The Amphorous and Crystalline Forms of Rubber Hydrocarbon

The coefficients of thermal expansion and the heat capacities of rubber hydrocarbon, both in an amorphous and in a so-called crystalline form, have been recently reported in two papers by Bekkedahl and Matheson. According to these investigators, the amorphous form undergoes a transition of the second order in the neighborhood of 199° K. Above this temperature they found a rather abrupt increase of approximately 205 per cent in the volume coefficient of thermal expansion, and one of about 38 per cent in the heat capacity. These phenomena are strikingly similar to those found in numerous studies on glasses in this laboratory, and especially in the recent investigation on polyisobutylene by Ferry and Parks. Thus, with the particular sample of polymerized isobutene employed, the transition region centered around 197° K. and the subsequent increases in volume coefficient and heat capacity were 200 and 32 per cent, respectively. Bekkedahl and Matheson found that, by cooling the amorphous rubber hydrocarbon to about 230° K. and then permitting it to warm up slowly over a period of days, their material could be obtained in a “crystalline” form. These “crystals” melted at 284° K. with a heat of fusion of 4.0 calories per gram. They also exhibited the previously mentioned second-order transition at about 199° K., but with somewhat smaller subsequent increases with rising temperature, i. e., about 165 per cent increase in the volume coefficient of thermal expansion and 28 per cent in the heat capacity. Two facts appear surprising and highly significant with these “crystals”: (1) the value of the heat of fusion which is extremely low compared with the figures of 20 to 54 calories per gram hitherto reported for various aliphatic hydrocarbons melting near room temperature, and (2) the duplication of the second-order transition found previously for amorphous rubber hydrocarbon.