Confinement effect on the adsorption from a binary liquid system near liquid/liquid phase separation

The preferential adsorption of one component of a binary system at the inner surfaces of mesoporous silica glasses was studied in a wide composition range at temperatures close to liquid/liquid phase separation. Confinement effects on the adsorption were investigated by using three controlled-pore glass (CPG-10) materials of different mean pore size (10 to 50 nm). For the experimental system (2-butoxyethanol+water), which exhibits an upper miscibility gap, strong preferential adsorption of water occurs, as the coexistence curve is approached at bulk compositions, at which water is the minority component. In this strong adsorption regime the area-related surface excess amount of adsorbed water decreases with decreasing pore width, while the shift in the volume-related mean composition of the pore liquid shows an opposite trend, i.e., greatest deviation from bulk composition occurring in the most narrow pores. A simple mean-field lattice model of a liquid mixture confined by parallel walls is adopted to rationalize these experimental findings. This model reproduces the main findings of the confinement effect on the adsorption near liquid/liquid phase separation.     

Structure and properties of polypropylene crystallized from the glassy state

Glassy isotactic propylene (PP) films of thickness up to 0.3 mm were obtained by an ultraquenching technique. The structure and properties of the as-quenched and subsequently crystallized samples were characterized by various techniques. Electron microscopy indicates the glass has no structure larger than 25 Å. X-ray diffraction shows PP crystallizes from the glass into a smectic structure at ca. ?20°C and then transforms to monoclinic microcrystals at ca. 40°C; a nodular structure (80 to 100 Å in diameter) was observed on the surface. The transformation temperature increases with the film thickness. Annealing above the α-relaxation temperature results in an increase in the nodule size. A correspondence was found between the diameter of the nodules observed on the surface and long spacings obtained by small-angle X-ray scattering from the bulk. Dynamic mechanical spectra show the presence of two relaxation-like peaks at ca. ?10°C and 10°C for the as-ultraquenched samples. X-ray scattering, differential scanning calorimetry (DSC), and torsion pendulum measurements show PP crystallizes from the glass at a temperature, depending on the rate of heating, that corresponds to the lower relaxation peak temperature.     

Microstructures in aqueous solutions of a polyoxyethylene trisiloxane surfactant and a cosurfactant studied by SANS and NMR self-diffusion

Microemulsion samples of a polyoxyethylene trisiloxane surfactant, water, and 1-decanol are investigated using pulsed field gradient NMR and small-angle neutron scattering (SANS) to determine the solution structure. The surfactant/decanol weight ratio has been kept constant at values of 10:1, 8:1, and 6:1 under variation of water content. The temperature was 32 degrees C for the measurement series at the weight ratio of 10:1 to avoid phase separation at high water content. Also, aqueous surfactant solution samples have been investigated as a function of composition and temperature. Water-rich samples consist of micelles that are close to spherical at very low surfactant concentration and grow into anisometric, that is, oblate formed aggregates, at higher surfactant (or surfactant and decanol) concentration. The aggregates grow with increasing temperature, most probably due to dehydration of the hydrophilic groups. In a concentration range around 50 wt % water, the systems form bicontinuous structures. SANS data are used to estimate surfactant film properties using a model developed for interpretation of neutron scattering data from related systems.     

Phase behavior and local structure of a binary mixture in pores: mean-field lattice model calculations for analyzing neutron scattering data

We investigate the phase behavior of an asymmetric binary liquid A-W mixture confined between two planar homogenous substrates (slit pore). Molecules of species W interact preferentially with the solid walls via a long-range potential. Assuming nearest-neighbor attractions between the liquid molecules, we employ a lattice-gas model and a mean-field approximation for the grand potential. Minimization of this potential yields the density profiles of thermodynamically stable phases for fixed temperature, chemical potentials of both species, pore width and strengths of attraction. This model is used to analyze experimental small-angle neutron-scattering (SANS) data on the microscopic structure of the binary system isobutyric acid (iBA)+heavy water (D2O) inside a mesoscopic porous matrix (controlled-pore glass of about 10 nm mean pore width). Confinement-independent model parameters are adjusted so that the theoretical liquid-liquid coexistence curve in the bulk matches its experimental counterpart. By choosing appropriate values of the pore width and the attraction strength between substrates and water we analyze the effect of confinement on the phase diagram. In addition to a depression of the liquid-liquid critical point we observe surface induced phase transitions as well as water-film adsorption near the walls. The temperature dependence of the structure of water-rich and iBA-rich phases of constant composition are discussed in detail. The theoretical predictions are consistent with results of the SANS study and assist their interpretation.     

