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Gravimetric determination of the diffusion characteristics of polymers using small specimens



An investigation has been undertaken of the factors controlling the rate of ingress of water into relatively small epoxy specimens. The application of a three-dimensional diffusion model, fully acknowledging that the specimen geometry controls the rate of ingress, has successfully resolved the problems of determining the diffusion characteristics—the diffusion coefficient D and the uptake M corresponding to saturation—and making theoretical predictions of the uptake Mt by small specimens. On the theoretical front, simulations have been carried out to demonstrate that (1) theoretical predictions require two equations to be used to obtain correct relative uptake Mt/M values for short and long exposure times; (2) a cube of the side h is saturated faster than an infinite slab of the thickness h, and (3) determination of the D value by applying the model derived for an infinite slab to specimens of finite dimensions results in an apparent value that may grossly exceed the true diffusion coefficient. On the experimental front, suggestions are made regarding (4) elimination of a systematic error that might be introduced by single weighing of a specimen prior exposure, (5) determination of the product MD1/2 from the gravimetry data for short exposure times, and (6) splitting this product into absolute values D and M by means of a minimization procedure. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42:2122–2128, 2004