PEEK is a semicrystalline [[thermoplastic]] with excellent mechanical and chemical resistance properties that are retained to high temperatures. The processing conditions used to moldmould PEEK can influence the crystallinity and hence the mechanical properties. Its [[Young's modulus]] is 3.6 GPa and its tensile strength is 90 to 100 MPa.<ref>[http://www.makeitfrom.com/material-data/?for=Polyetheretherketone-PEEK Material Properties Data: Polyetheretherketone (PEEK)], www.makeitfrom.com.</ref> PEEK has a [[glass transition temperature]] of around 143 °C (289 °F) and melts around 343 °C (662 °F). Some grades have a useful operating temperature of up to 250 °C (482 °F).<ref name="Polymers, High-Temperature">{{cite book|author=David Parker|author2=Jan Bussink|author3=Hendrik T. van de Grampe|author4=Gary W. Wheatley|author5=Ernst-Ulrich Dorf|author6=Edgar Ostlinning|author7=Klaus Reinking|title=Polymers, High-Temperature |journal=Ullmann's Encyclopedia of Industrial Chemistry |date=15 April 2012| doi=10.1002/14356007.a21_449.pub3|isbn=978-3527306732}} {{subscription required}}</ref> The thermal conductivity increases nearly linearly with temperature between room temperature and [[Solidus (chemistry)|solidus]] temperature.<ref>J. Blumm, A. Lindemann, A. Schopper, "Influence of the CNT content on the thermophysical properties of PEEK-CNT composites", ''Proceedings of the 29th Japan Symposium on Thermophysical Properties'', October 8–10, 2008, Tokyo.</ref> It is highly resistant to [[thermal degradation]],<ref>{{cite journal |last1=Patel |first1=Parina |last2=Hull |first2=T. Richard |last3=McCabe |first3=Richard W. |last4=Flath |first4=Dianne |last5=Grasmeder |first5=John |last6=Percy |first6=Mike |title=Mechanism of thermal decomposition of poly(ether ether ketone) (PEEK) from a review of decomposition studies |journal=Polymer Degradation and Stability |date=May 2010 |volume=95 |issue=5 |pages=709–718 |doi=10.1016/j.polymdegradstab.2010.01.024|url=http://clok.uclan.ac.uk/1919/1/Hull_Patel_PEEK_Decomposition_Review_1.pdf }}</ref> as well as to attack by both organic and aqueous environments. It is attacked by halogens and strong [[Brønsted acid|Brønsted]] and [[Lewis acid]]s, as well as some halogenated compounds and aliphatic hydrocarbons at high temperatures. It is soluble in concentrated sulfuric acid at room temperature, although dissolution can take a very long time unless the polymer is in a form with a high surface-area-to-volume ratio, such as a fine powder or thin film. It has high resistance to biodegradation.
