Chain microstructure of two highly impact polypropylene resins with good balance between stiffness and toughness

Journal Pre-proof Chain microstructure of two highly impact polypropylene resins with good balance between stiffness and toughness Wei Liu, Jiaqi Zhang, Mei Hong, Pei Li, Yanhu Xue, Quan Chen, Xiangling Ji PII:

S0032-3861(19)31150-4

DOI:

https://doi.org/10.1016/j.polymer.2019.122146

Reference:

JPOL 122146

To appear in:

Polymer

Received Date: 7 November 2019 Revised Date:

30 December 2019

Accepted Date: 31 December 2019

Please cite this article as: Liu W, Zhang J, Hong M, Li P, Xue Y, Chen Q, Ji X, Chain microstructure of two highly impact polypropylene resins with good balance between stiffness and toughness, Polymer (2020), doi: https://doi.org/10.1016/j.polymer.2019.122146. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Credit Author Statement

Wei Liu did experiments, collected the data, performed the analysis, and wrote the initial manuscript. Jiaqi Zhang did parts of the experiments. Mei Hong did parts of the experiments. Pei Li performed the analysis. Yanhu Xue performed the analysis. Quan Chen processed the data and wrote a section (rheological properties) of the manuscript. Xiangling Ji supervised the research.

Chain microstructure of two highly impact polypropylene resins with good balance between stiffness and toughness Wei Liu,a Jiaqi Zhang,a,b Mei Hong,a,b Pei Li,a Yanhu Xue,a Quan Chen,a,b,* Xiangling Jia,b,* a

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied

Chemistry, Chinese Academy of Sciences, Changchun 130022, China b

University of Science and Technology of China, Hefei 230026, China

ABSTRACT: For highly impact polypropylene (HIPP), a good balance between stiffness and toughness has attracted much attention from both industrial and academic research. Herein, two HIPP resins (A and B) with different ethylene contents and impact resistances especially at low temperatures are investigated. Their chain microstructure is studied by combining preparative temperature rising elution fractionation (P-TREF) with multiple characterization techniques such as high-temperature gel permeation chromatography (HT-GPC), Fourier transform infrared (FTIR) spectroscopy,

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nuclear magnetic resonance (13C NMR), wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), rheometer, and thermal fractionation. Both samples are mainly composed of four portions: amorphous ethylene−propylene random copolymer (EPR), ethylene−propylene (EP) segmented and blocky copolymer, and isotactic polypropylene homopolymer. Sample A contains ~30 wt% EPR fractions eluted at 35 °C, ~9 wt% EP segmented and blocky copolymers eluted at 80−100 °C, and ~19 wt% highly isotactic polypropylene fractions eluted at 125−140 °C, which are higher than the contents of corresponding fractions in sample B. Moreover, most

fractions of sample A have higher molecular weights than the corresponding fractions of sample B. The contribution of different components to stiffness and toughness is also discussed. The excellent impact resistance of sample A is demonstrated from the perspective of chain microstructure. Keywords: Chain microstructure; Highly impact polypropylene; Fractionation 1. Introduction Polypropylene (PP) has lots of excellent properties, such as high mechanical strength, chemical stability, and heat resistance, which make it one of the most widely used thermoplastic materials in food packaging [1] and automotive industry, etc. However, the low temperature brittleness of PP restricts some specific applications. Lots of work [2−5] has focused on improving the low-temperature impact resistance of PP. Two main ways of improving the toughness of PP include physical blending of rubber or elastomers with isotactic PP [6−8] and sequential multi-step polymerization to prepare impact polypropylene copolymers (IPC) [9−10]. IPC are heterophasic PP with improved impact resistance at low temperatures, which are usually produced by a multi-step polymerization involving the homopolymerization of PP and the following copolymerization with comonomers like ethylene [11]. The as-synthesized product is generally composed of isotactic PP and EP copolymers with a very broad distribution of ethylene comonomer, including random, segmented, or blocky copolymers [10,12]. These complex components coexist and generate phase-separation into rubbery phase of multilayer core–shell structure and PP matrix [13−15]. EPR is the major component contributing to toughness. Some literatures [16−17] have demonstrated

