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ABSTRACT: Polymer tracer diffusion in multiwall carbon nanotube (MWCNT)/polymer nanocomposites is reported. As previously reported for SWCNT/polystyrene (PS) nanocomposites, the tracer diffusion of 680k deuterated polystyrene (dPS) is strongly suppressed at low MWCNT concentrations and then increases at higher concentrations. In contrast, the tracer diffusion of 10k dPS and 75k dPS is independent of MWCNT loading. These results reveal an important criterion for exhibiting a minimum in the tracer diffusion coefficient (Dmin) with nanoparticle concentration, namely the relative size of tracer molecule and nanoparticle. Specifically, when the radius of gyration of the tracer polymer (Rg) is smaller than the radius of the nanotube particle (RCNT), the tracer diffusion is independent of nanoparticle concentration, while a Dmin is observed when Rg > RCNT. When the tracer molecule is large relative to the nanoparticle, the diffusion in the vicinity of the nanoparticle appears to become anisotropic, which leads to a Dmin with increasing nanoparticle loading.
11/2009;
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ABSTRACT: The transfer mechanism of applied stress in single-wall carbon nanotube (SWCNT)/poly(methyl methacrylate) (PMMA) nanocomposites was investigated using in situ Raman spectroscopy on composite fibers. These SWCNT/PMMA nanocomposite fibers have no specific SWCNT-polymer interactions and the high degree of nanotube alignment minimizes the contributions from nanotube-nanotube interactions. Although tensile testing found significantly improved overall mechanical properties of the fibers, effective stress transfer to SWCNTs is limited to a small strain regime (epsilon<0.2%). At higher strains, the stress on the SWCNTs decreases due to the slippage at the nanotube-polymer interface. Slippage was also evident in scanning electron micrographs of fracture surfaces produced by tensile testing of the composite fibers. Above epsilon = 0.2%, the strain-induced slippage was accompanied by irreversible responses in stress and Raman peak shifts. This paper shows that efficient stress transfer to nanotubes as monitored by Raman spectroscopy is crucial to improving the mechanical properties of polymer nanocomposites and to detecting internal damage in nanocomposites.
Nanotechnology 08/2009; 20(33):335703. · 3.98 Impact Factor
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ABSTRACT: Mechanical interactions between amorphous polymers and nanofillers were probed by measuring the tensile moduli of nanocomposite fibers with aligned single wall carbon nanotubes (SWNT) and carbon nanofibers (CNF). Polymer chains provide better load transfer and thereby higher tensile moduli when the polymer size is large relative to the diameter of the filler. The specific interfacial area of the filler is not sufficient to explain this observed increase in elastic modulus. The effective modulus of SWNT bundles was 250 GPa, as calculated using a short fiber model.
07/2007;
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ABSTRACT: The copolymer of beta-casein-graft-dextran was prepared using the Maillard reaction and the acidic solution properties of the copolymer were studied with dynamic light scattering. At pH range 4-5 where is close to the isoelectric point of beta-casein, the copolymer forms micelles which are spherical verified by atomic force microscopy imaging. The size and existent time of the micelles depend on the graft degree and the length of dextran side chains of the copolymers. During storage at pH 4.6, the micelles formed by the copolymers with short side chains and low graft degree tend to precipitate, while the micelles formed by the copolymers with long side chains and high graft degree tend to dissociate. The micellization of the copolymers can be suppressed by adding NaCl. Optical microscopy and turbidity studies show that the copolymers dissolved in molecular state and with higher hydrophilicity have better emulsifying ability.
Journal of Colloid and Interface Science 10/2006; 301(1):98-106. · 3.07 Impact Factor
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ABSTRACT: Casein is almost insoluble at around pH 4.6, which is its isoelectric point (pI). Grafting copolymer, casein-g-dextran, was prepared through the Amadori rearrangement of the Maillard reaction. The copolymer has a reversible pH sensitive property: micellization at the pI of casein forming a casein core and dextran shell structure and dissociation when pH differs from the pI. The micelles produced at pH 4.6 have a spherical shape and their size is dependent on the Maillard reaction: reaction time, molar ratio of casein to dextran, and molecular weight of dextran used. Typically, the hydrodynamic diameter of the micelles is about 100 nm and the critical micelle concentration is about 10 mg/L. The micelles are very stable in aqueous solution and can be stored as lyophiled powder. The micelles are able to encapsulate hydrophobic compounds such as pyrene.
Biopolymers 02/2006; 81(1):29-38. · 2.87 Impact Factor
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ABSTRACT: In this Letter we report a neat method for preparing polymeric giant vesicles. Self-assembly of a rigid oligomer, polyether imide (PEI) with amino groups at its two ends and stearic acid (SA) in chloroform/cyclohexane, which was a selective solvent for SA, led to the giant vesicles. However, PEI and low-molecular-weight polystyrene with a carboxyl end (CPS) formed micelles instead of vesicles in the solvent mixture. Furthermore, morphology transition from micelle to vesicle was observed when SA was added to the micellar solution of PEI/CPS.
10/2003;
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Journal of Polymer Science Part A Polymer Chemistry 03/2002; 40(9):1253 - 1266. · 3.92 Impact Factor
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ABSTRACT: We present a three-dimensional simulation and calculation of electrical conductivity above the filler percolation threshold for networks containing finite, conductive cylinders as a function of axial orientation (S) and aspect ratio (L/D). At a fixed volume fraction and L/D, the simulations exhibit a critical degree of orientation, S-c, above which the electrical conductivity decreases dramatically. With increasing filler concentration and aspect ratio, this critical orientation shifts to higher degrees of alignment. Additionally, at a fixed volume fraction and L/D, the simulated electrical conductivity displays a maximum at slight uniaxial orientation, which is less pronounced at higher volume fractions and aspect ratios. Our approach can be used as a predictive tool to design the optimal filler concentration and degree of orientation required to maximize electrical conductivity in polymer nanocomposites with conductive cylindrical fillers of finite dimension.
