High Solid Dispersion

248 Pages·9.207 MB·English

Preface
High solid dispersions refer to a broad class of materials comprising colloidal or
non-colloidalparticlesdispersedatveryhighvolumefractioninaliquidphase.They
areubiquitousindailylife,inindustry,inbiologyandingeophysics,encompassing
a great variety of systems at the frontier between polymers, colloidal glasses and
granularmaterials.Amongthese arepersonalcare products,paints,drillingmuds,
biologicaltissuesandvarioustypesofsyntheticornaturalslurries.Inspiteoftheir
huge diversity in composition,high solid dispersions have an amorphousjammed
structureincommon,whichisattheoriginofremarkableproperties.Athighvol-
ume fraction, the individual motion of particles is dramatically reduced unless a
sufficientforceisappliedtoovercomestericconstraints.Astrikingconsequenceis
thathighsoliddispersionsbehavelikesolidsatrestbutyieldandflowunderlarge
stresses.Theexistenceandthenatureofthissolid–liquidtransition,whichcantake
a variety of forms, have perplexed engineers and scientist for a long time. Over
the years, high solid dispersions have stimulated a lot of work both from an ap-
pliedandfundamentalperspective.Ontheappliedside,controllingtheflowability
ofhighsoliddispersionsiscentralto theformulationof a largerangeof commer-
cialproductsandtotheprocessingofhighperformancematerialssuchascoatings,
ceramics and serigraphic inks. On the fundamental side, understanding the com-
plex behaviourof high solid dispersionsis an outstandingchallenge for statistical
and condensed-matter physics as well as for materials science, biology and geo-
physics.Duringthelastdecade,ithasbecomeclearthatmacroscopicdescriptions
ofhighsoliddispersionshavereachedtheirlimits,andthatamicroscopicapproach
linking the structure and the dynamics at different scales is highly desirable. The
contributionscollectedinthisvolumereviewsomeresultsobtainedrecentlyinthis
direction.
The volumebeginswith a chapter byVlassopoulosandFytas who surveyhow
macromolecularandcolloidalchemistrycanbeusedtodesignhighsolidcolloidal
dispersionsexhibitingarichvarietyofphasestatesandmaterialproperties.Theau-
thorsdescribetwostrategies.Thefirststrategyuseswell-definedcolloidalparticles
with tunable interaction potentials much softer than the very short-range repul-
sion existing between solid particles. Soft particles, which can be as diverse as
graftedhardspheres,multiarmstarpolymers,microgels,blockcopolymermicelles,
dendrimersand dendritically branched polymers, display an extraordinaryvariety
ix
x Preface
of architectures and topologies. The second strategy consists in engineering soft
nanocompositesbyblendingdifferentclassesofcolloidalparticlesorpolymers.In
bothtypes of materials, interactionsand transitionsare manipulatedby tuningthe
chemical(composition,architectureand topology)and physical(temperature,sol-
vent,depletion,enthalpy–entropybalance)parameters.Thisinturnprovidesunique
toolstotailorthedynamicsandtherheologyofsoftparticledispersions.
Asexplainedabove,severalaspectsofthestructure,localdynamicsandmacro-
scopicrheologyofhighsoliddispersionsexhibitstronganalogieswiththephysics
of glasses. In Chap.2, Fuchs focuses on concentrated suspensions of colloidal,
slightlypolydispersehardspheresinteractingthroughexcludedvolumeinteractions,
whichconstituteoneofthesimplestsystemsundergoingaglasstransition.Hedis-
cussestheuniversalscenariooftheglasstransitionundershearandreviewsrecent
extensionsofModeCouplingTheorythatoffersamicroscopictheoryofthelinear
and non-linear rheology of colloidal fluids and glasses under shear, starting from
firstprinciples.
