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Ductile-to-Brittle Transition Behavior of Low Molecular Weight Polycarbonate Under Carbon Dioxide.

We investigated the stress-strain behavior of low molecular weight polycarbonate for optical disc grade (OD-PC) under carbon dioxide (C[O.sub.2]) at various pressures, and compared the results with that under ambient pressure at various temperatures. Elongation at break decreased sharply with increased C[O.sub.2] pressure at around 2 MPa, while the elastic modulus decreased gradually up to 6 MPa. These results indicate that the tensile property changed from ductile to brittle with increased C[O.sub.2] pressure, although the molecular motion is accelerated due to the plasticization effect of C[O.sub.2]. Such ductile-to-brittle transition is similar to that observed under elevated temperatures caused by chain disentanglement due to accelerated molecular motion. Although the changes of tensile properties were similar, the craze structure obtained by the brittle behavior was different, i.e., a filamented-craze structure was obtained under high-pressure C[O.sub.2], while a lace-like one was obtained under elevated temperatures.

INTRODUCTION

Polycarbonate is a transparent, ductile material. Because of its excellent optical and mechanical properties, polycarbonate is used for various goods such as optical discs, eyeglass lenses, window glass, transparent bottles, bulletproof glass, and so on. Low molecular weight polycarbonate is used for optical discs to improve flow in the mold and to reduce the residual birefringence [1], It is known in the manufacture that low molecular weight polycarbonate for optical disk grade (OD-PC) is ductile at low temperature below 50[degrees]C, but it turns into brittle by elevated temperature and is fractured at a small strain by stretching at high temperature below its glass transition temperature. That is, ductile-to-brittle transition occurs in OD-PC by elevated temperature.

The ductile-to-brittle transition by elevated temperature is opposite to that usually observed in other glassy polymers such as poly(methyl methacrylate) (PMMA) and polystyrene, i.e., the property is brittle at low temperature and becomes ductile with increasing temperature [2-4]. The brittle behavior is usually induced by craze propagation. A craze consists of a dense array of fibrils separated by voids, and there are two distinct mechanisms responsible for crazing [3-11]. One mechanism is scission crazing, which is caused by the breaking of molecular chains during fibril formation. Another is disentanglement crazing, which is caused by chain disentanglement due to accelerated molecular motion. Scission crazing usually occurs at low temperatures and at a high strain rate, while disentanglement crazing occurs at high temperatures and at a low strain rate. In polycarbonate, disentanglement crazing due to accelerated molecular motion was found to occur at high temperature [12-15], but the related brittle property was not reported [3, 12-18]. Hence, details of the brittle behavior have not been explained, though the ductile-to-brittle transition by elevated temperature has been known for OD-PC in the manufacture.

The molecular motion of polymers can be also accelerated by the plasticization effect of compressed gas [16-27]. Compressed gas is absorbed into the free volume between polymer chains, so that molecular motion is accelerated [28-32], glass transition temperature is depressed [27, 33-37], deformation is enhanced [38-44] with an increase in the amount of absorbed gas in the polymers associated with the increased gas pressure. Our recent study on the stress-strain behavior of PMMA under compressed gas revealed that molecular orientation is enhanced with increased pressure of carbon dioxide (C[O.sub.2]) due to the plasticization effect when the amount of gas absorbed into the polymers is large at high C[O.sub.2] pressure, while it is suppressed by the hydrostatic pressure effect when the amount of absorbed gas is small at low C[O.sub.2] pressure [44].

