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OBC blend
Polymer 55 (2014) 6409e6417
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Simultaneous the thermodynamics favorable compatibility and morphology to achieve excellent comprehensive mechanics in PLA/ OBC blend Meng Wu, Zhiqiang Wu, Ke Wang*, Qin Zhang, Qiang Fu* College of Polymer Science and Engineering, Sichuan University, State Key Laboratory of Polymer Materials Engineering, Chengdu 610065, People's Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 July 2014 Received in revised form 3 September 2014 Accepted 3 October 2014 Available online 12 October 2014
For the toughening of thermoplastics by using elastomeric components, the relatively high contents of elastomeric phase are commonly demanded to trigger the brittle-to-ductile transition. However, an obvious drawback of remarkably decreased strength and rigidity may arise after incorporation of large amount of elastomeric species. The main thinking in our present work is to achieve a good toughnessstrength balance in an elastomer-toughened thermoplastic system with less amount of elastomeric phase, i.e., the blend of poly(lactic acid) (PLA)/olefin block copolymer (OBC) 90/10 w/w. When both of the thermodynamics favorable compatibility and the thermodynamically stable morphology were realized, the impact toughness of PLA/OBC 90/10 blend was 25 multiples for that of pure PLA, while the tensile strength could preserve as a level of 87% (based on the value of pure PLA). The interfacial compatibilization between PLA and OBC achieved through adding EMA-GMA as a compatibilizer. On the other hand, a quiescent annealing process at 90 C resulted in a more thermodynamically stable morphology, which was characterized as high crystallinity, large size of elastomer-phase droplet, and thickening interfacial layer. Our study offers new insight into the optimization of properties of multicomponent blend that except for realizing the thermodynamics favorable compatibility, the transition from kinetics-dominated morphology to thermodynamically stable one also plays a crucial role. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Olefin block copolymer Toughness-strength balance Thermodynamically stable morphology
1. Introduction The increasing concern on environmental pollution caused by plastic wastes and the diminishing of oil resources have accelerated the boom of biodegradable materials derived from renewable resources [1e4]. Poly(lactic acid) (PLA) outstands of these green materials owning to its high strength and modulus, excellent biocompatibility, competitive cost, etc [5]. Unfortunately, one of its major drawbacks, the inherent brittleness, shown by low impact strength (IS) and poor tensile toughness, greatly confines its wide applications [5,6]. Over the past several years, extensive efforts have been made to surmount the brittleness of PLA. The most commonly used strategy was blending with flexible polymers. Various flexible polymers ranged from commercially available ones like linear low-density polyethylene (LLDPE) [7], thermoplastic polyolefin [8], hydrogenated styrene-butadiene-styrene block
* Corresponding authors. Tel.: þ86 28 85461795. E-mail addresses: [email protected] (K. Wang), [email protected] (Q. Fu). http://dx.doi.org/10.1016/j.polymer.2014.10.004 0032-3861/© 2014 Elsevier Ltd. All rights reserved.
copolymer (SEBS) [9], polyamide elastomer [10], polyurethane [11,12], acrylonitrile-butadiene-styrene (ABS) [13], ethylene-nbutylacrylate-glycidyl methacrylate (EBA-GMA) [14,15], poly(butylenessuccinate-co-adipate) [16,17], poly(b-hydroxybutyrate-co-b-hydroxyvalerat) [18], to those experimentally synthesized species such as expoxidized natural rubber [19], glycidyl methacrylate grafted poly(ethylene octane) [20] and polymerized soybean oil [21,22] have been employed as tougheners for PLA. However, when focused on the impact resistance, very high contents of flexible modifiers have been frequently used to trigger the brittle-toughness transition and obtain satisfactory level of IS. This inevitably resulted in the tremendous decrease of tensile strength and modulus. Study on the PLA toughening system with low content of flexible polymers and excellent toughness-strength balance is considerably lacking at present. Good interfacial adhesion is needed to achieve high impact strength, because poor interface would easily result in premature interfacial failure and hence rapid and catastrophic crack propagation through the whole material [23,24]. However, most polymer blends are thermodynamically immiscible due to the low entropy
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of mixing [25], and a sharp interface between the two phases results. Therefore, copolymers premade or formed in-situ has been widely employed to compatibilize two immiscible polymers. Ideally, the copolymer resides at the interface, resulting in decreased interfacial tension, smaller dispersed droplets, greater interfacial adhesion and increased energy transfer efficiency [26]. In addition, morphology factors like the crystallization of matrix polymer and the size and shape of dispersed phase also play crucial roles in effective toughening. Numerous experimental phenomena have suggested that toughening is easier to achieve in crystalline polymers than in amorphous ones [27e29]. What's more, a linear relationship between PLA matrix crystallinity and the notched impact strength has been reported in PLA/poly(ε-caprolactone) (PCL) blends [27]. The authors found the impact fracture process was dominated by crazing in amorphous matrix and shear yielding in the case of crystalline matrix, while the latter has been proved to be a more effective energy dissipation mechanism than crazing [30e32]. It is well accepted that moderate filler particle size is a requirement for effective toughening. Too small rubber particles are inefficient for cavitation and may delay the shear yielding [33e35], otherwise too large particles which resulted in too large interparticle distance at the same rubber content influence the efficiency for local yielding process to propagate and pervade over the entire deformation zone [36]. In principle, the morphology of polymer blends should be determined by thermodynamic issue, i.e., towards a lowest-energy equilibrium state. However, thermodynamic equilibrium is commonly difficult to be achieved due to high viscosities of polymers and disturbance of shearing fields, i.e., the kinetics effects. To manipulate the morphology of polymer blends, the method of quiescent thermal annealing was often used. The aim of thermal annealing is to drive the kinetics-dominated morphology to the thermodynamics equilibrium one. The structure/morphology changes during thermal annealing include secondary crystallization, lamellar perfection and thickening, rearrangement of molecular chain in constrained amorphous phase [37e39], coalescence of dispersed particles [40,41], etc. Since the toughened PLA blends are of promising application prospect, the commercialized cost-effective constitutes are highly desirable. In recent years, a novel thermoplastic polyolefin elastomer of alternative hard and soft polyethylene blocks, olefin block copolymer (OBC), has attracted wide attentions [42e45]. It is fabricated involving two catalysts and a “chain shuttling agent” (CSA) that switches the growing chain from one catalyst to another at random intervals [46]. The hard polyethylene blocks with very low level of a-olefin comonomer crystallized and dispersed in the amorphous soft blocks matrix which has relatively high a-olefin comonomer content. OBC keeps the good performances of polyethylene like flexibility, friendly processing conditions and excellent abrasion resistance and aging resistance, and is with the improved heat resistance, compared to the last generation of polyolefin elastomer (POE) [47]. Our attention in this study is focused on a blending system of PLA basal resin containing low content (10%) of OBC elastomer. For such blending system, its strength and modulus are comparable to the properties of pure PLA; whereas the increment in toughness is very limited. So we attempt to develop an effective strategy for dramatically improving the toughness of PLA/OBC 90/10 w/w blend. Moreover, the roles of thermodynamics and kinetics influences on the realizing of excellent toughness-strength balance will be demonstrated and discussed with taking into accounts of interfacial compatibility and multiphase morphology feature. Since PLA and OBC are thermodynamically immiscible, a commercially available random terpolymer, ethylene-(methyl acrylate)-(glycidyl methacrylate) (EMA-GMA), was introduced as the compatibilizer, in which -GMA has been widely reported as a reactive chain block
