High-Temperature Shape Memory Behavior of Semicrystalline Polyamide Thermosets

Abstract

We have explored semicrystalline poly(decamethylene terephthalamide) (PA 10T) based thermosets as single-component high-temperature (>200 °C) shape memory polymers (SMPs). The PA 10T thermosets were prepared from reactive thermoplastic precursors. Reactive phenylethynyl (PE) functionalities were either attached at the chain termini or placed as side groups along the polymer main chain. The shape fixation and recovery performance of the thermoset films were investigated using a rheometer in torsion mode. By controlling the M_n of the reactive oligomers, or the PE concentration of the PE side-group functionalized copolyamides, we were able to design dual-shape memory PA 10T thermosets with a broad recovery temperature range of 227–285 °C. The thermosets based on the 1000 g mol^–1 reactive PE precursor and the copolyamide with 15 mol % PE side groups show the highest fixation rate (99%) and recovery rate (≥90%). High temperature triple-shape memory behavior can be achieved as well when we use the melt transition (T_m ≥ 200 °C) and the glass transition (T_g = ∼125 °C) as the two switches. The recovery rate of the two recovery steps are highly dependent on the crystallinity of the thermosets and vary within a wide range of 74%–139% and 40–82% for the two steps, respectively. Reversible shape memory events could also be demonstrated when we perform a forward and backward deformation in a triple shape memory cycle. We also studied the angular recovery velocity as a function of temperature, which provides a thermokinematic picture of the shape recovery process and helps to program for desired shape memory behavior.

1. Introduction

Interest in shape memory polymers (SMPs) has grown rapidly since the 1980s.¹⁻⁵ Typical applications are heat-shrink tubing, temperature sensors and actuators, biomedical and surgical materials, and aerospace devices.^6,7 Thermoresponsive SMPs are the most investigated systems that polymers are able to adopt a temporary shape upon deformation and to revert back to the permanent shape upon exposure to heat.^7,8 They are generally composed of a polymer network to maintain the permanent shape and a reversible switch responsible for the shape fixation and recovery.⁸⁻¹¹ The glass transition temperature (T_g) and melting temperature (T_m) are the two most important thermal transitions for thermoresponsive SMPs. Polyurethanes, polyesters, and (methyl)acrylate-based polymer networks have been investigated as SMPs based on the glass transition. Cross-linked semicrystalline networks or (multi)block copolymer systems have been developed to design T_m-based SMPs, such as polyolefins, polyethers, and polyesters.⁶ However, most SMPs exhibit a switching temperature lower than 100 °C, which may not meet the requirements for high temperature aerospace, automotive, or electronic applications.

Recently, several examples of high-temperature (>200 °C) SMPs have been reported.¹²⁻²¹ Vaia and co-workers synthesized an amorphous fluorinated polyimide with a shape recovery temperature of 220 °C.²⁰ This was the first example of a single-component high-temperature SMP. However, this fluorinated polyimide can only be obtained as thin films, which excludes the possibility to produce complex shapes using melt processing methods. Weiss et al. have introduced ionic moieties into poly(ether ether ketone) (PEEK) leading to a T_g-based SMP with a switching temperature close to 200 °C, where the exact switching temperature depends on the metal counterions used.¹⁴ In a later publication, this system was further developed into a T_m-based SMP by incorporation of sodium oleate, which displays a higher switching temperature of 230–240 °C due to the melting of sodium oleate.¹⁵

In contrast to the dual SMPs mentioned above, triple SMPs featuring two independent temporary shapes were first reported by Lendlein’s group.²² They require two programming steps and show two recovery steps. Triple-shape memory effects can be realized in a polymer material possessing two distinct thermal transitions^15,22−24 or one single broad transition such as a broad glass transition.^16,25 High temperature triple SMPs based on a single-component thermotropic liquid crystalline poly(esterimide) thermosets have recently been reported by Guan et al. The polymer shows two glass transition temperatures at ∼120 and ∼200 °C, both of which can be used as switches for triple SMPs.²¹

