Molecular Weight Of Rubber Research Articles

Fractionation and Molecular Weight of Rubber and Gutta-Percha

Fractionation and Molecular Weight of Rubber and Gutta-Percha

The State of Rubber in Solutions, Based on the Surface Properties of the Solutions

Abstract The problem of the constitution of rubber and especially of its state in solutions has recently again become the subject of active discussion. Based on a series of investigations, Staudinger reached the conclusion that dilute solutions contain free colloidal rubber molecules which are composed of closed chains of an unusually large number of isoprene residues. Pummerer considers the molecular weight of rubber to be much less than does Staudinger. (According to Staudinger the molecular weight of rubber is 50,000 to 170,000; according to Pummerer it is 1200 to 1600.) Meyer and Mark assume for the primary valence chain of rubber a molecular weight of about 5000. These latter investigators are of the opinion that there are secondary aggregates of rubber molecules in a rubber solution and that the high values which were found by a number of investigators (Caspari, Kroepelin, and others) do not represent the molecular weight but the average micellar weight of rubber. In deciding the question of the state of rubber in solution the majority of investigators make use of the classic cryoscopic and osmotic methods of investigation. Staudinger has used viscosimetric measurements in an original way in his experiments. In the present work we have turned to a study of the surface properties of rubber solutions, assuming that data along this line might explain some peculiarities in the structure of these solutions.

Isoprene and Rubber. Part 33. End Groups in Rubber

Abstract In a recent work, Pummerer, Ebermeyer, and Gerlach attempted to identify end members in the rubber molecule by ozone decomposition, and in this way to determine the molecular weight of rubber by chemical means. In this connection they say: The basis for the validity of the conclusions derived from the cleavage fragments on the average length of the rubber chain is that this hydrocarbon contains no structurally foreign impurities, but is built up of homogeneous molecules, though of perhaps different lengths. Pummerer abandons his earlier view, according to which rubber is composed of a uniform base molecule, in favor of concepts held by me for years on the constitution of rubber and other high polymeric substances. As was first proved for synthetic products and as shown for balata and rubber, these substances consist of a mixture of similarly constituted molecules of various lengths and therefore of a mixture of homologous polymers. Pummerer has now adopted this concept, but on the other hand he does not agree on the question of the molecular weight.

Isoprene and Rubber. Part 32.The Constitution of Rubber

Abstract 1. The establishment of the molecular size of high molecular compounds which are composed of fiber molecules by the end-group method of determination is only possible if homologous polymeric series of similar type are concerned. 2. The end-group method assures reliable values with molecules up to a molecular weight of 1000 at the highest. With higher molecular products, like cellulose and rubber, the method is inexact. 3. The molecular weight of rubber and balata may be determined by viscosity determinations in the following two ways: (a) M=ηsp/cKm (b) M=Kc.Kcm. The constants Km and Kcm are determined with low molecular decomposition products. 4. Rubber and balata are composed of fiber molecules, which in one dimension have the magnitude of colloidal particles and in both the others, the dimensions of low molecular substances. 5. In highly viscous rubber solutions, there is the characteristic state of solution. As a result, the sphere of action of the dissolved molecule is greater than the volume at the disposal of the solution. This solution is midway between a true solution and a gel, and is therefore designated as a gel solution. It occurs only with high molecular substances, and is characteristic of them. 6. The readiness with which rubber solutions vary is explained by the fact that the rubber molecules are very sensitive to chemical influences and to changes in temperature as a result of the position of the double bonds. This sensitivity varies with the length of the molecules.

Isoprene and Rubber. Part 23. Cryoscopic Measurements of Rubber Solutions

Abstract Pummerer, Andriessen, and Gündel published a work with this title which contains a number of remarks about the communication of Staudinger, Asano, Bondy, and Signer. The following discussion deals with this subject. 1. Molecular Weight Determinations of Rubber in Camphor according to East Determinations of the molecular weight of rubber in camphor cannot explain the constitution of rubber because, as has been explained before, when rubber is heated in melted camphor, at 170°-180°, a very pronounced decomposition of the rubber to semi-colloidal cleavage products takes place. The rubber molecule is very unstable as a consequence of the peculiar position of the double bonds in the chain; cleavage takes place with extraordinary ease, and attention has already been called to the fact that the cleavage of hexaphenylethane into triphenylmethyl, of dicyclopentadiene into cyclopentadiene, as well as the migration of the ally group, e. g., in phenylallyl ethers, the mobility of the substituents in allyl residues and finally the extremely easy depolymerization of rubber, all have one and the same cause: namely, that a substituent in the ally! group is very loosely combined. These facts, which are of such importance in the chemistry of rubber, should not be ignored as happens in most works on rubber. In order to study the decomposition, we carried out viscosity determinations. As the following experiments show, the viscosity of rubber is much less after melting in camphor than before. There occurred a very great decomposition at 170°, as was to be expected, and the relation t1/t2 which characterizes the decomposition is about 15. If pure rubber is decomposed in boiling tetralin the relation t1/t2 = 6.2. The decomposition in camphor is, therefore, surprisingly great, possibly because of the greater concentration of the dissolved rubber. The camphor solution used was 10 per cent, that of tetralin on the contrary was only 1 per cent.

