Determination of plateau moduli and entanglement molecular weights of ultra-high molecular weight isotactic polypropylene synthesized by Ziegler-Natta catalyst

Polymer Testing 60 (2017) 260e265

Contents lists available at ScienceDirect

Polymer Testing journal homepage: www.elsevier.com/locate/polytest

Material Properties

Determination of plateau moduli and entanglement molecular weights of ultra-high molecular weight isotactic polypropylene synthesized by Ziegler-Natta catalyst Hui Niu a, *, Yuanjie Wang a, Xiaoyan Liu b, Yanshai Wang a, Yang Li a, * a

State Key Laboratory of Fine Chemicals, Department of Polymer Science and Engineering, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China Lanzhou Petrochemical Research Center, PetroChina Co. Ltd., Lanzhou, Gansu 730000, China

b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 28 February 2017 Accepted 5 April 2017 Available online 7 April 2017

Ziegler-Natta catalysis was employed to prepare isotactic polypropylenes (iPPs) with various viscosity average molecular weight Mh, where Mh exceeds 1 000 000 g/mol. The viscoelastic properties of the synthesized ultra-high molecular weight iPPs (UHMWiPPs) were investigated by means of oscillatory rheometry. Due to the polydispersity of these polymers that derived from multi-sites catalyst, the UHMWiPPs rheological parameters, including Arrhenius activation energy Ea, plateau modulus G0N , as well as the molecular weight between entanglements Me, were compared with that of the reported monodisperse iPPs. We critically evaluated the typical methods for the experimental determination of G0N for polydisperse UHMWiPPs with linear architecture and concluded that the preferred method for UHMWiPP was the extrapolation integration method, which gave an average value of G0N ¼ (4.9 ± 0.3)  105 Pa and Me ¼ (6300 ± 400) g/mol. The polydipersity was proved to be acceptable for the UHMWiPP system in this research. However, the rheological measurements should be operated with caution because the temperature window was very narrow and the UHMWiPPs were extremely sensitive to thermal oxidative degradation during the tests. © 2017 Elsevier Ltd. All rights reserved.

Keywords: Plateau modulus Isotactic polypropylene Ultra-high molecular weight Polydisperse

1. Introduction Polypropylene is one of the most important thermoplastic polymers owing to its relatively low manufacturing cost and rather versatile properties. Among the three known stereo-selectivity structures (isotactic-, syndiotactic- and atactic-) of polypropylenes, isotactic polypropylene (iPP) is the most fascinating because of its unique structure that endows iPP resins with high performance, such as improved strength and modulus, excellent tensile properties and heat resistance, which promote its widespread applications in modern life [1]. So far, current industrial iPP preparation is dominated by Ziegler-Natta catalysts, which operate as multi-site catalysts and form iPPs with broad molecular weight distributions (MWD). To obtain an excellent model iPP for the investigation of rheological properties, two methods are utilized. One is known as post-

* Corresponding authors. E-mail addresses: [email protected] (H. Niu), [email protected] (Y. Li). http://dx.doi.org/10.1016/j.polymertesting.2017.04.007 0142-9418/© 2017 Elsevier Ltd. All rights reserved.

preparation that usually includes peroxide-induced thermal degradation as well as extensive purification by means of solvent fractionation to achieve a narrow MWD iPP. Another method is catalytic preparation of polymer with single-site metallocene catalysis systems [2]. Even if narrow MWD has been achieved, it is still impossible to observe the high-frequency plateau region of iPP to determine the plateau modulus G0N as well as the molecular weight between entanglements Me, which are very important fundamental parameters describing the melt topological network of the tube models [3]. This is because iPP, as a semicrystalline polymer, has very narrow temperature window for rheological measurements. It can only be tested above its melting point rather than the glass transition and below its degradation temperature [4e7], so that there is no chance to vary the temperature of the isotherms towards the plateau region due to the beginning of crystallization of iPP. Although different methods for the determination of G0N have been reported [3], for example, according to the storage modulus at the frequency of the loss factor tand has a minimum, or at the frequency of the loss modulus has a minimum, they are not applicable for iPP melt due to its loss factor and loss

