Reversible Cross-Linking of Aliphatic Polyamides Bearing Thermo

Article pubs.acs.org/Macromolecules

Reversible Cross-Linking of Aliphatic Polyamides Bearing Thermoand Photoresponsive Cinnamoyl Moieties Deniz Tunc,†,‡,§ Cedric Le Coz,†,‡ Michael Alexandre,§ Philippe Desbois,∥ Philippe Lecomte,*,§ and Stephane Carlotti*,†,‡ †

Laboratoire de Chimie des Polymères Organiques, UMR 5629, IPB/ENSCBP, Univ. Bordeaux, 16 avenue Pey-Berland, F-33607 Pessac, Cedex, France ‡ Laboratoire de Chimie des Polymères Organiques, UMR 5629, Centre National de la Recherche Scientifique, F-33607 Pessac, Cedex, France § Center for Education and Research on Macromolecules, University of Liège, Sart-Tilman, B6a, 4000 Liège, Belgium ∥ Advanced Material and System Research, BASF SE, D-67056 Ludwigshafen, Germany ABSTRACT: Photosensitive polymers have gained strong attention in development of different polymeric systems for diverse applications. Particular attention is paid to polymers with cinnamoyl groups either in polymer backbone or in pendent position owing to their excellent photoreactivity and thermoreactivity upon UV light irradiation or heating. This work aims at synthesizing a novel photo- and thermosensitive aliphatic polyamide by anionic ring-opening copolymerization of εcaprolactam and α-cinnamoylamido-ε-caprolactam. The photochemical reversible cross-linking of solutions and thermal cross-linking of spin-coated films made up of these new cinnamoyl-functionalized polyamides were monitored by UV−vis spectroscopy. These aliphatic polyamide copolymers were characterized by magic angle spinning solid-state NMR, and their thermal behaviors were studied by thermogravimetric analysis, differential scanning calorimetry, and dynamic mechanical analysis.

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INTRODUCTION In recent decades, polymers containing photosensitive moieties proved a remarkable interest because of their potential use in a broad range of application fields such as photolithography,1,2 photocurable coatings,3 and photoresists.4 Cinnamoyl,5,6 coumarin,7−10 anthracene,11,12 and azobenzene13,14 are the mostly used chromophore groups for photoresponsive systems by dimerization or isomerization reaction. Photoreversibility of these groups can be set in the main chain or as pendant groups to enable controlling of some physical and chemical properties by cross-linking. Among photosensitive moieties, the photoreactive cinnamoyl group dimerization, occurring through the [2 + 2] thermal or photochemical cycloaddition reaction,15 was implemented for polymerization reactions and for cross-linking purposes.16 The so-obtained cyclobutane dimer combines thermal stability17,18 and photosensitivity.19 The photochemical reversibility of the cross-linking was, for instance, nicely exploited for the synthesis of self-healing polymers20,21 and tunable shape-memory22 materials. Recently, Oriol, Barner-Kowollik, and co-workers studied on miktoarm star polymer bearing azobenzene photoresponsive units and reported the self-assembly properties of this novel polymer in water.13 Quite recently, Yagci and co-workers reported self-healing strategy for poly(propylene oxide)s bearing coumarine−benzoxazine units based on lightinduced coumarine dimerization reactions.9 On the other hand, © XXXX American Chemical Society

thermoresponsive systems have gained great interest for the synthesis of reversible cross-linking polymers. As an example, the preparation of reversibly cross-linking polyurethane based on Diels−Alder reaction of furan and maleimide moieties were studied by Du Prez and co-workers.23 Experimental conditions as well as the nature of polymer chains have a significant influence on the efficiency of the dimerization reaction. Polymers bearing photosensitive groups show a higher photoreactivity in solution than in a solid-state coated film because the photosensitive groups are not in close vicinity enough in the solid state.24 Besides, the efficiency of the dimerization reaction of pendant cinnamoyl groups decreases when the chain length increases owing to a lower mobility.25 Polyamide 6 (PA6) is a high performance semicrystalline thermoplastic with large industrial applications. It is produced through (1) the bulk anionic ring-opening polymerization (AROP) of ε-caprolactam (CL)26−30 and (2) the hydrolytic ring-opening polymerization.26,27,31,32 PA6 has attained interest owing to its high strength, good fatigue resistance, moderate water absorption (about 8−10%), and good resistance to most common solvents and weak acids.27 Besides those attractive properties, polyamides are difficult to process due to poor Received: October 10, 2014 Revised: November 20, 2014

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dx.doi.org/10.1021/ma502083p | Macromolecules XXXX, XXX, XXX−XXX

