Structure and Optical Properties of a Thermoresponsive Polymer-Grafted, Lipid-Based Complex Fluid

Abstract

We report herein an example of a self-assembling, stimulus-responsive complex fluid that switches 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, lauryldi-methylamine 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 °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 (31P NMR) spectroscopy. The line shape of the 31P 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 Å. The topology of the phase has been found to be a normal hexagonal structure, H1, 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 Å. 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 α), providing evidence that gelation is not a consequence of alkyl chain ordering.

1H and13C NMR study of phase transition and molecular motion in ionic liquids forming lyotropic liquid-crystalline ionogels

“…Technological developments of so called 'smart' materials are continuously boosting over the whole past decade [1][2][3][4][5][6][7]. Materials are classified as 'smart' if they possess an intrinsic ability to sense and definitely to respond to various external stimuli in a predictable way ( [2] and references therein).…”

“…Materials are classified as 'smart' if they possess an intrinsic ability to sense and definitely to respond to various external stimuli in a predictable way ( [2] and references therein). The research of these features open new technological vista for various sensors, molecular machines, switches, controllers, etc.…”

A 2H solid-state NMR spectroscopic investigation of biomimetic bicelles containing cholesterol and polyunsaturated phosphatidylcholine

“…As the temperature is raised, the bilayer undergoes two thermal phase transitions leading to the lamellae liquid-crystalline phase (L ␣ ). In the liquid-crystalline phase, the phospholipid bilayer passes through many subphases characterized by different viscosities, hydrations, ordering, and motion, which can affect the formation and orientation of magnetically aligned phospholipid bilayers (Selig, 1976;Katsaras et al, 1997;Firestone et al, 1998;Bennett and Hess, 1999;Binnemans et al, 2000;Mangels et al, 2000;Raffard et al, 2000;Tiburu et al, 2001;Cardon et al, 2003). Kinks in the acyl chains cause the bilayer structure to become less tightly packed and more fluid increasing the rate of lateral diffusion of the phospholipids by two orders of magnitude (Dowhan, 1997).…”

Oligomer‐to‐Polymer Transition in Short Ethylene Glycol Chains Connected to Mobile Hydrophobic Anchors

“…The hexagonal phase has also been found for various types of glycolipids and lipopolymers, [27,[30][31][32][33] dependent on their hydrophobic/hydrophilic balance, as well as on the mismatch between cross-sectional areas of head groups and lipid anchors. The transition enthalpy of EG6 lipids, DH = 11 kcal mol À1 , is larger than the typical transition enthalpy from the fluid lamella (L a ) to hexagonal (H II ) phase, DH = 1-5 kcal mol À1 .…”