Printer Friendly

Annihilation kinetics of color center in polycarbonate irradiated with gamma ray at elevated temperatures.

INTRODUCTION

Polycarbonate (PC) is a high performance engineering thermoplastic, which has found broad applications in many industrial fields due to its high toughness, high impact strength, excellent optical clarity, heat resistance, and dimensional stability. In these applications, PC is exposed to outdoor agents, for example, sunlight, radiation, and so forth. Thus, the effect of ionic irradiation on the properties of PC has been paid much attention. Araujo et al. (1) showed a decrease in carbonyl groups in irradiated PC with an increase in radiation dose, using the FUR spectra. According to the study of Seguchi et al. (2), a small dose of gamma-ray irradiation at high temperature improved the Rockwell hardness and the resistance to wear of the PC. Wang et al. (3) studied the chemical modification of PC induced by 1.4 GeV Ar ions using FUR and UV/Vis spectroscopies. Singh and Prasher (4) explored the chemical and spectral responses of the gamma-ray irradiated Lexan PC using FTIR and UV/Vis spectroscopies. Neerja et al. (5) found that the activation energy of chemical etching of the Lexan PC at a given temperature and constant chemical concentration decreases with increasing gamma-ray dose. Instead of Lexan PC, Singh and Neerja (6) analyzed the chemical etching of Makrofol-KG PC and found that the activation energy of chemical etching decreases with increasing gamma-ray dose. Furthermore, Sapkal et al. (7) investigated the etching and optical response of Tuffak PC subjected to gamma-ray irradiation. In addition to gamma-ray irradiation, Nouh and Naby 18) also studied the effect of laser irradiation on the thermal properties of Markrofol-KG PC. Vujisie et al. (9) examined the influence of gamma-ray and neutron irradiation on the dissipation factor and capacitance of capacitors with PC dielectrics. Du and Liu (10) analyzed the dc tracking resistance of gamma-ray-irradiated PC using the recurrence plot method. Weber et al. (11) exposed PC to gamma-ray source accumulated to 340 kGy and realized that the gamma-ray irradiation did not modify significantly the strength of PC due to the increase in the [beta] transition temperature. The above studies were focused on the effect of gamma-ray irradiation on the optical, mechanical, chemical, and electric properties of PC, but the evolution of color centers in PC was not reported.

Color centers were first observed in pure alkali halide crystals (12), (13). For the alkali halide single crystals irradiated with the gamma ray, color centers were able to anneal out at elevated temperatures (14-17). Lin et al. (14) examined the discoloration phenomenon in irradiated LiF single crystals at elevated temperatures, but they did not analyze the transmittance data at the short annealing period. Color centers are the major product of gamma-ray irradiation on glassy polymers. Wallace et al. (18) found that the color centers in irradiated polystyrene could not be fully annealed out, which is different from those in alkali halide single crystals. Thus, they proposed the presence of both annealable and permanent color centers. Clough et al. (19), (20) collected the amounts of annealable and permanent color centers in 17 different polymers. They concluded that the color centers decreased with increasing annealing time. However, this phenomenon was not observed in all glassy polymers. Harmon et al. (21) and Lu et al. (22) found that color centers in polysiloxane and HEMA copolymer increased with increasing annealing time. This implies that the time dependence of color center in glassy polymers is not well understood. This prompted us to investigate the evolution of color centers in PC at elevated temperatures.

EXPERIMENTAL

Lexan 8010-type PC sheets were obtained from the General Electric Company (Compton, CA). Specimens of 15 x 10 x 1 [mm.sup.3] were cut from the PC sheets. They were ground with 800, 1200, and 4000 grit cabinet papers and polished with 1 and 0.05 [micro]m alumina slurries. Before the gamma-ray irradiation, the specimens were annealed in air at 100[degrees]C for 24 h and furnace cooled to 25[degrees]C. They were sealed in glass tubes and then exposed to the gamma-ray source with a dose rate 11.69 kGy/h at the Radioisotope Division, National Tsing Hua University. The accumulated gamma-ray doses are 200, 300, 400, and 500 kGy.

