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A zone drawing method to determine optimum polymer concentration and gelation temperature of high molecular weight polymer film
Polymer Testing 18 (1999) 299–311
Test Method
A zone drawing method to determine optimum polymer concentration and gelation temperature of high molecular weight polymer film B.C. Jia, G.S. Jeonga, W.S. Yoonb, S.S. Hanb, W.S. Lyooc,* a
Department of Dyeing and Finishing, Kyungpook National University, Taegu 702-701, South Korea b School of Textiles, Yeungnam University, Kyongsan 712-749, South Korea c Division of Polymer Research, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul, South Korea Received 2 March 1998; accepted 5 May 1998
Abstract The zone drawing method has been used to determine optimum initial concentration and gelation/crystallization temperature for suitable macromolecular entanglements. Ultra-high molecular weight (UHMW) polyethylene (PE) gel films were prepared from decalin solution with different polymer concentrations and different gelation temperatures and then drawn under various zone drawing conditions. It was found that the initial concentration and gelation temperature of PE solution caused significant changes in draw ratio of UHMW PE gel film. That is, at initial concentration of 0.5 g/dl and at gelation temperature of 25°C, the one-step zone draw ratio revealed a maximum and gradually decreased at higher or lower concentrations. In addition, the lower the initial concentration, the lower the gelation/crystallization temperature. This tendency was similar to the maximum draw ratio data. Conclusively, it was identified that the optimum polymer concentration and gelation temperature of UHMW PE solution could be obtained simply by the zone draw ratio. 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction Preparative methods for fibers having various functionalities have been much developed because of increased requirements for new materials. Among them, manufacturing methods for * Corresponding author. Tel.: ⫹ 82-2-2-958-5359; ⫹ 82-2-958-5309; E-mail: [email protected] 0142-9418/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 2 - 9 4 1 8 ( 9 8 ) 0 0 0 3 2 - 4
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high strength and high modulus fibers can be classified as follows [1]: first, synthesis and spinning of new polymers having rigid rod structure [1]; second, modification of the molecular structure of the polymeric fiber [2]; third, orientation of flexible chain polymers, each as polyethylene (PE) or poly(vinyl alcohol) along the fiber axis [3–12]. In the case of the orientation of flexible chain polymers, it is very important to maximise the draw of fibrous material. Thus, to enhance the drawabilities of film and fiber, gel drawings [3], single crystal mat drawings [4,5], and high temperature zone drawings [6–10] have been in progress actively. As is well known, molecular weight [11–14], molecular weight distribution [15], concentration of polymer solution [16,17], and gelation/crystallization temperature [18,19] have a marked influence on the ultra-drawability of polymers. In general, the ultra-drawability increases as the molecular weight increases [20]. Above a certain molecular weight, however, this property depends principally on the initial concentration and gelation/crystallization temperature of the polymer solution at which the gel was made [14,16]. This is due to a reduced number of entanglements per molecule in the solution cast or spun polymers in comparison with those obtained from the melt [21]. That is, the macromolecules are thought to form transient networks with entanglements acting as friction centers or nonlocalized junctions. Because high entanglement density at higher concentration or at lower gelation temperature impedes large deformation of solidified high molecular weight polymers, their drawability might be improved by reduction of the number of entanglements. However, in the case of dilute solution which has fewer entanglements, maximum draw ratio cannot be obtained owing to rare coil overlap and chain slippage occurring at the drawing step. Therefore, the proper level of entanglement is needed to increase the maximum draw ratio and this can be realized by the control of solution concentration and gelation/crystallization temperature. So, the question arises as to how this concentration and temperature can be measured. For preparing ultra-high molecular weight (UHMW) PE gel film with high drawability, Sawatari et al. [21] determined optimum entanglements by means of the solution concentration through measurement of solution viscosity. However, this viscosity method requires multiple steps and substantial time. Besides the viscosity method, there have been several methods to qualitatively determine optimum chain entanglements. Based on the assumption that, during film formation, one entanglement per chain should decrease drawability by 20% compared to the absence of entanglement, Smook and Pennings [22] have determined the number of entanglements of film using degree of shrinkage of fully drawn film and radius of gyration of the polymer. Also, Qin et al. [23] have calculated macromolecular entanglements of PE fibers utilizing swelling differential scanning calorimetric analysis. These two methods were not constructed on an experimental basis directly related to the draw ratio but on theoretical and analytical bases. The zone drawing technique [8–10] shows many advantages compared to hot drawing experiments, such as reduced probabilities of microcrystallite formation, of back folding of molecular chains, and of thermal degradation of the sample. It reveals that the maximum draw ratio is significantly related to the one-step zone draw ratio [6,7,24–26]. Thus, in this study, considering the relationships between maximum draw ratio of UHMW PE gel film in zone drawing and optimum solution concentration and gelation/crystallization temperature, a convenient and simple method is introduced for the determination of optimum initial concentration and gelation temperature of UHMW PE solution for maximum draw ratio using only the one-step zone draw ratio.
