Yield stress-zeta potential relationship of oxide dispersions with adsorbed polyacrylate — Steric effect and zeta potential at the flocculated-dispersed transition state

The yield stress-DLVO force relationship is obeyed by α-Al₂O₃ and alumina-coated TiO₂ dispersions with adsorbed polyacrylate only if the yield stress and its corresponding zeta potential data were collected in the positively charged region. In this region, the underlying surface positive charge density of the particles exceeds the negative charge density of the polyacrylate. At this state the adsorbed polyelectrolyte lies flat on the particle surface forming a steric layer of fixed thickness at a given polymer concentration. In the negative charge region, the steric layer thickness is not constant and hence yield stress-DLVO relationship is not obeyed. The (critical) zeta potential at the flocculated-dispersed transition state decreases with increasing polymer concentration. This result reflects a decreasing van der Waals force as the steric layer increases in thickness. A steric layer ensured that the surface or zeta potential is sufficiently low in the flocculated regime for the DLVO theory to remain valid. The ratio of the critical zeta potential square between alumina-coated TiO₂ and α-Al₂O₃ is an indication of their Hamaker constants ratio in water. The effect of alumina coating on the value of this ratio is presented and discussed.     

Controlling attractive interparticle forces via small anionic and cationic additives in kaolin clay slurries

Interparticle forces govern slurry behavior in flow, mixing, sedimentation and thickening. This study evaluates the use of small anionic and cationic additives with pH to control the interparticle forces in kaolin slurry via the yield stress parameter. Both phosphate and citrate additives were found to reduce the interparticle attractive force or yield stress in the moderate pH region of 4–12. These relatively low charged additives were unable to impart a sufficiently strong repulsive interparticle force to completely disperse the slurry. Three linear relationships between yield stress and the square of zeta potential were observed in slurry with and without these additives, indicating that the yield stress–DLVO force model is obeyed in each linear region. The mid-range zeta potential region yielded a positive slope which was attributed to heterogeneous charge attraction between clay particles. It is this heterogeneous charge attraction that was weakened by the adsorbed additives. In contrast, cationic Polyethylenimine (PEI) of Mw 70,000 increases the yield stress at all pH level via bridging. Charge reversal was also observed at high PEI concentrations. In two cases, the pH of maximum yield stress and zero zeta potential coincided. A single linear yield stress–zeta potential squared relationship was observed despite particle bridging interaction being the dominant interparticle force.     

Effect of graphene oxide on the molecules of a sodium alginate–krill protein composite system and characterization of the system's composite fiber morphology and mechanical properties

Graphene oxide (GO), sodium alginate (SA), and Antarctic krill protein (AKP) were blended to get functional fibers. These GO–SA–AKP composite fibers were obtained by conventional wet spinning with 5% calcium chloride as a coagulation solution. The intermolecular interactions of the GO–SA–AKP composite system were characterized by Fourier transform infrared spectroscopy. The morphology, crystallinity, thermal stability, and mechanical properties of the GO–SA–AKP composites were investigated with scanning electron microscopy, X‐ray diffraction, thermogravimetric analysis, and differential scanning calorimetry. The results show that the number of GO layers decreased gradually during the process of wet spinning. The addition of GO promoted an increase in intramolecular hydrogen bonding and a decrease in intermolecular hydrogen bonding in the composite system. With increasing GO mass fraction, the intermolecular hydrogen‐bond content, crystallinity, breaking strength, and thermal stability in the composite system increased first and then decreased. At the same time, the mass fraction of GO and the draw ratio had significant effects on the surface morphologies of the composite fibers. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46642.     

