Influence of TEMPO-oxidation on pulp fiber chemistry, morphology and mechanical paper sheet properties

In this contribution, we report on the TEMPO-mediated oxidation of pulp fibers used in the general context of papermaking and for the future design of tailor-made paper in advanced applications. We focus in our studies on properties of TEMPO-oxidized pulp fibers to explain the characteristics of the paper made thereof. 13C solid-state NMR analysis reveals that in particular amorphous regions of the fibers are being chemically oxidized, while at the same time the crystalline regions of the fibers are not significantly affected. Investigation of the fiber morphology before and after oxidation shows that the fiber length is not changed, yet the fibers do exhibit an increase in width if in contact with water, which is attributed to an increase in fiber swelling. In addition, fibrillation decreases due to the oxidative removal of loosely bound fines and fibrils, rendering the surface of the resulting oxidized fibers much smoother in comparison to the original fibers. Finally, we observe that both, dry and wet tensile strengths are also higher for paper made of oxidized fibers, most likely due to cross linkable aldehyde groups formed during oxidation (i.e. hemiacetal bond formation in the sheet during thermal drying). Our results of the oxidation of paper fibers thus offer a systematic study helpful for the design of tailor-made paper useful in several applications where a fiber-modification with fiber-immobilized functional motifs is crucial, such as for example in paper-based microfluidic sensors (µPADs) or lab-on a chip-devices.