Viscous-flow State Research Articles

Rheology of Entangled Polymers Not Far above Glass Transition Temperature: Transient Elasticity and Intersegmental Viscous Stress

This work studies the transient rheological responses of different entangled melts to startup deformation at unconventionally high rates, which can be accessed by working at sufficiently low temperature, e.g., at and below 130 °C for polystyrene. When the rate is as high as the second crossover frequency ωe observed from the small amplitude oscillatory shear (SAOS) data for storage and loss modulus G′ and G″, there is sizable viscous stress that dominates the initial mechanical response. It is shown based on the various polymer melts including PS, PMMA, SBR, PC, and PS mixtures that this viscous component of the stress cannot be neglected when characterizing such fast deformation in either extension or shear. This sizable frictional addition to the rubber elasticity component of the initial stress is founded to be preceded by solid-like deformation. The remarkable transient elasticity can be characterized by a modulus Ginterseg that grows well beyond the magnitude of the melt plateau modulus GN0 as the temperature lowers toward the glass transition temperature Tg. In the case of PS, Ginterseg reaches a level of 300 MPa at 110 °C and reduces to 2.0 MPa, i.e., 10GN0, at 130 °C. The initial elasticity is short-lived because the melts quickly transition to a state of viscous flow at a strain of just a few percent. The magnitude of the viscous stress at the solid-to-liquid transition defines a yield stress. This yield stress is found to scale with the applied rate in a power law, agreeing with the dependence of G″ on frequency ω from the SAOS data. Moreover, the intersegmental effect, i.e., the transient elasticity, is shown to also take place in unentangled and barely entangled PS melts as well as branched polyisoprene melts.

On the mechanisms of photogeneration of charge carriers in PEPC-C60 composites

Photogeneration of electric charge carriers at high strengths of the external electric field and the temperature of transition to the viscous-flow state (T visc) of thin PEPC-C60 composite films obtained by casting from a toluene solution have been studied. The rheology of the composite films has been investigated by the nondestructive optical method. The consistent correlated change in the values of T visc and the effective temperature in the expression for the photogeneration quantum yield (T 0) in the Meltz representation with variations in the C60 concentration has been established. The difference between T visc and T 0 in thin composite films does not exceed 2–3% of T visc (T 0).

Relation between elastic modulus and glass softening temperature in the delocalized atom model

The ratio of softening temperature (glass transition temperature) to elastic modulus (Tg/E) is mainly determined by the limiting elastic deformation of an interatomic bond, which characterizes the transition of a structural microregion from an elastic into a viscous-flow state. In silicate glasses, this transition is caused by the limiting deformation of directed ionic-covalent Si-O-Si bonds. In the case of amorphous hydrocarbons, it is related to the relatively weak intermolecular bonds between regions in chain macromolecules, and the Tg/E ratio is significantly higher than in inorganic glasses. In glassy systems of one class, this ratio turns out to be constant (Tg/E ≈ const), and a linear correlation is detected between softening temperature and elastic modulus, which can be explained in terms of the delocalized atom model. The values of Tg/E can be used to classify glasses similarly to the well-known Angell classification according to so-called fragility.

Effects of thermal treatment on the tribological characteristics of thermoplastic polymer film

Friction tests were carried out using a microtribometer to investigate the effect of thermal treatment on microscale friction and wear between poly(methyl methacrylate) (PMMA) film and a fused silica lens. Two films were examined: one that was baked at 413 K for 2 min and one that was baked at 433 K for 24 h. The friction forces on the PMMA films were measured under atmospheric conditions as the temperature of the films was increased from 300 K to 443 K. The contact area between the films and the lens was also examined. As the temperature increased, the friction force increased for both films. The slope of the friction force with temperature and the contact area varied, depending on the state of the film surface; glassy, rubbery, and viscous flow states. The baking conditions also affected the slope, contact area, and wear generation. For temperatures at which the samples were in a glassy state, wear particles were not generated on the sample baked for 24 h. The results demonstrate that the tribological characteristics can be altered by the thermal treatment of the PMMA film as well as the temperature. When the film contains some residual solvent, the residual solvent in the PMMA film can diffuse to the PMMA surface due to heating and thus decrease the friction force under room-temperature conditions.

