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Download fileEffect of Core Cross-linking on the Physical Properties of Poly(dimethylsiloxane)-Based Diblock Copolymer Worms Prepared in Silicone Oil
journal contribution
posted on 2019-08-30, 17:03 authored by Matthew J. Rymaruk, Cate T. O’Brien, Steven L. Brown, Clive N. Williams, Steven P. ArmesA trithiocarbonate-capped poly(dimethylsiloxane)
(PDMS) precursor
is chain-extended via reversible addition–fragmentation chain
transfer dispersion polymerization of 2-(dimethylamino)ethyl methacrylate
(DMA) in decamethylcyclopentasiloxane (D5) silicone oil at 90 °C.
For a fixed mean degree of polymerization (DP) of 66 for the PDMS
steric stabilizer block, targeting core-forming PDMA block DPs of
between 105 and 190 enables the preparation of either well-defined
worms or vesicles at a copolymer concentration of 25% w/w. The as-synthesized
linear PDMS66–PDMA100 worms exhibit thermoresponsive
behavior in D5, undergoing a worm-to-sphere transition on heating
to 100 °C. Variable temperature 1H NMR spectroscopy
indicates that this thermal transition is driven by reversible solvent
plasticization of the PDMA cores. This change in copolymer morphology
is characterized by transmission electron microscopy (TEM) studies,
variable temperature dynamic light scattering and small-angle X-ray
scattering experiments. Oscillatory rheology studies indicate that
degelation occurs at 32 °C, but shear-induced polarized light
imaging measurements suggest that full conversion of worms into spheres
requires significantly higher temperatures (∼110 °C).
1,2-Bis(2-iodoethoxy)ethane (BIEE) is evaluated as a cross-linker
for PDMS66–PDMAx diblock
copolymer nano-objects in D5. This bifunctional reagent quaternizes
the tertiary amine groups on the DMA residues within the worm cores,
introducing cross-links via the Menshutkin reaction. TEM studies confirm
that such covalently-stabilized worms no longer undergo a worm-to-sphere
transition when heated to 100 °C. Kinetic studies performed on
PDMS66–PDMA176 vesicles suggest that
cross-linking requires approximately 13 h at 20 °C to ensure
that these nano-objects remain intact when dispersed in chloroform,
which is a good solvent for both blocks. Oscillatory rheology studies
of a PDMS66–PDMA100 worm gel indicated
that covalent stabilization using a BIEE/DMA molar ratio of 0.15 increased
its dynamic elastic modulus (G′) by almost
two orders of magnitude. Furthermore, such cross-linked worms exhibit
a much lower critical gelation concentration (∼2% w/w) compared
to that of the linear precursor worms (∼12% w/w).