Dynamic glass—rubber relaxation

The crystalline phase affects the viscoelastic dynamic functions describing the glass-rubber relaxation. For example, the location of this absorption in the relaxation spectrum is displaced with respect to that of the amorphous polymer and greatly broadened. Consequently, the perturbing effects of crystal entities in dynamic experiments propagate throughout the amorphous fraction. The empirical Boyer-Beaman law (32)... 

Note r , dynamic mechanical peak temperature for glass transition (1 Hz) Tg, calorimetric glass transition temperature Pkww. KWW distribution parameter for glass-rubber relaxation FE V, fractional fiee volume based on density measurements DSC, differential scanning calorimetry. 

The fact that crystals can be formed and annealed at room temperature in these systems can also be cited as evidence that the glass transition temperature (Tg) of these materials is sufficiently below room temperature. If the amorphous material were glassy at room temperature, there could not be sufficient large-scale motion to allow the formation of such crystals. This information can be applied in the interpretation of the dynamic mechanical relaxations as will be seen. No indication of a glass-to-rubber relaxation was found in the DSC scans of these materials. 

Amorphous phase transition effects Dynamic mechanical analysis (DMA) on both a cold (20°C) compressed powder sample and a compression moulded (240°C) sheet sample was used to investigate the relaxation processes i.e. especially the glass-rubber transition, of the amorphous phase of PK terpolymer. 

A modulus value increase upon storage under ambient conditions is also reported for other semi-crystalline polymers like, for instance, polypropylene. Struik [11] measured for PP a continuously increasing dynamic stiffness at 20°C in combination with a decrease of the intensity of the glass-rubber (S) transition of PP (the temperature location of the S-transition did not change). Struik called this phenomenon an amorphous phase ageing effect a densification process of the amorphous PP phase due to a free volume relaxation effect. 

Fig. 4.10. Frequency-dependence of the dynamic shear modulus G

Frequency dependence of the dynamic shear modulus of polyisobulylene at different temperatures in the range -4S°C to This relaxation is the glass-to-rubber... 

---