Radiation induced crosslinking of poly(vinyl methylether) (PVME) has been investigated in aqueous solutions. The spectral and kinetic features of the transients involved in the crosslinking reaction have been studied by pulse radiolysis of dilute PVME solutions. H atoms reacts with PVME, like OH radicals, by abstracting an H atom predominantly from β-position with respect to ---OCH3 group, but the rate of reaction of H atom is an order of magnitude slower than that of OH reaction. The PVME radicals formed by H attack have been found to decay by usual 2nd-order kinetics unlike PVME radicals produced by OH attack that are reported to decay by a complex time-dependent kinetics that deviates strongly from 2nd-order kinetics. The rate constant of e−aq with PVME at pH 5.5 has been found to be 1.2×108 dm3 mol−1 s−1. From the decay behaviour of the transient species formed by reaction of e−aq with PVME, it has been shown that the transient initially reacts with solvent protons by a fast reaction to yield radical species which subsequently recombine by a slow mode. The dependence of gelation dose and radiation yields of crosslinking (Gx) of PVME on various factors such as polymer concentration, dose rate, pH, presence of oxygen and crosslinking agent has also been studied by steady-state radiolysis using an electron-beam accelerator. 相似文献
The present paper describes the formation of hydrogels by reaction of gelatins with periodate oxidized dextrans. The effect of a number of reaction parameters on the rate of gelation is investigated. It is demonstrated that the time to onset of gelation depends on the nature of the gelatin, the nature of the dextran dialdehyde, the type of buffer, the ionic strength and the presence of added salts. The gelation time decreases with increasing concentration of aldehydes, increasing pH and increasing molecular weight of the dextran dialdehyde. In general, the gelation occurs more rapidly with decreasing concentration of indifferent electrolytes, except for the reaction in phosphate buffer where an increase in gelation is observed with increasing buffer strength. Ammonium and calcium salts are found to retard significantly the crosslinking reaction. 相似文献
New hydrogels based on polyethylene glycol (PEG) and poly(vinyl alcohol) (PVA) of different degrees of hydrolysis were synthesized. To form the network the PEG was modified at their ends with acyl chloride groups to be used as the crosslinking agent. The compositions of the hydrogels were between 50% and 90% by weight of PEG and PVA of various degrees of hydrolysis were used. It was found that the degree of hydrolysis of the PVA and the PEG content influence the equilibrium water content of the hydrogel. The process of swelling of all the hydrogels prepared followed a second-order kinetics. 相似文献
We prepared thermoresponsive and microporous polymer hydrogels by γ-ray irradiation of aqueous solutions poly(vinyl methyl ether) (PVME) at different heating rates. Under all temperature programs, opaque and heterogeneous PVME gels formed, which swelled at temperatures below the lower critical solution temperature and shrank at temperatures above it. All of the samples contained porous and phase-separated structures. The shape and size of the gel pores varied depending on the temperature programs. Gels having a sponge-like continuous porous structure formed only when the radiation-induced crosslinking was carried out at an optimum heating rate, which we found to be 0.11–0.13°C min−1. For temperature changes between 10°C and 40°C, gels with this structure showed rapid volume transitions on a time scale of about a minute. 相似文献
A series of poly(vinyl alcohol)/chitosan (PVA/CTS) hydrogel thin films were prepared via ultraviolet (UV) irradiation, with acrylic acid (AA) monomer added as a crosslinker without the addition of any other photo-initiator. The swelling behaviors, intermolecular chemical bonds, molecular structures, thermal behaviors, degrees of crystallinity, morphologies of the surfaces and internal structure, and their relationship to the AA content were characterized using Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). Poly(acrylic acid) (PAA) and its chemical crosslinks formed in hydrogel films via free-radical reactions were confirmed using FTIR and DSC analyses. The XRD patterns indicated that the degree of crystallinity of the hydrogel films decreased as the PAA content was increased. SEM micrographs showed that a uniform interconnected pore structure was formed through the entire hydrogel structure, and a gradient in the crosslinking density through the film thickness was observed to result from extended irradiation times. The swelling behaviors revealed that the formation of PAA and its crosslinking in the hydrogel thin films improved the pH stability and controlled the degree of swelling while retaining a high swelling rate. The successful formation of chemical crosslinking without any specific photo-initiator improves the natural characteristics of CTS and PVA and imparts the resulting PVA/CTS hydrogel thin films with properties that make them very promising in biomedical applications. 相似文献
