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A polymer (; Greek '' poly-'', "many" + ''
-mer {{Short pages monitor The structure of a polymeric material can be described at different length scales, from the sub-nm length scale up to the macroscopic one. There is in fact a hierarchy of structures, in which each stage provides the foundations for the next one. The starting point for the description of the structure of a polymer is the identity of its constituent monomers. Next, the microstructure essentially describes the arrangement of these monomers within the polymer at the scale of a single chain. The microstructure determines the possibility for the polymer to form phases with different arrangements, for example through
crystallization Crystallization is the process by which solid forms, where the atoms or molecules are highly organized into a structure known as a crystal. Some ways by which crystals form are precipitating from a solution, freezing, or more rarely deposi ...
, the
glass transition The glass–liquid transition, or glass transition, is the gradual and reversible transition in amorphous materials (or in amorphous regions within semicrystalline materials) from a hard and relatively brittle "glassy" state into a viscous or rubb ...
or microphase separation. These features play a major role in determining the physical and chemical properties of a polymer.


Monomers and repeat units

The identity of the repeat units (monomer residues, also known as "mers") comprising a polymer is its first and most important attribute. Polymer nomenclature is generally based upon the type of monomer residues comprising the polymer. A polymer which contains only a single type of repeat unit is known as a homopolymer, while a polymer containing two or more types of repeat units is known as a
copolymer In polymer chemistry, a copolymer is a polymer derived from more than one species of monomer. The polymerization of monomers into copolymers is called copolymerization. Copolymers obtained from the copolymerization of two monomer species are some ...
. A terpolymer is a copolymer which contains three types of repeat units.
Polystyrene Polystyrene (PS) is a synthetic polymer made from monomers of the aromatic hydrocarbon styrene. Polystyrene can be solid or foamed. General-purpose polystyrene is clear, hard, and brittle. It is an inexpensive resin per unit weight. It is a ...
is composed only of styrene-based repeat units, and is classified as a homopolymer.
Polyethylene terephthalate Polyethylene terephthalate (or poly(ethylene terephthalate), PET, PETE, or the obsolete PETP or PET-P), is the most common thermoplastic polymer resin of the polyester family and is used in fibres for clothing, containers for liquids and foods ...
, even though produced from two different
monomer In chemistry, a monomer ( ; ''mono-'', "one" + '' -mer'', "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or three-dimensional network in a process called polymerization. Classification Mo ...
s (
ethylene glycol Ethylene glycol (IUPAC name: ethane-1,2-diol) is an organic compound (a vicinal diol) with the formula . It is mainly used for two purposes, as a raw material in the manufacture of polyester fibers and for antifreeze formulations. It is an odo ...
and
terephthalic acid Terephthalic acid is an organic compound with formula C6H4(CO2H)2. This white solid is a commodity chemical, used principally as a precursor to the polyester PET, used to make clothing and plastic bottles. Several million tonnes are produced annua ...
), is usually regarded as a homopolymer because only one type of repeat unit is formed.
Ethylene-vinyl acetate Ethylene-vinyl acetate (EVA), also known as poly (ethylene-vinyl acetate) (PEVA), is the copolymer of ethylene and vinyl acetate. The weight percent of vinyl acetate usually varies from 10 to 40%, with the remainder being ethylene. There are th ...
contains more than one variety of repeat unit and is a copolymer. Some biological polymers are composed of a variety of different but structurally related monomer residues; for example, polynucleotides such as DNA are composed of four types of
nucleotide Nucleotides are organic molecules consisting of a nucleoside and a phosphate. They serve as monomeric units of the nucleic acid polymers – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both of which are essential biomolecules wi ...
subunits. : A polymer containing ionizable subunits (e.g., pendant carboxylic groups) is known as a polyelectrolyte or
ionomer An ionomer () ('' iono-'' + ''-mer'') is a polymer composed of repeat units of both electrically neutral repeating units and ionized units covalently bonded to the polymer backbone as pendant group moieties. Usually no more than 15 mole percent ...
, when the fraction of ionizable units is large or small respectively.


Microstructure

The microstructure of a polymer (sometimes called configuration) relates to the physical arrangement of monomer residues along the backbone of the chain. These are the elements of polymer structure that require the breaking of a covalent bond in order to change. Various polymer structures can be produced depending on the monomers and reaction conditions: A polymer may consist of linear macromolecules containing each only one unbranched chain. In the case of unbranched polyethylene, this chain is a long-chain ''n''-alkane. There are also branched macromolecules with a main chain and side chains, in the case of polyethylene the side chains would be
alkyl groups In organic chemistry, an alkyl group is an alkane missing one hydrogen. The term ''alkyl'' is intentionally unspecific to include many possible substitutions. An acyclic alkyl has the general formula of . A cycloalkyl is derived from a cycloa ...
. In particular unbranched macromolecules can be in the solid state semi-crystalline, crystalline chain sections highlighted red in the figure below. While branched and unbranched polymers are usually thermoplastics, many
elastomer An elastomer is a polymer with viscoelasticity (i.e. both viscosity and elasticity) and with weak intermolecular forces, generally low Young's modulus and high failure strain compared with other materials. The term, a portmanteau of ''elastic p ...
s have a wide-meshed cross-linking between the "main chains". Close-meshed crosslinking, on the other hand, leads to
thermosets In materials science, a thermosetting polymer, often called a thermoset, is a polymer that is obtained by irreversibly hardening (" curing") a soft solid or viscous liquid prepolymer (resin). Curing is induced by heat or suitable radiation and ...
. Cross-links and branches are shown as red dots in the figures. Highly branched polymers are amorphous and the molecules in the solid interact randomly. :


Polymer architecture

An important microstructural feature of a polymer is its architecture and shape, which relates to the way branch points lead to a deviation from a simple linear chain. A
branched polymer In polymer chemistry, branching is the regular or irregular attachment of side chains to a polymer's backbone chain. It occurs by the replacement of a substituent (e.g. a hydrogen atom) on a monomer subunit by another covalently-bonded ch ...
molecule is composed of a main chain with one or more substituent side chains or branches. Types of branched polymers include
star polymer Star-shaped polymers are the simplest class of branched polymers with a general structure consisting of several (at least three) linear chains connected to a central core. The core, or the center, of the polymer can be an atom, molecule, or macr ...
s,
comb polymers In polymer chemistry, graft polymers are segmented copolymers with a linear backbone of one composite and randomly distributed branches of another composite. The picture labeled "graft polymer" shows how grafted chains of species B are covalent ...
, polymer brushes,
dendronized polymer Dendronized polymers (or dendronised polymers) are linear polymers to every repeat unit of which dendrons are attached. Dendrons are regularly branched, tree-like fragments and for larger ones the polymer backbone is wrapped to give sausage-like, cy ...
s,
ladder polymer In chemistry, a ladder polymer is a type of double stranded polymer with the connectivity of a ladder. In a typical one-dimensional polymer, e.g. polyethylene and polysiloxanes, the monomers form two bonds, giving a chain. In a ladder polymer the ...
s, and dendrimers. There exist also
two-dimensional polymer A two-dimensional polymer (2DP) is a sheet-like monomolecular macromolecule consisting of laterally connected repeat units with end groups along all edges. This recent definition of 2DP is based on Hermann Staudinger's polymer concept from the 1 ...
s (2DP) which are composed of topologically planar repeat units. A polymer's architecture affects many of its physical properties including solution viscosity, melt viscosity, solubility in various solvents, glass-transition temperature and the size of individual polymer coils in solution. A variety of techniques may be employed for the synthesis of a polymeric material with a range of architectures, for example living polymerization.


