Embed Size (px)
Citation preview
lable at ScienceDirect
Polymer 120 (2017) 176e188
Contents lists avai
Polymer
journal homepage: www.elsevier .com/locate/polymer
Thermophysical characterization of a reversible dynamic polymernetwork based on kinetics and equilibrium of an amorphous furan-maleimide Diels-Alder cycloaddition
M.M. Diaz a, *, G. Van Assche a, F.H.J. Maurer b, B. Van Mele a
a Department of Materials and Chemistry, Physical Chemistry and Polymer Science (FYSC), Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgiumb Department of Chemistry, Polymer & Materials Chemistry, Lund University, SE-22100 Lund, Sweden
a r t i c l e i n f o
Article history:Received 11 March 2017Received in revised form21 May 2017Accepted 24 May 2017Available online 27 May 2017
Keywords:Variable cross-link densityDe-gelationSelf-healing network
* Corresponding author.E-mail addresses: [email protected] (M.M
(G. Van Assche), [email protected] (F.H.J.(B. Van Mele).
http://dx.doi.org/10.1016/j.polymer.2017.05.0580032-3861/© 2017 Elsevier Ltd. All rights reserved.
a b s t r a c t
The equilibrium and kinetics of the furan-maleimide Diels-Alder (DA) reaction for the formation of areversible network were studied by calorimetry between 25 �C and 90 �C using an amorphous bisma-leimide eliminating solvent use. The relationship between the equilibrium conversion xeq with tem-perature and the effect of dilution were simulated. The glass transition-conversion relationship of thereversible network was established.
The thermophysical properties of the reversible network were linked to the kinetics and equilibrium ofthe DA system, and studied by dynamic mechanical analysis (DMA), dielectric analysis (DEA) and rhe-ometry. Below Tg the reversible network behaves like an irreversible network; however, above Tg, adecrease in the rubber tensile storage modulus was observed due to a reduction of cross-link densitycaused by the retro DA reaction. DEA revealed that above Tg, an interface is formed between releasedbismaleimide molecules and the remaining network. The rheological behavior is related to xeq and thelifetime of the reversible covalent bonds. When xeq is higher than the gelation conversion xgel (below90 �C), the system gels at constant xgel. An elastic strengthening effect is observed in a transition regionbetween 90 �C and 115 �C. Above 115 �C the system is reaching a viscous melt behavior. These obser-vations are important for the application of this network as a self-healing material and as a recyclableelastomer or thermoset.
© 2017 Elsevier Ltd. All rights reserved.
1. Introduction
Traditional polymer networks are irreversibly processed into aparticular shape by curing the reacting compounds. Due to theirreversible covalent cross-links these networks cannot be resha-ped. These materials are developed with a ‘damage prevention’instead of a ‘damage management’ concept in mind and in case offailure a repairing strategy is not contemplated in the materialdesign [1]. One of the existing alternatives to design a polymernetwork with a ‘damage management’ concept is the introductionof reversible cross-links. These cross-links can be ionic bonds, su-pramolecular interactions or reversible covalent bonds. Severalpolymers have been reported using ionic interactions [2e4] and
. Diaz), [email protected]), [email protected]
supramolecular interactions such as hydrogen bonding [5e7], hy-drophobic interactions [8e10], p-p stacking [11,12] and metal-ligand coordination [13,14]. Reversible covalent bonding is one ofthe strategies by which reversible polymer networks may be ob-tained. These kinds of networks can be used in several applicationssuch as self-healing, re-shapeable thermosets or tunable adhesivesfor bonding/de-bonding on demand. Several chemistries have beenproposed for reversible covalent bonds, for example ester exchange[15], C]N exchange [16], disulfide exchange [17], radical exchange[18,19] and the Diels-Alder (DA) reaction [20]. In the frame of theresearch on self-healing polymer networks the DA reaction hasbeen explored by several authors [20e25]. It consists of the forwardreaction between a conjugated diene and a dienophile to form a DAcycloadduct [26]. The reverse process is called the retro Diels-Alder(rDA) reaction, which converts the cycloadduct into the startingdiene and dienophile. Reversible cross-links give rise to specialnetwork properties which are mainly dictated by the kinetics andequilibrium of the reversible cross-linking reaction [27].
Fig. 1. Compounds for reversible DA and irreversible network.
M.M. Diaz et al. / Polymer 120 (2017) 176e188 177
The DA reaction is often carried out by using compounds withfuran F and maleimide M moieties that form a DA adduct:
½F� þ ½M� ) *kDA
krDA
½DA� (1)
The rate of formation of the DA adduct is dictated by the kineticconstant kDA of the forward reaction and the rate of formation ofthe initial components by the kinetic constant krDA of the retro DAreaction. The equilibrium of the reaction is expressed by the equi-librium constant K:
K ¼ ½DA�eq½F�eq½M�eq
¼ kDAkrDA
(2)
which is the ratio between both rate constants and also equal to theratio of the equilibrium concentrations between the adduct [DA]eq,and the furan and maleimide equilibrium concentrations, [F]eq and[M]eq, respectively.
In this paper the macroscopic properties of a reversible polymernetwork based on DA chemistry using a furan functionalizedcompound (F400) and an amorphous bismaleimide (ABMI400) areexplored. The advantage over other reversible DA polymer networksystems is that the actual network can be synthesized just bymixing the fully miscible reaction compounds without the use ofany solvent. This characteristic offers many advantages for an ac-curate investigation of the behavior of this homogeneous reversiblenetwork, especially for the study of the Tg-x relationship, the (de)gelation behavior of the system, and a successful design of self-healing applications. Interpretation of the thermophysical proper-ties is unambiguously based on the kinetics and equilibrium of theDA reaction.
2. Materials and methods
Furfuryl glycidyl ether (FGE) was purchased from Sigma Aldrich.Poly (propylene oxide) bis(2-amino propyl ether), commerciallyknown as Jeffamine D-series, was purchased from Huntsman, onewith an average degree of polymerization n¼ 2.5 and a molar massof 230 g mol�1 (Jeffamine D230 or J230) and one with n ¼ 6.1 and amolar mass of 430 g mol�1 (Jeffamine D400 or J400). Poly (pro-pylene oxide) 400 bismaleimide (ABMI400; n ¼ 6.1) was obtainedfrom Specific Polymers. Diglycidyl ether of bisphenol A (DGEBA;n¼ 0.7), commercially known as EPIKOTE 828, was purchased fromMomentive Specialty Chemicals. All materials were used asreceived (See Fig. 1).
2.1. Synthesis
Furan functionalized compound F400. The furan functionalizedcompound was synthesized by mixing a 50% excess of FGE withJ400 at room temperature. The mixture was left in a closed flask at80 �C during 5 days for the epoxy-amine reaction to take place.Afterwards, the excess of FGE was removed in vacuum at 110 �C fortwo days. The formation of the furan functionalized compound(F400) was confirmed using Nuclear Magnetic Resonance (NMR)showing a furan-functionality of 3.4.
DA network F400-ABMI400. For calorimetric studies (kineticstudies and Tg-x relationship) the networks were prepared bymixing stoichiometric amounts of F400 and ABMI400 with the aidof a mechanical mixer at ambient temperature during 1 min. Thefreshly made mixtures were placed in the microcalorimeter orDifferential Scanning Calorimeter to follow the DA reaction.
To study the properties of F400-ABMI400 freshly preparedmixtures were left to react via the DA reaction at ambient
temperature during 5 days. Teflon molds were used to make sam-ples of uniform thickness.
Irreversible epoxy-amine network DGEBA-J230. The irrevers-ibly cross-linked network was prepared by mixing DGEBA and J230in stoichiometric amounts with the aid of a mechanical mixerduring 1 min. The sample was cured at 90 �C during three days in ateflon mold to make a film of uniform thickness.
