E. Kontou

Physical and chemical cross-linking effects in polyurethane elastomers

A series of polyester-urethane block copolymers of various molecular weights was prepared via a two-step polymerization process. The prepolymer composition was kept constant in all the samples, while the NCO/OH ratio during the chain extension was varied from 0.9 to 1.2. Chemical and physical cross-linking effects were studied by means of F.T.I.R spectroscopy, swelling, and elastic behavior. Equilibrium stress-strain measurements and tensile-retraction tests were carried out to examine the elastomeric behavior of the materials tested. The extent of agreement between microscopic and macroscopic behavior was then evaluated.

Determination of the true stress–strain behaviour of polypropylene

The true tensile and compressive stress–strain curves for a typical semicrystalline polymer (polypropylene) were constructed with a new experimental technique. This is based on a non-contact method of strain measurement along the specimen gauge length. The viscoplastic behaviour obtained in this manner was then described with a nonlinear viscoelastic constitutive model mentioned analytically elsewhere. This consideration leads to a satisfactory description of yield and post yield behaviour, including strain softening, and strain hardening. The rate effect was also predicted with a high accuracy.

Experimental and theoretical description of the plastic behaviour of semicrystalline polymers

In this work, a new experimental technique, based on a non-contact method of strain measurement, has been applied in the case of semicrystalline polymers, where yielding occurs through inhomogeneous deformation (necking). It was then possible to construct the true stress-strain curves at various crosshead speeds tested. Furthermore, a constitutive model for the description of this behaviour has been introduced, grounded on a plasticity theory, which has been developed and initially applied on plastic behaviour of crystalline metals. This theoretical model was proved to predict satisfactorily the experimental results. Moreover, the rate effect and the post yield phenomena, as possible strain softening and strain hardening have also been described in terms of molecular parameters connected with the material tested.

The effect of moisture absorption on the thermomechanical properties of particulates

Water absorption in particulate composites at ambient temperature influences their thermomechanical properties. Second Ficks law of diffusion was used in this paper to predict the diffusion coefficient of the composite materials tested. In all cases the matrix material was a diglycidyl ether of bisphenol-A polymer cured with 8 phr triethylene tetramine and filled with iron particles with an average diameter 150 μm at five distinct volume fractionsvf=0, 0.05, 0.10, 0.16 and 0.20. The modification of the modulus of elasticity, ultimate stress, breaking strain and breaking energy due to moisture absorption was examined. Moreover, differential scanning calorimetry was used to study the influence of the time exposure into water and the filler concentration of the particulates on their glass transition temperature. Finally, the void occupancy in the composite was evaluated from free volume considerations.

Relaxation phenomena and morphology of polyurethane block copolymers

Abstract Five types of linear segmented polyurethanes based on a low molecular weight polyester, reacted with 4,4′-diphenylmethane diisocyanate (MDI), and extended with 1,4-butanediol (BDO), have been studied by dynamic mechanical analysis (DMA), thermally stimulated depolarization current method (TSDC), and differential scanning calorimetry (DSC). Different types of transitions and relaxations have been detected and related to the structure and morphology of the block copolymers studied.

The Effect of Filler-Volume Fraction and Strain Rate on the Tensile Properties of Iron- Epoxy Particulate Composites

The stress-strain behavior of metal-filled polymeric composites was studied at various tensile deformation rates. The dependency of the mechanical properties of iron-epoxy composites, such as elastic modulus, maximum load, breaking-strain and breaking-energy, on the rate of deform ation, as well as on the filler-volume fraction was examined. The results corroborate satisfactorily the theoretically predicted values.

Formulation of the Viscoplastic Behavior of Epoxy-Glass Fiber Composites

The tensile behavior of epoxy-glass fiber composites is examined experimentally and theoretically, in terms of a 3-D viscoplastic model, appropriate for material anisotropy. For the kinematic description, the multiplicative decomposition of the deformation gradient tensor has been used, while a flow rule for an orthotropic material, based on a dyadic of vectors parallel to the material axes, is introduced. Moreover, a functional form expressing the rate of plastic deformation, developed for polymeric materials in earlier works, is applied in the case of polymeric composites. A satisfactory agreement with the experimental results of the off-axis specimens is found, while the strain rate effect is described in terms of a scaling rule, valid in the viscoplasticity.

Structure-property interrelation of polypropylene-liquid crystalline polymer blends

The thermomechanical behaviour and the structure-property interrelation for blends based on a thermotropic liquid crystalline polymer (LCP) with a thermoplastic semicrystalline polymer as polypropylene (PP) was investigated. The polymeric materials were melt blended in a twin-screw extruder under the appropriate conditions in order to obtain blends with LCP in fibrous form. A series with four different LCP contents has been prepared, while another series with 5% LCP per weight, at high values of draw ratio and at the same temperature profile has also been investigated. All blends exhibit improved mechanical properties, while with DSC and DMA experiments further information about the morphology and distribution of the dispersed phase has been obtained. Finally, a simple percolation model, developed elsewhere, has been used to interpret the reinforcing effect of LCP and the interrelation between the two polymeric phases.

Dynamic Mechanical Properties of an Iron‐Epoxy Particulate Composite

The mechanical properties of iron‐epoxy particulate composites were studied from the glasslike state through the transition region to the rubberlike state in dynamic loading applied by a Dynastat and a Dynalizer apparatus. The effect of filler content on the viscoelastic properties was studied by using six distinct filler concentrations. Measurements were carried out at frequencies between 0.1 and 100 Hz and temperatures from 50 to 120 °C. Composite curves for both the storage and the loss moduli along a wide frequency range were made by applying the time‐temperature super‐position principle. Moreover, the Takayanagi model was used for the prediction of the filler contribution on the storage compliance (D′) in the transition region. Finally, the effect of filler content on the relaxation spectra H and the glass transition temperature was examined.

The Effect of Accelerator on the Crosslink Density and Mechanical Behaviour of an SBR-Filled Elastomer:

The effect of accelerator to sulfur ratio variation on the crosslink density of an SBR-carbon black filled elastomer has been studied. Moreover, the reinforcement ef fect of the inclusions as well as the type of rubber attachment on the filler surface with varying the accelerator content has also been examined. DSC tests and swelling measure ments have been made for the studying of the chemical crosslinking. Equilibrium stress- strain tests and tensile-retraction measurements have been carried out for the determina tion of the mechanical behaviour of the materials tested. The Mooney-Rivlin relation was used to describe the behaviour of the rubber matrix and values of constants C1, C2 have been calculated with the use of the strain amplification factor.