Electrically-Responsive Reversible Polyketone/MWCNT Network Through Diels-Alder Chemistry

polymers

Article Electrically-Responsive Reversible Polyketone/MWCNT Network through Diels-Alder Chemistry

Rodrigo Araya-Hermosilla 1,*, Andrea Pucci 2 , Patrizio Raffa 3 , Dian Santosa 3, Paolo P. Pescarmona 3 ,Régis Y. N. Gengler 4, Petra Rudolf 4 , Ignacio Moreno-Villoslada 5 and Francesco Picchioni 3,*

1 Programa Institucional de Fomento a la Investigación, Desarrollo e Innovación, Universidad Tecnológica Metropolitana, Ignacio Valdivieso 2409, P.O. Box 8940577, San Joaquín, Santiago 8940000, Chile 2 Department of Chemistry and Industrial Chemistry, University of Pisa, Via Moruzzi 13, 56124 Pisa, Italy; [email protected] 3 Department of Chemical Product Engineering, ENTEG, University of Groningen, Nijenborgh 4, 9747AG Groningen, The Netherlands; [email protected] (P.R.); [email protected] (D.S.); [email protected] (P.P.P.) 4 Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747AG Groningen, The Netherlands; [email protected] (R.Y.N.G.); [email protected] (P.R.) 5 Laboratorio de Polímeros, Instituto de Ciencias Químicas, Facultad de Ciencias, Universidad Austral de Chile, Valdivia 5110033, Chile; [email protected] * Correspondence: [email protected] (R.A.-H.); [email protected] (F.P.); Tel.: +56-2-27877911 (R.A.-H.); +31-50-3634333 (F.P.) Received: 7 September 2018; Accepted: 25 September 2018; Published: 28 September 2018

Abstract: This study examines the preparation of electrically conductive polymer networks based on furan-functionalised polyketone (PK-Fu) doped with multi-walled carbon nanotubes (MWCNTs) and reversibly crosslinked with bis-maleimide (B-Ma) via Diels-Alder (DA) cycloaddition. Notably, the incorporation of 5 wt.% of MWCNTs results in an increased modulus of the material, and makes it thermally and electrically conductive. Analysis by X-ray photoelectron spectroscopy indicates that MWCNTs, due to their diene/dienophile character, covalently interact with the matrix via DA reaction, leading to effective interfacial adhesion between the components. Raman spectroscopy points to a more effective graphitic ordering of MWCNTs after reaction with PK-Fu and B-Ma. After crosslinking the obtained composite via the DA reaction, the softening point (tan(δ) in dynamic mechanical analysis measurements) increases up to 155 ◦C, as compared to the value of 130 ◦C for the PK-Fu crosslinked with B-Ma and that of 140 ◦C for the PK-Fu/B-Ma/MWCNT nanocomposite before resistive heating (responsible for crosslinking). After grinding the composite, compression moulding (150 ◦C/40 bar) activates the retro-DA process that disrupts the network, allowing it to be reshaped as a thermoplastic. A subsequent process of annealing via resistive heating demonstrates the possibility of reconnecting the decoupled DA linkages, thus providing the PK networks with the same thermal, mechanical, and electrical properties as the crosslinked pristine systems.

Keywords: functionalised polyketone; MWCNT; electrically conductive plastic nanocomposite; reversible Diels-Alder; recycling; resistive heating annealing; reworkability

Polymers 2018, 10, 1076; doi:10.3390/polym10101076 www.mdpi.com/journal/polymers Polymers 2018, 10, 1076 2 of 15

1. Introduction Conductive polymer nanocomposites are an important class of materials used in a broad range of applications, including supercapacitors, solar cells, conductive adhesives, advanced electronic devices, among many others [1]. Such nanocomposites based on the direct mixing between carbon nanotubes (CNTs) and thermoplastic polymers often resulted in materials with outstanding performance due to the synergistic effect between the inherent features of CNTs and the type of interaction at the interface with the matrix [2]. The interface stability for CNT-reinforced thermoplastics is difficult to achieve due to the lack of effective interfacial adhesion between the components. As a consequence, for instance, thermoplastics display poor dispersion of CNTs during the melt state mixing process, thus resulting in poor exfoliation and aggregate structures [3]. New approaches, such as in-situ chemical and electrochemical polymerisation have substantially improved the effective exfoliation of entangled agglomerates of CNT bundles and ropes in thermoplastic matrices [4]. The latest progress in the field established that the dispersion, exfoliation, and stabilisation of CNTs are enhanced when an effective interfacial covalent bond is formed between filler and matrix [5]. Accordingly, the most common approach consists of grafting reactive moieties to the edges and sidewalls of CNTs, which then covalently interact with the polymer matrix [6,7]. However, this method presents disadvantages regarding the chemical oxidation of the sp2 graphitic arrangement of CNTs, thus diminishing their conductivity and mechanical performance [8,9]. An emerging alternative to the above problems is the direct mixing of CNTs with thermosetting resins that crosslink during conventional liquid processing. The employed chemical routes barely disrupt the graphitic structure of CNTs upon curing into solid structures [10,11]. CNT-polymer crosslinked nanocomposites offer many superior properties compared to thermoplastics-based ones, such as mechanical strength, dimensional stability at elevated temperature, solvent resistance, and reliable electric performance owing to the conductive network of well-dispersed CNTs [5]. Unfortunately, crosslinked polymer nanocomposites are generally not re-processable after their service life, thus hindering their recyclability in the “cradle to cradle” approach, a keystone in the current efforts for sustainability. A promising strategy currently employed in the fabrication of re-workable and re-processable crosslinked polymer systems is the incorporation of thermally reversible moieties used for reprocessing (and recycling) the network after the product’s life ends [12]. The most common approach is based on Diels-Alder (DA) chemistry (reversible covalent cycloaddition) [13–15]. Mainly, crack propagation in DA crosslinked polymers is more likely to take place at the DA linkages because DA covalent bonds (given their shear sensitive character) are weaker than regular covalent bonds [16]. After grinding, thermal mending procedures provide the energy to activate the functional moieties and the process of reshaping and reconnection of linkages [17–19]. As a next step, thermal annealing is commonly performed to improve the structural properties of the material [20,21]. The latter further promotes the intrinsic ability of thermally reversible polymers to re-connect local areas with fast kinetics. For these mechanisms to come into play, DA active groups have to be present along the backbone, and this often requires long and cumbersome polymerisations and/or modification strategies. An easier alternative is to introduce alternating aliphatic polyketones (PK) by copolymerisation of carbon monoxide, ethylene, and propylene via the Paal-Knorr reaction to insert furan groups directly attached to the backbone chain [20,22,23]. This chemical reaction proceeds in the bulk with high yields and relatively fast kinetics, producing water as the only by-product [24]. The grafted furan groups allow for the formation of a three-dimensional polymer network after crosslinking with aromatic bis-maleimide. The material indeed formed a thermally reversible and self-healing thermoset by means of a DA and retro-DA (r-DA) sequence employing conventional heating procedures and post-curing thermal annealing [20]. The combination of such materials with CNTs should then merge the advantages of thermosetting nanocomposites (see above) with the self-healing and recycling possibilities of this easily affordable (and thus industrially attractive) resin. Herein we report on an intrinsic (thermally reversible) crosslinked conductive nanocomposite that displays thermomechanical re-workability, stable electrical response, annealing by resistive Polymers 2018, 10, 1076 3 of 15 heating, and which behaves like a thermoplastic upon heating/pressing for recycling. This electrically conductive nanocomposite stems from the chemical modification of an alternating aliphatic polyketone grafted with furan groups (PK-Fu) via the Paal-Knorr reaction, crosslinked with B-Ma and reinforced with multi-walled carbon nanotubes (MWCNTs) via reversible Diels-Alder cycloaddition. Spectroscopic tools were employed to study the modification of the starting polyketone, the crosslinking of the resulting polymer, and the reversible chemical bonding with the nanofiller via the DA and retro-DA sequences. Thermomechanical tests allowed evaluating the mechanical performance, re-workability, and recyclability of the composite. The results concerning the material’s modulus and impedance measurements before and after recycling were contrasted with further electrical resistive heating annealing procedures in order to simultaneously improve the mechanical and electrical performance of the nanocomposite. The intrinsic chemical and resistive heating responses were studied by attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy and IR thermography, respectively. The morphology of the system and dispersion/exfoliation of MWCNTs were analysed by electronic microscopy before and after annealing by resistive heating.

2. Materials and Methods The alternating aliphatic polyketone (PK) was synthesised according to Mul et al. [25]. The resulting co- and ter-polymer of carbon monoxide presents a total oleﬁn content of 30% of ethylene and 70% of propylene (PK30, MW 2687 Da). Furfurylamine (Fu, Sigma Aldrich, ≥99%, Zwijndrecht, The Netherlands) was freshly distilled before use, and benzylamine (Bea) (Sigma Aldrich 99%), MWCNTs (O.D. 6–9 nm, average length 5 µm, Sigma Aldrich 95% carbon), DMSO-d6 (Laboratory-Scan, 99.5%, The Netherlands), (1,1-(methylenedi-4,1-phenylene)bis-maleimide (B-Ma, Sigma Aldrich 95%), 1-Methyl-2-pyrrolidone (NMP, Sigma Aldrich, 99.5%), Dimethylformamide (DMF, Sigma Aldrich 99.8%), tetrahydrofuran (THF, Laboratory-Scan, 99.5%), chloroform (CHCl3, Laboratory-Scan, 99.5%) and deuterated chloroform (CDCl3, Sigma Aldrich 99.8 atom% D) were purchased and used as received.

