Clem E. Powell

Effect of surfactant architecture on the properties of polystyrene-montmorillonite nanocomposites.

A series of surfactants were designed and synthesized for use as clay modification reagents to investigate the impact of their chemical structure on the nanocomposites morphology obtained following polymerization. The behavior of the surfactant-modified clays at three different stages were investigated: after ion exchange, following dispersion in styrene monomer, and once polymerization was complete. The propensity of the styrene monomer to swell the surfactant-modified clay was observed to be a useful indicator of compatibility and predictor of the resultant polystyrene nanocomposite morphology which was directly observed using small-angle X-ray scattering (SAXS) and cryogenic transmission electron microscopy (TEM). It was found that the key components of surfactant design driving exfoliated morphologies were (1) the position of the ammonium group, (2) the inclusion of a polymerizable group, (3) the solubility of the surfactant in the monomer, (4) the length of the alkyl chain, and (5) sufficient concentration of surfactant used to exchange the clay. This understanding should lead to better design of clay modifications for use in polymer nanocomposites.

Determination of dosimetric perturbations caused by aneurysm clip in stereotactic radiosurgery using gel phantoms and EBT‐Gafchromic films

Some radiotherapy patients are treated with titanium surgical aneurysm clips in the radiation field. This is of particular importance for stereotactic radiosurgery brain treatments, where the length of the blade of the clip may be comparable to the size of the radiation field. This study seeks to determine the extent of the dosimetric effects caused by surgical clips in stereotactic radiosurgery, using polyacrylamide gel phantoms and EBT type Gafchromic films. Using gel phantoms scanned with magnetic resonance imaging scanner, dose enhancement of around 20% was noted at distances less than 2 mm away from the clip surface. Gafchromic films showed about 6% variations in the dose up to few millimeters from the clip. These experimental results confirmed results predicted by Monte Carlo simulation techniques for higher density material surgical clips such as lead and platinum. Moreover, these experimental measurements clearly indicate dose reduction due to radiation attenuation behind the clip of about 4%.

Photonuclear activation of oxygen & copper with high-energy x-ray or electron beams

Electron or x-ray energies above the threshold for photonuclear activation of certain elements (such as oxygen and copper) can be achieved in some of the high energy beams used today in radiotherapy. Neutrons and gamma radiations are produced in such reactions. These secondary radiations, beside their implications on the treatment, they have potential to be used for quality testing of the radiation beam and can also be imaged with a PET scanner and hence employed for testing such imaging modalities. In this work applications and implications of such secondary radiations in radiotherapy is investigated through oxygen that is in the water molecules inside gel dosimeters and copper in its solid and solution format.

Microdosimetry and small field measurements in Polymer Gels

Dose measurements at small fields (around 1×1 cm) in radiotherapy is increasingly becoming important by the introduction of new technologies such as IMRT and it is a challenging task at the same time. Most if not all of the known dosimeters fail to measure the dose reliably at such small fields. Gel dosimeters are tissue equivalent and they are used as phantom and dosimeters at the same time. Their dose resolution is limited by the pixel sizes of the imaging modality used for their scanning, in this case an MRI scanner. Therefore reducing the pixel size increases the dose resolution since the contrast is very high in MRI images. In using normal scanners pixel sizes of around one millimeter at reasonable scanning times is usually obtained. In this work a small core and strong magnetic field scanners are employed which produces pixels of the order of 30 micrometers rendering them as micro-dosimeters.

SU‐FF‐T‐188: Dosimetry of Microbeam Radiotherapy Using Gel Dosimeters

Purpose:Microdosimetry (measurement of radiationdose on the μm scale) suitable for micro‐beam radiotherapy is a challenging task with known dosimeters such as ionization chambers and others dosimeters. The purpose of this work was to use geldosimeters for dose determination across micro‐beam radiotherapy fields. Method and Materials: A geldosimeter that was highly resistant to radiation was developed by using a free radical scavenger in the gel to reduce the rate of polymerisation. The samples were irradiated using the x‐ray collimated micro‐beam from a synchrotron in order to measure the synchrotrons high‐dose. Gel scanning was performed using special type MRI scanner, Raman spectroscopy and also attempted using phase contrast imaging. Results: It was found that using current equipment and techniques, that MRI with resolution down to 20 μm is possible, but given the size of the beam (30 μm), is unable to adequately describe the dose distribution across the beam. Raman spectroscopy measurements of gel have been shown to be possible, and work in this area is involved in gaining accurate measurements in this method. Comparative measurements made using CCD camera on the beams show similar results. Conclusions: This work shows the dose measurement in gel down to the μm range is possible and could be applied in many fields such as microbeam radiotherapy.

Thermal dependence of carbon dioxide transport through a dense polymeric membrane

Publisher Summary Polymeric gas separation membranes are currently used commercially for CO2/CH4 separation in a number of operations worldwide. However, a major technical hurdle associated with such membranes is the plasticization of the membrane with increasing concentrations of polar gases such as CO2. Significant research efforts are being conducted into characterizing and avoiding the plasticization phenomena. This chapter presents a study that investigates the permeability of polyimide membranes formed from a readily available commercial membrane Matrimid 5218 in carbon dioxide at temperatures up to 100°C and pressures up to 2500kPa. In this study, membranes were tested in both as cast and thermally annealed formats. The effect of thermal annealing on plasticization has also been examined. Thermal annealing was found to suppress plasticization of the membranes in this temperature and pressure range. It was shown that some degree of crosslinking occurred during annealing, leading to a reduction of carbon dioxide permeabilities by a factor of 2–3.