Article pubs.acs.org/IECR
Melt Temperature and Initial Polymorphs Dependencies of Polymorphs Selection during Subsequent Crystallization in Propylene-ethylene Random Copolymer Jiayi Zhao,† Yingying Sun,‡ and Yongfeng Men*,† †
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, University of Chinese Academy of Sciences, Renmin Street 5625, 130022 Changchun, P.R. China ‡ ExxonMobil Asia Pacific Research & Development Co., Ltd., 1099 Zixing Road, Minhang District, 200241 Shanghai, P.R. China ABSTRACT: Subsequent crystallization behavior during cooling for two random propylene-ethylene copolymers with 6 mol % ethylene co-units but different molecular weights initially in a pure α-form or pure γ-form processed at different melt temperatures was investigated using differential scanning calorimetry and wide-angle X-ray diffraction techniques. The onset crystallization temperature (Tonset) and fraction of the two polymorphs after crystallization depended on the molecular weight, initial polymorph, and melt temperature (Tmelt). At high Tmelt (120−160 °C), Tonset and the fraction of the γ-form (fγ) increased with decreasing Tmelt. For the high molecular weight sample initially in α-form, a higher Tonset and fγ than the one in the γ-form was observed because a higher Tmelt was needed to fully homogenize the melt for samples initially in the α-form due to the different morphology between the samples of different polymorphs. For the low molecular weight sample, the initial polymorph did not affect this process.
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curves at 20.07° (117) for the γ-form and 18.6° (130) for the αform. Recently, the crystallization behavior of different molten states for isotactic propylene has been investigated. In the case of iPP initially in β-form,25,26 the fraction of two polymorphs (α and β) was strongly dependent on the hold temperature and time. Lower hold temperatures and shorter hold times led to samples rich in the α-form, whereas higher temperatures and longer hold times favored the β-form followed by crystallization. Besides, a thermodynamically unstable structure in molten mesomorphic iPP5 also accelerated the spherulite growth of the α-form during subsequent cooling, while the investigation of the γ-form was not involved in a previous study since the γ-form is a kind of a polymorph which can only be generated under tough conditions such as crystallization at high pressure27,28 for isotactic polypropylene. However, a mixture of the γ-form and α-form is obtained during isothermal crystallization at atmosphere24 for propylene copolymers, and the fraction of the γ-form is regarded as a function of the defect concentration because such a sample with stereo and regio defects in the chain sequence has an advantage to form a low-energy ordered structure by chain tilting. The mechanical behavior of iPP is closely related to crystalline polymorphs obtained under specific conditions. Although the γform transforms into the α-form during stretching,29 the unique nonparallel structure of the γ-form can prevent chain slips
INTRODUCTION Generally, previous thermal history has a significant influence on the subsequent crystallization process of semicrystalline polymers, which is very significant in practical processing. Molten states with different thermal histories are always created in experiments to investigate the following crystallization behavior. “Memory effect” in the molten state at a temperature above the equilibrium melting point leads to higher crystallization rate, smaller spherulites, or polymorphic change1−6 in subsequent crystallization. The origin of memory effect is still under debate. Hikosaka and Yamazaki7,8 considered that the origin of memory effect relies on the change of entanglement state during annealing rather than the seeding. The existence of a disentangled region in the molten state has also been considered to be the reason,9 and sequence segregation in the heterogeneous melt has been observed in Monte Carlo simulations.10 For partial melting at lower melting temperature, self-nucleation11−15 increases the number of nuclei in the melt and the crystallization peak temperature during cooling, which provides more evidence in investigation of crystallization. Isotactic polypropylene (iPP) is a typical polymorphic polymer that shows three crystalline modifications including monoclinic (α), trigonal (β), orthorhombic (γ), and one smectic mesophase.16−23 The helical conformation for all the crystalline polymorphs is a 3-fold helix. Two characteristic diffraction peaks are expected in the wide-angle X-ray diffraction (WAXD) curves of β-iPP including 16.1° (300) and 21.4° (311). In addition, the α-form and γ-form can be distinguished by two specific diffraction peaks24 in the WAXD © 2016 American Chemical Society
Received: Revised: Accepted: Published: 198
October 23, 2016 December 11, 2016 December 16, 2016 December 16, 2016 DOI: 10.1021/acs.iecr.6b04087 Ind. Eng. Chem. Res. 2017, 56, 198−205
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FT-IR spectra of several samples in the molten states were recorded using a Nicolet iS50 FT- IR spectrometer (Thermo Scientific, America) operating at a resolution of 8 cm−1 equipped with continuum IR microscope.
