Showing papers on "Polymer published in 1979"

Electrical properties of polymers

Abstract: 1. Introduction 2. Dielectrics in static fields 3. Dielectric relaxation 4. Electronic conduction in polymers 5. Measurement of electrical properties 6. Dielectric breakdown 7. Static charges 8. Ionic conduction, particulate and molecular composites 9. Intrinsically conductive polymers 10. Applications of electro-active and conductive polymers 11. References 12. Index.

Thermal gelation properties of methyl and hydroxypropyl methylcellulose

Abstract: Aqueous solutions of methyl and hydroxypropyl methylcellulose are known to gel upon heating. These gels are completely reversible in that they are formed upon heating yet will liquefy upon cooling. The precipitation temperature, gelation temperature, and gel strength of these methylcellulose solutions were determined as a function of molecular weight, degree of methyl and hydroxypropyl substitution, concentration, and presence of additives. The precipitation temperature of these polymer solutions decreases initially with increasing concentration until a critical concentration is reached above which the precipitation temperature is little affected by concentration changes. The incipient gelation temperature decreases linearly with concentration. The strength of these gels is time dependent, increases with increasing molecular weight, decreases with increasing hydroxypropyl substitution, and depends on the nature of additives. Hydrophobe–hydrophobe interaction or micellar interaction is postulated to be the cause of gelation. This thermal gelation property of the polymers is utilized in many end uses including food, pharmaceuticals, ceramics, tobacco, and other industrial applications.

Dynamics of concentrated polymer systems. Part 4.—Rheological properties

Abstract: The constitutive equation for polymer melts and concentrated solutions derived in the previous papers is applied to two typical rheometrical flows: steady and transient shear flow, oscillatory shear flow superposed on steady flow, steady and transient uniaxial elongational flow. The stress responses predicted are qualitatively in good agreement with experiments except for one case (the first normal stress in the transient shear flow). A particularly interesting result is that the constitutive equation suggests instability in steady shear flow and uniaxial elongational flow.

Conjugated One and Two Dimensional Polymers

Abstract: The band structures of some existing and hypothetical one and two dimensional conjugated polymers are examined in this work. The aim is to explore how the band gap sizes and the occurrence of partially filled bands are related to unit cell constitution and the geometrical disposition of the atoms in the unit cell. Among the polymers explored are polyacetylene, polydiacetylene, polyphenylene, polyacene, graphite, boron nitride, polynitrile, polycyanonitrile, polypyridinopyridine, paracyanogen, the C$\_{3}$N net, (SN)$\_{x}$, (XCNSN)$\_{x}$, and C$\_{2}$S$\_{3}$N$\_{6}$ net.

Microscopic mechanisms and mechanics of craze growth and fracture

Abstract: Isolated air crazes have been produced in thin films of polystyrene (PS) bonded to copper grids by straining these in tension The craze thickness profile, r(x), was directly determined from a series of transmission electron micrographs taken along the craze Local values of the craze fibril volume fraction v 1 and fibril extension ratio λ were established at frequent intervals along the craze by optical densitometry of the micrographs The craze surface displacement profile w(x), craze surface stress profile S(x) and the true stress a t in the craze fibrils are computed from these parameters The λ(x) profile provides conclusive evidence that the craze increases in thickness as it grows in length by drawing new polymer from the craze surface into the fibrils rather than by creep of the existing fibrils The S profile exhibits a modest maximum at the craze tip but falls slowly over a distance of about 15 μm behind the craze tip to a value about 10% below the applied tensile stress The λ of the d

Gas Sorption and Transport in Glassy Polymers

Abstract: Gas sorption and transport in amorphous polymers above their glass transition temperature are adequately described phenomenologically by Henry's and Fick's laws. However, below the glass transition temperature an additional sorption mechanism develops which follows the Langmuir isotherm, and, thus, there are two populations of gas molecules which may be regarded to be in equilibrium with each other. The Langmuir mechanism evidently arises from the non-equilibrium nature of the glassy state. Molecules sorbed by this mechanism have less diffusional mobility than molecules sorbed by the Henry's law mode. Because of these differences, the analyses of transient permeation experiments are more complex for glassy polymers compared to those used widely for rubbery polymers. A more comprehensive phenomeno-logical model for transport of gases in glassy polymers is developed here which has been successfully used to interpret the observed dependence of the permeability coefficient and diffusion time lag on upstream gas pressure. The state of knowledge about gas sorption and transport in glassy polymers is reviewed and contrasted with that of rubbery polymers. It is seen that such observations provide a unique method to probe the physical structure of the glassy state and that knowledge in this area is increasing rapidly.

