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High-Temperature Shape Memory Behavior of Novel All-Aromatic (AB)n‑Multiblock Copoly(ester imide)s Qingbao Guan,† Stephen J. Picken,‡ Sergei S. Sheiko,§ and Theo J. Dingemans*,† †

Faculty of Aerospace Engineering, Delft University of Technology, Kluyverweg 1, 2629 HS Delft, The Netherlands Faculty of Applied Sciences, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands § Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290, United States ‡

ABSTRACT: Thermoplastic and thermoset all-aromatic liquid crystal (LC) (AB)n-multiblock copoly(ester imide)s based on N-(3′-hydroxyphenyl)trimellitimide (IM), 4-hydroxybenzoic acid (HBA), and 6-hydroxy-2-naphthoic acid (HNA) were investigated as single-component high-temperature (≥250 °C) shape memory polymers (SMPs). A high Tg (∼200 °C) HBA/IM block embedded in a low Tg (∼120 °C) HBA/HNA matrix creates a stable rubbery plateau that can be extended to ∼240 °C by cross-linking. The shape fixation (Rf) and shape recovery efficiency (Rr) of the thermoplastic and thermoset films were investigated using a rheometer in torsion mode. Thermoplastic LC copoly(ester imide) films showed excellent dual SM behavior (Rf and Rr ∼ 100%) at 170 °C. After cross-linking the thermoplastic films a single component system as obtained that exhibited high-temperature (≥250 °C) tunable triple SM and one-way reversible SM behavior.

1. INTRODUCTION Shape memory polymers (SMPs) are stimuli-responsive materials with the ability to undergo well-defined, programmable shape alterations upon the application of an external stimulus such as heat, light, solvent, electrical, and magnetic fields.1−4 So far, thermally responsive SMPs are the most extensively studied systems.5−9 The shape-shifting temperature is usually equal to the glass transition temperature (Tg) for an amorphous SMP or melting point (Tm) for a semicrystalline SMP. The development of thermally responsive SMPs has primarily been focused on relatively low to medium switching temperatures (100 °C). In recent years, there have been several reports on semicrystalline and amorphous high-temperature SMPs based on polyimides, poly(ether ether ketone)s, and cyanate esters.14−18 We note that most high-temperature SMPs are composed of multiple components (e.g., graphene/polyimide or SiC/epoxy) because the switching temperature of the neat polymer is relatively low. A recently reported amorphous polyimide (Figure 1) exhibits the highest shape-shifting temperature of 320 °C reported to date for a single-component © XXXX American Chemical Society

Figure 1. Structure of a high-temperature (shape-shifting temperature = 320 °C) dual SM polyimide reported by Xiao et al.17

system. In this case, the chain entanglements act as physical cross-links and combined with chemical cross-linking, excellent SMP properties with a shape fixation (Rf) of 99%, and shape recovery efficiency (Rr) of 98% were achieved.17 However, this single transition material does not allow for programming of multiple shapes. Multiple shape memory effect can remember one or more shapes in addition to its original permanent shape. SMPs with multiple shape memory effect have been developed because of the significant applications such as packaging and robot. Various multiple-component systems such as polymer blends or polymer composites were designed to enable triple-shape Received: March 16, 2017 Revised: May 7, 2017

A

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Figure 2. (A) Backbone composition of the thermoplastic (AB)n-multiblock copoly(ester imide) (22IM) used for this study and (B) schematic representation of the cross-linked (AB)n-multiblock copoly(ester imide) (22IM-5K). The reactive (cross-linkable) phenylethynyl end-groups are placed at both chain-ends of the oligomer (Mn = 5000 g mol−1).