2.1. Functionalisation of Polyketone with Furan and Benzyl Groups The reaction between PK and furfurylamine was carried out in bulk by the Paal-Knorr reaction, and the molar ratio between the reactants (i.e., the di-carbonyl group of polyketone and the amine group of furfurylamine) was established as percentages with a maximal conversion of 80% according to Zhang et al. and Toncelli et al. [20,21]. In order to establish more clearly the role of the furan motifs, a polymer grafted with benzyl groups was prepared by the Paal-Knorr reaction and used as a reference. This compound, called PK-Bea hereafter, displays the same backbone structure as PK-Fu, but has non-reactive pendant groups, at least in our conditions, via Diels-Alder cycloaddition (Figure1, see SI1 for experimental procedures).

2.2. Functionalisation of MWCNTs with PK-Fu or B-Ma via Diels-Alder Reaction PK-Fu (0.95 g) was dissolved in 5 mL of NMP, and added to 0.05 g of MWCNTs (previously sonicated for 30 min in 5 mL of NMP). The reaction was set under vigorous stirring at 50 ◦C for 24 h using an oil bath equipped with a temperature controller. The reaction mixture was repeatedly washed with THF and ﬁltered in order to recover the MWCNTs grafted with PK-Fu (see product recovered in Table S2A). The remaining solvent was removed under vacuum at 50 ◦C for 48 h. The same procedure was used to functionalise MWCNTs with bis-maleimide. Likewise, MWCNTs and PK-Bea were mixed following the same protocol to generate a reference sample (Table S2A and S2B).

2.3. Preparation of PK-Fu/B-Ma/MWCNT Composite The thermoset nanocomposite was prepared by one-pot solvent-mix containing equimolar amounts of PK-Fu and B-Ma (at a furan/maleimide ratio of 1:1) and 5 wt.% of MWCNTs. The reactants were previously dissolved in THF (≈10 wt.%) and bath sonicated for 30 min in a 150 mL round-bottomed ﬂask equipped with a magnetic stirrer. The reaction mixture was heated to 50 ◦C for Polymers 2018, 10, 1076 4 of 15

24 h to form the crosslinked network under reﬂux. After reaction, the solvent was removed under vacuum at 50 ◦C overnight. The resulting powder was divided into small portions of ≈500 mg that were moulded into rectangular bars at 150 ◦C for 30 min under a pressure of 40 bar. A neat thermoset sample without MWCNTs was also prepared for comparison. After moulding, the samples were cooled down to room temperature (30 min) and then stored at −17 ◦C for further analysis.

2.4. Characterisation The elemental composition of the samples was analysed using a Euro EA elemental analyser (Langenselbold, Germany). 1H NMR spectra were recorded on a Varian Mercury Plus 400 MHz apparatus (Agilent, Santa Clara, CA, USA) using deuterated chloroform as solvent. FT-IR spectra were collected using a Perkin-Elmer Spectrum 2000 (San Francisco, CA, USA). Sample pellets were prepared by mixing potassium bromide (KBr) with the polymer (≈1.5 wt.%). The powder was then kept under vacuum at 50 ◦C for 24 h to remove residual water. ATR-FTIR spectra were recorded using a Thermo Nicolet NEXUS 670 FTIR (Waltham, MA, USA). Differential Scanning Calorimetry (DSC) analysis was performed on a TA-Instrument DSC 2920 (Eschborn, Germany) under N2 atmosphere. The samples were weighed (10–17 mg) in an aluminium pan, which was then sealed. Then, the samples were heated from 0 ◦C or 40 ◦C to 180 ◦C and then cooled to 0 or 40 ◦C. Four heating-cooling cycles were performed at a rate of 10 ◦C/min. Gel Permeation Chromatography (GPC) measurements were performed with a HP1100 Hewlett-Packard (Wilmington, Philadelphia, PA, USA). The equipment consists of three 300 × 7.5 mm PLgel 3 µm MIXED-E columns in series and a GBC LC 1240 RI detector (Dandenong, Victoria, Australia). The samples were dissolved in THF (1 mg/mL) and eluted at a flow rate of 1 mL/min and a pressure of 100–140 bar. The calibration curve was made using polystyrene as standard and the data were interpolated using the PSS WinGPC software. Dynamic Mechanical Thermal Analyses (DMTA) were conducted on a rheometrics scientific solid analyser (RSA II) (TA Instruments, New Castle, DE, USA) under air environment using the dual cantilever mode at an oscillation frequency of 1 Hz and a heating rate of 3 ◦C/min. The samples for DMTA analysis were prepared by compression moulding of 500 mg of the composite into rectangular bars (6 mm wide, 1 mm thick, 54 mm long) at 150 ◦C for 30 min under a pressure of 40 bar to ensure full homogeneity. Resistive heating annealing was performed on the rectangular bars used for DMTA analysis. The setup consisted of a power supply (EA-PS 3150-04 B) and a multimeter (FLUKE 175). Electrical parameters were measured on samples connected to a conventional circuit using copper clamps holders. Thermal images of the samples subjected to electrical current were obtained using a FLUKE IR thermometer camera (VT02) (Everett, WA, USA). Impedance analysis was performed with a KEYSIGHT E4990A (Santa Rosa, CA, USA) impedance analyser in the frequency ranges from 100 to 30 MHz at room temperature. Samples with both sides displaying an area of 0.019 mm2 and 1 mm thick were connected directly to the electrodes for testing. Raman spectra were recorded on an Alpha 300 (Witec) (Ulm, Germany) using a laser at 532 nm; the spectral range from 1000 to 3500 cm−1 was investigated. At least 3 different points were analysed to reliably describe the bulk of each sample regarding possible structural inhomogeneities. The acquisition time was 50 s and 10 spectra were accumulated to reduce the signal to noise ratio. X-ray photoelectron spectroscopy (XPS) was carried out with a SSX-100 (Surface Science Instruments) (Mountain View, CA, USA) spectrometer equipped with a monochromatic Al Kα X-ray source (hν = 1486.6 eV) that operates at a base pressure of 3 × 10−10 mbar. The energy resolution was set at 1.45 eV, the photoelectron take-off angle was 37◦ with respect to the surface normal, and the diameter of the analysed spot was 600 µm. Spectra were collected at a minimum of two different spots on each sample, checked for consistency, and averaged for each spectral region. The data were fitted using the Winspec software (developed at the L.I.S.E. of the University of Namur, Belgium), applying the Shirley background and a linear combination of Gaussian and Lorentzian peaks. All samples were re-suspended in toluene and deposited dropwise on Au substrates. After the solvent was evaporated, the samples were transferred into ultra-high vacuum via a load-lock system. Samples containing the pure polymer display broadened peaks due to sample Polymers 2018, 10, 1076 5 of 15

chargingPolymers effects. 2018, 10 Therefore,, x FOR PEER REVIEW the ﬁtting width was set at 2.1 eV. The binding energy scale was5corrected of 15 by using the C=C sp2 signal at 284.4 eV as a reference. Scanning electron microscope micrographs werea taken reference. with a Scanning Philips XL30S electron Environmental microscope SEM-FEGmicrographs instrument were taken (Eindhoven, with a Philips The Netherlands). XL30S High-resolutionEnvironmental images SEM-FEG were instrument acquired on(Eindhoven, freshly broken The Netherlands) surfaces. . High-resolution images were acquired on freshly broken surfaces. 3. Results and Discussion 3. Results and Discussion 3.1. PK Functionalised with Furan and Benzyl Groups via Paal-Knorr Reaction 3.1. PK Functionalised with Furan and Benzyl Groups via Paal-Knorr Reaction The solvent-free Paal-Knorr reaction between PK and the amine compounds was carried out The solvent-free Paal-Knorr reaction between PK and the amine compounds was carried out according to different molar ratios between the 1,4-di-carbonyl groups of polyketone and furfurylamine according to different molar ratios between the 1,4-di-carbonyl groups of polyketone and or benzylamine aiming at a maximal conversion of 80% (Figure1). furfurylamine or benzylamine aiming at a maximal conversion of 80% (Figure 1).

Figure 1. Schematic representation of PK functionalised with furan (PK-Fu) or benzyl (PK-Bea) groups Figure 1. Schematic representation of PK functionalised with furan (PK-Fu) or benzyl (PK-Bea) groups via Paal-Knorrvia Paal-Knorr reaction. reaction.

The summaryThe summary of theof the experimental experimental results results isis displayed in in Table Table 1.1 .

