nkihil singha macromolecules43(7), 3193-3205 (2010)

7/27/2019 Nkihil Singha Macromolecules43(7), 3193-3205 (2010)

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pubs.acs.org/MacromoleculesPublished on Web 03/05/2010r 2010 American Chemical Society

Macromolecules 2010, 43, 31933205 3193

DOI: 10.1021/ma902203r

Smart All Acrylate ABA Triblock Copolymer Bearing ReactiveFunctionality via Atom Transfer Radical Polymerization (ATRP):Demonstration of a Click Reaction in Thermoreversible Property

A. Amalin Kavitha and Nikhil K. Singha*Rubber Technology Centre, Indian Institute of Technology, Kharagpur, India 721302

Received October 5, 2009; Revised Manuscript Received January 30, 2010

ABSTRACT: Tailor-made ABA triblock copolymers (poly(furfuryl methacrylate)-b-poly(2-ethylhexylacrylate)-b-poly(furfuryl methacrylate) (FEF)) bearing a reactive pendant furfuryl group were successfullysynthesized by atom transfer radical polymerization. The chemical compositions were calculated by 1HNMR, and molecular weights and molecular weight distributions were determined by gel permeationchromatography analysis. The tensile properties of the triblock copolymers such as tensile strength,elongation at break, and tension set were studied. Differential scanning calorimetry (DSC) analysis anddynamic mechanical analysis (DMA) show theexistence of twoglasstransition temperatures (Tg) andtherebythe presence of well-defined soft and hard phase. Thermoreversible self-healing material was successfullyprepared by using Diels-Alder reaction between this reactive furfuryl group (diene) of the ABA triblock

copolymer and a bismaleimide (dienophile). Thermoreversible property of the polymer was confirmed byFTIR and DSC analysis. The self-healing nature of the polymers was characterized by scanning electronicmicroscopic analysis. Viscoelastic behavior of the Diels-Alder polymer was thoroughly studied by DMAstudies.

Introduction

In recent years, there has been great interest to design and tosynthesize newer types of polymeric materials, with uniqueproperties to meet with demands and challenges for novelapplications.1 Smart polymers2-5 are the materials which canrespond toward external stimuli like the change of temperature,light, magnetic field, and electric field. Because of theirinteresting

properties, they are used in self-healing or self-repairing materi-als, in novel biomaterial for biomedical applications, in micro-electronic applications, and in different strategic applications likein shape memory polymers.2-4 The self-healing concept involvesthedevelopmentof a newclassof cross-linked polymer capable ofhealing internal cracks through thermoreversible covalent bonds.The concept of self-healing polymeric materials was proposed inthe 1980s6 as a means of healing invisible microcracks forextendingthe service life and safety of thepolymeric components.Recently, there are literatures which report that the self-healingpolymeric materials have the built-in capability to substantiallyrecover their load transferring ability after damage. Such reco-very can occur separately or by activation with heat or radiation.Self-healing polymers are finding applications in scratch resis-

tance coatings and in repairing the cracks developed in amaterial.7 Smart materials are expected to contribute greatly tothe safety and durability of polymeric components without highcosts of active monitoring or external repair.2-5

Homo- and copolymers of furfuryl methacrylate (FMA) areinteresting materials because of the presence of reactive furfurylfunctionality.8-11 This reactive diene functionality can be usedfor the Diels-Alder (DA) reaction using a suitable bismale-imide (BM) as dienophile. DA reaction is an interesting clickreaction. The concept click chemistry was coined by K. B.

Sharpless, who wasawarded theNobelPrizein 2001,12because ofhis outstanding contribution in this field. Use of click chemistrynowadays has become a powerful tool to design tailor-madepolymer architectures.13 Kyeremateng et al.14 reported the clickreaction of 1,3 dipolar cycloaddition reaction between azido-terminated AB diblock copolymer and R-alkyne fluorinatedcompound. Opsteen et al.15 demonstrated the combination of

