Biobased Polyamide Thermosets: From a Facile One-Step Synthesis

Aug 9, 2019 - Biobased polyamide (PA) thermosets composed of renewable ethylene brassylate were synthesized through a one-step reaction under ...
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Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

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Biobased Polyamide Thermosets: From a Facile One-Step Synthesis to Strong and Flexible Materials Charalampos Pronoitis, Geng Hua, Minna Hakkarainen, and Karin Odelius* Department of Fibre and Polymer Technology, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden

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ABSTRACT: Biobased polyamide (PA) thermosets composed of renewable ethylene brassylate were synthesized through a one-step reaction under solvent-free conditions, at 100 °C in the presence of an organocatalyst. Under these conditions, thermoset formation times as low as 10 min were achieved. The thermosets were easily prepared as thin, transparent films with high strength, flexibility, and high thermal stability. The ester-to-amine content and formation of ethylene glycol in situ as a byproduct of the reaction were studied and correlated with the final properties of the materials. Crystalline oligoester segments were identified as a result of ring-opening polymerization and were shown to have a beneficial effect on the mechanical properties of the thermosets and endowed shape-memory behavior. In contrast to other routes, employing multistep monomer preparation, harsh conditions, and chlorinated reagents, this procedure contributed to the development of sustainable, functional PA thermosets.

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biobased PA thermosets.20 Usually, the synthesis of PA thermosets takes place in two steps, initially with the synthesis of prepolymers through one of the two conventional routes followed for thermoplastic PAs, that is, the polycondensation between diacids or diacyl chlorides with diamines or the ringopening polymerization (ROP) of lactams. The monomers bear double or triple bonds and once prepolymers are formed, they are cross-linked in a second step, either by thermal-curing or photocuring.21−28 The PA thermosets synthesized through these routes retain high thermal stability and strength, and they can further exhibit interesting properties such as shape memory performance.11 Both synthetic routes, however, have downsides. High temperatures, typically above 200 °C, are needed for the polycondensation between diacids and diamines in combination with reduced pressure to remove the condensate. This renders the overall process highly energy demanding. More reactive acyl chlorides can be employed to carry out the reaction at room temperature, albeit, their preparation requires hazardous halogenated reagents such as thionyl chloride, and the cross-linking takes place at temperatures as high as 350 °C.21,22 In the case of functionalized lactams, photocuring overcomes the disadvantage of the harsh conditions and results in fast cross-linking reactions, but the preparation of the monomers is often multistep and consumes large amounts of solvents both during the synthesis and the purification.26−28 Another approach that could tackle the drawbacks of high temperatures, solvent consumption, and

INTRODUCTION Polyamides (PAs) are well known for the combination of high strength and thermal stability originating from the strong hydrogen bonds formed between the amide units. The numerous available monomers enabling structural diversity and their ability to be readily processed into fibers and films have rendered PAs a very useful class of materials found in several industries.1 To date, oil-based PAs prevail over biobased, with few exceptions such as the long-standing PA 11 which is commercially available from the 1950s2 and few others developed later such as the PA 13,133 and the PA 10,10.4 However, there is an increasing effort in exploiting renewable resources, especially plant oils and their derivatives, to obtain platform chemicals that can be transformed into monomers for fabrication of more sustainable polymers including PAs. However, the focus has mainly been on the well-established and most commonly encountered thermoplastic PAs.5−10 Yet, PA thermosets are systems of great potential. The fusion of a chemically cross-linked, rigid structure with the benefits of traditional thermoplastic PAs is appealing for many practical thermoset applications. For instance, PAs’ high mechanical strength, thermal stability, and insolubility in most common solvents are perfectly combined in membrane filters for purification where high chemical resistance is needed. Moreover, the crystallinity of PAs, if incorporated into a thermoset, is an advantage to create materials with shape memory effect, highly sought in many fields.11,12 Unfortunately, in contrast to the progress that has been made on developing biobased epoxy,13 polyurethane,14−16 and polyester thermosets,17−19 much less has been achieved on © XXXX American Chemical Society

Received: February 19, 2019 Revised: June 20, 2019

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DOI: 10.1021/acs.macromol.9b00359 Macromolecules XXXX, XXX, XXX−XXX

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respectively, the final ester carbonyl content (ECC) of the products was calculated through eq 1

halogenated monomers is the aminolysis of esters in the presence of a basic catalyst to generate amide bonds.29 In this case, PAs can be synthesized either from a difunctional monomer such as an ω-amino acid ester undergoing intermolecular aminolysis or a diamine and a diester reacting while a small molecule is removed as the condensate.30 With a proper design, a cross-linked structure could be formed to provide an approach of great value for the synthesis of more sustainable PA thermosets. Biobased thermoplastic PAs with a diverse structure and properties resembling their petroleum based analogues were recently reported utilizing ethylene brassylate (EB), a macrocyclic dilactone, and implementing the aminolysis approach under solvent-free conditions, at 100 °C.31 EB is used as a musk in the perfume industry and it is produced at an industrial scale through the reaction of ethylene glycol and brassylic acid (tridecanedioic acid) which is obtained from the ozonolysis of erucic acid, the main fatty acid contained in rapeseed oil. Utilization of biobased building blocks is a promising route in moving toward more sustainable polymers, and there are several natural resources that can be exploited to obtain monomers with various functionalities.32 The aim of the work was to design and demonstrate a route to novel biobased high-performance PA thermosets by a simple one-step reaction under solvent-free conditions utilizing EB as a biobased monomer. To gain a holistic understanding of the effect of the monomers’ structure and composition and the systems synthesis conditions, a series of control experiments were performed, and the structure−property relationship and shape-memory properties of the PA thermosets were determined.

