Temperature dependence of micelle shape transitions in copolymer solutions: the role of inter-block incompatibility.

The nature of the transition between worm-like and spherical micelles in block copolymer dispersions varies between systems. In some formulations, heating drives a transition from worms to spheres, while in other systems the same transition is induced by cooling. In addition, a sphere-worm interconversion can be accompanied either by an increase or a decrease in the core solvation, even if the direction of the temperature dependence is the same. Here, self-consistent field theory is used to provide a potential explanation of this range of behaviour. Specifically, we show that, within this model, the dependence of the transition on the incompatibility χBS of the solvophobic block B and the solvent S (the parameter most closely related to the temperature) is strongly influenced by the incompatibility χAB between B and the solvophilic block A. When χAB is small (χAB < 0.1), it is found that increasing χBS produces a transition from worm-like micelles to spheres (or, more generally, from less curved to more curved structures). When χAB is above 0.1, increasing χBS drives the system from spheres to worm-like micelles. Whether a transition is observed within a realistic range of χBS is also found to depend on the fraction of solvophilic material in the copolymer. The relevance of our calculations to experiments is discussed, and we suggest that the direction of the temperature dependence may be controlled not only by the solution behaviour of the solvophobic block (upper critical solution temperature-like versus lower critical solution temperature-like) but also by χAB.

Heterotelechelic homopolymers mimicking high χ - ultralow N block copolymers with sub-2 nm domain size.

Three fluorinated, hydrophobic initiators have been utilised for the synthesis of low molecular mass fluoro-poly(acrylic acid) heterotelechelic homopolymers to mimic high chi (χ)-low N diblock copolymers with ultrafine domains of sub-2 nm length scale. Polymers were obtained by a simple photoinduced copper(ii)-mediated reversible-deactivation radical polymerisation (Cu-RDRP) affording low molecular mass (<3 kDa) and low dispersity (D = 1.04-1.21) homopolymers. Heating/cooling ramps were performed on bulk samples (ca. 250 µm thick) to obtain thermodynamically stable nanomorpologies of lamellar (LAM) or hexagonally packed cylinders (HEX), as deduced by small-angle X-ray scattering (SAXS). Construction of the experimental phase diagram alongside a detailed theoretical model demonstrated typical rod-coil block copolymer phase behaviour for these fluoro-poly(acrylic acid) homopolymers, where the fluorinated initiator-derived segment acts as a rod and the poly(acrylic acid) as a coil. This work reveals that these telechelic homopolymers mimic high χ-ultralow N diblock copolymers and enables reproducible targeting of nanomorphologies with incredibly small, tunable domain size.

Synthesis and pH-responsive dissociation of framboidal ABC triblock copolymer vesicles in aqueous solution.

A series of pH-responsive all-methacrylic ABC triblock copolymer vesicles were prepared from precursor diblock copolymer vesicles via RAFT seeded emulsion polymerisation. Microphase separation between the two hydrophobic membrane-forming B and C blocks produced a distinctive framboidal morphology, for which the mean globule size can be tuned by adjusting the triblock copolymer composition. These vesicles remain intact at neutral pH, but undergo irreversible dissociation on addition of acid as a result of protonation of the tertiary amine groups located within the third block. Small-angle X-ray scattering (SAXS) was utilised to characterise the morphologies formed at pH 8 and pH 3. According to time-resolved SAXS studies, the acid-induced dissociation of these pH-responsive framboidal vesicles involves appreciable membrane swelling within 50 ms and is complete.

ABC Triblock Copolymer Worms: Synthesis, Characterization, and Evaluation as Pickering Emulsifiers for Millimeter-Sized Droplets.

Polymerization-induced self-assembly (PISA) is used to prepare linear poly(glycerol monomethacrylate)-poly(2-hydroxypropyl methacrylate)-poly(benzyl methacrylate) [PGMA-PHPMA-PBzMA] triblock copolymer nano-objects in the form of a concentrated aqueous dispersion via a three-step synthesis based on reversible addition-fragmentation chain transfer (RAFT) polymerization. First, GMA is polymerized via RAFT solution polymerization in ethanol, then HPMA is polymerized via RAFT aqueous solution polymerization, and finally BzMA is polymerized via "seeded" RAFT aqueous emulsion polymerization. For certain block compositions, highly anisotropic worm-like particles are obtained, which are characterized by small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). The design rules for accessing higher order morphologies (i.e., worms or vesicles) are briefly explored. Surprisingly, vesicular morphologies cannot be accessed by targeting longer PBzMA blocks-instead, only spherical nanoparticles are formed. SAXS is used to rationalize these counterintuitive observations, which are best explained by considering subtle changes in the relative enthalpic incompatibilities between the three blocks during the growth of the PBzMA block. Finally, the PGMA-PHPMA-PBzMA worms are evaluated as Pickering emulsifiers for the stabilization of oil-in-water emulsions. Millimeter-sized oil droplets can be obtained using low-shear homogenization (hand-shaking) in the presence of 20 vol % n-dodecane. In contrast, control experiments performed using PGMA-PHPMA diblock copolymer worms indicate that these more delicate nanostructures do not survive even these mild conditions.

Vermicious thermo-responsive Pickering emulsifiers.

Thermo-responsive vermicious (or worm-like) diblock copolymer nanoparticles prepared directly in n-dodecane via polymerisation-induced self-assembly (PISA) were used to stabilise water-in-oil Pickering emulsions. Mean droplet diameters could be tuned from 8 to 117 µm by varying the worm copolymer concentration and the water volume fraction and very high worm adsorption efficiencies (∼100%) could be obtained below a certain critical copolymer concentration (∼0.50%). Heating a worm dispersion up to 150 °C led to a worm-to-sphere transition, which proved to be irreversible if conducted at sufficiently low copolymer concentration. This affords a rare opportunity to directly compare the Pickering emulsifier performance of chemically identical worms and spheres. It is found that the former nanoparticles are markedly more efficient, since worm-stabilised water droplets are always smaller than the equivalent sphere-stabilised droplets prepared under identical conditions. Moreover, the latter emulsions are appreciably flocculated, whereas the former emulsions proved to be stable. SAXS studies indicate that the mean thickness of the adsorbed worm layer surrounding the water droplets is comparable to that of the worm cross-section diameter determined for non-adsorbed worms dispersed in the continuous phase. Thus the adsorbed worms form a monolayer shell around the water droplets, rather than ill-defined multilayers. Under certain conditions, demulsification occurs on heating as a result of a partial worm-to-sphere morphological transition.