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fig 3) and spectrometric methods such as infrared (IR) and ultraviolet visible (UV), were utilized to determine the mechanism. All the IR absorption bands were assigned to the different chemical groups of the wood surface (table IV). The absorption changes occurring during the solar-type exposure showed a decrease in the hydroxyl groups, an increase in the carbonyls and also desaromatization (table VII, fig 8). These changes were oxygen-dependent (fig 9). By cutting the UV or visible light emitted by the lamp with filters, it was possible to show that UV wavelengths were more important in the IR spectral modification (fig 10). Also, lignin photodegradation was similar in spectral evolution to fir degradation under solar-type irradiation. UV-visible spectroscopy was used to characterize the evolution of yellowing in fir, mainly due to the UV wavelengths (fig 13). At the beginning of light exposure, the yellowing rate seemed to be independent of the gaseous atmosphere. Lignin seemed to be responsible for the light absorption of wood and for discolouration (fig 15), and the main source of radicals. These gaiacoxy radicals appear to form coloured quinonoid structures with oxygen, and colourless products without it (fig 19).

The details of a new all‐atom force field designed to reproduce the phases of the native I‐α and I‐β forms found in crystalline cellulose I are reported in this article. The energy differences, densities, unit cell parameters, and moduli are in close agreement with experimental evidence. Analyses of the modular dynamics simulations also included thermodynamic data and angle distributions as well as characterization of the intrachain, intrasheet, and intersheet hydrogen‐bond networks for both phases under study. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 1939–1946, 2000