memory programming.19−22 Luo and Mather reported a SM composite, which involves an interpenetrating crystallizable thermoplastic poly(ε-caprolactone) microfiber mat, i.e., the “switching phase”, with an elastomeric silicon rubber matrix. This SM composite shows Rf and Rr values in excess of 97%, but the maximum temperature for triple SM behavior is only 80 °C.21 Tuning triple SME for multicomponent systems requires a change in material composition. It is worthy to note that there are several reports on triple SM behavior in single-component conventional thermoplastic elastomers that possess broad thermal transition (e.g., glass transition or polymer crystallization), but their highest shape-shifting temperature is limited to 140 °C.7,23−25 This brings up the question, can a hightemperature triple SMP be designed based on a singlecomponent polymer system? Our approach toward high-temperature SMPs makes use of all-aromatic thermotropic liquid crystal polymers (TLCPs). Main-chain TLCPs are a well-known class of high-temperature polymers. They possess excellent properties such as easy processing as well as high tensile strength and modulus, which make them attractive for aerospace, electronics, and automotive applications.26,27 However, to date there has been no report of high-temperature SMPs based on main-chain TLCPs. This is mainly attributed to a lack of a useful physical or chemical network in TLCPs to ensure that any macroscopic deformation results in entropic extension of the network strands, which in turn provides the necessary driving force for shape recovery.28 It is well-known that most linear TLCPs primarily consist of allaromatic rigid building blocks, which favor linear conformations. The chains are organized in a (local) parallel arrangement

with essentially no or very little chain entanglements.29 The absence of a useful physical (e.g., crystal, chain entanglement) or chemical network might allow the polymer chains to slip or contract, which can affect the shape fixation. Recently, we have introduced a new thermotropic (AB)nmultiblock copoly(ester imide) (labeled 22IM, Figure 2A) based on N-(3′-hydroxyphenyl)trimellitimide (IM), 4-hydroxybenzoic acid (HBA), and 6-hydroxy-2-naphthoic acid (HNA) with a stable intermediate elastic plateau between two distinct Tgs. The low-end Tg of ∼120 °C (Tg,A) corresponds to the HBA/HNA rich A-block, and the high-end Tg of ∼200 °C (Tg,B) corresponds to the HBA/IM rich B-block.30 When reactive phenylethynyl end-groups were introduced, the endgroups end-up exclusively at the high Tg HBA/IM block. Upon cure, the Tg of the HBA/IM block shifts to a higher temperature of ∼240 °C, and a thermoset (Figure 2B) is obtained with a broad elastic plateau between Tg,A and Tg,B and a stable rubbery plateau above Tg,B. The unique thermomechanical properties of these (AB)n-multiblock copolymers provide interesting opportunities for the design of hightemperature single-component SMPs. In this paper, besides the dual SME of the LC poly(ester imide) and cured thermoset will be investigated at elevated temperatures (>200 °C); more sophisticated SMP forms such as triple and one-way reversible SM behavior and kinetics will be explored as well.

2. EXPERIMENTAL SECTION Materials. The synthesis of the all-aromatic LC (AB)n-multiblock copolymer 22IM and reactive 5000 g mol−1 oligomer 22IM-5K (precursor to the thermoset) based on N-(3′-hydroxyphenyl)B

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Figure 3. Schematic illustration of the SM experiments performed using a rheometer in torsion-controlled mode: (A) dual SM, (B) triple SM, and (C) one-way reversible SM. Samples were twisted between 90° and 180°. In this setup samples were exposed to large deformations but experience little strain. trimellitimide (IM), 4-hydroxybenzoic acid (HBA), and 6-hydroxy-2naphthoic acid (HNA) was reported elsewhere.30 The backbone structure of 22IM and 22IM-5K is shown in Figure 2A. An all esterbased reference polymer with a HBA/HNA molar ratio of 0.73/0.27 was synthesized according to an identical procedure and labeled HBA/ HNA. Preparation of Thin Films. Melt pressed thin films were prepared using standard melt pressing techniques. LC poly(ester imide) 22IM powder was placed between two Kapton films and consolidated in a preheated Joos hot press at 320 °C for 20 min using a 5 kN force. The thermoset films were prepared from reactive oligomer 22IM-5K powder under similar conditions but cured at 370 °C for 45 min. Characterization. The thermal and mechanical properties of the all ester-based reference polymer HBA/HNA, (AB)n-multiblock copolymer 22IM, and cured 22IM-5K thermoset films were reported elsewhere.30 The viscoelasticity of the films was measured with a PerkinElmer Diamond dynamic mechanical thermal analyses (DMTA) in tension mode, using thin films (20.0 ± 0.2) mm × (5.0 ± 0.2) mm × (0.25 ± 0.05) mm under a nitrogen atmosphere and at a heating rate of 2 °C min−1. All experiments were performed at a frequency of 1.0 Hz, static tension force of 2000 mN, minimum tension force of 200 mN, tension gain of 1.5, and length amplitude of 5 μm. The minimum recordable storage modulus (E′) was set to 1 × 104 Pa. The shape memory (SM) torsion tests were performed using a Thermofisher Haake MARS III rheometer. The shape fixation (Rf) and recovery efficiencies (Rr) were determined from SM cycles carried out using rectangular thin films with the dimension of (25.0 ± 1.0) mm × (4.0 ± 0.2) mm × (0.25 ± 0.05) mm in controlled torsion mode. All experiments were performed under a nitrogen atmosphere and at a constant strain rate of 0.001% s−1, equivalent to a rotation speed of 0.9° s−1. Torsion Tests. Nematic LCPs are different from amorphous and semicrystalline polymers in that the molecular chains readily align above Tg, which results in plastic (unrecoverable) deformation at high strain.27 In order to circumvent this problem, we have performed shape programming in torsion mode (Figure 3), which allows for monitoring large shape transformations at small strain. Accordingly, from the cyclic SM torsion test Rf and Rr can be calculated using the following equations:

