Effect of Poly(acrylic acid)-Modified Poly(ethylene terephthalate) on

Apr 17, 2015 - The preparation of supertoughening poly(ethylene terephthalate) (PET) blends has always been a practical and valuable task. In our work...
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Effect of Poly(acrylic acid)-Modified Poly(ethylene terephthalate) on Improving the Integrated Mechanical Properties of Poly(ethylene terephthalate)/Elastomer Blend Lizhao Xie,† Yunyun Xie,† Qianghua Wu,*,† Mozhen Wang,† Qichao Wu,‡ Xiao Zhou,‡ and Xuewu Ge*,† †

CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China ‡ Guangdong Tianan New Material Co., Ltd., Foshan, Guangdong 528000, P. R. China ABSTRACT: The preparation of supertoughening poly(ethylene terephthalate) (PET) blends has always been a practical and valuable task. In our work, PET resins grafted with poly(acrylic acid) (PAA), termed as PET-g-PAA, were first prepared through γ-ray radiation induced graft polymerization and blended in a partially miscible PET/ethylene-methyl acrylate-glycidyl methacrylate random terpolymer (ST2000) system as the compatibilizer. The impact strength of the PET blends achieves the maximum at a 6 wt % of PET-g-PAA, but without the loss of tensile strength. Furthermore, much less of ST2000 is needed for the blends to possess high impact strength at the existence of PET-g-PAA. The SEM morphological analysis of the impactfracture surface implies a good interfacial adhesion between ST2000 and PET matrix, which should be ascribed to the effective compatibilization by the in situ formed PET-g-PAA/ST2000 graft copolymer through the reaction between the COOH groups and epoxy groups on ST2000.

1. INTRODUCTION As one of the most important synthetic polymer materials in current world, poly(ethylene terephthalate) (PET) has been applied in a wide range of areas including textile fabric, package, biomaterials, and so forth due to its good mechanical strength, thermostability, and radiation resistance.1−4 Unfortunately, the raw PET resin has two serious shortcomings, that is, poor impact resistance and low crystallization rate,5,6 which limits its application as an engineering plastic.7 Therefore, the research and development on the fabrication of PET engineering plastic through blending PET with various toughening agents is very necessary, practical and valuable.8 However, PET is thermodynamically immiscible with most toughening agents currently used in research works or practical applications due to the inner chemical structure feature, resulting in the poor dispersibility of the toughening agent phase in PET matrix. As a consequence, an unsatisfactory toughening effect often appears whatever the traditional elastomers,9 or some relatively rigid materials, such as polyolefin,10 polyester,11 polyamide,12 and inorganic fillers,13 have been used as the toughening agent. To enhance the interaction between PET matrix and the toughening agent, reactive blending between PET and the toughening agent modified with functional groups has been widely used in some research works.14,15 Coltelli et al. found that the transesterification reaction between poly(ethylene terephthalate) (PET) and dibutyl succinate functionalized polyethylene (POF) catalyzed by different Zn and Ti catalysts had great influence on the phase distribution of POF in PET matrix because all the metal derivatives resulted effective in determining the formation of not negligible amount of compatibilizing graft copolymer.15 However, the most common strategy to improve the compatibility between PET and immiscible additives is adding a block or graft copolymer, © 2015 American Chemical Society

namely compatibilizer, in which there are two parts of molecular chains, respectively, identical or miscible with PET and the toughening agents. Leong et al.16 investigated the compatibilization effect of styrene−ethylene−butadiene−styrene-based terpolymer on the recycled poly(ethylene terephthalate) (RPET) and polypropylene (PP) blends. The results show that the unnotched impact strength of RPET/PP (90/10) blends would increase 1.3 times at the existence of 1 phr of the compatibilizer, whereas the Young’s modulus decreases about 1.5%. But in fact, the effect of copolymer compatibilizer is often limited because the synthesis of a block or graft PET copolymers is difficult due to the special synthesis method and chemical stability of PET molecule chain.17 At the same time, it is also difficult to find an existing copolymer containing the segments which are actually miscible with PET molecular chains. On the other hand, some research work focused on introducing toughened PET blends with a substance which is miscible with the toughening agent but has some additional active groups to react with the terminal carboxyl or hydroxyl groups on PET molecular chains during the preparation process. The in-situ-produced copolymers are located at the phase interface, and act as a compatibilizer to improve the compatibility of PET and the toughening agent.18 For example, three maleic anhydride-grafted polypropylene (PP) derivatives were prepared by melt grafting and utilized to compatibilize PET/PP blends by Pang et al.19 The results showed the compatibilizing effects of the three PP grafts were very different and strongly dependent on the functional groups present. The impact strength of PET/PP (75/20) blend was improved two Received: Revised: Accepted: Published: 4748

