The influence of the adsorbent amount on the changes in molecular mass distribution of polymers under adsorption from mixtures

Changes in the molecular mass distribution (MMD) for polymer as a result of adsorption from binary and ternary solutions have been studied by the exclusion chromatography method. It was found that the affinity of polymer components to a surface has a crucial influence on the changes in MMD of polymers. The diminution of polydispersity in solutions after adsorption was observed for two polymers. In the case of the polar polymer poly(butyl methacrylate) (PBMA) the diminution of polydispersity is caused mainly by the preferential adsorption of low-molecular-mass fractions, whereas in the case of the nonpolar polymer polystyrene (PS) it is caused by the transition of the high-molecular-mass fractions onto the adsorbent surface. The analysis of experimental results indicates that the quantity of the adsorbent affects the composition of the adsorption layer formed by polymers of different chemical nature.     

Rheology of polymers that are initially in the disentangled state

An attempt was made to measure the effects of molecular entanglements on the rheological properties of polymer melts. Two classes of polymers were studied; glassy atactic polystyrene polymers covering a 60-fold range in molecular weight, and semicrystalline high-density polyethylene from two sources covering about a twofold range in molecular weight. The entanglements initially present were removed or greatly reduced in number by freeze drying the polystyrene polymers from dilute solutions below and above C*, the critical overlap concentration, and by slowly crystallizing the polyethylene from very dilute solutions. Since only minor rheological changes were observed with polystyrene, it would appear that the initially isolated coils interpenetrate more rapidly than is indicated by the results of Liu and Morawetz, or that the rheological behavior is rather insensitive to whether the flow obstacles are intermolecular or intramolecular. The enhancement of the viscosity and elasticity observed with polyethylene polymers indicate the importance of the crystallization step on the local melt topology of the polymer chains.     

Dependence of the adsorption of polymer mixtures from solution on the amount of an adsorbent

Simultaneous competitive adsorption from solutions of mixtures of poly(butyl methacrylate) and polystyrene and adsorption of each component from binary solutions have been studied for three ratios of the adsorbent mass to the solution volume, A/V. It was found that adsorption from both binary and ternary solutions strongly depends on the amount of an adsorbent, adsorption of poly(butyl methacrylate) being preferential. The characteristic adsorption isotherms of both polymers were constructed under conditions of equal equilibrium concentration of each component to estimate the parameters of preferential adsorption and their dependence on the A/V ratio. It was found that the A/V effect plays an important role in adsorption from polymer mixtures and determines the peculiarities of adsorption from polymer mixtures as well as from solution of single polymers. Changing the A/V ratio may be one way to regulate the composition of an adsorption layer consisting of two chemically different polymers. The reasons for the A/V effect are considered in the framework of the concept of the plurality of adsorption equilibria between two chemically different components and between fractions of different molecular mass of each component having various absorbability.     

Influence of solid surface on equilibria in polymer mixtures

The influence of solid disperse particles (aerosil) on phase equilibria in ternary (polymer-polymer-solvent) and binary (polymer-polymer) systems has been investigated using adsorption and gas chromatography techniques. The change in position and shape of the binodal for the ternary systems has been established. The region of thermodynamic compatibility of two polymers in a common solvent is broadened due to the selective adsorption of high molecular weight fraction of one of the polymers, this effect being dependent on the amount of solid particles introduced into the system. For binary systems, the thermodynamic interaction parameters χ₂₃ have been determined and increasing thermodynamic stability of the mixture in the presence of the solid phase has been discovered. The complicated dependences of the interaction parameters on mixture composition are connected with differences in selectivity of adsorption for various compositions. It is supposed that increased thermodynamic stability of a mixture of two incompatible polymers in the presence of solid is due to the transition of both polymers into adsorption and border layers.     

