ZDMA Shape-Memory

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Materials and Interfaces

Design of high strength XSBR/Fe3O4/ZDMA shape memory composite with dual responses Cong Liu, Jiarong Huang, Daosheng Yuan, and Yukun Chen Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 03 Oct 2018 Downloaded from http://pubs.acs.org on October 3, 2018

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Industrial & Engineering Chemistry Research

Design of high strength XSBR/Fe3O4/ZDMA shape memory composite with dual responses Cong Liu, Jiarong Huang, Daosheng Yuan*, Yukun Chen* Lab of Advanced Elastomer, South China University of Technology, Guangzhou 510640, China Corresponding Author: Yukun Chen ([email protected]); Daosheng Yuan ([email protected])

ABSTRACT: In this paper, dual responses shape memory polymers based on carboxylic styrene butadiene rubber (XSBR)/ferriferrous oxide (Fe3O4)/zinc methacrylate (ZDMA) were designed. Fe3O4 was incorporated to endow XSBR with magnetic property and improve its glass transition temperature (Tg). ZDMA was employed as in situ reinforcer and compatibilizer for Fe3O4 and XSBR, which further improved Tg of the SMPs by introducing ionic cross-linking points. With incorporation of 10 phr ZDMA, Tg increased to 28.3 oC, the shape fixation ratio (SF) of the SMP was ~100% at room temperature and shape recovery ratio (SR) were ~100% both in thermal field and alternating magnetic field, and the tensile strength reached to 30.26 MPa. The chemical and physical properties of the SMPs were characterized by curing tests, cross-linking density, scanning electron microscope (SEM), differential scanning calorimetric (DSC) and tensile tests, etc. Such dual-responses SMPs exhibit potential applications in intelligent biomedical devices and wide temperature range. Keywords: shape memory polymers, dual responses, carboxylic styrene butadiene rubber, ferriferrous oxide, in situ polymerization. 1. INTRODUCTION SMPs are a class of stimuli-responsive materials, which can switch freely between temporary and original shapes when receiving various external stimuli (heat, light, electricity, magnetism, and pH, etc).1-5 SMPs have been widely used in electronic devices, self-assembling structure, biomedical devices and other high-tech areas, etc.6-8

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As the most widely used SMP, the heat-triggered SMPs are most commonly triggered by ambient heat source directly.9-11 However, in some cases, such as medical instruments, traditional direct temperature heating procedure is seriously limited. Therefore, SMPs with different indirect heating ways have received wide attention in recent years,12 especially the magnetically-sensitive SMPs have shown great potential to be applied in intelligent biomedical field. As for magnetic SMPs, it is often prepared by adding a mass of magnetic fillers (such as ferrite particles,13,14 rubidium iron boron particle,15 etc) into matrices. Among these magnetic particles, Fe3O4 is found to be the most convenient and widely used dues to its high initial permeability, relatively large saturation magnetization, as well as low connectivity.16 But large amounts of Fe3O4 always show inferior dispersion in matrices, which will lead to deterioration of mechanical properties.17,18 Fortunately, an impressive method named “in situ polymerization” shows potential in solving this problem, especially the in situ polymerization of zinc dimethacrylate (ZDMA). Xu et al.19,20 systematically studied the enhancement of natural rubber (NR) incorporated with ZDMA and magnesium dimethacrylate (MDMA). During the in situ polymerization of ZDMA, the polymerized ZDMA (PZDMA) could be grafted with the unsaturated matrices and formed a large number of Zn2+ ion pairs. The strong affinity between Zn2+ ion pairs and Fe3O4 nanoparticles could greatly improve the compatibility between matrices and Fe3O4.21 Meanwhile, this approach could introduce reversible ion cross-linking bonds, which is beneficial for the preparation of functional materials, such as self-healing materials22 and shape memory materials23, etc. XSBR, as an elastomer, shows great potential to be used as matrix for SMPs owing to its ideal glass transition temperature (Tg)24,25 and excellent resilience. As a shape memory polymer, the trigger mechanism of XSBR is actually determined by its glass transition process. Its chains could be frozen to a temporary shape and released to original shape below and above Tg, respectively. Furthermore, as a kind of rubber, the Tg


