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Polymer Degradation and Stability 156 (2018) 245e251

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Polymer Degradation and Stability

journal homepage: www.elsevier .com/locate /polydegstab

Effects of g-radiation on the performances of optically transparentshape memory polyimides with a low glass transition temperature

Hui Gao a, Fang Xie b, Yanju Liu c, Jinsong Leng a, *

a Centre for Composite Materials and Structures, Harbin Institute of Technology (HIT), Harbin, 150080, PR Chinab School of Naval Architecture and Ocean Engineering, Harbin Institute of Technology (HIT), Weihai, 264209, PR Chinac Department of Astronautical Science and Mechanics, Harbin Institute of Technology (HIT), Harbin, 150001, PR China

a r t i c l e i n f o

Article history:Received 2 May 2018Received in revised form13 August 2018Accepted 17 September 2018Available online 18 September 2018

Keywords:Shape memory polyimidesg-radiationShape memory behaviorMechanical propertySpace active deformation structures anddevices

* Corresponding author.E-mail address: lengjs@hit.edu.cn (J. Leng).

https://doi.org/10.1016/j.polymdegradstab.2018.09.0140141-3910/© 2018 Elsevier Ltd. All rights reserved.

a b s t r a c t

Recently, shape memory polyimides (SMPIs) with varying glass transition temperature (Tg) and opticaltransparency have been developed widely. While, rare reports were focused on the evaluation of thedurability of SMPIs in the complex and volatile space environments. This paper presents the simulated g-radiation accelerated experiment of two kinds of optically transparent SMPIs with a Tg of 178 �C and196 �C, respectively. The effects of g-radiation on chemical structure, optical transmittance, shapememory behavior, thermomechanical property, thermal stability, and mechanical properties wereinvestigated. Although SMPIs have endured g-radiation for 1� 106 Gy, the chemical structure, Tg, opticaltransmittance, thermal stability, and the shape memory behavior remained unchanged. Also, SMPIs witha Tg of less than 200 �C could maintain their high tensile strengths and elongations at break (>109.5MPa,>7.6%) after being exposed to 1� 106 Gy g-radiation, which shows great potential application in spaceactive deformation structures and devices.

© 2018 Elsevier Ltd. All rights reserved.

1. Introduction

Shape memory polymers (SMPs) are smart polymeric materialsthat have the capabilities to recover from the pre-deformed shapesto the permanent shapes under external stimuli [1,2]. Many types ofstimulus can be used to actuate SMPs, such as light [3e5], elec-tricity [6,7], magnetism [8e10], heat [11e14], and solvent [15e19].Among these driving methods, heat stimulus has been studiedwidely. After decades of research and development, SMPs haveexhibited broad application prospects in smart textiles [20],shrinkable tubes [21], intelligent information carriers [22], flexibleelectronics [23e25], biomedical or surgical devices [26e28].Moreover, high heat-resistant SMPs and their composites can applyin space active deformation structures due to their excellent ad-vantages in mass, cost, storage bulk and reliability [29e31]. How-ever, before the practical application in space environments, theimpacts of space environments on SMPs and SMPs-based struc-tures, which are usually studied through the ground stimulationexperiments, are required to figure out [32e34].

