High-porosity aerogels of high specific surface area prepared from nanofibrillated cellulose (NFC)

Abstract

Low-density aerogels based on nanofibrillated cellulose (NFC) from wood pulp were prepared from NFC aqueous dispersions using solvent exchange from water to tert-butanol followed by tert-butanol freeze-drying. In the present study, the dispersion of NFC nanofibers in the hydrocolloid was very well preserved in the aerogels. The “effective” diameter of the NFC nanofibers in the aerogels is around 10–18 nm corresponding to specific surface areas as high as 153–284 m² g⁻¹. Aerogels based on different NFC nanofibers were studied by FE-SEM, BET analysis (nitrogen gas adsorption), and mechanical properties were measured in compression for different densities of aerogels. The properties are compared with polymer foams and inorganic aerogels. Compared with cellular NFC foams, the present nanofibrous aerogels have lower modulus and show lower stress in compression for a given strain. Tert-butanol freeze-drying can therefore be used to create “soft” aerogels.

Introduction

Aerogels is an important class of porous materials. They have been defined as nanoporous solids formed by replacement of liquid in a gel with gas. In addition, there should be no or limited shrinkage during this replacement process [1], and the volume of the solid phase should be only of few percent of the total volume (0.2–20%) [2]. For instance, silica aerogels can have a volume fraction of air as high as 99.8% and a density of only 0.004 g cm⁻³ [2]. Aerogel characteristics make them interesting in applications for gas or liquid permeation and adsorption, thermal and acoustic insulation, optical applications, carriers for catalysis or drug release. Aerogels may also function as low-density cores in sandwich structures or as templates for precipitation of inorganic nanoparticles [3].

The first aerogels were reported by Kistler in 1931 and produced from liquid gels [4]. In order to avoid shrinkage, the liquids were gradually transformed into supercritical fluids and evaporated using supercritical drying. In one case, Kistler replaced the liquid in solvent-swollen cellophane to propanol before subjecting the gel to supercritical drying [4]. This aerogel was from regenerated cellulose so that the cellulose I structure of native cellulose was dissolved and formed cellulose in altered crystal form (cellulose II) during regeneration from solution. More recently, aerogels from regenerated cellulose were prepared using a supercritical carbon dioxide process [5], [6]. This process is interesting since the low critical temperature and pressure of CO₂ (31 °C and 7.4 MPa respectively) are favourable since cellulose degradation is avoided. The supercritical CO₂ drying resulted in cellulose aerogels with a specific surface area as high as 500 m² g⁻¹ [5], [6]. However, the required processing steps of dissolution, gelation, and solvent exchange are very slow and can take several days. Moreover, the aerogel is not nanofibrous.

Aerogels based on nanofibrillated cellulose (NFC) may offer advantages from an environmental point of view since NFC is obtained from renewable resources and no harmful solvents are required during processing. NFC aqueous dispersion is prepared by disintegration of pulped wood fiber cell walls. In the plant cell wall, cellulose is the reinforcing constituent of high strength and modulus due to its extended chain structure [7]. It is present in the form of “microfibrils” of 5 nm diameter and up to several micrometers in length, and they are embedded in a matrix of hemicelluloses and lignin. During chemical processing of wood (“cooking”, “pulping”), a large fraction of the hemicelluloses and most of the lignin components are removed from the cell wall. During chemical processing, aggregation of cellulose microfibrils also takes place so that the several microfibrils are assembled and form cellulose fibril aggregates of increased diameter [8]. NFC is prepared by mechanical disintegration of wood pulp. Wood pulp of high cellulose content is often used. In order to reduce the energy required for mechanical disintegration of wood pulp to NFC, the pulp has been subjected to enzymatic pretreatment [9] or chemical pretreatment [10]. The resulting NFC (also sometimes termed microfibrillated cellulose, MFC) can have a diameter in the 10–30 nm range [9] or only of 3–5 nm [10] and is obtained as an aqueous dispersion.

Paakko et al. pioneered cellulose I nanofiber aerogels by freeze-drying of an NFC dispersion [11]. In principle, the water is removed from the hydrogel so that a porous low-density NFC aerogel is created. Mechanical properties were reported, the aerogels are quite ductile and mechanically robust. The aerogel was also used as template where a conducting polymer coated the NFC surfaces. The conductivity of the resulting aerogel was reported. Sehaqui et al. [12] then prepared a series of low-density foams based on NFC using freeze-drying with a lower degree of undercooling during the freezing stage. Mechanical properties were studied in detail, and positive effects were noted from the introduction of a polymer matrix (xyloglucan) to the foam cell walls. A limitation with these NFC materials is that the highest specific surface area reported is limited to 66 m² g⁻¹ [11]. Most likely, NFC agglomerates during the freezing and sublimation stages of the freeze-drying process. It is desirable to explore if we can increase the specific surface area of these materials, since it may help to realize the property advantages associated with nanostructured composites.

An alternative to conventional freeze-drying is tert-butanol freeze-drying [13]. This drying method has been applied to prepare regenerated cellulose aerogels [5], [14], [15]. Due to the nature of tert-butanol, a lower extent of surface tension effects (capillary action) are displayed during the drying process. This resulted in aerogels with higher specific surface area (max. of 332 m² g⁻¹ for regenerated cellulose) compared to those prepared by conventional freeze-drying [5], [14], [15], [16].

In the present study, NFC aerogels of ultra-high porosity (93–99%) are prepared by tert-butanol freeze-drying of NFC hydrocolloidal dispersions. A simple 1-step procedure is included in order to provide a fast preparation alternative. The objective is to increase the specific surface area, study the structure and compare properties to other porous materials such as NFC foams [12].