The effects of femtosecond laser ablation, with 115 fs pulses at 1040 nm wavelength and 57 MHz repetition-rate, on the physical and chemical properties of polylactide (PLA) were studied in air and in water. The surface of the PLA sample ablated by high-repetition-rate femtosecond laser was analysed using field emission scanning electron microscopy, infrared spectroscopy, raman spectroscopy, as well as X-ray photoelectron spectroscopy. Compared with the experiments in the air at ambient temperature, melting resolidification was negligible for the experiments conducted under water. Neither in air nor under water did oxidation and crystallization process take place in the laser ablated surface. In addition, the intensity of some oxygen related peaks increased for water experiments, probably due to the hydrolysis. Meantime, the chemical shift to higher energies appeared in C1s XPS spectrum of laser processing in water. Interestingly, a large amount of defects were observed after laser processing in air, while no significant change was shown under water experiments. This indicates that thermal and mechanical effects by high-repetition-rate femtosecond laser ablation in water are quite limited, which could be even ignored.
© 2015 Optical Society of America
Polylactide (PLA) is a kind of biodegradable aliphatic polyester material with good biocompatibility, which is extensively used in biomedical applications. However, the physical and chemical properties of PLA are required to remain unaltered during micromachining of biomedical device. For example, its application in laser micromachining is very suitable for precision material processing . The effects of manufacturing on the basic properties such as biodegradability were recently studied by several research groups. P. Slepička et al. reported that KrF laser exposure would strongly influence the surface chemistry of PLA . Rytlewski et al. reported that ArF laser radiation could induce various changes through simultaneous processes such as photooxidation, photo- and thermal-degradation . Stępak et al. found that KrF excimer laser treatment would affect the degradation time and the mechanical properties of laser machined samples . Antończak et al. discovered that CO2 laser could lead to thermal decomposition of PLLA . Stępak et al. reported that CO2 laser micromachining of PLA could significantly affect its mechanical properties and the heat affected zone (HAZ) induced by CO2 laser could reach the order of 100 microns . To reduce the heat affected zone need a thorough understanding of the effects of laser process.
In order to understand the effects of laser machining, the experimental results at ambient temperature should be considered. For example, laser machining in water is the result of combined action of laser etching and surrounding liquid medium . When a laser heats target, a plasma forms in the interface of solid and liquid, and water confines the expanding plume of plasma, increasing the pressure and temperature . High temperature and pressure can enhance hydrolysis of polymers . The significant pressure effects can cause substantial damage to the polymer . After the laser is switched off, an adiabatic cooling of the plasma can increase the duration of the applied pressure over a longer period, which is about twice of the laser-pulse duration . S. Singh et al. demonstrated ablation process was through a combination of photo-thermal and photo-chemical mechanism . M. Malinauskas et al. reported on femtosecond laser-assisted micromachining of PLA in water [13,14 ], they used water as a media to generate light filaments by high laser intensity, and machine PLA by the light filaments. However, to the best of our knowledge, the effects of laser micromachining on the physical and chemical properties of PLA in water have not been reported.
In this work, we studied the effects of high-repetition-rate femtosecond laser ablation on the physical and chemical properties of PLA both in air and in water. The morphological features were observed with scanning electron microscope (SEM). To better understand this chemical process, laser ablated surface was investigated using an Infrared spectrometer, a micro-Raman spectrometer and an X-ray Photoelectron Spectroscopy (XPS).
The samples used were 0.3-mm-thick Polylactic acid (PLA) sheets (Changchun SinoBiomaterials, China). A homemade femtosecond laser provided 115 fs (FWHM) linear polarization pulses at a wavelength of 1040 nm and a repetition rate of 57MHz. Laser beam was focused on the sample surface through a long working distance objective lens with a numerical aperture of 0.40. The PLA target was fixed on the inner surface of a spectroscopy-type quartz cell. And the quartz cell was mounted on a three-dimensional translation stage with micron resolution, which was driven by a computer. The PLA target was perpendicular to the incident laser beam. Laser pulses scanned perpendicular to the polarization direction on PLA. Before the samples were ablated in water, the quartz cell was filled with deionized water. After laser ablation, samples were dried. The laser machined grooves were tested using field emission scanning electron microscopy (FESEM, Nova NanoSEM 430, FEI, USA). Specular Reflection Fourier Transform Infrared (SR-FTIR) spectra of PLA, before and after laser processing, were tested using a Bio-Rad FTS 6000FTIR spectrometer. Raman spectroscopy, measured with an InVia-Reflex system, was carried out to analyse chemical changes induced by femtosecond laser ablation of PLA. The XPS spectra were recorded using a Kratos Axis Ultra DLD multi-technique employing a monochromated Al-Ka X-ray source.
