Diffusion Coefficients of Small Molecules as Guests in Various Phases of Pluronic L64 Measured by One-Dimensional Electron Spin Resonance Imaging

Abstract

The diffusion coefficients of small nitroxide probes as guests in aqueous solutions of the triblock copolymer poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) EO13PO30EO13 (Pluronic L64) were measured at 300 K by one-dimensional electron spin resonance imaging (1D ESRI). The method is based on encoding the spatial distribution of the probes as a function of time in ESR spectra recorded in the presence of magnetic field gradients and simulation of these spectra in order to extract the diffusion coefficient, D. The rate of transport of each probe as a function of polymer content in the various phases (micellar, hexagonal, lamellar, and reverse micellar) of aqueous L64 depends on the probe location in the self-assembled system. The probe site was deduced from the analysis of the ESR spectra; the isotropic hyperfine splitting, a N, from the 14N nucleus of the >NO fragment in the probes was the polarity sensitive parameter. D values for the cationic probe 4-(N,N,N-trimethyl)ammonium-2,2,6,6-tetramethyl-piperidine-1-oxyl iodide that is known to reside in the water domains follow the expression D = D 0 exp(−aw 2), where D 0 is the diffusion coefficient in the neat solvent and w 2 is the weight fraction of the polymer. For the hydrophobic probe 5DSE, the methyl ester of doxylstearic acid where 5 indicates the carbon atom to which the doxyl group is attached, D is significantly lower and almost constant for w 2 in the range 0.20−0.80. For the probe perdeuterio-2,2‘,6,6‘-tetramethyl-piperidone-N-oxide that is located at the interface between water and the EO domains, D decreases with increase in the polymer content, but the decrease is more prominent for w 2 in the range 0.10−0.30; at w 2 = 0.90, D is similar to that of the polymer chains. The method of 1D ESRI enables the measurement of D values for guests present in small concentrations, typically ≤2 × 10-3 mol/L. This method and the results obtained in this study are relevant for assessing both the rate of transport of drugs in drug delivery systems and the partitioning of various guests in complex systems, for instance in biological membranes.

Microphase separation in poly(acrylonitrile–butadiene–styrene) (ABS) studied with paramagnetic spin probes. I. Electron spin resonance spectra

“…The stability of the VT unit was better than ±1 K. All samples were allowed to equilibrate for at least 10 min after approaching the desired temperature. Additional experimental details have been described elsewhere 8–10…”

“…In our studies of polymeric systems, we have taken advantage of the sensitivity of a N to the local polarity in ion‐containing polymers5–7 and nonpolar surfactants8–10 to demonstrate the presence of phase separation and water penetration in the hydrophobic aggregates of perfluorinated ionomers 7. For poly(styrene‐ co ‐maleic anhydride)/ poly(methyl vinyl ether) blends, ESR data based on a nitroxide chemically attached to the maleic anhydride (label) enabled the detection of phase heterogeneity that was not seen in differential scanning calorimetry (DSC) measurements 11.…”

Thermosensitive Self-Assembling Block Copolymers as Drug Delivery Systems

“…PPO is soluble in the temperature range of 2-15 °C, while for higher temperatures a precipitation cloud point exists [26]. Thus it is assumed that the hydrophobic PPO is located in the core of the micelle, while the PEO lyophilic block forms the outer corona, as assessed by electron paramagnetic resonance [27,28] (EPR) and the above mentioned techniques. With increasing temperature, micellization becomes more important, and at a definite temperature point, micelles come into contact and no longer move.…”

Fluorescence of commercial Pluronic F127 samples: Temperature-dependent micellization

“…Aspects of the structural and dynamic properties of micellar self-assembly of the PEO-PPO-PEO block copolymers have been investigated by a variety of experimental methods, including fluorescence spectroscopy [5,7], differential scanning calorimetry [8], transmission electron microscopy (TEM) [9,10], viscometry [11,12], light scattering [13][14][15], nuclear magnetic resonance (NMR) [16][17][18], UV-vis absorption of selected probes [19], surface tension [20,21], electron spin resonance (ESR) [22], small angle neutron scattering (SANS) [9,23,24], Fourier transform infrared spectroscopy (FTIR) [25,26], and Raman spectroscopy [27]. At room temperature (RT) both the PEO and the PPO blocks of the Pluronic F127 are hydrophilic and water soluble forming a transparent solution.…”