Structural and thermodynamic characterization of modified cellulose fiber-based materials and related interactions with water vapor

Abstract

In this study, the surface characteristics, water vapor interactions, and state of water adsorbed on unmodified and coated paper samples were investigated in an attempt to obtain a better understanding of the fundamental principles related to thermodynamics of this process, as well as to provide essential insight that could be used for further improvement of the papers’ barrier properties. Based on the BET measurement, the coated paper samples showed higher specific surface areas than unmodified paper; however, their mean pore diameters are smaller. The BJH method was used for pore size distribution analysis. Hydrophobic properties of the paper samples were determined from experimental isotherms, e.g., monolayer moisture content, and these results have been related to the water vapor transfer rates (WVTRs) showing a complex nature of these relations. The highest peak corresponding to the modified samples with smaller pore sizes was found to be in the range of 1–30nm, while it was in the 30–100nm pore size range for unmodified paper. The net isosteric heats of sorption for different unmodified and modified paper samples were determined from water vapor adsorption isotherms measured at 15, 25, and 35°C. The net isosteric heats of sorption decreased with an increase of moisture content after reaching the maximum values at 12.53, 15.25, 14.71, 23.2, and 22.77kJ/mol for unmodified, zein grafted, calendered coated, PLA, and PHBV coated papers, respectively. The state of adsorbed water and water-vapor interaction on paper surface were also studied by TGA–DSC and FT-IR spectroscopic techniques. The FT-IR results revealed the formation of water vapor clusters due to poly-layer formation at the higher relative humidity. This has a profound effect on the equilibrium properties, e.g., the intensity of interactions between water molecules and the paper surface decreases as the relative humidity (moisture content) increases. Percentage of bound and unbound water formation and also the dehydration energy at various relative humidities were determined for the paper samples using the TGA–DSC technique.