2.4 Molecular Weights and Their Distributions
2.4.1 Molecular Weight Effects
With the increase of molar masses from the lower end, some common properties are
enhanced, such as the melting points, the mechanical properties, the glass transition
temperatures, etc. However, the enhancement of properties will soon saturate with
further increasing of molar masses, as illustrated in Fig.2.4. A higher molar mass
leads to a larger viscosity, and as a result, the fluid processing becomes more
difficult. In most cases, the higher molar mass does not bring the better performance
of the product. Therefore, from the practical point of view, it is important to control
the molar mass in a proper range during polymer processing.
High or low molecular weights are also reflected in the concentration of chain
ends in the bulk phase. The chain ends often exhibit quite different behaviors from
those monomers locating in the middle of polymer chains. In the following are
listed the so-calledchain-end effects.
- High mobility
Short chains are viable to disentangle. The addition of plasticizer is actually adding
small compatible molecules for enhancing the high-mobility effect of chain ends.
The glass transition temperature will be lowered in this way.
- Defects in the crystal
Short-chain crystals have lower melting points, which can be regarded as a result of
high content of chain-end defects in the large-size crystal. The defects bring an
effective depression in the melting points.
Fig. 2.4Illustration for some
common propertiesP
increasing with the molecular
weightM, but the viscosity
increasing as well
2.4 Molecular Weights and Their Distributions 23