**Modifications of Thermoplastic Elastomers**

134 Thermoplastic Elastomers

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**8** 

 *Australia* 

**Thermoplastic Elastomers** 

*Applied Sciences, RMIT University, Melbourne* 

An elastomer is defined by mechanical response not by chemical structure. Elastomers comprise a diverse range of chemical structures although they are characterized as having weak intermolecular forces. An elastomer will undergo an immediate, linear and reversible response to high strain to an applied force. This response has a mechanical analogy with a spring according to Hooke's Law. Non-linear, time dependent mechanical response is distinguished as viscoelasticity according to the parallel spring and dashpot model. Time dependent irreversible response is a viscous response according to a dashpot model. An ideal elastomer will only exhibit an elastic response. Real elastomers exhibit a predominantly elastic response, however they also exhibit viscoelastic and elastic responses

The chemical structure and molecular architecture of elastomers is tightly related to elastomeric mechanical response. High strain requires a polymer with high molar mass preferred. Many materials can exhibit an elastic response, that is immediate, reversible and linear strain with stress, however only a polymer can exhibit additionally high strain. High strain is due to uncoiling of random molecular coils into more linear conformations. The limit to elastic response is when molecules are in fully extended conformations. This mechanism is due to uncoiling of chain segments. Molecules do not move relative to each

Reversibility and immediate response is obtained with macromolecules that have flexible chains with weak intermolecular forces. Rigid groups such a benzene, bulky side-chains such as isopropyl, polar groups such as ester and hydrogen bonding groups such as hydroxy are not desirable if a polymer is to be an elastomer. This description supposes elastomeric properties at ambient temperatures, since at elevated temperatures above the

At high extensions and when under strain for longer times viscous flow occurs, known as creep when over longer times. Chemical cross-linking prevents viscous flow, the movement of molecules relative to each other. Elastomers are cross-linked after moulding or shaping to fix molecules into their relative positions. Once cross-linked the unstrained shape of an elastomer cannot be altered and the elastomer cannot be reprocessed or recycled. The permanence brought about by cross-linking and the need to perform a cross-linking reaction

other, there are reversible random coiling not translational motions.

glass transition temperature many polymers become elastomers.

on elastomers are disadvantages for their applications.

**1. Introduction** 

especially at higher strains.

Robert Shanks and Ing Kong

Robert Shanks and Ing Kong

*Applied Sciences, RMIT University, Melbourne Australia* 

#### **1. Introduction**

An elastomer is defined by mechanical response not by chemical structure. Elastomers comprise a diverse range of chemical structures although they are characterized as having weak intermolecular forces. An elastomer will undergo an immediate, linear and reversible response to high strain to an applied force. This response has a mechanical analogy with a spring according to Hooke's Law. Non-linear, time dependent mechanical response is distinguished as viscoelasticity according to the parallel spring and dashpot model. Time dependent irreversible response is a viscous response according to a dashpot model. An ideal elastomer will only exhibit an elastic response. Real elastomers exhibit a predominantly elastic response, however they also exhibit viscoelastic and elastic responses especially at higher strains.

The chemical structure and molecular architecture of elastomers is tightly related to elastomeric mechanical response. High strain requires a polymer with high molar mass preferred. Many materials can exhibit an elastic response, that is immediate, reversible and linear strain with stress, however only a polymer can exhibit additionally high strain. High strain is due to uncoiling of random molecular coils into more linear conformations. The limit to elastic response is when molecules are in fully extended conformations. This mechanism is due to uncoiling of chain segments. Molecules do not move relative to each other, there are reversible random coiling not translational motions.

Reversibility and immediate response is obtained with macromolecules that have flexible chains with weak intermolecular forces. Rigid groups such a benzene, bulky side-chains such as isopropyl, polar groups such as ester and hydrogen bonding groups such as hydroxy are not desirable if a polymer is to be an elastomer. This description supposes elastomeric properties at ambient temperatures, since at elevated temperatures above the glass transition temperature many polymers become elastomers.

At high extensions and when under strain for longer times viscous flow occurs, known as creep when over longer times. Chemical cross-linking prevents viscous flow, the movement of molecules relative to each other. Elastomers are cross-linked after moulding or shaping to fix molecules into their relative positions. Once cross-linked the unstrained shape of an elastomer cannot be altered and the elastomer cannot be reprocessed or recycled. The permanence brought about by cross-linking and the need to perform a cross-linking reaction on elastomers are disadvantages for their applications.

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

Elastomers extend and contract by conformational change from a compact random coil to extended chain. The random coil can have many possible conformations resulting in a high entropy. A fully extended chain can only have one conformation resulting in low entropy. The extended chain will spontaneously contract into a random coil since the entropy of the process is favourable. Enthalpy is not a contributor for an ideal elastomer since intermolecular forces are minimal. Entropy is overcome by a mechanical force deforming the elastomer. Thermodynamic equations are applied to elastomeric deformation and recovery. By analogy with an ideal gas, elastomers that conform to the thermodynamics are called ideal elastomers. As in the case of real gases, real elastomers deviate from ideality.

 At high extensions elastomer chains become fully extended between cross-links, chemical or physical, and as the distribution of chains become fully extended the stress-

 At high extensions the extended chains can pack closely forming crystals that cannot be further extended. Crystallisation appears as stress whitening and it is only maintained with the deforming force since the intermolecular interactions are too weak to prevent

 Molecular entanglements prevent free molecular uncoiling and therefore cause deviation from a linear elastic response. Often entanglement caused deviations are more pronounced in a first extension-recovery cycle that differs from subsequent cycles

In compression deviations from ideality are caused by the finite size of the molecules

The aim of this review is to compile the updated knowledge on thermoplastic elastomers in general, practically their structures, syntheses, processing methods, mechanical properties

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

and the limited free volume available for molecules to occupy.

crystalline melt temperatures for major TPEs are given in Table 1.

**3. Thermodynamics of elasticity** 

Deviations of elastomers are the result of:

strain response becomes non-linear.

**4. Structure of thermoplastic elastomers** 

entropy-controlled recovery.

when performed together.

and applications.
