An Efficient β-Nucleating Agent for Isotactic ... - ACS Publications

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Chain Folding in Main-Chain Liquid Crystalline Polyester with Strong π−π Interaction: An Efficient β‑Nucleating Agent for Isotactic Polypropylene Rong Yang,*,† Lv Ding,† Weilong Chen,† Li Chen,‡ Xin Zhang,† and Jinchun Li*,† †

Jiangsu Key Laboratory of Environmentally Friendly Polymeric Materials, School of Materials Science and Engineering, Jiangsu Collaborative Innovation Center of Photovolatic Science and Engineering, Changzhou University, Changzhou 213164, China ‡ Center for Degradable and Flame-Retardant Polymeric Materials, College of Chemistry, National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan), Sichuan University, Chengdu 610064, China S Supporting Information *

ABSTRACT: The influence of the main-chain liquid crystalline polyester (PBDPS) on the crystallization and melting behaviors of isotactic polypropylene (iPP) has been investigated by differential scanning calorimetry (DSC), X-ray diffraction (WAXD), and polarized optical microscopy (POM). The results suggested that PBDPS is an efficient βnucleating agent for iPP. The melting crystallization temperature as well as β-crystal of iPP increased with increasing of PBDPS content. The relative content of β-iPP crystals (kβ) can reach 96.6% while adding 4% PBDPS during isothermal crystallization at 135 °C. The increased β-nucleation efficiency was ascribed to the adjacent phenyl rings in side groups of PBDPS stacked well together even when the temperature was higher than its clearing point (Ti). Moreover, the distance between stacked phenyls is 0.32 nm, which is half of the unit cell parameter in the c-direction of β-iPP, indicating a lattice match between β-iPP and PBDPS.

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low toxicity, and low cost. Yan et al. reported β-iPP can be induced by their homogeneity fibers with high orientation due to surface-induced transcrystallization by the identical chemical composition and perfect lattice matching.14−16 However, technically the iPP fiber cannot be counted as a nucleating agent. Recently, it was reported that polystyrene (PS),17 styrene−acrylonitrile (SAN),17 and isotactic polystyrene (iPS)18 can induce β-crystal during quiescent melt crystallization. Although there is no general agreement about the nucleation mechanism of these polymers, however, it is generally believed that the benzene rings stacking of the macromolecular nucleating agents would be a key factor.17,18 Thermotropic liquid crystalline polymers (TLCPs) have been the focus of attention for years because of their high strength and modulus, good thermal stability, excellent chemical resistance, etc.19 Moreover, in the liquid crystalline state, LCPs still retain their orientational order which means a large amount of ordered phenyls structure exist. In that case, LCPs could be considered as the most competitive candidates for macromolecular β-nucleating agents. In fact, Torre et al.20 found that liquid crystalline polymer (Vectra A950) can induce β-iPP during isothermal crystallization in 2004. After that, a series of side-chain LCPs21−23 were synthesized and used as β-

INTRODUCTION The crystal forms and crystallinity are very important for semicrystalline polymers, which have a significant effect on mechanical properties of polymers such as tensile stress, impact strength, elongation at break, heat deformation temperature, and so on. For isotactic polypropylene (iPP), it has at least four polymorphisms: α-form (monoclinic),1 β-form (trigonal),2 γform (orthorhombic),3 and smectic form (mesomorphic).4 It is well-known that β-iPP has a better impact strength, elongation at break, and heat deformation temperature than the other forms.5−7 However, iPP can only form α-phase crystal under quiescent melt crystallization. Therefore, enhance the relative β-form crystal of iPP to become hotspots. Generally, there are three methods to obtain β-PP such as crystallization in a temperature gradient,8 shear-induced crystallization,9 and addition of β nucleating agents.10 The most efficient and accessible method is adding highly effective β nucleating agents. So far, the low molecular weight organic β nucleating agents including polycyclic aromatic molecules,11 a certain group of IIA metal salts and mixtures with dicarboxylic acids12 (calcium stearate/pimelic acid), and aromatic amides13 (N,N′-dicyclohexyl-2,6-naphthalenedicarboxamide) have been wildly used due to their high efficiency. However, there are a few drawbacks of low molecular weight organic β nucleating agents, such as color, low thermal stability, and toxicity. Recently, an attempt was carried out to utilize the macromolecular nucleating agents due to their good thermal stability, compatibility, dispersibility, © XXXX American Chemical Society

