Morphology and performance of poly(vinylidene fluoride) flat sheet membranes: Thermodynamic and kinetic aspects

In this study, poly(vinylidene fluoride) (PVDF) membranes were prepared using two different solvents with various polymer concentrations to investigate the predominant kinetic or thermodynamic aspects of membrane preparation in a phase separation process. For this purpose, dimethyl sulfoxide (DMSO) as a weak solvent and N‐2‐methylpyrrolidone (NMP) as a strong solvent were used with polymer concentrations between 8 and 15 wt %. Scanning electron microscopy and water content, contact angle, and pore size measurements were used to assess the factors affecting the physicochemical properties of the prepared membranes. The results showed that in the case of NMP, the membrane structure is mainly controlled by thermodynamic parameters, while when using DMSO, kinetic parameters are predominant. According to the results, the prepared PVDF‐based membranes with DMSO exhibited a relatively denser top layer and less permeation compared to the NMP/PVDF membranes. The difference between the viscosities of the casting solutions with equal polymer concentrations in DMSO and NMP was considered to be the main effective factor in solvent/nonsolvent exchange, resulting in denser top layers in the DMSO/PVDF membranes. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46419.     

Effects of Lewis Acids on the Thermal Rearrangements of Chloromethylsilane: A Density Functional Theory Study

Understanding the thermal rearrangement mechanism of chloromethylsilane in the presence of Lewis acids is particularly important in industrial applications. In this study, the thermal rearrangements of chloromethylsilane under the catalysis of AlCl₃ and BF₃ were studied on the basis of density functional theory (DFT) at the B3LYP/6-311G (d, p) level. The structures of the reactants, transition states and products were located and fully optimized. The geometries of various stationary points and the harmonic vibrational frequencies were calculated at the same levels. The reaction paths were investigated and confirmed by intrinsic reaction coordinate (IRC) calculations. The results showed that the thermal rearrangement of chloromethylsilane in the presence of a Lewis acid occurred via two pathways (MX₃-1 and MX₃-2). Only pathway MX₃-1 had a catalytic effect. The determining factor of the reaction activation energies and rates was the formation of the transition state in each reaction. Among all the reactions, the Al atom could form an approximate tetrahedral coordination with four Cl atoms, resulting in the lowest energy barrier. The kinetic and thermodynamic calculations also proved this result.     

Introduction of novel kinetic approach to calculation of activation energy and its application to the sinter-crystallization of strontian feldspar

The kinetics, the mechanism and the thermodynamics of activated state of formation of primary strontian feldspar via sinter-crystallization of non-equilibrium melt during the thermal treatment of ceramic body was investigated in this work via differential thermal analysis using isoconversional Kissinger kinetic equation. The process of formation of non-equilibrium melt and subsequent crystallization of primary strontian feldspar requires the activation energy of 631±3 and 664±2 kJ mol⁻¹, respectively. The investigation of mechanism of formation of primary strontian feldspar reveals that the process is driven by the surface nucleation and diffusion controlled growth of the new phase. The nucleation rate decreases with the time of process and non-equilibrium melt can be formed only in metastable equilibrium with activated state of strontian feldspar. Deep consideration of kinetic data leads to the deduction of new kinetic approach that enables single calculation of activation energy and frequency factor of heterogeneous processes as well as the dependence of thermodynamic parameters of activated state on temperature. Further consideration of kinetic data reveals that the activation energy is directly proportional to the function of csch (z)+1. For z=e, this term enables to derive the value for the parameter B(x) in empirical equation for Arrhenius temperature integral p(x) proposed by Doyle to be 1.0642.     

Adhesion properties of phosphate‐ and siloxane‐containing polyurethane dispersions to steel: An analysis of the metal–coating interface

Polyurethane dispersions containing phosphate and siloxane groups in the main chain were investigated as possible self‐assembling metal coatings. Improved adhesion of the polymer to the metal was observed because of the formation of an insoluble metal phosphate layer at the metal–coating interface. The neutralizing amine of the dispersions affected the formation of this metal phosphate, and the metal phosphate formation was dependent on the curing temperature and boiling point of the amine used for neutralization. A crosscut comparative study of adhesion proved that the phosphate‐containing coatings had better adhesion because of the formation of ionic bonds at the metal–coating interface. A solid‐state adhesion prediction method based on thermodynamic considerations was used. The results of the solid‐state adhesion method correlated well with that obtained from the crosscut adhesion test method. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 900–907, 2003     

