Silylated CNC/PDMS Composite Membranes with Improved Selectivity for H₂O/Air Separation at Elevated Temperatures

Cellulose nanocrystals (CNCs) are purported as potential nanoreinforcements in polymer composites; however, the hydrophilic surface nature of CNCs limits their usage in composites if the polymers are hydrophobic, which results in poor compatibility. In this research, renewable biobased CNCs were modified using silylation, which is based on introducing a siloxane group to increase the compatibility between CNCs and the polydimethylsiloxane (PDMS) polymer. A silylated CNC/PDMS composite membrane was manufactured, which has applications in air dehydration. For this purpose, spray-dried CNC powder was used and modified. The surface modification of CNCs was carried out through the reaction between hydroxyl groups of the CNCs and the silylation agent. Finally, the silylated CNCs (SCNCs) were added to the PDMS solution to make SCNC/PDMS membrane samples with an SCNC weight concentration of 2%. Characterization analyses of FTIR and XRD of SCNCs confirmed the effectiveness of the silylation. SEM, AFM, and polarized light microscopy analyses indicated the improved dispersion of SCNCs in the polymer matrix compared to the not-modified CNC. The SCNC/PDMS membrane exhibited a 23% increase in water vapor permeability and a 79.6% increase in selectivity for water vapor over nitrogen gas at 25 °C compared to the pure PDMS membrane. Furthermore, the thermo-mechanical analysis (TMA) technique provided evidence that the addition of both CNC and SCNC resulted in a decrease in the coefficient of thermal expansion (CTE) of the PDMS membrane. These findings contribute to the development of cellulose-based materials with improved performance for their potential applications in various fields.