Order–Order Morphological Transitions for Dual
Stimulus Responsive Diblock Copolymer Vesicles
Joseph
R. Lovett
Nicholas J. Warren
Steven P. Armes
Mark J. Smallridge
Robert B. Cracknell
10.1021/acs.macromol.5b02470.s001
https://acs.figshare.com/articles/journal_contribution/Order_Order_Morphological_Transitions_for_Dual_Stimulus_Responsive_Diblock_Copolymer_Vesicles/2080750
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 <i>worms</i> [Lovett, J. R.; Angew. Chem. 2015, 54, 1279−1283], such diblock copolymer <i>vesicles</i> 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) being
required at 20 °C. Moreover, no order–order morphological
transitions were observed for vesicles comprising longer membrane-forming
blocks (e.g., PGMA<sub>43</sub>–PHPMA<sub>225–250</sub>) 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.
2016-02-09 00:00:00
pH 3.5
Dual Stimulus Responsive Diblock Copolymer VesiclesA series
transmission electron microscopy
PHPMA DPs
pH switch
100 mM KCl
pH 6.0.
diblock copolymer vesicles
transition
TEM
PHPMA block
dispersion polymerization
charge screening
target vesicle phase space
time scales
PGMA block
DLS studies
solution pH