2.2. Characterization techniques
NMR. A Bruker Avance II 500 spectrometer was used to studythe structure of the furan functionalized compound F400. Thesample was diluted in deuterated chloroform in a 5 mg ml�1
solution.FTIR spectroscopy. Spectroscopic studies of the DA reaction
were performed using a Thermo Fisher Scientific Nicolet 6700 FT-IRspectrometer equipped with a heating stage. Samples weremeasured in transmission on coated KBr disks.
Microcalorimetry. The calorimetric studies of the kinetics of theDA reaction and the partial curing for the Tg-x relationship of theF400-ABMI400 system were carried out in the Thermal ActivityMonitor (TAM III) from TA Instruments. The microcalorimeter wasoperated in isothermal mode and samples of 0.5 g were studied in4ml sized ampoules. Freshly preparedmixtures were introduced inthe microcalorimeter and left for thermal equilibration during15 min; afterwards they were lowered to the measuring position.For the kinetic studies of F400-ABMI400, the signal was followeduntil no further exothermicity was observed. For the study of theTg-x relationship, the measurements performed at 25 �C werestopped at different partial curing times.
Differential Scanning Calorimetry (DSC). The instrument usedis a TA Instruments Q2000 DSC equipped with an RCS coolingsystem. The kinetic studies of F400-ABMI400 at 80 �C and 90 �Cwere performed in the DSC. For the Tg-x relationship, the Tg of thesamples was measured by DSC at a scanning rate of 10 �C min�1.The samples to measure the Tg at equilibrium at 25 �C, 40 �C and60 �C, were cured in the TAM III at these temperatures andimmediately measured by DSC. The samples at equilibrium at 80 �Cand 100 �C were cured and measured inside the DSC. Tg∞ was
M.M. Diaz et al. / Polymer 120 (2017) 176e188178
measured by DSC using a sample cured initially at room tempera-ture and subsequently left in a fridge at 2 �C for three weeks.
Dynamic Mechanical Analysis (DMA). DMA measurementswere performed in a TA Instruments DMA Q800 equipped with aGas Cooling Accessory. Samples were measured in tension modeusing a strain of 0.1%, at a scanning rate of 2.5 �C min�1 and a fre-quency of 1 Hz.
Dielectric Analysis (DEA). Dielectric analysis measurementswere performed in a Novocontrol high-resolution dielectricanalyzer V 1.01S instrument. The experiments were conducted inparallel plate configuration with plates of 25 mm in diameter andsamples of 0.7 mm in thickness. A potential of 0.5 V was appliedand a stepwise isothermal protocol was followed every 5 �C with2 min of equilibration time between 10 �C and 100 �C; at eachisothermal step frequency sweeps were carried out.
Rheometry. Rheometrical studies were performed in a TA In-struments rheometer AR-G2 equipped with electrically heatedplates. Isothermal gelation experiments on freshly mixed sampleswere done using parallel plates of 25 mm in diameter and an initialstrain of 1% and 0.1% at the end of the experiments. Non-isothermalde-gelation experiments were performed using a strain of 0.5%. Allmeasurements were performed in the linear viscoelastic region.
3. Results
3.1. Kinetics and equilibrium of DA network F400-ABMI400
The studied network system consists of an amorphous multi-functional furan compound F400 and an amorphous bifunctionalbismaleimide ABMI400 that react via the DA reaction making anelastomeric network. The reaction of these two compounds isshown in Fig. 2.
FTIR spectroscopic studies of this system F400-ABMI400 at40 �C showed the reduction of the maleimide and furan absorbancepeaks and the increase of the absorbance peak corresponding to theDA adduct over time (see Supplementary Data Fig. S1). As expected,
Fig. 2. DA and retro DA reaction of the network forming system between F400 a
the absorbance values are small due to the dilution of functionalgroups. Therefore, the kinetics of the DA reaction for the networkforming system was studied isothermally by microcalorimetry anddifferential scanning calorimetry between 25 �C and 90 �C for amore reliable quantification. The reaction rate of the furan moietiesd [F]/dt can bewritten as a function of the kinetic constants kDA andkrDA of the forward and retro DA reaction, respectively, and theconcentration of furan [F], maleimide [M] and adduct [DA]:
d½F�dt
¼ �kDA½F�½M� þ krDA½DA� (3)
Stoichiometric mixing of both starting materials F400 andABMI400 leads to a simplification of this expression and as a resultthe conversion rate dx(t)/dt of furan or maleimide can be describedas:
dxðtÞdt
¼ kDA½F0�ð1� xðtÞÞ2 � krDAxðtÞ (4)
where [F0] ¼ [M0] is the initial concentration of furan or maleimideand x(t) is the furan or maleimide conversion.
The continuous heat flows dQ/dt in W g�1 obtained from thedifferent calorimetric experiments of isothermal curing (see Sup-plementary Data Fig. S2) were used to calculate the rate of con-version dx(t)/dt by means of the enthalpy of reaction DrH� in J g�1:
dxðtÞdt
¼ dQdt
1DrH� (5)
The conversions for specific reaction times at each reactiontemperature were calculated using Equations (4) and (5) for a givenset of the rate constants kDA and krDA. Fitting of the pre-exponentialfactors ADA and ArDA and the activation energies EDA and ErDA wasdone by minimizing the sum of squares of the difference betweenthe experimental and the modeled conversion data for all tem-perature profiles simultaneously, using the Solver tool of MSExcel.
nd ABMI400. The reversible covalent bonds are indicated with dotted lines.
Table 2Williams-Landel-Ferry parameters for the shift factor in F400-ABMI400 according toDEA and DMA.
WLF parameters aT (DEA) aT (DMA)
C1 [-] 16.9 23.5C2 [K] 47.1 51.3T0 [K] 288.15 (79.2 Hz) 278.15 (0.1 Hz)
M.M. Diaz et al. / Polymer 120 (2017) 176e188 179
The starting values for the kinetic parameters and the enthalpy ofreaction DrH� were those shown in Table 1 for previous studies [25].These Arrhenius parameters are linked to the thermodynamicequilibrium parameters of the DA reaction, i.e. the enthalpy of re-action△rH� and the entropy of reaction△rS� bymeans of the van'tHoff equation:
ln K ¼ ln�kDAkrDA
�¼ ln
xeq
½F0��1� xeq
�2!
¼ DrS�
R� DrH�
RT(6)
where K is the equilibrium constant, xeq is the equilibrium con-version, R is the gas constant, and T is the absolute temperature.
As the kinetic parameters of the DA and retro DA reaction arechanging during the fitting procedure, the reaction enthalpy DrH�
which is calculated as the difference between the activation energyof both reactions is also changing. As a result, the experimentalconversion rates (and conversions) derived from the heat flows arevarying during the iterations in the fitting process. The finalexperimental conversions and the fitting with the optimized pa-rameters are shown in Fig. 3. Themodel with optimized parameterssuccessfully describes the experimental data. It can be seen that athigher temperatures the DA reaction proceeds very fast but theretro DA reaction becomesmore important shifting the equilibriumand resulting in lower end conversions. On the contrary, for lowertemperatures the reaction advances slowly to higher end conver-sions. The optimized parameters for the network forming systemF400-ABMI400 are represented in Table 1. These parameters arecompared with parameters obtained previously for a networkforming system F2000-CBMI using a crystalline 1,1’-(methylenedi-4,1-phenylene) bismaleimide (CBMI) with a furan functionalizedcompound using a long jeffamine spacer [25], and with data foranother network forming system of the same CBMI and a trifunc-tional furan compound, pentaerythritol propoxylate tris(3-(furfur-ylthiol)-propionate (PPTF)) [24].
The data of Table 1 were also graphically presented in Arrheniusplots (see Supplementary Data Fig. S3) and van't Hoff plots (seeSupplementary Data Fig. S4). In the van't Hoff plots the equilibriumconstant K is shown in the temperature window in which the ki-netic study was carried out.
