Nanostructure and Interfacial Mechanical Properties of PEG-Treated Nanocomposites Studied with Molecular Dynamics Simulation

Polyethylene glycol (PEG) is widely used in the treatment of archeological wood to reinforce the decayed wood structure[1]. Understanding the mechanisms that allow PEG to strengthen archeological wood can provide fundamental insights into wood hygromechanical behavior. Inspired by the structure and components of the wood S2 cell wall layer, the composites composed of crystalline cellulose and PEG-treated amorphous cellulose are used to study the interfacial mechanical behavior of the fiber-matrix with the aim to reveal the impact of PEG molecules on the stick-slip behavior.<br/>GROMACS package[2] and the OPLS force field[3] are used for the MD simulations. The composites in the dry state are composed of infinitely long crystalline cellulose fiber embedded in a PEG-impregnated amorphous cellulose matrix. Seven levels of PEG loading and eleven levels of moisture loading are constructed resulting in a total of 77 initial configurations. The pulling-out test simulations are carried out to study the interfacial mechanical properties of the composites while recording the shear stress and the displacement of the crystalline cellulose.<br/>The initial densities of the dry systems range from 1.4 g/cm³ to 1.44 g/cm³ depending on the given PEG mass ratio. Three factors including, the atom number densities of the matrix atoms all exhibit peak values at certain distances to the surface of crystalline cellulose, a linear correlated relationship between the number density peak values of water atoms and hydrogen bond numbers between crystalline cellulose and water molecules with increasing the moisture content, and the chains of amorphous matrix near the fiber-matrix interface tends to be more aligned revealed by Herman orientation functions, all demonstrate that the ordered structure of crystalline cellulose has a significant impact on the distribution of matrix atoms. The pure amorphous cellulose swells linearly with PEG variation, which indicates that PEG molecules behave like integrated with the amorphous matrix. The swelling coefficients concerning the moisture content variation are found to be larger for the composites with a higher PEG mass ratio.<br/>The maximum shear forces are extracted from the pulling-out simulations which span all PEG mass ratios and moisture content constructed. The shear force map shows that the maximum shear force appears at around 10%-15% PEG mass ratio and 3%-6% moisture content, which indicates the consolidation of the interfacial mechanical property at low moisture content.<br/><br/>[1] R.J. Barbour, Treatments for Waterlogged and Dry Archaeological Wood, in: Archaeological Wood, American Chemical Society, 1989: pp. 177–192.<br/>[2] M. Abraham, T. Murtola, R. Schulz, S. Páll, J.C. Smith, B. Hess, E. Lindahl, GROMACS: High performance molecular simulations through multi-level parallelism from laptops to supercomputers, (2015).<br/>[3] M.J. Robertson, J. Tirado-Rives, W.L. Jorgensen, Improved Peptide and Protein Torsional Energetics with the OPLS-AA Force Field, J. Chem. Theory Comput. 11 (2015) 3499–3509.