Controlling the sp2/sp3 ratio of amorphous hydrogenated carbon layers (a-C:H) on polylactide acid (PLA) at constant distance but angle-dependent deposition

Abstract

The biopolymer polylactide acid (PLA) is suitable as an eco-friendly substitute for conventional polymers, but polymer surfaces often do not meet industrial requirements and thus severely limit applications. Surface adhesion, substrate related decomposition and barrier permeability can be modified with plasma-deposited thin amorphous hydrogenated carbon (a-C:H) coatings. Their properties depend significantly on the sp2/sp3 ratio, here to be controlled by changing the coating geometry between sample and plasma source. A 50 µm PLA film was a-C:H coated at different angle settings (30° intervals) for 2.5, 5.0, 7.5 and 10.0 min to analyze angle-dependent layer growth. Surface morphology was recorded by scanning electron and atomic force microscopy, and the surface free energy (SFE) and contact angle were determined. Modified barrier properties such as the water vapor transmission rate were investigated and showed a reduction in permeability of up to 47% for defined coating geometries and times. In addition, investigations of the top a-C:H layers by synchrotron assisted X-ray techniques revealed angle-dependent changes in the sp2/sp3 binding ratio. The angle-dependent coating not only changes the chemical composition of the layer, but also improves the barrier properties and extends the SFE of PLA by an increased polar fraction.