Fundamentals of Polymer-Clay Nanocomposites

193 Pages·2011·11.683 MB·English

by  G W Beall, Clois E Powell

Fundamentals of Polymer–Clay Nanocomposites
Written for graduate students, researchers, and practitioners, Fundamentals
of Polymer–Clay Nanocomposites provides a complete introduction to the
science, engineering, and commercial applications of this new class of
material.
Startingwithadiscussionofgeneralconcepts,theauthorsdefineindetail
the specific terms used in the field, providing newcomers with a strong
foundation in the area. Then, the physical and mechanical properties of
polymer–clay nanocomposites are described, with chapters on thermo-
dynamicsandkinetics,engineeringproperties,barrierproperties,andflame
retardancy. Mechanisms underpinning observed effects, such as UV resist-
ance,solventresistance,andhardness,arealsoexplained.Throughouteach
chapter, experimental results are combined with theory, ensuring that the
reader gains a full appreciation of the subject matter.
In-depth discussions of clay and clay surface treatment, fabrication, and
characterizationofnanocompositesareprovided,andparticularemphasisis
placedontheproperuseandinterpretationofanalyticaltechniques,helping
the reader to avoid artifacts in their own work. With commercial applica-
tions discussed throughout, this is an ideal reference for those working in
polymer science.
GaryW.BeallisaProfessorintheDepartmentofChemistryandBiochemistry
at Texas State University, San Marcos, and Director of the Center for
NanophaseResearch.HewasawardedhisPh.D.inPhysicalChemistryfrom
Baylor University in 1975, and has since gained over 20 years of industry
experience, co-edited the first book on polymer–clay nanocomposites, and
authorednumeroustechnicalpapersandtwobookchapters.
CloisE.PowellisAssociateDirectoroftheCenterforNanophaseResearchat
TexasStateUniversity,SanMarcos,wherehehasworkedsince2004.After
receiving his Ph.D. in Organic Chemistry from Rutgers University in 1979,
hegainedover20yearsofprofessionalexperiencewithcompaniesincluding
Southern Clay Products Inc. and The Sherwin WilliamsCompany.

