Fully Biobased Shape Memory Material Based on Novel

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Research Article pubs.acs.org/journal/ascecg

Fully Biobased Shape Memory Material Based on Novel Cocontinuous Structure in Poly(Lactic Acid)/Natural Rubber TPVs Fabricated via Peroxide-Induced Dynamic Vulcanization and in Situ Interfacial Compatibilization Daosheng Yuan,† Zhonghua Chen,† Chuanhui Xu,*,†,‡ Kunling Chen,† and Yukun Chen*,† †

The Key Laboratory of Polymer Processing Engineering, Ministry of Education, South China University of Technology, Guangzhou, 510640, China ‡ School of Chemistry and Chemical Engineering, Guangxi University, Nanning, 530004, China S Supporting Information *

ABSTRACT: Shape memory polymers (SMPs) based on fully biobased poly(lactide) (PLA)/natural rubber (NR) thermoplastic vulcanizates (TPVs) were fabricated via peroxide-induced dynamic vulcanization. Simultaneously, in situ reactive compatibilization was achieved by PLA molecule grafting onto NR chains. Differing from the general concept of spherical rubber particles being formed after dynamic vulcanization, the cross-linked NR was found to be a “netlike” continuous phase in the PLA matrix. This novel structure explained the surprising shape memory property of PLA/NR TPVs well (shape fixities ∼ 100%, shape recoveries > 95%, and fast recovery speed < 15 s at the switching temperature, ∼60 °C): the cross-linked NR continuous phase offers strong resilience and the PLA phase serves as the heat-control switch. We envision that the “green” raw materials and excellent shape memory properties of the dynamically vulcanized PLA/NR SMPs will open up a wide range of potential applications in intelligent medical devices. KEYWORDS: Dynamic vulcanization, Cocontinuous phase, Cross-link, Shape memory

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INTRODUCTION Shape memory polymers (SMPs) are a type of intelligent materials which can be programmed to be fixed at a temporary shape and then recover to the original shape by receiving a heat stimulus.1−6 A heat-triggered shape memory polymers (HSMP) usually consists of a fixing phase and a reversible phase.7,8 The temporary shape of the HSMP is typically performed at a temperature above the transition temperature (Ttrans), namely the glass transition temperature (Tg)9−11 or melting temperature (Tm)12 of the fixing phase. Shape fixation is then achieved by cooling the deformed SMPs below Ttrans, holding the deformed reversible phase. The shape memory performance is activated by reheating the SMPs back to Ttrans, the deformed reversible phase with stored elastic force provides the driving force for recovery. Shape memory performance is not an intrinsic property of SMPs, which can be designed and tailored by changing structure, morphology, shape memory programming, and processing conditions.13,14 Cross-linked networks and thermoplastic vulcanizates (TPVs) are the two well-known categories in HSMPs according to the nature of the heat-reversible transition.15 Although the cross-linked networks offer remarkable shape memory recovery performance, the expensive synthesis of complex cross-linked © XXXX American Chemical Society

networks impose major challenges to their broad utilization. Polymer blending is a more accessible method to achieve required shape memory functions,16 since it is easy to design new HSMPs by changing the components to tune the relationship between structure and properties. There are many reports about the shape memory effect of polymer TPVs such as PVDF/PMMA,17 PLLA/PVAc,12 HDPE/PCL,18 and so on. In these TPVs the domains of crystalline phases act as physical cross-links and the domains of amorphous phase serve as switching segments. However, the inherent low shape recovery driving force and recovery speed seriously limits their further developments and applications. To increase the shape recovery force, introducing a crosslinked rubber phase to form a dual-domain system is a necessary choice. It is easy to understand that a stretched crosslinked rubber phase certainly provides strong resilience to highlight the shape memory performance of TPVs. However, there are fewer successful SMPs based on plastic/rubber TPVs19 than the plastic/plastic TPVs since the majority of Received: July 31, 2015 Revised: September 12, 2015

