Increasing Polymer Molecular Weight Research Articles

New Poly(ethylene oxide)−Clay Composites

This paper reports a new mechanism for the formation of clay intercalates containing poly(ethylene oxide) (PEO). This mechanism permits formation of a two-dimensional PEO crystal phase. Under acidic conditions, polymer adsorption occurs through an ion-exchange process that is mediated by oxonium cation formation. A single phase exhibiting a plateau in the d001 reflections of 19 Å is formed at a polymer/clay stoichiometry of 0.5 g/g. This two-dimensional PEO crystal phase has a higher melting temperature than its three-dimensional counterpart because it is confined within the clay galleries. Unlike previously reported methods for forming PEO/clay intercalates, oxonium ion exchange produces structures whose basal spacings increase with increasing polymer molecular weight.

Study of the Adsorption Mechanism and the Structure of Adsorbed Layers of Polyelectrolytes at the Metal Oxide/Solution Interface

The influence of the molecular weight of polyacrylic acid (PAA) and polyacrylamide (PAM) as well as of inorganic contaminants on the ZrO2 surface on the adsorption and electrokinetic properties of the metal oxide/polyelectrolyte solution interface were studied for ZrO2 and Fe2O3 solid particles. The calculated concentrations of the various surface groups on ZrO2 and Fe2O3 enabled an investigation of the possible mechanism for the bonding of the studied polyelectrolytes with the surfaces of both oxides. PAA and PAM macromolecules bond with the solid surface mainly via the –OH groups of the oxides, which may interact with the carboxy groups of polyelectrolytes through hydrogen bridging. A comparison of the change in values of the diffuse layer charge with the surface charge enabled the principal factors responsible for the changes in the zeta potential of the oxides, e.g. pH, polymer molecular weight and concentration of the polymer solutions, to be evaluated. From such zeta potential values, it was possible to determine the free energies of adsorption of PAA and PAM on the surfaces of both oxides. The thicknesses of adsorbed polymer layers on Fe2O3 and ZrO2 were calculated on the basis of measurements of their suspension viscosities in the absence and presence of adsorbed polymer. It was shown that the thickness of the adsorption layer increased with increasing polymer molecular weight, pH and concentration of the polymer solution. Because the experimental determination of the number and the length of trains, loops and tails in such polyelectrolytes was not possible, the participation of such segments of polymer structure at the interface was computed using the Scheutjens–Fleer model of polymer adsorption. The polymer adsorption expressed as the number of equivalent monolayers was calculated and compared with the experimental data.

Crossover behavior of star polymers in good solvents

We perform Monte Carlo calculations for the mean-square center-to-end distance, mean-square radius of gyration, and second virial coefficient of f=3 to 41 arm star polymers composed of rigidly bonded hard spheres of varying diameters. As with linear chains, there are two different crossover regimes: (i) crossover from the Gaussian chain to the Kuhnian chain limit, where the penetration function Ψ(f) increases monotonically with increasing polymer molecular weight, and (ii) crossover from the rigid-rod to the Kuhnian chain limit, where the penetration function decreases with increasing molecular weight. We propose a phenomenological approach for the extension of our previous crossover theory for linear polymers to star polymers. We show that the theoretical crossover function obtained earlier by Douglas and Freed [Macromolecules 16, 1854 (1984)] fails to reproduce the simulation data for the penetration function with f⩾6, while the phenomenological crossover model is in good agreement with the simulation data up to f⩽41. We also obtain a generalized crossover equation for the penetration function for linear and star polymers in good solvents. The crossover equation is able to accurately describe the variation of the infinite molecular weight limit of the penetration function Ψ*(f) with the number of arms f on the star polymer, and it predicts that Ψ*(f) approaches 2.39 in the limit f→∞.

Surfactant-mediated gelation of hydrophobically modified hydroxyethyl cellulose: effects of polymer and surfactant structure

We have investigated the effects of polymer structure, surfactant structure, and system composition on the phase behavior of a nonionic hydrophobically modified cellulose ether (HMHEC) and the rheological properties of selected hydrogels. Specific conditions under which strong hydrogels phase separate out of these systems have been identified. Gel formation is favored when the side chain level on the polymer is ∼210 side chains/backbone. In general, gel strength increases with increasing polymer molecular weight but does not scale with side chain content on the polymer. A mechanism for this is proposed. Stronger gels formed in sodium dodecyl sulfate (SDS) than in sodium octyl sulfate (SOS) or sodium octyl sulfonate (SS), suggesting that the surface activity of a surfactant, which is proportional to the length of its tail group and inversely proportional to its critical micelle concentration (cmc), governs the extent of surfactant binding to the polymer as well as the extent of polymer/polymer hydrophobic association.

