Quantitative determination of the chemical composition of silica-poly(norbornene) nanocomposites

Nanoparticle hybrid materials consisting of a silica core surrounded by a poly(norbornene) brush have been prepared by ring opening metathesis polymerization (ROMP). A quantitative determination of each stage of composite formation has been accomplished, including a determination of the density of surface-bound functional groups, catalyst molecules, and polymer chains. This analysis has enabled the determination of the reaction efficiency between the catalyst and the surface-bound functional groups as well as the determination of the fraction of metal-mediating species that initiate a polymer chain. Control of the chain density was demonstrated by two methods: the use of controlled reaction times between the catalyst and the surface, and the variation of the surface functional group density. Polymer chain densities resulting from composites prepared with different tether structures will also be reported. The resulting brush densities were found to span a wide range, including those previously reported for polymer layers formed by adsorption, grafting of preformed polymer chains, and surface-initiated polymerization (SIP).     

Monodisperse micron-sized macroporous poly(styrene-co-divinylbenzene) particles by seeded polymerization

Macroporous poly(styrene-co-divinylbenzene) particles were produced in a micron-size range by two-stage swelling and continuous polymerization. The molecular weight of the polystyrene seed particles was controlled by incorporating a urethane acrylate. It was found that the porosity of the particles produced by the seeded polymerization was dependent on the molecular weight of the seed polymer. As the molecular weight of the polystyrene seed increased, the porous particles produced became macroporous. Interestingly, the high molecular weight of the polystyrene seed had a negligible influence on the change of porosity of the seeded polymerized particles. It is believed that the viscosity of the swollen droplet phase remained pretty high with the change in composition because the polystyrene seed copolymerized with urethane cacrylate had many side chains. Received: 16 December 1999 Accepted: 9 August 2000     

Characterization of selectively degraded poly(ethylene terephthalate)

To investigate the morphology of unoriented poly(ethylene terephthalate) (PET) films and the selective character of the aminolysis of PET, 67% crystalline polymer samples were degraded with 40% aqueous methylamine at room temperature. The aminolyzed PET samples were subjected to gel permeation chromatography (GPC), viscometry, electron microscopy, and small-angle x-ray diffraction (SAXD). Weight loss and density crystallinity measurements were also made. After 24 hr of aminolysis, the amorphous regions and chain folds were completely removed. The long molecular chains in the semi-crystalline polymer were reduced to monodisperse rods having a molecular weight of 1,800. The corresponding lamellar thickness was calculated to be 101 Å, consistent with the x-ray diffraction and electron microscope (EM) measurements. The EM photographs of “stripped” crystals show the lamellar structure previously found for other selectively degraded polymeric materials. The weight of crystalline debris remaining was consistent with the initial crystallinity. After degradation the crystallinity as determined by density was 96%.     

Modeling molecular weight distribution of comb-branched graft copolymers

Grafting one type polymer onto a different polymer type may yield a comb-branched copolymer. The branching density has a significant effect on its overall molecular weight distribution. A general model is derived to describe the bivariate distribution of molecular weight and branching density for such comb copolymers. The model is applicable for various grafting mechanisms provided the side chains are randomly grafted onto the backbone. The determining parameters are the molecular weight distributions of backbone and side chains, and the branching density. Analytical expressions are obtained for the cases of the side chains having uniform and Schulz–Zimm distributions. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 705–714, 1998     

High-resolution polymer separations in a four-pass hairpin thermal field-flow fractionation column

A thin-channel four-pass hairpin thermal field-flow fractionation (FFF) column is described, and the advantages of its unique dimensional characteristics are explained. The problem of isolating the performance of this and other separations columns from the ubiquitous polydispersity effects are discussed and treated theoretically. The discussion is extended to size exclusion chromatography, and it is shown that the 2 to 6 times lower selectivity of the latter compared to FFF leads to the requirement for 4 to 36 times more theoretical plates to encounter polydispersity effects and thereby obtain information on polymer molecular weight distribution. A fractogram of six narrow polystyrene samples obtained from the hairpin system is shown to imitate closely the fractograms obtained from two totally different thermal FFF columns, showing that polydispersity dominates and that molecular weight information is revealed for these samples with only a few hundred theoretical plates. Various experimental and theoretical attempts are made to isolate the polydispersity and column contributions to plate height, including cut-and-recycle experiments, the observation of plate height versus velocity curves, and the direct calculation of the contributing effects. The various methods are subject to moderate errors, but are in rough agreement. The plate height plots show that the polydispersity effect contributed 53% and 68% to the measured plate height for 51,000 and 160,000 molecular weight samples, respectively. The latter polymer is shown to emerge with ca. 1300 true column plates. It is suggested that much higher column efficiency will be observed in the future if higher retention levels can be experimentally realized.     

