Density measurements as a function of polyimide film thickness have revealed that the surface layers, each 1.5 μm in thickness, have a density lower than that of the bulk by a statistically significant 1%. Infrared measurements have confirmed that this is due to the presence of isoimide in these surface layers: because of its nonlinear nature, this impurity structure causes a decrease in the measured density.
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Chauvin et al. (1986) studied this question.
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