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ABSTRACT: Antifouling surfaces are critical for the good performance of functional materials in various applications including water filtration, medical implants, and biosensors. In this study, we synthesized amphiphilic triblock terpolymers (tri-BCPs, coded as KB) and fabricated amphiphilic nanofibers by electrospinning of solutions prepared by mixing the KB with poly(lactic acid) (PLA) polymer. The resulting fibers with amphiphilic polymer groups exhibited superior antifouling performance to the fibers without such groups. The adsorption of bovine serum albumin (BSA) on the amphiphilic fibers was about 10-fold less than that on the control surfaces from PLA and PET fibers. With the increase of the KB content in the amphiphilic fibers, the resistance to adsorption of BSA was increased. BSA was released more easily from the surface of the amphiphilic fibers than from the surface of hydrophobic PLA or PET fibers. We have also investigated the structural conformation of KB in fibers before and after annealing by contact angle measurements, transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), and coarse-grained molecular dynamics (CGMD) simulation to probe the effect of amphiphilic chain conformation on antifouling. The results reveal that the amphiphilic KB was evenly distributed within as-spun hybrid fibers, while migrated toward the core from the fiber surface during thermal treatment, leading to the reduction in antifouling. This suggests that the antifouling effect of the amphiphilic fibers is greatly influenced by the arrangement of amphiphilic groups in the fibers.
Biomacromolecules 04/2012; 13(5):1606-14. · 5.48 Impact Factor
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ABSTRACT: Nanofibers are synthesized by electrospinning highly loaded water-based precursor-polymer hybrid solutions followed by thermal treatment to control crystal structure. Electrical conductivity and magnetic coercivity, as shown, are tested displaying independent magnetic and electrical property control from coercive to superparamagnetic and resistive to near-bulk conductivity at room temperature.
Small 03/2012; 8(10):1510-4. · 8.35 Impact Factor
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ABSTRACT: Monoaxial silica nanofibers containing iron species as well as coaxial nanofibers with a pure silica core and a silica shell containing high concentrations of iron nanocrystals were fabricated via electrospinning precursor solutions, followed by thermal treatment. Tetraethyl-orthosilicate (TEOS) and iron nitrate (Fe(NO(3))(3)) were used as the precursors for the silica and iron phases, respectively. Thermal treatments of as-spun precursor fibers were applied to generate nanocrystals of iron with various oxidation states (pure iron and hematite). Scanning electron microscopy (SEM), x-ray diffraction (XRD), and transmission electron microscopy (TEM) were used to probe the fiber morphology and crystal structures. The results indicated that the size, phase, and placement of iron nanocrystals can be tuned by varying the precursor concentration, thermal treatment conditions, and processing scheme. The resulting nanofiber/metal systems obtained via both monoaxial and coaxial electrospinning were applied as catalysts to the alkaline hydrolysis of glucose for the production of fuel gas. Differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and bulk weight change in a furnace with residual gas analysis (RGA) were used to evaluate the performance of the catalysts for various ratios of both Fe to Si, and catalyst to glucose, and the oxidation state of the iron nanocrystals. The product gas is composed of mostly H(2) (>96 mol%) and CH(4) with very low concentrations of CO(2) and CO. Due to the clear separation of reaction temperature for H(2) and CH(4) production, pure hydrogen can be obtained at low reaction temperatures. Our coaxial approach demonstrates that placing the iron species selectively near the fiber surface can lead to two to three fold reduction in catalytic consumption compared to the monoaxial fibers with uniform distribution of catalysts.
Nanotechnology 08/2011; 22(32):325302. · 3.98 Impact Factor
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ABSTRACT: New organic−inorganic hybrid fibers (polyvinyl alcohol (PVA)/HfO2) were prepared by an electrospinning method using DI water as a solvent. The HfO2−acetate nanoparticles could be loaded up to 80% by weight in PVA polymer to fabricate uniform PVA/HfO2 electrospun hybrid fibers. After annealing at 140 °C for 12 h, electrospun PVA/HfO2 fibers were stable when soaked in water. Calcination of these hybrid fibers at temperatures ranging from 450 to 700 °C resulted in the formation of pure inorganic HfO2 fibers with diameters ranging from 0.1 to 0.8 μm depending on the concentration of HfO2−acetate nanoparticles in the spinning dope. Fourier transform infrared spectroscopy and Raman spectroscopy techniques were used to investigate the crystal structures of the HfO2 fibers, which were confirmed by X-ray diffraction (XRD) results. XRD results showed the presence of both monoclinic and tetragonal crystal structures in the HfO2 fibers. The type and size of the crystals formed depended on the concentration of HfO2−acetate nanoparticles in the initial spinning dope and on the calcination temperature. In particular, the formation of stable tetragonal crystal structures in the as-spun hybrid fibers with lower concentration of HfO2 was achieved at lower temperature than the monoclinic-to-tetragonal transition temperature.
