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thermoplastic poly(ether)urethane composites via carbon black self-networking induced co-continuous structure
Composites Science and Technology 139 (2017) 8e16
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Composites Science and Technology journal homepage: http://www.elsevier.com/locate/compscitech
Enhanced shape memory property of polylactide/thermoplastic poly(ether)urethane composites via carbon black self-networking induced co-continuous structure Xiaodong Qi 1, Hao Xiu 1, Yuan Wei, Yan Zhou, Yilan Guo, Rui Huang, Hongwei Bai**, Qiang Fu* College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 August 2016 Received in revised form 28 November 2016 Accepted 5 December 2016 Available online 8 December 2016
The preparation of elastomer/plastic blends with co-continuous structure is beneficial to achieve good shape recovery and fixing performances. In this work, carbon black (CB) nanoparticles with selfnetworking capability were introduced to tailor the phase morphology and shape memory properties of polylactide (PLA)/thermoplastic poly(ether) urethane (TPU) blend (70/30 by weight). A morphological change from sea-island structure to co-continuous structure was observed with increasing CB content. The strong affinities between CB nanoparticles and TPU as well as the self-networking capability of CB nanoparticles led to the formation of this co-continuous structure. With such novel structure, the PLA70/ TPU30/CB ternary composites owned an outstanding shape memory property because the continuous TPU phase provided stronger recovery driving force. Moreover, the selective localization of CB nanoparticles in the continuous TPU phase imparted the composites with enhanced mechanical properties and excellent electrical conductivities with low filler content. The composites then showed a good electroactive shape memory behavior, which could recover to their original shape within 80 s at 30 V. Our work provides a universal strategy via CB self-networking to prepare double percolated conductive polymer composites with optimal shape memory properties and excellent electrical conductivities, which may promote specific applications in intelligent devices. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Polymer-matrix composites (PMCs) Functional composites Strength Electrical properties
1. Introduction Shape-memory polymers (SMPs) represent a class of smart response materials that are capable to fix the temporary deformed shape and recover to their permanent shape upon external stimulus, including heat, light, electricity, magnetic field or moisture [1e5]. Such a ‘memorize’ property gives SMPs great opportunities to be applied in various fields, such as the packaging, textile, medical and aerospace industries [6,7]. On the basis of structure, SMPs usually contain two components: net-points and switching phases [8]. Net-points are the chemically or physically cross-linked points for preventing chain relaxation during deformation, which determine the permanent shape. Switching phases are responsible
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (H. Bai), [email protected] (Q. Fu). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.compscitech.2016.12.007 0266-3538/© 2016 Elsevier Ltd. All rights reserved.
for the fixation of the temporary shape, which are polymer chains with defined glass transition (Tg) or melting temperature (Tm). The ratio between net-points and switching phases has reached to a balance, which is considered to be necessary in producing SMPs with good shape recovery and shape fixation. Among various strategies developed to prepare SMPs, polymer blending provides a more accessible route because of its good processability [9e16]. SMP blends typically consist of an elastomer serving as permanent phase and an amorphous or a crystalline plastic playing the role of switching phase [14,17,18]. For example, Kurahashi et al. prepared the polyurethane (PU)/poly(oxyethylene) (POE) binary blends via controlling the composition ratio, and the results demonstrated that the co-continuous structure could show optimal recovery and fixing performances [10]. It can be explained from the perspective of mechanical knowledge that in a SMP blend both the shape recovery determined by net-point and the shape fixation determined by switching phase are in connection with the efficiency of stress transfer, in which the continuous degree of each
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component plays a key role [19]. Therefore, co-continuous morphology in a blend system is believed to be helpful in balancing shape recovery and fixing performances [20e23]. However, the relative high content of elastomer is needed to obtain cocontinuous structure, resulting in the decrease of the modulus and strength. Besides, the co-continuous morphology is usually obtained within a narrow range of compositions, which limits the developments of SMPs based on elastomer/plastic blends. In recent years, some nanoparticles with self-networking capability have been found effective in expanding the composition range for co-continuity of binary immiscible polymer blends [24e27]. Through adding a small amount of nanoparticles, such as silica dioxide (SiO2) and carbon black (CB) into binary immiscible polymer blends, a morphological transition from initial sea-island structure to a unique co-continuous structure was observed [28]. Wu et al. pointed out that the nanoparticles had a self-networking capability to form a continuous network structure and the adjacent droplets of the dispersed phase were drove to approach together to form continuous phase during melt blending [29]. Inspired from these interesting results, it's natural to think that the co-continuous structure, which is beneficial for realizing both good fixing and recovery performances, can be obtained by introducing the nanoparticles with the self-networking capability into the elastomer/ plastic blends. More importantly, if the nanoparticles used are conductive [30e32], such as CB and carbon nanotubes, SMP composites with electrical conductivity can be achieved. However, high content of conductive fillers is often needed to attain good electrical conductivity that triggers the electroactive shape recovery of SMPs, giving rise to the large agglomerates in polymer matrix and is unfavorable for practical processing. Forming the double percolated structure, that is to make conductive fillers selectively localize in one phase of polymer blend with the co-continuous structure, is a well-established way to obtain conductive composites with low filler content [33e36]. The term ‘‘double percolation’’ has been demonstrated to explain the reduction of the percolation threshold, which represents the percolation of the conductive filler distributed in one phase and the continuity of the filler-rich phase in the polymer blend occur simultaneously. Our previous study reported that a double percolated conductive composite was prepared through carbon black (CB) self-networking induced co-continuous-like morphology in polylactide (PLA)/thermoplastic poly(ether) urethane (TPU) blends [26]. In this work, we extend our interest in the electroactive shape memory properties of PLA/TPU/CB blends. Carbon black (CB) nanoparticles with the self-networking capability and electrical conductivity were introduced into PLA/TPU blends (70/30 by weight). In contrast to previous SMP blends in which elastomers occupy large proportion, the content of TPU in our study is only 30 wt%. It is expected that the addition of CB nanoparticles extends the composition range for co-continuity of PLA/TPU blends at relative low TPU content. The influence of phase morphology tailored by CB nanoparticles on shape recovery performances was investigated. Besides, the conductive CB nanoparticles could impart composites with electrical conductivities; thus the electroactive shape memory effect was studied as well. We aim to obtain a new type of double percolated conductive polymer composites with optimal shape memory properties, mechanical properties and excellent electrical conductivities. 2. Materials and methods 2.1. Materials PLA (4032D) with a density of 1.25 g/cm3 was purchased from
