Strategy for Fabricating Multiple-Shape-Memory Polymeric Materials

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A Strategy for Fabricating Multiple Shape Memory Polymeric Materials via the Multilayer Assembly of Co-Continuous Blend Yu Zheng, Xiaoying Ji, Min Yin, Jiabin Shen, and Shaoyun Guo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10345 • Publication Date (Web): 25 Aug 2017 Downloaded from http://pubs.acs.org on August 27, 2017

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ACS Applied Materials & Interfaces

A Strategy for Fabricating Multiple Shape Memory Polymeric Materials via the Multilayer Assembly of Co-Continuous Blend Yu Zheng, Xiaoying Ji, Min Yin, Jiabin Shen*, Shaoyun Guo Polymer Research Institute of Sichuan University, State Key Laboratory of Polymer Materials Engineering, Chengdu, Sichuan 610065, P. R. China

KEYWORDS: multiple-shape memory, co-extrusion, multilayer structure, interface, co-continuous

ABSTRACT: Shape memory polymeric materials containing alternating layers of thermoplastic polyurethane (TPU) and co-continuous poly (butylene succinate) (PBS)/polycaprolactone (PCL) blend (denoted as SLB) were fabricated through layermultiplying co-extrusion. Since there were two well-separated phase transitions caused by the melt of PCL and PBS, both the dual and triple shape memory effects were discussed. Compared with the blending specimen with the same components, the TPU/SLB multilayer system with multi-continuous structure and a plenty of layer interfaces was demonstrated to have higher shape fixity and recovery ability. When the layer numbers reached 128, both the shape fixity and recovery ratios were beyond 95% and 85% in dual and triple shape memory processes, respectively, which were difficult to be achieved through conventional melt-processing methods. Based on the

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classic viscoelastic theory, the parallel assembled TPU and SLB layers capable of maintaining the same strain along the deforming direction were regarded to possess the maximum ability to fix temporary shapes and trigger them to recover back to original ones through the interfacial shearing effect. Accordingly, the present approach provided an efficient strategy for fabricating outstanding multiple shape memory polymers, which may exhibit a promising application in the fields of biomedical devices, sensors and actuators, etc.

INTRODUCTION Shape memory polymers (SMPs), as a class of promising intelligent materials, have drawn increasing attention because of their specific capacity to fix temporary deformation and spontaneously recover their original shape in response to an external stimuli such as heat, light, moisture, electric or magnetic field, among others.1-6 Such an attractive ability endows SMPs great potential to be extensively applied in smart fabrics, intelligent packaging, biomedical devices, sensors and actuators.7-11 Most SMPs exhibit dual-shape memory effect (DSME) that is able to memory only one temporary shape in each shape memory cycle.12 Generally, the dual SMPs (DSMPs) depend on a single glass or melting transition (Tg or Tm) to fix the temporary shape which is formed after thermal deforming at the switching temperature (Ts) higher than the corresponding transition followed by a fast cooling. On the other hand, the optimal recovery of the original shape usually requires the fabrication of chemical or physical cross-linking structure to restrain the irreversible molecular slipping.13,14 To expand the application potential of SMPs, a great deal of research has been carried out to increase the number of temporary shapes a SMP can


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memorize in a shape memory cycle, and the triple shape memory polymers (TSMPs) are successfully produced which have been accelerated developed within the past decade or so. Unlike the conventional DSMPs, TSMPs are able to fix two temporary shapes and recover sequentially from one temporary shape to the other, and eventually to the original shape upon heating,15-18 thus can meet the more complex requirements of the applications with a demand of continuous shape change. Commonly, the TSMPs include polymers with broad phase transitions and multiphase polymers with numerous shape memory switches. By achieving a broad thermal transition, some polymers can demonstrate triple shape memory effect (TSME), even multiple shape memory property with more than two temporary shapes. For example, Xie.19 explored the shape memory performance of the pure perfluorosulphonic acid (PSFA) ionomer which shows one sufficient broad phase transition, and the results revealed that PSFA can efficiently recover to its original shape from two or more previously programmed temporary shapes. Additionally, Samuel et al.20 fabricated a new TSMP in a straight forward method by simply blending the miscible poly(l-lactide acid) (PLLA) and poly(methyl methacrylate) (PMMA) to broaden the Tg, and an excellent TSME was achieved for the amorphous blend with 50% PMMA. However, the use of a broad phase transition would result in unstable temporary shapes due to the relaxation of TSMPs occurring at any temperature within the broad phase transition. Moreover, the TSME is significantly affected by the choice of the thermal-mechanical treatment conditions,21 leading to that the greater care should be taken to create an optimal TSME. Thus, designing multiphase polymers with distinct elastic modulus plateaus caused by the wellseparated thermal transitions may be more efficient to prepare TSMPs.



