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This investigation was aimed to enhance the dispersibility of multi-walled carbon nanotubes (MWCNT) using sodium polystyrene sulfonate (Na-PSS) polyelectrolyte. Subsequently, electrically conducting, multi-layer thin films are prepared utilizing layer by layer assembly method with poly(vinyl alcohol) as a host matrix. The highest extent of MWCNT dispersion was observed in MWCNT:Na-PSS ratio of 1:9 (wt/wt), which was estimated from UV-Vis spectroscopic analysis. Zeta potential measurements of Na-PSS modified MWCNT dispersion showed large negative potentials ranging from -52 to -64 mV in the most stable pH range of 4 to 10, suggesting the colloidal stability is due to the long-range repulsive nature of electrostatic interactions from negatively charged sulfonate groups. Complementary molecular dynamics simulations showed that adsorption of Na-PSS imparts a large negative potential to the carbon nanotube surface, which increases with an increase in Na-PSS concentration. The multi-layer thin film of (1:9) MWCNT:Na-PSS exhibited a DC electrical conductivity of 2.96 * [10.sup.2] S/m. KEYWORDS conducting thin films, layer by layer assembly, multi-walled carbon nanotubes, poly(sodium 4-styrene sulfonate), zeta potential measurement, 1 | INTRODUCTION Carbon nanotubes (CNTs) with their high aspect ratio and an unique combination of its superior electrical, mechanical, thermal and elastic properties are considered as an ideal nanomaterial [...]

The role of hydrophobicity of phenylalanine-glycine nucleoporins (FG-Nups) in determining the transport of receptor-bound cargo across the nuclear pore complex (NPC) is investigated using Langevin dynamics simulations. A coarse-grained, minimal model of the NPC, comprising a cylindrical pore and hydrophobic–hydrophilic random copolymers for FG-Nups was employed. Karyopherin-bound receptor-cargo complexes (Kaps) were modeled as rigid, coarse-grained spheres without (inert) and with (patchy) FG-binding hydrophobic domains. With a sequence-agnostic description of FG-Nups and the absence of any anisotropies associated with either NPC or cargo, the model described tracer transport only as a function of FG-Nup hydrophobicity, f. The simulations showed the emergence of two important features of cargo transport, namely, NPC selectivity and specificity. NPC selectivity to patchy tracers emerged due to hydrophobic Kap–FG interactions and despite the sequence-agnostic description of FG-Nups. Furthermore, NPC selectivity was observed only in a specific range of FG-hydrophobic fraction, 0.05 ≤ f ≤ 0.20, resulting in specificity of NPC transport with respect to f. Significantly, this range corresponded to the number fraction of FG-repeats observed in both S. cerevisiae and H. sapiens NPCs. This established the central role of the FG-hydrophobic fraction in determining NPC transport, and provided a biophysical basis for conservation of the FG-Nup hydrophobic fraction across evolutionarily distant NPCs. Specificity in NPC transport emerged from the formation of a hydrogel-like network inside the pore with a characteristic mesh size dependent on f. This network rejected cargo for f > 0.2 based on size exclusion, which resulted in enhanced translocation probability for 0.05 ≤ f ≤ 0.20. Extended brush configurations outside the pore resulted in entropic repulsion and exclusion of inert cargo in this range. Thus, our minimal NPC model exhibited a hybrid cargo translocation mechanism, with aspects of both virtual gate and selective-phase models, in this range of FG-hydrophobic fraction. [ABSTRACT FROM AUTHOR]