The system F2000-CBMI shares the same type of furan func-tionalized compound as F400-ABMI400, showing difference only inthe nature of the bismaleimide which explains the similarity be-tween the kinetic parameters. Note that vitrification was inter-fering in the temperature range of the kinetic study of PPTF-CBMI. Itwas observed that the system became mobility-restricted below45 �C, which is affecting the rates of reaction and the equilibriumconversion. Different values for ErDA of other rDA reactions havebeen reported, such as 64.8 kJ mol�1 for a network containing furanand maleimide functionalities on the polymethacrylate copolymerbackbone [28], 55.25 kJ mol�1 for a polymethacrylate copolymer
Table 1Optimized kinetic and equilibrium parameters for F400-ABMI400 in comparisonwith other furan-maleimide DA network systems.
Kinetic/equilibrium F400-ABMI400 F2000-CBMI PPTF-CBMI
Parameter [Unit] (Calorimetry) [25],a [24],a
ln (ADA) [kg mol�1 s�1] 11.7 13.1 9.7EDA [kJ mol �1] 53.9 55.7 48.0ln (ArDA) [s�1] 28.3 25.8 22.4ErDA [kJ mol�1] 105.7 94.2 88.0
DrH� [kJ mol�1] �51.8 �38.6 �40.0DrS� [J mol�1 K�1] �138.0 �105.3 �106.0
a Based on IR data.
cross-linked with CBMI [29] and 111.6 kJ mol�1 for the rDA reactionof a fullerene-cyclopentadiene system [30]. It is expected thatdifferent ErDA values appear due to differences in the chemicalstructure of the DA cycloadducts. For similar chemistries the valuesof ErDA are quite close to each other as it has been observed for thepolymethacrylate systems.
The CBMI bismaleimide used in previous studies (Table 1 [25]) iscrystalline at room temperature and one of the difficulties of its useis to achieve a homogeneous reversible network structure. Thepresence of CBMI above its melting temperature may lead to twoundesired effects. Firstly, a competition upon cooling betweenincorporation of the molten bismaleimide into the reversiblenetwork by DA reaction and (partial) phase separation by aggre-gation (or recrystallization) might reduce the amount of maleimidefunctionalities for the DA reaction and result in a lower reactionenthalpy and a non-homogeneous network structure. Secondly, thebismaleimide might undergo homopolymerization above itsmelting temperature, consuming maleimide functional groups inthis side reaction [25,31].
Due to the aliphatic nature of ABMI400, it shows very goodmiscibility with the other reacting compounds without the need ofsolvents or high temperatures for sample preparation. For self-healing applications this amorphous bismaleimide is advanta-geous because self-healing can be done without the introduction ofa solvent. Other studies in the frame of self-healing [32,33] haveproposed the use of other bismaleimides that are solid at roomtemperature and are used dissolved in DMF, toluene or phenylacetate. In one of these studies [32] the aliphatic bismaleimideprovided the least healing efficiency due to its higher flexibilitywith respect to the other non-aliphatic bismaleimides, which couldmake it react only at one side of the defect and not across it. Itshould be noted that this could happen if the damaged surfaces arenot brought into contact soon enough in combination with fast DAkinetics [34]. This example illustrates the importance of the DAcycloaddition kinetics to design a successful self-healing reversiblenetwork.
A final remark can be made regarding the thermodynamics ofthese DA reactions. In view of the reaction enthalpy obtained forthe formation of the Diels-Alder adduct (�51.8 kJ mol�1 for theF400-ABMI400 system, see Table 1), it can be concluded that thenew s bonds in the cycloadduct are much weaker than the otheravailable single bonds, typically around 368 kJ mol�1 for C-C bonds,and therefore would be preferentially broken in case of mechanicaldamage [35]. In this case it is assumed that both new s bonds in theDA cycloadduct are broken in a concerted process to reform thefuran (diene) and maleimide (dienophile) functionalities.
3.2. Simulation of effect of temperature and heating rate on Diels-Alder equilibrium
The obtained kinetic and thermodynamic parameters were usedto predict the equilibrium conversion of the F400-ABMI400reversible network at different temperatures, also outside thetemperature interval of the kinetic study (see Fig. 4). Simulations ofdifferent heating and cooling rates show the response of the systemto an applied temperature program. According to Fig. 4, the
Fig. 3. Experimental conversion of the system F400-ABMI400 from isothermal calorimetric experiments at 25 �C, 30 �C, 40 �C, 50 �C, 60 �C, 70 �C, 80 �C, 90 �C (-). Fitted conversions(- - -) according to data of Table 1.
Fig. 4. Simulated equilibrium conversion as a function of temperature for F400-ABMI400 (-), simulated conversion at heating rates of 0.1 �C min�1, 0.5 �C min�1 and 5 �C min�1 (- --) and simulated conversions at cooling rates of 0.1 �C min�1, 0.5 �C min�1 and 5 �C min�1 (…) according to data of Table 1.
M.M. Diaz et al. / Polymer 120 (2017) 176e188180
response of the system to heating rates as low as 0.1 �C min�1 is notinstantaneous; the system deviates from equilibrium and remainsat the same initial conversion until conversion starts decreasingaround 40 �C and finally follows again the equilibrium line above80 �C. For cooling programs, when starting at high temperatures,the equilibrium is followed only for very slow cooling rates such as0.1 �C min�1 until 80 �C where the reaction becomes too slow tofollow the equilibrium even for that slow cooling rate. These sim-ulations give an insight in the materials response to temperaturestimuli and are important for understanding the thermophysicalproperties of the material depending on the experimental appli-cation or measuring conditions. In out of equilibrium conditionswith a conversion x(T) above xeq(T), the endothermic retro DA re-action will be dominant and will be the driving force to restore the
equilibrium condition at the chosen temperature; in case x(T) isbelow xeq(T), the driving force to reach the equilibrium will be theforward exothermic DA reaction. The reliability of these simula-tions based on the kinetics of Table 1 is illustrated for the recoveryof equilibrium by the endothermic retro DA reaction after a tem-perature jump to 80 �C of a F400-ABMI400 network initially inequilibrium at room temperature (see Supplementary Data Fig. S5).
3.3. Tg-x relationship
Due to the interest of the network forming system for severalapplications, such as self-healing or adhesion on demand, the Tg-xrelationship of the reversible network is of importance. The evo-lution of the glass transition of the F400-ABMI400 system
M.M. Diaz et al. / Polymer 120 (2017) 176e188 181
throughout the curing was studied at 25 �C at different reactionstimes (see Fig. 5). The Tg of the network forming system increases ina non-linear way from an initial value of�42 �C for conversion x¼ 0to an equilibrium value of 3 �C for conversion x ¼ 0.92. This equi-librium Tg indicates that the system cured at room temperatureyields an elastomeric material. The equilibrium Tg between 40 �Cand 80 �C was also determined exhibiting lower equilibrium Tgvalues for increasing curing temperatures. This is consistent with adecreasing equilibrium conversion linked to lower cross-link den-sity with increasing temperature. Fig. 5 shows that the equilibriumTg values found at different temperatures follow the same trend asthose determined by partial curing at 25 �C. It demonstrates thatthe conversion path has no influence on the final result of Tg, givingrise to a unique Tg-x relationship for the reversible DA networkconsidered.
Vitrification of the network forming system F400-ABMI400 canbe predicted from the crossing of the Tg-x relationship with theequilibrium conversion curve (T-xeq, see Fig. 5). At conversionsbeyond this crossing, e.g. at reaction temperatures below 5 �C,diffusion control is expected.
