polypropylene blends realized by in-situ compatibilization of sodium methacrylate

Journal Pre-proof Shape memory effect of dynamically vulcanized ethylene-propylene-diene rubber/ polypropylene blends realized by in-situ compatibilization of sodium methacrylate Chuanhui Xu, Rui Cui, Yukun Chen, Jianping Ding PII:

S1359-8368(19)33283-4

DOI:

https://doi.org/10.1016/j.compositesb.2019.107532

Reference:

JCOMB 107532

To appear in:

Composites Part B

Received Date: 10 July 2019 Revised Date:

10 October 2019

Accepted Date: 10 October 2019

Please cite this article as: Xu C, Cui R, Chen Y, Ding J, Shape memory effect of dynamically vulcanized ethylene-propylene-diene rubber/polypropylene blends realized by in-situ compatibilization of sodium methacrylate, Composites Part B (2019), doi: https://doi.org/10.1016/j.compositesb.2019.107532. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Shape memory effect of dynamically vulcanized ethylene-propylene-diene rubber/polypropylene blends realized by in-situ compatibilization of sodium methacrylate Chuanhui Xu1,3,*, Rui Cui2, Yukun Chen3, Jianping Ding2 1 Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology, School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, China 2 College of Material Science and Engineering, South China University of Technology, Guangzhou, 510640, China 3 Lab of Advanced Elastomer, South China University of Technology, Guangzhou, 510640, China Corresponding Author: Chuanhui Xu [email protected]

Abstract ： In this paper, the shape memory (SM) effect of dynamically vulcanized ethylene-propylene-diene rubber/polypropylene (DV-EPDM/PP) blends with typical sea-island structure is successfully fabricated by using sodium methacrylate (NaMAA) as compatibilizer and proper programming. The powerful EPDM/PP interface generated by in-situ compatibilization of NaMAA, as confirmed by FTIR, DMA, SEM and TEM, is the key to orchestrate the overall cooperation of EPDM and PP components to fulfill the SM behavior. The crystalline PP continuous phase holds the highly elongated EPDM particles via strong interface to fix the temporary shape perfectly, while the elongated EPDM particles restore elastic resilience as the driving force for the shape recovery. After a proper SM programming, e.g. shaping at 130 °C and triggering at 165 °C, the shape fixity ratio (Rf) and the shape recovery ratio (Rr) of DV-blends with NaMAA could exceed ~95% and ~95%, respectively. Keywords: A. Polymer-matrix composites (PMCs); B. Interface/interphase; D. Mechanical testing; E. Cure

1. Introduction Heat-responsive shape-memory polymers (SMPs), with the most convenient stimuli to trigger shape memory (SM) behavior, are becoming a particular interest to many applications such as smart devices, sensors and intelligent actuators [1,2]. This charming behavior is usually realized by the cooperation of two requisite components in the structure of SMPs: the fixed domains to sustain permanent shapes by restricting the strain, and the reversible domains to freeze/unfreeze temporary shapes at around transition temperatures (Ttrans) which are usually the melting temperature (Tm) [3,4] or glass transition temperature (Tg) [5]. From a micro perspective, the essence of SM behavior comes from the release of prestored strain-energy: the fixed domains under deforming state produce an elastic restoring force for the shape recovery, while the reversible domains unfreeze the temporary shape by triggering the strain relaxation at around Ttrans [6]. That is to say, the structural designing based on above essential mechanism of SM behavior guides to fabricating SMPs with desired performances. At present, the structural designing of SMPs can be achieved by grafting [7], 1

