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Effect of gamma ray on isothermal crystallization kinetics ofsyndiotactic polystyrene

Yi-Wen TingDepartment of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, Taiwan

Tinh NguyenTinh Nguyen Scientific Consulting, Inc., Gaithersburg, Maryland 20878

Chen-Ti HuDepartment of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, Taiwan

Chia-Chieh ChenInstitute of Nuclear Energy Research, Longtan, Taoyuan 32546, Taiwan

Sanboh Leea)

Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, Taiwan

(Received 27 August 2013; accepted 4 October 2013)

Isothermal crystallization kinetics of gamma-irradiated syndiotactic polystyrene (sPS) has beeninvestigated by differential scanning calorimetry. Amorphous sPS samples were irradiated in airwith gamma ray at various doses from 0 to 800 kGy, at a rate of 30 kGy/h, and melt-crystallizedat different temperatures and times. Kinetics parameters were determined using Avrami’smodel with Gaussian functions and a modified Arrhenius equation. Isothermally crystallizedsPS irradiated in air with gamma ray exhibited multiple endothermic melting peaks correspondingto various crystalline forms, and the radiation dose had a strong effect on their melting enthalpies,crystallinities, and crystallization kinetic parameters. The amount of the a-crystalline formincreased with increasing crystallization time and those of the b- and b9 forms had an oppositetrend. Both crystallization half time and crystallization activation energy of the a form ingamma-irradiated sPS increased with increasing radiation dose.

I. INTRODUCTION

Syndiotactic polystyrene (sPS) has received a greatdeal of interest because of its high melting temperature,high crystallization rate, low dielectric constant, goodchemical resistance, and good irradiation resistance. Thestructure, quantity, and form of crystalline materials in ansPS material are complex and vary greatly depending onthe thermal history and solvent treatment.1–16 Botha- and b-crystalline forms are obtained by cooling fromthe melt,1 whereas c and d are induced by a solvent.2

a and b crystals exist mainly between limit disordered(a9 and b9) and limit ordered (a0 and b0). The a form has ahexagonal structure,3,4 whereas the b form possesses anorthorhombic shape.2,6 Guerra et al.2 observed that thecrystalline structure of sPS that is produced by coolingfrom the melting point consists of both a0 and b9 forms.

The multiple endothermic peak phenomenon fre-quently observed for sPS cooling from the melt has alsobeen investigated. Woo et al.,7 Lin and Woo,8 and Honget al.9 identified the high, intermediate, and low melting

peak temperatures correspond to b9, a, and b forms,respectively. Wang et al.10 agreed the low and intermediatemelting peaks correspond to b and a forms, respectively,but they believed that the high melting peak is a mixture ofa and b. For isothermal crystallization of sPS, Zhou et al.11

and Rosa et al.12 found that the crystallization temperatureand annealing time increase with increasing amount of band decreasing amount of a. Adding to this morphologicalcomplexity of sPS is the transformation from one crystal-line form to another during crystallization under differentconditions. For example, Ho et al.13 reported a phasetransformation from a0 to b9 in sPS crystallized at 245 °Cfor different times. Recently, Lu et al.16 found that thelower nucleation energy is beneficial for the formation ofthe a form in melt-crystallized sPS.sPS is being used as a scintillator material in high-energy

radiation, and much research has focused on the effectsof gamma ray radiation on its mechanical and optical prop-erties.17–19 Our previous study on crystallization behaviorsfrom the melt of gamma-irradiated sPS in oxygen andvacuum revealed that the irradiation conditions have a strongeffect on the chemical properties and microstructure of thispolymer.15 In addition to causing extensive oxidation andchain scission, gamma radiation treatment also increasedthe crystallinity and decreased the melting peak temperatures

a)Address all correspondence to this author.e-mail: sblee@mx.nthu.edu.tw

DOI: 10.1557/jmr.2013.309

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of sPS. By using Gaussian function to resolve the multipleoverlapping peaks in the heat flow thermograms, we showedquantitatively that gamma irradiation at a high dose stronglyaccelerates the formation of the b9-crystalline form in sPSmaterials. The primary objective of the present study was toinvestigate the isothermal melt-crystallization kinetics ofsPS that had been irradiated in air with gamma radiation atdifferent doses. In addition, the effects of crystallization timeon the melting enthalpies and amounts of different crystal-line forms generated during the isothermal crystallization ofgamma-irradiated sPS were also examined. Differentialscanning calorimetry (DSC) technique was used to provideisothermal crystallization kinetics of sPS irradiated withgamma rays in air.

II. EXPERIMENTAL PROCEDURES

A. Materials and gamma ray irradiation

sPS pellets were obtained from Dow ChemicalCompany (Midland, MI). This material had a numberaverage molecular mass of 3.5 � 105 g/mol, a DSC glasstransition temperature (Tg) of 99.6 °C, and containedunknown additives having a concentration of approxi-mately 0.1–0.5 ppm. The sample preparation was the sameas given in Ref. 15. Briefly, sPS sheets were fabricated bymolding dry sPS pellets in a press at 300 °C, followed byquenching in 0 °C water to produce transparent samples.After grounding and polishing, sPS sheets were annealed at90 °C for 24 h to remove residual stresses.