that EP segmented or blocky copolymers act as compatibilizers to improve the interfacial adhesion between the disperse phase and the matrix. Santonja-Blasco et al. [18] studied five HIPPs with ethylene content of 8−11 mol% and revealed the influence of chain microstructure on the phase structure and crystallization behavior of HIPP. Since the content of rubbery component was identical for all five HIPPs, the difference in multi core−shell morphology was ascribed to the ethylene segment distribution in the EP copolymers. Recently, Tang et al. [15,19] established an atomic force microscopy-infrared (AFM-IR) technique to investigate the phase domain composition of HIPPs and found that the rigid cores of the rubber particles can be mainly composed of either polyethylene (PE) or PP. This structural difference maybe closely related to the chain microstructure of the raw resins. Therefore, in order to achieve desired impact properties, the chain microstructure needs to be clearly analyzed, which is the most important prerequisite to understand the macroscopic phase structure and the compatibility between rubbery phase and the matrix. Establishing the relationship between chain microstructure and mechanical properties is essential for both manufacture and application. As HIPP is a multi-phase and multi-component system, the measurements applied to the bulk samples usually give the average properties of all components. The major components may even dominate the measurement results. However, the neglected minor components could be necessary to the overall properties. Therefore, component separation is the basis of chain microstructure study. Preparative temperature rising elution fractionation (P-TREF) [20−23] is a powerful tool for fractionation of

polyolefin resins according to crystallizability. During a very slow cooling step, components with different crystallizability are sequentially loaded onto the support, and then these components dissolve in the solvent and are eluted during stepwise heating. The molecular heterogeneity is further studied by analyzing the fractions with multiple effective characterization techniques. Cheruthazhekatt et al. [24] combined P-TREF with high-temperature HPLC (high performance liquid chromatography) to fractionate one complex HIPP and identified three chemically different components including isotactic PP homopolymer, EP copolymers, and PE homopolymer by offline coupling with FTIR spectroscopy and high-performance DSC. Fernández et al. [25] fractionated HIPPs with ethylene content in the range of 6.5−15.5 wt% using P-TREF and analyzed the molecular structure by SEC (size exclusion chromatography), analytical TREF, DSC, and solid state NMR. The raw resins consisted of 10.3−21.7 wt% atactic PP and elastomeric EPR, 6.2−11.3 wt% EP copolymers with intermediate ethylene and propylene segments, 5.1−8.0 wt% propylene based polymers, and 80.4−60.4 wt% crystalline isotactic PP. Therefore, the combination of P-TREF with various characterization techniques can discern the subtle difference in resin composition and further establish a relationship between chain microstructure and low-temperature impact resistance of HIPP resins. In this study, two HIPP resins with different ethylene contents and impact resistances at low temperatures are fractionated into nine fractions using P-TREF. The obtained fractions are then thoroughly analyzed by HT-GPC, FTIR,

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C NMR, WAXD, and

DSC. By comparing the differences in the chain microstructures of the two samples,

the main factors, from the chain structure point of view, determining the stiffness-toughness balance are confirmed, which will provide guidance and suggestion for the production of HIPP resins. 2. Experimental 2.1. Materials Two industrial HIPP resins were coded as samples A and B, and their mechanical properties were listed in Table 1. Table 1. The mechanical properties of samples A and B. Notched Izod Impact Strength (J/m) 23 °C -18 °C

Sample

MFR (g/10 min)

Flexural Modulus (MPa)

A

18.9

1100

609.6

103.6

B

30

953

584.9

82.2

2.2. Preparative temperature rising elution fractionation (P-TREF) Fractionation by TREF includes two procedures: crystallization and elution. Firstly, 10 g sample was dissolved in 700 mL 1,2,4-trimethylbenzene (TMB) at 140 °C. 0.1 wt% 2,6-di-tert-butyl-4-methylphenol (BHT) was added into TMB as antioxidant. The polymer solution was transferred to a column packed with 60−80 mesh glass beads at 140 °C, and then the column was slowly cooled down to 35 °C at a rate of 2 °C/h to ensure the successive crystallization of polymers onto the glass beads. At the elution step, the column equilibrated at a preprogrammed temperature for 24 h. Then, the polymer solution was eluted with 1100 mL TMB and precipitated with twice its volume of acetone. The fractions were filtered and then dried in vacuum at 50 °C until to a constant weight.

2.3. High-temperature gel permeation chromatography (HT-GPC) The molecular weights and molecular weight distribution (MWD) of the two samples and their fractions were measured using a PL-GPC 220 high-temperature GPC (Polymer Laboratories Ltd.) at 150 °C. Three PLgel 10 µm Mixed-B LS columns (300 ×7.5 mm) were used. 1,2,4-Trichlorobenzene stabilized with 1.25×10-4 g/mL BHT was used as eluent. The sample concentration was 1−2 mg/mL. All the polymer solutions were prepared at 150 °C using a PL-SP 260 high-temperature sample preparation system (Polymer Laboratories Ltd.). The solutions were filtered before injection. The injection volume was 200 µL, and the flow rate was 1.0 mL/min. 2.4. Fourier transform infrared (FTIR) spectroscopy FTIR spectra were recorded on a Bruker ALPHA spectrometer. Thin films prepared by hot pressing were used for testing. 2.5. 13C Nuclear magnetic resonance spectroscopy (13C NMR) 13

