Current Opinion in and Interface Science 8 (2003) 103–108

Properties of polymer– composites

Gudrun Schmidt*, Matthew M. Malwitz

Department of Chemistry, Louisiana State University, Baton Rouge, LA 70803, USA

Abstract

An overview of properties of polymer–nanoparticle composites in bulk and in solution is presented along with a review of work 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 composites are emphasized. This relatively new area demands sophisticated experiments to augment pragmatic knowledge necessary to support theoretical descriptions of composite structures and properties. The complexity of this area guarantees that this will remain an active field for some time to come. 2003 Elsevier Science Ltd. All rights reserved.

Keywords: Nanoparticle; Polymer; Orientation; Shear

1. Introduction and solutions as well as polymer solutions. However, little is known about the influence of shear on combined In recent years, polymer–nanoparticle composite polymer–nanoparticle systems. Here we will focus on materials have attracted the interest of a number of some of the most recent results. researchers, due to their synergistic and hybrid properties This review highlights recent accomplishments and derived from several components. Whether in solution trends in the field of polymer–nanoparticle composites or in bulk, these materials offer unique mechanical w1x, which combine soft polymer components with rigid electrical w2x, optical w2,3●x and thermal properties w1,4x. inorganic . Reviewed articles examine the Such enhancements are induced by the physical presence unique chemical and physical aspects associated with of the nanoparticle and by the interaction of the polymer polymer based composites and show future directions with the particle and the state of dispersion w1,5,6x. and opportunities for the development of new materials. One advantage of nanoparticles, as polymer additives Unraveling the structure property relationships of poly- appear to have is that compared to traditional additives, mer–nanoparticle materials become a challenge and a loading requirements are quite low. Microsized particles new frontier in polymer–nanoparticle composites. used as reinforcing agents scatter light, thus reducing light transmittance and optical clarity. Efficient nanopar- 2. Nanotubes, fibers, rods ticle dispersion combined with good polymer–particle There has been increasing commercial interest in interfacial adhesion eliminates scattering and allows the polymer composites mainly due to exciting possibility of developing strong yet transparent their conductivity at very low loading levels. Driven in films, coatings and membranes. part by industrial relevance and concerns over intellec- Shear-induced structural changes in fluids containing tual property, fundamental scientific studies abound in anisotropic species are often encountered in polymer the literature. Recent reviews on nanotubes and polymers solutions in liquid crystalline materials, block cover aspects of mechanical and electrical properties of melts and in particle solutions. A large body of literature polymer composites w7,8x and survey approaches to the exists on the flow behavior of inorganic particle slurries chemical functionalization of nanotubes to tailor the w x q q interaction with polymers 9 as well as the attachment *Corresponding author. Tel.: 1-225-578-7375; fax: 1-225-578- w x 3458. of nanotubes to surfaces and polymer matrices 10 . E-mail address: [email protected] (G. Schmidt). Effective dispersion of nanotubes in polymer matrices

