Structure and features determined for pores created with femtosecond laser machining of polymers

Femtosecond laser pulses have the ability to roughen materials on both the nano- and microscale. As the technology used to produce these microstructures improves, recent work showcases the technique’s potential to overcome the low absorbance of polymer surfaces and to one day be used for better biomedical implants and superhydrophobic “self-cleaning” materials. Although relatively little is yet known about femtosecond laser machining of polymers, recent work looks to shed light on how irradiation affects the surfaces of these materials.

Researchers characterized the effects of femtosecond laser machining on the crystallographic structure of high-density polyethylene (HDPE), a common polymer with a straightforward chemical composition. Publishing their findings in the Journal of Applied Physics, the team used computed tomography, scanning electron micrography and grazing incidence X-ray diffractometry (GIXRD) to measure deformations to HDPE’s crystalline phase, investigate pores’ internal structures and determine how changes in a polymer’s molecular weight affected pore sizes.

GIXRD revealed that the orthorhombic phase typically observed in HDPE deforms to a monoclinic crystalline phase, with only one of the axes perpendicular to the other two, accompanied by a local decrease in crystallinity. This deformation confirmed that the rapid quenching of a superheated melt layer, undergoing explosive boiling, causes the appearance of porosity.

In order to optimize the porous structures for specific applications, the authors investigated the properties affecting pore size. Viscoelastic melt properties played a major role, as small angle oscillatory shear tests indicated that final pore size decreases with increasing average molecular weight or increasing melt viscosity.

According to the researchers, tailoring polymer pore sizes may one day lead to materials optimized to provide scaffolding for cell growth in biomedical devices, a key feature that would improve biocompatibilty.

Source: “Femtosecond laser-induced porosity on poly(ethylene) surfaces – A crystallographic and rheological study,” by Youssef Assaf, Mark Zhao, and Anne-Marie Kietzig, Journal of Applied Physics (2018). The article can be accessed at https://doi.org/10.1063/1.5039849 .