Further evidence for the effects of critical concentration fluctuations on the diffusion-controlled reaction kinetics in binary polymer mixtures

Summary Kinetics of the intramolecular photodimerization reaction of a molecular probe, 9-(hydroxymethyl)-10-[(Naphthylmethoxy)-methyl] anthracene (HNMA) was studied in the miscible region of binary polymer mixtures. Poly(ethylene oxide)/poly(methyl methacrylate)(PEO/PMMA), polystyrene/poly(vinyl methyl ether) (PSH/PVME) and deuterated polystyrene/poly(vinyl methyl ether) (PSD/PVME) mixtures were used as binary polymer blends. It was found that the reaction kinetics is strongly affected by the concentration fluctuations in the vicinity of the coexistence curve. These behavior are consistent with the magnitude of the binary interaction parameters X estimated from the small-angle neutron-scattering and the melting point depression data. These experimental results confirm the fact that the reaction kinetics of HNMA reflects the concentration fluctuations with the wavelength comparable to the dimension of the probe.     

Combined single particle growth and population balance modeling in stereoregular polymerization of styrene

Modeling and experimental analysis for syndiospecific polymerization of styrene over silica-supported metallocene catalyst was carried out. A detail model was developed by coupling the single particle growth model (PGM) with particle population balance equation. The model was employed to predict the effects of intraparticle mass transfer limitations and the initial catalyst particle size on the rate of polymerization and the particle size distribution (PSD) of syndiotactic polystyrene (sPS). The single PGM, based on a modified polymeric multigrain model, was first utilized to calculate the single particle growth rate and polymerization rate under intraparticle mass transfer limitations and different initial catalyst particle sizes. Then, the model was solved simultaneously with particle population balance equation to estimate the PSD of sPS under the same limitations. The single PGM results showed a significant radial distribution of styrene concentration across polymer growth. It was further noticed that the diffusion resistance was most intense at the beginning of the polymerization reaction and the effects of polymerization rate were stronger. Moreover, it appeared that increasing the initial catalyst particle size led to lower rate of polymerization. The PSD simulation results revealed that the mass transfer limitation, as well as the initial catalyst particle size made a strong impact on the PSD of sPS. In addition, the simulation results obtained from this model showed good agreement results with experimental data of sPS.     

Fluorescence densitometry at polymer-polymer interfaces: interdiffusion in polystyrene-poly(cyclohexyl methacrylate) blends

An experimental set-up is described to determine the concentration profile of a polymer labelled with a fluorescent dye at the interface with a compatible unlabelled polymer. This method is applied to a study of interdiffusion in blends of labelled polystyrene with poly(cyclohexyl methacrylate) and compared with tracer diffusion of labelled polystyrene in the same system.     

Effects of polymerization conditions on particle size distribution in styrene‐graphite suspension polymerization process

Suspension polymerization in the presence of graphite has been studied in order to determine the effects of some operational parameters on the particle size distribution (PSD). The results showed that, with increasing graphite content, the particle size of the polystyrene/graphite (PS/G) beads increased. Moreover, instability of the suspension polymerization system was found at high amounts of graphite. With increasing initiator concentration, the particle size of the polymer beads increased and the PSD became slightly narrower. Changing the concentration of the suspending agent proved to be an efficient way of controlling the particle size, although its increase led to a broadening of the PSD. Adding the suspending agent in two portions at different times decreased the particle size, maintained a lower concentration of suspending agent, and kept the suspension polymerization system stable. Adjusting the stirring speed proved to be a very efficient means of manipulating the PSD of the PS/G composite beads. The Sauter mean diameter decreased and the PSD was broadened with increasing stirring speed; 400 rpm was identified as an appropriate value to obtain polystyrene/graphite beads with desirable particle size and distribution. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44270.     

丁二烯气相聚合产物分子量分布和粒径分布模型研究

对非均相催化的丁二烯气相聚合,基于聚合物多层模型,考虑催化剂颗粒间活性位初始浓度和粒径分布对聚合物分子量分布和粒径分布的影响,建立了聚合物分子量分布和粒径分布的数学模型。模拟了反应温度、催化剂颗粒间活性位初始浓度和粒径分布等因素的影响,结果表明。随着温度升高,聚合物颗粒平均粒径变小,粒径分布变窄,聚合物分子量变小,分子量分布变宽;催化剂颗粒间的活性组分负载越均匀,聚合物分子量越大,分子量分布和粒径分布越窄;随着催化剂平均粒径变大,聚合物分子量变小,分子量分布变宽,不存在催化剂颗粒粒径分布和聚合物颗粒粒径分布间的复制现象。模型模拟结果与实验结果吻合较好,可用于预测丁二烯气相聚合产物的分子量、分子量分布和粒径分布。     