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Because of the ability to regulate the composition and design the molecular structure, graft polymerization and block copolymerization are usually proposed to obtain TSMPs with additional reversible phase transitions, such as grafting polybutylene succinate (PBS) to maleated-polystyrene-b-poly(ethylene-co-buty-lene)b-polystyrene (m-SEBS) backbone as another SC,22 synthesis of a block polymer owning two distinct shape memory switches by using poly(cyclohexylmethacrylate) and poly(ε-carpolactone) (PCL),23 and fabricating the copolymer network of poly(ωpentadecalactone) (PPD) and PCL with the separated Tms.24 Furthermore, the chemical cross-linking,25,26 side-chain liquid crystalline network,27 and sequence structure design28 are also utilized to realize TSME based on numerous thermal transitions. Recently, fabricating SMPs through melt blending has been focused due to its ease of processability.29,30 And several reports have substantiated that the optimal DSME can be easily achieved by incorporating one permanent component (PC) which acts as the memorizing phase and another switching component (SC) with reversible molecular mobility, especially when the co-continuous morphology is formed.31-34 Hence, introducing several distinct and well-defined glassy and/or melting domains as the SCs through the melt-processing technology seems like an effective approach to prepare TSMPs. Nevertheless, to the best of our knowledge, there are very few reports on producing TSMPs by the melt blending. Actually, the co-continuous structure with high phase continuity which would enhance the efficiency of stress delivery35 is crucial for realizing excellent TSME.36,37 However, the morphology of the polymer blend is easily affected by the viscosity ratio of components, interfacial tensions or the processing parameters,38-41 especially for the multiphase system, thereby it is too


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difficult to tailor the structure of the multiphase blend to fabricate a co-continuous TSMP with the melt-processing method. As a special co-continuous morphology, the multilayer structure is widely exiting in nature.42,43 In such architecture, the stable and well-defined continuous layer space of each component can be easily constructed without being influenced by the factors proposed in blending system,44 which is beneficial for introducing additional shape memory domains and avoiding phase morphology destruction. Additionally, the high phase continuity and numerous interfaces between the layers would promote the efficiency of stress transfer and load distribution, which contributes to the balance of shape fixation and shape recovery. Xie et al. 45 have reported a bilayer TSMP composed of two epoxy thermosets with different Tgs, and tunable TSME can be achieved through varying the thickness ratio between the two layers. Therefore, it is believed that the multilayer structure should be an excellent choice for fabricating multiphase co-continuous TSMP with outstanding shape memory performance. Among the various layer-assembly methods, the layer-multiplying co-extrusion technology recently has attracted tremendous scientific interests because of its capacity to enable the fabrication of multilayer structures become more controllable and higher efficient, and various unique properties have been produced through that technology.46-51 As schematically illustrated in Figure 1, a multilayer SMP containing alternating layers of PC and SC can be fabricated by combining an assembly of layermultiplying elements (LMEs) with two extruders. By increasing the number of LMEs, the numerous interfaces can be developed accompanied with the layer multiplication, while the overall thickness of the whole material is maintained. What’s more, it is worth noting that the PC or SC layers may contain multicomponents with co-



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continuous morphology, which provides a potential route to endow the whole material with multiple shape memory behaviors. In this work, the PBS/PCL blend with co-continuous morphology was first prepared (denoted as SLB) through conventional melt blending method, and then coextruded with neat thermoplastic polyurethane (TPU) through the layer-multiplying coextrusion. The multiple shape memory performances of the TPU/SLB multilayer system, including the DSME and TSME, were investigated, respectively. Considering the materials used in present work have been proved to possess excellent biocompatibility,34,52-53 such multilayer TSMP is of great potential to be applied in biomedical fields.

