Metastable glasses

Fig. 6 DSC thermograms showing representative thermal transitions in frozen solutions. The top thermogram is for a mannitol solution and is characteristic of a metastable glass-forming system.

Despite these precautions, a slow freezing process is not always sufficient to ensure a dimensionally stable pellet of good cosmetic appearance. In this respect, formulated solutions that produce interstitial metastable glasses on cooling are sometimes a source of problems. In such systems, devitrification followed by partial erratic recrystallization of the excipient may occur during lyophilization, thereby generating a pellet of poor powdery appearance. 

Glass formation, the other common fate of a compressed fluid, however, gives a much larger viscosity increase. For mixtures and fluids with nonspherical molecules, it is common to produce a metastable glass at some pressure in excess of the equilibrium crystallization point. Methanol is one example. It can be easily superpressed past its crystallization pressure of 3.5 GPa at T = 25°C towards its glass-transition pressure of 11.4 GPa and beyond. Being a... 

Both compounds are derivatives of triphenylamine, a well-known photoconductor. If thin films of TPD and TAPC are prepared by vacuum evaporation both compounds form metastable glasses. In such glasses carrier mobUities up to 10 cm A s for TPD [13] and 10 cm A s for TAPC [14] have been reported. But both TAPC and TPD glasses are metastable and have a strong tendency to crystallize. If the molecules are imbedded in a polymer matrix, e.g. polycarbonate or polystyrene, morphologically stable materials are formed, but the mobilities decrease drastically [13]. 

Further evidence of the presence of a glass-like mesophase was provided by the DSC data obtained at high temperatures for the quenched polymers. In general a change in heat capacity, characteristic of a glass transition, was found to occur at --100 K above the of the amorphous material in the semicrystalline samples. A typical example of this behaviour is represented in Fig. 2. Experimental results thus supported the conclusion that a metastable glass-like modification is formed by quenching from the LC... 

Calorimetric data supported the conclusion that, during the quenching of these materials from the anisotropic melt, the LC phase is prevented from crystallizing and remains as a highly supercooled mesophase. This metastable glass-like modification is subsequently transformed below 235 K into a more ordered, stable phase. 

The slight polydispersity in particle size allows the system to avoid the crystalline phase and reach the metastable glass state. Above ( ) = 0.58, the system is metastable with polydispersity, the random close-packing volume fraction shifts to higher values. 

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