Synthesis and characterization of PVA ferrogels obtained through a one-pot freezing�Cthawing procedure

Polyvinyl alcohol (PVA) ferrogels were easily obtained through a one-pot technique that involves co-precipitation of iron salts in the presence of a PVA solution, followed by freezing?Cthawing cycles of the resulting nanoparticles (NPs) dispersions. The protecting effect of PVA enabled the synthesis of small magnetic NPs that did not agglomerate in the initial solution allowing the synthesis of well-dispersed ferrogels by physical cross-linking. Physical properties of the physically cross-linked ferrogels, as swelling ability, melting temperature, and crystallinity, were barely affected by the presence of NPs, presenting similar or improved values when compared with chemically cross-linked systems. Ferrogels showed superparamagnetic properties at room temperature that combined with the absence of toxic residues arising from cross-linking agents make them ideal candidates for their use in biomedical applications (artificial muscles, drug delivery, and sensors among others).     

Gelation of cassia gum by freezing and thawing

Aqueous solution of cassia gum (CG), which is categorized as a galactomannan polysaccharide having mannose/galactose ratio = 5/1, forms hydrogels by freezing and thawing. When frozen CG aqueous solution was thawed, transparent sol was separated from a turbid gel, i.e. syneresis occurred. Gel concentration ({(Mass of dry gel) / (Mass of gel)} × 100) increased with increasing CG concentration. Viscoelastic properties of CG hydrogels formed by freezing and thawing were investigated by thermomechanical analysis (TMA) in water using an oscillation mode at 0.05 Hz. Dynamic modulus (E′) increased from 3 kPa to ca. 5 kPa with increasing freezing rate. In contrast, E′ maintained a constant value regardless of repeating number of freezing and thawing. From TMA results, it is concluded that the density of cross-linking network structure depends on the size of ice formed by freezing. At the same time, the low E′ value of CG gels is ascribed to the fact that association of galactosyl side group is disturbed by the stiff chain attributed to the unsubstituted region of CG.     

PVA微晶交联和SA/PAA双网络协效增强增韧SA纤维

为了提高海藻酸钠(SA)纤维的断裂强度和断裂伸长率,以丙烯酸(AA)为化学交联组分,SA为离子交联组分,聚乙烯醇(PVA)为微晶交联组分,采用湿法纺丝和冻融循环方法制备含有PVA微晶交联点和海藻酸钠/聚丙烯酸(SA/PAA)双网络结构的海藻酸钠/聚丙烯酸/聚乙烯醇(SA/PAA/PVA)复合纤维。通过流变性能、力学性能、红外光谱、X射线衍射仪(XRD)和扫描电子显微镜(SEM)测试研究了交联剂N,N-亚甲基双丙烯酰胺(MBA)含量和PVA微晶交联对SA/PAA/PVA纺丝原液和复合纤维的结构与性能的影响。结果表明,当MBA质量分数为0. 5%时,纺丝原液的损耗模量(G″)最小,可纺性最好,复合纤维的断裂强度达到2. 83 cN/dtex,断裂伸长率达到9. 38%,比再生SA纤维分别提高了15. 98%和38. 96%; PVA冷冻之后形成微晶交联点并且PAA和PVA已经复合到体系中; PAA和PVA的加入提高了复合纤维的结晶度;复合纤维的表面形貌趋于光滑和规整,纤维断面更加致密。     

Microstructure and mechanical properties of bacterial cellulose/chitosan porous scaffold

A family of polysaccharide based scaffold materials, bacterial cellulose/chitosan (BC/CTS) porous scaffolds with various weight ratios (from 20/80 to 60/40 w/w%) were prepared by freezing (−30 and −80 °C) and lyophilization of a mixture of microfibrillated BC suspension and chitosan solution. The microfibrillated BC (MFC) was subjected to 2,2,6,6-tetramethylpyperidine-1-oxyl radical (TEMPO)-mediated oxidation to introduce surface carboxyl groups before mixing. The integration of MFC within chitosan matrix was performed by 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide hydrochloride (EDC)-mediated cross-linking. The covalent amide bond formation was determined by ATR-FTIR. Because of this covalent coupling, the scaffolds retain their original shapes during autoclave sterilization. The composite scaffolds are three-dimensional open pore microstructure with pore size ranging from 120 to 280 μm. The freezing temperature and mean pore size take less effect on scaffold mechanical properties. The compressive modulus and strength increased with increase in MFC content. The results show that the scaffolds of higher MFC content contribute to overall better mechanical properties.     

