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Rao V.S.,CIPET | Samui A.B.,Naval Materials Research Laboratory
Journal of Polymer Science, Part A: Polymer Chemistry | Year: 2011

A series of linear and hyperbranched polyester epoxies, with varied structural parameters such as kinked structure and different dendritic architectures, were synthesized by A2 + B2, A2 + B3, A3 + B2, and A3 + B 3 approaches. The structures of synthesized monomers and polymers were confirmed by Fourier transform infrared, 1H NMR, and 13C NMR spectroscopic techniques. The effect of varied structural parameters on phase behavior and photoresponsive properties was investigated by using differential scanning calorimeter, thermal optical polarized microscope, UV-visible spectroscopy, photoviscosity, and refractive index studies. The transition temperatures of hyperbranched polymers were higher than that of the corresponding linear analogues. All the polymers showed nematic phase (nematic droplets) over a broad temperature range. The effect of kinked structural unit on photoresponsive property is less in both linear and hyperbranched architectures. Although the effect of architectural nature is highly considerable within the hyperbranched architectures, the polymer (HPE-33) synthesized by A3 + B3 approach showed highest rate of photocrosslinking, followed by HPE-I 32; HPE-T 32, and HPE-23, which were synthesized by A3 + B2 and A2 + B3 approaches, respectively. The findings in photoresponsive properties were further supported by molecular modeling studies. Substantial variation of refractive index (0.015-0.024) indicates that these polymers could be used for optical recording. © 2011 Wiley Periodicals, Inc.

Ratna D.,Naval Materials Research Laboratory | Simon G.P.,Monash University
Journal of Applied Polymer Science | Year: 2010

In this work, the phase separation of an epoxy- functionalized hyperbranched polymer (HBP) in a blend with a conventional epoxy resin is examined. Morphology development with the advancement of curing reaction was investigated by hot-stage polarized optical microscope, where it was found that HBP is miscible in epoxy resin solvent at 120°C and undergoes phase separation during the curing reaction, leading to a two-phase microstructure which maintains a dispersed morphology up to 20 wt % HBP. The degree of phase separation and morphology were also investigated using differential scanning calorimetry, and the resultant microstructure was confirmed by atomic force microscopy. The epoxy/HBP blends were characterized by positron annihilation lifetime spectroscopy for their free volume characteristics where behavior typical of miscible blends was seen, likely due to chemical bonding between the two phases. © 2010 Wiley Periodicals, Inc.

Ratna D.,Naval Materials Research Laboratory | Karger-Kocsis J.,Budapest University of Technology and Economics
Polymer | Year: 2011

Shape memory semi-interpenetrating polymer networks (semi-IPNs) composed of crystalline poly (ethylene oxide) (PEO) and crosslinked poly (methyl methacrylate) (x-PMMA) have been investigated. The selected compositions show shape memory property with a reasonable fast recovery (recovery time ∼1 min) and shape recovery ratio of 99%. Effects of composition (x-PMMA/PEO = 80/20...60/40) and crosslinker (triethyleneglycol dimethacrylate) concentration (up to 6 wt.%) on the creep property were also studied. The recovery time of the semi-IPNs increased and the creep compliance decreased with increasing crosslinker concentration. The network structure containing PEO crystal was characterized by scanning electron microscopy (SEM). Differential scanning calorimetry (DSC) indicated that the PEO, present confined in the semi-IPN, melts at a lower temperature compared to the pure PEO. Dynamic mechanical analysis (DMA) showed a decrease in the glass transition (Tg) of the semi-IPN due to the phase mixing of amorphous PEO and PMMA. Both the glassy and rubbery moduli (Eg and Er, respectively) were lower for the semi-IPNs than for the x-PMMA network. On the other hand, the E g/Er ratio was markedly increased for the semi-IPNs supporting an easy shaping along with a good shape fixing. © 2010 Elsevier Ltd. All rights reserved.

