Triple-Shape Memory Materials Based on Cross-Linked Poly(ethylene

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Triple-shape memory materials based on cross-linked poly(ethylene vinyl acetate) and poly(#-caprolactone) Zhi-xing Zhang, Xiao Wei, Jing-Hui Yang, Nan Zhang, Ting Huang, Yong Wang, and Xiao-ling Gao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03438 • Publication Date (Web): 07 Nov 2016 Downloaded from http://pubs.acs.org on November 8, 2016

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Industrial & Engineering Chemistry Research

Triple-shape memory materials based on cross-linked poly(ethylene vinyl acetate) and poly(ε-caprolactone) Zhi-xing Zhang1, Xiao Wei1, Jing-hui Yang1, Nan Zhang1, Ting Huang1, Yong Wang1*, Xiao-ling Gao2* 1. School of Materials Science and Engineering, Southwest Jiaotong University, Key Laboratory of Advanced Technologies of Materials, Ministry of Education of China, Chengdu 610031, P. R. China 2. Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang, 621900, P. R. China

ABSTRACT: In this work, chemically cross-linked poly(ethylene vinyl acetate) (EVA) was melt-compounded with poly(ε-caprolactone) (PCL) to prepare the EVA/PCL blends. The chemical cross-linking reaction of EVA was induced by dicumyl peroxide (DCP) during the melt compounding processing. The effects of the cross-linking degree of the EVA component on the rheological properties, morphology, and melting and crystallization behavior of samples were systematically investigated. The results demonstrated that a morphological change from the quasi cocontinuous structure to the typical cocontinuous structure was induced with increasing the cross-linking degree of the EVA components. Thermal properties measurements and dynamic mechanical properties measurements demonstrated that the EVA/PCL blends exhibited two independent thermal transitions. The shape memory behavior measurements showed that the samples exhibited excellent triple shape memory effects (triple-SME). The corresponding mechanisms were then analyzed. Keywords: EVA/PCL, cross-linking structure, microstructure, shape memory behavior

1. INTRODUCTION Shape memory polymers (SMPs) are smart polymeric materials which can realize the switch between different shapes upon exposure to external stimulus.1-3 From the general shape memory mechanism, SMPs are composed of two different structures. One is fixed domain which provides the recovery force during the shape recovery process, and it can be molecule entanglement,4 crystalline phase,5-7 and chemical cross-linking structure,8 etc. The other is reversible domain 1

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which determines the temporary shape and responses to external stimulus, and it is usually related to crystallization/melting transition,9 glass transition,10 reversible molecule cross-linking structure,11 etc. The combined action of fixed and reversible domains determines the shape memory performances of the SMPs. SMPs can be divided into dual-SMPs and multiple-SMPs based on the number of temporary shape.12 No matter dual-SMPs and multiple-SMPs, they all possess only one original shape, but the temporary shape can be one for the dual-SMPs and more for the multiple-SMPs. If these multiple-SMPs include two temporary shapes, they are so called the triple-SMPs. Compared to the dual-SMPs which remember only one shape, the multiple-SMPs have the ability to recover to their permanent shape step by step from two or more temporary shapes,13 which endows the multiple-SMPs with more potential applications, especially in the fields which need accurately controlling and continuous operation. Obviously, realization of multiple-shape memory effect (multiple-SME) is mainly determined by the characteristic of the reversible domain. So far, there are at least two kinds of strategies to acquire the multiple-SME. One kind of strategy to achieve the multiple-SME is to use polymers with a broad thermal transition. In this system, the broad thermal transition is treated as a set of multiple thermal transitions and temporary shapes are programmed at multiple temperatures across the broad transition.14-16 For example, Nöchel et al.15 prepared the covalently cross-linked poly(ethylene-co-vinyl acetate) (EVA) and found that the material exhibited the triple shape memory effects (triple-SME) due to the presence of the broad melting transitions. Moreover, Samuel et al.16 reported that the poly(L-lactide) (PLLA)/poly(methyl methacrylate) (PMMA) blends exhibited the interesting triple-SME. In this system, the blends exhibit the broad glass transition which varies with blend compositions because of the good miscibility between components, and the triple-SME is also greatly dependent upon the blend compositions. The other methods to achieve the broad thermal transition are related to graft or block copolymerization of different components,17,18 or the chemical cross-linking coupled with supramolecular bonding,19 etc. However, it is worth noting that the method based on chemical reaction to achieve the broad thermal transition is very complicated while the method based on the use of miscible polymer blends is very limited because most polymer blends are immiscible rather than miscible. The other kind of strategy is constructing several reversible domains with well separated 2


