High-Temperature Triple-Shape Memory Polymer with Full Recovery

Herein, without using solvent and high curing temperature (>300 °C), a new kind .... In addition, compared with the 1H NMR spectra of 3-EA and EP, th...
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High-Temperature Triple-Shape Memory Polymer with Full Recovery through Cross-Linking All-Aromatic Liquid Crystalline Poly(ester imide) under Reduced Molding Temperature Zhenjie Ding, Li Yuan, Ting Huang, Guozheng Liang,* and Aijuan Gu*

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State and Local Joint Engineering Laboratory for Novel Functional Polymeric Materials Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application Department of Materials Science and Engineering College of Chemistry, Chemical Engineering and Materials Science Soochow University, Suzhou, 215123, P. R. China S Supporting Information *

ABSTRACT: High thermal transition temperature, good processing feature, and full recovery of two temporary shapes are three necessary properties of triple-shape memory polymers (Tri-SMPs) for cutting-edge fields such as aerospace deployable structures, high temperature sensors, and actuators; however, it is still an interesting project with great challenges to prepare these functional polymers. Herein, without using solvent and high curing temperature (>300 °C), a new kind of thermally resistant Tri-SMP (LCPEI-EPEA) with two distinct glass transition temperatures (Tg = 93 and 218 °C) and 100% shape recovery ratios of two recovery processes was developed, which was prepared through building cross-linked networks based on acetylene terminated all-aromatic liquid crystalline poly(ester imide) (LCPEI) and reactive epoxy oligomer with acetylene side groups (EPEA). The relationship between cross-linked structures and integrated properties is intensively discussed. Unique copolymer molecular structures and chemical cross-linked points endow LCPEIEPEA with excellent triple-shape memory performance. The obvious advantages of high thermal transition temperature and good processing property make the preparation of LCPEI-EPEA highly competitive with that of other advanced polymers with triple-shape memory effects.

1. INTRODUCTION As one of several deformable smart materials, shape memory polymers (SMPs) are capable of memorizing temporary shapes and recovering to their permanent shapes upon external stimulations,1−5 and thus becoming key functional materials in many fields such as biomedical materials,6−10 electronics,11,12 and intelligent fibers.13,14 However, the fast development of some cutting-edge fields such as aerospace (self-deployable structures,15,16 satellites, solar arrays, and antennas17) as well as high-temperature sensors and actuators18,19 demand unique high-performance SMPs. High-performance SMPs not only show multishape memory effects, but also have high switching temperatures (one thermal transition temperature is higher than 200 °C) and good processing performances (the molding temperature is lower than 250 °C and the process uses no solvent). More and more attention is being paid to triple-shape memory polymers (Tri-SMPs). These polymers can recover their permanent shapes step-by-step from two preprogrammed temporary shapes under external stimuli, thus increasing the degree of intelligence of the SMPs by providing an additional dimension to the technical potential of SMPs.20−26 The shape memory transition temperature (Ttrans), which may be the glass transition temperature (Tg) or melting temperature (Tm), is © XXXX American Chemical Society

one of the most critical parameters for thermal-triggered TriSMP.27 Current strategies for obtaining Tri-SMPs include preparing polymers with a broad Ttrans or incorporating two different thermal transitions into polymers by copolymerizing, blending, or constructing multilayer structures.20,28−31 Up to now, some Tri-SMPs which have trigger temperatures that exceed 200 °C were reported.32−36 However, they have poor processing characteristics such as very high molding temperatures (>300 °C)35,36 and/or requiring harmful organic solvents with high boiling points (for example, N-methyl pyrrolidone).32−34 On the other hand, these Tri-SMPs exhibit poor shape recovery ratios (77−93%) because their physical cross-linked points allow polymer chains to slip.32−34 Therefore, developing high-temperature Tri-SMP with full recovery and a good processing property is still an interesting challenge. All-aromatic liquid crystalline poly(ester imide)s have unique structures and are attractive because of their easy processing as well as excellent mechanical, thermal, and chemical properties.37−40 Up-to-date only one paper reported Received: Revised: Accepted: Published: A

