Partially crystalline

A solid that contains both crystalline and amorphous areas ( domains ) is referred to as partially crystalline . The term essentially plays a role in polymer physics .

If the melt of a polymer is cooled , the chains move less and less and begin to arrange themselves regularly ( crystallize ). But as they each other looped are ( English entangled ), this process can not take place in the whole sample, but only in domains. In the rest of the sample, the chains solidify in a random manner (amorphous).

Influencing factors:

• The slower the melt is cooled, the more polymers crystallize.
• Tacticity : iso- or syndiotactic polymers crystallize; Atactic polymers only crystallize when the substituents are very small, as in the case of polyvinyl fluoride .
• Polymers with small side chains crystallize better than those with large ones.
• Crosslinked or branched polymers also do not crystallize .

Crystalline substances that do not exist as single crystals but as polycrystals are not referred to as partially crystalline, even if there is a thin amorphous film between the crystallites .

Crystallinity, degree of crystallinity, degree of crystallization

The degree of crystallinity describes the crystalline proportion of a partially crystalline solid. Usually it is given

• as a mass fraction :

${\displaystyle k={\frac {m_{\mathrm {krist} }}{m_{\mathrm {ges} }}}}$

or

• as a mole break :

${\displaystyle k={\frac {n_{\mathrm {krist} }}{n_{\mathrm {ges} }}}}$

The unit of measure used for crystallinity depends on the measurement method.

In the case of polymers, the degree of crystallinity depends on the thermal history of the material. Tempering , that is , heating the polymer to a temperature just below the melting point for a long time , increases the crystallinity. The polymer chains can then arrange themselves.

Typically, crystallinities of 10 to 80% are used industrially. Achieving higher crystallinities is only possible with low molecular weight materials or specially tempered samples. In the former case, this makes the material brittle , in the latter case the annealing is too expensive for use. Crystallinities below 10% lead to too high a tendency to creep if the glass transition is below the application temperature .

Methods for determining crystallinity in polymers are

• DSC
• Measurement of density
• Birefringence measurements with the polarizing microscope
• X-ray diffraction
• Infrared spectroscopy or
• Nuclear Magnetic Resonance (NMR).

As a rule, partially crystalline polymers are opaque ; H. cloudy. This is due to the refraction of light due to the different refractive indices of crystalline and amorphous areas. The degree of cloudiness increases with the crystallinity, but also depends on differences in the refractive index. So is z. B. syndiotactic polypropylene is almost completely transparent, while isotactic polypropylene with a comparable crystallinity of about 50% is highly opaque. This can be explained by the different crystal structure of these two modifications .

See also

• Spherulite

literature

• Bernd Tieke: Macromolecular Chemistry. An introduction. Wiley-VCH, Weinheim 2000, ISBN 978-3527293643 .