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thermal transitions.20-24 For example, Zhao et al.21 prepared the chemically cross-linked polyethylene (PE)/polypropylene (PP) blends and they found that the PE/PP blend with 50 vol% of PE showed the pronounced triple-SME. In the blends, the reversible domains are related to the two crystallization/melting transitions of PE and PP, which are well separated. Similar phenomenon can be also found in other blend systems, such as polycyclooctene (PCO)/PE 22 and polyurethane (PU)/polylactide–polytetramethylene (PLA-PTMEG)

23

. Moreover, the other blend

systems containing two glass transitions or one crystallization/melting transition and one glass transition are also developed to obtain the triple-SMPs.24 For example, Chatani et al.25 prepared a composite material composed of dual polymer networks uniquely formed from a single reaction type and catalyst but involving monomers with dramatically different reactivities. The results demonstrated that the composite exhibited two narrow glass transitions and the difference in glass transition temperatures ( Tg ) was about 50 oC and consequently, the material exhibited the triple-SME based on the dual polymer networks. Luo et al.

26

reported a composite composed of

nonwoven poly(ϵ-caprolactone) (PCL) fibers embedded in an epoxy resin matrix. The triple-SME was achieved based on the glass transition of the epoxy domain ( Tg of 30 oC) and the crystallization/melting transition of the PCL domain ( Tm of 65 oC). Obviously, the strategy to construct several reversible domains to achieve the multiple-SME is more exciting, because this strategy can be applied in more polymer blends. In other words, through the appropriate microstructural controlling, most polymer blends might be endowed with the multiple-SME. The aim of this work is further seeking an appropriate method to prepare the SMPs based on immiscible polymer blends. Poly(ethylene vinyl acetate) (EVA)/poly(ε-caprolactone) (PCL) blend is selected as the research object according to the following aspects. EVA has good melt processability, low-temperature flexibility, good tensile ductility, and outstanding oil resistance. 27 Specifically, EVA is a kind of semi-crystalline elastomer with a tunable melting temperature depending on the change of vinyl acetate (VA) content. For example, at VA content of 7.5 wt%, the melting temperature ( Tm ) is about 100 °C, while Tm is decreased to 74 °C at VA content of 25 wt%.28,29 So far, much work has been carried out to prepare the EVA-based SMPs.15, 28, 30 For example, Li et al.30 investigated the effect of cross-linking structure on the SME of EVA and they 3



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found that relatively high gel fraction endowed the sample with excellent SME. The triple-SME of chemically cross-linked EVA is mainly ascribed to the broad melting transition.15 However, it is worth noting that the shape recovery process cannot be accurately controlled because the recovery processes in different stages may interlap each other. PCL has a relatively low Tm of about 60 °C and good tensile ductility, and especially it is biocompatible and fully biodegradable.31,32 The presence of PCL in the EVA does not apparently deteriorate the ductility of the material. Specifically, the EVA/PCL blends can exhibit two independent crystallization/melting transitions, which is much different from the chemical cross-linked EVA sample which usually exhibits a broad melting transition.15 Therefore, the blends of EVA/PCL with multiple-SME possibly exhibit wide potential applications in many fields.

2. EXPERIMENTAL SECTION 2.1 Materials EVA (trade name of Elvax 3120) with VA content of 7.5 wt% and Tm of about 100 °C was obtained from DuPont Industrial Polymers (USA). It exhibited a melt flow rate (MFR) of 1.2 g/10 min (190°C /2.16 kg) and a density of 0.93 g/cm3. PCL (trade name of CAPA*6806) was purchased from Solvay (Belgium) and the MFR was 7.3 g/10 min (160 °C/2.16 kg). Dicumyl peroxide (DCP) was purchased from Chengdu Kelong Chemical Reagent Company (Sichuan, Chengdu, China). 2.2 Sample preparation EVA and PCL were dried in an oven at 40 °C for about 8 h before they were used. The melt compounding was conducted on an internal mixer ZJL-300 (Changchun Zhineng Instrument, China) at a melt temperature of 160 °C. The screw speed was set at 50 rpm and the mixing duration was 5 min. The blends were prepared through two steps. In the first step, DCP was introduced into EVA to prepare the chemically cross-linked EVA. The degree of cross-linking was tuned by changing the DCP content (ranging from 0, 0.1, 0.2 to 0.5 wt%). In the second step, the cross-linked EVA was compounded with PCL to prepare the EVA/PCL blends. The weight ratio of EVA and PCL was maintained at 60:40. The cross-linked EVA was named as Dx based on the DCP content. For example, D0.2 represented that EVA was cross-linked by 0.2 wt% DCP. Similarly, the 4