Rf =

Rr =

φf φp

× 100%

φp − φr φp − φr

(1) × 100% (2)

where φp, φf, and φr denote the torsion angles at the programming, fixation, and recovery steps of shape transformation, respectively. Shape Programming and Recovery. Dual SM. The procedure for the dual SM torsion test (Figure 3A) includes the following steps: (1) heating the sample to the programming temperature (Tprog) at 10 °C min−1, twisting the sample to a predetermined angle followed by an isothermal hold at Tprog for 15 min; (2) cooling the sample to 70 °C at 10 °C min−1 followed by removing the stress; (3) heating the sample to the recovery temperature (Tr) at 10 °C min−1. Triple SM. For the triple SM torsion test (Figure 3B): (1) heating the sample to Tprog(A→B) (250 °C) at 10 °C min−1, twisting the sample to a predetermined angle (temporary shape B) followed by an isothermal hold at Tprog(A→B) for 15 min and then cooled to 180 °C to fix the temporary shape B; (1′) a second torsion was then applied to program temporary shape C at Tprog(B→C) (180 °C) and isothermally held at Tprog(B→C) for 15 min; (2) cooling the sample to 70 °C at 10 °C min−1 to fix temporary shape C followed by removing the stress; (3 and 3′) subsequent heating the sample to Tr(C→B) and Tr(B→A) at 10 °C min−1. One-Way Reversible SM. For the one-way reversible SM test (Figure 3C): (1) heating the sample to Tprog(A→B) (250 °C) at 10 °C min−1, twisting the sample forward to 90° (temporary shape B, Δφ = +90°) followed by an isothermal hold at Tprog(A→B) for 15 min, and then cooled to 180 °C to fix the temporary shape B; (1′) twisting the sample back to 0° (temporary shape C, Δφ = −90°) at Tprog(B→C) (180 °C) and isothermally held at Tprog(B→C) for 15 min; (2) cooling the sample to 70 °C at 10 °C min−1 to fix temporary shape C followed by removing the stress; (3 and 3′) subsequent heating the sample to Tr(C→B) and Tr(B→A) at 10 °C min−1.

3. RESULTS AND DISCUSSION Thermomechanical Properties. The general requirement in the design of thermally activated shape memory polymers (SMPs) is to secure a balance between a permanent network allowing for large and reversible strain and a strong temporary C

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Figure 4. DMTA results of the reference polymer (HBA/HNA) and the block copoly(ester imide)s (22IM and 22IM-5K) melt pressed films. (A) Storage moduli (E′) and (B) loss moduli (E″) as a function of temperature. Heating rate 2 °C min−1/nitrogen atmosphere and a frequency of 1 Hz. The Tgs were defined as the maximum of the E″ peak.

