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TSAI, Hsing-Ying, [NAKAMURA, Yasuyuki](https://orcid.org/0000-0003-0078-6413), FUJITA, Takehiro, [NAITO, Masanobu](https://orcid.org/0000-0001-7198-819X)

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[Strengthening epoxy adhesives at elevated temperatures based on dynamic disulfide bonds](https://mdr.nims.go.jp/datasets/12a17248-abb3-423b-a319-5822c46de926)

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Strengthening epoxy adhesives at elevated temperatures based on dynamic disulfide bonds3182 | Mater. Adv., 2020, 1, 3182--3188 This journal is©The Royal Society of Chemistry 2020Cite this:Mater. Adv., 2020,1, 3182Strengthening epoxy adhesives at elevatedtemperatures based on dynamic disulfide bonds†Hsing-Ying Tsai,ab Yasuyuki Nakamura,a Takehiro Fujitaa and Masanobu Naito *abEpoxy resins incorporating aromatic disulfide bonds demonstratedimproved adhesive properties with increasing temperature belowtheir glass transition points. This behaviour was explained in terms ofthe relatively low topology freezing transition temperature of thesematerials as compared with their glass transition temperatures.Thermosetting structural adhesives have a wide range of appli-cations, including in the aerospace, automobile and infrastruc-ture industries, because they tend to add less weight thanconventional mechanical fastening methods. The majority ofsuch adhesives are composed of crosslinked network polymerssuch as epoxy resins, and provide excellent dimensional stabi-lity, good mechanical properties and superior creep andchemical resistance.1–6 However, although these substancesare often required to be stable under harsh high temperatureconditions, the adhesive strength of traditional thermosettingresins tends to be reduced with increases in temperature. Thedevelopment of materials with greater adhesive strength athigh temperatures would enable the reliable application ofenvironmentally-friendly adhesive joints in lightweight auto-mobiles or various infrastructure components.To this end, our group has focused on the use of dynamiccovalent chemistry (DCC) to develop adhesives that maintaintheir strength at higher temperatures. Network polymers withdynamic covalent bonds generally behave as conventional ther-moset resins but can exhibit thermoplastic characteristics as aresult of reversible bond exchange reactions in response tospecific external stimuli, such as heating or exposure to ultra-violent (UV) radiation or ultrasound.7–11 To date, Diels–Alderreaction,12–14 olefin metathesis,15 imine/amine exchange,16 silox-ane/silanol exchange,17 disulfide metathesis,18 transamination,19and transesterification20,21 have all been utilized to deviseexchangeable bond systems. The introduction of adaptablebonding into thermoset resins using these approaches providesa variety of unique properties, including self-healing and theability to reprocess, remold and recycle these materials. However,the adhesive properties of such resins have not yet been fullyexploited because these adaptable bonding systems (typicallybased on transesterification) require the use of insoluble cata-lysts that can damage metal substrates. In addition, the pro-longed reaction times that are required may induce thermaldegradation and poor mechanical strength, both of which canlimit the applications of resins as structural adhesives.Epoxy resins based on exchangeable aromatic disulfideexhibit exceptional thermal and mechanical stability and canbe synthesized using catalyst-free reactions. These materialsalso provide rapid exchange rates and healing efficiency com-pared with other dynamic bonding systems. Even though thedegradation, self-healing characteristics and electrical resis-tance properties of epoxy resins incorporating disulfide bondshave been examined,22–25 the adhesive characteristics of thesesubstances, especially the effects of temperature on adhesion,have not been fully elucidated. The present study thereforeperformed a detailed examination of the adhesive characteris-tics of epoxy resins having aromatic disulfide bonds. Theresults of this work demonstrate unusual thermal strengthen-ing of adhesive joints at elevated temperatures. These resultsare discussed herein with regard to the thermal properties ofepoxy resins with adaptable bonding systems, especially withregard to glass transition temperature (Tg) and topology freez-ing