The Degree Of Crystallization: Nucleation

Crystal formation begins at nucleation points and extends outwards into the bulk of the polymer. Nucleation starts with small, nanometer-sized areas where as a result of heat motion some chains or their segments occur parallel. Those seeds can either dissociate, if thermal motion destroys the molecular order, or grow further, if the grain size exceeds a certain critical value.

The nucleation itself can be one of the following:

Homogeneous nucieation--this is the case for a pure polymer, e.g. virgin PP and the result is a small number nucleation sites and a few large crystals.

Heterogeneous nucleation.where particles in the polymer (natural or added deliberately) act as nucleation sites and many small crystals are formed. This is the case for PP with certain coloring agents and results in higher shrinkage rates. Many of the good nucleating agents are metal salts of organic acids, which themselves are crystalline at the solidification temperature of the olymer solidification.

The type of nucleation process will also affect the properties of crystalline polymers. A product with relatively few large crystals will have different properties than one with more, but smaller, crystals. Examples of crystalline polymers are PP, PE (all varieties but some more so than others) PA and POM.