an equilibrium. The premise behindthese simulations was to examine the role, if any,of vdW interactions as a function of copolymercomposition in their ability to assume higher- orlower-density states. With an initial density of0.50 g/cm3, respective copolymer chains wereisothermally equilibrated. Only for self-healingcompositions (range II) did the density increaseto the0.529- to0.562-g/cm3 range,whereas fornon–self-healing compositions, the density decreased(fig. S9), supporting the hypothesis that enhancedvdW forces facilitate favorable interchain inter-actions and return to denser packing upon phys-ical separation. The question then arises fromthese experimental andmodeling exercises:Whatare the molecular entities within this narrow com-positional range that lead to stronger interchaininteractions and subsequent self-healing?To determine the role of the monomer se-

quences and the vdWcontributions to self-healing,we examined vdW forces and cohesive energies(CEp) for model pentads containing selected se-quences of M and Bmonomer units (where M andBrepresentMMAandnBAmonomers, respectively).Under NVTMD conditions, selected pentads wereplaced into one cell and equilibrated. Figure 3Aillustrates BMBMB/BMBMB, BMBMB/BMBBM,BMBMB/BMMBB, and BMBMB/MMBBB pentadpairs and the CEp values due to their interactions.The highest CEp value (313.6 kJ/mol) exhibits analternating BMBMB/BMBMB pair (1:1). By con-trast, more ‘“blocky-type’” MMBBB/MMBBBpentads (Fig. 3B, pair 4-4) have the lowest CEp

value (258.2 kJ/mol). Similarly, other “blocky-type” combinations (Fig. 3B) also exhibit lowerCEp values, thus indicating that the alternatingBMBMB-type monomer sequences of the neigh-boring chains favor overall higher CEp values.Notably, for alternating BMBMB-type segmentscomposed of MMA (M) and nBA (B) units, thereis an average ~120 Å3 space (~7.1 Å by 4.2 Å by4.0Å) between two neighboring nBAmonomersseparated by one MMA unit along one chain,

thus being spatially capable and energeticallyfavorable for hosting an nBA unit of an adjacentchain and thereby enabling the key-and-lockinteractions stabilized by vdW forces.Helix-like chain conformationsmay also con-

tribute to the high CEDeq values within self-healable compositions (range II; Fig. 2). Toexamine this hypothesis, we analyzed cohesive

energy densities for fixed helix-like conformations(CEDhl) as a function of copolymer composition.All copolymers across the compositional rangewere forced to retain a 34.0 ± 0.2 Å end-to-enddistance (Fig. 2C”) and extended helix-like chainconformations of the 45/55 self-healable co-polymer. The results are summarized in Table 1,and CEDhl and end-to-end distance values are

Urban et al., Science 362, 220–225 (2018) 12 October 2018 2 of 6

Fig. 1. Self-healing of copolymer films and their mechanical analysis. (A) Optical images ofdamaged p(MMA/nBA) copolymers with the following MMA/nBA molar ratios: 40/60 (A1 to A4),45/55 (B1 to B4), 50/50 (C1 to C4), and 55/45 (D1 to D4). The copolymers were allowed to repairunder ambient conditions. A video of the self-healing process is shown in movie S1. (B) A5 toD5: The corresponding stress-strain curves before damage and 14 hours after repair for eachcopolymer composition in (A).

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plotted in Fig. 2A (curves b and b’, respectively).As shown, regardless of p(MMA/nBA) compo-sition, the CEDhl values are higher compared totheir corresponding CEDeq counterparts, sug-gesting that the helix-like conformations result-ing from alternatingmonomer sequences are themain contributing factors to higher CEDeq andstrong vdW interchain forces. The monomer se-

quence contributions to self-healing (range II,Fig. 2) are also supported by the most negativeDHeq values (Table 1). Because pMMA-b-nBAblock copolymers do not self-heal and exhibitlower CEDb values (table S7B), these results fur-ther substantiate that the presence of alternating/random BMBMB-like sequences favors stronginterchain vdW interactions reflected in higher

