High-elastic State Research Articles

Transition of polymers from the fluid to the forced high-elastic and leathery states at temperatures above the glass transition temperature

The transition from the fluid to the forced high-elastic (rubber-like or quasi-vulkanizate) state and then to the leathery state (that is between rubbery and glassy states ∗∗ ∗∗ FHES and FLS in abbreviated form ) has been systematically studied by increasing stresses and deformation rates, using an uncured high molecular-mass 1, 2-polybutadiene characterized by a rather high glass transition temperature (−18°C). Investigation of polymers in simple shear is possible up to critical stresses corresponding to the transition of fluid polymers to FHES. Uniaxial extension offers wide possibilities for characterizing polymers in FHES and FLS since it covers the range of 4–6 decades of deformation rates over the range of simple shear. The deformability of polymers is limited by their fracture even in FHES, and the fracture process is determined only by recoverable deformation. The failure envelope covering FHES and transition to FLS has been constructed and the long-term durability in the range of 7 decades of time measured. The maximum extensibility corresponding to the transition of the polymer to FLS is determined by the ultimate extensibility of macromolecular coils. Over a wide range of stresses the polymer behaves like a linear viscoelastic body. This makes it possible to correlate the data obtained for uniaxial extension and low-amplitude sinusoidal shear deformation, which is important for the prediction of fracture phenomena by means of the low amplitude non-destructive dynamic method.

Relaxation transitions in polyethylene

Eight relaxation processes have been investigated below the melting point in high-density polyethylene (HDPE) by relaxation spectrometry, and the nature of the processes has been examined. Two relaxation processes ( β and β 1) correspond to small-scale motion of the polymer chains in amorphous and crystalline phases. Three relaxation processes ( α, α 1 and α 2) correspond to segmental mobility of the chains respectively in amorphous phase (glass transition process, in crystal→amorphous phase transition layers, and in amorphous parts of spherulite fibrils. The three high-temperature processes ( λ 1, λ 2 and λ 3) relate to molecular mobility of elements of supersegmental structure of HDPE amorphous phase and are similar to those in noncrystalline polymers that are in the high-elastic state (elastomers).

Relaxation nature and regularities of breakdown of crosslinked and non-crosslinked polymers in the high-elastic state

A study was made of the durability and strength of crosslinked and non-crosslinked elastomers with varying degrees of crosslinking. A power law is observed for durability and strength with exponents showing a slight dependence on the number of chemical crosslinks in 1 cm 3 polymer N. The activation energy of durability is independent of N and coincides with the activation energy of relaxation λ-transitions but strength and relaxation characteristics show a marked dependence on the degree of crosslinking passing through a maximum, which is explained by the effect chemical network on non-temperature factors of durability and relaxation times and therefore on dimensions of micro-ranges of supermolecular microstructure. Results confirm a unique relaxation type process of visco-elasticity and fracture.

Thermomechanical properties of ethylene-propylene copolymer-acrylonitrile (EPR-PAN) graft copolymers containing a polyconjugated system

The formation of a polyconjugated system during thermal treatment of EPR-PAN graft copolymers results in a change in their thermomechanical properties. The glass transition temperature remains constant, while the yield point is shifted into the higher temperature range. The more complete the process of formation of conjugated bonds in side PAN chains, and the higher the PAN content in the copolymer the wider the temperature range of the high elastic state. Thus the incorporation of polyconjugated systems as side blocks into elastomers provides an important general method for modifying their properties and creating thermoelastoplastic type materials.

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.

Fracture of uncured linear flexible-chain polymers of narrow molecular mass distribution in triaxial stress (behaviour of elastomers as adhesives)

Possibly the simplest method of studying polymers under conditions of triaxial stress is the investigation of the behaviour of thin films of adhesive between two rigid surfaces. The adhesives used were uncured linear flexible chain polymers-polybutadienes. These polymers were of narrow molecular mass distribution ( MMD). The use of high molecular polymers of narrow MMD allows the evaluation of the behaviour of elastomers as adhesive joints to be simplified. It has been shown recently that, depending on the rate of deformation, the behaviour of polymers of this kind may be similar to that of viscoelastic liquids or systems in the high-elastic state, which behave as quasi-cured systems, The transition from the fluid to the high-elastic state is a pure relaxation process. It would be expected that the behaviour of the adhesive would vary substantially, depending on its physical state. The fracture behaviour of the adhesive and the rupture of its contact with the solid surface must be dependent on this factor. Cohesive, cohesive-adhesive and adhesive fractures may be observed in such cases. Cohesive fracture is typical for regimes of polymer deformation upon transition from the fluid to the high-elastic state. The further the transition of the polymer to the high-elastic state, the more sharply pronounced is the adhesive fracture of the polymer in triaxial stress. The present work is concerned with the investigation of the temperature-time dependence of the parameters characterizing the fracture of the adhesive and its contact with the solid surface. The results of experiments carried out under conditions of triaxial stress are compared with the data obtained for the same polybutadienes by experiments involving simple shear and uniaxial extension.