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ECC (%) =

Aester carbonyl Aester carbonyl + A amide I + A amide II

(1)

The ring-opening aminolysis-cross-linking reaction for the formation of PA thermosets was monitored over a period of 4 h by real time infrared (RTIR) spectroscopy at 100 °C with the same instrumentation using a Specac WEST 6100+ Heated Golden Gate Controller. The scan number, range, and resolution were as previously stated, and the spectrum acquisition time was set to 7.8 s. Gel Content Measurements. A rectangular specimen of approximately 0.2 g of each film was immersed in HFIP for 24 h and thereafter dried under vacuum at 80 °C for 48 h upon which constant mass was reached. The gel content was then calculated using eq 2 m Gel content = f × 100% mi (2) where mf is the final mass of the specimen after drying and mi is the initial mass before the immersion, respectively. Differential scanning calorimetry (DSC) was performed with a Mettler Toledo DSC 1 instrument. Preweighed samples (4−6 mg) were sealed into 100 μL aluminum crucibles and subjected to two heating and one cooling scan, respectively, under a 50 mL min−1 N2 flow. The heating and cooling rates, respectively, was set to 10 K min−1. During the first heating scan, the temperature was increased from 25 to 120 °C and kept steady for 2 min prior to cooling down to −80 °C, where it was kept steady for 2 min. Finally, for the second heating scan, the temperature was increased again to 120 °C. The crystallization (Tc) and melting (Tm) temperatures are reported as the maximum of the crystallization and melting peaks of the cooling and second heating scan, respectively, and the glass transition temperature (Tg) is reported as the midpoint of the glass transition obtained from the second heating scan. All of the measurements are reported as average values from triplicate measurements. The evaluation of the data was carried out by Mettler Toledo STARe v. 15.00 software. The thermal stability of the thermosets was assessed with a Mettler Toledo TGA/DSC 1 instrument. Each sample (6−8 mg) was loaded into 70 μL alumina cups without lids and heated from 50 to 600 °C at a 10 K min−1 heating rate under a 50 mL min−1 N2 flow. The initial decomposition temperature (Td,5%) was calculated at 5% of mass loss. Tensile mechanical properties of the PA films were measured with an Instron 5944 tensile testing machine equipped with a 500 N load cell according to ASTM D882-18 Standard. Rectangle-shaped specimens of 70 ± 0.50 mm length, 5.0 ± 0.01 mm width, and 0.52 ± 0.03 mm thickness were cut from the initial films and conditioned at least for 40 h prior to testing at 23 ± 2 °C and 50 ± 10% relative humidity. The initial grip separation was 50 mm, and the rate of the grip separation was set to 500 mm min−1. The Young’s modulus (E), stress at break (σb), and elongation at break (εb %) are reported as an average of at least five measurements. Dynamic mechanical analysis (DMA) of the PA thermosets was performed using a Q800 (TA Instruments) dynamic mechanical analyzer. Specimens with dimensions 25 × 5 × 0.5 mm were cut from the original films and were subjected to a temperature sweep from −70 to 120 °C at a 3 °C min −1 heating rate, at 1 Hz frequency and 0.1% strain. The cross-linking density was calculated using eq 3

EXPERIMENTAL SECTION

Materials. EB (95%), diethyl sebacate (DSEBAC, 98%), tris2(aminoethyl)amine (TAEA, 96%), diethylenetriamine (DETA, 99%), and 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD, 98%) were purchased from Sigma-Aldrich. 1,1,1,3,3,3-Hexafluoropropan-2-ol (HFIP, 99%) was purchased from Apollo Scientific, UK. Chloroform-d (CDCl3, 99.8%) and 1,1,1,3,3,3-hexafluoropropan-2-ol-d2 (HFIP-d2, 98%) were purchased from Cambridge Isotope Laboratories. All chemicals were used as received without further purification. General Procedure for the Synthesis of the PA Thermosets. A typical synthesis of a PA thermoset was carried out as follows: the desired mol % of TBD to amine functionality (e.g. for ∼4 mol % TBD: 41.1 mg, 0.29 mmol) was mixed with TAEA (360 mg, 2.46 mmol) in a snap-on lid glass vial, and the mixture was heated to 100 °C in an oil bath until complete dissolution was achieved. The solution was subsequently allowed to cool down to room temperature, and then, EB (1 g, 3.7 mmol) or DSEBAC (0.95 g, 3.68 mmol) was added. The mixture was vortexed, and for the optimization reactions, it was placed in a preheated oil bath at predetermined temperature or, in the case of film preparation, it was poured in an aluminum mold and quickly placed in a thermostated oven at 100 °C for 24 h. Characterization. An Avance 400 (Bruker, USA) NMR spectrometer was utilized to record the 1H NMR (400.13 MHz) spectra at 25 °C. CDCl3 was used as a solvent and the chemical shift of residual CHCl3 (δ = 7.26 ppm) was used as an internal standard. Fourier transform infrared spectroscopy (FTIR) spectra of at least three different parts of the films were recorded using a PerkinElmer Spectrum 2000 FTIR spectrometer (Norwalk, CT, USA) equipped with a Specac Heated Golden Gate (Kent, UK) single reflectionattenuated total reflectance accessory. Sixteen scans with a 4 cm−1 resolution were performed for each spectrum in the range 4000−600 cm−1. Calculating the areas of the signals corresponding to the ester carbonyl (Aester carbonyl) and amide I (Aamide I) and II (Aamide II) bands,

