Structures and impact strength variation of chemically crosslinked high-density polyethylene: effect of crosslinking density

Highly elastic relaxor ferroelectric via peroxide crosslinking

Relaxor ferroelectrics are well-known for their high dielectric constants, low dielectric losses, and excellent electromechanical properties, making them valuable for various electronic devices. Despite recent efforts to enhance the durability of ferroelectrics through chemical cross-linking, achieving elasticity in relaxor ferroelectric materials remains a significant challenge. These materials inherently possess traits such as low crystallinity and small crystal size, while chemical crosslinking tends to diminish polymer crystallinity considerably. Thus, a key obstacle to making relaxor ferroelectric polymers elastic lies in safeguarding their crystalline regions from the effects of slight crosslinking. To tackle this issue, we selected P(VDF-CTFE-DB) with highly reactive C[double bond, length as m-dash]C double bonds as crosslinking sites, reducing the amount of cross-linking agents added and thereby lessening their impact on crystallinity. Through peroxide crosslinking, we transformed linear P(VDF-CTFE-DB) into a network structure, successfully producing a resilient relaxor ferroelectric material with maintained polarization intensity for ferroelectricity. Notably, this elastic relaxor ferroelectric was synthesized at relatively low temperatures, exhibiting a remarkable dielectric constant, superior resilience, fatigue resistance, and a stable ferroelectric response even under strains of up to 80%. Our approach paves the way for developing low-cost, high-dielectric-constant elastomers suitable for wearable electronics and related applications.

Bionic ordered structured hydrogels: structure types, design strategies, optimization mechanism of mechanical properties and applications

Natural organisms, such as lobsters, lotus, and humans, exhibit exceptional mechanical properties due to their ordered structures. However, traditional hydrogels have limitations in their mechanical and physical properties due to their disordered molecular structures when compared with natural organisms. Therefore, inspired by nature and the properties of hydrogels similar to those of biological soft tissues, researchers are increasingly focusing on how to investigate bionic ordered structured hydrogels and render them as bioengineering soft materials with unique mechanical properties. In this paper, we systematically introduce the various structure types, design strategies, and optimization mechanisms used to enhance the strength, toughness, and anti-fatigue properties of bionic ordered structured hydrogels in recent years. We further review the potential applications of bionic ordered structured hydrogels in various fields, including sensors, bioremediation materials, actuators, and impact-resistant materials. Finally, we summarize the challenges and future development prospects of bionic ordered structured hydrogels in preparation and applications.

From Bottle Caps to Frisbee-A Case Study on Mechanical Recycling of Plastic Waste towards a Circular Economy

This study demonstrates an open-loop recycling process of a specific post-consumer plastic waste stream. The targeted input waste material was defined as high-density polyethylene beverage bottle caps. Two methods of waste collection, informal and formal, were employed. Thereafter, materials were hand-sorted, shredded, regranulated, and then injection-molded into a flying disc (i.e., frisbee) as a pilot product. To observe the potential changes in the material throughout the entire recycling process, eight different test methods including melt mass-flow rate (MFR), differential scanning calorimetry (DSC), and mechanical tests were carried out on the various material states. The study showed that the informal collection led to a relatively higher purity in the input stream, which also appeared to have a 23% lower MFR value compared to that of the formally collected materials. The DSC measurements revealed a cross-contamination by polypropylene, which clearly affected the properties of all investigated materials. The cross-contamination led to a slightly higher tensile modulus in the recyclate, while the Charpy notched impact strength declined after processing by approximately 15% and 8% compared to those of the informal and formal input materials, respectively. All materials and the processing data were documented and stored online as a practical implementation of a digital product passport as a potential digital traceability tool. Furthermore, the suitability of the resulting recyclate to be used in transport packaging applications was also investigated. It was found that a direct replacement of virgin materials for this specific application is not possible without proper material modification.

Chitosan/glycyrrhizic acid hydrogel: Preparation, characterization, and its potential for controlled release of gallic acid

In the present work, chitosan (CHT) as a biodegradable polymer was crosslinked using various amounts of glycyrrhizic acid (GLA) as a novel crosslinking agent to prepare biocompatible hydrogels. The prepared hydrogels were used for the controlled release of gallic acid (GA) in transdermal therapy application. FTIR, XRD, and SEM were used to characterize the prepared gels. The results indicated that the carboxylic acid groups of GLA react with the amine groups of the CHT in the presence of activating coupling reagents to form covalent amide linkage between the polymer chains of CHT and construct CHT cross-linked hydrogel (CCH) network structure. The prepared CCH samples were characterized and used for the controlled release of a drug, i.e. (GA). For this purpose, the swelling kinetic, loading and encapsulation efficiency, in vitro drug release, drug release kinetics, cell viability assay, and anti-bacterial activity of the samples were evaluated. The swelling ratio of CCH samples were in the range of 455-37 % depending on the pH of environment. Swelling kinetic results showed an aggregate to the non-linear second-order kinetic model. Drug release results were fitted by kinetic models while the Korsmeyer-Peppas model was fitted better. The CCH samples exhibited high biocompatibility for 5 mg/ml hydrogel concentration. In addition, the CHT and CCH sample without the GA did not show anti-bacterial properties for 1200 and 150 μg/ml concentrations, respectively. The CCH sample containing the GA exhibited enough anti-bacterial activity on the S. aureus bacteria strain at 150 μg/ml concentration. In contrast, the CCH sample containing the GA has a light anti-bacterial effect on the E. coli bacteria strain. The calculated mesh size of hydrogel networks, drug size, and kinetics models revealed that the CCH samples could release GA based on a diffusion mechanism. In conclusion, the designed CCH samples have enough ability for controlled drug release in transdermal applications.

