Polymer Nanocomposition Approach to Advanced Materials - Journal

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Polymer Nanocomposition Approach to Advanced Materials Christopher O. Oriakhi Hewlett-Packard Company, 1000 NE Circle Blvd, Corvallis, OR 97330; [email protected]

What do bulletproof vests, golf clubs, skis, tennis rackets, vehicle tires, spacecraft, and missile parts have in common? They are all examples of composite materials. Composite materials are commonplace and have become indispensable in the construction industry, where they offer the advantage of low weight but high strength, stiffness, and durability. Notable examples include sandwich structures, laminates, reinforced polymers, concrete, and fiber-reinforced composites (fiberglass and carbon fiber). These are macroomposites or microcomposites and are formed by mixing two or more different components. The resulting mechanical properties depend largely on the interfacial interactions between the phases. Usually, one component serves as the matrix in which particles or fibers of the others are uniformly dispersed like blueberries in muffins. Less familiar, but of great research interest, is a new class of materials known as nanocomposites. These offer properties not attainable in the above conventional composites and allow materials scientists to prepare tailor-made advanced materials. Nanocomposites are multiphase materials containing two or more distinctly dissimilar components mixed at the nanometer scale. Particles in this region are about 100–1000 times the size of a typical atom. The phases may be inorganic– inorganic, inorganic–organic, or organic–organic and the resulting material may be amorphous, crystalline, or semicrystalline (1). Nanocomposites display new and sometimes improved mechanical, catalytic, electronic, magnetic, and optical properties not exhibited by the individual phases or by

their macrocomposite and microcomposite counterparts. The basic reason for the synergistic improvement in properties is not completely understood. However, scientists believe it is related to confinement, “quantum-size” effects, and sometimes coulombic-charging effects originating from the ultrafine sizes, morphology, and interfacial interactions of the phases involved (2–5). Biological polymer–based nanocomposites are widespread. Excellent examples include bones, cartilage, cobwebs, cuticles, scales, shells, teeth, and wood. These are derived from inorganic building blocks like carbonate and phosphate minerals and from biological polymers such as carbohydrates, lipids, and proteins in a highly controlled biomineralization process (Table 1) (6 ). These materials have superb mechanical properties. For example, the nacre of abalone shell (mother of pearl) has been studied in detail. It consists of alternating nanolayers of aragonite (CaCO3) platelets and a mixture of proteins and polysaccharides (7, 8). This nanocomposite combines great strength with remarkable hardness and toughness that are better than or comparable to those of some manmade advanced structural materials (8). Nature makes these well-defined nanomaterials using aqueous chemistry at ambient temperature and pressure. It receives help from the forces of molecular self-assembling and nonbonded interactions such as hydrogen bonding, hydrophobic interactions, coulombic interactions, and van der Waals forces. The polymer matrix is used as a template to initiate and control the nucleation, growth, and microstructure of the inorganic

Table 1. Some Biogenic Nanocomposites and Constituent Parent Phases Inorganic Phase

Organic Phase

Example

— Inorganic–Polymer Nanocomposites — CaCO3 /(Ca1 0 (PO4 )6 (OH)2 )a Collagen polypeptide Mucopolysaccharide/collagen fibril

Bones Tooth dentin & enamel

Calcite/aragoniteb

Polypeptide/protein

Mollusc shells (abalone, mother-of-pearl)

Calcite

Glycoprotein Polypeptide/protein

Sea urchin spine Egg shell

— Polymer–Polymer Nanocomposites — Polysaccharide (chitin), lipids, & protein

Insect cuticle

Protein fiber/polypeptides

Spider silk

Collagen fibrils (nano/micro)

Tendon

Glycoprotein and polysaccharide

Mucus

Collagenous connective tissues, protein–polysaccharide matrix

The elephant's trunk

— Nanoclusters — Magnetic particle in magnetosome—Fe3 O4 /Fe3 S4

Magnetotactic bacteria

Ferritin, barite, silica, CdS

Algae

a Compounds

of the type M10(PO 4 )6(OH)2, where M is Ca, Ba, or Sr, are known as

hydroxyapatite. bCalcite and aragonite are different crystalline forms of CaCO . 3