Our primary interest in this study is the plasticization effect of compressed C[O.sub.2] on the mechanical properties of the OD-PC, specifically, do the mechanical properties of the OD-PC become more ductile by enhancement of molecular orientation as observed in PMMA under C[O.sub.2] or do the ductile properties change to brittle by acceleration of the molecular motion as observed in the ODPC at elevated temperatures under ambient pressure? To understand the effect of C[O.sub.2] on the mechanical properties of the ODPC, we investigated the stress-strain behavior of the glassy ODPC under C[O.sub.2] at various pressures by using a specially designed in situ tensile-deformation instrument for measurement under C[O.sub.2]. The results were compared with those at various temperatures under air at ambient pressure, to understand the detail of the brittle behavior at elevated temperature and the characteristic tensile properties under C[O.sub.2]. The craze structure of the stretched-and-fractured specimen was also examined to discuss the plasticization effect of C[O.sub.2] on deformation behavior.

EXPERIMENTAL

The OD-PC specimen used in this study was a commercial low molecular weight polycarbonate for optical disc grade (Iupilon H-4000, [M.sub.v] = 1.5 x [10.sub.4] g/mol) supplied by Mitsubishi Gas Chemical Company, Inc. The OD-PC pellets were compression molded between metal plates at 200[degrees]C for 5 min to obtain film specimens with a thickness of about 200 [micro]m which were then cooled to room temperature.

To perform in situ tensile-deformation measurements of the film specimens under compressed gas, we designed a special stretching instrument with a stainless steel pressure vessel, as mentioned in the details of our previous article [44]. Schematic illustration for the top view of the stretching instrument is shown in Fig. 1. The crosshead of the stretching instrument traveled to a strain limit of 2.8 in the pressure vessel (Taiatsu Techno Corporation). Movement of the crosshead was regulated by a shaft connected to a linear motor (LD1208-244, Oriental Motor Co., Ltd.) outside the vessel. A strain gauge (KFR-2-120-C1-16, Kyowa Electronic Instruments Co., Ltd.) was attached to the surface of the crosshead to measure stress during stretching of the OD-PC film. The voltage caused by deformation of the strain gauge was recorded and then the applied stress was acquired with DCS-100A software (Kyowa Electronic Instruments Co., Ltd.).

A dumbbell-shaped film specimen was cut from the PC film and was mounted on the stretching instrument. After sealing, high-pressure C[O.sub.2] was injected into the pressure vessel with a syringe pump (NPKX-500, Nihon Seimitsu Kagaku Co., Ltd.) at room temperature and maintained for 1 h to dissolve any C[O.sub.2] in the specimen. Then, the specimen was stretched at a stretching speed of 0.05 [s.sup.-1] under C[O.sub.2]. Note here that the tensile properties of the OD-PC films were not changed by absorbing C[O.sub.2] for longer than 1 h, so that 1 h is sufficient for absorbing the C[O.sub.2] in the specimen. The pressure of C[O.sub.2] within the vessel was monitored with an output pressure transducer and was kept constant with a back-pressure regulator (TESCOM 26-1763-24). The temperature was set at 30[degrees]C by an autotune temperature controller unit with a thermocouple for measurement under C[O.sub.2], while it was set at a suitable temperature for measurement under air at ambient pressure.

The structure of each stretched-and-fractured specimen was observed by a polarized optical microscope (Olympus BX50) equipped with a sensitive tint plate having an optical path difference of 0.53 [micro]m. The microscopic images of the specimens were recorded by a digital camera (Olympus DP71) and stored on a personal computer. Each stretched-and-fractured specimen was also observed under a scanning electron microscope (SEM) (Hitachi S2100A). For observation, the surface of the specimen was sputter coated with platinum.

RESULTS AND DISCUSSION

Figure 2 shows the stress-strain curve of the low molecular weight polycarbonate for optical disc grade (OD-PC) at various temperatures under air at ambient pressure. At 30[degrees]C, the yield point appeared at around a strain of 0.1, the plasticized plateau region was observed at a strain above 0.2, and the specimen was elongated up to a large strain above 1.2. Such deformability is characteristic of ductile behavior. The elongation became shorter with increasing temperature. The specimen was fractured at a small strain below 0.2 at temperatures above 45[degrees]C (Fig. 2a), and the yield point disappeared at temperatures above 80[degrees]C (Fig. 2b). Fracture at a small strain is characteristic of brittle behavior. These results indicate that the ductile property turned into a brittle one with increasing temperature. That is, ductile-to-brittle transition occurred in the OD-PC at elevated temperatures. Since the ductile-to-brittle transition with increasing temperature is not observed in polycarbonate when the molecular weight is high [3, 12-18], it is characteristic on the low molecular weight polycarbonate such as OD-PC.