Effects of aggregation structure on rheological properties of thermoplastic polyurethanes

The effects of the molecular aggregation structure on the rheological properties of thermoplastic polyurethane (TPU) were investigated. The TPU was composed of poly{(tetramethylene adipate)-co-(hexamethylene adipate)} glycol as the soft segments, 4,4′-diphenylmethane diisocyanate and 1,4-butanediol as the hard segments. The TPU sheets prepared by injection molding were annealed at various temperatures from 23 to 120 °C to vary the molecular aggregation structure. Glass transition temperature of the soft segment and melting points of the hard segment domains of the TPUs decreased and increased, respectively, with increasing annealing temperature. The results of DSC, solid-state NMR spectroscopy and dynamic viscoelastic measurements revealed that the degree of micro-phase separation of the TPUs becomes stronger with increasing annealing temperature due to the progress of formation of well-organized hard segment domains. The dynamic temperature sweep experiments for molten TPUs revealed that the temperature at critical gel point, which is defined as the temperature at which the dynamic storage modulus coincides with the loss storage modulus, in the cooling process increased with the progress of aggregation of the hard segments in the TPUs observed in the solid state. The uniaxial elongational viscosity measurements showed that TPUs exhibited an obvious strain hardening behavior with strain rate owing to residual hard segment domains at an operating temperature. It was revealed that the formation of well-organized hard segment domains had a profound effect on the rheological properties of TPUs, in particular on their elongational viscosity.     

Effect of PEG additives on properties and morphologies of polyetherimide membranes prepared by phase inversion

This study investigated the effect of poly (ethylene glycol) (PEG) additive as a pore-former on the structure formation of membranes and their permeation properties connected with the changes in thermodynamic and kinetic properties in the phase inversion process. The membranes were prepared by using polyetherimide/N-methyl-2-pyrrolidone/PEG (PEI/NMP/PEG) casting solution and water coagulant. The resulting membranes, prepared by changing the ratio of PEG to PEI, were characterized by scanning electron microscope (SEM) observations, measurements of water flux and γ-globin rejection. The thermodynamic and kinetic properties of the membrane-forming system were studied through viscosity. The pore radius distribution curves were especially obtained by differential scanning calorimetry (DSC). Furthermore, the membranes were characterized for pure water flux and rejection of solute and by SEM observation. The filtration results agreed well with the SEM observations. As expected, PEG with a fixed molecular weight (PEG 600) acted as a pore forming agent, and membrane porosity increased as the PEG content of the casting solution increased.     

Effect of stirring on morphology and properties in a catalyst cured epoxy system

This article describes the effect of stirring on epoxy network morphology and physical properties. Several different mechanical stirrer speeds, from 45 rpm to 800 rpm, were employed for the mixing of diglycidyl ether of bisphenol A resin with 2-ethyl-4-methyl imidazole catalytic curing agent. The investigation indicated that the glass transition temperature, room temperature density, low temperature relaxation, tensile mechanical properties in the rubbery state, and nodular morphology are significantly affected by the stirrer speed used. Transmission electron microscopy has revealed that the extent of homogeneity for the dispersion of nodule domain in the low crosslinking matrix improved with increasing stirrer speed. However, an optimum stirrer speed condition is needed for achieving the most homogeneous system. In addition, it is shown that the room temperature tensile mechanical properties are independent of the mixing condition. This independence is ascribed to the fact that the glassy state mechanical properties are closely related to the intermolecular forces rather than to the network structure.     