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EVA/PCL blends were named as E6P4Dx. For example, E6P4D0.2 represented that the cross-linked EVA with 0.2 wt% DCP was blended with PCL. Finally, the cross-linked EVA and EVA/PCL blends were compression-molded at a melt temperature of 170 °C and a pressure of 5 MPa to obtain the plates with a thickness of 0.5 mm. 2.3 Fourier transform infrared spectroscopy (FTIR) A Fourier transform infrared spectroscope (FTIR) measurements were performed on a Thermo Scientific Nicolet 6700 spectrometer (Thermo Electron Corporation, USA). The measurements were carried out at a resolution of 2 cm-1 and a wavenumber range of 650-4000 cm-1 through a transmission mode. 2.4 Gel fraction measurement The gel fraction of sample was measured through a swelling equilibrium experiment. The initial weight ( M 0 ) of sample was accurately weighed, and then the sample was immersed in xylene at 65 °C for 24 h to reach a swelling equilibrium state. Subsequently, the solution was removed and the residual sample was carefully washed by fresh xylene for several times. After that, the residual sample was transferred in a vacuum oven at 60 °C for 4 h to remove the residual solvent. Finally, the weight of the residual sample ( M 1 ) was carefully weighed. The gel fraction ( G f ) of the sample was then calculated according to the following relation:

Gf =

M1 × 100% M0

(1)

For each sample, the measurements were repeated for three times and the average value of gel fraction was reported. 2.5 Scanning electron spectroscopy (SEM) The phase morphologies of the blends were characterized using a scanning electron microscope (SEM) Fei Inspect (FEI, the Netherlands) which was operated at an accelerating voltage of 20 kV. Before SEM characterization, sample was firstly cryogenically fractured in liquid nitrogen, and then the sample was immersed into N,N-Dimethylformamide (DMF) at 45 °C for 2 h to etch PCL component. Finally, the cryo-fractured surface was coated with a thin layer of gold. 2.6 Differential scanning calorimetry (DSC) The thermal properties of samples were investigated using a differential scanning calorimeter 5



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Pyris-1 (Perkin-Elmer, USA). The DSC scanning program was set as follows: the sample of about 5 mg which was cut from the compression-molded plate was firstly heated from 0 °C to 150 °C at a heating rate of 10 °C/min and maintained at 150 °C for 5 min, then the sample was cooled down to 0 °C at a cooling rate of 5 °C/min. All the measurements were carried out in nitrogen atmosphere. 2.7 Rheological measurement The rheological properties of samples were measured using a stress-controlled rheometer DHR-1 (TA Instrument, USA). The sample disk was prepared through a compression molding method, and it had a diameter and a thickness of 20 and 1 mm, respectively. The measurements were carried out at a melt temperature of 160 °C and a frequency range of 0.01–100 Hz. Specifically, the sample was protected in the nitrogen atmosphere to avoid the possible oxidative degradation. 2.8 Dynamic mechanical analysis (DMA) A dynamic mechanical analyzer Q800 (TA Instrument, USA) was used to measure the dynamic mechanical properties of samples. A rectangular cross-sectional bar (with a length of 25 mm, a width of 4 mm and a thickness of 0.5 mm) was used. During the measurements, a tensile mode was selected, and the sample was heated from 0 °C to 150 °C at a heating rate of 3 °C/min and a frequency of 1 Hz. 2.9 Shape memory measurements The shape memory performance measurement was conducted on the same DMA as mentioned previously. Experiments were carried out in a controlled force mode according to literature.21 A four-step program was employed on the DMA, as shown in Figure 1. In the first step (A), a rectangular sample was equilibrated at 115 °C, and then a constant stress was employed in 1 min and in this condition, the sample was stretched and a tensile strain of