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Nature Works Co. Ltd., USA. TPU (WHT-1570) with a density of 1.21 g/cm3 was purchased from Yantai Wanhua Co. Ltd., China. CB nanoparticles (Printex-XE2B, Degussa Co. Ltd, Germany) had a primary diameter of 30 nm and were used as received. 2.2. Sample preparation PLA/TPU/CB ternary composites were firstly mixed through melt blending of PLA, TPU and CB in a Haake mixer at 190 C and 60 rpm for 6 min. The composition ratio of PLA/TPU was fixed at 70/30 and the loading amount of CB was changed from 0 to 8 phr. After that, the ternary PLA/TPU/CB mixtures were compressed into plates at 190 C for 6 min under 10 MPa. For convenience, PLA/TPU/CB ternary composites are termed as PLA70/TPU30/CBx, where x represents the weight percent of CB. 2.3. Characterization The phase morphology of cryogenically fractured PLA/TPU/CB ternary composites were inspected by using an FEI Inspect F scanning electron microscope (SEM, USA). The morphological structure and the localization of CB in the PLA/TPU blends were observed through transmission electron microscope (TEM, JEM2010) under an acceleration voltage of 200 KV. The ultra-thin samples with a thickness of 100 nm were prepared via a Leica UCT microtome at 100 C. Dynamic mechanical analyzer (DMA Q800, TA Company) was employed to evaluate the dynamic mechanical behavior of PLA/TPU/CB ternary composites. The temperature were increased from 0 C to 120 C with a heating rate of 3 C min1. The shape memory properties of PLA/TPU/CB ternary composites were measured by using DMA. A four-step procedure was designed as follows. (1) First, the sample was heated to 85 C and stretched to a certain strain (ε) under a constant force. (2) Then, the deformed sample was quenched to room temperature (25 C) with keeping the force. (3) After the force was removed, the temporary strain (εload) was measured. (4) Lastly, the sample was reheated to 85 C, maintained at 85 C for 15 min and the recovery strain (εrec) was recorded. The samples were heated or cooled with a rate of 5 C min1. The shape fixing ratio (Rf) and shape recovery ratio (Rr) of PLA/TPU/CB ternary composites could be calculated according to the following Equations (1) and (2):
Rf ¼
εload ε
(1)
Rr ¼
ε εrec ε
(2)
The tensile tests at a high temperature of 85 C were carried out on DMA (Q800, TA Company, USA). The samples were kept at 85 C for 5 min and then stretched at 3 N min1 until fracture. The tensile tests at room temperature (25 C) were measured using an Instron 4302 universal tensile tester (SANSI, China) with a crosshead speed of 5 mm min1. The notched Izod impact toughness of PLA/TPU/CB ternary composites was evaluated on an impact testing machine (XJU-5.5, China). The electrical conductivities of PLA/TPU/CB ternary composites were measured using a Keithley 6487 picoammeter. The sides of rectangular samples were coated with silver paint to eliminate contact resistance before testing. The ITECH IT6700 was conducted to yield constant voltage/current on samples to achieve electroactive shape memory behavior. The surface temperature of PLA/TPU/CB ternary composites was recorded in the middle of the rectangular samples by using a digital thermocouple.
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3. Results and discussion 3.1. Effect of CB nanoparticles on phase morphology of PLA70/ TPU30 blend The phase morphology has an important impact on the shape memory property in polymer blend system [9,10]. The effects of CB nanoparticles on the phase morphology of PLA70/TPU30 blend were characterized through SEM and shown in Fig. 1. It can be clearly seen that the PLA70/TPU30 binary blend has a typical seaisland morphology, where TPU spherical droplets with several micron sizes are dispersed in the PLA matrix (Fig. 1a). With a low loading content of 1 phr CB nanoparticles in blend, it turns out that CB nanoparticles are selectively distributed in the TPU phase. There is no obvious morphologic change except for a slightly increase in the size of TPU domains (Fig. 1b). Very interestingly, when the content of CB nanoparticles is increased to 3 phr, some discrete TPU droplets tend to approach each other and a special co-continuous like structure is formed (as shown by red circles in Fig. 1c). This phenomenon indicates that 3 phr CB nanoparticles can induce the morphology transition of PLA70/TPU30 blend from the initial seaisland morphology to the unique co-continuous like morphology. Further raising the content of CB nanoparticles to 5 phr, a network structure with higher continuity is formed since more and more TPU droplets fuse together during melt blending (Fig. 1d). It's worth noting that the phase size of TPU shown in Fig. 1c is quite larger than that in Fig. 1d. According to our previous works [24,25], the morphological transition from sea-island to co-continuous like or co-continuous structure in PLA/TPU blends is mainly attributed to the self-networking behavior of used nanoparticles. Hence, the size of TPU phase is largely determined by the size of CB network. By increasing the CB content from 3 phr to 5 phr, a more compact and perfect CB network is expected to be formed, as a result the
decrease in size of the TPU phase is observed. Swelling experiments were further conducted to check the morphology transition of TPU phase (see Fig. S1). As for the PLA70/ TPU30/CB3, the sample was only slightly swollen after immersing in chloroform for 6 h (chloroform is a good solvent for PLA), which confirmed that the TPU phase indeed formed a continuous phase to support the sample in chloroform and was consistent with the SEM results. To confirm the morphological transition and the distribution of CB nanoparticles in PLA70/TPU30 blends, the representative PLA70/ TPU30/CB3 and PLA70/TPU30/CB5 composites were observed by using TEM. As shown in Fig. 2, PLA70/TPU30/CB3 exhibits a cocontinuous-like structure, in which the disconnected TPU clusters (darker parts) prevail through the PLA matrix (bright parts). A cocontinuous structure with higher continuous degree of TPU phase is formed in PLA70/TPU30/CB5. Moreover, CB nanoparticles are selectively localized in TPU phase, suggesting strong affinities between CB nanoparticles and TPU phase. In order to clear the role of selectively localized CB nanoparticles in the formation of co-continuous structure in PLA70/TPU30/CB nanocomposites, the dynamic viscoelasticity was investigated by rheology (see Fig. S2). The viscosity of TPU is largely improved by adding 5 phr CB nanoparticles, while the viscosity of PLA matrix is almost unchanged because CB is selectively dispersed in TPU phase. According to the classical Paule-Barlow theory [37], increasing the volume fraction or decreasing the viscosity would enhance the continuity of the minor phase in the matrix. Hence, the expected morphology of PLA70/TPU30/CB5 ternary composites should be sea-island structure same as the PLA70/TPU30 binary blend, rather than the co-continuous one as observed in Fig. 1d. The deviation implies that morphology change is not induced by the change of shear viscosity ratio of PLA and TPU/CB phase. Here, the selfnetworking effect of nanoparticles is confirmed to be closely
Fig. 1. SEM images of the cryo-fractured surfaces of PLA70/TPU30/CB composites with various contents of CB: (a) 0 phr, (b) 1 phr, (c) 3 phr, (d) 5 phr. Scale bar: 10 mm.
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Fig. 2. TEM images of PLA70/TPU30/CB3 (a, a0 ) and PLA70/TPU30/CB5 (b, b0 ).
