Mechanical properties of thermoplastic polyester elastomer controlled by blending with poly(butylene terephthalate)

Polymer Testing 55 (2016) 152e159

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Material Properties

Mechanical properties of thermoplastic polyester elastomer controlled by blending with poly(butylene terephthalate) Jing Huang a, Jun Wang b, Yaxin Qiu a, Defeng Wu a, c, * a

School of Chemistry & Chemical Engineering, Yangzhou University, Jiangsu, 225002, China Jinsen Photoelectric Material Co. Ltd., Yangzhou, Jiangsu, 225009, China c Provincial Key Laboratories of Environmental Engineering & Materials, Jiangsu, 225002, China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 12 July 2016 Received in revised form 15 August 2016 Accepted 24 August 2016 Available online 26 August 2016

Thermoplastic polyester elastomer (TPEE) blends with poly(butylene terephthalate) (PBT) were prepared by melt compounding for the phase morphology and mechanical property studies. Although PBT is immiscible with the continuous soft poly(tetramethylene glycol) (PTMEG) phase of TPEE, it is miscible with the discrete hard PBT one of TPEE. Therefore, PBT and TPEE are compatible and their blends reveal very low level of interfacial tension and very small size of discrete domains, as well as good interfacial adhesion between two phases, which provide high possibility to prepare TPEE alloys with controllable properties. Mechanical test results reveal that both the modulus and yield and tensile strengths increase with increasing weight ratios of PBT. The increased system rigidity and decreased system plasticity are further confirmed by the cyclic tensile tests. The main objective of this work is to provide useful information on the structure and property control of TPEE by simple mixing with aromatic polyesters. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Thermoplastic polyester elastomer Poly(butylene terephthalate) Blends Morphology Mechanical properties

1. Introduction As a new member in the thermoplastic elastomer (TPE) family, thermoplastic polyester elastomer (TPEE) has recently attracted much attention for the application in automotive, fluid power and electronic, as well as sporting goods [1]. Its hard segments are commonly crystalline polyesters such as poly(butylene terephthalate) or poly(ethylene terephthalate), and the soft segments are amorphous polyethers such as poly(tetramethylene ether glycol terephthalate) or poly(alkylene glycol) [2]. Similar with other members in the TPE family, the hard crystalline domains of TPEE determine elastomeric properties and thermal resistance, while the soft amorphous ones exhibit good low-temperature flexibility [3]. Accordingly, TPEE shows combination of high-temperature mechanical properties and good low-temperature flexibility [4,5]. Besides, it has better resistance to tear and impact, and higher resistance to oil and hydraulic fluids relative to the traditional TPEs [6,7]. Hybridization with micro-sized or nano-sized particles, such as clays, talc, mica, silica, fly ash, and carbon nanotubes [8e12], is a

* Corresponding author. School of Chemistry & Chemical Engineering, Yangzhou University, Jiangsu, 225002, China. E-mail address: [email protected] (D. Wu). http://dx.doi.org/10.1016/j.polymertesting.2016.08.020 0142-9418/© 2016 Elsevier Ltd. All rights reserved.

commonly used way to improve overall properties of TPEE. The reported results reveal that the composite technology is very effective for the reinforcement and stability improvement of TPEE. It is also an effective strategy to endow TPEE with new properties, for instance, electrical conductivity. Another convenient way of control of the structure and final properties of TPEE is to blend it with the polymers with various intrinsic properties, including crystalline and amorphous polyesters, and polyolefin [5e7,12e17]. Some new TPEE based blend materials and/or alloys with excellent flex fatigue and broadened service temperature range has been developed successfully using the blending technology. For instance, Hussain et al. [12] found a significant improvement in both the mechanical and thermal properties of TPEE using thermoplastic poly(butylene terephthalate) (PBT) as the blended component. With addition of a small amount of clay, there was further property improvement. But the reported work on the TPEE blend systems is still very limited because TPEE is a newly-developed material. Some key issues around phase structure and structure-property relationships in the TPEE based blends, for instance, how the presence of second polymer component affect phase separation structure of TPEE, and how the multiphase structure of TPEE blends affect their final properties, etc., are not very clear. Therefore, in this work, a widely used aromatic polyester, PBT, which has the same chain structure

