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Performance enhancement of shape memory poly(aryl ether ketone) via photodimerization of pendant anthracene units
European Polymer Journal 123 (2020) 109413
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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Performance enhancement of shape memory poly(aryl ether ketone) via photodimerization of pendant anthracene units ⁎
Yiyang Gu, Chunyu Ru, Zhen Zhao, Danming Chao , Xincai Liu
T
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College of Chemistry, Jilin University, Changchun 130012, People’s Republic of China
A R T I C LE I N FO
A B S T R A C T
Keywords: Shape memory polymer Poly(aryl ether ketone) Photocrosslinking Anthracene
Shape memory polymers have attracted much attention due to their promising applications in biomedicine, textiles, aerospace et al. Here, novel poly(aryl ether ketone)s bearing flexible alkyl segments and pendant anthracene groups were synthesized via nucleophilic copolycondensation coupled with post-polymerization functionalization. Their chemical structure, thermostability and mechanical properties were characterized through Fourier Transform infrared spectroscopy, nuclear magnetic resonance spectroscopy, thermal gravimetric analysis and dynamic mechanical analysis. The soft-rigid alternative segment structure endows these poly (aryl ether ketone)s with promising shape memory properties. After the photodimerization of pendant anthracene units, the resultant polymers exhibit enhanced shape memory performance in terms of satisfying shape fixity ratio and high shape recovery ratio, attributed to their photocrosslinked polymeric network architecture. This shape memory polymer with reversible photodimerization holds great promise for a deep understanding of the mechanism and future applications.
1. Introduction Shape memory polymers (SMPs) have received increasing attention for their wide potential applications such as aerospace field [1], biomaterials devices [2], functional textiles [3], sensors, actuators, and et al [4–6]. These polymeric materials possessing the capability of changing their shapes when exposed to external stimuli, including light, heat, magnetic field, moisture, or solvent are widely developed for SMPs [7–11]. Thereinto, thermally induced SMPs (TSMPs) have been intensively investigated for their easy design and synthesis [12–14]. Usually, the trigger temperature (Tt) is served for the switch of TSMPs, which may be the glass transition temperature (Tg) [15,16], melting temperature (Tm) [17–20], or isotropic temperature (Ti) [21–23] of these polymers. For example, polyethylene is considered as the most typical TSMP based on semi-crystalline phase switch with Tm [24]. Besides, a variety of Tg-type TSMPs such as epoxy [25], poly(arylene ether ketone)s (PAEKs) [26], and polynorbornene [27] are developed. Compared with Tm-type and Ti-type polymers, Tg-type TSMPs exhibit relatively unsatisfactory shape recovery behavior and mechanical properties, which would severely hinder their practical application in many fields [28]. Therefore, it is an urgent need to exploit new Tg-type shape memory polymers featuring outstanding shape memory performance [29].
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Among all Tg-type shape memory polymers, poly(aryl ether ketone) s (PAEKs) have attracted much attention in recent decades, because of their good mechanical properties, desirable chemical stability, and designable structure [30–33]. Some novel shape memory PAEKs bearing soft-rigid alternative segments have recently been designed and synthesized in our lab [34,35]. To improve their shape memory performance, carboxylic groups were also introduced in this polymeric architecture and generate physically crosslinking action through hydrogen bonds. However, these weak interactions from micro phase separation of soft-hard segments and/or hydrogen bonds are too weak to the practical application of these polymers. Therefore, chemically crosslinking action based on covalent bonds would be an efficient method for the improvement of shape memory performance. Among them, the dynamic covalent bonds would be ideal to construct crosslinking network architecture [36,37]. Thereinto, photocrosslinkable anthracene groups have been frequently utilized to generate dynamic covalent bonds due to their reversible cycloaddition reaction under UVlight illumination and/or thermal treatment [38–41]. Herein, photocrosslinkable anthracene groups were introduced in the soft-rigid alternative type of PAEKs using the post-polymerization functionalization technique. Then the photodimerization was triggered to form crosslinked network polymeric structure. Their mechanical capacities, thermal properties and shape memory performance were comparatively
Corresponding authors. E-mail addresses: [email protected] (D. Chao), [email protected] (X. Liu).
https://doi.org/10.1016/j.eurpolymj.2019.109413 Received 18 October 2019; Received in revised form 29 November 2019; Accepted 5 December 2019 Available online 10 December 2019 0014-3057/ © 2019 Elsevier Ltd. All rights reserved.
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2.3. Photocrosslinking PAEK-DF-x membranes by UV radiation
investigated in detail.
The uniform viscous PAEK-DF-x dimethylacetamide (DMAC) solution was cast onto a flat glass plate, and then transferred to the oven at 60 °C for 24 h. The obtained PAEK-DF-x membranes were cured in UVcuring machine (CEL-LAM500, Beijing Zhongjiaojinyuan technology co. LTD) equipped with a long arc mercury lamp (500 W, main wavelength 365 nm) in presence of air. Different exposure times were carried out in the photocrosslinking procedure. These resultant crosslinked polymers were defined as c-PAEK-DF-x.
2. Experimental section 2.1. Materials 3,3′,5,5′-Tetramethyl-4,4′-biphenyl (TMBP) and 4,4′-difluorobenzophenone (DFBP) were purchased from Yanjin Technology (Tianjin, China). Diphenolic acid, 4-fluorobenzoyl chloride, 1,14-tetradecanediol, and triethylamine were bought from Aladdin reagent Co. Ltd, China. N’-dicy-clohexylcarbodiimide (DCC), 2-aminoanthracene and 4-dimethylaminopyridine (DMAP) were purchased from Energy reagent Co. Ltd, China. In addition, all chemical reagents and solvents were analytical pure grade and used without further purification.
2.4. Characterization and measurements 2.4.1. Primary measurements 1 H NMR spectra of PAEK-DF and PAEK-DF-x were carried out on a Bruker 500 MHz NMR spectroscopy. Fourier transform infrared spectroscopies (FT-IR) were collected via Bruker Vector 22 spectrometer. UV–Visible spectra were recorded on a Hitachi U-1900 spectrophotometer (Varian, USA) in the wavelength range of 200–600 nm. Fluorescence data were collected on F97Pro fluorospectro photometer (Lengguang, Shanghai). All the thermogravimetric analyses (TGA) were characterized using a Perkin-Elmer thermal analyzer Pyris 1 TGA. The mechanical properties of samples were measured on Shimadzu AG-I universal tester. The dry samples for the tensile test were cut into 4 mm width and 35 mm length. The strain rate was controlled at 2 mm/min. Furthermore, both glass transition temperature (Tg) and storage modulus (G′) was obtained via a dynamic mechanical analysis (DMA, TA Q800) in the mode of Multi-Force-Strain.
