Tailored Architectures of Ammonium Ionenes

The synthesis and characterization of a variety of ammonium ionenes from water-soluble coatings to high-performance elastomers are discussed. Water-soluble random copolymer ionenes were synthesized using the Menshutkin reaction from 1,12-dibromododecane, N,N,Nâ ²,Nâ ²-tetramethyl-1,6-hexanediamine, and 1,12-bis(N,N-dimethylamino)dodecane. The absolute molecular weights were determined for the first time using a multiangle laser light scattering detector in aqueous size exclusion chromatography and the weight-average molecular weights of these ionenes were in the range of 17,000-20,000 g/mol. Charge density increased with increasing molar ratio of N,N,Nâ ²,Nâ ²-tetramethyl-1,6-hexanediamine and the glass transition temperature (Tg) increased from 69 °C to 90 °C as the charge density increased. Small angle x-ray scattering (SAXS) showed isotropic scattering patterns for these ionenes. A limited study on cytotoxicity of these ionenes showed no direct correlation between charge density and cell viability for human brain microvascular endothelial cell line.

A series of low hard segment (HS) content, poly(propylene glycol) (PPG)-based ammonium ionenes were synthesized using a Menshutkin reaction from bromine end-capped PPG oligomers (prepared using acid-chloride reactions) and N,N,Nâ ²,Nâ ²-tetramethyl-1,6-hexanediamine. Matrix assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry, titration analyses, and ¹H NMR spectroscopy, confirmed the difunctionality of bromine end-capped PPG oligomers. Thermal analysis revealed Tg's of -60 °C, comparable to pure PPG, using differential scanning calorimetery (DSC), dynamic mechanical analysis (DMA) confirming microphase separation, and an onset of degradation (Td) at 240 °C.

Synthesis of a series of random block copolymer ammonium ionenes with an aliphatic 1,12-dibromododecane as part of the hard segment (33 wt% HS) enhanced film formation and supported microphase separation property. The Td and Tg did not change compared to PPG-ionenes with lower HS content. DMA and tensile testing demonstrated the influence of soft segment (SS) molecular weight and hard segment (HS) content on the mechanical properties of segmented ammonium ionenes. DMA showed the onset of flow, ranging from 100-140 ºC for 1K and 2K g/mol PPG-based ionenes respectively. SAXS revealed a Bragg distance scaled with soft segment molecular weight and ranged from 6.6 to 23.4 nm for 1K to 4K g/mol PPG-based ionenes, respectively.

An investigation of the salt-responsive solubility property of random block copolymer PPG-ionenes revealed a dependence on PPG molecular weight. The 1K g/mol PPG-based ionenes with a hydrophilic (HPL)/hydrophobic (HPB) value ranging from one to three showed solubility in both water and one wt% NaCl aqueous solutions. The 2K g/mol PPG-based ionenes containing HPL/HPB value of two to 15 showed cloudy dispersions in water and one wt% NaCl solutions. The 4K g/mol PPG-based ionenes possessed the salt-responsive character; 4K g/mol PPG-based ionenes with HPL/HPB values of one to 12 showed milky dispersions in water, suspended particles in one wt% NaCl solutions and film precipitation at a HPL/HPB molar ratio of 19.