3. Results and discussion
3.1 Morphological and surface characterization
The features of laser ablated line at laser power 0.8 W, corresponding to 0.38 TW/cm2 (energy per pulse duration per focal spot ), at different scanning speeds are shown in Fig. 1 . At scanning speed of 0.04mm/s, corresponding to pulse-to-pulse overlap 4528571 pulses per focal spot, strong ablation occurred, with large bumps and cracks observed in laser ablated line at Fig. 1(a). There are a few microns cavities on the bump, which come from the gas release during thermal degradation of PLA. Laser heating area is larger and deeper in strong ablation. A mass of gas produced and released result in the cracks under large pressure. While at higher scanning speed of 0.16 mm/s, corresponding to pulse-to-pulse overlap 1132143 pulses per focal spot, subtle ablation appeared, with microns bumps and nano holes formed on the surface in Fig. 1(b). The feature of heat melting was clearly observed in the rim of laser ablated groove. Therefore, larger heat-affected zone formed during strong ablation, and limited heat-affected zone formed during the subtle ablation.
The process of melting and resolidification in laser ablated region depends on the material thermal properties. The polymer is thermally dissociated into volatile fragments, and the pressure developed during the phase separation creates cavitation bubbles . As a result, high repetition rate ultrashort pulse laser ablation of PLA was heterogeneous, with the formation of large bump in the strong ablation.
Regarding femtosecond laser treatment of PLA at a repetition rate of 1 kHz or 100 Hz , the appearance of the quasi-periodic structures was not found upon the induction of linear polarization laser at 57 MHz. The quasi-periodic structures were reported on the surface of polymer sample by Ezquerra et al . They believed that the accumulation of laser pulses on the surface could result in cycles of heating and cooling and imprint the interference pattern between the incident laser beam and the scattering light or Plasmon polaritons on the sample surface. However, Eaton et al. discovered that the cumulative heating in femtosecond laser processing was far stronger at high repetition rates, which were greater (>200 kHz) than 1 kHz or even less [19,20 ]. We think that the pulse interval in this case might be so short that the heat could not diffuse from the focal volume, and the cumulative heating at a repetition rate of 57 MHz would break the cycles of heating and cooling, thus preventing the formation of quasi-periodic structure.
Interestingly, laser ablation of PLA in water does not follow the behavior in air. There are clear differences in the nature of ablation.
The features of laser ablated line at laser power 1.4 W, corresponding to 0.68 TW/cm2 (energy per pulse duration per focal spot) at different scanning speeds under water are shown in Fig. 2 . At scanning speed of 0.01mm/s, corresponding to pulse-to-pulse overlap 18114285 pulses per focal spot, strong ablation produced groove with redeposition in the rim. This is because under the water confinement, transient melt-flow produced by laser heating ejected out and resolidified. With laser scanning, a deep groove formed with large volume resolidified melting in the rim. While at scanning speed of 0.16mm/s, corresponding to pulse-to-pulse overlap 1132143 pulses per focal spot, a distinctive morphology appeared in subtle ablation under water. There were some random distribution nano holes and microns craters on the surface of subtle laser ablation. As no melting trace around the nano holes, they could not come from gas releasing in melting. The plausible explaination was that the sample occurred dissolve in water during laser subtle ablation. The water confinement increased the pressure and temperature in the interface of water and PLA in the laser focal spot. At certain pressure and temperature, some sort of hydrothermal reaction could happen . The removal mechanism of PLA may be through the combination of hydrolysis with subtle laser ablation. When the PLA surface underwent random degradation and subtle laser ablation, the smooth surface changed into a rough surface with large craters . This suggests that rapid and permanent dissolution of polymer may be possible, yielding no trace of redeposited molten and resolidified material from laser ablation. But when the pressure and temperature are too high, strong eruption of melt would occur to give resolidification on the rim of the groove in Fig. 2(a).
