Water sorption tests were conducted on unfilled poly(methyl methacrylate) samples in distilled water at 5, 37, and 60
$\,^{\circ}$
C under three different tensile stress ratios
$(\sigma_{\rm appl}/\sigma _{\rm ys}= 0$
%, 5%, and 10%). Each sample was placed in a modified Hoffman open‐side tubing clamp and subjected to four‐point bending at pre‐determined stress level for 1 day, 3 days, 1 week, 2 weeks, and 4 weeks. Water sorption was measured by weight change calculations, without accounting for any weight loss due to solubility of uncured monomer. A generalized diffusion equation can be used to express both stress‐free and stress conditions;
$D=D_{\rm o}\exp[-E(\sigma)/kT]$
. It was found that the activation energy for water sorption diffusion was linearly related to applied stress ratios; i.e.,
$E=1.15\ \sigma_{\rm appl}/\sigma_{\rm ys} + 10.76$
(kJ/mol), with the correlation coefficient
$r =0.97$
. Since the proportional pre‐exponential constant,
$D_{\rm o}$
, is independent of temperature, it is speculated that the loading percentage of reinforcing filler elements in composite resin materials can be related to this constant.
