Role of Dicumyl Peroxide on Toughening PLLA via Dynamic

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Role of Dicumyl Peroxide on Toughening PLLA via dynamic vulcanization Lu Liu, Jiarui Hou, Liping Wang, Jianming Zhang, and Yongxin Duan Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b02122 • Publication Date (Web): 16 Aug 2016 Downloaded from http://pubs.acs.org on August 16, 2016

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Industrial & Engineering Chemistry Research

Role of Dicumyl Peroxide on Toughening PLLA via dynamic vulcanization Lu Liu, Jiarui Hou, Liping Wang, Jianming Zhang and Yongxin Duan*

Key Laboratory of Rubber-Plastics, Ministry of Education/Shandong Provincial Key Laboratory of Rubber-plastics, Qingdao University of Science & Technology, Qingdao City 266042, Shandong, China.

ABSTRACT Our previous work demonstrated that the toughness of PLLA could be improved dramatically by dynamic vulcanizing blends of PLLA/NBR with dicumyl peroxide (DCP) as crosslink agent. Herein, to reveal the toughening mechanism of DCP on the PLLA/NBR thermoplastic vulcanizates (TPVs), the distribution of DCP in PLLA matrix and NBR phase was modulated by changing feeding procedures. The crosslink density of NBR, phase morphology and mechanical properties of PLLA/NBR–TPVs were investigated thoroughly. It was found that impact toughness of the blends is dependent on the distribution of DCP, while the tensile properties almost keep unchanged with alteration of the feeding procedure. PLLA/NBR–TPV prepared by premixing DCP with PLLA and NBR separately, had the highest toughness and comparable tensile properties. This was attributed to synergistic effect between the improved PLLA molecular chain entanglement due to DCP initiated branch reaction of PLLA and improvement of the compatibility between two phases. 1. INTRODUCTION Poly(L-lactic acid)(PLLA) derived from renewable biomass, is considered as one of the most promising materials that can substitute the usual petroleum-based polymers.1-4 However, the inherent brittleness limits its application.5,6 Therefore, various toughening strategies for PLLA including plasticization, copolymerization, 1

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and melt blending with elastomers or nanoparticles have been extensively studied in last decades.7-9 Among these methods, blending with elastomers has been proved as an economic and effective routine to toughen PLLA.10 However, most of the blends with superior toughness are based on the addition of large amount of polymeric elastomers (≥20%) which will generally compromise the biodegradability and compatibility of PLLA, and lead to the sharp decrease of strength and modulus. Therefore, extending the practical application of PLLA relies on how to obtain superior tough PLLA by adding small amount of elastomer so that its high modulus, strength and biodegradability can be retained maximally. And the impact toughness of elastomer toughening PLLA prepared by simple blending is limited due to weak interfacial adhesion between the immiscible components and the low strength of uncrosslinked elastomer. Recently, blending PLLA with small amount of elastomer via dynamic vulcanization to fabricate thermoplastic-rich thermoplastic vulcanizate (TPV) attracted more and more interests.11-13 During dynamic vulcanization process, rubber was crosslinked, broken into small particles and dispersed in the thermoplastic matrix under the high temperature and strong shear field. The interfacial adhesion between plastics matrix and rubber particles also can be enhanced by compatibilizer during dynamic vulcanization. The special microstructure benefits PLLA-rich TPV high toughness, plasticity, and recyclability. For example, Liu et al. fabricated a supertoughed PLLA-TPV with ethylene/n-butyl acrylate/glycidyl methacrylate (EBA-GMA) terpolymer and zinc ionomer of ethylene/methacrylic acid (EMAA-Zn) through simultaneous dynamic vulcanization and interfacial compatibilization. Wang et al prepared supertoughened PLLA blends containing in situ crosslinked polyurethane via dynamic vulcanization. Recently, we found the TPV composed of PLLA and only 10% NBR with DCP as crosslinking agent has highly improved toughness compared to PLLA, meanwhile high modulus and tensile strength of PLLA were inherited.14 DCP has been often used as interface compatibilizer of polymer blends and also employed as crosslinking agent of rubber. For example, Wang et al. found that the 2


