Synthesis and derivatization of epoxy-functional sterically-stabilized diblock copolymer spheres in non-polar media: does the spatial location of the epoxy groups matter?

Epoxy-functional sterically-stabilized diblock copolymer spherical nanoparticles were synthesized via polymerization-induced self-assembly (PISA) in mineral oil. Epoxy groups were located either (i) in the nanoparticle cores or (ii) within the steric stabilizer chains. For the first system, reversible addition–fragmentation chain transfer (RAFT) dispersion polymerization of glycidyl methacrylate (GlyMA) was conducted using a poly(lauryl methacrylate) (PLMA63) precursor. The second system involved statistical copolymerization of GlyMA with lauryl methacrylate to produce a P(LMA50-stat-GlyMA9) precursor followed by chain extension using methyl methacrylate (MMA). 1H NMR studies and THF GPC analysis indicated that high monomer conversions (≥ 95%) and narrow molecular weight distributions (Mw/Mn ≤ 1.17) were obtained for both formulations. Dynamic light scattering indicated hydrodynamic diameters of 26 nm and 28 nm for P(LMA50-stat-GlyMA9)-PMMA67 and PLMA63-PGlyMA89 spheres, respectively. Transmission electron microscopy studies confirmed a well-defined spherical morphology in each case. Post-polymerization modification of these spherical nanoparticles was examined by reacting the epoxy groups with benzylamine. For the PLMA63-PGlyMA89 spheres, an [amine]/[epoxy] molar ratio of unity was sufficient to react all the epoxy groups. In contrast, the P(LMA50-stat-GlyMA9)-PMMA67 spheres required a fifty-fold excess of benzylamine for complete reaction. Furthermore, epoxy ring-opening reactions were conducted using either a trace amount of water or 50% v/v aqueous acetic acid at 110 °C. The extent of reaction was assessed using 1H NMR spectroscopy and THF GPC for the P(LMA50-stat-GlyMA9)-PMMA67 spheres and by FT-IR spectroscopy for the core-crosslinked PLMA63-PGlyMA89 spheres

Sulfate-based anionic diblock copolymer nanoparticles for efficient occlusion within zinc oxide

Occlusion of copolymer particles within inorganic crystalline hosts not only provides a model for understanding the crystallisation process, but also may offer a direct route for the preparation of novel nanocomposite materials with emergent properties. In the present paper, a series of new well-defined anionic diblock copolymer nanoparticles are synthesised by polymerisation-induced self-assembly (PISA) via reversible addition-fragmentation chain transfer (RAFT) aqueous emulsion polymerisation and then evaluated as crystal habit modifiers for the in situ formation of ZnO in aqueous solution. Systematic studies indicate that both the chemical nature (i.e. whether sulfate-based or carboxylate-based) and the mean degree of polymerisation (DP) of the anionic stabiliser block play vital roles in determining the crystal morphology. In particular, sulfate-functionalised nanoparticles are efficiently incorporated within the ZnO crystals whereas carboxylate-functionalised nanoparticles are excluded, thus anionic character is a necessary but not sufficient condition for successful occlusion. Moreover, the extent of nanoparticle occlusion within the ZnO phase can be as high as 23% by mass depending on the sulfate-based nanoparticle concentration. The optical properties, chemical composition and crystal structure of the resulting nanocomposite crystals are evaluated and an occlusion mechanism is proposed based on the observed evolution of the ZnO morphology in the presence of sulfate-based anionic nanoparticles. Finally, controlled deposition of a 5 nm gold sol onto porous ZnO particles (produced after calcination of the organic nanoparticles) significantly enhances the rate of photocatalytic decomposition of a model rhodamine B dye on exposure to a relatively weak UV source

Tuning the glass transition temperature of a core-forming block during polymerization-induced self-assembly: statistical copolymerization of lauryl methacrylate with methyl methacrylate provides access to spheres, worms, and vesicles

A series of poly(lauryl methacrylate)–poly(methyl methacrylate-stat-lauryl methacrylate) (PLMAx–P(MMA-stat-LMA)y) diblock copolymer nanoparticles were synthesized via RAFT dispersion copolymerization of 90 mol % methyl methacrylate (MMA) with 10 mol % lauryl methacrylate (LMA) in mineral oil by using a poly(lauryl methacrylate) (PLMA) precursor with a mean degree of polymerization (DP) of either 22 or 41. In situ1H NMR studies of the copolymerization kinetics suggested an overall comonomer conversion of 94% within 2.5 h. GPC analysis confirmed a relatively narrow molecular weight distribution (Mw/Mn ≤ 1.35) for each diblock copolymer. Recently, we reported an unexpected morphology constraint when targeting PLMA22–PMMAy nano-objects in mineral oil, with the formation of kinetically trapped spheres being attributed to the relatively high glass transition temperature (Tg) of the PMMA block. Herein we demonstrate that this limitation can be overcome by (i) incorporating 10 mol % LMA into the core-forming block and (ii) performing such syntheses at 115 °C. This new strategy produced well-defined spheres, worms, or vesicles when using the same PLMA22 precursor. Introducing the LMA comonomer not only enhances the mobility of the core-forming copolymer chains by increasing their solvent plasticization but also reduces their effective glass transition temperature to well below the reaction temperature. Copolymer morphologies were initially assigned via transmission electron microscopy (TEM) studies and subsequently confirmed via small-angle X-ray scattering analysis. The thermoresponsive behavior of PLMA22–P(0.9MMA-stat-0.1LMA)113 worms and PLMA22–P(0.9MMA-stat-0.1LMA)228 vesicles was also studied by using dynamic light scattering (DLS) and TEM. The former copolymer underwent a worm-to-sphere transition on heating from 20 to 170 °C while a vesicle-to-worm transition was observed for the latter. Such thermal transitions were irreversible at 0.1% w/w solids but proved to be reversible at 20% w/w solids

