Decrease In Crosslink Density Research Articles

Investigating the impact of manufacturing conditions on crack growth rate in nitrile butadiene rubber for enhanced service life – Part 02: Crosslink density via multi-stage swelling analysis

The importance of the state of cure of elastomer components with regard to fatigue behavior and thus also durability was already discussed in more detail in Part 01 of the paper. It was shown that crosslinking time and temperature have an enormous influence on the crack growth behavior in nitrile butadiene rubber (NBR). In the course of this, a fourfold deceleration in crack growth was observed with a 20 K increase in manufacturing temperature. To confirm this trend, test specimens were produced at 180 °C. Crack growth was found to be 40 times slower, especially in high tearing energy ranges, compared to 150 °C. To analyze the observed behavior in more detail, cylindrical samples were taken from the plain strain test specimens (used for the crack growth measurements), and multi-stage swelling was performed with them. This allowed a detailed analysis of the degree of cure and the sulfur chain composition. It was found that the crosslink density decreases with increasing manufacturing temperature due to decomposition and the proportion of polysulfidic crosslinks decreases with increasing heating time due to desulfurization. Furthermore, a correlation between the results found by means of the rubber process analyzer (RPA) and the swelling results could be established looking at the maximum transmitted torque during the isothermal crosslinking phase.

Hygrothermal aging mechanism of epoxy composites used for medium-frequency transformers

The integration of medium-frequency transformer design and the harsh service environment raise higher demands for the medium-frequency insulation performance of epoxy composites. As epoxy composite's frequency response characteristics and sensitivity to moisture intrusion are still unknown, its application as the insulation materials of medium-frequency transformers under hygrothermal conditions is limited. This paper investigates the mechanism of moisture intrusion damage to the molecular structure of the epoxy resin matrix and its cross-linking points with the curing agent. The regularity of medium-frequency insulation performance degradation of epoxy composites after hygrothermal aging is studied. Results indicate that after 40 days of hygrothermal aging, moisture together with high temperature damaged the main chain of the epoxy resin and its cross-linking points with the curing agent, resulting in a 23 °C decrease in the glass transition temperature and an 82 % decrease in cross-linking density of the epoxy composite. Moisture ionization and the increase of carriers due to epoxy resin hydrolysis lead to an increase in the amount of space charge. The temperature rise caused by dipole steering polarization facilitates the formation of internal conductive pathways in the epoxy composite, resulting in a 73 % decrease in the medium-frequency breakdown strength and a four-order magnitude decrease in the medium-frequency volume resistivity of the epoxy composite. This study can guide the insulation design of high-reliability medium-frequency transformers.

Fabrication of High-Performance Polyisocyanurate Aerogels through Cocyclotrimerization of 4,4'-Methylene Diphenyl Diisocyanate and Its Mono-Urethane Derivatives.

Aromatic polyisocyanurate (PIR) aerogels are recognized as advanced porous materials and extensively studied due to their lightweight nature, high porosity, and specific surface area, which attribute to their outstanding thermal insulation properties. The inherent thermal stability of the PIR moieties, combined with great insulating performance, renders PIR aerogels highly suitable for building insulation applications. Nevertheless, materials containing isocyanurate obtained through direct trimerization of aromatic isocyanates exhibit brittleness, resulting in inferior mechanical performance. In order to enhance the processability of the PIR aerogels, we propose a cocyclotrimerization approach involving mixtures of mono- and difunctional aromatic isocyanates. This approach is designed to develop a PIR network with decreased cross-linking density and brittleness. Herein, we developed an array of PIR aerogels from different alkyl chain-modified isocyanate mixtures. The resulting PIR aerogels exhibited high porosity (>89%), a large surface area (∼300 m2/g), superinsulating performance with ultralow thermal conductivity (∼16.8 mW m-1 K-1), notable thermal stability (Td5% ∼ 250 °C), improved mechanical performance, and intrinsic hydrophobicity without the need for postmodification. These high-performance organic aerogels hold significant promise for applications requiring superinsulating materials.

