- Email: [email protected]
Temperature Triggered Shape Memory Effect of Transpolyisoprene-based Polymer
Basic Research—Technology
Temperature Triggered Shape Memory Effect of Transpolyisoprene-based Polymer Gakuji Tsukada, DDS, PhD, Masayuki Tokuda, DDS, PhD, and Mitsuo Torii, DDS, PhD Abstract Introduction: Pure gutta-percha (trans-1, 4polyisoprene [TPI]) has been used extensively as a main component of gutta-percha for root canal filling. TPI has the interesting shape memory property by cross-linking, and this polymer was commercialized under the product name of SMP-2 (Kuraray Corp, Kashima, Japan). Therefore, the purpose of this study was to examine the thermal properties and the mechanism of the shape memory function of cross-linked SMP-2. Methods: The crystalline of the TPI was observed by x-ray diffraction. The effects of temperature on shape recovery, recovery stress, and relaxation modulus (Er[5]) were measured in cross-linked cylindrical specimens of SMP-2. Differential scanning calorimetry was used to monitor thermal events. Results: On heating, a pronounced increase in recovery stress, a marked decrease in Er(5), and endothermic DSC peaks were observed over the same temperature range (38 –51 C) with shape recovery. On the other hand, on cooling, a pronounced decrease in recovery stress, a marked increase in Er(5), and an exothermic DSC peak were observed over the same temperature range (27 –33 C). Conclusions: The shape memory property of TPI is derived from its crystallinity and cross-linking ability. Fixing the deformed shape and shape recovery from the deformed shape to the original shape is relatively easy to achieve by changing the temperature of SMP-2. The shape memory function of the cross-linked SMP-2 was expected to be very useful as a root canal filling material by the modification of its some thermal properties. (J Endod 2014;40:1658–1662)
Key Words Crystallinity, gutta-percha, root canal filling material, shape memory polymer, thermal property, trans-1, 4polyisoprene
From the Department of Restorative Dentistry and Endodontology, Kagoshima University Graduate School of Medical and Dental Sciences, Kagoshima, Japan. Address requests for reprints to Dr Gakuji Tsukada, Department of Restorative Dentistry and Endodontology, Kagoshima University Graduate School of Medical and Dental Sciences, 8-35-1 Sakuragaoka, Kagoshima, 890-8544, Japan. E-mail address: [email protected] 0099-2399/$ - see front matter Copyright ª 2014 American Association of Endodontists. http://dx.doi.org/10.1016/j.joen.2014.05.003
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T
he root canal filling treatment, which is a finishing procedure for endodontics, is 1 of the most important procedures. In addition to the traditional lateral condensation root canal filling technique, a number of other techniques using gutta-percha have been developed. Most take advantage of the thermoplasticity of the material (1, 2). The sealing ability of these techniques has been compared with the lateral condensation technique (3–5). However, most techniques using warmed guttapercha are not necessarily superior to the lateral condensation procedure (6, 7). Melted gutta-percha undergoes a large amount of shrinkage during setting (8, 9), and the use of sealer has been encouraged (10, 11). Additionally, the technical difficulty of these techniques is greater, and the risk of overextension of gutta-percha and sealer has been pointed out (12, 13). At present, warmed gutta-percha techniques have not completely replaced the lateral condensation methods. There is interest in a material that works effectively in response to various outside stimuli, such as heat, light, electricity or pressure (ie, a functional material). Such functional materials have been developed for applications in different fields. In endodontics, if the root canal filling material has an effective functionality, sealing the root canal using its inherent functionality is ideal. For the lateral condensation or warmed gutta-percha techniques, the components are pure gutta-percha (trans-1, 4-polyisoprene), zinc oxide, wax or resin, barium sulfate, and other additives (14–17). Gutta-percha has been used extensively as a root canal filling material and is thus familiar to the dentist. It has been difficult to find a substitute material for gutta-percha. Trans-1, 4-polyisoprene (TPI) is a diene rubber possessing a double bond in the main chain, and for this reason, it can be easily vulcanized by sulfur. The degree of crystallinity of frozen raw gutta-percha produced in Tjipetir of Indonesia is 55%–60% (18); the crystal structure of pure gutta-percha has been reported previously in detail (19, 20). Thus, TPI is a crystalline polymer, and its mechanical properties are affected by its crystallinity (8, 14, 15). Because of these properties, TPI has an interesting shape memory property (21), and this polymer was commercialized as the product name SMP-2 (Kuraray Corp, Kashima, Japan). SMP-2 was not developed in the dental arena, and its production is now discontinued. Therefore, the purpose of this study was to examine the thermal properties and mechanism of the shape memory function of cross-linked SMP-2.