υe =

E′ 3RT

(3)

where E′ is the tensile storage modulus obtained from the rubbery plateau at the respective temperature (T) and R is the universal gas constant (8.314 J K−1 mol−1). The alpha transition temperature (Tα) was calculated from the maximum of the loss modulus (E″) against temperature. Shape memory cycles were obtained with the same instrument on specimens with dimensions 10 × 6 × 0.45 mm. The sample was initially heated to 80 °C and equilibrated for 10 min. Then, a force of 5 N was applied at a 1 N min−1 rate, and the sample B

DOI: 10.1021/acs.macromol.9b00359 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Ring-Opening Aminolysis-Cross-Linking Reaction Forming PA Thermosets

Figure 1. (a) Thermoset formation time at room temperature and (b) compiled FTIR spectra after 24 h of the PA thermosets synthesized with 0, 2, 4, and 8 mol % TBD to amine functionality and stoichiometric balance between the ester and amine functional groups. was cooled down to 45 °C, where it was kept steady for another 10 min to adopt the temporary shape. The force was subsequently removed, and the sample was reheated back to 80 °C to return to its permanent shape. The cycle was repeated four times, and the temperature ramp was 5 °C min−1 in all cases. The fixicity ratio (Rf) and recovery ratio (Rr) were calculated through eqs 4 and 5, respectively Rf =

ε × 100% εload

(4)

Rr =

ε − εrec × 100% ε

(5)

exploited as a means to promote the aminolysis reaction which subsequently leads to cross-linking due to the release of ringstrain resulting from the ring-opening of EB.31 Reaction Optimization. To optimize the reaction conditions, the influence of temperature and catalyst loading were considered. Initially, the effect of TBD loading on the thermoset formation time at room temperature with a stoichiometric balance between ester and amine groups was determined. Three different TBD loadings, namely, 2, 4, and 8 mol % to the amine functionality, were evaluated and an uncatalyzed system was used as a reference, Figure 1. As a facile measure of the reaction rate, the time required for visual thermoset formation (solidification)28,33 was appraised and shown to decrease from 180 to 15 min with increasing the catalyst loading from 2 to 8 mol %. This is a considerably shorter time frame as compared to the 7 days needed for the uncatalyzed reaction, Figure 1a. The catalyzed reactions were allowed to proceed for 24 h, after which the mixtures had turned hard and white, and the products were insoluble in CDCl3 and HFIP. To confirm the formation of amide bonds, FTIR spectroscopy was employed to monitor the decrease in the intensity of EB’s ester carbonyl stretching at 1733 cm−1 and the appearance of amide I, amide II, and −NH stretching (H-bonding) bands at 1642, 1543, and 3290 cm−1 , respectively,34 Figure 1b. FTIR spectra of at least three different parts of the thermosets were recorded to ensure the homogeneity of the samples. For comparison, the ECC in the final products originating either from the EB or poly(ethylene brassylate) (PEB) was compared to the amide content and used to evaluate the relative degree of amide formation and calculated through the areas of the signals using eq 1. The

where εload is the deformation after the force is applied at 80 °C, ε is the deformation after cooling down at 45 °C, and εrec is the deformation at the recovered state when the force is released.

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RESULTS AND DISCUSSION Biobased, strong, and flexible PA thermosets were successfully synthesized through a one-step reaction without the use of hazardous reactants. This was achieved via ring-opening aminolysis-cross-linking between EB, a renewable dilactone, and TAEA, a triamine, with TBD as a catalyst forming ethylene glycol as the byproduct, Scheme 1. TBD was chosen as a catalyst for the PA thermoset synthesis as it has been proven efficient for the synthesis of purely amide, linear, functional PAs from EB and various diamines as building blocks.31 The reactions were performed in air, at merely 100 °C. The influence of catalyst loading, reaction temperature, and composition of the constituent functional groups were evaluated based on their effect on the mechanical and thermal properties of the created thermosets. In contrast to the polycondensation route, the ring structure of EB was herein C

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Figure 2. (a) Thermoset formation time and (b) compiled FTIR spectra after 24 h of the PA thermosets synthesized with 2 and 4 mol % TBD to amine functionality and stoichiometric balance between the ester and amine functional groups at 70, 100, and 130 °C.

opaque thermosets obtained at room temperature. The difference in the appearance of the thermosets at room and at elevated temperatures is probably due to the different crystalline contents between the thermosets. PEB, which formed as a result of ROP, can crystallize and make the thermoset opaque. At higher temperatures, however, the mobility of the chains and mobility of the functional groups is higher, promoting the aminolysis of the ester bonds which leads to a higher degree of cross-linking. Therefore, a lower amount of crystalline segments would be created, making the thermosets transparent. Similar behavior has been observed for biobased epoxy thermosets with varying length oligoamide segments.33 To summarize, the reactions carried out using 2 mol % TBD presented comparably long thermoset formation times even at 130 °C (50 min), whereas the use of 4 mol % of the catalyst allowed the formation of the thermoset within substantially shorter times, Figure 2a. The color of the thermosets at 130 °C becomes darker, and after a reaction at 70 °C, there were larger amounts of unreacted carbonyl esters in the system. Thus, 4 mol % of TBD and a reaction temperature of 100 °C were chosen as the optimal reaction conditions for further work. Ring-Opening Aminolysis-Cross-Linking Monitored by RTIR Spectroscopy. To obtain deeper insights into the thermoset formation, RTIR spectroscopy was employed to monitor the reaction between EB and TAEA with 4 mol % of TBD to amine functionality. The progress of the reaction was monitored over 4 h. A reaction mixture was prepared, a drop of it was placed on the preheated RTIR instrument, and acquisition began immediately after, Figure 3. Already after 7.8 s, the amide I and II bands appeared as broad signals at ∼1660 and ∼1540 cm−1, Figure 3. These bands gradually narrowed and shifted toward lower wave-

