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Unformatted text preview: Poly(ethylene oxide)/Silica Nanocomposites: Structure and Rheology Qiang Zhang and Lynden A. Archer* School of Chemical and Biomolecular Engineering, Cornell University, Ithaca, New York 14853 Received August 1, 2002. In Final Form: September 27, 2002 The effects of polymer- particle and particle- particle interactions on viscoelastic properties of nanocompositematerialsareinvestigatedusingnarrowmolecularweightdistributionpoly(ethyleneoxide) (PEO)containingisotropicsilicananospheres.Nanocompositesarepreparedusinga“freeze-drying”method to guarantee homogeneous dispersion of silica. The dispersion state of silica nanospheres is characterized by atomic force microscopy phase contrast imaging, and mechanical rheometry is used to study relaxation dynamics and viscoelastic properties of these materials in the melt state. Linear viscoelastic data indicate a transition to a solidlike response at low oscillation frequencies for particle volume fractions φ as low as 2%, dramatically lower than the theoretical percolation threshold ( φ ∼ 30%). Nanoparticle volume fraction, surface chemistry, and PEO molecular weight are all found to strongly influence nanocomposite structure and dynamics. A filler networking mechanism, wherein nanosized silica particles surrounded by an immobilized shell of PEO are bridged by much larger polymer molecules, is proposed to explain our observations. Introduction Many polymer materials are composites containing isotropicparticlessuchascarbonblackandsilica.Inthese systems, polymer volume fraction is typically large, so that the composite structure can be described as particle aggregates dispersed in a polymer matrix. 1 Interactions between individual particles within aggregates, as well as between aggregates, hinder relative motion between material planes, enhancing mechanical properties of the hostpolymer.Inafilledpolymercontainingparticleswith micron or submicron dimensions, two filler particles can only interact when their separation distance is small compared with the particle size. High particle volume fractions are therefore required to significantly change mechanical properties of the host material. For example, in highly filled polymers, filler interactions are so strong that solidlike yield phenomenon can be observed even at temperatures above the quiescent melting temperature or glass transition temperature of the polymer. 2 This behavior is often attributed to the existence of a filler networkthatspanslargesectionsofthepolymermatrix. 1,3 When filler particles become so small that their size (at least in one dimension) approaches the mean radius of gyration of host polymer chains, several new noncon- tinuum effects become important. For highly interactive polymer- particlesystemsinwhichweareinterested,the physisorption is strong enough to be considered irrevers- ible. Individual polymer molecules can adopt stretched configurations that allow them to simultaneously adsorb tothesurfacesofmanyparticles.Nanosizedfillerparticlestothesurfacesofmanyparticles....
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This note was uploaded on 03/16/2010 for the course AMS 203 taught by Professor Tucker during the Spring '10 term at SUNY Stony Brook.
- Spring '10