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notched impact strength of PLLA/PBS blend significantly increased after the addition of DCP, but the strength and modulus monotonically decreased with increasing DCP content.15 They ascribed the improved toughness of the blend to the better interfacial adhesion between PLLA and PBS, which is improved by DCP’s compatibilization effect. Hillmyer et al. toughened PLLA with polymerized soybean oil using DCP as initiator for the polymerization of soybean oil.16 Semba et al. relieved the brittleness of PLLA by blending PCL with DCP, and found that DCP can crosslink PLLA and PCL, thus improve interfacial adhesion between them, which yields a ductile material with five times better elongation at break than the corresponding blend without peroxide.17 A large amount studies showed the compatibilization effect of DCP. It is found that DCP can generally improve the impact toughness of PLLA but compromise its modulus and tensile strength. However, there is no reports regarding the detailed studies on the compatibilization mechanisms of DCP on PLLA based TPVs. Moreover, the influences of DCP distribution on the blends have not been investigated. At high temperature DCP decomposes into free radicals, which can react with PLLA and NBR. The distribution of DCP in PLLA and NBR should have great influence on its respectively reaction with different polymer components and subsequently influence the microstructure and mechanical properties of the blends. To understand the effect of DCP on the microstructure, crystallization of PLLA matrix and mechanical properties of the PLLA/NBR-TPV, the interaction of DCP with PLLA and NBR was investigated thoroughly by changing feeding procedures of DCP and modulating its distribution in PLLA and NBR phase. 2. EXPERIMENTAL SECTION 2.1 Materials and Sample preparation The Poly(L-lactic acid) (2003D) was purchased from Nature Works LLC, USA, with melt flow index (210 °C,2.16 kg) of 6g/10min and specific gravity of 1.24 g/cm3. NBR(4450F) is supplied by Lanxess, the initial Mooney viscosity(4±1) is 50±5, and the acrylonitrile content is 43.5±1.5%. DCP was purchased from Sinopharm Chemical Reagent limited corporation, China. 3



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In this work, the weight contents of NBR and DCP were kept at 10 wt% and 0.045 wt% respectively. PLLA was dried in drum wind dryer at 80 °C for 8 h before use. NBR bulks were simply crushed into sheets at the room temperature by using an open mill, then the NBR sheets were cut into long strips followed by putting into a mono-screw extruder at 80 °C and granulated. The granules of PLLA and NBR were put into a container and mixed manually, then were put into a melt mixer (HAAKE Polylaos, Bruker) at a rotation speed of 60 rpm under a fixed temperature of 160 °C. When the torque was equalized, 0.1g DCP (0.045 wt%) was put into the mixer. The total blending time is 10 min. After the melt mixing process, the bulks of the blending were grounded into thin sheets by an open mill. The composite sheets were hot pressed at 170 °C, 10 MPa for 1 min followed by cold compression at 15 MPa for 3 min. Thus prepared blend was designated as P/N/D in the following. The blend P/N-D was prepared by the following procedure. DCP dissolved in acetone was firstly mixed with the granulated NBR in the container. Then, PLLA was put into the container after the acetone evaporated thoroughly and mixed manually with the NBR granules attaching DCP. For P-D/N, DCP dissolved in acetone was firstly mixed with PLLA. Then, NBR was put into the container after the acetone evaporated thoroughly and mixed manually with the PLLA granules attaching DCP. For P-D/N-D blends, DCP was divided evenly into two containers, dissolved by acetone and mixed with PLLA and NBR respectively. After that, components of two containers were mixed manually. All these samples (P/N-D, P-D/N and P-D/N-D) prepared with different feeding procedure experienced the same melt mixing, grounding and pressing process with the blend of P/N/D. 2.2 Mechanical testing The tensile tests were performed at a cross-head speed of 20 mm/min at room temperature using an universal testing machine (REOLL 2005, ZWICK). The dimension of the samples was 75×4×1 mm3. Notched Izod impact strength was tested using a GT-7045-MDH. The thickness and the width are 4mm and 10 mm respectively, and the notch is 2 mm in depth. To ensure good measurement statistics, 5 specimens were prepared for each sample. 4


