A comprehensive investigation on modified cellulose nanocrystals and their films properties

https://doi.org/10.1016/j.ijbiomac.2022.08.057Get rights and content

Highlights

  • The films produced from different modified CNCs showed better thermal stability compared to the pristine CNC films.

  • The combination of periodate oxidation and EPTMAC cationization resulted in lower E-modulus and higher strain at break.

  • All the modification treatments can preserve the crystalline structure of the CNCs.

Abstract

In this work, we aimed to tune cellulose nanocrystals (CNCs) properties by introducing different functional groups (aldehyde, carboxyl, silane, and ammonium groups) on the surface through different chemical modifications. These functional groups were obtained by combining: the periodate oxidation with TEMPO-oxidation, aminosylation or cationization. CNCs produced and their films were characterized to elucidate their performances. The results showed that the properties of obtained CNCs varied depending on the grafted functionalities on the surface. The results reveal that after each modification a colloidal stability is preserved.

Interestingly, Periodate oxidation of cellulose nanocrystals results in film components that interact through intra- and intermolecular hemiacetals and lead to films with a tensile strength of 116 MPa compared to the pristine CNCs, in contrast the subsequent modifications led to lower tensile strength. Of note, remarkable thermal stability has been achieved after modifications reaching a maximum of 280 °C. The oxygen barrier properties of the films after modifications varied between 0.48 and 0.54 cm3μm/(m2d*kPa) at 50 % RH.

Introduction

Cellulosic nanomaterials have attracted a huge interest lately for their appealing features with potential applications in different fields. In this sense, cellulose nanocrystals (CNCs) are one of the promising cellulosic nanomaterials. These nanomaterials are produced from cellulosic sources via controlled acid hydrolysis, which lead to the breakage of most accessible glycosidic linkages within the cellulose structure, and the liberation of nano-sized and crystalline cellulose materials [1]. CNCs has excellent strength, high Young's modulus, biocompatibility, high surface area [2], tunable self-assembly, thixotropic, and photonic properties, which are essential for the applications of this material. Applications include functional paper, optoelectronics, antibacterial films [3], [4], packaging, mechanically reinforced polymer composites, tissue engineering scaffolds, drug delivery, biosensors, energy storage, catalysis, etc. [5]. There has been an increasing interest in the production of transparent films of CNCs. It is usually accepted that such films show high stiffness and brittleness which reflects difficulties to integrate in the industry [6], [7] and hinder applications in certain fields. Brittleness of CNC films arises from their intrinsic rigidity and the lack of highly energy-dissipative as matrix which is critical factor for achieving the toughness of the film [6]. One of the highly efficient methods to handle this challenge is to use different polyhydroxy-compounds serving as plasticizers to blend with CNCs before preparation of films [8]. Sorbitol and glycerol are the largely used plasticizers. Csiszar & Nagy [8] investigated how plasticizers with similar structure, but different molecular weight and concentration, affected the perceptible and measured properties of CNC films. The obtained results showed that the CNC films with 15, 20 and 25 wt% sorbitol added had significantly higher tensile strength and Young's modulus than the glycerol plasticized ones. Extensive work has also been done using plasticizers different from polyhydroxy-compounds such as ionic liquids (ILs) of 1-allyl-3-methylimidazolium chloride (AmimCl) [6]. It has been demonstrated that the plasticization effect of AmimCl makes the film soften and more ductile with a gradual red shift in the reflected color with increasing ILs contents in the film.

Another effective approach is the surface chemical modification of CNCs via a reaction between the hydroxyl groups located on the surface of the CNCs and the modifying agent. Changing the surface chemistry can address this while still maintaining the desired properties of the CNCs. Both covalent and non-covalent surface modifications techniques can be applied, with different advantages leading to the introduction of active functional groups for tuning the properties and function of the CNCs', making them useful in a plethora of applications.

Common routes to modify CNCs include oxidation [9], [10], acetylation [11], [12], cationization [13], [14], [15], silylation [16], [17] and polymer grafting reactions [18], [19]. In particular, oxidation has attracted an increasing attention from research and academy, owing to the vast range of the products that could be obtained, depending on the reactive site and employed reagents, leading the way for quite some new CNCs derivatives for industrial applications [20]. Oxidation is usually performed on CNCs to introduce carboxylic acid or aldehyde functionalities [21]. The two most common methodologies are periodate oxidation and (2,2,6,6-Tetramethylpiperidin1-yl)oxyl radical (TEMPO) oxidation. Tempo oxidation has been used to convert the hydroxymethyl groups on cellulose chains into carboxylic form [22]. The use of this technique has been the subject of several reports since its introduction by de Nooy et al., who were the first to show that only the hydroxymethyl groups of polysaccharides were oxidized, while the secondary hydroxyls remained unaffected [23]. Concerning the periodate oxidation, it is selective cleavage of vicinal diols leading to the formation of two aldehyde groups and breakage of the C2-C3 bond [21]. These resulting aldehyde groups can be further converted to carboxylic groups [24], [25], primary alcohols [26], or Imines (Schiff bases) with primary amines [27]. This can help the generation of new materials with new properties; self-standing nanocellulose films with advanced properties. Numerous research reported the periodate oxidation of cellulose/nanocellulose materials. For instance, Duran et al. [28] reported improved mechanical properties of films prepared from dialdehyde cellulose/cellulose nanofibrils. It was also reported by the same group that the periodate oxidization of cellulose fibers affected the stability of paper, presumably due to the possibility of covalent linkages generated between the aldehyde and hydroxyl functionalities [29].