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Table 1 Zone drawing conditions of UHMW PE gel film Drawing stress (MPa) Drawing temperature (°C) Heat band speed (mm/min)
1.5, 2.5, 3.5, 4.5 83, 98, 112, 127 1, 5, 10, 50, 100
Fig. 1. Scanning electron micrographs of fractured surfaces of UHMW PE gel films cast at six different gelation/crystallization temperatures of (a) − 180°C, (b) − 30°C, (c) 0°C, (d) 25°C, (e) 50°C and (f) 70°C.
2. Experimental 2.1. Preparation and zone drawing of UHMW PE gel film The concentrations of PE (viscosity–average molecular weight of 7.3 ⫻ 106 g/mol, Hoechst, Hostalen GUR 415) solution in decalin were 0.3, 0.5, 0.7, and 1.0 g/dl, respectively. The UHMW
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Fig. 1. Continued
PE/decalin mixture with anti-oxidant (di-t-butyl-p-cresol, 0.1% (w/w)) was heated with stirring at 85°C for 10 min, followed by 115°C for 70 min and finally 135°C for 40 min under nitrogen to form a uniform solution. The homogenized solution was poured into an aluminium tray and kept at ⫺ 180, ⫺ 30, 0, 10, 25, 50, and 70°C (gelation temperature) for 1 day, respectively, to form a gel. After decalin was removed from the gels, the dried gel film of 135 m thickness was obtained. PE gel films of 5 mm width and 10 cm length were zone-drawn at various zone drawing conditions for the preparation of one-step zone-drawn film. Also, to obtain maximum draw ratio, films having the same initial cross-section areas were zone-drawn through four steps. Draw ratio was regulated by varying the drawing stress, drawing temperature and heat band speed. Zone drawing was carried out by moving a pair of narrow band heaters with dimensions of length 7 cm, width 2.5 cm, and thickness l mm along the film. The film was drawn under a tension
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303
Fig. 1. Continued
controlled by different dead weights on an Instron model 4201 [6,7]. The zone drawing conditions are listed in Table 1. 2.2. Characterization of the UHMW PE gel film The scanning electron micrographs of the pre-drawn gel films which were fractured at liquid nitrogen temperature were taken on a Cambridge S-360 SEM at an acceleration voltage of 20 kV. The crystallinity was determined by differential scanning calorimeter (Perkin-Elmer, DSC 7) at a heating rate of 10°C/min. The crystallinity was calculated from heat of fusion of 294 J/g, which is for ideal PE crystals. Load–elongation curves were recorded on an Instron model 4201 using a sample length of 2 cm and a cross-head speed of 100 mm/min. The tensile strength and modulus of the UHMW PE gel film were the average values from 20 samples.
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3. Results and discussion Generally it has been known that the maximum draw ratio of solution-cast film is influenced by the solution concentration, and maximum draw ratio can be obtained at a certain solution concentration. Moreover, in the vicinity of this concentration the solution viscosity steeply increases. Thus, in this study, firstly, relative viscosity-measuring experiments were tried to predict optimum polymer concentration of UHMW PE. From the fact that all the measured data were approximated to two different linear lines, it was possible to determine critical concentration showing an abrupt change in the solution viscosity. This result is coincident with Sawatari et al.’s data [21]. However, optimum gelation/crystallization temperature with optimum initial concentration could not be determined at the same time by solution viscometry. For the above reasons, we tried to determine optimum polymer concentration accompanied by optimum gelation/crystallization temperature of UHMW PE solution by the zone drawing method. The structure and physical properties of zone-drawn film may vary with the conditions of preparation and drawing of film. That is, firstly, initial concentration and gel formation temperature of polymer solution during preparation of the gel film; secondly, processing parameters like drawing stress, drawing temperature and heat band speed during drawing of film have marked influences on the drawing behavior of film. In this study, effects of those factors on the draw ratios of films from different solution concentrations and gelation/crystallization temperatures were investigated. Fig. 1 shows the fractured surfaces of the pre-drawn PE gel films having the same initial concentration of 0.5 g/dl cast at six different gelation temperatures. From the fact that the pre-drawn PE gel films had structural differences in the fractured surfaces according to the gelation temperatures, it could be supposed that the gelation/crystallization temperature of the pre-drawn PE gel film had a marked influence on the deformability of UHMW PE gel film. Fig. 2 shows the draw ratio of one-step zone drawn PE gel film as a function of the initial concentration of the solution at various drawing temperatures under drawing stress of 4.5 MPa and heat band speed of 10 mm/min. As the drawing temperature increased, draw ratio increased. This implies that as drawing temperature reaches crystal melting temperature (129–132°C) of
Fig. 2. Draw ratio dependence on the initial solution concentration of UHMW PE gel films at a gelation/crystallization temperature of 25°C at gradual zone drawing temperature.
