Effect of Polymer Concentration on Sustained Release Microparticles of Metformin Hydrochloride Prepared by Using Spray Dryer

Metformin hydrochloride, polyacrylic acid and β-cyclodextrin were taken for the preparation of spray dried sustained released micro particles in a different ratios. These sustained release micro particles were used for the preparation of tablets. Hardness of tablet increases with increase in concentration of polymer while the percentage friability decreases with increase in polymer concentration in tablet. The encapsulated drug shows better sustained release than the conventional tablet. The kinetics of the dissolution process were studied by analyzing the dissolution data using three kinetic equations—the zero-order equation, the first-order equation and the Hixson–Crowell cube root law equation.     

Low-field NMR studies of polymer crystallization kinetics: Changes in the melt dynamics

The free induction decay in ¹H NMR experiments carried out for crystallizing polymers can be directly decomposed in contributions from crystals, melt-like regions and amorphous regions with a reduced mobility. Here, the results of time-dependent experiments conducted with the aid of a cost-efficient low-field NMR instrument are presented, obtained for sPP, P?CL and P(EcO). Crystallization isotherms are compared with those obtained by X-ray scattering and dilatometry. There are some minor systematic deviations which can be explained and accounted for. For all systems, a large fraction of amorphous chain parts in regions with a reduced mobility is found.     

Dissolution and rheological behaviour of hematite and quartz particles in aqueous media at pH 1

In this study we investigate isothermal, atmospheric acid dissolution behaviour of quartz and hematite minerals which constitute two of the predominant host gangue phases of typical low grade limonitic laterite ores. Batch dissolution tests were carried out on 57 wt.% solid dispersions for 4 h at pH 1 and 25 and 70 °C to establish the influential role of oxide mineralogy/chemistry on rheology and leaching behaviour. The results show that the two minerals displayed a weakly temperature and time-independent, non-Newtonian rheological behaviour with low shear yield stresses (<4 Pa) and viscosities (9–17 mPa s). Hematite dissolution rate was significantly higher compared with that of quartz under similar conditions. Quartz dissolution mechanism was substantially volume diffusion controlled at lower agitation rate (600 rpm) whilst for hematite it was both volume diffusion and chemical reaction controlled. These mechanisms reflected activation energies of 17.7 ± 0.9 and 28.5 ± 1.4 kJ/mol, respectively, for quartz and hematite. At 800 and 1000 rpm, dissolution of both minerals was chemical reaction-controlled with similar activation energies (32.6 ± 1.7 and 32.2 ± 1.6 kJ/mol). The findings underscore the need for higher agitation rates and elevated temperatures, to overcome both volume diffusion and chemical reaction limitations for enhanced acid leaching of these two fairly refractory oxides studied herein.     

Anodic behaviour of zinc in methanol solutions of lithium perchlorate

Anodic dissolution of poly- and single-crystals of zinc was performed in methanol solution of lithium perchlorate by means of cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The mechanism of anodic dissolution of zinc in organic solvents occurs in two oxidation steps. It is the first step that surface anodic product is created. Stability of the surface product is much better in anhydrous organic environments than in aqueous media; because the product is stable, a barrier layer composed of is formed at low anodic overvoltage. The formation of the layer is much stronger on the low index than on the high-index plane (0 0 0 1), where the adsorption of anodic product is more difficult.     

Recycling of polymers from plastic packaging materials using the dissolution–reprecipitation technique

In this work, results are presented on the application of the dissolution/reprecipitation technique in the recycling of polymers from waste plastic packaging materials used in food, pharmaceuticals and detergents. Initially, the type of polymer in each packaging was identified using FT-IR. Furthermore, experimental conditions of the recycling process (including type of solvent/non-solvent, initial polymer concentration and dissolution temperature) were optimized using model polymers. The dissolution/reprecipitation technique was applied in the recycling of a number of plastic materials based on polyethylene (LDPE and HDPE), polypropylene, polystyrene, poly(ethylene terephthalate) and poly(vinyl chloride). The recovery of the polymer was measured and possible structural changes during the recycling procedure were assessed by FT-IR spectroscopy. Potential recycling-based degradation of the polymer was further investigated by measuring the thermal properties (melting point, crystallinity and glass transition temperature), of the polymer before and after recycling, using DSC, their molecular properties (average molecular weight) using viscosimetry, as well as their mechanical tensile properties. High recoveries were recorded in most samples with the properties of the recycled grades not substantially different from the original materials. However, a slight degradation was observed in a few samples. It seems that this method could be beneficial in waste packaging recycling program.     