In Chap.3, Bonnecaze and Cloitre focus on high solid dispersions made of
elastic and deformable particles packed together at volume fractions well above
close-packing, they term soft glasses. Unlike hard sphere glasses, soft glasses in-
teract throughslowly varying repulsiveforces of elastic origin that develop at the
contactsbetweentheparticles.Thesolventplaysan importantroleintransmitting
the elastic interactions through the glass. After reviewing some generic features
ofsoftglasses, the authorsshow thatmanyoftheir propertiesresultfroma subtle
interplaybetweendisorderandsolvent-mediatedelastohydrodynamicinteractions,
which thus constitute the two basic ingredients of a micromechanical description
of soft glasses. The theory quantitatively accountsfor near-equilibriumproperties
(statistical distribution, osmotic pressure, shear modulus), for the slip phenomena
occurringwhensoftglassesareshearednearsmoothsurfaces,andforthenon-linear
rheologyofsoftglasses.
Generally, high solid dispersions do not flow homogeneously when they are
sheared; instead the macroscopic deformation is localized in slip zones, shear-
bands or fractures. In Chap.4, Isa, Besseling, Schofield and Poon review modern
advancesinfastconfocalmicroscopyimaginganddataanalysistechniques,which
enabletime-resolvedtrackingofindividualparticlesinBrownianandnon-Brownian
suspensionsandglasses.Afterdescribingthesampleenvironmentsandtheexperi-
mentaltechniqueneededto performthiskindofexperiments,theypresentseveral
applicationsoffastconfocalimagingasauniquetooltoprobetheflowresponseof
hardspheresuspensionsandglasses, givingemphasisto the relationbetweenpar-
ticlescaledynamicsandnon-linearrheologicalphenomenasuchasyielding,shear
localization,wallslipandshear-inducedordering.
Chapter5,byGongandOsada,isdevotedtobiologicaltissueswhichconstitute
oneparticularclassofhighsoliddispersions,consistingofwaterandvariousmacro-
molecularcomponents.Bio-tissueshaveexceptionalmechanicalpropertiessuchas
lowfriction,hightoughness,specificadhesionandshock-absorbancecapacity.The
authors discuss recent progress on the study and developmentof model synthetic
soft and wet hydrogels as substitutes to natural bio-tissues. A strong emphasis is
Preface xi
given to the rich and complex surface friction and lubrication properties of these
materials,whicharefoundtosharecommonfeatureswiththeslippropertiesofsoft
glassesreviewedinChap.3.
ItwasourintentiontoprovidetheSoftMattercommunitywithacomprehensive
reviewofsomerecentapproachesontherheologyofpolymer–colloiddispersions.
Wehopethatthereaderwillfeelthatthedifferenttopicsdiscussedinthisvolume,
which each address a particular facet of high solid dispersions, complement each
otherandhelptodrawabridgebetweenmicroscopicphenomenaandmacroscopic
rheology.
France MichelCloitre
Contents
From Polymers to Colloids: Engineering the Dynamic
PropertiesofHairyParticles...................................................... 1
DimitrisVlassopoulosandGeorgeFytas
Nonlinear Rheological Properties of Dense Colloidal
DispersionsClosetoaGlassTransitionUnderSteadyShear................. 55
MatthiasFuchs
MicromechanicsofSoftParticleGlasses ........................................117
RogerT.BonnecazeandMichelCloitre
QuantitativeImagingofConcentratedSuspensionsUnderFlow.............163
LucioIsa,RutBesseling,AndrewB.Schofield,
andWilsonC.K.Poon
SoftandWetMaterials:FromHydrogelstoBiotissues........................203
JianPingGongandYoshihitoOsada
Index.................................................................................247
xiii
AdvPolymSci
DOI:10.1007/12 2009 31
From Polymers to Colloids: Engineering
the Dynamic Properties of Hairy Particles
DimitrisVlassopoulosandGeorgeFytas
Abstract For many years, colloidal hard spheres and polymeric coils served as
modellimitingcasesofsoftmatterbehavior.Softeningthepotentialofinteractions
has been an obvious possibility for altering properties and has been explored in
greatdetailwithstericallyinteractingchargedcolloids.Asthebehaviorofsuch(and
technologically relevant) systems is very complex, it is desired to isolate the role
ofinteractionsandusewell-characterizedsystems.Today,itispossibletoachieve
this goal by taking advantage of the capabilities of macromolecularand colloidal
chemistry.Weshowhowtousewell-definedsoftspheresystemsinteractingviaex-
cluded volume repulsions to generate a rich variety of phase states and materials
properties.Thisapproachprovidesopportunitiesforbridgingthegapbetweenpoly-
mers and colloids in terms of property variation, and thus designing soft systems
withdesiredproperties.Atthesametime,previouslyunexploredaspectsofimpor-
tantopenproblemssuchasglasstransitionandeffectsofsolventonthedynamics
ofcorrelatedsystemsareaddressed.Appropriateblendingofthetwomainclasses
of softmatter andfurtherdirectingtheirassembly and dynamicresponsehas now
becomeachallengingnewtask.
