Effect of Composition of HPAM/Chromium(III) Acetate Gels on Delayed Gelation Time

A delayed crosslinking system employed with hydrolyzed polyacrylamide (HPAM) and chromium acetate was developed for high profile control in low-temperature reservoirs. The crosslinking system formed strong gel at polymer concentration range of 3000–5000 mg/L, and the gelation time was delayed to 8–30 days, which offered the crosslinking system enough time to flow into deep water-producing zones and plug large pore paths, forcing follow-up fluids to enter low-permeability layer and reduce disproportionate permeability. The effect of polymer hydrolysis degree, polymer concentration, pH, and crosslinker concentration on delaying gelation time was evaluated using bottle testing. Meanwhile, sand-packed tube displacement experiments revealed the plugging performance of delayed crosslinking system. According to Fourier transform infrared spectroscopy (FTIR) analysis, as well as crosslinking mechanism between polymer and chromium acetate, the delayed mechanism of crosslinked system demonstrated that due to stronger affinity of acetate complexes as a ligand, its substitution in situ by carboxylate group of polyacrylamide slowed down the initial rate-determining step of crosslinking reaction.     

Composite of SiO2 and Hydrogel Having Microphase Separated Structure: Physical Properties Dependence on pH

Abstract

Organic-inorganic composite gel was prepared by using PEG-modified urethane acrylate (PMUA) gel and tetraethoxysilane (TEOS). PMUA gel was prepared by the phase-inversion emulsion polymerization of PMUA emulsion. The gelation of PMUA emulsion using this method enables PMUA gel to swell with H₂O, TEOS, and ethanol. Hydrolysis and condensation reaction rates of the sol-gel process are strongly influenced by the pH controlled by catalysts such as HCl and NH₄OH. Additionally, the morphology on the cross section of composite and the amount of silica ingredient incorporated into the composite gel were dependent on solvent, the molar ratio of H₂O to TEOS, as well as the pH value.

As the silica content increased, due to hydrogen bonds interacting between PMUA gel and SiO₂, particles, the tensile strength of composites considerably increased, whereas the elongation at break decreased. The incorporation of silica ingredient in PMUA gel/silica composites was verified with FTIR/ATR and SEM. The amount of the silica component in the composite was indirectly investigated by using TGA thermal analysis.     

Key techniques to control porous microsphere morphology in S/O/W emulsion system

A solid-in-oil-in-water (S/O/W) emulsion system has been developed to prepare porous polymeric microspheres. The obtained microspheres showed unique core–shell structure with a dense core and a surface porous layer. The emulsion system has two processes. In the first process, S/O/W₁ viscous emulsion is prepared by dropping of S/O phase in the first water phase (W₁). In the second process, the S/O/W₁ emulsion is poured to another water phase (W₂) as S/O/W₁/W₂ emulsion. During the process, S/O/W₁ droplet becomes microsphere after organic medium completely diffusion. Emulsion techniques have various effective combinations such as additive and process conditions to design microsphere morphology. With regards to the proposed S/O/W system, addition of the solid phase in the system is a key factor to form the porous structure. When the medium diffusion starts, the solid makes W₁ phase kept inside the S/O/W₁ droplet. The remained W₁ phase changes surface porous layer after purification. Affinity between the solid and oil phase should be adjusted as well. In this study, an optimization of the emulsion system was attempted considering solubility parameter and polarity. Additionally, it is found that process conditions could help to design microsphere morphology such as pore size and porous layer thickness.     

Dynamic thickening investigation of the gelation process of PAM/PEI system at high temperature and high pressure

The gelation process of organically polymer gel was investigated by dynamic thickening measurements. Rheological measurements were used to evaluate the viscosity of the gel. During the gelation process, high temperature resulted in higher rate of crosslinking. Rigid and stable gel was formed in neutral and alkaline media, and the higher of the pH value, the faster of the gelation process. However, gel could not be formed in acid condition. Moreover, the rate of crosslinking increased with the increase of concentration of polymer and crosslinker. The addition of NH₄Cl elongated the gelation time significantly, but played a negative role in the gel strength, while a rigid gel was formed in the presence of Sodium acetate or trisodium citrate dehydrate. This paper summarizes the results and discusses how various parameters affect the gelation process of the gel.     

