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[[File:Spherulite1.jpg|thumb|Spherulites viewed between crossed [[polarizer]]s in an optical microscope.]]
<!--[[File:Spherulit in Mosaiktextur.jpg|thumb|A spherulite embedded into a mosaic [[mesogen]]]]-->
'''Crystallization of polymers''' is a process associated with partial alignment of their molecular chains. These chains fold together and form ordered regions called lamellae, which compose larger spheroidal structures named [[Spherulite (polymer physics)|spherulite]]s.<ref name=sp/> [[Polymer]]s can crystallize upon cooling from the melt, mechanical stretching or solvent evaporation. Crystallization affects optical, mechanical, thermal and chemical properties of the polymer. The degree of crystallinity is estimated by different analytical methods and it typically ranges between 10 and 80%, thus crystallized polymers are often called "semicrystalline". The properties of semicrystalline polymers are determined not only by the degree of crystallinity, but also by the size and orientation of the molecular chains.

==Crystallization mechanisms==
[[File:Polymerketten - amorph und kristallinEN.svg|thumb|The arrangement of molecular chains in amorphous and semicrystalline polymers.]]

===Solidification from the melt===
Polymers are composed of long molecular chains which form irregular, entangled coils in the melt. Some polymers retain such a disordered structure upon freezing and thus convert into amorphous solids. In other polymers, the chains rearrange upon freezing and form partly ordered regions with a typical size of the order 1 micrometer.<ref name=sp2>{{cite book|url=http://books.google.com/books?id=T8Krosds-1gC&pg=PA5|page=5|title=Polymer microscopy|author=Linda C. Sawyer, David T. Grubb, Gregory F. Meyers|publisher=Springer|year=2008|isbn=0387726276}}</ref> Although it would be energetically favorable for the polymer chains to align parallel, such alignment is hindered by the entanglement. Therefore, within the ordered regions, the polymer chains are both aligned and folded. Those regions are therefore are neither crystalline nor amorphous and are classified as semicrystalline. Examples of semi-crystalline polymers are linear [[polyethylene]] (PE), [[polytetrafluoroethylene]] (PTFE) or [[tacticity|isotactic]] [[polypropylene]] (PP).<ref name=dens/>

[[Image:Isotactic polypropylene.svg|thumb|The structure of isotactic polypropylene.]]
[[File:Atactic polypropylene.svg|thumb|The structure of atactic polypropylene.]]
Whether or not polymers can crystallize depends on their molecular structure – presence of straight chains with regularly spaced side groups facilitates crystallization. For example, crystallization occurs very much easier in [[Tacticity|isotactic]] than in the atactic polypropylene form. Atactic polymers crystallize when the side groups are very small, as in [[polyvinyl]] and don't crystallize in case of large substituents like in [[rubber]] or [[silicone]]s.<ref name="dens"/>

====Nucleation====
[[File:Lamellenbildung bei der Kristallisation von PolymerenEN.svg|thumb|Lamellae form during crystallization from the melt. The arrow shows the direction of temperature gradient.<ref name="Menges"/>]]
[[Nucleation]] starts with small, nanometer-sized areas where as a result of heat motion some chains or their segments occur parallel. Those seeds can either dissociate, if thermal motion destroys the molecular order, or grow further, if the grain size exceeds a certain critical value.<ref name="Menges">Georg Menges, Edmund Haberstroh, Walter Michaeli, Ernst Schmachtenberg: [http://books.google.com/books?id=R3p4KXQBNAAC&pg=PA68 Plastics Materials Science] Hanser Verlag, 2002, ISBN 3-446-21257-4</ref><ref name=dens/>

Apart from the thermal mechanism, nucleation is strongly affected by impurities, dyes, plasticizers, fillers and other additives in the polymer. This is also referred to as heterogeneous nucleation. This effect is poorly understood and irregular, so that the same additive can promote nucleation in one polymer, but not in another. Many of the good nucleating agents are metal salts of organic acids, which themselves are crystalline at the solidification temperature of the polymer solidification.<ref name="Menges"/> 