The isothermal crystallization behaviour of the polypropylene (PP) phase in PP/multi-walled carbon nanotubes (MWCNTs) composites has been investigated via differential scanning calorimetric analysis, which showed the influence of the varying dispersion level of MWCNTs in the respective PP matrix. PP/MWCNTs composites were prepared via melt-blending technique, wherein two different grades of MWCNTs of varying average 'agglomerate' size and varying entanglements (N-MWCNTs and D-MWCNTs) were utilized. Furthermore, the influence of melt-viscosity of the PP phase was investigated on the crystallization kinetics of the PP/MWCNTs composites. Heterogeneous nucleation ability of MWCNTs has resulted in a decrease in half time of crystallization ([t.sub.1/2]) from ~14 min for pure PP to ~6 min for PP/N-MWCNTs and ~11 min for PP/ D-MWCNTs composites at 1 wt% of MWCNTs at 132[degrees]C. Overall rate of crystallization (k) has significantly increased to 4.9 X [10.sup.-2] [min.sup.-1] for PP/N-MWCNTs composite as compared with 6.2 X [10.sup.-3] [min.sup.-1] for PP/D-MWCNTs composite at 0.5 wt% of MWCNTs at 132[degrees]C. Moreover, the effect of a novel organic modifier, Li-salt of 6-amino hexanoic acid along with a compatibilizer (PP-g-MA) has also been investigated on the crystallization kinetics of the PP phase in PP/ MWCNTs composites., INTRODUCTION The phenomenal discovery of carbon nanotubes (CNTs) in 1991 [1] has led to unprecedented interest in the industrial as well as in the academic community. The outstanding properties associated [...]

Helical intermediates appear to be crucial in the amyloid formation of several amyloidogenic peptides, including Aβ, that are implicated in different neurodegenerative diseases. Intermediate species of amyloid formation have been reported to be more toxic than mature amyloid fibrils. Hence, the current work focuses on understanding the mechanistic roles of the helical intermediates in the early stages of amyloid self-assembly in amyloidogenic peptides. Molecular dynamics (MD) simulations and the adaptive biasing force (ABF) method were utilized to investigate structural changes that lead to amyloid formation in amphibian peptide uperin-3.5 (U3.5), an antimicrobial and amyloidogenic peptide. Microsecond time-scale MD simulations revealed that peptide aggregation, into β-sheet dominated aggregates, is centred on two important factors; evolution of α-helical intermediates and the critical role of local peptide concentration inside these aggregates. Electrostatic attraction between the oppositely charged aspartate (D) and arginine (R) residues located near the N-terminus induced hydrogen bonding resulting in the formation of precursor 310-helices close to the N-terminus. The 310-helices transitioned into α-helices, thereby imparting partial helical conformations to the peptides. In the initial stages of aggregation, U3.5 peptides with amphipathic, partial helices were driven closer by hydrophobic interactions to form small clusters of helical intermediates. These helices imparted stability to the helical intermediates, which promoted the growth of clusters by further addition of peptides. This led to an increase in the local peptide concentration, which enabled stronger peptide–peptide interactions and triggered a β-sheet transition in these aggregates. Thus, this study emphasized that the helical intermediates may be crucial to the evolution of β-sheet-rich amyloid structures. [ABSTRACT FROM AUTHOR]

Multiwall carbon nanotubes (MWNTs) were melt-mixed in polyamide 6 (PA6) and acrylonitrile-butadiene-styrene (ABS) copolymer blends using a simultaneous mixing protocol in order to investigate the state of dispersion of MWNTs in PA6/ABS blends. The blend composition was varied from 40/60 (wt/wt) to 60/40 (wt/wt) in PA6/ABS blends, which showed 'co-continuous' morphology in the presence of MWNTs. State of dispersion of MWNTs in these blends was assessed through bulk electrical conductivity measurements, morphological analysis, solution experiments, and UV-vis spectroscopic analysis. MWNTs were subsequently modified with a novel organic modifier, sodium salt of 6-aminohexanoic acid (Na-AHA), to improve the state of dispersion of MWNTs. Blends with unmodified MWNTs exhibited the DC electrical conductivity in the range [~10.sup.-11] to [~10.sup.-5] S/cm, whereas blends with Na-AHA-modified MWNTs exhibited DC electrical conductivity in the range [~10.sup.-7] to [~10.sup.-5] S/cm. The reduction in MWNTs 'agglomerate' size (~73.7 µm for 40/ 60 blend with unmodified MWNTs to ~59.9 µm in the corresponding blend with Na-AHA-modified MWNTs) was observed through morphological analysis. The rheological studies showed increased complex viscosity and storage moduli in lower frequency region in case of blends with Na-AHA-modified MWNTs confirming a refined 'network-like' structure of MWNTs., INTRODUCTION Polymer blends are of immense commercial interest as high-performance alloys, due to their excellent properties, which can be suited for applications where materials with high thermal stability, good chemical [...]

