Thermoplastic elastomers crystalline amorphous block copolymers

Figure 6.21 Model of crystallizable triblock copolymer thermoplastic elastomer. The center block, amorphous, is rubbery, whereas the end blocks are crystalline (64).

Block copolymers of the A—B—A type where A is a thermoplast and B an elastomer can have properties at ambient temperatures which would normally be expected from a crosslinked rubber. The cause of this phenomenon are the physical crosslinks produced by the thermoplastic blocks which may be either crystalline or amorphous (glassy). Above the melting temperature of the hard phase such materials flow and can be processed by the usual thermoplastic processing techniques. 

Thermoplastic tri-block copolymers are interesting since they possess novel properties different from those of the homo- or copolymers. The thermoplastic elastomers have many of the physical properties of rubbers, i.e., softness, resilience, and flexibility. The unique properties of this kind of copolymer are due to the microphase separation of the hard crystalline domains dispersed in a continuous amorphous matrix (Fig. 6). Such phase morphology provides a physical network of flexible chains cross-linked by crystalline microdomains. The advantages over natural vulcanized rubbers are that thermoplastic elastomers are readily soluble in an appropriate solvent and can be processed as thermoplastics [109],... 

Figure 7-6. Simplified morphology of a thermoplastic elastomer. A-B-A triblock copolymer has amorphous elastomeric B blocks (solid curves) between high Tg or crystalline outer blocks (jagged lines). Circles represent crystallites or domains of high Tg outer blocks.

Block copolymers made from ethylene and propylene are valuable industrial materials. They can be used as thermoplastic elastomers and as compatibOizing agents for homopolymer blends. The properties of this type of copolymer depend on the microstructure of the blocks, the relative lengths of the blocks, and the overall molecular v eight. An ABA triblock copolymer structure containing crystalline A blocks and an amorphous B block can exhibit elastomeric behavior. The crystalline "hard" blocks can consist of isotactic or syndiotactic polypropylene (iPP or sPP) units or linear polyethylene (PE). The amorphous "soft" blocks can consist of atactic polypropylene (aPP) or ethylene-propylene copolymer (ethylene-propylene rubber, EPR). 

Later development of vulcanization technology has involved peroxide crosslinking and thermoplastic elastomers. The latter consist of block copolymers with hard segments (physical crosslinks) and flexible segments (Fig. 3.6). The crosslink domains are either glassy amorphous or crystalline. These materials can be processed by conventional thermoplastic process-... 

The thermal transitions and the relaxation processes observed in multiblock terpolymers allow to evaluate their phase morphology. At room temperature, these polymers are composed of three phases hard, soft, and strongly expanded interphase. The two latter phases are amorphous and form a matrix (continuous phase), whereas the hard (crystalline) phase is the dispersed phase. The thermal transition and relaxation processes occurring in the interphase of the multiblock copolymers are not detected by the DSC and DMTA methods. The incorporation of the third short block into the copolymer chain causes an increase in the volume of the interphase. This facilitates the establishment of the processes occurring in this phase at various temperatures. Moreover, it enables the evaluation of the influence of the dimension and composition of this phase on the polymer properties. (About the number of phases in poly(ether ester) thermoplastic elastomers, see also Chapter 6.)... 

Sky pel is the registered trademark of a polyester-based thermoplastic engineering elastomer manufactured by SK Chemicals [9]. It can be processed in a wide variety of products from ultra small precision parts to spread sheets by injection, extrusion, and others. Skypel is a block copolymer consisting of hard crystalline phase and soft amorphous phase, and the relative proportions of the two phases determine the material properties. Skypel , ranging over a broad hardness spectrum, offers excellent features such as toughness, resilience, resistance to creep, impact and flexural fatigue, flexibility at low temperatures, thermal stability at elevated temperatures, and resistance to many industrial chemicals, oils, and solvents. Table 9 shows some typical properties of Skypel . 

Polycondensation with sebacoyl chloride yielded the desired multiblock copolyester. Such multiblock copolymers consisting of a crystalline hard block and an amorphous soft block show the typical mechanical properties of thermoplastic elastomers [53, 55]. 

Multiphase or multicomponent polymers can clearly be more complex structurally than single phase materials, for there is the distribution of the various phases to describe as well as their internal structure. Most polymer blends, block and graft copolymers and interpenetrating networks are multiphase systems. A major commercial set of multiphase polymer systems are the toughened, high impact or impact modified polymers. These are combinations of polymers with dispersed elastomer (rubber) particles in a continuous matrix. Most commonly the matrix is a glassy amorphous thermoplastic, but it can also be crystalline or a thermoset. The impact modified materials may be blends, block or graft copolymers or even all of these at once. 

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