with PLA [9,14,15,20,29] (reaction between the epoxide groups of -GMA and the terminal carboxyl and hydroxyl groups of PLA) and ethylene segment (-E) was expected to entangle with the ethylene blocks in OBC. 2. Experimental section 2.1. Materials and sample preparation A poly(L-lactic acid) resin (4032D, Nature Works LLC) with 1.2e1.6 % D-isomer lactide and density of 1.25 g/cm3 was used as the matrix polymer. The weight-average molecular weight (Mw) and molecular weight distribution is 207 kDa and 1.74, respectively. Olefin block copolymer (INFUSE 9507) containing 14.6 mol% of octane comonomer and 12 wt% of hard blocks was supplied by Dow Chemical Company (Mw ¼ 90.8 kg/mol, r ¼ 0.867 g/cm3, melt index of 5 g/10 min). Random terpolymer of ethylene, methyl acrylate and glycidyl methacrylate used as reactive precursor for the compatibilization of PLA and OBC is LOTADER® AX8900 from Arkema (France), having 24 wt% methyl acrylate, 8 wt% glycidyl methacrylate, a density of 0.94 g/cm3and melt index of 6 g/10 min (190 C, 2.16 kg). Melt blending of the materials was carried out in a 69 ml mixing chamber of the Haake Rheometer (HAAKE Polylab OS, USA) with a rotation speed of 60 rpm at 190 C for 5 min. Prior to mixing, the three components, OBC, PLA and EMA-GMA, were vacuum dried at 60 C for 12 h. A series of binary and ternary blends were prepared as the following composition: PLA/OBC 90/10 (w/w), 70/30 (w/w); PLA/OBC/EMA-GMA 90/(10 x)/x (w/w) (x ¼ 1, 2, 3, 5, where x represents the weight content of EMA-GMA). The obtained samples were injection-molded into specimens using a HAAKE MiniJet (Thermo Scientific, USA), at a barrel temperature of 190 C and a mold temperature of 50 C. Some of the blends (PLA/OBC 90/10, PLA/OBC/EMA-GMA 90/7/3) were further quiescent annealed at 90 C for various time in vacuum oven. 2.2. Mechanical tests Notched izod impact tests were carried out according to ISO180/ 179, using a VJ-40 impact tester at room temperature (23 ± 2 C). Tensile tests were performed using dumbbell-shape samples on a SANS Universal tensile testing machine following ISO 527-2. The measurements were conducted at room temperature (23 ± 2 C) with a crosshead speed of 5 mm/min. At least five specimens were tested for each sample. 2.3. Differential scanning calorimetry (DSC) DSC measurements were performed on a PerkineElmer pyris-1 DSC instrument in nitrogen atmosphere. The instrument was calibrated prior to testing by indium. Samples (about 5 mg) cut from the center of the molded bar were heated from 30 C to 210 C at a speed of 10 C/min to record the melt curves. Crystallinity of PLA (Xc,PLA) was evaluated as following:
XC ¼
DHm DHc DHt 0 wf DHm
(1)
where DHm, DHc, and DHt represent the enthalpies of melting, cold crystallization, crystalline phase transition of PLA, respectively. 0 is the melting enthalpy of 100% crystalline PLA, which was DHm reported to be 93.7 J/g [48]. wf is the weight fraction of the component. The crystallinity of OBC (Xc,OBC) was also calculated using a simplified equation:
M. Wu et al. / Polymer 55 (2014) 6409e6417
XC ¼
DHm 0 wf DHm
(2)
0 (the melting enthalpy of 100% crystalline where the value of DHm polyethylene) is 290 J/g [49]. The isothermal cold crystallization curves of PLA, PLA/OBC 90/ 10, PLA/OBC/EMA-GMA 90/7/3 at 90 C were recorded. Samples was first heated to 200 C at 30 C/min and kept for 5 min to eliminate the thermal history, then cooled to 30 C at 100 C/min and kept for 2 min. Then the samples were heated to 90 C at 100 C/min and kept for various time to record the crystallization curves.
2.4. Phase morphology characterization Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were employed to characterize the phase morphology of the PLA/OBC and PLA/OBC/EMA-GMA blends. Cryofractured surface from the cross section of impact bar was observed using an FEI Inspect F field emission scanning electron microscope (FE-SEM, USA) at an accelerating voltage of 5 kV. The surface was sputter-coated with a gold layer before testing. The obtained digital micrographs were analyzed for the determination of the size of dispersed phase using an Image-Pro Plus software. At least 200 particles from 3 to 4 micrographs were analyzed. The number average diameter (dn), weight-average diameter (dw), particle size distribution parameter (s) were calculated according to the equation below:
P nd dn ¼ P i i ni
(3)
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using a cryo-ultramicrotome (Leica EM FC7, Germany) equipped with a diamond knife. These sections were stained with RuO4 for 10 min. 2.5. Interfacial tensions and spreading coefficient Interfacial tensions between PLA, OBC and the graft copolymer of EMA-GMA and PLA (EGMA-PLA) were calculated by measuring surface energies. The EGMA-PLA was extracted from PLA/EMAGMA blend by 1,4-dioxane which can selectively dissolve PLA. Contact angles of the three materials were measured on KRUSS DSA100 with water and diiodo-methane at 25 C. The surface tension (g), disperse part (gd) and polar part (gp) of the materials were calculated from the measured contact angles according to Wu's method. Then the interfacial tension (gAB) of the polymer pairs can be estimated using the following equation [50,51]:
1=2 1=2 gAB ¼ gA þ gB 2 gdA gdB 2 gpA gpB
(6)
where gA and gB represent the surface energies of the two materials in contact, gA ¼ gdA þ gpA , gB ¼ gdB þ gpB . To correlate the phase morphology and the interfacial tensions, the spreading coefficient, which can be used to predict the tendency for one phase to encapsulate another, was calculated by the following equation [52]:
l31 ¼ g12 g32 g13
(7)
where l31 is spreading coefficient of component 3 on component 1, and component 2 represents the matrix, in our system, OBC ¼ 1, PLA ¼ 2, EGMA-PLA ¼ 3. 3. Results and discussion
P ni d2i dw ¼ P ni d i
(4)
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi P ni ðln di ln dn Þ2 P ln s ¼ ni
(5)
where ni is the number of particles having the particle diameter of di. Precise feature of phase morphology was characterized using a transmission electron microscope (JEOL JEM-2100F, Japan) at an accelerated voltage of 200 kV. Ultrathin sections from the center of the molded bar with a thickness around 80 nm were cut at 100 C
3.1. Uncompatibilized PLA/OBC binary blends The mechanical properties and phase morphology of the uncompatibilized PLA/OBC blends are given attention first. Fig. 1 shows the results of tensile test and notched izod impact test of the binary blends. As expected, without interfacial compatibilization, the tensile properties of the blends are very poor and the increments of the impact toughness are not significant. The breaking strains of PLA/OBC 90/10 and 70/30 are even lower than that of neat PLA. Additionally, although increasing OBC from 10% to 30% leads to a slight improvement of impact strength, the sharply drop of tensile strength is undesirable. To realize the excellent toughness-strength
Fig. 1. Stress-strain curves and notched impact strength of PLA and PLA/OBC blends with OBC weight contents of 10% and 30%.
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M. Wu et al. / Polymer 55 (2014) 6409e6417 Table 1 Morphology parameters of PLA/OBC 90/10 binary blends and PLA/OBC/EMA-GMA 90/7/3 ternary blends. Sample
Annealing time (h)
dn (mm)
dw (mm)
s
PLA/OBC 90:10
0 5 0 0.25 0.5 5 24
0.97 0.96 0.38 0.38 0.42 0.76 0.77
1.12 1.06 0.41 0.40 0.45 0.80 0.80
1.47 1.43 1.14 1.15 1.34 1.30 1.26
PLA/OBC/EMA-GMA 90:7:3
Fig. 2. Notched impacted strength of neat PLA and PLA/OBC 90/10 blends with various annealing time.