Figure 1. Schematic of layer-multiplying coextrusion system: (a and b) single screw extruder, (c) co-extrusion block, (d) layer-multiplying elements (LMEs), (e) exiting block, (f) rolling and cooling block, (g) extrudate with an alternating multilayer structure.


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2. EXPERIMENTAL SECTION 2.1 Materials. Polyester-based TPU (Elastogran® S85A11), with a Shore A hardness of about 85 and a density of 1.23 g/cm3, was purchased from BASF group (Germany). The 4,4’-methane diisocyanate (MDI) acts as the hard segments. Commercial PCL (CAPA6800), with a density of 1.11 g/cm3 and a molecular weight distribution of 1.22 (Mn = 7.22 × 105 g/mol, Mw = 8.84 × 105 g/mol), was purchased from Perstorp Corp. (UK). PBS (1001MD) with a density of 1.26 g/cm3 was obtained from Showa Denko Corp. (Japan). The materials were used without further purification and the basic structures of TPU, PCL, and PBS are illustrated in Figure S1. 2.2. Specimen Preparation. TPU and PBS were dried at 80 °C for 40 hrs and PCL was dried at 40 °C for 24 hrs in a vacuum oven before melt processing. Prior to coextrusion, PBS/PCL blend pellets (denoted as SLB) contained 50 vol % PCL were prepared using a SHJ-20 twin-screw extruder (screw diameter is 21.7 mm, L/D = 40:1, Nanjing Giant Co. Ltd., China) with an extrusion temperature of 185 °C and extrusion rate of 150 rpm. Subsequently, the multilayer SMPs consisting of alternating TPU and SLB layers were prepared through the layer-multiplying coextrusion. As schematically illustrated in Figure 1, TPU and SLB were simultaneously extruded from different extruders, and combined as 2-layer melt in the co-extrusion block, then flowed through an assembly of LMEs. In a LME, the melt was sliced into two left and right sections by a divider, and then recombined vertically leading to the double of layer numbers (see Figure S2). When n LMEs were applied, the materials with 2(n+1) layers can be produced eventually. In this work, 8-, 32- and 128-layer materials were



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fabricated by applying 2, 4 and 6 LMEs, respectively. By controlling the co-extruding speed, the total thickness of each extrudate was maintained about 1.0 mm, and the thickness ratio of TPU and SLB layers was kept at 1:1. The processing conditions were the same as the preparation of SLB pellets. For comparison, TPU/SLB blend with the same composition as that in multilayer system was produced by extruding from one of extruders of the above coextrusion system, for keeping a similar processing history. 2.3. Differential Scanning Calorimetry (DSC). For determining the Ts, the heating curves of the TPU/SLB blending and multilayer samples were recorded on a DSC (Q20, TA Instrument, USA) under nitrogen. Each specimen with 5-7 mg was heated from 20 to 160 °C at a heating rate of 10 °C/min. 2.4. Morphological Observation. The microstructure of each specimen was observed using an Olympus BX51 polarizing light microscope (PLM) equipped with a camera. A thin slice about 10 µm in thickness was obtained by a microtome from each sample perpendicular to the extruding direction. The morphologies of SLB and TPU/SLB blend were observed using a scanning electron microscope (SEM, JEOL JSM-5900LV) under an accelerating voltage of 20 kV. The specimens were first cryofractured in liquid nitrogen, then SLB and TPU/SLB

were

selectively

etched

by

immersing

in

tetrahydrofuran

and

dichloromethane to remove the dispersed phase, respectively. Prior to visualization, the surface of each specimen was coated with a layer of gold in a vacuum chamber. 2.5. Shape Memory Test. The samples for shape memory test were cut from the extruded films into rectangular specimens with dimensions of 10 mm (length) × 2 mm (width) × 1.0 mm (thickness). DSME tests were carried out on the dynamic


