A review on versatile applications of blends and composites of CNC with natural and synthetic polymers with mathematical modeling

Highlights

• The CNC can be used as nanofiller to reinforce several synthetic and natural polymers.

• Comprehensive overview of modification using CNCs as reinforcer in respective polymers matrix

• The immense activities of CNCs are successfully utilized to enhance the mechanical properties.

• Technical issues highlighting the recent advancement in biomedical and packaging field.

Abstract

Cellulose is world's most abundant, renewable and recyclable polysaccharide on earth. Cellulose is composed of both amorphous and crystalline regions. Cellulose nanocrystals (CNCs) are extracted from crystalline region of cellulose. The most attractive feature of CNC is that it can be used as nanofiller to reinforce several synthetic and natural polymers. In this article, a comprehensive overview of modification of several natural and synthetic polymers using CNCs as reinforcer in respective polymer matrix is given. The immense activities of CNCs are successfully utilized to enhance the mechanical properties and to broaden the field of application of respective polymer. All the technical scientific issues have been discussed highlighting the recent advancement in biomedical and packaging field.

Introduction

The world's most abundant, recyclable and biodegradable polysaccharide is cellulose [1,2]. It is structural component of cell wall of plants. It is hydrophilic, odorless and tasteless in nature [3]. It has wide range of applications in paper and cardboard industry due to its biodegradability, biocompatibility and renewability [4]. Nanocellulose is produced by delaminating cellulosic fibers [5]. The extraction of CNCs can be done by hydrolysis process in which biopolymer is treated with strong acid then resulting product would be needle like crystallites of cellulose chain which are commonly called nanocrystals (CNCs) or whiskers [6]. By adding cellulose nanoscale fillers in different polymers we can generate cellulosic nanocomposites resulting in mechanical reinforcement and alternation of other properties [7].

The primary source of cellulose is plants because they are cheap and abundantly available other sources are like bacteria, algae and tunicate (marine animals) [8]. Cellulose in its pure form found in fibers of textile plants such as cotton, ramie and jute [9]. Cellulose is present with lignin, hemicellulose and small amount of extractives in woody plants (soft and hard wood, wheat, straw, bamboo etc) [10]. Many types of algae which are highly crystalline contain cellulose as chief component of their cell wall, green algae is most favored ones for extraction of cellulose common cellulose producing algae are valonia and cladophora [11] which possess extremely high degree of crystallinity which can be as high as 95% [12]. Species of bacteria such as Komagataeibacter xylinus are well-known for producing cellulose [13]. Large amount of cellulose is also present in tunicates (marine animal) these animals are good source of cellulose because they have thick, leathery mantle [14]. CNCs are sourced from natural fibers like wood [15,16], sisal [17], ramie [18], cotton stalks [19], wheat straw [20], bacterial cellulose [21,22], sugar beet [23], chitin [24], potato pulp [25,26] as well as sea animals called tunicin [27,28].

Cellulose is a naturally occurring long chain polysaccharide composed of β 1,4-linked d-glucose rings [29,30,31,32,33]. Chemical formula of cellulose (biopolymer) is (C₆H₁₀O₅)n. It is a linear homopolymer with quite complex structure. Three structural level can be considered for cellulose [34,10], first one is molecular level in this level cellulose is treated as single macromolecule, next is super molecular level in which gathering of cellulose macromolecule into elementary fibril and macrofibril is discussed and last is morphological level which explains the association of microfibril into layers. Cellulose organizes in quite dense fashions due to presence of inter & intramolecular networks of H-bonds and Van der Waals interactions [[35], [36], [37], [38], [39], [40], [41], [42]].