So far, someworks have reported the SMPs' durability under the

stimulated g-radiation, vacuum, ultraviolet, atomic oxygen, andthermal cycles. For example, Leng et al. [35] characterized theproperties of epoxy-based SMPs after irradiated by varying dosageof g-radiation. Epoxy-based SMPs could keep their good shape re-covery performances, although the thermal and mechanical per-formances illustrated a downward trend as the increase of radiationdosage. Arzberger et al. [36,37] investigated the vacuum out-gassing performances of epoxy-based SMPs, and the test resultsmet the requirements for space applications (weight loss<1.0%,condensed out-gas products<0.1%). Xie et al. [2] manifested thedurability of cyanate-based SMPs under ultraviolet radiation envi-ronments and investigated their vacuum out-gassing behaviors at aconstant temperature (23 �C) and humidity (50%) environments for24 h. Cyanate-based SMPs maintained their good thermal, shapememory and mechanical performances. Xiao et al. [38] studied theeffects of thermal cycling on the thermomechanical, shapememorybehaviors and the thermal stabilities of shape memory polyimide(SMPI) with a Tg of 240 �C. However, the mechanical propertieswere not mentioned. Then, in our previous work [30], we investi-gated the mechanical performances changes of colorless andtransparent SMPIs with a Tg of less than 200 �C under thermalcycling, atomic oxygen and ultraviolet radiation, respectively. Theperformances of colorless and transparent SMPIs are comparable

Fig. 1. ATR-FIIR spectra of A1 (a) and A2 (b) before and after g-irradiation.

H. Gao et al. / Polymer Degradation and Stability 156 (2018) 245e251246

with that of the UPILEX-S polyimide films that have been served asthe base film for spacecraft [39].

It is well known that g-radiation can be used to evaluate thepolyimides' ability to withstand the cosmic particle rays comparingwith ultraviolet and thermal cycling, etc., especially when they areutilized in high and low earth orbital environments. Furthermore,few works focus on investigating the space environmental adapt-ability of SMPIs other than traditional polyimides over the pastseveral decades [40e43]. To better promote the practical applica-tion of SMPIs with a Tg of less than 200 �C in space environments,the thermal, mechanical and shape memory performances changesof SMPIs under stimulated g-radiation environments are furtherinvestigated in detail in this paper. Herein, two kinds of colorlessand transparent SMPIs with Tg of 178 �C and 196 �C were synthe-sized through two-step polycondensation procedures. The SMPIwith a Tg of 178 �C was the precious work of our research group,which was synthesized by using flexible 1,3-bis(3-aminophenoxy)benzene (BAB) and bisphenol A dianhydride (BPADA) [12]. Theother was a new novelty colorless and transparent SMPI synthe-sized by adding 4,40-(1,10-biphenyl-4,40-diyldioxy)dianiline (BAPB)into the BAB-BPADA system and controlling the molar ratio of BABto BAPB at 1:2/3. Dynamic mechanical analysis (DMA), thermo-gravimetric analysis (TGA) and tensile test were used to charac-terize the thermomechanical properties, shape memory perfor-mances, thermal stabilities and mechanical properties of these twokinds of colorless and transparent SMPIs before and after g-irra-diation. Notably, the work is just a basic assessment of the dura-bility of SMPIs as controllable deployment structures and thermalprotective materials in the complex and volatile spaceenvironments.

2. Experimental

2.1. Preparation of two kinds of colorless and transparent shapememory polyimides

Firstly, 1,3-bis(3-aminophenoxy)benzene (BAB; Tokyo ChemicalIndustry Co., Ltd., Japan) and 4,40-(1,10-Biphenyl-4,40-diyldioxy)dianiline (BAPB; Aladdin Industrial Co., Ltd., China) were mixeduniformly according to the molar ratio of BAB to BAPB of 1:0 and1:2/3 respectively. The mixture was then added into N-methyl-pyrrolidone solvent (NMP; Aladdin Industrial Co., Ltd., China) andmagnetically stirred for 20e30min at the atmosphere of nitrogen.Subsequently, bisphenol A dianhydride (BPADA; Sigma-Aldrich Co.,Ltd., USA) was fed into the above solution in several batches within1 h and magnetically stirred for 24 h at room temperature underthe protection of nitrogen. Therefore, a highly viscous poly(amicacid) (PAA) was formed successfully after the above (first-step)polycondensation procedure. In this procedure, the molar ratio ofdiamine and dianhydride was controlled at 1:1 and the solid con-tents was kept at 15wt%. The viscous PAA was poured on the glasssubstrates and the second-step polycondensation procedure wascarried out at 40 �C/3 h, 80 �C/4 h, 110 �C/2 h, 160 �C/2 h, 190 �C/2 h,230 �C/2 h and 250 �C/2 h to complete the imidization. Lastly, glasssubstrates were peeled off in warm water to obtain the colorlessand transparent SMPIs. The samples with the molar ratio of BAB toBAPB of 1:0 were defined as A1, the other samples were named asA2. The weight of A1 and A2 after step-wise curing processes was3.7154 g and 3.8879 g respectively, while the total mass of diamineand dianhydride added during synthesis was 4.0873 g and 4.2325 grespectively. Hence, the yield of A1 and A2 in this work was 90.9%and 91.9% respectively. The thickness of all samples was deter-mined by the amount of PAA solution and was controlled at around0.13mm in this work.