UV/Vis transmittance in the range of 230-800 nm was performed using a Hitachi U-3410 spectrophotometer (Chiyoda, Tokyo, Japan) with a scanning speed and resolution of 1200 nm/min and 2 nm, respectively. The specimens were preheated in a thermostated water bath to 50, 60, 70, 80, and 90[degrees]C. respectively, and then moved to a thermos cup at the same temperature. The transmittance at wavelength 450 nm was measured periodically until the transmittance was leveled off.

RESULTS AND DISCUSSION

The curves of ultraviolet and visible transmittance of irradiated PC before annealing versus wavelength under different doses are shown in Fig. 1. It can be seen from Fig. 1 that the knee-shaped section, which is present in the irradiated PC, is absent in the nonirradiated specimen ([PHI] = 0). The knee section shifts toward longer wavelength range from 360 to 415 nm with increasing dose. The transmittance decreases as the dosage increases. The UV cutoff wavelengths are 285, 345, 360, 370, and 379 nm for doses 0, 200, 300, 400, and 500 kGy, respectively. For a given temperature, the transmittance increases with annealing time and finally reaches a steady state. The transmittance of irradiated PC at the steady state is lower than that of nonirradiated ones in the range of ultraviolet and visible wavelengths. Figure 2 shows the curves of transmittance of irradiated PC annealed at 90[degrees]C for 30 h. The knee sections are absent under all doses. Comparing Fig. 1 with Fig. 2, at a given dose, the transmittance is greater for the annealed specimen than for the prean-nealed one. The UV cutoff wavelength of annealed specimen is 285, 340, 344, 347, and 348 nm for doses 0, 200, 300, 400, and 500 kGy, respectively. For a given dose, the UV cutoff wavelength is shorter for the annealed specimens than for the preannealed ones.

The transmittance of irradiated PC as a function of annealing time at 80[degrees]C for [PHI] = 200 kGy is plotted in Fig. 3. Similar curves of transmittance versus annealing time at 80[degrees]C for [PHI] = 300, 400, and 500 kGy were also obtained (not shown). It can be seen from Fig. 3 that the transmittance at given radiation dose and temperature increases with increasing annealing time until it approaches a plateau. Although the transmittance decreases with increasing dose before annealing, the annealing period to reach the steady state is shorter under a higher dose at a given temperature. It is worthwhile to point out that at a given dose the transmittance profiles are similar for annealing temperatures in the range from 50 to 90[degrees]C. The transmittance of irradiated PC in UV/Vis wavelength range is attributed to the color centers. The present analysis follows the previous works (23), (24). It is reasonable to assume that the optical absorption A is proportional to the concentration of color centers n (23), (24)

A = [beta][n.sup.p] (1)

where [beta] is the pre-exponent factor and p is the exponent. The sum of transmittance, reflectance, and absorbance is equal to unity. The reflectance depends on the surface morphology and the presence of other defects (excluding the color center). In this study, the surface is flat and the optical beam is normal to the surface so that the reflectance is negligible. Even if reflectance is not zero, the conclusion will be changed insignificantly. Thus, (23), (24)

A = 1 - I (2)

where A and I are the absorbance and transmittance of PC, respectively. Note that when I is close to unity, A is equal to-Log I (or follows the Beer-Lambert law). It is known that both annealable and permanent color centers exist in the specimen. The permanent color centers are not expected to be annealed out. The annealable color centers are assumed to follow a first-order kinetic process

dn/dt = -[alpha](n - [n.sub.[infinity]]) (3)

where n[infinity] is the concentration of color center at infinite time and a is the rate constant. Solving Eq. 3, we obtain

n = [n.sub.[infinity]] + ([[n.sub.0] - [n.sub.[infinity]]) exp(-[alpha]t) (4)

where [n.sub.0] is the concentration of color centers at the initial annealing time t = 0. Equation 4 can be rewritten with Eq. l as