TableTable 1. Experimental 1. Experimental results results of of PK PK modiﬁed modified with furfurylamine furfurylamine (PK (PK-Fu)-Fu) and andbenzylamine benzylamine (PK-Bea). (PK-Bea).

a a b b ◦ c c d d RunRun C COC(%)CO (%) ηη(%) (%) TTg g(°(C)C) PDI PDI PK-FuPK-Fu 7474 9292 31 31 2.3 2.3 PK-BeaPK-Bea 7373 9191 42 42 2.2 2.2 a b c d a CarbonylCarbonyl conversion conversion;; b conversionconversion efficiency efﬁciency;; c glass transition temperature d polydispersity index.index.

Notably,Notably, functionalised functionalised polyketones polyketones display display a carbonyl conversion conversion (Cco (Cco,, measured measured by elemental by elemental analysis)analysis) close close to theto the target target conversion conversion (efficiency,(efficiency, η > 90 90%,%, see see Table Table 1).1 ).This This result result validates validates the the robustnessrobustness and and versatility versatility of of the the Paal-Knorr Paal-Knorr reactionreaction with with polyketones polyketones [23 []23. For]. Forbrevity, brevity, the spectral the spectral characterisation of PK-Fu is displayed in SI1B, which shows the 1H NMR spectrum of the polymer characterisation of PK-Fu is displayed in SI1B, which shows the 1H NMR spectrum of the polymer mixed with furfurylamine before and after the chemical reaction. After modification, the mixed with furfurylamine before and after the chemical reaction. After modification, the characteristic characteristic peaks of the pyrrole appear in the spectrum, namely the proton signal ascribed to CH2 peaksgroups of the between pyrrole the appear pyrrole in theand spectrum, the furan groups namely at the 4.9 protonppm, the signal one related ascribed to the to CHpyrrole2 groups group between at the pyrrole5.8 ppm andand theproton furan signals groups of the at furan 4.9 ppm,moieties the at one 5.9, 6.2 related and 7.3 to ppm the pyrrole[20,23]. The group FT-IR at spectrum 5.8 ppm and protonof the signals same ofsample the furan is displayed moieties in Figure at 5.9, 2B. 6.2 It and is possible 7.3 ppm to notice [20,23 the]. The appearance FT-IR spectrum of C-H stretching of the same samplerelated is displayed to the heterocyclic in Figure 2 groupsB. It is aroundpossible 3150 to notice–3115 cm the−1 appearance(pyrrole and of furan C-H groups), stretching the related C=O to the heterocyclicstretching of groupsthe residual around carbonyl 3150–3115 groups cm at −17071 (pyrrole cm−1, the and C=C furan stretching groups), of the the heterocyclic C=O stretching groups of the residualat 1507 carbonyl cm−1, the groups pyrrole at C 1707–N stretching cm−1, the at C=C1345 cm stretching−1, the furan of theC–O heterocyclic–C stretching groupsat 1073 cm at −11507, and cm −1, −1 the pyrroleeventually C–N, the stretching out-of-plane at 1345bending cm of−1 the, the furan furan ring C–O–C C–H bo stretchingnds at 735 cm at 1073. cm−1, and eventually, The Tg of the functionalised polymers was measured by DSC (Table 1). As expected, the chemical the out-of-plane bending of the furan ring C–H bonds at 735 cm−1. modification of the di-carbonyl arrangement into pyrrole groups increases the rigidity of the The Tg of the functionalised polymers was measured by DSC (Table1). As expected, the chemical backbone. Indeed, PK-Fu and PK-Bea show a Tg of 31 °C and 42 °C, respectively, i.e., much higher modificationthan that ofof thePK di-carbonylbefore modification arrangement (−12 °C). into GPC pyrrole measurements groups increases showed no the significant rigidity ofdifferences the backbone. ◦ ◦ Indeed,in the PK-Fu PDI of and the PK-Bea two systems, show indicating a Tg of 31 theC absence and 42 ofC, significant respectively, irreversible i.e., much sidehigher reactions. than The that of ◦ PK beforecomparison modification of the two (− systems12 C). suggests GPC measurements that benzyl pendant showed groups no confer significant stronger differences supramolecular in the PDI of theinteraction two systems, among indicating the macromolecular the absence chains of significant with respect irreversible to that provided side reactions.by furan moieties. The comparison This of the two systems suggests that benzyl pendant groups confer stronger supramolecular interaction Polymers 2018, 10, 1076 6 of 15 among the macromolecular chains with respect to that provided by furan moieties. This highlights the Polymers 2018, 10, x FOR PEER REVIEW 6 of 15 versatility of the PK system, whose modification with different amino compounds grafted on the same Polymers 2018, 10, x FOR PEER REVIEW 6 of 15 backbonehighlights allows the tuning versatility the of Tg thevalue PK of system, the ultimate whose modification polymer. with different amino compounds grafted on the same backbone allows tuning the Tg value of the ultimate polymer. highlights the versatility of the PK system, whose modification with different amino compounds 3.2. Functionalisation of MWCNTs with PK-Fu or B-Ma via Diels-Alder Reaction and PK-Bea via grafted on the same backbone allows tuning the Tg value of the ultimate polymer. Physical3.2. Functionali Interactionssation of MWCNTs with PK-Fu or B-Ma via Diels-Alder Reaction and PK-Bea via Physical Interactions 3.2Figure. Functionali2 displayssation schematically of MWCNTs with the PK reaction-Fu or B- betweenMa via Diels MWCNTs-Alder Reaction and PK-Fuand PK- orBea B-Ma via (Table S2A). AfterPhysical functionalisation,Figure Interactions 2 displays schematically the MWCNTs the were reaction repeatedly between MWC washedNTs withand PK THF-Fu or in B order-Ma (Table to remove S2A). the After functionalisation, the MWCNTs were repeatedly washed with THF in order to remove the unreactedFigure PK-Fu, 2 displays PK-Bea schematically and B-Ma whilethe reaction the amount between ofMWC theNTs grafted and PK component-Fu or B-Ma was (Table evaluated S2A). by unreacted PK-Fu, PK-Bea and B-Ma while the amount of the grafted component was evaluated by elementalAfter functionali analysis (EA)sation, (Table the MWCNTs S2B). were repeatedly washed with THF in order to remove the elemental analysis (EA) (Table S2B). unreacted PK-Fu, PK-Bea and B-Ma while the amount of the grafted component was evaluated by elemental analysis (EA) (Table S2B).

Figure 2. Functionalisation of MWCNTs with (A) PK-Fu (diene) and (B) bis-maleimide (dienophile) Figure 2. Functionalisation of MWCNTs with (A) PK-Fu (diene) and (B) bis-maleimide (dienophile) via Diels-Aldervia Diels-Alder reaction. reaction. Figure 2. Functionalisation of MWCNTs with (A) PK-Fu (diene) and (B) bis-maleimide (dienophile) via Diels-Alder reaction. TheThe results results obtained obtained clearly clearly establish establish the presence of of nitrogen nitrogen in all in allthe thefunctionali functionalisedsed MWCNT MWCNT samples, whereas only traces (0.01 wt.%) were found for pristine MWCNTs. The values of nitrogen samples,The whereas results only obtained traces clearly (0.01 establish wt.%) werethe presence found forof nitrogen pristine in MWCNTs. all the functionali The valuessed MWCNT of nitrogen measured for the grafted MWCNT/PK-Fu showed the highest N content close to 34 wt.%, whereas measuredsamples for, whereas the grafted only MWCNT/PK-Fu traces (0.01 wt.%) showedwere found the for highest pristine N contentMWCNTs. close The to values 34 wt.%, of nitrogen whereas this this drops to about 8 wt.% for MWCNT/B-Ma accordingly. In the case of MWCNTs mixed with PK- dropsmeasured to about for 8 wt.%the grafted for MWCNT/B-Ma MWCNT/PK-Fu accordingly.showed the highest In the N case content of MWCNTs close to 34 mixed wt.%, whereas with PK-Bea, Bea, no covalent interaction is expected; thus, the N content of 13 wt.% arises from physical no covalentthis drop interactions to about 8 wt.% is expected; for MWCNT/ thus,B the-Ma N accor contentdingly of. In 13 the wt.% case arises of MWCNTs from physical mixed with interactions PK- interactions between PK-Bea and the filler, due to π-π stacking [26,27]. The results clearly suggest a betweenBea, PK-Bea no covalent and the interaction filler, due is to expectedπ-π stacking; thus, [the26, 27 N]. content The results of 13 clearly wt.% suggest arises from a strong physical chemical strong chemical interaction between MWCNTs and PK-Fu different to a physical π-π stacking interactions between PK-Bea and the filler, due to π-π stacking [26,2π7].π The results clearly suggest a interactionobserved between in the case MWCNTs of MWCNT/ andPK PK-Fu-Bea. different to a physical - stacking observed in the case strong chemical interaction between MWCNTs and PK-Fu different to a physical π-π stacking of MWCNT/PK-Bea.In literature, the formation of covalent bonds between cyclopentadiene, furan and bis-maleimide observed in the case of MWCNT/PK-Bea. withIn literature, CNTs via theDA formationchemistry has of covalentbeen already bonds proved between by means cyclopentadiene, of X-ray photoele furanctron and spectroscopy bis-maleimide In literature, the formation of covalent bonds between cyclopentadiene, furan and bis-maleimide with(XPS) CNTs [2 via8,29 DA]. Inchemistry connectionhas with been those already findings, proved XPS analysis by means of the of X-rayMWCNTs photoelectron functionalis spectroscopyed with with CNTs via DA chemistry has been already proved by means of X-ray photoelectron spectroscopy PK-Fu clearly confirmed the effectiveness of the DA grafting mechanism (Figure 3). (XPS)(XPS) [28, 29[28].,29 In]. connectionIn connection with with those those findings,findings, XPS XPS analysis analysis of ofthe the MWCNTs MWCNTs functionali functionalisedsed with with PK-FuPK clearly-Fu clearly confirmed confirmed the the effectiveness effectiveness ofof the DA grafting grafting mechanism mechanism (Figure (Figure 3). 3).