Chain length

A common means of expressing the length of a chain is the
degree of polymerization The degree of polymerization, or DP, is the number of monomeric units in a macromolecule or polymer or oligomer molecule. For a homopolymer, there is only one type of monomeric unit and the ''number-average'' degree of polymerization is given by ...
, which quantifies the number of monomers incorporated into the chain.Rubinstein, p. 3 As with other molecules, a polymer's size may also be expressed in terms of
molecular weight A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioch ...
. Since synthetic polymerization techniques typically yield a statistical distribution of chain lengths, the molecular weight is expressed in terms of weighted averages. The
number-average molecular weight The molar mass distribution (or molecular weight distribution) describes the relationship between the number of Mole (unit), moles of each polymer species (Ni) and the molar mass (Mi) of that species. In linear polymers, the individual polymer chain ...
(''M''n) and weight-average molecular weight (''M''w) are most commonly reported.Rubinstein, pp. 23–24 The ratio of these two values (''M''w / ''M''n) is the
dispersity In chemistry, the dispersity is a measure of the heterogeneity of sizes of molecules or particles in a mixture. A collection of objects is called uniform if the objects have the same size, shape, or mass. A sample of objects that have an inconsi ...
(''Đ''), which is commonly used to express the width of the molecular weight distribution. The physical properties of polymer strongly depend on the length (or equivalently, the molecular weight) of the polymer chain.Rubinstein, p. 5 One important example of the physical consequences of the molecular weight is the scaling of the
viscosity The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the inte ...
(resistance to flow) in the melt. The influence of the weight-average molecular weight (M_w) on the melt viscosity (\eta) depends on whether the polymer is above or below the onset of
entanglements ''Entanglements'' is the third full-length album from indie rock ensemble Parenthetical Girls. Track listing #"Four Words" - 3:10 #"Avenue of Trees" - 3:17 #"Unmentionables" - 1:51 #"Gut Symmetries" - 3:58 #"A Song for Ellie Greenwich Ele ...
. Below the entanglement molecular weight, \eta \sim ^, whereas above the entanglement molecular weight, \eta \sim ^. In the latter case, increasing the polymer chain length 10-fold would increase the viscosity over 1000 times. Increasing chain length furthermore 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 ''Entanglements'' is the third full-length album from indie rock ensemble Parenthetical Girls. Track listing #"Four Words" - 3:10 #"Avenue of Trees" - 3:17 #"Unmentionables" - 1:51 #"Gut Symmetries" - 3:58 #"A Song for Ellie Greenwich Ele ...
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.


Monomer arrangement in copolymers

Copolymers are classified either as statistical copolymers, alternating copolymers, block copolymers, graft copolymers or gradient copolymers. In the schematic figure below, and symbolize the two repeat units. : *Alternating copolymers possess two regularly alternating monomer residues:Painter, p. 14 . An example is the equimolar copolymer of styrene and maleic anhydride formed by free-radical chain-growth polymerization.Rudin p.18-20 A step-growth copolymer such as Nylon 66 can also be considered a strictly alternating copolymer of diamine and diacid residues, but is often described as a homopolymer with the dimeric residue of one amine and one acid as a repeat unit.Cowie p.104 *Periodic copolymers have more than two species of monomer units in a regular sequence. *Statistical copolymers have monomer residues arranged according to a statistical rule. A statistical copolymer in which the probability of finding a particular type of monomer residue at a particular point in the chain is independent of the types of surrounding monomer residue may be referred to as a truly random copolymer.Painter, p. 15 For example, the chain-growth copolymer of
vinyl chloride Vinyl chloride is an organochloride with the formula H2C=CHCl. It is also called vinyl chloride monomer (VCM) or chloroethene. This colorless compound is an important industrial chemical chiefly used to produce the polymer polyvinyl chloride (PVC ...
and vinyl acetate is random. *Block copolymers have long sequences of different monomer units. Polymers with two or three blocks of two distinct chemical species (e.g., A and B) are called diblock copolymers and triblock copolymers, respectively. Polymers with three blocks, each of a different chemical species (e.g., A, B, and C) are termed triblock terpolymers. *Graft or grafted copolymers contain side chains or branches whose repeat units have a different composition or configuration than the main chain. The branches are added on to a preformed main chain macromolecule. Monomers within a copolymer may be organized along the backbone in a variety of ways. A copolymer containing a controlled arrangement of monomers is called a
sequence-controlled polymer A sequence-controlled polymer is a macromolecule, in which the sequence of monomers is controlled to some degree. This control can be absolute but not necessarily. In other words, a sequence-controlled polymer can be uniform (its dispersity Ð is ...
. Alternating, periodic and block copolymers are simple examples of
sequence-controlled polymer A sequence-controlled polymer is a macromolecule, in which the sequence of monomers is controlled to some degree. This control can be absolute but not necessarily. In other words, a sequence-controlled polymer can be uniform (its dispersity Ð is ...
s.


Tacticity

Tacticity describes the relative
stereochemistry Stereochemistry, a subdiscipline of chemistry, involves the study of the relative spatial arrangement of atoms that form the structure of molecules and their manipulation. The study of stereochemistry focuses on the relationships between stereois ...
of chiral centers in neighboring structural units within a macromolecule. There are three types of tacticity:
isotactic Tacticity (from el, τακτικός, taktikos, "relating to arrangement or order") is the relative stereochemistry of adjacent chiral centers within a macromolecule. The practical significance of tacticity rests on the effects on the physical p ...
(all substituents on the same side), atactic (random placement of substituents), and syndiotactic (alternating placement of substituents). :