Effects of Temperature on the Microscale Adhesion Behavior of Thermoplastic Polymer Film

Adhesion tests were carried out under various temperature conditions to investigate the effects of temperature on adhesion behavior between a fused silica lens and a poly(methylmethacrylate) (PMMA) film, materials used for molds and as thermoplastic polymer films in nanoimprint lithography (NIL). The pull-off force and the contact area were measured as the temperature of the PMMA film was increased from 300 to 443 K, and as it was decreased back to 300 K. As the temperature increased, the PMMA film changed from a glassy state to a viscous flow state, and the adhesion behavior showed significant variation corresponding to this change in state. In the glassy state (below 363 K), the pull-off force did not change noticeably as the temperature increased. In the rubbery state (383–413 K), the pull-off force increased significantly as the temperature increased. In addition, the contact area was enlarged. In the viscous state (above 423 K), fingering instability was observed in the contact area and the pull-off force decreased, while the contact area increased, as the temperature increased. The adhesion behavior was also found to vary markedly with thermal history of the PMMA film. The causes of these changes are discussed.

Comparative evaluation of the efficiency of a series of commercial antioxidants studied by kinetic modeling in a liquid phase and during the melt processing of different polyethylenes

AbstractAn antioxidant response in condensed polymeric environments is often ambiguous and may vary strongly depending on the nature of the polymer and the conditions of polymer storage, processing, and use. The impact of polymeric environments during melt processing on the intrinsic efficiency of a set of commercial antioxidants was studied. The antioxidative activity of primary antioxidants Lowinox CPL, Lowinox 22IB46, Naugard 445, hydroxylamine Genox EP, and secondary phosphite Weston TNPP were determined by using two versions of the model reaction of cumene initiated (2,2′‐azobisisobutyronitrile, AIBN, and cumyl hydroperoxide, ROOH) oxidation. The melt stabilizing efficiency of the antioxidants was also studied during multipass extrusion testing in HDPE (Phillipstype), metallocene LLDPE, and (Ziegler‐Natta) LLDPE. The kinetic measurements showed that each of the three functional hydroxyl groups of Lowinox CPL is consumed in the model reaction (version 1) with the same high inhibition rate constant (k7), whereas the two functional groups of Lowinox 22IB46 have different activity stipulated by hydrogen bonding between the hydroxyls. All the primary stabilizers involved afforded transformation products with additional antioxidative activity. For phenolic Lowinox CPL and amine Naugard 445, these products exhibited lower inhibition rate constants than that of the main functionality, but for Lowinox 22IB46, the discrepancy was not observed. Genox EP revealed three inhibition centers with different rate constants which, however, have low values of the inhibition coefficients (f). This effect is presumably due to the versatility of the inhibition pathways for the antioxidant and its intermediates, including the path of active interception of cumylalkyl (R•) radicals. The secondary stabilizer Weston TNPP, tested by means of the second version of the model system, along with the expected decomposition of hydroperoxide appeared to be an effective radical scavenger. Kinetic parameters of the antioxidizing activity of the stabilizers – inhibition rate constants, coefficients of the oxidation chain termination, and total antioxidative activity {A = ∑[k7(i) (fn[InH])(i)]} — were determined for each functional group and for the whole antioxidant molecule. The phenolic stabilizers manifested powerful antioxidative activity. Their strongest functional groups have very high inhibition rate constant values: (log k7) = 5.4 ± 0.15 (Lowinox 22IB46) and 5.2 ± 0.1 M−1s−1 (Lowinox CPL). In terms of the total inhibiting activity in the liquid system the antioxidants can be ordered as: Lowinox CPL > Lowinox 22IB46 > Naugard 445 > Genox EP > Weston TNPP. The effect of stabilizers during multipass extrusion experiments was assessed via melt flow rate and yellowness index measurements conducted as a function of the number of passes. Phenolic antioxidants and Genox EP significantly improved the melt stability of the polyethylenes in terms of melt viscosity retention and in partial compliance with the data from kinetic modeling measurements. According to the melt stabilizing efficiency data, the antioxidants can be arranged as: Lowinox 22IB46 > Lowinox CPL > Genox EP > Naugard 445 > Weston TNPP. The Lowinox 22IB46 with relatively lower molecular weight exhibited the best results among the primary stabilizers because of the unrestricted molecular dynamics in the viscous‐flow state of the polymer. Yellowness index measurements made after multiple extruder passes indicated that Weston TNPP effectively decreased the color development caused by the phenolic antioxidants. Genox EP displayed high efficiency as an antioxidant and melt‐processing stabilizer and additionally provided good color protection. Generally, we received a good correlation between the activity of the antioxidants in the model system and their melt stabilization performance in HDPE, metallocene LLDPE, and LLDPE. The model reaction of cumene‐initiated oxidation has demonstrated excellent applicability as an effective tool for preliminary quantitative assessment of antioxidant radical‐scavenging efficiency. J. VINYL ADDIT. TECHNOL., 2010. © 2009 Society of Plastics Engineers