====Crystal growth from the melt====
[[File:Spherulite2.PNG|thumb|Schematic model of a spherulite. Black arrows indicate direction of molecular alignment]]
Crystal growth is achieved by the further addition of folded polymer chain segments and only occurs for temperatures below the [[melting point|melting temperature]] T<sub>m</sub> and above the [[glass transition temperature]] T<sub>g</sub>. Higher temperatures destroy the molecular arrangement and below the glass transition temperature, the movement of molecular chains is frozen.<ref name="Becker">GW Becker, Ludwig Bottenbruch, Rudolf Binsack, D. Braun: [http://books.google.com/books?id=S3t_ZOMVvQ0C&pg=PA340 ''Engineering Thermoplastics. Polyamides.''] (in German) Hanser Verlag, 1998 ISBN 3-446-16486-3</ref> Nevertheless, secondary crystallization can proceeds even below T<sub>g</sub>, in the time scale of months and years. This process affects mechanical properties of the polymers and decreases their volume because of a more compact packing of aligned polymer chains.<ref name=dens/><ref name="Woebcken">Wilbrand Woebcken, Klaus Stöckhert, HBP Gupta: ''Plastics Encyclopedia.'' (in German) Hanser Verlag, 1998, ISBN 3-446-17969-0</ref>

The chains interact via various types of the [[van der Waals force]]s. The interaction strength depends on the distance between the parallel chain segments and it determines the mechanical and thermal properties of the polymer.<ref name="Weißbach">Wolfgang Weissbach: [http://books.google.com/books?id=dcenroTkuLgC&pg=PA259 ''Materials science and materials testing.''] Vieweg + Teubner Verlag, 2007, ISBN 3-8348-0295-6</ref>

The growth of the crystalline regions preferably occurs in the direction of the largest [[temperature gradient]] and is suppressed at the top and bottom of the lamellae by the amorphous folded parts at those surfaces. In case of strong gradient, the growth has a unidirectional, dendritic character.<ref name="IWF">[http://www.iwf.de/iwf/do/mkat/listing.aspx?Action=Quicklink&Search=Dendrit&SearchIn=Keywords Dendrite] in the IWF Knowledge and Media gGmbH (videos and articles on the dendritic crystallization of polypropylene)</ref> However, if temperature distribution is isotropic and static then lamellae grow radially and form larger quasi-spherical aggregated called spherulites. Spherulites have a size between about 1 and 100 micrometers<ref name=sp2/> and form a large variety of colored patterns (see, e.g. front images) when observed between crossed polarizers in an optical microscope, which often include the "maltese cross" pattern and other polarization phenomena caused by molecular aignment within the individual lamellae of a spherullite.<ref name=sp>{{cite book|url=http://books.google.com/books?id=Jg_l8B7-4ngC&pg=PA45&dq=polymer+crystallization+spherulite+lamellae&lr=&num=100&as_brr=3&cd=1#v=onepage&q=polymer%20crystallization%20spherulite%20lamellae&f=false|pages=43–45|title=Seymour/Carraher's polymer chemistry|author=Charles E. Carraher, Raymond Benedict Seymour|publisher=CRC Press|year=2003|isbn=0824708067}}</ref><ref name="Menges"/><ref name=dens/>

===Crystallization by stretching===
[[File:Kristallisation durch Verstreckung (Polymer)EN.svg|thumb|The arrangement of the molecule chains upon crystallization by stretching.<ref name="dens"/>]]
The above mechanism considered crystallization from the melt, which is important for [[injection]] molding of plastic components. Another type of crystallization occurs upon [[extrusion]] used in making fibers and films. 

In this process, the polymer is forced through, e.g., a nozzle that creates [[tensile stress]] which partially aligns its molecules. Such alignment can be considered as crystallization and it affects the material properties. For example, the strength of the fiber is greatly increased in the longitudinal direction, and optical properties show large anisotropy along and perpendicular to the fiber axis. Polymer strength is increased not only by extrusion, but also by blow molding, which is used in the production of plastic tanks and [[Polyethylene terephthalate|PET]] bottles.<ref name="Weißbach"/> Some polymers which do not crystallize from the melt, can be partially aligned by stretching.<ref name="Thielen">Michael Thielen, Klaus Hartwig, Peter Gust: [http://books.google.com/books?id=JNY1FOutvycC&pg=PA13 Blow molding of plastic articles] Hanser Verlag, 2006, ISBN 3-446-22671-0</ref>

===Crystallization from solution===
Polymers can also be crystallized from a solution or upon evaporation of a solvent. This process depends on the degree of dilution: in dilute solutions, the molecular chains have no connection with each other and exist as a separate polymer coils in the solution. Increase in concentration which can occur via solvent evaporation, induces interaction between molecular chains and a possible crystallization as in the crystallization from the melt.<ref name="Lehmann"/> Crystallization from solution may results in the highest degree of polymer crystallinity. For example, highly linear polyethylene can form platelet-like single crystals with a thickness on the order 10–20&nbsp;nm when crystallized from a dilute solution. The crystal shape can be more complex for other polymers, including hollow pyramids, spirals and multilayer dendritic structures.<ref name=sp/>