Morphology

Polymer morphology generally describes the arrangement and microscale ordering of polymer chains in space. The macroscopic physical properties of a polymer are related to the interactions between the polymer chains. * Disordered polymers: In the solid state, atactic polymers, polymers with a high degree of branching and random copolymers form
amorphous In condensed matter physics and materials science, an amorphous solid (or non-crystalline solid, glassy solid) is a solid that lacks the long-range order that is characteristic of a crystal. Etymology The term comes from the Greek ''a'' ("wi ...
(i.e. glassy structures).Bernd Tieke: ''Makromolekulare Chemie.'' 3. Auflage, Wiley-VCH, Weinheim 2014, S. 295f (in German). In melt and solution, polymers tend to form a constantly changing "statistical cluster", see freely-jointed-chain model. In the solid state, the respective conformations of the molecules are frozen. Hooking and entanglement of chain molecules lead to a "mechanical bond" between the chains. Intermolecular and intramolecular attractive forces only occur at sites where molecule segments are close enough to each other. The irregular structures of the molecules prevent a narrower arrangement. * Linear polymers with periodic structure, low branching and stereoregularity (e. g. not atactic) have a
semi-crystalline Crystallinity refers to the degree of structural order in a solid. In a crystal, the atoms or molecules are arranged in a regular, periodic manner. The degree of crystallinity has a big influence on hardness, density, transparency and diffusion ...
structure in the solid state. In simple polymers (such as polyethylene), the chains are present in the crystal in zigzag conformation. Several zigzag conformations form dense chain packs, called crystallites or lamellae. The lamellae are much thinner than the polymers are long (often about 10 nm). Wolfgang Kaiser: ''Kunststoffchemie für Ingenieure.'' 3. Auflage, Carl Hanser, München 2011, S. 84. They are formed by more or less regular folding of one or more molecular chains. Amorphous structures exist between the lamellae. Individual molecules can lead to entanglements between the lamellae and can also be involved in the formation of two (or more) lamellae (chains than called tie molecules). Several lamellae form a superstructure, a spherulite, often with a diameter in the range of 0.05 to 1 mm. :The type and arrangement of (functional) residues of the repeat units effects or determines the crystallinity and strength of the secondary valence bonds. In isotactic polypropylene, the molecules form a helix. Like the zigzag conformation, such helices allow a dense chain packing. Particularly strong intermolecular interactions occur when the residues of the repeating units allow the formation of
hydrogen bond In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing a ...
s, as in the case of ''p''-aramid. The formation of strong intramolecular associations may produce diverse folded states of single linear chains with distinct circuit topology. Crystallinity and superstructure are always dependent on the conditions of their formation, see also:
crystallization of polymers Crystallization of polymers is a process associated with partial alignment of their molecular chains. These chains fold together and form ordered regions called Lamella (materials), lamellae, which compose larger spheroidal structures named Spheru ...
. Compared to amorphous structures, semi-crystalline structures lead to a higher stiffness, density, melting temperature and higher resistance of a polymer. * Cross-linked polymers: Wide-meshed cross-linked polymers are
elastomer An elastomer is a polymer with viscoelasticity (i.e. both viscosity and elasticity) and with weak intermolecular forces, generally low Young's modulus and high failure strain compared with other materials. The term, a portmanteau of ''elastic p ...
s and cannot be molten (unlike
thermoplastic A thermoplastic, or thermosoft plastic, is any plastic polymer material that becomes pliable or moldable at a certain elevated temperature and solidifies upon cooling. Most thermoplastics have a high molecular weight. The polymer chains associate ...
s); heating cross-linked polymers only leads to
decomposition Decomposition or rot is the process by which dead organic substances are broken down into simpler organic or inorganic matter such as carbon dioxide, water, simple sugars and mineral salts. The process is a part of the nutrient cycle and is e ...
. Thermoplastic elastomers, on the other hand, are reversibly "physically crosslinked" and can be molten. Block copolymers in which a hard segment of the polymer has a tendency to crystallize and a soft segment has an amorphous structure are one type of thermoplastic elastomers: the hard segments ensure wide-meshed, physical crosslinking.


Crystallinity

When applied to polymers, the term ''crystalline'' has a somewhat ambiguous usage. In some cases, the term ''crystalline'' finds identical usage to that used in conventional
crystallography Crystallography is the experimental science of determining the arrangement of atoms in crystalline solids. Crystallography is a fundamental subject in the fields of materials science and solid-state physics (condensed matter physics). The wor ...
. For example, the structure of a crystalline protein or polynucleotide, such as a sample prepared for
x-ray crystallography X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...
, may be defined in terms of a conventional unit cell composed of one or more polymer molecules with cell dimensions of hundreds of
angstrom The angstromEntry "angstrom" in the Oxford online dictionary. Retrieved on 2019-03-02 from https://en.oxforddictionaries.com/definition/angstrom.Entry "angstrom" in the Merriam-Webster online dictionary. Retrieved on 2019-03-02 from https://www.m ...
s or more. A synthetic polymer may be loosely described as crystalline if it contains regions of three-dimensional ordering on atomic (rather than macromolecular) length scales, usually arising from intramolecular folding or stacking of adjacent chains. Synthetic polymers may consist of both crystalline and amorphous regions; the degree of crystallinity may be expressed in terms of a weight fraction or volume fraction of crystalline material. Few synthetic polymers are entirely crystalline. The crystallinity of polymers is characterized by their degree of crystallinity, ranging from zero for a completely non-crystalline polymer to one for a theoretical completely crystalline polymer. Polymers with microcrystalline regions are generally tougher (can be bent more without breaking) and more impact-resistant than totally amorphous polymers. Polymers with a degree of crystallinity approaching zero or one will tend to be transparent, while polymers with intermediate degrees of crystallinity will tend to be opaque due to light scattering by crystalline or glassy regions. For many polymers, crystallinity may also be associated with decreased transparency.


Chain conformation

The space occupied by a polymer molecule is generally expressed in terms of radius of gyration, which is an average distance from the center of mass of the chain to the chain itself. Alternatively, it may be expressed in terms of pervaded volume, which is the volume spanned by the polymer chain and scales with the cube of the radius of gyration. The simplest theoretical models for polymers in the molten, amorphous state are ideal chains.


Properties

Polymer properties depend of their structure and they are divided into classes according to their physical bases. Many physical and chemical properties describe how a polymer behaves as a continuous macroscopic material. They are classified as bulk properties, or
intensive properties Physical properties of materials and systems can often be categorized as being either intensive or extensive, according to how the property changes when the size (or extent) of the system changes. According to IUPAC, an intensive quantity is one ...
according to
thermodynamics Thermodynamics is a branch of physics that deals with heat, work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed by the four laws of the ...
.


Mechanical properties

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.


Tensile strength

The
tensile strength Ultimate tensile strength (UTS), often shortened to tensile strength (TS), ultimate strength, or F_\text within equations, is the maximum stress that a material can withstand while being stretched or pulled before breaking. In brittle materials t ...
of a material quantifies how much elongating stress the material will endure before failure. This is very important in applications that rely upon a polymer's physical strength or durability. For example, a rubber band with a higher tensile strength will hold a greater weight before snapping. In general, tensile strength increases with polymer chain length and
crosslinking Cross-linking may refer to *Cross-link In chemistry and biology a cross-link is a bond or a short sequence of bonds that links one polymer chain to another. These links may take the form of covalent bonds or ionic bonds and the polymers ca ...
of polymer chains.


Young's modulus of elasticity

Young's modulus Young's modulus E, the Young modulus, or the modulus of elasticity in tension or compression (i.e., negative tension), is a mechanical property that measures the tensile or compressive stiffness of a solid material when the force is applied leng ...
quantifies the
elasticity Elasticity often refers to: *Elasticity (physics), continuum mechanics of bodies that deform reversibly under stress Elasticity may also refer to: Information technology * Elasticity (data store), the flexibility of the data model and the cl ...
of the polymer. It is defined, for small strains, as the ratio of rate of change of stress to strain. Like tensile strength, this is highly relevant in polymer applications involving the physical properties of polymers, such as rubber bands. The modulus is strongly dependent on temperature.
Viscoelasticity In materials science and continuum mechanics, viscoelasticity is the property of materials that exhibit both viscous and elastic characteristics when undergoing deformation. Viscous materials, like water, resist shear flow and strain linearly wi ...
describes a complex time-dependent elastic response, which will exhibit
hysteresis Hysteresis is the dependence of the state of a system on its history. For example, a magnet may have more than one possible magnetic moment in a given magnetic field, depending on how the field changed in the past. Plots of a single component of ...
in the stress-strain curve when the load is removed.
Dynamic mechanical analysis Dynamic mechanical analysis (abbreviated DMA) is a technique used to study and characterize materials. It is most useful for studying the viscoelastic behavior of polymers A polymer (; Greek '' poly-'', "many" + ''-mer'', "part") is a subst ...
or DMA measures this complex modulus by oscillating the load and measuring the resulting strain as a function of time.