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(Specialty Coating Systems, P67080 spin coater) on glass plates from 1% solutions in HFIP at 2000 rpm for 5 min and then dried at 25 °C overnight under vacuum. The UV irradiation of the spin-coated film and solution was performed by using a UV lamp TQ 150 (150 W) between λ = 200−280 nm and 450HHL halogen (325−380 nm, 400 W), respectively. UV absorbances of de-cross-linked samples were followed by a Hitachi U-3300/130-0614 model UV−vis spectrophotometer. Synthesis of α-Cinnamoylamido-ε-caprolactam [(2E)-N-(2Oxoazepan-3-yl)-3-phenylprop-2-enamide]. To the solution of cinnamoyl chloride (5.2 g, 30 mmol) and triethylamine (5.05 g, 50 mmol) in 50 mL of tetrahyrofuran (THF) cooled in an ice bath, αamino-ε-caprolactam (4 g, 30 mmol) was added dropwise while purging the reaction flask with argon. After 15 min, the ice bath was removed, and the reaction was allowed to proceed overnight at room temperature. The precipitate was removed by filtration, then THF was evaporated, and the product was resolubilized in chloroform. The solution was transferred to an extraction funnel and washed with dilute HCl(aq) (1 × 50 mL), distilled water (1 × 50 mL), and brine solution (1 × 50 mL). The organic phase was separated, dried over anhydrous Na2SO4, filtered, and evaporated. The product was further purified by washing with diethyl ether to give a white solid (80% yield). 1H NMR (CDCl3) δ (ppm) = 7.26−7.49 (m, 5H), 7.64 (d, 1H), 7.07 (s, 1H), 6.5 (d, 1H), 4.6 (m, 1H), 3.3 (m, 2H), 2.3−1.2 (m, 6H). 13C NMR (CDCl3) δ (ppm) = 175.8, 164.5, 140.8, 135.1, 129.9, 135.1, 129.9, 129.0, 127.8, 120.5, 52.7, 41.7, 31.7, 29.0, 27.7. Synthesis of Polyamide 6. Polyamide 6 was prepared in bulk by anionic ring-opening polymerization. ε-Caprolactam (8.35 g, 73.8 mmol) and C10 initiator (1.15 g) (or 1.53 mmol of sodium εcaprolactamate) were added in a glass reactor at 140 °C under a nitrogen atmosphere. After C20 activator (0.5 g) (or 0.21 mmol of reactive functions) was added to the molten mixture, the polymerization started with solidification. After reaction, the polymer obtained was crushed and refluxed in water for 24 h and then dried in an oven overnight at 90 °C under vacuum before analysis. Conversion of the monomer was determined gravimetrically. 1H NMR (CDCl3) δ (ppm) = 6.0 (s, 1H), 3.1 (m, 2H), 2.0 (m, 2H), 1.25−1.75 (m, 6H). 13C NMR (solid-state) δ (ppm) = 169.8, 39.7, 32.9, 26.6, 22.7. Synthesis of Polyamide 6 Bearing Pendant Cinnamoyl Groups. Polyamide 6 with pendant cinnamoyl groups was prepared by a one-step pathway by anionic ring-opening polymerization. As a typical example, α-cinnamoylamido-ε-caprolactam (0.2 g, 0.77 mmol), ε-caprolactam (8.15 g, 72 mmol), and C10 initiator (1.15 g) (or 1.53 mmol of sodium ε-caprolactamate) were added to a glass reactor under a nitrogen atmosphere and were molten at 140 °C with continuous stirring after hexamethylene-1,6-dicarbamoylcaprolactam (C20) (0.5 g) (or 0.21 mmol of reactive functions) was added to the mixture to start the polymerization. After reaction, all polymers were refluxed in water first and followed by washing with dichloromethane for 24 h each. The polymers were dried in an oven overnight at 90 °C under vacuum before any analysis. Conversions of the monomers were determined gravimetrically. 13C NMR (solid-state) δ (ppm) = 169.9, 131.9, 125.7, 39.8, 36.6, 33.0, 26.6, 18.4. Synthesis of α-Truxillic Acid.39 trans-Cinnamic acid (Aldrich, 99%) was recrystallized from acetone (Aldrich) to obtain the αpolymorph. The crystals (2 g, 13.5 mmol) were put in a reactor and distributed in 8 mL of heptane. The powder sample in solid state was exposed to 325−380 nm irradiation using a light source which is a 450HHL halogen (400 W) for 24 h to obtain truxillic acid. 13C NMR (solid-state) δ (ppm) = 178.9, 175.5, 134.7, 125.6, 123.4, 121.9, 47.8, 42.3, 40.9, 37.4. De-Cross-Linking of Polyamide 6. The 1.2 × 10−4 mol L−1 gel− solution of cinnamoyl-containing cross-linked polyamide 6 (10% CinPA6) in HFIP was prepared before irradiation. To induce the de-crosslinking reaction, the sample was irradiated with a UV lamp TQ 150 (150 W, λ = 200−280 nm) at 25 °C for 7 h, and the distance between the samples and the light source was 4 cm. The increasing absorbance of the cinnamoyl units was followed by UV absorbance. Re-Cross-Linking of Polyamide 6. After 7 h, de-cross-linking of cinnamoyl-pendant polyamide 6 solution (10% Cin-PA6), solvent was