Figure 3 shows the temperature dependence of elongation at break under air at ambient pressure. Here, elongation at break was obtained from Fig. 2 at a strain in which the stress started to decrease for fracture. Elongation at break decreased sharply from 1.2 to 0.05 with increasing temperature from 30 to 60[degrees]C, respectively, and then it decreased gradually at temperatures above 60[degrees]C. This result indicates that ductile-to-brittle transition occurred at temperatures between 30 and 60[degrees]C by elevated temperature. The ductile-to-brittle transition with increasing temperature observed in the OD-PC is opposite to that observed in PMMA, i.e., the tensile properties change from brittle to ductile by elevated temperature in PMMA [2-4].

The temperature dependence of the elastic modulus under air at ambient pressure is shown in Fig. 4. Here, the elastic modulus was obtained from the initial slope in the elastic region of the stress-strain curves in Fig. 2b. The elastic modulus decreased gradually and almost linearly with increasing temperature at temperatures below 120[degrees]C, and then decreased sharply at temperatures above 120[degrees]C by approaching the glass transition temperature associated with increased molecular motion. Thus the decrease of the elastic modulus is attributed to the accelerated molecular motion with increasing temperature.

As shown in Figs. 3 and 4, deformability decreased with increasing temperature, though the elastic modulus decreased by accelerated molecular motion. The sharp decrease of deformability at temperatures between 30 and 60[degrees]C shown in Fig. 3 was not associated with the gradual decrease of the elastic modulus up to 120[degrees]C, shown in Fig. 4. The difference might be attributed to the different contributions of the two molecular mechanisms of deformation: distortion and molecular orientation [3, 4, 45-48]. Distortion is a local change of the intersegment distance by torsion around the main-chain bonds or local displacement of the interchain spacing. On the other hand, molecular orientation is relatively long-range conformational rearrangements by orientation of the main-chain segments. The elastic modulus in the elastic region is mainly attributed to distortion, while deformability is attributed to molecular orientation. Hence, the molecular orientation decreases sharply at temperatures between 30 and 60[degrees]C while distortion gradually increases up to 120[degrees]C due to the accelerated molecular motion with increasing temperature.

The decrease of the molecular orientation and the ductile-to-brittle transition with increasing temperature is opposite to that observed in PMMA, i.e., molecular orientation is enhanced in PMMA by accelerated molecular motion and the breaking of molecular chains is suppressed at elevated temperatures [2-4]. As mentioned before, the embrittlement of polycarbonate with increasing temperature is not observed when the molecular weight is high [3, 12-18]. Thus, the ductile-to-brittle transition by the decrease of the molecular orientation at elevated temperature is attributed to the disentanglement of molecular chains by accelerated molecular motion for low molecular weight polycarbonate such as OD-PC.

