Table Of ContentTopics in Current Chemistry 369
Roman Boulatov Editor
Polymer
Mechanochemistry
369
Topics in Current Chemistry
Editorial Board
H. Bayley, Oxford, UK
K.N. Houk, Los Angeles, CA, USA
G. Hughes, CA, USA
C.A. Hunter, Sheffield, UK
K. Ishihara, Chikusa, Japan
M.J. Krische, Austin, TX, USA
J.-M. Lehn, Strasbourg Cedex, France
R. Luque, C(cid:1)ordoba, Spain
M. Olivucci, Siena, Italy
J.S. Siegel, Tianjin, China
J. Thiem, Hamburg, Germany
M. Venturi, Bologna, Italy
C.-H. Wong, Taipei, Taiwan
H.N.C. Wong, Shatin, Hong Kong
V.W.-W. Yam, Hong Kong, China
S.-L. You, Shanghai, China
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Roman Boulatov
Editor
Polymer Mechanochemistry
With contributions by
(cid:1) (cid:1) (cid:1) (cid:1)
R.D. Astumian A. Balan A. Barge M.J. Buehler
(cid:1) (cid:1) (cid:1) (cid:1) (cid:1)
B. Cheng P. Cintas J.M. Clough G. Cravotto S. Cui
(cid:1) (cid:1) (cid:1)
A.P. Haehnel G.S. Heverly-Coulson G. Jung
(cid:1) (cid:1) (cid:1) (cid:1) (cid:1)
G.S. Kochhar Y. Li Y. Lin K. Martina N.J. Mosey
(cid:1) (cid:1) (cid:1) (cid:1) (cid:1)
Z. Qin Y. Sagara M.J. Serpe S.S. Sheiko R.P. Sijbesma
(cid:1) (cid:1) (cid:1) (cid:1) (cid:1)
Y.C. Simon C. Weder W. Weng Y. Xu H. Zhang
Q.M. Zhang
Editor
RomanBoulatov
DepartmentofChemistry
UniversityofLiverpool
Liverpool,UnitedKingdom
ISSN0340-1022 ISSN1436-5049 (electronic)
TopicsinCurrentChemistry
ISBN978-3-319-22824-2 ISBN978-3-319-22825-9 (eBook)
DOI10.1007/978-3-319-22825-9
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Preface
Few people realize that when they stretch a rubber band – or inflate tires on their
cars – they do chemistry. In this example of vulcanized rubber, the chemistry is
simple – homolysis of C–C, C–S, or S–S bonds, but the underlying principle is
anything but simple. The probability of ethyl disulfide spontaneously dissociating
intoradicalsatroomtemperatureisnegligible.Yetthisprobabilitycanincreaseby
many orders of magnitude for the same molecular moiety when it is as part of
amorphous material under mechanical load. In other words, translation of macro-
scopicobjects(e.g.,ourhands)thatcompress,stretch,ortwistpolymericmaterial
can directly control reaction rates of material building blocks. Such control of
course is not seen in the vast majority of reactions studied by chemists and as a
resultisnotaccommodatedinanyoftheexistingmodelsofchemicalkinetics.
Thiscouplingbetweenmacroscopicmotion(ormechanicalloads)andchemical
reactivity is called mechanochemistry [1]. Load-induced fragmentation of poly-
merswasdiscoveredalmostassoonasthenatureofpolymershadbeenrecognized
[2]. Polymer mechanochemistry is thought to be an important (but poorly under-
stood)determinantofhowpolymericmaterialsrespondtomechanicalloads[3–5].
Because polymers are subject to such loads throughout their lifecycles, from
production to recycling, polymer mechanochemistry pervades our everyday lives.
