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Direct observation of polymer crystallization process under shear by a shear flow observation system
Polymer Testing 22 (2003) 101–108 www.elsevier.com/locate/polytest
Test Apparatus
Direct observation of polymer crystallization process under shear by a shear flow observation system K. Watanabe a, W. Nagatake a, T. Takahashi a, Y. Masubuchi b, J. Takimoto a, K. Koyama a,∗ b
a Department of Polymer Science and Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa 992-8510, Japan Department of Computational Science and Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
Received 20 March 2002; accepted 13 May 2002
Abstract Isothermal crystallization of polybutene-1 melt under shear flow was observed with an optical microscope by using a device newly developed by our group. Firstly, our coaxial cylinder-type apparatus (shear flow direct observation system (SF-DOS)) to observe crystallization directly under shear flow is explained in detail. Secondly to examine the accuracy of the measurement with the new device and to establish an appropriate procedure for the elimination of overshoot in temperature profiles, various examinations were performed in terms of temperature distribution, stability, and the velocity gradient by comparing the results from isothermal DSC measurements. The precise investigation suggested that the temperature distribution was proved to be very small (within 0.5°C) and the establishment of the procedure eliminated the overshooting of temperature profiles. Finally, we observed isothermal crystallization of polybutene-1 with the new device at various temperatures between 102°C and 140°C under shear rates of up to 0.7 s–1. When the shear strain became higher and the temperature became lower, narrow bands of oriented region formed after a certain time along the flow direction even at high temperature. This tendency was also observed in a polystyrene melt; therefore, it was interpreted that the narrow bands were considered to be the highly deformed region, which was generated by high shear stress. In the narrow bands, spherulites nucleated. After a certain amount of time, spherulites were also generated randomly outside of the narrow bands. Under very low shear rate, where the narrow bands did not exist, the spherulites appeared randomly. 2002 Elsevier Science Ltd. All rights reserved. Keywords: Shear flow direct observation system; Crystallization; Polybutene-1; Isothermal
1. Introduction In the 1970s, investigations showed that the crystallization of polymers are induced and accelerated by flow [1–10]. This idea has been practically applied in the melt spinning of fiber, film molding, etc. [11]. In recent years, in our laboratory, computer-aided engineering (CAE) including flow-induced crystallization and experimental investigations for polymer crystallization have been carried out in the injection molding process [12–16]. A
∗
Tel.: +81-238-26-3055; Fax: +81-238-26-3411. E-mail address: [email protected] (K. Koyama).
shear flow thermal rheometer (SFTR) having differential thermal analysis capability has been successfully developed using a rotational shear rheometer to enable the direct determination of crystallization fraction under shear flow [17,18]. This mechanism should be related to the molecular orientation in the melt. The effect of flow on the meso-scale morphology, such as the spherulite structure, is a very important factor for polymer processing and molding performance; however, morphology and crystallization kinetics under shear are not yet well understood. Due to flow, polymer chains can crystallize with morphologies different from those encountered under static conditions (observation of shish-kebabs or row-nucleated structures rather than the usual spherulites).
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Various devices able to apply shear have been built in order to study crystallization under shear or after shear. Many studies have been concerned with the measurement of the induction time of crystallization [1– 5,18] because the onset of crystallization is relatively easy to characterize by an increase of the force or the transmitted torque. Some studies [2,10,21] have determined the fraction of transformed material versus time and have reported anomalous values of the Avrami exponent under shear, but few in situ measurements concern the density of nuclei, the growth rate [7] and morphology development [4,8] during shear flow. Recently, Monasse [19,20], Tribout et al.[21] and Jay et al.[22] developed experimental apparatus to observe the crystallization kinetics with a flow generated by a plane–plane device [19,21] and pull-out fiber [20–22]. They make it possible to perform isothermal crystallization after rapid cooling, under an optical microscope. The appearance and development of crystalline morphologies could be observed during crystallization. They measured the growth rate of polyethylene [19], polypropylene [20,22] and the nucleation rate, density and growth rate of polypropylene copolymer [21]. They revealed that both the nucleation rate and the growth rate are affected by the flow. The plane–plane device can apply a constant shear rate (velocity gradient was not discussed), but the direct observation of the morphologies in the shear plane (parallel to the flow (X), sample thickness (Z), perpendicular to the flow (Y), and shear plane (XZ)) was not possible [19]. On the contrary, the fiber pull-out device can observe the morphologies in the shear plane but shear flow was localized near the fiber and not constant. Numerous spherulites grow from the fiber [22]. Neither device can apply long shearing times to the sample; the construction of the plane–plane device limited the shearing time to 60 s for a shear rate of 2 s–1 and to 3 s for the maximum shear rate of 26 s–1 [21]. In this study, we developed an originally designed coaxial cylinder-type device (shear flow direct observation system (SF-DOS)) to directly observe the crystallization process in the shear plane under shear flow. It was possible to observe the crystalline morphologies during shear flow. The purpose of this study is to develop a new device which can apply constant shear rate, stable temperature and long shearing times. In isothermal crystallization experiments of polybutene-1, the results of the velocity profile and thermal data by using SF-DOS are presented. 2. Experimental 2.1. Development of shear flow direct observation system Fig. 1 shows (a) a photograph and (b) a schematic diagram of the newly developed device for the direct
Fig. 1. (a) The photographs of the developed SF-DOS apparatus. (b) Its schematic diagram.
