Surface treatment of biopolymer films Polylactic acid and Polyhydroxybutyrat with angular changing oxygen plasma ‒ More than just gradual purification

Abstract

Bio-based and biodegradable polymers such as polylactic acid (PLA) and polyhydroxybutyrat (PHB) are developing into increasingly important raw materials. Especially when it comes to replacing conventional polymers in order to save resources and reduce environmental impact. Such biopolymers often conform to mechanical requirements of industry, but inappropriate surface characteristics such as poor barrier properties and increased degradation limit their application. Appropriate surface modification, such as the deposition of thin amorphous hydrogenated carbon films (a-C:H) produced by low-temperature mediated radio frequency plasma-enhanced chemical vapor deposition (RF-PECVD) can provide a remedy here. Prior to this, the surfaces are mostly cleaned/activated with oxygen plasma to enable better deposition of the subsequent a-C:H layer. It has been noticed that the O2 treatment, strongly changes the surface properties. It is now to be determined how a change in the O2 plasma treatment geometry affects the surface properties and whether this can be used accordingly. Surface morphology was analyzed by AFM, surface free energy (SFE) was determined using a contact angle goniometer, and changed barrier properties were investigated via the water vapor transmission rate. Top surfaces were studied by synchrotron supported X-ray spectroscopy, revealing angle-dependent changes in C and O content and binding conditions. The present angle-dependent O2 treatment not only changed the surface roughness and its chemical composition, but also, contrary to expectations, improved the barrier properties of PLA up to 18% at highest value, while the transmittance of PHB increased by more than 10%. A similar change in the mobility of the contact angle hysteresis is observed for both materials.