Epitaxial Recrystallization of IPBu in Form II on an Oriented IPS Film

6 days ago - Epitaxy between isotactic polystyrene (iPS) and isotactic poly(1-butene) (iPBu) has been studied. The epitaxial crystallization of iPS on...
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Epitaxial Recrystallization of IPBu in Form II on an Oriented IPS Film Initially Induced by Oriented Form I IPBu Yunpeng Li,† Zhixin Guo,† Meiling Xue,‡ and Shouke Yan*,†,‡ †

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China Key Laboratory of Rubber-Plastics, Ministry of Education/Shandong Provincial Key Laboratory of Rubber-plastics, Qingdao University of Science & Technology, Qingdao 266042, China

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ABSTRACT: Epitaxy between isotactic polystyrene (iPS) and isotactic poly(1-butene) (iPBu) has been studied. The epitaxial crystallization of iPS on the oriented form I iPBu film was realized through cold crystallization from its amorphous state at 130 °C for 10 h. The epitaxial crystallization of iPBu on the oriented iPS substrate induced initially by form I iPBu crystals, that is, the vice versa process, was achieved simply by heating the sample coldcrystallized at 130 °C up to 190 °C to melt the iPBu crystals completely and followed by melt crystallization of iPBu. The results show that the epitaxy of iPS on iPBu form I crystals is based on two-dimensional lattice matching, which results in a parallel chain alignment of both polymers. On the other hand, the oriented iPS thin film produced by the form I iPBu ordered film in turn generates still a chain parallel epitaxy of iPBu but in its form II, even though the matching of iPS with form II iPBu is much poorer than with form I iPBu. This demonstrates that polymer epitaxy is not solely caused by geometric matching as widely accepted up to date. In the present case, the form II crystallization of iPBu on the iPS substrate is clearly controlled by the crystallization kinetics, while the chain orientation is associated with less pronounced one-dimensional lattice matching. These results shed more light toward a better understanding of polymer epitaxy.

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INTRODUCTION Owing to many advantages of polymeric materials, such as light weight, easy processibility, and high specific strength, they have been used in advanced frontier science and hold an eternal place of many sophisticated applications. The effective utilization of polymeric materials rests also on the fact that they can meet the requirements of some specific applications more easily because their physical and mechanical properties, and even functionality, depend directly on their multiscale structures.1−5 Therefore, precise control on the multiscale structure of polymers provides an efficient way to manipulate the property and functionality of polymers. Consequently, many sophisticated techniques have been developed to control the structures of polymers on a different scale. Among them, epitaxial crystallization of semicrystalline polymers provides a simple and efficient way to fabricate special structures with improved properties and even introduce new functionality for polymeric materials. It can control multiscale structures of semicrystalline polymers, including the crystal structure of polymorphic polymers, orientation of molecular chains or chain segments, and spatial arrangement of planar backbone molecular chains.6−17 It is generally accepted that crystallographic matching is very important for the occurrence of polymer epitaxy. This has been particularly well illustrated by the epitaxy of polymers on low molecular weight organic or inorganic substrates because of their nearly endless possible variations in the crystallographic parameters.6 Even though matching between epitaxial polymer © XXXX American Chemical Society

pairs is usually less pronounced, it exhibits nevertheless an evident effect on the nucleation of the overgrowth polymers. For example, a recent study on the epitaxial crystallization of poly(ε-caprolactone) (PCL) on fiber-oriented isotactic polypropylene (iPP) and polyethylene (PE) demonstrated that the better lattice matching between PCL and PE than that between PCL and iPP leads to a stronger nucleation ability of PE than iPP. Moreover, the matching fulfilled between every (hk0) lattice plane of both PCL and PE, but only between the (100)PCL and (010)iPP lattice planes, there is a much higher nucleation density of PCL on PE than on iPP.7 On the other hand, in an early work, we found that the orientation of PCL chains on the oriented PE substrate could be achieved at ca. 60 °C.8 This can hardly be explained by the crystallographic matching because PCL is in the molten state at 60 °C. Furthermore, preferred orientation of amorphous isotactic poly(methyl methacrylate) and poly(L-lactide) on oriented PE or iPP substrates has also been identified by the polarized IR study.9−11 Therefore, further studies are needed for a better understanding of the origin of polymer epitaxy. In the present work, the epitaxy of isotactic polystyrene (iPS) on an oriented form I isotactic poly(1-butene) (iPBu) substrate was first realized by cold crystallization from the amorphous state at 130 °C. The thus obtained iPS ordered Received: March 28, 2019 Revised: May 20, 2019

A

DOI: 10.1021/acs.macromol.9b00627 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules film was subsequently used as the substrate for melt crystallization of iPBu. Keeping in mind that the iPBu exhibits pronounced polymorphic behavior, the polymorphism selection of iPBu in the vice versa epitaxial crystallization process, that is, epitaxy of iPBu on the ordered iPS substrate initially induced by the oriented form I iPBu film, will shed more light on the polymer epitaxial crystallization. The purpose of this paper is to present detailed experimental results about the epitaxial relationship of iPS cold-crystallized on iPBu and iPBu melt grown on iPS. The important factors in governing polymer epitaxy are discussed according to the obtained results.

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study was conducted with Agilent Technologies 5500 operated under a tapping model. Silicon tips with Al-coating, a resonance frequency of 303 kHz, and a spring constant of about 50 N/m were hired.