transition temperature (Tv).Ester and disulfide bonds were introduced into aromaticepoxy networks to obtain model epoxy adhesives with adaptablebonding (Scheme 1). The disulfide system comprised bis(4-glycidyloxyphenyl)disulfide (BGPDS, termed the 1a herein) anddithiodianiline (DTDA, 2a) as the epoxy monomer and diaminehardener, respectively. The diglycidyl ether of bisphenol Aa Data-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.jpb Program in Materials Science and Engineering, Graduate School of Pure andApplied Sciences, University of Tsukuba, 1-1-1, Tenodai, Tsukuba,Ibaraki 305-8571, Japan† Electronic supplementary information (ESI) available: Synthesis, experimentaldetails and characterization (NMR, FTIR, DMA). See DOI: 10.1039/d0ma00714eReceived 17th September 2020,Accepted 19th November 2020DOI: 10.1039/d0ma00714ersc.li/materials-advancesMaterialsAdvancesCOMMUNICATIONOpen Access Article. Published on 21 November 2020. Downloaded on 11/17/2021 2:35:04 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article OnlineView Journal  | View Issuehttp://orcid.org/0000-0001-7198-819Xhttp://crossmark.crossref.org/dialog/?doi=10.1039/d0ma00714e&domain=pdf&date_stamp=2020-11-26http://rsc.li/materials-advanceshttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d0ma00714ehttps://pubs.rsc.org/en/journals/journal/MAhttps://pubs.rsc.org/en/journals/journal/MA?issueid=MA001009This journal is©The Royal Society of Chemistry 2020 Mater. Adv., 2020, 1, 3182--3188 | 3183(DGEBA, 1b) and diaminodiphenyl methane (DDM, 2b) wereemployed as controls analogues of BGPDS and DTDA, respec-tively, without disulfide bonding. The chemical reactivities ofthe aromatic disulfide in the epoxy monomer and amine hard-ener were assumed to be almost identical. The epoxy adhesiveswere synthesized by combining the epoxy monomer (either 1aor 1b) and the diamine hardener (2a or 2b) followed by mixingat 90 1C for 30 min in stoichiometric ratios (2 : 1) of theingredients. The mixture of the epoxy monomer and hardenerwas transferred onto the pre-treated aluminium substrates.This was followed by curing at 120 1C for 2 h, 140 1C for 2 hand 160 1C for 2 h. Aluminium substrates were employedbecause these are chemically inert with regard to thiols andbecause aluminium is an important lightweight structuralmaterial.26,27 To systematically elucidate the effects of adapta-ble bonding, the epoxy monomer and amine hardener with/without disulfide bonding were prepared in all combination, togive specimens referred to herein as SS (4a), SC (4b), CS (4c) andCC (4d) (Scheme 1). Note that the extent of disulfide bonding inthe SS, SC and CS samples was in the ratio of 3 : 2 : 1, whichmeant the most content of disulfide bonding in the SS sample.For comparison purposes, a previously reported epoxy networkwith ester bonding (referred to as CA sample (4e) herein) wasprepared by mixing citric acid and sebacic acid followed by theaddition of DGEBA together with 1-methylimidazole as acatalyst. This mixture was subsequently precured at 120 1Cfor 3 h, followed by full curing at 160 1C for 6 h. The ratio of CA(4e) for DGEBA, citric acid, sebacic acid, and 1-methylimidazolewas 1 : 0.45 : 0.45 : 0.05. An excess of the epoxy monomer wasadded to this composition so as to increase the crosslinkingdensity.28 Details of the sample preparation conditions areprovided in Table S1 (ESI†). Prior to the adhesive tests, thethermomechanical properties of these epoxy adhesives wereevaluated. Specifically, Tg, Tv, activation energy of stress relaxa-tion (Ea) and storage modulus (E0) were determined based ondynamic mechanical analysis (DMA) (Table 1).It is important to note that the viscoelastic behaviour ofpolymer networks having adaptable bonding systems can becharacterized by two transition temperatures. One is associatedwith the transition between glassy and rubbery states, which isrelated to intramolecular chain motions. Segmental chainmovements will occur above Tg, resulting in dramatic decreaseof mechanical strength. On the other hand, rigid structurebelow Tg will limit the movement, so that the mechanicalstrength only slightly decreases with increasing temperaturewithin this region. The other transition mode is associated withthe topology freezing transition, which occurs at the point atwhich the material exhibits a viscosity of 1012 pa s. Tv isgenerally considered as the critical point at which the exchangerate becomes