Abstract
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 [LovettJ. R.; Angew. Chem.2015, 54, 1279−128325418214], 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) 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.
Highlights
Over the past 50 years or so, there have been many reports of AB diblock copolymer self-assembly in solvents that are selective for only one block.[1−12] In principle, varying the relative degrees of polymerization (DP) of each block enables a wide range of morphologies to be obtained in dilute solution, including spherical micelles, cylindrical micelles, or vesicles
A low-polydispersity poly(glycerol monomethacrylate) (PGMA) macro-chain transfer agent (CTA) (Mw/Mn = 1.20) containing a terminal carboxylic acid was prepared in ethanol at 70 °C by reversible addition−fragmentation transfer (RAFT) solution polymerization of Glycerol monomethacrylate (GMA) using PETTC
We demonstrate that PGMA−PHPMA diblock copolymer vesicles prepared using a carboxylic acid-functionalized RAFT agent exhibit complex stimulus-responsive behavior in aqueous solution
Summary
Over the past 50 years or so, there have been many reports of AB diblock copolymer self-assembly in solvents that are selective for only one block.[1−12] In principle, varying the relative degrees of polymerization (DP) (and volume fractions) of each block enables a wide range of morphologies to be obtained in dilute solution, including spherical micelles, cylindrical micelles (e.g., rods or worms), or vesicles. PGMA43 macro-CTA with 175 units of HPMA via RAFT aqueous dispersion polymerization of HPMA is as follows: PGMA43 macroCTA (0.143 g, 0.020 mmol), HPMA monomer (0.50 g, 3.5 mmol), and ACVA (1.9 mg, 0.006 mmol; PGMA43 macro-CTA/ACVA molar ratio = 3.0) were added to a 25 mL round-bottomed flask, prior to addition of water to produce a 10% w/w solution. This reaction solution was purged with nitrogen gas for 30 min at 20 °C prior to immersion into an oil bath set at 70 °C. Transmittance measurements were recorded every minute immediately after this solution pH was increased to pH 9.0 using KOH
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