(365e) Characterizations of Nano-Structures of Polyurea Synthesized in Soft Ionic Liquids
Room temperature ionic liquids (ILs) are molten organic salts with melting temperature normally below 100oC. This class of soft materials exhibits very complicated molecular interactions such as ionic interactions, hydrogen bonding, π-π interactions, and amphiphilic polarization, rendering various molecular structures from merely local orderness up to macroscopic thermotropic or lyotropic liquid crystalline phases.
Recent reports on nano-size morphological features such as flower-like or shuttle-like crystals or interconnected nano-porous structures of inorganic compounds synthesized in ILs, including ZnO, TiO2, and silica, have received increasing attentions for understanding the molecular interactions mediated by the ILs during the synthetic reactions and for the possible applications. Some explanations based on hydrogen-bonding/π-π ring staking were given to interpret the observed porous structures for a length scale below 3nm.
On the organic counterpart, we have firstly found exotic nano-porous structures in polyurea synthesized by interfacial polymerization between n-hexane (with 2,4-toluene diisocyanate, TDI) and a series of 1-alkyl-3-methylimidazolium tetrafluoroborates and 1-alkyl-3-methylimidazolium hexafluorophosphates (with ethylene diamine or 1,4 diaminobutane). These polymeric materials exhibit volcano-like, coral-like, sphere (~100nm), or polymer fibrils of sizes around 50 nm forming porous structures with pore size ~ 300 nm - depending on the ionic liquids and diamines. Results from small angle x-ray scattering (SAXS, synchrotron source, Brookhaven Laboratories) depict characteristic lengths around 30~40 nm. Micrographs from transmission electron microscopy (TEM) have been used to confirm the structures. These structures clearly show that the kinetics of phase separation is affected by the ILs with a length scale at least an order of magnitude larger than the reported interpretation based on local molecular structures.
We will present the morphological features of polyurea and discuss the physical origins based molecular interactions, particularly, hydrogen bonding, π-π interactions, and molecular orientations due to ionic interactions and polarity that may extend to a quasi long range beyond 10 nm. Evidences from our recent results in neutron diffraction (ND, Chalk River Laboratories, NRC, Canada) and wide angle x-ray scattering (WAXS) will be incorporated in the discussion to link the scenarios from local molecular scale to the mesoscale of interests. Attempts to understanding the structures and chemicophysical properties of ILs can facilitate their potential applications in chemical industry.
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