The potential of advanced biobased materials made from cellulose nanofibrils (CNF) has been demonstrated in various applications with impressive mechanical properties and simplicity for chemical modifications. The chemical properties are largely dependent on the surface area of the CNF, which in turn requires knowledge of their cross-sectional distribution. Although small-angle X-ray scattering (SAXS) is an ideally suited technique to target this, previous studies have concluded that variations of extraction conditions yield ribbon-shaped CNF with dimensions significantly different from the now widely accepted 18-chain microfibril synthesized in plant cell walls. In our recent work [1], we studied dispersed CNF extracted from wood with two different oxidation methods including variations of degree of oxidation and high-pressure homogenization. The SAXS results were analyzed assuming a quantized cross-section distribution containing different aggregates of a pseudo-square unit. We find that the extraction conditions only influence the aggregation level, and that the unit fibril size is consistent with the expected dimensions of an 18-chain microfibril; a result that provides a comprehensive explanation to the long-standing puzzle of varying CNF cross-sections from different sources or via different processing conditions. Complementary wide-angle X-ray diffraction measurements of the freeze-dried samples also suggest that the aggregates have a preferred cohesion of phase boundaries parallel to the (110)-diffraction planes of the cellulose I_β crystal, leading to the resemblance of a ribbon-shaped cross-section on average.

Cellulose nanofibers (CNFs) are analyzed through X-ray scattering and assuming cross-sectional aggregation of 18-chain elementary microfibrils.
Image from T. Rosén et al., ACS Nano (2020), 14(12), 16743–16754

Tom Lindström

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