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ApproachestoPolymericMechanochromicMaterials

CélineCalvino#,LauraNeumann#,ChristophWeder*,StephenSchrettl*

AdolpheMerkleInstitute,UniversityofFribourg,ChemindesVerdiers4,1700Fribourg,Switzerland

#Theseauthorscontributedequally.

*Correspondenceto:ChristophWeder(christoph.weder@unifr.ch),StephenSchrettl(stephen.schrettl@unifr.ch)

INTRODUCTION

Thereiswidespreadfundamentalinterestinthedevelopmentofmaterialsthateitherreversiblyor permanently change their color in responseto an external stimulus.1,2 Mechanochromicmaterials, i.e.,materials that display anopticalresponse in reaction to the application of amechanical stimulus,3-7 represent an importantsubset of this class ofmaterials. The ability toselectively and reliably signal mechanicalstresses is not only of great fundamentalinterest, but also potentially useful fortechnological applications in fields that rangefrom packaging to structural healthmonitoring.4,7 Among the most prominentexamples are tamper-proof packagingmaterials, structural components that help topredict mechanical failure by displaying visualwarning signs when elements are highly

strained, or stress-sensitive sensors that allowfor an in situ signaling of deformation andfailure. The visualization of the effects ofmechanical stresses on polymeric materials isalsouseful fordevelopingadeeperunderstan-dingofstresstransfer inpolymericobjectsandtheunderlyingprocessesofmechanicalfailure.8

The optical responses associated withmechanochromicmaterialscaninvolvechangesof the absorption, emission, and reflection oflight.9 Alterations of the reflectivity aregenerally the result of mechanical manipu-lations of defined nanostructures andassociated photonic effects.10-12 By contrast,mechanically induced modifications of theabsorption or emission properties, which arethe main subject of this highlight, almostexclusivelyoriginatefromvariationsofmolecu-lar structure, conformational rearrangements,

ABSTRACT

Mechanochromicmaterialsrespondtomechanicalstimuliwithachangeoftheiropticalproperties.Suchmaterialsareofinterestformanytechnologicalapplicationsandsupportfundamentalresearchas they help improving the understanding of stress transfer in polymeric objects and aid in theidentificationoftheprocessesthatleadtomechanicalfailure.Inthishighlight,differentapproachesare discussed that permit the design of polymeric materials, which signal mechanical stressesthroughachromic response.Thishighlight emphasizesmaterials thatexhibitmechanically inducedchanges of their intrinsic absorption or emission properties. These responses almost exclusivelyoriginate from changes of molecular structure, conformational rearrangements, or disruption ofintermolecularinteractions.

KEYWORDS:chromogenic,mechanochromic,polymer,stimuli-responsive,colorchange

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and/or changes of intermolecular interactions.These principles are at play in small moleculecompounds that show mechanochromicbehavior,aswellasinpolymericmaterials.Theformer typically take the form of crystallinesolids without notable mechanical propertiesand are, as such, beyond the scope of thishighlight.Readerswithspecificinterestsinsuchcompoundsaredirectedtowardsrecentreviewarticles.13-16

When polymers are placed under mechanicalstress, a variety of processes may take placethat range from subtle adjustments, such as areshuffling of the conformation of polymerchains, to chemical reactions, such as thecleavage of covalent chemical bonds, to re-arrangements involvingmultiplemolecules.17-22Taking advantage of these processes hasallowed researchers to devise a plethora ofresponsive materials that show a variety ofmechanically triggered functions, includingcatalysis,pHchanges,andhealing.17-19 Ifstress-induced physical or chemical rearrangementsaffect chromogenic moieties, changes of amaterial’s absorption and emission propertiesmay occur. Provided that the spectral changesare significant and that the properties of asufficiently large fraction of the chromophoresin the system are changed, macroscopicallydetectable mechanochromic responses can beachieved. Whether a mechanochromicresponse is reversible or permanent usuallydepends on the reversibility of the physical orchemicalchangestothechromogenicmoieties.Another important determinant is thedeformation behavior of the polymer systemundertheappliedstrainorstress, i.e.,whetherthedeformationiselasticorplastic.7

The different approaches pursued to designmechanochromic materials can be classified

according to the processes that occur uponapplication of the mechanical stimulus.Conformationalchangesofpolymerchains, thedispersion of dye aggregates embedded inpolymer matrices, cleavage of non-covalentinteractions, or the scission of covalent bondsare the main mechanisms that have beenemployed as the basis for the creation ofpolymer-based mechanochromic materials. Inthe following, these different approaches arehighlighted by means of selected illustrativeexamples that showcase the general principlesat play and we provide an overview of thedesignprinciplesofmechanochromicmaterials.

CONFORMATIONALCHANGESOFPOLYMERS

Atthemolecularlevel,polymersreacttotensiledeformation by extending through stretchingand uncoiling of the individual chains, whichinvolves rotations around the covalent bondswith an adjustment of the bond angles.23Similarly, polymers adjust to compressivestresses through rearrangements of the chainsinorder toachieveaclosepacking in thebulk.Such conformational changes of the polymerbackbone can provoke a change of theelectronic properties, notably in conjugatedpolymers,andthiscanbeexploitedtoachieveamacroscopic mechanochromic response. Thus,conjugated polymers interact with light inaccordance to their effective conjugationlength, i.e., the structurally integer and fullyconjugated segments of the polymer chainacross which electron delocalization takesplace.24-26 As the effective conjugation lengthand the lowest energy optical transition ofconjugated polymer chains are related, stress-induced changes of the conformation of thepolymer backbone such as a transition from aplanar to a non-planar conformation usuallychange the conjugation along a given segment

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andthisstructuralperturbation leadstoashiftof the corresponding absorption or emissionband.

