Editing Polypropylene (section)

==== Crystal structure of polypropylene ====
Isotactic polypropylene has a high [[Crystallization of polymers#Degree of crystallinity|degree of crystallinity]], in industrial products 30–60%. Syndiotactic polypropylene is slightly less crystalline, atactic PP is [[Amorphous solid|amorphous]] (not crystalline).<ref name="Domininghaus">{{Cite book|title=Kunststoffe : Eigenschaften und Anwendungen|last=Hans.|first=Domininghaus|date=2011|publisher=Springer Berlin|isbn=9783642161728|edition=8., bearb. Aufl|location=Berlin|oclc=706947259}}</ref>{{rp|251}}

===== Isotactic polypropylene (iPP) =====
Is polypropylene can exist in various crystalline modifications which differ by the molecular arrangement of the polymer chains. The crystalline modifications are categorized into the α-, β- and γ-modification as well as mesomorphic (smectic) forms.<ref>{{cite journal |doi=10.1002/macp.1964.020750113 |title=Crystalline forms of isotactic polypropylene |year=1964 |last1=Jones |first1=A. Turner |journal=Die Marco Chain |volume=75 |issue=1 |pages=134–58 |last2=Wood |first2=Jean M |last3=Beckett |first3=D. R. }}</ref> The α-modifications is predominant in iPP. Such crystals are built from lamellae in the form of folded chains. A characteristic anomaly is that the lame are arranged in the so-called "cross-hatched" structure.<ref>{{cite book |first1=G. |last1=Fischer |title=Deformations- und Versagensmechanismen von isotaktischem Polyp(i-PP) obrhalb dr Glasübergangstemperatur |trans-title=Deformations- und Versagensmechanismen von isotaktischem Polypropylen (i-PP) oberhalb der Glasübergangstemperatur |language=de |type=PhD Thesis |publisher=Universität Stuttgart |year=1988 |oclc=441127075 }}{{page needed|date=July 2018}}</ref> The melting point of α-crystalline regions is given as 185<ref name="melting behavior">{{cite journal |doi=10.1002/pol.1975.180130713 |title=Quantitative structural characterization of the melting behavior of isotactic polypropylene |journal=Journal of Polymer Science: Polymer Physics Edition |volume=13 |issue=7 |pages=1417–46 |year=1975 |last1=Samuels |first1=Robert J |bibcode=1975JPoSB..13.1417S }}</ref><ref>{{cite journal |doi=10.1016/0032-3861(86)90130-8 |title=Melting behaviour of isotactic polypropylene isothermally crystallized from the melt |journal=Polymer |volume=27 |issue=5 |pages=721–7 |year=1986 |last1=Yadav |first1=Y.S |last2=Jain |first2=P.C }}</ref> to 220&nbsp;°C,<ref name="melting behavior" /><ref name="ReferenceA">{{cite journal |doi=10.1002/pen.760070412 |title=Morphological transformations of polypropylene related to its melting and recrystallization behavior |journal=Polymer Engineering and Science |volume=7 |issue=4 |pages=309–16 |year=1967 |last1=Cox |first1=W. W |last2=Duswalt |first2=A. A }}</ref> the density as 0.936 to 0.946 g·cm<sup>−3</sup>.<ref>{{cite journal |doi=10.1016/0032-3861(84)90076-4 |title=On the lamellar morphology of isotactic polypropylene spherulites |journal=Polymer |volume=25 |issue=7 |pages=935–46 |year=1984 |last1=Bassett |first1=D.C |last2=Olley |first2=R.H }}</ref><ref>{{cite book |doi=10.1002/0471532053.bra025 |chapter=Physical Constants of Poly(propylene) |title=The Wiley Database of Polymer Properties |year=2003 |last1=Bai |first1=Feng |last2=Li |first2=Fuming |last3=Calhoun |first3=Bret H |last4=Quirk |first4=Roderic P |last5=Cheng |first5=Stephen Z. D |isbn=978-0-471-53205-7 }}</ref> The β-modification is in comparison somewhat less ordered, as a result of which it forms faster<ref name="Guan-yi Shi 1993">{{cite journal |doi=10.1002/macp.1993.021940123 |title=Melting behavior and crystalline order of ß-crystalline phase poly(propylene) |year=1993 |last1=Shi |first1=Guan-yi |journal=Die Makromolekulare Chemie |volume=194 |issue=1 |pages=269–77 |last2=Zhang |first2=Xiao-Dong |last3=Cao |first3=You-Hong |last4=Hong |first4=Jie }}</ref><ref>{{cite journal |doi=10.1002/macp.1995.021960125 |title=A stereochemical and statistical analysis of metallocene-promoted polymerization |journal=Macromolecular Chemistry and Physics |volume=196 |issue=1 |pages=353–67 |year=1995 |last1=Farina |first1=Mario |last2=Di Silvestro |first2=Giuseppe |last3=Terragni |first3=Alberto }}</ref> and has a lower melting point of 170 to 200&nbsp;°C.<ref name="melting behavior" /><ref name="Varga 1992">{{cite journal |doi=10.1007/BF00540671 |title=Supermolecular structure of isotactic polypropylene |journal=Journal of Materials Science |volume=27 |issue=10 |pages=2557–79 |year=1992 |last1=Varga |first1=J |bibcode=1992JMatS..27.2557V |s2cid=137665080 }}</ref><ref>{{cite journal |doi=10.1002/pol.1977.180150405 |title=Studies on the α and β forms of isotactic polypropylene by crystallization in a temperature gradient |journal=Journal of Polymer Science: Polymer Physics Edition |volume=15 |issue=4 |pages=641–56 |year=1977 |last1=Lovinger |first1=Andrew J |last2=Chua |first2=Jaime O |last3=Gryte |first3=Carl C |bibcode=1977JPoSB..15..641L }}</ref><ref name="ReferenceA"/> The formation of the β-modification can be promoted by nucleating agents, suitable temperatures and shear stress.<ref name="Guan-yi Shi 1993" /><ref>{{cite journal |doi=10.1016/0032-3861(68)90006-2 |title=Morphology of polypropylene crystallized from the melt |journal=Polymer |volume=9 |pages=23–40 |year=1968 |last1=Binsbergen |first1=F.L |last2=De Lange |first2=B.G.M }}</ref> The γ-modification is hardly formed under the conditions used in industry and is poorly understood. The [[mesophase|mesomorphic]] modification, however, occurs often in industrial processing, since the plastic is usually cooled quickly. The degree of order of the mesomorphic phase ranges between the crystalline and the amorphous phase, its density is with 0.916 g·cm<sup>−3</sup> comparatively. The mesomorphic phase is considered as cause for the transparency in rapidly cooled films (due to low order and small crystallites).<ref name="Domininghaus" />