sufficiently rapid such that bond breaking andbond rearrangement can occur dynamically. This transitiontemperature is calculated by varying the sample temperatureduring a stress relaxation test, as shown in Fig. 1. In thisprocess, the relaxation times at different temperatures areobtained based on the Maxwell model, defined as the timerequired for a 63% drop in the effective modulus, after whichthese times are plotted as a function of temperature in themanner of an Arrhenius plot (Fig. 2). The Tv values determinedfor the epoxy networks having disulfide and ester bonds in theScheme 1 Synthesis and chemical structure for all epoxy-based network in this study.Table 1 Thermal and mechanical properties of all epoxy networks inthis workItem Tg (1C) Tv (1C)Ea (stressrelaxation)(kJ mol!1)Storagemodulus(25 1C) (GPa)Storage modulus(Tg + 30 1C)(MPa)SS 133.7 !26 37.7 1.93 15SC 144.0 67 65.5 2.22 30CS 160.5 91 85.9 1.88 20CC 177.9 — — 1.88 200CA28 73 105 106 1.5–1.8 4–5Communication Materials AdvancesOpen Access Article. Published on 21 November 2020. Downloaded on 11/17/2021 2:35:04 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d0ma00714e3184 | Mater. Adv., 2020, 1, 3182--3188 This journal is©The Royal Society of Chemistry 2020present work are summarized in Table 1,28 while the Tv and Tgvalues for various adaptable bonding epoxies reported in theliterature are presented in Table S2 (ESI†). It should be notedthat the viscoelastic properties of the dynamic covalent systemscould be classified into two categories based on the relation-ship between the Tv and Tg values of the materials.8,10 In thecase of the Tv value was much greater than the Tg value, (that is,the CA sample), as shown in Fig. 3(a), the heating of thethermosetting epoxy to temperatures within the intermediateregion II (between Tg and Tv) was expected to induce a transi-tion from a glassy to rubbery state and so provide increasedchain mobility. As a consequence, the material would acquirethe properties of an elastomer, but would not present theexchange reaction such as transesterification in this region.Upon further heating above Tv (region III), the exchange reac-tion started so that the polymer would change from anelastomer to a viscoelastic liquid. In contrast, the epoxy sam-ples with aromatic disulfide bonding (that is, the SS, SC and CSspecimens) had Tg values higher than their Tv values, as can beseen from Fig. 3(b). Here, the intrinsically rapid exchangereaction associated with disulfide bonding was able to takeplace in the epoxy network even within the intermediatetemperature region II (above Tv and below Tg). In this scenario,although the exchange reaction involving the disulfide bondswas able to proceed at temperatures in excess of Tv, a lack ofchain motion below Tg would be expected to limit the segmen-tal movement of the epoxy network, resulting in rigid andglassy structure. Above Tg (region III), these samples alsotransitioned to viscoelastic liquids because the chain motionswere able to proceed along with active exchange reactionsFig. 1 Normalized stress relaxation curves of dynamic epoxy networks SSat different temperatures.Fig. 2 Fitting line of the relaxation time to Arrhenius equation for dynamicepoxy networks SS, SC, and CS (R-square = 0.9579, 0.9877, and 0.9653,separately).Fig. 3 Comparison for the effect of glass transition temperature (Tg) andtopology freezing transition temperature (Tv) on viscosity and viscoelasticbehavior of dynamic systems (a) Tv 4 Tg (b) Tg 4 Tv.Materials Advances CommunicationOpen Access Article. Published on 21 November 2020. Downloaded on 11/17/2021 2:35:04 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d0ma00714eThis journal is©The Royal Society of Chemistry 2020 Mater. Adv., 2020, 1, 3182--3188 | 3185associated with the disulfide bonds. The data show that ahigher content of aromatic disulfide bonding in the networklowered the Tv value. This effect is attributed to the increasedprobability of exchange reactions with higher densities ofdisulfide bonds, which accelerated the bond cleavage and bondreorganization. This behaviour can also be explained by con-sidering activation energy, and the activation energy value forthe stress relaxation of each composition was obtained fromthe data in Fig. 2 based on the Arrhenius equation. Theresulting values are presented in Table 1. Although theexchange reaction of dynamic bonds would result in relaxation,significant