In this work polymeric samples of polylactic acid (PLA) were used, which has been the subject of research for over a century. In the beginning, PLA films (20x20 cm) with a thickness of around 1 mm were produced. The samples were created as the film was initially engraved on the marked points and then by applying pressure on the edges it breaks into smaller pieces. More information about how the PLA film were prepared can be found in our previously published paper .
Our main focus was to see how the time that the samples were under treatment affects their surface properties. Therefore, the samples have been treated separately via UVA irradiation and ultrasonication of High (860 kHz) and Low Frequency (20 kHz) for 1, 3 and 6 hours. Initially, 6 PLA samples are immersed in demineralized water. After 1 hour, the experiment was stopped and 2 samples were taken out, dried under ambient conditions, and kept in dark environment. Similarly, we took samples at 3 and 6 hours of the experiment. In another experiment, samples have been treated with atmospheric DBD plasma for 5, 10, 20 and 60 seconds.
The PLA samples were treated with UVA irradiation by immersing them in demineralized water in a UVA transparent baker and placing them on a stirring plate in the self-made UV reactor. The reactor includes three 11 W UVA lamps on each side, resulting to 66 W UVA irradiation while being stirred continuously. In the UV treatments, no photocatalytically active materials were included.
The ultrasonic processor UIP500hdT (20 kHz, 500 W) from Hielscher Ultrasound Technology, Germany, was used to generate low-frequency ultrasounds (20 kHz). The power intensity was set at 100 W/cm2 and delivered through a 2.2 cm diameter tip. The Ultrasound Multifrequency Generator fitted with the Ultrasound Transducer E805/T/M and an adapted glass reactor UST 02/500-03/1500 from Meinhardt® Ultrasonics, Germany, produced high-frequency ultrasound with a maximum output power of 400 W. The frequency was set to 860 kHz, and the power amplitude was set at 40%. An external Julabo recirculating cooler set to 20 °C was used for both low and high frequency to prevent the polymers from exceeding the glass transition temperature (Tg), which is low (about 55-60 oC) and to shield the high-frequency equipment, which can be destroyed at temperatures over 50 °C. Temperature was kept at 25±1 °C for ultrasonication treatments.
DBD plasma treatment
The dielectric barrier discharge (DBD) reactor, which was used for the experiments, consists of two parallel stainless-steel electrodes with 30x50 mm2 dimensions and a 7 mm thick alumina ceramic plate es dielectric. For the experiments the DBD plasma settings at ambient air were 16 KV at a frequency of 7 kHz with a spacing of 9 mm between the electrodes and the samples (including the 7 mm thick dielectric).
After the treatments and in order to see how they affect the surface of the PLA, the samples were analyzed using various characterization methods. Using X-Ray photoelectron spectroscopy (XPS) we can observe the change of the elemental surface composition, while with FTIR the concentration of the active surface groups can be estimated. Drop Contour Analysis (DCA) is used to characterize the wettability of the surface and Confocal Laser Scanning Microscopy (CLSM) to measure the surface roughness and observe the morphology of the surface, as CLSM provides 3-dimensional surface profiles, by capturing multiple two-dimensional images at various depths. Combining the results of each characterization method we are able to scrutinize how each pre-treatment method affects the surface of PLA samples, both chemically and morphologically.
X-Ray photoelectron spectroscopy (XPS)
XPS spectroscopy is used for chemical analysis and identification of the chemical state of the elements on the surface of a solid. In XPS spectroscopy the sample is exposed, under ultra-high vacuum conditions, to a monochromatic or non-monochromatic X-ray beam with defined energy (energy hv) that causes photionization and emission of photoelectrons. The XPS spectrum reflects the energy spectrum of the emitted photoelectrons and consists of a series of distinct responsive bands in the characteristic layers of the electronic structure of the atom .
