Properties Of Polymer–nanoparticle Composites
Current Opinion in Colloid and Interface Science 8 (2003) 103–108Properties of polymer–nanoparticle compositesGudrun Schmidt*, Matthew M. MalwitzDepartment of Chemistry, Louisiana State University, Baton Rouge, LA 70803, USAAbstractAn overview of properties of polymer–nanoparticle composites in bulk and in solution is presented along with a review ofwork performed during the last 3 years. The review is particularly focused on organic–inorganic materials such as polymer–nanospheres, tubes, rods, fibers and nanoplatelets. Fundamental studies on flow-induced structures in polymer–particle compositesare emphasized. This relatively new area demands sophisticated experiments to augment pragmatic knowledge necessary tosupport theoretical descriptions of composite structures and properties. The complexity of this area guarantees that this will remainan active field for some time to come.䊚 2003 Elsevier Science Ltd. All rights reserved.Keywords: Nanoparticle; Polymer; Orientation; Shear1. IntroductionIn recent years, polymer–nanoparticle compositematerials have attracted the interest of a number ofresearchers, due to their synergistic and hybrid propertiesderived from several components. Whether in solutionor in bulk, these materials offer unique mechanical w1x,electrical w2x, optical w2,3 x and thermal properties w1,4x.Such enhancements are induced by the physical presenceof the nanoparticle and by the interaction of the polymerwith the particle and the state of dispersion w1,5,6x.One advantage of nanoparticles, as polymer additivesappear to have is that compared to traditional additives,loading requirements are quite low. Microsized particlesused as reinforcing agents scatter light, thus reducinglight transmittance and optical clarity. Efficient nanoparticle dispersion combined with good polymer–particleinterfacial adhesion eliminates scattering and allows theexciting possibility of developing strong yet transparentfilms, coatings and membranes.Shear-induced structural changes in fluids containinganisotropic species are often encountered in polymersolutions in liquid crystalline materials, block copolymermelts and in particle solutions. A large body of literatureexists on the flow behavior of inorganic particle slurries*Corresponding author. Tel.: q1-225-578-7375; fax: q1-225-5783458.E-mail address: gudrun@lsu.edu (G. Schmidt).and solutions as well as polymer solutions. However,little is known about the influence of shear on combinedpolymer–nanoparticle systems. Here we will focus onsome of the most recent results.This review highlights recent accomplishments andtrends in the field of polymer–nanoparticle compositeswhich combine soft polymer components with rigidinorganic nanoparticles. Reviewed articles examine theunique chemical and physical aspects associated withpolymer based composites and show future directionsand opportunities for the development of new materials.Unraveling the structure property relationships of polymer–nanoparticle materials become a challenge and anew frontier in polymer–nanoparticle composites.2. Nanotubes, fibers, rodsThere has been increasing commercial interest incarbon nanotube polymer composites mainly due totheir conductivity at very low loading levels. Driven inpart by industrial relevance and concerns over intellectual property, fundamental scientific studies abound inthe literature. Recent reviews on nanotubes and polymerscover aspects of mechanical and electrical properties ofpolymer composites w7,8x and survey approaches to thechemical functionalization of nanotubes to tailor theinteraction with polymers w9x as well as the attachmentof nanotubes to surfaces and polymer matrices w10x.Effective dispersion of nanotubes in polymer matrices1359-0294/03/ - see front matter 䊚 2003 Elsevier Science Ltd. All rights reserved.doi:10.1016/S1359-0294Ž03.00008-6
104G. Schmidt, M.M. Malwitz / Current Opinion in Colloid and Interface Science 8 (2003) 103–108is being investigated as a means of deriving new andadvanced engineering materials w11x. The synthesis ofpolymer films and fibers containing oriented single wallnanotubes employing shear orientation at the nanoscalestill poses a special challenge w12,13x. Recent resultsshow that flow induced alignment of nanotubes in apolymer matrix can lead to preferential orientation ofthe tubes, into either ribbons or fibers w14 x. Ramanspectroscopy is able to determine the degree of shearorientation and the polarization direction of the nanotubes w15x. Studies of nanotube incorporation cover awide range of matrices, flows and techniques. However,consistently successful approaches to production ofshear-induced supramolecular structures containing inorganic nanotubes, rods or fibers remain elusive.Films with tunable colors