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Shhyam Khairkkar, [Amol V. Pansare](https://orcid.org/0000-0001-8133-1685), Shubham V. Pansare, Shraddha Y. Chhatre, Junji Sakamoto, Michel Barbezat, Giovanni P. Terrasi, Vishwanath R. Patil, Amit A. Nagarkar, [Masanobu Naito](https://orcid.org/0000-0001-7198-819X)

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[Adhesive-less bonding of incompatible thermosetting materials](https://mdr.nims.go.jp/datasets/056d3f13-dd50-488d-961a-e67a035443a8)

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Adhesive-less bonding of incompatible thermosetting materialsRSCApplied PolymersPAPERCite this: RSC Appl. Polym., 2025, 3,247Received 18th September 2024,Accepted 3rd December 2024DOI: 10.1039/d4lp00288arsc.li/rscapplpolymAdhesive-less bonding of incompatiblethermosetting materials†Shhyam Khairkkar,‡a Amol V. Pansare, *‡b Shubham V. Pansare,‡cShraddha Y. Chhatre,d Junji Sakamoto,e Michel Barbezat,b Giovanni P. Terrasi,bVishwanath R. Patil,c Amit A. Nagarkar*f and Masanobu Naito *aWe show that dynamic covalent exchange at the interface of two thermosetting polymers results instrong bonding between the materials via creation of a new material at the interface. Thus, polymers ofsignificantly different polarities can be bonded without the use of adhesives. We also show that suchdynamic covalent exchange is not only limited to the interface but also penetrates into the bulk material(ca. 20 microns), thereby creating a strong bond. The creation of a new material at the interface wasconfirmed by Energy Dispersive X-ray (EDX) elemental mapping as well as a new glass transition tempera-ture at the interface. Using this phenomenon, we show that hydrophobic, compliant polymers can also beused as adhesives for polar, stiff materials. We also show that such dynamic exchange also takes place inthe presence of fillers like nano-silica. Lastly, using this technique, we demonstrate the adhesive-less fab-rication of layered materials where each layer has vastly different polarities and mechanical properties,thereby tuning the failures modes of the resulting composite material.1. IntroductionAdhesives have enabled complex manufacturing processes viabonding between materials such as composites, polymers,metals etc.1–4 New adhesive technologies have generated hugeinterest in the field of composite structure bonding, biologicaltissue bonding, etc.5 Good interfacial compatibility is extre-mely important to obtain a strong bond.6–9 In this report, weshow that dynamic covalent bond exchange can occur betweentwo thermosetting polymers of significantly different polaritiesas well as mechanical properties (Fig. 1). Bond exchangebetween such “incompatible” polymeric networks leads to anexchange zone, i.e. a region where bond exchange creates anew material that penetrates into the bulk region of the ther-mosets. This type of adhesive-less bonding is fundamentallydifferent from other bonding techniques such as melt-bonding of thermoplastic polymers10,11 which melt and createbonds due to van der Waals attractive forces, intercalation ofpolymer chains at the interface,12 or partially uncuredadhesives which cure on heating.13Bonding of thermosetting polymers of significantlydifferent polarities as well as mechanical properties is challen-ging and an unsolved problem in the field14 and hence, use ofexternal adhesives is necessary. Surface wetting of the appliedadhesive is a key parameter for obtaining a strong bond.15–17However, when polymeric materials differ drastically in theirsurface properties (e.g. hydrophobic, low surface energy silox-anes and hydrophilic, high surface energy epoxy thermosets),obtaining adhesion between these dissimilar materials is notstraightforward. Moreover, unlike bonding of thermoplasticpolymers where heat assisted bonding can be used, thermoset-ting polymers do not go through a melting phase needed forheat assisted bonding.18Here, we show strong adhesive-less bonding between twoanhydride-cured epoxy thermosetting networks significantlydiffering in mechanical and chemical properties usingdynamic covalent bond exchange. In recent years, epoxy ther-mosets have become the materials of choice to fabricate com-†Electronic supplementary information (ESI) available: Experimental methods,figures, tables and videos. See DOI: https://doi.org/10.1039/d4lp00288a‡These authors contributed equally.aData–Driven Polymer Design Group, Research and Services Division of MaterialsData and