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 3,3′-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 networks’ 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 3,3′-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 networks’ 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 University’s Center for Advanced Materials’ 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

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

Morphology and Magnetic Properties of Sulfonated Poly[styrene-(ethylene/butylene)-styrene]/Iron Oxide Composites

α-Fe2O3 structures were initiated in the sulfonated polystyrene block domains of poly[styrene–(ethylene/butylene)–styrene] (SEBS) block copolymers via a domain-targeted in-situ chemical precipitation method. The crystal structure of these particles was determined using wide-angle X-ray diffraction and selected area electron diffraction using a transmission electron microscope (TEM). TEM revealed that for less sulfonated SEBS (10 mole%), nanoparticles were aggregated with aggregate size range of 100–150 nm whereas for high sulfonation (16 and 20 mole% sSEBS) there were needle-like structures with length and width of 200–250 nm and 50 nm, respectively. Dynamic mechanical analyses suggest that initial iron oxide nanoparticle growth takes place in the sulfonated polystyrene block domains. The magnetic properties of these nanocomposites were probed with a superconducting quantum interference device magnetometer at 5 and 150 K as well as with an alternating gradient magnetometer at 300 K. The materials exhibited superparamagnetism at 150 K and 300 K and ferrimagnetism at 5 K

Simulation Study of Sulfonate Cluster Swelling in Ionomers

We have performed simulations to study how increasing humidity affects the structure of Nafion-like ionomers under conditions of low sulfonate concentration and low humidity. At the onset of membrane hydration, the clusters split into smaller parts. These subsequently swell, but then maintain constant the number of sulfonates per cluster. We find that the distribution of water in low-sulfonate membranes depends strongly on the sulfonate concentration. For a relatively low sulfonate concentration, nearly all the side-chain terminal groups are within cluster formations, and the average water loading per cluster matches the water content of membrane. However, for a relatively higher sulfonate concentration the water-to-sulfonate ratio becomes non-uniform. The clusters become wetter, while the inter-cluster bridges become drier. We note the formation of unusual shells of water-rich material that surround the sulfonate clusters.Comment: 24 pages, 15 figure

Open system effects on slow light and electromagnetically induced transparency

The coherence properties of a three-level $\Lambda$-system influenced by a Markovian environment are analyzed. A coherence vector formalism is used and a vector form of the Lindblad equation is derived. Together with decay channels from the upper state, open system channels acting on the subspace of the two lower states are investigated, i.e., depolarization, dephasing, and amplitude damping channels. We derive an analytic expression for the coherence vector and the concomitant optical susceptibility, and analyze how the different channels influence the optical response. This response depends non-trivially on the type of open system interaction present, and even gain can be obtained. We also present a geometrical visualization of the coherence vector as an aid to understand the system response.Comment: Several changes; journal reference adde

Phytophthora cactorum and Colletotrichum acutatum: Survival and Detection

Phytophthora cactorum and Colletotrichum acutatum are pathogens which are transported with plant material as latent infections and can also survive in soil and plant debris. Since the beginning of 1990’s P. cactorum caused losses in strawberries in Finland and increased culling of silver birch seedlings in forest nurseries because of stem lesions. In this study primers specific for the pathogen were designed, and in a simple PCR they gave an amplification product from pure cultures only when P. cactorum was used as a template. No cross reactions were found with other Phytophthoras in group I or other microbes. Inoculated strawberry plants gave also a clear band in PCR-analyses when the template concentration was diluted. However, amplification was not always reproducible with birch seedlings. With soil samples the best result was gained by a combination of baiting and isolation. C. acutatum is a quarantine pathogen on strawberry in the European Union and thus the infected plants are destroyed in Finland to avoid further spread of the pathogen. The pathogen has earlier been found to survive over one winter in infected plant debris and soil. In the survival test (2003-2005) done in this study, specific amplification products were obtained from test plants inoculated with artificially infected plant residues after 20 months of storage outdoors on soil surface. More positive results were achieved from bait plants grown in soil collected from the field where infected plants had been destroyed two years before, than from samples collected a year after the plant destruction