Thermoplastic elastomers crystalline structures

PEBA exhibit a two-phase (crystalline and amorphous) structure and can be classified as a flexible nylon. Physical, chemical, and thermal properties can be modified by appropriate combination of different amounts of polyamide and polyether blocks [149], Hydrophilic PEBAs can be prepared which can have specific applications in medical devices. Similarly to other thermoplastic elastomers, the poiyamide-based ones find applications in automotive components, sporting goods conveyor belting, adhesives, and coatings [150]. In recent years the world consumption was approximately 6400 tons per year with about 80% in Western Europe and the rest equally split between the United States and Japan [143],... 

Figure 6.4. Schematic model of the structural changes in drawn semicrystalline thermoplastic elastomers of the PEE type at different stages of deformation (a) no stress applied, (b)- e) under increasing stress (/) after stress removal (only crystalline regions are depicted). (From Fakirov et al, 1991.)...

The first commercial thermoplastic elastomers (TPE) were the thermoplastic urethanes (TPU). Their general structure is A-B-A-B, where A represents a hard crystalline block derived by chain extension of a diisocyanate with a glycol. The soft block is represented by B and can be derived from either a polyester or a polyether. Figure 5.1 shows typical TPU structures, both polyester and polyether types. 

Steller, R., Zuchowska, D., Meissner, W., Paukszta, D., Garbarczyk, J., Crystalline structure of polypropylene in blends with thermoplastic elastomers after electron beam irradiation. Radiation Physics and Chemistry 2006, 75, 259-267. 

Yamauchi et al. investigated the structures of NR/high density polyethylene (HDPE) thermoplastic elastomers (TPEs) and their composites with carbon black using SANS. The extremely low contrast between crystalline and amorphous HDPE for SANS enabled us to measure the interface thickness between NR and HDPE in the TPE (5 mn) as well as that between NR and carbon black in the composite (2.4 mn). The interface thickness and fractal dimension of the blends and composites were earefully analysed using SANS even though both polymers were not deuterated. It was revealed that NR and HDPE are immiscible blends. 

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). 

In the SAXS regime, typical nanostructures (e.g., domain sizes and long periods in semicrystalline materials, thermoplastic elastomers) are observed, while the ultra small angle X-ray scattering (USAXS) extends the accessible structure towards the micrometer range (e.g., spherulites). In addition to size and orientation of domains as determined by WAXS, the arrangement of crystalline domains within SMPs could be characterized by SAXS. The programming and recovery processes performed... 

In the case where the polymer forms a triblock copolymer of the structure A—B—A, or a multiblock copolymer -(-A—BAh. where A is crystalline at use temperature and B is rubbery (above Tg), then a thermoplastic elastomer is formed (see Figure 6.21). The material exhibits some degree of rubber elasticity at use temperature, the crystallites serving as cross-links. Above the melting temperature of the crystalline phase, the material is capable of flowing that is, it is thermoplastic. It must be pointed out that a very important kind of thermoplastic elastomer is when the A polymer is glassy rather than crystalline (see Section 9.16). 

The side chain length strongly affects the physical properties of PHAs ic/-PHAs are rigid and brittle as semi-crystalline thermoplastics, while mcZ-PHAs behave as thermoplastic elastomers. The chemical structures of PHAs presented in this chapter are listed in Figure 6.1. 

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