Dynamic modeling of crosslinking and gelation in continuous ethylene-propylene-diene polymerization reactors using the pseudo-kinetic constant approach

A dynamic model for continuous ethylene-propylene-diene terpolymerization reactors in which crosslinking and gel formation are attributable to reactions between the pendant double bonds of diene units has been developed. The model is applicable to other types of crosslinking reactions such as those due to aging, polymer blending, and vulcanization. The polymer properties at the gel point and in the post-gel region are computed using the numerical fractionation method. Direct application of this method to the prediction of terpolymer properties in the gel or post-gel region can lead to severe numerical problems, due to large differences in order of magnitude of various moments across the generations. These problems are overcome by applying a pseudo-kinetic rate constant method, i.e., by constructing a moment model for a pseudo-homopolymer that approximates the behavior of the actual terpolymer under the long chain and quasi-steady state assumptions. The pseudo-homopolymer model is then used as the basis for application of the numerical fractionation method. We show that the proposed dynamic model is capable of predicting realistic polydispersities and molecular weight distributions even near the gel point with as few as 11 generations, and in the post-gel region with as few as five generations. The largest steady-state polydispersities of the soluble polymer are obtained when the crosslinking rate just exceeds the critical value for gelation. The steady-state polydispersity decreases exponentially in the post-gel region at higher values of the rate constant, while the sol fraction decreases in a more linear fashion. The overall molecular weight distribution (MWD) of the sol is constructed assuming a Schulz two parameter distribution for each generation. For the industrial case of a small number of crosslinks, the first two generations contribute the most to the MWD, which is unimodal. The tail of the MWD is longest near the initial gelation time; the tail is shortened in the post-gel region as higher generations are consumed.     

Progress in Drying Technology for Nanomaterials

Abstract:

Production of nanomaterials is becoming increasingly important as a result of diverse industrial applications. Nanomaterials can be prepared by a dry process or a wet process; the latter involves drying, which affects the quality of the products. This article presents an overview of recent research and development activities in drying of nanomaterials. Current drying methods for such applications are classified and compared with the help of some examples. Selection criteria for drying methods as well as future research requirements are suggested.     

Structure–fracture relationships in gas-filled gelatin gels

Food aeration has become one of the fastest growing unit operations practiced in the food industry. Dispersed air (or other gases) provides an additional phase within the gel that may accommodate new textural and functional demands. This paper addresses the relationships between structural characteristics and fracture properties of gas-filled gelatin gels (GGG), and compare these properties with those of control gelatin gels (CGG). Three gases were used in the fabrication of GGG: air, nitrogen and helium. Experimental methods to determine density, gas hold-up, bubble sizes and bubble size distributions as well as fracture properties of GGG are presented. Increasing protein concentration produced higher density, lower gas hold-up and decreased polydispersity of bubbles due to its effect on increased solution viscosity. Type of gas affected density and gas hold-up due to the different diffusivities of gases and structures (bubble size, size distribution and number of bubbles per area) formed in GGG. Fracture values increased for both GGG and CGG with increasing protein concentration for the three gases used. GGG were weaker and less ductile than CGG, the decrease in stress and strain at fracture being between 70 and 80%, and 40 and 65%, respectively. A power law relationship (σ_f = 2.73 × 10⁻¹²ρ_G^4.76) was found between the fracture stress and gel density for the three gases studied. This study shows that the presence of bubbles in gel-based food products results in unique textural properties conferred by the additional gaseous phase.     

Maize processing waste water arabinoxylans: Gelling capability and cross-linking content

Water-soluble feruloylated arabinoxylans were extracted from maize processing waste water (nejayote) generated from tortilla-making industries. Nejayote arabinoxylans (NAX) presented a ferulic acid content of 0.23 μg/mg, an arabinose to xylose ratio (A/X) of 0.65, an intrinsic viscosity, [η], of 183 ml/g and a molecular weight (Mw) of 60 kDa. Laccase-induced gels were obtained from 4% (w/v) NAX solution while, at lower concentrations, no gelation was observed. Laccase covalent cross-linking of NAX led to the formation of diferulic (di-FA) and triferulic (tri-FA) acid structures. 4% (w/v) NAX gel presented di-FA and tri-FA contents of 0.02 and 0.01 μg/mg NAX, respectively, and a G′ value of 2 Pa. Recuperation of this gum from a low-value maize by-product could represent a commercial advantage over other gums commonly used in the food industry.     