H. Niu et al. / Polymer Testing 60 (2017) 260e265

modulus do not show a minimum. Hence, the extrapolation of the terminal peak at the high-frequency side becomes a difficult problem [5,8], and significant guesswork is necessary to extrapolate the terminal peaks [5,9]. The most widely agreed theory for semicrystalline iPP is the integration method [3,5], which calculates G0N by numerical integration over the terminal relaxation peak of loss moduli G00 (u):

G0N ¼

2

p

Zþ∞

G00 ðuÞdlnu

(1)

∞

In some cases, the terminal relaxation spectrum of polymers with low molecular weight components is broad for polydisperse systems, so that the loss modulus peak is often treated as symmetrical. Therefore, the integration method can be simplified by taking twice the area of the peak up to the frequency of the maximum, thereby avoiding integration over the problematic highfrequency region:

G0N

¼

4

p

umax Z

00

G ðuÞdlnu

(2)

∞

In order to obtain approximately accurate and convincing results, data processing and extrapolating at the high-frequency side have to be done very carefully to avoid wide deviation arising from data insufficiency of conventional iPP with molecular weight of 104~105 g/mol. It is known that the experimental detection of the viscoelastic properties is usually easier for higher molecular mass iPP, because of the increase of viscosity. Therefore, to get a maximum of loss modulus G00 at high-frequency for accurate extrapolation and integration, iPPs with molecular weight as high as ~106 g/mol are preferred. Polymers with molecular mass higher than 1 000 000 g/mol are usually known as ultra-high molecular weight polymers, represented by the ultra-high molecular weight polyethylene (UHMWPE). Synthesis and properties of UHMWPE have been fully discussed, especially in the rheological behaviour which provide guidance on UHMWPE analysing and processing, and also enriches our understanding of the UHMWPE melt characteristics [10]. We note however, that, for the ultra-high molecular weight iPP (UHMWiPP), there has not been a single example reporting their synthesis technique via industrial Ziegler-Natta catalysts or their melt rheological behaviour. The challenge, in our opinion, lie in the fact that most of the catalysts for propylene isotactic polymerization present severe chain transfer reactions, which result in the polymer molecular weight decreasing. Literature [11,12] that reported UHMWiPP preparation were based upon metallocene catalysis strategies without exception, but most of them performed unsatisfactorily in catalytic activity and required very low reaction temperature. In fact, current industrial iPP processes and those of the foreseeable future will continue to be dominated by traditional Ziegler-Natta catalysts from the supported MgCl2/TiCl4 family, which is well-accepted for both economic and performance reasons [13,14]. Therefore, simple and highly-efficient preparation of UHMWiPP with traditional Ziegler-Natta catalysts is greatly desired. The purpose of this work is twofold. First, for the UHMWiPPs, our main goal is to realize their preparation via traditional ZieglerNatta catalysts. Indeed, relatively broad MWD is usually a favourite character for polymer processing, although these polymers are opposed to the rheological theory model systems. Our second objective is concerned with polydisperse UHMWiPP. It is generally believed that precise measurements on narrow disperse polymers are essential for testing the predictions of tube models in general.

