Dynamic Imine Bonds Research Articles

Makromolekulare Hydrazon Hauptkettenphotoschalter

AbstractHydrazone – bestehend aus einer dynamischen Iminbindung und einem sauren NH‐Proton – haben sich in letzter Zeit aufgrund ihrer Fähigkeit zur Bildung thermisch bistabiler (Z) bzw. (E) Isomere als vielseitige Potoschalter erwiesen. In diesem Artikel stellen wir zwei lichtsensitive Homopolymere vor, die strukturell unterschiedliche Hydrazone als sich wiederholende Einheiten in der Hauptkette enthalten und durch „Kopf‐Schwanz“‐Polymerisation mittels azyklischer Dien‐METathese (ADMET) synthetisiert wurden. Der Hauptunterschied der beiden Polymere liegt dabei im Design des Hydrazon‐Photoschalters, insbesondere in der Lage des aliphatischen Arms, der den Rotor mit dem aliphatischen Polymerrückgrat verbindet. Es zeigt sich, dass sich das hydrodynamische Volumen der Polymere in verdünnter Lösung während der Photoisomerisierung (λ=410 nm) in entgegengesetzter Weise ändert. Darüber hinaus zeigt sich nach der Photoisomerisierung eine Veränderung der Glasübergangs‐temperatur (Tg) um fast 10 °C bei beiden Polymeren, unabhängig vom Design des jeweiligen Hydrazon‐Monomers. Die hier vorgestellte Designstrategie erlaubt es, makromolekulare Eigenschaften durch Strukturänderungen photochemisch zu kontrollieren.

Main-chain Macromolecular Hydrazone Photoswitches.

Hydrazones-consisting of a dynamic imine bond and an acidic NH proton-have recently emerged as versatile photoswitches underpinned by their ability to form thermally bistable isomers, (Z) and (E), respectively. Herein, we introduce two photoresponsive homopolymers containing structurally different hydrazones as main-chain repeating units, synthesized via head-to-tail Acyclic Diene METathesis (ADMET) polymerization. Their key difference lies in the hydrazone design, specifically the location of the aliphatic arm connecting the rotor of the hydrazone photoswitch to the aliphatic polymer backbone. Critically, we demonstrate that their main photoresponsive property, i.e., their hydrodynamic volume, changes in opposite directions upon photoisomerization (λ=410 nm) in dilute solution. Further, the polymers-independent of the design of the individual hydrazone monomer-feature a photoswitchable glass transition temperature (Tg ) by close to 10 °C. The herein established design strategy allows to photochemically manipulate macromolecular properties by simple structural changes.

Reversible emulsification based on pH-responsive surfactant with dynamic imine bond

Efficient demulsification of emulsions is crucial in heavy oil transportation, interfacial reactions, and emulsion polymerization applications. Surfactants that are responsive to the environment have the ability to alter their surface activity in a dynamic manner. However, these types of surfactants typically require a significant amount of energy and may be less efficient. The present study involved the preparation of a surfactant with dynamic covalent bonding and pH-responsive property. This was achieved through the creation of dynamic imine bonds between decyloxybenzaldehyde (DOBA) and sodium 3-amino-1-propanesulfonate (AP) via a Schiff base reaction to synthesize dynamic covalent surfactant CB. The present study aimed to investigate the impact of various pH environments on the conversion rate and interfacial tension of surfactant CB. To achieve this objective, 1H NMR, FTIR, and IFT tests were conducted. The results confirmed the reversible conversion of the dynamic imine bond with pH. Furthermore, the dynamic imine bonded surfactant demonstrated the ability to complete at least three emulsification-breakage cycles. The present investigation introduces a novel approach for the synthesis of pH-responsive surfactants through the utilization of dynamic covalent bonds.

