Shape-shifting thermoreversible diblock copolymer nano-objects via RAFT aqueous dispersion polymerization of 4-hydroxybutyl acrylate. Issue 41 (7th October 2021)

• Record Type: Journal Article
• Authors: Deane, Oliver J.; Jennings, James; Armes, Steven P.
• Is Part Of: Chemical science. Volume 12:Issue 41(2021)
• Page Start: 13719
• Page End: 13729
• Publication Date: 2021-10-07
• Subjects: Chemistry -- Periodicals; 540.5
• DOI: 10.1039/d1sc05022b
• Languages: English
• ISSNs: 2041-6520

Abstract

Abstract : RAFT aqueous dispersion polymerization of 4-hydroxybutyl acrylate (HBA) affords shape-shifting thermoresponsive diblock copolymer nano-objects. 1 H NMR studies suggest that such behavior involves uniform plasticization of the PHBA block. 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 –PHBA x spheres, worms or vesicles to be reconstituted from freeze-dried powders on addition of water at 20 °C. Moreover, variable temperature 1 H 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 PHPMA x chains within PEG113 –PHPMA x 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.