Polymer Degradation and Stability最新文献

A circular economy requires that plastic packaging should be recyclable or compostable as well as reusable. Compostable/biodegradable poly(lactic acid) (PLA) is an alternative to conventional packaging materials for films, bags, and containers. Packaging is not only for food and beverages but also for medicine, agricultural chemicals, industrial chemicals, and waste solvents such as chlorinated solvents, which sometimes contain water. This study determined that PLA films were completely soluble in dichloromethane and chloroform, insoluble but strongly swollen in trans-1,2-dichlorocycrohexane, o-dichlorobenzene, and carbon tetrachloride, and insoluble with retained film shape in tetrachloroethylene (TCE), 1,2,4-trichlorobenzene (1,2,4-TCB), and 1-bromonaphthalene (1-BN). The equilibrium mass uptake values of pure insoluble solvents in PLA films were 0.977 ± 0.219 wt% for TCE, 1.716 ± 0.631 wt% for 1,2,4-TCB, and 3.351 ± 1.936 wt% for 1-BN. After sorption of the three insoluble pure solvents, the α’-type crystals of PLA films changed to α-type crystals. This phenomenon was based on the molecular size and electrostatic potential value of the solvents. When insoluble solvents were mixed with water, the water-in-oil mixture enhanced the mass uptake for TCE and 1,2,4-TCB but reduced it for 1-BN. The oil-in-water mixture distinctly reduced the solubility for all solvents. The α-type crystal structure was stable in TCE and 1-BN. If an industrially appropriate method of α-type crystal structure formation could be realized selectively, then PLA could be used as packaging materials for films, bags, and containers for these solvents without any further modification.

Deconstruction of polyolefins into functionalized macromonomers presents a compelling strategy for polyolefin upcycling by creating macromonomers through dehydrogenation/depolymerization. We show that nitrous oxide (N₂O), a greenhouse gas waste product from the production of nylon, mediates the deconstruction of polycyclooctene (PCOE) and generates carbonyl-functionalized macromonomers. Carbonyl incorporation and macromonomer molar mass were well controlled by reaction time, and subsequent hydrogenation readily removed residual carbon-carbon double bonds. We also demonstrated that the reaction could progress efficiently with substrates of moderate levels of unsaturation, closely mimicking partially dehydrogenated polyethylene. Such carbonyl-functionalized macromonomers could serve as feedstock for preparing vitrimers and other functional polymers.

Liquid silicone rubber (LSR) exhibits excellent thermal stability and has been selected for use in a variety of applications where thermal stability, chemical resistance and fire-retardant are required. The enhancement of the organic-to-inorganic conversion of LSR to improve their flame-retardant properties represents a significant area of research. The thermal stability of the platinum catalysts and the crosslinked network structure of the LSR have a considerable influence on the organic-to-organic conversion behavior of LSR. The present study demonstrates the efficacy of N-heterocyclic carbene (NHC) ligand-modified Karstedt's catalysts as catalysts for the curing of LSR by hydrosilylation at room temperature and for the organic-to-inorganic conversion of LSR at elevated temperatures. The catalyst was employed in the preparation of three LSRs with varying network structures, utilizing four polysiloxanes with differing degrees of functionality. The pyrolytic behavior and organic-to-organic conversion rate of these LSRs were investigated using a thermogravimetric analyzer (TG) coupled with a Fourier transform infrared spectrometer (FTIR). The findings indicated that LSRs with the highest crosslink density exhibited the highest organic-to-inorganic conversion rate; however, they demonstrated the lowest fire-resistance. The anomalous behavior has been subjected to further analysis with respect to the mechanical properties of the LSRs and the characteristics of their network structure. LSR coatings with enhanced hardness and fire-resistance are then produced by combining the advantageous properties of both LSRs in a layer-by-layer (LBL) assembly.

Atomic oxygen (AO) is the most common gas species in the Low-Earth-Orbit (LEO) and responsible for material degradation of the outer shell of spacecrafts within this space region. Due to their similar properties, low temperature oxygen plasmas are suited for material degradation studies taking place on earth instead of quite expensive space studies. Here we focus on the long-term degradation of Polytetrafluoroethylene (PTFE), which is often employed on the outside of spacecrafts. Up to date, there is no complete understanding of the degradation process on molecular level, which is necessary for materials improvement and new materials development.

