Influence of stoichiometry on physical properties of clay epoxy nanocomposites

The reinforcement of epoxy resins by the dispersion of exfoliated clay platelets is discussed. The extent and nature of the reinforcement is dependent on the nature of the interaction of the resin with the clay surface. It is shown that varying the ratio of epoxy to hardener, as well as the chemical structure both of the epoxy resin and the hardener have a significant effect on whether or not reinforcement is observed. The creation of intercalated structures at high clay loadings does not significantly influence the extent of enhancement in those cases where there is a strong connectivity between the resin and the clay. However, in some cases the chemical structure of the resin appears to inhibit significant transfer of stress between the resin and the clay and virtually no enhancement in the physical Properties is observed

Low coefficient of thermal expansion of thermoset composite materials

Selection of the correct resin – filler combination is important in achieving materials which have a low coefficient of thermal expansion (CTE). A study of nanosilica-modified resin, in combination with silica fillers, allows low levels of CTE to be achieved. In this study, a novel curing agent, ytterbium triflate, is reported. This curing agent provides a stable catalyst system which can be used to create the viscous composite mixtures but has the facility of effective cure over a relatively narrow temperature range from 70 °C to 100 °C. A series of formulations were examined based on incorporation of fillers with nanoscale silica particles into either the pure resin or a resin which contained nanoscale functionalized silica particles. The filler incorporation leads to a significant increase in the glass transition temperature as determined by dynamic mechanical thermal analysis. The CTE was observed to be lower below T g than above it. Changing the nanosilica particle size and distribution produced significant changes in the values. However, the CTE scaled according to the total silica content; and the values were in general lower than those calculated theoretically. The use of a highly functional o-cresol epoxy novolac demonstrated how increasing functionality raised the T g and lowered the CTE, but the use of too high a post-cure temperature reversed this trend. Very good results were achieved using 3,4-epoxycyclohexylmethyl, 3,4 epoxycyclohexanecarboxylate, which has a low viscosity and allowed high levels of silica to be readily achieved. This article indicates how the adjustment of the epoxy selected to be used as the base material and the type of silica particles used allows the values of T g and CTE to be modified in a composite material

Influence of the epoxy structure on the physical properties of epoxy resin nanocomposites

The preparation and physical properties of a series of nanocomposites based on dispersions of Montmorillonite clays in thermoset epoxy resins are reported. The effects of the variation of the concentration of the clay and the influence of a change of the functionality of the epoxy compounds and the amine curing agent are reported. The effects of the method of dispersion of the clay are studied, and it was found that ultrasound provides an effective aid to dispersion of the clay platelets. In general, the addition of clay platelets leads to an increase in the glass-rubber transition, but in the case of a highly cross-linked system, the reverse effect was observed. The effects observed are discussed in the context of the way in which the chemical structure of the monomers influence the dispersion process and the structure of the final resin system

Investigating the ageing behavior of polysiloxane nanocomposites by degradative thermal analysis

The ageing behavior of novel polysiloxane nanocomposite elastomers is reported. A series of model polysiloxane nanocomposites has been prepared incorporating the montmorillonite nanoclay Cloisite 6A. The nanoclay dispersion has been characterized with X-ray Diffraction (XRD) and Transmission Electron Microscopy (TEM). TGA has been utilized to study the effects of ageing on the non-oxidative stability of the nanocomposite systems. The complex evolution of volatiles that occurs during ageing has been studied using Sub-Ambient Thermal Volatilization Analysis (SATVA). Results indicate that significant chemical changes take place within the nanocomposites upon ageing; acid catalyzed hydrolysis, chain backbiting and recombination reactions are re-structuring the polymer-filler network into a more thermodynamically stable form