related to the formation of the co-continuous structure [24e26]. CB nanoparticles tend to self-aggregate to form a continuous network in polymer melts. These CB nanoparticles are selectively located in TPU phase and drive adjacent TPU droplets fuse together to form continuous phase in PLA matrix. 3.2. Shape memory properties of PLA70/TPU30/CB composites with various contents of CB Based on the above morphological analysis, CB nanoparticles are capable to induce the phase morphology of PLA70/TPU30 blend change from sea-island structure to co-continuous structure, which is helpful for both shape fixing and recovery performances. The large modulus decline around the transition temperature (Ttrans) is necessary for the material to show shape memory effect [11]. PLA70/TPU30 blend has a transition temperature range (60 Ce80 C) where the storage modulus (E0 ) suddenly decreases (see Fig. S3). Thus, the Ttrans is set at 85 C, which is about 15 C above the glass transition temperature (Tg) of PLA. The dual shape memory performances of PLA70/TPU30/CB blends with various contents of CB were quantitatively measured on DMA (see Fig. 3). The programming procedure were conducted with a uniaxial stretching above the Tg of PLA followed by a rapid cooling to 25 C while keeping the stretched state. For example, the PLA70/TPU30 binary blend was heated to 85 C and then stretched to a strain of 68% with a constant stress of 2.25 MPa. The stretched state could be well maintained after fast quenching and subsequent unloading stress at 25 C. Upon reheating above the Tg of PLA (85 C), a large proportion of fixed strain was recovered while a
final (unrecovered) strain of about 30% still remained, indicating the PLA70/TPU30 binary blend could not fully recover to its original shape. In contrast, with adding 3 phr CB in blend, the PLA70/TPU30/ CB3 ternary blend exhibited much better capability to recovery strain, i.e., the final strain recovered to about 12% at 85 C. Further increasing the content of CB to 5 phr or higher, the final strain diminished to 10% and kept almost constant, suggesting a saturation phenomenon. The shape fixing ratio (Rf) and shape recovery ratio (Rr) are two important parameters in quantitatively evaluating the shape memory properties of SMPs. The Rf and Rr of PLA70/TPU30/CB composites with various contents of CB are shown in Fig. 4. Obviously, all the composites with or without CB exhibit a high Rf nearly 90%, suggesting that the content of CB has little effect on the Rf. This is ascribed to most of the retractions resulting from the elongated TPU phase are prevented by the totally rigid PLA continuous phase at 25 C. When the temperature is heated higher than the Tg of PLA, the amorphous PLA chains begin to move, and release the restricted TPU phase. The strong resilience of the stretched TPU phase provides the recovery driving force, which is the major contributor for the Rr. In the case of the Rr, the PLA70/TPU30 binary blend only has a Rr of 59%, whereas the PLA70/TPU30/CB3 ternary composite has a largely enhanced recovery capability, as revealed by its Rr of 80.2%. Differing from the sea-island morphology of PLA70/TPU30 binary blends, a co-continuous like structure is formed in PLA70/TPU30/ CB3 ternary composite because of the self-networking ability of CB nanoparticles as well as the strong affinities between CB nanoparticles and TPU phase. The formed consecutive TPU phase offers a strong recovery driving force to achieve the outstanding shape
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Fig. 3. The thermal-mechanical tensile curves of PLA70/TPU30/CB composites with various contents of CB: (a) 0 phr, (b) 1 phr, (c) 3 phr, (d) 5 phr, (e) 6 phr, (f) 8 phr.
Fig. 4. Shape recovery ratio (Rr) and shape fixing ratio (Rf) of PLA70/TPU30/CB composites with various contents of CB.
recovery performance. The strong dependence of the shape recovery property on phase morphology can be further demonstrated by the variation of Rr of PLA70/TPU30/CB composites as a function of CB content. PLA70/TPU30/CB5, PLA70/TPU30/CB6, and PLA70/ TPU30/CB8 all exhibit a good Rr of 84.5%, 85%, and 85.9%, respectively. The Rr keeps almost constant with increasing CB content from 5 phr to 8 phr because the co-continuous structure are all well
formed in these composites. The phase morphology and thermal-mechanical tensile curve of binary PLA50/TPU50 blend are shown in Fig. S4. PLA50/TPU50 displays a co-continuous morphology and reveals good shape memory property with not only high Rf (92.1%) but also Rr (85.7%). In this PLA/TPU blend system, the powerful recovery driving force of the elongated TPU phase is the main contributor for the shape recovery performance, especially the elastic resilience of continuous TPU phase is larger than that of dispersed TPU domains. It is believed that the increase in the content of TPU could further enhance the Rr of PLA/TPU blends. However, the relative high content of TPU would decrease the modulus and strength of PLA/ TPU blends, which limits the practical applications. In this study, we aim to obtain PLA/TPU blends with good stiffness-toughness balance and shape memory properties. Thus, the content of TPU in our study is only 30 wt%. The self-networking capability of CB nanoparticles plays a key role, which leads to the morphological change from a typical sea-island structure to a unique co-continuous structure, and thus endowing PLA70/TPU30/CB ternary composites with remarkably improved shape recovery property as compared to PLA70/TPU30 binary blend.
3.3. Shape memory mechanism of PLA70/TPU30/CB composites Based on the aforementioned considerations, we propose a possible shape memory mechanism for PLA70/TPU30/CB
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composites in Fig. 5. The PLA70/TPU30/CB composites can be easily stretched to certain strains above the Tg of PLA. Then the amorphous PLA chains are frozen below its Tg by rapid cooling to room temperature (25 C). No instant shrinkage is observed after releasing the tensile force at ambient temperature, because the modulus of the blends originated form the continuous PLA phase is much more than the resilience force of the elongated TPU phase. Therefore, the temporary shape can be well held. The recovery driving force mainly derives from the stored resilience of elongated TPU phase, which is hindered by the stiff PLA continuous phase in temporary shape. PLA70/TPU30 binary blend shows a sea-island morphology, in which spherical TPU domains are dispersed in the PLA matrix (Fig. 5a). The discrete and small TPU droplets are not powerful enough to fully drive the material back to the original shape, which is the reason for low Rr. With the addition of CB nanoparticles (above 3 phr) into the PLA70/TPU30 blend, these CB nanoparticles with self-networking ability are selectively located in the TPU phase and drive adjacent TPU domains fuse together to form continuous phase in the PLA matrix (Fig. 5b). The elastic resilience of continuous TPU phase is clearly larger than that of dispersed TPU particles. This indicates that the continuous TPU phase experienced large deformation absorbs more energy and generates a stronger recovery driving force. When the temperature is reheated above the Tg of PLA, the elongated continuous TPU phase is released without much prevention of the PLA phase and then drives the composite recover to its original shape. At this moment, both the powerful recovery driving force of the elongated TPU phase and the intrinsic recovery of the orientated PLA chains contribute to an excellent shape recovery performance. Generally, to achieve the desired shape memory properties, the elastomer/plastic ratio is often larger than 50/50 (wt/wt%) [9,10,20], whereas the content of elastomer in our study is only 30 wt%. Our work demonstrates the introduction of nanoparticles with selfnetworking ability into binary polymer blends to be a highly flexible strategy to design their shape memory properties via tuning
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the phase morphology over a much larger composition range. 3.4. Mechanical properties of PLA70/TPU30/CB composites Although SMPs have already been applied in some fields, there are still some limitations such as small recovery stress due to low modulus at Ttrans and inertness to electricity [38]. From above analysis, a unique co-continuous structure is obtained by introducing CB nanoparticles into PLA70/TPU30 blend. Compared with PLA70/TPU30 binary blend (sea-island structure), the PLA70/ TPU30/CB blends shows a superior shape recovery property because the continuous TPU phase provides a strong resilience. In the following work, we mainly pay attention to the superior mechanical property and electrical conductivity of CB nanoparticles. It's anticipated that the CB nanoparticles selectively localization in continuous TPU phase can lead to an improvement in the recovery force as well as good electrical conductivities with the double percolation conductive network structure. It is necessary to measure the tensile properties of SMPs at Ttrans, because the recovery stress at Ttrans is an important element in the application of SMPs. The stress-strain curves of PLA70/TPU30/CB composites with various contents of CB at Ttrans (85 C) are shown in Fig. 6. It is noteworthy that the modulus (the stress at initial 10% strain) and tensile strength of PLA70/TPU30 blends is remarkably increased with the addition of CB nanoparticles. For example, the tensile strength of PLA70/TPU30 is 1.16 MPa. With the addition of 5 phr CB, the tensile strength of PLA70/TPU30/CB5 is increased to 2.65 MPa, showing about 130% improvement compared with that of PLA70/TPU30. However, the strain at break is decreased with the incorporation of CB nanocomposites. This is ascribed to that the mobility of polymer chains are restricted by the CB nanocomposites. Moreover, the strength and toughness at room temperature (25 C) of PLA70/TPU30/CB composites are also needed to be considered. As shown in Fig. S5 and Table S1, the impact toughness of PLA70/TPU30 blends is greatly enhanced with increasing the CB
Fig. 5. Schematic illustration of the shape memory mechanism for (a) PLA70/TPU30 binary blends and (b) PLA70/TPU30/CB5 ternary blends.
