Featured Crystallization Polymorphism and Memory Effect in Novel

Article pubs.acs.org/Macromolecules

Featured Crystallization Polymorphism and Memory Effect in Novel Butene-1/1,5-Hexadiene Copolymers Synthesized by PostMetallocene Hafnium Catalyst Leilei He,† Bin Wang,† Fei Yang,† Yuesheng Li,†,‡ and Zhe Ma*,† †

Tianjin Key Laboratory of Composite and Functional Materials and School of Materials Science and Engineering, Tianjin University, Tianjin 300072, P. R. China ‡ Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, P. R. China S Supporting Information *

ABSTRACT: New butene-1/1,5-hexadiene copolymers with 0−2.15 mol % methylene-1,3-cyclopentane (MCP) co-units were synthesized by dimethylpyridylamidohafnium/organoboron catalyst, which for the first time introduces structural unit of five-carbon ring type into the polybutene-1 main chain. The effects of these novel co-units on the crystallization polymorphism, and the featured memory effect that often appears above the equilibrium melting point in copolymers, were investigated with differential scanning calorimetry, wide-angle X-ray diffraction, and polarized optical microscopy. First of all, it was found that the hexagonal form I′ can directly crystallize from the melt in the copolymers possessing the two highest co-unit concentrations of 0.65 and 2.15 mol %. With just 2.15 mol % co-unit, pure form I′ crystallites were obtained by the isothermal crystallization at 65 °C, whereas pure form II appeared at 25 °C and a crystallite mixture of form I′ and form II was generated in the intermediate temperature region. Such dependence in butene-1/1,5hexadiene copolymer that high temperature favors form I′ formation is opposite to the case of butene-1/propylene copolymer, where low temperature induces more form I′. Second, the crystallization kinetics depends on the temperature of initial melt, referred to a memory effect due to the local segregation of long crystallizable segments in the heterogeneous melt. The critical temperature for observing this memory effect decreases with increasing the incorporation. Interestingly, this memory effect has no influence on the modification of the formed crystallites, though the crystallization kinetics is significantly accelerated.

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INTRODUCTION Polybutene-1 (PB-1) is a very valuable polyolefin material with remarkable physical and mechanical properties, such as high toughness, tear strength, thermal endurance, and excellent creep and impact resistance, which allow applications in highpressure tanks, pumps, and hot-water pipes.1−3 The outstanding properties of PB-1 originate from the specific structures formed within the processing processes. PB-1 is the typical polymorphic semicrystalline polymer, which can form various helical conformations and pack into different types of unit cell depending on the solidification conditions. First, from the thermodynamic point of view, the stable modification is the hexagonal crystals with 31 helical conformation.4,5 This PB-1 hexagonal crystal may exist in two variants of forms I and I′,6 which have the identical helical conformation and packing unit cell but differentiate in formation routines. Form I is that hexagonal crystal obtained by crystal−crystal phase transformation from the metastable form II via aging,7 whereas form I′ refers to the hexagonal crystal that directly crystallizes from the melt or solution.5 Although form I/I′ is the most thermodynamically stable, it is the tetragonal form II with 113 helical conformation8 that is often directly obtained from the common melt crystallization9 because of its faster kinetics than © 2016 American Chemical Society

form I. Moreover, form III has an orthorhombic unit cell with 41 helical conformation10,11 and can crystallize from dilute solution.5 Obviously, the complex polymorphism of the semicrystalline polymer is of great importance to application of final products. To control the modification of crystallites formed, copolymerization of foreign co-units into the macromolecule is a rather useful approach, which eventually is available for many polymers.12−16 For instance, Laihonen et al.14 investigated the crystallization behavior of ethylene/propylene copolymers with 2.7−11.0 mol % ethylene unit. They found that new γ phase can be formed in the copolymers, and the proportion of γ form with respect to α form (the dominant modification in the isotactic polypropylene homopolymer17) increased with ethylene content. For PB-1, the approach of copolymerization of foreign co-unit to vary the crystal modification also works. Stolte et al.18 found that in the random butene-1/propylene copolymer with more than around 10 mol % propylene co-units form I′ can directly crystallize Received: July 7, 2016 Revised: August 18, 2016 Published: September 1, 2016 6578

DOI: 10.1021/acs.macromol.6b01457 Macromolecules 2016, 49, 6578−6589

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Macromolecules from the melt. Very recently, De Rosa et al.15 showed that in the butene-1/ethylene copolymers an amount of 6 mol % ethylene unit favored the direct crystallization of form I′ from the melt state. The direct crystallization of form I′ was tentatively interpreted by the reason that the presence of ethylene co-unit can enhance the flexibility of PB-1 chain while reduce the thermal stability of metastable form II, which leads to the crystallization rate of form I′ modification competitive with that of the metastable form II in some specific conditions.2 Also using a butene-1/ethylene copolymer but with a higher ethylene unit of 9.88 mol %, the work of Wang et al. showed that form I′ appears at 50 °C if the melt temperature is 130 °C.16 It was suggested that the polymorphic selection is determined by the interplay between size of the domain segregating long crystallizable sequences (the residual heterogeneous distribution of long crystallizable sequences that were packed within lamellar crystallite but were not homogenized during the melting) and the critical sizes required for potential forms II and I′ nuclei.19 Actually, even form II is still the dominant modification in the formed crystallites; addition of extra co-unit may also significantly accelerate the transition from kinetically favored form II to thermodynamically stable form I.12,20−22 As Azzurri et al. reported,21 the time requiring the complete phase transition reduced from more than 10 days for homopolymer to few hours for butene-1/ethylene copolymers with 5.5 wt % co-unit. However, until present, the comonomers copolymerized with butene-1 all are only alkenes.12,15,20,23,24 This means that with different type of alkene comonomer, the varying structural parameter of copolymers is the length of branch introduced. Then, the followed question concerns how other type of co-unit influences polymorphism of the resultant copolymer. Therefore, in present work, a series of new butene-1 copolymers containing methylene-1,3-cyclopentane (MCP) units are specially synthesized through cyclization of 1,5-hexadiene. To the best of our knowledge, this butene-1/1,5-hexadiene (1,5HD) copolymer, even a similar butene-1 copolymer with the cyclic unit, has not been reported in the literature. In the meantime, it is recently reported that copolymerization of foreign co-unit into the main chain can introduce a strong memory effect, which appears above the equilibrium melting point of polymer and can enhance the crystallization kinetics largely.16,25,26 The experimental and simulation works performed by Reid et al.25 and Gao et al.,26 respectively, suggest that the strong memory effect above equilibrium melting point is associated with the local segregation of long crystallizable segments. In other words, copolymer crystallization selects long crystallizable sequences to build up lamellae, between which the more defective segments are aggregated. During subsequent melting of crystallites such an inhomogeneous distribution of the various crystallizable sequences requires time to diffuse back to the initial homogeneous state, so the high concentration of long crystallizable sequence raises the melting point and consequently favors the crystallization nucleation. Unexpectedly, in a butene-1/ethylene copolymer with 9.88 mol % ethylene, Wang et al.16 found the strong memory effect appeared to not only affect the crystallization kinetics largely but also vary the crystal modification, i.e., formation of I′ and its relative fraction with respect to the form II. Therefore, the intrinsic crystallization behaviors of polymorphism and memory effect need to be understood before being applied to practical products, which may in turn provide innovative ideas for the design of processing and control of product properties.27,28

On the other hand, post-metallocene hafnium catalyst can effectively diminish the introduction of stereodefect during incorporation of co-unit into the main chain29−32 and achieve close molecular weight and similar polydispersity among various polymers synthesized. Therefore, the novel butene-1/ 1,5-hexadiene random copolymers synthesized by post-metallocene hafnium catalyst in this work provide excellent model materials to focus on the influence of the type and concentration of comonomer on the polymer crystallization.

