Polyolefin Elastomer

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Materials and Interfaces

Fabricated Biobased Eucommia Ulmoides Gum/ Polyolefin Elastomer Thermoplastic Vulcanizates into a Shape Memory Material Hailan Kang, Ming Gong, Mingze Xu, Haoyu Wang, Yushi Li, Qinghong Fang, and Liqun Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b04710 • Publication Date (Web): 08 Apr 2019 Downloaded from http://pubs.acs.org on April 9, 2019

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Industrial & Engineering Chemistry Research

Partially biobased thermoplastic vulcanizates with shape memory properties were prepared by in situ dynamic vulcanization eucommia ulmoides gum and polyolefine elastomer. 49x28mm (300 x 300 DPI)

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Industrial & Engineering Chemistry Research 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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Fabricated Biobased Eucommia Ulmoides Gum/ Polyolefin Elastomer Thermoplastic Vulcanizates into a Shape Memory Material Hailan Kang,†‡ Ming Gong, †‡ Mingze Xu, †‡ Haoyu Wang, †‡ Yushi Li, †‡ Qinghong Fang,*, †‡ Liqun Zhang*,§

†

College of Materials Science and Engineering, Shenyang University of Chemical Technology, Shenyang, 110142, China

‡

Key Laboratory for Rubber Elastomer of Liaoning Province, Shenyang University of Chemical Technology, Shenyang, 110142, China

§

State Key Laboratory of Organic–Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China

Correspondence to: Q. Fang (E-mail: [email protected]) and L. Zhang (E-mail: [email protected]).

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ABSTRACT：Partially biobased thermoplastic vulcanizates (TPVs) with shape memory properties were prepared by in situ dynamic vulcanization of eucommia ulmoides gum (EUG) and polyolefin elastomer (POE). The crosslinked EUG phase dispersed in POE matrix acted as a framework structure throughout the POE phase, which limited irreversible deformation of POE and provided sufficient resilience force to drive the POE molecular chains recover to its initial shape. The shape memory results demonstrated that EUG/POE TPVs exhibited excellent shape memory properties with high shape fixity and shape recovery rate all above 95%. The EUG with the dimension of 0.4 to 1.6 m dispersed in the POE matrix, and the more EUG phase resulted in large EUG aggregates. The crosslinked EUG phase considerably improved the permanent deformation of POE, and tensile set linearly decreases from 134% to 4% with increasing EUG contents. The mechanical and shape memory properties of EUG/POE TPVs changed little after reprocessed twice, exhibiting good reprocessability. All of these remarkable advantages make EUG/POE TPVs good candidates for shape memory materials. Keywords: Eucommia ulmoides gum ； Polyolefin elastomer ； Dynamic vulcanization ； Shape memory properties; Reprocessability.

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1. INTRODUCTION Shape memory polymers (SMPs) are a class of stimuli-responsive smart materials that are programmed to change their shape upon external stimulus, such as temperature, light, magnetism, moisture, solvent, and so on[1-7]. Among these SMPs, thermo-responsive SMP (TSMP) has been extensively investigated and widely applied in many fields such as textile[8], biomedicine[9], aerospace[10] and so on. TSMPs generally contain permanent shape, which is obtained by physical or chemical crosslinks, and temporary shape, which is formed by vitrification, crystallization, or some other physical interaction. The temporary shape is obtained by deforming the samples above switching temperature (Ttrans), namely the glass transition temperature (Tg) or melting temperature (Tm). The temporary shape is fixed by cooling the deformed samples below Ttrans. Then the shape recovery is achieved by reheating the samples above Ttrans, and the driving force for recovery was provided by the deformed reversible phase with stored elastic force. Some synthetic block copolymers usually present shape memory effect (SME), in which hard segments containing physically or chemically crosslinked create the permanent shape and soft segments generate temporary shape.[11] However, the synthesis of block copolymers with the required SME is very expensive, limiting their broad utilization more or less.[12, 13] Among various technologies to design and fabricate TSMPs, polymer blending recently attracts more attentions because of the ease of processing.[14-18] The memory shape blends obtain the adjustable structure and properties by changing the components. Generally, shape memory blends consist of an elastomer serving as the memory shape and a crystalline polymer acting as the