February 1, 2019 March 17, 2019 April 30, 2019 April 30, 2019 DOI: 10.1021/acs.iecr.9b00662 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Scheme 1. Structures of LCPEI (a) and EPEA (b)

washed thoroughly using methanol and hot water for 3 times, followed by filtration and drying at 100 °C under vacuum for 24 h, successively. The resultant product was acetyleneterminated all-aromatic liquid crystalline poly(ester imide), coded as LCPEI (8.38 g, yield: 93%) (Scheme 1a). The characteristic signals of acetylene groups were observed at 3.1− 3.2 ppm in 1H NMR spectrum and 81 ppm in 13C NMR spectrum, and the number molecular weight (Mn) of LCPEI was 4925 g mol−1.41 EP (10 g, 26 mmol) and 3-EA (3.65 g, 31.2 mmol) were added into a round bottomed flask. The reaction was conducted at 80 °C under nitrogen for 6 h. After that, the reaction liquid was poured out for cooling to get a light yellow transparent solid, which was the reactive epoxy oligomer with acetylene and hydroxyl groups, coded as EPEA (Mn = 1687 g mol−1) (Scheme 1b). LCPEI and EPEA were ground into fine powders using a mortar and pestle. LCPEI and EPEA powders with a mass ratio of 1:1 were placed between two metal plates covered by Kapton films, and then consolidated at 250 °C for 15 min with a pressure of 20 MPa in a preheated hot press. After that, the temperature was cooled down to 30 °C, and the obtained film was cured with the temperature program of 230 °C/3 h + 250 °C/3 h + 270 °C/3 h in an oven to get a cross-linked film, coded as LCPEI-EPEA. 2.3. Characterization. Fourier transform infrared (FTIR) spectra were recorded at room temperature on a Bruker Vertex 70 spectrometer (USA) covering a spectral range of 600−4000 cm−1 using 16 scans with a resolution of 4 cm−1. Gel permeation chromatography (GPC) was conducted on a Waters 1525 instrument (USA) with a refractometric detector. N,N-Dimethylformamide was employed as the eluent at a flow rate of 1 mL min−1, and the calibration was done with monodisperse polystyrene. 1 H NMR and 13C NMR spectra were obtained using a nuclear magnetic resonance spectrometer (Bruker Avance 300, Germany). DMSO-d6 was used as the solvent. The signals of DMSO-d6 in 1H NMR and 13C NMR spectra appear at δ 2.50 ppm and δ 39.5 ppm, respectively. Differential scanning calorimetry (DSC) (Q200 TA Instrument, USA) was used to investigate thermal behaviors. Each sample was heated from 50 to 350 °C under a nitrogen atmosphere at a heating rate of 10 °C min−1. Thermotropic liquid crystalline transition behaviors were observed using a polarizing microscope (POM) equipped with

a Tri-SMP based on all-aromatic liquid crystalline poly(ester imide);35 however, its processing temperature was as high as 350 °C. Additionally, its triple-shape memory behavior was limited to torsion deformation and the fixing ratio of the first temporary shape was ignored. A new high-temperature Tri-SMP (LCPEI-EPEA) was developed by building cross-linked networks with acetylene terminated all-aromatic liquid crystalline poly(ester imide) (LCPEI) and reactive epoxy oligomer with acetylene side groups (EPEA). The integrated performances of LCPEI-EPEA, including dynamic thermomechanical performances, thermal properties, melting transition (thermotropic liquid crystalline transition) behaviors, and dual-shape and triple-shape memory properties were intensively studied, and the mechanism behind the network was revealed.