1359-0294/03/$ - see front matter 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S1359-0294Ž03.00008-6 104 G. Schmidt, M.M. Malwitz / Current Opinion in Colloid and Interface Science 8 (2003) 103–108 is being investigated as a means of deriving new and strongly adsorbed polymers w21x. The effect of physical advanced engineering materials w11x. The synthesis of adsorption of polymers on the viscosity of concentrated polymer films and fibers containing oriented single wall dispersions of charged silica particles has been found to nanotubes employing shear orientation at the nanoscale be an interplay between rheology, adsorption and surface still poses a special challenge w12,13x. Recent results charge w22x. Adsorption studies provide information on show that flow induced alignment of nanotubes in a the polymer layer density and conformation, that is polymer matrix can lead to preferential orientation of whether the polymer is pancake-like or brush-like the tubes, into either ribbons or fibers w14●x. Raman adsorbed to the silica particles w22x. Shear disruption spectroscopy is able to determine the degree of shear processes of fractal polydisperse silica clusters can be orientation and the polarization direction of the nano- monitored via online ultrasonic measurements w23x. tubes w15x. Studies of nanotube incorporation cover a There is also extensive interest in morphological wide range of matrices, flows and techniques. However, features of films and membranes provided from com- consistently successful approaches to production of posites of hard inorganic particles and soft polymeric shear-induced supramolecular structures containing inor- materials. Enhancing membrane separation processes are ganic nanotubes, rods or fibers remain elusive. attractive because they contribute to low cost, energy Films with tunable colors can be produced depending efficient, green technology where nanoparticles enhance on the nature and size of a nanorod w3●x. nanorods the selectivity and permeability in glassy amorphous are especially interesting because they exhibit high membranes w24x. electrical capacitance and the color of a colloid is A recent study on latex–silica nanocomposite films affected by the effective charge of the particle w3●x. has shown that structure and morphology can be con- Solid state polymerization of molecularly oriented trolled by systematically varying synthetic parameters silicaymonomer matrices lead to unique conductive in solution. The authors suggest that a competition composites. The architecture and extent of self assembly between silica aggregation and the solidification of the of conductive polymer–silica hybrids as films or fibers film is responsible for the aggregation kinetics w25x. is suitable for integration into different kinds of devices These films show considerable reinforcement when sub- and microsystems w16●x. The nanostructured inorganic jected to small deformations, whereas at high elonga- host alters the monomer polymerization behavior, and tions, the rheology approaches that of the pure nanolatex the resulting nanocomposite exhibits unusual chromatic film w26x. changes in response to thermal, mechanical and chemical Measurements by Kobayashi et al. on polymer– stimuli. The inorganic framework serves to protect, colloidal silica films show that the polymer interdiffu- stabilize, and orient the polymer, and to mediate its sion depends on the silica filler size. The high surface function w16●x. of the silica raises the effective glass transition temper- ature of the polymer matrix w27x. Either small particles 3. Polymers and nanospheres can act as obstacles to increase the tortuosity of the diffusion path or the silica surface can make the adjacent Combining the ease of processability of an organic polymer matrix more rigid, which is the traditional polymer with the improved mechanical and optical ‘filler effect’. The authors suggest that the polymer properties of an inorganic nanoparticle is of practical adsorption onto the silicate surface plays an important use for the fabrication of many new devices. For role in affecting the diffusion rate of the polymer w27x. example, Brott et al. developed a method for creating Nanoparticles may not only raise the glass transition of an ordered array of silica spheres on a polymer substrate. polymers w27x but also change the phase behavior of To obtain this they incorporated a peptide nanocomposite blends w28x, nanoparticles enhance the into a polymer hologram. When the nanopatterned struc- selectivity and permeability in glassy amorphous mem- ture is exposed to silicic acid, an ordered array of branes w24x, or lead to new electro-optical properties in biocatalytically formed silica nanospheres is deposited polymer films w2x. onto the polymer substrate w17●●x. Other studies describe Recent theoretical studies deal with chain dimensions well structured silica nanospheres with incorporated in nanosphere-filled polymers w29x, while molecular polymer w18x, gold nanospheres coated with polymer dynamic simulations cover the polymer–nanoparticle brushes w19x, or nanoparticle formation within a dendri- interactions and the structure and dynamics of the mer matrix w20x. Many different structures can be polymer melt containing nanoparticles w30x. Investiga- prepared by using superstructures in organogels as a tion on the flow dichroism of colloidal particle systems template. The synthesis of polymer coated silica nano- suggests that the polymer induces depletion attractions, particles and the strength of interactions and aggregation which give rise to a ‘gas liquid critical point’. Dichroic in solution was studied by Fong et al. They contrast the behavior is used to test theoretical predictions (mean weakness of physical polymer–silica interactions which field) on microstructural order under shear conditions can be ‘washed away’ vs. the covalently attached or w31x. G. Schmidt, M.M. Malwitz / Current Opinion in Colloid and Interface Science 8 (2003) 103–108 105