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2.3 Scanning Electron Microscopy (SEM) Scanning electron microscope (SEM, 7500F, JEOL) was used to observe the morphology. The tensile samples were fractured along the tensile direction after quenching in the liquid nitrogen for about 10 min. The images of broken plane of the impact specimens were scanned after the impact test. The surfaces were all vacuum-plated with gold-platinum. The dispersed rubber particles diameter and the inter particle distance were analyzed with Smile View software. For each samples, at least 300 particles from five independent micrographs were measured. The number average particle diameter (dn), weight – average particle diameter (dw) and particle diameter distribution parameter (DDP) was calculated from the equation (1), (2) and (3) respectively. The number – average surface-to-surface inter particle distance Ln, which is also referred as the average matrix ligament thickness, was calculated from equation (4). N

dn

∑ = ∑ ∑ = ∑

i =1 N

ni d i

N

dw

(1)

n i =1 i

i =1 N

n 2i d i

(2)

nd i =1 i i

DDP =

dw dn

(3)

N

Ln =

∑i N ∑i

=1

ni Li

L =1 i

(4)

2.4 Differential Scanning Calorimetry (DSC) DSC measurements were performed with a Q20 (TA, America) in nitrogen atmosphere. Specimens about 6 mg were cut from the compression sheets. All specimens tested were first heated to 200 °C at a rate of 10 °C /min and maintained for 5 min, and then cooled down to different temperature with a rate of 50 °C /min. After that, the second heating scans were monitored at a heating rate of 10 °C /min. 2.5 Polarized Optical Microscope (POM) The samples were first heated to 200 °C for 2 min, then cooled down to 120 °C and kept for 4hrs. Then the crystalline morphology was observed by polarized optical 5



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microscopy (POM, BX51, Olympus). 2.6 Measurement of the crosslink density The blends prepared by dynamic vulcanization were enveloped in a PTFE film, and refluxed with boiled chloroform through Soxhlet apparatus until the weight keep unchanged. Then the residue was weighted and the weight is designated as W1. And the residue was immersed in toluene for 72 hrs. The residue was taken out and the solvent on its surface was wiped carefully by filter paper. After that the residue was weighted again and got the weight W2. The crosslink density Ve was calculated according to the following function 5, 6 and 7. δ1,δ2 are solubility parameters of toluene and NBR,V1 is the molar volume of toluene and χ is the Flory/Huggins interaction parameter between toluene and NBR ,η is the density ratio of NBR to toluene.18 （5））

（6））

（7））

3. RESULTS AND DISCUSSION 3.1 Mechanical Properties The stress – strain curves of pure PLLA and PLLA/NBR vulcanizates with DCP are shown in Figure 1, parts a. As could be seen from the stress-strain curves, the pure PLLA exhibits a brittle behavior as revealed by the only 3% elongation at break. While the four kinds of dynamic vulcanizates with DCP prepared by different feeding procedure have much similar stress-stain behavior, all exhibit greatly increased tensile elongation (above 300% strain), the stress-strain curves take on the yielding of PLLA matrix with the appearance of nucleation and propagation of a neck, and an obvious strain hardening stage is observed after the complete propagation of the neck prior to sample break, which is the typical ductile fracture. The thoroughly observation on the 6


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tensile curves found that the change of feeding procedure has no obvious influence on the modulus, tensile strength and elongation. The tensile elongation of all the blends, no matter with or without DCP, was over 300%.

