POE-g-GMA Elastomer through

Article Cite This: Ind. Eng. Chem. Res. 2019, 58, 12650−12663

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Toughening of PBT by POE/POE‑g‑GMA Elastomer through Regulating Interfacial Adhesion and Toughening Mechanism Mengyao Shang,† Yijian Wu,‡ Baoqing Shentu,*,† and Zhixue Weng† †

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State Key Lab of Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, 38 Zheda Road, Hangzhou 310027, China ‡ National Engineering Laboratory for Plastic Modification and Processing, KingFa Sci & Tech. Co., Ltd., Guangzhou 510663, China ABSTRACT: Supertough poly(butylene terephthalate) (PBT) blends were prepared by melting with poly(ethyleneoctene) (POE) and glycidyl methacrylate grafted POE (POEg-GMA), and the toughening mechanism was systematically analyzed. Scanning electron microscopy (SEM) and rheological measurements identified that POE-g-GMA effectively improved the interaction between PBT and elastomer. The compatibility between phases improved gradually, while the notched impact strength increased at first and then decreased with the increase of POE-g-GMA content. The blends containing 10 wt % POE-g-GMA showed the highest impact strength, which was 18.0-fold compared to that of neat PBT. The study of the toughening mechanism indicated that a suitable compatibility was significant for obtaining supertough PBT blends because the good elastomers dispersion and suitable interfacial adhesion can be obtained simultaneously. The weak interface and the large particle size led to unstable crack propagation, while too strong interfacial adhesion prevented interface debonding and arrested matrix shear yielding. Microvoiding generated by both the debonding and internal cavitation of elastomers followed by matrix shear yielding was the main toughening mechanism in toughened PBT blends.

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INTRODUCTION Poly(butylene terephthalate) (PBT) as one of the important engineering plastics possesses high mechanical strength, excellent electrical insulation, and dimensional stability, which make it widely used in the fields of automotive, electronics, and industrial parts.1−3 However, the notched impact resistance of PBT is poor. The common method to enhance the fracture toughness of PBT is melt blending with elastomers. It has been reported that the toughening effect of PBT depends on the phase morphology, elastomer content, interfacial adhesion, etc.3−5 Generally, PBT and elastomer are incompatible due to the difference of polarity. The poor interaction between phases results in poor dispersion of elastomer and unsatisfactory mechanical properties.6 Therefore, improving the compatibility and increasing interfacial adhesion between phases is crucial in enhancing fracture toughness of PBT. Generally, the way to enhance the interaction between PBT and elastomer is blending PBT with functionalized elastomers or adding compatibilizers.7−9 The functional groups of the modified elastomers are usually maleic anhydride and epoxy, because grafting reactions can occur between maleic anhydride or epoxy groups and the terminal carboxyl and hydroxyl groups of PBT during melt blending. The generated graft copolymer can effectively enhance interfacial adhesion and improve phase dispersion, and hence desirable properties are achieved.10−15 © 2019 American Chemical Society

For example, Aróstegu et al. introduced maleic anhydride (MAH) modified poly(ethylene-octene) copolymer (POE) as the impact modifier for PBT. They found that when the grafting ratio of the modified elastomers was 0.32%, the notched impact strength of PBT/POE-g-MAH (80/20) blends was 20-fold compared to that of neat PBT.16 Moreover, they reported that difunctional epoxy resin was an effective compatibilizer for PBT/POE blends. With only 1.0 wt % epoxy, the impact strength of PBT/POE (80/20) blends was increased 17-fold compared to that of neat PBT.17 Besides, other functionalized elastomers, such as styrene-(ethylene-cobutadiene)-styrene copolymer (SEBS),8,18 ethylene-propylene copolymer (EPR),19 and ethylene-propylene-diene copolymer (EPDM),20 were also used to toughen PBT. It is generally accepted that a compatibilization reaction can improve the interfacial adhesion between phases and reduce elastomer particle size. However, too strong interfacial adhesion and too small elastomer particle size are not always favorable for toughening. As we know, microvoiding is essential in an elastomer toughened polymer system, which can induce matrix shear yielding and absorb large impact Received: Revised: Accepted: Published: 12650