4. Polymer–platelet interactions mer–clay composites by Balazs and Ginzburg, who combined self-consistent field theory with density func- The colloidal and rheological properties of polymer– tional theory to calculate the equilibrium behavior of nanoplatelet composites in bulk and in solution have nanocomposites w44,45x. Their theory models the phase received considerable attention, and good reviews are behavior for a mixture of polymers and solid, thin discs available w6,32●● ,33x. One recent focus has been on the and takes into account the possible nematic ordering of supramolecular organization of polymers and nanopar- the discs within the polymer matrix. Recent computer ticles. A transition from a liquid crystalline hexagonal simulation studies by Hackett et al. use Monte Carlo to a lamellar phase has been observed in an aqueous and molecular dynamics to explore the atomic scale mixture of Pluronic type block copolymer and clay. The structure of intercalated polymer–nanoplatelet compos- adsorption of polymer to clay was important to structure ites. Particular attention is paid to the configuration of formation w34x. A small angle neutron scattering the polymer within confinement w46x. (SANS) study by Lal and Auvray described the adsorp- tion of PEO polymer chains to Laponite clay platelets 5. Flow effects in polymer–platelet systems at low concentrations w35,36x. It was observed that gelation of clay was prevented or extremely retarded, Understanding polymer solutions, liquid crystalline depending on the polymer weight, and the polymer and materials, block copolymer melts and nanoparticle-con- clay concentration. For low concentrations Lal and taining composite materials is complicated by shear- Auvray have been able to separate the SANS contribu- induced structural changes. Industrial processes involve tions from bulk and adsorbed polymer chains using non-equilibrium as well as equilibrium states, where contrast variation methods w35,36x. While their results shear affects both preparation and product application. were not sensitive to the shape of the polymer concen- Real-time investigations are quite relevant to such proc- tration profile, Smalley et al. and Swenson et al. use essing applications. On-line scattering studies of flowing neutron diffraction to address the interlayer and ordered systems provide insight into the real-time states of structure around each clay platelet as well as the mech- matter. anism of bridging flocculation w37,38●●x. Ordering platelets at the nanometer length scale is a Below the threshold for complete saturation of clay challenging and active research area in materials science. particles by polymer, ‘shake gels’ can be generated with Approaches developed so far, range from manipulation the consistency of ‘a half cooled gelatine dessert’. of individual particles to exploitation of self assembly Zebrowski et al. observe these suspensions to undergo in w47x. The large aspect ratio of platelets a dramatic change in shear thickening when subjected promotes a supramolecular organization similar to other to vigorous shaking w39x. The shake gels are reversible, mesoscopic systems such as liquid crystalline polymers, relaxing back to a fluid with a relaxation time that surfactants or block . Multicomponent sys- depends on the PEO concentration. Shear induces a tems may generate new structures with hybrid properties. bridging between the colloidal particles resulting in Here we focus on the shear orientation of polymer– some kind of temporary gel network that spans the nanoplatelet systems in solution as well as the bulk. system. For the same polymer–clay system, however, at A two-dimensional object can align under flow along slightly higher concentrations and higher polymer three primary directions, often referred as a, b, and c molecular weight, polymer chains were found to be in orientation in the literature (Fig. 1). In the perpendicular, dynamic adsorption–desorption equilibrium with the or ‘a’ orientation, the surface normals align parallel to clay particles to form a permanent network w40–42x. the vorticity direction so that the particles lie in the flow These highly elastic networks behave more like a soft shear gradient plane. In the transverse, or ‘b’ orientation, ‘chewing gum’ than gelatin. In this case SANS contrast the surface normals align parallel to the flow direction matching methods cannot distinguish between the inten- so that the particles lie in the vorticity shear gradient sity contribution of network active PEO, adsorbed PEO plane. Finally, in the parallel or ‘c’ direction, the surface or excess PEO inside the network w40–42x. normals align parallel to the shear gradient direction and Despite all the obvious differences between gelatin the particles lie in the flow vorticity plane w48x. desserts and chewing gums, the interactions between The general or intuitive response of clay platelets in polymer and clay are intriguing and a clearer understand- a polymer matrix or network is to align in the parallel ing of polymer–particle interactions as well as more or ‘c’ orientation. This is nicely described by early quantitative knowledge is necessary for proper interpre- studies on nylon based nanocomposites as reviewed by tation of the experimental results. Theoretical approaches Krishnamoorti and Yurekli w32●●x. Recent studies such by Nowicki look at the structure and entropy of polymer as by Lele et al. w49x using in situ X-ray diffraction chains in the presence of colloidal particles and may aid experiments provide a direct evidence for rheology- in understanding systems discussed above w43x. There microstructure linkages in polypropylene nano- also has been some recent theoretical interest in poly- composites. 106 G. Schmidt, M.M. Malwitz / Current Opinion in Colloid and Interface Science 8 (2003) 103–108

