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An important new class of polymers, the thermoplastic elastomers, was announced by the Shell Chemical Company (U.S.A.) in 1965. These new products may be formed into useful articles by modern rapid thermoplastics processing techniques, such as injection molding, and without any chemical vulcanization step provide most of the useful physical properties of vulcanized rubber. High resilience, high tensile strength, highly reversible elongation, and abrasion resistance are obtained. The thermoplastic elastomers consist of ordered, block copolymers of the general structure A‐B‐A, where A is a thermoplastic block polymer and B is an elastomeric block polymer. Choice of monomers, block length, and the weight fractions of A and B are crucial in achieving elastomeric performance. An example is the polystyrene–polybutadiene–polystyrene block copolymer (S‐B‐S) where the molecular weights of S and B and the weight fraction of S are restricted. A two‐phase system is formed, with the middle‐block phase constituting a continuous three‐dimensional elastomeric network and the dispersed end‐block phase serving as multijunction points for the ends of the middle blocks. These systems, without vulcanization, have rubber‐like properties similar to those of conventional rubber vulcanizates but flow as thermoplastics at temperatures above the glass transition of the end block. The behavior is fully temperature reversible. Melt viscosity behavior, measured as a function of shear and temperature, is similar to that of conventional thermoplastics. Activation energies obtained at constant shear stress vary with temperature: at high temperatures they are between those of the pure homopolymers and at low temperatures they approach that of the thermoplastic part of the molecule. Melt viscosities, however, are very much higher than those of either homopolymer of the same total molecular weight. An additional energy term is indicated in the flow process which arises from the transfer of the end block from one aggregate to another, in the process being forced to pass through the elastomeric matrix with which it is thermodynamically incompatible. The classical kinetic theory of rubber elasticity can be applied to these polymers, treating the end‐block phase as hard discrete particles which do not contribute to the elastic network For example, equilibrium modulus or swelling measurements are used to calculate the concentration of elastically effective chains or effective crosslink density. The resulting effective elastic chain length (Mc) is thus identified, not with the middle‐block segment length, but with the normal entanglement length. Hence, normal entanglement junctions in the elastomeric matrix behave as strong effective crosslinks because the ends of the middle block are securely anchored in the end‐block aggregates. High tensile strengths, in the absence of reinforcing fillers or crystallization, may be attributed to a highly perfected network (Case theory) or to the inertial masses of the discrete end‐block aggregates (Bueche the...

Thermoplastic elastomers have many of the physical properties of vulcanized rubbers but can be processed as thermoplastics. This unique behavior has opened a new field of polymer science and technology. Since their commercial introduction in the 1960s, thermoplastic elastomers have become a significant part of the polymer industry. They are used in applications as diverse as adhesives, footwear, medical devices, automobile parts, and asphalt modification. Many of them are block copolymers with alternating hard and soft segments. The thermoplastic elastomers with polystyrene hard segments are the larget group and probably the most versatile; they can be produced over a wide range of hardness values and applied either in the molten state or from solution. Other examples are block copolymers with elastomers, either vulcanized or unvulcanized, in which the two components are intimately mixed together, but in most cases still remain as separate phases. Thermoplastic elastomers of this type are generally used as replacements for vulcanized rubbers. This article reviews the synthesis, morphology, structure–property relationships, and commercial applications of thermoplastic elastomers of all types.

Material imperfections usually play a substantial role in the early stages of fatigue cracking. This article presents some observations concerning fatigue crack initiating flaws and early crack growth in 7050‐T7451 aluminium alloy specimens and in full‐scale fatigue test articles with a production surface finish. Equivalent initial flaw size (EIFS) approaches used to evaluate the fatigue implications of metallurgical, manufacturing and service‐induced features were refined by using quantitative fractography to acquire detailed information on the early crack growth behaviour of individual cracks; the crack growth observations were employed in a simple crack growth model developed for use in analysing service crack growth. The use of observed crack growth behaviour reduces the variability which is inherent in EIFS approaches which rely on modelling the whole of fatigue life, and which can dominate EIFS methods. The observations of realistic initial flaws also highlighted some of the significant factors in the fatigue life‐determining early fatigue growth phase, such as surface treatment processes. Although inclusions are often regarded as the single most common type of initiating flaw, processes which include etching can lead to etch pitting of grain boundaries with significant fatigue life implications.

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