Preparation and Properties of Poly(Ethylene Glycol-Co-Cyclohexane-1,4-Dimethanol Terephthalate)/Polyglycolic Acid (PETG/PGA) Blends

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Article Preparation and Properties of Poly(ethylene glycol-co-cyclohexane-1,4-dimethanol terephthalate)/Polyglycolic Acid (PETG/PGA) Blends

Kai Wang 1, Jianing Shen 1, Zhao Ma 1, Yipeng Zhang 1, Nai Xu 1,* and Sujuan Pang 2

1 Department of Polymer Materials and Engineering, School of Materials Science and Engineering, Hainan University, Haikou 570228, China; [email protected] (K.W.); [email protected] (J.S.); [email protected] (Z.M.); [email protected] (Y.Z.) 2 Department of Public Chemistry Teaching, School of Science, Hainan University, Haikou 570228, China; [email protected] * Correspondence: [email protected]; Tel.: +86-1313-602-3445

Abstract: Polyglycolic acid (PGA) is used as a reinforcing component to enhance the mechanical properties of poly(ethylene glycol-co-cyclohexane-1,4-dimethanol terephthalate) (PETG). The tensile performance, micromorphology, crystallinity, heat resistance, and melt mass ﬂow rates (MFRs) of PETG/PGA blends with varying PGA contents were studied. Both the tensile yield strength and tensile modulus of the PETG/PGA blends increased gradually with an increase in the PGA content from 0 to 35 wt%. The tensile yield strength of the PETG/PGA (65/35) blend increased by 8.7% (44.38 to 48.24 MPa), and the tensile modulus increased by 40.2% (1076 to 1509 MPa). However, its tensile ductility decreased drastically, owing to the poor interfacial compatibility of PETG/PGA and the oversized PGA domains. A multiple epoxy chain extender (ADR) was introduced into the Citation: Wang, K.; Shen, J.; Ma, Z.; PETG/PGA (65/35) blend to improve its interfacial compatibility and rheological properties. The Zhang, Y.; Xu, N.; Pang, S. tensile performance, micromorphology, rheological properties, crystallinity, and heat resistance of Preparation and Properties of PETG/PGA (65/35) blends with varying ADR contents were studied. The strong chain extension Poly(ethylene effect of ADR along with its reactive compatibilization improved the rheological properties and glycol-co-cyclohexane-1,4-dimethanol tensile ductility. By carefully controlling the ADR concentration, the performance of PETG/PGA terephthalate)/Polyglycolic Acid blends can be regulated for different applications. (PETG/PGA) Blends. Polymers 2021, 13, 452. https://doi.org/10.3390/ polym13030452 Keywords: PETG; PGA; tensile performance; interfacial compatibility; rheological property

Academic Editor: Dimitrios Bikiaris Received: 10 January 2021 Accepted: 26 January 2021 1. Introduction Published: 31 January 2021 Poly(ethylene glycol-co-cyclohexane-1,4-dimethanol terephthalate) (PETG) is an amorphous copolyester, synthesized by the polycondensation of 1,4-cyclohexanedimethanol Publisher’s Note: MDPI stays neutral (CHDM), ethylene glycol (EG), and terephthalic acid (TPA) in certain proportions [1]. with regard to jurisdictional claims in PETG has excellent properties, such as high ductility, high chemical resistance, pro- published maps and institutional afﬁl- cessability, and recyclability [2–4]. PETG, owing to its versatility, has been widely used for iations. packaging. In recent years, PETG has also become a material of choice for 3D printing [5,6]. However, with its increased applications, the mechanical performance of PETG, such as tensile strength, modulus, and heat resistance, need to be boosted. Seong-Hun et al. used carbon black (CB) as a reinforcement to enhance the mechanical Copyright: © 2021 by the authors. properties of PETG [7]. The results revealed that when the CB content was increased from Licensee MDPI, Basel, Switzerland. 0 to 3 wt%, the tensile yield strength and tensile modulus of the composite increased by This article is an open access article 11.2 and 5.9%, respectively. This could be attributed to the excellent mechanical properties distributed under the terms and and three-dimensional crystal structures of CB particles, which increased the strength of conditions of the Creative Commons the PETG. Attribution (CC BY) license (https:// Yuhsin Tsai et al. studied the inﬂuence of the organic clay content on the physical creativecommons.org/licenses/by/ properties of PETG [8]. It was found that with an increase in the organic clay content from 0 4.0/).

Polymers 2021, 13, 452. https://doi.org/10.3390/polym13030452 https://www.mdpi.com/journal/polymers Polymers 2021, 13, 452 2 of 20

to 3 wt%, the tensile yield strength increased from 47.9 to 53.3 MPa, and the tensile modulus increased from 1849 to 2154 MPa. This was mainly due to the reinforcing effect of the clay nanosheets, which were rigid and had a high aspect ratio. However, the tensile ductility (in terms of the elongation at break) of the PETG/organoclay nanocomposites significantly declined with an increase in organic clay content. For example, when 3 wt% organic clay was introduced into the PETG matrix, the elongation at break decreased from 112.9% (neat PETG sample) to 9.2%. As well-known, for most polymer blends and composites, how to tailor their properties to achieve a stiffness–ductility balance is still an interesting and challenging issue. Besides rigid inorganic particles, rigid organic polymers are also potential reinforce- ments for the PETG matrix. To date, few studies have reported on the introduction of rigid polymers into the PETG matrix as the reinforcing material. Polyglycolic acid (PGA) is a kind of aliphatic polyester, mainly synthesized by the ring-opening polymerization of glycolide or by the direct polycondensation of glycolic acid [9,10]. PGA possesses excellent mechanical strength and rigidity [11–13] along with good biocompatibility and biodegradability [14,15]. Owing to its high strength and rigidity (tensile strength = 115 MPa and tensile modulus = 7 GPa) [11,12], PGA can be considered as a candidate for enhancing the mechanical performance of PETG. To date, very little research has been reported on the reinforcing effect of PGA on PETG. In this paper, PGA was used as a reinforcing component to improve the mechanical properties of PETG. PETG/PGA binary blends with different PGA contents were prepared by melt-blending. The tensile performance, micromorphology, crystallinity, heat resistance, and melt mass flow rates (MFRs) of the PETG/PGA blends were studied in detail. It was found that the tensile yield strength and modulus of the binary blend increased with an increase in the PGA content. With 35 wt% PGA, the PETG/PGA (65/35) blend showed the maximum tensile yield strength and modulus. However, the poor interfacial compatibility between the PETG matrix and the PGA domains caused the tensile ductility of the PETG/PGA (65/35) blend to drastically decrease. Moreover, the high melt fluidity of PGA increased the MFR of the PETG/PGA blend, which was not conducive to its processing for some common processing technologies, such as extrusion, blown film, thermoforming, etc. The interfacial compatibility and the rheological properties of the PETG/PGA (65/35) blend could be improved by introducing a multiple epoxy chain extender into the PETG/PGA (65/35) blend. The effect of the chain extender on the PETG/PGA melt was evaluated by torque rheometry and MFR testing. In addition, the effect of adding the chain extender on the tensile properties, micromorphology, crystallinity, and heat resistance of the PETG/PGA (65/35) blend was studied in detail. Finally, a modified PETG/PGA (65/35) blend with improved tensile performance and heat resistance, and good rheological properties was successfully prepared.

2. Materials and Methods 2.1. Materials PETG (K2012), with a density of 1.27 g/cm3 and MFR of 5.1 g/10 min (230 ◦C, 2.16 kg), was supplied by SK Chemicals Co. Ltd. (Seoul, Korea). PGA, with a density of 1.53 g/cm3 and MFR of 28.4 g/10 min (230 ◦C, 2.16 kg), was obtained from Inner Mongolia Pujing Polymer Materials Technology Co. Ltd. (Bao Tou, China). The multiple epoxy chain extender (Joncryl ADR 4370s, abbreviated as ADR in this paper) was purchased from BASF (Ludwigshafen, Germany). Its molecular weight and epoxy equivalent weight were 6800 and 285 g/mol, respectively. The chemical structures of PETG, PGA, and ADR are shown in Figure1. Polymers 2021, 13, x FOR PEER REVIEW 3 of 22

Polymers 2021, 13, 452 3 of 20 and 285 g/mol, respectively. The chemical structures of PETG, PGA, and ADR are shown in Figure 1.

(a) (b)

(c)

FigureFigure 1. Chemical 1. Chemical structures structures of (a of) poly(ethylene (a) poly(ethylene glycol-co-cyclohexane-1,4-dimethanol glycol-co-cyclohexane-1,4-dimeth terephthalate)anol terephthalate) (PETG), (PETG), (b) polygly- (b) pol- colic acid (PGA), and (c) Joncryl ADRyglycolic 4370s (ADR). acid (PGA), and (c) Joncryl ADR 4370s (ADR).