Transport properties

Transport properties such as
diffusivity Diffusivity is a rate of diffusion, a measure of the rate at which particles or heat or fluids can spread. It is measured differently for different mediums. Diffusivity may refer to: *Thermal diffusivity, diffusivity of heat *Diffusivity of mass: ...
describe how rapidly molecules move through the polymer matrix. These are very important in many applications of polymers for films and membranes. The movement of individual macromolecules occurs by a process called
reptation A peculiarity of thermal motion of very long linear macromolecules in ''entangled'' polymer melts or concentrated polymer solutions is reptation. Derived from the word reptile, reptation suggests the movement of entangled polymer chains as bein ...
in which each chain molecule is constrained by entanglements with neighboring chains to move within a virtual tube. The theory of reptation can explain polymer molecule dynamics and
viscoelasticity In materials science and continuum mechanics, viscoelasticity is the property of materials that exhibit both viscous and elastic characteristics when undergoing deformation. Viscous materials, like water, resist shear flow and strain linearly wi ...
.


Phase behavior


Crystallization and melting

Depending on their chemical structures, polymers may be either semi-crystalline or amorphous. Semi-crystalline polymers can undergo crystallization and melting transitions, whereas amorphous polymers do not. In polymers, crystallization and melting do not suggest solid-liquid phase transitions, as in the case of water or other molecular fluids. Instead, crystallization and melting refer to the phase transitions between two solid states (''i.e.'', semi-crystalline and amorphous). Crystallization occurs above the glass-transition temperature (''T''g) and below the melting temperature (''T''m).


Glass transition

All polymers (amorphous or semi-crystalline) go through
glass transition The glass–liquid transition, or glass transition, is the gradual and reversible transition in amorphous materials (or in amorphous regions within semicrystalline materials) from a hard and relatively brittle "glassy" state into a viscous or rubb ...
s. The glass-transition temperature (''T''g) is a crucial physical parameter for polymer manufacturing, processing, and use. Below ''T''g, molecular motions are frozen and polymers are brittle and glassy. Above ''T''g, molecular motions are activated and polymers are rubbery and viscous. The glass-transition temperature may be engineered by altering the degree of branching or crosslinking in the polymer or by the addition of plasticizers. Whereas crystallization and melting are first-order
phase transition In chemistry, thermodynamics, and other related fields, a phase transition (or phase change) is the physical process of transition between one state of a medium and another. Commonly the term is used to refer to changes among the basic states of ...
s, the glass transition is not. The glass transition shares features of second-order phase transitions (such as discontinuity in the heat capacity, as shown in the figure), but it is generally not considered a thermodynamic transition between equilibrium states.


Mixing behavior

In general, polymeric mixtures are far less
miscible Miscibility () is the property of two substances to mix in all proportions (that is, to fully dissolve in each other at any concentration), forming a homogeneous mixture (a solution). The term is most often applied to liquids but also applies ...
than mixtures of
small molecule Within the fields of molecular biology and pharmacology, a small molecule or micromolecule is a low molecular weight (≤ 1000 daltons) organic compound that may regulate a biological process, with a size on the order of 1 nm. Many drugs ar ...
materials. This effect results from the fact that the driving force for mixing is usually
entropy Entropy is a scientific concept, as well as a measurable physical property, that is most commonly associated with a state of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields, from classical thermodynam ...
, not interaction energy. In other words, miscible materials usually form a solution not because their interaction with each other is more favorable than their self-interaction, but because of an increase in entropy and hence free energy associated with increasing the amount of volume available to each component. This increase in entropy scales with the number of particles (or moles) being mixed. Since polymeric molecules are much larger and hence generally have much higher specific volumes than small molecules, the number of molecules involved in a polymeric mixture is far smaller than the number in a small molecule mixture of equal volume. The energetics of mixing, on the other hand, is comparable on a per volume basis for polymeric and small molecule mixtures. This tends to increase the free energy of mixing for polymer solutions and thereby making solvation less favorable, and thereby making the availability of concentrated solutions of polymers far rarer than those of small molecules. Furthermore, the phase behavior of polymer solutions and mixtures is more complex than that of small molecule mixtures. Whereas most small molecule solutions exhibit only an
upper critical solution temperature The upper critical solution temperature (UCST) or upper consolute temperature is the critical temperature above which the components of a mixture are miscible in all proportions. The word ''upper'' indicates that the UCST is an upper bound to a tem ...
phase transition (UCST), at which phase separation occurs with cooling, polymer mixtures commonly exhibit a
lower critical solution temperature The lower critical solution temperature (LCST) or lower consolute temperature is the critical temperature below which the components of a mixture are miscible in all proportions. The word ''lower'' indicates that the LCST is a lower bound to a t ...
phase transition (LCST), at which phase separation occurs with heating. In dilute solutions, the properties of the polymer are characterized by the interaction between the solvent and the polymer. In a good solvent, the polymer appears swollen and occupies a large volume. In this scenario, intermolecular forces between the solvent and monomer subunits dominate over intramolecular interactions. In a bad solvent or poor solvent, intramolecular forces dominate and the chain contracts. In the
theta solvent In a polymer solution, a theta solvent (or θ solvent) is a solvent in which polymer coils act like ideal chains, assuming exactly their random walk coil dimensions. Therefore, the Mark–Houwink equation exponent is 1/2 in a theta solvent. Thermo ...
, or the state of the polymer solution where the value of the second virial coefficient becomes 0, the intermolecular polymer-solvent repulsion balances exactly the intramolecular monomer-monomer attraction. Under the theta condition (also called the Flory condition), the polymer behaves like an ideal random coil. The transition between the states is known as a
coil–globule transition In polymer physics, the coil–globule transition is the collapse of a macromolecule from an expanded coil state through an Ideal chain, ideal coil state to a collapsed globule state, or vice versa. The coil–globule transition is of importance ...
.


Inclusion of plasticizers

Inclusion of plasticizers tends to lower Tg and increase polymer flexibility. Addition of the plasticizer will also modify dependence of the glass-transition temperature Tg on the cooling rate. The mobility of the chain can further change if the molecules of plasticizer give rise to hydrogen bonding formation. Plasticizers are generally small molecules that are chemically similar to the polymer and create gaps between polymer chains for greater mobility and fewer interchain interactions. A good example of the action of plasticizers is related to polyvinylchlorides or PVCs. A uPVC, or unplasticized polyvinylchloride, is used for things such as pipes. A pipe has no plasticizers in it, because it needs to remain strong and heat-resistant. Plasticized PVC is used in clothing for a flexible quality. Plasticizers are also put in some types of cling film to make the polymer more flexible.


