Surface chemical functionalization of cellulose nanocrystals by 3-aminopropyltriethoxysilane

https://doi.org/10.1016/j.ijbiomac.2017.08.136Get rights and content

Highlights

  • The surface of CNCs was functionalized with 3-aminopropyltriethoxysilane (APTES).

  • The EDX, NMR and XPS results confirmed that APTES was successfully deposited on CNC.

  • Crystalline structure of CNCs was not altered by modification.

  • Thermal stability was significantly enhanced after modification.

Abstract

Surface functionalization of cellulose nanocrystals (CNCs) is valuable option to tailor properties as well as increase opportunities for their application. In this study, the surface of CNCs was functionalized with 3-aminopropyltriethoxysilane (APTES), without using hazardous solvents and by a direct, simple and straightforward method. APTES was firstly hydrolyzed in water and then adsorbed onto CNC through hydrogen bonds, finally the chain hydrocarbon was covalently linked to the surface of CNC through Sisingle bondOsingle bondC bonds which formed via the condensation reaction between hydroxyl and silanol groups. The chemical modification of the CNCs surface was confirmed by ATR-IR and NMR spectroscopy. Experiments conducted by AFM and XRD showed no significant change in the CNC dimensions and crystalline structure as a result of the modification. The EDX and XPS results confirmed the exsistence of APTES onto the CNCs. Silylated CNC exhibited good thermal stability and a greater amount of residual char was formed at 500 °C compared to non-chemically modified CNC. Thus, The silylation of CNCs may offer applications in composite manufacturing, where these nanoparticles have limited dispersibility in hydrophobic polymer matrices, and as nano-adsorbers due to the presence of amino groups attached on the surface.

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 OHsingle bondrich surface of CNC through hydrogen bonding between silanol and single bondOH groups of cellulose; and (iii) chemical condensation leading to siloxane bridges (Sisingle bondOsingle bondSi) and to grafting onto CNCs surface through Sisingle bondOsingle bondC 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 Csingle bondH 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.

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