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EXPERIMENTAL SECTION

Materials. A series of new butene-1/1,5-hexadiene random copolymers were synthesized using dimethylpyridylamidohafnium catalyst and [Ph3C][B(C6F5)4] and triisobutylaluminum Al(iBu)3 cocatalysts, as illustrated in Scheme 1. The synthesis method has

Scheme 1. Illustrated Copolymerization of the Butene-1/1,5Hexadiene Copolymers Using the Hafnium/Organoboron Catalyst

been described before.30 Concerning the samples used in present work, the detailed synthesis procedures and NMR characterizations of microstructure are given in the Supporting Information. Clearly, the cyclization of 1,5-hexadiene introduces the methylene-1,3-cyclopentane (MCP) units into the main chain. Incorporation of MCP co-unit into the PB-1 main chain disturbs the regularity of macromolecules and consequently decreases the ° ). The equilibrium melting temperature of the copolymer (Tm,copo dependence of T°m,copo on co-unit concentration can be calculated with the Flory equilibrium theory as follows:34 1 1 R = ln NA o − o Tm Tm,copo ΔHu

(1)

° and Tm,copo ° are the equilibrium melting temperatures of PBwhere Tm 1 homopolymer and butene-1/1,5-HD random copolymer, respectively, R is gas constant, ΔHu is the enthalpy of fusion per mole of ideal crystalline repeating unit, and NA is the mole fraction of the crystallizable component in the random copolymer. It is known that T°m of form I of PB-1 homopolymer is 140.9 °C.35 The detailed molecular characteristics and properties are given in Table 1.

Table 1. Molecular Characteristics and Properties of PB-1 Homopolymer and Butene-1/1,5-HD Random Copolymers sample code

MCP co-unita (mol %)

Mn (kg/mol)

Mw/Mn

Xcb (%)

Tm° (form I)c (°C)

PB-1 MCP0.17 MCP0.65 MCP2.15

0 0.17 0.65 2.15

204 206 208 208

2.13 2.20 2.05 2.12

48.1 43.9 29.6 22.6

140.9 140.5 139.7 137.0

a

MCP co-unit incorporation was determined by 13C NMR. bThe melting enthalpies (Hm) and melting temperatures (Tm) of form II are obtained from the second heating scans from DSC analysis. Crystallinity is determined using Xc = Hm/ΔHmo(II) with the melting enthalpy of ideal form II crystal ΔHmo(II) = 62 J/g.33 The melting temperatures measured at 5 °C/min are 110, 98, 85, and 69 °C for PB1, MCP0.17, MCP0.65, and MCP2.15, respectively. cThe equilibrium melting temperature (Tm°) is calculated from eq 1. 6579


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Macromolecules Methods. Differential Scanning Calorimetry. Differential scanning calorimetry (DSC) measurements were carried out with a DSC1 Star System (Mettler Toledo Instruments, Switzerland) under a nitrogen atmosphere. Films with a thickness of 0.5 mm were prepared by compression molding at 180 °C for 10 min and then moved into a cold press to cool down. All film samples are aged at room temperature for around 5 months to allow the transformation from the kinetically favored form II to the thermodynamically stable form I crystals. For DSC experiments, all sample weights were approximately 5 mg. Thermal protocols employed are shown in Scheme 2. To

Scheme 2. Schematic Illustrations of Thermal Protocols for (a) Dynamic Cooling Crystallization and (b) Isothermal Crystallization

Figure 1. WAXD curves of PB-1 and butene-1/1,5-HD random copolymers obtained by compression molding after aging at room temperature for 5 months. (311) reflections at 2θ = 11.9°, 16.9°, and 18.3° are observed in polymers except MCP2.15. Clearly, the form II−I transition occurs in all samples during aging at room temperature, and the complete phase transition from form II to form I has been achieved for copolymer MCP2.15. The fraction of residual form II crystals in total crystals is estimated by

fII =

R × A(200)II × 100% A(110)I + R × A(200)II

(2)

in which A(110)I and A(200)II are the area of integrated diffraction peak of form I at 2θ = 9.9° and form II at 2θ = 11.9°, respectively, and a correction factor R = 0.36.15 For the PB-1 after aging at room temperature for 5 months, the fraction of residual form II is 13%. While for the random copolymers, the fraction of form II decreases from 10% to 5% with increasing co-unit from 0.17 to 0.65 mol %. Therefore, Figure 1 clearly shows that the presence of MCP co-unit can accelerate the phase transformation from the metastable form II to the thermodynamically stable form I crystals. Actually, it has been reported that the commercial PB-1 can complete the form II−I transformation within 10−20 days22 and incorporation of co-unit can even accelerate transformation kinetics.20,21 For polymers studied in present work, the slow transformation, which did not complete with 5 months, may be attributed to lack of sufficient driving force (internal stress along chain direction) and the resulting very slow nucleation rate of the new phase form I within the original phase form II.36

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RESULTS AND DISCUSSION Nonisothermal Crystallization. The effect of MCP counit on crystallization capability is first investigated with the DSC dynamic cooling measurements. Prior to cooling, polymers are annealed at 180 °C for 10 min to erase all sample preparation histories. This annealing temperature 180 °C is well above the equilibrium melting temperatures of the homopolymer and copolymers and is demonstrated high enough for clearance of any memory effect (data presented in next section). Figure 2a shows the DSC cooling results of homopolymer PB-1 and three random copolymers with 0.17, 0.65, and 2.15 mol % MCP incorporation. The crystallization temperature (peak temperature) of homopolymer PB-1 is around 67 °C. In the copolymer MCP0.17, incorporating only 0.17 mol % 1,5-HD comonomer into the main chain can decrease the crystallization temperature by 9 °C. The presence of MCP co-units destroys the macromolecular regularity and consequently decreases the polymer crystallization capability significantly; even the co-unit concentration is as low as 0.17 mol %. As increase co-unit from 0.17 to 0.65 mol %, the crystallization temperature is decreased from 58 to 32 °C. Moreover, copolymer MCP2.15 is not able to crystallize during

investigate the effect of melt temperature on crystallization, the initial melting temperature was varied from 110 to 180 °C, but the annealing time was fixed at 10 min. Both dynamic cooling and isothermal crystallization protocols were applied, where the former employed a mild cooling rate of 10 °C/min and the latter utilized a higher cooling rate of 30 °C/min to prevent crystallization during cooling. The isothermal crystallization was performed in the temperature range from 25 to 90 °C. In this work, the cooling process after melting is referred to as “first cooling”, and the subsequent reheating process of the formed crystallites is referred to as “second heating” (see Scheme 2a). Wide-Angle X-ray Diffraction. Wide-angle X-ray diffraction (WAXD) measurements were performed by an in-house X-ray setup employing an image plate detector (Mar 345, 2300 × 2300 pixels). The X-ray wavelength was 0.154 nm, and the sample-to-detector distance was 300 mm. For off-line WAXD measurement, the measuring time of each WAXD image was 10 min, which included exposure and record of data. For in situ WAXD measurements, collection of one WAXD image took 140 s. Figure 1 shows 1D WAXD profiles of all samples aged at room temperature for 5 months. The distinct diffraction peaks observed at 2θ = 9.9°, 17.3°, and 20.5° correspond to the crystallographic planes of (110), (300), and (220) + (211) of form I, respectively. However, the diffraction signals of form II characteristic (200), (220), and (213) + 6580


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Figure 2. (a) DSC cooling scans of homopolymer PB-1 and PB-1/1,5HD random copolymers with different comonomer contents and (b) DSC heating scan of the amorphous MCP2.15 obtained by cooling the melt after melting at 180 °C (the heating rate is 5 °C/min).

Figure 3. (a) 1D WAXD curves of PB-1, MCP0.17, and MCP0.65 samples crystallized after melt at 180 °C. (b) WAXD curves of reheating the amorphous MCP2.15 sample.

