Chain Structure, Aggregation State Structure, and Tensile Behavior of

Article pubs.acs.org/JPCB

Chain Structure, Aggregation State Structure, and Tensile Behavior of Segmented Ethylene−Propylene Copolymers Produced by an Oscillating Unbridged Metallocene Catalyst Zai-Zai Tong,† Yao Huang,‡ Jun-Ting Xu,*,† Zhi-Sheng Fu,† and Zhi-Qiang Fan† †

MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science & Engineering, Zhejiang University, Hangzhou 310027, China ‡ State Key Laboratory of Chemical Engineering, Department of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China S Supporting Information *

ABSTRACT: Segmented ethylene-propylene copolymers (SEPs) with different propylene contents were prepared by an unbridged metallocene bis(2,4,6-trimethylindenyl)zirconium dichloride [(2,4,6-Me3Ind)2ZrCl2] catalyst. Due to oscillation of the unbridged ligands in the catalyst, the SEPs are composed of segments with low propylene contents, alternated by the segments with high propylene contents. Such a chain structure was verified by 13C NMR and successive selfnucleation and annealing (SSA). As the propylene/ethylene feed ratio during copolymerization increases, the comonomer contents in both segments are increased, leading to noncrystallizability of the high propylene segments and smaller crystallinity of the low propylene segments. Consequently, SEPs may be used as thermoplastic elastomers (TPEs). The aggregation state structures at nano- and micro-scales were characterized with small angle X-ray scattering, transmission electron microscopy and polarized optical microscopy, and compared with those of ethylene−octene multiblocky copolymers (OBCs) with similar crystallinity. It is found that SEPs form thinner lamellar crystals with a lower melting temperature due to shorter length and higher comonomer content of the low propylene segments. Moreover, the short length of the high propylene segments in SEPs results in an evidently thinner amorphous layer among the lamellar crystals, thus lots of amorphous phases are excluded out of the interlamellae. Accordingly, ill-developed spherulites or even bundle crystals are formed in SEPs, as compared with the well-developed spherulites in OBCs. SEPs exhibit the tensile property of typical TPEs with diffused yielding and large strain at break. content,16 which limits the application of POEs at high temperatures.21,22 Olefinic blocky copolymers (OBCs) are a type of recently emerging TPO synthesized by the chain shuttling method, which consist of alternating hard blocks (crystalline blocks with low octene content) and soft blocks (amorphous blocks with high octene content).14 OBCs exhibit not only excellent elastomeric properties but also high thermal stability due to the high melting temperature of the hard blocks.23−35 However, synthesis of OBCs requires two catalysts with matching polymerization rates and high activity at a high polymerization temperature. Waymouth et al. also synthesized PP-based TPO with a blocky fashion using an unbridged metallocene.36 They designed a metallocene catalyst that can isomerize between achiral and chiral geometries during polymerization to produce the atactic-isotactic stereoblock

1. INTRODUCTION Thermoplastic elastomers (TPEs) are fascinating materials and have drawn much attention in both industry and science.1,2 TPEs usually have a physically cross-linking structure, and thus exhibit many advantages in processing and recycling, as compared with natural or synthetic rubbers. Due to the low cost and wide availability of olefinic monomers, much effort is directed toward the development of thermoplastic polyolefinic elastomers (TPOs). Advances in polyolefin synthesis enable to prepare three new kinds of polyolefin elastomers, including the ethylene-α-olefin copolymers,3−7 stereoblock polypropylene (PP)-based materials,8−12 and olefinic blocky copolymers.13−15 Ethylene-octene random copolymers (POEs) with low crystallinity are a subcategory of TPO, in which the crystalline segments act as the physically cross-linking points.16−20 In POEs, the comonomer units are statistically distributed along the polymer chains. As a result, the length of the crystalline segments and the melting temperature of the formed crystals are dramatically reduced with increasing the commoner © 2015 American Chemical Society

Received: February 24, 2015 Revised: April 14, 2015 Published: April 23, 2015 6050

DOI: 10.1021/acs.jpcb.5b01845 J. Phys. Chem. B 2015, 119, 6050−6061

Article

The Journal of Physical Chemistry B

Scheme 1. (a) The Two Most Possible Conformations of Bis(2,4,6-trimethylindenyl) Zirconium Dichloride upon Oscillation of the Indenyl Ligand, (b) the Structure of the Segments Produced by Different Isomers of the Metallocene Catalyst, and (c) the Resulting Chain Structure of SEPs

PP.36 The isotactic blocks serve as crystalline domains, and the atactic blocks contribute to the elastic property. The PP-based TPO is more thermally stable due to the higher melting temperature of isotactic PP than that of PE. Nevertheless, PP has a higher glass transition temperature (Tg ∼ 10 °C) than PE, which limits the application of PP-based TPO at low temperatures. Inspired by Waymouth’s work,36 we designed another unbridged metallocene catalyst, i.e., bis(2,4,6-trimethylindenyl)zirconium dichloride [(2,4,6-Me3Ind)2ZrCl2].37−39 As shown in Scheme 1, the ligands in this metallocene cannot rotate freely but oscillate due to the steric hindrance of three methyl groups, forming the chiral and achiral geometries. In our previous studies, we mainly focused on homopolymerization of propylene or copolymerization of ethylene with 1-hexene using this catalyst, but the aggregation state structure and its correlation with the chain structure were not investigated. Herein, this catalyst was used for ethylene/propylene copolymerization, instead of propylene homopolymerization or ethylene/1-hexene copolymerization. The chiral and achiral geometries of the metallocene produce segments of low and high comonomer contents, respectively, leading to a segmented structure of the obtained polymers (Scheme 1). If the segments of low comonomer content can crystallize and the ones of high comonomer content are amorphous, it is expected that such a polymer material is also a type of TPO, and we may use a single catalyst without addition of chain transfer agent to prepare TPOs having a chain structure like OBCs. Because of the low Tg of the amorphous ethylene-α-olefin copolymers, this type of TPO can be used at low temperatures. In the present work,

four segmented ethylene−propylene copolymers (SEPs) with different propylene contents were prepared by this unbridged metallocene. The chain structure, aggregation state structures at different scales, and tensile behavior of SEPs were studied, and compared with those of OBCs.28 It was found that SEPs indeed exhibited typical tensile behavior of TPEs. We also demonstrated that, due to the different chain structures, the aggregation state structures of SEPs at nano- and micrometer scales are quite different from those of OBCs. These findings are helpful for understanding the relationship between the chain structure and aggregation state structure as well as macroproperties of TPOs.