2.2.2.2. Sample Sample Preparation Preparation Prior to compounding, pellets of both PETG and PGA were dried under vacuum at ◦ Prior to compounding, pellets of both PETG and PGA were dried under vacuum at 4545C °C for for 12 12 hours hours to removeto remove moisture. moisture. An An internal internal mixer mixer (XS-60, (XS-60, Shanghai Shanghai Kechuang Kechuang RubberRubber and and Plastic Plastic Machinery Machinery Equipment Equipment Co. Co. Ltd., Ltd., Shang Shang Hai, Hai, China) China) equipped equipped with with a a 6060 mLmL mixingmixing chamber was used for for i) (i) the the melt-blending melt-blending of of PETG/PGA PETG/PGA mixtures mixtures with with different PGA contents, and (ii) the reactive melt-blending of PETG/PGA/ADR different PGA contents, and ii) the reactive melt-blending of PETG/PGA/ADR (65/35/x) (65/35/x) blends with different ADR contents. The compositions of the PETG/PGA (y/z) blends with different ADR contents. The compositions of the PETG/PGA (y/z) and and PETG/PGA/ADR (65/35/x) samples are presented in Table1, where y/z and 65/35 PETG/PGA/ADR (65/35/x) samples are presented in Table 1, where y/z and 65/35 denote denote the weight ratio of the PETG/PGA, and x denotes the ADR content in parts per the weight ratio of the PETG/PGA, and x denotes the ADR content in parts per hundred hundred of total resin (phr). The melt blending was conducted at 230 ◦C for 8 min at a of total resin (phr). The melt blending was conducted at 230°C for 8 min at a rotational rotational speed of 50 rpm. Then, the obtained blends were compression-molded using speed of 50 rpm. Then, the obtained blends were compression-molded using a hot press a hot press at 230 ◦C under 20 MPa for 8 min. After hot pressing, the closed mold with at 230°C under 20 MPa for 8 min. After hot pressing, the closed mold with the in-mold the in-mold melt was quickly transferred into a water-cooling system with a clamping melt was quickly transferred into a water-cooling system with a clamping pressure of 20 pressure of 20 MPa and cooled down to room temperature at about 100 ◦C/min. Thus, MPa and cooled down to room temperature at about 100°C/min. Thus, compres- compression-molded sheets of 0.5 mm thickness were obtained. These sheets were cut into sion-molded sheets of 0.5 mm thickness were obtained. These sheets were cut into dumbbell-shaped specimens for the subsequent tensile test. dumbbell-shaped specimens for the subsequent tensile test. Table 1. Compositions of PETG/PGA (y/z) and PETG/PGA/ADR (65/35/x) samples. Table 1. Compositions of PETG/PGA (y/z) and PETG/PGA/ADR (65/35/x) samples. Sample Number PETG (wt.) PGA (wt.) ADR (phr) Sample number PETG (wt.) PGA (wt.) ADR (phr) 1# 100 0 0 2#1# 95100 50 0 0 3# 85 15 0 4#2# 7595 255 0 0 5# 65 35 0 3# 85 15 0 6# 65 35 0.3 7#4# 6575 3525 0.6 0 8# 65 35 0.9 5# 65 35 0 2.3. Tensile Testing 6# 65 35 0.3 A universal tensile testing machine (WDW-1, Yinuo Century Testing Instrument Co. Ltd., Ji Nan, China)7# was used to determine65 the tensile performance35 of the samples.0.6 Prior to tensile testing, the samples were stored in an electronic drying oven for 48 h. The 8# 65 35 0.9 tensile yield strengths, tensile moduli, and elongation at break values of the samples were measured at a tensile rate of 5 mm/min. Each result presented in Table2 is an average of ﬁve valid test results per sample, along with the standard deviation.

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Table 2. Tensile performance of neat PETG and PETG/PGA (y/z) samples.

Samples Tensile Yield Strength (MPa) Tensile Modulus (MPa) Elongation at Break (%) Neat PETG 44.38 ± 0.62 1076 ± 35 223.0 ± 10.2 PETG/PGA (95/05) 45.26 ± 0.62 1145 ± 17 227.2 ± 8.9 PETG/PGA (85/15) 46.33 ± 0.20 1254 ± 07 260.4 ± 14.2 PETG/PGA (75/25) 47.57 ± 0.77 1370 ± 50 29.0 ± 6.5 PETG/PGA (65/35) 48.24 ± 0.72 1509 ± 31 11.1 ± 1.8

2.4. Scanning Electron Microscopy (SEM) SEM (Verios G4 UC, Thermo Fisher Scientiﬁc Brno Co. Ltd., Waltham, USA) was used to study the morphological evolutions of the specimens during the tensile deformation process. The tensile fractured specimens were collected after the tensile testing. Some tensile fractured specimens were immersed in liquid nitrogen for a sufﬁcient time, and then broken quickly to obtain cryofractured surfaces along the tensile direction. The cryofractured surfaces of the specimens were sputter coated with gold powder in order to obtain better SEM images. Both the stretched and unstretched regions on the cryofractured surface of each specimen were photographed.

2.5. Differential Scanning Calorimetry (DSC) DSC curves were obtained using a DSC instrument (Q100, TA Instruments, New Castle, DE, USA). The DSC thermograms were used to determine the crystallinities of the samples. The speciﬁc steps were as follows: each sample (around 5–8 mg) was heated from ◦ ◦ 20 to 250 C at 10 C/min in an N2 atmosphere to obtain the heating DSC thermogram. The relevant parameters were determined from the DSC thermogram, and the crystallinity (Xc) of the PGA component was calculated according to Equation (1).

|∆H | m × Xc = 0 100% (1) ωPGA × ∆Hm

where ωPGA is the mass fraction of the PGA component in the sample, ∆Hm is the en- 0 dothermic melting enthalpy of the PGA component, and ∆Hm is the ideal melting enthalpy of 100% crystalline PGA (−198.14 J/g) [16,17]. In this study, the endothermic enthalpy was negative in the DSC measurements.

2.6. Wide-Angle X-ray Diffraction (WAXD) The crystalline structures of the samples were determined by WAXD (Smart Lab, Rigaku Co., Akishima, Japan). The dimensions of the specimens were 40 × 40 × 2 mm (length × width × thickness). A CuKα radiation source (λ = 1.542Å) was used at a 40 kV voltage and 40 mA current. WAXD data were recorded in the 2θ range of 5−40◦ with a scanning speed of 5◦/min.

2.7. Vicat Softening Temperature (VST) A VST tester (CZ-6005, Changzhe Testing Machinery Co., Ltd., Yang Zhou, China) was used to determine the heat resistance of the samples. The VST test was conducted according to GB/T 1633–2000 at a heating rate of 120 ◦C/h and under a load of 10 N. The dimensions of the specimens were 10 × 10 × 2 mm (length × width × thickness).

2.8. Torque The curves for the torque vs. mixing time of the PETG/PGA/ADR (65/35/x) blends were plotted from the results obtained by torque rheometry (XSS-300, Kechuang Rubber & Plastic Equipment Co. Ltd., Shang Hai, China). A steady temperature of 230.0 ± 1.0 ◦C was maintained, and the rotation speed was set at 50 rpm. The total mixing time was set as 30 min. Polymers 2021, 13, 452 5 of 20

2.9. Melt Mass Flow Rate (MFR) The MFRs of the samples were determined using a melt ﬂow rate apparatus (XNR- 400B, Desheng Testing Equipment Co. Ltd., Cheng De, China) at 230 ◦C with a 2.16 kg load.

2.10. Transmission Electron Microscopy (TEM) Transmission electron microscopy (JEM-2100, JEOL, Akishima, Japan) was employed to investigate the inﬂuence of adding ADR on the domain size of PGA at an accelerating voltage of 200 kV. TEM images of microtomed sections of 70–80 nm thickness were obtained.

3. Results and Discussion 3.1. PETG/PGA Binary Blends Polymers 2021, 13, x FOR PEER REVIEW 3.1.1. Tensile Performance 6 of 22 The tensile yield strengths, tensile moduli, and elongation at break values of PETG/PGA binary samples with various PGA contents are listed in Table2. The typical stress–strain curves are shown in Figure2.

1- PETG 50 2- PETG/PGA (95/05) 3- PETG/PGA (85/15) 4- PETG/PGA (75/25) 40 5- PETG/PGA (65/35)

Stress (MPa) 20 5 4 1 2 3

0 0 30 60 90 120 150 180 210 240 270 300 Strain (%)

Figure 2. Tensile strain–stress curves of PETG/PGA samples. Figure 2. Tensile strain–stress curves of PETG/PGA samples. As shown in Table2, with an increase in PGA content from 0 to 35 wt%, the tensile 3.1.2. Scanningyield strength Electron of the Microscopy PETG/PGA (SEM) binary sample increased from 44.38 MPa (neat PETG) to To48.24 study MPa. the micromorphological Meanwhile, the tensile evolution modulus of increasedthe binary from samples 1076 during to 1509 the MPa, tensile indicating process,that the PGA cryofractured has a good surfaces reinforcing of the effect tensile on thefractured PETG specimens matrix. However, along the when tensile the PGA direction,content including was more the stretching-oriented than 15 wt%, the tensile parts ductilityand the unstretched (in terms of theparts, elongation are shown at break)in of Figure the3. binary sample decreased remarkably. With an increase in PGA content to 35 wt%, the elongation at break decreased from 223.0% (neat PETG) to 11.1%. This could be attributed to the poor interfacial compatibility between PETG and PGA. With an increase in PGA content, a serious combination of the PGA dispersed phase was inevitable as a result of the poor interfacial compatibility. The oversized PGA particles with poor interfacial adhesion caused defects in the PETG matrix that induced the formation of cracks, which developed rapidly under tensile stress. Similar experimental results have also been reported for many other blends [18–20].

3.1.2. Scanning Electron Microscopy (SEM) To study the micromorphological evolution of the binary samples during the tensile process, the cryofractured surfaces of the tensile fractured specimens along the tensile

direction, including the stretching-oriented parts and the unstretched parts, are shown in Figure3.

Polymers 2021, 13, x FOR PEER REVIEW 6 of 22

1- PETG 50 2- PETG/PGA (95/05) 3- PETG/PGA (85/15) 4- PETG/PGA (75/25) 40 5- PETG/PGA (65/35)

Stress (MPa) 20 5 4 1 2 3

0 0 30 60 90 120 150 180 210 240 270 300 Strain (%)

Figure 2. Tensile strain–stress curves of PETG/PGA samples.

3.1.2. Scanning Electron Microscopy (SEM) To study the micromorphological evolution of the binary samples during the tensile Polymers 2021, 13, 452 process, the cryofractured surfaces of the tensile fractured specimens along the6 of 20tensile direction, including the stretching-oriented parts and the unstretched parts, are shown in Figure 3.

Polymers 2021, 13, x FOR PEER REVIEW 7 of 22

FigureFigure 3. SEM 3. SEMimages images (×3000) (×3000) of cryofractured of cryofractured surfaces surfaces of of PETG/PGA PETG/PGA tensile fractured fractured specimens specimens along along tensile tensile direction. direction. (A) unstretched(A) unstretched part part (A1-95/5, ((A1)-95/5, A2-85/15, (A2)-85/15, A3-75/25, (A3)-75/25, A4-65/35), (A4)-65/35), (B) stretching-oriented (B) stretching-oriented part part (B1-95/5, ((B1)-95/5, B2-85/15, (B2)-85/15, B3-75/25), and (C)(B3 sketching)-75/25), and diagram (C) sketching of cryofractu diagramred of surfaces cryofractured along surfaces tensile along direction. tensile direction.