within crystals under stress except for the (001) transverse slips.30,31 In this case, research on the nonisothermal crystallization behavior of different molten states of the iPP copolymer is of high importance in controlling the crystallization behavior of the γ-form and the mechanical behavior in practical process. In this article, using differential scanning calorimetry (DSC) and WAXD techniques, we report the influence of initial polymorphs on the subsequent crystallization behavior of different molten states of random propylene-ethylene copolymers with different molecular weights. The results are helpful for a comprehensive understanding of the γ-form formation under practical nonisothermal processing.
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RESULTS AND DISCUSSION The polymorphs of iPPcoE6H and iPPcoE6L at the required crystallization temperature consisted of the α-form and γ-form. A peak fitting method32 was designed to fit the 1D WAXD curve into several Gaussian peaks. According to a wellestablished method, the relative fraction of the γ-form ( fγ) was calculated by24
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fγ = Iγ(117)/[Iγ(117) + Iα(130)]
EXPERIMENTAL SECTION The random propylene-ethylene copolymers produced using a metallocene catalyst were supplied by ExxonMobil Asia Pacific Research & Development Co., Ltd. The content of ethylene counits is 6 mol % for both samples. Information, such as melt flow rate (MFR), molecular weight, and polydispersity, is shown in Table 1.
where Iγ(117) and Iα(130) are the areas of the corresponding diffraction peaks. To find a suitable isothermal crystallization temperature for preparing samples with the pure γ-form or α-form, the relative fractions of the γ-form in isothermally crystallized samples for iPPcoE6H and iPPcoE6L are shown in Figure 1. The results
Table 1. Random Propylene-Ethylene Copolymers Information sample iPPcoE6H iPPcoE6L
MFR (g/10 min)
Mw (g/mol)a
Mn (g/mol)a
Mw/Mna
8 18420
2.1 × 10 2.8 × 104
9.9 × 10 1.2 × 104
2.15 2.36
5
4
a
Molecular weight and Mw/Mn of samples was obtained by gel permeation chromatography (GPC) in 1,2,4-trichlorobenzene.
The films with a thickness of 0.5 mm were melted at 180 °C and then rapidly cooled to the required crystallization temperature and kept for time long enough on a custommade HCS 412W (Instec, America) hot stage attached with liquid nitrogen pump which was installed in a micro-WAXD system. WAXD patterns were measured at crystallization temperature with the aid of a semiconductor detector with a resolution of 487 × 195 (pixel size = 172 μm) (Pilatus100 K, DECTRIS, Swiss) attached to a multilayerd mirror (FOX3D 21−21, Xenocs SA, France) with a focused Cu Kα X-ray source (GeniX3D Xenocs SA, France), generated at 50 kV and 0.6 mA. The wavelength of X-ray radiation is 0.154 nm, and sample-todetector distance is 39.8 mm. The beam was focused onto the sample position with a size of 40 μm × 60 μm. Each pattern was collected in 60 s and the background subtracted. According to the WAXD results obtained during isothermal crystallization at different temperatures, a suitable crystallization temperature has been chosen for preparation of samples with the pure α-form or γ-form. Samples initially with the γ-form or α-form were heated to different melt temperatures (Tmelt) ranging from 105 to 180 °C and held for 5 min followed by cooling to 25 °C. The crystallization behaviors were measured by a DSC1 Stare System (Mettler Toledo Instruments, Swiss) under a nitrogen atmosphere (50 mL/min) which had been calibrated for temperature and melting enthalpy by using indium as a standard. To investigate the evolution of the polymorph structure in the crystallization process, WAXD experiments have been carried out for the samples during cooling after being heated to different Tmelt. Both the cooling rates for DSC and WAXD measurements were 10 K/min. Each pattern was collected at 30 s with the background subtracted.