Themeta-substituted groups and flexible ether linkages that are

provided by BAPB, BAB and BPADA could greatly influence themolecular chain arrangements and charge transfer complex in-teractions, improving the optical transparency of samples. Inaddition, the introduction of flexible segments could adjust theratio of the hard segments and the flexible segments in the poly-imides' chains, so that a good shape memory function was suc-cessively obtained.

2.2. g-radiation experiments

Two kinds of colorless and transparent SMPI samples (A1 andA2) were firstly put into a 60Co isotope exposure chamber. Duringaccelerated test, the irradiation rate was controlled at 5 Gy/s andthe chamber temperature was maintained at 25 �C. The total radi-ation dosages were 1� 104 Gy, 1� 105 Gy and 1� 106 Gy, respec-tively, and the whole experiments were last for 55.6 h.

2.3. Characterizations

The organic functional groups of the samples before and after g-irradiation were characterized by Fourier transform infrared (FTIR;Spectrum Two, Perkin-Elmer, USA) spectra with the accessory ofuniversal attenuated total refraction (ATR). The scanning wave-number ranged from 650 to 4000 cm�1 at 4 cm�1 resolution. Theoptical transparency of the original samples as well as the irradi-ated samples with 0.13mm thick was tested on UV/Vis/NIR spec-trometer (Perkin-Elmer Lambda 950, USA) in 200e2000 nmwavelength. TGA (Mettler-Toledo TGA/DSC 1 STARe System, USA)was accepted for testing all samples' thermal stabilities. All runsoperated from 20 to 1000 �C at a temperature gradient of 10 �C/minunder the protection of dry nitrogen.

Storage modulus (E0) and Tg of the samples before and afterirradiated by g-radiation were characterized by DMA (TA INSTRU-MENT DMA Q800, USA). All samples with the thickness of 0.13mmwere cut into rectangular shape with dimensions of 20mm long by4mm wide and tested at a temperature gradient of 3 �C/min over25e250 �C. In addition, the frequency was performed at 1 Hz dur-ing all the experiments. Here, the corresponding temperature of thepeak values of the loss factors (tan d) was identified as Tg.

Fig. 2. Optical transmittances of A1 (a) and A2 (b) before and after g-irradiation.

Fig. 3. Thermal stabilities of A1 and A2: (a)

H. Gao et al. / Polymer Degradation and Stability 156 (2018) 245e251 247

The shape memory behaviors of original samples as well as theirradiated samples were also characterized by DMA Q800 in amodule of DMA controlled force. The detailed testing procedureswere as follows: (1) heating samples to approach their Tg at aheating rate of 5 �C/min; (2) applying a stress to extend A1 and A2at a loading rate of 0.1MPa/min for 7 and 9min, respectively; (3)cooling samples to 25 �C at a temperature gradient of 5 �C/min; (4)removing stress; (5) heating samples again at the same heating rateto approach their Tg; (6) repeating step (2) to step (5) for threetimes. It is important to note that the first cycle was set to eliminateresidual strains in the samples which might be generated duringthe preparation processes [44].