Citric acid-loaded W1/O/W2 multiple emulsions efficiently remove colonic ammonia both in vitro and in vivo

Hepatic encephalopathy (HE) is a neuropsychiatric disorder caused by chronic and acute liver diseases. It is believed that ammonia played an important role on the pathogenesis of the disease. Herein, acid-loaded water-in-oil-in-water (W₁/O/W₂) multiple emulsions with over 95% encapsulation efficiency were prepared and used to absorb colonic ammonia. The study of citric acid release from W₁ to W₂ in bulk deionized water indicated that only 17% acid released in 8 hours, which proved the stability of the multiple emulsions and hence prevented the intestine from being irritated by acid burst release. In vitro, the W₁/O/W₂ emulsion could remove around 90% ammonia in 1.5 hours from either the 3 mmol/L ammonia solution or the artificial colonic fluid with 1 and 0.5 mmol/L ammonia without acidifying the artificial colonic fluid. In vivo, compared with lactulose, W₁/O/W₂ emulsions could efficiently reduce the blood ammonia to the similar level in the rat models with HE without increasing the water content in feces. All these results indicated a potential application of W₁/O/W₂ multiple emulsions for the treatment of HE.     

Gel growth in free radical crosslinking copolymerization: Effect of inactive gel radicals

A kinetic model is presented for the post‐gelation period of free‐radical crosslinking copolymerization. The model takes into account the trapped radical centers in the gel forming system. It was shown that the weight fraction of sol, W_s, relates to the number of crosslinked units per weight‐average primary molecule, ε, through the equation where n = 2 for Flory's most probable molecular weight distribution, and n = 3 for primary molecules formed by radical combination. Calculation results demonstrate that the existence of trapped radicals significantly affects the growth rate of the gel molecule. It increases the total radical concentration and accelerates the gel growth. The difference in the predictions with and without considering the trapped radicals becomes significant as the crosslinker concentration decreases or, as the vinyl group reactivity on the crosslinker or on the polymer decreases.     

A study of the catalytic behavior of phosphotungstic acid-modified Janus particles in a heterogeneous oil/water pickering emulsion system

An emulsion interface materialization method was used to obtain amphiphilic silica Janus nanoparticles. Reducing the photosynthesis of aquatic organisms after water pollution. PW₁₂O₄₀³⁻ was introduced onto Janus particles by ion exchange, and an amphiphilic particle emulsion catalyst (PWO-J) was prepared. Hydrogen peroxide was used as the oxygen source, and the amphiphilicty of the catalyst was used to assemble the catalyst at the Pickering emulsion interface. The PWO-J catalyst was found to exhibit very high catalytic activity toward the oxidation of oleic acid in water-in-oil systems. The results showed that PWO-J catalysis of oxidation had similar results as CTAB and phosphotungstic acid (control system) under the same conditions. The azelaic acid recovery rate was 86.7%, and PWO-J could be reused 4 times. A reaction mechanism was proposed, and the constructed model was used to calculate a reaction rate constant of 15.32 × 10⁻⁵L•mol⁻¹•s⁻¹ for the PWO-J system. The PWO-J system had a lower activation energy than the control system, showing that the catalytic oxidation of oleic acid into azelaic acid was more likely to occur in the PWO-J system.     

