Use of Commercial Block Copolymers as Soot Dispersants in Engine Oil

The micellar self-assembly behaviour of a near-monodisperse linear poly(styrene-b-hydrogenated isoprene) (PS-PEP) diblock copolymer is examined in n-alkanes. Direct dissolution leads to formation of polydisperse colloidal aggregates that are kinetically frozen artefacts of the solid-state morphology. Dynamic light scattering (DLS) and transmission electron microscopy (TEM) studies indicate that heating such copolymer dispersions up to 90 - 110oC gives well-defined spherical micelles that persist on cooling to 20oC. These observations are also consistent with small-angle X-ray scattering (SAXS) studies, which indicate the formation of star-like micelles in n-heptane and n-dodecane following a thermal cycle. Variable temperature 1H NMR studies in deuterated n-alkanes confirm partial solvation of the polystyrene micelle cores occurs on heating. Increased mobility of the core-forming polystyrene chains is consistent with the evolution in morphology via exchange of individual copolymer chains, as observed by DLS. Adsorption of this diblock copolymer onto a model colloidal substrate (carbon black) has been investigated using X-ray photoelectron spectroscopy (XPS). A Langmuir-type adsorption isotherm has been constructed using a supernatant depletion assay based on the aromatic chromophore in the polystyrene block. Comparable results were obtained using thermogravimetric analysis (TGA) to directly determine adsorbed amounts. Based on maximum adsorbed amounts at 20oC, these studies strongly suggest that individual copolymer chains adsorb onto carbon black from chloroform (a non-selective solvent), whereas micellar adsorption occurs from n-alkanes. This is important, because such copolymers are used as soot dispersants for engine oils. A near-monodisperse PS-PEP star diblock copolymer is examined in n-alkanes. Variable temperature 1H NMR studies using deuterated n-dodecane confirm that the outer polystyrene blocks are only partially solvated at 25oC, and solvation remains essentially constant on heating to 100oC. Physical adsorption of this copolymer onto carbon black is examined, with particular attention being paid to the effect of copolymer concentration on colloidal stability. Isotherms are constructed for copolymer adsorption onto carbon black at 20oC using a supernatant depletion assay based on UV spectroscopy analysis of the polystyrene aromatic chromophore. In addition, TGA is used to directly determine the amount of adsorbed copolymer on carbon black. Analytical centrifugation, optical microscopy (OM) and TEM studies indicate that the star copolymer acts as a flocculant for the carbon black particles at low concentration, with steric stabilisation observed above a certain solvent-dependent critical copolymer concentration. This is attributed to the spatial location of the polystyrene block, which enables copolymer adsorption onto multiple carbon black particles at low coverage, whereas all polystyrene ‘stickers’ adsorb onto single carbon black particles at high coverage, leading to steric stabilisation. SAXS is used to characterise copolymer-coated carbon black particles, providing complementary insights regarding changes in the fractal morphology that occur with increasing copolymer concentration. Moreover, SAXS also provided direct evidence for the presence of the copolymer chains at the particle surface. The effect of copolymer composition on both micelle diameter and dispersant performance (for carbon black particles) has been assessed for PS-PEP and poly(styrene-b-hydrogenated butadiene) diblock copolymers in n-dodecane. Direct dissolution at 20oC produces kinetically-frozen polydisperse aggregates, and higher polystyrene contents accentuate such an effect. Heating to 110oC produces relatively small, well-defined spherical micelles that persist on cooling to 20oC. Physical adsorption of these diblock copolymer micelles onto carbon black has been investigated by constructing Langmuir-type adsorption isotherms based on UV spectroscopy, which were also supported by TGA. Stokes’ law is used to calculate particle velocities in two very similar solvents (n-dodecane and d26-dodecane). Although each copolymer forms micelles with similar DLS and SAXS diameters, subtly different effective densities (0.92-1.02 g cm-3) are observed for the micelle-stabilised carbon black particles, which are substantially lower than the solid-state density of carbon black (1.89 g cm-3). Since the rate of sedimentation of sterically-stabilised carbon black particles depends on the density difference between the particles and the solvent, significant errors can be incurred in analytical centrifugation studies unless care is taken to determine accurate effective particle densities. Finally, the carbon black used in this project is assessed for its suitability as a mimic for diesel soot. Particle size, morphology, density and surface composition are assessed using BET surface area analysis, TEM, helium pycnometry and XPS. The extent of adsorption of a poly(ethylene-co-propylene) (dOCP) statistical copolymer or a PS-PEP diblock copolymer onto these two substrates is compared indirectly using a supernatant depletion assay based on UV spectroscopy. TGA is also used to directly determine the extent of copolymer adsorption. Degrees of dispersion are examined using OM, TEM and analytical centrifugation. SAXS reveals some structural organisation differences between carbon black and diesel soot particles: for example, the mean radius of gyration for soot is significantly smaller. Soot particle suspensions in n-dodecane comprise relatively loose mass fractals compared to the corresponding carbon black suspensions. SAXS also provides evidence for copolymer adsorption and indicates that addition of either copolymer transforms the initially compact agglomerates into relatively loose aggregates, while the primary particles remain unchanged. It is believed that this is also the case for diesel soot. In favourable cases, similar experimental data is obtained for carbon black and diesel soot with both copolymer dispersants. However, it is not difficult to identify certain copolymer-particle-solvent combinations for which substantial differences can be observed. Such observations are most likely the result of dissimilar surface chemistries, which can have a profound effect on the colloidal stability.