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RESULTS AND DISCUSSION The morphology of melt-drawn iPBu films was first checked by AFM. The AFM height and phase images of thus prepared iPBu are shown in Figure 1a,b, respectively. We can see that

EXPERIMENTAL SECTION

Materials. iPS used in this study was purchased from SHANGHAI ZZBIO Co. Ltd, China. The tacticity of iPS is 90%, and the weightaverage molar mass is 400 kg·mol−1. iPBu with a weight-average molar mass of 170 kg·mol−1 was provided by Mitsui Chemicals. The polydispersity index of iPBu is 5.3. Its melting temperature is measured by differential scanning calorimetry to be approximately 130 °C. However, the used highly oriented ultrathin films exhibit only a partial melting at 155 °C as confirmed by transmission electron microscopy (TEM) observation, which is much higher than the nominate melting temperature and even higher than its equilibrium melting temperature. The reason for the high thermal stability of the oriented thin films is not clear at the moment but will be studied in the follow-up work. The analytical xylene and chloroform solvents were purchased from Beijing Chemical Reagent Co. Ltd., which were used without further purification. Sample Preparation. Highly oriented iPBu thin films were prepared according to the melt-draw technique introduced by Petermann and Gohil.18 During the melt-draw procedure, a small amount of iPBu 0.6 wt % solution in xylene was first poured on a preheated glass plate at 124 °C and spread uniformly by a glass rod. After evaporation of the xylene solvent, the thin molten iPBu layer was then picked up by a motor-driven cylinder with a drawing speed of about 10 cm·s−1. The thickness of the as-prepared films ranges from 40 to 60 nm (as measured by Dektak XT). The specimens for TEM and atomic force microscopy (AFM) observations were prepared by spin-coating a 0.2 wt % iPS chloroform solution on the oriented iPBu substrate. While those for the polarized Fourier transform infrared (FTIR) spectroscopy detection were prepared by spin-coating 0.2 wt % iPS chloroform solution on the oriented iPBu substrate supported by the KBr tablet. In order to enhance the infrared signal of iPBu and iPS, three layers of the thus prepared iPS/iPBu films with same iPBu orientation were used. The spin-coated iPS layers are originally in the amorphous state.19 The epitaxial crystallization of iPS on the oriented iPBu substrate was realized through cold crystallization of it from the amorphous state by annealing the sample for 10 h at 130 °C, which is well below the melting point of the used iPBu substrate film. The vice versa process, that is, the epitaxial crystallization of iPBu on the oriented iPS substrate, has been achieved by heating the sample used for checking the epitaxial crystallization of iPS on iPBu up to 190 °C for 5 min to ensure the complete melting of the original iPBu crystals and subsequent annealing at 130 °C for 2 h and 55 °C for 1 h, respectively. Characterization. For FTIR analysis, a Spectrum 100 FTIR spectrometer (PerkinElmer) was used. Polarized FTIR was used to identify the mutual chain orientation of iPS and iPBu. FTIR spectra in the wavenumber ranging from 450 to 4000 cm−1 were obtained by averaging 16 scans at 4 cm−1 resolutions. For TEM observation, a JEOL JEM-2100 transmission electron microscope operated at 200 kV was used in this study. Phase contrast bright-field (BF) electron micrographs were obtained by defocus of the objective lens. To minimize radiation damage by the electron beam, focusing was carried out on an area; then, the specimen film was translated to an adjacent undamaged area for recording the images immediately. The AFM

Figure 1. AFM height (a) and phase (b) images of a melt-drawn iPBu thin film. A phase contrast BF electron micrograph and its corresponding electron diffraction pattern of the melt-drawn iPBu thin film are presented in parts (c,d), respectively. The BF image and the electron diffraction were taken with the same orientation. The white arrows in the AFM images and BF electron micrograph indicate the drawing directions of the films during preparation. The crystallographic a*, b*, and c* orientation of iPBu crystals has been indicated in parts (d).

the melt-drawn iPBu film is composed with parallel-aligned fibrillar crystals, demonstrating the high orientation of it. The fibrils are oriented along the drawing direction during film preparation. This has been further confirmed by electron microscopy observation. As presented in Figure 1c, the BF electron micrograph shows a similar parallel-aligned fibrillar structure. The corresponding electron diffraction pattern (Figure 1d), which is taken with exactly the same orientation as the BF image, confirms a high orientation of iPBu molecular chains along the drawing direction. The sharp and well-defined reflection spots shown in Figure 1d can all be indexed by the trigonal (or hexagonal) unit cell with parameters a = b = 17.53 Å and c = 6.48 Å.20 This means that the iPBu crystallizes in form I under stretching. Moreover, the appearance of several (hk0) diffractions indicates that the melt-drawn iPBu thin film exhibits only a fiber orientation with the c-axis arranged along the drawing direction, whereas the crystallographic a- and baxes rotated randomly about the c-axis, as illustrated in Figure 1d. Figure 2 shows the AFM images of iPS cold-crystallized isothermally from the amorphous state at 130 °C on the highly oriented form I iPBu thin film. It was found that dewetting of iPS solution on the highly oriented iPBu thin film takes place B

