Thermally and Electrically Triggered Triple-Shape Memory Behavior of

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Thermally and Electrically Triggered Triple-shape Memory Behavior of Poly(vinyl acetate)/Poly(lactic acid) Due to Graphene-induced Phase Separation Mohammad Sabzi, Masoud Babaahmadi, and Mohammadreza Rahnama ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 22 Jun 2017 Downloaded from http://pubs.acs.org on June 22, 2017

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ACS Applied Materials & Interfaces

Thermally and Electrically Triggered Triple-shape Memory Behavior of Poly(vinyl acetate)/Poly(lactic acid) Due to Graphene-induced Phase Separation Mohammad Sabzia*, Masoud Babaahmadia, Mohammadreza Rahnamaa a

Department of Chemical Engineering, Faculty of Engineering, University of Maragheh, Maragheh 5518183111, Iran

Abstract This work aimed to develop a facile and broadly applicable method for fabricating multistimuli responsive triple-shape memory polymers (SMPs). Hence, herein the SMPs were prepared through the simple physical blending of two commercially available biopolymers, poly(lactic acid) (PLA) and poly(vinyl acetate) (PVAc), in the presence of robust and conductive graphene nanoplatelets. Interestingly, atomic force microscopy (AFM) observations and thermal analyses revealed that the presence of nanofillers led to phase separation and appearance of two well-separated transition temperatures in the blend of these two miscible polymers. Consequently, shape memory results showed that the unfilled blend of PLA/PVAc with a single thermal transition can only show moderate heat triggered dual-shape memory behavior. While, PLA/PVAc/graphene nanocomposite blends demonstrated excellent thermally and electrically actuated triple-shape memory effects besides their remarkable dual-shape memory behavior. In addition, electrical conductivity of the blend was enhanced by ∼14 orders of magnitude in the presence of graphene. More interestingly, electroactive shape recovery experiments exhibited that

*

Corresponding author. E-mail address: [email protected]

ACS Paragon Plus Environment


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depending on the applied voltage, temporary shapes in each region of sample can be either individually or simultaneously recovered. Keywords: Poly(vinyl acetate); poly(lactic acid); graphene; phase separation; thermal and electrical actuation; triple-shape memory polymer.

1. Introduction Shape memory polymers (SMP) are a type of smart materials that exhibit the ability to switch from their temporary shape(s) to original shape upon exposure to external stimuli, such as heat 1-6, electricity 7-13, alternating magnetic field 14, radio frequency 15

, light radiation 16, solvent 17 and pH 18.

Dual-shape polymers are the most often investigated type of SMPs. A limitation to conventional dual-SMPs has been their restriction to two shapes: one temporary shape and one permanent shape. Addressing this limitation, recently “triple-SMPs” have drawn much attention of the current researches. Triple-SMPs can memorize two temporary shapes and one permanent shape. The SMPs with multi-shape memory effects are desired for many applications that require adjustable complex shapes, such as precisely smart medical devices, smart valves and assembly systems, deployable space structures 19-24. Generally, multi-shape recovery processes can be achieved via two or even more distinct thermal transition temperatures in multi-phase SMPs 25 or SMP possessing a wide thermal transition region associated with the switching domains

26

. These strategies can be realized by grafting,


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copolymerization, blending or preparation of interpenetrating polymer network (IPN) 25-27. Polylactide (PLA) is a biocompatible, biodegradable polymer with high mechanical strength

28

. Very recently, we have shown that dual-shape memory properties of

PLA can be improved through the incorporation of graphene nanoplatelets

4-5

.

However, the shape memory effect of PLA is mostly limited to small deformations. Moreover, the glass transition temperature (Tg) of PLA is around 60 °C, which limits its direct use in biomedical applications. A series of thermally induced SMPs based on PLA were reported in the literature 7. For example, Zhang and co-workers fabricated an immiscible SMP by blending Poly(styrene-butadiene-styrene) (SBS) and poly(D,L-lactic acid) (dl-PLA) [18]. Zhang et al. also

29

prepared a

PLA/biodegradable polyamide elastomer (PAE) partial miscible blend with good shape memory capability at high deformations. On the other hand, we recently observed that poly(vinyl acetate) PVAc/graphene nanocomposites ehibit good thermally and electrically induced dual-shape memory behavior with a fast recovery rate

6, 13

. This was correlated to high stored elastic

strain energy in nanocomposites, which provided a high driving force for subsequent quick and complete shape recovery. Nevertheless, the Tg of the developed SMPs based on PVAc was below the room temperature, which can limit their use in many applications. Considering that both the PLA and PVAc demonstrated good shape memory effect in the presence of graphene nanoparticles 6, 13

, hence, in the work reported here we aimed to prepare a blend of PLA and PVAc



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and evaluate the effect of graphene incorporation on its shape memory properties. Also, it was reported that these two polymers are miscible and their blends show a single Tg