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DOI: 10.1021/acssuschemeng.5b00788 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. (a) Torque changes during dynamic vulcanization. (b) SEM images of toluene-etch cryogenically fractured surface of B65/35. SEM images of dichloromethane-etch cryogenically fractured surfaces of (c) B65/35, (d) D65/35, (e) dichloromethane D40/60 (×5000) and (f) D40/60 (×10 000). TEM images of D40/60 (g) ×5000 and (h) ×10 000. (China). Dicumyl peroxide (DCP) was purchased from Sinopharm Chemical Reagent Co. Ltd. (China). Irganox 1010, antioxidant, of industrial grade, was obtained in the open market. Preparation of Dynamically Vulcanized PLA/NR SMPs. The dynamically vulcanized PLA/NR SMPs were prepared by reactive blending in a Haake Rheocord 90 at 150 °C with a rotor speed of 60 rpm. PLA with quantitative Irganox 1010 (0.2% weight of (PLA + NR)) was first shear-melted and then the masticated NR was added. After the mixing torque was stable, DCP (1.5% weight of NR component) was added and the mixing was continued to achieve the final stable torque. Subsequently, the prepared SMPs were cooled down to the room temperature and were chopped into granules. The specimens for shape memory testing were prepared via injection molding machine (TTI-160F, Welltec Machinery & Equipment Co. Ltd., China). We defined the sample codes according to the PLA/NR ratio. Fox example, the dynamically vulcanized PLA/NR (60/40) SMP was defined as D60/40, the physical blending of PLA/NR (60/40) SMP was defined as B60/40. Characterization. The morphology of dichloromethane (DCM)etch cross-section was observed using a Nova NanoSEM 430 (FEI Company, USA). The TEM observations were carried out on a JEM100CX II transmission electron microscope (JEOL, Japan) with an accelerating voltage of 100 kV. The samples were ultramicrotomed into thin sections of about 100 nm in thickness with Leica EMUC6 under liquid nitrogen atmosphere. The differential scanning calorimetry (DSC) experiments were carried out by using a NETZSCH DSC 204 F1 (German) from 20 to 180 °C. The FTIR test was done by using a Tensor 27 Spectrometer (Bruker, Germany) with a resolution of 4 cm−1 and 32 scans. The FTIR spectrum of blend films was directly measured using the attenuated total reflectance (ATR) model. To get the FTIR spectrum of the cross-linked NR phase, the blend sample was first extracted with DCM at ambient temperature for 72 h to selectively remove the free PLA completely, then the residual was compressed into disks for the FTIR test. All FTIR samples were oven-dried under a vacuum to eliminate the effects of residual solvent and moisture before test. Room temperature tensile behaviors were conducted on a Shimadzu AG-1 (Japan) at a crosshead speed of 50 mm/min. High temperature tensile behaviors were carried out using an Instron testing machine Mod 5500R fitted with a heating oven of accuracy to ±1 °C at a crosshead speed of 50 mm/min. The test temperature was selected at 25, 50, 60, and 70 °C. The test samples were placed in the oven at the

plastic/rubber TPVs are immiscible systems and it is hard to achieve a 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. In addition, in most cases, the cross-linking of the rubber phase during melt mixing with a plastic, namely dynamic vulcanization, inevitably results in a two-phase blend material in which the cross-linked rubber is in particulate forms dispersed in the plastic matrix.20 The resilience of dispersed rubber particles is obviously inferior to that of a continuous rubber phase, which also limited the developments of SMPs based on plastic/rubber TPVs. In recent years, biodegradable SMPs are of increasing interest for applications in biomedical areas such as sutures and bandage-fixing materials.21,22 Among numerous biodegradable polymers, poly(lactide) (PLA) and its copolymers have been reported to exhibit mild thermoresponsive shape memory effect.23,24 We recently developed a super toughened biobased PLA/natural rubber (NR) TPV, which was prepared via a dynamic vulcanizing technique. It was found that the crosslinked NR phase had a continuous network-like morphology structure in PLA phase.25 This novel structure was reported for the first time in the field of dynamic vulcanization, which broke the traditional concept of sea−island morphology. According to this novel structure, the dynamically vulcanized PLA/NR TPVs own a surprising shape memory property, since the cross-linked NR continuous phase provides a strong resilience and the PLA phase can serve as the heat-control switch. In this paper, we report the significant shape memory performance of this particular PLA/NR TPV and hope it pushes development in HSMPs based on plastic/rubber TPVs as well as opens up specific applications in intelligent medical devices.