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mechanical analyzer (Q800, TA Instrument, USA) in controlled force mode. The test was performed in following steps: (1) stretching the sample at the Ts to a certain strain (εm) with a constant force, (2) cooling to 25 °C while keeping the sample at εm, (3) unloading at 25 °C to allow the film to elastically recover to the temporary strain (εf), and (4) heating to the Ts, causing the sample return to the final strain (εr). Then the shape fixity ratio (Rf) and shape recovery ratio (Rr) were respectively calculated by using the following equations (1) and (2), and N is the given cycle number.54,55 εf(N)

Rf = ε

× 100%

(1)

m (N)

εf(N) - εr(N)

Rr = ε

f(N) - εr(N-1)

× 100%

(2)

TSME experiments were also conducted by using the DMA Q800 from TA Instruments in controlled force mode. Typical thermo-mechanical cycle involves following steps: (1) stretching the sample at the Ts1 to a certain strain (εm1) with a constant stress, (2) cooling to a lower Ts2, then removing the load for 5 min to obtain the first temporary strain (εf1), (3) a second force-controlled stretching was performed at Ts2 to reach the maximum strain (εm2), (4) cooling to 25 °C, and the stress is again removed to gain the second temporary strain (εf2), (5) a free-strain recovery was first performed at Ts2 for 20 min to the recovered strain (εr2), subsequently performed at Ts1 for 20min to the final strain (εr1). Rr and Rf were respectively calculated by using the following equations (3) and (4):20 εf(x)

Rfx = ε

× 100%

(3)

m(x)

εf(x) - εr(x)

Rrx = ε

f(x)

- εf(x-1)

× 100%


(4)


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2.6. Creep-recovery Test. The creep-recovery curves were measured using the DMA Q800 from TA Instruments at the Ts. Each sample was first stretched to a certain strain under the constant stress and then the load was released instantaneously. The creeping process lasted for 20 min and the recovered strain as a function of the creeping time was recorded.

3. RESULTS AND DISCUSSION 3.1. Microstructure. Figure 2 shows the microstructure of the TPU/SLB multilayer specimen obtained through PLM observation. The bright and dark layers are corresponding to the semicrystalline SLB and amorphous TPU components, respectively. For the individual SLB, the SEM image exhibited in Figure S3 demonstrates the typical co-continuous blending morphology. Thus, by alternately assembling with TPU, a special multi-continuous multilayer structure is successfully constructed. Additionally, with increasing the number of layers, numerous interfaces are created between SLB and TPU layers, and the thickness of each layer decreases proportionally because the total thickness of the extrudate keeps constant in the layermultiplying process. For comparison, the microstructure of the conventional blend with the same composition as that in the multilayer specimens was displayed as well. It can be clearly observed that the bright crystals of PCL and PBS are dispersed in the amorphous TPU matrix. By selectively etching PCL and PBS phases with dichloromethane,56 the typical sea-island structure in which the PCL and PBS act as the islands can be found through the SEM observation (Figure S4). Consequently, the different assembly forms of SLB and TPU would result in distinct phase structure, which is believed to make different contributions to the shape memory performances.


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Figure 2. Structural observation of TPU/SLB multilayer and blending specimens through PLM.

3.2 Thermal Behaviors. On the basis of the classic shape memory mechanism of thermal-induced SMPs, the Ts of a SMP is commonly determined by the Tm or Tg of the SC. Figure 3 displays the heating curves of TPU/SLB blending and multilayer specimens measured in a DSC instrument. In the measuring temperature range, semicrystalline PCL exhibits a similar melting transition process and has the same Tm at about 57 °C as that of pure PCL, irrespective of the structural distinctions. At higher temperature side, PBS in the multilayer samples shows a dual melting behavior with two Tm at 103 and 113 °C, the lower one is resulted from the meltrecrystallization previously explained by Wang et al.57 However, there is only a single endothermic peak around 107 °C corresponds to the melt of PBS in the blending specimen, which is consistent with the case of TPU/PBS blend recorded in Figure S5. Different from the assembly of the components in the multilayer system, the TPU is directly blended together with SLB in the blending specimen. Hence, it is possible that the hydrogen-bonding interaction between TPU and PBS limits the occurrence of melt-recrystallization process.58 Although different melting behaviors of PBS phase may exist between the multilayer and blending systems, it still can be confirmed that there are two well-separated thermal transitions in the range of 40 and 140 °C, which