30

. Thus, the blending of PVAc with PLA can simultaneously increase

their toughness and decrease the Tg value without deteriorating shape memory efficiency 30-31. Mather et al. qualitatively demonstrated that a blend of PLA/PVAc, with a weight ratio of 30:70, has a quick and excellent shape recovery upon immersing in a hot water bath

31

. Thus, herein we prepared SMPs based on

PLA/PVAc/graphene nanocomposites with the same weight ratio to quantify their shape memory properties with thermomechanical analysis. Interestingly, we observed two distinct transition temperatures for the prepared nanocomposites, implying that the presence of graphene led to phase separation in the blend. Consequently, the PLA/PVAc nanocomposites are expected to display triple-shape memory behavior besides their dual-shape memory effect. Furthermore, direct heating is the most common method to trigger SMPs. While, these heating methods might not be applicable in all cases, such as in-vivo and aerospace devices. A straight forward

approach for indirect heating is the

application of heat by electricity. When electrical conductive fillers were incorporated into the SMPs, shape memory effect could be actuated indirectly by the generated internal Joule heating 12-13. Surprisingly, triple-SMPs that can be actuated by both direct heating and electricity stimuli are rare in the literature

32

. Hence, here we report a new and broadly

applicable method for designing and fabricating triple-SMPs with both thermal and


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electric actuations. The SMPs were prepared through the simple physical blending of two commercially available biopolymers (i.e. PLA and PVAc) in the presence of graphene nanoplatelets. We expect that graphene nanoplatelets with ultra-high modulus and elasticity, exceptional thermal and electrical conductivity, high specific surface area and aspect ratio will endow remarkable thermally and electrically activated shape memory responses to the matrix. 2. Experimental 2.1 Materials PLA (NatureWorks 3251D) was obtained from Jamplast Inc. (Ellisville, MO). PVAc (Mw = 125000-175000 g/mol) was provided from Wacker Chemie AG. Chloroform (99.8%) was purchased from Merck Co. Commercially available graphene nanoplatelets, grade of N002-PDR (with average number of graphene layers < 3, average X & Y dimensions ≤ 10 um, true density ≥ 2.2 g/cm-3, specific surface area = 400–800 g/m2, carbon content

≥ 95.0%,

hydrogen content ≤ 2.0%, oxygen content ≤ 2.5% and nitrogen content ≤ 0.5%) were prepared from Angstron Materials, Inc. (Dayton, OH). 2.2 Preparation of samples PLA/PVAc/graphene nanocomposites containing 3 and 4.5 wt% graphene were prepared via solution casting method. Desired amount of graphene powder was poured into chloroform as the solvent and stirred at room temperature for 8 h with aid of a magnetic stirrer, and then sonicated (Misonix



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S3000 sonicator) for 5 min. Afterward, PLA and PVAc (with a weight ratio of 30:70) were added to the homogenous graphene suspension and dissolved by stirring for another 8 h at room temperature, followed by more dispersing of graphene particles in the solution by sonication for 5 min. The unfilled PLA/PVAc (30:70) blend was prepared with the same procedure, except no nanofiller was added. The prepared solutions were then poured into a silicone mold and the solvent was allowed to evaporate at room temperature for 24 h, followed by further drying in a vacuum oven at 90 °C for 12 h. Finally, sheets of the samples with a thickness of ∼0.5 mm were prepared by a hot press machine at 180 °C. 2.3 Characterization of materials Wide angle X-ray diffraction (WAXRD) profile of samples was obtained using a JEOL X-ray diffractometer (JDX-8030) apparatus with CuKa radiation operating at voltage of 40 kV and current of 44 mA. The height and phase-mode images of the surface of samples were taken at room temperature using a Dualscope/Rasterscope C26, DME, Denmark, Atomic Force Microscope (AFM) equipped with a DS 95-50-E scanner. Differential scanning calorimetry (DSC) was performed using a DSC200F3 (Netzsch Instruments, Germany) under nitrogen atmosphere. Samples were first heated from -20 to 120 °C with a heating rate of 10 °C /min, and then kept isothermally at 120 °C for 3 min to erase their previous thermal history. Afterwards, they were cooled to -20 °C at 10 °C /min followed by scanning


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from -20 to 120 °C with the same rate. The Tg of samples was chosen as the middle point of the step transition peak. Dynamic mechanical analysis (DMA) of samples was performed on a Triton (Tritec 2000 DMA) instrument with dynamic strain of 0.1% under tensile mode. Rectangular specimens (25 × 10 × 0.5 mm3) were heated from 0 to 90 °C with a rate of 3 °C/min and operating frequency of 1 Hz. Thermally induced dual-shape property of samples was quantitatively measured using the same DMA instrument under controlled force mode as follows

33-34

: (i)

heating with a rate of 3 °C/min and isothermal holding at 80 °C, above the switching temperature of samples, (ii) ramping the force to a strain of ∼60-70%, (iii) cooling to -20 °C at 3 °C/min, and equilibrating at -20 °C for 20 min while keeping the external load constant, (iv) fixing the temporary shape by unloading the force, and (v) recovering the permanent shape with continuous heating to 80 °C at 3 °C/min. This cycle was then conducted consecutively for one more time on the same sample. The dual-shape memory properties of samples were obtained in terms of the fixing ratios (Rf) and recovery ratios (Rr) through the equations (1) and (2), respectively 34: % =

% =

× 100 1

− × 100 2 − − 1

where εload, ε, εrec and N stand for the maximum strain before unloading, the strain upon unloading, the permanent strain after shape recovery, and the cycle number, respectively.



