Issue 1, 2017

Stimulus-responsive non-ionic diblock copolymers: protonation of a tertiary amine end-group induces vesicle-to-worm or vesicle-to-sphere transitions

Abstract

A well-defined poly(glycerol monomethacrylate) (PGMA) macromolecular chain transfer agent (macro-CTA) with a mean degree of polymerisation (DP) of 43 was prepared by reversible addition–fragmentation chain transfer (RAFT) polymerisation using a morpholine-functionalised trithiocarbonate-based chain transfer agent (MPETTC). Chain extension of this macro-CTA by RAFT aqueous dispersion polymerisation of 2-hydroxypropyl methacrylate (HPMA) at pH 7.0–7.5 produced a series of four MPETTC-PGMA43-PHPMAy vesicles (where y = 190, 200, 220 or 230). Protonation of the morpholine end-group increases the hydrophilic character of the PGMA stabiliser block, which leads to a reduction in the packing parameter for the diblock copolymer chains. However, such pH-responsive behaviour critically depends on the value of y. For y = 190 or 200, lowering the solution pH to pH 3 induces a vesicle-to-worm transition at 20 °C according to dynamic light scattering, aqueous electrophoresis, transmission electron microscopy and turbidimetry studies. This order–order transition is suppressed in the presence of added electrolyte, which screens the cationic end-groups. In addition, no change in copolymer morphology was observed on lowering the solution temperature at neutral pH, regardless of the y value. The diblock copolymer nano-objects obtained at pH 3 were also cooled to 4 °C to examine their dual stimulus-responsive behaviour to both pH and temperature triggers. In all four cases, a change in morphology from either worms or vesicles to afford spheres (or spheres plus relatively short worms) was observed. Temperature-dependent oscillatory rheology experiments performed on cationic worms at pH 3 indicated a worm-to-sphere transition on cooling from 20 °C to 4 °C, which leads to reversible degelation. In summary, spheres, worms or vesicles can be obtained for MPETTC-PGMA-PHPMA diblock copolymers on first lowering the solution pH to pH 3, followed by cooling from 20 °C to 4 °C.

Graphical abstract: Stimulus-responsive non-ionic diblock copolymers: protonation of a tertiary amine end-group induces vesicle-to-worm or vesicle-to-sphere transitions

Supplementary files

Article information

Article type
Paper
Submitted
21 jun. 2016
Accepted
20 jul. 2016
First published
21 jul. 2016
This article is Open Access
Creative Commons BY license

Polym. Chem., 2017,8, 272-282

Author version available

Stimulus-responsive non-ionic diblock copolymers: protonation of a tertiary amine end-group induces vesicle-to-worm or vesicle-to-sphere transitions

N. J. W. Penfold, J. R. Lovett, P. Verstraete, J. Smets and S. P. Armes, Polym. Chem., 2017, 8, 272 DOI: 10.1039/C6PY01076H

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