C NMR spectra were recorded at 130 °C on a Bruker AV400 NMR spectrometer at

100.58 MHz. About 80 mg sample was dissolved in 0.5 mL of o-dichlorobenzene-d4 at 130 °C. In all measurements, the inverse gated decoupling was used to remove NOE and

13

C−1H couplings, and the pulse angle was 90°. The number of scans was

more than 5000, and the delay time was 8 s. 2.6. Wide-angle X-ray diffraction (WAXD) WAXD measurements were performed on powdered samples at ambient temperature on Bruker D8 Advance X-ray diffractometer using Cu Kα radiation (the wavelength is 1.54 Å) at 40 kV and 40 mA. Scattered intensities were measured in the range of 2θ =

5−50°. 2.7. Differential scanning calorimetry (DSC) DSC thermograms were recorded using DSC Q20 (TA Instruments). 5−6 mg sample was first heated from 20 to 200 °C at a rate of 10 °C /min, and then kept at 200 °C for 5 min to erase thermal history. Then, the samples were cooled down to 20 °C at a rate of 10 °C /min, and held at 20 °C for 5 min. Finally, the samples were heated to 200 °C at a rate of 10 °C/min. The crystallization temperature (Tc) and melting temperature (Tm) were obtained during the cooling and reheating steps, respectively. The successive self-nucleation and annealing (SSA) thermal fractionation was carried out according to literature [26]. The annealing time at each temperature step was 15 min and the heating and cooling rate is 10 °C/min. The first Ts temperature was determined from DSC thermograms using the method reported by Fillon et al. [27]. 2.8. Rheological measurements Linear viscoelasticity was measured for the samples A and B with a strain-controlled rheometer ARES-G2 utilizing 25 mm parallel plates. Frequency sweep measurements were conducted with small strains (≤ 10%) within linear regime, as confirmed by the strain sweep measurements. All measurements were conducted at temperature above Tc via utilizing heated stream of nitrogen for temperature control and prevention of thermal degradation. The two HIPP samples were loaded on the rheometer at 230 °C (higher than the Hoffman-Weeks equilibrium melting temperature of 208 °C) and annealed there for 10 min to remove the thermal and flow histories. After that, temperature was

decreased 20 °C each step to 150 °C, which is above Tc (cf. Table 2), frequency sweep measurement was conducted at each step temperature, and time temperature superposition (TTS) principle was employed to construct the master curves. Shear rate sweep measurements were conducted utilizing 25 mm cone-and-plate geometry. Samples were loaded on the rheometer at 230 °C and annealed there for 10 min to remove the thermal and flow histories. After that, the samples were cooled down to 190 °C and shear rate sweep measurement was conducted in a range in between 0.001 s-1 to 100 s-1.

3. Results and discussion 3.1. Basic characterization of the original samples As listed in Table 1, the impact strengths of samples A and B at 23 °C are 609.6 J/m and 584.9 J/m, respectively. Sample A has higher impact strength of 103.6 J/m at -18 °C than that of sample B (82.2 J/m). In addition, the flexural modulus of sample A is 1100 MPa, which is also higher than that of sample B (953 MPa). Therefore, sample A shows better balance between stiffness and toughness than sample B. The MWD profiles of the two samples are shown in Fig. 1. The detailed molecular weight data and thermal properties of the two samples are listed in Table 2. Sample A shows slightly higher weight-average molecular weight (Mw) of 19.73×104 g/mol and narrower MWD of 4.33 than sample B (Mw = 19.15×104 g/mol, MWD = 4.43).

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NMR measurements are carried out to calculate the triad distribution, isotacticity (mm), and the number-average sequence lengths of propylene segment (nP) and

ethylene segments (nE) [28−29] as summarized in Table 3. Sample A has higher ethylene content of 29.0 mol% than sample B of 19.1 mol%. The isotacticity of sample A is 79.5 mol%, which is lower than 81.3 mol% for sample B. The triad PPP content of sample A is 58.9 mol%, which is also lower than that of sample B (67.4 mol%). The insertion of more ethylene monomers into the PP chains decreases the isotacticity of sample A. The nE of samples A and B are very short around 2 and 3, suggesting that short ethylene segments are copolymerized into the PP chains. The DSC measurements show that the two samples have very similar melting temperatures around 165 °C and crystallization temperatures around 131 °C. Sample A shows lower crystallinity (34.0 %) than sample B (35.3 %) because of its lower isotacticity.