balance, less elastic component and an effective strategy for dramatically improving the toughness are expected. Fig. 2 demonstrates the notched impact strength of PLA and PLA/OBC 90/10 blend after quiescent annealing at 90 C for 0, 0.25, 0.5 and 5 h. With the increasing of annealing time, the impact strengths keep rising and reach about 1.5 times of the original ones at 5 h for both the pure sample and the binary blend. Obviously, annealing can improve the brittleness of the samples, but the improved amplitudes are not impressive enough. SEM micrographs of PLA/OBC 90:10 blends are presented in Fig. 3. Typical sea-island morphology is obtained as spherical OBC particles with diameter around 0.96 mm dispersed in PLA matrix. After experiencing thermal annealing, the size and size distribution of the dispersed phase of OBC do not change significantly, as listed in Table 1. Melting behaviors revealed by DSC (the first heating scan) are shown in Fig. 4. For neat PLA without annealing (Fig. 4(a)), the melting curve exhibit multiple transitions: a cold crystallization peak at 90e120 C and a melting peak at 155e175 C. The cold crystallization peak diminishes for sample annealed for 0.25 h and is absent when longer anneal time is employed, indicating the increased crystalline level of PLA. In addition, except original PLA, a small exothermic peak is observed just before the melting peak, 0 which reflects the phase transition from disordered a form crystal to ordered a form one. The melting behaviors and crystallinity variation of PLA/OBC binary blends resemble that of neat PLA, as
shown in Fig. 4(b). Note that the cold crystallization peak of PLA in the blends is about 10 C lower than that of the neat PLA, which may be attributed to the enhanced chain mobility at the phase interface resulting from the poor miscibility between the matrix and minor phase [53]. The endothermic peak at 120 C reflects the melting of OBC. Due to the low hard block content (12%), the crystalline degree of the OBC utilized is very low (4e5%). Unlike PLA, it changes little after experiencing the thermal annealing process, indicating that the OBC does not crystallize during annealing. In general, through thermal annealing, the thermodynamically stable states are obtained for both the neat PLA and binary PLA/OBC blend, as characterized by increased PLA crystallinities. As mentioned above, crystalline polymers are intrinsically easier to shear yield and fracture upon more effective energy dissipation mechanism, thus resulting in higher IS. Unfortunately, limited by the poor interfaces of the two components, the utilizing of thermal annealing alone does not cause the dramatic improvement of impact strength. 3.2. PLA/OBC blend compatibilized by EMA-GMA For preliminary exploration, the total content of OBC and EBAGMA was fixed at 10 wt%, and the EBA-GMA content changed as 0, 1, 2, 3, 5 and 10 wt%. Results of notched impact strength are shown in Fig. 5. Unexpectedly, the impact strength reduces abruptly from 14.2 kJ/m2 to 7.2 kJ/m2 with the addition of only 1% EBA-GMA and increases slightly at 2 wt%, then declined afterwards. The PLA/OBC blend with medium EMA-GMA content of 3 wt% (PLA/OBC/EMA-GMA 90/7/3) was picked out for micro-morphology analysis and performing further annealing treatment. The corresponding SEM micrographs are shown in Fig. 6. Compared to the blend of PLA/OBC 90/10, a significant diminishment in dispersed droplet size is clearly observed in ternary system, i.e., from 0.96 to 0.38 mm (Table 1). This could be directly related to the decrease of interfacial tension [54]. Besides, a remarkably narrower particle
Fig. 3. SEM micrographs of PLA/OBC 90/10 blends: (a) before annealing, and (b) after annealing for 5 h.
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Fig. 4. DSC melting curves of (a) neat PLA, (b) PLA/OBC 90/10 blends with various annealing time. In the term PLA-a-5 h, “a” represents annealing and 5 h is the annealing time. The crystallinities of PLA (Xc,PLA) and OBC (Xc,OBC) are also presented in the profile.
Fig. 5. Notched impact strength of PLA/OBC/EMA-GMA blends with different compositions.
size distribution of 1.14 also indicates the effective interfacial compatibilization. Nevertheless, the impact strength still decreased to a very low level. Since dispersed size of elastomeric species plays a crucial role in the efficiency of toughing, the sharp reduction of OBC particle size may be the main reason. An optimum size region of 0.5e0.9 mm has been reported in glassy PLA/polymerized soybean oil blend [22]. In these SEM images, many spherical cavities
are observed for the blends with and/or without compatibilizer. The occurrence of debonding of dispersed phase spheres was caused by cryo-fracture, in which two new fractured surfaces were generated. These cavities indicate that partial of dispersed phase spheres might stick on the other side surface. TEM micrographs shown in Fig. 7 illustrate the morphology of the binary and ternary blends. In Fig. 7(a), for the binary PLA/OBC blend, dark OBC domains dispersed individually in the light PLA matrix. Note that the dispersed phases exhibit an elliptical or irregular shape rather than the thermally stable spherical one in SEM image, this may be caused by the deformation during cyroultrathin microtome process, because of the mismatch of modulus/rigidity between PLA resin and OBC elastomer. As for the PLA/OBC/EMA-GMA 90/7/3, besides the much smaller OBC droplets, a large number of fine, nanometer-sized (50e100 nm) microdomains slightly darker than the background are clearly observed in Fig. 7(b). Similar nanoscale morphologies have been reported in many other reactive polymer blending systems [55e57]. It is now well known that the nanoparticles are composed of in-situ formed copolymers resulting from the mutual contact of the reactive constituents of the blend. According to the “interfacial erosion” mechanism [57], viscosity mismatch between graft copolymer of EMA-GMA and PLA (simplified as EGMA-PLA) formed in-situ and the base components lead to its departure from the interface region, to form nanoscale micelles in the bulk. On the other hand, it has been noted in literature that the affinity of the copolymer for the matrix material dominated its migration efficacy to the interface [58]. Due to the asymmetry interaction between the polymer pairs, i.e., chain entanglement between OBC and EMA-GMA whereas chemical reaction between PLA and EMA-GMA, the
Fig. 6. SEM micrographs of (a) PLA/OBC 90/10, and (b) PLA/OBC/EMA-GMA 90/7/3.
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Fig. 7. TEM micrographs of (a) PLA/OBC 90/10, and (b) PLA/OBC/EMA-GMA 90/7/3.
Table 2 Contact angles and surface energies of PLA, OBC and EGMA-PLA. Sample
PLA OBC EGMA-PLA
Contact angle ( )
Surface tension (mN/m)
Water
Diiodo-methane
Disperse part (gd)
Polar part (gp)
Total (g)
62.5 ± 1.4 102.6 ± 1.7 78.3 ± 1.1
27.6 ± 1.4 45.9 ± 0.2 22.0 ± 0.9
42.11 36.95 44.20
16.09 0.53 8.52
58.20 37.49 52.72
compatibilizer which has better affinity with the matrix shown a strong tendency to form micelles directly in the PLA base component. The results of surface tensions are listed in Table 2 and the calculated interfacial tensions in Table 3. The interfacial tension of PLA and OBC is considerably high, indicating a poor interfacial interaction between this component pair; whereas the values of PLA/EGMA-PLA and OBC/EGMA-PLA counterparts are much lower. The calculated spreading coefficient of EGMA-PLA on OBC is positive, which predicates that a core-shell dispersed phase structure with OBC cores completely encapsulated by EGMA-PLA shells is thermodynamically favorable. However, due to the fast mixing process, the morphology observed with a large number of EGMAPLA micro-domains in the matrix is far from the thermodynamic stable state. 3.3. PLA/OBC/EMA-GMA ternary blend after thermal annealing
of PLA at ~90 C disappears after 0.5 h quiescent annealing. Correspondingly, the crystallinity of PLA increases with the increasing treatment time and reaches 32.6% at 0.5 h, then remains unchanged upon the longer annealing time. Similar to the binary blends, OBC shows almost the same and low value of crystallinity (4e5%) during annealing. Fig. 9(b) demonstrates that the cold crystallization kinetics of PLA/OBC/EMA-GMA is significantly slower than PLA/OBC. The retardance of matrix crystallization in compatibilized system was also observed by other researchers [59]. This is attributed to the expelling of the compatibilizer and small OBC domains accommodated in PLA chains from the crystalline regions. The SEM micrographs of samples upon different annealing time were analyzed, and the particle sizes and distributions were calculated according to the equations presented in experimental part. As shown in Fig. 10, a significant increase of the dispersed phase particle size is observed when the long times were utilized (Fig. 10(d, e)). In particular, as depicted in Fig. 10 and Table 1, the dn keeps at ~0.4 mm before 0.25 h and slightly increases at 0.5 h. Then, it experiences a dramatic rise to 0.76 mm for the sample annealed upon 5 h and do not change with longer time of thermal annealing. In addition, the s increases obviously upon the annealing time of 0.5 h resulting from the forming of a few large particles. A well accepted mechanism for rubber toughening is that the cavitation of soft rubber phase causes local relaxation of triaxial stresses surrounding the particles, and triggers matrix shear yielding, thereby resulting in considerable energy dissipation [60e63]. Dompas et al. [34] pointed out that the cavitation resistance of rubber particles
Fig. 8 shows the notched impact strength of PLA/OBC/EMA-GMA 90/7/3 blends with various annealing time of 0, 0.25, 0.5, 5 and 24 h at 90 C. Interestingly, different from PLA/OBC binary blends, the compatibilized samples experience sharp brittle-ductile transition with only 0.5 h annealing, followed by a continuing toughness growing in the next few hours. The impact strength increased from 6.9 kJ/m2 at 0 h to 49.2 kJ/m2 at 0.5 h, and to 64.2 kJ/m2 at 5 h. After that, the impact toughness keeps at this high level when the sample was quiescent annealed for 24 h. Since crystallization of the matrix plays a positive role in vanquishing of the brittleness, the crystallinities of the ternary blends with various annealing times were calculated according to the DSC melting curves presented in Fig. 9(a). The cold crystallization peak
Table 3 Interfacial tensions calculated with surface energy of PLA/EMA-GMA/OBC ternary composites. Polymer pairs
Interfacial tension (mN/m)
PLA/OBC PLA/EGMA-PLA OBC/EGMA-PLA
13.05 1.22 5.15
Fig. 8. Notched impact strength of PLA/OBC/EMA-GMA 90/7/3 blends with various annealing time.