can be produced dependingon the nature and size of a nanorod w3 x. Metal nanorodsare especially interesting because they exhibit highelectrical capacitance and the color of a colloid isaffected by the effective charge of the particle w3 x.Solid state polymerization of molecularly orientedsilicaymonomer matrices lead to unique conductivecomposites. The architecture and extent of self assemblyof conductive polymer–silica hybrids as films or fibersis suitable for integration into different kinds of devicesand microsystems w16 x. The nanostructured inorganichost alters the monomer polymerization behavior, andthe resulting nanocomposite exhibits unusual chromaticchanges in response to thermal, mechanical and chemicalstimuli. The inorganic framework serves to protect,stabilize, and orient the polymer, and to mediate itsfunction w16 x.3. Polymers and nanospheresCombining the ease of processability of an organicpolymer with the improved mechanical and opticalproperties of an inorganic nanoparticle is of practicaluse for the fabrication of many new devices. Forexample, Brott et al. developed a method for creatingan ordered array of silica spheres on a polymer substrate.To obtain this nanostructure they incorporated a peptideinto a polymer hologram. When the nanopatterned structure is exposed to silicic acid, an ordered array ofbiocatalytically formed silica nanospheres is depositedonto the polymer substrate w17 x. Other studies describewell structured silica nanospheres with incorporatedpolymer w18x, gold nanospheres coated with polymerbrushes w19x, or nanoparticle formation within a dendrimer matrix w20x. Many different structures can beprepared by using superstructures in organogels as atemplate. The synthesis of polymer coated silica nanoparticles and the strength of interactions and aggregationin solution was studied by Fong et al. They contrast theweakness of physical polymer–silica interactions whichcan be ‘washed away’ vs. the covalently attached orstrongly adsorbed polymers w21x. The effect of physicaladsorption of polymers on the viscosity of concentrateddispersions of charged silica particles has been found tobe an interplay between rheology, adsorption and surfacecharge w22x. Adsorption studies provide information onthe polymer layer density and conformation, that iswhether the polymer is pancake-like or brush-likeadsorbed to the silica particles w22x. Shear disruptionprocesses of fractal polydisperse silica clusters can bemonitored via online ultrasonic measurements w23x.There is also extensive interest in morphologicalfeatures of films and membranes provided from composites of hard inorganic particles and soft polymericmaterials. Enhancing membrane separation processes areattractive because they contribute to low cost, energyefficient, green technology where nanoparticles enhancethe selectivity and permeability in glassy amorphousmembranes w24x.A recent study on latex–silica nanocomposite filmshas shown that structure and morphology can be controlled by systematically varying synthetic parametersin solution. The authors suggest that a competitionbetween silica aggregation and the solidification of thefilm is responsible for the aggregation kinetics w25x.These films show considerable reinforcement when subjected to small deformations, whereas at high elongations, the rheology approaches that of the pure nanolatexfilm w26x.Measurements by Kobayashi et al. on polymer–colloidal silica films show that the polymer interdiffusion depends on the silica filler size. The high surfaceof the silica raises the effective glass transition temperature of the polymer matrix w27x. Either small particlescan act as obstacles to increase the tortuosity of thediffusion path or the silica surface can make the adjacentpolymer matrix more rigid, which is the traditional‘filler effect’. The authors suggest that the polymeradsorption onto the silicate surface plays an importantrole in affecting the diffusion rate of the polymer w27x.Nanoparticles may not only raise the glass transition ofpolymers w27x but also change the phase behavior ofnanocomposite blends w28x, nanoparticles enhance theselectivity and permeability in glassy amorphous membranes w24x, or lead to new electro-optical properties inpolymer films w2x.Recent theoretical studies deal with chain dimensionsin nanosphere-filled polymers w29x, while moleculardynamic simulations cover the polymer–nanoparticleinteractions and the structure and dynamics of thepolymer melt containing nanoparticles w30x. Investigation on the flow dichroism of colloidal particle systemssuggests that the polymer induces depletion attractions,which give rise to a ‘gas liquid critical point’. Dichroicbehavior is used to test theoretical predictions (meanfield) on microstructural order under shear conditionsw31x.