Surface force arising from adsorbed graphene oxide in alumina suspensions with different shape and size

The effects of graphene oxide (GO) on the yield stress‐pH of α‐Al₂O₃ (alumina) suspensions were investigated. For micron‐sized platelet alumina suspensions, micron‐sized GO additive increased the maximum yield stress by as much as six‐folds. This was attributed to GO‐mediated bridging interactions between the platelet particles. This type of bridging interactions was much less effective with submicron‐sized, spherical, and irregular shape alumina. Adsorption of the anionic GO reflected by the shift of pH of zero zeta potential to a lower pH is particularly high for platelet alumina. The 1.0 dwb % GO concentration added is sufficient to reinforce each platelet particle–particle bond, assisted by a directed GO–platelet interaction configuration. This is, however, not true with submicron‐sized particles as the particle concentration increases sharply with the inverse of the particle diameter to power of 3. Moreover, a GO sheet can adsorb several submicron‐sized particles and this does not produce the right interaction configuration. © 2013 American Institute of Chemical Engineers AIChE J, 59: 3633–3641, 2013     

Stability of kaolinite dispersions in the presence of sodium and aluminum ions

The stability of colloidal kaolinite dispersions in the presence of NaCl and AlCl₃ was studied by measuring turbidity, electrophoretic mobility and adsorption. The kaolinite particles coagulated at pH 2.5–3.5 and were dispersed at pH >4.5. These results well obeyed the classic DLVO theory if the mean zeta potential of the kaolinite particles in aqueous solutions was taken into account in the computation of potential energy of electrical double layer repulsion, which suggests that the kaolinite particles might coagulate in the same way as normal colloidal particles. The kaolinite particles in aqueous aluminum salt solution only coagulated at a medium AlCl₃ concentration, and formed a stable dispersion at a high salt concentration. This is caused by Stern-layer adsorption of hydrolyzed aluminum species, probably adsorbed on the kaolinite surfaces through hydrogen bonds between the hydroxyl groups of the aluminum species and the oxygen atoms on the kaolinite surfaces.     

Critical zeta potential and the Hamaker constant of oxides in water

The critical zeta potential characterises the flocculated-dispersed state transition of a colloidal dispersion. For many colloidal dispersions, yield stress displayed a linear relationship with the square of zeta potential, indicating that they obeyed the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. From this relationship, the critical zeta potential is obtained from the intercept at the zeta potential axis at a yield stress of zero. The critical zeta potential is a measure of the repulsive potential required to exactly counter the maximum attractive potential between particles in dispersion in the flocculated state. When the forces of interaction between particles in the dispersion are only the van der Waals and electrostatic forces, then the critical zeta potential is indirectly a measure of the van der Waals attractive potential and, hence, it may be used to determine the Hamaker constant of solids in water. This potential is proportional to the square root of the solids Hamaker constant in water. At present, only the ratio of Hamaker constant between two oxides was obtained and compared with that obtained by other techniques. These oxides were ultrapure anatase TiO₂ and γ-Al₂O₃, and they displayed a linear relationship between yield stress and the square of zeta potential. At the conductivity (or ionic strength) of about 3000 μS/cm, the critical zeta potential for both TiO₂ and Al₂O₃ is ∼47 and ∼32 mV, respectively. These critical zeta potential data give a value of 2.2 for the ratio of Hamaker constant of anatase TiO₂/H₂O/TiO₂ to γ-Al₂O₃/H₂O/γ-Al₂O₃. This ratio compares well with a value ranging from 1.0 to 2.18 for rutile TiO₂/H₂O/TiO₂ to α-Al₂O₃/H₂O/α-Al₂O₃ where their Hamaker constants were calculated from the Lifshitz theory using full optical spectral data.     

Microstructure‐property relation in alumina ceramics during post‐annealing process after laser shock processing

Laser shock processing (LSP) is a new surface engineering approach to introduce significant compressive residual stress into ceramics to improve their mechanical properties. However, LSP of ceramics may induce microcracks, which limit the further improvement of mechanical properties of ceramics. In this research, the effect of a post‐LSP annealing process on α‐Al₂O₃ ceramics was investigated. The annealing treatment can cause thermal relaxation of compressive residual stress generated by LSP while still maintain the positive attribute of LSP. The compressive residual stress was stabilized after annealing after 10 hours at 1100‐1300°C. The healing of microcracks in α‐Al₂O₃ ceramics was observed during the post‐LSP annealing process, which is caused by diffusion bonding mechanisms and accompanied by dislocation and void formation. The combination of the stabilized compressive residual stress and microcrack healing can improve the cracking resistance of α‐Al₂O₃ ceramics to mechanical impact on the surface by 69%.     