2θ = 11.9°, 16.9°, and 18.3°, respectively. The observation of only tetragonal form II in copolymers MCP0.17 and MCP0.65 for the dynamic cooling crystallization is consistent with the literature that form II is a kinetically favorable modification, and it is often formed when polymer crystallizes from melt.2,5,38 Since MCP2.15 did not crystallize in the cooling process, the subsequent heating process was monitored with WAXD (see Figure 3b), about which DSC results show the occurrence of cold crystallization. Prior to heating, the broad diffraction halos confirm the amorphous structure in MCP2.15 after cooling. Afterward, substantial diffraction peaks at 2θ = 11.9°, 16.9°, and 18.3° appear during heating, which belong to form II. Now it can be concluded that the cold crystallization of MCP2.15 is also in the metastable form II. Thus, for the butene-1/1,5-HD copolymers of interest in present work, form II is the crystal modification generated during the dynamic crystallization process, regardless of heating and cooling. However, it should be noted that within the dynamic (cooling or heating) process crystallization happens only in a limited temperature range, which is narrower than the whole range covering all available crystallization temperatures. Thus, above crystallite modifications shown in Figure 3 only correspond to the specific temperature range for dynamic (cooling/heating) crystallization, which may depend on the temperature variation rate. To reveal the polymorphism for other crystallization temperatures, isothermal crystallizations were performed at those temperatures higher than aforementioned dynamic crystallization temperatures. The isothermal crystallization of copolymer MCP2.15 will be discussed as representative. The results in Figure 3b have already shown that

this cooling process at a rate of −10 °C/min, indicated by the absence of exothermic peak during cooling to even 0 °C (see Figure 2a). The co-unit concentration of 2.15 mol % is enough to completely inhibit polymer crystallization within the dynamic cooling process at the cooling rate of 10 °C/min. It is suggested that 1,5-HD is a quite effective copolymerization comonomer to disturb the crystallization capability. Since copolymer MCP2.15 does not crystallize during the dynamic cooling process, its crystallization ability is examined by the subsequent DSC reheating experiment. Figure 2b shows the DSC result of reheating the amorphous MCP2.15 that is obtained by aforementioned cooling. With heating, an exothermic peak was identified at 17 °C, indicating the occurrence of cold crystallization. Thus, although copolymer MCP2.15 does not crystallization during a cooling process at −10 °C/min, it can crystallize in the way of cold crystallization during the subsequent heating. Polymorphism. It is known that PB-1 homopolymer and butene-1 copolymers with ethylene or propylene co-unit are the typical polymorphic crystalline polymers.12,15,16,37 In present work, the incorporation of MCP co-unit into the main chain not only can decrease the crystallization capability but also may vary the modification of crystallites formed. The crystallization modifications of PB-1, MCP0.17, and MCP0.65 crystallites that are obtained by cooling from the melt are examined by WAXD, and the results are shown in Figure 3a. With cooling process, PB-1, MCP0.17, and MCP0.65 all crystallize in tetragonal form II, indicated by the presence of three characteristic diffraction peaks of (200)II, (220)II, and (213)II + (311)II of only form II at 6581


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Macromolecules for MCP2.15 cold crystallization completes below 20 °C and generates pure form II. Now, crystallites isothermally crystallized at relatively higher temperatures from 25 to 65 °C are examined with WAXD (see Figure 4). Clearly, temperature has

Figure 5. 1D WAXD curves of isothermal crystallization at 40 °C of initial amorphous MCP2.15 melt.

Figure 5 also shows that as isothermal crystallization proceeds, the characteristic diffraction peaks (200), (220), and (213) + (311) of form II all can be identified and become more intensive with that of form I′. Clearly, both form I′ and form II crystals can grow during the isothermal process. It should be pointed out that as soon as form II appears, the spontaneous transition from form II to form I becomes possible to occur. Therefore, the increase of hexagonal phase may be complemented by (1) form I′ solely, or (2) form I via spontaneous phase transformation of form II, or (3) both. In other words, the subsequent growth of hexagonal phase during isothermal process is either a direct melt−solid crystallization or a solid−solid phase transition, or both. To distinguish the origin of increase of hexagonal phase, an annealing experiment at 40 °C was proposed for form II to check the possibility of form II−I transformation. The annealing process at 40 °C was monitored with WAXD for a long period of 50 min (see Supporting Information). This 50 min annealing duration is designed according to the crystallization time of aforementioned isothermal process at 40 °C shown in Figure 5. Neither diffraction signal of form II decreases nor characteristic diffraction peak of newly transformed form I can be observed during the 50 min annealing at 40 °C. This means that the form II crystallites, once formed, cannot transform into form I within the experimental duration of 50 min. Actually, a recent work of Qiao et al. has decomposed the PB-1 II−I phase transition into nucleation and growth two steps.36 It has been demonstrated that during the isothermal process at 40 °C the rare nucleation, associated with the lack of internal stress along the chain direction, results in a very slow transition kinetics, which is consistent with the unobservable phase transition at 40 °C in the present work. Therefore, the substantial increase of hexagonal phase at 40 °C is due to the continuous growth of form I′ from amorphous melt rather than the form II−I transition. For MCP2.15, the hexagonal phase crystallized at further higher temperature of 60 °C was directly demonstrated form I′ (see Supporting Information). Form I′ is the thermodynamically stable crystal, but form II crystallizes directly from the amorphous melt because of the kinetic reason. Under dynamic cooling process or relatively low isothermal temperature, crystallization of form II is so fast that the crystallization is finished before crystallization of form I′ starts. For MCP2.15, form II is induced with cooling crystallization as well as the isothermal crystallization at temperatures less than or equal to 25 °C. While at elevated temperatures, the difference in crystallization kinetics between form I′ and form II is not the dominant factor anymore,

Figure 4. Integrated 1D WAXD curves of copolymer MCP2.15 crystallized at different isothermal crystallization temperatures.

a great effect on the polymorphic behavior of MCP2.15. For 25 °C, the 1D WAXD profile shows strong diffraction peaks only coming from (200), (220), and (213) + (311) crystal planes of form II crystalthe same with cold crystallization below 20 °C. Increasing the isothermal temperature to 30 °C, characteristic diffraction peaks at 9.9°, 17.3°, and 20.5°, belonging to (110), (300), and (220) + (211), respectively, of hexagonal form can be identified. This means that stable hexagonal form I/I′ and unstable tetragonal form II both are generated. Moreover, the diffraction intensity of form I/I′ with respect to form II increases with increasing temperature. As soon as isothermal temperature reaches 65 °C, it is hard to distinguish form II anymore, and all crystallites are in form I/I′. According to crystal modification observed, it seems that crystallization temperatures are divided into three regimes: low temperature with only metastable form II (T ≤ 25 °C), intermediate temperature range possessing the mixed modification of forms II and I/I′ (30 ≤ T ≤ 60 °C), and high temperature generating unique stable form I/I′ (T = 65 °C). Actually, PB-1 has two types of hexagonal modification,6 termed form I or form I′ depending on the formation paths. The form I is obtained by spontaneous crystal−crystal phase transformation from metastable form II generated first,7 while form I′ corresponds to the crystallites directly from the melt or solution without the intermediate step of form II formation.5 Owing to the identical crystallographic structure, it is impossible to distinguish between form I and form I′ with the WAXD results of the final crystallites. In order to identify whether the formed crystallite is form I that is transformed from metastable form II or form I′ that is crystallized directly from melt, in situ WAXD measurements were performed to visualize the isothermal process from the amorphous polymer melt. Figure 5 shows the time-resolved WAXD results of isothermal crystallization at 40 °C, in which condition a mixture of form II with form I/I′ was found after complete crystallization. The WAXD results show that during the isothermal process at 40 °C the diffraction signal of the crystal that is first formed corresponds to the (110) crystallographic plane of hexagonal phase, without observation of form II. Thus, it can be concluded that the crystal first formed at 40 °C is form I′. 6582