A prominent family of polymers that exhibitmechanochromism on the basis of this effectare poly(diacetylene)s.27-29 Wegner pioneeredtheUV-inducedpolymerizationof suitably pre-organized1,4-diacetylenesthatyieldspolymerswith a backbone of alternating double andtriple bonds (Figure 1).30,31 The conjugationalong this conformationallyhinderedbackboneis very sensitive to perturbations. Indeed,poly(diacetylene)s were found to displaydistinct color changes from blue to red(sometimes yellow) upon application of astimulus.28,32-34 The chromism of poly(di-acetylene)s has been extensively investigatedand is associated with conformationalrearrangements of the polymer backbone,35-38although these effects are apparentlysuperimposedwithacontributionfromchangesintheintermolecularinteractions.32,39,40

The characteristic red-to-blue chromism ofpoly(diacetylene)s can be induced by a varietyof stimuli, including mechanical force. Müllerand Eckhardt reported that the visual color ofpoly(diacetylene) single crystal specimenchangedfromgoldentogreenafterexposuretocompressive stresses.41 Polymerized mono-layers of poly(diacetylene)s were also exposedto compressive stresses at the air-waterinterface.42 The strain of the applied surfacepressure was sufficient to cause a reversibletransition fromablue color at lowerpressuresto a red colored film at higher surfacepressures. Similarly, shear forces were appliedtomultilayeredpoly(diacetylene) filmson solidsubstrates by means of an AFM tip.43,44 Thisapproach permitted creating locally strainedregions with dimensions as small as 30nm, asevidencedbythecharacteristicredemissionof

perturbed poly(diacetylene)s among a non-fluorescentbluedomain.

FIGURE 1 Mechanochromic poly(diacetylene)scan be embedded in poly(dimethylsiloxane) aselastomeric matrix and a chromic response isobserved upon deformation. (a) Chemicalstructure of the diacetylene derivative 10,12-pentacosadiynoicacid.(b)Photographicand(c)microscopic image of a polymer film withembedded poly(diacetylene) crystallites. (d)Schematic depiction of the poly(diacetylene)structure in the crystallites. (e) Opticalmicroscopyimageofacrystallitebeforeand(f)after swellingof thepolymer filmwithoctane.(Reproduced and adapted from [49], withpermissionfromJohnWiley&Sons.)

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In addition to investigations involving the neatpoly(diacetylene)s, poly(diacetylene) segmentswere also incorporated into different polymermatricesasforce-sensitiveprobes.Forexample,NallicheriandRubnerreportedthepreparationof poly(urethane)s that featured polymerizablediacetylenes in the hard segments.45-47 Afterprocessingthepoly(urethane)s,thediacetylenepolymerization was performed in situ. Theresulting poly(diacetylene) domains wereshowntoserveasmechanochromicprobesthatcan signal mechanical stress to the harddomains through a color change. The uniaxialdeformation of the elastomers thus preparedcaused clearly visible mechanochromism, withcolor changes from blue to red to yellow. Forsamples with crystalline hard domains, themechanicallyinducedcolorchangeswerefoundto be reversible below a strain level of 250%;above this threshold, the deformation of thehard domains as well as the color changesbecame irreversible. Poly(diacetylene)s werealso embedded in poly(ethylene oxide) wiresandexposedtooscillatoryforcesthatinducedachromic response.48 Moreover, apoly(dimethylsiloxane)elastomerwasusedasamatrix and the strain of deformation uponhydrocarbon-induced swelling led toconformational changes of embeddedpoly(diacetylene)s that gave rise to a chromicresponse (Figure1).49 The intensity of theresponsewasfoundtobedirectlyproportionalto the swelling rate and thereby allowed for avisualdifferentiationofdifferentlinearalkanes.

Other polymers with π-conjugated backboneshave been reported to display mechanically-induced chromic responses, as well. Forexample, the effect of compressive stress onfilms of poly(3-dodecylthiophene)s has beeninvestigatedbyYoshinoandcoworkers.50,51Thecorrespondingabsorptionandemissionspectra

displayedareversibleredshiftathighpressuresandtheresultsweretentativelyexplainedbyareduced torsion of the polymer backbone thatled toan increasedconjugation.These findingswerelaterconfirmedfordifferentotherpoly(3-alkylthiophene)derivatives.52-55Detailedinvesti-gations of the mechanochromic responseindicatedthatthespectralshiftsweretheresultofthecombinedeffectsofaplanarizationofthebackbone and intermolecular interactions atincreased packing densities. Compressivestresseswere shown tohave similareffectsonpoly(acetylene)s,56,57poly(phenylene)s,58poly(p-phenylenevinylene)s,59poly(pyrrole)s,60aswellaspoly(anthraquinone)s.55,61Inadditiontosuchstudies involving neat π-conjugated polymers,Shiga and coworkers dispersed poly(thio-phene)s in a matrix of poly(methyl meth-acrylate) and observed a relation between theapplied tensile load and the fluorescencebehaviorofthesamples.62-64Elasticdeformationinfluenced the fluorescencedecay times,whileplastic deformation was accompanied by areduced intensity and red-shift of thefluorescencesignal.

The preceding examples highlight how theconformation of the conjugated backbone ofpolymerscanbe,typicallyreversibly,alteredbyapplication ofmechanical stresses. As a result,the conjugation is either disrupted or atransition to conformations with an increasedconjugation was observed. In both cases, therearrangement can trigger a significant changeof the absorption and emission properties ofthese materials. In addition to theconformational changes, intermolecularaggregation was often observed as asuperimposed effect when investigating thesepolymers. In the following section we willdiscuss mechanochromic materials whose

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response predominantly originates from theformationordispersionofsuchaggregates.