Themechanical properties of tensile standard samples (ISO 527-3-2003, Type I) before and after g-irradiation were tested on anInstron 5500R Universal Testing Machine (Instron Corp., USA) witha load cell of 5 kN. Tensile tests were carried out at the elongationrate of 5mm/min at ambient temperature.

3. Results and discussion

3.1. Chemical structure and optical transmittance

FTIR-ATR spectra of samples before and after irradiated by g-radiation (Fig. 1) illustrate that the characteristic peaks of typicalgroups, including the stretching vibration of CeNeC at 1365 cm�1,symmetric stretching of C]O at 1714 cm�1, and asymmetricstretching of C]O at 1781 cm�1, remain unchanged [12,30].Moreover, the locations and kinds of other organic functionalgroups have no changes with the increase of g-radiation dosage. Nonew functional groups were produced after varying dosage of g-radiation. In other words, after irradiated by 1� 106 Gy g-radiation,SMPIs' chemical structures stability is still good.

Fig. 2 shows the UVevis transmittance of the original and irra-diated samples over a wavelength range of 200e2000 nm. Theoptical transmittance of A1 shows a little fluctuation with the in-crease dosage of g-radiation, whereas A2 could better maintaintheir optical properties than A1 under g-radiation. The stable

, (c) TG curves and (b), (d) DTG curves.

Table 1Properties of the samples before and after g-irradiation.

Title Dosage (Gy) E'G (MPa) at 25 �C E'I (MPa) at Tg-20 �C E'R (MPa) at Tgþ20 �C Tg (�C) by DMA Tg (�C) by DSC Td (�C) Tmax (�C) Char yield (%)

A1 0 1878 1390 4 178 165 516 540 531� 104 1861 1389 5 179 165 516 540 521� 105 1851 1391 4 178 165 516 539 521� 106 1859 1381 4 179 165 516 540 54

A2 0 1864 1253 5 196 183 513 537 541� 104 1867 1296 4 196 183 513 538 531� 105 1861 1200 4 195 182 513 537 521� 106 1875 1240 4 195 182 513 538 54

H. Gao et al. / Polymer Degradation and Stability 156 (2018) 245e251248

optical transparency indicates that these SMPI samples have greatpotential application for employing as the substrates for spaceflexible electronics, which can bring in the shapememory functionswithout affecting their good electrical performances. It is worth tonote that the transmittance of A2 was 86% at the wavelength of450 nm, better than that of A1 (60% at the wavelength of 450 nm).

3.2. Thermal stability

The TGA and derivative thermogravimetric (DTG) curves over25e1000 �C of samples before and after g-irradiation were illus-trated in Fig. 3. Comparing the original and irradiated samples,there is no obvious change of the decomposition temperature (Td)at the weight loss of 5wt% and the temperature at the maximumweight loss (Tmax). The Td and Tmax of A1 are stayed at 516 �C and540 �C, respectively. The Td and Tmax of A2 are about 513 �C and537 �C, respectively. The detailed information of Td and Tmax of A1and A2 are listed in Table 1. The only difference between the

Fig. 4. E0 and tan d of A1 (a) and A2 (b) before and after g-irradiation.

original and irradiated samples was that the char yield at600e1000 �C. From Fig. 3(a) and (c), we can conclude that theamount of char yield firstly decreased and then increased with theincrease of g-radiation dosage. While, the char yield of all sampleswas still over 50wt%, as listed in Table 1, indicating good durabilityin harsh g-radiation environments.

3.3. Thermomechanical property

Tg is one of the most important parameters to characterizewhether the polymer materials are suitable for utilizing in thepractical applications or not. Differential scanning calorimetry(DSC) and DMA are usually utilized to detect Tg. The DSC curves ofA1 and A2 before and after g-irradiation were shown in Fig. S1,which were tested by Mettler/Toledo DSC 1 STARe System at the

Fig. 5. Mechanism illustration of SMPIs before and after g-irradiation.