Materials and Methods X-ray Diffraction Measurement of TPI After 1.2 g TPI (TP-301, Kuraray Corp) was melted on the heated metal plate at 80 C for 10 minutes, the melted TPI was pressed by another heated metal plate to achieve a 1.0-mm thickness. Then, TPI with metal plates was rapidly cooled down by immersing in water at 0 C for 10 minutes. This film shape TPI was allowed to warm to 24 C, was cut into 18 20 mm, and was used for the measurement of x-ray diffraction. An x-ray diffractometer (Ultima; Rigaku Corp, Tokyo, Japan) with 40 kV and 40 mA using CuKa radiation at the continuous 2q scanning was used to measure the crystalline of this film shape TPI. Additionally, the degree of crystallization of TPI was calculated from the ratio of the scattering intensity of the crystalline part to the total scattering intensity by using analyzing software (PDXL2; Rigaku Corp, Tokyo, Japan). In the measurements of x-ray diffraction, the median value is reported because similar results were obtained from triplicate determinations.
JOE — Volume 40, Number 10, October 2014
Basic Research—Technology
Figure 1. (A) Photograph of the metal mold and (B) a schematic representation of the metal mold. Shape deformation and recovery of the test specimen: (C) original shape, (D) deformed shape, and (E) recovery shape. (F) The x-ray diffraction pattern of TPI.
Shape Memory Polymer Preparation Kuraray developed a shape memory polymer under the product name SMP-2. This polymer was not intended to be developed in the dental arena, and, therefore, its production has been discontinued. For such a reason, a non–cross-linked SMP-2 compound with the following composition was specially obtained from the supplier (Kuraray Corp): 100 parts by weight of TPI (TP-301), 30 parts by weight of light calcium carbonate, 5 parts by weight of zinc oxide, 1 part by weight of stearic acid, 5 parts by weight of titanium oxide, 0.3 parts by weight of sulfur, 2 parts by weight of dibenzothiazyl disulfide, 1 part by weight of tetramethylthiuram disulfide, and 1 part by weight of 2,20 -methylene bis (4-methyl-6-tert-butylphenol). Sulfur was used as the cross-linking agent. This non–cross-linked SMP-2 compound had been prepared by Kuraray by kneading the ingredients together using an open roller at 75 5 C. The non–cross-linked SMP-2 compound was heated at 90 C for 10 minutes to soften the material, placed in a metal mold (Fig. 1A), and pressed at 2.94 MPa (Fig. 1B). The metal mold containing the non–cross-linked SMP-2 compound was transferred to an oven; the temperature of the metal mold was increased from 24 –150 C for 60 minutes and then held at 150 C for 60 minutes. This affected the cross-linking of the TPI. The metal mold was taken out of the oven, cooled down to room temperature, and the cross-linked SMP-2 was removed from it. By following such a procedure, a cylindrical test specimen having the shape memory property (6.0-mm diameter, 9.5-mm length, and 297-mg weight) was prepared. The following measurements were performed by using the cylindrical test specimen removed from the metal mold. Fixation of a Deformed Shape and Shape Recovery from a Deformed Shape Fixation of a deformed shape and shape recovery from a deformed shape are shown in Figure 1C–E. The test specimen (Fig. 1C) was JOE — Volume 40, Number 10, October 2014
softened by heating in a water bath at 90 C for 3 minutes; after this, the softened test specimen was axially compressed and then cooled in a water bath at 0 C for 10 minutes. The specimen was removed from the water bath and allowed to warm to 24 C. This procedure fixed the dimensions of the compressed specimen (Fig. 1D). The deformed test specimen could recover to the original shape (Fig. 1E) by raising the temperature of the test specimen. We examined the thermal properties and the mechanism of this shape memory function by the following measurement. In these measurements, the median value is reported because similar results were obtained from triplicate determinations.
Measurement of Shape Recovery from the Deformed Shape as a Function of Temperature Four test specimens were softened by heating in a water bath at 90 C for 3 minutes and then axially compressed by 0%, 14%, 28%, or 42%. After this compressive deformation, the dimensions were fixed by cooling in a water bath at 0 C. The test specimen was removed from the water bath, allowed to warm to 24 C, and then heated from 24 –80 C at 1.0 C/min. Changes in length were measured using a laser displacement meter (LC-2100; Keyence Corp, Osaka, Japan) at various temperatures to examine the shape recovery from the deformed shape. Measurement of Shape Recovery Stress The stress generated by shape recovery from the deformed shape was measured using a compression testing machine (AG-100; Shimadzu Corp, Kyoto, Japan). After the test specimen was softened by heating in a water bath at 90 C for 3 minutes, it was axially compressed by 30%. The compressive deformation was fixed by cooling in a water bath at 0 C for 10 minutes. The test specimen was removed from the water bath and allowed to warm to 24 C. The test specimen was then placed on the table installed in the testing machine, which was Shape Memory Effect of Transpolyisoprene-based Polymer
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Basic Research—Technology surrounded by a box whose temperature could be controlled. The indenter, which was connected to the load cell, was placed in contact with the top of the deformed test specimen. The position of the indenter was fixed while the test specimen was heated or cooled at the rate of 1.0 C/min from 24 –65 C. The stresses generated by heating and cooling were measured.
Measurement of Relaxation Modulus The relaxation modulus was measured using a compression testing machine (TG-50 KN; Minebea Corp Ltd, Nagano, Japan). The specimen was placed on a platen that was enclosed in a thermostatted box; the temperature of the specimen was changed by changing the temperature of the box. Measurements were made at temperatures from 14 –80 C. The compressive strain (g) was 0.05. The measurement time was 10 seconds, and the relaxation modulus (Er[t]) was calculated as follows: ErðtÞ ¼ f ðtÞ=g where f(t) is the compressive stress after t seconds from the start of the measurement. In this study, f(5) was used.
