Polymers at surfaces: nanostructures and adhesion studied by atomic force microscopy
Understanding and characterising the behaviour of polymers at surfaces is of great fundamental interest, in addition to being vitally important for many applications. Composite materials, films and coatings, functional membranes, and nanoelectronics are only a few examples of applications which rely on polymers functioning at surfaces. The interactions between polymers and surfaces are extremely influential in governing the overall bulk properties of materials and products. Despite this, the behaviour of polymers at surfaces is not fully understood and there are many unexplored areas in this field of research. Atomic force microscopy (AFM) is a technique which can image features with a high spatial resolution down to a sub-nanometre scale. It can accurately image a variety of polymer nanostructures on surfaces such as droplets, networks, thin films, and even single chains. AFM can also be used in a mode of operation called force spectroscopy which generates information regarding the strength of adhesion between different materials with a piconewton force resolution. It can be used to measure the magnitude of interaction forces between single polymer chains and surfaces. The primary aim of this study was to characterise the behaviour of poly(styrene-cobutadiene) random copolymers on various surfaces at the nanoscale using AFM techniques. Poly(styrene-co-butadiene) is heavily utilised within industry, particularly in the manufacturing of automotive tyres where it is mixed with carbon black to form a robust composite material. This study is the first work to provide a comprehensive report on the morphology of poly(styrene-co-butadiene) nanostructures on various surfaces, under different experimental parameters. Furthermore, it is the first time where the specific interactions and adhesion between poly(styrene-co-butadiene) and various surfaces have been examined using AFM force spectroscopy. A systematic study was carried out which investigated the structural behaviour of adsorbed poly(styrene-co-butadiene) random copolymers on mica and graphite surfaces using AFM imaging. A large range of concentrations and molecular weights allowed investigations and discussions of many phenomena such as thin film formation (and dewetting), networks, spherical cap nanodroplets, and single chain conformations. Polymer morphology was generally more consistent on the mica, and varied significantly on graphite. The contact angles of the nanodroplets on the mica surface were shown to be size dependent by a specific trend irrespective of molecular weight. A minimum contact angle was observed for droplets with radii ranging from 100 - 250 nm across each molecular weight. This was due to influences from line tension, changes in elastic modulus, and surface heterogeneities. On the graphite, the nanostructures exhibited distinct ordering at the nanoscale. The features reflected the crystalline symmetry of the graphite by orientating themselves at intervals of 60° due to π-π stacking interactions. The ordering was extremely precise at the lowest concentration and became less defined at higher concentrations, but remained statistically significant. An AFM force spectroscopy study was implemented in order to investigate the adhesion and specific interactions between poly(styrene-co-butadiene) and mica, silicon, and graphite substrates. AFM tips were dip coated into polymer solutions to physically adhere polymer chains to the surface of the tips at varying molecular weights and surface coverages. Polymer chains were also adhered to AFM tips using force spectroscopy techniques. The results showed that capillary forces were increasing polymer/substrate adhesion on the more hydrophilic substrates. Single chain desorption events did occur, but had a very low probability. The experimental system was redesigned to reduce capillary effects and increase desorption events. Thin polymer films were deposited onto each substrate using dip coating and the AFM tips were left blank. The results revealed that capillary forces were eliminated using this system and the probability of single chain desorption events occurring was extremely high. It was demonstrated that the specific interactions between poly(styrene-co-butadiene) and graphite were the strongest of the three substrates due to π-π stacking interactions and van der Waals forces.