Gelation theory for solutions of polymers with loci of strong interaction

This paper presents a statistical mechanical analysis of gelation in certain solutions ofA/B copolymers. Repeat unitsA, the major units, must constitute about 90 mole% or more of the copolymer, and are compatible with the solvent. Repeat unitsB, the minor units, must be totally incompatible with the solvent and represent loci of strong interaction between the macromolecular chains. If these interactions are sufficiently strong they can lead to gelation of the mixture. The following simplifications of the model afford results that can be expressed in simple, closed form: (1) TheB units are uniformly distributed along the chain backbone. (2) Polymer concentration must be greater then that for chain overlap. (3)B–B interactions lead to dimerization ofB units only. Examples include: (1) Any aqueous polymer solution where theA repeat units are hydrophilic and theB repeat units are hydrophobic;B–B interactions are thus by hydrophobic bonding. (2) Neutralized acrylic acid/acrylamide copolymers where theB are acrylamide repeat units which associate by hydrogen bonds. (3) Ionomer solutions whereB–B bonding is by dipole-dipole interaction. The results show that there is a critical copolymer concentration for gel formation, C _c . It turns out that C _c decreases withB content for low dimerization energies and increases withB content for higher dimerization energies. At intermediate dimerization energies, C _c is insensitive to theB content. The two extreme regimes of behavior are similar to contradictory predictions of previously published theories. Received: 18 September 2000 / Reviewed and accepted: 20 September 2000     

Effect of high pressure processing on the gel properties of salt-soluble meat protein containing CaCl2 and κ-carrageenan

The effects of high pressure processing (HPP) on the water-binding capacity and texture profile (TPA) of salt-soluble meat protein (SSMP) containing 0.2% CaCl₂ and 0.6% κ-carrageenan (SSMP-CK) gels were investigated. The results showed that 300–400 MPa improved water-binding capacity and decreased TPA parameters of SSMP-CK gels (P < 0.05), while 100 MPa could increase hardness and chewiness of the gels. The thermal transition temperature peak for the myosin head (T_peak1) of SSMP disappeared on addition of CaCl₂ and κ-carrageenan. 300 MPa produced a new peak, and caused a shift of the NH-stretching left peak and amide I and the disappearance of NH-stretching right peak. The destruction of network structure and the weakening of molecular interaction within the pressurized gels could result in the decrease of TPA parameters. Thus gelling properties could be modified by HPP, κ-carrageenan and Ca^2 +. It is of interest to develop low-fat and sodium-reduced meat products.     

Ultrasonic generation of aerated gelatin gels stabilized by whey protein β-lactoglobulin

Dispersed air provides an additional phase within gel-type foods may accommodate new textural and functional demands. This paper addresses the effect of using whey protein β-lactoglobulin (β-lg), with different degrees of denaturation, as stabilizing agent in the formation of aerated gelatin gels using ultrasound as a novel method to incorporate bubbles in model foods. The heat denaturation, aggregate formation and surface properties of β-lg dispersions were studied at three pHs (6.0, 6.4 and 6.8) and at a heating temperature of 80 °C. β-Lg dispersions with four degrees of denaturation (0%, 20%, 40% and 60%) were used to stabilize bubbles generated by high intensity ultrasound in aerated gelatin gels. Experimental methods to determine gas hold-up, bubble size distributions and fracture properties of aerated gelatin gels stabilized by β-lg (AG), as well as control gels (CG), aerated gelatin gels without β-lg, are presented. Gas hold-up of AG peaked at a degree of denaturation of 40% when AG were fabricated using β-lg heated at pH 6.4 and 6.8, whereas using β-lg heated at pH 6.0 gas hold-up decreased constantly with increasing degree of denaturation. The use of β-lg as surfactant at pH 6.8 and 6.4 reduced the bubble sizes of AG compared with CG, but no effect was observed at pH 6.0. AG showed values of stress and strain at fracture lower than CG (5.86 kPa and 0.62), probably because of the lower gas hold-up of CG. However, both type of aerated gels were weaker and less ductile than non-aerated gels, with a decrease in stress and strain at fracture for AG between 56–71% and 33–43%, respectively. This study shows that the presence of bubbles in gel-based food products results in unique rheological properties conferred by the additional gaseous phase.     

