Abstract
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.
Highlights
Polymerization-induced self-assembly (PISA) is wellestablished as a robust protocol for the convenient synthesis of diblock copolymer nano-objects.[1−6] Typically, reversible addition-fragmentation chain transfer (RAFT) polymerization[7−10] is used to prepare a soluble precursor block
A representative kinetic experiment was conducted for the RAFT dispersion polymerization of methyl methacrylate (MMA) at 90 °C when targeting PLMA19−PMMA100 nano-objects at 20% w/w solids in mineral oil using the onepot protocol
A series of PLMAy−PMMAx nano-objects were prepared via RAFT dispersion polymerization of MMA using a PLMA22, PLMA30 or PLMA41 precursor at 90 °C in mineral oil at 20% w/w solids. 1H NMR spectroscopy studies indicated that more than 97% MMA conversion was achieved for such PISA syntheses, while THF GPC confirmed relatively narrow molecular weight distributions (Mw/Mn ≤ 1.39)
Summary
Polymerization-induced self-assembly (PISA) is wellestablished as a robust protocol for the convenient synthesis of diblock copolymer nano-objects.[1−6] Typically, reversible addition-fragmentation chain transfer (RAFT) polymerization[7−10] is used to prepare a soluble precursor block. This leads to an increase in the volume fraction of the PLMA22 stabilizer block relative to the PMMA69 core-forming block, which results in a reduction in the critical packing parameter (P) and favors the formation of spheres.[40] It is perhaps noteworthy that the synthesis temperature of 115 °C examined in Figure 8 exceeds the critical gelation temperature (CGT) observed for the 20% w/w dispersion of PLMA22−PMMA69 nano-objects (see Figure 11) Given that this CGT is associated with a worm-to-sphere transition, this explains why worms cannot be produced at this relatively high temperature: this morphology is thermodynamically unstable with respect to spheres under such conditions.
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