Hydrogen-bonding structures and glass transitions in 1,2-propanediamine mixed with inorganic salts

Solvation structures and glass transitions in binary systems of 1,2-propanediamine (12PDA) and some inorganic salts are investigated using Raman scattering and differential scanning calorimetry. The NH2 stretching vibrational modes in the Raman spectrum of 12PDA with added salts shift to higher wavenumbers compared to those of pure 12PDA. At low salt concentrations, the peaks shift to lower wavenumbers with decreasing temperature. The dependence of salt concentration on the glass transition temperature, Tg, for the (NaClO4)x(12PDA)1−x system shows a sigmoidal curve. We discuss the change in the hydrogen-bonding structure and molecular dynamics in the (NaClO4)x(12PDA)1−x system based on a two-state model in which the NH2 groups are either strongly hydrogen bonded with other solvent molecules or weakly interact with solvent molecules or ions. It is suggested that the dominant interactions in the system changes from strong hydrogen bonds to weak interionic interactions with variations in salt concentrations, and the anomalous Tg behavior is caused by a crossover between the two different interactions.