H. Gao et al. / Polymer Degradation and Stability 156 (2018) 245e251 249

atmosphere of nitrogen over 25e300 �C. The thermomechanicalproperties of A1 and A2 characterized by DMA Q800 were illus-trated in Fig. 4(a) and (b) respectively. It can be seen from Fig. 4 thatE0 and Tg of samples after irradiated by varying dosage of g-radia-tion remained unchanged. Similar results could be obtained fromDSC results (illustrated in Fig. S1) where Tg also had no obviouschanges after 0e106 Gy g-radiation. The performances of original

Fig. 6. The shape recovery curves of A1 (I) and A2 (II) irradiated by (a)

samples and the irradiated samples, including the E0 at glass state(E'G, 25 �C), intermediate state (E'I, Tg-20 �C), rubbery state (E'r,Tgþ20 �C), and Tg, are listed in Table 1. From Table 1 we can see thatthere are large differences in E0 at glassy state and rubbery state,which guaranteed the good shape memory performances forSMPIs. With the increase of g-radiation dosage, A1 and A2 with a Tgof 178 �C and 196 �C could maintain their own good

0 Gy, (b) 1� 104 Gy, (c) 1� 105 Gy, and (d) 1� 106 Gy g-radiation.

Fig. 7. Mechanical properties of A1 (a) and A2 (b) irradiated by varying dosage of g-radiation.

H. Gao et al. / Polymer Degradation and Stability 156 (2018) 245e251250

thermomechanical properties. Moreover, the samples with a Tg ofless than 200 �C could be used in the election and proton radiationenvironment with no detectable degradation of thermomechanicalproperties.

3.4. Shape memory behavior

The mechanism of the shape memory effect of SMPIs is illus-trated in Fig. 5. The shape memory effect of thermoplastic SMPIsrelies on the phase transformation of glass-rubber transitions.SMPIs (permanent length: L) were programmed at the rubberystate through mechanically stretching when the temperature wasabove the samples' Tg. These stretched SMPIs (stretched length:L þ L0) could fix the temporary shapes when temperature wentbelow the Tg (fixed length: L þ L00). At this moment, SMPIs sampleswere in glassy state. The physically cross-linked SMPIs could returnto their permanent shapes (permanent length: L) under the ther-mal actuation (a temperature above the Tg). Two crucial factors areutilized to characterize the shape memory effects of SMPIs, i.e.shape recovery ratio (Rr) and fixation ratio (Rf). Rr represents thecapacity of SMPIs to recover the permanent shapes, and Rf standsfor the ability of SMPIs to hold the temporary shapes.

The shape memory behaviors of original samples as well as theirradiated samples are illustrated in Fig. 6, and the Rf and Rr werecalculated by equations (1) and (2), respectively. The test resultsincluding the average values are listed in Table 2. The average Rf andRr of the samples are over 97% and 95% respectively even if theywere exposed to g-radiation at the dosage range of0 Gye1� 106 Gy. In other words, A1 and A2 with a Tg of 178 �C and196 �C still have high Rf and Rr in the stimulated g-radiation envi-ronments. The sharp decrease of E0 between glass state (E'G, 25 �C)and rubbery state (E'R, Tgþ20 �C) is responsible for the excellentshape memory performances of SMPIs. In addition, the stablethermomechanical properties guarantee the good shape memoryperformances of the irradiated samples.

Rf ðNÞ ¼εiðNÞεload

� 100% (1)

RrðNÞ ¼ εloadðNÞ � εrecðNÞεloadðNÞ � εrecðN � 1Þ � 100% (2)

Here εload, εi and εrec are the strain in the stretched shape above Tg,the fixed shape below Tg, and the recovered shape during eachcycle, respectively, and N is the cycle number.

3.5. Mechanical properties

The mechanical properties of A1 and A2 after irradiated by

Table 2Shape memory performances of A1 and A2 before and after g-irradiation.