January 7, 2015 March 29, 2015 April 17, 2015 April 17, 2015 DOI: 10.1021/acs.iecr.5b00091 Ind. Eng. Chem. Res. 2015, 54, 4748−4755

Article

Industrial & Engineering Chemistry Research

homopolymer PAA and unreacted AA monomers. The product was dried in an oven at 90 °C until a constant weight. 2.3. Preparation of Ternary PET/PET-g-PAA/ST2000 Blends. Prior to the melt blending process, the raw PET and PET-g-PAA granules were dried at 90 °C for 24 h, and ST2000 was dried at 50 °C for 12 h. Then, the three ingredients were mixed at specific ratios in a corotating twin-screw extruder (TE35, China) with a screw diameter of 35 mm and an overall L/D of 37. The feed rate was 300 rpm. The temperatures of the first to seventh regions were set as 140, 200, 260, 260, 260, 260, and 260 °C. The die temperature was also 260 °C. The screw speed was 200 rpm. The extrudate was cooled in water and then pelletized. 2.4. Characterization. The grafting degree of AA monomers (GAA) on PET was measured by weight method using the following equation:

times after the addition of 5 phr of PP graft copolymer containing hydroxyl groups, which can react with carboxyl end groups of PET or undergo transesterification with PET main chains during blending. But the terminal groups on PET molecular chains are very few,20 resulting in a limited improvement on the mechanical properties of PET. Therefore, it is significant to search a new way to in situ synthesize PET block or graft compolymers as the compatibilizer to obtain PET blends with high mechanical performance. In our previous work, 60Co γ-ray radiation has been utilized to conveniently and effectively induce the grafting copolymerization of various vinyl monomers, such as acrylic acid,21 butyl acrylate,22,23 and methyl acrylate24 on PET film or resins. This grafting method needs no chemical initiators and can be operated at room temperature. Further studies showed that the toughness of PET/ethylene-methyl acrylate-glycidyl methacrylate random terpolymer elastomer (ST2000) blend would be improved to a certain degree at the existence of the prepared PET resins grafted with poly(methyl acrylate) (PMA), although there was no chemical interactions between PMA and ST2000 except the common nonpolar interactions. This encouraging result opens a new direction to obtain high-performance PET blends. Therefore, in this paper, we first modified PET resins by grafting poly(acrylic acid) (PET-g-PAA) induced by γ-ray radiation since the reactivity of carboxylic terminal groups with epoxy groups has been proved in many literatures.25−27 The effect of PET-g-PAA on the phase interfacial compatibility of PET/ST2000 blend was then investigated in detail by the fracture morphological analysis. The impact strength of the PET blends achieves the maximum at a 6 wt % of PET-g-PAA, but without the loss of tensile strength. The toughness mechanism was also discussed with the microvoiding and plastic deformation theories.

GAA =

Wg − W0 W0

× 100%

(1)

