Real-time characterization of polymer-chain orientation
10.1002/spepro.002584
Real-time characterization of polymer-chain orientation Jiang Li, Congmei Lin, Shan Wang, Huimin Sun, Jian Huang, and Shaoyun Guo
Ultrasound is a powerful tool for monitoring orientation and disorientation behavior of polymer melts during processing. A certain level of shear stress and rate during processing results in polymer chains melting along the direction of the external force, with the chains transforming from coils to extended configurations. When the shear is halted, the molecular chains recoil. The non-negligible friction between molecular chains affects the relaxation of both a chain’s orientation and disorientation. Orientation plays a key role in final product characteristics, such as mechanical, optical, barrier, and thermal properties.1, 2 Therefore, many efforts have focused on measuring degrees of orientation. In the past decade, various approaches have been developed to characterize orientation, including wide-angle x-ray diffraction and scattering, optical birefringence, IR dichroism, and nuclear magnetic resonance.3–5 However, most of these methods operate off-line and are costly and time-consuming. They cannot be used to continually inspect orientation-relaxation behavior. We have developed a novel ultrasonic technique to study in-process orientation and disorientation relaxation of polymer melts. Ultrasound has been widely applied to characterize injection molding and extrusion processes in real time6, 7 because of its robustness, safety, simplicity, fast response, nondestructiveness, noninvasiveness, cost-effectiveness, and high sensitivity to material properties and processing conditions. However, research aimed at gaining a fundamental understanding of the relationship between ultrasonic measurements and orientationrelaxation behavior is relatively scarce.8, 9 We have, therefore, investigated the effects of molecular weight, shear rate, and temperature on orientation and disorientation relaxation of polymer melts. We designed and employed a special experimental apparatus (see Figure 1). An instrumental slit die (flow-channel dimensions: 20 2 250mm3 ) was fitted to the barrel exit of a capillary rheometer. (The latter is used to preheat samples and induce a certain level of shear stress.) We equipped the slit die with two ultrasonic sensors, two pressure/temperature transducers, and a die-temperature sensor.
Figure 1. Schematic of our experimental apparatus. US: Ultrasonic sensors. PT2, PT3: Pressure/temperature transducers. Td i e : Dietemperature sensor. A: Capillary rheometer. B: Connector. C: Instrumental die. I, II, III: Cross-sections at equal distances from the exit of the slit die.
The velocity of longitudinal waves .C / through the polymer material is a function of the bulk modulus .K/ and density ./, s K C D (1) During orientation, a decrease in bulk modulus perpendicular to the shear flow results in a velocity decrease. (The opposite applies to
Continued on next page
10.1002/spepro.002584 Page 2/3
Figure 2. Relaxation processes for JHC7260 (a type of high-density polyethylene resin) at a shear temperature of 185ı C and a shear rate of 22.1s 1 . (a) Orientation. (b) Disorientation. CR : Relative ultrasonic velocity. t: Time.
disorientation.) The evolution of the ultrasonic velocity during orientation and disorientation obeys, respectively (see Figure 2), CR1 .t/ D C1 .1
t = 1
e
/
(2)
and CR0 .t/ D C0 e
t = 0
;
Figure 3. Effects of shear rate and molecular weight on the maximum degree of orientation, and on the relaxation time of orientation and disorientation. 5000S, DGDB2480: Types of high-density polyethylene resin. C1 , C0 : Maximum degree of orientation and disorientation.
(3)
where the subscripts n D 1; 0 represent orientation and disorientation, respectively, Cn and CR n are the maximum relative velocity and the relative ultrasonic velocity, respectively, is the relaxation time, and C.t/ is the maximum velocity at time t with respect to that in the unoriented state, C.t0 /. Figures 3 and 4 show the effects of shear rate, temperature, and molecular weight on orientation and disorientation. The maximum degree of orientation (C1 ) and disorientation (C0 ) increases with increasing shear rate and temperature, while the orientation-relaxation time simultaneously decreases. This can be explained by considering that the presence of an external force thermodynamically favors molecular-chain motion. (The interactions of molecular chains could be reduced by increasing the temperature.) Note that no pronounced variation in disorientation time is seen during disorientation because molecular disorientation is a spontaneous process. The sample characterized by a low melt index exhibits a significant increase in both the maximum degree of orientation and the relaxation times of orientation and disorientation. In summary, we have demonstrated how orientation and disorientation relaxation behavior can be measured using ultrasound. Our results reveal that the ultrasonic velocity is sensitive to orientation and disorientation relaxation during processing. Our future work will focus on high-density polyethylene and other polymer composites to study the influences of compatibilizer concentration and component ratio on their orientation and disorientation relaxation behavior.