Matrix-assisted laser desorption ionization time-of-flight mass spectrometry for the analysis of polydienes

An analytical method based on matrix-assisted laser desorption ionization (MALDI) time-of-flight mass spectrometry has been developed to provide information on oligomer structure, average molecular weight, and molecular weight distributions of polydienes (e.g., polybutadiene and polyisoprene), an important class of industrial polymers. This MALDI method involves the use of all-trans-retinoic acid as the matrix, copper (II) nitrate as the cationization reagent, and tetrahydrofuran as the solvent. The incorporation of this copper salt generates Cu+ adducts with the polymer chains. It also improves the signal strength and extends the upper mass range when used with all-trans-retinoic acid, as compared to silver nitrate. With this formulation, it is demonstrated that polybutadienes of narrow polydispersity with masses up to 300,000 u and polyisoprenes of narrow polydispersity with masses up to 150,000 u can be analyzed. The upper molecular weight limit is set by the requirement of using higher matrix-to-polymer ratios with increasing polymer molecular weight, to the point where the instrument can no longer detect the small quantity of polymer present in the matrix host. It is also shown that this sample preparation generates previously unreported adduction behavior. The practical implications of this adduction behavior on polymer structural analysis, accuracy of molecular weight determination, and the upper molecular weight limit of oligomer resolution are discussed. It is illustrated that, in a linear time-lag focusing MALDI instrument, oligomer resolution can be obtained for polydienes with molecular weights up to 24,000, providing structural confirmation of the end-groups and the repeat unit. The average molecular weights of a number of polydienes of narrow polydispersity determined by MALDI are compared to those obtained by gel permeation chromatography, and discrepancies are noted.

Characterization of Poly(dimethylsiloxane)s by Time-of-Flight Secondary Ion Mass Spectrometry

A series of poly(dimethylsiloxane)s (PDMS) terminated by trimethylsilyl groups, and having molecular weights from 600 to 20 000, were characterized by time-of-flight secondary ion mass spectrometry. The fragmentation patterns of the PDMS were examined. The predominant fragmentation pathway involves formation of cyclic fragments, probably via an intermediate with a four-membered ring. The relative intensities of the clusters in the PDMS fragmentation patterns were investigated as a function of sample molecular weight. The relative intensities of cyclic fragments increase with increasing polymer molecular weight. Poly(dimethylsiloxane)s containing different end groups were also investigated to determine the effect of the terminal groups on fragmentation. The terminal groups in PDMS were identified by analysis of low mass range spectra. Effects of nitrogen-containing end groups were found on the fragmentation of PDMS. Large fragments with intrinsic positive charge were observed in the spectra of PDMS terminated by (aminopropyl)dimethylsiloxy and (amidopropyl)dimethylsiloxy groups.

A friction factor analysis of the coupling between polymer/solvent self- and mutual-diffusion: polystyrene/toluene

The Bearman statistical mechanical theory, which couples the mutual-diffusion and self-diffusion coefficients via friction factors, has been applied to polystyrene/toluene solutions with polystyrene molecular weights of 18 kDa and 900 kDa. Toluene and polystyrene self-diffusion coefficients, obtained from the literature and measured here, along with polystyrene/toluene binary mutual-diffusion coefficients and thermodynamic data, were employed to independently calculate the three friction coefficients (ζ12, ζ11, and ζ22) required to describe transport within binary solutions. Results reveal that the frequently used geometric mean approximation (GMA) for relating the friction coefficients consistently underestimates ζ22 and worsens with increasing polymer molecular weight. The GMA is found to be appreciably more accurate than the arithmetic mean approximation. A formalism recently proposed by Vrentas et al. has also been evaluated for this system and is found to describe the concentration dependence of ζ11 very well. The Vrentas model, therefore, can accurately predict mutual-diffusion coefficients from solvent self-diffusion coefficients for polystyrene/toluene solutions. © 1996 John Wiley & Sons, Inc.