may afford two shape memory switches in the shape memory process. Additionally, the thermal transition temperature of TPU was examined through the DSC heating curve as shown in Figure S6. The Tg of the amorphous soft domains is about -28 °C, whereas the Tm of the crystalline hard domains is 167 °C. Accordingly, the TPU is capable of acting as a PC to memory the original shape even when the Ts of the TPU/SLB multilayer system is chosen around the Tm of PBS.

8L 32L

Endo

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128L Blend

20

30

40

50

60

70

80

90 100 110 120 130 140 150 160

Temperature, °C

Figure 3. DSC heating curves of TPU/SLB multilayer and blending specimens.

3.3 DSME. If the whole SLB domain acts as SC and the TPU acts as PC, a DSME may be realized when the Ts is chosen at 125 °C above the Tm of PBS. Figure 4 exhibits the detailed shape memory testing curves of the TPU/SLB blending and multilayer specimens with the same compositions. The shape fixity and recovery values, Rf and Rr which aim to quantitatively characterize the efficacy of the shape fixing and recovering steps, are calculated and shown in each curve. For the blend, its Rf and Rr are 90.5 and 81.7%, respectively. In contrast, both of them are distinctly enhanced as the two components are alternately assembled in parallel forming


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multilayer structure. When the layer numbers are increased to 128, Rf and Rr can reach 96.3 and 94.7%, respectively. Intuitively, the original, temporary and permanent shapes of 128-layer specimen were captured by a digital camera. As displayed in Figure 5, the specimen could be well fixed to an expected shape temporarily at room temperature and turned to quickly recover back to a permanent shape which almost entirely copied from the original one as soon as be reheated to 125oC. Based on the calculated values and the visual observation, it is demonstrated that the multilayer assembly with multi-continuous morphology plays a significant role in optimizing the dual shape memory performance. Moreover, considering the SLB layers are composed of PCL and PBS phases, the morphological evolution is tracked in original, temporary, and permanent shapes. It can be observed in Figure S7 that the cocontinuous blending morphology is stably maintained in all stages. Since the component ratio of PCL and PBS is 50/50 (vol.%/vol.%), a dynamic balance between the phase aggregation and breakage could be obtained during the thermally deforming process.

40

T e m p e ratu re ,

120 100 80

30

60 20

Rr:81.7%

10

0 0

10

20

30

Time, min

40

50

60

20 15

120

Rf:91.6%

40

100

30

80

20

60

10

40

40 20

0

25

140

5 0

10

Rr:87.8% 20

0

0 0

10

20

30

Time, min


40

50

60

30 25 20 15 10 5 0

S tre s s, M p a

Rf:90.5%

50

(b)50

Te m pe ra ture ,

30 140

S tre s s, M p a S train, %

(a) 60

S tra in, %

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



30

80 60 15

40

Rr:91.1% 0

20 0

10

20

30

40

50

60

Time, min

(d) 45

140

Rf:96.3%

25 20 15 10

100

30

80 60 15

40

Rr:94.7% 20

5 0

0

0 0

10

20

30

30

120

40

50

25 20 15

S tres s, M p a

100

T em p e ratu re ,

Rf:94.3%

0

30

120

Tem pe rature ,

140

S tre ss , M p a S train, %

(c)45

S train , %

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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10 5 0

60

Time, min

Figure 4. Evolution of strain, stress, and temperature during the thermomechanical cycle at the Ts of 125 °C for (a) TPU/SLB blend, and (b) 8-, (c) 32-, (d)128-layer TPU/SLB specimens.

Figure 5. Original, temporary, and permanent shapes of 128-layer TPU/SLB specimen recorded by a digital camera in a dual shape memory progress.