CONTRIBUTIONS TO THE THEORY OF GLASS FORMATION AND THE GLASSY STATE*

An attempt has been made to assimilate the most important results of recent glass research into a theory of glass formation and the glassy state and to discuss it in relation to present knowledge of physics, chemistry, and colloidal chemistry. The formation of glass from the liquid state, in contrast to crystallization, is a continuous path from the liquid, through the viscous (supercooled) to the brittle state. The transformation point (Tjp) represents the boundary temperature between viscous and brittle glass, at which boundary the property-temperature coefficients more or less suddenly change. Property values are influenced by thermal history which can not be explained by mechanical strains. Therefore, while a melt is cooled, an inner change of state, dynamic aggregation, is assumed which takes more and more time as the transformation point is approached and is completely checked at lower temperatures. The question of the thermodynamic stability of the glassy state thus becomes a problem. The difference between “unchecked” (lasting) and “checked” displacements of state is explained with respect to a series of properties. From measurements of the viscosity and of the electric conductivity at temperatures from 1300 down to 300°C, it is shown that the liquid or viscous state follows a simple hyperbolic law, according to which these properties can only continue down to a lower limiting temperature (7V) above the absolute zcro point. The brittle state begins at the transformation point and follows a new law, knowledge of which is made more difficult by sooner or later “checking” the displacements of state. From the previous two different laws, it follows that brittle glass is something other than a supercooled liquid. The transformation point can not be thermodynamically explained. Molecular kinetic considerations make it appear to be that temperature at which densest packing with a cessation of free molecular kinetic motion of the particles is reached. Changes in the brittle state are explained from an atom-kinetic point of view (shrinking of particle volumes) provided no changes in the state of aggregation occur. Dynamic aggregation takes place by the formation of primary and secondary particles. The structure of secondary particles is compared to a roll of coins and this picture is used to explain negative expansion under pressure, peculiarities in expansion curves, and the beginning of brittleness. According to colloidal chemical considerations, the glassy or viscous states represent a solution of secondary particles as the disperse phase in a dispersing medium of primary particles and individual molecules, and the transformation point represents the boundary temperature between viscous and brittle glass paralleled by the gelatinizing temperature characteristic of the change from a sol to a gel. Support for the colloidal-chemical conception is obtained from observations on rapidly cooled glass melts and from reaction kinetic. Changes of property in the brittle and viscous states are proportional to the logarithm of the time, z, a law which is also found in elastic after-working, in isothermal dissociation of carbonates, and in some other instances. This law can he deduced from v. Smoluchowski's theory of coagulation. The disappearance of, double refraction of chilled glasses due to annealing (cooling) can also be linked to the log Z law.     

Effect of PEG additives on properties and morphologies of polyetherimide membranes prepared by phase inversion

This study investigated the effect of poly(ethylene glycol) (PEG) additive as a pore-former on the structure formation of membranes and their permeation properties connected with the changes in thermodynamic and kinetic properties in the phase inversion process. The membranes were prepared by using polyetherimide/N-methyl-2-pyrrolidone/PEG (PEI/NMP/PEG) casting solution and water coagulant. The resulting membranes, prepared by changing the ratio of PEG to PEI, were characterized by scanning electron microscope (SEM) observations, measurements of water flux and γ-globin rejection. The thermodynamic and kinetic properties of the membrane-forming system were studied through viscosity. The pore radius distribution curves were especially obtained by differential scanning calorimetry (DSC). Furthermore, the membranes were characterized for pure water flux and rejection of solute and by SEM observation. The filtration results agreed well with the SEM observations. As expected, PEG with a fixed molecular weight (PEG 600) acted as a pore forming agent, and membrane porosity increased as the PEG content of the casting solution increased.     

The formation mechanism of phase inversion membranes

The formation of asymmetric and symmetric membranes obtained by precipitation of a polymer solution is discussed. The different membrane structures and especially the formation of the “skin” is rationalized on the basis of thermodynamic and kinetic aspects of phase separation processes.     

Structure-property behaviour of segmented polyether-MDI-butanediol based urethanes: effect of composition ratio

The structure-property relationship of a systematic series of segmented polyurethanes was investigated segment was 4,4′-diphenylmethane diisocyanate (MDI) extended with 1,4-butanediol, and the soft segment was 2000 M_w poly (tetramethylene oxide) ether. X-ray studies reveal that some hard segment segment was 2000 mW poly (tetramethylene oxide) ether. X-ray studies reveal that some hard segment crystallization occurs at high hard segment content (45%). In addition, other morphological changes take place as the hard segment fraction is increased. The texture changes from that in which little such domain content exists at low hard segment levels (15%), to that in which the polymer has an interlocking domain morphology at high hard segment content (35 and 45%). Preferable elastomeric properties (low hysteresis, high extension) can be obtained when isolated hard segment domains exist (25% hard segment). Thermal treatment of the samples results in domain disruption and hard-soft segment mixing. However, this phenomenon, and the consequent time-dependent structure recovery as the material is allowed to age, is composition dependent. In general, crystalline domains, when present, are disrupted the least while the fastest recovery is displayed by samples with noncrystalline domain texture. This behaviour can be explained qualitatively in terms of kinetic diffusion effects relative to the thermodynamic driving forces for phase separation.     

