A rapid method for determining the degree of crosslinking in elastomers is described which involves measuring equilibrium compression modulus at very small deformations on solvent‐swollen samples. The elastomer used was a polyether urethane vulcanized by an accelerated sulfur recipe. The degree of crosslinking as measured by the compression method agreed with that measured by the extension method within 1–3%. Both methods agreed reasonably well with the theoretical values calculated from the sulfur level, polymer molecular weight, and crosslink structure.
The present work was undertaken to assess the effect of crosslink structure on the properties of an elastic network derived from a linear polyetherurethane elastomer. In order to study this effect, a polyetherurethane was synthesized which contained two types of reactive sites suitable for establishing crosslinks : pendant hydroxyl groups for reaction with diisocyanate curatives and pendant double bonds for vulcanization with sulfur. Incorporation of both types of curing sites within the same linear polymer served to hold constant variables which might otherwise influence vulcanizate properties. Thus, the elastic networks formed by both curing systems were produced from the same polymer, and such factors as polymer molecular weight and molecular weight distribution, interchain forces and cure site distribution remained constant. Furthermore, the curing sites have been placed on pendant groups well removed from the main polymer chain in order to avoid degradation of the polymer by side reactions which may accompany sulfur vulcanization.It is recognized that common elastomers with internal unsaturation, such as SBR and natural rubber, can be crosslinked by more than one method (e.g., with sulfur, peroxides, or high energy radiation), but the extent and nature of side reactions which may occur is not known with certainty. There are possibilities of polymer degradation reactions with all of these curing systems, and the occurence of such degradation reactions would cloud any conclusions concerning the relation between crosslink structure and vulcanizate properties.Polyetherurethane I was synthesized for the present study : n n
The trimethylethylene was distilled with the ether and titrated with bromine by the method of Stanerson and Levin39 which indicated that a 41% yield was formed (based on nitrosourethan). Addition of excess bromine to the solution, washing with bisulfite and two, successive distillations gave a 25% yield of 2-methyl-2,3-dibromobutane, b.p. (16 mm.) 57-59°, w"p 1.5091 (lit.40 b.p. (19 mm.) 63°, »*>o 1.5090). The residue from the distillation of the trimethylethylene consisted of 1.7 g. of a mixture of dinitrobenzoate esters. From 1.35 g. of such a mixture obtained in a similar experiment, was isolated by fractional crystallization 0.52 g. of ¿-amyl 3,5-dinitrobenzoate, m.p. and mixed m.p. with an authentic sample described below, 115.5-116°, and 0.12 g. of neopentyl 3,5-dinitrobenzoate. m.p. and mixed m.p. 91.5-92.5°.(b) In Ligroin.-When the ether in the above experiment was replaced by ligroin the yield of trimethylethylene by titration was 33% and theyield of esters, 0.69 g., m.p. llS-HS®, and 1.2 g., m.p. 57-72°.Reaction of III with Dilute Sulfuric Acid.-III was prepared as above from 50 g. of nitrosourethan and collected in 100 cc. of 0.045 A sulfuric acid cooled in an ice-salt-bath.A second trap cooled by acetone and Dry Ice was attached to the first. When the reaction was finished the aqueous solution was warmed to drive the trimethylethylene formed (39) B.
Correlation of the physical properties of elastomers with their molecular structure depends basically upon the availability of a systematic array of appropriate model polymers. Differences in molecular weight and its distribution, degree of linearity, and structure and distribution of backbone constituents (in copolymers) among others are variables which can affect the physical properties of high polymers. In the classical vinyl polymerization systems control of these factors is a t best, crude. However, the recently developed technology for the preparation of urethane block polymers provides the means for the reasonable control of structural details. The chemical reactions involved in the synthesis of polyurethanes are well defined and readily adaptable to the preparation of a variety of elastomers differing systematically in specific structural features. Models necessary for a detailed experimental correlation of physical properties with molecular structure are now a pract,ical reality.In the present work polyurethane technology has been used to investigate the effects of chain branching on polymer physical properties. Two aspects of chain branching phenomena have been examined: first, the influence of various short chain aromatic branches on the properties of polymers with fixed backbone structure; and second, the influence of "loose ends," or long chain branches, on polymer vulcanizate properties.For the study of short chain branching, polyether urethanes (I) and (11) were synthesized according to the method described previously. 0 0 II
An improved differential thermal analytical technique which permits the rapid, convenient characterization of the thermal behavior of crystalline polymers free of any influence of prior thermal history is presented. Characterization of both crystallization and fusion phenomena is described for ethylene/propylene copolymers subjected to well‐controlled thermal scanning techniques. Parameters describing these phenomena are derived. While they are nonequilibrium parameters, they are reproducible and capable of correlation with polymer composition. the crystallization onset temperature determined by this cooling technique was found to relate to the molar ethylene content of the copolymers by an equation similar to the one derived by Flory5 based on equilibrium melting point. The relationship was found to hold true for a number of ethylene copolymers, including samples of linear and branched polyethylene, commercial EPDM, and ethylene/vinyl acetate copolymers.
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