03/2011;
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Macromolecular Materials and Engineering 07/2010; 295(8):763 - 773. · 1.99 Impact Factor
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ABSTRACT: Coarse-grained, molecular dynamics (MD) simulations have been conducted to study the effect of shear flow on polymer nanocomposite systems. In particular, the interactions between different components have been tuned such that the nanoparticle-nanoparticle attraction is stronger than nanoparticle-polymer interaction, and therefore, the final equilibrium state for such systems is one with clustered nanoparticles. In the current study, we focus on how shear flow affects the kinetics of particle aggregation at the very initial stages in systems with polymers of different chain lengths. The particle volume fraction and size are kept fixed at 0.1 and 1.7 MD units, respectively. Through this work, shear has been shown to significantly slow down nanoparticle aggregation, an effect that was found to be a strong function of both polymer chain length and shear rate. To understand our findings, a systematic study on effect of shear on particle diffusion and an analysis of relative time scales of different mechanisms causing particle aggregation have been conducted. The aggregation rate obtained from the time scale analysis is in good agreement with that determined from the aggregation time derived from the pair correlation function monitored during simulations.
The Journal of chemical physics 01/2010; 132(2):024901. · 3.09 Impact Factor
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ABSTRACT: A comprehensive model for the stable jet region in electrospinning of crystallizing polymer melts has been presented. First, the conventional flow-induced crystallization (FIC) model by Ziabicki was coupled with the non-isothermal melt electrospinning model. The modeled initial jet profiles were compared to digitized experimental images of the stable Nylon-6 melt jet near the spinneret. The final jet diameters were also compared to the average thickness of collected fibers. The results were in good agreement with the flow visualization experiments for various melt temperatures and flow rates. The modeled crystallinity predictions were also in agreement with experimental data from collected fiber mats. Then, a new FIC model that can provide microstructure information, such as crystallite number density and average size, has been proposed and validated under isothermal and non-isothermal conditions in the bulk as well as in the confined geometry of the polymer melt jet in electrospinning. Nylon-6,6 was used as the model polymer in this crystallization study, and the results are in good agreement with the widely-used Ziabicki FIC model.
Polymer. 01/2010; 51(1):274-290.
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ABSTRACT: Symmetric diblock copolymer/nanoparticle (NP) systems under planar elongational flow have been modeled and simulated using coarse-grained nonequilibrium molecular dynamics. The aim of our present study is to understand how the dispersion of NPs in a block copolymer system is influenced by elongational flow and how the presence of NPs changes the rheology and flow-induced morphology transition in block copolymers. We consider two different kinds of spherical NPs categorized with respect to their interaction potential with the polymeric blocks: (1) selective NPs that show a preference toward one of the blocks of a model diblock copolymer and (2) nonselective NPs that show equal attraction toward both blocks. For unrestricted simulation times during elongational flow, spatially and temporally periodic boundary conditions devised by Kraynik and Reinelt [Int. J. Multiphase Flow 18, 1045 (1992)] have been implemented. Our results show that the concentration peak of both selective NPs at the center of the preferred domain and nonselective NPs at the domain interface becomes broader with increasing elongation rate, suggesting that elongational flow can be used as another parameter to control nanocomposite self-assembly. In addition, our results reveal that the onset of flow-induced transition from lamellar to disordered morphology is greatly influenced by particle-particle and particle-polymer interactions.