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electrical conductivity of PLA70/TPU30/CB3 increases to 2.9 103 S m1, which is about four orders of magnitude higher than that of PLA70/TPU30/CB1. It is well known that the double percolation in conductive filler-added polymer blends plays an important role in improving the electrical conductivity, which represents the percolation of the conductive filler distributed in one phase and the continuity of the filler-rich phase in the polymer blend happen at the same time [33]. The continuous degree of TPU phase is reasonable to be high enough to form the perfect conductive path in PLA70/TPU30/CB3. The value is almost the same as the critical CB loading observed from SEM images, at which the continuous TPU phase forms in the PLA matrix. Therefore, it can be concluded that the electrical conductivities of PLA70/TPU30/CB composites are dependent on the CB contents and phase morphology, and both effects give rise to the construction of the conductive path. Fig. 6. Typical stress-strain curves of PLA70/TPU30/CB composites with various contents of CB at 85 C.
content due to the formation of continuous TPU phase. There is no obvious decrease in the Young's modulus and tensile strength. Thus, the prepared PLA70/TPU30/CB composites with good stiffness-toughness balance and shape memory properties have great application prospects in intelligent devices. 3.5. Electrical conductivities of PLA70/TPU30/CB composites CB is a kind of commonly used fillers for conductive polymer composites because of its outstanding electrical conductivity. The electrical conductivities of PLA/CB binary composites and PLA70/ TPU30/CB ternary composites with various contents of CB are shown in Fig. 7. It can be seen that the electrical conductivities of the PLA70/TPU30/CB ternary composites are higher than those of the PLA/CB binary composites at the same CB content. In addition, except for CB content, the phase morphology is another predominant factor in the electrical conductivity of PLA70/TPU30/CB ternary composites. The continuous degree of the TPU phase has a significant impact on constructing a conductive network through the whole matrix, since CB nanoparticles are selectively distributed in the TPU phase. For PLA70/TPU30/CB1 with the discontinuous TPU droplets, its electrical conductivity is only 4.4 107 S m1. However, when increasing the CB content to 3 phr, the cocontinuous-like structure is formed in PLA70/TPU30/CB3. The
Fig. 7. Electrical conductivity of PLA/CB binary composites and PLA70/TPU30/CB ternary composites with various contents of CB.
3.6. Electroactive shape memory behavior of PLA70/TPU30/CB composites The high electrical conductivities of ternary PLA70/TPU30/CB composites is helpful for fast electroactive shape recovery behavior. The surface temperature of ternary PLA70/TPU30/CB composites under a voltage of 30 V is shown in Fig. 8. The surface temperature increases consistently as time goes by, which could be attributed that the sample is heated by Joule heat derived from the electric current. According to the equation of Q ¼ (U2 T)/R [39,40], the generated Joule heat (Q) is determined by voltage (U), time (T) and resistance (R) of the sample. It's clear that the sample with a higher voltage and lower resistance of will get more heat at the same time. The PLA70/TPU30/CB3 retains a stable state at only 35 C in 120 s. This temperature is about 35 C less than the Tg of PLA (nearly 70 C), indicating that the PLA70/TPU30/CB3 cannot be successfully actuated at 30 V. Further increasing the content of CB, the PLA70/TPU30/ CB8 is rapidly heated to 70 C within 30 s and finally reaches an equilibrium state of about 95 C. The electrical conductivity of PLA70/TPU30/CB8 (2.87 S/m) is approximately one order higher than that of PLA70/TPU30/CB3 (0.34 S/m). As a result, more Joule heat is produced and thus the surface temperature of PLA70/TPU30/ CB8 can be fast increased. This result demonstrates that PLA70/ TPU30/CB composites are capable of conducting the electrical current by adding CB as conductive filler. Meanwhile, the electrical conductivity of composites is high enough to be used as stimulation in the electroactive recovery of the deformed composites.
Fig. 8. The variation of surface temperature of PLA70/TPU30/CB blends with various contents of CB as a function of time under the constant voltage of 30 V.
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Acknowledgements We would like to express our sincere thanks to financial support of the National Natural Science Foundation of China (Grant No. 51421061 and 51210005). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.compscitech.2016.12.007. References
Fig. 9. Shape recovery ratio with the increase of time for PLA70/TPU30/CB6 and PLA70/TPU30/CB8 under the voltage of 30 V. Inset pictures display the electroactive shape recovery behavior of PLA70/TPU30/CB6.
The bending test method was utilized to evaluate the recovery process, and the shape recovery ratio was measured based on the variation of deformation angle. The shape recovery ratio with the increase of time for PLA70/TPU30/CB6 and PLA70/TPU30/CB8 under 30 V is displayed in Fig. 9. It can be seen that the electrical conductivity of the sample has a great influence on the rate of electroactive shape recovery process. The PLA70/TPU30/CB8 shows faster response of shape recovery at the same time and finally achieves a Rr of 90% in 80 s, while the PLA70/TPU30/CB6 reaches a Rr of 90% in 150 s. As shown in inset photos of Fig. 9, the visual electroactive shape recovery behavior of PLA70/TPU30/CB6 was actuated under the constant voltage of 30 V and recorded with a camera. The initially rectangular strip-shaped sample was bended into an “n”-like (temporary shape) at 85 C and then rapidly cooled to room temperature (25 C). After being kept at 25 C for 2 h, the temporary shape was still well fixed. And with the voltage loaded at 30 V, the sample started to return to its original shape in 150 s. This consistent result was obtained after the sample experiment being repeated for more than three times. 4. Conclusions In this work, CB nanoparticles were used to tune the phase morphology and the shape memory properties of PLA/TPU blends (70/30 by weight). A phase morphology change from a common sea-island structure to a special co-continuous structure was observed. The CB nanoparticles with the self-networking capability had a tendency to self-aggregate to form a network during melt blending and drove dispersed TPU droplets fuse together to form continuous phase. With such novel co-continuous structure, the PLA70/TPU30/CB blends exhibited a superior shape memory property because the continuous TPU phase offered a strong recovery driving force. Moreover, the CB nanoparticles selectively localization in continuous TPU phase led to an improvement in the mechanical properties as well as good electrical conductivities with the double percolation conductive network structure. The PLA70/ TPU30/CB8 showed good electroactive shape memory behavior, which could recover to their original shape in 80 s at 30 V. Our work provides a simple strategy to prepare polymer/elastomer/fillers ternary composites with a good balance of shape memory properties, mechanical properties and excellent electrical properties, which will proceed some promising applications in intelligent devices.
