Figure 4: DSC Results obtained for all conditions.
With respect to the fatigue results, it should be noted that as the time
of exposure to UV radiation increases, the number of cycles changes,
probably due to the surface degradation of the samples. This behavior
results from the embrittlement of irradiated PS due to its high
crystallinity and reduction of molecular weight produced by the
degradation caused by UV radiation [20].
SEM micrographs of the fracture surfaces of the fatigue-tested specimens
are shown in Figure 5. According to the equivalent stress amplitude
levels, in which all evaluation groups were subjected, the PS in the
non-irradiated condition supports a much higher number of cycles than
the UV degraded material. All samples exposed to UV radiation displayed
a predominantly brittle fracture, indicating that the main failure mode
for the mechanical fatigue mechanism. However, when analyzing the
fracture surface of the non-irradiated PS, regions of brittle behavior
and ductile appearance are observed, which is typical of mixed fractures
[28].
The brittleness of the degraded surface, confirmed by the reduction in
molecular mass, is evidenced in the micrographs of the UV degraded
groups. In addition, the progressive recovery of resistance observed in
the fatigue test is noteworthy because the fracture surface of the PS
575 UV group has ductile regions. These regions could be associated with
molecular chain movements such as rotation and stretching.