Microplastics are formed by the degradation of plastic wastes under the action of physicochemical mechanisms in environment, and they are contaminants of emerging concern that have been received considerable attention in recent years due to their adverse impact on living organisms and the environment. However, research on the aging characteristics and mechanism of microplastics is limited. For example, common polymers exposed to the environment are adversely affected by solar radiation (primarily ultraviolet (UV) UV-B), which initiates photooxidative degradation. Thus, exposure to ultraviolet UV radiation may provoke significant degradation of their structure since it results in breaking of the polymer chains, produces free radical and reduces their molecular weight, causing though the deterioration of their mechanical properties after an unpredictable time. In the present study, to improve understanding of aging process of microplastics, four of the most widely used polymers covering a wide spectrum of applications, due to their excellent chemical inertness and high processability in the present study, such as low-density polyethylene (LDPE), high-density polyethylene (HDPE), polypropylene (PP) and polystyrene (PS) in the form of thin films were exposed to UV radiation at 254 nm with constant temperature (25 °C) and constant relative humidity (50%) for several times. After exposure, the films were removed from the chamber (after 5, 10, 20, 30, 45 and 60 days of radiation) and UV irradiation influence was evaluated by using FTIR (Fourier-Transform Infrared) Spectroscopy, DSC (Differential Scanning Calorimetry) measurements, XRD (X-Ray Diffraction), Py-GC/MS (Pyrolysis-Gas Chromatography/Mass Spectroscopy), SEM (Scanning Electron Microscopy), while their mechanical properties were also evaluated. From FTIR spectroscopy it was found that new carbonyl, vinyl, and hydroxyl/hydroxyperoxide groups were formed during UV exposure, while XRD, DSC and mechanical measurements boosted the obvious effect of UV irradiation in their crystalline, thermal and mechanical properties. SEM micrographs revealed the significant morphological alterations at the irradiated samples, due to the appearance of defects and holes at their surface, revealing extended decomposition after just 30 days of UV exposure. Finally, the mechanism of thermal degradation of the four polymers before and after UV exposure was studied by Py-GC/MS.

Acknowledgments
This research was financially supported by the Greek Ministry of Development and Investments (General Secretariat for Research and Technology) through the research project “Intergovernmental International Scientific and Technological Innovation-Cooperation. Joint declaration of Science and Technology Cooperation between China and Greece” (Grant no: T7ΔKI-00220).