Photocrosslinked hydrogels based on copolymers of poly(ethylene glycol) and lysine

A group of new, water-soluble poly(ether-urethane)s, derived from poly(ethylene glycol) and the amino acid L -lysine, provide pendent carboxylic acid groups along the polymer backbone at regular intervals. The carboxylic acid groups were utilized for the attachment of acrylate and methacrylate pendent chains (hydroxyethyl acrylate, hydroxyethyl methacrylate, aminoethyl methacrylate, and aminoethyl methacrylamide), leading to functionalized polymers. The pendent chains were attached via ester and/or amide bonds having different degrees of hydrolytic stability. The attachment reactions proceeded with high yields (up to 95%). The functionalized polymers were subsequently photopolymerized (UV irradiation) to obtain crosslinked hydrogels. Crosslinked membranes with the highest degree of mechanical strength were obtained when the crosslinking reaction was performed in dioxane with benzoin methyl ether (0.1 wt %) as the initiator. the crystallinity, thermomechanical properties, and hydrolytic stability of the crosslinked membranes were studied. All membranes were transparent and highly swellable (equilibrium water content: 64–88%). The tensile strength in the swollen state ranged from 0.15 to 1.09 MPa. Under physiological conditions (phosphate buffered water, 0.1M, pH 7.4, 37°C) the hydrolytic stability of the hydrogels varied depending on the bonds used in the attachment of the acrylate pendent chains: Hydrogels with hydroxyethyl acrylate pendent chains dissolved within 30 days, while hydrogels containing aminoethyl methacrylamide pendent chains remained unchanged throughout a 30 day period. Using high molecular weight FITC-dextrans as model compounds, complete release from the swollen hydrogels required between 60 and 150 h. Overall, the evaluation of poly(ethylene glycol)-lysine derived, photocrosslinked hydrogels indicated that these materials provide a range of potentially useful properties. © 1994 John Wiley & Sons, Inc.     

Molecular weight distribution of metallocene polymerization with long chain branching using a binary catalyst system

In metallocene polymerization, termination by β-hydride elimination generates polymer chains containing unsaturated vinyl groups at their chain ends. Further polymerization of these macromonomers produces branched polymers. Material properties of the branched polymers not only depend on molecular weight and branching density, but also on chain structure. This work presents analytical expressions to predict the bivariate distribution of molecular weight and branching density for polymer chains having dendritic and comb structures. It is shown that when a single metallocene catalyst is used the formation of dendritic polymers is favored with only a very small fraction of highly branched chains assuming comb structure. The use of a binary catalyst system is therefore proposed to obtain high content of comb polymers. One catalyst generates macromonomers and the other yields in-situ branching. It is found that the comb polymers give much narrower molecular weight distributions than dendritic polymers with same branching densities.     

Aging of Transformer Insulation Paper

Abstract

As a transformer ages, the chemical and physical properties of the cellulose insulation materials in the transformer change, and the paper loses its strength and becomes brittle. The average molecular weight of the cellulose chains decrease with age, and degradation products are formed, including water, carbon monoxide, carbon dioxide, and furans. The molecular weight changes in the cellulose have been studied by several methods, but the GPC method for determination of the molecular weight has been shown to offer advantages over other methods because it yields the total molecular weight distribution. The tensile strength of the cellulose insulation in transformers also changes with age as a result of the changing molecular weight of the cellulose. Work carried out in our laboratory on the aging of cellulose insulation is reviewed in this paper. Our studies have included investigations of insulation materials from retired transformers as well as accelerated aging of insulation paper in transformer oil in the temperature range of 129–166°C under vacuum. In the study the relationships between the molecular weight of the cellulose and the furan degradation products and tensile strength have been delineated, and they have been correlated with information on the kinetics of degradation of the insulation paper.     

Surface anchored polymer: Role in adhesion and friction

We present an investigation of thin polymer layers formed by either strong adsorption or end grafting on a surface or an interface. Depending on the kind of surface attachment, different internal organisations of the chains are observed: either polydisperse loops for adsorbed layers, or almost monodisperse tails in the case of grafting. The molecular parameters of the layer (length and surface density of anchored chains) and the molecular organisation inside the layer govern the ability of the surface anchored chains to be swollen by a good solvent or to penetrate into a bulk polymer, either a melt or a cross-linked elastomer, three properties which have been characterised through neutrons reflectivity techniques. We then analyse how the ability of the surface anchored chains to penetrate into a bulk polymer, entangle with it, and then be deformed when this bulk polymer is mechanically solicitated, are key parameters which govern adhesion and friction properties.     