J. Huang et al. / Polymer Testing 55 (2016) 152e159

with the hard segments of TPEE, was chosen as the second component to be blended with TPEE to prepare the TPEE/thermoplastic polyester blends for the phase structure-property relation detected. The immiscible morphology and interfacial structure of the blend systems were studied, aiming at figuring out the relations between phase separation structure of TPEE and additional component phase. Then, the viscoelastic and mechanical properties of the blend system were explored in detail by the step cycle tensile tests. The main objective of this work is to provide useful information on the structure and property control of TPEE by the simple blending approach using the polyesters with similar chain structure with the polyester segments of TPEE. 2. Experimental 2.1. Material preparation Thermoplastic polyester elastomer (KP3340) was purchased from KOLON Co. Ltd., Korea. Its number-average molecular weight (Mn) is 36,000 g mol
1, with poly(butylene terephthalate) (PBT, 50.69 wt%) as the hard segments, and poly(tetramethylene glycol) (PTMEG, 49.31 wt%) as the soft segments (molar ratio ¼ 25/75, measured by the approach of nuclear magnetic resonance (NMR) [18]). The nanosized domains of hard PBT phase of TPEE, with the average size of about 100 nm, are dispersed in its continuous soft PTMEG phase (Fig. 1), which was studied in the previous work [10]. Its density is about 1.17 g cm
3. Poly(butylene terephthalate) (1097A, Mn ¼ 23,200 g mol
1) with the density of 1.31 g cm
3 was purchased from Nantong Xinchen Synthetic Material Co. Ltd., P. R. China. Its end eCOOH values lower than 0.03 mmol g
1. The TPEE/PBT blends were prepared by melt mixing using a Haake Polylab Rheometer (Thermo Electron Co., USA) at 240  C and 50 rpm for 6 min. For better comparison, the neat TPEE and PBT were also processed to keep the same thermal histories. All materials were dried under vacuum for 24 h before using. The sheet samples with the thickness of about 1 mm for morphological and rheological tests were prepared by compression molding at 240  C and 15 MPa. The rectangular specimens (30 mm  5 mm  1 mm) and the dog-bone shaped ones (32 mm  4 mm  2 mm) were prepared by a Haake mini-jet (Thermo Scientific Co., USA) for the mechanical tests. The injection molding was performed at the cylinder temperature of 240  C, with the injection pressure 600 bar

153

and holding pressure 500 bar. Hereafter the blends are referred as to TPEEm/PBTn, where m and n (m þ n ¼ 10) denote the weight ratio of TPEE and PBT, respectively.

2.2. Morphological characterizations The phase morphologies of the blend samples were studied using a Philips XL-30ESEM (Netherlands) and a Zeiss SUPRA55 (Germany) scanning electron microscope (SEM) with 20 kV accelerating voltage. The sheet samples were frozen in liquid nitrogen and fractured. The fractured surfaces were then coated with gold using an SPI sputter coater for enhanced conductivity. The number average radii (Rn) of the domains were determined respectively according to the following relations:

P

ni Ri Rn ¼ iP ni

(1)

i

where ni is the number of the dispersed domains with radii Ri counted from the SEM images. The total number of domains analyzed was about 100. The highest deviations of the calculated average radii were ±0.08 mm. The phase separation morphology of TPEE were detected at the room temperature using an Icon atom force microscope (AFM, Bruker Co. Ltd., USA) in the quantitative nanomechanical mapping mode (QNM). The silicon probes were used (tip radius<8 nm, force constant 40 N/m, resonance frequency 300 kHz) to acquire images. Image processing and mechanical properties analysis were performed with the NanoScope Analysis software.

2.3. Deformed drop retraction experiments A modified deformed drop retraction method developed by Bousmina and coworkers [19] was used here to estimate the interfacial tension between TPEE and PBT. This method consists of studying the kinetics of relaxation of a deformed droplet, in this case a short PBT fiber between two TPEE thin films. The relaxation kinetics is based on a theoretical equation describing the shape evolution of an ellipsoidal liquid drop suspended in an infinite fluid domain. Upon cessation of the flow,

Fig. 1. AFM (a) height and (b) modulus images (10 mm  10 mm) of neat TPEE. The inset graphs are 3D images of the scanned area.

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Fig. 2. SEM images of the etched blend samples: (a) TPEE9/PBT1, (b) TPEE8/PBT2, (c) TPEE1/PBT9 and (d) TPEE2/PBT8 with the scale bar of 10 mm.

under a nitrogen atmosphere. The images were taken every 3 s. Detailed experimental explanation for the measurements performed was reported in the previous work [20,21].