AGGREGATION-BASEDRESPONSES

The absorption and emission properties ofchromophores with conjugated π-electronsystems are usually highly sensitive to theintermolecular distance and the geometricorientationwithinaggregatesof the respectivemolecules.65-68 Accordingly, changing themolecular packing of such chromophoresthroughmechanical forcescanprovideadirectpathway toward responsive mechanochromicmaterials.

While the aggregation of organicmolecules orpolymers typically causes strong fluorescencequenching,6,69 Ben Zhong Tang and coworkerspioneered the development of chromophoresthat display an increased emission in theaggregated state.70 In early studies, thepropeller-shaped, non-planar 1-methyl-1,2,3,4,5-pentaphenylsilolewasexplored,whichdoes not luminesce in dilute solutions due torapidintramolecularrotationsthatfacilitatethethermal relaxation of excited states. Bycontrast, the aggregation of these molecules

leads to a coplanarization with restrictedintramolecular rotations and an increasedelectrondelocalizationacrosstheconjugatedπ-system. As a result, an intense fluorescenceemissioncanbeobserved.

After these initial findings, materials thatdisplay such an aggregation-induced emission(AIE) have received considerable attentiondueto potential applications in photovoltaic cells,organic light-emitting diodes, or responsivesensing materials.71,72 Thus, series of AIEchromophores were developed,6,16 some ofwhich display luminescent properties thatchange in response to mechanical grinding ofthe solids.14,73,74 In a recent example by Tangandcoworkers,thepreparationofafluorescent1,3-indandione substituted tetraphenylethenewas reported (Figure2). This chromophoredisplays AIE activity in combination with avisible mechanochromic response.75 In thesolid-state, grinding of the yellow powderproducedanorangesolidandthefluorescencespectra showed a shift of the yellowish-greenemissionat510nmtoared-orangeemissionat562nmwhenexcitedat365nm. Investigationsby X-ray diffraction confirmed that thechromism was associated with mechanically

FIGURE2(a)Chromophoressuchasthe1,3-indandione-modifiedtetraphenylethyleneshowaggrega-tion-induced emission. (b) Reversible mechanochromic transitions between the amorphous andcrystalline state are observed for these compounds, as can be seen in photographs taken underambientlight(upperrow)andunderUVlight(lowerrow,λex=365nm)beforeandaftergrindingofthesolids.(Reproducedandadaptedfrom[75],withpermissionfromJohnWiley&Sons.)

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inducedrearrangementsofthemoleculesandatransitionfromthecrystallinetotheamorphousstate.Whiletheseexamplesarepromising, thecomplex interplay of the interactions betweenchromophoresandapolymermatrixhavesofarstifled thewidespread implementation of suchchromophoresinmechanoresponsivepolymericmaterials that exploit the AIE effect.6 Anexample reported by Rowan and coworkersmight be considered as a notable exception.76Theauthorsemployedsquare-planarcomplexesof platinum with 4-dodecyloxy-2,6-bis(N-methylbenzimidazol-20’-yl)pyridine anddispersed these in different acrylic polymerssuchaspoly(methylmethacrylate)orpoly(butylmethacrylate). Films of these polymersdisplayedapronouncedred-shiftedemissionofincreased intensity upon application ofcompressive stresses, which was explained bystructural rearrangements that led to theformation of short-range, intermolecular Pt-Ptinteractions. Apparently, only a minor fractionof the intermolecular aggregates with shortcontactswereformed,buttheirhighlyemissivenature dominated the spectra. Moreover,Sottos,Moore and coworkers recently demon-strated the potential benefits of theAIE effectby employing core-shell microcapsules filledwith dilute solutions of 1,1,2,2-tetraphenyl-ethylene.77 The mechanical disruption ofcapsulesembeddedinpolymermatricesreliablyled to an aggregation of the released chromo-phores and a strong, turn-on fluorescencesignal facilitated the visual detection ofmicro-scopicdamagestothematerial.

Aggregationdependentchangesoftheemissionproperties of excimer forming chromophoreswere employed byWeder and coworkers whopioneeredtheuseofexcimer-formingdyesandtheirdispersioninsuitablepolymermatricesforthepreparationofmechanochromicfluorescent

materials. Thus, cyano-substituted oligo(p-phenylene vinylene) derivatives (cyano-OPV)were employed as luminescent chromophoresthatshowapronouncedbathochromicshift,forexamplederivativeswithmethoxy substituentsonthecentralphenylringdisplayedashiftfroma red emission centered at 644nm in thecrystallinestate toagreenemissionat538nmin dilute solution.78,79 The chromophores werefirst dispersed in linear low-densitypoly(ethylene) through a swelling-induceddiffusion process. The fluorescence of thesepolymer films exhibited a pronounced redexcimer emission that was associated toaggregatesofthedyemoleculesinthepolymermatrix. Tensile deformation of the filmsdisrupted the aggregated chromophores andresulted in a chromic response as thefluorescence shifted to the green, monomer-dominatedemission.Theseinitialfindingsweresubsequently expanded and the conditions oftheblendingprocedurewereoptimizedtoallowfor direct melt processing (Figure3a,b).80 Fastquenching of the blends and plasticization ofthe polymer matrix was key to prevent large-scale phase segregation when higher dyeconcentrations were employed, as blends thatfeatured microscopic aggregates on the orderof 10µm did not show mechanochromism.Moreover, cyano-OPV derivatives with longeralkyl chains were synthesized in order todecrease their solubility in polar matrices andfurnish higher nucleation rates that promotethe formation of small aggregates. Thismodification allowed for the preparation ofexcimer-containing blends with other polymermatrices such as poly(ethylene terephthalate)and poly(ethylene terephthalate glycol).81,82Gratifyingly, all these blends showed a changeof the fluorescence color upon application ofmechanical stresses as the chromophoreaggregatesweredisrupted.Selectedcyano-OPV

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derivatives also displayed a significantbathochromic shift of the absorption bandsuponaggregation.81However,mechanochromicmaterials that displayed a macroscopicallydiscerniblechangeof theabsorptionwereonlyobtained when mechanical stresses led to avery efficient disruption of the aggregatedchromophores,(almost)exclusivelyleavingwelldispersed monomeric species in the strainedregions of the polymer films. Thus, apronounced change of the optical propertiesneeds to arise in a large fraction of thechromophores to obtain materials withdesirableproperties.