Table 2. Molecular weight and thermal properties of samples A and B. Sample

Mw (104)

Mn (104)

Mw/Mn

Tm (°C)

Tc (°C)

Wc,h (%)

A

19.73

4.55

4.33

165.9

131.0

34.0

B

19.15

4.33

4.43

165.5

131.4

35.3

Table 3. Tacticity, ethylene content, and triad sequence distribution in samples A and B. Sample

P

E

PPP

PPE

EPE

PEP

PEE

EEE

nP

nE

mm (mol%)

A

0.710 0.290 0.589 0.071 0.050 0.043 0.096 0.153 7.8 3.2

79.5

B

0.809 0.191 0.674 0.098 0.037 0.049 0.073 0.076 9.5 2.3

81.3

3.2. Rheological properties of the original samples Fig. 2a compares master curves of linear storage and loss moduli, G' and G", plotted against the angular frequency ω of the samples A and B reduced at Tr = 190 °C. To construct the master curves, G' and G" measured at T = 150−230 °C were multiplied by an intensity factor bT = Tr/T, and shifted horizontally by a shift factor aT until the best superposition was achieved. Fig. 2a shows that time temperature superposition holds for G' and G" obtained at 150−230 °C. In Fig. 2a, we noted G" > G' in a wide angular frequency range ω < 102 rad/s, and G' and G" cross at high frequency 102 rad/s < ω < 103 rad/s. This feature means that the linear viscoelasticity reflects mainly disentanglement of the isotactic PP chains in HIPP. To compare the relaxation mode distribution, G' and G" of the sample B are shifted horizontally until the terminal tail of G", G" ~ ω, coincides with that of the sample A in Fig. 2b. After the shift, the samples A and B exhibit similar relaxation mode distribution at low ω, but the high ω moduli are slightly lower for sample B. This result is in accordance with the molecular weight distribution profiles shown in Fig. 1; Since the high Mw ends (in Fig. 1) are similar for A and B, the terminal tails governed by the high Mw ends are quite similar for the samples A and B (in Fig. 2b). However, the remaining part at the right side of the elution peak is lower for B (in Fig. 1), leading to the weaker increase of both G' and G" with increasing the frequency (in Fig. 2b).

Fig. 3 tests the Cox-Merz rule for the samples A and B, where the filled symbols show the shear viscosity η plotted against the shear rate

, and the unfilled symbols

show the complex viscosity η*, calculated from storage and loss moduli shown in Fig. 2a as: η*(ω) = [G'(ω)2 + G"(ω)2]0.5/ω, plotted against the angular frequency ω. η is shown only up to

= 10 s-1 because edge fracture was noted at higher

≥ 10s-1

[30]. In Fig. 3, we found that the Coz-Merz rule, specifying an agreement between η( ) as function of

and η*(ω) as function of ω, holds for the samples A and B in

this study. Particularly, the zero-shear viscosity is higher for A, in accordance with the slower terminal relaxation of A seen in Fig. 2a. Instead of evaluation on the average properties of the original samples as shown above, more detailed information on chain microstructure is needed to clearly elucidate the relationship between structure and performance. Comprehensive analysis on chain microstructure of the two samples can be achieved by fractionation and subsequent characterization of the fractions by multiple techniques.

3.3. Fractionation of the two samples using P-TREF Both samples A and B are fractionated into nine fractions (fractions 1−9) using P-TREF, respectively. The fractionation results are summarized in Table S1 (supporting information). The weight percentages of each fraction versus the elution temperatures are depicted in Fig. 4. Both samples have two major components, fractions 1 and 7, which are collected at 35 °C and 120 °C, respectively. It is noteworthy that the content of A-1 (30.14 wt%) is higher than B-1 (28.85 wt%). The

fraction eluted at room temperature is basically EP random copolymer, which mainly contributes to the toughness of resins. The contents of fractions eluted below 100 °C (fractions 1−3) and above 125 °C (fractions 8 and 9) in sample A are all higher than those in sample B. For fractions 4−7 eluted in the medium temperature range of 105−120 °C, the content of each fraction in sample B is higher than that in sample A. Both samples contain small amount of fractions eluted at the highest temperature of 140 °C (fraction 9). But this fraction (A-9) occupies 1.24 wt% of sample A, which is higher than the content of B-9 in sample B (0.63 wt%). The fractions eluted at high temperatures are usually composed of highly isotactic PP, which generally contribute to the stiffness of the resins.

