Impact of an Irreversibly Adsorbed Layer on Local Viscosity of Nanoconfined Polymer Melts

We report the origin of the effect of nanoscale confinement on the local viscosity of entangled polystyrene (PS) films at temperatures far above the glass transition temperature. By using marker x-ray photon correlation spectroscopy with gold nanoparticles embedded in the PS films prepared on solid substrates, we have determined the local viscosity as a function of the distance from the polymer-substrate interface. The results show the impact of a very thin adsorbed layer ( 7 nm in thickness) even without specific interactions of the polymer with the substrate, overcoming the effect of a surface mobile layer at the air-polymer interface and thereby resulting in a significant increase in the local viscosity as approaching the substrate interface.T. K. acknowledges the financial support from NSF Grant No. CMMI-084626. Uses of the Advanced Photon Source and the National Synchrotron Light Source were supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contracts No. DE-AC02- 06CH11357 and No. DE-AC02-98CH10886, respectively

Measurement of the Interior Structure of Thin Polymer Films Using Grazing Incidence Diffuse X-Ray Scattering

A method is developed for calculating the small-angle x-ray scattering originating from within the interior of a thin film under grazing incidence illumination. This offers the possibility of using x-ray scattering to probe how the structure of polymers is modified by confinement. When the diffuse scattering from a thin film is measured over a range of incident angles, it is possible to separate the contributions to scattering from the interfaces and the contribution from the film interior. Using the distorted-wave Born approximation the structure factor, S q , of the film interior can then be obtained. We apply this method to analyze density fluctuations from within the interior of a silicon supported molten polystyrene PS film. Measurements were made as a function of film thickness ranging from one to ten times the polymer radius of gyration Rg . The compressibility, calculated by extrapolating the measured S q to q=0, agrees well with that of bulk PS for thick films, but thinner films exhibit a peak in S q near q=0. This peak, which grows with decreasing thickness, is attributed to a decreased interpenetration of chains and a consequent enhanced compressibility.This work is supported by NSF Grant No. DMR-0209542. Use of the Advanced Photon Source at Argonne National Laboratory was supported by U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under DOE Contract No. DE-AC02-06CH11357

Substrate Suppression of Thermal Roughness in Stacked Supported Bilayers

We have fabricated a stack of five 1,2-dipalmitoyl-sn-3-phosphatidylethanolamine (DPPE) bilayers supported on a polished silicon substrate in excess water. The density profile of these stacks normal to the substrate was obtained through analysis of x-ray reflectivity. Near the substrate, we find the layer roughness and repeat spacing are both significantly smaller than values found in bulk multilayer systems. The reduced spacing and roughness result from suppression of lateral fluctuations due to the flat substrate boundary. The layer spacing decrease then occurs due to reduced Helfrich repulsion.This work was partially supported by NSF Grants No. DMR-0706369 and No. DMR-0706665. Use of the Advanced Photon Sourcewas supported by theUSDepartment of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. SKS and ANP wish to acknowledge support from the Office of Basic Energy Sciences, US Department of Energy, via Grant No. DE-FG02- 04ER46173. We would also like to thank Suresh Narayanan for his support of the experimental work at Sector 8-ID

Observation of a Low-viscosity Interface Between Immiscible Polymer Layers

この論文は国立情報学研究所の電子図書館事業により電子化されました。研究会報告X-ray Photon Correlation Spectroscopy (XPCS) was employed in a surface standing wave geometry in order to resolve the thermally diffusive in-plane dynamics at both the surface/vacuum (top) and polymer/polymer (bottom) interfaces of a thin Polystyrene (PS) film on top of Poly (4-bromo styrene) (PBrS) and supported on a Si substrate. The top vacuum interface shows two relaxation modes: one fast and one slow, while the buried polymerpolymer interface shows a single slow mode. The slow mode of the top interface is similar in magnitude and wavevector dependence to the single mode of the buried interface. The dynamics are consistent with a low-viscosity mixed layer between the PS and PBrS and coupling of the capillary wave fluctuations between this layer and the PS

Reduced Viscosity of the Free Surface in Entangled Polymer Melt Films

By embedding ‘‘dilute’’ gold nanoparticles in single polystyrene thin films as ‘‘markers’’, we probe the local viscosity of the free surface at temperatures far above the glass transition temperature (Tg). The technique used was x-ray photon correlation spectroscopy with resonance-enhanced x-ray scattering. The results clearly showed the surface viscosity is about 30% lower than the rest of the film. We found that this reduction is strongly associated with chain entanglements at the free surface rather than the reduction in Tg.T. K. acknowledges financial support from NSF Grant No. CMMI-0846267 and ChemMatCARS, APS. The use of the APS was supported by the DOE, Office of Basic Energy Science, under Contract No. DE-AC02- 06CH11357

Thickness Induced Structural Changes in Polystyrene Films

Changes to the structure of polystyrene melt films as measured through the spectrum of density fluctuations have been observed as a function of film thickness down to the polymer radius of gyration (Rg). Films thicker than 4Rg show bulklike density fluctuations. Thinner films exhibit a peak in S(q) near q = 0, which grows with decreasing thickness. This peak is attributed to a decreased interpenetration of chains resulting in an enhanced compressibility. Measurements were made using small angle x-ray scattering in a standing wave geometry designed to enhance scattering from the interior of the film compared to interface scattering.This work is supported by NSF Grant No. DMR-0209542. Use of the Advanced Photon Source at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357

Cholesterol Partition and Condensing Effect in Phase-Separated Ternary Mixture Lipid Multilayers

The cholesterol partitioning and condensing effect in the liquid-ordered (Lo) and liquid-disordered (Ld) phases were systematically investigated for ternary mixture lipid multilayers consisting of 1:1 1,2-dipalmitoyl-sn-glycero-3-phosphocholine/1,2-dioleoyl-sn-glycero-3-phosphocholine with varying concentrations of cholesterol. X-ray lamellar diffraction was used to deduce the electron density profiles of each phase. The cholesterol concentration in each phase was quantified by fitting of the electron density profiles with a newly invented basic lipid profile scaling method that minimizes the number of fitting parameters. The obtained cholesterol concentration in each phase versus total cholesterol concentration in the sample increases linearly for both phases. The condensing effect of cholesterol in ternary lipid mixtures was evaluated in terms of phosphate-to-phosphate distances, which together with the estimated cholesterol concentration in each phase was converted into an average area per molecule. In addition, the cholesterol position was determined to a precision of (±0.7Å) and an increase of disorder in the lipid packing in the Lo phase was observed for total cholesterol concentration of 20∼30%

Accurate calibration and control of relative humidity close to 100% by X-raying a DOPC multilayer.

<p>In this study, we have designed a compact sample chamber that can achieve accurate and continuous control of the relative humidity (RH) in the vicinity of 100%. A 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) multilayer can be used as a humidity sensor by measuring its inter-layer repeat distance (d-spacing) via X-ray diffraction. We convert from DOPC d-spacing to RH according to a theory given in the literature and previously measured data of DOPC multilamellar vesicles in polyvinylpyrrolidone (PVP) solutions. This curve can be used for calibration of RH close to 100%, a regime where conventional sensors do not have sufficient accuracy. We demonstrate that this control method can provide RH accuracies of 0.1 to 0.01%, which is a factor of 10-100 improvement compared to existing methods of humidity control. Our method provides fine tuning capability of RH continuously for a single sample, whereas the PVP solution method requires new samples to be made for each PVP concentration. The use of this cell also potentially removes the need for an X-ray or neutron beam to pass through bulk water if one wishes to work close to biologically relevant conditions of nearly 100% RH.</p