Cellulose in its pure form consists of glucose residues with D configuration linked by β-(1–4) glycosidic linkage between C-1 and C-4 adjacent glucose units [34,[43], [44], [45]]. Three reactive hydroxal groups are present in each anhydroglucose unit at C-2, C-3 and C-6 positions showing typical behavior of primary and secondary alcohols and reducing and non-reducing end groups are present on C-1 and C-4 positions respectively. Degree of polymerization (DP) of cellulose relies on source of cellulose, extraction method and techniques used for measurement [46]. For example cotton has DP >10,000, wood pulp has DP between 300 & 1700, plant fiber have DP varying from 800 to 10,000 and DP of regenerated cellulose fibers is 250–500 [47]. First fibrillar unit known as “elementary fibril” is formed when glucan chains are combined, five or more elementary fibrils combine and form microfibrils and when elementary fibril aggregate into micro fibril forms crystalline unit's called “elementary crystallites” [48] known as cellulose nanowhiskers, whiskers, nanocrystals, nanofibers, nanoparticles, microcrystallites, or microcrystals having diameter 20–40 nm [49]. DP of whiskers prepared by acid hydrolysis is in between 100 and 200 [34].

CNCs can be extracted from variety of sources such as wood [[50], [51], [52]], cotton [50,[53], [54], [55]], ramie [50,[53], [54], [55]], bacteria [56,57], tunicates [58,59,[54], [55]], tunicate cellulose [[60], [61], [62], [63], [64]], bacterial cellulose [65], softwood kraft pulp [16,[66], [67], [68]] and recycled pulp [69]. CNCs can be easily collected from bulk of cellulose through mechanical or chemical treatments to separate out crystalline phase, normally in use techniques are acid hydrolysis in which sulfuric acid is commonly used [58,18,21,[70], [71], [72], [73], [74], [75], [76], [77], [78],53], (2,2,6,6-tetramethylpiperidin-1-yl)oxy (TEMPO) mediated oxidation [12,[79], [80], [81]], enzymatic hydrolysis [82], homogenization and grinding or mechanical disintegration [[83], [84], [85]] among all of these acid hydrolysis is more preferable as in this technique amorphous (disordered) regions of cellulose are removed or hydrolyzed by keeping crystalline (ordered) regions yielding rod like crystalline residues [[86], [87], [88], [89], [90], [91]].

Cellulose nanocrystals (CNCs) CNCs possess potential array of applicatiobals due to remarkable properties such as biocompatibility, biodegradability [92,93], non-toxicity [93,74] and human and environmental safety [94,95]. It also has high aspect ratio [96], high modulus [47,97,98], high crystallinity [99,100], high mechanical strength [72,[101], [102], [103], [104]], high stiffness and young's modulus [105]. It is worth to mention that CNC is a biobased polymer [106,107] with unique strength [92], low density [108], optical transparency [[97], [98], [99], [100]] and with high modulus of elasticity [109]. Some exclusive properties of CNCs are that, it is electroactive polymer [109], can act as nucleating agent [110], anisotropy [55,92,[95], [96], [97], [98]] and its low cost [[111], [112], [113], [114]]. All these properties of CNC make it attractive filler for polymer matrix.

Small angle X-Ray scattering SAXS is able to resolve the shape and organization of nanoobjects by measuring the intensity of scattered radiation as a function of angle with respect to the incident beam. Two dimensional SAXS data was analyzed to understand the anisotropy within CNC suspensions in a magnetic field, and order was quantified in a variety of ways. The I(χ) curve can be fit phenomenologically to yield an ad-hoc parameter by fitting the curve to Maier-Saupe distribution as given (Eq. (1)).

$$I\left(\chi \right)=\frac{1}{c}exp\left[\mathit{mcos}cos2\left(\chi -\mathit{\chi o}\right)\right]$$

With knowledge of the orientation distribution (f(χ)), the order parameter is computed as described (Eq. (2))

$$S=\left\{P2\left(\mathit{COS\chi }\right)\right\}=\frac{\left[3\left\{{\mathit{COS}}^2\chi \right\}-1\right]}{2}$$

The form of Equation (Eq (3)) is constructed with the long-axis is along the z-direction; to predict the scattering for other CNC orientations, the corresponding reciprocal-space is rotated.

$$I\left(q_x,q_y,q_z\right)=\mathit{Cexp}[-\left(\sqrt{q{x}^2+q{y}^2}-\frac{\mathit{qo}){}^2}{2{\sigma }_{\mathit{qxy}}^2}\right]exp\left[-\frac{q_z^2}{2{\sigma }_{\mathit{qz}}^2}\right]$$