Choo et al. reported that water could carry away the debris and eliminate the redeposition of molten materials during laser silicon micromachining in water . Water may take away the hydrolysis products and give a clean groove after laser processing under water.
The appearance of resolidified melting material may result from the nature of polymer. Tokarev et al. reported that the polymeric viscous liquid in the transient melt bath produced with laser exposure is expelled out by the ablation pressure and long fiber form . In this case, a quick increase of jet viscosity might cause more efficient cooling during jet propagation in water. Therefore, resolidification occurred on the rim of the groove.
3.2 Infrared spectroscopy (FTIR) measurements
The infrared spectra of PLA, before and after femtosecond laser ablation in air, were overlapped for comparison in Fig. 3(a) . Stępak  and Vasanthan  suggested that the bands at 921 and 955 cm−1 of PLA could be used to characterize the crystalline and amorphous phases, respectively. The baselines of laser ablated sample were larger than that of the pristine sample in IR spectra owe to the rough surface of laser ablated samples. Therefore, the intensity of band at 955 cm−1 was equal before and after laser process approximately, which indicated that amorphous state of PLA samples was maintained. No peak appears at 921 cm−1 during laser process, indicating crystallization process did not occur. A lack of crystallization may result from the high cooling rate due to large temperature gradient by laser ablation, i.e. the heat effect is not significant for high-repetition-rate femtosecond laser ablation in air.
A new weak peak around 1647 cm−1 for the carbon-carbondouble bond (C = C) appears after laser exposure, implying the thermal decomposition of PLA .
Oxygen related peaks: –C = O bend (1269 cm−1), –C = O carbonyl stretch (1750 cm−1), –C–O– stretch (1184, 1088 cm−1), –OH bend (1045 cm−1) are also observed . A decrease in the intensity of these peaks occurs. The significant decrease of peak intensity at 1750 cm−1 implies the number of aliphatic ester segments decreased . The photooxidation of the surface did not occur in spite of the presence of oxygen in the ambient atmosphere. Therefore, we conclude the degradation in high-repetition-rate femtosecond laser ablation in air likely went through a nonoxidative thermal process.
The infrared spectra of PLA, before and after femtosecond laser ablation in water, were overlapped for comparison in Fig. 3(b). The unchanged band intensity at 955 cm−1, as well as no band at 921 cm−1, indicates crystallization process did not occur, i.e. the heat effect is weaker in water for high-repetition-rate femtosecond laser ablation.
Compared with laser ablation in air, the changes in the intensity of the oxygen related peaks decrease for laser ablation in water. The band intensities at 1750, 1088, and 1184 cm−1 all decrease, however, the intensities of –C = O bond (1269 cm−1) and –OH bond (1045 cm−1) increase. Such an increase in the intensity of some oxygen related peaks may result from hydrolysis. Compared with micromachining in air, the peak at 1647 cm−1 for the carbon-carbon double bond (C = C) disappeared, indicating that the mechanism may be due to hydrolysis rather than thermal degradation  of PLA in high-repetition-rate femtosecond laser ablation under water.
3.3. Micro-Raman measurements
The raman spectra of PLA before and after laser ablation in air are shown in Fig. 4(a) . The strong background photoluminescence was observed in laser ablated regions. Since femtosecond laser ablated PLA led to ultrafast heating and cooling, structural defects and voids were produced. As a result, significant amount of defects appeared after a quick thermal quenching and resolidification . The raman spectra of the PLA bottom upon laser ablation is stronger than that of the rim of the groove. Due to the major role that the basic structure plays in determining mechanical properties of the bulk polymer, such laser treatment would affect the final outcome.
The peaks at 873 and 1454 cm−1 are the νC-COO and δCH3 asymmetric modes , respectively. We removed the intensity of the background photoluminescence from total intensity of Raman peaks. The intensity ratio of these two peaks (I873/I1454 = 2.56) in the bottom of laser ablated groove is smaller than that (I873/I1454 = 2.65) of pristine sample, and the intensity ratio (I873/I1454 = 2.66) in the edge of laser ablated groove is the same as that of pristine sample. Taddei et al.  reported that the intensity ratio decreased with degradation of PLA. Therefore, the degradation occurred in the bottom of laser ablated line only.
