Glasses Denser than Supercooled Liquid

When aged below the glass transition temperature, T_g, the density of a glass cannot exceed that of the metastable supercooled liquid (SCL) state, unless crystals are nucleated. The only exception is when another, high-density SCL (HD-SCL) states exists, with a density between that of the ordinary SCL and the crystal states. Experimentally, such HD-SCL states have not been observed in simple molecular liquids, either in aged or vapor-deposited stable glasses, even near their Kauzmann temperatures. Here, we report that thin vapor-deposited films of N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine (TPD) have densities that exceed their corresponding SCL density by as much as 3%. These glasses are also more isotropic than their thick film counterparts, and do not show any signs of crystallization. We demonstrate that these states are achieved at a range of thicknesses (25 – 55 nm) where thin films have large mobility gradients due to enhanced surface mobility, allowing them to access states that are not accessible in bulk liquids, due to kinetic barriers. We hypothesize that another HD-SCL state exists in TPD that has a liquid-liquid phase transition temperature below T_g and as such it has not been previously observed.