February 3, 2019 June 13, 2019 June 18, 2019 June 18, 2019 DOI: 10.1021/acs.iecr.9b00691 Ind. Eng. Chem. Res. 2019, 58, 12650−12663

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Industrial & Engineering Chemistry Research energy.21,22 Microvoiding can occur through matrix-elastomer debonding or internal cavitation of the elastomers.23−25 Too strong interfacial adhesion, on the one hand, may lead to very small elastomer particles. According to the literature, internal cavitation of elastomers cannot be produced if elastomer particles are too small. This is because shear yield stress is lower than critical stress for internal cavitation.26 For instance, Wang et al. employed epoxidized poly(styrene-b-butadiene-bstyrene) (ESBS) to toughen polylactide (PLA).27 They found that when the epoxy group content in ESBS was too high, the dispersed particles were too small to effectively toughen PLA. Similar results have also been reported in other research.28,29 On the other hand, too strong interfacial adhesion may prevent the interfacial debonding process which plays an important role in many toughening systems.30 For instance, Sue et al. reported that for polycarbonate/polyethylene (PC/PE) blends, debonding at the interface was the major toughening mechanism, which released triaxial stress around microvoids and caused matrix shear yielding.31 Therefore, in order to achieve supertoughness of polymer, the compatibilization should be kept within the optimal range. Although a great amount of research has reported melt blending with elastomers to enhance fracture toughness of PBT, very little work has thoroughly studied the influence of compatibilization and interfacial adhesion on morphology and mechanical properties, especially toughness of PBT/elastomer blends. In this study, poly(ethylene-octene) copolymer (POE) and glycidyl methacrylate grafted POE (POE-g-GMA) were employed as the impact modifiers for PBT. The weight ratio of PBT/elastomer was kept at 80/20, and POE-g-GMA concentration in elastomer phase was changed to regulate the interaction between PBT matrix and elastomer phase. The relation between compatibility, phase morphology, and fracture toughness of different samples was studied. Meanwhile, toughening mechanisms of the blends with different interfacial adhesions were comprehensively analyzed through differential scanning calorimetry (DSC), instrumented impact tests, the essential work of fracture (EWF), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) tests.

Table 1. Composition of Different Samples sample

PBT (wt %)

POE (wt %)

POE-g-GMA (wt %)

B0 B3 B6 B10 B15 B20

80 80 80 80 80 80

20 17 14 10 5 0

0 3 6 10 15 20

Ulm, Germany) at room temperature. The tensile speed was 20 mm/min. Izod notched impact experiments were carried out using an instrumented impact tester (9050, CEΛST, Italy) at room temperature with 5.5 J of the impact hammer. The impact force-time (F-t) and impact force-deflection (F-f) curves can be obtained by the test machine. The total impact energy (Et) was obtained by integrating the area under the F-f diagram. In principle, Et consists of initiation energy (Ei) and propagation energy (Ep). Ei is the energy integrated into the area from beginning to the maximum impact force (Fmax), and Ep corresponds to the energy from Fmax to complete the fracture of the sample.32,33 The bar specimens with a V notch of 2 mm were made according to GB 1843-2008. The mean results of five tested samples were calculated. The essential work of fracture (EWF) tests were carried out using a universal testing machine (Zwick/Roell Z020, Zwick, Ulm, Germany) at room temperature. The crosshead speed was 5 mm/min. The double-edged-notched tension (DENT) specimens were prepared according to the following steps: (i) the extruded samples were compression molded into boards with a thickness of 0.5 mm, (ii) specimens of 100 mm × 35 mm were cut, and (iii) the notching of the specimens was made by fresh razor blades. For each sample, ligament lengths of the specimens were 3−11 mm. In the EWF procedure, the total fracture work (Wf) was obtained by integrating the load− displacement diagram area and is split into the essential work of fracture (We) and the nonessential work (Wp).34−36 We is the measure of fracture work consumed for the inner fracture and surface-related. Wp corresponds to fracture work dissipated for the outer plastic deformation and volume-related. Therefore, Wf is represented as

