Melt crystallization of behenic acid in microspheres dispersed in polymer

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ELSEVIER

CRYSTAL QIROWTH

Journal of Crystal Growth 163 (1996) 440-444

Melt crystallization of behenic acid in microspheres dispersed in polymer Kyoji Tsutsui a,*, Yoshihiko Hotta a, Kiyotaka Sato b a Chemical Products R & D Center, Ricoh Co., Ltd., Honda-machi, Numazu 410, Japan b Faculty of Applied Biological Science, Hiroshima University, Kagamiyama, Higashi-Hiroshima 724, Japan

Received 11 April 1995; accepted 27 September 1995

Abstract

Kinetic studies were made for melt crystallization of behenic acid, which was dispersed in thin films of a polymer, poly(vinyl chloride-co-vinyl acetate), as microspheres having varying sizes in a range of 0.2-2.8 /xm. Irrespective of the size of the microspheres and the crystallization procedure, the melting of behenic acid occurred at 79°C. However, complicated crystallization behavior was observed, depending both on the thermal treatment before cooling and on the size of the microspheres. When cooling started from a temperature above the melting point, crystallization occurred at 36-52°C, decreasing with decreasing size of the microsphere. By contrast, three stages of crystallization occurred, just below the melting point, around 60°C and about 40°C, in the case that the cooling started from the temperature range of melting. The first two stages of crystallization did not depend on the size of the microspheres. A mechanistic interpretation is presented to explain the complicated crystallization processes.

1. Introduction

A rewritable recording device of a novel type has been developed by the present authors [1-3]. It employs long-chain fatty acids as crystallizing materials and polymers in which microspheres of the fatty acids are dispersed. The key processes revealing the light scattering (opaque) and transparent states, are illustrated in Fig. 1, involving: (a) the formation of the microspheres of crystallizing materials in the dispersed film; (b) the cooling process starting from a temperature above the melting point of fatty acid to produce light scattering bodies (Fig. 1, A ~ B); (c)

* Corresponding author.

the heating and re-cooling processes starting from the temperature of initial melting of fatty acid to annihilate the light scattering bodies (Fig. 1, B ~ C D); (d) the re-cooling process starting from a partially melting state leads to a half transparent state (Fig. l a, B ~ E ~ F). It is important to select the polymers having an optimal glass transition temperature (Tg), which should be lower than the melting point of the fatty acid. The light scattering bodies are the microspheres including polycrystals of fatty acids, and void spaces, which are formed when crystallization occurs far below Tg by process (b), because of the volume changes associated with the crystallization of the fatty acid. This does not occur when the crystallization takes place above Tg by process (c), since the softened polymers do not form a void after

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K. Tsutsui et al./ Journal of Crystal Growth 163 (1996) 440-444

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crystallization of behenic acid. In this system, a regulation of the crystallization temperature of the dispersed fatty acid is critically important. The present paper aims at precise analysis of the crystallization kinetics of behenic acid (BA), a longchain fatty acid having 22 carbon atoms and a melting point (Tm) of 79.5°C. BA was dispersed in poly(vinyl chloride-co-vinyl acetate) (PVCA) with Tg at 70°C. Special attention was paid to the effects of the size of microspheres and the starting temperature of cooling on the crystallization behavior of BA. The event of crystallization was monitored in-situ by DSC (differential scanning calorimetry), X-ray diffraction and FF-IR techniques.

2. E x p e r i m e n t a l procedure

Poly(vinyl chloride-co-vinyl acetate) (Mn = 27000 and co-polymerization ratio= 86/14) was purchased from Union Carbide Chemicals and Plastics Co., Ltd. Behenic acid (99% purity) was avail-

