A Robust Cross-Linking Strategy for Block Copolymer Worms Prepared via Polymerization-Induced Self-Assembly.

J R Lovett,N J Warren,M J Smallridge,L P D Ratcliffe,S P Armes,R B Cracknell,B R Saunders

doi:10.1021/acs.macromol.6b00422

J R Lovett, N J Warren + Show 5 more

Open Access

https://doi.org/10.1021/acs.macromol.6b00422

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Abstract

A poly(glycerol monomethacrylate) (PGMA) chain transfer agent is chain-extended by reversible addition–fragmentation chain transfer (RAFT) statistical copolymerization of 2-hydroxypropyl methacrylate (HPMA) with glycidyl methacrylate (GlyMA) in concentrated aqueous solution via polymerization-induced self-assembly (PISA). A series of five free-standing worm gels is prepared by fixing the overall degree of polymerization of the core-forming block at 144 while varying its GlyMA content from 0 to 20 mol %. 1H NMR kinetics indicated that GlyMA is consumed much faster than HPMA, producing a GlyMA-rich sequence close to the PGMA stabilizer block. Temperature-dependent oscillatory rheological studies indicate that increasing the GlyMA content leads to progressively less thermoresponsive worm gels, with no degelation on cooling being observed for worms containing 20 mol % GlyMA. The epoxy groups in the GlyMA residues can be ring-opened using 3-aminopropyltriethoxysilane (APTES) in order to prepare core cross-linked worms via hydrolysis-condensation with the siloxane groups and/or hydroxyl groups on the HPMA residues. Perhaps surprisingly, 1H NMR analysis indicates that the epoxy–amine reaction and the intermolecular cross-linking occur on similar time scales. Cross-linking leads to stiffer worm gels that do not undergo degelation upon cooling. Dynamic light scattering studies and TEM analyses conducted on linear worms exposed to either methanol (a good solvent for both blocks) or anionic surfactant result in immediate worm dissociation. In contrast, cross-linked worms remain intact under such conditions, provided that the worm cores comprise at least 10 mol % GlyMA.

Highlights

Over the past fifty years or so, block copolymer self-assembly has become a well-recognized and widely adopted route for the production of organic nanoparticles in a wide range of solvents
A well-defined nearmonodisperse poly(glycerol monomethacrylate) (PGMA) macro-CTA was prepared via reversible addition−fragmentation chain transfer (RAFT) solution polymerization of Glycerol monomethacrylate (GMA) in ethanol at 70 °C using 2-cyano-2-propylbenzodithioate (CPDB) as the chain transfer agent (CTA).70,71 1H NMR spectroscopy studies suggested a mean degree of polymerization (DP) of 56, as judged by end-group analysis
Increasing the glycidyl methacrylate (GlyMA) content in such linear copolymers affords weaker gels, as judged by rheology studies. 1H NMR studies of the kinetics of statistical copolymerization of water-immiscible GlyMA with watermiscible hydroxypropyl methacrylate (HPMA) indicate that the former comonomer is more reactive than the latter

Summary

■ INTRODUCTION

Over the past fifty years or so, block copolymer self-assembly has become a well-recognized and widely adopted route for the production of organic nanoparticles in a wide range of solvents. A series of hydroxyl-functional methacrylic diblock copolymer worms containing varying amounts of glycidyl methacrylate (GlyMA) in the core-forming block are prepared in aqueous solution via PISA. Such worms are covalently stabilized via cross-linking of the core-forming block using 3aminopropyltriethoxysilane (APTES) (see Figure 1). (3.70 mg, 0.011 mmol; PGMA56 macro-CTA/VA-044 molar ratio = 4.0) were added to a 25 mL round-bottomed flask, prior to addition of sufficient water to afford a 15% w/w aqueous solution This reaction solution was purged under nitrogen for 30 min at 20 °C prior to immersion into an oil bath set at 50 °C. A cone-and-plate geometry (40 mm 2° aluminum cone) was used for these measurements

■ RESULTS AND DISCUSSION

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES