Improving the colloidal stability of Cellulose nano-crystals by surface chemical grafting with polyacrylic acid

Cellulose nano-crystals (CNC) can be tailored for various value-added applications. However, its use in aqueous systems is hampered by its limited dispersability, especially at a high CNC concentration. In this study, the improvement of CNC colloidal stability by surface chemical grafting with polyacrylic acid (PAA) was investigated, and the zeta potential and the charge density of the chemically modified CNC were analyzed. The results showed that an acrylic dosage of 1% (based on the dry weight of CNC) was sufficient to significantly enhance the colloidal stability. CNC, after chemical grafting with PAA, showed better stability against the increase in storage time or solid content of the aqueous medium, compared with the un-modified CNC.

For the production of CNC from various cellulosic sources, usually a key step is the acid hydrolysis of cellulose fibers to remove their amorphous regions, 13,14 leading to the formation of rod-like CNC.In this step, sulfuric acid is most commonly used, [15][16][17][18] although other acids such as phosphoric acid, 19 hydrochloric acid 20 and nitric acid 21 , have also been investigated.Due to the reaction between sulfuric acid and the surface hydroxyl groups of cellulose, the cellulose hydrolysis results in the formation of negatively charged surface sulfate ester groups, facilitating the dispersion of the resulted CNC in aqueous systems via electrostatic repulsion. 22,23 owever, the resulting sulfate groups on the surface of CNC are rather labile, which are easy to remove under mild alkaline conditions. 24,25 oreover, the adjacent hydrophilic particles/rods of CNC can easily form hydrogen bonds, which hurdles the dispersion of CNC. 19There is practical interest in improving the dispersion of CNC, particularly at a high CNC concentration.
7][28] For example, Winter et al. 29 investigated the effect of carboxymethyl cellulose (CMC) and xylogucan (XG) on the stability of CNC suspension, and found that the incorporation of CMC and XG can significantly enhance the dispersion of CNC suspension when the concentration ratios of CMC/CNC and XG/CNC exceeded 0.05 and 0.5, respectively.In another paper, CNC was rendered cationic by grafting with glycidyltrimethylammonium chloride (GTMAC), and the result showed that the cationically modified CNC was were well dispersed and stable in aqueous media due to enhanced cationic surface charge density. 30Acrylic acid is also a versatile monomer that has been widely used for the grafting of various matrices. 31,32 n the literature, Zhou and Ye 33 studied the graft copolymerization of acrylic acid on the surface of CNC and found that the hydrophilic group content of (sum of sulfonic and carboxylic groups) of the grafted CNC was significantly higher (five times more than the original CNC).In another study, hybrid nanoparticles consisting of CNC tethered with polyelectrolyte brushes were prepared by a three-step process: a) surface esterification of CNC, b) Cu-Mediated surface initiated controlled radical polymerization (SI-CRP) of poly tert-butyl acrylate, c) subsequent hydrolysis of hydrophobic poly t-butyl acrylate to form hydrophilic PAA.The resulting polymers have a very high polymer brush grafting density (ca.0.3 chains/nm), which may be further utilized for many applications. 34n this study, the improvement of the colloidal stability of CNC by surface chemical grafting with poly acrylic acid (PAA) was investigated.It was proposed that the colloidal stability of CNC particles could be improved due to: 1) increased negative surface charge resulting from the introduction of carboxyl groups, and 2) steric hindrance arising from the grafted polymer molecules.Zeta potential and charge density of the grafted products were measured to analyze the surface charge characteristics.Also, particle size measurements and TEM analyses were conducted to evaluate the colloidal stability and the PAA grafting effects.

Materials
The CNC sample was provided by Tianjin Woodelf Biotechnology Co., Ltd., China, which was produced from H 2 SO 4 hydrolysis process.Acrylic acid (99 wt%, analytical grade) was supplied by Tianjin Chemical Reagent Co. Ltd., China.Ammonium persulfate and sodium bisulfite were obtained from Shanghai Chemical Reagent Co. Ltd., China.Deionized water was used throughout the experiments.Dialysis membrane (MD77), with a molecular weight cut-off of 14,000, was purchased from Beijing Henghui Technology Co. Ltd., China.

