Effects of periodate oxidation on cellulose polymorphs

Since periodate oxidation selectively creates (masked) aldehyde groups that can serve as anchors for further modification steps, this method is suitable for modifying and functionalizing cellulose. Although numerous studies deal with that topic, there are still knowledge gaps regarding periodate oxidation. In our study, we focused on examining how the type of cellulose allomorph influences the reaction. We compared the oxidation of two allomorphs, namely cellulose I, cellulose II and mixtures of cellulose I and II, and examined changes in crystallinity and thermal decomposition behavior. Generally, periodate oxidation proceeded faster in the case of cellulose II samples, followed by the mixed cellulose I/II samples; cellulose I was the slowest. Based on our results, the major influencing factor is the overall crystallinity of the sample. The influence of the allomorph was minor. Crystallinity decreased upon oxidation, but no significant differences were found between the different cellulose polymorphs. Following the crystallinity during the oxidation reaction proved to be very difficult. Determining crystallinity with solid-state nuclear magnetic resonance (NMR) was largely hampered by superposition with new resonances that interfere with crystallinity determination. Structural changes during oxidation as evident from solid-state NMR are discussed in detail. Alternative methods for crystallinity analysis, such as near infrared spectroscopy, attenuated total reflection infrared spectroscopy, and Raman spectroscopy, had similar problems but to a lesser extent, with Raman being the method of choice. Thermogravimetric analysis showed thermal decomposition of oxidized cellulose I and II to be similar. An anomaly was found in the case of oxidized viscose fibers. Slightly oxidized samples showed increased mass loss in the temperature range up to 360 °C whereas higher oxidized samples and all pulp samples showed decreased mass loss.