1,139 research outputs found
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 (â90 °C) and accounts for motions of the side groups within the repeating unit such as âNH2, âOH, and âCH2OH in chitosan and poly(vinyl alcohol). The second process gets activated around â10 °C and it is thought to be related to the local main chain segmentsâ 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. KramersâKrĂśnig integral transformation of the Îľâ˛â˛ 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
Molecular Scale Cure Rate Dependence of Thermoset Matrix Polymers
This manuscript demonstrates the molecular scale cure rate dependence of di-functional epoxide based thermoset polymers cured with amines. A series of cure heating ramp rates were used to determine the influence of ramp rate on the glass transition temperature (Tg) and sub-Tg transitions and the average free volume hole size in these systems. The networks were comprised of 3,3â˛-diaminodiphenyl sulfone (33DDS) and diglycidyl ether of bisphenol F (DGEBF) and were cured at ramp rates ranging from 0.5 to 20 °C/min. Differential scanning calorimetry (DSC) and NIR spectroscopy were used to explore the cure ramp rate dependence of the polymer network growth, whereas broadband dielectric spectroscopy (BDS) and free volume hole size measurements were used to interrogate networksâ molecular level structural variations upon curing at variable heating ramp rates. It was found that although the Tg of the polymer matrices was similar, the NIR and DSC measurements revealed a strong correlation for how these networks grow in relation to the cure heating ramp rate. The free volume analysis and BDS results for the cured samples suggest differences in the molecular architecture of the matrix polymers due to cure heating rate dependence
Molecular scale cure rate dependence of thermoset matrix polymers
This manuscript demonstrates the molecular scale cure rate dependence of di-functional epoxide based thermoset polymers cured with amines. A series of cure heating ramp rates were used to determine the influence of ramp rate on the glass transition temperature (Tg) and sub-Tg transitions and the average free volume hole size in these systems. The networks were comprised of 3,3â˛-diaminodiphenyl sulfone (33DDS) and diglycidyl ether of bisphenol F (DGEBF) and were cured at ramp rates ranging from 0.5 to 20°C/min. Differential scanning calorimetry (DSC) and NIR spectroscopy were used to explore the cure ramp rate dependence of the polymer network growth, whereas broadband dielectric spectroscopy (BDS) and free volume hole size measurements were used to interrogate networksâ molecular level structural variations upon curing at variable heating ramp rates. It was found that although the Tg of the polymer matrices was similar, the NIR and DSC measurements revealed a strong correlation for how these networks grow in relation to the cure heating ramp rate. The free volume analysis and BDS results for the cured samples suggest differences in the molecular architecture of the matrix polymers due to cure heating rate dependence.Qatar Universityâs Center for Advanced Materialsâ Start-Up grant. AFOSR Award Number FA9550-13-1-0103; and Dr. Gregg Bogucki and Dr. Stephen Heinz from Boeing Research and Technology for their kind financial support and collaboration
Glasgow Coma Scale score at intensive care unit discharge predicts the 1-year outcome of patients with severe traumatic brain injury
OBJECTIVE: To analyse the association between the Glasgow Coma Scale (GCS) score at intensive care unit (ICU) discharge and the 1-year outcome of patients with severe traumatic brain injury (TBI). DESIGN: Retrospective analysis of prospectively collected observational data. PATIENTS: Between 01/2001 and 12/2005, 13 European centres enrolled 1,172 patients with severe TBI. Data on accident, treatment and outcomes were collected. According to the GCS score at ICU discharge, survivors were classified into four groups: GCS scores 3â6, 7â9, 10â12 and 13â15. Using the Glasgow Outcome Scale (GOS), 1-year outcomes were classified as âfavourableâ (scores 5, 4) or âunfavourableâ (scores <4). Factors that may have contributed to outcomes were compared between groups and for favourable versus unfavourable outcomes within each group. MAIN RESULTS: Of the 538 patients analysed, 308 (57 %) had GCS scores 13â15, 101 (19 %) had scores 10â12, 46 (9 %) had scores 7â9 and 83 (15 %) had scores 3â6 at ICU discharge. Factors significantly associated with these GCS scores included age, severity of trauma, neurological status (GCS, pupils) at admission and patency of the basal cisterns on the first computed tomography (CT) scan. Favourable outcome was achieved in 74 % of all patients; the rates were significantly different between GCS groups (93, 83, 37 and 10 %, respectively). Within each of the GCS groups, significant differences regarding age and trauma severity were found between patients with favourable versus unfavourable outcomes; neurological status at admission and CT findings were not relevant. CONCLUSION: The GCS score at ICU discharge is a good predictor of 1-year outcome. Patients with a GCS score <10 at ICU discharge have a poor chance of favourable outcome
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 3,3â˛- and 4,4â˛-diaminodiphenyl sulfone isomer crosslinkers. Secondary relaxations and the glass transitions of resultant networks were probed using broadband dielectric spectroscopy (BDS). A sub-TgÎł 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. 4,4â˛-based networks exhibited higher Tg relative to 3,3â˛-based networks as per dynamic mechanical as well as BDS analyses. The VogelâFulcherâTammannâHesse equation fitted well to relaxation time vs. temperature data and comparison of Vogel temperatures suggests lower free volume per mass for the 3,3â˛-based network. The KramersâKrĂśnig transformation was used to directly calculate dc-free Éâł vs. f data from experimental ÉⲠvs. f data. Distribution of relaxation times (DRT) curves are bi-modal for the 3,3â˛-crosslinked resin suggesting large-scale microstructural heterogeneity as opposed to homogeneity for the 4,4â˛-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
Representativeness assessment of the pan-Arctic eddy-covariance site network and optimized future enhancements
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