2. EXPERIMENTAL SECTION 2.1. Raw Materials. Diglycidyl ether of bisphenol A epoxy resin (EP) with an epoxy value of 0.51 mol/100 g was purchased from Nantong Xingchen Synthetic Material Co., Ltd., China. 3-Ethynylaniline (3-EA) with a purity of 98% was obtained from Beijing Yinuokai Technology Co., Ltd., China. p-Hydroxybenzoic acid (HBA), 6-hydroxy-2-naphthoic acid (HNA), and diphenyl phosphoryl chloride (DPCP) were bought from Shanghai Aladdin Biochemical Technology Co., Ltd., China. Pyridine and methanol were obtained from Jiangsu Qiangsheng Functional Chemical Co. Ltd., China, and used without further purification. Anhydrous lithium chloride (LiCl) was bought from J&K Scientific Ltd., China, and dried under vacuum at 150 °C for 15 h prior to use. 2-(3Ethynylphenyl)-1,3-dioxoisoindoline-5-carboxylic acid and N(3′-hydroxyphenyl)-trimellitimide were synthesized in our laboratory.41 2.2. Preparation of LCPEI-EPEA Film. DPCP (17.2 g) and LiCl (2 g) were dissolved in pyridine (100 mL) with stirring at 25 °C for 30 min to obtain a clear solution A. 3-EA (0.20 g, 1.7 mmol), HBA (3.52 g, 25.5 mmol), HNA (2.54 g, 13.5 mmol), N-(3′-hydroxyphenyl)-trimellitimide (3.12 g, 11 mmol), and 2-(3-ethynylphenyl)-1,3-dioxoisoindoline-5-carboxylic acid (0.50 g, 1.7 mmol) were added into pyridine (50 mL) to get a mixture, which was heated and held at 120 °C for 5 min to get a clear solution B, into which solution A was added dropwise within 1 h. The obtained suspension was refluxed for 3 h at 120 °C. After that, white polymer powders were filtered from pyridine and mixed with 200 mL of methanol. The obtained precipitate was then B

DOI: 10.1021/acs.iecr.9b00662 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research a hot stage at a heating rate of 50 °C min−1 under an air atmosphere. Dynamic thermomechanical analyses (DMA) were performed on a TA Q800 instrument (USA) in “multi-frequencystrain” mode with a frequency of 1 Hz and a heating rate of 3 °C min−1 using a film clamp. The dimensions of specimens were (20 ± 0.10) mm × (5 ± 0.10) mm × (0.15 ± 0.05) mm. Shape memory properties of all samples were evaluated on a TA Q800 instrument (USA) using a tensile film clamp and the strain rate mode with a preload force of 0.001 N to keep the sample taut. The dimensions of specimens were (20 ± 0.10) mm × (5 ± 0.10) mm × (0.15 ± 0.05) mm. Dual-shape memory procedures were conducted as follows. A film sample was first heated from room temperature to programming temperature (Tprog = 173 or 248 °C) with a heating rate of 10 °C min−1. After the sample equilibrated at Tprog, it was stretched isothermally to 8% using a certain force. Once the strain equilibrated, the sample was cooled rapidly under load to 0 °C to fix the temporary shape, and the force was then removed. The sample was reheated to Tprog at a heating rate of 10 °C min−1 without force, and was maintained isothermally at Tprog for 30 min to allow the strain to recover. The shape fixing ratio (Rf) and shape recovery ratio (Rr) of the dual-shape memory behavior were calculated using eqs 1 and 2, respectively:42 ε R f = unload × 100% εload (1) Rr =

εunload − εrec × 100% εunload

Figure 1. FTIR spectra of 3-EA, EP, and products with different reaction time.

while the characteristic absorption of the epoxy group is observed at 914 cm−1 in the spectrum of EP. These peaks disappear with the progress of reaction; moreover, a peak reflecting the stretching vibration of N−H appears at 3377 cm−1, confirming that imine has been formed and the epoxy group has fully reacted with amine to produce the linear oligomer.43 From the GPC result (Figure 2), the Mn of EPEA

(2)

where εload represents the maximum strain under load, εrec is the recovered strain, and εunload is the fixed strain after cooling and restriction removal. Triple-shape memory procedures were carried out using a tensile mode and the strain rate mode of DMA. First, the film sample with permanent shape (S0) was stretched at Tprog2 (248 °C) under a certain stress and fixed at Tprog1 (173 °C) to yield the first temporary shape (S1). Second, the film sample was further stretched at Tprog1 under a high stress and fixed at 0 °C to yield the second temporary shape (S2). When the sample was reheated to Tprog1, the first temporary shape (S1rec) was obtained. When the sample was further heated to Tprog2, the shape of the film sample recovered to the initial shape (S0rec). The Rf and Rr of the triple-shape memory behavior were calculated using eq 3 and eq 4, respectively:33 εy − εx × 100% R f(x → y) = εy ,load − εx (3) R r(y → x) =