Effect of temperature on the structure of adsorption layers formed by adsorption of polymers in the transition region from dilute to semidilute solutions

The measurements of adsorption from solutions of polymers (poly(butyl methacrylate) and polystyrene) and their mixtures at various temperatures in the wide concentration region from dilute to semidilute solution have been made. The adsorption isotherms and fraction of bound segments confirm the existence of the transition concentration region near the critical concentration of the chain overlapping when the change of adsorption mechanisms take place. The effect of temperature on adsorption from the mixtures is different for both the polymers depending on the concentration regime. For PBMA, which adsorbs from the mixtures preferentially, the adsorption isotherms are of the same shape as for adsorption from binary solutions and are characterized by the presence of a small forepart in the transition region and by increasing adsorption with temperature. For less adsorbing PS by transition from dilute to semidilute regime the inversion of the temperature effect on adsorption is observed. These results are confirmed by the estimations of the parameters of preferential adsorption at various temperatures. The data on the fraction of bound segments for dilute regime corresponds to the extended conformation of chains at the surface. The transition to semidilute regime leads to the diminishing of the fraction of bound segments as a result of simultaneous adsorption of macromolecular aggregates. Values of adsorption layer thickness have been calculated for various solution regimes and concentration. The dependence of the adsorption layer thickness on the temperature and on the solution regime at which adsorption occurs was established.     

Temperature Dependence of Adsorption of Polymer Mixtures from Solutions

Adsorption from binary (poly(butyl methacrylate)-CCl(4), poly-styrene-CCl(4)) and ternary (poly(butyl methacrylate)-polystyrene-CCl(4)) solutions has been studied at 10, 25, and 60 degrees C. It was found that with increasing temperature the values of adsorption grew due to worsening of the thermodynamic quality of the common solvent. Worsening of the quality of the solvent leads to a decrease in size of the macromolecular coils and to an increase in the critical concentrations of overlapping macromolecules in solution. As a result, the state of macromolecules in solution depends on temperature and determines adsorption values. From the temperature dependence of adsorption using the Clapeyron-Clausius equation, the differential enthalpy of adsorbate (polymer) DeltaH was calculated for each polymer, by adsorption both from binary solution and from the mixture. Determining DeltaH values from the temperature dependence of adsorption allows us to find this value simultaneously for each polymer in the polymer mixture. It was established that transition of a polymer from solution onto the surface leads to an increase in its enthalpy in the case of adsorption from both binary and ternary solutions. DeltaH increases with increasing coverage of the surface. By transition from solution onto the surface, the enthalpy of the adsorbed polymer increases; i.e., the polymer transits in an energetically less favorable state. This effect is more pronounced for adsorption from the mixture, which may be connected with the presence of the second polymer in the adsorption layer. Copyright 2000 Academic Press.     

Influence of poly(ethylene glycol) molecular mass on separation and ordering in solutions of CiEj nonionic surfactants: depletion interactions and steric effects

We study ternary mixtures of nonionic surfactants C(i)E(j) (i = 12; j = 5, 6, 8) and poly(ethylene glycol) (PEG) in water. For sufficiently large molecular mass of PEG (M >M(sep) approximately 600), we observe a lowering of phase separation temperature with an increase in polymer concentration. The value of M(sep) is consistent with the analysis based on depletion interactions between micelles induced by polymer chains. We also demonstrate that there is another critical molecular mass of PEG (M = M* approximately 2000) necessary to induce ordering in the surfactant-rich phase. This critical molecular mass follows from two requirements: (a) PEG has to reduce the separation temperature below a temperature of hexagonal-isotropic phase transition in a binary surfactant-water mixture and (b) the PEG radius of gyration has to be larger than the size of the water channels in the hexagonal phase.     

Monitoring the hygrothermal response of poly(vinyl methyl ether) submicron films using AFM

Films with submicron thickness of the thermoresponsive polymer poly(vinyl methyl ether) (PVME) are prepared by spin-coating and measured with AFM. The change of film thickness, work of adhesion (WOA), and modulus of elasticity (MOE) are monitored below and above the lower critical solution temperature (LCST) in controlled atmosphere. The film thickness decreases above the LCST at high relative humidity, but not at low humidity. Simultaneously, the MOE increases by 3 orders of magnitude due to conformational transition. The WOA shows a hysteretic behavior due to changes in surface wettability and film compressive strength. Hygrothermal cycling is found to facilitate the relaxation of initially arrested entanglements. The proposed hypothesis of hygrothermally induced partial disentanglement is further supported with cyclic DSC measurements of concentrated aqueous PVME solutions. This work is an important contribution to understand the local and macroscopic behavior of thermoresponsive polymers in dry and humidified atmosphere.     

