Polymer Dynamics in an Interface-Confined Space: NMR Study of Poly(hexyl ethacrylate)-block-poly(acrylic acid) and Poly(dodecyl methacrylate)-block-poly(acrylic acid) Micelles in D2O

A nanocell for quartz crystal microbalance and quartz crystal microbalance with dissipation-monitoring sensing

A novel device for nanometer-confinement of soft matter in one dimension (1D) is presented. This nanocell, with very large (up to 10(6):1) cell-radius to cell-height ratio, is tailored as an accessory for quartz crystal microbalance (QCM) and QCM with dissipation-monitoring (QCM-D) sensing to study internal and interfacial energy dissipation phenomena in highly confined (in 1D) soft matter and fluid films (patent pending). The cell consists of two macroscopic plates (diameter of 9 mm), a top (the "lid") and a bottom (the QCM-D sensor), separated by appropriate spacers with heights ranging from below 100 nm up to 10 microm. The surfaces of both the lid and the bottom plate can be mechanically or/and chemically modified, prior to cell assembly, in order to tailor desired interfacial properties for the experiment. The cell is mounted on a standard QCM-D sensor, an AT-cut quartz crystal (the quartz crystal is cut at an angle of 35 degrees from its ZX-plane), forming the bottom plate. We illustrate theoretically and experimentally, as application examples, the use of this device for studies of dynamic mass loading and internal energy dissipation processes in thin films of ethylene glycol respective thin liquid crystal films around the nematic-isotropic phase transition.

NMR and theoretical study of the cooperative interaction of hydrated proton with dibenzo-24-crown-8

Interaction of hydrated protons (HPs) with dibenzo-24-crown-8 (DBC in nitrobenzene-d(5) was studied by (1)H and (13)C NMR under assistance of ab initio-density functional theory (DFT) quantum calculations. HPs were afforded by hydrogen bis(1,2-dicarbollyl) cobaltate (HDCC) with 3.5 M excess of H(2)O. The forming of a complex between HP and DBC leads to marked and additive relative shifts of both (1)H and (13)C signals. This was utilized for the estimation of the stabilization constant K of the complex. Its value is at least 10(6) l/mol, which agrees with the result of independent extraction method (log K = 6.3). Using absolute integral intensities of the HP signal in a water-saturated system, it was shown that the form of HP present in the complex must be H(5)O(2)(+), in accord with formerly published structure of the complex in crystalline form. The investigation of the dynamics of exchange between bound and free DBC by transverse relaxation using variably spaced pulses in the Carr-Purcell-Meiboom-Gill (CPMG) sequence or on-resonance rotating-frame relaxation with variable spin-lock field intensity was partly hampered by the fast relaxation of some signals in the complex because of relative immobilization of its internal motions. In order to remove these effects, a pulse sequence dipolar interaction-free transverse relaxation (DIFTRE) for static DIFTRE was devised and the MLEV17 pulse sequence with high intensity of electromagnetic field was used in a separate series of experiments. Using the results of these latter experiments, the correlation time of exchange was established to be about 0.8 ms, which complied with the shape of the spectra. The accompanying ab initio DFT calculations showed that the apparent symmetry of the molecules of both DBC and its complex with H(5)O(2)(+) was probably the result of averaging of energetically close conformations (five for DBC and four for the complex). Both NMR and the calculations show that the basic mode of binding of the ion in the complex is analogous to that found in crystal but the most pronounced conformation is slightly different.

(1)H NMR and small-angle neutron scattering investigation of the structure and solubilization behavior of three-layer nanoparticles

Three-layer nanoparticles were prepared by radiation-induced polymerization of 1-10 g/L of methyl methacrylate dissolved in a 0.1 wt % D(2)O solution of polystyrene-poly(methacrylic acid) (PS-PMA) micelles. According to NMR and small-angle neutron scattering (SANS), most of the poly(methyl methacrylate) (PMMA) is adsorbed at the core-shell interface of the particles. A small fraction of shorter PMMA probably sticks to outer parts of the PMA chains. The absorption kinetics and equilibria of benzene and chloroform were studied by NMR and SANS time-resolved experiments. The diffusion front in the PS core is very narrow but quite broad in the PMMA sheet suggesting, thus, a less compact state of PMMA. According to SANS, the diffusion kinetics is almost independent of the PMMA sheet thickness. In contrast to it, the absorption capacity, reflected by both SANS and NMR, increases markedly with the PMMA content in the particle. The maximum amount of solubilized compound depends on its positive interaction with PMMA (expressed by the chi parameter) but is restricted by the growing interface tension between swollen PMMA and D(2)O. In accordance with this conclusion, a particle saturated with benzene can absorb chloroform only at the expense of a part of benzene expelled into the surrounding medium and vice versa. Starting with 10 g PMMA/L (10 times the weight of the original micelles), the particles become unstable when being swollen with a good solvent.

Diffusion of molecules confined in semipenetrable nanoscale carriers probed by pulsed field gradient NMR

Diffusion of three low-molecular-weight compounds cyclohexane (CX), benzene (BZ), and chloroform (CL) preferentially confined in the cores of nanoscale carriers was probed by pulsed field gradient (PFG) NMR methods. The carriers were monolayer micelles of sodium dodecyl sulfate (SDS), bilayer micelles of poly(hexyl methacrylate)-block-(acrylic acid) (M2), and trilayer micelles of poly(2-ethylhexyl methacrylate)-block-(methyl methacrylate)-block-(acrylic acid) (M3) in D(2)O at 300 K. Although the radius of the confinement space was 10(-)(8) m or lower, the course of the PFG signal attenuation in pulsed gradient spin-echo or stimulated echo experiments under varied diffusion time corresponds to apparently unrestricted diffusion, which is slowed down compared to that of the compound dissolved in D(2)O. Analysis using approximate relations reveals that the response of the system to PFG NMR consists of three independent components, namely (i) diffusion of the carrier as a whole, (ii) hindered escape of a confined molecule and its diffusion in the medium, and (iii) diffusion of the molecules dissolved in the medium. If process ii is fast enough, exchange of the compound between the carrier and the medium includes the influence of iii as a component of a monoexponential PFG decay; otherwise, two sets of signals are observed with different diffusion responses, or biexponential PFG is observed. According to the results of this study, the only barrier of the diffusion of the inspected compounds CX, BZ, and CL out of their confinement in the carriers SDS or M2 is a thermodynamic one, that is, the resistance of the saturated solution to accept surplus molecules of the solute. In a three-layer micelle M3, the additional polymer sheet around the confinement area forms an additional diffusion barrier for CX, however. The study shows that PFG NMR, though unable to observe directly restricted diffusion on the nanoscale, can be useful in studying systems designed, for example, for a controlled release of low-molecular-weight substances.