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The structural properties of a polymer relate to the physical arrangement of monomer residues along the backbone of the chain. Structure has a strong influence on the other properties of a polymer. For example, a linear chain polymer may be soluble or insoluble in water depending on whether it is composed of polar monomers (such as ethylene oxide) or nonpolar monomers (such as styrene). On the other hand, two samples of natural rubber may exhibit different durability, even though their molecules comprise the same monomers. Polymer scientists have developed terminology to describe precisely both the nature of the monomers as well as their relative arrangement.

Monomer identity

The identity of the monomers comprising the polymer is generally the first and most important attribute of a polymer. The repeat unit is the constantly repeated unit of the chain and is also characteristic of the polymer. Polymer nomenclature is generally based upon the type of monomers comprising the polymer. Polymers that contain only a single type of monomer are known as homopolymers, while polymers containing a mixture of monomers are known as copolymers. Poly(styrene), for example, is composed only of styrene monomers, and is therefore classified as a homopolymer. Ethylene-vinyl acetate, on the other hand, contains more than one variety of monomer and is thus a copolymer. Some biological polymers are composed of a variety of different but structurally related monomers, such as polynucleotides composed of nucleotide subunits.

The repeating unit of the polymer may be different from the starting monomer(s), for example in condensation polymerization. A simple example is PET polyester. The monomers are terephthalic acid (HOOC-C6H4-COOH) and ethylene glycol (HO-CH2-CH2-OH) but the repeating unit is (-OC-C6H4-COO-CH2-CH2-O-), which corresponds to the combination of the two monomers with the loss of two water molecules.

A polymer molecule containing ionizable subunits is known as a polyelectrolyte. An ionomer is a subclass of polyelectrolyte with a low fraction of ionizable subunit.

Polymer properties

Types of polymer properties can be broadly divided into several categories based upon scale. At the nano-micro scale there are properties that directly describe the chain itself, and can be thought of as polymer structure. At an intermediate mesoscopic level there are properties that describe the morphology of the polymer matrix in space. At the macroscopic level properties describe the bulk behavior of the polymer.

The bulk properties of a polymer are those most often of end-use interest. These are the properties that dictate how the polymer actually behaves on a macroscopic scale.

Relationship between chain length and polymer properties

Polymer bulk properties are strongly dependent upon their structure and mesoscopic behavior. A number of qualitative relationships between structure and properties are known.

Increasing chain length tends to decrease chain mobility, increase strength and toughness, and increase the glass transition temperature (Tg). This is a result of the increase in chain interactions such as Van der Waals attractions and entanglements that come with increased chain length. These interactions tend to fix the individual chains more strongly in position and resist deformations and matrix breakup, both at higher stresses and higher temperatures. Chain length is related to melt viscosity roughly as 1:103.2, so that a tenfold increase in polymer chain length results in a viscosity increase of over 1000 times.