where W0 and Wg are the weights of PET granules before and after being grafted with PAA induced by γ-ray radiation, respectively. The attenuated total reflectance Fourier-transform infrared (ATR−FTIR) spectra of PET samples were recorded on a Nicolet-8700 infrared spectrometer (Thermo Scientific Instrument Co. U.S.A) and then scanned 32 times at a resolution of 1 cm−1. The samples were prepared by pressing the raw PET or as-synthesized PET-g-PAA granules under the pressure of 5 MPa. 1 H nuclear magnetic resonance (1H NMR) spectra were measured on Bruker ADVANCE 300-MHz NMR instrument with CF3COOH and CDCl3 as the solvents and tetramethylsilane (TMS) as the internal reference. The static contact angles (CA) of water and glycidyl methacrylate (GMA) on PET and PET-g-PAA films were measured on Contact Angle Meter SL200B (Solon Tech. Co., Ltd.). A droplet of water or GMA (2 μL) was injected onto the film. The mean value of the CAs recorded at three different points on each sample was taken as the final result. The PET and PET-g-PAA films were prepared, respectively, by meltpressing 10 mg of raw PET and as-synthesized PET-g-PAA granules between two cover glasses on a hot stage at 260 °C. To test the mechanical properties of raw PET and the asprepared PET blends, the samples were injection-molded into the standard specimens for the tensile and notched Izod impact strength measurement using an injection-molding machine (HTF80 × 1, China) after being dried at 90 °C for 24 h. The temperatures of the first to sixth regions were set as 265, 260, 260, 260, 255, and 25 °C, respectively. The injection pressure was 80 MPa. The screw rate was 20 rpm. The retention time was 35 s. The notched Izod impact strengths of all samples were tested on a Memory Impact Test machine (JJ-20, Intelligent Instrument Equipment Co., Ltd.) at room temperature according to GB/T 1843-2008 (ISO 180:2000). The tensile properties of all samples were conducted on an Electronic Universal Testing machine (WSM-20KB, Intelligent Equipment Co., Ltd.) at room temperature. The dumbbellshaped specimens were stretched until they broke at a crosshead rate of 50 mm·min−1 according to GB/T1040.22006 (ISO 527-2:1993). A minimum of five tensile and impact specimens were tested for each reported value.

2. EXPERIMENTAL SECTION 2.1. Materials. PET resin (CB651) ([η] = 0.75 dL/g) and ethylene-methyl acrylate-glycidyl methacrylate random terpolymer (ST2000, 1.5 wt % of glycidyl methacrylate) were purchased from Far Eastern Industry (Shanghai, China) and Shanghai Xiuhu Chemical Co., Ltd., respectively. Acrylic acid (AA) was provided by Shanghai Chemical Reagents Co., China, and refined through vacuum distillation to remove the inhibitor before use. Analytical grade ferrous sulfate hydrate (FeSO4· 7H2O) and acetone were all purchased from Shanghai Chemical Reagents Co., China, and used as received. The distilled water was used in all experiments. 2.2. Preparation of PET Resins Grafted with Poly(acrylic acid) (PET-g-PAA) by γ-ray Radiation Induced Graft Polymerization. The raw PET resin granules were first washed with acetone and then dried in an oven at 90 °C for 24 h. The dried granules (200 g) were immersed in 500 mL of an aqueous solution containing 35 wt % of AA and 0.35 wt % of FeSO4. After being bubbled with nitrogen for 10 min to remove the dissolved oxygen, the system was sealed and stood for 24 h to let PET granules be swollen by AA monomers as far as possible. Then the system was irradiated by 60Co γ-ray radiation at a dose rate of 18.2 Gy/min and an absorbed dose of 25 kGy. The 60Co source with a radioactivity of 2 × 1014 Bq is located in University of Science and Technology of China. The temperature was controlled at 25 °C by a surround circulating water system. Finally, the treated PET granules were taken out, and washed with hot water for five times to remove the 4749

DOI: 10.1021/acs.iecr.5b00091 Ind. Eng. Chem. Res. 2015, 54, 4748−4755

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Industrial & Engineering Chemistry Research The dispersibility of ST2000 in the PET matrix before and after the impact test was observed by field-emission scanning electron microscopy (SEM, JEOL JSM-6700, Japan, 5 kV). For observing the morphologies of the samples before impact test, the samples were fractured in liquid nitrogen. The fractured surfaces were etched by chloroform at 75 °C for 12 h to remove the elastomer phase. The samples were then dried in a vacuum oven at 50 °C for 12 h and sputter coated with gold. The average diameter of the dispersed phase (d̅) and the standard deviations (σ) were calculated by the following equations with the diameters of at least 100 particles measured in the SEM images

d̅ =

σ=

∑i nidi ∑i ni 1 × ∑i ni

(2) Figure 1. ATR−FTIR spectra of raw PET, PET-g-PAA, and PET-gPAA resins treated with the aqueous solution of NaOH (0.1 M).