Fundamentals of Polymer–
Clay Nanocomposites
GARY W. BEALL AND
CLOIS E. POWELL
TexasStateUniversity,SanMarcos
cambridge university press
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G.BeallandC.Powell2011
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thewrittenpermissionofCambridgeUniversityPress.
Firstpublished2011
PrintedintheUnitedKingdomattheUniversityPress,Cambridge
AcatalogrecordforthispublicationisavailablefromtheBritishLibrary
LibraryofCongressCataloging-in-PublicationData
Beall,G.W.(GaryW.),author.
FundamentalsofPolymer-ClayNanocomposites/GaryW.BeallandCloisE.Powell.
p. cm
ISBN978-0-521-87643-8(Hardback)
1. Nanocomposites(Materials) 2. Polymerclay. I. Powell,CloisE.(CloisElbert),1944–,
author. II. Title.
TA418.9.C6B432011
620.1092–dc22
2010044487
ISBN978-0-521-87643-8Hardback
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Contents
1 Introduction page1
2 Thermodynamics and kinetics ofpolymer–clay nanocomposites 4
2.1 Clay surface compatibility withpolymers 4
2.1.1 Smectiteclay structure 4
2.1.2 Turbostratic natureofsmectite clays 6
2.1.3 Intercalation chemistry 8
2.1.4 Intercalation ofwater-solublepolymers 9
2.1.5 Hydrophobic intercalation 12
2.1.6 Intercalation via ion exchange 12
2.1.7 Alternativeintercalation chemistries 13
2.1.8 Intercalation via ion–dipole bonding 13
2.1.9 Hydrophobic polymer intercalation 16
2.1.10 Edge treatment with silane couplingagents 17
2.2 Thermodynamics of polymer–clay interactions 17
2.2.1 The enthalpic role inexfoliation 17
2.2.2 The entropicrole inexfoliation 19
2.2.3 Kineticsofintercalation–exfoliation 19
References 20
3 Analytical methods utilized in nanocomposites 23
3.1 Wide-angle X-raydiffraction 23
3.2 Transmission electron microscopy(TEM) 27
3.3 Scanning electronmicroscopy (SEM) 31
3.4 Atomic force microscopy (AFM) 32
3.5 Indirectmethods 32
References 33
4 Gas diffusion characteristics ofpolymer–clay nanocomposites 35
4.1 Potentialof polymer–clay nanocomposites as barrier materials 35
v
vi Contents
4.2 Modelsforgas transportin polymer–claynanocomposites 36
4.2.1 The tortuous pathmodel for barrier in nanocomposites 36
4.2.2 Experimental data on nanocomposite barrier performance 38
4.2.3 Data supporting the constrained polymer model 44
References 46
5 Engineering properties ofpolymer–claynanocomposites theory
and theory validation 49
5.1 Mechanics 49
5.2 Properpreparation and analysisof polymer–clay nanocomposites 50
5.3 Theoryof anisotropic dispersed-phase reinforcement of polymers 51
5.4 Genesis:anisotropicdispersed-phase reinforcement of metal alloys 51
5.5 Transitionfrom anisotropicdispersed-phase reinforcement
inmetalalloys toanisotropic dispersed-phasereinforcement
inpolymers 53
5.6 Validation ofthe morphology of montmorillonite as anisotropic
dispersed-phase reinforcement inpolymers 55
5.7 Refinement ofthe mechanism ofmontmorillonite reinforcement
ofpolymers 58
5.8 Conclusions 63
References 66
6 Variables associatedwithpolymer–clay processing in relation
toreinforcement theory 68
6.1 The polymer asa significant independentvariablein
themechanical performance ofpolymer–clay nanocomposites 68
6.2 Processing asa significant independentvariablefor polymer–clay
nanocompositepreparation 71
6.3 Hydrophilic–hydrophobicbalance of the surface of
montmorilloniteas a significantindependent variablefor
polymer–claynanocompositepreparation 74
6.4 Examination ofthe historical revelation ofpolymer–clay
nanotechnology 78
6.5 Examination ofpolymer–clay composites with complex
processing issues 83
6.6 Polymer chain engineeringin relation to montmorillonite
incorporation asa nanoparticle 86
6.7 Conclusions 90
References 91
Contents vii
7 The relationships of polymer type specificity tothe production
ofpolymer–clay nanocomposites 95
7.1 Complexity of polyolefin–montmorillonite nanocomposites 95
7.2 Difficulties associated withthe preparation ofpolyimide–clay
nanocomposites 121
7.3 The conundrum ofpolystyrene–clay nanocomposites 124
7.4 Mysteries associated with elastomer–clay nanocomposites 130
7.5 Dichotomyof crystallineand amorphous polyester–clay
nanocomposites 135
7.6 Two-phase engineered polymer(polyurethane) synergy withclay
nanocomposite reinforcement 140
7.7 Elastomers that crosslink withclay nanocompositereinforcement 145
7.8 Conclusions 149
References 151
8 Flame retardancy 156
8.1 Enhanced thermal stabilityprovided by polymer–clay
nanocomposites 156
8.2 Relationships between enhanced thermal stabilityof
polymer–clay nanocomposites and flameretardancy 165
8.3 Evaluationsof potentialsynergies between traditional
flame retardantsforpolymers and polymer–clay nanocomposites 174
8.4 Summary and conclusions 177
References 178
Index 183

