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Polysaccharide Nanocrystals: Current Status and Prospects in Material Science

Jin Huang, Peter R. Chang, and Alain Dufresne

1.1 Introduction to Polysaccharide Nanocrystals

Native polysaccharides usually consist of crystalline and amorphous regions; to produce highly crystalline polysaccharide nanocrystals (PNs) the amorphous component is removed through acid hydrolysis. The morphologies and dimensions of PNs strongly depend on the different sources of biomass and different extraction methods. Figure 1.1 depicts the transmission electron microscope (TEM) images of rod-like cellulose and chitin nanocrystals, and platelet-like starch nanocrystals [1]. It is worth noting that the surface properties of PNs are influenced by the extraction methods. The H2SO4 hydrolysis protocol usually produces sulfate groups on the surface of PNs, resulting in improved dispersibility in water and lower thermal stability [2, 3]. By comparison, PNs with higher thermal stability may result from HCl hydrolysis, but the resultant suspension aggregates easily in water and shows poor dispersibility [4, 5]. Moreover, PNs with improved dispersibility in water and thermal stability can also be successfully obtained using an acid mixture consisting of hydrochloric acid and an organic acid, such as acetic or butyric acid [6]. In exploring economical routes for enhancing the efficiency and yield of PN production many approaches have been attempted including pretreatments [7], hydrothermal methods [5], microwave- and ultrasonic-assisted technologies [8, 9], and so on.

Figure 1.1 TEM images of cellulose nanocrystals from cotton linter with 200-300 nm length and 10-15 nm width (a); chitin nanocrystals from crab shell with 200-600 nm length and 10-20 nm width (b); and pea starch nanocrystals 6-8 nm thick, 40-60 nm in length, and 15-30 nm in width (c).

The rod-like nanocrystals of cellulose and chitin show a predominant characteristic of high aspect ratios, and thus their suspensions display many unique properties, such as cholesteric liquid crystallinity and flow birefringence. These properties showed a dependence on concentration, and phase behavior was affected by electrolytes [10-14]. When the suspensions reached a critical concentration, the rod-like nanocrystals exhibited an ordered phase displaying flow birefringence and nematic or chiral nematic structures. Cellulose nanocrystals (CNs) exhibit birefringence not only in aqueous suspensions but also in organic solvents such as dimethylsulfoxide (DMSO), dimethyl formamide (DMF), cyclohexane, and toluene [15, 16]. At the same time, it has been found that surface-modified CNs, that is, carboxymethylated, tetramethyl-piperidin-1-oxyl (TEMPO) oxidized [17], or silylated CNs, displayed birefringence in suspension with tetrahydrofuran (THF) [18]. CN suspensions possess distinctive rheological properties [19] and may form gels from aqueous glycerol suspensions with careful evaporation at 70 °C, when the CN concentration is below 3 wt% [20]. Starch nanocrystals have a unique platelet-like structure and often tend to aggregate in aqueous solution [21], although a stable suspension can be achieved by adjusting the pH [22]. Detailed descriptions of the structure and properties of PNs can be found in Chapter 2.

The abundant hydroxyl groups on the surface of PNs contribute to their positive surface chemical properties and are therefore an essential route to altering surface structure, regulating surface properties, and developing functional materials. The common methodologies for chemical modification of PNs, presented in Figure 1.2, can be classified in three categories: small molecule conjugation, "graft onto," and "graft from" strategies for polymers. There are three main mechanisms of small molecule conjugation, including isocyanation [23], silylation [18, 24], and esterification [25], depending on the availability and activity of the surface hydroxyl groups. In addition, alkynylation and azidation can be used to import alkynyl or azide groups for click chemistry using the Huisgen reaction [26]. The "graft onto" strategy for polymer grafting abides by the mechanism for small molecule conjugation, but the grafting efficiency may be inhibited by the steric hindrance of large polymer chains in contrast with the conjugation efficiency of small molecules [27-29]. In addition, hydroxyl groups on the surface of PNs can directly initiate ring opening polymerization (ROP) of lactones [30] and free radical polymerization (FRP) of olefins to graft polymer chains based on the "graft from" strategy [31]. At the same time, by using small molecule conjugation to import initiating groups, controlled radical polymerization, that is, atom transfer radical polymerization (ATRP), can be achieved using the functionalized Br atom as the initiating point [32]. Detailed surface chemical modification of PNs is described in Chapter 3.