In previous publications we reported on the synthesis and (thermo)mechanical properties of semiaromatic polyamide thermosets based on poly(decamethylene terephthalamide) (PA 10T).^26,27 We showed that this class of polymers can be processed into semicrystalline thermoset films where the degree of crystallinity can be controlled. The melting/crystallization of the crystalline phase and the covalent network can be used as the high-temperature switch and permanent scaffold, respectively, for a dual shape memory effect (SME). More sophisticated triple and one-way reversible SME can be designed with the glass transition as the second switch. To the best of our knowledge, this is the first demonstration of a high-temperature SMP based on a single-component semicrystalline polyamide thermoset.

2. Experimental Section

2.1. Materials

The syntheses of phenylethynyl (PE) end-capped PA 10T oligomers (Scheme 1A) and PE side-group-functionalized copolyamides (Scheme 1B) have been described in detail elsewhere.^26,27 In order to obtain a covalently cross-linked polyamide network, the reactive precursors were thermally cured. A standard melt compression technique was used to prepare the thermoset films according to the following procedure: The precursor polymers were ground into a fine powder using a mortar and pestle. The powder was placed between two metal plates lined with Kapton film, and this stack was consolidated in a Joos hot-press using a predetermined temperature program and a 5 kN force. The temperature program was set to heat to 350 °C at 5 °C min^–1, hold for 15 min, and cooled to 50 °C at 20 °C min^–1. The obtained cured films were used for further characterization.

Scheme 1. Structures of Precursors via (A) Reactive End-Group Approach and (B) Reactive Side-Group Approach.

The film samples prepared from PA 10T reactive oligomers (M_n of 1000 and 3000 g mol^–1) are denoted as PE-1K and PE-3K. TPE-5, TPE-10, and TPE-15 represent the film samples prepared from reactive TPE-copolyamides with 5, 10, and 15 mol % of reactive TPE comonomer, respectively. A PA 10T thermoplastic sample (M_n = 7500 g mol^–1) was used as the reference in this paper and denoted as Ref. The ¹H NMR spectra of the synthesized precursors (Supporting Information, Figure S1) confirms that the concentration of reactive functionalities, either as end-caps or as side groups, is consistent with the feed ratio of the monomers.

2.2. Characterization

¹H NMR spectra were recorded on a 400 MHz Bruker WM-400 at 25 °C using trifluoroacetic acid-d as solvent. DSC was conducted on a PerkinElmer Sapphire DSC under a nitrogen atmosphere at a heating/cooling rate of 20 °C min^–1. DMTA was performed on a PerkinElmer Diamond DMTA with film samples (0.2–0.3 mm thick) at a heating rate of 2 °C min^–1 under a nitrogen atmosphere. Data were collected at a frequency of 1 Hz.

The SME was characterized in a cyclic torsion mode as illustrated in Figure 1. Compared to the traditional extension or bending tests,²⁸ torsion tests involve nonhomogeneous strains and stresses in the cross section of a rectangular bar and enable the SMPs to reach large deformations with moderate strains, which are believed to be more representative of practical applications of SMPs.²⁹⁻³²

Figure 1.

Illustration of a shape deformation, fixation, and recovery cycle of the dual SME in torsion mode.

The shape programming and recovery cycles were performed using a Thermofisher Haake MARS III rheometer equipped with a solid clamp geometry under a N₂ atmosphere. Rectangular thin films with a width of ∼2.5 mm and thickness of ∼0.25 mm were loaded into the setup with a length of 15 mm between the clamps. The samples were deformed in a torsion mode at a constant strain rate of 0.1% s^–1 equivalent to a rotation speed of 3.4° s^–1.