2.4. Mechanical property characterizations The tensile properties of the TPEE blends were determined by an Instron Mechanical Tester (ASTM D638) at a crosshead speed of 50 mm min
1 at 25  C using the dog-bone shaped specimens. Strength and modulus values reported here represent an average of the results for tests run on six specimens. The step cycle tensile tests were performed at room temperature, with the stretching rate of 1 mm s
1 using rectangular specimens. The sample was extended step-by-step up to different strains. Once the sample reached the appropriate strain, the crosshead direction was reversed and the sample strain was decreased at the same crosshead velocity until zero stress was achieved. Then, the sample was extended again at the same constant crosshead speed until it reached the next targeted strain. The unloading-reloading cycles were recorded in this process until the sample fractured (ε ¼ 1, 2, 3, 4 …, where ε is the nominal tensile strain). Detailed experimental methods can be found elsewhere [22,23].

Fig. 3. Viscosity curves of neat TPEE and PBT obtained from steady sweep.

 D ¼ D0 exp


40ðl þ 1Þ s t ð2l þ 3Þð19l þ 6Þ hm R0

 (2)

where D is the drop deformation parameter defined as D ¼ (L
B)/ (LþB), where L and B are the major and minor axis of the ellipsoidal drop, respectively; D0 is an initial deformation parameter, l the viscosity ratio, s the interfacial tension, t the time, hm the viscosity of the matrix phase, and R0 the radius of the drop at equilibrium. The retraction process of a PBT thread embedded in TPEE matrix was recorded at 230  C using a DMLP optical microscope (Leika, Germany) equipped with a LTM350 hot stage (Linklam, England)

2.5. Rheological measurements Rheological tests were performed on a Haake RS600 rheometer (Thermo Electron Co., USA) using a parallel-plate geometry with a diameter of 20 mm. The sheet samples were melted at 230  C for 3 min to eliminate the residual thermal histories, and then experienced shear flow. The stress and viscosity response to the shear rate were recorded during steady shear flow.

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2.6. Thermal property characterizations

10 K/min

TPEE TPEE9/PBT1

endo up

TPEE8/PBT2

Tg TPEE TPEE9/PBT1

3. Results and discussion

TPEE8/PBT2

3.1. Phase morphology of the TPEE/PBT blends

10 K/min

(a) -120 -100

-80

-60

-40

-20

0

o

20

40

60

80

temperature ( C) TPEE

Tc

TPEE9/PBT1 TPEE8/PBT2 TPEE7/PBT3

endo up

Thermal behavior of neat TPEE and PBT, and their blend samples were recorded using a differential scanning calorimeter (DSC, Netzsch DSC-204F1, Germany). The sheet samples (2.8e3.0 mg, with thickness of 180e190 mm) were molten at 240  C for 3 min to eliminate previous thermal histories, and cooled to
120  C at predetermined cooling rate, then heated again to 240  C. The thermal response of heat flow as a function of temperature was recorded in this process. All tests were carried out under nitrogen.

TPEE6/PBT4 TPEE5/PBT5 TPEE4/PBT6 PBT

10 K/min

(b) 50

100

150

o

temperature ( C)

200

250

Fig. 4. DSC traces for neat TPEE and its blend samples at various temperature ranges: (a)
120  Ce80  C and (b) 50  Ce250  C.

Fig. 2 gives SEM images of the etched blend samples. The twophase structure can be seen on all samples, including the TPEErich and PBT-rich ones. This indicates that the two polymers are immiscible thermodynamically. However, the average size of discrete domains is very small (for instance, the size of PBT domains in TPEE9/PBT1 is only 1.6 mm), suggesting that the two phases are compatible to a certain extent in the blends. This will be discussed in detail later. Moreover, the PBT domains show smaller size in the TPEE-rich samples relative to the TPEE ones in the PBT-rich samples. This is reasonable because the less viscous phase is broken down more easily during melt mixing process [24e26] (PBT has lower viscosity than TPEE, as can be seen in Fig. 3). Fig. 4 shows the DSC thermograms of neat TPEE and its blend samples. There are no evident thermal events in the lower temperature range, except for the glass transition of amorphous PTMEG phase of TPEE (Fig. 4a) (no evident glass transition is observed on the hard PBT phase because PBT is crystallized with high degree). The presence of discrete PBT phase nearly has no influence on the glass transition temperature (Tg) of PTMEG segments of TPEE, indicating that the additional PBT component is immiscible with soft phase of TPEE (only the traces of TPEE-rich samples are given because the Tgs of PTMEG segments in the PBT-rich samples are hard to be detected). However, the blends show thermal behavior far different with the neat TPEE in the higher temperature range (Fig. 4b). The crystallization temperatures (Tcs) of TPEE-rich blends are located between those of neat TPEE and PBT, and increase with increasing PBT weight ratios. This is indicative of cocrystallization of the PBT segments of TPEE and the blended PBT component [27e30], suggesting that the blended PBT is miscible with hard