3


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switching phase. For example, Zheng at al.[15] prepared a kind of TSMPs by thermoplastic polyurethane (TPU) and polycaprolactone (PCL). The TPU/PCL blends exhibited higher shape fixing ratios and recovery ratios through layer-multiplying extrusion. Zhang at al.[19] employed styrene–butadiene–styrene tri-block copolymer and PCL to fabricate a novel TSMP with controllable shape memory performance. Compared with the above mentioned plastic/rubber blends, a crosslinked elastomer components can provide strong driving force for shape recovery to improve the shape memory effect of TSMPs[20, 21]. Thermoplastic vulcanizate (TPV) is a special thermoplastic elastomer prepared by a dynamically vulcanizing technique, which consists of crosslinked rubber and thermoplastic. In most cases, the crosslinked rubber is sheared to particles and tends to form dispersed in the thermoplastic matrix under the high-speed shear force[22]. The resilience of the dispersed rubber phase is obviously inferior to that of a continuous rubber phase, since the typical sea−island structure is hard to achieve strong interfacial adhesion to hold the highly elongated rubber phase to its temporary shape. Without the strong interfacial adhesion, the retraction of stretched rubber phase causes the voids, reducing shape recovery[20, 21]. Few successful SMPs based on plastic/rubber TPVs are reported, and thus it is urge for us to develop a new type SMPs based on plastic/rubber TPVs. Eucommia ulmoides gum (EUG) is a natural rubber resource derived from the eucommia ulmoides oliv in central and southern China. It chemical structure is trans-1,4-polyisoprene and an isomer of natural rubber (NR).[23, 24] With both flexibility and plasticity, EUG can be processed into thermoplastic materials, thermoelastic materials, and high elastic materials. EUG has been used

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as the plastic product, submarine cable, rubber blend, medical material and so on.[25-27] Xia’ groups prepared foamed EUG shape memory composites with multiple [28] and triple shape memory effect[29]. However, the studies about SMEs based on EUG or its blends are still rare. In the present work, EUG and polyolefin elastomer (POE) are chosen as the two components to develop a new-typed TPV with shape memory properties. In our design, the important reason for choosing POE and EUG are follows: i) POE served as matrix, providing the elasticity for TPV; ii) semicrystalline EUG crosslinked and dispersed in POE matrix, acted as a framework structure throughout physical entanglement between EUG and POE, providing sufficient resilience force to drive the POE molecular chains recover to its initial shape. The effects of EUG content and its crosslinking degree on the properties of SMPs based on EUG/POE TPVs SMPs, such as thermal properties, mechanical properties, microstructures, shape-memory behavior and reprocessability, were detailedly investigated. To data, there is no reported about EUG based TPVs.

2. EXPERIMENTAL SECTION 2.1 Materials Eucommia ulmoides gum (EUG), with a number-weight molecular weight (Mn) of 1.9×105 g/mol and a density of 0.91 g/cm-3, was kindly denoted by Xiangxi Laodie Biotechnology Co., Ltd. Polyolefin elastomer (POE, Engage 8842), with a melt flow index (MFI) of 1.0 g/10 min (190ºC, 2.16 kg) and a density of 0.86 g/cm-3, was purchased from DuPont-Dow Company. The sulfur, zinc oxide (ZnO), stearic acid (SA), accelerant tetramethylthiuram disulfide (TMTD), and antioxidant N-isopropyl-Nꞌ-phenyl-p-phenylene diamine (4010NA) and antioxidant pentaerythritol tetrakys 5