TheThe unstretched unstretched regions regions of of the the PETG/PGA PETG/PGA (95/5) (95/5) and and (85/15) (85/15) samples samples (see (see Figure µ 3A1,Figure A2)3 (A1,A2))showed showedthat the that PGA the particles PGA particles with with finer ﬁner particle particle sizes sizes (<5 (<5 μmm for mostmost of of the PGA particles) uniformly dispersed in the PETG matrix. However, as the PGA content increased to 25 and 35 wt%, a serious combination of PGA domains appeared, as shown in Figure 3-A3 and –A4. Most of the PGA particles in the PETG/PGA (75/25) and (65/35) samples showed oversized diameters (more than 5 μm). The formation of such oversized PGA particles could be attributed to the poor interfacial compatibility in the PETG/PGA blend and increased PGA content. For the PETG/PGA (95/5) and (85/15) samples (see Figure 3-B1 and -B2), the regions oriented in the stretching direction showed the debonding of PGA particles from the PETG matrix along with highly oriented cavities. The tensile performance shown in Table 2 confirmed that the PGA component had an obvious reinforcing effect on the PETG matrix. In addition, when the PETG/PGA (95/5) and (85/15) specimens were subjected to tensile loads, finer PGA particles showed stronger stress concentration around these particles, which led to the formation of massive shear zones in the PETG matrix. How- ever, no plastic deformation occurred in the PGA domains, due to their superior rigidity. As a result, interfacial debonding in PETG/PGA was observed in the SEM images. The cavities formed due to interfacial debonding deformed along the tensile direction. This was followed by the plastic deformation and stretching of the PETG matrix along the tensile direction, as shown in Figure 3-B1 and -B2. Compared to that of the neat PETG sample, the presence of fine particles of PGA induced a higher number of shear zones in the matrix under a tensile load. Therefore, the improvement in the fracture toughness was due to energy absorption, caused by the matrix deformation. This was the result of the centralization of external stress around the PGA particles. Furthermore, these shear zones could effectively terminate crazes and prevent crack propagation. Consequently, the tensile yield strength, modulus, and elongation at break of the PETG/PGA (85/15) sample were improved. With an increase in the PGA content to 25 wt%, many oversized PGA particles dispersed in the PETG matrix, as shown as Figure 3-B3. As we know, dis-

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the PGA particles) uniformly dispersed in the PETG matrix. However, as the PGA content increased to 25 and 35 wt%, a serious combination of PGA domains appeared, as shown in Figure3(A3,A4). Most of the PGA particles in the PETG/PGA (75/25) and (65/35) samples showed oversized diameters (more than 5 µm). The formation of such oversized PGA particles could be attributed to the poor interfacial compatibility in the PETG/PGA blend and increased PGA content. For the PETG/PGA (95/5) and (85/15) samples (see Figure3(B1,B2)), the regions oriented in the stretching direction showed the debonding of PGA particles from the PETG matrix along with highly oriented cavities. The tensile performance shown in Table2 confirmed that the PGA component had an obvious reinforcing effect on the PETG matrix. In addition, when the PETG/PGA (95/5) and (85/15) specimens were subjected to tensile loads, finer PGA particles showed stronger stress concentration around these particles, which led to the formation of massive shear zones in the PETG matrix. However, no plastic deformation occurred in the PGA domains, due to their superior rigidity. As a result, interfacial debonding in PETG/PGA was observed in the SEM images. The cavities formed due to interfacial debonding deformed along the tensile direction. This was followed by the plastic deformation and stretching of the PETG matrix along the tensile direction, as shown in Figure3(B1,B2). Compared to that of the neat PETG sample, the presence of fine particles of PGA induced a higher number of shear zones in the matrix under a tensile load. Therefore, the improvement in the fracture toughness was due to energy absorption, caused by the matrix deformation. This was the result of the centralization of external stress around the PGA particles. Furthermore, these shear zones could effectively terminate crazes and prevent crack propagation. Consequently, the tensile yield strength, modulus, and elongation at break of the PETG/PGA (85/15) sample were improved. With an increase in the PGA content to 25 wt%, many oversized PGA particles dispersed in the PETG matrix, as shown as Figure3(B3). As we know, dispersed particles with excessive domain sizes have relatively less ability to induce the formation of shear zones in the matrix that weaken the craze termination ability of the matrix. Meanwhile, the oversized PGA particles with poor interfacial compatibility became defect points and induced the formation of massive cracks under a tensile load. This led to the formation of an unstable and premature fracture. As a result, the PETG/PGA (75/25) sample showed a considerable decrease in its tensile ductility. A further increase in PGA content to 35 wt% led to the formation of more oversized PGA particles in the PETG matrix. This led to a further decrease in the tensile ductility of the binary sample that even caused a ductile–brittle transition in the tensile behavior. Owing to its poor tensile ductility, the PETG/PGA (65/35) sample (with a very low elongation at break of 11.1%) showed no obvious tensile yield and orientation behavior before its rupture during the tensile testing. Hence, an SEM image of the stretching-oriented part for the PETG/PGA (65/35) sample cannot be provided in this paper.

3.1.3. DSC and WAXD Measurements It is well-known that the crystallization properties of thermoplastic polymers, such as the crystallinity, crystalline structures, etc., can signiﬁcantly inﬂuence the physical performance of the polymers and their blends [21–24]. DSC and WAXD analysis were used to evaluate the crystallization properties of PETG/PGA blends with different PGA contents. The heating DSC thermograms of neat PETG, PGA, and PETG/PGA binary samples are shown in Figure4. The corresponding thermodynamic parameters are summarized in Table3. Polymers 2021, 13, x FOR PEER REVIEW 8 of 22

persed particles with excessive domain sizes have relatively less ability to induce the formation of shear zones in the matrix that weaken the craze termination ability of the matrix. Meanwhile, the oversized PGA particles with poor interfacial compatibility became defect points and induced the formation of massive cracks under a tensile load. This led to the formation of an unstable and premature fracture. As a result, the PETG/PGA (75/25) sample showed a considerable decrease in its tensile ductility. A further increase in PGA content to 35 wt% led to the formation of more oversized PGA particles in the PETG matrix. This led to a further decrease in the tensile ductility of the binary sample that even caused a ductile–brittle transition in the tensile behavior. Owing to its poor tensile ductility, the PETG/PGA (65/35) sample (with a very low elongation at break of 11.1%) showed no obvious tensile yield and orientation behavior before its rupture during the tensile testing. Hence, an SEM image of the stretching-oriented part for the PETG/PGA (65/35) sample cannot be provided in this paper.

3.1.3. DSC and WAXD Measurements It is well-known that the crystallization properties of thermoplastic polymers, such as the crystallinity, crystalline structures, etc., can significantly influence the physical performance of the polymers and their blends [21–24]. DSC and WAXD analysis were used to evaluate the crystallization properties of PETG/PGA blends with different PGA contents. The heating DSC thermograms of neat PETG, PGA, and PETG/PGA binary samples are shown in Figure 4. The corresponding thermodynamic parameters are summarized in Table 3.

Table 3. Differential scanning calorimetry (DSC) parameters of neat PGA, PETG, and PETG/PGA (y/z) samples.

Samples Tg (℃) (PETG) Tm (℃) (PGA) △Hm (J/g) (PGA) X c (%) (PGA)

Neat PGA N/A 224.9 -74.67 37.7

Neat PETG 74.9 N/A N/A N/A

PETG/PGA (95/05) 74.7 221.2 -2.91 29.4

PETG/PGA (85/15) 74.1 221.6 -9.48 31.9

PETG/PGA (75/25) 72.3 221.1 -17.49 35.3 Polymers 2021, 13, 452 8 of 20 PETG/PGA (65/35) 73.1 221.0 -24.96 36.0

10°C/min PETG

PETG/PGA (95/05) EXO PETG/PGA (85/15) ) -1 PETG/PGA (75/25)

PETG/PGA (65/35) PGA Heat Flow (W·g Flow Heat

30 60 90 120 150 180 210 240 Temperature (°C) FigureFigure 4.4. HeatingHeating DSCDSC thermographsthermographs ofof PETG/PGAPETG/PGA (y/z) samples. samples.

Table 3. Differential scanning calorimetry (DSC) parameters of neat PGA, PETG, and PETG/PGA (y/z) samples. ◦ ◦ Samples Tg ( C) (PETG) Tm ( C) (PGA) ∆Hm (J/g) (PGA) Xc (%) (PGA) Neat PGA N/A 224.9 −74.67 37.7 Polymers 2021, 13, xNeat FORPETG PEER REVIEW 74.9 N/A N/A 9 of 22 N/A PETG/PGA (95/05) 74.7 221.2 −2.91 29.4 PETG/PGA (85/15) 74.1 221.6 −9.48 31.9 PETG/PGA (75/25) 72.3 221.1 −17.49 35.3 PETG/PGA (65/35) 73.1 221.0 −24.96 36.0 The DSC thermogram of the neat PETG sample displayed a heat jump to approximately 74.9°C, which corresponded to its glass transition temperature (Tg). Moreover, the The DSC thermogram of the neat PETG sample displayed a heat jump to approxi- absence of an endothermic◦ peak of melting proved that PETG was an amorphous poly- mately 74.9 C, which corresponded to its glass transition temperature (Tg). Moreover, ester withoutthe absence crystallization of an endothermic ability. A similar peak conclusion of melting was proved also thatdrawn PETG from was its WAXD an amorphous analysispolyester (see Figure without 5). crystallization ability. A similar conclusion was also drawn from its WAXD analysis (see Figure5).

