Polymer Degradation and Stability最新文献

The development of recyclable epoxy thermoset is a major area of research interest today due to their environmental threats and non-sustainability. Recyclable products have recently gained a lot of interest as an intriguing class of regenerating thermoset due to their capability to display strength, durability, and corrosion resistance approaching that of conventional thermosets, while displaying end-of-life recyclability. This review article summarizes the most recent and significant advances in different reversible dynamic bonds such as Diels–Alder, ester, phosphate-ester, boronic-ester, vinylogous urethane, disulfide, imine, diselenide, thioester, acetal, urea, and hemiaminals/hexahydrotriazines containing curing agents that can cure traditional or recently developed epoxy resins for producing reversible epoxy thermosets. Particular emphasis is given to synthesis approaches and curing performances of intrinsically recyclable epoxy curing agents for the development of next-generation epoxy thermosets. The mechanical, thermomechanical, thermal, and recycling properties of the epoxy thermosets cured by dynamic adaptable networks (DANs) containing curing agents are also investigated. Finally, challenges, opportunities and emerging trends in the field are also addressed. Therefore, it would be desirable for both industry and academia to appropriately formulate distinctive curing agents for epoxy resins adopting various chemistries.

The continuous consumption of fossil resources and the resulting environmental pollution problems make the synthesis of polymers from renewable resources more and more attractive. Herein, a bio-based epoxy curing agent, 3,6-epoxide hexahydrophthalic anhydride (EHPA), was synthesized successfully from renewable furan and maleic anhydride by combining Diels-Alder and hydrogenation reactions and can cure diglycidyl ether of bisphenol A (DGEBA) to produce a sustainable thermosetting resin (EEP). Tensile properties and dynamic mechanical analysis show that EEP is a high-strength, high-toughness, and high-modulus material, showing higher glass transition temperature (201 °C), tensile strength (104.51 MPa), and storage modulus (4.14 GPa) than those of epoxy thermosets (MEP) cured with the petroleum-based methyl tetrahydrophthalic anhydride (MeTHPA). Besides its high performance, EEP can be degraded via the cleavage of its ester bonds at mild conditions (150 °C, 5 wt.% NaOH aqueous solution), making it an environmentally friendly alternative to petroleum-based epoxy resins. Our results provide a way to synthesize bio-based curing agents to fabricate high-performance and degradable epoxy resin polymers that are expected to be applied in the aerospace fields.

In this study, the relationship between the crystal orientation and enzymatic degradation rate of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH) was investigated. The enzymatic degradation rates of melt-spun PHBH fibers were quantified via PHB depolymerase from Ralstonia pickettii T1, and the degradation rate decreased with increasing drawing ratio. The solid structure was investigated via differential scanning calorimetry and small-angle X-ray scattering, and kinetic analysis was performed, indicating that the degradation rate was affected not only by crystallinity and lamellar thickness but also by highly ordered structure. The highly ordered structure was examined via wide-angle X-ray diffraction. The enzymatic degradation rate from the fiber cross-sections was then quantified for the first time, and it increased with drawing ratio and was larger than that from the fiber sides. This result showed that the degradation rate varied depending on the crystal orientation. Furthermore, an enzymatic degradation test of PHBH ring-banded spherulites was conducted, and it was found that the degradation rate of flat-on lamellar crystals was greater than that of edge-on lamellar crystals. This is the first study to quantitatively evaluate the relationship between the crystal orientation and the enzymatic degradation rate.

Elastic materials with high biodegradability should replace those with low biodegradability in some applications. We previously reported biodegradable thermoplastic elastomers (TPEs) composed of poly(l-lactide) (PLLA) as a hard segment and aliphatic polyesters from 2-methyl-1,3-propanediol (MP) and short aliphatic dicarboxylic acids as soft segments. In this study, we synthesized the biodegradable thermoplastic elastomers using longer aliphatic dicarboxylic acids to develop the TPEs with lower glass-transition temperature (T_g) and high biodegradability. A series of aliphatic polyesters were prepared by polycondensation of MP and aliphatic dicarboxylic acids bearing 7–10 carbons. Among them, poly(2-methyl-1,3-propylene azelate) (PMP9) was found to have lower T_g, amorphous nature, and high biodegradability in seawater. The ring-opening polymerization of l-lactide (LLA) using PMP9 as a macroinitiator afforded triblock copolymers, PLLA-b-PMP9-b-PLLA (TPE9). The successive addition of LLA to in-situ generated PMP9 resulted in the successful one-pot synthesis of high molecular weight TPE9. The TPE9s showed both low T_g of the PMP9 segment and high melting temperature of the PLLA segment. The TPE9s exhibited elastic behavior showing elongation at break of up to 2500 % in tensile tests and also high biodegradability in seawater. Thus, we developed potentially practical TPE with tunable physical properties and high biodegradability obtainable from easily available and renewable starting materials.