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of XSBR can be reflected by the mobility of rubber chains, which indicates that the trigger temperature of XSBR can be tuned by its cross-link density. Recently, we have reported a reprocessable XSBR composites, in which the ion cross-linking bonds of Zn2+ salt-bondings were introduced in this system and the cross-link density of XSBR could be manipulated by the amount of Zn2+ salt-bondings.21 Therefore, in this paper, in order to design a high strength SMP with dual respones based on XSBR. We added nanoscale magnetic packing of Fe3O4 to promote its Tg and endow the composite with magnetic properties. The introduction of ZDMA was employed as in situ reinforcer and compatibilizer for Fe3O4 and XSBR, and to further control the Tg of the SMPs. The chemical and physical properties of the composite were characterized by curing test, cross-linking density, scanning electron microscope (SEM), differential scanning calorimetric (DSC) and tensile tests, etc. This kind of SMP we prepared in this paper could maintain its impressive properties in a wide range of temperatures which exhibits great potential in biomedical applications.

2. EXPERIMENTAL SECTION 2.1. Materials In this study, XSBR was used with a solid content of 48% and a random copolymer consisting of 43 wt% butadiene, 57 wt% styrene, and 3 wt% of carboxylate functional monomer, which was obtained from Fujian (China). ZDMA was purchased from Qiangda New Materials Technology Company Guangdong (China). Fe3O4 (average particle size was 250 nm, its density was 5.7 g/cm3 and specific surface was 13 m2/g), it was purchased from Shanghai ST-NANO Science Technology Company. The curing agent of dicumyl peroxide (DCP) was purchased from Sinopharm Chemical Reagent. 2.2. Samples preparation Prior to blending, Fe3O4 was dried for at least 5 h at 60 oC in a vacuum oven, and XSBR was masticated for about 5 min in a two-roller mill at room temperature.



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In our experiments, XSBR/Fe3O4/ZDMA compounds consist of 100 weight parts of XSBR, 2 phr (parts per 100 parts of rubber) DCP, 70 phr Fe3O4 and 0, 5, 10, 15 phr ZDMA, respectively. At first, XSBR and Fe3O4 was mixed homogeneously. Then, different dosages of ZDMA were added. DCP was filled in at last. In the end, we got the composites of XSBR/Fe3O4/ZDMA. They were cured at 170 oC for their optimum cure time (Tc90) which was determined by UR-2010 Rotorless Rheometer produced by U-CAN Dynatex. The obtained vulcanized samples which were named by XSBR, XF70, XF70Z5, XF70Z10, XF70Z15, respectively. 2.3. Characterization The cross-linking density of the rubber is an important factor to evaluate the molecular structure of the vulcanized rubber. The curing characteristics include the minimum torque (ML), maximum torque (MH), and vulcanization time (Tc90). In our work, the curing process of the composite was tested by using a UR-2010 Rheometer (U-CAN Dynates Inc. China) under the temperature of 170 oC. Equilibrium swelling experiments were executed to measure the apparent cross-link density. To calculate the cross-link density of the composites, samples with different weights were soaked in toluene at about 25 oC for a period of 120 h in a shading environment keeping the swollen sample from light. Then, the samples were wiped off the surface by filter paper to remove the excess solvent and immediately weighed accurately. At last, the samples were dried in a vacuum oven at 80 oC for 72 h until a constant weight. The volume fraction of rubber swollen in the gel (Vr) was used to represent the cross-link density of the composites, which could be was calculated by the following equation,

Vr 

mo (1   )  r 1 1 mo (1   )  r  (m1  m2 )  s

where (1)

m0 is the sample mass before swollen, m1 and m2 are the swollen sample masses before


