Surface modification of cellulose nanocrystals (CNC) by a cationic surfactant

https://doi.org/10.1016/j.indcrop.2014.11.027Get rights and content

Highlights

  • Hexadecyltrimethylammonium (HDTMA) can be used in surface modification of cellulose nanocrystals (CNC).

  • Surface modification of CNC by HDTMA affected negatively neither crystalline structure nor dimensions of CNC.

  • The concentration of HDTMA controlled the extent of modification.

Abstract

Surface modification of cellulose nanocrystals (CNC) is very important to explore new applications for these CNC. In this study, a cationic surfactant, hexadecyltrimethylammonium (HDTMA) bromide, was used to modify the surface of CNC. Chemical and structural characterizations of CNC were conducted before and after modification. Results revealed that the chemical reaction of CNC with HDTMA led to the presence of new chemical groups (CH2, CH3 and quaternary ammonium groups) on the surface of the CNC. Experiments conducted by X-ray diffraction indicated no change in the crystalline structure as a result of the modification. Modified CNC had different suspension stabilities and dispersibility compared to unmodified CNC. The results of this study demonstrated that HDTMA can be used for a simple and straightforward surface modification with no negative effects on CNC.

Introduction

Given that cellulose has a natural nano-structure, different methods can be used to explore its nano-structure, leading to the introduction of various nano-celluloses (namely cellulose nanocrystal (CNC) and microfibrillated cellulose (MFC)). CNCs, also known as whiskers, consist of rod like cellulose crystals with widths of 5–70 nm and lengths between 100 nm and several micrometers. CNC is abundant, renewable and non-toxic. Compared to mineral fillers, it has a lower density, a high form factor of about 70 and a high specific area of around 150 m2/g. It has been shown that CNC has remarkable reinforcing properties in a range of different matrices (Favier et al., 1995, Angles and Dufresne, 2011, Dubief et al., 1999, Azizi Samir et al., 2004).

CNC is of a hydrophilic nature, as is the case for cellulose itself. This causes difficulties in mixing CNC with hydrophobic polymers and its utilization has thus been restricted to hydrophilic polymers. Broadening its field of application would require developing different degrees of surface hydrophobicity. Our experience with CNCs indicates that even commercially available CNCs are too hydrophilic to be used in emulsions with so-called water based polymers, since some agglomeration occurs (Kaboorani et al., 2012).

As is the case for essentially all cellulose derivatizations, hydroxyl groups are the primary reactive sites for cellulose surface modification. CNC has a high surface area to volume ratio, which means that it has a highly reactive and easy to functionalize surface. All chemical functionalizations of CNCs have been primarily conducted to (1) introduce stable negative or positive electrostatic charges on the surface to obtain better dispersion, and (2) tune their surface energy characteristics to improve compatibility, especially when used in conjunction with nonpolar or hydrophobic matrices in nano-composites (Eichhorn, 2010).

Various covalent and non-covalent modification approaches have been developed for CNCs to make functional cellulose nanocrystal particles or to improve their dispersion in hydrophobic polymer matrices (Peng et al., 2011). Functional groups such as initiators for living radical polymerizations (Xu et al., 2008, Majoinen et al., 2011, Morandi et al., 2009), alkyne and azide groups (Filpponen and Argyropoulos, 2010), and epoxy and amine groups (Dong and Roman, 2007) have been previously grafted onto the surface of CNCs, which facilitates further reactions, such as graft polymerization. Industries may be reluctant to use these modifications because they are time-consuming and often involve use of organic solvents. Alternatively, surface modification of CNC can be made via adsorption of surfactants, which is considered a non-covalent surface modification. A surfactant is defined as a material that can greatly reduce the surface tension of water when used in very low concentrations. CNCs possess negative charges, at neutral pH, on their surface due to the formation of sulphate ester groups during sulfuric acid treatment, which enhances their stability in aqueous solutions, but restricts its dispersibility in most non-polar solvents. One possible route to improve this is to coat the CNC surface with a surfactant, a process which has been successfully introduced by Heux et al. (2000). They used surfactants consisting of mono- and di-esters of phosphoric acid bearing alkylphenol tails. It has been shown that surfactants coated CNC surfaces and were effective in improving CNC dispersion in organic solvents and poly(propylene) (Heux et al., 2000, Heux et al., 2002, Ljungberg et al., 2005).

