**2. Thermoplastic elastomer**

A thermoplastic elastomer has all the same features as described for an elastomer except that chemical cross-linking is replaced by a network of physical cross-links. The ability to form physical cross-links is the opposite to the chemical and structural requirements of an elastomer just described. The answer to this dilemma is that thermoplastic elastomers must be two-phase materials, and each molecule must consist of two opposite types of structure, one the elastomeric part and the second the restraining, physical cross-linking part. Thermoplastic elastomers are typically block copolymers.

The elastic block should have high molar mass and possess all of the others characteristic required of an elastomer. The restraining block should resist viscous flow and creep. One restraining block can be used per macromolecule, giving a diblock copolymer (AB), or one restraint block at each of the elastomer can be used giving a triblock copolymer (ABA). Specific polymers will be described in the context of these general principles in the following sections. To provide an example of thermoplastic elastomer block copolymer structures the monomers butadiene and styrene are chosen.


Notes:

aIn blends containing polypropylene

bThe values are for polyester and polyether respectively cThe values are presumably the result of the short length of polyethylene and polypropylene segments dEPDM, EPR, butyl rubber and natural rubber

Source: Holden, 2011; Drobny, 2007

Table 1. Glass transition and crystalline melt temperatures of major TPEs

Polymerisation of butadiene via 1,4-addition gives the elastomer poly(1,4-butadiene). This polymer is a hydrocarbon with low intermolecular forces, no rigid or bulky groups and a relatively flexible chain, except for the double bond between carbons 2-3. The cis stereoisomer of the double bond is preferred over trans since this decreases chain regularity.

A thermoplastic elastomer has all the same features as described for an elastomer except that chemical cross-linking is replaced by a network of physical cross-links. The ability to form physical cross-links is the opposite to the chemical and structural requirements of an elastomer just described. The answer to this dilemma is that thermoplastic elastomers must be two-phase materials, and each molecule must consist of two opposite types of structure, one the elastomeric part and the second the restraining, physical cross-linking part.

The elastic block should have high molar mass and possess all of the others characteristic required of an elastomer. The restraining block should resist viscous flow and creep. One restraining block can be used per macromolecule, giving a diblock copolymer (AB), or one restraint block at each of the elastomer can be used giving a triblock copolymer (ABA). Specific polymers will be described in the context of these general principles in the following sections. To provide an example of thermoplastic elastomer block copolymer

SBS -90 95 (Tg) SIS -60 95 (Tg) SEBS -55 95 (Tg) and 165 (Tm)a SIBS -60 95 (Tg) and 165 (Tm) Polyurethane elastomers -40 to -60b 190 (Tm) Polyester elastomers -40 185 to 220 (Tm) Polyamide elastomers -40 to -60b 220 to 275 (Tm) Polyethylene-poly(-olefin) -50 70 (Tm)c Polypropylene/poly(ethylene-propylene) -50 50 to 70 (Tm)c Poly(etherimide)-polysiloxane -60 225 (Tg) Polypropylene/hydrocarbon rubberd -60 165 (Tm) Polypropylene/nitrile rubber -40 165 (Tm) PVC-(nitrile rubber+DOP) -30 80 (Tg) and 210 (Tm) Polypropylene/poly(butylacrylate) -50 165 (Tm) Polyamide or polyester/silicone rubber -85 225 to 250 (Tm)

bThe values are for polyester and polyether respectively cThe values are presumably the result of the short length of polyethylene and polypropylene segments dEPDM, EPR, butyl rubber and natural rubber

Polymerisation of butadiene via 1,4-addition gives the elastomer poly(1,4-butadiene). This polymer is a hydrocarbon with low intermolecular forces, no rigid or bulky groups and a relatively flexible chain, except for the double bond between carbons 2-3. The cis stereoisomer of the double bond is preferred over trans since this decreases chain regularity.

Table 1. Glass transition and crystalline melt temperatures of major TPEs

Tg (°C)

Hard phase, Tg or Tm (°C)

**2. Thermoplastic elastomer** 

Notes:

aIn blends containing polypropylene

Source: Holden, 2011; Drobny, 2007

Thermoplastic elastomers are typically block copolymers.

structures the monomers butadiene and styrene are chosen.

Elastomer Type Soft phase,

The transform is more regular and crystallinity can occur, which will prevent elastomeric response. Poly(butadiene) would need to be cross-linked to be a useful elastomer. Polystyrene is a glassy polymer with glass transition temperature =100 °C so it will resist flow and creep at ambient temperatures, but it can flow and be moulded at temperatures above Tg. A diblock copolymer of butadiene and styrene will provide the combination of properties required for a thermoplastic elastomer when the butadiene content is higher. Poly(butadiene-b-styrene) (BS) has two separate phases, a continuous polybutadiene phase with dispersed poly(styrene) phase. The matrix phase gives the overall elastomeric response while the dispersed islands are the restraining physical cross-links. Glass transition and crystalline melt temperatures for major TPEs are given in Table 1.