Figure 4. Effects of temperature on the maximum degree of orientation, and on the relaxation time of orientation and disorientation for JHC7260. The authors thank the National Natural Science Foundation of China (grant 50973075) and acknowledge the project sponsored by the Scientific Research Foundation for Returned Overseas Chinese Scholars, State Education Ministry (2008890-19-8) for providing scholarships and financial support. Continued on next page
10.1002/spepro.002584 Page 3/3
Author Information Jiang Li, Congmei Lin, Shan Wang, Huimin Sun, Jian Huang, and Shaoyun Guo The State Key Laboratory for Polymer Materials Engineering Sichuan University Chengdu, China References 1. A. D. Francesco and R. A. Duckett, Effects of orientation on mechanical properties of uniaxially oriented polystyrene films, Polymer 45, pp. 8005–8011, 2004. 2. R. H. Somani, B. S. Hsiao, A. Nogales, S. Srinivas, A. H. Tsou, I. Sics, F. J. Balta-Calleja, and T. A. Ezquerra, Structure development during shear flow-induced crystallization of i-PP: in-situ small-angle x-ray scattering study, Macromol. 33 (25), pp. 9385–9394, 2000. 3. A. Fujimori, N. Ninomiya, and T. Masuko, Structure and mechanical properties in drawn poly(L-lactide)/clay hybrid films, Polym. Adv. Technol. 19, pp. 1735–1744, 2008. 4. A. D. Schwab, D. M. G. Agra, J. H. Kim, S. Kumar, and A. Dhinojwala, Surface dynamics in rubbed polymer thin films probed with optical birefringence measurements, Macromol. 33 (13), pp. 4903–4909, 2000. 5. G. Wiberg, M. L. Skytt, and U. W. Gedde, Shear-induced alignment and relaxation of orientation in smectic side-chain liquid-crystalline polymers, Polymer 39 (13), pp. 2983–2986, 1998. 6. Z. Sun, C. K. Jen, C. K. Shih, and D. A. Denelsbeck, Application of ultrasound in the determination of fundamental extrusion performance: residence time distribution measurement, Polym. Eng. Sci. 43, pp. 102–111, 2003. 7. Z. Sun, C. K. Jen, J. Yan, and M. Y. Chen, Application of ultrasound and neural networks in the determination of filler dispersion during polymer extrusion processes, Polym. Eng. Sci. 45, pp. 764–772, 2005. 8. J. Li, Z.Sun, J. Tatibou¨et, and C. K. Jen, An in-process ultrasonic approach to investigating the relaxation of orientation and disorientation of polymer melts, Polym. Eng. Sci. 48, pp. 987–994, 2008. 9. C. M. Lin, H. M. Sun, S. Wang, J. Huang, J. Li, and S. Y. Guo, In-process ultrasonic measurements of orientation and disorientation relaxation of high density polyethylene/polyamide-6 composites with compatibilizer, J. Appl. Polym. Sci. 116, pp. 320–327. doi:10.1002/app.31566
c 2010 Society of Plastics Engineers (SPE)
Real-time characterization of polymer-chain orientation Jiang Li, Congmei Lin, Shan Wang, Huimin Sun, Jian Huang, and Shaoyun Guo
Ultrasound is a powerful tool for monitoring orientation and disorientation behavior of polymer melts during processing. A certain level of shear stress and rate during processing results in polymer chains melting along the direction of the external force, with the chains transforming from coils to extended configurations. When the shear is halted, the molecular chains recoil. The non-negligible friction between molecular chains affects the relaxation of both a chain’s orientation and disorientation. Orientation plays a key role in final product characteristics, such as mechanical, optical, barrier, and thermal properties.1, 2 Therefore, many efforts have focused on measuring degrees of orientation. In the past decade, various approaches have been developed to characterize orientation, including wide-angle x-ray diffraction and scattering, optical birefringence, IR dichroism, and nuclear magnetic resonance.3–5 However, most of these methods operate off-line and are costly and time-consuming. They cannot be used to continually inspect orientation-relaxation behavior. We have developed a novel ultrasonic technique to study in-process orientation and disorientation relaxation of polymer melts. Ultrasound has been widely applied to characterize injection molding and extrusion processes in real time6, 7 because of its robustness, safety, simplicity, fast response, nondestructiveness, noninvasiveness, cost-effectiveness, and high sensitivity to material properties and processing conditions. However, research aimed at gaining a fundamental understanding of the relationship between ultrasonic measurements and orientationrelaxation behavior is relatively scarce.8, 9 We have, therefore, investigated the effects of molecular weight, shear rate, and temperature on orientation and disorientation relaxation of polymer melts. We designed and employed a special experimental apparatus (see Figure 1). An instrumental slit die (flow-channel dimensions: 20 2 250mm3 ) was fitted to the barrel exit of a capillary rheometer. (The latter is used to preheat samples and induce a certain level of shear stress.) We equipped the slit die with two ultrasonic sensors, two pressure/temperature transducers, and a die-temperature sensor.
Figure 1. Schematic of our experimental apparatus. US: Ultrasonic sensors. PT2, PT3: Pressure/temperature transducers. Td i e : Dietemperature sensor. A: Capillary rheometer. B: Connector. C: Instrumental die. I, II, III: Cross-sections at equal distances from the exit of the slit die.
