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Abstract: Binary mixtures of anionic and non-ionic macromolecular chain transfer agents (macro-CTAs) are utilized in order to rationally design diblock copolymer nanoparticles with tunable morphologies and anionic character via pseudo-living radical polymerization. More specifically, poly(methacrylic acid) (PMAA) and poly(glycerol monomethacrylate) (PGMA) macro-CTAs are pre-mixed prior to reversible addition–fragmentation chain transfer (RAFT) aqueous dispersion polymerization of 2-hydroxypropyl methacrylate (HPMA). This strategy facilitates the formation of PHPMA-based diblock copolymer spheres, worm-like micelles and vesicles via polymerization-induced self-assembly (PISA). The presence of the anionic PMAA stabilizer block has a dramatic impact on the resulting copolymer morphology, particularly if the degree of polymerization (DP) of the PMAA stabilizer chains is longer than that of the PGMA. Two phase diagrams have been constructed to investigate the effect of the relative proportion and molar mass of the two macro-CTAs. Such a systematic approach is essential for the reproducible synthesis of pure worm-like micelles, which occupy relatively narrow phase space. The rheological behavior of a series of soft, free-standing worm gels is investigated. Finally, such gels are examined as model matrices for the growth of biomimetic calcite crystals and the role of the anionic PMAA stabilizer chains in directing crystal growth is evaluated.

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Abstract: A series of model sterically stabilized diblock copolymer nanoparticles has been designed to aid the development of analytical protocols in order to determine two key parameters: the effective particle density and the steric stabilizer layer thickness. The former parameter is essential for high resolution particle size analysis based on analytical (ultra)centrifugation techniques (e.g., disk centrifuge photosedimentometry, DCP), whereas the latter parameter is of fundamental importance in determining the effectiveness of steric stabilization as a colloid stability mechanism. The diblock copolymer nanoparticles were prepared via polymerization-induced self-assembly (PISA) using RAFT aqueous emulsion polymerization: this approach affords relatively narrow particle size distributions and enables the mean particle diameter and the stabilizer layer thickness to be adjusted independently via systematic variation of the mean degree of polymerization of the hydrophobic and hydrophilic blocks, respectively. The hydrophobic core-forming block was poly(2,2,2-trifluoroethyl methacrylate) [PTFEMA], which was selected for its relatively high density. The hydrophilic stabilizer block was poly(glycerol monomethacrylate) [PGMA], which is a well-known non-ionic polymer that remains water-soluble over a wide range of temperatures. Four series of PGMAx–PTFEMAy nanoparticles were prepared (x = 28, 43, 63, and 98, y = 100–1400) and characterized via transmission electron microscopy (TEM), dynamic light scattering (DLS), and small-angle X-ray scattering (SAXS). It was found that the degree of polymerization of both the PGMA stabilizer and core-forming PTFEMA had a strong influence on the mean particle diameter, which ranged from 20 to 250 nm. Furthermore, SAXS was used to determine radii of gyration of 1.46 to 2.69 nm for the solvated PGMA stabilizer blocks. Thus, the mean effective density of these sterically stabilized particles was calculated and determined to lie between 1.19 g cm–3 for the smaller particles and 1.41 g cm–3 for the larger particles; these values are significantly lower than the solid-state density of PTFEMA (1.47 g cm–3). Since analytical centrifugation requires the density difference between the particles and the aqueous phase, determining the effective particle density is clearly vital for obtaining reliable particle size distributions. Furthermore, selected DCP data were recalculated by taking into account the inherent density distribution superimposed on the particle size distribution. Consequently, the true particle size distributions were found to be somewhat narrower than those calculated using an erroneous single density value, with smaller particles being particularly sensitive to this artifact.

Open Access

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Abstract: This article describes an experimentally versatile strategy for producing inorganic/organic nanocomposites, with control over the microstructure at the nano- and mesoscales. Taking inspiration from biominerals, CaCO3 is coprecipitated with anionic diblock copolymer worms or vesicles to produce single crystals of calcite occluding a high density of the organic component. This approach can also be extended to generate complex structures in which the crystals are internally patterned with nano-objects of differing morphologies. Extensive characterization of the nanocomposite crystals using high resolution synchrotron powder X-ray diffraction and vibrational spectroscopy demonstrates how the occlusions affect the short and long-range order of the crystal lattice. By comparison with nanocomposite crystals containing latex particles and copolymer micelles, it is shown that the effect of these occlusions on the crystal lattice is dominated by the interface between the inorganic crystal and the organic nano-objects, rather than the occlusion size. This is supported by in situ atomic force microscopy studies of worm occlusion in calcite, which reveal flattening of the copolymer worms on the crystal surface, followed by burial and void formation. Finally, the mechanical properties of the nanocomposite crystals are determined using nanoindentation techniques, which reveal that they have hardnesses approaching those of biogenic calcites.

Open Access

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Abstract: 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.