The main challenge for the chemical functionalization of CNCs is to conduct the process in such a way that it only changes their surface, while preserving their original morphology so as to avoid any polymorphic conversion and maintain the integrity and strength of the crystal. Reduction in the crystalline structure of CNCs during modification leads to inferior mechanical properties of composites. Another challenge in modifying CNCs is to design a simple modification system with no or minimum negative impact on the environment.

In this research, a cationic surfactant was used to modify CNC surfaces. Cationic surfactants, such as quaternary ammonium compounds, have been employed in the past to exchange the extra structural cations of zeolites and clays, enhancing the surface affinity for organic species such as nonionic organic compounds (NOCs) (Li and Bowman, 1998) and hydrophobic organic compounds (HOCs) (Wagner et al., 1994). Quaternary ammonium salts have also been used to modify CNC surfaces (Salajkova et al., 2012). The main disadvantage of using a surfactant for the surface modification of CNC is that large amounts are required due to the high specific area of CNC, preventing use of this technique for composites processing in organic solvents (Kamel, 2007). In the present study, an attempt was made to develop a method for the surface modification of CNC via adding a reduced amount of quaternary ammonium compound.

Hexadecyltrimethylammonium (HDTMA) bromide is a tetra-substituted ammonium cationic surfactant with permanently charged quaternary nitrogen and a long straight alkyl (C16) chain that imparts it with a high degree of hydrophobicity. The intercalation of HDTMA, into swollen layer silicates has been studied to evaluate their potential use as sorbents for aromatic hydrocarbons in environmental technology (Jaynes and Boyd, 1997, Bonczek et al., 2002).

HDTMA has some advantages over other materials used so far to modify the surface of CNC. HDTMA is readily available, inexpensive (Bonczek et al., 2002) and can be degraded by microorganisms. It has been shown that microorganisms with a capacity to degrade oily contaminants in the environment (Cho et al., 1997) are able to grow very well when HDTMA loading is less than 0.5 times the cation exchange capacity (CEC) of modified montmorillonite (Gao et al., 2001).

Although different modification methods have been developed for CNC, little attention has been paid to controlling the degree of hydrophobicity of the resulted CNC. Controlling the hydrophobicity of CNC is very important to achieve the best compatibility between CNC and the polymer.

The main objective of this research was to modify the surface of CNC by using a cationic surfactant. This is possible since CNC is covered with a large amount of charged anionic sites at neutral pH. The modifications were conducted under different conditions so as to impose different degrees of hydrophobicity.

Section snippets

Materials and methods

Cellulose nanocrystals (CNCs) were kindly provided by Forest Products Laboratory, Madison, WI, USA. The CNC contained 0.81 wt% sulfur on a dry cellulose basis as reported by the supplier. Hexadecyltrimethylammonium (HDTMA) bromide, 99% purity was purchased from Sigma–Aldrich.

Nitrogen content

As naturally there are no, or very little, molecules containing nitrogen in cellulose, any trace of nitrogen in modified CNC can be an indication that molecules containing nitrogen are present on the CNC. The results of nitrogen content measurements are presented in Table 1. As expected, unmodified CNC had no nitrogen in its compositions. Modification by HDTMA resulted in significant amounts of nitrogen. The amount of nitrogen content was proportional to the concentration of HDTMA. The duration

Conclusions

The results of this study showed that HDTMA has a capability of changing the surface of CNC. HDTMA modified the CNC surface without compromising its reinforcing effects since no changes in its crystallite structure and dimensions occurred. Studying the chemical structure of modified CNC by several methods showed that new groups including CH2, CH3 and Csingle bondN (quaternary ammonium groups) appeared on the surface of CNC as a result of the modification. Dispersibility and state of suspension were also

Acknowledgements

The authors acknowledge financial support from Canadian Forest Nano Products Network (ArboraNano). The authors would like to express their thanks to Forest Products Laboratory – USDA Forest Service for so kindly providing cellulose nanocrystal (CNC) for this research. Thanks are also extended to research center for advanced materials of Laval University for providing the equipment for characterizations of CNC.

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