261

We, therefore, want to investigate the possible extension of methods for G0N determination to UHMWiPP systems with broad MWD. An important example concerns universal methods for relating iPP structure to macroscopic properties, including G0N and Me [15,16]. Fetters et al. has suggested that viscoelastic properties can be correlated with chain dimensions, in particular the packing length [17,18]. Again, definitive G0N data are a prerequisite to test such approaches. It is also believed that the plateau modulus from Eq (1) is unaffected by MWD, even although MWD alters the shape of the terminal relaxation. However, this practically requires molecular weights of at least 50 times Me. Based on the UHMWiPPs synthesized, we critically evaluate and compare two typical methods aforementioned for the experimental determination of the plateau modulus G0N for polydisperse iPP. 2. Experimental section 2.1. Materials All O2 and moisture-sensitive manipulations were carried out inside an argon-filled vacuum atmosphere dry-box equipped with a dry train. Cyclohexyldimethoxymethylsilane (ED-C), dicyclopentyldimethoxysilane (ED-D), diisopropyl dimethoxysilane (EDP), AlEt3 (1.0 M in heptane), and AliBu3 (1.0 M in heptane) were from Aldrich and used without further purification. Polymerization grade propylene was supplied by Lanzhou Petrochemical Co. of PetroChina. Ziegler-Natta catalyst, with the composition of Ti 2.60 wt%, Mg 18.5 wt%, dibutyl phthalate 5.25 wt%, diethyl phthalate 1.18 wt% and butyl ethyl 1,2-benzenedicarboxylate 1.26 wt%, was prepared according to the literature [19,20]. A commercial polypropylene P20 was kindly donated by Tianjin Petrochemical Co. of China. 2.2. Preparation of UHMWiPPs via Ziegler-Natta catalyst Propylene liquid polymerization was conducted in a 10 L stainless steel autoclave reactor equipped with a mechanical stirrer. The reactor was dried before being charged with 2 kg of liquid propylene. A typical reaction involved the sequential addition of external donor, cocatalyst solution and Ziegler-Natta catalyst into the reactor. The polymerization temperature was set at 70  C and the reaction was allowed to proceed for 60 min before the reactor was cooled to ambient temperature and the residual propylene was flashed. The resulting polymer was finally dried under vacuum at 60  C for 24 h. 2.3. Measurements The molecular weight of the polymers was determined by viscosity analysis. The measurement was performed at 150  C with naphthane as the solvent using glass capillary viscometers. Polymer thermal properties were measured by differential scanning calorimetry (DSC) using a TA Q2000 instrument controller at a heating and cooling rate of 10  C/min under a nitrogen atmosphere. Data were collected from the second scan curves. Dynamic shear rheological measurements were made with a TA Instruments AR2000ex rheometer equipped with 25 mm parallel stainless plates. All the samples were compression-moulded at 210  C to form small disks of 25 mm in diameter and 1 mm in thickness. For all samples, 0.3 wt% Irganox 1010 was added before moulding, and the measurements were performed under a nitrogen atmosphere to prevent thermo-oxidative degradation. To verify that no degradation had occurred during experiments, polymer viscosity average molecular weight at the beginning and the end of the experiments were compared. The rheological behaviour was

262

H. Niu et al. / Polymer Testing 60 (2017) 260e265

studied using dynamic oscillatory tests, with the angular frequency u varied from 0.01 to 500 rad/s and strains of 3%. Because of high melt stiffness, relatively high temperature was employed ranging from 210 to 260  C. The isotherms were shifted to obtain master curves at the reference temperature T0 ¼ 210  C.