Integrated System (MaDIS), National Institute for Materials Science(NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan.E-mail: NAITO.Masanobu@nims.go.jpbComposites Group, Mechanical Systems Engineering, Swiss Federal Laboratories forMaterials Science and Technology-Empa ETH Domain, 8600 Dübendorf, Switzerland.E-mail: amol.pansare@empa.chcDepartment of Chemistry, University of Mumbai, Santacruz (E), Mumbai 400098,IndiadNational Chemical Laboratory (NCL), Dr Homi Bhabha Road, Pune- 411008, IndiaeAdvanced Material Technology Center, Technology Division, Panasonic HoldingsCorporation, 1006 Kadoma, Kadoma City, Osaka 571-8508, JapanfDatacule Inc., 529 Main St., Boston, Massachusetts, 02129, USA.E-mail: amit@datacule.com© 2025 The Author(s). Published by the Royal Society of Chemistry RSCAppl. Polym., 2025, 3, 247–256 | 247http://rsc.li/rscapplpolymhttp://orcid.org/0000-0001-8133-1685http://orcid.org/0000-0001-7198-819Xhttps://doi.org/10.1039/d4lp00288ahttps://doi.org/10.1039/d4lp00288ahttp://crossmark.crossref.org/dialog/?doi=10.1039/d4lp00288a&domain=pdf&date_stamp=2025-01-18posites reinforced with carbon fibres, glass fibres, nano-particles, etc.19–22The push towards green energy in automotive and aero-space sectors has led to widespread adoption of fiberreinforced composites which have a favorable strength-to-weight ratio.23 Adhering thermosetting epoxy polymers typi-cally requires adhesives; however, conventional adhesivesoften fail to establish a strong bond between materials withsignificantly different polarities and mechanical properties.Recently, there has been a shift towards innovative adhesiontechnologies, such as induction welding and microwave curingetc.24–27 to replace traditional adhesives.Leibler et al. introduced the concept of vitrimers, wheredynamic covalent bond exchange leads to reprocessable ther-mosetting polymers.28,29 In subsequent works on vitrimers,research groups applied the concept of dynamic covalentexchange between thermosets to create mechanically strongmaterials that can be recycled and reprocessed.30 Otsuka et al.recently published a report where they used intercalation ofpolymer chains to create bonding between thermosets usingdisulfide bond exchange.12 The mechanism of transesterifica-tion in vitrimers has been subject to many studies, and thecurrent assumption is that neighbouring group participationby the –OH group lowers the energy barrier fortransesterification.31Dynamic covalent bonds, essential for enabling reversiblebond formation and exchange, play a critical role in self-healing materials by facilitating recovery after damage. Thesebonds, often utilized in adhesive materials, allow self-healingpolymers to autonomously repair or re-bond under specificconditions, such as changes in temperature or pH.32,33 Forinstance, Wang et al. discuss how imine-based dynamiccovalent bonds contribute to self-healing in polymerichydrogels, highlighting their flexibility and durability inapplications that require robust yet adaptable bonding,similar to adhesive systems. The reversible nature of thesebonds provides both adhesion and cohesion, allowingmaterials to regain strength and maintain structural integ-rity post-damage.34 Similarly, Liu et al. focus on the appli-cation of dynamic covalent chemistry in creating materialswith both self-healing and recyclable properties, emphasiz-ing how these reversible reactions enable materials torespond dynamically to environmental stressors. Such adap-tability underlines the parallels between self-healing andadhesive systems, as both benefit from dynamic covalentmechanisms that manage stress, reduce crack propagation,and allow for material reformation. Together, these studiesreinforce the multifunctional potential of dynamic covalentbonds in both adhesion and self-healing, supporting therobustness and reusability of these materials.35Here, we show that dynamic covalent bond exchangebetween two thermosetting polymers is possible even in thecase of chemically and mechanically dissimilar thermosettingpolymeric networks, leading a strong bond between the twothermosetting polymers. An exchange zone of ca. 20 micronsis created at the interface of the dissimilar materials, and thiszone creates an adhesive bond between these “incompatible”polymers.Fig. 1 (A) General scheme for curing of the epoxy resins with an anhydride curing agent, hexahydrophthalic anhydride (HHPA), using 2-ethyl-4-methyl imidazole (EMI) as a catalyst. In this work, three polar thermosetting