Poly(vinyl alcohol) (PVA) thin films are widely used for humidity sensing due to their high sensitivity towards water but have some drawbacks such as instability at high levels of humidity, nonlinearity and presence of hysteresis. In this study, in order to improve PVA sensing properties hybrids comprising hydrophobically modified PVA copolymers and silica particles were utilized as humidity sensing media.

Series of poly(vinyl alcohol-co-vinyl acetal) copolymers (PVAac) with acetal content in the range 18-28 % were synthesized by partial acetalization of hydroxyl groups of PVA with acetaldehyde. Copolymer solutions of concentration 1 wt % in mixed water-methanol solvent (20:80 volume ratio) were used for thin film deposition via spin-coating method. To obtain hybrid polymer-silica thin films, SiO₂ particles were in situ generated in copolymer solutions via the sol-gel method. Properties of PVAac and PVAac-SiO₂ films were compared in terms of optical constants, sensing behaviour toward different levels of humidity and sensor element characteristics like sensitivity, hysteresis and reflectance change (ΔR or color change). It was shown that acetal modified PVA films doped with appropriate amount of SiO₂ particles offers linearity in the entire humidity range. The feasibility of hydrophobically modified PVA thin films for optical sensing of humidity is demonstrated and discussed.

Acknowledgments: S. Bozhilova acknowledge the National Scientific Program for young scientists and postdoctoral fellows, funded by Bulgarian Ministry of Education and Science with PMC № 271/2019. This work was partially supported by the European Regional Development Fund within the Operational Programme “Science and Education for Smart Growth 2014 - 2020” under the Project CoE “National center of mechatronics and clean technologies“ BG05M2OP001-1.001-0008-С01.

In this paper, we report the fabrication of polyvinyl alcohol (PVA) and polyethylene oxide (PEO) blended polymer nanocomposite (PNC) films loaded with different percentages of carbon black (CB) using the stencil printing method. The effect of the PEO-PVA blend weight ratio on the existence of the dual-phase was studied. The fabricated films were characterized using the high-resolution scanning electron microscope (HRSEM), the surface profilometer, and the four-point probe resistive measurements. HRSEM analysis showed the homogenous dispersion of carbon black fillers in the polymer blend matrix. It has also revealed the formation of carbon black agglomerations in the polymer matrix. The topographical properties of the films such as the surface roughness and thickness were obtained. The CB concentration in the polymer blend had been varied from 0 to 14wt%. A typical PNC film loaded with 14wt% CB had a thickness of 120 ± 0.24 µm. The I-V characteristics of the PNC films were obtained to investigate the effect of thickness and CB content variation. An electrical conductivity of 0.417 S/m was achieved with 14wt% CB loading. The percolation threshold and critical exponent of the PNC films was estimated to be 0.2 vol% and 1, respectively. The latter indicated the presence of a two-dimensional conductive network. In general, the CB-polymer composite films with improved structural and electrical properties were fabricated and characterized for potential sensor applications.

The microencapsulation method is an exhaustively used technique for phase change material (PCM) shape-stabilization. This technique has been used in a broad spectrum of applications such as building, medical, electronics, food, etc. Polymeric encapsulation is characterized with high toughness and good heat transfer property due to large surface area of capsules. In this paper, phenol-formaldehyde shelled PCM microcapsules (MPCM) were fabricated and their processing parameters were analysed with Taguchi method. Core to shell ratio, surfactant concentration and speed of mixing are the parameters which were optimized in five levels. The optimized values for surfactant concentration, core to shell ratio and agitation speed were 3%, 1:1 and 800 rpm. The obtained microcapsules were spherical in shape. The melting enthalpy of MPCM synthesized with optimized processing parameters was 148.93 J/g in the range of 35-62 ⁰C. The obtained temperature range of phase transition temperature can be used for storing different food articles such as chocolate and hot served foods.

