Order–Order Morphological Transitions for DualStimulus Responsive Diblock Copolymer Vesicles

摘要

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 vesicle-to-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) beingrequired at 20 °C. Moreover, no order–order morphologicaltransitions were observed for vesicles comprising longer membrane-formingblocks (e.g., PGMA43–PHPMA225–250) on raising the pH from pH 3.5 to pH 6.0. However, in such casesthe application of a dual stimulus comprising the same pH switch immediatelyfollowed by cooling from 20 to 5 °C, induces an irreversiblevesicle-to-sphere transition. Finally, TEM and DLS studies conductedin the presence of 100 mM KCl demonstrated that the pH-responsivebehavior arising from end-group ionization could be suppressed inthe presence of added electrolyte. This is because charge screeningsuppresses the subtle change in the packing parameter required todrive the morphological transition.