Critical regimes of oscillatory deformation of polymeric systems above glass transition and melting temperatures

High molecular weight linear polymers and their concentrated solutions were investigated over a wide range of frequencies and amplitudes of oscillatory deformation. At definite critical deformation and stress amplitudes, the resistance to deformation drops abruptly as a result of the rupture of continuity of polymer specimens in the region of action of the highest shear stresses. The lowest critical values of deformation rate amplitudes are inversely proportional to the initial viscosity and correspond quantitatively to the critical shear rates at which the spurt occurs during the flow of polymeric systems in ducts. The spurt effect is due to the transition of the polymer systems to the forced high-elastic state, in which they behave like quasi-cured polymers whose deformability is always limited. Up to the critical values of the stress amplitudes, narrow-distribution high molecular weight linear flexible-chain polymers behave like Hookean bodies, whereas the broad-distribution polymers show a sharply defined nonlinear behavior which asymptotically passes to a spurt. The amplitude dependence of the dynamic characteristics of the high molecular weight linear polymers, as well as their non-Newtonian behavior, is due to polymolecularity. An increase in deformation amplitudes reduces the frequency at which the spurt, and hence the transition of the polymer systems to the high-elastic state, is observed. Therefore, under conditions of oscillatory deformation the physical state (fluid or high-elastic) is determined not only by the frequency but also by the value of deformation. In the high-elastic state region (estimated at low amplitude deformation), the critical deformation amplitude is frequency independent and has an unambiguous relationship with the molecular mass of the chain (Me) between the entanglements. For the bulk polymers studied, the spurt in the high-elastic state occurs at stress amplitudes of the order of 105 N/m2 irrespective of frequency, molecular mass, or polymolecularity. In concentrated polymer solutions, in the high-elastic state the critical stress amplitudes decrease with reducing polymer content, whereas the critical deformation amplitudes increase.

Polarization-optical investigation of polymers in fluid and high-elastic states under oscillatory deformation

The relationship was investigated between birefringence and oscillatory shear deformation of linear high molecular mass polymers exemplified by narrow- and broad-distribution polybutadienes and polyisoprenes. Polymer deformation at different frequencies and amplitudes was carried out in an annular gap. The stress field uniformity was not below 95%. It was shown that in oscillatory deformation of polymers in the fluid and high-elastic states, birefringence contains a time-independent steady component and an oscillatory component with a frequency equal to that of the assigned oscillation. A linear interrelation was found to exist between the amplitude of the oscillatory component of birefringence and that of the shear stresses, with a proportionality factor equal to the stress-optical coefficient of the polymers. The phase of the oscillatory component of birefringence coincides with that of the shear stresses. Measurements of the steady component of the birefringence make it possible to find the steady component of the first normal stress difference resulting from the assignment of shear oscillations to the polymer. On the basis of the experimental data obtained for polybutadienes and polyisoprenes, and the literature data for polystyrene solutions, a master curve was constructed that generalizes the dependence of the steady component of the first normal stress difference in the linear and nonlinear deformation regimes on the product of the square of the deformation amplitude and the storage modulus measured at low amplitudes. This dependence is valid in the linear and nonlinear deformation regimes. It is invariant with frequency, amplitude deformation, molecular mass, and molecular mass distribution of the polymers. It is shown by visual observation of deformation that the abrupt drop in resistance of polymer to shear in large-amplitude deformation is due to polymer rupture near the surface of the inner cylinder and is accompanied by a slip-stick process. This is the phenomenon of spurt early observed in capillary viscometers at high shear stresses and recently investigated in coaxial cylinder devices at large amplitude deformation.

Decay of free induction in polyvinylchloride in the high elastic state

An experimental study was made of the form of decay of free induction (ID) in PVC samples with different molecular weights in the high elastic state. It was found that the form of ID differs from the form predicted by a model of rotary Brownian motion with a single correlation time. For the theoretical description of the form of ID an attempt was made to use the existing theory, which assumes anisotropic motion of the polymer chain.

The viscoelastic properties of linear polymers in the state of flow and the transition to the high elastic state. Review

The results of experimental research on the viscoelastic properties of linear polymers with a narrow MWD, and of mixtures and solutions of these, are discussed and generalized. A rational classification of macromolecular compounds that lays down a clear boundary between oligomers and polymers is proposed. In comparison of polymers of different homologous series, their molecular weights must be standardized with respect to the number of monomer units between points of molecular entanglement, and for this the reference temperature is taken as 100° above the glass temperature. The transition of polymers from the flow state to the high elastic state, brought about by increase in deformation rate (frequency), is studied systematically. The transition of linear high polymers to the high elastic state takes place within a narrow range of critical stresses (in the region of 0·1 → 1·0 × 10 7 dyne/cm 2) and is not dependent on molecular weight, whereas the critical rates of strain (frequencies) are dependent on molecular weight and viscosity, these rates falling very rapidly with increase in these characteristics. Polydispersity affects the properties of polymeric systems to a great extent. As the rate of deformation is increased, components of higher molecular weight pass successively into the high elastic state. This gives rise to nonlinear, especially non-Newtonian, behaviour of systems comprising components with linear characteristics. The durability of raw rubbers in the high elastic state exhibits the same relationships as for vulcanized rubbers. This means that the three dimensional network of uncrosslinked macromolecules, behaves similarly to networks with chemical crosslinkages.