For scattering at higher q, we instead use the form factor for a prolate spheroid (i.e. an ellipsoid of revolution)

$$I\left(q_x,q_y,q_z\right)=C\frac{\mathit{sin}\left(q{R}_{\theta }\right)-q{R}_{\theta }\mathit{cos}\left(q{R}_{\theta }\right)}{{\left(\mathit{qR\theta }\right)}^3}$$

$$R_{\theta }=R_z^2{\mathit{cos}}^2\theta +R_r^2\left(1-{\mathit{cos}}^2\theta \right)$$

where Rθ is the projected spheroid size, Rz = 61 nm is the half-length of the CNC, Rr = 4 nm is the short-axis radius of the CNC rods, and

$$\theta ={\mathit{tan}}^{-1}\left(\sqrt{q{x}^2+q{y}^2}/q_z\right)$$

CNC alone and in combination with other polymers show wide range of applications like it is good nanomaterial for variety of applications such as for production of antimicrobial and medical materials, for synthesis of drug carrier material in therapeutic and diagnostic medicine [115,116] e.g. CNCs obtained from softwood which can be used to bind ionizable drugs like tetracycline and doxorubicin [117]. The most advantageous application of CNC is that it can be used as reinforcing filler for preparation of bionanocomposites (BNC) [118]. CNCs can be used for nanocoatings, nanobarriers, food packaging, cosmetic and pharmaceutical products [119]. Recent studies show that CNCs can also be used in various fields such as solar cells [114], optical devices [120], biopackaging materials [121], chiral materials [122], well-drilling fluid [123] and templates for the synthesis of metal nanospheres [124].

It has been proved with experiments that exchange of heat is drastically increased with the help of nanoparticles in the many fields like in cooling and transportation industry etc. While changing the temperature the tendency of a fluid to retain its shape in response. For a nanofluid this coefficient is approximated in the following equation:

$${\left(\mathit{\rho \gamma }\right)}_{\mathit{nf}}=\left(1-\varphi \right)({\left(\mathit{\rho \gamma }\right)}_b+\varphi ({\left(\mathit{\rho \gamma }\right)}_n$$

where, γn and γb are respectively the thermal expansion coefficients of nano and base fluids.

Since viscosity of a nanofluid is of great importance while studying its flow and much work has been done in the recent years. Generally, the base fluid is less viscous than that of a nanofluid, also viscosity of a fluid is greatly affected by changing concentration and temperature of a fluid. Furthermore, the viscosity of base fluid is greatly affected by the suspension of nanofluids as a function of its volume fraction while flowing. Einstein and Brinkman gave the mathematical formulas for viscosity of nanofluids respectively in the following equations [125,126].

$${\mu }_{\mathit{nf}}=\left(1-2.5\varphi \right){\mu }_b$$

$${\mu }_{\mathit{nf}}=\frac{{\mu }_f}{{\left(1-\varphi \right)}^{5/2}}$$

where μb, μnf and ф are respectively the viscosities of base fluid, nanofluid and volume fraction of nanofluids.

Furthermore, Batchelor [127] gave the relation for viscosity of nanofluid in the following equation while taking into account the effect of Brownian motion and the interaction among the spherical nanoparticles

$${\mu }_{\mathit{nf}}=\left(1-{\beta }_1\varphi +{\beta }_2{\varphi }^2+..\dots \right){\mu }_b$$

where, β1 = 2.5, the Intrinsic viscosity; β2 = 6.5, Huggins coefficient.

Viscosity for non-spherical particles can be given by the following formula

$${\mu }_{\mathit{nf}}=\left(1-{\beta }_1\varphi \right){\mu }_b$$

$${\beta }_1=\frac{0.312r_1-0.5}{{ln}_2{r}_1-1.5}+2-\frac{1.872}{r_1}$$

where r₁ is the aspect ratio of nanoparticles.

CNC is hydrophilic in nature because of cellulose hydrophilic nature and restricts the dispersion of CNCs in hydrophobic polymer matrix [128,129] or in nonpolar media [130], such as in poly(lactic acid) (PLA) [131,132] which happens due to ability of CNCs to form aggregates due to presence of polar groups [133,134]. These problems of CNCs limit its use and badly affect its mechanical properties [135].