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3-(3,5-ditert-butyl-4-hydroxyphenyl)propionate (1010) were commercial products. 2.2 Preparation of EUG/POE TPVs EUG Premix. EUG premix was mixed by a 6-inch two-roll mill at 60ºC. The formula of EUG premix were as follows (in phr (parts per hundred of polymer)): EUG, 100; ZnO, 4; SA, 2; sulfur, 0-3; TMTD, 1.2; 4010NA, 2. ZnO, SA together with sulfur and TMTD, constitute the ‘vulcanizing system’ for the formulation. The reaction product (Zinc stearate) of ZnO and SA, together with TMTD speeds up the rate of sulfur vulcanization. POE/EUG TPV. The dynamically crosslinked EUG/POE TPV blends were prepared by melt-mixing EUG, antioxidant 1010 and POE for 8 min by using an internal mixer (XSS-300, Shanghai Kechuang Rubber and Plastic Machinery Equipment Co., Ltd, China) at 160 º C with a rotational speed of 80 rpm. The crosslinked samples were hot-pressed at 180 ºC for 6 min to form 2 mm thick sheets. For brevity, the sample with 40/60 composition ratio of EUG/POE was abbreviated to E4P6. The chemical structures of POE and EUG and the vulcanizing schematic of EUG are shown in Figure 1. The POE could not be crosslinked by sulfur, so the vulcanization reaction only occurs on EUG macromolecule chains. Along with the vulcanization, the EUG macromolecule chains link by polysulfur bonds to form reticulate structures. 2.3 Characterization Differential Scanning Calorimetry (DSC). DSC measurements were conducted using a DSC-Q200 (TA Instruments, USA) under nitrogen atmosphere. The test conditions were as follows： i) the temperature was heated to 100 ºC at 10 ºC /min and kept isothermal for 5 min to eliminate any 6



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thermal history; ii) then the temperature was cooled to -80 ºC at 10 ºC /min and finally reheated to 100 ºC at 10 ºC /min. The crystalline degree of the EUG was calculated by the following equation, 𝐶rystallinity(%) =

∆𝐻𝑚 ∆𝐻𝑚∗

× 100%

(1)

where ΔHm and ΔHm* are melting enthalpy of polymer portion and that of its pure polymer (ca. 186.8 J g−1 for EUG [28] ), respectively. Dynamic mechanical thermal analysis (DMTA). DMTA was carried on a DMA Q800 (TA Instruments, USA) with a tension mode at 1Hz and from -100 ºC to 100 ºC at a heating rate of 5 ºC /min. Scanning Electron Microscopy (SEM). Morphology of EUG/POE TPV was observed by a SU8010 SEM (Hitachi Co., Ltd, Japan). The scanning electron microscopy (SEM) samples were fractured in liquid nitrogen, and then surface-coated with a thin gold layer. Nano Measurer 1.2 was adopted to calculate the number-average particle diameters. Mechanical Property Measurements. Tensile tests were carried out according to ASTM D412 by using a universal material testing machine (Instron 3365) with a crosshead speed of 500 mm/min at 25±2 ºC. At five samples were tested to get the average. Shore A hardness was tested according to ASTM 2240. The tensile set (H) of the samples was determined by H(%) =

𝐿1 ― 𝐿0 𝐿0

∗ 100

(2)

L1: the gauge length of sample after 3 min in the tensile test, and L0: the gauge length of original sample. 7


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Gel content. Soxhlet extraction method was used to determine the EUG gel content as a measure of the degree of crosslinking of the EUG phase. Approximately 0.5 g of samples was weighed (M1) and placed in an 80-mesh coppery pouch. The pouch was also weighted (M2) and then immersed in toluene for 24 h. After the sample was dried in a vacuum oven at 60 °C for 12 h, the weight of the pouch (M3) was determined. The EUG gel content was calculated by

Gel 

M3  M 2  100% M 1  WEUG

(3)

WEUG : the mass fraction of the EUG. Shape memory properties. Shape memory test was performed on the tensile test instrument. First, the dumbbell-shaped sample was stretched to the strain of 100% (ε1) at 60°C with a crosshead speed of 50 mm/min. Second, the elongated sample was cooled down to 0°C in the extended state for 5 min. Then the stress was unloaded to zero, recording the strain (ε2). Last, the specimen was reheated up to 60°C or other temperature and hold for 5min, recording the strain (ε3). Shape recovery ratio (Rr) and shape fixing ratio (Rf) were calculated by the following equations, respectively [30, 31].