Figure 3. X-ray photoemission spectra of the C1s, N1s and O1s core level regions of MWCNTs Figurefunctionali 3. sed with PK-Fu. Data and fitting lines are shown. Figure X-ray 3. X-ray photoemission photoemission spectra spectra of of the the C1s, C1s, N1s N1s and and O1s O1s core core level level regions regions of MWC of MWCNTsNTs

functionalisedfunctionalis withed with PK-Fu. PK-Fu. Data Data and and ﬁttingfitting lines are are shown. shown.

Polymers 2018, 10, 1076 7 of 15 Polymers 2018, 10, x FOR PEER REVIEW 7 of 15

It is is worth worth noting noting that that in the in the carbon carbon 1s spectra, 1s spectra, differences differences between between the modified the modified and un-modified and un- modifiedMWCNTs MWCNTs surfaces cansurfaces be identified; can be identified; for the for un-modified the un-modified MWCNTs, MWCNTs the main, the main peak peak at 284.5 at 284.5 eV, eV,accompanied accompanied by theby the shake-up shake-up feature feature at at about about 6.0 6.0 eV eV higher higher binding binding energyenergy characteristiccharacteristic of ππ-π 2 conjugated systems, systems, perfectly perfectly fits fits with with C C sp sp2 reportedreported in inthe the literature literature [28,29 [28,].29 On]. Onthe theother other hand, hand, the C1sthe C1sspectrum spectrum of the of pure the pure polymer polymer shows shows a broad a broad feature feature peaked peaked at 286 at.0 286.0eV and eV resulting and resulting from three from 2 3 three components, i.e. C–C2 sp , –CH –CH3 sp and CH –C–CH , which are too closely spaced in components, i.e. C–C sp , –CH2 –CH3 2sp and3 CH2–C–CH23, which3 are too closely spaced in binding energybinding [ energy29] to be [29 resolved] to be resolved in our experimental in our experimental conditions. conditions. The spectrum The spectrum for the for pure the purepolymer polymer also showsalso shows O1s components O1s components attributed attributed to O=C to O=Cat 532.4 at 532.4eV and eV to and O– toC at O–C 533.8 at 533.8eV, as eV, well as as well a N1s as a peak N1s peakat 401.5 at eV 401.5. All eV. these All are these in remarkabl are in remarkablee agreement agreement with the with PK-Fu the structure. PK-Fu structure. For the product For the obtained product afterobtained the grafting after the PK grafting-Fu to PK-Futhe MWCNTs, to the MWCNTs, the presence the presenceof the polymer of the is polymer evident is from evident the fromN1s and the O1sN1s andintensities O1s intensities,, but can butbe also can beclearly also clearlyobserved observed in the carbon in the carbon spectra, spectra, namely namely by the by presence the presence of a broadof a broad peak peak,, which which was fitted was fitted here with here a with linear a linear combination combination of the ofpure the polymer pure polymer and MWCNT and MWCNT signal (respectivelysignal (respectively in dotted in dotted and dashed and dashed line). While line). While it is difficult it is difficult to conclude to conclude from fromthe C1s the spectrum C1s spectrum that thethat sidewall the sidewall functionali functionalisationsation of MWCNTs of MWCNTs with with PK PK-Fu-Fu proceeds proceeds via via C C–C–C bonding, bonding, in in the the O1s spectrum the O O-C-C component component results results shifted shifted by by 1 1eV eV to to higher higher binding binding energy, energy, clearly clearly pointing pointing at ata charga chargee redistribution redistribution resulting resulting from from the the furan furan attachment attachment on on the the MWCNT MWCNT surface. surface. The Raman spectra of pristine and PK-Fu-functionalisedPK-Fu-functionalised MWCNTs areare displayeddisplayed inin FigureFigure4 .4.

−1 Figure 4. 4. RamanRaman spectra spectra (normali (normalisedsed at the G band, at the 1580 G cm band,−1) of pristine 1580 and cm PK)-Fu of-functionali pristine andsed MWCNTsPK-Fu-functionalised. MWCNTs.

For brevity, only few spectra are displayed (see supporting information Figure S3 for PK-Bea For brevity, only few spectra are displayed (see supporting information Figure S3 for PK-Bea and B-Ma functionalised MWCNTs). It is widely known that Raman spectroscopy provides useful and B-Ma functionalised MWCNTs). It is widely known that Raman spectroscopy provides useful information regarding the degree of crystallinity, lattice defects, and graphitic ordering of MWCNTs. information regarding the degree of crystallinity, lattice defects, and graphitic ordering of MWCNTs. As shown in Figure4, the G band, which testiﬁes to the graphitic crystalline arrangement ( 1580 cm−1), As shown in Figure 4, the G band, which testifies to the graphitic crystalline arrangement (1580 cm−1), the D band related to defects (1345 cm−1) and the D-band overtone, G’ band, indicative of the the D band related to defects (1345 cm−1) and the D-band overtone, G’ band, indicative of the multi- multi-walled structure (2684 cm−1) are detected. The band centred at 2995 cm−1 (C-H vibration walled structure (2684 cm−1) are detected. The band centred at 2995 cm−1 (C-H vibration of alkanes) is of alkanes) is only detected in the presence of the polymer. The intensity ratio between D and G only detected in the presence of the polymer. The intensity ratio between D and G bands is also a bands is also a useful standard measurement of the amount of defects in carbon nanotubes [30]. useful standard measurement of the amount of defects in carbon nanotubes [30]. Accordingly, the Accordingly, the D/G ratio decreases from 1.49 for pristine MWNTs to 1.29 for PK-Fu functionalised D/G ratio decreases from 1.49 for pristine MWNTs to 1.29 for PK-Fu functionalised MWCNTs. This MWCNTs. This suggests that after the cycloaddition step subsequent rehybridisation restores the suggests that after the cycloaddition step subsequent rehybridisation restores the sp2 state, recovering sp2 state, recovering the aromaticity of the system in the defect region of MWCNTs, as recently the aromaticity of the system in the defect region of MWCNTs, as recently reported [31]. Highly reported [31]. Highly defective graphene layers can also be followed by the key analysis of intensity defective graphene layers can also be followed by the key analysis of intensity ratio between G’ and ratio between G’ and D bands, that increases from 0.17 for pristine MWCNTs to 0.27 for the less D bands, that increases from 0.17 for pristine MWCNTs to 0.27 for the less entangled PK-Fu functionalised MWCNTs (see Figure S4). In summary, even though PK-Fu modifies the graphitic

Polymers 2018, 10, 1076 8 of 15

Polymers 2018, 10, x FOR PEER REVIEW 8 of 15 entangled PK-Fu functionalised MWCNTs (see Figure S4). In summary, even though PK-Fu modiﬁes surfacethe graphitic of MWCNTs, surface of its MWCNTs, presence itspossibly presence improves possibly the improves long-range the long-range graphitic graphitic order of order pristine of pristineMWCNTs MWCNTs.. DSC analysis analysis of of the the PK PK-Fu-Fu nanocomposite nanocomposite (Figure (Figure 5) 5evidences) evidences two two endothermal endothermal transitions: transitions: one ◦ aroundone around 75–80 75–80 °C, associatedC, associated with the with glass the transition glass transition of the material of the,material, and the second and the from second 120 to from 170 ◦ ◦ 120°C, centred to 170 atC, 160 centred °C and at 160associatedC and to associated the retro-DA to the reaction. retro-DA The reaction. energy required The energy for the required retro-DA for processthe retro-DA reach processes 11.6 J/g reaches, a value 11.6 that J/g, perfectly a value that correlates perfectly with correlates the energy with associated the energy to associated retro-DA reactionto retro-DA for reaction conventional for conventional thermally reversible thermally furane/maleimide reversible furane/maleimide crosslinked systems crosslinked (see systems below) (see[21,3 below)2]. Overall, [21,32 the]. Overall, MWCNT/PK the MWCNT/PK-Fu-Fu represents a represents consistent a consistentexample of example reversible of reversible grafting between grafting abetween graphitic a graphitic electrically electrically-conductive-conductive material material and a and polymer a polymer matrix matrix;; new new investigations investigations are are now now in progressin progress to tomodulate modulate the the grafting grafting extent extent through through the the choice choice of ofthe the proces processingsing parameters. parameters.