copolymerization [8], interpenetrating polymer network (IPN) constructing [9] or polymer blending [10], with the aim to obtain a multiphase material displaying at least two distinct transition temperatures. Among the above preparation techniques, the strategy of blending two immiscible polymers is an effective, convenient and economical method to achieve a blend with two-phase structure [11]. In this case, the continuous phase can be amorphous or crystalline polymer acting as reversible domain and the other one serves as fixed phase to provide recuperation force for shape recovery. For example, blending one or two polymers into a SM matrix will generate new SMPs blends. The most successful cases of SMPs blends are the PLA blended with various polymers, including NR [12], ENR [13], PU [14], PCL [15], etc. The introduced polymers mainly assist the SM matrix to improve its SM effect, as well as the mechanical performances. When the matrix, namely the continuous phase of the blend, has no SM characteristic, to realize the SM behavior for the blend will be more difficult. Designing a co-continuous phase structure is an ideal method to achieve SM behavior for the blend. The large phase interface in co-continuous structure earns a high efficiency of stress delivery between the two phases which is critical to SM behavior. For instance, Zheng et.al [16] reported a TPU/SLB multilayer material with excellent SM effect. They found that the SLB layers with a co-continuous PCL and PBS could further improve the SM behavior. Ji et.al [17] also developed a PVDF/PMMA multilayer material through layer-multiplying coextrusion, which had a broad and continuous Ttrans range from PVDF to PMMA layers. The good compatibility between PVDF and PMMA made their molecular chains diffuse easily into each other at layer interfaces and the resultant 1024-layer specimen exhibited a better triple- and quadruple-SM capacity. Unfortunately, the sea-island structure is the common option from an immiscible plastic/rubber blending system, e.g. ethylene-propylene-diene rubber/polypropylene (EPDM/PP) [18]. The drawback of sea-island morphology structure lies in that the elastic restoring force of dispersed rubber particles is inferior to that of co-continuous ones and a strong enough adhesive force between the plastic/rubber phases is not easily obtained to prevent abscission during deformation [19]. The slippage and abscission occurring at plastic/rubber interface during shape deformation seriously deteriorate the stress delivery efficiency and even eliminate SM effects. From this point of view, generating a strong interfacial adhesion between plastic and rubber phases is the critical issue for structural designing of SMPs blends. In our previous work [20,21], we employed zinc dimethacrylate (ZDMA) as an in-situ compatibilizer for dynamically vulcanized EPDM/PP (DV-EPDM/PP) blends. When using peroxides as crosslinker, ZDMA could be graft-polymerized with EPDM and PP chains, which resulted in a strong phase interface to promote the efficiency of stress delivery between PP phase and EPDM particles. In this paper, based on our previous work, we used methacrylic acid (MAA) and sodium hydroxide (NaOH) to in-situ form sodium methacrylate (NaMAA) as the reactive compatibilizer for DV-EPDM/PP blends and successfully programmed blends to materialize SM effect. It is reported for the first time that NaMAA, a monovalent metal salt coming from the acid-base neutralization of MAA and NaOH, could also significantly improve the interfacial adhesion between EPDM and PP in peroxide-induced DV blending. Our results substantiated that the NaMAA not only reinforced EPDM particles, but also provided stronger elastic restoring force. Simultaneously, the strengthened EPDM/PP interface promoted the stress delivery, which were responsible for the SM effect of DV-EPDM/PP blends. We hope our work could enrich the 2

structural designing of SMPs which possess a sea-island phase structure.

2. Experimental section 2.1 Materials. PP with a melting point 167 °C was got from CNOOC & Shell Petrochemicals Company Limited. EPDM was purchased from Jilin Chemical Co. (China), its ethylene content was 56% and the third monomer was 7.5%. Dicumyl peroxide (DCP) was purchased from Shanghai aladdin Biochemical Technology Co., Ltd. MAA (analytical pure) was purchased from Sinopharm Chemical Reagent Co. Ltd (China). NaOH (analytical pure) was purchased from Fuchen Chemical Reagents Co., Ltd (China). 2.2 In-situ formation of NaMAA in EPDM. As shown in Figure 1, NaMAA (MAA/NaOH was 1/1) was in-situ formed during EPDM mastication in an internal mixer (Haake Rheocord 90). The rotor speed and the mixing temperature were set at 32 rpm and 60 °C, respectively. EPDM was first added into the internal mixer and masticated for 3 min until a stable torque. Then, NaOH were added. 2 min later, MAA was instilled into the chamber of internal mixer by an injector. The dropping process was about 5 min. After that, the chamber was closed and the EPDM compound was mixed for another 5 min. The frictional/shear heating could increase the temperature of inner chamber to about 80°C. The high mixing temperature and the long mixing time helped to promote the neutralization of MAA and NaOH to their utmost to form NaMAA [22]. More details about the in-situ formation of metallic salts of unsaturated carboxylate via neutralization of MAA and metal oxides/hydroxides during mastication can be found in references [23]. Then, the rubber compounds with two EPDM/NaMAA ratios (100/2.5 and 100/5) were prepared.

Figure 1 Schematic of in-situ formation of NaMAA during EPDM mastication 2.3 Dynamic vulcanization of EPDM/PP/NaMAA blends. In our experiment, EPDM/PP weight ratio was fixed at 60/40. Dynamic vulcanization of EPDM/PP/NaMAA blends was also conducted in Haake Rheocord 90. The rotor speed and the mixing temperature were set at 70 rpm and 165 °C, respectively. PP was firstly shear-melted for 3 min, then EPDM/NaMAA compound was added. 3 min later, DCP (1.5 wt% of the EPDM component) was added to initiate the dynamic vulcanization of the EPDM phase. The mixing was continued for another 4 min. Then, the DV-EPDM/PP blends were removed from the chamber and cut into small pieces immediately when they were still soft. To prepare test specimens, the small pieces were compression-molded into films (~1.2 mm thick) at 190 ℃ under 15 MPa pressure. For simplicity, DV-EPDM/PP blends were named according to the EPDM/NaMAA ratio. For example, simple blend represented a simple EPDM/PP 3