Adsorption interaction parameter of polyethers in ternary mobile phases: The critical adsorption line

It is shown that in LC of polymers, the interaction parameter in ternary mobile phases can be described by a plane, which is determined by the dependencies in binary mobile phases. Instead of a critical adsorption point, critical conditions are observed along a straight line of composition between the two critical points in binary mobile phases. Consequently, a separation of block copolymers under critical conditions for one block by an adsorption mechanism for the other block can be achieved in ternary mobile phases of different compositions, which allows an adjustment of the retention of the adsorbing block.     

Modeling the adsorption of Cd(II) onto goethite in the presence of citric acid

The adsorption of cadmium onto goethite in the presence of citric acid was measured as a function of pH and cadmium concentration at 25 degrees C. Potentiometric titrations were also performed on the system. Cadmium adsorption onto goethite was enhanced above pH 4 in the presence of 50 microM, 100 microM and 1 mM citric acid. While there was little difference between the enhancements caused by 50 and 100 microM citric acid below pH 6, above pH 6 further enhancement is seen in the presence of 100 microM citric acid. When 1 mM citric acid was present, the enhancement of cadmium adsorption was greater below pH 6, with increased Cd(II) adsorption down to pH 3.5. However, above pH 6, 1 mM of citric acid caused slightly less enhancement than the lower citric acid concentrations. ATR-FTIR spectra of soluble and adsorbed citrate-cadmium species were measured as a function of pH. At pH 4.6 there was very little difference between the ternary Cd(II)-citric acid-goethite spectrum and the binary citric acid-goethite spectrum. However, spectra of the ternary system at pH 7.0 and 8.7 indicated the presence of additional surface species. Further analysis of the spectra suggested that these were metal-ligand outer-sphere complexes. Data from the adsorption experiments and potentiometric titrations of the ternary Cd(II)-citric acid-goethite system were fitted by an extended constant-capacitance surface complexation model. The spectroscopic data were used to inform the choice of surface species. Three reactions in addition to those for the binary Cd(II)-goethite and citric acid-goethite systems were required to describe all of the data. They were [formula in text], [formula in text], and [formula in text]. Neither the spectroscopy nor the modeling suggested the formation of a ternary inner-sphere complex or a surface precipitate under the conditions used in this study.     

氰乙基纤维素溶致性液晶的研究

氰乙基纤维素在二甲基甲酰胺、二甲基乙酰胺、二甲基亚砜、乙腈以及丙酮等溶剂中可以形成溶致性液晶。随浓度增加,溶液从各向同性状态经两相共存态转变成为完全的液晶态。升高温度到T_c,液晶相消失;降低温度到T′_c,液晶相再生成。T_c总大于T′_c。而且,浓度越高,过冷温度△T=T_c—T′_c越小。在各向同性,两相共存或完全的液晶状态,溶液平均折射率和消光度均与浓度呈线性关系。但在两相间相互转变时,即在C_1~*和C_2~*处,n-C和 A-C 曲线上出现转折点。高聚物与溶剂的相互作用参数X_(12)愈小,临界浓度C_1~*愈小。把描述大分子链柔顺性的参数f与X_(12)联系起来,可用 1956年 Flory的理论定性地解释溶剂对高聚物溶致性液晶形成的影响。     

Vapor-liquid equilibrium properties of sodium n-heptyl sulfonate in Water and in aqueous solutions of poly(ethylene glycol) at different temperatures

The vapor-liquid equilibria properties of sodium n-heptyl sulfonate (C₇SO₃Na) in pure water and in aqueous poly(ethylene glycol) (PEG) solutions were determined at different temperatures below and above the critical micelle concentration (CMC). Vapor-liquid equilibrium data such as water activity, vapor pressure, osmotic coefficient, activity coefficient and Gibbs free energies were obtained through isopiestic method. The concentration dependence of all investigated thermodynamic properties exhibit a change in slope at the concentration in which micelles are formed. It was found that the vapor pressure depression for a ternary aqueous C₇SO₃Na + PEG system is more than the sum of those for the corresponding binary solutions and, at higher temperatures, the higher concentration of PEG is in equilibrium with a certain concentration of surfactant.     

