- Chemical activity
Polyoxymethylene (POM) and polycarbonate (PC) often start thermal degradation
from the chain ends. If their plastic products contain too many short chains, the
products are liable to turn yellow in color, losing their good quality.
- Specific interactions
Poly(ethylene oxide) (PEO) contains -OH groups at its chain ends, which are able to
form association clusters via hydrogen bonding, leading to high apparent molecular
weights in the measurements.
- Fixed chain ends
Vulcanization effectively fixes the ends of flexible polymer chains to form three-
dimensional networks. By this way, the stress relaxation of stretched polymer
chains can be avoided, and the high entropy elasticity of the rubber can be
produced. Moreover, the fixed chain ends also increase both melting points and
glass transition temperatures of short flexible chains.
In the synthesis process of polymer chains, polymerization is not able to
synchronize the initiation, the propagation and the termination steps in all
polymers, and therefore leads to a certain width of molecular weight distributions
in the product. Such a character is called thepolydispersity of polymers.
The production of polymeric materials has to pay close attention to both higher
and lower ends of molecular-weight distributions, which often play important roles
in determining the processing technology as well as the product quality of polymers.
For examples, some low molecular weight fractions of poly(vinyl chloride) can help
the fluid processing of high molecular weight fractions, like the additives of
plasticizers. The low molecular weight fractions of PC could speed up chemical
degradation, causing air bubbles and darkening in the plastic product. Adding a
minute amount of ultra-high molecular weight fraction of iPP could significantly
improve the crystal nucleation behavior of polypropylene product. Due to the
heterogeneous character of Ziegler-Natta catalysts, the low molecular weight frac-
tion of the low-density polyethylene (LDPE) usually contains more short branches,
and thus exhibits a difficulty to crystallize with high molecular weight fractions.
2.4.2 Characterization of Molecular Weights
A polydisperse polymer exhibits a certain width in its molecular weight distribu-
tion. We assume the number of polymer moleculesNiin the fraction of the
molecular weightMi, and the total weight of this fractionWi¼NiMi, then we
define thenumber-average molecular weightby
MN
SNiMi
SNi(2.17)
24 2 Structure–Property Relationships