Editing Polymer (section)

==Structure==
<!-- Polymer properties are broadly divided into several classes based on the scale at which the property is defined as well as upon its physical basis.<ref>{{cite journal|author=S.A. Baeurle|doi=10.1007/s10910-008-9467-3|title=Multiscale modeling of polymer materials using field-theoretic methodologies: a survey about recent developments|journal=Journal of Mathematical Chemistry |volume=46|issue=2|pages=363–426|year=2009}}</ref> -->
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.<ref>Sperling, p. 29</ref>
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 of polymers|crystallization]], the [[glass transition]] or [[Copolymer#Microphase separation|microphase separation]].<ref name="Bower">{{cite book |last1=Bower |first1=David I. |title=An introduction to polymer physics |date=2002 |publisher=Cambridge University Press |isbn=9780511801280}}</ref>
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]]'''.<ref>Rudin, p. 17</ref> A '''terpolymer''' is a copolymer which contains three types of repeat units.<ref>Cowie, p. 4</ref>

[[Polystyrene]] is composed only of [[styrene]]-based repeat units, and is classified as a homopolymer. [[Polyethylene terephthalate]], even though produced from two different [[monomer]]s ([[ethylene glycol]] and [[terephthalic acid]]), is usually regarded as a homopolymer because only one type of repeat unit is formed. [[Ethylene-vinyl acetate]] 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, [[polynucleotide]]s such as DNA are composed of four types of [[nucleotide]] subunits.

:{| class="wikitable" style="text-align:left; font-size:90%;" width="80%"
|-
| class="hintergrundfarbe6" align="center" colspan="4" |'''Homopolymers and copolymers (examples)'''
|- style="vertical-align:top" class="hintergrundfarbe2"
| [[File:Polystyrene skeletal.svg|80px|center]]
| [[File:Poly(dimethylsiloxan).svg|100px|center]]
| [[File:PET.svg|200px|center]]
| [[File:Styrol-Butadien-Kautschuk.svg|180px|center]]
|- style="vertical-align:top"
| Homopolymer [[polystyrene]]
| Homopolymer [[polydimethylsiloxane]], a [[silicone]]. The main chain is formed of silicon and oxygen atoms.
| The homopolymer [[polyethylene terephthalate]] has only one [[repeat unit]].
| Copolymer [[styrene-butadiene rubber]]: The repeat units based on [[styrene]] and [[1,3-Butadiene|1,3-butadiene]] form two repeating units, which can alternate in any order in the macromolecule, making the polymer thus a random copolymer.
|}

A polymer containing ionizable subunits (e.g., pendant [[carboxylic acid|carboxylic groups]]) is known as a [[polyelectrolyte]] or [[ionomer]], when the fraction of ionizable units is large or small respectively.

===Microstructure===
{{Main|Microstructure}}
The microstructure of a polymer (sometimes called configuration) relates to the physical arrangement of monomer residues along the backbone of the chain.<ref>Sperling, p. 30</ref> 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 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]]s have a wide-meshed cross-linking between the "main chains". Close-meshed crosslinking, on the other hand, leads to [[thermosets]]. 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.

:{| class="wikitable" style="text-align:center; font-size:90%;" width="60%"
|- class="hintergrundfarbe2"
|[[File:Polymerstruktur-linear.svg|130px]]<br /> Linear, unbranched macromolecule
|[[File:Polymerstruktur-verzweigt.svg|130px]]<br /> Branched macromolecule
|[[File:Polymerstruktur-teilkristallin.svg|150px]]<br />Semi-crystalline structure of an unbranched polymer
|[[File:Polymerstruktur-weitmaschig vernetzt.svg|130px]]<br /> Slightly [[cross-link]]ed polymer ([[elastomer]])
|[[File:Polymerstruktur-engmaschig vernetzt.svg|130px]]<br /> Highly cross-linked polymer ([[Thermosetting polymer|thermoset]])
|}

====Polymer architecture{{Anchor|Intermolecular forces}}====
{{Main|Polymer architecture}}
[[File:Polymer Branch.svg|thumb|right|upright=0.9|Branch point in a polymer]]

