A molecular dynamics model to measure forces between cellulose fibril surfaces: on the effect of non-covalent polyelectrolyte adsorption

This study describes the development of representative models of cellulose fibril surface (CFS) as a first approximation to the study of the molecular interactions that are developed between cellulose fibres. In order to assess its sensitivity and representativeness towards the main factors affecting the bonding properties at the fibre scale, these models were non-covalently surface modified with two types of polyelectrolytes, sodium carboxymethyl cellulose (CMC-ONa) and a cationic polyacrylamide (CPAM). From the analysis of pair correlation functions (g(r)) it was possible to assess the main interactions of adsorption of polyelectrolytes towards the (1-10) hydrophilic cellulose, which were due to electrostatic interactions coupled with hydrogen bonding. Besides, the bond strength between fibril surfaces through the (100) hydrophobic surface was calculated from pull out simulations (using steered molecular dynamics). Using a rate of change of force of 0.159 nNps(-1), the calculated bond strength for the neat CFS model (nanometer scale) was two to three orders of magnitude higher than the experimental values observed at the fibre scale (micrometer scale). The results for the polyelectrolyte modified setups supported the validity of the CFS models to reproduce the expected behavior of inter-fibre joints in terms of the specific bond strength and the relative bonded area at the fibre scale in cellulose materials, and thereby the CFS models are a suitable complement, in conjunction with other techniques, for the systematic study of the effect (in qualitative terms) of chemical or physical factors on the bond strength properties of cellulosic materials. [GRAPHICS] .