Effect of monomer type and dangling end size on polymer network synthesis

The design of novel biomaterials for applications in biological recognition, drug delivery, or diagnostics requires a judicious choice of preparation conditions and methods for the production of well‐characterized 3‐dimensional structures, preferably by benign processes. In this work, the polymerization of poly(ethylene glycol) (PEG) methacrylates was examined by kinetic gelation modeling and kinetic analysis in order to ascertain the factors affecting the resulting structure. The kinetics of the polymerization and structure of the final polymer network are strongly affected by the length of the PEG graft chain. The propagation of the polymer chains becomes increasingly diffusion limited with the incorporation of longer PEG grafts. In addition, a more heterogeneous network consisting of numerous microgel regions is produced as the length of the PEG graft is increased. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 3506–3519, 2003     

Cyclopolymerization of divinyl monomer with manganese (VII)—thiol redox pair

Polymerization of non-conjugated divinyl monomer N,N′-methylene bisacrylamide was studied with a novel redox pair of manganese (VII)–thiol in a homogeneous gel-free state, under varying conditions of concentration, temperature, pH and ionic strength, and the kinetic results were interpreted by a suitable mechanistic sequence from among various possibilities. Absence of gelation is attributed to the formation of a cyclic polymer. An attempt is made to correlate the structure of the reductant–oxidant complex with the observed kinetic results.     

Studies on preparation of immersion‐type polypropylene fragrant fiber. I. Formation of matrix fiber in the melt‐spinning process and its technique of immersion essential oil

Polypropylene/ethylene vinyl acetate (PP/EVA) blends were prepared in a plastic extruder with a static mixer. The thermodynamic compatibility, morphology, crystal form, and rheological behavior of PP/EVA blends were investigated by SEM, DSC, and rheology instruments. The results showed that PP and EVA were thermodynamically incompatible, the viscosity of the PP/EVA blends decreased with increase of shear rate in a range of temperature, the PP/EVA blends had a sea‐islands structure, and the crystalline zones remained in their original state and could not form mixed crystals in the PP/EVA blends. The PP/EVA blends were melt spun to prepare matrix fibers and the spinning conditions such as EVA content, the matching factor between pump delivery and winding velocity, and the melt‐spinning temperature were also determined. The sorption process of a matrix fiber for essential oils, adsorbed under various sorption conditions such as sorption time, sorption temperature, and EVA content, was also studied. The results revealed that the composite isotherm of the adsorption of matrix fiber for essential oil was characteristic of a U model. Through adsorbing essential oil, the immersion‐type PP fragrant fibers could be prepared with the matrix fiber. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 1970–1979, 2003     

Thermo-Responsive Ultrafiltration Block Copolymer Membranes Based on Polystyrene-block-poly(diethyl acrylamide)

Within the present work, a thermo-responsive ultrafiltration membrane is manufactured based on a polystyrene-block-poly(diethyl acrylamide) block copolymer (BCP). The poly(diethyl acrylamide) block segment features a lower critical solution temperature (LCST) in water, similar to the well-known poly(N-isopropylacrylamide), but having increased biocompatibility and without exhibiting a hysteresis of the thermally induced switching behavior. The BCP is synthesized via sequential “living” anionic polymerization protocols and analyzed by ¹H-NMR spectroscopy, size exclusion chromatography, and differential scanning calorimetry. The resulting morphology in the bulk state is investigated by transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) revealing the intended hexagonal cylindrical morphology. The BCPs form micelles in a binary mixture of tetrahydrofuran and dimethylformamide, where BCP composition and solvent affinities are discussed in light of the expected structure of these micelles and the resulting BCP membrane formation. The membranes are manufactured using the non-solvent induced phase separation (NIPS) process and are characterized via scanning electron microscopy (SEM) and water permeation measurements. The latter are carried out at room temperature and at 50 °C revealing up to a 23-fold increase of the permeance, when crossing the LCST of the poly(diethyl acrylamide) block segment in water.     