Two new dinuclear oxo-bridged peroxo complexes of tungsten with coordinated dipeptides of the type, Na2[W2O3(O2)4(glycyl-glycine)2] · 3H2O (1) and Na2[W2O3(O2)4(glycyl-leucine)2] · 3H2O (2) have been synthesized from the reaction of H2WO4, 30% H2O2 and the respective dipeptide at pH ca. 2.5. Synthesis of the compounds, in addition to pH, is sensitive to reaction temperature and concentrations of the components. The compounds were characterized by elemental analysis, spectral and physico-chemical methods including thermal analysis. In the dimeric complexes the two W(VI) centres with edge bound peroxo groups are bridged by an oxo group. The dipeptides occurring as zwitterions bind the metal centers through O (carboxylate) atoms leading to hepta co-ordination around each W(VI). Thermal stability of the compounds as well as their stability in solution were determined. The compounds are highly stable toward decomposition in solutions of acidic as well as physiological pH. These compounds, besides another similar dimeric compound Na2[W2O3(O2)4(cystine)] · 4H2O (3) efficiently oxidized bromide to a bromination competent intermediate in phosphate buffer at physiological pH, a reaction in which only two of the peroxide groups of the complex species were found to be active. The complexes could also mediate bromination of organic substrate in aqueous-organic media. 相似文献
Hydrogel precursors that crosslink within minutes are essential for the development of cell encapsulation matrices and their implementation in automated systems. Such timescales allow sufficient mixing of cells and hydrogel precursors under low shear forces and the achievement of homogeneous networks and cell distributions in the 3D cell culture. The previous work showed that the thiol-tetrazole methylsulfone (TzMS) reaction crosslinks star-poly(ethylene glycol) (PEG) hydrogels within minutes at around physiological pH and can be accelerated or slowed down with small pH changes. The resulting hydrogels are cytocompatible and stable in cell culture conditions. Here, the gelation kinetics and mechanical properties of PEG-based hydrogels formed by thiol-TzMS crosslinking as a function of buffer, crosslinker structure and degree of TzMS functionality are reported. Crosslinkers of different architecture, length and chemical nature (PEG versus peptide) are tested, and degree of TzMS functionality is modified by inclusion of RGD cell-adhesive ligand, all at concentration ranges typically used in cell culture. These studies corroborate that thiol/PEG-4TzMS hydrogels show gelation times and stiffnesses that are suitable for 3D cell encapsulation and tunable through changes in hydrogel composition. The results of this study guide formulation of encapsulating hydrogels for manual and automated 3D cell culture. 相似文献
N‐Vinylformamide (NVF) free‐radical polymerization was found to form polymer gels at high conversions both in bulk and in solution. The polymerization was conducted at different temperatures, monomer and initiator concentrations to show the gelation conditions. Gel fractions and gel swelling ratios were also measured after separating the gel from the polymer samples. In order to confirm the crosslinking unit, a series of hydrolysis experiments were conducted on the gel samples. The hydrolysis results showed that the crosslinks in PNVF gels could be destroyed by alkaline hydrolysis. The most appropriate explanation to this fact is that crosslinking takes place via the amide group. 相似文献
The lanthanum complex of a hexa-aza macrocycle is shown to be an effective catalyst for the hydrolysis of the water soluble phosphate triester 2,4-dinitrophenyl diethyl phosphate. At pH 9, using a catalyst concentration of 2.5 × 10−3 mol dm−3 at 25°C, the rate enhancement is ca 103 fold. The reaction is shown to be catalytic rather than stoichiometric and possible mechanisms involving a metal-bound hydroxide nucleophile are considered to account for the catalysis. The macrocyclic complex is alswn to be an effective catalyst for the hydrolysis of the anticholinesterase agent O-isopropyl methylfluorophosphate (Sarin). 相似文献
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. 相似文献
The gelation of buffered aqueous Cr+3/polyacrylamide (PAAm) solutions has been studied over the pH range 2-11. With Cr+3 supplied as Cr(NO3)3 and a 7.5% hydrolyzed PAAm the upper limit for gelation was about pH 7, but Cr(acetate)3/PAAm solutions gelled as high as pH 9. Evidence is presented that the formation of unreactive colloidal Cr(OH)3(H2O)3, rather than polymer crosslinking, is favored thermodynamically for Cr+3/PAAm solutions at pH > 7. The crosslinks formed in Cr(acetate)3/PAAm solutions at pH 7–9 are deduced to be kinetically controlled products. Independent evidence for the kinetic stability of Cr+3/PAAm gels above pH 7 has also been obtained. Other observations relevant to the mechanism of crosslinking of PAAm with Cr(acetate)3 are described. 相似文献
A new alcohol dehydration membrane, poly(vinyl alcohol)—chitosan blended composite membrane (PVA-CS) has been prepared. This membrane has high selectivity and promising permeability, especially in separating ethanol—water near the azeotropic region (Jt > 200 g/m2 h, w/e > 500, 70°C). The separating characteristics, which vary with feed composition, operating temperature and the surface structure of the membrane, are determined and the results agree well with the theoretical predictions.