Yield stress and zeta potential of washed and highly spherical oxide dispersions — Critical zeta potential and Hamaker constant

The linear relationship between yield stress and the square of the zeta potential based on the yield stress-DLVO force model, was used to determine the zeta potential at the point of transition from flocculated to dispersed state of a range of oxide dispersions. The critical zeta potential at this point for washed α-Al₂O₃, TiO₂ and ZrO₂ dispersions was of magnitude 40, 49 and 52 mV respectively. For highly spherical silica and alumina dispersions, this value was 23 and 38 mV respectively. The square of the critical zeta potential is proportional to the Hamaker constant of the oxide in water when the van der Waals force is the only attractive force in play. Thus the critical zeta potential data obtained allowed the Hamaker constant ratio between the three oxide dispersions to be determined. This ratio between rutile TiO₂/water and α-Al₂O₃/water was 1.50. In comparison, a similar value of 1.46 was obtained for the ratio calculated from Hamaker constant value determined via Lifshitz theory. The ratio between rutile TiO₂/water and ZrO₂/water is ∼ 0.90. Using the Hamaker constant of rutile TiO₂/water of 61.2 zJ as the standard, the Hamaker constant determined by our method is 41 zJ for α-Al₂O₃, 68 zJ for ZrO₂ and 13.6 zJ for silica.     

Adhesion aspects of levulinic-acid-modified furan polymers to crystalline zinc phosphate metal surfaces

The nature of the interfacial interactions between functional levulinic-acid-modified furan resin coatings and crystalline zinc phosphate hydrate films deposited on carbon steel surfaces has been systematically investigated. The typical surface topography of the highly crystallized zinc phosphate films was found to be characterized by the presence of a dendritic microstructure array of interlocking triclinic crystals. This structure acts significantly to develop mechanical interlocking bonds with the functional blend polymer which penetrates into the open surface structure of the films. Both the thickness of deposition film and the polar H₂O molecules of hydrate at the outermost film surface sites play essential roles in wetting by the functional liquid resin. When the polarized furan polymers spread on the oxide film surfaces, the carboxylate groups derived from the levulinic ester and acid molecules react to form strong hydrogen bonds with the crystallized H₂O molecules on the hopeite film. This formation of hydrogen bonding was shown to be a major factor affecting the chemical intermolecular attractions. A formulation consisting of 95 parts furan to 5 parts levulinic acid was found to yield the optimum protective coating. More than 5 parts levulinic acid resulted in the transformation of the characteristics of the polymer film from hydrophobic to hydrophilic.     

Preparation and properties of castor oil‐based polyurethane/α‐zirconium phosphate composite films

Novel castor oil‐based polyurethane/α‐zirconium phosphate (PU/α‐ZrP) composite films with different α‐ZrP loading (0–1.6 wt %) and different NCO/OH molar ratios were synthesized by a solution casting method. The characteristic properties of the PU/α‐ZrP composite films were examined by Fourier transform infrared spectroscopy (FTIR), thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), X‐ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and tensile testing. The results from Fourier transform infrared spectroscopy indicated that strong intermolecular hydrogen bonding formed between α‐ZrP and PU, XRD and SEM results revealed that the α‐ZrP particles were uniformly distributed in the PU matrix at low loading, and obvious aggregation existed at high loading. Because of hydrogen bonding interactions, the maximum values of tensile strength were obtained with 0.6 wt % α‐ZrP loading and 1.5 of NCO/OH molar ratio in the matrix. Evidence proved that the induced α‐ZrP used as a new filler material can affect considerably the mechanical and thermal properties of the composites. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Electrochemical oxidation of adsorbed alkenoic acids as a function of chain length at Pt(111) electrodes. Studies by cyclic voltammetry,EELS and Auger spectroscopy