Figure 1. (a) Stress-strain curves and (b) the impact toughness of PLLA, N/P and the NBR/PLLA-TPV. Figure 1, parts b showed the Izod impact strength of PLLA and the blends. The impact strength of pure PLLA is only 4 KJ/m2. All the blends with DCP prepared by dynamic vulcanization process have much higher impact strength compared to pure PLLA and the impact toughness of the dynamic vulcanizates is dependent on the feeding procedure. As shown in Figure 1, parts b, P-D/N-D has the highest impact toughness, then is P/N/D, the third one is P-D/N, while P /N-D has the lowest impact toughness among the blends. Worth noting is that the tensile properties (tensile elongation, tensile strength and modulus) of all the dynamic vulcanizates with DCP are nearly same and have comparable modulus with pure PLLA, while the impact strength and tensile elongation of the vulcanizates, is much higher than that of PLLA. By comparing the tensile properties of the vulcanizates in this work with that of the simple blending PLLA/NBR in our former work, it is found that the all the blends of PLLA/NBR with same content of NBR have almost same tensile properties no matter with or without DCP.14 This suggests that DCP and its appropriate distribution in PLLA and NBR phase can improve the Izod impact strength, without deteriorating the tensile strength and high elongation at break compared with the simple blending PLLA/NBR, and at the same time the P/N-TPVs can keep high modulus of PLLA. This is very interesting 7



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and commendable, since in most of cases the addition of DCP can improve the impact strength but deteriorates modulus and tensile strength. For example, Ma et al. found that the notched impact strength of PLLA/PBS blend significantly increased after the addition of DCP, but the strength and modulus monotonically decreased with increasing DCP content.19 Wang et al. also found that the uncompatibilized PLLA/PBS blend showed much higher elongation than PLLA, but only slightly higher notched Izod impact strength, while addition of 0.1 phr DCP greatly increased the impact strength of the blend, but both strengths and modulus invariably decreased with increasing DCP content.15 The difference may be attributed to the weak strength of PBS compared with NBR. This also indicates that the tensile toughness and impact toughness is determined by different factors. 3.2 Microstructure

Figure 2. SEM micrograph of cryogenic fractured surfaces of the blends: (a) P /N-D (b) P-D/N (c) P /N/D (d) P-D/N-D. The magnification is 10000. The dispersed state, size of elastomer particles and ligament thickness of polymer matrix have direct relationship with mechanical properties of polymer blend materials. The microstructure of PLLA/NBR-TPVs was investigated by SEM. As showed in Figure 2, regardless of the feeding procedure the average rubber particles size was under 1 µm, though the dn of P/N-D was 0.62 µm, which was bigger than that of other blends. The other three vulcanizates have much close dn, which is around 0.4 µm. Closely inspection on the SEM micrograph of the cryo-fractured surface of 8


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different blends found more difference. For vulcanizate of P/N-D, the interface between rubber and PLLA matrix is very sharp, while for the other three vulcanizates, the interface between two phases is blur and it’s difficult to distinguish the NBR rubber particles from PLLA matrix, which may indicate the difference in the compatibility between rubber and matrix caused by the different feeding procedure of DCP. The particle diameter distribution parameters of different samples (as shown in table 2) have no much difference and fall in a narrow range from 1.12 to 1.30, which means that all the samples have finely dispersed NBR particles. The average PLLA matrix ligament thickness of all the blends was under 0.4 µm. As several reports found that, for toughening PLLA, the critical value of the PLLA matrix ligament thickness is around 1 µm, below this value, the toughness of the blends increases.19,20 So, all the blends of NBR/PLLA studied in this work should have high toughness. And this was verified by the mechanical properties showed in Figure 1. All of the blends have almost same ductile stress – strain behavior and improved impact toughness and tensile elongation compared to pure PLLA, though the impact strength is dependent on the feeding procedure of components. In addition, as showed in Figure 2, the cross-linked NBR phase distributed much denser in P/N/D, P-D/N-D than in P/N-D, and interconnections between cross-linked NBR phases also seemed increased, which means that NBR phases could occupy more volumes by reducing particle size after change the distribution of DCP. As reported, the phase morphology and dispersion particle size of plastics/rubber blends are dependent on the difference in viscosity of two components and the crosslink density of the rubber phase.21 Generally, the viscosity of rubber phase is much higher than that of plastics due to the higher molecular weight of rubber, so increasing the viscosity of PLLA can decrease the viscosity difference between PLLA and NBR, which makes the two phases blended more homogeneously. And during melt blending process, the breakup of the rubber is dependent on the extensibility of the rubber network and its strength.22 When the crosslink density of NBR is increased, the break strength and modulus of NBR increase too, at same time the finite extensibility of network decrease as the molecular weight between crosslink points decreases. The stress and strain exerted on 9