solubility and to high softening and melting temperatures caused by high crystallinity. On the other hand, adding a functional groups to tune PA6 properties is a key issue. To date, scarce examples of functionalization and copolymerization of CL with its derivatives were reported either by anionic or hydrolytic ring-opening polymerization. For example, Reimschuessel et al. reported on the copolymerization of CL and βcarboxymethylcaprolactam33 with linear polyimide structure. Overberger et al.34 studied the ring-opening polymerization of γ-tert-butoxymethylcaprolactam by obtaining soluble copolyamide owing to hydroxymethyl pendant groups. The hydrolytic ring-opening polymerization γ-phenylsulfonatecaprolactam was successfully reported by Nijenhuis et al.35 γ-Carboxyethylcaprolactam and γ-aminoethylcaprolactam were used for the synthesis of hyperbranched aliphatic polyamides.36 It is worth noting that the synthesis of these monomers requires several or complex reaction steps with moderate yields. Recently, Mathias et al. reported the AROP of γ-ethylene ketal-ε-caprolactam.37 After deprotection of pendant ketal groups into ketones, the polyamide was successfully thermally and photochemically cross-linked. In our previous work, a controlled cross-linking of polyamide 6 was proposed by AROP of CL and a new bismonomer synthesized from α-amino-ε-caprolactam for different rheological behaviors of such a material.38 The aim of the present work is to synthesize novel reversibly cross-linked aliphatic polyamide by implementing the photoand thermodimerization reactions of pendant cinnamoyl units. Toward this end and starting from cyclic lysine, the one-pot synthesis of α-cinnamoylamido-ε-caprolactam was carried out followed by its anionic ring-opening copolymerization with CL.

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EXPERIMENTAL SECTION

Materials. DL-α-Amino-ε-caprolactam (3-aminoazepan-2-one) (>98%, BASF) was purified by solubilization in toluene and filtration. After evaporation of toluene a white product was obtained. Cinnamoyl chloride [(2E)-3-phenylprop-2-enoyl chloride)] (98%) and triethylamine (N,N-diethylethanamine) (>99.5%) were purchased from Sigma-Aldrich and used without further purification. ε-Caprolactam (azepan-2-one) (CL) (BASF, 99%) was recrystallized from dry cyclohexane prior to use. Brüggolen C20 (hexamethylene-1,6dicarbamoylcaprolactam [N,N′-hexane-1,6-diylbis(2-oxoazepane-1-carboxamide] in CL, 17% w/w of isocyanate in CL, Brüggemann Chemical) and Brü g golen C10 (∼18% w/w of sodium εcaprolactamate [sodium 2-oxoazepan-1-ide] in CL, Brüggemann Chemical) were used as received. Tetrahydrofuran, chloroform, acetone, heptane, and diethyl ether were purchased from SigmaAldrich. 1,1,1,3,3,3-Hexafluoropropan-2-ol (HFIP) (Acros Organics, ≥99.7%) was used without any further purification. Characterization. 1H NMR spectra of reactants were recorded by using a Bruker AC250 instrument at a proton frequency of 250 MHz at room temperature. 13C solid state NMR spectra were recorded on a Bruker 400 Avance I 400 MHz spectrometer at room temperature. During experiments, the magic angle spinning (MAS) rate was 30 kHz using a Bruker 2.5 mm HX probe. The number of scans was 256, and recycle delay was 5 s. Single-pulse experiment (90° pulse of duration) was 1.8 μs, and FID acquisition time was 41 ms. The FTIR spectra were recorded using a Nicolet Nexus with an ATR, accessory by continuum microscope provided with an ATR germanium (Ge) crystal. Differential scanning calorimetry (DSC) was performed on a PerkinElmer Diamond DSC with a heating rate of 10 °C min−1 under nitrogen flow (10 mL min−1). Thermogravimetric analysis (TGA) was performed on a PerkinElmer Diamond TA/TGA with a heating rate of 10 °C min−1 under nitrogen flow. The storage modulus (E′) and loss factor (tan δ) for each sample were automatically recorded by a computer throughout the test. For surface re-cross-linking by heating and UV irradiation, thin films of PA6 precursors were spin-coated B


