Polymer

Polymers (from the Greek poly and meros, “many parts”) are made up of macromolecules that form natural or synthetic substances generally of organic type. The macromolecules that make up polymers are of high size and weight, similar to each other (but not necessarily identical), in turn formed by a repetition of structural units (molecular groups also called repetitive units) equal or different, smaller and joined “chain” through the repetition of the same type of bond (covalent) to form, in fact, the polymer chain. According to the international IUPAC definition, the structural units of the polymer chain are called Constitutional, Repeating Unit (CRU).

The terms repetitive unit and monomer are not synonymous: in fact, a repetitive unit is a part of a molecule or macromolecule, while a monomer is a molecule composed of a single repetitive unit. Therefore, when we talk about “monomers” we mean the reagents from which the polymer is formed through the polymerization reaction, while the term “repetitive units” means the molecular groups that make up the polymer (which is the product of the polymerization reaction).

Although strictly speaking also the typical macromolecules of living systems (proteins, nucleic acids, polysaccharides) are polymers, in the field of industrial chemistry the term “polymers” is commonly used to refer to macromolecules of synthetic origin: plastics, synthetic rubbers and textile fibers (e.g. nylon), but also biocompatible synthetic polymers widely used in the pharmaceutical, cosmetic and food industries, including polyethylene glycols (PEGs), polyacrylates and synthetic polyamino acids. The most important inorganic polymers are silicon-based (colloidal silica, silicones, polysilanes).

Alongside the Chemistry of Polymeric Materials has also developed the Science and Technology of Polymeric Materials, a discipline that studies the correlation between the structure and properties of polymers and their practical application, based both on the laws governing physics, chemistry and engineering, and on the technological principles applicable to the treatment of metals. The chemistry of polymers and macromolecules is a multidisciplinary science that studies chemical synthesis and the chemical and physicochemical properties of polymers and macromolecules. According to IUPAC guidance, the term “macromolecule” refers to high molecular weight compounds, which can be characterized either by repetition of monomeric units or by the presence of a macrocycle.

A real revolution was given by the discovery of the stereospecific polymerization, through the use of catalysts, with the obtainment of polymers with very high steric regularity. The structures of proteins and DNA double helix are also clarified, laying the foundation for the understanding of the genetic code.

Polymer nomenclature

IUPAC (International Union of Pure and Applied Chemistry) and CAS (Chemical Abstracts Service) have agreed on a set of guidelines for the identification, orientation, and naming of polymers based on the structural repeat unit (SRU). IUPAC names polymers as “poly” (constitutional repeat units) while CAS uses “poly” (structural repeat units). These two approaches generally yield similar results. Typically, polymers are named using the name of the monomer from which they are formed preceded by the prefix poly. For example: polyethylene (made from ethylene) and polypropylene (made from propylene).

If the name of the monomer is composed of more than one name, then it should be put in parenthesis as in poly(vinyl chloride). To represent a polymer it is necessary to know, in addition to the monomer, the monomeric unit (what is left of the monomer inside the polymer) and the repeating unit, that is the structure that is repeated “n” times in the polymer. The polymer is represented by placing the repeating unit in square brackets with the letter “n” in subscript.

Polymer properties

The properties of polymers depend on their microstructure, with particular reference to the geometric arrangement in space of the constituent monomers along the polymer chain; consequently: mechanical strength, toughness, hardness, ductility, corrosion resistance, behavior at different temperatures, fatigue strength, etc. are all very variable properties depending on the type of polymer.

Salient chemical properties of polymers are: the degree of polymerization, the average molecular weight, the tacticity, the monomeric succession in copolymers, the degree of branching, the terminal groups, the presence of cross-links. Other properties to be considered are chemical-physical ones such as crystallinity, melting point and glass transition temperature; moreover, for polymers in solution, solubility, viscosity and tendency to gel are also considered. In the study and characterization of polymers, as for the determination of shape, size and average molecular weight, are commonly used techniques such as: X-ray diffraction, osmometry, laser light scattering, viscosimetry, ultracentrifugation, gel permeation chromatography and MALDI-TOF spectrometry. Also widely used in this field, particularly for biological polymers, is electrophoresis.

The multiplicity of products obtainable by combining different polymers with different additives is therefore very high. Attention has been focused on some special properties that these charged polymers can assume such as thermal and/or electrical conductivity (traditionally associated with metals) and eco-compatibility. Polymers, traditionally considered as insulating materials, become today capable of transferring heat from a hot source to a colder one, similarly to what happens with metals, although not to the same extent. However, they have all the advantages linked to the production methods of plastic materials, so that, in some applications, they can represent an excellent substitute for traditional thermal conductors.

