Materials
Calotropis procera fiber as raw cellulose materials was kindly supplied by World Agroforestry Centre, Kunming institute of Botany, CAS. Acetic acid and hydrochloride acid were purchased from Sinopharm Chemical Reagent Co., Ltd. (Beijing, China). Sulfuric acid was purchased from Pinghu Chemical Reagent Co., Ltd (Pinghu, China). All the chemicals used in this research were of analytical grade and applied without further purification.
Determination of the chemical composition of Calotropis procera fiber
The chemical compositions of Calotropis procera fiber (CPF) and the fiber after each treatment were determined according to Technical Association of the Pulp and Paper Industry (TAPPI) standard. The lignin content was determined according to the TAPPI norm T222 om-88 (de Oliveira et al. 2016; Espino et al. 2014). Briefly, 1 g of CPF was added into the 15-mL H2SO4 solution (72 wt%), and maintained at room temperature for 2 h. Then, the distilled water, 560 mL, was added and boiled the mixture for 4 h before the centrifuge to get the insoluble lignin. The obtained lignin was oven-dried and weighted. The lignin quantity was determined using Eq. (1).
$${\text{Lingin }}\left( \% \right) = \frac{{M_{1} }}{M} \times 100,$$
(1)
where M1 was the obtained lignin mass, M was the initial sample mass.
Hemicellulose content was determined according to the TAPPI T257 om-09. Holocellulose content was quantified with sodium chlorite treatment according to the reported procedure (dos Santos et al. 2013). Fiber, 1 g, was added into the solution, 30 mL, containing acetic acid 0.25 mL, sodium chlorite 0.3 g and kept at 75 °C for 1 h. The mixture is then cooled down and the residue is filtered and washed thoroughly with water. The residue was finally dried and weighted. The holocellulose content was calculated using Eq. (2).
$${\text{Holocellulose }}\left( \% \right) = \frac{{M_{2} }}{M} \times 100,$$
(2)
where M2 was the obtained residue mass, M was the initial sample mass.
Cellulose content was determined by extracting holocellulose with the aqueous sodium hydroxide (17.5%) for 5 h before quenching the reaction with ice. The obtained white powder was washed with copious amount of water until filtrate becoming neutral (Xu and Hanna 2010). The cellulose content was calculated using Eq. (3).
$${\text{Cellulose }} \left( \% \right) = \frac{{M_{3} }}{M} \times 100,$$
where M3 was the obtained white powder mass, M was the initial sample mass.
Pretreatment of Calotropis procera fiber
Calotropis procera fiber (CPF) was first washed and dried. The pretreatment process of CPF included alkali treatment, delignification and bleaching treatment (Ray et al. 2001; Neto et al. 2013). Firstly, the cleaned CPF, 1 g, was immersed in NaOH solution (2 wt%), 100 mL, at room temperature and stirred for 3 h to finish the alkali treatment to remove the impurities including pectin, wax. The alkali-treated fiber was washed thoroughly till neutral. The delignification process was carried out with dewaxed CPF, 1 g, suspended in acetic acid (93% v/v), 50 mL, and hydrochloride acid solution (0.3% v/v) under strong stirring for 3 h at 90 °C and then washed till neutral pH value. The bleaching procedure was performed by adding the obtained delignined CPF into the mixture of H2O2 (5 wt%) and NaOH (3.8 wt%) at a ratio of 1:50 and stirred at room temperature for 3 h before washing thoroughly.
Extraction of cellulose nanocrystal
Cellulose nanocrystal (CNC) was obtained by sulfuric acid hydrolysis according to the reported method (Lin et al. 2012). The pretreated CPF powder, 1 g, was slowly added into H2SO4 solution (63 wt%), 30 mL, under vigorously stirring at room temperature for 1 h. After that, the hydrolysis was quenching by adding iced water, 300 mL, into the mixture. The resultant mixture was first centrifuged at 1000 rpm for 10 min to remove large particles, and then centrifuged at 11,000 rpm for 15 min to obtain cellulose nanocrystal. The obtained cellulose nanocrystal was washed and centrifuged repeatedly for 3 times before dialysis against distilled water for 2 days. The obtained CNC was processed by ultrasonic processor (VCX 500:500 W, Sonics & Materials, Newton, CT) to suspension better before further application.
Particle size and zeta potential measurement of the obtained cellulose nanocrystal
The particle size and zeta potential of the obtained CNC were measured by Nano-Z5 Analyzer (Malvern Instruments Co., Ltd., UK). The obtained CNC suspension was diluted to 0.1–0.6 wt% concentration firstly. Then, the suspension was put into a container for measurement.
Morphology analysis
Morphologies of the obtained CNC were measured by Hitachi S-4800 field emission scanning electron microscope (Hitachi Co., Ltd., Japan). CNC powder was obtained by freeze drying of CNC suspension and the powder was sputter coating with a layer of gold firstly before SEM analysis. Also, TEM analysis was carried out for morphology measurement. The obtained CNC suspension was diluted to 1–4 mg/mL concentration before deposited into copper grid. TEM analysis was carried out at 100 kV. The diameter and length distribution of the obtained CNC was analysis using Image J software (National Institutes of Health Co., Ltd., USA).
Characterization of the obtained cellulose nanocrystal
FTIR analysis of the obtained CNC was performed by Nicolet 6700 spectrometer (Thermo Fisher Scientific, USA) using KBr pellet methods. TG analysis of CNC was performed by TGA-50 thermal stability analyzer at a heating rate of 15 °C/min form room temperature to 600 °C using N2 atmosphere. The crystallinity index of the obtained CNC was analyzed by Bruker D4 X-ray diffractometer. The measurement was carried out at 40 kV under Cu Kα radiation. CrI of the samples was calculated by Eq. (3) (Mariano et al. 2016).
$${\text{CrI}} = \frac{{I_{002} - I_{\text{am}} }}{{I_{002} }} \times 100,$$
(3)
where I002 was the intensity of 200 peak (I200) between 2θ = 22–23° and Iam was the minimum intensity between the peaks at 200 and 110 (Iam) 2θ = 18–19°.