observation of crystallization under shear flow. It consists of a shallow cylinder and a rotating disk made from phosphor bronze. The sample is placed in the gap (0.5 mm) between the inner disk and the outer cylinder. The inner disk (diameter 109 mm and thickness 2 mm) is driven by a stepping motor (Oriental Motor Co. Ltd.) through a worm gear and a worm wheel to apply shear
K. Watanabe et al. / Polymer Testing 22 (2003) 101–108
flow to the sample. The outer cylinder is 110 mm internal diameter and is fixed on a glass pedestal. To stabilize temperature, the disk and the cylinder are wrapped in an insulating jacket made from MIOLEX (Ryoden Kasei Co. Ltd.). Mica heaters (Hakko Co. Ltd.) are installed into the inner disk, the outer cylinder and the pedestal, and the thermocouples (Thermocoax Company) are inserted in the edge of the inner disk (positions B, D, E ), the outer cylinder (position C) and the sample (position A). The thermocouple at position A was only used before and after the measurements, because the thermocouple disturbs the uniform shear flow. They are connected to a thermal control system (Shimaden Co. Ltd.). The positions, A, B, C, are controlled by the thermal control system. A part of the gap was illuminated through a transparent bottom plate, and the crystallization observed by an optical polarizing microscope (Leica Company, type DMR HC) equipped with a CCD camera (Simazu Co. Ltd., type CCD-SII). Micrographs of the morphologies were recorded on videotape, and imported by a personal computer. 2.2. Samples Polybutene-1 (PB-1) was used as a test polymer for observing crystallization behavior. The weight-averaged molecular weight Mw was 180,000 and the molecular weight distribution index (Mw/Mn) was 3.3. Atacticpolystyrene (PS, PS666) was supplied by Asahikasei Co. Ltd. and used as a reference amorphous polymer. The weight-averaged molecular weight was 210,000 and the Mw/Mn was 2.2. Disk-like samples with 110 mm diameter and 0.5 mm thickness were made from pellets using a hot-press machine. These disk samples were used for the measurements in the SF-DOS. 2.3. DSC curves of polybutene-1 under isothermal crystallization The isothermal crystallization characteristics of PB-1 under quiescent state were studied with a differential scanning calorimetry (DSC) from Perkin-Elmer Co. Ltd. (type DSC-7). The samples (7.2 mg of weight) were melted at 180°C for 5 min in order to eliminate any thermal history of material, then rapidly cooled to the crystallization temperature, Tc. 2.4. Measurement by shear flow direct observation system To eliminate thermal history, considerable attention was paid to the establishment of the measurement procedure by the new device. First, the sample was kept at 180°C for 5 min, and then cooled to a temperature 10°C higher than the measurement temperature. Second, the sample was kept for 5 min, then cooled to a temperature
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5°C higher than the measurement temperature and finally kept for 5 min and then cooled to the measurement temperature to avoid overshooting of temperature. Before observation, the measurement temperature was held for 10 min to stabilize temperature. The observation was carried out at isothermal conditions under steady shear rates of 0, 0.025, 0.05, 0.1, 0.5 and 0.7 s–1. The velocity profile was measured by using a dispersion which consists of polyethylene particles (particle size 25 µm, Mipelon from Mitsui Chemical Ltd.) and silicon oil ( PE/PDMS, weight ratio is 5/95).
3. Results and discussion 3.1. DSC curves of polybutene-1 crystallization isotherms To separate the crystallization induced by shear from the crystallization without shear, it is important to find at which temperature crystallization occurs even without shear. Isothermal DSC measurements were performed to determine the critical temperature. Fig. 2 shows the crystallization isotherms of PB-1 at different temperatures obtained by DSC under quiescent state. Below 98°C, crystallization took place. Cooling speed to the isothermal temperature was 200°C/min, so that a very slight overshoot might have taken place. At 100°C, the isothermal DSC measurements were conducted at two conditions. One is after normal rapid cooling (200°C/min). The other is two-step cooling, rapid cooling to 105°C and slow cooling from 105 to 100°C (1°C /min) to aim at eliminating the overshoot. Above 102°C, crystallization under quiescent state did not take place within 2.5 hours. It is considered that 100°C is a critical temperature for crystallization. Therefore, it would be safe to conclude that we should observe the effect of shear-induced crystallization above 102°C, where no crystallization occurred without shear. 3.2. A shear flow direct observation system 3.2.1. Temperature distribution and stability in the shear flow direct observation system To measure and to minimize the temperature distribution in the shear flow direct observation system is important. We used five thermocouples. The thermocouples’ location were A (sample), B, D, E (rotating disk), C (outer cylinder), as shown in Fig. 1 (b). The position A was located in the sample, before and after shear observation to keep uniform shear flow. Fig. 3 represents temperature change as a function of time from 180°C at various points before observation. The temperature profile has already been mentioned in the experimental section. The positions, A, B, C, are open to the air and controlled by the thermal control sys-
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Fig. 2.
