Elsevier

Polymer

Volume 46, Issue 23, 14 November 2005, Pages 10221-10225
Polymer

All-cellulose nanocomposite

https://doi.org/10.1016/j.polymer.2005.08.040Get rights and content

Abstract

Cellulose-based nanocomposite films with different ratio of cellulose I and II were produced by means of partial dissolution of microcrystalline cellulose powder in lithium chloride/N,N-dimethylacetamide and subsequent film casting. The mechanical and structural properties of the films were characterised using tensile tests and X-ray diffraction. The films are isotropic, transparent to visible light, highly crystalline, and contain different amounts of undissolved cellulose I crystallites in a matrix of regenerated cellulose. The results show that, by varying the cellulose I and II ratio, the mechanical performance of the nanocomposites can be tuned. Depending on the composition, a tensile strength up to 240 MPa, an elastic modulus of 13.1 GPa, and a failure strain of 8.6% were observed. Moreover, the nanocomposites clearly surpass the mechanical properties of most comparable cellulosic materials, their greatest advantage being the fact that they are fully biobased and biodegradable, but also of relatively high strength.

Introduction

The importance of ‘green’ properties such as biodegradability and favourable CO2 balance grows with the awareness of consumers and engineers for sustainability in the use of materials [1]. Reinforcement of polymer composites with plant fibre instead of glass fibre is a way of improving these properties, yet such composites are often of modest strength when both fibre and matrix are biobased and biodegradable [2], [3], [4].

Cellulosic fibre from wood, annual plants, and agricultural by-products is an abundant renewable resource [5], [6]. Cellulose is a straight carbohydrate polymer chain consisting of several 1000 β 1–4 glucopyranose units. In cellulosic plant fibres, cellulose is present in amorphous state, but also associates to crystalline domains through intramolecular hydrogen bonding [7]. The elastic modulus of the cellulose I crystallite, which is the crystalline cellulose form typical for plant fibres, has been measured to 128 GPa [8], and estimates for the strength of the cellulose I crystallite lie in the order of 10 GPa [9]. In spite of the good mechanical properties of cellulose, the strength of cellulosic fibre-reinforced composites remains far below the potential provided by cellulose. Due to the heterogeneous structure and composition of plant fibres [10], and insufficient fibre-matrix compatibility [11], [12], [13], [14], typical random-oriented plant fibre-reinforced composites show a tensile strength of 15–140 MPa and an elastic modulus of 1–13 GPa [15], [16], [17], [18], [19], [20].

Recently, cellulose fibre-reinforced phenol–formaldehyde composites with high bending strength of up to 400 MPa were produced using cellulose nanofibrils obtained by microfibrillation of wood pulp [21] or from bacterial cellulose [22]. High-strength cellulosic composites were also obtained by self-reinforcement, embedding unidirectionally aligned ramie fibres in a matrix of regenerated cellulose [23]. Being chemically homogeneous, self-reinforced composites are easy to recycle.

In this study, we aim to combine advantages of nanofibre reinforcement and self-reinforcement in order to obtain high-strength random-oriented biobased, easily recyclable and biodegradable composites. For this purpose, microcrystalline cellulose will be partly dissolved in lithium chloride/N,N-dimethylacetamide solvent and films will be cast from the solution. Tensile tests and X-ray diffraction will be used to characterise the films and evaluate the reinforcing effect of microcrystalline cellulose.

Section snippets

Production of all-cellulose composite films

Aldrich microcrystalline cellulose (31,069-7, MCC) was chosen as raw material for the production of all-cellulose films. This microcrystalline powder (Fig. 1) is produced by acid hydrolysis of amorphous domains in cotton linters, which results in high crystallinity (∼65%). MCC (2 g for composite A, 3 g for composite B, and 4 g for composite C) was activated for 6 h in distilled H2O at room temperature. Subsequently, the cellulose was dehydrated in ethanol, acetone, and N,N-dimethylacetamide (DMAc)

Structural characterisation

The term all-cellulose composite was first introduced by Nishino et al. [23] for a material consisting of ramie fibres embedded in a matrix of regenerated cellulose. The films obtained in the present study by partly dissolving MCC in LiCl/DMAc are referred to as composites because regenerated cellulose is supposed to serve as matrix, which is reinforced by cellulose I crystallites originating from undissolved MCC. Although regenerated cellulose and reinforcing crystallites are chemically

Conclusion

It is shown that by means of partial dissolution of microcrystalline cellulose powder in LiCl/DMAc and subsequent film casting, all-cellulose nanocomposite films consisting of regenerated cellulose reinforced with undissolved cellulose I crystallites can be produced. Such random-oriented nanocrystallite reinforced films are transparent to visible light and of high strength and stiffness with regard to comparable cellulosic materials. However, the biggest advantage of the new nanocomposite films

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