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Fig. 3. Draw ratio dependence on the gelation/crystallization temperature and initial solution concentration of UHMW PE gel films drawn at (a) 127°C, (b) 112°C and (c) 98°C.
undrawn UHMW PE gel film, degree of freedom of PE chains increases. The maximum value of draw ratio appeared at solution concentration of 0.5 g/dl at all drawing temperatures. At lower or higher concentration, draw ratio decreased gradually. This corresponds to a suitable number of entanglements for the gel film prepared at a solution concentration of 0.5 g/dl. Generally, for effective drawing, gel film must have a suitable number of entanglements that can be evaluated by determination of optimum solution concentration. Optimum initial concentration of polymer solutions varies with molecular weight, linearity, and stereoregularity of polymers and type of solvent, etc. In this study, however, those effects were negligible because the same type of polymer with the same molecular weight and the same solvent were used in all experiments. Therefore, it can be supposed that an initial concentration of 0.5 g/dl is the optimum concentration which contains suitable entanglements at a gelation/crystallization temperature of 25°C. This result was similar to other research by the viscometric method [3]. The general viscometric method for a determination of optimum solution concentration is complicated and requires a long time, thus, a zone drawing method could be better.
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Fig. 4. Draw ratio dependence on the heat band speed in one-step zone drawing of UHMW PE gel films prepared at four different gelation/crystallization temperatures and at three different initial solution concentrations of (a) 0.3 g/dl, (b) 0.5 g/dl, and (c) 0.7 g/dl.
Fig. 3 shows the effect of gelation/crystallization temperature and initial concentration on onestep zone draw ratios of PE gel film drawn at 127°C (a), 112°C (b), and 98°C (c) using a drawing stress of 4.5 MPa and heat band speed of 10 mm/min. From the results in Fig. 2, it was identified that a concentration of 0.5 g/dl was the optimum initial concentration when solution was gelled and crystallized at 25°C. So, draw ratio of film prepared by gelation/crystallization at 0.5 g/dl and 25°C was the highest and draw ratio decreased at higher (50 and 70°C) and lower ( ⫺ 180, ⫺ 30, 0, and 10°C) gelation/crystallization temperatures. This effect can be explained by the number of chain entanglements of gel film prepared at ⫺ 180, ⫺ 30, 0, and 10°C being more than that at 25°C. Also, those prepared at 50 and 70°C had fewer than that at 25°C due to loosening of chain entanglements. This tendency is nearly the same irrespective of different drawing temperatures. It is interesting to see that optimum gelation temperature was shifted according
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to the initial polymer concentration. That is, at initial concentrations of 0.3 g/dl and 0.7 g/dl, these were found to be below 10°C and ca. 50°C, respectively. From this fact, it was found that a higher chain entanglement density should be required at a higher gelation temperature. In this study, the gel films were drawn at different drawing stresses. The draw ratio increased with an increase in the drawing stress. This result can be explained by deformation of molecular chains increasing with increasing drawing stress above the glass transition temperature. The maximum value of draw ratio appeared at solution concentration of 0.5 g/dl and gelation temperature of 25°C. Fig. 4 shows plots of draw ratios of gel films prepared at different initial concentrations and at different gelation/crystallization temperatures drawn under a drawing temperature of 127°C, and drawing stress of 4.5 MPa, against heat band speeds. It was shown that the slower the heat band speed, the larger the draw ratio. This may indicate that a softening of gel film becomes easier owing to longer residence time for gel film between two heat bands. As identified in the above figures, the one-step zone draw ratio of gel film prepared at a concentration of 0.5 g/dl and at a gelation temperature of 25°C was the largest among all the films of different concentrations
Fig. 5. Plots of multi-step draw ratios of gel films prepared at initial concentrations of (a) 0.3 g/dl, (b) 0.5 g/dl and (c) 0.7 g/dl vs. gelation/crystallization temperatures of UHMW PE solutions.
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Fig. 6. Crystallinity of one-step zone drawn gel film drawn at 127°C with gelation/crystallization temperature.