Diels-Alder Network Blends as Self-Healing Encapsulants for Liquid Metal-Based Stretchable Electronics.

Two dynamic covalent networks based on the Diels-Alder reaction were blended to exploit the properties of the dissimilar polymer backbones. Furan-functionalized polyether amines based on poly(propylene oxide) (PPO) FD4000 and polydimethylsiloxane (PDMS) FS5000 were mixed in a common solvent and reversibly cross-linked with the same bismaleimide DPBM. The morphology of the phase-separated blends is primarily controlled by the concentration of backbones. Increasing the PDMS content of the blends results in a dilute droplet morphology at 25 wt %, with a growing size and concentration of droplets and the formation of two separate PPO- and PDMS-rich layers at 50 wt %. Further increasing the PDMS content to 75 wt % leads to larger droplets and a thicker layer of the secondary phase. The hydrophobic PDMS phase creates a barrier against water, while the more hydrophilic PPO phase enhances the resistance against oxygen diffusion. Lowering the maleimide-to-furan stoichiometric ratio resulted in a decrease in cross-link density and thus more flexible and stretchable encapsulants. Changes in the stoichiometric ratio also affected the phase morphology due to resulting changes in phase separation and network formation kinetics. Lowering the stoichiometric ratio also resulted in enhanced self-healing properties of 96% at room temperature as a consequence of the increased chain mobility in the blended networks. The self-healing blends were used to encapsulate liquid metal circuits to create stretchable strain sensors with a linear electro-mechanical response without much drift or hysteresis, which could be efficiently recovered by 90% after the damage-healing cycles.

Influence of Lowering the pH Value on the Generation of Fibrous Structures of Protein Gels with Different Network Types.

High-moisture extrusion of plant proteins to create meat-like structures is a process that has met with increasing attention in the recent past. In the process, the proteins are thermomechanically stressed in the screw section of the extruder, and the resulting protein gel is structured in the attached cooling die. Various protein sources, notably soy protein isolate (SPI) and wheat gluten, are used to form gels with different networks: SPI creates a physical, non-covalent network, while gluten forms a chemical, covalent one. The food industry frequently adds weak acids to modify taste and shelf life. However, it is known that a change in pH affects the gelation behavior of proteins because the repulsive forces within and between the proteins change. The research reported here was carried out to investigate for the two proteins mentioned the influence of pH modification by the addition of citric acid and acetic acid on gel formation and the meat-like structures produced. For this purpose, materials and parameters were screened using a closed cavity rheometer, followed by extrusion trials at pH 7.36-4.14 for SPI and pH 5.83-3.37 for gluten. The resulting extrudates were analyzed optically and mechanically, and protein solubility was tested in a reducing buffer. For both protein systems, the addition of acid results in less pronounced meat-like structures. At decreasing pH, the complex viscosity of SPI increases (from 11,970 Pa·s to 40,480 Pa·s at 100 °C), the generated gel becomes stronger (strain decreased from 0.62 to 0.48 at 4.5 × 105 Pa), and the cross-linking density grows. For gluten, a decreasing pH results in altered reaction kinetics, a more deformable resulting gel (strain increased from 0.7 to 0.95 at 4.5 × 105 Pa), and a decreased cross-linking density. Solubility tests show that no additional covalent bonds are formed with SPI. With gluten, however, the polymerization reaction is inhibited, and fewer disulfide bonds are formed.

