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

Morphology of Poly(styrene-co-butadiene) Random Copolymer Thin Films and Nanostructures on a Graphite Surface

We studied the morphology of poly­(styrene-<i>co</i>-butadiene) random copolymers on a graphite surface. Polymer solutions were spin coated onto graphite, at various concentrations and molecular weights. The polymer films and nanostructures were imaged using atomic force microscopy. Above the overlap concentration, thin films formed. However, total wetting did not occur, despite the polymers being well above their <i>T</i>_g. Instead, dewetting was observed, suggesting the films were in a state of metastable equilibrium. At lower concentrations, the polymers formed networks, nanoislands, and nanoribbons. Ordered nanopatterns were observed on the surface; the polymers orientated themselves due to π–π stacking interactions reflecting the crystalline structure of the graphite. At the lowest concentration, this ordering was very pronounced. At higher concentrations, it was less defined but still statistically significant. Higher degrees of ordering were observed with poly­(styrene-<i>co</i>-butadiene) than polystyrene and polybutadiene homopolymers as the copolymer’s aromatic rings are distributed along a flexible chain, which maximizes π–π stacking. At the two lowest concentrations, the size of the nanoislands and nanoribbons remained similar with varying molecular weight. However, at higher concentrations, the polymer network features were largest at the lowest molecular weight, indicating that in this case, a large proportion of shorter chains stay on top of the adsorbed ones. The contact angles of the polymer nanostructures remained mostly constant with size, which is due to the strong polymer/graphite adhesion dominating over line tension and entropic effects

Effect of Glass Fibre Sizing on the Interfacial Properties of Composites Produced using In-Situ Polymerised Polyamide-6 Transfer Moulding

The fibre-matrix interfacial properties of glass-fibre/polyamide-6 (GF/PA-6) composites produced by thermoplastic resin transfer moulding (TP-RTM) were investigated. Two different fibre sizings were compared, a specially-developed novel reactive fibre sizing and a standard silane glass fibre sizing. Scanning electron microscopy, atomic force microscopy and a number of mass-loss techniques were employed to study the form, distribution, quantity and degradation temperature of the fibre sizings. Activated PA-6 monomer precursor materials with viscosities of ∼10 mPa s were injected into the glass-fibre fabrics, contained between heated press platens, and polymerisation occurred in-situ within several minutes. Glass-fabric laminates with fibre volume fractions of ∼53% and low void content were produced at a pressure of ∼4 bar, with the low viscosity of the monomer negating the need for expensive high-pressure injection. Similar quality between the laminates was demonstrated by measuring density, thickness, fibre volume fraction, void content and fibre bundle distribution. Transverse mechanical properties of the composites reinforced with the novel reactive sizing were 20–28% higher than those with the standard fibre sizings, demonstrating improved fibre-matrix interfacial properties. Average mode I fracture toughness was also measured to be 10–30% higher than with the standard fibre sizing. Scanning electron microscopy and 3D depth composition were used to investigate fracture surfaces and determine the surface roughness. The novel reactive fibre sizing resulted in improved fibre-matrix adhesion and improved fracture toughness

Thermomechanical Properties of Virgin and Recycled Polypropylene—High-Density Polyethylene Blends

This paper provides evidence and discusses the variability in the thermomechanical behaviour of virgin and recycled polypropylene/high-density polyethylene blends without the addition of other components, which is sparse in the literature. Understanding the performance variability in recycled polymer blends is of critical importance in order to facilitate the re-entering of recycled materials to the consumer market and, thus, contribute towards a circular economy. This is an area that requires further research due to the inhomogeneity of recycled materials. Therefore, the thermal and mechanical properties of virgin and recycled polypropylene/high-density polyethylene blends were investigated systematically. Differential scanning calorimetry concludes that both the recycled and virgin blends are immiscible. Generally, recycled blends have lower overall crystallinity and melting temperatures compared with virgin blends while, remarkably, their crystallisation temperatures are compared favourably. Dynamical mechanical analysis showed little variation in the storage modulus of recycled and virgin blends. However, the alpha and beta relaxation temperatures are lower in recycled blends due to structural deterioration. Deterioration in the thermal and mechanical properties of recycled blends is thought to be caused by the presence of contaminants and structural degradation during reprocessing, resulting in shorter polymeric chains and the formation of imperfect crystallites. The tensile properties of recycled blends are also affected by the recycling process. The Young’s modulus and yield strength of the recycled blends are inferior to those of virgin blends due to the deterioration during the recycling process. However, the elongation at break of the recycled blends is higher compared with the virgin blends, possibly due to the plasticity effect of the low-molecular-weight chain fragments

Principal Component Analysis to Determine the Surface Properties That Influence the Self-Cleaning Action of Hydrophobic Plant Leaves

It is well established that many leaf surfaces display self-cleaning properties. However, an understanding of how the surface properties interact is still not achieved. Consequently, 12 different leaf types were selected for analysis due to their water repellency and self-cleaning properties. The most hydrophobic surfaces demonstrated splitting of the νs CH2 and ν CH2 bands, ordered platelet-like structures, crystalline waxes, high-surface-roughness values, high-total-surface-free energy and apolar components of surface energy, and low polar and Lewis base components of surface energy. The surfaces that exhibited the least roughness and high polar and Lewis base components of surface energy had intracuticular waxes, yet they still demonstrated the self-cleaning action. Principal component analysis demonstrated that the most hydrophobic species shared common surface chemistry traits with low intra-class variability, while the less hydrophobic leaves had highly variable surface-chemistry characteristics. Despite this, we have shown through partial least squares regression that the leaf water contact angle (i.e., hydrophobicity) can be predicted using attenuated total reflectance Fourier transform infrared spectroscopy surface chemistry data with excellent ability. This is the first time that such a statistical analysis has been performed on a complex biological system. This model could be utilized to investigate and predict the water contact angles of a range of biological surfaces. An understanding of the interplay of properties is extremely important to produce optimized biomimetic surfaces

Novel Carbon-Fibre Powder-Epoxy Composites: Interface Phenomena and Interlaminar Fracture Behaviour

Carbon fibres with three different sizing agents were used to manufacture unidirectional composites based on a powder epoxy resin. Powder epoxy processing was investigated as a route for fast, cost-effective manufacturing of out-of-autoclave composites compared to more time-consuming vacuum infusion technologies. In this work, a heat-activated epoxy powder was used as a resin system in low-cost vacuum-bag-only prepregs for thick composite parts that are required in the renewable energy industry (e.g. wind turbine blade roots). The importance of interfacial bonding between fibres and the matrix is shown and the impact on the ultimate mechanical performance of the manufactured composites demonstrated. The surface characteristics of the sizing on the carbon fibres were investigated using atomic force microscopy (AFM) and Raman spectroscopy. Results showed that the amount of sizing on the fibres' surfaces was inextricably linked with surface roughness and coverage. This in turn influenced the mechanical and chemical interlocking phenomena occurring at the fibre/matrix interface. The composites’ mechanical performance was evaluated using tensile, flexural and interlaminar fracture toughness tests. Fractographic analysis using optical and scanning electron microscopy (SEM) was likewise employed to analyse the fracture surfaces of the tested/failed composites. Interlaminar fracture toughness testing (DCB Mode-I) revealed that the interfacial adhesion differences could alter the fracture resistance of the composites, hence emphasizing the importance of the interfacial bonding strength between the polymer matrix and the carbon fibres