Effects of water adsorption in hydrophilic polymers

The origin of the adsorption-induced phenomena in hydrophilic polymers, such as cellulose, lies at the atomistic scale. We use molecular dynamic simulations to investigate the mechanisms by which the adsorbing polymers change their properties upon water adsorption. Despite of their limited spacial and temporal capabilities MD simulations are able to reproduce the hygroscopic swelling and moisture softening effects in several natural polymeric systems, such as crystalline and amorphous cellulose, hemicellulose, or lignin. The revealed picture of adsorption shows a complex dependency between system porosity, water concentration, chemical potential and number of hydrogen bonds. Clear correlation between the number of hydrogen bonds, porosity and mechanical moduli implies that the dynamics of hydrogen bonds is an underlying mechanism of the adsorption process in hydrophilic polymers. The adsorbed water molecules, attracted by sorption sites tend to break the hydrogen bonds between the polymer residues that are responsible for the mechanical stability of the system. As a result, the number of hydrogen bonds decreases with water concentration which causes a substantial decrease in elastic modulus. This plasticization effect causes the polymer matrix to yield under swelling pressure exerted by the water molecules clustered in the nanopores under higher-than-bulk density. Thus, as the adsorbed amount increases, the pore structure of the polymer evolves in such a way that the average pore size increases and the pores merge. The evolution of the pores topology makes that the diffusion of water changes non-linearly with water content. Highlighting the central role of hydrogen bonds in adsorption process helps understanding the concept of hydrophilicity in polymers and enables further research in a more application-oriented direction, e.g. wood industry, moisture sensors/actuators.