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and after dried, respectively, φ is the mass fraction of XSBR rubber in the samples, α is the mass loss of the vulcanization after swollen, and the rubber density is ρr (ρr=0.95 g/cm3), and toluene density (ρS=0.865 g/cm3). To distinguish ionic cross links from covalent cross-links, the above samples were swollen in a mixture of toluene and chloroacetic acid once again for 120 h to destroy ionic cross links, followed by swelling in toluene for 72 h and weighed, then dried in a vacuum and reweighed. Finally, Vr1 was calculated from the equation, which represents the covalent cross link density. Vr2, which was calculated by subtracting Vr1 from Vr, was used to represent the ionic cross link density. A dumbbell shaped sample of 6 mm width, 75 mm length, and about 2 mm thickness was prepared before. Tensile tests were carried out at room temperature. The mechanical properties of the samples were measured on a UCAN UT-2080 universal tensile testing machine, equipped with a 1000 N load cell. The tests were conducted at ambient conditions using dumbbell samples with a gauge length of 20 mm and strain rate of 500 mm/min. Scanning electron microscopy (SEM) was carried out using ZEISS Merlin field emission microscopy working at 10 kV voltages. The tensile fractured surfaces were coated with a thin layer of gold before observation. Differential scanning calorimetric (DSC) analysis was carried out on a NETZSCH 204 F1 under nitrogen atmosphere in the range from -10 oC to 60 oC, and at a heating rate of 10 oC /min. The shape memory behaviors were studied by a rectangular specimens. At first, we bent the sample into a U-like shape at a temperature beyond Tg. Then, the deformed samples were fixed at ice water and room temperature respectively observing their fixed behavior. Finally, the temperature rose to Tg to induce active shape recovery via directly heating and magnetic stimulation. The shape fixity ratio (SF) and shape-recovery ratio



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(SR) were obtained as flowing equations:26,27 SF 

SR 

180o    100% 180o

 180o

100%

(2)

(3)

where φ is the recovery angle fixed at room temperature, θ is the recovery angle under directly heating and magnetic stimulation.

3. RESULTS AND DISCUSSION 3.1. In situ compatibilization of ZDMA for Fe3O4 and XSBR Because of rubbers’ typical resilience, they could be used as good matrices for SMPs. Nevertheless, they have been rarely reported as SMPs due to their low glass transition temperature (Tg).28 Here, XSBR with high Tg was employed though its Tg was still not yet reach room temperature. In this paper, to further increase the Tg of XSBR, 70 phr Fe3O4 nanoparticles were added. Considering the fact that large number of nano-sized Fe3O4 was easy to agglomerate due to its electrostatic and van der Waals forces,29 in situ polymerization of ZDMA was therefore introduced to increase both compatibility and Tg of XSBR/Fe3O4. The compatibility effect could be intuitively seen in the SEM images.


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Figure 1. SEM images of the cross-section of tensile fractured surfaces (a) XF70, (b) XF70Z5, (c) XF70Z10, (d)XF70Z15.

SEM images of the cross-section of tensile fractured surfaces were shown in Figure 1. It could be clearly seen that there are many gaps between Fe3O4 and XSBR at the interface and an obvious aggregation of Fe3O4 is also observed in Figure 1(a). This directly reflected the poor dispersion of Fe3O4 in XSBR. When ZDMA was added, these defects were gradually disappeared by in situ compatibilization of ZDMA as shown in Figure 1(b) and (c). Interfacial compatibility was greatly improved by the introduction of ZDMA, where the samples exhibited smooth fracture surface and no aggregated particles were observed. In addition, the gap between two phases significantly reduced. There was no doubt that ZDMA achieved improved interfacial compatibilization between Fe3O4 and XSBR as the dosage of ZDMA ranged from 0 phr to 10 phr. Nevertheless, some plastic deformation zones appeared when ZDMA reached to 15 phr in Figure 1(d). It may be due to that the Tg of the composite has exceeded room temperature when 15 phr ZDMA was



used, which could be confirmed by DSC. Therefore, in order to balance a high good compatibility and other properties, we preliminarily selected the dosage of ZDMA as 10 phr in this paper. To understand the mechanism of in situ polymerization of ZDMA in XSBR, especially the in situ compatibilization of ZDMA, the schematics of chemical structures and reactions about the in situ compatibilization of ZDMA for Fe3O4 is illustrated in Figure 2.

Figure 2. Schematic of reaction of XSBR, Fe3O4 and ZDMA.

