Glass transition temperature and brittleness

Seveik A, Klepal V, Betakova S, Sionova Z. Contribution to method for measurement of glass transition temperature and brittle temperature in rubber materials. In IRC Conference, Prague, 1-4 July, 2002 p. 9. Proceeding Paper 69. 

Fig. 7. Blends of Perbunan N grades with different acrylonitrile contents. Glass transition temperature and brittleness temperature versus the acrylonitrile content.

With plastics there is a certain temperature, called the glass transition temperature, Tg, below which the material behaves like glass i.e. it is hard and rigid. As can be seen from Table 1.8 the value for Tg for a particular plastic is not necessarily a low temperature. This immediately helps to explain some of the differences which we observe in plastics. For example, at room temperature polystyrene and acrylic are below their respective Tg values and hence we observe these materials in their glassy state. Note, however, that in contrast, at room temperature, polyethylene is above its glass transition temperature and so we observe a very flexible matoial. When cooled below its Tg it then becomes a hard, brittle solid. Plastics can have several transitions. 

Membrane design and fabrication requires more optimization than the synthesis of the right type of polymer. For example, those phosphazene polymers that contained the highest ratios of methylamino groups were too brittle to be used as membranes (because of the high glass-transition temperatures) and too soluble in aqueous media. However, the polymers could be made insoluble in water by radiation cross-linking as shown in reaction (54). 

Below T polymers are stiff, hard, brittle, and glass-like above 7. if the molecular weight is high enough, they are relatively soft, limp, stretchable, and can be somewhat elastic. At even higher temperatures they flow and are tacky. Methods used to determine glass-transition temperatures and the reported values for a large number of polymers may be found in References 7—9. Values for the T of common acrylate homopolymers are found in Table 1. 

PVC, however, suffers from an inherent susceptibility to thermal degradation and hence must invariably be processed with heat stabilizers and careful control of processing temperature. The other important drawbacks of PVC are its brittleness in the absence of a plasticizer (low notched Izod impact strength) and its low heat distortion temperature (ca. 60°C) originating from its low glass transition temperature and essentially... 

It is realized that even at this higher temperature, organic materials of this nature are below their glass transition temperature and their utility is dependent upon a lack of brittleness and high impact resistance below this transition. The problem, therefore, is to find a curable elastomer that will resist, as much as possible, bend stresses and shock and vibration loading without exhibiting brittle failure. 

Not all polymers show cold drawing there are requirements such as a minimum molecular weight for strain hardening, and this has been extensively discussed elsewhere (see for example Ref. 5, pp. 271 and 322). It is clearly necessary for the polymer to be above its brittle-ductile transition, but this is a necessary rather than a necessary and sufficient condition for cold drawing. It should perhaps also be emphasised that there is no immediately obvious relationship between the glass transition temperature and the brittle-ductile transition temperature Tb (see for example Ref. 6). 

Brittle temperature is very closely related to the glass transition temperature and determines the minimum temperature at which a semi-crystalline polymer could be used without significant loss of its impact properties. 

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