εy − εx ,rec εy − εx

Figure 2. GPC curve of EPEA.

is found to be 1687 g mol−1. In addition, compared with the 1 H NMR spectra of 3-EA and EP, the 1H NMR spectrum of EPEA shows signals reflecting protons of acetylene but no signals for protons of epoxy groups (Figure S1). The 13C NMR spectrum of EPEA also confirms that EPEA has been synthesized successfully (Figure S2). Figure 3a shows DSC curves of uncured EPEA, LCPEI, and LCPEI/EPEA blend as well as cured LCPEI-EPEA. The curve of EPEA displays an endothermic peak at 60 °C, and the curve of LCPEI displays a board endothermic peak at 180−220 °C, corresponding to melting transition of EPEA and LCPEI. Each curve of EPEA, LCPEI, and LCPEI/EPEA shows a strong exothermic peak ranging from 230 to 320 °C due to the polymerization of acetylene. Whereas, the curve of cured LCPEI-EPEA film does not show an exothermic peak, indicating that the reaction between LCPEI and EPEA has completely processed.44 In addition, this statement can be further confirmed by comparing FTIR spectra of EPEA, LCPEI, and LCPEI-EPEA. Specifically, as shown in Figure 3b, in both spectra of LCPEI and EPEA, there is an asymmetric stretching vibration peak reflecting acetylenic C−H bond (3284 cm−1), which is not

× 100% (4)

where x and y denote two different shapes, εy,load is the maximum strain after applying the load, εy and εx are fixed strains after cooling to low temperature and unloading, respectively, and εx,rec is the strain after recovery.

3. RESULTS AND DISCUSSION 3.1. Structures of LCPEI-EPEA. FTIR spectra of EP, 3-EA, and products with different reaction times are shown in Figure 1. The stretching vibration of the N−H of the primary amine is found at around 3357 and 3436 cm−1 in the spectrum of 3-EA, C

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Figure 3. DSC curves of EPEA, LCPEI, LCPEI/EPEA blend, and LCPEI-EPEA (a); FTIR spectra of EPEA, LCPEI, and LCPEI-EPEA (b).

Figure 4. Schematic cross-linking mechanism of acetylene groups.

Figure 5. Hot-stage polarized micrographs of LCPEI/EPEA blend at different curing stages with a heating rate of 50 °C min−1: (a) 60 °C; (b) 200 °C; (c) 200 °C/1 h; (d) 200 °C/1 h + 230 °C/1 h; (e) 200 °C/1 h + 230 °C/1 h + 270 °C/1 h; (f) 200 °C/1 h + 230 °C/1 h + 270 °C/1 h + 300 °C/1 h; (g) ramping sample to 350 °C; (h) ramping sample to 400 °C; scale bar, 50 μm.

found in the spectrum of LCPEI-EPEA film, indicating that the curing process is sufficient for accomplishing the curing reaction, and the obtained cross-linked networks are formed through the reaction of acetylene groups.45 Figure 4 shows the cross-linked mechanism of reactive acetylene groups between LCPEI and EPEA, which belongs to copolymerization.44 Conjugated polyenes and benzene-ring structures are formed due to the reaction of acetenyl

groups,44,46 which provide permanent chemical cross-linked points of LCPEI-EPEA (Figure 4). 3.2. Melting Transition (Thermotropic Liquid Crystalline Phase Transition) Behaviors of LCPEI/EPEA Blend during Curing Process. Melting transition behaviors are accompanied by liquid crystal phase transition behaviors of thermotropic liquid crystalline polymers. A blend of LCPEI and EPEA, LCPEI/EPEA, was investigated under a hot-stage polarizing microscope. The photographs of LCPEI/EPEA at D

DOI: 10.1021/acs.iecr.9b00662 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 6. Tan δ−temperature plots and fitting curves of cured EPEA (a), cured LCPEI (b), and LCPEI-EPEA film (c).