Received: November 21, 2016 Revised: January 27, 2017

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DOI: 10.1021/acs.macromol.6b02521 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

Variable temperature WAXD was carried out with a D8 ADVANCE (BRUKER, Germany) diffractometer using Cu Kα radiation at 10 °C min−1 heating rate from 5° to 45.0° at a speed of 2° min−1. The spherulite morphology of pure iPP and iPP/PBDPS blends was observed via a Nikon 50I polarizing optical microscope (POM) equipped with a hot stage. The samples were first melted at 190 °C for 5 min and then quenched to 135 °C. Dynamic oscillatory rheological properties of liquid crystalline polyesters were performed by using a parallel-plate rheometer (Physica MCR301, Anton Paar) with oscillatory shear mode.

nucleating agents for iPP. Compared to other polymeric agents, LCP showed much higher efficiency, and the value of kβ can reach 70%. However, it is not yet comparable with low molecular weight organic agents. In this paper, we report an efficient TLCP β-nucleating agent (PBDPS) with benzene on both the main chain and side group. The influence of PBDPS on the crystallization and melting behaviors of isotactic polypropylene (iPP) and the probable nucleation mechanism have been investigated in detail.

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EXPERIMENTAL SECTION

RESULTS AND DISCUSSION Figure 1 shows the first DSC cooling scans (a) and the subsequent heating scans (b) of iPP/PBDPS blends, and the detailed data are summarized in Table 1. It is observed that iPP shows one exothermic peak at 117.5 °C with the crystallization enthalpy of 89.0 J g−1 and one endothermic peak at 163.5 °C with the melting enthalpy of 84.7 J g−1. For iPP/PBDPS blends, the temperature of crystallization peak of iPP increases from 117.5 to 123.6 °C, with increasing the content of PBDPS on the first cooling scan. It indicated that PBDPS can accelerate the rate of crystallization of iPP. However, the crystallization enthalpy of iPP/PBDPS blends decreases with the introduction of PBDPS. For one thing, the percentage of iPP decreased with increasing the content of PBDPS; therefore, the crystallization enthalpy of iPP decreased. For another, the enthalpy of β-iPP (ΔHβ = 168.5 J g−1) is lower than of α-iPP (ΔHα = 177 J g−1).25 As a β-nucleating agent, PBDPS can induce β-form crystal of iPP which lead to less crystallization enthalpy of iPP. On the second heating scans, iPP/PBDPS blends show two or three melting peaks. The first one appears at around 150 °C, and the melting enthalpy increases with an increase in content of PBDPS. According to references, this melting peak belongs to the melting of β-formed crystal of iPP, which indicated that PBDPS can induce β-formed crystal for iPP. The second melting peak (α-crystal) appears at around 163 °C, which is consistent with pure iPP. However, it shows another melting peak (α′-crystal) at around 169 °C with incorporating PBDPS. In addition, the intensity of α′-crystal melting peak increases with increasing the content of PBDPS. Finally, there was only α′-crystal melting peak left while the addition of PBDPS is higher than 3%. It can be observed that the changed trend of the intensity of α′-crystal melting peak is in accordance with βcrystal melting peak. It indicated that α′-crystal melting peak

Materials. Isotactic polypropylene used in this work was purchased from Sinopec Zhenhai Refining & Chemical Company (PPH-T03) with a melt flow rate 2.5 g 10 min−1. PBDPS was synthesized by melting transesterification from biphenyl-4,4′-diylbis(oxy)dihexanol and 2-phenylsuccinic acid, with Mn = 49 000 and PDI = 1.99 as determined at 30 °C in THF solvent. The chemical structure of PBDPS is shown in Scheme 1, and the synthesis and properties were fully described in ref 24.

Scheme 1. Chemical Structure of PBDPS

Preparation of iPP/PBDPS Blends. iPP/PBDPS blends were prepared by using an internal mixer (Changzhou Suyan Science and Technology Co., Ltd., China, SU-70C) at 190 °C with a rotation speed of 20 rpm for 5 min. Characterization. Thermal transition temperatures of pure iPP, PBDPS, and iPP/PBDPS blends were measured by using a differential scanning calorimeter (DSC, TA Q20) at a heating rate of 10 °C min−1 from 0 to 190 °C in a continuous nitrogen flow. Isothermal Crystallization. Samples were heated to 190 °C for 3 min and then quenched to the desired crystallization temperature and held at a given temperature until the crystallization was complete. After that, samples were heat to 190 °C at 10 °C min−1. Wide-angle X-ray diffraction (WAXD) was carried out with a D/ MAX2500 (Rigaku) diffractometer using Cu Kα radiation at room temperature scanned from 5° to 45.0° at a speed of 2° min−1. The isothermal melt-crystallization samples were first held at 200 °C for 3 min and then transferred to an oven at 135 °C for 2 h.