and temperatures. Thus, it was supposed that the gel film prepared at this concentration with 25°C had suitable entanglements to attain effective draw ratio. These results are in agreement with Sawatari et al.’s results[13]. Matsuo et al. [27] reported that as gelation/crystallization temperature increased during film formation, the number of chain entanglements decreased by enfeeblement of molecular chain restriction. In the present work, considering this point, we investigated the relationship between maximum draw ratio and gelation/crystallization temperature. Fig. 5 shows the effects of gelation/crystallization temperature and initial concentration on stepwise draw ratios (four-step draw ratio is the maximum) of PE gel film drawn at a drawing temperature of 127°C, drawing stress of 4.5 MPa, and heat band speed of 10 mm/min prepared with concentrations of (a) 0.3, (b) 0.5 and (c) 0.7 g/dl. Maximum draw ratio (draw ratio of 223) of UHMW PE gel film prepared at a gelation/crystallization temperature of 25°C and at an initial solution concentration of 0.5 g/dl was the highest. As presented in the figure, the effects of concentration and temperature on the draw ratio showed a similar tendency in every drawing step. Thus, it was identified that the prediction of maximum drawing behavior was possible by one-step zone draw ratio only. Consequently, the zone drawing method is appropriate for determining optimum initial concentration and gelation/crystallization temperatures for gel film formation. Fig. 6 shows plots of the crystallinities of one-step zone drawn PE gel films against the initial concentration and gelation temperature of the solution under a drawing temperature of 127°C, drawing stress of 4.5 MPa and heat band speed of 10 mm/min. Two things are worth noting in the figure. Firstly, maximum crystallinity appeared at a solution concentration of 0.5 g/dl with a temperature of 25°C. At lower or higher concentrations, crystallinity decreased. The reason is because the difference in draw ratio by one-step zone drawing might cause variation of crystallinity. Secondly, crystallinity of gel film varied with respect to gelation temperature at each concentration. Figs. 7 and 8 show the effect of the same draw ratio on the tensile strength and tensile modulus of one-step zone drawn gel film prepared at different gelation/crystallization temperatures and different initial solution concentrations with a drawing temperature of 127°C, drawing stress of 4.5 MPa, and heat band speed of 10 mm/min. It was shown that the tensile strength and the tensile
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Fig. 7. Tensile strength of one-step zone drawn gel film prepared at four different gelation/crystallization temperatures and at three different initial solution concentrations of (a) 0.3 g/dl, (b) 0.5 g/dl and (c) 0.7 g/dl.
modulus increased with an increase in the draw ratio and tensile properties had a dependence on gelation temperature. That is, in the case of 0.5 g/dl, maximum tensile strength and tensile modulus were obtained at a gelation temperature of 25°C, which were 2.88 GPa and 39.5 GPa, respectively, in spite of one-step zone drawing. In contrast, those for film from initial concentrations of 0.3 and 0.7 g/dl revealed maximum values at gelation/crystallization temperatures of 10 and 50°C, respectively. 4. Conclusions Considering the effects of various parameters on the zone draw ratio of UHMW PE gel film prepared at different initial solution concentrations and gelation/crystallization temperatures, we may conclude the following. The one-step zone draw ratio of the gel film prepared at 25°C was the largest at the initial concentration of 0.5 g/dl and gradually decreased at higher or lower
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Fig. 8. Tensile modulus of one-step zone drawn gel film prepared at four different gelation/crystallization temperatures and at three different initial solution concentrations of (a) 0.3 g/dl, (b) 0.5 g/dl and (c) 0.7 g/dl.
concentrations and temperatures. That is, a relationship between initial concentration and gelation temperature was found. This tendency was similar to the results observed for the maximum draw ratio data. The concentration of 0.5 g/dl at 25°C is the optimum initial concentration and gelation/crystallization temperature from this study. The tensile properties revealed their maximum at these points. Consequently, it was identified that the optimum concentration and gelation/crystallization temperature of initial UHMW PE solutions could be determined by the draw ratio obtained by simple one-step zone drawing. References [1] Ciferri A, Valenti B. In: Ciferri A, Ward IM, editors. Ultra-high modulus polymers. London: Applied Science, 1984. [2] Donnet JB, Bansal RC. In: Lewin M, Dekker M, editors. Carbon fibers. New York: Marcel Dekker, 1984. [3] Smith P, Lemstra PJ. Colloid Polym Sci 1980;258:89.