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Fig. 9. (a) DSC melting curves of PLA/OBC/EMA-GMA 90/7/3 blends with various annealing time, (b) DSC curves of isothermal crystallization of PLA, PLA/OBC 90/10 and PLA/OBC/ EMA-GMA 90/7/3 at 90 C.
decreased with increasing particle size. Therefore, the enlarged OBC particles may induce the matrix shear yielding more easily, thus contribute to the further increment of toughness after 0.5 h thermal annealing. Detailed phase morphologies of the ternary PLA/OBC/EMA-GMA 90/7/3 blends annealed for 0.5 and 5 h are revealed by TEM micrographs (Fig. 11(a and b), respectively). Similar to the TEM result before, elongated or angular shaped domain rather than the spherical one is observed due to the deformation during croymicrotome process. Consistent with the SEM result, the OBC particle size of sample annealed for 5 h is much larger than that of sample annealed for only 0.5 h. Besides, for sample annealed for 5 h, some of the OBC rich domains are packaged by a thickened interfacial layer with intermediate contrast. The areas are indicated by white arrows in Fig. 11(b). This is supposed to be caused by the enrichment of EGMA-PLA rich component (a small amount of unreacted EMA-GMA may also exist) to the interface. The spreading coefficient indicates a strong trend for EGMA-PLA to locate at the interface. However, the migration of dispersed phase was not homogeneous everywhere, due to the constraint effects of high viscosity, crystalline phase of PLA and OBC. So only a partial of OBC domains were encapsulated by EGMA-PLA shell, and small OBC
domains and EGMA-PLA domains still exist. Anyway, the enrichment of copolymer compatibilizer at the interface region may improve the interfacial adhesion and stress transfer efficiency. The results of SEM and TEM indicate that the coarsening of dispersed phase particles happened during quiescent thermal annealing. As discussed in literature, the driving force for dispersed phase coarsening is to reduce interfacial energy [40]. The most commonly used mechanisms for coarsening are coalescence and Ostwald ripening. The former considers the dispersed particles move through the matrix and collide with each other to form larger droplets [40]. The latter describes the dissolving of small droplets into the surrounding due to their larger curvature and higher energy, and then separating out onto the larger droplets [40]. Although some researchers have reported the suppression of coalescence with the existence of interfacial compatibilizer [64,65], owning to the elastic repulsive force required for compression of the copolymer [64], the effect seems not obvious in our system. This can be attributed to the relatively low content of EGMA-PLA compatibilizer at the interface and the strong driving force to reduce the interfacial energy. On the other hand, it is noted worthy that the dramatic domain enlarging happened after 0.5 h at which the crystallization of PLA had almost completed. As discussed above,
Fig. 10. SEM micrographs of PLA/OBC/EMA-GMA 90/7/3 blends with various annealing time: (a) 0 h, (b) 0.25 h, (c) 0.5 h, (d) 5 h, (e) 24 h and the number average particle size as a function of annealing time.
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Fig. 11. TEM images of PLA/OBC/EMA-GMA 90/7/3 blend annealing for (a) 0.5 h, and (b) 5 h.
the crystallization of PLA would expel the other components from the original position, thus induces the phase migration and coalescence of OBC rich domains at amorphous PLA region. According to the discussions above, the multiphase morphology evolution of ternary PLA/OBC/EMA-GMA blends during quiescent annealing is clarified by the schematic diagram of Fig. 12. Before annealing, small OBC particles with average diameter of ~0.4 mm and EGMA-PLA nanometer-scale domains dispersed individually in the amorphous PLA matrix. When the blend was thermally treated, the matrix crystallized and the dispersed OBC and EGMA-PLA domains started to coarsen, as exhibited by the migration of EGMAPLA micelles to the surface of OBC, the coalescence of OBC rich domains and the coalescence of EGMA-PLA domains. The final multiphase morphology as characterized by higher PLA crystallinity, enlarged OBC particles (~0.8 mm) surrounded by thick EGMAPLA cover-layer, in our opinion, is responsible for the dramatic enhancement of impact toughness. The observed morphology should be arisen from the interplay of two issues: minimization of the interfacial free energy and crystallization of PLA. However, due
to the low annealing temperature utilized, crystallization process of PLA is not in the equilibrium state. The coarsening of the dispersed phase is under the constraints of the crystallized phases of PLA and OBC, and depends on the thermal pathway from the melt state. The mechanism of morphology development suggested in Fig. 12 is acceptable in the temperature range from Tg,PLA to Tm,PLA, but the kinetics difference between various temperatures is obvious, due to the effects of crystallization rate and viscosity. Fig. 13 displays the tensile properties and impact toughness of the neat PLA and PLA/OBC/EMA-GMA blends annealed for 5 h. The ternary blend with 10% rubbery phase exhibits the tensile strength and modulus as high as 53.6 MPa and 2135.8 MPa, respectively, which are slightly lower than that of neat PLA. However, its break elongation is much larger than that of pure PLA. Moreover, the impact strength is dramatically increased as compared to the neat one and even higher than that of the blend containing 30% rubbery component. As to the sample containing 30% rubber phase, the sharp drop of tensile strength is an inevitable disadvantage. Therefore, an excellent toughness-strength balance is achieved in
Fig. 12. Schematic diagram of micro-morphology evolution of PLA/OBC/EMA-GMA blends during annealing.
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Fig. 13. Stress-strain curves and impact strength of neat PLA, PLA/OBC/EMA-GMA blends with total OBC/EMA-GMA weight content of 10% and 30% after 5 h annealing.