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The triple-shape properties of nanocomposites were further quantified by measuring the fixing ratios (Rf) and recovery ratios (Rr) through the following equations 12: % =

,

→ % =

× 100 3

− , × 100 4 −

where εx,load, εx, and εx,rec represent the maximum strain before unloading, the strain after unloading, and the obtained strain after recovery of shape x (x can be A, B or C, the same for y). For each sample, two Rf values corresponding to the fixing of temporary shapes B and C, and three Rr values, including R C → B, R B → A and R C → A were reported. A scheme of the triple-shape memory experiment is illustrated in Figure 1. Four-probe system (Jandel RM3000) was also used to measure the electrical conductivity of samples at room temperature.

Figure 1. Schematic description of quantitative measurement of triple-shape memory behavior of samples.

In order to evaluate electroactive shape memory behavior of nanocomposites, a sheet of samples was cut into a specific geometry. Afterward, they were


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bended into a ‘‘U’’ like shape at 80 °C and then cooled down quickly to shape fixing temperature (5 °C). The shape recovery of samples was investigated using a DC electrical power source at constant applied voltages of 40, 50 and 70 V. The shape recovery process was recorded using a digital video camera, and snapshots were then analyzed with ImageJ software to obtain the deformation angle θ(t) at a time of t. The recovery ratio, R, is defined as 12:

% =

$% − $& × 100 5 180 − $&

where θ0 is the deformation angle at the time of 0 s.

3. Results and discussion 3.1 Microstructure analyses XRD analysis was used to investigate the dispersion state of graphene nanoplatelets in the matrix. Figure 2 compares the XRD patterns of the pristine graphene, neat PLA/PVAc and nanocomposites containing 3 and 4.5 wt% graphene. The primary graphene powder showed a very weak and broad peak centered at ∼26.4° corresponding to the coexistence of single graphene layers and some graphene layers organized in stacks 35. The PLA/PVAc/graphene nanocomposites with various graphene contents show a typical WAXD pattern of the neat matrix, while there are no peaks of layered structure of graphene. The lack of the diffraction peaks of graphene could be attributed to the exfoliation and random distribution of the graphene platelets within the matrix during composite processing 36.



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Figure 2. Comparative XRD patterns of the as-received graphene, neat PLA/PVAc, and PLA/PVAc nanocomposites with 3 and 4.5 wt% graphene.

The AFM images of neat PLA/PVAc and PLA/PVAc nanocomposite containing 4.5 wt% graphene are shown in Figure 3 (a) and (b), respectively. The left images were for height and the right images were for phase. A homogeneous morphology can be observed in the phase image of the neat PLA/PVAc blend (Figure 3 (a)). While, Figure 3 (b) shows that the presence of graphene nanoplatelets led to heterogeneity and phase separation in the nanocomposite. As the lighter areas in the phase image of the nanocomposite are PLA-rich domains, while the darker areas represent PVAc-rich phase. Similar morphology changes with embedding nano fillers have been previously reported in the literature

37-39

. For example, carbon nanotubes

(CNTs) induced a morphological change from co-continuous morphology to sea-island morphology in PLA/thermoplastic polyurethane (TPU) shape


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memory blend composites 37. The phase separation phenomenon in the current study can be explained in a manner similar to that obtained by Bose et al

38

.

They have also found that loading of 2 wt% CNTs reduces the macromolecular mobility of the blend components and induces phase separation at lower temperatures as compared with the neat blends.

Figure 3. The AFM images of the (a) neat PLA/PVAc and (b) PLA/PVAc nanocomposite containing 4.5 wt% graphene. The left images were for height and the right images were for phase. The ranges of each image were all 10 µm × 10 µm.



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3.2 Thermomechanical analyses Analyzing the dynamic mechanical properties of SMPs is important because it can predict the shape memory behavior and provide perspective for programming thermomechanical conditions for shape fixing and shape recovery processes. Hence, the dynamic mechanical properties of the neat PLA, PVAc, PLA/PVAc blend and its corresponding nanocomposites were examined by DMA. The storage modulus (E′) and loss factor (tan δ) as a function of temperature are displayed in Figure 4, and the results are listed in Table 1. Unfilled PLA/PVAc blend exhibits a wide decrease of E′ at 10-55 °C and a broad tan δ peak centered at 33 °C attributed to the Tg of the blend. Interestingly, as it can be seen from Figure 4, both of the nanocomposites show two well-separated thermal transitions, with the step transition temperatures at 25-26 °C and 45-47 °C corresponding to the glass transitions of PVAc-rich phase and PLA-rich domains, respectively. Accordingly, one glassy modulus plateau (E′5

