Self-healing high temperature shape memory polymer

European Polymer Journal 120 (2019) 109279

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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj

Macromolecular Nanotechnology

Self-healing high temperature shape memory polymer a

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b

a

Deyan Kong , Jie Li , Anru Guo , Xintong Zhang , Xinli Xiao

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T

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MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, No. 92 West Dazhi Street, Harbin 150001, PR China Aerospace Research Institute of Materials & Processing Technology, No. 1 South Dahongmen Road, Fengtai District, Beijing 100076, PR China

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ARTICLE INFO

ABSTRACT

Keywords: Smart materials High-temperature properties Mechanical properties

Self-healing can improve reliability and extend service lifetime of materials while shape memory polymers (SMPs) have potential in many smart applications, and here self-healing shape memory polyimide (SHSMPI) that can heal damages at high temperature is fabricated for the first time. SHSMPI is synthesized by incorporating fusible thermoplastic polystyrene (PS) at 8% weight content into shape memory polyimide (SMPI) matrix, and it exhibits excellent shape memory effect. SHSMPI can self-heal crack, pierced hole, and cut effectively at healtemperature of 243 °C. Self-healing mechanism of SHSMPI is discussed, and SMPI matrix produces spatial proximity through shape memory effect while PS acts as healing agent to heal the damages. SHSMPI can produce stored energy of 0.105 J g−1 at the efficiency of 33.1% upon triggering. The designing of SHSMPI at macroscale makes it suitable for mass production, and the self-healing performance will greatly expand application areas of SMPI.

1. Introduction Self-healing polymers (SHPs) have become a hot topic in recent decades due to their capability to self-heal damages, which will greatly improve the reliability, extend service lifetime and reduce maintenance cost of the materials [1–4]. Self-healing is mainly achieved through intrinsic or extrinsic methods, and intrinsic methods with reversible reactions such as non-covalent bonds and dynamic covalent bonds can heal the microcracks without healing agents [5–8]. While extrinsic methods mainly depend on healing agents encapsulated in microcapsules or vascular networks, which can be released and then heal the microcracks upon crack intrusion [9]. Therefore, extrinsic method is more likely to be realized in commercial polymers since structural modification of matrix molecules is not required [10]. Temperature is an important stimulus for self-healing, and the healing temperature (Th) of different SHPs may vary greatly. For example, the host-guest interaction and thiuram disulfide can produce self-healing effect at ambient temperature [11]. Diels-Alder (D-A) moiety can be activated for selfhealing at intermediate temperature of about 80 °C, while hydroxylurethane bonds were activated at relatively high temperature of about 130 °C [12,13]. Although high temperature self-healing is strongly desired in many edge-cutting applications such as aerospace and electronic industries, there is no report about SHP with Th higher than 200 °C until now [9–13].

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Among the various polymers [14–18], shape memory polymers (SMPs) are smart polymers that can fix the temporary shapes and then return to the original shapes under suitable stimuli such as heat, electricity, light and magnetic fields [19–25]. SMPs have great potentials in various fields such as biomedical devices, smart fabrics, deployable structures and actuators [26–28]. Self-healing SMPs that combine shape memory effect with self-healing performance have attracted more and more attentions in recent years [29–31]. Various methods such as combining SMP fibers with healing agent embedded in microcapsules, bearing thermo-reversible ionical netpoints and dynamic covalent bonds in the main chain or side chains, and incorporating fusible healing agent in host matrix have been employed to prepare selfhealing SMPs [32–35]. Shape memory assisted self-healing effect is a common phenomenon and the majority of self-healing SMPs are focused upon samples with low to medium Ths, while high temperature self-healing SMPs are still lacking now [30,32,36,37]. High temperature SMPs are developed for applications in harsh environments such as high temperature actuators, self-deployable aerospace structures and smart jet propulsion systems [38–41]. In these applications, high reliability and long service lifetime cause by selfhealing effect are of crucial importance. Therefore, it is very necessary to fabricate self-healing high temperature SMPs as they can expand and develop the application areas of both SHPs and SMPs greatly. Shape memory polyimide (SMPI) is a typical kind of high temperature SMP,

Corresponding author. E-mail address: [email protected] (X. Xiao).

https://doi.org/10.1016/j.eurpolymj.2019.109279 Received 26 July 2019; Received in revised form 25 September 2019; Accepted 29 September 2019 Available online 30 September 2019 0014-3057/ © 2019 Elsevier Ltd. All rights reserved.

