Isolation of thermostable cellulose II nanocrystals and their molecular bridging for electroresponsive and pH-sensitive bio-nanocomposite

Highlights

• ADES system was developed to produce cellulose Ⅱ nanocrystals under mild condition.

• ADES system was green, sustainable and could be recycled easily.

• CNCs had a high yield of 82.3 %, excellent thermostability, and high crystallinity.

• Electroresponsive and pH-sensitive nanocomposite films were built with CNCs assist.

• CNCs played molecular bridging and nano-sized role in nanocomposite film building.

Abstract

Sustainable, economical, and green extraction of cellulose Ⅱ nanocrystals from lignocellulose under mild conditions is still challenging since the highly recalcitrant nature of lignocellulose. Herein, a sustainable system using ammonium persulfate (APS)-catalyzed deep eutectic solvent (ADES) was developed to produce functional cellulose Ⅱ nanocrystals (CNCs) with a high yield of 82.3 %. The obtained CNCs had excellent thermal stability (>350 ℃), high crystallinity of 76.6 % with surface carboxyl groups, and superior dispersion stability. CNCs played a role of both molecular bridging and nano-sized effect in gelatin matrix, which combined with gelatin tightly through multiple hydrogen bonds to prepare bio-nanocomposite films with outstanding electric-responsiveness and pH-sensitivity. It was proved that CNCs had remarkable reinforcing potential and effective stress transfer capacity, contributing to a 187 % enhancement in tensile strength and an approximately 110 % increase in Young’s modulus of the formed CNCs/gelatin nanocomposite films when 0.5 wt % CNCs was loaded. Therefore, the study may open the door for developing a green and sustainable avenue to produce functional CNCs, and is of great significance for the building of CNCs-based functional nanocomposites.

Introduction

Cellulose Ⅱ nanocrystals (CNCs), as renewable, biodegradable, and nontoxic type-II crystal form nanomaterials have been widely used in biomaterials (Du et al., 2019; Tong et al., 2018), electronic materials (Zhang et al., 2018), optical device (Kang et al., 2018), sensors (Dai et al., 2017), and nanocomposites (Liu, 2018) due to their excellent mechanical performance, low density, and low thermal expansion (Rajinipriya et al., 2018). However, CNCs composed of both amorphous and crystalline regions are tightly embedded in the cell wall through strong hydrogen bonding, causing the obstacle of efficiently fractionate CNCs from lignocellulose using a mild condition. In addition, CNCs are usually prepared by mercerization of cellulose Ⅰ nanocrystals (Jin et al., 2016), or sulfuric acid hydrolysis (Sèbe et al., 2012), but the crystal structure and thermal performance of the obtained CNCs are easily impaired, and the yields are low. Therefore, developing green and mild methods is a key aspect to realize the large-scale production of high performance CNCs.

Existing fabrication methods of nanocellulose include mineral acid hydrolysis (Yu et al., 2019, 2013), pure mechanical disintegration (Velásquez-Cock et al., 2016), organic acid hydrolysis (Bian et al., 2019; Chen et al., 2016), enzymatic hydrolysis (Chen et al., 2018; Nie et al., 2018), oxidation (Leung et al., 2011; Patankar and Renneckar, 2017), or ionic liquid treatment (Abitbol et al., 2018; Miao et al., 2016). Unfortunately, pure mechanical disintegration is energy-intensive, mineral acids usually cause equipment corrosion and environmental pollution, enzymatic hydrolysis is low-yield, and ionic liquid is toxic and high-cost (Hu et al., 2015; Du et al., 2016). Organic acids are recyclable, low corrosive, and green, but their weak acidity leads to lengthy reaction and low yield of hydrolysis. Recently, because of the low price, good solvent ability, and recyclability, deep eutectic solvents (DESs) have been used as green solvents to convert biomass into platform compounds and pretreat lignocellulose to extract nanocellulose sustainably (Shen et al., 2018; Sirviö et al., 2016; Tang et al., 2017; Guo et al., 2019). However, further mechanical fibrillation (e.g., ultrasonication, shearing or ball milling) is usually indispensable in the DESs processing, implying that the hydrolysis capacity of DES is weak. It was reported that FeCl₃ has been combined with DES system to improve the hydrolysis efficiency of cellulose (Yang et al., 2019), nevertheless, the recycle of FeCl₃ and the post-processing were tedious. As far as the cellulose structure is concerned, nanocellulose generally retains its native cellulose Ⅰ polymorph. To the best of our knowledge, the direct and rapid extraction of functional CNCs with high thermostability and crystallinity from lignocellulose has been rarely reported.

In the present study, a sustainable and green hydrothermal-assisted ammonium persulfate (APS)-catalyzed oxalic acid/choline chloride (ADES) system was employed to directly produce functional CNCs from a cellulose Ⅰ substrate. It was found that a high yield of 82.3 % of CNCs with high thermal stability and crystallinity was obtained at 80 ℃ for 2 h. The oxalic acid and choline chloride of ADES were recovered by recrystallization and reduced pressure distillation respectively, which can be reused to extract CNCs for the next cycle. Therefore, few wastes were generated in the whole process, benefiting to the environment, and implying the establishment of a sustainable and economic avenue for the mass production of functional CNCs. Moreover, the fabricated CNCs was proved to have molecular bridging and reinforcing effects on gelatin, which combined with gelatin to form bio-nanocomposite films with electroresponsive and pH-sensitive capacity. The significance of the present study lies in the sustainable and recyclable ADES, which induces the green and low-cost production of CNCs, contributing to their plethora of diversified applications in nanocomposites.