Abstract
Polymerization-induced self-assembly (PISA) has become a widely used technique for the rational design of diblock copolymer nano-objects in concentrated aqueous solution. Depending on the specific PISA formulation, reversible addition–fragmentation chain transfer (RAFT) aqueous dispersion polymerization typically provides straightforward access to either spheres, worms, or vesicles. In contrast, RAFT aqueous emulsion polymerization formulations often lead to just kinetically-trapped spheres. This limitation is currently not understood, and only a few empirical exceptions have been reported in the literature. In the present work, the effect of monomer solubility on copolymer morphology is explored for an aqueous PISA formulation. Using 2-hydroxybutyl methacrylate (aqueous solubility = 20 g dm–3 at 70 °C) instead of benzyl methacrylate (0.40 g dm–3 at 70 °C) for the core-forming block allows access to an unusual “monkey nut” copolymer morphology over a relatively narrow range of target degrees of polymerization when using a poly(methacrylic acid) RAFT agent at pH 5. These new anisotropic nanoparticles have been characterized by transmission electron microscopy, dynamic light scattering, aqueous electrophoresis, shear-induced polarized light imaging (SIPLI), and small-angle X-ray scattering.
Highlights
In recent years, polymerization-induced self-assembly (PISA) has become a widely recognized route for the synthesis of many types of diblock copolymer nano-objects.[1−5] Compared to post-polymerization processing techniques, PISA is much more efficient and can be performed at relatively high solids (10−50% w/ w).[3,6−8] This approach involves growth of an insoluble block from a soluble homopolymer in a suitable solvent to give welldefined sterically stabilized diblock copolymer nanoparticles
Reversible addition-fragmentation chain transfer (RAFT) aqueous dispersion polymerization involves polymerization of a water-miscible monomer such as 2-hydroxypropyl methacrylate (HPMA) from a water-soluble stabilizer, e.g. poly(glycerol monomethacrylate).[9,10]. Such formulations enable the production of various copolymer morphologies such as spheres, worms or vesicles.[11−19] reversible addition-fragmentation chain transfer (RAFT) aqueous emulsion polymerization has received significant attention.[2,6,7,20−24] In this case, a water-immiscible monomer is used to produce the hydrophobic core-forming block, but according to many literature reports only kinetically-trapped spheres can be obtained.[6,7,24−31] Exceptionally, Charleux and co-workers reported the synthesis of diblock copolymer worms and vesicles, as well as spheres.[23,32−35] Recent empirical experiments have undoubtedly provided some useful insights,[36] but the critical synthesis parameters that determine whether only kinetically-trapped spheres are obtained or the full range of morphologies are observed have not yet been established
Truong et al recently synthesized novel “filomicelle nanomaterials” directly in water by employing RAFT aqueous emulsion polymerization followed by temperature-induced morphological transition
Summary
Polymerization-induced self-assembly (PISA) has become a widely recognized route for the synthesis of many types of diblock copolymer nano-objects.[1−5] Compared to post-polymerization processing techniques (solvent exchange, film rehydration, or pH switch), PISA is much more efficient and can be performed at relatively high solids (10−50% w/ w).[3,6−8] This approach involves growth of an insoluble block from a soluble homopolymer in a suitable solvent to give welldefined sterically stabilized diblock copolymer nanoparticles. X-ray scattering data were reduced using standard routines from the beamline and were further analyzed using Irena SAS macros for Igor Pro.[40] The SAXS pattern for PMAA56−PHBMA50 was obtained using a Bruker AXS Nanostar laboratory instrument modified with a microfocus X-ray tube (GeniX3D, Xenocs) and motorized scatterless slits for the beam collimation (camera length = 1.46 m, Cu Kα radiation, and HiSTAR multiwire gas detector) In this case the SAXS pattern was recorded for a 1.0 % w/w aqueous dispersion at pH 5 over a q range of 0.08 nm−1 < q > 1.6 nm −1 using a glass capillary of 2.0 mm diameter and an exposure time of 1.0 h. All SAXS patterns were analyzed (background subtraction, data modeling and fitting) using Irena SAS macros for Igor Pro.[40]
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