ε 1 was achieved. And then

the sample was cooled down to a mid-temperature of 75 °C under the invariant stress. The stress was unloaded over 5 min, and the sample was kept at this temperature for 5 min to obtain the first temporary strain ε 1−1 . In the second step (B), the other constant stress was applied for the sample at 75 °C in 1 min and the sample was stretched again to achieve the tensile strain of

ε 2 , then the

sample was further cooled down to 20 °C and kept at this temperature for 5 min, after that the

6


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stress was released and the sample had the second temporary strain of ε 2 −1 . In the third step (C), the sample was heated to 80 °C and kept at this temperature for 25 min to obtain the second recovered strain ε 2 − 2 . In the fourth step (D), the sample was further heated to 115 °C and kept at 115 °C for 10 min to obtain the first recovered strain ε 1− 2 . Shape fixing ratio ( R f ) and shape recovery ration ( Rr ) were evaluated according to the following relations:

R f −1 =

ε 1 −1 × 100% ε1

(2)

Rf −2 =

ε 2 −1 × 100% ε2

(3)

Rr − 2 =

ε 2 −1 − ε 2 − 2 × 100% ε 2 −1

(4)

Rr −1 =

ε 2 − 2 − ε 1− 2 × 100% ε 2− 2

(5)

Where R f -1 and R f -2 represented the shape fixing ratios obtained in the first (Step A) and second (Step B) deformation processes, respectively, while Rr − 2 and Rr −1 represented the shape recovery ratios of sample obtained in the first (Step C) and second (Step D) recovery processes, respectively.

3. RESULTS AND DISCUSSION 3.1 Determination of the chemical cross-linking reaction of EVA induced by DCP The chemical cross-linking reaction of EVA induced by DCP was firstly confirmed through FTIR characterizations and gel fraction measurements. Figure 2 shows the FTIR spectra of pure EVA without DCP and chemically cross-linked EVA with different DCP contents. From Fig. 2a one can see several characteristic absorption bands. The characteristic absorption bands at 2915 and 2848, 1463 and 1370 cm-1 are mainly attributed to the stretching vibration of –CH2(symmetrical and asymmetrical stretching), the deformation vibration of –CH2-, and the flexural vibration of –CH3, respectively.33 Furthermore, the characteristic absorption bands at 1738, 1238 and 1019 cm-1 can be ascribed to the stretching vibration of -C=O, the asymmetrical stretching 7



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vibration of -CO-O-, and the symmetric stretching vibration of -O-CH- of ester groups,34 respectively. Compared with the pure EVA without DCP, although the characteristic absorption bands of cross-linked EVA samples do not show apparent shift, the intensity of the absorption bands of carbonyl and ester groups is greatly reduced. This demonstrates that the chemical cross-linking reaction mainly occurs among the chain segments of the VA component rather than among the chain segments of the ethylene component. The presence of cross-linking structure in EVA can be further demonstrated by gel fraction measurements as shown in Table 1. For pure EVA sample without DCP, only after being immersed into xylene at 65 °C for 6 h, it is completely dissolved and therefore, there is not any cross-linking structure in the pure EVA sample. However, a large quantity of cross-linking structure is induced in the EVA by adding DCP. The D0.1 sample exhibits G f of 91.0%, while for the D0.5 sample

G f is further increased up to 96.8%. The similar results have been reported by Nöchel U et al.15 In their work, 0.5 wt% DCP was introduced into EVA and G f was about 85%. The difference in

G f between this work and literature is most likely attributed to the difference in VA content. In this work, the VA content is only 7.5 wt% while in the literature, the VA content is 18 wt%. This indicates that most of VA component in this work can be cross-linked due to the relatively high concentration of DCP relating to VA component. 3.2 Microstructure and morphology of the EVA/PCL blends The processing flowability of the materials was investigated through rheological measurements. Figure 3 shows the rheological properties of all samples, including complex viscosity ( η * ), storage modulus ( G ′ ), and loss modulus ( G ′′ ). As shown in Fig. 3a, all samples show the shear-thinning characteristics on one hand. On the other hand,

η * increases with increasing DCP

content at low frequencies, which indicates that the flowability of the material is reduced due to the presence of a large quantity of cross-linking structure. Because the chemical cross-linking reaction occurs in the EVA component, the increase of to the increase of

η * of the blends can be mainly attributed

η * of the EVA component. In other words, the viscosity ratio between EVA

and PCL component increases with increasing DCP content. From Fig. 3b and 3c one can also see 8


