Heat Capacity, Entropy, and Free Energy of Rubber Hydrocarbon

Abstract

The best method for obtaining the free energy of formation of rubber is by making use of the third law of thermodynamics. This makes necessary the determination of heat-capacity values of rubber from room temperature down to temperatures sufficiently low to apply an empirical formula for obtaining the values below this lower temperature. From these heat-capacity values the entropy may be obtained. Then from this latter value, along with the entropy values of carbon (graphite) and gaseous hydrogen and the heat of formation of rubber, a reliable value for the free energy of formation of rubber may be calculated. Several investigators have previously determined the heat capacities of rubber, but their observations were not made at temperatures sufficiently low to permit accurate extrapolation to the absolute zero in order to apply the third law. Furthermore, in the previous work the possibility that rubber at low temperatures might exist either as a metastable amorphous form or as a crystalline form was not clearly recognized. In the present investigation the aim was not only to extend the temperature range but also to obtain data of a higher order of accuracy than that previously reported.

Specific heat and increases of entropy and enthalpy of the synthetic rubber GR-S from 0 degrees to 330 degrees K

“…The difference between the entropy of the natural rubber hydrocarbon in its crystalline and amorphous states at 0° K is less than 1 percent of the value of the entropy at 25° C [1]. It is probable that the entropy of random orientation of amorphous GR-S, for each polymerization unit, is of the same order of magnitude as for the natural rubber hydrocarbon.…”

“…Values of the specific heat are given at 5-degree intervals in This sample of GR-S showed no evidence of the crystallization reported for natural rubber [1], but it exhibited a transition of the second order characterized by a sudden rise of about 40 percent in specific heat. In figure 1 the specific heat curve for GR-S is shown with the specific-heat curves of amorphous natural rubber hydrocarbon [1] and the synthetic rubber Hycar OR-15 [4].…”

“…In figure 1 the specific heat curve for GR-S is shown with the specific-heat curves of amorphous natural rubber hydrocarbon [1] and the synthetic rubber Hycar OR-15 [4]. The natural rubber hydrocarbon is a polymer of 2-methyl-l,3-butadine (isoprene), GR-S is a copolymer of 1,3-butadiene and styrene, and Hycar OR-15 is a copolymer of 1,3-butadiene and acrylonitrile.…”

“…the specific heat of natural rubber [1] and of Hycar OR-15 [4]). The apparatus for th e measurement of t emperature and energy input is described in a paper on butadiene [5].…”

“…Such data have been obtain ed for unvulcanized natural rubber [1)/ its monomer, isoprene [2], the equilibrium relationship between the two [3], Hycar OR-I5 [4], and 1,3-butadiene [5]. In this paper specific h eat measuremen ts on a 1 Figures in brackets indicate the literatnre references at the end of this paper. 63 sample of GR-S synthetic rubber are described, and the increases in entropy, enthalpy, and free energy from 0 0 to 330 0 K calculated from them, are included in a table at 5-degree intervals.…”