3. Results and discussion A typical Ziegler-Natta catalyst, with 2.60 wt% of Ti, 18.5 wt% of Mg, and 7.69 wt% of hybrid internal donors (5.25 wt% of dibutyl phthalate, 1.18 wt% of diethyl phthalate and 1.26 wt% of butyl ethyl 1,2-benzenedicarboxylate, respectively), was utilized to synthesize UHMWiPPs. Varied external donors (ED), namely, cyclohexyldimethoxymethylsilane (ED-C), dicyclopentyldimethoxysilane (ED-D) and diisopropyl dimethoxysilane (ED-P), were accompanied, respectively, with cocatalysts (AlEt3 or AliBu3) to implement the propylene liquid polymerization at 70  C. The polymerization conditions and results are summarized in Table 1. Overall, Ziegler-Natta catalyst performed efficiently in the polymerization, with moderate activities higher than 6000 gPP/ (gcat,h). All these iPPs behaved as high isotactic polymers with melting temperature of 162e165  C. In Table 1, data were categorized according to the external donors. The combination ingredient of catalyst, external donor and cocatalyst is one of the most complicated factors for polymer molecular weight control. In the case of Ziegler-Natta catalyst applied in this study, polymerizations with ED-C as external donor resulted in relatively low viscosity average molecular weight Mh of iPPs, whereas that with ED-P and ED-D gave UHMWiPPs with Mh higher than 1 500 000 g/mol. Moreover, cocatalyst AliBu3 performed more effectively in achieving high molecular weight comparing with AlEt3. The effect of cocatalyst (Al)/external donor (Si) mole ratio on the productivity and molecular mass for iPP was also investigated. Increasing Al/Si ratio resulted in higher activity but lower Mh, as Runs P146 and P134 shows in Table 1. By conducting the polymerization with AliBu3/ED-P/Ti mole ratio of 400/20/1, UHMWiPP with Mh as high as 2 040 000 g/mol was prepared. To determine the plateau modulus G0N of polydisperse UHMWiPP, melt rheometry was applied on these polymers. The samples used for theological studies are derived directly from the reaction pot without any further purification or solvent fractionation. Generally, the rheology of polymer melts depends strongly on temperature. It is well known that, in the case of thermorheological simplicity, isotherms of storage and loss moduli G0 (u) and G00 (u) can be superimposed by horizontal shifts along the frequency axis u:

G0ðuaT ; T0 Þ ¼ G0ðu; TÞ

(3)

G00 ðuaT ; T0 Þ ¼ G00 ðu; TÞ

(4)

where aT is the horizontal shift factor and T0 the reference temperature. For UHMWiPPs, there is only a very small window (210e260  C) useful for rheological measurements, because of oxidative degradation above 260  C and a sudden increase of viscosity below 210  C. The specimens were photographed after the experiments and shown in Fig. 1. Although all experiments were performed under a nitrogen atmosphere to prevent thermooxidative degradation, specimens undergoing the oscillatory test at 260  C turned to slight yellow, indicating thermal aging took place during the measurement. To verify that no degradation had occurred in these specimens, their viscosity molecular weight Mh at the beginning and the end of the experiments were compared. The horizontal shift factor aT was found to range from 0.3 to 2.3 as the reference temperature T0 was set at 210  C and the measuring temperature was varied over 50  C. These values of aT for UHMWiPPs are comparatively small in contrast to those of other thermoplastic polymers such as polystyrene and poly (methyl methacrylate) [3], but they are in the range of iPPs' aT reported in other literature [5,6,18]. Usually, a semi-empirical equation, the Arrhenius equation, is used to evaluate the temperature dependence of the shift factors:

log aT ¼

  Ea 1 1  2:303R T T0

(5)

where R is the universal gas constant (R ¼ 8.314 J/(mol,K)) and Ea is

Fig. 1. Digital photographs of the specimens after the rheological dynamic shearing at different temperatures: (a) specimen P107, (b) specimen P125, (c) specimen P168, and (d) specimen P204.

Table 1 Propylene polymerizations catalyzed by Ziegler-Natta catalyst and the polymers characterization.a. Polymers

External donors

Cocatalyst

Al/Si/Ti (mol)

Activity (gPP/(gcat,h))

Mh (g/mol)

Tc ( C)

Tm ( C)

DHm (J/g)

P204 P168 P146 P182 P174 P134 P125 P107 P20 (control)

ED-P ED-P ED-P ED-D ED-D ED-D ED-C ED-C e

AliBu3 AlEt3 AliBu3 AliBu3 AlEt3 AliBu3 AlEt3 AliBu3 e

400/20/1 400/20/1 400/10/1 400/20/1 400/20/1 400/10/1 400/20/1 400/20/1

6471 6060 9429 6829 6534 12286 7381 9928 e

2 040 000 1 680 000 1 460 000 1 820 000 1 740 000 1 340 000 1 250 000 1 070 000 200 000