The empirical DiBenedetto equation was used to model the Tg-xrelationship of this system:
Tg � Tg0Tg0 � Tg∞
¼ lx1� ð1� lÞx (7)
where Tg0 is the Tg of the mixture of unreacted components, Tg∞ isthe highest attainable Tg of the system if the reaction proceeds tofull conversion, x is the conversion and l is the ratio of the change inheat capacity at the Tg of the fully cured and the uncured system[36,37]. After fitting with the experimental Tg data, optimizedvalues of l ¼ 0.53 and Tg∞ ¼ 8 �C were found (see Fig. 5). The fittedvalue of l fairly compares with the experimental ratio of the step inthe heat capacity at Tg∞ and Tg0. For Tg∞ the step in the heat ca-pacity for a sample left for three weeks at 2 �C was used, leading toa l value of 0.64, which is close to the result obtained by fitting.Note that in a classical irreversible polymer network Tg∞ can beexperimentally determined. However, according to Fig. 5, fullconversion could only be attained at temperatures even lowerthan�30 �C; therefore, the experimental determination of Tg∞ for a
Fig. 5. Tg-x relationship for F400-ABMI400: Tg after partial curing at 25 �C (B), equilibriumEquilibrium conversion of F400-ABMI400 is also partly shown (solid curve).
fully cured sample is not possible due to diffusion limitations.
3.4. Thermo-mechanical properties
The thermo-mechanical properties of the reversible DA networkF400-ABMI400 were investigated using DMA at a single frequencyto demonstrate the appropriate temperature window for their useas reversible elastomer or thermoset. For comparison thermo-mechanical studies were also conducted on an irreversible epoxyamine model network made of DGEBA and J230. The DMA mea-surement of this irreversible system (see Supplementary DataFig. S6) shows the common traits of a thermoset. At low temper-atures the network shows a glassy behavior with a tensile storagemodulus E0 in the GPa level, around 85 �C the Tg region is observedas a drop in the storage modulus, a maximum in the loss angle andin the tensile loss modulus E”. Afterwards the rubber plateau isreached with a modulus in the MPa level which increases slightlywith increasing temperature.
The DMA measurement of the reversible network (see Fig. 6)shows a very different profile with respect to that of DGEBA-J230.The reversible network F400-ABMI400 exhibits a glassy behaviorat low temperatures below Tg, which is found here at 0 �C(maximum in E00) or 10 �C (maximum in loss angle). Above Tg therubber plateau is found and it extends between 30 �C and 70 �C. Athigher temperatures, however, the level of the rubber plateau islost and a significant decrease in E0 is observed. Theoretically thelevel of the rubber plateau is proportional to the cross-link densityand both the shear modulus G and the tensile modulus E can becalculated for simple test geometries using the Poisson's ratiowhich varies experimentally between 0.25 for thermosets and 0.49for rubbers [38]. A prediction of the E0 value of the rubber plateaufor the reversible network F400-ABMI400 was made assuming thatJ400 is the cross-linking point and calculating the number cross-link density as the number of J400 molecules per volume of sam-ple. The theoretical calculation gives a value of 4.2 MPa against avalue of 2.6 MPa observed experimentally at 30 �C. The slightlyhigher theoretical value is expected taking into account that thereversible network system F400-ABMI400 is not fully cross-linkedbecause even at room temperature full conversion is never reached(see Fig. 4). Additionally, the decrease of the tensile storage
Tg at 2 �C (▬), 25 �C (>), 40 �C (D), 60 �C (,), 80 �C (þ) and DiBenedetto model (- - -).
Fig. 6. DMA of F400-ABMI400 at 1 Hz: tensile storage modulus E’ (-), tensile loss modulus E” (…), and loss angle (-∙∙-).
M.M. Diaz et al. / Polymer 120 (2017) 176e188182
modulus E0 at high temperatures for this network can be explainedby the fact that increasing temperature further decreases conver-sion due to the increasing importance of the retro DA reaction andtherefore leads to a reduction of cross-link density. Although thenetwork F400-ABMI400 loses mechanical properties significantlyabove 80 �C due to the retro DA reaction, the system maintains itsgeometrical integrity without flow up to 113 �C (see Flow Proper-ties). The good agreement between the theoretical and experi-mental storage modulus at 30 �C is a confirmation of thehomogeneous nature of the F400-ABMI400 network as a result ofthe fully miscible amorphous bismaleimide, in contrast with thenon-homogeneous network F2000-CBMI with different thermo-mechanical properties due to the crystalline bismaleimide (seeSupplementary Data Fig. S7).
3.5. Dielectric analysis: effect of frequency
Dielectric analysis (DEA) was used to study the changesobserved in the reversible network F400-ABMI400 with increasingtemperature in a broad range of frequencies. A comparison wasmade with DMA at different frequencies (see Supplementary DataFigs. S8 and S9). Frequency sweeps in DEAwere performed during astepwise heating procedure. Since in the temperature window ofstudy the material is in the rubber state and in this condition themeasured polarization losses ε” (u,T) are usually masked by con-ductivity losses, the obtained data set was treated to isolate onlythe polarization losses ε
00pol (u,T) using the following expression
[39,40]:
ε}polðu; TÞ ¼ ��p2
� vε0ðu; TÞvðln uÞ (8)
where ε0 (u,T) is the permittivity (see Supplementary Data Fig. S10)and u is the angular frequency. Additionally the loss angle d wascalculated by the following expression taking into account only thepolarization losses:
tan d ¼ ε}polðu; TÞε0ðu; TÞ (9)
The loss angle data are shown in Fig. 7. From these data and the
permittivity ε0 (u,T) two frequency dependent phenomena are
observed. One of them is the glass transition temperature Tg whichis seen as a peak in the loss angle with an intensity of 6� (thin lines),and as a stepwise increase in the permittivity ε’ (u,T) (see Sup-plementary Data Fig. S10). The second transition occurs around40 �C above Tg and shows an increasing intensity in the loss angle(thick lines). This second transition is not observed in DMA. Addi-tional information was obtained from the conductivity s(u,T),which was calculated based on the following equation:
sðu; TÞ ¼ ε}ðu; TÞ � ε}polðu; TÞðε0uÞ
(10)
where ε0 is the permittivity in vacuum. All the extracted data fromDEA and DMA is collected in a frequency-temperature correlationmap (Fig. 8). For both Tg and the second transition the maximum ofthe loss angle was used. Since the discrete data points were notnecessarily indicating the exact position of the maximum of theloss angle, a continuous Gaussian curve with parameters a, b and cwas fitted to the data:
gðTÞ ¼ a,exp
� 12
�T � cb
�2!
(11)
where c is the center of the peak, b controls the width and a is theheight of the peak. The c value was used as the value of the tem-perature corresponding to the maximum of the loss angle (seeSupplementary Data Fig. S9). The frequency-temperature correla-tion map shows how the transitions vary with temperature andfrequency. In the left of the graph the second transition measuredin DEA is displayed; notice that the points from this transition donot follow a straight line in the whole temperature window whichindicates that this process is not a simple Arrhenius process.
The conductivity in the low temperature range follows a linearbehavior and around 45 �C it starts deviating from its linear path.This deviation indicates a change in activation energy probably dueto the gain in mobility as the cross-link density of the network isdecreased, as seen in the DMA measurement. It can be noticed thatboth the conductivity and the second transition follow a similartemperature dependency. Considering the data points between
Fig. 7. DEA of F400-ABMI400. Calculated loss angle of stepwise isothermal measurements for different frequencies: 1.81E-1, 9.16E-1, 6.95E0, 5.28E1, 1.19E2, 6.01E2, 1.35E3, 6.85E3,5.2E4, 1.76E5, 5.93E5 Hz. Tg (thin curves) and second transition (thick curves).
Fig. 8. Frequency-temperature correlation map of F400-ABMI400. Tg (C) observed from DMA, Tg (B), conductivity (þ), the second transition (>) seen in DEA. Tg prediction usingWLF parameters of Table 2 according to DEA and DMA (- - -). Trend lines for conductivity (between 25 �C and 50 �C) and for second transition (between 30 �C and 50 �C) accordingto calculated activation energies (-).
