The effects of tribochemical impacts--sonochemical treatments and milling variants--on cellulose were studied, employing multi-detector size exclusion chromatography with group-selective fluorescence labelling ("CCOA method") and diagnostic beta-elimination as the analytical tools. Milling at different temperatures was compared to sonochemical degradation of cellulose by a 24 kHz ultrasound probe system in homogeneous solution. Chain cleavage was generally accompanied by random oxidation of cellulose. The degree of oxidation increased with increasing temperature; at 77K oxidation occurs only to a minor extent. Degradation proceeded towards a final value, the limiting molecular weight (Mw(lim)), beneath which no further decrease of the chain length occurred even at prolonged treatment times. Regardless of the lignin or hemicellulose content of the pulps, the Mw(lim) reached in a specific milling process was largely constant and showed little dependence on the substrate used. The formation of radicals during tribochemical treatments under different conditions and with different substrates was discussed based on theoretical considerations and EPR data. The overall radical content increased with increasing time of milling until a plateau is reached. Here, the mechano-radical content largely depends on the lignin content in the pulp as anticipated. The formation of different radical species and their precursor structures were discussed.
Keywords: Cellulose; Degradation; EPR; Free radical; Lignocellulosics; Milling; Oxidation; Tribochemistry.
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