M. A. Guseva

The effect of the physicomechanical characteristics of the polymer matrix and structure of a filler on the stress-strain behavior of polymer-montmorrilonite nanocomposites

The stress-strain behavior of PE-Na+-montmorillonite nanocomposites is studied. The stress-strain characteristics of the composites are shown to be controlled by the structure of their nanofiller, which is formed upon melt mixing of the polymer and layered silicate (intercalated or exfoliated), the profile of the stress-strain curve of PE matrix, and the fracture mechanism (either adhesive or cohesive). In nanocomposites with a strong adhesive bonding between matrix and clay particles (under cohesive fracture), both the modulus and yield point are found to be markedly increased. In the case of adhesive fracture, mechanical characteristics are less improved due to debonding between matrix and filler results. For nanocomposites, experimental stress-strain characteristics are compared with theoretical estimates calculated according to the models proposed for predicting the characteristics of filled thermoplastic polymers. In some cases, experimental values of modulus and elongation at break appear to differ appreciably from theoretical estimates. The applied models should take into account the orientation of anisodiametric inclusions in a polymer matrix and the character of separation in the composite (adhesive or cohesive).

Simulation of the elastic-plastic behavior of polyolefin-based nanocomposites with a different structure of layered filler

A phenomenological model is proposed for the correct description of the mechanical behavior of nanocomposites based on a polymer matrix and a layered silicate under finite elastic—plastic deformations. The constitutive equations are constructed according to the approach based on an interpretation of the mechanical behavior of the material in terms of symbolic circuits. This model makes it possible to describe not only strain hardening of the material but also its loss of strength or weakening. This model takes into account the specific features of yielding and allows one to model the accumulation of structural changes upon loading. At high strains, stress in intercalated systems appears to be lower than that in exfoliated nanocomposites with the same filler content. This is probably related to the fact that intercalated tactoids (crystallites of layered silicates), whose interplanar spaces are occupied by polymer molecules, move in the material as individual large-sized particles, which are strongly anchored to the matrix, and this process requires higher stresses. Changes in the shape of isolated silicate platelets (in the exfoliated nanocomposites) and tactoids in the course of deformation are studied. Calculations show that various silicate platelets lose their stability, and this loss in stability is shown to depend on their orientation in the composite with respect to the direction of tensile drawing, on the number of platelets in each stack, and on the external force applied. Upon loading, the edges of silicate platelets in the tactoids are able either to come closer or move apart, depending on the orientation of platelets with respect to the direction of the external force.

Quantitative characterization of the orientation of macromolecules in intercalated nanocomposites of polyolefins/layered silicates by Raman spectroscopy

Raman spectroscopy is used to study variations in the orientational order of macromolecules in the uniaxially drawn intercalated nanocomposites based on two polymer matrices (polyethylene (PE) and isotactic polypropylene (PP)) and a filler (modified clay (MC)). The orientation parameters of macromolecules measured using Raman spectroscopy are compared with the X-ray data. It is demonstrated that, for the uniaxially drawn PE-MC and PP-MC intercalated nanocomposites, the filler impedes the orientation along the draw direction for the macromolecules localized in the noncrystalline phase of the polymer matrix. The orientational ability of the PE and PP crystallites in nanocomposites is not affected by the filler.

An efficient route for design of luminescent composite materials based on polyethylene containing europium dibenzoylmethanate

Luminescent composite materials based on linear polyethylene doped by a well dispersed tetrakis dibenzoylmethanate europium complex have been developed. The anion of the latter serves as an efficient light harvesting antenna and possesses the desired photophysical properties. The targeted modification of the Eu complex, namely introduction of a long hydrocarbon chain into the tetraalkylammonium cation [RNEt3]+, has ensured the compatibility of the complex with the polyethylene matrix. The [RNEt3]+ cation has been obtained by using long-chained 1-iodoalkanes synthesized via the Nd-catalyzed ethylene oligomerization process. The photophysical properties and the homogeneity of the obtained composites have been controlled by optical spectroscopy, luminescence intensity mapping as well as scanning electron microscopy.

A study of the elastoplastic properties of polymer-silicate nanocomposites with allowance for the change in their volumes during deformation

The results of experimental and theoretical studies of the elastoplastic properties of medium-density polyethylene and nanocomposites formed on its basis with a filler of layered clay minerals are described. It is found that the deformation of these materials is accompanied by a significant change in their volumes, which is primarily caused by the development of internal damage of the system (the formation of pores and cracks at the microlevel). A component that takes into account the change in volume during deformation is introduced into the previously proposed model of a heterogeneous medium that can undergo significant non-linear elastic and plastic deformations. For this purpose, we use a differential approach to the construction of constitutive equations and an additive decomposition of the total strain-rate tensor of the medium into strainrate tensors of the elastic and plastic components. Using an improved model, we describe the experimental curves of uniaxial stretching for pure PE and two PE-based polymer-silicate nanocomposites. The introduction of filler particles into the polymer matrix leads to enhancement of the role of two structural mechanisms in the formation of plastic properties: The presence of the filler contributes to the development of internal structural damage; the same particles decrease the orientation ability of the PE matrix, thereby hindering the development of plastic deformations in it. The use of tensor variables in the model makes it appropriate for describing not only stretching but also other types of loading of the material.

Atomic Force Microscopy of Structural-Mechanical Properties of Polyethylene Reinforced by Silicate Needle-Shaped Filler

The paper presents the results of experimental studies of polyethylene-based composites reinforced with silicate needle-shaped filler (palygorskite) of different mass fraction (0, 5, 10, and 15%). These composites are less flammable and fire toxic than unfilled polyethylene. The structure (size, shape, and agglomeration of filler) and local mechanical properties of composites in nonstretched and elongated states were investigated by AFM. In stretched samples palygorskite takes a wavy shape, and at extremely high elongation the filler is orthogonal to the axis of tension. The smooth surfaces of the samples, required for AFM, were prepared using the heating/cooling procedure.

Using Raman spectroscopy to determine the structure of copolymers and polymer blends

We present Raman structural study of two grades of random propylene/1-octene copolymers with low and high molecular weights and the 1-octene content up to 4.5 mole %. The copolymer spectra are compared with the spectra of the α, γ, and smectic modifications of isotactic polypropylene. Raman investigation has showed that the degree of crystallinity and conformational order of the copolymer macromolecules slightly decrease with the growth of the 1-octene content. The degree of crystallinity is slightly higher for the samples of the high-molecular-weight grade compared to the low-molecular-weight one. Furthermore, we present Raman spectra of polyethylene/polypropylene (90/10) blends, mixed in the polyethylene melt at three different temperatures, corresponding to three different states of polypropylene macromolecules. It was concluded that the degree of crystallinity and conformational order of the polyethylene macromolecules in the blends are the highest for the temperature, at which polypropylene macromolecules have lost their packing and conformational order.