Departmental Papers (MSE).
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Minfang Mu
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ABSTRACT: Since the discovery two decades ago, fullerene family has drawn remarkable attention because of their unique electrical, thermal, optical, mechanical and flammable properties. They have been widely used to improve polymer properties. These nanofillers produce huge interfacial areas between the polymer and the fillers. Despite the intensive research on fullerene nanocomposites, understanding of the importance of the filler-polymer interface is still limited and further investigation of the structure-property relationships is needed. This dissertation probed influence of nanoparticles on polymer tracer diffusion and molecular weight dependence of composite mechanical properties, and developed a coated particle process to obtain composites with high electrical conductivity. Deuterated polystyrene (dPS) diffusion in nanoparticle/polystyrene (PS) nanocomposites was measured by an elastic recoil detection method. We used single wall carbon nanotubes (SWCNTs), multiwall carbon nanotubes (MWCNTs) and C60 as nanofillers and found that the nanofillers have a significant influence on polymer tracer diffusion. When the tracer molecules ( Rg ) are larger than the fillers ( RCNT ), the tracer diffusion coefficient exhibits a minimum as a function of filler concentration. In contrast, the tracer diffusion in nanocomposites is constant when the tracer chains are smaller than the fillers. A trap model simulation was developed to understand the minimum diffusion coefficient. The load transfer mechanism from polymer matrix to fillers were studied by tensile testing and Raman spectroscopy in SWCNT/poly(methyl methacrylate) (PMMA) nanocomposite fibers. Without strong filler-polymer interactions, effective load transfer is limited to small strains, and Raman peak shift and stress-strain curve of composite fibers are reversible, suggesting an elastic deformation. Beyond this strain region, the load transfer is nonlinear because of a slippage at the polymer-filler interface. The stress on nanotubes reaches a maximum and then decreases with further increase in the strain. A coated particle process (CPP) method was developed to prepare nanocomposites with a cellular filler structure. The fillers are SWCNTs and PMMA is used as polymer matrix. Compared with the coagulated nanocomposites with well dispersed SWCNTs, the CPP-made nanocomposites have a higher electrical conductivity (2 orders higher), a smaller percolation threshold (50%).
Dissertations available from ProQuest.
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ABSTRACT: The thermal conductivity of single-walled carbon nanotube (SWCNT)/polystyrene composites, prepared by a method known to produce a uniform distribution of SWCNT bundles on the micrometer length scale, was measured in the temperature range from approximately 140 to 360 K. The thermal conductivity enhancement (50% for 1 mass % at 300 K) is reasonably constant above room temperature but is reduced at the lower temperatures. This result is consistent with the expected, large contribution of interfacial thermal resistance in SWCNT/polymer composites. Enhancements in electrical conductivity show that 1 mass % loading is in the region of the electrical percolation threshold.
Departmental Papers (MSE).
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ABSTRACT: Nanoparticles present a new frontier for understanding polymer dynamics in complex, nanoscale environments. We report that the addition of single-walled carbon nanotubes (SWCNTs) produces a minimum in the diffusion coefficient with increasing nanoparticle concentration, φ. Initially, tracer diffusion coefficients (D) are suppressed with increasing φ and then increase beyond a critical concentration, φ crit < 1 vol %. Shorter tracer chains exhibit a greater slowing down than longer chains, whereas longer matrix chains decrease the value of φ crit . The experimental results are discussed in terms of locally anisotropic diffusion perpendicular and parallel to the nanotube filler and simulated using a trap model that defines a trap size and the extent of slowing perpendicular to the cylindrical trap. The simulated diffusion coefficients capture both the initial decrease in D attributed to isolated traps and the recovery of D above φ crit corresponding to trap percolation. Nanoparticles influence polymer diffusion in fascinating ways and will refine our understanding of polymer reptation and might also inform the study of biopolymer diffusion in living systems.
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ABSTRACT: A new processing method has been developed to combine a polymer and single wall carbon nanotubes (SWCNTs) to form electrically conductive composites with desirable rheological and mechanical properties. The process involves coating polystyrene (PS) pellets with SWCNTs and then hot pressing to make a contiguous, cellular SWCNT structure. By this method, the electrical percolation threshold decreases and the electrical conductivity increases significantly as compared to composites with well-dispersed SWCNTs. For example, a SWCNT/PS composite with 0.5 wt% nanotubes made by this coated particle process (CPP) has an electrical conductivity of ∼3 × 10−4 S/cm, while a well-dispersed composite made by a coagulation method with the same SWCNT amount has an electrical conductivity of only ∼10−8 S/cm. The rheological properties of the composite with a macroscopic cellular SWCNT structure are comparable to PS, while the well-dispersed composite exhibits a solid-like behavior, indicating that the composites made by this new CPP are more processable. In addition, the mechanical properties of the CPP-made composite decrease only slightly, as compared with PS. Relative to the common approach of seeking better dispersion, this new fabrication method provides an important alternative means to higher electrical conductivity in SWCNT/polymer composites. Our straightforward particle coating and pressing method avoids organic solvents and is suitable for large-scale, inexpensive processing using a wide variety of polymers and nanoparticles.
Polymer.