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are 1:1:1:2 and support the synthesis and purification of the αcinnamolyamido-ε-caprolactam. First of all, neat PA6 was prepared by AROP in a reactor system which provides inert atmosphere and easy way to follow the rate of polymerization (temperature/time diagram). The polymerization was carried out in bulk at 140 °C. εCaprolactam (CL, monomer) and sodium caprolactamate (C10, initiator) were added to a glass reactor at 140 °C under a nitrogen atmosphere to activate the monomer and were molten with continuous stirring after hexamethylene-1,6dicarbamoylcaprolactam (C20, activator) was added to mixture to start the polymerization. In 15 min, solidification completed and exothermy slowed down; PA6 was refluxed in water to remove unreacted reagents and then dried in an oven overnight at 90 °C under vacuum before any analysis. Conversion of the monomer (98%) was determined gravimetrically. Syntheses of PA6 bearing different amount of cinnamoyl units were conducted exactly under the same polymerization conditions as the one of PA6 was carried out. Addition to that, Cin−CL comonomer was added to the reactor at the same time with CL. Results are summarized in Table 1, and the proposed reaction is depicted in Scheme 2. After the polymerization step, all obtained copolymers were washed by refluxing first from water and then with dichloromethane (CH2Cl2) to remove unreacted Cin−CL comonomer and other reagents. Different molar ratios of α-cinnamoylamido-ε-caprolactam were used in each copolymerization with the same amount of activator and initiator at the same reaction temperature of 140 °C. As the relative amount of cinnamoyl unit was increased from 2% to 10%, the obtained polymers turned to gel formation in hexafluoroisopropanol (HFIP) for 6% and 10% of Cin-PA6 (runs 3 and 4). Those polymers, which have higher quantity of cinnamate units, were thermally cross-linked because of the reduced distance and mobility of dimerizable groups. It has already been shown that the dimerization reaction has a favorable encounter of the cinnamoyl groups due to polymer chain mobility through heating and irradiation.15,24,25 During the PA6 synthesis, an exothermic reaction, polymerization and crystallization occur almost simultaneously (Figure 2). These temperature/time data were followed by a thermocouple which is installed in the calorimeter to measure the reaction temperature. On the other hand, the copolymerization of CL with its derivative leads to a decrease of polymerization and crystallization exothermic temperatures (run 2, Table 1, and Figure 2). The flat evolution of temperature with time and limited conversion were observed by using 10 wt % of the cinnamated monomer (run 4, Table 1). Figure 2 indicates clearly a decreased reactivity of the overall polymerization process and the introduction limit of cinnamated groups into PA6. The copolymerization of an increasing amount of α-cinnamoylamido-ε-caprolactam slows down the polymerization time and decreases the monomer conversion. Trying to introduce much more comonomer was not successful. This is an expected behavior as ring-opening polymerization of acyl derivative of CL is concerned. The presence of amide N anions along the polymer chain may produce imide groups and lead to polymer branching in strongly basic medium.40 A similar effect can be a concern for this anionic copolymerization. In addition, a competitive hydrogen abstraction, due to difference of hydrogen acidities, from the exocyclic and endocyclic amides by C10 initiator may occur on Cin-CL (Scheme 3). The deprotonation of the exocyclic amide consumes C10 amount, decreasing its

evaporated, and 1% w/v of HFIP solutions of cinnamoyl-containing de-cross-linked polyamide 6 was prepared and spin-coated on a quartz plate. Irradiations under re-cross-linking conditions were carried out placing the samples at 10 cm from the light source which is a 450HHL halogen (325−380 nm, 400 W). Following 7 h irradiation, re-crosslinking was completed with 1 h heating at 140 °C. Absorbance decreasing of cinnamoyl unit double bonds was followed by UV absorbance. The coated sample was not soluble in HFIP anymore.

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RESULTS AND DISCUSSION A successful synthesis of α-cinnamoylamido-ε-caprolactam (Cin-CL) monomer was achieved by a one-step amidation reaction (Scheme 1) from α-amino-ε-caprolactam (ACL) Scheme 1. One-Step Amidation Reaction of Cinnamoyl Chloride with α-Amino-ε-caprolactam

which can be obtained from ring closure reaction of L-lysine. After 24 h reaction in an inert atmosphere and then purification steps, a white solid product was obtained with 80% yield. The structure of the α-cinnamoylamido-ε-caprolactam was confirmed by 1H NMR analysis, and Figure 1a exhibits the

Figure 1. 1H NMR spectra of (a) α-amino-ε-caprolactam and (b) αcinnamolyamido-ε-caprolactam in CDCl3.

typical proton signals attributed to ethylene groups of the αamino-ε-caprolactam structure between 1.25 and 2.30 ppm (−CH2), primary amine at 2.10 ppm, and amide proton (CO− NH−C) at 6.75 ppm. After reaction (Figure 1b), endocyclic and exocylic protons were observed at 6.10 and 7.10 ppm, respectively. Cinnamoyl aromatic protons appeared between 7.2 and 7.5 ppm. At the same time cinnamoyl alkene protons were shown at 6.5 and 7.6 ppm. Indeed, integral ratios of i:h:c:b C


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Table 1. Synthesis of Polyamide 6 and Polyamide 6 Bearing Pendant Cinnamoyl Groups at 140 °C in Bulk and Their Solubility in HFIP run 1, 2, 3, 4,

PA6 2% Cin-PA6 6% Cin-PA6 10% Cin-PA6

activator (mol/L)

initiator (mol/L)

