Glass transition temperatures stress related

Argon and co-workers (95,96) have developed an atomistic mechanics model of polypropylene and related it to experiments performed at a temperature of 10°C below the glass-transition temperature. Stress-strain curves calculated after small strain increments showed a series of generally monotonically increasing... 

Original iPP is a highly cohesive polymer in the crystalline state (see Section 15.1), a property resulting from low intermolecular distances in the crystalline phase. It is much more impact-sensitive than PE, particularly at temperatures below ambient. This can be due to a relatively difficult flow of the chains under sudden stress near the glass transition temperature, in relation with their helical structure. 

From the practical point of view, the glass transition is a key property since it corresponds to the short-term ceiling temperature above which there is a catastrophic softening of the material. For amorphous polymers in general, and thus for thermosets, one can consider that the glass transition temperature, Tg, is related to the conventional heat deflection temperature (HDT) (usually, HDT is 10-15°C below Tg, depending on the applied stress and the criterion selected to define Tg). 

True liquids, in contrast, do not show an extensive order. Even with very slight stresses applied for a short time, they deform so completely that they very quickly adopt the form of the surrounding container. Low-molar-mass liquids thus behave in a purely viscous way under normal conditions. When stress is applied, the molecules are displaced irreversibly in relation to one another. In high-molar-mass substances above the glass transition temperatures, flow can be produced relatively easily. Deformations are much more difficult below the glass transition temperature of amorphous polymers. For this reason, and because of their lack of order, amorphous substances below their glass-transition temperatures are often termed supercooled liquids. 

Figure 2.39 shows the response of the modulus to stress. The nylon fibers are characterized by an initial modulus at elongation —> 0 that is proportional to the ratio at which the fibers were drawn. This initial modulus is related to the glass transition temperature of the amorphous region and consequently to the mobility of the chain segments in these regions. It depends therefore on factors such as temperature and water content, which affect the mobility of the chain segments. 

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