3.4. Molecular weight and molecular weight distribution of the fractions. The MWD profiles of the fractions and their sums are displayed in Fig. 5. Corresponding data are listed in Table 4. In Fig. 5b and 5d, the sums of GPC curves of the fractions in both resins are very consistent with the GPC curves of the pristine samples A and B, indicating that the fractionation procedure is quite efficient without degradation. As shown in Fig. 5a and 5c, the fractions eluted below 110 °C have very broad MWD, such as fractions 2−5 have a bimodal MWD. In both samples, the fractions eluted at 80 °C have the broadest MWD larger than 12. As for fractions eluted above 115 °C, the MWD become narrower around 2. The molecular weights of the fractions vary in the range of 3.97×104−54.87×104 g/mol for sample A and 3.04×104−48.01×104 g/mol for sample B, respectively. In

both samples, Mw first decreases with elution temperature and the fractions eluted at 105 °C (A-4 and B-4) have the lowest Mw, and then Mw increases with elution temperature and the fractions eluted at the highest temperature of 140 °C (A-9 and B-9) have the highest Mw. Fractions eluted in the temperature range of 105−115 °C have lower Mw than other fractions. These results indicate that the fractionation proceeds depending on crystallizability rather than molecular weights. Fractions eluted at 35 °C (fraction 1) occupying more than 28 wt% of both samples have medium molecular weights among all the fractions. Fraction B-1 shows a higher Mw of 30.73×104 g/mol than the corresponding fraction A-1 with Mw = 25.19×104 g/mol. Fractions 2−9 of sample A always have higher Mw than the corresponding fractions of sample B, which will enhance the stiffness of resin A [31]. Moreover, for PP homopolymers, increasing molecular weight may cause more entanglements in the inter-lamellar region and further improve the toughness [32]. Therefore, the higher molecular weight of fractions in sample A than those in sample B is one of the reasons for the higher stiffness and impact resistance of sample A.

Table 4. Molecular weights of the fractions in samples A and B.

Fraction

Sample A

Elution temperature (°C)

Sample B

Mw (104)

Mn (104)

Mw/Mn

Mw (104)

Mn (104)

Mw/Mn

1

35

25.19

6.85

3.68

30.73

9.79

3.14

2

80

18.79

1.47

12.78

12.49

0.68

18.27

3

100

12.68

1.51

8.38

4.47

0.83

5.42

4

105

3.97

1.33

2.99

3.04

1.14

2.68

5

110

5.41

2.23

2.43

4.69

2.08

2.26 1.83

6

115

7.32

3.96

1.85

6.45

3.53

7

120

15.65

7.58

2.06

14.00

6.35

2.20

8

125

30.37

13.63

2.23

27.01

12.3

2.20

9

140

54.87

24.76

2.22

48.01

24.7

1.94

Summation

20.31

4.85

4.19

18.94

3.93

4.82

Original Resin

19.73

4.55

4.33

19.15

4.33

4.43

3.5. Composition analysis of the fractions by FTIR spectra All fractions of the two resins were analyzed by FTIR as shown in Fig. 6. Absorptions at 998 and 841 cm-1 are attributed to the crystalline PP sequences [9,33]. The band at 973 cm-1 is the feature of isolated propylene units and can represent the amorphous PP sequence. As for crystalline ethylene sequence, doublet in the range of 720−740 cm-1 can be observed. In both samples, fractions eluted at 35 °C show single band at 722 cm-1. Meanwhile, the absorptions at 998 and 841 cm-1 are very weak. These observations indicate that both propylene and ethylene segments in fraction 1 are not long enough to crystallize. Therefore, fractions A-1 and B-1 are amorphous EPR. For fractions 2 and 3, bands at 998 and 841 cm-1 become obvious, proving the presence of crystalline PP segments. Moreover, doublet associated with crystalline PE segments appears at 720−730 cm-1. In particular, the doublets in fractions A-3 and B-3 are much stronger than those in fractions A-2 and B-2, implying that the crystalline PE

sequences in fraction 3 are much longer than those in fraction 2. Based on these observations, fraction 2 is considered as EP segmented copolymers; whereas fraction 3 may be EP blocky copolymers. Moreover, by comparing the relative intensities of PE and PP absorptions in the two samples, it can be found that the relative ethylene contents in fractions 1−3 of sample A are all higher than those of sample B. The spectra of fractions 4−8 exhibit very strong characteristic bands of crystalline PP and almost invisible PE absorptions. These fractions are mainly composed of isotactic PP homopolymers.