Rr 

Rf 

2  3 2 2 1

(4)

(5)

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3. RESULTS AND DISCUSSION 3.1 Effect of sulfur content on the properties of EUG/POE TPVs Crosslinking degree. The degree of crosslinking has great influence on the crystallinity and mechanical properties. Consequently, the EUG gel content was adopted to characterize the degree of crosslinking of the EUG phase. A previous study demonstrated that the extraction samples in a good solvent would be suitable for determining the degree of crosslinking[32]. The dissolving experiment shows that EUG and POE dissolved fully in toluene at 60 °C. Because POE phase could not be crosslinked by sulfur, the EUG gel content was determined by extraction in toluene for 24 h at 60 °C to remove the non-crosslinked EUG and POE. The EUG gel content as a function of sulfur content is shown in Figure 2 for E4P6 TPVs. The EUG gel content of the E4P6 blend with no sulfur is 0%, as expected. The EUG gel content increases with increasing sulfur content, indicating the increase of the degree of crosslinking. The increasing of sulfur content above 2 phr do not lead to further increases in EUG gel content, indicating the formation of fine crosslinked network. Thermal Properties. We further study the effect of sulfur content on the thermal properties, and the DSC curves and the relevant data are shown in Figure 3 and Table 1, respectively. POE exhibits a crystallizing peak (Tc) at 22°C (Peak 1), a glass transition (Tg) at -50 °C and a broad melting peak (Tm) at 30-60 °C; while EUG shows a Tc at 26°C (Peak 2) upon cooling, Tg at -51 °C and an obvious Tm at 50 °C upon heating. The E4P6 blend without sulfur shows two Tcs at 22°C (Peak 1) and 26°C (Peak 2) corresponding to that of POE and EUG, and a broad melting peak at 30-60°C owing to the overlapped melting peaks of POE and EUG. For E4P6 TPVs, the crystallization enthalpy and the 9


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crystallization temperature (Peak 1) of POE remain almost unchanged; while the crystallization enthalpy of EUG decreases and the crystallization temperature (Peak 2) shifts to the low temperature as the sulfur content increased. This indicates that the crosslinking reaction only occurs in EUG phase, and the crosslinking restrains chain mobility of EUG, resulting in a decreased crystallization ability of EUG. As can be seen from Figure 3b, E4P6 TPVs show a decreased the melting enthalpy and a declined melting temperature compared with those of EUG/POE blend owing to the decreased crystallization ability of EUG. Mechanical properties. The stress-strain curves of pure EUG, pure POE and E4P6 TPVs with different sulfur content are shown in Figure 4. As shown in Figure, pure EUG exhibits distinct yielding and strain hardening during stretching with high tensile strength of 18.7 MPa and elongation at break of ~560%, while POE behaves elastic stretching with tensile strength of 12.8 MPa and high elongation at break of ~1150%. Compared with that of POE, the tensile stress of E4P6 blend is higher at the same tensile strain, indicating that the EUG can strengthen POE. The E4P6 blend and E4P6 TPVs all exhibit an elastic stretching behavior of soft and tough, attributing to elastomeric materials. The tensile strength of E4P6 TPVs further decreases with increasing of sulfur content since the crosslinking restricts the chain mobility of EUG and thus reduces their crystallinity as confirmed by the DSC results. Shape memory properties. The shape memory properties were performed on a typical tensile model. To have a well understanding of the shape memory properties, the results of shape recovery ratio (Rr) and shape fixing ratio (Rf) as a function of sulfur content is shown in Figure 5. According

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to the DSC curves, the crystalized and melting temperature of EUG and POE phase are very close. For the thermally induced shape memory polymer, the switching temperature (Ts) generally corresponds to Tg or Tm. According to the DSC curves, the melting peak of POE is very wide, so Ts is set at 60 °C, which is above Tm value of the TPV. The shape recovery ratio of a function of temperature would be discussed in the following section 3.3. The Rf decreases with increasing of sulfur content. The blend with a higher crosslinking degree has a lower crystallinity as confirmed by DSC results. The decreased crystallinity increases the content of non-crystalline part in the EUG phase, which is reversible phase. The blend with more reversible phase needs more energy for deformation, and stores more energy with the loss of elastic entropy when cooled. Thus, the blends are more prone to return to their original shapes, resulting in a lower Rf. Without crosslinking, the EUG/POE blend also shows shape recovery behavior with ~45% Rr, indicating that the polymer chains of EUG and POE form the strong physical entanglement networks. Such low Rr for EUG/POE blend displays that EUG/POE blend cannot used as shape memory materials directly. After crosslinking, Rr, all above 90%, increases with sulfur content and then slightly declines. One hand, the increase in crosslinking density would further limit irreversible deformation of POE, and thus enhances the Rr of the TPVs. On the other hand, the much higher crosslinking density inhibits large relaxations of the EUG chains between crosslinks, resulting in a declined Rr. Considering the shape memory properties and mechanical properties, in the latter investigations we used 2 phr as sulfur content to prepare EUG/POE TPVs.