DOI: 10.1021/acs.macromol.9b00627 Macromolecules XXXX, XXX, XXX−XXX

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diffraction arcs of iPS, even though diffused to some extent, can be more clearly observed after electron beam radiation damage of the iPBu crystals as shown in Figure 3c. This demonstrates the occurrence of epitaxial crystallization of iPS on the highly oriented form I iPBu film. As shown in Figure 3c, all of the observed iPS diffraction spots can be accounted for by the hexagonal unit cell with axes a = b = 21.9 Å and c = 6.65 Å.19 It should be pointed out that the (00l) electron diffraction spots of iPS are located at the same layer lines of the iPBu owing to the almost identical periodicity of iPS and form I iPBu along the c-axis (ciPS = 6.65 Å and ciPBu = 6.48 Å). Consequently, they are covered by the strong diffraction spots of iPBu and clearly displayed when the diffraction spots of iPBu disappeared. This indicates that the molecular chains of iPS are oriented parallel to the chain direction of form I iPBu. The appearance of (110) and (220) diffraction spots of iPS demonstrates that these lattice planes of iPS are arranged preferentially parallel to the electron beam. According to the electron diffraction results, the parallel chain alignment of iPS and iPBu originates first from the almost identical periodicity along the backbone of them (ciPS = 6.65 Å and ciPBu = 6.48 Å), with a mismatching of only 2.6%. Owing to the fiber orientation of the iPBu substrate, the contact plane of iPBu cannot be definitely decided. Nevertheless, matching between the interplane distance of (300) or (030) iPBu (5.11 Å) and the interplane distance of (220) iPS (5.434 Å) can also be found with a mismatching of 6.3%, which is well below the upper limit for occurrence of epitaxy (15%).21 These diffraction spots are really observed and located very close to each other in the electron diffraction pattern of Figure 3b. Taking all those into account, the epitaxial crystallization of iPS on form I iPBu is caused by a two-dimensional matching between them. FTIR is a versatile and important tool in characterizing semicrystalline polymers, which is not only sensitive to structural conformation and molecular environment but also beneficial for quantitatively analyzing the orientation of molecular chains when measured with polarized light.22,23 It has therefore also been used to characterize the epitaxial crystallization of iPS on the oriented iPBu substrate. Figure 4 shows the spectra of an iPS/iPBu double-layered film coldcrystallized at 130 °C for 10 h in the 1510−1425, 935−910, and 572−558 cm−1 regions measured with electron vectors parallel (0°) and perpendicular (90°) to the drawing direction of the iPBu film, respectively. The oriented structure of both the iPBu substrate and iPS overgrowth layer causes obvious anisotropic IR absorption of them. The assignments of the

Figure 2. AFM height (a) and phase (b) images of iPS coldcrystallized isothermally at 130 °C for 10 h on a melt-drawn iPBu thin film. The white arrow in the picture indicates the drawing direction of the iPBu thin film during preparation.

during spin-coating. This is ideal to clearly disclose the influence of the iPBu substrate on the cold crystallization of iPS at the boundary of the dewetted iPS domains. In Figure 2, the strips located at the left part of the pictures are the dewetted iPS domains. It is clear that keeping the sample at 130 °C for 10 h does not change the morphology of the iPBu substrate film, comparing the right part of Figure 2a with 1b. On the other hand, cold crystallization of iPS on the iPBu substrate leads to the formation of the highly oriented lamellar structure. This demonstrates the occurrence of epitaxial crystallization of iPS on the iPBu oriented film. The iPS crystalline lamellae are perpendicular to the iPBu fibrillar crystals. Taking the chain folding feature of iPS lamellae into account, the AFM image tells us the epitaxial crystallization of iPS on the highly oriented iPBu substrate in terms of a parallel chain alignment of both polymers. This has been further confirmed by the electron microscopy observation. The above AFM results are infusive because there are no reports about epitaxial crystallization of polymers on the oriented iPBu thin film up to now. To establish the epitaxial relationship between iPS and iPBu, the phase contrast BF electron micrograph and corresponding diffraction patterns are obtained and shown in Figure 3. In the BF image, Figure 3a, the gray areas represent oriented iPBu regions, while the darker area is the iPS/iPBu double-layered region because of its high mass-thickness contrast. The BF electron micrograph shows essentially the same morphology as that revealed by AFM. It contains edge-on iPS lamellae aligned perpendicular to the iPBu fibrils, indicating a parallel chain alignment of iPS and iPBu. In the electron diffraction pattern, Figure 3b, except for the sharp and well-defined diffraction spots of iPBu, slightly arced and weak diffraction spots of iPS can be identified. The

Figure 3. Phase-contrast BF electron micrograph (a) and corresponding electron diffraction patterns (b,c) of iPS cold-crystallized isothermally at 130 °C for 10 h on a melt-drawn iPBu thin film. The electron diffraction pattern shown in part (c) was taken after iPBu crystals being destroyed by electron radiation. The white arrow in part (a) indicates the drawing direction of the iPBu thin film during preparation. The crystallographic orientation of iPBu is the same as that shown in Figure 1d. The crystallographic orientation of iPS has been indicated in parts (c). C

