Morphologies of a solvent cast polystyrene‐polybutadiene‐polystyrene (S‐B‐S) triblock copolymer characterised by TEM , AFM and Energy Filtered SEM

Abstract

The most common method of characterisation of the morphologies of block copolymers is transmission electron microscopy (TEM) of ultrathin sections because of the nanoscale of the domains in the morphologies of this block copolymer[1]. The contrast of polymer phases is often enhanced by heavy metal element staining using, typically, OsO 4 . TEM provides imaging at high resolution at the cost of time‐consuming and challenging specimen preparation by cryo‐ultramicrotoming and the negative staining which is not only toxic but also introduces artifacts[2]. Furthermore, the 2D projection by TEM imaging makes direct interpretation of a nanoscale complex material difficult when the specimen sections thickness (~100nm) is considered. Atomic force microscopy (AFM) can characterise a multiphase material by detecting localised variation in mechanical properties, e.g. friction, adhesion and modulus, and has been used to identify the morphology in block copolymers in some recent studies without staining [3]. Combined AFM and TEM characterisation of multiphase copolymers has been developed to characterise the complex microstructures of block polymers in a complementary way [3,4]. Novel holders were designed to facilitate sample preparation for both techniques and to direct correlation of the topographical information provided by the AFM better to interpret the two‐dimensional images provided by TEM. However, the lateral resolution of AFM is significantly worse than TEM limited by the scanning tip although the depth resolution is super. An alternative approach is to use the recent energy‐filtered scanning electron microscopy (EFSEM) technique, which is based upon the energy spectroscopy of detected secondary electrons (SE)[5]. In this work, the S‐B‐S triblock copolymer cast from toluene, cyclohexane and ethyl acetate was characterised in order to understand the effect of casting solvent on the solid state morphology of this triblock copolymer. The morphology varies from continuous to spherical and lamella structures ranging from a few nm to ~100nm. Preliminary results have shown complementary morphologies provided by AFM and TEM, as shown in Figures 1 and 2. The contrast mechanisms from the three methods and consequences for the morphology determination will be discussed, as well as the sample preparation methods.