The physicochemical characteristics of oxidized polysaccharides

The paper presents the results obtained in studies of the physicochemical characteristics of modified polysaccharides prepared by the catalytic oxidation of starch with molecular oxygen in an alkaline medium. The amorphous-crystalline structure of starch was partially retained after oxidation. The differential scanning calorimetry data were used to determine the specific heat capacity of oxidized polysaccharides in the vitreous state over the temperature range 10–160°C and the enthalpy and temperature characteristics of fusion of amorphous-crystalline lamellae. The structural and thermodynamic parameters of crystallites (surface free energy, enthalpy, and entropy) were calculated. The segmental mobility of oxidized polysaccharide macromolecules and the transition from the vitreous to high-elasticity state were observed at Tg = 187.7°C, and the transition to the viscous-flow state, above 209°C. The pyrolysis of the polymer began after the unfreezing of its segmental mobility and was accompanied by the release of water and carbon dioxide and the formation of a large amount of coke foam.

Status and Prospects for Development of Polyacrylonitrile Fibre Production. A Review

Sixty years of experience in manufacture of fibres from acrylonitrile polymers and copolymers suggest favorable prospects for the development of this kind of textile stock. The characteristics of the primary and supermolecular structure of this class of fibre-forming polymers, the ability to widely vary the methods of converting them into the viscous-flow state, and the methods of spinning, orientational drawing, modification and thermal fixation treatments form the technological base for creation of PAN fibres with defined properties. The basic trends in scientific and technical progress in development of production (volumes and assortment) of PAN fibres are: increasing the unit capacities for production of large-tonnage assortments of fibres while minimizing power and materials consumption; creating multivariant production lines to manufacture fibres for special applications and with specific performance characteristics; increasing the economy of production while minimizing environmental pressure. The important prospects for expanding the areas of application of PAN fibres are due to the reactivity of the functional groups in the polymer substrate. Industrial implementation of technologies based on polymer-analog transformations will allow creating new kinds of materials for use in industry, agriculture, and construction and for data collection, storage, and transfer.

The Relationship between the State of Active Species in a Ni/Al2O3Catalyst and the Mechanism of Growth of Filamentous Carbon

The formation of active particles and their changes in the course of 1,3-butadiene decomposition on a Ni/Al2O3catalyst at temperatures from 400 to 800°C were studied by high-resolution electron microscopy. It was found that carbon filaments of different types were formed at 400–800°C. The growth of thin filaments (20–30 nm in diameter) takes place at 400–600°C on a conical Ni particle located at the growing end of the filament, whereas di-symmetrical filaments 50–100 nm in diameter grow on biconical metal particles. As the carbonization temperature was increased to 700–800°C, graphite nanotubes 5–20 nm in diameter were formed. It was found that the mechanism of formation and the structure of filaments are related to the state of catalytically active species, which consist of a solid solution of carbon in the metal. It is suggested that the metastable surface nickel carbide Ni3C1 – xis an intermediate compound in the catalytic formation of graphite filaments from 1,3-butadiene. Upon termination of the reaction, the metastable Ni3C1 – xmicrophase is decomposed with the formation of hexagonal nickel microinclusions. The role of epitaxy in the nucleation and growth of a graphite phase on the metal is discussed. Models are presented for the growth of structurally different carbon filaments depending on the formation of active metal species at various temperatures. Considerable changes in the structure of carbon and the formation of nanotubes at 700–800°C are related to the appearance of a viscous-flow state of metal–carbon particles.

