Editing Polymer (section)

====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.