矿化度对疏水缔合聚合物溶液溶解时间的影响研究

疏水缔合聚合物是在亲水的聚丙烯酰胺大分子主链上引入少量疏水基团合成的新型水溶型聚合物。其有耐温耐盐的良好特性。但是疏水基的引入降低了聚合物的水溶性,增加了溶解时间。本文以渤海某油田地层水的离子构成作为参考,通过配制不同矿化度溶液来溶解疏水缔合聚合物,探索矿化度对疏水缔合聚合物溶液粘度及溶解时间的影响,并从原理上解释了出现这些现象的原因。最终发现,随着矿化度升高,疏水缔合聚合物溶液粘度降低,溶解时间增大。这种现象是由于矿化度升高,溶液中电解质增多,溶液极性增强,影响了疏水缔合聚合物分子的展开和缔结。     

Monte Carlo study of structure and kinetics of formation of end-linked polymer networks

The kinetics of formation and structural properties of end-linked polymer networks were studied by Monte Carlo simulations on a three-dimensional cubic lattice. Networks were generated from the solutions of linear polymer chains with functional end groups and tetrafunctional cross linkers. Systems of 10-, 20-mer precursor polymer chains with values of the ratio of cross linkers to polymer chains end groups r ranging from 0.9 to 1.4 were studied. Polymer volume fraction ? was varied from 0.1 to 0.5. Different ways of polymer network formation are possible, namely, gel creation process can proceed as homogeneous gelation as well as microgel separation. In addition to those limiting ways of polymerization process, intermediate cases were observed. All varied parameters (length of precursor polymer chains N, r and ?) influence the kinetics of the cross-linking. An algorithm to determine the soluble fraction in solution and the amount of loops and pendent structures in the polymer network was proposed. In agreement with experimental observations, simulations show that networks with lower soluble fractions which are more defect-free result from long precursor polymer chains (N=20) for approximately polymer melt densities (?≈0.4) at higher than stoichiometric r values (r≈1.2).     

Recent advances in polymer reaction engineering: Modeling and control of polymer properties

Complex reaction kinetics and mechanisms, physical changes and transport effects, non-ideal mixing, and strong process nonlinearity characterize polymerization processes. Polymer reaction engineering is a discipline that deals with various problems concerning the fundamental nature of chemical and physical phenomena in polymerization processes. Mathematical modeling is a powerful tool for the development of process understanding and advanced reactor technology in the polymer industry. This review discusses recent developments in modeling techniques for the calculation of polymer properties including molecular weight distribution, copolymer composition distribution, sequence length distribution and long chain branching. The application of process models to the design of model-based reactor optimizations and controls is also discussed with some examples. This paper is dedicated to Professor Hyun-Ku Rhee on the occasion of his retirement from Seoul National University.     

Biodegradable vesicular nanocarriers based on poly(?-caprolactone)-block-poly(ethyl ethylene phosphate) for drug delivery

Biodegradable polymer vesicle for drug delivery is reported. Poly(?-caprolactone)-block-poly(ethyl ethylene phosphate) with well-defined structure (PCL₁₅₀-b-PEEP₃₀) has been prepared by ring-opening polymerization. It forms vesicles in aqueous solution using the thin-film hydration method and further exclusion of the as-formed vesicles results in vesicles at nano-size, demonstrated by confocal laser scanning microscope (CLSM) and transmission electron microscopy observations. Doxorubicin (DOX) has been loaded into the vesicles with a loading content of 4.38% using an acid gradient method. The release of DOX from the vesicles is accelerated in the presence of an enzyme phosphodiesterase I that is known to catalyze the degradation of polyphosphoester, achieving 83.8% release of total loaded DOX in 140 h. The DOX-loaded vesicles can be successfully internalized by A549 cells, and it results in enhanced inhibition to A549 cell proliferation, likely owning to the sustained intracellular release of DOX as observed by CLSM. With these properties, the vesicles based on the block copolymer of PCL and PEEP are attractive as drug carriers for pharmaceutical application.     

Hydrogels in drug delivery: Progress and challenges

There has been considerable progress in recent years in addressing the clinical and pharmacological limitations of hydrogels for drug delivery applications but substantial challenges remain. Here we discuss recent progress in overcoming these challenges, particularly with regards to effectively delivering hydrogels inside the body without implantation, prolonging the release kinetics of drugs from hydrogels, and expanding the nature of drugs which can be delivered using hydrogel-based approaches.     

On the upscaling of reaction-transport processes in porous media with fast or finite kinetics

We show that for reaction-transport processes with fast kinetics (in the limit of thermodynamic equilibrium), conventional volume averaging for determining effective kinetic parameters applies only when the macroscopic variable approaches its equilibrium value. Even under such conditions, computing the effective mass transfer coefficient requires solving an eigenvalue problem, which couples the local microstructure with the global. Two examples, one involving a simple advection-dissolution problem and another a drying problem in a pore network, illustrate the theoretical predictions. Similar considerations apply for the case of finite kinetics, when the macroscale concentration approaches an equilibrium value. In that case, the effective kinetic parameter is not equal to the local, as typically assumed, but it becomes a function of the local Thiele modulus.     