A very different process is precipitation; it uses a solvent which dissolves individual monomers but not the resulting polymer. When a certain degree of polymerization is reached, the polymerized and partially crystallized product precipitates out of the solution. The rate of crystallization can be monitored by a technique which selectively probes the dissolved fraction, such as [[nuclear magnetic resonance]].<ref name="Lehmann">{{cite journal|author = J. Lehmann|title = The observation of the crystallization of high polymer substances from the solution by nuclear magnetic resonance|journal = Colloid & Polymer Science|volume = 212|number = 2|year = 1966|pages = 167–168|doi = 10.1007/BF01553085}}</ref>

==Degree of crystallinity==
The fraction of the ordered molecules in polymer is characterized by the degree of crystallinity, which typically ranges between 10 and 80%.<ref name=dens/>Higher values are only achieved in materials having small molecules, which are usually brittle, or in samples stored for long time at temperatures just under the melting point. The latter procedure is costly and is applied only in special cases. 

Most methods of evaluating the degree of crystallinity assume a mixture of perfect crystalline and totally disordered areas; the transition areas are expected to amount to several percent. These methods include [[density]] measurement, [[differential scanning calorimetry]] (DSC), [[X-ray diffraction]] (XRD), [[infrared spectroscopy]] and [[nuclear magnetic resonance]] (NMR). The measured value depends on the method used, which is therefore quoted together with the degree of crystallinity.<ref name="dens"/>

In addition to the above integral methods, the distribution of crystalline and amorphous regions can be visualized with microscopic techniques, such as [[polarized light microscopy]] and [[transmission electron microscopy]]).

<div style="float:right;margin-left:0.5em;">
{| Class = "wikitable float-right"
|+Degree of crystallinity (D, %) and densities of crystalline (ρ<sub>c</sub>) and amorphous (ρ<sub>a</sub>, g/cm<sup>3</sup>) polymers.<ref name=dens>{{cite book|url=http://books.google.com/books?id=_ad2mQ-b5cUC&pg=PA67|pages=67–78|title=Polymeric materials: structure, properties, applications|author=G. W. Ehrenstein, Richard P. Theriault|publisher=Hanser Verlag|year=2001|isbn=1569903107}}</ref>
|- 
! Polymer|| D || ρ<sub>c</sub>||ρ<sub>a</sub>
|-
| [[Nylon]] (PA66 and PA6) ||35–45 || 1.24 ||1.08
|-
| [[Polyoxymethylene]] (POM)||70–80 || 1.54|| 1.28
|-
| [[Polyethylene terephthalate]] (PET)|| 30–40|| 1.50|| 1.33
|-
| [[Polybutylene terephthalate]] (PBT)|| 40–50 || – || –
|-
| [[Polytetrafluoroethylene]] (PTFE) || 60–80|| 2.35|| 2.00
|-
| isotactic [[polypropylene]] ||70–80 || 0.95 ||0.85
|-
| atactic polypropylene|| ~0 || – || –
|-
| [[High-density polyethylene]] ||70–80 || 1.0 ||0.85
|-
| [[Low-density polyethylene]] ||45–55 || 1.0 ||0.85
|}
</div>
; Density measurements: Crystalline areas are generally more densely packed than amorphous areas. This results in a higher density, up to 15% depending on the material. For example, polyamide 6 (nylon) has crystalline density ρ<sub>c</sub> = 1.24 g/cm<sup>3</sup> and amorphous density ρ<sub>a</sub> = 1.08 g/cm<sup>3</sup>). However, moisture which is often present in the sample does affect this type of measurement.<ref name="Becker"/>

; Calorimetry: Additional energy is released upon melting a semicrystalline polymer. This energy can be measured with [[differential scanning calorimetry]] and compared with that released upon melting of the standard sample of the same material with known crystallization degree.<ref name="Becker"/><ref name="Ehrenstein2">Gottfried W. Ehrenstein, Gabriela Riedel, Pia Trawiel: [http://books.google.com/books?id=xythvkmwJxIC&pg=PA148 ''Practice of thermal analysis of plastics.''] Hanser Verlag, 2003, ISBN 3-446-22340-1</ref>