Traditional handmade papers, as the carriers of paper-based cultural relics, inevitably undergo various deteriorations during long-term preservation. Establishing a reasonable degradation kinetic model of handmade paper under the synergistic action of multiple factors is the crucial to evaluate paper aging and lifespan of paper. This study explores the moisture content and diffusion behavior of water molecules in traditional handmade paper to identify the interaction between fibers and water molecules at the accessible sites by two-dimensional infrared spectroscopy (2D-COS). Additionally, the optimized second-order kinetic models for the degradation of three types of Kaihua handmade papers was presented based on the time-temperature-humidity superposition method. A time-temperature-humidity translation factor is incorporated into the dynamic model to quantitatively analyze the synergistic effect of temperature and humidity on the degradation rate of handmade paper. The degradation rates of handmade paper with different raw materials and handcraft processes demonstrated significant effects of the cooking and bleaching processes on the aging degradation process and the durability of the paper. The improved second-order degradation kinetic model, considering the cooperation process with multi-factors and mechanisms, enables the extrapolation of paper aging properties at arbitrary temperature and humidity effectively, which provides a more reasonable estimation of handmade paper's lifespan.

This study investigates the solvent-based free-radical graft copolymerization of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) with C₁₁ (undecenoic acid, UDA), C₁₂ (dodecene, DD), and C₁₈ (octadecene, OD) alkene substrates using dicumyl peroxide as an initiator. Comprehensive Nuclear Magnetic Resonance (NMR) spectroscopy was employed to confirm and quantify grafting, which was achieved at up to 5.8 mol% of monomer units. Thermal analysis revealed a significant reduction in melting temperature for all modified PHBVs, ranging from 10-20°C decrease in comparison to virgin PHBV. A decrease in crystallisation temperature and a reduction in glass transition temperature was measured for the UDA and DD grafted PHBV. Additionally, molecular weight analysis showed a marked decrease in weight average molecular weight (${\overline{M}}_w$) and number average molecular weight (${\overline{M}}_n$) for PHBV grafted with UDA, which was attributed to the presence of carboxylic acid functionality, which accelerates chain scission. The molecular weight profiles of the dodecene and octadecene grafted PHBV were only marginally reduced (32% and 18% reductions in ${\overline{M}}_w$, respectively), which is a much less severe reduction than has been previously observed for similar chemistries. Overall, this work demonstrates that solvent-based free-radical graft copolymerization can reduce the melting temperature and disrupt the crystallinity of PHBV, hence improving its processability and progressing the field of PHA modification and application.

Polyhydroxyalkanoate (PHA) copolymers consisting of 3-hydroxybutyrate, 2-hydroxy-4-methylthiobutyrate (2H4MTB), and 2-hydroxy-4-methylvalerate were biosynthesized by recombinant Escherichia coli using L-methionine as the 2H4MTB precursor. The 2H4MTB unit contains a sulfide group in its side chain that can be oxidized to sulfoxide and sulfone groups by oxidants, thereby increasing the hydrophilicity of the polymer. The PHAs were biosynthesized at 3.9, 9.2, and 13.9 mol% 2H4MTB, and their polymer films were oxidized with hydrogen peroxide. The surface of the oxidized film was characterized using Fourier-transform infrared spectroscopy, Raman spectroscopy, and contact angle analysis. The oxidized films with 13.9 mol% 2H4MTB showed a 30° lower water contact angle than the non-oxidized films. To assess their marine biodegradability, the PHA films were immersed for 114 days in seawater continuously pumped from two depths (24 and 397 m) in Suruga Bay, Shizuoka, Japan. The weight loss of the films immersed in deep seawater was higher than that of those immersed in surface seawater. Additionally, there was a tendency for higher degradation of the oxidized films than that of the non-oxidized films, which was also confirmed by the biochemical oxygen demand test. In the surface morphology analysis by scanning electron microscopy, irregularities were observed in the degraded films, but their morphologies differed between the oxidized and non-oxidized films. Based on these observations, the biodegradation of the PHA films in seawater is discussed.

