Effect of the Thermal History of the Melt on the Degradation of the Surface Mechanical Properties of Poly(aryl-ether-ether-ketone) as Measured by Microindentation

In this study, the effect of the thermal history of the melt on the morphology and the mechanical properties of the surface of crystallized poly(aryl-ether-ether-ketone) (PEEK) films was followed by density measurements, X-ray diffraction, differential scanning calorimetry and microindentation. The study has shown a decrease in microhardness of the polymer surface as the temperature at which the polymer is kept in the molten state increases and as the time spent in the molten state becomes longer. The study suggests that keeping PEEK for long periods of time in the molten state, in the presence of oxygen, results in chemical changes (presumably crosslinking involving oxygen atoms) which are responsible for drastic changes in the morphological characteristics of the samples and consequently on the mechanical properties of the surface of the solidified material. Above 400 °C, in air, a total change in surface morphology takes place and the mechanical properties of the surface are degraded. The study also suggests that the negative effects of degradation can be avoided to a large extent if the samples are melted at temperatures below 385 °C for short periods of time and kept from oxygen exposure.