different curing stages are shown in Figure 5. At 60 °C, LCPEI/EPEA is yellow solid and shows a certain birefringence property (Figure 5a); when LCPEI/EPEA is heated to 200 °C, the brightness of polarized field increases and a liquid crystalline (nematic) phase transition behavior appears (Figure 5b); as the temperature increases, the nematic texture becomes more obvious. This typical nematic texture is maintained at 230 °C for 1 h (Figure 5c,d). The melting temperature is about 180−220 °C (Figure 3a), which indicates that LCPEI/EPEA has a board molding temperature window (about 3 h) at 200− 250 °C. With continuous heating to 270 °C, thermal copolymerization between acetenyl groups of LCPEI/EPEA occurs (Figure 5e,f) which is further maintained at 300 °C for 1 h. At this temperature, the cross-linked network gradually forms, and almost no change is found even when the temperature reaches 400 °C (Figure 5g,h), suggesting that the liquid crystalline phase of LCPEI is fixed in the cross-linked network during the heating process. 3.3. Dynamic Thermomechanical Properties of LCPEIEPEA. For a cross-linked polymer, Tg is often regarded as the peak temperature of tan δ−temperature curve from DMA tests.47,48 The tan δ−temperature plots and corresponding fitting curves of cured LCPEI-EPEA film, cured LCPEI, and cured EPEA are shown in Figure 6. Cured EPEA shows a symmetry tan δ peak centered at 202 °C (Figure 6a), while cured LCPEI has a broad peak that can be divided into three symmetry peaks centered at 99, 171, and 196 °C (Figure 6b). This occurs because LCPEI is a block copolymer, and different parts of the molecular chains have the ability to move. The cross-linked network of LCPEI-EPEA film contains structures of both EPEA and LCPEI as well as the copolymer between EPEA and LCPEI, so it is not surprising to observe that the plot of LCPEI-EPEA film is broad and can be regarded as a combined curve consisting of three fitting curves. Each of them shows a symmetry peak centered at 89, 145, and 223 °C (Figure 6c), belonging to relatively flexible polyester segments in LCPEI (peak 1), aromatic imide segments and liquid crystalline segments (peak 2), rigid cross-linked backbone segments (peak 3), respectively. The tan δ−temperature curve of LCPEI-EPEA film (Figure 6c) shows that there are two well separated thermal transitions with peaks centered at 93 °C (Tg1) and 218 °C (Tg2). These peaks can be assigned to contributions of aromatic linear copolymer segments (peak 1, peak 2) of LCPEI and the rigid cross-linked backbone segments (peak 3) of conjugated polyenes and benzene-ring structures in cross-linked LCPEI-EPEA, respectively. The temperature gap between the lowest and highest Tg values is up to 125 °C, such a broad temperature window is superior to those of other high-temperature Tri-SMPs reported

(Table S1),32−36 which is an important progress and attractive feature for practical applications. These excellent performances are attributed to rigid cross-linked backbone segments (conjugated polyenes and benzene ring structures) of the cross-linked networks and the all-aromatic linear copolymer segments of LCPEI. From DMA tests, storage modulus−temperature plots are also obtained. As shown in Figure S3, LCPEI-EPEA shows two distinct drops of storage modulus during their respective glass transition region (Tg1 and Tg2), the storage modulus of LCPEIEPEA at 30 °C, 173 and 248 °C is 3789, 288, and 17.2 MPa, respectively, indicating that LCPEI-EPEA possesses good shape memory behaviors.49 3.4. Dual-Shape and Triple-Shape Memory Properties of LCPEI-EPEA. When the temperature is higher than Tg, molecular segments become movable and thus act as molecular switches that are responsible for temporary shape fixation, while cross-linked points acting as hard segments are responsible for permanent shape recovery, so dual-shape memory behaviors are tested at the temperature within the glass transition region. Figure 7 and Figure 8 severally show

Figure 7. Dual-shape memory behavior of LCPEI-EPEA at Tprog1 (173 °C) obtained from DMA tests.