Phase Transitions and Pattern Formation in Chemo-Responsive Gels and Composites

Design of multifunctional materials often requires the chemical functionalization enabling the material to respond to a variety of stimuli. Herein, we focus on modeling of functionalized chemo-responsive gels and composites; functionalization alters the volume phase transitions allowing system to undergo structural reconfiguration. For instance, we consider the volume phase transitions in photo-sensitive hydrogels functionalized with two different types of chromophores that either alter the hydrophobicity of the illuminated sample or cause these samples to heat up. We then focus on gels that are driven by the oscillatory Belousov-Zhabotinsky reaction and show that chemically mediated interactions among these gels drive them to self-aggregate in a well-defined manner. Finally, we focus on the self-oscillating composite that does not contain oscillatory chemically reactions. In all these examples, effective control over the volume phase transitions is achieved through the combination of chemical functionalization and external stimuli, extending the potential utility of the host materials.     

Synthesis and kinetics of microgel in inverse emulsion polymerization of acrylamide

The kinetic behavior is studied in inverse emulsion polymerization of acrylamide. The polymerization rate in a micelle is proportional to the monomer concentration and the effective average radical concentration. When concentrations of monomer and radical are too low to proceed the polymerization only, some finite final conversion is obtained. The micelles of various sizes have different effective average radical concentrations at the same monomer concentration. Therefore, the polymerization rate and the final conversion are different for various sized micelles. The minimum amounts of emulsifier required to get stable state during reaction is 10 and 7 wt% (based on oil phase) in toluene and n-heptane systems, respectively. Phase inversion is observed with the increase of viscosity during polymerization. The final latex coagulates obviously in the toluene system even with 10 wt% of emulsifier, however, it can be improved more in the n-heptane system with 10 wt% of emulsifier. The crosslinking agent which copolymerizes with the monomer was supposed to remain in the interface of micelles. Such interfacial copolymerization not only hardens particles but improves the stability of the system also, therefore avoiding coagulation among particles. An increase in the content of crosslinking agent leads to a more uniform size of final particles. The microgel added in the alkyd resin decreases the drying time. The greater the amount of microgel added, the shorter the drying time.     

An improved general kinetic analysis of non-linear irreversible polymerisations

A method to predict average molecular weights before and after gelation for general irreversible non-linear polymerisations forming tree-like molecules is described. Recently developed numerical methods for solving two point boundary value problems are essential for the success of these calculations after gelation and open the way to eventually be able to efficiently predicting chain length distributions. Anionic and free-radical polymerisation of vinyl monomers in the presence of divinyl monomers or with transfer to polymer are taken as case studies. Comparison to experimental data and with simulation results obtained through “numerical fractionation” confirms the usefulness of current approach.     

Organogels and Low Molecular Mass Organic Gelators

Nanostructured materials have many forms. One that has been somewhat neglected until recently is organogels, especially those whose three‐dimensional structural networks are based on the self‐assembly of low molecular‐mass organic gelators (LMOGs). These thermoreversible materials consist of a small amount of LMOG and an organic liquid. Because of the wide diversity of structures of molecules known to function as LMOGs and the dearth of direct information concerning how they pack in their gel assemblies, it has been difficult to decipher the salient features that constitute an LMOG. The stepwise simplification of LMOG structures and the development of methods to determine their packing in organogels at the micrometer‐to‐angstrom distance regimes are discussed for the simplest known LMOGs to the more complex, such as CEP (see Figure), which is known to form molecular wires when gelling chloroform. Work from the laboratory of the authors is emphasized. In addition, an overview of current and potential applications for these materials is presented.     

调驱用RSP3抗盐聚合物弱凝胶研制

目前使用的调驱用弱凝胶抗盐性差.分析了常用或推荐的改善弱凝胶调驱体系抗盐性的措施,包括清水预冲洗地层,使用钙镁离子络合剂,使用有机交联剂,使用粘土胶液和无机凝胶,认为均不能令人满意.合成了RSP系列抗盐聚合物(类梳状聚合物),用中等分子量(9.5×106)、低水解度(10%)的一种抗盐聚合物RSP3和有机铬交联剂CL-2配制了弱凝胶,并与几种常用HPAM进行了比较,结果如下.用模拟长庆油田高矿化度、高钙镁含量地层水(2.3%NaCl+1.7%CaCl2溶液)配制的0.3%RSP3溶液的粘度,在30℃下为37.5mPa·s,65℃下为23.0mPa·s,在65℃下放置24小时后为20.5mPa·     

Fluorescent Liquid‐Crystal Gels with Electrically Switchable Photoluminescence

Fluorescent self‐assembled liquid gels have been prepared for the first time through the use of a gelator showing aggregation‐induced enhanced emission. The photoluminescence, arising from the fibrous aggregates of the gelator, can be repeatedly switched by an electric field. Modulating the excitation light through electric‐field‐induced liquid‐crystal orientation offers new ways to achieve and explore electrically controllable photoluminescence.