| Although low molecular weight gelators were discovered in the early 19th century,the advent of supramolecular gels has reinvigorated the attention of scientists towards this area of soft matter research and development.As of late,numerous types of functional supramolecular gels have been synthesized,and reported.Our group has most recently shown that donor-acceptor(D-A)π-electron charge transfer(CT)interactions between the π-electron deficient acceptors and π-electron rich donors can not only direct the addition polymerization of monomers but also enhance the rheological properties of the resulting untemplated hydrogels.Based on this previous work,I developed a new kind of hydrogel formed by combining a π-electron deficient bipyridinium-based polyacylhydrazone and various π-electron rich indole-based templates to further study the mechanism of the donor-acceptor π-stacking templated gelation process.With the help of nuclear magnetic resonance(NMR)spectroscopy,high-resolution mass spectroscopy,Fourier transform infrared(FT-IR)spectroscopy,UV-Vis absorption spectroscopy,gel permeation chromatography(GPC),and oscillatory rheological analyses,I comprehensively analyzed the molecular structures,properties,and hydrogelation mechanism of my hydrogels.The addition of templates brought changes to both the polymer polyacylhydrazone and the hydrogels which they formed in-situ.Based on the template-assisted synthetic strategy,the templated polyacylhydrazone was synthesized with larger molecular weights than polymers obtained via the non-templated synthetic protocol.For example,the templated polyacylhydrazone’s Mw and Mn increased by 113%and 95%,respectively,when compared with the non-templated polyacylhydrazone.The critical hydrogelation concentration for the templated polymers decreased from 19 mM to less than 10 mM following the introduction of indole-based templates to the system.Moreover,the templated hydrogels exhibited stronger rheological properties and faster recovery capabilities following shear-induced breakdown than the non-templated gels.It was discovered that hydrogen-bonding played a major role in the hydrogelation of these polyacylhydrazones at lower monomer concentrations,whereas at the higher concentrations,the donor-acceptor(DA)π-electron charge transfer(CT)interaction dominated the driving formation of the hydrogels.Thus,DA π-electron CT interactions played dual roles in enhancing the rheological properties of hydrogels:1)to elongate the polymer chains of polyacylhydrazones and 2)strengthen the hydrogel and speed up the recovery of the gel following a sheer-induced breakdown.The combination of non-covalent interactions and the dynamic covalent bond chemistry in this work resulted in a smart hydrogel that showed responses to temperature,mechanical force,and saturated urea solutions.The construction and analyses of this novel hydrogel can help us to better understand supramolecular hydrogelating soft matter systems and how to design increasingly complex polymeric compounds using template-assisted polymerization strategies. |