Differential Scanning Calorimeter Measurement A differential scanning calorimeter (DSC) (DSC-7000, Shimadzu Corp) was used to monitor thermal events. A sample (30 mg) was removed from a cylindrical test specimen that had been previously
used for mechanical properties testing. The sample was heated or cooled at a rate of 1.0 C/min from 24 –90 C.
Results X-ray Diffraction Analysis The x-ray diffraction pattern of TPI is shown in Figure 1F. The crystalline peaks were observed. The average value of the degree of crystallization was 38.3%. Shape Recovery from the Deformed Shape as a Function of Temperature The changes in shape recovery for SMP-2 are shown in Figure 2A. The length increased slightly for each compaction condition as the temperature was increased from 20 –37 C. Marked increases in length were observed from 38 –45 C in cases of axial fixed deformations of 14%, 28%, and 42%. The shape recovery was complete at 50 C in these 3 cases. Above 50 C, deformed and nondeformed specimens behaved similarly. Recovery Stress Changes in the recovery stress generated by the shape recovery from the deformed shape are shown in Figure 2B. The recovery stress increased slightly up to 37 C and then increased markedly from 37 –50 C. On cooling, the stress decreased slightly until 34 C and then decreased markedly from 33 –30 C.
Figure 2. Thermal properties of cross-linked SMP-2. (A) Change in shape recovery from deformation as a function of temperature, (B) change in shape recovery stress as a function of temperature, (C) change in relaxation modulus (Er[5]) as a function of temperature, and (D) DSC thermogram.
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Basic Research—Technology Relaxation Modulus Changes in the relaxation modulus, Er(5), of SMP-2 as a function of temperature are shown in Figure 2C. Er(5) decreased slightly until 37 C and then markedly decreased from 38 –48 C. Er(5) was almost unchanged above 50 C. On cooling from 80 –33 C, Er(5) remained almost unchanged but then increased markedly from 32 –27 C. DSC Measurement The DSC results are shown in Figure 2D. On heating, 2 peaks were identified at 44.0 and 51.0 C. However, on cooling, a single peak at 28 C was detected.
Discussion The schematic representation of the behavior of SMP-2 is shown in Figure 3. The main component of non–cross-linked SMP-2 is TP-301 having a melting point of 67 C (Fig. 3A) (22). Heating the non– cross-linked SMP-2 compound at 90 C for 10 minutes was sufficient to soften the material enough to fill the metal mold (Fig. 3A and B). Heating the non–cross-linked SMP-2, to which sulfur had been added, by raising the metal mold temperature from 24 –150 C for 60 minutes followed by maintaining it at 150 C for 60 minutes allowed the material to cross-link and memorize its shape (Fig. 3B and C). The cross-linked SMP-2 was removed from the metal mold (Fig. 3D). At room temperature, the cross-linked test specimen had a high Er(5) (Fig. 2C), probably because of the crystallinity of the TPI (Fig. 3D). The degree of the crystallization of TP-301, which was melted at 80 C and then rapidly cooled down at 0 C, was reported to be 36% as measured by densitometry (22). In this study, the degree of the crystallization of TP-301 under the same condition that was calculated from the x-ray diffraction pattern was 38.3% (Fig. 1F). The cross-linked SMP-2 could be softened by heating at 90 C for 3 minutes and could be deformed easily (Fig. 3E and F). The marked decrease in Er(5) (Fig. 2C) and the endothermic DSC peaks on heating (Fig. 2D) are consistent with melting of the TPI crystalline regions (Fig. 3D and E). When the deformed SMP-2 was cooled below the transition temperature, the deformation remained (Fig. 1D and Fig. 3F and G). A pronounced decrease in recovery stress
(Fig. 2B), a marked increase in Er(5) (Fig. 2C), and the exothermic DSC peak on cooling (Fig. 2D) are consistent with crystallization of the TPI. DSC measurement can detect a change of the heat flow with exothermic and endothermic effects and determine crystallization and crystalline melting by observing the crystallization peak and crystalline melting peak in its DSC curve. The crystallization of TPI could resist the forces trying to return it to its original shape, which had been memorized in the cross-linking reaction. The deformed shape could be fixed using this mechanism. The result that recovery stress was not detected at all on cooling shows that the deformed shape was completely fixed (Fig. 2B). Unless the temperature of the deformed SMP-2 exceeds the transition temperature, there is little possibility of relapse to the memorized shape. The temperature that is lower than a transition temperature is preferable for the dimension stability of deformed cross-linked SMP-2. Therefore, the refrigerated condition is thought to be preferred for long-term preservation of the deformed SMP-2. Heating deformed SMP-2 above its transition temperature causes the crystalline regions to melt and form an amorphous polymer. The crystallinity, which fixes any deformation, disappears, and the deformed shape then tries to recover its original shape, which had been memorized in the cross-linking reaction (Fig. 3G and H). This explains our finding that regardless of the amount of compressive deformation, the recovery from the deformed shape to the original memorized shape occurred at the same temperature (Fig. 2A). A pronounced increase in recovery stress (Fig. 2B), a marked decrease in Er(5) (Fig. 2C), and endothermic DSC peaks (Fig. 2D) were observed over the same temperature range. It is presumed that melting of the TPI crystalline regions would occur over this temperature range. Because the previous deformed shape (Fig. 3G) cannot be memorized, the recovered shape (Fig. 3H) cannot return to the previous deformed shape (Fig. 3G) by cooling. Therefore, the shape memory effect of cross-linked SMP-2 is not reversible. The mechanism for the shape memory behavior of SMP-2 is now more clearly understood. Fixing the deformed shape and shape recovery from the deformed shape to the original shape is relatively easy to achieve. The shape recovery of the gutta-percha point has the shape memory function expected to fit its shape to the internal shape of the
Figure 3. A schematic representation of the behavior of SMP-2: (A) non–cross-linked SMP-2, (B) molding, (C) cross-linking, (D) crystallization, (E) amorphous state, (F) deformation, (G) fixation of deformation by crystallization, and (H) shape recovery.