ECC decreased from 30 to 8% when the TBD loading increased from 0 to 8 mol % with a higher amide content observed in the catalyzed reactions as compared to the uncatalyzed reaction. The thermosets were extracted with CDCl3, and the extract was analyzed by 1H NMR spectroscopy. The analysis showed that EB in the uncatalyzed system had reacted significantly less compared to all of the catalyzed reactions. A very small fraction of PEB was also observed in the 1 H NMR spectra of all of the products as a result of the competing ROP initiated by hydroxyl- (from the side-product ethylene glycol) or amine-groups. Thus, TBD was considered necessary to achieve a robust reaction proceeding within a practical time frame. To rule out the possibility of having an efficient uncatalyzed reaction driven by the removal of ethylene glycol, which is formed as a byproduct, an uncatalyzed reaction at 200 °C was carried out as the boiling point of ethylene glycol is in that vicinity. Although a solidification point was observed after approximately 30 min, the reaction mixture quickly turned dark brown and the thermoset was inhomogeneous, Figure S1. We aimed at a fast reaction with as high amide bond formation in the thermosets as possible, while maintaining a low catalyst loading and a low reaction temperature. To achieve this, three different reaction temperatures, 70, 100, and 130 °C, were evaluated for the 2 and 4 mol % catalyst loadings, and the ester-to-amine ratio was kept stoichiometrically balanced, Figure 2. The time needed for solidification to occur was greatly reduced for both TBD loadings at the elevated temperatures. For the reaction with 2 mol % TBD, 120, 90, and 50 min were needed at 70, 100, and 130 °C, respectively, while for the synthesis at room temperature, the time needed was 180 min. The effect of temperature was even more pronounced for the reaction with 4 mol % TBD, where the time was further reduced to 30, 10, and 5 min at 70, 100, and 130 °C, respectively, showing a very fast thermoset formation. The gel content of the thermosets, calculated using eq 2, increased for the 2 mol % TBD loading from 88 to 90 and 93.5% when cured at 70, 100, and 130 °C, respectively, while it remained constant at 94% for the 4 mol % TBD at all tested temperatures. As the reaction temperature was increased, the conversion of EB was higher and the ECC, calculated using eq 1, was below 1% for all of the temperatures for the 4 mol % of TBD, while for the 2 mol % TBD systems, it was 6, 3, and 1% at 70, 100, and 130 °C, respectively, Figure 2b. All of the thermosets synthesized were transparent, but at elevated temperatures, they became first slightly yellow and then acquired a browner color at 130 °C in contrast to the white,

Figure 3. 1800−1500 cm−1 region of selected RTIR spectra showing the progressive decrease of EB’s −CO stretching (1733 cm−1) and increase of amide I (∼1642 cm−1) and II (∼1537 cm−1) bands, respectively, during 4 h of acquisition time. D

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Figure 4. (a) Compiled FTIR spectra after 24 h of the PA thermosets prepared as films with a decreasing ratio between ester and amine functional groups (E/A), 4 mol % of TBD-to-amine functionality at 100 °C as well as using DSEBAC, instead of EB; (b) transparency of the films obtained.

Table 1. Thermal Properties and Gel Content of the PA Thermosets Prepared as Films with EB or DSEBAC and with Decreasing Ester-to-Amine Stoichiometric Ratiosa E/Ab

Tg (°C)c

Tm (°C)c

ΔHm (J g−1)c

Tc (°C)d

ΔHc (J g−1)d

gel content (%)e

1:1 1:1.2 1:1.5 1:1f

25 23 3.0 5.7

± ± ± ±

67 ± 0.6 65 ± 1.0 66 ± 0.4

19 ± 1.3 18 ± 0.5 24 ± 2.8

49 ± 0.4 46 ± 0.4 42 ± 0.2

20 ± 1.3 18 ± 0.8 24 ± 2.7

93 95 87 89

1.6 0.4 0.5 1.4

a All of the values are reported as averages from triplicate measurements. bEster-to-amine stoichiometric ratio. cCalculated from the second heating scan. dCalculated from the cooling scan. eCalculated using eq 2. fPrepared with DSEBAC instead of EB.