Rheology and structure of liquid crystalline alkylenearomatic polyesters

The rheological, thermodynamic and structural properties of two liquid crystalline polyesters—poly(oxyfumaroyloxy- 1,4-phenylene carbonyloxyalkyleneoxycarbonyl- 1,4-phenylene)s with six and ten methylene groups (PES-6 and PES- 10, respectively) and a statistical copolyester (co-PES) have been investigated. It has been shown that they all form a smectic LC-phase. The change from homopolyesters to co-PE S expands the temperature range of the LC-state. The high sensitivity of the structure to the prehistory in the melt state is typical for such copolymers. Prolonged heating leads to rearrangement of the co-PES crystalline structure and a drastic increase in viscosity. The orientation of these polymers during flow was investigated. It was shown that a high level of molecular orientation of the smectic layers, perpendicular to the direction of flow, was realized at high shear stresses. In the melt region, the existence of a metastable network with junctions of crystalline nature hinders the flow of the homo- and copolyesters. The structure of the unit cell of the crystallites depends on the composition and thermal history.     

Estimation of thermodynamic stability conditions and perspectives for synthesis of covalent carbon nitride

The present work is devoted to the determination of conditions of thermodynamic stability of carbon nitride having structure of β-C₃N₄. The thermodynamic functions of crystalline covalent carbon nitride required for thermodynamic reckoning of parameters of formation of carbon nitride were determined on the base of the Debye theory with the characteristic temperature varied in the range from 1000 K to 2500 K. The formation enthalpy of carbon nitride was estimated on the base of the energy of atomization and formation enthalpy of a mixture of atomized carbon and nitrogen. The resulted quantity of the standard formation enthalpy of covalent carbon nitride at 298.15 K made up 4.47 kcal/mol. Thermodynamic computations were accomplished with the use of the Automated System of Thermodynamic Reckonings and Algorithms ASTRAL. Behavior of the gaseous phase of a chemical system was described by the BKW-RR equation of state. Carbon nitride was considered to be incompressible. The region of thermodynamic stability of covalent carbon nitride is computed. It is shown that in contrast to the carbon condensation into graphite the pressure of condensation of β-C₃N₄ when adding other chemical elements to a thermodynamic system can not only increase but decrease as well. Consideration of detonation and explosion processes in high explosives shows a way for practical synthesis of covalent carbon nitride.     

Rate and oxygen activity oscillations during hydrogen oxidation on nickel films in a CSTR

The catalytic oxidation of hydrogen on polycrystalline nickel films was studied in a CSTR at 510-681°K and atmospheric pressure. The technique of Solid Electrolyte Potentiometry (SEP) was used to monitor the thermodynamic activity of oxygen adsorbed on the catalyst surface. To this end the reaction was studied in an yttria-stabilized zirconia solid electrolyte cell. Both steady state kinetics and potentiometric results indicate formation and reduction of a surface nickel oxide. In a certain range of temperature and gas composition sustained oscillatory phenomena were observed on both the reaction rate and the surface oxygen activity. A dynamic model of six differential equations that takes into account transient changes of the gas phase concentration of H₂, O₂ and H₂O as well as of the surface coverage of adsorbed hydrogen, oxygen and nickel oxide has been developed. The model explains quantitatively the stable steady state kinetic and SEP results. The same model predicts oscillatory behavior and explains qualitatively the observed unsteady state phenomena.     

化学交联与热处理对聚乙烯醇膜性能的影响

采用相转化法制备聚乙烯醇膜,分别研究了化学交联以及热处理对膜结构和性能的影响,利用红外光谱获得膜的交联度和结晶度。结果表明随着处理时间的延长,膜的交联度或者结晶度增加;溶涨试验结果表明热处理后膜的孔隙率下降,而化学交联对膜的孔隙率影响不大;力学性能测定表明膜在干态与湿态下,力学性能差别巨大,化学交联的膜在干态下的性能要大大好于湿态下的力学性能,而热处理的膜正好相反,热处理后膜的机械性能湿态要好于干态。