K. Watanabe et al. / Polymer Testing 22 (2003) 101–108
DSC curves of PB-1 crystallization isotherms at (a) 96°C, (b) 97°C, (c) 98°C, (d) 100°C, (e) 100°C (slow cooling), and (f) 102°C.
is about 2°C higher than the other positions, B, C, D, E. This difference would be due to the difference in terms of the thermal conductivity and the heat capacity between the metal and the polymer. This fact suggests that it is difficult to control the polymer temperature and we have to measure the sample temperature precisely. We have found an overshoot problem (about 2–4°C below the target isothermal temperature) when cooling to the measurement temperature in the newly developed device. To eliminate the overshoot phenomenon, therefore, considerable attention was paid to the establishment of the measurement procedure. First, the sample was kept at 180°C for 5 min, and then cooled to a temperature 10°C higher than the measurement temperature. Second, it was kept for 5 min, then cooled to a temperature 5°C higher than the measurement temperature, kept for 5 min and then finally cooled to the measurement temperature to avoid overshooting of temperature. Before observation, the measurement temperature was maintained for 10 min to stabilize temperature. Fig. 4 shows temperature profiles (four different target temperatures) of the melt (position A) as a function of time before observation according to the procedure. It is clearly shown that by using the step-down cooling procedure (except 100°C) in Fig. 4, the overshoot problem disappeared and the stability of temperature was within 0.5°C. Fig. 3. Temperature change as a function of time at various points before observation. A sample (䊐), B rotating disk (䊏), C outer cylinder (䉬), D rotating disk (쎲), E rotating disk (왖).
tem, therefore, the temperature of the positions, D, E are higher than the positions, B, C before reaching the equilibrium stage. However, about 10 min after setting to the measurement temperature, temperature stability was achieved except at position A (sample). The position A
3.2.2. Velocity profile of the polyethylene particle The device is new, so that it was necessary to examine the velocity profiles and whether pure shear flow is generated or not. We have used silicone oil including 5 wt% of polyethylene particle (PE/PDMS = 5/95) between the rotating disk and outer cylinder to calculate the velocity profiles under shear. Fig. 5 (a) shows the optical micrograph of the 5/95 PE/PDMS under a shear rate of 0.05 s–1, and Fig. 5 (b) shows the velocity profile of the polyethylene particle under various shear rates. The velocity
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profile of the shear was not disturbed. These results allowed us to conclude that pure shear flow can be generated in the newly developed device.
Fig. 4. Sample temperature change as a function of time before observation. 112°C (쎲), 105°C (䊏), 102°C (䉬), 100°C, overshoot (왕).
Fig. 5. (a) Optical micrographs of the 5/95 PE/PDMS ( g˙ ⫽ 0.05s⫺1). (b) Velocity profile of the polyethylene particle.
3.2.3. Direct observation of polymer under shear flow Fig. 6 shows typical snapshots of the observation under crossed-Nicols (polarizer and analyzer were at 45° to the flow direction). The rotating disk was located on the lower side. The images of PB-1 observed under shear flow are shown in Fig. 6 ((a) at 112°C, (b) γ˙ = 0–0.05 s–1 at 102°C, (c) γ˙ = 0.1–0.7 s–1 at 102°C). The crystallization of PB-1 did not take place within 10 min at 112°C under shear rates below 0.1 s–1. At a shear rate of 0.5 s– 1 or higher, on the other hand, bands of oriented region appeared along the flow direction, which were seen as bright bands under crossed-Nicols conditions, as shown in Fig. 6 (a). This implies that highly deformed regions were generated at high shear rates after a certain amount of time. Under no shear flow, crystallization did not take place within 2 hours at 102°C (Fig. 6 (b)); this is consistent with the results of DSC (Fig. 2). The nucleation rate and growth rate of PB-1 under quiescent state at 105°C has been reported by Wolkowicz [7], the crystallization of PB-1 has taken place 10 hours after the beginning of the measurement. Under shear rates more than 0.1 s–1, bright bands were also observed at 102°C. Soon after detecting the bands, the distorted (slightly anisotropic) spherulites were also observed in the bands at 102°C (Fig. 6 (c)), for a shear rate of 0.5 s–1 or higher. In addition, spherulites were also generated outside of the bands. Wolkowicz [7] has measured the nucleation and growth rates of PB-1 under higher shear rate (0.2–12 s–1) but the morphological and kinetic behaviors were not observed directly. Two types of nuclei were exhibited: randomly dispersed [7] and row nuclei [4,8,9,19]. Kobayashi et al. [9] reported calculations to show the effects of chain elongation on crystal thickness, nucleation and growth rates in terms of the change in birefringence on orientation. They revealed the morphological observation of transverse growth from row nuclei by using electron micrographs. Monasse [19] observed polyethylene crystallization during shear by using a parallel-plate device. It is reported that the row-nucleated structure observed in the direction perpendicular to the flow direction, and some of these initial nuclei turned from the perpendicular direction to the flow direction during shear. Ulrich et al. [8] observed the row-nucleated structure along to the flow direction and random nuclei of poly(ethylene oxide) by using a rotational plate–plate. They observed the rownucleated structure on the surface of the glass plate and supposed that the row-nucleated structure formed along fine scratches or heterogeneities on the glass plate. These results were quite similar to those of Hass et al. for PB1[4], but they supposed that the spherulites cause a disturbance resulting in a local increase in stress of the melt
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Fig. 6. Typical snapshots of the morphology of PB-1 crystallizing under various shear flow at (a) 112°C, (b) 102°C (g˙ ⫽ 0⫺ 0.05s⫺1), (c) 102°C (g˙ ⫽ 0.1 ⫺ 0.7s⫺1), (d) typical snapshots of PS. The polarizer and analyzer were kept at 45°. Arrows indicate the flow direction.