Postgel properties in the statistical crosslinking of heterochains. I. Systems with N types of chains

The heterochain crosslinking model describes nonrandom crosslinking of polymer chains and is an extension of the classical Flory/Stockmayer gelation theory. We consider the postgelation relationship for the system consisting of N types of polymer chains, in which the probability that a crosslink point on an i‐type chain is connected to a j‐type chain is explicitly given by p_ij. The analytical solutions for the weight fraction of the sol, the number‐average and weight‐average molecular weights within the sol fraction, and the crosslinking density within the sol and gel fractions are derived for the systems, with each type of chain conforming to the Schulz–Zimm distribution. Illustrative calculations are shown for the systems consisting of two and three types of chains, and the obtained results agree with those from the Monte Carlo method. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2333–2341, 2000     

Methods for the preparation of doxycycline-loaded phb micro- and nano-spheres

Natural and synthetic biodegradable polymers have been investigated for controlled drug release. Poly(3-hydroxybutyrate) can be produced by bacteria and is remarkable for this application due to its excellent biocompatibility and biodegradability. The objective of this work was to study different drug-entrapment and emulsification methods for the obtaining of doxycycline-loaded PHB micro- and nano-spheres. The micro-/nano-particles were prepared by polymer precipitation via dialysis, simple emulsion (O/W) or multiple emulsion (W₁/O/W₂) applying solvent evaporation in the last two cases. This was carried out either by ultrasonication, dripping and/or high speed stirring. Different processing conditions were varied in order to evaluate their influence on morphology, size, and drug entrapment capabilities. The highest drug loading was obtained by single emulsion with high speed stirring. In the case of multiple emulsion, the combination of ultrasound with high speed stirring resulted in the most elevate process yield and drug loading capability.     

pH-responsive microgels containing hydrophilic crosslinking co-monomers: shell-exploding microgels through design

pH-responsive microgels are crosslinked polymer colloids that swell when the pH approaches the pK _a of the particles. They have potential application for injectable gels for tissue repair and drug delivery systems. This study focuses on the pH-triggered gelation behaviour of a series of poly (EA/MAA/X) microgels. EA and MAA are ethylacrylate and methacrylic acid. Here, we investigate the effect of crosslinking monomer type (X) on microgel properties. The crosslinking monomers used were poly (ethyleneglycol) dimethacrylate (PEGD), ethyleneglycol dimethacrylate (EGD) and butanediol diacrylate (BDD). The microgel containing PEGD (m-PEGD) is a new system. The microgel containing BDD (m-BDD) was used as a control system. The concentrated microgel dispersions formed physical gels when the pH was increased to 5.3?C6.7, and the polymer volume fractions (? _p ) were above about 0.05. Evidence from photon correlation spectroscopy (PCS) and dynamic rheology was presented for abrupt pH-triggered increases, and then decreases of the hydrodynamic diameters for m-PEGD and the microgel prepared using EGD (m-EGD). This appears to be tuneable through crosslinker structure. An unexpected gelation behaviour, which may involve a new gel state for microgels, was found for m-PEGD dispersions. Uniquely, those dispersions formed gels at pH values less than the microgel's pK _a . This behaviour was linked to an outer-shell electrostatic repulsive interaction. The data point to a phenomenon, whereby the m-PEGD shells appear to explode at pH values above 7.0. The control microgel prepared, using BDD (m-BDD), did not show any evidence of shell fragmentation at any pH. That microgel has potential as a model pH-responsive microgel system in that the properties measured by PCS and rheology agreed well. To probe that system in more detail, the rheological data for m-BDD was analysed using scaling theory. The variation of the storage modulus (G') with ? _p gave a scaling exponent of 2.0.     

Markovian approach to nonlinear polymer formation: Free-radical crosslinking copolymerization