1.5 PK-Fu functionalized MWCNTs 1.0

0.5

0.0

-0.5

-1.0

-1.5

Heat flow (W/g)

-2.0

-2.5 40 60 80 100 120 140 160 180 Temperature C

Figure 5 5.. DCS thermal cycles of the PK PK-Fu-Fu functionali functionalisedsed MWCNTs MWCNTs..

3.3.3.3. PK PK-Fu/B-Ma/MWCNT-Fu/B-Ma/MWCNT Composite The MWCNT/PK-FuMWCNT/PK-Fu nanocomposite nanocomposite was was further further crosslink crosslinkeded with with B B-Ma-Ma via Diels-AlderDiels-Alder reversible reaction aiming at the production of electrically conductive soft and chemically reversible polymer networks.networks. The The crosslink crosslinkeded nanocomposite nanocomposite was was then then obtained obtained in in the the form form of highly homogeneous rectangular rectangular bars bars by by one one-pot-pot solvent solvent-mix-mix containing containing equimolar equimolar amounts amounts of PK of-Fu PK-Fu and Band-Ma B-Ma (ratio (ratio 1:1 between 1:1 between Fu Fu and and Ma Ma groups) groups) and and 5 5 wt.% wt.% of of MWCNTs MWCNTs fol followedlowed by compression mouldingmoulding.. The The neat neat thermoset thermoset and and the nanocomposite were were characteri characterisedsed b byy DSC ( (FigureFigure S5 and Figure 66,, respectively) respectively) to to study study their their thermal thermal behaviour, behaviour, reversibility, reversibility and, and the the exo-/endo-thermal exo-/endo-thermal processes related to the DA and retro-DAretro-DA sequence respectively. The thermograms display a broad ◦ ◦ endothermic transition in the range of temperature between 120 °CC and 180 °CC for each consecutive thermal cycle. The The similarity similarity between between the the three three thermal thermal cycles clearly indicates the re-workablere-workable character of of the the crosslinked crosslink PK-Fu,ed PK- evenFu, wheneven reinforced when reinforced with the MWCNTswith the [MWCNTs20,21]. The endothermic[20,21]. The endothermictransition corresponds transition to corresponds the r-DA process. to the Therefore, r-DA process. the peak Theref of theore, curves the peak corresponds of the curves to the correspondsaverage temperature to the average at which temperature the majority at ofwhich the DA the adducts majority are of broken,the DA andadducts the area are broken under the, and curve the areaassociated under tothe this curve peak associated is related to to this the energypeak is absorbedrelated to duringthe energy the cleavage absorbed of during the DA the adducts. cleavage of the DAThe adducts. DSC analysis of the nanocomposite shows that the presence of MWCNTs has no apparent effectThe on theDSC thermal analysis behaviour of the nanocomposite of the thermoset shows [22]. Indeed,that the the presence energy of associated MWCNTs to thehas r-DAno apparent process waseffect calculated on the thermal from the behaviour DSC heating of the trace thermoset to be 11.1 [22] J/g. Indeed, for the nanocomposite the energy associated and 10.9 to J/g the for r-DA the processneat thermoset, was calculated thus suggesting from the that DSC the heating presence trace of to the be ﬁller 11.1 does J/g for not the interfere nanocomposite with the r-DA and process10.9 J/g forof the the matrix. neat thermoset, Namely, eventhus suggesting if a covalent that interaction the presence between of the matrix filler anddoes ﬁller not mightinterfere occur with via the the r-DA processmechanism, of the the matrix corresponding. Namely, thermal even if transitiona covalent is interaction not visible between in the DSC matri traces,x and due filler to either might overlap occur withvia the the DA one mechanism associated, tothe adducts corresponding of Fu and thermal Ma, or transition simply relatively is not visible low concentration. in the DSC traces, due to either overlap with the one associated to adducts of Fu and Ma, or simply relatively low concentration.

Polymers 2018, 10, 1076 9 of 15

Polymers 2018, 10, x FOR PEER REVIEW 9 of 15

Cycle 0 1 2 3 -2

Heat flow (W/g)

Heat flow (W/g) -4

0 20 40 60 80 100 120 140 160 180 o Temperature (oC)

FigureFigure 6 6.. DSCDSC thermal thermal cyc cyclesles of of PK PK-Fu-Fu crosslink crosslinkeded with with B B-Ma-Ma and and reinforced reinforced with with MWCNTs. MWCNTs.

TheThe thermo thermomechanicalmechanical features features of of PK PK-Fu-Fu crosslink crosslinkeded with with B B-Ma-Ma and and reinforced reinforced with with MWCNTs MWCNTs ◦ werewere also determined determined after after compression compression a grinded a grinded sample sample at 150 at 150C and °C 40and bar 40 for bar 30 for min. 30 This min. process This processwas supposed was supposed to favour to r-DA favour mechanism r-DA mechanism and the consequent and the consequent de-crosslinking de-crosslinking of the thermoset of the thermosetnanocomposite. nanocomposite. The latter aimsThe latter at building aims at aproof building of concept a proof related of concept to the related reversibility to the of reversibility crosslinked ofsystems crosslink as highed systems performance as high conductive performance and recyclable conductive thermoplastics. and recyclable DMTA thermoplastics. analysis performed DMTA on analysisrectangular performed bars of on this rectangular material (obtained bars of this after material compression (obtained moulding) after compression shows that moulding) the ﬁller leads shows to thatclear the enhancements filler leads to of clear softening enhancements point (tan( ofδ) softening peak), loss point and ( elastictan(δ) modulipeak), loss with and respect elastic to themoduli neat withcrosslinked respect systemto the neat (Figure crosslink7). ed system (Figure 7).

0.25 PK-Fu/B-Ma 1E10 PK-Fu/B-Ma/MWCNT before annealing PK-Fu/B-Ma/MWCNT after annealing 0.20

1E9 0.15

Tan Tan

0.10



 1E8

E'E" (Pa) 1E8

E'E" (Pa) 0.05

1E7 0.00

40 60 80 100 120 140 160 Temperature (oC) Temperature ( C) FigureFigure 7 7.. DMTADMTA of of PK PK-Fu-Fu crosslink crosslinkeded with with b b-Ma-Ma ( (PK-Fu/B-Ma,PK-Fu/B-Ma, filled ﬁlled red red squares) squares) and and PK PK-Fu-Fu crosslinkcrosslinkeded with with B B-Ma-Ma and and reinforced reinforced with with 5 wt.% wt.% of MWCNTs before (PK-Fu/B-Ma/MWCNT,(PK-Fu/B-Ma/MWCNT, filled ﬁlled blackblack squares) squares) and and after after resistive resistive heating heating annealing annealing ( (PK-Fu/B-Ma/MWCNT,PK-Fu/B-Ma/MWCNT, black black empty empty squares) squares) for for 2424 h h using using 35 35 V V..

Notably, the softening point (tan(δ)) is increased from ~130 °C for the PK-Fu crosslinked with B- Ma to ~140 °C for the PK-Fu/B-Ma/MWCNT nanocomposite, which, after resistive heating, increased to ~155 °C. This can be explained by the fact that MWCNTs reinforce the structure of the matrix,

Polymers 2018, 10, 1076 10 of 15

PolymersPolymers 20182018,, 1010,, xx FORFOR PEERPEER REVIEWREVIEW 1010 ofof 1515 Notably, the softening point (tan(δ)) is increased from ~130 ◦C for the PK-Fu crosslinked with B-Ma ◦ whichtowhich ~140 consequentlyconsequentlyC for the PK-Fu/B-Ma/MWCNT displaysdisplays increaseincreasedd rigidity.rigidity. nanocomposite, This This effect effect which, is is afteraided aided resistive by by the the heating, effective effective increased interfacial interfacial to ◦ ~155interactionsinteractionsC. This betweenbetween can be explained thethe matrixmatrix by andand the thethe fact fillerfiller that viavia MWCNTs DADA cycloaddition,cycloaddition, reinforce the asas structuresuggestedsuggested of byby the XPSXPS matrix, andand RamanRaman which analyses.consequentlyanalyses. AsAs aa displays result,result, thethe increased fillerfiller helpshelps rigidity. inin dispersingdispersing This effect thethe is appliedapplied aided by forceforce the,, effectiveandand cconsequentlyonsequently interfacial improvesimproves interactions thethe mechanicalbetweenmechanical the performance performance matrix and the of of thefiller the composite. composite. via DA cycloaddition, Another Another phenomenonphenomenon as suggested that that by possibly possibly XPS and contributes contributes Raman analyses. toto thethe Assofteningsoftening a result, pointpoint the filler enhancementenhancement helps in dispersing isis thethe potentialpotential the applied catalyticcatalytic force, effecteffect and exertedexerted consequently byby MWCNTsMWCNTs improves onon thethe the DADA mechanical reactionreaction performancebetweenbetween PK PK-- ofFuFu the and and composite. BB--MaMa adducts, adducts, Another which which phenomenon l leadseads to to thataa higherhigher possibly crosslinking crosslinking contributes density density to the softeninguponupon resistive resistive point heatingenhancementheating [3[333]].. is the potential catalytic effect exerted by MWCNTs on the DA reaction between PK-Fu and B-MaItIt is is worth worthadducts, noting noting which that that leads the the to elastic elastic a higher modulus modulus crosslinking (E’) (E’) anddensity and the the upon softening softening resistive temperature temperature heating [33 ( (t].tanan((δδ)))) are are significantlysignificantlyIt is worth improvedimproved noting afterafter that resistive theresistive elastic heatingheating modulus forfor 2424 (E’) hh andaatt 3535the VV,, as softeningas comparedcompared temperature withwith thethe systemsystem (tan(δ)) afterafter are mouldingsignificantlymoulding.. TheThe improved electricalelectrical after currentcurrent resistive effectivelyeffectively heating inducesinduces for 24 heatheat h at dissipationdissipation 35 V, as compared fromfrom MWCNTs,MWCNTs, with the whichwhich system triggerstriggers after themoulding.the reconnectionreconnection The electrical betweenbetween current decoupleddecoupled effectively DADA adductsadducts induces,, andand heat thusthus dissipation,, rere--establisestablis fromheshes MWCNTs, thethe crosslinkcrosslink whicheded networknetwork triggers [3the[344]] reconnection.. A A schematic schematic between representation representation decoupled of of DA the the adducts, proposed proposed and m m thus,eechanismchanism re-establishes is is reported reported the incrosslinked in Figure Figure 8 8. network. InIn order order [34 to to]. Aconfirmconfirm schematic thisthis representationidea,idea, wewe monitoredmonitored of the thethe proposed temperaturetemperature mechanism ofof thethe is samplesample reported duringduring in Figure thethe 8applicationapplication. In order to ofof confirm thethe voltagevoltage this (Figureidea,(Figure we 9).9). monitored the temperature of the sample during the application of the voltage (Figure9).

