Volume 31, Issue 5

Biodegradable sheath-core structured nanofibers, with chitosan as the sheath material and polylactide (PLA) at the core, were developed for the removal of metal ions. For the electrospinning of sheath-core nanofibers, predetermined weight percentages of chitosan were first dissolved in trifluoroacetic acid (TFA) with sonication for 30 min, and then mixed by a magnetic stirrer for 12 h. The same procedure was conducted with polylactide (PLA). The chitosan and polylactide solutions were then fed into two different capillary tubes with needles of small diameter, respectively, for co-axial electrospinning. The delivery rates of the solutions were controlled by two independent pumps for the sheath chitosan solutions and the core PLA solution. The morphology of the electrospun nanofibers was examined by a scanning electron microscope (SEM). The average diameter of the electrospun nanofibers was found to range from 234 nm to 562 nm. The influence of various process conditions on the metal removal was also investigated. The removal efficiency of the sheath-core chitosan/PLA nanofibers was measured and compared with the efficiency of blended chitosan/PLA nanofibers. The experimental results suggested that the electrospun sheath-core nanofibers exhibited superior metal ion removal efficiency compared to the blended nanofibers. The removal efficiency of the nanofibrous membranes increased with the initial metal ion concentrations and the pH value and decreased with the temperature of the solutions.

In biodegradable polymer world nanobiocomposites represent a new group of materials filled with inert nanoparticles that shows very interesting properties and the biodegradability of the matrix. In this work we have studied the effect of the polarity of the organomodified montmorillonite and of the elongational flow on the morphology and the rheological and mechanical properties of a new nanobiocomposite with a matrix of biodegradable PLA based blend. The elastic modulus enhances in presence of the nanofiller and this increase is larger and larger with the increment of the orientation. The tensile strength does not show any significant change at the same level of orientation. Moreover, a brittle-to-ductile transition is observed in the anisotropic sample and this effect is again more evident for the nanocomposite. The raise of the interlayer distance is higher for the more polar montmorillonite, even if the two nanocomposites show about the same final interlayer distance and morphology. Some exfoliation is also observed as a result of the application of the elongational flow.

Graphene stacks/epoxy nanocomposites were produced and characterized in order to analyse the effect of different graphene precursors on cure reaction of a model epoxy matrix. A kinetic analysis of the cure mechanism of the epoxy resin associated to the catalytical activity of the graphite based fillers was performed by isothermal DSC measurements. The DSC results showed that the addition of all graphite based fillers greatly increased the enthalpy of epoxy reaction and the reaction rate, confirming the presence of a catalytic activity of graphitic layers on the crosslinking reaction between the epoxy resin components (epoxide oligomer and di-amine). A kinetic modelling analysis, arising from an autocatalyzed reaction mechanism, was finally applied to isothermal DSC data, in order to predict the cure mechanism of the epoxy resin in presence of the graphite based nanofiller.

In this study thermoplastic polyurethane (TPUs) nanocomposites incorporating carbon nanotubes (CNTs) and graphene nanoplatelets (GNPs) were prepared via melt blending and compression molding and CNT dispersion was optimized by using non-covalent surface modification (surfactant). Filler dispersion was further improved by combining two fillers with different geometric shape and aspect ratio in hybrid filler nanocomposites. Synergistic effects were observed in the TPU-GNP-CNT hybrid composites, especially when combining GNP and CNT at a ratio of 6 : 4, showing higher tensile modulus and strength with respect to the systems incorporating individual CNTs and GNPs at the same overall filler concentration. This improvement was attributed to the interaction between CNTs and GNPs limiting GNP aggregation and bridging adjacent graphene platelets thus forming a more efficient network. Hybrid systems also exhibited improved creep resistance and recovery ability. Morphological analysis carried out by scanning electron microscopy (SEM) indicated that the hybrid nanocomposite presented slightly smaller and more homogeneous filler aggregates. The well-dispersed nanofillers also favored higher phase separation in TPU, as indicated by atomic force microscopy (AFM), resulting in a better microstructure able to enhance the load transfer and maximize the mechanical and viscoelastic properties.

New welding technologies have been implemented in the polymer processing industry in the past few years. Additionally, the most used polymer in complex products is polypropylene. This material can also be mixed with several additives, namely glass fiber, carbon nanotubes, etc. This compatibility with different additives allows products with characteristics that range from impact absorption to electrical conductivity. When it is necessary to join components in PP, they can be welded by hot plate, ultrasonic or laser welding. The present research aims at studying the influence of variables capable of influencing the final quality of the seam, namely those related to injection molding, such as mold temperature and cooling time, laser welding variables and materials, and how laser welding interacts with PP. The results showed that the most influential variables were mold temperature, laser velocity and laser diameter. The seams were analyzed using optical microscopy. They showed perfect contact between the components, despite the high standard variation present in the mechanical tests.