Glassy crystalline state and water sorption of alkyl maltosides

A differential scanning calorimetric and sorption calorimetric study of two alkyl maltosides, C8G2 and C10G2, was performed. In the dry state, C8G2 and C10G2 do not form solid crystals but undergo a glass transition upon temperature change. The glass is partly ordered and has the same lamellar structure as the liquid crystals formed by the two maltosides. To reflect the presence of the glass transition and the structure, the terms "glassy crystals" and "glassy liquid crystals" can be used. A mechanism of the relaxation of the glassy crystals based on the results of small-angle X-ray scattering experiments is proposed. Experiments on water sorption showed that the glassy crystals turn into lyotropic liquid crystals upon sorption of water at constant temperature. This isothermal glass transition can be characterized by water content and change of partial molar enthalpy of mixing of water. A method to calculate the phase diagram liquid crystals-glassy liquid crystals is proposed.     

Porosity in ancient glass from Syria (c. 800 AD) using gas adsorption and atomic force microscopy

Gas adsorption and atomic force microscopy have been used to quantify and compare the porosity, surface area and surface characteristics of five samples from Raqqa in northern Syria dating to ca. 8th century AD . High‐quality high‐pressure liquid chromatography (HPLC) glass was also analysed to compare the results with these characteristics of modern glass. All five ancient samples were found to have some porosity across the microporous (up to 2 nm) and the mesoporous (2–50 nm) range. The reasons for this porosity are not entirely clear at present but are likely to be due to the manufacturing methods used, including the raw materials. A correlation was found between glass chemical composition (especially the total alkali) and pore size, inferring that the melting temperature is one of the parameters that affects the development of pores. The work in this paper presents the preliminary findings from the study of porosity in ancient glass. Copyright © 2004 John Wiley & Sons, Ltd.     

Kinetics of thermally induced phase separation in ternary polymer solutions. II. Comparison of theory and experiment

Light‐scattering measurements and spinodal decomposition modeling have been used to quantify the kinetics of pore growth in thermally quenched polymer‐solvent–nonsolvent [poly(methyl methacrylate) (PMMA)/1‐methyl‐2‐pyrrolidinone (NMP)/glycerin] solutions. Solutions of fixed composition were quenched to a series of temperatures and light‐scattering measurements and model calculations were performed to determine the temperature dependence of the pore growth rate. Both the experimental results and the model calculations show that the growth rate exhibits a maximum at an intermediate quench temperature that is related to an interplay between the thermodynamic and transport effects that govern pore growth. A similar growth‐rate maximum is also observed when a series of solutions of varying nonsolvent composition are all quenched to the same temperature. The relevance of these experiments to the dynamics of pore growth and the eventual locking‐in of the two‐phase structure that forms during nonsolvent‐induced phase inversion is discussed. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1461–1467, 1999     

Complex study of reorientational dynamics of the liquid crystal in PDLC films

Morphological, electro-optical and switching properties of polyester resin/nematic liquid crystal composite films have been studied for varying composition (10-40 wt% of LC), temperature (20-50 C), film thickness (10-75 mum) and UV curing time of the matrix (0.25-12 min). The PDLC films were formed by LC separation in a UV polymerization process of the thin layer of oligoester resin (liquid crystal mixture) between ITO coated glass plates. The electro-optical and response behaviour based on the electric field controlled light scattering of the composite films was observed. The results were interpreted in terms of effective anchoring strength at the interface of the polymer and liquid crystal depending either on the area fraction of the interface in the composite film (dependent on the size and shape of the liquid crystal droplets) or the stiffness and resistivity of the polyester resin changing in the course of the crosslinking polymerization.     

