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Biomacromolecules 2009, 10, 334–341
Single-Step Method for the Isolation and Surface Functionalization of Cellulosic Nanowhiskers Birgit Braun† and John R. Dorgan*,‡ PolyNew, Inc., Golden, Colorado 80401, and Department of Chemical and Biochemical Engineering, Colorado School of Mines, Golden, Colorado 80401 Received September 30, 2008; Revised Manuscript Received November 16, 2008
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.
Introduction Polymer nanocomposites containing inorganic or organic nanoparticles are attracting interest for a wide range of applications.1 Whisker-like nanostructures are particularly effective even at low loading levels for mechanical property enhancement; when well dispersed they form a percolating network resulting in excellent reinforcement.2 While the preparation of man-made nanorods is successfully demonstrated for various inorganic materials, synthesis of polymeric nanowhiskers has proven difficult.3 However, a number of living organisms synthesize cellulosic nanocrystals with high aspect ratios. Microfibrils produced by plants and some animals contain regions of highly ordered polysaccharide chains with only a few defects.4 Carefully conducted acid treatment of cellulose source materials allows the isolation of crystalline cellulose nanowhiskers (CNW) by taking advantage of the lower hydrolysis kinetics for crystal structures.5 The recoverable mass fraction is determined by the ratio of amorphous to crystalline mass, which is characteristic for the cellulose source, as are the whisker dimensions.5-9 Due to the ultimately finite nature of fossil resources, their environmental impacts, and their historically volatile pricing, alternative renewable based materials are increasingly attractive. CNW offer the benefit of being a nanofiller derived from renewable biomass in addition to possessing exceptional mechanical properties (tensile modulus is 143 GPa for CNW from tunicin,10 a value that is 100 times greater than a typical glassy polymer). Isolation of colloidal suspensions of CNW by sulfuric acid hydrolysis was first documented by Rånby in 1951.11 Battista * To whom correspondence should be addressed. Email: jdorgan@ mines.edu. † PolyNew, Inc. ‡ Colorado School of Mines.
discovered that the formation of stable suspensions can also be achieved by hydrolysis of cellulose using hydrochloric acid followed by mechanical disintegration.12 During sulfuric acid hydrolysis, sulfate ester groups are introduced randomly on the surface resulting in nonflocculating suspensions,11,13-21 which have been investigated as model systems for rod-like polyelectrolytes and their ability to form chiral nematic liquid crystal phases.13,16,17,21-26 For composite applications, these sulfate groups are problematic due to the decreased thermal stability after drying, precluding typical polymer meltprocessing.18 However, it was shown that the thermal stability of spherical cellulosic nanocrystals can be recovered if the counterion of the sulfate ester groups is exchanged to the sodium form.27 The hydrophilic nature of native cellulose generally limits the formation of CNW nanocomposites to water-soluble polymers; in hydrophobic matrices repulsive forces lead to aggregation and poor interfacial contact. Recently, the preparation of a three-dimensional network of well-individualized, unmodified CNW that can subsequently be filled with any polymer was demonstrated.28 Though the approach leads to good filler dispersion, composites cannot be obtained by simply melt blending the filler with the polymer as desired in industrial processes. Efforts to overcome this limitation are documented in the literature, and include acetylation of the CNW surface using mixtures of acetic acid and anhydride,29 use of surfactant30,31 and coupling agents,3,32 polymer grafting,33 and acylation by drying aqueous emulsion.34 In this study, a one-step acid-catalyzed esterification (known as Fischer esterification) is established as an alternative to the more complex approaches described in the previous paragraph. The reaction is a well-known method for the preparation of esters by heating a carboxylic acid in alcohol containing a small amount of catalyst, usually a strong acid.35 The reaction scheme applied to cellulose is illustrated in Figure 1 for an acid mixture
10.1021/bm8011117 CCC: $40.75 2009 American Chemical Society Published on Web 12/22/2008
Functionalization of Cellulosic Nanowhiskers
Biomacromolecules, Vol. 10, No. 2, 2009
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Figure 1. Reaction scheme illustrating the simultaneous occurrence of cellulose hydrolysis and esterification of hydroxyl groups using a mixture of acetic and hydrochloric acid as example.
consisting of hydrochloric and acetic acid. The hydronium ions resulting from the hydrochloric acid dissociation hydrolyze amorphous cellulose domains and also catalyze the esterification of hydroxyl groups on the exposed cellulose chains. The better atom economy compared to the use of anhydrides is preferable according to the “12 Principles of Green Chemistry”.36 In addition, less expensive and lower toxicity reagents, which potentially can be derived using renewable resources, are used. Foremost, isolation and functionalization of CNW can occur simultaneously, thus eliminating the need for multiple reaction steps. An additional advantage is reduced aggregation upon drying due to disruption of hydrogen bonding between nanowhiskers.
Materials and Methods A. Materials. Cotton linter sheets were obtained from Of The Earth, Auburn, WA. Reagent grade glacial acetic, butyric, and hydrochloric acid were obtained from Sigma Aldrich and used as received. For multiangle laser-light scattering, a polystyrene calibration standard with a molecular weight of 30000 g/mol and a polydispersity index of