vdWdensities that facilitate self-healing (Table 1,italicized rows). Average MMA and nBA reac-tivity ratios (r1 = 1.75 to 3.15 and r2 = 0.2 to 0.39)indicate that it is unlikely that copolymers inrange IIwill formpurely alternating copolymers.However, 1H NMR analysis shows the presenceof minute homopolymer blocks manifested bythe presence of CH3 protons due to MMA triads(fig. S6 and table S5) for self-healing composi-tions (range II), but their content is small com-pared to non–self-healing compositions. MDsimulations conducted for average r1 and r2 val-ues (2.61 and 0.36, respectively) showed that themaximum CED values are still reached for self-healing compositions (table S7), the probabilityof finding alternating topologies are also greater,and chain conformations follow the same trend.A lack of interfacial fluidity attributed to the

elevated Tg at damage on theMMA-rich compo-sitional end (range III), and limited quantities ofvdW interactions on theMMA-poor end (range I),inhibit self-healing outside the 50/50 to 45/55region (range II). Because the increase in theCEDeq values parallels the increasing numberof neighboring MMA/nBA units (table S7C),the formation of key-and-lock configurationsbetween adjacent chains will be favorable foralternating/random copolymer topologies with-in region II, as reflected by higher junctiondensities. Assuming that chain entanglements(E) and side-by-side (S) chains are the primarycontributors to enhanced junction densities (nj)experimentally obtained in DMA measurements(table S6), we extracted both types of interac-tions from MD simulations and examined thedistribution of the induced dipoles due to vdWinteractions that contribute to the enhancednj values. Figure 4A-1 illustrates that withinthe self-healing range II, d+- d−–induced dipoleinteractions dominate the entanglement (MD-E’) and side-by-side (MD-S’) chain interactions.By contrast, Fig. 4A-2 shows extracted co-polymer chains just outside the self-healablerange (range III) in which randomized orien-tation of induced dipoles for entangled (MD-E”)and side-by-side (MD-S”) chains dominate. nj val-ues significantly increased for self-healing com-positions, clearly supporting MD predictions.Enhanced segmental chain mobility within in-terfacial regions generated during damage mayalso aid the self-healing process, owing to lowerTg values near surfaces (24, 25) which can beboosted by collective structural rearrangementsat the interfacial regions (26).Further evidence for interchain interactions

can be found in determining the flexibility pa-rameter (feqÞ (27), defined as the fraction of bondscapable of bending out of the collinear directionof previous segments expressed as feq = rmax/[req

2(l (2 − f ))], where: rmax is fully extendedchain length, req is the end-to-end distance ob-tained from MD simulations, and l is length ofthe repeat unit. The feq values as a function of co-polymer composition are summarized in Table 1.When chains are in the equilibrium state (feq),the chain flexibility is the smallest for self-healingcompositions, indicating that if chains are

Urban et al., Science 362, 220–225 (2018) 12 October 2018 3 of 6

Fig. 2. The results of MD simulations as a function of copolymer composition. (A) Cohesiveenergy densities at equilibrium (CEDeq) (curve a), end-to-end equilibrium distances (req) (curve a’),cohesive energy densities (CEDhl) of forced helical conformations (curve b), and end-to-end chaindistances for forced helical conformations (rhl) (curve b’) as a function of molar % of MMA inp(MMA/nBA) copolymers. (B) Representative examples of copolymer morphologies in range I(MMA/nBA molar ratio: 30/70), range II (MMA/nBA molar ratio: 45/55), and range III (MMA/nBAmolar ratio: 70/30); circles denote examples of non-interwinding chains. (C) Average end-to-enddistances for macromolecular chains extracted from MD simulations in (B). (D) Average end-to-end distances for single isolated chains (reqs) in range I (MMA/nBA molar ratio: 30/70), range II(MMA/nBA molar ratio: 45/55), and range III (MMA/nBA molar ratio: 70/30). The req and reqs valueswere measured from 3D chain images and may appear not to scale.