Keywords Colloidaldispersions·Colloidalglasses·Dynamics·Graftedparticles
·Hairyparticles·Micelles·Nanoparticle-polymerhybrids·Phasediagrams·Poly-
mers·Rheology·Softcolloids·Softness·Stars
Contents
1 WhySoftColloids?
2 ModelSoftSpheres
2.1 ColloidalStarPolymers
2.2 BlockCopolymerMicelles
B
D.Vlassopoulos( )andG.Fytas
FORTH,InstituteofElectronicStructure&Laser,andUniversityofCrete,
DepartmentofMaterialsScience&Technology,71110Heraklion,Crete,Greece
[email protected]
(cid:2)c Springer-VerlagBerlinHeidelberg2009
D.VlassopoulosandG.Fytas
2.3 GraftedColloidalParticles
2.4 MicrogelParticles
2.5 SelectionofModelSystems
3 TuningtheSoftness:FromPolymerstoHardSpheres
4 Form,StructureandDiffusionDynamics
4.1 ComparingDifferentSystems
4.2 DynamicsofInteractingColloidalStarPolymers
4.3 DynamicsofCore-CoronaSystems:BlockCopolymer
MicellesandGraftedParticles
4.4 RemarksonCrystallization
5 Vitrification
5.1 SoftColloidsintheGlassyState
5.2 SignaturesofTransitionsandRheologyManipulation
6 HybridSystemsandOtherEmergingApplications
7 ConclusionsandOutlook
References
1 WhySoftColloids?
Inthelasttwodecades,withtheevolutionofsoftcondensedmatterphysics[1–8]the
manipulationofmaterialspropertieshasemergedasathemeofscientificandtech-
nologicalsignificance. The ultimate goalof such an activity is the rationaldesign
ofmaterialswithdesiredpropertiesforparticularapplications.Asafirstapproach,
this challenge can be met by combining properties of different, well-understood
classes of materials. Linear flexible polymers and hard sphere colloids represent
two of the most studied, and thus better understood classes of soft materials
[6,9–11](seeFig.1).Theyalsoreflecttwoextremecasesinthespatialorganization
ofalargenumberofmolecularunits:intheformercase(acontiguoussequenceofN
monomerswhicharecovalentlybonded)flexiblerandomcoilsandinthelattercase
solid, compact assemblies with a well-defined shape (here spherical). Moreover,
due to this architectural disparity, there are different characteristic length scales:
for polymers, the monomer size is typically 1nm whereas a colloidal sphere can
reach a diameter of 1μ m. These distinct features impart considerable differences
inthestructureanddynamicsandconsequentlyinthepropertiesofthesesystems.
Forexample,semidilutesolutions(atconcentrationcabovetheoverlapconcentra-
∗
tionc )ofhomopolymersexhibitcooperativeconcentrationfluctuationswhichare
controlledbytheosmoticpressureofthesystem[10,11].Thisreflectsweakinter-
actionsat monomericscales, of O(k T) or less, with a relevantcorrelationlength
B
ξ of O(nm) independentof N; very high concentrationsare therefore needed for
c
short-rangeordering,ifattainableatall.Ontheotherhand,colloidaldispersionsof
solid particles with radius R in a host fluid exhibit a size-dependentdynamic be-
haviorgovernedbycollectivethermalnumberdensityfluctuations,withcorrelation
lengthofO(μ m);theorderingoccursatrelativelylownumberdensitiesandislong-
ranged[12,13]. The stress is transmittedin bothsystems dueto their entropy,but
viadifferentchannels(chainelasticityandBrownianmotion,respectively).
