Sergei Bronnikov

A recently proposed method called “controlled swelling of monolithic films” was implemented to prepare ultra-high-molecular-weight polyethylene (UHMWPE) ultrafiltration membranes. For the first time, the effect of UHMWPE molecular weight (MW) on the structure and properties of the membranes prepared via this special case of thermally induced phase separation was studied in detail. The morphology and properties of the membranes were studied using SEM, DSC, liquid–liquid displacement porometry, and standard methods for the evaluation of mechanical properties, permeance, rejection, and abrasion resistance. High-quality membranes with a tensile strength of 5.0–17.8 MPa, a mean pore size of 25–50 nm, permeance of 17–107 L m⁻² h⁻¹ bar⁻¹, rejection of model contaminant (blue dextran) of 72–98%, and great abrasion resistance can be prepared only if the MW of the polymer in the initial monolithic film is sufficiently high. The properties of the membranes can effectively be controlled by changing the MW of the polymer and the mass fraction of the latter in the swollen film. Shrinkage is responsible for the variation in the membrane properties. The membranes prepared from a higher-MW polymer are more prone to shrinking after the removal of the solvent. Shrinkage decreases before rising again and minimizes with an increase in the polymer content in the swollen film.

Polyacrylamide-based hydrogel filled with carbon quantum dots (PAA@CQDs) exhibiting intensive photoluminescence (PL) under UV irradiation was prepared. The exceptional capability of the Fe3+ ions to quench PAA@CQDs PL in aqueous solution was found. The PL recovery after repeated quenching was detected when hydrochloric acid was used as a regenerant, thus ensuring the PAA@CQDs reusability as a chemosensor for the Fe3+ ions detection in aqueous media.

Oriented polyvinylidene fluoride (PVDF) films were obtained in multistage process based on melt extrusion of polymer. We investigated the results of the polymorphic α→β crystalline phase transformation in the PVDF films subjected to uniaxial stretching. During uniaxial extension both appearance of a polar piezo active crystalline structure and significant changes in the samples morphology were observed. Variations in the PVDF films morphology, polymorphic composition, and supramolecular structure upon transformation were detected with wide-angle X-ray scattering and scanning electron microscopy techniques. Broadband dielectric spectroscopy was used to ascertain a change in molecular mobility of the polymer chains during α→β phase transformation. The relaxation processes, γ-, α_а-, α_с-, and interfacial polarization, in both α- and β-phases of PVDF were identified in the dielectric loss spectra and described with either Arrhenius or Vogel-Fulcher-Tammann equations. The analysis of the equations parameters allowed concluding that that initiation of a polymorphic α→β transition through uniaxial extension results in hindering of the relaxators mobility in the β-phase of PVDF samples, except γ-relaxators. This finding confirms a proposal that γ-relaxators are located in the amorphous part of PVDF. Uniaxial extension resulted in a substantial increase in the interfacial polarization, which can be attributed to the emergence of new interface boundaries.

A new method of fabricating porous membranes based on ultra-high molecular weight polyethylene (UHMWPE) by controlled swelling of the dense film was proposed and successfully utilized. The principle of this method is based on the swelling of non-porous UHMWPE film in organic solvent at elevated temperatures, followed by its cooling and further extraction of organic solvent, resulting in the formation of the porous membrane. In this work, we used commercial UHMWPE film (thickness 155 μm) and o-xylene as a solvent. Either homogeneous mixtures of the polymer melt and solvent or thermoreversible gels with crystallites acting as crosslinks of the inter-macromolecular network (swollen semicrystalline polymer) can be obtained at different soaking times. It was shown that the porous structure and filtration performance of the membranes depended on the swelling degree of the polymer, which can be controlled by the time of polymer soaking in organic solvent at elevated temperature (106 °C was found to be the optimal temperature for UHMWPE). In the case of homogeneous mixtures, the resulting membranes possessed both large and small pores. They were characterized by quite high porosity (45–65% vol.), liquid permeance of 46–134 L m⁻² h⁻¹ bar⁻¹, a mean flow pore size of 30–75 nm, and a very high crystallinity degree of 86–89% at a decent tensile strength of 3–9 MPa. For these membranes, rejection of blue dextran dye with a molecular weight of 70 kg/mol was 22–76%. In the case of thermoreversible gels, the resulting membranes had only small pores located in the interlamellar spaces. They were characterized by a lower crystallinity degree of 70–74%, a moderate porosity of 12–28%, liquid permeability of up to 12–26 L m⁻² h⁻¹ bar⁻¹, a mean flow pore size of up to 12–17 nm, and a higher tensile strength of 11–20 MPa. These membranes demonstrated blue dextran retention of nearly 100%.

PbS quantum dots do not affect the mobility of the branched ethylhexyloxy side chains of the PTB7 polymer in the photovoltaic blend, whereas they facilitate the mobility of the fluorinated thienothiophene units of PTB7.

Using a broadband dielectric spectrometry we studied the effect of carbon nanofillers (CN) with various aspect ratio (fullerene C₆₀, multi-walled carbon nanotubes (MWCNT), reduced graphene oxide (rGO) and hybrid rGO:MWCNT nanofillers) on the electrical conductivity of the polyazomethine-based nanocomposites. One-dimensional MWCNTs with high aspect ratio were shown to be the most effective CN for fabrication of polymer-based nanocomposites with enhanced electrical conductivity.