Although epoxy asphalt (EA) mixtures have been widely used for the pavement in tunnels, it was limited by its flammability, poor compatibility and low mechanical property. To solve the above problems, a compatibilized and toughening flame retardant (ESO-AA-DOPO) was prepared via the epoxidized soybean oil (ESO), arachidonic acid (AA) and 9,10-Dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (DOPO). The presence of ESO-AA-DOPO significantly improved the flame retardancy, in which passing the UL-94 V-2 rating. The peak heat release rate (PHRR) reducing by 46.2 % for EA/40A-ESO-AA-DOPO composites comparing to that of pure EA. The analysis of char residue confirmed that the catalytic formation of dense and continuous char layer residue with good thermal oxidation stability originating from the ESO-AA-DOPO. The initial stage viscosity of ESO-AA-DOPO modified EA was lower than that of pure EA. Under the action of aliphatic epoxy resin and double bond, the molecular chain of epoxy resin presented a network structure, the base asphalt was divided into micelles, and the overall distribution of EA system was a 3D networks “sea-island”, which ensuring the increase in the glass transition temperature (Tg) of the EA system. The tensile strength and elongation at break of EA/40A-ESO-AA-DOPO composites were 123 % and 207 % higher than those of pure EA attributing to the improved compatibility and the formation of 3D networks “sea-island” structure.

Plastics when exposed to UV radiation start to degrade via photooxidative aging including free radical formation, oxidation, chain scission and/or crosslinking reactions. These chemical changes can cause loss in mechanical strength, surface embrittlement, and eventually surface erosion. The eroded particles are microplastics (MPs), which have been identified as a potentially serious threat to the environment and its inhabitants. In general, photodegradation of virgin plastics has been studied extensively, but there is not much literature on the degradation of recycled plastics. The goal of the study was to investigate the changes caused by photodegradation in recycled plastics and assess the potential risks of MPs formation. And eventually, knowing the chemical and physical transformations occurring on the surface understand the mechanism behind surface microcracking, which is the first step of MPs formation. Pellets of five industrially recycled plastics (low-density polyethylene (rLDPE), linear low-density polyethylene (rLLDPE), high-density polyethylene (rHDPE), and two polypropylenes (rPP)) from different waste sources were analysed. UV irradiation was performed in an accelerated weathering chamber for milled (< 400 µm) plastic powder to ensure homogeneous changes throughout the sample. The properties were investigated by ATR-FTIR, HT-SEC, XPS and DSC. Formation of microcracks was studied on plastic pellets by SEM. The results showed that the degradation significantly differed between the recycled plastics, and the waste source was more important than the plastic type. rLDPE and one of the rPP samples showed a significant increase in carbonyl index as well as decrease in molar mass during the first 500 h of UV exposure. The other rPP and rHDPE samples showed first considerable signs of degradation only after 1000 h of UV exposure. Minor changes were observed for the rLLDPE sample during the whole test. The SEM revealed microcracking on the surface of all samples, which also had noticeable degradation identified by other methods. These recycled plastics can be considered the ones with the highest potential of MPs formation. From the chemical and physical transformations identified on the surface, the mechanism leading to microcracking, which is the first step in the formation of MPs, is proposed.

The study of fabrics’ flame retardant properties has always been a hot topic in the field of textiles. In this paper, a flame-retardant fabric with a sandwich structure was obtained by combining hydrogel patch method and in-situ polymerization technology via a two-step method. The thermogravimetric analysis (TGA) showed that the hydrogel could improve PLA materials’ thermal stability effectively. The hydrogel treated fabrics’ limiting oxygen index (LOI) value increased from 21.05 % to 80.00 %. The cone calorimeter test demonstrated that the pHRR and THR of the hydrogel treated fabrics decreased to 51.48 % and 55.97 %, respectively. After adding a mixture of ferric oxide and mica during the in-situ polymerization of the hydrogel, the pHRR and THR of the obtained fabric decreased to 43.77 % and 46.00 %, respectively, compared to the original PLA fabric. Furthermore, the mechanical strength test showed that the introduction of hydrogel patch had almost no effect on the mechanical properties of PLA materials. In addition, by using the way that metal ions can be arranged into a special array on the magnetic induction line, under the synergistic effect between ferric oxide and mica, the array formed by the ferric oxide particles on the magnetic induction line was designed with a series of patterns on its surface. Finally, a kind of textile with flame retardant properties and different patterns was obtained.