2.2. Synthesis of soft-rigid alternative type of PAEKs with anthracene pendants. A new difluorobenzene monomer containing a flexible alkyl chain was synthesized in our lab according to the procedure described in the literature [34]. 1,14-Tetradecanediol (23.04 g, 0.1 mol) and triethylamine (20.00 g) were dissolved in 150 mL tetrahydrofuran under mechanical stirring for 4 h. 4-Fluorobenzoyl chloride (15.86 g, 0.2 mol) was then added dropwise into the above solution in the ice bath. After the addition of acylation reagent, the esterification reaction was carried out at room temperature for 3 h, followed by pouring into deionized water to obtain the crude product. The product was washed with deionized water and recrystallized using ethanol, and then dried under the vacuum at 40 °C for 24 h. The resultant 1,14-tetradecanediol-pfluorobenzoyl diester (DFAD) was characterized using 1H NMR. 1H NMR (d6-DMSO): δ = 8.00 ppm (H-Ar), δ2 = 7.34 ppm (H-Ar), δ = 4.20 ppm, (eCH2e next to the ester group), δ = 1.67 ppm (eCH2e), δ = 1.20–1.40 ppm (other eCH2e in alkyl chain). The soft-rigid alternative type of PAEKs was prepared through nucleophilic copolycondensation as depicted in Scheme 1. Diphenolic acid (8.58 g, 30.0 mmol), TMBP (7.26 g, 30.0 mmol), DFBP (7.86 g, 36.0 mmol), DFAD (11.37 g, 24.0 mmol), K2CO3 (11.4 g, 83.0 mmol), dimethyl sulfoxide (105 mL), and toluene (20 mL) were added into a 250 mL three-necked flask. Then the mixture was heated with mechanical stirring under nitrogen atmosphere. The azeotropic distillation was employed to remove the moisture generated during the copolycondensation. After releasing of the toluene, the copolycondensation temperature was elevated to 180 °C to ensure the completion of the reaction. Eight hours later, the viscous solution was cooled to the room temperature and then poured into deionized water. The resultant precipitate was washed and activated with deionized water and 0.5 M H2SO4, respectively, followed by drying under vacuum at 60 °C for 24 h. This obtained polymer was defined as PAEK-DF. The photocrosslinkable anthracene groups were introduced in PAEK-DF using the post-polymerization functionalization technique. A typical synthesis procedure of PAEK-DF-20, where 20 refers to the feed ratio of 2-aminoanthracene to diphenol monomers, was as follows. PAEK-DF (0.800 g) and 2-aminoanthracene (0.030 g) were dissolved in 10 mL tetrahydrofuran (THF). DMAP (0.005 g) THF solution was then added dropwise to the aforementioned solution with magnetic stirring in an ice bath. After the addition, DCC (0.16 g) THF solution was transferred to the above mixture with magnetic stirring. The postpolymerization functionalization proceeded for 48 h in dark circumstances. The resultant mixture was centrifugalized to remove the insoluble impurity, and then poured into ethanol. The precipitate was washed using ethanol three times, followed by drying under vacuum at 60 °C for 24 h. The compared PAEK-DF-10 and PAEK-DF-30 were also prepared using a similar procedure as shown in Scheme 1.
2.4.2. Characterization of shape memory properties The shape memory properties of these polymers were performed on DMA Q800 with a force-controlled mode. The dimension of the samples was about 4 mm × 35 mm. The heating and cooling rates for the DMA procedures were fixed at 10 °C/min. The successive procedures including deformation, cooling, fixing and recovery were also accomplished on DMA as follows. Firstly, the samples were equilibrated at the trigger temperature (Tt) for 3 min with a preload of 0.005 N to eliminate thermal history (ε0). Next, ramping stress (~0.14 MPa) on the samples to deform a temporary shape (εmax), and the shape was fixed when the temperature cools down to Tlow (35 °C). Finally, the force was reduced to 0.005 N (ε1) and then the samples were heated to Tt again for 20 min to recover the permanent shape (εre). Thus the key indicators to evaluate the shape memory properties, shape fixity ratio (Rf) and shape recovery ratio (Rr), can be calculated by the following equations [25,30],
Rf =
ε1 − ε0 × 100% εmax −ε0
(1)
Rr =
ε1 − εre × 100% ε1 − ε0
(2)
3. Result and discussion 3.1. Characterization of PAEK-DF and PAEK-DF-x These shape memory PAEKs bearing dynamic covalent bonds were designed and prepared via delicate synthetic strategy as depicted in Scheme 1. Firstly, a series of soft-rigid alternative types of PAEKs containing carboxylic acid groups were prepared through nucleophilic copolycondensation. Then the photocrosslinkable anthracene groups were introduced in the structure of the aforementioned PAEKs via amidation reaction by post-polymerization functionalization tactic. Finally, the crosslinked network polymeric structure was achieved through the photodimerization under the UV radiation. The resultant PAEK-DF and PAEK-DF-20 were characterized using 2
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Scheme 1. The synthesis procedure and photocrosslinking structure of PAEK-DF-x. 1 H NMR spectra. As shown in Fig. 1, the characteristic signals of eCH2e groups from DFAD segments have been found at 1.31 ppm, 1.67 ppm and 4.20 ppm, respectively. The peak at 2.19 ppm could be attributed to the methyl groups and methylene groups from phenol groups. As expected, the protons of aryl groups appeared between 6.80 ppm and 8.00 ppm. Some characteristic signals have been identified and labeled in Fig. 1. After the post-polymerization functionalization of anthracene groups, some new proton signals from anthracene groups were observed in the range of 6.80–7.80 ppm. Furthermore, a new signal peak was disclosed at 10.90 ppm, ascribed to amide groups (eCONHe), which indicates that the photocrosslinkable anthracene groups have been successfully introduced in the architecture of shape memory PAEK. According to the integral ratios of amide signal (10.90 ppm) to methylene signal (4.20 ppm), the graft ratios of anthracene were calculated about 9.5%, 19.2% and 28.6%, respectively, indicating the availability of post-polymerization functionalization tactic. In addition, the structure of PAEK-DF and PAEK-DF-20 were also examined using FT-IR spectra. As shown in Fig. 2a, the infrared absorptions of alkyl groups were found at 2920 cm−1 and 2851 cm−1. The stretching vibration absorption of the carbonyl group from carboxylic acid appeared at 1709 cm−1. The absorption around 1600 cm−1 and 1500 cm−1 could be ascribed to the stretching vibration absorption of aryl groups in the molecular structure. The
Fig. 1. 1H NMR spectra of PAEK-DF and PAEK-DF-20.