2. EXPERIMENTAL SECTION 2.1. Materials. The segmented SEPs were prepared by the unbridged metallocene bis(2,4,6-trimethylindenyl)zirconium dichloride [(2,4,6-Me3Ind)2ZrCl2] catalyst with modifiedmethylaluminoxane (MMAO) as a cocatalyst in our laboratory. Copolymerization was carried out at 50 °C under 0.4 MPa pressure in a 300 mL Büchi reactor equipped with a propellerlike stirrer and thermostat water bath. n-Heptane (100 mL) was introduced into the nitrogen-purged reactor and stirred, then the ethylene/propylene mixed gas was rapidly bubbled through the reactor. The flow rate of ethylene and propylene was much larger than the monomer consumption rate during the polymerization, so the monomer concentrations in the solution could be kept at a constant level. Polymerization was initiated by adding an n-heptane solution of MMAO (1 M, 2.5 mL) and then a toluene solution of catalyst (0.1 mM, 25 mL) into the reactor under stirring. Polymerization was terminated after 30 6051


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where ΔHm0 is the fusion enthalpy of PE with 100% crystallinity, which is 290 J g−1. For the measurements of SPEs’ glass transition temperature (Tg), the samples were first heated to 150 °C and held for 5 min to eliminate thermal history, and then cooled to −90 °C at a rate of 20 °C/min. Finally, the samples were heated to 0 °C at a rate of 20 °C/min, and the Tgs were determined. 2.4. Wide Angle X-ray Diffraction. Wide angle X-ray diffraction (WAXD) patterns were obtained on an X’Pert Pro instrument (PANalytical B.V.) at room temperature using Nifiltered CuKα radiation with wavelength of 1.54 Å as the X-ray source. The scanning increment is 0.02°. By resolving the multiple-peak pattern into individual crystalline peaks and an amorphous halo, the weight fraction crystallinity (XcWAXD) can be calculated based on the following equation:40

min, and then the mixture of ethanol (500 mL) and concentrated HCl (2 mL) was introduced. After filtration, the obtained product was washed with ethanol and deionized water, respectively, and then dried in vacuum at 50 °C overnight. Copolymers with various propylene contents were prepared by changing the ethylene/propylene feed ratio. The weight-average molecular weight (Mw) and the molecular weight distribution (Mw/Mn) were determined by gel permeation chromatography (GPC) on a PL-220 GPC instrument. The GPC measurements were performed at 150 °C with 1,2,4-trichlorobenzene as the eluent at a flow rate of 1.0 mL/min. Polystyrene samples with a narrow molecular weight distribution were used as standards for calibration. The compositions of SEP samples were determined by 13C-nuclear magnetic resonance (NMR). The 13C NMR experiments were carried out on a 300 MHz Varian Mercury plus instrument employing o-dichlorobenzene-d4 as solvent at 120 °C. The SEP samples are coded as SEPn, where n represents the approximate molar percentage of propylene. Two olefinic blocky copolymers (OBC-A and OBC-B in ref 28) and an ethylene-hexene-1 random copolymer prepared by conventional Ziegler−Natta catalyst (ZNEH, in ref 29) were used for comparison, in which OBC-A and ZNEH have similar crystallinity with SEP14, and OBC-B and SEP26 also have similar crystallinity. The chain structure and morphology of OBCs and ZNEH were reported in our previous work,28,29 and the related data are given in Table S1 in the Supporting Information. 2.2. Successive Self-Nucleation and Annealing. Successive self-nucleation and annealing (SSA) experiments were conducted on a TA Q200 instrument. However, in the SSA experiment, the most important parameter is the first selfnucleation temperature (Ts1). In the present work, the Ts1 temperatures for samples SEP8, SEP14, SEP23, and SEP26 were determined as 119, 116, 108, and 98 °C, respectively (refer to Figure S1 in the Supporting Information). The samples were first heated to 150 °C and held for 5 min, then cooled to 0 °C at a rate of 10 °C/min for crystallization. The crystallized samples were heated to the first self-nucleation temperature (Ts1). After staying at Ts1 for 5 min, the samples were cooled to 0 °C at a rate of 10 °C/min, and then the samples were heated to the second self-nucleation temperature and held for 5 min. The above thermal treatment procedures were repeated, but the Ts became 5 °C lower gradually until the final Ts of 40 °C. After SSA thermal treatment, the samples were heated to 150 °C, and the melting traces were recorded. In all temperature ramps, the heating or cooling rate was 10 °C/min. All the measurements were performed under a nitrogen atmosphere with a flow rate of 50 mL/min. The temperature was calibrated with indium. 2.3. Differential Scanning Calorimetry. Specimens weighing 3−10 mg were cut from compression-molded films for thermal analysis. The differential scanning calorimetry (DSC) experiments were carried out on a TA Q200 calorimeter. The samples were first heated to 150 °C and held for 5 min to eliminate thermal history. Subsequently, the samples were cooled to 0 °C at a rate of 10 °C/min and the nonisothermal crystallization DSC curves were recorded. Finally, the samples were again heated to 150 °C at a rate of 10 °C/min to obtain the melting curves. The crystallinity (XcDSC) can be determined from the melting enthalpy (ΔHm): XcDSC = ΔHm/ΔHm0

XcWAXD =

A110

A110 + 1.46A 200 + 1.46A 200 + 0.75Aa

(2)

where A110 and A200 are the areas of the (110) and (200) crystalline peaks, respectively, and Aa is the area of the amorphous halo. 2.5. Small Angle X-ray Scattering and Data Analysis. Small angle X-ray scattering (SAXS) experiments were performed at the BL16B1 beamline at the Shanghai Synchrotron Radiation Facility (SSRF) in China. The wavelength was 1.24 Å, and the sample-to-detector distance was set as 5100 mm. Two-dimensional (2D) SAXS patterns at various temperatures were recorded. The average exposure time was 150 s for each scan. The bull tendon was used as standard material for calibrating the scattering vector. The 2D SAXS patterns were converted into one-dimensional (1D) SAXS profiles using Fit2D software. The structural parameters describing the lamellar stacking of polymer crystals were derived from a one-dimensional correlation function, which is obtained through Fourier transformation of the Lorentz-corrected SAXS profile based on the following equation:41−43 ∞

γ (r ) =

∫0 I(q)q2 cos(qr ) dq ∞

∫0 I(q)q2 dq

(3)

where I(q) is the scattering intensity, q is the scattering vector defined as q = 4π sin θ/λ, and λ is the X-ray wavelength. The details of this method have been reported in our previous works.28,29 Useful structural parameters such as the thickness of the transition layer dtr, the lamellar crystal thickness lc, the long period L, and the thickness of the amorphous layer (la = L − lc) can been obtained from the curve of correlation function. The linear crystallinity Xl is governed by lc and L, which is defined as

Xl = lc /L

(4)

On the other hand, the bulk volume fraction crystallinity, Xvol, can be calculated from the weight fraction crystallinity measured from WAXD (XcWAXD): ρa X vol = WAXD ρc /Xc − ρc + ρa (5) where ρc and ρa are the densities of the crystalline and amorphous phases (for PE, ρc = 1.000 g/cm3 and ρa = 0.855 g/ cm3),44 respectively. The volume fraction crystallinity is also the product of linear crystallinity and the volume fraction of the lamellar stacks in the sample, Φs:

(1) 6052


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The Journal of Physical Chemistry B Table 1. Molecular Characteristics of Four SEP Samples dyads (%)

triads (%)

sample

P/E feed ratio

Mw (kg/mol)

Mw/Mn

[P] (mol %)

EE

EP

PP

PPP

PPE

EPE

EEE

EEP

PEP

SEP8 SEP14 SEP23 SEP26

30/70 50/50 60/40 70/30

97 54 40 43

4.6 4.2 4.0 4.3

7.8 14.0 23.4 25.9

85.2 74.5 59.7 57.2

14.0 23.1 33.9 33.8

0.8 2.4 6.4 9.0

0 0 0.6 2.6

1.6 4.7 11.5 12.8

6.2 10.1 12.9 12.1

78.8 65.6 48.3 46.1

12.4 17.7 22.7 22.2

1.0 1.9 4.0 4.2

Figure 1. Plots of [EEE][PEP] versus ([EEP]/2)2 for a first-order Markov process (a) and [EEEE][PEEP] versus ([EEEP]/2)2 for a second-order Markov process in SEPs.

