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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. Source

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. Source

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. Source

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. Source

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. Source

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