Following a similar approach, Pucci andcoworkers explored the use of stilbenederivatives to create polymers withmechanoresponsive luminescent behavior.83Commerciallyavailable4,4’-bis(2-benzoxazolyl)-stilbene featured an excellent dispersibility inaddition to a high melting point (360°C) anddegradation temperature (380°C),making it anideal candidate as additive for thermoplastics.

Thus, different concentrations of thechromophore were well dispersed in theamorphous phase of poly(propylene) filmsthroughamelt-mixingprocessat260°C.Aboveadyeconcentrationof0.2wt%,theabsorptionof the blends was found to be concentrationindependent and a new emission band at500nm emerged in the fluorescence spectra.This suggested the successful formation ofexcimer-type aggregates.When these polymerfilmswereexposed to tensiledeformation, thegreenemissioncolorchangedtothebluecolorof the monomer (Figure3c). In addition to adisruption of the aggregates, measurementswith polarized light indicated that thechromophores adopted a uniaxial alignmentalong the stretching direction. Thesechromophores were subsequently alsoemployed with other thermoplastics such asaliphatic polyesters and poly(1,4-butylenesuccinate).84Pucciandcoworkersalsoexploredthe use of aggregates of perylene derivativesfor the preparation of mechanochromicmaterials.85,86 Similarly to the previous

FIGURE3 Selectedaggregachromicdyesand images of thecorrespondingpolymer filmsafter tensiledeformation recordedunderUV light. (a)1,4-Bis(α-cyano-4-methoxystyryl)benzeneand (b) 1,4-bis(α-cyano-4-methoxystyryl)-2,5-dimethoxybenzeneinlinearlow-densitypoly(ethylene).(c)4,4’-Bis(2-benz-oxazolyl)stilbene in poly(propylene). (Reproduced from [80, 83], with permission from AmericanChemicalSociety,JohnWiley&Sons.)

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examples,linearlow-densitypoly(ethylene)wasmelt-processed with different amounts ofperylene and the prepared films exhibited apale yellow color with well-defined absorptionand emission bands characteristic for theformation of dye aggregates. Tensiledeformation disrupted the aggregates and thefilms showed the blue luminescence ofindividualperylenemolecules.

Similar principles are at play in mechano-chromic materials based on macromoleculesthat carry the corresponding chromophores inthepolymerbackboneoraspendantgroups.87-89 In all cases, the mechanical stress impartedby tensile deformation could be transferred toaggregates of the polymer-linked chromo-phores.Thedisruptionoftheseaggregateswasthen observed through changes of theabsorption and emission properties. Sufficientforces to disrupt the aggregateswere typicallyachievedintheregimeofplasticdeformationofthe polymer matrices. While tailored blendswith such chromophore-containing polymerscan be readily prepared, aggregate formationand stress-transfer to the aggregates can bechallenging, especially in polymers with highsegmental mobility such as thermoplasticpoly(urethane)s. The dispersion of chromo-phores in a polymer melt is, by comparison,synthetically less complex and lower dyeconcentrations are sufficient to achievesuccessfulaggregateformation.

Dispersions of two different chromophoresmight also be used to probe mechanicalstresses in polymeric specimen. Thus, theefficiencyofthefluorescenceresonanceenergytransfer (FRET) process between donor andacceptormolecules is highlydependenton theintermolecular distances.90,91 Sijbesma andKarthikeyanexplored theuseofnaphthalimide

(acceptor) and 4-methylcoumarin (donor)derivativesasaFRETpair todetectmechanicalstressesinthermoplasticelastomerswithalter-nating blocks of soft poly(tetramethyleneoxide)s and hard bisureas.92 The donormoleculeswereanchoredtothehardblocksofthepolymers throughhydrogenbonding,whilethe acceptor molecules were either randomlydispersedorplacedatthecenterofatelechelicpolymer carrying two donor molecules at thetermini. The latter systems displayed superiorstress-sensing properties as tensile elongationofthesamplesby100%ledtoadecreaseoftheFRET intensity ratio by 35%. For comparison,polymers with randomly dispersed acceptorsonlyshowedsimilar responsesafterelongationby more than 300%. The alteration of thedonor-acceptordistanceofaFRETpairwasalsoused todetect compressive stresses. Thus, Leeand Jee dispersed rhodamine 123 (donor) andazulene (acceptor) in an ethylene/1-octenecopolymer, exposed the films to compressivestresses in nanoindentation experiments withanAFM,andmonitoredthefluorescencedecaytimesofthedonorchromophoresasameasureof the efficiency of the FRET process.93 Theenergy transfer efficiency was enhanced by afactor of two as the donor-acceptor distanceswere reduced with increasing indentationdepth.

In summary, the formation or disruption ofintermolecular aggregates has emerged as anefficient approach for the preparation ofmechanochromic materials. However, most ofthe reported examples involve fluorescentpolymers, which rely on the often very largedifference of the emission properties ofaggregated and isolated chromophores orenergytransferpairs.Theexploitationofsimilarprinciples for the creation of materials thatdisplay similarly pronounced non-fluorescent

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mechanochromicresponses,e.g.,largechangesin the absorption properties, appears moredifficult. Unless charge-transfer effects are atplay, the changes of the absorption spectrumresulting from the (de)aggregation of chromo-phores are usually small and therefore a largefractionofthesemoleculesmustbe“switched”to elicit an appreciable optical change. Itappearsthatthedevelopmentandinvestigationofsuchmaterialsdeservesfurtherattention,asthe mentioned obstacles are not offundamental nature and because for manyapplications color changing (as opposed tofluorescence color changing) mechanochromicmaterialsseempreferable.

