Structures of carboxylated cellulose fibers – fates of S1, S2, and S3 layers

Many of the cellulose chemistries involve various ways of carboxylation and the carboxyl groups offer readily available potential reaction sites. Upon carboxylation, both electrostatic repulsion and the disrupted interfibrillar hydrogen bonding facilitate the swelling process. With increased degree of substitution, the carboxylated fibers become highly swollen, which leads to an irreversible structural deformation. Changes in fiber structures may vary depending on the type of pulp fibers, pretreatments, and chemistries applied. In this study, we will discuss how the microfibrils in secondary wood cell walls respond to the environment and how they attribute to some of the challenges in cellulose chemistries such as low yield and non-uniform fiber fragmentations.

In order to elucidate the structural changes of carboxylated fibers, softwood kraft fibers were modified by both chemically and mechanically. Balloon-like structures were formed when non-fibrillated fibers were carboxymethylated. The innermost S3 layers became visible inside the balloons and they were effectively isolated for further characterizations. The carboxymethylated S2 microfibrils that fill up the balloons have enough mobility to rearrange themselves upon mechanical breakup, thereby resulting in the formation of spherical particles. The S1 layers are largely responsible for the ballooning phenomenon as they create tightly wound collars that are mechanically resistant. The ballooning can be avoided by grinding the fiber exterior – mostly S1 layers – prior to the chemical treatment, or by applying a chemistry which results in more uniform conversion than carboxymethylation.