K. Watanabe et al. / Polymer Testing 22 (2003) 101–108
and row-nucleated structure formation. Our experimental results are different from previous results. The bands have not been reported until now and the spherulites occurred randomly in the bands, not located in the same line in the bands. Temperature distribution in the SFDOS is almost zero, as mentioned earlier. We have eliminated the overshoot problems of the temperature profiles. The observation was started substantially after reaching the equilibrium stage of the temperature. When the shear strain became higher and the temperature became lower, the narrow bands of oriented region formed after a certain time along the flow direction even at high temperature. The bright bands seemed to be related to highly deformed regions. To get more insight into the origin of the bright bands, amorphous a-PS was tested with the SF-DOS (Fig. 6 (d)). Under shear flow, bands similar to PB-1 were observed at 140°C under higher shear rates; therefore, these bands would be related to molecular orientation along the flow direction, but not crystallization. The bright bands were observed at lower temperature with higher shear rates after a certain amount of time, even with stable temperature without overshooting and uniform shear rates. Taking these results into consideration, the bright bands were generated above a certain amount of critical shear stress, where it is difficult to deform uniformly. We consider that the spherulites develop more easily in the highly oriented bands rather than in the matrix region. On the other hand, when the shear rate is very low (0.025 s–1) at 102°C, spherulites (slightly anisotropic) appeared randomly without having bright band structures (Fig. 6 (b)). As the shear stress and the viscosity become higher, uniform shear flow was not formed completely, resulting in the separated bands appearing and a nucleated structure was developing in the bright bands, which are illustrated in Fig. 6 (c). As the shear rate increased, the generation of nuclei became accelerated. We intend to measure the nucleation rate and growth rate of isolated structures under slow shear rate (below 0.1 s–1 at 102°C) quantitatively, which will be published elsewhere.
4. Conclusions The present study provided the following new findings: 1. To observe the crystallization under shear flow, a shear flow direct observation system (SF-DOS) has been built. To examine the accuracy of the measurement with the new device and to establish an appropriate procedure for the elimination of overshoot in temperature profiles, various examinations, such as temperature distribution, stability, and the velocity
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gradient, were carried out by comparing the results from isothermal DSC measurements. The precise investigation suggested that the temperature distribution is very small (within 0.5°C) and the establishment of the procedure (cooling down the temperature stepwise) eliminated the overshooting of temperature targets. 2. By using SF-DOS, bands of more oriented regions along the flow direction appeared in both polybutene1 and PS at lower temperatures with higher strain rates. It was considered that the bands were generated above a certain amount of critical shear stress where it was difficult to deform uniformly. The spherulites of PB-1 developed more easily in the molecular oriented bands rather than in the matrix. Under very low shear rates, spherulites of PB-1 developed randomly without forming the band structures.
Acknowledgements We thank Mr Y Seino for his precise machinery work in making the device for direct observation of crystallization under shear flow.
References [1] D. Krueger, G.S.Y. Yeh, J Appl Phys 43 (1972) 4339. [2] A.K. Fritzsche, F.P. Price, Polym Engng Sci 14 (6) (1974) 401. [3] R.R. Lagasse, B. Maxwell, Polym Engng Sci 16 (3) (1976) 189. [4] T.W. Haas, B. Maxwell, Polym Engng Sci 9 (4) (1969) 225. [5] V. Tan, C.G. Gogos, Polym Engng Sci 16 (1) (1976) 512. [6] P.G. Andersen, S.H. Carr, Polym Engng Sci 18 (3) (1978) 215. [7] M.D. Wolkowicz, J Polym Sci Polym Symp 63 (1978) 365. [8] R.D. Ulrich, F.P. Price, J Appl Polym Sci 20 (1976) 1077. [9] K. Kobayashi, T.J. Nagasawa, Macromol Sci Phys B4 (2) (1970) 331. [10] C.H. Sherwood, F.P. Price, R.S. Stein, J Polym Sci Polym Symp 63 (1978) 77. [11] G.C. Alfonso, M.P. Verdona, A. Wasiak, Polymer 19 (1978) 711. [12] H. Ito, Y. Tsutsumi, K. Minagawa, K. Tada, K. Koyama, J Jpn Soc Polym Proc 6 (4) (1994) 265. [13] H. Ito, K. Minagawa, J. Takimoto, K. Tada, K. Koyama, Int Polym Proc 11 (4) (1996) 363. [14] H. Ito, J. Takimoto, K. Tada, K. Koyama, Kobunshi Ronbunshu 53 (6) (1996) 333. [15] J. Takimoto, H. Ito, S. Muto, K. Tada, K. Koyama, J Jpn Soc Polym Proc 8 (19) (1996) 675. [16] M. Okamoto, Y. Shinoda, N. Kinami, T. Okuyama, J Appl Polym Sci 57 (1995) 1055. [17] W. Nagatake, T. Takahashi, Y. Masubuchi, J. Takimoto, K. Koyama, Polymer 41 (2000) 523.