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Shape Memory Effect of Transpolyisoprene-based Polymer
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Acknowledgments The authors thank Kuraray Corp for providing the SMP-2 sample. Supported by Grants-in-Aid for Scientific Research #14571818. The authors deny any conflict of interest related to this study.
References 1. Schilder H. Filling root canals in three dimensions. Dent Clin North Am 1967;11: 723–44. 2. Yee FS, Marlin J, Kradow AA, Gron P. Three-dimensional obturation of the root canal using injection-molded, thermoplasticized dental gutta-percha. J Endod 1977;3: 168–74. 3. Mann SR, McWalter GM. Evaluation of apical seal and placement control in straight and curved canals obturated by laterally condensed and thermoplasticized guttapercha. J Endod 1987;13:10–7.
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4. Veis AA, Molyvdas IA, Lambrianidis TP, Beltes PG. In vitro evaluation of apical leakage of root canal fillings after in situ obturation with thermoplasticized and laterally condensed gutta-percha. Int Endod J 1994;27:213–7. 5. Goldberg F, Massone EJ, Artaza LP. Comparison of the sealing capacity of three endodontic filling techniques. J Endod 1995;21:1–3. 6. LaCombe JS, Campbell AD, Hicks ML, Pelleu GB Jr. A comparison of the apical seal producted by two thermoplasticized injectable gutta-percha techniques. J Endod 1988;14:445–50. 7. Hata G, kawazoe S, Toda T, Wein FS. Sealing ability of thermafil with and without sealer. J Endod 1992;18:322–6. 8. Tsukada G, Tanaka T, Torii M, Inoue K. Shear modulus and thermal properties of gutta percha for root canal filling. J Oral Rehabil 2004;31:1139–44. 9. Lottanti S, Taub€ock TT, Zebnder M. Shrinkage of backfill gutta-percha upon cooling. J Endod 2014;40:721–4. 10. Evans JT, Simon JHS. Evaluation of the apical seal produced by injected thermoplasticized gutta-percha in the absence of smear layer and root canal sealer. J Endod 1986;12:101–7. 11. Skinner RL, Himel VT. The sealing ability of injection-molded thermoplasticized gutta-percha with and without the use of sealers. J Endod 1987;13:315–7. 12. Al-Dewani N, Hayes SJ, Dummer PMH. Comparison of laterally condensed and low-temperature thermoplasticized gutta-percha root fillings. J Endod 2000;26: 733–8. 13. Peng L, Ye LY, Tan H, Zbou X. Outcome of root canal obturation by warm guttapercha versus cold lateral condensation: a meta-analysis. J Endod 2007;33: 106–9. 14. Friedman CM, Sandrik JL, Heuer MA, Rapp GW. Composition and mechanical properties of gutta-percha endodontic points. J Dent Res 1975;54:921–5. 15. Friedman CE, Sandrik JL, Heuer MA, Rapp GW. Composition and physical properties of gutta-percha endodontic filling materials. J Endod 1977;3:304–8. 16. Marciano J, Michailesco PM. Dental gutta-percha: chemical composition, X-ray identification, enthalpic studies, and clinical implication. J Endod 1989;15: 149–53. 17. Gurgel-Filho ED, Andrade Feitosa JP, Teixeira FB, et al. Chemical and X-ray analyses of five brands of dental gutta-percha cone. Int Endod J 2003;36:302–7. 18. Goppel JM, arlman JJ. On the degree of crystallinity in natural rubber IV. The degree of crystallization in frozen raw rubber and stretched vulcanized rubber. Appl Sci Res 1948;A1:462–74. 19. Bunn CW. Molecular structure and rubber-like elasticity, part 1. The crystal structure of beta gutta-percha, rubber and polychloroprene. Proc R Soc Lond Biol Sci 1942;A180:40–66. 20. Fisher D. Crystal structure of gutta percha. Proc Phys Soc 1953;66B:7–16. 21. Ishii M. Shape memory plastics, polyisoprene based plastics. Plastic Age 1989;35: 158–64. 22. Yoshikawa R. Transpolyiospurene. J Soc Rubber Ind 1984;57:723–7.