Figure 1. DSC curves of pure iPP and iPP/PBPDP blends on the first cooling (a) and the second heating scan (b) in N2. B

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Macromolecules Table 1. Crystallization and Melting Behaviors of Pure IPP and iPP/PBDPS Blends Obtained from DSC first cooling scan PBDPS content (%)

Tc (°C)

ΔHc (J g−1)

0 0.1 0.3 0.5 1 2 3 4 5

117.5 117.6 118.6 119.2 119.6 120.9 122.9 123.4 123.6

89.0 88.7 80.6 78.5 77.5 76.5 75.6 75.1 72.3

second heating scan Tm‑β (°C) 149.3 149.5 149.7 150.2 151.3 151.7 151.9 151.7

ΔHm‑β (J g−1)

Tm‑α (°C)

ΔHm‑α (J g−1)

ΔHm (J g−1)

14.7 23.2 30.3 31.9 51.6 68.7 71.6 71.8

163.5 163.7 163.4 163.4 163.6 164.2 169.2 169.2 169.1

84.7 70.7 53.6 45.1 42.7 23.5 15.7 15.3 15.4

84.7 85.4 76.8 75.4 74.6 75.1 84.4 86.9 87.2

Xβ (%)

Xα (%)

XAll (%)

Φβ (%)

8.7 13.7 17.9 18.8 30.4 40.5 42.2 42.3

47.6 39.7 30.1 25.3 24.0 13.2 8.8 8.6 8.7

47.6 48.4 43.7 43.2 42.8 43.6 49.2 50.7 50.9

0 17.9 31.2 41.4 43.9 69.7 82.1 83.1 83.1

originated from the melting and recrystallization of partial βcrystal. The crystallinity of the α-form and β-form and the percentage of β-crystal form (Φβ) are shown in Figure 2

Figure 3. DSC melting curves of iPP and iPP/PBDPS blends after isothermal crystallization at 135 °C.

Figure 2. Percentage of β-crystal form (Φβ), the crystallinity of αcrystal (Xα), and β-crystal (Xβ), evaluated from the DSC curves.

crystallization at 135 °C for 2 h are shown in Figure 4. The relative content of β-form crystal of iPP was calculated according to the references (see Supporting Information for

which was calculated according to the references (see Supporting Information for detailed equations).26,27 It could be observed that PBDPS can induce amount of β-crystals for iPP. According to calculation, with increase of PBDPS content, the degree of α crystallinity of iPP (Xα) decreases from 47.6 to 8.6% and the degree of β crystallinity of iPP (Xβ) increases from 0 to 42.3%. The value of Φβ increases monotonously first and then keeps nearly the same with increasing the content of PBDPS. The iPP/PBDPS blend reaches the highest Φβ value of 83.1% with addition of 4% PBDPS. Figure 3 shows the melting DSC curves of iPP/PBDPS blends after isothermal crystallization at 135 °C. For pure iPP, there is only one melting peak appears at 164 °C, which is the melting of α crystal of iPP. However, it shows another melting peak at lower temperature around at 156 °C for iPP/PBDPS blends, which belongs to the melting of β crystal of iPP. Meanwhile, the melting peak of β crystal increases, and the melting peak of α crystal decreases with increasing the PBDPS content. When the addition of PBDPS is higher than 3%, there is no obvious melting peak for the α crystal. It indicated that PBDPS can induce only β crystal for iPP under quiescent melt crystallization. The isothermal crystallization of iPP/PBDPS blends was further determined on the WAXD instrument. The WAXD patterns of pure iPP and iPP/PBDPS blends after isothermal

Figure 4. WAXD patterns of iPP and iPP/PBDPS blends after isothermal crystallization at 135 °C for 2 h. C