ATRP with click reaction of Cu(I)-catalyzed azide-alkyne cyclo-addition for the modular synthesis of block copolymers. The DAclick reaction is a very versatile, efficient, and environment-friendly process. This reaction occurs at very high yield and atrelatively simple reaction condition in no solvent or in benignsolvent. Importantly, DA reaction can be made thermoreversiblevia the retro-DA (rDA) reaction. It is very challenging to prepareABA triblock copolymers bearing reactive functionality (e.g.,furfuryl functionality group)16 because of the several side reac-tions involving reactive diene during the polymerization of thisfunctional monomer.8-11 The preparation of well-defined rever-sible functional triblock copolymers is an active field of researchbecause of their applications in the thermoplastic elastomers(TPE),17-19 self-healing polymers,20,21 and biomedical applica-tions.22,23 TPEs are a family of polymers that can be repeatedlystretched without permanently deforming the shape of thematerial.24,25 ABA triblock copolymers composed of an elasto-meric middle block (B block) and glassy end blocks (A block) canact as TPE. The glassy microdomains act as physical or chemicalcross-links. It can also actas reinforcing filler rendering attractivethermal and mechanical properties without vulcanization andwithout addition of any filler extraneously.26

Block copolymers based on styrene (e.g., SBS, SEBS, etc.) andbased on acrylates (e.g., PMMA-PBA-PMMA) are usefulTPEs and are prepared by anionic polymerization. Ionic poly-merizations are very much susceptible to impurities present inthe monomer, solvents, and the initiator.26 Controlled radical

*Corresponding author: Tel 91 3222 283178; Fax 91 3222 255303;e-mail [email protected].


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3194 Macromolecules, Vol. 43, No. 7, 2010 Kavitha and Singha

polymerizations (CRP) have several advantages over ionic poly-merization. It eliminates the stringent reaction of ionic polymeri-zation but maintains versatility of ionic polymerization. Amongdifferent types of CRP, transition metal halide-catalyzed CRP,widelyknown as atom transfer radical polymerization(ATRP), iswidely used to prepare different tailor-made polymers withcomplex architectures like block copolymers, star polymers,hyperbranched polymers, etc.27-29 Interestingly, ATRP can beapplied to a great variety of monomers, especially functionalmonomers which are very difficult to polymerize via ionicpolymerizations. ATRP has been used to prepare different blockcopolymers based on acrylates.30 Recently, we have reported

ATRP of furfuryl-containing monomers and studied the DAreaction in the resultant polymers.31 Preparation of well-definedblock copolymer with controlled block length is very challen-ging, especially because of the presence of the reactive furfurylfunctionality.

To the best of our knowledge use of click chemistry of DAreaction is not explored in the tailor-made block copolymerbearing reactive functionality prepared by any method of con-trolled radical polymerization. The objective of this present workis to prepare tailor-made thermoreversible ABA triblock copo-lymers (PFMA-b-PEHA-b-PFMA) bearing reactive furfurylfunctionality with controlled molecular weight via ATRP andto apply this polymer in DA and retro-DA reaction by usingbismaleimide as dienophile. In this 2-ethylhexyl acrylate (EHA)was used as a middle block. PEHA has very low Tg (-58 C), andit has very good film formation properties. It is a useful com-ponent for paints, coatings, and adhesive applications.32 Thesynthetic outline of this investigation is shown in Scheme1. In thistriblock copolymer PFMA block (Tg = 65 C) and PEHA(Tg = -50 C) can act as hard and soft phase, respectively.Thermoreversible nature of the Diels-Alder block copolymers(i.e., block copolymer-bismaleimide adduct) was characterizedby FTIR and by a heating-cooling cycle in differential scanningcalorimetry (DSC). Thethermal and mechanicalproperties of thepolymers were characterized by thermogravimetric analysis anddynamic mechanical analysis. The self-healing nature of thetriblock polymers is characterized by scanning electronic micro-scopic analysis (SEM) and differential scanning calorimetryanalysis (DSC).

Experimental Section

Materials. Furfuryl methacrylate (FMA) (97%, Aldrich) and2-ethylhexyl acrylate (EHA) (98%, Aldrich) were purified byvacuum distillation prior to use. CuCl and CuBr (98%, Aldrich)were purified by washing with glacial acetic acid, followed bydiethyl ether, and then dried under vacuum. Toluene (SD FineChemicals, Mumbai, India) was purified by vacuum distillationover CaH2. Triethylamine (TEA, SD Fine Chemicals, Mumbai,India) was dried over KOH and distilled before use. Dichloro-methane (DCM, SD Fine Chemicals, Mumbai, India) was driedover anhydrous CaCl2 and was distilled over anhydrous P2O5before use. Ethylene glycol (99.8%), 2-bromoisobutyryl bromide

(98%), 1,1,4,7,10,10-hexamethyltriethylenetetramine (HMTETA)(97%), 2,20-bipyridine (bpy) (99%), and 1,1-(methylenedi-4,1-phenylene)bismaleimide (BM) were obtained from Aldrich andwere used as received.