°C)

and two rubbery plateaus (E′35

°C

and E′60

°C)

were observed; one lying between the Tg of PVAc-rich phase and Tg of PLA– rich domains, where the material exists as a rubbery PVAc with glassy PLA, and the other rubbery modulus existing above the Tg of PLA-rich phase, where both PVAc and PLA are in rubbery state. Hence, it can be expected that the two-step modulus decrement could lead to the release of deformed strain in two steps, and PLA/PVAc/graphene nanocomposites could demonstrate two-step strain recovery responses

40

. The observed two distinct


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transition temperatures for nanocomposites can imply that the presence of graphene led to phase separation of these two miscible polymers. In our case, this is advantageous since it prerequisites to achieve triple-shape memory behavior from PLA/PVAc nanocomposites. 10000

2.5

Neat PVAc 0 wt% 3 wt% 4.5 wt% Neat PLA

(b)

(a) 1000

2.0

100

Tan δ

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


10 Neat PVAc 0 wt% 3 wt% 4.5 wt% Neat PLA

1 0 5

15

25

1.5 1.0 0.5 0.0

35

45

Temperature (oC)

55

5

65

15

25

35

45

55

65

75

Temperature (oC)

Figure 4. (a) Storage modulus and (b) loss factor of the neat PVAc, PLA and PLA/PVAc blends with 0, 3 and 4.5 wt% graphene.

Table 1. DMA data for PLA/PVAc and PLA/PVAc/graphene nanocomposites. Graphene Content (wt%)

Tg (°C)

E′ (MPa) 5 °C 35 °C

Tan δmax

Electrical Conductivity (S m-1)

60 °C

0

1964

-

1.9

33

1.80

3.0 × 10-13

3

2563

90

2.3

26 and 47

0.54 and 0.45

5.2

2711 185

18.9

25 and 45

0.42 and 0.48

31.3

4.5

Another noticeable feature in the DMA results is the fact that the E' of the matrix over the whole temperature region is remarkably increased with the addition of graphene nanoplatelets. For example, with embedding 4.5 wt%



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graphene, E′5

°C

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enhanced by ∼140% from 1964 to 2711 MPa, and E′60

°C

increased by ∼1000% from 1.9 to 18.9 MPa as compared with the neat polymer matrix (Table 1). We have previously established that these graphene nanofillers, with high surface area and aspect ratio (10 000), can build a stiff space-filling network in the matrix

41

. In this case, this can account for the

noticeable increase in the storage modulus of PLA/PVAc nanocomposites, which can potentially provide large recovery stresses. Furthermore, from Figure 4 (b) and Table 1 one can see that the loading of graphene nanoplatelets led to noticeable decrease in both the intensity (tan δmax) and the area of loss factor, which are direct indicative of the much lower damping of nanocomposites as compared with the unfilled polymers. This is consistent with our previous rheological studies regarding that most of the backbones in the percolated network of graphene act as elastic elements

41-42

.

For example, with embedding 4.5 wt% graphene, tan δmax of PLA/PVAc decreased from 1.80 to 0.42 and 0.48 (Table 1), which can be correlated to the formation of graphene network that resists against the viscous flow of polymer chains 8. In other words, more energy can be stored and less energy is dissipated during the deformation of nancomposites as compared with the neat polymer matrix, which is very favorable for the shape recoverability. Thus, these thermomechanical results indicate that PLA/PVAc/graphene nanocomposites are very promising material for shape memory application.


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The thermal transitions of samples were also determined using DSC with the second heating thermograms, and the results are shown in Figure 5. A single broad glass transition step is observed at 29 °C in the thermogram of PLA/PVAc, as an indication of a homogeneous blend. In contrast, PLA/PVAc nanocomposites containing 3 and 4.5 wt% graphene showed two wellseparated glass transitions, which demonstrates that the presence of graphene nanoplatelets caused heterogeneity in the blend. For instance, PLA/PVAc nanocomposite with 3 wt% graphene showed two transitions at ∼21.5 and 43 °C, corresponding to the Tg of PVAc-rich phase and Tg of PLA–rich domains, respectively.

Figure 5. DSC thermograms of the unfilled PLA/PVAc and its nanocomposites obtained in second heating scan.



