Nadka Tzankova Dintcheva

New phosphazene-based chain extenders containing allyl and epoxide groups

In this paper we present the synthesis and the characterization of cyclophosphazenes substituted with allyl groups, their transformation in epoxide-containing cyclophosphazenes and the final utilization of these compounds as chain extenders in combination with polyamides. The reaction at high temperature of Nylon-6 with epoxy-functionalized cyclophosphazenes leads to the opening of the epoxy units by the action of both amino (—NH2) and carboxylic (—COOH) end-groups of the polymer to enhance the final molecular weight of this material. The consequences of this fact on the thermal, mechanical and visco-elastic properties of treated Nylon-6 have been also evaluated and compared to those of the pristine material. Possible utilization of these cyclophosphazenes in recycling processes for degraded polyamides could be envisaged.

The role of the matrix-filler affinity on morphology and properties of polyethylene/clay and polyethylene/ compatibilizer/clay nanocomposites drawn fibers

Abstract In this work the structural variations and mechanical performance of polyethylene/clay nanocomposite drawn fibres, also in the presence of compatibilizer, such as a commercial maleic anhydride grafted polyethylene, PEgMA, was studied. In the isotropic state both systems show intercalated morphology. After spinning and cold drawing, by adding the nanoparticles, the tensile strength as a function of the draw ratio increases and this rise is more pronounced for the filled compatibilized system. The reduction of the elongation at break, on the contrary, is about the same for all the examined samples. The orientation of the macromolecules, evaluated by measurements of the birefringence and calorimetric analysis, is similar for all the samples, but the filled, drawn fibres show a higher level of intercalation and, in particular, some exfoliation, more and more pronounced with the draw ratio and in presence of compatibilizer, as a consequence of the application of the extensional (at low and high temperature) flow. For the three components system with greater affinity between the polymer matrix and clay, the extensional flow is more efficient. The initial intercalated morphology changes to some more intercalation and finally, at the highest anisotropic condition in the presence of the PEgMA, evolves to delaminated clay structure.

Nitrogen-Doped Carbon Nanodots-Ionogels: Preparation, Characterization, and Radical Scavenging Activity

Hybrid diimidazolium-based ionogels were obtained by dispersing nitrogen-doped carbon nanodots (NCNDs) in ionic liquid (IL) solutions and by using dicationic organic salts as gelators. The properties of the NCND-ionogels were studied in terms of thermal stability, mechanical strength, morphology, rheological, and microscopic analyses. Insights into the formation of the hybrid soft material were attained from kinetics of sol-gel phase transition and from estimating the size of the aggregates, obtained from opacity and resonance light-scattering measurements. We demonstrate that, on one hand, NCNDs were able to favor the gel formation both in the presence of gelating and nongelating ILs. On the other hand, the gelatinous matrix retains and, in some cases, improves the properties of NCNDs. The NCND-ionogels showed the typical fluorescence emission of the carbon dots and a notable antiradical activity, with higher efficiency as compared to the single components. The presented hybrid materials hold great promise for topical applications in antioxidant fields.

Advanced nano-hybrids for thermo-oxidative-resistant nanocomposites

In the present work, trisilanol phenyl polyhedral olygomeric silsesquiosane (TSPh-POSS) has been physically immobilized onto carbon nanotubes (CNTs) bearing covalently linked Br-terminated long-alkyl chain (Br-alkyl-f-CNTs), and the so obtained hybrid nanoparticles (Br-alkyl-f-CNTs/TSPh-POSS) have been used to prepare ultra-high molecular weight polyethylene (UHMWPE)-based nanocomposites with enhanced thermo-oxidative resistance. The effective immobilization of the TSPh-POSS molecules has been confirmed by spectroscopic and thermo-gravimetric analyses. Besides, the influence of the hybrid nanoparticles on the rheological and mechanical behaviour and morphology of the nanocomposites have been fully investigated. The obtained results show that the rheological and thermo-mechanical behaviour of UHMWPE/Br-alkyl-f-CNTs/TSPh-POSS nanocomposite is affected by two different opposite contributions: reinforcement effect of CNTs and plasticizing action induced by TSPh-POSS molecules. The unexpected excellent thermo-oxidative resistance of the nanocomposite containing hybrid nanoparticles seems to be due to a synergistic effect of TSPh-POSS and CNTs. Moreover, TSPh-POSS molecules, upon thermo-oxidative treatment, are able to migrate toward film surface, forming a TSPh-POSS-rich protective layer that hinders the oxygen diffusion.

Immobilization of natural anti-oxidants on carbon nanotubes and aging behavior of ultra-high molecular weight polyethylene-based nanocomposites

The use of natural antioxidants is an attractive way to formulate nanocomposites with extended durability and with potential applications in bio-medical field. In this work, Vitamin E (VE) in the form of α-tocopherol and Quercetin (Q) are physically immobilized on the outer surface of multi-walled carbon nanotubes (CNTs). Afterward, the CNTs-VE and CNTs-Q are used to formulate thermally stable ultra high molecular weight polyethylene based nanocomposites. The obtained results in the study of the thermo-oxidation behavior suggest a beneficial effect of the natural anti-oxidant carbon nanotubes systems. The unexpected excellent thermo-resistance of the nanocomposites seems to be due to a synergistic effect of the natural anti-oxidant and carbon nanotubes, i.e. strong interaction between CNT surface and anti-oxidant molecules. Particularly, these interactions cause the formation of structural defects onto outer CNT surfaces, which, in turn, increase the CNT radical scavenging activity.