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Contents lists available at ScienceDirect
Composites Science and Technology journal homepage: http://www.elsevier.com/locate/compscitech
Enhanced shape memory property of polylactide/thermoplastic poly(ether)urethane composites via carbon black self-networking induced co-continuous structure Xiaodong Qi 1, Hao Xiu 1, Yuan Wei, Yan Zhou, Yilan Guo, Rui Huang, Hongwei Bai**, Qiang Fu* College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 August 2016 Received in revised form 28 November 2016 Accepted 5 December 2016 Available online 8 December 2016
The preparation of elastomer/plastic blends with co-continuous structure is beneficial to achieve good shape recovery and fixing performances. In this work, carbon black (CB) nanoparticles with selfnetworking capability were introduced to tailor the phase morphology and shape memory properties of polylactide (PLA)/thermoplastic poly(ether) urethane (TPU) blend (70/30 by weight). A morphological change from sea-island structure to co-continuous structure was observed with increasing CB content. The strong affinities between CB nanoparticles and TPU as well as the self-networking capability of CB nanoparticles led to the formation of this co-continuous structure. With such novel structure, the PLA70/ TPU30/CB ternary composites owned an outstanding shape memory property because the continuous TPU phase provided stronger recovery driving force. Moreover, the selective localization of CB nanoparticles in the continuous TPU phase imparted the composites with enhanced mechanical properties and excellent electrical conductivities with low filler content. The composites then showed a good electroactive shape memory behavior, which could recover to their original shape within 80 s at 30 V. Our work provides a universal strategy via CB self-networking to prepare double percolated conductive polymer composites with optimal shape memory properties and excellent electrical conductivities, which may promote specific applications in intelligent devices. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Polymer-matrix composites (PMCs) Functional composites Strength Electrical properties
1. Introduction Shape-memory polymers (SMPs) represent a class of smart response materials that are capable to fix the temporary deformed shape and recover to their permanent shape upon external stimulus, including heat, light, electricity, magnetic field or moisture [1e5]. Such a ‘memorize’ property gives SMPs great opportunities to be applied in various fields, such as the packaging, textile, medical and aerospace industries [6,7]. On the basis of structure, SMPs usually contain two components: net-points and switching phases [8]. Net-points are the chemically or physically cross-linked points for preventing chain relaxation during deformation, which determine the permanent shape. Switching phases are responsible
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (H. Bai), [email protected] (Q. Fu). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.compscitech.2016.12.007 0266-3538/© 2016 Elsevier Ltd. All rights reserved.
for the fixation of the temporary shape, which are polymer chains with defined glass transition (Tg) or melting temperature (Tm). The ratio between net-points and switching phases has reached to a balance, which is considered to be necessary in producing SMPs with good shape recovery and shape fixation. Among various strategies developed to prepare SMPs, polymer blending provides a more accessible route because of its good processability [9e16]. SMP blends typically consist of an elastomer serving as permanent phase and an amorphous or a crystalline plastic playing the role of switching phase [14,17,18]. For example, Kurahashi et al. prepared the polyurethane (PU)/poly(oxyethylene) (POE) binary blends via controlling the composition ratio, and the results demonstrated that the co-continuous structure could show optimal recovery and fixing performances [10]. It can be explained from the perspective of mechanical knowledge that in a SMP blend both the shape recovery determined by net-point and the shape fixation determined by switching phase are in connection with the efficiency of stress transfer, in which the continuous degree of each
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component plays a key role [19]. Therefore, co-continuous morphology in a blend system is believed to be helpful in balancing shape recovery and fixing performances [20e23]. However, the relative high content of elastomer is needed to obtain cocontinuous structure, resulting in the decrease of the modulus and strength. Besides, the co-continuous morphology is usually obtained within a narrow range of compositions, which limits the developments of SMPs based on elastomer/plastic blends. In recent years, some nanoparticles with self-networking capability have been found effective in expanding the composition range for co-continuity of binary immiscible polymer blends [24e27]. Through adding a small amount of nanoparticles, such as silica dioxide (SiO2) and carbon black (CB) into binary immiscible polymer blends, a morphological transition from initial sea-island structure to a unique co-continuous structure was observed [28]. Wu et al. pointed out that the nanoparticles had a self-networking capability to form a continuous network structure and the adjacent droplets of the dispersed phase were drove to approach together to form continuous phase during melt blending [29]. Inspired from these interesting results, it's natural to think that the co-continuous structure, which is beneficial for realizing both good fixing and recovery performances, can be obtained by introducing the nanoparticles with the self-networking capability into the elastomer/ plastic blends. More importantly, if the nanoparticles used are conductive [30e32], such as CB and carbon nanotubes, SMP composites with electrical conductivity can be achieved. However, high content of conductive fillers is often needed to attain good electrical conductivity that triggers the electroactive shape recovery of SMPs, giving rise to the large agglomerates in polymer matrix and is unfavorable for practical processing. Forming the double percolated structure, that is to make conductive fillers selectively localize in one phase of polymer blend with the co-continuous structure, is a well-established way to obtain conductive composites with low filler content [33e36]. The term ‘‘double percolation’’ has been demonstrated to explain the reduction of the percolation threshold, which represents the percolation of the conductive filler distributed in one phase and the continuity of the filler-rich phase in the polymer blend occur simultaneously. Our previous study reported that a double percolated conductive composite was prepared through carbon black (CB) self-networking induced co-continuous-like morphology in polylactide (PLA)/thermoplastic poly(ether) urethane (TPU) blends [26]. In this work, we extend our interest in the electroactive shape memory properties of PLA/TPU/CB blends. Carbon black (CB) nanoparticles with the self-networking capability and electrical conductivity were introduced into PLA/TPU blends (70/30 by weight). In contrast to previous SMP blends in which elastomers occupy large proportion, the content of TPU in our study is only 30 wt%. It is expected that the addition of CB nanoparticles extends the composition range for co-continuity of PLA/TPU blends at relative low TPU content. The influence of phase morphology tailored by CB nanoparticles on shape recovery performances was investigated. Besides, the conductive CB nanoparticles could impart composites with electrical conductivities; thus the electroactive shape memory effect was studied as well. We aim to obtain a new type of double percolated conductive polymer composites with optimal shape memory properties, mechanical properties and excellent electrical conductivities. 2. Materials and methods 2.1. Materials PLA (4032D) with a density of 1.25 g/cm3 was purchased from