polymers were fabricated (based on bisphenol A (BisA), butane diol(Bdiol) and polypropylene glycol (PPG)) and their dynamic covalent exchange with a non-polar thermoset of epoxy-terminated polydimethylsiloxane(PDMS) was studied. (B) Schematic representation of dynamic covalent exchange taking place between ester bonds at the interface of materials withdrastically different surface energies. (C) Mechanism of dynamic covalent exchange between ester bonds.Paper RSC Applied Polymers248 | RSCAppl. Polym., 2025, 3, 247–256 © 2025 The Author(s). Published by the Royal Society of ChemistryAnhydride-cured epoxy materials are well-known for theirexcellent mechanical properties and UV stability,36 makingthem widely studied and increasingly popular for consumer-grade applications, such as composite automobile wheels thatreduce axle weight and improve fuel efficiency.37–39 In thisstudy, we employ similar anhydride-cured epoxy polymers tohighlight their versatility and applicability in both researchand commercial contexts. When epoxide resins are crosslinkedwith anhydride agents (hexahydrophthalic anhydride (HHPA)),they undergo an exothermic ring-opening reaction, formingester bonds and free hydroxyl groups, which generate a highsurface energy in the resulting thermosetting polymer.40 Thishigh surface energy, however, poses a significant challengewhen bonding with low surface energy materials, such as sili-cone polymers.412. Materials and instrumentationMaterialsAll chemicals were commercially purchased and were used asreceived without further purification. Bisphenol A diglycidylether (DER 332) (epoxide equivalent weight, 172–176), poly(di-methylsiloxane) diglycidyl ether (Mn = 800), 1,4-butanedioldiglycidyl ether, poly-(propylene glycol) diglycidyl ether (Mn =380), hexahydrophthalic anhydride (HHPA), 2-ethyl-4-methyl-imidazole, 4-(dimethylamino)pyridine (DMAP) and dimethylaniline were purchased from Sigma-Aldrich. Nano-silica(AEROSIL-RX-200) was purchased from Evonik OprationGmbH.InstrumentationHeat press. AS-ONE heat press machine was used to performadhesive-less bonding, typically at 160 °C for 4 hours andapplied a weight of 200 kg over 100 mm2 overlap area whichcorresponds to approximately 19.6 MPa pressure on thepolymers.Scanning electron microscopy (SEM) and EDAX analysis.Low vacuum SEM analysis was conducted using TM 3000(Hitachi High-Technologies Co., Japan). Carbon and siliconmaps were generated using the in-built EDX gun.Differential scanning calorimetry (DSC). A SHIMADZUDSC-60 plus instrument was used to monitor the thermalproperty and glass transition temperatures (Tg) of all polymersunder nitrogen atmosphere in a temperature range was of−40 °C to 200 °C with a heating and cooling rate of 10 °Cmin−1. Aluminium pans were used as sample holders. The Tgvalues were determined as the midpoint of the slope of phasetransition.Mechanical analysis. Mechanical analysis of softer sampleswas performed with a Texture Analyser (Stable Micro Systems)with a load cell of 5 N and that of harder samples was per-formed with a Shimadzu AG-X plus lap-shear tester with a loadcell of 10 kN. Mechanical analysis of samples requiring higherloads was performed with a Shimadzu autograph AG-X plusinstrument with a 10 kN load cell. The crosshead speed was2 mm min−1 for all cases. The reported values are the averageof four measurements with standard error for each study. Theadhesive strength was determined using following equation:Adhesive strength ðMPaÞ ¼ Load ðNÞ=Area ðmm2Þ:Here, area = 10 mm × 10 mm = 100 mm2.Contact angle. Water contact angles were measured using acontact angle meter (Kyowa Interface Science, Co Ltd, Japan,Model DMs-401). Distilled water (5 µL) was used as the probeliquid. The images were analyzed for the measurement of thecontact angles in Surftens software version 4.5. The measure-ment of the contact angle was given by measurements of thediameter of the base of the drop and the height of the drop,where each drop was measured five times.Three point bending tests. Texture Analyzer (Stable MicroSystem, TA.XTplusC Texture Analyser) was used with a three-point probe with a 50 N load cell for 3-point bending tests.3. Experimental proceduresSynthesis of BisA thermosetting polymer (Fig. S1 and S2†)A 9.0 g of epoxy resin and 7.5 g of hexahydrophthalic anhy-dride (HHPA) were thoroughly mixed in a Teflon beaker withaddition of 0.450 g of 2-ethyl-4-methylimidazole (5% by weightof epoxy resin) as a catalyst. This mixture was placed in avacuum