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that at relatively low frequencies, G ′ and G ′′ increase with increasing DCP content. It is well known to all that G′ is mainly related to the elasticity of microstructure in the melt while G′′ is related to the molecular interactions and molecular mobility. Generally, the increase of

G′ indicates that the elastic response of the melt under the shear condition becomes more apparent. The intensified elastic response clearly demonstrates the presence of the cross-linking structure in the samples. And similarly, the higher the G′′ , the stronger the molecular interaction is or the smaller the mobility of the molecular chains is. Generally, the variation of viscosity ratio between components results in the morphological change of the blends. For the SMPs based on polymer blends, it has been demonstrated that the morphologies are also one of the main factors which determine the SME of the SMPs.28 In the present work, the morphological change of the blends was also characterized using SEM. The typical SEM images of samples are shown in Figure 4. In these images, the black zones represent the PCL component which was removed by DMF. The E6P4 sample shows a quasi cocontinuous structure. Besides the continuous PCL phase domain, the dispersed PCL particles are also observed in the continuous EVA phase domain. Once the chemically cross-linked EVA is used, although the weight ratio between components maintains invariant, an apparent morphological change from quasi cocontinuous structure to typical cocontinuous structure is observed. The morphological change can be attributed to the change of the viscosity ratio of EVA and PCL components due to the presence of cross-linking structure in the EVA component, which results in the increase of the viscosity of the EVA component. This indicates that the PCL component with continuous phase domain possibly exhibits more important role in shape memory process. As mentioned earlier, the realization of the triple-SME can be achieved if the SMPs exhibit a broad thermal transition or several reversible domains with well separated shape thermal transitions. Here, to confirm the independent crystallization/melting transitions of the blends, the melting and crystallization behaviors of samples were then investigated using DSC. Figure 5 shows the DSC thermograms of the EVA/PCL blends with different DCP contents during the first heating and cooling process, respectively. From Fig. 5a it can be seen that all of the samples show two independent endothermic peaks at about 55 °C and 100 °C, attributing to the melting temperature of the PCL ( Tm − PCL ) and EVA ( Tm − EVA ) components, respectively. Similarly, two 9



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independent exothermic peaks are observed at about 25-32 °C and 89-91 °C, which are attributed to the crystallization temperature of the PCL ( Tc − PCL ) and EVA ( Tc − EVA ) components, respectively. Different from the invariant Tm − PCL and Tm − EVA of samples with and without DCP, one can see that the samples with chemically cross-linked EVA component shows lower Tc − EVA but higher

Tc − PCL compared with the sample without DCP. The slight decrease of Tc − EVA is possibly related to the presence of cross-linking structure in the EVA component which restricts the nucleation and growth of crystallites. Obviously, the more the cross-linking structure is, the higher the degree of the restriction on crystallization. For the PCL component, it is interesting to observe that

Tc − PCL increases gradually with increasing DCP content. There are two possibilities which contribute to the gradually increase of Tc − PCL . One possibility is related to the enhanced heterogeneous nucleation effect of the EVA crystallites on the PCL crystallization and the other is possibly related to the morphological change of the blends. Considering the restriction effect of the cross-linking structure on crystallization of the EVA component, which usually results in the decrease of crystallinity, it is believed that the morphological change is the main reason for the enhanced crystallization ability of the PCL component. As demonstrated in Fig. 4, a morphological change from quasi cocontinuous structure to typically cocontinuous structure is observed with increasing DCP content from 0.1 wt% to 0.5 wt%. The increase of phase domain is favorable for the nucleation and growth of the PCL spherulites,35 which accordingly promotes the increase of Tc − PCL . Whatever, the above DSC results confirm that the EVA/PCL blends with selectively cross-linked EVA have two independent crystallization/melting transitions, which provide the prerequisite for the preparation of the triple-SMPs. Either for the shape fixing ability or for the shape recovery ability of the SMPs during the shape memory process, it is directly or indirectly related to the elastic modulus of sample. Therefore, investigating the modulus change of the SMPs is necessary. Figure 6 shows the dependence of elastic modulus of samples on temperature. From Fig. 6 one can see that all the samples exhibit three regions and two transitions. At temperature below 65 °C (Region 1), PCL and EVA components are in the crystalline state and these samples exhibit high storage modulus. The 10