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crystal (6 ). Some of the more interesting structures that develop may not have any major technological uses, but an understanding of the underlying biomineralization chemistry can guide materials scientists toward a rational design and synthesis of advanced materials with predictable properties, in a reproducible manner. There are several reasons why chemists are interested in preparing nanocomposites containing polymers and inorganic solids. A judicious selection and combination of organic polymers and inorganic materials may yield new properties. For example, the flexibility and processing attributes of a polymer such as polyethylene or polystyrene can be introduced into an otherwise brittle inorganic glass. Also, new catalytic, electronic, magnetic, optical, and superconducting properties can be obtained by nanometer-level mixing of conjugated polymers and specialty inorganic materials (1–4). Nanocomposites containing polymers are commonly made by two synthetic strategies. These are the sol–gel process and the guest–host inclusion chemistry process. The first process, the sol–gel process, is based on inorganic polymerization reactions. It allows low molecular weight organic polymers to be included into the inorganic matrix. In a typical reaction, the alkoxide derivatives of one or more inorganic precursors are allowed to undergo controlled hydrolysis in the presence of the polymer. This is followed by polymerization, gel formation, densification, and drying of the resulting solid (9). The sol–gel method offers several advantages. For example, the nanocomposite so obtained is very pure and homogeneous. The process occurs at a relatively low temperature that will not degrade the polymer. Particle size and morphology can be controlled. In the wet form, the material can be spin-coated onto desired surfaces. This approach has been used to prepare several polymer nanocomposites (10–12). The second process, the guest–host inclusion chemistry process, employs intercalation reactions involving the insertion of polymer or polymer precursors into suitable preformed

host structures, or template nucleation and growth of the desired inorganic host crystal in a solution of the polymer. The term intercalation describes the insertion of extra time (day, week, or month) into the calendar year. When used by chemists, it refers to the insertion of guest ions or molecules into the vacant site of a crystalline host lattice. There are four types of host lattice structures for intercalation reactions: three-dimensional (framework), two-dimensional (layer or lamella), one-dimensional (chain), and zero-dimensional (molecular or clusters). Examples are shown in Table 2. The 3-D and 2-D host lattices offer the richest intercalation chemistry because of their stability and well-defined vacant sites for intercalation reactions. For example, the 3-D framework consists of well-defined nanometer-size pores and channels as is found in microporous zeolites and mesoporous solids. The pores and channels can be filled with polymer without destroying their connectivity or framework structure. The 2-D layered hosts are characterized by strong covalent bonds within the layers and weak van der Waals forces between the layers. Consequently, the interlayer space can be separated considerably to incorporate the guest polymer while preserving the integrity of the layer structure. The rest of this article will focus on polymer nanocomposites based on 3-D and 2-D inorganic host materials. Design and Synthesis of Polymer Nanocomposites

Nanocomposites Derived from 3-D Inorganic Structures Mesoporous and microporous inorganic solids are excellent 3-D host lattices for intercalation reactions because they contain well-defined stable empty pores and channels with diameter ranging from 2 to 500 Å. Examples include zeolites and zeolite-like materials such as MCM-41 and MCM-48 solids (13). Guest monomeric ions or molecular polymer precursors that have the appropriate size and geometry can be

Table 2. Host Lattices for Intercalation Reaction Host Lattice Structure 3-D (framework)

Type of Material

Examplecccccccccccccccccccccccccc c

Zeolite

Zeolite MFI (ZSM-5) Zeolite MTW (ZSM-12) Zeolite MOR (mordenite) Zeolite LTL (L) Zeolite FAU (fajusite) MCM-41 MCM-48 MCM-58

Mesoporous material

2-D (layered)

Elemental Metal chalcogenide Metal oxide Metal phosphate Metal oxy-halide Smectite clay/silicate Titanate/niobate Layered double hydroxide

cPore Size/nm

Layer Charge

0.55 0.60 0.68 0.72 0.74 ca. 3.0 ca. 3.5

Graphite; black phosphorus MX2 (M = Ta, Ti, Zr, Nb, Mo, W; X = S, Se, Te) MPX3 (M = Cd, Fe, Mn, Ni, Zn; X = S, Se) MoO3, Mo18O52, V2O5 M(HPO4)2 (M = Ti, Zr, Ce, Sn) MOX (M = Ti, V, Al, La, Cr, Fe; X = Cl, Br) Hectorite, kaolinites, montmorillinite KNbO3; K4Nb6O17; K2Ti4O9, H2Ti3O7; KTiNbO5 LiAl2(OH)6OH⭈2H2O; Mg4Al2(OH)12[anion]y⭈H2O

1-D (chain)

AMO3X3 (A = In, Tl, or alkali metal; X = S or Se) MX3 (M = Nb, Ta, Zr, Ti; X = S, Se) AFeS2 (A = Na, K, Rb, Cs)

0-D (molecular)

Fullerene (C60); complex anions (e.g., [Na9Fe20Se38]9᎑, [As4Mo6V7O39(SO4)]4᎑)