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similar to MCP0.17 and homopolymer. However, a faint signal of (110) diffraction at 2θ = 9.9° of form I can be observed when isothermal crystallization temperature reaches 60 or 70 °C. The designed isothermal protocols, combining the isothermal process of amorphous melt crystallization with annealing process of form II, demonstrate that the hexagonal crystal generated is form I′ (see Supporting Information). The appearance condition of form I′ in MCP0.65 is consistent with that in MCP2.15, where the high temperature is favorable for formation of form I′. So far, we have found that MCP co-unit content has a great influence on polymorphic modification. In butene-1/1,5-HD random copolymers, high co-unit content tends to directly crystallize copolymer into form I′, and high temperature favors the formation of form I′. The direct formation of form I′ from the amorphous melt has been reported in the butene-1 copolymers with alkene counit (e.g., ethylene or propylene) and homopolymer of low stereoregularity.2,15,16,18,22 Stolte et al. found that a butene-1/ propylene copolymer with around 11 mol % propylene co-unit can form a hexagonal form I′ crystal from the melt. In that study, form I′ is more likely to appear at subambient temperatures below 310 K, and the amount of generated form I′ increases with decreasing the isothermal temperature.18 Differently, De Rosa et al.2 studied the crystal modifications of a series of PB-1 homopolymer with varying concentrations of stereodefect. They found that low cooling rate favors the formation of form I and the polymer with 3−4 mol % rr defect crystallizes directly into form I from the melt at low cooling rates. In the cooling crystallization process, the lower cooling rate is the higher crystallization temperature range is. Thus, low cooling rate in the dynamic cooling crystallization is equivalent to relatively high isothermal crystallization. However, the present work demonstrates that high temperature favors form I′ formation, and the relative fraction of form I′ on the total crystals increases with increasing the isothermal temperature. Such temperature dependence of MCP2.15 form I′ formation is opposite to that of butene-1/propylene copolymer. For any counit, the general effect is to disturb the chain regularity. However, different types of co-units may possess various sizes, which influence the packing of co-unit in the crystal lattice. Actually, whether molecular defects of co-units are incorporated into crystal unit can be understood by looking at the variation of lattice parameters with changing amounts of comonomer copolymerized. For butene-1/1,5-HD copolymer, the lattice parameter of a0 of form I is estimated using the formula by Androsch et al.,22 i.e., a0 = 3 × d(300)/cos 30°, where d(300) is the interplanar spacing of the (300) crystallographic plane of form I. Meanwhile, the lattice parameter a0 of form II is obtained from the interplanar spacing d(200) of the (200) crystal planes of tetragonal form II crystal. As shown in Figure 7, the lattice parameter a0 of form I is independent of the co-unit content, indicating the exclusion of co-unit from the crystal. Such dependence of lattice parameter a0 in forms I and II on the counit concentration in the butene-1/1,5-HD copolymer is similar to in the copolymer with ethylene as co-unit,23,24 but different from the propylene as co-unit, where the lattice parameter a0 of form I decreases with increasing propylene comonomer concentration.22,24 Form II is not the thermodynamically stable modification, but its advantage of faster growth with respect to form I causes the melt crystallization following the Ostwald’s rule of stages,40 i.e., first crystallizes into the kinetically favored form II and

whereas the thermodynamic factor is. Therefore, form I′ appears as isothermal crystallization temperature increases and even becomes the unique crystal modification for Tc = 65 °C, as shown by Figure 4. The concomitant crystallization of forms II and I′ was also observed in butene-1/propylene copolymers.22 It was suggested that copolymerization of propylene comonomer may both lower the energy barrier for direct crystallization of form I′ from the amorphous melt and accelerate the II−I transformation driven thermodynamically.22,39 For the butene-1/1,5-hexadiene studied in present work, our annealing experiments show that the II−I transition at 40 °C is too slow to be detected within the experimental duration of 50 min, even for the copolymer with the highest counit concentration of 2.15 mol %. Thus, it seems that the concomitant crystallization of forms II and I′ mainly results from the lowering of energy barrier for direct evolution of form I′ from the amorphous melt. For other two copolymers MCP0.17 and MCP0.65, Figure 6 shows isothermal crystallization at the temperatures over the

Figure 6. 1D WAXD curves of (a) MCP0.17 and (b) MCP0.65 obtained by the isothermal crystallization at different temperatures after melt at 180 °C.

dynamic cooling crystallization range. Only metastable form II is found in MCP0.17 after completion of isothermal crystallizations at 70, 80, and 90 °C. Further higher temperature, which is already located in the melting region of MCP0.17 (DSC result given in the Supporting Information), was not studied. It seems that MCP0.17 crystallizes only into form II at its available crystallization temperatures, of which the crystallization polymorphism is more like the homopolymer2,5,38 but different from the MCP2.15 with a higher counit concentration. For copolymer MCP0.65 at 55 °C, still only form II is observed as in the dynamic cooling crystallization, 6583


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kinetics is just mildly over form II kinetics, so form II can also appear within the experimental duration before form I′ completes the crystallization. Above crystallization mechanism of random copolymers is consistent with that of direct formation of PB-1 form I′ in the ultrathin films.41 The film is so thin that film thickness for formation of metastable form II may not be satisfied especially for high isothermal crystallization temperatures. So the formation of tetragonal form II is restricted, while much thinner and thermodynamically stable form I′ crystal is favor to develop. Also interesting is that for the cyclic co-unit the lattice parameter a0 of form II slightly decreases with increasing the concentration, suggesting the co-unit may be incorporated into the crystal lattice of form II. This means that after including the cyclic co-unit into the main chain, the lattice dimension is reduced. Apparently, arrangement size of MCP co-unit is even smaller than the butene-1 unit, though the MCP co-unit has two carbons more than the butene-1 unit. The above in situ WAXD measurement reveals that at 40 °C the crystallization kinetics of form I′ is faster than form II. The following question concerns the heating resistance of forms I′ and II. It is widely accepted that form II is metastable, but form I′ is also thought to have a poor stability because of the low level of crystal perfection as well as very thin lamellar crystals.42 To understand their heating stabilities, the mixture of form I′ and form II was heated (see Figure 8), which was obtained by

Figure 7. Dependence of lattice parameter a0 on the concentration of MCP co-unit for form I crystals aged for 5 months (circle) and 10 months (triangle) and form II crystal (square) in the butene-1/1,5-HD random copolymers.

subsequently transforms into form I at the end. The main reason for form II crystallization from the melt is its fast growth kinetics. However, copolymerizing co-units into the PB-1 main chain may vary the kinetic advantage of form II from the following aspects. First of all, in the copolymer molecular regularity is disturbed, and consequently the crystallization kinetics of form II can be largely decreased. This was already demonstrated by the substantial drop of dynamic cooling crystallization temperature in Figure 2. The significant depression of crystallization kinetics is likely to diminish the kinetic superiority of form II with respect to form I. Second, the presence of co-unit in the PB-1 main chain shortens the average length of crystallizable segments. The higher the co-unit concentration is the shorter the average length of crystallizable chain length is. With shortening the molecular crystallizable segments, both nucleation and crystal growth processes need to look for those crystallizable segments long enough to meet the requirement of the minimum lamellar thickness. So the collection of sufficiently long crystallizable segments among all chains may slow down nucleation and growth processes and even determine the occurrence of crystallization, especially for high co-unit concentration and high temperatures. An extreme case, which most likely appears in the high co-unit concentration and at high temperature, is that the length of crystallizable segments is too short to build critical nuclei/lamellae with minimum thickness, and as a consequence, this type of crystal cannot form at all. The theoretical calculation made by De Rosa et al. 2 and experimental study done by Wang et al.16 both suggest that at the same temperature the minimum lamellar thickness required for crystallization is thicker for form II than for form I′. In addition, this difference in minimum lamellar thickness enlarges with rising the crystallization temperature. Then it seems that formation of the kinetically favored metastable form II is more vulnerable to the addition of comonomer and increase of isothermal temperature than formation of form I′ crystallites that is thermodynamically stable. Thus, with increasing the content of MCP co-units and rising isothermal temperature, form I′ is more favored to crystallize directly from the melt, as shown by Figures 4 and 6. The observation of form I′ (110) diffraction prior to those of form II demonstrates that at 40 °C the reduced growth of form II is already slower than that of form I′. Now the form II has no kinetic superiority anymore with respect to form I′, whereas it is form I′ that grows faster. Of course, at 40 °C the form I′ crystallization