Title Dosage (Gy) Rf (%) Rr (%)

Shape memorycycles

Average Shape memorycycles

Average

1 2 3 1 2 3

A1 0 97.3 98.5 97.4 97.7 97.5 98.4 97.8 97.91� 104 97.2 97.2 97.2 97.2 94.0 96.3 97.4 95.91� 105 97.5 97.6 97.6 97.6 94.4 95.8 96.8 96.71� 106 97.3 97.3 97.4 97.3 94.0 96.8 97.2 96.0

A2 0 97.4 97.6 97.7 97.6 94.2 95.6 96.2 95.31� 104 97.9 98.2 98.1 98.1 96.4 97.4 98.1 97.31� 105 97.8 97.9 98.0 97.9 94.2 96.5 96.7 95.81� 106 98.1 98.1 98.2 98.1 92.2 97.5 97.7 95.8

varying dosage of g-radiation are shown in Fig. 7, all the samplesare tested at room temperature. The tensile stresses of A1 sampleswere 122 ± 6MPa, 120± 2MPa, 117± 1MPa and 116± 1MPa afterexposing to 0 Gy, 1� 104 Gy, 1� 105 Gy, and 1� 106 Gy g-radiation,respectively. With the increase of the g-radiation dosage, theelongation at break of A1 was 9.0± 1.4%, 8.5± 1.0%, 7.9± 2.1% and7.9± 0.8%, respectively. Similarly, with the increase of the g-radia-tion dosage, the tensile stresses of A2 were 111± 5MPa,113± 3MPa, 111± 4MPa and 114± 1MPa, respectively. The elon-gation at break were 9.7± 1.2%, 8.7± 1.0%, 8.2± 0.9% and 7.6± 0.1%,respectively, showing a downward trend with the increase of the g-radiation dosage. While, these tensile stress values of A1 and A2stay within 116e122MPa and 111e114MPa, respectively, showingno significant changes. These results indicate that SMPIs with a Tg ofless than 200 �C have minimal mechanical degradation under g-radiation environments. Similar results have been found by Nielsenet al. [41] and Hill et al. [42] that g-radiation has little impact onmechanical properties of LaRC CP-2 polyimide and Kapton andUltem polyimides.

4. Conclusions

This work demonstrates a systematic research of the durabilityof two kinds of optically transparent SMPIs with a Tg of less than200 �C in a stimulated g-radiation environment. SMPI samples kepttheir original chemical structures under the g-radiation conditions,where no new functional group was produced and locations of allthe characteristic peaks were not changed. The optical trans-parency of A1 shows a little fluctuation after irradiated by g-radi-ation, while, the optical transparency of A2 remained unchanged. Tg

H. Gao et al. / Polymer Degradation and Stability 156 (2018) 245e251 251

detected by DSC and DMA had no changes after irradiated byvarying dosage of g-radiation, and the corresponding E0 determinedby DMAQ800 was in good stability. A1 and A2 with a Tg of less than200 �C showed a stable Td and Tmax, and the char yields were allover 50wt%, indicating excellent thermal stabilities. After irradi-ated by varying dosage of g-radiation, these two kinds of SMPIscould maintain their good shape memory behaviors with Rf>97%and Rr>95%. The elongation at break had a little decrease with theincrease of the g-radiation dosage, however, the tensile stressvalues of A1 and A2 stayed within 116e122MPa and 111e114MPa,respectively. Thus, SMPIs with a Tg of less than 200 �C could bedurably utilized in space active deformation structures anddeployable electronics with no detectable damages of molecularstructures, optical transparency, shape memory behaviors, thermaland even mechanical properties.

Acknowledgments

This work was supported by the National Natural ScienceFoundation of China [Grant No. 11632005, 11672086] and by theFoundation for Innovative Research Groups of the National NaturalScience Foundation of China [Grant No. 11421091].

Appendix A. Supplementary data

Supplementary data to this article can be found online athttps://doi.org/10.1016/j.polymdegradstab.2018.09.014.

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