Glass engineering of aminotriazine-based materials with sub-ambient Tg and high kinetic stability

Abstract

A challenge in glass engineering is the design of molecular glasses combining a high glass kinetic stability (GS) of the amorphous phase with a low (sub-ambient) glass transition temperature (T_g). Triazine derivatives with arylamino substituents readily form glassy phases that can show outstanding resistance to crystallization. In the present study, a series of 12 analogous compounds incorporating phenylamino and cyclohexylamino groups was synthesized, and their thermal properties and intermolecular interactions were studied. All compounds possess an excellent glass-forming ability, a low T_g ranging from 32 °C to as low as −19 °C, and a high GS. While the cyclohexyl derivatives show higher T_g, the phenyl derivatives possess a higher GS with some compounds remaining completely amorphous for over three years despite their sub-ambient T_g. X-ray diffraction, infrared spectroscopy and DFT calculations reveal that the higher volume occupancy and rotational energy barrier of cyclohexyl groups are the main factors responsible for the compounds' higher T_g values but that they also contribute to their higher propensity to crystallize. In counterpart, the planarity of phenyl groups leads to poorer packing and enhances their GS while keeping their T_g well below ambient. The formation of hydrogen bonds or competing interactions provides an additional handle to tune the T_g of the compounds. Taken together, these studies provide guidelines for the design of molecular glasses with readily tunable thermal properties in view of their functionalization.