Figure 8. 1D WAXD curves of MCP2.15 during second heating process after crystallized at 40 °C with the melting at 150 °C.

isothermal crystallization at 40 °C. The diffraction peaks at 2θ = 9.9° and 11.9° belonging to form I′ (110)I plane and form II (200)II plane, respectively, both gradually decrease with heating. Unexpectedly, when heated to 79 °C, the (200) diffraction of form II completely disappears and only (110) diffraction for form I′ survives, demonstrating a higher heating stability of form I′ than form II. Very often form I′ is thought to be less stable than form II,18,43 whereas in this work form I′ is distinctly more resistant to heating than the metastable form II in specific copolymers. However, it should be pointed out that difference in crystal heating resistance cannot directly compare thermodynamic stabilities for the original forms I′ and II generated at 40 °C because the melting and recrystallization can occur during heating and consequently improve the heating stability of initial crystallites. In other words, whether the surviving form I′ is the original crystallites generated at 40 °C or the newly developed crystallites by recrystallization cannot be distinguished by the in situ WAXD experiment with a relatively low temperature resolution employed in the present 6584


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equilibrium melting temperature of form I of copolymer MCP0.17 estimated, Tm°(MCP0.17), is only 140.5 °C, much lower than the aforementioned critical temperature causing enhancement of crystallization kinetics, 165 °C. Thus, the memory effect above equilibrium melting temperature appears in this butene-1/1,5-HD copolymers. For MCP2.15, the amorphous melt relaxed completely does not crystallize during the dynamic cooling process at −10 °C/ min. Interestingly, Figure 9b shows that its cooling crystallization can occur after melt at temperature ≤130 °C. As the melt temperature is further lowered, the crystallization peak becomes more prevalent and the peak temperature increases. Note that for form I Tm°(MCP2.15) estimated with Flory equilibrium model is 137 °C. Thus, the memory effect of MCP2.15 appears below the equilibrium melting point. It should be pointed out that absence of crystallization peak during the cooling after melt above 135 °C cannot exclude the existence of memory effect, since possibly memory effect is too weak to trigger crystallization during the dynamic cooling process considering the poor crystallization ability of copolymer MCP2.15. In this case, the cold crystallization temperature may be an alternative indicator for the occurrence memory effect because amorphous MCP2.15, which does not crystallize during cooling, may crystallize during the followed heating process. Thus, the heating processes of sample cooled after melt at various temperatures were examined (see results in Supporting Information). For the cold crystallization kinetics, a critical temperature of 130 °C was identified to deviate from that of the completely relaxed melt, which is the same with above determined from cooling crystallization. Now it is clear that the content of co-unit has an influence on the occurrence of memory effect. Thus, for all polymers synthesized in present work, the dependence of crystallization temperature on melt temperature is summarized in Figure 10.

work. However, this heating resistance comparison between form II and form I′ presented in this work is very crucial for researchers to interpret thermal behaviors during heating, such as the DSC heating results. Memory Effect. Recently, a memory effect above the equilibrium melting point (Tm°) was found in the random copolymer.16,25,26 It was reported that the local segregation of crystallizable segments in crystal region leads to a heterogeneous distribution of crystallizable segments when melted and as a consequence, enhances crystallization kinetics. Whether and how the memory effect occurs may provide a theoretical foundation for processing and structure designs; therefore, the specific memory effect in the new copolymers synthesized for present work is explored using DSC and WAXD measurements, of which the experimental protocols are shown in Scheme 2. Figure 9 illustrates the DSC cooling results of typical

Figure 10. Nonisothermal crystallization temperature is plotted as a function of initial melt temperature for homopolymer PB-1 (square) and butene-1/1,5-HD random copolymers with 0.17 mol % (circle), 0.65 mol % (triangle), and 2.15 mol % (diamond) co-unit. The filled symbols represent the cooling crystallization, and open ones represent cold crystallization during heating. Short vertical lines are the equilibrium melting temperatures calculated for form I of various copolymers.

Figure 9. DSC cooling scans of (a) MCP0.17 and (b) MCP2.15 after melt at different temperatures. The cooling rate is 10 °C/min.

copolymers MCP0.17 and MCP2.15 after melt at different temperatures. For copolymer MCP0.17, when the melt temperatures are 165 °C or higher, the crystallization temperatures are all approximately 58 °C, independent of the melt temperature, indicating that all previous sample histories are erased. However, in the temperature range below 165 °C, the crystallization temperature begins to increase with decreasing the melt temperature. The acceleration of crystallization kinetics is associated with the memory effect, which is some residual ordering or local segregation of the crystallizable segments due to the incomplete clearance of previous crystallites. Using the Flory equilibrium method, the

It can be seen that for PB-1, if melt temperature exceeds its equilibrium melting point for form I, Tm°(PB-1) = 140.9 °C, the cooling crystallization temperature Tc is almost the same, around 69 °C. The observation of identical crystallization temperature indicates that melting PB-1 at high temperature above the equilibrium melting temperature destroys all possible orderings, which are associated with the former crystallites and 6585


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compares the nucleation densities of MCP0.17 crystallization at 85 °C after melt at 180 and 150 °C, i.e., without and with memory effect. Clearly, nucleation density after melt at 150 °C is much larger than that after melt at 180 °C, demonstrating that the memory effect significantly increases the nucleation density. In the meantime, it is known that the crystallization includes nucleation and growth two steps. Thus, the linear growth rates of spherulites at 85 °C are also measured for the above two crystallization processes, and the linear growth rates estimated are very close for melt temperatures of 180 and 150 °C, respectively (see the microscopy data in Supporting Information). It is not clear yet why the linear growth rate is not varied significantly in the heterogeneous melt, since increased local melt temperature and segregation of long crystallizable sequences seem also likely to favor the crystal growth. Anyway, the microscopy result in Figure 11 does safely clarify that the enhancement of crystallization kinetics is mainly contributed by the raise of nucleation density. Second, the critical temperature for observing the memory effect (indicated by crystallization temperature deviation) decreases with increasing co-unit concentration. This co-unit concentration dependence of memory effect in butene-1/1,5HD is consistent with finding by Reid et al.25 on ethyl branched polyethylene. It was speculated that with increasing co-unit concentration the crystallinity and lamellar thickness of crystallites formed decrease, as well reported in the literature.50−53 Thus, the restriction effect of these crystallites within lower crystallinity and thinner lamellae on the homogenization of molecular segments becomes weaker, which of course requires a relatively lower temperature to be erased. Third, the origin of memory effect is more complex for copolymer MCP2.15, considering that its critical temperature for memory effect of 130 °C is lower than form I Tm°(MCP2.15) = 137 °C. At temperatures lower than Tm°, crystalline ordering like tiny crystals and helical conformation may survive and contribute to the memory effect as well as the heterogeneous distribution of the crystallizable segments. Actually, PB-1 is a featured model polymer for study on memory effect, since existence of residual ordering can vary its crystal modification. Su et al. utilized FTIR to study the memory effect from form I and found that the residual form I ordering favors a formation of form I, where the completely relaxed melt crystallizes into form II.54 In addition, Cavallo et al. also found that the residual form I self-nuclei can induce formation of form I′.55,56 Therefore, the modifications of MCP2.15 crystallites formed with and without memory were examined to see whether the memory effect is caused by the residual crystallites ordering. The isothermal protocols at various temperatures from 25 to 60 °C were employed to form crystallites, instead of the dynamic cooling crystallization protocol. The reason for choosing isothermal protocol is to rule out the influence of kinetic enhancement, i.e., the increase of crystallization temperature, on the crystal modification. The crystallization kinetics is significantly accelerated by lowing the melt temperature from 150 to 110 °C for all isothermal temperatures applied; see the half-time of crystallization (t1/2) shown in the Supporting Information. It is clear that some memory effect is caused by melting the crystallites at a relatively low temperature of 110 °C.