9
Nature Works Co. Ltd., USA. TPU (WHT-1570) with a density of 1.21 g/cm3 was purchased from Yantai Wanhua Co. Ltd., China. CB nanoparticles (Printex-XE2B, Degussa Co. Ltd, Germany) had a primary diameter of 30 nm and were used as received. 2.2. Sample preparation PLA/TPU/CB ternary composites were firstly mixed through melt blending of PLA, TPU and CB in a Haake mixer at 190 C and 60 rpm for 6 min. The composition ratio of PLA/TPU was fixed at 70/30 and the loading amount of CB was changed from 0 to 8 phr. After that, the ternary PLA/TPU/CB mixtures were compressed into plates at 190 C for 6 min under 10 MPa. For convenience, PLA/TPU/CB ternary composites are termed as PLA70/TPU30/CBx, where x represents the weight percent of CB. 2.3. Characterization The phase morphology of cryogenically fractured PLA/TPU/CB ternary composites were inspected by using an FEI Inspect F scanning electron microscope (SEM, USA). The morphological structure and the localization of CB in the PLA/TPU blends were observed through transmission electron microscope (TEM, JEM2010) under an acceleration voltage of 200 KV. The ultra-thin samples with a thickness of 100 nm were prepared via a Leica UCT microtome at 100 C. Dynamic mechanical analyzer (DMA Q800, TA Company) was employed to evaluate the dynamic mechanical behavior of PLA/TPU/CB ternary composites. The temperature were increased from 0 C to 120 C with a heating rate of 3 C min1. The shape memory properties of PLA/TPU/CB ternary composites were measured by using DMA. A four-step procedure was designed as follows. (1) First, the sample was heated to 85 C and stretched to a certain strain (ε) under a constant force. (2) Then, the deformed sample was quenched to room temperature (25 C) with keeping the force. (3) After the force was removed, the temporary strain (εload) was measured. (4) Lastly, the sample was reheated to 85 C, maintained at 85 C for 15 min and the recovery strain (εrec) was recorded. The samples were heated or cooled with a rate of 5 C min1. The shape fixing ratio (Rf) and shape recovery ratio (Rr) of PLA/TPU/CB ternary composites could be calculated according to the following Equations (1) and (2):
Rf ¼
εload ε
(1)
Rr ¼
ε εrec ε
(2)
The tensile tests at a high temperature of 85 C were carried out on DMA (Q800, TA Company, USA). The samples were kept at 85 C for 5 min and then stretched at 3 N min1 until fracture. The tensile tests at room temperature (25 C) were measured using an Instron 4302 universal tensile tester (SANSI, China) with a crosshead speed of 5 mm min1. The notched Izod impact toughness of PLA/TPU/CB ternary composites was evaluated on an impact testing machine (XJU-5.5, China). The electrical conductivities of PLA/TPU/CB ternary composites were measured using a Keithley 6487 picoammeter. The sides of rectangular samples were coated with silver paint to eliminate contact resistance before testing. The ITECH IT6700 was conducted to yield constant voltage/current on samples to achieve electroactive shape memory behavior. The surface temperature of PLA/TPU/CB ternary composites was recorded in the middle of the rectangular samples by using a digital thermocouple.
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3. Results and discussion 3.1. Effect of CB nanoparticles on phase morphology of PLA70/ TPU30 blend The phase morphology has an important impact on the shape memory property in polymer blend system [9,10]. The effects of CB nanoparticles on the phase morphology of PLA70/TPU30 blend were characterized through SEM and shown in Fig. 1. It can be clearly seen that the PLA70/TPU30 binary blend has a typical seaisland morphology, where TPU spherical droplets with several micron sizes are dispersed in the PLA matrix (Fig. 1a). With a low loading content of 1 phr CB nanoparticles in blend, it turns out that CB nanoparticles are selectively distributed in the TPU phase. There is no obvious morphologic change except for a slightly increase in the size of TPU domains (Fig. 1b). Very interestingly, when the content of CB nanoparticles is increased to 3 phr, some discrete TPU droplets tend to approach each other and a special co-continuous like structure is formed (as shown by red circles in Fig. 1c). This phenomenon indicates that 3 phr CB nanoparticles can induce the morphology transition of PLA70/TPU30 blend from the initial seaisland morphology to the unique co-continuous like morphology. Further raising the content of CB nanoparticles to 5 phr, a network structure with higher continuity is formed since more and more TPU droplets fuse together during melt blending (Fig. 1d). It's worth noting that the phase size of TPU shown in Fig. 1c is quite larger than that in Fig. 1d. According to our previous works [24,25], the morphological transition from sea-island to co-continuous like or co-continuous structure in PLA/TPU blends is mainly attributed to the self-networking behavior of used nanoparticles. Hence, the size of TPU phase is largely determined by the size of CB network. By increasing the CB content from 3 phr to 5 phr, a more compact and perfect CB network is expected to be formed, as a result the
decrease in size of the TPU phase is observed. Swelling experiments were further conducted to check the morphology transition of TPU phase (see Fig. S1). As for the PLA70/ TPU30/CB3, the sample was only slightly swollen after immersing in chloroform for 6 h (chloroform is a good solvent for PLA), which confirmed that the TPU phase indeed formed a continuous phase to support the sample in chloroform and was consistent with the SEM results. To confirm the morphological transition and the distribution of CB nanoparticles in PLA70/TPU30 blends, the representative PLA70/ TPU30/CB3 and PLA70/TPU30/CB5 composites were observed by using TEM. As shown in Fig. 2, PLA70/TPU30/CB3 exhibits a cocontinuous-like structure, in which the disconnected TPU clusters (darker parts) prevail through the PLA matrix (bright parts). A cocontinuous structure with higher continuous degree of TPU phase is formed in PLA70/TPU30/CB5. Moreover, CB nanoparticles are selectively localized in TPU phase, suggesting strong affinities between CB nanoparticles and TPU phase. In order to clear the role of selectively localized CB nanoparticles in the formation of co-continuous structure in PLA70/TPU30/CB nanocomposites, the dynamic viscoelasticity was investigated by rheology (see Fig. S2). The viscosity of TPU is largely improved by adding 5 phr CB nanoparticles, while the viscosity of PLA matrix is almost unchanged because CB is selectively dispersed in TPU phase. According to the classical Paule-Barlow theory [37], increasing the volume fraction or decreasing the viscosity would enhance the continuity of the minor phase in the matrix. Hence, the expected morphology of PLA70/TPU30/CB5 ternary composites should be sea-island structure same as the PLA70/TPU30 binary blend, rather than the co-continuous one as observed in Fig. 1d. The deviation implies that morphology change is not induced by the change of shear viscosity ratio of PLA and TPU/CB phase. Here, the selfnetworking effect of nanoparticles is confirmed to be closely
Fig. 1. SEM images of the cryo-fractured surfaces of PLA70/TPU30/CB composites with various contents of CB: (a) 0 phr, (b) 1 phr, (c) 3 phr, (d) 5 phr. Scale bar: 10 mm.
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Fig. 2. TEM images of PLA70/TPU30/CB3 (a, a0 ) and PLA70/TPU30/CB5 (b, b0 ).