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2. Materials and methods 2.1. Materials 1,3-Bis(3-aminophenoxy) benzene (TPE-R, 99.5%) and 3,3′,4,4′Diphenyl ether tetracarboxylic acid dianhydride (ODPA, 99.5%) were purchased from Changzhou Sunlight Pharmaceutical Co., Ltd and used directly. PS with melting temperature (Tm) of 240 °C was purchased from Aladdin Industrial Corporation, and it was used without further purification. The solvent of N,N-Dimethylacetamide (DMAc, 99%) was purchased from Sigma-Aldrich Co., and it was distilled with molecular sieves to remove the residual water. 2.2. Methods 2.2.1. Synthesis of native SMPI and SHSMPI 2.9110 g TPE-R was added into DMAc in 100 ml three-necked bottle, and it was dissolved by violent stirring under dry N2 atmosphere. 3.0890 g ODPA was added into TPE-R solution and stirred for 8 h to form poly(amic acid) (PAA). After being heated at 40 °C under vacuum to remove the bubbles, PAA was cast onto clean glass substrate and experienced step-wise curing at 80, 120, 170, 210 and 260 °C for 1 h, respectively. Native SMPI was obtained by removing the film from glass substrate in deionized water and then drying it at 100 °C. 0.5217 g PS was added into another bottle of the above-mentioned PAA and stirred for 6 h to produce uniform PAA/PS blend, and the blend underwent the same curing procedure as native SMPI to obtain SHSMPI.

Fig. 1. Synthesis procedures of native SMPI and SHSMPI, and the inlet is SEM image of SHSMPI.

2.2.2. Characterizations Fourier Transform Infrared (FTIR) spectra were examined with Nicolet IS50 spectrometer, Thermo Scientific, USA. Scanning electron microscopy (SEM) images were obtained through ZEISS SUPRA55 (Carl Zeiss AG, Germany) with accelerating voltage of 10 kV. Mechanical properties were examined using Shimadzu Precision Universal Tester AG-X plus (Shimadzu Corporation, Japan) with crosshead speed of 5 mm/min at room temperature, and the average of five independent measurements was taken as the value of mechanical property. The analysis of variance (ANOVA) on the results was performed, and Tukey’s test was used to obtain means comparisons. The ASTM D882-02 standard technique was used in the determination of mechanical properties, and both ends of the film were bonded to aluminum (Al) tabs with glue. Thermal stability was characterized with thermogravity analysis (TGA) in N2 atmosphere using Mettler-Toledo TGA/SDTA851 (Mettler-Toledo International Inc., Switzerland) at heating rate of 10 °C/min from room temperature to 800 °C. Thermomechanical

and it combines shape memory effect with the outstanding properties of polyimide such as superior mechanical properties, extraordinary radiation resistance and excellent thermal stability [42,43]. Here selfhealing SMPI (SHSMPI) that can heal the damages of SMPI at high temperature is reported for the first time. In the current study, SHSMPI is synthesized by incorporating fusible thermoplastic polystyrene (PS) into a new SMPI matrix as healing agent, wherein SMPI and PS construct the phase separation blend. SHSMPI can heal the crack, pierced hole, and cut effectively at 243 °C, as shape memory effect of SMPI can close the damages while the molten PS can flow to heal them. SHSMPI is expected to expand application areas of shape memory polyimide with its unique self-healing performance.

Fig. 2. Stress-strain curves (a) and TGA spectra (b) of native SMPI and SHSMPI.

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Fig. 3. Tan δ (a) and storage modulus (b) versus temperature of native SMPI and SHSMPI.