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[18] Y. K. [19] B. [20] B.
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Masubuchi, K. Watanabe, W. Nagatake, J. Takimoto, Koyama, Polymer 42 (2001) 5023. Monasse, J Mater Sci 30 (1995) 5002. Monasse, J Mater Sci 27 (1992) 6047.
[21] C. Tribout, B. Monasse, J.M. Haudin, Colloid Polym Sci 274 (1996) 197. [22] F. Jay, J.M. Haudin, B. Monasse, J Mater Sci 34 (1999) 2089.
Test Apparatus
Direct observation of polymer crystallization process under shear by a shear flow observation system K. Watanabe a, W. Nagatake a, T. Takahashi a, Y. Masubuchi b, J. Takimoto a, K. Koyama a,∗ b
a Department of Polymer Science and Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa 992-8510, Japan Department of Computational Science and Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
Received 20 March 2002; accepted 13 May 2002
Abstract Isothermal crystallization of polybutene-1 melt under shear flow was observed with an optical microscope by using a device newly developed by our group. Firstly, our coaxial cylinder-type apparatus (shear flow direct observation system (SF-DOS)) to observe crystallization directly under shear flow is explained in detail. Secondly to examine the accuracy of the measurement with the new device and to establish an appropriate procedure for the elimination of overshoot in temperature profiles, various examinations were performed in terms of temperature distribution, stability, and the velocity gradient by comparing the results from isothermal DSC measurements. The precise investigation suggested that the temperature distribution was proved to be very small (within 0.5°C) and the establishment of the procedure eliminated the overshooting of temperature profiles. Finally, we observed isothermal crystallization of polybutene-1 with the new device at various temperatures between 102°C and 140°C under shear rates of up to 0.7 s–1. When the shear strain became higher and the temperature became lower, narrow bands of oriented region formed after a certain time along the flow direction even at high temperature. This tendency was also observed in a polystyrene melt; therefore, it was interpreted that the narrow bands were considered to be the highly deformed region, which was generated by high shear stress. In the narrow bands, spherulites nucleated. After a certain amount of time, spherulites were also generated randomly outside of the narrow bands. Under very low shear rate, where the narrow bands did not exist, the spherulites appeared randomly. 2002 Elsevier Science Ltd. All rights reserved. Keywords: Shear flow direct observation system; Crystallization; Polybutene-1; Isothermal
1. Introduction In the 1970s, investigations showed that the crystallization of polymers are induced and accelerated by flow [1–10]. This idea has been practically applied in the melt spinning of fiber, film molding, etc. [11]. In recent years, in our laboratory, computer-aided engineering (CAE) including flow-induced crystallization and experimental investigations for polymer crystallization have been carried out in the injection molding process [12–16]. A
∗
Tel.: +81-238-26-3055; Fax: +81-238-26-3411. E-mail address: [email protected] (K. Koyama).
shear flow thermal rheometer (SFTR) having differential thermal analysis capability has been successfully developed using a rotational shear rheometer to enable the direct determination of crystallization fraction under shear flow [17,18]. This mechanism should be related to the molecular orientation in the melt. The effect of flow on the meso-scale morphology, such as the spherulite structure, is a very important factor for polymer processing and molding performance; however, morphology and crystallization kinetics under shear are not yet well understood. Due to flow, polymer chains can crystallize with morphologies different from those encountered under static conditions (observation of shish-kebabs or row-nucleated structures rather than the usual spherulites).