The procedure of one cycle for a dual SME test includes the following steps: (1) heat the sample to the programming temperature (T_prog); (2) rotate the sample to the predetermined angle (ϕ_d); (3) keep the angle constant, cool to the fixation temperature (T_f) at 10 °C min^–1 and stabilize for 10 min; (4) remove the stress; (5) heat the sample to T_prog at 10 °C min^–1 in a stress-free condition followed by an isothermal hold at T_prog for 10 min to stabilize. This cycle was repeated multiple times to characterize the reproducibility of the shape memory performance. The rotation angle of the sample was monitored and recorded during the whole test. For triple SME measurements, the sample was consecutively deformed in two steps in one programming cycle. Two different T_progs were used, one being above T_m and the other between T_g and T_m.

3. Results and Discussion

3.1. Melting and Thermomechanical Properties

In general, either the T_g or the T_m transition of a polymer can be applied to trigger a dual-shape memory event; however, to achieve high-temperature triple-shape memory behavior in a single-component polymer is challenging, as two high-temperature temporary networks with distinct rubbery plateaus are required.^15,22−24 One solution to realize this type of structure is to introduce moderate cross-links into a high-T_g semicrystalline polymer. In our previous work, we have explored a semiaromatic polyamide, PA 10T (T_g = ∼125 °C, T_m = ∼315 °C), as the base polymer to prepare semicrystalline polyamide thermosets. Phenylethynyl (PE) shows high cure-temperature of 330–370 °C and thus is able to provide a proper processing window for the PA 10T-based precursors. Therefore, we have synthesized a reactive PE-based end-cap and a reactive PE-based comonomer, which were copolymerized with terephthalic acid and 1,10-diaminodecane to yield curable PE end-capped oligomers and PE side-group functionalized copolymers. The polyamide thermosets were obtained after a subsequent thermal cure at a temperature of 350 °C for 15 min. PE-1K and PE-3K refer to the cured end-capped oligomers with M_n = 1000 and 3000 g mol^–1, respectively. TPE-5, TPE-10, and TPE-15 represent the cured side-group functionalized copolyamides with 5, 10, and 15 mol % PE side groups, respectively.

The melting curves and properties of a thermoplastic PA 10T reference polymer (Ref) and the resultant polyamide thermosets are shown in Figure 2 and Table 1. The thermoset samples after cure are semicrystalline showing a T_m of 227–288 °C and ΔH_m of 8–33 J g^–1. These values are much lower than the T_m and the ΔH_m of the reference polymer (Ref) (318 °C and 82 J g^–1), which means the crystallizability of the polymer chains in the thermosets is strongly suppressed.

Figure 2.

DSC heating scans (first heat) of (A) reference polymer (Ref), PE-3K, and PE-1K and (B) reference polymer (Ref), TPE-5, TPE-10, and TPE-15. The asterisk refers to melting peaks (N₂ atmosphere and heating rate of 20 °C min^–1).

Table 1. Thermal Properties of the Thermoplastic Reference Polymer (Ref) and the Cured Thermoset Samples.

sample	Tma (°C)	ΔHm (J g–1)	Tgb (°C)	cross-linking densityc (kmol m–3)
Ref	318	82	127
PE-3K	283	33	127	0.68
PE-1K	244	8	129	0.72
TPE-5	285	33	123	0.94
TPE-10	250	27	123	1.79
TPE-15	227	20	125	2.34

^aT_m refers to the max of the melting peak as observed in DSC experiments.

^bT_g refers to the max of E″ as observed in DMTA experiments.

^cCross-linking density (υ) was calculated using , where E′ is the storage modulus of cured films at 350 °C (T = 623 K) and R the universal gas constant (8.314 J K^–1 mol^–1).

Figure 3 shows the thermomechanical behavior of the reference polymer (Ref) and the thermosets. The Ref film exhibits a T_g at 127 °C, but this film fails at 297 °C because it has reached the melting point (T_m). In contrast to the reference polymer (Ref), the thermoset samples show two plateau regions (T_g – T_m and >T_m) in the DMTA profiles. The second plateau of the thermosets is stable up to ∼400 °C, which confirms the presence of a network structure. The T_gs of both thermoset samples remain virtually unchanged (123–129 °C) when compared to that of the reference polymer (Ref).