Fig. 4. Shape memory processes of native SMPI (a–d) and SHSMPI (e–h).

properteis were examined with dynamic mechanical analysis (DMA) using TA-Q800 (TA Instruments, New Castle, DE, USA) at heating rate of 3 °C/min in tensile mode according to ASTM D5026 standard technique. Differential scanning calorimetry (DSC) measurements were performed from room temperature to 250 °C with TA Instruments Q20 (TA Instruments, New Castle, DE, USA) under N2 atmosphere at the heating rate of 5 °C/min. Shape memory effect was determined by bending test as follows: the sample set at shape transition temperature (Ts) was bent to bending angle (θb), then it was cooled at room temperature for 5 min to fix the temporary shape. Fixity angle (θf) was obtained after the release of bending stress, and shape fixity (Rf) is calculated with Eq. (1).

Rf =

f

× 100%

b

with damage on hot-stage, and the process was monitored with Zeiss Axio Imager A2m optical microscopy (OM; Carl Zeiss AG, Germany) equipped with Linkam THMS-600 heating/freezing stage (Linkam Scientific Instruments Ltd, England). In order to demonstrate the selfhealing of cut more clearly, the sample was placed on slide glass for observation. Recovery force and mechanical work performed by SHSMPI upon triggering were measured through DMA Q800 with the following methods: (1) Increase temperature of SHSMPI to 240 °C and isothermal for 3 min, (2) Increase force to 1.0 N and stretch the sample at 240 °C, (3) Remove force and decrease temperature to 140 °C, (4) Increase force to 0.55 N and increase temperature, (5) Constrained shape recovery at 240 °C and remove force.

(1)

3. Results and discussion

When reheated to Ts, the sample would return to its initial shape with recovery angle (θr), and the shape memory process was recorded with Canon VIXIA HF R500. The shape recovery (Rr) is calculated with Eq. (2).

Rr =

f

r f

× 100%

3.1. Synthesis and structures The native SMPI is prepared through two-step condensation polymerization of TPE-R and ODPA, while SHSMPI is a polymer blend comprising SMPI and PS, as shown by their synthesis procedures in Fig. 1. The native SMPI exhibits smooth and neat surface like other SMPIs reported, [22,24,38] while SHSMPI possesses microscale phase separation structure wherein thermoplastic PS particles with sizes of several micrometers are distributed in SMPI matrix, as shown in Fig. 1. IR is often used to characterize the structure of polymer [44,45], and IR spectrum of SHSMPI is similar to that of native SMPI (Fig. S1). They both exhibit characteristic peaks of polyimide at 1778, 1712, 1369 and 1255 cm−1 that correspond to the asymmetric and symmetric

(2)

Self-healing performance was visually demonstrated by the healing of crack, pierced hole, and cut, respectively. The crack perpendicular to the length direction of the film across its surface was produced with a Swiss Army Knife, the pierced hole was produced with safety-pin needle, and the cut separating part of the film in two was produced with scissors. Self-healing was accomplished through heating the sample

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Fig. 5. Self-healing of crack in SHSMPI at 243 °C for 0 (a), 1 (b), 2 (c) and 4 min (d).

stretching of imide carbonyl, stretching of eCeN and asymmetric stretching of AreCeO, respectively [38,40]. These results indicate that there is only physical reaction between PS and SMPI in SHSMPI, which is the basis for microscale phase separation [46,47].