∑ (di − d ̅ )2 i

(3)

cm−1) has been enhanced due to the introduction of PAA chains. The change of static water contact angle (CA) displayed in Figure 2 can also reflect whether PET resins have been grafted

where ni is the number of ST2000 particles with a diameter of di. The crystallization behavior of raw PET and as-prepared ternary blends were investigated by differential scanning calorimetry (DSC, Shimadzu DSC-60) in N2 atmosphere. The sample (about 5−10 mg) cut from the tensile specimen was heated from 25 to 290 °C at a heating rate of 10 °C/min, and kept for 5 min. Subsequently, it was cooled to −70 °C at 50 °C/min. Both the heating and cooling scans were recorded. A POM (Olympus BX51, Japan) equipped with a CCD camera (Tucsen TCC-3.3N, China) and a hot stage was also used to observe the isothermal crystallization of PET and its blends. A sample of about 2 mg was heated to 270 °C, and pressed into a slice with a thickness of about 20 μm. The slice was kept at 270 °C for 5 min, and then quickly moved to another hot stage with the preset temperature (180 °C) for isothermal crystallization for 20 min.

Figure 2. Static contact angles of water and glycidyl methacrylate (GMA) on the PET and PET-g-PAA films.

3. RESULTS AND DISCUSSION 3.1. Characterization of PET-g-PAA Resins through γray Radiation Induced Graft Polymerization. The grafting of PAA on PET resin granules through γ-ray radiation was conveniently conducted based on our previous work on the graft polymerization of AA on PET films.23 However, the GAA of PET granules measured by weight method was about 0.25%, much less than that of PET films under the same conditions. It could be attributed to the much smaller specific surface area and the lower degree of swelling by monomers of PET granules compared with those of PET film. The successful grafting of PAA can be proved by the ATR−FTIR spectrum of PET-gPAA resins, as shown in Figure 1. Because the GAA is very low, it is difficult to detect the increased intensities for νOH and νCO caused by the grafted PAA. Therefore, to clearly show the existence of PAA, we also got the FTIR spectrum of the PET-gPAA samples after being treated with 0.1 M NaOH solution because the νCO will shift to lower energy after the COOH is deprotonated, giving rise to an asymmetric feature between 1540 and 1650 cm−1.28 It is clearly seen in Figure 1 that besides the normal characteristic absorbance of νCO in PAA and PET molecular chains at 1730 cm−1, a new absorbance located at 1575 cm−1 appears on the ATR−FTIR spectrum of PET-g-PAA resins treated by NaOH. It should be induced by the  COO−Na+ groups on PAA chains. In addition, it is also can be seen that the intensity of νCH of CH2 groups (2950

with PAA. The film made by raw PET granules has a hydrophilic surface with a CA of 65°. In contrast, the CA on the film made by PET-g-PAA granules decreases to 54°, indicating an improved hydrophilicity caused by the grafting of hydrophilic PAA chains. Further, the contact angle of glycidyl methacrylate (GMA) was also shown in Figure 2. It is clearly seen that GMA can spread evenly on the surface of PET-g-PAA, which implies that PET-g-PAA has better affinity with ST2000 compared with PET because there is 1.5 wt % of GMA in ST2000. 3.2. Effect of PET-g-PAA on the Morphologies and Mechanical Properties of PET/PET-g-PAA/ST2000 Blends. Previous research shows that the COOH groups will react with epoxy groups during the high-temperature melt blending process.25−27,29 As shown in Figure 2, GMA has a very good affinity with PET-g-PAA, which favors the contact of  COOH and GMA groups. In order to prove whether these two groups react during the blending process, the 1H NMR spectrum of the PET/PET-g-PAA/ST2000 blend after being extracted with THF at 50 °C for 24 h was performed, as shown in Figure 3A. It clearly shows the characteristic peaks for epoxy groups at δ = 4.02, 2.06, and 1.39 ppm, compared with the spectrum of raw PET (Figure 3B). It means that a part of ST2000 has been grafted on PET so that they cannot be removed by THF. The in situ produced graft copolymer should 4750