Figure 3.

DMTA of (A) reference polymer (Ref), PE-1K, and PE-3K films and (B) reference polymer (Ref), TPE-5, TPE-10, and TPE-15 films. Heating rate 2 °C min^–1 under N₂ atmosphere and a frequency of 1 Hz. The insets show E″ at the glass transition temperature.

The molecular structure of the resultant semicrystalline thermosets is depicted in Figure 4. PE groups combine at the cure temperature resulting in cross-links with multiple after-cure chemical structures depending on cure temperature, time, and PE concentration. We attempted to investigate the cure chemistry using Raman and FTIR spectroscopy.^33,34 However, Raman spectroscopy failed due to a strong fluorescence background, and FTIR cannot detect the acetylene bond of PE because of IR insensitivity. Hence, direct evidence is not available to confirm the chemical change of the PE functionalities. The DMTA results of the cured samples show a rubbery plateau above T_m, and this plateau is stable up to ∼400 °C, which confirms that cross-linking has taken place during cure. The two-plateau thermal behavior enables triple-shape memory properties by taking both T_g and T_m transitions as the switches. The T_m transition can be solely used as a switch to design a high-temperature (>200 °C) dual-shape memory polymer.

Figure 4.

Molecular representation of the semicrystalline PA 10T thermosets. Crystalline polymer (dark blue) is embedded in an amorphous matrix (light blue), and the red dots represent covalent cross-link points.

3.2. Dual-Shape Memory Behavior

Because of the semicrystalline nature of the synthesized polyamide thermosets, their melting and crystallization processes can be used as the thermoresponsive switch for dual-shape memory behavior. Three consecutive deformation, fixation, and recovery cycles in a torsion mode were conducted for each sample to test the shape memory performance over multiple cycles.

Shape fixation rate (R_f) and shape recovery rate (R_r) are the most important parameters to characterize the shape memory performance.³⁵R_f describes how accurately the temporary shape can be fixed, and R_r quantifies the ability of the polymer to memorize its permanent shape. When performing the measurements in torsion mode, R_f and R_r can be calculated using eqs 1 and 2.

1

2

where φ_d, φ_f, and φ_r denote the rotational angle after deformation, at the fixed temporary shape at T_f, and after recovery, respectively. The instantaneous recovery velocity V_r can be calculated as the time derivative of the angle as shown in eq 3.

3

Plotting V_r against temperature reveals the temperature range corresponding to the shape recovery process.²¹ This provides a clear thermokinematic view of the shape recovery and helps to program for desired SME.

The thermoplastic reference film (ref) cannot be deformed above the melting temperature because of the absence of a rubbery plateau, thus the glass transition was used as the switch for the SME. As shown in Figure 5A, the sample was deformed at 220 °C, which lies between T_g (127 °C) and T_m (318 °C), and the shape was fixed at 100 °C where the sample is in a glassy state. Therefore, the crystalline domains act as the scaffold, and the deformation of the polymer takes place in the amorphous state.

Figure 5.

Three consecutive dual-shape memory cycles for (A) reference polymer (Ref), (B) PE-3K, and (C) PE-1K. Shape recovery velocity as a function of temperature in the second cycle for (D) reference polymer (Ref), (E) PE-3K, and (F) PE-1K. Cooling/heating rate 10 °C min^–1 and N₂ atmosphere.

In contrast to the reference polymer (Ref), PE-3K and PE-1K thermoset films are stable up to ∼400 °C in DMTA experiments and exhibit two plateau regions (T_g – T_m and >T_m), as shown in Figure 3. This allows for deforming the film samples in the second plateau region (>T_m). Figures 5B and 5C show that PE-3K and PE-1K were deformed at 315 and 275 °C, respectively, which is about 30 °C higher than their T_ms (283 and 244 °C). At this temperature the covalent network acts as the scaffold, and the temporary shape is fixed by crystallization and vitrification at the fixation step. At the recovery step, the angle shows a slight decrease when the sample passes through the glass transition, which is associated with the release of the stress trapped in the mobile amorphous region. A subsequent slow increase in the angle is observed, which can be explained by thermal expansion.³⁶ When the sample starts melting, the angle recovers rapidly due to the fast release of the trapped stress.