Grafting of PAA onto CNC
The prepared CNC and water were transferred to a plastic bag so that a desired CNC concentration was reached.Nitrogen gas was purged into the system for 10 min.The initiators (ammonium persulfate and sodium bisulfite) were then added.Subsequently, acrylic acid was added dropwise.The polymer grafting reaction was carried out at 50 °C for 3 h.The aqueous solution was mixed by hand kneading to obtain a homogeneous reaction system.After the completion of the polymer grafting reaction, the PAAgrafted CNC was purified with centrifugation for three times.Then, the suspension was purified by dialysis for one week to remove the unreacted monomer and byproducts.

Determination of particle size, zeta potential, and charge density
The particle size and zeta potential of the original CNC and the PAA-grafted CNC in aqueous medium were determined by using Brookhaven Zeta-Plus Instrument (USA) in combination with 90 plus/BI-MIAS software. 35he charge density of CNC and the PAA-grafted CNC in aqueous medium were measured based on the colloidal titration using a Mütek PCD 04 Particle Charge Detector (Germany). 30A standard cationic polyelectrolyte (poly DADMAC) was used as the titrant.

TEM observation
Aqueous suspension of CNC and the PAA-grafted CNC with solids content of 0.4 wt% was diluted to 0.01 wt% prior to observation.20 uL of the suspension was transferred to a carbon-coated copper grid using a pipette.The grid was air-dried at room temperature overnight.The TEM observation was conducted on JEOL 2010 STEM instrument (Japan) operated at an accelerating voltage of 200 keV.

Improvement of CNC colloidal stability by surface chemical grafting with PAA
][38][39][40] The improvement of CNC colloidal stability by surface chemical grafting with PAA was illustrated in Fig. 1.In the presence of initiators (ammonium persulfate and sodium bisufite), radical-induced polymerization occurred in the presence of CNC and polyacrylic acid (PAA) would be grafted onto the surface of the CNC particles.The grafted PAA could result in the formation of a "hairy" surface structure.The negatively charged carboxyl groups of acrylic acid can be introduced to the surface of CNC particles, leading to increased net negative charges.With the increased net negative charge on the CNC surface, the particle-to-particle electrostatic repulsion can be enhanced.These carboxyl groups, together with the sulfate ester groups, would jointly contribute to the colloidal stability of the CNC particles.Additionally, the steric hindrance arising from the grafted PAA may be another contributing factor of the enhanced colloidal stability.The combination of enhanced electrostatic repulsion and the steric hindrance can decrease the aggregation tendency of CNC particles in aqueous system.