A comparison of block copolymer surfactant gels

The aqueous gel forming properties of three series of block copolymer surfactants, poloxamers, poloxamines, and Butronic® polyols, are briefly reviewed and compared. The differences in their gel formation properties are explained. Aqueous gel formation of these polymers is attributed to the aggregation of extended linear coil micelles in which the hydrophilic ends become entangled as the temperature rises, due to dehydration. The failure of the Butronic polyol gels to exhibit the same reverse thermal behavior shown by the other two polyol series is attributed to a more lipophilic hydrophobe and to the larger number of moles of water associated with the Butronic hydrophile. The reverse thermal characteristics of aqueous poloxamer and poloxamine solutions has led to the development of aerosol detergent systems which can form foamy gels, thereby expanding the usefulness of these surfactants.     

Fabrication of thick gel-like films by anodizing iron in a novel electrolyte based on dimethyl sulfoxide and H2SiF6

The thick bi-layered gel-like film has been grown by anodizing iron in a novel electrolyte composition based on dimethyl sulfoxide and silica hexafluoride acid (H₂SiF₆) and examined. The thickness of the anodic film composed of the inner orange-colored nanoporous layer and the outer cherry-colored gel-like layer increased at a constant rate up to 40–50 μm depending on the bath composition and anodizing conditions. Under drying and subsequently annealing, the gel layer shrinks and cracks producing hierarchical morphology of thick films mainly composed of hematite (a-Fe₂O₃). Scanning electron microscopy, Mössbauer, Raman and energy dispersive X-ray spectroscopies, and X-ray diffraction have been applied to reveal the composition and morphology of these novel films before and following various post-treatments.     

All solid-state electrochromic window of electrodeposited WO3 and prussian blue film with PVC gel electrolyte

An all-solid-state electrochromic window (SEW) of prussian blue and electrodeposited WO₃ film with poly(vinyl chloride) PVC gel electrolyte with high conductivity (2 mS/cm) at room temperature has been fabricated. The SEW has been found to be excellent for electrochromism and memory characteristics. W 4f core level of WO₃ film as a function of the injected charge has been investigated using X-ray photoelectron spectroscopy (XPS). W 4f peaks become broader after coloration.     

Applications of freeze-fracture replication to problems in materials and colloid science

Understanding the relationship between the molecular structure and the macroscopic properties of polymer solutions and gels, oil-water-surfactant emulsions, lyotropic and thermotropic liquid crystals, colloidal dispersions, detergents, and other such “microstructured fluids” is essential to the optimal use of these commercially important materials. Modern rapid-freezing methods followed by freeze-fracture replication techniques are ideally suited to allow the direct visualization of the three-dimensional structure of the particles or units that make up the dispersion, while simultaneously revealing their orientation and distribution with molecular resolution. This paper reviews the necessary experimental conditions required to successfully exploit the freeze-fracture technique as it applies to microstructured fluid systems. The benefits and limitations of structural studies by freeze-fracture techniques as opposed to the more commonly used light, X-ray, and neutron-scattering methods are discussed. Freeze-fracture replicas can also be imaged by scanning tunneling microscopy to reveal directly three-dimensional fracture contours with improved resolution.     

有机锆交联剂OZ—1应用研究

合成了具有延迟交联特性的有机锆交联剂ＯＺ１；介绍了ＯＺ１交联的已在胜利油田应用的３种压裂液的性能；探讨了ＯＺ１在水基聚合物冻胶堵剂和弱凝胶或胶态分散凝胶中的应用。     

间苯二酚在聚合物凝胶堵水技术中的应用

间苯二酚作为聚丙烯酰胺的优良交联剂已成功应用于油田聚合物凝胶堵水技术。本文综述了将间苯二酚应用于近井地带小剂量堵水作业、地层深部大剂量堵水作业以及地层内就地交联聚合物弱凝胶驱替作业的室内研究和现场应用成果。