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Macromolecules detailed equations).14,28 For pure iPP, there are five peaks at 2θ of approximate values 14°, 16.8°, 18.5°, 21.1°, and 21.8°, which correspond to the (110), (040), (130), (111), and (−131) plane of α-crystal reflections, respectively.29 It indicated that pure iPP can form only α-crystal in this condition. With incorporating PBDPS into iPP, it shows a new reflection at 15.9°, corresponding to the (300) plane of β-crystal. Besides that the peak at 21.1° is another reflection (311) of β-crystal, however, it overlapped with the (111) plane of α-crystal. Moreover, the intensity of characterization peak of β-crystal increases with increasing the content of PBDPS. Meanwhile, the intensity of characterization peak of α-crystal decreases with increasing the content of PBDPS. The characterization peaks of α-crystal are almost disappeared when the addition of PBDPS is higher than 2 wt %. Figure 5 shows the relative content of β-crystal which was calculated from the melting DSC curves (Φβ) and X-ray

Figure 6. Polarized optical microscope photographs of pure iPP and iPP/PBDPS blends isothermal crystallization at 135 °C (magnification 200×).

size decreases with the incorporation of PBDPS. When the loading of PBDPS is higher than 2%, it is hard to find a complete spherulite, which means that PBDPS can accelerate the effect of nucleation and crystallization of iPP. Figure S2 shows the photographs of pure iPP and iPP/PBDPS blends melted at 170 °C after crystallization at 135 °C. Because of the strong interfacial interaction between iPP and PBDPS, the size and dispersion of PBDPS can be observed with POM. It can be seen that PBDPS disperses very well in iPP melt with an average size of 1−2 μm while the addition of PBDPS is higher than 1 wt %. Meanwhile, there is no observable aggregation of PBDPS in iPP melt with increase of PBDPS content. Figure 7 shows the DSC melting curves of iPP/PBDPS4 after isothermal at indicated temperature. It is obvious that iPP/ Figure 5. Relative content of β-crystal evaluated from the DSC (Figure 3) and WAXD (Figure 4) measurements.

diffraction patterns (kβ) as shown in Figures 3 and 4. The results show that the values of Φ β and k β increase monotonously first with the increasing the PBDPS percentage and reach a maximum value when incorporating 4% PBDPS. Then, it decreases slightly with further increase of PBDPS content. The maximum kβ values of DSC and XRD tests are 99.2% and 96.6%, respectively. Polarized optical microscope (POM) is a useful tool to observe crystalline morphologies, nucleating process, and spherulitic growth. Moreover, POM can identify α-crystal and β-crystal of iPP because of different optical properties between α- and β-crystal.30 Figure 6 shows the crystalline morphologies of pure iPP and iPP/PBDPS blends which were observed by the polarized optical microscope at 135 °C, respectively. As shown in Figure 6, the spherulites are formed with clear Maltese cross for both pure iPP and iPP/PBDPS blends after isothermal crystallization. It is clear that the spherulites of iPP show much stronger birefringent while incorporating PBDPS. As is well-known, β-crystal form spherulites have a much higher birefringent than α spherulites; therefore, β spherulites are brighter than α ones. In that case, it is evident that PBDPS can induce the formation of β spherulites for iPP and the β spherulites increases with increasing the content of PBDPS. Meanwhile, the nucleation density increases and the spherulite

Figure 7. Melting curves of iPP/PBDPS4 isotherm crystallization at indicated temperatures.

PBDPS4 shows two melting peaks at 151.0 and 163.9 °C, which is in accordance with the melting of β-iPP and α-iPP, respectively. The temperatures of melting peaks increase with increasing temperature of isothermal crystallization due to the more perfection crystal can obtain at higher temperature. The D

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Macromolecules melting of β-iPP increase whiles the decline of α-iPP with increase of isothermal temperature. When the isothermal crystallization temperature is higher than 130 °C, the melting peak of α-iPP is disappeared, suggesting that PBDPS can induce only β-iPP at the higher temperature of isothermal crystallization. It indicated that PBDPS are more efficient at higher crystallization temperature. Compared with α-iPP, β-iPP is metastable on thermodynamics; therefore, a higher temperature is beneficial for formation of β-iPP. Generally, polymer melting consists of melt, recrystallization, and polymorphic transformation processes. In that case, it is hard to evaluate melting process of iPP/PBDPS by using DSC measurement alone. Therefore, in situ XRD was utilized to investigate the melting behavior of iPP/PBDPS blends. Figure 8 shows the XRD patterns of iPP/PBDPS4 at indicated

Figure 9. Crystallinity (α-form, β-form, and all) of iPP/PBDPS4 obtained from Figure 8.