Φs = X vol /Xl

switches among different geometries, among which the achiral meso-like and chiral racemic are stable and energetically preferable. The chiral racemic geometry tends to form polymer segments with a high propylene content due to its open structure, while the achiral meso-like geometry may produce the segments of a low propylene content (Scheme 1b). When the ligands oscillate quickly, the switch among different geometries may occur in a period shorter than the lifetime of the growing polymer chain, leading to a segmented or multiblocky structure (Scheme 1c), which is quite like that of OBCs. The propylene contents in both low and high propylene segments vary with the feed ratio during copolymerization. When the P/E feed ratio is small, both segments may be crystallizable, though there is a difference in propylene content between these two types of segment. When the P/E feed ratio is increased to a certain level, the high propylene segment may become noncrystallizable, whereas the low propylene segment remains crystallizable. The SEPs prepared by the oscillating unbridged metallocene at different P/E feed ratios were characterized with GPC and 13 C NMR, and the results are summarized in Table 1. One can see that all the samples have a broad molecular weight distribution, as compared with the value of Mw/Mn ∼2.0 for the polymers produced by most of the metallocene catalysts. This shows that oscillation of the ligands may also broaden the molecular weight distribution. As the P/E feed ratio increases, the overall propylene content increases from 7.8 mol % for EP8 to 25.9 mol % for SEP26, showing that more propylene units are incorporated into both segments. The dyad and triad distributions are calculated from the 13C NMR and are also given in Table 1. Due to the resolution of the 13C NMR instrument, only partial tetrads (such as EEEE, EEEP and PEEP) can be observed in the 13C NMR spectra (Figure S2 of Supporting Information).47 The ratio of these three tetrads can be calculated, as shown in Table S2 of Supporting Information.

(6)

Φs can serve as a measurement for the extent of interlamellar segregation.45,46 2.6. Polarized Optical Microscopy. Polarized optical microscopy (POM) observations were carried out on an Olympus Microscope (BX51) equipped with a hot stage. Thin film specimens of about 0.05 mm thickness were used for observation. 2.7. Transmission Electron Microscopy. Specimens for transmission electron microscopy (TEM) observation were prepared by dropping 0.15% xylene solution of the polymers on the carbon film supported on a glass slide at 120 °C. After evaporation of the solvent, the samples were placed on a hot stage and heated to 150 °C for 5 min, then cooled to room temperature at a rate of 10 °C/min. After thermal treatment, the glass slide was corroded by hydrofluoric acid (40% w/w), and the floating samples were captured by copper grids. Observations were carried out on a JEOL JEM-1200EX instrument at an acceleration voltage of 80 kV. 2.8. Mechanical Property. The stress−strain behavior under uniaxial tension was performed on a CMT 4204 instrument. The tensile specimens were cut from the compression molded films with a thickness of about 0.5 mm. The distance of two grips was 20 mm and the specimen width was 2.0 mm. A strain rate of 20 mm min−1 was applied to uniaxial tension. The tests were repeated five times for each sample, and the averaged data were reported. For recovery measurements, specimens were stretched to 100% strain, the lower grip was released, and the recovered length was measured after 10 min.

3. RESULT AND DISCUSSION 3.1. Chain Structure. Since the catalyst for synthesis of SEPs is an unbridged metallocene, the 2,4,6-trimethylindenyl ligands can oscillate (Scheme 1a). As a result, the catalyst 6053


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Figure 2. Melting traces of (a) SEPs and (b) OBCs after SSA thermal treatment. The detailed information on OBCs is reported in ref 28, and the molecular characteristic is summarized in Table S1 in the Supporting Information.

For a first-order Markov process of binary copolymerization, there is following relationship among the triads:48 (1/2[EEP])2 = [EEE][PEP]

have a broad composition distribution. Even for SEP8, in which the overall propylene content is the lowest, small melting peaks can still be observed around the temperature as low as 60 °C. The melting behavior of SEP samples prepared by the oscillating metallocene catalyst after SSA is quite different from that of OBCs. As shown in Figure 2b, the strongest melting peak of OBCs is always located at the highest temperature, irrespective of the comonomer content (Supporting Information Table S1). In addition, OBCs usually exhibit a major melting peak, i.e., the peak at the highest temperature, together with some weak peaks. This is due to the nearly constant comonomer content of the hard block in OBCs (crystalline block) resulting from the weak copolymerizability of the catalyst producing the hard block (Table S1). The increase of the overall comonomer content in OBC is the result of increase of the comonomer content in the soft block (noncrystalline block) or the increase of the fraction of the soft block. By contrast, in the SEPs prepared by the oscillating metallocene, both the low and high propylene segments may crystallize at a relatively low overall propylene content. As a result, the composition distribution of the SEPs is broader and more heterogeneous than that of OBCs to some extent. Moreover, since the meso-like geometry of the oscillating metallocene also exhibit a certain activity toward propylene, the most probable ethylene sequence in the low propylene segments of SEPs is not the longest ethylene sequence and its length is reduced as the P/E feed ratio increases. The multiple melting peaks after SSA can be deconvoluted into discrete peaks using multiple Lorentz functions. Based on the areas of the deconvoluted peaks, the weight percentage of each melting peak can be calculated. On the other hand, the lamellar crystal thickness (lc) can be calculated from the melting temperature (Tm) based on the Thomson-Gibbs equation:57

(7)

On the other hand, if the binary copolymerization follows a second Markov process, the following relationship is established for the tetrads:48 (1/2[EEEP])2 = [EEEE][PEEP]

(8)

The plots of [EEE][PEP] versus ([EEP]/2) for a first-order Markov process and [EEEE][PEEP] versus ([EEEP]/2)2 for a second-order Markov process are illustrated in Figure 1. One can see that all the data of these four samples evidently deviated from the diagonal line (Figure 1a,b). There are two possible reasons for such a deviation. (1) The copolymerization is neither a first-order Markov nor a second-Markov process. (2) There exists more than one type of active species in the catalyst. The first reason is highly unlikely because copolymerization of ethylene with alkene-1 catalyzed by most of the single-site metallocene catalysts is usually a first-order Markov process49 or a second Markov process.50 As a result, analysis of the 13C NMR result verifies that there exist plural active species in the metallocene catalyst used in the present work, which may result from the oscillation of the ligands. 13 C NMR can usually characterize the sequence distribution of copolymers in a short length, such as triads and tetrads in ethylene-propylene copolymers. By contrast, longer sequences are frequently related to crystallization behavior, such as the lamellar crystal thickness, thus can be characterized by DSC.51−54 The successive self-nucleation and annealing (SSA) technique can be used to evaluate the distribution of the lamellar crystal thickness, which may be further correlated to the distribution of crystallizable ethylene sequences.55,56 Using this method, the ethylene sequences up to tens of nanometers can be measured. Figure 2a shows the DSC melting traces of four SEP samples after SSA thermal treatment. Like other ethylene-α-olefin copolymers, multiple melting peaks are observed for all SEPs, which correspond to the melting of PE crystals with different lamellar thicknesses. With increasing the overall propylene content, the first melting peak at high temperature shifts to lower temperature. This indicates that the longest crystallizable sequence in the samples becomes shorter gradually when more propylene units are enchained. The strongest melting peak is located between the highest and the lowest melting peaks. It is also noticed that all the SEPs 2

lc =

2σeTm0 0 ΔHm(Tm0 −

Tm)

(9)

−2

where σe (0.09 J m ) is the surface free energy of the folding surface,58 ΔHm0 is the fusion enthalpy of PE crystal of 100% crystallinity (287.3 × 106 J m−3),58 Tm0 is the equilibrium meting temperature of linear polyethylene (141 °C),59 and Tm is the measured melting temperature of SEPs. The calculated distributions of the lamellar crystal thickness of four SEP samples are shown in Figure 3. One can see that the sample SEP8 has a broad distribution of lc. As the overall 6054