Mechanochemical phenomena are thought to affect the generation, growth, and
propagationofmicrocrackswhichareresponsibleforcatastrophicfailureofpoly-
meric materials, desalination membranes, impact-resistant materials (e.g., bullet-
proof vests), and tires, and the stabilities of surface-anchored polymers in
microfluidic diagnostics and high-performance chromatography. Polymer mecha-
nochemistrymaybeimportantinjetinjection(e.g.,duringinkjetprintingoforganic
electronics),polymermeltprocessing,high-performancelubrication,enhancedoil
recovery (e.g., polymer flooding), and turbulent drag reduction. Exploiting mech-
anochemical phenomena may yield remarkable new materials and processes,
including polymer photoactuation (i.e., direct conversion of light into motion to
power autonomous nanomechanical devices, control information flow in optical
computing,positionmirrorsorphotovoltaiccellsinsolarcaptureschemes)[6,7]),
v
vi Preface
efficient capture of waste mechanical energy, materials capable of autonomous
reportingofinternalstressesandself-healing,andtoolstostudypolymerdynamics
atsub-nanometerscales[8].
Untilabout10yearsago,unambiguousexamplesofmechanochemicalreactions
were limited to simple backbone fragmentations that resulted when biological or
syntheticpolymersweresubjecttotensileloadsinsolids,melts,orsolutions.Early
attempts to interpret rates of bulk polymer failures under various conditions as
governed by mechanochemical acceleration or inhibition of reactions of their
monomers, such as amide hydrolysis, are now considered unreliable and unlikely
to reflect true load-induced changes in the intrinsic kinetic stabilities of stretched
polymerchains[9,10].Incontrast,thepastdecadehasseenimpressiveprogressin
designingpolymerswhosestretchingatthesingle-chainlevelacceleratesreactions
more complex than simple bond hemolysis with the ultimate goal of both under-
standingthefundamentalaspectsofmechanochemicalenergycouplinganddesign-
ingstress-responsivematerialsofthetypesdiscussedabove.
Tounderstandthecurrentstateofpolymermechanochemistryandtotrytoguess
the directions of its evolution, it is useful to divide the phenomena studied by
polymermechanochemists(Fig.1)intothosewhere:
1. Thecouplingbetweenmacroscopicmechanicaleffectsandatomisticallylocal-
izedreactivityismediatedbyinteractionsbetweenmultiplepolymerchainsasin
amorphousmaterials,melts,andload-bearingbiologicaltissues
Fig.1 Hierarchyof
mechanochemical
phenomenaandthevolume
chaptersdevotedtospecific
categories
Preface vii
2. Individualpolymerchainsarestretchedeitherbyinteractionwithnon-polymeric
environment(e.g.,solventflows,seechapters“MechanochemistryofTopolog-
ically Complex Polymer Systems”, “Force induced Reactions and Catalysis in
Polymers”, and “The Interplay of Mechanochemistry and Sonochemistry”) or
because they are bound directly to translating macroscopic objects, such as in
single-molecule force spectroscopy (see chapter “Supramolecular Chemistry
andMechanochemistryofMacromoleculesattheSingleChainLevel”)
Molecular interpretation of mechanochemical phenomena in amorphous mate-
rialsandmeltsremainslargelyqualitativeandthemaineffortandsuccesstodatein
this area has been primarily in empirical exploration and some tentative exploita-
tion of materials obtained by incorporating force-sensitive reactive sites in other-
wise inert polymer chains and matrices (see chapters “Mechanochemistry in
Polymers with Supramolecular Mechanophores” and “Responsive Polymers as
Sensors, Muscles, and Self-Healing Materials”). This work has the potential to
expand greatly our presently primitive quantitative understanding of mechano-
chemistryofentangledpolymerchainsbyprovidingexperimentaltoolstoquantify
howandhowfastmechanicalloadspropagatethroughamorphouspolymermatri-
ces to reactive sites, and the range of local forces (or molecular strains) and their
temporaspatial distributions that reactive sites experience in response to macro-
scopic load. In comparison, the mechanism and dynamics of mechanochemical
energy transduction that underlies the operation of motor proteins is well under-
stood(seechapter“Understanding theDirectionality ofMolecularMachines:The
ImportanceofMicroscopicReversibility”)asistheresponseofbiologicaltissueto
mechanicalloads.Thereasonis,atleastinpart,theprimarilynon-covalentnature
of mechanochemistry of biological tissues, which makes it amenable to usefully
accurate large-scale computational simulations, as described in the chapter
“MechanicalPropertiesandFailureofBiopolymers:AtomisticReactionstoMac-
roscaleResponse”.