Fig. 5. Schematic diagrams of phase structure in neat TPEE and its blend samples (light blue sphere represents hard PBT domain, and medium blue one discrete PBT one enriched by additional PBT phase, and gray continuous part soft PTMEG phase). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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J. Huang et al. / Polymer Testing 55 (2016) 152e159

Fig. 6. SEM images of (a) TPEE9/PBT1 and (b) TPEE1/PBT9 samples with the scale bar of 20 mm.

Fig. 7. Retraction process of a PBT thread embedded in the TPEE matrix at 240  C.

50

0.0

40

stress (MPa)

ln (D ) (μm)

-0.4 -0.8 -1.2 -1.6

30

20

10

-2.0 -3

-2.4

TPEE TPEE9/PBT1 TPEE8/PBT2 TPEE7/PBT3 TPEE6/PBT4 TPEE5/PBT5 PBT

-1

σ =7.12× 10 mN m 50

100

150

200

time (s)

250

300

350

Fig. 8. Variation of the deformation parameter (ln D) of the PBT fibers versus time (t) in the TPEE matrix.

0 0

2

4

6

strain (100%)

8

Fig. 9. Stress-strain curves for neat TPEE and its blend samples.

10

J. Huang et al. / Polymer Testing 55 (2016) 152e159

157

Table 1 Mechanical parameters of the neat TPEE and its blends. Samples

Young's modulus (MPa)

Yield strength (MPa)

Tensile strength (MPa)

TPEE TPEE9/PBT1 TPEE8/PBT2 TPEE7/PBT3 TPEE6/PBT4 TPEE5/PBT5 PBT

56.09 ± 1.12 101.05 ± 3.82 187.61 ± 4.69 278.63 ± 13.68 464.57 ± 15.50 530.51 ± 21.80 1550.67 ± 25.76

8.27 ± 0.20 10.75 ± 1.53 12.75 ± 0.40 16.95 ± 0.60 20.28 ± 0.90 e e

16.81 18.25 18.77 20.28 e 23.38 45.84

± ± ± ±

2.38 1.76 2.37 0.92

± 0.65 ± 1.58

Elongation at break (%) 899.10 ± 22.91 676.80 ± 15.96 617.04 ± 21.58 288.36 ± 5.08 53.28 ± 2.54 e 5.30 ± 0.40

Fig. 10. Cyclic tensile deformation with maximum strains of 1/3, 2/3, 3/3, 4/3 and so on up to be broken up for neat TPEE and its blend samples.

phase of TPEE [29,30]. Therefore, it is reasonable to propose that the blended PBT is enriched around the hard phase of TPEE in a TPEErich blend, forming phase-separated structure with larger scale than neat TPEE, as schematically illustrated in Fig. 5. In other words, physically mixing between TPEE and PBT, in a way, is equivalent to increase the content of hard phase of TPEE. Good compatibility between PBT and hard phase of TPEE would result in good interfacial adhesion, as shown in Fig. 6. Actually the two-phase structure is hard to be seen were the samples not etched. Small domain size and vague phase interface are also indicative of lower interfacial tension level between two polymer components, which can be evaluated by the deformed drop retraction tests. The retraction process of a PBT thread embedded in a TPEE sheet matrix is shown in Fig. 7, and the variation of the deformation parameter of PBT thread with time is shown in Fig. 8. Assuming that the zero shear viscosity equals approximately to the viscosity at the Newtonian flow region (the viscosity of matrix TPEE (hm) and the viscosity ratio (l) can be obtained by the steady shear sweep (Fig. 3)), the interfacial tension s can be calculated according to eq. 2, and is about 7.12  10
3 mN m
1. This value is of very lower tension level. Favis et al. [31,32] proposed three types of interface in the immiscible polymer blends. Type I systems are described as being

binary compatible, that is to say, immiscible but with strong interactions at the phase interface, namely with lower interfacial tension levels. Type II systems are immiscible with high interfacial tension, and the blends are described as binary incompatible systems. Type III systems are ternary compatibilized blends using the third component as the compatibilizer. Clearly, the TPEE/PBT blends in this work are type I systems. Three characteristics is found in the current systems, including 1) very small size of discrete phase, and 2) very lower levels of interfacial tension between two polymers, as well as 3) good interfacial adhesion between two phases. Clearly, simple physical mixing with PBT provides high possibility to fabricate TPEE alloys or TPEE-rich blend materials with tailorable properties.