G. Schmidt, M.M. Malwitz / Current Opinion in Colloid and Interface Science 8 (2003) 103–1084. Polymer–platelet interactionsThe colloidal and rheological properties of polymer–nanoplatelet composites in bulk and in solution havereceived considerable attention, and good reviews areavailable w6,32 ,33x. One recent focus has been on thesupramolecular organization of polymers and nanoparticles. A transition from a liquid crystalline hexagonalto a lamellar phase has been observed in an aqueousmixture of Pluronic type block copolymer and clay. Theadsorption of polymer to clay was important to structureformation w34x. A small angle neutron scattering(SANS) study by Lal and Auvray described the adsorption of PEO polymer chains to Laponite clay plateletsat low concentrations w35,36x. It was observed thatgelation of clay was prevented or extremely retarded,depending on the polymer weight, and the polymer andclay concentration. For low concentrations Lal andAuvray have been able to separate the SANS contributions from bulk and adsorbed polymer chains usingcontrast variation methods w35,36x. While their resultswere not sensitive to the shape of the polymer concentration profile, Smalley et al. and Swenson et al. useneutron diffraction to address the interlayer and orderedstructure around each clay platelet as well as the mechanism of bridging flocculation w37,38 x.Below the threshold for complete saturation of clayparticles by polymer, ‘shake gels’ can be generated withthe consistency of ‘a half cooled gelatine dessert’.Zebrowski et al. observe these suspensions to undergoa dramatic change in shear thickening when subjectedto vigorous shaking w39x. The shake gels are reversible,relaxing back to a fluid with a relaxation time thatdepends on the PEO concentration. Shear induces abridging between the colloidal particles resulting insome kind of temporary gel network that spans thesystem. For the same polymer–clay system, however, atslightly higher concentrations and higher polymermolecular weight, polymer chains were found to be indynamic adsorption–desorption equilibrium with theclay particles to form a permanent network w40–42x.These highly elastic networks behave more like a soft‘chewing gum’ than gelatin. In this case SANS contrastmatching methods cannot distinguish between the intensity contribution of network active PEO, adsorbed PEOor excess PEO inside the network w40–42x.Despite all the obvious differences between gelatindesserts and chewing gums, the interactions betweenpolymer and clay are intriguing and a clearer understanding of polymer–particle interactions as well as morequantitative knowledge is necessary for proper interpretation of the experimental results. Theoretical approachesby Nowicki look at the structure and entropy of polymerchains in the presence of colloidal particles and may aidin understanding systems discussed above w43x. Therealso has been some recent theoretical interest in poly-105mer–clay composites by Balazs and Ginzburg, whocombined self-consistent field theory with density functional theory to calculate the equilibrium behavior ofnanocomposites w44,45x. Their theory models the phasebehavior for a mixture of polymers and solid, thin discsand takes into account the possible nematic ordering ofthe discs within the polymer matrix. Recent computersimulation studies by Hackett et al. use Monte Carloand molecular dynamics to explore the atomic scalestructure of intercalated polymer–nanoplatelet composites. Particular attention is paid to the configuration ofthe polymer within confinement w46x.5. Flow effects in polymer–platelet systemsUnderstanding polymer solutions, liquid crystallinematerials, block copolymer melts and nanoparticle-containing composite materials is complicated by shearinduced structural changes. Industrial processes involvenon-equilibrium as well as equilibrium states, whereshear affects both preparation and product application.Real-time investigations are quite relevant to such processing applications. On-line scattering studies of flowingsystems provide insight into the real-time states ofmatter.Ordering platelets at the nanometer length scale is achallenging and active research area in materials science.Approaches developed so far, range from manipulationof individual particles to exploitation of self assemblyin colloids w47x. The large aspect ratio of plateletspromotes a supramolecular organization similar to othermesoscopic systems such as liquid crystalline polymers,surfactants or block copolymers. Multicomponent systems may generate new structures with hybrid properties.Here we focus on the shear orientation of polymer–nanoplatelet systems in solution as well as the bulk.A two-dimensional object can align under flow alongthree primary directions, often referred as a, b, and corientation in the literature (Fig. 1). In the perpendicular,or ‘a’ orientation, the surface normals align parallel tothe vorticity direction so that the particles lie in the flowshear gradient plane. In the transverse, or ‘b’ orientation,the surface normals align parallel to the flow directionso that the particles lie in the vorticity shear gradientplane. Finally, in the parallel or ‘c’ direction, the surfacenormals align parallel to the shear gradient direction andthe particles lie in the flow vorticity plane w48x.The general or intuitive response of clay platelets ina polymer matrix or network is to align in the parallelor ‘c’ orientation. This is nicely described by earlystudies on nylon based nanocomposites as reviewed byKrishnamoorti and Yurekli w32 x. Recent studies suchas by Lele et al. w49x using in situ X-ray diffractionexperiments provide a direct evidence for rheologymicrostructure linkages in polypropylene nanocomposites.