Figure 5 shows the stress-strain curve of the OD-PC under C[O.sub.2] of various pressures at 30[degrees]C. At C[O.sub.2] pressures below 2 MPa, the yield point appeared at around a strain of 0.1, the plasticized plateau region was observed at a strain above 0.2, and the specimen was elongated up to a large strain above 0.8. Such deformability is characteristic of ductile behavior. As C[O.sub.2] pressure increased, the elongation became shorter and the specimen was fractured at a small strain below 0.2 at a C[O.sub.2] pressure above 6 MPa (Fig. 5a). Fracture at a small strain is characteristic of brittle behavior. Thus, the ductile-to-brittle transition, as observed with increasing temperature shown in Fig. 2, was also found to occur under C[O.sub.2] with increased C[O.sub.2] pressure. The stress at a C[O.sub.2] pressure of 8 MPa was quite small (Fig. 5b) because the measurement temperature under C[O.sub.2] was close to the glass transition temperature by depression of the glass transition temperature associated with an increase in the absorbed C[O.sub.2] with increased C[O.sub.2] pressure [33, 35]. Although the glass transition temperature was close to the measurement temperature under C[O.sub.2] of 8 MPa, the specimen could not be elongated, but was fractured at a small strain of about 0.05. Since the yield point shifted to a smaller strain with increased C[O.sub.2] pressure, the yield point and plasticized plateau region were observed up to a C[O.sub.2] pressure of 6 MPa even though the material was brittle under C[O.sub.2], while the yield point and plateau plasticized region disappeared when the material was brittle at elevated temperatures under air at ambient pressure.

Figure 6 shows elongation at break as a function of C[O.sub.2] pressure. Here, elongation at break was obtained from Fig. 5 at a strain in which the stress started to decrease for fracture. The elongation at break decreased sharply from 1.2 to 0.2 with increased C[O.sub.2] pressure at pressures from 0.1 to 3 MPa, respectively, and then it decreased gradually at pressures above 3 MPa. The result indicates that the ductile-to-brittle transition occurred at C[O.sub.2] pressures between 0.1 and 3 MPa with increased C[O.sub.2] pressure. Ductile-to-brittle transition with increased C[O.sub.2] pressure in the OD-PC is similar to that observed at the elevated temperatures shown in Fig. 3, while it is opposite to that observed under C[O.sub.2] in PMMA, i.e., the tensile properties change from brittle to ductile with increased C[O.sub.2] pressure in PMMA [44],

Figure 7 shows the elastic modulus obtained from the initial slope of the stress-strain curves in Fig. 5 as a function of C[O.sub.2] pressure at 30[degrees]C. The elastic modulus decreased gradually and almost linearly with increased C[O.sub.2] pressure at pressures up to 6 MPa, and it decreased sharply at C[O.sub.2] pressures above 6 MPa by depression of the glass transition temperature associated with an increase of absorbed C[O.sub.2] with increased C[O.sub.2] pressure. This behavior is similar to that observed at elevated temperatures under air at atmospheric pressure, as shown in Fig. 4. As mentioned in our previous paper [44], the elastic modulus decreases due to the enhancement of distortion by the plasticization effect of C[O.sub.2] as the amount of absorbed C[O.sub.2] increases with increased C[O.sub.2] pressure. Thus, the decrease of the elastic modulus with increased C[O.sub.2] pressure is attributed to the accelerated molecular motion by the plasticization effect of C[O.sub.2].

As shown in Figs. 6 and 7, deformability decreased with increased C[O.sub.2] pressure and ductile-to-brittle transition occurred, though the elastic modulus decreased by accelerated molecular motion. The sharp decrease of deformability observed at C[O.sub.2] pressures between 0.1 and 3 MPa was not associated with the gradual decrease of the elastic modulus up to 6 MPa, as observed in the temperature dependence shown in Figs. 3 and 4. Since the elastic modulus is mainly attributed to distortion while deformability is attributed to molecular orientation [48], the molecular orientation decreases sharply at C[O.sub.2] pressures between 0.1 and 3 MPa while distortion is gradually enhanced up to C[O.sub.2] pressures of 6 MPa due to accelerated molecular motion with increased C[O.sub.2] pressure. Thus, the ductile-to-brittle transition of the OD-PC with increased C[O.sub.2] pressure by the decrease of the molecular orientation might be attributed to the disentanglement of the molecular chains by accelerated molecular motion. This behavior is similar to that suggested in the observation at elevated temperatures demonstrated in Figs. 2-4.