Ionic liquids gels: Soft materials for environmental remediation

HYPOTHESIS Nanostructured sorbents and, in particular, supramolecular gels are emerging as efficient materials for the removal of toxic contaminants from water, like industrial dyes. It is also known that ionic liquids can dissolve significant amounts of dyes. Consequently, supramolecular ionic liquids gels could be highly efficient sorbents for dyes removal. This would also contribute to overcome the drawbacks associated with dye removal by liquid-liquid extraction with neat ionic liquids which would require large volumes of extractant and a more difficult separation of the phases. EXPERIMENTS Herein we employed novel supramolecular ionic liquid gels based on diimidazolium salts bearing naturally occurring or biomass derived anions, to adsorb cationic and anionic dyes from wastewaters. We also carried out a detailed investigation of thermal, structural, morphological and rheological features of our gels to identify which of them are key in designing better sorbents for environmental remediation. FINDINGS The most effective gels showed fast and thorough removal of cationic dyes like Rhodamine B. These gels could also be reused up to 20 times without any loss in removal efficiency. Overall, our ionic gels outperform most of gel-based sorbents systems so far reported in literature.

Rheological Percolation Threshold in High-Viscosity Polymer/CNTs Nanocomposites

AbstractPolystyrene (PS)/multi-walled carbon nanotubes (CNT) nanocomposites have been prepared through melt mixing processing aiming at obtaining a uniform and homogeneous dispersion of the used nanoparticles within the polymeric matrix. Optical and scanning electron microscopy has been employed to determine the dispersion and distribution of CNTs at different length scales. Furthermore, the linear viscoelastic behavior of formulated nanocomposites has been deeply investigated. As a result of CNTs added, the nanocomposites experience a transition from liquid-like to solid-like rheological behavior, and a disappearance of relaxation processes at low frequency can be noticed. By plotting G’ versus CNTs loading and fitting with a power-law function, the rheological threshold of PS-based nanocomposites has been calculated. Moreover, the mechanical behavior of nanocomposites has been investigated and related to the formed CNTs network within polymer matrix.

Biopolyester-based systems containing naturally occurring compounds with enhanced thermo-oxidative stability

Background This work presents a sustainable approach for the stabilization of polylactic acid (PLA) against thermo-oxidative aging. Methods Naturally occurring phenolic and polyphenolic compounds, such as ferulic acid (FerAc), vanillic acid (VanAc), quercetin (Querc) and vitamin E (VitE), were introduced into PLA. Results The preliminary characterization of the systems formulated containing different amounts of natural stabilizers showed that all compounds used acted as plasticizers, leading to a decrease in rheological functions with respect to neat PLA, without significantly modifying the crystallinity of the raw material. The study of the thermo-oxidative behavior of neat PLA and PLA/natural compound systems, performed by spectrometric and thermal analyses, indicated that all stabilizers considered were able to exert a remarkable antioxidant action against thermo-oxidative phenomena. Conclusions All natural compounds considered are thus proposed as ecofriendly stabilizers, to get fully bio-based polymer systems with enhanced thermo-oxidative stability, suitable for biomedical applications.

Sonication-Induced Modification of Carbon Nanotubes: Effect on the Rheological and Thermo-Oxidative Behaviour of Polymer-Based Nanocomposites

The aim of this work is the investigation of the effect of ultrasound treatment on the structural characteristics of carbon nanotubes (CNTs) and the consequent influence that the shortening induced by sonication exerts on the morphology, rheological behaviour and thermo-oxidative resistance of ultra-high molecular weight polyethylene (UHMWPE)-based nanocomposites. First, CNTs have been subjected to sonication for different time intervals and the performed spectroscopic and morphological analyses reveal that a dramatic decrease of the CNT’s original length occurs with increased sonication time. The reduction of the initial length of CNTs strongly affects the nanocomposite rheological behaviour, which progressively changes from solid-like to liquid-like as the CNT sonication time increases. The study of the thermo-oxidative behaviour of the investigated nanocomposites reveals that the CNT sonication has a detrimental effect on the thermo-oxidative stability of nanocomposites, especially for long exposure times. The worsening of the thermo-oxidative resistance of sonicated CNT-containing nanocomposites could be attributed to the lower thermal conductivity of low-aspect-ratio CNTs, which causes the increase of the local temperature at the polymer/nanofillers interphase, with the consequent acceleration of the degradative phenomena.

Double bond-functionalized POSS: dispersion and crosslinking in polyethylene-based hybrid obtained by reactive processing

Polyethylene-based organic inorganic hybrids were prepared by one-step reactive melt mixing using a mono-functionalized nanofiller, i.e., allyl-heptaisobutyl-substituted polyhedral oligomeric silsesquioxane (1POSS) and a multi-functionalized octavinyl polyhedral oligomeric silsesquioxane (8POSS). The hybrids were also prepared in dicumyl peroxide (DCP) presence and morphological, spectroscopical and calorimetric analysis were carried out. Moreover, rheological measurement and Sohxlet extraction were performed on investigated samples. It was inferred that double bonds of POSS functional groups were triggered by radicals coming from the peroxide decomposition or from the degradation reactions occurring during preparation. The type of the functional groups (mono- or multi-reactive) of the POSS is a leading factor, along with radicals content in the systems, in the formation of a polymeric network. In particular, the presence of multi-reactive groups in 8POSS molecules results in a successful POSS grafting/crosslinking in the polymeric backbone: the covalent bonds formation between nanofillers and matrix during processing improves POSS dispersion within the polyolefin matrix and leads to a network structure formation.