106G. Schmidt, M.M. Malwitz / Current Opinion in Colloid and Interface Science 8 (2003) 103–108Fig. 1. SANS detection of possible shear-orientation of clay platelets. (a) Assume plates align with surface normal along neutral direction: verticalstreak in radial pattern (y) and vertical streak in tangential pattern (x). (b) Assume plates align with surface normal in shear plane: horizontalstreak in radial SANS pattern (y direction) and isotropic tangential pattern (x direction). (c) Assume plates align with surface normal alongvelocity direction: isotropic radial pattern (y) and horizontal streak in tangential pattern (x). An imperfect orientation of platelets is assumed.An unexpected case of ‘a’ orientation was observedby Schmidt et al. w40,41x for aqueous polymer–claysolutions containing poly(ethyleneoxide) and Laponiteclay. The polymer and the clay interact in a dynamicadsorption–desorption equilibrium to form a networkw42x. SANS on samples in D2O measured the shearinduced orientation of polymer and platelets. SANS oncontrast matched samples detected the orientation of thepolymer alone. With increasing shear rate clay particlesorient first then polymer chains start to stretch. As theshear distorts and ruptures the transient gel, couplingbetween composition and shear stress leads to theformation of spatially modulated macrodomains w50 x.Lin-Gibson et al. suggested that the clay orients inresponse to the biaxial stress arising from the shear andelastic forces w50 x.Recent TEM analysis by Okamoto et al. w51 x hasrevealed a house of cards structure in polypropyleneyclay nanocomposite melts under elongational flow.Strong strain induced hardening and rheopexy featuresat higher deformation originated from the perpendicularalignment of the silicate to the stretching direction (‘b’orientation). Although TEM is not an in situ techniqueit did reveal the difference in the shear flow induced vs.elongational internal structures of the nanocompositemelt. Similar to the polymer–clay solutions discussedby Lin-Gibson, Okamoto’s nanocomposites do havestrong interactions between the polymer matrix and thesilicate layers.A dispersion of nickel hydroxide platelets stabilizedwith a polymer was studied by Brown and Rennie w52x.Their main aim was to look at flow of dispersions ratherthan polymer–particle interactions. Adsorption of lowmolecular weight charged polyacrylate onto the plateletsprovided steric repulsion. They found direct evidencefor a shear-induced phase transition with a change inalignment of the particles. At low shear rates the discsaligned with the normals in flow direction (‘c’ orientation) while at higher shear rates the discs aligned withtheir normals in the gradient direction (‘a’ orientation).At intermediate shear rates, they observed a phasetransition from a columnar to a smectic phase. Thissystem appears to use its finite two-dimensional size toarrange itself so that it can respond to low shear as asystem of uniaxial particles, while at high shear rates itprefers to respond like lamellar phase w48,52x.6. ConclusionsFrom the point of view of materials chemistry, fundamental studies described here should eventually leadto the discovery of a new class of oriented polymer–particle materials. We expect that in the future it will bepossible to orient particles in any desired way. Nanoparticle composites contribute to the development of futuredata storage, optical and electro-rheological materials,or display devices.AcknowledgmentsWe thank Dr Bill Daly and Dr Paul Russo for reviewof this manuscript and acknowledge financial supportby the Louisiana Board of Regents Support Fund:LEQSF (2002-05)-RD-A-09 and by our IGERTprogram.References and recommended reading of special interest of outstanding interestw1x Krishnamoorti R, Vaia RA. Polymer nanocomposites, vol.804. Washington, DC: ACS, 2002.
G. Schmidt, M.M. Malwitz / Current Opinion in Colloid and Interface Science 8 (2003) 103–108w2x Chapman R, Mulvaney P. Electro-optical shifts in silvernanoparticle films. Chem Phys Lett 2001;349:358 –62.w3x Wilson O, Wilson GJ, Mulvaney P. Laser writing in polarized silver nanorod films. Adv Mater 2002;14:1000.w4x Yoon PJ, Fornes TD, Paul DR. Thermal expansion behaviorof nylon 6 nanocomposites. Polymer 2002;43:6727 –41.w5x Lagaly G. Introduction: from clay mineral–polymer interactions to clay mineral–polymer nanocomposites. Appl ClaySci 1999;15:1 –9.w6x Luckham PF, Rossi S. The colloidal and rheological properties of bentonite suspensions. Adv Coll Interface Sci1999;82:43 –92.w7x Bernholc J, Brenner D, Nardelli MB, Meunier V, Roland C.Mechanical and electrical properties of
between silica aggregation and the solidification of the film is responsible for the aggregation kinetics w25 x. These films show considerable reinforcement when sub-jected to small deformations, whereas at high elonga-tions, the rheology approaches that of the pure nanolatex film w26 x. Measurements by Kobayashi et al. on polymer–
as reinforcements for polymer composites. This replacement could be again synthetic, petroleum-based polymer but prepared as fibers, micro- or nanofibrils. Of course, this approach is not as advantageous as using natural fibers that are biodegradable and eco-friendly. At the same time, the synthetic polymer-polymer composites seem to be much
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