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Bundell DJ, Keller A. J Macromol Sci-Phys 1968;B(2):301. Kanamoto T, Kiyooka S, Tovmasyan Y, Sano H, Narukawa H. Polymer 1990;31:2039. Han SS, Yoon WS, Choi JH, Kim SY, Lyoo WS. J Appl Polym Sci 1997;66:1583. Lyoo WS, Han SS, Choi JH, Cho YW, Ha WS. J Korean Fib Soc 1995;32:1023. Kunugi T, Akiyama I. Polymer 1982;23:1199. Garrett PD, Grubb DT. Polym Commun 1988;29:60. Kunugi T, Kawasumi T, Ito T. J Appl Polym Sci 1990;40:2101. Smith P, Lemstra PJ, Pijpers JPL. J Polym Sci Polym Phys 1982;20:2229. Hallam MA, Cansfield DLM, Ward IM, Pollard G. J Mater Sci 1986;21:4199. Sawatari C, Okumura T, Matsuo M. Polym J 1986;18:741. Bastiaansen CWM. J Polym Sci Polym Phys 1990;28:1475. Liu BL, Murakami N, Sumita M, Miyasaka K. J Polym Sci Polym Phys 1989;27:2441. Smith P, Lemstra PJ, Booji HC. J Polym Sci Polym Phys 1981;19:877. Kanamoto T, Tsurata A, Tanaka K, Porter RS. Polym J 1983;15:327. Ogita T, Suzuki N, Kawahara Y. Polymer 1992;33:698. Ogita T, Kawahara Y, Soga Y, Matsuo M. Colloid Polym Sci 1992;270:833. Nunes RW, Marhn, JR, Johnson JF. Polym Eng Sci 1982;22:205. Douglas MJF, Freed F. Macromolecules 1983;16:741. Pennings AJ, Smook J. Colloid Polym Sci 1984;26:712. Qin J, Hu P, Zhao Z, Wu Z, Qian B. J Appl Polym Sci 1994;51:1433. Kamezawa M, Yamada Y. J Appl Polym Sci 1979;24:1227. Yamada K, Takayanagi M. J Appl Polym Sci 1982;27:2091. Kunugi T, Ohmori S. Polymer 1988;29:814. Matsuo M, Sawatari C, Iida M, Yoneda M. Polym J 1985;11:1197.
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Test Method
A zone drawing method to determine optimum polymer concentration and gelation temperature of high molecular weight polymer film B.C. Jia, G.S. Jeonga, W.S. Yoonb, S.S. Hanb, W.S. Lyooc,* a
Department of Dyeing and Finishing, Kyungpook National University, Taegu 702-701, South Korea b School of Textiles, Yeungnam University, Kyongsan 712-749, South Korea c Division of Polymer Research, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul, South Korea Received 2 March 1998; accepted 5 May 1998
Abstract The zone drawing method has been used to determine optimum initial concentration and gelation/crystallization temperature for suitable macromolecular entanglements. Ultra-high molecular weight (UHMW) polyethylene (PE) gel films were prepared from decalin solution with different polymer concentrations and different gelation temperatures and then drawn under various zone drawing conditions. It was found that the initial concentration and gelation temperature of PE solution caused significant changes in draw ratio of UHMW PE gel film. That is, at initial concentration of 0.5 g/dl and at gelation temperature of 25°C, the one-step zone draw ratio revealed a maximum and gradually decreased at higher or lower concentrations. In addition, the lower the initial concentration, the lower the gelation/crystallization temperature. This tendency was similar to the maximum draw ratio data. Conclusively, it was identified that the optimum polymer concentration and gelation temperature of UHMW PE solution could be obtained simply by the zone draw ratio. 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction Preparative methods for fibers having various functionalities have been much developed because of increased requirements for new materials. Among them, manufacturing methods for * Corresponding author. Tel.: ⫹ 82-2-2-958-5359; ⫹ 82-2-958-5309; E-mail: [email protected] 0142-9418/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 2 - 9 4 1 8 ( 9 8 ) 0 0 0 3 2 - 4
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B.C. Ji et al. /Polymer Testing 18 (1999) 299–311
high strength and high modulus fibers can be classified as follows [1]: first, synthesis and spinning of new polymers having rigid rod structure [1]; second, modification of the molecular structure of the polymeric fiber [2]; third, orientation of flexible chain polymers, each as polyethylene (PE) or poly(vinyl alcohol) along the fiber axis [3–12]. In the case of the orientation of flexible chain polymers, it is very important to maximise the draw of fibrous material. Thus, to enhance the drawabilities of film and fiber, gel drawings [3], single crystal mat drawings [4,5], and high temperature zone drawings [6–10] have been in progress actively. As is well known, molecular weight [11–14], molecular weight distribution [15], concentration of polymer solution [16,17], and gelation/crystallization temperature [18,19] have a marked influence on the ultra-drawability of polymers. In general, the ultra-drawability increases as the molecular weight increases [20]. Above a certain molecular weight, however, this property depends principally on the initial concentration and gelation/crystallization temperature of the polymer solution at which the gel was made [14,16]. This is due to a reduced number of entanglements per molecule in the solution cast or spun polymers in comparison with those obtained from the melt [21]. That is, the macromolecules are thought to form transient networks with entanglements acting as friction centers or nonlocalized junctions. Because high entanglement density at higher concentration or at lower gelation temperature impedes large deformation of solidified high molecular weight polymers, their drawability might be improved by reduction of the number of entanglements. However, in the case of dilute solution which has fewer entanglements, maximum draw ratio cannot be obtained owing to rare coil overlap and chain slippage occurring at the drawing step. Therefore, the proper level of entanglement is needed to increase the maximum draw ratio and this can be realized by the control of solution concentration and gelation/crystallization temperature. So, the question arises as to how this concentration and temperature can be measured. For preparing ultra-high molecular weight (UHMW) PE gel film with high drawability, Sawatari et al. [21] determined optimum entanglements by means of the solution concentration through measurement of solution viscosity. However, this viscosity method requires multiple steps and substantial time. Besides the viscosity method, there have been several methods to qualitatively determine optimum chain entanglements. Based on the assumption that, during film formation, one entanglement per chain should decrease drawability by 20% compared to the absence of entanglement, Smook and Pennings [22] have determined the number of entanglements of film using degree of shrinkage of fully drawn film and radius of gyration of the polymer. Also, Qin et al. [23] have calculated macromolecular entanglements of PE fibers utilizing swelling differential scanning calorimetric analysis. These two methods were not constructed on an experimental basis directly related to the draw ratio but on theoretical and analytical bases. The zone drawing technique [8–10] shows many advantages compared to hot drawing experiments, such as reduced probabilities of microcrystallite formation, of back folding of molecular chains, and of thermal degradation of the sample. It reveals that the maximum draw ratio is significantly related to the one-step zone draw ratio [6,7,24–26]. Thus, in this study, considering the relationships between maximum draw ratio of UHMW PE gel film in zone drawing and optimum solution concentration and gelation/crystallization temperature, a convenient and simple method is introduced for the determination of optimum initial concentration and gelation temperature of UHMW PE solution for maximum draw ratio using only the one-step zone draw ratio.