Star Diblock Copolymer Concentration Dictates the Degree of Dispersion of Carbon Black Particles in Nonpolar Media: Bridging Flocculation versus Steric Stabilization

The solution behavior of a polystyrene–hydrogenated polyisoprene star diblock copolymer (Mn ∼ 384 K; 6 mol % polystyrene) is examined in nonpolar media. Variable temperature 1H NMR studies using deuterated n-dodecane confirm that the outer polystyrene blocks are only partially solvated in n-dodecane at 25 °C: the apparent polystyrene content of 3.2 ± 0.2 mol % remains essentially constant on heating up to 100 °C. Physical adsorption of this star diblock copolymer onto carbon black particles is examined, with particular attention being paid to the effect of copolymer concentration on colloidal stability. An isotherm is constructed for copolymer adsorption onto carbon black from n-dodecane at 20 °C using a supernatant depletion assay based on UV spectroscopy analysis of the aromatic chromophore in the polystyrene block. Langmuir-type adsorption is observed with a maximum adsorbed amount, Γ, of ∼2.2 ± 0.1 mg m–2. In addition, thermogravimetric analysis is used to directly determine the amount of adsorbed copolymer on the carbon black particles, which are essentially incombustible under an inert atmosphere. Analytical centrifugation, optical microscopy, and transmission electron microscopy studies indicate that the star diblock copolymer acts as an effective flocculant at low concentration, with steric stabilization only being observed above a certain critical copolymer concentration (∼5.5% w/w based on carbon black). This is attributed to the spatial location of the polystyrene block and the star copolymer architecture, which enables copolymer adsorption onto multiple carbon black particles at low coverage, leading to bridging flocculation. Above 5.5% w/w copolymer, the surface coverage is sufficiently high that all of the polystyrene “stickers” adsorb onto single carbon black particles, resulting in colloidally stable, sterically stabilized carbon black dispersions. Small-angle X-ray scattering (SAXS) is also used to characterize the copolymer-coated carbon black particles: this technique provides useful complementary insights regarding the rather subtle changes in the fractal morphology that occur with increasing copolymer concentration. Moreover, SAXS also provides direct evidence for the presence of the copolymer chains at the particle surface

Enhanced adsorption of epoxy‐functional nanoparticles onto stainless steel significantly reduces friction in tribological studies

Epoxy-functional sterically-stabilized diblock copolymer nanoparticles (ca. 27 nm) are prepared via RAFT dispersion polymerization in mineral oil. Nanoparticle adsorption onto stainless steel is examined using a quartz crystal microbalance. Incorporating epoxy groups within the steric stabilizer chains results in a two-fold increase in the adsorbed amount, Γ, at 20 °C (7.6 mg m−2) compared to epoxy-core functional nanoparticles (3.7 mg m−2) or non-functional nanoparticles (3.8 mg m−2). A larger difference in Γ is observed at 40 °C; this suggests chemical adsorption of the nanoparticles rather than merely physical adsorption. A remarkable near five-fold increase in Γ is observed for ca. 50 nm epoxy-functional nanoparticles compared to non-functional nanoparticles (31.3 vs. 6.4 mg m−2, respectively). Tribological studies confirm that chemical adsorption of the latter epoxy-functional nanoparticles leads to a significant reduction in friction between 60 °C and 120 °C

RAFT dispersion polymerization of methyl methacrylate in mineral oil : high glass transition temperature of the core-forming block constrains the evolution of copolymer morphology