Improvement of the surface of polyethylene foamed sheet by irradiation

The manufacturing methods of cross-linked polyethylene foams are classified into two categories based on a type of cross-linking. One is chemical cross-linking by using peroxide as a cross-linking agent. The other method is cross-linking by irradiation. As for chemical cross-linking, a fairly thick foam sheet can be produced, and a comparatively high degree of cross-linking can be achieved. This means chemical cross-linking excels in thermo-forming but, due to a rough surface, the product is lacking in adhesive property and printability. We studied how to improve the surface condition of foam sheet without damaging the features proceeding from chemical cross-linking. As a result, it has been revealed that at the pre-stage of foaming, and by irradiating the surface at low voltage, the resultant foamed sheet with smooth surfaces and excelling in mechanical properties can be produced.     

NMR characterization of styrene-divinylbenzene gel beads using freezing point depression of benzene

Freezing processes of benzene in the presence of styrene-divinylbenzene gel beads were investigated by proton nuclear magnetic resonance spectroscopy. Some portion of benzene in the pore did not freeze below its freezing point, which was detected until ?80°C. Temperature dependence of the amount of unfrozen benzene was discussed in relation to the pore size and the cross-linking density of gel beads. The small pore size and high crosslinking density increased the amount of unfrozen benzene, whereas the polymer matrix concentration showed little effect on the freezing processes. © 1995 John Wiley & Sons, Inc.     

可控交联聚醚醚酮的动态力学性能

动态力学分析技术(DMA)是研究聚合物性能的重要方法之一．动态力学实验可以检测聚合物的玻璃化转变温度和次级松弛过程,直接与聚合物的链结构和聚集态结构密切相关,聚合物的化学组成、支化和交联、结晶和取向、增塑和共混等结构因素的变化,都与分子运动状态的变化密切相关,而分子运动的变化又能灵敏地反映在动态力学性能上,     

UV-Induced Nucleic Acid–Protein Cross-Linking: Manual on Planning of Irradiation Experiments and Calculation of Absorbed Dose and Quantum Yield

We have developed methods of photochemical quantitation of photobiological studies on UV-induced nucleic acid–protein cross-linking. Cases relating to incoherent low-intensity UV sources, laser UV sources and high-intensity laser UV sources are considered. In the case of low-intensity UV radiation the most important point is the correct determination of absorbed dose. The laser UV pulse energy is easily measured and the short-pulse irradiation also has the advantage of "freezing" the conformation of complexes under study. However, the use of high-intensity laser UV irradiation leads to realizations of two-quantum processes both in nucleic acid chromophores–bases and in solvent–water, which complicates singificantly the processing of results. In this paper methods for calculating the absorbed dose and the quantum yield of cross-linking for all above-mentioned cases are given as well as practical advice.     

Study of cryostructurization of polymer systems

The influence of freezing on reaction systems consisting of a polymer and suitable cross-linking agent in organic media has been studied. It was shown that the cryostructurization in organic medium displays mainly the same regularities as the processes of gelation at the freezing of aqueous polymer solutions in the presence of cross-agent. The study of two reaction systems was carried out: A — alternative N-viniylpyrrolidone — maleic anhydride and 4,4-diaminodiphenyloxide in dimethylsulphoxide; B — polystyrene and p-xylylene dichloride in the presence of SnCl₄ in nitrobenzene medium.     