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deformed as a result of external forces, they willstore energy and act like mechanical springs ca-pable of returning to the original state. As wasshown for pentadmodelMD simulations (Fig. 3),these interactions are stabilized by BMBMB/BMBMB key-and-lock junctions between neigh-boring chains, resulting in recovery upon dis-placement. Similar behavior is observed formethylmetacrylate/n-pentyl acrylate (PA)– andmethylmetacrylate/n-hexyl acrylate) (HA)–basedpentads (table S8), in which also alternating co-polymer compositions favor enhanced CEp. The

optical images (fig. S10) of selected copolymercompositions show similar self-healing behav-ior, and stress-strain curves recorded before andafter damage are strong indicators ofmechanicalproperty recovery after ~14 hours (fig. S10).Comparison of mechanical properties beforedamage and after self-healing for selectedp(MMA/nBA), p(MMA/nPA), and p(MMA/nHA)copolymers is summarized in tables S10 and S11.To illustrate that vdW interactions can be highlyeffective in self-healing of thermoplastic ma-terials, we severed and physically reattached

~200-mm-thick 46/54 p(MMA/nBA) film. Afterreattachment, self-healing occurred within a fewminutes, but to regain ~70 to 85% mechanicalproperties took ~80 hours under ambient con-ditions. The tensile strength of these materialsbefore damage andafter self-healing is in the rangeof 6 to 9MPa (fig. S11). Repetitive damage and self-healing bymaking parallel cuts over the same areadoes not affect self-healing efficiency (fig. S12).The presence of strong vdW interchain forces

for predominantly alternating/randomcopolymercompositions forming helix-like conformations

Urban et al., Science 362, 220–225 (2018) 12 October 2018 4 of 6

Table 1. Cohesive energy density of equilibrated (CEDeq) and forced helix-like (CEDhl) p(MMA/nBA) copolymer conformations, van der Waals(vdW) density, end-to-end distance (req), flexibility parameter (feq), and enthalpy changes (DHeq) as a function of MMA/nBA molar ratios [italicsindicate self-healing (range II of Fig. 2) copolymer compositions].

MMA/nBA

molar ratio

CEDeq ± 0.05

(105 kJ/m3)

CEDhl ± 0.05

(105 kJ/m3)

vdWeq density ± 0.05

(105 kJ/m3)

req ± 0.2

(Å)feq ± 0.1

DHeq ± 0.08

(103 kJ/mol)

100/0 1.95 2.08 1.87 27.7 0.696 −6.64.. .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .

70/30 1.71 1.84 1.53 25.8 0.762 −6.58.. .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .

55/45 1.58 1.86 1.35 29.6 0.64 −6.45.. .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .

50/50 1.99 1.99 1.91 34.1 0.521 −8.11.. .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .

45/55 2.03 2.01 1.96 34.0 0.523 −8.32.. .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .

40/60 1.41 1.92 1.30 30.0 0.625 −5.88.. .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .

30/70 1.72 1.98 1.44 28.8 0.66 −7.26.. .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .

0/100 1.67 1.68 1.49 25.9 0.758 −7.93.. .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .

Fig. 3. Cohesive energies (CEp) for selected pentad pair combinations. (A) CEp values for (1–1), (1–2), (1–3), (1–4). (B) (4–2), (4–5), (4–2), (4–4)pentad pair interactions (1, BMBMB; 2, BMBBM; 3, BMMBB; 4, MMBBB 5, MBMBM).

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Urban et al., Science 362, 220–225 (2018) 12 October 2018 5 of 6

Fig. 4. Visual representation of MD simulations and proposed self-healing mechanism. (A1) Extracted interchain interactions from MDsimulations for self-healable compositions (range II) of entangled (MD-E’)and side-by side (MD-S’) chains. (A2) Extracted interchain interactionsfrom MD simulations for non–self-healable compositions (range III; 55/45MMA/nBA ratio) of entangled (MD-E″) and side-by side (MD-S″) chains.To visually differentiate copolymers, the neighboring chains were coloredin orange and blue. The color scale represents relative distributions of

induced dipoles (red, high; blue, low). (B) Proposed self-healingmechanism responsible for the restoration of vdW interactions; thepresence of key-and-lock associations (red) facilitates chain recoveryupon mechanical damage. (C1) Pictorial representation of thedistribution of induced dipole moments in self-healable entangled (E’)and side-by side (S’) chains. (C2) Pictorial representation of distributionof induced dipole moments for none self-healable entangled (E’’) andside-by side (S’’) chains.