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the rubber particles must match the break strength and the finite extensibility of the rubber particles to significantly deform and breakup the rubber particles. In a word, improvement of PLLA viscosity and moderate crosslink of rubber phase favor the fine dispersion of NBR particles and decrease the NBR particle size. And crosslinking of rubber phase can prevent small rubber particles formed in the strong shear field to coalescence and forms bigger size NBR phase after the melt blending. Table 1. The average rubber particle size and size distribution in different blends Samples P/N-D P-D/N N/P/D P-D/N-D

dn/µm 0.62 0.44 0.34 0.36

dw/µm 0.75 0.49 0.45 0.42

MWD=dw/dn 1.21 1.12 1.30 1.16

Ln/µm 0.36 0.24 0.32 0.24

For P/N-D, DCP was premixed with NBR and almost all of DCP was enveloped in NBR phase. The crosslink of NBR started when the temperature reached 170 ºC, and the rapid increased crosslink density and strength of NBR make it difficult to be broken into small particles and lead to bigger NBR particles in the dynamic vulcanizates. And this was supported by the fact that the crosslinking density of NBR in P/N-D is 15.0×10-4 mol/cm3 (table 1), which is the highest among all the blends. In the case of P-D/N, DCP was pre-mixed with PLLA, but during melt blending a small amount of DCP will immigrate to the NBR phase due to the preferentially disperse of DCP in NBR phase, since the solubility parameters of PLLA, NBR and DCP are 21.0, 20.8 and 14.7 (J/cm3)1/2, respectively. In this case, the small amount of DCP in NBR phase crosslinked NBR and prevented the broken NBR particles to coalescence into big NBR particles. In addition, DCP can branch PLLA and decrease the viscosity difference between PLLA and NBR phase, which make two components blended more homogenous. And in the melt blending process, active free radical of PLLA and NBR coexist, two kinds of radicals can couple terminate and form copolymer, which located in the interface region and improve the compatibility of PLLA and NBR. This explanation is supported by the SEM micrograph shown in Figure 2, parts b, the interface is quite blur in P-D/N blend. But the amount DCP immigrated into NBR is quite small, so the crosslink density of NBR is low (8.0×10-4 mol/cm3) compared to 10


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that in P/N-D, as shown in table 2. For P/N/D, PLLA and NBR were melt blended at first and when the torque equilibrated, DCP was introduced. In this case, most of DCP dispersed in PLLA matrix and the interface region, because the weight percent of NBR is only 10%. In the dynamic vulcanization process, the broken of rubber particles occurs in the early stage,23 and the rubber particles formed in the early stage do not further break up into smaller particles. Most of DCP contributes to branch PLLA and improve the compatibility of two phases, the SEM micrograph shown in Figure 2, parts d also verified the strong adhesion between PLLA and NBR. In addition, for the P/N/D, the DCP was introduced much later than other cases, so the relative short reaction time may lead to the low crosslink density of NBR. But the crosslink of NBR prevent the small NBR particles to coalescence into big aggregates. For P-D/N-D, DCP was divided evenly into two parts and premixed with PLLA and NBR respectively. Branching of PLLA and crosslink of NBR started at early stage of the melt blending process, active free radical of PLLA and NBR had more time and opportunities to couple terminate and form copolymer. Thus on the fracture surface of P-D/N-D vulcanizates, few NBR particles can be observed, most of NBR particles were covered by PLLA matrix, which indicated the improved compatibility between two phases. Thus P-D/N-D has relative high crosslinking density and improved interface adhesion. Table 2. The crosslink density of TPVs Samples Crosslink density (×10-4mol/cm3)