MECHANICALLY INDUCED CLEAVAGE OFCOVALENTANDNON-COVALENTBONDS

The development of polymeric materials thatrespond to mechanical stresses through theideally selective cleavage of predefined weaklinks – so called mechanophores – is anotherapproach toward chromic materials that hasrecently seensignificantprogress.18,19,21 Insuchmaterials,macroscopicmechanical stressesaredirectly translated into responses on themolecular level,wherechemical reactionsalterthe chemical structure of the mechanophore.Such mechanically triggered reactions may beexploited tocreateabroad rangeof functions,which depend on the specific design of themechanophore,andcanincludeachangeoftheabsorption or emission properties, renderingthese materials mechanochromic. To enablestresstransfertotheforce-responsivemoieties,theymustbe integratedwithapolymermatrixand examples of suitable covalent as well asnon-covalent mechanophores that display achromicresponsehavebeenreported.

ChromicResponsesofPolymerswithCovalentMechanophores

The first reports of mechanochemistry dateback to Staudinger and the observation thathigh shear forces can induceadecreaseof themolecularweightofpoly(styrene)onaccountofthe mechanically induced scission of covalentbonds.94 Thedeliberate force-induced cleavageof polymers at specific active sites is a muchmore recent development.18,19,21 Severalmechanophores that undergo a well-definedchemical reaction upon application of asufficiently high mechanical force have beenexplored in this context. Thebond scissioncanoccur homo- or heterolytically, but pericyclicreactionshavebeenreported,aswell.Whereasmechanochemical transformations of polymersin dilute solutions have become routine,95similar mechanochemical transductions ofpolymers that support useful functions in solidmaterialshavebeenmoredifficulttoachieve.96In the case of chromic mechanophores, thiswouldprovidetheultimatemeansofdetectingandunderstandingtheeffectsofdeformationinpolymericmaterials.

Moore, Sottos,White and coworkerswere thefirsttoreportsyntheticpolymersthatdisplayedmechanochromic behavior induced bydeliberate mechanical activation of chemicalreactions.97 In their pioneering study, a spiro-pyranmoietywascovalentlyembeddedintothebackboneofpoly(methylacrylate)oremployedas cross-linker in poly(methyl methacrylate).Tensile deformation or compressive stressespermit a (reversible) electrocyclic ring-openingreaction of the colorless spiropyran to affordthehighlycoloredmerocyanine(Figure4a).Thespiropyran, hence, acted as a molecular forcesensor thatwas embedded in the investigatedpolymers. The distinct color changes providedthe possibility for a visible detection and

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mappingofmechanicalstresseswithinthebulkpolymeric materials. Indeed, the most intensecoloration was localized in regions of thehighestdeformation.

Several spiropyran derivatives have beenthereafterutilized toprepareabroad rangeofpolymer systems that display a chromicresponse to mechanical stimuli.98 Recentexamples include their incorporation in thecenter of the poly(n-butyl acrylate) block oftriblock copolymers of poly(styrene)-b-poly(n-butyl acrylate)-b-poly(styrene).99 In the lattermaterials, thepoly(styrene)blocks formahardphase,andtheirfractiondeterminedthestressat which a mechanochromic response wasobserved. This example nicely demonstratesthat the choice of the general design permitstailoringthepropertiesofthemechanochromic

materials. Spiropyran derivatives have alsobeen embedded in the backbone ofsupramolecularpolymersbasedonmetal-ligandinteractions or hydrogen bonding to createpolymers that combine good mechanicalperformance,self-healingproperties,andstresssensing capabilities.100-103 For example,spiropyranswereincorporatedinthebackboneof telechelic, ureidopyrimidinone-terminatedpolymers and tensile deformation led to anefficientstresstransfertothemechanochromicspiropyrans.102 Moreover, the use of thespiropyran-motif in soft robotics has also beenreported. Thus, the mechanophore was curedintoamoldedpoly(dimethylsiloxane)softrobotwalker and gripper.104 The color change wasthen employed to detect high mechanicalstresses and places where failure was mostprobable to occur displayed the most intense

FIGURE4Selectedmechanophoresthatdisplayachromicresponseaftermechanicalactivation.(a)Thespiropyran moiety can be embedded in polymer chains such as poly(methyl methacrylate) andmechanical activation leads to the formationof the coloredmerocyanine. (b) Diarylbibenzofuranonemechanophores were embedded in poly(urethane)s and display a chromic response upon tensiledeformation.(c)Bis(adamantyl)1,2-dioxetanewasembeddedinpoly(methylacrylate)andmechanicalactivation causes a pericyclic reaction of the strained four-membered ring with concomitant bright-blue luminescence. (Reproduced and adapted from [8, 97, 112], with permission from [AmericanAssociationfortheAdvancementofScience,NaturePublishingGroup,RoyalSocietyofChemistry].)

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coloration.Inanefforttopreparematerialsthatdisplay load-triggered cross-linking in additionto a chromic response, Weng and coworkersembedded spirothiopyran derivatives in thebackbone of poly(urethane)s andpoly(ester)s.105 Isomerization of thespirothiopyran mechanophores undermechanical stress yielded the correspondingthiomerocyanines, whose thiolate moietiesunderwent addition reactions with suitablealkenes. Thus, reaction with 1,6-bismaleimido-hexane in solution led to a cross-linking of thepolymer chains. While a similar reactivity hasyet to be demonstrated in the solid state,corresponding materials promise to signal aswell as reinforce sample regions thatexperiencemechanicalstresses.

