Polymer networks of different cross-linking densities were prepared by copolymerisation of hydroxyethyl acrylate and ethylene
glycol dimethacrylate. The average molecular weight between cross-links as well as the polymer chain mobility were characterised
by means of dynamic–mechanical spectroscopy. Equilibrium sorption isotherms and the water uptake in immersion in liquid water
allowed the determination of the Flory–Huggins interaction parameter between water molecules and polymer chain segments, which
decreased with the water activity in the hydrogel and increased with the cross-linking density as a consequence of the hydrophobic
character of the cross-linking agent. Dynamic sorption and desorption experiments were used to determine the diffusion coefficient.
Received: 11 November 1999 Accepted: 28 July 2000 相似文献
In this paper, a novel composite hydrogel was prepared by the use of dialdehyde konjac glucomannan (DAK) as macromolecular cross-linking agent for chitosan (CS). This biocompatible material cross-links and gels in minutes. The structure and morphology were characterized by various analyses. The results indicate that the hydrogels formed through the Schiff-base reaction between the amino groups of CS chains and the aldehyde groups of DAK. The cross-link density (rho(x)) increases with the enhancement of DAK content in hydrogels, while equilibrium swelling ratio (SR) and the average molecular weight between cross-links (Mc) value decrease. Drug release was evaluated by varying the pH of the release medium, reversed dependence of release rate on the equilibrium SR of hydrogel indicated that drug release may be impeded by the association of drug with the polymer. Importantly, this process offers an entirely new window of materials preparation when compared with the traditional preparation of CS-based hydrogels with small molecules cross-linking agent. 相似文献
The degradation pathway of a cross-linked polyethyleneimine (c-PEI) suitable as a gene carrier was studied at different pH values by real-time 1H NMR and diffusion-weighted 1H NMR spectroscopy. The c-PEI was synthesized by cross-linking PEI segments with 2,4-pentanediol diacrylate (PDDA). The experimental results show that under basic, neutral and acidic conditions, the degradation of c-PEI occurs through hydrolysis of the ester moieties of the PDDA linkers. The degradation half-life of the polymer is 22, 48 and more than 720 h at pH 10.2, 7.4 and 4.6, respectively, showing that the degradation of c-PEI is highly pH sensitive. By using a modified version of diffusion-weighted 1H NMR experiment, the variation of the apparent molecular weight of c-PEI in the degradation process was monitored. Furthermore, a Monte Carlo simulation was applied to simulate the relation between the average molecular weight and the number of chain ends during the degradation process of a model system of c-PEI. By comparing the NMR results with those obtained from simulation, the mechanism of degradation under various pH conditions is discussed. The present work demonstrates that the combination of real-time 1H NMR, diffusion-weighted 1H NMR spectroscopy and Monte Carlo simulation is a useful strategy for characterizing the degradation process of degradable polymers. 相似文献
The ring-closing metathesis (RCM) mediated cross-linking of dendrimer 1 and it subunits was examined. Through the combined use of MALDI-MS-TOF, SEC, NMR, and X-ray crystallographic analysis it was shown that the cross-links are formed with the following order of preference: type d >/= type c > type b > type a. 相似文献
Nanoparticles have been prepared by dispersion polymerisation of n-butyl cyanoacrylate in acidified water, with and without the inclusion of insulin. The molecular weight of the polymerising material increases by a stepwise process, in which chains are initiated, terminated, and reinitiated, until an equilibrium molecular weight is reached. This equilibrium molecular weight is higher at lower dispersion pH. The reaction is complete within two hours. Insulin is capable of initiating polymerisation, but if introduced after all of the monomer has been incorporated into the growing nanoparticles it has no effect on polymer molecular weight. A drug loading of 72% was achieved in particles produced at 25 °C and pH 3.0, with insulin introduced one hour after monomer initiation. Particle degradation characteristics were assessed using solutions of esterase in phosphate buffered saline at pH 7.0, with butanol release monitored as a measure of polymer degradation. Insulin release was monitored under the same conditions. Both butanol production and insulin release showed a similar biphasic mechanism, indicating that the drug release rate is determined by polymer degradation characteristics. An initial burst release of both materials is associated with the degradation of surface species, and this is then followed by a steady-state release from sub-surface material.