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3.3 Direct thermally induced dual- and triple-shape memory effects The dual-shape memory experiment involved first loading the sample to ∼6070% strain at 80 °C (above the Tg, PLA/PVAc). The temperature was then reduced continuously to -20 °C, during which time both the PLA and PVAc vitrifications took place. Following this step, the applied force was released and thus the temporary shape was fixed. Afterwards, the sample was continuously reheated to 80 °C with PVAc and PLA passing from glassy state to a rubbery state and recovery of the original shape. The amorphous PVAc and amorphous PLA segments in the PLA/PVAc blend act as reversible phases, while physical entanglement of the amorphous PVAc and the amorphous phase of PLA as well as the crystallites of PLA act as the fixed phase, all of which can be considered as physical netpoints. Figure 6 presents the typical dual-shape memory results of the unfilled PLA/PVAc blend and PLA/PVAc blend containing 4.5 wt% graphene investigated by cyclic thermomechanical analysis. The corresponding data are also summarized in Table 2. The unfilled PLA/PVAc blend exhibited a relatively low shape recovery of ∼56-60% as shown in Figure 6 (a). This is because the chain entanglements in the stretched sample were not sufficiently high to provide enough physical netpoints and elasticity required to restore the original shape

43

. In contrast, nanocomposites exhibited excellent recovery

ratio, as the nanocomposite with 4.5 wt% graphene achieved a high Rr of 99.5% in the second cycle (Figure 6 (b)).


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(b)

(a)

Figure 6. The dual-shape memory behavior recorded for two consecutive cycles of the (a) unfilled PLA/PVAc blend and (b) PLA/PVAc nanocomposite containing 4.5 wt% graphene. Table 2. Shape fixing and shape recovery values of samples obtained from two consecutive dual-

shape memory cycles. Graphene Content (wt%)

Rf (1)%

Rf (2)%

Rr (1)%

Rr (2)%

0

84.5

86.5

60.1

56.7

3 4.5

92.1 98.6

92.6 98.9

95.2 98.8

95.7 99.5

It should be noted that these nanocomposites have much higher graphene concentrations than the required percolation threshold of 0.51-0.70 wt%

41

.

Hence, the stiff and elastic network of the graphene nanoplatelets can be considered as the second physical netpoints in conjunctions with the first physical netpoints of the matrix

41-42

. Thus, it can be noted here that the

observed remarkable improvement in Rr with incorporation of graphene is originated from the high physical crosslink density strain energy

44

43

and large stored elastic

in the nanocomposites, which provide a high driving force for

the subsequent complete shape recovery. In other words, the stiff network of nanofillers can be elastically deformed and recovered during the shape fixing



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and shape recovery steps, respectively, and thus inhibits the viscous flow of the polymer molecular chains, which can account for the observed full shape recovery behavior of nanocomposites. Also, it can be seen from Figure 6 (b) that the unfilled PLA/PVAc blend presented moderate Rf values located within 84%−87%, while with incorporation of graphene into the blend, the shape fixing of nanocomposites was noticeably improved, as the nanocomposites with 3 and 4.5 wt% graphene exhibited high Rf values of 92.6% and 98.9% in the second cycle, respectively. It is worth noting that a certain fraction of the stretched polymer is unlocked even at a temperature below Tg, hence there is an instantaneous retractive force upon loading removal, which causes incomplete shape fixing in SMPs 33, 45. While, almost no instantaneous retraction occurred immediately after unloading the nanocomposite containing 4.5 wt% graphene. Because the stiff space-filling graphene network completely restricted the stretched frozen polymer molecular chains, hence, inhibited any possible mobility of the polymer molecular chains, and thus provided high shape fixing for the matrix. Furthermore, DMA and DSC results revealed that nanocomposites possess two distinct transition temperatures. Hence, it can be expected that PLA/PVAc nanocomposites display triple-shape memory behavior. Thus, their triple-shape memory properties were quantitatively investigated with DMA; a rectangular sample, as permanent shape (A), was first deformed at 80 °C (above the Tg, PVAc and Tg, PLA) until a strain of ∼50% was achieved. The


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sample was then cooled to 40 °C (below the Tg,

PLA

and above the Tg,

PVAc)

while holding the external force constant, followed by unloading to the small preload. This finished the fixing of the first temporary shape (B). The tripleshape memory experiment continued by reloading the sample at 40 °C until a strain of ∼90% was achieved. The temperature was further reduced to -20 °C followed by the second release of external force. This resulted in fixing of the second temporary shape (C). During continuous heating to 80 °C, the strain recovered in two separated steps, first from shape C to B and then from B to A, as schematically demonstrated in Figure 7 (a). This is an indicative of a triple-shape memory behavior of SMPs

12, 26

. When the deformed sample is cooled from 80 to 40

°C, the strain (∼50%) is fixed by vitrification of the amorphous PLA segments. Further decreasing the temperature from 40 to -20 °C led to vitrification of PVAc chains and, as result, the second temporary shape with ∼90% strain is fixed. When the temperature is raised to Tg, PVAc < T < Tg, PLA, the devitrified PVAc chains attempt to elastically recover to its equilibrium state. However, in the same temperature range PLA still exists as rigid, glassy state and is resisting this matrix recovery. As a consequence, the overall composite only recovers to the point at which the two competing factors balance each other. This accounts for the first step-recovery seen in Figure 7 (b). The remaining strain could only recover upon further heating to T > Tg, PLA.



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(b)

Figure 7. (a) Schematic illustration of the triple-shape memory response of samples at two staged temperatures during the stretching and recovery processes, and (b) the triple-shape memory behavior recorded for PLA/PVAc nanocomposite with 4.5 wt% graphene.