1 Introduction
Canoneimaginethe utilityofadispersed-phasereinforcementforpoly-
mers that has a thickness of 1nm, a platelike morphology with minimal
dimensions of 150 to 200 nm, robust with a modulus of 180 GPa,
nontoxic (FDA classification of GRAS; generally regarded as safe for a
majority of applications), a surface area in excess of 750 m2/g, a charge
suitable for altering its hydrophobic–hydrophilic balance at will, and a
refractive index similar to polymer so that the nanoparticles will appear
transparent in the polymer composite? How difficult would it be to
prepare such a particle?
This particle is naturally occurring and found around the world. It is
easilyminedandpurified.Thereactorfortheparticlewasavolcano.The
ashfrommanyvolcanoeswasspreadaroundtheearthduringanintense
period of activity many millions of years ago. This ash was transformed
into clay (montmorillonoids or smectites) by natural processes, into
uncharged species (talc and pyrophyllite) and charged species through
isomorphicsubstitutionofthecrystalstructure(hectorite,montmorillon-
ite, saponite, suconite, volchonskoite, vermiculite, and nontronite).
Montmorillonite serves as the principle mineral for the development
of polymer–clay nanocomposites discussed in this book. A misunder-
standing of the terms bentonite (the ore or rock) and montmorillonite
(the mineral) are pervasive in the literature. We will focus on utilizing
themineralname.Thecompositionofmontmorillonitecanbedescribed
by imagining a sandwich structure with the top and bottom layers com-
posedofsilicadioxidetetrahedralstructures.Thecenterlayeriscomposed
of a metal oxide octahedral layer. Metal in this octahedral structure
originally was completelyAlþ3and uncharged(pyrophilite). Isomorphic
substitution of Mgþ2 for Alþ3 in the octahedral layer produced
montmorillonite.
Notice that a substitution of a þ2 Mg for a þ3 Al will produce a net
negative charge. The amount of substitution will dictate the charge on
the particle. Nature will not abide a net charge development without
1
2 Introduction
supplyingcounterionsforthatcharge.Theseareusuallysodium,calcium,
potassium,etc.ionswhere the montmorilloniteisnatural.
HectoriteistheisomorphicsubstitutionofLiforMg(fromtalc).Some
isomorphicsubstitutionisobservedinthetetrahedralsilicadioxidelayer
withAlsubstitutingforSi.Theseparticlesarefoundinnatureinstacked
arrays similar to a pad of paper. They can be easily mined and purified
by dispersing the montmorillonite into water and removing the larger,
heavier particles (sand, gravel, etc.) by centrifugation.
Individual montmorillonite particles in water must be obtained by a
highshearenvironment.Thenatureofthecounterionscaninterferewith
ultimate separation of single particles (full exfoliation).
Sodium has the greatest utility as a counterion for the preparation of
fullyexfoliatedmontmorillonitedispersions.Sodiumcanbeexchangedfor
MgandCacounterionsbystandardionexchangemethods(forexample,
water-softening equipment). These fully exfoliated montmorillonite par-
ticles can be exchanged with positively charged organic molecules (for
example, quaternary ammonium ions) to form very stable individually
organicallymodifiedmontmorilloniteparticles.Whenthemodificationis
significantly hydrophobic, the organomontmorillonite will separate from
thewater.Thismaterialcanbefilteredfromthewater,dried,andgroundto
apowder.
Differentorganomontmorillonitesareemployedextensivelyforawide
rangeofmarketareas.Aninternationalawarenessthatorganomontmor-
illonite could be successfully utilized as a dispersed-phase reinforcement
in polymers began in the early 1990s.
Many descriptive terms that describe the morphology of montmoril-
lonite in polymers have been misused. The galleries of the organomont-
morillonite are composed of the organic treatment separating the
individual montmorillonite particles. These galleries begin to fill with
polymer and swell. The particle separation up to about 7 nm will allow
polymer chains to interact with two particles simultaneously. These
structures are referred to as intercalated. Assemblies of these particles
are referred to as tactoids and behave as a collection of particles in
polymer composites. Separation of montmorillonite particles past this
threshold precludes the interaction of polymer chains interacting with
more than one particle. These assemblies are referred to as exfoliated
polymer–montmorillonite nanocomposites.Atthisstage,the fullbenefit
of dispersed-phase reinforcement by montmorillonite in the polymer is
realized. Because the particles are anisotropic, they also must be aligned
to achieve this benefit.