The shape-memory performance of the first few cycles is usually not representative, as can be seen in Figure 5B,C. This is generally attributed to residual strain from the processing history of the sample.^13,35Table 2 shows the fixation and recovery results of the first three cycles of the reference thermoplastic polymer (Ref), the PE-3K, and PE-1K thermoset films. The PE-3K film shows a very low R_r of 36% in the first cycle compared to the following cycles (R_r = 72% and 76%), whereas the R_r of the PE-1K film shows a medium change over cycles (R_r = 81%, 90%, and 88%). Such significant difference can originate from the different cross-linking densities of these two samples. PE-3K has a lower cross-linking density and therefore longer polymer chains between cross-links, which is more likely to store residual strain. The recovery of both samples cannot reach 100% because the irreversible chain-segment orientation and the relaxation effect in the polymer network partly dissipate the stored entropy.²⁰

Table 2. Dual-Shape Fixation and Recovery Results of the Reference Thermoplastic Polymer (Ref), PE-3K, and PE-1K Thermoset Films.

			cycle 1		cycle 2		cycle 3
sample	Tproga (°C)	Trb (°C)	Rf (%)	Rr (%)	Rf (%)	Rr (%)	Rf (%)	Rr (%)
Ref	220	210	89	86	89	82	89	84
PE-3K	315	285	93	36	94	72	94	76
PE-1K	275	144, 240	99	81	98	90	95	88

^aT_prog refers to the programming temperature.

^bT_r refers to the temperature at the maximum recovery velocity.

The shape recovery velocity of the second cycle is plotted as a function of temperature as shown in Figure 5D–F. The reference polymer (Ref) shows a low velocity (<20° min^–1) through the recovery process because the shape recovery is triggered by the activation of the amorphous chain segments, which are strongly restricted by the crystalline domains. A low recovery velocity is generally observed in T_g-based shape-memory polymers.⁸ In contrast, PE-3K shows a quick recovery in the temperature range of 260–310 °C and reaches a maximum V_r of ∼60° min^–1 at 285 °C, which is close to its T_m (283 °C). The shape recovery is triggered by melting of the crystalline domains, which is a relatively fast process.

It is worthy to note that the PE-1K film was deformed at 275 °C in one step; however, this polymer shows two distinct recovery steps with a maximum recovery velocity of 20–30° min^–1 at around 144 and 240 °C, respectively. The first recovery takes place when the sample passes through the glass transition and reaches an R_r of ∼42% at 200 °C. The second recovery is triggered by the melting of the crystalline domains. These two recovery steps indicate that the crystalline domains in PE-1K are not able to form a penetrating scaffold throughout the sample and thus cannot completely lock the amorphous chains. This allows the sample to partially recover at the T_g – T_m range. These results demonstrate that the degree of crystallinity, which is reflected in the ΔH_m values in Table 1, strongly affects the shape memory behavior of our semicrystalline PAs.

The dual-shape memory behavior of the TPE-based thermosets were investigated using the same method. Table 3 shows the shape fixation and recovery results for these samples. All samples reveal good shape fixation with R_f values of 93–99%. The TPE-15 samples with the highest cross-linking density shown in Table 1 exhibit excellent recovery efficiency (R_r > 90%), whereas the other samples show R_r values of 62–66% in the first cycle and 78–82% in the second and third cycles.