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EXPERIMENTAL SECTION Materials. PBT (S600F10) was provided by DuPont (USA), and POE (Engage EG8200) with octene contents about 24 wt % was from DuPont-Dow (USA). POE-g-GMA with GMA contents about 1.5 wt % was provided by Fineblend Compatilizer Co., Ltd. (China). Xylene was from Sinopharm Chemical Reagent Co., Ltd. (China). Sample Preparation. Prior to blending, PBT and elastomer were dried in a vacuum oven at 120 and 60 °C, respectively. The samples were melt blended using a twin screw extruder (HAAKE Polylab OS, Thermo Electron GmbH, Germany). The temperature of the barrel was 230− 250 °C, and the screw speed was 300 rpm. Pure PBT was extruded with the same procedure. The extruded pellets after drying were molded to prepare standard specimens using an injection molding machine (PNX40III-2A, NESSI Plastic Industrial Co. Ltd., Japan). The barrel temperature was 230−255 °C. The weight ratio of PBT/elastomer was kept at 80/20. Throughout the article the blends were defined as Bx (Table 1), where x represented POE-g-GMA content. Mechanical Properties. The tensile properties were tested using a universal testing machine (Zwick/Roell Z020, Zwick,

Wf = We + Wp = wetl + βwptl 2

(1)

Wf = we + βwpl (2) tl where wf, we, and wp are the specific fracture work, specific essential work, and specific nonessential work, respectively, t is the specimen thickness, l is the ligament length, and β is the shape factor of the plastic deformation zone. According to eq 2, we (the y-intercept) and wp (the slope) can be given by plotting wf versus l. Morphology Characterization. The morphology of PBT blends was characterized by scanning electron microscopy (SEM) (SU-3500, Hitachi, Japan). Before SEM observation, the cryo-fractured surfaces were etched with xylene at 75 °C for 2 h to remove the free elastomer phase. The numberaverage particle size (dn) and volume-average particle size (dv) of dispersed phase were calculated using an image analysis software (Image-Pro PLUS) from a minimum of 500 particles according to eqs 3 and 4 Wf =

dn = 12651

∑ n i di / ∑ n i

(3) DOI: 10.1021/acs.iecr.9b00691 Ind. Eng. Chem. Res. 2019, 58, 12650−12663

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Industrial & Engineering Chemistry Research dv =

∑ nidi4/∑ nidi3

Thermal Analysis. Differential scanning calorimeter (DSC) tests were performed using a differential scanning calorimeter (TA, Q200, USA). First, samples (about 6−8 mg) were heated from room temperature to 260 °C at a rate of 40 °C/min and held for 5 min to eliminate heat history. Second, the samples were cooled down to 0 °C at a rate of 20 °C/min and held for 5 min. Last, the samples were reheated up to 260 °C at a rate of 20 °C/min. According to eq 6, crystallinity (Xc) of PBT in the sample was calculated

(4)

where ni is the number of particles with size di. The impact fractured surfaces were characterized by SEM after gold coating. The deformation behavior was observed using a transmission electron microscope (TEM) (JEM-1200EX, JEOL, Japan). The samples with a thickness of about 100 nm were cut from the stress-whitening zone using a cryoultramicrotome and then stained with ruthenium tetroxide (RuO4). Infrared Spectroscopy. Fourier transform infrared (FTIR) spectra were obtained with a spectrometer (Nicolet5700, Thermo Scientific). Samples were compression molded into films at 250 °C and tested after drying. Soxhlet Extractor Method. The samples (about 2.0 g) were dried in a vacuum oven and weighed as m1. Then dried samples were extracted with boiled xylene and m-cresol sequentially using a Soxhlet extractor for 24 h to remove the un-cross-linked elastomer and PBT, respectively. The extracted samples were washed, dried, and then reweighed as m2. The cross-linked material content (CMC) was calculated according to eq 5: m CMC = 2 × 100% m1 (5)