The characteristics of mechanical strength, stability and resistance to water were also determined. The results show that they are considerably enhanced by blending and crosslinking, in comparison with PVA composite membranes. 相似文献
Summary: To mathematically describe crosslinking kinetics for polymers, we have proposed a novel method that accounts for the number of crosslinks, that is, partitioning according to number of crosslinks (PANC). By contrast, the well‐known method of numerical fractionation tracks generations of crosslinked molecules, defined to include a range of crosslinks. The proposed crosslinking kinetics yield a population balance model that provides moments and hence measurable average properties such as number average molecular weight and weight average molecular weight, polydispersity and average crosslink number. The gel points for batch and continuous‐flow stirred‐tank reactors are derived. Because the usual closure methods do not yield satisfactory convergence, new representations for post‐gelation moments are proposed. The results realistically show how the moments change with time in the post‐gel region.
The average number of crosslinks in the bulk and sol versus time in a batch reactor; the gel point is the dashed line. 相似文献
In a previous paper a theory was presented concerning gels formed by polyfunctional crosslinks in a polydisperse primary polymer. The equilibrium shear modulus as well as the crosslinking index were obtained as functions of the functionality of the crosslinks and of the sol fraction. In this paper the theory is applied to gelatin gels (known functionality of crosslinks) and to poly(vinyl chloride) gels (known number of crosslinks). 相似文献
A new bulk acoustic wave (BAW) cellulase sensing technique, which is based on the enzymatic hydrolysis process of sodium carboxymethylcellulose (CMC) by cellulase, was established. The frequency shift curves of BAW sensor indicated that the viscosity of the tested solutions decreased during the hydrolysis process. The hydrolysis rate of CMC by cellulase was calculated from the frequency shift curves. The hydrolysis rate of CMC under different pH conditions at 30°C showed that cellulase had high hydrolysis ability approximately at pH 5.0. Kinetic parameters (the Michaelis constant Km and the maximum rate Vmax) of the process were estimated by using a linear method of Lineweaver–Burk plot. Km is 1.95±0.25 mg ml−1 and Vmax is −(4.25±0.58)×10−3 g1/2 cm−3/2 cP1/2 min−1. Also the activation energy (Ea) of the enzymatic hydrolysis, with a value of 51.99±1.26 kJ mol−1, was estimated in this work. 相似文献
The effects of catalysts, pH and reaction conditions on the course of the hydrolysis and condensation of ETS40 (ethyl silicate 40), and on the composition of the reaction products were studied with the aid of gas and gel chromatography, potentiometry and gelation tests. Strong acids (HCl, HClO4, HNO3, H2SO4, p-toluenesulphonic acid), weak acids (Cl3, CCOOH, ClCH2COOH, (COOH)2, CH3COOH and HCOOH) and bases (LiOH, NH4,OH) were used as catalysts.
The hydrolysis rate increased with increasing temperature, catalyst concentration, initial water concentration and initial ethyl silicate concentration, whereas it decreased with increasing number of Si atoms in the ethyl silicate molecules. At pH 0–7 the hydrolysis was acid catalysed, but at pH above 7.0 it was base catalysed. Simultaneously with the hydrolysis, condensation occurred at a rate which increased with increasing temperature, catalyst concentration, ETS40 concentration and, above all, with increasing initial water concentration. The condensation rate depended on the pH. The condensation was at its slowest for pH around 2.0. For pH below 2.0, the condensation increased with increasing hydrogen ion concentration; for pH above 2.0 the condensation increased with decreasing hydrogen ion concentration. Phosphoric acid and hydrofluoric acid increased the rate of condensation considerably. The reaction of ETS40 with water at pH around 2.0 gave rise during the hydrolysis to solutions of ethoxyhydroxysiloxanes with an average of 14–20 Si atoms in a molecule, which displayed long-term stability. 相似文献