A Neurological Wake-Up Test in the Neurointensive Care Unit: Pros and Cons

Traumatic brain injury (TBI) induces a marked systemic biochemical stress response with the release of several stress-related hormones including cortisol and the catecholamines. A major aim of using continuous sedation in the neurointensive care unit (NIC) unit is to attenuate the TBI-induced stress response via reduction of the cerebral energy metabolic demands. In the era of modern multimodality monitoring and neuroimaging for patients with severe TBI, what is the role for neurological evaluation, a neurological wake-up test (NWT), of patients on continuous sedation and mechanical ventilation? In particular, does the information obtained by the NWT outweigh the risk of inducing a substantial stress response? The additional use of NWTs in NIC is controversial and is not mentioned in any recent TBI guidelines. Although daily interruption of continuous sedation is suggested for patients in general intensive care, reasons for not using the NWT in NIC may be a fear of an NWT-induced stress response and uncertainty to the additional value of NWTs in patients monitored with multimodality monitoring and frequent neuroradiological examinations. A recent survey showed that use of NWT varies markedly in Scandinavians’ NIC units where half of the evaluated centres never use the NWT, whereas others use the NWT up to six times daily. In a series of studies characterising the NWT-induced stress response, the NWT was found to induce a significant increase in ICP and CPP in severe TBI patients on controlled ventilation. Additionally, the NWT caused an increase in adrenocorticotrophic (ACTH) hormone, catecholamine and cortisol levels. In the absolute majority of patients, the ICP and CPP changes were modest and transient and the absolute increases in stress hormone levels were small. However, the stress response was marked in a small subset of patients. These studies suggest that the NWT is safe in the majority of patients but that the test should be individualised and avoided in patients reacting with markedly increased ICP and/or decreased CPP. Although important clinical information may be obtained from the NWT, future studies need to evaluate the risk-benefit ratio of the NWT in TBI management

Critical ischemia time in a model of spinal cord section. A study performed on dogs

Vascular changes after acute spinal cord trauma are important factors that predispose quadriplegia, in most cases irreversible. Repair of the spinal blood flow helps the spinal cord recovery. The average time to arrive and perform surgery is 3 h in most cases. It is important to determine the critical ischemia time in order to offer better functional prognosis. A spinal cord section and vascular clamping of the spinal anterior artery at C5–C6 model was used to determine critical ischemia time. The objective was to establish a critical ischemia time in a model of acute spinal cord section. Four groups of dogs were used, anterior approach and vascular clamp of spinal anterior artery with 1, 2, 3, and 4 h of ischemia and posterior hemisection of spinal cord at C5–C6 was performed. Clinical evaluation was made during 12 weeks and morphological evaluation at the end of this period. We obtained a maximal neurological coordination at 23 days average. Two cases showed sequels of right upper limb paresis at 1 and 3 ischemia hours. There was nerve conduction delay of 56% at 3 h of ischemia. Morphological examination showed 25% of damaged area. The VIII and IX Rexed’s laminae were the most affected. The critical ischemia time was 3 h. Dogs with 4 h did not exhibit any recovery

The Future of Neurocritical Care Research: Proceedings and Recommendations from the Fifth Neurocritical Care Research Network Conference

The Fifth Neurocritical Care Research Network (NCRN) Conference held in Boca Raton, Florida, in September of 2018 was devoted to challenging the current status quo and examining the role of the Neurocritical Care Society (NCS) in driving the science and research of neurocritical care. The aim of this in-person meeting was to set the agenda for the NCS\u27s Neurocritical Care Research Central, which is the overall research arm of the society. Prior to the meeting, all 103 participants received educational content (book and seminar) on the Blue Ocean Strategy, a concept from the business world which aims to identify undiscovered and uncontested market space, and to brainstorm innovative ideas and methods with which to address current challenges in neurocritical care research. Three five-member working groups met at least four times by teleconference prior to the in-person meeting to prepare answers to a set of questions using the Blue Ocean Strategy concept as a platform. At the Fifth NCRN Conference, these groups presented to a five-member jury and all attendees for open discussion. The jury then developed a set of recommendations for NCS to consider in order to move neurocritical care research forward. We have summarized the topics discussed at the conference and put forward recommendations for the future direction of the NCRN and neurocritical care research in general