CLa (mol/L)

Cin-CLb (mol %)

Cin-CLb (wt %)

time (min)

convc (%)

sold

0.021 0.021 0.021 0.021

0.153 0.153 0.153 0.153

7.38 7.20 6.84 6.50

1.03 3.19 5.45

2 6 10

15 15 30 24

98 90 60 60

S S NS NS

a ε-Caprolactam. bα-Cinnamoylamido-ε-caprolactam. cDetermined gravimetrically; overall conversion. dSolubility in HFIP (hexafluoroisopropanol); S = soluble, NS = not soluble.

concentration in the reaction medium, and leads to a new amidate able to react with lactam chain ends. The new chain end is then a nonreactive one and stops the propagation. Cross-linking of 2% Cin-PA6 did not occur during the AROP as the final polymer was soluble in HFIP. It is mostly probable that a low amount of the cinnamated monomer (2 wt %) leads to a high distance between the reactive double bonds as well as faster AROP than the dimerization reaction. On the other hand, 6% Cin-PA6 synthesis and cross-linking, owing to higher amount of cinnamated pendant units, were completed in a short polymerization time (15 min.). Only temperature/time diagrams of 2% and 6% of cinnamoyl pendant PA6 (runs 2 and 3, Table 1) were able to be observed and compared with PA6 (Figure 2). 10% Cin-PA6 (run 4, Table 1) has a flat temperature/time plot with a quite longer polymerization time (24 h) to reach 60% yield. After the copolymerization reaction, 6% Cin-PA6 and 10% Cin-PA6 were not soluble (gel formed) in HFIP due to the cross-linking occurrence. The decelerating effect of the comonomer on the copolymerization process also reflects a lower monomer conversion compared to homopolymerization of CL (Table 1). The exact amount of comonomer incorporated in the copolymer could not be determined by molecular characterizations due to its nonsolubility in any organic compounds. The soluble 2% sample was not significant enough. Solid-state 13C MAS NMR was helpful to confirm the structure of PA6 bearing cinnamoyl groups (Figure 3). The dimerization−cross-linking (for more than 2% of α-cinnamoylamido-ε-caprolactam, Table 1) which occurs during AROP of CL and Cin-CL was attributed to the cyclobutane ring formation. To further explore and compare possible influences of dimerized cinnamoyl units on polyamides, α-truxillic acid was prepared from cinnamic acid dimerization by UV irradiation.41 As the dimerization proceeds during the copolymerization, the new carbon signals attributed to the aromatic carbons indicate the successful copolymerization. As expected, the new chemical shift of aromatic carbon peaks appeared between 115 and 140 ppm, as shown elsewhere.41 The solid-state chemical shifts of PA6 and PA6 with pendant cinnamoyl groups were similar for aliphatic carbon and carbonyl groups. One signal in the carbonyl region around 170 ppm and four different signals of aliphatic carbons of PA6 appeared and overlap the cyclobutane ring (around 40 ppm) and its carbonyl functions. In addition to PA6 aliphatic carbon peaks, there is one more signal which may belong to overlapping cyclobutane ring carbons. Thermal and thermomechanical characterizations of PA6 bearing cinnamoyl units and neat PA6 were investigated by differential scanning calorimetry (DSC), thermogravimetry (TGA), and dynamic mechanical analysis (DMA). DSC measurements of PA6 copolymers having 2, 6, and 10 wt % of cinnamoyl units were performed in nitrogen (Table 2). The

Scheme 2. Anionic Ring-Opening Copolymerization and Thermal Dimerization of ε-Caprolactam and αCinnamoylamido-ε-caprolactam

Figure 2. Temperature−time diagrams of PA6 (solid), 2% Cin-PA6 (dashed), 6% Cin-PA6 (dotted), and 10% Cin-PA6 (dashed-dotted).

D


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Scheme 3. Competitive Proton Abstractions from Exo- and Endocyclic Amide Protons of α-Cinnamoylamido-ε-caprolactam

Figure 3. 13C MAS NMR spectra of 10% Cin-PA6 (a), PA6 (b), truxillic acid (c), and cinnamic acid (d).

amorphous phase and related to the pendant cinnamoyl moieties. This trend was also confirmed by the measurement of the maximum of tan δ by DMA with an increase from 72 to 84 °C. Melting temperatures (Tm) and melting enthalpies (ΔHm) of the copolymers measured during the second analysis

copolymers of PA6 bearing bulky cinnamoyl pendant groups, either cross-linked PA6 (10% Cin-PA6 and 6% Cin-PA6) or non-cross-linked (2% Cin-PA6), demonstrated significant increase in Tg up to 62 °C as compared to neat PA6 with a Tg of 53 °C. It is due to a decrease of chain mobility in the E


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Table 2. Thermal and Thermomechanical Properties PA6 and PA6S Containing Pendant Cinnamoyl Moieties DSC 2nd run sample PA6 2% Cin-PA6 6% Cin-PA6 10% Cin-PA6

