Figure 5: SEM photomicrograph of fracture surface: a) 0h exposure, b)
336h exposure and c) 575h exposure.
Conclusions
The evaluation of the experimental results obtained along with the
information contained in the literature indicates that PS when exposed
to UV-B radiation in an oxidizing atmosphere exhibits macromolecular
modification that influences its mechanical properties.
- The UV degradation of PS occurs preferably by cleavage of the
molecular chains, as indicated by chromatography and gel content. No
crosslinking is produced; however, there is a reduction in molecular
mass as a function of exposure time in the surface region.
- Exposure to UV radiation at exposure times of 336 and 575 h was not
sufficient to change the decomposition temperature of PS, as this
remained virtually unchanged. However, significant changes in the
degree of crystallinity were obtained and attributed to a possible
crystalline rearrangement after cleavage of the chains.
- Regarding the mechanical behavior of PS, UV radiation alters its
mechanical fatigue, resulting in changes in specimen properties, such
as the reduction in the number of cycles accumulated until fracture.
This behavior is possibly related to the surface conditions of the
specimen and its degradation thickness.
- Relating the fracture surface characteristics, it appears to be a
mixture of brittle cleavage of molecular chains with ductile rotation
and stretching of these chains.