increased and achieved a maximum value (33.1%) at 145 °C because of the polymorphic transformation from β-iPP to αiPP. Finally, the Xall, Xα, and Xβ decreased to 0 with further heating to 175 °C. The in situ WAXD results demonstrated that a polymorphic transformation from β- to α-phase happened at 130−145 °C during the heating process, and the transformed α-form crystal showed a higher melting temperature which is in accordance with melting behavior of nonisothermal crystallization. Combined DSC, XRD, and POM results, it can be draw a conclusion that PBDPS is an efficient macromolecular β nucleating agent for iPP. As shown in Figure 10, compared to

Figure 8. WAXD patterns of iPP/PBDPS4 at indicated temperatures with a 10 °C min−1 heating rate.

temperature with a 10 °C min−1 heating rate. It can be seen that both characterization peaks of α-iPP and β-iPP appeared at room temperature (25 °C), and the kβ is about 69%. The intensity of those characterization peaks keeps nearly the same while the temperature heated to 130 °C. However, the intensity of characterization peak (300) of β-iPP decreases while the temperature is higher than 130 °C due to the melting of β-iPP. Meanwhile, the intensity of characterization peaks (110, 040, 130) of α-iPP increased, which is an evidence of polymorphic transformation from β-iPP to α-iPP. With increasing temperature, the characterization peak (300 β) disappeared at 165 °C which means β-iPP melted completely, and the transformed αiPP shows a higher melting temperature at 175 °C. Figure 9 shows the total crystallinity (Xall) of iPP/PBDPS4, and crystallinity of α-form (Xα) and β-form (Xβ) crystals at different temperatures, which were calculated based on WAXD results of Figure 8. As it can be seen that the Xall, Xα, and Xβ of iPP kept nearly the same while the temperature of 25−130 °C. With increasing temperature, β-iPP melted and the Xβ decreased dramatically. However, the Xα of iPP/PBDPS4

Figure 10. Summary of reported kβ values of macromolecular βnucleating agents.17,18,20−23,31 PBDPS shows the highest kβ value.

other macromolecular β nucleating agents, PBDPS shows the best efficiency, and the value of kβ can achieve 96.6%, which is comparable with the small molecular organic nucleating agent for iPP. PBDPS shows a SmA phase, and its isotropic temperature (Ti) is 73.5 °C, which was confirmed by DSC (Figure S3), POM (Figure S4), and SAXS (Figure S5) tests. Therefore, a question should be put forward: how the isotropic state of PBDPS acts as a nucleating agent and why it can induce β-form crystal for iPP? First, variable temperatures WAXD of PBDPS E

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Macromolecules were carried out to investigate the structure of PBDPS during isotropic state. Figure 11a shows the WAXD patterns of

Figure 11. (a) WAXD patterns of PBDPS at the indicated temperature, (b) chain folding packing model of PBDPS, and (c) parallel displaced configuration stacking model of the phenyl side group of PBDPS.

Figure 12. Complex viscosity (η), storage modulus (G′), and loss modulus (G″) plotted against oscillation frequency at 100 °C for PBDSP and 170 °C for PBDS.

modulus (G′), and loss modulus (G″) with different oscillation frequency at 100 °C for PBDSP and 170 °C for PBDS. The tested temperatures were almost 30 °C higher than their isotropic temperature. It can be seen that PBDS shows a Newonian plateau in the low-frequency region and a shear thinning behavior in the high-frequency region. However, PBDPS exhibits an obvious shear thinning behavior in the whole frequency region which can be attributed to the long relaxation time because of the presence of strong π−π interactions. Meanwhile, PBDPS shows a much higher complex viscosity than PBDS. It should be ascribed to the zipper-like stacking benzene rings which can act as the physical crosslinking and restrict the chain mobility. Consequently, the storage modulus (G′) and loss modulus (G″) of PBDPS were much higher than PBDS. For PBDPS, when the frequency is higher than 0.63 Hz, G′ was higher than G″, indicating that a reversible deformation was in the dominant position.32 The rheological results suggested that strong π−π interactions exist in molten PBDPS which result from the π−π stacking of the side phenyl group of PBDPS. In this case, the ordered phenyl groups of PBDPS act as nucleation for iPP. As shown in Figure 13, the lattice constants of β-iPP are a = b = 1.101 nm and c = 0.65 nm,33 and the dspacing of the 27.6° value is 0.32 nm which is exactly half of the unit cell parameter in the c-direction of β-iPP. The ordered