Study of the kinetics of mushroom-to-brush transition of charged polymer chains

The grafting of thiol-terminated poly[(2-dimethylamino)ethyl methacrylate] (HS-PDMEM) chains to a gold surface from a solution was investigated with quartz crystal microbalance with dissipation (QCM-D) in real time. The frequency and energy dissipation responses revealed a three-regime-kinetics of the grafting. The chains are quickly grafted in regime I forming a random mushroom. In regime II, the grafted chains have a rearrangement and form an ordered mushroom structure. The grafting is accelerated in regime III. As the grafting density increases, the chains form brushes. From regime II to III, the mushroom-to-brush transition occurs. The results were further supported by atomic force microscopy (AFM) imagines.     

Notes on the dissolution rate of gas hydrates in undersaturated water

An elementary model for the dissolution of pure hydrate in undersaturated water is proposed that combines intrinsic decomposition within a desorption film and the subsequent diffusion of the released hydrate guest species into bulk water. Applying the proposed approach to recently published measurements of the decomposition rates of methane (CH₄) and carbon dioxide (CO₂) hydrates in deep seawater suggests that the concentration of the hydrate guest species at the interface between desorption film and diffusive boundary layer may be much lower than ambient solubility. Calculations, however, fail to account for the observed proportionality of decomposition rate with solubility for both CH₄ and CO₂ hydrates. This may indicate a limitation in the range of applicability of published formulas for intrinsic hydrate decomposition rates.     

Relations between apparent and intrinsic kinetics of “programmable” drug release in human plasma

To better understand and “program” the drug release in human plasma, a kinetic characterization of the process is necessary for every drug ligand and macromolecular support. Due to few observed species and limited standard information, process modelling based on extended kinetic structures is difficult. Laboratory experiments that mimic the natural conditions and the bio-environment, together with statistical estimators allow identification of reduced (apparent) kinetic models. The scope of this paper is to present a more systematic approach to interpret the kinetic models and to link the identified apparent rate/equilibrium constants to kinetic characteristics of an extended “intrinsic” reaction path. The developed stoichiometric analysis and linear lumping rules are exemplified for a case study from literature, that considers a four-ligands (“drug” dansyl groups) release from a dendrimeric support structure in a reducing environment that mimic the human plasma.     

Asymptotic analysis of a complex reaction scheme in solid-liquid system

Asymptotic analysis of a complex reaction, Claisen condensation, is discussed. Under certain assumptions related to experimental observations the kinetics scheme of the reaction can be considerably reduced. Additional complications in the problem arise due to the fact that one of the substances is present in a sparingly soluble solid phase. We show how reductions for increasingly complex systems with kinetics, mass transfer and feed can be systematically handled by employing the methodology of asymptotic analysis. Generalizations of the reduction approach for the case of an arbitrary number of fast intermediate reactions are also considered.The identification of model parameters is considered with aid of the reduced systems. Questions of parameter estimation and optimal design of experiments are discussed using the MCMC methods.     

熔融插层法制备聚合物复合材料的研究进展

对近年来熔融插层法制备聚合物复合材料的研究进展进行了综述。从动力学、热力学方面对插层结果进行了分析,总结了影响插层效果的因索;并归纳出插层以后复合材料性质的改善,最后展望了熔融插层法制备聚合物／粘土纳米复合材料。     

A Model of Controlled Release of Polymer-dispersed Drug Systems Containing Regulatory Particles

Abstract

A model of controlled release systems using a polymer matrix with mixtures of dispersed drug particles and regulatory particles is considered. The regulatory particles are fillers that release at a slower rate than the drug and open more paths for the passage of the drug. Mathematical analysis shows that both the drug and regulatory particles have Fickian diffusion behaviors. The use of regulatory particles increases the release rate and can make the system responsive to physiological change.     

Numerical Simulation of Crystallization Morphological Evolution Under Nonisothermal Conditions

In order to obtain more information about the process of crystallization, the Monte Carlo method was used to simulate crystallization morphological evolution of semicrystalline polymers, such as isotactic polypropylene (iPP), and calculate the development of crystallinity. With remarkable progress, the morphological evolution of crystallization and development of crystallinity were obtained simultaneously during the process of the simulation. The simulated results are in good agreement with the experimental results under nonisothermal quiescent conditions when the induce time is explicitly considered during the process of simulation.