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The effect of water absorption on the dielectric properties of epoxy nanocomposites
In this research, the influence of water absorption on the dielectric properties of epoxy resin and epoxy micro-composites and nano-composites filled with silica has been studied. Nanocomposites were found to absorb significantly more water than unfilled epoxy. However, the microcomposite absorbed less water than unfilled epoxy: corresponding to reduced proportion of the epoxy in this composite. The glass transition temperatures of all the samples were measured by both differential scanning calorimetry and dielectric spectroscopy. The Tg decreased as the water absorption increased and, in all cases, corresponded to a drop of approximately 20K as the humidity was increased from 0% to 100%. This implied that for all the samples, the amount of water in the resin component of the composites was almost identical. It was concluded that the extra water found in the nanocomposites was located around the surface of the nanoparticles. This was confirmed by measuring the water uptake, and the swelling and density change, as a function of humidity as water was absorbed. The water shell model, originally proposed by Lewis and developed by Tanaka, has been further developed to explain low frequency dielectric spectroscopy results in which percolation of charge carriers through overlapping water shells was shown to occur. This has been discussed in terms of a percolation model. At 100% relative humidity, water is believed to surround the nanoparticles to a depth of approximately 5 monolayers. A second layer of water is proposed that is dispersed by sufficiently concentrated to be conductive; this may extend for approximately 25 nm. If all the water had existed in a single layer surrounding a nanoparticle, this layer would have been approximately 3 to 4 nm thick at 100%. This "characteristic thickness" of water surrounding a given size of nanoparticle appeared to be independent of the concentration of nanoparticles but approximately proportional to water uptake. Filler particles that have surfaces that are functionalized to be hydrophobic considerably reduce the amount of water absorbed in nanocomposites under the same conditions of humidity. Comments are made on the possible effect on electrical aging
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A "water shell" model for the dielectric properties of hydrated silica-filled epoxy nano-composites
The electrical properties of epoxy resin have been studied as a function of hydration. The epoxy was studied in an un-filled state, filled with 40 µm SiO2 particles, and filled with 50 nm SiO2 particles. The relative humidity was controlled by saturated salt solutions at ambient temperatures from 298-353 K. Measurements were made using dielectric spectroscopy over the frequency range 10-3-105 Hz. The hydration isotherm (i.e. the mass uptake of water) was established by measuring the mass as a function of relative humidity (RH). It was found that the nanocomposites absorb up to 60% more water than the unfilled and micro-filled epoxies. Dielectric spectroscopy shows different conduction and quasi-DC behaviours at very low frequencies (<10-2 Hz) with activation energies dependent on the hydration and temperature. These observations have led to the development of a “water shell” model to explain this phenomenon
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Influence of absorbed water on the dielectric properties and glass-transition temperature of silica-filled epoxy nanocomposites
Work on dielectric spectroscopy of epoxy resin filled with nano-SiO2 at different relative humidities and temperatures is reported. Above the glass-transition temperature (Tg), dc-like imperfect charge transport (QDC or LFD) dominates the low frequency dielectric spectrum. Another mid-frequency relaxation process was found in the non-dried composites. Water also induces glass-transition temperature decreases, which can be measured both by dielectric spectroscopy and DSC. Both theory and experiment demonstrated that a higher water content could exist in nanocomposites than unfilled epoxy suggesting a bigger free volume when nanostructured. In our system, the hydrophilic surface of silica is likely to cause water to surround and lead to delamination of the epoxy from SiO2. This is a potential mechanical and dielectric weakness in the nanocomposites, which may lead to an ageing phenomenon. Hydrophobic surface group may reduce the water adsorption in nanocomposites
The Impact of the 2006 Massachusetts Healthcare Reform on Spine Surgery Patient Payer-Mix and Age
OBJECTIVE
Several similarities exist between the Massachusetts health care reform law of 2006 and the Affordable Care Act (ACA). The authors’ prior neurosurgical research showed a decrease in uninsured surgeries without a significant change in surgical volume after the Massachusetts reform. An analysis of the payer-mix status and the age of spine surgery patients, before and after the policy, should provide insight into the future impact of the ACA on spine surgery in the US.
METHODS
Using the Massachusetts State Inpatient Database and spine ICD-9-CM procedure codes, the authors obtained demographic information on patients undergoing spine surgery between 2001 and 2012. Payer-mix status was assigned as Medicare, Medicaid, private insurance, uninsured, or other, which included government-funded programs and workers’ compensation. A comparison of the payer-mix status and patient age, both before and after the policy, was performed. The New York State data were used as a control.
RESULTS
The authors analyzed 81,821 spine surgeries performed in Massachusetts and 248,757 in New York. After 2008, there was a decrease in uninsured and private insurance spine surgeries, with a subsequent increase in the Medicare and “other” categories for Massachusetts. Medicaid case numbers did not change. This correlated to an increase in surgeries performed in the age group of patients 65–84 years old, with a decrease in surgeries for those 18–44 years old. New York showed an increase in all insurance categories and all adult age groups.
CONCLUSIONS
After the Massachusetts reform, spine surgery decreased in private insurance and uninsured categories, with the majority of these surgeries transitioning to Medicare. Moreover, individuals who were younger than 65 years did not show an increase in spine surgeries, despite having greater access to health insurance. In a health care system that requires insurance, the decrease in private insurance is primarily due to an increasing elderly population. The Massachusetts model continues to show that this type of policy is not causing extreme shifts in the payer mix, and suggests that spine surgery will continue to thrive in the current US health care system
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