the ternary blend with relatively low rubbery phase content (10%), realizing by a combination of thermodynamics favorable interface compatibility and the morphology developed during annealing. 4. Conclusions The good comprehensive mechanical property was obtained in the PLA/OBC blend with elastomer content as low as 10%, via simultaneously realizing the thermodynamically favorable compatibility and morphology. For the ternary blend of PLA/OBC/ EMA-GMA 90/7/3 experienced quiescent thermal annealing at 90 C for 5 h, the impact strength increased for 25 times as comparing to that of the pure PLA, and the tensile strength can preserve as 87% of pure PLA. The thermodynamically stable morphology favorable for excellent toughness-strength balance is characterized as high crystallinity, large dispersed phase droplet size, and thick interface layer. Our present study indicates that except for the thermodynamics favorable compatibility, the transition from kinetics-dominated morphology to thermodynamically stable one also plays a crucial role on the optimization of properties of multicomponent blends. Acknowledgment Financial supports from NSFC (51373108), the Ministry of Education of China (NCET-11-0348), the Science & Technology Department of Sichuan Province (2013TD0013) and Sichuan University (2011SCU04A12) are gratefully appreciated. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
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Simultaneous the thermodynamics favorable compatibility and morphology to achieve excellent comprehensive mechanics in PLA/ OBC blend Meng Wu, Zhiqiang Wu, Ke Wang*, Qin Zhang, Qiang Fu* College of Polymer Science and Engineering, Sichuan University, State Key Laboratory of Polymer Materials Engineering, Chengdu 610065, People's Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 July 2014 Received in revised form 3 September 2014 Accepted 3 October 2014 Available online 12 October 2014
For the toughening of thermoplastics by using elastomeric components, the relatively high contents of elastomeric phase are commonly demanded to trigger the brittle-to-ductile transition. However, an obvious drawback of remarkably decreased strength and rigidity may arise after incorporation of large amount of elastomeric species. The main thinking in our present work is to achieve a good toughnessstrength balance in an elastomer-toughened thermoplastic system with less amount of elastomeric phase, i.e., the blend of poly(lactic acid) (PLA)/olefin block copolymer (OBC) 90/10 w/w. When both of the thermodynamics favorable compatibility and the thermodynamically stable morphology were realized, the impact toughness of PLA/OBC 90/10 blend was 25 multiples for that of pure PLA, while the tensile strength could preserve as a level of 87% (based on the value of pure PLA). The interfacial compatibilization between PLA and OBC achieved through adding EMA-GMA as a compatibilizer. On the other hand, a quiescent annealing process at 90 C resulted in a more thermodynamically stable morphology, which was characterized as high crystallinity, large size of elastomer-phase droplet, and thickening interfacial layer. Our study offers new insight into the optimization of properties of multicomponent blend that except for realizing the thermodynamics favorable compatibility, the transition from kinetics-dominated morphology to thermodynamically stable one also plays a crucial role. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Olefin block copolymer Toughness-strength balance Thermodynamically stable morphology
1. Introduction The increasing concern on environmental pollution caused by plastic wastes and the diminishing of oil resources have accelerated the boom of biodegradable materials derived from renewable resources [1e4]. Poly(lactic acid) (PLA) outstands of these green materials owning to its high strength and modulus, excellent biocompatibility, competitive cost, etc [5]. Unfortunately, one of its major drawbacks, the inherent brittleness, shown by low impact strength (IS) and poor tensile toughness, greatly confines its wide applications [5,6]. Over the past several years, extensive efforts have been made to surmount the brittleness of PLA. The most commonly used strategy was blending with flexible polymers. Various flexible polymers ranged from commercially available ones like linear low-density polyethylene (LLDPE) [7], thermoplastic polyolefin [8], hydrogenated styrene-butadiene-styrene block
* Corresponding authors. Tel.: þ86 28 85461795. E-mail addresses: [email protected] (K. Wang), [email protected] (Q. Fu). http://dx.doi.org/10.1016/j.polymer.2014.10.004 0032-3861/© 2014 Elsevier Ltd. All rights reserved.
copolymer (SEBS) [9], polyamide elastomer [10], polyurethane [11,12], acrylonitrile-butadiene-styrene (ABS) [13], ethylene-nbutylacrylate-glycidyl methacrylate (EBA-GMA) [14,15], poly(butylenessuccinate-co-adipate) [16,17], poly(b-hydroxybutyrate-co-b-hydroxyvalerat) [18], to those experimentally synthesized species such as expoxidized natural rubber [19], glycidyl methacrylate grafted poly(ethylene octane) [20] and polymerized soybean oil [21,22] have been employed as tougheners for PLA. However, when focused on the impact resistance, very high contents of flexible modifiers have been frequently used to trigger the brittle-toughness transition and obtain satisfactory level of IS. This inevitably resulted in the tremendous decrease of tensile strength and modulus. Study on the PLA toughening system with low content of flexible polymers and excellent toughness-strength balance is considerably lacking at present. Good interfacial adhesion is needed to achieve high impact strength, because poor interface would easily result in premature interfacial failure and hence rapid and catastrophic crack propagation through the whole material [23,24]. However, most polymer blends are thermodynamically immiscible due to the low entropy
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of mixing [25], and a sharp interface between the two phases results. Therefore, copolymers premade or formed in-situ has been widely employed to compatibilize two immiscible polymers. Ideally, the copolymer resides at the interface, resulting in decreased interfacial tension, smaller dispersed droplets, greater interfacial adhesion and increased energy transfer efficiency [26]. In addition, morphology factors like the crystallization of matrix polymer and the size and shape of dispersed phase also play crucial roles in effective toughening. Numerous experimental phenomena have suggested that toughening is easier to achieve in crystalline polymers than in amorphous ones [27e29]. What's more, a linear relationship between PLA matrix crystallinity and the notched impact strength has been reported in PLA/poly(ε-caprolactone) (PCL) blends [27]. The authors found the impact fracture process was dominated by crazing in amorphous matrix and shear yielding in the case of crystalline matrix, while the latter has been proved to be a more effective energy dissipation mechanism than crazing [30e32]. It is well accepted that moderate filler particle size is a requirement for effective toughening. Too small rubber particles are inefficient for cavitation and may delay the shear yielding [33e35], otherwise too large particles which resulted in too large interparticle distance at the same rubber content influence the efficiency for local yielding process to propagate and pervade over the entire deformation zone [36]. In principle, the morphology of polymer blends should be determined by thermodynamic issue, i.e., towards a lowest-energy equilibrium state. However, thermodynamic equilibrium is commonly difficult to be achieved due to high viscosities of polymers and disturbance of shearing fields, i.e., the kinetics effects. To manipulate the morphology of polymer blends, the method of quiescent thermal annealing was often used. The aim of thermal annealing is to drive the kinetics-dominated morphology to the thermodynamics equilibrium one. The structure/morphology changes during thermal annealing include secondary crystallization, lamellar perfection and thickening, rearrangement of molecular chain in constrained amorphous phase [37e39], coalescence of dispersed particles [40,41], etc. Since the toughened PLA blends are of promising application prospect, the commercialized cost-effective constitutes are highly desirable. In recent years, a novel thermoplastic polyolefin elastomer of alternative hard and soft polyethylene blocks, olefin block copolymer (OBC), has attracted wide attentions [42e45]. It is fabricated involving two catalysts and a “chain shuttling agent” (CSA) that switches the growing chain from one catalyst to another at random intervals [46]. The hard polyethylene blocks with very low level of a-olefin comonomer crystallized and dispersed in the amorphous soft blocks matrix which has relatively high a-olefin comonomer content. OBC keeps the good performances of polyethylene like flexibility, friendly processing conditions and excellent abrasion resistance and aging resistance, and is with the improved heat resistance, compared to the last generation of polyolefin elastomer (POE) [47]. Our attention in this study is focused on a blending system of PLA basal resin containing low content (10%) of OBC elastomer. For such blending system, its strength and modulus are comparable to the properties of pure PLA; whereas the increment in toughness is very limited. So we attempt to develop an effective strategy for dramatically improving the toughness of PLA/OBC 90/10 w/w blend. Moreover, the roles of thermodynamics and kinetics influences on the realizing of excellent toughness-strength balance will be demonstrated and discussed with taking into accounts of interfacial compatibility and multiphase morphology feature. Since PLA and OBC are thermodynamically immiscible, a commercially available random terpolymer, ethylene-(methyl acrylate)-(glycidyl methacrylate) (EMA-GMA), was introduced as the compatibilizer, in which -GMA has been widely reported as a reactive chain block