Tg (°C) 53 59 62 61

Tm (°C) 216 214 204 201

DMA ΔHm (J/g)

TGA

tan δ (°C)

E′ (Pa)

72 74 78 84

× × × ×

70 60 47 44

2.1 7.4 7.8 6.2

9

10 108 108 108

T5a (°C)

T10b (°C)

Tdc (°C)

290 307 308 361

300 321 336 390

308 330 346 407

a Decomposition temperature at a 5% weight loss. bDecomposition temperature at a 10% weight loss. cMaximum of the peak decomposition temperature.

were monitored at different wavelengths. After exposing the 10% Cin-PA6 gel formed in HFIP to 220−280 nm light irradiation for de-cross-linking (decomposition of the dimer), the gel became a solution as represented in Scheme 4. Polymers having cinnamoyl groups are known to be insolubilized16 after photoirradiation between 325 and 380 nm or thermal treatment due to the [2 + 2] cycloaddition to form cross-linkages. As depicted in Figure 6, the cinnamate residues, after cleavage, reached to their highest absorption state in 6 h irradiation at 295 nm. The double signal at t = 0 is characteristic of the truxillic acid moiety but shifted to slightly higher wavelength probably due to its polyamide environment. Possible degradation of polyamide may occur upon irradiation, but it was not observed by TGA, DMA, or size exclusion chromatography in HFIP in the conditions used. After de-cross-linking, spectral changes of the spin-coated thin films upon UV irradiation at 325−380 nm or following heating at 140 °C for re-cross-linking are shown in Figure 7. Absorption spectra of spin-coated polymer film decreased gradually upon further irradiation with time. A more efficient decrease was also observed by heating at 140 °C for 1 h after following the irradiation at 25 °C. Re-cross-linking was also confirmed from the insolubility of the thin film after irradiation and heating at 140 °C. A difference in terms of efficiencies of re-cross-linking by UV irradiation and heating was clearly observed. The reason for this difference in efficiency relies in the much higher mobility degree43 of chains bearing pendant cinnamoyl units at high temperature during the thermal treatment compared to the lower temperature used during irradiation. It was already reported that, whenever photosensible groups are in close proximity in the condensed state compared to solution,24 heating increases the mobility of chains and thus favors the diffusion of cinnamoyl units to each one another. Restricted solubility of cinnamoyl pendant PA6 allows only spin-coated film irradiation which is also required due to polar solvent effect of HFIP on dimerization. It is also noted for another system24 that the UV spectrum of the re-cross-linked content on the glass plate is broader than de-cross-linked samples in HFIP solutions. The percentages of reacted chromophore groups are related to the accessibility. Once the network starts to form, the encounter between two chromophores on different molecules is increasingly hindered.44 Cross-linking rate of 10% Cin-PA6 was measured by following UV absorbance intensity of polymer film after UV irradiation and 1 h heating at 140 °C, according to following expression: rate of cross-linking (%) = [(A0 − AT)/A0] × 100, where A0 and AT are absorption intensities of the CC chromophore after irradiation or heating at times t = 0 and t = T, respectively. The rate of cross-linking of cinnamated PA6 is 33% after 6 h UV irradiation and 42% after 1 h heating. After UV irradiation and heating of 10% Cin-PA6, the polymer

by DSC were shown to decrease with increasing cinnamoyl content, as compared to neat PA6, due to chain reorganizations. It is worth noting that those values were not changed during the first run, i.e., right after the polymerization. The single melting endotherms tend to conclude on a statistical copolymerization. The elastic modulus (E′) measured below Tg (at 25 °C) by DMA exhibited a decrease down to 6 × 108 or 7 × 108 Pa, instead of 2 × 109 Pa for neat PA6, in agreement with a reduced crystallinity. Thermal stability of the polyamides was investigated by thermogravimetric analysis under a nitrogen atmosphere. The TGA thermograms of PA6 and PA6 bearing cinnamoyl units are shown in Figure 4, and the results are summarized in Table

Figure 4. TGA thermograms of PA6 (dashed-dotted), 2% Cin-PA6 (dotted), 6% Cin-PA6 (dashed), and 10% Cin-PA6 (solid).

2. Thermal stabilities of the cross-linked (10% Cin-PA6 and 6% Cin-PA6) and non-cross-linked (2% Cin-PA6) polyamides are higher than neat PA6. For PA6, the initial decomposition temperatures of 5 and 10% weight losses (T5 and T10) are measured at 290 and 300 °C, respectively. For 10% Cin-PA6, T5 and T10 are increased until 361 and 390 °C. The improvement of the thermal stability of modified PA6s could be explained by minimizing the depolymerization process, which is known to occur by different scission or elimination reactions.42 During the process, the electronic effect of the cinnamoyl group attached to CL chain end may be different than the one in CL chain end. The bond scissions and formation of cyclic compounds are certainly disfavored at 300− 350 °C. Photosensitivity of PA6 bearing cinnamoyl groups were studied by means of UV absorbance spectroscopy. Both photocleavage reactions and photo- or thermodimerization F


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Scheme 4. From Gel to Solution Form of 1.23 × 10−4 mol L−1 of PA6 Bearing Pendant Cinnamoyl Groups in HFIP Solution after Irradiation at 254 nm for 6 h

Figure 6. UV absorption spectra of 1.23 × 10−4 mol L−1 of 10% CinPA6 bearing pendant cinnamoyl group in HFIP solution at irradiation times from 1 to 6 h between 200 and 280 nm.