As shown in Fig. 8, the elastic modulus with C[O.sub.2] pressure P (Fig. 7) could be superimposed to that with temperature T (Fig. 4) by proportional extension from the same reference data for 0.1 MPa and 30[degrees]C using the following relationship:

(P - 0.1)/(6 - 0.1) = (T - 30)/(120 - 30) (1)

The superposition of Figs. 4 and 7 suggests that the origin of the decrease of the elastic modulus by the increased C[O.sub.2] pressure is same as that of the elevated temperature. This result supports the above concept obtained from Fig. 5 that the decrease of the elastic modulus by increased C[O.sub.2] pressure is attributable to accelerated molecular motion. The molecular motion under C[O.sub.2] is accelerated due to the plasticization effect associated with the increased amount of absorbed C[O.sub.2] with increased C[O.sub.2] pressure [16-44].

On the other hand, as shown in Fig. 9, the elongation at break with C[O.sub.2] pressure (Fig. 6) could not be superimposed to that with temperature (Fig. 3) by proportional extension using Eq. 1, which was used for the superposition of the elastic modulus shown in Fig. 8. The elongation at break with C[O.sub.2] pressure was larger than that with temperature. This deviation might be attributable to the different contributions of elevated temperature and high-pressure C[O.sub.2] on distortion and molecular orientation. Since deformability is mainly attributed to molecular orientation, the result suggests a different molecular orientation under high-pressure C[O.sub.2] and elevated temperature, i.e., the reduction of molecular orientation by disentanglement is smaller under high-pressure C[O.sub.2] than that under elevated temperature.

Figure 10 is a polarized optical micrograph of the stretched-and-fractured OD-PC obtained by stretching under ambient pressure at 100 and 120[degrees]C in which the tensile property was brittle. Since the elongation occurred locally at elevated temperatures, a craze structure was only seen around the fractured edge. Small numbers of distorted-shaped crazes having a size of several ten micrometers were obtained at 100[degrees]C. The number of crazes increased with increasing temperature, and large ellipsoidal lacelike crazes having a size of several ten micrometers were obtained at 120[degrees]C. The crazes were long and perpendicular to the stretching one. Since the crazing occurred at temperatures in which the tensile property was brittle, the ductile-to-brittle transition by elevated temperature demonstrated in Figs. 2 to 4 is attributed to the development of the crazes by accelerated molecular motion with increasing temperature. Since crazing occurs because of elevated temperatures, the crazing is caused by intermolecular separation due to disentanglement of molecular chains by accelerated molecular motion, i.e., disentanglement crazing occurs as suggested in Refs. 3 to 15.

Figure 11 is a polarized optical micrograph of the stretched-and-fractured OD-PC obtained by stretching under C[O.sub.2] at 5 and 6 MPa in which the tensile properties were brittle and the elastic modulus was almost same as those under air at ambient pressure at 100 and 120[degrees]C, respectively. The interesting result here is that the craze structure obtained by the increased C[O.sub.2] pressure was quite different from that caused by elevated temperatures. This result supports the above concept suggested from Fig. 9 that the molecular orientation under high-pressure C[O.sub.2] and elevated temperatures are different although the changes of the tensile properties with C[O.sub.2] pressure and temperature are similar. A filamented-craze structure like window blind was obtained under high-pressure C[O.sub.2]. The craze was long and perpendicular to the stretching direction, i.e., the length was more than several hundred micrometers while the width in the parallel direction was several micrometers. A periodic structure with different interference colors was observed along the filemanted-craze structure, suggesting that the craze structure develops uniformly with periodic distance. The existence of periodic and long craze was confirmed by long grooves observed by SEM, as shown in Fig. 12.

