Accounting for Substrate Interactions in the Measurement of the Dimensions of Cellulose Nanofibrils

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.

Figure 1

Schematic illustration of the methodology used to assess CNF dimensions, including (a) preparation of the supported sample and AFM imaging to obtain the cross-sectional dimensions of CNFs. In the evaluation, the effects of tip convolution should be considered. The effect of drying stresses leading to a substantial flattening of CNFs is also demonstrated and accounted for. (b) Width measurements are cross-correlated with the corresponding height. An equivalent cylindrical cross section can be obtained from the observed noncylindrical cross-sectional area measured for adsorbed CNFs. (c) From the approximated cross-sectional area of the flattened CNF, a significantly different value is obtained for the diameter resulting from the equivalent circular cross section.

Figure 2

(a) Representative AFM images obtained from dip casting a 0.01 mg mL^–1 CNF suspension, followed by rapid blotting onto PEI-coated mica used as a solid support. (b) Correlation between the lateral and height dimensions of CNFs obtained from AFM. Distributions of (c) height and (d) width obtained from AFM imaging. The exponential decay fit shown in (b) is given by “width = −257.4 × exp(−height/(29.8)) + 290.7”.

Figure 3

(a) Geometrical considerations and associated fittings governing the maximum tip convolution on the width measured by AFM (tip radius of 8 nm and half cone angle of 20°). (b) Curve of tip convolution as a function of the height of the CNF sample. (c) Correlation between the cross-sectional dimensions of the CNF obtained from AFM measurements, including distributions expected after subtraction of maximum tip convolution (light green). The exponential decay fits shown in (c) are given by “width = −257.4 × exp(−height/(29.8)) + 290.7” for the as-measured values and “width = −122.5 × exp(−height/(13.9)) + 144.7” for the values after tip-convolution correction.

Figure 4

(a) Effect of substrate affinity between the highly adhesive PEI-coated surface and the lower affinity mica surface on adsorption induced spreading of CNFs depicted schematically. (b) Correlation between width and height for CNFs measured on PEI-coated mica or bare mica, highlighting a less oblate cross-sectional area for CNF supported on bare mica. The linear fits in (b) are given by “width = 36.1 + 2.9 × height” for bare mica and “width = 45.3 + 5.7 × height” for PEI-coated mica.

Figure 5

(a) Sample preparation using CNFs that are first adsorbed onto a support followed by carting with a thin metal layer (left) and sample preparation whereby the metal coating is first applied onto the substrate followed by deposition of the CNFs (right). (b,c) images obtained with samples prepared as described in panel (a), left and right, respectively. The insets highlight contrast differences.

Figure 6

Distribution of (a) length and (b) width of CNF obtained by NegC SEM as described in Figure 5, as well as (c) their linear correlation. The linear fit shown in (b) is given by “width = 16.7 × length”.