blend suitable for high temperature applications. 3.3. Thermomechanical propertie The peak value of loss factor (tan δ) is usually taken as Tg of polyimide, and it is observed in Fig. 3a that Tgs of native SMPI and SHSMPI are 235 °C and 218 °C, respectively. The only one Tg indicates that although SHSMPI possesses microscale phase separation, its thermomechanical properties are rather uniform at macroscale [46]. Glass transition process is related with chain segments motion and chain mobility, and the incorporation of flexible PS chains will increase the overall chain mobility in SHSMPI. As a result, SHSMPI exhibits lower Tg than that of native SMPI due to the weaker intermolecular interactions [39,46]. The intensity of tan δ increases from 0.28 of native SMPI to 1.21 of SHSMPI with the incorporation of PS, which indicates that SHSMPI can relax and dissipate energy more effectively than native SMPI. DSC is another common way to determine Tg of polymer, and DSC spectra of native SMPI and SHSMPI are shown in the supplementary information as Fig. S5 [40,43]. It is observed that there is only one Tg in SHSMPI, which further confirms that it is uniform at macroscale despite the microscale phase separation. The native SMPI mainly exhibits two plateaus corresponding to the glassy state at low temperature and rubbery state at high temperature, as shown in Fig. 3b. Storage modulus (E') of native SMPI decreases slowly with the increase of temperature in glassy state, and a sudden

3.2. Mechanical and thermal properties The tensile stress-strain curves of native SMPI and SHSMPI are shown in Fig. 2a, and it is observed that native SMPI exhibits maximum tensile strength ( max ) and elongation at break ( R ) of 117 MPa and 21.5%, respectively. While SHSMPI manifests max and R of 102 MPa and 11.0%, respectively. The detailed information of stresses and elongations at break of native SMPI and SHSMPI are shown in the supplementary information as Fig. S2, and the high mechanical properties of SHSMPI indicate that it is suitable for practical applications. SHSMPI manifests lower max and R than native SMPI due to the incorporation of PS, and tensile stress-strain curves of PS are shown in the supplementary information as Fig. S3 [38,48]. TGA spectra are shown in Fig. 2b and native SMPI exhibits one thermal decomposition corresponding to the weight loss of polyimide, while SHSMPI exhibits two thermal decompositions corresponding to the weight loss of PS and polyimide, respectively. The native SMPI and SHSMPI show carbonaceous chars of 60% and 55% at 800 °C, respectively. TGA of PS is shown in the supplementary information as Fig. S4, and TGA results further confirm that SHSMPI is a phase separation

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Fig. 6. Self-healing of pierced hole in SHSMPI at 243 °C for 0 (a), 1 (b), 2 (c) and 4 min (d).

drop can be observed directly between the two plateaus. The E's of native SMPI at 210 °C (Tg − 25 °C, glassy state) and 260 °C (Tg + 25 °C, rubbery state) are 1.20 GPa and 67.8 MPa, respectively. The addition of PS reduces E' of SHSMPI obviously, and its E's at 193 °C (Tg − 25 °C) and 243 °C (Tg + 25 °C) are 977 MPa and 3.66 MPa, respectively. The declined E' of SHSMPI is due to the low E' of PS, as well as the lower steric hindrance caused by the incorporation of PS chains.

E' at glassy and rubbery states (Fig. 4e), and the images of its shape recovery on 243 °C hot-stage at 12 s and 18 s are manifested in Fig. 4f and g, respectively. SHSMPI can also return to its initial shape with Rr of 100% (Fig. 4h), and the video recording this process is supplied in the supplementary information as Video S2.

3.4. Shape memory effects

The damaged specimen was placed on hot-stage with healing temperature of 243 °C, which is higher than both Tg of SMPI matrix and Tm of PS. As shown in Fig. 5a, the original crack gap, i.e., the distance between crack surfaces is about 170 µm. The crack was healed nicely with almost undetectable traces, as manifested by its images on 243 °C hot-stage at 1, 2 and 4 min (Fig. 5b–d), respectively. Pierced hole is a more serious damage than crack, and its selfhealing process was also studied here. As shown in Fig. 6a, the original hole possesses diameter of about 520 µm. Heating can heal the hole effectively with faint traces left, as manifested by the images on 243 °C hot-stage at 1, 2 and 4 min (Fig. 6b–d), respectively. Cut is one of the most serious damages in self-healing polymer, since it separates part of the sample in two [32]. The cut produced with scissors possesses width of about 120 µm (Fig. 7a), and heating at 243 °C can heal the cut, as demonstrated by the images subjected to heating for 5, 10 and 15 min (Fig. 7b–d), respectively. Healing efficiency ( h ) of SHPs is usually defined as the ratio of