Phase diagram and glass transition of confined benzene

We used differential scanning calorimetry, neutron scattering, and proton NMR to investigate the phase behavior, the structure, and the dynamics of benzene confined in a series of cylindrical mesoporous materials MCM-41 and SBA-15 with pore diameters, d, between 2.4 and 14 nm. With this multitechnique approach, it was possible to determine the structure and, for the first time to our knowledge, the density of confined benzene as a function of temperature and pore size. Under standard cooling rates, benzene partially crystallizes in SBA-15 matrixes (4.7 相似文献   

Phase separation of polymer blends in thin films

Films of polystyrene-poly(vinylmethyl ether) blends of various compositions are formed by a dip-coating procedure, the thickness of the film being controlled by the concentration of the solution. The substrates used are glass and gold. The phase separation process is followed by a laser light scattering experiment in which the total forward scattering intensity is monitored as a function of temperature. Morphological examination shows that phase separation occurs by a spinodal decomposition mechanism. A thickness effect on the phase separation temperature is noticeable when film thickness is smaller than 1 μm. This effect is substrate dependent. In all films formed on gold the spinodal temperature increases as film thickness decreases. Films formed on glass exhibit a destabilizing effect on decreasing film thickness. This effect is slight in films of composition poorer in polystyrene than the critical composition, and is enhanced in films richer in polystyrene. The stabilizing effect of decreasing the thickness of films formed on the gold substrate is considered to reflect mainly a purely geometrical effect. The decreasing dimensionality is shown by simple theoretical considerations to increase the phase-separation temperature. However, the phase separation behavior of thin films on glass appears to be the result of two kinds of substrate-polymer interactions in addition to the geometrical effect: (a) electrostatic interaction of the charged glass surface (a destabilizing effect at all film compositions) and (b) selective adsorption of polystyrene on glass.     

Characterization of a novel continuous supermacroporous monolithic cryogel embedded with nanoparticles for protein chromatography

A novel continuous supermacroporous monolithic cryogel embedded with nanometer-size particles was prepared by the radical cryogenic co-polymerization of acrylamide (AAm), N,N'-methylene-bis-acrylamide (MBAAm), allyl glycidyl ether (AGE) and the dispersed surfactant-stabilized Fe3O4 nanoparticles under the freezing-temperature variation condition in a glass column. This special separation matrix has interconnected supermacropores with pore size of 10-50 microm, which permit the free-passage of microbial cells or cell debris in the culture fluids and then is interest in downstream processes. The axial liquid dispersion coefficients of the new continuous supermacroporous monolithic bed at different liquid flow rates were obtained by measuring residence time distributions (RTDs) using tracer pulse-response method. The experimental results showed that the axial liquid dispersion within the bed was weak in a wide water flow rate of 0.5-15 cm/min. The axial dispersion coefficient was found to be increased exponentially with the increase of liquid flow rate. Chromatographic process of bovine serum albumin (BSA) in the cryogel monolithic bed was carried out to reveal the protein breakthrough and elution characteristics. Compared with other reported cryogel beds in literature, the protein adsorption capacity of the present cryogel bed was improved due to the embedded nano-sized solid adsorbents in the gel matrix. Microstructure morphology of the embedded nanoparticles in the cryogel and the gel matrix structure were also analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) in this paper.     

Determination of Benzo(a)Pyrene and other Polynuclear Aromatic Hydrocarbons in Airborne Particulate Material by Ultrasonic Extraction and Reverse Phase High Pressure Liquid Chromatography

Abstract

Ultrasonic extraction of airborne particulate material on Hi-Vol filters is described. Almost all of the polar compounds are removed during the extraction by adsorption on the surface of the shredded glass fibers and controlled pore glass powder (CPG). The non-polar polynuclear aromatic hydrocarbons (PAH) in the extract are separated at room temperature by high pressure liquid chromatography (HPLC) on reverse phase Vydac using acetonitrile:water (70:30 v/v) as the chromatographic solvent. There is baseline separation of benzo(a)pyrene (BaP), benzo(k)fluoranthene (BkFt) and of benzo(e)pyrene (BeP), benzo(b)fluoranthene and perylene, the latter three present in one band. Extracts of airborne particulates show the same peaks. BaP elutes in approximately 14 minutes. Precision and accuracy measurements indicate full recovery of PAH and good extraction reproducibility. The detection limit of BaP at F 290/389 is less than 10 pg. Total analysis time is approximately 1 1/2 hr, most of which is waiting time.     