Quasi-independence of dissipative processes in vitreous materials

The dissipative phenomena in vitreous materials of different chemical nature have been studied. It is demonstrated that the loss maxima in the internal friction spectra are observed in the viscous-flow state and the elastic solid state. The loss maxima observed in the spectra of glasses at the temperaturesT ≪ T g cannot be described using the viscosity of the vitreous material, because it is not experimentally determined at these temperatures. It is postulated that these low-temperature losses are brought about by the presence of a set of structural subsystems characterized by their own local dissipative and local elastic characteristics. The relaxation time of any dissipative process can be represented as the ratio between the local dissipative and local elastic parameters. For the α-relaxation process occurring at the temperaturesT > T g, the local dissipative parameter and the viscosity of the vitreous material as a continuous medium coincide. The relation between the number of loss peaks in the internal friction spectra and the number of structural subsystems comprising the studied material as a continuous medium is considered. The quasi-independence of the response of structural sub-systems to the external action is analyzed.

Physical transitions in polymers plasticized by interacting penetrant

Plasticization by a penetrant of a polymer with the functional groups, specifically interacting (donor-acceptor type of interactions) with the penetrant is analyzed by using the systems: ammonia which is an electron-donor/aromatic polyamides, where amide group is as an electron-acceptor; sulfur dioxide which is an electron-acceptor/polymers with electron-donor groups. It is shown that there is the plasticization of polymer by penetrant as the result of the sorption of penetrant molecules on the active functional groups of a polymer. There are four sections (regions) on the concentration dependencies of the diffusion coefficient. The boundaries between them correspond to the transition of the polymer (or rigid phase of block-copolymer) from one glassy state to another (β-transition), from glassy state to rubbery state (α-transition), and from rubbery state to the viscous-flow state. Essentially, the concentrations of a penetrant on the active functional groups, n, (or the degree of fullness of the effective sorptive capacity of the functional groups, ϕ), which corresponds to the α- and β- transitions in the polymer, have the ratio: n α n β⋍ρα ρβ≈4± . Concentration barriers of the activation, corresponding to the α- and β-transitions also have the same ratio. It seems, that the process of α-relaxation in the polymer includes the intermolecularly correlated displacement of neighboring segments of the same size as in the β-transition, i.e. close to the Kuhn's segment, but it is close to the correlated displacement of 3–5 segments. Relative concentration activation barriers of the physical transitions depend on the energy of interaction between penetrant and functional group of polymer.

Main features of the viscoelastic behavior of carbon fiber-reinforced plastics with a polymer matrix: Model study and calculation

It was shown that below the glass transition temperature of the amorphous matrix, its viscoelastic behavior in a composite is described by the Halpin-Tsao equation for composites, as modified by Nielsen, with a sufficient degree of reliability. However, when the matrix passes into the highly elastic and especially the viscous flow state characteristics of thermoplastic matrices, a significant difference is observed in the experimental and calculated temperature curves of the elastic and matrix loss moduli. It is possible to hypothesize that this is due to hindrance of plastic deformation of the matrix in the composite not accounted for in the Halpin-Tsao model. Using simple models of the one-sided coating and sandwich type, it was shown that consideration of the stress state of a matrix in contact with a rigid coating can result in the same kind of effects as in a composite, namely, apparent toughening of the matrix in the highly elastic region and shifting of the maximum of the mechanical loss modulus to the high-temperature region.

An Overview of the Viscous and Viscoelastic Behavior of Ionomers in Bulk and Solution

Abstract The subject of the viscous and viscoelastic behavior of ionomer systems is addressed from a general point of view. Following some introductory material concerning terminology and characteristics of ionomers that are pertinent for later discussion, the format will be to first briefly consider the glassy state followed with a more complete discussion of the rubbery and viscous flow (melt) states. The viscous and viscoelastic response of ionomer systems when in solution will then be discussed. We have also attempted to compare the so-called “random copolymer” ionomers to the telechelic systems. Emphasis is given to single component materials in contrast to blends or filled systems, but some brief remarks are made concerning the latter topics.