Nucleophilic amino acids as a renewable alternative to petrochemically-derived amines in glycerol epoxy resins

The standard epoxy resin curing agents revealed are from unsustainable petroleum-based sources, which produce poisonous exhaust when cured. Amino acids, a bio-based epoxy curing agent with amino and carboxyl groups, are another potential curing agent. Water-soluble epoxy resins cured with lysine (Lys), glutamic acid (Glu), leucine (Leu), and serine (Ser) as amino acids were investigated. The results showed that the water-soluble epoxy resin (glycerol epoxy resins, GER) was cured with Lys and Glu after reacting. Fourier transform infrared (FT-IR) spectroscopic analysis of the GER-Lys showed that the amino and carboxyl groups of Lys primarily reacted with the epoxy groups of GER. The elongation at break of Lys-cured GER (GER-Lys) cured at 70 ℃ with a molar ratio of 1꞉0.75 was 75.32%. The fact that elongations at break of GER-Lys (79.43%) were higher than those of GER-Glu (17.33%), respectively supports the decrease of crosslinking density by the amino acid-cured GER reaction. The potential of Lys and Glu alternatives for petrochemical amines is demonstrated and provides promising opportunities for industrial application.

Effect of localized control of cross-link density on mechanical properties of bicontinuous cubic lyotropic networks via copolymerization with different singly-polymerizable monomers

Two singly-polymerizable analogs of a bicontinuous cubic (Q) phase, lyotropic liquid crystal (LLC) (i.e., amphiphilic) cross-linking monomer were designed and synthesized as phase-compatible, non-cross-linkable co-monomers. These were copolymerized with the parent, single-head/single-tail cross-linking monomer to locally control the cross-link density in either the hydrophobic tail or hydrophilic headgroup regions of the Q phase and study their effect on the mechanical properties of the resulting polymer networks. Structure-property relationships were elucidated: (1) Blends with smaller amounts of the parent cross-linking monomer relative to non-cross-linkable analog exhibit progressively lower endpoint stress and Young's modulus values. (2) A decrease in cross-link density also influences the pliability of each sample as there appears to be a threshold (i.e., minimum) cross-link density that must be exceeded before stretchability is significantly affected. (3) Each LLC-based structural motif tolerates different levels of non-cross-linkable analog, shifting the stress-strain behavior of these cross-linked Q-phase materials.

Probing the Solubility of Imine-Based Covalent Adaptable Networks.

Covalent adaptable networks (CANs) are polymer materials that are covalently cross-linked via dynamic covalent bonds. The cross-linked polymer network is generally expected to be insoluble, as is seen for traditional thermosets. However, in recent years, it has become apparent that-under certain conditions-both dissociative and associative CANs can be dissolved in a good solvent. For some applications (e.g., those that require long-term (chemical) stability), the solubility of CANs can be problematic. However, many forget that (selective) solubility of CANs can also be applied advantageously, for example, in recycling or modification of the materials. In this work, we provide results and insights related to the tunable solubility of imine-based CANs. We observed that selected CANs could be fully dissolved in a good solvent without observing dissociation of imines. Only in an acidic environment (partial) dissociation of imines was observed, which could be reverted to the associated state by addition of a base. By adjusting the network composition, we were able to either facilitate or hamper solubility as well as control the size of the dissolved particles. DLS showed that the size of dissolved polymer particles decreased at lower concentrations. Similarly, decreasing cross-linking density resulted in smaller particles. Last, we showed that we could use the solubility of the CANs as a means for chemical recycling and postpolymerization modification. The combination of our studies with existing literature provides a better understanding of the solubility of CANs and their applications as recyclable thermosets.

Toward soluble high-performance electrochromic polymer: Optimizing cross-link density

Cross-linked conjugated polymers attract great attentions in optoelectronic field due to their unique properties resulting from the existence of intramolecular pore and network structure. However, the poor solubility restricts the application of cross-linked polymers in photoelectric device. Here, the strategy of changing cross-link density is employed to optimize solubility and properties of cross-linked conjugated polymer. Five polythiophene derivatives with different cross-link densities were synthesized through direct arylation reaction. The cross-linked conjugated polymers achieve good solubility in organic solvent through decreasing cross-link density, and possess similar optical absorption regions compared to conjugated polymer without cross-linked structure. Through optimizing cross-link density, the electrochromic properties of conjugated polymers could be enhanced, including optical contrast (55 %), response time (0.6 s), coloration efficiency (CEox = 510 cm2 C-1; CEred = 3567 cm2 C-1), and stability, which is more excellent than that of reported cross-linked polymers. And, the flexible electrochromic device based on PET-(1,9) was constructed by simple solution processing and could achieve fast color change between wine red and light blue in low voltage driving force, which indicating the applicability of soluble cross-linked conjugated polymer. Besides, the mechanism about ionic transport and stability improvement of cross-linked conjugated polymers is discussed. These results demonstrate that changing cross-link density is an effective strategy to optimize the properties of cross-linked conjugated polymers.