Neutral Neutral Neutral Neutral Negative Neutral Negative Negative Positive

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inserted into the vacant nanopores or channels by ion exchange chemistry or by direct inclusion from gas or liquid phase. Polymerization in the presence of a suitable initiator produces polymer molecular wires, nanofilaments, fibrils, or thin-film coatings confined within the hosts. The resulting hybrid material is a 3-D polymer nanocomposite. The strategy is illustrated in Figure 1. New materials with novel electronic structures not accessible otherwise can be designed and synthesized this way, by a judicious selection of functional monomers. Polymer Nanocomposites Based on Zeolitic Materials Zeolite molecular sieves are crystalline porous aluminosilicates with pore sizes ranging from 2 to 10 Å. They have attracted great interest as catalysts and adsorbent materials because of their highly uniform acidity and excellent thermal stability. Like clay, zeolites have anionic oxide framework structures that require the presence of cations to achieve charge neutrality. They are therefore excellent host materials for intercalation chemistry by virtue of their ability to insert ionic and molecular species into their nano-sized channels and pores. Figure 2 is a schematic representation of the structure of zeolite (mordenite) intercalated by a thiophene molecule. Several porous zeolite-like materials have pore sizes in the range of 10–100 Å. Examples are the gallophosphate “cloverite”, molybdenum and vanadium phosphates, and mesoporous MCM-41 and MCM-48 (14, 15). A unique feature of zeolites is their ability to discriminate between shapes and sizes of reactant molecules. Only species with an appropriate size and shape can intercalate into the vacant pores. Therefore a successful design of new materials with zeolite requires a judicious pairing of reactant geometry and host’s pore sizes. Two basic synthetic strategies can be used to insert polymer into the 3-D host lattice. One is the direct threading of preformed polymer through the host’s channel. This approach is suitable for soluble polymers and melt-processable polymers. However, direct insertion of preformed high molecular weight polymers into the host’s nanopores and channels is difficult because of their larger size, conformation, and diffusion limitations. The second strategy offers the greatest flexibility for designing nanocomposites of polymers and zeolitic materials. It involves the inclusion of suitable monomers into the host’s nanospace either from the gaseous or liquid phase, followed by in situ polymerization. If possible, redox-active species and other polymerization initiators are loaded into the host material by direct adsorption or by ion-exchange reactions. Otherwise, polymer formation can be induced externally in the presence of air, heat, or high-energy radiation sources. Therefore it is essential to understand the conditions and mechanism of polymerization of the inserted monomer. Using this strategy, zeolite nanocomposites containing both structural and electronically conducting polymers have been prepared. Representative examples are shown in Table 3. Nanocomposites Derived from Structural Polymers. Poly(acrylonitrile) (PAN) has been confined within the channels of mordenite and Y zeolite (16, 17 ). The strategy involves the introduction of acrylonitrile (vapor) into the channels of zeolite NaY. The resulting acrylonitrile–zeolite complex is evacuated to remove any acrylonitrile adsorbed on the zeolite surface. Gamma irradiation of the complex polymerizes the acrylonitrile within the zeolite channels to afford the nanocomposite material. 1140

Monomer

+ Monomer (e.g. aniline)

Intercalation

Mesoporous host (e.g. MCM 41)

Monomerintercalated host Intra-channel polymerization

Intercalated polymer nanocomposite

Figure 1. In situ formation of polymer within the channels of a mesoporous host lattice.

Figure 2. Polyhedral representation of zeolite (mordenite) and its guest–host compound with thiophene.

Table 3. Some Common Conducting Polymers Polymer Poly(aniline)

Abbreviation

Structure

PANI N H

Poly(pyrrole)

PPY N H

Poly(furan)

PFU O

Poly(thiophene)

PTH S

Poly(phenylene vinylene)

PPV

Poly(p-phenylene)

PPP



A similar method is to prepare poly(furfuryl alcohol)– zeolite nanocomposites (17). Furfuryl alcohol is intercalated into the zeolite channels from the liquid phase. The encapsulated polymer can further be converted to conjugated ladder structure; then conducting graphite-like carbon is formed by controlled pyrolysis under nitrogen at temperatures in the range of 200 to 700 °C. Figure 3 is a schematic illustration of intrazeolite formation and carbonization or pyrolysis of polyacrylonitrile. The dc conductivity of the extracted pyrolyzed PAN is in excess of 10᎑5 S cm᎑1 (16 ). Poly(methylmethacrylate) (PMMA) has also been entrapped within zeolite NaY, mordenite, beta, and ZSM-5 as well as in the mesoporous MCM-41 and MCM-48 (18). The methyl methacrylate monomer is adsorbed and polymerized within the channels. The amount of polymer in the nanocomposites increases with increasing pore volume or channel diameter. An interesting observation is that the entrapped polymer within the 6–35-Å diameter channels does not exhibit the characteristic glass transition temperature. In addition to methyl methacrylate, Llewellyn and coworkers have polymerized styrene and vinyl acetate within the pores of MCM-41 using free-radical initiator (19). The degree of polymerization and the molecular weight of the confined polymer can be controlled by varying the pore diameter of the host lattice and the monomer-to-initiator ratio (19). Generally, the physical properties of the confined polymers in the nanocomposites are significantly different from those of the pristine polymers. This is the motivation for research in the design and synthesis of polymer nanocomposites. Nanocomposites Derived from Conducting Polymers. Electronically conducting organic polymers have attracted interest ever since their discovery in the early 1970s. Some examples of conducting polymers are presented in Table 3. Technological applications include all-plastic transistors, rechargeable batteries, light-emitting diodes (LEDs), antistatic coatings, electromagnetic shielding and infrared polarizers, nonlinear optics, solar cells, sensors, and nerve guides (20). Nanocomposition with 3-D host materials may further expand or enhance the application properties of conducting polymers by allowing researchers to better control conformation, crosslinking, orientation or tacticity, interchain electronic processes, and environmental stability. Zeolite nanocomposites containing conducting polymers have been actively investigated as a way of making highly oriented “molecular or nanowires” that could potentially lead to molecular electronics. Bein and coworkers (15, 21) and other researchers (22–24) have prepared molecular wires by incorporating conducting polymers into several zeolite and zeolite-like materials. Table 4 illustrates examples of some conducting-polymer-based nanocomposites. Poly(aniline), poly(pyrrole), and poly(thiophene) chains have been encapsulated within the channels of various forms of zeolite by incorporation of the corresponding monomers followed by oxidative polymerization in the presence of Cu(II), Fe(III), or persulfate (21). The electronic conductivity of the resulting nanocomposites is quite low. This is consistent with the low dielectric and highly insulating nature of the host material and suggests that polymer chains are electronically isolated within the framework. Cox and Stucky reported the successful incorporation and polymerization of methylacetylene within the pores of

