synopsisThe effect of monomer sequence on physical properties was investigated for butadienestyrene solution copolymers made by organolithium initiation. The polymers varied from random copolymers of uniform composition along the polymer chain to ideal block polymers of specific block sequence arrangement and included rubbers of intermediate degrees of randomness. Uniform composition random copolymers exhibit a single glaw transition temperature and a very narrow dynamic loss peak corresponding to this transition. The glass transition can be predicted from the styrene content and the microstructure of the butadiene portion of the rubber. Random copolymers in which compe sition varies along the polymer chain, and to some extent between molecules, exhibit a single glass transition, but the dynamic loss peak is broadened. The extent of this broadening is shown to be compatible with the sequence distribution, polymer segments of various compositions losing mobility a t different temperatures. This indicates a tendency for association between segments of different temperatures. This indicates a tendency for association between segments of different chains which are similar in composition. Block copolymers display two transitions, corresponding to T, for each type of block. The position and width of the dynamic loss peaks are related to block length and compositional purity of the blocks. 1581
Stress softening of carbon black‐reinforced butadiene‐styrene rubber was studied as a function of the rate and temperature of the original tensile deformation. To a good approximation, stress softening depends on the product of the extension rate and a temperature function which is analytically well represented by the familiar Williams‐Landel‐Ferry relationship. When the elongation of the original deformation is also varied, a good correlation is obtained between stress softening and the maximum stress attained in the original extension, irrespective of the particular combination of strain, strain rate and temperature used to achieve this stress. Variables which tend to increase the stiffness of the vulcanizate, such as increased degree of crosslinking or carbon black chain structure, also increase stress softening; dilution by plasticizers decreases it. Prestressing at high strain rates and low temperatures affects the stress–strain curve of the softened vulcanizates beyond the elongation of the original extension. Connections are established between stress softening and viscoelastic and failure behavior. The evidence presented favors the contribution of several mechanisms to the general phenomenon of stress softening. These are thixotropy of transient filler structures, network chain rupture, and breakage of “permanent” filler structure. The latter appears to be most important at high strain rates, low temperatures, and with highly reticulated “structure” blacks.
Polymer molecules containing chemically dissimilar segments have been investigated for many years. Their synthesis and characterization are challenging, and it is evident that their properties are often unusual and useful as witnessed by several books and reviews published on block and graft polymers. In this article we intend to review parts of this rapidly expanding area of polymer research of particular interest in rubber chemistry and technology, namely linear elastomeric block polymers. Sometimes the literature specifies the elastomeric nature of the products, on other occasions it does not. We have arbitrarily included those monomer combinations in which at least one monomer is known to form rubbery homopolymer. Urethan elastomers have been excluded because they have been extensively described elsewhere. In the broadest sense, block polymers are molecules in which two or more chemically dissimilar segments are joined end to end, a feature which distinguishes them from graft polymers. Each segment or block is usually a long sequence of units of a single monomer (A or B or C, etc.), but it may also be a long sequence of randomly copolymerized units (A plus B or C, etc.). Both types are known.
A rapid and inexpensive method has been devised for measuring diffusion coefficients of substances in solution. The procedure consists essentially of soaking a porous disc in the solution of interest and then suspending the disc in a bath of pure solvent. The rate of diffusion of solute from the disc is then ascertained by measuring the apparent weight of the suspended disc at various times. The rate of change of the apparent weight can be directly related to the loss of solute and hence to the diffusion coefficient of the material. The method has been thoroughly tested for aqueous solutions of inorganic salts whose diffusion coefficients are known. The process is sufficiently rapid to permit easy determination of (he diffusion coefficients of slowly diffusing substances like high polymers.
In butadiene styrene copolymers containing long block sequences chain segments associate with like segments to form a two phase structure. Properties of such polymers are dependent not only on composition and molecular weight but also on block sequence along the chain. Polymers containing two or more polystyrene blocks per molecule form networks and exhibit elastomeric properties in the uncured state resembling those of filler reinforced vulcanizates. This behavior is shown both by linear styrene-butadiene-styrene elastomers and multichain block copolymers branched in the polybutadiene blocks. A prominent loss tangent peak was observed around —40° C for the multichain polymers. Stress strain following prestretching and stress relaxation measurements indicate some shifting of polystyrene associations during stretching. Tensile strength is reduced by increasing temperature and addition of plasticizers. Reinforcement by polystyrene domains in vulcanized block copolymers is evident from tensile strength, dynamic modulus, and swelling measurements, but decreases with increased crosslinking. The number of styrene sequences in the primary molecules is less important after vulcanization as crosslinking destroys the individuality of the original polymer chains.
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