As mentioned above, the crazing occurred at C[O.sub.2] pressures above 3 MPa in which the tensile property was brittle and the molecular motion was accelerated due to the plasticization effect of C[O.sub.2]. Thus, the ductile-to-brittle transition with C[O.sub.2] pressure demonstrated in Figs. 5 to 7 is attributable to disentanglement crazing caused by accelerated molecular motion with increased C[O.sub.2] pressure. The craze structure obtained under C[O.sub.2] was much longer than that obtained under elevated temperatures. Elongation occurred uniformly and crazing occurred macroscopically throughout the whole specimen under high-pressure C[O.sub.2], while crazing occurred locally only around the fractured edge under elevated temperature. The difference might be attributed to the different molecular orientations demonstrated in Fig. 9, i.e., the reduction of molecular orientation by disentanglement is smaller under high-pressure C[O.sub.2] than that under elevated temperature.

The criteria for craze initiation is demonstrated by the relationship between craze stress [[sigma].sub.cr] and yield stress [[sigma].sub.y]. The craze stress [[sigma].sub.cr] is described by

[[sigma].sub.cr] [varies] [square root of [[sigma].sub.y][GAMMA]] (2)

[GAMMA] = [gamma] + 1/4 [v.sub.e]dU (3)

where [GAMMA] is the effective surface energy at the void tips for crazing, [gamma] is the van der Waals surface energy, [v.sub.e] is the entanglement density, d is the entanglement distance, U is the energy for breaking a single backbone bond. It is considered that disentanglement crazing occurs when the [[sigma].sub.cr] becomes lower than the [[sigma].sub.y] at elevated temperature [3, 4, 7, 8, 11]. When the chain disentanglement occurs, [GAMMA] in Eq. 3 is dominated by [gamma] and the [[sigma].sub.cr]] is described by

[[sigma].sub.cr] [varies] [square root of [[sigma].sub.y][gamma]] (4)

The yield stress [[sigma].sub.y] was obtained from the peak appeared after the initial slope of the stress-strain curves in Figs. 2 and 5. In Fig. 13, the [[sigma].sub.y] was plotted as a function of temperature and C[O.sub.2] pressure by proportional extension using Eq. 1, which was used for the superposition of the elastic modulus shown in Fig. 8. The [[sigma].sub.y] decreased with increasing temperature and C[O.sub.2] pressure. Since the surface tension of polymers decreases with increasing temperature [7] and C[O.sub.2] pressure [49, 50], the [[sigma].sub.cr] also decreases with increasing temperature and C[O.sub.2] pressure. Crazing occurred at the temperature above 60[degrees]C and at the C[O.sub.2] pressure above 4 MPa, as indicated by open circle and filled circle, respectively. On the other hand, crazing did not ocur at the temperature below 45[degrees]C and at the C[O.sub.2] pressure below 3 MPa, as indicated by open square and filled square, respectively. The C[O.sub.2] pressure for the craze initiation was higher than that expected from the superposition with the temperature. These results suggest that the craze initiation is suppressed under high-pressure C[O.sub.2] comparing with that under elevated temperature due to small reduction of molecular orientation by disentanglement under high-pressure C[O.sub.2]. Since the craze initiation was suppressed under high-pressure C[O.sub.2] and the disentanglement crazing occurs when the [[sigma].sub.cr] becomes lower than the [[sigma].sub.y], it is considered that the [[sigma].sub.cr] with temperature ([[sigma].sub.cr](T)) is lower than that with C[O.sub.2] pressure ([[sigma].sub.cr](C[O.sub.2])), as schematically shown by broken line and dotted one in Fig. 13, respectively.