DOI: 10.1021/acs.macromol.9b00627 Macromolecules XXXX, XXX, XXX−XXX

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Besides, the band at 1466 cm−1 is assigned to the CH3 asymmetric deformation modes of iPBu, which behaves in the same way as the 924 cm−1 band.24 On the other hand, the absorptions at 1493 and 565 cm−1 are the characteristic bands of iPS in the crystalline phase.29 The bands at 565 and 1493 cm−1 are related to the C−H out-of-plane mode and the C−C stretching vibrations of the phenyl ring, respectively.30,31 While the absorption at 565 cm−1 exhibits a parallel transition moment with respect to the main chain of iPS, the transition moment of the 1493 cm−1 band is perpendicular to the main chain. Taking these into account, the higher absorption of the 565 cm−1 band measured with the electron vector parallel (0°) to the molecular chain direction of iPBu indicates a parallel chain alignment of iPBu and iPS molecular chains. Moreover, opposite response of the 1493 cm−1 band with respect to the 565 cm−1 band measured with the electron vector parallel (0°) and perpendicular (90°) to the molecular chain direction of iPBu further confirms the occurrence of parallel chain epitaxy of iPS on the oriented iPBu substrate. The polarized IR results are clearly in good accordance with the TEM. The advantage of polarized IR is to characterize the orientation of polymers quantitatively through calculation of the orientation function (f), which is given by

Figure 4. Polarized FTIR spectra of iPS/iPBu double-layered films, which has been annealed at 130 °C for 10 h. The measurements were performed with the electron vectors parallel (0°) and perpendicular (90°) to the draw direction of the iPBu substrate film, respectively.

characteristic absorption bands observed in the polarized FTIR spectra for iPS and iPBu are summarized in Table 1. Table 1. Characteristic Infrared Bands of iPBu and iPS24−31 bands (cm−1)

dichroism

assignments

polymer

1493

⊥

iPS

1466 1439

⊥ ⊥

1439

∥

924 565

⊥ ∥

C−C stretching vibration of the phenyl ring CH3 asymmetric deformation mode bending modes of main chain CH2 and ethyl group C−C stretching mode of the phenyl ring and CH2 scissor vibration rocking modes of main chain CH2 and CH3 C−H out-of-plane mode

f=

D−1 2 × D+2 3 cos2 α − 1

(1)

where D is the dichroic radio, and α is the angle between the transition moment vector and the chain axis. The dichroic radio D can be easily calculated by A∥/A⊥ (A∥ and A⊥ are the intensities of a characteristic band measured with electron vectors parallel (0°) and perpendicular (90°) to the reference direction, respectively). However, the α angle depends on the selected band. Here, in this work, to compare the orientation status of iPBu and iPS, the bands at 924 cm−1 for iPBu and 565 cm−1 for iPS were used. Therefore, the corresponding α of 924 and 565 cm−1 bands should be first determined before calculation. To this end, the molecular chain direction of iPBu was selected as the reference direction. Polarized FTIR spectra measured with the angle between the electron vector and the iPBu chain direction (θ) ranging from 0° to 175° with a 5° interval were first acquired to obtain the angle-dependent absorbance intensity of each band. Figure 5 shows the plots of absorbance against the measurement angle of 565 and 924 cm−1, respectively. The fitted results indicated that the bands at 565 and 924 cm−1 reach maximum absorbance at 30.25° and 68.08°, respectively. In other words, the α values related to the

iPBu iPBu iPS iPBu iPS

From Table 1, it is clear that the band at 924 cm−1 is contributed by iPBu, corresponding to the coupling of rocking vibration of CH2 in the main chain and the CH3 side group. This band is characteristic of form I iPBu.24−28 The appearance of this band in the spectrum confirms that the melt-drawn iPBu film is composited by oriented form I fibrillar crystals. From Figure 4, it is clear that the intensity of 924 cm−1 measured with the electron vector perpendicular (90°) to the molecular chain direction of iPBu is much higher than with the electron vector parallel (0°) to the chain direction. This is reasonable because the 924 cm−1 band exhibits a perpendicular transition moment with respect to the main chain of iPBu.

Figure 5. Plots of band intensities of 565 cm−1 for iPS (a) and 924 cm−1 for iPBu (b) as a function of the angle (θ) between the electron vector and drawing direction of iPBu. The red lines are the fitted curves with the equations inserted. D

DOI: 10.1021/acs.macromol.9b00627 Macromolecules XXXX, XXX, XXX−XXX

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Figure 6. Polarized FTIR spectra of iPS/iPBu double-layered films, which has been annealed at 130 °C for 10 h, in the 572−558 (a) and 935−910 cm−1 (b) regions. The spectra were obtained with the electron vector parallel (0°) and perpendicular (90°) to the draw direction of the iPBu substrate film, respectively.