Table 1. Thermomechanical Properties of the HBA/HNA Reference Polymer and the Block Copoly(ester imide)s, 22IM, and 22IM-5K after Cross-Linking sample

behaviora

Tg,Ab (°C)

Tg,Bb (°C)

E′ (MPa)b at 25 °C

E′ (MPa) at 180 °C

E′ (MPa) at 300 °C

νc (mol m−3)

HBA/HNA 22IM 22IM-5K

TP TP TS

111 124 126

200 243

5195 6102 10607

306 722 1461

58

4060

TP = thermoplastic; TS = thermoset. bTg and storage modulus (E′) data were obtained from DMTA experiments using melt pressed films. The Tg is defined by the maximum of the loss modulus (E″) peak. Heating rate 2 °C min−1/nitrogen atmosphere and a frequency of 1 Hz. cCross-linking density calculated using ν = E′/3RT, where E′ is the storage modulus at rubbery plateau (T = 300 °C). a

network for shape fixation, which can be released at a predetermined temperature. For programming of triple-shape memory, two temporary networks with distinct rubbery plateaus are required, which is challenging for high-temperature polymeric single-component materials. To accomplish hightemperature single-component programming, we have synthesized two all-aromatic block copoly(ester imide)s: one with thermoplastic behavior (labeled 22IM) and the second polymer is a cross-linked version of the first polymer (labeled 22IM5K).30 Their properties were contrasted with a thermoplastic all ester-based LCP reference (HBA/HNA). The results of dynamic mechanical thermal analysis (DMTA) are summarized in Figure 4 and Table 1. HBA/HNA has a glass transition temperature (Tg) at 111 °C followed by a continuous decrease in modulus until the crystalto-nematic transition at TK−N = 280 °C, where the polymer film fails. The viscoelastic behavior of HBA/HNA suggests that it may exhibit shape memory (SM) behavior by taking vitrification as the dual SMP mechanism.7 However, the switching temperature for HBA/HNA is too low to be useful for high-temperature SM applications. To enhance the switching temperature, we introduced a high Tg (∼200 °C) ester imide-based block (HBA/IM), which can act as a temporary physical network resulting in a well-defined rubbery plateau in a high temperature range. Furthermore, we can introduce chemical cross-linking to secure the second rubbery plateau due to the chemical network at even higher temperatures. We first consider the un-cross-linked system. The (AB)nmultiblock copolymer 22IM consists of HBA/HNA as block A and 4-hydroxybenzoic acid/N-(3′-hydroxyphenyl)trimellitimide (HBA/IM) as block B (Figure 2A). As shown in Figure 4, this polymer shows two distinct Tgs at 124 and 200 °C, assigned to blocks A and B, respectively. Between the two Tgs we observe a well-defined rubbery plateau with E′ ∼ 1 GPa of the temporary network, which allows for programming a dual-shape memory

transition. The programming temperature (Tprog) is set between the two Tgs. Also, an efficient shape recovery process may be anticipated because the E′ of 22IM drops rapidly above Tg,B, which can be the recovery temperature (Tr). In order to introduce cross-linking, a reactive 5000 g mol−1 phenylethynyl-terminated B(AB)n-reactive 22IM oligomer (22IM-5K) was processed into thermoset films with two characteristic glass transition temperatures. The Tg of the HBA/HNA block (Tg,A) remains virtually unchanged, and the Tg of the HBA/IM (Tg,B) increases from 200 to 243 °C. As a result, the rubbery plateau between the two Tgs (E′ of ∼2 GPa) is broadened compared to thermoplastic 22IM. Moreover, a second rubbery plateau (E′ ∼ 60 MPa) above Tg,B can be observed with a cross-linking density (ν) of 4060 mol m−3. This suggests that 22IM-5K may not only exhibit dual SM behavior but possibly also triple SM using physical and chemical crosslinks as the permanent network structure. Dual-Shape Memory Behavior. As highlighted above, the designed polymers allow programming different types of hightemperature SM transformations including (i) conventional dual SM, i.e., a transformation between a temporary and permanent shape; (ii) triple SM, i.e., one-way sequential transformation between two temporary and one permanent shape; and (iii) one-way reversible SM, i.e., triple shape memory, where the second temporary shape is identical to the permanent shape. We first present our findings on the dual SM behavior. Figure 5 compares the SM behavior of 22IM and the reference polymer (HBA/HNA). In both cases, the programming process consists of three steps: (i) deformation (torsion), (ii) isothermal annealing, and (iii) quenching (fixation). After fixation, shape recovery was triggered by increasing the temperature above the programming temperature. Rf and Rr can be calculated according to eqs 1 and 2, and the results are summarized in Table 2. Our reference polymer HBA/HNA exhibits a moderate Rf of 77% and high Rr of 93%. The low fixation ratio is explained by D