可控交联聚醚醚酮动态流变学行为的研究

聚芳醚酮类材料因其优异的综合性能在许多领域得到广泛应用 [1,2 ] .许多研究者通过提高聚芳醚酮分子链的刚性度来实现进一步提高其使用温度 ,但由于其在高温时流动性下降 ,熔体粘度增大 ,给加工及应用带来很大困难[3] .基于此 ,我们将可在高温或辐照条件下发生交联反应的硫醚结构作为交联点引入到聚醚醚酮主链中 ,合成了可控交联的聚醚醚酮[4 ,5] .聚合物的分子结构及其熔体中分子的内部作用可以用流变学进行研究 .因此 ,我们用动态流变学实验监测跟踪聚合物的交联反应过程 ,研究可控交联聚醚醚酮的交联反应动力学 ,为设计改造分子结构以满…     

Cross-linking of functionalised siloxanes with alumatrane: Reaction mechanisms and kinetics

Cross-linking of functionalised polysiloxanes is an important tool to adjust their viscoelastic properties and can be achieved by the reaction with alumatrane. The cross-linking reaction has been found to proceed only with hydrolyzed alumatrane species. Siloxane model compounds with different functional groups such as alkoxy, siloxy, and hydroxy groups were considered in order to optimise the rheological properties of the polymer. The activation energy barriers of the related reactions were analysed using the density functional theory under the assumption of the presence of Al-OH groups formed by the hydrolysis of alumatrane. The cross-linking involving hydroxy groups of siloxane and hydrolyzed alumatrane has been found to have the lowest activation energy (−14 kJ/mol). As the reaction of the Si-O-Si-polymer backbone with the hydroxy groups of the hydrolyzed alumatrane turned out to have the very low activation energy of +2 kJ/mol, this type of reaction is predicted to play a key role for the cross-linking of polysiloxanes with hydrolyzed alumatrane. The involved water molecules are formed back in the course of subsequent polycondensation reactions, therefore H₂O can be considered as a cross-linking catalyst in these systems.     

Colloid probe AFM investigation of the influence of cross-linking on the interaction behavior and nano-rheology of colloidal droplets

The repulsive forces between a glass sphere and immobilized colloidal droplets of poly(dimethylsiloxane) (PDMS) (with various levels of internal cross-linking) have been determined in aqueous solution using colloid probe atomic force microscopy. On initial surface approach, droplet deformation is negligible and interaction forces resemble those expected for electrical double layer interaction of rigid spheres. Upon further approach, droplet flattening results in forces that deviate below rigid body electrical double layer interaction. The extent of droplet deformation has been determined in terms of the deviation from hard-sphere interaction. Droplet deformability is strongly dependent on the droplet cross-linking level and hence controlled by some combination of the bulk rheological and interfacial properties of the droplets. Droplet nano-rheology has been determined from the extent of force curve hysteresis. For liquidlike droplets, with low levels of cross-linking, no force curve hysteresis is observed and the elastic deformation may be described by a single spring constant, which is controlled by the interfacial properties. For highly cross-linked droplets, the extent of deformation is controlled by the droplet's bulk rheology rather than the interfacial properties. Upon retraction of the surfaces, force curve hysteresis is observed and is due to the viscoelastic response of the PDMS. The extent of hysteresis is dependent on the rate of approach/retraction and the loading force and has been theoretically analyzed to determine nano-rheological parameters that describe droplet relaxation processes. Elastic moduli and relaxation times of the PDMS droplets vary over several orders of magnitude as a function of cross-linking.     

Calorimetric analysis of the cross-linking reaction of epoxidized polybutadienes

Liquid polybutadienes were epoxidized with hydrogen peroxide-acetic acid confirming the influence of the microstructure on the epoxy content. Thermal cross-linking of the epoxidized products was studied by DSC and a different behaviour due to epoxy content as well as to microstructure was found. A correlation between enthalpy/epoxy ratio and vinyl content was also observed. The addition of a radical initiator as dicumyl peroxide (DCP) yielded exothermal effects of cross-linking in the original polybutadienes, confirming the higher reactivity of the vinyl groups. In the epoxidized products, the introduction of DCP gave a peak split, which was explained by the overlap of two cross-linking mechanisms, due to residual double bonds and epoxy groups. From the DSC data, the isothermal curves of cross-linking conversion were calculated as a function of time, for a better comparison of the performances and properties of the epoxidized polybutadienes.     

