Surface chemical functionalization of cellulose nanocrystals by 3-aminopropyltriethoxysilane
Introduction
Cellulose nanocrystals (CNCs) are unique renewable, biodegradable and non-toxic materials with impressive mechanical properties. These rigid rod-like cellulose crystals, obtain by acid hydrolysis of cellulose fibres or fibrils, largely removing non-crystalline moieties. CNCs exhibit unique features, such as high aspect ratio (10–70) [1], high strength and modulus (10 and 150 GPa, respectively) [2], low density (1.6 g/cm3) [3], and high specific surface area (150 m2/g) [4]. This bio-based nanomaterial is considered to have great potential applications in different fields, such as reinforcement agent for films and nanocomposites, drug delivery systems, medical implants, conducting polymer nanocomposites, functional hydrogels, components in tissue engineering materials, protective coatings, supports for enzyme immobilization, etc. [5], [6]. The unique features and various applications of CNCs as well as future commercialization prospects have recently led to the industrial production of CNCs in Canada, the USA and in Europe [3]. However, the development of high-performance CNC-based materials is restricted by some limitations. One of the main drawbacks associated with the utilization of CNCs as high value-added materials is their poor dispersibility within a non-polar polymeric matrix and a strong tendency for self-agglomeration because of the omnipresence of interacting surface hydroxyl groups as well as formation of inter- and intra-molecule hydrogen bonds [2], [7]. Surface chemical modification via the reaction between the hydroxyl groups located at the surface of CNCs and a functional group from the organomodifying agents has been proposed as an approach to overcome this shortcoming. In this regard, various surface chemical modification techniques including acetylation [8], [9], [10], cationisation [11], [12], oxidation [13], [14], [15], silylation [16], [17], [18], [19], polymer grafting reactions [20], [21], [22] and etc. have been reported. The general strategy of all chemical functionalizations is to (1) hydrophobize the CNCs surface to promote their dispersion in non-polar organic media and/or impart better compatibility with hydrophobic polymers; (2) introduce stable negative or positive charges on the surface of CNC, to obtain better electrostatic repulsion induced dispersion [4]. A desirable modification practice not only is environmentally friendly, cheap and easily done but also has no bad effect on mechanical properties and degree of crystallinity of CNC.
Silylation, also known as silane grafting, has proven to be an efficient way to modify CNC surfaces [16], [17], [18], [19]. Silanes used for treatment of CNCs have different functional groups at either end such that interaction at one end can occur with OH groups of the CNCs whilst the other end can interact with functional groups in the matrix to form a bridge between them [23], [24], [25]. APTES is one of the most commonly used silanes due to its simplistic structure and minimal cost [26]. The chemical grafting of APTES onto CNC surfaces (Scheme 1) normally involves three steps, which is true for all types of alkoxysilane chemical modifications under hydrolytic conditions: (i) the hydrolysis of the alkoxy groups of the silane in the presence of water to give the respective silanols; (ii) the adsorption of the silanol groups onto OHrich surface of CNC through hydrogen bonding between silanol and OH groups of cellulose; and (iii) chemical condensation leading to siloxane bridges (SiOSi) and to grafting onto CNCs surface through SiOC bonds. The siloxane bridges resulting from self-condensation contribute to the formation of a polysiloxane network on the CNCs surface [27], [28], [29], [30].
Different researchers have attempted to graft silane onto CNCs. Goussé et al. [16] partially silylated cellulose whiskers resulting from the acid hydrolysis of tunicate by a series of chlorosilanes and found that the silylated cellulose whiskers were not able to be dispersed in solvents such as toluene, with polarity lower than that of tetrahydrofuran (THF) [16]. A similar study performed by Pei et al. [17] that functionalized cellulose nanocrystals by partial silylation through reactions with n-dodecyldimethylchlorosilane in toluene and then suspended in organic solvents such as THF and chloroform and form stable homogeneous suspensions [17]. In addition, in a study performed by Taipina et al. [19] cotton nanocrystals silylated with isocyanatepropyltriethoxysilane (IPTS) in dimethylformamide (DMF) in order to improve the dispersion of filler in polymeric matrices [19]. The previous studies are good attempts. However, the major drawback in classical grafting processes with silane to increase the hydrophobicity of nanocellulose is the tedious solvent exchange process and the use of organic solvents in these reactions [31]. Recently, a direct process for surface silylation of cellulose nanocrystals was developed by Raquez et al. [18]. They introduced amino and methacrylate groups onto CNC surfaces by direct silylation of CNCs in citrate buffer [18].
