Investigation of mass transport properties of microfibrillated cellulose (MFC) films
M. Minelli, M. Giacinti Baschetti, F. Doghieri, M. Ankerfors, T. Lindström, I. Siró, D. Plackett
Cellulose-based nanocomposites are now emerging as promising new materials suitable for a variety of applications. One source for such new nanocomposites is microfibrillated cellulose (MFC), produced by delamination of cellulosic fibers in high-pressure homogenizers. MFC is an organic and biodegradable reinforcement in polymer nanocomposites because of its high aspect ratio, good mechanical properties and ability to form networks: promising properties that may make this material suitable for use in a number of industrial fields and products.
The structure and transport properties of a four different films based on two different generations of microfibrillated cellulose (MFC), alone or in combination with glycerol as plasticizer, were investigated through FE-SEM analysis and sorption or permeation experiments. FE-SEM revealed the existence of complex structures in the different samples. A porous, closely packed fiber network, more homogeneous in the samples containing glycerol, was characteristic of the surface of MFC films; while film cross-sections presented a dense layered structure with no evidence of porosity. Water vapor sorption experiments confirmed the hydrophilic character of these cellulosic materials and showed a dual effect of glycerol, which reduced the water uptake at low water activity while enhancing it at high relative humidity, as showed in Figure 1. The observed trends of water sorption can be described considering that both physical adsorption on fiber surfaces and absorption in the cellulose amorphous phase takes place and therefore a dual-model sorption model was employed.
The water diffusivity (in the panel in Figure 1) in dry samples was remarkably low for a porous material (10-11-10-12 cm2/s), confirming the existence of complex structures below the film surface; however, when water is present in the system, D rapidly increases with an exponential trend. Diffusivity is also definitely affected by plasticization, being higher for glycerol-containing samples.
Similar behavior was observed in permeation experiments. Dry MFC films showed excellent oxygen barrier properties, comparable with those of polymers usually considered suitable for ultra-high barrier applications. Indeed, when dry, the cellulose network presents a particularly compact and stiff structure, causing the observed barrier effect. Furthermore, although MFC films are characterized by a certain porosity, FE-SEM images of sample surfaces suggests that the pores are not substantially interconnected
When the water content in the membrane is raised, however, a dramatic decrease in these properties was observed (Figure 2).
Figure 1. Water vapor sorption isotherms in MFC samples at 35°C, dotted lines are given by the best fitting of the data with the dual-mode model equation. In the panel: water diffusivity at 35°C in the four different MFC samples as function of the average water concentration.
Figure 2. Oxygen permeability in the four MFC samples at 35°C as a function of water