Adsorption C

C

C

C

C

N

N

N

N

N

Acrylonitrile monomer

Monomer intercalate

Zeolite channel Radical polymerization

Pyrolysis C N

N

N

Intrazeolite ladder polymer

C N

C N

N

PAN-Zeolite nanocomposite

Figure 3. Polymerization of acrylonitrile within the channels of a zeolite host material.

Table 4. Selected Polymers and 3-D Inorganic Host Lattices for Nanocomposite Synthesis Polymer Type

Polymer

Structural

Poly(acrylonitrile)

Conducting

Inorganic Framework

Ref

Zeolite-Y (NaY) Mordenite

16, 17

Poly(furfuryl alcohol)

Zeolite-Y

17

Polystyrene

MCM-41

19

Poly(vinyl acetate)

MCM-41

19

Poly(methyl methacrylate)

Zeolite-Y Mordenite ZSM-5 MCM-41 MCM-48

18 18 18 18, 19 18, 19

Poly(ferrocenyl silane)

MCM-41

45

Poly(phenol-formaldehyde)

MCM-41

46

Polyaniline

Mordenite Zeolite-Y MCM-41

21a 21a 21d

Polypyrrole

MCM-41 Zeolite-L Mordenite

21b 21b 21b

Polythiophene

Zeolite

21c, 23

Poly(methylacetylene)

Mordenite ZSM-5 SAPO-5

22 22 22

acidic forms of several zeolites (mordenite, omega, L, Y, beta, ZSM-5, SAPO-5) at room temperature with the goal of producing materials with improved nonlinear optical properties (22). Detailed chemical, thermal, and spectroscopic characterization of the resulting materials suggested the formation of conjugated oligomers within the pores of the zeolites. However, the exact nature of this oligomer in terms of the extent of branching, cyclization, conjugation length, and the degree of oligomerization remains to be understood. The presence of mesoporous channels in transition-metalcontaining aluminosilicate host MCM-41 has been exploited to form conducting nanocomposite materials. Polyaniline filaments in the conducting emeraldine salt form resulted from the in situ polymerization of intercalated aniline (21). The presence of intrachannel polymer was confirmed by contactless microwave conductivity measurement and by the large drop in the host’s pore volume. The molecular weight of the intrachannel poly-


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mer was approximately 34,000 Da. Now that molecular wires can be synthesized, further research is needed to define processing conditions and application properties.

Nanocomposites Derived from 2-D Inorganic Structures The insertion of guest species such as an organic molecule or polymer between inorganic sheets of layered inorganic solids is often referred to as intercalation, and the reaction has been extensively investigated as a way of preparing complex organic– inorganic nanocomposites (1, 25). Table 5 lists some of the types of layered host materials available for intercalation reactions.

Layered host lattices are 2-D structures that offer greater flexibility in the design and synthesis of multifunctional advanced materials than 3-D host lattices for three reasons: (i) a larger number of layered host solids with properties ranging from those of insulators to semiconductors to semimetals are available; (ii) several layered host materials are capable of intercalating both neutral and charged species; (iii) the interlayer spacing of many 2-D hosts can be expanded almost infinitely to accommodate the size and steric requirements of the guest ions or molecules without destroying covalent bonds in the host lattice.

