Benzyl methacrylate (BzMA) is polymerized
using a poly(lauryl methacrylate)
macromolecular chain transfer agent (PLMA macro-CTA) using reversible
addition–fragmentation chain transfer (RAFT) polymerization
at 70 °C in n-dodecane. This choice of solvent
leads to an efficient dispersion polymerization, with polymerization-induced
self-assembly (PISA) occurring via the growing PBzMA block to produce
a range of PLMA–PBzMA diblock copolymer nano-objects, including
spheres, worms, and vesicles. In the present study, particular attention
is paid to the worm phase, which forms soft free-standing gels at
20 °C due to multiple inter-worm contacts. Such worm gels exhibit
thermo-responsive behavior: heating above 50 °C causes degelation
due to the onset of a worm-to-sphere transition. Degelation occurs
because isotropic spheres interact with each other much less efficiently
than the highly anisotropic worms. This worm-to-sphere thermal transition
is essentially irreversible on heating a dilute solution (0.10% w/w)
but is more or less reversible on heating a more concentrated dispersion
(20% w/w). The relatively low volatility of n-dodecane
facilitates variable-temperature rheological studies, which are consistent
with eventual reconstitution of the worm phase on cooling to 20 °C.
Variable-temperature 1H NMR studies conducted in d26-dodecane confirm partial solvation of the
PBzMA block at elevated temperature: surface plasticization of the
worm cores is invoked to account for the observed change in morphology,
because this is sufficient to increase the copolymer curvature and
hence induce a worm-to-sphere transition. Small-angle X-ray scattering
and TEM are used to investigate the structural changes that occur
during the worm-to-sphere-to-worm thermal cycle; experiments conducted
at 1.0 and 5.0% w/w demonstrate the concentration-dependent (ir)reversibility
of these morphological transitions.
ABSTRACT. Well-defined poly(lauryl methacrylate-benzyl methacrylate) (PLMA-PBzMA) diblock copolymer nanoparticles are prepared in n-heptane at 90°C via reversible addition-fragmentation chain transfer (RAFT) polymerization. Under these conditions, the PLMA macromolecular chain transfer agent (macro-CTA) is soluble in n-heptane, whereas the growing PBzMA block quickly becomes insoluble. Thus this dispersion polymerization formulation leads to polymerization-induced self-assembly (PISA).
10Using a relatively long PLMA macro-CTA with a mean degree of polymerization (DP) of 37 or higher leads to the formation of well-defined spherical nanoparticles of 41 to 139 nm diameter, depending on the DP targeted for the PBzMA block. In contrast, TEM studies confirm that using a relatively short PLMA macro-CTA (DP = 17) enables both worm-like and vesicular morphologies to be produced, in addition to the spherical phase. A detailed phase diagram has been elucidated for this more asymmetric diblock 15 copolymer formulation, which ensures that each phase can be targeted reproducibly.1 H NMR spectroscopy confirmed that high BzMA monomer conversions (> 97 %) were achieved within 5 h, while GPC studies indicated that reasonably good blocking efficiencies and relatively low diblock copolymer polydispersities (M w /M n < 1.30) were obtained in most cases. Compared to prior literature reports, this allmethacrylic PISA formulation is particularly novel because: (i) it is the first time that higher order 20 morphologies (e.g. worms and vesicles) have been accessed in non-polar solvents and (ii) such diblock copolymer nano-objects are particularly relevant to potential boundary lubrication applications for engine oils.
There is considerable current interest in polymerization-induced self-assembly (PISA) via reversible addition-fragmentation chain transfer (RAFT) polymerization as a versatile and efficient route to various types of block copolymer nano-objects. Many successful PISA syntheses have been conducted in water using either RAFT aqueous dispersion polymerization or RAFT aqueous emulsion polymerization. In contrast, this review article is focused on the growing number of RAFT PISA formulations developed for non-aqueous media. A wide range of monomers have been utilized for both the stabilizer and core-forming blocks to produce diblock copolymer nanoparticles in either polar or non-polar solvents via RAFT dispersion polymerization. Such nanoparticles can exhibit spherical, worm-like or vesicular morphologies, often with controllable size and functionality. Detailed characterization of such sterically-stabilized diblock copolymer dispersions provide important insights into the various morphological transformations that can occur both during the PISA synthesis and also subsequently on exposure to a suitable external stimulus (e.g. temperature).
Diblock copolymer spheres, worms and vesicles are prepared via RAFT dispersion polymerization of benzyl methacrylate in either mineral oil or a poly(α-olefin) using polymerization-induced self-assembly; an efficient ‘one-pot’ protocol is reported for spheres at 30% solids in mineral oil.
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.
A new amphiphilic diblock copolymer prepared via polymerization-induced self-assembly forms spheres, worms, vesicles or lamellae in aqueous solution on adjusting the temperature.
Thermo-responsive vermicious (or worm-like) diblock copolymer nanoparticles prepared directly in n-dodecane are used to stabilise water-in-oil Pickering emulsions.
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