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storage modulus of samples dramatically decreases in the temperature range of 65-80 °C (Transition I), which can be attributed to the melting of the PCL crystallites. In the range of 80-100 °C (Region 2), the PCL component of the blends is in the molten state while the EVA component is still in the crystalline state. Due to the presence of chemically cross-linking structure in the EVA component, one can see that the samples containing cross-linked EVA component exhibit relatively high storage modulus compared with the sample without cross-linking structure. Further increasing temperature, the melting of EVA crystallites occurs and the storage modulus also dramatically decreases (Transition II). In Region 3, it is very difficult for the E6P4 sample to maintain the enough geometric stability and the measuring cannot be further carried out. However, the presence of cross-linked EVA component greatly improves the stability of the samples, and especially the larger the degree of the cross-linking is, the higher the stability of the sample. Obviously, the relatively high modulus and high stability endows the sample with SME at relatively high temperature. 3.3 Shape memory performances From the above microstructural characterizations, it can be known that there are two separate crystalline phases in the EVA/PCL blend with melting transitions at distinctly different regions. Specifically, the presence of cross-linking structure in the EVA component endows the sample with relatively high storage modulus and geometric stability. According to the general shape memory mechanisms, the above results suggest that the EVA/PCL blends with cross-linked EVA components can show the triple-SME. Here, the triple-shape memory behavior of the EVA/PCL blends was examined by DMA measurements, and the shape memory properties are summarized. Figure 7 presents the variations of tensile strain of samples with varied stress, temperature and measuring time. Here, only the samples containing cross-linked EVA component were measured. The corresponding shape memory performances are calculated and listed in Table 2. From Table 2 one can see that all samples show R f −1 of about 91%-92% during the first deformation process. Here, to obtain the first temporary shape, the deformation temperature was set at 115 °C which exceeds Tm − EVA of the EVA component and in this condition, both EVA and PCL components are in the molten state with relatively low modulus as demonstrated in Fig.6. This indicates that samples have higher ability to be deformed. Once the deformed sample is 11



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cooled down to 75 °C which is lower than Tc − EVA of the EVA component, the crystallization of EVA occurs and the deformation of sample can be fixed. In other words, the melting/crystallization of the EVA component can be the reversible switch during the shape memory measurements. It is worth noting that R f −1 is smaller than 100%, which is possibly related to the volume shrinkage induced by crystallization on one hand. On the other hand, the temperature (75 °C) is still higher than Tm − PCL of the PCL component and in this condition, the spontaneous recovery of the deformed PCL component occurs to a certain extent once the external force is unloaded. In the second deformation process, the EVA component is in the crystalline state while the PCL component is still in the molten state and in this condition, the deformation of the sample is simultaneously related to the deformation of the PCL component and the slight deformation of the crystalline EVA component. Once the sample is deformed at 75 °C and cooled at 20 °C, the deformation of the sample can be further fixed through the crystallization of the PCL component. In this condition, the melting/crystallization of the PCL component is also the reversible switch while the crystallites and cross-linking structure of EVA are the fixed phase during the shape memory measurements. Consequently, all samples exhibit high R f − 2 in this deformation process. R f − 2 is slightly larger than R f −1 , which is possibly related to the different deformation mechanisms and the different deformation temperatures. Furthermore, it is worth noting that either for R f −1 or for R f − 2 , all the samples show the similar values, which indicates that the effect of the cross-linking degree on shape fixing ratios of samples is inconspicuous. For the recovery process, Rr − 2 and Rr −1 are used to characterize the shape recovery ability of samples during the first and second shape recovery processes. It can be seen that the shape recovery ability of all samples during the first shape recovery process are rather low. Rr − 2 is only about 50% for the E6P4D0.1 sample, and further increasing the cross-linking degree of the EVA component results in lower Rr − 2 . This can be explained as follows. In the first recovery process, the recovery temperature (80 °C) is only higher than Tm − PCL of the PCL component and therefore, the shape recovery of sample is mainly related to the recovery of the deformed PCL component 12


