Cryoscopic and Viscosity Studies of Polyisobutylene. Cryoscopic Deviation of Polyisobutylene Solutions from Raoult's Law

1. The cryoscopic method is not satisfactory for polyisobutylenes having chain lengths much over 40 isobutylene units, on account of the failure of their solutions to obey Raoult's law. 2. The present work has led to the selection of a Kcm value of 0.75×104 for n-hexane solutions of polyisobutylene for use in the equation M=(log ηr×Kcm)/C. 3. Extrapolation of cryoscopic data obtained on nonideal solutions to infinite dilution gives inordinately high molecular weight values compared with those based on freezing point measurements of ideal solutions. 4. Of several solvents studied, n-hexane was found to be the best for viscosity—molecular weight measurements of polyisobutylene. 5. Fractionation of polyisobutylene by diffusion into mixtures of n-hexane and acetone resulted in the separation of the lower polymer fractions.

Viscosity and Cryoscopic Data on Polystyrene. Discussion of Staudinger's Viscosity Rule

1. The cryoscopic method is not satisfactory for polystyrenes containing more than twelve styrene units in the chain on account of the failure of their solutions to obey Raoult's law. 2. The present work has established a new Kcm value of 0.45×104 for benzene solutions of polystyrene to be used in the equation: A Kcm value of 0.6×104 has been similarly established for benzene solutions of polyindene. 3. Increased confidence in the viscosity-molecular weight procedure is derived from the data presented for the new Kcm equivalent covering a wide range of different polymers.

Fractionation and Molecular Weight of Rubber and Gutta-Percha

1. Evaporated latex films and Bolivian fine para contain a chloroform-soluble fraction of about 62 per cent, whereas the soluble portion of crepe and smoked sheets is about 86 per cent. The average molecular weight of these sols, which contain the low polymers and “resin”, range from about 130,000 to 180,000, the more soluble type having the lower value. 2. Diffusion and precipitation methods were employed to fractionate R. C. M. A. crepe, and the following data indicate the approximate composition and molecular weight range of the soluble hydrocarbon fractions separated: 3. The chloroform-insoluble portion of crepe contains a nonlinear type of hydrocarbon, judged by x-ray and viscosity studies. These methods show that the chloroform-insoluble portion of ammonia-preserved latex films contain extremely high-molecular rubber hydrocarbon which is easily oriented to give the regular x-ray crystalline pattern for rubber. 4. Freshly tapped latex contains a large proportion of petroleum-ether-soluble fraction, which becomes insoluble in this solvent on standing in the presence of ammonia. Ammonia-preserved latex films are soluble in chloroform or in hexane containing alcohol-acetone or acetic acid; this insolubility is thus an association effect, which is overcome by the addition of a polar solvent. 5. Viscosity studies on fractions of balata and gutta-percha show that their hydrocarbons have an average molecular weight of about 42,000 and cover a narrow polymeric range.

Viscosity-Molecular Weight of Rubber. Cryoscopic Deviation of Rubber Solutions from Raoult's Law

1. The cryoscopic method is not satisfactory for polyprenes with chain lengths much greater than about 20 isoprene units on account of the failure of their solutions to obey Raoult's law. 2. Since Staudinger's viscosity-molecular weight constants were in many cases determined on nonideal solutions, this subject will be further investigated. 3. The present work has led to the selection of a Kcm value of 0.75 × 104, which is 25 per cent lower that the value chosen by Staudinger, who used higher polymers which do not behave ideally in solution. 4. The use of Staudinger's equation, M=ηsp/CKm, leads to large deviations with the variations in viscosity usually employed, and this deviation is largely avoided by employing the equation M=(log ηr/C)Kcm.

Sol and Gel in Hevea Latex and Crude. Rubber Influence of Oxidation on Gel-Sol Transformation

An apparatus and procedure to determine the amounts of sol and gel rubber in dried latex films and crude rubber have been described. It is shown that whereas plantation smoked sheet and crepe contain large and varying quantities of rubber soluble in petroleum ether, dried rubber films from latex are practically free from sol rubber when oxidation has been avoided. Latex increases in sol rubber content when subjected to various treatments permitting oxidation of the hydrocarbon. It appears from this work that the rubber in the form of latex is very susceptible to oxidation, which however involves only minute quantities of combined oxygen. When latex film rubber is subjected to creeping, it shows an increased content of sol rubber corresponding to that of crepe. A short milling of crepe was found to convert it completely into sol rubber. This confirms the gel-sol transformation mechanism of breakdown. Gel rubber from dried latex films is swelled to a maximum of 1500 to 7800 per cent but is not dissolved when placed in benzene, chloroform, carbon tetra-chloride or carbon disulfide and allowed to stand for several days. Gel rubber becomes soluble and therefore diffusible in these solvents as a result of oxidation. It was found that dried latex film and various types of crude rubber swelled to a maximum of 1000 to 2400 per cent in petroleum ether and that this swelling was not proportional to the sol content. This investigation strengthens the belief that sol rubber is an oxidation product of gel rubber. The formula for sol rubber is suggested as (C5H8)nO2, and a relationship between the molecular weight and the oxygen content is postulated on the basis that oxygen acts as a molecular splitting agent for both gel and sol.