Spherulite (polymer physics)

The term spherulite (from Greek σφαῖρα sphaira, ball ' and Greek λίθος líthos ' stone' ) generally refers to a spherical or strahliges crystal aggregate. In polymer physics is known as a typical spherulite for thermoplastics spherical superstructure unit, are arranged radially symmetric in the crystallites and solidly connected amorphous intermediate areas. The size and the number of spherulites in a workpiece influenced quite considerably its mechanical and optical properties.

Formation

Spherulites can form during cooling of melts of thermoplastics. The tendency to crystallization of the polymer type, i.e., the arrangement of the atoms or functional groups in the molecular chain depends. Examples of polymers which can form spherulitic, include polyamide 6, polypropylene (PP), polyoxymethylene (POM ), polyethylene (PE) or polybutylene terephthalate ( PBT). In these polymers, the molecular chains can be arranged regularly with each other and thus form crystallites.

The crystalline structures are preferentially formed of nuclei and grow from its center uniformly in all directions outward. Therefore form spherical, radially symmetrical arrangements. Whether the formation of spherulites and how big they are depends on the type of polymer and the cooling conditions in the melt from.

At slow cooling rate less spherulites form. At the same time they have but a lot of time to grow and are therefore relatively large. With rapid cooling crystallization in many places uses the same time. Since the crystallization temperature is below faster if rapid cooling, the spherulites remain comparatively small. For some polymers such as polyamide (PA) or polybutylene terephthalate (PBT ) can cause a relatively rapid cooling at the surface to form a layer with lower crystallinity or even amorphous structures in the edge region.

Spherulites can only grow as long as they are surrounded by amorphous material. If the spherulites so large that they touch each other, so they can not expand further in this direction. It then arise between the spherulites flat surfaces.

A decisive influence on the spherulite have foreign substances and impurities. They can act as a nucleating agent, thus ensuring an increased spherulite. In practice, therefore, the polymer can be added to the part of a nucleating agent in order to significantly accelerate the crystallization. At the same time, the solidification occurs at a higher temperature, which is beneficial to process times in injection molding. Typical concentrations of nucleating agents are at 0.1-0.5 %. Insoluble inorganic fillers as a nucleating agent such as metal oxides, metal salts, silicates or boron nitride are commonly mixed with particle sizes of from about 3 microns to the polymer. , Fillers and reinforcing agents and coloring agents can act nucleating. There are also so-called " clarifier " described nucleating agents, which are dissolved in the melt. The nucleation density here is orders of magnitude higher than that of non-dissolved additives, so that, visually much more transparent materials. Nucleating agents are usually found empirically, and optimized for a particular polymer, i.e., a nucleating agent for polypropylene, for example, do not necessarily work for polyethylene.

Design and Structure

Spherulites are not themselves crystals in the crystallographic sense, but represent aggregates ( clusters ) of very many, smaller crystalline regions dar. This could be detected by X-ray diffraction for individual materials. The size of the spherulites alone indicates neither about the crystallinity of the material ( fraction crystalline to amorphous ) or the size of the actual crystals. The spherulite size is rather an indication of the crystallization conditions in the polymer.

The crystallites are arranged radially symmetrically around the center. X-ray diffraction experiments of very small areas have shown that this procedure the polymer chains are arranged more or less tangentially into the spherulites. The mechanism of growth in which are gradually deposited sequentially chains alongside, would correspond to the mechanism of crystallization of short chain paraffins. The parallel arrangement of the chains occurs in the crystals to the birefringent properties ( form birefringence ), that is, the refractive index in the radial direction is different from the tangential direction.

In addition to spherulitic structures are in some polymers (eg polypropylene) and dendritic superstructures known. They form when a strong temperature gradient is present in the sample. The crystallization begins in the cooler areas and the crystalline areas grow in the direction of the areas with a higher temperature. This leads to directed, dendritic crystal growth.

Effects

Spherulites affect the thermal properties of the polymer ( for example, melting point, heat resistance, shrinkage), the mechanical strength, and partially also the chemical resistance and optical properties.

The crystalline fractions are harder, more brittle and have a higher density, while the amorphous portions are more ductile and less dense and take on the task of elasticity in the component. They also have a higher melting point, which leads to better heat resistance of the component.

The spherulites differ in the optical properties of the amorphous regions. Since they are much smaller than the wavelength of visible light only in rare cases, semi-crystalline polymeric materials generally appear milky to opaque (opaque ).

Depending on the time, crashing to the spherulites produces very firm or even no connections, so that the separation surfaces between the spherulites can form distinct structural weaknesses. Large spherulites are therefore not desirable. To avoid them by nucleation and / or hypothermia, so that many spherulites grow simultaneously.

A decrease in the spherulite size by increasing nucleation leads to a greater bending modulus and increased yield strength and a lower elongation at break and ductility. Through a nucleation ( higher proportion of nucleating agents ), the heat resistance is increased.

Proof

Since spherulites contain crystalline regions and thus are birefringent, they can be detected using polarized light microscopy. The appearance is different depending on the polymer used. Usually, it can be recognized on the basis of the typical pattern ( ' Maltese cross ' ) the dark bars are aligned parallel to the polarization direction of polarizer and analyzer of the microscope. If you turn the object, the orientation of the contrast still remains in the same direction in space exist, so do not rotate with the sample.

The spherulite diameter means the largest diameter of the three -dimensional spherulites. The light- microscopically detectable size is between 1 micron and several 100 microns. For very small spherulites the pattern described above in the microscope is no longer recognizable. It recognizes only a diffuse scattering of light.