Dielectric investigations of the N-SmB transition in a porous glass

Dielectric measurements in the frequency range 0.01 Hz to 10 MHz on a liquid crystal material with SmB/N dimorphism confined to randomly oriented and interconnected mesoporous (pore diameter 2-10 nm) and macroporous glasses (pore diameter 86 nm) were carried out. In the macroporous glasses the liquid crystal behaves similarly to the bulk phase, but the phase transition temperature SmB-N is suppressed. In the mesoporous glass the transition SmB-N is completely suppressed.     

Dielectric investigations of the N-SmB transition in a porous glass

Dielectric measurements in the frequency range 0.01 Hz to 10 MHz on a liquid crystal material with SmB/N dimorphism confined to randomly oriented and interconnected mesoporous (pore diameter 2-10 nm) and macroporous glasses (pore diameter 86 nm) were carried out. In the macroporous glasses the liquid crystal behaves similarly to the bulk phase, but the phase transition temperature SmB-N is suppressed. In the mesoporous glass the transition SmB-N is completely suppressed.     

Self-assembly via a complex phase transition in mixtures of polystyrene-block -polybutadiene block copolymer and poly(methyl vinyl ether)

The self-assembly of a binary mixture of polystyreneblock-polybutadiene (SB) and poly(methyl vinyl ether) (PVME) was studied by transmission electron microscopy and time-resolved light scattering. The self-assembly studied involved first microphase separation, in which a microdomain structure composed of polybutadiene block chains (PB) was formed in a matrix composed of polystyrene block chains (PS) and PVME homopolymers, and subsequently macrophase separation of the PVME from the microdomain phase of SB. The microphase separation was induced in a film preparation process using a solution cast method at room temperature. The macrophase separation was induced by rapidly heating the film specimens to above a critical temperature where PVME and PS undergo spinodal decomposition (SD). This complex phase transition, involving microphase separation followed by macrophase separation, was found to generate a superlattice structure (or a modulated structure) with two characteristic spacings: A_macro associated with the SD and A_micro associated with the microphase separation, both being generally time-dependent. The growth of the “macrodomains” was found to be pinned at A_macro ˜ 840 nm due to the elastic effect of the microdomain structure. The microdomain structure with A_micro ˜ 57 nm was found to undergo a morphological transition (a transition between two ordered phases of block copolymers) as a consequence of the local composition change of the two polymers induced by the SD.     

2H-solid state NMR and DSC study of isobutyric acid in mesoporous silica materials

Solid state deuterium NMR has been used to study the molecular motion of d(6)-isobutyric acid (d(6)-iBA) in the pure (unconfined) state and confined in the cylindrical pores of two periodic mesoporous silica materials (MCM-41, pore size 3.3 nm and SBA-15, pore size 8 nm), and in a controlled pore glass (CPG-10-75, pore size ca. 10 nm). The line shape analysis of the spectra at different temperatures revealed three rotational states of the iBA molecules: liquid (fast anisotropic reorientation of the molecule), solid I (rotation of the methyl group) and solid II (no rotational motion on the time scale of the experiment). Transition temperatures between these states were determined from the temperature dependence of the fraction of molecules in these states. Whereas the solid I-solid II transition temperature is not affected by confinement, a significant lowering of the liquid-solid I transition temperature in the pores relative to the bulk acid was found for the three matrix materials, exhibiting an unusual dependence on pore size and pore morphology. Complementary DSC measurements on the same systems show that the rotational melting (solid I-liquid) of d(6)-iBA in the pores occurs at a temperature 20-45 K below the thermodynamic melting point. This finding indicated that the decoupling of rotational and translational degrees of freedom in phase transitions in confined systems previously found for benzene is not restricted to molecules with non-specific interactions, but represents a more general phenomenon.     

SANS and DSC study of water distribution in epoxy-based hydrogels

Distribution of water in stoichiometric hydrophilic epoxy network swollen in heavy water to different degrees (epoxy-based hydrogels) at 25 °C has been investigated by small-angle neutron scattering (SANS) and differential scanning calorimetry (DSC). Nanophase separated structure of the hydrogels consisting of water-rich and water-poor domains was revealed by SANS. Two regimes for hydrogel structure were found: (a) at low water content hydrogel consists of isolated water-rich domains dispersed in continuous water-poor phase and (b) at high water content the water-rich domains form another continuous phase. Isosbestic point of scattering curves was found by SANS in the latter region and attributed to conservation of Porod’s length of the nanophase separated structure. Thermal properties of the system are qualitatively different in the two regions: in the former one the glass transition temperature decreases with growing water content while in the latter one it remains constant. Percolation threshold separating both regimes is reflected in a jump of glass transition temperature and inversion of the dependence of the specific heat difference at glass transition.     