In modern fruit supply chain a common method to trigger ripening is to keep fruits inside special chambers and initiate the ripening process through administration of ethylene. Ethylene is usually administered through cylinders with inadequate control of its final concentration in the chamber. The aim of this study is the development of intelligent polymeric supports able to accurately regulate ethylene concentration in the atmosphere where fruits are preserved. Two different technologies were proposed: a polymeric (PEGDA) film and a polymeric (PLA) bag filled with inclusion complex of ethylene/α-cyclodextrin. The complex was prepared by molecular encapsulation which allows the entrapment of ethylene into the cavity of α-cyclodextrin. After encapsulation, ethylene can be gradually released from the inclusion complex and its release rate can be regulated by temperature and humidity. Intelligent polymeric film was prepared by dispersing inclusion complex into a thin polymeric film produced by UV-curing. Intelligent polymeric bag was made by inserting inclusion complex into heat sealed bag. The kinetics of ethylene release was studied for both systems, showing that it can effectively possible to control the release of ethylene within confined volume. Furthermore, modelling and simulations of ethylene release in a food container were made, demonstrating that it is possible to modulate release rate and, thus, control ripening.

The aim of this work is the analysis of the suitability of cardanol derivatives as primary plasticizer for polyvinyl chloride (PVC). Cardanol derived plasticizers (CDP) were obtained by acetylation of cardanol, followed by epoxidation of the side chain double bonds. The developed procedures allowed to achieve high yields of acetylated cardanol, whereas plasticizers characterized by different yields of epoxidation were obtained, according to the used synthesis procedure. Rheological analysis was used in order to study the gelation process of PVC plastisols with CDP. A comparison of commercial phthalate and natural derived plasticizers showed that the CDP are characterized by adequate gelation properties, involving a complete absorption of the plasticizers at relatively low temperatures. The gelation properties showed a strong dependence on the yield of the epoxidation. In particular, lower gelation temperatures, higher plasticizer absorption and faster gelation were observed for higher epoxy content in the plasticizer. Finally, ageing tests were performed on soft PVC produced by the CDP. The weight loss of soft PVC with CDP is comparable to that of commercial plasticizers, particularly at high yields of epoxidation.

Traditional methods for polymer processing involve the use of hazardous organic solvents which may compromise the biological function of scaffolds in tissue engineering. Indeed, the toxic effect of them on biological microenvironment has a tremendous impact on cell fate so altering the main activities involved in in vitro tissue formation. To date, extensive researches focus on seeking newer methods for bio-safely processing polymeric biomaterials to be implanted in the human body. Here, we aim at over viewing two approaches based on solvent free or green solvent based processes in order to identify alternative solutions to fabricate bio-inspired scaffolds to be successfully used in regenerative and degenerative medicine.

Polyoxymethylene composites with octakis (dimethylsiloxy, ethyl epoxycyclohexy)octasilsesquioxanes and octakis(dimethylsiloxy, vinyl)octasilsesquioxanes contents (0.5 and 1 wt.%) were studied. The effect of silsesquioxanes (POSS) on crystallization and melting behaviour of polyoxymethylene (POM) was investigated by polarized light microscope (PLM), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM), with the aim to determine the structural properties like crystallization temperature, degree of crystallinity and lamellar thickness distributions. It was found that POSS as nucleating agent affects the crystallization process of POM, leading to a homogeneous structure, improving the thermo-mechanical properties of polyoxymethylene composites. Based on SEM observations it was found that the POSS particles in a concentration of 0.5 wt.% are uniformly dispersed in polyoxymethylene matrix. The storage modulus and relaxations temperatures of POM with POM/0.5 wt.% ehPOSS and vinPOSS are slightly increased.

The thermo-optical behavior of immiscible PS/PC blends filled with silica nanoparticles was studied in order to get some information upon the location of nanoparticles in the polymer blend phases and interface using optical microscopy. The systems were designed taking into account rheological and optical requirements, having droplet-matrix morphology, with particle size in the range of the visible light wavelength. The melt blending procedure helped to set the nanoparticles at specific locations including within the PC minor phase, PS matrix phase and PS/PC interphase, which was confirmed via transmission electron microscopy. The light scattering was measured via the normalized transmitted light intensity over temperature, encompassing the Tg of the two polymers. The PS/PC blends showed an increase in the light scattering as compared to the pure polymers, which is magnified upon increasing the PC content. The addition of the nanosilica forming PS/PC/Nanosilica systems greatly reduces the light scattering, particularly above the Tg of the PS phase. The use of hydrophilic nanosilica does not show any appreciable hysteresis upon comparing data from heating and cooling cycles. This type of silica stays mainly trapped within the PC dispersed phase, little interfering with the light scattering, which happens at the polymer-polymer interface. On the other hand, the use of hydrophobic nanosilica does show a clear hysteresis. The hydrophobic silica located at the PS/PC interphase, interfere with the light scattering intensity at this interface, and can be used to identify its presence. The proposed procedure can help control the mixing process, thus improving the effective action of the nanoparticles in the final properties of polymer systems.