Particle size distribution and molecular size distribution of polymers in soap-free emulsion polymerization of styrene

Polystrene (PS) particles, generated from soap-free emulsion polymerization of styrene monomer, water, and potassium persulfate, were investigated by photon correlation spectroscopy (PCS) and TEM. The particle size distribution (PSD) was quite uniform. From the data of PCS, it could be said that lots of particles flocculated in the final stage of reaction. It was also deduced from the molecular weight distribution (MWD), measured by GPC, that, during the early stage of reaction, molecules with low molecular weight (<4000) might exist and the particles were perhaps formed through micellar-type nucleation mechanism. When initiator concentration increased, reaction rate increased but weight average molecular weight, tensile modulus, and elongation decreased. The number density of particles was found to be proportional to 0.49 power of initiator concentration. However, monomer concentration did not seem to have any great effect on all of them above.     

Phase separation kinetics and morphology in a polymer blend with diblock copolymer additive

The phase diagram for a low molecular weight blend of deuterated polystyrene (PSD) and polybutadiene (PB) was determined by temperature jump light scattering (TJLS) measurements and phase contrast optical microscopy (PCOM). The PSD/PB blend exhibited upper critical solution temperature behavior, and the critical temperatures measured by these two techniques were consistent. Upon addition of 0 to 0.12 mass fraction of a comparable molecular weight PSD-PB symmetric diblock copolymer, a linear decrease in the phase transition temperature was observed with increasing diblock copolymer content. At a constant, shallow quench depth, the kinetics of phase separation via spinodal decomposition as measured by TJLS were greatly retarded by the presence of the copolymer. Additionally, the time dependence of the concentration fluctuation growth did not seem to follow a universal functional form anywhere in the accessible q range when the diblock was present. The results of morphology study of the blends in the late stage of phase separation by PCOM also indicated that the phase separated domain sizes did not grow to the same size for a given annealing time as diblock content increased.     

Measurement of thermal diffusion parameters of polystyrene using a thermal field flow fractionation method

Thermal diffustor parameters such as the thermal diffusion factor and the thermal diffusion coefficient for polystyrene in toluene are measured by using a thermal field flow fractionation (TFFF) method The dependence of these parameters upon temperature concentration and polymer molecular weight is ex-amined. The thermal diffusion coefficien is found to be independent of the molecular weight for sufficiently large polymer molecules but on the other hand the ordinary diffusion coefficient is known to be pronouncedly dependent on the molecular weight. This result indicates that the fractionation effect is primarily governed by the differences in ordinary dif-fusion coefficients.     

Population balance modelling for high concentration nanoparticle sizing with ultrasound spectroscopy

Ultrasound particle sizing is attracting an increasing attention from academic research and industrial applications as it offers non-invasive, suitable for highly turbid and concentrated nanoparticle suspensions and potentially no sample dilution needed features. The main challenge to this technique is thought to be its capability of dealing with high concentration. Most ultrasound particle sizing techniques employ ECAH (Epstein, Carhart, Allegra and Hawley) theory based models for the inversion of ultrasound spectra to particle size distribution (PSD). However, this theory is based on “single particle scattering”, namely a single particle immersed in an infinite medium, it is therefore only valid when ultrasound attenuation and particle concentration are linearly related. With the increase of particle concentration, due to the interactions between particles, the relation between attenuation and concentration may become nonlinear for solid-liquid suspensions. This paper demonstrates a method using population balance (PB) modelling to deal with the high concentration PSD problem for silica suspensions. It concludes that with a de-aggregation model, it is possible to convert attenuation inverted PSDs (ECAH model based inversion) at high concentrations into the PSD that is thought to be the correct PSD at a critical low concentration by a PB simulation.     

Single-Molecule AFM Study of Polystyrene Grafted at Gold Surfaces

Single-molecule studies under poor solvent conditions show that atomic force microscopy (AFM) measurements of contour lengths of polystyrene 12K and 17K relate well to gel permeation chromatography (GPC) data when grafting concentrations are low (not greater than 0.7 mM). Contact angles and ellipsometry have been used to characterize the surfaces and indicate low grafting densities at this grafting concentration. Persistence lengths (p) of different length polystyrene chains, when fitted to a WLC (worm-like chain) model, show values of p = 0.23 nm±0.10 nm and p = 0.25 nm±0.13 nm, for 12 K and 17 K polystyrene, respectively, when the persistence lengths are fitted to log-normal distributions. These values are close to the expected theoretical value of 0.23 nm and further confirm that mostly single molecules were studied on these polystyrene surfaces. Higher grafting concentrations (≥1 mM) resulted in pulling multiple molecules.     