Studies of the electrochemical oxidation of a series of straight-chain terminal alkenoic acids adsorbed at a Pt(111) electrode surface are reported. Compounds adsorbed were: propenoic acid (acrylic acid, PPA); 3-butenoic acid (vinylacetic acid, 3BTA); 4-pentenoic acid (allylacetic acid, 4PTA); 6-heptenoic acid (6HPA); and 10-undecenoic acid (10UDA). Vibrational spectra of adsorbed layers were obtained by use of electron energy-loss spectroscopy (EELS). Molecular packing densities were measured by use of Auger spectroscopy. Electrochemical oxidation of each adsorbed layer was explored by means of cyclic voltammetry in aqueous inert electrolyte (KF/HF). As the analogous aliphatic acids are not chemisorbed at Pt under the same conditions, the alkenoic acids evidently adsorb at Pt(111) predominantly through the C=C double bond. Molecular packing densities indicate that the carboxylic acid moiety is in contact with the Pt surface only in the case of PPA. EELS spectra also indicate that the carboxylate groups (other than in PPA) are present as pendants. The carboxylic acid O-H stretching bands of most of the adsorbed acids are red-shifted and broadened, evidently due to extensive intermolecular hydrogen bonding; the exceptions are PPA, for which the interaction is primarily with the Pt surface, and 3BTA, for which intermolecular interaction between the carboxylic acid pendants is apparently prevented by steric considerations. The surface-attached carboxylic acid moieties react with KOH solution, leading to retention of K⁺ ions, detected by Auger spectroscopy, and to changes in the vibrational spectra indicative of carboxylate anions; reactivity toward KOH decreases with chain length. Adsorbed alkenoic acids at Pt(111) surfaces are stable in water and in vacuum. Oxidation of the adsorbed short-chain acids PPA and 3BTA proceeds to completion, forming CO₂ as the principal product. Oxidation of the adsorbed long-chain acids converts the C=C moiety to 2CO₂, and transforms the remainder of the molecule to an unadsorbed diacid (likely possibilities are malonic acid from 4PTA; glutaric acid from 6HPA; and heptane-1,7-dioic acid from 10UDA).     

Influence of hydrogen bonding interaction on the damping properties of poly(n‐butyl methacrylate)/small molecule hybrids

The hydrogen bonding interactions between poly(n‐butyl methacrylate) and a series of low molecular weight phenols containing two to four hydroxyl groups with different steric effects were investigated by differential scanning calorimetry and Fourier‐transform infrared spectroscopy. Results showed that the hydrogen bonding strength between the two components varies greatly according to the steric effects of the phenolic hydroxyl group. As the size of the group beside the hydroxyl increases, the hydrogen bond strength weakens. The glass transition temperature of binary hybrid systems was put into relation with the corresponding hydrogen bonding interaction strength. Strong hydrogen bonding strength increased T_g to higher values than that predicted by the linear additivity rule; by contrast, T_g of hybrid systems with weak hydrogen bonds showed linear changes. All of the samples showed linear variations at low concentrations of small molecules. The damping properties of five systems were analyzed by dynamic mechanical analysis. Either the loss factor or area of tan δ peak of the five systems increased compared with that of the pure polymer, thereby showing great improvements in the damping properties of the poly(n‐butyl methacrylate)/small molecule hybrid material. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41954.     

Effect of N-substituents on the surface characteristics and hydrogen bonding network of polybenzoxazines

In our study, the effect of N-substituents on the surface free energy and hydrogen bonding network structure of polybenzoxazine has been studied systematically. The contact angle measurement results show the surface energies of the polybenzoxazines decrease with the increase of alkyl chain length, and are not affected by the steric factor of the tert-butyl group. However, the FTIR curve fitting results show that both the chain length and bulkiness of alkyl group have an effect on the hydrogen bonding network of the polybenzoxazines, and facilitate the formation of intermolecular hydrogen bonding during the progress of cure. This indicates that both alkyl group and the fraction of intermolecular hydrogen bonding have an effect on the surface energy of the polybenzoxazines. Additionally, the transformation mechanism of the intermolecular and intramolecular hydrogen bonding during the progress of cure is proposed for the first time.     