Absorption of low molecular weight compounds in aqueous dispersions of polymer-oligomer particles, 2. A two step swelling process of polymer particles giving an enormous increase in absorption capacity

Abstract: A novel method for preparation of stable emulsions of slightly water soluble compounds is described. The method implies that the slightly water soluble compound diffuses through water and becomes absorbed into polymer particles which in a previous step have been brought to absorb a water insoluble, relatively low molecular weight compound. By this two step swelling process the polymer particles can be brought to absorb more than 100 times their own volume of the low molecular weight compounds to form stable o/w emulsions of the latter, with high oil content, and with a droplet size and size distribution which is completely determined by the size of the polymer particles in the latex applied initially. Monodisperse emulsions with large droplet size could easily be prepared. When the slightly water soluble compound added in the second step is a vinyl monomer which may subsequently be polymerized, the method represents a seed technique which is especially favourable for preparation of latexes with large particles size, including monodisperse latexes, with high solid content.

Polymerization of diacetylenes in multilayers

Abstract: The synthesis of extended and oriented very thin layers of polymers by solid-state polymerization of multilayers built up from monomolecular films of suitable monomers by means of the Langmuir-Blodgett (LB) technique has been felt to be a particular challenge for a long time. Attempts to make use of radiation-induced free radical polymerization of long-chain vinyl derivatives have been successful only in part (1–5).

Silicone Elastomer Developments 1967–1977

Abstract: Silicone rubber for purposes of this review will cover materials based on the polymer polydimethylsiloxane (PDMS). Of necessity, groups other than methyl as substituents on silicon will be included, and changes in the backbone structure from that of pure siloxane will be discussed. Rubbers in which polydimethylsiloxane is only a minor constituent will not be a part of this paper. The definition of what constitutes a rubber becomes blurred as one considers elastoplastic materials and rubbery resins. In this review, a rubber is a material whose properties and function when cured depend principally upon an elastic response to stress. Curing means that a network of crosslinked polymer is established by any one of a number of different vulcanization reactions. Silicone rubber is unique in the large number of choices available for forming crosslinks. A crosslinked network of silicone rubber polymers is relatively weak; hence reinforcement by small-particle active fillers, such as silicas, is essential....

The origin of residual internal stress in solvent‐cast thermoplastic coatings

Abstract: Experiments on polystyrene and poly(isobutyl methacrylate) coatings cast from toluene have shown that residual internal stress is independent of dried coating thickness and initial solution concentration. A theory is presented that predicts the magnitude of the stress and shows that it has no dependence on thickness or initial solution concentration. Internal strain is calculated from the volume of solvent lost after the coating has solidified. This solidification point is identified with the solvent concentration that is sufficient to depress the glass transition of the polymer to the prevailing experimental temperature. A one-to-one correlation is confirmed between the volume of solvent lost from solution during drying and the volume change of the polymer film.

Thermal effects on the optical properties of single crystals and solution‐cast films of urethane substituted polydiacetylenes

Abstract: The optical properties of single crystals and solution‐cast films of two polydiacetylenes, poly3BCMU and poly4BCMU where the substituent group is– (CH2)3.4OCONHCH4COOC4H9, are characterized. Visible absorption and reflection spectra for the polymer crystals are typical of those observed for other urethane substituted polydiacetylenes. The optical properties of the polymer films are controlled primarily by intramolecular hydrogen bonding between the N–H and C=O of the urethane functionalities on adjacent substituent groups. Hydrogen bonding stabilizes the planar, fully conjugated conformation of the individual polymer chains in the films. Increased temperature causes a disruption of the hydrogen bond network and a destabilization of the planar polymer conformation. Dramatic color changes result because of the sensitivity of the optical properties to backbone conformation.

