MOLECULAR DESIGN FOR NONPOLYMERIC ORGANIC-DYE GLASSES WITH THERMAL-STABILITY - RELATIONS BETWEEN THERMODYNAMIC PARAMETERS AND AMORPHOUS PROPERTIES

The molecular structures of low-molecular-weight organic compounds and their amorphous properties have been investigated to obtain a design rule for uniform amorphous films with high thermal stability. The glass transition temperature (T(g)/K), maximum crystal-growth velocity (MCV/m s-1), and maximum crystal-growth temperature (T(c,max)/K) are key parameters for characterizing the amorphous properties of organic materials. Some quantitative relations between these parameters and thermodynamic parameters were examined from both theoretical and experimental viewpoints. The equation for T(g) of various aromatic compounds expressed as T(g) = a - bSIGMADELTAS(tr,m)/N has been found to be nearly established, where SIGMADELTAS(tr,m) is the sum of the entropies of fusion and of phase transitions between T(g) and the melting point (T(m)/K), N is the number of heavy atoms per molecule except hydrogen atoms, and a and b are constants. The relation can be successfully explained by using the Adam-Gibss theory on the viscosity of supercooled liquids. It has also been found that MCV for aromatic compounds nearly followed the equation log (MCV) = c - dN/(T(m)SIGMADELTAH(tr,m)), where c and d are constants and SIGMADELTAH(tr,m) is the sum of the enthalpies of fusion and of phase transitions between T(c,max) and T(m). This can be explained by a potential barrier model for molecular diffusion both at a crystal/supercooled liquid interface and in a bulk supercooled liquid. Consequently, molecules preferably used for amorphous films should have a symmetric globular structure with a large molecular weight and small intermolecular cohesion. According to these findings, it has been revealed that high T(g) and T(c,max) and low MCV yield stable organic glasses with high thermal stability.