The experimental investigations of the untreated and treated PLA samples were performed in an UHV chamber with a base pressure of below 5×10-10 mbar. The films were characterized by core level spectroscopy (XPS) using Al K radiation with 1486.6 eV photon energy of a non-monochromatic X-ray source (Omicron DAR 400). Emitted electrons were detected by a hemispherical analyzer (Omicron EA125) under an angle of 45° to the surface normal. The analyzer was operated with a constant pass energy of 50 eV for survey spectra and 20 eV for detail spectra. Since degradation of polymer films, that are exposed to non-monochromatic X-ray radiation, was observed before [22, 23] the XPS analysis was performed as fast detail scan of the C 1s, O 1s, regions and the survey spectra with a total exposure time of about 1 h. All spectra were displayed as a function of the binding energy with respect to the Fermi level. The XPS spectra have been charge-corrected by fixing the C 1s component of the aliphatic C-C/C‑H group to 285.0 eV. For quantitative XPS analysis, a Shirley-background-subtraction was employed. Photoelectron peak areas were calculated by fitting Gauss-type profiles optimized by the Levenberg-Marquard algorithm with the CasaXPS software. Photoelectric cross-sections calculated by Scofield  and asymmetry factors calculated by Yeh and Lindau  as well as the transmission function of the hemispherical analyzer have been considered for stoichiometric calculations.
Confocal Laser Scanning Microscopy (CLSM)
CLSM is an optical imaging technique widely used in the field of Materials Science. The principal of CLSM is quite similar to fluorescence microscopy however it can provide better vertical and lateral optical resolution and observation precision by combining the colour and laser intensity information from the camera and from the laser light photoreceptor, respectively. In the case of Confocal Laser Scanning Microscopy (CLSM) a laser beam is focussed on the surface of the sample. The reflected light is detected behind a pinhole aperture. This technique enables a lateral resolution that is one third better in comparison to a classic wide field microscope.
The measurements were performed using the microscope VK-X200K from KEYENCE. The good lateral resolution (approx. 160 nm) enables the accurate mapping of many samples. On the software side, several images can be combined to produce one large image, meaning that even when significantly enlarged, wide areas of the sample can be mapped. Because the topography of the sample is recorded, it is possible to conduct roughness analyses or profile sections. A wide field microscopic image is also captured at the same time , .
Drop Contour Analysis (DCA)
Drop contour analysis enables contact angle measurement in case of configurable pressures and temperatures. The contact angle θ is the most important value in order to characterise the wetting of surfaces with a liquid (Fig. 1). In accordance with the Young equation, it is directly dependent on the involved surface tensions between the solid and the drop, the solid and the atmosphere, and the drop and the atmosphere. The greater the surface tension between the solid and the atmosphere, the greater surface energy of the solid. The drop will attempt to spread out, which results in a small contact angle. The DCA measurements were performed using the Dataphysics OCA. The Dataphysics OCA enables contact angle measurements and the determination of associated material parameters under various conditions. Different atmospheres, such as nitrogen or vacuum conditions (pmin = 1 mbar) can be configured. The temperature can vary between room temperature and 1800°C. The video recording enables the recording of changes in the contact angle over time and the determination of the melt temperature. Due to the geometry of the furnace, the size of the drop is limited to a diameter of approximately 2 cm , .
Infrared Spectroscopy (IR)
Infrared spectroscopy (IR) is one of the basic spectroscopic techniques. Molecules can absorb electromagnetic radiation of the infrared range, resulting in molecular vibrations. Infrared spectroscopy is based on this principle. Vibrational modes of chemical bonds can be assigned to specific absorption energies. Each spectrum of infrared radiation is characteristic of each sample and is its fingerprint with absorption peaks which result respectively from the frequencies of vibrations between the bonds of the atoms that make up the material.
The characterization of the treated PLA samples by IR spectroscopy was carried out using the Bruker ATR-FTIR-spectrometer ALPHA-T FTIR , which is coupled with an Attenuated Total Reflexion (ATR) asseccory. A diamond crystal serves as the internal reflection element. The measurements were performed by pressing the sample onto the ATR-crystal. In a spectral range from 4000 to 400 cm‑1, 16 scans with a spectral resolution of 2 cm-1 were recorded for each sample .