Multifunctional hydrogel for synergistic reoxygenation and chemo/photothermal therapy in metastatic breast cancer recurrence and wound infection

Metastatic recurrence and postoperative wound infection are two major challenges for breast cancer patients. In this study, a multifunctional responsive hydrogel system was developed for synergistic reoxygenation and chemo/photothermal therapy in metastatic breast cancer and wound infection. The hydrogel system was obtained by cross-linking Prussian blue-modified N-carboxyethyl chitosan (PBCEC) and oxidized sodium alginate using the amino and aldehyde groups on the polysaccharides, resulting in the formation of responsive dynamic imine bonds. Conditioned stimulation (e.g., acid microenvironment) enabled the controlled swelling of hydrogels as well as subsequent slow release of loaded doxorubicin (DOX). Additionally, this hydrogel system decomposed endogenous reactive oxygen species into oxygen to relieve the hypoxic tumor microenvironment and promote the healing of infected-wounds. Both in vitro and in vivo experiments demonstrated the synergistic reoxygenation and chemo/photothermal effects of the PB/DOX hydrogel system against metastatic breast cancer and its recurrence, as well as postoperative wound infection. Thus, the combination of reoxygenation and chemo/photothermal therapy represents a novel strategy for treating and preventing tumor recurrence and associated wound infection.

A self-healable and highly flame retardant TiO2@MXene/P, N-containing polyimine nanocomposite for dual-mode fire sensing

Scalable early fire warning sensors that integrate passive flame-retardant strategies and active fire alarm sensing are highly requirable in diverse areas. However, the single-mode fire response and performance degradation or even malfunction under mechanical damages remain key challenges for current early fire warning sensors. Herein, a self-healable and flame retardant TiO2@MXene/P, N-containing polyimine nanocomposite (TMPNP) is designed as repeatable dual-mode fire warning sensors. On the one hand, the TMPNP exhibits a high temperature coefficient of resistance of −2.21 %/℃, which endows nanocomposite with an extraordinarily sensitive non-contacting fire warning performance; on the other hand, the TMPNP demonstrates outstanding fire contact warning features of responsive time being less than 1 s and repeatable alarming response, as well as excellent flame-retardant functionality of LOI reaching up to 32.0 %. Moreover, the TMPNP can be effectively recovered after 30 cutting-and-healing cycles with a healing efficiency of 89.3 % via dynamic imine bonds. Particularly, its flame retardancy and fire warning properties in contact/non-contact mode are well maintained after self-healing. These merits support the scalable assembly TMPNP onto cotton textile to develop a wireless household fire early warning sensor linked to the Internet of Things, contributing to construct fire-safety smart buildings. This work provides a scalable strategy for preparing healable sensitive fire warning sensors, opening up a new opportunity for the practical application of eliminating indoor fire hazards.

All-natural chitosan-based polyimine vitrimer with multiple advantages: A novel strategy to solve nondegradable plastic waste pollution

The increasing amount of nondegradable petroleum-based plastic waste releases chemical hazards, posing a significant threat to the environment and human health. Chitosan, derived from marine wastes, is an attractive feedstock for the preparation of plastic replacement due to its renewable and degradable nature. However, in most cases, complex chemical modifications of chitosan or hybridization with chemicals from fossil resources are required. Herein, we present a high-performance chitosan-based polyimine vitrimer (CS-PI) through a mild and catalyst-free Schiff base reaction between chitosan and vanillin. The CS-PI were formed by integrating dynamic imine bonds into the polymer networks, resulting in superior thermo-processability and mechanical performances. The tensile strength and Young’s modulus of the CS-PI films reached 38.72 MPa and 3.20 GPa, respectively, which was significantly higher than that of both commercial petroleum-based plastics and bioplastics. Additionally, the CS-PI films exhibited good light transmittance, self-healing ability, reprocess capacity, water resistance, and durability to various organic solvents. Moreover, the CS-PI films could be completely degraded under both acidic and natural conditions, enabling a sustainable circulation. Therefore, this work offers a new design strategy for developing all-natural environmentally friendly polymers as sustainable replacements for petroleum-based plastics, thus reducing the accumulation of nondegradable plastic waste.

Fully Biobased and Mechanically Robust Elastomeric Vitrimer based on Epoxidized Natural Rubber and Dynamic Imine Bonds

Fully Biobased and Mechanically Robust Elastomeric Vitrimer based on Epoxidized Natural Rubber and Dynamic Imine Bonds

The Preparation and Properties of Amino-Carboxymethyl Chitosan-Based Antibacterial Hydrogel Loaded with ε-Polylysine.