Synthesis,separation and characterization of knotted ring polymers

Knotted ring polystyrene (PS) with molecular weight of 380 k was successfully synthesized by intramolecular cyclization reaction in cyclohexane under extremely diluted condition. Crude product was confirmed to include linear precursor molecule, single ring molecules, and various intermolecular-reacted byproducts by SEC and interaction chromatography characterizations. The crude product was fractionated repeatedly several times by high performance interaction chromatography and finally highly-purified knotted ring molecules were obtained. It has been found by SEC-MALS that the chain dimension of the knotted ring polymers is evidently smaller than those of linear and the trivial ring polymer, while knotted polymer molecules have the same absolute molecular weight as the corresponding counterparts.     

Molecular conformation and interactions in oligomeric mixtures of polystyrene with polybutadiene: a small-angle neutron scattering study

In this preliminary study we report values of the interaction parameter determined from small angle neutron measurements at two temperatures and three concentrations for two blends of oligomeric polystyrene with polybutadiene. One blend contained labelled (deuterated) polystyrene with hydrogenous polybutadiene and the other deuterated polybutadiene with hydrogenous polystyrene. The results from the latter are compared with predicted values.     

Quality Control Certification of RNA Aptamer‐Based Detection

Aptamers are single‐stranded DNA or RNA molecules with a defined tertiary structure for molecular recognition. Numerous RNA aptamers with excellent binding affinity and specificity have been reported; they constitute an attractive reservoir of molecular recognition elements for biosensor development. However, RNA is relatively unstable owing to spontaneous hydrolysis and nuclease degradation. Thus, RNA aptamer‐based biosensors are prone to producing false‐positive signals. Here, we present an RNA aptamer biosensor design strategy that utilises an internal control to distinguish target binding from false‐positive signals. The sequence of a chosen RNA aptamer is expanded so that it can form three consecutive short RNA–DNA duplexes with 1) a quencher‐labelled DNA strand (Q₁DNA), 2) a dual‐fluorophore‐labelled DNA strand (F₁DNAF₂) and 3) another quencher‐labelled DNA strand (Q₂DNA). The addition of a target releases Q₂DNA from the duplex assembly, and produces the expected positive signal from F₂. However, the authenticity of target recognition is validated only if no signal is generated from F₁. We have successfully engineered two fluorescent reporters by using an RNA aptamer that binds thrombin and one that binds theophylline. Both reporters show the expected binding affinity and specificity, and are capable of reporting system malfunction when treated with nucleases and chemical denaturants. This strategy provides a simple and reliable way to ensure high‐quality detection when RNA aptamers are employed as molecular‐recognition elements.     

Inferential closed‐loop control of particle size and molecular weight distribution in emulsion polymerization of styrene

In this work, simultaneous inferential control of particle size distribution (PSD) and molecular weight distribution (MWD) in a semi‐batch emulsion polymerization reactor of styrene has been addressed. Using a comprehensive dynamic model for PSD and MWD predictions and performing a sensitivity analysis, it has been revealed that free surfactant and chain transfer agent (CTA) concentrations in the reactor are the most suitable candidates for inferential control of PSD and MWD, respectively. To control concentrations of these species in the reactor, their inlet feed flow rates are used as manipulated variables. It is assumed that the concentration of CTA is measured infrequently and therefore an open‐looped observer, based on the reaction calorimetry, has been designed to estimate the CTA concentration. The infrequent measurements of CTA concentration are used to correct its estimation. As the online measuring of the ionic free surfactant concentration is also difficult, solution conductivity which is a good indication of free surfactant concentration is used for control purposes. Simulation results show that the performance of the proposed control scheme is satisfactory even in the presence of model mismatch. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers     

Solvent effect on TEMPO-mediated living free radical dispersion polymerization of styrene

Living free radical dispersion polymerization of styrene in the presence of 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) and 2,2-azobisisobutyronitrile (AIBN) was conducted in various glycol media having different solubility parameters. Genuine heterogeneous polymerization nature was observed from the initial stage of the reaction. The solubility parameter of the media was found to significantly influence the living characteristics of the polymerization including the polymerization kinetics and the molecular weight evolution at a fixed concentration of TEMPO. A grafting of stabilizer molecules onto monomer and a possible partitioning of TEMPO between the continuous medium and particle phase were postulated for the deviation of the experimentally obtained molecular weight from the calculated value. For a poor medium for styrene, such as diethylene glycol, the living nature was achieved by increasing the amount of TEMPO. The polystyrene (PS) microsphere was well obtained in all tested glycol media and the average size was increased with enhanced affinity of the media to styrene and with decreasing concentration of TEMPO.     