Insulin release as a function of time at an esterase concentration of 2.0 mg · ml−1. 相似文献
Rubbers are long-chain molecules which are plastic in nature in the raw or unvulcanized state. Vulcanization is an irreversible process by which the predominantly plastic rubber is converted to predominantly elastic and a three-dimensional network structure through the anchoring between two polymer chains. Chemically, this is done by an intermolecular cross-linking reaction. Cross-linking increases the retractive force and reduces the amount of permanent deformation remaining after removal of the deforming force. These links between polymer chains may be chains of sulfur atom or atoms, carbon-carbon bonds, polyvalent metal ions, etc., depending on the nature of the vulcanizing system. Properties of elastomers depend on how efficiently this cross-linking has been achieved and also which types of cross-linking agents are used. For example, the retractive force resisting a deformation is proportional to the number of network supporting polymer chains per unit volume of elastomers [1]. Again the strength of elastomers depends on the stress relaxation mechanism in the cross-linked material [2]. The modulus of a vulcanizate is proportional to the number of cross-links formed, while the tensile strength normally passes through a maximum with an increase in the number of cross-links. The resilience, heat build-up, and fatigue properties depend on the chemical nature of the cross-links and on the chemical structure of the base polymer [3]. Different types of cross-links have both advantages and disadvantages with respect to technical properties. To cite one example, a sulfur cross-linking system produces good tensile strength but poor aging properties. On the other hand, carbon-carbon cross-links produce good aging properties but poor tensile properties. Thus there are reasons for using a mixed cross-link system in order to obtain the right compromise. 相似文献
Many of the attractive properties in polymers are a consequence of their high molecular weight and therefore, scission of chains due to mechanochemistry leads to deterioration in properties and performance. Intramolecular cross-links are systematically added to linear chains, slowing down mechanochemical degradation to the point where the chains become virtually invincible to shear in solution. Our approach mimics the immunoglobulin-like domains of Titin, whose structure directs mechanical force towards the scission of sacrificial intramolecular hydrogen bonds, absorbing mechanical energy while unfolding. The kinetics of the mechanochemical reactions supports this hypothesis, as the polymer properties are maintained while high rates of mechanochemistry are observed. Our results demonstrate that polymers with intramolecular cross-links can be used to make solutions which, even under severe shear, maintain key properties such as viscosity. 相似文献
The topology of the GCAP-2 homodimer was investigated by chemical cross-linking and high resolution mass spectrometry. Complementary conducted size-exclusion chromatography and analytical ultracentrifugation studies indicated that GCAP-2 forms a homodimer both in the absence and in the presence of Ca2+. In-depth MS and MS/MS analysis of the cross-linked products was aided by 15? N-labeled GCAP-2. The use of isotope-labeled protein delivered reliable structural information on the GCAP-2 homodimer, enabling an unambiguous discrimination between cross-links within one monomer (intramolecular) or between two subunits (intermolecular). The limited number of cross-links obtained in the Ca2+-bound state allowed us to deduce a defined homodimeric GCAP-2 structure by a docking and molecular dynamics approach. In the Ca2+-free state, GCAP-2 is more flexible as indicated by the higher number of cross-links. We consider stable isotope-labeling to be indispensable for deriving reliable structural information from chemical cross-linking data of multi-subunit protein assemblies.
Highly branched polyethylene (HBPE) was developed into practical application as highly efficient sorbent material for oil-spill cleanup and oil/water separation. To obtain large-scale production of HBPE, a thermal stable Ni(II)-α-diimine catalyst for ethylene polymerization in our previous work has been employed to prepare the polyolefin material in a 10-liter polymerization reactor with a high activity (>106 gPE/mol[Ni] h). The structure, molecular weight and distribution, thermal and mechanical properties were systematically characterized by NMR, ATR-FTIR, GPC, DSC, DMA, and TGA, respectively. Through simple but feasible cross-linking process, the HBPE-based oil-absorption materials with contact angles up to 111.5° were directly applied into absorption test using various oil and pure hydrocarbons. Reusability and recovery of the absorption materials and oil or solvents were probed by drying or using the distillation method. Oil/water separation was made to determine the hydrophobic and oleophilic nature of this material. Flory-Rehner polymer swelling theory is employed to study the structure–property relationship via determining the network structure including cross-linking density ρc and average molecular weight Mc between two cross-links. The mixture of sorbent material after absorbing oil was regarded as crude oil component to simulate oil refinery process by thermogravimetric analysis, providing an alternative approach for oil-collection. 相似文献