3
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Fig. 2. (a) FT-IR spectra of PAEK-DF and PAEK-DF-20. (b) UV–vis spectra of PAEK-DF-20 under UV radiation with different exposure time. (c) Fluorescence spectra of PAEK-DF-20 membrane and the UV-dimerized c-PAEK-DF-20 membrane. The excitation wavelength is 474 nm.
absorption of ether linkage was observed at 1224 cm−1. The bending vibration absorption of carbon-hydrogen bond was found at 839 cm−1 and 748 cm−1. Moreover, specified carbonyl absorption at 1680 cm−1 presented in the spectrum of PAEK-DF-20 revealed should be attributed to the amide groups, which indicates the success of post-polymerization functionalization using 2-aminoanthracene. Furthermore, the photocrosslinking procedure of PAEK-DF-20 was monitored through UV–vis spectroscopy as shown in Fig. 2b. Due to the high thickness and strong absorbance of these specimen membranes, their liquid tetrahydrofuran solution was measured using UV–vis spectroscopy under the UV radiation. Before the radiation, the PAEKDF-20 exhibited characteristic absorption between 350 nm and 450 nm. After the radiation of UV (365 nm), the resultant c-PAEK-DF-20 displayed decreased characteristic absorption with increasing of exposure time. Finally, the characteristic absorption disappeared after 5 min radiation, which indicates the accomplishment of photodimerization of anthracene groups. In addition, this photodimerization procedure was also monitored using fluorescent spectrometry. As shown in Fig. 2c, cPAEK-DF-20 membrane exhibited an obvious emission centered at 540 nm, ascribed to the characteristic emission of anthracene groups. After the photodimerization, the resulting c-PAEK-DF-20 membrane revealed a dramatic decreased fluorescent emission centered at 550 nm, due to the formation anthracene dimers. These results indicated the availability of photodimerization of anthracene groups in this synthesized polymeric system.
photodimerization, these resultant partially crosslinked polymers exhibited elevated Tg values from 85.2 °C to 87.4 °C, and to 89.3 °C as the anthryl grafting ratio raised from 10% to 30%. Obviously, this partially crosslinked architecture leads to the increase of Tg value. In addition, the thermal stability of these polymers was also evaluated by TGA under nitrogen atmosphere. As shown in Fig. 3b, the softrigid alternative PAEK-DF displayed good thermostability with a high Td5% of 407 °C. However, these crosslinked polymers exhibited decreased Td5% values in the range of 329 °C-372 °C, attributed to the introduction of huge rigid side anthracene groups. Actually, these crosslinked c-PAEK-DF-x polymers would transform to their original PAEK-DF-x under the high temperature, due to the reversibility of dynamic bonds from cycloaddition reaction. Therefore, these dynamic covalent bonds from photodimerization could not enhance their thermostability. Nonetheless, these as-synthesized polymers could completely satisfy the requirement of shape memory behavior. In addition, the final residual weights of c-PAEK-DF-x at 800 °C were 42–44%, more than that of PAEK-DF (36%), due to the high thermostability of polycyclic aromatic hydrocarbon.
3.3. Mechanical properties The mechanical properties of these polymers were evaluated on Shimadzu AG-I universal tester. Their stress-strain curves were presented in Fig. 4a. The soft-rigid alternative PAEK-DF showed good mechanical strength with a tensile strength of 28.8 MPa and an elongation rate of 4.7%. After crosslinking under UV radiation, the resultant c-PAEK-DF-x displayed enhanced tensile strength (30.6 MPa for cPAEK-DF-10, 34.0 MPa for c-PAEK-DF-20, 43.9 MPa for c-PAEK-DF-30), ascribed to their robust network structure. Moreover, c-PAEK-DF-x showed decreased elongation rate from 6.1%, 4.9%, 3.7% with the increase of anthracene content in the molecular structure. Storage modulus (G’) could visually reflect the ability of a material to store the energy of elastic deformation; therefore, it is an important
3.2. Thermal properties To determine the trigger temperature for shape memory behavior, Tanδ-temperature curves were acquired by DMA. As shown in Fig. 3a, the Tg value of PAEK-DF determined from the peak on the curve is 82.5 °C, which is much lower than the traditional PAEK. This low Tg value could attribute to the flexible alkyl segments in the main chain, which requires less energy to reach the high-elastic state. After the
Fig. 3. (a) Tanδ-temperature curves and (b) TGA curves of PAEK-DF and c-PAEK-DF-x. 4
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Fig. 4. Temperature dependence of storage modulus (a) and stress-strain curves (b) of PAEK-DF-x.
Fig. 5. The fix-recover curves with temperature of PAEK-DF and c-PAEK-DF-x.
it more deformable. After crosslinking through photodimerization, these resultant polymers exhibited improved storage modulus of 956.6 MPa for c-PAEK-DF-10, 1009.7 MPa for c-PAEK-DF-20, and 1054.1 MPa for c-PAEK-DF-30, respectively, again attributed to their networked architecture (see Table 1).
Table1 The shape memory behaviors of PAEK-DF and c-PAEK-DF-x. Sample
Rf(%)
Rr(%)
PAEK-DF c-PAEK-DF-10 c-PAEK-DF-20 c-PAEK-DF-30
98.6 99.0 98.9 98.7
83.1 87.8 100 98.9
3.4. Shape memory behaviors The force-controlled mode of DMA is the most popular way to investigate the shape memory behavior of temperature responsive shape memory polymers. In this study, the trigger temperature (Tt) was set as Tg + 10 °C, and the force for deformation was fixed at 0.14 MPa. PAEKDF and c-PAEK-DF-x films were deformed to a temporary shape and cooled down for fixing. Then they could recover to the initial shape at Tt when the deformation force was removed. The shape fix-recover curves of these films were collected using DMA and shown in Fig. 5.
indicator to evaluate the shape memory behavior of materials. Hence, the storage modulus-temperature curves of all samples were collected using DMA and shown in Fig. 5b. It could be found that PAEK-DF possessed a low G′ value of 917.7 MPa compared with those conventional PAEKs, ascribed to the flexible alkyl segments in the main chain. The more flexible structure weakens the rigidity of polymer and makes 5
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Fig. 6. (a) The cyclic fix-recover curve and (b) photographs about the shape memory processes of c-PAEK-DF-20.