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.<ref name="PP6">{{cite book |last1= Rubinstein |first1= Michael |last2= Colby |first2= Ralph H. |title= Polymer physics |url= https://archive.org/details/polymerphysics00rubi_825 |url-access= limited |year= 2003 |publisher= Oxford University Press |location= Oxford; New York |isbn= 978-0-19-852059-7 |page= [https://archive.org/details/polymerphysics00rubi_825/page/n14 6]}}</ref> A [[branching (polymer chemistry)|branched polymer]] molecule is composed of a main chain with one or more substituent side chains or branches. Types of branched polymers include [[star polymer]]s, [[comb polymers]], [[polymer brush]]es, [[dendronized polymer]]s, [[ladder polymer]]s, and [[dendrimer]]s.<ref name="PP6"/> There exist also [[two-dimensional polymer]]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|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]], which quantifies the number of monomers incorporated into the chain.<ref>McCrum, p. 30</ref><ref name="PP33">Rubinstein, p. 3</ref> As with other molecules, a polymer's size may also be expressed in terms of [[molecular weight]]. 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]] (''M''<sub>n</sub>) and [[weight-average molecular weight]] (''M''<sub>w</sub>) are most commonly reported.<ref>McCrum, p. 33</ref><ref name="PP233">Rubinstein, pp. 23–24</ref> The ratio of these two values (''M''<sub>w</sub> / ''M''<sub>n</sub>) is the [[dispersity]] (''Đ''), which is commonly used to express the width of the molecular weight distribution.<ref>Painter, p. 22</ref>

The physical properties<ref>{{cite book |last1= De Gennes |first1= Pierre Gilles |title= Scaling concepts in polymer physics |year= 1979 |publisher= Cornell University Press |location= Ithaca, N.Y. |isbn= 978-0-8014-1203-5}}</ref> of polymer strongly depend on the length (or equivalently, the molecular weight) of the polymer chain.<ref name="PP5">Rubinstein, p. 5</ref> One important example of the physical consequences of the molecular weight is the scaling of the [[viscosity]] (resistance to flow) in the melt.<ref>McCrum, p. 37</ref> The influence of the weight-average molecular weight (<math>M_w</math>) on the melt viscosity (<math>\eta</math>) depends on whether the polymer is above or below the onset of [[reptation|entanglements]]. Below the entanglement molecular weight{{clarify|date=December 2018}}, <math>\eta \sim {M_w}^{1}</math>, whereas above the entanglement molecular weight, <math>\eta \sim {M_w}^{3.4}</math>. In the latter case, increasing the polymer chain length 10-fold would increase the viscosity over 1000 times.<ref>Introduction to Polymer Science and Chemistry: A Problem-Solving Approach By Manas Chanda</ref>{{page needed|date=December 2018}} Increasing chain length furthermore tends to decrease chain mobility, increase strength and toughness, and increase the glass-transition temperature (T<sub>g</sub>).<ref>{{cite journal |last1=O'Driscoll |first1=K. |last2=Amin Sanayei |first2=R. |date=July 1991 |title=Chain-length dependence of the glass transition temperature |journal=Macromolecules |volume=24 |issue=15 |pages=4479–4480 |doi= 10.1021/ma00015a038|bibcode=1991MaMol..24.4479O}}</ref> This is a result of the increase in chain interactions such as [[Van der Waals force|van der Waals attractions]] and [[reptation|entanglements]] that come with increased chain length.<ref>{{cite book|last1=Pokrovskii|first1=V. N.|year=2010|title=The Mesoscopic Theory of Polymer Dynamics|series=Springer Series in Chemical Physics|volume=95|doi=10.1007/978-90-481-2231-8|isbn=978-90-481-2230-1|bibcode=2010mtpd.book.....P|url=https://cds.cern.ch/record/1315698}}</ref><ref>{{cite journal|last1=Edwards|first1=S. F.|year=1967|title=The statistical mechanics of polymerized material|journal=Proceedings of the Physical Society|volume=92|issue=1|pages=9–16|bibcode=1967PPS....92....9E|doi=10.1088/0370-1328/92/1/303}}</ref> 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====
{{Main|Copolymer}}

Copolymers are classified either as statistical copolymers, alternating copolymers, block copolymers, graft copolymers or gradient copolymers. In the schematic figure below, <span style="color:#F46C2C">Ⓐ</span> and <span style="color:#00AAC5">Ⓑ</span> symbolize the two [[repeat unit]]s.

