Nanomaterials 2022 , 12 , 790
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CMFs/CNFs have been tested in strength and coating applications, failing without any apparent reason. This suggests that some key parameters are not taken into consideration as, for example, the dispersion degree of nanofibrils before its application or the mixing of CNFs within the studied matrix [25]. Some researchers have studied the importance of dispersion and uniformity of nanocellulose on the reinforcement performance, e.g., in paper [26] or in polymeric matrices [27]. Although several methods for measuring dis- tribution size or homogeneity, such as dynamic light scattering, turbidity, self-assembly, and shear birefringence have been reported [23], there are currently no methodologies for determining dispersion degree on the CMF/CNF suspensions despite being an important parameter for the use of nanocelluloses at industrial scale [21]. Our hypothesis is that the stirring methods used to prepare the CMF/CNF suspen- sions may have an impact on the entanglement network affecting the aspect ratio (AR) of CMFs/CNFs as well as the separation between the nanofibers and, therefore, their final behavior. AR determination is based on several sedimentation methodologies: on the one hand, the use of the gel point or connectivity threshold (Ø g ) [28] and, on the other hand, the use of the differences in light transmission with time due to sedimentation [29]. In this study, the Ø g method is proposed to study the dispersion of CMF/CNF suspensions and validated by the morphological characterization of the CMFs/CNFs using Transmission Electronic Microscopy (TEM) images to determine both the CMF/CNF structure and the mean diameter of the nanofibrils. Ø g describes the compressibility and structure of sediments and is defined as the volume concentration of a suspension in the boundary between semi-dilute and dilute region which depends on time [6,28]. This volume fraction is also considered the lowest volume fraction, at which all primary fibres and fibre flocs are interconnected throughout the container, forming a self-supporting three-dimensional network. Below the Ø g con- centration, the material suspended does not contribute to the mechanical strength of the suspension [30]. Figure 1 shows the progress with time in a sedimentation experiment. During the experiment, the volume fraction on the top of the sediment is considered to be unchanged from C o and, above this fraction, a clear liquid region appears [31]. As time passes, a concentrated deposit is formed in the lower part of the measured cylinder, whose volume becomes constant with time.
Figure1. Progression of a self-supporting region with time.
WhereC o is the initial concentration of solids, and H s /H o is the relation between the sediment height and the initial suspension height.
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