The rheological properties of polytrifluorochloroethylene (PTFCE)

The possibilities of using various rheological methods to study the behaviour of polytrifluorochloroethylene in the state of viscous flow are elaborated. PTFCE has been found to change to a state of viscous flow in which it has the properties typical of a melt over a very limited temperature range. The melt viscosity is very high and the MWD of industrial batches is narrow. PTFCE therefore changes to the forced highly elastic state already at low rates of shear and loses its flow properties. The flow possibilities are limited at low temperatures and long deformation by orientational crystallization, and by thermal degradation at high temperatures.

The synthesis and structural investigation of polymers and copolymers of the cholesterol ester of methacryloyl-ω-hydroxypentadecanoic acid

The results of the synthesis and structural analysis and study of certain physicochemical properties of the cholesterol ester of methacryloyl-ω-hydroxypentadecanoic acid are presented, as well as those of the polymers and copolymers of the latter monomer with butyl methacrylate and butyl acrylate. The transition temperatures have been identified and the polymer and copolymer structures investigated. The fact of the formation of a liquid crystalline structure in the glass-like, high elastic and viscous flow states is proved. Boundary conditions have been found under which the liquid-crystalline state is realized for the copolymers, and a study has been made of the effect of the conditions of formation of films of the homopolymers on their ability to display optical anisotropy.

The mechanical properties of gels in dimethyl phthalate of cellulose acetate of different degrees of substitution

The dependence of the temperature of transition to the high elastic and viscous flow state, on the degree of substitution and molecular weight of cellulose acetate has been found, using thermomechanical and dynamic mechanical methods. The molecular weight of the mechanical segment of the polymer was found from the temperature range of the high elastic state. The relaxational transitions in cellulose acetate gels in dimethyl phthalate are classified as segmental or local types, in accordance with the calculated apparent energies of activation and the lengths of the segments between branch points. Increase in the degree of substitution and in the concentration of the polymer raises the transition temperatures and increases the dynamic modulus and the apparent energy of activation for the relaxational processes in these systems.

High-frequency periodic deformation of polymers in the viscous-flow state

The propagation of high-frequency periodic vibrations in melts and solutions is considered in the case where the acoustic characteristics of the material are determined by the deformation amplitude. It is shown that in sufficiently large specimens the consequences of periodic deformation are concentrated near the vibration source, as a result of which a layer of material with strongly modified mechanical characteristics develops. It is suggested that when periodic high-frequency deformation is superimposed on steady-state flow, owing to the presence of a boundary layer there is effective slippages of the material at the vibration-emitting surfaces.

The strengthening of oriented polyethylene by graft polyvinylidenechloride, polymethacrylic acid and polyacrylonitrile

High and low density polyethylene films were modified by radiation graft polymerization of acrylonitrile, vinylidene chloride and methacrylic acid. The structures, strength and deformation properties of the grafted films were investigated in relation to various factors in the temperature range — 196 to 180° which covers alll the states of PE. It was found that in the high elastic and the viscous flow states the strength and the initial elastic modulus of PE are increased as the graft polymer content in the PE rises. It is shown that peculiarities in the behaviour of the grafted films may all be accounted for through a mechanism of PE reinforcement by the graft structures acting as active particles preventing the relaxation of the PE oriented structures.

Periodic axial and shear deformation of polymers in the viscous-flow state

Results are given of an experimental investigation of the combined periodic axial and shear deformation of polymers in the viscous-flow state. Specimens in the form of right cylinders were disposed between two parallel plates to which the material adhered completely. The experimental data are compared with those obtained under simple shear. The results indicate that, with combined periodic axial and shear deformation, nonlinear shear deformations affect the axial viscoelastic properties and nonlinear axial deformations affect the shear properties of the material.

Variation of the work of brittle fracture in the impact bending of the prestressed polymer material 40K60V

Tensile stresses in a polymer in the viscous-flow state affect the work of brittle fracture in subsequent impact bending, with the formation of a maximum on the A-σ curves. The effect of crystallinity, preliminary molecular orientation, and the magnitude of the stresses on the position of this maximum has been studied in relation to possible conformational changes in the molecules. The experiments were performed on a polymer material consisting of an alloy of wax and rosin.