Figure 7. UV absorption of 1% w/v spin-coated PA6 bearing pendant cinnamoyl groups (10% Cin-PA6) film before UV irradiation (solid), after UV irradiation 6 h (dashed) between 325 and 380 nm, and after heating at 140 °C for 60 min (dotted).

became insoluble in HFIP due to the efficient but not complete cross-linking.

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observed. Crystallinity was shown to decrease after heat treatment. The cross-linking reaction was shown to be reversible, and the functionalized aliphatic polyamides can be cross-linked thermally (at 140 °C) or photochemically (at 364 nm) and de-cross-linked photochemically (at 254 nm). This work highlight also the potential of α-amino-ε-caprolactam as an interesting precursor of comonomers in the anionic ringopening polymerization of ε-caprolactam.

CONCLUSION In this work, a reversible thermal and photochemical crosslinked polyamide was synthesized. The strategy was based on the anionic copolymerization of ε-caprolactam and αcinnamoylamido-ε-caprolactam, a monomer prepared from biosourced L-lysine. During the anionic ring-opening polymerization at 140 °C in bulk, the thermal dimerization of cinnamoyl pendant groups took place to end-up with crosslinked polyamides, whenever the amount of functionalized comonomer is 6 or 10 wt %. The thermal and thermomechanical properties of these cross-linked polyamides were investigated. An increase of both Tg and thermal stability was

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected], tel +33540002734, fax +33540008487 (S.C.). G


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Reference; Matyjaszewski, K., Möller, M., Eds.; Elsevier: Amsterdam, 2012; pp 331−396. (27) Roda, J. Polyamides. In Handbook of Ring-Opening Polymerization; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, 2009; pp 165−195. (28) Hashimoto, K. Prog. Polym. Sci. 2000, 25 (10), 1411−1462. (29) Ivin, K. J.; Saegusa, T. In General Thermodynamic and Mechanistic Aspects of Ring-Opening Polymerization; Elsevier: New York, 1984; Vol. 1. (30) Šebenda, J. In Comprehensive Chemical Kinetics; Bamford, C. H., Tipper, C. F. H., Eds.; Elsevier: Amsterdam, 1976; Vol. 15, Chapter 6, pp 379−471. (31) Hanford, W. E.; Joyce, R. M. J. Polym. Sci. 1948, 3 (2), 167−172. (32) Kiss, L.; Karger-Kocsis, J. J. Therm. Anal. 1980, 19, 139−141. (33) Reimschuessel, H. K. Macromol. Synth. 1982, 8, 37−40. (34) Overberger, C. G.; Kozlowski, J. H.; Radlmann, E. J. Polym. Sci., Part A-1: Polym. Chem. 1972, 10 (8), 2265−2289. (35) Nijenhuis, A. J. WO02/44246A1, 2002. (36) Chikh, L.; Arnaud, X.; Guillermain, C.; Tessier, M.; Fradet, A. Macromol. Symp. 2003, 199 (1), 209−222. (37) Tarkin-Tas, E.; Mathias, L. J. Macromolecules 2009, 43 (2), 968− 974. (38) Bouchekif, H.; Tunc, D.; Le Coz, C.; Deffieux, A.; Desbois, P.; Carlotti, S. Polymer 2014, 55, 5991−5997. (39) Abdelmoty, I.; Di, L.; Guo, C.; Foxman, B. M.; Buchholz, V.; Kowitz, K.; Enkelmann, V.; Wegner, G. Cryst. Growth Des. 2005, 5 (6), 2210−2217. (40) Č efelín, P.; Stehlíček, J.; Šebenda, J. Collect. Czech. Chem. Commun. 1974, 39, 2212−2220. (41) Khan, M.; Brunklaus, G.; Enkelmann, V.; Spiess, H.-W. J. Am. Chem. Soc. 2008, 130 (5), 1741−1748. (42) Levchik, S. V.; Weil, E. D.; Lewin, M. Polym. Int. 1999, 48, 532− 557. (43) Nakayama, Y.; Matsuda, T. J. Polym. Sci., Part A: Polym. Chem. 1992, 30 (11), 2451−2457. (44) Froimowicz, P.; Belgacem, M. N.; Gandini, A.; Strumia, M. C. Eur. Polym. J. 2008, 44 (12), 4092−4097.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Erasmus Mundus−IDSFunMat program for the financial support of this work. CREMAN Laboratory (Liege/Belgium) and BASF (Germany) are also acknowledged for conducting the solid state NMR measurements and supplying the materials, respectively.