Compared to PAEK-DF, c-PAEK-DF-x exhibited large εmax and ε1 values, indicating the greater deformation. This enhanced deformation should be attributed to the increase of free volume stemmed from these pendant anthracene groups. However, the deformation of c-PAEK-DF-x decreased obviously with the increase of crosslinking degree. A more crosslinking structure would restrict the movement of molecules, resulting in a lower deformation. The evaluation of shape memory behavior involving Rf and Rr are calculated according to the established equations. As shown in Fig. 5a, the soft-rigid alternative PAEK-DF exhibited approving shape memory behavior with good shape fixity ratio (Rf = 98.6%) and acceptable shape recovery ratio (Rr = 83.1%). After crosslinking through photodimerization, these resultant polymers exhibited enhanced shape memory behavior (c-PAEK-DF-10 with Rf of 99.0% and Rr of 87.8%; c-PAEK-DF-20 with Rf of 98.9% and Rr of 100%; c-PAEK-DF-30 with Rf of 98.7% and Rr of 98.9%). The crosslinking structure not only improved the ability of deformation, but also significantly enhanced shape recovery capacity. Overall, c-PAEK-DF-20 film disclosed optimal shape memory behaviors with approving deformation and good shape fixity and recovery. Therefore, the cyclic fixrecover measurement was carried out with c-PAEK-DF-20 film. As shown in Fig. 6a, the c-PAEK-DF-20 film kept high deformation with satisfying Rf value above 98.0%. However, the Rr value decreases gradually during the cyclic fix-recover process. Finally, the Rr value decreased to 75.8% after the four cycles. This decrease of shape recovery could be due to the cleavage of the crosslinking bonds under the long-term high temperature (Tt is about 97 °C.) The detailed and clear cause is still explored in our lab. The shape memory process of c-PAEKDF-20 film was recorded using photographs in Fig. 6. Firstly, c-PAEKDF-20 film was deformed into a temporary shape with an external force at Tt and then fixed at room temperature. It could recover to its original shape completely at Tt. In addition, c-PAEK-DF-20 film can be secondarily deformed and recovered to permanent shape, which indicated good repeatability of shape memory behavior.
applications of smart mechatronics, multistimuli-responsive mechanical energy conversion devices, and so on. Author statement We assure you that the results in this manuscript are original and have not been disclosed elsewhere. Acknowledgements We graciously acknowledge the National Natural Science Foundation of China for funding (grant No. 21774046) and Jilin Provincial Science and Technology Department, China (Grant No. 20190201075JC). Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. References [1] J. Flanagan, R. Strutzenberg, R. Myers, J. Rodrian, Development and flight testing of a morphing aircraft, the NextGen MFX-1, Aerospace. Eng. (2007) 1–3. [2] C. Yakacki, R. Shandas, C. Lanning, B. Rech, A. Eckstein, K. Gall, Unconstrained recovery characterization of shape-memory polymer networks for cardiovascular applications, Biomaterials 28 (2007) 2255–2263. [3] J. Hu, Adaptive and functional shape memory polymers, textiles and their applications, Imperial College Press Publisher, London England, 2011, p. 392. [4] A. Lendlein, R. Langer, Biodegradable, elastic shape-memory polymers for potential biomedical applications, Science 296 (2002) 1673–1676. [5] M. Kohl, D. Brugger, M. Ohtsuka, T. Takagi, Adaptive and functional shape memory polymers, textiles and their applications, Sens. Actuat., A 114 (2004) 445–450. [6] J. Kunzelman, T. Chung, P. Mather, C. Weder, Shape memory polymers with builtin threshold temperature sensors, J. Mater. Chem. 18 (2008) 1082–1086. [7] Z. Mohadeseh, P. Molamma, P. Nader, R. Seeram, Thermally-induced two-way shape memory polymers: mechanisms, structures, and applications, Chem. Eng. J. 374 (2019) 706–720. [8] Binjie Jin, Huijie Song, Ruiqi Jiang, Jizhou Song, Qian Zhao, Tao Xie, Programming a crystalline shape memory polymer network with thermo- and photo-reversible bonds toward a single-component soft robot, Sci. Adv. 4 (1) (2018) eaao3865, https://doi.org/10.1126/sciadv.aao3865. [9] T. Xie, I.A. Rousseau, Facile tailoring of thermal transition temperature of epoxy shape memory polymers, Polymer 50 (2009) 1852–1856. [10] Z. Lan, X. Chen, X. Zhang, C. Zhu, Y. Yu, J. Wei, Transparent, high glass-transition temperature, shape memory hybrid polyimides based on polyhedral oligomeric silsesquioxane, Polymers 11 (2019) 1058. [11] V. Anand, K. Vimal, S. Varughese, Asme, Solvent induced shape memory behavior of Sulfonated Poly Ether Ether Ketone (SPEEK), Proceedings of the Asme Conference on Smart Materials, Adaptive Structures and Intelligent Systems, Vol. 1, Amer. Soc. Mechanical Engineers: New York, 2012, pp. 27–34. [12] X. Fu, Y. Yuan, Z. Liu, P. Yan, C. Zhou, J. Lei, Thermoplastic shape memory polymers with tailor-made trigger temperature, Eur. Polym. J. 93 (2017) 307–313. [13] R. Zhao, T. Zhao, X. Jiang, X. Liu, D. Shi, C. Liu, S. Yang, E. Chen, Thermoplastic high strain multishape memory polymer: side-chain polynorbornene with columnar
4. Conclusion In this work, we have introduced photocrosslinkable anthracene groups in soft-rigid alternative poly(aryl ether ketone)s though postpolymerization functionalization. Then the intermolecular photodimerization of pendant anthracene groups was carried out under the UV radiation, resulting in a dynamic crosslinked network polymeric architecture. The resulting crosslinked polymers exhibited good thermostability and improved mechanical properties. Enhanced shape memory performance with satisfying shape fixity ratio and shape recovery ratio was disclosed for those polymer films, attributed to their networked molecular structure. Considering the reversibility of photocrosslink reaction of anthracene groups, this dynamic crosslinking shape memory polymers would play an important role in intelligent 6
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Performance enhancement of shape memory poly(aryl ether ketone) via photodimerization of pendant anthracene units ⁎
Yiyang Gu, Chunyu Ru, Zhen Zhao, Danming Chao , Xincai Liu
T
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College of Chemistry, Jilin University, Changchun 130012, People’s Republic of China
A R T I C LE I N FO
A B S T R A C T
Keywords: Shape memory polymer Poly(aryl ether ketone) Photocrosslinking Anthracene
Shape memory polymers have attracted much attention due to their promising applications in biomedicine, textiles, aerospace et al. Here, novel poly(aryl ether ketone)s bearing flexible alkyl segments and pendant anthracene groups were synthesized via nucleophilic copolycondensation coupled with post-polymerization functionalization. Their chemical structure, thermostability and mechanical properties were characterized through Fourier Transform infrared spectroscopy, nuclear magnetic resonance spectroscopy, thermal gravimetric analysis and dynamic mechanical analysis. The soft-rigid alternative segment structure endows these poly (aryl ether ketone)s with promising shape memory properties. After the photodimerization of pendant anthracene units, the resultant polymers exhibit enhanced shape memory performance in terms of satisfying shape fixity ratio and high shape recovery ratio, attributed to their photocrosslinked polymeric network architecture. This shape memory polymer with reversible photodimerization holds great promise for a deep understanding of the mechanism and future applications.