3.6. Ethylene content, tacticity, and sequence distribution in the fractions calculated from 13C NMR spectra. The chemical composition of the fractions is further confirmed by

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C NMR

measurements. The ethylene content, triad sequence distributions, tacticity, nE, and nP in the fractions are calculated from 13C NMR spectra and shown in Table 5. Fractions A-1 and B-1 contain 57.4 mol% and 43.7 mol% ethylene comonomer, respectively. The mole fractions of all six triads (PPP, PPE, EPE, PEP, PEE, and EEE) in A-1 and B-1 are in the range of 0.093−0.251, which are relatively homogeneous. Moreover, the nP and nE of A-1 and B-1 are very short, only between 2 and 3. These results confirm that fraction 1 is amorphous EP random copolymers, i.e., EPR. In fraction 2, the molar fractions of all six triads become heterogeneous. The sums of PPP and EEE triads in A-2 and B-2 are 0.820 and 0.849, respectively. Besides, both nP and nE become longer than those in fraction 1. The nP and nE increase to 5.8 and 11.5 for A-2,

whereas 12.0 and 8.3 for B-2, respectively. The above results indicate that fraction 2 is EP segmented copolymers. The ethylene contents in A-3 and B-3 are 43.7 mol % and 11.5 mol %, which are lower than those in A-2 (66.2 mol %) and B-2 (51.5 mol %). Both nP and nE of fraction 3 (nP = 77.8 and nE = 60.3 for A-3; nP = 267.7 and nE = 34.6 for B-3) are much longer than those of fraction 2. The sums of PPP and EEE triads in A-3 and B-3 reach up to 0.970 and 0.981, respectively. Moreover, fractions A-3 and B-3 contain certain amounts of triads such as PPE, EPE, PEP, and PEE. Therefore, fraction 3 is EP blocky copolymer. The fractions eluted above 105 oC (fractions 4−8) contain a small amount of ethylene comonomer. Meanwhile, the isotacticity [mm] is high enough (93.3−99.1 mol%). These fractions are isotactic PP. The comonomer distributions in the polymer chains of fractions in the two samples are illustrated in Fig. 7. In both samples, the ethylene content almost decreases, whereas nP and isotacticity of the fractions increase with the elution temperature. The insertion of more ethylene comonomers will interrupt the long PP sequence and reduce the tacticity and crystallizability. Most fractions in sample B have slightly higher isotacticity than the corresponding fractions collected at the same elution temperatures in sample A due to the lower ethylene content in sample B. But, the difference in the isotacticity between fractions eluted at the same temperatures especially at high temperatures is really small. The composition analysis combined with the fractionation and molecular weight results give the difference in chain microstructure between the two samples. Sample A contains more amorphous EPR (fraction 1), EP segmented and blocky

copolymers (fractions 2 and 3), and highly isotactic PP with high molecular weights (fractions 8 and 9) than sample B. The amorphous EPR content is a key parameter affecting the toughness of resins since the dispersed rubbery phase can absorb impact energy when suffering impact [34]. Furthermore, the highly isotactic PP fractions with high molecular weights can enhance the stiffness of resins. EP segmented and blocky copolymers can improve the compatibility between rubbery phase and the PP matrix. Therefore, the more reasonable distribution of components in sample A generates a better balance between stiffness and toughness than sample B.

Table 5. Tacticity, ethylene content, and triad sequence distribution in fractions of samples A and B. mm

Fraction

P

E

PPP

PPE

EPE

PEP

PEE

EEE

nP

nE

A-1

0.426

0.574

0.145

0.165

0.115

0.093

0.235

0.251

2.0

2.7

31.2

A-2

0.338

0.662

0.251

0.048

0.039

0.018

0.079

0.569

5.8

11.5

71.8

A-3

0.563

0.437

0.550

0.008

0.005

0.002

0.011

0.420

77.8

60.3

90.7

A-4

0.985

0.015

0.977

0.008

0

0.002

0

0.012

596.7

8.7

93.3

A-5

0.992

0.008

0.989

0.004

0

0.002

0

0.004

660.9

4.5

95.5

A-6

0.990

0.010

0.990

0

0

0

0

0.010

-

-

98.0

A-7

0.990

0.010

0.990

0

0

0

0

0.010

-

-

96.5

A-8

1

0

1

0

0

0

0

0

-

-

99.0

B-1

0.563

0.437

0.238

0.224

0.101

0.123

0.182

0.135

2.6

2.0

42.1

B-2

0.591

0.409

0.515

0.049

0.027

0.023

0.054

0.334

12.0

8.3

79.2

B-3

0.885

0.115

0.873

0.011

0.002

0.001

0.004

0.108

267.7

34.6

92.0

B-4

0.993

0.007

0.986

0.007

0

0

0

0.006

-

-

94.5

B-5

0.993

0.007

0.991

0.002

0

0

0

0.006

-

-

96.2

B-6

1

0

1

0

0

0

0

0

-

-

97.5

B-7

1

0

1

0

0

0

0

0

-

-

98.2

B-8

1

0

1

0

0

0

0

0

-

-

99.1

(mol%)

