A series of non-ionic poly(glycerol monomethacrylate)−
poly(2-hydroxypropyl methacrylate) (PGMA−PHPMA) diblock copolymer
vesicles has been prepared by reversible addition−fragmentation
chain transfer (RAFT) aqueous dispersion polymerization of HPMA at
70 °C at low pH using a carboxylic acid-based chain transfer agent. The
degree of polymerization (DP) of the PGMA block was fixed at 43, and
the DP of the PHPMA block was systematically varied from 175 to 250 in
order to target vesicle phase space. Based on our recent work describing
the analogous PGMA−PHPMA diblock copolymer worms [Lovett, J. R.;
et al. Angew. Chem. 2015, 54, 1279−1283], such diblock copolymer vesicles were expected to undergo an order−order
morphological transition via ionization of the carboxylic acid end-group on switching the solution pH. Indeed, irreversible vesicleto-sphere
and vesicle-to-worm transitions were observed for PHPMA DPs of 175 and 200, respectively, as judged by
turbidimetry, transmission electron microscopy (TEM), and dynamic light scattering (DLS) studies. However, such
morphological transitions are surprisingly slow, with relatively long time scales (hours) being required at 20 °C. Moreover, no
order−order morphological transitions were observed for vesicles comprising longer membrane-forming blocks (e.g., PGMA43−
PHPMA225−250) on raising the pH from pH 3.5 to pH 6.0. However, in such cases the application of a dual stimulus comprising
the same pH switch immediately followed by cooling from 20 to 5 °C, induces an irreversible vesicle-to-sphere transition. Finally,
TEM and DLS studies conducted in the presence of 100 mM KCl demonstrated that the pH-responsive behavior arising from
end-group ionization could be suppressed in the presence of added electrolyte. This is because charge screening suppresses the
subtle change in the packing parameter required to drive the morphological transition.
CORE (COnnecting REpositories)
CORE (COnnecting REpositories)