Merlyn X. Pulikkathara

Medium density polyethylene composites with functionalized carbon nanotubes

A strong interface between the single-walled carbon nanotubes (SWNTs) and polymer matrix is necessary to achieve enhanced mechanical properties of composites. In this work a series of sidewall-functionalized SWNTs have been investigated in order to evaluate the effect of functionalization on SWNT aspect ratio and composite interfacial chemistry and their role on mechanical properties of a medium density polyethylene (MDPE) matrix. Fluorinated nanotubes (F-SWNTs) were used as precursors for subsequent sidewall functionalization with long chain alkyl groups to produce an F-SWNT- C(11)H(23) derivative. The latter was refluorinated to yield a new perfluorinated derivative, F-SWNT- C(11)F(x)H(y). The functionalized SWNTs as well as the pristine SWNTs were integrated into an MDPE matrix at a 1 wt% loading. The nanotubes and composite materials were characterized with FTIR, Raman spectroscopy, NMR, XPS, AFM, SEM, TGA, DSC and tensile tests. When incorporated into polyethylene, the new perfluorinated derivative, F-SWNT- C(11)F(x)H(y), yielded the highest tensile strength value among all nanotube/MDPE composite samples, showing a 52% enhancement in comparison with the neat MDPE. The 1 wt% SWNT/MDPE composite contained nanotubes with a larger aspect ratio but, due to a lack of interfacial chemistry, it resulted in less improvement in mechanical properties compared to the composites made with the fluorinated SWNT derivatives.

Neutron radiation effects on the nonlinear current–voltage characteristics of ilmenite-hematite ceramics

Ilmenite-hematite (IH) is a wide-band gap semiconducting material with a potential for low-voltage varistor applications. The nonlinear characteristics of this material were investigated before and after exposure to high-energy neutron radiation. The typical current–voltage characteristics of a varistor device are retained in all our samples subjected to neutron radiation. However, the effect is more pronounced on the device parameters like the nonlinear coefficient (α) and switching field (Es). The crystallinity of the ceramic remains unaffected after irradiation. Our observations confirm that IH-based varistor devices can perform satisfactorily even in radiation dominant environments such as in space and nuclear reactors.

Fluorinated Single Wall Nanotube/Polyethylene Composites for Multifunctional Radiation Protection

Abstract : Fluorinated Single Wall Nanotubes (f-SWNTs) have been processed in polyethylene by an incipient wetting technique to achieve a well dispersed nanocomposite for radiation protection. In some cases, samples were further processed using the rapid prototyping method of extrusion freeform fabrication. Composites were exposed to 40 MeV proton radiation with a flux of about 1.7x10(exp 7) protons/sq cm/sec to a total fluence of 3x10(exp 10) protons/sq cm. This exposure is consistent with a long-term space mission in low earth orbit. The samples were evaluated by means of Raman spectroscopy and thermogravimetric analysis (TGA). These results were compared to the unexposed composite and unfilled polymer samples. This study has focused on the stability of the nanotube composites when exposed to radiation and prior to hydrogen exposure. It was shown that the stability of the functional group is not constant with SWNTs produced by different processes and that radiation exposure is capable of defluorinating SWNTs in polyethylene.

Synthesis of carbon nanotube—nanodiamond hierarchical nanostructures and their polyurea nanocomposites

Hierarchical carbon nanostructures (HCNs) comprising functionalized nanodiamond particles (ND) covalently bonded to carbon nanotubes (CNTs) through urea or ethylene diamine linkers were synthesized using wet chemistry technique. Atomic force microscopy, transmission electron spectroscopy, and scanning electron spectroscopy reveal the pearl-necklace-like morphology of new HCNs with up to 50% of the CNT surface decorated by ND particles. Nanocomposites fabricated using polyuria/polyurethane hybrid polymer matrix and 0.2 wt.% of HCNs as a reinforcing filler show a 64% increase in tensile and elongation strength at break relatively to neat polymer.

Raman Study of Hydrogenated and Fluorinated Single-walled Carbon Nanotubes

Hydrogenated and fluorinated single‐walled carbon nanotubes (SWNT‐H and SWNT‐F) were investigated by micro‐Raman spectroscopy. The as‐prepared SWNT‐H exhibits giant structureless hot luminescence that screens the Raman peaks. The luminescence background, attributed to random binding of the hydrogen atoms, gradually decreases upon annealing of the sample. In contrast, the as‐prepared SWNT‐F shows no luminescence but the Raman peaks are rather broad. The annealing of SWNT‐F results in structural ordering due to fluorine removal.