PBDPS at different temperature. It can be observed that there are three peaks appeared at 20.0°, 27.6°, and 32.0°. When the temperature is higher than Ti, the peak of 20° becomes broad which means PBDPS become amorphous; however, the intensity and shift of peaks at 27.6° (d = 0.32 nm) and 32.0° (d = 0.28 nm) remain the same even at the temperature of 160 °C. As shown in Figure 11b, the chain of PBDPS is folding packing because of the introduction of phenyl pendant group.24 Therefore, the benzene rings in side groups from different domains stacked well together which assembled like a zipper (Figure 11c) and formed strong π−π interactions. The zipperlike π−π stacking structure was stable enough even when the temperature was higher than Ti which can protect the structure of π−π stacking order. To further prove that π−π stacking was induced by the side phenyl group of PBDPS, a controlled LCP was also synthesized by melting transesterification from biphenyl-4,4′-diylbis(oxy)dihexanol and succinic acid, named as PBDS (without side phenyl group). First, it blended with iPP; however, it cannot act as nucleating agent and induce β-iPP as expected (Figure S6). The rheology of polymer melt is a crucially way to investigate the molecular structures such as branching, entanglements, and cross-linking. Therefore, the effect of zipper-like stacking of side phenyl groups on the rheological properties of PBDPS was investigated. Figure 12 shows the complex viscosity (η), storage F

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Figure 13. Schematic of β-iPP induced by macromolecular β-nucleating agent PBDPS.

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phenyl structure can act as growth surface, thereby inducing the helix chains of iPP epitaxial to crystallize on it.34 Second, mesomorphic iPP is formed through a nucleation process initiated by chains self-folding.35 For iPP/PBDPS blends, it exists as noncovalent interaction involving the CH/π interaction between the hydrogen atoms of polypropylene (H donor) and the π-face of PBDPS (H acceptor).36 Therefore, the folding chains of PBDPS as epitaxial nucleation adsorbed the flexible iPP chains which can significantly accelerate the nucleation process of iPP. Moreover, the lattice matching between the c-axis periodicity of iPP and a corresponding distance of adjacent phenyl rings of PBDPS induced the β-iPP polymorph.37

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b02521. Calculation of the relative of β-iPP crystal (Φβ/kβ), POM photographs of iPP/PBDPS blends, thermal transition property, polarized optical microscope photographs, small-angle X-ray scattering (SAXS) of PBDPS, and DSC curves of PBDS and iPP/PBDS2 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (R.Y.). *E-mail: [email protected] (J.L.).

CONCLUSIONS

In summary, we developed a novel efficient liquid crystalline polyester (PBDPS) β-nucleating agent for iPP. The crystallization rate of iPP was significantly accelerated with incorporating PBDPS. When adding 4% PBDPS, the value of kβ can reach 83.1% under nonisothermal crystallization and 96.6% under isothermal crystallization, which is the best efficiency macromolecular nucleating agent as far as we know. Meanwhile, the kβ of iPP/PBDPS4 increased with increase of isothermal crystallization temperature, and it can form all β-iPP while the isothermal crystallization is higher than 130 °C. The in situ WAXD results provided obvious evidence that iPP/ PBDPS blends show a polymorphic transformation from βcrystal to α-form crystal at 130−145 °C during the heating process. The transformed α-form crystal by melting and recrystallization of β-form crystal are stable and show a high melting temperature. Unlike other LCP nucleating agents, PBDPS showed not a liquid crystal state but an isotropic state at the temperature of crystallization of iPP. The side phenyl groups of PBDPS stack like a zipper and form strong π−π interactions which can keep the phenyl stacking well even the temperature is higher than isotropic temperature. As a consequence, PBDPS melt shows isotropic state in macro; however, it keeps ordered benzene rings structure in micro. In that case, these zipper-like phenyl groups can act as nucleating agent and induce crystallization of iPP. The excellent nucleating effect was mainly ascribed to the lattice matching between the unit cell parameter in the c-direction of β-iPP and a corresponding distance of stacked phenyl rings of PBDPS.

ORCID

Rong Yang: 0000-0002-0193-6949 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of Jiangsu Province (BK20150257).