Polyamide6 (PA6)/acrylonitrile butadiene styrene copolymer (ABS) blends with unmodified multiwall carbon nanotubes (MWNTs) were prepared via melt-blending in a conical twin-screw micro-compounder with varying melt-mixing time. To improve the state of dispersion of MWNTs, non-covalent organic modifiers for MWNTs have been utilized: sodium salt of 6-amino hexanoic acid (Na-AHA) and 1-pyrene-carboxaldehyde (PyCHO). PA6/ABS blends with MWNTs have shown a phase morphology transition from 'matrix-dispersed droplet' type to 'co-continuous' type as a function of melt-mixing time with the exception of 40/60 PA6/ABS blend with PyCHO-modified MWNTs. Non-isothermal crystallization studies revealed the heterogeneous nucleating action of MWNTs through the presence of double crystallization exothermic peaks (at ~192°C and >200°C) while pure PA6 shows bulk crystallization peak at ~192°C. 40/60 and 60/40 (wt/wt) PA6/ABS blends with 5 wt% unmodified MWNTs exhibited electrical conductivity values of ~3.9 x [10.sup.-11] S/cm and ~4.36 x [10.sup.-6] S/cm, respectively. A significant enhancement in electrical conductivity was observed with Na-AHA and PyCHO-modified MWNTs (order of [~10.sup.-6] and [~10.sup.-4] S/cm, respectively)., INTRODUCTION The incorporation of multi wall carbon nanotubes (MWNTs) in the polymer blends via melt-mixing is a well-established strategy to make electrically conducting composites [1-7]. The influence of interfacial tension, [...]

State of dispersion of purified multiwall carbon nano-tubes (p-MWNT) in the presence of neutralized MWNT (n-MWNT) in aqueous solution was assessed through UV-Visible spectroscopy, dynamic light scattering measurements, and solution experiments. Raman spectroscopic analysis revealed that debundling of p-MWNT in the presence of p-MWNT in aqueous solution was persistent even in the solid mixture, which was supported by transmission electron microscopic analysis. The proposed mechanism behind the improved dispersion of p-MWNT in the presence of n-MWNT in aqueous solution has been based on the electrostatic charge repulsion between negatively charged n-MWNT. The state of dispersion of p-MWNT in the presence of n-MWNT in 45/55 polypropylene/acrylonitrile butadiene styrene (PP/ABS) blends was assessed through Raman spectroscopic analysis, bulk electrical conductivity measurements, solution experiment, and crystallization studies. Raman spectroscopic analysis indicated that the state of dispersion of MWNT was improved with increasing n-MWNT content of the mixture. This strategy led to a remarkable increase in the bulk electrical conductivity of 45/55 PP/ABS blends at 3 wt% MWNT content and was strongly dependent on the concentration of n-MWNT in the mixture. Differential scanning calorimetric measurements along with solution experiments revealed the subsequent migration of MWNT from the PP phase to the ABS phase in the blends during melt-mixing in the presence of higher fraction of n-MWNT. POLYM. ENG. SCI., 51:1891-1905, 2011. ©2011 Society of Plastics Engineers, INTRODUCTION The outstanding properties associated with carbon nanotubes (CNT) attract them toward its use in polymer/CNT composites and are exploited for various potential applications viz. electrostatic discharge materials, conducting composites, [...]

Melt‐mixing was employed to prepare multiwall carbon nanotubes (MWCNTs) based polypropylene (PP) composites, wherein MWCNTs vary in "agglomerate" size and also in terms of "agglomerate" structure. Morphological analysis revealed MWCNTs‐D type exhibits bigger "agglomerate" size with compact "agglomerate" structure with respect to MWCNTs‐N type, that show smaller "agglomerate" size with porous "agglomerate" structure in the corresponding PP/MWCNTs composites. Melt‐rheological analysis showed a rheological percolation threshold of 2 to 3 wt% in PP/MWCNTs‐N composites, whereas PP/MWCNTs‐D composites exhibited a rheological percolation of 3 to 4 wt% of MWCNTs. AC electrical conductivity measurements exhibited an electrical percolation threshold of 0.5 to 1 wt% of MWCNTs‐N, whereas MWCNTs‐D type depicts an electrical percolation threshold of 2 to 3 wt% of MWCNTs in the respective composites. Further, an unique dispersant; Li salt of 6‐aminohexanoic acid (Li‐AHA) was utilized to "deagglomerate" MWCNTs. Moreover, PP‐g‐MA was also utilized in combination with Li‐AHA encapsulated MWCNTs in the corresponding composites. An extensive morphological, rheological, and electrical conductivity measurements suggested the influence of Li‐AHA encapsulated MWCNTs and PP‐g‐MA on the transformation of MWCNTs "agglomerate" into "individualized" and smaller MWCNTs "agglomerate" in PP/MWCNTs composites. [ABSTRACT FROM AUTHOR]