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Temperature Triggered Shape Memory Effect of Transpolyisoprene-based Polymer Gakuji Tsukada, DDS, PhD, Masayuki Tokuda, DDS, PhD, and Mitsuo Torii, DDS, PhD Abstract Introduction: Pure gutta-percha (trans-1, 4polyisoprene [TPI]) has been used extensively as a main component of gutta-percha for root canal filling. TPI has the interesting shape memory property by cross-linking, and this polymer was commercialized under the product name of SMP-2 (Kuraray Corp, Kashima, Japan). Therefore, the purpose of this study was to examine the thermal properties and the mechanism of the shape memory function of cross-linked SMP-2. Methods: The crystalline of the TPI was observed by x-ray diffraction. The effects of temperature on shape recovery, recovery stress, and relaxation modulus (Er[5]) were measured in cross-linked cylindrical specimens of SMP-2. Differential scanning calorimetry was used to monitor thermal events. Results: On heating, a pronounced increase in recovery stress, a marked decrease in Er(5), and endothermic DSC peaks were observed over the same temperature range (38 –51 C) with shape recovery. On the other hand, on cooling, a pronounced decrease in recovery stress, a marked increase in Er(5), and an exothermic DSC peak were observed over the same temperature range (27 –33 C). Conclusions: The shape memory property of TPI is derived from its crystallinity and cross-linking ability. Fixing the deformed shape and shape recovery from the deformed shape to the original shape is relatively easy to achieve by changing the temperature of SMP-2. The shape memory function of the cross-linked SMP-2 was expected to be very useful as a root canal filling material by the modification of its some thermal properties. (J Endod 2014;40:1658–1662)
Key Words Crystallinity, gutta-percha, root canal filling material, shape memory polymer, thermal property, trans-1, 4polyisoprene
From the Department of Restorative Dentistry and Endodontology, Kagoshima University Graduate School of Medical and Dental Sciences, Kagoshima, Japan. Address requests for reprints to Dr Gakuji Tsukada, Department of Restorative Dentistry and Endodontology, Kagoshima University Graduate School of Medical and Dental Sciences, 8-35-1 Sakuragaoka, Kagoshima, 890-8544, Japan. E-mail address: [email protected] 0099-2399/$ - see front matter Copyright ª 2014 American Association of Endodontists. http://dx.doi.org/10.1016/j.joen.2014.05.003
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T
he root canal filling treatment, which is a finishing procedure for endodontics, is 1 of the most important procedures. In addition to the traditional lateral condensation root canal filling technique, a number of other techniques using gutta-percha have been developed. Most take advantage of the thermoplasticity of the material (1, 2). The sealing ability of these techniques has been compared with the lateral condensation technique (3–5). However, most techniques using warmed guttapercha are not necessarily superior to the lateral condensation procedure (6, 7). Melted gutta-percha undergoes a large amount of shrinkage during setting (8, 9), and the use of sealer has been encouraged (10, 11). Additionally, the technical difficulty of these techniques is greater, and the risk of overextension of gutta-percha and sealer has been pointed out (12, 13). At present, warmed gutta-percha techniques have not completely replaced the lateral condensation methods. There is interest in a material that works effectively in response to various outside stimuli, such as heat, light, electricity or pressure (ie, a functional material). Such functional materials have been developed for applications in different fields. In endodontics, if the root canal filling material has an effective functionality, sealing the root canal using its inherent functionality is ideal. For the lateral condensation or warmed gutta-percha techniques, the components are pure gutta-percha (trans-1, 4-polyisoprene), zinc oxide, wax or resin, barium sulfate, and other additives (14–17). Gutta-percha has been used extensively as a root canal filling material and is thus familiar to the dentist. It has been difficult to find a substitute material for gutta-percha. Trans-1, 4-polyisoprene (TPI) is a diene rubber possessing a double bond in the main chain, and for this reason, it can be easily vulcanized by sulfur. The degree of crystallinity of frozen raw gutta-percha produced in Tjipetir of Indonesia is 55%–60% (18); the crystal structure of pure gutta-percha has been reported previously in detail (19, 20). Thus, TPI is a crystalline polymer, and its mechanical properties are affected by its crystallinity (8, 14, 15). Because of these properties, TPI has an interesting shape memory property (21), and this polymer was commercialized as the product name SMP-2 (Kuraray Corp, Kashima, Japan). SMP-2 was not developed in the dental arena, and its production is now discontinued. Therefore, the purpose of this study was to examine the thermal properties and mechanism of the shape memory function of cross-linked SMP-2.