rearrangement of whole polymer network wouldalso be required for stress release. In the present work, the SSspecimen was found to have the lowest activation energy forstress relaxation among all the compositions. Both the Tvvalues and activation energies demonstrate that higher con-centrations of aromatic disulfide bonds promoted the stressrelease because of the easier reorganization of the polymernetwork and the increased possibility of exchange occurring.Most importantly, the viscoelastic properties derived fromthe Tg and Tv of these materials had a significant effect on theirperformance as adhesives. The single lap shear test is astandard means of assessing adhesive strength and so theroom temperature lap shear strength of each epoxy samplewas ascertained. In these trials, each epoxy was applied to apretreated aluminium substrate followed by curing at tempera-tures that exceeded both the Tv and Tg of the epoxies withdynamic covalent bonding (see ESI,† for details). In addition, toevaluate recycling and rebonding ability, the used aluminiumspecimens were rebonded to their original configurations byhot-pressing. Each epoxy showed a relatively high initial shearstrength in the range of 13 to 19 MPa (Fig. 4). Thus, introducingdisulfide bonds into the epoxy adhesive did not significantlyaffect the adhesive strength, regardless of whether such bond-ing resulted from the phenolic epoxy or the aromatic aminehardener. After rebonding, the shear strength of the epoxy withadaptable bonding was found to be 83 to 95% of the initialshear strength, while the sample without adaptable bonding(that is, the CC specimen) did not exhibit any adhesion whenrebonding was attempted. These results can be explained interms of both the viscoelastic nature of the epoxy network andthe exchange reaction associated with the adaptable bondingsystem. In the case that an epoxy adhesive with adaptablebonding was heated above both its Tg and Tv, the materialbehaved as a viscoelastic liquid within region III (see Fig. 3).Simultaneously, the bond exchange reaction proceeded amongthe adaptable bonds at a rapid rate to restore the polymernetwork, resulting in recovery of the shear strength. Evidencefor these effects is provided by the correlation between therecovery of shear strength and the amount of disulfide bond-ing, such that the shear strength of the SS sample was 95% ofthe original value. This was higher than the recovery percen-tages showed by the other two compositions (the SC and CS).Moreover, the CC sample (without exchangeable dynamicbonding) could not adhere again once it was cleaved. Theseresults confirm that a high degree of adaptable bonding isrequired to allow the efficient recovery of the polymer network,leading to restoration of the adhesive strength. Similarly, theCA sample also showed good recovery of shear strength whenheated to a viscoelastic state because of the presence ofdynamic ester bonds, although a longer recovery time wasneeded. As a result, there were no significant differencesbetween the rebonding strengths of the epoxies with the dis-ulfide and ester bonds when the specimens were repaired byheating above both Tg and Tv. However, there were obviousdifferences in adhesive behaviour after heating to within regionII, intermediate between Tv and Tg. For this reason, the adhe-sive properties of the epoxy specimens with adaptable bondingwere evaluated in this intermediate region. Although there havebeen several reports concerning the adhesive characteristics ofadaptable bonding systems to date, the effect of temperature onthese properties has not yet been fully established, especially inthe range between Tv and Tg. Even so, this temperature range isimportant because it may coincide with temperatures appliedduring industrial bonding processes. Variations in adhesiveproperties in this temperature range were examined by prepar-ing two SS and CA film samples and adhering these specimensat 80 1C using a compressive force of 10 kN. Here it is importantto note again that the SS sample had a Tg (134 1C) higher thanits Tv (!26 1C), whereas the CA sample had a Tg (73 1C) lowerthan its Tv (105 1C). The pressing temperature of 80 1C waschosen so as to be intermediate between the Tv and Tg of bothmaterials (Fig. 3). The subsequent testing of these specimensindicated that the SS showed good adhesive properties (Fig. 5a)while the CA film pieces did not adhere but simply deformedfrom their original shapes during testing (Fig. 5b). When the SSfilm was heated to 80 1C, the bond exchange reaction wasinitiated but chain motions of the epoxy network could notproceed efficiently because the material was below its Tg.Therefore, the film shape could be maintained such that itexhibited solely good adhesion. At the same temperature, thetransesterification process in this polymer is thought to havebeen dormant, while chain motions would have been able toFig. 