M.M. Diaz et al. / Polymer 120 (2017) 176e188 183
30 �C and 50 �C, the activation energy for both processes wascalculated, giving a value of 124 kJ mol�1 and 114 kJ mol�1 for theconductivity and second transition, respectively. These quitesimilar values are confirming the idea that the temperature de-pendency is shared for both processes. The fact that the secondtransition is not observed in DMA suggests the existence of in-terfaces or fractions of the material with different dielectric con-stants [41]. Since the chemistry of the components of the networkis of the same nature, the only possible source of interfaces mightoccur at high temperatures when the amorphous bismaleimideABMI400 disconnects from the network. This disconnection occursgradually; first at one side giving a network with dangling chains,and then fully at both sides generating a kind of sol-gel structure inwhich the gel is the remaining network and the sol the discon-nected moieties of bismaleimide. This disconnection of amorphousbismaleimide occurs at high temperatures.
In the frequency-temperature correlation map Tg is plotted formeasurements conducted by DEA and DMA. They both appear closeto each other, but not overlaying due to the different nature of themeasuring techniques. The Tg data points show the characteristiccurvature of Tg relaxations according to the Williams-Landel-Ferry(WLF) relationship [42]. The WLF parameters C1 and C2 for thereversible network F400-ABMI400 around Tg were determinedfrom both DEA and DMA data using the following expression:
ln aT ¼ � C1ðT � T0ÞC2 þ ðT � T0Þ
(12)
where aT is the shift factor and T0 a reference temperature. Datapoints were chosen between 10 �C and 50 �C for DEA and between5 �C and 25 �C for DMA. This choice of data points at low temper-atures guarantees that the predicted WLF parameters are not
M.M. Diaz et al. / Polymer 120 (2017) 176e188184
influenced by the reversible nature of the network. The referencetemperature was 15 �C at 79.2 Hz for DEA data and 5 �C at 0.1 Hz forDMA data. The results for the parameters C1 and C2 shown inTable 2 are close to the universal constants C1 ¼17.4 and C2 ¼ 51.6 K[43], obtained if the value of Tg is chosen for T0. The predictionsbased on these WLF parameters are shown in the frequency-temperature correlation map as dashed lines. Note that for theDEA data the upper Tg points at the highest frequencies are slightlydeviating from the predicted dashed line and shifted to lowertemperatures. Indeed, at increasing temperatures the cross-linkdensity of the reversible network starts lowering, and the Tgvalue no longer stays invariant during the measurement. In thisrespect, it might be interesting to perform DEA at even higherfrequencies to amplify this deviation and highlight the reversiblenature of the network.
3.6. Flow properties and gelation
Since an important goal of reversible DA networks is to be usedas self-healing materials and one of the desired features in suchmaterials is the generation of a mobile phase for the defects to besealed, the influence of gelation and de-gelation on flow propertieswas studied. The curing and network formation of the DA systemF400-ABMI400 was studied by rheometry for different isothermaltemperatures and the variation of the loss angle with time is shownin Fig. 9 (only profiles from 30 �C to 80 �C are plotted for clarity). Forall experiments the same trend is observed: an initial value of theloss angle close to 90� indicating a predominant viscous behavior,followed by a frequency independent region at the gelation pointaccording to the Winter criteria [44] and further on a region ofelastic behavior where cross-link density increases with decreasingvalues of the loss angle towards zero degrees. From these data thetime of gelation can be extracted and converted into a gelationconversion xgel with the use of the kinetics of Table 1. In Table 3 itcan be noticed that xgel for isothermal experiments up to 90 �C staysalmost constant; however, for higher temperatures this trend is lostand xgel decreases considerably for 100 �C and 110 �C. The sameconclusions can be drawn if the conversion at which the shearstorage modulus G0 equals the shear loss modulus G00 is taken as analternative definition for the gelation point, i.e. when the loss angle
Fig. 9. Dynamic rheometry of F400-ABMI400: Loss angle at 0.1 Hz showing isothermal gefrequencies of 0.1, 0.4 and 2.5 Hz, increasing as indicated by arrows.
gets 45� at an arbitrarily chosen frequency (see Table 3 for 0.1 and2.5 Hz).
Interestingly, it can be concluded that the gelation conversionfor all isothermal experiments stays constant up to 90 �C, i.e. as longas xeq exceeds xgel. This observation is consistent with curing inirreversible systems for which the gelation conversion is inde-pendent of temperature if the reaction mechanism is invariant. Incase of step growth polymerization, the Flory-Stockmayer equationcan be used to predict xgel as a function of the mixing ratio r and thefunctionality of the involved reacting compounds fM and fF [45,46]:
xgel ¼1ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
rðfM � 1Þðf F�1Þq (13)
If it is assumed that the mechanism of the DA process resemblesa step growth polymerization mechanism, a value of xgel of 0.63would be predicted taking into account a functionality of 3.4 for thefuran functionalized compound F400, as determined by NMRspectroscopy, a functionality of 2 for the bismaleimide ABMI400and a mixing ratio of 1. This prediction lies above the experimentalvalues of xgel and the difference stays constant for temperaturesbelow 90 �C. It must be noted that a value of 0.57 would be pre-dicted in case of stoichiometric mixing if the functionalities of thefuran functionalized compound and the maleimide were 4 (insteadof 3.4) and 2, respectively.
At higher temperatures between 90 and ca. 113 �C, xgel stays justslightly lower than xeq and follows exactly the trend of the equi-librium conversion line (see Table 3 and Fig. 10). In this limitedthermal interval the observation of ‘gelation’ and the correspond-ing xgel, according to rheological gelation criteria, is probably anelastic effect caused by the dynamics and lifetime of the reversiblebonds (see section 3.8). True gelation occurs for xgel < xeq (below90 �C). Gelation beyond 115 �C surely no longer occurs and thesystem at equilibrium behaves like a liquid according to theconsidered rheological criteria.
3.7. Simulation of solvent effect on Diels-Alder equilibrium and de-gelation
The effect of concentration of functional groups in the DA
lation at 30 �C (,), 40 �C (▪), 50 �C (△), 60 �C (:), 70 �C (C), 80 �C (>). Other
Table 3Isothermal gelation time tgel, the corresponding conversion xgel according to different gelation criteria, time to reach equilibrium teq (and xeq) and lifetime of the reversiblebond t(T).
T/�C tgela/min
tgelb/min
tgelc/min
xgela xgelb xgelc teq/min
xeq t(T)/s
30 181.3 191.0 208.0 0.552 0.568 0.588 7116.3 0.90 8.2Eþ0540 90 93.9 100.5 0.547 0.560 0.574 2978.0 0.87 2.1Eþ0550 48 49.7 52.6 0.547 0.559 0.572 1253.3 0.82 6.1Eþ0460 27.5 28.4 28.9 0.558 0.565 0.570 337.8 0.77 1.9Eþ0470 16.3 17.8 18.4 0.565 0.575 0.581 232.0 0.71 6.2Eþ0380 12.5 13.2 13.8 0.566 0.576 0.582 103.7 0.64 2.2Eþ0390 15.8 16.7 18.4 0.569 0.570 0.571 45.0 0.57 8.0Eþ02100 16.7 18.1 18.7 0.498 0.498 0.498 21.3 0.50 3.1Eþ02110 �8 �8 �8 �0.420 �0.420 �0.420 8.0 0.42 1.3Eþ02120 e e e e e e 2.7 0.35 5.6Eþ01
a Gelation time/conversion according to Winter criteria [44].b Gelation time/conversion according to loss angle equals 45� at 0.1 Hz.c Gelation time/conversion according to loss angle equals 45� at 2.5 Hz.
Fig. 10. Simulated equilibrium conversion of F400-ABMI400 (-), isothermal gelation conversion (B) and Flory-Stockmayer conversion prediction (d. d). Dilution effect: diluted 10times (- - -) and diluted 100 times (…) according to data of Table 1.