In our study, ZDMA contained in the system underwent the polymerization initiated by DCP and formed PZDMA which could be grafted with XSBR and brought about ionic cross-linking structure. Thus, there were two different kinds of bonds acting as cross-linking points, covalent cross-linking of XSBR itself and ionic cross-linking structure obtained from the PZDMA Ionic cross-linking structure in this study called Zn2+ salt-bindings, which was mainly formed by the reaction between Zn2+ and −COO- groups.21 During the curing


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process, polymerized ZDMA (PZDMA) and XSBR-graft-PZDMA generated a large number of Zn2+ ion pairs. The strong interaction between Fe3O4 nanoparticles and Zn2+ ionic pairs led to the encapsulation of Fe3O4 nanoparticles by Zn2+ ionic pairs, which limited the reunion of Fe3O4 nanoparticles. Besides, when two PZDMA radicals met or one PZDMA radical abstracted hydrogen from a rubber chain, a free (ungrafted) PZDMA molecule would also appear. Theoretically, ion clusters of Zn2+ ionic pairs grown as ZDMA usage increased. Due to the strong interaction between massive Zn2+ ion pairs and Fe3O4, both graft-PZDMA and Zn2+ ion pairs improved the compatibility between XSBR and Fe3O4 greatly. During the in situ polymerization of ZDMA, Tg of the composites increased as well. 3.2. Preparation of rubber-based SMP with higher Tg

Figure 3. (a) DSC curves of the composites, (b) Illustration for the interfacial interactions of the composites.

As shown in Figure 3(a), when 70 phr Fe3O4 was added alone, the composite of XSBR/Fe3O4 possessed a higher Tg of 20.5 oC than neat XSBR. This result confirmed that the addition of Fe3O4 nanoparticles played a role in the increase of Tg. We speculated that the XSBR chains with carboxyl groups of were attracted to the surface of nano-Fe3O4 and formed coordination bonds,30,31 thus inducing the interactions between Fe3O4 and



XSBR and improving the cross-link network and promoting Tg of the composite materials, which would be also reflected in curing tests. However, the Tg of 20.5 oC was not up to our requirements. Thanks to the in situ polymerization of ZDMA in XSBR/Fe3O4, the Tg of the composite further increased. After different contents (5 phr, 10 phr, 15 phr) of ZDMA were added, the Tg of the composites reached to 24.2 oC, 28.3 oC

and 32.3 oC respectively. The addition of ZDMA has successfully raised the Tg to a

temperature near the room temperature. It is known that Tg is associated with the mobility of rubber chains as exhibited in Figure 3(b). The mobility of rubber chains was hindered by grafted PZDMA and cross-link networks originated from the Zn2+ ion cluster and the covalent bonds, and thereby leading to an increase of Tg. The mobility of rubber chains could be explained by the degree of cross-link networks.32,33,34 So as to monitor the cross-linking network of the composite we depicted its cure curves and cross-link density as shown in Figure 4. It is well accepted that the promotion of torque value represents the formation of cross-linking in rubbers.15 Generally speaking, the higher of maximum torque (MH) is, the higher degree of cross-linking density is.19 This assumption could be supported by cross-link density via the dissolution/swell experiment in toluene as shown in Figure 4(b). The equilibrium swelling tests were therefore in agreement with DSC measurements of the composites discussed earlier where the Tg of composites exhibited an increase with increasing the content of ZDMA.


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Figure 4. (a) Elastic torque of XSBR and XSBR/Fe3O4 with various concentrations of ZDMA at 170℃, (b) Cross-link density of XSBR and XSBR/Fe3O4 with various concentrations of ZDMA

For comparison, 70 phr Fe3O4 was added to neat XSBR. It is interesting to find that there was a slight increase of MH suggesting the some interactions of XSBR/Fe3O4 occurred during the curing process and made a contribution to higher Tg. However, the covalent cross-link density of XSBR/Fe3O4 actually decreased compared with pure XSBR as shown in Figure 4(b). This is because some DCP might be adsorbed by abundance of Fe3O4 fillers.35,36 Therefore, the covalent cross-linking density decreased slightly and the curing time improved correspondingly. With the addition of different dosage of ZDMA, a continuous increase of MH was achieved as shown in Figure 4(a). At the same time, Figure 4(b) showed that with the increase of ZDMA content, the covalent cross-linking density of XSBR/Fe3O4/ZDMA composites changed little. The increase of the total cross-linking density mainly came from the increase of ion cross-linking density. This phenomenon strongly confirmed that in situ polymerization of ZDMA in XSBR during curing process occurred. As the content of ZDMA increased, dual network structure of covalent cross-link and ionic cross-link was formed. Ultimately, due to the improvement of ionic cross linking structure, a continuous increase of Tg was achieved. The dispersion between nano-sized magnetic fillers of Fe3O4 and XSBR was successfully improved, and the Tg of the composite has reached room temperature. Therefore, this kind of composite has been already suitable for the preparation of SMPs with dual responses. Ahead of it, it is necessary to analyze the components of SMPs we obtained. Generally, SMP based on glass transition was mainly controlled by its two-phase structure, where the stationary phase maintained its original shape and the reversible phase controlled the deformation and recovery.37 In our work, the cross-linking points of both covalent cross-link and ionic cross-link bonds could be regarded as fixed phases,