Instrumentation. Nuclear Magnetic ResonanceSpectroscopy(NMR). 1H NMR characterization of polymers was performedat room temperature (25 C) with a Bruker 400-MHz spectro-meter using CDCl3 as solvent which had a small amount oftetramethylsilane (TMS) as internal standard.

FTIR Spectroscopy. FTIR spectra were recorded on a Perkin-Elmer, Inc., version 5.0.1 spectrophotometer. The polymersample dissolved in chloroform was film cast over KBr cells,and then the IR spectra were recorded. The powder samples(Diels-Alder cross-linked polymer) were ground with spectro-scopic grade KBr and made into pellets according to the

specified sample preparation procedure. FTIR spectra wererecorded in the range of 4000-400 cm-1.Gel Permeation Chromatography (GPC). GPC spectra were

recorded on a Viscotek GPC equipped with a refractive indexdetector (model VE 3580) at ambient temperature. The polymersolution was passedthrough twoViscoGELGPC columns (modelGMHHR-M #17392) connected in series. The columns weremixed bed column having pore size of 30-650 A and were suitablefor the analysis of polymer of medium to high molecular weights(exclusion limit Mn = 1 10

6). Tetrahydrofuran (THF) was usedas eluent at room temperature with a flow rate of 1.0 mL/min.Linear poly(methyl methacrylate) (PMMA) standards (PolymerLaboratories) were used as calibration standard. Data acquisi-tion and processing were performed using Viscotek OMNI-01software.

Scheme 1. General Presentation of Self-Healing ABA Triblock Polymer by Diels-Alder Click Chemistry


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Scheme 2. (a) Synthesis of Difunctional Initiator; (b) ATRP of 2-Ethylhexyl Acrylate (EHA) To Prepare DifunctionalMacroinitiator

Scheme 3. Synthesis of ABA (Poly(furfuryl methacrylate)-b-poly(2-ethylhexyl acrylate)-b-(polyfurfuryl methacrylate)[PFMA-b-PEHA-b-PFMA]) Triblock Copolymer


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5-20-5 (5000-20000-5000), 10-20-10, and 20-20-20,respectively. The final composition and the incorporation ofFMA were confirmed by 1H NMR spectroscopy. Figure 3shows the 1H NMR spectra of the PFMA-b-PEHA-b-PFMAtriblock copolymer of FEF-1. The composition of the blockcopolymers was determined from integral area of the reso-nances of-OCH2 protons (= 5.0 ppm) in the PFMA blocks(designated as 5 in Figure 3) and that of-OCH2- protons( = 4.0 ppm) in the PEHA blocks (designated as 6 inFigure 3). The compositions of the block copolymers as well astheir molecular weightscalculated from1H NMR spectra had agood agreement with those determined by GPC analysis(Table 1).

Controlled polymerization and incorporation of FMAinto the copolymer is very challenging because of the sidereaction involving the reactive functionality.8,31 Conven-tional radical polymerization of FMA leads to gel formationdue to the excessive chain transfer involving the C4 proton

(designated as 4) and-OCH2 protons (designated as 5)in FMA (Figure 3).8 In this investigation we have success-fully prepared FEF triblock copolymers with controlledmolecular weight without any gel formation. The resultantblock copolymer was soluble in many organic solvents. Thepolymers were dissolved in THF and were film cast to studytheir mechanical properties.

Thermal and Mechanical Studies of ABA Triblock Copolymer.DSC. DSC analysis showed that FEF triblock copolymer(PFMA-b-PEHA-b-PFMA) has two Tgs (Figure 4). The Tgsof the polymers are shown in Table 2. Tg for the PFMAhomopolymer was 64 C,9 and that for the homopolymerPEHA was-58 C.32 In the case of FEF-2 block copolymer weobserved two Tgs: one of hard segment at65 C (PFMA) andanother of soft segment at-50C (PEHA). Also, it isfound thatwith increase in hard segment content Tg of the block copolymerslightlyincreases. Theincreasein Tg forthe differentblocks in theblock copolymers, especially for the soft phase, is due to therestriction in the free chain mobility of the polymer molecules.