FigureFigure 8.88.. SchematicSchematic repre representationrepresentationsentation of of PK PK-FuPK--FuFu crosslink crosslinkedcrosslinkeded withwith BB B-Ma--MaMa andand reinforcedreinforced withwith MWCNTs.MWCNTs. ArrowsArrows displaydisplay thethe processprocess of:of: (1)((1)1) crosslinkingcrosslinkcrosslinkinging (DA),(DA), (2)((2)2) mouldingmoulding (partial(partial r-DA)rr--DA)DA) andand (3)((3)3) annealingannealing (DA)(DA) byby resistiveresistive heating.heating.heating.

AA BB CC

FigureFigure 9.99.. (((AA)) PhotographPhotographPhotograph ofofof thethethe PK-Fu/B-Ma/MWCNT PKPK--Fu/Fu/BB--Ma/MWCNTMa/MWCNT compositecomposite composite samplesample sample withwith with 5 5 wt.%wt.% ofof MWCNTs connectedconnected to to the the electricalelectrical circuitcircuit circuit andand and itsits its thermalthermal thermal imagesimages images beforebefore before ((BB (B)) )andand and duringduring during thethe the applicationapplication application ofof of aa voltageavoltage voltage ofof of 3535 35 VV V ((CC (C).). ).

Notably,Notably, thethe well-distributedwellwell--distributeddistributed redred colourcolour allall alongalong thethe ﬁlmfilmfilm surfacesurface (Figure(Figure9 99C)C)C) clearly cleaclearlyrly suggests suggestssuggests thethe homogeneous homogeneous distribution distribution of of MWCNTs MWCNTs MWCNTs in in in the the the crosslinkedcrosslinkcrosslinkeded nanocomposite.nanocomposite.nanocomposite. The The temperature temperature reachedreached byby by thethe the networknetwork network duringduring during thethe the applicationapplication application ofof thethe of electrical theelectrical electrical currentcurrent current correspondscorresponds corresponds toto aboutabout to about5555––6060 ◦ °C,55–60°C, i.e.,i.e., C aa , temperaturetemperature i.e., a temperature atat whichwhich at the whichthe directdirect the DADA direct reactionreaction DA reaction isis predominantpredominant is predominant eveneven ifif nn evenotot exclusivelyexclusively if not exclusively presentpresent present[[2020,3,355,3,366 [20]].. AlthoughAlthough,35,36]. Although higherhigher temperaturestemperatures higher temperatures mightmight bebe might possiblepossible be possible byby usingusing by aa usingdifferentdifferent a different fillerfiller loadload ﬁller oror load higherhigher or voltagevoltage,, thethe equilibriumequilibrium naturenature ofof thethe DADA reactionreaction ((i.e.,i.e., bothboth directdirect andand retroretro--DADA takingtaking place)place) mightmight bebe stillstill observedobserved atat thesethese temperatures,temperatures, thusthus possiblypossibly resultingresulting inin relativelyrelatively lowlow temperaturetemperature selselff-- healinghealing behaviourbehaviour..

Polymers 2018, 10, 1076 11 of 15 higher voltage, the equilibrium nature of the DA reaction (i.e., both direct and retro-DA taking place) Polymers 2018, 10, x FOR PEER REVIEW 11 of 15 might be still observed at these temperatures, thus possibly resulting in relatively low temperature self-healingThe sequence behaviour. of crosslinking in solution, de-crosslinking during moulding, and annealing by resistiveThe heating sequence of ofthe crosslinking nanocomposite in solution, was also de-crosslinking monitored by duringATR-FTIR moulding, spectroscopy and annealing (Figure S6 by). resistiveThe normali heatingsed spectra of the nanocompositeat 2930 cm−1 (C– wasH stretc alsoh, monitored Figure S6) by and ATR-FTIR at 1145 cm spectroscopy−1 (C–N stretching (Figure band S6). −1 −1 The[37], normalisedFigure S6B) spectraevidence at the 2930 intensity cm (C–H variations stretch, of Figurethe C–N S6)–C andpeak at at 1145 1185 cm cm−1 (C–Nattributed stretching to the −1 bandstretching [37], Figureof the succinimide S6B) evidence ring the in intensity the DA variationsadduct. It ofis theworth C–N–C noting peak that at the 1185 DA cm bandattributed decreased to thein intensity stretching after of the moulding succinimide (decoupling ring in the of DA DA adduct. adducts, It isred worth curve), noting along that with the DA the band appearance decreased of infuran intensity and maleimide after moulding moieties (decoupling (736 cm−1 of and DA 672 adducts, cm−1, redrespectively curve), along), whereas with the it promptly appearance recovered of furan −1 −1 andafter maleimide annealing moieties(blue curve) (736 cm to intensitiesand 672 cm similarly, respectively), collected from whereas the it nanocomposite promptly recovered thermally after annealingcrosslinked (blue in solution curve) to(black intensities curve). similarly collected from the nanocomposite thermally crosslinked in solutionAccording (black to curve). the results displayed in Figure S6, part of the DA adducts are broken after mouldingAccording, which to the could results possibly displayed favour in Figure re-agglomeration S6, part of the DA of MWCNTs adducts are, brokenthus leading after moulding, to poor whichconductivity could. To possibly support favour this hypothesis, re-agglomeration impedance of measurements MWCNTs, thus and leadingmorphological to poor studies conductivity. by SEM Towere support combined this in hypothesis, order to figure impedance out the degree measurements of dispersion and morphologicalof MWCNTs in studiesthe matrix by ( SEMFigures were 10 combinedand 11). in order to ﬁgure out the degree of dispersion of MWCNTs in the matrix (Figures 10 and 11).

0.34 PK-Fu/B-Ma PK-Fu/B-Ma/MWCNT before annealing ) PK-Fu/B-Ma/MWCNT after annealing



0.32

0.30

0.02

0.01

Absolute ImpedanceAbsolute IZI ( 0.00 105 106 107 Frequency (Hz)

Figure 10.10. Variation ofof absoluteabsolute impedanceimpedance ofof PK-FuPK-Fu crosslinkedcrosslinked withwith B-MaB-Ma (PK-Fu/B-Ma)(PK-Fu/B-Ma) and of the crosslinkedcrosslinked PK-Fu/B-Ma/MWCNTPK-Fu/B-Ma/MWCNT crosslink crosslink composite composite before before and and after after resistive heating annealingannealing using 35 V for 24 h.h.

Figure 1010 indicatesindicates that that the the impedance impedance (i.e., (i.e.,the total the resistance total resistance of the material) of the material) of the PK of-Fu/ theB- PK-Fu/B-Ma/MWCNTMa/MWCNT crosslinked crosslinked system decreases system one decreases order of onemagnitude order of as magnitude compared to as the compared un-reinforced to the un-reinforcedsystem. Even more system. remarkable Even more is remarkablethe fact that isthe the material fact that reduces the material its impedance reduces its by impedance half and becomes by half andmore becomes electrically more conductive electrically upon conductive annealing upon (compare annealing red (compare and blue redcurves and in blue Figure curves 10) in, as Figure is clearly 10), asnoticed is clearly for noticedfrequencies for frequencies >5 MHz, when >5 MHz, the whenimpedance the impedance becomes becomesnearly constant nearly constant.. This can This be d canue beto duethe effective to the effective interfacial interfacial adhesion adhesion between between the components the components and good and gooddispersion dispersion of the of MWCNTs the MWCNTs that thatalso alsosuggests suggests less matrix/filler less matrix/ﬁller mobility mobility upon upon different different frequencies frequencies [38,39 [38]. ,39 ].