A Markovian model is used to extend the Flory/Stockmayer gelation theory to nonequilibrium reaction systems, by taking free-radical crosslinking copolymerization of vinyl and divinyl monomer as an example. Free-radical polymerizations are kinetically controlled; therefore, each primary polymer molecule experiences a different history of crosslinked structure formation. By assuming that the primary chains with identical birth time conform to the same chain connection probabilities, the nonlinear structural development can be viewed as a system in which the primary chains formed at different birth times are combined into nonlinear polymers in accordance with the first-order Markov chain statistics. According to the present Markovian model, the weight-average chain length, P̄_w is given by a matrix formula, P̄_w = W _p( E — Q )⁻¹ l where W _p is the row vector that concerns the weight contribution of a primary chain, E is a unit matrix, Q is the transition matrix representing the chain connection statistics, and I is a column vector whose elements are all unity. For an equilibrium system, W _p = P̄_wp (weight-average chain length of the primary chains), E = 1, Q = ρP̄_wp (ρ is the crosslinking density), and I = 1; therefore, the present formula reduces to the Flory/Stockmayer equation, P̄_w = P̄_wp/(1 − ρP̄_wp). The criterion for the onset of gelation is simply stated as a point at which the largest eigenvalue of the transition matrix Q reaches unity, i.e., det( E − Q ) = 0. The present Markovian approach elucidates important characteristics of the kinetically controlled network formation, and provides greater insight into nonequilibrium gelling systems.     

Characterization of resulting network polymer precursors in stepwise crosslinking diepoxide/diamine polymerization

Our previous mechanistic discussion of network formation in chainwise crosslinking multiallyl polymerization was extended to stepwise crosslinking diepoxide/diamine polymerization, typically including bisphenol‐A diglycidyl ether (BADGE) and 4,4′‐diaminodiphenylmethane (DDM). In allyl polymerization a monomer chain transfer is an essential termination reaction, providing only oligomeric primary polymer chains. Therefore, crosslinking multiallyl polymerization could be in the category of a classical gelation theory. Thus, the gelation behavior was discussed by comparing the actual gel point with the theoretical one. Then the resulting network polymer precursors (NPPs) were characterized by size‐exclusion chromatography‐multiangle laser light scattering‐viscometry to clarify the stepwise crosslinking BADGE/DDM polymerization mechanism. Notably, the intrinsic viscosity ratio [η]_NPP/[η]_Linear tended to decrease with the progress of crosslinking and finally, it reached less than 0.2. This suggests that the structure of resulting NPP becomes dendritic at a conversion close to the gel point. These dendritic NPPs can collide with each other to form crosslinks between NPPs, eventually leading to gelation as a reflection of the high concentration of NPP. The dilution effect on gelation was marked in polar solvent; no gelation was observed at a dilution of 1/5. However, in nonpolar solvent the gelation was promoted by dilution; this is ascribed to enhanced crosslink formation between NPPs through hydrogen bonding due to abundant hydroxyl groups in the NPP generated by the polyaddition reaction. Finally, the subject of “Is cured epoxy resin inhomogeneous?” is briefly discussed. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Gel formation in cationic polymerization of divinyl ethers. II. 2,2-Bis{4-[(2-vinyloxy)ethoxy]phenyl}propane

Cationic polymerization of 2,2-bis{4-[(2-vinyloxy)ethoxy]phenyl}propane [CH₂CH O CH₂CH₂O C₆H₄ C(CH₃)₂ C₆H₄ OCH₂CH₂ O CHCH₂; 2], a divinyl ether with oxyethylene units adjacent to the polymerizable vinyl ether groups and a bulky central spacer, was investigated in CH₂Cl₂ at 0°C with the diphenyl phosphate [(C₆H₅O)₂P(O)OH]/zinc chloride (ZnCl₂) initiating system. The polymerization proceeded quantitatively and gave soluble polymers up to 85% monomer conversion. In the same fashion as the polymerization of 1,4-bis[2-vinyloxy(ethoxy)]benzene (CH₂CH O CH₂CH₂O C₆H₄ OCH₂CH₂ O CHCH₂; 1) that we already studied, the content of the unreacted pendant vinyl ether groups of the produced soluble polymers decreased with monomer conversion, and almost all the pendant vinyl ether groups were consumed in the soluble products prior to gelation. Alternatively, endo-type double bonds were gradually formed in the polymer main chains by chain transfer reactions and other side reactions as the polymerization proceeded. The polymerization behavior of isobutyl vinyl ether (3), a monofunctional vinyl ether, under the same conditions, showed that the endo-type olefins in the polymer backbones are of no polymerization ability with the growing active species involved in the present polymerization systems. These results indicate that the intermolecular crosslinking reactions occurred primarily by the pendant vinyl ether groups, and the final stage of crosslinking process leading to gelation also may occur by the small amount of the residual pendant vinyl ether groups (supposedly less than 2%). The formation of the soluble polymers that almost lack the unreacted pendant vinyl ether groups is most likely due to the frequent occurrence of intramolecular crosslinking reactions. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1931–1941, 1999     