Depending on the type of filler used, it is possible to obtain thermally conductive polymers in both electrically insulating and conductive versions; polymer composites are available on the market mainly based on polypropylene (PP), polyphenylene sulfide (PPS) and polyamide (PA).

Thanks to their thermal conductivity, they are used in all those applications where it is necessary to allow easy heat dissipation, thus creating effective cooling systems. They are therefore a valid solution for the production of components for the electrical and electronic sector – encapsulation, overmoulding of coils, winding support, heat sinks for electric motors, circuits, processors and lamps – as well as systems for the thermo-technical sector, such as, for example, heat exchangers to be used in chemically aggressive working environments, where the use of metallic materials can often be problematic.

The growing attention to renewable energy sources, energy saving and recovery of waste materials means that polymers filled with natural-based materials have developed strongly in recent years. In particular, these are composites based on polymer (mainly Polyethylene, Polypropylene and PVC) and powder or wood fiber fillers, whose growing use is justified by the great advantage of being able to use traditional and typical production processes of plastics processing, such as injection, extrusion and hot compression, combined with that of obtaining products with unique characteristics. In these cases the thermoplastic often acts as a “binder” for the wood fiber filler since the polymer/charge percentage can reach a value of 20/80 and therefore the final product is mainly made of filler and not of plastic matrix.

Classification of polymers

Based on their origin, polymers can be classified into: natural, man-made and synthetic. Over time, polymer chemistry has gradually shifted its focus towards the production of fully synthetic polymers from low molecular weight materials. In particular, synthetic polymers represent today a large part of the materials to be processed; they can be classified, in turn, according to the type of reaction used in their synthesis, the type of growth of the macromolecules, the structure of the chains (depending on their structure, they can be classified in linear, branched or cross-linked polymers), the most important properties they manifest. Synthetic polymers find application in the most diverse sectors: from elastomers for the production of tires, to polymers used as insulators or components in the electrical-electronic sector, in automotive components, in packaging and medical devices.

Therefore, to define a polymer it is necessary to know:

  • the nature of the repeating unit;
  • the nature of the terminal groups;
  • the presence of branching and/or cross-linking;
  • any defects in the structural sequence that may alter the mechanical properties of the polymer.

Classification according to the stress-strain diagram

In relation to the properties of mechanical deformation, each polymeric material, as a result of a stress responds with a deformation, which is associated with a greater or lesser percentage elongation; they are differentiated into: fibers, thermosetting polymers, thermoplastic polymers and elastomers.

From the stress-strain diagram it is possible to obtain the following parameters:

  • modulus of elasticity: increases as the crystallinity of the polymer increases;
  • percentage elongation at break: decreases with the increase of the polymer crystallinity;
  • tensile stress at break: increases with increasing crystallinity of polymer;
  • yield strength: increases with increasing polymer crystallinity.

Classification of polymers by structure and configuration

The structure of polymers is defined at various levels, all interdependent and decisive in forming the rheological properties of the polymer, on which depend the applications and industrial uses. Even if two polymers are formed from the same monomer, they can have very different properties depending on the mode of polymerization, in fact chains with different structure give the polymer different physical characteristics. The chain structure of a polymer can be linear, branched or cross-linked.

Polymers with linear chain are the simplest ones, and they are formed by an ordered succession of monomeric units that can assume different conformations and therefore the chain can be arranged in a straight way or it can be folded several times on itself. This influences the way in which the chains can be packed: tangled chains produce a disorderly situation and the polymer is called amorphous, straight chains produce an orderly packing with the chains arranged in parallel and the polymer is called crystalline.

Most polymers, of course, are in intermediate situations between the two extremes and have disordered areas interspersed with more ordered, crystalline ones, which are called crystallites. The intensity of intermolecular forces between chains depends on the percentage of crystallinity. A highly crystalline polymer has the chains packed more efficiently, so it has a higher density and will be more rigid and heat resistant. An example of a linear polymer is Teflon, which is made up of tetrafluoroethylene. It is a single strand of units made up of two carbon atoms and four fluorine atoms.

Polymers with branched chains are characterized by additional small chains of monomeric units that protrude (branch off) from the main chain. These branches are known as side chains and can also be very long groups of repeating structures. These branches decrease crystallinity because they hinder the orderly packing of the chains and thus make the polymer less dense and worsen its mechanical and heat resistance properties.

