A study of photothermal laser ablation of various polymers on microsecond time scales
To analyze the photothermal ablation of polymers, we designed a temperature measurement setup based on spectral pyrometry. The setup allows to acquire 2D temperature distributions with 1 μ m size and 1 μ s time resolution and therefore the determination of the center temperature of a laser heating process. Finite element simulations were used to verify and understand the heat conversion and heat flow in the process. With this setup, the photothermal ablation of polystyrene, poly( α -methylstyrene), a polyimide and a triazene polymer was investigated. The thermal stability, the glass transition temperature T g and the viscosity above T g were governing the ablation process. Thermal decomposition for the applied laser pulse of about 10 μ s started at temperatures similar to the start of decomposition in thermogravimetry. Furthermore, for polystyrene and poly( α -methylstyrene), both with a T g in the range between room and decomposition temperature, ablation already occurred at temperatures well below the decomposition temperature, only at 30 – 40 K above T g . The mechanism was photomechanical, i.e. a stress due to the thermal expansion of the polymer was responsible for ablation. Low molecular weight polymers showed differences in photomechanical ablation, corresponding to their lower T g and lower viscosity above the glass transition. However, the difference in ablated volume was only significant at higher temperatures in the temperature regime for thermal decomposition at quasi-equilibrium time scales.
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