Polymers 2018, 10, 1076 12 of 15 Polymers 2018, 10, x FOR PEER REVIEW 12 of 15 A B

C D

FigureFigure 11.11. SEMSEM micrographs of PK-Fu/B-Ma/MWCNTs PK-Fu/B-Ma/MWCNTs reversible reversible crosslink crosslinkeded composite composite after after mouldingmoulding ( A,,C)) and and after after annealing by resistive heating ( B,D).

CContrastontrast mode mode SEM SEM micrographs micrographs [4 [400]] corroborate the the results obtained from electrical measurementsmeasurements on on the the same sample after being moulded and successively annealed by resistive heatingheating (Figure (Figure 1111).). On On the the freshly freshly broken broken surfaces surfaces of of the the composite composite after after moulding moulding (Figure (Figure 11A,11A,C),C), bundlesbundles of of MWCN MWCNTsTs (indicated by white arrows) arrows) can can be be observed. observed. Conversely, Conversely, after after resistive resistive heating heating ((FigureFigure 11B,B,D)D) MWCNTs are better distributed distributed,, thus confirming conﬁrming the ability of the electric current to restorerestore a more effective percolative network. The The re re-distribution-distribution of of th thee MWCNTs MWCNTs in the crosslink crosslinkeded matrixmatrix is is also also evidence evidencedd by by the the average average outer outer diameter diameter analysis analysis of of the the MWCNTs MWCNTs before before and and after annealingannealing (Figure(Figure S7).S7). TheThe analysisanalysis of of the the diameter diameter average average of of the the graphitic graphitic assemblies assemblies reported reported in thein theSEM SEM micrograph micrograph suggests suggests an effective an effective modiﬁcation modification of the MWCNT of the MWCNT distribution distribution within the within polymer the polymermatrix after matrix the after annealing the annealing procedure procedure (Figure S7). (Figure During S7).resistive During resistive heating, theheating increased, the increased polymer polymermobility mobility possibly possibly favours thefavour redistributions the redistribution of macromolecular of macromolecular chains, thus chains increasing, thus increasing the MWCNT the MWCNTdispersion dispersion (average diameter(average passeddiameter from passed about from 15 toabout about 15 11to nm)about aided 11 nm) by theaided better by interfacialthe better interfacialadhesionbetween adhesion the between components. the components.

44.. Conclusions Conclusions WeWe have demonstrated that furan-functionalisedfuran-functionalised polyketone ( (PK-Fu)PK-Fu) become becomess electrical electricallyly conductiveconductive through through addition addition of of moderate moderate amounts amounts of of MWCNTs MWCNTs (5 (5 wt.%) wt.%),, and and that, that, in in this this way, way, intrinsicintrinsic self self-healing-healing can can be be promoted by thermally reversible Diels Diels-Alder-Alder crosslinks crosslinks activated activated by by resistiveresistive heating. Spectroscopy Spectroscopy investigations investigations show showeded that that MWCNTs MWCNTs contribute contribute in in the the reversible crosslinkingcrosslinking of the PK PK-Fu-Fu with B B-Ma-Ma due to the diene/dienophile character character of of the the MWCNT MWCNT and and their their increasedincreased graphitic graphitic order order after after network network formation formation.. In In addition, addition, electronic electronic microscopy investigations revealrevealeded the the effective effective distribution of M MWCNTs.WCNTs. By means of the the presented presented approach, it was possible toto reali realisese a a crosslink crosslinkeded nanocomposite nanocomposite displaying displaying a a relatively relatively high high softening softening point point ( (tan(tan(δδ)))) at at 155 155 °C,◦C, i.e.,i.e., about 25 °C◦C higher than that without MWCNTs. After grinding the electrically conductive PK PK network,network, compression compression moulding moulding at at 150 150 °C◦C activates activates the the retro retro-DA-DA process process,, thus thus making making it it possible to to reshapereshape the the nanocomposite nanocomposite as thermoplastic. thermoplastic. A A subsequent subsequent process process of of annealing annealing via via resistive resistive heating heating at 35 V for 24 h favoured the reconnection of the decoupled DA linkages, and provided the

Polymers 2018, 10, 1076 13 of 15 at 35 V for 24 h favoured the reconnection of the decoupled DA linkages, and provided the crosslinked PK nanocomposite with the same thermal, mechanical, and electrical characteristics as the undamaged polymer network. In summary, we believe that our results may pave the way towards the cost-effective industrial production of electrically-conductive crosslinked and recyclable nanocomposites endowed with intrinsic re-connection features triggered by resistive heating activation of reversible Diels-Alder adducts.

Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4360/10/10/1076/ s1, Table S1: Synthesis of PK-Fu and PK-Bea, Table S2: Calculation of grafted MWCNTs with PK-Fu, PK-Bea and bis-maleimide, Figure S3: Raman spectra of pristine and PK-Fu, B-Ma, PK-Bea functionalized MWCNTs, Figure S4: SEM micrographs of (A) pristine MWCNTs and (B) PK-Fu functionalized MWCNTs, Figure S5: DSC thermal cycles of crosslinked PK-Fu with B-Ma, Figure S6: ATR-FTIR spectra of PK-Fu crosslinked with B-Ma and reinforced with MWCNTs, Figure S7: Statistical analysis of PK-Fu/B-Ma/MWCNT micrographs. Author Contributions: R.A.-H.: Idea developer, experimental design, laboratory work, results processing, article writing. A.P.: Idea developer, writing corrections, literature recommendation. P.R.: Laboratory guidance, discussion of results, writing corrections. D.S.: synthesis and characterisation of materials. P.P.P.: Orientation of the Idea, writing corrections. (P.R.) Discussion of results, writing corrections. R.Y.N.G.: XPS analysis, discussion of results. I.M.-V.: Writing corrections. F.P.: Polymer design, idea developer, writing corrections. Acknowledgments: We gratefully acknowledge the support of Valentina Dini, Judit Lisoni, José Tapia Cáceres, Pablo Druetta, Lorenzo Polgar, Mario E. Flores, Franck Quero, Felipe Oyarzún Ampuero, Rodrigo Espinoza, Francisco Melo and Victor Carrasco. Rodrigo Araya-Hermosilla gratefully acknowledges the support of FONDECYT-CHILE; grant number: 3170352 and FONDEQUIP, EQM130149. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Leng, J.; Kin-Tak Lau, A. Multifunctional Polymer Nanocomposites; Taylor and Francis Group: Boca Raton, FL, USA; London, UK; New York, NY, USA, 2011. 2. McNally, T.; Pötschke, P. Polymer-Carbon Nanotube Composites; Woodhead Publishing Limited: Philadelphia, PA, USA, 2011. 3. Kasaliwal, G.R.; Villmow, T.; Pegel, S.; Pötschke, P. Inﬂuence of material and processing parameters on carbon nanotube despersion in polymer melts. In Polymer-Carbon Nanotube Composites; McNally, T., Pötschke, P., Eds.; Woodhead Publishing Limited: Cambridge, UK, 2011; p. 92. 4. Xie, X.; Mai, Y.; Zhou, X. Dispersion and Alignment of Carbon Nanotubes in Polymer Matrix: A Review. Mater. Sci. Eng. R-Rep. 2005, 49, 89–112. [CrossRef] 5. Imtiaz, S.; Siddiq, M.; Kausar, A.; Muntha, S.T.; Ambreen, J.; Bibi, I. A Review Featuring Fabrication, Properties and Applications of Carbon Nanotubes (CNTs) Reinforced Polymer and Epoxy Nanocomposites. Chin. J. Polym. Sci. 2018, 36, 445–461. [CrossRef] 6. Wang, Q.; Wen, G.; Chen, J.; Su, D.S. Reinforcing Epoxy Resin with Nitrogen Doped Carbon Nanotube: A Potential Lightweight Structure Material. J. Mater. Sci. Technol. 2018, 34, 2205–2211. [CrossRef] 7. Zhang, Q.; Liu, L.; Pan, C.; Li, D.; Gai, G. Thermally Sensitive, Adhesive, Injectable, Multiwalled Carbon Nanotube Covalently Reinforced Polymer Conductors with Self-Healing Capabilities. J. Mater. Chem. C 2018, 6, 1746–1752. [CrossRef] 8. Guadagno, L.; De Vivo, B.; Di Bartolomeo, A.; Lamberti, P.; Sorrentino, A.; Tucci, V.; Vertuccio, L.; Vittoria, V. Effect of Functionalization on the Thermo-Mechanical and Electrical Behavior of Multi-Wall Carbon Nanotube/Epoxy Composites. Carbon 2011, 49, 1919–1930. [CrossRef] 9. Yang, Y.; Xie, X.; Mai, Y. Functionalization of Carbon Nanotubes for Polymer Nanocomposites. In Polymer–Carbon Nanotube Composites; Woodhead Publishing: Sawston/Cambridge, UK, 2011; pp. 55–91. 10. Irzhak, T.F.; Irzhak, V.I. Epoxy Nanocomposites. Polym. Sci. Ser. A 2017, 59, 791–825. [CrossRef] 11. Kulakov, V.; Aniskevich, A.; Ivanov, S.; Poltimae, T.; Starkova, O. Effective Electrical Conductivity of Carbon Nanotube-Epoxy Nanocomposites. J. Compos. Mater. 2017, 51, 2979–2988. [CrossRef] 12. Ma, S.; Webster, D.C. Degradable Thermosets Based on Labile Bonds or Linkages: A Review. Prog. Polym. Sci. 2018, 76, 65–110. [CrossRef] Polymers 2018, 10, 1076 14 of 15