The polymorphism of isotactic polypropylene (iPP) in combination with the strong response of this polymer to nucleation can be utilized for expanding the application range of this versatile polymer. Based on three “case studies” related to β-iPP pressure pipes, ethylene-propylene (EP) random copolymers for thin-wall injection molding and transparency and sterilization resistance of cast films we demonstrate ways of combining polymer composition, nucleation and process settings to achieve the desired application performance. The importance of considering interactions between polymer design, nucleation and processing parameters for designing application properties is highlighted.

Prediction of the supermolecular structure and with that of properties of crystallizable polymers requires in-depth knowledge about the crystallization behavior, in particular the temperature-dependence of the nucleation kinetics. Typically, at low supercooling of the melt the nucleation rate/nuclei density often is assessed by optical microscopy, through an analysis of the evolution of the spherulitic superstructure. This approach fails if the nuclei density is too high, or if nuclei formation is not followed by growth due to chain-mobility constraints. In such cases, Tammann's two-stage crystal nuclei development method can be applied. It includes the formation of crystal nuclei at high supercooling of the melt, and their detection at higher temperature, after their growth to crystals. Though initially developed for analysis of low molecular mass compounds, this approach has recently also successfully been employed for analysis of the nucleation behavior of polymers, which is demonstrated here on the examples of poly (L-lactic acid) (PLLA), and poly (∊-caprolactone) (PCL). While in case of PLLA the ability to gain information about isothermal and non-isothermal nucleation is explained, in case of PCL new information about the thermal stability of nuclei is presented. The importance of such analyses in the context of understanding structure formation of polymers at processing-relevant cooling conditions is discussed.

The demand for small-diameter blood vessel substitutes has been increasing due to a shortage of autograft vessels and problems with thrombosis and intimal hyperplasia with synthetic grafts. In this study, hybrid small-diameter vascular grafts made of thermoplastic polyurethane (TPU) and silk fibroin, which possessed a hybrid fibrous structure of an aligned inner layer and a random outer layer, were fabricated by the electrospinning technique using a customized striated collector that generated both aligned and random fibers simultaneously. A methanol post-treatment process induced the transition of fibroin protein conformation from the water-soluble, amorphous, and less ordered structures to the water-insoluble β-sheet structures that possessed robust mechanical properties and relatively slow proteolytic degradation. The methanol post-treatment also created crimped fibers that mimicked the wavy structure of collagen fibers in natural blood vessels. Ultrafine nanofibers and nanowebs were found on the electrospun TPU/fibroin samples, which effectively increased the surface area for cell adhesion and migration. Cyclic circumferential tensile test results showed compatible mechanical properties for grafts made of a soft TPU/fibroin blend compared to human coronary arteries. In addition, cell culture tests with endothelial cells after 6 and 60 days of culture exhibited high cell viability and good biocompatibility of TPU/fibroin grafts, suggesting the potential of applying electrospun TPU/fibroin grafts in vascular tissue engineering.

The three-phase arrangement of random copolymers of butene-1 with ethylene was investigated and compared with isotactic poly(butene-1) homopolymer (iPB-1). In all the analyzed compositions, isothermal crystallization leads to a three-phase structure, made of one crystal phase and two amorphous fractions that differ in mobility: the mobile amorphous fraction (MAF), made of the polymer chains that relax at the glass transition, and a rigid amorphous fraction (RAF) made of the amorphous segments coupled with the crystal phase. Copolymerization with ethylene leads to a drop in crystal fraction and to a sizable increase of both the RAF, and of the specific RAF, i.e. of the RAF normalized to crystallinity. Analysis of crystal growth rate allowed quantifying the fold surface free energy, which increases of about 50 to 100% in the copolymers, compared to iPB-1 homopolymer. In the butene-1/ethylene random copolymers, ethylene units are mostly excluded from the crystals and accumulate at the crystal/amorphous interphase, thus affecting the rigid amorphous area. The varied composition and higher mobility of the rigid amorphous fraction of the copolymers affects also the Form II to Form I transformation of poly(butene-1) crystals, which occurs with enhanced kinetics in the copolymers, compared to iPB-1 homopolymer.

UNISA code, a software for the analysis and modeling of injection molding, was born at the University of Palermo in Italy in the 1980s. Afterwards, in the 1990s, it was rewritten and expanded at the University of Salerno (Italy) and continuously improved over the years. It is a study code, aimed at understanding rather than simulating. It has the unique characteristic of describing, since the early versions, the morphology of the molded samples. Furthermore, it always implemented the interrelationships among the different material properties (crystallinity, viscosity, density). In this work, the evolution of the software is reviewed, placed in the background, underlining the contribution given to the understanding of polymer processing and morphology evolution. Eventually, the future challenges of modeling are presented.