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EXPERIMENTAL METHODS

Raw Materials. PLA, REVODE101 grade, MI (190 °C, 2.16 kg) = 5−8 g/10 min, average molecular weight (Mw) ≈ 150 000 g/mol, ρ = 1.25 g/cm3, was provided by Zhejiang Hisun Biomaterials Co., Ltd. NR, SMR CV60, Mooney viscosity (ML(1 + 4)100 °C) = 60 ± 5, was kindly supplied by Guangzhou rubber industry research institute B

DOI: 10.1021/acssuschemeng.5b00788 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 2. Digital photographs of swelling experiments for a PLA, PP/EPDM(60/40), NR, and D40/60 (PLA/NR = 40/60). target temperature for 15 min to achieve thermal equilibrate before test. Thermomechanical cycle experiments was similar to the documents,5,16 which were carried out using a TA DMA Q800 instrument to evaluate the shape memory performance of the dynamically vulcanized PLA/NR SMPs. Rectangular samples (8.0 mm × 6.0 mm × 1.0 mm) were first heated to 60 °C and kept isothermal for 10 min to eliminate the stress history. Then the stress-controlled cyclic thermal mechanical testing was launched. The cycle contained four stages: (1) the sample was stretched from its original shape to the target shape. The applied force was increased from a preloaded value of 0.005 N to the designated value achieving 100% strain at a rate of 0.5 N/min. (2) The samples were then cooled to 20 °C with the stress kept constant, and the sample length was recorded as εF. (3) the applied force was unloaded to the preloaded value of 0.005 N at a rate of 0.5 N/min and the sample was held at 20 °C for 10 min to ensure the full shape fixing. Then the sample strain was recorded as εU. (4) In the final step, samples were reheated at a rate of 3 °C/min to 60 °C and held there for 10 min to recover any possible residual strain. The final permanent strain was recorded as εP. The above four stages cycle was repeated three times for each sample. The shape fixity (SF) and shape recovery (SR) in a given cycle N were determined by using the following formulas: SF =

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εU × 100%, εF

SR =

εF − εP(N ) εF − εP(N − 1)

as shown in the followed SEM images, without further tearing to be small particles. It is well-known that the uncross-linked NR can be dissolved in toluene, thus we used toluene to etch the cryo-fracture surface of B65/35 for 24 h, and the result of SEM image is shown Figure 1b. The deep and dark holes or cracks represented the NR phase, which suggest a cocontinuous morphology phase in the B65/35. For the clearer SEM image in large magnification, please see Supporting Information Figure S1. Then, we used DCM to etch the PLA layer on the surfaces of the B65/35, D65/35, and D40/60 carefully at room temperature and took the SEM photographs for the etched surface of the samples. The results in Figures 1c−f clearly showed that the remaining NR phase on the surface layer had a continuous morphological structure,25 not the particles as reported in other polymer system such as PP/EPDM.27 After removing the rigid PLA phase, the shrinkage of NR was not avoided due to its soft nature. This resulted in a high dense NR phase (D40/60) in a viewing window in Figure 1e. The TEM images of D40/60 are also provided in Figures 1g and h, showing a cocontinuous structure in which the darker and lighter parts represented the NR and PLA phases, respectively. Swelling Experiments to Support the Cross-Linked NR Continuous Phase. Although we have reported the detail structure of cross-linked NR continuous phase in this system in our previous work,25 we employed a similar swelling experiment to support the NR continuous structure in PLA. This seemingly repeat work is a necessary guarantee to discuss the shape memory mechanism of the dynamically vulcanized PLA/ NR TPVs in this paper. As shown in Figure 2, the immersed part of PLA was dissolved completely in DCM within 10 min at ambient temperature. If the NR phase was dispersed particles, the DCM-etched residue should be dispersed in DCM or precipitated at the bottom of the test tube, like the situation of the PP/EPDM (60/40) sample shown in Figure 2. As expected, without the support of PLA phase, the immersed part of the D40/60 (PLA/NR = 40/60) crimped within 10 min rather than disappeared. Furthermore, the solution of D40/60 still kept clear and transparent. All these experimental results strongly suggested that the NR phase indeed had a continuous morphological structure in PLA. In order to further prove that the continuous NR phase was cross-linked, the DCM-etched residue (D40/60 etched by DCM for 15 min) was transferred into toluene for 2 days. The result showed that the NR was not dissolved but further swollen, which strongly indicated that the NR was cross-linked.28,29 Shape Memory Performance of Dynamically Vulcanized PLA/NR TPVs. We conducted stress-controlled cyclic