CONCLUSIONS

We found that the mechanical properties of low molecular weight polycarbonate for optical disc grade (OD-PC) changed from ductile to brittle under compressed C[O.sub.2] with increased C[O.sub.2] pressure, i.e., elongation at break decreased sharply from 1.2 at 0.1 MPa to 0.2 at 3 MPa, while the elastic modulus decreased gradually up to 6 MPa. Such brittle-to-ductile transition behavior was similar to that observed at elevated temperatures under air at ambient pressure. Thus, brittle-to-ductile transition under C[O.sub.2] might be attributable to the chain disentanglement caused by accelerated molecular motion for low molecular weight polycarbonate, as suggested by the stress-strain behavior at elevated temperatures. The accelerated molecular motion under C[O.sub.2] is caused by the plasticization effect of C[O.sub.2] associated with the increased amount of absorbed C[O.sub.2] with increased C[O.sub.2] pressure. Although the change in tensile property was similar, the elongation at break could not be superimposed and the reduction of the molecular orientation under high-pressure C[O.sub.2] was smaller than that under elevated temperatures. Because of the different molecular orientation, the craze structure thus obtained was different. A filamented-craze structure with periodic distance was obtained macroscopically throughout the whole specimen under high-pressure C[O.sub.2], while a lace-like craze structure was obtained locally only around the fractured edge under elevated temperatures. Owing to the smaller reduction of the molecular orientation under high-pressure C[O.sub.2], the craze initiation under high-pressure C[O.sub.2] pressure was suppressed comparing with that under elevated temperature. In situ observation of the development of the craze structure in polycarbonate under high-pressure C[O.sub.2] will be reported in the near future.

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Tomoaki Taguchi, Ramu Miike, Tomoe Hatakeyama, Hiromu Saito (iD)

Department of Organic and Polymer Materials Chemistry, Tokyo University of Agriculture and Technology, Koganei-shi, Tokyo 184-8588, Japan

Correspondence to: H. Saito; e-mail: hsaitou@cc.tuat.ac.jp

Contract grant sponsor: Japan Society for the Promotion of Science (Grant in-Aid for Scientific Research (C)); contract grant number: 15K05620.

DOI 10.1002/pen.24599

Caption: FIG. 1. Schematic illustration for the top view of a stretching instrument for tensile-deformation measurements under compressed gas.

Caption: FIG. 2. Stress-strain curve of PC at various temperatures under ambient pressure at a stretching speed of 0.05 [s.sup.-1]: (a) large-strain region, (b) small-strain region. (Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 3. Elongation at break of PC under ambient pressure as a function of temperature.

Caption: FIG. 4. Elastic modulus of PC under ambient pressure as a function of temperature.

Caption: FIG. 5. Stress-strain curve of PC under C[O.sub.2] of various pressures at 30[degrees]C at a stretching speed of 0.05 [s.sup.-1]: (a) large-strain region, (b) small-strain region. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 6. Elongation at break of PC at 30[degrees]C as a function of C[O.sub.2] pressure.

Caption: FIG. 7. Elastic modulus of PC at 30[degrees]C as a function of C[O.sub.3] pressure.

Caption: FIG. 8. Elastic modulus of PC as a function of temperature and C[O.sub.2] pressure by proportional extension using Eq. 1.

Caption: FIG. 9. Elongation at break of PC as a function of temperature and C[O.sub.2] pressure by proportional extension using same equation (Eq. 1) for elastic modulus used in Fig. 8.

Caption: FIG. 10. Polarized optical micrographs of stretched-and-fractured PC obtained by stretching under atmospheric pressure at various temperatures. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 11. Polarized optical micrographs of stretched-and-fractured PC obtained by stretching at 30[degrees]C under C[O.sub.2] of various pressures. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 12. SEM micrograph of surface of stretched-and-fractured PC obtained by stretching at 30[degrees]C under C[O.sub.2] at 5MPa.

Caption: FIG. 13. Yield stress of PC as a function of temperature and C[O.sub.2] pressure by proportional extension using same equation (Eq. 1) for elastic modulus used in Fig. 8. (a)(b), temperature; (c)(d), C[O.sub.2] pressure; (b)(d), crazing; (a)(c), without crazing; . . .. . ..., assumed crazing stress with temperature; - - -, assumed crazing stress with C[O.sub.2] pressure.
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Author:Taguchi, Tomoaki; Miike, Ramu; Hatakeyama, Tomoe; Saito, Hiromu
Publication:Polymer Engineering and Science
Article Type:Report
Date:May 1, 2018
Words:5597
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