Editing Polymer (section)

===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
 |last1= Capponi
 |first1= S.
 |last2= Alvarez
 |first2= F.
 |last3= Racko
 |first3= D.
 |title= Free Volume in a PVME Polymer–Water Solution
 |journal= Macromolecules
 |volume= XXX
 |pages= XXX-XXX
 |year= 2020
 |issue= XXX
 |doi= 10.1021/acs.macromol.0c00472 |bibcode= 2020MaMol..53.4770C
 |hdl= 10261/218380
 |s2cid= 219911779
 |hdl-access= free
 }}
</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.