Effect of chain extender dichlorodimethylsilane (DDS) on the mechanical properties of hydroxyl terminated polydimethylsiloxane (HOPDMS)

The effect of the chain extender dichlorodimethylsilane (DDS) on the mechanical properties of hydroxyl terminated polydimethylsiloxane (PDMS) was investigated. Different amounts of DDS ranges from 0.5 to 5.0 phr were added to the base polymer. After degassing (degasification) the prepared solution of chain extender (DDS) for 5h, viscosity was measured. It was observed that viscosities were increased from 1189 to 212246 cst (almost 180 times), while their molecular weights increased from 13899 to 206019 g/mol (almost 15 times) at 3.5 phr DDS concentration. Crosslinking agents such as tetraethoxysilane [tetraethyl orthosilicate (TEOS)] and tin catalyst were utilized for fabricating the sheet. Mechanical properties were evaluated by using universal testing machine. Tear strength was increased from 0.057 to 0.352 N/mm while Tensile strength was decreased from 0.40 to 0.23 MPa % and elongation at break was increased from 38.87 to 552.20%. Durometer shore A was utilized to measure hardness of the thin films which was decreased from 25 to 10 shore A. Swelling measurement indicated that crosslinking density was decreased from 3.6x10−4 to 1.1x10−5 mol/cm3, while molecular weight between crosslinks was increased from 2979 to 89597 g/mol. Increase in viscosity, molecular weight and average molecular weight between junctions and decrease in crosslinking density with the addition of dichlorodimethylsilane as chain extender confirmed the chain extension of low molecular weight PDMS. Results showed that maximum outcome in physical and mechanical properties of cured PDMS were obtained at 3.5 phr DDS concentration. Relation between tear strength and average molecular weight between junctions obeyed the lake and Thomas equation.

Improvement in the toughness and adhesive properties of cyanate ester resin by incorporation of fluorinated poly(aryl ether nitrile)

AbstractCyanate ester (CE) resins possess excellent thermal and mechanical characteristics, yet their brittleness and high cross‐link density limit their practical application. To overcome this limitation, the modification of CE with thermoplastic resins has been employed as an effective strategy to enhance their toughness and mechanical properties. In this study, a novel fluorinated poly(aryl ether nitrile) (PFPEN) with excellent solubility and thermal properties was synthesized through a nucleophilic substitution reaction. The effect of the addition of PFPEN on a blend composed of bisphenol A cyanate ester (ACE) and a mixed catalyst was investigated. Differential scanning calorimetry results showed that the addition of PFPEN facilitated the curing reaction of ACE, resulting in a lower exothermic peak temperature. Compared to the pure ACE, the addition of PFPEN at 20 phr significantly improved the tensile and impact strengths of the resin, as well as enhanced the lap shear strength at room temperature by 36% and the peel strength by 320%, respectively. The scanning electron microscope results showed a ductile fracture mechanism. However, it should be noted that the decrease in crosslinking density resulting from the addition of PFPEN may affect the lap shear strength at 300°C and the glass transition temperature (Tg) of the material.