Materials and Methods X-ray Diffraction Measurement of TPI After 1.2 g TPI (TP-301, Kuraray Corp) was melted on the heated metal plate at 80 C for 10 minutes, the melted TPI was pressed by another heated metal plate to achieve a 1.0-mm thickness. Then, TPI with metal plates was rapidly cooled down by immersing in water at 0 C for 10 minutes. This film shape TPI was allowed to warm to 24 C, was cut into 18 20 mm, and was used for the measurement of x-ray diffraction. An x-ray diffractometer (Ultima; Rigaku Corp, Tokyo, Japan) with 40 kV and 40 mA using CuKa radiation at the continuous 2q scanning was used to measure the crystalline of this film shape TPI. Additionally, the degree of crystallization of TPI was calculated from the ratio of the scattering intensity of the crystalline part to the total scattering intensity by using analyzing software (PDXL2; Rigaku Corp, Tokyo, Japan). In the measurements of x-ray diffraction, the median value is reported because similar results were obtained from triplicate determinations.
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Basic Research—Technology
Figure 1. (A) Photograph of the metal mold and (B) a schematic representation of the metal mold. Shape deformation and recovery of the test specimen: (C) original shape, (D) deformed shape, and (E) recovery shape. (F) The x-ray diffraction pattern of TPI.
Shape Memory Polymer Preparation Kuraray developed a shape memory polymer under the product name SMP-2. This polymer was not intended to be developed in the dental arena, and, therefore, its production has been discontinued. For such a reason, a non–cross-linked SMP-2 compound with the following composition was specially obtained from the supplier (Kuraray Corp): 100 parts by weight of TPI (TP-301), 30 parts by weight of light calcium carbonate, 5 parts by weight of zinc oxide, 1 part by weight of stearic acid, 5 parts by weight of titanium oxide, 0.3 parts by weight of sulfur, 2 parts by weight of dibenzothiazyl disulfide, 1 part by weight of tetramethylthiuram disulfide, and 1 part by weight of 2,20 -methylene bis (4-methyl-6-tert-butylphenol). Sulfur was used as the cross-linking agent. This non–cross-linked SMP-2 compound had been prepared by Kuraray by kneading the ingredients together using an open roller at 75 5 C. The non–cross-linked SMP-2 compound was heated at 90 C for 10 minutes to soften the material, placed in a metal mold (Fig. 1A), and pressed at 2.94 MPa (Fig. 1B). The metal mold containing the non–cross-linked SMP-2 compound was transferred to an oven; the temperature of the metal mold was increased from 24 –150 C for 60 minutes and then held at 150 C for 60 minutes. This affected the cross-linking of the TPI. The metal mold was taken out of the oven, cooled down to room temperature, and the cross-linked SMP-2 was removed from it. By following such a procedure, a cylindrical test specimen having the shape memory property (6.0-mm diameter, 9.5-mm length, and 297-mg weight) was prepared. The following measurements were performed by using the cylindrical test specimen removed from the metal mold. Fixation of a Deformed Shape and Shape Recovery from a Deformed Shape Fixation of a deformed shape and shape recovery from a deformed shape are shown in Figure 1C–E. The test specimen (Fig. 1C) was JOE — Volume 40, Number 10, October 2014
softened by heating in a water bath at 90 C for 3 minutes; after this, the softened test specimen was axially compressed and then cooled in a water bath at 0 C for 10 minutes. The specimen was removed from the water bath and allowed to warm to 24 C. This procedure fixed the dimensions of the compressed specimen (Fig. 1D). The deformed test specimen could recover to the original shape (Fig. 1E) by raising the temperature of the test specimen. We examined the thermal properties and the mechanism of this shape memory function by the following measurement. In these measurements, the median value is reported because similar results were obtained from triplicate determinations.
Measurement of Shape Recovery from the Deformed Shape as a Function of Temperature Four test specimens were softened by heating in a water bath at 90 C for 3 minutes and then axially compressed by 0%, 14%, 28%, or 42%. After this compressive deformation, the dimensions were fixed by cooling in a water bath at 0 C. The test specimen was removed from the water bath, allowed to warm to 24 C, and then heated from 24 –80 C at 1.0 C/min. Changes in length were measured using a laser displacement meter (LC-2100; Keyence Corp, Osaka, Japan) at various temperatures to examine the shape recovery from the deformed shape. Measurement of Shape Recovery Stress The stress generated by shape recovery from the deformed shape was measured using a compression testing machine (AG-100; Shimadzu Corp, Kyoto, Japan). After the test specimen was softened by heating in a water bath at 90 C for 3 minutes, it was axially compressed by 30%. The compressive deformation was fixed by cooling in a water bath at 0 C for 10 minutes. The test specimen was removed from the water bath and allowed to warm to 24 C. The test specimen was then placed on the table installed in the testing machine, which was Shape Memory Effect of Transpolyisoprene-based Polymer
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Basic Research—Technology surrounded by a box whose temperature could be controlled. The indenter, which was connected to the load cell, was placed in contact with the top of the deformed test specimen. The position of the indenter was fixed while the test specimen was heated or cooled at the rate of 1.0 C/min from 24 –65 C. The stresses generated by heating and cooling were measured.
Measurement of Relaxation Modulus The relaxation modulus was measured using a compression testing machine (TG-50 KN; Minebea Corp Ltd, Nagano, Japan). The specimen was placed on a platen that was enclosed in a thermostatted box; the temperature of the specimen was changed by changing the temperature of the box. Measurements were made at temperatures from 14 –80 C. The compressive strain (g) was 0.05. The measurement time was 10 seconds, and the relaxation modulus (Er[t]) was calculated as follows: ErðtÞ ¼ f ðtÞ=g where f(t) is the compressive stress after t seconds from the start of the measurement. In this study, f(5) was used.