Statistical description of relaxation processes in polymers

A theoretical description of the deformation of elastomers, in which supermolecular structure is taken into account, is proposed. Relaxation in the transition region between the glassy and high elastic states is considered. Here motion of the macromolecules is of the diffusional type. A one dimensional model of the diffusion processs is studied mathematically. It is shown that the temperature and frequency relationships of small, reversible strains are described by the fractional integration operators proposed previously from phenomenological considerations. Equations relating stress and strain are derived and an expression for the complex elasticity modulus is obtained. The results so obtained are compared with experimental findings.

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.

Three dimensional network study and investigation of molecular motion in copolymers of unsaturated polyesters using the method of free torsion oscillations

The visco-elastic properties and relaxation processes in the copolymers of unsaturated polyesters (UPC) have been investigated using the method of free torsion oscillations. It has been found that the effect of the degree of crosslinking on visco-elastic parameters could be dissimilar in character in the high elastic and glass-like states. The molecular mechanism of relaxation processes has been investigated.

Special features of the variation of dipole-segmental loss in plasticized polyvinylchloride during heat treatment

During the heat treatment of quenched plasticized PVC samples the maximum value of dipole-segmental losses tan δ max decreases. Most considerable variations in tan δ max take place at temperatures of heat treatment, corresponding to the range of transition from the glassy to the high elastic state. The extent to which the non-equilibrium structure approximates to the equilibrium, and in the region of high elasticity—the degree to which the structure of quenched samples deviates from the equilibrium state, are factors determining the variations of tan δ max during heat treatment in the glassy state.

A molecular model of isothermal volume relaxation in polymeric glasses

On the basis of a critical examination of the existing theories of the relaxational behaviour of polymers in the region of transition from the glassy to the high elastic state, it is suggested that the kinetics of isothermal structural relaxation in this region can be described by a proposed model of interacting, structural holes, caused by transgauche transitions of units. By analysis of the kinetic equation thus obtained for the average fraction of gauche (trans) isomers in this system, it is shown that the temperature dependence of the relaxation time has an anomalous section, which coincides with a corresponding section found experimentally in the transition region. The effect of the molecular parameters of the model on the position and width of this section is discussed.

Ultimate regimes of deformation of linear flexible chain fluid polymers

At temperatures far from the glass transition temperature linear flexible chain high polymers of narrow MMD when being deformed may undergo a transition from the fluid to the forced high elastic (and the glass) state. In simple shear, uniaxial extension and in the triaxial stressed state the transition of polymers to the forced high elastic state is accompanied by their rupture. Of decisive importance for this transition is the combination of the rate and amount of deformation. On the other hand, such a transition occurs at stresses which vary by 10–20 times for the various homologous series of linear flexible chain polymers. We have established unexpectedly simple relations connecting the parameters that characterize the rheological properties of polymers at deformation rates and stresses tending to zero with their ultimate strength and long term durability. This is determined by the fact that the polymers under consideration behave like linear bodies until they are fractured (in terms of the theory of linear viscoelasticity). From this it may be inferred that the selection of the simplest (as to composition) polymers having a highly homogeneous entanglement network plays a decisive role in the elucidation of the fundamental specificity of the rheological properties of polymeric systems.

Relaxational transitions and the structure of poly-N-n-octadecylacrylamide

The variation in dielectric relaxation, refractive index, IR absorption and thermomechanical behaviour of poly-N-n-octadecylacrylamide, is studied in the temperature range of −170° to +160°. Transitions have been detected in the region of melting of the side chains of the polymer, close to the glass temperature and in the high elastic state. A molecular interpretation of these processes is proposed.

Role of Autohesion During Film Formation from the Latex

Abstract As it is known not all dispersions of synthetic polymers can produce solid latex films upon drying. Only dispersion with a polymer in high elastic or flow state have such ability. Otherwise the film formation temperature should be either increased, or plasticizer or solvent should be introduced into the system

PMR study of the effect of active fillers and vulcanization on macromolecular mobility of isoprene rubber

Studied were made of the temperature dependences of line width, second moment and the form of PMR line in the temperature range of −190+60° using isoprene rubber, crude and vulcanized systems (unfilled and containing carbon black, an active technical carbon). It was shown that vulcanization displaces transition from the glassy state to the high elastic state by 10° in the direction of higher temperatures. Crude carbon produces oriented ranges, in which macromolecular mobility is higher than in inhibited regions of unfilled vulcanizates. A narrow component is observed in spectra of samples containing untreated carbon, which is due to groups containing proton on the surface of untreated carbon particles.

Nonequilibrium thermodynamics and rheology of viscoelastic polymer media

Nonlinear constitutive equations for viscoelastic polymer media have been derived with the help of irreversible thermodynamical methods. These equations contain a small number of constants which have obvious physical meaning. The work is based on the hypothesis that the high-elasticity state characterized by large elastic strains is the local equilibrium thermodynamical state of these media. A theoretical description is given to explain the kinetic transition of fluid polymer media into high-elasticity state at temperatures above the flow temperature.