desiccator for 10 min to remove air bubbles. Themixture was then poured in a rectangular shaped PTFE mold.The thickness of the parts was kept at 2 mm (similar withindustry standards). Curing conditions: 120 °C, 4 h.Synthesis of Bdiol thermoseting polymer (Fig. S3 and S4†)A 5.0 g of epoxy resin and 7.5 g of hexahydrophthalic anhy-dride were thoroughly mixed in a Teflon beaker with additionof 0.250 g of 2-ethyl-4-methylimidazole (5% by weight of epoxyresin) as a catalyst. This mixture was placed in a vacuum desic-cator for 10 min to remove air bubbles. The mixture was thenpoured in a rectangular shaped PTFE mold. The thickness ofthe parts was kept at 2 mm (similar with industry standards).Curing conditions: 120 °C, 4 h.Synthesis of PPG thermosetting polymer (Fig. S5 and S6†)A 3.2 g of epoxy resin and 1.54 g of hexahydrophthalic anhy-dride were thoroughly mixed in a Teflon beaker with additionof 0.160 g of 2-ethyl-4-methylimidazole (5% by weight of epoxyresin) as a catalyst. This mixture was placed in a vacuum desic-cator for 10 min to remove air bubbles. The mixture was thenpoured in a rectangular shaped PTFE mold. The thickness ofthe parts was kept at 2 mm (similar with industry standards).Curing conditions: 120 °C, 4 h.Synthesis of PDMS thermosetting polymer (Fig. S5 and S6†)8 g of poly(dimethylsiloxane) diglycidyl ether and 3.2 g of hexa-hydrophthalic anhydride were taken in a Teflon beaker withaddition of 0.40 g of 2-ethyl-4-methylimidazole (5% by weightof epoxy resin) as catalyst. This mixture was placed in aRSC Applied Polymers Paper© 2025 The Author(s). Published by the Royal Society of Chemistry RSCAppl. Polym., 2025, 3, 247–256 | 249vacuum desiccator for 30 min to remove air bubbles. Themixture was then poured in a rectangular shaped PTFE mold.The thickness of the parts was kept at 2 mm. Curing con-ditions: 140 °C, 4 h.4. Results and discussionAcid and anhydride curing agents are extensively used indust-rially to crosslink epoxy resins.42 These curing agents, whenused with a catalyst, lead to a nucleophilic ring opening reac-tion to generate an ester bond. Transesterification in acidcured crosslinked epoxy networks has been used to demon-strate recyclability of these “vitrimers” – a class of thermoset-ting polymers that undergo dynamic bond exchange withoutgoing through a phase transition.28,29,43In this report, commercially available epoxy resins werecured with HHPA (hexahydro phthalic anhydride) using EMI(2-ethyl-4-methyl-imidazole) as the catalyst (Fig. 1).30 Todemonstrate dynamic covalent exchange in incompatible poly-mers, we chose a siloxane-based epoxide that, when cured, hasa significantly different water contact angle and mechanicalproperties as compared to the other epoxy resins (Fig. 2, S9–S11 and Table S1†). Siloxanes are widely used in materialscience, industrial components and in biological devices.44,45Due to the immiscibility of siloxanes with other commoncarbon-rich polymers, they are incompatible for processingwith other polymers and commonly phase separate from poly-meric mixtures.46,47 Bonding such siloxanes to common poly-mers is very difficult due to the hydrophobic nature of thesiloxane groups.48,49 While this property is an advantage formany applications such as soft-lithography,50 it is detrimentalin cases where bonding of these siloxane polymers to othersubstrates is required. Another property of siloxane containingpolymers is their mechanically compliant behaviour, whichhas given rise to rapidly emerging field of soft robotics.51While the epoxy resins cured with anhydrides incorporateester groups, an should, in-theory, possess vitrimer-like pro-perties by themselves, the temperatures required for transes-terification in these neat polymers is too high and degradationis commonly observed before vitrimeric transitions. In thisstudy, the siloxane-based epoxy polymer provides a low-Tg poly-meric network for such vitrimeric exchange and lowers theenergy barrier for transesterification, and hence, this low-Tgsiloxane polymer was chosen to demonstrate this concept ofvitrimeric bonding between incompatible polymers.The individual epoxy resins were cured separately with thesame curing agent HHPA, and the same catalyst, EMI, to gene-rate crosslinked polymeric parts containing ester groupswithin the thermosets which were confirmed by FTIR (ESIFig. S35†). When these thermosets were placed in contact withthe siloxane-based epoxide polymer at elevated temperature(160 °C for 4 hours) and pressure (200 kgf over 100 mm2overlap area which corresponds to approximately 19.6 MPaFig. 