Optical and chemical effects governing femtosecond laser-induced structure formation on polymer surfaces

Abstract

With the emergence of femtosecond technology, laser machining has recently led to the creation of novel porous structures on polymers. However, the mechanism behind their formation is yet to be understood. In this study, the dependence of femtosecond laser-induced surface structure on processing parameters is established at two distinct wavelengths (800 nm and 275 nm) for six different polymer films: LDPE, PC, PET, PLA, PMMA, and PTFE. All of the observed structures are then optically and chemically characterized as a first step towards elucidating their formation mechanism. The threshold fluence at operating conditions was determined to be the main parameter affecting porosity formation during machining. Furthermore, for transparent films, a transition from multiphoton to linear absorption is observed to occur at 800 nm but not at 275 nm. This shift in optical properties was determined to be a major contributor to incubation effects.These observations are also in agreement with UV/VIS analysis as measurements show that polymers with a cut-off wavelength lower than that of the laser beam undergo a shift in absorption behavior. Finally, some polymers experience a continuous darkening of their surface with increasing fluence due to an increasing degree of photo-oxidation.

Femtosecond laser-induced porosity on poly(ethylene) surfaces—A crystallographic and rheological study

“…The theoretical plausibility of this hypothesis on PMMA was also validated by the numerical simulations performed by Guay et al [20]. In our previous work, we further suggested that this phenomenon can occur on any polymer as long as its threshold fluence is large enough so that the melt layer is heated past Tc [12]. In summary, during femtosecond laser polymer ablation, energy delivery to the surface and subsequent boiling happens in the order of picoseconds [21].…”

“…This phenomenon was initially observed on the side walls of inscribed lines and holes on poly(methyl methacrylate) (PMMA) and poly(carbonate) (PC) [10,11]. Since then, homogeneous polymer irradiation under certain conditions has been shown to cause the formation of a porous microstructure over the whole surface [7,12,13]. The resulting increase in surface area can be greatly beneficial for a number of applications such as omniphobic materials [13] or biomedical implants [14].…”

“…With increasing power, more pores were observed to appear but their size diminished. In our two recent studies, the effect of fluence on laser-induced microstructure on several polymers was investigated [12,25]. While some polymers had their texture affected at high fluence, pore size was observed to be mostly unaffected by this parameter.…”

Laser Surface Structuring of Polymers and Functionalization

“…Finally, in a recent publication, Assaf et al have characterized the optical and chemical properties of polymer surfaces machined at two distinct wavelengths (λ = 800 nm and λ = 275 nm) (Assaf and Kietzig 2018). They have observed that polymer behavior depends on its cutoff wavelength λ c , below which the energy of a single photon is larger than the bandgap energy.…”

Ablation thresholds and morphological changes of poly‐l‐lactic acid for pulse durations in the femtosecond‐to‐picosecond regime

“…Recently, there have been reports on femtosecond laser irradiation in which laser-induced periodic surface structures unique to polymers, for avoiding hydrophobicity, have been confirmed. 1,6,12 It is also known that ablation thresholds are reduced when irradiation with shorter pulse duration is used, regardless of the number of laser pulses to which the sample is exposed. 13,14 Data on the basic characteristics of femtosecond laser processing are required to form PLLA into three-dimensional structures.…”