Temperature Measurements During Laser-Induced Graphitization of Polydimethylsiloxane

Laser-induced graphitization of polymers has attracted attention as a technique to simultaneously form and pattern graphitic carbon on and/or inside polymers. Depending on the laser parameters, the morphology and the crystallinity of the formed graphitic carbon vary with photochemical and photothermal effects. However, the mechanism of laser-induced graphitization of polymers has not been revealed in detail. In this study, we measure the temperature of a polymer’s surface during laser-induced graphitization to investigate the relationship between the laser parameter and the characteristics of the formed structure with regard to photothermal effects. An organosilicon polymer, polydimethylsiloxane (PDMS), was irradiated with high-repetition-rate femtosecond laser pulses at different laser scanning speeds. The temperature of the PDMS surface was measured using a thermography camera. Plano-convex lenses were located between the irradiated surface and the thermography camera for micro-thermographic measurements with a spatial resolution of ~40 µm. The morphology of the formed structure was observed using scanning electron microscopy (SEM), and the crystallinity of the structure was analyzed using Raman spectroscopy, respectively. SEM observations showed that the cross-sectional area of the formed structure increased with the decrease in scanning speed, suggesting that the amount of the energy absorbed by PDMS was positively correlated with the number of irradiated pulses per spot. Furthermore, for significantly low scanning speeds, the formation of cracks was observed in the formed structures, which may be attributable to the laser ablation induced by the excessive amount of energy absorbed by PDMS. Raman spectra showed peaks at ~1350, ~1600, and ~2700 cm^-1 that corresponds to the D, G, and 2D bands, respectively, indicating the formation of graphitic carbon. Also, a peak at ~520 cm^-1was observed in the obtained Raman spectra in the case of significantly low scanning speeds, indicating the formation of crystalline silicon. Taking the results of SEM observations into account, it can be assumed that crystalline silicon was formed owing to the high temperature induced by the excessive amount of energy absorbed by PDMS. It was confirmed in temperature measurements that the measured temperature increased with the decrease in the scanning speed, suggesting the increase in the amount of energy absorbed by PDMS with the increase in the number of irradiated pulses per spot. However, in the case of significantly low scanning speeds, the measured temperatures kept constant regardless of the scanning speed, indicating the saturation of the temperature of the laser-irradiated area. The saturation of the temperature at significantly low scanning speeds cannot be explained solely by the change in the amount of the energy absorbed by PDMS. One possible explanation for the saturation is the change in the energy diffusion property of the laser-irradiated area with the change in the morphology and the material composition. It can be suggested from the obtained SEM images that the bulk density of the laser-irradiated area may be changed with the formation of cracks, changing the energy diffusion property of the laser-irradiated area. Moreover, considering the obtained Raman spectra, the formation of crystalline silicon in addition to the graphitic carbon may change the energy diffusion property of the laser-irradiated area.