related to the formation of the co-continuous structure [24e26]. CB nanoparticles tend to self-aggregate to form a continuous network in polymer melts. These CB nanoparticles are selectively located in TPU phase and drive adjacent TPU droplets fuse together to form continuous phase in PLA matrix. 3.2. Shape memory properties of PLA70/TPU30/CB composites with various contents of CB Based on the above morphological analysis, CB nanoparticles are capable to induce the phase morphology of PLA70/TPU30 blend change from sea-island structure to co-continuous structure, which is helpful for both shape fixing and recovery performances. The large modulus decline around the transition temperature (Ttrans) is necessary for the material to show shape memory effect [11]. PLA70/TPU30 blend has a transition temperature range (60 Ce80 C) where the storage modulus (E0 ) suddenly decreases (see Fig. S3). Thus, the Ttrans is set at 85 C, which is about 15 C above the glass transition temperature (Tg) of PLA. The dual shape memory performances of PLA70/TPU30/CB blends with various contents of CB were quantitatively measured on DMA (see Fig. 3). The programming procedure were conducted with a uniaxial stretching above the Tg of PLA followed by a rapid cooling to 25 C while keeping the stretched state. For example, the PLA70/TPU30 binary blend was heated to 85 C and then stretched to a strain of 68% with a constant stress of 2.25 MPa. The stretched state could be well maintained after fast quenching and subsequent unloading stress at 25 C. Upon reheating above the Tg of PLA (85 C), a large proportion of fixed strain was recovered while a
final (unrecovered) strain of about 30% still remained, indicating the PLA70/TPU30 binary blend could not fully recover to its original shape. In contrast, with adding 3 phr CB in blend, the PLA70/TPU30/ CB3 ternary blend exhibited much better capability to recovery strain, i.e., the final strain recovered to about 12% at 85 C. Further increasing the content of CB to 5 phr or higher, the final strain diminished to 10% and kept almost constant, suggesting a saturation phenomenon. The shape fixing ratio (Rf) and shape recovery ratio (Rr) are two important parameters in quantitatively evaluating the shape memory properties of SMPs. The Rf and Rr of PLA70/TPU30/CB composites with various contents of CB are shown in Fig. 4. Obviously, all the composites with or without CB exhibit a high Rf nearly 90%, suggesting that the content of CB has little effect on the Rf. This is ascribed to most of the retractions resulting from the elongated TPU phase are prevented by the totally rigid PLA continuous phase at 25 C. When the temperature is heated higher than the Tg of PLA, the amorphous PLA chains begin to move, and release the restricted TPU phase. The strong resilience of the stretched TPU phase provides the recovery driving force, which is the major contributor for the Rr. In the case of the Rr, the PLA70/TPU30 binary blend only has a Rr of 59%, whereas the PLA70/TPU30/CB3 ternary composite has a largely enhanced recovery capability, as revealed by its Rr of 80.2%. Differing from the sea-island morphology of PLA70/TPU30 binary blends, a co-continuous like structure is formed in PLA70/TPU30/ CB3 ternary composite because of the self-networking ability of CB nanoparticles as well as the strong affinities between CB nanoparticles and TPU phase. The formed consecutive TPU phase offers a strong recovery driving force to achieve the outstanding shape
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Fig. 3. The thermal-mechanical tensile curves of PLA70/TPU30/CB composites with various contents of CB: (a) 0 phr, (b) 1 phr, (c) 3 phr, (d) 5 phr, (e) 6 phr, (f) 8 phr.
Fig. 4. Shape recovery ratio (Rr) and shape fixing ratio (Rf) of PLA70/TPU30/CB composites with various contents of CB.
recovery performance. The strong dependence of the shape recovery property on phase morphology can be further demonstrated by the variation of Rr of PLA70/TPU30/CB composites as a function of CB content. PLA70/TPU30/CB5, PLA70/TPU30/CB6, and PLA70/ TPU30/CB8 all exhibit a good Rr of 84.5%, 85%, and 85.9%, respectively. The Rr keeps almost constant with increasing CB content from 5 phr to 8 phr because the co-continuous structure are all well
formed in these composites. The phase morphology and thermal-mechanical tensile curve of binary PLA50/TPU50 blend are shown in Fig. S4. PLA50/TPU50 displays a co-continuous morphology and reveals good shape memory property with not only high Rf (92.1%) but also Rr (85.7%). In this PLA/TPU blend system, the powerful recovery driving force of the elongated TPU phase is the main contributor for the shape recovery performance, especially the elastic resilience of continuous TPU phase is larger than that of dispersed TPU domains. It is believed that the increase in the content of TPU could further enhance the Rr of PLA/TPU blends. However, the relative high content of TPU would decrease the modulus and strength of PLA/ TPU blends, which limits the practical applications. In this study, we aim to obtain PLA/TPU blends with good stiffness-toughness balance and shape memory properties. Thus, the content of TPU in our study is only 30 wt%. The self-networking capability of CB nanoparticles plays a key role, which leads to the morphological change from a typical sea-island structure to a unique co-continuous structure, and thus endowing PLA70/TPU30/CB ternary composites with remarkably improved shape recovery property as compared to PLA70/TPU30 binary blend.
3.3. Shape memory mechanism of PLA70/TPU30/CB composites Based on the aforementioned considerations, we propose a possible shape memory mechanism for PLA70/TPU30/CB
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composites in Fig. 5. The PLA70/TPU30/CB composites can be easily stretched to certain strains above the Tg of PLA. Then the amorphous PLA chains are frozen below its Tg by rapid cooling to room temperature (25 C). No instant shrinkage is observed after releasing the tensile force at ambient temperature, because the modulus of the blends originated form the continuous PLA phase is much more than the resilience force of the elongated TPU phase. Therefore, the temporary shape can be well held. The recovery driving force mainly derives from the stored resilience of elongated TPU phase, which is hindered by the stiff PLA continuous phase in temporary shape. PLA70/TPU30 binary blend shows a sea-island morphology, in which spherical TPU domains are dispersed in the PLA matrix (Fig. 5a). The discrete and small TPU droplets are not powerful enough to fully drive the material back to the original shape, which is the reason for low Rr. With the addition of CB nanoparticles (above 3 phr) into the PLA70/TPU30 blend, these CB nanoparticles with self-networking ability are selectively located in the TPU phase and drive adjacent TPU domains fuse together to form continuous phase in the PLA matrix (Fig. 5b). The elastic resilience of continuous TPU phase is clearly larger than that of dispersed TPU particles. This indicates that the continuous TPU phase experienced large deformation absorbs more energy and generates a stronger recovery driving force. When the temperature is reheated above the Tg of PLA, the elongated continuous TPU phase is released without much prevention of the PLA phase and then drives the composite recover to its original shape. At this moment, both the powerful recovery driving force of the elongated TPU phase and the intrinsic recovery of the orientated PLA chains contribute to an excellent shape recovery performance. Generally, to achieve the desired shape memory properties, the elastomer/plastic ratio is often larger than 50/50 (wt/wt%) [9,10,20], whereas the content of elastomer in our study is only 30 wt%. Our work demonstrates the introduction of nanoparticles with selfnetworking ability into binary polymer blends to be a highly flexible strategy to design their shape memory properties via tuning
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the phase morphology over a much larger composition range. 3.4. Mechanical properties of PLA70/TPU30/CB composites Although SMPs have already been applied in some fields, there are still some limitations such as small recovery stress due to low modulus at Ttrans and inertness to electricity [38]. From above analysis, a unique co-continuous structure is obtained by introducing CB nanoparticles into PLA70/TPU30 blend. Compared with PLA70/TPU30 binary blend (sea-island structure), the PLA70/ TPU30/CB blends shows a superior shape recovery property because the continuous TPU phase provides a strong resilience. In the following work, we mainly pay attention to the superior mechanical property and electrical conductivity of CB nanoparticles. It's anticipated that the CB nanoparticles selectively localization in continuous TPU phase can lead to an improvement in the recovery force as well as good electrical conductivities with the double percolation conductive network structure. It is necessary to measure the tensile properties of SMPs at Ttrans, because the recovery stress at Ttrans is an important element in the application of SMPs. The stress-strain curves of PLA70/TPU30/CB composites with various contents of CB at Ttrans (85 C) are shown in Fig. 6. It is noteworthy that the modulus (the stress at initial 10% strain) and tensile strength of PLA70/TPU30 blends is remarkably increased with the addition of CB nanoparticles. For example, the tensile strength of PLA70/TPU30 is 1.16 MPa. With the addition of 5 phr CB, the tensile strength of PLA70/TPU30/CB5 is increased to 2.65 MPa, showing about 130% improvement compared with that of PLA70/TPU30. However, the strain at break is decreased with the incorporation of CB nanocomposites. This is ascribed to that the mobility of polymer chains are restricted by the CB nanocomposites. Moreover, the strength and toughness at room temperature (25 C) of PLA70/TPU30/CB composites are also needed to be considered. As shown in Fig. S5 and Table S1, the impact toughness of PLA70/TPU30 blends is greatly enhanced with increasing the CB
Fig. 5. Schematic illustration of the shape memory mechanism for (a) PLA70/TPU30 binary blends and (b) PLA70/TPU30/CB5 ternary blends.
