Nanoscale Chemical Characterization and Imaging of a Recycled Post-Consumer Waste Polyolefin Blend using AFM-IR

Abstract

The recycling of polyolefins is a key aspect to reduce the amount of improperly disposed plastic waste. Polyolefins, such as polypropylene (PP) and polyethylene (PE), are the most produced polymer group in Europe, it is thus of great interest ensure efficient recycling. The need to sort plastic waste to avoid the effect that the presence of other polymers has on the mechanical properties of a blend increases the cost of the process. Therefore, blends deriving from heterogeneous waste streams that retain desireable properties could have a cost-advantage. To better understand these blends, we must also understand their nanoscale structure and chemical composition. Currently, nanoscale characterization of polymers is carried out using techniques that do not have chemical sensitivity – without prior staining or relying on previous good knowledge of its chemical composition. Examples of such techniques include SEM, TEM and AFM. These approaches often rely on a the predictable chemistry ensured by the tight process control existant in the production of virgin polymers. Thus, they may prove insufficient for the analysis of complex recyclates. AFM-IR (also known as PTIR), is a rather recent scanning probe-based nanoscale IR technique that combines the resolution of an AFM with the chemical information provided by IR spectroscopy. In addition to reaching lateral resolutions of ≈ 10 nm in tapping mode, AFM-IR benefits from the long established spectra-structure correlations established for bulk IR spectroscopy. In this work, we demonstrate how tapping mode AFM-IR can be applied to the analysis of a PE/PP/rubber blend derived entirely from post-consumer waste collected at the municipal level. In addition to the presence of sub-micron particles of other polymers (polyamide and polyurethane), we also imaged small (≈500 nm diameter) PP droplets that are present inside the PE phase. Furthermore, the presence of an interface between the PE and PP was detected with the help of a gaussian mixture model and identified as EPR rubber. The presence of the interface is observable in both the IR maps and full AFM-IR spectra obtained in the same area, and is further confirmed by reference TEM measurements. We believe this work demonstrates the applicability and usefulness of AFM-IR to the analysis of complex polymers at the nanoscale.