Cross linking and rheological characterization of adsorbed protein layers at the oil-water interface

The dilatational rheological properties of cross-linked protein layers adsorbed at the oil-water interface were investigated with help of a modified drop tensiometer allowing successive replacements of the external phase. This setup enables one to perform cross-linking reactions at the interface only, that is, without any contact between the cross-linking agent and protein molecules in solution, under continuous monitoring of the interfacial tension. The mechanical properties of the resulting interface were investigated with dilatational large strain experiments. Measured rheological properties were related to the expected stability of an emulsion against disproportionation by considering the ratio of the interfacial elasticity to the interfacial tension. In an attempt to increase this ratio to improve the resistance against disproportionation, experiments were performed with densified protein layers obtained via reduction of the droplet area prior to cross linking. To highlight the influence of the protein morphology on the dilatational rheological properties of the cross-linked adsorbed layers, experiments were performed with random coil (beta-casein) as well as globular (beta-lactoglobulin) proteins. Glutaraldehyde was used as a cross-linking agent. Experiments were performed at 55 degrees C and pH 7.0 in 20 mM imidazole buffer for later comparison with enzymatically cross-linked adsorbed protein layers. The present work demonstrated substantial qualitative and quantitative differences in the interfacial rheological properties of cross-linked random coil and globular proteins.     

Diepoxide treatment of softwood kraft pulp: influence on absorption properties of fibre networks

Many different approaches to the introduction of intra-fibre cross-links in fibres for use in absorption products are described in the patent literature, but relatively little has been done in terms of academic research. In this study, the long fibre fraction of a Scandinavian softwood kraft pulp has been cross-linked with the diepoxide 1,4-butanediol diglycidyl ether (BDDE). The fibre properties and the performance of the pulp in low density fibre networks were analyzed. In experiments, the cross-linking agent (BDDE) was dissolved in acetone and then mixed with the pulp. It was shown that a prior wash with NaOH (0.1 M) activates the polysaccharides and increases the extent of cross-linking, detected as a decrease in the water retention value. Chemical analysis of modified fibres also gave a clear indication of that cross-linking reactions actually occurred. The pulp properties could furthermore be influenced by varying time, temperature and the amount of BDDE in the reaction. It was also shown that it is possible to use water as a solvent for the cross-linking agent, even though the reactivity of water towards diepoxide must be considered to be high. The cross-linked pulps showed wet bulk under load comparable to that of the mechanical pulps and enhanced properties than unmodified Scandinavian softwood kraft pulp. It was also observed that fibre network test pads of the cross-linked pulp fibres, after being tested in the wet state, regained a substantially large part of their low-density structure when air-dried.     

Synthesis and properties of hybrid materials obtained via additive cross-linking of liquid polybutadiene rubber with H-Si containing reagents

Herein, we present the results describing the synthesis and thermal, mechanical and surface properties of liquid polybutadiene rubber cross-linked with organosilicon reactants. The differences in the structure of cross-linking agents, as well as a number of reactive H-Si species, have a significant impact on the above-mentioned properties of synthesized via Rh(I) catalyzed additive cross-linking of hybrid organic-inorganic coatings.     

Determining landscape-based criteria for freezing of liquids

The correlation between statistical properties of the energy landscape and the number of accessible configurational states, as measured by the exponential of the excess entropy (e(Se)), are studied in the case of a simple Lennard-Jones-type liquid in the neighborhood of the thermodynamic freezing transition. The excess entropy Se is defined as the difference between the entropy of the liquid and that of the ideal gas under identical temperature and pressure conditions and is estimated using the pair correlation contribution, S2. Landscape properties associated with three categories of configurations are considered: instantaneous configurations, inherent saddles, and inherent minima. Landscape properties studied include the energy and the key parameters of the Hessian eigenvalue distribution as well as the mean distances between instantaneous configurations and the corresponding inherent saddles and minima. The signatures of the thermodynamic freezing transition are clearest in the case of inherent structure properties which show, as a function of e(S2), a pronounced change in slope in the vicinity of the solid-liquid coexistence. The mean distance between instantaneous and saddle configurations also shows a similar change in slope when the system crosses from the stable to the supercooled regime. In the case of inherent saddles, the minimum eigenvalue acts as a similar indicator of the thermodynamic freezing transition but the average and maximum eigenvalues do not carry similar signatures. In the case of instantaneous configurations, a weak indicator of the thermodynamic freezing transition is seen in the behavior of the fraction of negative curvature directions as a function of the exponential of the excess entropy.     