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in all samples could not be detected clearly on DSC curves due to the crystallization of PET. Because the crystallization rate of PET is low, the cold crystallization peak of PET appears at about 130 °C on the melting curves of all samples and shifts a little to a low temperature compared with that of the raw PET resins, which implies that the mobility of PET molecular chains is improved at the existence of ST2000. The melting peak of PET also shifts to a low temperature after blending with ST2000. However, the melting temperature of PET rises again after the addition of PET-g-PAA. The crystallization peaks of PET during the cooling process exhibit the same change trend as the melting peaks, as shown in Figure 5b. It can be inferred from the above results that the elastomer phase disturbs the regular arrangement of the PET chains, resulting in the formation of the imperfect crystals with a low melt temperature.30 On the contrary, the existence of PET-g-PAA results in the production of the graft copolymer of PET-g-PAA and the elastomer, which increases the intermolecular force of PET matrix, thus the melt temperature of PET increases. What’s more, the interaction between PET-g-PAA and ST2000 promotes the enrichment of PET on the surface of ST2000. This regular arrangement of PET chains facilitates the crystallization of PET. The crystallization structure of PET in blends was also observed by polarization optical microscope, as shown in Figure 6. Small spherulitic structures are clearly observed in raw PET (Figure 6a). The addition of ST2000 makes the crystallization rate of PET decrease so much that the spherulitic structure cannot be observed in PET/ST2000 blend in the same crystallization temperature (Figure 6b). However, the spherulites can be observed again when PET-g-PAA is added into PET/ST2000 blend (Figure 5c). Undoubtedly, the improvement of the dispersibility of ST2000 and the crystallization properties of PET at the existence of PET-g-PAA will favor the mechanical properties of the PET blends. 3.2.3. Mechanical Properties of PET/PET-g-PAA/ST2000 Blends. The mechanical properties including the tensile strength, the elongation at break, and the notched Izod impact strength of the prepared blends were investigated. The results are shown in Table 1. The raw PET has a high tensile strength but a very low impact strength and elongation at break, indicating that PET has a bad toughness. When it is blended with 20 wt % of ST2000, the impact strength of PET/ST2000 blend increases by 1 order of magnitude, whereas the tensile strength falls by about 22%. The results are in accord with the common cases when plastic is blended with an elastomer alone.31 It is noted that the toughness of PET/ST2000 has been further improved after a small amount of PET is replaced by the prepared PET-g-PAA. At the same time, no obvious loss

Figure 3. 1H NMR spectra of (A) PET/PET-g-PAA/ST2000 blend after being extracted with tetrahydrofuran (THF) at 50 °C for 24 h and (B) raw PET.

be located at the phase interface between PET matrix and ST2000, acting as a good compatibilizer, as illustrated in Scheme 1. Therefore, it is necessary to investigate the effect of PET-g-PAA on the morphologies and mechanical properties of PET/PET-g-PAA/ST2000 blends. 3.2.1. Morphologies of PET/PET-g-PAA/ST2000 Blends. After PET-g-PAA was melt-blended with PET and ST2000, the dispersibility of ST2000 in PET was studied by SEM. Figure 4 exhibits the morphologies of the cryofractured samples of the prepared PET/PET-g-PAA/ST2000 ternary blends with different weight content of PET-g-PAA. The samples were etched by chloroform to remove ST2000 particles. The number-average domain diameters and the interdomain distances of the dispersed ST2000 (defined in Figure 4g) in each sample were measured using the image processing software (ImageJ, NIH, U.S.A.) and listed in Figure 4h. Obviously, the size of the dispersed ST2000 particles reduces dramatically with the addition of PET-g-PAA until the weight content is up to 6 wt % and then rises slightly with the increase of the content of PET-g-PAA. But in general, the size of ST2000 phase will decrease after the addition of PET-g-PAA. At the same time, the interdomain distance of the dispersed ST2000 particles drops nearly linearly with the weight content of PET-g-PAA, indicating that ST2000 particles disperse more and more homogeneously in PET matrix. All these results prove that a better phase compatibility between PET matrix and ST2000 has been achieved at the existence of PET-g-PAA. 3.2.2. Crystallization Behavior of PET in PET/PET-g-PAA/ ST2000 Blends. It is well known that the phase compatibility has a great influence on the crystallization behavior of a semicrystalline polymer in a blend system. Figure 5a shows the melting curves of the prepared PET/PET-g-PAA/ST2000 blends. The change of the glass transition temperature (Tg)

Scheme 1. In-Situ Compatibilization Mechanism of PET-g-PAA on PET and ST2000 Elastomer

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Figure 4. SEM images of PET/PET-g-PAA/ST2000 with different weight content of PET-g-PAA: (a) 0 wt %; (b) 2 wt %; (c) 4 wt %; (d) 6 wt %; (e) 8 wt %; (f) 10 wt %. The domain diameters and the interdomain distances of the dispersed ST2000 phase in each samples are defined and listed in (g) and (h), respectively. The weight content of ST2000 is 20 wt % for all the samples.

