Rajeeb Kumar Jena

Facile growth of heparin-controlled porous polyaniline nanofiber networks and their application in supercapacitors

Herein, we present the synthesis of porous polyaniline (PANI) nanofiber networks by a chemical oxidative polymerization method using a biomolecule, heparin. Long uniform nanofibers were formed when the weight ratio of heparin to aniline was 0.25 (PANIH-0.25), where the average nanofiber diameter was about 80–110 nm. No such PANI nanofibers were formed at other weight ratios. A comparison of the morphology of PANIH-0.25 was made with the pure PANI which was synthesized without any template. We further studied the potential of PANIH-0.25 as a nanostructured electrode material for the supercapacitor applications. The PANIH-0.25 nanofiber electrode yielded a six-fold improvement in specific capacitance when compared with pure PANI. The PANIH-0.25 electrode exhibited a specific capacitance of 732.18 ± 24.1 F g−1 and a longer cycle life with the capacitance retention of 72.28% after 1000 cycles. The observed capacitance was elucidated and justified based on theoretical considerations.

Development of 3D Urchin-Shaped Coaxial Manganese Dioxide@Polyaniline (MnO2@PANI) Composite and Self-Assembled 3D Pillared Graphene Foam for Asymmetric All-Solid-State Flexible Supercapacitor Application

We have fabricated high-energy-density all-solid-state flexible asymmetric supercapacitor by using a facile novel 3D hollow urchin-shaped coaxial manganese dioxide@polyaniline (MnO2@PANI) composite as positive electrode and 3D graphene foam (GF) as negative electrode materials with polyvinyl alcohol (PVA)/KOH gel electrolyte. The coaxial MnO2@PANI composite was fabricated by hydrothermal route followed by oxidation without use of an external oxidant. The formation mechanism of the 3D hollow MnO2@PANI composite occurs first by nucleation and growth of the MnO2 crystal species via dissolution-recrystallization and oriented attachment mechanisms followed by the oxidation of aniline monomers on the MnO2 crystalline template. The self-assembled 3D graphene block was synthesized by hydrothermal route using vitamin C as a reducing agent. The microstructures of the composites are analyzed by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. The morphology is characterized by field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM), which clearly showed the formation of urchin-shaped coaxial MnO2@PANI composite. The electrochemical studies are explored by cyclic voltammetry, electrochemical impedance spectrometry, and cyclic charge-discharge tests. The symmetric all-solid-state flexible MnO2@PANI//MnO2@PANI and GF//GF supercapacitors exhibit the specific capacitance of 129.2 and 82.1 F g-1 at 0.5 A/g current density, respectively. The solid-state asymmetric supercapacitor shows higher energy density (37 Wh kg-1) with respect to the solid-state symmetric supercapacitors MnO2@PANI//MnO2@PANI and GF//GF, where the obtained energy density are found to be 17.9 and 11.4 Wh kg-1, respectively, at 0.5 A/g current density. Surprisingly, the asymmetric supercapacitor shows a high energy density of 22.3 Wh kg-1 at a high current density of 5 A g-1. The solid-state asymmetric supercapacitor shows a good cyclic stability in which ∼11% capacitance loss was observed after 5000 cycles.

A novel high performance bismaleimide/diallyl bisphenol A (BMI/DBA)–epoxy interpenetrating network resin for rigid riser application

A new class of interpenetrating network (IPN) resin system was developed by mixing tetrafunctional epoxy resin (TGDDM) with diallyl bisphenol A (DBA) modified bismaleimide (BMI) for rigid riser applications at high temperatures of more than 280 °C. The curing kinetics of the resins were assessed by differential scanning calorimetry (DSC). Epoxy resin with DBA modified BMI (BMI/DBA–epoxy IPN) showed low activation energy which is due to the autocatalytic effect of DBA modified BMI in the curing process. The thermal stability of the cured resins as evaluated using thermogravimetric analysis (TGA) showed that they were stable up to 325 °C. Dynamic mechanical analysis (DMA) showed that the BMI/DBA–epoxy IPN resin had a glass transition temperature (Tg) equal to that of neat epoxy of around 280 °C. The incorporation of DBA modified BMI into the epoxy resin enhanced the mechanical properties such as tensile, flexural and impact strength by 25%, 30% and 45%, respectively compared to neat epoxy resin. Furthermore, the viscosity of the IPN was 0.3–1 Pa s which is within the filament winding range required for rigid risers. Prototype rigid risers were made using the BMI/DBA–epoxy IPN resin as matrix and carbon fiber as the reinforcement using the filament winding technique. The approach presented here represents a good approach to developing high performance high thermal stability resins for rigid risers in oil field applications.

Development of a 3D graphene aerogel and 3D porous graphene/MnO2@polyaniline hybrid film for all-solid-state flexible asymmetric supercapacitors

There is an increasing demand for safe, environmentally benign energy storage devices in portable electronic appliances, wearable gadgets, flexible displays, and other personal multimedia devices. In this study, we have fabricated an all-solid-state flexible asymmetric supercapacitor using a novel 3D porous reduced graphene oxide/manganese dioxide@polyaniline (RGO/MnO2@PANI) hybrid film as the positive electrode and a self-assembled 3D pillared graphene aerogel as the negative electrode material with a polyvinyl alcohol/potassium hydroxide (PVA/KOH) gel electrolyte. The flexible composite film was synthesized by vacuum filtration of GO and a MnO2@PANI mixture followed by chemical reduction of the resulting film in a hydrothermal autoclave. The 3D graphene aerogel was synthesized by a hydrothermal route using a solution of the nonionic triblock copolymer Pluronic F-68 as a soft template and vitamin C as a reducing agent. Herein, the Pluronic copolymer played dual roles: first, it enabled the effective dispersion of graphene oxide in water, and second, it assisted the formation of a stable 3D pillared hydrogel assembly. The RGO/MnO2@PANI-based symmetric supercapacitor shows a high energy density of 18.33 W h kg−1 at a power density of 0.388 kW kg−1. An asymmetric supercapacitor (graphene aerogel//RGO/MnO2@PANI), which was fabricated by optimizing the individual electrode materials, exhibited a very high energy density of 38.12 W h kg−1 at a power density of 1.191 kW kg−1 utilizing a large potential window of 1.5 V. Moreover, 3 cells connected in series successfully lit up a red LED for 45 s and displayed similar performance under bending conditions.

