Phase identification and interfacial transitions in ternary polymer blends by ToF-SIMS

Abstract

Phase identification and the study of the interphase region in multi-component polymer blends with a chemically similar structure using conventional techniques is a challenge. In this work, the detailed morphological analysis of such systems is examined. A ternary blend comprised of poly butylene succinate (PBS); poly lactic acid (PLA); and polycaprolactone (PCL) with a partial wetting morphology is carefully selected since all three components are polyesters with low interfacial tensions. It will be shown that a novel technique by applying multivariate analysis (MVA) on time-of-flight secondary ion mass spectrometry (ToF-SIMS) data can effectively identify the complex phase structure, especially in blends with chemically similar components. Furthermore, for the first time for such systems, this technique provides detailed information about interfacial thicknesses and transitions. By employing the principal component analysis (PCA) method on the ToF-SIMS data of pure polymers, specific peaks with a certain molecular ion mass related to each polymer are determined. Using overlaid mappings on the surface of the blend by ToF-SIMS and selected ion masses to identify each polymer results in the differentiation of the various phases represented as a morphological image. In a second step, the multivariate curve resolution (MCR) method is used as a “self modeling curve resolution” for the recovery of pure components from a multi-component mixture when little or no prior information is available. Total pseudo-color RGB images of PBS/PLA/PCL show that PLA droplets unambiguously partially wet the PBS and PCL phases. Since each pixel from the analysis in the high lateral resolution image represents a 200 nm diameter, the interfacial transitions can also be studied for both PLA/PBS and PLA/PCL interfaces. The results show the concentration variation of phases across the interfaces. A complete trace line across the two interfaces (PLA/PBS and PLA/PCL) allows for the quantitative determination of interfacial thickness for the first time for such systems.