Tensile Properties. Figure 5 depicts the stress-strainplots for FEF triblock copolymers having varying lengthof hard segment. FEF-1, FEF-2, and FEF-3 represent thetriblock copolymers having various length of hard-soft-hard segment of 5-20-5, 10-20-10, and 20-20-20, res-pectively. The increase in final polymer molecular weight hasa direct relationship with the chain length of hard segments,as the length of soft segment is fixed at 20 000. Table 3 showsthe tensile properties of the triblock copolymers havingvarying lengths of hard segment.

The stress-strainplot (Figure 5) for FEF-1 resembles thatof a nonstress crystallizing elastomer. The strain shows veryhigh value even at low stresses. The ultimate tensile strengthand elongation at break for FEF-1 triblock copolymer are3.0 MPa and 42.8%, respectively, which are in the range ofthat of an unfilled synthetic elastomer. The tensile modulusvalue of 8.2 MPa shows less resistance to flow when anexternal force is applied to it. This is an indicative of theprominence of elstomeric PEHA block present in the blockcopolymer, which takes the majority of the applied stressand deforms into high elongations. On the other hand,the stress-strain plot for FEF-2, which has equal blocklengths of both elastomeric PEHA and thermoplastic PFMA

Figure 3.1H NMR spectra of poly(furfuryl methacrylate)-b-poly(2-ethylhexyl acrylate)-b-poly(furfuryl methacrylate) triblock copolymer.

Figure 4. Differential scanningcalorimetry (DSC) thermograph of poly-(furfuryl methacrylate)-b-poly(2-ethylhexyl acrylate)-b-poly(furfurylmethacrylate) (FEF) triblock copolymer.

Table 2. Glass Transition Temperature (Tg) of the Poly-(furfuryl methacrylate)-b-poly(2 ethylhexyl acrylate)-b-poly-

(furfuryl methacrylate) (FEF) Triblock Copolymers

DSC DMA

polymerfeed

composition Tg I (C) Tg II (C) Tg I (C) Tg II (C)

FEF-1 5-20-5 -52 63 -35 88FEF-2 10-20-10 -50 65 -33 90FEF-3 20-20-20 -46 67 -30 92


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breathing and ether linkage in the furfuryl ring, respectively.Figure 7b shows the mixture of FEF-2 and BM at t =0h.The

peak at 1395 cm-1 due to the -C-N-C- of maleimide andthe characteristic peaks (i.e., peaks at 1012 and 1153 cm-1) for

Scheme 4. Synthesis of Diels-Alder (DA) Cross-Linked Smart Polymer

Figure 6. Dynamic mechanical analysis (DMA) of the poly(furfuryl methacrylate)-b-poly(2-ethylhexyl acrylate)-b-poly(furfuryl methacrylate) (FEF)triblock copolymers: (a) storage modulus; (b) tan .


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thefurfuryl groups are in thesame positionand arenot affectedat t = 0 h. Parts c, d, and e of Figure 7 show the reactionbetween FEF-2 and BM at t = 4, 12, and 24 h, respectively.Figure 7c,d shows the disappearance of peak at 1012 cm-1 andthe formation of new peak at 1636 cm-1, which indicates theformation ofCdC due to the DA cross-link between FEF-2triblock copolymer and BM. The shifting ofCdO peak from1727 to 1711 cm-1 is dueto the interaction between PFMA andBM.37

Thermoreversible Studies of the Self-Healing TriblockCopolymer. DSC Studies. Thermoreversible study wasfurther confirmed by DSC analysis. Figure 8 shows theheating and cooling curves of DSC analysis of the FEF-2-BM and DA cross-linked polymer. Figure 8a shows the firstheating curve of the cross-linked polymer heated at-100 to200 C. The curve A shows the first heating cycle. It has anendothermic peak at 150 C, which indicated the cleavageof the DA cross-links. The curve B shows the coolingcurve of the polymer. It shows a broad exothermic peak at

122-180 C due to the covalent bond formation betweenfuran and maleimide moieties, as the samples come closer oncooling and then finally reconnect very efficiently. The curveC shows the second heating cycle of the DA cross-linkedpolymer. It also shows the same observation in the firstheating cycle. It clearly indicates that the Diels-Alder cross-linked block polymer (FEF-2-BM) is thermoreversible. TheTgs of the polymer were determined from the second heatingcycle of the polymers and are shown in Figure 8b. The

transition at 114 C is due to the Tg of the hard segment(PFMA-BM). The shifting of hard segment Tg from 65 to114 C is due to the DA cross-linked between PFMA andBM. The transition at -44 C (Figure 8b) is due to the Tg of(PEHA) the soft segment. The shifting of Tg from -50 to-44 C is due to the restriction of the chain mobility in DAcross-links.