1. Introduction Shape memory polymers (SMPs) have received increasing attention for their wide potential applications such as aerospace field [1], biomaterials devices [2], functional textiles [3], sensors, actuators, and et al [4–6]. These polymeric materials possessing the capability of changing their shapes when exposed to external stimuli, including light, heat, magnetic field, moisture, or solvent are widely developed for SMPs [7–11]. Thereinto, thermally induced SMPs (TSMPs) have been intensively investigated for their easy design and synthesis [12–14]. Usually, the trigger temperature (Tt) is served for the switch of TSMPs, which may be the glass transition temperature (Tg) [15,16], melting temperature (Tm) [17–20], or isotropic temperature (Ti) [21–23] of these polymers. For example, polyethylene is considered as the most typical TSMP based on semi-crystalline phase switch with Tm [24]. Besides, a variety of Tg-type TSMPs such as epoxy [25], poly(arylene ether ketone)s (PAEKs) [26], and polynorbornene [27] are developed. Compared with Tm-type and Ti-type polymers, Tg-type TSMPs exhibit relatively unsatisfactory shape recovery behavior and mechanical properties, which would severely hinder their practical application in many fields [28]. Therefore, it is an urgent need to exploit new Tg-type shape memory polymers featuring outstanding shape memory performance [29].
⁎
Among all Tg-type shape memory polymers, poly(aryl ether ketone) s (PAEKs) have attracted much attention in recent decades, because of their good mechanical properties, desirable chemical stability, and designable structure [30–33]. Some novel shape memory PAEKs bearing soft-rigid alternative segments have recently been designed and synthesized in our lab [34,35]. To improve their shape memory performance, carboxylic groups were also introduced in this polymeric architecture and generate physically crosslinking action through hydrogen bonds. However, these weak interactions from micro phase separation of soft-hard segments and/or hydrogen bonds are too weak to the practical application of these polymers. Therefore, chemically crosslinking action based on covalent bonds would be an efficient method for the improvement of shape memory performance. Among them, the dynamic covalent bonds would be ideal to construct crosslinking network architecture [36,37]. Thereinto, photocrosslinkable anthracene groups have been frequently utilized to generate dynamic covalent bonds due to their reversible cycloaddition reaction under UVlight illumination and/or thermal treatment [38–41]. Herein, photocrosslinkable anthracene groups were introduced in the soft-rigid alternative type of PAEKs using the post-polymerization functionalization technique. Then the photodimerization was triggered to form crosslinked network polymeric structure. Their mechanical capacities, thermal properties and shape memory performance were comparatively
Corresponding authors. E-mail addresses: [email protected] (D. Chao), [email protected] (X. Liu).
https://doi.org/10.1016/j.eurpolymj.2019.109413 Received 18 October 2019; Received in revised form 29 November 2019; Accepted 5 December 2019 Available online 10 December 2019 0014-3057/ © 2019 Elsevier Ltd. All rights reserved.
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2.3. Photocrosslinking PAEK-DF-x membranes by UV radiation
investigated in detail.
The uniform viscous PAEK-DF-x dimethylacetamide (DMAC) solution was cast onto a flat glass plate, and then transferred to the oven at 60 °C for 24 h. The obtained PAEK-DF-x membranes were cured in UVcuring machine (CEL-LAM500, Beijing Zhongjiaojinyuan technology co. LTD) equipped with a long arc mercury lamp (500 W, main wavelength 365 nm) in presence of air. Different exposure times were carried out in the photocrosslinking procedure. These resultant crosslinked polymers were defined as c-PAEK-DF-x.
2. Experimental section 2.1. Materials 3,3′,5,5′-Tetramethyl-4,4′-biphenyl (TMBP) and 4,4′-difluorobenzophenone (DFBP) were purchased from Yanjin Technology (Tianjin, China). Diphenolic acid, 4-fluorobenzoyl chloride, 1,14-tetradecanediol, and triethylamine were bought from Aladdin reagent Co. Ltd, China. N’-dicy-clohexylcarbodiimide (DCC), 2-aminoanthracene and 4-dimethylaminopyridine (DMAP) were purchased from Energy reagent Co. Ltd, China. In addition, all chemical reagents and solvents were analytical pure grade and used without further purification.
2.4. Characterization and measurements 2.4.1. Primary measurements 1 H NMR spectra of PAEK-DF and PAEK-DF-x were carried out on a Bruker 500 MHz NMR spectroscopy. Fourier transform infrared spectroscopies (FT-IR) were collected via Bruker Vector 22 spectrometer. UV–Visible spectra were recorded on a Hitachi U-1900 spectrophotometer (Varian, USA) in the wavelength range of 200–600 nm. Fluorescence data were collected on F97Pro fluorospectro photometer (Lengguang, Shanghai). All the thermogravimetric analyses (TGA) were characterized using a Perkin-Elmer thermal analyzer Pyris 1 TGA. The mechanical properties of samples were measured on Shimadzu AG-I universal tester. The dry samples for the tensile test were cut into 4 mm width and 35 mm length. The strain rate was controlled at 2 mm/min. Furthermore, both glass transition temperature (Tg) and storage modulus (G′) was obtained via a dynamic mechanical analysis (DMA, TA Q800) in the mode of Multi-Force-Strain.
