Crystalline structure, morphology and mechanical properties of β-nucleated controlled-rheology polypropylene random copolymers

Polymer Testing 30 (2011) 899–906

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Material properties

Crystalline structure, morphology and mechanical properties of b-nucleated controlled-rheology polypropylene random copolymers Jing Cao*, Qiu-Feng Lü College of Materials Science and Engineering, Fuzhou University, Fuzhou 350108, Fujian, People’s Republic of China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 18 July 2011 Accepted 30 August 2011

b-nucleated controlled-rheology polypropylene random copolymers (CRPPRs) were prepared by peroxide-initiated degradation via adding dicumyl peroxide (DCP) to bnucleated PPR. Melt flow index (MFI) and mechanical properties of b-nucleated PPR and CRPPRs were analyzed and compared with PPR. Wide-angle X-ray diffractometer (WAXD), differential scanning calorimeter (DSC) and polarized light microscope (PLM) were used to study the crystalline structure and morphology. The results indicate that the MFI of bnucleated CRPPR increases linearly with increasing DCP content. The strain hardening rate and impact strength of b-nucleated CRPPRs well reflect the degree of degradation and bphase content. No significant change of yield strength and strain at break is seen with increasing DCP content, except for an increase of the latter when DCP content reaches 0.15 wt%. The maximum crystallinity is observed at 0.05 wt% DCP. b-nucleated CRPPRs show increased crystallization temperature and decreased b-crystal content and crystal size as DCP content increases. Ó 2011 Elsevier Ltd. All rights reserved.

Keywords: Controlled-rheology Polypropylene random copolymer Structure Morphology Mechanical property

1. Introduction Polypropylene random copolymer (PPR) is a semicrystalline polymer with a number of desirable properties. PPR plays an important role in film, rigid packaging and pipe applications due to its satisfying balance of clarity, flexibility and mechanical strength with respect to polypropylene homopolymer (PPH) [1,2]. This is because the random insertion of a small amount of ethylene along the PP chains promotes the formation of shorter stereoregular blocks and decreases the melting temperature and crystallinity [3]. The properties of PPR are related to the ethylene content in the PP chains. However, the applications of commercial PPR are restricted by its high melt viscosity and elasticity, as commercial PPR resins are commonly polymerized by conventional Ziegler–Natta

* Corresponding author. Tel./fax: þ86 591 2286 6531. E-mail address: [email protected] (J. Cao). 0142-9418/$ – see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2011.08.016

catalyst systems and have a high molecular weight (Mw) and a broad molecular weight distribution (MWD). These features restrict the applications of the resin in high-speed spinning and injection molded thin walled products. A “controlled-rheology polypropylene” (CRPP) technology which has been widely used for PPH [4–8] and PP block copolymers (PPB) [9,10] provides a feasible approach to solve the above problems. CRPP is obtained by a post reactor procedure that consists of degradation with organic peroxide. Peroxide initiated scission reactions could efficiently result in polymers with superior properties, i.e. improved melt flow characteristics, less shear sensitivity, high elongation at break and surface smoothness [8]. PP chains can organize into different spatial arrangements, giving rise to three basic crystalline polymorphs: a, b and g phases, of which a phase has higher strength, while b phase yields better impact toughness. The formation of b phase can be simply obtained by introducing an effective b nucleating agent, which could easily shorten the time of inducing nuclei during crystallization

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and give rise to improved mechanical properties, especially toughness [11–13]. The effect of CRPP technology on the chemical structure, rheological and crystallization behavior and mechanical properties of PPH has been widely studied in previous works [4,5,14–16], but the effect of CRPP technology on bnucleated PP has seldom been studied [17]. Furthermore, the related work has focused on the crystal structure of bnucleated CRPP, whereas the mechanical properties and the effect of peroxide content on the b-crystallinity have not been researched. In addition, it was noted that the MWD of PP copolymer was narrower compared with the homopolymer [16], thus the introduction of CRPP technology into the former is of more significance. Therefore, in this paper, a moderate amount of b nucleating agent is introduced into PPR to obtain b-nucleated PPR. Furthermore, the CRPP technology was applied to prepare b-nucleated CRPPR via the addition of peroxide, and the effect of peroxide content on the crystal structure, morphology and mechanical properties of b-nucleated CRPPR is studied in detail. 2. Experimental

