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Surface modification of cellulosic nanowhiskers (CNW) is of great interest, especially to facilitate their use as polymer reinforcements. Generally, alteration of the surface chemistry is performed using multiple reaction steps. In contrast, this study demonstrates that the needed hydrolysis of amorphous cellulose chains can be performed simultaneously with the esterification of accessible hydroxyl groups to produce surface functionalized CNW in a single step. The reaction is carried out in an acid mixture composed of hydrochloric and an organic acid (acetic and butyric are both demonstrated). Resulting CNW are of similar dimensions compared to those obtained by hydrochloric acid hydrolysis alone; sizes are verified by multiangle laser-light scattering and transmission electron microscopy. However, narrower diameter polydispersity indices indicate that surface groups aid the individualization of the nanowhiskers (Px = 2.5 and 2.1 for acetic and butyric acid, Px = 3.0 for hydrochloric acid). More than half of the hydroxyl groups located on the CNW surface are substituted under the employed reaction conditions as determined by quantitative Fourier-transform infrared-spectroscopy. The resulting surface modified CNW are dispersible in ethyl acetate and toluene indicating increased hydrophobicity and thus are presumably more compatible with hydrophobic polymers when used as a reinforcing phase.
Braun et al. (Mon,) studied this question.