Mahkota Dewa Subcritical Water Extraction Process: Experimental and Molecular Dynamics Simulation Study

Subcritical water extraction and molecular dynamics simulation were performed to investigate mangiferin and antioxidant extraction from Mahkota Dewa fruits at different operating temperatures. The mangiferin yield and antioxidant activity were analyzed using high‐performance liquid chromatography and 2‐diphenyl‐1‐picrylhydrazyl assay method, respectively. The diffusivity and intermolecular interactions were determined by mean squared displacement (MSD) and radial distribution function (RDF) analysis, respectively. The temperature exerts a momentous effect on mangiferin yield and antioxidant activity of the extract. The MSD result showed an increment in diffusivity coefficient of mangiferin with temperature. The RDF analysis revealed hydrogen bonding formations between mangiferin and water through O_H2O^??H_MR4(OH1) interaction that plays a role in the extraction process.     

The Effect of Macromolecular Architecture on Functional Group Accessibility: Hydrogen Bonding in Blends Containing Phenolic Photoresist Polymers

Summary An investigation of phenolic functional group accessibility in linear poly(4-hydroxy styrene) (PHS) and novel, branched poly(4-hydroxystyrene) (PHS-B) is presented. The phase behavior and extent of hydrogen bonding in blends of PHS-B with complimentary Lewis base polymers are calculated from glass transition temperature (T_g) enhancements measured using differential scanning calorimetry (DSC) techniques. The fraction of PHS-B hydroxyls accessible for hydrogen bonding were compared to linear PHS chains and small molecular weight analogs to help establish a molecular architecture-functional group accessibility property relationship for use with dissolution inhibition in candidate microlithographic photoresist binders. Design approaches to macromolecular structure and architecture for tailored intermolecular interactions in phenolic systems are discussed with respect to the effects of local steric screening as well as an intriguing thermodynamic competition between inter- and intra-molecular hydrogen bonding via molecular mechanics modeling.     

α‐Crystallin Modulates its Chaperone Activity by Varying the Exposed Surface

The α‐crystallin family of small heat shock proteins possesses chaperone activity in response to stress and is involved in several neurological, muscular, and ophthalmic pathologies. This family includes the vertebrate lens protein α‐crystallin, associated with cataract disease. In this study, by combining small‐angle X‐ray and light scattering techniques, the structure and shape of α‐crystallin was revealed in its native state and after a transition caused by heat stress. Below critical temperature (T_c), α‐crystallin appears as an ellipsoid with a central cavity; whereas at high temperatures the cavity almost disappears, and the protein rearranges its structure, increasing the solvent‐exposed surface while retaining the ellipsoidal symmetry. Contextually, at T_c, α‐crystallin chaperone binding shows an abrupt increase. By modelling the chaperone activity as the formation of a complex composed of α‐crystallin and an aggregating substrate, it was demonstrated that the increase of α‐crystallin‐exposed surface is directly responsible for its gain in chaperone functionality.     

Preparation and property of poly(vinyl alcohol) grafted with butyl glycidyl ether

The crosslinked polyvinyl alcohol (CPVA) and alkyl chain grafted CPVA (CPVA‐g‐BGE) were prepared through the addition reaction of epoxy group of epichlorohydrin and butyl glycidyl ether (BGE) with the hydroxyl group of PVA. By FTIR and ¹HNMR analysis, BGE was confirmed to be grafted onto the molecular chain of PVA successfully. By grafting with BGE, the area of the stress–strain curves of CPVA increased, and the elongation at break increased remarkably with little drop of the tensile strength. Much rougher fractured surface with folds was observed, indicating the increased toughness of CPVA. The relaxation peak corresponding to the glass transition temperature (T_g) of CPVA shifted to low temperature with increasing grafting ratio of BGE. When compared with CPVA, the crystallization ability of CPVA‐g‐BGE decreased, indicating that although the intermolecular hydrogen bonding of PVA was weakened by grafting with alkyl chain, appropriate intermolecular association of alkyl chain facilitated the formation of physical entanglement of molecular chains to strengthen and toughen the PVA matrix. Ink contact angles of CPVA‐g‐BGE decreased with increasing grafting ratio of BGE, indicating the increasing compatibility of CPVA with ink, which was advantageous for PVA to be used as surface sizing agent in papermaking. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Terminal functionalized hydroxyl‐terminated polybutadiene: An energetic binder for propellant