M.M. Diaz et al. / Polymer 120 (2017) 176e188 185
network F400-ABMI400 was also simulated, for example in caseswhere the system is diluted as it may occur in the presence of asolvent. The outcome is a decreased equilibrium conversion as seenin Fig. 10. If the system is diluted the equilibrium conversion de-creases significantly, as illustrated for a dilution of 10 or 100 times.Eventually if the network is too diluted the equilibrium conversionat room temperature xeq may drop below the gel conversion xgelleading to de-gelation of the network and the system gets molec-ularly dissolved.
A rheometry experiment in non-isothermal conditions wasperformed from 105 �C on, starting from the network cured at roomtemperature and then equilibrated at 105 �C, to check if the initialDA network F400-ABMI400 could be transformed again into aliquid and flow in the high temperature region, and at whichtemperature de-gelation occurs (see Fig. 11). The experiment wasconducted at different frequencies and the scanning rate of 0.5 �Cmin�1 was slow enough to guarantee that the network remained inequilibrium throughout the experiment. Initially a low loss angleindicating elastic behavior is observed, in agreement with theisothermal measurements of Fig. 9 and confirming the gelled
structure of F400-ABMI400 at equilibrium. The loss angle startsincreasing beyond 45� around 113 �C until it goes through a fre-quency independent region around 115 �C, and finally reaches avalue close to 90� exhibiting a viscous response. This non-isothermal experiment clearly shows the reversible nature of thenetwork and how it can be de-gelled to a liquid state again beyond115 �C.
Using all rheological and DMA data the T-dependence of thedynamic modulus at xeq (jG*
eqj) and the corresponding loss anglewas plotted in Fig. 12. It can be noticed how jG*eqj decreases forincreasing temperatures and how it drops abruptly above 100 �C.Above this temperature, the loss angle shows a tremendous in-crease indicating the transition of the system from elastic to viscousbehavior. The dynamic modulus at xgel (jG*
gelj), using the Wintercriteria, is also shown in Fig. 12; jG*
gelj decreases slightly forincreasing temperatures. At high temperatures jG*
gelj and jG*eqj
come close to each other, showing how the properties of thestrengthenedmelt (jG*
eqj) become very similar to those observed atgelation (jG*
gelj) between 110 �C and 115 �C.
Fig. 11. De-gelation of F400-ABMI400 at scanning rate of 0.5 �C min�1 and frequency of 0.46 Hz (△), 2.154 Hz (,), and 4.64 Hz (B).
Fig. 12. T-dependence of dynamic modulus at xgel (jG*gelj; A) and at xeq (jG*
eq j; C), and of loss angle at xeq (B).
M.M. Diaz et al. / Polymer 120 (2017) 176e188186
3.8. Dynamics of reversible bonds
The interpretation of the results in the high temperature rangebeyond 90 �C can be linked to the dynamics of the reversiblenetwork. At higher temperatures the rates of the forward and retroDA reaction increase conferring the network with a dynamiccharacter. This can be quantified by calculating the lifetime t(T) ofthe reversible bonds at different temperatures using the followingequation, where [DA] is the concentration of the DA adduct andkrDA is the rate constant for the retro DA reaction:
tðTÞ ¼ ½DA�½DA�krDA
¼ 1krDA
(14)
The lifetime of a bond is defined by the concentration of thebond divided by its rate of disappearance, a definition which hasalso been used to describe the lifetime of supramolecular
interactions in supramolecular polymers [47]. The lifetime of thesupramolecular interactions has a clear influence on the macro-scopic properties of these materials [48,49]. The lifetime of thereversible bonds in the DA network F400-ABMI400 decreases withincreasing temperature (see Table 3 and Supplementary DataFig. S11); at temperatures beyond 90 �C the lifetime is only a fewhundreds or tens of seconds. In this temperature region, the life-time of the covalent bond is getting too short to be considered as‘permanent’, in contrast with a classical network with irreversiblecross-links and permanent covalent bonds of ‘infinite’ lifetime, forwhich the gel point conversion can be predicted by the Flory-Stockmayer equation in case of step growth polymerization. In atransition zone, between 90 and 115 �C, the system still shows adominant elastic character and flow of the material is not noticedwhile the equilibrium conversion progressively decreases. Thesystem gets dynamic but the lifetime of the bond is still long
M.M. Diaz et al. / Polymer 120 (2017) 176e188 187
enough to strengthen the liquid state, and the system behaves like a‘strong’ liquid in this narrow temperature window. At highertemperatures from 115 �C on the lifetime becomes so short that thisstrengthening effect is lost. This ‘strong’ liquid effect is typical forvitrimers [15]. Similar effects have been noticed in the melt of su-pramolecular thermoplastics and thermoplastic elastomers madeby the electrostatic association between two types of triblock co-polymers [50]. A characteristic of strong liquids, as often seen ininorganic materials, is that their viscosity above the glass transition(Tg) is changing with temperature in a gradual manner following anArrhenius law, as opposed to ‘fragile’ liquids for which the viscosityis changing more abruptly with temperature according to the WLFbehavior [51,52].
It is important to note that the transient temperature zone fromelastic network properties to viscous liquid properties depends onthe time scale of the dynamic measurement in relation to thetemperature dependent lifetime t (T) of the reversible bond.
4. Conclusions
The reversible DA network F400-ABMI400 was synthesizedusing an amorphous bismaleimide which facilitated the study ofdifferent aspects of the network such as the DA cycloaddition ki-netics, the Tg-x relationship and the (de)gelation properties. Thekinetic and thermodynamic parameters of the furan-maleimidereversible DA cycloaddition model as shown in Fig. 2 were opti-mized to fit the experimental calorimetric data between 25 �C and90 �C. The optimized parameters of the kinetic study were used tosimulate (i) the equilibrium DA conversion for different tempera-tures, (ii) the effects of different heating/cooling rates and (iii) thechanges in conversion due to dilution of the network. At moderatetemperatures (around room temperature) the system is slowlyresponding to an applied temperature program and under verydilute conditions the network can be completely dissolved. The Tg-xrelationship of F400-ABMI400 was found to be independent of theconversion path followed.
The optimized DA kinetic and thermodynamic datawere used tofurther evaluate important thermophysical properties of thereversible network F400-ABMI400, showing clear differences inthermo-mechanical properties with respect to traditional irre-versible networks. At room temperature the network has elasto-meric properties with a glass transition Tg of 3 �C. Due to thereduction of cross-link density with increasing temperatureinduced by the increasing importance of the retro DA reaction, adecrease in the rubber tensile storage modulus E0 was observedabove 80 �C, and this drop showed no frequency dependency, asopposed to the behavior at Tg. It was interpreted as the macroscopiceffect of the breaking down of the network.
Dielectric studies of F400-ABMI400 revealed how at increasingtemperatures an interface is created between the still existingnetwork and disconnected bismaleimide moieties as the systemstarts de-cross-linking. The study of the WLF parameters around Tgshowed a very close resemblance with the universal constants andalso evidenced the decrease in Tg at increasing temperatures due toa reduction in cross-link density.
The reversibility of the F400-ABMI400 polymer network wasfurther demonstrated by showing that the network can be de-gelled at increasing temperatures to reach again a viscous flowbehavior. The rheological study showed how the lifetime of thereversible bonds, which is connected to the kinetics of the retro DAreaction, affects the flowbehavior. In particular it was observed thatthe lifetime of the DA adduct between 90 �C and 115 �C is still longenough to confer the system with an elastic strengthening effect.For higher temperatures the lifetime of the DA adduct is too shortand the strengthening effect is lost.