× 100%

RESULTS AND DISCUSSION Preparation and Cocontinuous Structure in PLA/NR TPVs. We first show the typical torque curves of B65/35 and D65/35 during dynamic vulcanization. As shown in Figure 1a, the first and second peaks in each torque curve corresponded to the melting of PLA (containing Irganox 1010) and NR, respectively. After about 6 min, both the torque of B65/35 and D65/35 exhibited a retarded decrease, which showed that the blend melt was approaching to a homogeneous distribution. After that, adding DCP resulted in an abrupt increase in the torque of the D65/35, suggesting the successful cross-linking of NR phase.26,27 Then, the torque of D65/35 was almost unchanged until the end of mixing. It was worth noting that the torque value before adding DCP was similar to that before adding NR (∼50 N m), this suggested that the viscosity of the blend melt was mainly dominated by the PLA phase. In our previous report,25 we have proved that the PLA had a much lower melt viscosity than the NR at 150 °C. The PLA matrix with quite low viscosity was beneficial to the rotor slipping smoothly between the NR zonal phases which had much high melt strength. The role of the PLA melt at this time served as a lubricant which made the rotors slipped easily without any hindrance of NR phase, resulting in a continuous rubber phase C

DOI: 10.1021/acssuschemeng.5b00788 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 3. Stress-controlled cyclic thermal mechanical testing for (a) PLA, (b) B65/35, (c) D65/35, and (d) D40/60.

Figure 4. SF and SR values of PLA, B65/35, D65/35 and D40/60.

thermal mechanical testing at 60 °C (∼Tg of PLA30) to characterize the shape memory performance of the B65/35, D65/35, and D40/60, using neat PLA as a control sample. Two continuous cycles were performed to examine repeatability.5 The results are shown in Figures 3a−d. The calculated shape fixity (SF) and shape recovery (SR) values for the first and second cycles are shown in Figure 4. Remarkably, the NR phase had little effect on the SF. All the samples exhibited a high SF nearly 100%. Generally, in a traditional HSMP based on crosslinked network, a certain fraction of the stretched chains or chain segments (usually the soft segment) still preserved mobility in the cooling stage, hence generating an instantaneous retract without the tensile loading.19 However, in the dynamically vulcanized PLA/NR TPVs, the continuous PLA

phase was an absolute rigid plastic at room temperature, absolutely restricting all the possible instantaneous retractions caused by elongated NR phase. As shown in Figure 4, the dynamically vulcanized PLA/NR TPVs also showed excellent SR: first-SR and second-SR of B65/35 are ∼88% and ∼86%, respectively; first-SR and secondSR of D65/35 are high up to ∼96% and ∼95%, respectively; first-SR and second-SR of D40/60 are ∼98% and ∼97%, respectively. The high SR might be attributed to at least two aspects: (1) SRNR, the strong resilience of the elongated NR phase. The cross-linked NR phase with continuous phase morphology served as a framework structure throughout the PLA phase, which was the main contributor for the D

DOI: 10.1021/acssuschemeng.5b00788 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

Figure 5. Digital photographs of the shape recovery sequence (spiral and V shapes) of neat PLA, D65/35, and D40/60 at 60 °C.

