2.1. Materials
CF11 (cotton-origin cellulose powder) sample was purchased from Whatman (UK), dextran T10 (average molecular weight 10,500, purity 95–100%) was purchased from Pharmacia (Denmark), hydrochloric acid (concentration 35–37%), 2,2,6,6-tetramethyl-1-piperidinyl radical (TEMPO), sodium hypochlorite (nominal concentration 4%+), sodium chloride (purity 99.5% or more), sodium sulfite (purity 97% or more), starch (corn), sodium hydroxide (0.1 mol L− 1), ammonium chloride (purity 99.5% or more), sodium cyanoborohydride (purity 80% or more), sodium bicarbonate (purity 99.5-100.3%), 2,4,6-trinitrobenzenesulfonic acid (TNBS, purity 98%), N-hydroxysuccinimide (NHS, purity 98–102%), 1-ethyl-3-3-dimethylaminopropyl carbodiimide (EDC, purity unknown) and calcium chloride (purity 95%) were purchased from Fuji Wako Pure Chemicals (Japan).
2.2. Preparation and oxidation of cellulose nanocrystals
Cellulose nanocrystal (CNCs) was prepared by partial hydrolysis of the cellulose powder (CF11 10 g), which was dispersed in 100 ml of 2.5 N hydrochloric acid and boiled for 15 min. Dispersed nanocrystal was collected by suction filtration and washed repeatedly with deionized water to reach neutrality with pH paper. The filter cake was processed with a Waring blender for 30 minutes. The large aggregate particles were removed by centrifugation (1600 G for 5 min) [8]. Collected CNCs were rodlike particles 10 ~ 20 nm wide 50 ~ 100 nm long and irregular aggregates thereof (Fig. 1).
The solids concentration of the cellulose nanocrystals water suspension was determined by oven drying (98°C.). One gram of TEMPO, 10 g of NaBr and 90 g of 9% NaClO were added to 1 L of 1% CNCs. The pH was kept to 10–11 by adding 3 N NaOH and stirred at room temperature for 2 h. The resulting TEMPO-oxidized cellulose nanocrystals (TOCNCs)l were recovered by addition ~ 30 g sodium (1 M) chloride centrifugation (8000 rpm 15 min) and washed twice by centrifugation to remove reagents. A small amount of 0.1 N HCl was added to the suspension to convert the carboxylic acid from sodium salt to free acid. The product was recovered by centrifugation. After repeated centrifugation, the suspension was dialyzed against deionized water to reach neutrality by pH paper. The solid concentration was determined by oven drying.
The oxidation level, i.e. the amount of introduced carboxylic acid was determined by conductometric titration [9]. The titration was performed as follows. 5 mL of 0.01 M NaCl solution and 2 mL of 0.1 N HCl were added to 45 mL of 1.0% nanocrystal suspension. Under monitoring of conductivity and pH, 0.1 N NaOH each was added to the suspension to determine the amount of carboxylic acid.
5 mL of 0.01 M NaCl solution was added to disrupt part of the Donan equilibrium so that the moving ions were more evenly distributed and not trapped in the fibers. Also, 2 mL of 0.1 N HCl solution was added to clarify the first inflection point of the graph
2.3. Amination of dextran
Dextran is a mainly linear homo-polysaccharide glucan with one reducing group at the end. The aldehyde group can be converted to amine by reductive amination [10]. We dissolved 10 g of dextran T10 (Pharmacia, nominal average molecular weight 10,500) in deionized water to make 200 mL of solution. 2 g of sodium cyanoborohydride and 11 g of ammonium chloride were added and constantly agitated at pH 7.4 (adjusted by 0.1 N NaOH) and 37°C for 6 days. The product was purified by dialysis and freeze-dried. To determine the amount of amino groups, 3 mL of 4% sodium bicarbonate solution and 3 ml of 0.1% 2,4,6-trinitrobenzenesulfonic acid solution were added to 3 mL (0.6-1 mg mL-1) of the amino-dextran solution, and allowed to stand for 2 hours at 40°C at pH 9 (adjusted by 0.1 N NaOH. The 1.5 mL of 1 N hydrochloric acid was added, and the absorbance at 335 nm was measured by a Shimadzu UV-1200. The amine content was calibrated using ammonium chloride [11]. The result showed that more than 90% of dextran’s reducing end was aminated. The product was used without further purification. The aminated dextran is referred to as D-NH2 hereafter.
2.4. Grafting of dextran on cellulose nanocrystals
Freeze-dried D-NH2 (28.6 g), N-hydroxysuccinimide (0.346 g) and 1-ethyl-3-3-dimethylaminopropyl carbodiimide (EDC, 0.573 g, 1.5 equivalent to the 2 mmol carboxyl group) of were added to TOCNCs suspension (1.4%, 139 g) and stirred at pH 7.5–8.0 (adjusted by adding 0.5 N NaOH or 0.5 N HCl) at room temperature overnight. 0.5 N HCl solution was added to stop the reaction at pH ≈ 1. The reaction formula is shown in Fig. 2 [2, 12]. The product was dialyzed against deionized water for 4 days. The tube (CE membrane, Spectrum brand) has cut-off molecular weight of 100,000, and expected to remove un-grafted dextran. The solid weights before and after grafting were determined gravimetrically (dried at 98°C). The same operation was carried out on unmodified dextran and unmodified CNCs suspension and resulted in 97% removal of dextran and 99% recovery of CNCs. The dextran grafted TOCNCs is referred to as D-TOCNCs hereafter.
2.5. Dispersibility test
The viscosity of the suspension was measured using a Brookfield viscometer (Brookfield LVDV-I +) at 25°C and shear rate of 61 s− 1 after 120 sec of shearing. The effect of salt was examined by adding 0.05 M CaCl2 for varied concentration of CNCs.