The Journal of chemical physics 12/2009; 131(21):214904. · 3.09 Impact Factor
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ABSTRACT: Multiaxial (triaxial/coaxial) electrospinning is utilized to fabricate block copolymer (poly(styrene-b-isoprene), PS-b-PI) nanofibers covered with a silica shell. The thermally stable silica shell allows post-fabrication annealing of the fibers to obtain equilibrium self-assembly. For the case of coaxial nanofibers, block copolymers with different isoprene volume fractions are studied to understand the effect of physical confinement and interfacial interaction on self-assembled structures. Various confined assemblies such as co-existing cylinders and concentric lamellar rings are obtained with the styrene domain next to the silica shell. This confined assembly is then utilized as a template to guide the placement of functional nanoparticles such as magnetite selectively into the PI domain in self-assembled nanofibers. To further investigate the effect of interfacial interaction and frustration due to the physically confined environment, triaxial configuration is used where the middle layer of the self-assembling material is sandwiched between the innermost and outermost silica layers. The results reveal that confined block-copolymer assembly is significantly altered by the presence and interaction with both inner and outer silica layers. When nanoparticles are incorporated into PS-b-PI and placed as the middle layer, the PI phase with magnetite nanoparticles migrates next to the silica layers. The migration of the PI phase to the silica layers is also observed for the blend of PS and PS-b-PI as the middle layer. These materials not only provide a platform to further study the effect of confinement and wall interactions on self-assembly but can also help develop an approach to fabricate multilayered, multistructured nanofibers for high-end applications such as drug delivery.
Small 07/2009; 5(20):2323-32. · 8.35 Impact Factor
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ABSTRACT: Coaxial nanofibers with poly(styrene-block-isoprene) (PS-b-PI)/magnetite nanoparticles as core and silica as shell are fabricated using electrospinning.1-4 Thermally stable silica helps to anneal the fibers above the glass transition temperature of PS-b-PI and form ordered nanocomposite morphologies. Monodisperse magnetite nanoparticles (NPs; 4 nm) are synthesized and surface coated with oleic acid to provide marginal selectivity towards an isoprene domain. When 4 wt% nanoparticles are added to symmetric PS-b-PI, transmission electron microscopy (TEM) images of microtomed electrospun fibers reveal that NPs are uniformly dispersed only in the PI domain, and that the confined lamellar assembly in the form of alternate concentric rings of PS and PI is preserved. For 10 wt% NPs, a morphology transition is seen from concentric rings to a co-continuous phase with NPs again uniformly dispersed in the PI domains. No aggregates or loss of PI selectivity is found in spite of interparticle attraction. Magnetic properties are measured using a superconducting quantum interference device (SQUID) magnetometer and all nanocomposite fiber samples exhibit superparamagnetic behavior.
Small 11/2008; 4(11):2067-73. · 8.35 Impact Factor
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ABSTRACT: Polyacrylonitrile (PAN) solution containing the iron oxide precursor iron (III) acetylacetonate (AAI) was electrospun and thermally treated to produce electrically conducting, magnetic carbon nanofiber mats with hierarchical pore structures. The morphology and material properties of the resulting multifunctional nanofiber mats including the surface area and the electric and magnetic properties were examined using various characterization techniques. Scanning electron microscopy images show that uniform fibers were produced with a fiber diameter of ~600 nm, and this uniform fiber morphology is maintained after graphitization with a fiber diameter of ~330 nm. X-ray diffraction (XRD) and Raman studies reveal that both graphite and Fe3O4 crystals are formed after thermal treatment, and graphitization can be enhanced by the presence of iron. A combination of XRD and transmission electron microscopy experiments reveals the formation of pores with graphitic nanoparticles in the walls as well as the formation of magnetite nanoparticles distributed throughout the fibers. Physisorption experiments show that the multifunctional fiber mats exhibit a high surface area (200–400 m2 g−1) and their pore size is dependent on the amount of iron added and graphitization conditions. Finally, we have demonstrated that the fibers are electrically conducting as well as magnetically active.
Nanotechnology 10/2008; 19(45):455612. · 3.98 Impact Factor
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ABSTRACT: We present molecular dynamics simulations coupled with a dissipative particle dynamics thermostat to model and simulate the behavior of symmetric diblock copolymer/nanoparticle systems under simple shear flow. We consider two categories of nanoparticles, one with selective interactions toward one of the blocks of a model diblock copolymer and the other with nonselective interactions with both blocks. For the selective nanoparticles, we consider additional variants by changing the particle diameter and the particle-polymer interaction potential. The aim of our present study is to understand how the nanoparticles disperse in a block copolymer system under shear flow and how the presence of nanoparticles affects the rheology, structure, and flow behavior of block copolymer systems. We keep the volume fraction of nanoparticles low (0.1) to preserve lamellar morphology in the nanocomposite. Our results show that shear can have a pronounced effect on the location of nanoparticles in block copolymers and can therefore be used as another parameter to control nanocomposite self-assembly. In addition, we investigate the effect of nanoparticles on shear-induced lamellar transition from parallel to perpendicular orientation to further elucidate nanocomposite behavior under shear, which is an important tool to induce long-range order in self-assembling materials such as block copolymers.