Figure 1.2 Chemical modification methodology and typical examples toward the surface of polysaccharide nanocrystals.

1.2 Current Application of Polysaccharide Nanocrystals in Material Science

PNs have been widely used as a reinforcing biomass-based filler to modify polymeric materials with matrixes of rubber, polyolefin, polyurethane and waterborne polyurethane, polyester, and natural polymer plastics of starch and protein. The reinforcing mechanism depends mainly on the formation of a three-dimensional PN network and the interfacial miscibility between the PN surface and matrix. The former abides by the percolation model and requires PN content to be higher than a critical concentration; the latter can be improved by the formation of strong interfacial interaction or by a co-continuous phase mediated by surface-grafted polymer chains that alter the surface chemical structure through surface modification of PNs [33-37]. Casting/evaporation processing is the most suitable method for development of the three-dimensional PN network because it provides enough freedom to form hydrogen bonds among PNs before and during solidification of the nanocomposite [37-41]; however, it requires high dispersibility of PNs in the aqueous or organic blending medium during compounding and solvent evaporation. Conversely, melt-compounding and thermoforming methods, such as intensive blending, extrusion, compression molding, and injection molding, may inhibit hydrogen bonding because of the relatively high melting viscosity, and may cleave associations between PNs by shear force and hence fail to construct a three-dimensional network. Furthermore, the commonly used method that uses sulfuric acid hydrolysis for extraction of cellulose and starch nanocrystals results in low thermal stability, which does not match the requirements of thermoprocessing. It is fortunate that physical and chemical modification of the PN surface can significantly enhance thermal stability to contribute to the application of industrial-scale production of PN-modified nanocomposites by thermoprocessing. As mentioned previously, surface physical and chemical modification of PNs play key roles in improving dispersibility in solvent, regulating miscibility with the polymer matrix [37], and enhancing thermal stability to facilitate the enhanced performance of PN-filled nanocomposites together with extension of processing methods and applications [42]. New processing technologies have also been used to develop structural and functional PN-based materials. For example, mats and nanofibers reinforced by PNs have been successfully prepared using electrospinning technology [43-46], while layer-by-layer assembly has been attempted for the preparation of well-defined multilayer films [47-49]. In addition, the sol-gel method can be used to make bulk nanocomposites that inherit the chiral nematic structure of the PN suspension for further development of functional materials. A detailed introduction and relevant preparation methods of PN-modified nanocomposites can be seen in Chapters 4 and 5, respectively. Furthermore, characterization of PN-based materials is key to understanding the structure-properties relationship, as elaborated in Chapter 7.

Many advanced applications for PN-based nanocomposites that depend on surface modification and compounding technologies have been explored. Figure 1.3 depicts the main achievements of PN-based functional materials [36]. PNs with unique rod and platelet structures can act as templates for the controlled preparation of inorganic nanoparticles. For example, TiO2 nanocubes with high crystallinity and uniform size were synthesized using a template of rod-like CNs [50]. Moreover, via the sol-gel method, the chiral nematic structure inherited from cellulose and chitin nanocrystals in suspension has been used to develop tunable optical biomimetic nanomaterials that are similar to the brilliant iridescent colors of beetle exoskeletons [51]. At the same time, the chiral nematic structure of the CN component in nanocomposites also acted as the template in the preparation of many kinds of silica [52], carbon [53], and titanium-based [54] mesoporous materials. Except for those smart materials with tunable optical functions based on the chiral nematic structure, the moisture sensitivity of the three-dimensional PN network is the driving force for the preparation of mechanically adaptive biomimetic nanomaterials [36, 55]. The moisture-sensitive behavior of the nanocomposites, is ascribed to the formation and decoupling of a...