3.5. Self-healing performance

In order to make the molecule segments in an activated status, shape recovery temperature of the specimen is set at its Tg + 25 °C. The native SMPI shows very nice shape memory effect, and it can be deformed into the temporary shape easily at 260 °C. The large difference in E' at glassy and rubbery states is efficient in freezing the chain mobility and thus fixing the temporary shape with Rf of 100% when cooled, as shown in Fig. 4a [40]. The images of its shape recovery on 260 °C hot-stage at 8 s and 12 s are shown in Fig. 4b and c, respectively. It can return to its initial shape with Rr of 100% after 16 s (Fig. 4d), and the video recording this shape recovery process is supplied in the supplementary information as Video S1. The high Rr indicates that the permanent phase composed of chain entanglements and strong π-π intermolecular interactions is large enough for native SMPI to recover to its original shape like other SMPIs reported [40]. SHSMPI also exhibits high Rf of 100% due to the large difference in

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Fig. 7. Self-healing of cut in SHSMPI at 243 °C for 0 (a), 5 (b), 10 (c) and 15 min (d).

heal maximum tensile stress after self-healing ( max ) to that of the initial init specimen ( max ), as expressed in Eq. (3) [49].

h

=

heal max init max

× 100%

2.27 GPa of the virgin to 1.51 GPa of the damaged SHSMPI, while selfhealing increased E to 2.18 GPa. These results indicate that SHSMPI can self-heal the damages simply through heating at 243 °C, which paves the way for self-healing high temperature SMPs.

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3.6. Self-healing mechanism

Stress-strain behaviors of the sample with crack across surface perpendicular to the length direction before and after self-healing are shown in Fig. 8a, and it is observed that crack led to decreased mechanical properties with max of 70.1 MPa. The max increased to 87.7 MPa with h of 86.0% after self-healing, indicating that SHSMPI can self-heal the crack efficiently. The stress-strain behaviors of the sample with pierced hole before and after self-healing are shown in Fig. 8b, and the pierced hole resulted in decreased mechanical properties with max of 72.1 MPa. The max increased to 76.3 MPa with h of 74.8% after self-healing, suggesting that SHSMPI can self-heal the pierced hole effectively. Stress-strain behaviors of the sample with cut before and after self-healing are shown in Fig. 8c, and the cut reduced mechanical properties with max of 39.5 MPa. The max enhanced to 64.9 MPa with h of 63.6% after self-healing, demonstrating that SHSMPI can also self-heal the cut. The stresses of SHSMPI samples with different damages before and after self-healing are summarized in Fig. 8d. The damage of cut decreased Young’s modulus (E) from

The native SMPI with crack similar to that of SHSMPI was placed on 260 °C hot-stage (Fig. 9a), and heating resulted in significant spatial proximity of the crack due to shape memory effect, as illustrated by its images at 120 s and 300 s (Fig. 9b and c). However, the crack is still observable after thermal treatment, indicating that native SMPI is not a self-healing polymer [35,36]. The images of pierced hole in native SMPI on 260 °C hot-stage at 0 s, 120 s and 300 s are shown in Fig. 9d to f, and the spatial proximity led to much more obvious flaw than that of SHSMPI. The images of cut in native SMPI exposed to 260 °C at 0 min, 8 min and 15 min are manifested in Fig. 9g to i, and the cut was still obvious despite its closure caused by spatial proximity. These results indicate that shape memory effect alone is unable to heal the damages of native SMPI. PS plays the role of healing agent, and the healing efficiency gets higher with the increase of PS content until 8% in SHSMPI. However, the SMPI/PS blend with higher content of PS will lose shape memory

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Fig. 8. Stress-strain behaviors before and after self-healing of crack (a), pierced hole (b), and cut (c); together with the summary of stresses (d).

