Effects of Crystal Growth Conditions on Morphology of Crystalline Syndiotactic 1,2-Polybutadiene Ye
Chen,†
Decai
Yang,*,†
Yanming
Hu,‡
and Xuequan
Zhang‡
State Key Laboratory of Polymer Physics and Chemistry, and Polymer Engineering Laboratory, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China Received July 3, 2003;
CRYSTAL GROWTH & DESIGN 2004 VOL. 4, NO. 1 117-121
Revised Manuscript Received August 21, 2003
ABSTRACT: The crystalline syndiotatic 1,2-polybutadiene was synthesized with a catalyst consisting of iron acetylacetonate (Fe(acac)3)-triisobutylaluminum (Al(i-Bu)3)-diethyl phosphite (DEP), and the effects of crystal growth conditions on morphology of thin films of the polymer were investigated by transmission electron microscopy (TEM) and electron diffraction (ED) techniques. The polymer with melting point 179 °C was found to have 89.3% 1,2content and 86.5% syndiotacticity by 13C NMR measurement. The results of electron microscopic studies indicate that the solution-cast thin films of the syndiotatic 1,2-polybutadiene consist of lath-like lamellae with the c-axis perpendicular to the film plane, while a- and b-axes are in the film plane. The morphology of isothermally crystallized thin films of the polymer is temperature dependent. At lower crystallization temperatures (130 °C), a spherulitic structure consisting of flat-on lamellae is formed. With an increase in the crystallization temperature (e.g., at 140 °C), the spherulites and single faceted crystals coexist. At higher crystallization temperatures (150 °C), single crystals with a hexagonal prismatic shape are produced. When the crystallization temperature is above 155 °C, no crystalline structure is observed because of the thermal cross-linking reaction of the double bonds. The melt-draw films contain highly oriented lamellae with its growing direction perpendicular to the drawing direction. According to the orthorhombic packing of planar zigzag chains, the unit cell parameters calculated from the electron diffraction patterns are a ) 1.102 nm, b ) 0.664 nm, and c ) 0.513 nm. Introduction Crystalline syndiotatic 1,2-polybutadiene was first obtained by Natta and Porri in 1955,1 and its crystal structure was determined also.2 Since then, considerable attention has been paid to it. In past few decades, however, most of the research work was focused on the preparation, physical properties, and application of the polymer,3-14 while almost no investigation on its crystallization and morphology has been reported. The main reason is due to the large amount of double bonds contained in the side chains of the polymeric molecules, which will cross-link at higher temperatures (>150 °C).5 Especially for the polymers with high syndiotacticity, the thermal cross-linking reaction will take place very easily during the melt crystallization process because of their high melting temperature (>200 °C). This makes the morphologic study of the polymer very difficult. The crystal structure of the crystalline syndiotatic 1,2polybutadiene was determined using a wide-angle X-ray diffraction fiber pattern and an electron diffraction diagram of solution-cast thin film (about 10 nm thick) containing very small crystallites, in combination with a film-tilted technique with respect to the electron beam.2 The crystal is characterized by chains in the planar zigzag conformation with an identity period of 0.51 nm along the fiber axis. The chains are packed in * Corresponding author: Decai Yang, State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China. Telephone: 86-431-5262139. Fax: 86-431-5262126. E-mail: dcyang@ ciac.jl.cn. † State Key Laboratory of Polymer Physics and Chemistry. ‡ Polymer Engineering Laboratory.