Degradation of hybrid material l,d-PLA : 5CB : SWCN under the influence of neutral, acidic, and alkaline environments

The aim of the study was to investigate the influence of the environment's pH on the degradation of the layers of the ternary composite l,d-PLA : 5CB : SWCN (10 : 1 : 0.5, w/w/w), where l,d-PLA (poly(lactic acid)) is a biodegradable polymer, 5CB is a well-known liquid crystal (4'-pentyl-4-biphenylcarbonitrile), and SWCN are single-walled carbon nanotubes. For this purpose, the samples were stored in air, distilled water, and solutions of 0.1 M NaOH and 0.1 M HCl, for up to 62 days. Using differential scanning calorimetry, atomic force microscopy, and infra-red spectroscopy methods it was observed that for both neat l,d-PLA and composite layers there was a poor degradation process after the storage under standard air conditions, distilled water, and 0.1 M HCl solution, while the erosion of the surface layer kept in 0.1 M NaOH solution was revealed just after 6 days. The longer storage in 0.1 M NaOH solution resulted in complete degradation of the l,d-PLA polymer layer, while the composite layer survived for up to 62 days. The solubilization of the polymeric l,d-PLA matrix in the composite after 62 days was so severe that it resulted in the vanishing of thermal effects on the DSC curve except for one that was probably connected with the glass transition of the residual quantity of the polymer that remained in the layer or the isotropisation of 5CB. As a result, we have shown that admixtures of 5CB and SWCN accelerate the degradation of l,d-PLA in the composite layer due to the hydrophilic/hydrophobic interface in the layer and act as plasticizers. The mechanism of the degradation process is also discussed.

The Effect of Dialkyl Peroxide Crosslinking on the Properties of LLDPE and UHMWPE

Peroxide has been considered a chemical agent that can be used to tune the properties of polymeric materials. This research evaluated the influence of different concentrations of dialkyl peroxides on the mechanical, thermal, and morphological properties of linear low-density polyethylene (LLDPE) and ultra-high molecular weight polyethylene (UHMWPE). The neat polymer, as well as those with the addition of 1% and 2% by mass of dialkyl peroxides, were subjected to compression molding and immersion in water for 1 h, under controlled temperatures of 90 °C. The values of the gel content found in the samples indicated that the addition of peroxide to the LLDPE and to the UHMWPE promoted the formation of a reticulated network. The structure obtained by the crosslinking led to less reorganization of the chains during the crystallization process, resulting in the formation of imperfect crystals and, consequently, in the reduction in melting temperatures, crystallization and enthalpy. The mechanical properties were altered with the presence of the crosslinker. The polymers presented had predominant characteristics of a ductile material, with the occurrence of crazing with an increased peroxide content.

Radiation-Based Crosslinking Technique for Enhanced Thermal and Mechanical Properties of HDPE/EVA/PU Blends

Crosslinking of polyolefin-based polymers can improve their thermal and mechanical properties, which can then be used in various applications. Radiation-induced crosslinking can be done easily and usefully by irradiation without a crosslinking agent. In addition, polymer blending can improve thermal and mechanical properties, and chemical resistance, compared to conventional single polymers. In this study, high-density polyethylene (HDPE)/ethylene vinyl acetate (EVA)/polyurethane (PU) blends were prepared by radiation crosslinking to improve the thermal and mechanical properties of HDPE. This is because HDPE, a polyolefin-based polymer, has the weaknesses of low thermal resistance and flexibility, even though it has good mechanical strength and machinability. In contrast, EVA has good flexibility and PU has excellent thermal properties and wear resistance. The morphology and mechanical properties (e.g., tensile and flexure strength) were characterized using scanning electron microscopy (SEM) and a universal testing machine (UTM). The gel fraction, thermal shrinkage, and abrasion resistance of samples were confirmed. In particular, after storing at 180 °C for 1 h, the crosslinked HDPE-PU-EVA blends exhibited ~4-times better thermal stability compared to non-crosslinked HDPE. When subjected to a radiation dose of 100 kGy, the strength of HDPE increased, but the elongation sharply decreased (80%). On the other hand, the strength of the HDPE-PU-EVA blends was very similar to that of HDPE, and the elongation was more than 3-times better (320%). Finally, the abrasion resistance of crosslinked HDPE-PU-EVA was ~9-times better than the crosslinked HDPE. Therefore, this technology can be applied to various polymer products requiring high heat resistance and flexibility, such as electric cables and industrial pipes.