favorable for the subsequent nucleation. In this case, crystallization kinetics is determined by the intrinsic heterogeneous nucleation of polymer. For MCP0.17, the critical temperature for memory effect is 165 °C. Further lowering melt temperature from 165 to 130 °C, crystallization temperature continuously shifts from 58 to 62 °C, showing that the strength of memory effect varies with melt temperature. However, for MCP0.65, the critical temperature for deviating from the completely relaxed melt is 145 °C, much lower than 165 °C for copolymer MCP0.17. Obviously, the critical temperature for MCP2.15 is lowest. Also note that the acceleration of crystallization kinetics increases cooling crystallization temperature but decreases cold crystallization temperature, so the filled and open symbols in Figure 10 deviate toward the opposite directions. In summary, the memory effect is observed above form I Tm° for MCP0.17 and 0.65 but below form I Tm° for MCP2.15. Note that above critical temperatures for observing memory effect are based on the fixed annealing time of 10 min. In a butene-1/ethylene copolymer containing 9.88 mol % ethylene co-units, Wang et al. have already found that at 150 °C the memory effect after melting for 600 min is weaker than that for 10 min.16 This means that the annealing duration can influence the strength of memory effect. Thus, for various annealing durations, the critical temperatures may be different. First of all, for copolymers MCP0.17 and MCP0.65, the critical temperature of memory effect is higher than the corresponding Tm° of form I. In principle, any crystallites without external field like stretching cannot survive above the equilibrium melting temperature. The origin of aforementioned memory effect observed above Tm° in random copolymer is different from those below Tm° in the literature that is associated with the remains of ordering from previous crystallites,28,44−49 which may be the residual crystallite, segmental conformation, and even local segregation of crystallizable sequences.25,26 The experimental and modeling studies have been respectively carried out by Reid et al.25 and Gao et al.26 on the molecular mechanism of memory effect above the equilibrium melting temperature for butene-1/ ethylene copolymers. It was speculated that the crystallizable sequences are segregated locally, resulting in a heterogeneous distribution of crystallizable sequences. The high concentration of long crystallizable segments causes an increase of the local melting temperature from the thermodynamic point of view and also raises the contact possibility of the long crystallizable sequences from the kinetic aspect. These two effects resulting from local segregation of long crystallizable segments favor nucleation. Therefore, the resultant higher concentration of crystallizable sequences accelerates the nucleation before recovering back to the ideal homogeneous melt. Figure 11

Figure 11. Micrographs of MCP0.17 crystallization at 85 °C for 11 min after melt at 180 and 150 °C. 6586


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identical temperature, even the melt temperatures are different. Unexpectedly, it was indicated that memory effect has no influence on polymorphism of isothermal crystallization, though the crystallization kinetics was largely accelerated (shown in the Supporting Information). In other words, the memory effect is only concerned about the crystallization kinetics, and the polymorphism is the intrinsic property of random copolymer, independent of the thermal histories. In PB-1, Su et al.54 found that the “memorized ordered melt” from incomplete melting of form I crystallites can lead to the recovery of the original crystal modification of form I, and the physical reason was associated with the residual orientation and 3/1 helical conformation. If the conformation ordering is also the reason for memory effect in present work, the polymer melt with memory effect should be more likely to crystallize into form I with crystallization. However, the results in Figure 12b demonstrate that polymorphism of isothermal crystallization does not depend on the memory effect, though the memory effect does significantly accelerate kinetics. It may be inferred that the memory effect appeared in present random copolymers is not due to the conformation ordering. Thus, it is reasonable to infer that the heterogeneous distribution of crystallizable segments may still be the main cause to memory effect in MCP2.15, though the critical temperature for appearance of memory effect is below Tm° of form I. In the ethyl branched polyethylene, Reid et al.25 found that the highest concentration of branches has a critical memory effect temperature that is below Tm°. Because of the fact that in polyethylene the contributions from heterogeneous melt and survival crystal ordering are hard to be distinguished, the memory effect that results from heterogeneous melt was thought to vanish. With the butene-1 polymers in present work, it is reasonable to infer the memory effect of heterogeneous melt still occurs. Actually, this is also consistent with the physical mechanism how heterogeneous melt works as a memory effect. The heterogeneous melt in random copolymer is mainly related to the local segregation of crystallizable segments. In the ideal homogeneous melt of the random copolymer, the distribution of co-units should be random as along the main chain. However, when the random copolymers crystallize, the crystallizable segments pack into the crystal lattice and consequently form local regions rich of these crystallizable segments, which after melting results in a heterogeneous melt in the sense of spatial distribution of crystallizable units and counits. In case that due to the poor mobility or/and insufficient relaxation time these local segregation of crystallized segments cannot be erased completely, the segments accumulated in these segregations have the longer length crystallizable than the average chains, and consequently the local melting point may be raised. The resulting raise of melting point causes an enhancement of nucleation, accelerating crystallization kinetics. So this means that the memory effect associated with heterogeneous melt only changes the distribution of long crystallizable segments but is not able to vary their length, which is the intrinsic property of the molecules. For a specific temperature, the minimum lamellar thickness required for crystallization is determined by the temperature, as discussed in previous section. Therefore, crystal modification, i.e., the crystallite polymorphism, is not influenced by occurrence of memory effect.

The corresponding WAXD curves of crystallites isothermally crystallized at various temperatures from 25 to 65 °C, after melt at 110 °C (i.e., with the memory effect melt), are given in Figure 12a. It can be seen that temperature has a significant

Figure 12. (a) Selected 1D WAXD curves of copolymer MCP2.15 isothermally crystallized at different crystallization temperatures after melt at 110 °C and (b) the relative fraction of MCP2.15 form I′ on total crystals after melt at 110 °C (triangle), 150 °C (circle), and 180 °C (diamond).

influence on crystal modification formed, where low temperature favors formation of metastable form II crystals. Pure form II crystals are formed at the lowest isothermal temperature applied of 25 °C, and form I′ crystals appear when the temperature is raised to 30 °C. The relative intensity of characteristic diffraction peak at 2θ = 9.9° corresponding to form I′ is strengthened with the increase of isothermal crystallization temperature with respect to that at 2θ = 11.9° for form II. This means that with further increasing the isothermal crystallization temperature, the fraction of form II in the total crystal decreases gradually. At 60 °C, no form II crystals can be identified anymore, and all crystals are in form I′. Even with memory effect, the formation trend of form II decreases as raise isothermal temperature, which is consistent with the case of no memory effect after melt at 150 °C (see Figure 4). The quantified fractions of form I′ in the total crystal are compared between various melt temperatures of 110 and 150 °C in Figure 12b. The temperatures 110 and 150 °C are the typical melt states with and without memory effect, respectively. Besides, with the melt temperature of 180 °C, three typical low, intermediate, and high crystallization temperatures of 25, 40, and 60 °C, respectively, are examined for a check on the crystallization with no memory effect. Interestingly, the fractions of form II are almost the same at the 6587