PGA

PETG/PGA (65/35)

PETG/PGA (75/25) Intensity PETG/PGA (85/15)

PETG/PGA (95/05)

PETG

5 10152025303540 2θ (°)

Figure 5. Wide-angle X-ray diffraction (WAXD) patterns of PETG, PGA, and PETG/PGA Figure 5.(y/z) Wide-angle samples. X-ray diffraction (WAXD) patterns of PETG, PGA, and PETG/PGA (y/z) samples. On the other hand, the DSC thermogram of the neat PGA sample showed its melting On the other hand, the DSC thermogram of the neat◦ PGA sample showed its melting temperature (Tm) to be approximately 224.9 C, along with a sharp endothermic melting temperaturepeak. (T Them) to DSC be thermogramsapproximately of 224.9°C, the binary along PETG/PGA with a sharp samples endothermic showedno melting obvious shift- peak. Theing DSC of peaks thermograms for the Tg ofof thethe PETG binary matrix PETG/PGA and Tm samplesof the PGA showed component. no obvious Moreover, in shifting contrastof peaks tofor the the amorphous Tg of the PETG nature matrix of the and neat T PETGm of the sample, PGA component. the neat PGA Moreover, sample showed in contrast to the amorphous nature of the neat PETG sample, the neat PGA sample showed a semicrystalline structure with a relatively high crystallinity of 37.7%. For the PETG/PGA binary samples with lower PGA contents (≤15 wt%), the crystallinity of the PGA component was decreased compared to that of the neat PGA, as seen in Table 3. The reason for this is as follows: with a lower PGA content, some of the PETG and PGA macromolecular chains or segments could interpenetrate each other. As a result, the crystallization ability of the PGA component was restrained, which led to a decrease in the crystallinity of the PGA component. However, with an increase in the PGA content (≥25 wt%), a serious combination of the PGA domains caused by the microphase separation in the binary blending system occurred, which weakened the interactions between the PETG and PGA chains and segments. Hence, the crystallinity of the PGA increased with an increase in the PGA content. Compared to the crystallinity (37.7%) of the neat PGA sample, the crystallinity of the PGA component in the PETG/PGA (65/35) sample was restored to 36.0%. The WAXD patterns of the PETG/PGA binary samples with various PGA contents are shown in Figure 5. For comparison, the WAXD patterns of the neat PETG and neat PGA are also provided. Consistent with the DSC results, the WAXD pattern of the neat PETG showed a dispersive diffraction peak in the range of 2θ = 10° to 35°, indicating that the neat PETG was amorphous. Neat PGA showed two sharp diffraction peaks at 2θ = 21.8° and 28.4°, corresponding to the (110) and (020) planes of PGA crystals, respectively [25–27]. In the WAXD pattern of PETG/PGA (95/5), two sharp diffraction peaks at 22.2° and 28.9° were characteristic of PGA crystals. Moreover, the dispersive diffraction peak from 2θ = 10° to 35° indicated that the PETG matrix was completely amorphous. With an

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a semicrystalline structure with a relatively high crystallinity of 37.7%. For the PETG/PGA binary samples with lower PGA contents (≤15 wt%), the crystallinity of the PGA component was decreased compared to that of the neat PGA, as seen in Table3 . The reason for this is as follows: with a lower PGA content, some of the PETG and PGA macromolecular chains or segments could interpenetrate each other. As a result, the crystallization ability of the PGA component was restrained, which led to a decrease in the crystallinity of the PGA component. However, with an increase in the PGA content (≥25 wt%), a serious combination of the PGA domains caused by the microphase separation in the binary blending system occurred, which weakened the interactions between the PETG and PGA chains and segments. Hence, the crystallinity of the PGA increased with an increase in the PGA content. Compared to the crystallinity (37.7%) of the neat PGA sample, the crystallinity of the PGA component in the PETG/PGA (65/35) sample was restored to 36.0%. The WAXD patterns of the PETG/PGA binary samples with various PGA contents are Polymers 2021, 13, x FOR PEER REVIEW shown in Figure 5 . For comparison, the WAXD patterns of the neat 10 PETGof 22 and neat PGA are also provided. Consistent with the DSC results, the WAXD pattern of the neat PETG showed a dispersive diffraction peak in the range of 2θ = 10◦ to 35◦, indicating that the neat PETG was amorphous. Neat PGA showed two sharp diffraction peaks at 2θ = 21.8◦ and 28.4◦, corresponding to the (110) and (020) planes of PGA crystals, respectively [25–27]. increase in PGA content, the two diffraction peaks characteristic of PGA crystals in the In the WAXD pattern of PETG/PGA (95/5), two sharp diffraction peaks at 22.2◦ and 28.9◦ binarywere samples characteristic increased of PGA gradually, crystals. Moreover,as expected. the dispersive diffraction peak from 2θ = 10◦ to 35◦ indicated that the PETG matrix was completely amorphous. With an increase in 3.1.4.PGA Vicat content, Softening the two Temperature diffraction peaks(VST) characteristic of PGA crystals in the binary samples Theincreased results gradually, of the DSC as expected. and WAXD analyses confirmed that the PETG matrix was amorphous, regardless of the PGA content. By contrast, the PGA component was semi- 3.1.4. Vicat Softening Temperature (VST) crystalline, with higher crystallinity. Hence, theoretically, the presence of PGA domains The results of the DSC and WAXD analyses conﬁrmed that the PETG matrix was could improve the heat resistance of PETG. amorphous, regardless of the PGA content. By contrast, the PGA component was semicrys- Thetalline, VST with is higherone of crystallinity.the most important Hence, theoretically, parameters the for presence evaluating of PGA the domains heat resistance could of a thermoplasticimprove the heat polymer. resistance For of PETG.an amorphous thermoplastic resin, the VST is strongly affected byThe its VST Tg. isDue one to of its the low most Tg important (ca. 75.0°C), parameters neat PETG for evaluating showed poor the heat heat resistance resistance, whichof limits a thermoplastic its applications polymer. at high For an temperatures. amorphous thermoplastic The relationship resin, between the VST isthe strongly VST and ◦ PGA affectedcontent by is itsshownTg. Due in to Figure its low 6.T gAs(ca. expe 75.0 cted,C), neat the PETGheat showedresistance poor of heat the resistance,PETG/PGA blendswhich increased limits itswith applications an increase at high in PGA temperatures. content. When The relationship the PGA content between increased the VST and from 0 to 35PGA wt%, content the isVST shown increased in Figure from6. As 81.0°C expected, (neat the PETG) heat resistance to 99.2°C. of theThis PETG/PGA could be at- blends increased with an increase in PGA content. When the PGA content increased from 0 tributed to the reinforcement of the PGA particles on the amorphous PETG matrix, owing to 35 wt%, the VST increased from 81.0 ◦C (neat PETG) to 99.2 ◦C. This could be attributed to theto rigidity the reinforcement and semicrystalline of the PGA nature particles of onPGA. the amorphous PETG matrix, owing to the rigidity and semicrystalline nature of PGA.

100

VST (°C) VST 88

0 5 10 15 20 25 30 35 PGA content (wt %)

Figure 6. Inﬂuence of PGA content on Vicat softening temperature (VST) of PETG/PGA samples. Figure 6. Influence of PGA content on Vicat softening temperature (VST) of PETG/PGA samples.

3.1.5. Melt Mass Flow Rate (MFR) The MFR values of binary blends with varying PGA contents are shown in Figure 7. The MFR increased with an increase in PGA content, due to the high melt fluidity of PGA. When the PGA content increased from 0 to 35 wt%, the MFR value (230°C, 2.16 kg) increased rapidly from 9.7 to 55.3 g/10 min. It should be noted that the PETG/PGA blends with high melt fluidity were not suitable for some of the important processing methods, such as extrusion, blown film, thermoforming, etc.

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3.1.5. Melt Mass Flow Rate (MFR) The MFR values of binary blends with varying PGA contents are shown in Figure7 . The MFR increased with an increase in PGA content, due to the high melt fluidity of Polymers 2021, 13, x FOR PEER REVIEW PGA. When the PGA content increased from 0 to 35 wt%, the11 of MFR 22 value (230 ◦C, 2.16 kg) increased rapidly from 9.7 to 55.3 g/10 min. It should be noted that the PETG/PGA blends with high melt fluidity were not suitable for some of the important processing methods, such as extrusion, blown film, thermoforming, etc.

48 ) -1 40

MFR (g·10 min (g·10 MFR 24

0 5 10 15 20 25 30 35 PGA content (wt%)