and the rubber chains between cross-linking points were reversible phases. The shape of the reversible rubber chains could be frozen below the Tg, while the polymer network was driven to recover to the initial state by entropic elasticity of XSBR. A higher rubbery modulus produced a higher elasticity stress and hence a faster recovery speed, which has been reported for other SMPs.38,39 3.3. Shape fixing behaviors at room temperature

Figure 5. (a) The samples were fixed at ice bath and room temperature for 0-2 h respectively, (b) The fixed ratio at room temperature for 1 h and 2 h.

During the experiments of shape memory, we firstly fixed the deformed sample in cold water with temperature of ~8 oC for 0-2 h. The samples all showed an excellent fixed ratio (SF) closed to 100%, which could be seen in Figure 5(a). However, to satisfy its practical application, shape fixation behavior was also explored at room temperature of ~27 oC as shown in Figure 5(b). Obviously, pure XSBR and XSBR/Fe3O4 showed a poor shape fixation behavior at room temperature. And in particular, compared with neat XSBR, owing to the driving force of the interaction between Fe3O4 and XSBR, the SF of XSBR/Fe3O4 at room temperature was worse. When ZDMA was added, accounting for the dual cross-link network structures leading to higher Tg, the composite showed a high SF when cross-linking points increased. From Figure 5(b), after ZDMA increased to 10 phr, the SF of XSBR/Fe3O4/ZDMA was ~100%, 88.87% at room temperature fixed for 1 h and 2 h, respectively. It may account for a slow creep recovery happened during this


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time.40 This result provided a precondition for the application of the SMP at room temperature. 3.4. Shape recovery behavior under directly heating and magnetic stimulation

Figure 6. (a) Shape memory behavior under directly heating, (b) Shape memory behavior under magnetic stimulation, (a′) Shape recovery ratio of different time under directly heating, (b′) Shape recovery ratio of different time under magnetic stimulation.

Shape recovery behavior was presented in Figure 6. Obviously, according to the shape recovery experiment via direct heating and magnetic stimulation, XSBR based SMPs with dual responses were prepared successfully. As exhibited in Figure 6(a), the shape memory recovery behavior of the composite was studied in hot water with the temperature of 50 oC by direct heating. All samples exhibited a complete shape recovery ratio (SR) of 100% at steady state, but the shape recovery speed was different as shown in Figure 6(a'). Compared with pure XSBR, the shape recovery speed of XSBR/Fe3O4 increased. It might because of the interaction



between Fe3O4 and XSBR, which provided a driving force for shape memory recovery. However, when 10 phr ZDMA was added, shape memory recovery speed further decreased. The reason was that the dissociation and instantaneous reconstruction of partial ionic cross-linking points happened during the deformation process,41 which resulted in a failure to obtain the deformation chains of ionic cross-linking networks. Therefore, the driving force of the shape recovery mainly came from the deformation of covalent cross-linked networks and partial ionic cross-linking points. At the same time, the reformed ion cross-linked bonds might have a limitation on this recovery behavior oppositely. This indicates that the recovery speed could be controlled by the amount of ZDMA. In this study, to characterize the magnetic shape memory behavior of the SMP, shape recovery behavior under the same alternating magnetic field was exhibited in Figure 6(b). Owning to the magnetic particles of Fe3O4, it could be induced heating indirectly under the action of alternating magnetic field, and achieved a shape memory recovery. It is legible that pure XSBR cannot show a magnetic shape recovery, while the behaviors were outstanding by the incorporation of Fe3O4. As illustrated in Figure 6(b′), the SR of all composites stimulated by magnetism were closed to 100%. When 10 phr ZDMA was added, the shape recovery speed dropped significantly. On the one hand, it might result from the dissociation and instantaneous reconstruction of ion cross-linking networks. On the other hand, Zn2+ ionic pairs produced by PZDMA had interactions with Fe3O4, leading to a shielding effect on Fe3O4 nanoparticles in the magnetic field, and resulting in a reduction of heating efficiency. Besides, magnetic shape memory effect was mainly influenced by the stimulation of magnetic field and magnetic particles. When the strength of magnetic field was fixed, the mass fraction of magnetic particles determined the magnetic driving.42 So when the dosage of ZDMA increased from 0 phr to 10 phr, the mass fraction of Fe3O4 decreased from 40.70% to 38.46% oppositely. The relative reduction of Fe3O4 contents might cause a slight decline in the saturation magnetization