===== Syndiotactic polypropylene (sPP) =====
Syndiotactic polypropylene was discovered much later than isotactic PP and could only be prepared by using [[metallocene catalysts]]. Syndiotactic PP has a lower melting point, with 161 to 186&nbsp;°C, depending on the degree of tacticity.<ref name="DerosaAuriemma2006">{{cite journal |last1=De Rosa |first1=C |last2=Auriemma |first2=F |year=2006 |title=Structure and physical properties of syndiotactic polypropylene: A highly crystalline thermoplastic elastomer |journal=Progress in Polymer Science |volume=31 |issue=2 |pages=145–237 |doi=10.1016/j.progpolymsci.2005.11.002}}</ref><ref>{{cite book |doi=10.1021/bk-1992-0496.ch008 |chapter=Structure and Morphology of Highly Stereoregular Syndiotactic Polypropylene Produced by Homogeneous Catalysts |title=Catalysis in Polymer Synthesis |volume=496 |pages=104–20 |series=ACS Symposium Series |year=1992 |last1=Galambos |first1=Adam |last2=Wolkowicz |first2=Michael |last3=Zeigler |first3=Robert |isbn=978-0-8412-2456-8 }}</ref><ref>{{cite journal |doi=10.1016/0032-3861(94)90978-4 |title=Crystallization, melting and morphology of syndiotactic polypropylene fractions: 1. Thermodynamic properties, overall crystallization and melting |journal=Polymer |volume=35 |issue=9 |pages=1884–95 |year=1994 |last1=Rodriguez-Arnold |first1=Jonahira |last2=Zhang |first2=Anqiu |last3=Cheng |first3=Stephen Z.D |last4=Lovinger |first4=Andrew J |last5=Hsieh |first5=Eric T |last6=Chu |first6=Peter |last7=Johnson |first7=Tim W |last8=Honnell |first8=Kevin G |last9=Geerts |first9=Rolf G |last10=Palackal |first10=Syriac J |last11=Hawley |first11=Gil R |last12=Welch |first12=M.Bruce }}</ref>

===== Atactic polypropylene (aPP) =====
Atactic polypropylene is amorphous and has therefore no crystal structure. Due to its lack of crystallinity, it is readily soluble even at moderate temperatures, which allows to separate it as by-product from isotactic polypropylene by [[Extraction (chemistry)|extraction]]. However, the aPP obtained this way is not completely amorphous but can still contain 15% crystalline parts. Atactic polypropylene can also be produced selectively using metallocene catalysts, atactic polypropylene produced this way has a considerably higher molecular weight.<ref name="Domininghaus" />

Atactic polypropylene has lower density, melting point and softening temperature than the crystalline types and is tacky and rubber-like at room temperature. It is a colorless, cloudy material and can be used between −15 and +120&nbsp;°C. Atactic polypropylene is used as a sealant, as an [[Insulating materials|insulating material]] for [[automobiles]] and as an additive to [[bitumen]].<ref>{{Cite book |title=Kunststoffchemie für Ingenieure: von der Synthese bis zur Anwendung |trans-title=Plastics chemistry for engineers: from synthesis to application |language=de |last=Wolfgang |first=Kaiser |date=2007 |publisher=Hanser |isbn=978-3-446-41325-2 |edition=2nd |location=München |oclc=213395068 |page=251 }}</ref>