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and partly related to the recovery of the deformed EVA component, and the recovery driving force is provided by the relaxation mobility of PCL segments when the temperature is upon the transition temperature and the elasticity of the EVA component. Specifically, the deformed EVA component obtained during the second deformation process cannot be recovered completely because the deformation of EVA component is possibly related to the destroy and orientation of the crystalline phase, which only can be recovered at temperature higher than Tm − EVA . Besides, the poor interfacial interaction which is unfavorable for the stress transferring from one component to the other36.37 and the constrained interface domain induced by the component with high transition temperature 26,38,39 also decrease the recovery capability. So, the relatively low Rr − 2 is obtained in the first recovery process. More interesting phenomena are observed during the second shape recovery process. The E6P4D0.1 sample shows Rr −1 of only about 57.5%. However, further increasing the degree of cross-linking in the EVA component, largely increased Rr −1 is obtained and the value is increased up to 100%. This indicates that the deformation of the E6P4D0.2 and E6P4D0.5 samples obtained during the previous two deformation processes can be completely recovered. In this process, the crystallized EVA melts and the amorphous EVA segments are activated and therefore, samples have a tendency to recover to the original shapes. The cross-linking structure in the EVA component is the fixing phase which provides the driving force for the recovery of the deformation. Obviously, increasing the amount of cross-linking structure facilitates the enhancement of Rr −1 . Furthermore, it has been demonstrated that with the increase of cross-linking structure, samples exhibit the morphological change from quasi cocontinuous structure to the typical cocontinuous structure, which is possibly the other reason for the increased

Rr −1 with increasing cross-linking structure,21,22 although the relationship between morphology and shape recovery ratio is still not very clear so far. To better understand the shape memory behavior of the EVA/PCL samples, the temporary shape and the recovered shape of the representative E6P4D0.5 sample obtained during the shape memory measurements are shown in Figure 8. The sample with original shape (A) is stretched at 115 °C and fixed at 75 °C to obtain the first temporary shape (B), then the sample is further 13



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bended at 75 °C and fixed at 20 °C to obtain the second temporary shape (C). Upon heating, the bended sample is heated to 80 °C and the sample is recovered to the first temporary shape (B). Further increasing temperature to 115 °C, the stretched sample is completely recovered to the original shape (A). These images clearly demonstrate the excellent triple-SME of the EVA/PCL blends with the cross-linked EVA component. Furthermore, to test the stability of the triple-SME of the blends, the E6P4D0.5 sample was also selected and the shape memory measurement was repeatedly carried out on a same sample for five times at the completely same conditions as shown in Fig. 8. The first and second temporary shape and shape recovery processes in each cycle were then recorded, and the results are shown in Figure 9. It can be clearly seen that the sample exhibits the consistent shape memory behaviors in each cycle and there is no apparent degeneration of the shape memory performance. Even if the measurements have been carried out for 5 times, the sample can still maintain its original shape. This indicates that the sample exhibits relatively high stability in shape memory performance. It is then believed that the blends may exhibit great potential applications in many fields, such as temperature sensor or actuating elements.

4. CONCLUSIONS In summary, we have fabricated the EVA/PCL blends with cross-linked EVA components. The results demonstrate that introducing DCP into EVA through melt compounding processing induces the occurrence of the cross-linking reaction. The more the DCP in the sample is, the more the cross-linking structure. For the EVA/PCL blends, the cross-linking degree of the EVA component greatly influences the microstructure and morphology of the EVA/PCL blends. For example, the crystallization temperature of the PCL component is apparently enhanced and the morphological change from quasi cocontinuous structure to the typical cocontinuous structure is induced with increasing the cross-linking degree of the EVA component, although the weight ratio between EVA and PCL maintains invariant in all samples. The shape memory behavior measurements demonstrate that the EVA/PCL blends with separated thermal transitions exhibit the typical triple-SME. Specifically for the samples with high cross-linking degree in the EVA component, the deformation obtained during the two shape deformation processes can be completely recovered at the appropriate conditions.

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AUTHOR INFORMATION

Corresponding Author *Tel.: +86-28-87603042. E-mail: [email protected] (Wang Y) and [email protected] (Gao XL).

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS

Authors express their sincere thanks to the National Natural Science Foundation of China (51473137).

REFERENCES

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Table 1: Variation of gel fraction of the chemically cross-linked EVA with different contents of DCP. Sample

EVA

D0.1

D0.2

D0.5

0

91.0

91.7

96.8

G f (%)

Table 2: Shape fixing ratios and shape recovery ratios of the EVA/PCL blends. Sample