A = [A.sub.[infinity]][{1 + [[([A.sub.0]/[A.sub.[infinity]]).sup.1/p] - 1][e.sup.-[alpha]t]}.sup.p] (5)

where [A.sub.0] and [A.sub.[infinity]] are the absorbance at t = 0 and t = [infinity], respectively. As [A.sub.0] is greater than [A.sub.[infinity]], the absorbance decreases with increasing time and finally reaches the steady state. Note that the data are poorly fitted to a second-order kinetics equation-like behavior (23).

As shown in Fig. 3, the transmittance of the irradiated PC at wavelength 450 nm decreases pronouncedly with increasing annealing time. Thus, we use this wavelength to detect the in situ data of the transmittance. With Eq. 2, we show in Fig. 4 the absorbance data of PC irradiated with [PHI] = 200 kGy at wavelength 450 nm as a function of annealing time at different annealing temperatures. Note that the absorbance data of PC irradiated with [PHI] = 300, 400, and 500 kGy and wavelength 450 nm are similar to those with [PHI] = 200 kGy. It can be seen from Fig. 4 that at all temperatures, the absorbance decreases monotonically with increase of annealing time until it reaches a plateau. The period to reach the steady state decreases with increasing annealing temperature and radiation dose. The absorbance at the steady state increases with increasing dose but is independent of the annealing temperature. It implies that the permanent color centers exist in irradiated PC during the annealing. This behavior was also observed in the case of the annealing of irradiated syndiotactic polystyrene (sPS) (18-20), (24). However, it is contrary to the case of the annealing of irradiated poly(methyl methacrylate) (PMMA) in which no permanent color centers exist (25) and of the annealing of irradiated hydroxyethyl methacrylate copolymer (HEMA) in which the color centers increase with an increase of annealing time (i.e., no annealable color centers exist) (22).

The solid lines in Fig. 4 are obtained using Eq. 5 with the best curve fitting. The theoretical predictions are in good agreement with the experimental data. The corresponding values of [alpha] and p are listed in Tables 1 and 2. Note that the values of [alpha] and p for [PHI] = 300, 400, and 500 kGy are also tabulated in Tables 1 and 2. From Table 1, we found that the rate constant satisfies the Arrhenius equation, as shown in Fig. 5, that is,

[alpha] = [[alpha].sub.0] exp(-Q/RT) (6)

TABLE 1. The values of (1/h) of irradiated PC as functions
of gamma-ray dose, [PHI], and annealing temperature, T.

T ([degrees]C)/[PHI] (kGy)     200      300     400     500

50                          0.0318   0.0327  0.0329  0.0345
60                          0.0551   0.0587  0.0671  0.0737
70                          0.1020   0.1160  0.1614  0.1727
80                          0.2479   0.2742  0.3086  0.3270
90                          0.5030   0.5649  0.5912  0.7238
TABLE 2. The activation energies for color centers and
parameter, p, in irradiated PC with different doses.

[PHI] (kGy)    200    300    400    500

Q (kJ/mol)   68.27  70.41  71.29  73.86
P             1.35    1.4    1.6    1.8


where [[alpha].sub.0], R, and Q are the pre-exponent factor, gas constant, and activation energy of color center, respectively. We obtained the slope of straight line as shown in Fig. 5 and calculated the activation energies of the color centers at the different doses, which are listed in Table 2. It can be seen that the activation energy for the kinetic process of color centers in irradiated PC increases with increasing dose. It implies that the gamma-ray irradiation enhances the energy barrier of color center in PC under annealing. It also enhances the concentration of color centers. Conversely, it can be seen from Table 2 that p increases from 1.35 to 1.8 as the radiation dose increases from 200 to 500 kGy. This infers that the ability of the color center to absorb light increases with increasing dose.