Figure 7. Influence of PGA content on MFR of PETG/PGA samples. Figure 7. Influence of PGA content on MFR of PETG/PGA samples. With the incorporation of 35 wt% PGA into the PETG matrix, the binary sample Withshowed the theincorporation maximum of tensile 35 wt% yield PGA strength into the and PETG modulus. matrix, However, the binary the sample PETG/PGA showed(65/35) the maximum blend displayed tensile very yield low strength tensile and ductility, modulus. which However, could be attributedthe PETG/PGA to the over- (65/35)sized blend PGA displayed particles very and low poor tensile interfacial ductil compatibilityity, which could in PETG/PGA. be attributed Moreover, to the over- the addi- sized tionPGA of particles PGA with and high poor melt interfacial fluidity resulted compatibility in an excessively in PETG/PGA. high MFRMoreover, for the the PETG/PGA addition(65/35) of PGA blend with that high significantly melt fluidity weakened resulted its processingin an excessively stability. high Hence, MFR a multiple for the epoxy PETG/PGAchain extender(65/35) blend (ADR) that was significantly used to increase weakened the melt its viscosityprocessing of stability. the PETG/PGA Hence, (65/35)a multipleblend epoxy through chain a chainextender extension/branching (ADR) was used to reaction increase between the melt the viscosity polyester of chains the and PETG/PGAADR. This(65/35) facilitated blend through further processing,a chain extension/branching since a high melt viscosity reaction is between a prerequisite the for polyesterprocesses chains such and ADR. as extrusion, This facilitated blown film, further thermoforming, processing, since etc. a Moreover, high melt ADRviscosity was also is a prerequisiteexpected to for act processes as a reactive such compatibilizeras extrusion, blown to improve film, thermoforming, the interfacial compatibilityetc. More- of over, ADRPETG/PGA, was also decrease expected the to PGAact as domain a reacti size,ve compatibilizer and thus improve to improve the tensile the interfacial ductility of the compatibilityPETG/PGA of PETG/PGA, (65/35) blend. decrease the PGA domain size, and thus improve the tensile ductility of the PETG/PGA (65/35) blend. 3.2. PETG/PGA/ADR Ternary Blend 3.2. PETG/PGA/ADR3.2.1. Torque and Ternary MFR Blend 3.2.1. TorqueRheological and MFR tests of neat PETG, neat PGA, and PETG/PGA (65/35) blends with and without ADR were carried out by torque rheometry, and the results are shown in Figure8 . Rheological tests of neat PETG, neat PGA, and PETG/PGA (65/35) blends with and Experiments were conducted to study the chain extension reaction of the polyesters as without ADR were carried out by torque rheometry, and the results are shown in Figure a function of the mixing time in the molten state. Figure8(A,B) show that the torque 8. Experiments were conducted to study the chain extension reaction of the polyesters as curves of neat PETG and neat PGA gradually shifted upward with an increase in ADR a functioncontent. of the According mixing totime previous in the research molten [state.20,28– Figure30], the 8 multifunctional (A) and (B) show epoxide that groupsthe of torquethe curves chain of extender neat PETG could and react neat with PGA both gradually the carboxyl shifted and upward hydroxyl with end an increase groups ofin many ADR kindscontent. of polyester.According Theto previous resultant research chain extension/branching [20,28–30], the multifunctional reactions are epoxide responsible groupsfor of the the increase chain extender in melt could viscosity react of with polyesters both the during carboxyl melt and processing. hydroxyl Theoretically,end groups the of manymultifunctional kinds of polyester. epoxide The groups resultant of ADR chain could extension/branching react with both the carboxylreactions and are hydroxylre- sponsibleend for groups the increase of PETG in or melt PGA viscosity chains. of This polyesters led to chain during extension/branching melt processing. Theo- reactions retically,between the multifunctional the polyesters andepoxide ADR. groups Hence, of the ADR increase could in viscosityreact with could both be the explained carboxyl by the and hydroxylformation end of groups chain extended/long-branched of PETG or PGA chains. PETG This led or PGA to chain chains extension/branching with ADR modification. reactions between the polyesters and ADR. Hence, the increase in viscosity could be explained by the formation of chain extended/long-branched PETG or PGA chains with ADR modification. The torque value of the PETG/PGA (65/35) blend increased upon the addition of ADR, as shown in Figure 8(C). However, the chain extension/branching reactions of the PETG/PGA/ADR (65/35/x) blends were more complicated. In addition to PETG–ADR– PETG and PGA–ADR–PGA with chain extension/long-branched structures, the PETG–

Polymers 2021, 13, x FOR PEER REVIEW 12 of 22

Polymers 2021, 13, 452 ADR–PGA copolymer could also be generated in situ through chain extension/branching11 of 20 reactions during blending.

50 20 250 250 (A) (B)

16 40 PETG 200 200 PETG/ADR (100/0.3) PGA PETG/ADR (100/0.6) 12 PGA/ADR (100/0.3) 30 150 PETG/ADR (100/0.9) 150 PGA/ADR (100/0.6) PGA/ADR (100/0.9) 8 100 20 100 (Nm) Torque Temperature (°C) Torque (Nm) Torque

Temperature (°C) 4 50 10 50 0

0 0 0 0 300 600 900 1200 1500 1800 2100 0 300 600 900 1200 1500 1800 2100 Time (s) Time (s)

40 （C） 250 35

30 200 PETG/PGA (65/35) 25 PETG/PGA/ADR (65/35/0.3) PETG/PGA/ADR (65/35/0.6) PETG/PGA/ADR (65/35/0.9) 150 20

Torque (Nm) 15 100 Temperature (°C) 10

5 50

0 0 0 300 600 900 1200 1500 1800 2100 Time (s)

Figure 8. Torque vs. time curves for (A) PETG/ADR (100/x), (B) PGA/ADR (100/x), and (C) PETG/PGA/ADR (65/35/x) Figure(x = 0, 8. 0.3, Torque 0.6, and vs. 0.9).time curves for (A) PETG/ADR (100/x), (B) PGA/ADR (100/x), and (C) PETG/PGA/ADR (65/35/x) (x = 0, 0.3, 0.6, and 0.9). The torque value of the PETG/PGA (65/35) blend increased upon the addition of ADR,MFR as testing shown was in Figure conducted8C. However, to further the evalua chainte extension/branchingthe impact of the ADR reactions concentration of the onPETG/PGA/ADR the melt fluidity of (65/35/x) the PETG/PGA/ADR blends were (6 more5/35/x) complicated. samples. A In plot addition of MFR to versus PETG–ADR– ADR contentPETG andis shown PGA–ADR–PGA in Figure 9. The with addition chain extension/long-branched of ADR resulted in a pronounced structures, decrease the PETG– in theADR–PGA MFR of the copolymer PETG/PGA/ADR could also beblends. generated The in introduction situ through of chain0.9 phr extension/branching of ADR into the blendreactions led to during a decrease blending. in the MFR value from 55.3 g/10 min for the PETG/PGA (65/35) binaryMFR sample testing to 12.7 was conductedg/10 min. toThis further could evaluate be mainly the impactattributed of the to ADRthe chain concentration extension/branchingon the melt fluidity reactions of the between PETG/PGA/ADR the polyesters (65/35/x) and ADR. samples. Based A on plot the of rheological MFR versus characteristicsADR content isduring shown polymer in Figure processing9. The addition and studies of ADR on resulted PLA/PBAT in a pronouncedblends [20], decreaseit could bein confirmed the MFR ofthat the the PETG/PGA/ADR increase in molecular blends. weight The and introduction the formation of 0.9 of phr long-branched of ADR into structuresthe blend were led tokey a factors decrease that in increased the MFR the value melt from viscosity 55.3 g/10of the minpolyesters. for the These PETG/PGA were a (65/35)result of binary modification sample with to 12.7 multiple g/10 epoxy min. Thischain could extenders. be mainly attributed to the chain extension/branching reactions between the polyesters and ADR. Based on the rheological characteristics during polymer processing and studies on PLA/PBAT blends [20], it could be confirmed that the increase in molecular weight and the formation of long-branched structures were key factors that increased the melt viscosity of the polyesters. These were a result of modification with multiple epoxy chain extenders. Obviously, an increase in the viscosity of a PETG/PGA/ADR (65/35/x) sample could facilitate its further processing, since a high melt viscosity is a prerequisite for processes such as extrusion, blown film, thermoforming, etc. Moreover, since the multiple epoxy groups of ADR could react with the terminal –OH and –COOH groups of PETG and PGA, theoretically, ADR could be used as a reactive compatibilizer to improve the interfacial compatibility in PETG/PGA blends. In this manner, the tensile performance and micromorphology of the PETG/PGA blend could be regulated by controlling the ADR content.

Polymers 2021, 13, x FOR PEER REVIEW 13 of 22

Polymers 2021, 13, 452 12 of 20

40 ) -1

20 MFR min (g·10 10

0.0 0.3 0.6 0.9 ADR content (phr)

Figure 9. Effect of ADR addition on MFR of PETG/PGA/ADR (65/35/x) samples. Figure 9. Effect of ADR addition on MFR of PETG/PGA/ADR (65/35/x) samples. 3.2.2. Tensile Performance The tensile properties of the neat PETG and PETG/PGA/ADR (65/35/x) blends are Obviously,listed in Table 4an. The increase typical tensilein the stress–strain viscosity curvesof a PETG/PGA/ADR of these samples are shown(65/35/x) in sample could facilitateFigure its 10 further. Compared processing to neat PETG,, since the PETG/PGA a high melt (65/35) viscosity blend showed is a prerequisite poor tensile for processes such ductility,as extrusion, due to a blown drastic decrease film, thermoformin in the elongation atg, break etc. fromMoreover, 223.0% (neat since PETG) the to multiple epoxy 11.1%. It is worth noting that the elongation at break of the PETG/PGA/ADR (65/35/x) groupssamples of ADR increased could with react an increase with in the the ADRterminal content. –OH Upon and the addition –COOH of 0.3 groups phr of of PETG and PGA,ADR, theoretically, the elongation ADR at break could increased be used from 11.1%as a forreactive the binary compatibilizer sample to 123.2%, to an improve the in- terfacialincrease compatibility of 1010%. With in the PETG/PGA further addition blends. of 0.9 phr In ADR, this themanner, elongation the at breaktensile of theperformance and ternary sample reached 160.8%. This improvement in tensile ductility could be attributed micromorphologyto the reactive compatibilization of the PETG/PGA effect of ADR blend on the co PETG/PGAuld be regulated interface. On by the controlling other the ADR content.hand, both the tensile yield strength and the tensile modulus of the PETG/PGA (65/35) binary sample were markedly higher than those for neat PETG, due to the reinforcing effect of PGA. Furthermore, compared to the PETG/PGA (65/35) binary sample, the tensile yield 3.2.2. strengthsTensile ofPerformance the PETG/PGA/ADR (65/35/x) samples remained unchanged, with only Thea slight tensile decrease properties in tensile modulus of the withneat an PETG increase and in ADR PETG/PGA/ADR content from 0 to 0.9 (65/35/x) phr. blends are From Table4, it can be observed that the PETG/PGA (65/35) blend modiﬁed with 0.9 listedphr in ofTable ADR showed4. The thetypical most balancedtensile tensilestress–s performance.train curves Compared of these to neat samples PETG, are shown in Figurethe 10. PETG/PGA/ADR Compared to (65/35/0.9) neat PETG, sample the showed PETG/PGA an enhanced (65/35) tensile blend yield strength showed poor tensile ductility,and tensile due to modulus, a drastic which decrease corresponded in the to increases elongation of 9.1 at and break 30.4%, from respectively. 223.0% At (neat PETG) to the same time, the ternary sample also showed relatively good tensile ductility, with an 11.1%.elongation It is worth at break noting of 160.8%. that the elongation at break of the PETG/PGA/ADR (65/35/x) samples increased with an increase in the ADR content. Upon the addition of 0.3 phr of TableADR, 4. Tensile the performanceelongation of neat at PETGbreak and increased PETG/PGA/ADR from (65/35/x) 11.1% samples. for the binary sample to 123.2%, an Samplesincrease Tensileof 1010%. Yield Strength With (MPa)the further Tensile addition Modulus (MPa)of 0.9 phr Elongation ADR, atthe Break elongation (%) at break of Neat PETGthe ternary sample 44.38 ± reached0.62 160.8%. This 1076 ± improvement35 in 223.0 tensile± 10.2 ductility could be at- PETG/PGA (65/35) 48.24 ± 0.72 1509 ± 31 11.1 ± 1.8 PETG/PGA/ADR (65/35/0.3)tributed to the 48.73reactive± 0.69 compatibilization 1512 ± ef15fect of ADR on 123.2 the± PETG/PGA11.4 interface. On PETG/PGA/ADR (65/35/0.6)the other hand, 48.06 both± 0.80the tensile yield streng 1450 ± 20th and the tensile 135.4 modulus± 5.1 of the PETG/PGA ± ± ± PETG/PGA/ADR (65/35/0.9)(65/35) binary sample 48.43 1.28 were markedly 1403 higher58 than those for 160.8 neat9.4 PETG, due to the reinforcing effect of PGA. Furthermore, compared to the PETG/PGA (65/35) binary sample, the tensile yield strengths of the PETG/PGA/ADR (65/35/x) samples remained unchanged, with only a slight decrease in tensile modulus with an increase in ADR content from 0 to 0.9 phr. From Table 4, it can be observed that the PETG/PGA (65/35) blend modified with 0.9 phr of ADR showed the most balanced tensile performance. Compared to neat PETG, the PETG/PGA/ADR (65/35/0.9) sample showed an enhanced tensile yield strength and tensile modulus, which corresponded to increases of 9.1 and 30.4%, respectively. At the same time, the ternary sample also showed relatively good tensile ductility, with an elongation at break of 160.8%.