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CONCLUSIONS A series of well-defined butene-1/1,5-hexadiene random copolymers with varying co-unit concentrations from 0 to 2.15 mol % and high tacticity and narrow polydispersity were synthesized by using the post-metallocene hafnium catalyst. The presence of methylene-1,3-cyclopentane co-units disturbed the chain regularity and significantly decreased the crystallization kinetics. Even in a copolymer with just 2.15 mol % MCP co-units, the crystallization was completely inhibited during the dynamic cooling process at the cooling rate of 10 °C/min and can occur in the way of cold crystallization with the subsequent heating. The WAXD measurements revealed that the polymorphism of crystallization from the melt, i.e., kinetically favored form II or thermodynamically stable form I′, depends on the concentration of MCP co-unit and crystallization temperature. For polymers with 0−0.65 mol % co-units, the dynamic cooling crystallization at the cooling rate of 10 °C/min is in tetragonal form II, of which only the isothermal crystallization of MCP0.65 at relatively high temperatures of 60 and 70 °C leads to appearance of hexagonal form I′ of very low fraction. Interestingly, when the co-unit concentration reaches 2.15 mol %, the pure form II or form I′ is obtained by isothermal crystallization at temperatures ≤25 °C or =65 °C, respectively. Between 30 and 60 °C, a mixture of form II and form I′ is found, and the relative fraction of form I′ increases with rising temperature. Moreover, at the intermediate temperature of 40 °C, form I′ appears earlier than form II during the isothermal crystallization. Copolymerizing MCP co-unit into the main chain also introduces a memory effect in copolymers, even above the equilibrium melting temperature. The critical occurrence temperatures of this memory effect are 165 and 145 °C in copolymer MCP0.17 and MCP0.65, respectively. The microscopy results demonstrate that the resulting acceleration of crystallization kinetics from the memory effect is mainly associated with the increase of nucleation density. For MCP2.15, the memory effect appears below equilibrium melting temperature. It was found that in MCP2.15 the memory effect only accelerates crystallization kinetics but does not vary polymorphism of crystallites generated, indicating that the memory effect in MCP2.15, though below Tm° of form I, does not contain the residual crystal or helical ordering.

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and discussions. Z.M. also thanks Prof. Gerrit W. M. Peters for the manuscript reading.

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(1) Luciani, L.; Seppälä, J.; Löfgren, B. Poly-1-butene: Its preparation, properties and challenges. Prog. Polym. Sci. 1988, 13, 37−62. (2) De Rosa, C.; Auriemma, F.; Ruiz de Ballesteros, O.; Esposito, F.; Laguzza, D.; Di Girolamo, R.; Resconi, L. Crystallization Properties and Polymorphic Behavior of Isotactic Poly(1-Butene) from Metallocene Catalysts: The Crystallization of Form I from the Melt. Macromolecules 2009, 42, 8286−8297. (3) Liu, Y. P.; Cui, K. P.; Tian, N.; Zhou, W. Q.; Meng, L. P.; Li, L. B.; Ma, Z.; Wang, X. L. Stretch-Induced Crystal−Crystal Transition of Polybutene-1: An in Situ Synchrotron Radiation Wide-Angle X-ray Scattering Study. Macromolecules 2012, 45, 2764−2772. (4) Natta, G.; Corradini, P.; Bassi, I. W. Crystal structure of isotactic poly-alpha-butene. Nuovo Cimento 1960, 15, 52−67. (5) Holland, V. F.; Miller, R. L. Isotactic Polybutene-1 Single Crystals: Morphology. J. Appl. Phys. 1964, 35, 3241−3248. (6) Goldbach, G.; Peitscher, G. Infrared investigations of the polymorphic modifications of polybutene-1. J. Polym. Sci., Part B: Polym. Lett. 1968, 6, 783−788. (7) Danusso, F.; Gianotti, G. Isotactic polybutene-1: Formation and transformation of modification 2. Makromol. Chem. 1965, 88, 149− 158. (8) Jones, A. T. Polybutene-1 − type II crystalline form. J. Polym. Sci., Part B: Polym. Lett. 1963, 1, 455−456. (9) Gohil, R. M.; Miles, M. J.; Petermann, J. On the molecular mechanism of the crystal transformation (tetragonal-hexagonal) in polybutene-1. J. Macromol. Sci., Part B: Phys. 1982, 21, 189−201. (10) Dorset, D. L.; McCourt, M. P.; Kopp, S.; Wittmann, J. C.; Lotz, B. Direct determination of polymer crystal structures by electron crystallography − isotactic poly(1-butene), form (III). Acta Crystallogr., Sect. B: Struct. Sci. 1994, 50, 201−208. (11) Lotz, B.; Thierry, A. Spherulite Morphology of Form III Isotactic Poly(1-butene). Macromolecules 2003, 36, 286−290. (12) Gianotti, G.; Capizzi, A. Butene-1/propylene copolymers. Influence of the comonomerie units on polymorphism. Makromol. Chem. 1969, 124, 152−159. (13) Turner-Jones, A. Development of the γ-crystal form in random copolymers of propylene and their analysis by dsc and x-ray methods. Polymer 1971, 12, 487−508. (14) Laihonen, S.; Gedde, U. W.; Werner, P. E.; Martinez-Salazar, J. Crystallization kinetics and morphology of poly(propylene-statethylene) fractions. Polymer 1997, 38, 361−369. (15) De Rosa, C.; Ruiz de Ballesteros, O.; Auriemma, F.; Di Girolamo, R.; Scarica, C.; Giusto, G.; Esposito, S.; Guidotti, S.; Camurati, I. Polymorphic Behavior and Mechanical Properties of Isotactic 1-Butene−Ethylene Copolymers from Metallocene Catalysts. Macromolecules 2014, 47, 4317−4329. (16) Wang, Y. T.; Lu, Y.; Zhao, J. Y.; Jiang, Z. Y.; Men, Y. F. Direct Formation of Different Crystalline Forms in Butene-1/Ethylene Copolymer via Manipulating Melt Temperature. Macromolecules 2014, 47, 8653−8662. (17) Brückner, S.; Meille, S. V.; Petraccone, V.; Pirozzi, B. Polymorphism in isotactic polypropylene. Prog. Polym. Sci. 1991, 16, 361−404. (18) Stolte, I.; Cavallo, D.; Alfonso, G. C.; Portale, G.; Drongelen, M. V.; Androsch, R. Form I′ crystal formation in random butene-1/ propylene copolymers as revealed by real-time X-ray scattering using synchrotron radiation and fast scanning chip calorimetry. Eur. Polym. J. 2014, 60, 22−32. (19) Wang, Y. T.; Liu, P. R.; Lu, Y.; Men, Y. F. Mechanism of polymorph selection during crystallization of random butene-1/ ethylene copolymer. Chin. J. Polym. Sci. 2016, 34, 1014−1020. (20) Jones, A. T. Cocrystallization in copolymers of α-olefins II Butene-1 copolymers and polybutene type II/I crystal phase transition. Polymer 1966, 7, 23−59.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b01457. Figures S1−S8 (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (Z.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (21234006, 51573132) and Tianjin Natural Science Foundation (16JCQNJC02700). We thank Prof. Yongfeng Men for the help in DSC measurements 6588