3.7. Crystallization properties of the fractions. The crystallization behavior of the fractions is studied using WAXD and the corresponding diffractograms are shown in Fig. 8. Fractions A-1 and B-1 eluted at 35 °C exhibit broad halos in the WAXD patterns, consistent with their composition as amorphous EP random copolymers. The other fractions show characteristic peaks at 2θ values of 14.00° (110), 16.81° (040), 18.41° (130), 21.00° (111), and 21.51° (131 and 041) for α-form crystal of isotactic PP [35]. Fractions A-2, A-3, B-2 and B-3 show characteristic peak of crystalline PE at 2θ = 23.21°, indicating that these four fractions have long crystalline PE sequences. These observations agree well with the composition confirmed by FTIR and 13C NMR spectra.

3.8. Thermal properties of the fractions. Thermal properties of all the fractions are studied by DSC to further verify their composition, as shown in Fig. 9. The detailed data are listed in Table S2 (supporting information). For both samples A and B, the melting temperatures of the fractions increase gradually with the elution temperature. Fraction 1 has no melting peaks, which agrees well with FTIR,

13

C NMR and WAXD measurements and proves that

fractions A-1 and B-1 are amorphous EPR. Fractions 2 and 3 show two or three melting peaks in the temperature range of 90−135 °C on the DSC traces. The multiple melting peaks imply the existence of long ethylene and propylene segments in these fractions, which is in good agreement with FTIR,

13

C NMR and WAXD results that

A-2 and B-2 are EP segmented copolymers, and A-3 and B-3 are EP blocky

copolymers. Moreover, the melting peaks of A-2 (94.0, 109.6 and 131.0 °C) and B-2 (96.2, 108.2, and 134.6 °C) all shift to higher temperatures in A-3 (124.2, 150.8, and 160.1 °C) and B-3 (123.2 and 149.5 °C). Besides, B-3 has a much stronger melting peak at 149.5 °C than A-3 at 150.8 °C because of the presence of longer PP segments in B-3 as confirmed by 13C NMR spectra. Fractions 4−9 of both A and B have single strong melting peak higher than 154 °C, and their melting temperatures increase with the elution temperature. The melting points of fractions 8 and 9 are even higher than 166 °C. These results are consistent with the 13C NMR results that fractions 4−9 are isotactic PP with increased isotacticity. The successive self-nucleation and annealing (SSA) thermal fractionation [26,36] is also conducted to analyze the chain microstructure of samples A and B. In general, a broad thermogram with multiple melting peaks suggests a rather broad crystallizable sequence length distribution. As presented in Fig. 10, fractions A-1 and B-1 show no melting peaks during SSA treatment indicating that they are mainly amorphous. Fractions A-2 and B-2 display multiple melting peaks in a broad temperature range from 70 to 150 °C. The peaks above 130 °C can be assigned to PP of different crystallizable sequence lengths. However, melting peaks below 130 °C generally result from complicated sequence distributions. According to FTIR, WAXD and

13

C

NMR measurements, these multi-melting peaks at low temperatures are generated from PP and PE sequences with a rather broad distribution. Fraction 3 has longer PP and PE sequences than fraction 2, so the melting peaks of fraction 3 shifted to higher temperatures between 110−170 °C. Compared with the broad distribution and

comparable strength of melting peaks in fraction A-3, fraction B-3 mainly shows strong melting peaks above 150 °C and few weak peaks around 120 °C. These differences are caused by the lower ethylene content and longer PP segments in fraction B-3 than those in fraction A-3. Fractions eluted above 105 °C (fractions 4−9) mainly show single strong melting peak at 150−180 °C, which shifts to higher temperatures with the increase of elution temperature. These fractions are mainly composed of long isotactic PP sequences.

3.9. TREF-GPC cross-fractionation Cross-fractionation of TREF and GPC can reflect the subtle difference in chain microstructure of samples A and B, which are illustrated by contour plots as shown in Fig. 11. In the contour plots, the logM axis shows the Mw obtained from GPC measurements, whereas the temperature axis represents the components eluted at different temperatures. The contour plots provide the relative amount of fractions with a specific molecular weight and composition, and clearly show the great heterogeneous distribution of the chain microstructure in each sample. They reveal two main differences in microstructure distribution of samples A and B. First, the contents of fractions collected at low temperatures (below 100 °C) and high temperatures (above 125 °C) in sample A are all higher than those in sample B. Second, most fractions in sample A have higher Mw than the corresponding fractions collected at the same elution temperatures in sample B.