Table 3. Dual-Shape Fixation and Recovery Results of the TPE Thermoset Films.

			cycle 1		cycle 2		cycle 3
sample	Tproga (°C)	Trb (°C)	Rf (%)	Rr (%)	Rf (%)	Rr (%)	Rf (%)	Rr (%)
TPE-5	315	277	93	62	93	82	94	82
TPE-10	295	242	98	66	96	78	96	79
TPE-15	275	140, 227	99	93	99	93	97	96

^aT_prog refers to the programming temperature.

^bT_r refers to the temperature at the maximum recovery velocity.

Figure 6 shows the dual-shape memory cycles and the recovery velocity of the TPE thermoset films. Similar to the PE-3K sample, TPE-5 and TPE-10 exhibit one recovery step in the temperature ranges of 260–300 and 230–270 °C, respectively. Interestingly, the TPE-15 film also displays two recovery steps, which is similar to that of the PE-1K film. The two recovery steps originate from the low degree of crystallinity in TPE-15 and PE-1K, where the amorphous chain segments cannot be completely fixed when the temperature is above T_g. TPE-5, TPE-10, and PE-3K, on the other hand, show higher degrees of crystallinity, and this prevents the T_g-induced shape recovery.

Figure 6.

Three consecutive dual-shape memory cycles for the TPE series (A) TPE-5, (B) TPE-10, and (C) TPE-15. Shape recovery velocity as a function of temperature in the second cycle for the TPE series (D) TPE-5, (E) TPE-10, and (F) TPE-15. Cooling and heating rate 10 °C min^–1 and N₂ atmosphere.

The difference between these samples clearly indicates that the shape memory behavior strongly depends on the degree of crystallinity of the semicrystalline thermosets. By changing the concentration of the PE side groups in the reactive copolyamides, the cross-linking density of the thermosets can be controlled, consequently leading to adjustable crystallinity and shape memory behavior.

3.3. Triple-Shape Memory Behavior

Based on our study above, two reversible processes, the glass transition and the melting process, can both act as the switches to trigger shape recovery. Unlike the traditional high-temperature triple-SMP composed of multiple components, we were able to design a single-component triple-SMP using both switches to achieve two distinct recovery processes.

Figure 7A shows the three consecutive triple-shape memory cycles for PE-3K. The film sample was first heated up to 315 °C, which is above the T_m (283 °C) and rotated by 90° from the original shape A (φ_A) to a temporary shape B (φ_B). The sample was cooled to 200 °C, a temperature between T_m and T_g, to fix the shape B. A second rotation of 90° was then applied to reach the temporary shape C (φ_C). The sample was subsequently cooled to 60 °C, which is below the T_g of 127 °C, to fix the shape C. The external stress was then removed, leading to the final fixed temporary shape (φ_f). A shape fixation rate (R_f) of ∼90% was calculated using eq 4.

4

Figure 7.

Three consecutive triple-shape memory cycles for the PE series (A) PE-3K and (B) PE-1K. Shape recovery velocity as a function of temperature in the second cycle for the PE series (C) PE-3K and (D) PE-1K. Cooling and heating rate 10 °C min^–1 and N₂ atmosphere.

In the subsequent recovery step, the sample was heated up to 315 °C in a stress-free state to monitor the recovery of the rotational angle. The shape recovery was accomplished in two distinct steps, as shown in Figure 7A. This means the thermoset can memorize two temporary shapes in one single shape memory cycle. The shape recovery rate R_r(C→B) and R_r(B→A) were calculated based on eqs 5 and 6, respectively.²¹

5

6

where φ_A, φ_B, and φ_C denote the rotational angle of shape A (φ_A = 0°), shape B (φ_B = 90°), and shape C (φ_C = 180°); φ_B/rec and φ_A/rec are the rotational angles of the first and second recovered shapes, respectively. Figure 7B shows the triple-shape memory cycles of PE-1K. This polymer exhibits a lower T_m (244 °C) compared to PE-3K (283 °C); therefore, lower programming temperatures (275 and 150 °C) were used.