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Table 2. Shape Fixation and Recovery Efficiencies of the LC Reference Polymer (HBA/HNA) and the Linear (22IM) and Cross-Linked (22IM-5K) LC Block Copoly(ester imide) Films sample HBA/ HNA 22IM 22IM5K

Tprog (°C)

Tr (°C)

E′ at Tprog (MPa)

E′ at Tr (MPa)

rotation deg (deg)

Rf (%)

Rr (%)

140

220

827

118

90

77

93

170 250

225 285

820 400

50 75

180 120

98 90

100 90

5B). Investigating the recovery kinematics provides information with respect to the kinetics of recovery (Figure 5C). The shape fixation was significantly improved due to the incorporation of rigid HBA/IM-blocks, which formed a strong physical scaffold to hold the temporary shape. Upon cooling below Tg,A, the shape could be fixed by both the low Tg,A and high Tg,B blocks. When heating the 22IM film, the HBA/HNA blocks become mobile as the temperature approaches Tg,A. At this temperature shape recovery starts with a low angular velocity and then reaches a maximum at Tg,B where the rigid HBA/IM-blocks are softening as well. These results show that the shape recovery is triggered by the activation of molecular mobility, which is consistent with a change in viscoelastic properties of the polymers.32 The better SM behavior of 22IM is attributed to its more distinct thermodynamic transitions and the rubbery plateau. Finally, we have looked at the experimental reproducibility. For 22IM we performed five consecutive SM cycles under said conditions and found an identical Rf of 95% and Rr of 100% for each SM cycle (Figure 6).

Figure 5. SM torsion test of 22IM and reference polymer (HBA/ HNA) melt pressed films. (A) HBA/HNA; the degree of rotation = 90°, Tprog = 140 °C, Tr = 220 °C. Torsion larger than 90° for HBA/ HNA would result in unrecoverable strain. (B) 22IM; the degree of rotation = 180°, Tprog = 170 °C, Tr = 225 °C. To compare the viscoelasticity of 22IM and HBA/HNA, the samples were isothermally held at Tprog for 1 h. For practical purposes, the samples of the torsion tests in all other experiments were isothermally held at Tprog for 10−15 min, which did not affect the test results. (C) Angular velocity of shape recovery as a function of temperature during stress-free heating for 22IM and HBA/HNA. Cooling and heating rate 10 °C min−1/ nitrogen atmosphere.

Figure 6. Five consecutive shape memory cycles for 22IM. The degree of rotation = 180°, Tprog = 170 °C, isothermal hold at Tprog for 10 min, Tr = 210 °C. Cooling and heating rate 10 °C min−1/nitrogen atmosphere.

The SM program and recovery temperatures were improved even further by introducing chemical (covalent) cross-linking. In the 22IM-5K thermoset, both the physical and chemical cross-links act as a scaffold and prevent polymer chain slippage. When the 22IM-5K film was programmed at 250 °C (above Tg,B), both HBA/HNA- and HBA/IM-blocks yield under the applied stress field. The high storage modulus at the glassy state gives rise to a large increase in internal stress during cooling, resulting in a relaxation of the torsional angle when the external stress is removed (Figure 7A). Upon heating (the recovery phase), a small portion of the initial shape is recovered at 125 °C, which is driven by the entropy stored in the HBA/HNAblocks whereas the main shape recovery at 230−285 °C (Figure

the lack of a chemical network or a strong physical scaffold (e.g., chain−chain entanglements). The nematic order of HBA/ HNA and vitrification help to hold the temporary shape. However, this is not sufficient to fix the deformation (torsion) as the internal stress increases, followed by a large relaxation of the torsional angle. This was also observed in other systems, which are only based on physical scaffolds.31 In contrast to HBA/HNA, the 22IM torsion test was performed at higher Tprog (170 °C) and a larger rotation (180°). This polymer displayed much higher fixation and recovery ratios, Rf = 98% and Rr = 100%, respectively (Figure E

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Figure 7. SM torsion test of a cured 22IM-5K thermoset film. (A) The degree of rotation = 120°, Tprog = 250 °C, and Tr = 285 °C. (B) Angular velocity of shape recovery as a function of temperature during stress-free heating. Cooling and heating rate 10 °C min−1/nitrogen atmosphere.