Inactivation of Staphylococcus aureus in gelatin nanoparticles using supercritical carbon dioxide

Nanoparticles (NPs) are widely studied for biomedical applications. Generally, NPs and NP-based materials should be effectively sterilized without altering their structure and function before clinical translation can be considered. However, research on industrially scalable sterilization methods of nanoparticles is still scarce. Therefore, we explored if treatment of gelatin nanoparticles (GNPs) using supercritical carbon dioxide (scCO2) resulted in effective inactivation of Staphylococcus aureus as frequent pathogen causing infections. After scCO2-treatment the morphology, size and surface charge of GNPs remained unchanged, although a slight decrease in crosslinking density was observed. ScCO2 successfully inactivated Staphylococcus aureus in GNPs, as evidenced by a 12-log reduction in bacterial burden. However, cytocompatibility of GNPs was lower after scCO2 treatment, indicating that the process still requires further optimization. Nevertheless, the work presented herein highlights the strong potential of scCO2 for inactivation of pathogens in nanoparticles.

Novel photosensitive resin simultaneously with outstanding cryogenic strength and toughness for digital light processing three-dimensional Printing

With the increased demand for three-dimensional (3D) printing technology in various fields, it is important to develop high-performance resin that could withstand temperature changes to expand their application potential. A new photosensitive oligomer (BDM–DDM–ETPS–GMA) based on epoxy-terminated polyether siloxane (ETPS) and bismaleimide diphenylmethane/4, 4′-diaminodiphenylmethane (BDM–DDM) resin was synthesized and then mixed with other oligomers, reactive diluents, and photoinitiators to prepare a novel 3D printing resin. The results show that the resulting resins exhibit good fluidity and rapid photopolymerization ability, which satisfies the rheological prerequisites of 3D printing resin. Moreover, the incorporation of BDM–DDM–ETPS–GMA can simultaneously improve the cryogenic stiffness and toughness of commercial resin. Specifically, the tensile strength, elongation at break, flexural strength, impact strength, and storage modulus at −30 °C of modified resin with 15% BDM–DDM–ETPS–GMA are 151.2 MPa, 10.9%, 146.2 MPa, 9.8 kJ/m2, and 4,131 MPa, respectively, which are about 2.81, 1.70, 1.37, 1.81, and 1.54 times of that of commercial resin. A synergistic enhancement mechanism is believed to be attributed to these results, which includes the introduced flexible siloxane chain and the rigid bismaleimide structure as well as decreased cross-linking density. These attractive features of modified resins suggest that the method proposed herein is a new approach to develop high-performance 3D printing photosensitive resin simultaneously with outstanding cryogenic strength and toughness and thus has wide application potential in the aerospace, military industry, and other cutting-edge fields.

Improving toughness of epoxy resin by in‐situ formed secondary network during tertiary amine initiated curing

AbstractThe mechanical properties of epoxy resin cured by tertiary amine initiated ring‐open reaction were investigated. Special curing procedures that first at 130°C for different times (facilitating chain transfer/reducing crosslinking density) and then 80°C/2 h–130°C/2 h were used. For cured specimens, the glass transition temperature showed a continuous decrease with increasing time at the first step of curing (t1st step), and the swelling ratio also increased. These changes pointed to a monotonic decrease in crosslinking density. Mechanical properties were characterized by three‐point bending and tensile test. Both the deflection and strain at break showed a sudden increase when t1st step reached 30 min, corresponding well to occurrence of gelation at 130°C. The fracture surface after tensile test was further examined. Rough surface with rupture of fibrillar structure was found when t1st step reached 30 min. These observations all suggest an abrupt increase of toughness related to gelation (t1st step = 30 min). It is explained by formation of network at the first step of curing, which has lower crosslinking density and serves as a secondary toughening network.

The role of molecular architecture on the viscoelastic properties of thermoreversible polyurethane adhesives

New polyurethane (PUR) adhesives containing covalent Diels-Alder (DA) bonds that can break and form with temperature have been recently developed for sustainable multilayer packaging. The understanding of the mechanical properties and in particular of creep in terms of molecular architecture appears of outmost importance in view of their applications and potential reutilization. Oscillatory shear rheometry shows a modulus decrease with temperature associated to bond breakage and the enhancement found upon cooling can be associated with network reconstruction. Analysis of storage modulus values above the glass transition and below the retro-DA reaction suggests a lower network crosslinking density with increasing adduct content. Room temperature indentation and shear rheometry tests reveal that DA bonds improve the modulus and reduce the resistance to linear viscoelastic flow. The modulus improvement can be associated to the enhanced chain rigidity introduced by the DA moieties and the reduced creep resistance seems to be related to the decrease of crosslinking density.