numbers at 1642 and 1537 cm−1 similar to the FTIR spectra of the PAs synthesized during optimization. The fact that the two characteristic amide bands are detected already at the earliest stages of the reaction denotes that the nucleophilic attack of deprotonated amine species of TAEA on EB begins immediately when the two components (the TAEA−TBD mixture and the EB) come into contact with each other. This behavior resembles the commercial two component adhesives, glues, etc. where EB represents the resin part and the TAEA− TBD mixture represents the second part, the hardener. The reaction proceeds rapidly within the first 10−20 min as seen by the fast decrease of EB’s −CO stretching and becomes slower afterward as evidenced by the small differences observed among the spectra recorded at 60 and 240 min, Figure 3. This decrease of the reaction rate is most probably associated with the formation of the thermoset’s network which as it proceeds, hinders the diffusion of the monomers and the mobility of the amine moieties and thereby the progression of the ring-opening aminolysis-cross-linking. Ultimately, longer times are needed for the completion of the reaction, for example, 24 h as followed herein. Despite the longer time, the reaction is facile as only one step is required for the synthesis which further takes place at much milder temperature of 100 °C compared to the stringent curing conditions reported previously for other PA thermosets.21,22 Thermal Properties and Structure Elucidation of the Thermosets. Under the optimized conditions, that is, 4 mol % of TBD to amine functionality and the reaction temperature of 100 °C, the PA thermosets were prepared as transparent and bubble-free films. These were tested for their thermal and mechanical properties, taking into account the presence of the byproduct ethylene glycol formed in situ. Two important parameters, the stoichiometry between ester and amine functional groups and the structure of the monomer, were varied to realize the effect on the properties of the thermosets.

The ester-to-amine stoichiometry was varied from 1:1 to 1:1.2 and 1:1.5, respectively, to prepare thermosets with excess of TAEA, resulting in more amide formation and varying crosslinking density. Further, films with excess of EB were prepared with 1.05:1, 1.2:1, and 1.5:1 ester-to-amine ratio as higher EB content could promote the ROP of the monomer and result in the formation of oligoester segments in the final PA thermoset. Also, the structure of the monomer was studied to determine whether it affects the reaction and the properties of the final thermoset utilizing DSEBAC as a linear analogue of EB, Figure 4a, with an 1:1 ester-to-amine stoichiometry. In all cases, flexible and transparent films were obtained, Figure 4. The reaction yielded PA thermosets for all of the different ester-to-amine ratios with excess of TAEA as well as for the linear DSEBAC monomer, although in the latter, 9% of ECC was calculated using eq 1, Figure 4a. Possibly, the reaction is slower in this case compared to the thermosets synthesized using EB, the cyclic structure of which could be promoting the first amidation taking place to a larger extent due to the release of the ring strain and enhanced reactivity of the ester groups.35 The films prepared with an excess of EB presented the characteristic amide I and II bands and as expected higher ECC values of 5, 9, and 18% for the 1.05:1, 1.2:1, and 1.5:1, respectively, Figure S2. The effect of DSEBAC and excess of TAEA on the thermal properties of the thermosets was found depending on the ester-to-amine ratio and monomer structure with a range of Tg between 3 and 25 °C. Crystallization and melting were observed only for the EB thermosets, Table 1, Figure 5. The 1:1 and 1:1.2 ester-to-amine ratio thermosets displayed similar behavior with a Tg of 25 and 23 °C and high gel content of 93 and 95%, respectively, while for the 1:1.5 and the DSEBAC thermosets, the Tg values were much lower, 3 and 6 °C with gel contents of 87 and 89%, respectively. These Tg values explain the apparent flexibility of the thermosets at E

DOI: 10.1021/acs.macromol.9b00359 Macromolecules XXXX, XXX, XXX−XXX

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atures (Tm) were similar to PEB homopolymer37,38 which melts at 69 °C and has a low Tg of −33 °C due to the long flexible −CH2− backbone.39 Therefore, we assumed that the Tc and Tm are related to oligoEB not oligoamides, which usually, of course depending on the structure, melt above 100 °C.31,33,36,40,41 It has been previously shown that within less than 2 h, at 100 °C, when amine groups are available, oligoester amides will undergo intense aminolysis until only pure amide products remain.31 Nevertheless, in this case, because of the presence of the thermoset’s network, the monomer’s diffusion and groups’ mobility are greatly hindered; thus, some oligoEB segments could remain intact and further crystallize. These segments could be found either between two consecutive amide linkages, as dangling chains or both, Scheme 1. The possibility of any PEB existing free, untethered to the rest of the network, was excluded by experiments in which the films were extracted with CDCl3 and analyzing the extracted fractions with 1H NMR spectroscopy. No PEB was detected, rather, only ethylene glycol, small amounts of residual monomers and catalyst, Figure S4. This can be explained by the fact that the combination of low TBD loading and −NH2 groups as the initiating species is not an effective polymerization system as evidenced by using TAEA as a trifunctional initiator for the ROP of EB in a 30:1:0.12 EB/ TAEA/TBD ratio at 100 °C. The conversion of EB reached merely 13.7% after 4 days of reaction, Figure S5. For comparison, when benzyl alcohol and TBD are used as an initiator−catalyst system at 80 °C in a 1:1 ratio, the conversion of EB to PEB reaches 99% within 15 min.39,42 The absence of linear oligoamides, not attached to the thermoset, was confirmed as well by extracting the films with HFIP-d2 and analyzing the extracted fractions with 1H NMR spectroscopy, where no oligoamide signals were observed. The linear oligoamides not being responsible for the crystallinity and melting observed is better illustrated with the DSEBAC thermoset which did not crystallize at all as the linear structure of the monomer deprived the opportunity of developing consecutive oligoester segments. Hence, the absence of such segments in combination with the reduced mobility caused by the surrounding network impedes crystallinity completely, resulting in a fully amorphous thermoset. Even when an excess of TAEA was used with DSEBAC (1:1.5 ester to amine) to promote formation of oligoamides, no crystallization or melting was observed, rather, only an increase in the Tg to 22 ± 0.8 °C which can be explained by the higher density of amide linkages forming more hydrogen bonds and therefore, resulting in a more rigid structure. Moreover, it is noteworthy that the Tm and Tc of the dried samples were not significantly affected while both the ΔHm and ΔHc decreased, Table S1. This observation also supports the speculated presence of crystalline EB sequences that remained intact because the surrounding network made them inaccessible to the −NH2 groups, yet, at the high temperature applied for drying, the mobility of the chains is higher and aminolysis of the ester bonds is promoted. A detailed analysis of the thermal properties of the films prepared with an excess of EB instead of TAEA, also clearly showed that oligoEB sequences are formed and they are the primary source of the crystallinity observed, Table S2 and Figure S6. The Tg of these samples was lower than their excess amine counter analogues, between 7 and 22 °C, denoting a higher fraction of EB oligomers between cross-links, dangling chains, and possibly a plasticizing effect from the residual