Figure 5. Melting (a) and crystallization (b) curves of raw PET and PET/PET-g-PAA/ST2000 blends with different weight content of PET-g-PAA.

Figure 6. Morphologies of spherulites formed in the raw PET (a), PET/ST2000 blend with 20 wt % of ST2000 (b), and PET/PET-g-PAA/ST2000 with 8 wt % of PET-g-PAA and 20 wt % of ST2000 (c). All the samples crystallized at 180 °C for 20 min.

weight content of PET-g-PAA with the size and distribution of ST2000 particles. Furthermore, the dependence of impact strength of PET/ ST2000 blends on the weight content of ST2000 at the presence or absence of PET-g-PAA were also studied, as shown in Figure 7. If PET is blended with ST2000 alone, only a weak rapid increase of impact strength of the blend when the content of ST2000 is higher than 15 wt %. However, a dramatic increase of impact strength (nearly 3-fold) occurs only when the content of ST2000 is above 10 wt % at the existence of 6 wt % PET-g-PAA. These inspiring results not only show the great influence of PET-g-PAA on the improvement of the interfacial compatibility between PET and ST2000 but also have a

of tensile strength is observed. The impact strength of PET/ PET-g-PAA/ST2000 with only 6 wt % PET-g-PAA is even twice that of PET/ST2000. It is so interesting that the impact strength of the PET blends will drop off when the content of PET-g-PAA is beyond 6 wt %. The variation of the elongation at break of the PET blends also has the same trend with the impact strength as seen from Table 1. Referring to the morphologies of PET/PET-g-PAA/ST2000 blends shown in Figure 4, it can be concluded that the enhancement of the impact strength of the blends should be attributed to the improvement of the dispersibility of ST2000 in the PET matrix because the impact strength has the same dependence on the 4752

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used impact modifier, and the extrinsic ones such as temperature, strain rate, or notch radius. From Figure 4h, the value of the interdomain distance decreases linearly with the content of PET-g-PAA. Compared with the mechanical properties listed in Table 1, it can be deduced that the critical IDc for the ternary blends was about 0.85 μm, that is, the corresponding interdomain distance at 2−4 wt % of PET-gPAA, from which the impact strength starts a sharp increase. 3.3. Discussion on the Toughening Mechanism in PET/PET-g-PAA/ST2000 Blends. Toughening mechanisms of plastics with elastomer particles have been widely used in the past decades.33 It is generally accepted now that if the elastomer particles have a good interfacial adhesion with the plastic matrix, the elastomer particles can induce the generation of a large amount of crazing and shear bands in the plastic matrix under the external impact force, which exhausts impact energy massively. At the same time, the shear bands can terminate the crazes and prevent them from developing into cracks. Therefore, the key to supertoughness is to enhance the interfacial adhesive property between the elastomer particles and the brittle matrix so as to induce crazes and shear bands effectively. Study on the impact-fractured surface morphology is favorable to the understanding of the toughening mechanisms.34 As illustrated in Figure 8, the impact fracture surface can be divided into three zones: zone A, zone B, and zone C, according to the crack evolution process under the impact force.35,36 Zone A stands for the early stage, that is, crack initiation zone. Zone B is the transmission stage, that is, the early stage of crack propagation. Zone C is the later stage of crack propagation. The impact fractured surfaces of the impact specimens were observed by SEM, and the results are also shown in Figure 8. For pure PET, a smooth surface morphology without any obvious plastic deformation is observed on the whole impactfractured surfaces, indicating a typical brittle-fracture (Figure 8a). Figure 8b−d show the three zones of fractured surface of PET/PET-g-PAA/ST2000 respectively. In the early stage of

Table 1. Mechanical Properties of PET/PET-g-PAA/ST2000 Blends with Different Weight Content of PET-g-PAAa weight content of PETg-PAA (wt %) raw PET resins 0 2 4 6 8 10 a

impact strength (kJ/m2) 2.5 34.2 39.8 55.1 68.6 53.7 43.4

± ± ± ± ± ± ±

0.5 1.6 1.2 1.4 2.5 1.8 1.2

tensile strength (MPa) 47.5 36.9 37.6 37.9 36.8 37.1 37.4

± ± ± ± ± ± ±

1.1 0.2 0.1 0.3 0.4 0.2 0.2

elongation at break (%) 2.5 11.8 12.1 15.5 18.2 14.0 13.6

± ± ± ± ± ± ±

0.1 0.1 0.2 0.3 0.5 0.5 0.2

The weight content of ST2000 is 20% for all the samples.