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REFERENCES

(1) Padden, F. J., Jr.; Keith, H. D. Spherulitic crystallization in polypropylene. J. Appl. Phys. 1959, 30 (10), 1479−1484. (2) Turner-Jones, A.; Cobbold, A. J. The β crystalline form of isotactic polypropylene. J. Polym. Sci., Part B: Polym. Lett. 1968, 6 (8), 539−546. (3) Kardos, J. L.; Christiansen, A. W.; Baer, E. Structure of pressurecrystallized polypropylene. J. Polym. Sci. Part A-2: Polym. Phys. 1966, 4 (5), 777−788. (4) Piccarolo, S.; Saiu, M.; Brucato, V.; Titomanlio, G. Crystallization of polymer melts under fast cooling. II. High-purity iPP. J. Appl. Polym. Sci. 1992, 46 (4), 625−634. (5) Fujiyama, M. Structures and Properties of Injection Moldings of β-Crystal Nucleator-Added Polypropylenes: Part 1 Effect of β-Crystal Nucleator Content. Int. Polym. Process. 1995, 10 (2), 172−178. (6) Varga, J. Supermolecular structure of isotactic polypropylene. J. Macromol. Sci., Part B: Phys. 2002, 41 (10), 1121−1171. (7) Luo, F.; Geng, C. Z.; Wang, K.; Deng, H.; Chen, F.; Fu, Q.; Na, B. New understanding in tuning toughness of β-polypropylene: the role of β-nucleated crystalline morphology. Macromolecules 2009, 42 (23), 9325−9331. G

DOI: 10.1021/acs.macromol.6b02521 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules (8) Lovinger, A. J.; Chua, J. O.; Gryte, C. C. Studies on the α and β forms of isotactic polypropylene by crystallization in a temperature gradient. J. Polym. Sci., Polym. Phys. Ed. 1977, 15 (4), 641−656. (9) Dragaun, H.; Hubeny, H.; Muschik, H. Shear-induced β-form crystallization in isotactic polypropylene. J. Polym. Sci., Polym. Phys. Ed. 1977, 15 (10), 1779−1789. (10) Stocker, W.; Schumacher, M.; Graff, S.; Thierry, A.; Wittmann, J. C.; Lotz, B. Epitaxial crystallization and AFM investigation of a frustrated polymer structure: isotactic poly (propylene), β phase. Macromolecules 1998, 31 (3), 807−814. (11) Leugering, V. H. J. Influence of Crystal Structure and Overstructure on Some Properties of Polypropylene. Makromol. Chem. 1967, 109 (1), 204−216. (12) Varga, J.; Mudra, I.; Ehrenstein, G. W. Crystallization and melting of β-nucleated isotactic polypropylene. J. Therm. Anal. Calorim. 1999, 56 (3), 1047−1057. (13) Varga, J.; Menyhárd, A. Effect of solubility and nucleating duality of N, N′-dicyclohexyl-2, 6-naphthalenedicarboxamide on the supermolecular structure of isotactic polypropylene. Macromolecules 2007, 40 (7), 2422−2431. (14) Li, H.; Jiang, S.; Wang, J.; Wang, D.; Yan, S. Optical microscopic study on the morphologies of isotactic polypropylene induced by its homogeneity fibers. Macromolecules 2003, 36 (8), 2802−2807. (15) Sun, X.; Li, H.; Zhang, X.; Wang, J.; Wang, D.; Yan, S. Effect of fiber molecular weight on the interfacial morphology of iPP fiber/ matrix single polymer composites. Macromolecules 2006, 39 (3), 1087−1092. (16) Li, H.; Yan, S. Surface-induced polymer crystallization and the resultant structures and morphologies. Macromolecules 2011, 44 (3), 417−428. (17) Su, Z. Q.; Dong, M.; Guo, Z.; Yu, J. Study of polystyrene and acrylonitrile-styrene copolymer as special β-nucleating agents to induce the crystallization of isotactic polypropylene. Macromolecules 2007, 40 (12), 4217−4224. (18) Phillips, A.; Zhu, P. W.; Edward, G. Polystyrene as a versatile nucleating agent for polypropylene. Polymer 2010, 51 (7), 1599−1607. (19) Shiota, A.; Ober, C. K. Rigid rod and liquid crystalline thermosets. Prog. Polym. Sci. 1997, 22 (5), 975−1000. (20) Torre, J.; Cortazar, M.; Gomez, M.; Ellis, G.; Marco, C. Melting behavior in blends of isotactic polypropylene and a liquid crystalline polymer. J. Polym. Sci., Part B: Polym. Phys. 2004, 42 (10), 1949−1959. (21) Hu, J. S.; Kong, B.; Chao, C. Y.; Sun, J. Study on liquidcrystalline copolymers as a new β-nucleator to induce crystallization structure of isotactic polypropylene. Chem. J. Chin. U. 2009, 30, 1253− 1255. (22) Hu, J. S.; Kong, B.; Sun, J.; Lu, H. L.; Li, H. Effect of nematic liquid crystalline copolymer as a β-form nucleator on crystallization structure and thermal properties of isotactic polypropylene. Gaofenzi Xuebao 2010, 10 (9), 1100−1107. (23) Sun, J.; Li, Q.; Yao, X. J.; Hu, J. S.; Qi, Y. A nematic liquid crystalline polymer as highly active novel β-nucleating agent for isotactic polypropylene. J. Mater. Sci. 2013, 48 (11), 4032−4040. (24) Yang, R.; Chen, L.; Ruan, C.; Zhong, H. Y.; Wang, Y. Z. Chain folding in main-chain liquid crystalline polyesters: from π−π stacking toward shape memory. J. Mater. Chem. C 2014, 2 (30), 6155−6164. (25) Li, J. X.; Cheung, W. L.; Jia, D. A study on the heat of fusion of β-polypropylene. Polymer 1999, 40 (5), 1219−1222. (26) Li, J. X.; Cheung, W. L. On the deformation mechanisms of βpolypropylene: 1. Effect of necking on β-phase PP crystals. Polymer 1998, 39 (26), 6935−6940. (27) Liu, M.; Guo, B.; Du, M.; Chen, F.; Jia, D. Halloysite nanotubes as a novel β-nucleating agent for isotactic polypropylene. Polymer 2009, 50 (13), 3022−3030. (28) Feng, M.; Gong, F. L.; Zhao, C. G.; Chen, G. M.; Zhang, S. M.; Yang, M. S.; Han, C. C. The β-crystalline form of isotactic polypropylene in blends of isotactic polypropylene and polyamide-6/ clay nanocomposites. J. Polym. Sci., Part B: Polym. Phys. 2004, 42 (18), 3428−3438.

