PVA/Chitosan/Silver Nanoparticles Electrospun Nanocomposites: Molecular Relaxations Investigated by Modern Broadband Dielectric Spectroscopy

In this study, we used broadband dielectric spectroscopy to analyze polymer nanofibers of poly(vinyl alcohol)/chitosan/silver nanoparticles. We also studied the effect of incorporating silver nanoparticles in the polymeric mat, on the chain motion dynamics and their interactions with chitosan nanofibers, and we calculated the activation energies of the sub-Tg relaxation processes. Results revealed the existence of two sub-Tg relaxations, the first gets activated at very low temperature (−90 °C) and accounts for motions of the side groups within the repeating unit such as –NH2, –OH, and –CH2OH in chitosan and poly(vinyl alcohol). The second process gets activated around −10 °C and it is thought to be related to the local main chain segments’ motions that are facilitated by fluctuations within the glycosidic bonds of chitosan. The activation energy for the chitosan/PVA/AgNPs nanocomposite nanofibers is much higher than that of the chitosan control film due to the presence of strong interactions between the amine groups and the silver nanoparticles. Kramers–Krönig integral transformation of the ε′′ vs. f spectra in the region of the chitosan Tg helped resolve this relaxation and displayed the progress of its maxima with increasing temperature in the regular manner

Polymer chain dynamics in epoxy based composites as investigated by broadband dielectric spectroscopy

Epoxy networks of the diglycidyl ether of bisphenol A (DGEBA) were prepared using 3,3′- and 4,4′-diaminodiphenyl sulfone isomer crosslinkers. Secondary relaxations and the glass transitions of resultant networks were probed using broadband dielectric spectroscopy (BDS). A sub-Tgγ relaxation peak for both networks shifts to higher frequencies (f) with increasing temperature in Arrhenius fashion, both processes having the same activation energy and being assigned to phenyl ring flipping in DGEBA chains. A β relaxation is assigned to local motions of dipoles that were created during crosslinking reactions. 4,4′-based networks exhibited higher Tg relative to 3,3′-based networks as per dynamic mechanical as well as BDS analyses. The Vogel–Fulcher–Tammann–Hesse equation fitted well to relaxation time vs. temperature data and comparison of Vogel temperatures suggests lower free volume per mass for the 3,3′-based network. The Kramers–Krönig transformation was used to directly calculate dc-free ɛ″ vs. f data from experimental ɛ′ vs. f data. Distribution of relaxation times (DRT) curves are bi-modal for the 3,3′-crosslinked resin suggesting large-scale microstructural heterogeneity as opposed to homogeneity for the 4,4′-based network whose DRT consists of a single peak.U.S. Office of Naval Research, Award N00014-07-1-1057 and fellowship support from the Department of Education Graduate Assistance in Areas of National Need Award P200A090066. Qatar University's Center for Advanced Materials' Start-Up grant

Energy Management for Mobile Devices through Computation Outsourcing within Pervasive Smart Spaces

Abstract — In this work we explore the opportunity Pervasive Spaces could provide as supplemental energy sources. We utilize the nature of pervasive smart spaces to outsource computation that would normally be performed on a mobile device to a surrogate server within the smart space. The decision to outsource a computation depends on whether its energy cost on the device is larger than the cost of communicating its data to the surrogate and receiving the results back. We propose an approach by which the outsourcing decision is made at runtime, while the intelligence that makes that decision is inserted at compile-time as logic that modifies the application code. The merit of our approach is that it is application-independent and requires minimal programmer energy awareness. We utilized a methodology from real-time systems to aid us in constructing the decision making logic. Additionally, we implemented a runtime support on top of Linux to facilitate for testing and experimenting with the client/server outsourcing approach. Our experimental validation and benchmarks shows significant energy saving on the mobile device, which validates our approach as a viable and novel approach to power saving and management for mobile devices

Broadband Dielectric Spectroscopic Studies of Molecular Motions in a Nafion® Membrane vs. Annealing Time and Temperature

The dynamics of macromolecular motions as coupled to proton migration in annealed Nafion® membranes were explored at temperatures above 100 °C using broadband dielectric spectroscopy. Loss permittivity vs. frequency spectra of both β and α relaxations showed increased relaxation times with annealing which was rationalized in terms of water de-sorption and diminished free volume. The α relaxation time increases with increased annealing temperature. A parameter N reflecting connectedness of charge hopping pathways was extracted and indicated that the conductivity network accumulates more charge traps, presumably due to free volume decrease, with increased annealing time. Conductivity increases with increased annealing time at all temperatures despite the decrease in N; this suggests a change in the nature of proton hopping on annealing at high temperatures to be between sulfonic acid groups rather than by hopping across H2OH+—OH2 bonds

PVA/Chitosan/Silver Nanoparticles Electrospun Nanocomposites: Molecular Relaxations Investigated by Modern Broadband Dielectric Spectroscopy

In this study, we used broadband dielectric spectroscopy to analyze polymer nanofibers of poly(vinyl alcohol)/chitosan/silver nanoparticles. We also studied the effect of incorporating silver nanoparticles in the polymeric mat, on the chain motion dynamics and their interactions with chitosan nanofibers, and we calculated the activation energies of the sub-Tg relaxation processes. Results revealed the existence of two sub-Tg relaxations, the first gets activated at very low temperature (−90 °C) and accounts for motions of the side groups within the repeating unit such as ⁻NH2, ⁻OH, and ⁻CH2OH in chitosan and poly(vinyl alcohol). The second process gets activated around −10 °C and it is thought to be related to the local main chain segments’ motions that are facilitated by fluctuations within the glycosidic bonds of chitosan. The activation energy for the chitosan/PVA/AgNPs nanocomposite nanofibers is much higher than that of the chitosan control film due to the presence of strong interactions between the amine groups and the silver nanoparticles. Kramers⁻Krönig integral transformation of the ε′′ vs. f spectra in the region of the chitosan Tg helped resolve this relaxation and displayed the progress of its maxima with increasing temperature in the regular manner

Analysis of Nafion® Fuel Cell Membrane Chemical Durability Using Broadband Dielectric Spectroscopy

Changes in macromolecular dynamics associated with the T g - related β relaxation of Nafion®, with chemical degradation, were investigated using broadband dielectric spectroscopy. Loss permittivity vs. frequency spectra showed peak splitting reflecting microstructural heterogeneity. Spectra were analyzed using the conductivity-modified Havriliak-Negami equation and the Kramers-Krönig integral transform, both of which showed broadening and peak splitting for degraded samples. Peak broadening and bi and tri-modal character of the spectra of degraded samples was also manifest in distributions of relaxation times which reflected a broadening of molecular weight distribution and related microstructural heterogeneity induced by chemical degradation. © 2011 ECS - The Electrochemical Society