7B) is driven by the entropy stored in the cross-linked HBA/ IM-blocks. The 22IM-5K film exhibits good Rf and Rr (≥90%). To the best of our knowledge, this is the first demonstration of a high-temperature LC dual SMP. More importantly, this twostep recovery behavior implies that 22IM-5K can be used to program triple shape memory behavior. Triple Shape Memory Behavior. Figure 3B outlines a protocol for setting up a triple SM torsion test for 22IM-5K, performed with various rotation partitionings (e.g., 30° + 90°, 60° + 60°, and 90° + 30°) at a constant total rotation of 120°. The experimental results, as shown in Figure 8, suggest that 22IM-5K can memorize two metastable shapes in a single shape memory cycle, which means triple shape memory is indeed possible.33 For instance, Figure 8A shows a shape fixation (Rf) of 95%, shape recoveries Rr(C→B) of 94%, and Rr(B→A) of 98% as calculated using eqs 3−534 for the triple SM test with rotation partitionings of 30° + 90°. φ R = f × 100% φC (3) R r(C → B) = R r(B → A) =

φC − φB/rec φC − φB φB − φA/rec φB − φA

Figure 8. Triple SM torsion test using a cross-linked 22IM-5K film with various partitionings. (A) The degree of rotation = 30° + 90°, Tprog(A→B) = 250 °C, Tprog(B→C) = 180 °C, Tr(C→B) = 200 °C, Tr(B→A) = 280 °C. (B) The degree of rotation = 60° + 60°, Tprog(A→B) = 250 °C, Tprog(B→C) = 180 °C, Tr(C→B) = 200 °C, Tr(B→A) = 295 °C. (C) The degree of rotation = 90° + 30°, Tprog(A→B) = 250 °C, Tprog(B→C) = 180 °C, Tr(C→B) = 180 °C, Tr(B→A) = 300 °C. Cooling and heating rate 10 °C min−1/nitrogen atmosphere.

Unlike traditional high-temperature triple SMP composed of multiple components, 22IM-5K is a single-component system; multiple phase switches are possible because of two welldefined rubbery plateaus (see DMTA results, Figure 4). Because of microphase separation, this copolymer forms a dense temporary network, which is used to encode shape B. Then, we introduced cross-links forming a loose permanent network, which is used to encode shape C. This provides controllable and complex shape morphing ability, which is clearly demonstrated by the triple SM torsion tests using different partitioning schemes (Figures 8B and 8C). The triple SM test results are summarized in Table 3. The triple SME of 22IM-5K, therefore, is highly tunable within a broad temperature regime (110−300 °C), as shown in Figure 9A. It is

× 100% (4)

× 100% (5)

where φA is the degree of permanent shape A (0°), φB and φC denote the degree of rotation of the two temporary shapes B and C, respectively, and φA/rec and φB/rec are the degree of recovery. F

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Macromolecules Table 3. Summary of the Triple SM Test Results Using a Cross-Linked 22IM-5K Film partitioning (deg/deg)

Tprog(A→B) (°C)

Tprog(B→C) (°C)

Tr(C→B) (°C)

Tr(B→A) (°C)

rotation deg(A→B) (deg)

rotation deg(B→C) (deg)

Rf (%)

Rr(C→B) (%)

Rr(B→A) (%)

30/90 60/60 90/30

250 250 250

180 180 180

200 200 180

280 295 300

30 60 90

90 60 30

95 88 93

94 100 100

98 89 93

Figure 10. One-way reversible shape memory test of 22IM-5K. The degree of rotation = +90°/−90°, Tprog(A→B) = 250 °C, Tprog(B→C) = 180 °C, Tr(C→B) = 210 °C, Tr(B→A) = 280 °C. Cooling and heating rate 10 °C min−1/nitrogen atmosphere.