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3.2 The Effect of EUG/POE ratio on Morphology and Mechanical Properties of EUG/POE TPVs Morphology. The effect of EUG/POE ratio on the morphology of EUG/POE TPVs was investigated, and the SEM micrographs are depicted in Figure 6. It can be seen that a number of irregular particles or voids are caused by the dispersion phase of EUG, revealing typical phase separation in the microstructures. As the EUG phase is in situ vulcanization during high shear forces, the EUG phase was broken up into nanoparticles and dispersed in the POE matrix. For EUG/POE TPV with low EUG content (Figure 6a, 6b), crosslinked EUG particles with the diameters of 0.4 to 1 m show the homogeneous dispersion in POE phase. The diameters of EUG particles and their distribution are increasing with the increase of EUG content, as shown in Figure 6d-e. For EUG/POE TPV with more EUG phase (Figure 6c), the more obvious aggregation in the EUG phase is observed and the diameters of EUG particles range from 0.8 to 1.6 m, resulting in larger particle diameters and wider particle distribution. This is attributed to the enhanced probability of agglomerates of EUG particles that “collide” with neighboring particles and the oppositely reduced probability of the split of existing ones to smaller[33]. The increase in diameters of dispersed phase with the rubber/plastic composition ratio also been found in other TPVs, such as EPDM/PP TPV[34], XNBR/PA12 TPV[35] and so on. Mechanical properties. The tensile properties of EUG/POE TPVs with different EUG/POE composition ratios are shown in Figure 7. As shown in Figure 7a, all the stress-strain curves show a typical elastomer stretching character of soft and tough, indicating that the dynamically vulcanized

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EUG/POE blends can be attributed to elastomers. The tensile strength (i.e., breaking strength) of EUG/POE TPVs decreases with increasing EUG content, range from 11.6 MPa to 5.1 MPa; while the elongation at break obviously decreases with the EUG contents, range from 1030% to 150%. For EUG/POE TPVs, the crosslinked EUG particles dispersed in POE matrix would inevitably fragment the continuity of POE, limit polymer chain mobility of POE, and thus destroy strain-hardening of POE polymer chains, resulting in the decreased in tensile strength and elongation at break. Moreover, the tensile stress at the same tensile strain increases with EUG content, which is due to the reinforced by the crosslinked rigid EUG particles and uniformly dispersed of EUG particles in POE matrix. In addition, the further decrease in tensile strength of TPVs with more EUG contents is also ascribed to nonuniform distribution of the crosslinked EUG particles in the POE matrix (as shown in Figure 6c). Besides, Shore A hardness of EUG/POE TPVs increases with EUG content from 62° to 90°, which also indicates the high strength and stiffness of crosslinked EUG particles. Interestingly, the tensile set linearly decreases with increasing EUG content, decreasing from 134% to 4%, indicating the permanent deformation of POE is obviously improved by dynamic vulcanization with EUG, as confirmed by POE/EPDM TPVs[36]. The crosslinked EUG particles dispersed in the POE matrix generate more molecular chain entanglements with POE chains, which could drive the EUG/POE TPVs to elastically recover from a highly deformed state. The dynamic mechanical analysis curves of EUG/POE TPVs and POE are depicted in Figure 8. EUG/POE TPVs all show high modulus (~109 MPa) at low temperature corresponding to glassy state and low modulus (~106 MPa) at high temperature corresponding to rubbery state expect pure POE. The storage modulus (E') drops due to glass transition of EUG and POE at -60 °C to -30 °C, and 13


