D. Acierno

A non-linear viscoelastic model with structure-dependent relaxation times: I. Basic formulation

Abstract A non-linear constitutive equation for polymer melts and concentrated solutions is presented. Based on known results of network theories, the model contains a distinctive feature: that of letting the relaxation times depend upon the existing structure. The model extends the constitutive equation of linear viscoelasticity to the non-linear region in a well-defined way, with the uncertainty of just a single adjustable parameter. Predictions of the model for common cases of non-linear response are derived and discussed.

A non-linear viscoelastic model with structure-dependent relaxation times: II. Comparison with l.d. polyethylene transient stress results

Abstract A model of non-linear viscoelasticity with relaxation times which depend on the structure is compared with experimental results reported in the literature for a L.D. polyethylene. The single parameter of the model is determined by comparison with steadystate shear results. The model is then used to interpret various transient data. These are: tangential and normal stress growth in shear, stress growth in elongation, normal stresses in shear creep. The comparison shows a good general agreement, thus supporting the suggestion of relating the change in time of the relaxation spectrum to structural variables.

Mechanical properties of blends of low density with linear low density polyethylene

Abstract Torque values and mechanical properties are reported for blends of low density with linear low density polyethylene (PE). The torque values show that incompatible mixtures in the molten state are formed when the torques of the pure polymers are almost equal. Semicompatible behaviour is shown in the other cases. The mechanical properties indicate that semicompatible mixtures are formed in the solid state. The mechanical properties of these blends are strongly influenced by the linear low density polyethylene only for contents exceeding 25%.

Influence of the structure of linear density polyethylene on the rheological and mechanical properties of blends with low density polyethylene

Abstract Rheological and mechanical data on LLDPE/LDPE blends are presented in order to evaluate the influence of the structure of LLDPE on the properties of these blends. The influence of the comonomer type is negligible, while the molecular weight exerts an important effect on properties. For both rheological and mechanical properties, the blends made from LLDPE of large MFI seem the more interesting.

A non-linear viscoelastic model with structure-dependent relaxation times: III. comparison with l.d. polyethylene creep and recoil data

Abstract The equations of a model previously proposed by the authors to describe the non-linear viscoelastic behaviour of polymer melts are elaborated here to predict creep and recoil responses. The comparison with existing data for both shear and elongational flow shows good agreement and supports the assumption of a material which is destructured by a prolonged application of high deformation rates.

Tensile properties of γ-irradiated LDPE-iPP blends

Abstract Low density polyethylene-isolatic polypropylene have been irradiated in vacuum in the dose range 0–150 Mrad. The structural modifications induced by irradiation were the subject of a previous work. Tensile tests have been performed on the same materials and the results are here discussed in the light of their physico-chemical properties.

Thermomechanical degradation of blends of isotactic polypropylene and high density polyethylene

Abstract Blends of isotactic polypropylene (iPP) and high density polyethylene (HDPE) at various compositions have been prepared in a mixer. Two different temperatures and various rotational speeds have been used in the experiments. Thermomechanical effects have been investigated through rheological measurements on the above blends. At the higher temperature, degradation occurs in all the samples and a synergistic effect of the thermal and mechanical degradations is observed. At the lower temperature iPP and iPP-rich blends also show degradative effects; HDPE and rich blends increase in viscosity, revealing slight crosslinking.

A comprehensive experimental study of the rheological behaviour of HDPE.

Rheological properties of six high-density-polyethylene (HDPE) samples with known average molecular weight and molecular weight distribution have been determined with the aid of two capillary viscometers over a wide range of temperatures and shear rates. Generalized relationships have been found for the entrance correction, the zero-shear viscosity and the shift factors of the whole viscosity curve.

Transient birefringence of polymer melts in intermittent shear flow: Model analysis of the non-linear viscoelastic behaviour

SummaryA non-linear viscoelastic model has been used to interpret transient flow birefringence in changing shear flow for a polymer melt. It is shown how the new model is consistent with the basic hypothesis of the linear stress-optical law. Stress growth in shear flow and relaxation after different amounts of shearing are compared with the predictions of the non-linear model. A good agreement between experimental data and theoretical predictions is found.ZusammenfassungEin nicht-lineares viskoelastisches Modell wird zur Interpretation der zeitabhängigen Strömungsdoppelbrechung verwendet, die bei wechselnder Scherung an einer Polymer-Schmelze zu beobachten ist. Es wird gezeigt, daß das neue Modell mit der Grundannahme eines linearen spannungsoptischen Gesetzes verträglich ist. Das Anwachsen der Spannung in der Scherströmung sowie ihre Relaxation in Abhängigkeit von der Größe der vorangegangenen Scherung wird mit den Voraussagen des nicht-linearen Modells verglichen. Es wird eine gute Übereinstimmung zwischen experimentellen Ergebnissen und theoretischen Voraussagen gefunden.

The rheological behaviour of HDPE/LDPE blends

The rheological behaviour of three types of HDPE/LDPE blends at several compositions (various weight percentages of LDPE) has been studied with the aid of a capillary rheometer and three different capillaries.In particular, entrance effects and shear viscosities have been determined over a wide shear rate region and at different temperatures. Thus activation energies could also be evaluated. Synergistic effects are evidenced when the relevant properties of the homopolymers parents are not too different from one another.