2.4. Mechanical characterization Tensile tests were carried out following ISO 527-2: 1997 in order to obtain the load-displacement curves and to measure the yield strength and stress and strain at break. Dumbbell shaped specimens of dimensions 10 mm  115 mm  4 mm in the narrow section were tested using an electromechanical testing machine (CMT6104) at a crosshead speed of 50 mm/min. Charpy impact tests were performed following ISO 1791982 to determine the impact strength. Rectangular samples with dimensions of 10 mm  80 mm  4 mm and an injection molded single-edge 45 notch were tested. 2.5. WAXD tests The WAXD tests of the samples were carried out on an Ultima III instrument. A conventional CuKa X-ray tube at a voltage of 40 kV and a filament current of 40 mA was used to obtain the WAXD spectra. The scanning 2q range was 5 – 40 with a scanning rate of 4 /min. The analysis method used on WAXD spectra was described elsewhere by Huo et al. [18]. The overall crystallinity was determined by the following equation

2.1. Materials The material under study was a PPR with a melt flow rate of 0.2 g/10 min (230  C/2.16 kg), Mw of 72.2  104 g/mol, Mw/Mn of 4.5 and an ethylene content of 3.79 wt%, supplied by Korea Petrochemical Ind. Co., LTD. Dicumyl peroxide (DCP) was obtained from China National Medicine (Group) Shanghai Reagent Corporation. b nucleating agent with the trademark WBG-Ⅱ, was a rare earth organic compound supplied by Guangdong Winner Functional Materials Corporation (China). 2.2. Sample preparation Firstly, b nucleating agent was mixed with PPR pellets at a fixed content of 4 wt% in a high-speed mixer for 5 min. The mixture was extruded in a twin-screw extruder (SJSH30, Shijiazhuang Xingshuo Ind. Co., LTD., China) with a temperature profile of 180–220  C and then pelletized and dried to obtain b masterbatch. The masterbatch was mixed with PPR pellets and DCP and extruded with the same temperature as above and then pelletized and dried. The resulting pellets were b CRPPRs with a b nucleating agent content of 0.2 wt% and DCP contents of 0.05 wt%, 0.1 wt% and 0.15 wt%, which were marked as NA-P5, NAP10 and NA-P15, respectively. Pure PPR and b-nucleated PPR pellets were also prepared as comparison, being marked as PPR and NA-P0, respectively. The pellets were then injection molded with an injection machine (SZ550NB, Ningbo plastic machine company, China) with a temperature profile of 180–225  C. 2.3. Melt index measurement Melt flow index (MFI) was measured following ISO 1133-97 using a MFI tester (XNR-400B) at 230  C/2.16 kg.

Xc ¼

Ac  100% Ac þ Aa

(1)

whereAc andAa are the areas under the crystalline peaks and amorphous halo, respectively. And the b-form crystal content was calculated by the Turner–Jones equation [19]

Kb ¼

Ibð300Þ Ibð300Þ þ Iað110Þ þ Iað040Þ þ Iað130Þ

(2)

where Kb is the relative b-form content, and Ibð300Þ , Iað110Þ , Iað040Þ and Iað130Þ are the diffraction peak intensity of the (300) crystal plane of the b-form, the peak intensities of the (110), (040) and (130) crystal planes of the a-form, respectively. 2.6. DSC measurements DSC scanning of the samples was performed on a SDT Q600 differential scanning calorimeter (TA Instruments, USA) in a nitrogen atmosphere. To reduce the b/a transformation during heating, samples of about 5 mg were heated from room temperature to 220  C at a rate of 20  C/min, and held for 5 min to eliminate thermal history. Then, the samples were cooled to room temperature at 10  C/min. DSC tests were also carried out after tensile testing on specimens taken from near the break region of the tensile test pieces. Scanning was also performed from room temperature to 220  C at 20  C/min for the purpose of comparison. The crystallinity was determined by

Xc ¼ DH=DH 0  100% in which DH and DH0 were the melting enthalpy of the samples and 100% pure crystalline PP, respectively, and DH0 ¼ 209 J/g [20].