Monitoring Flocculation in Situ in Sterically Stabilized Silica Dispersions Using Rheological Techniques

Flocculation of sterically stabilized colloidal dispersions is a technologically important and complex phenomenon. Often, a state of controlled, weak flocculation is desired to slow the aging of dispersions due to irreversible flocculation and subsequent sedimentation. Using rheological techniques, we have characterized flocculation in situ in sterically stabilized fumed silica dispersions toward identifying those variables important to the control of the flocculated network strength. Both poly(methyl methacrylate) homopolymers and polystyrene/poly(methyl methacrylate) block copolymers have been used as stabilizers. Dynamic oscillatory measurements have been used to quantify the elastic moduli and hence the strength of the flocculated networks as a function of polymer surface coverage, molecular weight, architecture, and solvency as well as particle volume fraction. The results show that the flocculated network strength is weaker for complete surface coverage and when diblock copolymers are used as stabilizers, as expected. In addition, a scaling of the elastic modulus with particle volume fraction demonstrates weaker flocculation with increasing polymer molecular weight. This work illustrates the utility of rheological techniques in studying the dynamic flocculation process

Effect of protic compounds on ethylene polymerization by HfTi supported catalysts

The effect was investigated of the addition of different protic compounds, such as water, n-butanol, butanoic acid and methyl ethyl ketone, to triisobutylaluminium used as the cocatalyst for an HfTi bimetallic catalyst supported on magnesium dichloride (Ti 1.0Hf 1.3Mg 2.0Al 0.85Cl 12.45) in the polymerization of ethylene. All the above compounds show a similar behaviour by decreasing catalyst activity, increasing polymer molecular weight and narrowing polymer molecular weight distribution. Minor differences are observed in the end-group unsaturations. An attempt has been made to correlate these effects to the interaction of hafnium and titanium sites with the reaction products from AlR 3 and the different protic compounds.

Influence of polymer molecular weight and temperature on phase composition in aqueous two-phase systems

In an effort to understand the molecular weight and temperature dependence of protein partition coefficients, we have conducted experiments to determine how the binodal curve and the difference in polymer concentrations between the top and bottom phases (▵ PEG  [PEG] T — [PEG] B and Δ Dx  [Dx] B — [Dx] T); depend on polymer molecular weight and temperature. We report the equilibrium compositions at 4°, 25° and 40°C for 16 different aqueous two-phase PEG-Dx systems which consist of all possible combinations of the four PEG molecular weights (4000, 6000, 10 000, and 20 000) and the four Dx molecular weights (10 000, 40 000, 110 000 and 500 000). Four tie lines were measured for each system. We confirm that increasing polymer molecular weight increases the tie line length and depresses the binodals to lower polymer concentrations, and that this effect will be greater when the ratio of this molecular weight increase to the molecular weight of the other species is large. The increase in the tie line length and in Δ Dx and Δ PEG with increasing polymer molecular weight tends to level off at high polymer molecular weight and at high total polymer concentrations. It is shown that the effect of temperature is not symmetric, i.e. changes in temperature produce a change in Δ Dx while Δ PEG remains nearly constant. The effect of polymer molecular weight on both Δ Dx and Δ PEG is magnified with increasing temperature. The results found here suggest a series of experiments which could be used to isolate (or highlight) the effect of the second virial coefficient on the partition coefficient.

Protein partitioning in two-phase aqueous polymer systems. 1. Novel physical pictures and a scaling thermodynamic formulation

A new molecular thermodynamic description of the interactions of globular proteins and flexible nonionic polymers, which influence the partition of proteins in two-phase aqueous polymer systems is presented. The changing nature of the polymer solution phase from one of identifiable polymer coils to one of an entangled web or net of polymers, with increasing polymer molecular weight is described. In addition to the novel physical pictures proposed for the interactions of polymers and proteins in two-phase aqeous polymer systems, descriptions are proposed for the free energy of interaction within each of the pictures and this quantity is related through a statistical thermodynamic framework to the experimentally measured protein partition coefficient

Helical pitch of ferroelectric liquid-crystalline polymers and copolymers

Abstract Ferroelectric liquid-crystalline polymers and copolymers were synthesized from acrylate polymers with side chains consisting of a flexible spacer unit, a mesogenic unit and an optically active substituent. In the chiral smectic C (S∗c) phases typical fan-shaped textures with equidistant lines (caused by the helical structures) were observed. It was found that the helical pitch increased with increasing polymer molecular weight. The helical pitch also increased in copolymers containing mixtures of right- and left-handed chiral side chains.