Table 5. Method of Preparation of Selected Polymer Nanocomposites Polymer Type

Polymer

Electronic conductor Polyaniline

Poly(pyrrole)

Ionic conductor

Structural polymer

Layered host a

Preparation method

Ref

MoS2 MoO3 RuCl3 FeOCl V2 O5 HTaWO6 VOPO4 Zr(HPO4 )2

Exfoliation/adsorption In situ polymerization In situ polymerization In situ polymerization In situ polymerization In situ polymerization In situ polymerization In situ polymerization

47 27g, 43b 27d 48 27b 49 50 51

V2 O5 FeOCl VOPO4

In situ polymerization In situ polymerization In situ polymerization

27c 27f 52

Poly(thiophene)

V2 O5

In situ polymerization

27c

Poly(phenylenevinylene)

MoO3

Thermal conversion

33

Poly(phenylenevinylene)

Clay

Chemical conversion

34

Poly(p-phenylene)

MoO3


43b

Poly(ethylene oxide)

MoS2 , MoSe2 , TiS2 NbSe2 MnPS3 , CdPS3 MoO3 Clay Clay Graphite oxide

Exfoliation/adsorption Exfoliation/adsorption Exfoliation/adsorption Exfoliation/adsorption Melt intercalation Exfoliation/adsorption Exfoliation/adsorption

29a, 29b 29e 31 55 35 53 54

Poly(ethylenimine)

MoS2 , MoSe2 , TiS2 Exfoliation/adsorption Clay Exfoliation/adsorption

29d 29d

Poly(ethylene glycol)

NbSe2

Exfoliation/adsorption

29e

Poly(phosphazene)

Clay


42b

Poly(acrylonitrile)

Kaolinite-clay


56

Poly(acrylamide)

Kaolinite-clay


57

Nylon 6

Clay Organoclay

In situ polymerization Melt intercalation

28a 58

Epoxy resin

Clay


28c

Poly(dimethylsiloxane)

Organoclay


41c

Polystyrene

MoS2 Organoclay

In situ polymerization Melt intercalation

59 27g

Polyimide

Clay


60

Poly(urethane)

Organoclay


28

Poly(styrene sulfonate)

Ca-Al-LDH Mg-Al-LDH Zn-Al-LDH

Template synthesis Template synthesis Template synthesis

39 39 39

Poly(acrylic acid)



39 39 39

Poly(vinyl sulfonate)



39 39 39

Poly(amino acids)

Mg-Al-LDH Mg-Al-LDH

In situ polymerization Template synthesis

40c 40c

Poly(vinyl alcohol)

Ca-Al-LDH Kaolinite

Template synthesis 40a Direct polymer intercalation 61

Poly(ethylene terephthalate) Clay aLDH

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28d

indicates layered double hydroxide.



Two general kinds of polymer nanocomposites can be derived from layered host materials: intercalated (ordered) and exfoliated (delaminated) layered nanocomposites (26 ). In intercalated nanocomposites, one or more molecular layers of the polymer are inserted and confined between the sheets or galleries of the inorganic host, forming an ordered multilayer with interlayer separation between 5 and 50 Å. In exfoliated nanocomposites the host lattice is delaminated into single nanolayers, which are then homogeneously dispersed throughout the polymer matrix. Compared to their intercalated counterpart, exfoliated nanocomposites contain a small weight percent of the host layers with no particular structural order. The interlayer separation may be related to a statistical average concentration of host layers or is comparable to the radius of gyration of the polymer, which could be a few hundreds of nanometers (26 ). Synthesis of Intercalated Nanocomposites There are several ways to introduce a polymer between the sheets of a host lattice: in situ polymerization of intercalated monomers, the exfoliation–adsorption process, chemical or ther-

NH3+ MoO3

+

MoO3−

Intercalation reaction

−

NH3+

MoO3− MoO3−

Lix MoO3

Anilimium ion

Aniline intercalate

In situ oxidative polymerization

MoO3− NH

MoO3

−

Poly(aniline)/MoO3 nanocomposites

Figure 4. Schematic diagram of nanocomposite synthesis employing the in situ polymerization approach.

n-Butyl Li

Polar solvent (e.g. H2O)

Dry ether

Sonication

MS2 (M = Mo, Ta, Ti)

Single MS2x − sheets

Lix MS2

Add PEO solution

O

Inter-layer distance

O

O O

O O

Ordered intercalated nanocomposite

Figure 5. Preparation of layered nanocomposites containing poly(ethylene oxide) using the exfoliation–adsorption process.