Figure 1. fγ of iPPcoE6H and iPPcoE6L as a function of isothermal crystallization temperature Tc.
did not include the data at lower crystallization temperatures since the crystallization has already finished before the temperature reached the given value which was limited by the power of the cooling pump. Previous detailed studies of metallocene-catalyzed iPP homopolymers33 and random propylene copolymers34,35 have found that the γ-form is favored with increasing crystallization temperature. As the critical nucleus sequence length increases with crystallization temperature at a fixed defects content, chain tilting is an effective way to form a low-energy three-dimensional ordered structure.33 At a fixed crystallization temperature and fixed counit content, fγ was higher for iPPcoE6L with low molecular weight.33 The pure γ-form was prepared by melting first and then quenching in an oil bath at 80 and 90 °C for iPPcoE6L and iPPcoE6H, respectively. In addition, the lack of segmental mobility at lower crystallization temperatures makes the formation of the γ-form a disadvantage. Thus, the pure αform was prepared by quenching the melt into a water bath at room temperature not only to obtain a large cooling rate and low crystallization temperature but also to avoid the generation of mesophase at too low temperatures.36 199
DOI: 10.1021/acs.iecr.6b04087 Ind. Eng. Chem. Res. 2017, 56, 198−205
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Industrial & Engineering Chemistry Research Integrated 1D WAXD curves of the pure γ-form and pure αform for iPPcoE6H are shown in Figure 2. The two
and the premelting temperature. Selected DSC data during the cooling process and second heating process and WAXD data at 25 °C for iPPcoE6H are shown in Figure 4. On decreasing Tmelt, the peak temperature of crystallization started to increase as shown in Figure 4(a) and (b). The melting peak temperature which was located at around 104 °C shifted slightly under different Tmelt shown in Figure 4(c) and (d), while the relative fraction of the γ-form increased a lot with decreasing Tmelt as identified by the WAXD data shown in Figure 4(e) and (f). When Tmelt was below 118 °C, the DSC melting curves showed two melting peaks representing newly formed crystals and original unmolten crystals. To investigate the mechanism of the nonisothermal crystallization behavior, the onset of crystallization temperature Tonset obtained from DSC cooling curves and fγ obtained from fitting WAXD curves for iPPcoE6L and iPPcoE6H initially in the pure α-form and γ-form processed at different Tmelt are given in Figures 5 and 6. As shown in the Figure 5, the temperatures of the molten state for both two polymorphs of iPPcoE6L had an effect not only on the subsequent crystallization process but also on the component of the crystallized samples. Both fγ and Tonset remained the same for both samples of the two initial polymorphs at high Tmelt (higher than 155 °C), indicating that memory of original crystallite was absent in the melt structure at high Tmelt. At such conditions, fγ remained low due to the crystallization process which occurred at low temperature regions. Here, Tonset for both polymorphs started to increase at Tmelt of 155 °C due to the accelerated nucleation rate during cooling by the memory effect of the molten state at Tmelt below 155 °C. Simultaneously, fγ increased at this point showing that the components of the crystallized samples have also been influenced by Tmelt. The reason for increasing in fγ is that the nonisothermal crystallization process begins at a higher temperature which is discussed in detail in the following part. Newly formed crystals and original unmolten crystals existed after crystallization when Tmelt was lower than 118 °C. Here, Tonset and fγ were both different for samples initially in the pure α-form and γ-form at the same Tmelt in this region. A maximum Tonset was observed as shown in a previous study,37 and Tonset for samples initially in the α-form were always higher than those initially in the γ-form. Additionally, a mixture of two polymorphs and the pure γ-form were obtained for samples initially in the pure α-form and γ-form, respectively. For samples initially in the α-form, fγ reduced at lower Tmelt. The crystallinities of the α-form left in the molten state and in the
Figure 2. Integrated 1D WAXD curves of samples initially in γ-form and α-form for iPPcoE6H, respectively.