Xc =

PBT B0 B3 B6 B10 B15 B20

47.5 29.8 30.5 31.5 31.9 30.8 32.4

± ± ± ± ± ± ±

1.2 0.3 0.4 0.2 0.4 0.3 0.5

εb (%) 248.0 44.4 130.0 157.2 190.8 153.8 102.3

± ± ± ± ± ± ±

7.2 6.5 5.8 4.4 8.4 6.1 5.2

± ± ± ± ± ± ±

(6)

RESULTS AND DISCUSSION Mechanical Properties. Table 2 summarizes mechanical properties of different samples, and Figure 1 shows the stress− strain curves and notched impact strength. Pure PBT showed a pronounced yield region followed by necking, which was the typical characteristic of a pseudoductile semicrystalline polymer. Pure PBT showed a high tensile strength (σt). As can be seen, with the addition of elastomer, the σt value of all blends decreased compared to that of neat PBT, because of elastomeric characteristics.16,30 It seemed that the σt of all blends was not significantly influenced by the POE/POE-gGMA ratio. These results were similar to other compatibilized polymer blends.16,17,34 In the case of elongation at break (εb), it was very low in B0 blends despite the rubbery nature of POE, which was due to the very weak interaction between PBT and POE. The addition of POE-g-GMA clearly increased the εb value of blends compared to that of B0 blends, reflecting that the interaction between PBT matrix and elastomer phase was enhanced by POE-g-GMA. However, larger POE-g-GMA loadings (>10 wt %) resulted in a decrease in the εb. This result was similar to other compatibilized polymer blends.16,30,38 The impact strength of different samples was depicted in Figure 1(b). Neat PBT has the impact strength of only 2.2 kJ/ m2, while the impact strength of B0 blends just rose to 4.4 kJ/ m2. The result showed that with poor interaction, the enhancement of impact resistance was not obvious. The

notched impact strength (kJ/m2) 2.2 4.4 11.0 34.2 39.7 30.5 18.0

× 100%

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Table 2. Mechanical Properties of Different Materials σt (MPa)

wf ΔHm0

where ΔHm was the melting enthalpy, wf was the weight fraction of PBT in the sample, and ΔH0m was the reference ΔHm (142 J/g) for 100% crystalline PBT.37

Rheological Measurement. The rheological characterization was tested on a rotational rheometer (RS6000, HAAKE, Germany) at 240 °C in parallel plate mode. The pellets were compression molded into disks with a diameter of 25 mm and a thickness of 1 mm. Strain amplitude of 1% was used so that the rheological behavior was in the linear viscoelastic region. The scanning frequency (ω) was from 0.1 to 100 rad/s.

sample

ΔHm

0.3 0.1 0.3 0.7 1.4 1.2 1.7

Figure 1. Mechanical properties of different samples: (a) stress−strain curves and (b) Izod notched impact strength. 12652

DOI: 10.1021/acs.iecr.9b00691 Ind. Eng. Chem. Res. 2019, 58, 12650−12663

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Industrial & Engineering Chemistry Research

Figure 2. SEM pictures of the cryo-fractured surfaces of different samples: (a) B0, (b) B3, (c) B6, (d) B10, (e) B15, and (f) B20.

Table 3. dn and dv of Dispersed Phase in PBT Blends sample name

dn (μm)

dv (μm)

B0 B3 B6 B10 B15 B20

2.17 1.34 0.85 0.69 0.66 0.62

3.03 1.70 1.06 0.89 0.88 0.85

impact strength of B3 blends was enhanced to 11.0 kJ/m2. POE-g-GMA effectively improved impact resistance of PBT blends indicating, as in the case of εb, compatibilization. As more POE-g-GMA was introduced, it was interesting to find that the impact strength of samples increased first and then decreased. B10 blends possessed the highest impact strength, 39.8 kJ/m2, which was 18.0-fold compared to that of neat PBT, while B20 blends revealed an impact strength of only 18.0 kJ/ m2. It should be noticed that the test specimens of B10 blends were not fully broken after impact tests, as shown in Figure 1(b). Obviously, the ratio of POE/POE-g-GMA has a significant influence on toughness of blends. Morphology. The micrographs of cryo-fractured surfaces were shown in Figure 2, and dispersed particle size was listed in Table 3. As can be seen, all blends displayed a typical sea− island morphology with the dispersed elastomer spherical