effect, as demonstrated by the images of specimens with 9% and 12% PS on Tg + 25 °C hot-stage in the supplementary information (Fig. S6). As a result, SHSMPI with 8% PS content offers the balance between shape memory effect and self-healing performance. Therefore, self-healing of SHSMPI comprises two parts, i.e., spatial proximity caused by shape memory effect of SMPI matrix and healing of damage by molten PS. The activated SMPI matrix will release the internal stress generated from the test and bring the surfaces of damages together, which is necessary for the occurrence of self-healing. The movements of SMPI chains are beneficial to the flowing of PS, and the molten PS can flow to the damages and then heal them. Accordingly, self-healing mechanism of crack, pierced hole and cut in SHSMPI can be schematically illustrated in Fig. 10.

to SHSMPI and the temperature was increased to 240 °C again in path ③. The elastically stored energy (Wstored) during “Constrained shape recovery at 240 °C” is also equal to Eq. (4) in the recovery path ④. The stretching energy and stored energy of SHSMPI at 240 °C are 0.317 J g−1 and 0.105 J g−1, respectively. Efficiency is defined as the ratio of stored energy to stretching energy, and SHSMPI exhibits efficiency of 33.1%. 4. Conclusion In summary, self-healing high temperature shape memory polymer has been fabricated by incorporating 8% PS into a new SMPI matrix. SHSMPI with microscale phase separation exhibits uniform macroscale thermomechanical properties and excellent shape memory effects. SHSMPI can self-heal the damages such as crack, pierced hole and cut at 243 °C effectively. Spatial proximity caused by shape memory effect of SMPI matrix is the basic process for self-healing, and the molten PS can flow to the damages and then heal them. Upon triggering, SHSMPI can produce stored energy of 0.105 J g−1 at the efficiency of 33.1%. The designing of SHSMPI structure at macroscale rather than synthesizing at molecular level makes it suitable for mass production, and this study will stimulate further exploration on high temperature SHPs and SMPs.

3.7. Energy-storage potential of SHSMPI SMPs also store energy besides strain during shape memory process, and they can produce recovery force to perform mechanical work upon triggering [50]. The energy-storage potential of SHSMPI was studied with DMA, and stretch of the specimen with two clamps at 240 °C corresponds to “Shape deformation at 240 °C” in Fig. 11. The stretching energy (Wstretch) in path ① is calculated by numerical integration of the load versus clamp travel curves with Eq. (4).

W=

l 0

Fdl

(4)

Declaration of Competing Interest

Then the load was released, the temperature was decreased to 140 °C and the stretched shape was fixed in path ②. A load was applied

There are no conflicts to declare.

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Fig. 9. The spatial proximity of damages caused by shape memory effect in native SMPI at 260 °C. Images of crack at 0 s (a), 120 s (b) and 300 s (c); images of pierced hole at 0 s (d), 120 s (e) and 300 s (f); and images of cut at 0 min (g), 8 min (h) and 15 min (i).

Fig. 11. Stretching and stored energy of SHSMPI during a shape memory cycle. Fig. 10. Illustration of self-healing mechanism of crack, pierced hole, and cut in SHSMPI.

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Acknowledgements [17]

The authors thank Prof. Shaoqin Liu and Dr. Jinzhi Sun from School of Life Science and Technology, Harbin Institute of Technology for their assistance in the characterization of OM. This work is supported by Natural Science Foundation of Heilongjiang Province (LC2018023), Science and Technology Innovation Program for Talents of Harbin (2017RALXJ004), and Startup Foundation of Postdoctoral Research in Heilongjiang Province (AUGA4120003016).

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Appendix A. Supplementary material

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IR spectra of native SMPI and SHSMPI, ANOVA of the stresses and elongations at break of native SMPI and SHSMPI, tensile stress-strain curves and TGA spectra of PS, DSC spectra of native SMPI and SHSMPI, images of SMPI/PS blends with 9 % and 12 % PS on hot-stage, as well as the videos recording shape recovery processes of native SMPI and SHSMPI are shown in the supplementary information. Supplementary data to this article can be found online at https://doi.org/10.1016/j. eurpolymj.2019.109279.

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