an orthorhombic unit cell with a ) 1.098 nm, b ) 0.660 nm, and c ) 0.514 nm. The space group proposed is Pacm.2 In this paper, the morphologic characteristics of thin films of the crystalline syndiotatic 1,2-polybutadiene crystallized at various conditions are reported for the first time, which will support further understanding of the relationship between crystal growing conditions and resulting morphologies. Experimental Section The crystalline syndiotatic 1,2-polybutadiene was prepared by polymerizing butadiene with a catalytic system consisting of iron acetylacetonate (Fe(acac)3), triisobutylaluminum (Al(i-Bu)3), and diethyl phosphite (DEP) in a hexane solution at 70 °C for 1 h. The polymer obtained with melting point 179 °C was found to have 89.3% 1,2-content, and 86.5% sydiotactic regularity [r], 84.5% triads [rr], and 83.9% pentads [rrrr], respectively, based on 13C NMR measurement. Thin films of the crystalline syndiotatic 1,2-polybutadiene were prepared by casting 0.1 wt % xylene solution of the polymer on the surface of glycerol at 130 °C. After evaporation of the solvent, the thin films were transferred onto the surface of water, and then collected on copper grids and shadowed with heavy metal (Pt) for TEM observations. To study the morphology of the polymer isothermally crystallized at different temperatures, the thin films of the crystalline syndiotatic 1,2-polybutadiene were at first prepared by casting 0.1 wt % solution of the polymer in xylene on carboncoated mica at about 100 °C. After evaporation of the solvent, the thin films were heated to 200 °C to eliminate previous thermal history. Subsequently, the thin films were cooled rapidly to a given temperature and isothermally crystallized at the temperature for 2 h, and then quenched to room temperature. The films were floated on the surface of water, and then collected on copper grids and shadowed with Pt for TEM studies.
10.1021/cg0341151 CCC: $27.50 © 2004 American Chemical Society Published on Web 09/27/2003
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Figure 1. BF electron micrograph (a) and corresponding ED pattern (b) of solution-cast films of the syndiotactic 1,2-polybutadiene.
Figure 2. BF electron micrograph (a) and corresponding selected area ED pattern (b) of the thin films isothermally crystallized at 130 °C for 2 h. Highly oriented thin films of the crystalline syndiotatic 1,2polybutadiene were prepared by a special melt-drawn technique.15 According to this method, a 0.5 wt % solution of the polymer in xylene was spread uniformly on a preheated glass slide (130 °C). After evaporation of the solvent, the thin melt film was drawn up vertically with a drawing speed about 4 cm/s. Subsequently, the melt-drawn films were transfer onto the surface of water, and collected on copper grids for TEM observations. A JEOL 2010 TEM operated at 200 kV was used in this study. Bright-field (BF) electron micrographs were obtained by defocus of the objective lens.
Results and Discussion Lath-Like Lamellar Structure of the SolutionCast Films. Figure 1 shows a BF electron micrograph and its corresponding ED pattern of the solution-cast films of the crystalline syndiotatic 1,2-polybutadiene. In the BF electron micrograph (Figure 1a), the dark lines represent the crystalline lamellae. Obviously, the solution-cast films of the polymer consist of the lamellae with lateral sale in micrometer size. It should be noted that the lamellae have no branching, and they are almost parallel each other in the localized areas. The lamellar growing direction is along the crystallographic b-axis, as revealed by the corresponding ED pattern (Figure 1b). According to the orthorhombic packing of the planar zigzag chains proposed by Natta and Corradini,2 all of the reflections can be indexed (Figure 1b). Surprisingly, the ED pattern of the solution-cast film only exhibits strong (hko) reflections, which indicate that the lamellae possess a single crystal-like orientation (see below), i.e., with their c-axis (the chain direction) perpendicular to the film plane, while a- and b-axes are in the film plane. This kind of lamellar structure
with a single crystal-like orientation is called a “lathlike” structure, which not only has been observed for the first time in the solution-cast films of the syndiotatic 1,2-polybutadiene but is less formed in other polymers.16 Spherulitic Structure Formed at Lower Temperatures. Figure 2a shows a BF electron micrograph of the thin films of the crystalline syndiotatic 1,2polybutadiene isothermally crystallized from the melt at 130 °C for 2 h. Apparently, the thin films contain a spherulitic structure consisting of lamellae. Actually, however, it is different from the typical polymeric spherulite structure with edge-on lamellae. The lamellae exhibit a single crystal-like crystallographic orientation, as revealed by the corresponding selected area ED pattern (Figure 2b). The (hko) reflections imply a flaton structure of the lamellae with the molecular chains (c-axis direction) oriented perpendicular to the film plane, and the a- and b-axes in the film plane. Comparing the BF electron micrograph (Figure 2a) and corresponding selected area ED pattern (Figure 2b), it is apparent that the lamellar growing direction is along the crystallographic b-axis. This kind of lamellar structure with the single crystal-like crystallographic orientation, to some extent, is similar to that of solution-cast films. The reason for the formation of the flat-on lamellar structure is not clear yet. It should be pointed out that there are lamellar twists to some extent, especially in the regions where there exist less lamellae. In addition, it seems that there are some flat-on lamellae or single-faceted crystals on the edge of the spherulite. However, no electron diffraction was observed in these regions. It should be pointed out that the crystalline morphology of the syndiotatic 1,2-polybutadiene thin films
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Figure 3. BF electron micrograph (a) and the ED pattern in the area shown by the circle (b) of the thin films isothermally crystallized at 140 °C for 2 h.