P/N-D

P-D/N

N/P/D

N-D/P-D

15.0

8.0

6.6

8.8

3.3 Thermal behavior and Miscibility It is well known that the crystallinity and crystalline morphology of PLLA matrix has great influence on the physical and mechanical properties of the blends. Consequently, it is important to study the influence of DCP on the thermal behavior and crystallization of PLLA. Figure 3 are the first heating DSC curves of the as prepared blends and second heating DSC curves after melting quenched from 200 ℃. The DCS curves of pure PLLA experienced same melt blending and compressing 11



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procedure was also provided. The blends with DCP prepared by dynamic vulcanization have much lower crystallization temperature than PLLA and the crystallization of the vulcanizates span narrower temperature range than that of pure PLLA. This indicated that the crystallization ability and crystallization rate of PLLA was promoted by dynamic vulcanization with NBR and DCP. Table 3 showed the thermal property parameters of the blends and PLLA. Though the initial crystallinity of all the samples are quite low, but that of PLLA is lowest, which also verified that dynamic vulcanization promoted the crystallization of PLLA. Figure 4 showed the Polarized optical micrograph of PLLA and N/P/D. PLLA forms big and few spherulites with obvious Maltes cross, while many small crystal particles impinging on one another are observed in N/P/D. Other vulcanizates with DCP have similar crystalline morphology with N/P/D. In a word, the addition of DCP increased the crystallization rate, improved the crystallinity and decreased the crystal sizes of the PLLA matrix. This may benefit the improvement of the blends toughness, since the crystallized PLLA matrix around the soft rubber particles is easier to trigger shear yielding deformation by NBR particles compared to amorphous ones. As reported, the deformation mechanism of PLLA changed from crazing to shear yielding as crystallization occurs.24 Fu et al. studied the effect of crystalline content on the toughness of PLLA / PCL blend by adding different amount of nucleating agent to modulate the crystallinity of PLLA. They found that the blends with high crystalline content had high toughness and ascribed the higher toughness to the shear yielding of the crystalline region.25

Figure 3. The first (a) and second (b) heating DSC curves of the blends. 12


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Table 3. The thermal properties parameters of the blends and pure PLLA Samples

Initial Xc/%

Total Xc/%

Tg-NBR /℃ ℃

Tg-PLLA /℃ ℃

PLLA

1.3

16.3

_

60.6

P/N-D

4.2

39.2

-11.1

59.3

P-D/N

2.8

40.9

-10.9

59.8

N/P/D

3.4

38.8

-10.9

59.7

N-D/P-D

5.0

39.8

-10.8

59.2

Figure 4. Polarized optical micrograph of (a) PLLA and (b) P/N/D. 3.4 Toughening mechanism and role of DCP For all of the blends studied in this work, necking process was observed during stretching process. Figure 5 is the SEM micrograph of the necking region along the stretching direction，it is very obvious that all the samples showed PLLA ligaments with elongated cavities along stretching direction. Additionally, PLLA matrix shearing and stretching combined with internal NBR voiding make the dispersions and the PLLA matrix difficult to be discriminate, obviously indicating excellent ductility of the blends during stretching. The elongated voids with hollow cylinder-like shapes of micro scale revealed the tensile toughening mechanism of microvoids promoting shear yielding, other than crazing. Void formation could prevent the strain softening and promote the matrix shear yielding by alteration from the triaxial stress state to plane stress state.26 For all the blends with DCP, the thickness of PLLA ligaments and deformed cavities are much small (around 0.7 µm) and their dispersion is very dense. This means that crosslinked NBR particles have high ability to initiate cavities and 13



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make PLLA ligament experience ductile deformation. Fu et al. proposed that toughening of semicrystalline polymers is sufficient if elastomer particles can initiate surrounding matrix yielding and bring average matrix ligament thickness under below the characteristic length scale of matrix heterogeneity.25 In our case, cavities and surrounding matrix yielding can be initiated in all the blends. All the blends, regardless of the feeding procedure, have high ductile stress-strain behavior.

a

b

c

d

Figure 5. SEM micrograph of necking region along the stretching direction of the blends: (a) P/N-D, (b) P-D/N, (c) N/P/D, (d)N-D/P-D .The magnification is 6000. Though all the blends have ductile stress-strain behavior, their impact toughness is dependent on the distribution of DCP and the feeding procedure. Figure 6 showed the SEM micrograph of the impact fracture surface of pure PLLA and the blends. The impact fracture surface of pure PLLA is quite smooth indicating the typical brittle fracture. The impact fracture surface SEM micrograph of P/N-D was a little bit rough and there were some NBR dislodged locations, but no obvious plastic matrix deformation was observed, which indicated poor compatibility between PLLA and crosslinked NBR particles. The P/N-D has lowest impact toughness among all the vulcanizates. In this case, most of DCP contributed to crosslink NBR, but the 14


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compatibility between two phases is still poor. For N/P-D, N-D/P-D and N/P/D, which have superior impact toughness, the PLLA matrix have obvious plastic deformation, and highly stretched PLLA fibrous ribbons surrounding some holes, characteristic of tough failure, were observed. The holes should be caused by the dislocation of NBR particles, which absorbed much impact energy and induced the high extent matrix plastic deformation thanks to the strong adhesion of two phases. The high extent matrix plastic deformation may be related to the branch of PLLA initiated by DCP in the dynamic vulcanization process. On improving the ductility of PLLA matrix, branch of PLLA has similar effect with increasing entanglement of PLLA molecular chains. As reported, the toughness of PLLA increases with increasing its chain entanglement and some researchers have developed PLLA with hyper-branched structures and high molecular chain entanglement density in order to improve the toughness of PLLA.27,28 Among the blends, P-D/N-D has the highest impact toughness, the distribution of dislocation holes is most densely and plastic deformation of PLLA matrix was most popular, indicating improved impact toughness through absorbing more impacting energy by the formation of such rougher morphologies. And this may caused by the modest crosslink density of NBR, branch of PLLA and improvement of the compatibility between two phases. In another word, the improved impact toughness was attributed to synergistic effect among the improved PLLA molecular chain entanglement due to the branch reaction of PLLA initiated by DCP, the improvement of the compatibility between two phases and the moderate crosslink density of NBR particles.

b

a

1µm

15



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d

c

e

Figure 6. SEM micrograph of the impact fracture surface of different systems (a) PLLA, (b) P/N-D, (c) P-D/N, (d) N/P/D (e) N-D/P-D. The magnification is 10000.

Figure 7. The second heating DSC curves of the blends after reflux with chloroform and pure PLLA, NBR. To investigate the effect of DCP, blends prepared by different feeding procedures were refluxed with boiled chloroform until the weight kept unchanged. Free PLLA molecular chains were removed by chloroform, while crosslinked NBR and PLLA chains attached tightly to NBR were kept in the residue. Then the residue materials experienced DSC scans. The materials were first heating to 200 °C at a heating rate of 10 °C/min, kept for 5 min, then quickly quenched to -30 °C and the

