Glass transition temperature rubber

Plasticisers are added to a polymer to reduce the glass rubber transition temperature drastically (e.g. with PVC), so that the polymer behaves as a rubber at ambient temperature rather than as hard glassy thermoplast. 

An atactic structure is in both cases not crystallisable. Atactic PP is because of its glass-rubber transition temperature (Tg = -15 °C) rubbery and technically of no use. Isotactic PP is able to crystallise and can, therefore, be used in practice. For PS atacticity is no objection its properties as a glassy polymer are retained up to its Tg (95 °C). 

The majority of synthetic polymers can be thin sectioned by microtomy for transmitted light purposes [2]. For optimum results, sectioning should be carried out at a temperature just below the glass/rubber transition temperature, Tg. 

In the case of any transition occurring at a temperature lower than the glass-rubber transition temperature, Tg, the frequency of the involved process has a temperature dependence which obeys an Arrhenius law ... 

As mentioned in Sect. 2.2.1.2, the yield point originates from a conformational change of the main chain, leading to an increase of conformations corresponding to what would happen at a temperature above the glass-rubber transition temperature. 

The thin film technique described in Sect. 2 has been applied to PMMA [37, 38] from room temperature to the glass-rubber transition temperature. 

The increase of nSSA in the low temperature part to about - 20 °C corresponds to the softening of the medium by the fi transition motions. The leveling observed at higher temperatures is consistent with the fact that, for MT Ii, copolyamides, there are no new motions until the glass-rubber transition temperature is reached, in contrast to the case of xTy -y copolyamides for which there is still an co transition in the range 20-80 °C (Fig. 84b). 

The chain flexibility has, a.o., a large influence on the glass-rubber transition temperature, and (if applicable) on the melting point. 

The height of the glass-rubber transition temperature is, in the first instance, governed by the competition between thermal motion and the attraction forces between the chains. 

E is also independent of chain stiffness and chain interactions, these factors play a role in the height of the glass-rubber transition temperature and the melting point. A stiffer chain, therefore, does not result in a stiffer polymer except, sometimes, in an indirect way, namely when stiff chains enable the formation of high orientation, such as in liquid-crystalline polymers (see 4.6). 

The situation for amorphous linear polymers is sketched in Fig. 2.8a. If a polymeric glass is heated, it will begin to soften in the neighbourhood of the glass-rubber transition temperature (Tg) and become quite rubbery. On further heating the elastic behaviour diminishes, but it is only at temperatures more than 50° above the glass-rubber transition temperature that a shear stress will cause viscous flow to predominate over elastic deformation. 

The glass-rubber transition temperature, commonly known as glass transition temperature (Tg), is a phase change reminiscent of a thermodynamic second-order transition. In the case of a second-order transition a plot of a primary quantity shows an abrupt change in slope, while a plot of a secondary quantity (such as expansion coefficient and specific heat) then shows a sudden jump. 

If the molar mass is sufficiently high, the glass-rubber transition temperature is almost independent of the molar mass. On the other hand, the very diffuse rubbery-liquid transition... 

Below the glass-rubber transition temperature glassy polymers also show other, secondary transitions. Their effects are smaller and often less obvious, although they are important to the mechanical behaviour (to diminish brittleness). Secondary transitions can be detected by studies of mechanical damping, by NMR or by electric loss measurements over a range of temperatures. 

In this chapter it will be demonstrated that the two main transition temperatures, viz. the glass-rubber transition temperature and the crystalline melting temperature can be correlated with the chemical structure by means of a method based on group contributions. 

In contradiction to the melting point, the glass-rubber transition temperature is not a thermodynamic transition point. It shows some resemblance, however, to a second order transition. For a second-order transition, the following relationships derived by Ehrenfest (1933) hold ... 

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