441

able from Sigma Chemical Co, BA and PVCA are insoluble to each other. To form the dispersed film, PVCA and BA were dissolved in tetrahydrofuran at 18 wt% of PVCA and BA in solution. The solution was coated on a transparent poly(ethylene terephthalate) film, a base film. The coated film was dried at 110°C to form the dispersed film of about 10/zm in thickness. The size of the BA microspheres was controlled by changing the weight ratio of B A / P V C A ( 1 / 6 and 1/3), followed by thermal annealing. For example, the B A / P V C A = 1 / 6 initially formed microspheres in a size range of 0.2-1.0 /zm (see below). Thermal annealing of this film at 140°C for 30 min enlarged the microspheres to a size range of 0.7-1.4/zm. The dispersed film formed thus was peeled off from the base film and subjected to in-situ DSC (MAC Science DSC3100), X-ray diffraction (Rigaku RINT1100 diffractometer, scintillation counter type) and Fr-IR (Perkin Elmer 1725X) measurements. The influence of the PVCA matrix on DSC and X-ray diffraction was negligible. The rate of heating and cooling was 2°C/rain for DSC or l°C/min for X-ray diffraction measurement. As to FT-IR, complicated absorption spectra of PVCA overlapped with many absorption bands of BA, yet CH 2 scissoring band of BA at 1460-1480 cm -l of BA was not affected. Hence, this band was employed to monitor in-situ the crystallization process. A transmission electron microscopic observation was done using film slices of about 0.1 /zm in thickness, which were carefully cut by a microtome vertically to the dispersed film surface.

3. Results and discussion

Fig. 2 shows a transmission electron micrograph of the dispersed film containing the B A / P V C A = 1/6. This shows that most of the BA microspheres are in a size range of 0.2-1.0 /zm. DSC heating and cooling thermograms are shown in Fig. 3 for three dispersed films with different size ranges of the BA microspheres, and two bulk samples of BA with and without PVCA. In the first, the melting of BA occurred at the same temperatures at 79.8 + 0.1°C for the film samples. As to the cooling of the dispersed films, the temperature was lowered

442

K. Tsutsui et aL / Journal of Crystal Growth 163 11996) 440-444

Fig. 2. A n electron m i c r o g r a p h o f a dispersed film (ratio o f B A / P V C A = 1 / 6 ) . Bright i m a g e s : b e h e n i c acid microspheres; dark images: P V C A matrix.

from 82°C, well above the melting state of BA, to form the light scattering bodies. Crystallization temperatures (T~), defined by DSC exothermic peak tops, are 45°C (size range: 1.1-2.8 /xm), 40°C (0.51.8 /~m) and 39°C (0.2-1.0 /xm). It is manifest that T~ was lowered with reducing size of the microspheres, particularly in the range of above 1 /~m. For the comparison, a bulk mixture containing B A / P V C A = 5//1 was measured to examine the

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effect of the addition of PVCA on T~ of BA (Fig. 3d). In this case, BA crystallized at 76.5°C, which is very close to that of pure behenic acid, 76.7°C. Hence, it is evident that the lowering in Tc of BA in the dispersed film is due to encapsulation in the microsphere, not due to the impurity effect. Accordingly, the lowered Tc enables the crystal formation of behenic acid far below Tg of PVCA. Fig. 4 shows the DSC thermograms of the dispersed films during re-cooling processes started from the temperature range of melting (state C and D in Fig. 1) after heating from state B in Fig. 1. This process is related to the annihilation procedure of the light scattering bodies. Two remarks should be noted, (a) the recrystallization behavior is sensitive to the starting temperature of cooling (T~t), and (b) two additional DSC exothermic peaks appear, which were absent in the process of cooling started from 82°C (Fig. 3). The first peak appears just below the melting point, and the second one at 60°C. The two crystallization temperatures did not depend on the size of the microspheres. It should be noted that the bulk samples of pure BA and the mixture BA/PVCA did not reveal the second crystallization processes upon cooling from the partially melting state (Figs. 4f and 4g). It should be noted here that the complete annihilation of the scattering bodies is attained by the recrys-

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K. Tsutsui et a l . / Journal of Crystal Growth 163 (1996) 440-444