Finally, it should be noticed that the thermodynamics of the DAreaction suggest that the new covalent s bonds in the cycloadductare much weaker than the other covalent single bonds in thenetwork system, which makes them prone to be broken mechan-ically. It means that when the F400-ABMI400 network suffersdamage it is more likely to have the DA adducts broken rather thanany other network bonds. This effect of preferential cleavage ofbonds which can reversibly be reformed, in combination withsuited thermo-mechanical and rheological properties, is of impor-tance for sealing/healing micro-cracks at application temperaturein reversible polymer networks based on DA chemistry. The ki-netics and equilibrium of the DA cycloaddition in the F400-ABMI400 network are suitable for this purpose: (i) a mild heatingbetween 90 �C and 115 �C should be sufficient to generate mobilityto seal micro-sized defects in the material, (ii) heating above 115 �Ccauses the loss of the geometrical integrity of the piece and isrecommended for healing of macro-cracks and for recycling or re-shaping the network, (iii) even at room temperature the ratherslow DA kinetics should allow healing of fresh micro-crack surfacesin a time frame of a few hours. The sealing/healing properties ofF400-ABMI400 will be investigated in future work.
Acknowledgements
M.M.D. gratefully acknowledges SIM (Strategic Initiative Mate-rials in Flanders) in the framework of the SHE-NAPROM (SIM 2009-1, SBO 3) project for the financial support. We also thank MatildaLarsson for her assistance with the dielectric analysis.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.polymer.2017.05.058.
References
[1] S. van Der Zwaag, An introduction to material design principles: damageprevention versus damage management, in: Self Heal. Mater. Altern.Approach to 20 Centuries Mater. Sci, Springer, 2007, pp. 1e18, http://dx.doi.org/10.1007/978-1-4020-6250-6_1.
[2] J. Odent, J.-M. Raquez, C. Samuel, S. Barrau, A. Enotiadis, P. Dubois,E.P. Giannelis, Shape-Memory behavior of polylactide/silica ionic hybrids,Macromolecules 50 (2017) 2896e2905, http://dx.doi.org/10.1021/acs.macromol.7b00195.
[3] T.L. Sun, T. Kurokawa, S. Kuroda, A. Bin Ihsan, T. Akasaki, K. Sato, M.A. Haque,T. Nakajima, J.P. Gong, Physical hydrogels composed of polyampholytesdemonstrate high toughness and viscoelasticity, Nat. Mater 12 (2013)932e937, http://dx.doi.org/10.1038/nmat3713.
[4] J.-Y. Sun, X. Zhao, W.R.K. Illeperuma, O. Chaudhuri, K.H. Oh, D.J. Mooney,J.J. Vlassak, Z. Suo, Highly stretchable and tough hydrogels, Nature 489 (2012)133e136, http://dx.doi.org/10.1038/nature11409.
[5] K.P. Nair, V. Breedveld, M. Weck, Complementary hydrogen-bonded ther-moreversible polymer networks with tunable properties, Macromolecules 41(2008) 3429e3438, http://dx.doi.org/10.1021/ma800279w.
[6] Y. Chen, Z. Guan, Multivalent hydrogen bonding block copolymers self-assemble into strong and tough self-healing materials, Chem. Commun. 50(2014) 10868e10870, http://dx.doi.org/10.1039/c4cc03168g.
[7] T. Ware, K. Hearon, A. Lonnecker, K.L. Wooley, D.J. Maitland, W. Voit, Triple-shape memory polymers based on self-complementary hydrogen bonding,Macromolecules 45 (2012) 1062e1069, http://dx.doi.org/10.1021/ma202098s.
[8] D.C. Tuncaboylu, M. Sari, W. Oppermann, O. Okay, Tough and self-healinghydrogels formed via hydrophobic interactions, Macromolecules 44 (2011)4997e5005, http://dx.doi.org/10.1021/ma200579v.
[9] A. Ustinov, H. Weissman, E. Shirman, I. Pinkas, X. Zuo, B. Rybtchinski, Supra-molecular polymers in aqueous Medium: rational design based on directionalhydrophobic interactions, J. Am. Chem. Soc. 133 (2011) 16201e16211, http://dx.doi.org/10.1021/ja2066225.
[10] E. Obert, M. Bellot, L. Bouteiller, F. Andrioletti, C. Lehen-Ferrenbach, F. Bou�e,Both water- and organo-soluble supramolecular polymer stabilized byhydrogen-bonding and hydrophobic interactions, J. Am. Chem. Soc. 129(2007) 15601e15605, http://dx.doi.org/10.1021/ja074296l.
[11] Q. Wang, Y. Bai, Y. Chen, J. Ju, F. Zheng, T. Wang, High performance shapememory polyimides based on pep interactions, J. Mater. Chem. A 3 (2015)
M.M. Diaz et al. / Polymer 120 (2017) 176e188188
352e359, http://dx.doi.org/10.1039/c4ta05058d.[12] S. Burattini, B.W. Greenland, D.H. Merino, W. Weng, J. Seppala,
H.M. Colquhoun, W. Hayes, M.E. Mackay, I.W. Hamley, S.J. Rowan, A healablesupramolecular polymer blend based on aromatic p - p stacking andhydrogen-bonding interactions, J. Am. Chem. Soc. 132 (2010) 12051e12058,http://dx.doi.org/10.1021/ja104446r.
[13] G.R. Whittell, M.D. Hager, U.S. Schubert, I. Manners, Functional soft materialsfrom metallopolymers and metallosupramolecular polymers, Nat. Mater 10(2011) 176e188, http://dx.doi.org/10.1038/nmat2966.
[14] C.-F. Chow, S. Fujii, J.M. Lehn, Metallodynamers: neutral dynamic metal-losupramolecular polymers displaying transformation of mechanical andoptical properties on constitutional exchange, Angew. Chem. - Int. Ed. 46(2007) 5007e5010, http://dx.doi.org/10.1002/anie.200700727.
[15] D. Montarnal, M. Capelot, F. Tournilhac, L. Leibler, Silica-like malleable ma-terials from permanent organic networks, Science 334 (2011) 965e968,http://dx.doi.org/10.1126/science.1212648.
[16] J.M. Lehn, Dynamers: dynamic molecular and supramolecular polymers, Prog.Polym. Sci. 30 (2005) 814e831, http://dx.doi.org/10.1071/CH10035.
[17] J. Canadell, H. Goossens, B. Klumperman, Self-healing materials based on di-sulfide links, Macromolecules 44 (2011) 2536e2541, http://dx.doi.org/10.1021/ma2001492.
[18] Y. Higaki, H. Otsuka, A. Takahara, A thermodynamic polymer cross-linkingsystem based on radically exchangeable covalent bonds, Macromolecules 39(2006) 2121e2125, http://dx.doi.org/10.1021/ma052093g.
[19] C. Yuan, M.Z. Rong, M.Q. Zhang, Z.P. Zhang, Y.C. Yuan, Self-healing of polymersvia synchronous covalent bond fission/radical recombination, Chem. Mater 23(2011) 5076e5081, http://dx.doi.org/10.1021/cm202635w.
[20] X. Chen, M.A. Dam, K. Ono, A. Mal, H. Shen, S.R. Nutt, K. Sheran, F. Wudl,A thermally Re-mendable cross-linked polymeric material, Science 295 (2002)1698e1702, http://dx.doi.org/10.1126/science.1065879.
[21] Y.-L. Liu, C.-Y. Hsieh, Crosslinked epoxy materials exhibiting thermal remen-dablility and removability from multifunctional maleimide and furan com-pounds, J. Polym. Sci. Part A Polym. Chem. 44 (2006) 905e913, http://dx.doi.org/10.1002/pola.21184.
[22] M. Wouters, E. Craenmehr, K. Tempelaars, H. Fischer, N. Stroeks, J. van Zanten,Preparation and properties of a novel remendable coating concept, Prog. Org.Coatings 64 (2009) 156e162, http://dx.doi.org/10.1016/j.porgcoat.2008.09.023.