The Journal of Chemical Physics 05/2008; 128(16):164909. · 3.33 Impact Factor
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ABSTRACT: Using Brownian dynamics simulations of wormlike chain bead-spring models, the dynamics of linear and star-branched polyelectrolyte molecules traveling through an array of entropic traps during electrophoresis have been investigated. First, the effectiveness of using coarse-grained bead-spring systems for linear molecules to model the electrophoretic process was demonstrated and compared to previous bead-rod (Kramers) chain simulations by Panwar and Kumar [Macromolecules 39, 1297 (2006)]. Second, the coarse-grained bead-spring model has been extended to investigate the effect of branching on the dynamics of molecules through the entropic trap array. Initial studies indicate the reduced mobility of star-branched molecules as compared to equivalent linear molecules. The radius of gyration of the polymer molecule appears to be the dominating factor governing the time scales encountered during traversal of the entropic trapping array.
The Journal of Chemical Physics 10/2007; 127(12):124902. · 3.33 Impact Factor
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ABSTRACT: Formation of various domain shapes in submicron scale fibers of poly(styrene-block-isoprene) (PS-b-PI) has been investigated via electrospinning. Monodisperse PS-b-PI block copolymers with 29 and 53 vol % of PI were synthesized using two-step anionic polymerization and were dissolved in tetrahydrofuran (THF). Solutions of block copolymer with varying concentrations in THF were electrospun, and fibers with average diameters from 200 nm to 5 μm were obtained. Small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) studies revealed that cylindrical and lamellar morphology can be formed in electrospun fibers of 29% and 53% PI copolymers, respectively. We note that these domain structures in fibers are not as well developed as those in films possibly due to the short residence time and strong elongational deformation involved in the electrospinning process. For both systems we find that the d spacing in electrospun fibers is smaller than that in the cast film. This could also be attributed to the elongational deformation and fast solvent evaporation during electrospinning. The domain structures of electrospun fibers from the symmetric (53% PI) copolymer exhibit the influence of fiber morphology such as confinement and curvature due to its high molecular weight. More uniform domain structures in the fibers and increase in d spacing are observed after the annealing process.
07/2006;
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ABSTRACT: Submicron scale vanadia/silica hybrid nanofiber mats have been produced by electrospinning silica sol-gel precursor containing vanadium oxytriisopropoxide (VOTIP), followed by calcinations at high temperature. The properties of the resulting inorganic hybrid nanofiber mats are compared to those of electrospun pure silica nanofibers. SEM images show fibers are submicron in diameter and their morphology is maintained after calcination. Physisorption experiments reveal that silica nanofiber mats have a high specific surface area of 63 m2/g. FT-IR spectra exhibit Si—O vibrations and indicate the presence of V2O5 in the fibers. XPS studies reveal that the ratio of Si to O is close to 0.5 on the surface of fibers and the amount of vanadium on the surface of fibers increases with calcination. XRD diffraction patterns show that silica nanofibers are amorphous and orthorhombic V2O5 crystals have formed after calcination. EFTEM images demonstrate the growth of crystals on the surface of fibers containing vanadium after calcination. SEM images of fibers with high-vanadium content (50 mol% V : Si) show that vanadia crystals are mostly aligned along the fiber axis. XPS shows an increase in vanadium contents at the surface, and XRD patterns exhibit an increase in the degree of crystallinity. A coaxial electrospinning scheme has successfully been employed to selectively place V2O5 in the skin layer.