mal conversion, melt intercalation, and template synthesis (1, 3). The choice of a synthesis approach is governed by consideration of the properties of both the guest polymer and the host lattice. Table 5 provides some examples of polymers in layered nanocomposites and how they are made. In situ Intercalative Polymerization. Many conjugated and structural polymers have limited solubility and may not have soluble polymeric intermediates. Like the 3-D host structures, nanocomposites containing these polymers may be prepared by the insertion of monomeric precursors followed by in situ polymerization within the interlayer space of the host. Advantage may be taken of the hosts’ oxidizing properties to induce polymerization in the interlayer space. Where the host lattice has no oxidizing properties, one can use external chemical initiators or apply heat or appropriate radiation sources to initiate polymerization. This method has been used to prepare novel nanocomposites containing conjugated polymer such as poly(aniline), poly(pyrrole), or poly(thiophene) (27). Figure 4 illustrates the incorporation and polymerization of aniline in MoO3. Structural nanocomposites based on this strategy and containing epoxide resins, nylon 6, poly(ethylene terephthalate) (PET), polystyrene, or poly(urethane) have been reported (28). Nylon 6 and PET nanocomposites have desirable dielectric and thermal properties needed in electronic packaging applications. Exfoliation–Adsorption. The exfoliation–adsorption technique is particularly attractive if the polymer of interest is soluble in a solvent and the host material can be separated into single-sheet colloid. Layered host lattices such as clay minerals, transition metal dichalcogenides (MS2), niobates, or titanates can be delaminated into single inorganic sheet colloids by suspending them in suitable solvents. Where appropriate, swelling agents such as long-chain alkyl amine or ammonium salts can be used to separate the interlayer considerably so that intercalation of the polymer occurs with ease. Some layered host lattices such as MoO3 and MS2 may be preintercalated with highly reactive alkaline metal ions such as lithium by means of chemical or electrochemical reaction, to enhance their delamination in a given solvent. The exfoliation process can be assisted further through mechanical stirring or by exposure to ultrasound. When a solution of the polymer of interest is added, the delaminated sheets spontaneously reconstitute to form ordered nanocomposites with the polymer trapped between the sheets. Layered nanocomposites containing poly(ethylene oxide) (PEO), poly(ethylenimine) (PEI), poly(vinylpyrrolidone), and polyaniline in various host lattices have been prepared by the methods described above (29) (see Table 5). Figure 5 illustrates the preparation of nanocomposites derived from layered metal disulfide and PEO using the exfoliation–adsorption process. These materials may be used as solid electrolyte and electrodes in rechargeable lithium batteries because of their ionic and electronic conductivities and their interfacial and mechanical stability (30). The interlayer spaces of many host crystals are very reactive (contain Brønsted–Lowry or electroactive sites), but this does not seem to be a problem for the intercalated polymer in this method. For example, Oriakhi and Lerner employed a displacement reaction to remove and characterize nanocomposited PEO from the interlayer of several layered host lattices including K x MPS3 (M = Cd, Mn) (31). The results show that the reactive host sheets did not degrade the polymer.


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Chemical/Thermal Conversion. Many conjugated polymers are solvent intractable and are therefore difficult to process by conventional plastic processing technology. However, considerable research effort has resulted in the syntheses of soluble derivatives of these intractable polymers, which may be processed from solution before they are converted to the desired intractable polymer. For example, the electronically conducting polymers polyacetylene, poly(p-phenylene), poly(phenylene sulfide), and poly(phenylene vinylene) and thermoplastics like poly(ether ether ketone) and poly(arylene sulfide ketone) have been synthesized from their high molecular weight precursor (32). With this chemistry in mind, the exfoliation–adsorption process can be utilized to prepare nanocomposites containing the precursor and the precursor is then converted to the target polymer between the sheets by chemical or thermal reactions. Nazar and her coworkers used this strategy to prepare nanocomposites of MoO3 and poly(phenylene vinylene) (PPV) by first intercalating poly(xylylene dimethysulfonium) cation between the sheets of MoO3, after which in situ thermal elimination of dimethylsulfide at 250 °C afforded the PPV–MoO3 nanocomposite (33). In a related study, Oriakhi et al. reported an alternate ambient temperature method to prepare PPV–clay nanocomposites in which the elimination of dimethylsulfide was achieved by a chemical reaction (34 ). This nanocomposite displays interesting luminescence properties. Melt Intercalation. The direct melt intercalation of polymers into layered host lattices is an environmentally benign strategy to synthesize polymer nanocomposites. The approach pioneered by Giannelis and coworkers involves heating a mixture of the polymer and the host lattice at a temperature above the melting or glass transition temperature for crystalline or amorphous polymer. This method has been used to prepare PEO–silicate nanocomposite materials (35). Under the synthesis conditions, the polymer has unrestricted local segmented motion and is able to diffuse into the gallery of the host lattice. For relatively nonpolar polymers, favorable interaction between the guest polymer and the host silicate may be enhanced by preintercalating the host with long-chain alkyl ammonium ions so that they become organophilic. Giannelis and his group used this method to prepare nanocomposites derived from organoclay and technologically important polymers such as polystyrene (PS), poly(vinyl pyridine), poly(dimethyl siloxane), and poly(vinylidene fluoride) (26, 36 ). The resulting nanocomposite materials can be processed with conventional plastic extrusion technology. Recently we have extended the melt intercalation process to non-silicate host lattices. Nanocomposites of PEO and layered LiMoO3, K xCdPS3, and K x MnPS3 have been prepared and characterized (37 ). The intercalation reaction is fast and completed in less than one hour at 70 °C for the MPS3 (M = Cd, Mn) host lattice. The reaction mechanism is not completely understood, but preliminary evidence from X-ray diffraction suggests local exfoliation of the host within the polymer matrix. Template Synthesis. This approach differs from the methods described above because a preformed host crystal is not used. Instead, ordered layered structures are crystallized from a homogeneous aqueous solution of the polymer and metal ion precursors by a process of self-assembly. The polymer is trapped within the layers and assists in the nucleation and growth of the inorganic host crystals. 1144

