Abstract
Mechanically fibrillated cellulose nanofibrils (CNFs) have attracted special attention as building blocks for the development of advanced materials and composites. A correlation exists between CNF morphology and the properties of the materials they form. However, this correlation is often evaluated indirectly by process-centered approaches or by accessing a single dimensionality of CNFs adsorbed on solid supports. High-resolution imaging is currently the best approach to describe the morphological features of nanocelluloses; nevertheless, adsorption effects need to be accounted for. For instance, possible deformations of the CNFs arising from capillary forces and interactions with the substrate need to be considered in the determination of their cross-sectional dimensions. By considering soft matter imaging and adsorption effects, we provide evidence of the deformation of CNFs upon casting and drying. We determine a substantial flattening associated with the affinity of CNFs with the substrate corresponding to a highly anisotropic cross-sectional geometry (ellipsoidal) in the dried state. Negative-contrast scanning electron microscopy is also introduced as a new method to assess the dimensions of the CNFs. The images obtained by the latter, a faster imaging method, were correlated with those from atomic force microscopy. The cross-sectional area of the CNF is reconstructed by cross-correlating the widths and heights obtained by the two techniques.
Highlights
Alongside developments in colloidal science, bio-based colloids have emerged as promising building blocks for the preparation of materials for a wide range of applications
On the basis of the precedent discussion, we argue that the cellulose nanofibrils (CNFs) aspect ratio obtained from atomic force microscopy (AFM) imaging has been overestimated in the literature[40] because the measured height is often used as a measure of the diameter
We introduce the combination of AFM and NegC SEM to and efficiently assess the morphology and dimensions of CNFs
Summary
Alongside developments in colloidal science, bio-based colloids have emerged as promising building blocks for the preparation of materials for a wide range of applications. Sample preparation and skilled operation are of fundamental importance as the solid support is generally of similar nature of the lowcrystallinity, “disordered” cellulose.[4,27,28] atomic force microscopy (AFM) relies on proper protocols for the deposition on the solid supports but, in addition, it is subjected to substantial measurement artifacts such as tip convolution, which prevents an absolute assessment of CNF dimensions This prevents comparison of results obtained between studies. Width and width−length correlations, respectively, for AFM and NegC SEM For such intrafibril (comparison of the dimensions within the same fibril) descriptions, a broad spectrum of sizes is preferable to describe the dimensionalities scaling as a function of the global size of the CNFs; we selected 200 CNFs of visible different sizes.
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