From the perspective of reproducible applications, the stability of a SMP should be required in cycling use. Herein, the 128-layer specimen was chosen to experience three successive thermo-mechanical cycles, and the Rf and Rr of each cycle were recorded in Figure 6. It can be found that its Rf is basically maintained around 96%, while the Rr is slightly increased to 96.4% after performing the third cycles, which is similarly reported in other systems14,28. This indicates that the multilayer system is


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capable of enduring multiple thermo-mechanical cycles for repetitively memorizing the same shape.

f1

: 96.3%

R

f2

: 95.9%

R

f3

140

: 95.6%

20

120 100

30

80 60 15

40

15

10

Stress, Mpa

R

Temperature, ° C

45

Strain, %

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


5 20

R

0 0

20

r1

: 94.7%

40

60

R

r2

: 96.6%

80

100

R 120

r3

: 96.4% 140

0

0

160

Time, min

Figure 6. Evolution of strain, stress, and temperature during three consecutive thermo-mechanical cycles of 128-layer specimens.

With regard to a multi-component SMP, DSME would be governed by the geometric organization of components.31,32 Accordingly, the shape fixing and recovery mechanisms related to the morphological distinction between blending and multilayer systems as schematically illustrated in Figure 7 deserve to be discussed.

Figure 7. Schematic of the phase structures of the TPU/SLB blending and multilayer specimens.



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Stage 1: Fixing to a temporary shape. After being thermally stretched to the εm at 125 °C, the specimen is quickly fixed at 25 °C where the crystallized SLB has a higher modulus than soft TPU. Thus, the shape fixity of the whole specimen would be primarily determined by SLB. The authors’ previous study has demonstrated that the phase continuity plays a crucial role in providing the mechanical support.59 As reported in Figure 8a, the storage modulus of the 128-layer specimen is distinctly larger than that of the blend, which indicates that the continuous SLB layers in multilayer system could possess the maximum ability to maintain the temporary shape of the whole specimen.

Stage 2: Recovering to a permanent shape. Due to have an extremely low modulus in molten state, the SLB would fail to store the strain energy when reheated to 125 ºC, so that the recovery ability of each specimen turns to be mainly determined by TPU. For the multilayer system, the parallel assembled SLB and TPU layers would have the same strain in the deforming process. Hence, the recovery of SLB can be efficiently driven by the recovery of adjacent TPU layers through the interfacial shearing effect. With regard to the blending system where the SLB is dispersed in the TPU matrix, it can be also considered to consist of many parallel organized layers but the components in each layer are assembled in series. According to the classic viscoelastic theory,60 the components assembled in series would receive the same stress along the deforming direction, so that the strain of each component is mainly determined by its own viscoelastic behaviors. This implies that the recovery of TPU would have less influence on that of SLB. As a result, more irreversible deformation is finally maintained in the blending specimen than that in the multilayer system, which is further evidenced through a creep experiment. Figure 8b compares the


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dependence of recovered strain of the TPU/SLB blend and 128-layer specimen on creep time at 125 ºC. After creeping for 20min, the 128-layer specimen has a recovered strain of 89.4 %, while the final strain of the blend is only 76.1 %. This signifies that the multilayer system has a less irreversible deformation, which is regarded as the critical reason for obtaining a higher Rr during the shape memory process. (a) 1000

(b) 100

128-layer blend

100

Recovered strain, %

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

128-layer blend

80

60

T = 125 °C

1 0

20

40

60

80

100

120

140

40 0

5

o

Temperature, C

10

15

20

Time, min

Figure 8. (a) Dependence of storage modulus of TPU/SLB blending and 128layer specimens on temperature; (b) Dependence of recovered strain of TPU/SLB blending and 128-layer specimens on creeping time at 125 ºC.

Moreover, it is noticed that the DMSE of the multilayer system can be tailored through the layer multiplication. With the increase of layer numbers, more interfaces between TPU and SLB layers are created and induce a stronger molecular interaction. Hence, the elastic recovery of TPU could be more efficiently depressed by adjacent crystallized SLB layers at the shape fixity stage leading to the increase of Rf. When the specimen is reheated to 125oC, the strain energy stored in TPU would trigger the recovery of the molten SLB layers through the interfacial shearing effect. Accordingly, the increased layer interfaces are regarded to be capable of strengthening the shape



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recovery ability, which enables the specimen to almost entirely recover back to the original shape and exhibit a higher Rr.