3.2. Tensile properties of TPEE nanocomposites Fig. 9 shows the tensile curves for neat TPEE and its blends. The obtained mechanical parameter values are summarized in Table 1. Both the yield and tensile strengths increase with increasing contents of PBT, accompanied by gradually decreased elongation level at break. It is seen that the presence of 10 wt% PBT can increase the modulus of TPEE from 56 MPa to 101 MPa by about 80%, and yield strengths from 8.27 MPa to 10.75 MPa by about 30%. Clearly, PBT

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J. Huang et al. / Polymer Testing 55 (2016) 152e159

16

cycle area

12

8

4

TPEE9/PBT1

TPEE

0 16

cycle area

12

8

4

TPEE8/PBT2

0 0

2

4

TPEE7/PBT3 6

0

2

4

6

unrecoverable strain

unrecoverable strain

Fig. 11. The dependence of cycle area on unrecoverable strain for each cycle.

can be used as good “reinforcement” to TPEE. It is notable that as the PBT weight ratios achieve up to 40 wt%, the elongation level decreases sharply. This is indicative of phase inversion in TPEE6/ PBT4, namely that PBT is continuous phase in this blend, instead of TPEE. It is well accepted that phase behavior of the polymer blend is basically governed by the relative melt viscosities of two components. The viscosity ratio (l ¼ 11.89) of the blend systems in the current work is lower than 4. That means that the breakup of dispersed phase can occur in the TPEE/PBT systems during melt mixing [33]. The Paul and Barlow model [34] can hence be used here to predict phase inversion point:

41 h1 ¼ 42 h2

(3)

where 4i and hi are the volume fraction and melt viscosity of component i, respectively. The predicted inversion point for this system is about 62.8 wt% TPEE (Fig. 3), which means that as PBT contents achieve close to 40 wt%, it becomes continuous phase in the TPEE/PBT blends. This is clearly in accordance with the tensile test observation. Therefore, to use PBT to control the mechanical properties of TPEE, or to prepare TPEE-rich blends, the contents of PBT should be no more than 30 wt%. Cyclic tensile tests can reveal more information on the system plasticity and elasticity of a multiphase blend system. Fig. 10 gives the nominal stress-nominal strain response during cyclic tensile deformation of neat TPEE and three TPEE-rich blends with maximum strain values increased sequentially from 1/3, 2/3, 3/3, 4/ 3 and so on up to be broken. The shape of these curves is commonly obtained in the case of high content amorphous polymer and results from the plastic deformation gives rise to progressively larger residual strain at zero stress [22,23]. Clearly, the amounts of tensile

cycles reduce monotonously with increasing contents of PBT component, confirming decreased plasticity of the blends. This agrees well with the decreased elongation levels of the blends. At the initial stage of cyclic tensile tests, the blends shows higher stress levels as compared with neat TPEE (see the arrows in Fig. 10), which increase with increasing PBT contents. This confirms increased system rigidity because of higher modulus of PBT component and good interfacial adhesion between two phases. Besides, to achieve to the same levels of unrecoverable strain, the blend samples require higher level of work (calculated from cycle area), as can be seen in Fig. 11. This also indicates decreased system plasticity with increased contents of hard PBT phase. Thus, one can adjust composition ratios to obtain the TPEE/PBT blend materials with optimized rigidity and plasticity according to the requirement of practical applications.

4. Conclusions TPEE and PBT are immiscible because PBT is immiscible thermodynamically with the continuous soft PTMEG phase of TPEE. However, these two polymers are compatible because PBT is miscible with the discrete hard PBT phase of TPEE. Therefore, the discrete PBT domains is of small size in the TPEE-rich blends, with good interfacial adhesion to the continuous PTMEG phase. The interfacial tension between two polymers is of very low levels (about 7.12  10
3 mN m
1). As a result, TPEE/PBT blends show far higher modulus and yield and tensile strengths than neat TPEE. The increased system rigidity and decreased system plasticity are further confirmed by cyclic tensile tests. This work provides a simple way to prepare TPEE alloys with high performance. But to obtain TPEE-rich blends, the contents of PBT should be no more than 30 wt%.

J. Huang et al. / Polymer Testing 55 (2016) 152e159

Acknowledgements Financial support from the National Natural Science Foundation of China (51573156), the Prospective Joint Research Program of Jiangsu Province (BY2014117-01) are gratefully acknowledged.

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