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Table 3. Shape fixing and shape recovery values of samples obtained from triple-shape memory measurements. Graphene

Rf (B)%

Rf (C)%

Rr (C → B)%

Rr (B → A)%

Rr (C → A)%

3

87.5

91.8

92.7

90.9

93.2

4.5

89.3

98.5

96.1

94.5

96.7

Content (wt%)

It can be seen from Table 3 that PLA/PVAc/graphene nanocomposites, especially the nanocomposite containing 4.5 wt% graphene, have high shape fixating (Rf (C) value close to 100%). However, it should be noted that Rf (B) values of samples are slightly lower than their corresponding Rf (C) ones. The Rf (B) and Rf (C) are determined by the freezing the amorphous segments of PLA and PVAc, respectively. Some instantaneous strain recovery at 40 °C upon unloading is observed in Figure 7 (b) and thus led to incomplete fixing of shape B. This phenomenon reveals that the glassy PLA phase with a droplet morphology in the blend (as seen in Figure 3) is not powerful enough to resist all the stored resilience of elongated rubbery PVAc phase at Tg, PVAc < T < Tg, PLA. On the other hand, the raising graphene content from 3 to 4.5 wt% had a little effect on Rf (B), while Rf (C) was noticeably increased with increasing graphene content, which likely resulted from higher stiffening effect of graphene nanoparticles on the PVAc component compared with PLA phase in the blend. Finally, we display a visual demonstration of the triple-shape recovery effect of PLA/PVAc/graphene nanocomposite with 4.5 wt% graphene. A rectangular straight



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specimen of the nanocomposite, as permanent shape A, was first immersed in a water bath at 80 °C, bent into a “ɣ” like shape and quickly put in a second water bath at 40 °C to fix temporary shape B. Afterards, the sample was further deformed to a “V” like shape at 40 °C, and transfered to a third water bath (5 °C). This led to fixing of the second temporary shape C as shown in Figure 8 (a) . After immersing the sample with shape C back into the water bath at 40 °C, it quickly recovered to shape B as shown in Figure 8 (b) , which further recovered to its permanent shape A just within few second upon immersing into the water bath at 80 °C (Figure 8 (c)).

Figure 8. Photographs displaying the sequential recovery of PLA/PVAc nanocomposite containing 4.5 wt% graphene (a) from temporary shape C, (b) to temporary shape B, and (c) to permanent shape A.

3.4 Electrically induced dual- and triple-shape memory effects Figure 9 exhibits electrical conductivity of PLA/PVAc blends as a function of graphene content. It can be seen that the incorporation of graphene significantly increased electrical conductivity of the matrix, as the electrical conductivity of PLA/PVAc blend enhanced by ∼14 orders of magnitude from 3×10-13 to 31.3 S m-1 (corresponding to a volumetric resistivity of 0.03 Ω m) by inclusion of 4.5 wt% graphene. The obtained electrical conductivity value is higher than the most reported values for polymer nanocomposites based on


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Page 23 of 34

the inclusion of conductive filler particles

8-11

. The structural origin of the

observed superior electrical conductivity can be assigned to the well exfoliation of graphene nanoplatelets with high aspect ratio (10 000) and surface area, which form a dense conductive network throughout the PLA/PVAc matrix.

1.E+02

Electrical Conductivity (S m-1)

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.E-01

1.E-04

1.E-07

1.E-10

1.E-13 0

1

2

3

4

5

Graphene Content (wt%)

Figure 9. Variation of the electrical conductivity of samples versus graphene content.

Electrically triggering the dual-shape memory effect of nanocomposites was evaluated under three different DC voltages (40, 50 and 70 V) and the resulting recovery ratio was measured using equation (5) and plotted as a function of recovery time in Figure 10. It can be obviously seen that the increasing graphene content from 3 to 4.5 wt% results in a dramatically faster recovery response. Moreover, Figure 10 shows that the nanocomposite containing 4.5 wt% graphene demonstrated almost full electrically induced shape recovery at all the applied voltages, whereas the blend containing 3



wt% graphene could only recovery to its original shape when the high voltage of 70 V was applied for 40 s, which is ∼14 times longer than that of the blend with 4.5 wt% graphene.

(b)

80 60 40 V

40

50 V 20

70 V

100

Recovery Ratio (%)

100

Recovery Ratio (%)

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

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(c)

80 60 40 V

40

50 V 20 70 V 0

0 0

10

20

30

40

50

60

70

0

5

10

15

20

Time (s)

Time (s)

Figure 10. (a) Photographs displaying fast shape recovery of the nanocomposite containing 4.5 wt% at 70 V. Electrically induced shape recovery ratio as a function of time under various triggering voltage for the nanocomposites with (b) 3 wt%, and (c) 4.5 wt% graphene. Inset scheme demonstrating the sample geometry.

