G. Spathis

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.

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.

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.

Thermomechanical behavior of metallocene ethylene-α-olefin copolymers

Abstract The thermal, dynamic mechanical and stress–strain properties of a set of commercial LLDPEs prepared by metallocene catalysts were studied and compared with two LLDPEs obtained with traditional Ziegler–Natta catalysts. The type and amount of comonomer strongly affects the degree of crystallinity and branching, resulting in different material morphology and macroscopic thermomechanical behavior. Within each set of ethylene-α-olefin copolymers a gradual decrease in the percentage crystallinity and position and intensity of β- and γ-transition is observed, with respect to the comonomer content. In addition, the studied materials are characterized by the absence of an α-transition. This behavior is attributed to the high comonomer content (5–24)%. Tensile behavior changes from the typical necking, cold drawing and strain hardening to the uniform elastomeric deformation, as the comonomer content increases. The experimental methods used, in combination with scanning electron microscopy, lead to converged results, while tensile testing proved to be sensitive to the crystallite size and distribution.

Mechanism of plastic deformation for polycarbonate under compression by a laser extensometer technique

The large strain behavior of a glassy polymer was studied in terms of compression tests at various rates, while the local deformation was evaluated with a noncontact laser extensometer. The main features of yield and postyielding were described with a constitutive model from nonlinear viscoelasticity, combined with a kinematic formulation for the separation of total deformation into a viscoelastic and a plastic part, respectively. The concept of plastic shear transformations introduced elsewhere was used to develop a mathematical model for the rate of plastic deformation. The entire experimental true stress–strain curves (including strain hardening and strain softening) could, through this model, be identified in a self-consistent manner.

Theory for the plastic deformation of glassy polymers

We present a theory to account for how a stress field can induce plastic flow in glassy polymers. We consider a molecular model in which chains are constituted from isotropic oriented elementary links embedded in a deforming continuum. The shear-stress field causes a redistribution of such links governed by a balance equation in orientation space. After detail calculations of the number of oriented units in certain directions, the net jump rate of the molecular links is given by an exponential form, according to the transition state theories of Adam and Gibbs [6]. These considerations are compared with the study of Boyce et al. [5] that included the effects of deformation rate, pressure and strain softening. The calculation of the plastic properties for polymethyl methacrylate and polycarbonate for various types of deformation has been made, and the corresponding results fitted the experimental data reasonably well.

An experimental and analytical study of the large strain response of glassy polymers with a noncontact laser extensometer

The yield behavior of an amorphous glassy polymer has been studied with true tensile stress-strain curves, obtained at various crosshead speeds by means of a new experimental method. A constitutive equation from nonlinear viscoelasticity has been used, with the further assumption that the material, during deformation, subsequently follows the following two distinct paths: a nonlinear viscoelastic, and a plastic one. The maximum strain, where this distinction is manifested, has been treated as a control parameter, while the strain rate was experimentally evaluated. The decomposition of deformation has been made with a suitable kinematic formulation, proposed in the literature. The theoretical results describe the experimental curves in detail.

A non-linear viscoelastic model for predicting the yield stress of amorphous polymers

A non-linear viscoelastic model is introduced by employing a new formulation where the relaxation time of rate processes is directly dependent on the induced deformation state applied to the material. The plastic flow of amorphous polymers is considered as a continuous structural change, and the yield phenomenon is modelled through the corresponding viscoelastic constitutive equations. All material constants necessary to predict the yield stress, the rate and temperature dependence are non-adjustable parameters depending on the particular deformation mode. To verify the results of the proposed model, two independent series of experiments have been performed on specific polymeric materials, and have been proved to satisfy adequately the proposed analysis.

Non-Gaussian stress-strain constitutive equation for crosslinked elastomers

A stress-strain equation is studied which uses a non-Gaussian statistical mechanics model for large deformation of crosslinked rubbers. The formulation is based on the work introduced by Doi and Edwards for the description of polymer dynamics in a fixed network. The resulting constitutive equation is applied in the entire network of elastomers. Uniaxial and biaxial tensile and compressive experiments are explained in detail. Comparison with other theoretical work reveals that the present model could be applied equivalently in any kind of physical or chemical network junction.

Non-linear constitutive equations for viscoelastic behaviour of elastomers at large deformations

Abstract A non-linear theory of viscoelasticity for large deformation of elastomers has been developed. The concept of an internal variable stress which is governed by a rate equation, has been used to account for the viscoelastic behaviour of incompressible rubbers. Following a molecular approach the parameters of the model were extracted by the inverse Langevin approximation of rubber elasticity. Qualitative applications were made in the case of uniaxial elongation and simple shear deformations.