Chemical properties

The attractive forces between polymer chains play a large part in determining the polymer’s properties. Because polymer chains are so long, they have many such interchain interactions per molecule, amplifying the effect of these interactions on the polymer properties in comparison to attractions between conventional molecules. Different side groups on the polymer can lend the polymer to ionic bonding or hydrogen bonding between its own chains. These stronger forces typically result in higher tensile strength and higher crystalline melting points. The intermolecular forces in polymers can be affected by dipoles in the monomer units. Polymers containing
amide In organic chemistry, an amide, also known as an organic amide or a carboxamide, is a compound with the general formula , where R, R', and R″ represent organic groups or hydrogen atoms. The amide group is called a peptide bond when it is ...
or carbonyl groups can form
hydrogen bond In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing a ...
s between adjacent chains; the partially positively charged hydrogen atoms in N-H groups of one chain are strongly attracted to the partially negatively charged oxygen atoms in C=O groups on another. These strong hydrogen bonds, for example, result in the high tensile strength and melting point of polymers containing urethane or
urea Urea, also known as carbamide, is an organic compound with chemical formula . This amide has two amino groups (–) joined by a carbonyl functional group (–C(=O)–). It is thus the simplest amide of carbamic acid. Urea serves an important r ...
linkages.
Polyester Polyester is a category of polymers that contain the ester functional group in every repeat unit of their main chain. As a specific material, it most commonly refers to a type called polyethylene terephthalate (PET). Polyesters include natural ...
s have dipole-dipole bonding between the oxygen atoms in C=O groups and the hydrogen atoms in H-C groups. Dipole bonding is not as strong as hydrogen bonding, so a polyester's melting point and strength are lower than Kevlar's (
Twaron Twaron (a brand name of Teijin Aramid) is a para-aramid. It is a heat-resistant and strong synthetic fibre developed in the early 1970s by the Dutch company Akzo Nobel's division Enka BV, later Akzo Industrial Fibers. The research name of the para ...
), but polyesters have greater flexibility. Polymers with non-polar units such as polyethylene interact only through weak
Van der Waals force In molecular physics, the van der Waals force is a distance-dependent interaction between atoms or molecules. Unlike ionic or covalent bonds, these attractions do not result from a chemical electronic bond; they are comparatively weak and th ...
s. As a result, they typically have lower melting temperatures than other polymers. When a polymer is dispersed or dissolved in a liquid, such as in commercial products like paints and glues, the chemical properties and molecular interactions influence how the solution flows and can even lead to self-assembly of the polymer into complex structures. When a polymer is applied as a coating, the chemical properties will influence the adhesion of the coating and how it interacts with external materials, such as superhydrophobic polymer coatings leading to water resistance. Overall the chemical properties of a polymer are important elements for designing new polymeric material products.


Optical properties

Polymers such as
PMMA PMMA may refer to: * para-Methoxymethamphetamine, a stimulant drug * Philippine Merchant Marine Academy The Philippine Merchant Marine Academy ( fil, Akademiya sa Bapor Pangkalakalan ng Pilipinas) also referred to by its acronym PMMA) is a ma ...
and HEMA:MMA are used as matrices in the gain medium of
solid-state dye laser Solid-state dye lasers (SSDL) were introduced in 1967 by Soffer and McFarland. In these solid-state lasers, the gain medium is a laser dye-doped organic matrix such as poly(methyl methacrylate) (PMMA), rather than a liquid solution of the dye. An ...
s, also known as solid-state dye-doped polymer lasers. These polymers have a high surface quality and are also highly transparent so that the laser properties are dominated by the laser dye used to dope the polymer matrix. These type of lasers, that also belong to the class of organic lasers, are known to yield very narrow linewidths which is useful for spectroscopy and analytical applications. An important optical parameter in the polymer used in laser applications is the change in refractive index with temperature also known as dn/dT. For the polymers mentioned here the (dn/dT) ~ −1.4 × 10−4 in units of K−1 in the 297 ≤ T ≤ 337 K range.


Electrical properties

Most conventional polymers such as polyethylene are
electrical insulators An electrical insulator is a material in which electric current does not flow freely. The atoms of the insulator have tightly bound electrons which cannot readily move. Other materials—semiconductors and conductors—conduct electric current ...
, but the development of polymers containing π-conjugated bonds has led to a wealth of polymer-based
semiconductor A semiconductor is a material which has an electrical resistivity and conductivity, electrical conductivity value falling between that of a electrical conductor, conductor, such as copper, and an insulator (electricity), insulator, such as glas ...
s, such as polythiophenes. This has led to many applications in the field of
organic electronics Organic electronics is a field of materials science concerning the design, synthesis, characterization, and application of organic molecules or polymers that show desirable electronic properties such as conductivity. Unlike conventional inorga ...
.