In this paper, amino-carboxymethyl chitosan (ACC) was prepared through amino carboxymethylation, which introduces -COOH and -NH2 groups to the chitosan (CS) chains. Meanwhile, dialdehyde starch (DAS) was produced by oxidizing corn starch using sodium periodate. To attain the optimal loading and long-time release of ε-polylysine (ε-PL), the ACC/DAS hydrogels were synthesized through the Schiff base reaction between the amino group on ACC and the aldehyde group in DAS. The molecular structure, microcosmic properties, loading capacity, and bacteriostatic properties of the four types of hydrogels containing different mass concentrations of ACC were investigated. The results showed that the dynamic imine bond C=N existed in the ACC/DAS hydrogels, which proved that the hydrogels were formed by the cross-linking of the Schiff base reaction. With the increasing mass concentration of the ACC, the cross-sectional morphology of the hydrogel became smoother, the thermal stability increased, and the swelling behavior was gradually enhanced. The tight network structure improved the ε-PL loading efficiency, with the highest value of 99.2%. Moreover, the loading of ε-PL gave the hydrogel good antibacterial properties. These results indicate that ACC/DAS hydrogel is potential in food preservation.

Development of vanillin-based crosslinking agent with phase-locked dynamic imine bonds for shape-memory polyurethanes

Scientists have gained so much attention due to one of polyurethane's tuneable and unique characteristics, i.e., the shape-memory effect. But, the simplistic fabrication of such materials persists a challenging work. In the current work, we report the synthesis and detailed characterization of shape-memory polyurethane obtained by incorporating lower concentrations of imine bonds, which readily undergo phase-locking systems beneficial for microphase separation. A symmetric imine moiety (DIM) was designed through the simple Schiff-base reaction of naphthalene diamine and vanillin. The tensile strength of the films reached up to 30 MPa with an %elongation of 87% by tuning the hard segment content and the concentration of imine bonds. Also, in the meantime, the films showed excellent shape memory effect, where the shape could recover its original form within 2 s at the highest loading of DIM. The remarkable balance between the mechanical properties and the shape memory effect could be associated with the two main factors. Firstly, incorporating symmetrical and rigid CN along with the naphthalene promoted the microphase separation and smoothened the orientation of the chain segments at the strain-induced crystallization at lower temperatures. Secondly, the phase locking occurred at lowered temperatures due to dynamic imine exchange reaction and H-bonding. The phase recovery started, and chains settled at their original positions once the temperature was elevated.

Dual-network hydrogels based on dynamic imine and borate ester bonds with antibacterial and self-healing properties.

Polymeric hydrogel materials with multiple functions are in great demand in practical biomedical scenarios. In this work, a self-healing hydrogel with both antimicrobial properties was prepared using a strategy that combines dynamic imine and borate ester bonds. In this hydrogel, polyvinyl alcohol (PVA) is used as the base network, and borax solution as the cross-linking agent, and borate ester bonds can be formed between these two. Dialdehyde carboxymethyl cellulose (DCMC) was selected to cross-link with the amino groups in carboxymethyl chitosan (CMCS) and polyethyleneimine (PEI) to form dynamic imine bonds. The PVA/PEI/DCMC/CMCS hydrogels prepared by double chemical cross-linking have good mechanical properties (maximum tensile strength up to 289 KPa and strain at the break up to 1025%). Due to the uniqueness of the two chemical bonds, the hydrogel material is self-healing at room temperature without additional stimulation. In addition, the inherent antibacterial properties of the raw materials in this hydrogel confer antibacterial properties, with a kill rate of up to 99% against E. coli and S. aureus. The multifunctional hydrogels developed in this study provide more ideas and references for the future application of hydrogel materials in practical scenarios.

Enhancing hydrolytic stability of dynamic imine bonds and polymers in acidic media with internal protecting groups

Imine bonds are among the most explored building motifs in dynamic chemistry, polymers, and materials, and yet, their acid-resistance remains a longstanding issue. Herein we demonstrate a concept of internal protecting groups for improving the kinetic stability of dynamic imine bonds and polymers. Systematic examination of structure-reactivity relationship of a series of aldehydes/imines bearing a neighboring carboxyl allowed uncovering of required structural features for dynamically masking imine bonds with cyclic structures. Mechanistic studies indicated that noncovalent interactions along with sterics control the ring-chain equilibrium and the stability of imine bonds. The incorporation of internal protecting groups into imine polymers further enabled their controlled stability in acidic media. Moreover, a combination of dynamic covalent network and coordination supramolecular network provided a facile means for the modulation of luminescent and mechanical properties of polymers. The strategies and results reported should be beneficial to molecular assemblies, dynamic polymers, biological delivery, and intelligent materials.