SEM Studies of the Self-Healing Cross-Linked SmartPolymer. The self-healing behavior of the thermoplasticFEF-2-BM triblock DA cross-linked filmswas characterizedby SEM studies (Figure 9). The cross-linked materials showvery smooth planar surface as shown in Figure 9a. Adeliberate scratch was made on the FEF-2-BM film surfaceusing a knife (Figure 9b). Theknife-cut sample was thermallyheated at 120 C for 4 h and kept at 25 C for 24 h. At a

different time interval the samples were characterized bySEM analysis. Parts c-f of Figure 9 show the FEF-2-BMsamples heated at 120 C at different time intervals of 1, 2, 3,and 4 h, respectively. It shows that the cleavage of thecovalent bonds occurs slowly due to rDA in the cross-linkedpolymer. Since the cleavage of bonds between furfuryl groupand BM, the polymeric materials slowly get enhancedmobility and bonded again via DA reaction as shown inFigure 9c-e. Finally Figure 9f shows that the polymer self-heals to show a complete notch recovery to regain its earlierstructure (Figure 9a).

Solution Properties of the Cross-Linked Smart Polymer.The solution properties of the thermoreversible cross-linkedDA polymer (FEF-2-BM) were studied using toluene as

solvent at 25 and 100 C. Scheme 5 shows the generalschematic representation of the solution behavior of theFEF-2-BM cross-linked polymer. The solid FEF-2-BMpolymer (polymer a) was taken in a 25 mL round-bottomedflask followed by the addition of toluene at 25 C (sample b).

Figure 8. DSC analysis of the polymer. (a) DSC curve for the cross-linked polymer poly(furfuryl methacrylate)-b-poly(2-ethylhexyl acrylate)-b-poly(furfuryl methacrylate)-2-bismaleimide(FEF-2-BM): (A) firstheating curve,(B) cooling curve,and (C) secondheating curve.(b) Tg of thecross-linked polymer.

Figure 7. FTIR spectra of theDA polymer: (a)poly(furfuryl methacry-late)-b-poly(2-ethylhexyl acrylate)-b-poly(furfuryl methacrylate) (FEF-2);(b) FEF-2 BM at t = 0; (c, d, e) FEF-2 BM at t = 4, 12, and 24 h,respectively.


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It was closed and kept at room temperature for 2 h. It wasobserved that the cross-linked polymer was insoluble at25 C (sample c) and got swollen, indicating the cross-linking reaction between FEF-2 and BM. When it washeated at 100 C for 9 h, it becomes soluble (Scheme 5d).It indicates the retro-DA (rDA) reaction which led to thecleavage of furfuryl ring (in the FEF triblock copolymer)and bismaleimide. This rDA was also confirmed by FTIRanalysis. Figure 10 shows the FTIR spectra of the sampled (in Scheme 5). It shows the disappearance of the CdCpeak at 1636 cm-1 and the emergence of new peaks at 1501and 1395 cm-1 which are attributed to the furan ringstretching and -C-N-C- of bismaleimide, respectively.It indicates the clear evidence of rDA reaction. The solublepolymer (sample d in Scheme5) was kept at 25Cfor2days.The solvent was evaporated slowly from the solution. Theresultant solid FEF-2-BM adducts was cross-linked again(sample e). This reformed DA polymer was further con-firmedby FTIR analysis. TheFTIR spectrum of sampleeis shown in Figure 10. It shows the emergence of the peak at1636 cm-1 attributed to the formation of the CdC peakwhich is due to the re-formation of DA cross-linked poly-mer. Further addition of toluene into the re-formed DApolymer (sample e) at 25 C had shown no solubility(sample b). Repetition of this cycle supports the thermo-reversibility of the DA adduct of ABA triblock copolymerand BM.