Differential Scanning Calorimeter Measurement A differential scanning calorimeter (DSC) (DSC-7000, Shimadzu Corp) was used to monitor thermal events. A sample (30 mg) was removed from a cylindrical test specimen that had been previously
used for mechanical properties testing. The sample was heated or cooled at a rate of 1.0 C/min from 24 –90 C.
Results X-ray Diffraction Analysis The x-ray diffraction pattern of TPI is shown in Figure 1F. The crystalline peaks were observed. The average value of the degree of crystallization was 38.3%. Shape Recovery from the Deformed Shape as a Function of Temperature The changes in shape recovery for SMP-2 are shown in Figure 2A. The length increased slightly for each compaction condition as the temperature was increased from 20 –37 C. Marked increases in length were observed from 38 –45 C in cases of axial fixed deformations of 14%, 28%, and 42%. The shape recovery was complete at 50 C in these 3 cases. Above 50 C, deformed and nondeformed specimens behaved similarly. Recovery Stress Changes in the recovery stress generated by the shape recovery from the deformed shape are shown in Figure 2B. The recovery stress increased slightly up to 37 C and then increased markedly from 37 –50 C. On cooling, the stress decreased slightly until 34 C and then decreased markedly from 33 –30 C.
Figure 2. Thermal properties of cross-linked SMP-2. (A) Change in shape recovery from deformation as a function of temperature, (B) change in shape recovery stress as a function of temperature, (C) change in relaxation modulus (Er[5]) as a function of temperature, and (D) DSC thermogram.
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Basic Research—Technology Relaxation Modulus Changes in the relaxation modulus, Er(5), of SMP-2 as a function of temperature are shown in Figure 2C. Er(5) decreased slightly until 37 C and then markedly decreased from 38 –48 C. Er(5) was almost unchanged above 50 C. On cooling from 80 –33 C, Er(5) remained almost unchanged but then increased markedly from 32 –27 C. DSC Measurement The DSC results are shown in Figure 2D. On heating, 2 peaks were identified at 44.0 and 51.0 C. However, on cooling, a single peak at 28 C was detected.
Discussion The schematic representation of the behavior of SMP-2 is shown in Figure 3. The main component of non–cross-linked SMP-2 is TP-301 having a melting point of 67 C (Fig. 3A) (22). Heating the non– cross-linked SMP-2 compound at 90 C for 10 minutes was sufficient to soften the material enough to fill the metal mold (Fig. 3A and B). Heating the non–cross-linked SMP-2, to which sulfur had been added, by raising the metal mold temperature from 24 –150 C for 60 minutes followed by maintaining it at 150 C for 60 minutes allowed the material to cross-link and memorize its shape (Fig. 3B and C). The cross-linked SMP-2 was removed from the metal mold (Fig. 3D). At room temperature, the cross-linked test specimen had a high Er(5) (Fig. 2C), probably because of the crystallinity of the TPI (Fig. 3D). The degree of the crystallization of TP-301, which was melted at 80 C and then rapidly cooled down at 0 C, was reported to be 36% as measured by densitometry (22). In this study, the degree of the crystallization of TP-301 under the same condition that was calculated from the x-ray diffraction pattern was 38.3% (Fig. 1F). The cross-linked SMP-2 could be softened by heating at 90 C for 3 minutes and could be deformed easily (Fig. 3E and F). The marked decrease in Er(5) (Fig. 2C) and the endothermic DSC peaks on heating (Fig. 2D) are consistent with melting of the TPI crystalline regions (Fig. 3D and E). When the deformed SMP-2 was cooled below the transition temperature, the deformation remained (Fig. 1D and Fig. 3F and G). A pronounced decrease in recovery stress
(Fig. 2B), a marked increase in Er(5) (Fig. 2C), and the exothermic DSC peak on cooling (Fig. 2D) are consistent with crystallization of the TPI. DSC measurement can detect a change of the heat flow with exothermic and endothermic effects and determine crystallization and crystalline melting by observing the crystallization peak and crystalline melting peak in its DSC curve. The crystallization of TPI could resist the forces trying to return it to its original shape, which had been memorized in the cross-linking reaction. The deformed shape could be fixed using this mechanism. The result that recovery stress was not detected at all on cooling shows that the deformed shape was completely fixed (Fig. 2B). Unless the temperature of the deformed SMP-2 exceeds the transition temperature, there is little possibility of relapse to the memorized shape. The temperature that is lower than a transition temperature is preferable for the dimension stability of deformed cross-linked SMP-2. Therefore, the refrigerated condition is thought to be preferred for long-term preservation of the deformed SMP-2. Heating deformed SMP-2 above its transition temperature causes the crystalline regions to melt and form an amorphous polymer. The crystallinity, which fixes any deformation, disappears, and the deformed shape then tries to recover its original shape, which had been memorized in the cross-linking reaction (Fig. 3G and H). This explains our finding that regardless of the amount of compressive deformation, the recovery from the deformed shape to the original memorized shape occurred at the same temperature (Fig. 2A). A pronounced increase in recovery stress (Fig. 2B), a marked decrease in Er(5) (Fig. 2C), and endothermic DSC peaks (Fig. 2D) were observed over the same temperature range. It is presumed that melting of the TPI crystalline regions would occur over this temperature range. Because the previous deformed shape (Fig. 3G) cannot be memorized, the recovered shape (Fig. 3H) cannot return to the previous deformed shape (Fig. 3G) by cooling. Therefore, the shape memory effect of cross-linked SMP-2 is not reversible. The mechanism for the shape memory behavior of SMP-2 is now more clearly understood. Fixing the deformed shape and shape recovery from the deformed shape to the original shape is relatively easy to achieve. The shape recovery of the gutta-percha point has the shape memory function expected to fit its shape to the internal shape of the
Figure 3. A schematic representation of the behavior of SMP-2: (A) non–cross-linked SMP-2, (B) molding, (C) cross-linking, (D) crystallization, (E) amorphous state, (F) deformation, (G) fixation of deformation by crystallization, and (H) shape recovery.