Following a similar approach, a rhodaminederivative has been recently embedded in thebackbone of poly(urethane)s and the polymerfilmspreparedfromthesematerialsexhibitedareversible mechanochromic response tostresses applied through shearing orcompression.106 This response is related to aring-opening isomerization of the rhodaminefrom a twisted spirolactam to a planarzwitterionic amide derivative. In an effort tofurther improve the optical stress-sensing ofmechanophores, Sijbesma and coworkersrecently reported a 9-π-extendedanthracene.107WhenthemaleimideDiels-Alderadductswere embedded asmechanophores inpoly(hexyl methacrylate), they could bemechanically activated in the solid statethrough compression. Solution-phaseexperimentsservedtodemonstratethatthe9-π-extended anthracenes that were obtainedafterthestress-inducedcycloreversionreactionfeatured a fluorescence quantum yield Φf of0.72 thatwas almost twoordersofmagnitudehigher than the values of spiropyran

mechanophores. Moreover, embedding base-sensitive chromophores into amine-containingepoxides allowed for the preparation ofmechanochromic materials that were highlysensitive to compressive stresses.108 Anelimination reaction was triggered by thecoordination of the amines to thechromophore, which led to an increasedconjugationinthelatterandavisiblechangeinthe absorption as well as emission spectra ofthematerial.Lowcompressivestrains(ε=0.14)were sufficient to trigger a visually detectableresponse, rendering this system highly usefulforthedetectionofimpactdamages.

The formation and propagation of cracks is adominant failure mode upon application ofmechanical stresses and Chung and coworkersemployed films of fluorescent mechano-responsive polymers to facilitate the detectionand analysis of cracks.109,110 Thus, monomersequipped with three cinnamoyl groups weredrop-cast onto the surface of polymericsubstrates. Subsequent irradiation led to apolymerization through [2+2]photocyclo-addition reactions between the cinnamoylmoieties, furnishing cyclobutane-containingpolymer films on the surface of thesubstrates.109 The application of mechanicalstresses by grinding or crack formation led tocycloreversion reactions that yielded thestronglyfluorescentcinnamoylmotifs.Similarly,dimeric anthracenederivativeswere employedtocross-linkpoly(vinylalcohol).Thecross-linkedpolymers were then used to coat plasticsubstrates.110Crack formation throughbendingof the substrates gave rise to a strongfluorescencesignalwithmaximaintherangeof500–600nm.

Another option to visualize and quantifycovalent bond scission is the stress-induced

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formation of stable, colored radicals. Forexample, Otsuka and coworkers reported adiarylbibenzofuranone mechanophore thatcould be embedded in the backbone ofpoly(urethane)s when its diol derivative wascopolymerized with poly(ethylene glycol) andhexamethyldiisocyanate.111 Alternatively, cross-linked poly(urethane)s were obtained when asimilar tetraol derivative was employed in thesamepolyadditionreaction.Thecleavageofthemechanophore in cross-linked gels wasachieveduponfreezingofthesample,resultinginbluecoloredstableradicals thatrecombinedafter thawing. The activation of themechanophores was found to be solvent-dependent.Presumably, thecoagulationof thesolvent upon freezing exerts mechanical forceonthepolymernetwork. Intriguingly,however,mechanical strain applied through tensile,compressive,orshearforcesaswellasfractureorswellingdidnot leadtoachromicresponse.The authors suggest that fast relaxation andrecombination processes of formed radicalsimpede their detection. By contrast, when thesame mechanophore was incorporated inpoly(urethane)-based thermoplastic elasto-mers, a reversible formation of blue radicalsupon tensile deformation was observed(Figure4b).112 Performing tensile deformationinmultiplecyclesshowedarepeatedcolorationand fading of the mechanophores and theamount of stress in the samples could bequantified by means of in situ electronparamagneticresonancemeasurements.113

Structural motifs that respond to mechanicalstimulation through the emission of lightwerepioneered by Sijbesma and coworkers,114 whoincorporated a bis(adamantyl) 1,2-dioxetane inthecenterofpoly(methylacrylate).Mechanicalactivation in solution by means of sonicationcausedapericyclicreactionofthestrainedfour-

membered ring, concomitant with bright-blueluminescence. Addition of suitable acceptorchromophores led to energy transfer andallowedfora tuningof theemissioncolor.Thescission of this mechanophore could also beachieved in the solid state and mechano-luminescent responses in thermoplasticelastomers could be demonstrated.115 Themechanofluorescence of 1,2-dioxetanesprovided an effective means to monitor thechain-scissioneventsthatoccurredupontensiledeformationofthesepolymersinrealtime.Theabilityofthesemotifstoserveasstress-sensorswas also exploited in a recent study in whichthe mechanical reinforcement of elastomerswas investigated.8 The toughening of theelastomers was achieved by preparing tripleinterpenetrating networks. The first networkfeatured shorted chainsofpoly(ethyl acrylate),whilethesecondandthirdnetworkconsistedoflongerpoly(methylacrylate)segments.Thefirstnetwork provided resistance to initialdeformationandpreferentiallybroke, resultingin energy dissipation and preventing crackpropagation. These details of the failuremechanismofthematerialswereconfirmedbyincorporatingdioxetanes into the first network(Figure4c). The mechanochromism of thedioxetanesfacilitatedtheanalysisofthefailuremodes upon tensile deformation by visualizingthe bond-breakage in the vicinity of thepropagating crack. Recently, Sijbesma andcoworkers reported the use ofmechanoluminescent dioxetanes to investigatethe processes of stress-softening andmechanomemory in filled elastomers.116 Thesereports showcase how suitablemechanochromic moieties can serve asanalytical tools for the investigation of thefailure of soft materials and aid in thedevelopment of better models, which might

13

ultimately lead to the preparation of novelmaterialswithimprovedproperties.