blend without NaMAA and dynamic vulcanization; DV-0NaMAA represented a DV-EPDM/PP blend without NaMAA and DV-5NaMAA represented a DV-EPDM/PP blend in which EPDM/NaMAA ratio=100/5. 2.4 Materials Characterization. FTIR spectra were recorded by a Bruker Tensor 27 spectrometer (Germany) with resolution of 4cm-1 and 32 consecutive scans using attenuated total reflectance (ATR) model. Morphology of cryo-fractured surface were studied using a ZEISS Merlin (Germany) with an acceleration voltage of 5 kV. To observe the crosslinked EPDM particles, the PP component of the cryo-fractured surface was etched by hot xylene. The etched samples were dried completely and then sprayed with gold before scanning. TEM observation was carried out by using a JEM-2100F with an acceleration voltage of 200 kV in high vacuum mode. The test samples were ultramicrotomed into thin sections with about 100 nm in thickness using an ultramicrotome Leica EMUC6 at -100 °C (liquid nitrogen atmosphere). Dynamic mechanical properties were determined by a dynamic mechanical analyzer (NETZSCH DMA 242C, Germany) with tensile mode of 10 Hz, heating rate of 3 ℃/min and scanning temperature from -100 to +100 °C. Test specimens were rectangular shape with 8mm×4mm×1.2mm. XRD was conducted in a D/max-Ultima IV X-ray diffractometer by using Cu Kα (1.5418Å) X-rays with a current of 30 mA and a voltage of 40 kV. At room temperature, the date was collected in angular range of 5~60° at the rate of 2 °/min in steps of 0.02°. 2.5 Crystallinity of the PP phase in DV-blends. Thermal analysis was carried out using a DSC (NETZSCH DSC 204 F1, Germany) under a nitrogen atmosphere. About 8-10 mg sample was heated to 200 ℃ (20 ℃/min) and held at 200℃ for 5 min to eliminate the heat history. Then, the sample was cooled to 20 ℃ (20 ℃/min) and held at 20 ℃ for 5 min, and then heated again to 200 ℃ (20 ℃/min). The heat of fusion of 100% isotactic PP was 209 J/g and the crystallinity of the PP phase in DV-blend was calculated by the following equation (1):

Xc 

Ηc 100% Ηc*

(1)

where the heat of crystallization (△Hc) was determined from the area under the exotherm, the △Hc*= 209 J/g and φ was the mass fraction of PP in the blends. 2.6 Polarized optical microscopy (POM). The crystal growth of the PP phase was observed with a Stemi 2000 POM (ZEISS, Germany) equipped with a hot stage under crossed polarizers. The sample was sandwiched between two cover slips and mounted on a hot stage for temperature control. Samples were first melted at 200 ℃ and held for 5 min to eliminate the thermal history, and then were cooled to 130℃ for isothermal crystallization. The POM micrographs were recorded. 2.7 Tensile test Stress-strain curves of DV-blends (dumbbell shaped sheets with 1mm of thicknesses) were obtained by a UT-2080 (U-CAN, Taiwan) under a tensile mode at a crosshead speed of 500 4

mm/min. 2.8 Shape memory effect SM behavior was realized according to followed programming [24]. (1) 2 cm was marked at the center of a dumbbell specimen by two distance lines (L0). (2) The specimen achieved its thermal equilibrium at programming temperature for 10 min. (3) The specimen was elongated to 4 cm (L1) by an Instron 5500R machine (tensile speed was 50 mm/min), the maximum strain was defined as max. (4) The elongated specimen under loading was cooled down immediately and then the loading was removed. 24 h later, the distance between the two lines was recorded (L2). The corresponding strain was defined as F. (5) The shape recovery of specimen was conducted at triggering temperature. After the shape changed, the distance between the two lines was recorded (L3), and the corresponding strain was defined as R. Rf and Rr were calculated according to the followed equation (2). Every sample was measured for 5 individual specimens to obtain the average value. 

𝑅𝑓 = m × 100%

𝑅𝑟 =

 − 

× 100%

(2)

SM thermomechanical cycles were evaluated by using DMA Q800 instrument. Test samples were rectangular shape with the size of 8.0 × 6.0 × 1.0 mm. Samples were first annealed at 130 °C for 10 min to reach thermal equilibrium. Then, the sample was applied a preloaded load of 0.005 N and stretched to 100% strain by a linearly increased stress with a rate of 0.5 N/min. Next, the sample was cooled to 30 °C rapidly under the loading force. Followed, the applied force was unloaded to the preloaded value at a rate of 0.5 N/min and the sample was held at 30 °C for 10 min. At last, the sample was reheated to 165 °C at a rate of 3 °C/min and holding at 165 °C for 10 min. The Rf and Rr obtained from above programming can be calculated by equation (3) [25]. 