Poly(1,4-butadiene) networks obtained by a 4-functional random cross-linking reaction over a broad range of polymer concentration were studied by small angle neutron scattering(SANS), 2H NMR and Monte Carlo(MC) simulation in the isotropic and uniaxially deformed state. The defect structure of the networks has been characterized by MC simulation of the cross-linking reaction. The anisotropy of the radius of gyration in deformed networks determined from SANS has been analyzed by the theory of Ullman. It was found that the number of active cross-links per chain is in agreement with MC and that the chain deformation follows phantom behaviour. The local orientation as measured by 2H NMR is related to the global anisotropy of the network by a MC calculation of oriented chains. The 2H NMR line shape of the deformed network is analyzed in terms of two relaxation processes arising from interior parts of the chains and from segments at chain ends. The mobility of both decrease with strain. It was found that the orientation connected to the first process shows the classical strain dependence of rubber elasticity, whereas the second exhibits a weaker dependence on strain. 相似文献
Gelation mechanisms of lithium-doped Siloxane-Poly(oxyethylene) (PEO) hybrids containing polymer of two different molecular weight (500 and 1900 g/mol) were investigated through the evolution of the electrical properties during the solgel transition. The results of electrical measurements, performed by in-situ complex impedance spectroscopy, were correlated with the coordination and the dynamical properties of the lithium ions during the process as shown by 7Li NMR measurements. For both hybrids sols, a decrease of the conductivity is observed at the initial gelation stage, due to the existence of an inverted percolation process consisting of the progressive separation of solvent molecules containing conducting species in isolated islands during the solid network formation. An increase of conductivity occurs at more advanced stages of gelation and aging, attributed to the increasing connectivity between PEO chains promoted by the formation of crosslinks of siloxane particles at their extremities, favoring hopping motions of lithium ions along the chains. 相似文献
Poly(propylene carbonate)/poly(β-hydroxybutyrate-co-β-hydroxyvalerate) (PPC/PHBV) blends were prepared via the solution casting method at different proportions. Their thermal characteristics were studied by means of differential scanning calorimetry (DSC) and thermogravimetry (TG). The degradability of the blends was investigated in soil suspension cultivation and in vitro degradation testing. The changes of structure and molecular weight for blends were also studied by 1H nuclear magnetic resonance spectroscopy (1H NMR), scanning electron microscopy (SEM) and gel permeation chromatography (GPC) before and after degradation. Although the PPC/PHBV blends were immiscible, the addition of PHBV could improve the thermal stability of PPC. PHBV was degraded mainly by the action of microbial enzymes in the soil suspension, which biodegraded it more rapidly than PPC in a natural environment. PPC was degraded mainly by chemical hydrolysis and random hydrolytic scission of chains in the PBS solution in vitro, and degradation of PPC was more rapid than that of PHBV in a simulated physiological environment. 相似文献
The biodegradable poly(5-methyl-5-methoxycarbonyl-1,3-dioxan-2-one-co-d,l-lactide) [poly(MMTC-co-d,l-LA)] copolymers were synthesized by the ring-opening copolymerization. The results show that the yield and molecular weight of copolymers are significantly influenced by reaction conditions. The chemical structure of the resultant copolymers was characterized by FTIR, 1H NMR and 13C NMR methods. Their molecular weight was measured by gel permeation chromatography (GPC). Study of monomer coreactivity ratios indicates that d,l-LA reacts faster than MMTC in the copolymerization. The enzymatic degradation of the polymers with various compositions was studied at 37 °C in pH = 8.6 Tris-HCl buffer solution in the presence of proteinase K. Their mechanical properties were also preliminarily investigated. 相似文献
Summary: Poly(alkylene hydrogen phosphonate)s with a number‐average molecular weight of about 3 000 Da were obtained by a transesterification of dimethyl hydrogen phosphonate with poly(ethylene glycol) (PEG 400) under microwave irradiation with a very short reaction time (55 min) relative to that of classical thermal heating (9 h). The structure of the resulting polymer was confirmed by 1H, 31P, and 13C NMR spectroscopy. The molecular weight was determined by 1H, 31P{H} NMR spectroscopy, MALDI‐TOF, and GPC.
The transesterification of dimethyl hydrogen phosphonate with poly(ethylene glycol). 相似文献
Abstract As a transformer ages, the chemical and physical properties of the cellulose insulation materials in the transformer change, and the paper loses its strength and becomes brittle. The average molecular weight of the cellulose chains decrease with age, and degradation products are formed, including water, carbon monoxide, carbon dioxide, and furans. The molecular weight changes in the cellulose have been studied by several methods, but the GPC method for determination of the molecular weight has been shown to offer advantages over other methods because it yields the total molecular weight distribution. The tensile strength of the cellulose insulation in transformers also changes with age as a result of the changing molecular weight of the cellulose. Work carried out in our laboratory on the aging of cellulose insulation is reviewed in this paper. Our studies have included investigations of insulation materials from retired transformers as well as accelerated aging of insulation paper in transformer oil in the temperature range of 129–166°C under vacuum. In the study the relationships between the molecular weight of the cellulose and the furan degradation products and tensile strength have been delineated, and they have been correlated with information on the kinetics of degradation of the insulation paper. 相似文献