Branched polymers can be further classified based on the way they branch off the main chain. Polymers with many branches are known as dendrimers. Branched polymers are often obtained by polyaddition reactions, which originate side chains that are longer or shorter than the main high molecular weight chain. Branched polymers can be “comb” or “dendridic” in structure if the branching and has a well-defined symmetry.

Polymers with cross-linked chains have the chains joined together by bridges that create a three-dimensional network structure that gives the polymer greater rigidity and great resistance to heat that does not allow it to melt. Cross-linking can occur during polymerization using di-, tri- or tetra-functional monomers, such as in the case of phenolic resins. The bridges between the chains can be made by the same components of the polymer, as in the phenol formaldehyde resin, or by special hardening agents such as diethylenetriamine in epoxy resins or finally they can be made of atoms other than carbon such as sulfur which is used in the vulcanization of rubber.

Finally, the mechanical properties of a polymer are also influenced by its configuration. When, during the polymerization reaction, asymmetric carbons are formed, they generally assume random configurations and so attactic polymers are formed, i.e. without tax or steric regularity. The first to obtain an isotactic (stereoregular) polymer was Giulio Natta who, in 1954, synthesized isotactic polypropylene (i.e. with all chiral centers of the same configuration) with a coordinated anionic polyaddition reaction using an innovative catalyst called Ziegler-Natta.

The parameter that most influences the mechanical properties of cross-linked polymers is the density of cross-linked nodes. When the number of nodes is high, we can talk about three-dimensional structure (network). With a high number of cross-linking nodes, polymers with high stiffness and dimensional stability are obtained, which are maintained even in the presence of mechanical stresses.

Syndiotactic polypropylene, i.e., with the chiral centers alternating in a regular R-S-R-S manner, is obtained by conducting the reaction at a very low temperature. The excellent mechanical properties of isotactic polypropylene arise from the fact that its chains tend to wrap around each other in a helical fashion and can be tightly packed only if the helical structure is regular and straight. This also happens with proteins (which are formed by L-amino acids): the presence of even a single R-amino acid interrupts the regularity of the alpha-helix. This fact can also be understood by imagining to build a spiral staircase that goes down always turning to the left. If we insert by mistake a section of staircase that turns right this would make the staircase deviate off axis.

According to the chemical structure, excluding the functional groups directly involved in the polymerization reaction, any other functional groups present in the monomer retain their chemical reactivity also in the polymer. In the case of biological polymers (proteins) the chemical properties of the groups arranged along the polymer chain (with their affinities, attractions and repulsions) become essential to model the three-dimensional structure of the polymer itself, a structure on which the biological activity of the protein itself depends.

Based on the stereochemical structure, the absence or presence of a regularity in the position of the side groups of a polymer with respect to the main chain has a significant effect on the rheological properties of the polymer and consequently on its possible industrial applications. A polymer whose side groups are distributed in no particular order is less likely to form crystalline regions than one that is stereochemically ordered. A polymer whose side groups are all on the same side of the main chain is called isotactic, one whose groups are alternated regularly on both sides of the main chain is called syndiotactic and one whose side groups are randomly positioned attactic.

The discovery of a catalyst able to guide the polymerization of propylene in order to give an isotactic polymer was worth the Nobel prize to Giulio Natta. The industrial importance is considerable, isotactic polypropylene is a rigid plastic, attactic polypropylene a rubber with almost no practical applications. Two new classes of polymers are comb polymers and dendrimers.

Classification according to molecular weight

Polymers (as opposed to molecules with low molecular weight or proteins) do not have a defined molecular weight (because they are not formed by all the same molecules), but variable in relation to the length of the polymer chain that constitutes them. Each macromolecule is characterized by a certain molecular weight. During the polymerization process, not all polymer chains will have the same length: this results in a distribution of molecular weights. Therefore an average molecular weight is considered. The value of the average molecular weight influences the mechanical properties of the polymer, which, in general, improve as it increases.

Lots of polymers are characterized by a typical parameter of these macromolecular substances, namely the polydispersity index (PI), which takes into account the distribution of molecular weights referable to a synthesis. It also makes use of the degree of polymerization, which indicates the number of repetitive units constituting the polymer, and which can be:

  • low: under 100 repetitive units;
  • medium: between 100 and 1000 repetitive units;
  • high: over 1000 repetitive units.

From the degree of polymerization depend the physical and rheological properties of the polymer, as well as possible applications. In the case in which the degree of polymerization is very low we speak more properly of oligomer (from the Greek “oligos-“, “few”).