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creates a viscoelastic response that energeticallyfavors self-recovery upon chain separation be-cause of key-and-lock associations of neighbor-ing chains (Fig. 4B). In the presence of theseinteractions, vdW forces stabilize key-and-lockneighboring junctions reflected in the enhancedCEDeq values. When chains are separated as aresult of mechanical damage and an externalforce is removed, copolymer chains return totheir initial conformations by restoring helix-like chain conformations in a spring-likemannerand reforming key-and-lock junctions mani-fested by increased CEDeq and req distances forself-healing compositions (range II). Outside self-healing compositions (ranges I and III), irrevers-ible chain dislocations and insufficient interchainvdW forces inhibit complete chain recovery. Thus,the presence of directional vdW forces due to in-duced dipole interactions enhances CEDeq of en-tangled or side-by-side chains (Fig. 4C).For comparison with vdW forces, when supra-

molecular interactions, such as H-bonding, wereemployed in self-healing of rubber, the tensilestrength ~3.5 MPa at similar elongation levelswas reached (7). Although the underlying mech-anisms responsible for self-healing using su-pramolecular and vdW forces are substantiallydifferent, they may result in somewhat similarresponses. Considering directionality and polar-ity as commonly accepted differences betweenH-bonding and vdW interactions, the formerfacilitates localized bonding directionality be-cause of the orientation of interacting molec-ular orbitals and high polarity (hydrophilicity).The main feature of vdW interactions is highpolarizability (hydrophobicity) with a tendencyto form ubiquitous nondirectional contacts be-tween neighboring macromolecular segments.However, in layered systems with large individ-

ual atomic planes, individual weak vdW attract-ive forces in two-dimensional materials (e.g.,graphene, others) are directional and becomecollectively strong. In amorphous polymers, atfirst approximation, vdW interactions are non-directional, but the magnitude of vdW forceswill strongly depend on the proximity of theneighboring units (28). As extended semihelixmacromolecules are in closer proximity to theiralternating/random copolymer neighbors, vdWforces will increase because of the preferablebearings of the side groups, resulting in inter-digitated key-and-lock interchain morphologiesthat facilitate self-healing.

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ACKNOWLEDGMENTS

We thank K. Ivey for technical assistance in GPC, DSC, and DMAmeasurements. Funding: This work was supported by the NationalScience Foundation under Award DMR 1744306 and partiallyby the J.E. Sirrine Foundation Endowment at Clemson University.Author contributions: The experiment was designed by M.W.U.,D.D., Y.Y., and L.C. (EPR). Experimental work was conductedby D.D., T.D., and Y.Z. Data analysis was performed by M.W.U., Y.Y,D.D., and L.C. M.W.U. wrote the manuscript. Competing interests:None declared. Data and materials availability: All data neededto evaluate the conclusions in the paper are present in the paper orthe supplementary materials. Patent application no. 62/702,410was filed 24 July 2018; contact: C. Gesswein, Clemson UniversityResearch Foundation (CURF); email: [email protected].

SUPPLEMENTARY MATERIALS

www.sciencemag.org/content/362/6411/220/suppl/DC1Materials and MethodsFigs. S1 to S12Tables S1 to S11References (29–40)Movies S1 and S2

27 February 2018; accepted 30 August 201810.1126/science.aat2975

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Key-and-lock commodity self-healing copolymersMarek W. Urban, Dmitriy Davydovich, Ying Yang, Tugba Demir, Yunzhi Zhang and Leah Casabianca

DOI: 10.1126/science.aat2975 (6411), 220-225.362Science 

, this issue p. 220; see also p. 150Sciencerepair.system is that it relies on van der Waals interactions rather than the reformation of hydrogen or covalent bonds for

-butyl acrylate show repeatable self-healing properties (see the Perspective by Sumerlin). A key characteristic of thisndemonstrate that for a very narrow range of compositions, simple vinyl polymers based on methyl methacrylate and

et al.Although self-repair has been demonstrated for some polymers, it usually required specialized monomers. Urban Biology provides many routes for self-healing or repair, but this trait is hard to endow into engineering materials.

Simple routes to self-healing

ARTICLE TOOLS http://science.sciencemag.org/content/362/6411/220

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REFERENCES

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