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High-Performance Isotactic Poly(1-butene)/Isotactic Polypropylene Alloys with in-Situ-Synthesized Poly(propylene-co-butene) Huarong Nie, Weijia Xiao, Yaping Ma, Chenguang Liu,* and Aihua He* Shandong Provincial Key Laboratory of Olefin Catalysis and Polymerization, Key Laboratory of Rubber-Plastics (Ministry of Education), School of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao, Shandong 266042, China
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S Supporting Information *
ABSTRACT: An admixture of isotactic poly(1-butene) (iPB) with isotactic polypropylene (iPP) enhances the scope of consumer applications and the commercialization of iPB because of the reduced price. However, the properties of the mixtures are not superior to but are intermediate between those of the components. Achieving high-performance iPB materials by blending without sacrificial advantages of each component is still a great challenge. In this work, the iPB/iPP in-reactor alloys, as novel polyolefin materials, are differentiated from the traditional mixtures by the existence of the in-situ-synthesized poly(propylene-co-butene). The copolymer enables improved compatibility between iPB and iPP and increased interfacial entanglements between their spherulites during crystallization. Compared with iPB/iPP blends, all of the iPB/iPP alloys exhibit higher overall performance, even exceeding the respective characteristics of iPB and iPP materials.
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INTRODUCTION Isotactic poly(1-butene) (iPB) is a special polyolefin with remarkable physical and mechanical properties, such as excellent creep, crack, and impact resistance.1 iPB as a stereoregular macromolecule crystallizes into chain-folded lamellae that melt near 130 °C,2,3 and it has been used in pressured tanks, tubes, and hot-water pipes.1,4−6 However, compared with isotactic polypropylene (iPP), the high price7 and the relatively low tensile strength of iPB limit its applications.8,9 The iPP has achieved a high reputation in the area of plastics for its low price.10 Polymer blending helps us to realize new, economically viable materials with improved properties.11,12 However, despite iPB and iPP having similar chemical structures, these two crystalline polymers are thermodynamically incompatible.3,13 Thus the phase-separated structure may lead to a weak interface and unsatisfactory properties of the products. A large number of studies have reported that the addition of copolymers to phase-separated polymer blends enhances the compatibility, and consequently the performance, provided that the associated chain segments entangle with the corresponding homopolymers at the interface.14−16 However, the difficulty in the synthesis of olefin copolymers with high stereoregularity makes such a strategy daunting.14,17−19 Fortunately, we have recently fabricated iPB/iPP alloys by sequential two-stage polymerization, where a few copolymers, named poly(propylene-co-butene), are documented to exist in the products.20,21 The iPP content in the alloys could be regulated by controlling the reaction time of the iPP synthesis. Herein we further investigated the properties of iPB/iPP alloys that have different propylene unit contents and interpreted the © XXXX American Chemical Society
phase morphology of the alloys. To understand the function of poly(propylene-co-butene) in alloys, the iPB/iPP blends with the same composition as the alloys were also employed as the controls.
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RESULTS AND DISCUSSION In this work, three alloys were used. As shown in Table 1, A20.8 denotes the alloy consisting of 20.8% propylene unit that originated from both the iPP homopolymer and poly(propylene-co-butene) and so on. Obviously, A-20.8 and A13.8 are accompanied by a high concentration of poly(propylene-co-butene). We hypothesize that the copolymers could improve the miscibility of iPP and iPB and increase the interfacial strength in crystalline/crystalline polymer blends. B20.8 is defined as the iPB/iPP mixture that contains 20.8% iPP homopolymer. It is noted that the amount of poly(propyleneco-butene) in alloys is calculated according to differential scanning calorimetry (DSC) after the alloys undergo temperature rising elution fractionation. For copolymers, two melting peaks arising from the PP chain segments and PB chain segments could be observed in DSC curves (data not shown here). Furthermore, the detailed chemical structures are analyzed according to 13C nuclear magnetic resonance (13CNMR) (Figures S1−S2, Tables S1−S2). It is found that the random copolymer, poly(propylene-co-butene), contains relatively long PP and PB chain segments (Table S3). Received: Revised: Accepted: Published: A
January 10, 2019 March 25, 2019 March 29, 2019 March 29, 2019 DOI: 10.1021/acs.iecr.9b00178 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research Table 1. Compositions of the Three Synthesized Alloys sample
Mw (K)a
PDI
isotacticity (wt %)b
isotacticity (wt %)c
PP (wt %)d
poly(propylene-co-butene) (wt %)e
A-6.3 A-13.8 A-20.8
942 790 892
7.4 7.8 7.5
95.4 97.8 96.4
92.3 93.1 95.4
6.3 13.8 20.8
1.8 4.9 4.6
a
Mw was obtained by GPC. bIsotacticity of iPB was obtained by boiling extraction. cIsotacticity of iPB was obtained by 13CNMR. dConcentration was obtained by 13CNMR. eConcentration was obtained by temperature rising elution fractionation.