Table 4. Tensile performance of neat PETG and PETG/PGA/ADR (65/35/x) samples.

Samples Tensile yield strength Tensile modulus Elongation at break

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(MPa) (MPa) (%)

Neat PETG 44.38 ± 0.62 1076 ± 35 223.0 ± 10.2

PETG/PGA (65/35) 48.24 ± 0.72 1509 ± 31 11.1 ± 1.8

PETG/PGA/ADR (65/35/0.3) 48.73 ± 0.69 1512 ± 15 123.2 ± 11.4

PETG/PGA/ADR (65/35/0.6) 48.06 ± 0.80 1450 ± 20 135.4 ± 5.1

Polymers 2021, 13, 452 PETG/PGA/ADR (65/35/0.9) 48.43 ± 1.28 1403 ± 58 160.8 ± 9.413 of 20

80 1- PETG 70 2- PETG/PGA (65/35) 3- PETG/PGA/ADR (65/35/0.3) 60 4- PETG/PGA/ADR (65/35/0.6) 5- PETG/PGA/ADR (65/35/0.9) 50

30 Stress (MPa) Stress

20 2 3 4 5 1

0 0 30 60 90 120 150 180 210 Strain (%) Figure 10. Tensile stress–strainstress–strain curvescurves forfor neatneat PETGPETG andand PETG/PGA/ADRPETG/PGA/ADR (65/35/x) (65/35/x) samples. samples.

In summary,summary, with with ADR ADR chain chain extension extension and compatibilization,and compatibilization, the rheological the rheological proper- tiesproperties and tensile and ductilitytensile ductility of the PETG/PGA/ADR of the PETG/PGA/ADR (65/35/x) (65/35/x) blend blend were greatly were greatly improved. improved. 3.2.3. TEM Observation 3.2.3.Microstructural TEM Observation evolution is known to greatly inﬂuence the mechanical performance of a blend.Microstructural Hence, TEM evolution analysis is was known used to to grea investigatetly influence the impact the mechanical of adding performance ADR on the micromorphologyof a blend. Hence, TEM of PETG/PGA/ADR analysis was used (65/35/x) to investigate samples the impact with varying of adding ADR ADR contents. on the Thismicromorphology would help to furtherof PETG/PGA/ADR elucidate the relationship(65/35/x) samples between with the tensilevarying performance ADR contents. and Polymers 2021, 13, x FOR PEER REVIEWmicromorphology of the samples with different ADR contents. 15 of 22 This would help to further elucidate the relationship between the tensile performance The typical TEM images of the PETG/PGA/ADR (65/35/x) samples at 1000x magni- and micromorphology of the samples with different ADR contents. ﬁcation are presented in Figure 11. The range of the particle size distribution and apparent The typical TEM images of− the PETG/PGA/ADR (65/35/x) samples at 1000x magni- number-averageficationADR–PGA are copolymerspresented particle inin sizes Figuresitu. (Thesedapp 11.) ofcopolyThe PGA rangmers fore theofcould the ternary improveparticle samples thesize PETG/PGA distribution were obtained interfa- and using ap- averagecial adhesion diameter and analysisthus significantly software re (Nanoduce_ the Measurer size of the 1.2), PGA as shown domains. in Figure 12. parent number-average particle sizes (dapp) of PGA for the ternary samples were obtained

using average diameter analysis software (Nano Measurer 1.2), as shown in Figure 12. TEM analysis showed the presence of many oversized PGA particles dispersed in the PETG/PGA (65/35) binary sample. This was due to the serious combining of the PGA domains as a result of the poor interfacial compatibility in the PETG/PGA blending sys- _ tem. However, upon the addition of 0.3 phr of ADR, the dapp of the PGA domains de- _ creased significantly. When the amount of ADR increased from 0.3 to 0.6 phr, the dapp further decreased from 1.24 to 0.29 μm. As the amount of added ADR increased to 0.9 _ phr, the value of dapp no longer decreased. That could be explained by the fact that the chain extension/branching reactions of PETG, PGA, and ADR could form some PETG–

Figure 11. TEM images (×1000) of PETG/PGA/ADR (65/35/x; x = 0 (A), 0.3 (B), 0.6 (C), and 0.9 (D)) samples. Figure 11. TEM images (×1000) of PETG/PGA/ADR (65/35/x; x = 0 (A), 0.3 (B), 0.6 (C), and 0.9 (D)) samples.

Figure 12. Statistical results for apparent particle sizes of PETG/PGA/ADR (65/35/x; x = 0.3 (A), 0.6 (B), and 0.9 (C)) samples.

Polymers 2021, 13, x FOR PEER REVIEW 15 of 22

ADR–PGA copolymers in situ. These copolymers could improve the PETG/PGA interfacial adhesion and thus significantly reduce the size of the PGA domains.

Polymers 2021, 13, 452 14 of 20 Figure 11. TEM images (×1000) of PETG/PGA/ADR (65/35/x; x = 0 (A), 0.3 (B), 0.6 (C), and 0.9 (D)) samples.

Figure 12. Statistical resultsFigure for appare 12. Statisticalnt particle results sizes of for PETG/PGA/ADR apparent particle (65/35/x; sizes of PETG/PGA/ADRx = 0.3 (A), 0.6 (B), (65/35/x; and 0.9 (C) x) = sam- 0.3 (A), 0.6 (B), and 0.9 (C)) samples. ples.

3.2.4. TEMSEM analysisObservation showed the presence of many oversized PGA particles dispersed in the PETG/PGA (65/35) binary sample. This was due to the serious combining of the PGAIn domains order to as research a result ofthe the micromorpholog poor interfacial compatibilityical evolution in of the the PETG/PGA PETG/PGA blending (65/35) samples with and without ADR during the tensile process, the− cryofractured surfaces of thesystem. tensile However, fractured upon specimens the addition along the of tens 0.3 phrile direction of ADR, were the d photographed,app of the PGA including domains the stretching-oriented parts and the unstretched parts, as shown in Figure 13. The− un- decreased signiﬁcantly. When the amount of ADR increased from 0.3 to 0.6 phr, the dapp further decreased from 1.24 to 0.29 µm. As the amount of added ADR increased to 0.9 phr, − the value of dapp no longer decreased. That could be explained by the fact that the chain extension/branching reactions of PETG, PGA, and ADR could form some PETG–ADR– PGA copolymers in situ. These copolymers could improve the PETG/PGA interfacial adhesion and thus signiﬁcantly reduce the size of the PGA domains.

3.2.4. SEM Observation In order to research the micromorphological evolution of the PETG/PGA (65/35) samples with and without ADR during the tensile process, the cryofractured surfaces of the tensile fractured specimens along the tensile direction were photographed, including the stretching-oriented parts and the unstretched parts, as shown in Figure 13. The unstretched part of the PETG/PGA (65/35) sample (see Figure 13A) showed many oversized PGA particles (with diameters more than 5 µm) dispersed in the matrix. This could be attributed to the poor compatibility at the PETG/PGA interface and excessive amount of PGA. Owing to their brittle fracturing behavior (in terms of their low elongations at break), there was no obvious tensile yielding and necking observed in the PETG/PGA (65/35) specimens before they ruptured during the tensile testing process. Hence, SEM images of the part oriented in the tensile direction of the PETG/PGA (65/35) sample could not be included in Figure 13. In the case of the unstretched parts of the PETG/PGA/ADR (65/35/x) samples (see Figure 13(A2–A4), the PGA particles were seen embedded in the matrix with a blurred interface. Moreover, the domain size of the PGA decreased signiﬁcantly upon the addition of ADR. The distribution of the diameters of the PGA particles and the apparent number- − average particle sizes (dapp) of PETG/PGA (65/35) with different ADR contents were determined from the TEM images (see Figures 11 and 12). The reactive compatibilization by ADR at the PETG/PGA interface was responsible for the decrease in the size of the PGA domains with a blurred interface, as discussed in the section “TEM observation”. Polymers 2021, 13, x FOR PEER REVIEW 16 of 22

stretched part of the PETG/PGA (65/35) sample (see Figure 13-A) showed many oversized PGA particles (with diameters more than 5 μm) dispersed in the matrix. This could be attributed to the poor compatibility at the PETG/PGA interface and excessive amount of PGA. Owing to their brittle fracturing behavior (in terms of their low elongations at break), there was no obvious tensile yielding and necking observed in the PETG/PGA (65/35) specimens before they ruptured during the tensile testing process. Hence, SEM images of the part oriented in the tensile direction of the PETG/PGA (65/35) sample could not be included in Figure 13. In the case of the unstretched parts of the PETG/PGA/ADR (65/35/x) samples (see Figure 13-A2, A3, and A4), the PGA particles were seen embedded in the matrix with a blurred interface. Moreover, the domain size of the PGA decreased significantly upon the addition of ADR. The distribution of the diameters of the PGA _ particles and the apparent number-average particle sizes (dapp) of PETG/PGA (65/35) with different ADR contents were determined from the TEM images (see Figures 11 and 12). The reactive compatibilization by ADR at the PETG/PGA interface was responsible for the decrease in the size of the PGA domains with a blurred interface, as discussed in the section “TEM observation”. Figure 13-B2, B3, and B4 show the SEM photographs of the stretching-oriented parts Polymers 2021, 13, 452 15 of 20 of the PETG/PGA/ADR (65/35/x) samples with different ADR contents. For the PETG/PGA/ADR (65/35/0.3) sample, as shown in Figure 13-B2, many PGA particles were seen debonded from the PETG matrix, accompanied by a highly oriented PETG matrix.