2.2. Synthesis of soft-rigid alternative type of PAEKs with anthracene pendants. A new difluorobenzene monomer containing a flexible alkyl chain was synthesized in our lab according to the procedure described in the literature [34]. 1,14-Tetradecanediol (23.04 g, 0.1 mol) and triethylamine (20.00 g) were dissolved in 150 mL tetrahydrofuran under mechanical stirring for 4 h. 4-Fluorobenzoyl chloride (15.86 g, 0.2 mol) was then added dropwise into the above solution in the ice bath. After the addition of acylation reagent, the esterification reaction was carried out at room temperature for 3 h, followed by pouring into deionized water to obtain the crude product. The product was washed with deionized water and recrystallized using ethanol, and then dried under the vacuum at 40 °C for 24 h. The resultant 1,14-tetradecanediol-pfluorobenzoyl diester (DFAD) was characterized using 1H NMR. 1H NMR (d6-DMSO): δ = 8.00 ppm (H-Ar), δ2 = 7.34 ppm (H-Ar), δ = 4.20 ppm, (eCH2e next to the ester group), δ = 1.67 ppm (eCH2e), δ = 1.20–1.40 ppm (other eCH2e in alkyl chain). The soft-rigid alternative type of PAEKs was prepared through nucleophilic copolycondensation as depicted in Scheme 1. Diphenolic acid (8.58 g, 30.0 mmol), TMBP (7.26 g, 30.0 mmol), DFBP (7.86 g, 36.0 mmol), DFAD (11.37 g, 24.0 mmol), K2CO3 (11.4 g, 83.0 mmol), dimethyl sulfoxide (105 mL), and toluene (20 mL) were added into a 250 mL three-necked flask. Then the mixture was heated with mechanical stirring under nitrogen atmosphere. The azeotropic distillation was employed to remove the moisture generated during the copolycondensation. After releasing of the toluene, the copolycondensation temperature was elevated to 180 °C to ensure the completion of the reaction. Eight hours later, the viscous solution was cooled to the room temperature and then poured into deionized water. The resultant precipitate was washed and activated with deionized water and 0.5 M H2SO4, respectively, followed by drying under vacuum at 60 °C for 24 h. This obtained polymer was defined as PAEK-DF. The photocrosslinkable anthracene groups were introduced in PAEK-DF using the post-polymerization functionalization technique. A typical synthesis procedure of PAEK-DF-20, where 20 refers to the feed ratio of 2-aminoanthracene to diphenol monomers, was as follows. PAEK-DF (0.800 g) and 2-aminoanthracene (0.030 g) were dissolved in 10 mL tetrahydrofuran (THF). DMAP (0.005 g) THF solution was then added dropwise to the aforementioned solution with magnetic stirring in an ice bath. After the addition, DCC (0.16 g) THF solution was transferred to the above mixture with magnetic stirring. The postpolymerization functionalization proceeded for 48 h in dark circumstances. The resultant mixture was centrifugalized to remove the insoluble impurity, and then poured into ethanol. The precipitate was washed using ethanol three times, followed by drying under vacuum at 60 °C for 24 h. The compared PAEK-DF-10 and PAEK-DF-30 were also prepared using a similar procedure as shown in Scheme 1.
2.4.2. Characterization of shape memory properties The shape memory properties of these polymers were performed on DMA Q800 with a force-controlled mode. The dimension of the samples was about 4 mm × 35 mm. The heating and cooling rates for the DMA procedures were fixed at 10 °C/min. The successive procedures including deformation, cooling, fixing and recovery were also accomplished on DMA as follows. Firstly, the samples were equilibrated at the trigger temperature (Tt) for 3 min with a preload of 0.005 N to eliminate thermal history (ε0). Next, ramping stress (~0.14 MPa) on the samples to deform a temporary shape (εmax), and the shape was fixed when the temperature cools down to Tlow (35 °C). Finally, the force was reduced to 0.005 N (ε1) and then the samples were heated to Tt again for 20 min to recover the permanent shape (εre). Thus the key indicators to evaluate the shape memory properties, shape fixity ratio (Rf) and shape recovery ratio (Rr), can be calculated by the following equations [25,30],
Rf =
ε1 − ε0 × 100% εmax −ε0
(1)
Rr =
ε1 − εre × 100% ε1 − ε0
(2)
3. Result and discussion 3.1. Characterization of PAEK-DF and PAEK-DF-x These shape memory PAEKs bearing dynamic covalent bonds were designed and prepared via delicate synthetic strategy as depicted in Scheme 1. Firstly, a series of soft-rigid alternative types of PAEKs containing carboxylic acid groups were prepared through nucleophilic copolycondensation. Then the photocrosslinkable anthracene groups were introduced in the structure of the aforementioned PAEKs via amidation reaction by post-polymerization functionalization tactic. Finally, the crosslinked network polymeric structure was achieved through the photodimerization under the UV radiation. The resultant PAEK-DF and PAEK-DF-20 were characterized using 2
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Scheme 1. The synthesis procedure and photocrosslinking structure of PAEK-DF-x. 1 H NMR spectra. As shown in Fig. 1, the characteristic signals of eCH2e groups from DFAD segments have been found at 1.31 ppm, 1.67 ppm and 4.20 ppm, respectively. The peak at 2.19 ppm could be attributed to the methyl groups and methylene groups from phenol groups. As expected, the protons of aryl groups appeared between 6.80 ppm and 8.00 ppm. Some characteristic signals have been identified and labeled in Fig. 1. After the post-polymerization functionalization of anthracene groups, some new proton signals from anthracene groups were observed in the range of 6.80–7.80 ppm. Furthermore, a new signal peak was disclosed at 10.90 ppm, ascribed to amide groups (eCONHe), which indicates that the photocrosslinkable anthracene groups have been successfully introduced in the architecture of shape memory PAEK. According to the integral ratios of amide signal (10.90 ppm) to methylene signal (4.20 ppm), the graft ratios of anthracene were calculated about 9.5%, 19.2% and 28.6%, respectively, indicating the availability of post-polymerization functionalization tactic. In addition, the structure of PAEK-DF and PAEK-DF-20 were also examined using FT-IR spectra. As shown in Fig. 2a, the infrared absorptions of alkyl groups were found at 2920 cm−1 and 2851 cm−1. The stretching vibration absorption of the carbonyl group from carboxylic acid appeared at 1709 cm−1. The absorption around 1600 cm−1 and 1500 cm−1 could be ascribed to the stretching vibration absorption of aryl groups in the molecular structure. The
Fig. 1. 1H NMR spectra of PAEK-DF and PAEK-DF-20.