Cavitation during Drawing of Crystalline Polymers

Summary: Cavitation in low molecular weight liquids under tension and in crystalline polymers during tensile drawing was recapitulated. The review indicated that the amorphous phase of crystalline polymers at temperature above its glass transition temperature differs markedly from low molecular weight liquids. Cavitation in polymers seemingly is not of a heterogeneous character, unlike in unpurified low molecular weight liquids. The most probable reasons are: confinements of amorphous layers between crystalline lamellae and macromolecular chain entanglements, the factors that are absent in low molecular weight liquids.     

Reversible protein adsorption and bioadhesion on monolayers terminated with mixtures of oligo(ethylene glycol) and methyl groups

Surface-grafted, environmentally responsive polymers have shown great promise for controlling adsorption and desorption of macromolecules and cells on solid surfaces. In the paper, we demonstrate that certain mixed self-assembled monolayers (SAMs) of oligo(ethylene glycol) (OEG) and methyl-terminated alkanethiolates on gold form surfaces with switchable hydrophobicity and tendency for protein adsorption and cellular attachment. At temperatures above 32 degrees C, SAMs with a surface density of approximately 50% OEG adsorbed significant amounts of pyruvate kinase and lysozyme, whereas below this temperature, these same SAMs were resistant to the adsorption of these proteins. Furthermore, protein layers adsorbed to these SAMs above 32 degrees C were removed upon rinsing with water below this temperature. Similar results were seen for attachment and release of the marine bacterium, Cobetia marina. The change from nonresistance to adsorptive state of the SAMs was concomitant with a change in advancing water contact angle. Vibrational sum frequency generation spectroscopy suggests that the temperature-induced changes coincide with a disorder-to-partial order transition of the hydrated methylene chains of the OEG moieties within the SAMs. Mixed OEG-methyl SAMs represent both a convenient means of controlling macromolecular and cellular adsorption within the laboratory and a useful tool for relating adsorption properties to molecular structures within the SAMs.     

Supercritical phase equilibria involving solids

The solubility of solids in supercritical solvents is reviewed in a phenomenological discussion of binary and ternary systems containing one highly volatile component. Solubility and selectivity are greatly determined by the course of the binary critical curves, the ternary critical end-point curves, and the locations of the triple points of the solids. The mean-field lattice-gas model is used to review some important molecular parameters.     

Chromatographic study of adsorption from binary and ternary solutions

Adsorption of benzene, anisole, and nitrobenzene on hydroxylated silica gel from binary and ternary solutions (adsorbate--n-hexane and adsorbate--n-hexane--terahydrofuran) was studied by HPLC. The equation that describes the adsorbate retention as a function of its concentration in binary and ternary solutions was proposed. The equation makes it possible to calculate the adsorption isotherms and adsorption equilibrium constants directly from chromatograms using the retention parameters and equilibrium concentrations of the adsorbate in the mobile phase.     

Thermodynamic characterization of ternary solutions of uncharged polymers

The thermodynamic interactions in aqueous solutions of uncharged polymers were studied. Using a gel-deswelling method, the water activities (chemical potentials) in binary and ternary (two polymers in one solvent) solutions of methylcellulose (MC), polyvinyl alcohol (PVA) and polyvinyl pirrolidone (PVP), respectively were determined at various polymer volume fractions (1.0 × 10⁻² < v₂ < 1.0 × 10⁻¹). On the theoretical basis of the Flory–Huggins approximations, the relevant solvent–segment (χ₁₂ or χ₁₃) and segment–segment pair interaction parameters (χ₂₃) have been calculated.

