Thermoplastic elastomers block sequences

Thermoplastic elastomers contain sequences of hard and soft repeating units in the polymer chain. Elastic recovery occurs when the hard segments act to pull back the more soft and rubbery segments. Cross-linking is not required. The six generic classes of TPEs are, in order of increasing cost and performance, styrene block copolymers, polyolefin blends, elastomeric alloys, thermoplastic urethanes, thermoplastic copolyesters, and thermoplastic polyamides. 

Another important class of copolymers synthesized by chain polymerisation are block (or sequenced) copolymers diblock and triblock copolymers being the most important ones. They are very useful as compatibilisers (emulsifiers) in immiscible polymer blends. Another major use is as thermoplastic elastomers. Both uses are best explained through the example of butadiene-styrene block copolymers. 

Sequential addition of different monomer charges to a living anionic polymerization system is useful for producing well-defined block copolymers. Thermoplastic elastomers of the triblock type are the most important commercial application. For example, a styrene-isoprene-styrene triblock copolymer is synthesized by the sequence... 

The properties of block copolymers are dependent on the length of the sequences of repeating units, or domains. The domains in typical commercial block copolymers of styrene and butadiene are sufficiently long such that the products are flexible plastics. They are called thermoplastic elastomers (TPE). It should be noted that although the Ts for random copolymers is between the T/s of the respective homopolymers, the repeating sequences in block copolymers exhibit their own characteristic Ff s. 

Thanks to their multiphase constitution, block copolymers have the originality to add advantageously the properties of their constitutive sequences. These very attractive materials can display novel properties for new technological applications. In this respect, thermoplastic elastomers are demonstrated examples (l, 2, 3) they are currently used without any modification as elastic bands, stair treads, solings in the footwear industry, impact resistance or flexibility improvers for polystyrene, polypropylene and polyethylene whereas significant developments as adhesives and adherends are to be noted (5.). 

Order-disorder transitions and spinodals were computed for linear multi block copolymers with differing sequence distributions by Fredrickson et al. (1992). This type of copolymer includes polyurethanes, styrene-butadiene rubber, high impact polystyrene (HIPS) and acrylonitrile-butadiene-styrene (ABS) block copolymers. Thus the theory is applicable to a broad range of industrial thermoplastic elastomers and polyurethanes. The parameter... 

The statistical anionic copolymerization of acrylates and methacrylates is also controlled in the presence of LiOEEM (30), as testified by the copolymerization of MMA and tBuA in THF at —78°C. Block copolymers were also prepared by the sequential polymerization of at least two methacrylates and acrylates. For instance, PMMA- >/c cA -PbBuA and PMMA-fcZocA -PnNonA were synthesized . The addition order of the comonomers is important. Indeed, when living PnBuA is the macroinitiator of the MMA polymerization, the expected block copolymer is contaminated by homo-PnBuA, which is not the case when the polymerization sequence is reversed. A fuUy acrylic-based thermoplastic elastomer, PMMA-fcZocA -P(2EtHA)-fcZocA -PMMA, was prepared by the sequential LiOEEM-ligated polymerization of MMA, 2-EtHA and MMA. ... 

The segmented block copoly(ether ester)s, based on poly(butylene terephthalate) (PBT) and on polyoxytetramethylene (POTM) are typical examples for this class of thermoplastic elastomers . They can be described as random copolyesters of tereph-thalic acid with 1,4-butanediol and a-hydro-hard phase whereas the remaining ester segments mix with the ether segments and build up the soft matrix. 

Modification of polymers by incorporating block sequences having low glass transition tenperatures is a means of changing the mechanical properties and is especially useful for the formation of thermoplastic elastomers if the basic pol3nner is semi-crystalline. These rubber-like blocks are usually formed by ionic or transition metal catalyzed reactions. Radical pol3nmeri-zation on the other hand is experimentally simpler and applicable to a wide variety of monomers. 

A cationic to ATRP transformation was also used in the synthesis of triblock copolymers with polyisobutylene (PIB) as the middle sequence. These materials are particularly useful as thermoplastic elastomers. In this case, a few units of styrene were added to living difunctional PIB after the isobutylene had reacted. The isolated PIBs could act as bifunctional macroinitiators for ATRP [86]. A similar strategy was used by Batsberg et al. [87] for the synthesis of block copolymers of isobutylene with p-acetoxystyrene (PlB-fo-PAcSt) or styrene (PlB-b-PSt). 

In contrast, thermoplastic elastomers vulcanize by a physical cross-linking, that is, by formation of hard domains in a soft matrix. Here, hard and soft refer to glass transition temperatures relative to application temperatures. The properties of these thermoplastic elastomers follow directly from their structures. All thermoplastic elastomers (TPEs, plastomers) are copolymers with long sequences of hard and soft blocks. They can be block polymers, segment polymers, or graft polymers. 

In addition to the relative ratio of the monomers, the arrangement of the units in the chain is important. This arrangement is referred to as the copolymer sequence distribution. In the previous discussion, the assumption was made that the comonomer units were well mixed in the polymer chain. If this is not the case, parts of the chain can reflect properties of the corresponding homopolymer. It is thus possible to produce polymers that have significantly different properties in different parts of the polymer chain. A most dramatic example of this can be found in styrene-butadiene-styrene or styrene-isoprene-styrene thermoplastic elastomers. These triblock polymers behave as cured elastomers at room temperature. The polystyrene blocks have sufficiently different solubility from the polydiene portions that they phase-separate... 

The term polyester thermoplastic elastomers is widely used for segmented poly(ether-ester) block copolymers with alternating/random length sequences... 

In a pure PB-PS block copolymer, both sequences are immiscible and the microstructure will be diphasic (at a supramolecular or nanoscopic level). If the ratio is such that PS particles are dispersed into a matrix of PB, below the Tg of PS, the system behaves like crosslinked PB, hence as an elastomer. However, above the Tg of PS, the system becomes viscous and can be processed like a thermoplastic. 

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