Structure of thermoplastic elastomers

Figure 4.131(a) displays schematic structures of thermoplastics and crosslinked rubbers, while Figure 4.131(b) displays two possible schematic structures of thermoplastic elastomers. 

FIGURE 2.3 Structure of thermoplastic elastomers from ABA triblock copolymers. (From Morton, 1971.)... 

A brief review is given of the important qualitative features of thermoplastic elastomers. Particular emphasis is given to the molecular structure, bulk morphology and interfacial character of these materials. Both equilibrium and nonequilibrium structures are discussed... 

Reversible network structure is the single most important characteristic of a thermoplastic elastomer. This novel property generally arises from the presence of a phase-separated morphology in the bulk material which in turn is dictated by the molecular structure, often of a block copolymer nature. A wide variety of synthetic methods can, in principle, produce endless varieties of thermoplastic elastomers this fact coupled with the advantageous processing characteristics of these materials suggest that the use of thermoplastic elastomers will continue to grow in the 1980 s. 

Before briefly discussing each type it is necessary to consider the performance of thermoplastic elastomers, and the problem of defining service temperature limits for them. The structural features that convey the ability to be processed as a thermoplastic are also a limiting factor in their use. Since it is the pseudocrosslinks that allow these materials to develop elastomeric behaviour, any factor which interferes with the integrity of the pseudocrosslinks will weaken the material, and allow excessive creep or stress relaxation to occur under the sustained application of stress and strain. Temperature is obviously one such factor. 

The development of anionic chemistry over the past 30 years has led to the emergence of new processes and products of Industrial Importance, the most significant being a family of thermoplastic elastomers. These unique elastomers are presently commercialized by Shell Chemical Company as Kratons and by Phillips Chemical Company as Solprenes. Their uniqueness is the result of deliberate design of the polymeric structure and composition. 

Health and Safety Issues. Polyesterether elastomers derived from dimethyl terephthalate, butanediol, and Ptmeg are not hazardous according to the published Materials Safety Data Sheets (MSDS) for this elastomer. Polymers of a similar structure containing isophthalic acid are also not considered hazardous. For other copolymer elastomers, the MSDS put out by suppliers should be consulted by potential users before evaluation. One environmental advantage of thermoplastic elastomers of this type is that they are melt-reprocessible and thus scrap and off-specification material and even obselete parts can be easily recycled. Up to 25% by weight of recycled material can be incorporated (see Recycling, plastics). 

Jalali-Arani A, Katbab AA, Nazockdast H (2003) Preparation of thermoplastic elastomers based on silicone rubber and polyethylene by thermomechanical reactive blending Effects of polyethylene structural parameters. J Appl Poly Sci 90(12) 3402-3408... 

The same approach applied to thermoplastic elastomers of the PEE-type fails to explain the large discrepancy (up to 100 MPa when the measured H values are in the range 20-40 MPa) between the experimental values and those calculated according to eq. (3.4) (Fakirov el al, 1998 Apostolov et al, 1998). For this reason one has to look for other factors which may be responsible for such a discrepancy. Before disclosing these let us recall some of the characteristic features in the structure and morphology of thermoplastic elastomers of PEE-type which are closely related with the problem discussed. 

Summarizing, it can be concluded that a relatively sharp (within 2-4% of deformation) drop in H is observed for copolymers of PBT but in comparison with homo-PBT this transition occurs at much higher deformations (between 25 and 30%). This difference as well as the following increase and decrease of H are related to the structural peculiarities of thermoplastic elastomers - the presence of a soft amorphous phase which first deforms and the existence of a physical network. The very low H values obtained for PEE are related to the fact that the PBT crystallites are floating in an amorphous matrix characterized by a low viscosity. 

To ascertain control of the molecular weight, structure and composition, block copolymers are usually synthesized in anionic polymerization. The block copolymers of commercial interest are specifically prepared from monomers that upon polymerization yield immiscible macromolecular blocks, a smaller one rigid and the other flexible. The rigid blocks form physical crosslinks that upon heating above the transition point make the copolymer to flow. Thus, these materials belong to the growing family of thermoplastic elastomers. 

This interesting behavior of the ABA triblock copolymers is not a unique feature of the styrene-diene stmcture, but can be found in the case of other analogous chemical structures. Thus thermoplastic elastomers have been obtained from other triblock copolymers, where the dienes have been replaced by cyclic sulfides (Morton et al., 1971), cyclic siloxanes (Morton et al., 1974), or alkyl acrylates (Jerome, 2004) poly(alkyl methacrylate) end blocks have also been investigated (Jerome, 2(X)4). 

FIGURE 4.2 Continuous structures of polymer nanocomposites. (A) SEM image of thermoplastic elastomer/CNT composite with CNT dispersed in polymer matrix. (B) SEM image of PPy/Au nanocomposite with ordered porous structure. (C) SEM image of P(VDF-HFP)/Si02 composite with porous structure. (D) TEM image of PEDOT/graphene composite with a multilayer structure. (A) Reproduced with permission from reference Koemer, H., Price, G., Pearce,... 

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. 

The most relevant outcome here has to do of course with the structure- ropraty relationships gathered from this investigation which gave clear-cut information about Tg, Tm and the corresponding extent of crystaUization, thermal stability, mechanical properties and phase separation phenomena associated with a series of thermoplastic elastomers, like 26, prepared together with all the other linear materials. 

Block polymers and polymer blends deserve now a great intere because of their multiphase character and their related properties. The thermodynamic immiscibility of the polymeric partners gives rise indeed to a phase separation, the extent of which controls the detailed morphology of the solid and ultimately its mechanical behavior. The advent of thermoplastic elastomers and high impact resins (HIPS or ABS type) illustrates the importance of the industrial developments that this type of materials can provide. In selective solvents, and depending on molecular structure, concentration and temperature, block polymers form micelles which influence the rheological behavior and control the morphology of the material. 

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