Suchsimulationsareoftenperformedby“attaching”avirtualspringbetweenthe
terminalatomsofabiopolymer,changingtheparameter(s)ofthisspringtoimpose
a time-varying tensile force on the biopolymer, and “watching” how the polymer
evolvesunderthisforce.Theexperimentalanalogofthisset-upissingle-molecule
force spectroscopy (SMFS) in which a single polymer chain bridges a tip of an
atomic force microscope and a retracting surface (see chapter “Supramolecular
ChemistryandMechanochemistryofMacromolecules”).SMFSistheleastintrac-
table manifestation of polymer mechanochemistry and is responsible for some of
the most important conceptual developments in polymer mechanochemistry [11].
Unfortunately,SMFSistechnically demandingandonly ahandful oflaboratories
worldwidecombinesufficientexpertiseofsyntheticpolymerchemistry,microma-
nipulation techniques, and physical analysis to design, perform, and interpret
cutting-edgeSMFexperiments.UnlikeSMFS,mechanochemistryofisolatedpoly-
merchainsinflowfieldsofdilutepolymersolutionshavebroadindustrialapplica-
tions (e.g., see chapter “Mechanochemistry of Topologically Complex Polymer
Systems”). The absence of chain entanglement potentially makes these systems
viii Preface
atomistically more tractable than mechanochemistry in amorphous polymers and
melts. Indeed, the value of planar elongational flow fields for studying the funda-
mental aspects of polymer dynamics in solution has long been recognized [12].
However, the technical challenges of achieving sufficient strain-rate gradients to
induce mechanochemistry in moderately long macrochains (with the contour
lengths below 10 μm) are daunting and planar elongational flows remain largely
unexploitedinpolymermechanochemistry.
In contrast to SMFS and planar elongational flows, which provide perhaps the
bestopportunitiestodevelopaphysicallysound,general,andpredictivemodelof
mechanochemicalkinetics(seebelow),sonicationofdilutepolymersolutionsisa
verysimpleandpopulartechniqueroutinelyusedtomimictheresponseofpolymer
chainstostretching(seechapters“ForceInducedReactionsandCatalysisinPoly-
mers” and “The Interplay of Mechanochemistry and Sonochemistry”). Sonication
createstransientelongationalflowswhenbubblesgeneratedbypropagatingsound
waves suddenly collapse. Because the solvent flow rate in close proximity to a
collapsingbubbledecreasesveryrapidlywithdistancefromthebubblesurface,the
two termini of the same chain located at different distances from this surface
experienceverydifferentflowratesandthechainbecomesstretched.Thedynamics
of bubble collapse, which can be extraordinarily complex [13], determines the
temporaspatial flow rate gradients and hence loading rates and maximum forces
thatpolymerchainsexperience,thusdirectlyaffectingtheapparent(macroscopic)
mechanochemical kinetics. However, little is known about how this dynamics is
governedbymacroscopiccontrolparameters(soundfrequencyandpowerdensity,
temperature,durationsofon/offcycles),solventandpolymercharacteristics(vapor
pressure,viscosity,andsolvationcapacityfortheformer;molecularmassdistribu-
tion for the latter), and environmental variables (shape and size of the ultrasound
hornandofthereactionvessel).Consequently,studiesofpolymermechanochem-
istry in sonicated solutions should be viewed as at best qualitative and the results
may vary from one laboratory to another simply because of the difficulty of
controllingkeykineticvariables(orevenidentifyingthem).Quantitativeinterpre-
tations of sonication experiments are further complicated by the very modest
chemicalselectivityofmechanochemicalreactions,whichmanifestsitselfinreac-
tionsofverydifferentstrain-freeactivationenergies(e.g.,C–Cbondhomolysisand
ring-openingofdichlorocyclopropanes)occurringatcompetitiveratesduringson-
ication. They are also complicated by the contribution of radicals from solvent
sonolysistoanyobservedpolymerchemistry,bytheuncertaindistributionofstrain
(or equivalently, restoring force, see below) along individual stretched polymer
chains,andbythe(presumably)strongdependenceoftheforcesexperiencedbya
stretched polymer chain on its contour length. These limitations of sonication-
induced mechanochemistry have long been acknowledged in the literature [4],
butthenumberofreportedstudiesdesignedtoclarifythemremainsdisappointingly
small[14,15].