Figure 3. Infrared spectra of PBT, B0, and B20 blends.

droplets distributed in the PBT matrix. The elastomer particle diameter of B0 blends was the largest, and the distribution was broad, which was due to poor compatibility of PBT and POE. After addition of POE-g-GMA, the morphology of PBT blends changed significantly. The dispersed particle became small and distributed uniformly. According to previous research, in polyester/epoxidized polymer blends, the graft reaction may occur between the epoxidized polymer and PBT during melt blending, leading to enhanced interfacial adhesion.5,39,40 In 12653


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(3.01%), indicating more cross-linked structures in B20 blends. With high content of POE-g-GMA in B10, B15, and B20 blends, both strong interaction between PBT and elastomer and cross-linking reactions occurred simultaneously. In this case, the diffusion of the graft copolymer formed by the compatibilization reaction was limited, and the increased effect of suppressing the coalescence of dispersed phase was not as remarkable as before. Rheological Properties. Rheology is a powerful tool in the study of polymer blends because rheological tests can not only study the processability of materials but also effectively characterize the internal structure of the sample.42 The complex viscosities (η*) of different samples were exhibited in Figure 4. Compared with neat PBT, the η* of all blends were higher and rose with increasing POE-g-GMA loadings. This was caused by intense interaction between PBT and POEg-GMA. The strong interaction formed a graft copolymer between phases that enhanced interfacial adhesion. This result was similar to other systems and showed that phase interaction could increase the melt viscosity.42−44 The storage modulus (G′) and loss modulus (G″) of different samples were revealed in Figure 5. Pure PBT complied with the linear viscoelasticity models at low ω, that is, G′ ∝ ω2 and G″ ∝ ω, whereas the blends did not.45,46 G′ and G″ of blends were affected by elastomer and rose with increasing POE-g-GMA loadings. The deviation from the linear viscoelasticity models and higher absolute values of G′ were considered to be a result of the generation of entangled structures in the blends.47−49 Since the molecular chain of POE was flexible and easily entangled, reversible elastic deformation of blends increased. Moreover, G′ increased with increasing POE-g-GMA loadings, because more graft copolymer and cross-linked material generated resulting in decreased mobility of the molecular chains. From the increase of η*and G′, it can be inferred that the reaction of PBT and POE-g-GMA was unsaturated. Toughening Mechanism. As we know, crystal structure and crystallinity of polymer have a great influence on mechanical properties of crystalline polymers.16 So crystal structure and crystallinity of PBT were investigated. The DSC results of the cooling scan and the second heating scan were revealed in Figure 6, and the melting temperature (Tm), melting enthalpy (ΔHm), crystallization temperature (Tc), crystallization enthalpy (ΔHc), and percent crystallinity (Xc) were listed in Table 4. It was observed from Figure 6(b) that

Figure 4. Complex viscosities (η*) of different samples.