Several studies on periodate oxidation of cellulose/nanocellulose materials have been reported in the literature, but very little is known about their use in conception of self-standing CNC films with different functionalities. Recently, Nagy et al. [30] and his co-workers prepared cellulose nanocrystal/aminoaldehyde films, and investigated the effect of wet cross-linking of cellulose on the structure and properties of CNC-based thin films, and the interaction of films with water as a function of cross-linking. The obtained results showed that the cross-linking agent made the interactions between CNCs stronger and changed the optical and tensile properties as well as the morphology of the films. Another work was reported [31] on CNC films functionalized with aldehyde functionality on one side and carboxyl functionality on the other. Therein, it was executed by combining periodate oxidation of CNCs prior to film formation with a gas-phase post-treatment of one of the film sides with ozone. The obtained results demonstrated that the periodate-oxidized CNCs resulted in cross-linking of the films providing strength in the range of 66 MPa. In the present work, we aimed to use periodate oxidation of CNCs as an asset for further chemical modifications to create tuned self-standing CNC films with different functional groups that can be vigorous for the properties of the resulting film. The modification with periodate oxidation formed aldehyde groups that served as platform for the following reactions; silylation with 3-aminopropyltriethoxysilane, TEMPO-oxidation and cationization. All of these modifications affected the properties of the CNC films in a greater or lesser varying degree by introducing new functional groups on the surface. Thus, we also intended to obtain further knowledge on the influence of the modifying agent over the structure and the properties of the resulting films. Each successful modification was validated using several analytic techniques, i.e., Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), X-Ray diffraction (XRD), thermogravimetric analysis (TGA), Atomic force electron microscopy (AFM), and zeta potential. Subsequently, films with the differently modified CNCs were prepared using a vacuum filtration method. Mechanical performance, thermal gravimetric analysis and oxygen barrier properties of the films were also investigated.

Section snippets

Materials

Bleached softwood dissolving pulp was supplied by Domsjö Fabriker (Sweden). The fibers were first grounded and hydrolyzed with sulfuric acid according to an earlier reported procedure [32]. All chemicals such as sulfuric acid, hydrochloric acid, sodium metaperiodate (99 %), 4-acetamido-2,2,6,6-tetramethylpiperidine-1-oxyl (≥98.0 %), sodium bromide, sodium hydroxide, sodium hypochlorite solution (10–15 % available chlorine), ethylene glycol, hydroxylamine hydrochloride, AgNO3, glacial acetic

Suspension stability and morphologies of CNCs and modified CNCs

Visual inspection of unmodified CNCs showed a transparent, stable, and homogeneous suspension (Fig. 1A.a). This stable suspension is mainly due to the negatively charged sulfate ester groups (OSO3) formed on the surface of CNCs during sulfuric acid hydrolysis. These negatively charged groups stabilize the suspension by electrostatic repulsion. By modifying the CNCs with sodium periodate, the resulting suspension change a little, becoming more turbid while maintaining the colloidal stability (

Conclusion

In this study, CNCs were produced by acid hydrolysis and modified using varying routes to introduce different functional groups on the surface of the CNCs, and their properties were investigated using advanced characterization techniques. We found that the functionalization did not significantly affect the surface morphology characteristic of the nanocrystals. In addition we revealed that the crystalline nature of the CNCs was preserved, even though the CrI decreased for the DCNC-APTES and

CRediT authorship contribution statement

Conception and design of study: N. El miri.

Acquisition of data: N. El miri.

Analysis and/or interpretation of data: N. El miri, E. Bævre Heggset, Kristin Syverud.

Drafting the manuscript: N. El miri.

Revising the manuscript critically for important intellectual content: N. El miri, M. Norgren.

Approval of the version of the manuscript to be published: Ellinor Bævre Heggset, Sara Wallsten, Anna Svedberg, Kristin Syverud, Magnus Norgren.

Acknowledgments

The authors gratefully acknowledge the Swedish Research Council FORMAS [Grant No. 942-2015-251] and Interreg Sverige-Norge [Grant No. 20201315] for the financial support.

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