R f −1 /%

R f − 2 /%

Rr − 2 /%

Rr −1 /%

E6P4D0.1 E6P4D0.2 E6P4D0.5

92.1 90.9 91.1

96.4 96.5 95.6

50 39 44.9

57.5 100 100

19



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Figure captions: Figure 1: Schematic representations showing the programs set for the shape memory measurements in this work. Figure 2: (a) Comparison of FTIR spectra of pure EVA and cross-linked EVA samples with different DCP contents, (b) and (c) showing the detail of the FTIR spectra in local wavenumber ranges. Figure 3: Rheological properties of the blends obtained in this work. (a) Complex viscosity, (b) storage modulus, and (c) loss modulus. Figure 4: SEM images showing the morphologies of the different samples. (a) E6P4, (b) E6P4D0.1, (c) E6P4D0.2, and (d) E6P4D0.5. Figure 5: DSC heating (a) and cooling (b) curves showing the melting and crystallization behaviors of all samples. Figure 6: Variations of storage modulus of all samples with increasing measuring temperature. Figure 7: The triple-shape memory behavior of the different samples recorded during the DMA measurements. (a) E6P4D0.1, (b) E6P4D0.2, and (c) E6P4D0.5. Figure 8: Images showing the triple shape memory process of the representative E6P4D0.5 sample. A: Original shape, B: the first temporary shape, C: the second temporary shape. Figure 9: Images showing the triple shape memory process of the representative E6P4D0.5 sample. The same sample was repeatedly measured for 5 times. A: Original shape, B: the first temporary shape, C: the second temporary shape. The x of Ax (or Bx and Cx) represents the cycle times. For example, B2 represents the first temporary shape in the second cycle. A0 represents the original sample.

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Figure 1

21



(a)

Absorbance

D0.5

D0.2

D0.1

EVA 3200

2800

2400

2000

1600

1200

800

-1

Wavenumbers (cm )

(b)

1239.64

(c)

1740.31

D0.5

D0.5 1239.85

1740.12

D0.2

D0.2

Absorbance

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 30

1740.29

D0.1 1738.77

1239.87

D0.1 1238.92

EVA

1760

1755

1750

1745

1740

1735

1730

1725

EVA

1720

1270

1260

1250

1240

1230 -1

Wavenumbers (cm )

-1

Wavenumbers (cm )

Figure 2

22


1220

1210

Page 23 of 30

5

10

Complex viscosity (Pa⋅s)

(a)

4

10

3

10

E6P4 E6P4D0.1 E6P4D0.2 E6P4D0.5 2

10

0.1

1

10

100

Frequency (Hz) 10

5

10

4

10

3

(b)

E6P4 E6P4D0.1 E6P4D0.2 E6P4D0.5 0.1

1

10

Loss modulus (Pa)

Storage modulus (Pa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10

5

10

4

10

(c)

E6P4 E6P4D0.1 E6P4D0.2 E6P4D0.5

3

100

0.1

Frequency (Hz)

1

Frequency (Hz)

Figure 3

23


10

100


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(a)

Page 24 of 30

(b)

50µm

50µm

(c)

(d)

50µm

50µm

Figure 4

24


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(a)

Tm-PCL

(b)

Tm-EVA

E6P4D0.5

E6P4D0.5

Exothermic

E6P4D0.2

Endothermic

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


E6P4D0.2

E6P4D0.1

E6P4

E6P4D0.1

Tc-PCL E6P4

20

Tc-EVA 40

60

80

100

120

20

o

40

60

80 o

Temperature ( C)

Temperature ( C)

Figure 5

25


100

120


Region 1 100

Storage modulus (MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Transition Ι 10

Region 2 1

Transition ΙΙ E6P4 E6P4D0.1 E6P4D0.2 E6P4D0.5

0.1

0.01 0

20

40

Region 3

60

80

100

120

o

Temperature ( C)

Figure 6

26


140

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Page 27 of 30

1.0 120

60

(a)

50

0.8

80 30 60

Strain/%

0.4

40

o

0.6

Temperature/ C

Stress/MPa

100

20 40

0.2

10 20

0.0

0 0

20

40

60

80

Time/min 1.0

1.0

60 120

60 120

(b) 50

0.8

0.8

(c) 50

100

20

0.4

40

80 60

30

40

20

20

10

40 0.2

0.2

10 20

0.0

0 0

20

40

60

0.0

80

0

0 0

20

Time/min

40

Time/min

Figure 7

27


60

80

Strain/%

30

60

o

Stress/MPa

o

0.6

80

Strain/%

0.4

40

Temperature/ C

0.6

Temperature/ C

100

Stress/MPa

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 8

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29



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For Table of Contents Only

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Triple-Shape Memory Materials Based on Cross-Linked Poly(ethylene

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