Figure 1. Mechanical properties of samples: (A) impact strength and (B,C) tensile strength at room temperature and high temperature of 100 °C, respectively.
molten state.13 When the concentration of iPP increases to 20% in blends, demixing between iPP and iPB inevitably takes place. As shown in Figure 2A,B, the biphasic structures on the micrometer scale are observed in iPP/iPB blends. However, the phase-separated structure cannot be seen in alloys. This means that iPP and iPB in A-20.8 may be miscible or the phase domains in A-20.8 are below the microscale, which cannot be observed by an optical micrograph. For a homogeneous material, the physical properties of the mixtures are between those of the two components. On the basis of the fact that A20.8 behaved with a higher impact strength than iPB and a higher tensile strength than iPP as well, it is believed that A20.8 should have a phase-separated structure, which contributes to the high performance. Furthermore, given that A-20.8 obtained a smaller phase domain than B-20.8, it is considered that the copolymer should exert its function as a compatibilizer between iPP and iPB; thereby, the improved miscibility of iPP and iPB enables the smaller iPP domains in alloys. Because the iPP and the iPB are both crystalline polymers, the ultimate properties of their alloys or their blends are directly dependent on the crystal morphology, which is indeed controlled by the crystallization process. Figure 2C,D shows the melting and crystallization behaviors of the mechanical test samples. The crystallinities, the melting temperatures, and the
Figure 1 shows the mechanical properties of iPB, iPP, alloys, and blends with the same mass fraction of PP. Clearly, iPB shows excellent ductility, whereas iPP behaves as a brittle plastic. The iPB/iPP blends prepared by direct mixing exhibit fair impact strength that is somewhere in between that of iPP and iPB. The increase in iPP concentration results in a decrease in impact strength. In contrast, all of the tested alloys maintain or even exceed the impact resistance of iPB. Unlike the iPP/iPB blends, the impact strength of the alloys increases somewhat with increasing PP concentration. Similar trends are found for the tensile strength of the alloys, irrespective of whether the measurements are performed at room temperatures or at high temperature (100 °C). For all of the tested samples, the A-20.8 preserves not only the best impact performance but also the highest tensile strength. Meanwhile, compared with the other pairs of blend and alloy that have the same content of propylene unit, the difference in performance is also the most obvious between A-20.8 and B-20.8. The priority in the physical properties of alloys is naturally attributed to the advantages of their structure and morphology, which are established by phase separation in the melt state and the melt−solid transition, crystallization.22−26 Our previous work has found that iPP/iPB blends show low critical solution temperature (LCST)-like behavior in the B
DOI: 10.1021/acs.iecr.9b00178 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research
Figure 2. (A,B) Phase contrast optical micrographs of blend-20.8 and alloy-20.8 that annealed at 200 °C for 1 h. (C,D) DSC curves of sample melting and recrystallization processes.