𝑅𝑓 = lo

d

× 100%

𝑅𝑟 =

− 

× 100%

(3)

where εload is the equilibrium strain under load after the cooling step, ε is the strain at unloading step before reheating, and εrec is the strain when the reheating and final annealing process is finished.

3. Results and discussion 3.1 In-situ compatibilization of NaMAA at EPDM/PP interface

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Figure 2 Schematic of in-situ compatibilization of NaMAA at EPDM/PP interface via DCP-induced dynamic vulcanization The in-situ compatibilization of NaMAA during DCP-induced dynamic vulcanization is schematically illustrated in Figure 2. The part of unsaturated carboxylic acid in NaMAA molecule is highly reactive in the presence of free radicals and could react with PP and EPDM at their interface [26]. According to the Eisenberg–Hird Moore (EHM) model [27], although Na+ is monovalent cation, it can also form ion pairs in the polymerized NaMAA due to the strong electrostatic interaction, which is similar to Zn2+ [28] and Mg2+ [29]. At the same time, EPDM and PP macromolecule radical were also formed by hydrogen abstraction in the presence of DCP [30]. As a result, the polymeric chains-attached ions functioned as multifunctional compatibilizer at the EPDM/PP interface and the complex reactions between poly-NaMAA, EPDM and PP macromolecular radicals turned out a strengthened PP/EPDM interface. Then, the compatibilized rubber/plastic interface improved the efficiency of stress delivery and prevented the possible slippage and abscission during shape deformation, which is the key feature of our approach to fulfill SM behavior of the DV-EPDM/PP blends. On the other hand, NaMAA has been reported as an effective reactive compatibilizer to improve the mechanical properties of rubbers [31]. After dynamic vulcanization, the NaMAA reinforced EPDM particles provided higher elastic restoring force to drive the shape recovery, which made additional contributions to the SM effect of DV-EPDM/PP blends.[22,23,32]

Figure 3 (a) Torque changes during dynamic vulcanization; (b) FTIR spectra of DV-blends extracted by hot xylene; (c) FTIR spectra of DV-blends after acidolysis treatment; (d) schematic of acidolysis of poly-NaMAA at EPDM/PP interface; (e) tan δ-temperature curve As shown in Figure 3a, the first two torque peaks were responded to the melting of PP pellets and the blending with EPDM/NaMAA compound, respectively. The addition of DCP initiated the crosslinking of EPDM under shearing state, resulting in a sharp increase in the torque. It is worth noting that the final stable torque of the DV-blends with NaMAA had an apparent increase compared with the ones before adding DCP, as marked by the red arrow heads in Figure 3a. For example, the final torque of DV-0NaMAA was 10.5 dN.m which was almost the same as that before adding DCP, while the final torques of DV-2.5NaMAA and DV-5NaMAA increased from 6

