Melting and crystallization transitions in organically modified layered silicates studied with differential scanning calorimetry

Abstract

Melting and crystallization behaviors of the monolayers in the surface modified layered silicates were studied with differential scanning calorimetry. The layered silicates with different layer charge densities were used for the purpose. The ammonium ions forming the monolayer were also varied according to their chemical architecture, chain length and number of chains in the molecular structure. The quality of surface modification of the mineral was quantified using high resolution thermo-gravimetric analysis. At an optimum combination of the cross-sectional area of the ions, chain length as well as chain density, ordered monolayers are formed on the surface which turn to disordered state at a temperature corresponding to the melting of the ordered structure. These transitions are reproducible in nature but may require longer cooling times to regenerate again. The heat flow to the modified mineral (corresponding to melt enthalpy) increased on increasing the chain length as well as chain density, however, the transition temperatures decreased with increasing chain density in the surface modification molecules. The modified minerals were also hygroscopic in nature with first heating cycles showing the evaporation of water, however, with increasing chain length and chain density in the surface modification, the extent of hydrophobicity of the minerals increased. The effect of chemical architecture was not significant as it seemed to be driven by the chain length and capability of the modification molecules to crystallize in general or on the processing conditions used.

Keywords:

• phase transition
• monolayer
• melting
• DSC
• TGA
• crystallization
• layer charge density
• montmorillonite

Acknowledgments

The experimental work was carried out at the Institute of Polymers, ETH Zurich, Switzerland.