Polymers 2021, 13, x FOR PEER REVIEW 17 of 22

Figure 13. SEMSEM imagesimages ((×3000)×3000) of cryofracturedcryofractured surfacessurfaces ofof PETG/PGA/ADRPETG/PGA/ADR tensil tensilee fractured fractured specimens specimens along along tensile direction. (A) (A )unstretched unstretched part part (A1—65/ ((A1)—65/35/0;35/0; A2—65/35/0.3; (A2)—65/35/0.3; A3—65/35/0.6; (A3)—65/35/0.6; A4—65/35/ (A40.9);)—65/35/0.9); (B) stretching-oriented (B) stretching- part (B2—65/35/0.3;oriented part (( B2B3—65/35/0.6;)—65/35/0.3; B4—65/35/0.9). (B3)—65/35/0.6; (B4)—65/35/0.9).

AsFigure compared 13(B2–B4) to the show oversized the SEM PGA photographs particles of of the the PETG/PGAstretching-oriented (65/35) binary parts ofsam- the ple,PETG/PGA/ADR the finer PGA (65/35/x)particles of samples the PETG/PGA/ADR with different ADR (65/35/0.3) contents. sample For the showed PETG/PGA/ADR significant ability(65/35/0.3) to induce sample, the asshear shown yielding in Figure of the 13 surrounding(B2), many PGA matrix. particles The formation were seen of debonded massive shearfrom thebands PETG was matrix, conducive accompanied to the termination by a highly of oriented craze and PETG a stable matrix. orientation. As a result, theAs comparedtensile ductility to the of oversized the ternary PGA samp particlesle was of significantly the PETG/PGA improved. (65/35) With binary a sample,further increasethe ﬁner in PGA the particlesaddition ofof the ADR PETG/PGA/ADR to 0.6 and 0.9 phr, (65/35/0.3) the degree sample of the showed orientation signiﬁcant of the PETGability matrix to induce increased, the shear accompanied yielding of theby the surrounding formation matrix. of a more The intensively formation oforientated massive structure. Moreover, for the parts oriented along the tensile direction, it is worth noting that the interfacial debonding of the PGA particles could be effectively restrained with an increase in ADR content. This demonstrates the efficient reactive compatibilization effect of ADR on the increase in the PETG/PGA interfacial adhesion. Overall, with an increase in ADR content, the domain size of the PGA decreased significantly. Additionally, the increased interfacial adhesion was more conducive to an increase in the tensile ductility of the PETG/PGA samples modified with ADR. However, compared with the oversized PGA particles of the PETG/PGA (65/35) binary sample, the finer PGA particles of the PETG/PGA/ADR (65/35/x) samples could result in more stress concentration around these particles under tensile loads, leading to a decrease in the inflexibility of the PETG matrix. As a result, the tensile moduli of the PETG/PGA/ADR (65/35/x) samples decreased slightly from 1509 to 1403 MPa when the ADR content increased from 0 to 0.9 phr, as shown in Table 4. In summary, the SEM results are consistent with the findings of the tensile testing results and TEM analysis. It was also confirmed that ADR had a pronounced reactive compatibilization effect, which tailored the interfacial adhesion and micromorphology of the PETG/PGA blends. As a result, a series of PETG/PGA/ADR blends with improved tensile performance were prepared.

3.2.5. DSC and WAXD Measurements The effect of adding ADR on the crystallization properties of the PETG/PGA (65/35) samples with varying ADR contents was investigated by DSC and WAXD measurements.The heating DSC thermograms of the PETG/PGA/ADR samples are presented in Figure 14, and their corresponding thermodynamic parameters are summarized in Table 5.

Table 5. DSC parameters of neat PGA, PETG, and PETG/PGA/ADR (65/35/x) samples.

Tg (℃) Tm (℃) △Hm (J/g) Xc (%) Samples (PETG) (PGA) (PGA) (PGA)

Polymers 2021, 13, 452 16 of 20

Polymers 2021, 13, x FOR PEER REVIEW 18 of 22 shear bands was conducive to the termination of craze and a stable orientation. As a result, the tensile ductility of the ternary sample was significantly improved. With a further increase in the addition of ADR to 0.6 and 0.9 phr, the degree of the orientation of the PETG matrix increased, accompanied by the formation of a more intensively orientated structure. Neat PGA Moreover, for the N/A parts oriented along 224.9the tensile direction, it is− worth74.67 noting that the 37.7 interfacial debonding of the PGA particles could be effectively restrained with an increase in Neat PETG ADR content. This 74.9 demonstrates the efficient N/A reactive compatibilization N/A effect of ADR on the N/A increase in the PETG/PGA interfacial adhesion. Overall, with an increase in ADR content, PETG/PGA (65/35) the domain size73.1 of the PGA decreased significantly. 221.0 Additionally, the−24.96 increased interfacial 36.0 adhesion was more conducive to an increase in the tensile ductility of the PETG/PGA PETG/PGA/ADR (65/35/0.3)samples modified 73.3 with ADR. However, 220.4 compared with the oversized−22.15 PGA particles of 32.0 the PETG/PGA (65/35) binary sample, the finer PGA particles of the PETG/PGA/ADR PETG/PGA/ADR (65/35/0.6)(65/35/x) samples 71.9 could result in more stress 220.1 concentration around−16.25 these particles under 23.6 tensile loads, leading to a decrease in the inflexibility of the PETG matrix. As a result, the PETG/PGA/ADR (65/35/0.9)tensile moduli of 72.6 the PETG/PGA/ADR (65/35/x) 219.9 samples decreased−11.28 slightly from 1509 to 16.4 1403 MPa when the ADR content increased from 0 to 0.9 phr, as shown in Table4. InThe summary, DSC thermogram the SEM results of are PETG/PGA consistent with (65/35) the findings showed of thea small tensile step testing at 73.1°C and a strongresults andendothermic TEM analysis. peak It was at also221.0°C. confirmed They that were ADR attributed had a pronounced to the reactiveglass transition tem- peraturecompatibilization of the effect,amorphous which tailored PETG the matrix interfacial and adhesion melting and peak micromorphology of the semicrystalline of PGA component,the PETG/PGA respectively. blends. As a result, It was a series confirmed of PETG/PGA/ADR by the data blendsin Table with 5 improved that there was no sig- tensile performance were prepared. nificant shift in the Tg of the PETG matrix and Tm of the PGA component. However, the crystallinity3.2.5. DSC and of WAXD the PGA Measurements component in the PETG/PGA/ADR (65/35/x) samples decreased fromThe 36.0 effect to 16.2% of adding with ADR an on increase the crystallization in ADR propertiescontent from of the PETG/PGA0 to 0.9 phr. (65/35) The increase in the irregularitysamples with varyingof the ADR PGA contents macromolecular was investigated segments by DSC and caused WAXD by measure- the chain exten- ments.The heating DSC thermograms of the PETG/PGA/ADR samples are presented in sion/branchingFigure 14, and their reactions corresponding between thermodynamic PGA, PETG, parameters and are ADR summarized were responsiblein Table5 . for the decrease in the crystallinity of PGA component.

10°C/min PETG

PETG/PGA/ADR (65/35/0.9)

PETG/PGA/ADR (65/35/0.6) EXO PETG/PGA/ADR (65/35/0.3) ) -1 PETG/PGA (65/35) PGA Heat Flow (W·g Flow Heat

50 100 150 200

Temperature(°C) Figure 14. Heating DSC thermograms for neat PETG, PGA, and PETG/PGA/ADR Figure(65/35/x) 14. samples. Heating DSC thermograms for neat PETG, PGA, and PETG/PGA/ADR (65/35/x) samples.

The WAXD patterns of the PETG/PGA/ADR (65/35/x) samples are shown in Figure 15. Consistent with the DSC results, the WAXD patterns of neat PETG showed a dispersive diffraction peak in the range of 2θ = 10° to 35°, suggesting an amorphous nature for neat PETG. The PETG/PGA (65/35) sample showed two shaper peaks at 22.2° and 28.8°, corresponding to the (110) and (020) crystal planes of PGA crystals, respectively. Fur- thermore, the PETG/PGA (65/35) samples modified with ADR showed a gradual decrease in the peak intensities of the PGA component with an increase in ADR content. This could be attributed to the decrease in the crystallinity of the PGA component resulting from chain extension/branching reactions between PGA, PETG, and ADR.

Polymers 2021, 13, 452 17 of 20

Table 5. DSC parameters of neat PGA, PETG, and PETG/PGA/ADR (65/35/x) samples.