[23] J. K€otteritzsch, S. Stumpf, S. Hoeppener, J. Vitz, M.D. Hager, U.S. Schubert, One-Component intrinsic self-healing coatings based on reversible crosslinking bydiels-alder cycloadditions, Macromol. Chem. Phys. 214 (2013) 1636e1649,http://dx.doi.org/10.1002/macp.201200712.
[24] B.J. Adzima, H.A. Aguirre, C.J. Kloxin, T.F. Scott, C.N. Bowman, Rheological andchemical analysis of reverse gelation in a covalently cross-linked diels - alderpolymer network, Macromolecules 41 (2008) 9112e9117, http://dx.doi.org/10.1021/ma801863d.
[25] G. Scheltjens, M.M. Diaz, J. Brancart, G. Van Assche, B. Van Mele, A self-healingpolymer network based on reversible covalent bonding, React. Funct. Polym.73 (2013) 413e420, http://dx.doi.org/10.1016/j.reactfunctpolym.2012.06.017.
[26] F. Fringuelli, A. Tattichi, Dienes in the Diels-alder Reaction, Wiley, New York,1990.
[27] C.J. Kloxin, C.N. Bowman, Covalent adaptable networks: smart, reconfigurableand responsive network systems, Chem. Soc. Rev. 42 (2013) 7161e7173,http://dx.doi.org/10.1039/c3cs60046g.
[28] R.K. Bose, J. K€otteritzsch, S.J. Garcia, M.D. Hager, U.S. Schubert, S. van derZwaag, A rheological and spectroscopic study on the kinetics of self-healing ina single-component diels-alder copolymer and its underlying chemical reac-tion, J. Polym. Sci. Part A Polym. Chem. 52 (2014) 1669e1675, http://dx.doi.org/10.1002/pola.27164.
[29] A.A. Kavitha, N.K. Singha, “Click chemistry” in tailor-made polymethacrylatesbearing reactive furfuryl functionality: a new class of self-healing polymericmaterial, ACS Appl. Mater. Interfaces 1 (2009) 1427e1436, http://dx.doi.org/10.1021/am900124c.
[30] L.M. Giovane, J.W. Barco, T. Yadav, A.L. Lafleur, J.A. Marr, J.B. Howard,V.M. Rotello, Kinetic stability of the C-60-cyclopentadiene Diels-Alder adduct,J. Phys. Chem. 97 (1993) 8560e8561, http://dx.doi.org/10.1021/j100135a004.
[31] M.-F. Grenier-Loustalot, L.D.A. Cunha, Sterically hindered bismaleimideMonomer: molten state reactivity and kinetics of polymerization, Eur. Polym.J. 34 (1998) 95e102, http://dx.doi.org/10.1016/S0014-3057(97)00046-3.
[32] P.A. Pratama, A.M. Peterson, G.R. Palmese, The role of maleimide structure inthe healing of furan-functionalized epoxyeamine thermosets, Polym. Chem. 4(2013) 5000e5006, http://dx.doi.org/10.1039/c3py00084b.
[33] P.A. Pratama, M. Sharifi, A.M. Peterson, G.R. Palmese, Room temperature self-healing thermoset based on the diels � alder reaction, ACS Appl. Mater. In-terfaces 5 (2013) 12425e12431, http://dx.doi.org/10.1021/am403459e.
[34] P. Cordier, F. Tournilhac, C. Souli�e-Ziakovic, L. Leibler, Self-healing and ther-moreversible rubber from supramolecular assembly, Nature 451 (2008)977e980, http://dx.doi.org/10.1038/nature06669.
[35] M.M. Caruso, D.A. Davis, Q. Shen, S.A. Odom, N.R. Sottos, S.R. White, J.S. Moore,Mechanically-induced chemical changes in polymeric materials, Chem. Rev.109 (2009) 5755e5798, http://dx.doi.org/10.1021/cr9001353.
[36] A.T. DiBenedetto, Prediction of the glass transition temperature of polymers: amodel based on the principle of corresponding states, J. Polym. Sci. Part BPolym. Phys. 25 (1987) 1949e1969, http://dx.doi.org/10.1002/polb.1987.090250914.
[37] J.P. Pascault, R.J.J. Williams, Glass transition temperature versus conversionrelationships for thermosetting polymers, J. Polym. Sci. Part B Polym. Phys. 28(1990) 85e95, http://dx.doi.org/10.1002/polb.1990.090280107.
[38] L.J. Fetters, D.J. Lohse, R.H. Colby, Chain dimensions and entanglement spac-ings, in: J.E. Mark (Ed.), Phys. Prop. Polym. Handb, second ed., Springer, NewYork, 2007, pp. 447e454.
[39] P.A.M. Steeman, J. van Turnhout, Fine structure in the parameters of dielectricand viscoelastic relaxations, Macromolecules 27 (1994) 5421e5427, http://dx.doi.org/10.1021/ma00097a023.
[40] C.J.F. B€ottcher, P. Bordewijk, The empirical description of dielectric relaxation,in: second ed.Theory Electr. Polariz. Dielectr. Time-dependent Fields, vol. II,Elsevier, Amsterdam, 1996, pp. 45e138.
[41] A. Vassilikou-Dova, I.M. Kalogeras, Dielectric analysis (DEA), in: J.D. Menczel,R.B. Prime (Eds.), Therm. Anal. Polym, Wiley, 2009.
[42] D.W. Van Krevelen, K.T. Nijenhuis, Properties of Polymers, fourth ed., Elsevier,2009.
[43] K.L. Ngai, D.J. Plazek, Temperature dependences of the viscoelastic response ofpolymer systems, in: J.E. Mark (Ed.), Phys. Prop. Polym. Handb, second ed.,Springer, New York, 2007, pp. 455e478.
[44] H. Winter, Can the gel point of a cross-linking polymer Be detected by the G’and G" crossover? Polym. Eng. Sci. 27 (1987) 1698e1702, http://dx.doi.org/10.1002/pen.760272209.
[45] J. Flory, Molecular size distribution in three dimensional polymers. I. Gelation,J. Am. Chem. Soc. 63 (1941) 3083e3090, http://dx.doi.org/10.1021/ja01856a061.
[46] W.H. Stockmayer, Theory of molecular size distribution and gel formation inbranched-chain polymers, J. Chem. Phys. 11 (1943) 45, http://dx.doi.org/10.1063/1.1723803.
[47] L. Leibler, M. Rubinstein, R.H. Colby, Dynamics of reversible networks, Mac-romolecules 24 (1991) 4701e4707, http://dx.doi.org/10.1021/ma00016a034.
[48] H. Hofmeier, R. Hoogenboom, M.E.L. Wouters, U.S. Schubert, High molecularweight supramolecular polymers containing both terpyridine metal com-plexes and ureidopyrimidinone quadruple hydrogen-bonding units in themain chain, J. Am. Chem. Soc. 127 (2005) 2913e2921, http://dx.doi.org/10.1021/ja042919e.
[49] T. Aida, E.W. Meijer, S.I. Stupp, Functional supramolecular polymers, Science335 (2012) 813e817, http://dx.doi.org/10.1126/science.1205962.
[50] L. Voorhaar, M.M. Diaz, F. Leroux, S. Rogers, A.M. Abakumov, G. Van Tendeloo,G. Van Assche, B. Van Mele, R. Hoogenboom, Supramolecular thermoplasticsand thermoplastic elastomer materials with self-healing ability based onoligomeric charged triblock copolymers, NPG Asia Mater 9 (2017), http://dx.doi.org/10.1038/am.2017.63.
[51] C.A. Angell, Relaxation in liquids, polymers and plastic crystals d strong/fragile patterns and problems, J. Non. Cryst. Solids 131e133 (1991) 13e31,http://dx.doi.org/10.1016/0022-3093(91)90266-9.
[52] C.A. Angell, formation of glasses from liquids and biopolymers, Science 267(1995) 1924e1935, http://dx.doi.org/10.1126/science.267.5206.1924.
