Introduction

Polybutene-1 (PB1) has outstanding mechanical properties. However, its applications are limited compared to those of the lighter olefin polymers such as polyethylene and polypropylene. One main reason is the complication introduced by crystal structure transformations. PB1 is polymorphous, with a trigonal form and a tetragonal form as the most common structures. Trigonal form is stable and tetragonal form is metastable, but despite the metastability it is always tetragonal form which is obtained when PB1 crystallizes from the melt. The transformation to trigonal form then takes place after cooling to room temperature, accompanied by strain of crystallized samples. Because of this transformation and strain, direct melt crystallization of PB1 trigonal form has long been a big issue.

In 1990’s, Kopp et al. reported that the trigonal phase can crystallize in the melt via epitaxy on aromatic acids or salts [1]. At an elevated temperature of 110°C, Zhang et al. found that trigonal crystals can be obtained from molten ultrathin films under atmospheric pressure [2]. We present another solution to this issue. Using solution-grown trigonal crystals as nuclei, we observed that the trigonal phase can grow in the melt via self-seeding at atmospheric pressure. In this paper we present some experimental results regarding the in-situ observation of trigonal crystals growing from the melt at several different temperatures and a comparison between the crystal growths of the trigonal and tetragonal phases..

Experimental

The PB1 used in this study was purchased from Scientific Polymer Products (Mw=185,000; the melt index is 20 g/10 min). Thin PB1 films were prepared by casting a 0.1 wt% p-xylene solution onto carbon-coated mica kept at 60°C on a hot plate. The films were dried in air, an appropriate film thickness of ca. 80 nm being judged by a gold interference color.

Crystallization was carried out on a hot stage (Mettler FP82). The PB1 films were heated at 128-135°C for 2 min and cooled to a crystallization temperature between 65°C and 87.8°C at a rate of 15 K min^-1. In-situ observations of the crystallization process were performed using an optical microscope (OM; Nikon OPTIPHOT2); the growth rate was determined from the time dependence of the radius or the major axis of crystals observed by OM.

Transmission electron microscopy (TEM; JEOL JEM-1200EXII) was used to identify crystal structures. Samples were examined immediately after crystallization and quenching. The PB1-carbon films were floated on a water surface, picked up on electron microscope grids and used as samples.

Results and Discussions

A sequence of the isothermal crystallization process at 75°C after heat-treatment at 132°C for 2 min is shown by successive optical micrographs in Figure 1. Faint circular crystals emerge from the melt in Figure 1a. The crystals slowly increase in size (Figures 1b, c) and finally some of them impinge upon each other (Figure 1d).

Figure 2 shows a transmission electron micrograph of an PB1 film that was heat-treated at 132°C for 2 min and then isothermally crystallized at 75°C, and its corresponding electron diffraction pattern. A round crystal similar to those observed in the OM images can be seen. The electron diffraction pattern shows a net pattern with hexagonal symmetry, and all the Bragg reflections could be indexed with the trigonal form of PB1. The electron micrograph and diffraction pattern demonstrate that the round crystal grown from the melt is a single crystal in the trigonal form.

Trigonal crystals are known to be obtained in solutions. The PB1 films used in this study are prepared by casting an PB1 solution onto a carbon-coated mica, and therefore contain abundant trigonal crystals grown from the solution. When the films are heated up to a temperature near but below the equilibrium melting point of PB1, 136°C, melting of the trigonal crystals is incomplete, which eventually prepares numbers of trigonal nuclei; we can utilize the incompletely melted trigonal crystals for self-seeding. On cooling to a crystallization temperature, we can observe trigonal crystals growing in the melt. These facts are considered to enable the trigonal crystals to grow in the melt even at atmospheric pressure and lower crystallization temperatures.

The radius R of the circular crystals increased linearly with crystallization time t for all the crystallization temperatures investigated as shown in Figure 3. The linearity indicates that the crystal growth is controlled not by diffusion, but by kinetics at the interface. The growth rate G was determined from the slope of the time-radius curve. The logarithm of G is plotted against crystallization temperature in Figure 4. The values of the growth rate G of tetragonal crystals we observed in our previous work [3] are also included for comparison. It should be noted that the growth rate of trigonal crystals is one hundredth that of tetragonal crystals. In 1965, Powers et al. used trigonal crystals obtained by solid-state transformation from the tetragonal phase as nuclei and attempted to observe the growth of trigonal crystals in the melt. This was, unfortunately, not successful and they hypothesized that the growth rate of trigonal crystals is “exceedingly” slower than that of tetragonal crystals [4]. The result obtained in this work is consistent with the prediction made by Powers et al.

Summary

Crystal growth of the it-PB1 trigonal form was successfully observed in the melt at atmospheric pressure. The growth rate of trigonal crystals was obtained by in-situ optical microscopy. The growth rate G of trigonal crystals is one hundredth that of tetragonal crystals.

References

[1] Kopp, S.; Wittmann, J. C.; Lotz, B. Polymer 1994, 35, 916.

[2]Zhang, B.; Yang, D.; Yan, S. J Polym Sci Part B: Polym Phys 2002, 40, 2641.

[3]Yamashita, M.; Miyaji, H.; Izumi, K.; Hoshino, A. Polym J 2004, 36, 226.

[4]Powers, J.; Hoffman, J. D.; Weeks, J. J.; Quinn Jr., F. A. J Res Nat Bur Std (U.S.) 1965, 69A, 335.

Fig1. In-situ optical micrographs of PB1 trigonal crystals taken at 75°C at intervals of 3 min.

Fig 2. (a) Electron micrograph and (b) its corresponding selected area electron diffraction pattern of an PB1 single flat-on crystal in the trigonal form grown at 75 °C. A round PB1 crystal is indicated by an arrow. The arc in the upper part of (a) is an area-selecting aperture.

Fig 3. Time dependence of radius R of trigonal crystals at several crystallization temperatures.

Fig 4. Growth rate G versus crystallization temperature T for the trigonal and tetragonal phases.

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