quantitative demonstration of dual-shape memory behaviors tested by DMA at two programming temperatures (Tprog1 = 173 °C, and Tprog2 = 248 °C). A dual-shape memory curve is divided into two parts, they are temporary shape fixing and permanent shape recovery. Both Rf1 and Rr1 (calculated from dual-shape memory cycle with programming temperature at Tprog1) are almost 100%, whereas Rf2 and Rr2 (calculated from dual-shape memory cycle with programming temperature at Tprog2) are 100% and 66%, respectively, meaning that the excellent fixing ability of the temporary shape is maintained. E

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(Table S1). The fixability of the first temporary shape is affected by the second heating process to a higher temperature, and the first temporary shape shows a trend to recover to the original shape, so Rf(S0→S1) is relatively lower than Rf(S1→S2). The recovery process of the permanent shape is driven by the entropy elasticity of rereactive molecular segments with mobility above Ttrans.50 High Rr is attributed to the stable cross-linked network and high entropy elasticity. Chemical cross-linked points guarantee the stability of permanent shape, while aromatic linear copolymer segments and rigid crosslinked backbone segments endow LCPEI-EPEA with higher entropy elasticity. Therefore, LCPEI-EPEA possesses outstanding triple-shape memory behavior and shows great potential applications in multilevel deformation intelligent structures. The triple-shape memory behaviors are visually illustrated in Figure 10 and Video S1. The straight film sample (permanent

Figure 8. Dual-shape memory behavior of LCPEI-EPEA at Tprog2 (248 °C) obtained from DMA tests.

The somewhat reduced ability to recover the permanent shape at Tprog2 is attributed to conjugated polyenes and benzene ring structures that constrain the mobility of rigid cross-linked backbone segments at a high temperature (Tprog2 > 200 °C), resulting in a relatively low recovery ratio. Previous research suggests that a polymer with two wellseparated Ttrans probably exhibited triple-shape memory behavior. 20,21 DMA curves of LCPEI-EPEA reflecting double-stage fixing and recovery at two programming temperatures (Tprog1 = 173 °C, Tprog2 = 248 °C) are provided in Figure 9. The triple-shape memory behavior of LCPEI-EPEA is

Figure 10. Whole process reflecting triple-shape memory behaviors of LCPEI-EPEA (Scale bar: 1 cm).

shape A) is deformed into “L” shape (temporary shape B) at 248 °C, and then the “L” shape sample is further deformed into “U” shape (temporary shape C) at 173 °C. The sample is then recovered from “U” to “L” and straight shape through heating to 173 and 248 °C, successively. The whole process reflects a complete triple-shape memory behavior (shape C → shape B → shape A). The molecular mechanism about triple-shape memory behavior of LCPEI-EPEA is shown in Figure 11. For a polymer to display shape memory effect, it needs a cross-linked network for setting the permanent shape as well as a reversible thermal transition (or shape memory transition) for temporary shape fixing and recovery.42 Besides, the recovery of Tri-SMPs is mainly dependent on reversible entropy elasticity of polymer chains.27 LCPEI-EPEA has two thermal transition-phases: they are a low-temperature transition phase belonging to aromatic linear copolymer segments (Tg1 = 93 °C) and a hightemperature transition phase belonging to rigid cross-linked backbone segments (Tg2 = 218 °C), responsible for fixing and recovering two temporary shapes, respectively. The molecular chains of the sample with shape A have the highest entropy, so the molecular chains of both linear copolymer segments and rigid backbone segments exhibit a thermodynamic stable state. When a sample is heated to the temperature above Tg2 (Tprog2 > Tg2), the mobility of both linear copolymer segments

Figure 9. Triple-shape memory behavior of LCPEI-EPEA obtained from DMA.