Journal of Nanomaterials. 01/2006;
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ABSTRACT: The continuous production of ultra-high-molecular-weight polyethylene (UHMWPE) filaments was studied by the direct roll forming of nascent reactor powders followed by subsequent multistage orientation drawing below their melting points. The UHMWPE reactor powders used in this study were prepared by the polymerization of ethylene in the presence of soluble magnesium complexes, and they exhibited high yield even at low reaction temperatures. The unique, microporous powder morphology contributed to the successful compaction of the UHMWPE powders into coherent tapes below their melting temperatures. The small-angle X-ray scattering study of the compacted tapes revealed that folded-chain crystals with a relatively long-range order were formed during the compaction and were transformed into extended-chain crystals as the draw ratio increased. Our results also reveal that the drawability and tensile and thermal properties of the filaments depended sensitively on both the polymerization and solid-state processing conditions. The fiber drawn to a total draw ratio of 90 in the study had a tensile strength of 2.5 GPa and a tensile modulus of 130 GPa. Finally, the solid-state drawn UHMWPE filaments were treated with O2 plasma, and the enhancement of the interfacial shear strength by the surface treatment is presented. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 718–730, 2005
Journal of Applied Polymer Science 10/2005; 98(2):718 - 730. · 1.29 Impact Factor
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ABSTRACT: Cellulose nonwoven mats of submicron-sized fibers (150 nm–500 nm in diameter) were obtained by electrospinning cellulose solutions. A solvent system based on lithium chloride (LiCl) and N,N-dimethylacetamide (DMAc) was used, and the effects of (i) temperature of the collector, (ii) type of collector (aluminum mesh and cellulose filter media), and (iii) postspinning treatment, such as coagulation with water, on the morphology of electrospun fibers were investigated. The scanning electron microscopy (SEM) and X-ray diffraction studies of as-spun fibers at room temperature reveal that the morphology of cellulose fibers evolves with time due to moisture absorption and swelling caused by the residual salt and solvent. Although heating the collector greatly enhances the stability of the fiber morphology, the removal of salt by coagulation and DMAc by heating the collector was necessary for the fabrication of dry and stable cellulose fibers with limited moisture absorption and swelling. The presence and removal of the salt before and after coagulation have been identified by electron microprobe and X-ray diffraction studies. When cellulose filter media is used as a collector, dry and stable fibers were obtained without the coagulation step, and the resulting electrospun fibers exhibit good adhesion to the filter media. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1673–1683, 2005
Journal of Polymer Science Part B Polymer Physics 05/2005; 43(13):1673 - 1683. · 1.53 Impact Factor
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ABSTRACT: Using Brownian dynamics (BD) simulations of FENE bead-spring models, the dynamics of star-branched polymers in dilute solutions under extensional flow have been investigated. Studies on star polymers in transient extensional flow reveal that the initial transient stress response at low strains is governed by both the number of arms and the shortest arm. On the other hand, the steady-state behavior of star polymers in elongational flow is limited by the maximum effective “contour” length of the molecules. The distribution of arm extension and birefringence of the star-branched molecule are broader and the mean is shifted to lower values, when compared to equivalent linear systems. As a result, the degree of arm extension at steady-state decreases as the number of arms in the star increases. Both an analysis of individual ensembles in Brownian dynamics simulations and a study of a simple force balance indicate that the constraint imposed on the star arms by the central branch point and contributions from “asymmetric” arm arrangements give rise to overall less extended and oriented star-branched molecules with broader arm extension and birefringence distributions. The results obtained from stress-conformation hysteresis simulation indicate that less-stretched arms exhibit more retarded relaxation, as the number of arms increases in star-branched molecules. The effect of excluded volume (EV) interactions, incorporated through the Lennard–Jones potential, on the dynamics of star polymers in extensional flow appears unimportant.
Journal of Non-Newtonian Fluid Mechanics.
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ABSTRACT: Non-woven mats of submicron-sized cellulose fibers (250–750 nm in diameter) have been obtained by electrospinning of cellulose solutions. Cellulose are directly dissolved in two solvent systems: (a) lithium chloride (LiCl)/N,N-dimethyl acetamide (DMAc) and (b) N-methylmorpholine oxide (NMMO)/water, and the effects of (i) solvent system, (ii) the degree of polymerization of cellulose, (iii) spinning conditions, and (iv) post-spinning treatment such as coagulation with water on the miscrostructure of electrospun fibers are investigated. The scanning electron microscope (SEM) images of electrospun cellulose fibers show that applying coagulation with water right after the collection of fibers is necessary to obtain submicron scale, dry and stable cellulose fibers for both solvent systems. X-ray diffraction studies reveal that cellulose fibers obtained from LiCl/DMAc are mostly amorphous, whereas the degree of crystallinity of cellulose fibers from NMMO/water can be controlled by various process conditions including spinning temperature, flow rate, and distance between the nozzle and collector. Finally, electrospun cellulose fibers are oxidized by HNO3/H3PO4 and NaNO2, and the degradation characteristics of oxidized cellulose fibers under physiological conditions are presented.
Polymer. 47(14):5097-5107.