Chromic Responses of Polymers with Non-CovalentMechanophores

Many of the examples of mechanochromicmaterials that respond with the scission ofcovalentbondsrequirerelativelyhighforcestotrigger a response. By contrast, mechano-transduction in biological systems is oftenactivated by small forces, which is at least inpart related to the fact that many naturalmaterials are soft and in most cases containsignificant amounts of water.117-119 Examplesthat are found in nature are based onconformational rearrangements in proteins orchanges in supramolecular architecture.Researchers are increasingly taking inspiration

from thedesignprinciples found innatureandhave sought toexploit thedissociationofnon-covalent interactions for the preparation ofmechanochromicmaterials.22

EarlynanoscaleforcesensorswerepreparedbyBrunsetal.whocombineddonorandacceptortype proteins that display fluorescenceresonance energy transfer with so-calledthermosome proteins that unfold along theirmechanicallyweakest sectionuponmechanicalstimulation.120 The protein complex wasembeddedinpoly(acrylamide)sandmechanicaldeformationledtoalossintheintensityofthefluorescence signal as the donor and acceptorproteinswere separated.More recently, Brunsand coworkers reported the preparation offiber-reinforced composites that utilizedbiomolecules to signal damages at the

FIGURE 5 Non-covalent mechanochromic materials based on metallosupramolecular polymers. (a)Chemicalstructureofthetelechelicpolymersandschematicrepresentationoftheirself-assemblyintoa network by complex formation with Eu3+ ions. (b) Reversible dissociation of the complexes uponsonication as well as irreversible metal exchange with Fe2+ ions. (c) Films imbedded with Fe(ClO4)2change their color after puncturing with a needle. (Reproduced from [125], with permission fromAmericanChemicalSociety.)

14

micrometer scale.121 This was achieved byplacinganenhancedyellowfluorescentproteinat the interface between glass fibers and anepoxyresinmatrix.Thematerialssignaledfiberfracture as well as debonding between fibersandmatrixastheforcesatthe interface ledtoan unfolding of the embedded proteins thatwas accompanied by a loss of its yellowfluorescence. Schaaf, Schiller, and coworkerscovalently grafted genetically modified greenfluorescent proteins onto a poly(dimethylsiloxane) surface and showed that uniaxialmechanicalstressesresultedinafullyreversiblelinear decrease of the fluorescence intensitythatwas attributed to conformational changesof the protein.122 Moreover, fluorescentproteins were also employed in polymermatricesandtheirfluorescencesignalwasusedto facilitate the detection of compressivestresses.123 These mechanochromic materialsrely on the unfolding of complex proteins toinduce a color change anddemonstratehowachromicresponsecanbeachievedonthebasisofnon-covalentinteractions.

Also relying on the mechanically induceddisassembly of supramolecular interactions,non-covalent mechanochromic materials weredeveloped that employ metallosupramolecularpolymers with metal-ligand complexes as the

mechanophores. Inspired by shake-responsivebehavior of metallosupramolecular polymersreported by Rowan et al.,124 Weder andcoworkers relied on the complex formationbetween europium salts and telechelicpoly(ethylene-co-butylene) with 2,6-bis(1'-methylbenzimidazolyl)pyridine ligands at thetermini to obtain a supramolecular network(Figure 5).125 The metal-ligand complexesserved as mechanophores that displayed asimultaneouschromicresponse.Reversibleanddose-dependent disassembly of the complexeswas induced for solutions of these polymersthrough application of ultrasound pulses. Thefluorescence signal of the complex therebyservedasaninternalopticalprobethatallowedfor a direct monitoring of the state of(dis)assembly.Moreover,when iron saltswereimbedded in films of this polymer,mechanicaldisruptionwith a needle led to disassembly ofthe europium complexes, as evidenced by theformation of the thermodynamically favoredironcomplexesandavisiblecolorchange.

Insimilarmetal-ligandsystemsaterpyridyl-end-capped four armpoly(ethyleneglycol) polymerwascomplexedwith lanthanide ions.126Awiderange of stimuli-responsive behaviors of thesemetallosupramolecular polymer gels wasobserved, including a chromic response to

FIGURE 6 A mechanoresponsive, white luminescent metallosupramolecular polymer gel is obtainedwhen a four-arm star polymer with terpyridyl ligands forms complexes with Eu3+ and Tb3+ ions.Sonication leads to a gel-sol phase transition and the initially white gel becomes a blue liquid.(Reproducedandadaptedfrom[126],withpermissionfromAmericanChemicalSociety.)

15

mechanicalstress(Figure6).Thus,sonicationofself-assembledwhite luminescentgels resultedinabreakdownofthesupramolecularnetworkandthebluefluorescenceoftheobtainedfluidindicated the presence of unbound ligand.Spectroscopic investigations confirmed partialcleavageofthecomplex-basedcross-links.Suchsupramolecular polymers are envisioned as aversatile platform for the development ofmechanochromic polymeric materials that canbenefit from the tunable interaction strengthand the dynamic, reversible nature of thesupramolecularinteractions.

CONCLUSIONS

Thedevelopmentofmechanochromicmaterialshas been inspired by the potential benefits ofpredicting mechanical failure as well as thedesire to signal and detect the impact ofmechanical stress and wear on polymericobjects. In this context, an advancedunderstandingof theprocesses that takeplacein polymeric specimen upon mechanicaldeformation has been both accompanied andperpetuated by the development ofmechano-chromic materials with improved properties.The approaches toward such materials haveevolved from the use of conjugated polymersthat undergo conformational changes of theirbackbone upon application of mechanicalstimuli to the stress-induced dispersion ofchromophore aggregates in polymer matrices.More recently, the controlled scission ofcovalent bonds or cleavage of non-covalentinteractions have become mechanisms ofchoice for the preparation of polymer-basedmechanochromicmaterials.