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Table 1 Zone drawing conditions of UHMW PE gel film Drawing stress (MPa) Drawing temperature (°C) Heat band speed (mm/min)
1.5, 2.5, 3.5, 4.5 83, 98, 112, 127 1, 5, 10, 50, 100
Fig. 1. Scanning electron micrographs of fractured surfaces of UHMW PE gel films cast at six different gelation/crystallization temperatures of (a) − 180°C, (b) − 30°C, (c) 0°C, (d) 25°C, (e) 50°C and (f) 70°C.
2. Experimental 2.1. Preparation and zone drawing of UHMW PE gel film The concentrations of PE (viscosity–average molecular weight of 7.3 ⫻ 106 g/mol, Hoechst, Hostalen GUR 415) solution in decalin were 0.3, 0.5, 0.7, and 1.0 g/dl, respectively. The UHMW
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Fig. 1. Continued
PE/decalin mixture with anti-oxidant (di-t-butyl-p-cresol, 0.1% (w/w)) was heated with stirring at 85°C for 10 min, followed by 115°C for 70 min and finally 135°C for 40 min under nitrogen to form a uniform solution. The homogenized solution was poured into an aluminium tray and kept at ⫺ 180, ⫺ 30, 0, 10, 25, 50, and 70°C (gelation temperature) for 1 day, respectively, to form a gel. After decalin was removed from the gels, the dried gel film of 135 m thickness was obtained. PE gel films of 5 mm width and 10 cm length were zone-drawn at various zone drawing conditions for the preparation of one-step zone-drawn film. Also, to obtain maximum draw ratio, films having the same initial cross-section areas were zone-drawn through four steps. Draw ratio was regulated by varying the drawing stress, drawing temperature and heat band speed. Zone drawing was carried out by moving a pair of narrow band heaters with dimensions of length 7 cm, width 2.5 cm, and thickness l mm along the film. The film was drawn under a tension
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303
Fig. 1. Continued
controlled by different dead weights on an Instron model 4201 [6,7]. The zone drawing conditions are listed in Table 1. 2.2. Characterization of the UHMW PE gel film The scanning electron micrographs of the pre-drawn gel films which were fractured at liquid nitrogen temperature were taken on a Cambridge S-360 SEM at an acceleration voltage of 20 kV. The crystallinity was determined by differential scanning calorimeter (Perkin-Elmer, DSC 7) at a heating rate of 10°C/min. The crystallinity was calculated from heat of fusion of 294 J/g, which is for ideal PE crystals. Load–elongation curves were recorded on an Instron model 4201 using a sample length of 2 cm and a cross-head speed of 100 mm/min. The tensile strength and modulus of the UHMW PE gel film were the average values from 20 samples.