(29) Somani, R. H.; Hsiao, B. S.; Nogales, A.; Fruitwala, H.; Srinivas, S.; Tsou, A. H. Structure development during shear flow induced crystallization of i-PP: in situ wide-angle X-ray diffraction study. Macromolecules 2001, 34 (17), 5902−5909. (30) Lotz, B. α and β phases of isotactic polypropylene: a case of growth kinetics p̀ hase reentrency’ in polymer crystallization. Polymer 1998, 39 (19), 4561−4567. (31) Cui, L.; Zhang, Y.; Zhang, Y. Crystallization, melting behavior, and morphology of PP/Novolac blends. J. Polym. Sci., Part B: Polym. Phys. 2006, 44 (22), 3288−3303. (32) Ruan, C.; Chen, L.; Yang, R.; Zhong, H. Y.; Wang, Y. Z. Mainchain liquid crystalline ionomers with a nonplanar ionic segment. RSC Adv. 2015, 5 (60), 48541−48550. (33) Meille, S. V.; Ferro, D. R.; Brückner, S.; Lovinger, A. J.; Padden, F. J. Structure of. beta.-isotactic polypropylene: a long-standing structural puzzle. Macromolecules 1994, 27 (9), 2615−2622. (34) Mathieu, C.; Thierry, A.; Wittmann, J. C.; Lotz, B. Specificity and versatility of nucleating agents toward isotactic polypropylene crystal phases. J. Polym. Sci., Part B: Polym. Phys. 2002, 40 (22), 2504− 2515. (35) Yuan, S.; Li, Z.; Hong, Y.; Ke, Y.; Kang, J.; Kamimura, A.; Otsubo, A.; Miyoshi, T. Folding of Polymer Chains in the Early Stage of Crystallization. ACS Macro Lett. 2015, 4 (12), 1382−1385. (36) Yamate, T.; Kumazawa, K.; Suzuki, H.; Akazome, M. CH/π Interactions for Macroscopic Interfacial Adhesion Design. ACS Macro Lett. 2016, 5 (7), 858−861. (37) Wittmann, J. C.; Lotz, B. Epitaxial crystallization of polymers on organic and polymeric substrates. Prog. Polym. Sci. 1990, 15 (6), 909− 948.

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DOI: 10.1021/acs.macromol.6b02521 Macromolecules XXXX, XXX, XXX−XXX