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2.7. Polarized light microscope (PLM) observations To investigate the crystalline morphology, a specimen was cut from the injected sample and hot-compressed into thin film at 230  C, held for 3 min and then cooled to room temperature at a rate of 5  C/min. The crystal morphology was observed by PLM (XP-213, Nanjing Jiangnan Novel Optics Co., Ltd., China). 3. Results and discussion 3.1. Melt flow properties The DCP induced post reactor procedure can result in the decrease of molecular weight and molecular weight distribution [21–23]. As a result, the MFI would increase with the addition of DCP, which is in accordance with the results shown in Table 1. The addition of b nucleating agent has little effect on the MFI of PPR, while the MFI increases linearly with the increase of DCP content, resulting in improved processability and lower energy consumption during process. 3.2. Mechanical properties Fig. 1 shows the Charpy impact strength of the samples. As expected, the addition of 0.2 wt% b nucleating agent largely improves the impact strength of PPR, increasing from 27.4 kJ/m2 for PPR to 42.1 kJ/m2 for NA-P0. The enhancement of toughness is attributed to the nucleating agent induced formation of b-phase. Compared with NAP0, the impact strength of b-nucleated CRPPRs are much lower. The toughness of b-nucleated CRPPR diminishes with the increase of the amount of DCP included. The magnitude of NA-P15 is even less than that of PPR. One possible explanation is that the DCP-induced break of molecular chains leads to the decrease of the entanglement density, giving rise to lower energy consumption during impact failure. Since the excellent impact property of bnucleated PPR is ascribed to the toughening effect of the bphase, another important factor influencing the impact strength of b-nucleated CRPPR is the b-phase content, which will be investigated in the following section. The results of tensile tests are displayed in Fig. 2 and Fig. 3. Fig. 2 shows the load-displacement curves of the samples. It is evident that all samples experience yielding and cold drawing, and all samples except pure PPR exhibit subsequent strain hardening. In other words, all samples with b nucleating agent show strain hardening, while pure PPR breaks before strain hardening could occur. From the load-displacement curves for samples with b nucleating agent, it seems that the strain hardening rate decreases as the amount of DCP increases. Previous researches on various polymers have revealed that the strain hardening

Fig. 1. Charpy impact strength of various samples.

rate reduces with decreasing molecular weight, because the conformational changes become easier and thus accelerate the relaxation of entanglement network [24– 26]. Moreover, results found in Xu’s [27] compression experiments and Karger-Kocsis’ [28] tension observations have revealed that the rate of strain hardening for b-PPH exceeded that of a-iPP, and the authors attribute the phenomenon to the b-a phase transformation. To further verify this speculation, the crystal structure of samples both before and after tensile tests will be explored by DSC. The yield strength, strain and stress at break obtained from tensile tests are listed in Fig. 3. The addition of b nucleating agent decreases the yield strength slightly and increases the strain and stress at break obviously, which is attributed to the better toughness and lower strength of bphase compared with a-phase. One observes in Fig. 3 (a) that the yield strength shows a slight decrease with a small addition of DCP but seems not to be influenced by the amount included. In Fig. 3 (b), compared with NA-P0, no

Table 1 MFI of the samples.

MFR (g/10 min)

Pure PPR

NA-P0

NA-P5

NA-P10

NA-P15

0.2

0.23

1.14

1.95

2.62 Fig. 2. Load-displacement curves of the samples.

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impact strength. The changes in the mechanical properties with molecular weight are in agreement with Salazar’s [29] observations. Authors of the related study attribute the changes in mechanical properties to the reduction of molecular weight in the amorphous phase, thus the strain and stress at break, which are predominantly governed by this amorphous region, varies with the decrease of molecular weight. However, the strain at break tends to decrease (expect an increase in CRPP with the minimal amount of peroxide) with increasing peroxide amount in Alicia Salazar’s research, which is different from the present result. Considering the much higher MFI values of CRPP used in Alicia Salazar’s study, the discrepancy could be explained by the entanglement density which is directly related to the MFI. For super-low MFI PP (as in this paper), the high entanglement density obstructs the motion of molecular chains and chain segments, thus a lower elongation at break is shown. An appropriate level of DCPinitiated degradation of PP decreases the entanglement density and favors the slippage of the chains during elongation, giving rise to a higher strain at break. In the case of a much higher MFI PP (as in Alicia Salazar’s research) a further decrease of molecular weight mainly decreases the intermolecular interactions instead of the entanglement density. Therefore, the mechanism of failure may change from ductile to brittle. That is maybe why the strain at break increases first and decreases later as the amount of DCP increases in Azizi’s research [4]. 3.3. Crystal structure The mechanical properties of semi-crystalline polymers greatly depend on their crystalline structure. Fig. 4 shows the diffraction patterns of PPR and b-nucleated PPR and CRPPRs. Obviously, all samples with b nucleating agent exhibit the characteristic diffraction peak at 2q ¼ 16.0 , corresponding to the (300) crystal plane of b-phase. A higher relative intensity of the (300) peak reflects a higher content of the b-form. The crystallinity and Kb values are