Figure 7. Dependence of impact strength of PET/ST2000 and PET/ PET-g-PAA/ST2000 blends on the weight content of ST2000. The content of PET-g-PAA is 6 wt %.

practical significance on the preparation of supertoughening PET blends. For many semicrystalline thermoplastics, the transition to a supertoughness is mostly characterized by the critical interparticle distance (IDc).31,32 The parameters that influence IDc include the internal ones such as the type of the

Figure 8. SEM images of the impact-fractured surface morphologies: (a) raw PET; (b)−(d) represent zone A, B, C on the surface of impact specimens of PET/PET-g-PAA/ST2000 with 6 wt % of PET-g-PAA and 20 wt % of ST2000. 4753

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(WK2060200012, 2014), and Foshan Scientific and Technological Innovation Team Project (No. 2013IT100041).

zone A, there is no considerable plastic deformation of the matrix. The fractured surface becomes coarser and some microvoids appear due to the interfacial debonding of the ST2000 particles from the PET matrix. In zone B (Figure 8c), besides the microvoids in the matrix, we can see clearly the slight plastic deformation of PET matrix, which indicates large quantity of impact energy has been absorbed in this zone so as to improve the resistance to the crack propagation. In the late stage zone C (Figure 8d), much more plastic deformation of PET matrix along with bigger microvoids can be observed. Both of the plastic deformation and microvoids can dissipate the impact energy. Evidently, the great improvement on the impact strength after the addition of PET-g-PAA into the PET/ elastomer blend should be attributed to the effective enhancement in the interfacial adhesion caused by the compatibilization of the in situ formed PET-g-PAA/ST2000 graft copolymer for PET matrix and ST2000.

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4. CONCLUSIONS In this work, PET resins were grafted with PAA through γ-ray radiation induced graft polymerization. The as-prepared PET-gPAA granules were melt blended with PET and ST2000. It was found that the addition of PET-g-PAA will greatly improve the compatibility between the PET matrix and the ST2000 elastomer, resulting in the decreased domain size and a homogeneously distribution of ST2000 particles. The crystallization properties of PET will also be improved at the existence of PET-g-PAA. The mechanical properties of PET/PET-gPAA/ST2000 blends have an essential relationship with the corresponding morphologies of ternary blends. The impact strength of PET/PET-g-PAA/ST2000 achieves the maximum at 6 wt % PET-g-PAA without the loss of tensile strength, and then drop off when the content of PET-g-PAA is beyond 6 wt %. The size and distribution of ST2000 particles have the same dependence on the weight content of PET-g-PAA. On the basis of the analysis of the fractured surface morphologies of the impact specimens, it can be concluded that the high toughness of PET/PET-g-PAA/ST2000 blends is achieved through the microvoiding and plastic deformation mechanism, which should be attributed to the effective compatibilization of the in situ formed PET-g-PAA/ST2000 graft copolymer for PET matrix and ST2000. The results also indicated that only the addition of 10−15 wt % of ST2000 can achieve a dramatic increase of impact strength at the existence of 6 wt % PET-gPAA. This work may open a new direction to prepare PET products with high mechanical performance in the industrial, taking advantage of γ-ray radiation technique.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Tel.: 86-551-63600843. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Prof. Yuan Hu, Prof. Zhigang Wang, and Dr. Bibo Wang of the University of Science and Technology of China, for their helpful advice and assistance. This work was supported by the National Natural Science Foundation of China (Nos. 51073146, 51103143, and 51173175), the Fundamental Research Funds for the Central Universities 4754

DOI: 10.1021/acs.iecr.5b00091 Ind. Eng. Chem. Res. 2015, 54, 4748−4755

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DOI: 10.1021/acs.iecr.5b00091 Ind. Eng. Chem. Res. 2015, 54, 4748−4755