FromPolymerstoColloids:EngineeringtheDynamicPropertiesofHairyParticles
Fig.1 Cartoonillustrationofdifferentsystemswhichcanbeobtainedusingappropriatechemistry
andspanthegapbetweensoft(limitingcasebeingapolymercoil)andhard(limitingcasebeing
ahardparticle)interactions.TheseinteractionsarecharacterizedbythepairpotentialsU,which
areplottedschematicallyagainstdistancerfromareferenceparticle.Thehorizontaldoublearrow
indicatesthepossiblewaystosoftenthehardsphereinteractionsviagraftedparticles,multiarm
stars,microgelsandeventuallypolymercoils,totheleft.Theverticalarrowsindicatevariouspos-
sibilitiesofincreasingcomplexityand/orchanginginteractionpotential(asshownschematically),
fromright:mixinghardsphereswithlinearpolymercoilsorotherspheresofsmallersize,grafting
particleswithnetworks(crosslinkingthegraftedlayer),blockcopolymermicelleswhosecorecan
bealsocrosslinked(thusimpartingstability),dendrimersanddendriticallybranchedpolymers
Therefore, it becomes evident that the intermediate behavior between short-
range polymeric and long-range colloidal interactions should be a rich area of
researchbecauseofthegreatpotentialtocombinepolymericwithcolloidalmeso-
scopic characteristics. A great dealof researchefforthas already been investedin
this direction. The key is the interdisciplinarity via a synergy of synthesis, phys-
ical experiment, and theoretical rationalization or predictive power. One way of
achieving optimum performanceis by using chemical means to combine material
properties,e.g.,obtainingvariousmacromolecularobjectsofmorecomplexarchi-
tecture.SomeexamplesofthedifferentpossibilitiesareillustratedinFig.1,where
startingfroma colloidalhardspherethe interactionpotentialcanbe progressively
softened with grafted hard particles, multiarm stars, microgels, and, eventually,
polymer coils. Moreover, different architectures and complexity such as particles
grafted with chemical networks, block copolymer micelles, dendrimers, and den-
dritically branched polymers can be achieved. Another way is simple mixing of,
e.g., polymers and hard or soft particles (Fig.1). This is a very efficient means
to introduce attractions and represents an avenue to obtain a variety of colloidal
gels[14–17]orglasses[16–20].However,itwillnotbediscussedhere.Moreover,
weemphasizethatthemethodologydiscussedheretotunethecolloidalinteractions
D.VlassopoulosandG.Fytas
fromhardspheretosoftisofcoursenottheonlypossibility.Forexample,recentlya
colloidalsystemwasdevisedwhosesoftnesscouldbeeffectivelytunedbyvarying
theappliedelectricfield,thesolventsaltconcentration,andthevolumefractionof
theparticles[7].
2 Model SoftSpheres
To meet the above-mentionedchallengeof rationallycombiningdifferentfeatures
ofdifferentsystems,theuseofmodelsystemsbecomesnecessary.Whereasthedis-
cussedcombinationswouldinprincipleleadto differentmaterialproperties,there
isaneedtodeveloppredictivepowerinordertocoordinatedifferenttypesofinter-
actionsina singlecomplexsystem, andhencecorrelatemacroscopicpropertiesto
interactions. To do this with reasonable precision, one has to resort to model sys-
tems,i.e.,systemsofwell-characterizedfeaturesandbehavior.Intheparticularcase
ofinterestwithreferencetoFig.1,thechallengeistodesign,prepare,andusemodel
sphericalparticlesoftunablesoftness.Somepopularexamplesarediscussedbelow.