11.9 and 13.5 dN.m to 12.4 and 15.1 dN.m, respectively. Besides the reinforce effect of the crosslinked EPDM phase, the increased torque should be contributed by the strengthened interactions between EPDM and PP melt by in-situ compatibilization of NaMAA. Then, the DV-blends with NaMAA, DV-2.5NaMAA and DV-5NaMAA were extracted by hot xylene for 5 d in a Soxhlet extractor to remove the free PP. The FTIR spectra of extracted DV-blends with NaMAA are shown in Figure 3b. As expected, the FTIR spectra of extracted DV-2.5NaMAA and DV-5NaMAA are quite similar to the un-extracted DV-0NaMAA. However, compared with EPDM, PP and DV-0NaMAA, the extracted DV-blends with NaMAA showed a new absorption peak at around 1565 cm-1 which represented the asymmetric stretching vibration of ―COO― due to the strong coupling occurred by the stretching vibrations of C=O and C―O in the NaMAA [33]. These results strongly suggested that the in situ compatibilization was successfully achieved, involving the complex EPDM-NaMAA-PP based graft products. To further understand the chemical structure at the EPDM/PP interface, the above extracted samples were soaked in the toluene/chloroacetic acid (95/5 vol%) mixture for 3 d to cut off the ionic interactions by replacing the Na+ with H+. Then, the acidolysis-treated samples were extracted with toluene/methanol mixture (31/69 wt%) for 2 d [34]. After vacuum-dried completely, the above samples were conducted FTIR test and resultant spectra are shown in Figure 3c. The schematics of the acidolysis of poly-NaMAA at EPDM/PP interface is also given in Figure 3d. As expected, new absorptions emerging at 3415 and 1721 cm-1 were attributed to the hydroxy group [35,36] and the carboxyl group [37] respectively, which strongly confirmed the existence of grafted methacrylic acid. In addition, the intensity of the peak at 1721 cm-1 of DV-5NaMAA was higher than that of DV-2.5NaMAA, which demonstrated that the degree of compatibilization could be improved at higher NaMAA load. The successful in-situ compatibilization between EPDM and PP can be further evidenced from the shifting of their tan δ peaks determined by DMA. The storage modulus (E’) vs temperature curves are provided in Figure S1 (Supporting Information). As shown in Figure 3e, two distinct tan δ peaks at around -39 °C and 15 °C are associated with the glass transition for EPDM phase and PP phase [38], respectively. It is reported that metallic salt of unsaturated carboxylate is an effective crosslinking coagent which can significantly increase the crosslink density of rubbers [39]. This made the tan δ peak of EPDM phase shift from -40.7°C of the simply blend to -38.9°C of the DV-5NaMAA [40]. On the other hand, in-situ compatibilization involving poly-NaMAA, EPDM and PP macromolecular radicals hindered the ordered arrangement of PP chains (this will discuss later) and resulted in more amorphous area at EPDM/PP interface [41]. This made the tan δ peak of PP phase shift from 16.4 °C of the simply blend toward 13.8°C of the DV-5NaMAA. Compared with the simple blend and DV-0NaMAA, the two tan δ peaks of DV-2.5NaMAA and DV-5NaMAA closing up to each other represented that the EPDM/PP interfacial compatibilization was improved by NaMAA, which is a critical step to develop the SM effect for DV-EPDM/PP blends.

3.2 Morphology study of the DV-blends

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Figure 4 SEM images of the cryo-fracture surface of DV-blends: (a) and (b) DV-0NaMAA; (c) and (d) DV-2.5NaMAA; (e) and (f) DV-5NaMAA As shown in Figure 4a and 4b, some pulling-out of EPDM particles were observed in the DV-0NaMAA. The sharp boundary of EPDM particles suggested that there was improving potential of the interfacial adhesion [42] between EPDM particles and PP matrix. As expected, NaMAA significantly improved their compatibility as shown for the DV-2.5NaMAA (Figure 4c, 4d) and DV-5NaMAA (Figure 4e, 4f). In this case, the rubber particles were not pulled-out on the cryogenically fractured surface and some irregular coarse regions representing the rubber phase could be distinguished from the relative smooth PP phase, as marked by blue dashed line in Figure 4d and 4f. The inset in Figure 4f clearly shows that the rubber phases were well embedded in the PP phase, leaving a blurry boundary on the cryogenically fractured surface. This strongly suggested that the interfacial adhesion between EPDM particles and PP matrix was promoted remarkably by the in-situ compatibilization of NaMAA.

Figure 5 SEM images of the DV-blends etched by hot xylene for 5min: (a), (b) and (c) DV-2.5NaMAA; (d), (e) and (f) DV-5NaMAA

8

Figure 6 TEM images of the DV-blends: (a) DV-0NaMAA; (b)DV-2.5NaMAA and (c) DV-5NaMAA

Figure 7 SEM images of DV-5NaMAA fractured without complete cryogenically frozen To observe the morphology of crosslinked EPDM particles in the DV-blends with NaMAA, the cryo-fracture surfaces of samples were etched by hot xylene to remove the free PP. It was found that the morphology of crosslinked EPDM particles in the DV-2.5NaMAA (Figure 5a, 5b and 5c) was similar to that in the DV-5NaMAA (Figure 5d, 5e and 5f). The crosslinked EPDM particles possessed a regular spherical shape with size of about 2~3 μm. However, from the magnified images of Figure 5c and 5e, it is clearly seen that the EPDM particles possessed a coarse surface with mass concavo-convex folds, which is quite different from the relative smooth surface of the EPDM particles in the DV-0NaMAA (Figure S2, Supporting Information). Such a coarse surface might originate from the complex reactions between PP, poly-NaMAA and EPDM on the surface on EPDM particles [40]. In addition, the coarse surface of the EPDM particles is of great benefit to increase their contact area with PP phase, improving the interfacial adhesion and efficiency of stress delivery. The results of TEM were in great agreement with above SEM characterization. As shown in Figure 6, the EPDM particles have a clear outline in the DV-0NaMAA (Figure 6a), while the acuity of EPDM/PP phase interface in the DV-2.5NaMAA (Figure 6b) and DV-5NaMAA (Figure 6c) is significantly reduced. The blurry interphase observed in TEM images firmly suggested that the EPDM/PP interface was successfully in-situ compatibilized by NaMAA, and a large number of nanoparticles emerging in Figure 6b and 6c were associated with the Poly-NaMAA [43]. Figure 7 shows the morphology of a crack which was found on a brittle fractured DV-5NaMAA without complete cryogenically frozen. The highly elongated rubber-like substances were associated with the stretched EPDM phase combined with partial PP component. No signs of debonding were found on the root segment of the highly elongated substances, showing excellent interfacial adhesion of the EPDM/PP interface at deformation state. It can be imaged that the extensive plastic deformation of the surround PP phase involving heterogeneous stress field forced the EPDM particles to deform via the powerful EPDM/PP interface, which generated essential driving force for SM behavior. Unfortunately, as 9