Fig. 3. Yield strength (a), strain at break (b) and stress at break (c) of the samples.

significant change of the strain at break of b-nucleated CRPPR is seen up to 0.1 wt% of DCP, but beyond 0.1 wt%, the value increases obviously. The stress at break of b-nucleated CRPPR tends to diminish linearly as the amount of DCP increases (Fig. 3 (c)), which is similar to the tendency of

Fig. 4. WAXD patterns of various samples.

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listed in Table 2. It is seen that the crystallinity shows no evident change with the addition of b nucleating agent. However, it increases slightly with the introduction of a small amount of DCP and decreases with the further addition of DCP, and the optimal DCP content is 0.05 wt%. That is maybe because PPR has a very low MFI value and the addition of a small amount of DCP leads to moderate degradation of PPR, especially the super-high molecular weight chains, thus facilitating the diffusion and arrangement of molecular chains during cooling, and results in elevated crystallinity. There is a critical molecular weight at 0.05 wt% DCP when the mobility of the molecular chains and the active sites are optimal. When the content exceeds 0.05 wt%, the molecular chains broke into much shorter ones and branching or micro-cross-linking occurs, thus there are not enough active sites for crystallization and, due to the strong mobility of the molecular chains, it may be difficult to maintain the ordered chain structure required for the crystallization process. As for the Kb value in Table 2, it is shown that the introduction of 0.2 wt% b nucleating agent induced a b-form content of 36% in PPR, which is much lower than that of b-nucleated PPH [30]. This divergence is in agreement with the previous works [31–33] which have found that PPR has a reduced tendency to bcrystallization because of the partially reduced regularity caused by incorporation of comonomer units. Kb decreases evidently with the addition of 0.05 wt% DCP but shows only slight changes as the amount of DCP increases, implying that in the researched range higher molecular weight and broader molecular weight distribution are beneficial for the formation of b-phase in PPR in the presence of b nucleating agent. A similar conclusion has been reported in Marigo’s research [34] on PPH without nucleating agent. The values of Kb for b-nucleated CRPPRs also provide support for the changes of impact strength (in Fig. 1) and strain hardening rate during tensile elongation (in Fig. 2). The decrease of b-form content in b-nucleated CRPPRs results in the reduction of impact strength as well as the rate of strain hardening, even when a small amount of DCP is included (the decrease of b-form content diminishes the extent of ba phase transformation during tensile elongation), and the further DCP induced decrease of molecular weight is the main reason for the decrease of impact strength and strain hardening rate with increasing DCP content. Fig. 5 (a) and (b) show the DSC melting and crystallization curves of PPR and b-nucleated PPR and CRPPRs, respectively. It is seen that b-nucleated PPR and CRPPRs exhibit two melting peaks, Tm1 and Tm2, corresponding to the b- and a-crystals, respectively. It seems that the peak intensity of b-crystal for b-nucleated CRPPRs is lower compared with that of b-nucleated PPR, indicating lower b-crystal content, which is in accordance with WAXD results. Crystalline parameters of the samples calculated from DSC results are listed in Table 3. The melting Table 2 Crystalline parameters of various samples calculated from WAXD.

Xc (%) Kb

PPR

NA-P0

NA-P5

NA-P10

NA-P15

38.2 0

38.9 0.36

41.0 0.23

39 0.28

34.6 0.24

Fig. 5. DSC melting curves before (a) and after (c) tensile test and crystallization curves (b) of various samples.