Layered double hydroxides (LDH) are interesting host materials for intercalation reactions because of their interesting application properties (38). Preformed LDH crystal does not exfoliate readily and anion exchange or direct insertion of species between the sheets is difficult because of the strong coulombic attraction for carbonate ions between the layers. Oriakhi et al. (39) and other researchers (40) have used this method to prepare several LDH nanocomposites containing anionic and neutral polymers. Evidence from X-ray diffraction indicates that the polymer is inserted between the LDH sheets as a bilayer (39). These nanocomposites display interesting microstructural and phase changes as well as enhanced thermal stability relative to both parent phases. Synthesis of Exfoliated Polymer Nanocomposites The in situ polymerization of preintercalated monomer and the exfoliation–adsorption strategies discussed above can also be employed to synthesize exfoliated nanocomposites in which nanometer-thick platelets of the host are dispersed uniformly within the polymer matrix. Research and development on exfoliated nanocomposites has so far concentrated on the use of clay minerals as host lattice (26, 41). The clay host lattice is hydrophilic and must be chemically modified to make the platelet surfaces organophilic. This ensures compatibility with many technologically important monomers and polymers. Remarkably, only 2–6 wt % of clay nanoplatelets is required to obtain desirable property enhancement. The polymer nanocomposites prepared by this approach include nylon 6, nylon 66, epoxy resin, polyester, poly(ethylene terephthalate), polyimide, polypropylene, polystyrene, and polyurethanes (26, 28, 41). Properties and Applications of Polymer Nanocomposites Polymer nanocomposition with framework or layered inorganic solids offers a smart approach to the design and synthesis of advanced functional materials with unique and predetermined properties. The properties of these materials depend not only on the properties of parent phases but on their morphology and interfacial characteristics. These materials promise new commercial and technological applications in the aerospace, automotive, biotechnology, electronic, energy, medical, and optical industries. Ionic conducting polymers such PEO, PEI, and poly(bismethoxyethoxyethoxy phosphazene) (MEEP), have been investigated as solid electrolytes for various electrochemical applications such as rechargeable lithium ion batteries, electrochromic windows, and sensors (30, 42). In the solid state, they are able to dissolve or form complexes with lithium salts and exhibit temperature-dependent ionic conductivity. At ambient temperatures their conductivity is generally low because of partial crystallization of the polymer. For example, the conductivity of PEO–Li salt complexes reaches a practically useful value (ca. 10᎑4 S cm᎑1) only at temperatures above 50 °C. Several researchers have prepared nanocomposites derived from PEO, PEI, and MEEP and layered inorganic host materials. Characterization studies showed that nanocomposition of these polymers resulted in lowered operational temperature and enhanced fast ion conduction, thermal and mechanical stability, and interfacial stability toward the electrode. Several