565 and 924 cm−1 bands are 30.25° and 68.08°, respectively. On the other hand, as shown in Figure 6, the D values of 924 and 565 cm−1 were calculated to be 0.347 and 3.51, respectively. Thus, the obtained orientation function is of 0.96 for iPBu and 0.734 for iPS. The above-polarized IR results confirm that the epitaxial crystallization of iPS on the highly oriented iPBu film leads to a parallel alignment of iPS and iPBu molecular chains. The orientation status of iPS is, however, somewhat poorer than the iPBu. This is in good agreement with the electron diffraction result, which shows the slightly arced diffraction spots of iPS. It may be associated with the cold crystallization at a relatively lower temperature owing to the limited melting point of iPBu. Another thing should be mentioned here is that iPBu exhibits pronounced polymorphic behavior. The form I iPBu exhibits the attractive properties, such as stiffness, puncture resistance in films, temperature resistance, environmental stress cracking resistance, low creep, good abrasion resistance, and so on. However, crystallization from the melt leads usually to the formation of kinetically favored form II, which transforms spontaneously into the thermodynamically stable form I during storage at room temperature.32−35 Because the solid-phase transition results in deformation and volume change of products, efforts have been made for obtaining form I iPBu directly. It was found that direct formation of the form I iPBu crystal from the melt can be achieved by ultrathin film crystallization at very low supercoiling.36 Moreover, direct crystallization of form I iPBu from the melt has been observed for stereo-irregular samples.37−39 In the present work, considering that form I iPBu can induce epitaxial crystallization of iPS based on a two-dimensional lattice matching, epitaxial crystallization of iPBu form I on the iPS substrate, that is, reverse epitaxy, is highly expected. If this is true, the iPS can therefore be used as a nucleation agent for form I crystallization of iPBu. Consequently, melt crystallization of iPBu on the oriented iPS substrate, which has been initially induced by the oriented iPBu thin film, has been performed by melting the samples used for Figures 2 and 3 at 190 °C for 5 min and cooling down to and keeping at 130 °C for 2 h and then at 55 °C for 1 h. Figure 7 shows the electron diffraction patterns of the thus prepared samples. From Figure 7a, we can see that oriented recrystallization of iPBu from the melt indeed takes place. However, all electron diffraction spots of iPBu are now accounted for by tetragonal form II of iPBu with unit cell parameters a = b = 14.6 Å and c = 21.2 Å.20,34,40 This indicates that the melt recrystallization of iPBu on the oriented iPS

Figure 7. Electron diffraction patterns of melt-recrystallized iPBu on the oriented iPS substrate taken (a) before and (b) after 1 min exposure. The sample used is the same one as shown in Figure 3 but heated up to 190 °C for 5 min, then cooled down to and kept at 130 °C for 2 h, and finally cooled down to and kept at 55 °C for 1 h. The crystallographic orientations of iPBu and iPS have been indicated in parts (a,b), respectively. For iPS, there are two populations of crystals with different orientations, which have been described as 1 and 2 in part (b), respectively.

substrate, which is induced initially by the form I iPBu oriented film, occurs in its form II. Nevertheless, the location of (0011)iPBu and (003)iPS in the same direction indicates that the molecular chains of iPBu and iPS are still parallel to each other after melt recrystallization of iPBu, even though changed from form I to form II. Moreover, the appearance of very strong (200) and strong (400) diffractions of iPBu form II crystals demonstrates that, unlike the melt-drawn iPBu thin films with uniaxial fiber orientation, a double orientation of recrystallized iPBu form II crystals has been achieved with crystallographic aand c-axes in the film plane, while c-axis aligned in the molecular chain direction of iPS (see Figure 7a), that is, with a (010) lattice plane in contact with the iPS substrate. To clearly display the diffraction of iPS, an electron diffraction pattern of the sample recorded after 1 min exposure is shown in Figure 7b. Now, all of the diffraction spots of iPBu disappeared because of electron radiation damage. Comparing Figure 7b with 3c, we can find that the two diffraction patterns are essentially the same except for the increased diffraction intensity and the newly appeared (300) diffraction spot of iPS, which has been covered by the very strong (200) diffraction of iPBu in Figure 7a. This implies that further crystallization of iPS takes place by heating the sample up to 190 °C and annealing the sample at 130 °C for another 2 h. The secondary crystallization of iPS results in the increased crystallinity of the crystals with the (220) lattice plane along the electron beam, E

DOI: 10.1021/acs.macromol.9b00627 Macromolecules XXXX, XXX, XXX−XXX

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melting and recrystallization conditions. This has been well explained in terms of the interplay between the domain size of originally aligned chain segregates included in the crystal lattice as well as the size and energy barrier of the critical nucleus corresponding to different crystalline forms. In the present case, iPBu is molten at 190 °C before isothermal crystallization. Therefore, crystallization of iPBu in form II is acceptable because the nucleation barrier for form II is much lower than that of form I. On the other hand, the oriented epitaxial crystallization of iPBu on the ordered iPS substrate indicates the existence of heterogenous nucleation ability of iPS toward iPBu, which governs the chain orientation direction of iPBu in form II crystals. The selection of kinetically favored form II rather than the better-matched form I of iPBu grown on the iPS surface can be understood. As reported by Cavallo and co-workers,44,45 crystallizing iPBu from the melt on the pre-existing form I spherulites always generates its form II crystals, a phenomenon referred to as cross-nucleation. Li et al.26 also confirms that the recrystallization of partially molten form I iPBu spherulites restores only a small amount of the form I crystal. These results demonstrated that, even on the surface of its form I counterpart, crystallization of iPBu from the melt produces only the kinetically favored form II. It should be noted here that the matching of iPS with form I iPBu is indeed better than with form II iPBu; it is nevertheless much less pronounced than the same iPBu form I crystals with perfect matching. Taking all these into account, the crystallization of the iPBu melt on the oriented iPS substrate in its form II is then not hard to be understood. Consequently, it is concluded that the kinetic factor plays a very import role in the epitaxial crystallization of the iPBu on the iPS substrate. The selection of polymorphism of iPBu is determined by the kinetic factor. From the melt, iPBu crystallizes generally in form II regardless of the substrate because its crystallization rate is 100 times faster than the form I.46 On a substrate absence of some special geometric matching, it will crystallize in randomly oriented form II lamellar crystals. On the other hand, when some crystallographic matching is fulfilled, the kinetically driven form II crystallization of iPBu will be in a unique way and produce ordered organization of the form II crystals. In the present case, the existence of one-dimensional matching provides a favorable interaction circumstance of form II iPBu with iPS. As a result, epitaxial crystallization of form II iPBu takes place with molecular chains aligned along the chain direction of iPS. In other words, the form II crystallization of iPBu is governed by the kinetic factor, while the orientation of the form II iPBu crystals is caused by geometric matching of it with iPS.