Research on modification mechanism and performance of an innovative bio-based polyurethane modified asphalt: A sustainable way to reducing dependence on petroleum asphalt

• An innovative BPMA was synthesized by BPU and VB. • BPU and VB are chemically crosslinked by urethane bond, resulting in satisfactory compatibility of BPMA. • BPU exhibits an appropriate crosslinking density, which results in an improved ductility of BPMA. • BPMA shows a suitable construction allowance time and good working performance. Sustainable bio-oil, as a modifier and partial substitute for asphalt binder, is potentially reducing dependence on non-renewable petroleum asphalt in pavement engineering. However, unsatisfactory performance of bio-oil asphalt dramatically hinders the application of bio-oil in pavement engineering. In this paper, bio-oil based polyurethane modified asphalt (BPMA), an innovative way of modifying bio-oil asphalt binder, was explored to prepare a sustainable asphalt binder with satisfactory performance. BPMA was synthesized by virgin asphalt and bio-based polyurethane (BPU), in particular, BPU was pre-assembled by castor oil, isosorbide, and isocyanate. The formulation and synthesis process of BPU were investigated via mechanical property test and fourier transform infrared spectroscopy (FTIR). Attributed to the addition of isosorbide, BPU exhibited a significant decrease in crosslinking density and a remarkable improvement in ductility. Furthermore, BPMA containing various contents of BPU at different curing duration were systematically studied by rotational viscosity, storage stability, fluorescence microscopy, FTIR, dynamic shear rheometer and bending beam rheometer to evaluate workability, compatibility, modification mechanism, working performances. Results showed that BPMA exhibited suitable workability, good compatibility, and satisfactory working performance. Moreover, the microstructure of BPMA transformed from VB-phase dominated island structure to BPU-phase dominated spatial network structure as BPU content increased. FTIR results indicated a combination modification mechanism of physical modification (effect of BPU) and chemical modification (chemical crosslinking between BPU and VB).

Mechanochemical modification of crosslinked low‐density polyethylene: Effect of solid‐state shear pulverization on crosslinks, branches, and chain lengths

AbstractCrosslinked low‐density polyethylene (XLLDPE) is widely used in several specialty plastics industries. However, the permanent chemical crosslinks cause high‐melt viscosity and poor processability, preventing the material from being reused and recycled effectively. This study investigates solid‐state shear pulverization (SSSP) as a continuous, commercially viable mechanochemical processing technique to initiate the decrosslinking of XLLDPE for mechanical recycling. Post‐industrial XLLDPE scrap material was processed using SSSP with a range of pulverization conditions, which were correlated with universal processing covariants of specific mechanical energy and particle size distribution. The physical properties of SSSP‐processed materials were compared to as‐received XLLDPE and uncrosslinked low‐density polyethylene. While gel content tests confirm a gradual decrease in crosslinking density with a more energy‐intensive SSSP process, melt rheology and dynamic mechanical analysis characterization revealed additional chain architecture modifications such as branching and chain scission. Based on differential scanning calorimetry and thermogravimetric analysis, the SSSP‐processed XLLDPE retained its thermal stability and crystallinity; tensile testing results showed improved stiffness, strength, and toughness. These results indicate that tunable SSSP can transform XLLDPE into a decrosslinked, branched, and melt‐processable recycled polyethylene.