Water-in-Oil-in-Water Double Emulsions as Protective Carriers for Sambucus nigra L. Coloring Systems

The use of natural colorants is needed to overcome consumer concerns regarding synthetic food colorants′ safety. However, natural pigments have, in general, poor stability against environmental stresses such as temperature, ionic strength, moisture, light, and pH, among others. In this work, water-in-oil-in-water (W₁/O/W₂) emulsions were used as protective carriers to improve color stability of a hydrophilic Sambucus nigra L. extract against pH changes. The chemical system comprised water and corn oil as the aqueous and oil phases, respectively, and polyglycerol polyricinoleate (PGPR), Tween 80, and gum Arabic as stabilizers. The primary emulsion was prepared using a W₁/O ratio of 40/60 (v/v). For the secondary emulsion, W₁/O/W₂, different (W₁/O)/W₂ ratios were tested with the 50/50 (v/v) formulation presenting the best stability, being selected as the coloring system to test in food matrices of different pH: natural yogurt (pH 4.65), rice drink (pH 6.01), cow milk (pH 6.47), and soy drink (pH 7.92). Compared to the direct use of the extract, the double emulsion solution gave rise to higher color stability with pH change and storage time, as corroborated by visual and statistical analysis.     

A synthetic hydrogel with thermoreversible gelation,III: an NMR study of the sol–gel transition

The sol–gel transition mechanism of a thermoreversible hydrogel composed of a copolymer comprising poly(N-isopropylacrylamide) and poly(ethylene glycol) (PNIPAAm–PEG) was studied by NMR. The ¹H– and ¹³C–NMR spectra measured on a PNIPAAm–PEG solution in 99.9% D₂O showed a remarkable line width broadening of the PNIPAAm block of more than that of the PEG block, during thermally induced hydrogel formation. This result suggested that the mobility of the PNIPAAm block is more restricted than that of the PEG block during gelation. A crosslinked polymer network formation was ascertained by a sudden reduction in the spin-lattice relaxation time (T₁) of the residual HDO proton during gelation. The temperature dependency of the T₁ values for the PNIPAAm and PEG blocks revealed that the microscopic condition of the PNIPAAm block in water was drastically changed during gelation, while that of the PEG block was unchanged. The experimental results from NMR supported the following gelation mechanism; that an aggregation of PNIPAAm blocks in the separate copolymers caused by hydrophobic interaction forms crosslinking points to give an infinite three-dimensional network structure. The hydrated PEG chains in the copolymers provide the network with a swelling property in water, and prevent the aggregation from causing a macroscopic phase separation.     

Highly porous poly(lactide‐co‐glycolide) microparticles for sustained tiotropium release

In this study, porous poly(lactide‐co‐glycolide) (PLGA) microparticles with low mass density and large particle size were developed for chronic obstructive pulmonary disease treatment using anticholinergic drug (tiotropium). The porous PLGA microparticles were prepared by the water‐in‐oil‐in‐water (W₁/O/W₂) multi‐emulsion method using PLGA polymer and ammonium bicarbonate (as a porogen). Herein, soluble starch was incorporated in porous PLGA microparticles for long‐term tiotropium release. In vitro drug release studies determined that the rapid release of tiotropium from porous PLGA microparticles was reduced because of the high viscosity of the incorporated starch. Tiotropium release from porous PLGA microparticles continued up to 3 days. Furthermore, the inhaled microparticles showed longer drug residence in in vivo lung epithelium. Copyright © 2013 John Wiley & Sons, Ltd.     