Isoprene and Rubber. Part 20. The Colloidal Nature of Rubber, Gutta-Percha, and Balata

Abstract I. The Molecular Weight of Rubber, Gutta-Percha, and Balata In the preceding work the molecular weight of rubber and balata was calculated on the basis of relations between specific viscosity ηsp and molecular weight which are shown by semi-colloidal decomposition products, on the assumption that this relation is also true for eucolloids. The value ηr−1 was taken as the specific viscosity, i. e., the characteristic viscosity increase of a substance of definite concentration and known solvent. The expression “specific viscosity” has already been used by J. Duclaux. In viscosity investigations of nitrocellulose solutions he represents this by a constant K which is calculated from the relations of the change of viscosity at various concentrations derived by Arrhenius: Based on these constants, nitrocelluloses show different average molecular weights for the increase in viscosity, that is, this constant K is greater with high molecular products than with low. In the following, this constant represents not the specific viscosity, but the viscosity-concentration constant Kc; the earlier constant Km which, on the basis of the formula: expressed the relation between the specific viscosity and the molecular weight, is called the viscosity-molecular weight constant.

Studies in the Vulcanization of Rubber. III—Kinetics of Vulcanization of Rubber with Sulfur and Selenium

Abstract A procedure for the determination of combined selenium in rubber has been evolved. The rate of combination of selenium and rubber has been ascertained under certain conditions and shown to follow a first-order equation. A minimum value for the molecular weight of rubber has been estimated. The formation of hard rubber under chosen experimental conditions has been put on a mathematical basis and has been shown to follow a second-order reaction. The soft- and hard-rubber reactions have been shown qualitatively to be successive reactions and the function of accelerators has been discussed. The theory explains the anomalous results obtained by previous investigators.

Studies in the Vulcanization of Rubber. II—Vulcanization of Rubber with Nitro Compounds

Abstract The Kjeldahl method for nitrogen analysis can be adapted to the determination of combined nitrogen in rubber vulcanized with nitro compounds. The combination of nitro compounds with rubber has been followed in several cases. Strong evidence is given that the vulcanization is a chemical reaction. The density of rubber has been shown to change during vulcanization with dinitrobenzene. The change approximates the progress of the combination of the vulcanizing agent with rubber. The vulcanization of rubber with dinitrobenzene and trinitrobenzene is monomolecular. A theory of the mechanism of the vulcanization is advanced. The value of the stoichiometric method in estimating the molecular weight of rubber is discussed. Values of this constant are suggested by the data. Nitro compounds appear to be incapable of producing hard rubber. The amount of combined reagent is only a small fraction of that required for ebonite formation.

An Investigation of the Viscosity of Rubber Solutions

Abstract Viscosity measurements have frequently been made with rubber, and very many suggestions have been put forward to explain the cause of the changes of viscosity in rubber solutions. According to Staudinger (Kautschuk, 5, 128 (1929)), if measured under certain conditions, e. g., in dilute solutions where no irregularities are found, the viscosity can be used as a measure of the molecular weight of the dissolved substance. Fickentscher and Mark (Kolloid-Z., 49, 140 (1929)) even calculate from viscosity measurements the length of the molecule and hence relative molecular weights. It is well known that the viscosities of rubber solutions differ greatly and that mechanical treatment of the rubber decreases the viscosity of its solutions to a very large extent. The latter effect has been explained by Staudinger as well as by Fickentscher and Mark (loc. cit.) as a depolymerization. The latter authors calculate that the molecular weight of rubber decreases to one-third of the original if it is masticated for 225 minutes. It has further been pointed out recently by Herschel and Bulkley (Kolloid-Z., 39, 291 (1926)) that rubber solutions show irregularities in their viscosity, e. g., the viscosity is not linearly proportional to the pressure. (According to Poiseuille's formula for the rate of flow of a liquid through a capillary, the viscosity coefficient:

Isoprene and Rubber. XIII. The Constitution of Rubber

Abstract It is of special interest in connection with the structure of rubber as well as that of all natural products of high molecular weight that two basically different viewpoints exist, and that both have been supported by experimental investigations. Both these concepts concerning the constitution of rubber are discussed in the following paper. I. The Constitution of Rubber According to Pummerer Pummerer defends the correctness of the basic concept of Harries “that a parent rubber hydrocarbon exists, which as a result of some condition tends to association and as a result gives the impression of a rubber of gigantic molecular weight. The question of whether this point of view is justified is of the greatest interest to colloid chemistry. The tendency to association may depend upon various factors, for example, upon the state of saturation, upon the size, or even upon the form of the molecules”. Pummerer expresses similar ideas, as does Bergmann about the constitution of polysaccharides and albumins. This concept is supported by Pummerer by important experiments.