George S. Parks

Studies on Glass XIII. Glass Formation by a Hydrocarbon Polymer

Polyisobutylene, with an average molecular weight of about 4900, is at room temperature a highly viscous liquid. When cooled, it has been found to form a glass, with the same transitions in thermal properties characteristic in the vitrification of substances of low molecular weight. The heat capacity of polyisobutylene has been measured from 118°K to 295°K. It increases by 32 percent between 192°K and 202°K. The thermal expansion coefficient of polyisobutylene has been measured from 160°K to 300°K. It increases by 200 percent between 185°K and 205°K. The mean temperature of transition corresponds to a viscosity of about 1013 poises, in agreement with a rule observed generally for glass‐forming materials. The factors on which the transition temperature depends are discussed.

Studies on Glass. XI. Some Thermodynamic Relations of Glassy and Alpha‐Crystalline Glucose

Although the properties of a glass depend to some extent upon its thermal history, the view is expressed that fairly satisfactory thermodynamic data can be obtained for well‐annealed glasses. In the present study the difference between the heat contents of glucose glass and α‐crystals has been determined calorimetrically at 20°C and with the aid of this result the corresponding differences in entropy and free energy have also been computed. The value for this last quantity is ΔF°293 = 1670 cal. per mol. Solubility measurements have been made for glucose α‐crystals and glass in methyl, ethyl and isopropyl alcohols at 20°. The free energy values thereby obtained for the conversion of α‐crystals into glassy glucose are in good agreement with the preceding value derived from the thermal data.

The Heats of Solution of Hexamethylbenzene, Cetyl Alcohol, and Dicetyl in Related Liquids; Heats of Fusion by an Extrapolation Process

The heats of solution of crystalline hexamethylbenzene, cetyl alcohol, and dicetyl in various related liquids have been obtained at 25°C. Benzene, toluene, xylene, and mesitylene have been used as solvents for hexamethylbenzene, and a value for the heat of fusion of this substance has been deduced from these heats of solution by an extrapolation process. Similarly, a series of alcohols (methyl, ethyl, n‐butyl, n‐heptyl, and n‐dodecyl) have been used for the solution of cetyl alcohol. Solubility limitations, however, made it impractical to employ any liquid paraffin except n‐heptane in the case of dicetyl. The heats of fusion at 25°C, as estimated from the present solution experiments, are per gram: hexamethylbenzene, 29.0±0.3 cal.; cetyl alcohol, 48.2±0.7 cal.; and dicetyl (dotriacontane), 58.7±1.2 cal.

Studies on Glass XII. Some New Heat Capacity Data for Organic Glasses. The Entropy and Free Energy of dl‐Lactic Acid

Heat capacity measurements by the Nernst method have been made upon samples of secondary butyl alcohol and 3‐methylhexane in the glassy and liquid condition and upon a sample of dl‐lactic acid (1) in an incompletely crystallized state, (2) in the form of an acid glass and liquid, and (3) in the form of a mixed glass and liquid. From these results the specific heats and heat of fusion of pure crystalline dl‐lactic acid also have been derived. The data for the several glasses and undercooled liquids show a rapid rise from the heat capacity characteristic of a crystalline solid to that for the liquid state within a transition region of about 10° and the nature of this transition region has been discussed. A calculation of the molal entropy and free energy of formation of liquid dl‐lactic acid at 298.1°K yields 45.9 (±1.0) e.u. and —124,300±(2300) cal., respectively.

Some Viscosity Data for Boron Trioxide

The viscosities of four samples of pure molten boron trioxide have been measured between 267° and 443°C with a concentric cylinder viscometer. The results, representing true viscosity, range from 2.1×1011 to 2.1×105 poises and are consistent with earlier data obtained at much higher temperatures. A viscosity value in the neighborhood of 1013 to 1014 poises appears to be associated with the so‐called transition region in this and other glassy, or amorphous, materials.

Vapor Pressure and Other Thermodynamic Data for n‐Hexadecane and n‐Dodecylcyclohexane near Room Temperature

Vapor pressure measurements, by the Knudsen effusion method, have been obtained for n‐hexadecane and n‐dodecylcyclohexane within the temperature range 299−324°K. From these the following vapor pressure equations have been derived: C16H34;  log10p (in mm)=11.2822–4212/T,C18H36;  log10p (in mm)=12.4190–4880/T. Thence, values at 298.16°K have been calculated for (a) the changes in enthalpy and free energy in the standard‐state vaporization of these hydrocarbons and (b) the corresponding enthalpies and free energies of formation in the gaseous state.

Studies on Glass. XVIII. The Heats of Solution of Crystalline and Glassy Glucose; the Heat of Mutarotation of α‐Glucose

A calorimeter has been developed for the rapid solution of a powdered solid in a suitable solvent and the accurate determination of the heat of solution during the process. This calorimeter has now been used to determine the heats of solution of two preparations of crystalline α‐d‐glucose in water, both with and without mutarotation, and also the heats of solution of two preparations of glucose glass. Relatively accurate thermodynamic data for the crystal‐to‐glass transformation have been computed from these solution heats. Also small, but significant, differences per mole in enthalpy (ΔH298=102±18 cal.), in entropy (ΔS298=0.24±0.06 cal./deg.), and in free energy (ΔF298°=31±10 cal.) have been found for these two glass preparations. A value of ΔH298=−175 (±5) calories has been obtained for the mutarotation of a mole of α‐glucose, dissolved in water, to yield the equilibrium mixture of the alpha‐ and beta‐forms.

The Entropies of n‐Butane and Isobutane, with Some Heat Capacity Data for Isobutane

The entropies of n‐butane (S298° = 75.8 e.u.) and isobutane (S298° = 70.0 e.u.) have been calculated from thermal data by the third law of thermodynamics. In this connection, heat capacity data for isobutane over the temperature range 79°—261°K have been obtained by the Nernst method and also, as a partial basis for these calculations, the mode of extrapolating entropies below liquid air temperatures has been critically examined. The discrepancies between these new results and the corresponding entropies obtained by statistical methods appear too large to be accounted for by errors in the thermal data or extrapolations involved. The thermodynamics of the isomerization reaction is briefly considered.

The Heats of Combustion of Polythene and Polyisobutylene

Values, with uncertainties of about 0.03 percent, are here presented for the heats of combustion of polythene and polyisobutylene. The results for polythene indicate that this polymer is crystalline at least to the extent of about 50 percent at 25°C. The results for polyisobutylene are abnormally high and can be explained on the basis of a steric interference effect within the molecules of this liquid polymer.

Studies on Glass XIV. Note on the Compressibility of Glucose Glass

The coefficient of cubical compressibility of glucose glass at 24°C has been found to be 18.8 (10—6) atmos.—1 for the pressure range 7–25 atmos. The relatively large intermolecular forces in polyhydroxy substances, such as glassy glucose and liquid glycerol, are indicated by a comparison of the compressibility and expansion coefficients for these and other typical organic compounds.