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electrical conductivity of PLA70/TPU30/CB3 increases to 2.9 103 S m1, which is about four orders of magnitude higher than that of PLA70/TPU30/CB1. It is well known that the double percolation in conductive filler-added polymer blends plays an important role in improving the electrical conductivity, which represents the percolation of the conductive filler distributed in one phase and the continuity of the filler-rich phase in the polymer blend happen at the same time [33]. The continuous degree of TPU phase is reasonable to be high enough to form the perfect conductive path in PLA70/TPU30/CB3. The value is almost the same as the critical CB loading observed from SEM images, at which the continuous TPU phase forms in the PLA matrix. Therefore, it can be concluded that the electrical conductivities of PLA70/TPU30/CB composites are dependent on the CB contents and phase morphology, and both effects give rise to the construction of the conductive path. Fig. 6. Typical stress-strain curves of PLA70/TPU30/CB composites with various contents of CB at 85 C.
content due to the formation of continuous TPU phase. There is no obvious decrease in the Young's modulus and tensile strength. Thus, the prepared PLA70/TPU30/CB composites with good stiffness-toughness balance and shape memory properties have great application prospects in intelligent devices. 3.5. Electrical conductivities of PLA70/TPU30/CB composites CB is a kind of commonly used fillers for conductive polymer composites because of its outstanding electrical conductivity. The electrical conductivities of PLA/CB binary composites and PLA70/ TPU30/CB ternary composites with various contents of CB are shown in Fig. 7. It can be seen that the electrical conductivities of the PLA70/TPU30/CB ternary composites are higher than those of the PLA/CB binary composites at the same CB content. In addition, except for CB content, the phase morphology is another predominant factor in the electrical conductivity of PLA70/TPU30/CB ternary composites. The continuous degree of the TPU phase has a significant impact on constructing a conductive network through the whole matrix, since CB nanoparticles are selectively distributed in the TPU phase. For PLA70/TPU30/CB1 with the discontinuous TPU droplets, its electrical conductivity is only 4.4 107 S m1. However, when increasing the CB content to 3 phr, the cocontinuous-like structure is formed in PLA70/TPU30/CB3. The
Fig. 7. Electrical conductivity of PLA/CB binary composites and PLA70/TPU30/CB ternary composites with various contents of CB.
3.6. Electroactive shape memory behavior of PLA70/TPU30/CB composites The high electrical conductivities of ternary PLA70/TPU30/CB composites is helpful for fast electroactive shape recovery behavior. The surface temperature of ternary PLA70/TPU30/CB composites under a voltage of 30 V is shown in Fig. 8. The surface temperature increases consistently as time goes by, which could be attributed that the sample is heated by Joule heat derived from the electric current. According to the equation of Q ¼ (U2 T)/R [39,40], the generated Joule heat (Q) is determined by voltage (U), time (T) and resistance (R) of the sample. It's clear that the sample with a higher voltage and lower resistance of will get more heat at the same time. The PLA70/TPU30/CB3 retains a stable state at only 35 C in 120 s. This temperature is about 35 C less than the Tg of PLA (nearly 70 C), indicating that the PLA70/TPU30/CB3 cannot be successfully actuated at 30 V. Further increasing the content of CB, the PLA70/TPU30/ CB8 is rapidly heated to 70 C within 30 s and finally reaches an equilibrium state of about 95 C. The electrical conductivity of PLA70/TPU30/CB8 (2.87 S/m) is approximately one order higher than that of PLA70/TPU30/CB3 (0.34 S/m). As a result, more Joule heat is produced and thus the surface temperature of PLA70/TPU30/ CB8 can be fast increased. This result demonstrates that PLA70/ TPU30/CB composites are capable of conducting the electrical current by adding CB as conductive filler. Meanwhile, the electrical conductivity of composites is high enough to be used as stimulation in the electroactive recovery of the deformed composites.
Fig. 8. The variation of surface temperature of PLA70/TPU30/CB blends with various contents of CB as a function of time under the constant voltage of 30 V.
X. Qi et al. / Composites Science and Technology 139 (2017) 8e16
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Acknowledgements We would like to express our sincere thanks to financial support of the National Natural Science Foundation of China (Grant No. 51421061 and 51210005). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.compscitech.2016.12.007. References
Fig. 9. Shape recovery ratio with the increase of time for PLA70/TPU30/CB6 and PLA70/TPU30/CB8 under the voltage of 30 V. Inset pictures display the electroactive shape recovery behavior of PLA70/TPU30/CB6.
The bending test method was utilized to evaluate the recovery process, and the shape recovery ratio was measured based on the variation of deformation angle. The shape recovery ratio with the increase of time for PLA70/TPU30/CB6 and PLA70/TPU30/CB8 under 30 V is displayed in Fig. 9. It can be seen that the electrical conductivity of the sample has a great influence on the rate of electroactive shape recovery process. The PLA70/TPU30/CB8 shows faster response of shape recovery at the same time and finally achieves a Rr of 90% in 80 s, while the PLA70/TPU30/CB6 reaches a Rr of 90% in 150 s. As shown in inset photos of Fig. 9, the visual electroactive shape recovery behavior of PLA70/TPU30/CB6 was actuated under the constant voltage of 30 V and recorded with a camera. The initially rectangular strip-shaped sample was bended into an “n”-like (temporary shape) at 85 C and then rapidly cooled to room temperature (25 C). After being kept at 25 C for 2 h, the temporary shape was still well fixed. And with the voltage loaded at 30 V, the sample started to return to its original shape in 150 s. This consistent result was obtained after the sample experiment being repeated for more than three times. 4. Conclusions In this work, CB nanoparticles were used to tune the phase morphology and the shape memory properties of PLA/TPU blends (70/30 by weight). A phase morphology change from a common sea-island structure to a special co-continuous structure was observed. The CB nanoparticles with the self-networking capability had a tendency to self-aggregate to form a network during melt blending and drove dispersed TPU droplets fuse together to form continuous phase. With such novel co-continuous structure, the PLA70/TPU30/CB blends exhibited a superior shape memory property because the continuous TPU phase offered a strong recovery driving force. Moreover, the CB nanoparticles selectively localization in continuous TPU phase led to an improvement in the mechanical properties as well as good electrical conductivities with the double percolation conductive network structure. The PLA70/ TPU30/CB8 showed good electroactive shape memory behavior, which could recover to their original shape in 80 s at 30 V. Our work provides a simple strategy to prepare polymer/elastomer/fillers ternary composites with a good balance of shape memory properties, mechanical properties and excellent electrical properties, which will proceed some promising applications in intelligent devices.
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