which exhibit the same orientation as that shown in Figure 3b and has been indicated as 1 in Figure 7b. Moreover, the appearance of (300) diffraction spots suggests the formation of some new crystals with their (300) lattice plane parallel to the electron beam. In other words, some new iPS crystals with crystallographic a* and c* in the film plane (indicated as 2 in Figure 7b) are generated by secondary crystallization. According to the electron diffraction results, the matching between iPS and form II iPBu can be analyzed as below. First, taking the (010) contact plane of iPBu form II into account, the matching between the interplane distances of (003)iPBu (7.07 Å) and (001)iPS (6.65 Å) with a mismatching of ca. 6.3% is below the upper limit of epitaxial crystallization of polymers.21 On the other hand, in the [hk0] direction, best matching can be found between the interplane distance of (200)iPBu (7.3 Å) and (300)iPS (6.32 Å) with a mismatching of ca. 15.5%, which is beyond the accepted upper limit for polymer epitaxy.21 Moreover, judged from the electron diffraction pattern, the amount of iPS crystals with (300) lattice planes parallel to the electron beam direction is very small. It should be also not sufficient in producing the resultant epitaxial crystallization of iPBu form II. Therefore, the observed epitaxial crystallization of form II iPBu on the oriented iPS substrate can only be based on a one-dimensional lattice matching in the molecular chain direction between them. The above experimental results demonstrate unambiguously the ability of mutual epitaxy between iPS and iPBu. Cold crystallization of iPS from the glass state on an ordered iPBu thin film in form I results in an oriented structure of iPS, which in turn induces epitaxial crystallization of iPBu from the melt in its form II. Based on these results, the mechanism of polymer epitaxy and the polymorphic selection of iPBu during melt crystallization should be addressed. Epitaxial crystallization is generally believed to occur when there is some lattice matching between a host (substrate) and guest (overgrowth) polymers. For iPS and iPBu, two-dimensional matching between form I iPBu and iPS can be found along the chain direction of both polymers as well as in the [300] direction of iPBu and [220] direction of iPS. It is this two-dimensional matching that enables the oriented epitaxial crystallization of iPS on ordered iPBu form I crystals. Based on this analysis, the thus produced iPS oriented film should be able to induce the epitaxial crystallization of iPBu in its form I again with the same mutual orientation, unless there exists better matching between iPS and other form iPBu. Actually, the epitaxial crystallization of iPBu on this kind of the iPS substrate in its form II with less pronounced matching takes place. This means that epitaxy between polymers is not solely governed by crystallographic matching between host and guest crystals. As mentioned above, crystallization of iPBu from the melt leads usually to the formation of kinetically favored form II of it, which transforms spontaneously into the thermodynamically stable form I during storage at room temperature. Because shrinkage happens during phase transition, which results in the shape change of the products, the polymorph selection of iPBu has been extensively studied in order to bypass the form II crystallization. Recently, Men and co-workers have performed detailed studies on the phase selection of iPBu copolymers.41−43 It was demonstrated that form II has always been obtained after melting at high temperature (e.g., 180 °C) with completely relaxed molecular chains. However, direct crystallization of form I′ could be achieved through controlled

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CONCLUSIONS In summary, epitaxial crystallization of iPS on the oriented iPBu substrate and its vice versa process have been studied by infrared spectroscopy, AFM, and TEM combined with electron diffraction. Using highly oriented melt-drawn iPBu ultrathin film as the substrate, the epitaxial crystallization of iPS on the ordered form I iPBu film has been realized by cold crystallization from its amorphous glassy state. The epitaxial crystallization results in a parallel chain alignment of both polymers based on a two-dimensional lattice matching. A quantitative analysis of the orientation status of both iPS and iPBu by polarized IR indicates that the used iPBu melt-drawn films exhibit an orientation function as high as 0.96. The orientation function of epitaxially crystallized iPS layer is F

DOI: 10.1021/acs.macromol.9b00627 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