113.1 113.5 113.1 113.2 113.9 112.4 112.2 109.5 120.0

164.6 164.8 163.8 165.6 165.3 165.0 162.7 161.5 166.3

96.9 96.8 96.3 97.8 100.8 93.9 97.7 84.8 123.2

a Polymerization conditions: propylene 2 kg, catalyst 60 mg, cocatalyst AliBu3 or AlEt3 with Al/Ti mole ratio of 400, polymerization temperature 70  C, polymerization time 60 min.

H. Niu et al. / Polymer Testing 60 (2017) 260e265

the activation energy of flow. The shift factors were plotted in terms of the Arrhenius equation, giving the Eas of all samples, as listed in Table 1. The calculated Eas are independent of UHMWiPP molecular weight and an average value was achieved as Ea ¼ 32.7 kJ/mol. This is slightly lower than that of typical iPP (~39 kJ/mol) in earlier observation [5], but is reasonable at a temperature 20  C higher than the usual reference T0 of 190  C. Master curves were then obtained with a relatively small range of frequencies, as a result of the small experimental window and the slight temperature dependence of iPP melts. In Fig. 2, G0 (u) and G00 (u) for all the UHMWiPPs are shown as functions of reduced frequency (uaT) at the reference temperature T0 ¼ 210  C. These master curves for G0 and G00 show a significant dependence on polymer molecular weight. It is obvious that measurements of all samples with viscosity average molecular weight Mh > 1 000 000 g/ mol displayed a plateau region in G00 (u) curves at high frequency. Furthermore, the loss modulus G00 is proportional to uaT in the terminal zone and, consequently, the slope is 1 in a double logarithmic plot. However, the storage modulus G0 curves show a deviation from the slope 2, implying the molecules have not fully relaxed so that the terminal zone is not finally reached because of the high molecular weight of all these iPPs, which is similar to the results in literature [5,6]. For iPP with general molecular weight, more data of G00 (u) toward higher frequencies are desirable but difficult to obtain. The molecular weight limitations are a result of the catalyst technology, which was resolved in this work by preparing iPPs with molecular weights two or three times higher than those of the iPPs usually used in previous investigations [5e7]. A clear maximum of the loss moduli G00 (u) was reached in all UHMWiPPs toward higher frequencies. Therefore, determination of the plateau modulus G0N is possible by using Eq (1). In Fig. 3, all UHMWiPPs loss moduli G00 (u) in a linear scale are shown as a function of reduced frequency (uaT) on a natural logarithmic scale at the reference temperature T0 ¼ 210  C, in which maxima can be observed in all the specimens. Using these data, we accomplished the linear extrapolation of G00 (u) to zero at high frequencies, on the basis of the theory that there is a linear dependence of the flow transition at high angular frequencies. The assumed high-frequency portions of the terminal peak are shown by the solid line. It is also observed that the variation of the integration areas between the UHMWiPPs with different molecular weight is not pronounced. The plateau modulus G0N was calculated by using Eq (1), and the values are listed in Table 2. The integration areas of all the curves are close, giving a G0N range from 4.6  105 to 5.4  105 Pa

263

Fig. 3. Loss moduli G00 (u) of the UHMWiPPs versus reduced angular frequency uaT in a linear, natural logarithm plot at the reference temperature T0 ¼ 210  C.

for these UHMWiPPs (0.49 ± 0.03 MPa), except for an increased value of sample P107 (7.5  105 Pa) due to its lack of sufficient data at high-frequency for extrapolation, and an abnormality in sample P174. However, it is generally claimed that, due to the lack of data, the moduli at the high-frequency region are uncertain, so that some

Table 2 Plateau moduli G0N s and entanglement molecular weight Mes of UHMWiPPs with different molecular weighta. Samples

