Abstract
2-Hydroxypropyl methacrylate (HPMA) is a useful model monomer for understanding aqueous dispersion polymerization. 4-Hydroxybutyl acrylate (HBA) is an isomer of HPMA: it has appreciably higher aqueous solubility so its homopolymer is more weakly hydrophobic. Moreover, PHBA possesses a significantly lower glass transition temperature than PHPMA, which ensures greater chain mobility. The reversible addition–fragmentation chain transfer (RAFT) aqueous dispersion polymerization of HBA using a poly(ethylene glycol) (PEG113) precursor at 30 °C produces PEG113–PHBA200–700 diblock copolymer nano-objects. Using glutaraldehyde to crosslink the PHBA chains allows TEM studies, which reveal the formation of spheres, worms or vesicles under appropriate conditions. Interestingly, the partially hydrated highly mobile PHBA block enabled linear PEG113–PHBAx spheres, worms or vesicles to be reconstituted from freeze-dried powders on addition of water at 20 °C. Moreover, variable temperature 1H NMR studies indicated that the apparent degree of hydration of the PHBA block increases from 5% to 80% on heating from 0 °C to 60 °C indicating uniform plasticization. In contrast, the PHPMAx chains within PEG113–PHPMAx nano-objects become dehydrated on raising the temperature: this qualitative difference is highly counter-intuitive given that PHBA and PHPMA are isomers. The greater (partial) hydration of the PHBA block at higher temperature drives the morphological evolution of PEG113–PHBA260 spheres to form worms or vesicles, as judged by oscillatory rheology, dynamic light scattering, small-angle X-ray scattering and TEM studies. Finally, a variable temperature phase diagram is constructed for 15% w/w aqueous dispersions of eight PEG113–PHBA200–700 diblock copolymers. Notably, PEG113–PHBA350 can switch reversibly from spheres to worms to vesicles to lamellae during a thermal cycle.
Highlights
The PEG113–PHBAx diblock copolymers used in this study were synthesized via reversible addition–fragmentation chain transfer (RAFT) aqueous dispersion polymerization of Hydroxybutyl acrylate (HBA) using a previously reported trithiocarbonate-based PEG113 precursor[53] (Scheme 1). 1H NMR studies (CD3OD) of the molecularly-dissolved PEG113-PHBAx diblock copolymers con rmed that high HBA conversions (>99%) were routinely achieved within 2 h at 30 C
PEG113–PHBA200 was a transparent free- owing uid at 20 C, whereas PEG113–PHBA400 formed a free-standing gel and the PEG113–PHBA700 dispersion was free owing but highly turbid. Comparing these observations with those reported by Warren et al for PEG113–PHPMAx dispersions suggested the successful synthesis of spheres, worms and vesicles, respectively
Warren et al reported the formation of PEG113–PHPMA260 vesicles at 15% w/w solids, rather than the weakly anisotropic PEG113–PHPMA260 worms shown in Fig. S4b.†52 the former polymerizationinduced self-assembly (PISA) syntheses were conducted at 50 C, whereas the latter worms were prepared at 30 C and it is well known that the precise reaction conditions can signi cantly in uence the nal copolymer morphology for this aqueous PISA formulation.[38,52]
Summary
Diblock copolymer self-assembly in a solvent that is selective for one of the two blocks enables the preparation of stericallystabilized nano-objects that have been utilized for a broad range of applications.[1,2,3,4,5,6,7,8] Traditionally, living anionic polymerization has been used to prepare molecularly-dissolved copolymer chains prior to their isolation, puri cation and self-assembly via various post-polymerization techniques, typically in diluteEdge Article hydroxypropyl methacrylate (HPMA; Fig. 1a) to produce a weakly hydrophobic core-forming PHPMA block.[28,32,52,53,54]Essentially, the HPMA repeat units nearest the block junction become hydrated, which leads to a shi in the effective block junction. Variable temperature 1H NMR spectroscopy is used to monitor the degree of hydration of the PHBA block and the reconstitution of freeze-dried PEG113– PHBAx powders to form aqueous dispersions of diblock copolymer nano-objects at neutral pH is examined at 20 C.
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