Chitosan-Based Self-Healing Hydrogel: From Fabrication to Biomedical Application.

Biocompatible self-healing hydrogels are new-generation smart soft materials that hold great promise in biomedical fields. Chitosan-based self-healing hydrogels, mainly prepared via dynamic imine bonds, have attracted broad attention due to their mild preparation conditions, excellent biocompatibility, and self-recovery ability under a physiological environment. In this review, we present a comprehensive overview of the design and fabrication of chitosan-based self-healing hydrogels, and summarize their biomedical applications in tissue regeneration, customized drug delivery, smart biosensors, and three/four dimensional (3D/4D) printing. Finally, we will discuss the challenges and future perspectives for the development of chitosan-based self-healing hydrogels in the biomedical field.

Sustainable biobased epoxy thermosets with covalent dynamic imine bonds for green composite development

The widespread use of thermoset composites has raised great environmental and economic awareness, due to the lack of recyclability of the resins and fibres. This work is focused on the development of a sustainable, recyclable and reprocessable epoxy vitrimer by using an epoxidized linseed oil resin and a vanillin derived Schiff base as dynamic hardener. Different epoxy:dynamic hardener ratios have been employed, analyzing its influence in the final physico-chemical properties of the thermoset resin. The reprocessability and the recyclability of the vitrimers have been demonstrated. This work also describes the manufacturing and characterization of greener composites obtained with these vitrimers and flax yarn fibres. A good adhesion between the fibres and the vitrimer is observed. Moreover, the characterization of developed composites indicated that their thermal and mechanical properties allow their use in lightweight engineering applications. Finally, chemical recyclability and compostability of the flax fibre vitrimer-based composite have been demonstrated.

Preparation and properties of UV curable room temperature repair bio‐based poly(siloxane‐urethane) coating

AbstractIn order to improve the problems of traditional self‐healing polyurethane coating, such as harsh curing and repair conditions, poor hydrophobicity, thermal stability and human affinity, the prepolymer was synthesized by bio‐based polyol castor oil (CO), self‐made alkyl aryl modified silicone polyol (SiOH) and isophorone diisocyanate (IPDI), and dimethylglyoxime (DOU) was used as chain extender containing dynamic imine bond, methacrylic acid 2‐(tert‐butylamino)ethyl ester (MABE) was used as a capping agent containing acrylate and high steric hindrance amino structure, and a UV curable room temperature repair bio‐based poly(siloxane‐urethane) coating (PUESi) was prepared by stepwise polymerization. In this article, the effect of the amount of DOU on the properties of PUESi was studied. The results showed that PUESi with different dosages of DOU could achieve self‐healing at room temperature. The maximum repair efficiency of full cut could reach 91.8%, and the scratch repair efficiency could reach 99.3%; The contact angle meter and synchronous thermal analyzer proved that the introduction of SiOH significantly improved the hydrophobicity, oil repellency and thermal stability of PUESi. The water contact angle increased from 78° to 104°, the oil contact angle increased from 63° to 91°, and the 5% thermal decomposition temperature increased from 179 to 196°C. The coating has broad application prospects in the fields such as antifouling and anticorrosion, building material protection, and automotive instruments.

Highly Efficient One-Pot Synthesis of Hexakis(m-phenyleneimine) Macrocyle Cm6 and the Thermostimulated Self-Healing Property through Dynamic Covalent Chemistry.

Highly efficient one-pot synthesis of hexakis(m-phenyleneimine) macrocycle Cm6 from acetalprotected AB-type monomer, m-aminobenzaldehyde diethylacetal, was successfully achieved based on imine dynamic covalent chemistry and precipitation-driven cyclization. The structure of Cm6 in the solid state was determined using CP/MAS NMR, X-ray single crystallographic analysis, and WAXD. Macrocycle Cm6 is composed of six phenylene and imine bonds facing the same direction, with nitrogen atoms arranged on the outside of the ring, and has a chair conformation, as predicted from DFT calculation. The macrocycle forms π-stacked columnar aggregates and hexagonally closest-packed structure. The cyclization process was investigated using MALDI-TOF MS and NMR. A mechanism of precipitation-driven cyclization based on imine dynamic covalent chemistry and π-stacked columnar aggregation is proposed. Both the nature of imine linkage and the shape anisotropy of the macrocycle played an important role in the single one-pot synthesis. The water-mediated mutual conversion between macrocycle Cm6 and linear oligomers driven by thermal stimulation was analyzed using MALDI-TOF MS and GPC methods. Macrocycle Cm6 with a dynamic covalent imine bond exhibited self-healing properties when stimulated using heat.