Ea (kJ/mol)

G0N (Pa) Eq (1)

P204 P182 P174 P168 P146 P134 P125 P107 a b c d

32.6 32.6 33.6 27.1 29.7 35.5 35.5 33.5

461 460 426 488 547 496 504 745

b

352 210 087d 415 643 781 731 055d

Z ¼ Mh/Meb

Me (g/mol) c

Eq (1)

716 400 926d 890 215 566 690 265

6700 6700 7200d 6300 5600 6200 6100 4100d

Eq (2) 359 351 315 393 400 376 362 418

b

Eq (2) 8600 8800 9700d 7800 7700 8200 8500 7400

c

306 272 241 267 260 216 205 145c

T0 ¼ 210  C, r ¼ 0.766 g/cm3. Calculated from Eq (1). Calculated from Eq (2). Not included in the calculation of average values of G0N or Me.

Fig. 2. Mastercurves of (a) the storage moduli G0 (u) and (b) the loss moduli G00 (u) of UHMWiPPs versus reduced angular frequency uaT at the reference temperature T0 ¼ 210  C.

264

H. Niu et al. / Polymer Testing 60 (2017) 260e265

guesswork is needed for extrapolating G00 to zero at high angular frequencies in a linear vs natural logarithm plot using Eq (1). Therefore, these values must be regarded as only tentative. It was found that no serious error was introduced when the value of umax (at which the loss modulus G00 takes its maximum) was taken as one of the integration limits or when the whole curve was assumed to be symmetrical (Eq (2)). Thus, the G0N s estimated by Eq (2) are also listed in Table 2. Unfortunately, Eq (2) gives a G0N value (average value of G0N ¼ 0.38 ± 0.02 MPa) that is systematically smaller than that calculated from Eq (1), because the true terminal peak is always skewed toward higher frequencies. Sample P107 fitted well in this case, indicating that Eq (2) can be used as a replacement when there are not enough experimental data at high frequencies, or as a supplement and confirmation of the results obtained by Eq (1). The plateau modulus G0N is connected with the entanglement molecular weight Me, which is defined as the molecular weight between adjacent temporary entanglement points:

G0N ¼

rRT Me

(6)

R is the universal gas constant (R ¼ 8.314 J/(mol,K)) and r is the density of the polymer at temperature T, at which the plateau modulus was measured. The datum for melt density r ¼ 0.766 g/ cm3 was taken from literature [5,6]. The calculated entanglement molecular weights Mes are listed in Table 2, with average Me value calculated from Eq (1) and Eq (2) of 6300 ± 400 g/mol and 8100 ± 500 g/mol, respectively. In the case of Mh > 1 200 000 g/mol, the Me value of polydisperse UHMWiPPs is consistent with that of narrow MWD iPPs (6200e6900 g/mol) [5,18]. It is usually believed that a broad MWD prevents the maximum of G00 (u) with respect to higher frequencies, because the transition between the plateau zone and the terminal flow region takes place over decades of frequencies. Nevertheless, the plateau zone is extremely pronounced when the number of entanglements per molecule Z (Z ¼ Mh/Me) is large enough (Z > 200 in this work as shown in Table 2). This implies that the influence of MWD on the plateau modulus can be neglected for UHMWiPPs. Fig. 4 shows the frequency dependence of loss factor (tan d ¼ G00 /G0 ) for the UHMWiPPs and the commercial polymer P20. As shown in Fig. 4a, tan d value of P20 ranged from 0.5 to 15 at 210  C, indicating a typically viscous contribution to the melt behaviour of iPP. In contrast, tan d of the UHMWiPPs was much less frequency sensitive at 210  C, though it did vary from ~0.1 to 2 over the range of frequencies studied. This should be ascribed to

the high entanglement of UHMWiPPs (with Z higher than 200). Raising the experiment temperature to 260  C, tan d increased finitely to 0.3e6, as shown in Fig. 4b, whereas the specimens were thermally aged and turned slightly yellow. This implies that a more effective method should be provided for the processing of UHMWiPPs. The use of the reciprocal of the cross-over modulus as a measure of MWD was proposed specifically for iPP made by Ziegler-Natta catalysis [21].