Pandey S.,Naval Materials Research Laboratory | Rath S.K.,Naval Materials Research Laboratory | Samui A.B.,Naval Materials Research Laboratory
Industrial and Engineering Chemistry Research | Year: 2012

A series of B 3 core-terminated highly branched siloxane-urethane polymers was synthesized through the A 2 + B 3 route. Isophorone diisocyanate (IPDI)-terminated polydimethylsiloxane (PDMS) was used as the A 2 unit and triethanol amine as the B 3 core. Size exclusion chromatography (SEC) studies revealed decreasing number-average molecular weights for the branched polymers and increasing tendency toward lower molecular weight species formation with increased proportion of B 3 core in the branched polymers. The degree of branching and fraction of dendritic units, evaluated from 1H NMR, increased monotonically with increasing B 3 core in the branched polymers. Cross-linked networks of the highly branched polymers were prepared by reaction of the terminal hydroxyl groups with tetraethoxysilane (TEOS) at room temperature. The sol fractions obtained for the networks from solvent extraction studies were consistent with the non-network-forming low molecular weight fractions obtained from the deconvoluted SEC traces. The solubility parameter, Flory-Huggins interaction parameter, and crosslink density of the networks were evaluated from swelling studies. FTIR spectroscopy was used to evaluate the degree of hydrogen bonding of the branched networks. The thermomechanical properties of the networks were evaluated by stress-strain measurements and dynamic mechanical analysis, and the results were correlated with the structural parameters, such as degree of branching, extent of hydrogen bonding, and cross-link density. © 2012 American Chemical Society.

Jagtap S.B.,Naval Materials Research Laboratory | Ratna D.,Naval Materials Research Laboratory
Express Polymer Letters | Year: 2013

Epoxy/multiwall carbon nanotubes (MWCNT) composites were prepared using sodium salt of 6-aminohexanoic acid (SAHA) modified MWCNT and its effect properties of related composites were investigated. The composite prepared using a polar solvent, tetrahydrofuran exhibits better mechanical properties compared to those prepared using less polar solvent and without using solvent. The tensile properties and dynamic storage modulus was found to be increased as a result of modification of MWCNT with SAHA. This improvement in the tensile properties and dynamic mechanical properties of epoxy/MWCNT composite is a combined effect of cation-! interaction and chemical bonding. Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy were used to explain cation-! interaction between SAHA with MWCNT and chemical bonding of SAHA with epoxy resin. The effect of modification of MWCNT on morphology of a nanocomposite was confirmed by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The present approach does not disturb the! electron clouds of MWCNT as opposed to chemical functionalization strategy. © BME-PT.

Somaiah B.,Naval Materials Research Laboratory | Agarwal V.,Indian Institute of Technology Bombay
IEEE Transactions on Power Electronics | Year: 2013

Extraction of maximum power from a fuel cell (FC) power source (PS) is essential for its optimum and economical utilization. However, the maximum extractable power from an FCPS varies dynamically during the fuel cell operation for varying load current requirements as the system parameters are also changing. One such example is the use of an FCPS in vehicular applications, where the power requirement varies dynamically during the drive cycle. This makes maximum power extraction a challenging task. As the load varies, the equivalent resistance $(R-{\rm EFC})$ appearing across the FC varies too. The maximum power point (MPP) appears prominently on the power versus $R\rm EFC curve. This paper presents a novel MPP tracking (MPPT) scheme using nonlinear curve fitting and recursive least-squares estimation (RLSE). A current controlled Cuk converter is used due to its low ripple feature. RLSE determines the MPP online, which is used as a reference for the control of the Cuk converter. The performance of this MPPT scheme for a typical vehicular drive cycle is compared with the popular Perturb and Observe and Incremental conductance methods. All the analytical and simulation results are included. Experimental results are also presented to validate the proposed scheme for the drive cycle load profile. © 1986-2012 IEEE.

Jagtap S.B.,Naval Materials Research Laboratory | Kushwaha R.K.,Naval Materials Research Laboratory | Ratna D.,Naval Materials Research Laboratory
Journal of Applied Polymer Science | Year: 2013

An investigation was carried out to improve the dispersion of multiwall carbon nanotubes (MWCNTs) in the poly(ethylene oxide) (PEO) matrix using a half-neutralized sodium salt of dicarboxylic acid with various number of carbon atoms. The effects of nature of various modifiers on mechanical properties of PEO were investigated. Among various dicarboxylic acid salts, half neutralized adipic acid (HNAA) is found to be highly effective in achieving the improvement in mechanical and dynamic mechanical properties due to improved dispersion of MWCNT in the PEO matrix. The physical interaction of HNAA with MWCNT (cation-π interaction) has been established using Fourier transform infrared and Raman spectroscopic analyses. Scanning electron microscope and transmission electron microscope (TEM) studies clearly indicate the improvement in the level of dispersion of MWCNT due to the addition of HNAA. Crystallization behavior of the PEO/MWCNT composites made with unmodified and modified MWCNT were studied by differential scanning colorimetry. Our approach is a noncovalent one and does not destroy the π-electron clouds of MWCNT as opposed to chemical functionalization techniques and particularly attractive because of possibility of preserving the structural integrity of nanotubes as well as improved phase adhesion with polymer matrix. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2013 Copyright © 2012 Wiley Periodicals, Inc.