order to confirm the interaction between PBT and POE-gGMA, the infrared measurement was performed. Figure 3 revealed the FTIR spectra of PBT, B0, and B20 blends. The characteristic peaks of PBT in pure PBT and B0 blends appeared at 1710 cm−1 (stretching of the CO) and 1267 and 1099 cm−1 (stretching of the C−O). The slight shift of these bands observed in B20 blends could be indicative of the interactions between PBT and POE-g-GMA. It was found that elastomer particle size further decreased in B6 blends. Since the number of epoxy functional groups was increased, the interaction between PBT and POE-g-GMA became stronger resulting in smaller interfacial tension. However, elastomer particle size decreased slightly at higher loadings of POE-gGMA. This was probably due to the cross-linking reactions and the reduction of diffusion of the interfacial graft copolymer. According to previous research, in addition to compatibilization reactions, cross-linking reactions may also occur in polyester/epoxy functionalized polymer blends.5,19,41 The Soxhlet extractor method was used to determine the formation of cross-linked material. As POE and POE-g-GMA were soluble in boiled xylene and m-cresol dissolved PBT rapidly, all the un-cross-linked elastomers and PBT could be extracted. For B3 and B6 blends, there was no insoluble fraction after the Soxhlet extractor experiment, indicating that cross-linking reactions did not occur in these systems. However, for B10, B15, and B20 blends, there existed an insoluble fraction. Moreover, the insoluble fraction content of B20 blends (4.75%) was higher than that of B10 (1.88%) and B15 blends

Figure 5. Storage modulus (G′) and loss modulus (G″) of different samples. 12654


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Figure 6. DSC curves of different samples: (a) cooling scan and (b) second heating scan.

Table 4. Thermal Properties of Different Samples during Cooling Scan and Second Heating Scan sample Tm1 (°C) Tm2 (°C) ΔHm (J/g) PBT B0 B3 B6 B10 B15 B20

214.8 214.6 214.4 215.0 214.4 214.4 214.3

222.6 223.0 223.2 223.4 223.0 222.8 223.0

45.7 36.7 32.9 33.2 33.7 32.8 34.3

Tc (°C) ΔHc (J/g) 192.5 190.2 189.9 190.3 189.7 190.6 189.8

47.6 46.0 43.8 47.8 44.1 43.2 47.5

Xc (%) 32.2 32.3 29.4 29.2 30.0 28.9 30.2

all samples exhibited two melting peaks, which was a characteristic of crystalline polyesters. This was because the degree of perfection of crystal in samples is different. The imperfect crystals melted at low temperature due to the poor stability and then recrystallized to more perfect crystals. Finally, all more perfect crystals melted at high temperature.50−52 The crystallinity for PBT/POE/POE-g-GMA blends was around 30%, and Tm showed no visible change, which suggested that crystallinity of PBT was not significantly affected by POE or POE-g-GMA. This result was similar to other PBT/elastomer blends and semicrystalline matrix/ elastomer blends.3,16,21,38 Therefore, the different toughness of the materials was not dependent on the change of crystallinity of PBT. As we know, fracture toughness of elastomer toughened polymers strongly depends on elastomer particles size.16 The Izod notched impact strength and the elastomer volumeaverage particle size (dv) of different samples were shown in Figure 7. With the introduction of POE-g-GMA, dv decreased significantly, and impact strength was enhanced. When POE-gGMA loadings were 10 wt %, dv reduced to 0.76 μm, and notched impact strength was maximized. Generally, there are optimal elastomer particle sizes for effective toughening. The use of POE-g-GMA resulted in reduced elastomer particle size, which was good for improving the impact strength. At higher POE-g-GMA loadings, dv changed slightly, but the impact strength significantly decreased. Therefore, it can be reasonably inferred that in B10, B15, and B20 blends, the different impact strength was not due to the influence of elastomer particle size. To further analyze fracture mechanisms, the results of instrumented impact tests were used to quantitatively evaluate crack initiation and crack propagation behaviors of PBT blends. Figure 8 exhibited the force-time (F-t) diagrams of different samples. From F-t curves, maximum impact force (Fmax ), total energy (Et ), initiation energy (E i ), and

Figure 7. Volume-average particle size and impact strength of different samples.