Polymer morphology

The morphology of polymers is very different from that of organic molecules in the solid state. Polymers simultaneously exhibit the characteristics of amorphous or semi-crystalline solids. The terms crystalline and amorphous phase are used to denote the ordered and disordered regions present in a solid polymer matrix. The morphology of crystalline domains was first described with the fringed-micelle theory (fringed-micelle model) developed in 1930. This theory considers polymers in the solid state to be formed by ordered domains called crystallites immersed in a continuous phase, called an amorphous polymer matrix.

The term “crystallites” refers to crystalline domains formed by crystals that are imperfect due to the nature and complexity of macromolecules. Since polymer chains are longer than 100 Å, it was hypothesized that the chains themselves would fold back and forth during crystallization, this folding would be the best compromise to form crystal structures with high stability.

In the following years another model was thought to indicate the existence of defects in the polymer chains that are not able to maintain the regularity of the folds. In fact one of the reasons why crystallinity in polymers reaches values of 60-65% at most can be attributed to these imperfections or defects that reduce the possibility of packing in the formation of flakes. Crystal formation occurs in various cores and crystal growth occurs in radial form.

Spherulites have a shape that is dependent on the conditions of crystallization. The core of the spherulite grows into a single crystal and multilayer structures are originated from it, forming lamellar fibrils. The lamellar fibrils diverge, distort and branch out from the core.

The crystalline state of polymers is characterized by three-dimensional ordering of at least some of the chains, regardless of the details of the structure. They are known as semi-crystalline since they crystallize only partially giving a mixed structure characterized by a crystalline melting temperature Tf and a glass transition temperature Tg always much lower than the melting temperature.

The degree of crystallinity of polymers, which because of their regular structure and the flexibility of their chains, have a greater tendency to crystallize, depends on the conditions of crystallization. A crystalline resin when it solidifies its chains tries to form structures called crystallites. Crystalline resins are opaque because there is no space for light to pass between the molecules.

  • polyamides
  • polyethylene
  • polypropylene
  • polyacetal
  • polyethylene terephthalate

In contrast to amorphous resins, crystalline resins have a good shrinkage percentage, which accounts for some considerations in their processing to achieve good dimensional stability and avoid problems such as impact and deformation due to shrinkage. In general, the degree of crystallinity:

  • is very high in polymers with unbranched linear structure
  • decreases drastically for strongly
  • asymmetric, strongly branched, or with frequent cross-links, i.e., cross-linked

A high degree of crystallinity determines a greater packing of the chains with a consequent increase in density, stiffness, hardness, resistance to friction, wear, environmental aggression and creep, i.e. a slow and progressive deformation of the material subjected to a constant stress.

The difference between the crystallinity of low molecular weight compounds and that of high molecular weight compounds consists essentially in the fact that, while in the former, the elemental cell generally includes more molecules or more ions or more atoms (depending on whether it is a molecular solid, or an ionic or metallic element), in polymers, the same molecule includes more elemental cells and sometimes even more crystals; in fact, since the crystallinity of polymers is never complete but partial, and therefore crystalline regions alternate with amorphous areas, generally the same molecule includes crystalline and amorphous parts, in several successive sections. This leads to a marked irregularity of the crystals, which are generally referred to as crystallites; therefore, for a polymer, the degree of crystallinity is defined as the ratio of the weight of a substance in crystalline form to the total weight and is generally expressed as a percentage.

Degrees of crystallinity range from zero or a few units, in many polymers, to very high percentages for polymers with a linear, unbranched structure. The degree of crystallinity also depends on the same type of mechanical thermal history configuration of the sample. Especially in the case of long linear chains, lamination, stretching and extrusion, especially when carried out at appropriate temperatures, at which the mobility of macromolecules is high, tend to be oriented parallel to each other. Remaining at a sufficiently high temperature (annealed), favoring macrobrownian movements of macromolecules, on the other hand tends to disorient them, while cooling, especially if abrupt (tempered), tends to stabilize the pre-existing structure, whether it is mainly amorphous or disordered.

The parts of the macromolecule that are part of the crystalline zones are not naturally free to move as those of the amorphous parts and therefore their conformation remains determined, although the individual elements may oscillate harmoniously around a point that is assumed to be representative of the position of the element of the crystal itself. The degree of crystallinity is defined as the percentage by weight of the substance in the crystalline state to the total weight. It depends on:

  • structure of the component molecules;
  • mechanical and thermal history of the substance.
  • Acrylic and methacrylic polymers
  • Thermoplastic (thermoplastic polymer)
  • Thermosetting polymer
  • Allyl polymer
  • Amine polymer
  • Heterocyclic polymer
  • Fluoropolymer

Leave a Comment