A comparison of the transmittance between the PC and other materials is worth being made. Lu et al. (25) found that the absorbance by the PMMA was proportional to the concentration of color center as [n.sup.p], where p is 2, 1.5, 1.2, and 1 for [PHI] = 400, 600, 800, and 1000 kGy, respectively. This trend is opposite to the case of PC. The annihilation process of color centers in PMMA follows the second-order kinetics. The energy barrier to overcome the recombination of color centers in PMMA decreases with increasing gamma-ray dose. No permanent color center was observed in PMMA. Lu et al. (22) studied the kinetic process of color center in HEMA copolymer irradiated with gamma ray at elevated temperatures. They obtained that the parameter p and activation energy Q of the first-order kinetic process are 2 and 17.37 kJ/mol for the doses from 400 to 1000 kGy. However, they did not detect the annealable color centers in HEMA during annealing. Liu et al. (24) examined the color centers in sPS irradiated with gamma ray at elevated temperatures. They found that the anneal able color center followed a first-order annihilation process and both annealable and permanent color centers coexisted during annealing, similar to this study. However, the parameter p is greater for PMMA than for sPS. Lin et al. (14) could not analyze the whole transmittance data of LiF single crystals irradiated with gamma ray because of the complication involved. In this study, the annealable color centers in gamma-ray-irradiated PC follow a first-order annihilation process and both permanent and annealable color centers are present during annealing.

CONCLUSIONS

The annihilation kinetics of color centers in PC irradiated with gamma ray at elevated temperatures has been investigated. The absorbance of gamma-ray-irradiated PC is attributed to the presence of both permanent and annealable color centers. The latter can be annihilated by thermal annealing. It was assumed that the absorbance, A, of the irradiated PC at wavelength 450 nm is proportional to the concentration, n, of annealable color centers, expressed as A = [beta][n.sup.p], where [beta] and p are the pre-exponent factor and the exponent, respectively. The measured absorbance data are in good agreement with the theoretical predictions in which the annealable color centers follow a first-order kinetic process during the thermal annealing. The activation energy of the kinetic process of color centers in PC increases with an increase of gamma-ray dose. The permanent color center increases with an increase of radiation dose and is independent of the annealing temperature. This behavior of PC is different from those of the following polymeric materials. For the irradiated poly(methyl methacrylate), the permanent color center does not exist, and the annealable color center obeys the first-order kinetic process in which the concentration of centers is nonlinearly proportional to the absorbance (25). For the irradiated hydroxyethyl methacrylate copolymer, annealable color center does not exist, and the permanent color center follows the first-order generation process (22). For the irradiated syndiotactic polystyrene, the permanent color center increases with an increase in radiation dose, and the annealable color center follows the first-order annihilation process in which the concentration of the centers is linearly proportional to the absorbance (24).

Correspondence to: Sanboh Lee; e-mail: sblee@mx.nthu.edu.tw

Contract grant sponsor: National Science Council (Taiwan).

DOI 10.1002/pen.23202

Published online in Wiley Online Library (wileyonlinelibrary.com).

[C] 2012 Society of Plastics Engineers

REFERENCES

(1.) E.S. Araujo, H.J. Khoury, and S.V. Silveira, Radiat. Phys. Chem., 53, 79 (1999).

(2.) T. Seguchi, T. Yagi, S. Ishikawa, and Y. Sano, Radiat. Phys. Chem., 63, 35 (2002).

(3.) Y. Wang, Y. Jin, Z. Zhu, C. Liu, Y. Sun, Z. Wang, M. Hou, X. Chen, C. Zhang, J. Liu, and B. Li, Nucl. Instrum. Methods Phys. Res. Sect. B, 164-165, 420 (2000).

(4.) S. Singh and S. Prasher, Radiat. Meas., 40, 50 (2005).

(5.) X. Neerja, S. Prasher, and S. Singh, Radiat. Meas., 42, 135 (2007).