with PLA [9,14,15,20,29] (reaction between the epoxide groups of -GMA and the terminal carboxyl and hydroxyl groups of PLA) and ethylene segment (-E) was expected to entangle with the ethylene blocks in OBC. 2. Experimental section 2.1. Materials and sample preparation A poly(L-lactic acid) resin (4032D, Nature Works LLC) with 1.2e1.6 % D-isomer lactide and density of 1.25 g/cm3 was used as the matrix polymer. The weight-average molecular weight (Mw) and molecular weight distribution is 207 kDa and 1.74, respectively. Olefin block copolymer (INFUSE 9507) containing 14.6 mol% of octane comonomer and 12 wt% of hard blocks was supplied by Dow Chemical Company (Mw ¼ 90.8 kg/mol, r ¼ 0.867 g/cm3, melt index of 5 g/10 min). Random terpolymer of ethylene, methyl acrylate and glycidyl methacrylate used as reactive precursor for the compatibilization of PLA and OBC is LOTADER® AX8900 from Arkema (France), having 24 wt% methyl acrylate, 8 wt% glycidyl methacrylate, a density of 0.94 g/cm3and melt index of 6 g/10 min (190 C, 2.16 kg). Melt blending of the materials was carried out in a 69 ml mixing chamber of the Haake Rheometer (HAAKE Polylab OS, USA) with a rotation speed of 60 rpm at 190 C for 5 min. Prior to mixing, the three components, OBC, PLA and EMA-GMA, were vacuum dried at 60 C for 12 h. A series of binary and ternary blends were prepared as the following composition: PLA/OBC 90/10 (w/w), 70/30 (w/w); PLA/OBC/EMA-GMA 90/(10 x)/x (w/w) (x ¼ 1, 2, 3, 5, where x represents the weight content of EMA-GMA). The obtained samples were injection-molded into specimens using a HAAKE MiniJet (Thermo Scientific, USA), at a barrel temperature of 190 C and a mold temperature of 50 C. Some of the blends (PLA/OBC 90/10, PLA/OBC/EMA-GMA 90/7/3) were further quiescent annealed at 90 C for various time in vacuum oven. 2.2. Mechanical tests Notched izod impact tests were carried out according to ISO180/ 179, using a VJ-40 impact tester at room temperature (23 ± 2 C). Tensile tests were performed using dumbbell-shape samples on a SANS Universal tensile testing machine following ISO 527-2. The measurements were conducted at room temperature (23 ± 2 C) with a crosshead speed of 5 mm/min. At least five specimens were tested for each sample. 2.3. Differential scanning calorimetry (DSC) DSC measurements were performed on a PerkineElmer pyris-1 DSC instrument in nitrogen atmosphere. The instrument was calibrated prior to testing by indium. Samples (about 5 mg) cut from the center of the molded bar were heated from 30 C to 210 C at a speed of 10 C/min to record the melt curves. Crystallinity of PLA (Xc,PLA) was evaluated as following:
XC ¼
DHm DHc DHt 0 wf DHm
(1)
where DHm, DHc, and DHt represent the enthalpies of melting, cold crystallization, crystalline phase transition of PLA, respectively. 0 is the melting enthalpy of 100% crystalline PLA, which was DHm reported to be 93.7 J/g [48]. wf is the weight fraction of the component. The crystallinity of OBC (Xc,OBC) was also calculated using a simplified equation:
M. Wu et al. / Polymer 55 (2014) 6409e6417
XC ¼
DHm 0 wf DHm
(2)
0 (the melting enthalpy of 100% crystalline where the value of DHm polyethylene) is 290 J/g [49]. The isothermal cold crystallization curves of PLA, PLA/OBC 90/ 10, PLA/OBC/EMA-GMA 90/7/3 at 90 C were recorded. Samples was first heated to 200 C at 30 C/min and kept for 5 min to eliminate the thermal history, then cooled to 30 C at 100 C/min and kept for 2 min. Then the samples were heated to 90 C at 100 C/min and kept for various time to record the crystallization curves.
2.4. Phase morphology characterization Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were employed to characterize the phase morphology of the PLA/OBC and PLA/OBC/EMA-GMA blends. Cryofractured surface from the cross section of impact bar was observed using an FEI Inspect F field emission scanning electron microscope (FE-SEM, USA) at an accelerating voltage of 5 kV. The surface was sputter-coated with a gold layer before testing. The obtained digital micrographs were analyzed for the determination of the size of dispersed phase using an Image-Pro Plus software. At least 200 particles from 3 to 4 micrographs were analyzed. The number average diameter (dn), weight-average diameter (dw), particle size distribution parameter (s) were calculated according to the equation below:
P nd dn ¼ P i i ni
(3)
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using a cryo-ultramicrotome (Leica EM FC7, Germany) equipped with a diamond knife. These sections were stained with RuO4 for 10 min. 2.5. Interfacial tensions and spreading coefficient Interfacial tensions between PLA, OBC and the graft copolymer of EMA-GMA and PLA (EGMA-PLA) were calculated by measuring surface energies. The EGMA-PLA was extracted from PLA/EMAGMA blend by 1,4-dioxane which can selectively dissolve PLA. Contact angles of the three materials were measured on KRUSS DSA100 with water and diiodo-methane at 25 C. The surface tension (g), disperse part (gd) and polar part (gp) of the materials were calculated from the measured contact angles according to Wu's method. Then the interfacial tension (gAB) of the polymer pairs can be estimated using the following equation [50,51]:
1=2 1=2 gAB ¼ gA þ gB 2 gdA gdB 2 gpA gpB
(6)
where gA and gB represent the surface energies of the two materials in contact, gA ¼ gdA þ gpA , gB ¼ gdB þ gpB . To correlate the phase morphology and the interfacial tensions, the spreading coefficient, which can be used to predict the tendency for one phase to encapsulate another, was calculated by the following equation [52]:
l31 ¼ g12 g32 g13
(7)
where l31 is spreading coefficient of component 3 on component 1, and component 2 represents the matrix, in our system, OBC ¼ 1, PLA ¼ 2, EGMA-PLA ¼ 3. 3. Results and discussion
P ni d2i dw ¼ P ni d i
(4)
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi P ni ðln di ln dn Þ2 P ln s ¼ ni
(5)
where ni is the number of particles having the particle diameter of di. Precise feature of phase morphology was characterized using a transmission electron microscope (JEOL JEM-2100F, Japan) at an accelerated voltage of 200 kV. Ultrathin sections from the center of the molded bar with a thickness around 80 nm were cut at 100 C
3.1. Uncompatibilized PLA/OBC binary blends The mechanical properties and phase morphology of the uncompatibilized PLA/OBC blends are given attention first. Fig. 1 shows the results of tensile test and notched izod impact test of the binary blends. As expected, without interfacial compatibilization, the tensile properties of the blends are very poor and the increments of the impact toughness are not significant. The breaking strains of PLA/OBC 90/10 and 70/30 are even lower than that of neat PLA. Additionally, although increasing OBC from 10% to 30% leads to a slight improvement of impact strength, the sharply drop of tensile strength is undesirable. To realize the excellent toughness-strength
Fig. 1. Stress-strain curves and notched impact strength of PLA and PLA/OBC blends with OBC weight contents of 10% and 30%.
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M. Wu et al. / Polymer 55 (2014) 6409e6417 Table 1 Morphology parameters of PLA/OBC 90/10 binary blends and PLA/OBC/EMA-GMA 90/7/3 ternary blends. Sample
Annealing time (h)
dn (mm)
dw (mm)
s
PLA/OBC 90:10
0 5 0 0.25 0.5 5 24
0.97 0.96 0.38 0.38 0.42 0.76 0.77
1.12 1.06 0.41 0.40 0.45 0.80 0.80
1.47 1.43 1.14 1.15 1.34 1.30 1.26
PLA/OBC/EMA-GMA 90:7:3
Fig. 2. Notched impacted strength of neat PLA and PLA/OBC 90/10 blends with various annealing time.