Molecular weight distribution in free radical polymerization with long-chain branching

A new theory to predict the molecular weight distribution in free radical polymerization that includes chain transfer to polymer is proposed. This theory is based on the branching density distribution of the primary polymer molecules. The branching density distribution provides the information on how each chain is connected to other chains, and therefore, a full molecular weight distribution can be calculated by application of the Monte Carlo simulation. The present theory accounts for the history of the generated branched structure and can be applied to various reaction systems that involve branching and crosslinking regardless of the reactor types used. The present simulation confirmed the validity of the method of moments in a batch polymerization proposed earlier. It was shown clearly why gelation never occurs by chain transfer to polymer without the assistance of other interlinking reaction such as bimolecular termination by combination. © 1993 John Wiley & Sons, Inc.     

Uniform oligomers as reference materials for molecular weight determination of polymers

 Uniform oligomers are special oligomers having no molecular weight distribution. They have been used as reference materials for polymer characterization, especially for accurate molecular weight determination in lower molecular weight ranges. These oligomers are used for different purposes. First, they are used in the calibration for chromatographic or spectroscopic data. Second, they are applied to identifying peaks appearing in chromatographies such as supercritical fluid chromatography (SFC) of oligomeric mixtures. Finally, they could give accurate instrumental or physicochemical constants used in polymer characterizations. The uniform oligomers have been prepared by using preparative chromatographies as separation methods. In some cases, step-wise syntheses have been applied together with the chromatographic methods. Recently, we prepared uniform oligostyrenes and oligo(oxyethylene)s whose molecular weights are greater than 1000 by using preparative size exclusion chromatography. More recently, using preparative SFC, some Japanese researchers have prepared high molecular weight and uniform oligomers such as oligo(methylmethacrylate)s. In this article, we briefly review recent progress mainly made by Japanese researchers in the field of preparation, analysis and applications of these pure oligomers. Received: 20 September 1996 Accepted: 3 December 1996     

Dielectric properties of poly(4-methylphenylisocyanate) and poly(4-methoxyphenylisocyanate) in solution

In the range 200 Hz-2 MHz, the dielectric behaviour of two polyarylisocyanates depends on the solvent. In certain solvents, such as toluene, there is only one high frequency absorption for poly(4-methylphenylisocyanate) and it is independent of molecular weight. Its character corresponds to chains having a low kinetic flexibility. In other solvents, this polymer, as well as poly(4-methoxyphenylisocyanate), shows two regions of dielectric absorption. The low frequency relaxation is molecular weight dependent: a polarization due to solvent polymer interactions occurs along the main chain. Despite its solvent-dependence, the total dipole moment of these polyarylisocyanates which behave like coils is much lower than that of polyalkylisocyanates, which are known to be rod-like.     

Functionality and Functionality Distribution Measurements of Binder Prepolymers

Abstract

The functionality of a prepolymer, which is defined as the ratio of molecular weight to equivalent weight, is probably the most important single parameter that determines the properties of the cross-linked polymer network. The determination of prepolymer functionality therefore requires accurate knowledge of both number average molecular weight and equivalent weight. Ideally, a suitable prepolymer for propellant binder applications has terminal functionality (OH or COOH). Such a prepolymer theoretically has a functionality of 2.0. Because of uncontrolled chain termination reactions during the prepolymer synthesis, however, not all polymer chains have the desired functional end group. As a result, prepolymers generally have a distribution of functionalities, including onfunctional, monofunctional, and the desired difunctional prepolymer.     

Thermal degradation of poly(lactic acid) (PLA) and poly(butylene adipate-co-terephthalate) (PBAT) and their blends upon melt processing

Poly(lactic acid) (PLA) and poly(butylene adipate-co-terephthalate) (PBAT) are biodegradable aliphatic polyesters, which being semicrystalline and thermoplastic can be processed by conventional methods. Their blends give interesting materials for industrial packaging applications, due to their increased ductility as PBAT content increases. However, like many aliphatic polyesters, the PLA matrix degrades upon melt processing thus affecting the thermo-mechanical features of the blended material. In this work, we studied the effect of processing at high temperature on the molecular weight distribution, morphology, and thermo-mechanical properties of both homopolymers, as well as the PLA/PBAT 75/25 blend. Notably, different processing conditions were adopted in terms of temperature (range 150-200 °C) and other relevant processing parameters (moisture removal and nitrogen atmosphere). Analysis of PLA/PBAT blends indicated that intermolecular chain reactions took place under strong degradative conditions of PLA, yielding PLA/PBAT mixed chains (copolymers). Increasing amounts of copolymers resulted in improved phase dispersion and increased ductility, as SEM and mechanical tests indicated. Conversely, reduced PLA degradation with less copolymer formation, afforded higher modulus materials, owing to poorer dispersion of the soft phase (PBAT) into the PLA matrix.     