immiscibility of iPP and iPB. Compared with the blends, the poly(propylene-co-butene) in the alloys could act as a compatibilizer to enable the uniform dispersion of iPP domains in the iPB matrix, and thus a single iPB crystallization is reasonable. However, iPP and iPB are not miscible in the blends, and thus the iPP domains are nonuniformly distributed in the iPB matrix. As a result, the iPB near iPP domains and the iPB far from iPP domains show different crystallization behaviors. The iPB near iPP domains could crystallize at a higher temperature because of the heterogeneous nucleation. Figure 3 describes the nucleation and crystal growth of iPB following the complete crystallization of iPP in alloy, with A13.8 as the representative sample. It is found that the previously formed iPP crystals (white circles) are at work in iPB crystallization, in particular, the nucleation step, which could take place in the frame of iPP spherulites. Subsequently, the growth of iPB crystals (green circles) could also occur in iPP spherulites, probably because the copolymer, poly(propylene-co-butene), acting as a compatibilizer, increases the interface layer thickness as iPB with the relatively high concentration locates in the spherulites of iPP. Upon iPB crystallization, the iPP crystals induced the nucleation of iPB, and poly(propylene-co-butene) as a tie molecule contributes to the appearance of iPB crystals around iPP spherulites. As a result, the tie molecules of poly(propylene-co-butene) between iPP and iPB spherulites increase the interfacial bonding and mechanical properties. Figure 4 shows the crystal morphology of the studied alloys and blends. It is found that the alloys have a smaller spherulite size than the blends if both contain the same propylene unit content. Meanwhile, one observes that the higher the concentration of the propylene unit, the smaller the crystalline
crystallization temperatures are all listed in Table 2. Obviously, the melting peaks of both iPB and iPP are observed, and there Table 2. Crystallinities, Melting Temperatures, and Crystallization Temperatures of iPP and iPB in Alloys and Blends iPP
iPB
sample
Tm (°C)
Tc (°C)
Xc (%)
PP B-20.8 A-20.8 B-13.8 A-13.8 B-6.3 A-6.3 PB
165.6 164.4 165.4 164.2 162.3 163.3 155.0
118.2 100.0 108.3
54.1 42.7 44.4 44.7 31.1 42.2 38.5
Tm (°C)
Tc (°C)
Xc (%)
129.8 128.8 130.2 127.2 128.3 129.2 128.3
78.2, 88.0 82.7 74.8, 85.7 79.0 72.0, 85.6 78.4 69.8
67.1 64.5 68.2 66.9 66.2 66.8 64.5
is no obvious difference in melting temperature between the alloys and the blends, indicating their same crystal forms of iPB and iPP, respectively. When the temperature subsequently drops to the crystallization temperature of the sample, the iPP in alloys or blends crystallizes at a lower temperature than neat iPP due to the dilution of iPB components. Thereafter, iPP crystals could act as nucleating agents,7,27 resulting in iPB crystallization at a higher temperature than that of neat iPB samples. It is observed that the higher the concentration of PP, the higher the crystallization temperature of iPB. An obvious difference between the alloys and the blends is that the blends have two iPB crystallization temperatures, and the alloys only have one. This is considered to be the result of the heterogeneous distribution of iPP components due to the C
DOI: 10.1021/acs.iecr.9b00178 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research
Figure 3. Phase contrast optical micrographs of A-13.8 with iPB crystallization at 95 °C for (A) 10 and (B) 13 min after the isothermal crystallization of iPP at 130 °C. The white circles indicate iPP spherulites, and the green circles indicate iPB crystals.
Figure 4. Polarization optical micrographs of samples that were heated to 200 °C rapidly and kept for 5 min before being rapidly cooled to room temperature: (A) A-6.3, (B) A-13.8, (C) A-20.8, (D) B-6.3, (E) B-13.8, (F) B-20.8, and (G) iPB. The scale bar is 50 μm.
alloys. For all tested samples, the A-20.8, showing the highest overall performance, is closely related to the combined contributions, including the two components admixing, the refined spherulites, and the copolymers acting as the compatibilizer and tie molecules to enhance the interfacial bonding between iPP and iPB spherulites. For blends, the copolymer poly(propylene-co-butene) is absent. The immiscibility of iPP and iPB results in a weak and thin interface between iPP and iPB spherulites, which could cause the failure of the mechanical performance in comparison with the alloys. Moreover, the higher the concentration of iPP, the larger the size of dispersed phase domains and the weaker the interfacial bonding force that occurs. This is the reason that the biggest performance difference between A-20.8 and B-20.8
size in alloys and blends. This phenomenon is consistent with the statement that iPP could induce the nucleation of iPB. In addition, the alloys have a smaller iPP domain than the blends in the case of both containing the same propylene units; thereby, the easy nucleation of iPB in alloys contributes to the smaller spherulites.28−30 However, the refined spherulites in this case cannot directly account for the high performance of the alloys because of the fact that the iPB/iPP blends also show a smaller spherulite size than iPB, and the latter has a higher impact strength than the former. Resultantly, it is believed that the poly(propylene-co-butene) copolymer may act as a tie molecule to enhance the interfacial entanglements between the iPP and iPB spherulites, which, combined with the spherulite refinement, accounts for the outstanding properties of the D
DOI: 10.1021/acs.iecr.9b00178 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
The liquid−liquid phase separation and crystal morphology were observed on an Olympus (BX51) optical microscope in phase contrast mode and in a polar manner. A Linkam (THMS600) hot stage was used to control the experimental temperature, and fresh nitrogen gas was circulated in the hot stage to avoid possible aging of the samples. Differential Scanning Calorimetry Analysis. Thermograms were obtained on a differential scanning calorimeter (DSC-8500, PerkinElmer) with 5−10 mg of samples. Scans were taken from 20 to 200 °C with a heating rate of 10 °C/ min. For the subsequent crystallization process, samples were further kept at 200 °C for 5 min before being cooled to 20 °C with a cooling rate of 10 °C/min. All of the measurements were performed under a nitrogen atmosphere with a flow rate of 50 mL/min. Mechanical Properties. Tensile testing was performed on a GT-TCS-2000 Gotech tensile apparatus at a strain rate of 50 mm/min according to GB/T 1040-2006. Impact testing was performed on a GT-7045-MDH Gotech impact testing machine according to GB/T 1843-2008. Because the crystal transition from form II to form I for iPB samples would take around 7 days, the mechanical properties of all samples were tested after 10 days.
