Development of the Glassy State of Benzophenone and Effect of Heating Rate from the Glassy State on Solidification
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DEVELOPMENT OF THE GLASSY STATE OF BENZOPHENONE AND EFFECT OF HEATING RATE FROM THE GLASSY STATE ON SOLIDIFICATION Paul E. Thoma and John J. Boehm Johnson Controls, Inc., Central Research, P.O. Box 591, Milwaukee, WI 53201-0591 USA ABSTRACT Benzophenone supercools to a glass when cooled to -100 0C. In fact, it is difficult to freeze benzophenone on cooling. In this investigation, the effect of cooling rate and the minimum cooling rate to obtain benzophenone as a glass is determined. From the glassy state, the influence of heating rate on the solidification temperature of benzophenone is determined. When heated at 3°C/min., solidification starts at about -29°C. Upon additional heating, melting usually starts0 at about +24°C, which is 23"C lower than the solid equilibrium structure melting temperature of 47 C. Occasionally the solid that forms at about -29°C undergoes a solid state phase transformation at about +22"C, when heated at 3°C/min. If this solid state phase transformation occurs, then the solid benzophenone starts to melt at 47°C. When solid benzophenone with the equilibrium structure is cooled to -100°C, no solid state phase transformation occurs. It appears that the structure that solidifies at -29°C is metastable. INTRODUCTION Many polymers, such as polyethylene terephthalate (PET), have a solidification temperature that is dependent on cooling rate. As the cooling rate increases, the solidification temperature of PET decreases. At a cooling rate greater than about 50°C/min., homopolymer PET does not crystallize, but instead becomes a glass at low temperatures. When heated from the glassy state, spontaneous solidification (crystallization) occurs. Solidification starts around 140°C when the heating rate is 30°C/min. Most simple organic compounds, when cooled, do not supercool very much before solidification occurs. However, Thoma [1] shows that a class of simple organic compounds having a three-dimensional partially charged pocket and a partially charged projection of opposite charge do supercool significantly. When in the liquid state, the attraction between neighboring molecules is significant and the probable reason for supercooling. The partially charged projection of one molecule is attracted to the oppositely charged pocket of another molecule and forms an ordered structure that hinders solidification. When the structure of a molecule promotes a strong attractive interaction between neighboring molecules, the amount of supercooling is so great that a glass forms when the liquid is cooled. Thoma [1] shows that 4-methylbenzophenone supercools and forms a glass at low temperatures (-56°C). Benzophenone also supercools and forms a glass at low temperatures. The purpose of this investigation is to determine the influence of cooling rate on the ability of benzophenone to form a glass. It is also a purpose of this investigation to determine the effect of heating rate from the glassy state on the solidification of benzophenone and the subsequent melting of the solid phase. MATERIAL ANDTEST METHODS The experi