T (◦C) T (◦C) ∆H (J/g) X (%) Samples g m m c (PETG) (PGA) (PGA) (PGA) Neat PGA N/A 224.9 −74.67 37.7 Neat PETG 74.9 N/A N/A N/A PETG/PGA (65/35) 73.1 221.0 −24.96 36.0 PETG/PGA/ADR (65/35/0.3) 73.3 220.4 −22.15 32.0 PETG/PGA/ADR (65/35/0.6) 71.9 220.1 −16.25 23.6 PETG/PGA/ADR (65/35/0.9) 72.6 219.9 −11.28 16.4

The DSC thermogram of PETG/PGA (65/35) showed a small step at 73.1 ◦C and a strong endothermic peak at 221.0 ◦C. They were attributed to the glass transition temperature of the amorphous PETG matrix and melting peak of the semicrystalline PGA component, respectively. It was confirmed by the data in Table5 that there was no significant shift in the Tg of the PETG matrix and Tm of the PGA component. However, the crystallinity of the PGA component in the PETG/PGA/ADR (65/35/x) samples decreased from 36.0 to 16.2% with an increase in ADR content from 0 to 0.9 phr. The increase in the irregularity of the PGA macromolecular segments caused by the chain extension/branching reactions between PGA, PETG, and ADR were responsible for the decrease in the crystallinity of PGA component. The WAXD patterns of the PETG/PGA/ADR (65/35/x) samples are shown in Figure 15. Consistent with the DSC results, the WAXD patterns of neat PETG showed a dispersive diffraction peak in the range of 2θ = 10◦ to 35◦, suggesting an amorphous nature for neat PETG. The PETG/PGA (65/35) sample showed two shaper peaks at 22.2◦ and 28.8◦, corre- Polymers 2021, 13, x FOR PEER REVIEW sponding to the (110) and (020) crystal planes of PGA crystals, 19 of 22 respectively. Furthermore, the PETG/PGA (65/35) samples modified with ADR showed a gradual decrease in the

peak intensities of the PGA component with an increase in ADR content. This could be attributed to the decrease in the crystallinity of the PGA component resulting from chain extension/branching reactions between PGA, PETG, and ADR.

PGA

PETG/PGA/ADR (65/35/0.9)

PETG/PGA/ADR (65/35/0.6)

Intensity PETG/PGA/ADR (65/35/0.3)

PETG/PGA (65/35)

PETG

5 10152025303540 2θ (°)

Figure 15. WAXD patterns of neat PETG, PGA, and PETG/PGA/ADR (65/35/x) samples. Figure 15. WAXD patterns of neat PETG, PGA, and PETG/PGA/ADR (65/35/x) samples. 3.2.6. Vicat Softening Temperature (VST) 3.2.6. Vicat SofteningThe VST Temperature values of the (VST) neat PETG and PETG/PGA (65/35) samples modiﬁed with ◦ Thedifferent VST values ADR of contents the neat are PETG listed and in Table PETG/PGA6. The VST (65/35) of neat samples PETG wasmodified only 81.0with C. This ◦ differentcould ADR becontents attributed are listed to the in lower TableT 6.g (74.9The VSTC) of of neat the amorphousPETG was only PETG 81.0°C. matrix. This The three could beknown attributed ways to to the enhance lower theTg heat(74.9°C) resistance of the ofamorphous thermoplastic PETG polymers matrix. includeThe three increasing known waysthe T tog, increasingenhance the the heat crystallinity, resistance of and thermoplastic reinforcement polymers with rigid include inorganic/organic increasing particles [31]. However, both increasing the T and increasing the crystallinity of the PETG the Tg, increasing the crystallinity, and reinforcementg with rigid inorganic/organic parti- matrix are inapplicable for the amorphous PETG resin, which cannot be crystallized. Hence, cles [31]. However, both increasing the Tg and increasing the crystallinity of the PETG matrix are inapplicable for the amorphous PETG resin, which cannot be crystallized. Hence, in this work, increasing the heat resistance of the PETG matrix mainly depended on the reinforcement effect of rigid PGA particles. As expected, the addition of 35 wt% PGA caused the VST of the PETG/PGA (65/35) sample to increase to 99.2°C, which was 18.2°C higher than that of neat PETG. Based on the DSC results, the PGA particles dispersed in the PETG/PGA (65/35) sample had a semicrystalline nature with a higher Tm (221.0°C) and crystallinity (36.0%). These rigid PGA particles having a higher Tm and crystallinity could prevent the deformation of the PETG matrix during heating. However, with an increase in the ADR concentration, the VST of the PETG/PGA/ADR (65/35/x) samples decreased gradually. For example, when the ADR content increased to 0.6 phr, the VST of the ternary sample decreased from 99.2 (PETG/PGA (65/35) binary sample) to 94.1°C. As the ADR content increased to 0.9 phr, the VST further dropped to 90.5°C. The weakening of reinforcing effect of PGA particles on the heat resistance of the PETG/PGA/ADR samples could be mainly attributed to the decreased crystallinity of the PGA particles. This was the result of chain extension/branching reactions between PGA, PETG, and ADR.

Table 6. VSTs of neat PETG and PETG/PGA/ADR (65/35/x) samples.

Samples VST (°C)

Neat PETG 81.0

PETG/PGA (65/35) 99.2

PETG/PGA/ADR (65/35/0.3) 95.0

PETG/PGA/ADR (65/35/0.6) 94.1

Polymers 2021, 13, 452 18 of 20

in this work, increasing the heat resistance of the PETG matrix mainly depended on the reinforcement effect of rigid PGA particles. As expected, the addition of 35 wt% PGA caused the VST of the PETG/PGA (65/35) sample to increase to 99.2 ◦C, which was 18.2◦C higher than that of neat PETG. Based on the DSC results, the PGA particles dispersed in ◦ the PETG/PGA (65/35) sample had a semicrystalline nature with a higher Tm (221.0 C) and crystallinity (36.0%). These rigid PGA particles having a higher Tm and crystallinity could prevent the deformation of the PETG matrix during heating. However, with an increase in the ADR concentration, the VST of the PETG/PGA/ADR (65/35/x) samples decreased gradually. For example, when the ADR content increased to 0.6 phr, the VST of the ternary sample decreased from 99.2 (PETG/PGA (65/35) binary sample) to 94.1 ◦C. As the ADR content increased to 0.9 phr, the VST further dropped to 90.5 ◦C. The weakening of reinforcing effect of PGA particles on the heat resistance of the PETG/PGA/ADR samples could be mainly attributed to the decreased crystallinity of the PGA particles. This was the result of chain extension/branching reactions between PGA, PETG, and ADR.

Table 6. VSTs of neat PETG and PETG/PGA/ADR (65/35/x) samples.

Samples VST (◦C) Neat PETG 81.0 PETG/PGA (65/35) 99.2 PETG/PGA/ADR (65/35/0.3) 95.0 PETG/PGA/ADR (65/35/0.6) 94.1 PETG/PGA/ADR (65/35/0.9) 90.5

4. Conclusions In this paper, PGA was used as a reinforcing component to improve the mechanical properties of PETG. When the PGA content increased to 35 wt%, the tensile yield strength/tensile modulus of the PETG/PGA (65/35) sample increased from 44.38/1076 MPa for the neat PETG to 48.4/1509 MPa. Meanwhile, the VST increased from 81.0 to 99.2 ◦C. However, due to the poor interfacial compatibility between the PETG matrix and PGA domains, the tensile ductility of the PETG/PGA (65/35) sample decreased drastically. In addition, the high melt fluidity of PGA largely increased the MFR of the PETG/PGA blend, which was not conducive to its processing by some common processing technologies, such as extrusion, blown film, thermoforming, etc. In order to improve the interfacial compatibility and rheological properties of the PETG/PGA (65/35) blend, ADR was introduced as a reactive modifier. The torque and MFR testing showed that the melt viscosity of the PETG/PGA blend increased significantly with an increase in ADR content. In addition, micromorphological analysis indicated that ADR had a strong reactive compatibilizing effect on the blend, which increased the interfacial adhesion in the PETG/PGA, accompanied by a decrease in the PGA domain size. As a result, the tensile ductility of the PETG/PGA (65/35) blend modified with ADR was significantly improved, in terms of the elongation at break. DSC and WAXD analyses showed that the PETG matrix was amorphous and that the PGA component was semicrystalline in nature. Moreover, the crystallinity of the PGA decreased gradually with an increase in ADR content. The decrease in the crystallinity of the PGA component due to the addition of ADR reduced the reinforcing effect of PGA on the heat resistance of the PETG/PGA/ADR (65/35/x) samples. In summary, by carefully controlling the ADR concentration, the micromorphology, mechanical performance, and rheological properties of PETG/PGA blends modified with ADR can be effectively regulated and balanced for meeting the requirements of different applications in a better manner.

Author Contributions: Conceptualization, K.W., N.X., and S.P.; methodology, K.W. and N.X.; val- idation, K.W.; formal analysis, K.W., J.S., and N.X.; investigation, K.W., J.S., Z.M., Y.Z., and N.X.; resources, N.X.; data curation, K.W. and N.X.; writing—original draft preparation, K.W.; writing— Polymers 2021, 13, 452 19 of 20

Table 10: tipa Phonetic Symbols review and editing, K.W., N.X., and S.P.; visualization, K.W.; supervision, N.X. and S.P.; project È \textbabygamma P \textglotstopadministration, N.X. Allï authors\textrtailn have read and agreed to the published version of the manuscript. b \textbarb ; \texthalflengthFunding: This researchó received\textrtailr no external funding. c \textbarc ż \texthardsign ù \textrtails Institutional Review Board Statement: d \textbard # \texthooktop ú \textrtailtNot applicable for studies not involving humans or animals. é \textbardotlessj á \texthtbInformed Consent Statement:ü \textrtailzNot applicable for studies not involving humans. g \textbarg ê \texthtbardotlessj $ \textrthook Data Availability Statement: The data presented in this study are available on request from the Ü \textbarglotstop Á \texthtc À \textsca corresponding author and the first author. 1 \textbari â \texthtd à \textscb ł \textbarl ä \texthtgAcknowledgments: Theď authors\textsce gratefully acknowledge support from the Analytical & Testing 8 \textbaro H \texththCenter of Hainan Universityå \textscg and Shiner National & Local Joint Engineering & Research Center of Ý \textbarrevglotstop Ê \texththengShiner Industrial Co.,Ë Ltd.,\textsch Haikou, China. 0 \textbaru Î \texthtk @ \textschwa Conflicts of Interest: The authors declare no conflict of interest. ì \textbeltl Ò \texthtp I \textsci B \textbetaReferences Ó \texthtq ĺ \textscj ò \textbullseye č \texthtrtaild Ï \textscl 1. 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