V. A. Gerasin

Composite biodegradable materials based on polyhydroxyalkanoate

Conditions for the processing and mixing of biodegradable polymers at temperatures less than their thermal destruction (130–150°C) using standard equipment have been identified. The structure of the polyhydroxybutyrate/valerate (PHB/V) copolymer has been revealed and peculiarities of the crystal phase formation at different monomer ratios have been investigated. It was shown that pure PHB with molecular mass 180–270 kDa has elastic module approximately 1.2 GPa, strength approximately 25 MPa, and elongation at break approximately 10%. The most active biodestructors of PHB, PHB/V, and their composites have been selected (Aspergillus caespitosus), and the ability of basidiomycete Panus tigrinus to biodegrade polyalkanoates was demonstrated for the first time. It was shown that A. caespitosus degraded PHB/V and Biopol films along with the PHB with the destruction rate depending on the technology of the film production, on the molecular mass, and on the extend of the polymer crystallinity.

Raman Spectroscopic Characterization of the Interlayer Structure of Na+-Montmorillonite Clay Modified by Ditetradecyl Dimethyl Ammonium Bromide

Raman spectroscopy has been applied for the rapid and nondestructive monitoring of the interlayer structure of sodium montmorillonite (MMT) clay modified by ditetradecyl dimethyl ammonium (DDA+) bromide. This work demonstrates that a detailed analysis of Raman spectra in the fingerprint region (600-1600 cm(-1)), in combination with model simulation, allows one to distinguish different conformational states of DDA+ in the interlayer space of the modified clay, namely, a liquidlike state but rich in trans conformers, disordered conformational states, and a crystallike conformation appearing at increasing modifier content. These conformations differ in the angle between their alkyl chains, the relative content of trans and gauche conformers and the relative length of trans segments. The shape and width of the Raman band at 1300 cm(-1) and the peak intensity ratio I1088/I1064 can be used for a qualitative analysis of the ratio of gauche/trans conformers. The integral intensity ratios I*1064/I*1300 and I*1300/I*705 help to determine the proportion of trans conformers and the content of the modifier in the clay, respectively, thereof providing quantitative characterization of the modified clay (conformational reorganization and modifier content). Noteworthy, the transition from a liquidlike to crystal-like conformation is further supported by the splitting of the symmetric C-C stretching Raman band of the trans segments within the alkyl chains at 1133 cm(-1) (liquidlike conformation) into two modes at 1124 and 1135 cm(-1) corresponding to two parallel trans chains of nonequivalent lengths (crystal-like conformation).

Nanocomposites based on layered silicates and ultrahigh-molecular-mass polyethylene prepared via in situ polymerization

Nanocomposites based on ultrahigh-molecular-mass PE and layered silicates are prepared via in situ polymerization. The process is conducted in the suspension mode under mild conditions with the use of a conventional Ziegler-Natta catalyst and a bifunctional complex containing a nickel oligoallene component along with a titanium component. The fillers are four types of montmorillonite: nonmodified montmorillonite and three montmorillonites modified with various organic intercalants. In their presence, the activities of the catalysts appreciably increase (by a factor of 2–3). Composites containing 10–40 wt % aluminosilicates are prepared. The structures of the polymer matrix and composites are studied via X-ray diffraction. It is found that the degree of exfoliation of the filler depends not only on its amount but also on the type of modifier and the type of catalytic system. The PE matrix with a molecular mass of (1.5–1.6) × 106 has a melting point of T m = 141–143°C, a high enthalpy of melting, and a high degree of crystallinity. The IR-spectral studies of the polymers synthesized with the bifunctional complex reveal the presence of branches in a chain, thereby suggesting the formation of linear PE with a medium density (0.938–0.946 g/cm3). Nanocomposites containing 10% aluminosilicate possess the best complex of mechanical properties relative to that of unfilled polyethylene.

Nanocomposites and high-modulus fibers based on ultrahigh-molecular-weight polyethylene and silicates: Synthesis, structure, and properties

Nanocomposites based on ultrahigh-molecular-weight polyethylene and inorganic fillers—such as organomodified layered aluminosilicates, aerosil, and diatomite—are prepared via polymerization filling. The polymerization of ethylene was conducted in the suspension mode with the use of a conventional Ziegler-Natta catalyst, TiCl4 + Al(i-Bu)3, under mild conditions (a temperature of 30°C and a pressure of 0.1MPa). The structure and properties of the composites are studied via X-ray diffraction analysis and DSC. The polyethylene matrix features a high enthalpy, a high melting temperature (up to 143°C), a crystallinity of 70–80%, a content of the monoclinic phase of 12–15%, and a bulk density of 0.05–0.15 g/cm3; the molecular mass is (1.5–1.6) × 106. High-modulus, high-strength fibers with an elastic modulus of 25–28 GPa and a strength of 0.65–0.70 GPa are prepared via direct solvent-free molding of nascent reactor powders based on ultrahigh-molecular-weight polyethylene filled (7 wt %) with aerosol or montmorillonite modified with vinyltrimethoxysilane.

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.

Symmetric C-C stretching mode splitting versus CH2-chain conformation order in sodium montmorillonite modified by cetyltrimethylammonium bromide.

Exploiting Raman spectroscopy and computational modeling, for the first time, we report and explain an interesting phenomenon in clay modified by cetyltrimethylammonium bromide. A splitting of the CH(2)-chains symmetric C-C stretching Raman mode found at ~1128 cm(-1) in cetyltrimethylammonium bromide into two bands at 1128 and 1139 cm(-1) in clay modified by cetyltrimethylammonium bromide is observed. We demonstrate that this splitting appears if two types of trans-segments with nonequivalent lengths and terminal groups coexist in the CH(2)-chain of the alkylammonium ion embedded into the clay interlayer space. We report Raman experimental evidence for a CH(2)-chain bending within the clay galleries, resulting in the symmetric C-C stretching band splitting, as was also suggested by computational modeling. Noteworthy, we postulate that this unique behavior based on CH(2)-chain bending provides a general understanding of conformation reorganization and switching within long CH(2)-chain molecules confined within modified clay interlayer galleries. For all modifier concentrations, we show that the intercalated cetyltrimethylammonium ions exist in a liquid-like state, consisting mainly of trans conformations (~86%) of two types in approximately equal proportions. Moreover, we demonstrate that the integral Raman intensity ratio I(1295)(CH(2))/I(705)(clay) provides a rapid nondestructive quantification of the relative content of alkylammonium ions in modified clays. These results demonstrate that a simple direct monitoring of specific modifier-dependent interlayer conformational states is possible, which is of great importance for a tunable fabrication of modified clays-based nanocomposites with desired properties.

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.

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.