Figure 5. Typical DSC heating scans for the PA thermosets prepared as films with an excess of TAEA and DSEBAC instead of EB. The inset graphs present the glass transition of the respective thermosets.

room temperature as they could be easily bended without breaking and resembled other PA thermosets with long methylene spacers between the cross-links.36 The variation of the Tg among the thermosets prepared with an excess of TAEA could be explained by an incomplete network formation with covalently attached dangling chains not contributing to crosslinking, especially in the case of the 1:1.5 ratio thermoset, in combination with the different amounts of ethylene glycol formed as a byproduct, resulting in a plasticizing effect, which would be more pronounced at higher concentrations of TAEA. The ethylene glycol content of the final thermosets depends on the ester-to-amine stoichiometry applied; higher amine excess will lead to more ethylene glycol and consequently to lower Tg. The DSEBAC thermoset presents a similar effect but in this case, it is probably the monomer residue that causes this behavior as DSEBAC is a viscous liquid of high boiling point resembling ethylene glycol. Because it is less reactive than EB, a considerable amount is also left in the final material and the likelihood of uncrosslinked dangling chains is higher, Figure 4a. The addition of plasticizers is a not an uncommon method to modify the properties of a thermoset. However, the plasticizer is usually an additional molecule included in the reaction mixture after thermoset formation. Herein, the possibility of a plasticizer being produced directly during the synthesis and automatically incorporated in the final material eliminates the possible need for an additional molecule and further, it could make use of the byproduct which would otherwise be removed as waste. To confirm if removal of ethylene glycol was possible and whether it would affect the Tg of the thermosets, rectangular pieces of the films were dried under vacuum at 130 °C for 24 h and then analyzed with DSC. Even after drying, no bubbles were observed; however, in all cases, the Tg value increased to 37, 30, and 11.5 °C for the 1:1, 1:1.2, and 1:1.5 ester-to-amine ratio thermosets, respectively, indicating stiffer thermosets. The same trend was observed for the DSEBAC thermoset, where its Tg increased to 29 °C after drying denoting the plasticizing effect of the residual monomer, Table S1 and Figure S3. We speculated that the crystallization and melting observed for the TAEA-rich thermosets could be either due to linear oligoamides or linear oligoEB sequences. The oligoamides would be formed in the case where only two of the amine groups of TAEA had reacted through the aminolysis route while the third could be uncrosslinked. On the other hand, linear oligo-EB sequences would be the result of ROP of EB initiated by the amine species of TAEA taking place concomitantly as a competitive reaction to the ring-opening aminolysis-cross-linking toward the formation of the amide thermoset. Both the crystallization (Tc) and melting temperF

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Macromolecules monomer, especially for the high 1.2:1 and 1.5:1 ester-toamine ratios which presented low gel contents 89.4 and 85.5%, respectively. Interestingly, an increase of the ΔHm and ΔHc with the increasing excess of EB was observed while the Tm and Tc remained similar, Table S2 and Figure S6. Even for the lowest ratio of 1.05:1, the ΔHm and ΔHc increased compared to the 1:1 stoichiometry thermoset, Table 1, implying that even slight excess of EB is sufficient to further promote the ROP of EB and consequently the formation of crystalline segments. Finally, to further corroborate our hypothesis that the Tc and Tm was a consequence of oligoEB segments, we synthesized linear oligoamides with a structure similar to the thermosets, through the reaction between DETA and EB on a 1:1 ester-toamine ratio and evaluated their thermal properties. The catalyst loading and reaction temperature were kept identical to the thermosets’ synthesis. The reaction exhibited characteristics similar to the thermoset formation as solidification took place at a very early stage resulting in a white, hard product which was insoluble in common solvents yet soluble in HFIP, indicating a linear structure. FTIR spectroscopy of samples taken at different time points during the reaction confirmed that EB reacts fast with DETA under TBD catalysis, and the ECC reaches approximately 2−3% after 1 h of reaction and thereafter remains, Figure S7. The molecular weight of the obtained product should not increase significantly because the condensate, that is, the ethylene glycol, was not removed to shift the equilibrium toward polymer formation. This means that the expected product is an oligoamide. The DSC revealed a semicrystalline product with Tg and Tm at 55 ± 2.3 and 139 ± 0.7 °C, respectively, and enthalpy of fusion (ΔHm) of 39.5 ± 0.8 J g−1. All of the values and especially the Tm are much higher than those found for the thermosets, Table 1; hence, the Tc and Tm observed for the EB thermosets should be associated with the presence of consecutive oligoEB sequences which could potentially crystallize despite their short length and constrain of free movement imposed by the surrounding network.33 The low ECC, however, implies that the number of these sequences should be relatively small, otherwise the ester carbonyl absorption would be much stronger in the FTIR spectra. Mechanical Behavior of the Thermoset Films. The mechanical properties of the thermosets were tested as a function of the ester-to-amine stoichiometry and were also compared with the thermoset synthesized with DSEBAC. Typical stress−strain curves for each thermoset synthesized with excess of TAEA and DSEBAC instead of EB are presented in Figure 6, and an overview of the Young’s modulus (E), stress (σb), and elongation at break (εb) is provided in Figure 7. Crystalline segments can be of great advantage for the mechanical properties of a thermoset because they can provide further strength and toughness.33 The Young’s modulus (E) of the PA thermosets synthesized with EB decreased from 330 to 150 MPa with increasing amine content resulting in more flexible materials. The thermosets, except for the one with 1:1.5 ester-to-amine ratio, underwent large plastic deformation prior to breaking as evidenced by the high stress (σb) and elongation at break (εb) values which were highest for the 1:1.2 thermoset, 17 MPa, and 176%, respectively, outmatching other PA thermosets reported in the literature.21,22 The poorer behavior of the 1:1.5 thermoset could be attributed to the higher amount of dangling EB chains which can crystallize as