The solvent activity curves (ln a₁ versus polymer volume fraction) can be well described by a polynomial of third-degree in both the binary and the ternary solutions of the polymers. The solvent–segment interaction parameters exhibit a slight dependence on the polymer concentration. For each binary solution, the χ₁₂−v₂ function can be fitted by a straight line wich has a small positive slope. In the mixtures of two polymers, the values of the segment–segment (χ₂₃) interaction parameters were close to zero or sligthly negative (χ₂₃ 0 ± 0.03), indicating that under the studied conditions, the polymers in the ternary solutions are compatible.     

Topological transformation of the phase diagram for the ternary system cesium nitrate-water-acetonitrile

In the ternary system cesium nitrate-water-acetonitrile, in which liquid-liquid phase separation with an upper critical solution point (UCSP) exists in the liquid binary subsystem, was studied in the range from ?5 to 120°C using visual polythermal analysis. Liquid-liquid phase separation in the ternary system is observed above 96.0°C and below 2.8°C. Acetonitrile distribution coefficients between the aqueous and organic phases of monotectic equilibrium were calculated for various temperatures. Phase isotherms of the system confirm the general scheme of the topological transformation of phase diagrams in salt-binary solvent ternary systems with salting out.     

A simple FTIR spectroscopic method for the determination of the lower critical solution temperature of N‐isopropylacrylamide copolymers and related hydrogels

Linear and crosslinked polymers based on N‐isopropylacrylamide (NIPAAm) exhibit unusual thermal properties. Aqueous solutions of poly(N‐isopropylacrylamide) (PNIPAAm) phase‐separate upon heating above a lower critical solution temperature (LCST), whereas related hydrogels undergo a swelling–shrinking transition at an LCST. A linear copolymer made of NIPAAm/acryloxysuccinimide (98/2 mol/mol) and two hydrogels with different hydrophilicities were prepared. Fourier transform infrared (FTIR) spectroscopy was employed to determine the transition temperature and provide insights into the molecular details of the transition via probing of characteristic bands as a function of temperature. The FTIR spectroscopy method described here allowed the determination of the transition temperature for both the linear and crosslinked polymers. The transition temperatures for PNIPAAm and the gel resulting from the crosslinking with polylysine or N,N′‐methylenebisacrylamide (MBA) were in the same range, 30–35 °C. For the gels, the transition temperature increased with the hydrophilicity of the polymer matrix. The spectral changes observed at the LCST were similar for the free chains and the hydrogels, implying a similar molecular reorganization during the transition. The C H stretching region suggests that the N‐isopropyl groups and the backbone both underwent conformational changes and became more ordered upon heating above the LCST. An analysis of the amide I band suggests that the amide groups of the linear polymer were mainly involved in hydrogen bonding with water molecules below the LCST, the chain being flexible and disordered in a water solution. During the transition, around 20% of these intermolecular hydrogen bonds between the polymer and water were broken and replaced by intramolecular hydrogen bonds. Similar changes were also observed at the LCST of a gel crosslinked with MBA. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 907–915, 2000     

On the resistance to the interpenetration between macromolecular coils of different segment densities

Using viscometry techniques on polymer fractions, we determine the critical concentrationc ^* (separating the dilute and semi dilute solutions). The same measurements have been conducted with mixtures of these fractions (mixtures 1:1 by weight of fractions differing in molecular mass and chemical nature, or fractions differing only in molecular mass). The determined values of critical concentrationc ^* of the mixtures are higher than the values calculated based on the critical concentrations of the corresponding fractions. This deviation from the additivity rule is attributed to the resistance in the interpenetration (delay to the attainment of the homogeneous state) between macromolecular coils of different chemical nature or of the same chemical nature but of different molecular mass. Higher values of the reduced viscosities of the mixture of the fractions, compared to the values calculated using the reduced viscosities of the corresponding fractions, are observed above the critical concentrationc ^*. In this concentration region the interaction parameter between two different polymers is calculated.