Relaxation softening point

The difference in softening temperatures for amorphous and semi-crystalline polymers becomes also clear from Fig. 13.3, where the Young moduli of amorphous and of semicrystalline polystyrene are illustrated. For amorphous polystyrene the two HDTs appear to be 92 and 97 °C and for the semi-crystalline polystyrene 99 and 114 °C. It has to be mentioned, however, that the curves in Fig. 13.3 are the so-called 10 s moduli, i.e. measured after 10 s of stress relaxation, every point at a specific temperature. The measurements in the softening experiments are not in agreement with the determination of the standard Young modulus. 

A wide variety of tests is performed in TMA, which are adapted from physical tests that were used before the instrument became commonly available. These tests may also be modeled or mimicked in TMA, such as heat distortion (Fig. 9) and softening points. Methods to obtain the modulus, compressive viscosity, and penetrative viscosity have been developed. Many of these methods, such as ASTM D648 for example, will specify the stress the sample needs to be exposed to during the run. In D684, a sample is tested at 66 and 264 psi. Most TMAs on the market today have software available that allows them to generate stress—strain curves and to run creep—recovery experiments. Some are also capable of limited types of stress relaxation studies (for example a constant gauge length test " ). 

A second source for accidental orientation is deformation of the material in the solid state, e.g. with cold-forming or solid-phase forming, in which a plastic sheet or billet is formed into an object at a temperature below the softening point of the plastic. The relaxation process can hardly take place under these conditions and although the article may be stable at room temperature, the deformation is (almost) completely recoverable at elevated temperatures. ... 

The calculated isocyanurate/oxazolidone ratio, as described above, is not enough to evaluate the properties of the resin, and so, their softening points and glass transition temperatures were determined and related to the reaction conditions used to prepare the films. The reaction parameters were studied systematically to establish the influence of the following variables on the relaxation temperatures of the resultant resins ... 

One disadvantage of SPPF is the level of residual stress and orientation remaining in the final product. This arises from stretching the sheet prior to its softening point. The presence of greater orientation enhances the clarity and stiffness of the formed article. One undesirable consequence of this, however, is the unsuitability of SPPF in the production of containers for high temperature applications. If used at elevated temperatures, e.g. in microwave oven trays, the frozen-in orientation and stresses have an opportimity to relax, causing distortion of the container. 

The temperature dependence of also shows additional inflections as a result of superposition of the relaxation processes associated with the glass transitions of the soft phase, the interphase, and the amorphous phase of the hard blocks, as well as with melting of the P04 block crystalline phase. The inflections of all curves at high temperatures, associated with the rapid decrease of modulus or increase of damping, characterize the softening point (plasticity),... 

Figure 10.13 Schematic of a thermomechanical analysis (TMA) thermal expansion curve of an inorganic compound glass (solid curve). The geometrical construction to obtain the dilatometric softening point, A4g, at 10 i =Pas is given. The analogous TMA thermal expansion curve is shown (dotted curve) when a high-fictive-temperature glass sample is heated under load relaxation to a lower-fictive-temperature configurational arrangement can produce an observable contraction as illustrated.

To continue the a measiuement beyond the softening point requires that the supercooled glass-forming liquid be enclosed in containment such that volume change can be measured. But a above Mg is not usually relevant to the performance of a glass material in normal service since above Mg the glass melt cannot support sustained stress and undergoes viscous flow and stress relaxation. 

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