Exploring the effects of cardanol-based co-curing agents with different phosphorus structures on the mechanical and flame-retardant properties of bismaleimide resin

Aiming to address the problems of flammability and brittleness of BMI and reduce the use of petroleum-based materials, three kinds of cardanol-based co-curing agents (CDPO, CDPC, CDCP) were synthesized and applied to prepare advanced BMI. Compared to the neat BMI, the modified BMI exhibited better thermal stability with higher decomposition temperatures and char yields. The BMI/CDCP-10 containing 4.5 wt% CDCP possessed the best flame retardancy with 50.4% reduction in PHRR, 27.5% reduction in THR and 21.2% reduction in TSP compared to those of neat BMI. Moreover, with the 4.5 wt% incorporation of CDPO, CDPC and CDCP, the impact strength of BMI was enhanced by 95%, 133.8% and 127.5% respectively, due to the flexible fatty chains and the decreased crosslinking density. The reason for different modification effects among modified BMI systems is studied from the structure of modifiers, finding that the phosphorylated cardanol with higher content of O and higher valence of P can be utilized to improve the mechanical property and fire safety of BMI. This work provides a new way to reduce the dependence on the petroleum-based materials and expand the application areas of BMI.

Synthesis, physicochemical, and antimicrobial characteristics of novel poly(urethane-siloxane) network/silver ferrite nanocomposites

In situ polymerization was used to produce novel AgFeO2@PEG/polyurethane network nanocomposites (NP-PUs) with 30–60 wt% of soft poly(dimethylsiloxane) segments in polyurethane (PU), containing 1 wt% of PEG-coated AgFeO2 nanoparticles, AgFeO2@PEG. Physicochemical properties and in vitro biological activity of the NP-PUs were systematically evaluated in terms of AgFeO2@PEG (NP) addition and soft segment content. High-angle annular dark-field transmission electron microscopy showed that the nanoparticles were generally uniformly distributed in the PU matrix. Increased soft segment content caused significantly increased intensity of the broad, amorphous X-ray diffraction peaks of crystalline AgFeO2, probably because the chemical composition of PU affected the distribution of nanoparticles. The Young modulus, hardness, and plasticity of the NP-PUs were higher than for pure PU and increased with decreasing soft segment content. Decreased soft segment content induced higher microphase separation, increased hydrophilicity and swelling ability, but decreased cross-linking density. Additionally, NP-PUs had higher glass transition temperatures, improved thermal stability, and enhanced nanomechanical performance over pure PU. The NP-PUs demonstrated good selective inhibition of Candida albicans and Candida parapsilosis (30–55%) and no pronounced cytotoxicity to MRC5 human lung fibroblasts. Among the investigated AgFeO2@PEG/PUs, the best antifungal activity was shown by composites with 30 and 40 wt% soft segments. Consequently, the novel AgFeO2@PEG/polyurethane network nanocomposites could be further optimized to be used as biocompatible surfaces that also prevent formation of fungal biofilms.Graphical abstract

The effect of temperature and peroxides content on cross-linking and properties of EPDM-based rubber matrix

In this work, dicumyl peroxide (DCP), di-tert-butyl peroxide (DTBP), di(tert-butylperoxyisopropyl)benzene (DTBPIB), and 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane (DDTBPH) were used for cross-linking of rubber matrix based on Ethylene-propylene-diene monomer rubber (EPDM). The aim was to investigate the influence of curing temperature and the type and content of peroxide on curing process of rubber compounds, cross-link density, and physical-mechanical properties of vulcanizates. The results revealed that higher curing temperatures promote faster decomposition of organic peroxides, leading to the faster reactions of peroxide radicals with rubber chains. The increase in temperature resulted also in the increase of cross-linking degree. Although, side reactions as chain scission take over the main cross-linking reactions at very high temperatures resulting in the decrease of cross-link density. The increasing content of peroxides resulted in the acceleration of curing reactions and to increase of cross-link density. The fastest curing kinetics exhibited rubber compounds cured with DCP. However, vulcanizates cured with DCP were found to have the lowest cross-link density. In generally, the dependences of modulus, hardness, and elongation at break of vulcanizates were in line with the dependences of the cross-link densities, while higher tensile strength were found to have vulcanizates cured at higher temperatures and lower peroxides content.