A simple methodology for the manufacture and calibration of polyacrylamide gel (PAG) for magnetic resonance imaging (MRI) radiation dosimetry is presented to enable individuals to undertake such work in a routine clinical environment. Samples of PAG were irradiated using a linear accelerator and imaged using a 0.5 T (22 MHz) Philips Gyroscan MRI scanner. The mean spin-lattice relaxation rate was measured using a 'turbo-mixed' sequence, consisting of a series of 90 degrees pulses, each followed by acquisition of a train of spin echoes. The mean sensitivity for five different batches of PAG in the range up to 10 Gy was calculated to be 0.0285 s-1 Gy-1 for the mean spin-lattice relaxation rate with a percentage standard deviation of 1.25%. The overall reproducibility between batches was calculated to be 2.69%. This methodology, which introduces the novel use of pre-filled nitrogen vials for calibration, has been used to develop techniques for filling anatomically shaped anthropomorphic phantoms.

SynopsisThe rate at which an additive is lost from the surface of its solution in a polymer is considered to depend upon three factors-the solubility of the additive, the rate at which it volatilizes from the polymer surface, and its diffusion coefficient within the bulk of the polymer. By adapting the mathematics of heat flow in a solid, the loss of additive from a polymer is mathematically modeled in terms of these three variables for bulk polymer and for film and fiber samples. Two cases are considered-loss of additive by volatilization or dissolution from the polymer surface and loss by precipitation on the surface from a supersaturated solution of the additive. The results are applied to a discussion of the relative importance of the three parameters under various conditions occurring or expected in practical application of additives as oxidation stabilizers for polyethylene and polypropylene.

SynopsisThe solubilities of six phenolic antioxidants in poiypropylene and polyethylene were measured between 5OoC and 100OC. Measurements were also made of their solubilities in a range of liquid alkanes. The measured solubilities at 7OoC were consistent with regular solution theory and could be used to predict values to within one half of an order of magnitude. No simple correlation wm found when regular solution theory was applied to predict the dependence of solubility on either temperature or the molar volume of the solvent. Hence, contrary to previous reports, the relative solubilities of antioxidants, measured at high temperatures or in alkane solvents, cannot be used to estimate their relative solubilities in polymers at room temperature.

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The thermo‐oxidative stability of high‐density polyethylene (HDPE) prepared by silica supported Ziegler‐Natta, bis(triphenylsilyl)chromate and bis(cyclopentadienyl)‐chromium(II) catalyst systems was examined by chemiluminescence (CL) techniques and thermogravimetric (TG) analysis. In particular, the technique of dynamic CL in nitrogen allows the concentration of polymer hydroperoxides and the relative stability of these to be simultaneously determined. Polymer hydroperoxides in Ziegler‐Natta catalyzed HDPE are relatively stable because titanium and aluminum residues are poor pro‐oxidants compared with chromium residues. HDPE produced by bis(cyclopentadienyl)chromium(II) has a low intrinsic thermo‐oxidative stability due to the chromium‐catalyzed conversion of polymer hydroperoxides into degradation products during thermal aging. Concentrations of residual chromium as low as 1 ppm can profoundly affect the oxidative stability of the polymer and exert a much greater influence than either the extent of branching or the degree of unsaturation. The appearance of the silica catalyst support before and after gas‐phase polymerization was studied by scanning electron microscopy. During polymerization the silica support shatters and the resulting submicron fragments are dispersed throughout the polymer particles. On exposure of the nascent polymer to the atmosphere, bis(cyclopentadienyl)chromium(II) is converted to a trivalent chromium species which remains associated with the silica substrate. The presence of trivalent chromium in the poisoned chromium catalyst was confirmed by UV/visible spectrophotometry and the nature of the Cr(III) species was investigated by diffuse reflectance Fourier transform infrared (FTIR) spectroscopy. The high surface area of the catalyst residue renders it a powerful pro‐oxidant despite its low concentration. © 1992 John Wiley & Sons, Inc.

A computer‐controlled chemiluminescence (CL) instrument incorporating a differential scanning calorimeter was used to simultaneously acquire photochemical and enthalpic data for poly(N‐vinyl‐2‐pyrrolidone) (PVP). Samples were subjected to a linear temperature ramp under nitrogen and their luminescence response recorded. The resultant dynamic CL peak is attributable to the decomposition of hydroperoxide groups and the subsequent mutual termination of secondary polymer peroxyl radicals. It is shown that dynamic CL can be used to characterize the level of oxidation in PVP, which in commercial samples, may be partly related to the level of residual polymerization inititor and to the drying process. The temperature at which maximum CL emission occurs correlates with the glass transition temperature (Tg) of the polymer and increases with increasing molecular weight. A marked increase in the Tg of PVP occurs after it is aged in air for 24 h at 120°C. This is due to the loss of adsorbed moisture from the polymer which was confirmed by thermogravimetric analysis. Oxidation profiles of PVP were obtained by plotting the integrated CL peak area as a function of aging time. The profiles are compared with data obtained from isothermal CL and viscosity measurements. Gas perturbation experiments suggest that when drying PVP under nitrogen at elevated temperatures significant populations of longlived macroalkyl radicals are formed which can peroxidize the polymer on exposure to air. © 1993 John Wiley & Sons, Inc.