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Macromolecules

(41) Zhang, B.; Yang, D. C.; Yan, S. K. Direct formation of form I poly(1-butene) single crystals from melt crystallization in ultrathin films. J. Polym. Sci., Part B: Polym. Phys. 2002, 40, 2641−2645. (42) Boor, J.; Youngman, E. A. Polymorphism in poly-1-butene: Apparent direct formation of modification I. J. Polym. Sci., Part B: Polym. Lett. 1964, 2, 903−907. (43) Lee, K. H.; Snively, C. M.; Givens, S.; Chase, D. B.; Rabolt, J. F. Time-Dependent Transformation of an Electrospun Isotactic Poly(1butene) Fibrous Membrane. Macromolecules 2007, 40, 2590−2595. (44) Supaphol, P.; Lin, J. Crystalline memory effect in isothermal crystallization of syndiotactic polypropylenes: effect of fusion temperature on crystallization and melting behavior. Polymer 2001, 42, 9617− 9626. (45) Massa, M. V.; Lee, M. S. M.; Dalnoki-Veress, K. Crystal nucleation of polymers confined to droplets: Memory effects. J. Polym. Sci., Part B: Polym. Phys. 2005, 43, 3438−3443. (46) Maus, A.; Hempel, E.; Thurn-Albrecht, T.; Saalwächter, K. Memory effect in isothermal crystallization of syndiotactic polypropylene –Role of melt structure and dynamics? Eur. Phys. J. E: Soft Matter Biol. Phys. 2007, 23, 91−101. (47) Xu, J. J.; Ma, Y.; Hu, W. B.; Rehahn, M.; Reiter, G. Cloning polymer single crystals through self-seeding. Nat. Mater. 2009, 8, 348− 353. (48) Martins, J. A.; Zhang, W.; Brito, A. M. Origin of the melt memory effect in polymer crystallization. Polymer 2010, 51, 4185− 4194. (49) Li, X. Y.; Su, F. M.; Ji, Y. X.; Tian, N.; Lu, J.; Wang, Z.; Qi, Z. M.; Li, L. B. Influence of the memory effect of a mesomorphic isotactic polypropylene melt on crystallization behavior. Soft Matter 2013, 9, 8579−8588. (50) Alamo, R.; Domszy, R.; Mandelkern, L. Thermodynamic and structural properties of copolymers of ethylene. J. Phys. Chem. 1984, 88, 6587−6595. (51) Alamo, R. G.; Mandelkern, L. Thermodynamic and structural properties of ethylene copolymers. Macromolecules 1989, 22, 1273− 1277. (52) Alamo, R. G.; Mandelkern, L. Crystallization kinetics of random ethylene copolymers. Macromolecules 1991, 24, 6480−6493. (53) Alamo, R. G.; Viers, B. D.; Mandelkern, L. Phase structure of random ethylene copolymers: a study of counit content and molecular weight as independent variables. Macromolecules 1993, 26, 5740−5747. (54) Su, F. M.; Li, X. Y.; Zhou, W. M.; Zhu, S. S.; Ji, Y. X.; Wang, Z.; Qi, Z. M.; Li, L. B. Direct Formation of Isotactic Poly(1-butene) Form I Crystal from Memorized Ordered Melt. Macromolecules 2013, 46, 7399−7405. (55) Cavallo, D.; Gardella, L.; Portale, G.; Müller, A. J.; Alfonso, G. C. Self-nucleation of isotactic poly(1-butene) in the trigonal modification. Polymer 2014, 55, 137−142. (56) Cavallo, D.; Zhang, L.; Sics, I.; Alfonso, G. C.; Dumas, P.; Marco, C.; Ellis, G. The morphology and polymorphism of selfnucleated trigonal isotactic poly(1-butene) studied by synchrotron IR microspectroscopy. CrystEngComm 2016, 18, 816−828.

(21) Azzurri, F.; Alfonso, G. C.; Gómez, M. A.; Martì, M. C.; Ellis, G.; Marco, C. Polymorphic Transformation in Isotactic 1-Butene/ Ethylene Copolymers. Macromolecules 2004, 37, 3755−3762. (22) Androsch, R.; Hohlfeld, R.; Frank, W.; Nase, M.; Cavallo, D. Transition from two-stage to direct melt-crystallization in isotactic random butene-1/propene copolymers. Polymer 2013, 54, 2528−2534. (23) Stolte, I.; Androsch, R. Kinetics of the melt − Form II phase transition in isotactic random butene-1/ethylene copolymers. Polymer 2013, 54, 7033−7040. (24) Stolte, I.; Androsch, R. Comparative study of the kinetics of non-isothermal melt solidification of random copolymers of butene-1 with either ethylene or propylene. Colloid Polym. Sci. 2014, 292, 1639− 1647. (25) Reid, B. O.; Vadlamudi, M.; Mamun, A.; Janani, H.; Gao, H.; Hu, W.; Alamo, R. G. Strong Memory Effect of Crystallization above the Equilibrium Melting Point of Random Copolymers. Macromolecules 2013, 46, 6485−6497. (26) Gao, H. H.; Vadlamudi, M.; Alamo, R. G.; Hu, W. B. Monte Carlo Simulations of Strong Memory Effect of Crystallization in Random Copolymers. Macromolecules 2013, 46, 6498−6506. (27) Khanna, Y. P.; Kumar, R.; Reimschuessel, A. C. Memory effects in polymers. III. Processing history vs. crystallization rate of nylon 6 comments on the origin of memory effect. Polym. Eng. Sci. 1988, 28, 1607−1611. (28) Alfonso, G. C.; Ziabicki, A. Memory effects in isothermal crystallization II. Isotactic polypropylene. Colloid Polym. Sci. 1995, 273, 317−323. (29) Shi, X. C.; Wang, Y. X.; Liu, J. Y.; Cui, D. M.; Men, Y. F.; Li, Y. S. Stereospecific Cyclopolymerization of α,ω-Diolefins by Pyridylamidohafnium Catalyst with the Highest Activity. Macromolecules 2011, 44, 1062−1065. (30) Shi, X. C.; Tang, X. Y.; Li, Y. S. Random copolymers of propylene with 1,5-hexadiene containing only cyclopentane units in main chain and tailoring structure and mechanical properties of the copolymers. Polymer 2011, 52, 3053−3058. (31) Wang, B.; Wang, Y. X.; Cui, J.; Long, Y. Y.; Li, Y. G.; Yuan, X. Y.; Li, Y. S. Cyclopolymerization of Si-Containing α,ω-Diolefins by a Pyridylamidohafnium Catalyst with High Cyclization Selectivity and Stereoselectivity. Macromolecules 2014, 47, 6627−6634. (32) Wang, X. Y.; Wang, Y. X.; Shi, X. C.; Liu, J. Y.; Chen, C. L.; Li, Y. S. Syntheses of Well-Defined Functional Isotactic Polypropylenes via Efficient Copolymerization of Propylene with ω-Halo-α-alkenes by Post-metallocene Hafnium Catalyst. Macromolecules 2014, 47, 552− 559. (33) Alfonso, G. C.; Azzurri, F.; Castellano, M. Analysis of Calorimetric Curves Detected During the Polymorphic Transformation of Isotactic Polybutene-1. J. Therm. Anal. Calorim. 2001, 66, 197−207. (34) Flory, P. J. Principles of Polymer Chemistry; Cornell University Press: Ithaca, NY, 1953; p 570. (35) Powers, J.; Hoffman, J. D.; Weeks, J. J.; Quinn, F. A., Jr. Crystallization kinetics and polymorphic transformations in poly-Ibutene. J. Res. Natl. Bur. Stand., Sect. A 1965, 69A, 335−345. (36) Qiao, Y. N.; Wang, Q.; Men, Y. F. Kinetics of Nucleation and Growth of Form II to I Polymorphic Transition in Polybutene-1 as Revealed by Stepwise Annealing. Macromolecules 2016, 49, 5126− 5136. (37) Stolte, I.; Fischer, M.; Roth, R.; Borreck, S.; Androsch, R. Morphology of Form I′ crystals of polybutene-1 formed on meltcrystallization. Polymer 2015, 63, 30−33. (38) Tosaka, M.; Kamijo, T.; Tsuji, M.; Kohjiya, S.; Ogawa, T.; Isoda, S.; Kobayashi, T. High-Resolution Transmission Electron Microscopy of Crystal Transformation in Solution-Grown Lamellae of Isotactic Polybutene-1. Macromolecules 2000, 33, 9666−9672. (39) Cavallo, D.; Alfonso, G. C. Concomitant Crystallization and Cross-Nucleation in Polymorphic Polymers; Springer: Berlin, 2015. (40) Ostwald, W. Z. Studies on the formation and transformation of solid compounds: Report I. Supersaturation and practicing cooling. Phys. Chem. 1897, 22, 289−330. 6589


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