PE-3K shows moderate R_r(C→B) values of 67–80% in the first recovery step and R_r(B→A) values of 60–73% in the second recovery step, as listed in Table 4. Compared to PE-3K, PE-1K shows much higher R_r(C→B) values of 128–138% and lower R_r(B→A) values of 40–49%. The first recovery is triggered by the glass transition and is due to the release of stress in the amorphous phase. The major recovery takes place in the first step, indicating that the crystalline phase cannot completely lock the stress in the amorphous phase. The stress, which is supposed to release in the second recovery step, is actually partially released in the first recovery step. This, together with the stress induced by the second deformation, drives the sample to reach a high recovery rate (>100%) in the first step. This result is consistent with the result we obtained from the dual-shape memory experiments and originates from the low degree of crystallinity of the PE-1K thermoset.

Table 4. Triple-Shape Fixation and Recovery Results of the PE- and TPE-Series Thermoset Films.

					cycle 1			cycle 2			cycle 3
sample	Tprog1a (°C)	Tprog2a (°C)	Tr1b (°C)	Tr2b (°C)	Rf (%)	Rr(C–B) (%)	Rr(B–A) (%)	Rf (%)	Rr(C–B) (%)	Rr(B–A) (%)	Rf (%)	Rr(C–B) (%)	Rr(B–A) (%)
PE-3K	315	200	137	289	91	67	60	93	80	66	89	79	73
PE-1K	275	150	144	240	97	128	49	97	135	44	93	139	40
TPE-5	315	200	132	285	88	64	69	86	74	79	86	79	79
TPE-10	295	175	138	250	92	79	91	92	89	80	91	91	82
TPE-15	275	150	133	236	93	97	84	92	123	55	93	136	45

^aT_prog1 and T_prog2 refer to the first and second programming temperature, respectively.

^bT_r1 and T_r2 refer to the temperature at the first and second maximum recovery velocity, respectively.

Figures 7C and 7D show the shape recovery velocity of PE-3K and PE-1K samples as a function of temperature. The first recovery step of PE-3K shows a lower V_r (13° min^–1) than the second step (28° min^–1) because of the limited mobility of the amorphous chain segments restricted by the crystalline domains. In contrast, the PE-1K sample shows much higher V_r (54° min^–1) in the first step, which is due to its low degree of crystallinity.

The triple-shape memory cycles and recovery velocity of the TPE thermosets are shown in Figure 8. The TPE-5 and TPE-10 films show moderate R_r(C→B) and R_r(B→A) (64–91%) and higher V_r values in the second recovery step, which is similar to the behavior of PE-3K, whereas the TPE-15 sample, similar to PE-1K, exhibits high R_r(C→B) (97–136%) and V_r values (38° min^–1) in the first recovery step, because this polymer has the lowest degree of crystallinity.

Figure 8.

Three consecutive triple-shape memory cycles for (A) TPE-5, (B) TPE-10, and (C) TPE-15. Angular velocity of shape recovery as a function of temperature in the second cycle for (D) TPE-5, (E) TPE-10, and (F) TPE-15. Cooling and heating rate 10 °C min^–1 and N₂ atmosphere.

The temperatures at the first maximum recovery velocity (T_r1) of all samples are within a narrow range of 132–144 °C because the T_g of all samples is around 125 °C. However, the temperatures of the second maximum recovery velocity (T_r2), which depend on the T_m of the samples, display a broad variation between 236 and 289 °C. Therefore, the triple-shape memory behavior of the thermosets is highly tunable over a wide temperature regime.

Besides deforming the sample in one direction only, more interestingly, we can involve both forward and backward deformations in a triple-shape memory cycle; therefore, the corresponding temporary shapes would recover in a reverse direction. To demonstrate this, the TPE-10 thermoset sample, which shows a high R_f (>90%), R_r(C–B) (∼90%), and R_r(B–A) (∼80%), was selected as a representative example.