13. Yang, Y.; Ding, X.; Urban, M.W. Chemical and Physical Aspects of Self-Healing Materials. Prog. Polym. Sci. 2015, 49–50, 34–59. [CrossRef] 14. Chen, J.; Ober, C.; Poliks, M. Characterization of Thermally Reworkable Thermosets: Materials for Environmentally Friendly Processing and Reuse. Polymer 2002, 43, 131–139. [CrossRef] 15. An, S.Y.; Arunbabu, D.; Noh, S.M.; Song, Y.K.; Oh, J.K. Recent Strategies to Develop Self-Healable Crosslinked Polymeric Networks. Chem. Commun. 2015, 51, 13058–13070. [CrossRef][PubMed] 16. Gandini, A. The Furan/Maleimide Diels–Alder Reaction: A Versatile Click–unclick Tool in Macromolecular Synthesis. Prog. Polym. Sci. 2013, 38, 1–29. [CrossRef] 17. Polgar, L.M.; van Duin, M.; Broekhuis, A.A.; Picchioni, F. The use of Diels-Alder Chemistry for Thermo-Reversible Cross-Linking of Rubbers: The Next Step Towards Recycling of Rubber Products? Macromolecules 2015, 48, 7096–7105. [CrossRef] 18. Ursache, O.; Gaina, C.; Gaina, V.; Tudorachi, N.; Bargan, A.; Varganici, C.; Rosu, D. Studies on Diels-Alder Thermoresponsive Networks Based on Ether-Urethane Bismaleimide Functionalized Poly(Vinyl Alcohol). J. Therm. Anal. Calorim. 2014, 118, 1471–1481. [CrossRef] 19. Liu, Y.; Chuo, T. Self-Healing Polymers Based on Thermally Reversible Diels-Alder Chemistry. Polym. Chem. 2013, 4, 2194–2205. [CrossRef] 20. Zhang, Y.; Broekhuis, A.A.; Picchioni, F. Thermally Self-Healing Polymeric Materials: The Next Step to Recycling Thermoset Polymers? Macromolecules 2009, 42, 1906–1912. [CrossRef] 21. Toncelli, C.; De Reus, D.C.; Picchioni, F.; Broekhuis, A.A. Properties of Reversible Diels-Alder Furan/Maleimide Polymer Networks as Function of Crosslink Density. Macromol. Chem. Phys. 2012, 213, 157–165. [CrossRef] 22. Araya-Hermosilla, R.; Lima, G.; Raffa, P.; Fortunato, G.; Pucci, A.; Flores, M.E.; Moreno-Villoslada, I.; Broekhuis, A.; Picchioni, F. Intrinsic Self-Healing Thermoset through Covalent and Hydrogen Bonding Interactions. Eur. Polym. J. 2016, 81, 186–197. [CrossRef] 23. Araya-Hermosilla, R.; Broekhuis, A.A.; Picchioni, F. Reversible Polymer Networks Containing Covalent and Hydrogen Bonding Interactions. Eur. Polym. J. 2014, 50, 127–134. [CrossRef] 24. Zhang, Y.; Broekhuis, A.A.; Stuart, M.C.A.; Picchioni, F. Polymeric Amines by Chemical Modiﬁcations of Alternating Aliphatic Polyketones. J. Appl. Polym. Sci. 2008, 107, 262–271. [CrossRef] 25. Mul, W.P.; Dirkzwager, H.; Broekhuis, A.A.; Heeres, H.J.; van der Linden, A.J.; Orpen, A.G. Highly Active, Recyclable Catalyst for the Manufacture of Viscous, Low Molecular Weight, CO-Ethene-Propene-Based Polyketone, Base Component for a New Class of Resins. Inorg. Chim. Acta 2002, 327, 147–159. [CrossRef] 26. Migliore, N.; Polgar, L.M.; Araya-Hermosilla, R.; Picchioni, F.; Raffa, P.; Pucci, A. Effect of the Polyketone Aromatic Pendent Groups on the Electrical Conductivity of the Derived MWCNTs-Based Nanocomposites. Polymers 2018, 10, 618. [CrossRef] 27. Araya-Hermosilla, R.; Pucci, A.; Araya-Hermosilla, E.; Pescarmona, P.; Raffa, P.; Polgar, L.; Moreno-Villoslada, I.; Flores, M.; Fortunato, G.; Broekhuis, A. An Easy Synthetic Way to Exfoliate and Stabilize MWCNTs in a Thermoplastic Pyrrole-Containing Matrix Assisted by Hydrogen Bonds. RSC Adv. 2016, 6, 85829–85837. [CrossRef] 28. Chang, C.; Liu, Y. Functionalization of Multi-Walled Carbon Nanotubes with Furan and Maleimide Compounds through Diels-Alder Cycloaddition. Carbon 2009, 47, 3041–3049. [CrossRef] 29. Zydziak, N.; Huebner, C.; Bruns, M.; Barner-Kowollik, C. One-Step Functionalization of Single-Walled Carbon Nanotubes (SWCNTs) with Cyclopentadienyl-Capped Macromolecules via Diels-Alder Chemistry. Macromolecules 2011, 44, 3374–3380. [CrossRef] 30. Sreekanth, M.; Ghosh, S.; Srivastava, P. Tuning Vertical Alignment and Field Emission Properties of Multi-Walled Carbon Nanotube Bundles. Appl. Phys. A-Mater. Sci. Process. 2018, 124, 52. [CrossRef] 31. Paoletti, C.; He, M.; Salvo, P.; Melai, B.; Calisi, N.; Mannini, M.; Cortigiani, B.; Bellagambi, F.G.; Swager, T.M.; Di Francesco, F.; et al. Room Temperature Amine Sensors Enabled by Sidewall Functionalization of Single-Walled Carbon Nanotubes. RSC Adv. 2018, 8, 5578–5585. [CrossRef] 32. Araya-Hermosilla, R.; Fortunato, G.; Pucci, A.; Broekhuis, A.A.; Raffa, P.; Lima, G.M.R.; Pourhossein, P.; Polgar, L.; Picchioni, F. Thermally Reversible Rubber-Toughened Thermoset Networks Via Diels-Alder Chemistry. Eur. Polym. J. 2006, 74, 229–240. [CrossRef] 33. Mauro, M.; Acocella, M.R.; Corcione, C.E.; Maffezzoli, A.; Guerra, G. Catalytic Activity of Graphite-Based Nanoﬁllers on Cure Reaction of Epoxy Resins. Polymer 2014, 55, 5612–5615. [CrossRef] Polymers 2018, 10, 1076 15 of 15

34. Park, J.S.; Darlington, T.; Starr, A.F.; Takahashi, K.; Riendeau, J.; Thomas Hahn, H. Multiple Healing Effect of Thermally Activated Self-Healing Composites Based on Diels–Alder Reaction. Compos. Sci. Technol. 2010, 70, 2154–2159. [CrossRef] 35. Lee, J.; Stein, I.Y.; Kessler, S.S.; Wardle, B.L. Aligned Carbon Nanotube Film Enables Thermally Induced State Transformations in Layered Polymeric Materials. ACS Appl. Mater. Interfaces 2015, 7, 8900–8905. [CrossRef] [PubMed] 36. Sosa, E.D.; Darlington, T.K.; Hanos, B.A.; O’Rourke, M.J.E. Multifunctional Thermally Remendable Nanocomposites. J. Compos. 2014, 2014, 12. [CrossRef] 37. Silverstein, R.M.; Bassler, G.C.; Morrill, T.C. Spectrometric Identiﬁcation of Organic Compounds; John Wiley and Sons, Inc.: Hoboken, NJ, USA, 1991. 38. Bode, S.; Enke, M.; Hernandez, M.; Bose, R.K.; Grande, A.M.; van der Zwaag, S.; Schubert, U.S.; Garcia, S.J.; Hager, M.D. Characterization of Self-Healing Polymers: From Macroscopic Healing Tests to the Molecular Mechanism. Self-Heal. Mater. 2016, 273, 113–142. 39. Paramane, A.S.; Kumar, K.S. A Review on Nanocomposite Based Electrical Insulations. Trans. Electr. Electron. Mater. 2016, 17, 239–251. [CrossRef] 40. Deng, H.; Skipa, T.; Bilotti, E.; Zhang, R.; Lellinger, D.; Mezzo, L.; Fu, Q.; Alig, I.; Peijs, T. Preparation of High-Performance Conductive Polymer Fibers through Morphological Control of Networks Formed by Nanoﬁllers. Adv. Funct. Mater. 2010, 20, 1424–1432. [CrossRef]

© 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).