4 Single lap shear strength for initial and rebonded dynamic epoxyadhesive networks at room temperature.Communication Materials AdvancesOpen Access Article. Published on 21 November 2020. Downloaded on 11/17/2021 2:35:04 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d0ma00714e3186 | Mater. Adv., 2020, 1, 3182--3188 This journal is©The Royal Society of Chemistry 2020occur. Therefore, only deformation of the film occurred withoutadhesion between the CA films.The effect of temperature on the Young’s modulus andadhesive strength values of the adaptable bonding specimenswas further assessed by performing single lap shear strengthtests at temperatures ranging from ambient to 200 1C (Fig. 6 and 7).As can be seen from Fig. 6, the Young’s modulus values of allcompositions were slightly decreased with increases in tempera-ture below the glass transition point but dramatically declinedupon heating above Tg. This was the case for both the dynamicsystems and the conventional thermoset polymers. Conse-quently, the heat-induced change in the mechanical propertiesof the adhesive resins is primarily ascribed to the glass–rubbertransition. However, when epoxy resins are applied as adhesives,their adhesive strength shows more complicated behaviour thatcan be placed into two categories. In the case of a standardthermosetting adhesive, the adhesive strength tends to graduallydecrease with increases in temperature and then drastically dropat the Tg of the material, as is also observed for bulk resins (seeFig. 6).29–31 This effect originates from the transition from aglassy to rubbery state. Similar to these typical thermoset adhe-sives, the lap shear strength of the CC sample transitioned frommoderate to very poor over the temperature range of 150 to200 1C, which contains the Tg of the sample CC (178 1C).Conversely, the epoxy samples with disulfide bonds (the SS,SC and CS) exhibited unusual increases in lap shear strength onheating from room temperature to 100 1C. In particular, the SSspecimen showed a 30% increase in lap shear strength at100 1C as compared with room temperature (Fig. 7). In general,epoxy adhesives accumulate internal stresses in their three-dimensional network structures as a result of volume shrinkageduring the curing process and the cooling process after curing.When external forces are applied to these polymer networks atroom temperature, these materials therefore tend to exhibitbrittle fracture because of cracks caused by internal stress and/or decreases in the elongation of the three-dimensional net-work structure, resulting in the cleavage of chemical bonds. Inaddition, considering that thiols will not readily react withaluminium/aluminium oxide surfaces, the exchange reactionof the disulfide bonds is thought to be the primary means ofrelieving localized internal stress via rearrangement of thepolymer network. To confirm the occurrence of efficientexchange reactions among the aromatic disulfide bonds, diphe-nyl disulfide and dithiodianiline were dissolved in dimethylsulfoxide (DMSO) as low molecular weight model compounds.The nuclear magnetic resonance (NMR) spectra of these solu-tions (Fig. S5, ESI†) indicate that the exchange reactions ofthese compounds proceeded even at room temperature.32Although it should be noted that the rapid exchange of dis-ulfide bonds may be more limited in a rigid polymeric structureat room temperature, we assumed that the dissociation andassociation of disulfide bonds could still occur within suchfixed networks. Increasing the temperature would be expectedto enhance both the bond cleavage and recombination rates asFig. 