:{| class="wikitable" style="text-align:center; font-size:90%;"
|- class="hintergrundfarbe2"
| [[File:Statistical copolymer 3D.svg|270px|Statistisches Copolymer]]<br />Random copolymer
| [[File:Gradient copolymer 3D.svg|270px|Gradientcopolymer]]<br />Gradient copolymer
| rowspan="2" | [[File:Graft copolymer 3D.svg|270px|Pfropfcopolymer]]<br /> [[Graft copolymer]]
|- class="hintergrundfarbe2"
| [[File:Alternating copolymer 3D.svg|270px|Alternierendes Copolymer]]<br /> Alternating copolymer
| [[File:Block copolymer 3D.svg|250px|Blockcopolymer]]<br /> [[Block copolymer]]
|}

*'''Alternating copolymers''' possess two regularly alternating monomer residues:<ref name="PC14">Painter, p. 14</ref> {{chem|(AB)|n}}. An example is the equimolar copolymer of [[styrene]] and [[maleic anhydride]] formed by free-radical chain-growth polymerization.<ref name=Rudin18>Rudin, p. 18–20</ref> 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.<ref name=Cowie104>Cowie, p. 104</ref>
*'''Periodic copolymers''' have more than two species of monomer units in a regular sequence.<ref>{{cite journal |title=Periodic copolymer |url=https://goldbook.iupac.org/terms/view/P04494 |website=IUPAC Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). |year=2014 |publisher=International Union of Pure and Applied Chemistry |doi=10.1351/goldbook.P04494 |access-date=9 April 2020|doi-access=free }}</ref>
*'''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'''.<ref name="PC15">Painter, p. 15</ref><ref>Sperling, p. 47</ref> For example, the chain-growth copolymer of [[vinyl chloride]] and [[vinyl acetate]] is random.<ref name=Rudin18/>
*'''Block copolymers''' have long sequences of different monomer units.<ref name=Rudin18/><ref name=Cowie104/> 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.<ref name=Cowie104/> The branches are added on to a preformed main chain macromolecule.<ref name=Rudin18/>

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]].<ref>{{cite journal|last1=Lutz|first1=Jean-François|last2=Ouchi|first2=Makoto|last3=Liu|first3=David R.|last4=Sawamoto|first4=Mitsuo|date=9 August 2013|title=Sequence-Controlled Polymers|journal=Science|language=en|volume=341|issue=6146|pages=1238149|doi=10.1126/science.1238149|issn=0036-8075|pmid=23929982|s2cid=206549042}}</ref> Alternating, periodic and block copolymers are simple examples of [[sequence-controlled polymer]]s.

====Tacticity====
{{Main|Tacticity}}
Tacticity describes the relative [[stereochemistry]] of [[chirality (chemistry)|chiral]] centers in neighboring structural units within a macromolecule. There are three types of tacticity: [[isotactic]] (all substituents on the same side), [[atactic]] (random placement of substituents), and [[syndiotactic]] (alternating placement of substituents).

:{| class="wikitable" style="text-align:center; font-size:90%;" width="60%"
|- class="hintergrundfarbe2"
|[[File:Isotactic-A-2D-skeletal.png|240px]]<br />Isotactic
|[[File:Syndiotactic-2D-skeletal.png|280px]]<br /> Syndiotactic
|[[File:Atactic-2D-skeletal.png|240px]]<br /> Atactic (i. e. random)
|}

===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.