shown in Figure 7, the PP continuous phase was broken that could not maintain its integrity during deformation, eliminating the potential SM effects of the test sample.

3.3 Crystallization behavior of PP phase

Figure 8 (a) heating DSC curves; (b) cooling DSC curves; (c) crystallinity and (d) XRD curves of DV-blends According to the structural designing of our strategy to realize the SM effect of DV-blends, the continuous crystalline PP phase plays an important role in fixing the deformed EPDM particles to stabilize the temporary shape. Therefore, the crystallization behavior of DV-blends was investigated by DSC. The heating and cooling DSC curves are shown in Figure 8a and 8b, respectively. Incorporation of NaMAA decreased the melt temperature (Tm) but increased the onset crystallization temperature (To) and crystallization temperature (Tc) of DV-blends. Compared with the DV-0NaMAA, the DV-5NaMAA showed that its Tm decreased from 166.2 °C to 162.6 °C, while the To and Tc increased from 119.3 and 109.1°C to 122.2 and 113.0°C, respectively. This can be explained by the two conflict effects of the in-situ compatibilization of NaMAA on the crystallization of PP phase [44]: poly-NaMAA and the compatibilized EPDM/PP interface provided more platforms for the heterogeneous nucleation of PP, while the strengthened EPDM/PP interactions also restricted and hindered the crystal growth of PP. This turned out a reduction of crystallinity (Xc) from 45.3% of the neat PP to 38.1% of the DV-5NaMAA (Figure 8c). However, these changes in crystallization behavior of PP phase are beneficial to the SM effect that the rapid crystallization of PP phase at higher To and Tc would promote the shape fixing and the reduced Xc could improve the shape recovery of the blends. The crystal structures of PP phase were also investigated by XRD. As shown in Figure 8d, the typical α-crystal diffraction peaks at 2θ values of approximately 14.2°, 16.9°, 18.5°, 21.4° and 21.9° corresponded to the (110), (040), (130), (111), and (131) planes of PP phase [45], respectively. Compared with the neat PP, the intensities of above crystal planes in DV-blends showed irregular variations, which confirmed that the crystal growth of PP phase was interfered by the strengthened phase interactions. To address 10

this, the evolution of crystal growths of neat PP, DV-0NaMAA and DV-5NaMAA were investigated by POM, as shown in Figure 9a, 9b and 9c, respectively. It is clearly seen that the spherulites of PP in DV-blends were squeezed during crystal growth, which generated defects in the crystal structure and reduced the spherulite size. Compared with the DV-0NaMAA, the DV-5NaMAA showed a faster crystal growth which facilitated it to fix the temporary shape quickly during the SM programming. All these changes in crystal structures of PP phase agreed well with the DSC results. The final crystal morphology of neat PP, DV-0NaMAA, the DV-5NaMAA are provide in Figure S3, S4 and S5 (Supporting Information), respectively.

Figure 9 POM images of the crystal growths of (a) neat PP, (b) DV-0NaMAA and (c) DV-5NaMAA

3.4 Shape memory behaviors of the DV-blends

Figure 10 Snapshots of the shape recovery sequence of samples spiral-shaped at 165°C: (a) simple blend; (b) DV-0NaMAA; (c) DV-2.5NaMAA and (d) DV-5NaMAA Snapshots of the shape recovery sequence of samples spiral-shaped at 165°C are presented in Figure 10 showing a preliminary SM effect. Doubtlessly, the simple blend exhibited the worst recovery. DCP-induced dynamic vulcanization possibly improved the interfacial interaction between EPDM and PP [46], which remarkably improved the shape recovery of DV-0NaMAA. As expected, NaMAA compatibilized EPDM/PP interface further promoted the shape recovery of DV-2.5NaMAA and DV-5NaMAA. However, all of the spiral-shaped samples were unable to recover fully due to this improper SM programming. Obviously, besides the essential strong 11

