Complementary light scattering and synchrotron small-angle X-ray scattering studies of the micelle-to-unimer transition of polysulfobetaines.

Kay E B Doncom,Nigel Kirby,Rachel K O'Reilly,Annhelen Lu,Beulah E Mckenzie,Helen Willcock,Anaïs Pitto-Barry

doi:10.1039/c5sm00602c

Kay E B Doncom, Nigel Kirby + Show 5 more

Open Access

https://doi.org/10.1039/c5sm00602c

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Abstract

AB and ABA di- and triblock copolymers where A is the hydrophilic poly(oligoethylene glycol methacrylate) (POEGMA) block and B is a thermo-responsive sulfobetaine block [2-(methacryloyloxy) ethyl] dimethyl-(3-sulfopropyl) ammonium hydroxide (PDMAPS) were synthesised by aqueous RAFT polymerisation with narrow dispersity (ĐM ≤ 1.22), as judged by aqueous SEC analysis. The di- and triblock copolymers self-assembled in salt-free water to form micelles with a PDMAPS core and the self-assembly of these polymers was explored by SLS and TEM analysis. The micelles were shown, by DLS analysis, to undergo a micelle-to-unimer transition at a critical temperature, which was dependent upon the length of the POEGMA block. Increasing the length of the third, POEGMA, block decreased the temperature at which the micelle-to-unimer transition occurred as a result of the increased hydrophilicity of the polymer. The dissociation of the micelles was further studied by SLS and synchrotron SAXS. SAXS analysis revealed that the micelle dissociation began at temperatures below that indicated by DLS analysis and that both micelles and unimers coexist. This highlights the importance of using multiple complementary techniques in the analysis of self-assembled structures. In addition the micelle-to-unimer morphology transition was employed to encapsulate and release a hydrophobic dye, Nile Red, as shown by fluorescence spectroscopy.

Highlights

IntroductionPolymeric betaines are a class of zwitterionic polymers in which the cationic and anionic functional groups are located on the same monomer unit.[9]
AB and ABA di- and triblock copolymers where A is the hydrophilic poly(oligoethylene glycol methacrylate) (POEGMA) block and B is a thermo-responsive sulfobetaine block [2-(methacryloyloxy) ethyl] dimethyl-(3-sulfopropyl) ammonium hydroxide (PDMAPS) were synthesised by aqueous reversible additionfragmentation chain-transfer (RAFT) polymerisation with narrow dispersity (ÐM r 1.22), as judged by aqueous Size exclusion chromatography (SEC) analysis
The hydrophilic POEGMA was subsequently used as a macroCTA for the chain extension with dimethyl(methacryloylethyl) ammonium propane sulfonate (DMAPS) in 0.5 M NaCl yielding a responsive diblock copolymer, 2, with Mn Nuclear magnetic resonance (NMR) = 209 kDa, Mn SEC = 106.4 kDa, ÐM = 1.16

Summary

Introduction

Polymeric betaines are a class of zwitterionic polymers in which the cationic and anionic functional groups are located on the same monomer unit.[9]. An increase in temperature caused the Rh of these polymers in water to increase, as determined by DLS analysis.[37] In another example Tian et al synthesised a diblock copolymer consisting of POEGMA and a tertiary amine acrylamide monomer by RAFT polymerisation.[39] This polymer exhibited both LCST and UCST behaviour. The UCST of the betaine block aggregation was observed by light scattering, for above the LCST cloud point of the PNIPAM block; further characterisation of the solution self-assembly was not provided. Between these two temperatures the polymer was molecularly dissolved. Scattering length densities used for the calculations are 1.02 Â 10À5 ÅÀ2 (core), 1.04 Â 10À5 ÅÀ2 (shell) and 9.46 Â 10À6 ÅÀ2 (solvent)

Synthetic procedures

Results and discussion

Mw þ q2Rg2 3 þ

Conclusions