BAB amphiphilic triblock copolymers comprising poly(methoxyethyl
acrylate) (PMEA) B blocks of variable degrees of polymerization (x = 50, 100, or 200) and a central poly(dimethyl acrylamide)
(PDMAc) hydrophilic A block with a degree of polymerization y = 400 were synthesized by reversible addition fragmentation
chain transfer polymerization, and their self-assembly in water was
studied by light scattering and rheology. The BAB triblocks form a
transient network in aqueous solution consisting of hydrophobic B
cores bridged by hydrophilic A blocks. The exchange of B blocks is
fast at low temperature (10 °C) and/or low x values because PMEA is not too hydrophobic. However, the PMEA blocks
become more hydrophobic with increasing temperature, leading to a
decrease of the exchange rate of the B blocks and to tunable thermo-thickening
properties, opposite to the classical Arrhenius behavior. In contrast
to many thermosensitive BAB triblocks that undergo an abrupt sol–gel
transition above the critical solubility temperature of the B blocks,
the hydrophobicity of PMEA gradually increases with T, leading to a progressive increase of the viscosity of the polymer
dispersion. In addition, an unprecedented reorganization of the network
was observed by rheology at a critical temperature, leading to a further
decrease of the exchange rate of the B blocks that is not fully reversible.
These features were attributed to the progressive variation of the
hydrophobic character and hydration of PMEA with temperature.
Polymerization-induced self-assembly (PISA) has established itself as a powerful and straightforward method to produce polymeric nano-objects of various morphologies in (aqueous) solution. Generally, spheres are formed in the early stages of polymerization that may evolve to higher order morphologies (worms or vesicles), as the solvophobic block grows during polymerization. Hitherto, the mechanisms involved in these morphological transitions during PISA are still not well understood. Combining a systematic study of a representative PISA system with rheological measurements, we demonstrate that-unexpectedly-unimer exchange is not necessary to form higher order morphologies during radical RAFT-mediated PISA. Instead, in the investigated aqueous PISA, the monomer present in the polymerization medium is responsible for the morphological transitions, even though it slows down unimer exchange.
Polymerization-induced self-assembly (PISA) has established itself as a powerful and straightforward method to produce polymeric nano-objects of various morphologies in (aqueous) solution. Generally, spheres are formed in the early stages of polymerization that may evolve to higher order morphologies (worms or vesicles), as the solvophobic block grows during polymerization. Hitherto, the mechanisms involved in these morphological transitions during PISA are still not well understood. Combining a systematic study of a representative PISA system with rheological measurements, we demonstrate that-unexpectedly-unimer exchange is not necessary to form higher order morphologies during radical RAFT-mediated PISA. Instead, in the investigated aqueous PISA, the monomer present in the polymerization medium is responsible for the morphological transitions, even though it slows down unimer exchange.
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