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Acknowledgments The authors thank Kuraray Corp for providing the SMP-2 sample. Supported by Grants-in-Aid for Scientific Research #14571818. The authors deny any conflict of interest related to this study.
References 1. Schilder H. Filling root canals in three dimensions. Dent Clin North Am 1967;11: 723–44. 2. Yee FS, Marlin J, Kradow AA, Gron P. Three-dimensional obturation of the root canal using injection-molded, thermoplasticized dental gutta-percha. J Endod 1977;3: 168–74. 3. Mann SR, McWalter GM. Evaluation of apical seal and placement control in straight and curved canals obturated by laterally condensed and thermoplasticized guttapercha. J Endod 1987;13:10–7.
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4. Veis AA, Molyvdas IA, Lambrianidis TP, Beltes PG. In vitro evaluation of apical leakage of root canal fillings after in situ obturation with thermoplasticized and laterally condensed gutta-percha. Int Endod J 1994;27:213–7. 5. Goldberg F, Massone EJ, Artaza LP. Comparison of the sealing capacity of three endodontic filling techniques. J Endod 1995;21:1–3. 6. LaCombe JS, Campbell AD, Hicks ML, Pelleu GB Jr. A comparison of the apical seal producted by two thermoplasticized injectable gutta-percha techniques. J Endod 1988;14:445–50. 7. Hata G, kawazoe S, Toda T, Wein FS. Sealing ability of thermafil with and without sealer. J Endod 1992;18:322–6. 8. Tsukada G, Tanaka T, Torii M, Inoue K. Shear modulus and thermal properties of gutta percha for root canal filling. J Oral Rehabil 2004;31:1139–44. 9. Lottanti S, Taub€ock TT, Zebnder M. Shrinkage of backfill gutta-percha upon cooling. J Endod 2014;40:721–4. 10. Evans JT, Simon JHS. Evaluation of the apical seal produced by injected thermoplasticized gutta-percha in the absence of smear layer and root canal sealer. J Endod 1986;12:101–7. 11. Skinner RL, Himel VT. The sealing ability of injection-molded thermoplasticized gutta-percha with and without the use of sealers. J Endod 1987;13:315–7. 12. Al-Dewani N, Hayes SJ, Dummer PMH. Comparison of laterally condensed and low-temperature thermoplasticized gutta-percha root fillings. J Endod 2000;26: 733–8. 13. Peng L, Ye LY, Tan H, Zbou X. Outcome of root canal obturation by warm guttapercha versus cold lateral condensation: a meta-analysis. J Endod 2007;33: 106–9. 14. Friedman CM, Sandrik JL, Heuer MA, Rapp GW. Composition and mechanical properties of gutta-percha endodontic points. J Dent Res 1975;54:921–5. 15. Friedman CE, Sandrik JL, Heuer MA, Rapp GW. Composition and physical properties of gutta-percha endodontic filling materials. J Endod 1977;3:304–8. 16. Marciano J, Michailesco PM. Dental gutta-percha: chemical composition, X-ray identification, enthalpic studies, and clinical implication. J Endod 1989;15: 149–53. 17. Gurgel-Filho ED, Andrade Feitosa JP, Teixeira FB, et al. Chemical and X-ray analyses of five brands of dental gutta-percha cone. Int Endod J 2003;36:302–7. 18. Goppel JM, arlman JJ. On the degree of crystallinity in natural rubber IV. The degree of crystallization in frozen raw rubber and stretched vulcanized rubber. Appl Sci Res 1948;A1:462–74. 19. Bunn CW. Molecular structure and rubber-like elasticity, part 1. The crystal structure of beta gutta-percha, rubber and polychloroprene. Proc R Soc Lond Biol Sci 1942;A180:40–66. 20. Fisher D. Crystal structure of gutta percha. Proc Phys Soc 1953;66B:7–16. 21. Ishii M. Shape memory plastics, polyisoprene based plastics. Plastic Age 1989;35: 158–64. 22. Yoshikawa R. Transpolyiospurene. J Soc Rubber Ind 1984;57:723–7.
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