Figure 6. Representative stress−strain curves of the PA thermosets prepared as films with an excess of TAEA and DSEBAC instead of EB.

evidenced by the highest crystalline content, Table 1, yet they do not offer mechanical support to the rest of the material. The beneficial effect of the crystalline regions is underlined in the thermosets prepared with an excess of EB, where although there was a decrease of the Young’s modulus, their σb and εb values were comparable or slightly higher in than those for the thermosets synthesized with excess of TAEA, Table S2. In contrast, the DSEBAC thermoset presented a tremendous difference with almost no ability for plastic deformation and Young’s modulus of 2 orders of magnitude lower than the EB thermosets, Figures 6 and 7. Obviously, the presence of crystalline segments within the thermoset’s network contributed to their mechanical behavior by offering superior strength and malleability while the absence of it deprived the material of any plastic deformation which is exemplified by the DSEBAC thermoset. To obtain a better understanding of the effect of ethylene glycol on the mechanical properties, the dried specimens were subjected to tensile testing. Unfortunately, the deformation of the specimens, due to the extensive drying, led to a large variation of their performance especially regarding σb and εb, Table S1. In the elastic region, a slight increase on the Young’s modulus was observed as expected for stiffer materials, for example, for the 1:1 thermoset, the value was 510 ± 80 MPa compared to 320 ± 16 MPa after and prior to drying, respectively, Table S1. The thermomechanical properties of the thermosets prepared with excess of TAEA were evaluated with DMA. The results are summarized in Table 2, and the curves of storage modulus (E′) and loss modulus (E″) against temperature are presented in Figure 8. The thermosets with 1:1 and 1:1.2 ester-to-amine ratio have the highest storage modulus (E′), cross-linking density (υe), and gel content, Tables 1 and 2, which further explains the high tensile strength they displayed. The lower tensile strength of the 1:1.5 ester-to-amine ratio thermoset can be attributed to the combination of particularly low υe (0.015 kmol m−3) and gel content (87%) which rendered it less strong. The DSEBAC thermoset has a gel content of 89% and relatively high υe of 0.37 kmol m−3; however, the low E′ value at 25 °C (5.5 MPa) denotes its poor elastic solid behavior at ambient temperature compared to the EB containing thermosets. This variation among the thermosets is directly reflected on the considerably different tensile mechanical behavior that they exhibited, Figures 6 and 7. Thus, it seems that the ring structure of EB is necessary to promote the aminolysis-cross-linking reaction and to obtain thermosets with mechanical integrity that can undergo large plastic deformation prior to breaking. The alpha transition temperature (Tα) of the thermosets was G

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Figure 7. Mechanical properties of the PA thermosets prepared as films with excess of TAEA and DSEBAC instead of EB; (a) Young’s modulus (E), (b) stress at break (σb), and (c) elongation at break (εb).

Thermal Stability of the Thermosets. The thermosets were stable during a large temperature range, up to 300 °C, as indicated by thermal gravimetric analysis (TGA), Figure 9.

Table 2. Thermomechanical Properties of the PA Thermosets Prepared as Films with Excess of TAEA and DSEBAC Instead of EB E/Aa

E25°C ′ (MPa)b

Tα (°C)c

υe × 10−3 (mol m−3)d

1:1 1:1.2 1:1.5 1:1e

600 393 206 6

17 16 −6 −5

0.58 0.51 0.015 0.21

Ester-to-amine stoichiometric ratio. bStorage modulus at 25 °C. Obtained from the maximum of the E″ curve. dCalculated using eq 3. e Prepared with DSEBAC instead of EB. a c

Figure 9. Typical TGA curves of the pristine and dried PA thermosets prepared as films with different monomers and ester-to-amine stoichiometric ratios.

dependent on the ester-to-amine ratio as determined from the maximum of the E″ curve against temperature, Figure 8b. The 1:1 and 1:1.2 thermosets presented the highest Tg at 17 and 16 °C, respectively, whereas for the 1:1.5 and DSEBAC thermosets, the values were decreased to −6 and −5 °C, respectively, similar to the Tg trend observed with the DSC. This analogous behavior supports the speculation about the more rigid crystalline structures of the 1:1 and 1:1.2 thermosets and is connected to the higher Young’s and elastic modulus they exhibited. The low Tα values of 1:1.5 and DSEBAC thermoset are associated with the higher amount of flexible, dangling EB chains in the former and the absence of crystallinity in the latter, implying that the amorphous structure even though cross-linked does not exhibit any constraints from the crystals; hence, the mobility is higher. Additionally, the plasticizing effect of ethylene glycol and DSEBAC monomer residue, respectively, contribute to the low Tα values as explained before.

