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In the second part of this general study, the carbon fiber–PEEK interfacial shear strength is measured by means of a fragmentation test on single‐fiber composites. Different thermal treatments (continuous cooling from the melt, isothermal treatments and long melting temperature time) are applied to these model composites prior to testing. The results are systematically compared with the previously determined reversible work of adhesion between carbon fiber and PEEK. It is shown that physical interactions at the interface determine, to a large extent, the magnitude of the interfacial shear strength between both materials. However, it appears that the magnitude of the stress transfer from the matrix to the fiber is affected either by the existence of an interfacial layer or by a preferential orientation of the polymer chains near the fiber surface. The results obtained on systems that have been subjected to isothermal treatments (isothermal crystallization of PEEK) seem to confirm the existence of a transcrystalline interphase, the properties of which are dependent upon the crystallization rate of the matrix and the interfacial adhesion energy.

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The aim of this general study is to determine the physicochemical characteristics and mechanical properties of carbon fiber–PEEK interfaces. In the first part, the dispersive component of the surface energy and the electron acceptor–donor (acid–base) characteristics of PEEK polymer and different types of untreated and surface‐treated carbon fibers are determined by means of inverse gas–solid chromatography at infinite dilution. It appears, in particular, that the acid–base surface properties of PEEK and, consequently, the orientation of macromolecules near the surface, depend on the processing of this polymer. Moreover, according to previous work, an estimation of the adhesion energy, corresponding to physical interactions (London and acid–base interactions) at carbon fiber–PEEK interfaces is proposed. Whatever the surface characteristics of PEEK, the highest level of carbon fiber–PEEK adhesion is achieved in systems involving oxidized or sized carbon fibers.