Abstract
We report the synthesis of poly(N-(2-acryloyloxyethyl)pyrrolidone)-poly(4-hydroxybutyl acrylate) (PNAEP85-PHBAx) diblock copolymer nano-objects via reversible addition–fragmentation chain transfer (RAFT) aqueous dispersion polymerization of 4-hydroxybutyl acrylate (HBA) at 30 °C using an efficient two-step one-pot protocol. Given the relatively low glass transition temperature of the PHBA block, these nano-objects required covalent stabilization prior to transmission electron microscopy (TEM) studies. This was achieved by core crosslinking using glutaraldehyde. TEM analysis of the glutaraldehyde-fixed nano-objects combined with small-angle X-ray scattering (SAXS) studies of linear nano-objects confirmed that pure spheres, worms or vesicles could be obtained at 20 °C in an acidic aqueous solution by simply varying the mean degree of polymerization (x) of the PHBA block. Aqueous electrophoresis, dynamic light scattering and TEM studies indicated that raising the dispersion pH above the pKa of the terminal carboxylic acid group located on each PNAEP chain induced a vesicle-to-sphere transition. 1H NMR studies of linear PNAEP85-PHBAx nano-objects indicated a concomitant increase in the degree of partial hydration of PHBA chains on switching from pH 2-3 to pH 7-8, which is interpreted in terms of a surface plasticization mechanism. Rheological and SAXS studies confirmed that the critical temperature corresponding to the maximum worm gel viscosity could be tuned from 2 to 50 °C by adjusting the PHBA DP. Such tunability is expected to be useful for potential biomedical applications of these worm gels.
Highlights
It is well known that AB diblock copolymers can self-assemble in solution to form a range of sterically stabilized nanoparticles.[1−4] Depending on the relative volume fractions of the two blocks,[1] the copolymer morphology can be spheres,[1] worms[4,5] or vesicles.[6]
To minimize the wellknown problem of chain transfer to polymer during acrylic polymerization,[60] a low-temperature redox initiator based on KPS and AsAc61−63 at pH 3 or KPS and N,N,N′,N′tetramethylethylenediamine (TMEDA) at pH 7 was employed at a [DDMAT]/[KPS] molar ratio of 5.0. 1H NMR studies confirmed good reproducibility when conducting this first step at pH 3: the PNAEP precursor had a mean DP of 85 ± 1 and 96 ± 1% N-(2-acryloyloxyethyl) pyrrolidone (NAEP) conversion was achieved within just 10 min at 30 °C; these data are averaged over the 30 PNAEP precursors that were used to construct the pseudo-phase diagram
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Summary
It is well known that AB diblock copolymers can self-assemble in solution to form a range of sterically stabilized nanoparticles.[1−4] Depending on the relative volume fractions of the two blocks,[1] the copolymer morphology can be spheres,[1] worms[4,5] or vesicles.[6]. Variable temperature 1H NMR spectroscopy studies indicate that this remarkable behavior is the result of subtle changes in the degree of hydration of the weakly hydrophobic structure-directing block,[18,56] which was chosen to be poly(2-hydroxypropyl methacrylate) (PHPMA) in the first study and a statistical copolymer comprising 80 mol % 4hydroxybutyl acrylate (HBA) and 20 mol % diacetone acrylamide (DAAM) in the second study In the latter case, significantly greater chain mobility exhibited by the HBA-rich block led to thermoreversible sphere/worm and worm/vesicle transitions, whereas the corresponding transitions observed for PHPMA-based nano-objects exhibited strong hysteresis. The maximum thermoresponsive character should be obtained by omitting the DAAM comonomer from the PISA formulation to produce purely PHBA-based nano-objects This approach requires the identification of a suitable alternative crosslinking protocol to enable the assignment of copolymer morphologies by conventional TEM. Worms be tuned over a wide range of temperature by systematic variation of the diblock copolymer composition? Given that each steric stabilizer chain contains a terminal carboxylic acid, do these nano-objects exhibit pH-responsive behavior via end-group ionization? can a robust new crosslinking strategy be developed to enable conventional TEM studies of such nano-objects?
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