4. Conclusions By combining P-TREF with high-temperature GPC, FTIR,

13

C NMR, WAXD, DSC

analysis, and SSA thermal fractionation, the chain microstructure of two HIPP resins is revealed. Both the resins mainly consist of four components: amorphous EP random copolymers, EP segmented copolymers, EP blocky copolymers, and isotactic PP homopolymers. In both resins, the major component is amorphous EP random copolymers available at 35 °C (fraction 1), which occupies 30.14 wt% of sample A and 28.85 wt% of sample B as shown in Fig. 12. The higher content of amorphous EP random copolymers improves the toughness of sample A. The content of highly isotactic PP factions with extremely high isotacticity (higher than 99 mol%), melting temperature (higher than 166 oC), and molecular weights (Mw = 30.37−54.87×104 g/mol) collected at 125−140 °C in sample A is 19.27 wt%, which is higher than that in sample B (15.31 wt%), contributing to the higher stiffness of sample A. Meanwhile, most fractions in sample A have higher molecular weight than those in sample B, which can facilitate the entanglement of polymer chains and improve the stiffness and possibly toughness to some extent. Fractions eluted at 80 °C (A-2 and B-2) and 100 °C (A-3 and B-3) are EP segmented and blocky copolymers, respectively. They occupy a total weight fraction of 9.14 wt% of sample A and 6.65 wt% of sample B. EP segmented and blocky copolymers can improve the compatibility between dispersed rubbery phase and the PP matrix. Thus, the higher contents of amorphous EP random copolymers, highly isotactic PP with high molecular weights, and EP segmented and blocky copolymers all contribute to a better balance between stiffness

and toughness for sample A than sample B.

Author contributions Wei Liu did experiments, collected the data, performed the analysis, and wrote the initial manuscript. Jiaqi Zhang did parts of the experiments. Mei Hong did parts of the experiments. Pei Li performed the analysis. Yanhu Xue performed the analysis. Quan Chen processed the data and wrote a section (rheological properties) of the manuscript. Xiangling Ji supervised the research.

Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments We are grateful to the financial support from ExxonMobil Asia Pacific Research and Development Company, Ltd. Appendix A. Supplementary data Table of P-TREF data of A and B samples; Table of DSC data of the fractions of samples A and B. References [1] F. Himma Nurul, S. Anisah, N. Prasetya, I.G. Wenten, Advances in preparation, modification, and application of polypropylene membrane, J. Polym. Eng. 36 (2016) 329. [2] L. Moballegh, S. Hakim, J. Morshedian, M. Nekoomanesh, A new approach to

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Captions

Fig. 1.

Molecular weight distribution profiles of samples A and B.

Fig. 2.

(a) Comparison of linear viscoelastic master curves of storage and loss moduli, G' and G", reduced at reference temperature Tr = 190 °C for A and B. G' and G" were multiplied by an intensity factor bT = Tr/T, and plotted against reduced angular frequency ωaT, with aT being the horizontal shift factor. (b) Comparison of the relaxation mode distribution of the samples A and B. The master curves of G' and G" of B are shifted horizontally until the terminal tail, G" ~ ω, coincides with that of A.

Fig. 3.

Test of Cox-Merz rule in between through comparison plots of complex viscosity η* against angular frequency ω, and shear viscosity η against

shear rate Fig. 4.

at 190 °C.

(a) Weight percent and (b) accumulative weight percent of the fractions in samples A and B versus the elution temperatures.

Fig. 5.

Molecular weight distribution profiles of fractions in samples A (a) and B (c), and their sums (b and d).

Fig. 6.

FTIR spectra of fractions in samples A (a) and B (b).

Fig. 7.

Schematic diagram of comonomer distributions in the polymer chains of fractions in the two samples.

Fig. 8.

WAXD diffractograms of fractions in samples A (a) and B (b). The inset in (b) is the partially magnified diffractogram of fraction B-3.

Fig. 9.

DSC melting thermograms of fractions in samples A (a) and B (b).

Fig. 10.

SSA melting curves of the fractions in sample A (a) and B (b).

Fig. 11.

Contour plots of samples A (a) and B (b).

Fig. 12.

Chain microstructure of samples A and B.

Highlights 1.

Chain microstructure of two highly impact polypropylene resins is revealed by

combining preparative temperature rising elution fractionation with multiple characterization techniques. 2.

The main components of the resins are ethylene−propylene random copolymers,

ethylene−propylene segmented copolymers, ethylene−propylene blocky copolymers, and isotactic polypropylene homopolymers. 3.

The good balance between stiffness and toughness of sample A is demonstrated

from the perspective of chain microstructure.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: