Editing Polymer (section)

==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 and extensive properties#Intensive properties|intensive properties]] according to [[thermodynamics]].

===Mechanical properties===
[[File:Stable neck MDPE.jpg|upright=0.5|thumb|right|A polyethylene sample that has [[necking (engineering)|necked]] under tension]]
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]] of a material quantifies how much elongating stress the material will endure before failure.<ref>{{cite book|author-link=M. F. Ashby|last1=Ashby|first1=Michael|last2=Jones|first2=David|title=Engineering Materials|url=https://archive.org/details/engineeringmater01jone|url-access=limited|pages=[https://archive.org/details/engineeringmater01jone/page/n199 191]–195|publisher=Butterworth-Heinermann|year=1996|edition=2|isbn=978-0-7506-2766-5}}</ref><ref>{{cite book|first1=M. A.|last1=Meyers|first2=K. K.|last2=Chawla|title=Mechanical Behavior of Materials|publisher=Cambridge University Press|year=1999|page=41|url=http://www.toodoc.com/Mechanical-Behavior-of-Materials-ebook.html|isbn=978-0-521-86675-0|access-date=31 December 2018|archive-url=https://web.archive.org/web/20131102010325/http://www.toodoc.com/Mechanical-Behavior-of-Materials-ebook.html|archive-date=2 November 2013|url-status=dead}}</ref> 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 [[cross-link|crosslinking]] of polymer chains.

====Young's modulus of elasticity====
[[Young's modulus]] quantifies the [[elasticity (physics)|elasticity]] of the polymer. It is defined, for small [[deformation (mechanics)#Strain|strain]]s, 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]] describes a complex time-dependent elastic response, which will exhibit [[hysteresis]] in the stress-strain curve when the load is removed. [[Dynamic mechanical analysis]] or DMA measures this complex modulus by oscillating the load and measuring the resulting strain as a function of time.

===Transport properties===
[[Transport phenomena|Transport properties]] such as [[mass diffusivity|diffusivity]] 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]] 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]].<ref>{{cite book |last1=Fried |first1=Joel R. |title=Polymer Science & Technology |publisher=Prentice Hall |isbn=0-13-018168-4 |pages=155–6 |edition=2nd|year=2003}}</ref>

===Phase behavior===

====Crystallization and melting====
[[File:Thermal transitions in amorphous and semicrystalline polymers.tif|thumb|300x300px|Thermal transitions in '''(A)''' amorphous and '''(B)''' semicrystalline polymers, represented as traces from [[differential scanning calorimetry]]. As the temperature increases, both amorphous and semicrystalline polymers go through the [[glass transition]] (''T''<sub>g</sub>). Amorphous polymers '''(A)''' do not exhibit other phase transitions, though semicrystalline polymers '''(B)''' undergo crystallization and melting (at temperatures ''T''<sub>c</sub> and ''T''<sub>m</sub>, respectively).]]
Depending on their chemical structures, polymers may be either semi-crystalline or amorphous. Semi-crystalline polymers can undergo [[crystallization of polymers|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''<sub>g</sub>) and below the melting temperature (''T''<sub>m</sub>).

====Glass transition====
All polymers (amorphous or semi-crystalline) go through [[glass transition]]s. The glass-transition temperature (''T''<sub>g</sub>) is a crucial physical parameter for polymer manufacturing, processing, and use. Below ''T''<sub>g</sub>, molecular motions are frozen and polymers are brittle and glassy. Above ''T''<sub>g</sub>, 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 [[plasticizer]]s.<ref>{{cite book|title=Polymer Handbook|last1=Brandrup|first1=J.|last2=Immergut|first2=E.H.|last3=Grulke|first3=E.A.|publisher=Wiley-Interscience|year=1999|isbn=978-0-471-47936-9|edition=4}}</ref>

Whereas crystallization and melting are first-order [[phase transition]]s, the glass transition is not.<ref>{{cite journal|url=https://www.degruyter.com/downloadpdf/j/pac.2011.83.issue-10/pac-rec-10-11-13/pac-rec-10-11-13.pdf|title=Definitions of terms relating to crystalline polymers (IUPAC Recommendations 2011)|journal=Pure and Applied Chemistry|volume=83|issue=10|pages=1831–1871|last1=Meille|first1=S.|last2=Allegra|first2=G.|doi=10.1351/PAC-REC-10-11-13|access-date=31 December 2018|last3=Geil|first3=P.|last4=He|first4=J.|display-authors=3|year=2011|s2cid=98823962}}</ref> 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====
[[File:LCST-UCST plot.svg|thumb|upright=1.4|Phase diagram of the typical mixing behavior of weakly interacting polymer solutions, showing [[spinodal]] curves and [[binodal]] coexistence curves]]
In general, polymeric mixtures are far less [[miscible]] than mixtures of [[small molecule]] materials. This effect results from the fact that the driving force for mixing is usually [[entropy]], 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]] phase transition (UCST), at which phase separation occurs with cooling, polymer mixtures commonly exhibit a [[lower critical solution temperature]] 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]], 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 [[Paul J. Flory|Flory]] condition), the polymer behaves like an ideal [[random coil]]. The transition between the states is known as a [[coil–globule transition]].

====Inclusion of plasticizers====
Inclusion of plasticizers tends to lower T<sub>g</sub> and increase polymer flexibility. Addition of the plasticizer will also modify dependence of the glass-transition temperature T<sub>g</sub> on the cooling rate.<ref>{{Citation
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</ref> 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 [[dipole]]s in the monomer units. Polymers containing [[amide]] or [[carbonyl]] groups can form [[hydrogen bond]]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 [[carbamate|urethane]] or [[urea]] linkages. [[Polyester]]s have [[intermolecular force#Dipole-dipole interactions|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]]), but polyesters have greater flexibility. Polymers with non-polar units such as polyethylene interact only through weak [[Van der Waals force]]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 [[poly(methyl methacrylate)|PMMA]] and HEMA:MMA are used as matrices in the gain medium of [[solid-state dye laser]]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 laser]]s, are known to yield very narrow [[laser linewidth|linewidths]] which is useful for spectroscopy and analytical applications.<ref name="D1999">{{cite journal|doi=10.1364/AO.38.006347|pmid=18324163|title=Multiple-prism grating solid-state dye laser oscillator: optimized architecture|year=1999|last1=Duarte|first1=F. J. |journal=Applied Optics|volume=38|issue=30|pages=6347–6349|bibcode= 1999ApOpt..38.6347D}}</ref> 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<sup>−4</sup> in units of K<sup>−1</sup> in the 297 ≤ T ≤ 337 K range.<ref>{{cite book |title= Tunable Laser Optics |publisher= Elsevier Academic |location=New York |year=2003 |isbn=978-0122226960 |author= Duarte, F. J. |author-link= F. J. Duarte}}</ref>

===Electrical properties===
Most conventional polymers such as polyethylene are [[Insulator (electricity)|electrical insulators]], but the development of polymers containing [[Conjugated system|π-conjugated bonds]] has led to a wealth of polymer-based [[semiconductor]]s, such as [[polythiophene]]s. This has led to many applications in the field of [[organic electronics]].