Abstract We consider the possibility of stabilising a polymer by using a low‐molecular‐weight, mobile and soluble antioxidant, trapped in a small volume fraction of encapsulating material so that it is released into the surrounding polymer to compensate for consumption or loss. Two models are discussed. In the “matrix” model, the additive is dissolved in an encapsulant and its release is controlled entirely by the diffusion coefficient in the encapsulant. In the “balloon” model, the pure additive is surrounded by a wall of encapsulant, through which it permeates into the surrounding polymer. Modelling of the two cases allows the required diffusion or permeation coefficients to be calculated for any chosen loss time. It is found that purely diffusion‐controlled loss would require improbably low diffusion coefficients, whereas the required permeabilities for balloon models are accessible. It is difficult to imagine a system for which the temperature coefficient of loss rate would be low enough to allow elevated temperature processing without loss of the additive.
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 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 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.
Abstract An apparatus is described for the measurement of oxygen uptake into a polymer sample at constant oxygen pressures in the range 20–1000 mm Hg. Measurements of the rate of oxygen uptake into poly‐4‐methylpentene‐1 show that the rate is accurately first‐order in oxygen pressure over the range 50–800 mm pressure for temperatures ranging from 122 to 154°C and film thickness in the range 0.001–0.025 cm. A theoretical treatment of the kinetics of a reaction in which oxygen diffuses into both faces of a thin film, in which it is consumed by a first‐order reaction shows that the oxidation rate ρ per unit area of film surface is given by ρ = ρ ∞ tanh ß L /2 where ρ ∞ is the limiting oxidation rate for a thick film, L is the film thickness, and ß = ( k / D ) 1/2 , k being the oxidation rate constant and D the diffusion constant. Values of D and the activation energy for diffusion calculated from autoxidation data are in good agreement with values determined directly.
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