Influence of process conditions for PAA grafting on zeta potential and charge density of PAA-grafted CNC
The zeta potential and charge density can effectively be used for the evaluation of the stability of the colloidal particles. 41,42 or this purpose, the process conditions, including the initiator dosage, CNC concentration, and acrylic acid dosage during the PAA grafting process were studied.
For the initiator dosage effect, with the increase in initiator dosage, the zeta potential and charge density initially increased, then reached a plateau at the dosage of 0.014 mmol/L (as shown in Fig. 2 (a)) due to the fact that the increased initiator dosage can result in more active sites to free radicals on the CNC backbone, which lead to greater PAA grafted on the surface of CNC, thus increasing the zeta potential and charge of CNC.However, a further increase in the initiator dosage failed to further enhance the zeta potential and charge density.This is because the excessive initiator could inhibit free radical polymerization.Similar observation was reported by Singha and Rana 43 , who found that excessive initiator dosage can decrease the grafting percentage (P g ) and efficiency percentage (P e ) in the graft copolymerization of acrylic acid onto fibers.In another paper, Thakur etal. 44ynthesized the butylacrylate-g-cellulose by microwave radiation assisted method, and found that the P g increased while increasing the initiator concentration, where further increase in molar ratio, would tend to decrease the P g due to the fact that an excess of initiator would result in the termination of the growing chains.
Increasing the CNC concentration during the PAA grafting process showed a positive impact on the charge characteristics of the resulting PAA grafted CNC, as shown in Fig 2 .(b).It can be seen that the zeta potential increased from -43 mv to -50 mv when the CNC concentration was increased from 0.8 to 10 wt% in the grafting process.Correspondingly, a significant increase in charge density of the modified CNC was observed (0.41 vs 0.79 mmol/g).In literature, Witono et al. 45 investigated the grafting copolymerization of cassava starch with acrylic acid using a free radical initiator system and found that a high starch concentration (10 wt%) resulted in relatively higher grafting efficiency.
The effect of acrylic acid dosage on the grafting copolymerization was presented in Fig. 2(c).It can be seen that the dosage of acrylic acid exhibited a marked effect on the zeta potential and charge density.Compared to the unmodified CNC (-34 mv zeta potential and 0.31 mmol/g and charge density), the zeta potential and charge density of the modified CNC increased to -43mv and 0.47 mmol/g at 0.5 wt% acrylic acid dosage, and further increased to -51 mv and 0.8 mmol/g when the acrylic acid dosage increased to 1%.The higher monomers concentration leads to a higher grafted carboxyl group on cellulose chain, resulting in the increase of zeta potential and charge density.However, both the zeta potential and charge density of the modified CNC had a slight increase when the acrylic acid addition exceeded 1 wt% due to the fact that the homopolymerization dominated over the grafting polymerization.In literature, guar gum was modified by graft copolymerization with acrylic acid in aqueous medium, and the results showed that the P e , P g , and add-on initially increased with increasing the acrylic acid concentration, but beyond the optimal dosage (0.02 mol/dm 3 ) these parameters decreased. 46

Colloidal stability
For any colloidal or coarse-colloidal system (e.g., the cellulosic fiber suspension), the stability of the particles or colloids is related to their capability against the particle-to-particle aggregation induced by physical or chemical interactions. 47In the case of CNC, the surface hydroxyl groups on the CNC rods tend to form hydrogen bonds, leading to the formation of aggregates. 19Thus, the natural affinities among the CNC particles impair their colloidal stability or water-dispersability.The particle size of CNC particles/rods present in the aqueous medium can be an indication of colloidal stability. 30The aggregation of particles/rods under various conditions (e.g., high solids content) can lead to the increase in particle size.The average particle sizes of the PAA-grafted CNC (prepared under the optimum conditions) and CNC under varied conditions are shown in Fig. 3.The average particle size of the PAA-grafted CNC showed significantly better stability against the increase in storage time or solids content of the aqueous medium, in comparison with that of the original CNC.The TEM images also confirmed that PAA-grafting significantly enhanced the dispersion of CNC (Fig. 4).These results agreed well with the concept of improving CNC colloidal stability by grafting PAA, as shown in Fig. 1.The improvement in the CNC colloidal stability can be attributed to the enhanced particle-to-particle repellence arising from the increased net negative charge and the PAA-induced steric hindrance.

CONCLUSIONS
Grafting PAA onto the surface of CNC improved its colloidal stability in aqueous medium.The PAA surface grafting led to the increases in the zeta potential and charge density of the CNC particles/rods.An acrylic acid dosage of 1% (based on the weight of CNC) was sufficient to achieve improved colloidal stability.The optimum grafting conditions were: initiator dosage of 0.014 mmol/L, CNC concentration of 10%, and acrylic acid dosage of 1%.Under these conditions the zeta potential and charge density increased from about -34 mv and 0.31 mmol/g to -51 mv and 0.8 mmol/g, respectively.The obtained PAA-grafted CNC exhibited stable particle size during a 30-day storage period at 0.8% CNC concentration, or at 6% CNC concentration.In contrast, the un-modified CNC showed substantial aggregation in both cases.

Fig. 1
Fig. 1 Schematic of the improvement of CNC colloidal stability in an aqueous medium by chemical grafting

Fig. 4
Fig. 4 TEM images of CNC and modified CNC: (a) the original CNC (b) the original CNC after 30 days, and (c) the modified CNC after 30 days Note: the PAA grafting conditions were: CNC concentration: 10%; initiator dosage: 0.014 mmol/L; acrylic acid dosage: 1%