Figure 6. (a) Storage modulus (E′) and loss modulus (E″) and (b) loss factor (Tan δ) of neat PLA, B65/35, D65/35, and D40/60.

mechanism of the dynamically vulcanized PLA/NR TPVs. The E′, E″, and Tan δ as a function of temperature are shown in Figure 6a and b. Two transitions corresponded to the glass transition of NR (∼−60 °C)34 and PLA (∼60 °C).30 Note that the NR phase was consecutive in the PLA. Therefore, it served as a “steel reinforced concrete” structure when the NR molecular was frozen at glass state (900%) at 60 °C, which was not broken even reaching the maximum elongation limit of the test instrument. The digital pictures of the samples being stretched to the maximum elongation limit are shown in the inset of Figure 9c. When the test temperature was heat to70 °C (Figure 9d) which was above the Tg of PLA, the tensile property of the PLA/NR TPVs was mainly contributed by the NR phase, since

the PLA phase almost served no tensile stress. Because of the cross-linked NR phase, D65/35 showed a higher tensile stress than the B65/35, which again implied a faster SR speed in D65/35 than B65/35. The remarkable continuous increase in the tensile stress and the super elongation without break for the D65/35 implied an unbroken continuous NR phase inside the PLA.40,41 This shows that such HSMPs may have potential in application which demands super high deformation. Effect of the Cross-link Density of NR Phase on the Shape Memory Performance. To further investigate the effect of the cross-link density in NR phase on the shape memory performance, we prepared a series of dynamically vulcanized PLA/NR (65/35) TPVs with different DCP dosage. The snapshots of the “V” shape recovery sequence of the TPVs are presented in Figure 10a. By comparing the recovery state of V samples at recorded time, two pieces of information can be summarized as follows: (1) The cross-link density increased with increasing DCP dosage, which offered stronger elastic resilience for the recovery driving force. This resulted in an enhanced recovery speed of the TPVs (see the cross-link density shown in Figure 10b). When the DCP dosage exceeded 1.5% weight of NR component, the cross-link density showed a limited slight increase at 2.0% weight of NR component. However, the increase in recovery speed was not apparent, which might be due to that the continuous networklike morphological structure of NR phase owned inherent good elasticity. (2) The residual deformation was reduced with increasing cross-link density of the NR phase. These results were in consistence with the former discussion that the NR phase offered the main recovery force. Further reinforcing the NR phase by adding nanoparticles42,43 may offer routes to the design of higher performance PLA/NR TPVs.

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CONCLUSION We have presented fully biobased PLA/NR TPVs with excellent shape memory performance. The cross-linked NR phase is a continuous morphology structure in PLA continuous phase, which is quite different from the dispersed rubber particles morphology in conventional dynamically vulcanized TPVs. This novel structure of cross-linked NR phase owns stronger resilience and generates higher SR driving force. The H

DOI: 10.1021/acssuschemeng.5b00788 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering continuous PLA phase acted as a rigid “container” to restrict the deformed rubber phase below its Tg. At the same time, the improved interface originated from in situ reactive compatibilization during DCP-induced dynamic vulcanization keeps the deformed NR phase in the temporary shape firmly, storing the sufficient elastic resilience. When the temperature was reheated to the Tg of PLA phase, the PLA frees the restricted NR phases and resulted in a perfect shape memory performance. The dynamically vulcanized PLA/NR TPVs possesses excellent shape fixities (∼100%), shape recoveries (>95%), and fast recovery speed (