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Supporting Information. However, our previous result showed that the crystallizability of the ethylene sequences is also affected by the local propylene content.60 As a result, the distribution of lc in the SEPs may still reflect the composition distribution to some extent. The decrease of I or σw2 indicates a much more uniform crystallizable length in SEP26. Such a result is reasonable. With increasing the overall propylene content, the long ethylene sequences produced by the mesolike geometry become shorter but still crystallizable, whereas the short crystallizable sequences produced by the chiral racemic geometry tend to be noncrystallizable, which cannot be measured by DSC. 3.2. Thermal Behavior and Crystallinity. The existence of two different segments in SEPs can also be identified from the melting curves. Figure 4 shows the second-run melting

Figure 3. Distributions of the lamellar crystal thickness for SEP samples.

propylene content increases, the distribution of lc becomes narrower. This is because the high propylene segments tend to become noncrystalline, and the noncrystalline sequences cannot be detected by DSC. Moreover, the weight- and number-average lamellar crystal thicknesses (lw̅ and ln̅ ) and the polydispersity (I = lw̅ /ln̅ ) or square error (σw2) of the lamellar thickness can be calculated as follows:

∑ f wi li

(10)

ln = 1/∑ (f wi /li)

(11)

I = l w / ln

(12)

lw =

σw2 =

∑ f wi ( lw − li)2

Figure 4. Second run melting curves of SEPs. The heating rate is 10 °C/min.

(13)

Where li is the lamellar thickness corresponding to the melting peak i, and f wi is the weight fraction of li, which is the fraction of the ith melting peak area in the overall melting peak area. The calculated result is listed in Table 2. One can see that both the values of lw̅ and ln̅ decreases with increasing propylene

curves of four SEP samples. Two evident melting peaks are observed for SEP8 and SEP14, which have relatively lower overall propylene contents. One narrow peak is located at high temperature, while the other broad melting peak appears at low temperature, as indicated in the dashed boxes. These two melting peaks can be assigned to the segments produced by the two most probable geometries depicted in Scheme 1. The segments produced by achiral meso-like geometry is reflected by the high temperature melting peak, and the broad melting peak at low temperature is attributed to the segments produced by the chiral racemic-like geometry. As the P/E feed ratio increases, the low temperature melting peak becomes insignificant. There exists only one melting peak belonging to the segments produced by the achiral meso-like geometry in SEP23 and SEP26, while the segments produced by the chiral racemic geometry become noncrystallizable due to the high local propylene content. The melting behavior of SEPs is quite different from that of OBCs. In OBCs, the melting temperature remains high, and the melting peak is always unimodal, regardless of the overall comonomer content.28 Comparing the melting behaviors of SEP14 and OBC-A that have a similar crystallinity (Figure S4), one can see that the high temperature melting peak of SEP14 is evidently lower than that of OBC-A, indicating that the comonomer content in the long ethylene sequences is much higher than that in the hard blocks of OBCA. Therefore, we can rationalize that the difference of the comonomer content between two kinds of segments in SEP14 is not as large as that between the soft and hard blocks in OBCA.

Table 2. Weight- and Number-Average Lamellar Crystal Thicknesses, and Polydispersity and Square Error of the Thickness sample

lw̅ (nm)

ln̅ (nm)

lw̅ /ln̅

σw2


6.5 5.0 4.6 4.0

5.8 4.6 4.3 3.8

1.13 1.09 1.07 1.03

5.54 2.33 1.37 0.53

content. It is also observed that the polydispersity decreases from 1.13 for SEP8 to 1.03 for SEP26, and the σw2 is reduced from 5.54 for SEP8 to 0.53 for SEP26. These indicate that the lamellar crystal thickness in SEP26 is more uniform than that in SEP8. Assuming that the comonomer units are excluded from the PE crystal lattice, the lamellar crystal thickness in ethylene-αolefin copolymers can be viewed as the length of the crystallizable ethylene sequences, thus the distribution of lc can be further correlated to the composition distribution. Ethylene−propylene copolymer is slightly different from other ethylene-α-olefin copolymers, since the propylene unit may be included into the PE crystal lattice.60 This can be confirmed by the WAXD result, which is shown in Figure S3 in the 6055


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The Journal of Physical Chemistry B The melting and crystallization temperature (see Figure S5), fusion enthalpy, and crystallinity determined by DSC (XcDSC) are summarized in Table 3. It is found that the sample SEP26, Table 3. Parameters of Thermal Property and Crystallinity for SEPs sample

Tg (°C)

Tm (°C)

Tc (°C)

ΔHm (J/g)

XcDSC (%)

XcWAXD (%)


−66 −66 −64 −62

111.5 102.4 95.4 88.3

95.8 84.8 75.2 66.6

90.0 46.4 27.8 19.8

30.7 16.9 9.5 6.8

38.2 25.0 14.0 ∼7

in which the propylene content is as high as 25.9 mol %, is still crystalline. The ethylene random copolymers with such a high comonomer content are usually completely amorphous. This verifies the heterogeneity of composition distribution in SEPs. Figure 5 shows the WAXD patterns of SEPs. Two diffraction

Figure 6. (a) Lorentz-corrected SAXS profiles, and (b) onedimensional correlation functions for SEPs at room temperature. The curves in panel b are vertically shifted for clarity.

SEP samples at room temperature. The long period (L) can be calculated from the scattering vector at peak position, qmax, by L = 2π/qmax. Analysis of 1D correlation function can yield various useful structural parameters, including the long period, the thickness of the transition layer between the crystalline and amorphous phases (dtr), the thickness of the lamellar crystals (lc), the thickness of the amorphous layer (la = L − lc) and linear crystallinity (Xl). The obtained structural parameters at room temperature are summarized in Table 4. One can see that the L obtained from 1D correlation function is slightly smaller than the values calculated from Bragg’s equation for all SEPs (Figure 6a) due to the systematic error between the two methods. With increasing the comonomer content, the qmax shifts to larger value, and the peak becomes weaker. Similarly, the first maximum in the plot of γ(r) versus r shifts to a small value (Figure 6b), indicating a smaller L. Moreover, the lc is obviously decreased as well when more propylene units are enchained. Since lc is related to the melting temperature, there is good agreement between the lamellar crystal thickness determined by SAXS and the melting temperature measured by DSC. Comparing the lc’s of SEP14, OBC-A, and ZNEH with similar crystallinity (∼16% determined from DSC), one can see that the lc of SEP14 (5.3 nm) is smaller than that of OBC-A (6.8 nm), but larger than that of ZNEH (3.6 nm). This shows that the length of the crystallizable segments in SEP is shorter than that in OBC-A, but longer than that in ZNEH. Similarly, both SEP26 and OBC-B have a value of XcDSC about 7%, but the lc of SEP26 is smaller than that of OBC-B. The thickness of the transition layer or interphase between the crystalline and amorphous phases, dtr, in which the density is lower than that of the crystalline phase but greater than that of the amorphous phase, can reflect the comonomer distribution between the crystalline and amorphous segments. Usually, the crystallizable segments adjacent to the amorphous segments tend to form the transition layers, in which the segments have weaker crystallizability due to restriction of the amorphous segments or transitional composition. The presence of dtr verifies the blocky fashion of the chain structure in all the SEP samples, since our previous result shows that the transition layer is absent or very thin in ZNEH sample (random copolymer).29 The values of la for SEPs are around 10 nm (Table 4). It is striking that the la decreases slightly as the propylene content increases. This is out of our expectation, since enchainment of more comonomer units usually leads to longer noncrystalline