Therefore, the results show that both the recovery time and recovery level of the two nanocomposites have strong dependence on the applied voltage. For instance, with increasing the applied voltage from 40 to 70 V, the recovery time of the nanocomposite containing 4.5 wt% graphene reduced by ∼153%, as it recovered its permanent shape in 2.77 s under 70 V, demonstrating a fast recovery performance (snapshots are shown in Figure 10 (a)).


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The high electrical conductivity of nanocomposites allows more Joule heat generation in unit time and, hence, the temperature of sample quickly reaches above the transition temperature of the polymer matrix, where the fast shape recovery happens

12-13

. Besides that, existence of a large amount of stored

elastic energy in the nanocomposites can provide high driving force for their subsequent quick and complete shape recovery when the temperature of sample reaches above the Tg 44. It is interesting to note that most of the current generated triple-SMPs are based on direct heating methods. Surprisingly, to the best of our knowledge, the triggering triple-shape memory behavior of SMPs by electricity is still quite rare in the literature, while it is very useful for many applications, such as artificial muscles, deployable and actuating devices. Hence, in this study the

electrically

triggered

triple-shape

recovery

performance

of

the

nanocomposite containing 4.5 wt% graphene was typically investigated, and the results are presented in Figure 11. For electroactive triple-shape memory experiments, a longer specimen, with a length of 60 mm and similar geometry shown in the inset of Figure 10 (c), was used for the programming process; the right side of a straight specimen, as permanent shape A, was bended at 80 °C and fixed the deformed state at 40 °C to obtain temporary shape B (“cane” like shape) due to the freezing of the amorphous PLA segments. Then, the left side of the obtained specimen was bended at 40 °C and cooled down quickly to 5 °C to obtain second temporary



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shape C (“S” like shape as shown in Figure 11 (a) at t=0 s) through the vitrification of PVAc chains. A series of photos taken during the electricity induced triple-shape memory process of sample are shown in Figure 11 (a). The sample was not activated when the applied voltage was below 40 V. While interestingly, one can see from Figure 11 (a) and (b) that upon applying a voltage of 40 V, the left side of the sample (deformed at 40 °C) was selectively actuated and recovered to the shape B within 7 s, followed by activation of the right side of the sample (deformed at 80 °C) and recovery to the permanent shape A in the time limit of 7 to 12 s. Also, Figure 11 (c) exhibits that raising the voltage from 40 to 50 V led to simultaneous recoveries of the two sides of sample. Once a voltage of 40 V was applied on the PLA/PVAc/graphene nanocomposite, the Joule heating is generated and the temperature of sample increases over the Tg of PVAc, which triggers the shape recovery from shape C to B due to the release of stored entropic energy and relaxation of the oriented chains of PVAc to a higher entropy state

22, 45

. With the passing of

time, the temperature of sample exceeds the Tg of PLA and, hence, the other side of the sample is activated, which led to recovery from shape B to A. Raising the voltage from 40 to 50 V allows more Joule heat generation in unit time and, as a result, the temperature of sample quickly reaches above the Tgs of both PVAc and PLA phases, where almost simultaneous recoveries of the two sides of sample were happened (Figure 11 (c)). Therefore, the results


Page 27 of 34

show that the temporary shapes in each region of sample can be either individually or simultaneously triggered by tuning the applied voltage.

100

(b)

80 60

40 V

40

Recovery from shape C to B

20

Recovery from shape B to A

Recovery Ratio (%)

100

Recovery Ratio (%)

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


(c)

80 60

50 V

40

Recovery from shape C to B

20

Recovery from shape B to A

0

0 0

2

4

6

8

10 12 14 16

0

2

4

6

Time (s)

8

10

12

14

16

Time (s)

Figure 11. (a) Photographs displaying the triple-shape memory behavior of the nanocomposite with 4.5 wt% graphene at 40 V. Recovery ratio of the nanocomposite with 4.5 wt% graphene versus time at voltage of (b) 40 V, and (c) 50 V.

It is worth noting that almost all the PLA-based SMPs are merely capable of memorizing one temporary shape (Table S1). As far as we know, there are few reports until now achieving a multi-shape memory effect for PLA based materials 46

26-27, 46

, and most of them are based on complex synthetic routes

27,

. To the best of our knowledge, this is the first report for the preparation of



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 28 of 34

biopolymer-based triple-SMPs through a simple one-pot method with both thermal and electric actuations. In addition, many types of works on the triggering dual-shape memory behavior by indirect heating methods have been reported to date

8-13

.