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continues to drop due to the melting of crystallization of the blends at 20 °C to 50 °C, which is consistent with the results of tan delta (Figure 8b). EUG/POE TPVs exhibit high storage modulus ratio (E'high/E'low) in excess of 100 and the modulus at high temperature are all at the scale of 106 Pa. The high storage modulus ratio is essential for fixing the temporary shape at low temperature, and the E' at high temperature benefits shape recovery, which make these blends suitable for SMPs application.[37, 38] As shown in Figure 8, the E' of the EUG/POE TPVs increases with increasing EUG contents, ascribing to the reinforcing of crosslinked EUG. The EUG/POE TPVs with high E' in the rubbery state require more deforming energy, and the more energy would be stored with the loss of elastic entropy when cooled. Thus, the samples own larger recovery stress to return to their original shapes, resulting in a faster shape recovery speed, which favors their applications in shape memory polymers. Rheological properties. Since the melt processability is an essential property of TPVs, the influence of EUG/POE composition ratios on the rheological properties of TPVs was carried out. Figure 9 displays the complex viscosity (η*) and elastic shear modulus (G′) as a function of the angular frequencies at 180 °C. The complex viscosity all decreases and elastic shear modulus increases with increasing of frequency. As shown in Figure 9a, pure POE and EUG all exhibit the characteristic of pseudoplastic behavior, i.e. shear thinning behavior; while pure EUG shows a higher viscosity and more shear thinning behavior compared with those of pure POE. Before vulcanization, EUG tends to form “sea” due to the higher viscosity than POE. The η* of TPV increases with EUG content, owing to the higher viscosity of vulcanized EUG and the formation of larger vulcanized EUG particles[34]. For TPV with higher EUG content, the curve of η* shows a steeper slope, 14



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exhibiting a more significant shear thinning behavior. Figure 9b shows that the elastic shear modulus (G′) increase with an increase of the EUG content, mirroring the increase in η*. TPVs with high EUG contents have high G′, and the G′ curves change gently at low frequency region. The increase in EUG contents gradually enhances the interaction between different crosslinked EUG particles, thus leading to the formation of an interconnected structure of EUG particles. However, at high values of EUG content (E6P4), the high value of G′ at low frequency region is due to the aggregation of EUG particles, as conformed by SEM. 3.3 Shape memory properties of EUG/POE TPVs Figure 10 illustrates the transformation of EUG/POE TPV samples from a temporary spiral shape to the permanent linear shape. It can be seen that POE cannot recover to its initial shape after about 60 min. However, the recovery time of EUG/POE TPV becomes shorter with increasing of EUG content, and EUG/POE TPV can recover to its initial shape when the EUG/POE ratio is above 30/70, indicating crosslinked EUG phase plays a leading role in shape memory effect. Figure 11a shows the shape recovery curves of the EUG/POE TPVs as a function of temperature. All the curves exhibits S-shaped, and the values of Rr all increase with increasing of temperature and EUG/POE ratio. On one hand, the EUG/POE TPVs with more EUG content display more pronounced temperature-response. In other words, for EUG/POE TPVs with more EUG content, the temperature of shape recovery shifts to lower temperature. For examples, the E2P8 displays Rr of 80% when heated over 57 °C, and complete shape recover when over 60 °C; while the E7P3 displays the same Rr when over 43°C, and complete shape recover when over 53°C. On the other hand, the EUG/POE

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TPVs with more EUG content display higher Rr, such as Rr of 98% for E7P3. This indicates a sufficient crosslinked EUG phase further limit irreversible deformation of POE phase, bringing in high Rr of above 95%. The calculated Rf for EUG/POE TPVs are shows in Figure 11b. The Rf values are all above 92% and increasing with the increase in EUG/POE ratio, owing to high storage modulus ratio (E'high/E'low) as mentioned above. These results suggest that EUG/POE TPV is an ideal candidate for shape memory materials. As mentioned above, the typical sea−island structure of TPV is unable to provide enough resilience force to obtain high shape recovery ratio. Based on the above research result, a possible shape memory mechanism of the EUG/POE TPVs was proposed and shown in Figure 12. The crosslinked EUG phase disperses in POE matrix and serves as a framework structure throughout the POE phase through physical entanglement between EUG and POE (Figure 12a). Above the Ts, the melting of crystal in the TPV endows the sample with desired shape under external force (Figure 12c). When the deformed sample is cooled down to below the Ts, the temporary shape is fixed by crystalized of the blend (Figure 12d). Since the crystallization of both POE and EUG restricts the stretched molecular chains, no shrinking occurred immediately when the tensile load released. When the temperature is raised above Ts again, the sample would recover to its initial shape (Figure 12b). When the crosslinked EUG phase is rare, the less crosslinked EUG phase results in the less revertible phase and the less chain entanglements between POE and EUG. So it is not strong enough to drive POE phase to recover to its initial shape. When the content of crosslinked EUG phase is above 40%, the more crosslinked EUG phase not only provides more revertible phase and stronger resilience force, but also limits irreversible deformation of POE phase. Moreover, a sufficient crosslinked EUG 16