Applications

Nowadays, synthetic polymers are used in almost all walks of life. Modern society would look very different without them. The spreading of polymer use is connected to their unique properties: low density, low cost, good thermal/electrical insulation properties, high resistance to corrosion, low-energy demanding polymer manufacture and facile processing into final products. For a given application, the properties of a polymer can be tuned or enhanced by combination with other materials, as in composites. Their application allows to save energy (lighter cars and planes, thermally insulated buildings), protect food and drinking water (packaging), save land and lower use of fertilizers (synthetic fibres), preserve other materials (coatings), protect and save lifes (hygiene, medical applications). A representative, non-exhaustive list of applications is given below. * Clothing, sportswear and accessories:
polyester Polyester is a category of polymers that contain the ester functional group in every repeat unit of their main chain. As a specific material, it most commonly refers to a type called polyethylene terephthalate (PET). Polyesters include natural ...
and PVC clothing, spandex, sport shoes, wetsuits,
footballs A football is a ball inflated with air that is used to play one of the various sports known as football. In these games, with some exceptions, goals or points are scored only when the ball enters one of two designated goal-scoring areas; football ...
and billiard balls, skis and
snowboard Snowboards are boards where the user places both feet, usually secured, to the same board. The board itself is wider than most skis, with the ability to glide on snow."snowboarding." Dictionary.com Unabridged (v 1.1). Random House, Inc. 17 Mar ...
s,
rackets Racket may refer to: * Racket (crime), a systematised element of organized crime ** Protection racket, a scheme whereby a group provides protection to businesses or other groups through violence outside the sanction of the law * Racket (sports equ ...
,
parachute A parachute is a device used to slow the motion of an object through an atmosphere by creating drag or, in a ram-air parachute, aerodynamic lift. A major application is to support people, for recreation or as a safety device for aviators, who ...
s,
sail A sail is a tensile structure—which is made from fabric or other membrane materials—that uses wind power to propel sailing craft, including sailing ships, sailboats, windsurfers, ice boats, and even sail-powered land vehicles. Sails may ...
s, tents and shelters. * Electronic and photonic technologies: organic
field effect transistors The field-effect transistor (FET) is a type of transistor that uses an electric field to control the flow of current in a semiconductor. FETs (JFETs or MOSFETs) are devices with three terminals: ''source'', ''gate'', and ''drain''. FETs control ...
(OFET), light emitting diodes (OLED) and
solar cells A solar cell, or photovoltaic cell, is an electronic device that converts the energy of light directly into electricity by the photovoltaic effect, which is a physics, physical and Chemical substance, chemical phenomenon.television components,
compact disc The compact disc (CD) is a Digital media, digital optical disc data storage format that was co-developed by Philips and Sony to store and play digital audio recordings. In August 1982, the first compact disc was manufactured. It was then rele ...
s (CD), photoresists, holography. * Packaging and containers:
films A film also called a movie, motion picture, moving picture, picture, photoplay or (slang) flick is a work of visual art that simulates experiences and otherwise communicates ideas, stories, perceptions, feelings, beauty, or atmosphere ...
, bottles,
food packaging Food packaging is a packaging system specifically designed for food and represents one of the most important aspects among the processes involved in the food industry, as it provides protection from chemical, biological and physical alterations ...
,
barrel A barrel or cask is a hollow cylindrical container with a bulging center, longer than it is wide. They are traditionally made of wooden staves and bound by wooden or metal hoops. The word vat is often used for large containers for liquids, ...
s. * Insulation: electrical and
thermal insulation Thermal insulation is the reduction of heat transfer (i.e., the transfer of thermal energy between objects of differing temperature) between objects in thermal contact or in range of radiative influence. Thermal insulation can be achieved with s ...
,
spray foam Spray foam is a chemical product created by two materials, isocyanate and polyol resin, which react when mixed with each other and expand up to 30-60 times its liquid volume after it is sprayed in place. This expansion makes it useful as a specia ...
s. * Construction and structural applications: garden furniture, PVC windows, flooring, sealing, pipes. * Paints, glues and lubricants:
varnish Varnish is a clear transparent hard protective coating or film. It is not a stain. It usually has a yellowish shade from the manufacturing process and materials used, but it may also be pigmented as desired, and is sold commercially in various ...
,
adhesive Adhesive, also known as glue, cement, mucilage, or paste, is any non-metallic substance applied to one or both surfaces of two separate items that binds them together and resists their separation. The use of adhesives offers certain advant ...
s,
dispersant A dispersant or a dispersing agent is a substance, typically a surfactant, that is added to a suspension of solid or liquid particles in a liquid (such as a colloid or emulsion) to improve the separation of the particles and to prevent their settl ...
s, anti-graffiti coatings, antifouling coatings, non-stick surfaces,
lubricant A lubricant (sometimes shortened to lube) is a substance that helps to reduce friction between surfaces in mutual contact, which ultimately reduces the heat generated when the surfaces move. It may also have the function of transmitting forces, t ...
s. * Car parts:
tire A tire (American English) or tyre (British English) is a ring-shaped component that surrounds a Rim (wheel), wheel's rim to transfer a vehicle's load from the axle through the wheel to the ground and to provide Traction (engineering), t ...
s, bumpers,
windshield The windshield (North American English) or windscreen (Commonwealth English) of an aircraft, car, bus, motorbike, truck, train, boat or streetcar is the front window, which provides visibility while protecting occupants from the elements. Mo ...
s,
windscreen wiper A windscreen wiper, windshield wiper, wiper blade (American English), or simply wiper, is a device used to remove rain, snow, ice, washer fluid, water, or debris from a vehicle's front window. Almost all motor vehicles, including cars, truck ...
s, fuel tanks, car seats. * Household items: buckets,
kitchenware :'' For a record label, see Kitchenware Records'' Kitchenware are the tools, utensils, appliances, dishes, and cookware used in food preparation, or the serving of food. Kitchenware can also be used in order to hold or store food before or aft ...
, toys (e.g., construction sets and
Rubik's cube The Rubik's Cube is a Three-dimensional space, 3-D combination puzzle originally invented in 1974 by Hungarians, Hungarian sculptor and professor of architecture Ernő Rubik. Originally called the Magic Cube, the puzzle was licensed by Rubik t ...
). * Medical applications:
blood bag Whole blood (WB) is human blood from a standard blood donation. It is used in the treatment of massive bleeding, in exchange transfusion, and when people donate blood to themselves. One unit of whole blood (~517 mls) brings up hemoglobin level ...
,
syringe A syringe is a simple reciprocating pump consisting of a plunger (though in modern syringes, it is actually a piston) that fits tightly within a cylindrical tube called a barrel. The plunger can be linearly pulled and pushed along the inside ...
s, rubber gloves,
surgical suture A surgical suture, also known as a stitch or stitches, is a medical device used to hold body tissues together and approximate wound edges after an injury or surgery. Application generally involves using a needle with an attached length of threa ...
,
contact lens Contact lenses, or simply contacts, are thin lenses placed directly on the surface of the eyes. Contact lenses are ocular prosthetic devices used by over 150 million people worldwide, and they can be worn to correct vision or for cosmetic ...
es,
prosthesis In medicine, a prosthesis (plural: prostheses; from grc, πρόσθεσις, prósthesis, addition, application, attachment), or a prosthetic implant, is an artificial device that replaces a missing body part, which may be lost through trau ...
, controlled drug delivery and release, matrices for cell growth. * Personal hygiene and healthcare:
diaper A diaper /ˈdaɪpə(r)/ (American and Canadian English) or a nappy ( Australian English, British English, and Hiberno-English) is a type of underwear that allows the wearer to urinate or defecate Defecation (or defaecation) follows dig ...
s using
superabsorbent polymer A superabsorbent polymer (SAP) (also called slush powder) is a water-absorbing hydrophilic homopolymers or copolymers that can absorb and retain extremely large amounts of a liquid relative to its own mass. Water-absorbing polymers, which are cla ...
s,
toothbrush A toothbrush is an oral hygiene tool used to clean the teeth, gums, and tongue. It consists of a head of tightly clustered bristles, atop of which toothpaste can be applied, mounted on a handle which facilitates the cleaning of hard-to-reach are ...
es,
cosmetics Cosmetics are constituted mixtures of chemical compounds derived from either natural sources, or synthetically created ones. Cosmetics have various purposes. Those designed for personal care and skin care can be used to cleanse or protect ...
,
shampoo Shampoo () is a hair care product, typically in the form of a viscous liquid, that is used for cleaning hair. Less commonly, shampoo is available in solid bar format. Shampoo is used by applying it to wet hair, massaging the product into the ...
,
condom A condom is a sheath-shaped barrier device used during sexual intercourse to reduce the probability of pregnancy or a sexually transmitted infection (STI). There are both male and female condoms. With proper use—and use at every act of in ...
s. * Security:
personal protective equipment Personal protective equipment (PPE) is protective clothing, helmets, goggles, or other garments or equipment designed to protect the wearer's body from injury or infection. The hazards addressed by protective equipment include physical, elec ...
,
bulletproof vest A bulletproof vest, also known as a ballistic vest or a bullet-resistant vest, is an item of body armor that helps absorb the impact and reduce or stop penetration to the torso from firearm-fired projectiles and fragmentation from explosions. T ...
s, space suits,
rope A rope is a group of yarns, plies, fibres, or strands that are twisted or braided together into a larger and stronger form. Ropes have tensile strength and so can be used for dragging and lifting. Rope is thicker and stronger than similarly ...
s. * Separation technologies: synthetic membranes, fuel cell membranes,
filtration Filtration is a physical separation process that separates solid matter and fluid from a mixture using a ''filter medium'' that has a complex structure through which only the fluid can pass. Solid particles that cannot pass through the filter ...
, ion-exchange resins. * Money:
polymer banknote Polymer banknotes are banknotes made from a synthetic polymer such as biaxially oriented polypropylene (BOPP). Such notes incorporate many security features not available in paper banknotes, including the use of metameric inks. Polymer banknote ...
s and payment cards. *
3D printing 3D printing or additive manufacturing is the Manufacturing, construction of a three-dimensional object from a computer-aided design, CAD model or a digital 3D modeling, 3D model. It can be done in a variety of processes in which material is ...
.