polymorphs have been verified by the two specific diffraction peaks, (117)γ and (130)α. Thus, samples with the pure γ-form and pure α-form were obtained for further investigation. The DSC melting curves for the pure α-form and γ-form are shown in Figure 3. For the pure α-form, one endothermic peak was located at around 100 °C and the other located at 40 °C were observed. The one at low melting temperatures for both the α-form and γ-form was due to the slow crystallization of some amorphous chain segments at room temperature according to a previous study.32 The final melting peak temperature for the pure γ-form was higher than that of the αform due to higher isothermal crystallization temperature. Furthermore, the melting peaks at middle temperatures were caused by crystallization during cooling33 from isothermal crystallization temperature. Samples initially in the pure γ-form or α-form were heated to different melt temperatures Tmelt ranging from 105 to 180 °C, held for 5 min, then cooled to 25 °C at 10 K/min. The thusobtained samples were subjected to WAXD measurements at room temperature to calculate the fraction of the γ-form in the crystals. DSC heating runs on these samples were also performed to check the relationship between the final structure
Figure 3. DSC melting curves of iPPcoE6H (a) and iPPcoE6L (b) initially in γ-form and α-form, respectively. 200
DOI: 10.1021/acs.iecr.6b04087 Ind. Eng. Chem. Res. 2017, 56, 198−205
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Figure 4. DSC cooling curves of iPPcoE6H initially in α-form (a) and γ-form (b) and corresponding heating curves of crystallized samples (c) and (d). WAXD curves of crystallized samples at 25 °C initially in α-form (e) and γ-form (f).
behavior for the two polymorphs, which was observed in the low molecular weight sample iPPcoE6L, still occurred in the sample with the high molecular weight. The nonisothermal crystallization mechanism is very complicated since the crystallization temperature changes all the time. To further investigate the crystallization mechanism during cooling under different Tmelt, change tendencies of fγ during cooling for iPPcoE6H of both polymorphs are shown as an example in Figure 7. Once cooled from Tmelt of 145 °C, the α-form appeared first, and then, the fraction of the α-form quickly decreased accompanied by an increase in fγ, which was in accordance with the epitaxy crystallization mechanism38 of the γ-form on the α-form during isothermal crystallization. Experimental evidence of the advantage of special epitaxy for the γ-form on the α-form has been reported39 that the α-form substrate has extensive cross-hatching resulting in spherulite
crystallized sample were the same as evidenced by WAXD and DSC results, indicating that most of the newly generated crystals were in the γ-form for samples with both original polymorphs. Although the initial polymorphs have no effect on the subsequent crystallization behavior for iPPcoE6L at Tmelt above 118 °C, the results were distinct for iPPcoE6H shown in Figure 6. Increase in Tonset and fγ started at Tmelt below 160 °C for the sample initially in the α-form, while it happened at Tmelt below 145 °C for the sample initially in the γ-form, which was the evidence for the difference between the molten states of the two polymorphs at high Tmelt in this high molecular weight sample. The Tonset and the fraction of the γ-form in the crystallized samples were always higher for iPPcoE6H initially in the α-form under the same Tmelt in this region. For the partial molten samples, the variance of the subsequent crystallization 201
DOI: 10.1021/acs.iecr.6b04087 Ind. Eng. Chem. Res. 2017, 56, 198−205
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then increased to a constant value. Although growth of the γform relied on a small amount of the α-form substrate, crystallization of the α-form was restricted once the γ-form developed largely in the late stage of crystallization. When cooling from Tmelt of 130 °C, the newly formed α-form and γform appeared simultaneously once cooled from a melt for the sample initially in the α-form, whereas only the α-form crystallites were generated at the beginning of cooling from melt for the sample initially in the γ-form. Combined with the results under two Tmelt, a higher fraction of the γ-form for the sample initially in the α-form was ascribed to the higher onset crystallization temperature during cooling. Since growth of the γ-form preferred higher temperatures as shown in Figure 1, more crystallizable chains tended to produce crystallites at higher temperature resulting in a higher fraction of the γ-form crystals after crystallized. When melted at 119 °C, the pure γ-form was obtained for both initial polymorphs because the crystallinity of the α-form substrate was little and the fast grown γ-form at high temperature occupied almost the whole crystallinity. For partially molten samples at 107 °C, the unmolten original αform and γ-form crystallites led to different crystallization processes which are shown in Figure 5. The unmolten α-form in the melt acted as a substrate directly for crystallization of the γ-form, while the α-form substrate was generated during cooling from the melt in another situation. Thus, Tonset for the sample initially in the α-form was