when one considers the fact that the low Tg HBA/HNA-block that deformed together with the high Tg HBA/IM-block at 250 °C was still mobile when the system was cooled to 180 °C. At this temperature, a backward rotation was applied to the HBA/ HNA-block, which results in a deformation of the film that is less than the programmed value of 90°. Despite the Rr(C→B) value of 80% our results demonstrate that 22IM-5K could work in a one-way reversible SM system. Note that the reversible shape transformations in this experiment occurred without application of any external mechanical force. Shape transformations were driven by internal stress of the oppositely strained subnetworks of glassy domains formed during the first and second fixation steps. Our results clearly demonstrate that tunable high-temperature SMEs can be designed based on our all-aromatic block copolyesterimide concept; both thermoplastic 22IM and the cross-linked analogue 22IM-5K are useful candidates.

Figure 9. Kinetics of the shape recovery as a function of temperature during stress-free heating for triple SM torsion tests of 22IM-5K. (A) Absolute angular velocity of shape recovery and (B) relative shape recovery rate.

interesting to note that the absolute angular velocity (Va = dφ/ dt) of each triple SM test cycle is proportional to the programmed rotation. This can be confirmed by the near identical relative recovery rate (Vr) values in Figure 9B. The Vr is defined as dφ /dt Vr = φp

4. CONCLUSIONS In conclusion, we have demonstrated that all-aromatic liquid crystal (AB)n-multiblock copoly(ether imide)s consisting of low Tg HBA/HNA and high Tg HBA/IM blocks can be used as high-temperature (≥250 °C) single-component shape-memory polymers. The thermoplastic (AB)n-multiblock copolymer (22IM) film with two Tgs (Tg,A ∼ 124 °C and Tg,B ∼ 200 °C) can be rotated by 180° and recovering the original shape results in a Rf of 98% and Rr of 100% (Tprog = 170 °C and Tr = 225 °C). The rigid HBA/IM-blocks play the role of a physical scaffold and ensures that the temporary shape can be recovered. 22IM-5K, a cross-linked analogue of 22IM, exhibits two distinct Tgs at ∼126 °C (Tg,A) and ∼242 °C (Tg,B), respectively. The two well-defined rubbery plateaus above Tg,A and Tg,B allowed us to explore more elaborate SM designs. We were able to successfully demonstrate high-temperature (≥250 °C) dual SM with good shape fixation and recovery efficiency in excess of 90% (rotation of 120°, Tprog = 140 °C and Tr = 220 °C). Finally, tunable high-temperature triple SM and one-way

(6)

where φp is the programmed rotation. This provides a convenient method to program the triple SM behavior for a range of desired applications. One-Way Reversible Shape Memory Behavior. The SM behavior discussed so far dealt with shapes that were programmed to deform in one direction, either forward or backward, and then the shapes were recovered by heating in the reverse direction. When both forward and backward deformations are programmed into one shape memory cycle, theoretically the corresponding temporary shapes would recover in the reverse direction. Figure 3C outlines a protocol for 22IM-5K to perform a one-way reversible transition in the shape memory cycle. As shown in Figure 10, the shape recovery Rr(C→B) and Rr(B→A) are 80% and 90%, respectively. This can be understood G

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Macromolecules reversible SM with large torsion angles at 250 °C could also be demonstrated using this single-component 22IM-5K thermoset.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (T.J.D.). ORCID

Qingbao Guan: 0000-0002-3384-3229 Sergei S. Sheiko: 0000-0003-3672-1611 Theo J. Dingemans: 0000-0002-8559-2783 Present Address

T.J.D.: Department of Applied Physical Sciences, University of North Carolina at Chapel Hill, 1113 Murray Hall, 121 South Road, Chapel Hill, NC 27599-3050. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded in part by the Dutch Polymer Institute under grant #761. Q.G. gratefully acknowledges funding from the Chinese Scholarship Council (#2011692002). S.S. gratefully acknowledges funding from the National Science Foundation, DMR 1407645. We thank Dr. Ranjita Bose and Mr. Ming Li for valuable discussions on shape memory torsion experiments.

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REFERENCES

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DOI: 10.1021/acs.macromol.7b00569 Macromolecules XXXX, XXX, XXX−XXX