balance, less elastic component and an effective strategy for dramatically improving the toughness are expected. Fig. 2 demonstrates the notched impact strength of PLA and PLA/OBC 90/10 blend after quiescent annealing at 90 C for 0, 0.25, 0.5 and 5 h. With the increasing of annealing time, the impact strengths keep rising and reach about 1.5 times of the original ones at 5 h for both the pure sample and the binary blend. Obviously, annealing can improve the brittleness of the samples, but the improved amplitudes are not impressive enough. SEM micrographs of PLA/OBC 90:10 blends are presented in Fig. 3. Typical sea-island morphology is obtained as spherical OBC particles with diameter around 0.96 mm dispersed in PLA matrix. After experiencing thermal annealing, the size and size distribution of the dispersed phase of OBC do not change significantly, as listed in Table 1. Melting behaviors revealed by DSC (the first heating scan) are shown in Fig. 4. For neat PLA without annealing (Fig. 4(a)), the melting curve exhibit multiple transitions: a cold crystallization peak at 90e120 C and a melting peak at 155e175 C. The cold crystallization peak diminishes for sample annealed for 0.25 h and is absent when longer anneal time is employed, indicating the increased crystalline level of PLA. In addition, except original PLA, a small exothermic peak is observed just before the melting peak, 0 which reflects the phase transition from disordered a form crystal to ordered a form one. The melting behaviors and crystallinity variation of PLA/OBC binary blends resemble that of neat PLA, as
shown in Fig. 4(b). Note that the cold crystallization peak of PLA in the blends is about 10 C lower than that of the neat PLA, which may be attributed to the enhanced chain mobility at the phase interface resulting from the poor miscibility between the matrix and minor phase [53]. The endothermic peak at 120 C reflects the melting of OBC. Due to the low hard block content (12%), the crystalline degree of the OBC utilized is very low (4e5%). Unlike PLA, it changes little after experiencing the thermal annealing process, indicating that the OBC does not crystallize during annealing. In general, through thermal annealing, the thermodynamically stable states are obtained for both the neat PLA and binary PLA/OBC blend, as characterized by increased PLA crystallinities. As mentioned above, crystalline polymers are intrinsically easier to shear yield and fracture upon more effective energy dissipation mechanism, thus resulting in higher IS. Unfortunately, limited by the poor interfaces of the two components, the utilizing of thermal annealing alone does not cause the dramatic improvement of impact strength. 3.2. PLA/OBC blend compatibilized by EMA-GMA For preliminary exploration, the total content of OBC and EBAGMA was fixed at 10 wt%, and the EBA-GMA content changed as 0, 1, 2, 3, 5 and 10 wt%. Results of notched impact strength are shown in Fig. 5. Unexpectedly, the impact strength reduces abruptly from 14.2 kJ/m2 to 7.2 kJ/m2 with the addition of only 1% EBA-GMA and increases slightly at 2 wt%, then declined afterwards. The PLA/OBC blend with medium EMA-GMA content of 3 wt% (PLA/OBC/EMA-GMA 90/7/3) was picked out for micro-morphology analysis and performing further annealing treatment. The corresponding SEM micrographs are shown in Fig. 6. Compared to the blend of PLA/OBC 90/10, a significant diminishment in dispersed droplet size is clearly observed in ternary system, i.e., from 0.96 to 0.38 mm (Table 1). This could be directly related to the decrease of interfacial tension [54]. Besides, a remarkably narrower particle
Fig. 3. SEM micrographs of PLA/OBC 90/10 blends: (a) before annealing, and (b) after annealing for 5 h.
M. Wu et al. / Polymer 55 (2014) 6409e6417
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Fig. 4. DSC melting curves of (a) neat PLA, (b) PLA/OBC 90/10 blends with various annealing time. In the term PLA-a-5 h, “a” represents annealing and 5 h is the annealing time. The crystallinities of PLA (Xc,PLA) and OBC (Xc,OBC) are also presented in the profile.
Fig. 5. Notched impact strength of PLA/OBC/EMA-GMA blends with different compositions.
size distribution of 1.14 also indicates the effective interfacial compatibilization. Nevertheless, the impact strength still decreased to a very low level. Since dispersed size of elastomeric species plays a crucial role in the efficiency of toughing, the sharp reduction of OBC particle size may be the main reason. An optimum size region of 0.5e0.9 mm has been reported in glassy PLA/polymerized soybean oil blend [22]. In these SEM images, many spherical cavities
are observed for the blends with and/or without compatibilizer. The occurrence of debonding of dispersed phase spheres was caused by cryo-fracture, in which two new fractured surfaces were generated. These cavities indicate that partial of dispersed phase spheres might stick on the other side surface. TEM micrographs shown in Fig. 7 illustrate the morphology of the binary and ternary blends. In Fig. 7(a), for the binary PLA/OBC blend, dark OBC domains dispersed individually in the light PLA matrix. Note that the dispersed phases exhibit an elliptical or irregular shape rather than the thermally stable spherical one in SEM image, this may be caused by the deformation during cyroultrathin microtome process, because of the mismatch of modulus/rigidity between PLA resin and OBC elastomer. As for the PLA/OBC/EMA-GMA 90/7/3, besides the much smaller OBC droplets, a large number of fine, nanometer-sized (50e100 nm) microdomains slightly darker than the background are clearly observed in Fig. 7(b). Similar nanoscale morphologies have been reported in many other reactive polymer blending systems [55e57]. It is now well known that the nanoparticles are composed of in-situ formed copolymers resulting from the mutual contact of the reactive constituents of the blend. According to the “interfacial erosion” mechanism [57], viscosity mismatch between graft copolymer of EMA-GMA and PLA (simplified as EGMA-PLA) formed in-situ and the base components lead to its departure from the interface region, to form nanoscale micelles in the bulk. On the other hand, it has been noted in literature that the affinity of the copolymer for the matrix material dominated its migration efficacy to the interface [58]. Due to the asymmetry interaction between the polymer pairs, i.e., chain entanglement between OBC and EMA-GMA whereas chemical reaction between PLA and EMA-GMA, the
Fig. 6. SEM micrographs of (a) PLA/OBC 90/10, and (b) PLA/OBC/EMA-GMA 90/7/3.
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Fig. 7. TEM micrographs of (a) PLA/OBC 90/10, and (b) PLA/OBC/EMA-GMA 90/7/3.
Table 2 Contact angles and surface energies of PLA, OBC and EGMA-PLA. Sample
PLA OBC EGMA-PLA
Contact angle ( )
Surface tension (mN/m)
Water
Diiodo-methane
Disperse part (gd)
Polar part (gp)
Total (g)
62.5 ± 1.4 102.6 ± 1.7 78.3 ± 1.1
27.6 ± 1.4 45.9 ± 0.2 22.0 ± 0.9
42.11 36.95 44.20
16.09 0.53 8.52
58.20 37.49 52.72
compatibilizer which has better affinity with the matrix shown a strong tendency to form micelles directly in the PLA base component. The results of surface tensions are listed in Table 2 and the calculated interfacial tensions in Table 3. The interfacial tension of PLA and OBC is considerably high, indicating a poor interfacial interaction between this component pair; whereas the values of PLA/EGMA-PLA and OBC/EGMA-PLA counterparts are much lower. The calculated spreading coefficient of EGMA-PLA on OBC is positive, which predicates that a core-shell dispersed phase structure with OBC cores completely encapsulated by EGMA-PLA shells is thermodynamically favorable. However, due to the fast mixing process, the morphology observed with a large number of EGMAPLA micro-domains in the matrix is far from the thermodynamic stable state. 3.3. PLA/OBC/EMA-GMA ternary blend after thermal annealing
of PLA at ~90 C disappears after 0.5 h quiescent annealing. Correspondingly, the crystallinity of PLA increases with the increasing treatment time and reaches 32.6% at 0.5 h, then remains unchanged upon the longer annealing time. Similar to the binary blends, OBC shows almost the same and low value of crystallinity (4e5%) during annealing. Fig. 9(b) demonstrates that the cold crystallization kinetics of PLA/OBC/EMA-GMA is significantly slower than PLA/OBC. The retardance of matrix crystallization in compatibilized system was also observed by other researchers [59]. This is attributed to the expelling of the compatibilizer and small OBC domains accommodated in PLA chains from the crystalline regions. The SEM micrographs of samples upon different annealing time were analyzed, and the particle sizes and distributions were calculated according to the equations presented in experimental part. As shown in Fig. 10, a significant increase of the dispersed phase particle size is observed when the long times were utilized (Fig. 10(d, e)). In particular, as depicted in Fig. 10 and Table 1, the dn keeps at ~0.4 mm before 0.25 h and slightly increases at 0.5 h. Then, it experiences a dramatic rise to 0.76 mm for the sample annealed upon 5 h and do not change with longer time of thermal annealing. In addition, the s increases obviously upon the annealing time of 0.5 h resulting from the forming of a few large particles. A well accepted mechanism for rubber toughening is that the cavitation of soft rubber phase causes local relaxation of triaxial stresses surrounding the particles, and triggers matrix shear yielding, thereby resulting in considerable energy dissipation [60e63]. Dompas et al. [34] pointed out that the cavitation resistance of rubber particles
Fig. 8 shows the notched impact strength of PLA/OBC/EMA-GMA 90/7/3 blends with various annealing time of 0, 0.25, 0.5, 5 and 24 h at 90 C. Interestingly, different from PLA/OBC binary blends, the compatibilized samples experience sharp brittle-ductile transition with only 0.5 h annealing, followed by a continuing toughness growing in the next few hours. The impact strength increased from 6.9 kJ/m2 at 0 h to 49.2 kJ/m2 at 0.5 h, and to 64.2 kJ/m2 at 5 h. After that, the impact toughness keeps at this high level when the sample was quiescent annealed for 24 h. Since crystallization of the matrix plays a positive role in vanquishing of the brittleness, the crystallinities of the ternary blends with various annealing times were calculated according to the DSC melting curves presented in Fig. 9(a). The cold crystallization peak
Table 3 Interfacial tensions calculated with surface energy of PLA/EMA-GMA/OBC ternary composites. Polymer pairs
Interfacial tension (mN/m)
PLA/OBC PLA/EGMA-PLA OBC/EGMA-PLA
13.05 1.22 5.15
Fig. 8. Notched impact strength of PLA/OBC/EMA-GMA 90/7/3 blends with various annealing time.