Brownian dynamics simulations are used to study the adsorption of an isolated polyelectrolyte molecule onto an oppositely charged flat surface in the absence and the presence of an imposed shear flow. The polyelectrolyte is modeled as a freely jointed bead-rod chain where excluded volume interactions are incorporated by using a hard-sphere potential. The total charge along the backbone is distributed uniformly among all the beads, and the beads are allowed to interact with one another and the charged surface through screened Coulombic interactions. The simulations are performed by placing the molecule a fixed distance above the surface, and the adsorption behavior is then studied as a function of screening length. In the absence of an imposed flow, the chain is found to lie flat and extended on the adsorbing surface in the limit of weak screening, whereas in the limit of strong screening it desorbs from the surface and attains free-solution behavior. For intermediate screening, only a small portion of the chain adsorbs and it becomes highly extended in the direction normal to the surface. An imposed shear flow tends to orient the chain in the direction of flow and also leads to increased contact of the chain with the surface. [ABSTRACT FROM AUTHOR]

We use two-dimensional Brownian dynamics simulations to study the electrophoresis of a bead-rod chain through a narrow slit. A constant electric field is assumed to act inside and outside of the slit, and each bead on the chain is assigned a constant uniform charge. We calculate the dependence of the polymer transit velocity on chain length, slit dimensions (width-to-length ratio), and electric-field strength. For sufficiently narrow slits, the transit velocity increases nonlinearly with the applied field for low-field strengths, whereas it increases linearly for high-field strengths. In the low-field strength region and for sufficiently narrow slits, the transit velocity decreases rapidly for small chain lengths and then decreases slowly beyond a critical chain length. As the slit width increases, the transit velocity decreases with chain length in more continuous manner, and for sufficiently large slits the transit velocity becomes independent of chain length as expected. Distributions of the chain end-to-end distances and the translocation times depend strongly on the relative size of the chain to the slit. These results show the sensitivity of the transit velocity vs chain length relationship to the slit dimensions and applied electric-field strength, and suggest that there may be an optimal slit width for a given field strength and vice versa. The results may be useful for microfluidic separations and for understanding the motion of biological polymers through narrow constrictions.© 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Brownian dynamics simulations are performed to study the stretching and transport of flexible polymers in complex electroosmotic flows. The flows are generated by prescribing a spatially periodic charge distribution on the walls of a parallel-plate channel and applying an electric field parallel to the direction of charge modulation. The polymer molecules are modeled as bead-spring chains and the model parameters are chosen to be representative of DNA. Simulations are performed for the cases of uncharged and charged polymers. For the uncharged case, it is found that the stagnation point in the center of the flow is not effective at stretching the polymers due to the fact that the flow has an inhomogeneous velocity gradient. It is observed that polymers tend to become trapped in the recirculation rolls near that stagnation point, and that the amount of stretching that does occur is directly proportional to the amount of time that the polymer spends near the stagnation point at the wall. For the charged case, it is found that the trapping persists below a critical charge density, but that above this threshold the polymer escapes from the rolls. Our observations suggest that while these complex flows may not be useful for stretching polymers far away from the channel walls, they may be useful for localizing the position of Brownian particles in microfluidic devices. In addition, they illustrate the rich dynamics that arise when polymers are placed in flows with inhomogeneous velocity gradients and when electroosmotic flow competes with electrophoresis. [ABSTRACT FROM AUTHOR]

A Langevin dynamics simulation study is presented for enzyme-modulated translocation of a single-stranded DNA molecule through a cylindrical nanopore. The studies have shown that the translocation velocity is directly proportional to the rate of DNA release.