0142-9418/02/$ - see front matter 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 2 - 9 4 1 8 ( 0 2 ) 0 0 0 5 7 - 0
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K. Watanabe et al. / Polymer Testing 22 (2003) 101–108
Various devices able to apply shear have been built in order to study crystallization under shear or after shear. Many studies have been concerned with the measurement of the induction time of crystallization [1– 5,18] because the onset of crystallization is relatively easy to characterize by an increase of the force or the transmitted torque. Some studies [2,10,21] have determined the fraction of transformed material versus time and have reported anomalous values of the Avrami exponent under shear, but few in situ measurements concern the density of nuclei, the growth rate [7] and morphology development [4,8] during shear flow. Recently, Monasse [19,20], Tribout et al.[21] and Jay et al.[22] developed experimental apparatus to observe the crystallization kinetics with a flow generated by a plane–plane device [19,21] and pull-out fiber [20–22]. They make it possible to perform isothermal crystallization after rapid cooling, under an optical microscope. The appearance and development of crystalline morphologies could be observed during crystallization. They measured the growth rate of polyethylene [19], polypropylene [20,22] and the nucleation rate, density and growth rate of polypropylene copolymer [21]. They revealed that both the nucleation rate and the growth rate are affected by the flow. The plane–plane device can apply a constant shear rate (velocity gradient was not discussed), but the direct observation of the morphologies in the shear plane (parallel to the flow (X), sample thickness (Z), perpendicular to the flow (Y), and shear plane (XZ)) was not possible [19]. On the contrary, the fiber pull-out device can observe the morphologies in the shear plane but shear flow was localized near the fiber and not constant. Numerous spherulites grow from the fiber [22]. Neither device can apply long shearing times to the sample; the construction of the plane–plane device limited the shearing time to 60 s for a shear rate of 2 s–1 and to 3 s for the maximum shear rate of 26 s–1 [21]. In this study, we developed an originally designed coaxial cylinder-type device (shear flow direct observation system (SF-DOS)) to directly observe the crystallization process in the shear plane under shear flow. It was possible to observe the crystalline morphologies during shear flow. The purpose of this study is to develop a new device which can apply constant shear rate, stable temperature and long shearing times. In isothermal crystallization experiments of polybutene-1, the results of the velocity profile and thermal data by using SF-DOS are presented. 2. Experimental 2.1. Development of shear flow direct observation system Fig. 1 shows (a) a photograph and (b) a schematic diagram of the newly developed device for the direct
Fig. 1. (a) The photographs of the developed SF-DOS apparatus. (b) Its schematic diagram.
observation of crystallization under shear flow. It consists of a shallow cylinder and a rotating disk made from phosphor bronze. The sample is placed in the gap (0.5 mm) between the inner disk and the outer cylinder. The inner disk (diameter 109 mm and thickness 2 mm) is driven by a stepping motor (Oriental Motor Co. Ltd.) through a worm gear and a worm wheel to apply shear
K. Watanabe et al. / Polymer Testing 22 (2003) 101–108
flow to the sample. The outer cylinder is 110 mm internal diameter and is fixed on a glass pedestal. To stabilize temperature, the disk and the cylinder are wrapped in an insulating jacket made from MIOLEX (Ryoden Kasei Co. Ltd.). Mica heaters (Hakko Co. Ltd.) are installed into the inner disk, the outer cylinder and the pedestal, and the thermocouples (Thermocoax Company) are inserted in the edge of the inner disk (positions B, D, E ), the outer cylinder (position C) and the sample (position A). The thermocouple at position A was only used before and after the measurements, because the thermocouple disturbs the uniform shear flow. They are connected to a thermal control system (Shimaden Co. Ltd.). The positions, A, B, C, are controlled by the thermal control system. A part of the gap was illuminated through a transparent bottom plate, and the crystallization observed by an optical polarizing microscope (Leica Company, type DMR HC) equipped with a CCD camera (Simazu Co. Ltd., type CCD-SII). Micrographs of the morphologies were recorded on videotape, and imported by a personal computer. 2.2. Samples Polybutene-1 (PB-1) was used as a test polymer for observing crystallization behavior. The weight-averaged molecular weight Mw was 180,000 and the molecular weight distribution index (Mw/Mn) was 3.3. Atacticpolystyrene (PS, PS666) was supplied by Asahikasei Co. Ltd. and used as a reference amorphous polymer. The weight-averaged molecular weight was 210,000 and the Mw/Mn was 2.2. Disk-like samples with 110 mm diameter and 0.5 mm thickness were made from pellets using a hot-press machine. These disk samples were used for the measurements in the SF-DOS. 2.3. DSC curves of polybutene-1 under isothermal crystallization The isothermal crystallization characteristics of PB-1 under quiescent state were studied with a differential scanning calorimetry (DSC) from Perkin-Elmer Co. Ltd. (type DSC-7). The samples (7.2 mg of weight) were melted at 180°C for 5 min in order to eliminate any thermal history of material, then rapidly cooled to the crystallization temperature, Tc. 2.4. Measurement by shear flow direct observation system To eliminate thermal history, considerable attention was paid to the establishment of the measurement procedure by the new device. First, the sample was kept at 180°C for 5 min, and then cooled to a temperature 10°C higher than the measurement temperature. Second, the sample was kept for 5 min, then cooled to a temperature
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5°C higher than the measurement temperature and finally kept for 5 min and then cooled to the measurement temperature to avoid overshooting of temperature. Before observation, the measurement temperature was held for 10 min to stabilize temperature. The observation was carried out at isothermal conditions under steady shear rates of 0, 0.025, 0.05, 0.1, 0.5 and 0.7 s–1. The velocity profile was measured by using a dispersion which consists of polyethylene particles (particle size 25 µm, Mipelon from Mitsui Chemical Ltd.) and silicon oil ( PE/PDMS, weight ratio is 5/95).