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3. Results and discussion Generally it has been known that the maximum draw ratio of solution-cast film is influenced by the solution concentration, and maximum draw ratio can be obtained at a certain solution concentration. Moreover, in the vicinity of this concentration the solution viscosity steeply increases. Thus, in this study, firstly, relative viscosity-measuring experiments were tried to predict optimum polymer concentration of UHMW PE. From the fact that all the measured data were approximated to two different linear lines, it was possible to determine critical concentration showing an abrupt change in the solution viscosity. This result is coincident with Sawatari et al.’s data [21]. However, optimum gelation/crystallization temperature with optimum initial concentration could not be determined at the same time by solution viscometry. For the above reasons, we tried to determine optimum polymer concentration accompanied by optimum gelation/crystallization temperature of UHMW PE solution by the zone drawing method. The structure and physical properties of zone-drawn film may vary with the conditions of preparation and drawing of film. That is, firstly, initial concentration and gel formation temperature of polymer solution during preparation of the gel film; secondly, processing parameters like drawing stress, drawing temperature and heat band speed during drawing of film have marked influences on the drawing behavior of film. In this study, effects of those factors on the draw ratios of films from different solution concentrations and gelation/crystallization temperatures were investigated. Fig. 1 shows the fractured surfaces of the pre-drawn PE gel films having the same initial concentration of 0.5 g/dl cast at six different gelation temperatures. From the fact that the pre-drawn PE gel films had structural differences in the fractured surfaces according to the gelation temperatures, it could be supposed that the gelation/crystallization temperature of the pre-drawn PE gel film had a marked influence on the deformability of UHMW PE gel film. Fig. 2 shows the draw ratio of one-step zone drawn PE gel film as a function of the initial concentration of the solution at various drawing temperatures under drawing stress of 4.5 MPa and heat band speed of 10 mm/min. As the drawing temperature increased, draw ratio increased. This implies that as drawing temperature reaches crystal melting temperature (129–132°C) of
Fig. 2. Draw ratio dependence on the initial solution concentration of UHMW PE gel films at a gelation/crystallization temperature of 25°C at gradual zone drawing temperature.
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Fig. 3. Draw ratio dependence on the gelation/crystallization temperature and initial solution concentration of UHMW PE gel films drawn at (a) 127°C, (b) 112°C and (c) 98°C.
undrawn UHMW PE gel film, degree of freedom of PE chains increases. The maximum value of draw ratio appeared at solution concentration of 0.5 g/dl at all drawing temperatures. At lower or higher concentration, draw ratio decreased gradually. This corresponds to a suitable number of entanglements for the gel film prepared at a solution concentration of 0.5 g/dl. Generally, for effective drawing, gel film must have a suitable number of entanglements that can be evaluated by determination of optimum solution concentration. Optimum initial concentration of polymer solutions varies with molecular weight, linearity, and stereoregularity of polymers and type of solvent, etc. In this study, however, those effects were negligible because the same type of polymer with the same molecular weight and the same solvent were used in all experiments. Therefore, it can be supposed that an initial concentration of 0.5 g/dl is the optimum concentration which contains suitable entanglements at a gelation/crystallization temperature of 25°C. This result was similar to other research by the viscometric method [3]. The general viscometric method for a determination of optimum solution concentration is complicated and requires a long time, thus, a zone drawing method could be better.
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Fig. 4. Draw ratio dependence on the heat band speed in one-step zone drawing of UHMW PE gel films prepared at four different gelation/crystallization temperatures and at three different initial solution concentrations of (a) 0.3 g/dl, (b) 0.5 g/dl, and (c) 0.7 g/dl.
Fig. 3 shows the effect of gelation/crystallization temperature and initial concentration on onestep zone draw ratios of PE gel film drawn at 127°C (a), 112°C (b), and 98°C (c) using a drawing stress of 4.5 MPa and heat band speed of 10 mm/min. From the results in Fig. 2, it was identified that a concentration of 0.5 g/dl was the optimum initial concentration when solution was gelled and crystallized at 25°C. So, draw ratio of film prepared by gelation/crystallization at 0.5 g/dl and 25°C was the highest and draw ratio decreased at higher (50 and 70°C) and lower ( ⫺ 180, ⫺ 30, 0, and 10°C) gelation/crystallization temperatures. This effect can be explained by the number of chain entanglements of gel film prepared at ⫺ 180, ⫺ 30, 0, and 10°C being more than that at 25°C. Also, those prepared at 50 and 70°C had fewer than that at 25°C due to loosening of chain entanglements. This tendency is nearly the same irrespective of different drawing temperatures. It is interesting to see that optimum gelation temperature was shifted according
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to the initial polymer concentration. That is, at initial concentrations of 0.3 g/dl and 0.7 g/dl, these were found to be below 10°C and ca. 50°C, respectively. From this fact, it was found that a higher chain entanglement density should be required at a higher gelation temperature. In this study, the gel films were drawn at different drawing stresses. The draw ratio increased with an increase in the drawing stress. This result can be explained by deformation of molecular chains increasing with increasing drawing stress above the glass transition temperature. The maximum value of draw ratio appeared at solution concentration of 0.5 g/dl and gelation temperature of 25°C. Fig. 4 shows plots of draw ratios of gel films prepared at different initial concentrations and at different gelation/crystallization temperatures drawn under a drawing temperature of 127°C, and drawing stress of 4.5 MPa, against heat band speeds. It was shown that the slower the heat band speed, the larger the draw ratio. This may indicate that a softening of gel film becomes easier owing to longer residence time for gel film between two heat bands. As identified in the above figures, the one-step zone draw ratio of gel film prepared at a concentration of 0.5 g/dl and at a gelation temperature of 25°C was the largest among all the films of different concentrations
Fig. 5. Plots of multi-step draw ratios of gel films prepared at initial concentrations of (a) 0.3 g/dl, (b) 0.5 g/dl and (c) 0.7 g/dl vs. gelation/crystallization temperatures of UHMW PE solutions.