RAFT dispersion polymerization of a prototypical methacrylic monomer, methyl methacrylate (MMA), is performed in mineral oil using various poly(lauryl methacrylate) (PLMA) precursors prepared with a trithiocarbonate-based RAFT agent. GPC analysis indicated reasonably narrow molecular weight distributions (Mw/Mn ≤ 1.39) for all diblock copolymers, with 1H NMR studies indicating high MMA conversions (≥95%) for all syntheses. An efficient one-pot synthesis protocol enabled high blocking efficiencies to be achieved when targeting higher PMMA DPs. However, the relatively high glass transition temperature (Tg) of the corresponding core-forming PMMA block unexpectedly constrains the evolution in copolymer morphology during polymerization-induced self-assembly (PISA). More specifically, well-defined PLMA22–PMMAx spheres (x = 19–39) and relatively short worms (x = 69–97) can be obtained at 90 °C when using a PLMA22 precursor but targeting higher x values (x ≥ 108) invariably leads to colloidally unstable aggregates of spheres, rather than long worms or vesicles. Interestingly, similar constraints were observed when targeting higher solids, when using n-dodecane instead of mineral oil, or when employing an alternative steric stabilizer block. Raising the PISA synthesis temperature from 90 to 115 °C (i.e., from below to above the Tg of the final PMMA block) does not alleviate this unexpected problem. Moreover, only spherical nanoparticles can be obtained at 115 °C when targeting PMMA DPs between 50 and 400 with the same PLMA22 precursor. This suggests that nanoparticle formation may occur by a chain expulsion/insertion mechanism at this relatively high reaction temperature. PLMA22–PMMAx nanoparticles were characterized in terms of their particle size and morphology using dynamic light scattering (DLS), transmission electron microscopy (TEM), and small-angle X-ray scattering (SAXS). DLS and TEM studies of a 0.1% w/w dispersion of PLMA22–PMMA69 short worms indicated an irreversible worm-to-sphere transition on heating from 20 to 150 °C. Oscillatory rheology and TEM studies indicated that this thermal transition was only partially reversible for a 20% w/w dispersion of PLMA22–PMMA69 short worms

Time-resolved small-angle neutron scattering studies of the thermally-induced exchange of copolymer chains between spherical diblock copolymer nanoparticles prepared via polymerization-induced self-assembly

Sterically-stabilized diblock copolymer nanoparticles (a.k.a. micelles) are prepared directly in non-polar media via polymerization-induced self-assembly (PISA). More specifically, a poly(lauryl methacrylate) chain transfer agent is chain-extended via reversible addition–fragmentation chain transfer (RAFT) dispersion polymerization of methyl methacrylate (MMA) to form sterically-stabilized spheres at 20% w/w solids in n-dodecane at 90 °C. Both fully hydrogenous (PLMA39–PMMA55 and PLMA39–PMMA94) and core-deuterated (PLMA39–d8PMMA57 and PLMA39–d8PMMA96) spherical nanoparticles with mean core diameters of approximately 20 nm were prepared using this protocol. After diluting each dispersion in turn to 1.0% w/w with n-dodecane, small-angle X-ray scattering studies confirmed essentially no change in spherical nanoparticle diameter after thermal annealing at 150 °C. Time-resolved small angle neutron scattering was used to examine whether copolymer chain exchange occurs between such nanoparticles at elevated temperatures. Copolymer chain exchange for a binary mixture of PLMA39–PMMA55 and PLMA39–d8PMMA57 nanoparticles produced hybrid (mixed) cores containing both PMMA55 and d8PMMA57 blocks within 3 min at 150 °C. In contrast, a binary mixture of PLMA39–PMMA94 and PLMA39–d8PMMA96 nanoparticles required 8 min at this temperature before no further reduction in neutron scattering intensity could be observed. These observations suggest that the rate of copolymer chain exchange depends on the degree of polymerization of the core-forming block. Relatively slow copolymer chain exchange was also observed at 80 °C, which is below the Tg of the core-forming PMMA block as determined by DSC studies. These observations confirm rapid exchange of individual copolymer chains between sterically-stabilized nanoparticles at elevated temperature. The implications of these findings are briefly discussed in the context of PISA, which is a powerful technique for the synthesis of sterically-stabilized nanoparticles

Application of scattering and diffraction techniques for the morphological characterization of asphaltenes

Asphaltenes are an important class of complex carbon-rich molecules found in crude oil. Their chemical structure varies depending on the geological source but generally comprises fused aromatic rings, aliphatic substituents and heteroatom functionality, which results in a strong tendency to aggregate and phase separate within crude oil. Asphaltene ‘drop-out’ owing to phase separation is a major problem spanning crude oil extraction, refining and application. More specifically, the build-up of asphaltene deposits can reduce the permeability of porous rock formations, block oil pipelines, and compromise the efficiency of marine engines. This major technical problem is compounded by the fact that the chemical composition, structure and colloidal behavior of asphaltenes varies significantly depending on the origin of the crude oil and the conditions employed for its refinement. As a result, there has been a concerted effort to (i) understand the morphology of asphaltene dispersions, (ii) identify the underlying mechanism(s) that lead to asphaltene ‘drop-out’ and hence (iii) design stabilizers to maintain colloid stability and/or minimize ‘drop-out’. In principle, imaging techniques can be used to visualize the asphaltene aggregates while light scattering can provide particle size information, but these techniques only provide rather limited structural information. Asphaltene aggregation involves several steps and results in highly hierarchical structures including primary nanoaggregates, clusters, and fractal structures, with characteristic length scales ranging from a few angstroms to several microns. In this review article, the use of small-angle scattering (SAS) and X-ray diffraction (XRD) to characterize asphaltene powders, dispersions and aggregates over the past six decades is summarized. These powerful techniques provide a wealth of structural information about molecular stacking, particle size and morphology, and fractal dimensions