Figure 4. BF electron micrograph (a) and corresponding ED pattern (b) of the single crystal produced from isothermal crystallization of the thin film at 150 °C for 2 h.
produces some changes with the increase of the isothermally crystallized temperature. Figure 3a shows the BF electron micrograph of the thin films of the polymer isothermally crystallized from the melt at 140 °C for 2 h. In addition to the spherulitic structure, which is similar to that shown in Figure 2a, a single-faceted crystal was observed on the boundary of the spherulite (as shown by the arrows). Figure 3b is the ED pattern of the single-faceted crystal region (as shown by the circle), which is similar to that of single crystals (see below). The sharp (hko) reflections imply that the singlefaceted crystal possesses excellent crystallographic orientation, i.e., with the molecular chains perpendicular to the film plane. The possible developing process of the coexisting morphologies of the spherulites and the single-faceted crystals is that the spherulites consisting of flat-on lamellae are produced at first during the isothermal crystallization of the polymer. Subsequently, the side surface plane ((100) plane) of the flat-on lamellae (especially those located at the boundary of the spherulite) are used as the nucleation sites (self-nuclei), resulting in a homoepitaxial growth of the single-faceted crystals with the c-axis parallel to that of the flat-on lamellae. The contact planes of the homoepitaxy should be (100) planes for both the flat-on lamellae and the single-faceted crystals. Single-Crystal Structure Produced at Higher Temperatures. With the further increase of the crystallization temperature, a single crystal of the crystalline syndiotatic 1,2-polybutadiene was achieved. Figure 4 shows the BF electron micrograph and its corresponding ED pattern of the single crystal produced from
isothermal crystallization of the melt film at 150 °C for 2 h. The single crystal is near a hexagonal prismatic shape (Figure 4a) with its long axis along the crystallographic b-axis, as revealed by the corresponding ED pattern. It should be pointed out that in the single crystal growing some defects exists, especially in the long axis side, as shown by the arrow in Figure 4a. Many experiments have been done to try to prepare a single crystal with a regular hexagonal prismatic shape. Unfortunately, we failed to obtain it. The main reason is due to the higher crystallization temperature and the longer crystallization time. Under these conditions, the cross-linking reactions will take place during the isothermal crystallization process, which will prevent the single crystal from growing in some localized areas. In addition, just because of the cross-linking reactions, which prevent the polymer from crystallization, no crystalline morphology has been observed when the crystallization temperature is above 155 °C. Figure 4b shows the electron diffraction pattern of the single crystal of the syndiotatic 1,2-polybutadiene, and the reflections can be indexed according to the orthorhombic packing of the planar zigzag chains proposed by Natta and Corradini.2 Clearly, the strong (hko) reflections indicate the single crystal exhibits a perfect crystallographic orientation with the c-axis (the chain direction) perpendicular to the film plane, while the aand b-axes are in the film plane. The unit cell parameters calculated from the electron diffraction pattern are a ) 1.102 nm and b ) 0.664 nm, respectively, which are completely consistent with the data reported by Natta and Corradini.2 To obtain the unit cell parameter
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Figure 5. Model of orthorhombic packing of planar zigzag chains of the syndiotactic 1,2-polybutadiene.2
Figure 6. Sketch of the expected single-crystal shape of the syndiotactic 1,2-polybutadiene.