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REFERENCES

(1) Feng, L.; Romulus, J.; Li, M.; Sha, R.; Royer, J.; Wu, K.-T.; Xu, Q.; Seeman, N. C.; Weck, M.; Chaikin, P. Nat. Mater. 2013, 12 (8), 747−753. (2) Reichmanis, E.; Nalamasu, O.; Houlihan, F. M.; Novembre, A. E. Polym. Int. 1999, 48 (10), 1053−1059. (3) Jang, J.; Nam, S.; Hwang, J.; Park, J.-J.; Im, J.; Park, C. E.; Kim, J. M. J. Mater. Chem. 2012, 22 (3), 1054−1060. (4) Liaw, D.-J.; Wang, K.-L.; Huang, Y.-C.; Lee, K.-R.; Lai, J.-Y.; Ha, C.-S. Prog. Polym. Sci. 2012, 37 (7), 907−974. (5) Shi, D.; Matsusaki, M.; Akashi, M. Bioconjugate Chem. 2009, 20 (10), 1917−1923. (6) Sandholzer, M.; Bichler, S.; Stelzer, F.; Slugovc, C. J. Polym. Sci., Part A: Polym. Chem. 2008, 46 (7), 2402−2413. (7) He, J.; Tong, X.; Zhao, Y. Macromolecules 2009, 42 (13), 4845− 4852. (8) Biyani, M. V.; Weder, C.; Foster, E. J. Polym. Chem. 2014, 5, 5716−5724. (9) Kiskan, B.; Yagci, Y. J. Polym. Sci., Part A: Polym. Chem. 2014, 52 (20), 2911−2918. (10) Biyani, M. V.; Weder, C.; Foster, E. J. Polym. Chem. 2014, 5 (18), 5501−5508. (11) Zheng, Y.; Micic, M.; Mello, S. V.; Mabrouki, M.; Andreopoulos, F. M.; Konka, V.; Pham, S. M.; Leblanc, R. M. Macromolecules 2002, 35 (13), 5228−5234. (12) Sako, Y.; Takaguchi, Y. Org. Biomol. Chem. 2008, 6 (20), 3843− 3847. (13) Blasco, E.; Schmidt, B. V. K. J.; Barner-Kowollik, C.; Piñol, M.; Oriol, L. Macromolecules 2014, 47 (11), 3693−3700. (14) Zhang, Q.; Schattling, P.; Theato, P.; Hoogenboom, R. Eur. Polym. J. 2014, DOI: 10.1016/j.eurpolymj.2014.06.029. (15) Sung, S.-J.; Cho, K.-Y.; Yoo, J.-H.; Kim, W. S.; Chang, H.-S.; Cho, I.; Park, J.-K. Chem. Phys. Lett. 2004, 394 (4−6), 238−243. (16) Lee, S. W.; Kim, S. I.; Lee, B.; Choi, W.; Chae, B.; Kim, S. B.; Ree, M. Macromolecules 2003, 36 (17), 6527−6536. (17) Zhu, J.; Wu, L.-P.; Zhang, Y.-Y.; Jin, X.; He, S.-J.; Shi, K.-Y.; Guo, X.; Du, Z.-J.; Zhang, B.-L. J. Appl. Polym. Sci. 2006, 102 (5), 4565−4572. (18) Kawatsuki, N.; Takatsuka, H.; Yamamoto, T.; Sangen, O. Macromol. Rapid Commun. 1996, 17 (10), 703−712. (19) Coqueret, X. Macromol. Chem. Phys. 1999, 200 (7), 1567−1579. (20) Chung, C.-M.; Roh, Y.-S.; Cho, S.-Y.; Kim, J.-G. Chem. Mater. 2004, 16 (21), 3982−3984. (21) Froimowicz, P.; Klinger, D.; Landfester, K. Chem.Eur. J. 2011, 17 (44), 12465−12475. (22) Garle, A.; Kong, S.; Ojha, U.; Budhlall, B. M. ACS Appl. Mater. Interfaces 2012, 4 (2), 645−657. (23) Rivero, G.; Nguyen, L.-T. T.; Hillewaere, X. K. D.; Du Prez, F. E. Macromolecules 2014, 47 (6), 2010−2018. (24) Ommer, H. J.; Ritter, H. Macromol. Chem. Phys. 1996, 197 (3), 797−809. (25) Sung, S.-J.; Kim, D.-H.; Kim, M.; Cho, K. Macromol. Res. 2010, 18 (6), 614−617. (26) Russo, S.; Casazza, E. 4.14 - Ring-Opening Polymerization of Cyclic Amides (Lactams). In Polymer Science: A Comprehensive H


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