Structure and optical properties of a thermoresponsive polymer-grafted, lipid-based complex fluid

We report herein an example of a self-assembling, stimulus-responsive complex fluid that snitches between two distinct structural (two-dimensional (2-D) to one-dimensional supramolecular architecture) and functional (fluid to gel and nonbirefringent to birefringent) states in response to modest temperature changes. This complex fluid is formed by the noncovalent self-assembly of a quaternary mixture of a phospholipid, dimyristolyphosphatidylcholine (DMPC), a lipopolymer consisting of poly(ethylene oxide) terminally grafted onto the phosphate headgroup of DMPE (DMPE-EO45), and a cosurfactant, lauryldimethylamine N-oxide (LDAO), dispersed in water. The supramolecular assembly formed by this mixture has been found to undergo a reversible transformation from a liquid-crystalline gel to a nonbirefringent fluid upon reduction in temperature. The onset of the thermotropic phase transition has been determined to be 16 degrees C by differential scanning calorimetry(DSC). The structural organization(i.e., the supramolecular architecture) of the noncovalent aggregate above and below this phase transition has been probed on a range of length scales (i.e., macrosopic, mesoscopic, and nanoscopic) using polarized optical microscopy (POM), small angle neutron scattering (SANS), transmission Fourier transform infrared (FT-IR) spectroscopy, and phosphorus-31 nuclear magnetic resonance (P-31 NMR) spectroscopy. The line shape of the P-31 NMR resonance of the fluid phase indicates hexagonal structural ordering. Low-resolution crystallographic structure analysis using SANS confirms the 31P NMR findings, indicating that the structure consists of a 2-D hexagonal (space group p6m) array of cylinders with the distance between the center of adjacent tubes (i.e., lattice spacing) of 345 Angstrom. The topology of the phase has been found to be a normal hexagonal structure, H-1, in which the hydrocarbon chains fill the interior of the cylinders, and the cylinders are immersed in a water continuum. The absence of birefringence in the fluid phase under crossed polarizers is believed to arise from the shortened axis of the cylinders. Structural characterization by 31P NMR and SANS demonstrates that the gel phase comprises microdomains of lamellae with a lattice spacing of 153 Angstrom. The lamellar structure, however, is a high defect structure, as evidenced by POM and the broad Bragg peaks in the small-angle neutron diffraction profile. Evaluation of the conformational state of the hydrocarbon chains by transmission FT-IR spectroscopy indicates high gauche content (type alpha), providing evidence that gelation is not a consequence of alkyl chain ordering.