The Build-Up of Polyelectrolyte Multilayers of Microfibrillated Cellulose and Cationic Polyelectrolytes

Abstract

A new type of nanocellulosic material has been prepared by high-pressure homogenization of carboxymethylated cellulose fibers followed by ultrasonication and centrifugation. This material had a cylindrical cross-section as shown by transmission electron microscopy with a diameter of 5-15 nm and a length of up to 1 microm. Calculations, using the Poisson-Boltzmann equation, showed that the surface potential was between 200 and 250 mV, depending on the pH, the salt concentration, and the size of the fibrils. They also showed that the carboxyl groups on the surface of the nanofibrils are not fully dissociated until the pH has reached pH = approximately 10 in deionized water. Calculations of the interaction between the fibrils using the Derjaguin-Landau-Verwey-Overbeek theory and assuming a cylindrical geometry indicated that there is a large electrostatic repulsion between these fibrils, provided the carboxyl groups are dissociated. If the pH is too low and/or the salt concentration is too high, there will be a large attraction between the fibrils, leading to a rapid aggregation of the fibrils. It is also possible to form polyelectrolyte multilayers (PEMs) by combining different types of polyelectrolytes and microfibrillated cellulose (MFC). In this study, silicon oxide surfaces were first treated with cationic polyelectrolytes before the surfaces were exposed to MFC. The build-up of the layers was monitored with ellipsometry, and they show that it is possible to form very well-defined layers by combinations of MFC and different types of polyelectrolytes and different ionic strengths of the solutions during the adsorption of the polyelectrolyte. A polyelectrolyte with a three-dimensional structure leads to the build-up of thick layers of MFC, whereas the use of a highly charged linear polyelectrolyte leads to the formation of thinner layers of MFC. An increase in the salt concentration during the adsorption of the polyelectrolyte results in the formation of thicker layers of MFC, indicating that the structure of the adsorbed polyelectrolyte has a large influence on the formation of the MFC layer. The films of polyelectrolytes and MFC were so smooth and well-defined that they showed clearly different interference colors, depending on the film thickness. A comparison between the thickness of the films, as measured with ellipsometry, and the thickness estimated from their colors showed good agreement, assuming that the films consisted mainly of solid cellulose with a refractive index of 1.53. Carboxymethylated MFC is thus a new type of nanomaterial that can be combined with oppositely charged polyelectrolytes to form well-defined layers that may be used to form, for example, new types of sensor materials.

A Fast Method to Produce Strong NFC Films as a Platform for Barrier and Functional Materials

“…Recently the development in cellulosic fibrillation methods has facilitated the production of nano-scaled NFC from cellulosic sources like wood and crops with reasonable low energy consumption. [1][2][3] The width of the nanofibrils depends on the production method but is typically around 5-20 nm. The length of the fibrils 3 is more challenging to determine but may exceed 5 µm and thus the aspect ratio of NFC is more than 250.…”

Addition of silica nanoparticles to tailor the mechanical properties of nanofibrillated cellulose thin films

“…Similar to thickness development (figure 4), the buckling wavelengths show a linear propagation as well (figure 5a), with an increasing rate for PVAm-SiO 2 -PVAm-NFC than for PVAm-NFC. It has been reported elsewhere [1][2][3][4][5] that the buckling wavelength increases with increasing the number of layers. The Young' modulus, however, should not be affected by thickness over a certain thickness range [9][10][11][12][13] which is also demonstrated here in figure 5b.…”

“…It has for example been shown that the CNC can selforganize [2] to give a chiral nematic arrangement of the CNC film as water is evaporated from the CNC dispersion. It has also been shown that both CNC and NFC can be incorporated into Layer-by-Layer (LbL) self assemblies to yield films with controlled thickness and hence controlled optical properties depending on the composition of the film [3,4]. Recent investigations have also shown that it is possible to prepare freestanding films on NFC [5,6] with controlled barrier properties and with very low oxygen permeabilities at low relative humidities (RH).…”

Sustainable Design for the Direct Fabrication and Highly Versatile Functionalization of Nanocelluloses

“…NCC is made by hydrolysis, generally using inorganic acids (e.g., sulfuric acid and phosphoric acids), and produces highly crystalline and rigid nanoparticles. [13][14][15][16] Since the amorphous region of the cellulose fibril is degraded in the process, the yield is low (30-50% yield, <7 MJ kg −1 energy consumption of the nano conversion). NFC fabrication from wood-derived pulp fibers was first achieved by mechanical agitation (high energy consumption, generally >30 000 kWht −1 , 700-1400 MJ kg -1 ) [7,13,14] and chemical-mechanical methods, e.g., the carboxymethylation route (high energy, generally >30 000 kWht −1 , 700-1400 MJ kg −1 ).…”