The conditions under which a material will beemployedmightbethemostcriticalparameterthat needs to be considered when choosingbetween different design approaches. Thus,

manyofthechromogenicmoietiesthatrespondto mechanical deformation can also beactivated by other means. When the chromo-genicunitsare,forexample,responsivetolightor temperature changes, practical issues and acomplication of the analysis might ensue.Another important criterion is the envisionedmode of detection,which directly results fromthe type of response, i.e., whether changes inemissionor absorptionpropertiesoccur.Manyofthereportedmaterialsrelyonchangesintheemission properties that are typically morestraightforward to discern when bulk samplesare investigated. Provided that the spectralchangesaresignificantandasufficient fractionof thechromophores in thesystemcontribute,macroscopically detectable mechanochromicresponses based on the absorption propertiescanalsobeachieved.However,thebackgroundcolorofembeddedchromophoresthathavenotbeen activated can impede the detection of achromic response that is based on changes ofthe absorption. At the same time, it would behighly desirable to be able to detect the colorchanges without the necessity to excite therespective materials throughout the analysis.Anintriguingopportunityinthisregardthathasonly been scarcely investigated is the use ofcharge-transfercomplexes.127Thetypicallylargecolor changes between the complex and theindividualdonorandacceptormoietiesrendersthemidealcandidates.

Whetheraresponseisreversibleorpermanentis determined by the nature of the physical orchemical changes that provoke a chromicresponse in polymeric materials, both withrespect to the chromogenic moieties and thedeformationbehaviorofthematrix.Indeed,themechanical properties and the deformationbehaviorofthesurroundingmatrixexertalargeinfluence on themagnitude of the forces that

16

thechromogenicmoietiesmayexperience.Thisappears to make the realization of mechano-chromic effects particularly challenging in rigidmaterials,specificallyinglassypolymers,wherethe selective mechanical activation of anembedded chemical motif (as opposed torandomevents)maybedifficulttoachieve.

To conclude, the field of mechanochromicmaterials has seen significant progress andmany opportunities remain. Especially therecent developments in mechanochemistryhave led to a surge in the development ofpolymericmaterialsthatrespondtomechanicaldeformationinausefulmanner.

ACKNOWLEDGEMENTS

The authors gratefully acknowledge financialsupport through the National Center ofCompetence in Research (NCCR) Bio-InspiredMaterials, a research instrument of the SwissNational Science Foundation (SNF) as well asfunding from the AdolpheMerkle Foundation.The research leading to these results hasreceived funding from the European ResearchCouncil under the European Union's SeventhFramework Programme (FP7/2007-2013) / ERCgrant agreement n° ERC-2011-AdG 291490-MERESPO.

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CélineCalvino

CélineCalvinoiscurrentlyaPhDstudentinProf.ChristophWeder’sresearchgroupattheAdolpheMerkleInstituteoftheUniversityofFribourg,Switzerland.ShegraduatedfromtheUniversityFribourgin2014,withafocusonorganicsynthesis,polymerchemistry,andsurfacescience.Herongoingresearchfocusesonthesynthesisofmechanochromicpolymersbasedonnon-covalentinteractions.

LauraNeumann

LauraNeumanniscurrentlyaPhDstudentinProf.ChristophWeder’sresearchgroupattheAdolpheMerkleInstituteoftheUniversityofFribourg,Switzerland.ShegraduatedfromtheEindhovenUniversityofTechnologyin2015,withafocusonsupramolecularchemistry.Herongoingresearchfocusesonthesynthesisandcharacterizationofmechanoresponsivesupramolecularpolymers

ChristophWeder

Prof.ChristophWederisDirectoroftheAdolpheMerkleInstitute(AMI)attheUniversityofFribourgandleadstheSwissNationalCenterofCompetenceinResearchBio-InspiredMaterials.ChriswaseducatedatETHZurichandheldpositionsattheMassachusettsInstituteofTechnology,ETHZurich,andCaseWesternReserveUniversity,beforejoiningtheAMIasProfessorforPolymerChemistryandMaterialsin2009.Hisresearchinterestsincludethedesign,synthesisandinvestigationofstimuli-responsivepolymersandnanomaterials.

StephenSchrettl

Dr.StephenSchrettlisapostdoctoralresearcherwithProf.ChristophWederattheAdolpheMerkleInstituteinFribourg,Switzerland.HereceivedhisChemistrydegreefromFreieUniversitätBerlinin2009andhisPhDinMaterialsSciencefromtheEcolePolytechniqueFédéraledeLausanne(EPFL)in2014.AftercontinuingasapostdoctoralresearcheratEPFL,hejoinedtheAdolpheMerkleInstitutein2015toinvestigatemechanicallyresponsivematerials.

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Groupphotooftheauthors.Fromleft:StephenSchrettl,CélineCalvino,LauraNeumann,andChristophWeder.Ifpreferred,individualimagescanbeprovidedinstead.

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GRAPHICALABSTRACT

CélineCalvino#,LauraNeumann#,ChristophWeder*,StephenSchrettl*

ApproachestoPolymericMechanochromicMaterials

Mechanochromicmaterialsdisplayachangeof theiropticalpropertiesuponmechanicaldeformation.Suchmaterialsareofinterestfortechnologicalapplicationsaswellasfundamentalresearchandcanaidinthe identificationofprocessesthat leadtomechanical failure inpolymericobjects. Inthishighlight,approachestowardpolymericmaterialsthatsignalmechanicalstressesthroughachromicresponsearediscussed. These responses almost exclusively originate from changes of molecular structure,conformationalrearrangements,ordisruptionofintermolecularinteractions.