Study of the polymer concentration and polymer/crosslinker ratio effect on gelation time of a novel grafted polymer gel for water shutoff using a central composite design method

In this research article, a hydrogel was prepared by crosslinking of carboxymethyl cellulose‐g‐polyacrylamide copolymer aqueous solution with chromium(III) acetate for the purpose of a water shutoff job in the oil reservoir. The experiments were conducted to investigate the main effects of copolymer concentration and crosslinker/copolymer ratio on gelation time of the hydrogel system. Then the effects of these two factors and their interactions on the gelation time were determined by using a central composite design (CCD) of the response surface method. CCD was used to generate the quadratic mathematical model for the gelation time response as a function of copolymer concentration, crosslinker/copolymer ratio, and their interaction. Furthermore, the analysis of variance (ANOVA) was used to evaluate the quality of the quadratic model. The ANOVA result of the developed model showed that the model was highly significant. The result also showed that the crosslinker/polymer ratio had more effects on the gelation time than did the polymer concentration and their interaction. A response surface method provides an optimum gel formulation. Core flooding experiments reveal that a significant permeability reduction on the sand pack cores can be achieved at reservoir conditions, when it is treated with an optimum gel formulation. Hence, this gel system may be suitable in the water shutoff job required for enhanced oil recovery from the oil fields. Copyright © 2015 John Wiley & Sons, Ltd.     

Reversible crosslinking and decrosslinking of polymers containing alcohol moiety using an acyclic bifunctional vicinal triketone

In this article, we report reversible crosslinking and decrosslinking systems of commercially available alcoholic polymers, using a bifunctional vicinal triketone (bistriketone) as a crosslinking reagent. A small amount of the bistriketone (0.2 equiv of the tricarbonyl moiety relative to the OH group of the polymer) was added to a solution of poly(2‐hydroxyethyl methacrylate) (PHEMA) in dimethyl sulfoxide (DMSO). After 4 days, the networked polymer was obtained as a yellow‐colored gel in 97% yield. Conversely, the resulting gel could be decrosslinked by treatment with MeOH for 4 days to recover the original PHEMA in 94% yield as well as the bistriketone in 54% yield. The network formation and dissociation behaviors were investigated in detail by ¹H NMR and size exclusion chromatography experiments. Similarly to PHEMA, reversible crosslinking and decrosslinking of poly(vinyl alcohol) were achieved by addition of the bistriketone and by treatment of the resulting gel with H₂O/DMSO (1/9, v/v), respectively. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 921–928     

A NOVEL EOR POLYMER (Ⅱ)——INVESTIGATION ON IN-SITU GELATION OF SMRF SYSTEM IN BEREA CORE

In-situ gelation of aqueous sulfomethylated resorcinol formaldehyde (SMRF) system inBerea core has been investigated. Two sets of displacement experiments were conducted with thissystem (containing 5% NaCl, 0. 036% CaCl_2. 2H_2O). The brine permeabilities of the coreswere reduced significantly from about 600 to 0.1 md. The in-situ gelation in Berea core occurreda little bit earlier than gelation anticipated from bulk test in the experiments. The gel time waseasier to control at initial pH between 6 and 8. During injection of SMRF system, the apparentviscosity was less than 1 mPa·s at 41℃.     

Polymer coupling and theory of on-random crosslinking: an analytical solution

Gel formation is an important feature in free-radical polymer coupling. Due to the different possible combination reactivities of each polymer backbone radical, polymer chains are crosslinked in a non-random manner. Equations of the moments have been derived to predict the pregel molecular weight development and the crosslink density at gel point. This work provides an analytical solution for the differential equations. The model agrees with the Flory-Stockmayer gelation theory under the condition of random crosslinking. The magnitude of deviations from the classical theory for non-random crosslinking depends on the product of the radical termination reactivity ratios (r₁r₂), the ratio of the rate constants of backbone radical generation (k), the ratio of the weight-average chain lengths of primary polymers (y), and the polymer weight fractions (w₂).