Elsevier

Carbohydrate Polymers

Volume 88, Issue 2, 2 April 2012, Pages 789-792
Carbohydrate Polymers

Short communication
Cellulose nanowhisker foams by freeze casting

https://doi.org/10.1016/j.carbpol.2011.12.035Get rights and content

Abstract

Cellulose nanowhisker foams with uniform layer structure were successfully prepared via freeze casting method. Here, we investigate the relationship between freezing rate, slurry concentration and the microstructure of the porous nanowhisker architecture. The results indicate that the above freezing parameters significantly influence the microstructure and morphological features and the structures obtained could have numerous applications, including scaffolds, filters and specifically as a template for dense multilayered composites after infusion with a second phase.

Highlights

Cellulose nanowhiskers as a raw material for fabricating porous architecture. ► Freeze casting technique provides long range ordered porous nanowhisker foams. ► Freezing parameters influence the microstructure and pore morphology of foams.

Introduction

Fabrication of materials with homogeneous and well defined architectures has received increasing research interest owing to their broad applications such as tissue engineering, delivery matrices, green packaging, nanocomposites, and automotive industry (Hoepfner et al., 2008, Kim et al., 2005, Rosenau et al., 2007, Xinhong et al., 2008). Several methods including spin coating, layer-by-layer, freeze casting, and eutectic growth in two phase system has been utilized to organize micron/nano size particles to obtain ordered structures. Among the different techniques used, freeze casting has been shown as a versatile, easily implemented, and promising technique to build structures such as scaffolds, porous nanocomposites, and microwire networks with well aligned and controlled porosity (Barr and Luijten, 2010, Deville et al., 2006a, Deville et al., 2007).

Freeze casting technique involves freezing a liquid suspension and sublimation of the solvent there after under reduced pressure. During the freezing process, the suspended particles are organized by rejection from the growing ice crystal front to the intervening space, which results in an ordered structure after sublimation. Fabrication of various materials by this technique suggests that the underlying principle of freeze casting is strongly dependent on simple physics of ice crystals and the physical interaction between the growing solidification front and the inert particles of the slurry (Kawata et al., 2003, Tsioptsias et al., 2008, Zhang et al., 2005). Depending on the choice of solvent, slurry formulation, and solidification conditions, the final porosity and pore morphologies can be readily tuned. However, the solidification conditions remain as the key factor since all the features of porosity are created during this stage and thereby, controlling the formation and growth of ice crystals would yield materials with specific microstructure. For instance, in case of unidirectional freezing, a porous structure with unidirectional channel is obtained. In fact, this approach has been utilized to prepare variety of ceramic structures such as silica fiber bundles, tubular supports with radially aligned pores, micro-honeyecombs as well as polymeric scaffolds (Mahler and Bechtold, 1980, Moon et al., 2003, Mukai et al., 2004).

Cellulose nanowhiskers, derived sustainably from biomass represent a relatively new raw material that has gained significant attention due to their intrinsically appealing physical, chemical as well as mechanical properties (Azizi Samir et al., 2005, Eichhorn, 2011, Gavillon and Budtova, 2008, Habibi et al., 2010). Cellulose nanowhiskers designate a class of rod like nanoparticles which are mainly prepared by controlled acid hydrolysis of native cellulose fibers. The size and properties of nanowhiskers depend on the source and hydrolysis conditions of cellulose fibers and typically are 5–10 nm in width and 100–300 nm in length for wood-based nanowhiskers (Beck-Candanedo et al., 2005, Bondeson et al., 2006). A number of non-periodic highly porous structures known as aerogels from micro and nano cellulose fibers as well as cellulose derivatives has been reported in literature and they are commonly prepared by solvent exchange of a wet gel followed by supercritical CO2 drying (Aaltonen and Jauhiainen, 2009, Fischer et al., 2006, Gavillon and Budtova, 2008, Heath and Thielemans, 2010, Sehaqui et al., 2010). However, porous structures with regular pattern can be obtained by controlling the freezing temperature of the slurry followed by subsequent freeze drying.

Recently, Lee and Deng (2011) reported the preparation of layered cellulose foams through directional freezing technique emphasizing the effect of fiber concentration and freezing temperature on the microstructure and mechanical properties of microfibril foams. They have also measured the compressive strength of cellulose nanowhisker foams but a detailed examination on their microstructure is lacking. Utilizing the facile freeze casting technique, we attempt to fabricate aligned porous cellulose nanowhisker structures and investigate the relationship between the freezing conditions and the microstructures obtained, which has not been reported so far. We expect that ice growth strategy of freeze casting technique will allow the fabrication of well ordered cellulose nanowhisker structures, opening their use as a template for layered composites, filters, and storage material.

Section snippets

Materials

A fully bleached commercial softwood Kraft pulp was used as a source for cellulose nanowhiskers preparation. Polyvinyl alcohol (PVA) was purchased from VWR International (Mw: 15,000, degree of hydrolysis: 87–89%).

Preparation of H2SO4-hydrolyzed cellulose nanowhiskers

The cellulose nanowhiskers were prepared by sulfuric acid hydrolysis of a bleached softwood pulp based on a literature procedure (Bondeson et al., 2006). In brief, 60.00 g (oven dried weight) of the pulp was mixed with H2SO4 solution (64%, w/w, 1:10 g mL−1) with continuous stirring at 45 °C for 45 min. The hydrolysis reaction was halted by adding excess (10-fold) of distilled water followed by the removal of acidic solution through successive centrifugation at 12,000 rpm for 10 min until the

Results and discussion

Cellulose nanowhisker suspension was mixed with PVA and solidification of the slurry was carried out by two ways: (i) quenching the slurry in liquid nitrogen (ii) freezing the slurry at two different cooling rates, i.e., 13 °C min−1 and 4.5 °C min−1. Freeze drying of the solvent created the final nanowhisker porous structure as a replica of ice crystals that generated during freezing. PVA was used as a binder due to its solubility in water and its compatibility with cellulose nanowhiskers (Kvien &

Conclusion

In conclusion, our results illustrate a simple approach to produce long range ordered porous cellulose nanowhisker structure with PVA as a support material. The resultant microstructure and pore morphology can be controlled by modifying the freezing rate and the slurry concentration. Finally, considering their lamellar and interconnected pore structure combined with the renewable nature of cellulose, ordered cellulose nanowhisker based materials with designed orientation of pore channels can be

Acknowledgments

The authors would like to acknowledge DOE (DE-EE0003144) and PSE Fellowship program at IPST@GT for the financial support of this study.

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