was observed in this case. Upon crystallization of alloys, the relatively long PP segments of poly(propylene-co-butene) may entangle with iPP chains in the amorphous state. Not surprisingly, the solidified PP segments of the copolymers enable iPB segment orientation at the interface, which favors the nucleation of iPB. Moreover, the amphiphilicity of the copolymers endows them with the interfacial agent function by acting as tethers to increase the interfacial entanglements between iPP and iPB spherulites, resulting in mechanical toughness.
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CONCLUSIONS We recommend novel polyolefin mixtures, in-reactor iPB/iPP alloys with in-situ-synthesized poly(propylene-co-butene), and evaluate their performance by comparison with iPB/iPP blends. The alloys benefit from the existence of poly(propylene-co-butene), characterized by higher mechanical performance than the blends. Because of the compatibilization of poly(propylene-co-butene), the alloys show not only smaller phase domains in the molten state but also smaller spherulites in the solid state. In particular, the poly(propylene-co-butene) acts as a tie molecule to increase the interfacial entanglements between the iPP and iPB spherulites, which mainly contributes to the high impact resistance and tensile strength of the alloys.
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ASSOCIATED CONTENT
* Supporting Information S
EXPERIMENTAL SECTION Materials. iPB (Mw = 679 K, Mw/Mn = 5.3, isotactic index = 96.0%) was purchased from DongFangHongYe Chemical, China. iPP (Mw = 718 K, Mw/Mn = 8.5, isotactic index = 96.9%) was purchased from Maoming Petro, China. Three kinds of iPB/iPP alloys, named A-6.3, A-13.8, and A-20.8, were synthesized by sequential polymerizations.20 In the first polymerization stage, the agents and reactants including nheptane, AlEt3, an external electron donor, and propylene were successively added into a stainless reactor at constant pressure. With the introduction of solid catalyst powder, the slurry polymerization of propylene was started and continued at 60 °C for the given time. Before the addition of the distilled liquid butane-1 for the second polymerization, the n-heptane and the unreacted propylene monomers were removed by a vacuum pump. With the quenching of the polymerization by ethanol containing 1% HCl solution, the powdery alloys were obtained by washing with plenty of ethanol, filtering, and drying in a vacuum oven at 70 °C. Note here that the reaction time of the iPP synthesis was controlled to obtain the alloys that consist of different propylene unit contents. Their microstructures are shown in Table 1. All samples that we used for the mechanical test, optical microscopy observation, and DSC analysis were prepared by twin-screw extrusion. Structure Characterization. The molecular weights of the alloys, iPP and iPB, were measured by an Agilent (PL-220) high-temperature gel permeation chromatograph with trichlorobenzene as the solvent. The test temperature was 150 °C, and the sample concentration was 0.10 mg/mL. A boiling extraction was carried out with ether for 24 h to achieve the isotacticity of samples. 13CNMR spectra were acquired on a Bruker VERTEX70 spectrometer to know the concentration of PP and isotacticity of samples. Also, to determine the concentration of the copolymers, the alloys were fractionalized with temperature rising elution fractionation;31 then, 13C NMR 32−35 and DSC were used to characterize the fractionations that were collected at different temperatures.
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.9b00178. 13
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C NMR of A-13.8 and the copolymer fractions collected at the temperature ranging from 110 to 140 °C after temperature rising elution fractionation and the analysis of the composition and chemical structure of copolymers (PDF)
AUTHOR INFORMATION
Corresponding Authors
*C.L.: E-mail:
[email protected]. *A.H.: E-mail:
[email protected];
[email protected]. Tel.: +86-0532-84022951. Fax: +86-0532-84022951. ORCID
Huarong Nie: 0000-0003-0007-5865 Aihua He: 0000-0002-7535-8379 Author Contributions
The manuscript was written through contributions of all authors. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the Shandong Provincial Key R&D Program, the National Natural Science Foundation of China (no. 21174074), and the Taishan Scholar Program.
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DOI: 10.1021/acs.iecr.9b00178 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.iecr.9b00178 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
