Elsevier

Carbohydrate Polymers

Volume 86, Issue 1, 1 August 2011, Pages 154-161
Carbohydrate Polymers

A “click-chemistry” approach to cellulose-based hydrogels

https://doi.org/10.1016/j.carbpol.2011.04.031Get rights and content

Abstract

Cellulose derivatives bearing azide- and alkyne moieties were prepared by conversion of cellulose p-toluenesulphonic acid ester with sodium azide, on one hand, and propargylamine, on the other. The products obtained were carboxymethylated to yield water soluble multifunctional cellulose derivatives. Elemental analysis, FTIR- and NMR spectroscopy were applied to prove the structure of the polymers. SEC of the hydrogel components revealed values of the degree of polymerization (DP) between 43 and 200 that are acceptable values after this multi-step reaction starting from celluloses with DP 600. The copper(I)-catalyzed 1,3-dipolar cycloaddition reaction (Huisgen-reaction) was applied for the cross-linking. Gel formation occurred within 55 and 1600 s after mixing of the aqueous solutions of both components and copper(I) catalyst. The gelation time was found to depend on both the degree of functionalization and the amount of copper(I) catalyst. FTIR spectroscopy revealed incomplete conversion of the reactive sites. The gels contain up to 98.4% water. Freeze-drying led to spongy materials with a porous structure as visualised by SEM.

Highlights

Cellulose derivatives as precursors for hydrogels. ► Gelation via azide/alkyne click-reaction. ► Fast gelation in aqueous solution within 55 and 1600 s. ► Freeze-drying leads to gels with porous structure.

Introduction

Hydrogels are three-dimensional networks of hydrophilic polymers that are cross-linked either chemically via covalent bonds or physically by ionic-, van der Waals-, hydrophobic, or hydrogen bond interactions (Yu & Ding, 2008). Their most important property is the capability of remarkable solvent uptake. Such materials play an enormous role in our daily lives as, e.g., superabsorbers in diapers and other hygiene products (Buchholz, 1996). Due to their high water content and tissue like structure, hydrogels are valuable materials for advanced applications in fields where biocompatible devices in chemical and biochemical applications are needed (Bajpai et al., 2008, Calvert, 2009, Lee and Mooney, 2001, Park and Park, 1996, Yu and Ding, 2008).

Due to its inherent hydrophilicity the renewable resource cellulose is a valuable starting material for hydrogels. It is already reported that physical gels from hydroxypropylmethyl cellulose, methyl cellulose, and hydroxypropyl cellulose possess thermoresponsive properties and are therefore only stable above the lower critical solution temperature (LCST). Moreover, chemical cross-linking can be achieved by either difunctional molecules (e.g. divinyl sulphone) or by radical cross-linking with high-energy irradiation. It is also possible to prepare hydrogels from solutions of unmodified cellulose (Chang & Zhang, 2011). Yang et al. dissolved cellulose in an ionic liquid followed by coagulation with water yielding a gel that is physically cross-linked by hydrogen bonds (Li, Lin, Yang, Wan, & Cui, 2009). Zhang et al. prepared gels by dissolving cellulose in an aqueous 6 wt% NaOH solution with 4 wt% urea and subsequent cross-linking with epichlorohydrin (Zhou, Chang, Zhang, & Zhang, 2007). Most of the cellulose-based hydrogels are hybrid systems. Biocompatibile cellulose and cellulose derivatives are combined with a variety of synthetic polymers, e.g., poly(acrylonitrile) (Pourjavadi, Zohuriaan-Mehr, Ghasempoori, & Hossienzadeh, 2007), poly(N-isopropylacrylamide) (pNIPAAm) (Ma, Zhang, Fan, Xu, & Liang, 2008), and poly(vinyl alcohol) (Alupei, Popa, Hamcerencu, & Abadie, 2002) in order to tune the properties. The disadvantage of most of these gels is that hazardous cross-linking agents like epichlorohydrin or glutaraldehyde have to be used (Shang, Shao, & Chen, 2008).

An alternative preparation method would be the cross-linking by 1,3-dipolar cycloaddition reaction of alkynes and azides. One example is a gel composed of 6-azido-6-deoxy cellulose and pNIPAAm-co-hydroxyethylmethacrylate (HEMA). The HEMA block is modified with an alkyne moiety and the gelation is accomplished by the 1,3-dipolar cycloaddition (Zhang, Xu, Wu, Zhang, & Zhuo, 2009). Because of the LCST of pNIPAAm, the gels produced are thermo-sensitive.

It was interesting to study hydrogels solely composed of cellulose derivatives that can be prepared in aqueous solution. This paper describes the synthesis of water-soluble cellulose derivatives capable of undergoing the copper-catalyzed 1,3-dipolar cycloaddition reaction. Hydrogels were prepared and characterized regarding their cross-linking- and swelling behaviour as well as their morphology in the dried state.

Section snippets

Materials

Spruce sulphite pulp was purchased from Fluka. Other chemicals were supplied by Aldrich, Merck and Fluka. Cellulose was dried in vacuum at 100 °C for 2 d over KOH and LiCl was dried in vacuum for 1 d at 150 °C.

Measurements

FTIR spectra were recorded on a Nicolet Avatar 370 spectrometer using the KBr-technique. The 1H- and 13C-NMR spectra were acquired on a Bruker AVANCE 400 spectrometer in dimethyl sulphoxide (DMSO)-d6 or D2O at 70 °C. For 1H-NMR spectra up to 200 scans and for 13C-NMR spectra up to 26,000

Preparation of the cellulose derivatives

The starting materials for the hydrogel preparation were synthesized from tosyl cellulose (Scheme 1), which can be easily prepared by conversion of cellulose dissolved in N,N-dimethyl acetamide (DMA)/LiCl with p-toluenesulphonic acid (tosyl) chloride in the presence of triethylamine according to (Rahn et al., 1996). Tosyl celluloses with different DS were used (sample 1, DSTos 0.33; sample 2, DSTos 0.94). A DSTos < 1 was chosen to ensure a preferred tosylation of position 6 with negligible

Conclusions

It was shown that copper mediated 1,3-dipolar cycloaddition of water soluble cellulose derivatives decorated with azide- and alkyne functions yields novel cellulose hydrogels. The gels were investigated via FTIR spectroscopy, oscillation rheology, SEM; also, the degree of swelling were measured. The gelation time can be controlled by the DS of azide- and alkyne groups as well as by the catalyst concentration. FTIR spectroscopy revealed remaining alkyne- and azide groups in the gels that might

Acknowledgements

The research leading to these results has received funding from the European Community's Seventh Framework Programme (FP7/2007-2013) under grant agreement no. 214653 (surface functionalization of cellulose matrices using coatings of functionalized polysaccharides with embedded nano-particles, SurFunCell). Cellulose p-toluenesulphonic acid ester (tosyl cellulose) 1 with a DS of 0.33 was a kind gift of M. Schöbitz (Thuringian Institute for Textile and Plastics Research, Rudolstadt, Germany).

References (22)

  • K.Y. Lee et al.

    Hydrogels for tissue engineering

    Chemical Reviews

    (2001)
  • Cited by (84)

    View all citing articles on Scopus
    View full text