Figure 5. WAXD patterns of SEPs. The inset is the enlargement of the dashed box.

peaks are observed at 2θ around 21.4° and 23.4°, which correspond to the (110) and (200) reflections of orthorhombic PE crystals, respectively. The WAXD also shows that SEP26 is still crystalline. The crystallinity can also be calculated from the WAXD patterns in terms of eq 2, and the data of XcWAXD are listed in Table 3. It is observed that XcWAXD decreases with increasing propylene content. For the same sample, the value of XcWAXD is a little larger than that of XcDSC, which is frequently reported for other ethylene copolymers.28,29 The DSC curves of SEPs at low temperature range are shown in Figure S6 of the Supporting Information, and the obtained glass transition temperatures (Tgs) are given in Table 3. It is found that, SEPs exhibit a broad glass transition, possibly due to the heterogeneous composition distribution. This leads to difficulty in determination of accurate Tgs. However, the Tgs of all SEPs are between those of the neat PE (∼ −100 °C) and PP (∼10 °C), which are far below room temperature. Therefore, SEPs may be used at low temperature. The Tg increases slightly as more propylene units are enchained. 3.3. Stacking of Lamellar Crystals. Since the chain structure of SEPs is quite different from that of OBCs, the aggregation state structures of SEPs and OBCs at a larger scale will differ accordingly. The stacking of lamellar crystals around tens of nanometers was first investigated utilizing SAXS. Figure 6 shows the Lorentz-corrected SAXS profiles and the corresponding one-dimensional correlation function of four 6056


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Table 4. Structural Parameters of the SEPs Obtained from One-Dimensional Correlation Functions of SAXS at Room Temperature

a

sample

dtr (nm)

lc (nm)

la (nm)

L (nm)

L*a (nm)

Xl

Xvol

Φs


1.2 1.0 0.8 0.5

5.7 5.3 4.4 3.2

10.9 8.6 8.2 8.1

16.6 13.9 12.6 11.3

18.0 16.0 16.2 14.3

0.343 0.381 0.349 0.283

0.345 0.222 0.122 0.060

1.00 0.583 0.349 0.212

Obtained from Bragg’s equation.

segments and a thicker amorphous layer. This may be because more amorphous segments are excluded out of the interlamellae at high propylene content, which will be further discussed in the following paragraph. Comparing the las of SEPs, OBCs and ZNEH, we notice that the las of SEPs are evidently smaller than those of OBCs (∼20 nm)28 and ZNEH (∼15 nm).29 This reveals that the noncrystalline segments in SEPs are shorter than those in both OBCs and ZNEH, which may originate from the rapid oscillation of the unbridged metallocene. The linear crystallinity Xl, which reflects the relative thickness of the crystal and amorphous phases in the lamellar stacking, can be calculated based on eq 4. On the other hand, the volume fraction crystallinity Xvol can be calculated from the weight fraction crystallinity determined by WAXD in terms of eq 5. We can see from Table 4, that the values of Xl and Xvol are similar for SEP8. This shows that all the amorphous phases are almost located at the interlamellae in this sample. With increasing the propylene content in SEPs, both Xl and Xvol decrease, but Xvol decreases more rapidly. As a result, the Xl is evidently larger than Xvol for the other three SEPs with higher propylene contents. The larger Xl than Xvol indicates that there exist some amorphous phases outside the lamellar stacking, which cannot be measured by SAXS. The amount of the amorphous phase excluded out of the interlamellae can be evaluated by the parameter Φs, which is equal to Xvol/Xl (eq 6). The smaller the Φs, the larger amount of the amorphous phase excluded out of the interlamellae. As can be seen from Table 4, Φs decreases gradually with increasing propylene content, showing that more amorphous phases are excluded out of the interlamellae. Moreover, because of the evidently smaller la of SEPs, the Φs of SEPs is much smaller than that of OBCs and ZNEH at a comparable level of crystallinity. For example, the Φs of EP14 is only 0.583, whereas it is 0.813 for OBC-A28 and 0.990 for ZNEH.29 Likewise, SEP26 has a much smaller Φs than OBC-B. Therefore, there is a remarkable difference in the location of the amorphous phase among SEPs, OBCs and ZNEH, which will affect the macroscopic morphology and mechanical properties. 3.4. Macroscopic Morphology. The macroscopic morphologies of SEPs were characterized by POM and TEM, respectively. Figure 7 shows the POM photographs of four SEPs after crystallization from melt. Small spherulites with typical negative birefringence (Figure 7a,b,c) are observed in all samples except for SEP26. The birefringence intensity also decreases with increasing the propylene content. The effect of propylene content on the crystal size is difficult to discuss because of the small crystal size in SEPs. The morphologies of these SEP samples are quite different from those of OBCs. Well-developed spacing-filling spherulites with a large size are observed for OBC-B, even if the crystallinity is as low as 7%.28 By contrast, no spherulites are formed in SEP26 with similar crystallinity (Figure 7d). It is also observed that, the crystals in

Figure 7. POM micrographs of (a) SEP8, (b) SEP14, (c) SEP23, and (d) SEP26 after crystallization from melt at 10 °C/min. The scale bar in the figure is 20 μm.

ZNEH are relatively smaller, and the birefringence intensity is lower, as compared with SEP14.28 The crystalline structure of SEPs can be identified more clearly by TEM. The TEM images of SEPs are shown in Figure 8. As we can see, the morphology of SEPs changes dramatically with propylene content. Regular banded spherulites with radius of about 3 μm are formed in SEP8 (Figure 8a). Although spherulites can also be identified in SEP14, the formed spherulites are irregular and inhomogeneous (Figure 8b). For SEP23, the spherulites become more ill-developed, and bundlelike crystals, which are embryos of spherulites, can be observed (Figure 8c). As the propylene content is increased to 25.9 mol %, rod-like crystals instead of spherulites are randomly dispersed in thin film (Figure 8d). The ill-developed spherulites with an inhomogeneous structure in SEP14 and SEP23 and discrete rod-like crystals in SEP26 show that there exist lots of amorphous phases outside the interlamellae, which is consistent with the small values of Φs in these three samples (Table 4). The TEM observation further confirms that the crystal size of SEPs decreases with increasing propylene content. In contrast to the ill-developed spherulitic structure in SEP14, fibril lamellar stacks within well-developed spherulites and obvious spherulitic boundaries are observed in OBC-A.28 On the other hand, no clear spherulitic structure can be identified in ZNEH, though the lamellae are densely stacked.29 The POM and TEM characterizations reveal that the crystals of SEP14 are less perfect than those of OBC-A, but more ordered than those in ZNEH. Similarly, the crystals of SEP26 are not so ordered as those in OBC-B. Such a difference in crystal morphology is tightly related to the chain structure of these three types of 6057


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Table 5. Data of Tensile Property for SEPs and OBCs sample