PI ¼

105 Gc

(7)

where Gc is the cross-over modulus of G0 (u) and G00 (u) curves. However, it is generally believed that a single rheological parameter Gc cannot adequately determine the polydispersity without any precondition. Bafna [22] compared two different iPP samples with similar molecular weight and MWD but reached dramatically different PI values (4.10 and 2.72, respectively). Hence, he drew the conclusion that the cross-over modulus sometimes was a reliable measure of MWD but sometimes was not, depending on the specific case. In this work, the cross-over point of G0 and G00 is depicted pictorially in Fig. 5 and the data are summarized in Table 3, in which the frequency u value at the cross-over point was also displayed. Calculated from Eq (7), PI data of all the UHMWiPPs are near the average value of 4.35 with minor deviation (±0.24). Considering all the UHMWiPPs are derived from same catalyst, the same active sites should give approximately same MWD, despite their molecular weights varying on a large scale. Although the results are insufficient to give any a firm conclusion, qualitative measurement of MWD with the empirical formula Eq (7) seemed acceptable in this research. However, commercial sample P20, which was synthesized from different Ziegler-Natta catalyst, gave a different PI value 3.43. In Fig. 5, the cross-over point frequencies of polymers with different molar masses are compared vertically. Widely used commercial iPPs usually present a cross-over point in the u range of 100~102 rad/s, as sample P20 showed in Fig. 5f, but for high molar masses, this point shifted to lower frequency. Samples with Mh between 1 000 000 to 1 700 000 g/mol showed cross-over points around u ¼ 101~100 rad/s; higher Mh (1 700 000e2 000 000 g/ mol) induced a much lower frequency cross-over point at u ¼ 102~101 rad/s.

Fig. 4. Loss factor tan d as a function of frequency at (a) 210  C and (b) 260  C for UHMWiPPs with different molecular weight.

H. Niu et al. / Polymer Testing 60 (2017) 260e265

265

4. Conclusions By tuning polymerization Ziegler-Natta catalyst, choosing the appropriate external donor and cocatalyst, polydisperse UHMWiPPs can be obtained with high viscosity average molecular weight Mh of up to 2  106 g/mol. The viscoelastic behaviour of the synthesized UHMWiPPs was investigated by rheological measurements. The activation energy of flow Ea, plateau modulus G0N , and entanglement molecular weight Me of these UHMWiPPs were studied. Polydispersity introduces a large uncertainty in the estimation of G0N , which is quite significant from the practical point of view, since numerous polymers cannot be synthesized with MWD <2, especially in the case of iPPs. In this work, samples with ultrahigh molecular weight enabled the measurement of G0 (u) and G00 (u) over a range of necessary to obtain a distinct onset of the plateau zone. We have analyzed the extension to polydisperse iPP with ultra-high molecular weight of the methods validated for monodisperse systems. The plateau moduli G0N s were determined successfully by integration of G00 (u) function, which was not possible for conventional iPP samples with a broad MWD or much lower molecular weight. The plateau moduli of UHMWiPPs with Mh higher than 1 200 000 g/mol are very close, giving an average value of G0N ¼ 4.9  105 Pa, whereas that of iPP with Mh of ~1 000 000 g/ mol is larger. The calculated Me in the molten state of UHMWiPPs is 6300 g/mol. This is consistent with the results derived from narrow MWD reported in literature. Overall, agreement within small error could be achieved as a result of the correct use of the methods, among which the preferred scheme is the extrapolation integration method. A prerequisite is relatively higher measuring temperature, as 210e260  C used in this research, which also indicates that the melt processing of UHMWiPPs might be a challenge.