For the degradation studies, a self-constructed capacitively driven 13.56 MHz RF reactor was used to generate an oxygen plasma for the simulation of LEO conditions. PTFE was characterised in the pristine state and after AO treatment at different times by ToF-SIMS, XPS and SEM. During plasma treatment, the samples show a linear mass loss behaviour. ToF-SIMS surface analysis reveal mass fragments which show a clear chemical reaction of oxygen species with PTFE. The presence of these molecular indicators was verified by XPS, where additional carbon species were found after plasma treatment. SEM micrographs showed an inhomogeneous degradation on the surface in the first hours similar to actual LEO exposure. For a complete understanding of the degradation progress, operando mass spectrometric studies of the plasma composition were carried out to detect volatile degradation products.

In summary, a steady degradation has been observed that leads to constant mass loss, defluorination, chain shortening and insertion of oxygen into the polymer.

In recent years, the excessive consumption of fossil energy leads to the depletion of petroleum resources and environmental pollution. Therefore, biomass which is renewable and easy availability has been exploited in the past few decades to replace petroleum resources and to design bio-based epoxy resins. Through molecular design and synthesis, alternative bio-based products with close properties to petroleum-based epoxy resins were exploited, and then bio-based epoxy resins with excellent and unique properties were developed. This present review mainly summarizes the synthetic strategies of bio-based epoxy resins through the chemical modification of various bio-based precursors, such as eugenol, vanillin, cardanol, furan, plant oil, and so forth. And then their inherent and superior properties relating to the unique structures and potential applications are discussed. Finally, the challenges and opportunities in the development of sustainable epoxy thermosets from renewable biomass are presented. It is hoped that this review will provide a framework for further design of bio-based epoxy thermosetting materials.

Simultaneously elevating fire safety and mechanical property is of great significance in flame retardation of polymers. Herein, a series of ethylene vinyl acetate (EVA) composites flame-retarded by expandable graphite (EG) were prepared and studied respecting their fire-retardant, mechanical, rheological and electrical properties. Results evince that combination of microencapsulated expandable graphite/microencapsulated red phosphorus (MEG/MRP) shows efficient fire-retarding effect on EVA in that adding merely 8 wt.% MEG/MRP can raise the polymer's limiting oxygen index to 26.6 % and UL-94 test to V-0 rating, whereas up to 20 wt.% EG and 10 wt.% EG/MRP are required for EVA to attain V-0 rating, respectively. The high efficiency is caused by more sizeable expansion ratio of MEG and high-quality expanded char resulting from MEG/MRP. Proper crosslinking of EVA matrix can augment strength and elasticity of EVA/MEG/MRP composite without lowering its fire retardance. Overall, the crosslinked EVA/MEG/MRP composite with 8 wt.% MEG/MRP shows good fire safety and mechanical property concurrently in contrast with virgin EVA.

The yellowing behavior of aliphatic polyamides at high temperatures considerably affects their performance and usability. Herein, this is the case for PA56, a typical bio-based odd–even polyamide with a structure similar to PA66. Furthermore, it exhibits the yellowing behavior during fiber- and film-thermal setting processes more severely than PA66. However, the mechanism behind this yellowing remains to be elucidated. The yellowing of aliphatic polyamides such as PA66 is usually considered to be caused by the oxidation of the N-vicinal methylene group and subsequent formation of chromophores such as pyrrole-type groups, α,β-unsaturated aldehydes, and conjugated azomethines. However, the above yellowing mechanisms were inconsistent with the experimental results observed during studies on the yellowing of PA56 during thermal oxidation. Herein, to shed some light on the yellowing mechanism of aliphatic polyamides, PA56 was oxidized at 180 °C and the resulting yellow substances were extracted with solvents and separated. The characterization of thermo-oxidized PA56 and generated yellow substances via ¹H NMR, ¹³C NMR, FTIR, UV–vis and fluorescence spectroscopies, and mass spectrometry revealed that conjugated N-acylamide was the chromophore responsible for the yellowing of PA56 during the thermal oxidation process, thereby proposing the mechanism behind the yellowing phenomenon. Although the same chromophore structure was deduced for PA56 and PA66 under identical conditions, conjugated N-acylamide produced via the thermal oxidation of PA56 was more stable and accumulated easily than PA66 because of the presence of hyper-conjugation, rendering PA56 more susceptible to yellowing during thermal oxidation. This study offers new insights into the thermo-oxidative transformation of aliphatic AB-type polyamides and their yellowing mechanism, thereby helping in the development of strategies that inhibit the yellowing of aliphatic polyamides.