Dynamic Mechanical Studies of Thermoreversible Self-Healing Triblock Copolymer: Effect of Frequency. Figure 11ashows the variation in storage modulus of FEF-2-BM, the DAcross-linked polymer with frequency of oscillation. The testwas carried out at different temperatures (40 C aroundtemperature of the thermoreversible cross-link formation.As the frequency increases, the storage modulus of thesamples increases irrespective of the temperatures beingused for testing. This behavior can be explained on the basisof the time-dependent molecular relaxations of polymers. Assinusoidal stress is applied at lower frequencies, polymermolecules get enough time to relax or rearrange accordingto the applied stress. The polymer behaves like a low viscousfluid that easily flows with an applied stress. When thefrequency increases, the stress applied on the polymericchains is too fast to have molecular rearrangements or rela-xations. Hence, polymer flow gets restricted, and it behaveslike a solid material. The increased storage modulus orstiffness at higher frequency is an outcome of the decreasedmolecular relaxations. There is decrease in storage moduluswith increase in test temperature. As the temperatureincreases, the free volume within the polymeric materialincreases. This is due to the scission of thermoreversiblecross-links on the DA polymer promoting more flow.The reduced stiffness (storage modulus) at higher tem-perature is an outcome of the cross-link scission thatresults in more free volume for deformations within the

Figure 9. Self-healing study of poly(furfuryl methacrylate)-b-poly(2-ethylhexyl acrylate)-b-poly(furfuryl methacrylate)-2-bismaleimide (FEF-2-BM)triblock DA cross-linked polymer. SEM images of (a) cross-linked polymer and (b) knife-cut sample. (c, d, e, f) Heating of FEF-2-BM triblock DA

cross-linked polymer at different times of 1, 2, 3, and 4 h, respectively.


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polymer. The sample tested at 160 C has no considerableincrease in storage modulus with increase in frequency.This can be attributed to the complete breakdown ofthermoreversible bonds at 160 C.

Figure 11b shows the variation of loss modulus withincrease in frequency tested at different temperatures. Theloss modulus has a directrelationship with frequencyand thetrend is similar to that of storage modulus with frequency.The lower loss modulus values at lower frequencies are

attributed to the promoted flow and significant molecularrelaxations. As the frequency increases, the loss modulusincreases which is attributed to the restricted flow behaviordue to the reduction in the time available for molecularrelaxations. The molecular relaxations are also a function oftemperature. The temperature range (80-160 C) selectedfor the test contains the temperature for cleavage of cross-links in the DA polymer. Above this temperature polymericchains arefree forintermolecularslippagewhichcauses flow.

This is the reason for the lower loss modulus values at highertest temperatures.Figure 11c demonstrates the effect of frequency on damp-

ing factor (tan ) at different temperatures ranging from80 to 160 C. At lower frequencies, a higher value of tan corresponds to linear chain and a low value indicatesbranching or cross-linking in the polymers.38 The samplestested at 160 C gave the highest damping factor at lowerfrequency as compared to its lower temperature scans. Itindicates that the DA cross-links break down at highertemperatures, giving a relatively linear structure. Also, thepeak height of damping factor decreases with decreasein temperature. At higher temperatures more damping isexpected because of the scission of cross-links. However, athigher frequencies higher tan values are obtained for

samples tested at lower temperatures. This maybe attributedto the increased effective molecular weight of the cross-linked polymers at lower temperatures which shows higherdamping as compared to easily flowable linear chains thatdeforms easily at higher frequencies. This study also drawsconclusions that the DA cross-links break at higher tem-peratures, transforming the cross-linked structure to a morelinear structure.

Scheme 5. General Schematic Representation of the Solution Behavior of the Poly(furfuryl methacrylate)-b-poly(2-ethylhexyl acrylate)-b-poly-(furfuryl methacrylate)-2-bismaleimide (FEF-2-BM) Cross-Linked Polymer: (a) FEF-2-BM Cross-Linked Polymer, (b) Addition of Toluene at 25 C;

(c) Insoluble Cross-Linked Polymer at 25 C for 2 h; (d) Soluble FEF-2-BM Cross-Linked Polymer at 100 C for 9 h; and (e) Soluble FEF-2-BMPolymer Cross-Linked Again at 25 C after 2 days

Figure 10. FTIR spectra of sample d and sample e (as shown inScheme 5).


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