XcDSC (%)

recovery from 100%

modulus (MPa)

fracture stress (MPa)

fracture strain

SEP8 SEP14 SEP23 SEP26 OBC-A OBC-B

30.7% 16.9% 9.5% 6.8% 15.7% 7.0%

24% 85% 90% 92% 88% 93%

47.1 14.6 5.0 3.4 8.2 4.3

13.7 4.1 2.0 1.3 7.6 5.4

4.5 6.6 6.5 6.4 13.8 18.5

elastic recovery property of SEPs, and the data are listed in Table 5. As we can see, except for the sample SEP8 with a high crystallinity, all the other three SEPs (SEP14, SEP23 and SEP26) exhibit a better recovery, which is comparable to that of OBCs. Comparing the tensile behaviors of SEPs and OBCs, one can see two remarkable differences. The first is the strain-hardening phenomenon. Strain-hardening is hardly observed for SEP TPEs, and stress is nearly constant after yielding, but strainhardening can be immediately observed after yielding for OBCA and OBC-B (Figure 9b). This difference may originate from the different aggregation state structure of SEPs and OBCs. In SEPs there are a larger amount of amorphous phases excluded out of the interlamellae and the value of Φs is smaller. Therefore, only amorphous phases form the continuous framework, and crystalline phases are dispersed in the amorphous phase. Upon tensile drawing, deformation is mainly contributed by the amorphous phase before fracture, while the crystalline phases in SEPs only act as physically cross-linking points and barely deform. By contrast, in OBCs, both amorphous and crystalline phases may form continuous framework because only a smaller amount of amorphous phases are excluded out of the interlamellae. As a result, the amorphous and crystalline phases deform simultaneously when tensile drawing is exerted on OBCs. The second difference lies in the fracture strain. Although the fracture strain of SEPs is quite high (>600%), it is still much smaller than that of OBCs, which can reach 1300%−1800%. The elastic of TPEs is mainly determined by the molecular weight, especially the molecular weight of segments between the cross-linking points, since the rubber elasticity of polymers mainly originates from entropic change, i.e., conformational change. The longer the polymer chain and the segments between the cross-linking points, the larger the elasticity.

Figure 8. TEM micrographs of (a) SEP8, (b) SEP14, (c) SEP23, and (d) SEP26 after being cooled from melt at a rate of 10 °C/min. The scale bar in the figure is 2 μm.

samples. The regularity of the chain structure of SEPs is intermediate between that of OBCs and ZNEH. 3.5. Tensile Behavior. The uniaxial stress−strain curves at room temperature for four SEP samples and two OBCs are shown in Figure 9. The data of mechanical property as well as the crystallinity (Xc) are summarized in Table 5. It is observed that SEP8 exhibits a typical tensile behavior of plastic with localized yielding and necking. This is in accordance with its chain structure, in which both the low and high propylene segments are crystallizable at a low P/E feed ratio. As the propylene content increases, SEPs become an elastomer, since the yielding point upon deformation is diffused and not obvious. With increasing propylene content, the modulus and fracture stress of SEPs also decrease, and the yielding point becomes more diffused. As a result, the tensile property of the SEPs can vary in a wide range from plastic to TPE, depending on the propylene content. The fracture strain of SEPs can be larger than 600% (Figure 9a). Moreover, we also examined the

Figure 9. Representative stress−strain curves of SEPs (a) and OBCs (b). 6058



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However, so far we cannot precisely determine the length of the amorphous segment between two crystalline segments in SEPs, just like in OBCs. In spite of such a difficulty, we can still quantitatively compare the overall molecular weight and the length of the amorphous segments in SEPs and OBCs. As we can see from Table 1 and Table S1, the overall molecular weight of SEPs is a little lower than those of OBCs. Moreover, the amorphous segments alternated by the crystalline segments in SEPs are shorter than the soft blocks in OBCs, which can be revealed by the evidently smaller la of SEPs (Table 4). We also notice that the fracture strains of three SEPs (SEP14, SEP23 and SEP26) are similar. This is possibly due to the similar lengths of the high propylene segments in these SEPs. In SEPs, the lengths of low and high propylene segments are mainly determined by the oscillation rate of the ligands in the metallocene catalyst, which depends on temperature but hardly varies with the P/E feed ratio. From above result we can see the fact that, as compared with commercialized OBCs, the SEPs show far less elongation at break and lack of strain-hardening behavior, so SEPs are far from practical application. The result of this work provides guidance for improving the chain structure of SEPs and thus their mechanical properties as a TPO. For example, the overall molecular weight of SEP needs to be enhanced, which may be achieved at an elevated polymerization pressure and/or with a modified cocatalyst. The lengths of the amorphous and crystalline segments should be optimized as well. This is related to the oscillation frequency of the catalyst between different geometries, which may be regulated by polymerization temperature and ligand structure. The above-mentioned work is in progress in our lab, and further results will be reported elsewhere.

Article

ASSOCIATED CONTENT

S Supporting Information *

Partial 13C NMR spectra of four SEP samples, including the molar ratio of the tetrads; determination of the first selfnucleation temperature; molecular characteristic of OBC-A, OBC-B and ZNEH; crystal cell parameters of SEPs; comparison of the melting curves of SEP14 and OBC-A; nonisothermal crystallization curves of four SEPs. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.5b01845.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Tel./Fax: +86-571-87952400. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (973 Program) (2011CB606005). The authors would also like to thank beamline BL16B1 (Shanghai Synchrotron Radiation Facility) for providing the beam time.

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REFERENCES

(1) Legge, N. R.; Holden, G.; Schmeder, H. E. Thermoplastic Elastomers: A Comprehensive Review; Oxford University: New York, 1988. (2) Müller, G.; Rieger, B. Propene Based Thermoplastic Elastomers by Early and Late Transition Metal Catalysis. Prog. Polym. Sci. 2002, 27, 815−851. (3) Domski, G. J.; Rose, J. M.; Coates, G. W.; Bolig, A. D.; Brookhart, M. Living Alkene Polymerization: New Methods for the Precision Synthesis of Polyolefins. Prog. Polym. Sci. 2007, 32, 30−92. (4) Alt, H. G.; Köppl, A. Effect of the Nature of Metallocene Complexes of Group IV Metals on Their Performance in Catalytic Ethylene and Propylene Polymerization. Chem. Rev. 2000, 100, 1205− 1222. (5) Alt, H. G.; Licht, E. H.; Licht, A. I.; Schneider, K. J. Metallacyclic Metallocene Complexes as Catalysts for Olefin Polymerization. Coord. Chem. Rev. 2006, 250, 2−17. (6) Hustad, P. D.; Kuhlman, R. L.; Arriola, D. J.; Carnahan, E. M.; Wenzel, T. T. Continuous Production of Ethylene-Based Diblock Copolymers Using Coordinative Chain Transfer Polymerization. Macromolecules 2007, 40, 7061−7064. (7) Ostoja Starzewski, A.; Steinhauser, N.; Xin, B. S. Decisive Progress in Metallocene-Catalyzed Elastomer Synthesis. Macromolecules 2008, 41, 4095−4101. (8) Cherian, A. E.; Rose, J. M.; Lobkovsky, E. B.; Coates, G. W. A C2-Symmetric, Living α-Diimine Ni (II) Catalyst: Regioblock Copolymers from Propylene. J. Am. Chem. Soc. 2005, 127, 13770− 13771. (9) Brintzinger, H. H.; Fischer, D.; Mülhaupt, R.; Rieger, B.; Waymouth, R. Stereospecific Olefin Polymerization with Chiral Metallocene Catalysts. Angew. Chem. 1995, 107, 1255−1283. (10) Cobzaru, C.; Hild, S.; Boger, A.; Troll, C.; Rieger, B. “Dual-Side” Catalysts for High and Ultrahigh Molecular Weight Homopolypropylene Elastomers and Plastomers. Coord. Chem. Rev. 2006, 250, 189− 211. (11) Harney, M. B.; Zhang, Y. H.; Sita, L. R. Discrete, Multiblock Isotactic−Atactic Stereoblock Polypropene Microstructures of Differing Block Architectures through Programmable Stereomodulated Living Ziegler-Natta Polymerization. Angew. Chem., Int. Ed. 2006, 45, 2400−2404. (12) Rose, J. M.; Deplace, F.; Lynd, N. A.; Wang, Z. G.; Hotta, A.; Lobkovsky, E. B.; Kramer, E. J.; Coates, G. W. C2-Symmetric Ni (II) αDiimines Featuring Cumyl-Derived Ligands: Synthesis of Improved