Interestingly, Table S2 shows that only few triple- and multiple-SMPs were developed that can be actuated by indirect heating methods, such as applying magnetic field

40, 47-48

, radiofrequency

15

and near infrared (NIR) light

49

. It is

worth noting that these SMPs mostly require synthesis of complex structures. Surprisingly, there is only one report on the triggering triple- and multipleSMPs with electricity up to now, while the electrical stimulation owns many advantages such as the remote controllability, uniform heating profile, simple manipulation and wide applications and

thermally

actuated

50

32

. Wang et al.

triple-SMPs

of

developed electrically

chemically

crosslinked

polycyclooctene (PCO)/polyethylene (PE)/multi-walled carbon nanotube (MWCNT) nanocomposites. The developed SMPs could actuate and their recover original shapes at a temperature of 120 °C and voltage of 150 V in 2 min. Such a high triggering temperature can strongly limit their direct use in biomedical applications due to concerns over potential tissue damage. Whereas, our designed biobased triple-SMPs demonstrated not only moderate switching temperatures (i.e. ∼25 and 45 °C), but also rapid shape recovery behavior (within 12 s). Furthermore, the results of the current work showed


Page 29 of 34

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that the temporary shapes in each region of sample can be either individually or simultaneously triggered by tuning the applied voltage.

4. Conclusions Herein we have successfully developed a facile and broadly applicable method for fabricating thermal and electrical responsive triple-SMPs through the blending of two commercially available biopolymers, PLA and PVAc, in the presence of robust and conductive graphene nanoplatelets. DMA analysis showed that the modulus of the matrix was remarkably improved in the presence of graphene. Interestingly, atomic force microscopy (AFM) observations revealed that the presence of nanofillers led to phase separation and heterogeneity in the blend of these two miscible polymers. Furthermore, two distinct Tgs were obtained from DMA and DSC results for nanocomposite blends, while there was a single Tg in the unfilled PLA/PVAc blend. Consequently, the unfilled blend could only exhibit a moderate heat triggered dual-shape memory behavior. Whereas, PLA/PVAc/graphene nanocomposites demonstrated excellent thermally and electrically actuated triple-shape memory effects besides their remarkable dualshape memory behavior. It is conclusive to note that graphene nanoplatelets inhibited possible mobility of the polymer molecular chains, and also provided large stored elastic strain energy during the shape fixing process, which revealed in the high shape fixing and shape recovery properties of nanocomposites, respectively. Also, electroactive shape recovery experiments exhibited that depending on the applied voltage, temporary



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shapes in each region of sample can be either individually or simultaneously recovered. This work can open an avenue in designing multi-stimuli responsive triple-SMPs from commercially available biopolymers desired for many applications that require adjustable complex shapes, such as smart aerospace and invivo devices. Acknowledgements Authors express their sincere thanks to the University of Maragheh (94112) for financial support. References (1) Yang, X.; Wang, L.; Wang, W.; Chen, H.; Yang, G.; Zhou, S. Triple Shape Memory Effect of Star-shaped Polyurethane. ACS Appl. Mater. Interfaces 2014, 6, 6545-6554. (2) Zhao, J.; Chen, M.; Wang, X.; Zhao, X.; Wang, Z.; Dang, Z.-M.; Ma, L.; Hu, G.-H.; Chen, F. Triple Shape Memory Effects of Cross-linked Polyethylene/Polypropylene Blends with Cocontinuous Architecture. ACS Appl. Mater. Interfaces 2013, 5, 5550-5556. (3) Wei, M.; Zhan, M.; Yu, D.; Xie, H.; He, M.; Yang, K.; Wang, Y. Novel Poly(tetramethylene ether) Glycol and Poly(ε-caprolactone) Based Dynamic Network via Quadruple Hydrogen Bonding with Triple-shape Effect and Self-healing Capacity. ACS Appl. Mater. Interfaces 2015, 7, 2585-2596. (4) Keramati, M.; Ghasemi, I.; Karrabi, M.; Azizi, H.; Sabzi, M. Dispersion of Graphene Nano Platelets in Poly Lactic Acid with the Aid of a Zwitterionic Surfactant: Evaluation of the Shape Memory Behavior. Polym.-Plast. Technol. Eng. 2016, 55, 1039–1047. (5) Keramati, M.; Ghasemi, I.; Karrabi, M.; Azizi, H.; Sabzi, M. Incorporation of Surface Modified Graphene Nanoplatelets for Development of Shape Memory PLA Nanocomposite. Fibers Polym. 2016, 17, 1062-1068. (6) Babaahmadi, M.; Sabzi, M.; Mahdavinia, G. R.; Keramati, M. Preparation of Amorphous Nanocomposites with Quick Heat Triggered Shape Memory Behavior. Polymer 2017, 112, 26-34. (7) Xie, M.; Wang, L.; Ge, J.; Guo, B.; Ma, P. X. Strong Electroactive Biodegradable Shape Memory Polymer Networks Based on Star-shaped Polylactide and Aniline Trimer for Bone Tissue Engineering. ACS Appl. Mater. Interfaces 2015, 7, 6772-6781. (8) Chen, J.; Zhang, Z.-x.; Huang, W.-b.; Yang, J.-h. Wang, Y.; Zhou, Z.-w.; Zhang, J.-h. Carbon Nanotube Network Structure Induced Strain Sensitivity and Shape Memory Behavior Changes of Thermoplastic Polyurethane. Mater. Des. 2015, 69, 105-113.


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Table of Contents


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Thermally and Electrically Triggered Triple-Shape Memory Behavior of

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