examples of PEO intercalation into MoO3, MoS2, MoSe2, TiS2, and V2O5 have been reported (Table 4). By using a semiconducting host lattice, nanocomposites with mixed ionic and electronic conductivity can be synthesized, thus expanding the scope of applications of these materials. Numerous layered transition metal dichalcogenides may be classified as semiconductors (TiS2, MoS2), semimetals (TiSe2, VSe2), and superconductors (TaS2, NbSe2) with varying degrees of electronic, magnetic, and optical properties. Polymer nanocomposites derived from them and electronically conducting polymers may find applications in areas dependent on their electronic properties (e.g., as rechargeable battery electrodes, electrocatalysts, or photoconductors). The synthesis of a plastic-like superconductor that hybridizes the superconducting properties of NbSe2 with the processable properties of poly(vinyl pyrrolidinone) (PVP), PEO, and poly(ethylene glycol) has been described (29). The metallic polymer–NbSe2 nanocomposites displayed electrical conductivity ranging from 140 to 250 S cm᎑1 and a metal-to-superconducting transition temperature (Tc) from 6.5 to 7.1 K. Potential applications include NMR tomography, MRI, energy storage, and highfrequency technology. The challenge is to design nanocomposites with higher Tc. Polyaniline and poly(p-phenylene) have been intercalated between the layers of MoO3. The resulting nanocomposites have been evaluated for use as electrode materials in rechargeable lithium ion batteries (43). Nanocomposites derived from structural polymers such as the nylon family, polyester, polypropylene, polystyrene, fluoropolymers, and epoxy and layered silicates have promising barrier, flame-retardant, mechanical, and thermal properties (26, 41). They are already finding commercial applications as light and tough automobile parts, barrier films and coatings for packaging, and fire-retarding materials. In the early 1990s, Toyota Central Research laboratories reported their pioneering work on nylon–clay nanocomposites. They have since built a broad-based proprietary patent technology. Toyota uses timing belt covers made from nylon–clay nanocomposites in automotive applications. Worldwide, industrial research and development efforts (41) are focusing on using polymer nanocomposite parts for interior and exterior automotive and aircraft applications, as well as electronic packaging where advantage can be taken of the enhanced mechanical properties. Polymer–clay nanocomposites have superior gas-barrier properties that are desirable in packaging applications. This is due to an increased effective path length for diffusion as the layered inorganic filler forces permeating gas molecules to travel a tortuous path (26 ). In Europe, ICI has commercialized two grades of nanocomposite films based on PET, whose barrier performance is superior to that of some existing products (41). Scientists at the National Institute of Standards and Technology (44) and Giannelis and coworkers (26 ) have demonstrated the outstanding flame resistance of nanocomposites derived from nylon, epoxy, and vinyl ester polymers and nanoclay materials. For example, the average heat release rate of nylon 6 nanocomposite is reduced by 60% without increasing the heat of combustion or amount of soot or carbon monoxide produced. The multilayered clay structure in the nanocomposite is believed to act as an insulator and a mass transport barrier, hindering the release of volatile polymer decomposition products and boosting the fire-retardant property (44). Polymer nanocomposites offer additional

advantages over conventional fire-retarding polymers. The silicate fillers can be regenerated and recycled after char formation, and the benefits conferred by the nanocomposite’s structure do not diminish the strength or flexibility of the material compared with that of the polymer. Conclusion Nanocomposite synthesis is an innovative route to complex advanced materials that opens a new stage of materials chemistry. Parent components can be selected from a pool of natural or synthetic polymers and inorganic host lattices with framework or layered structures. This allows scientists to tailor the properties of materials predictably and reproducibly by judicious matching of the organic and inorganic phases. The quantum size effects associated with the ultrafine nature of the parent phases and the characteristic interfacial interactions result in new or improved chemical, electronic, magnetic, mechanical, and optical properties of nanocomposites. The examples discussed in this article indicate that nanocomposites have a bright future in technological advancement. However, more work is needed to develop synthetic processes for these materials so that there will be a method for almost any kind of polymer nanocomposite. Developing synthetic strategies is only part of the challenge. More research is also needed in developing nanotechnology to produce and process nanocomposites into useful products. This requires a multidisciplinary effort of scientists and engineers. Acknowledgments I want to thank Mike Lerner, Jim Krueger, and David Landman for their encouragement and helpful discussion during the preparation of this article. Literature Cited 1. Oriakhi, C. Chem Br. 1998, 11, 59. 2. Ozin, G. A. Adv. Mater. 1992, 4, 612. 3. Lerner, M.; C. Oriakhi. In Handbook of Nanophase Materials; Goldstein, A., Ed.; Dekker: New York, 1997; p 199. 4. Komarneni, S. J. Mater. Chem. 1992, 2, 1219. 5. Alivisatos, A. P.; Barbara, P. F.; Castleman, A. W.; Chang, J.; Dixon, D. A.; Klein, M. L.; McLendon, G. L.; Miller, J. S.; Ratner, M. A.; Rossky, P. J., Stupp, S. I.; Thompson, M. E. Adv. Mater. 1998, 10, 1297. 6. Mann, S. Nature 1993, 365, 499. 7. Weiner, S.; Addadi, L. J. Mater. Chem. 1997, 7, 689. 8. Sarkaya, M.; Liu, J.; Aksay, I. A. In Biomimetics: Design and Processing of Materials; Sarikaya, M.; Aksay, I. A., Eds.; AIP: Woodbury, NY, 1995; p 35. 9. Novak, B. M. Adv. Mater. 1993, 5, 422. 10. Judeinstein, P.; Sanchez, C. J. Mater. Chem. 1996, 6, 511. 11. Beecrof, L. L.; Ober, C. K. Chem. Mater. 1997, 6, 1302. 12. Wen, J.; Wilkes, G. L. Chem. Mater. 1996, 8, 1667. 13. Schollhorn, S. Chem. Mater. 1996, 8, 1747. 14. Akporiaye, D. E. Angew. Chem., Int. Ed. Engl. 1998, 37, 2456. 15. Moller, K.; Bein, T. Chem. Mater. 1998, 10, 2950. 16. Enzel, P.; Bein, T. Chem. Mater. 1992, 4, 819. 17. Kyotani, T.; Nagai, T.; Inoue, S.; Tomita, A. Chem. Mater. 1997, 9, 609.


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