(6.) S. Singh and X. Neerja, Radiant. Eff. Defect Solids, 161, 377 (2006).

(7.) J.A. Sapkal, P.C. Kalsi, C. Agarwal, M. Thanamani, and S. Murali, Radiat. Phys. Chem., 78, 81 (2009).

(8.) S.A. Nouh and A.A. Naby, Radiat. Eff Defects Solids, 162, 109 (2007).

(9.) M. Vujisic, K. Stankovic, E. Dolieanin, and B. Jovanovic, Nucl. Technol. Radiant. Protect., 24, 209 (2009).

(10.) B. Du and H. Liu, IEEE Trans. Dielect. Electr. Insul., 16, 17 (2009).

(11.) R.P. Weber, K.S. Vecchio, and J.C.M. Suarez, Rev. Mater., 15, 218 (2010).

(12.) W.B. Fowler, Physics of Color Centers, Academic Press, Waltham, MA (1968).

(13.) C. Kittel, Introduction to Solid State Physics, Wiley, New York 522 (1991).

(14.) H.Y. Lin, Y.Z. Tsai, and S. Lee, J. Mater. Res., 7, 2833 (1992).

(15.) D. Sugak, A. Matkovskii, A. Durygin, A. Suchocki, I. Sol-skii, S. Ubizskii, K. Kopczynski, Z. Mierczyk, and P. Potera, J. Lumin., 82, 9 (1999).

(16.) Q. Deng, Z. Yin, and R.Y. Zhu, Nucl. lnstrum. Methods Phys. Res. Sect. A, 438, 415 (1999).

(17.) J.S. Nadeau and W.G. Johnston, J. Appl. Phys., 32, 2563 (1961).

(18.) J.S. Wallace, M.B. Sinclair, K.T. Gillen, and R.L. Clough, Radiat. Phys. Chem., 41, 85 (1993).

(19.) R.L. Clough, K.T. Gillen, G.M. Malone, and J.S. Wallace, Radiat. Phys. Chem., 48, 583 (1996).

(20.) R.L. Clough, G.M. Malone, K.T. Gillen, J.S. Wallace, and M.B. Sinclair, Polym. Degrad. Stab., 49, 305 (1995).

(21.) J.P. Harmon, A.G. Taylor, G.T. Schueneman, and E.P. Goldberg, Polym. Degrad. Stab., 41, 319 (1993).

(22.) K.P. Lu, S. Lee, and C.C. Han, J. Mater. Res., 17, 2260 (2002).

(23.) I.J. Chiang, C.T. Hu, and S. Lee, Mater. Chem. Phys., 70, 61 (2001).

(24.) C.K. Liu, C.J. Tsai, C.T. Hu, and S. Lee, Polymer, 46, 5645 (2005).

(25.) K.P. Lu, S. Lee, and C.P. Cheng, Appl. Phys., 88, 5022 (2000).

J.S. Peng, (1) C.M. Hsu, (2) S.H. Yeh, (1) Sanboh Lee (1)

(1.) Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, Taiwan

(2.) Department of Chemical Engineering, National Tsing Hua University, Hsinchu 300, Taiwan
COPYRIGHT 2012 Society of Plastics Engineers, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 2012 Gale, Cengage Learning. All rights reserved.

Article Details
Printer friendly Cite/link Email Feedback
Author:Peng, J.S.; Hsu, C.M.; Yeh, S.H.; Lee, Sanboh
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:9TAIW
Date:Nov 1, 2012
Words:3111
Previous Article:Nonlinear determination of the equilibrium melting temperature from initial nonreorganized crystals of poly(3-hydroxybutyrate).
Next Article:Morphology, wettability, and mechanical properties of polycaprolactone/hydroxyapatite composite scaffolds with interconnected pore structures...
Topics:

Terms of use | Privacy policy | Copyright © 2022 Farlex, Inc. | Feedback | For webmasters |