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Fig. 9. (a) DSC melting curves of PLA/OBC/EMA-GMA 90/7/3 blends with various annealing time, (b) DSC curves of isothermal crystallization of PLA, PLA/OBC 90/10 and PLA/OBC/ EMA-GMA 90/7/3 at 90 C.
decreased with increasing particle size. Therefore, the enlarged OBC particles may induce the matrix shear yielding more easily, thus contribute to the further increment of toughness after 0.5 h thermal annealing. Detailed phase morphologies of the ternary PLA/OBC/EMA-GMA 90/7/3 blends annealed for 0.5 and 5 h are revealed by TEM micrographs (Fig. 11(a and b), respectively). Similar to the TEM result before, elongated or angular shaped domain rather than the spherical one is observed due to the deformation during croymicrotome process. Consistent with the SEM result, the OBC particle size of sample annealed for 5 h is much larger than that of sample annealed for only 0.5 h. Besides, for sample annealed for 5 h, some of the OBC rich domains are packaged by a thickened interfacial layer with intermediate contrast. The areas are indicated by white arrows in Fig. 11(b). This is supposed to be caused by the enrichment of EGMA-PLA rich component (a small amount of unreacted EMA-GMA may also exist) to the interface. The spreading coefficient indicates a strong trend for EGMA-PLA to locate at the interface. However, the migration of dispersed phase was not homogeneous everywhere, due to the constraint effects of high viscosity, crystalline phase of PLA and OBC. So only a partial of OBC domains were encapsulated by EGMA-PLA shell, and small OBC
domains and EGMA-PLA domains still exist. Anyway, the enrichment of copolymer compatibilizer at the interface region may improve the interfacial adhesion and stress transfer efficiency. The results of SEM and TEM indicate that the coarsening of dispersed phase particles happened during quiescent thermal annealing. As discussed in literature, the driving force for dispersed phase coarsening is to reduce interfacial energy [40]. The most commonly used mechanisms for coarsening are coalescence and Ostwald ripening. The former considers the dispersed particles move through the matrix and collide with each other to form larger droplets [40]. The latter describes the dissolving of small droplets into the surrounding due to their larger curvature and higher energy, and then separating out onto the larger droplets [40]. Although some researchers have reported the suppression of coalescence with the existence of interfacial compatibilizer [64,65], owning to the elastic repulsive force required for compression of the copolymer [64], the effect seems not obvious in our system. This can be attributed to the relatively low content of EGMA-PLA compatibilizer at the interface and the strong driving force to reduce the interfacial energy. On the other hand, it is noted worthy that the dramatic domain enlarging happened after 0.5 h at which the crystallization of PLA had almost completed. As discussed above,
Fig. 10. SEM micrographs of PLA/OBC/EMA-GMA 90/7/3 blends with various annealing time: (a) 0 h, (b) 0.25 h, (c) 0.5 h, (d) 5 h, (e) 24 h and the number average particle size as a function of annealing time.
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Fig. 11. TEM images of PLA/OBC/EMA-GMA 90/7/3 blend annealing for (a) 0.5 h, and (b) 5 h.
the crystallization of PLA would expel the other components from the original position, thus induces the phase migration and coalescence of OBC rich domains at amorphous PLA region. According to the discussions above, the multiphase morphology evolution of ternary PLA/OBC/EMA-GMA blends during quiescent annealing is clarified by the schematic diagram of Fig. 12. Before annealing, small OBC particles with average diameter of ~0.4 mm and EGMA-PLA nanometer-scale domains dispersed individually in the amorphous PLA matrix. When the blend was thermally treated, the matrix crystallized and the dispersed OBC and EGMA-PLA domains started to coarsen, as exhibited by the migration of EGMAPLA micelles to the surface of OBC, the coalescence of OBC rich domains and the coalescence of EGMA-PLA domains. The final multiphase morphology as characterized by higher PLA crystallinity, enlarged OBC particles (~0.8 mm) surrounded by thick EGMAPLA cover-layer, in our opinion, is responsible for the dramatic enhancement of impact toughness. The observed morphology should be arisen from the interplay of two issues: minimization of the interfacial free energy and crystallization of PLA. However, due
to the low annealing temperature utilized, crystallization process of PLA is not in the equilibrium state. The coarsening of the dispersed phase is under the constraints of the crystallized phases of PLA and OBC, and depends on the thermal pathway from the melt state. The mechanism of morphology development suggested in Fig. 12 is acceptable in the temperature range from Tg,PLA to Tm,PLA, but the kinetics difference between various temperatures is obvious, due to the effects of crystallization rate and viscosity. Fig. 13 displays the tensile properties and impact toughness of the neat PLA and PLA/OBC/EMA-GMA blends annealed for 5 h. The ternary blend with 10% rubbery phase exhibits the tensile strength and modulus as high as 53.6 MPa and 2135.8 MPa, respectively, which are slightly lower than that of neat PLA. However, its break elongation is much larger than that of pure PLA. Moreover, the impact strength is dramatically increased as compared to the neat one and even higher than that of the blend containing 30% rubbery component. As to the sample containing 30% rubber phase, the sharp drop of tensile strength is an inevitable disadvantage. Therefore, an excellent toughness-strength balance is achieved in
Fig. 12. Schematic diagram of micro-morphology evolution of PLA/OBC/EMA-GMA blends during annealing.
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Fig. 13. Stress-strain curves and impact strength of neat PLA, PLA/OBC/EMA-GMA blends with total OBC/EMA-GMA weight content of 10% and 30% after 5 h annealing.
the ternary blend with relatively low rubbery phase content (10%), realizing by a combination of thermodynamics favorable interface compatibility and the morphology developed during annealing. 4. Conclusions The good comprehensive mechanical property was obtained in the PLA/OBC blend with elastomer content as low as 10%, via simultaneously realizing the thermodynamically favorable compatibility and morphology. For the ternary blend of PLA/OBC/ EMA-GMA 90/7/3 experienced quiescent thermal annealing at 90 C for 5 h, the impact strength increased for 25 times as comparing to that of the pure PLA, and the tensile strength can preserve as 87% of pure PLA. The thermodynamically stable morphology favorable for excellent toughness-strength balance is characterized as high crystallinity, large dispersed phase droplet size, and thick interface layer. Our present study indicates that except for the thermodynamics favorable compatibility, the transition from kinetics-dominated morphology to thermodynamically stable one also plays a crucial role on the optimization of properties of multicomponent blends. Acknowledgment Financial supports from NSFC (51373108), the Ministry of Education of China (NCET-11-0348), the Science & Technology Department of Sichuan Province (2013TD0013) and Sichuan University (2011SCU04A12) are gratefully appreciated. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
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