Regardless of the monomer structure (ring vs linear) or the ester-to-amine stoichiometry applied, the thermosets presented similar decomposition behaviors with minor differences. Two trends were observed; the 1:1 and 1:1.2 ratio thermosets had the highest thermal stability due to the highest gel content with the onset of mass loss at 5% (Td,5%) calculated at 330 and 320 °C, respectively, while the 1:1.5 and the DSEBAC thermosets presented slightly lower thermal stability at 305 and 310 °C, respectively, Figure 9. Initially, the samples were only run as prepared and early mass loss was observed approximately between 150 and 265 °C, but up to these temperatures, PAs as well as the ester bonds of the oligoEB sequences were stable.33,37,38 Therefore, the mass loss in this range is associated with the loss of moisture, ethylene glycol, and possible residual TAEA which should evaporate at temper-

Figure 8. Thermomechanical properties of the PA thermosets prepared as films with excess of TAEA and DSEBAC instead of EB; (a) storage modulus (E′) and (b) loss modulus (E″) against temperature. H

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curing conditions. EB’s cyclic structure led to thermosets with crystalline segments compared to the completely amorphous thermosets obtained when linear DSEBAC was used. The thermosets’ structure was elucidated, their film formation ability was proven, and a combination of desired characteristics such as transparency, strength, flexibility, high thermal stability, and shape memory was achieved, rendering them interesting candidates for practical thin film thermoset applications. In the quest for biobased PA thermosets, this approach overcomes the common problems associated with the use of high temperatures and harmful reactants during the synthesis of PAs and provides the incentive for further exploiting other renewable monomers and implementing milder methods to produce more sustainable, functional materials.

atures higher than their normal boiling points due to extensive H-bonding.43 The same initial mass loss was observed for the samples synthesized with excess of EB, Figure S8. After drying the samples, it became clear that the PA network is stable and its actual decomposition begins at higher temperatures, as presented in the inset graph of Figure 9. Hence, although the lowest ester-to-amine ratio (1:1.5) and the linear structure of DSEBAC seem to induce a slight drop in the initial decomposition temperature; overall, the thermosets presented good thermal stability up to at least 300 °C. Shape Memory Effect. It is known that the combination of crystalline regions within a chemically cross-linked structure can result in shape memory behavior by taking advantage of the melting transition as a trigger.12 Above the Tm, the thermoset becomes softer and deformation to a temporary shape, which is kept when the temperature is decreased below the Tm, can take place. Once the thermoset is reheated above the Tm, the permanent shape is regained as a result of higher molecular mobility. This phenomenon was observed for all of the synthesized PA thermosets, and the higher the crystallinity the more pronounced was the shape-memory effect. Utilizing the thermoset with 1:1.2 ester-to-amine ratio as an example and testing it in a temperature range between 45 and 80 °C, a shape-memory phenomenon was observed during four consecutive shape memory cycles, Figure 10.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b00359.

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FTIR spectra of the uncatalyzed reaction mixture at 200 °C, FTIR spectra of the films prepared with excess of EB; thermal and mechanical properties of the films after drying; thermal, mechanical, and TGA curves of the films prepared with an excess of EB; 1H NMR spectra of the extracted fractions of the films; 1H NMR spectra of the PEB synthesized using TAEA as an initiator; and FTIR spectra of the linear oligoamides synthesized with DETA and visual test of the shape-memory behavior (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +46-8 790 80 76. ORCID Figure 10. Consecutive shape memory cycles of the EB thermoset with 1:1.2 ester-to-amine stoichiometry recorded with DMA between 45 and 80 °C, applying a 5 N force.

Geng Hua: 0000-0001-7304-6737 Minna Hakkarainen: 0000-0002-7790-8987 Karin Odelius: 0000-0002-5850-8873 Author Contributions

The temperature range chosen was such that the thermoset was constantly above its Tg (23 °C) and its Tm at 65 °C was employed as the transition temperature to induce the shapememory effect. The fixicity ratio (Rf) and recovery ratio (Rr) calculated through eqs 4 and 5, respectively, were above 92 and 96% after all consecutive cycles indicating a strong shapememory behavior. A sharp increase of the strain reaching almost 75% during the cooling step and while maintaining the stress constant was observed during all cycles and among different specimens tested, but it did not seem to affect the rest of the process because once the force was released, it decreased and the temporary strained shape was retained. A visual testing of the 1.5:1 ester-to-amine ratio thermoset’s ability to return to its original shape is provided in Figure S9.

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the Swedish Research Council Formas (2016-00700_3). REFERENCES

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CONCLUSIONS Biobased PA thermosets with excellent combination of properties were synthesized through a solvent-free, benign route beginning from renewable EB as the monomer. The route was robust and easily applicable as the reaction temperature was merely 100 °C, and there was no need for monomer preparation, use of halogenated reagents or harsh I

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K

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