; X-ray diffraction: Regular arrangement of atoms and molecules produce sharp diffraction peaks whereas amorphous regions result in broad halos. The diffraction pattern of polymers usually contains a combination of both. Degree of crystallinity can be estimated by integrating the relative intensities of the peaks and halos.<ref name="Becker"/>

; Infrared spectroscopy (IR): Infrared absorption or reflection spectra from crystalline polymers contain additional peaks which are absent in amorphous materials with the same composition. These signals may originate from deformation vibrations of the regular arrangement of molecular chains. From the analysis of these bands, the degree of crystallinity can be estimated.<ref name="Becker"/>

; Nuclear magnetic resonance (NMR): crystalline and amorphous areas differ by the mobility of protons. The latter can be monitored through the line shape of NMR signals and used to estimate the degree of crystallinity.<ref name="Becker"/>

==Properties of semicrystalline polymers==
===Thermal and mechanical properties===
Below their glass transition temperature, amorphous polymers are usually hard and brittle because of the low mobility of their molecules. Increasing the temperature induces molecular motion resulting in the typical rubber-elastic properties. A constant force applied to a polymer at temperatures above T<sub>g</sub> results in a [[viscoelasticity|viscoelastic]] deformation, i.e., the polymer begins to [[Creep (deformation)|creep]]. Heat resistance is thus given for amorphous polymers just below the glass transition temperature.<ref name="Nentwig">Joachim Nentwig: [http://books.google.com/books?id=sIdhvOEdkvcC&pg=PA17 ''Plastic films''] (in German) Hanser Verlag, 2006, ISBN 3-446-40390-6</ref>

Relatively strong intermolecular forces in semicrystalline polymers prevent softening even above the glass transition temperature. Their elastic modulus changes significantly only at high (melting) temperature.<ref name=dens/>It also depends on the degree of crystallinity: higher crystallinity results in a harder and more thermally stable, but also more brittle material, whereas the amorphous regions provide certain elasticity and impact resistance.<ref name=sp/><ref name="Menges"/> Another characteristic feature of semicrystalline polymers is strong anisotropy of their mechanical properties along the direction of molecular alignment and perpendicular to it.<ref name="Bonnet">Martin Bonnet: [http://books.google.com/books?id=uGipGKUG3A0C&pg=PA149 ''Plastics in engineering applications: properties, processing and practical use of polymeric materials.''] (in German) Vieweg+Teubner Verlag, 2008 ISBN 3-8348-0349-9</ref> 

Plastics are [[viscoelasticity|viscoelastic]] materials meaning that under applied stress, their deformation increases with time (creep). The elastic properties of plastics are therefore distinguished according to the time scale of the testing to short-time behavior (such as tensile test which lasts minutes), shock loading, the behavior under long-term and static loading, as well as the vibration-induced stress.<ref>{{cite book|url=http://books.google.com/books?id=tj5TAUVIEXAC&pg=PA168|pages=168–169|title=Introduction to Materials Science for Engineers|author=James F. Shackelford|publisher=Prentice Hall|year=2009|isbn=0136012604}}</ref>

===Optical properties===
Semicrystalline polymers are usually opaque because of light scattering on the numerous boundaries between the crystalline and amorphous regions. The density of such boundaries is lower and thus the transparency is higher either for low (amorphous polymer) or high (crystalline) degree of crystallinity.<ref name="Menges"/> For example, atactic polypropylene is usually amorphous and transparent while [[tacticity|syndiotactic]] polypropylene, which has crystallinity ~50%, is opaque.<ref>{{cite book|url=http://books.google.com/books?id=FPrfQJuDE3YC&pg=PA287|pages=286–287|title=Polymer chemistry: properties and applications|author=Andrew J. Peacock, Allison R. Calhoun|publisher=Hanser Verlag|year=2006|isbn=1569903972}}</ref> Crystallinity also affects dyeing of polymers: crystalline polymers are more difficult to stain than amorphous ones because the dye molecules penetrate much easier through amorphous regions.<ref>{{cite book|url=http://books.google.com/books?id=YSjkkVI1QGQC&pg=PA11|page=11|title=Chemical principles of textile conservation|author=Ágnes Tímár-Balázsy, Dinah Eastop|publisher=Butterworth-Heinemann|year=1998|isbn=0750626208}}</ref>

==See also==
*[[Liquid crystal polymer]]

==References==
{{reflist|2}}

[[Category:Chemical processes]]
[[Category:Phase changes]]
[[Category:Polymers|Polymers]]
[[Category:Separation processes]]

[[de:Kristallisation (Polymer)]]
[[Category:2Fix]]