2.1 ColloidalStarPolymers
These are chemically homogeneous multiarm star polymers (usually homopoly-
mers)withonlyexcludedvolumeinteractions[21,22].Duetotheirsynthesisproce-
dure(highvacuumanionicpolymerization),theyarestableandnearlymonodisperse
[23]. Their softness can be tuned at the synthesis level(numberand size of arms)
[23,24]and/orbyvaryingthetemperatureindifferentsolvents[25,26].Moreover,
these systems can be functionalized in various ways [27]. What made these sys-
temstrulyidealsoftcolloidswerethebreakthroughsinboththeoreticaldescription
andsynthesis.Theformerreferstotheabilitytodescribeanalyticallytheirinternal
structure[28]andtheirsoftnessintermsofaneffectiveinteractionpotential[24,29].
The majority of the experimental studies have been carried out with multiarm
1,4-polybutadienestars,whichweresynthesizedbyRooversandco-workersviatwo
distinctroutes.(1) Using chlorosilanechemistry,centraldendriticcoresof spheri-
calshapeanddifferentgenerationsweresynthesized,onwhichthedesirednumber
of polymericarms were grafted[23,30,31]. With this approachregularstars with
typical nominal functionality (i.e., number of arms, f) in the range 18–128 and
nominalarm molar mass, M , in the range 10–80kg mol−1 were synthesized. (2)
a
Alternatively, a short 1,2-polybutadiene backbone chain was hydrosilylated with
HSi(CH )Cl yieldingtwocouplingsitespermonomerunit,whichweresubstituted
3 2
with 1,4-polybutadieneby addition of poly(butadienyl)lithium[32]. Such “irregu-
lar”stars(withouttrulysphericalcentralcore)withnominal f of270andM inthe
a
range11–42kgmol−1 weresynthesized.
Severalothereffortsforsynthesizinghigh-functionalitystarpolymershavebeen
reported in the literature. Stars of intermediate functionality (typically below 64)
FromPolymerstoColloids:EngineeringtheDynamicPropertiesofHairyParticles
[33–39] or high functionality (typically up to 250) [34] were prepared, whereas
ultra-high functionalities (reaching nominal values of 6,400) were achieved using
methologies similar to (2) above, leading to block copolymer stars (arborescent
copolymersconsistingofpolyisoprenechainsgraftedonpolystyrenehyperbranched
polymers)[40].Ofcourse,therearealwaysissueswiththecharacterizationofsuch
complexsystems, and differentmethodsof preparationhave their own merits and
problems as well. For example, whereas the atom transfer radical polymerization
methodstypicallyyieldstarsofhighpolydispersity[35–37],recentlyanimproved
methodologydrastically reducedthis problem[38,39]. Whereasthere is nodoubt
thatRoovers’starsareprobablythebestcharacterized,truemodelsystems,theprice
onepaysisthattheyareavailableinsmallamounts,andthereforetherecentprogress
inthefieldasoutlinedaboveisveryencouraging.