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Table 3 Crystalline parameters of various samples calculated from DSC. Samples Melting properties Before tensile test

After tensile test

Crystallization properties

Tm1 ( C) Tm2 ( C) Xc (%) Tm2 ( C) Xc (%) Tco( C) Tcp( C) PPR NA-P0 NA-P5 NA-P10 NA-P15

– 130.7 130.9 130.9 130.5

145.9 145.1 145.9 145.9 145.7

21.2 21.5 22.0 21.0 19.2

145.6 146.1 145.4 146.0 145.5

25.2 29.1 27.8 25.2 21.0

102.8 113.7 114.6 115.2 114.6

98.7 100.5 100.2 100.6 101

temperatures of both phases are almost constant with the addition of DCP. Changes in crystallinity with the addition of b nucleating agent and DCP coincide well with the results derived from WAXD. The crystallization peak temperature (Tcp) and crystallization onset temperature (Tco) of b-nucleated PPR is about 2  C and more than 10  C higher than that of PPR, respectively, as a result of the nucleation effect of b nucleating agent. With the addition of DCP, both the Tcp and Tco tend to increase slightly, which is consistent with previous reports [10,17], indicating that due to the degradation of molecular chains, especially the ultra-high molecular weight component, the improved mobility of the molecular chains favors the diffusion and arrangement of macromolecules into crystal cells,

Fig. 6. PLM photographs of crystal morphologies for PPR (a), NA-P0 (b), NA-P5 (c), NA-P10 (d) and NA-P15 (e).

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resulting in elevated crystallization rate and higher crystallization temperature. Melting curves of the samples after tensile testing are shown in Fig. 5 (c). All samples display the unique and constant melting peak corresponding to the a-crystal, indicating that the b-phase detected in Fig. 5 (a) has transformed to a-phase after tensile testing. From the related data in Table 3, one observes that the crystallinity of all samples clearly increases after tensile testing, which is due to the rearrangement and orientation of the molecular chain segments during stretching allowing more chains to pack into crystals.

induced degradation of PPR is unfavorable for the formation of b-phase. The crystallization temperature of bnucleated CRPPRs increases slightly and the crystal size decreases with increasing addition of DCP.

3.4. Crystal morphology

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The crystal morphology of PPR, b-nucleated PPR and bnucleated CRPPRs is revealed by PLM pictures in Fig. 6. The spherulites of original PPR are well developed, resulting in larger spherulite size and a broader distribution of the spherulite dimensions. Furthermore, one observes clear boundaries between the spherulites of PPR. The size of the crystals in Fig. 6 (b) becomes much smaller and more even, and the quantity is much larger as a result of the nucleation effect of b nucleating agent, which gives rise to numerous nuclei at the initial stage of crystallization. These smaller and finely dispersed crystal structures, combined with the formation of b-phase, contribute to the elevated impact strength of NA-P0. Compared with NA-P0, b-nucleated CRPPRs exhibit similar crystalline morphology but slightly decreased crystal size with the increasing addition of DCP. This is probably due to the small number of molecular chains with branching and micro-cross-linking structure acting as nucleating agents [35,36] to induce larger numbers of crystallites. Since more crystallites grow in a confined space, they tend to squeeze together, resulting in smaller size. These results of PLM are clearly in accordance with the DSC results. 4. Conclusions

b-nucleated PPR was prepared by adding b nucleating agent to PPR, then various amount of DCP was introduced to initiate controlled degradation and obtain b-nucleated CRPPRs. It was found that the MFI of b-nucleated CRPPR is higher than that of b-nucleated PPR, and increases as the amount of DCP increases. Due to the decreased content of b-form and molecular weight, the impact strength of bnucleated CRPPR and the rate of strain hardening diminish with increase of DCP content. The addition of b nucleating agent leads to improved toughness and slightly decreased strength of PPR. The yield strength keeps almost constant, the stress at break decreases linearly, while the strain at break remains constant at first and then increases with increase of the amount of DCP. The changes in stress and strain at break are mainly attributed to the reduced molecular weight and entanglement density in the amorphous phase. WAXD and DSC results are in good agreement with each other and demonstrated the similar changing trend in crystallinity, i.e. increases first and decreases later with increasing DCP content, and the reduced b-form content in b-nucleated CRPPRs. It is concluded that DCP

Acknowledgements This work was supported by the Foundation for Development of Science and Technology of Fuzhou University (Grant No. 2011-XY-1). References

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