In advanced material science, we explore the potential of boratrane, a promising agent for enhancing thermal stability. By combining rigorous Density Functional Theory (DFT) calculations with groundbreaking experimental analyses, we reveal the intricate interplay of radical intermediates and mechanisms underlying the thermal evolution of boratrane-infused materials. Our innovative approach illuminates dynamic structural transformations and elucidates boratrane's pivotal role in fortifying thermal resilience. The DFT calculations identify radical intermediates and mechanisms of thermal degradation, highlighting the role of borate bridges in delocalizing π-electrons in aromatic rings through Gibbs free energy (∆G_RXN), Highest Occupied Molecular Orbital (HOMO), Lowest Unoccupied Molecular Orbital (LUMO), and electrostatic potential (ESP) analyses. Fukui function analysis provides insights into the reactivity of these structures towards free radical attacks. Our findings demonstrate that boratrane-modified resins exhibit a stable BO₄^- structure, which prevents self-condensation of boratrane to B₂O₃ and enhances the thermal stability of oxygenated resins. This improvement is due to the formation of intramolecular hydrogen bonds, contributing to helix-like structures that strengthen the resin. The mechanism by which the BO₄^- structure terminates radical agents and transforms into carbonaceous material is elucidated through thermodynamic values, revealing the plausible reactions and chemical structure of boron in the resulting material.

Biobased flame retardants have attracted great attention owing to their environmental friendliness and easy availability. Urushiol, extracted from natural resin-raw lacquer, has been capped with diphenylphosphoryl chloride to afford a biobased flame retardant, namely urushiol-based phosphonate (U-DC). As an additive this material is effective in reducing flammability while maintaining good mechanical strength for epoxy resins (EP). Impressively, an EP/9 %U-DC blend exhibits a UL-94 V-0 rating, and an LOI of 37.1 % for combustion. Further, compared to that for EP, the peak heat release rate (pHRR) and total heat release (THR) for the EP/9 %U-DC blend was lower by 40.5 % and 26.6 %. Through analysis of volatile products formed and char residue produced during polymer degradation, a mode of flame retardant action for U-DC has been proposed. Further, EP/U-DC blends display better mechanical properties than those of unmodified EP. Tensile strength of EP containing 7 wt% U-DC (81.6 MPa) is greater than that for unmodified EP (64.4 MPa). This may be ascribed to the presence of abundant rigid benzene units in U-DC and chain entanglement between the long side chain of U-DC and the epoxy macromolecular chain. These observations provide a basis for the utilization of new urushiol derivatives in the production of high-performance EP.

Metal organic frameworks (MOFs) are a novel class of multidimensional nanoscale substances that exhibit highly controllable structures and possess large specific surface areas and porosities. In recent years, MOFs have garnered significant attention as flame retardants. This paper elucidates the construction method of MOFs-based flame retardant nanostructures and the flame retardant mechanism. Meanwhile, it provides a comprehensive review of recent advancements in utilizing pristine MOFs, MOFs composites incorporating phosphorus-nitrogen-based, carbon-based, silicon-based, and other materials, as well as MOFs-based multicomponent hybrids as flame retardant in polymeric materials. This study presents the modification of MOFs for their application as flame retardants in terms of both structure and chemical composition and outlines their flame retardant mechanisms and flame retardant efficiencies. The emphasis is placed on elucidating the flame retardant mechanism achieved through multi-component synergy. Finally, the paper summarizes the existing challenges as well as the prospects of MOFs-based flame retardants, with a view to providing guidance for the design of innovative flame retardant materials.