Characterisation of Biodegradable Microspheres Containing Dehydro-Iso-Androsterone

AbstractPoly-DL-lactide (PLA) and poly-DL-lactide-co-glycolide (DL-PGA) micromatrices (PharmazomesTM) containing dehydro-isoandrosterone (DHEA), a weak androgen, were prepared by a solvent evaporation process. Micromatrices with increasing drug loading as well as increasing polymer molecular weight were prepared. The morphology of these systems depended on the drug loading, the polymer molecular weight and polymer composition. Increasing the drug loading or polymer molecular weight resulted in increasingly irregular microparticles being formed. DSC thermograms did not reveal the presence of crystalline DHEA in micromatrices containing 10 to 50% DHEA loading. However crystalline DHEA was observed in microspheres heated to above the glass transition temperature of the polymer. Xray analysis of 30% DHEA micromatrices established the presence of crystalline DHEA in the micromatrices. The percent release of DHEA from the micromatrices, into 40% ethanol at 37°C, increased with increasing DHEA loading. The disso...

Polymer Characteristics and Attachment Sites in the Sludge Matrix

This study identified polymer attachment sites in three types of sludge for different molecular weight and charge density cationic polymers. Conditioned and unconditioned sludge samples were treated with cationized ferritin (CF) to label anionic charged sites on the surface of sludge particles. Sludge surfaces were examined using transmission electron microscopy. The presence of CF indicates an anionic site not attached to polymers. For increasing polymer molecular weight, comparison of micrographs of samples conditioned with similar polymer doses showed an increase in CF attachment. Therefore polymer attachment was seen to decrease with increasing polymer molecular weight. At extremely high polymer dosages, the presence of CF indicates that anionic sites have not been saturated by polymers. These results imply the molecular weight (chain length) and structure of polymers may determine the mechanisms of polymer action. No appreciable differences were observed for sludges conditioned with polymers of various charge densities.

Interfacial Phenomena in Change-of-Phase Heat Transfer: Low Concentration Polymer Solutions

Interferometry was used to measure in situ the thickness profiles of evaporating thin films as a function of heat flux for low concentration polystyrenel/cyclohexane solutions. Evaporative heat flow rates per unit width of contact line were calculated based on polynomial fits of interference data, combined with a fluid flow model. Temperature profiles obtained from thermocouple measurements were employed in evaluating macroscopic interline region heat sink capacities using a one-dimensional heat conduction model. Reasonable agreement between evaporative heat fluxes derived from both methods was established. Pure solvent integral heat fluxes were not significantly altered through polymer addition. Fluid flow toward the triple interline to replace that which evaporated resulted in an apparent non-equilibrium phase separation of concentrated polymer solution from the bulk phase; the separation increased with increasing polymer molecular weight and concentration. A substantial rise in the reflectivity beyond ...

Stability of non-aqueous microgel dispersions in the presence of free polymer

A series of cross-linked polystyrene microgel particles have been prepared and characterised in terms of their surface carboxyl-group concentration and their unswollen (in water) and swollen (in ethyl benzene) diameters. From the values for these two diameters the average segment volume fraction in the swollen microgel particles has been calculated (values range from 0.10 to 0.63). The effect on adding homopolymer polystyrene samples covering a wide molecular-weight range (1800 to 2 × 106) has been studied. Critical polymer/solvent volume fractions (ϕ+ps) have been established, at different microgel volume fractions, below which the microgel dispersion is stable, but above which weak, reversible flocculation is observed. For a fixed microgel volume fraction, ϕ+ps decreased with increasing polymer molecular weight. For a given molecular weight, ϕ+ps increased with increasing average segment concentration in the swollen microgel particles. Both of these trends are predicted by the recent theory of Vincent, Luckham and Waite, which analyses the effect of free polymer on the stability of sterically stabilised dispersions. By a suitable choice of the form of the segment concentration distribution in the swollen microgel particles, good agreement between experimental and theoretical ϕ+ps values may be obtained.

