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Received Jan 20, 2018; Accepted Feb 21, 2018
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1. Introduction
Cellulose is the most abundant natural polymer and can be sustainably produced from various biomass. The global annual output of cellulose is approximately 75 to 100 billion tonnes [1]. The development and utilization of cellulose is of great significance to the sustainable development of human society, especially the energy scarcity in the future. Nanocelluloses (NCs) could be produced by the degradation of cellulose from various sources of biomass and have many unique properties, such as high specific surface area, high aspect ratio, high tensile strength and stiffness, low density, and low thermal expansion coefficient [2]. Recently, the application of NCs has attracted great attention [2–5]. NCs include cellulose nanocrystals (CNCs), cellulose nanofibrils (CNFs), bacterial cellulose (BC), and electrospun cellulose nanofibers (ECNFs). CNCs and CNFs are more common than the other NCs. This is because CNCs and CNFs are gained by disintegration of raw cellulose materials into nanosize particles (top-down process), while BC and ECNFs are produced by a build-up of nanofibers from low molecular weight sugars by bacteria and electrospinning, respectively. This review will focus on CNCs and CNFs, while BC and ECNFs will not be covered. There is a significant difference between CNCs and CNFs in terms of dimension and crystallinity. CNCs are highly crystalline and usually have a length less than 500 nm, while CNFs are made up of amorphous and crystalline cellulose chains and have a length of up to several microns. Both of them have immense potential in the manufacture of renewable and biodegradable materials [6, 7]. They have been applied in the fields of biomedical engineering, food, sensor, packaging, optical and electronic devices, and so on [8–10]. Previous reviews have summarized their source, chemistry, and applications [2, 11–25]. The present review mainly focuses on the advances in preparation of CNCs and CNFs in order to display a clear development direction for the preparation of CNCs and CNFs.
2. Preparation of Cellulose Nanocrystals
CNCs have high crystallinity with diameter less than 100 nm and length less than 500 nm, which are formed though intra-...