(9) Tu, C.; Jiang, S.; Li, H.; Yan, S. Origin of Epitaxial Cold Crystallization of Poly(l-lactic acid) on Highly Oriented Polyethylene Substrate. Macromolecules 2013, 46, 5215−5222. (10) Guan, G.; Zhang, J.; Sun, X.; Li, H.; Yan, S.; Lotz, B. Oriented Overgrowths of Poly(l -Lactide) on Oriented Isotactic Polypropylene: A Sequence of Soft and Hard Epitaxies. Macromol. Rapid Commun. 2018, 39, 1800353. (11) Guo, Z.; Li, S.; Liu, X.; Zhang, J.; Li, H.; Sun, X.; Ren, Z.; Yan, S. Epitaxial Crystallization of Isotactic Poly(methyl methacrylate) from Different States on Highly Oriented Polyethylene Thin Film. J. Phys. Chem. B 2018, 122, 9425−9433. (12) Kopp, S.; Wittmann, J. C.; Lotz, B. Epitaxial crystallization and crystalline polymorphism of poly(1-butene): forms III and II. Polymer 1994, 35, 908−915. (13) Kopp, S.; Wittmann, J. C.; Lotz, B. Epitaxial crystallization and crystalline polymorphism of poly(1-butene): form I. Polymer 1994, 35, 916−924. (14) Bonev, I. On the terminology of the phenomena of mutual crystal orientation. Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. 1972, 28, 508−512. (15) Sun, Y.; Li, H.; Huang, Y.; Chen, E.; Zhao, L.; Gan, Z.; Yan, S. Epitaxial Crystallization of Poly(butylene adipate) on Highly Oriented Polyethylene Thin Film. Macromolecules 2005, 38, 2739− 2743. (16) Sun, Y.; Li, H.; Huang, Y.; Chen, E.; Gan, Z.; Yan, S. Epitaxial crystallization of poly(butylene adipate) on highly oriented isotactic polypropylene thin film. Polymer 2006, 47, 2455−2459. (17) Liu, J.; Wang, J.; Li, H.; Shen, D.; Zhang, J.; Ozaki, Y.; Yan, S. Epitaxial Crystallization of Isotactic Poly(Methyl Methacrylate) on Highly Oriented Polyethylene. J. Phys. Chem. B 2006, 110, 738−742. (18) Petermann, J.; Gohil, R. M. A new method for the preparation of high modulus thermoplastic films. J. Mater. Sci. 1979, 14, 2260− 2264. (19) Natta, G.; Corradini, P.; Bassi, I. W. Crystal structure of isotactic polystyrene. Nuovo Cimento 1960, 15, 68−82. (20) Tashiro, K.; Hu, J.; Wang, H.; Hanesaka, M.; Saiani, A. Refinement of the Crystal Structures of Forms I and II of Isotactic Polybutene-1 and a Proposal of Phase Transition Mechanism between Them. Macromolecules 2016, 49, 1392−1404. (21) Wittmann, J. C.; Lotz, B. Epitaxial crystallization of polymers on organic and polymeric substrates. Prog. Polym. Sci. 1990, 15, 909− 948. (22) D’Esposito, L.; Koenig, J. L. Application of fourier transform infrared spectroscopy to the study of semicrystalline polymers: Poly(ethylene terephthalate). J. Polym. Sci., Polym. Phys. Ed. 1976, 14, 1731−1741. (23) Duan, Y.; Zhang, J.; Shen, D.; Yan, S. In Situ FTIR Studies on the Cold-Crystallization Process and Multiple Melting Behavior of Isotactic Polystyrene. Macromolecules 2003, 36, 4874−4879. (24) Ukita, M. The Vibrational Spectra and Vibrational Assignments of Isotactic Polybutene-1. J. Chem. Soc. Jpn. 1966, 39, 742−749. (25) Su, F.; Li, X.; Zhou, W.; Chen, W.; Li, H.; Cong, Y.; Hong, Z.; Qi, Z.; Li, L. Accelerating crystal-crystal transition in poly(1-butene) with two-step crystallization: An in-situ microscopic infrared imaging and microbeam X-ray diffraction study. Polymer 2013, 54, 3408− 3416. (26) Su, F.; Li, X.; Zhou, W.; Zhu, S.; Ji, Y.; Wang, Z.; Qi, Z.; Li, L. Direct Formation of Isotactic Poly(1-butene) Form I Crystal from Memorized Ordered Melt. Macromolecules 2013, 46, 7399−7405. (27) 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. (28) He, A.; Xu, C.; Shao, H.; Yao, W.; Huang, B. Effect of molecular weight on the polymorphic transformation of isotactic poly(1-butene). Polym. Degrad. Stab. 2010, 95, 1443−1448. (29) Painter, P. C.; Koenig, J. L. A normal vibrational analysis of isotactic polystyrene. J. Polym. Sci., Polym. Phys. Ed. 1977, 15, 1885− 1903.

estimated to be 0.73, which is slightly lower than the used substrate film. Using thus prepared iPS oriented thin film as the substrate, the vice versa process, that is, epitaxial crystallization of iPBu on the iPS film, has been achieved by recrystallization of iPBu after melting of its form I crystals at 190 °C. The intriguing finding of this process is that the oriented iPS substrate initially induced by ordered form I iPBu in turn induces the oriented crystallization of form II iPBu with the same mutual chain orientation. Considering that the crystallographic matching of iPS with form II iPBu is poorer than that with form I iPBu, it is concluded that the epitaxial crystallization of polymers is not solely governed by the existing geometric matching between the polymer pairs. The kinetic factor also plays a very important role in determining the structure of the overgrowing polymer. The polymorphism selection of iPBu, here the form II, is clearly determined by its crystallization kinetics. Nevertheless, the existence of onedimensional lattice matching between form II iPBu and iPS in the molecular chain direction, even though not as good as that between iPS and form I iPBu, can still trigger the oriented epitaxial crystallization of form II iPBu on the iPS substrate.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. ORCID

Shouke Yan: 0000-0003-1627-341X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

The financial support of the National Natural Science Foundations of China (nos. 21434002 and 51521062) is gratefully acknowledged.

