Nanostructuring the Interior of Stimuli-Responsive Microgels by N-Vinylimidazoles Quaternized with Hydrophobic Alkyl Chains

The functionality of stimuli-responsive microgels can be tailor-made by manipulating their internal nanostructure induced by the chemical composition and morphology of the polymer network. Microgels with phase-separated domains on a nanoscopic length scale were synthesized by copolymerization of N-vinylcaprolactam (VCL) and amphiphilic-to-hydrophobic 1-vinyl-3-alkylimidazolium (VIM+CnH2n+1) bromides (Br-) of different alkyl chain lengths (n = 12, 14, 16) as comonomers. These quaternized imidazoles provide dual functionality for immobilization of payload by electrostatic and hydrophobic interactions. The morphologies and properties of synthesized poly(VCL-co-VIM+CnH2n+1Br-) microgels with 10 mol % comonomer were investigated systematically by H-1 and C-13 high-resolution NMR spectroscopy and relaxometry. Chemical side-selective information about the monomers' volume-phase transition temperatures, width of transition, and change in transition entropy was reported and correlated to the alkyl chain length of the VIM+CnH2n+1Br- comonomer. C-13 NMR spectroscopy reveals the existence of trans and gauche conformers of alkyl chains, which depends on the alkyl chain length and temperature. Morphologies and dynamic contrasts of alkyl chain domains and VCL moieties of poly(VCL-co-VIM+CnH2n+1Br-) microgels were investigated by H-1 transverse magnetization relaxation (T-2-relaxation). Finally, the microgels were successfully applied in the uptake of the hydrophobic dye Nile red, proving their ability to solubilize hydrophobic substances. In addition, the poly(VCL-co-VIM+CnH2n+1Br-) microgels were utilized in electrostatic interactions, as well as simultaneous addition of hydrophobic and negatively charged payload as a proof of concept for dual functionality. This investigation will allow for a better understanding of the internal nanophase structure of complex poly(N-vinylcaprolactam) (PVCL)-based microgels comprising pH-independent positive charges, as well as hydrophobic compartments, which have potential application as dual-functional delivery systems.