propagation energy (Ep) were obtained.32,33 If a polymer system can obtain high Ei and Ep values, implying a good effect of preventing crack initiation and crack propagation, then a toughened polymer may be obtained. As seen, pure PBT and B0 blends exhibited a brittle fracture behavior. After the impact force reached Fmax, it rapidly dropped to zero. After the incorporation of POE-g-GMA, the fracture behavior of samples changed significantly, and the impact force slowly dropped to zero. Figure 9 showed Ei, Ep, and the impact strength of different samples. Both Ei and Ep were very low in PBT and B0 blends, indicating these samples had poor ability to inhibit crack initiation and crack propagation. As a result, the impact strength of PBT and B0 blends was very low. The Ei of the blends increased at first and then was maintained at about 0.2 J with increasing POE-g-GMA loadings. It was observed that the tendency of Ep and the impact strength of samples exhibited high similarity. The value of Ep increased significantly with increasing POE-g-GMA loadings to 10 wt %, implying better ability in resisting crack propagation and greatly enhanced plastic deformation. However, as more POE-g-GMA was added, the Ep decreased which corresponded well with the decreased impact strength. The fracture characteristic of different samples was further evaluated via EWF tests. Figure 10 exhibited the load− displacement curves of different samples. As can be seen, all load−displacement curves with different ligament lengths of each sample showed good self-similarity, which is a necessary prerequisite for the EWF approach.34 Besides, the Hill’s criterion is required to ensure the fracture process in plane stress conditions.34 According to Hill’s theory, the maximum net section stress (σn = Fn/lt) for each ligament length is 12655


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Figure 8. Typical instrumented impact force-time curves of different materials: (a) PBT, (b) B0, (c) B3, (d) B6, (e) B10, and (f) B20.

almost constant (1.15σy) and independent of ligament length, where Fn is the maximum force of the load−displacement curves with different ligament lengths and σy represents tensile yield stress of sample. As shown in Figure 11, the σn values for all samples remained nearly constant, indicating that all fracture tests met the requirements of the EWF method. The specific total fracture work (wf) was plotted versus l as exhibited in Figure 12, and then the y-intercept and slope represented the value of specific essential work (we) and specific nonessential work (βwp), respectively. According to previous research, we is an inherent property of the material unaffected by specimen shape and reflects crack resistance, while βwp is a geometry related value corresponding to work consumed for the outer plastic zone. Thus, the values of we can

Figure 9. Instrumented impact results of different samples. 12656


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Figure 10. Load−displacement curves in EWF tests.

impact test. This suggested a better effect of crack resistance of B10 blends. Moreover, it can be observed that stress whitening at the tip of crack was more intense in B10 blends (Figure 13), which was caused by shear yielding and voiding at this zone. Therefore, the fracture toughness of B10 blends was significantly increased. To further study the fracture behavior, the impact fracture surface morphology of PBT and PBT blends was identified. Figure 14 exhibited the morphology of the representative samples obtained at relatively low magnifications. Pure PBT exhibited a brittle fracture behavior, and the impact fractured surface was smooth without visible plastic deformation, which corresponded to the low impact strength. There was also no sign of plastic deformation for B0 blends. However, with the introduction of POE-g-GMA, coarse surface morphology and

Table 5. EWF Fracture Parameters (we) of PBT and PBT Blends sample name PBT B0 B3 B6 B10 B15 B20

we (kJ/m2) 2.05 4.54 8.13 10.05 15.35 9.42 8.31

± ± ± ± ± ± ±

0.03 0.18 0.27 0.31 0.32 0.28 0.11

represent the inherent material toughness and were summarized in Table 5. As seen, B10 blends possessed the highest we, which corresponded with the results of the instrumented 12657


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Figure 11. Plots of σn versus l of different samples.

unstable crack propagation in B0 blends thus reduced fracture resistance. For B3 blends, there were not only empty holes but also obvious plastic deformation, indicating that matrix shear yielding occurred. The plastic deformation became more intensive in B10 blends. The formation of this surface dissipated large impact energy, thereby super toughness was obtained.5 However, for B20 blends, a more smooth fracture and slight plastic deformation were observed, corresponding to the decreased impact strength.

