Variations in the fibre repeat between samples of cellulose I from different sources

doi:10.1016/j.carres.2004.10.005

Carbohydrate Research

Volume 339, Issue 18, 27 December 2004, Pages 2889-2893

https://doi.org/10.1016/j.carres.2004.10.005 Get rights and content

Abstract

A powder X-ray diffractometer was used to measure the fibre repeat in cellulose I with sufficient precision to detect variations between samples from different sources. The variations were correlated with the lateral dimensions of the crystallites and were attributed to different minimum-energy fibre repeats for chains in the interiors and on the surfaces of crystallites. Results were interpreted in terms of a model for internal mechanical stress in which the interior chains were under compression and the surface chains under tension to ensure identical fibre repeats for all chains. The model was used to extrapolate the fibre repeat to a value of 1.043 nm for a hypothetical, infinitely large crystal, and to 1.029 nm for a crystallite so narrow that all chains were exposed on surfaces.

Graphical abstract

Introduction

Fibre repeat is defined as the length of the repeating unit in a chain of a fibrous polymer.¹ The repeating unit in cellulose I is a pair of β-(1→4)-linked d-glucosyl residues (Fig. 1).² Nine measurements of the cellulose I fibre repeat between 1967 and 1991 gave a mean value of 1.036 nm.² The standard deviation of 0.002 nm was larger than the experimental uncertainty commonly attached to diffraction experiments in the latter half of the 20th century. We set out to test whether there might be natural variations in the fibre repeat between samples from different sources. In particular, we sought variations that might be associated with disruption of hydrogen bonding at the surfaces of crystallites.

Ioelovich and Larina studied 11 samples of cellulose I and found that the unit cell dimension a varied between 0.785 and 0.793 nm, while b varied between 0.816 and 0.821 nm.³ These variations were much larger than experimental uncertainties, and were correlated with the lateral dimensions L of the cellulose crystallites. The unit cell dimension c, that is, the fibre repeat, did not show any detectable variations. The mean value was 1.034 nm, with a standard deviation of 0.001 nm. All 11 samples were prepared from cotton, flax or ramie.

We drew from a wider diversity of sources in our search for variations. Our experimental approach was similar to that of Ioelovich and Larina³ in that we used a powder diffractometer to obtain wide-angle X-ray scattering (WAXS) diffractograms. An internal standard was mixed with powders, dusted on fibres before construction of fibre bundles or sandwiched between thin slices in laminated composites. Consistent use of the same internal standard throughout our work eliminated one of the sources of uncertainty in compilations of literature values for the cellulose I fibre repeat.

Section snippets

Results

We used the cellulose I (0 0 4) reflection because it was well resolved, whereas the (0 0 2) and (0 0 6) reflections could not be fully resolved from neighbouring reflections.⁴ We chose MgO as the internal standard because of a strong reflection close to the cellulose I (0 0 4) reflection. WAXS diffractograms showed minor variations in the position of the MgO reflection, attributed to differences in sample geometry, but more obvious variations in the position of the cellulose I (0 0 4) reflection (Fig. 2

Cellulose I_α and I_β

Variations in relative proportions of cellulose I_α and I_β allomorphs could not account for the variations in fibre repeat. We included in the study four samples previously characterised by solid-state ¹³C NMR spectroscopy.⁷ The two samples with the sharpest (2 0 0) reflections (Table 1) were an algal cellulose containing 57% I_α and 43% I_β and a tunicate cellulose containing 10% I_α and 90% I_β.⁷ The fibre repeats were indistinguishable despite different proportions of allomorphs. Two of the

Conclusions

We have shown that variations in the cellulose I fibre repeat did not arise from mechanical stress associated with drying samples, or from variations in the relative proportions of I_α and I_β allomorphs. The variations were consistent with a model in which the minimum-energy fibre repeat was 1.043 nm in crystallite interiors and 1.029 nm on surfaces. Hydrogen bonding between adjacent chains forces a uniform fibre repeat, so that interior chains are compressed and surface chains are stretched. We

Samples for fibre repeat measurements

Samples of wood (P. radiata and C. sativa) were cut into fragments of dimensions <1 mm. Algal (Chaetomorpha coliformis) and tunicate (P. pachydermatina) tissue was air dried and ground in a Wiley mill to 40 mesh. Characterisation of cellulose I_α and I_β allomorphs in those four samples was described elsewhere.¹⁷ Commercial cellulose powder (Sigma C6663) was used as received. These samples were mixed with MgO powder and spread on a sample holder made from a silicon wafer cut along (4 0 0) planes so

Acknowledgements

The authors thank the New Zealand Marsden Fund for Contract MF902.

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Variations in the fibre repeat between samples of cellulose I from different sources

Abstract

Graphical abstract

Introduction

Section snippets

Results

Cellulose Iα and Iβ

Conclusions

Samples for fibre repeat measurements

Acknowledgements

Polym. Sci.

Solid-State Nucl. Magn. Reson.

X-ray Analysis of Organic Structures

Cellulose

Cellul. Chem. Technol.

Mokuzai Gakkaishi

Holzforschung

Polym. Sci. Part B, Polym. Phys.

Holzforschung

Macromolecules

J. Am. Chem. Soc.

J. Am. Chem. Soc.

Cellulose I_α and I_β