2 Chemical structures of the cured epoxy materials with pictures of a rectangular strip (50 mm × 10 mm × 2 mm) of the materials on which a10 g weight is placed. BisA (A) and Bdiol (B) thermosets are relatively hard and do not display mechanical deformation on placing the weight. PPG (C)and PDMS (D) thermosets are softer thermosets and show mechanical deformation on placement of the weight. Inset pictures show the watercontact angles of the polymers. (E) Image of a dynamic covalently bonded composite system of BisA thermoset and PDMS thermoset before (left)and after (right) application of tensile force. The PDMS substrate fails due to stress concentration at the stiffness step before failure of adhesion (F)tensile force vs. displacement graphs of the four different composite systems. (G) The stress at failure values of the four thermoset pairs are similaras, in each case, the substrate fails before failure of adhesion.Paper RSC Applied Polymers250 | RSCAppl. Polym., 2025, 3, 247–256 © 2025 The Author(s). Published by the Royal Society of Chemistrypressure), strong adhesion between these polymers wasobserved (Fig. 2 and ESI Fig. S23†). Individually, both theadhering parts are crosslinked polymers that contain no reac-tive groups within their network. However, when heat andpressure is applied to these polymers above their glass tran-sition temperatures (Fig. S11–S18†), ester groups from onenetwork undergo transesterification reactions with the estergroups from the second network, ensuring a strong covalentbond at the interface (Fig. 1C). This dynamic covalentadhesion phenomenon is catalyst-independent (ESI Table S2and Fig. S24†) and a variety of tertiary amino catalysts can beused (e.g. 1-ethyl 4-methaylimidazoles, N,N-dimethyl-aminopyridine, N,N-dimethylaniline, etc.).Strong adhesion is observed without use of any externaladhesives which, when measured by tensile elongation, leadsto failure of the siloxane substrate without any failure at theinterface (Fig. 2F). Bonding with all three carbon-based epoxyresins showed a strong adhesion with similar failure-at-breakvalues. Thus, dynamic covalent bond exchange between dis-similar polymers takes place even when the constituent poly-mers have vastly different chemical and mechanical properties.Strictly speaking, this failure-at-break value is not the strengthof the adhesion because the soft substrate (PDMS) fails beforefailure of the dynamic covalent adhesion.As this adhesion based on dynamic covalent exchange ispredominantly an interfacial phenomenon, we analyzed theinterface of these bonded materials with scanning electronmicroscopy (SEM) and subsequent elemental mapping usingenergy dispersive X-ray (EDX) analysis. The elemental compo-sition across the interface of polar epoxy thermosets and non-polar epoxy thermosets was measured by averaging over a linescan of the EDX elemental map across the cross section of theadhesive bond. All the interfaces show a gradient-transitionfrom the siloxane-rich phase to the carbon-rich phase (Fig. 3).Elemental mapping of silicon confirms that there is no bulk-diffusion of silicon from one phase to the other (Fig. 3 and ESIFig. S25–S27†).Further evidence of extensive dynamic covalent exchange atthe interface was provided by differential scanning calorimetry(DSC). When thin films (50 micrometers) of the two dissimilarpolymers were bonded together using the typical adhesive-lessbonding procedure, DSC traces (Fig. 4A, B and S20–22†) shownot just two glass transition temperatures (of the constituentpolymers, i.e. the Tg for BisA thermoset is 115 °C, and that ofPDMS thermoset is −25 °C), but also a third glass transitiontemperature (Tg = 27 °C) occurring between the individual Tgs.Similarly, new glass transition temperatures are observed inthe case of Bdiol – PDMS thermoset as well as PPG – PDMSthermoset. Thus, the interfacial dynamic covalent exchangecreates an exchange zone which is a new material at the inter-face with different thermal properties than those of the con-stituent polymers.Commercial thermosetting materials consist not only of theneat polymeric network, but also fillers/plasticizers, etc. Fillerssuch as nano-silica, titanium dioxide, etc. are extensively usedto improve the mechanical properties of the crosslinked net-works like impact resistance, improvement in modulus,etc.52,53 To investigate the influence of fillers in dynamiccovalent exchange at