New Universal Extensional Rheometer for Polymer Melts. Measurements on a Polystyrene Sample

Abstract: A new universal extensional rheometer, which makes it possible to investigate the elongational properties of polymer melts, is described. Measurements under a wide variety of constant stretching rates and constant tensile stresses can be undertaken and recoil and relaxation experiments can be performed on a small amount of material. The operation modes of the apparatus are demonstrated on a polystyrene sample. For this polystyrene the viscosity and the recoverable strain in the steady state of elongation were measured over more than three decades of the tensile stress. At small stresses the recoverable strain is proportional to the stress, i.e., the compliance is constant and the viscosity gives a constant value which is three times the zero‐shear viscosity. At higher stresses the recoverable strain seems to level off and the viscosity runs through a maximum which lies 30% above the stress‐independent value.

Mechanical, thermal, and electrical properties of metal fiber‐filled polymer composites

Abstract: The critical concentration at which a metal-filled composite becomes electrically conductive can be dramatically reduced by adding the metal as randomly dispersed fibers. The higher the aspect ratio of the fibers, the lower the concentration needed to induce electrical conductance. Composites exhibiting resistivities below 20 ohm-cm have been produced with less than 8 volume percent aluminum fibers, having an aspect ratio of 24:1. At low fiber loadings the tensile strength of the composites is similar to that of the unfilled polymer. The thermal properties of these composites are shown to increase monotonically in accordance with the theoretical development of Nielsen.

Ultrahigh-strength polyethylene filaments by solution spinning/drawing, 2. Influence of solvent on the drawability†

Abstract: High-modulus structures of linear polyethylene with Young’s moduli of 70 GPa may be generated, for example, by drawing meltspun fibers to very high draw ratios’), solid-state extrusion (e. g., cf.’)) and by the solution-crystallization technique referred to as surface growth3). The tensile strength of the drawn and extruded materials is usually found below 1 GPa, whereas the longitudinal crystals of Zwijnenburg and Pennings may have a strength as high as 3 GPa. The superior strength of the latter filaments, which have comparable Young’s moduli, is due to the high molecular weight of the polymer sample used4*’). Previous attempts to draw or extrude polyethylene of similar molecular weight to high draw ratios only succeeded at relatively high temperatures, in fact above the melting point, where the effectiveness of the chain-extension process is known to be low (see6)). The structures thus produced, therefore, exhibited but moderately improved mechanical properties6). In a wide-ranging study on the processability of high molecular weight polymers it has been found that the effective drawability of these materials is drastically enhanced by spinning” or casting from dilute solutions. This effect will be illustrated in the present paper for polyethylene, and this improved drawability will be discussed in terms of a favourable intermolecular topology of the polymer.

Dynamics of polymers in dilute solutions : an explanation of anomalous indices by cross-over effects

Abstract: 2014 It is shown that the dynamical properties of long polymers made of N links, in solutions, converge much more slowly to the asymptotic limit (N ~ oo) than the static properties. This effect, which has a simple origin, explains why dynamical measurements of critical indices lead to results which seem to disagree with static measurements and recent theories. LE JOURNAL DE PHYSIQUE TOME 40, JANVIER 1979, 1 Classification Physics Abstracts 61.40K 05.40 47.90

The structure, magnetic characterization, and oxidation of colloidal iron dispersions

Abstract: Colloidal iron dispersions have been prepared by the thermolysis of Fe(CO)5 in solutions of functional polymers. The structure of the particles was very disordered at sizes ≲100 A but changed to become single crystal with a disordered core as the size increased to 100–200 A. Particles ≲100 A were superparamagnetic, and particles in the 100–200‐A range had a time‐dependent hysteresis. On exposure to the atmosphere an ∼30‐A‐thick γ‐Fe2O3 oxide film was produced on the surface of the particles. This is the ’’passive oxide film’’ detected previously by a number of techniques but never before imaged directly in situ. As water was absorbed from the atmosphere the chlorinated solvent‐based dispersions reacted further to give β‐FeOOH. This reaction was promoted by chloride‐ion impurity. The magnetic moment decayed with oxidation roughly in proportion to the quantities of Fe, γ‐Fe2O3, and β‐FeOOH present. Disorder in the structure of ≲100‐A particles formed in nonchlorinated solvent‐based dispersions produced an i...