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phase generates more molecular chain entanglements with POE matrix. Therefore, the strong resilience force is prone to drive the POE molecular chains recover to its initial shape, leading to a perfect shape memory performance. 3.4 Reprocessability of EUG/POE TPVs One of the most notable distinctions of thermoplastic vulcanizate from traditional vulcanizate is reprocessability. To investigate the reprocessability of the E4P6 TPVs, the pressed scrap and the tested specimens were remixed by an open mill at 160 °C and hot-pressed to form a 1 mm thick sheet. The mechanical properties and shape memory properties of the E4P6 TPVs are measured after reprocessed one and two times. Figure 13a shows the tensile strength the elongation at break and tensile set change little after reprocessed, indicating that the E4P6 TPVs can be reprocessed with no reduction of mechanical properties. Moreover, the Rf and Rr of EUG/POE TPVs remain in the high values of 96% and 94% after reprocessed two times, exhibiting good shape memory properties after reprocessing.

4. CONCLUSION In this work, a series of shape memory materials based on TPVs was developed by eucommia ulmoides gum (EUG) and polyolefin elastomer (POE) through in situ dynamic vulcanization. The crosslinked EUG particles were dispersed in the POE matrix and entangled with POE molecular chains. The DSC results indicated that the crosslinking reaction only occurred in EUG phase, and the crosslinking further limited irreversible deformation of POE. The ensile strength and the elongation at break of EUG/POE TPVs ranged from 12.8 MPa to 5.1 MPa and from 1150% to 150%, 17


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respectively. The tensile set at break decreased with increasing EUG loading, decreasing from 134% to 4%, indicating the permanent deformation of POE was obviously enhanced by the vulcanized EUG particles. The high storage modulus at rubbery state and high storage modulus ratios (E'high/E'low) are benefit to shape memory properties, as confirmed by DMA results. The elastic shear modulus and complex viscosity of TPVs increase with an increase of the EUG content. When the content of crosslinked EUG phase is above 40%, shape fixity and shape recovery rate all above 95%, owing to the strong resilience force generated by sufficient crosslinked EUG phase. Besides, the EUG/POE TPVs showed no significant change in mechanical properties and shape memory properties after reprocessing two times.

Acknowledgements This work was supported by the National Key Research and Development Program of China (No.2017YFB0306902), the National Natural Science Foundation of China (Nos. 51703133 and 51573098) and the Program for Young and Middle-aged Scientific and Technological Innovative Talents of Shenyang , China (RC180154). .

18



Page 20 of 29

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Page 23 of 29

x

y n

Vulcanizing

POE

EUG

Sx

Sx

Sx

Crosslinking point

Sx

Crosslinking point

Crosslinking point

Sx

Sx

Sx

Sx

Figure 1 The chemical structures of POE and EUG and the vulcanizing schematic of EUG

100

EUG gel content (%)

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


80 60 40 20 0 0

1

2

3

Sulfur content (phr)

Figure 2 EUG gel content of E4P6 TPVs with different sulfur content.

22



(b)

(a) 1

2 POE

exo

POE

1

-60

-30

S1 S2

S2

1

0

S0

S1

1

2

EUG

EUG S0

2

1

2

S3

S3

30

exo

60

-60

-30

Temperature(oC)

0

30

60

Temperature(oC)

Figure 3 DSC traces of E4P6 TPVs with different sulfur content: (a) cooling curves and (b) heating curves.

20

EUG E4P6 S0 E4P6 S1 E4P6 S2 E4P6 S3 POE

18 16 14

Stress (%)

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

Page 24 of 29

12 10 8 6 4 2 0 0

200

400

600

800

1000

1200

Strain (%)

Figure 4 The stress-strain curves of EUG, POE and E4P6 TPVs with different sulfur content.

23


90

Page 25 of 29

(b)

(a)

100 100

95

Rr (%)

Rf (%)

80

60

90

85

0

1

2

40

3

0

1

Sulphur content (phr)

2

3

Sulphur content(phr)

Figure 5 (a) Shape fixing ratio (Rf) and (b) shape recovery ratio (Rf) of E4P6 TPVs with different sulfur content.