Kinetics of thermally induced phase separation in ternary polymer solutions. I. Modeling of phase separation dynamics

Simulations based on Cahn–Hilliard spinodal decomposition theory for phase separation in thermally quenched polymer/solvent/nonsolvent systems are presented. Two common membrane‐forming systems are studied, cellulose acetate [CA]/acetone/water, and poly(ethersulfone) [PES]/dimethylsulfoxide [DMSO]/water. The effects of initial polymer and nonsolvent composition on the structure‐formation dynamics are elucidated, and growth rates at specific points within the ternary phase diagram are quantified. Predicted pore growth rate curves exhibit a relative maximum with nonsolvent composition. For shallow quenches (lower nonsolvent content) near a phase boundary, the pore growth rate increases with increasing quench depth, whereas for deep quenches, where the composition of the polymer‐rich phase approaches that of a glass, the pore growth rate decreases with increasing quench depth. With increasing initial polymer concentration, the overall rate of structure growth is lowered and the growth rate maximum shifts to higher nonsolvent compositions. This behavior appears to be a universal phenomenon in quenched polymer solutions which can undergo a glass transition, and is a result of an interplay between thermodynamic and kinetic driving forces. These results suggest a mechanism for the locking‐in of the two‐phase structure that occurs during nonsolvent‐induced phase inversion. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1449–1460, 1999     

Towards stationary phases for chromatography on a microchip: molded porous polymer monoliths prepared in capillaries by photoinitiated in situ polymerization as separation media for electrochromatography

Photoinitiated free radical polymerization has been used for the preparation of porous polymer monoliths within UV transparent fused silica capillaries and quartz tubes. These formats were used as models for the preparation of the separation media within channels of microfabricated devices. A mixture of ethylene dimethacrylate, butyl methacrylate, and 2-acrylamido-2-methyl-1-propanesulfonic acid was polymerized in the presence of a porogenic solvent consisting of 1-propanol, 1,4-butanediol, and water at room temperature under UV irradiation. Modification of the porogen composition enables the tailoring of pore size within the broad range from ca. 100 to 4000 nm. Scanning electron micrographs confirmed the homogeneity of the porous structure of the materials prepared, even in a quartz tube with a diameter as large as 4 mm. Separation properties of the resulting capillary columns were tested in capillary electrochromatography (CEC) mode using a mixture of thiourea and eight aromatic compounds. Plate number as high as 210 000 plates/m were found for a capillary column with optimized porous properties. The monolithic columns were also able to separate mixtures of peptides.     

Variations of wettability and protein adsorption on solid siliceous carriers grafted with poly(N-isopropylacrylamide)

Poly(N-isopropylacrylamide), a thermally responsive polymer, was end-grafted to mercaptopropyl derivatives of silica gel, plane glass sheets and glass capillary tubing by free radical polymerization of the monomer in 1,4-dioxane at 100 degrees C. The polymer monolayer attached to the glass carriers provided them with thermally controlled wettability registered by two independent methods: direct measurements of the water contact angle and capillary rise. The water contact angle changed from 54+/-3 degrees to 68+/-3 degrees in the temperature range from 20 to 50 degrees C. The polymer grafting to silica gel (pore diameter 100 A, particle size 5 microm) resulted in 15-30-fold reduction in protein adsorption on the carrier at 35 degrees C. Adsorption isotherms of myoglobin indicate completely different characters of the protein adsorption to silica gel and its polyNIPAM-grafted derivative. Cooling of the grafted carrier containing adsorbed myoglobin to 9 degrees C led to a partial release of the protein to the contacting solution, whereas heating of the system to 35 degrees C resulted in reversible binding of the protein. Adsorption of myoglobin on polyNIPAM-coated silica was ca. 2-fold higher at 35 than at 9 degrees C, most probably due to steric repulsion displayed by the swollen copolymer at the lower temperature.