We report the functionalization of hydroxyl terminated polybutadiene (HTPB) backbone by covalently attaching 1‐chloro‐2, 4‐dinitrobenzene (DNCB) at the terminal carbon atoms of the HTPB. The modification of the HTPB by the DNCB does not alter the unique physico–chemical properties and the microstructure of the parent HTPB. IR, ¹H‐NMR, ¹³C‐NMR, size exclusion chromatography (SEC) and absorption spectroscopy studies prove that the DNCB molecules are covalently attached to the terminal carbon atoms of the HTPB. The π electron delocalization owing to long polymer chain, strong electron withdrawing effect of the DNCB molecule are the major driving forces for the covalent attachment of the DNCB at the terminal carbon atom of the HTPB. We are the first to observe the existence of intermolecular hydrogen bonding between the terminal hydroxyl groups of the HTPB. IR study shows that the attached DNCB molecules at the terminal carbon atoms of the HTPB breaks the intermolecular hydrogen bonding between the HTPB chains and forms a hydrogen bonding between the NO₂ groups of the DNCB and the OH groups of the HTPB. Absorption spectral study of the modified HTPB indicates the better delocalization of π electron of butadiene due to the strong electron withdrawing effect of the DNCB molecules. Theoretical calculation also supports the existence of hydrogen bonding between the OH and NO₂ groups. Theoretical calculation shows that the detonation performance of both the DNCB and the HTPB‐DNCB are promising. HTPB‐DNCB is the new generation energetic binder which has potential to replace the use of HTPB as binder for propellant.© 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Effects of comb copolymer PAA-g-MPEO on rheological and dispersion properties of aqueous CaCO<Subscript>3</Subscript> suspensions

Summary The comb copolymer poly(acrylic acid) (PAA) grafted methoxyl poly(ethylene oxide) (MPEO) (PAA-g-MPEO) as dispersant was used in aqueous CaCO₃ suspensions. The PAA-g-MPEO was adsorbed onto CaCO₃particle surfaces due to the electrostatic attraction. The adsorbed amount increased with increasing PAA-g-MPEO content. The CaCO₃ adsorbed PAA-g-MPEO displayed negative zeta potential. The zeta potential was more negative with increasing PAA-g-MPEO content. Addition of PAA-g-MPEO, the conductivity of aqueous CaCO₃ suspensions decreased firstly, and then increased with increasing PAA-g-MPEO content. Compared to that of aqueous CaCO₃ suspensions, the viscosity of aqueous CaCO₃/PAA-g-MPEO suspension reduced remarkably, and the liquidity of the suspensions was improved. The dispersion of CaCO₃ particles in aqueous CaCO₃/PAA-g-MPEO suspensions was significantly improved due to electrostatic repulsions and steric hindrance between CaCO₃ particles adsorbed PAA-g-MPEO.     

Investigation of PEG adsorption on the surface of zinc oxide nanoparticles

The influence of polyethylene glycol (PEG) on the adsorption at zinc oxide/polymer solution interface has been determined. PEG macromolecules bond with the solid surface mainly via the -OH group of the surface of ZnO nanoparticles, which may interact with PEG through hydrogen bonding. Adsorption isotherms demonstrate the marked influence of the PEG molecular weight and the concentration of polymer solution on the extent of adsorption. The thickness of the adsorbed polymer layer on ZnO nanoparticles was calculated on the basis of measurements of their suspension viscosities in the absence and presence of adsorbed polymer. Results show that the thickness of the adsorption layer increased with increasing polymer molecular weight and the concentration of polymer solution. The main factors responsible for the changes in zeta potential were determined on the basis of the data obtained. The shift of the slippage plane away from the surface of ZnO nanoparticles plays major role below pH_iep. Above pH_iep, the blockage of the adsorption sites is the predominant factor.