Acetylated cellulose nanocrystals with high-crystallinity obtained by one-step reaction from the traditional acetylation of cellulose

Highlights

• ACNCs are prepared by one-step reaction via the traditional cellulose acetylation.

• The crystallinity of ACNCs is up to 70%.

• This study provides a simply new way to prepare ACNCs with high crystallinity.

Abstract

One-step reaction instead of two-step or multi-step reaction is simple and feasible to prepare the acetylated cellulose nanocrystals (ACNCs). Herein, a common and moderate one-step reaction for obtaining the acetylated cellulose nanocrystals (ACNCs) with high-crystallinity via traditional acetylation of cellulose is first proposed. Morphology property analysis indicates that ACNCs with a rod-like structure have a length of 21-140 nm and a width of 12-20 nm. The ACNCs show an allomorph of cellulose II with a high-crystallinity of ∼70%. ¹³C NMR and FT-IR analysis indicate that the obtained ACNCs have been acetylated, and the DS of ACNC-1.5 and ACNC-2.0 are 0.18 and 0.42, respectively. By studying the reaction mechanism, it can be found that ACNCs were obtained in the synergetic process of hydrolysis and acetylation. They can be redispersed in both water and organic solvent systems. This study provides a simple new method to prepare the acetylated CNCs with high crystallinity.

Introduction

Cellulose has attracted immense interest from scientific researchers due to its great potential for producing a variety of high-value-added, renewable products (Habibi, Lucia, & Rojas, 2010; Qiu & Hu, 2013; Siró & Plackett, 2010). Cellulose nanocrystals (CNCs) are obtained by acid hydrolysis of cellulose under controlled conditions in which the cellulose chains are broken down and the amorphous regions are removed. Acids commonly used for cellulose hydrolysis mainly include sulfuric acid (Klemm et al., 2011; Roman & Winter, 2004), hydrochloric acid (Cheng et al., 2017; Yu et al., 2013) and phosphoric acid (Tang et al., 2015). CNCs got by hydrolysis of sulfuric acid can stably disperse in water (Abitbol, Kloser, & Gray, 2013; Lu & Hsieh, 2010; Wu, Xu, Gong, Li, & Mo, 2018). Because of this hydrophilic property, they are poorly dispersed in hydrophobic polymer substrates (Lin, Huang, Chang, Feng, & Yu, 2011; Yan et al., 2013). Therefore, surface hydrophobic modification of CNCs has been proposed as an available route to change surface hydrophobicity of CNCs (Habibi et al., 2010; Habibi, 2014; Klemm et al., 2011; Lin et al., 2011).

Acetylation which is simple and versatile can enhance the hydrophobicity of CNCs. Most of the preparation processes of ACNCs include two steps: the strong acid hydrolysis of CNCs and acetylation (Ávila Ramírez, Fortunati, Kenny, Torre, & Foresti, 2017; Lin et al., 2011; Wu et al., 2018; Yan et al., 2013; Zhao, Zhao, Wang, & Peng, 2016). However, the multiple steps of these modification processes are not only costly and time consuming, but also damaging to the morphology and crystallinity of CNCs (Abraham et al., 2016; Habibi et al., 2010; Wu et al., 2018). As we all known, the acetylation of CNCs in an acid system improve their thermal stability, but the crystallinity is greatly reduced from ∼80% to ∼45% (Abraham et al., 2016; Eyley & Thielemans, 2014; Wu et al., 2018). This illustrates that the crystalline structure of CNCs is inevitably destroyed in the process of modification.

One-step method to achieve the preparation and modification of CNCs can avoid the erosion of CNC crystalline structure in multi-step reaction to some extent. This may be an effective solution to obtain ACNCs with high-crystallinity. So far, only several studies on the surface modification of CNCs in one-step reaction have been reported. Dorgan’s group first reported a study on the acetylation of CNCs by single-step method in 2009 (Braun & Dorgan, 2009; Sobkowicz, Braun, & Dorgan, 2009). CNCs were isolated and surface modified through the Fischer esterification process by using a mixture of organic acid (acetic and butyric acid, respectively) and HCl. Subsequently, a study on the preparation of ACNCs by one-step process was demonstrated using phosphoric acid as a strong acid for hydrolysis and catalysis and acetic anhydride as an acetylation reagent, in which different degrees of substitution of ACNCs were obtained by controlling the reaction time (Yan et al., 2013). These works involving one-step methods to replace multistep methods are particularly beneficial for the environment and economy. However, these few works do not provide many details about the crystal allomorph, crystalline structure and properties of the obtained ACNCs. Therefore, it is necessary to develop some new one-step methods, from which to study the differences in crystal structure and performance compared with the two-step method in detail.

Herein, a common and moderate one-step method for obtaining the acetylated cellulose nanocrystals (ACNCs) with high-crystallinity is first proposed via traditional acetylation of cellulose. Isolation and functionalization of CNCs were simultaneously occurred from the traditional cellulose acetylation reactions with acetic anhydride as the acetylation reagent, sulfuric acid as the catalyst and acetic acid as the dispersal agent (as shown in Fig. 1) (Malm, Tanghe, Laird, & Smith, 1953). The sample treatments performed after the traditional acetylation of cellulose indicated both acetylated CNCs and acetylated cellulose (the residue) were the products. The properties of the ACNCs and acetylated cellulose were investigated, and the mechanism of concurrent acetylation and hydrolysis was exploited. In the general preparation method of CNCs, people focus on the quality of the CNCs and neglect the utilization of their residue. In this study, we not only obtained the desired CNCs in one-step reaction but also fully considered the utilization of residues in the preparation process of CNCs.