3. Results and discussion 3.1. DSC curves of polybutene-1 crystallization isotherms To separate the crystallization induced by shear from the crystallization without shear, it is important to find at which temperature crystallization occurs even without shear. Isothermal DSC measurements were performed to determine the critical temperature. Fig. 2 shows the crystallization isotherms of PB-1 at different temperatures obtained by DSC under quiescent state. Below 98°C, crystallization took place. Cooling speed to the isothermal temperature was 200°C/min, so that a very slight overshoot might have taken place. At 100°C, the isothermal DSC measurements were conducted at two conditions. One is after normal rapid cooling (200°C/min). The other is two-step cooling, rapid cooling to 105°C and slow cooling from 105 to 100°C (1°C /min) to aim at eliminating the overshoot. Above 102°C, crystallization under quiescent state did not take place within 2.5 hours. It is considered that 100°C is a critical temperature for crystallization. Therefore, it would be safe to conclude that we should observe the effect of shear-induced crystallization above 102°C, where no crystallization occurred without shear. 3.2. A shear flow direct observation system 3.2.1. Temperature distribution and stability in the shear flow direct observation system To measure and to minimize the temperature distribution in the shear flow direct observation system is important. We used five thermocouples. The thermocouples’ location were A (sample), B, D, E (rotating disk), C (outer cylinder), as shown in Fig. 1 (b). The position A was located in the sample, before and after shear observation to keep uniform shear flow. Fig. 3 represents temperature change as a function of time from 180°C at various points before observation. The temperature profile has already been mentioned in the experimental section. The positions, A, B, C, are open to the air and controlled by the thermal control sys-
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Fig. 2.
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DSC curves of PB-1 crystallization isotherms at (a) 96°C, (b) 97°C, (c) 98°C, (d) 100°C, (e) 100°C (slow cooling), and (f) 102°C.
is about 2°C higher than the other positions, B, C, D, E. This difference would be due to the difference in terms of the thermal conductivity and the heat capacity between the metal and the polymer. This fact suggests that it is difficult to control the polymer temperature and we have to measure the sample temperature precisely. We have found an overshoot problem (about 2–4°C below the target isothermal temperature) when cooling to the measurement temperature in the newly developed device. To eliminate the overshoot phenomenon, therefore, considerable attention was paid to the establishment of the measurement procedure. First, the sample was kept at 180°C for 5 min, and then cooled to a temperature 10°C higher than the measurement temperature. Second, it was kept for 5 min, then cooled to a temperature 5°C higher than the measurement temperature, kept for 5 min and then finally cooled to the measurement temperature to avoid overshooting of temperature. Before observation, the measurement temperature was maintained for 10 min to stabilize temperature. Fig. 4 shows temperature profiles (four different target temperatures) of the melt (position A) as a function of time before observation according to the procedure. It is clearly shown that by using the step-down cooling procedure (except 100°C) in Fig. 4, the overshoot problem disappeared and the stability of temperature was within 0.5°C. Fig. 3. Temperature change as a function of time at various points before observation. A sample (䊐), B rotating disk (䊏), C outer cylinder (䉬), D rotating disk (쎲), E rotating disk (왖).
tem, therefore, the temperature of the positions, D, E are higher than the positions, B, C before reaching the equilibrium stage. However, about 10 min after setting to the measurement temperature, temperature stability was achieved except at position A (sample). The position A
3.2.2. Velocity profile of the polyethylene particle The device is new, so that it was necessary to examine the velocity profiles and whether pure shear flow is generated or not. We have used silicone oil including 5 wt% of polyethylene particle (PE/PDMS = 5/95) between the rotating disk and outer cylinder to calculate the velocity profiles under shear. Fig. 5 (a) shows the optical micrograph of the 5/95 PE/PDMS under a shear rate of 0.05 s–1, and Fig. 5 (b) shows the velocity profile of the polyethylene particle under various shear rates. The velocity
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profile of the shear was not disturbed. These results allowed us to conclude that pure shear flow can be generated in the newly developed device.
Fig. 4. Sample temperature change as a function of time before observation. 112°C (쎲), 105°C (䊏), 102°C (䉬), 100°C, overshoot (왕).
Fig. 5. (a) Optical micrographs of the 5/95 PE/PDMS ( g˙ ⫽ 0.05s⫺1). (b) Velocity profile of the polyethylene particle.