(a)

(b)

E2P8

(e) 40 Percentage (%)

40 30 20 10 0 0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

Diameter of EUG particle (m)

(c)

2.0

(f)

E4P6

Percentage (%)

(d) 50 Percentage (%)

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 20 10 0 0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8


2.0

E6P4

40 30 20 10 0 0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8


Figure 6 SEM images of frozen-fracture surfaces of EUG/POE TPVs: (a) E2P8; (b) E4P6; (c) E6P4; (d), (e) and (f) the corresponding size distribution of EUG particles.

24


2.0


(b) E1P9

200

90 160

85

Stress (MPa)

o

Shore hardness ( )

12

95

E5P5 E4P6

9

E6P4 6

E2P8 E3P7

E7P3

3

80

120

75 80

70 65

40

60 0

0

300

600

55

900

Strain (%)

E0P10 E1P9 E2P8 E3P7 E4P6 E5P5 E6P4 E7P3

0

Figure 7 The tensile properties of EUG/POE TPVs.

(a)

(b)

POE E2P8 E4P6 E6P4 E7P3

9

10

8

7

10

POE E2P8 E4P6 E6P4 E7P3

0.5

Tan delta

10

0.6

0.4 0.3 0.2

6

10

0.1 0.0

5

10

-80

-40

0

40

80

-80

o

Temperature ( C)

-40

0

o

40

Temperature ( C)

Figure 8 DMA traces of POE and EUG/POE TPVs: (a) Storage modulus and (b) Tan delta.

25


80

Tension set (%)

(a)

Storage modulus (Pa)

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

Page 26 of 29

Page 27 of 29

(b)

(a)

4

10

5

10

4

G' (Pa)

Pure POE E2P8 E4P6 E6P4 Pure EUG

5

10

* (Pa.s)

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

Pure POE E2P8 E4P6 E6P4 Pure EUG

3

10 3

10

2

0.1

1

10

100

10

0.1

1

10

100

Frequency (Hz)

Frequency (Hz)

Figure 9 (a) Complex viscosity (η*) and (b) Elastic shear modulus (G′) as a function of the angular frequencies at 180 °C for EUG/POE TPVs.

Figure 10 Transition from the temporary spiral shape to the permanent linear shape for EUG/POE TPVs. The recovery process was recorded after heating the samples to 60 °C.

26



(a)

(b)

100

100 80

98

E1P9 E2P8 E3P7 E4P6 E5P5 E6P4 E7P3

40

20

0 20

Rf (%)

60

Rr(%)

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

Page 28 of 29

30

40

50

60

96 94 92

70

90

E1P9

E2P8

E3P7

E4P6

E5P5

E6P4

E7P3

o

Temperature( C)

Figure 11(a) Shape recovery ratio (Rr) and (b) shape fixing ratio (Rf) of EUG/POE TPVs.

Figure 12 Schematic illustration of shape memory mechanism for EUG/POE TPVs.

27


(a)

70

800

500 400 300 200

Tensile strength (MPa)

600

Elongation at break Tensile strength Tensile set 60

10

700

Elongation at break (%)

8

50 40

6

30 4 20 2

10

100

0

0

0

0

(b) 100

1

Reprocessing times Rf Rr

2

80

Rf Rr(%)

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


Tensile set (%)

Page 29 of 29

60

40

20

0 0

1

Processing times

2

Figure 13 Mechanical and shape memory properties of E4P6 TPVs after reprocessing.

Table 1 DSC parameters of EUG/POE TPVs. Sample

Tc,POE

∆Hc,POEa

Tc,EUG

∆Hc,EUGa

Tm,POE

Tm,EUG

POE

21.2

8.1

-

-

45.4

-

Crystallinity EUG(%) -

EUG E4P6 blend E4P6 S1

-

-

25.3

43.5

-

49.3

23.3

21.5

9.2

26.4

42.3

-

57.1

22.6

20.9

8.0

8.9

19.0

46.3

37.7

10.2

E4P6 S2

21.1

9.2

-4.4

17.0

47.4

30.3

9.1

46.9

21.7

-

E4P6 S3 21.4 9.1 a Calculated from cooling curves by peak separation method.

28


Polyolefin Elastomer

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