Flow of drag-reducing fluids through packed beds

An experimental investigation of the flow of drag-reducing fluids through beds packed with spheres, at flows beyond the range of validity of Darcy's law, is presented. Measurements were conducted in a constant flow rate apparatus. The Ergun equation used for comparison with the experimental data was modified to account for the wall effect of the column. Drag reduction was exhibited in flow through the packed beds. The onset of drag reduction occurs at Reynolds numbers in the range 150–250 and was found to depend on the size of the packing and on the type and molecular weight of the polymer additive. The level of drag reduction increased with increasing polymer molecular weight and decreasing particle diameter. Optimum polymer concentrations that yielded maximum drag reduction were observed. The optimum concentrations for dilute aqueous solutions of Polyox Coagulant and Polyox WSR-301 were found to be 20 and 30 ppm, respectively, in agreement with those reported for turbulent pipe flow.

Polymerization and reaction rate studies on substituted phenylmethylbis(dimethylamino)silanes

AbstractThe synthesis of seven para‐ or meta‐substituted phenylmethylbis(dimethylamino)‐silane monomers has been carried out. These silanes were polymerized with 1,4‐bis(hydroxydimethylsilyl)benzene in tetrahydrofuran at 30°C, and the polymerization kinetics were followed by monitoring dimethylamine evolution for 200 min. The polymers were quenched by precipitation in methanol and molecular weight data were obtained. The polymerizations followed second‐order kinetics in every case as evidenced by the linear plots of 1/(a − x) versus time. The molecular weight data generally correlated with the specific reaction rate constant k2 to show an increasing polymer molecular weight with increasing polymerization rate, although the range of k2 values obtained for the substituted aminosilanes was relatively small (2.50 × 10−5–6.67 × 10−5 l./mole‐sec). The value of k2 increased in the following order: p‐OCH3, p‐F, m‐CH3, H, m‐OCH3, p‐CF3, 3,5‐di(CF3). The logarithms of the rate constants correlated with the σ constants for the substituents, with a reaction constant, ρ of 0.391. The displacement at silicon in these reactions is discussed in terms of bimolecular mechanisms in which a four‐center transition state may participate.

The source of degradation during extrusion of polystyrene

The nature and magnitude of mechanical reactions of polystyrene in capillary flow has been examined in a model extrusion process. Studies on polystyrene quantify the sensitive increase in shear degradation tendency with increasing polymer molecular weight. A molecular weight spectrum caused by the shear stress profile was measured across the extrudate radius by the new technique of solvent coring. It was further determined that an appreciable fraction of the mechanical reaction is shear induced in the capillary reservoir. This is confirmed by precision determinations of molecular weights and distributions by gel permeation chromatography on samples taken from concentric layers in the capillary reservoir after 50% sample extrusion. These results, involving traces of oxygen as a chemical probe, describe the stress profile in the reservoir and in the capillary during the pressure extrusion of high molecular weight polystyrene. Thus, changes in molecular weight and distribution may be attributable to changes in different portions of the shear geometry rather than the uniform changes generally considered. Clear evidence is also presented showing the dramatic effects of oxygen on these shear-induced changes in molecular weight and distribution.

The effect of polymeric hydrocolloids on the partial coalescence of liquid drops at liquid-liquid interfaces

The coalescence behavior of organic drops at organic-aqueous hydrocolloid interfaces has been investigated. Synthetic water-soluble polymers of varying solution surface activity and molecular weight were employed as stabilizing agents for the organic drops. The number of steps required for complete coalescence of an organic drop, and the stability provided that drop, were a function of the hydrocolloid's molecular weight and solution surface tension. With weakly surface-active hydrocolloids, drop stability increased whereas the number of coalescence steps decreased with increasing polymer molecular weight. However, with strongly surface-active hydrocolloids greater drop stability was observed with lower molecular weight polymers. An aging phenomenon, which imparted stability, was noted with hydrocolloid increasing solution surface activity and decreasing molecular weight. This study, in general, relates stepwise coalescence and drop stability to the amount of hydrocolloid adsorbed and its interfacial reorientation capability. The results are believed to reflect the significance of a “dynamic interfacial viscosity parameter” in the coalescence phenomenon. In a related study, an attempt was made to examine the effect of poly(ethylene oxide) conformational changes on coalescence behavior. Copolymers of ethylene oxide and the effect of contraction in poly(ethylene oxide's) hydrodynamic volume, induced by interaction with inorganic salts, were examined. The significance of these studies is discussed.