Viscoelastic properties of adsorbed and cross-linked polypeptide and protein layers at a solid-liquid interface

The real-time changes in viscoelasticity of adsorbed poly(L-lysine) (PLL) and adsorbed histone (lysine rich fraction) due to cross-linking by glutaraldehyde and corresponding release of associated water were investigated using a quartz crystal microbalance with dissipation monitoring (QCM-D) and attenuated total reflection Fourier transform infrared spectroscopy (ATR/FTIR). The kinetics of PLL and histone adsorption were measured through changes in mass adsorbed onto a gold-coated quartz surface from changes in frequency and dissipation and using the Voigt viscoelastic model. Prior to cross-linking, the shear viscosity and shear modulus of the adsorbed PLL layer were approximately 3.0 x 10(-3) Pas and approximately 2.5 x 10(5) Pa, respectively, while after cross-linking, they increased to approximately 17.5 x 10(-3) Pas and approximately 2.5 x 10(6) Pa, respectively. For the adsorbed histone layer, shear viscosity and shear modulus increased modestly from approximately 1.3 x 10(-3) to approximately 2.0 x 10(-3) Pas and from approximately 1.2 x 10(4) to approximately 1.6 x 10(4) Pa, respectively. The adsorbed mass estimated from the Sauerbrey equation (perfectly elastic) and the Voigt viscoelastic model differ appreciably prior to cross-linking whereas after cross-linking they converged. This is because trapped water molecules were released during cross-linking. This was confirmed experimentally via ATR/FTIR measurements. The variation in viscoelastic properties increased substantially after cross-linking presumably due to fluctuation of the randomly cross-linked network structure. An increase in fluctuation of the viscoelastic properties and the loss of imbibed water could be used as a signature of the formation of a cross-linked network and the amount of cross-linking, respectively.     

可交联聚醚醚酮酮交替共聚物的合成与热交联研究

聚芳醚酮类特种工程塑料以其优异的机械性能、热稳定性、耐溶剂、耐辐照等特性在运输、航空航天、军事、电子、信息、核能等领域得到了广泛应用_{[1].聚醚醚酮的玻璃化转变温度(Tg)和熔点(Tm)分别为416和607 K,其长期使用温度为513 K,而其热分解温度在800 K以上,是热稳定性较好的聚合物之一.为了满足一些特殊需求,人们通过在聚芳醚酮的主链中引入刚性结构链,提高其主链的刚性程度,从而提高其T g和Tm,进而提高其使用温度[2～4].文献[5]报道的新型聚芳醚酮的T g和T m最高可达482和742 K,采用常规方法进行加工难度较大.为了在不提高加工温度的前提下提高聚芳醚酮类材料的使用温度,我们已成功地在聚醚醚酮的主链中引入可交联的硫醚结构,得到使用温度更高的可控交联聚醚醚酮材料,其可利用热塑性材料的加工方法进行加工,加工温度与聚醚醚酮相同,交联后的材料具有热固性材料的使用特性[6,7].为了拓宽可交联聚芳醚酮材料的种类,本文合成了一种类新型的可交联型聚醚醚酮酮材料,并对其热交联性能进行了研究.}     

Electrochemically active cross-linking reaction for fluorescent labeling of aliphatic alkenes

Although cross-linking reactions serve as a valuable tool for the integration of two or more functionalities or properties, the application of electrochemical synthesis to cross-linking reactions is restricted due to the difficulty of mass transfer. Thus, the primary purpose of this research is to explore electrochemical cross-linking systems to construct a fluorescent probe, triggered by the formation of a covalent linkage. The second purpose is to apply the probe to insoluble targets. Towards these goals, a combination of electrochemically active phenol derivatives and aliphatic alkenes were employed to form polycyclic compounds. Several of the dihydrobenzofuran derivatives formed through [3+2] cyclization reactions exhibited fluorescence. Furthermore, this approach allowed the effective modification of alkene-modified silica gel with electrochemically active species, which enables the construction of fluorescent probes that are triggered by C-C bond formation.