Self-Healable and Reprocessable Silicon Elastomers Based on Imine-Boroxine Bonds for Flexible Strain Sensor.

Silicon elastomers with excellent self-healing and reprocessing abilities are highly desirable for the advancement of next-generation energy, electronic, and robotic applications. In this study, a dual cross-linked self-healing polysiloxane elastomer was facilely fabricated by introducing an exchangeable imine bond and boroxine into polydimethylsiloxane (PDMS) networks. The PDMS elastomers exhibited excellent self-healing properties due to the synergistic effect of dynamic reversible imine bonds and boroxine. After healing for 2 h, the mechanical strength of the damaged elastomers completely and rapidly recovered at room temperature. Furthermore, the prepared PDMS elastomers could be repeatedly reprocessed multiple times under milder conditions without significant degradation in mechanical performance. In addition, a stretchable and self-healable electrical sensor was developed by integrating carbon nanotubes (CNTs) with the PDMS elastomer, which can be employed to monitor multifarious human motions in real time. Therefore, this work provides a new inspiration for preparing self-healable and reprocessable silicone elastomers for future flexible electronics.

Thermally Reversible Cross-Linking of Recyclable Polyamide Materials Based on Schiff Base and Diels-Alder Reactions.

Recyclability of cross-link polymer materials is essential to alleviate environmental pollution caused by discarded or damaged polymers. Herein, a facile method for producing recyclable polyamide materials is developed. Linear polymer chains are constructed by Schiff base reaction between glutaraldehyde (GD) and furandiamine (FD). The linear polymer chains are crosslinked by bismaleimide (BM) to give rise to polyamide material, named GF-BMs. The resulting GF-BMs polyamide material possesses strong tensile strength (78MPa) and good solvent resistance from room temperature to 135 °C. Especially, the thermally reversible Diels-Alder covalent bonds and dynamic imine bonds in the polymer network have a synergistic effect on fast-reprocessing, self-healing, and recyclability, which provides a new idea for recyclable materials.

Adsorption of Aldehyde-Functional Diblock Copolymer Spheres onto Surface-Grafted Polymer Brushes via Dynamic Covalent Chemistry Enables Friction Modification.

Dynamic covalent chemistry has been exploited to prepare numerous examples of adaptable polymeric materials that exhibit unique properties. Herein, the chemical adsorption of aldehyde-functional diblock copolymer spherical nanoparticles onto amine-functionalized surface-grafted polymer brushes via dynamic Schiff base chemistry is demonstrated. Initially, a series of cis-diol-functional sterically-stabilized spheres of 30-250 nm diameter were prepared via reversible addition-fragmentation chain transfer (RAFT) aqueous dispersion polymerization. The pendent cis-diol groups within the steric stabilizer chains of these precursor nanoparticles were then oxidized using sodium periodate to produce the corresponding aldehyde-functional spheres. Similarly, hydrophilic cis-diol-functionalized methacrylic brushes grafted from a planar silicon surface using activators regenerated by electron transfer atom transfer radical polymerization (ARGET ATRP) were selectively oxidized to generate the corresponding aldehyde-functional brushes. Ellipsometry and X-ray photoelectron spectroscopy were used to confirm brush oxidation, while scanning electron microscopy studies demonstrated that the nanoparticles did not adsorb onto a cis-diol-functional precursor brush. Subsequently, the aldehyde-functional brushes were treated with excess small-molecule diamine, and the resulting imine linkages were converted into secondary amine bonds via reductive amination. The resulting primary amine-functionalized brushes formed multiple dynamic imine bonds with the aldehyde-functional diblock copolymer spheres, leading to a mean surface coverage of approximately 0.33 on the upper brush layer surface, regardless of the nanoparticle size. Friction force microscopy studies of the resulting nanoparticle-decorated brushes enabled calculation of friction coefficients, which were compared to that measured for the bare aldehyde-functional brush. Friction coefficients were reasonably consistent across all surfaces except when particle size was comparable to the size of the probe tip. In this case, differences were ascribed to an increase in contact area between the tip and the brush-nanoparticle layer. This new model system enhances our understanding of nanoparticle adsorption onto hydrophilic brush layers.

