Volume retardation

A closely related phenomenon is the volume retardation in the glassy condition. When, after rapid cooling, point A has been reached (Figure 3.6), the volume will, at a constant temperature, gradually decrease to e.g. A. Possibilities for motion of chain parts, though on a very limited scale, cause the free volume to decrease gradually, though, as we now know, an equilibrium will never be reached. The rate of volume retardation is, as a matter of fact, smaller as the temperature is lower. 

In 3.3 the effect of cooling rate on the free volume in the glassy state has been discussed. Rapidly cooled glassy polymers have a greater free volume, but they show volume retardation. This volume change, though very small, has a considerable effect on the creep behaviour all relaxation times for creep are shifted towards higher values. This phenomenon has been studied extensively by Struik (thesis Delft 1977). 

Fully amorphous polymers may be transparent, such as PVC, PMMA, PC and PS. They can, in principle, be applied in the optical industry for spectacles, simple photographic lenses etc. For precision optics they are less suited, since because of volume retardation as well as by the fact that they are often manufactured by injection moulding, they cannot meet the requirements of narrow dimension tolerances. Moreover, their low resistance to scratching is a disadvantage in optical applications. 

Creep and stress relaxation of polymeric solids depend on the state of the polymer. Both effects are much faster and stronger for a material that was quenched from the liquid state (i.e. the state above the glass transition temperature) to room temperature than for a material that was cooled down slowly. This is due to the process of volume retardation. 

FIG. 13.34 Volume retardation after a jump of temperature at point A complete retardation is possible, whereas at point C only partial retardation is possible in reasonable time. 

After a temperature quench from T0 to T with AT = T, T0 > 0 an almost instantaneous volume decrease from V0 to V1 occurs, determined by the thermal expansion coefficient ag. Subsequently the volume gradually decreases (volume retardation) to UlDO as determined by the Aa = [Pg.429]

FIG. 13.38 Schematic diagram of volume retardation curve after a jump of temperature. 

The retardation curve is normalised by plotting [V(t)—lAJ/IVi—VoJ vs. In t. Because the volume retardation immediately starts after the temperature quench the volume V is difficult to determine, but it can be calculated by subtracting both Eqs. (13.100) ... 

Another problem is that several minutes are needed to obtain thermal equilibrium, whereas volume retardation immediately starts after the temperature quench. In order to overcome this equilibrium time volume retardation can also be measured following a pressure quench, which takes only a few seconds to reach equilibrium. The corresponding equations are... 

Classic studies concerning volume retardation were made by Kovacs (1963) and by Goldbach and Rehage (1966,1967). Illustrations of typical isothermal volume retardation curves are shown in Fig. 13.39 for atactic polystyrene, where the volume retardation is... 

The origin of the phenomenon of ageing lies in the fact that glasses are not in thermodynamic equilibrium their volume and entropy are too large, hence there is a tendency to volume reduction (volume retardation). Decreases of rates of stress relaxation and creep are consequences of this phenomenon. 

Introduction of Cl into a thermoplastic composition changes the kinetics of its thermal destruction processes. This is expressed in variations of the thermal resistance temperatures (Tt) and destruction initiation (Td) recorded by DTA. Tt and Td for the PE-based compositions shift to the region of lower temperatures by, correspondingly 1-50°C and 5-20°C. The activation energy (Ta) and macromolecular destruction found by DTA also diminish. Compositions based on HOPE show a reduction of Ta from 266 to 100-150kJ/mole, and the more efficient the antioxidizing effect of Cl towards HDPE the lower the Eg,. This is probably because the lower amount of oxide compounds in the polymer volume retards cross-linking of macromolecules and results in reduction. 

However, the slopes of curves c and a are practically identical (Le., a 7 could not be found) and all three curves, a, b and c, deviate strongly from the freeze in curve d. Based on these data and supporting evidence from volume retardation analysis. Brewer and Rehage (151) have concluded that a 2nd order transition within the limiting cases of high temperatures and pressures and infinitely slow cooling rates at normal pressures does not exist. 

---