c, a highly oriented thin film was prepared and the parameter c was calculated from its ED pattern (see below). According to the orthorhombic packing features of the planar zigzag chains shown in Figure 5, a single crystal shape bounded by the densely packed {210} prism faces should be expected. Namely, a hexagonal prismatic shape as shown by the thick lines in Figure 6 should be a more reasonable expected shape of the single crystal of the syndiotatic 1,2-polybutadiene. However, the expected single crystal shape was not completely confirmed by the experimental observation (Figure 4a). The
Chen et al.
angles between two growth planes showing in Figure 4a are different from that between two {210} and that between {210} and {100} as shown in Figure 6. According to the experimental result, the growth planes of the single crystal should be {110} instead of {210}. A possible reason to create the angle differences between the expected hexagonal prismatic shape and the experimentally observed single-crystal shape is the effect of the crystallization temperature, i.e., the higher crystallization temperature (150 °C) may cause the changes of the single-crystal shape or growth planes. It is known that in some of polymer single crystals such as polyethylene, with increasing the crystallization temperature, the shape of single crystal changes. We have tried to grow a single crystal at lower crystallization temperatures (130-140 °C) for a longer time (60 h). Unfortunately, we did not obtain it. To sum up, the above results indicate that the isothermal crystallization temperature has an important effect on the morphology of the thin films of the syndiotactic 1,2-polybutadiene. When the crystallization temperature is lower, e.g., at 130 °C, the spherulitic structure consisting of flat-on lamellae is formed. At intermediate temperatures (e.g., 140 °C), the coexisting morphologies of the spherulites and the single-faceted crystals are developed. If the crystallization temperature is higher (150 °C), the single crystals with a hexagonal prismatic shape are obtained. With a further temperature increase (above 150 °C), as a result of the cross-linking reaction, crystallization of the polymer has been inhibited. Highly Oriented Lamellar Structure in the MeltDrawn Thin Films. Figure 7 shows the BF electron micrograph and its corresponding ED pattern of highly oriented films of the crystalline syndiotatic 1,2-polybutadiene, prepared by the special melt-drawn tachnique.16 The arrow in Figure 7a represents the drawing direction of the film. Obviously, the crystalline lamellae, which appear as dark lines, are oriented, with the growing direction perpendicular to the drawing direction. The average thickness of the lamellae is about 18 nm. The corresponding ED pattern (Figure 7b) of the melt-drawn films reveals that the molecular chains are highly oriented, with the c-axis parallel to the drawing direction. All the reflections can be indexed based on the orthorhombic packing.2 The identity period along
Figure 7. BF electron micrograph (a) and corresponding ED pattern (b) of the melt-drawn thin films.
Crystalline Syndiotactic 1,2-Polybutadiene
the fiber axis, i.e., the unit cell parameter c, is 0.513 nm, as calculated from the (hkl) reflections in Figure 7b. Conclusions The solution-cast thin films of the crystalline syndiotatic 1,2-polybutadiene consist of “lath-like” lamellar structure with the c-axis perpendicular to the film plane. The crystalline morphology of the thin films is different at various isothermal crystallization temperatures. With the increase of the crystallization temperature, at least three kinds of crystalline morphologies, i.e., the spherulitic structure consisting of flat-on lamellae, the coexisting morphologies of the spherulites and the singlefaceted crystals, and the single crystals, are produced at lower, intermediate, and higher temperatures, respectively. The single crystals exihibit a hexagonal prismatic shape with its long axis along the crystallographic b-axis. The electron diffraction results confirm the orthorhombic packing of the planar zigzag chains. In addition, the melt-drawn films consist of highly oriented lamellae with its chain direction parallel to the drawing direction. Acknowledgment. The financial support from the National Science Foundation of China (No. 20234020) and the Special Funds for Major State Basic Research Projects are gratefully acknowledged.
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