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Fig. 2. (a) FT-IR spectra of PAEK-DF and PAEK-DF-20. (b) UV–vis spectra of PAEK-DF-20 under UV radiation with different exposure time. (c) Fluorescence spectra of PAEK-DF-20 membrane and the UV-dimerized c-PAEK-DF-20 membrane. The excitation wavelength is 474 nm.
absorption of ether linkage was observed at 1224 cm−1. The bending vibration absorption of carbon-hydrogen bond was found at 839 cm−1 and 748 cm−1. Moreover, specified carbonyl absorption at 1680 cm−1 presented in the spectrum of PAEK-DF-20 revealed should be attributed to the amide groups, which indicates the success of post-polymerization functionalization using 2-aminoanthracene. Furthermore, the photocrosslinking procedure of PAEK-DF-20 was monitored through UV–vis spectroscopy as shown in Fig. 2b. Due to the high thickness and strong absorbance of these specimen membranes, their liquid tetrahydrofuran solution was measured using UV–vis spectroscopy under the UV radiation. Before the radiation, the PAEKDF-20 exhibited characteristic absorption between 350 nm and 450 nm. After the radiation of UV (365 nm), the resultant c-PAEK-DF-20 displayed decreased characteristic absorption with increasing of exposure time. Finally, the characteristic absorption disappeared after 5 min radiation, which indicates the accomplishment of photodimerization of anthracene groups. In addition, this photodimerization procedure was also monitored using fluorescent spectrometry. As shown in Fig. 2c, cPAEK-DF-20 membrane exhibited an obvious emission centered at 540 nm, ascribed to the characteristic emission of anthracene groups. After the photodimerization, the resulting c-PAEK-DF-20 membrane revealed a dramatic decreased fluorescent emission centered at 550 nm, due to the formation anthracene dimers. These results indicated the availability of photodimerization of anthracene groups in this synthesized polymeric system.
photodimerization, these resultant partially crosslinked polymers exhibited elevated Tg values from 85.2 °C to 87.4 °C, and to 89.3 °C as the anthryl grafting ratio raised from 10% to 30%. Obviously, this partially crosslinked architecture leads to the increase of Tg value. In addition, the thermal stability of these polymers was also evaluated by TGA under nitrogen atmosphere. As shown in Fig. 3b, the softrigid alternative PAEK-DF displayed good thermostability with a high Td5% of 407 °C. However, these crosslinked polymers exhibited decreased Td5% values in the range of 329 °C-372 °C, attributed to the introduction of huge rigid side anthracene groups. Actually, these crosslinked c-PAEK-DF-x polymers would transform to their original PAEK-DF-x under the high temperature, due to the reversibility of dynamic bonds from cycloaddition reaction. Therefore, these dynamic covalent bonds from photodimerization could not enhance their thermostability. Nonetheless, these as-synthesized polymers could completely satisfy the requirement of shape memory behavior. In addition, the final residual weights of c-PAEK-DF-x at 800 °C were 42–44%, more than that of PAEK-DF (36%), due to the high thermostability of polycyclic aromatic hydrocarbon.
3.3. Mechanical properties The mechanical properties of these polymers were evaluated on Shimadzu AG-I universal tester. Their stress-strain curves were presented in Fig. 4a. The soft-rigid alternative PAEK-DF showed good mechanical strength with a tensile strength of 28.8 MPa and an elongation rate of 4.7%. After crosslinking under UV radiation, the resultant c-PAEK-DF-x displayed enhanced tensile strength (30.6 MPa for cPAEK-DF-10, 34.0 MPa for c-PAEK-DF-20, 43.9 MPa for c-PAEK-DF-30), ascribed to their robust network structure. Moreover, c-PAEK-DF-x showed decreased elongation rate from 6.1%, 4.9%, 3.7% with the increase of anthracene content in the molecular structure. Storage modulus (G’) could visually reflect the ability of a material to store the energy of elastic deformation; therefore, it is an important
3.2. Thermal properties To determine the trigger temperature for shape memory behavior, Tanδ-temperature curves were acquired by DMA. As shown in Fig. 3a, the Tg value of PAEK-DF determined from the peak on the curve is 82.5 °C, which is much lower than the traditional PAEK. This low Tg value could attribute to the flexible alkyl segments in the main chain, which requires less energy to reach the high-elastic state. After the
Fig. 3. (a) Tanδ-temperature curves and (b) TGA curves of PAEK-DF and c-PAEK-DF-x. 4
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Fig. 4. Temperature dependence of storage modulus (a) and stress-strain curves (b) of PAEK-DF-x.
Fig. 5. The fix-recover curves with temperature of PAEK-DF and c-PAEK-DF-x.
it more deformable. After crosslinking through photodimerization, these resultant polymers exhibited improved storage modulus of 956.6 MPa for c-PAEK-DF-10, 1009.7 MPa for c-PAEK-DF-20, and 1054.1 MPa for c-PAEK-DF-30, respectively, again attributed to their networked architecture (see Table 1).
Table1 The shape memory behaviors of PAEK-DF and c-PAEK-DF-x. Sample
Rf(%)
Rr(%)
PAEK-DF c-PAEK-DF-10 c-PAEK-DF-20 c-PAEK-DF-30
98.6 99.0 98.9 98.7
83.1 87.8 100 98.9
3.4. Shape memory behaviors The force-controlled mode of DMA is the most popular way to investigate the shape memory behavior of temperature responsive shape memory polymers. In this study, the trigger temperature (Tt) was set as Tg + 10 °C, and the force for deformation was fixed at 0.14 MPa. PAEKDF and c-PAEK-DF-x films were deformed to a temporary shape and cooled down for fixing. Then they could recover to the initial shape at Tt when the deformation force was removed. The shape fix-recover curves of these films were collected using DMA and shown in Fig. 5.
indicator to evaluate the shape memory behavior of materials. Hence, the storage modulus-temperature curves of all samples were collected using DMA and shown in Fig. 5b. It could be found that PAEK-DF possessed a low G′ value of 917.7 MPa compared with those conventional PAEKs, ascribed to the flexible alkyl segments in the main chain. The more flexible structure weakens the rigidity of polymer and makes 5
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Fig. 6. (a) The cyclic fix-recover curve and (b) photographs about the shape memory processes of c-PAEK-DF-20.