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as well.21 By comparing these two types of stimulus, we found that shape recovery speed under magnetic stimulation was slower at the beginning and faster in the end. This could be explained that indirectly heating by alternating magnetic field was a process, which was slow in the early and fast in the late. 3.5. Mechanical Properties

Figure 7. Typical tensile stress-strain curves of XSBR/Fe3O4/ZDMA

To study the reinforcing effect, typical tensile tests were performed as shown in Figure 7. As can be clearly seen, neat XSBR showed a moderate tensile strength of 15.12 MPa and an elongation of break of 83.56%. With the incorporation of 70 phr Fe3O4, the tensile strength and elongation at break increased slightly to 16.91 MPa and 92.32% respectively. When 70 phr Fe3O4 was incorporated, the nanoparticles of Fe3O4 were easy to aggregate and badly dispersed in XSBR, thus causing an inferior reinforcing effect. In addition, because of the interaction between Fe3O4 and XSBR, which also led to a certain reinforcing effect. However, this reinforcing effect could not meet the requirements of SMPs. The addition of ZDMA sufficiently contributed to the mechanical property of the composite. It is obvious that with the addition of ZDMA, the tensile strength of the composite increased greatly. When ZDMA was increased to 10 phr, the tensile strength



increased to 30.26 MPa, which was approximately 1.5 times of XSBR/Fe3O4. The reduced aggregation and uniform dispersion of Fe3O4 improved the mechanical properties of the composite greatly.43,44 In addition, the ion domains formed by ZDMA could also serve as effective reinforcers to enhance the tensile property of XSBR.45 But there were obvious changes in stress-strain profiles with the addition of ZDMA. A great decreased elongation at break occurred when tensile strength increased. In fact, it was a common phenomenon for an increase in tensile strength and a decrease in elongation at break in the ZDMA reinforce rubbers.46 For the composite with 15 phr ZDMA, because the Tg approached and even exceeded room temperature, an obvious plastic deformation appeared.

4. CONCLUSIONS In this paper, we presented a new SMP combined with high strength and dual responses. Fe3O4 was used as magnetic particles and ZDMA was employed as in situ reinforcer and compatibilizer for the composites. Polymerized ZDMA (PZDMA) could be grafted with XSBR (graft-PZDMA) and generated massive Zn2+ ion pairs. Because of the strong interaction of Zn2+ and Fe3O4, both graft-PZDMA and Zn2+ ion pairs greatly improved the compatibility between XSBR and Fe3O4. At the same time, the reduced aggregation and uniform dispersion of Fe3O4 as well as the ion domains formed by PZDMA made a significant reinforcement for XSBR. Moreover, the Tg of the SMP could be regulated from 20.5 oC to 32.3 oC by changing the dosage of ZDMA. Especially, when the content of ZDMA increased to 10 phr, the SMP exhibited a high tensile strength of 30.26 MPa and Tg of 28.3 oC, the SF at room temperature was ~100% at room temperature and SR was 100% driven by both directly heating and magnetic stimulus. The obtained SMP exhibited potential applications in intelligent biomedical devices with wide temperature range.


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ACKNOWLEDGEMENTS This work was supported by the Program of Guangdong Provincial Department of Science and Technology (Grant No. 2016A010103004), the Fundamental Research Funds for the Central Universities and the National Natural Science Foundation of China (Grant No. 21704028), the Natural Science Foundation of Guangxi Province (2015GXNSFBA139237), the Project Sponsored by the Scientific Research Foundation of Guangxi University (Grant No. XTZ140787).

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ZDMA Shape-Memory

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