interfacial adhesion between PP continuous phase and dispersed EPDM particles, the programming design, particularly the choice of programming temperature, determines the final SM effect of the DV-blends to a great degree. The powerful EPDM/PP interface works well in transferring the applied force from PP phase to EPDM particles and fixing the elongated EPDM particles in the temporary shape. That is to say, the PP continuous phase must not be destroyed and has to keep its integrity during the whole shaping process to fulfill the impeccable stress transfer. If the programming temperature is too low, for example when the blend is stretched rapidly at room temperature, the adjustment of PP chains cannot catch up with the shape deformation due to “frozen effect” of the perfect PP crystals [47], then the formation of cracks in PP continuous phase (even broken) makes it lost sufficient power to against the resilience of deformed EPDM particles, deteriorating and even losing the potential SM effect consequently. If the programming temperature is too high, for example above the Tm of PP phase, melting of crystal also makes PP phase lost enough powers to deform the EPDM particles effectively, reducing the SM effect. Therefore, the selection of programming temperature must be careful in SM programming for the DV-blends. According to the DSC melting curve (Figure 8a), we chose 100 to 170 °C as the temperature range for shape programming. The specimens were stretched to 100% strain at various temperatures and immediately immersed into water at room temperature to fix the temporary shape. The Rf was determined after 24h and then the elongated specimens were placed in an oven at 165 °C to trigger the shape recovery. The calculated Rf and Rr at various programming temperatures are shown in Figure 11a and 11b. At higher programming temperature, the activated PP chains could be rearranged quickly to adapt the stretching, which avoided the rupture of PP continuous phase. The intact PP continuous phase with sufficient strength held the elongated EPDM particles to fix the temporary shape, which turned out a better Rf for the DV-blends. As seen, when the programming temperature was above 130 °C, the Rf of DV-blends with NaMAA was higher than 95% (Figure 11a). However, when the programming temperature exceeded 160 °C, as mentioned before, the melting of partial crystal reduced the strength of PP phase, which could not effectively drive the deformation of EPDM particles. Therefore, the low deformation degree of EPDM particles provided inferior driving force for the shape recovery, resulting in an unfavorable Rr of 65% at 170 °C. Obviously, the DV-blends shaped at a suitable programming temperature is critical to the SM behavior. It was found that the DV-blends shaped at 130~150 °C possessed better Rr (above 93%) as marked by the gray zone in Figure 11b. Considering the facility of SM programming, 130°C might be an optimal programming temperature for this DV-EPDM/PP/NaMAA system: DV-2.5NaMAA of 95% SF and 95% Rr, DV-5NaMAA of 96% Rf and 98% Rr. Furthermore, it is evident that from Figure 11a and 11b, the NaMAA in-situ compatibilized EPDM/PP interface did endow DV-2.5NaMAA and DV-5NaMAA with better SM effect than DV-0NaMAA.

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Figure 11 (a) Shape fixity ratio and (b) shape recovery ratio of DV-blends shaped at various temperatures; (c) shape recovery rate of DV-blends at various triggering temperatures; (d) stress-strain curves The shape recovery rate of DV-blend was also evaluated by recording the angle changes of a U-shaped sample at various triggering temperature [48], as shown in Figure 11c. The rectangular sample (50mm×5mm×2mm) with a “U” temporary shape was placed at various temperatures. The time required for the angle changed from 0 to 160° was recorded to evaluate the shape recovery rate. As shown in Figure 11c, incorporation of NaMAA promoted the shape recovery rate of DV-blends. For example, when triggering at 165 °C, the required time of the DV-0NaMAA was 116 s, while that of the DV-2.5NaMAA and the DV-5NaMAA were decreased to 104 s and 98 s, respectively. Besides the effective stress-transfer of improved EPDM/PP interface, the reinforced EPDM particles by NaMAA [49] with stronger resilience contributed to the shape recovery. Figure 11d shows the stress-strain curves of the DV-blends. The improved tensile behavior of DV-blends with NaMAA earned superior driving force for shape recovery as expected. Based on the above analysis for SM programming, we proposed a suitable SM programming for this DV-EPDM/PP/NaMAA system to achieve an excellent SM effect as shown in Figure 12. After proper programming, DV-2.5NaMAA and DV-5NaMAA exhibited an impressive Rr of 98% and 99%, respectively. The shape memory behaviors of DV-2.5NaMAA and the DV-5NaMAA were also carried out by TA DMA Q800, and the results are shown in Figure S6 and S7 (Supporting Information), respectively. It is clearly seen that both the Rf and the Rr are above 99% and 92%, respectively, for the two samples.

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Figure 12 The excellent SM behaviors of DV-blends after programming: shaped at 130°C and recuperated at 165 °C.