Duetotheirnon-uniformmonomerdensitydistributionarisingfromtheirtopol-
ogy [28,41], colloidal star polymers can be considered as effective core-corona
particles with core radius r ∼ f1/2 [28] and softness defined as s=L/(L+r ),
c c
L being the corona thickness [42–44]. Typical value of s for the 128-arm stars is
about 0.89 for M around 80kg mol−1 [42]. Figure2 illustrates a cartoon repre-
a
sentation of a single such star in a good solvent along with the monomer density
profile[28]. Notethatthe topology(geometricconstraintsdue to curvature)is the
onlydifferencebetweenasphericalbrush[45]andasemidilutelinearflexiblepoly-
mer solution [10]: here, the blob size increases with the radial distance and three
monomerdensityregimescanbeobserved[24,28]:theinnermelt-likecoreregime,
theintermediatetheta-like(coatorunswollen)regimewheretheblobsareidealand
only solvent can penetrate in a dense suspension, and the outer excluded volume
1.0
f(r)
(r) 0.5
0.0
0 5 10 15 20
r
r
Fig.2 Left:Simulatedmonomerdensityprofile(distributionofintrastardensityaroundthecenter
ofmass)forameltofstarsofvaryingarmnumber f (fromleftstartorightopensquare:2(linear
chain), 4, 8, 16, 24, 36, 48, 64) [41]. The low intrastar density at low functionalities indicates
penetrabilitybyotherstars(tosatisfyincompressibilitycondition),whereasathighfunctionality
thestarcoreisformedwithconstantdensity.Inset:Cartoonillustrationofamultiarmstarwiththe
threeareasindifferent colors:melt-likeinnerblackcore,theta-likeintermediateblueunswollen
andexcluded-volumeouterredswollenregion.Right:ThepredictedDaoud–Cotton[28]monomer
densitydistribution.Thehorizontaldashedlineindicatestheaveragesolutionconcentration
D.VlassopoulosandG.Fytas
(orswollen)regimewheretheblobsareswollenandstar–starinterpenetrationcan
takeplaceindensesuspensions.TheDaoud–Cottondensityprofilewasconfirmed
experimentallyforpolyisopreneandpolybutadienestarpolymerswithfunctionali-
tiesintherange8–129[21,46,47]aswellasbycomputersimulations[21,24,41].
Naturally,thesestarpolymers(regularandirregularalike)aredistinctlydifferent
fromdendrimers:thelatterrepresentanotherclassofmodelcolloidalparticles[48],
whichhoweverarenotdiscussedinthisreview.
2.2 Block Copolymer Micelles
Block copolymers(such as diblocks and triblocks), when dispersed in a selective
solventforoneblock,self-assembleintosupramolecularstructurescalledmicelles
[49]. Dependingon the blockcomposition,the micelles can take differentshapes,
includingsphericalstar-like structures(the core is usually larger in micelles com-
paredtostars,andclearlyseparatedfromthecoronaduetotheenthalpicrepulsion
of the two blocks); thus, micelles can be structurally similar to stars [50–52]. In
fact, it was demonstrated [51–54] that the density profile of the star-like micelles
ingoodsolventforthecoronafollowstheDaoud–Cotton[28]scaling.However,in
contrasttothestarformationorothermeansofchemicalgrafting,themicellesare
formedthroughaphysicalprocess.Thishastheadvantageofsimpleandrelatively
inexpensivechemistry,andexplainsthefactthatmicellesrepresentbyfarthemost
studiedsoftspherecolloidalsystems.Ontheotherhand,themajordisadvantageis
thestabilityofthemicellesduetotheexchangekineticsofthechainsparticipating
inamicelle[55],especiallyincasesoftemperatureandconcentrationvariation,and
theattainabilityofequilibriumstructuresinthecaseofcoresofglassyblocks.One
waytoovercomethisimportantproblemisbyfreezingthecorebelowitsglasstran-
sitiontemperature[56,57]butthereremainofcourseissuesrelatedtoequilibrium.
Recently, a versatile class of poly(ethylene propylene)/poly(ethylene oxide)
blockcopolymermicelleswereintroduced;theywere stabledueto a combination
of high block incompatibility,kinetically frozen core, and high interfacial tension
between core and solvent [53,58]. Moreover, by using a co-solvent of varying
composition,theaggregationnumberwascontrolledandsoftspheresfromstar-like
to micelle-like could be obtained. Another way is core stabilization via chemical
crosslinking,saybyUVradiation[59–64].
It is not our intention to review the vast field of block copolymermicelles and
theirnanotechnologicalapplications.Thereareseveralrelevantreviewsforthein-
terestedreader[49,65].Wementiononlytheversatilityofthesesystemsasmodel
soft colloids and the variousaspects that need carefulconsiderationfor designing
appropriatesystems.
Theblockcomposition,i.e.,therelativesizeofthecoronatothecore,dictatesto
a greatextentthesoftnessof the micellesandthereforetheir propertiesand phase
behavior [52,66–71]. One particularly appealing feature of block copolymer mi-
cellesistheirtunability[49,72].ParameterssuchasthepHstrengthandamountof