Jagtap S.B.,Naval Materials Research Laboratory | Ratna D.,Naval Materials Research Laboratory
Journal of Applied Polymer Science | Year: 2013

We describe a simple and novel method for dispersing multiwalled carbon nanotubes (MWCNTs) in a flexible epoxy matrix. The MWCNTs were modified with half-neutralized dicarboxylic acids having different numbers of carbon atoms. The modified MWCNTs were prereacted with epoxy in the presence of triphenylphosphine. The dispersion of the MWCNTs and the enhancement in the tensile properties were found to be better for composites prepared with a solvent. Among the half-neutralized dicarboxylic acids used, half-neutralized adipic acid (HNAA) exhibited the best performance. Scanning electron microscopy and transmission electron microscopy studies clearly indicated an improvement in the level of dispersion of the MWCNTs with the addition of the modifier. The good dispersion of the MWCNTs and the resulting improvement in their properties were attributed to the cation-π interactions (the cation of HNAA and the π-electron clouds of the MWCNTs) between the HNAA and MWCNTs and the chemical bonding of COOH groups of HNAA and the epoxy resin. The cation-π interaction and chemical bonding was assessed with Fourier transform infrared spectroscopy and Raman spectroscopy. This approach did not destroy the π-electron clouds of the MWCNTs in contrast to a chemical functionalization strategy. © 2013 Wiley Periodicals, Inc.

Boddu S.,Naval Materials Research Laboratory | Agarwal V.,Indian Institute of Technology Bombay
IEEE Transactions on Power Electronics | Year: 2014

Fuel Cell stacks, due to their low voltage output, are usually connected in series to achieve higher voltages in typical high-power applications. Power from individual stacks varies dynamically during their operation because of variation in operating parameters such as temperature, humidity, flow rates, ageing, etc., which results in mismatch of electrical performance of the stacks. When one of the series-connected stacks is under performing, the current from the branch is affected leading to reduced power. This paper proposes a novel scheme, to extract maximum power from series-connected stacks by current compensation which also improves reactants economy. A minimal power-processing (compensating) power converter is used with each stack to achieve maximum power extraction. Since activation of compensation is not effective during low-power operation, an algorithm is developed to implement compensation only for the performance improvement zone. Compensating converters are designed using interleaved fly-back topology, for reduced ripple, with DSP TMS320F2812.The efficacy of the technique is analyzed using Simulink. Experimental results are presented to validate the proposed scheme. © 2014 IEEE.

Kiran D.V.,Indian Institute of Technology Bombay | Basu B.,Naval Materials Research Laboratory | De A.,Indian Institute of Technology Bombay
Journal of Materials Processing Technology | Year: 2012

Two-wire tandem submerged arc welding process involves simultaneous depositions from two electrode wires with the leading wire usually connected to a DC power source and the trailing wire connected to a pulsed AC power source. The weld bead profile and mechanical properties in the tandem submerged welding are significantly affected by the leading and trailing wire current transients and the welding speed. We present here a detailed experimental study on the influence of leading wire current, trailing wire current pulses, and welding speed on the weld bead dimensions and mechanical properties in single-pass tandem submerged welding of a typical HSLA steel. It is realized that the weld bead penetration is primarily influenced by the leading wire current while the weld bead width and the reinforcement height are sensitive to the trailing wire current pulses. Greater magnitude of trailing wire current pulses and shorter negative pulse duration increase the weld pool volume leading to reduced cooling rate and poor mechanical properties as the formation of the strengthening phases like acicular ferrite is inhibited. In contrast, increase in welding speed reduces the rate of heat input thereby enhancing the cooling rate and the weld bead mechanical properties. A set of empirical relations are developed to estimate the weld bead dimensions and mechanical properties as function of the welding conditions. The predictions from the empirical relations and the corresponding measured results are observed to be in fair agreement. © 2012 Elsevier B.V.

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