Pressure driven flow of polymer solutions in nanoscale slit pores

Polymer solutions subject to pressure driven flow and in nanoscale slit pores are systematically investigated using the dissipative particle dynamics approach. The authors investigated the effect of molecular weight, polymer concentration, and flow rate on the profiles across the channel of the fluid and polymer velocities, polymer density, and the three components of the polymers radius of gyration. They found that the mean streaming fluid velocity decreases as the polymer molecular weight and/or polymer concentration is increased, and that the deviation of the velocity profile from the parabolic profile is accentuated with increase in polymer molecular weight or concentration. They also found that the distribution of polymers conformation is highly anisotropic and nonuniform across the channel. The polymer density profile is also found to be nonuniform, exhibiting a local minimum in the center plane followed by two symmetric peaks. They found a migration of the polymer chains either from or toward the walls. For relatively long chains, as compared to the thickness of the slit, a migration toward the walls is observed. However, for relatively short chains, a migration away from the walls is observed.     

Complex formation and dynamics in polymer melts

The viscoelastic properties of polymer chains with a few number of pairwise associating functional groups are studied as a function of polymer molecular weight, degree of substitution and polymer concentration. As a model system, polybutadienes of narrow molecular weight distribution are used as starting materials. These polymer chains are modified by introducing few 1,2,4-triazolidine-3,5-dione (urazole) groups U. These form defined dimeric hydrogen bond complexes U + U → U₂.     

Effect of electron radiation and triallyl isocyanurate on the average molecular weight and crosslinking of poly(ε‐caprolactone)

Investigation of the effect of electron radiation on the flow rate and average molecular weight of poly(ε‐caprolactone) (PCL) as well as on formation of the gel fraction of this polymer being irradiated in the presence of triallyl isocyanurate (TAIC) was the aim of the present paper. It was found that PCL macromolecules upon the electron radiation underwent both degradation and linking, because of which the polymer molecular weight increased. The processes associated with elongation of the polymer chains prevailed over the degradation ones. It was also found that PCL irradiated in the presence of TAIC underwent crosslinking resulting in formation of a significant amount of the gel fraction. The largest amount of this fraction was created upon the radiation with the dose of 60 kGy, which was confirmed by the results of determination of the swelling index. Changes in properties of PCL, occurring because of the electron radiation, are important for controlling viscosity of polymeric materials during processing of these materials. Copyright © 2015 John Wiley & Sons, Ltd.     

Spherical polymer brushes under good solvent conditions: molecular dynamics results compared to density functional theory

A coarse grained model for flexible polymers end-grafted to repulsive spherical nanoparticles is studied for various chain lengths and grafting densities under good solvent conditions by molecular dynamics methods and density functional theory. With increasing chain length, the monomer density profile exhibits a crossover to the star polymer limit. The distribution of polymer ends and the linear dimensions of individual polymer chains are obtained, while the inhomogeneous stretching of the chains is characterized by the local persistence lengths. The results on the structure factor of both single chain and full spherical brush as well as the range of applicability of the different theoretical tools are presented. Finally, a brief discussion of the experiment is given.     

Mesure de la distribution des masses moléculaires entre ponts dans les réseaux

The classical theories of rubber elasticity allow a polymer network to be characterized by the mean molecular weight between crosslinks. This is measured by swelling or by determination of the elastic modulus. A method is described here that determines the distribution of the molecular weights between crosslinks, obtained by finding the spectrum of the freezing temperatures of a swelling solvent. It is known that the freezing point depends on the crystal size. It is proposed that the crystal size is equal to the distance between crosslinks in the swollen gel. The molecular weight between crosslinks and its distribution can be determined by the scaling laws.     

Determination of Morphological Properties of Swellable Solids by Size Exclusion Chromatography

Many technically interesting porous solids, e.g. ion exchangers or adsorbents for catalysis, are swellable polymers, i.e. the pore structure depends on the solvent medium. A method based on exclusion chromatography, permits determination of the pore size and pore size distribution in the swollen state.—Size exclusion chromatography, also referred to as gel permeation, gel filtration, or molecular sieve chromatography, is a widely employed method for the separation of dissolved substances—mostly polymer mixtures—according to their molecular size. Porous solids are used as stationary phase. Conversely, pore sizes and other structural data can be determined by exclusion chromatography. This application requires a series of standards (polymer samples) of known molecular weight. As a simple and rapid method, it has already proven valuable for such determinations in the case of rigid solids; in the case of swellable solids, this constitutes the sole method by which the pore structure can be characterized: classical methods require dry samples.