A synergistic effect of MXene/MWCNT enables self-healable and low percolation elastomer sensor: A combined experiment and all-atom molecular dynamics simulation

Flexible conductive composites have garnered significant interest as wearable strain sensors due to their potential in diverse application fields, such as next-generation robotics automation, electronic skin, and human body detection. Nevertheless, the practical usage scenarios often entail tear and fracture, posing a persistent challenge to their performance stability. Consequently, there exists a pressing demand to engineer conductive composites that not only exhibit flexibility, stretchability, and sensing capabilities but also demonstrate effective self-healing properties. This pursuit presents a formidable task, considering the complex requirements imposed on the materials. This study presents a novel methodology for fabricating a conductive silicone elastomer composite. The approach involves the initial utilization of an excess of amino-capped Polydimethylsiloxane (PDMS) to undergo a reaction with Toluene diisocyanate (TDI) and Isoflurane diisocyanate (IPDI), leading to the formation of amino-capped polyurea (TPU). Subsequently, the self-healing elastomer IPDI/TDI/TA (ITT) is synthesized through chain expansion with Terephthalaldehyde (TA) in tetrahydrofuran (THF). To complement the experimental investigations, all-atom molecular dynamics (AAMD) simulations are employed to develop a comprehensive model elucidating the mechanical characteristics and self-healing capabilities of the elastomers. To introduce hybrid fillers into the elastomer composite, MXenes are electrostatically modified with L-glutamine (negatively charged), while multi-walled carbon nanotubes (MWCNTs) are modified with cetyltrimethylammonium bromide (positively charged). These modified fillers self-assemble in water and are subsequently combined with the aforementioned rubber THF solution after drying. Through ultrasonication and drying, the ITT elastomers/MXene/MWCNT composite is successfully prepared. The formation of hydrogen bonds and dynamic imine bonds is verified using FT-IR and AAMD simulations. By maintaining a fixed ratio of TA to PDMS, we observe a positive correlation between TDI concentration and the fracture strength of the ITT elastomer, while the elongation at break decreases with increasing TDI. Furthermore, the successful modification of MXene and MWCNT is confirmed through FT-IR, XRD, and zeta potential measurements, and the resulting composites exhibit significant electrical conductivity and self-healing properties. Tensile and electrochemical measurements demonstrate that the composite possesses promising characteristics suitable for sensor applications, and its mechanical and electrical properties can recover after self-healing. Notably, the resulting composites exhibit a very low conductivity threshold (3.5 wt%). The composite system exhibits desirable tensile properties and efficient self-healing ability due to the reversibility of multiple hydrogen and imine bonds. Overall, this study aims to introduce a novel elastomer-based composite that possesses excellent properties in terms of self-healing, stretchability, sensing capability and flexibility.

Robust, waterproof, and degradable cellulose-based polyimine vitrimer for plastic replacement

The increasing application of non-degradable petroleum-based plastics has resulted in serious environmental pollution. Renewable and degradable cellulose-based materials are attractive candidates to replace petroleum-based plastics. However, the strong hydrogen bond (H-bond) interactions of cellulose result in its poor plastic formability such as thermoforming. Herein, a novel and effective thermoforming method of cellulose was developed by modifying cellulose into cellulose-based polyimine vitrimer (Cel-PI) via introducing dynamic imine bonds between cellulose molecular chains using Schiff-base reaction. The presence of dynamic imine bonds enables the heat-induced dynamic exchange and stress relaxation behavior of Cel-PI networks, making the excellent thermo-processability of Cel-PI. The as-synthesized Cel-PI powders can be easily thermoformed into Cel-PI film via heat-pressing approach without adding any plasticizers. The resulting Cel-PI film exhibits high Young’s modulus (2.9 GPa), high strength (46.3 MPa), good self-healing ability, excellent waterproof capability and organic solvent stability, and high thermal stability. Moreover, it can be chemically degraded, biodegraded, and recycled. Therefore, Cel-PI represents a promising and sustainable petroleum-based plastic replacement.