Polymers & Structure

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Polymers & structure (Author: Prof. Dr. G. H. Michler)

Structure and composition of polymers

Molecular Structures

Polymers consist of a large number of identical or different monomer units that are chemically linked together like pearls in a pearl necklace and form giant molecules – macromolecules. Hermann Staudinger (1881–1965) developed this concept as early as 1920 and was awarded the Nobel Prize for Chemistry in 1953 [1]. These substances made up of macromolecules are also known as polymers, although Staudinger never wanted to recognise the term ‘polymers’. Other synonyms are high polymers, plastics and elastomers. They exhibit great variability in molecular and supramolecular structure from around 0.1 nm to 100 µm (or 10^-10 m to 10^-4 m). The molecular or chemical structure of macromolecules is described by three parameters, the so-called ‘3Cs’, constitution, configuration and conformation. The simplest case is a linear-stretched chain of CH₂-sequences (example: polyethylene – abbreviation: (PE)). The number of monomers usually varies between 10³ and 10⁵.

Asymmetric monomers (with side groups or side chains) define the configuration, and the arrangement of side groups describes the tacticity and the side chains the branching (Fig. 1) [2, 3].

Fig. 1:

Configuration of homopolymer chains with side groups – tacticity and short or long side chains – branching

The arrangement of two or more monomers leads to copolymers with different configurations. In copolymers, the monomers follow each other randomly or alternately, while in graft or terpolymers there are various side chains on a main chain (Fig. 2). In block copolymers, blocks of monomers are arranged in linear or star-shaped structures.

Fig. 2:

Configuration of random copolymers

The shape and form of the macromolecules is described by the conformation, whereby the two limiting cases are a random arrangement (random, statistical coil – see Fig. 3) and a parallel arrangement (folding – see Fig. 4).

Other important characterisation parameters are the number N of monomers along a chain and the molecular weight M_w. Usual degrees of polymerisation N of 104 give PE molecular weights of M_w = 280,000 with a stretched length of the chain of 2.5 µm (in comparison: with a magnification of 106, this would be a fibre that is 0.5 mm thick and 2.5 m long). The macromolecules of a polymer usually vary in molecular weight, i.e. in their length, which is expressed in the molecular weight distribution.

Supramolecular structures, morphology

In the supermolecular structure, there are two limiting cases corresponding to the limiting cases of conformation: the amorphous and the crystalline state. Amorphous polymers are characterised by the random molecular arrangement and the complete absence of crystalline structures, as in polystyrene (abbreviation: PS), polymethyl methacrylate (abbreviation: PMMA) and polycarbonate (abbreviation: PC). In a polymer material, the macromolecules are tightly packed with several hundred segments of neighbouring macromolecules in the same area and close interpenetrations – topological, physical links. These so-called entanglements hold the macromolecules together. The entanglements or linking points form a network (responsible for the strength of the polymer materials) with meshes of a somewhat looser material, the so-called free volume (responsible for ductility or toughness) (Fig. 3).

Fig. 3:

Model of an amorphous polymer with entanglements (1) and network meshes (2) between the linkage points: Distance of entanglements d ~ Me1/2, le length of entanglements (Me Entanglement molecular weight)

Semi-crystalline polymers are based on a partial parallelisation of macromolecule sequences (Fig. 4). The characteristic elements of semi-crystalline structures are the crystalline lamellae, the lamellar boundary layers and the interlamellar amorphous regions.

Fig. 4:

Semi-crystalline structures in polymers Left – as they are visible in the electron microscope (see Fig. 5): crystalline bright, boundary layers and amorphous layers hatched; Right – schematic chain arrangements (a, d) crystalline nanoblocks, b) lamellar boundary layers (amorphous), c) interlamellar, amorphous layers, e) defect layers between the crystalline nanoblocks and f) tilted chains within the lamellae

The crystalline lamellae appear bright in the electron microscope image, they are stretched or bent, the amorphous areas appear dark due to selective contrasting (Fig. 5). The broad lamella in the centre of the image is seen from the broad side and the narrow lamellae from the narrow side. Within the lamellae, the dark lines mark amorphous interference layers. The lamellae can be arranged in different ways, in the form of spherulites, in bundle structures or various parallel arrangements. The spherulites can be up to about 100 µm in size, and with the arrangements of lamellae and the internal structure of lamellae, the morphological elements of the semi-crystalline polymers cover the range from 100 µm down to 0.1 nm.

Fig. 5:

Lamellae in flat-on and edge-on position in a selectively contrasted sample of high-density polyethylene (abbreviation: PE-HD) in the transmission electron microscope

Typical spherulites with a good overview of shape, size and distribution can be seen in optical images under polarised light (Fig. 6).

Fig. 6:

Spherulites in low and higher magnification of a low-density polyethylene (abbreviation: PE-LD), light microscope in polarised light

Most of the polymers used are so-called bulk polymers (the crystalline polymers PE, PP and the amorphous polymers PS, PVC and PC) [4]. In addition to varying the molecular structures (the 3 ‘C’), improved properties can be achieved in a variety of ways through polymer modifications:

• Combination of different monomers with defined arrangement of the monomers in copolymers, graft polymers or block copolymers

• Combination of different polymers with variation in composition in polymer blends or mixtures

• Combination of polymers with inorganic particles (see: particle-filled thermoplastics) or fibres (see: fibre-reinforced plastics) in composites or fibre composite polymers.

Such combinations have great potential for materials with significantly improved properties. Future challenges for materials research include the targeted realisation of combinations ranging from microstructural design to ‘molecular design’.

The scientific monographs [5, 6] provide a comprehensive overview of the structural and morphological diversity of plastics.

See also

• Plastics
• Microscopic structure
• Material Science & plastics
• Micromechanics & nanomechanics

References

[1]	Staudinger, H.: Makromolekulare Chemie und Biologie. Wepf & Co., Basel (1947)
[2]	Michler, G. H.: Werkstoffwissenschaft und Kunststoffe. Schriften der Sudetendeutschen Akademie der Wissenschaften und Künste. Band 43, Forschungsbeiträge der Naturwissenschaftlichen Klasse, Munich (2024) 27–58; see AMK-Library under F 33
[3]	Michler, G. H.: Mechanik–Mikromechanik–Nanomechanik. Vom Eigenschaftsverstehen zur Eigenschaftsverbesserung. SpringerSpektrum (2024), ISBN 978-3-662-66965-5; e-book: ISBN978-3-66966-2; https://doi.org/10.1007/978-3-662-66966-2; see AMK-Library under F 34
[4]	Rätzsch, M., Arnold, M.: Hochpolymere und ihre Herstellung. Fachbuchverlag Leipzig (1973), see AMK-Library under G 13
[5]	Michler, G. H.: Kunststoff-Mikromechanik: Morphologie, Deformations- und Bruchmechanismen; Carl Hanser Munich (1992); ISBN 3-446-17068-5, see AMK-Library under F 4
[6]	Michler, G. H.: Atlas of Polymer Structures: Morphology, Deformation and Fracture Structures, Carl Hanser Munich (2016); ISBN 978-1-56990-557-9 E-Book ISBN 978-1-56990-558-6, see AMK-Library under F 14

Weblinks

• Plastics Europe 2022, https://plasticseurope.org/de/
• Nova-Institut GmbH, Hürth, https://www.nova-institute.eu