the interface, we characterized theadhesion between carbon-rich thermosets and siloxane-richthermosets with nano-silica (1% by weight of the thermoset)(Fig. S28–S30†). As seen in Fig. 4C, D and Table S3† adhesionbetween these dissimilar thermosets is enhanced on additionof the reinforcements. In all cases, we observed cohesivefailure, i.e. the bulk-adhesive fails, and no interfacial delami-nation was observed.Fig. 3 SEM and superimposed energy dispersive X-ray maps of silicon and carbon of the two thermosetting networks after bonding of BisA ther-moset – PDMS thermoset (A), Bdiol thermoset – PDMS thermoset (B) and PPG thermoset – PDMS thermoset (C). Plots at the bottom show theintensity counts of silicon (blue trace) and carbon (black trace) across the cross section at the interface (yellow line). In all the cases, we see an“exchange zone” of ca. 20 microns where there is a gradient-transition from the silicon rich phase to the carbon rich phase. Note that the intensitycounts show a qualitative comparison when comparing different elements, as the counts are not normalized.RSC Applied Polymers Paper© 2025 The Author(s). Published by the Royal Society of Chemistry RSCAppl. Polym., 2025, 3, 247–256 | 251Film adhesives are thin polymeric films that are one of themany types of adhesives used commercially.54–56 The adhesivefilm can be cut into tapes of the required areas and can thenbe laid down by hand or by a robotic tape-laying machine toautomate adhesion of materials. Till date, such film adhesivesare thermoplastic films which melt on application of hightemperature or partially cured polymers which, after curing,adhere the materials that are to be bonded.57,58 Using ourconcept of adhesive-less bonding, we demonstrate a filmadhesive that comprises of a completely cured thermosettingpolymer (Fig. S31 and S32†).Thermosetting adhesives bring many advantages inadhesion, particularly solvent resistance, no creep behavior,thermal stability, etc.59,60 Moreover, adhesives require flexi-bility to be able to dissipate energy throughout the polymericnetwork without initiating cracks.61,62 We used our siloxane-epoxy polymer system to fabricate a thin (100 micron) silox-ane-rich thermosetting polymer film. When this thermosettingpolymer film was used to bond two carbon-rich epoxy poly-meric parts, strong adhesion (1.1 MPa, Fig. 5B) could beobserved for this adhesive. This adhesive strength is compar-able to commercially available silicone adhesives.63 Here,dynamic covalent exchange at two interfaces creates a strongadhesive bond. After failure, a cohesive failure mode isobserved where the adhering interfaces do not fail, butinstead, the siloxane material fails. This type of failure modesupports the fact of excellent interfacial dynamic covalentbond exchange, which is enhanced by application of externalpressure on the adhesive.64 In addition, these de-bonded partscan be re-bonded with heat and pressure, again by dynamiccovalent exchange (Fig. 5C). After the first failure of theseFig. 4 (A) Schematic representation of an exchange zone created at theinterface of the two dissimilar thermosetting polymers. (B) DSC traces ofthe respective materials show new glass transition temperatures due tocreation of dynamic covalent exchange zones. (C) Force – displacementcurves of adhered thermosets with and without nano-silica (1% byweight as a filler). Thus, this adhesive-less bonding technique can beapplied to reinforced polymeric materials as well. (D) Addition of nano-silica reinforces the polymeric networks, and the bonded materials fail athigher stress as compared to the unreinforced polymeric networks.Fig. 5 (A) Image of two BisA thermosets bonded together with a PDMS thermosetting film. The hydrophobic film is dry to touch but can success-fully bond two hydrophilic thermosetting polymers. (B) Characterization of the adhesion strength of the PDMS dry adhesive film. (C) The dryadhesive film can be recycled, and the parts can be re-bonded with good retention of the bonding strength. (D) A sandwich structure synthesizedusing the adhesive-less bonding technique. The soft PDMS thermoset is flanked by two hard BisA thermosets. The bottom image shows theSEM-EDX map of silicon. (E) The dog-bone shaped BisA-PDMS-BisA sandwich structure was subjected to a 3-point bending test to show the uniquebehavior of such structures on failure. (F) Force vs. strain graph for the 3-point bending test of the sandwich structure shows unique failure modesof the