always higher for partially molten samples. Although the subsequent crystallization behavior of the samples under different Tmelt has been clarified, the mechanism which led to the different behavior between the samples initially in two different polymorphs under high Tmelt of the high molecular weight sample iPPcoE6H is still obscure. FT-IR spectra have been recorded after the samples were held at different Tmelt for 5 min to identify the helical conformation. Such results are shown in Figure 8. Actually, no obvious differences were observed at Tmelt of 140 and 162.5 °C for both polymorphs. The regular band at 972 cm−1 which always persisted in the melt corresponded to helices made of three isotactic propene units.40 The other band at 1153 cm−1 was used to represent the amorphous fractions.41 At both temperatures, the bands at 808 and 900 cm−1 with low intensity indicated the existence of a slight amount of conformational ordered segments. In addition, all the crystals have been melted completely due to the absence of 841 cm−1 even at Tmelt of 140 °C. Since the conformational structure for iPPcoE6H initially in two polymorphs at molten states with or without memory effect was the same, other factors have to be taken into consideration. As reported in a previous study, sequence length segregation in the molten state caused by restrictions of the copolymer sequences to diffuse and to reach a homogeneous state10,42 was one of the reasons for the memory effect. Partially increasing the concentration of the long sequences lowered the free energy barrier for nucleation. For samples initially in the αform, critical Tmelt for the homogeneous molten state is higher because the iPPcoE6H samples with different initial polymorphs reached the molten state relaxed to different extents at the same Tmelt. This can be understood as a consequence of different morphologies in the α- and γ-form crystallites in iPPcoE6H being more regularly spherulite and defective crystalline aggregates, respectively. In addition, the high molecular weight sample naturally ends up with high viscosity
Figure 5. Melt temperature dependencies of Tonset during cooling (top) and fraction of γ-form after crystallization (bottom) for iPPcoE6L initially in γ-form and α-form.
Figure 6. Melt temperature dependencies of Tonset during cooling (top) and fraction of γ-form after crystallization (bottom) for iPPcoE6H initially in γ-form and α-form.
morphologies for the γ-form. In this case, the α-form substrate generated at higher temperatures for the sample initially in the α-form at Tmelt of 145 °C indicated that the crystallization process started earlier during cooling with the memory effect. The growth of the γ-form started at a higher temperature and 202
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Figure 7. Change tendencies of fγ during cooling for iPPcoE6H initially in γ-form and α-form at different Tmelt.
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CONCLUSIONS To summarize, the onset crystallization temperature and the fraction of two polymorphs after crystallization were dependent on the initial polymorph and the melt temperature in the propylene-ethylene random copolymers during nonisothermal crystallization. In the high Tmelt region, memory effect was observed in samples initially in both polymorphs which led to an increase in fγ and Tonset. The reason for the increase in fγ was that the growth of the γ-form started at higher temperature in the specific epitaxy crystallization process. The fγ and Tonset was higher at the same Tmelt for iPPcoE6H with high molecular weight initially in the α-form than in the γ-form due to the different morphology of the α-form and γ-form crystallites, while the initial polymorph did not affect this process in iPPcoE6L with low molecular weight. For partial melting at low Tmelt, most of the newly generated crystals were in the γ-form for samples with both original polymorphs. The observed results provide an opportunity to discuss mechanism of polymorphs selection in this propylene-ethylene copolymer with different initial crystallographic polymorphs.
Figure 8. FT-IR spectra for iPPcoE6H initially in α-form and γ-form at Tmelt of 140 and 162.5 °C.
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and longer relaxation times leading to much longer times for a full homogenization of the melt in the case of spherulitic structures (α-form) than the defective crystalline aggregates (γform). Such a difference is however not obvious in the iPPcoE6L samples. Both the α- and γ-form crystallites in iPPcoE6H show similar grain-like structures due to massive nucleation during crystallization for both forms. Thus, molten states for samples of both initial polymorphs in iPPcoE6L relaxed to a similar extent when heated at the same condition resulting in almost the same crystallization behavior after being processed at the same Tmelt.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Yongfeng Men: 0000-0003-3277-2227 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (21134006, 51525305) and ExxonMobil. We thank Dr. R. Wittenbrink at ExxonMobil Asia Pacific 203
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Research and Development Company, Ltd. for helpful discussions.
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