Hot served food items are maintained around 60⁰C temperature. The food delivery system should be quick to transport such items. If these food items once prepared and stored in container equipped with phase change material (PCM) slabs with phase change temperature around serving temperature, it can maintain temperature of food item for longer time in food delivery process while utilizing lesser amount of energy. Beeswax is a bioderived PCM with phase change temperature around 60⁰C. Beeswax should be shape stabilized to serve thermoregulation purpose. This report has used recycled paperboard as matrix for shape stabilizing beeswax. Optimization study revealed that 45% is the maximum amount of beeswax that can be incorporated in paper composite. Beeswax showed melting enthalpy of 216.09 J/g and melting enthalpy of composite with beeswax content 45% was 102.51 J/g. The enthalpy of melting is directly proportional to amount of beeswax in composite. Thermal conductivity of beeswax and composite with 45% beeswax calculated with T-history method as 0.285 and 0.157 respectively. The carton containing PCM sheets maintain temperature at higher level for longer time than for carton without PCM. To address concerns such as toxicity, environment friendly nature and recycling, beeswax-recycled paperboard composite should be considered as promising candidate.

Plastics are very useful for a wide range of applications, given the mechanical and chemical properties, and ease of manipulation. Unfortunately, because of the issue of non-degradability, plastic waste pollution poses significant threat to the ecological environment. This aspect has become particularly relevant in the sustainable development of industrial production. Nonetheless, additive manufacturing (AM), well-known as 3D printing, is emerging as a crucial industrial technology for rapid prototyping, to convert a numerical model into material deposition and 3D printed parts. Bio based polymers and recycling operation through circular use are representing alternative solutions to reduce plastic waste and limit the environmental impact of AM process. In this framework, this study investigates the possibility to adopt recycled polymers in the AM technology by replacing virgin matrices. At regards, two commercial filaments, made from polylactide acid (PLA), -the second (recycled) obtained from the production waste of the first one (virgin)-, were initially characterized using infrared (IR) spectroscopy, thermogravimetric analysis (TGA) and dynamic rheology. Then, the filaments were extruded in a 3D printer and characterized by dynamic mechanical analysis (DMA). Despite of a small reduction of the intensity in correspondence of typical absorption bands of PLA polymer in the case of recycled material compared to virgin one (as attested by IR spectra), the thermal-mechanical results allowed to attest very similar characteristics of recycled and neat filaments. The onset of the thermal degradation was found around 315°C in both systems. Both materials exhibited the same time-dependent trend of complex viscosity, with a reduction of approximately 50% after 900 seconds of testing. When the samples were dried at 80°C under vacuum for 10 hours, the stabilization of the rheological features against time was improved. There is no significant difference in the storage modulus (E') of 3D printed parts made with different types of PLA-based filaments.

A high efficiency, low band-gap photocatalyst is presented. The platform consists of gold nanoparticles (AuNPs) confined in the pores of a silicon substrate. Careful fabrication of the pores and choice of the particles allows for a maximum of two AuNPs within a single pore, preventing agglomeration. The nanoporous substrate is produced by first, creating a mask with block copolymer templating and metal infiltration on a silicon substrate, and second, reactive ion etching to remove the silicon, which has not been masked by the template. In this work, the pores are 60 nm in depth and 25 nm in diameter and the AuNPs are 18 nm in diameter. The AuNPs’ access to the analyte, provides more active sites for redox reaction, leading to enhanced efficiency. While proximity of nanoparticles enhances coupling efficiency, confinement prevents rapid recombination of photogenerated charge carriers, a major factor contributing to low efficiency of photocatalytic materials. Degradation of methyl orange (MO) is used to determine the photocatalytic efficacy of AuNSM compared to (i) bare silicon and (ii) AuNPs randomly dispersed on silicon. After 90 minutes exposure to UV light (λ = 353 nm) in the AuNSM, the MO absorption is <1%, indicating near complete degradation, while it is still 85% and 70% for systems (i) and (ii), respectively. Finite Element Method simulations of the confined structure suggest that the AuNPs act as a mediator/receptacle for photogenerated charges rather than a source of them at this wavelength and thus enhance the performance of the photocatalyst by creating more effective Schottky junctions—preventing recombination of electrons and holes—rather than by a localised surface plasmonic resonance effect.

In December 2019, a novel strain of coronavirus, SARS-CoV-2, was identified. Hoping to prevent transmission, many countries adopted a mandatory mask use in closed public spaces. However, most mask options display a passive-like action against COVID-19. To overcome such restrictions, this work proposes the incorporation of anti-viral essential oils (EOs) loaded onto a nanofibrous layer that can be adapted to both community and commercial masks.