{| class="wikitable floatright" style="text-align:center; font-size:90%;" width="30%"
|- class="hintergrundfarbe2"
| [[File:Statistischer Kneul.svg|100px]]<br />Randomly oriented polymer
| [[File:Verhakungen.svg|200px]]<br />Interlocking of several polymers
|}

* '''Disordered polymers:''' In the solid state, atactic polymers, polymers with a high degree of [[Branching (polymer chemistry)|branching]] and random copolymers form [[Amorphous solid|amorphous]] (i.e. glassy structures).<ref name="MakroChem">Bernd Tieke: ''Makromolekulare Chemie.'' 3. Auflage, Wiley-VCH, Weinheim 2014, S. 295f (in German).</ref> In melt and solution, polymers tend to form a constantly changing "statistical cluster", see [[Freely Jointed Chain|freely-jointed-chain model]]. In the [[Solid|solid state]], the respective [[Protein conformation|conformations]] of the molecules are frozen. Hooking and entanglement of chain molecules lead to a "mechanical bond" between the chains. [[Intermolecular force|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.

{| class="wikitable floatright" style="text-align:center; font-size:90%;" width="30%"
|- class="hintergrundfarbe2"
| [[File:Polyethylene-xtal-view-down-axis-3D-balls-perspective.png|180px]]<br />Polyethylene: zigzag conformation of molecules in close packed chains
| [[File:Lamellen.svg|180px]]<br /> Lamella with tie molecules
| [[File:Spherulite2de.svg|180px]]<br /> [[Spherulite (polymer physics)|Spherulite]]
|}
{| class="wikitable floatright" style="text-align:center; font-size:90%;" width="30%"
|- class="hintergrundfarbe2"
| [[File:Helix-Polypropylen.svg|100px]]<br />[[polypropylene]] [[helix]]
| [[File:P-Aramid H-Brücken.svg|200px]]<br />[[Aramide|''p''-Aramid]], red dotted: hydrogen bonds
|}

* '''Linear polymers''' with periodic structure, low branching and stereoregularity (e. g. not atactic) have a [[semi-crystalline]] structure in the solid state.<ref name="MakroChem" /> 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&nbsp;nm).<ref name="KunstChemIng">[[Wolfgang Kaiser (physicist)|Wolfgang Kaiser]]: ''Kunststoffchemie für Ingenieure.'' 3. Auflage, Carl Hanser, München 2011, S. 84.</ref> 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 (polymer physics)|spherulite]], often with a diameter in the range of 0.05 to 1&nbsp;mm.<ref name="KunstChemIng" />
: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]]s, as in the case of [[Aramid|''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]]. 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]]s and cannot be molten (unlike [[thermoplastic]]s); heating cross-linked polymers only leads to [[Thermal decomposition|decomposition]]. [[Thermoplastic elastomer]]s, 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.

{| class="wikitable centered" style="text-align:center; font-size:90%;" width="80%"
|- class="hintergrundfarbe2" valign="top"
| [[File:Polymerstruktur-weitmaschig vernetzt.svg|130px]] <br />Wide-meshed cross-linked polymer (elastomer)
| [[File:Polymerstruktur-weitmaschig vernetzt-gestreckt.svg|270px]] <br /><br />Wide-meshed cross-linked polymer (elastomer) under [[tensile stress]]
| [[File:Polymerstruktur-TPE-teilkristallin.svg|140px]] <br /> [[Crystallite]]s as "crosslinking sites": one type of [[thermoplastic elastomer]]
| [[File:Polymerstruktur-TPE-teilkristallin gestreckt.svg|250px]] <br /><br /> Semi-crystalline thermoplastic elastomer under tensile stress
|}

====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]]. For example, the structure of a crystalline protein or polynucleotide, such as a sample prepared for [[x-ray crystallography]], may be defined in terms of a conventional unit cell composed of one or more polymer molecules with cell dimensions of hundreds of [[angstrom]]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.<ref>{{cite book|last1=Allcock|first1=Harry R.|last2=Lampe|first2=Frederick W.|last3=Mark|first3=James E.|title=Contemporary Polymer Chemistry|publisher=Pearson Education|edition=3|year=2003|page=546|isbn=978-0-13-065056-6}}</ref> 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.<ref>Rubinstein, p. 13</ref> 
The simplest theoretical models for polymers in the molten, amorphous state are [[ideal chain]]s.