Rubbery polyethers

With these polymers hard blocks with T s well above normal ambient temperature are separated by soft bloeks which in the mass are rubbery in nature. This is very reminiscent of the SBS triblock elastomers discussed in Chapter 11 and even more closely related to the polyether-ester thermoplastic elastomers of the Hytrel type deseribed in Chapter 25. 

The Material of the Example. Poly(ether ester) (PEE) materials are thermoplastic elastomers. Fibers made from this class of multiblock copolymers are commercially available as Sympatex . Axle sleeves for automotive applications or gaskets are traded as Arnitel or Hytrel . Polyether blocks form the soft phase (matrix). The polyester forms the hard domains which provide physical cross-linking of the chains. This nanostructure is the reason for the rubbery nature of the material. 

In contrast, organophilic PV membranes are used for removal of (volatile) organic compounds from aqueous solutions. They are typically made of rubbery polymers (elastomers). Cross-linked silicone rubber (PDMS) is the state-of-the-art for the selective barrier [1, 43, 44]. Nevertheless, glassy polymers (e.g., substituted polyacetylene or poly(l-(trimethylsilyl)-l-propyne, PTMSP) were also observed to be preferentially permeable for organics from water. Polyether-polyamide block-copolymers, combining permeable hydrophilic and stabilizing hydrophobic domains within one material, are also successfully used as a selective barrier. 

Physical properties, which depend on molecular weight, the nature of the alkyl group, the nature of the initiator, stereospecificity, and crystallinity, range from viscous liquids, through sticky liquids and rubbery solids, to brittle solids. Polyethers with long alkyl side chains are waxy, however, as the alkyl group in such cases dominates physical properties. 

T inear polyurethane block copolymers have been objects of considerable -L/ research interest over recent years. These materials are composed of three parts, a diisocyanate, a low molecular weight chain extender (usually a diol but sometimes a diamine), and a low molecular weight rubbery polymer, frequently a polyether or polyester. The soft segment consists of the rubbery polymer while the diisocyanate and the chain extender form the hard segment These materials are classified as thermo-... 

A number of such combinations are mentioned in the literature, and may be offered by various manufacturers. They combine the soft flexible rubberiness of polyether or polyester elastomers with the high strength, heat-, oil-,... 

The polyether or polyester blocks provide the necessary rubberiness, whereas the accumulation of relatively stiff links with hydrogen bonding capacity between the elastomeric blocks increases resistance to high temperatures and the action of solvents. It also increases the modulus of elasticity and the ultimate tensile strength of the resulting fibers. 

Closely related to the polyether-ester thermoplastic elastomers are thermoplastic polyurethane elastomers, which consist of polyurethane or urethane terminated polyurea hard blocks, with Tg above normal ambient temperature, separated by soft blocks of polyol, which in the mass are rubbery in nature (see Section 4.11 for more details). The main uses of thermoplastic rubbers (e.g., Estane by Goodrich) are for seals, bushes, convoluted bellows, and bearings. 

Typical polyurethanes use a diol extender such as 1,4 butane diol. The NH groups form hydrogen bonds with C=0 groups on neighbouring chains, inside very small, hard blocks. The soft blocks are crosslinked polyethers or polyesters. A typical polyether is poly(propylene oxide) —[—O—CH2— C(CH2)H—] pre-polymerised to a molecular mass Mn of about 6000, a degree of polymerisation n = 140. The PPO has a glass transition temperature of about —60 °C, so the crosslinked PPO is a rubber. This rubbery phase is connected, via the polyurethane molecules, to the crystalline phase. 

In reaction injection moulding (RIM), two monomers are injected into a mould, where polymerisation and crosslinking occur. It is used mainly with polyurethanes to make large automotive panels. The chemistry of polyurethanes was described in Chapter 4. Other systems used include a block copolymer between a crystalline polyamide (nylon 6) and a rubbery polyether (polypropylene oxide). In principle, any polymerisation reaction that can be substantially completed after about 30 s in the mould is a candidate for RIM. 

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