16


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second heating process was performed. Figure 7 showed the second heating DSC curves of the blends with DCP and the pure polymers, it is found that the thermal behavior of residue after chloroform reflux is dependent on the feeding procedures of the components. Both of P-D/N and P-D/N-D have three glass transitions in their corresponding DSC curves. The lowest one for P-D/N and P-D/N-D at around -6.2 and -8.7 °C, respectively, can be attributed the crosslinked NBR in the vulcanizates, which is higher than that of pure NBR (-11.8 °C as shown in table 4). The highest Tg transition of both blends is at same temperature 62 °C belong to the glass transition of PLLA. The intermediate one at 16.1 °C for both of P-D/N and P-D/N-D should be attributed to the interphase, maybe the copolymer of PLLA and NBR formed in the dynamic vulcanization process. The increase of the NBR Tg and appearance of the intermediate Tg transition give strong evidence that the two phases have highly improved compatibility and some complex substance or copolymer was produced during the dynamic vulcanization process. There are two glass transitions in the DSC curve of P/N/D. The lower one at -2.8 °C is much higher than that of pure NBR and that in P-D/N and P-D/N-D. Generally, the Tg of polymer is proportional to its crosslink density, higher crosslink density and higher Tg. But as shown in table 3, the crosslink density of NBR in P/N/D is lowest among all the blends. One possibility for relative high Tg of NBR in P/N/D is that the slightly crosslinked NBR formed copolymer with PLLA. The chain mobility of NBR was much depressed by the rigid PLLA chains bonded with NBR and due to most of NBR molecules formed copolymer with PLLA, the Tg of NBR can not be discriminated from that of copolymer formed during dynamic vulcanization. Another glass transition of P/N/D is located at around 62.4 °C corresponding to the Tg of PLLA phase. The glass transition of NBR phase in P/N-D is at -8.8 °C. Though the NBR phase in P/N-D has the highest crosslink density among the dynamic vulcanizate blends, its Tg is lowest. This maybe caused by the lock of strong interaction between NBR and PLLA phase. This also supports the speculation that most of DCP is enveloped in NBR phase and initiated the crosslink of NBR particles, but its compatibilization is marginal. The surrounding PLLA matrix has no obvious influence on the glass transition of NBR phase, which 17



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indicates weak interaction between PLLA and NBR. And this is the stem for the lowest impact toughness of P/N-D among the vulcaniztes. Table 4. The thermal properties parameters of second heating DSC curves of the blends after reflux with chloroform and pure PLLA, NBR Samples NBR

P/N-D

P-D/N N/P/D

N-D/P-D

PLLA

Tg1(°C)

-11.8

-8.8

-6.7

-2.8

-8.7

_

Tg2(°C)

_

62.4

62.4

62.5

62.3

60.9

Tg3(°C)

_

_

16.1

_

16.1

_

4. CONCLUSIONS The P/N –TPVs prepared by dynamic vulcanization can reach higher impact toughness, while keep high tensile strength, modulus and strain at break. The phase morphology, interface adhesion, tensile properties and impact toughness of P/N–TPVs are dependent on the feeding procedures of the components and distribution of DCP in PLLA matrix and NBR phase. For all the blends studied in this work, when the size of NBR particles and PLLA ligament thickness are under critical value, high ductile property can be obtained. The branch of PLLA, crosslink density of NBR particles and the compatibility didn’t show obvious influence on the tensile properties of the blends. But for impact toughness, besides the small size of NBR particles and thin PLLA ligament thickness, the compatibility of two phases, the crosslink degree of NBR, branch of PLLA and crystallization of PLLA play important role. Branch of PLLA and strong interphase adhesion benefit from the compatibilization of DCP can improve the impact toughness of the blends more effectively than crosslink of NBR. AUTHOR INFORMATION Corresponding Author *Y. X. Duan. E-mail: [email protected]. Tel.: +86 0532-84022604. Notes The authors declare no competing financial interest.

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For Table of Contents Only

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Role of Dicumyl Peroxide on Toughening PLLA via Dynamic

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