3.4 Tunable Ts. For a DSMP, it is well recognized that a lower Ts can make it easier to achieve thermal actuation. In this study, the DSC curves in Figure 3 show a sufficient internal of the Tm of PCL and PBS. Therefore, it is believed that the DSME could also be realized when the Ts was chosen at 70 oC. The shape memory testing curves and calculated Rf and Rr of the TPU/SLB multilayer and blending specimens are presented in Figure 9. Similar to the previous case choosing 125 oC as Ts, both the Rf and Rr of the multilayer system are distinctly larger than those of the blending specimen, and show a rising trend with the increase of layer numbers. However, for the same specimen, comparatively smaller Rf and Rr are obtained, by thermally deforming and recovering at a lower Ts (i.e. 70 oC). Taking the blending specimen for instance, the Rf and Rr are remarkably decreased from 90.5 and 81.7% to 74.1 and 62.8%, respectively, when the Ts is reduced to 70oC from 125 oC. The reason can be explained according to the mechanism proposed in previous section. Since the Ts chosen at 70 oC is intermediate between the Tm of PCL and PBS, only the PCL phase acts as SC and the PBS turns to act as PC together with TPU. The decreased proportion makes the SC become more difficult to resist the elastic recovery of the whole specimen at the shape fixity stage leading to a lower Rf. When the shape recovery stage is triggered, the recovery of the molten SC has to be driven by PC which stores the strain energy, but the crystalline regions in PBS would inevitably restrict this process. As compared in Figure S8, it is further proved that a lower Ts would cause more irreversible deformation after creeping for the same period. Accordingly, the Rr is also reduced as the Ts is decreased below the Tm of PBS.


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Moreover, it is worth noting that the Ts exhibits a comparatively low influence on the Rf and Rr of 128-layer specimen, which indicates the multilayer assembly with a plenty of layer interfaces would be a promising strategy to achieve comparatively high shape memory performance at a low Ts.

20

Rr:62.8%

10

30 20 10

0

0 20

30

40

30 20

10

Rr:82.9%

10 5

0

80 70

T e m p e ra tu re , ℃

Rf:86.4%

10

20

40

0

50

80

50

25

20

15

30

Rr:85.2%

10 0

20 10 0

40

50

Time, min

10 5 0

S tra in , %

40

20

S tre s s , M p a

50

30

30

(d)60

30

60

40

50

30

40

20

30 20

Rr:87.2%

10

10

0

0 0

10

20

30

70

Rf:89.6%

60

30

20

5

0

0

10

20

Time, min

50

10

15

10 0

50

(c)

0

20

40 30

Time, min

40

50

30

40

50

25 20 15

S tre s s , M p a

10

15

25

60


0

20


40

Rf:83.2%

40

S tra in , %

50

30

30

70

25

60

Rf:74.1%

80

(b)

50

T e m p e ra tu re ,

40

S tra in , %

30

70

S tre s s , M p a

50

S tre s s , M p a

80

(a)

S tra in , %

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 0

60

Time, min

Figure 9. Evolution of strain, stress, and temperature during the thermomechanical cycle at the Ts of 70 °C for (a) TPU/SLB blend, and (b) 8-, (c) 32-, (d)128-layer TPU/SLB specimens.

3.5 TSME. Since the SLB contains two well-separated melting transitions, the TSME of TPU/SLB blending and 128-layer specimens is considered at last. Herein, a thermal-mechanical cycle with double consecutive stretching steps at different Tss (70 and 125 °C) and subsequent shape recovery under continuous heating was performed as exhibited in Figure 10. Compared with the blend, the 128-layer specimen shows



more favorable ability to fix the two temporary shapes and trigger both of them to recover back at the specific Ts, which is consistent with the tendency obtained in dual shape memory process. The corresponding Rf and Rr at each stage are detailedly collected in Table 1. Based on the studies on DSME, it has been demonstrated that both the Rf and Rr values would be reduced when the Ts is decreased from 125 to 70 o