Compared to PAEK-DF, c-PAEK-DF-x exhibited large εmax and ε1 values, indicating the greater deformation. This enhanced deformation should be attributed to the increase of free volume stemmed from these pendant anthracene groups. However, the deformation of c-PAEK-DF-x decreased obviously with the increase of crosslinking degree. A more crosslinking structure would restrict the movement of molecules, resulting in a lower deformation. The evaluation of shape memory behavior involving Rf and Rr are calculated according to the established equations. As shown in Fig. 5a, the soft-rigid alternative PAEK-DF exhibited approving shape memory behavior with good shape fixity ratio (Rf = 98.6%) and acceptable shape recovery ratio (Rr = 83.1%). After crosslinking through photodimerization, these resultant polymers exhibited enhanced shape memory behavior (c-PAEK-DF-10 with Rf of 99.0% and Rr of 87.8%; c-PAEK-DF-20 with Rf of 98.9% and Rr of 100%; c-PAEK-DF-30 with Rf of 98.7% and Rr of 98.9%). The crosslinking structure not only improved the ability of deformation, but also significantly enhanced shape recovery capacity. Overall, c-PAEK-DF-20 film disclosed optimal shape memory behaviors with approving deformation and good shape fixity and recovery. Therefore, the cyclic fixrecover measurement was carried out with c-PAEK-DF-20 film. As shown in Fig. 6a, the c-PAEK-DF-20 film kept high deformation with satisfying Rf value above 98.0%. However, the Rr value decreases gradually during the cyclic fix-recover process. Finally, the Rr value decreased to 75.8% after the four cycles. This decrease of shape recovery could be due to the cleavage of the crosslinking bonds under the long-term high temperature (Tt is about 97 °C.) The detailed and clear cause is still explored in our lab. The shape memory process of c-PAEKDF-20 film was recorded using photographs in Fig. 6. Firstly, c-PAEKDF-20 film was deformed into a temporary shape with an external force at Tt and then fixed at room temperature. It could recover to its original shape completely at Tt. In addition, c-PAEK-DF-20 film can be secondarily deformed and recovered to permanent shape, which indicated good repeatability of shape memory behavior.
applications of smart mechatronics, multistimuli-responsive mechanical energy conversion devices, and so on. Author statement We assure you that the results in this manuscript are original and have not been disclosed elsewhere. Acknowledgements We graciously acknowledge the National Natural Science Foundation of China for funding (grant No. 21774046) and Jilin Provincial Science and Technology Department, China (Grant No. 20190201075JC). Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. References [1] J. Flanagan, R. Strutzenberg, R. Myers, J. Rodrian, Development and flight testing of a morphing aircraft, the NextGen MFX-1, Aerospace. Eng. (2007) 1–3. [2] C. Yakacki, R. Shandas, C. Lanning, B. Rech, A. Eckstein, K. Gall, Unconstrained recovery characterization of shape-memory polymer networks for cardiovascular applications, Biomaterials 28 (2007) 2255–2263. [3] J. Hu, Adaptive and functional shape memory polymers, textiles and their applications, Imperial College Press Publisher, London England, 2011, p. 392. [4] A. Lendlein, R. Langer, Biodegradable, elastic shape-memory polymers for potential biomedical applications, Science 296 (2002) 1673–1676. [5] M. Kohl, D. Brugger, M. Ohtsuka, T. Takagi, Adaptive and functional shape memory polymers, textiles and their applications, Sens. Actuat., A 114 (2004) 445–450. [6] J. Kunzelman, T. Chung, P. Mather, C. Weder, Shape memory polymers with builtin threshold temperature sensors, J. Mater. Chem. 18 (2008) 1082–1086. [7] Z. Mohadeseh, P. Molamma, P. Nader, R. Seeram, Thermally-induced two-way shape memory polymers: mechanisms, structures, and applications, Chem. Eng. J. 374 (2019) 706–720. [8] Binjie Jin, Huijie Song, Ruiqi Jiang, Jizhou Song, Qian Zhao, Tao Xie, Programming a crystalline shape memory polymer network with thermo- and photo-reversible bonds toward a single-component soft robot, Sci. Adv. 4 (1) (2018) eaao3865, https://doi.org/10.1126/sciadv.aao3865. [9] T. Xie, I.A. Rousseau, Facile tailoring of thermal transition temperature of epoxy shape memory polymers, Polymer 50 (2009) 1852–1856. [10] Z. Lan, X. Chen, X. Zhang, C. Zhu, Y. Yu, J. Wei, Transparent, high glass-transition temperature, shape memory hybrid polyimides based on polyhedral oligomeric silsesquioxane, Polymers 11 (2019) 1058. [11] V. Anand, K. Vimal, S. Varughese, Asme, Solvent induced shape memory behavior of Sulfonated Poly Ether Ether Ketone (SPEEK), Proceedings of the Asme Conference on Smart Materials, Adaptive Structures and Intelligent Systems, Vol. 1, Amer. Soc. Mechanical Engineers: New York, 2012, pp. 27–34. [12] X. Fu, Y. Yuan, Z. Liu, P. Yan, C. Zhou, J. Lei, Thermoplastic shape memory polymers with tailor-made trigger temperature, Eur. Polym. J. 93 (2017) 307–313. [13] R. Zhao, T. Zhao, X. Jiang, X. Liu, D. Shi, C. Liu, S. Yang, E. Chen, Thermoplastic high strain multishape memory polymer: side-chain polynorbornene with columnar
4. Conclusion In this work, we have introduced photocrosslinkable anthracene groups in soft-rigid alternative poly(aryl ether ketone)s though postpolymerization functionalization. Then the intermolecular photodimerization of pendant anthracene groups was carried out under the UV radiation, resulting in a dynamic crosslinked network polymeric architecture. The resulting crosslinked polymers exhibited good thermostability and improved mechanical properties. Enhanced shape memory performance with satisfying shape fixity ratio and shape recovery ratio was disclosed for those polymer films, attributed to their networked molecular structure. Considering the reversibility of photocrosslink reaction of anthracene groups, this dynamic crosslinking shape memory polymers would play an important role in intelligent 6
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