Gel Permeation Chromatography with High Loads

Abstract

High loads on GPC columns usually lead to poor efficiency because of steep viscosity gradients. A great difference in density between solution and solvent can also cause excessive band broadening. However, under certain conditions good separations are achieved with loads of 150 mg/100 cc column volume and higher. Two mechanisms are proposed to explain this phenomenon. Secondary exclusion is caused by obstruction of pores to larger molecules by the more rapidly diffusing small molecules. It takes place predominantly with molecules of less than 2000 molecular weight in small pore gels. Incompatibility is caused by repulsive interaction between solute molecules and the polystyrene gel. It is observed with solutes which are chemically quite different from polystyrene, e.g., with polyvinyl acetate, and in a low to intermediate molecular weight range.     

Synthesis, characterization and photophysical properties of benzil-labelled polymers for studies of diffusion-limited interactions by phosphorescence quenching

A new technique for synthesizing benzil-labelled polystyrenes has been developed. This new procedure has advantages over an older synthesis by Horie and Mita in that it involves fewer and less complicated synthetic steps. In addition, the new synthetic pathway should allow for labelling of different sites on the polymer chain and for labelling of different polymers such as poly(methyl methacrylate). The phosphorescence lifetime of terminally benzil-labelled polystyrenes in cyclohexane was found to be approximately 85–90 μs at 25°C with the concentration of benzil at 10⁻⁵ M and was independent of labelled polystyrene molecular weight and polystyrene concentration up to 150gl⁻¹. When used in phosphorescence quenching studies, these materials were found to exhibit non-single-exponential lifetimes in the presence of anthracene and azulene quenchers for a concentration of benzil label 10⁻⁴ M. An explanation for this anomalous behaviour and a technique for decoupling these decays into two lifetimes has been proposed. This technique should allow for the study of diffusion-limited interactions occurring in higher-molecular-weight polymer systems where the label concentration 10⁻⁴ M.     

The role of adsorbent pore size distribution in multicomponent adsorption on activated carbon

The impact of adsorbent pore size distribution (PSD) on adsorption mechanism for the multi solute system was evaluated in this study. Anoxic and oxic adsorption equilibrium for the single solute (phenol), binary solute (phenol/2-methylphenol) and ternary solute (phenol/2-methylphenol/2-ethylphenol) systems on one granular activated carbon (GAC) F400 and two types of activated carbon fibers (ACFs), namely, ACC-10 and ACC-15, were determined. F400 has a wide PSD, while ACC-10 and ACC-15 have narrow PSD and their critical pore diameters are 8.0 Å and 12.8 Å, respectively. In single solute adsorption, the increase of adsorptive capacity under oxic conditions as compared to anoxic ones was related to the PSD of the adsorbent. Binary solute adsorption on ACC-10 and ternary solute adsorption on ACC-15 indicated no impact of the presence of molecular oxygen on the adsorptive capacity and the adsorption isotherms were well predicted by the ideal adsorbed solution theory (IAST). Significant differences between oxic and anoxic isotherms were noticed for other multicomponent adsorption systems. The narrow PSD of ACFs was effective in hampering the oligomerization of phenolic compounds under oxic conditions. Such a phenomenon will provide accurate predictions of fixed bed adsorbers in water treatment systems.     

Diffusion Coefficients and Molecular Weight Distributions of Humic and Fulvic Acids Determined by Flow Field-Flow Fractionation

Abstract

The ability to characterize molecules whose physical and chemical properties are intimately linked to their diffusion coefficients and molecular weight is important to further understanding of chemical transport in the environment. Flow field-flow fractionation (flow FFF) was used to obtain separations of water-soluble macromolecules of varying molecular weight, including polystyrene sulfonates and humic substances. The separation occurs due to differing diffusion rates for chemical species of differing molecular weight in aqueous solution. Flow FFF uses fluid flow as the mechanism of separation. A model that yields liquid phase diffusion coefficients as a function of molecular weight was utilized to determine molecular weights from degree of separation. Separations of polystyrene sulfonates, a humic acid, and two fulvic acids of known molecular weight were accomplished using flow FFF. The separations obtained were used to develop a relationship between flow FFF separation and species molecular weight. Separations were obtained for humic and fulvic acids of unknown molecular weight.     

A study of interpenetration using fluorescence quenching of chromophore-labelled polymers

Fluorescence quenching was used to investigate the onset and extent of overlap and interpenetration in poly(vinyl methyl ether) (PVME) solutions using polystyrene with a dilute level of anthracene labelling (APS) as a probe. A low, constant concentration of APS was added to solutions varying in PVME concentration. The critical overlap concentration, c*, was taken to be the PVME concentration at which quenching of the probe anthracene-labelled polymer molecules commenced. Variations in probe molecular weight and concentration produced no significant changes in c* or in the rate of decrease in fluorescence intensity above c*, lending credibility to the idea that the APS is simply probing the PVME system. The values of c* [η] from these experiments were found to lie in the range of 0.87–1.7 which agrees with the results from previous studies. The sensitivity of this method to small differences in solvent quality for PVME was also investigated.