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Fig. 6. Crystallinity of one-step zone drawn gel film drawn at 127°C with gelation/crystallization temperature.
and temperatures. Thus, it was supposed that the gel film prepared at this concentration with 25°C had suitable entanglements to attain effective draw ratio. These results are in agreement with Sawatari et al.’s results[13]. Matsuo et al. [27] reported that as gelation/crystallization temperature increased during film formation, the number of chain entanglements decreased by enfeeblement of molecular chain restriction. In the present work, considering this point, we investigated the relationship between maximum draw ratio and gelation/crystallization temperature. Fig. 5 shows the effects of gelation/crystallization temperature and initial concentration on stepwise draw ratios (four-step draw ratio is the maximum) of PE gel film drawn at a drawing temperature of 127°C, drawing stress of 4.5 MPa, and heat band speed of 10 mm/min prepared with concentrations of (a) 0.3, (b) 0.5 and (c) 0.7 g/dl. Maximum draw ratio (draw ratio of 223) of UHMW PE gel film prepared at a gelation/crystallization temperature of 25°C and at an initial solution concentration of 0.5 g/dl was the highest. As presented in the figure, the effects of concentration and temperature on the draw ratio showed a similar tendency in every drawing step. Thus, it was identified that the prediction of maximum drawing behavior was possible by one-step zone draw ratio only. Consequently, the zone drawing method is appropriate for determining optimum initial concentration and gelation/crystallization temperatures for gel film formation. Fig. 6 shows plots of the crystallinities of one-step zone drawn PE gel films against the initial concentration and gelation temperature of the solution under a drawing temperature of 127°C, drawing stress of 4.5 MPa and heat band speed of 10 mm/min. Two things are worth noting in the figure. Firstly, maximum crystallinity appeared at a solution concentration of 0.5 g/dl with a temperature of 25°C. At lower or higher concentrations, crystallinity decreased. The reason is because the difference in draw ratio by one-step zone drawing might cause variation of crystallinity. Secondly, crystallinity of gel film varied with respect to gelation temperature at each concentration. Figs. 7 and 8 show the effect of the same draw ratio on the tensile strength and tensile modulus of one-step zone drawn gel film prepared at different gelation/crystallization temperatures and different initial solution concentrations with a drawing temperature of 127°C, drawing stress of 4.5 MPa, and heat band speed of 10 mm/min. It was shown that the tensile strength and the tensile
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Fig. 7. Tensile strength of one-step zone drawn gel film prepared at four different gelation/crystallization temperatures and at three different initial solution concentrations of (a) 0.3 g/dl, (b) 0.5 g/dl and (c) 0.7 g/dl.
modulus increased with an increase in the draw ratio and tensile properties had a dependence on gelation temperature. That is, in the case of 0.5 g/dl, maximum tensile strength and tensile modulus were obtained at a gelation temperature of 25°C, which were 2.88 GPa and 39.5 GPa, respectively, in spite of one-step zone drawing. In contrast, those for film from initial concentrations of 0.3 and 0.7 g/dl revealed maximum values at gelation/crystallization temperatures of 10 and 50°C, respectively. 4. Conclusions Considering the effects of various parameters on the zone draw ratio of UHMW PE gel film prepared at different initial solution concentrations and gelation/crystallization temperatures, we may conclude the following. The one-step zone draw ratio of the gel film prepared at 25°C was the largest at the initial concentration of 0.5 g/dl and gradually decreased at higher or lower
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Fig. 8. Tensile modulus of one-step zone drawn gel film prepared at four different gelation/crystallization temperatures and at three different initial solution concentrations of (a) 0.3 g/dl, (b) 0.5 g/dl and (c) 0.7 g/dl.
concentrations and temperatures. That is, a relationship between initial concentration and gelation temperature was found. This tendency was similar to the results observed for the maximum draw ratio data. The concentration of 0.5 g/dl at 25°C is the optimum initial concentration and gelation/crystallization temperature from this study. The tensile properties revealed their maximum at these points. Consequently, it was identified that the optimum concentration and gelation/crystallization temperature of initial UHMW PE solutions could be determined by the draw ratio obtained by simple one-step zone drawing. References [1] Ciferri A, Valenti B. In: Ciferri A, Ward IM, editors. Ultra-high modulus polymers. London: Applied Science, 1984. [2] Donnet JB, Bansal RC. In: Lewin M, Dekker M, editors. Carbon fibers. New York: Marcel Dekker, 1984. [3] Smith P, Lemstra PJ. Colloid Polym Sci 1980;258:89.
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