measured using an appropriate programming step, that is, fixing two temporary shapes at Tprog2 and Tprog1, respectively, followed by two recovery processes at Tprog1 and Tprog2, successively. First, a rectangular film with its original shape (S0) is uniaxially stretched to a strain of 15% at Tprog2 (248 °C). This is followed by cooling to Tprog1 (173 °C), followed by removal of the external force to get the first fixed temporary shape (S1). The second stretching is performed at Tprog1 (173 °C) under a higher stress to obtain the second temporary shape (S2) with an additional strain of 6%; the sample is cooled to 0 °C and then the external force is removed. After that, the temperature is heated to Tprog1, and the sample shape changes from S1 to S0. From eqs 3 and 4, the first and second shape fixed ratios of LCPEI-EPEA, Rf(S0→S1) and Rf(S1→S2), are calculated to be 86% and 94%, respectively, values that are similar to those of reported high-temperature Tri-SMPs (Table S1). Interestingly, both shape recoveries at Tprog1 (173 °C) and Tprog2 (248 °C) of LCPEI-EPEA are as high as 100%, the highest values of high-temperature Tri-SMPs reported so far F

DOI: 10.1021/acs.iecr.9b00662 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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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.iecr.9b00662. 1 H and 13C NMR spectra; storage modulus−temperature and stress−strain curves; summary of thermal properties and shape memory properties of triple-shape memory polymers with high transition temperatures (PDF) Triple-shape memory behaviors of LCPEI-EPEA (MPG)

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86 512 65880967. Fax: +86 512 65880089. E-mail: [email protected] (G. Z. Liang). *Tel.: +86 512 65880967. Fax: +86 512 65880089. E-mail: [email protected] (A. J. Gu).

Figure 11. Molecular mechanism of triple-shape memory effect for LCPEI-EPEA.

ORCID

Zhenjie Ding: 0000-0002-2247-785X Li Yuan: 0000-0003-2059-5235 Ting Huang: 0000-0001-9450-6229 Guozheng Liang: 0000-0001-9690-7931 Aijuan Gu: 0000-0002-2235-1018

and rigid backbone segments is significantly activated. When an external deformation load is applied, the chain conformations are changed, leading to a lower entropy state and macroscopic shape changes (step 1). When the sample is cooled to Tprog1 (Tg1 < Tprog1 < Tg2), the temperature at which the rigid backbone segments are frozen, they cannot recover to the state with high conformational entropy, and shape B is fixed (step 2), whereas at Tprog1, linear copolymer segments are still active and their conformations can be changed by applying an external force (step 3). After that, cooling the sample to the temperature below Tg1 and removing external stress, kinetically traps the lower entropy state due to the frozen linear copolymer segments, resulting in the macroscopic shape fixation for the second temporary shape (shape C) (step 4). When the temperature is increased to a value above Tg1 but below Tg2 under a stress-free condition, the mobility of the linear copolymer segments is reactivated but the rigid backbone segments are still frozen. The molecular chains are allowed to return to their higher entropy state and recover to shape B (step 5). When the temperature is further increased to a value above Tg2, the mobility of rigid backbone segments is reactivated and allows the molecular chains to return to original highest entropy state, and shape A is obtained (step 6).

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was supported by National Natural Science Foundation of China (Grant No. 51873135), Key Major Program of Natural Science Fundamental Research Project of Jiangsu Colleges and Universities (18KJA430013), Priority Academic Program Development of Jiangsu Higher Education Institution (PAPD), and Postgraduate Research & Practice Innovation Program of Jiangsu Province (SJKY19_2279).

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REFERENCES

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4. CONCLUSIONS A novel cross-linked triple-shape memory polymer (LCPEIEPEA) with high Ttrans (above 200 °C), 100% recovery of two temporary shapes, and good processing property (a board molding temperature window about 3 h at 200−250 °C) has been developed through cross-linking reactive LCPEI and EPEA with acetylene groups. LCPEI-EPEA possesses good dual-shape and triple-shape memory properties. The high Ttrans of LCPEI-EPEA is attributed to a cross-linked network with aromatic copolymer linear main chains as well as cross-linked points formed by rigid conjugated polyenes and benzene ring structures. Excellent shape memory performances not only result from stable cross-linked networks and two reversible thermal transformation phases, but also from the relatively good mobility of the linear copolymer main chain compared to the rigid cross-linked backbone, endowing LCPEI-EPEI with outstanding deformation capacity. G

DOI: 10.1021/acs.iecr.9b00662 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.iecr.9b00662 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX