Main Chain Scission Research Articles

Not Cutting Corners: Bioderived Triggers Driving Oxidative Main Chain Scission of Poly(ethylene terephthalate)

Not Cutting Corners: Bioderived Triggers Driving Oxidative Main Chain Scission of Poly(ethylene terephthalate)

Operando Fluorescence Spectroscopy Setup for Radical Detection during Proton Exchange Membrane Water Electrolysis Operation

In a world with an ever-increasing energy demand, efficient and durable energy conversion devices for renewable energy sources are of great importance. Proton exchange membrane water electrolyzers (PEMWEs) are among the industry-established systems for converting electricity into hydrogen as a chemical energy carrier. In these systems, the proton exchange membrane (PEM) has to separate the two evolved gases (H2 and O2). The degradation of this membrane during operation, e.g., membrane thinning, can lead to system failure due to electrical shorts or dangerous explosive gas mixtures of H2 in O2 [1]. Therefore, it is important to gain a detailed understanding of the membrane degradation mechanisms in order to develop suitable mitigation strategies and extend the device lifetime. One possible cause of ionomer degradation is the evolution of reactive oxygen species (ROS), namely hydroxyl and hydroperoxyl radicals, during WE operation, causing main chain scission and leading to membrane thinning [2]. Due to the very short lifetime of these highly reactive species, direct detection within the PEM is challenging.However, there is an indirect detection method possible that has already been applied to PEM fuel cells [3]: The membrane is infused with a radical-sensitive fluorescent dye and an optical probe is embedded within this dyed membrane [Fig.1]. This way, a fluorescence signal can be measured upon excitation with a suitable light source. With the membrane being exposed to ROS, the radicals attack the membrane as well as the dye molecules, which can be measured by a change in fluorescence intensity. This setup was adapted for the application in PEMWEs to measure the effect of various operating conditions of the cell on ROS production and, hence, to detect stressors for membrane degradation.

A Model for Dry Electron Beam Etching of Resist

This paper presents a detailed physical model for a novel method of two- and three-dimensional microstructure formation: dry electron beam etching of the resist (DEBER). This method is based on the electron-beam induced thermal depolymerization of positive resist, and its advantages include high throughput and relative simplicity compared to other microstructuring techniques. However, the exact mechanism of profile formation in DEBER has been unclear until now, hindering the optimization of this technique for certain applications. The developed model takes into account the major DEBER phenomena: e-beam scattering in resist and substrate, e-beam induced main-chain scissions of resist molecules, thermal depolymerization of resist, monomer diffusion, and resist reflow. Based on the developed model, a simulation algorithm was implemented, which allowed simulation of the profile obtained in resist by DEBER. Experimental verification of the DEBER model was carried out, which demonstrated the reliability of the model and its applicability for theoretical study of this method. The ultimate DEBER characteristics were estimated by simulation. The minimum line width and the maximum profile slope that could be obtained by DEBER were approximately 300 nm and 70°, respectively.

Fast and Selective Main-Chain Scission of Vinyl Polymers Using the Domino Reaction in the Alternating Sequence for Transesterification.

This communication reports on vinyl polymers capable of selective and fast main-chain scission (MCS). The trick is the domino reaction in an alternating sequence of methyl 2-(trimethylsiloxymethyl)acrylate and 5,6-benzo-2-methylene-1,3-dioxepane, a cyclic ketene acetal for radical ring-opening polymerization. Removal of the trimethylsilyl group using Bu4N+·F- readily led to MCS via irreversible transesterification of the ester backbone, affording a five-membered lactone fragment. The molar mass decreased drastically within 5 min, and no side reactions were observed. Control experiments suggest that the formation of a five-membered ring via a domino reaction is critical for fast and selective MCS. The terpolymers with methyl methacrylate and styrene also exhibited a large decrease in molar mass within 5 min. In addition, MCS was also observed for the heterogeneous reaction system in acidic aqueous media; treatment of the binary copolymer in a 50 wt % acetic acid solution resulted in a significant decrease in molar mass after 30 min. These results suggest efficient construction of degradable sites using a binary monomer system corresponding to the pendant trigger and ester backbone. Because this molecular design using a binary monomer system provides selective and fast MCS for terpolymers containing other vinyl monomers, it can provide various degradable vinyl polymers.

Oxygen concentration regulated the efficient liquefaction of vulcanized natural rubber

Oxidative liquefaction represents a promising avenue for the homogeneous and high-value utilization of waste tire rubber. Given that truck tires predominantly comprise natural rubber (NR), this study investigated the efficient liquefaction of vulcanized NR regulated by oxygen concentration. Remarkably, the liquefaction of vulcanized NR was realized with an oxygen concentration of 75 % at 200 °C within 3 min. FTIR spectroscopy showed that carbonyl, ether and sulphoxide groups were produced during the liquification of NR vulcanizates. Oxygen concentration significantly affected the oxidative efficiency of both the main chains and crosslinks, with a more pronounced effect observed on the main chains. Nevertheless, the similar molecular weights and polydispersity indices across various degraded products suggested a selective oxidative cleavage of the NR backbone. Furthermore, online ATR-FTIR was used to monitor the dynamic changes of NR's molecular structure to elucidate the oxidative degradation mechanism. The oxidative cleavage of NR main chain primarily occurred at the C-C bonds connected with allyl groups, producing oxidized products enriched with conjugated carbonyl groups. Alternatively, the main chain scission may also occur due to the electronic rearrangement effect of the C=C bond and produce saturated carbonyl groups. Oxygen concentration modulated the degradation efficiency of vulcanized NR, which provided an efficient strategy for the upcycling of NR-based waste tires.

Efficient degradation of vulcanized natural rubber into liquid rubber by catalytic oxidation

Waste tire rubber based on natural rubber (NR) represents an abundant feedstock for the production of valuable products. This study investigated an efficient approach to convert NR vulcanizates into liquid rubber with the number-average molar masses of 1.3 × 104 g/mol. Compared to the anaerobic liquefaction at 300 °C for 3 min, NR vulcanizates were liquefied efficiently at 210 °C for 3 min by thermo-oxidative degradation catalyzed by manganese (II) stearate. The apparent activation energy of the NR degradation reaction decreased from 470.8 kJ/mol to 208.2 kJ/mol with catalysis, which was attributed to the formation of a coordination complex between catalysts and peroxides via the single-electron transfer redox reaction, leading to the catalytic oxidative degradation of NR vulcanizates. The main chain scission and crosslink scission occurred simultaneously during the degradation process, and the main chain scission was the primary catalytic oxidation reaction according to the Horikx theory. In addition, the obtained liquid rubber contained carbonyls, ether linkages, and sulfoxide groups, as proved by FTIR and 13CNMR . The catalytic oxidative degradation mechanism of NR vulcanizates was proposed based on the chemical structure of products and simulation results. The efficient method was proposed to highly facilitate the recycling and valorization of NR-based waste tire rubber.

Thermo-oxidative degradation behavior of natural rubber vulcanized by different curing systems

In this work, the effects of three types of curing systems: conventional vulcanization (CV), semi-efficient vulcanization (SEV) and efficient vulcanization (EV) on the degradation efficiency of sulfur cured natural rubber (NR) were investigated. NR vulcanizates were prepared and degraded at 210 °C in air for different periods of time. The sol fraction, molecular weight, element contents and degradation kinetics were studied in detail. The degradation efficiency of the NR vulcanizates was decreased in the order of CV, SEV and EV cured NR, which was caused by the lower crosslink density, apparent activation energy and bond dissociation energy of polysulfide bonds formed in the NR vulcanizates with CV, as compared to others. The results of Horikx theory and contents of carbonyl and sulfone group in degraded rubber indicated both sulfide scission and main chain scission occurred in the NR vulcanizates prepared with CV and SEV. Both the type of sulfidic crosslinks and crosslink density affect the degradative efficiency and products. The type of sulfidic crosslinks was the main factor affecting the degradation degree of NR vulcanizates, while the crosslink density mainly influenced the sol molecular weight of the degraded products. Moreover, the degradation mechanism of the three types of NR vulcanizates was proposed. This work demonstrated that the thermo-oxidative degradation efficiency of vulcanized rubber is affected by curing systems, which may provide guidance for upcycling waste rubber effectively.

Roles of hygrothermal aging temperature and time in degradation of the fracture toughness of highly cross‐linked epoxy

AbstractThe evolution of plane‐strain fracture toughness in highly cross‐linked epoxies was assessed for different hygrothermal aging temperatures (25, 50, and 75°C) and durations (0–75 days). The absorption of water increased with hygrothermal aging, but at a diminishing rate and without saturating. A fraction of the absorbed water persisted despite prolonged drying. Thermogravimetric analysis revealed that the epoxy‐decomposition temperature decreased with hygrothermal aging. 13C solid‐state nuclear magnetic resonance spectroscopy indicated an increase in the de‐shielding of carbon atoms in the vicinity of electronegative elements like oxygen with hygrothermal aging severity, indicating the development of intermolecular hydrogen bonding. X‐ray photoelectron spectroscopy tracked the chemical bonding state of carbon by using a narrow scan on the C1s binding energy region. Upon hygrothermal aging, there was a decrease in CC/CC groups, indicating main chain scission through hydrolysis. A simultaneous increase in the CO species indicates the formation of alcohols as hydrolysis reaction products. In this manner, retained water was involved in hydrogen bonding and chemical reactions with the epoxy. With increasing severity of hygrothermal aging, the fracture toughness decreased. This was consistent with fracture surface micrographs that showed a greater distance amongst crack stretch marks in specimens with less toughness.

Experimental study on the thermal decomposition of epoxy/anhydride thermoset matrix in composite insulator core rods

Epoxy/anhydride thermoset matrix within composite insulator core rods may thermally decompose by temperature rise caused by partial discharge. The thermal decomposition characteristics of the epoxy/anhydride thermoset was analyzed using pyrolysis-gas chromatography-mass spectrometry (PY–GC–MS) from 200 to 1000 °C, including the mechanisms of crosslinked network scission and the formation of gaseous or liquid products with potential secondary effects on surrounding materials. Below 360 °C, scission at crosslinking points predominates by ceiling temperature behavior, forming anhydrides. Above 380 °C, scission of main chains leads to the production of corresponding fragments. Beyond 600 °C, decomposition of anhydrides and fragments from main chains leads to the formation of lower molecular weight substances. During the pyrolysis process, organic acids are formed from 240 °C to 1000 °C. Above 600 °C, gases with high vapor pressures and liquids that can induce swelling are produced. These byproducts can have secondary effects on surrounding materials, including the failure of interfacial adhesion, swelling of silicone rubber sheaths, and a reduction in the mechanical strength of glass fibers. Consequently, these effects may accelerate the decay-like deterioration of core rods, characterized by a surface morphology reminiscent of rotting wood. These findings will inform the optimization of materials for enhanced electrical equipment performance and facilitate the evaluation of potential environmental impacts posed by the thermal decomposition of epoxy/anhydride thermosets in future applications.

Chemically Recyclable Vinyl Polymers by Free Radical Polymerization of Cyclic Styrene Derivatives.

To achieve a sustainable society supported by resource circulation, vinyl monomers that can radically polymerize and be recovered from vinyl polymers (VPs) are desirable. However, the chemical recycling of VPs remains challenging because of the difficulty in quantitative and selective main-chain scission or depolymerization. In this study, VPs of cyclic styrene derivatives, such as 3-methylene phthalide, were investigated to be chemically recyclable. The ring-opening of the pendant groups by saponification enhanced the steric hindrance of the pendants, which resulted in main-chain scission and depolymerization to the monomer precursors. Highly efficient chemical recycling was achieved by suspending the polymer in aqueous KOH. These results facilitate resource circulation toward achieving a sustainable society.

Bicyclic Guanidine Promoted Mechanistically Divergent Depolymerization and Recycling of a Biobased Polycarbonate.

We here report the organocatalytic and temperature-controlled depolymerization of biobased poly(limonene carbonate) providing access to its trans-configured cyclic carbonate as the major product. The base TBD (1,5,7-triazabicyclo[4.4.0]dec-5-ene) offers a unique opportunity to break down polycarbonates via end-group activation or main chain scission pathways as supported by various controls and computational analysis. These energetically competitive processes represent an unprecedented divergent approach to polycarbonate recycling. The trans limonene carbonate can be converted back to its polycarbonate via ring-opening polymerization using the same organocatalyst in the presence of an alcohol initiator, offering thus a potential circular and practical route for polycarbonate recycling.

Bicyclic Guanidine Promoted Mechanistically Divergent Depolymerization and Recycling of a Biobased Polycarbonate

AbstractWe here report the organocatalytic and temperature‐controlled depolymerization of biobased poly(limonene carbonate) providing access to its trans‐configured cyclic carbonate as the major product. The base TBD (1,5,7‐triazabicyclo[4.4.0]dec‐5‐ene) offers a unique opportunity to break down polycarbonates via end‐group activation or main chain scission pathways as supported by various controls and computational analysis. These energetically competitive processes represent an unprecedented divergent approach to polycarbonate recycling. The trans limonene carbonate can be converted back to its polycarbonate via ring‐opening polymerization using the same organocatalyst in the presence of an alcohol initiator, offering thus a potential circular and practical route for polycarbonate recycling.

Polymer Degradation by Synergistic Dual Stimuli: Base Interaction and Photocatalysis to Unlock a Boron Pendant Trigger for Main-Chain Scission

Polymer Degradation by Synergistic Dual Stimuli: Base Interaction and Photocatalysis to Unlock a Boron Pendant Trigger for Main-Chain Scission

Additive Free Crosslinking of Poly-3-hydroxybutyrate via Electron Beam Irradiation at Elevated Temperatures.

When applying electron or gamma irradiation to poly-3-hydroxybutyrate (P3HB), main chain scissions are the dominant material reactions. Though propositions have been made that crosslinking in the amorphous phase of P3HB occurs under irradiation, a conclusive method to achieve controlled additive free irradiation crosslinking has not been shown and no mechanism has been derived to the best of our knowledge. By applying irradiation in a molten state at 195 °C and doses above 200 kGy, we were able to initiate crosslink reactions and achieved gel formation of up to 16%. The gel dose Dgel was determined to be 200 kGy and a range of the G values, the number of scissions and crosslinks for 100 eV energy deposition, is given. Rheology measurements, as well as size exclusion chromatography (SEC), showed indications for branching at doses from 100 to 250 kGy. Thermal analysis showed the development of a bimodal peak with a decrease in the peak melt temperature and an increase in peak width. In combination with an increase in the thermal degradation temperature for a dose of 200 kGy compared to 100 kGy, thermal analysis also showed phenomena attributed to branching and crosslinking.

Mechanisms of Degradation for Polydisulfides: Main Chain Scission, Self-Immolation, Or Chain Transfer Depolymerization.

Organic polydisulfides hold immense potential for the design of recyclable materials. Of these, polymers based on lipoic acid are attractive, as they are based on a natural, renewable resource. Herein, we demonstrate that reductive degradation of lipoic acid polydisulfides is a rapid process whereby the quantity of added initiator relative to the polymer content defines the mechanism of polymer degradation, through the main chain scission, self-immolation, or "chain transfer" depolymerization. The latter mechanism is defined as the one during which a thiol group released through the decomposition of one polydisulfide chain initiates depolymerization of the neighbor macromolecule. The chain transfer mechanism afforded the highest yields of recovery of the monomer in its pristine form, and just one molecule of the reducing agent to initiate polymer degradation afforded recovery of over 50% of the monomer. These data are important to facilitate the development of polymer recycling and monomer reuse schemes.

Polythioethers bearing side groups for efficient degradation by E1cB reaction: reaction design for polymerization and main-chain scission.

We have previously reported the polycondensation by the tandem reactions of dithiols and α-(bromomethyl)acrylates, consisting of conjugate substitution (SN2' reaction) and conjugate addition (Michael addition) reactions. The resulting polythioethers underwent a main-chain scission (MCS) by E1cB reaction, which is the reverse reaction of conjugate addition, although it was not quantitative due to the equilibrium. Herein, the modification of the structures of polythioethers led to irreversible MCS, whereby the β-positions of ester moieties were substituted with a phenyl group. This slight modification in the polymer structure influenced the monomer structures and polymerization mechanisms. The understanding of reaction mechanisms by model reactions was required to obtain high molecular weights of polythioethers. It was clarified that the consequent additions of 1,4-diazabicyclo[2.2.2]octane (DABCO), 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), and PBu3 were effective to achieve high molecular weight. The resulting polythioethers decomposed by irreversible MCS via E1cB reaction with DBU.

Gaseous- and Condensed-Phase Activities of Some Reactive P- and N-Containing Fire Retardants in Polystyrenes.

Polystyrene (PS) was modified by covalently binding P-, P-N- and/or N- containing fire-retardant moieties through co- or ter-polymerization reactions of styrene with diethyl(acryloyloxymethyl)phosphonate (DEAMP), diethyl-p-vinylbenzyl phosphonate (DEpVBP), acrylic acid-2-[(diethoxyphosphoryl)methylamino]ethyl ester (ADEPMAE) and maleimide (MI). In the present study, the condensed-phase and the gaseous-phase activities of the abovementioned fire retardants within the prepared co- and ter-polymers were evaluated for the first time. Pyrolysis-Gas Chromatography/Mass Spectrometry was employed to identify the volatile products formed during the thermal decomposition of the modified polymers. Benzaldehyde, α-methylstyrene, acetophenone, triethyl phosphate and styrene (monomer, dimer and trimer) were detected in the gaseous phase following the thermal cracking of fire-retardant groups and through main chain scissions. In the case of PS modified with ADEPMAE, the evolution of pyrolysis gases was suppressed by possible inhibitory actions of triethyl phosphate in the gaseous phase. The reactive modification of PS by simultaneously incorporating P- (DEAMP or DEpVBP) and N- (MI) monomeric units, in the chains of ter-polymers, resulted in a predominantly condensed-phase mode of action owing to synergistic P and N interactions. The solid-state 31P NMR spectroscopy, Scanning Electron Microscopy/Energy Dispersive Spectroscopy, Inductively-Coupled Plasma/Optical Emission Spectroscopy and X-ray Photoelectron Spectroscopy of char residues, obtained from ter-polymers, confirmed the retention of the phosphorus species in their structures.

Dependence of Dissolution Kinetics of Main-Chain Scission Type Resists on Molecular Weight

The dissolution kinetics and molecular weight in main-chain scission type resists such as poly(methyl methacrylate) (PMMA) and ZEP520A were investigated using a quartz crystal microbalance (QCM) method and a Gel Permeation Chromatography (GPC) to clarify the effects of the molecular weight after polymer degradation on the dissolution kinetics. G-values of main chain scissions in PMMA and ZEP520A are estimated using GPC measurement. The G-value of ZEP520A showed higher G-values than that of PMMA. There are three molecular weight regions in PMMA, whereas there are only two molecular weight regions in ZEP 520A. The dissolution behavior depends on molecular weight after polymer degradation in both PMMA and ZEP 520A. These results indicate that ZEP520A is better to be better lithographic performance.

Spin-Trapping Analysis of the Thermal Degradation Reaction of Polyamide 66.

The radical mechanisms of the thermal degradation of polyamide 66 (PA66) occurring under a vacuum at a temperature range between 80 °C and 240 °C (which includes the temperature of practical applications) were investigated using a spin-trapping electron spin resonance (ST-ESR) technique, as well as FTIR, TG-DTA, and GPC methods. No significant weight loss and no sign of thermal degradation are observed at this temperature range under oxygen-free conditions, but a slight production of secondary amine groups is confirmed by FTIR. GPC analysis shows a small degradation by the main chain scission. ST-ESR analysis reveals two intermediate radicals which are produced in the thermal degradation of PA66: (a) a ●CH2- radical generated by main chain scission and (b) a -●CH- radical generated by hydrogen abstraction from the methylene group of the main chain. The ST-ESR result does not directly confirm that a -NH-●CH- radical is produced, although this reaction has been previously inferred as the initiation reaction of the thermal degradation of PA; however, the presence of -●CH- radicals strongly suggests the occurrence of this initiation reaction, which takes place on the α-carbon next to the NH group. The ST-ESR analysis reveals very small levels of reaction, which cannot be observed by common analytical methods such as FTIR and NMR.

Understanding radiation-thermal aging of polydimethylsiloxane rubber through molecular dynamics simulation

The effect of radiation-thermal aging on the structure and properties of polydimethylsiloxane (PDMS) rubber at the micro-scale was investigated through molecular dynamics simulation. The aged PDMS models were constructed by incorporating the aging-induced chemical changes (hydroxyl groups, cross-linking, and scission of main chain). The simulation results show that the introduction of hydroxyl groups and cross-linking in molecular chains lower the chain mobility and the diffusion of the chains and oxygen molecules owing to the strong intermolecular interactions and long-chain structure, respectively. The introduction of short chains caused by the scission of main chains can enhance the mobility, diffusion, and flexibility of the chains and the diffusion range of oxygen molecules, resulting in the decrease in the free volume and Tg. In addition, the hardening effect of cross-linking and the softening effect of scission of main chain collectively contribute to the degradation of mechanical properties of the PDMS rubber.