intensive plastic deformation were observed. The fractured surface of samples was further observed at relatively high magnifications as shown in Figure 15. It should be noted that all micrographs were taken from the early section of the crack propagation zone of the impact fractured surface, as is shown in Figure 14a. Different from the smooth impact fractured surface of pure PBT, POE and PBT were debonding in B0 blends, and no sign of plastic deformation could be seen. The large dispersed particles and weak interfacial adhesion initiated 12658


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Figure 12. Plots of wf versus l of different samples.

but also by the debonding at the interface. In comparison, in B20 blends, microvoids were only observed inside the dispersed phase. This is because the interfacial adhesion was very high in B20 blends, and the possibility of interfacial debonding was very low. Combining the results of the instrumented impact test, the EWF test, and the impact fracture surface, it can be concluded that a suitable compatibility was important for effective toughening. In the uncompatibilized PBT/POE blends, unfavorable interaction led to large dispersed particles and weak interfacial adhesion, which caused unstable crack propagation. When present in small amounts, POE-g-GMA

To further investigate internal deformation behavior of stress-whitening zones, B10 and B20 blends were selected for TEM analysis. In an elastomer toughened polymer system, microvoiding plays an important role which activates matrix shear yielding and absorbs large impact energy.21 Microvoiding can occur by matrix-elastomer debonding or internal cavitation of elastomers, and TEM analysis is the most reliable method for characterizing cavitation and debonding.22 Therefore, TEM was introduced to observe the stress-whitening zones. As exhibited in Figure 16, the deformations in B10 and B20 blends were different. The microvoids in B10 blends were formed not only by the internal cavitation of elastomers phase 12659


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interfacial adhesion, which might completely prevent interfacial voiding, thus matrix shear yielding was restricted.

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CONCLUSION Supertough PBT/POE/POE-g-GMA blends were obtained by melt blending through regulating interfacial adhesion, and the toughening mechanism was comprehensively analyzed. Strong interactions between PBT and POE-g-GMA occurred during melting. With an increase in POE-g-GMA loadings, the interaction between phases increased gradually, the dispersed particle size decreased significantly at first and then decreased slightly, while the impact strength of samples increased at first and then decreased. The blends containing 10 wt % POE-gGMA showed the highest impact strength. The supertoughness in PBT blends was not dependent on the crystallization of PBT. The analysis of the instrumented impact test, the EWF test, and the morphology of impact fractured surfaces and stress-whitening zones indicated that suitable compatibility was significant for obtaining supertough PBT blends because the good elastomers dispersion and suitable interfacial adhesion can be obtained simultaneously. When both the debonding at the matrix/elastomer interface and the internal cavitation were realized to produce microvoids, the fracture impact energy was

Figure 13. Photographs of the EWF tested samples.

provided sufficient interfacial adhesion to obtain the good dispersion of elastomer and promote the massive matrix shear yielding. The presence of excess POE-g-GMA led to too strong

Figure 14. SEM pictures of impact fractured surfaces of different samples at low magnifications: (a) PBT, (b) B0, (c) B3, (d) B6, (e) B10, and (f) B20. 12660


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Figure 15. SEM pictures of impact fractured surfaces of different samples at high magnifications: (a) PBT, (b) B0, (c) B3, (d) B6, (e) B10, and (f) B20.

Figure 16. TEM micrographs of the stress-whitened zones of PBT/POE/POE-g-GMA blends: (a) B10 and (b) B20.

dissipated the most, and then the highest impact strength was

ORCID

obtained.

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Baoqing Shentu: 0000-0002-5684-7468

AUTHOR INFORMATION

Notes

Corresponding Author

*Phone/Fax: +86-0571-87951612. E-mail: [email protected].

The authors declare no competing financial interest. 12661


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ACKNOWLEDGMENTS This study was supported by the Natural Science Foundation of China (20974100). We thank Jijiang Hu, Qun Pu, Na Zheng, Li Xu, Sudan Shen, and Jing He for their assistance in performing twin-screw extrude/mechanical properties tests/ SEM/DSC/TEM/rheological measurements, etc. analyses at the State Key Laboratory of Chemical Engineering (Zhejiang University).

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POE-g-GMA Elastomer through

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