Twenty EOs selected based on their antimicrobial nature were examined for the first time against the Escherichia virus MS2. The most effective were the lemongrass (LO), Niaouli (NO) and eucalyptus (ELO) with a minimum inhibitory concentration (MIC) of 356.0 mg/mL, 365.2 mg/mL and 586.0 mg/mL, respectively. Polycaprolactone (PCL) was prepared at 14 wt% in chloroform/dimethylformamide (DMF) and processed via electrospinning, with processing parameters being optimized to 23 kV, 0.7 mL/h and 26 cm. Uniform, beadless nanofibers were obtained. Mats were characterized as mechanically resilient, to endure movements arising from mask positioning, and hydrophobic in nature, to repel droplets coming from the exterior. Loading of the nanofibrous mats was accomplished via two ways: (1) physisorption and (2) by combining the EOs with the polymer solution. In both cases, EOs were loaded at 10% of MIC concentration (saturation) for 24 h. Presence of the EOs was confirmed along the mats (UV-visible). Antimicrobial testing via halo determination, verified their diffusion abilities. More importantly, time-kill kinetics testing of the loaded mats attested to the EOs capability to fight the virus MS2 even when bonded to the nanofibers at a smaller concentration than MIC. EOs-physisorbed mats were quicker in their action, while those entrapping the EOs in their polymeric matrix retained the antiviral activity of the mat for longer. Data demonstrated the potential of these EOs-loaded PCL nanofibers mats to work as COVID-19 active barriers for individual protection masks.

Hydrogels have recently been increasingly studied due to their similarity to natural soft tissues. However, the stable mechanical properties and elasticity required for hydrogels used in sensing and wearable devices remain challenging. Herein, a novel 3D microporous structure hydrogel with favorable stable mechanical properties and elasticity has been developed via a simple and economical method. The good resilience (94.5%) and less residual strain (11.5%) are realized based on the results after 20 successive cycles at a strain of 300%. The elasticity of the hydrogel is achieved by varying the effective network chain density. The prepared hydrogels have stable mechanical properties and high elasticity, resulting in remarkable performance when used in sensors. The hydrogel-based sensors can accurately and consistently record human activities when used as wearable sensors. This work provides a new way to simply and effectively prepare hydrogels, which has great potential to be widely applicated in sensing and flexible devices such as health-recording sensors, wearable devices, and artificial intelligence.

Polymers are widely used in applications where thermal losses greatly affect efficiency of performance. Improving thermal buffering capacity is a possible remedy for this problem. This objective can be achieved by incorporating phase change materials (PCMs) into polymers. PCMs can absorb, store or release latent heat while undergoing phase transition at predetermined temperature range and hence can be used in thermal energy storage applications. When a PCM is combined with a polymeric substrate, it melts from crystalline phase while temperature of the substrate is prevented from rising. Similarly, during crystallization process from molten phase, cooling of substrate is prevented. This phenomenon maintains the required application temperature in PCM incorporated polymers. The PCM addition methods vary as per application requirements and PCM characteristics. Herein, this review article describes popular strategies of incorporation of PCMs into polymers which are direct incorporation, encapsulation and chemical modification. This article highlights benefits achieved with the use of thermal energy storing polymers in some of the application areas, such as textile systems, building systems, electronic systems, packaging systems and solar energy storage devices.

One of the ways to provide new functionalities for a textile product is the insertion of natural ingredients into the process of its manufacture. One of methods for formation multifilament yarns is melt spinning. During melt spinning process is possible to use additives on/in polymer and to form multicomponent yarns with various functionalities. Melt spun yarns are ideal for application in medicine field [1,2].
Polylactide (PLA) polymer is produce from synthesized nonpetroleum sources (most often from corn). It has good biocompatibility, biodegradability by enzymes and hydrolysis under physiological conditions, and antibacterial activity. Due it is used for medical products[3,4].
Natural resin as Myrrh is an aromatic gum resin obtained from a small tree. It is consisted of alcohol-soluble resins and volatile oil together with a gum soluble in water containing polysaccharides, proteins, and long chain aliphatic derivatives. The lipophilic part of myrrh is composed of steroids, sterols, and terpenes. Myrrh for a long time was used as a natural medicine and as an antibacterial material for wound dressing [5,6].The aim of the research is to investigate the influence of myrrh extract on PLA pellets on the melt spinning process and formed multifilament yarns mechanical properties at different multifilament yarns draw ratio.
It was estimated that melt spun multifilament yarns from PLA with myrrh extract have higher linear density, lower breaking force and higher elongation at break comparing with pure PLA multifilament yarns. The increase of draw ration cause the decrease of multifilament yarns linear density, decrease breaking force, increase elongation at break.