4. CONCLUSIONS 13

C NMR and SSA results verify that there exist two types of segments, i.e., low and high propylene segments, in SEPs prepared by the oscillating unbridged metallocene catalyst. However, since the propylene contents in both segments increase with increasing the P/E feed ratio during copolymerization, the composition distribution of SEPs is quite different from that of OBCs, which results in different aggregation state structures at nano- and microscales. First, the lamellar crystal thickness is smaller, and the melting temperature is lower, due to the higher comonomer content and shorter length of the low propylene segments in SEPs, as compared with OBCs with similar crystallinity. Moreover, the amorphous layer among the lamellar crystals is also thinner, and thus more amorphous phases are excluded out of the interlamellae, leading to a smaller fraction of spherulite in SEPs. The macroscopic crystalline morphology of SEPs changes from ill-developed spherulites at low propylene content into bundle crystals at high propylene content. By contrast, OBCs can form well-developed spherulites, even if the crystallinity is very low. Upon tensile drawing, SEPs can exhibit large deformation without obvious yielding, which is the typical behavior of TPEs. However, as compared with OBCs having similar crystallinity, the strain-hardening is absent in SEPs of low crystallinity, and the tensile strength and the strain at break of SEPs are relatively smaller. 6059


Article

The Journal of Physical Chemistry B Elastomeric Regioblock Polypropylenes. Macromolecules 2008, 41, 9548−9555. (13) Tian, J.; Hustad, P. D.; Coates, G. W. A New Catalyst for Highly Syndiospecific Living Olefin Polymerization: Homopolymers and Block Copolymers from Ethylene and Propylene. J. Am. Chem. Soc. 2001, 123, 5134−5135. (14) Arriola, D. J.; Carnahan, E. M.; Hustad, P. D.; Kuhlman, R. L.; Wenzel, T. T. Catalytic Production of Olefin Block Copolymers via Chain Shuttling Polymerization. Science 2006, 312, 714−719. (15) Edson, J. B.; Wang, Z. G.; Kramer, E. J.; Coates, G. W. Fluorinated Bis(phenoxyketimine) Titanium Complexes for the Living, Isoselective Polymerization of Propylene: Multiblock Isotactic Polypropylene Copolymers via Sequential Monomer Addition. J. Am. Chem. Soc. 2008, 130, 4968−4977. (16) Bensason, S.; Minick, J.; Moet, A.; Chum, S.; Hiltner, A.; Baer, E. Classification of Homogeneous Ethylene−Octene Copolymers Based on Comonomer Content. J. Polym. Sci., Part B: Polym. Phys. 1996, 34, 1301−1315. (17) Androsch, R.; Blackwell, J.; Chvalun, S. N.; Wunderlich, B. Wide- and Small-Angle X-ray Analysis of Poly(ethylene-co-octene). Macromolecules 1999, 32, 3735−3740. (18) Androsch, R.; Stribeck, N.; Lüpke, T.; Funari, S. S. Investigation of the Deformation of Homogeneous Poly(ethylene-co-1-octene) by Wide- and Small-Angle X-ray Scattering Using Synchrotron Radiation. J. Polym. Sci., Part B: Polym. Phys. 2002, 40, 1919−1930. (19) Wang, H. P.; Chum, S. P.; Hiltner, A.; Baer, E. Comparing Elastomeric Behavior of Block and Random Ethylene−Octene Copolymers. J. Appl. Polym. Sci. 2009, 113, 3236−3244. (20) Wang, H. P.; Chum, S. P.; Hiltner, A.; Baer, E. Deformation of Elastomeric Polyolefin Spherulites. J. Polym. Sci., Part B: Polym. Phys. 2009, 47, 1313−1330. (21) Minick, J.; Moet, A.; Hiltner, A.; Baer, E.; Chum, S. P. Crystallization of Very-Low-Density Copolymers of Ethylene with Alpha-Olefins. J. Appl. Polym. Sci. 1995, 58, 1371−1384. (22) Alizadeh, A.; Richardson, L.; Xu, J.; McCartney, S.; Marand, H.; Cheung, Y. W.; Chum, S. Influence of Structural and Topological Constraints on the Crystallization and Melting Behavior of Polymers. 1. Ethylene/1-Octene Copolymers. Macromolecules 1999, 32, 6221− 6235. (23) Wang, H. P.; Khariwala, D. U.; Cheung, W.; Chum, S. P.; Hiltner, A.; Baer, E. Characterization of Some New Olefinic Block Copolymers. Macromolecules 2007, 40, 2852−2862. (24) Dias, P.; Lin, Y. J.; Poon, B.; Chen, H. Y.; Hiltner, A.; Baer, E. Adhesion of Statistical and Blocky Ethylene−Octene Copolymers to Polypropylene. Polymer 2008, 49, 2937−2946. (25) Park, H. E.; Dealy, J. M.; Marchand, G. R.; Wang, J. A.; Li, S.; Register, R. A. Rheology and Structure of Molten, Olefin Multiblock Copolymers. Macromolecules 2010, 43, 6789−6799. (26) Zuo, F.; Burger, C.; Chen, X. M.; Mao, Y. M.; Hsiao, B. S.; Chen, H. Y.; Marchand, G. R.; Lai, S. Y.; Chiu, D. An in Situ X-ray Structural Study of Olefin Block and Random Copolymers under Uniaxial Deformation. Macromolecules 2010, 43, 1922−1929. (27) Zuo, F.; Mao, Y. M.; Li, X. W.; Burger, C.; Hsiao, B. S.; Chen, H. Y.; Marchand, G. R. Effects of Block Architecture on Structure and Mechanical Properties of Olefin Block Copolymers under Uniaxial Deformation. Macromolecules 2011, 44, 3670−3673. (28) Tong, Z. Z.; Huang, J.; Zhou, B.; Xu, J. T.; Fan, Z. Q. Chain Microstructure, Crystallization, and Morphology of Olefinic Blocky Copolymers. Macromol. Chem. Phys. 2013, 214, 605−616. (29) Tong, Z. Z.; Xu, J. T.; Xia, S. J.; Fan, Z. Q. Comparison of Chain Structure and Morphology of an Olefinic Blocky Copolymer and a Ziegler−Natta-Based Ethylene Random Copolymer. Polym. Int. 2013, 62, 228−237. (30) Tong, Z. Z.; Zhou, B.; Huang, J.; Xu, J. T.; Fan, Z. Q. Olefinic Blocky Copolymer/Montmorillonite Nanocomposites with Collapsed Clay Layers. Compos. Sci. Technol. 2013, 85, 111−117. (31) Tong, Z. Z.; Zhou, B.; Huang, J.; Xu, J. T.; Fan, Z. Q. Regulation of Crystallization Kinetics, Morphology, and Mechanical Properties of Olefinic Blocky Copolymers. Macromolecules 2014, 47, 333−346.