C. Hence, the Rf2 and Rr2 obtained at the stages thermally stretching and recovering

at 70 oC are reasonably lower than Rf1 and Rr1 obtained at the stages thermally stretching and recovering at 125 oC, respectively. Moreover, it is clearly presented that the minimum Rf and Rr of the blending specimen are only 78.4 and 63.5%, respectively. For the multilayer specimen, all of the values are beyond 85%, which are difficult to be achieved through conventional melt-processing methods, to the best of

40 30 20 10 0 0

10

20

30

40

50

60

70

80

90

25

50

130 110 90

20 15 10 5 0

40 70 30

50

20 10

30 25 20 15

30

10

10

5

S tre s s , M p a

50

(b) 60


60

30

S tra in , %

130 120 110 100 90 80 70 60 50 40 30 20 10 0

Stress, M pa

(a)70

T em perature,

the authors’ knowledge.

S tra in , %

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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-10 0

0 0

10

20

30

40

50

60

70

80

90 100

Time, min

Time, min

Figure 10. Evolution of strain, stress, and temperature during triple shape memory cycles with Tss of 70 and 125 °C for the TPU/SLB (a) blending and (b) 128-layer specimens.

Table 1. Rf and Rr values of the TPU/SLB blending and 128-layer specimens at each stage in the triple shape memory progress.


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Sample

Rf1

Rf2

Rr2

Rr1

Blend

81.9%

78.4%

63.5%

82.4%

128-layer

93.4%

87.7%

85.0%

90.0%

Figure 11 intuitively displays the triple shape memory progress of the TPU/SLB 128-layer sample recorded by a digital camera. The stripe was first deformed to a hooklike shape at 125 °C and fixed at 70 °C for 5 min to obtain the temporary shape I; then it was further deformed to a “S” shape and cooled to 25 °C to fix the second temporary shape. After that, the temporary shape II was triggered to recover back towards the temporary shape I by placing the specimen into the 70 °C vacuum oven for 10 min. Subsequently, it was moved into another 125 °C vacuum oven and isothermally treated for another 10min. Finally, it can be found that the specimen is almost entirely recover back to the original shape. This reveals that the material with multi-continuous structure and well-separated transition temperatures possesses an outstanding capability to memorize multiple shapes at different Ts, which exhibits a promising application in the fields of biomedical devices, sensors and actuators, etc..



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Figure 11. The triple shape memory progress of the 128-layer TPU/SLB specimen recorded by a digital camera.

4. CONCLUSIONS TPU/SLB multilayer materials consisting of co-continuous PCL and PBS in SLB were fabricated through layer-multiplying co-extrusion. The two well-separated Tms of PCL and PBS were applied to determine the Ts of the dual and triple shape memory processes. The results demonstrate that the multilayer system has larger Rf and Rr than the blending specimen with the same components and the values could be further enhanced with the increase of layer numbers. Based on the classic viscoelastic theory, it is considered that the parallel assembled TPU and SLB layers capable of maintaining the same strain along the deforming direction possess the maximum ability to fix temporary shapes and trigger them to recover back to original ones through the interfacial shearing effect. Consequently, the multilayer-assembled system with multi-continuous structure and a plenty of layer interfaces exhibits a competitive advantage in achieving outstanding multiple shape memory performances.

ASSOCIATED CONTENT

Supporting Information.


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Molecular structures, schematic of layer-multiplying process, phase morphologies, DSC heating curves, creeping curves. The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author

∗ E-mail address: [email protected] (Jiabin Shen)

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors are grateful to the National Natural Science Foundation of China (51673136, 51227802, 51420105004, 51421061) for financial support of this work.

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For Table of Contents use only

A Strategy for Fabricating Multiple Shape Memory Polymeric Materials via the Multilayer Assembly of Co-Continuous Blend Yu Zheng, Xiaoying Ji, Min Yin, Jiabin Shen*, Shaoyun Guo Polymer Research Institute of Sichuan University, State Key Laboratory of Polymer Materials Engineering, Chengdu, Sichuan 610065, P. R. China


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Strategy for Fabricating Multiple-Shape-Memory Polymeric Materials

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