composite structure.Paper RSC Applied Polymers252 | RSCAppl. Polym., 2025, 3, 247–256 © 2025 The Author(s). Published by the Royal Society of Chemistryadhesives, the parts were again kept in contact with each otherin the original configuration and subjected to the dynamicbond-exchange conditions (160 °C, 4 hours, 20 MPa pressure,Fig. 5C). Due to inefficient material overlap, the adhesionstrength obtained on reuse decreased by 14.11%, 16.85% and28.68% for BisA-PDMS-BisA, Bdiol-PDMS-Bdiol andPPG-PDMS-PPG respectively, but was still quite strong (Fig. 5Cand ESI Fig. S33†).This dynamic covalent exchange of incompatible materialscan also be used to create novel anisotropic materials, i.e.sandwich structured thermosets wherein mutually incompati-ble polymers can be stacked on top of one another. We syn-thesized BisA-PDMS-BisA thermosets as well PDMS-BisA-PDMSthermosets to fabricate sandwich structures from incompatiblepolymers (Fig. 5D and ESI Fig. S34, S35†). To confirm goodinterfacial adhesion, we subjected these sandwich structuresto 3-point-bending tests until failure, and no interfacial dela-mination is observed (see video in ESI† and Fig. 5E, F, ESIFig. S36, S37†). Thus, such novel thermosetting materials areaccessible via this adhesive-less bonding approach, wherein,failure modes of these composite structures can be tuned onselection of appropriate materials.5. ConclusionIn conclusion, we show that dynamic covalent bond exchangecan be used to create a strong adhesive bond between poly-meric networks of vastly different polarities as well as differentmechanical properties, without use of adhesives. The adhesionis a result of an exchange zone created across the interface ofboth the polymeric networks, where formation of a new poly-meric network distinct from the constituent networks isobserved. SEM-EDX mapping as well as DSC traces indicatethat the new material in the exchange zone has differentthermal and chemical properties as compared to the bulk ther-mosetting materials.We also show that this strategy of bonding between chemi-cally incompatible thermosetting polymeric networks is com-patible with reinforcements such as fillers, which are com-monly used in polymer composites. Addition of nano-silica tothe individual polymeric networks led not only to an increasein mechanical strength of the individual polymeric networksbut also to an increase in the adhesion strength on dynamiccovalent exchange.In addition, we show that non-polar thermosetting poly-mers can be used for adhesion of two polar thermosettingpolymers. This is surprising and challenges the conventionalknowledge in adhesives. Due to resistance to crack initiationand propagation,65 flexible thermosetting adhesives may bevery well-suited as adhesives for high tensile strength poly-meric composites. Moreover, we also show that such a “poly-meric adhesive film” can be reused after adhesive failure forre-bonding of the thermosetting polymers.Such an adhesive-less bonding strategy could simplifycomplex manufacturing chains and impart new avenues ofresearch in adhesion in a variety of industrial applications,such as aerospace composites, automotive composites, compo-sites for renewable energy, adhesion to electronic coatings, etc.Author contributionsM. N. and A. N. designed the conceived the project anddesigned the experiments. All experiments were performed byS. R. K., A. V. P. and S. V. P. Polymer mechanical properties aswell as adhesives data was analyzed by S. R. K, A. V. P., andS. V. P. All authors helped in the interpretation of the data andcontributed to writing the manuscript.Data availabilityAuthors confirmed that the data supporting the findings ofthis study are available within the article and its ESI.†Conflicts of interestThe authors certify that there is no conflict of interest for ourwork.AcknowledgementsS. R. K. acknowledges funding from National Institute forMaterials Science (NIMS), Japan for post-doctoral funding.This work was partially supported by the Core Research forEvolutional Science and Technology (CREST) program“Revolutional material development by fusion of strong experi-ments with theory/data science” of the Japan Science andTechnology Agency (JST), Japan, under Grant JPMJCR19J3.A. V. P. acknowledges Composites Group, MechanicalSystems Engineering-304, Swiss Federal Laboratories forMaterials Science and Technology-Empa, 8600 Dübendorf,Switzerland.References1 C. Heinzmann, C. Weder and L. 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