E. S. Trofimchuk

Nanocomposites on the Basis of Crazed Polymers

Analysis of published data on the mechanism of structural rearrangements in solid polymers on their crazing in liquid media is presented. The experimental evidence characterize crazing not only as a kind of spontaneous polymer dispersion under joint action of a mechanical stress and an active liquid medium, but also as the method of colloidal dispersion of low-molecular substances in a polymer. In the process of crazing, active liquid fills the porous structure of crazes, thereby transporting various low-molecular substances to the polymer volume. Crazing is believed to open the ways for preparing various nanocomposites on the basis of a wide variety of glassy and crystalline polymers, on the one hand, and target additives on the basis of practically any low-molecular substances, on the other.

Crazing of isotactic polypropylene in the medium of supercritical carbon dioxide

Uniaxial tensile drawing of films based on semicrystalline isotactic PP in the medium of supercritical carbon dioxide at a pressure of 10 MPa and a temperature of 35°C is studied. The tensile drawing of PP is shown to proceed in the homogeneous mode without necking and is accompanied by intense cavitation. The maximum level of porosity is 60 vol %. The porous structure that develops owing to the tensile drawing of the polymer in supercritical CO2 is provided by formation of a set of crazes that are primarily localized in interlamellar regions. According to small-angle X-ray scattering data, the average diameter of fibrils that bridge craze walls changes slightly with an increase in tensile strain and is ∼10 nm; the specific surface of the craze fibrils is 100–150 m2/cm3.

Preparation method for noble metal-polymer matrix nanocomposites

A method is described for the preparation of new nanocomposites based on poly(ethylene terephthalate), poly(vinyl chloride), and polypropylene on the one hand and on noble metals (Ag and Pt) on the other. The method comprises the formation of nanoporous polymer matrices by crazing the polymers with simultaneous incorporation of noble metal precursors (AgNO3 or H2PtCl6) into the matrices. Subsequent in situ reduction of the precursors yields the metal-polymer nanocomposites. Prospects for the practical application of the developed method for the production of metal-polymer nanocomposites are discussed.

Crazing of polymers in the presence of hyperbranched poly(ethoxysiloxane)

The crazing of various polymers (PET, isotactic PP, and HDPE) in the presence of branched poly(ethoxysiloxane) and its low-molecular-mass analog—tetraethoxysilane—has been studied. The hyperbranched poly(ethoxysiloxane) is shown to be an effective adsorptionally active medium for crazing of various solid polymers and development of nanoporous structures with a volume porosity of up to 60%. Depending on the nature of polymers, two mechanisms of crazing (either classical or delocalized crazing) can take place. The reactions of hydrolysis (basic and acidic) within the pores leading to formation of solid silica have been performed. Electron microscopic observations provide evidence that the transformation of a viscous adsorptionally active liquid into a solid compound directly within the volume of a polymer matrix leads to the stabilization of a highly dispersed polymer structure that arises in the course of crazing.

Preparing film composites based on crazed polymers and silica sol nanoparticles

Films of polymer/silica nanocomposites based don isotactic polypropylene and high density polyethylene are prepared via solvent-crazing in the tetrahydrofurane solution of molecular silica sol. This method allows us to disperse SiO2 particles with a diameter of 5–15 nm in a volume of polymer matrices homogeneously and at the nanometer level. The possibility of producing a fragmented riffled silica coating onto polymer surfaces is presented.

Specific features of the formation of the silicon dioxide phase in porous poly(propylene) prepared through the crazing mechanism

A new technique is described for preparing poly(propylene)-silica nanocomposites with the use of crazing of polymers in reactive liquid media that exhibit an adsorption capacity with respect to the polymer and contain functional groups able to enter into different chemical reactions, in particular, hydrolytic condensation. The advantage of this technique over conventional mixing is that the components can be mutually dispersed at the nanolevel without using additional modifying additives. The hydrolytic condensation of tetraethoxysilane and hyperbranched poly(ethoxysiloxane) in the presence of acid or base catalysts with the formation of a silica gel in a crazed polymer matrix is investigated. It is established that the morphology of the prepared composites is determined by the structure of the crazed polymer matrix, the nature of the precursor, and the hydrolytic polycondensation conditions. Composites are prepared in which the silica phase is located either inside the poly(propylene) matrix (in the form of a continuous phase or discrete particles) or on the surface of the polymer. Porous silicon plates are produced through heat treatment of the poly(propylene)-silicate nanocomposites at a temperature of 700°C.

Novel approach to obtain composite poly-L-lactide based films blended with starch and calcium phosphates and their bioactive properties

A novel approach for preparation of composite materials based on the poly-L-lactide (PL), starch and calcium phosphates is provided, applying the polymer crazing process in liquid absorption active medium. For composite films preparation, PL and blends of PL and starch have been chosen as polymeric matrixes. Pores formation in the polymer films occurred during the process of uniaxial stretching in the presence of ethanol or water-ethanol mixtures via the solvent-crazing mechanism. The diameter of pores and fibrils in crazed PL was 20–30 nm. Porous polymer matrixes have been loaded with starch, using crazing mechanism, and filled with calcium phosphates (CP) by the countercurrent diffusion method, the content of starch and CP being up to 11.0 and 14.5 wt%, respectively. The obtained composites were investigated by XRD, SEM-EDS and TEM methods. It was found out that the starch filled the porous crazed structure of polymer, while the CP synthesis in the PL pores resulted in the formation of amorphous particles with the diameter of about 20 nm. These CP nanoparticles aggregated into the submicron particles of 100–300 nm in diameter. The bioactivity of the initial and porous poly-L-lactide films, and composites, based on PL with starch and calcium phosphates, was investigated. It was shown that the bioactivity depends on the chemical composition and surface morphology of the prepared materials. After 10 days of cultivation of the pre-osteoblastic MC3T3E1 cells, an intense, 6 times, growth of osteoblast population was achieved for the composite containing calcium phosphates. This result is perceptibly higher than those obtained for other samples (4 times growth) and for the control sample (5 times growth). Based on the results of in vitro tests, it can be concluded that the prepared materials are promising for biomedical applications.

Crazing of Polymers in a Supercritical Carbon Dioxide Fluid

ISSN 00125008, Doklady Chemistry, 2009, Vol. 428, Pa rt 2, pp. 238–241.

Preparation of nanoporous polyolefin films in supercritical carbon dioxide

The peculiarities of uniaxial deformation of semicrystalline polyolefins were studied using high-density polyethylene and isotactic polypropylene as an example in carbon dioxide at 35–75°C and pressures of 0.1–30 MPa. A nanoporous structure (with pore diameters of 3–7 nm) started to form in polymer films by the delocalized crazing mechanism at 4–5 MPa and higher. The increment of the pore volume depends on the parameters (pressure, temperature, and density) of the medium and on the deformation of the polymer, reaching 30–70%. The pore formation by the crazing mechanism was most effective when CO2 was in the supercritical state at a nearly critical temperature (35°C) and its density approached the density of a liquid.

Obtaining nanoporous inorganic sheets

An original technique to obtain porous sheets from various inorganic substances (silicon dioxide, hydroxyapatite, and silver) using polymer matrices whose high-disperse structure is formed by the crazing mechanism has been described. The structure of a porous sheet and its characteristics can vary in wide ranges (the diameter of pores can vary from several nanometers to several micrometers) and depend on the matrix morphology of the initial polymer, its strain degree, and the amount of the inorganic component and the method of its introduction. The conditions needed to form a sheet are formulated: the degree of the polymer film strain should be no less than 50% and the filler content should be no less than 10 vol %.