5 Demonstration of exchange and healing behaviour at 80 1C forformula (a) SS and (b) CA.Fig. 6 Temperature-dependent Young’s modulus for all adhesive epoxynetworks.Fig. 7 Temperature-dependent single lap shear strength for all adhesiveepoxy networks.Materials Advances CommunicationOpen Access Article. Published on 21 November 2020. Downloaded on 11/17/2021 2:35:04 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d0ma00714eThis journal is©The Royal Society of Chemistry 2020 Mater. Adv., 2020, 1, 3182--3188 | 3187well as the probability of exchange and rearrangement. As aresult, epoxy networks containing disulfide bonds will becometougher at higher temperatures because of the rapid exchangeand reorganization that occur above Tv. At the same time, if thesample is below its Tg, the mechanical strength of the adhesiveresin will be maintained owing to its rigid structure. As asample approaches its Tg, its adhesive strength will greatlydecrease and adhesive joints will tend to break, as is observedwith conventional polymeric materials. The present data estab-lish that a high density of aromatic disulfide bonds strength-ened the adhesive effect. However, when the CA sample washeated, the bond strength was drastically decreased. This effectcan likely be attributed to the frequent chain motions in thesample (because of its relatively low Tg of 73 1C) but lack ofexchange reaction, as transesterification did not occur.The Tg of a polymer reflects its transition from a rigid brittlesolid to a flexible rubbery state due to chain movement. Inaddition, the effect of temperature on polymers containingadaptable bonds is also often evaluated based on Tv. Both Tvand Tg are associated with changes in viscosity, but for entirelydifferent reasons. The Tg transition originates from changes inintramolecular chain motion, while Tv is attributed to inter-molecular bond exchange processes. This difference leads todissimilar phenomena in terms of adhesive properties. Speci-fically, bonding strength is greatly decreased above Tg butadhesion may be strengthened while the occurrence of frac-tures is minimized as a result of bond exchange reactions aboveTv. Compared with other dynamic covalent bonds, aromaticdisulfide systems tend to have low Tv and high Tg values, whichincrease the rates of dissociation, diffusion and association ofthe disulfide bonds and widen the temperature range overwhich these materials can be applied as industrial adhesives.On the contrary, adhesives with transesterification bonds willshow high Tv and low Tg values, and so undergo glass transitionbefore toughening.19,23–25,28,33,34In summary, this work represents the first observation ofimproved adhesion behaviour at elevated temperatures as aresult of adaptable aromatic disulfide bonding. An improvedunderstanding of disulfide-based networks may permit thedevelopment of new dynamic systems and broaden thepotential range of material design.ConclusionsThis study demonstrated the application of epoxy polymersincorporating aromatic disulfide bonds as adhesives. Theimproved adhesive properties of these materials at elevatedtemperatures below the glass transition region (compared withother dynamic systems and traditional thermosets) is ascribedto their extremely low Tv and activation energy values. Thesefactors resulted in tougher adhesion because the easier rear-rangement of aromatic disulfide bonds allowed the release ofinternal stress. This study also established that a high densityof disulfide bonds accelerated the exchange process by increas-ing the likelihood of bond cleavage and rearrangement, whichin turn improved adhesion.With regard to industrial applications, epoxy-based adhesivesystems with dynamic disulfide bonding represent anenvironmentally-friendly and economical option. These resinsnot only show good adhesion properties and permit simplerebonding procedures, but also demonstrate improved high-temperature adhesive performance as a result of the incorpora-tion of exchangeable disulfide bonds that expand their usabletemperature range. These factors could permit the develop-ment of next-generation adhesives based on dynamic covalentchemistry and should be further investigated in the future. Theknowledge gained from the present study should also beapplicable to other epoxy adhesive systems, especially becausebisphenol A is often used in epoxy adhesives.NotesThe Maxwell relationship viscosity = G"t was used to determinethe relaxation times for Tv calculations. Here, G is the shearmodulus, calculated as G = E0/2(1 + v) where v is Poisson’s ratiofor a rubber (with a value of 0.5) and E0 is the storage modulusfor each composition in a rubbery state as determined usingDMA. 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