tallization of type (a) in Fig. 4. Namely, the cooling started from the initial melting state, and the crystallization occurred just below the melting point, but above Tg of PVCA. By contrast, the annihilation was not completed when the crystallization predominantly occurred at 60 and 40°C below Tg of PVCA like type (b)-(d), as cooled from the partially melting state. X-ray diffraction provided the structural information associated with the crystallization processes (Fig. 5). All of the diffraction peaks indicate the presence of the C-form of behenic acid, a high-temperature polymorph. Neither transformation nor crystallization of low-temperature polymorphs (B form and A form) were detectable. Fig. 5a shows the (110) and (200) reflections of behenic acid, initiating to appear at 50°C during the cooling process starting from 82°C. Fig. 5b shows slight changes in the diffraction intensity of the (110) and (200) reflections during the re-cooling process starting from 79°C. The first increase occurred at just below the melting point of BA, meanwhile the second one occurred at 60°C and the third one at 45°C. Correspondingly, the d-values slightly decreased by 0.01 A at 60 and 40°C. These changes may be related to the changes in additional growth of the C-form, not to the polymorphic transition. These results are well consistent to the DSC exothermic peaks displayed in Figs. 3 and 4. Fig. 6 shows the FF-IR spectra of CH2-scissoring bands of BA obtained during the same thermal treatments as those of Fig. 5. Two absorption bands

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(1472 and 1464 cm - ] ) appear at 40°C during the cooling process starting from 82°C (Fig. 6a). The band at 1472 cm -1 is stronger than the band at 1464 cm -1. On the other hand, the re-cooling process from 78.5°C revealed a gradual increase in the two absorption bands (1472 and 1464 c m - l ) . Accordingly the latter case shows the crystallization of the C-form of behenic acid [4]. In the former case, the spectra change shows the crystallization of the Cform and another form which was not identified. Based on the experimental data described above, we discuss the three important stages of crystallization of BA dispersed in the PVCA matrix, referring to the model depicted in Fig. 1: (a) Tc around 40°C (Fig. la, A-B), which is dependent on the size of the microspheres. (b) T~ around 60°C (Fig. 1a, E - F ) and (c) Tc just below the melting points (Fig. la, C-D). The processes of (b) and (c) occur when the re-cooling is started from the temperature in a range of temperatures of initiation and completion of melting of behenic acid, after the heating of the crystallized samples. In particular, process (c) is the crystallization of the liquid fraction present in the space between the BA crystals due to surface melting. The present experiments directly proved the existence of process (a), as observed in Figs. 3a-3c, 5a and 6a. The processes of (b) and (c) were also observed by DSC (Fig. 4) and X-ray diffraction (Fig. 5b). As to the process (a), which plays a decisive role in the light scattering property, supercooling is quite large, ~ 40°C. A possible mechanism to explain this large supercooling is that the interaction between BA

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K. Tsutsui et a l . / Journal of Crystal Growth 163 (1996) 440-444

molecules and PVCA polymer chains may inhibit diffusion of BA molecules due to kinetic effects, increasing the supercooling up to 40°C. The reduction in the size of microspheres of BA below a few /zm may allow this kinetic effect to be observed. In this connection, we think that the simple size effect caused by the reduction in the crystallizing medium may not be determinative, since BA is a small molecule and the size of a microsphere is not small enough to reveal the simple size effect. One may note the fact that polymer crystallization was reduced when the size of a molten liquid droplet was decreased to 5 ~ m , increasing the supercooling [5]. This was interpreted to be ascribed to the molecular interactions occurring at the liquid/substrate interface. In the case of process (b), we speculate that the presence of the seed crystals may accelerate heterogeneous nucleation to some extent, under the kinetic influences from the PVCA molecules, raising T~ by 20°C compared to process (a). Further study is needed to clarify the kinetic interactions between BA and

PVCA molecules, which reduces the rate of heterogeneous nucleation of BA crystals.

Acknowledgements We are deeply indebted to Professor M. Kobayashi of the Faculty of Science, Osaka University, for helpful suggestions and discussions.

References [1] Y. Hotta and K. Kubo, Proc. 4th Symp. Non-lmpact Printing Technol., Tokyo (1987) 57. [2] Y. Hotta, T. Yamaoka, K. Morohoshi, T. Amano and K. Tsutsui, Chem. Mater., to be published. [3] K. Tsutsui, Y. Hotta and K. Sato, Nippon Kagaku Kaishi 8 (1995) 641 (in Japanese). [4] M. Kobayashi, Crystallization and Polymorphism of Fats and Fatty Acids, Eds. N. Garti and K. Sato (Dekker, New York, 1988) p. 146. [5] P.J. Barham, D.A. Jarvis and A. Keller, J. Polym. Sci. Polym. Phys. Ed. 20 (1982) 1733.