The results of test I are shown in Figure 9A,B. The sample was programmed forward and backward by 180° in two steps and reached a final temporary shape, which is identical to the original shape. The subsequent heating resulted in an increase in angle in the first recovery step achieving a maximum recovery velocity at 142 °C and an R_r(C–B) of 79–91% at 213 °C. By increasing the temperature further, the sample recovered in the reverse direction reaching a maximum recovery velocity at 243 °C and an R_r(B–A) of 81–87% (Table 5). These results demonstrate that our semicrystalline PA thermosets show one-way reversible shape memory behavior. The reversible shape changes in this experiment occurred without application of any external force. The shape changes were driven by internal stress of the oppositely strained networks formed in the first and second programming steps.

Figure 9.

Three consecutive triple-shape memory cycles for TPE-10 and shape recovery velocity as a function of temperature in the second cycle. (A) and (B) deformation of 180° and −180° in two steps, respectively; (C) and (D) deformation of 180° and −360° in two steps, respectively.

Table 5. Triple-Shape Fixation and Recovery Results of TPE-10 Deformed in Two Opposite Directions.

test	cycle no.	Δφd1a (deg)	Δφd2a (deg)	Rf (%)	Rr(C–B) (%)	Rr(B–A) (%)	Tmaxb (°C)
I	1	180	–180	97	91	81	213
	2	180	–180	98	85	81	211
	3	180	–180	99	79	87	209
II	1	180	–360	90	86	54	218
	2	180	–360	89	83	59	213
	3	180	–360	91	82	61	212

^aΔφ_d1 and Δφ_d2 refer to the deformation angles in the first and second deformation steps.

^bT_max refers to the temperature at the maximum angle.

We can adjust the second programming step from Δφ_d2 = −180° to Δφ_d2 = −360° in test II, which results in two opposite temporary shapes, as shown in Figure 9C,D. The sample shows a R_f of 89–91%, R_r(C–B) of 82–86%, and R_r(B–A) of 54–61% (Table 5). The maximum recovery velocity occurs at 142 and 244 °C in the first and second recovery steps, respectively, which is consistent with the results of Test I. Our results clearly demonstrate that high-temperature SME with tunable recovery directions and amplitudes can be designed based on our single-component semicrystalline polyamide thermosets.

4. Conclusions

We have investigated two novel series semicrystalline PA thermosets and demonstrated that they can be as single-component high-temperature (>200 °C) shape memory polymers (SMPs). Two molecular design approaches based on reactive phenylethynyl (PE) functionalities have been explored: cross-linked semicrystalline PA films were prepared by curing reactive thermoplastic PA 10T oligomers, and films were prepared by curing reactive thermoplastic side-group functionalized copolyamides. Compared to the thermoplastic PA 10T reference polymer, the PE-3K, TPE-5, and TPE-10 thermoset films show high-temperature dual-shape memory behavior (>200 °C) when T_m is used as the switching temperature. The densely cross-linked PE-1K and TPE-15 films show the highest fixation rate (99%) and recovery rate (≥90%) and two distinct recovery steps because the degree of crystallinity is too low and cannot provide a scaffold that locks the shape at temperatures between T_g and T_m. Triple-shape memory behavior can be demonstrated when the T_g (∼125 °C) is used as the second switching temperature. The recovery rate of the two recovery steps is highly dependent on the degree of crystallinity of the thermosets and vary within a broad range of 74%–139% and 40–82% for the first and second step, respectively. One-way reversible shape memory events can also be designed when we perform forward and backward deformation in a triple shape memory cycle. We also studied the recovery velocity as a function of temperature to elaborate the thermokinematics of the shape recovery process. The use of phenylethynyl reactive functionalities allows us to control the degree of crystallinity in the final polyamide thermosets and in turn tune their shape memory behavior in terms of recovery temperature, velocity, and efficiency. We believe that the design rules presented herein will help in designing new shape memory polymers based on well-known semicrystalline polyamide chemistries.

Supporting Information Available

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b03658.

• Figure S1 (PDF)

Author 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.

The authors declare no competing financial interest.