Laser-induced nanometer-nanosecond expansion and contraction dynamics of poly(methyl methacrylate) film studied by time-resolved interferometry

The expansion and contraction behavior of a neat poly(methyl methacrylate) film is induced by 248 nm excimer laser exitation and directly measured by nanosecond interferometry. Above the ablation threshold (1400 ml/cm²), the film expands during the excitation laser pulse, and after 25% expansion of the thickness, it undergoes fragmentation. Below the threshold, the expansion rate is still high (a few nm/ns), and permanent swelling is observed after transient expansion and succeeding contraction. By decreasing the laser fluence below 800 mJ/cm², permanent swelling was not observed, and at a few tens of mJ/cm², novel oscillatory expansion and contraction behavior was successfully detected. The expansion amplitude as a function of laser fluence indicates that the polymer film undergoes phase transition from glass to rubber upon excitation with a fluence higher than 450 mJ/cm². In the glass state, the interpenetrating structure of the polymer film and associated free volume distribution may be changed upon irradiation, and photochemical degradation should be involved. A dynamic view of these polymer properties explains well the unique expansion and contraction behavior. Furthermore, these laser-induced morphological changes are compared with those of polymer films doped with aromatic molecules and considered in general.