3.5 Discussion of the shape-memory mechanism

Figure 13 TEM images of elongated DV-2.5NaMAA and the DV-5NaMAA

Figure 14 SEM images of elongated DV-5NaMAA: (a) etched by hot xylene for 30s; (b), (c) and (d) etched by hot xylene for 120s Figure 13a and 13b show the TEM images for the intermediate section of elongated DV-2.5NaMAA and DV-5NaMAA, respectively. As seen, the PP continuous phase could hold the elongated EPDM particles tightly, which resulted in a perfect temporary shape. Figure 14a shows a SEM image caught from the DV-5NaMAA at elongated temporary shape that was slightly etched by hot xylene. Since the etch time was very short, majority of PP was remained on the observation surface. However, the PP continuous phase was destroyed, which could not fix the highly elongated EPDM particles. As a result, they retracted back to the spherical shape. The observed silk-like substances under stretching state between “large particles” were associated with the complex grafts involved with EPDM, PP and poly-NaMAA. After the residual PP was further 14

removed, more spherical EPDM particles were exposed as shown in Figure 14b. Nevertheless, elongated EPDM particles still kept their elongated state in deeper layers. More details of the retracted EPDM particles bonded with stretched rubbery fibers can be found in higher magnification images in Figure 14c and 14d. Therefore, during the temporary shaping, the orientation of PP molecules forced the elongation and orientation of EPDM particles along the stretching direction through the NaMAA compatibilized EPDM/PP interface. Then, the PP continuous phase fixed the elongated EPDM particles trough the compatibilized EPDM/PP interface in the temporary shape. When the strength of PP continuous phase was sharply reduced at triggering temperature, the resilience of elongated EPDM particles pulled back the PP phase via the compatibilized EPDM/PP interface again to perform the shape recovery.

4. Conclusion Dynamically vulcanized EPDM/PP blends were in-situ compatibilized by NaMAA. The strengthened phase interface, involving the complex EPDM-NaMAA-PP based graft products, was critical to the stress-transfer between EPDM particles and PP continuous phase. The in-situ compatibilization of NaMAA resulted in a coarse surface of the EPDM particles, which was of great benefit to improve the interfacial adhesion and efficiency of stress delivery. The incorporation of NaMAA reduced the whole crystallinity yet promoted the crystal growth of the PP phase, which facilitated the PP phase to fix the temporary shape quickly during the SM programming. As for the SM mechanism of this DV-EPDM/PP/NaMAA system, the crystalline PP continuous phase deformed EPDM particles during shape fixing and fixed the EPDM particles at their elongated state in the temporary shape. When the strength of PP continuous phase was sharply reduced at triggering temperature, the resilience of elongated EPDM particles pulled back the PP phase via the compatibilized EPDM/PP interface again to fulfill the shape recovery. After a proper SM programming, e.g. shaping at 130 °C and triggering at 165 °C, the Rf and Rr of DV-5NaMAA achieved 96% and 98%, respectively. ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (Grant No. 21664003) and Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology Foundation (2018Z006). References [1] Hu JL, Zhu Y, Huang HH, Lu J. Recent advances in shape-memory polymers: structure, mechanism, functionality, modeling and applications. Prog Polym Sci 2012; 37: 1720-1763. [2] Xie T. Recent advances in polymer shape memory. Polymer 2011; 52: 4985-5000. [3] Nie JD, Mou WJ, Ding JP, Chen YK. Bio-based epoxidized natural rubber/chitin nanocrystals composites: Self-healing and enhanced mechanical properties. Composites Part B 2019; 172: 152-160. [4] Hassanzadeh-Aghdam MK, Ansari R. Thermal conductivity of shape memory polymer nanocomposites containing carbon nanotubes: A micromechanical approach. Composites Part B 2019; 162: 167-177. [5] Wu WC, Xu CH, Zheng ZJ, Lin BF, Fu LH. Strengthened, Recyclable Shape Memory Rubber Films with Rigid Filler Nano-Capillary Network. J Mater Chem A 2019; 7: 6901-6910. [6] Du HY, Liu LW, Zhang FH, Leng JS, Liu YJ. Triple-shape memory effect in a 15

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Highlights • • • • •

In-situ compatibilization of NaMAA for EPDM/PP is first reported Shape memory effect was realized by the powerful EPDM/PP interface PP continuous phase holds the highly elongated EPDM particles Elongated EPDM particles restore elastic resilience as SR driving force SF and SR achieved 96% and 98%, respectively.

Conflict of Interest

We state that: The article is original. The article has been written by the stated authors (Chuanhui Xu, Rui Cui, Yukun Chen, Jianping Ding) who are all aware of its content and approve its submission. The article has not been published previously. No conflict of interest exists. If accepted, the article will not be published elsewhere in the same form, in any language, without the written consent of the publisher.