Rheological (visco-elastic behaviour) analysis of cyclic olefin copolymers with application to hot embossing for microfabrication

Transparent, amorphous cyclic olefin copolymers (COCs) have been frequently used for the fabrication of microfluidic devices using a hot embossing technique for numerous applications. In hot embossing, the polymer is deformed near its glass transition temperature (Tg), i.e. between Tg and Tg + 60 °C where the viscoelastic properties of the material are dominant. The proper characterization of the viscoelastic properties is of interest as this can lead to a better understanding of polymer flow behaviour during microfabrication. Furthermore, the ability to model its rheological behaviour will enable the prediction of the optimal hot embossing processing parameters. We performed small amplitude oscillatory shear experiments on four grades of COCs, TOPAS-8007, TOPAS-5013, TOPAS-6015 and TOPAS-6017, in order to characterize their flow behaviour. The experiments were conducted within the frequency range from 0.01 to 500 Hz at between Tg + 20 and Tg + 60 °C. The flow properties could be represented using a generalized Maxwell viscoelastic constitutive model with Williams–Landel–Ferry-type temperature dependence. Good fit of the experimental data was obtained over a wide range of temperatures. The model could be coupled with ABAQUS finite element software to predict the optimal conditions for fabricating a capillary electrophoresis micro-chip on a TOPAS-5013 substrate by hot embossing.

Effect of polymer orientation on pattern replication in a micro-hot embossing process: experiments and numerical simulation

The hot embossing process has been identified as a promising technique for fabricating micro- and nanostructures for polymer-based biological and chemical MEMS (micro electro mechanical systems). However, there has not been any investigation of the effect of polymer chain orientation in the base polymer substrate on replication during the micro-embossing process. Such effects could prove important because polymer chain orientation may develop in the polymer substrates during their production. In this investigation, it was observed that the degree and ease of microchannel replication are significantly influenced by the molecular chain orientation in injection-molded polymer substrates. Microchannels aligned along the flow direction of the polymer replicate easily compared to microchannels aligned across the flow direction of the polymer. The replication fidelity during hot embossing was investigated using a white-light confocal microscope. The anisotropy of injection-molded polymer plays a dominant role in the replication fidelity of microchannels, and the ability to model the anisotropic behavior of the material will enable understanding and prediction of the hot embossing process. Therefore, a material model that reflects the directionality was utilized to simulate the experimental embossing results obtained both along and across the flow direction of the polymer. By comparing experimental results with simulations, we observed that the model is reasonably realistic.

Effect of a CNT based composite micromold on the replication fidelity during the microfabrication of polymeric microfluidic devices

In recent years, polymer based microfluidic devices have become more widespread with the driving force being the development of inexpensive disposable analytical devices. Hot embossing and injection moulding are the two promising techniques that are widely used for micro-fabrication of polymeric substrates because identical devices can be mass produced using a master/mold. However, the major problem with these techniques is the time and expense needed to produce a stamping tool (mold) which can withstand the temperature and stress of the fabrication process over multiple cycles. To overcome this problem, we have developed an epoxy toughened nanocomposite mold material, which is relatively inexpensive compared to the commonly used metallic glass mold material. Our molds can be produced with ease, and are sufficiently durable to withstand multiple embossing cycles. Moreover, compared to other mold materials, very high aspect ratio microchannels can be replicated. We have characterized the morphological, physical and thermomechanical properties of this new nanocomposite mold material including its surface morphology, roughness and friction coefficient. Finally, the performance of the nanocomposite mold to fabricate microdevices using a cyclic olefin copolymer (COC) by the hot embossing and injection molding techniques was assessed. This mold material was found to be suitable for fabricating polymeric microdevices with high channel integrity.

Microfabrication (Hot-Embossing) of COC Microfluidic Devices: Effect of Norbornene Content on Replication Fidelity and Surface Modification by UV-Photografting

The fabrication of polymer based microfluidic devices using the hot embossing technique and their surface modification for easy fluid flow through the devices has been a growing field of research. During hot embossing, the replication fidelity on polymer substrate not only depends on the processing parameters such as temperature, pressure and time but also on their chemical structure which affects their thermo-dependent viscoelastic properties. For copolymers such as cyclic olefin copolymer (COC) which comprises ethylene and norbornene units, such properties depend on their relative ethylene and norbornene content. We report in this paper, a systematic study of replication fidelity and surface modification on COC polymer with varying norbornene content from 65 to 82 wt%. Replication fidelity which includes the surface morphology and cross-section profiles of the microchannel were characterized using SEM and Confocal microscope respectively. The modified surface was evaluated using Fourier transform infrared spectroscopy (FTIR spectroscopy) and water contact angle measurement. It was observed that in hot embossing, higher norbornene content contributed to good replication fidelity at identical experimental conditions. Furthermore, it was observed that with increase in norbornene content, the grafting efficiency decreases resulting in poor surface modification.