Standardized nomenclature

There are multiple conventions for naming polymer substances. Many commonly used polymers, such as those found in consumer products, are referred to by a common or trivial name. The trivial name is assigned based on historical precedent or popular usage rather than a standardized naming convention. Both the
American Chemical Society The American Chemical Society (ACS) is a scientific society based in the United States that supports scientific inquiry in the field of chemistry. Founded in 1876 at New York University, the ACS currently has more than 155,000 members at all d ...
(ACS) and
IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...
have proposed standardized naming conventions; the ACS and IUPAC conventions are similar but not identical. Examples of the differences between the various naming conventions are given in the table below: In both standardized conventions, the polymers' names are intended to reflect the monomer(s) from which they are synthesized (source based nomenclature) rather than the precise nature of the repeating subunit. For example, the polymer synthesized from the simple alkene ethene is called polyethene, retaining the ''-ene'' suffix even though the double bond is removed during the polymerization process: :→ :However, IUPAC structure based nomenclature is based on naming of the preferred constitutional repeating unit.


Characterization

Polymer characterization spans many techniques for determining the chemical composition, molecular weight distribution, and physical properties. Select common techniques include the following: * Size-exclusion chromatography (also called gel permeation chromatography), sometimes coupled with static light scattering, can used to determine the number-average molecular weight, weight-average molecular weight, and
dispersity In chemistry, the dispersity is a measure of the heterogeneity of sizes of molecules or particles in a mixture. A collection of objects is called uniform if the objects have the same size, shape, or mass. A sample of objects that have an inconsi ...
. *Scattering techniques, such as static light scattering and small-angle neutron scattering, are used to determine the dimensions ( radius of gyration) of macromolecules in solution or in the melt. These techniques are also used to characterize the three-dimensional structure of microphase-separated
block polymer In polymer chemistry, a copolymer is a polymer derived from more than one species of monomer. The polymerization of monomers into copolymers is called copolymerization. Copolymers obtained from the copolymerization of two monomer species are some ...
s, polymeric micelles, and other materials. *
Wide-angle X-ray scattering In X-ray crystallography, wide-angle X-ray scattering (WAXS) or wide-angle X-ray diffraction (WAXD) is the analysis of Bragg peaks scattered to wide angles, which (by Bragg's law) are caused by sub-nanometer-sized structures. It is an X-ray-diffr ...
(also called wide-angle X-ray diffraction) is used to determine the crystalline structure of polymers (or lack thereof). *Spectroscopy techniques, including Fourier-transform infrared spectroscopy,
Raman spectroscopy Raman spectroscopy () (named after Indian physicist C. V. Raman) is a spectroscopic technique typically used to determine vibrational modes of molecules, although rotational and other low-frequency modes of systems may also be observed. Raman sp ...
, and nuclear magnetic resonance spectroscopy, can be used to determine the chemical composition. *
Differential scanning calorimetry Differential scanning calorimetry (DSC) is a thermoanalytical technique in which the difference in the amount of heat required to increase the temperature of a sample and reference is measured as a function of temperature. Both the sample and ref ...
is used to characterize the thermal properties of polymers, such as the glass-transition temperature, crystallization temperature, and melting temperature. The glass-transition temperature can also be determined by
dynamic mechanical analysis Dynamic mechanical analysis (abbreviated DMA) is a technique used to study and characterize materials. It is most useful for studying the viscoelastic behavior of polymers A polymer (; Greek '' poly-'', "many" + ''-mer'', "part") is a subst ...
. * Thermogravimetry is a useful technique to evaluate the thermal stability of the polymer. *
Rheology Rheology (; ) is the study of the flow of matter, primarily in a fluid ( liquid or gas) state, but also as "soft solids" or solids under conditions in which they respond with plastic flow rather than deforming elastically in response to an appl ...
is used to characterize the flow and deformation behavior. It can be used to determine the
viscosity The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the inte ...
, modulus, and other rheological properties. Rheology is also often used to determine the molecular architecture (molecular weight, molecular weight distribution, branching) and to understand how the polymer can be processed.


Degradation

Polymer degradation is a change in the properties—tensile strength,
color Color (American English) or colour (British English) is the visual perceptual property deriving from the spectrum of light interacting with the photoreceptor cells of the eyes. Color categories and physical specifications of color are associ ...
, shape, or molecular weight—of a polymer or polymer-based product under the influence of one or more environmental factors, such as
heat In thermodynamics, heat is defined as the form of energy crossing the boundary of a thermodynamic system by virtue of a temperature difference across the boundary. A thermodynamic system does not ''contain'' heat. Nevertheless, the term is al ...
,
light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 tera ...
, and the presence of certain chemicals, oxygen, and
enzymes Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different molecule ...
. This change in properties is often the result of bond breaking in the polymer backbone (
chain scission Chain scission is a term used in polymer chemistry describing the degradation of a polymer main chain. It is often caused by thermal stress (heat) or ionizing radiation (e.g. light, UV radiation or gamma radiation), often involving oxygen. Durin ...
) which may occur at the chain ends or at random positions in the chain. Although such changes are frequently undesirable, in some cases, such as
biodegradation Biodegradation is the breakdown of organic matter by microorganisms, such as bacteria and fungi. It is generally assumed to be a natural process, which differentiates it from composting. Composting is a human-driven process in which biodegrada ...
and
recycling Recycling is the process of converting waste materials into new materials and objects. The recovery of energy from waste materials is often included in this concept. The recyclability of a material depends on its ability to reacquire the p ...
, they may be intended to prevent environmental
pollution Pollution is the introduction of contaminants into the natural environment that cause adverse change. Pollution can take the form of any substance (solid, liquid, or gas) or energy (such as radioactivity, heat, sound, or light). Pollutants, the ...
. Degradation can also be useful in biomedical settings. For example, a copolymer of
polylactic acid Polylactic acid, also known as poly(lactic acid) or polylactide (PLA), is a thermoplastic polyester with backbone formula or , formally obtained by condensation of lactic acid with loss of water (hence its name). It can also be prepared by rin ...
and polyglycolic acid is employed in hydrolysable stitches that slowly degrade after they are applied to a wound. The susceptibility of a polymer to degradation depends on its structure. Epoxies and chains containing aromatic functionalities are especially susceptible to UV degradation while polyesters are susceptible to degradation by
hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...
. Polymers containing an unsaturated backbone degrade via
ozone cracking Cracks can be formed in many different elastomers by ozone attack, and the characteristic form of attack of vulnerable rubbers is known as ozone cracking. The problem was formerly very common, especially in tires, but is now rarely seen in those ...
. Carbon based polymers are more susceptible to thermal degradation than inorganic polymers such as polydimethylsiloxane and are therefore not ideal for most high-temperature applications. The degradation of polyethylene occurs by random scission—a random breakage of the bonds that hold the
atom Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons. Every solid, liquid, gas, and ...
s of the polymer together. When heated above 450 °C, polyethylene degrades to form a mixture of hydrocarbons. In the case of chain-end scission, monomers are released and this process is referred to as unzipping or depolymerization. Which mechanism dominates will depend on the type of polymer and temperature; in general, polymers with no or a single small substituent in the repeat unit will decompose via random-chain scission. The sorting of polymer waste for recycling purposes may be facilitated by the use of the resin identification codes developed by the Society of the Plastics Industry to identify the type of plastic.