Thevastmajorityofreportedstudiesinpolymermechanochemistryareonlinear
polymers. Mechanochemistry of topologically complex polymers, which is of
increasingindustrialimportance(seechapter“MechanochemistryofTopologically
Preface ix
ComplexPolymerSystems”),isanareawheresonication,despiteallitslimitations,
hasachancetomakeanoutsizeimpact.Unfortunately,theveryfewreportsonthe
subjectarecontradictory,warrantingfurtherdetailedresearch.
Model studies have and continue to be critical for developing the conceptual
foundation of polymer mechanochemistry and for rationalizing and systematizing
the existing observations. Experimental model studies (see chapter “Mechano-
chemistryDrivenbyIntermolecularForces”)usemoleculeswhichattempttostrain
reactive sites by means of molecular architecture rather than the application of
mechanical loads. The most prominent examples include strained macrocycles
based on stiff stilbene and overcrowded polymers. They are distinct from the
large number of strained molecules which chemists have studied over the past
100 years in that their architectures are designed specifically to reproduce the
highly anisotropic molecular strain that localized reactive sites in polymers expe-
rienceinmechanochemicalphenomenaandtofacilitatequantitationofsuchlocal
strain as restoring force (as opposed to strain energy). Likewise, computational
mechanochemistry (see chapter “Theoretical Approaches for Understanding the
Interplay Between Stress and Chemical Reactivity”) uses models, such as small
reactivemoietiesinwhichonenon-bondinginternucleardistanceisconstrainedtoa
non-equilibriumvaluebyanexternalpotentialofvaryingstiffness,becausedirect
quantum-chemical calculation of mechanochemical response of polymer chains
remainsbeyondreach.Asisthecaseinanymodelstudies,anoutstandingproblem
with this approach is to learn how to map the measurements and trends observed
(or computed) in such models onto bona fide mechanochemical systems [16]. To
date, extrapolating results ofmodel studies to even the simplest manifestations of
polymer mechanochemistry (i.e., a single-molecule force experiment) has had
mixedsuccess[11,16].
With the exception of motor proteins (see chapter “Understanding the Direc-
tionality of Molecular Machines: The Importance of Microscopic Reversibility”),
thesignificantincreaseinthediversityofempiricaldatainpolymermechanochem-
istry and the development of quantum-chemical methods ofcalculating force-rate
correlationsinsmall-moleculereactantshavenotyetbeenmatchedbycomparable
progressindevelopingtheconceptualfoundationofmechanochemicalkinetics.A
keycomponentofthisfoundationisageneralandpredictiverelationshipbetween
the macroscopic control parameters that define mechanical loads (e.g., stress or
straintensorsandloadingrates)andthechangesinreactionratesinthesameway
thatthetransitionstatetheoryandtheEyringequationrelatereactiontemperature
and rate. One of the earliest (empirical) models of kinetics of mechanochemical
fragmentation of polymer chains was put forth by Eyring [17], who postulated a
direct proportionality between the activation energy of the fragmentation and the
force exerted on the stretched polymer chain by its surroundings. The model was
silent on what the proportionality constant might be or how to derive the single-
chain force from the flow rate gradient, which was the control parameter in the
system considered by Eyring. The same idea was subsequently applied by Bell to
celladhesion[18]andextendedtotime-varyingsingle-chainforcebyEvans[19],
who derived it within the Kramers formulation of chemical kinetics. This Evans
