The situation is different for the PCL/20%µHAp composite (Figure , 3C'). A clear weakening of the polymer-filler interface interactions can be observed. This fact can be attributed to two factors. Firstly, PCL has a very low glass transition temperature -around -60 °C . As a result, it exhibited the properties of a ductile material at the temperature used for sample preparation, which may have caused interphase weakening and plastic deformation of the matrix during microtome sectioning. Secondly, the main chain of PCL is more non-polar than that of PLA due to the presence of five CH2 groups. Consequently, PCL has a much lower affinity to form hydrogen bonds with HAp particles than PLA. Despite the poor adhesion, the hydroxyapatite particles appear to be well dispersed in the matrix, as in the case of PLA.