**4. Structure of thermoplastic elastomers**

Thermoplastic elastomers (TPEs) are defined as a group of polymers that exhibit instantaneous reversible deformation (to be an elastomer). Most of the TPEs consist of continuous phase that exhibit elastic behaviour and dispersed phase that represents the

elastomeric hydroxyl terminated polymer. Examples of a diisocyanate are methane 4,4' diphenyl diisocyanate (MDI), 2,4- and 2,6-toluene diisocyanate (TDI) and 1,6-hexane diisocyanate (HDI). A chain extender may be 1,4-butanediol. When MDI and butanediol react they form a polyurethane with alternating monomer units connected by urethane groups, though other functional groups also form as by products of the reaction. This polyurethane is not elastomeric and it constitutes the hard phase of a typical TPU. Hydroxyl terminated elastomers include polyethers: poly(oxyethylene), poly(oxybutylene), polyesters: poly(ethylene succinate), poly(butylene succinate), poly(ethylene adipate), poly(butylene), hydroxyl terminated polybutadiene and hydroxyl terminated poly(butadiene-*co*-

Soft segment Hard segment

Fig. 2. Schematic representation of TPUs composed of alternating hard segment and soft

These polymeric diols react with isocyanate and are linked into the TPU as a complete elastomer block. The hard and soft (elastomeric) chain segments phase separate with the hard segments as a dispersed minor phase, since the soft segments must form a continuous phase if elastomeric properties are to be displayed. The reactivity difference between the –OH groups of the polyol and the chain extender with different isocyanate groups affect the sequence of hard segments in the polymer chain. Thus, polyurethanes obtained by using this method have a more random sequence. However, the polymer is highly crystalline due to the favoured reaction between polyol and diisocyanate before

A chain growth polymerisation can be used to form a TPE in one step. An example is a poly(ethylene-*co*-butene) with a high butane content, polymerised using a single-site metallocene initiator. This polyethylene can undergo phase separation due to crystallisation, crystals are the physical crosslinks, and the highly branched structure will exhibit elastomeric properties. It may need to be blended with a less branched polyethylene to increase the physical cross-links. Alternatively it can be partially cross-linked by dynamically vulcanising by extrusion with a peroxide initiator. While chemical cross-links are formed this type of polyethylene can still be processed as a thermoplastic. Dynamic Vulcanization can be applied to poly(ethylene-*co*-propylene) rubber (EPR) that may be

TPU can be synthesised by a two-step method, which is known as prepolymer method. The reaction may be carried out in two steps where excess diisocyanate is added to the polymeric diol to form an isocycanate terminated pre-polymer with excess diisocyanate

blended with a thermoplastic polyethylene to provide a binding crystalline phase.

Hydrogen bond

acrylonitrile).

segment structures

**5.2 Two-step methods** 

extended polymer growth has occurred.

physical crosslinks. If the dispersed phase is elastic then the polymer is a toughened thermoplastic, not an elastomer. Elastomer reversibility must have physical cross-links, therefore these crosslinks must be reversible. Physical crosslinks do not exist permanently and may disappear with the increase of temperature.

Generally, thermoplastic elastomers can be categorized into two groups: multi-block copolymers and blends. The first group is copolymers consist of soft elastomers and hard thermoplastic blocks, such as styrenic block copolymers (SBCs), polyamide/elastomer block copolymers (COPAs), polyether ester/elastomer block copolymers (COPEs) and polyurethane/elastomer block copolymers (TPUs). TPE blends can be divided into polyolefin blends (TPOs) and dynamically vulcanized blends (TPVs).

Thermoplastic elastomers are known as two-phase system consisting of rubbery elastomeric (soft) component and rigid (hard) component. The soft phase can be polybutadiene, poly(ethylene-*co*-alkene), polyisobutylene, poly(oxyethylene), poly(ester), polysiloxane or any of the typical elastomers while the hard phase are polystyrene, poly(methyl methacrylate), urethane, ionomer – poly(ethylene-*co*-acrylic acid) (sodium, Mg, Zn salt), ethylene propylene diene monomer, and fluropolymers. The structure representing a styrenic TPE is shown schematically in Figure 1.

Fig. 1. Schematic of a styrene-butadiene-styrene block copolymer