Fig. 1. SANS detection of possible shear-orientation of clay platelets. (a) Assume plates align with surface normal along neutral direction: vertical streak in radial pattern (y) and vertical streak in tangential pattern (x). (b) Assume plates align with surface normal in shear plane: horizontal streak in radial SANS pattern (y direction) and isotropic tangential pattern (x direction). (c) Assume plates align with surface normal along velocity 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 observed alignment of the particles. At low shear rates the discs by Schmidt et al. w40,41x for aqueous polymer–clay aligned with the normals in flow direction (‘c’ orienta- solutions containing poly(ethyleneoxide) and Laponite tion) while at higher shear rates the discs aligned with clay. The polymer and the clay interact in a dynamic their normals in the gradient direction (‘a’ orientation). adsorption–desorption equilibrium to form a network At intermediate shear rates, they observed a phase w x 42 . SANS on samples in D2 O measured the shear- transition from a columnar to a smectic phase. This induced orientation of polymer and platelets. SANS on system appears to use its finite two-dimensional size to contrast matched samples detected the orientation of the arrange itself so that it can respond to low shear as a polymer alone. With increasing shear rate clay particles system of uniaxial particles, while at high shear rates it orient first then polymer chains start to stretch. As the prefers to respond like lamellar phase w48,52x. shear distorts and ruptures the transient gel, coupling between composition and shear stress leads to the 6. Conclusions formation of spatially modulated macrodomains w50●x. Lin-Gibson et al. suggested that the clay orients in From the point of view of materials chemistry, fun- response to the biaxial stress arising from the shear and damental studies described here should eventually lead elastic forces w50●x. to the discovery of a new class of oriented polymer– Recent TEM analysis by Okamoto et al. w51●●x has particle materials. We expect that in the future it will be revealed a house of cards structure in polypropyleney possible to orient particles in any desired way. Nanopar- clay nanocomposite melts under elongational flow. ticle composites contribute to the development of future Strong strain induced hardening and rheopexy features data storage, optical and electro-rheological materials, at higher deformation originated from the perpendicular or display devices. alignment of the silicate to the stretching direction (‘b’ orientation). Although TEM is not an in situ technique Acknowledgments it did reveal the difference in the shear flow induced vs. elongational internal structures of the nanocomposite We thank Dr Bill Daly and Dr Paul Russo for review melt. Similar to the polymer–clay solutions discussed of this manuscript and acknowledge financial support by the Louisiana Board of Regents Support Fund: by Lin-Gibson, Okamoto’s nanocomposites do have ( ) strong interactions between the polymer matrix and the LEQSF 2002-05 -RD-A-09 and by our IGERT silicate layers. program. A dispersion of nickel hydroxide platelets stabilized References and recommended reading with a polymer was studied by Brown and Rennie w52x. Their main aim was to look at flow of dispersions rather ● of special interest than polymer–particle interactions. Adsorption of low ●● of outstanding interest molecular weight charged polyacrylate onto the platelets provided steric repulsion. They found direct evidence w1x Krishnamoorti R, Vaia RA. Polymer nanocomposites, vol. for a shear-induced phase transition with a change in 804. Washington, DC: ACS, 2002. G. Schmidt, M.M. Malwitz / Current Opinion in Colloid and Interface Science 8 (2003) 103–108 107

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