Diffusion‐controlled formation of porous structures in ternary polymer systems

Abstract: We propose a theory for the appearance of two-phase structures during the formation of polymer membranes from a casting solution immersed in a coagulant bath Our model is based on diffusion induced phase separation at the spinodal in the ternary nonsolvent-solvent-polymer system A simplified treatment of the interdiffusion process by the diffusion layer method permits the formulation of criteria for the formation of two-phase structures in the course of the solvent-coagulant exchange Our criteria are expressed in terms of the composition dependence of the chemical potentials in the stable and metastable region of the ternary phase diagram Comparison with experimental results shows qualitative similarities with theoretical predictions

Imidized acrylic polymers

Abstract: Polymers containing imide units and a process of imidizing acrylic polymers to any desired degree of imidization without the use of added water or solvent.

Loss of additives from polymers: A theoretical model

Abstract: The rate at which an additive is lost from the surface of its solution in a polymer is considered to depend upon three factors—the solubility of the additive, the rate at which it volatilizes from the polymer surface, and its diffusion coefficient within the bulk of the polymer. By adapting the mathematics of heat flow in a solid, the loss of additive from a polymer is mathematically modeled in terms of these three variables for bulk polymer and for film and fiber samples. Two cases are considered—loss of additive by volatilization or dissolution from the polymer surface and loss by precipitation on the surface from a supersaturated solution of the additive. The results are applied to a discussion of the relative importance of the three parameters under various conditions occurring or expected in practical application of additives as oxidation stabilizers for polyethylene and polypropylene.

Preparation and properties of star-branched polymers

Abstract: The methods available for the preparation of star-branched polymers are described. After presentation of theoretical predictions of the effect of this type of branching on the properties of polymers, available experimental data are compared and critically discussed. Thermodynamic and visco-elastic properties at infinite dilution are first presented followed by visco-elastic properties in concentrated solution and the melt. Finally the effect of star-branching on T g of low molecular weight polymers is described.

First observation of the coil–globule transition in a single polymer chain

Abstract: The conformations of single polymer chains in solution have been studied extensively since the 1940s. At high temperatures and in good solvents, a polymer has an extended coil configuration, while at low temperatures and in poor solvents a polymer is in a collapsed globule state. The transition between the extended and collapsed state as the temperature or solvent composition is varied was thought to be smooth and continuous, and experiments supported this idea1. However, in the 1960s it was suggested2–4 that the transition between the two configurations, the coil–globule transition, was discrete. We now report the first observations of the single polymer coil–globule transition. The observations were made on polyacrylamide molecules dissolved in acetone–water mixtures.

Adsorption characteristics of certain polyoxyethylene-polyoxypropylene block co-polymers on polystyrene latex

Abstract: The adsorption of a series of commerical polyoxyethylenepolyoxypropylene block copolymers onto polystyrene latex has been studied. Adsorption was Langmuirian in all cases with maximum adsorption occuring after the apparent critical micelle concentration had been reached. Examination of molecular areas at the interface indicate that for high ethylene oxide content surfactants, the hydrophilic chains adsorb as loops. This was confirmed by measurements of the adsorbed layer thickness determined by intensity fluctuation spectroscopy and microelectrophoresis.

Properties of amorphous and crystallizable hydrocarbon polymers. I. Melt rheology of fractions of linear polyethylene

Abstract: The dynamic moduli G′(ω) and G″(ω) for two groups of linear polyethylene fractions (reported Mw/Mn < 1.2) were measured in the melt state using the eccentric rotating disk method. Values of zero shear viscosity η0 were obtained and compared with published results on similar fractions. Molecular weight data were converted to a common basis through intrinsic viscosities in trichlorobenzene (TCB) at 135°C. With recent data on Mw (light scattering) vs. [η]TCB, for linear polyethylene, the relationship at 190°C, η0 = 3.40 × 10−14(Mw)3.60, was obtained. The flow activation energy Ea was 6.4 kcal (T = 140–195°C). The plateau modulus G at 190°C was determined from the area under the loss modulus peak in one high-molecular-weight sample. The value obtained, G = 1.58 × 107 dyn/cm2, corresponds to an apparent molecular weight between entanglements of 1850. The storage compliance J′(ω) becomes anomalously large at low frequencies. The recoverable compliance J could not be determined for any of the fractions.