Polymerization-induced
self-assembly (PISA) is a powerful platform technology for the rational
and efficient synthesis of a wide range of block copolymer nano-objects
(e.g., spheres, worms or vesicles) in various media.
In situ
small-angle X-ray scattering (SAXS) studies of reversible addition–fragmentation
chain transfer (RAFT) dispersion polymerization have previously provided
detailed structural information during self-assembly (see
30155157
Chem. Sci.
2016
7
5078
5090
). However, conducting the analogous
in situ
SAXS studies during RAFT aqueous emulsion polymerizations
poses a formidable technical challenge because the inherently heterogeneous
nature of such PISA formulations requires efficient stirring to generate
sufficiently small monomer droplets. In the present study, the RAFT
aqueous emulsion polymerization of 2-methoxyethyl methacrylate (MOEMA)
has been explored for the first time. Chain extension of a relatively
short non-ionic poly(glycerol monomethacrylate) (PGMA) precursor block
leads to the formation of sterically-stabilized PGMA-PMOEMA spheres,
worms or vesicles, depending on the precise reaction conditions. Construction
of a suitable phase diagram enables each of these three morphologies
to be reproducibly targeted at copolymer concentrations ranging from
10 to 30% w/w solids. High MOEMA conversions are achieved within 2
h at 70 °C, which makes this new PISA formulation well-suited
for
in situ
SAXS studies using a new reaction cell.
This bespoke cell enables efficient stirring and hence allows
in situ
monitoring during RAFT emulsion polymerization for
the first time. For example, the onset of micellization and subsequent
evolution in particle size can be studied when preparing PGMA
29
-PMOEMA
30
spheres at 10% w/w solids. When targeting
PGMA
29
-PMOEMA
70
vesicles under the same conditions,
both the micellar nucleation event and the subsequent evolution in
the diblock copolymer morphology from spheres to worms to vesicles
are observed. These new insights significantly enhance our understanding
of the PISA mechanism during RAFT aqueous emulsion polymerization.
Physical gelation by block copolymer worms can be explained in terms of multiple inter-worm contacts using percolation theory, suggesting that worm entanglements are irrelevant in this context.
The environmentally-friendly synthesis of epoxy-functional spherical nanoparticles is achieved via RAFT aqueous emulsion polymerization of glycidyl methacrylate under mild conditions; derivatization of such nanoparticles with sodium azide or diamines is demonstrated.
High-pressure microfluidization
is used to prepare a series of
oil-in-water Pickering nanoemulsions using sterically-stabilized diblock
copolymer nanoparticles as the Pickering emulsifier. The droplet phase
comprised either n-octane, n-decane, n-dodecane,
or n-tetradecane. This series of oils enabled the
effect of aqueous solubility on Ostwald ripening to be studied, which
is the primary instability mechanism for such nanoemulsions. Analytical
centrifugation (LUMiSizer instrument) was used to evaluate the long-term
stability of these Pickering nanoemulsions over time scales of weeks/months.
This technique enables convenient quantification of the fraction of
growing oil droplets and confirmed that using n-octane
(aqueous solubility = 0.66 mg dm–3 at 20 °C)
leads to instability even over relatively short time periods. However,
using n-tetradecane (aqueous solubility = 0.386 μg
dm–3 at 20 °C) leads to significantly improved
long-term stability with respect to Ostwald ripening, with all droplets
remaining below 1 μm diameter after 6 weeks storage at 20 °C.
In the case of n-dodecane, the long-term stability
of these new copolymer-stabilized Pickering nanoemulsions is significantly
better than the silica-stabilized Pickering nanoemulsions reported
in the literature by Persson et al. (Colloids Surf., A,2014,459, 48–57). This is
attributed to a much greater interfacial yield stress for the former
system, as recently described in the literature (see P. J. Betramo
et al. Proc. Natl. Acad. Sci. U.S.A.,2017,114, 10373–10378).
.013) is observed. Self-assembly and encapsulation is also observed during aqueous nanoprecipitation of the hyperbranched materials, with nanoparticle size (diameters from 60-140 nm) controlled through modification of precipitation conditions and the generation of the ideally branched dendrons at one end of each primary chain. The aqueous nanoparticles are highly stable and offer considerable opportunities for tailored functionality and future advanced applications.
The one-pot synthesis of highly anisotropic epoxy-functional diblock copolymer worms is achieved directly in water using a single monomer (glycidyl methacrylate).
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