Acknowledgement This work was financially supported by National Natural Science Foundation of China (No. 21374121).

References [1] [2] [3] [4] [5] Fig. 5. Series of frequency sweep experiments on different molecular weight of iPPs at 210  C: (a) specimen P204, (b) specimen P168, (c) specimen P146, (d) specimen P125, (e) specimen P107, and (f) specimen P20.

[6] [7] [8] [9] [10]

Table 3 Moduli Gcs and frequencies ucs at G0 & G00 cross-over point.a

[11] [12]

Samples

uc (rad/s)

Gc (Pa)

PI ¼ 10 Gc

P204 P182 P174 P168 P146 P134 P125 P107 P20

0.0365 0.0336 0.0331 0.0928 0.1405 0.0489 0.1542 0.1354 74.827

21 23 21 23 23 22 25 22 20

4.64 4.25 4.64 4.23 4.22 4.48 3.94 4.43 4.89

a

Temperature ¼ 210  C.

533 551 546 626 694 320 359 561 444

5

[13] [14] [15] [16] [17] [18] [19] [20] [21] [22]

P. Galli, G. Vecellio, Prog. Polym. Sci. 26 (2011) 1287. L. Resconi, L. Cavallo, A. Fait, F. Piemontesi, Chem. Rev. 100 (2000) 1253. C. Liu, J. He, E. Ruymbeke, R. Keunings, C. Bailly, Polymer 47 (2006) 4461. S. Wu, J. Polym. Sci. Part B Polym. Phys. 27 (1989) 723. A. Eckstein, J. Suhm, C. Friedrich, R.D. Maier, J. Sassmannshausen, M. Bochmann, R. Mülhaupt, Macromolecules 31 (1998) 1335. A. Eckstein, C. Friedrich, A. Lobbrecht, R. Spitz, R. Mülhaupt, Acta Polym. 48 (1997) 41. H. Mavridis, R.N. Shroff, Polym. Eng. Sci. 32 (1992) 1778. P.M. Wood-Adams, J.M. Dealy, A.W. deGroot, O.D. Redwine, Macromolecules 33 (2000) 7489. C. Liu, J. Yu, J. He, W. Liu, C. Sun, Z. Jing, Macromolecules 37 (2004) 9279. S. Talebi, R. Duchateau, S. Rastogi, J. Kaschta, G.W.M. Peters, P.J. Lemstra, Macromolecules 43 (2010) 2780. M. Vathauer, W. Kaminsky, Polymer 42 (2001) 4017. €, B. Rieger, OrganoS. Deisenhofer, T. Feifel, J. Kukral, M. Klinga, M. Leskela metallics 22 (2003) 3495. K. Norio, J. Polym. Sci. Part A Polym. Chem. 42 (2004) 1. Y. Qin, N. Wang, Y. Zhou, Y. Huang, H. Niu, J.Y. Dong, Macromol. Rapid Commun. 32 (2011) 1052. W.W. Graessley, S.F. Edwards, Polymer 22 (1981) 1329. S.T. Milner, Macromolecules 38 (2005) 4929. L.J. Fetters, D.J. Lohse, D. Richter, T.A. Witten, A. Zirkel, Macromolecules 27 (1994) 4639. L.J. Fetters, D.J. Lohse, C.A. Garcia-Franco, P. Brant, D. Richter, Macromolecules 35 (2002) 10096. L. Lu, H. Niu, J.Y. Dong, J. Appl. Polym. Sci. 124 (2012) 1265. X. Wen, M. Ji, Q. Yi, H. Niu, J.Y. Dong, J. Appl. Polym. Sci. 118 (2010) 1853. G.R. Zeichner, P.D. Patel, Montreal, Quebec, Canada, in: Proceedings of the Second World Congress of Chemical Engineering, 1981, p. 333. S.S. Bafna, J. Appl. Polym. Sci. 63 (1997) 111.