Apart from this, several silane derivatives such as N-(β-aminoethyl)-γ-aminopropyl-trimethoxysilane (AEAPTMS) [32], [33], [34]; 3-(2-aminoethylamino)propyl-dimethoxymethylsilane (AEAPDMS) [35]; 3-isocyanatepropyltriethoxysilane (IPTS) [19], [36]; 3-aminopropyltriethoxysilane (APTES) [18], [37]; N-dodecyldimethylchlorosilane (DDMSiCl) [16], [17]; 3-glycidoxypropyltrimethoxysilane (GPTMS) [38], [39]; 3-methacryloxy-propyltrimethoxysilane (MPS) [18], [38] have already been used to functionalize CNC for applications such as the preparation of reinforcing elements for composites. However, relatively few studies have been published so far regarding surface functionalization of these nanocrystals to obtain new derivatives that can lead to new utilitarian applications.
In this study, surface of CNCs was functionalized with APTES, without using hazardous solvents and by a direct, simple and convenient method to develop novel applications of CNCs. APTES was chosen as the silane coupling agent due to its high reactivity, low toxicity, simplistic structure, minimal cost and amino group. The APTES-grafted CNC was characterized by attenuated total reflection infrared spectroscopy (ATR-IR), energy dispersive X-ray analysis (EDX), X‐ray diffraction (XRD), solid-state 13C and 29Si nuclear magnetic resonance (NMR), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), and atomic force microscopy (AFM) and compared with the pristine CNC material.
Section snippets
Materials
An 6.2 wt.% aqueous CNC suspension was purchased from the University of Maine (USA). The CNC contained 0.95 wt.% sulfur on a dry cellulose basis as reported by the supplier. 3-aminopropyltriethoxysilane (APTES; C9H23NO3Si, ≥98%); glacial acetic acid (C2H4O2) and ethanol (C2H6O) were purchased from Carl Roth GmbH & Co. KG (Karlsruhe, Germany) and used without further purification.
Surface modification of CNC
Surface modification of CNC with APTES was performed using the following methodology: APTES and water were added in a
Attenuated total reflection infrared spectroscopy (ATR-IR)
Fig. 1 shows ATR-IR spectra for CNC before and after modification by APTES. As expected, both neat and modified CNC displayed absorption peaks that were characteristic of cellulose.
Neat CNCs are characterized with absorbance peaks in the range 3600–3000 cm−1 assigned to the stretching vibration of hydroxyl groups of cellulose [40], the band between 3000 and 2800 cm−1 corresponds to asymmetric and symmetric CH stretching vibration [41], [42], and the peak at around 1640 cm−1 corresponds to bending
Conclusions
In this study, an environmental-friendly and simple method, has been developed for a solvent-free silylation of cellulose nanocrystals surface by using 3-aminopropyltriethoxysilane. Structure and chemical analyses of the modified CNC were performed using advanced characterization techniques. AFM demonstrated that the functionalization did not significantly affect the surface morphology characteristic of nanocrystals. The efficiency of grafting was confirmed by Attenuated total reflection
Conflict of interest
The authors declare that there is no conflict of interest.
References (70)
- et al.
Modification of cellulose nanocrystal via SI-ATRP of styrene and the mechanism of its reinforcement of polymethylmethacrylate
Carbohydr. Polym.
(2016) - et al.
Surface modification of cellulose nanocrystals (CNC) by a cationic surfactant
Ind. Crops Prod.
(2015) Nanocellulose: a new ageless bionanomaterial
Mater. Today
(2013)- et al.
Surface acetylation of cellulose nanocrystal and its reinforcing function in poly(lactic acid)
Carbohydr. Polym.
(2011) - et al.
Synthesis and characterization of cationically modified nanocrystalline cellulose
Carbohydr. Polym.
(2012) - et al.
Stable suspensions of partially silylated cellulose whiskers dispersed in organic solvents
Polymer
(2002) - et al.
Functionalized cellulose nanocrystals as biobased nucleation agents in poly(L-lactide) (PLLA) – crystallization and mechanical property effects
Compos. Sci. Technol.
(2010) - et al.
Surface-modification of cellulose nanowhiskers and their use as nanoreinforcers into polylactide: a sustainably-integrated approach
Compos. Sci. Technol.
(2012) - et al.
Grafting of cellulose by ring-opening polymerisation—a review
Eur. Polym. J.
(2012) - et al.
Short natural-fibre reinforced polyethylene and natural rubber composites: effect of silane coupling agents and fibres loading
Compos. Sci. Technol.
(2007)
A review of recent developments in natural fibre composites and their mechanical performance
Compos. Part A
Molecular layer deposition of APTES on silicon nanowire biosensors: surface characterization, stability and pH response
Appl. Surf. Sci.
Modification of cellulosic fibres with functionalised silanes: development of surface properties
Int. J. Adhes. Adhes.
Cellulose nanocrystal reinforced liquid natural rubber toughened unsaturated polyester: effects of filler content and surface treatment on its morphological, thermal, mechanical, and viscoelastic properties
Polymer
Surface-modified nano-cellulose as reinforcement in poly (lactic acid) to conform new composites
Ind. Crops Prod.
The modified nanocrystalline cellulose for hydrophobic drug delivery
Appl. Surf. Sci.
Soy protein isolate-based films reinforced by surface modified cellulose nanocrystal
Ind. Crops Prod.
Calcium hydroxyapatite microfibrillated cellulose composite as a potential adsorbent for the removal of Cr(VI) from aqueous solution
Chem. Eng. J.
Binary mixed homopolymer brushes tethered to cellulose nanocrystals: a step towards compatibilized polyester blends
Biomacromolecules
Extraction of cellulose nanocrystals from plant sources for application as reinforcing agent in polymers
Compos. Part B Eng.
Adsorption of polyethylene glycol (PEG) onto cellulose nano-crystals to improve its dispersity
Carbohydr. Polym.
Nanocomposites with functionalised polysaccharide nanocrystals through aqueous free radical polymerisation promoted by ozonolysis
Carbohydr. Polym.
Coupling onto surface carboxylated cellulose nanocrystals
Polymer
Catalytic oxidation of formaldehyde by ruthenium multisubstituted tungstosilicic polyoxometalate supported on cellulose/silica hybrid
Appl. Catal. A
Physicochemical characterization of organosilylated halloysite clay nanotubes
Micropor. Mesopor. Mat.
Cellulose surface grafting with polycaprolactone by heterogeneous click-chemistry
Eur. Polym. J.
Maleimide-grafted cellulose nanocrystals as cross-linkers for bionanocomposite hydrogels
Carbohydr. Polym.
Surface modification of microfibrillated cellulose for epoxy composite applications
Polymer
Process scale up and characterization of wood cellulose nanocrystals hydrolysed using bioethanol pilot plant
Ind. Crops Prod.
Physico-chemical characterization of lignins from different sources for use in phenol–formaldehyde resin synthesis
Bioresour. Technol.
Effect of different carboxylic acids in cyclodextrin functionalization of cellulose nanocrystals for prolonged release of carvacrol
Mater. Sci. Eng. C
Physical and/or chemical compatibilization of extruded cellulose nanocrystal reinforced polystyrene nanocomposites
Macromolecules
Surface peeling of cellulose nanocrystals resulting from periodate oxidation and reductive amination with water-soluble polymers
Cellulose
Cellulose nanomaterials review: structure, properties and nanocomposites
Chem. Soc. Rev.
Concurrent cellulose hydrolysis and esterification to prepare a surface-modified cellulose nanocrystal decorated with carboxylic acid moieties
ACS Sustain. Chem. Eng.
Cited by (237)
3D printing of cellulose nanocrystal-based Pickering foams for removing microplastics
2024, Separation and Purification TechnologyExploitation of function groups in cellulose materials for lithium-ion batteries applications
2024, Carbohydrate PolymersA gentle strategy to design amine-functionalized cellulose aerogel with tunable graft density for urea adsorption
2024, Chemical Engineering ScienceCellulose nanofiber aerogels modified with titanium dioxide nanoparticles as high-performance nanofiltration materials
2024, International Journal of Biological MacromoleculesA sustainable biomass-based electret for face mask and non-volatile transistor memory
2024, Organic Electronics