Amorphous polymers cooling rates

In the molten state polymers are viscoelastic that is they exhibit properties that are a combination of viscous and elastic components. The viscoelastic properties of molten polymers are non-Newtonian, i.e., their measured properties change as a function of the rate at which they are probed. (We discussed the non-Newtonian behavior of molten polymers in Chapter 6.) Thus, if we wait long enough, a lump of molten polyethylene will spread out under its own weight, i.e., it behaves as a viscous liquid under conditions of slow flow. However, if we take the same lump of molten polymer and throw it against a solid surface it will bounce, i.e., it behaves as an elastic solid under conditions of high speed deformation. As a molten polymer cools, the thermal agitation of its molecules decreases, which reduces its free volume. The net result is an increase in its viscosity, while the elastic component of its behavior becomes more prominent. At some temperature it ceases to behave primarily as a viscous liquid and takes on the properties of a rubbery amorphous solid. There is no well defined demarcation between a polymer in its molten and rubbery amorphous states. 

Parison cooling significantly impacts the cycle time only when the final parison thickness is large. In thin blown articles the mold is opened when the pinched-off parts have solidified so that they can be easily stripped off thus they are the rate-controlling element in the cooling process. For fast blow molding of even very thin articles, the crystallization rate must be fast. For this reason, HDPF, which crystallizes rapidly, is ideally suited for blow molding, as are amorphous polymers that do not crystallize at all. 

As the temperature of an amorphous polymer is lowered, there is a transition from rubberlike material with a low Young s modulus to a stiff glass with a high modulus. For example, the Young s modulus of PVC (measured at 1 s) increases from 0.15 to 1.2 GPa as the temperature is decreased from 90 to 75 °C. The glass transition temperature is in this range. The exact temperature depends on the rate of cooling. 

Comparison between the Crystallization from the Melt or the Glass. The crystallization studies at low temperatures can either be performed on samples cooled from the melt to the crystallization temperature or on quenched samples which are annealed to the crystallization temperature. Each method would be indicated for the particular region where equilibration to the crystallization temperature would be faster without having induced crystallinity in the amorphous polymer. In general, crystallization from the melt is better at temperatures above that which gives the maximum in growth rate, and crystallization from the glass is better for temperatures lower than the maximum. 

When an amorphous polymer is gradually cooled from above the glass transition temperature Tg its volume decreases (see Fig. 13.32) according to its thermal expansion coefficient aj. In the region around the Tg the volume decrease will lag behind, starting at temperature Tel because the rate of reorganisation process becomes too small. The polymer starts to vitrify and a temperature Tel will be reached where the reorganisation completely stops and where the vitrification process is completed. Decrease of volume is only the result of normal volume contraction with expansion coefficient ag. The relationship between both thermal expansion coefficients is... 

FIG. 13.33 Schematic diagram of volume-temperature relationship for an amorphous polymer around the glass transition temperature for three different cooling rates qi > <72 > <73-... 

Figure 3.1. Schematic illustration of the volume-temperature relationship of a typical polymer. When the polymer is prevented from crystallization, it is brought to temperatures below Tm in an amorphous state and is then turned into a glass at the glass-transition temperature Tg or Tg, which depends on the cooling rate. (From Roe, 1990.)...

A peculiarity of crystalline polymers, in contrast to the amorphous ones, is their ability to exist in different crystal modifications and thus to undergo polymorphic transitions (crystalline phase transitions). The various types of crystal modifications can be obtained by changing the crystallization conditions, such as temperature, cooling rate, etc. 

The level of sinkage/shrinkage is also influenced by other factors such as fillers, pigments, melt flow index, density and cooling rate. It should be noted that materials which are oriented, either purposely or accidentally (e.g. in a moulding, operation), will shrink significantly if they are heated above the temperature used for the orientation process. Amorphous materials show less shrinkage than crystalline polymers. 

When measuring thermal noise of carbon-black-filled PS and other amorphous polymers pronounced maxima were found near the vicinity of Tg, which are very similar to the maxima in resistivities reported earlier (J). The intensity of these peaks depended on the rate of cooling/heating of the sample prior to and during the measurements and also on the storage time. With HDPE, noise peaks were recorded in the vicinity of Tm, and the time dependence of the peaks was less pronounced. 

Figure 23.14 presents the relation between the volume and temperature at several pressure levels, showing the competition between compressibility and shrinkage for amorphous and semicrystalline polymers. These figures are valid for a certain rate of cooling. The effect of cooling rate and pressure are of importance. Higher pressures... 

Chang, R. Y, Chen, C. H., and Su, K. S., Modifying the Tait equation with cooling-rate effects to predict the pressure-volume-temperature behaviors of amorphous polymers modeling and experiments, Polym. Eng. Sci., 36, 1789-1795 (1996). 

---