The band of C = O stretch, located at 1771 cm−1 for the amorphous sample spectrum, does not change before and after laser processing, indicating that crystallization process (peak splitting) did not occur . This could be explained by the high cooling rate resulted from large temperature gradient by laser ablation, i.e. the heat effect is weaker in high-repetition-rate femtosecond laser ablation in air.
Raman spectra of PLA surface before and after laser ablation in water are shown in Fig. 4(b). The intensity ratio of these two peaks (I875/I1452 = 2.5) in the bottom of laser ablated groove is smaller than that of pristine sample, indicating hydrolysis occur during laser processing in water. A decrease in C = O groups would indicate the hydrolytic degradation of PLA, and the hydrolysis products leaked out as soluble oligomers within the polymer . The spectral background photoluminescence (PL) in laser ablated regions does not change significantly. The band of C = O stretch, located at 1771 cm−1 for the amorphous sample spectrum, also shows no change, indicating crystallization process (peak splitting) did not occur. Therefore, thermal and mechanical effects by high-repetition-rate femtosecond laser ablation in water are quite limited, which could even be ignored.
3.4 Surface Chemistry
The detailed XPS spectra of PLA samples before and after ablation with laser pulses are shown in Fig. 5(a) . The carbon XPS spectrum of pristine PLA sample consists of three peaks, corresponding to C-H group (284.6 eV), C-O group (286.6 eV) and C = O group (288.7 eV) as seen in Fig. 5(b). Compared to the pristine sample, the intensities of C-O and C = O groups after laser ablation slightly decreased, which could be explained by the breaking of the main chain in the area of ester groups  due to surface photodegradation by laser exposure. In addition, the right peak broadened clearly. Polzonettiet al. discovered that CO2 could interact with polymer molecular to form weakly bonded C = O groups . During the process of laser ablation, gas particles (CO2) released from thermal degradation or photodegadation, then interacted with the surface, causing the broad right peak of laser ablated sample.
The O/C ratios lightly decreased after laser ablation in air, from initial ratio of 55.3% to 44.9% upon laser exposure, it is consistent with the decrease in the intensity of total oxygen related peaks in IR spectroscopy. It indicated that no photooxidation occured on the sample surface in spite of the presence of oxygen in the ambient atmosphere. A similar deoxidation was also reported when PLA was ablated with ns laser pulses  and μs laser pulses .
C1s XPS spectrum shows a clear chemical shift, especially for the right peak to higher energy region (Fig. 5(c)). Wochnowski et al. [32,33 ] reported the same results in laser ablated PMMA sample, and explained this phenomenon by the existence of a free radical generated during laser processing. We assume that a free radical maybe produced during hydrolysis in our laser ablation under water. The hydrolysis products were not detected as water could carry them away during laser processing .
The O/C ratio also slightly decreased after laser ablation in water, from the starting ratio of 55.3% to 49.3% upon laser exposure under water. Compared with micromachining in air, the descend range of the O/C ratio is lower. Because the FTIR intensity of partial oxygen related peaks slight increased under water, the decrease extent of the O/C ratio should be smaller than that in air. The results obtained from the FTIR and XPS are coincident.
We described the surface and physico-chemical properties of PLA upon ablation with high-repetition-rate femtosecond laser, in air and in water, respectively. Two major differences between laser processing in air and under water were observed. The first one is the absence of laser damage in the rim of laser ablated groove under water while significant laser damage is present in air. The second one is the mechanism of the material removal. The ablated surface swells in the case of laser micromachining in air, while it shows random distribution nano holes and microns craters on the surface for micromachining under water. The surface of PLA ablated in air indicates that molten PLA could rapidly solidify with a nanofoam structure, while the surface of ablated PLA under water displays a clean groove through selective hydrolysis.
A nonoxidative process occurred in high-repetition-rate femtosecond laser ablation in air and under water, however, the mechanical strength could be modified by high-repetition-rate ultrashort pulse laser micromachining. High-repetition-rate femtosecond laser ablation of PLA in water may induce permanent dissolution of polymer, giving no trace of laser micromachining. These findings have important implications in the context of polymer and biomaterial processing.
This work was supported by the National Natural Science Foundation of China (NSFC) (Grant No. 61275131 and No. 61322502), and the National Basic Research Program of China (Grant No. 2011CB808101).
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