(1) Smith, P.; Lemstra, P. J. Ultra-high-strength polyethylene filaments by solution spinning/drawing. J. Mater. Sci. 1980, 15, 505−514. (2) Hu, J.; Xin, R.; Hou, C.; Yan, S. Preparation and Self-Repairing of Highly Oriented Structures of Ultrathin Polymer Films. Macromol. Chem. Phys. 2019, 220, 1800478. (3) Lovinger, A. J. Ferroelectric polymers. Science 1983, 220, 1115− 1121. (4) Zheng, Y.-R.; Zhang, J.; Sun, X.-L.; Li, H.-H.; Ren, Z.-J.; Yan, S.K. Enhanced αγ′ Transition of Poly(vinylidene fluoride) by Step Crystallization and Subsequent Annealing. Chin. J. Polym. Sci. 2018, 36, 598−603. (5) Wang, J.; Li, H.; Liu, J.; Duan, Y.; Jiang, S.; Yan, S. On the α → β Transition of Carbon-Coated Highly Oriented PVDF Ultrathin Film Induced by Melt Recrystallization. J. Am. Chem. Soc. 2003, 125, 1496−1497. (6) Wittmann, J. C.; Lotz, B. Epitaxial crystallization of polyethylene on organic substrates: A reappraisal of the mode of action of selected nucleating agents. J. Polym. Sci., Polym. Phys. Ed. 1981, 19, 1837− 1851. (7) Hu, J.; Xin, R.; Hou, C.-Y.; Yan, S.-K.; Liu, J.-C. Direct Comparison of Crystal Nucleation Activity of PCL on Patterned Substrates. Chin. J. Polym. Sci. 2019, 37, 1−7. (8) Yan, C.; Li, H.; Zhang, J.; Ozaki, Y.; Shen, D.; Yan, D.; Shi, A.C.; Yan, S. Surface-Induced Anisotropic Chain Ordering of Polycarprolactone on Oriented Polyethylene Substrate: Epitaxy and Soft Epitaxy. Macromolecules 2006, 39, 8041−8048. G

DOI: 10.1021/acs.macromol.9b00627 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules (30) Jasse, B.; Koenig, J. L. Fourier transform infra-red study of uniaxially-oriented isotactic polystyrene. Polymer 1981, 22, 1040− 1044. (31) Tadokoro, H.; Nishiyama, Y.; Nozakura, S. i.; Murahashi, S. Stereoregular Polymers. VII. Infrared Spectra of Isotactic Polystyrene, Isotactic Poly-α, β, β-trideuterostyrene and Isotactic Poly-pdeuterostyrene. Bull. Chem. Soc. Jpn. 1961, 34, 381−391. (32) Xin, R.; Zhang, J.; Sun, X.; Li, H.; Ren, Z.; Yan, S. Polymorphic Behavior and Phase Transition of Poly(1-Butene) and Its Copolymers. Polymers 2018, 10, 556. (33) Yamashita, M.; Ueno, S. Direct melt crystal growth of isotactic polybutene-1 trigonal phase. Cryst. Res. Technol. 2007, 42, 1222− 1227. (34) Qiao, Y.; Wang, H.; Men, Y. Retardance of Form II to Form I Transition in Polybutene-1 at Late Stage: A Proposal of a New Mechanism. Macromolecules 2018, 51, 2232−2239. (35) Qiao, Y.; Wang, Q.; Men, Y. 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. (36) Zhang, B.; Yang, D.; Yan, S. Direct formation of form I poly(1butene) single crystals from melt crystallization in ultrathin films. J. Polym. Sci., Polym. Phys. Ed. 2002, 40, 2641−2645. (37) De Rosa, C.; Auriemma, F.; Resconi, L. Metalloorganic Polymerization Catalysis as a Tool To Probe Crystallization Properties of Polymers: The Case of Isotactic Poly(1-butene). Angew. Chem., Int. Ed. 2009, 48, 9871−9874. (38) De Rosa, C.; Auriemma, F.; Ruiz de Ballesteros, O.; Esposito, D.; Di Girolamo, R.; Resconi, L.; 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. (39) De Rosa, C.; Auriemma, F.; Malafronte, A.; Scoti, M. Crystal structures and polymorphism of polymers: Influence of defects and disorder. Polym. Cryst. 2018, 1, No. e10015. (40) Hu, J.; Tashiro, K. Relation between higher-order structure and crystalline phase transition of oriented isotactic polybutene-1 investigated by temperature-dependent time-resolved simultaneous WAXD/SAXS measurements. Polymer 2016, 90, 165−177. (41) Wang, Y.; Lu, Y.; Zhao, J.; Jiang, Z.; Men, Y. Direct formation of different crystalline forms in butene-1/ethylene copolymer via manipulating melt temperature. Macromolecules 2014, 47, 8653− 8662. (42) 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. (43) Lai, Y.; Men, Y. Polymorph selection during crystallization of random copolymers. Eur. Polym. J. 2018, 101, 218−224. (44) Cavallo, D.; Gardella, L.; Portale, G.; Müller, A. J.; Alfonso, G. C. On cross- and self-nucleation in seeded crystallization of isotactic poly(1-butene). Polymer 2013, 54, 4637−4644. (45) Cavallo, D.; Gardella, L.; Portale, G.; Müller, A. J.; Alfonso, G. C. Kinetics of Cross-Nucleation in Isotactic Poly(1-butene). Macromolecules 2014, 47, 870−873. (46) Yamashita, M.; Hoshino, A.; Kato, M. Isotactic Poly(butene-1) Trigonal Crystal Growth in the Melt. J. Polym. Sci., Part B: Polym. Phys. 2007, 45, 684−697.

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DOI: 10.1021/acs.macromol.9b00627 Macromolecules XXXX, XXX, XXX−XXX