The velocity of longitudinal waves .C / through the polymer material is a function of the bulk modulus .K/ and density ./, s K C D (1) During orientation, a decrease in bulk modulus perpendicular to the shear flow results in a velocity decrease. (The opposite applies to
Continued on next page
10.1002/spepro.002584 Page 2/3
Figure 2. Relaxation processes for JHC7260 (a type of high-density polyethylene resin) at a shear temperature of 185ı C and a shear rate of 22.1s 1 . (a) Orientation. (b) Disorientation. CR : Relative ultrasonic velocity. t: Time.
disorientation.) The evolution of the ultrasonic velocity during orientation and disorientation obeys, respectively (see Figure 2), CR1 .t/ D C1 .1
t = 1
e
/
(2)
and CR0 .t/ D C0 e
t = 0
;
Figure 3. Effects of shear rate and molecular weight on the maximum degree of orientation, and on the relaxation time of orientation and disorientation. 5000S, DGDB2480: Types of high-density polyethylene resin. C1 , C0 : Maximum degree of orientation and disorientation.
(3)
where the subscripts n D 1; 0 represent orientation and disorientation, respectively, Cn and CR n are the maximum relative velocity and the relative ultrasonic velocity, respectively, is the relaxation time, and C.t/ is the maximum velocity at time t with respect to that in the unoriented state, C.t0 /. Figures 3 and 4 show the effects of shear rate, temperature, and molecular weight on orientation and disorientation. The maximum degree of orientation (C1 ) and disorientation (C0 ) increases with increasing shear rate and temperature, while the orientation-relaxation time simultaneously decreases. This can be explained by considering that the presence of an external force thermodynamically favors molecular-chain motion. (The interactions of molecular chains could be reduced by increasing the temperature.) Note that no pronounced variation in disorientation time is seen during disorientation because molecular disorientation is a spontaneous process. The sample characterized by a low melt index exhibits a significant increase in both the maximum degree of orientation and the relaxation times of orientation and disorientation. In summary, we have demonstrated how orientation and disorientation relaxation behavior can be measured using ultrasound. Our results reveal that the ultrasonic velocity is sensitive to orientation and disorientation relaxation during processing. Our future work will focus on high-density polyethylene and other polymer composites to study the influences of compatibilizer concentration and component ratio on their orientation and disorientation relaxation behavior.
Figure 4. Effects of temperature on the maximum degree of orientation, and on the relaxation time of orientation and disorientation for JHC7260. The authors thank the National Natural Science Foundation of China (grant 50973075) and acknowledge the project sponsored by the Scientific Research Foundation for Returned Overseas Chinese Scholars, State Education Ministry (2008890-19-8) for providing scholarships and financial support. Continued on next page
10.1002/spepro.002584 Page 3/3
Author Information Jiang Li, Congmei Lin, Shan Wang, Huimin Sun, Jian Huang, and Shaoyun Guo The State Key Laboratory for Polymer Materials Engineering Sichuan University Chengdu, China References 1. A. D. Francesco and R. A. Duckett, Effects of orientation on mechanical properties of uniaxially oriented polystyrene films, Polymer 45, pp. 8005–8011, 2004. 2. R. H. Somani, B. S. Hsiao, A. Nogales, S. Srinivas, A. H. Tsou, I. Sics, F. J. Balta-Calleja, and T. A. Ezquerra, Structure development during shear flow-induced crystallization of i-PP: in-situ small-angle x-ray scattering study, Macromol. 33 (25), pp. 9385–9394, 2000. 3. A. Fujimori, N. Ninomiya, and T. Masuko, Structure and mechanical properties in drawn poly(L-lactide)/clay hybrid films, Polym. Adv. Technol. 19, pp. 1735–1744, 2008. 4. A. D. Schwab, D. M. G. Agra, J. H. Kim, S. Kumar, and A. Dhinojwala, Surface dynamics in rubbed polymer thin films probed with optical birefringence measurements, Macromol. 33 (13), pp. 4903–4909, 2000. 5. G. Wiberg, M. L. Skytt, and U. W. Gedde, Shear-induced alignment and relaxation of orientation in smectic side-chain liquid-crystalline polymers, Polymer 39 (13), pp. 2983–2986, 1998. 6. Z. Sun, C. K. Jen, C. K. Shih, and D. A. Denelsbeck, Application of ultrasound in the determination of fundamental extrusion performance: residence time distribution measurement, Polym. Eng. Sci. 43, pp. 102–111, 2003. 7. Z. Sun, C. K. Jen, J. Yan, and M. Y. Chen, Application of ultrasound and neural networks in the determination of filler dispersion during polymer extrusion processes, Polym. Eng. Sci. 45, pp. 764–772, 2005. 8. J. Li, Z.Sun, J. Tatibou¨et, and C. K. Jen, An in-process ultrasonic approach to investigating the relaxation of orientation and disorientation of polymer melts, Polym. Eng. Sci. 48, pp. 987–994, 2008. 9. C. M. Lin, H. M. Sun, S. Wang, J. Huang, J. Li, and S. Y. Guo, In-process ultrasonic measurements of orientation and disorientation relaxation of high density polyethylene/polyamide-6 composites with compatibilizer, J. Appl. Polym. Sci. 116, pp. 320–327. doi:10.1002/app.31566
c 2010 Society of Plastics Engineers (SPE)