Product failure

Failure of safety-critical polymer components can cause serious accidents, such as fire in the case of cracked and degraded polymer
fuel line A fuel line is a hose or pipe used to transfer fuel from one point in a vehicle to another. The United States Environmental Protection Agency defines a fuel line as "all hoses or tubing designed to contain liquid fuel or fuel vapor. This includes ...
s. Chlorine-induced cracking of
acetal resin Polyoxymethylene (POM), also known as acetal, polyacetal, and polyformaldehyde, is an engineering thermoplastic used in precision parts requiring high stiffness, low friction, and excellent dimensional stability. As with many other synthetic pol ...
plumbing joints and
polybutylene Polybutylene (polybutene-1, poly(1-butene), PB-1) is a polyolefin or saturated polymer with the chemical formula (CH2CH(Et))n. Not be confused with polybutene, PB-1 is mainly used in piping. Production Polybutylene is produced by polymerisation ...
pipes has caused many serious floods in domestic properties, especially in the US in the 1990s. Traces of
chlorine Chlorine is a chemical element with the Symbol (chemistry), symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate betwee ...
in the water supply attacked polymers present in the plumbing, a problem which occurs faster if any of the parts have been poorly extruded or
injection molded Injection moulding (U.S. spelling: injection molding) is a manufacturing process for producing parts by injecting molten material into a mould, or mold. Injection moulding can be performed with a host of materials mainly including metals (for ...
. Attack of the acetal joint occurred because of faulty molding, leading to cracking along the threads of the fitting where there is
stress concentration In solid mechanics, a stress concentration (also called a stress raiser or a stress riser) is a location in an object where the stress is significantly greater than the surrounding region. Stress concentrations occur when there are irregularitie ...
. Polymer oxidation has caused accidents involving
medical device A medical device is any device intended to be used for medical purposes. Significant potential for hazards are inherent when using a device for medical purposes and thus medical devices must be proved safe and effective with reasonable assura ...
s. One of the oldest known failure modes is ozone cracking caused by chain scission when
ozone Ozone (), or trioxygen, is an inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , breaking down in the lo ...
gas attacks susceptible
elastomer An elastomer is a polymer with viscoelasticity (i.e. both viscosity and elasticity) and with weak intermolecular forces, generally low Young's modulus and high failure strain compared with other materials. The term, a portmanteau of ''elastic p ...
s, such as natural rubber and
nitrile rubber Nitrile rubber, also known as nitrile butadiene rubber, NBR, Buna-N, and acrylonitrile butadiene rubber, is a synthetic rubber derived from acrylonitrile (ACN) and butadiene. Trade names include Perbunan, Nipol, Krynac and Europrene. This rubber is ...
. They possess double bonds in their repeat units which are cleaved during ozonolysis. Cracks in fuel lines can penetrate the bore of the tube and cause fuel leakage. If cracking occurs in the engine compartment, electric sparks can ignite the
gasoline Gasoline (; ) or petrol (; ) (see ) is a transparent, petroleum-derived flammable liquid that is used primarily as a fuel in most spark-ignited internal combustion engines (also known as petrol engines). It consists mostly of organic co ...
and can cause a serious fire. In medical use degradation of polymers can lead to changes of physical and chemical characteristics of implantable devices. Nylon 66 is susceptible to acid hydrolysis, and in one accident, a fractured fuel line led to a spillage of diesel into the road. If
diesel fuel Diesel fuel , also called diesel oil, is any liquid fuel specifically designed for use in a diesel engine, a type of internal combustion engine in which fuel ignition takes place without a spark as a result of compression of the inlet air and t ...
leaks onto the road, accidents to following cars can be caused by the slippery nature of the deposit, which is like black ice. Furthermore, the
asphalt concrete Asphalt concrete (commonly called asphalt, blacktop, or pavement in North America, and tarmac, bitumen macadam, or rolled asphalt in the United Kingdom and the Republic of Ireland) is a composite material commonly used to surface roads, parkin ...
road surface will suffer damage as a result of the diesel fuel dissolving the asphaltenes from the composite material, this resulting in the degradation of the asphalt surface and structural integrity of the road.


See also

*
Biopolymer Biopolymers are natural polymers produced by the cells of living organisms. Like other polymers, biopolymers consist of monomeric units that are covalently bonded in chains to form larger molecules. There are three main classes of biopolymers, cl ...
* Ideal chain *
Catenation In chemistry, catenation is the bonding of atoms of the same element into a series, called a ''chain''. A chain or a ring shape may be ''open'' if its ends are not bonded to each other (an open-chain compound), or ''closed'' if they are bonded ...
* Inorganic polymer * Important publications in polymer chemistry *
Oligomer In chemistry and biochemistry, an oligomer () is a molecule that consists of a few repeating units which could be derived, actually or conceptually, from smaller molecules, monomers.Quote: ''Oligomer molecule: A molecule of intermediate relativ ...
*
Polymer adsorption Adsorption is the adhesion of ions or molecules onto the surface of another phase. Adsorption may occur via physisorption and chemisorption. Ions and molecules can adsorb to many types of surfaces including polymer surfaces. A polymer is a large mo ...
* Polymer classes * Polymer engineering *
Polymerization In polymer chemistry, polymerization (American English), or polymerisation (British English), is a process of reacting monomer, monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are ...
* Polymery (botany) *
Reactive compatibilization Reactive compatibilization is the process of modifying a mixed immiscible blend of polymers to arrest phase separation and allow for the formation of a stable, long-term continuous phase. It is done via the addition of a reactive polymer, miscible ...
*
Sequence-controlled polymer A sequence-controlled polymer is a macromolecule, in which the sequence of monomers is controlled to some degree. This control can be absolute but not necessarily. In other words, a sequence-controlled polymer can be uniform (its dispersity Ð is ...
* Shape-memory polymer *
Sol–gel process In materials science, the sol–gel process is a method for producing solid materials from small molecules. The method is used for the fabrication of metal oxides, especially the oxides of silicon (Si) and titanium (Ti). The process involves con ...
*
Supramolecular polymer The term "polymer" refers to large molecules whose structure is composed of multiple repeating units and the prefix "supra" meaning "beyond the limits of". Supramolecular polymers are a new category of polymers that can potentially be used for mat ...
*
Thermoplastic A thermoplastic, or thermosoft plastic, is any plastic polymer material that becomes pliable or moldable at a certain elevated temperature and solidifies upon cooling. Most thermoplastics have a high molecular weight. The polymer chains associate ...
* Thermosetting polymer


References


Bibliography

* * * *


External links


How to Analyze Polymers Using X-ray DiffractionPolymer Chemistry Hypertext, Educational resourceIntroduction to Polymers
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