(32) Tong, Z. Z.; Zhou, B.; Huang, J.; Xu, J. T.; Fan, Z. Q. Hierarchical Structures of Olefinic Blocky Copolymer/Montmorillonite Nanocomposites with Collapsed and Intercalated Clay Layers. RSC Adv. 2014, 4, 15678−15688. (33) Tong, Z. Z.; Huang, J.; Zhou, B.; Xu, J. T.; Fan, Z. Q. SelfNucleation Behaviors of Olefinic Blocky Copolymer/Montmorillonite Nanocomposites with Collapsed and Intercalated Clay Layers. J. Appl. Polym. Sci. 2015, 132, 41771. (34) Lin, Y. J.; Yakovleva, V.; Chen, H. Y.; Hiltner, A.; Baer, E. Comparison of Olefin Copolymers as Compatibilizers for Polypropylene and High-Density Polyethylene. J. Appl. Polym. Sci. 2009, 113, 1945−1952. (35) Li, Z.; Shi, Y. J.; Liu, H. L.; Chen, F.; Zhang, Q.; Wang, K.; Fu, Q. Effect of Melting Temperature on Interfacial Interaction and Mechanical Properties of Polypropylene (PP) Fiber Reinforced Olefin Block Copolymers (OBCs). RSC Adv. 2014, 4, 45234−45243. (36) Coates, G. W.; Waymouth, R. M. Oscillating Stereocontrol: A Strategy for the Synthesis of Thermoplastic Elastomeric Polypropylene. Science 1995, 267, 217−219. (37) Wang, W.; Fan, Z. Q.; Feng, L. X. Ethylene Polymerization and Ethylene/1-Hexene Copolymerization Using Homogeneous and Heterogeneous Unbridged Bisindenyl Zirconocene Catalysts. Eur. Polym. J. 2005, 41, 2380−2387. (38) Wang, W.; Fan, Z. Q.; Feng, L. X.; Li, C. H. Substituent Effect of Bisindenyl Zirconene Catalyst on Ethylene/1-Hexene Copolymerization and Propylene Polymerization. Eur. Polym. J. 2005, 41, 83−89. (39) Yasin, T.; Fan, Z. Q.; Feng, L. X. Synthesis of Unbridged Metallocene Catalyst for Propylene Polymerization. Eur. Polym. J. 2004, 40, 517−522. (40) Aggarwal, S. L.; Tilloy, G. P. Determination of Crystallinity in Polyethylene by X-ray Diffractometer. J. Polym. Sci. 1955, 18, 17−26. (41) Strobl, G. R.; Schneider, M. J. Direct Evaluation of the Electron Density Correlation Function of Partially Crystalline Polymers. J. Polym. Sci., Part B: Polym. Phys. 1980, 18, 1343−1359. (42) Strobl, G. R.; Schneider, M. J.; Voigt-Martin, I. G. Model of Partial Crystallization and Melting Derived from Small-Angle X-ray Scattering and Electron Microscopic Studies on Low-Density Polyethylene. J. Polym. Sci., Part B: Polym. Phys. 1980, 18, 1361−1381. (43) Vonk, C. G.; Kortleve, G. X-ray Small-Angle Scattering of Bulk Polyethylene. Kolloid Z. Z. Polym. 1967, 220, 19−24. (44) Khariwala, D. U.; Taha, A.; Chum, S. P.; Hiltner, A.; Baer, E. Crystallization Kinetics of Some New Olefinic Block Copolymers. Polymer 2008, 49, 1365−1375. (45) Chen, H. L.; Wang, S. F.; Lin, T. L. Morphological Structure of Crystalline Polymer Blend Involving Hydrogen Bonding: Polycaprolactone/Poly(4-vinylphenol) System. Macromolecules 1998, 31, 8924− 8930. (46) Chiu, H. J.; Chen, H. L.; Lin, T. L.; Lin, J. S. Phase Structure of Poly(3-hydroxy butyrate)/Poly(vinyl acetate) Blends Probed by Small-Angle X-ray Scattering. Macromolecules 1999, 32, 4969−4974. (47) Tynys, A.; Saarinen, T.; Hakala, K.; Helaja, T.; Vanne, T.; Lehmus, P.; Löfgren, B. Ethylene-Propylene Copolymerisations: Effect of Metallocene Structure on Termination Reactions and Polymer Microstructure. Macromol. Chem. Phys. 2005, 206, 1043−1056. (48) Koenig, J. L. Chemical Microstructure of Polymer Chains; John Wiley & Sons: New York, 1980. (49) Schneider, M. J.; Suhm, J.; Mülhaupt, R.; Prosenc, M. H.; Brintzinger, H. H. Influence of Indenyl Ligand Substitution Pattern on Metallocene-Catalyzed Ethene Copolymerization with 1-Octene. Macromolecules 1997, 30, 3164−3168. (50) Busico, V.; Cipullo, R.; Segre, A. L. Advances in the 13C NMR Characterization of Ethene/Propene Copolymers, 1: C2-Symmetric ansa-Metallocene Catalysts. Macromol. Chem. Phys. 2002, 203, 1403− 1412. (51) Xu, J. T.; Xu, X. R.; Feng, L. X. Short Chain Branching Distributions of Metallocene-Based Ethylene Copolymers. Eur. Polym. J. 2000, 36, 685−693. (52) Xu, X. R.; Xu, J. T.; Feng, L. X.; Chen, W. Effect of Short ChainBranching Distribution on Crystallinity and Modulus of Metallocene6060


Article

The Journal of Physical Chemistry B Based Ethylene−Butene Copolymers. J. Appl. Polym. Sci. 2000, 77, 1709−1715. (53) Zhang, F. J.; Fu, Q.; Lu, T. J.; Huang, H. Y.; He, T. B. Improved Thermal Fractionation Technique for Chain Structure Analysis of Ethylene/α-Olefin Copolymers. Polymer 2002, 43, 1031−1034. (54) Zhang, F. J.; Liu, J. P.; Fu, Q.; Huang, H. Y.; Hu, Z. J.; Yao, S.; Cai, X. Y.; He, T. B. Polydispersity of Ethylene Sequence Length in Metallocene Ethylene/α-Olefin Copolymers. I. Characterized by Thermal-Fractionation Technique. J. Polym. Sci., Part B: Polym. Phys. 2002, 40, 813−821. (55) Starck, P.; Rajanen, K.; Löfgren, B. Comparative Studies of Ethylene-α-Olefin Copolymers by Thermal Fractionations and Temperature-Dependent Crystallinity Measurements. Thermochim. Acta 2002, 395, 169−181. (56) Liu, L.; Tong, Z. Z.; Xu, J. T.; Fan, Z. Q.; Yu, K. M. Composition Heterogeneity of Tetrafluoroethylene-Hexafluoropropylene Random Copolymers Characterized by Successive SelfNucleation and Annealing. J. Therm. Anal. Calorim. 2013, 114, 573− 579. (57) Hosoda, S. Structural Distribution of Linear Low-Density Polyethylenes. Polym. J. 1988, 20, 383−397. (58) Hoffman, J. D.; Miller, R. L. Kinetics of Crystallization from the Melt and Chain Folding in Polyethylene Fractions Revisited: Theory and Experiment. Polymer 1997, 38, 3151−3212. (59) Wunderlich, B. Macromolecular Physics, Vol. 3: Crystal Melting; Academic Press: New York, 1980. (60) Du, Z. X.; Xu, J. T.; Dong, Q.; Fan, Z. Q. Thermal Fractionation and Effect of Comonomer Distribution on the Crystal Structure of Ethylene−Propylene Copolymers. Polymer 2009, 50, 2510−2515.

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