3.2.3. Direct observation of polymer under shear flow Fig. 6 shows typical snapshots of the observation under crossed-Nicols (polarizer and analyzer were at 45° to the flow direction). The rotating disk was located on the lower side. The images of PB-1 observed under shear flow are shown in Fig. 6 ((a) at 112°C, (b) γ˙ = 0–0.05 s–1 at 102°C, (c) γ˙ = 0.1–0.7 s–1 at 102°C). The crystallization of PB-1 did not take place within 10 min at 112°C under shear rates below 0.1 s–1. At a shear rate of 0.5 s– 1 or higher, on the other hand, bands of oriented region appeared along the flow direction, which were seen as bright bands under crossed-Nicols conditions, as shown in Fig. 6 (a). This implies that highly deformed regions were generated at high shear rates after a certain amount of time. Under no shear flow, crystallization did not take place within 2 hours at 102°C (Fig. 6 (b)); this is consistent with the results of DSC (Fig. 2). The nucleation rate and growth rate of PB-1 under quiescent state at 105°C has been reported by Wolkowicz [7], the crystallization of PB-1 has taken place 10 hours after the beginning of the measurement. Under shear rates more than 0.1 s–1, bright bands were also observed at 102°C. Soon after detecting the bands, the distorted (slightly anisotropic) spherulites were also observed in the bands at 102°C (Fig. 6 (c)), for a shear rate of 0.5 s–1 or higher. In addition, spherulites were also generated outside of the bands. Wolkowicz [7] has measured the nucleation and growth rates of PB-1 under higher shear rate (0.2–12 s–1) but the morphological and kinetic behaviors were not observed directly. Two types of nuclei were exhibited: randomly dispersed [7] and row nuclei [4,8,9,19]. Kobayashi et al. [9] reported calculations to show the effects of chain elongation on crystal thickness, nucleation and growth rates in terms of the change in birefringence on orientation. They revealed the morphological observation of transverse growth from row nuclei by using electron micrographs. Monasse [19] observed polyethylene crystallization during shear by using a parallel-plate device. It is reported that the row-nucleated structure observed in the direction perpendicular to the flow direction, and some of these initial nuclei turned from the perpendicular direction to the flow direction during shear. Ulrich et al. [8] observed the row-nucleated structure along to the flow direction and random nuclei of poly(ethylene oxide) by using a rotational plate–plate. They observed the rownucleated structure on the surface of the glass plate and supposed that the row-nucleated structure formed along fine scratches or heterogeneities on the glass plate. These results were quite similar to those of Hass et al. for PB1[4], but they supposed that the spherulites cause a disturbance resulting in a local increase in stress of the melt
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Fig. 6. Typical snapshots of the morphology of PB-1 crystallizing under various shear flow at (a) 112°C, (b) 102°C (g˙ ⫽ 0⫺ 0.05s⫺1), (c) 102°C (g˙ ⫽ 0.1 ⫺ 0.7s⫺1), (d) typical snapshots of PS. The polarizer and analyzer were kept at 45°. Arrows indicate the flow direction.
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and row-nucleated structure formation. Our experimental results are different from previous results. The bands have not been reported until now and the spherulites occurred randomly in the bands, not located in the same line in the bands. Temperature distribution in the SFDOS is almost zero, as mentioned earlier. We have eliminated the overshoot problems of the temperature profiles. The observation was started substantially after reaching the equilibrium stage of the temperature. When the shear strain became higher and the temperature became lower, the narrow bands of oriented region formed after a certain time along the flow direction even at high temperature. The bright bands seemed to be related to highly deformed regions. To get more insight into the origin of the bright bands, amorphous a-PS was tested with the SF-DOS (Fig. 6 (d)). Under shear flow, bands similar to PB-1 were observed at 140°C under higher shear rates; therefore, these bands would be related to molecular orientation along the flow direction, but not crystallization. The bright bands were observed at lower temperature with higher shear rates after a certain amount of time, even with stable temperature without overshooting and uniform shear rates. Taking these results into consideration, the bright bands were generated above a certain amount of critical shear stress, where it is difficult to deform uniformly. We consider that the spherulites develop more easily in the highly oriented bands rather than in the matrix region. On the other hand, when the shear rate is very low (0.025 s–1) at 102°C, spherulites (slightly anisotropic) appeared randomly without having bright band structures (Fig. 6 (b)). As the shear stress and the viscosity become higher, uniform shear flow was not formed completely, resulting in the separated bands appearing and a nucleated structure was developing in the bright bands, which are illustrated in Fig. 6 (c). As the shear rate increased, the generation of nuclei became accelerated. We intend to measure the nucleation rate and growth rate of isolated structures under slow shear rate (below 0.1 s–1 at 102°C) quantitatively, which will be published elsewhere.
4. Conclusions The present study provided the following new findings: 1. To observe the crystallization under shear flow, a shear flow direct observation system (SF-DOS) has been built. To examine the accuracy of the measurement with the new device and to establish an appropriate procedure for the elimination of overshoot in temperature profiles, various examinations, such as temperature distribution, stability, and the velocity
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gradient, were carried out by comparing the results from isothermal DSC measurements. The precise investigation suggested that the temperature distribution is very small (within 0.5°C) and the establishment of the procedure (cooling down the temperature stepwise) eliminated the overshooting of temperature targets. 2. By using SF-DOS, bands of more oriented regions along the flow direction appeared in both polybutene1 and PS at lower temperatures with higher strain rates. It was considered that the bands were generated above a certain amount of critical shear stress where it was difficult to deform uniformly. The spherulites of PB-1 developed more easily in the molecular oriented bands rather than in the matrix. Under very low shear rates, spherulites of PB-1 developed randomly without forming the band structures.
Acknowledgements We thank Mr Y Seino for his precise machinery work in making the device for direct observation of crystallization under shear flow.
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