Samples having a dimension of 5 � 5� 1 mm (about8 mg) were then placed in air-filled glass tubes havingouter and inner radii of 15 and 2.5 mm, respectively, forgamma radiation treatment. They were irradiated at 24 °C atdifferent doses from 200 to 800 kGy at a rate of 30 kGy/husing a Co-60 gamma source at Radiation ApplicationTechnology Center in Institute of Nuclear Energy Research(Tauyuan, Taiwan).

B. Measurements of isothermalcrystallization behaviors

Isothermal crystallization behaviors of sPS irradiatedwith gamma rays in air were followed using a NetzschDSC 200F3 differential scanning calorimeter (Netzsch,Germany). Except for the crystallization time, this studyused the same special heating/cooling/holding/heatingprocedure described previously.15 Briefly, after heatingto the melt temperature at 310 °C, the samples were cooleddown to a prespecified crystallization temperature (Tc) inthe 220–260 °C range, held at that specified Tc for a fixedcrystallization time, then heated up again. DSC thermo-grams during this isothermal crystallization cycle wererecorded. We found that this scheme can provide DSCthermograms containing resolvable melting endothermicpeaks that are tenable for analyzing the crystallization

behaviors of gamma-irradiated sPS. The heat flow char-acteristics and the thermal history for the samples used inthis study were the same with those presented in Fig. 1 ofRef. 15. The main thermal history difference between thepresent study and the previous work15 was the duration ateach crystallization temperature. The previous study useda fixed 10-min crystallization time, whereas the presentstudy varied the crystallization time. Another differencewas the material: the previous investigation irradiated sPSsamples in vacuum and oxygen, whereas this studyirradiated sPS samples in air.

In addition to DSC, Fourier transform infrared (FTIR)spectroscopy and x-ray diffraction (XRD) studies were alsoconducted to characterize chemical and microstructuralchanges of irradiated sPS. FTIR measurement was carriedout using a Nicolet AVATAR 320 FTIR spectrometer in theattenuation total reflection (ATR) mode (Thermo ElectronCorp., Madison, WI), and XRD used a Rigaku D/MAX-IIBdiffractometer (Shibuya-Ku, Tokyo, Japan). More completedetails of all three measurements were given in Ref. 15.

III. RESULTS AND DISCUSSION

A. Melting enthalpies and crystallinity asa function of crystallization time for differentcrystalline forms in sPS irradiated with gammarays in air

sPS is known to crystallize rapidly from the melt andtends to crystallize before the crystallization temperature(Tc) is reached. Further, depending on crystallizationconditions, crystalline materials in an sPS material maychange from one form to another during crystallization.Therefore, the heat flow curve of sPS crystallization is verycomplex, comprising of multiple overlapping peaks repre-senting various crystalline forms. This intricate behavioroften complicates the analysis of sPS crystallization data.When sPS is irradiated with gamma rays, its chemical andmorphological properties are greatly changed, and its melt-ing behaviors are markedly altered.15 Such a strong radia-tion effect on the microstructure and phase changes makesthe study of crystallization kinetics of gamma-irradiatedsPS even more challenging. As indicated in the experimen-tal section, to effectively study the crystallization character-istics of gamma-irradiated sPS, we have used a particularheating/cooling/holding scheme to generate resolvable en-dothermic peaks in the heat flow curves. Figure 1(a)representatively displays such curves in the 250–285 °Crange for sPS irradiated with gamma rays in air (air gamma-irradiated sPS). These are the DSC thermograms at fourdifferent crystallization times (t) for an sPS sample irradi-ated at a 400 kGy dose followed by isothermally crystal-lizing atTc5 230 °C. It is noted here that because numerousexperimental irradiation and crystallization variables wereincluded in this study, unless specified, only representative

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graphical results are presented in this paper to show theeffects of some key variables. Figure 1(a) shows that theDSC curves of air gamma-irradiated sPS material crystal-lized at 230 °C composed of three endothermic peaks.Similar three melting peak curves were also observed forother Tc values in the 220 °C # Tc # 242 °C range.However, only two melting peaks were detected for DSCcurves at Tc 5 250 °C and only one peak for Tc 5 260 °C.In general, crystallization time, crystallization temperature,and gamma radiation dose had a strong influence on theendothermic peak areas (proportional to crystalline frac-tions and heats of melting) and crystallization kineticparameters but imparted only a small effect on themelting peak temperatures of air gamma-irradiated sPS(see Tables I–V).

As seen in Fig. 1(a), the multiple endothermic peakscorresponding to the melting behavior of various crystal-line forms in air gamma-irradiated sPS overlapped witheach other. To provide quantitative data for accuratelyassessing the effects of crystallization and irradiationconditions on the melting enthalpy and the amount of eachcrystalline form in the gamma-irradiated sPSmaterial, theseendothermic peaks were resolved through the use of Eq. (1).The melting enthalpy change with temperature of theendothermic envelop composing of multiple Gaussiandistributions is written as:

DHDT

¼ +n

i¼1Ai exp � T � Ti

ci

� �2" #

þ B ; ð1Þ

where DH, Ai, Ti, and ci denote the melting enthalpyfraction corresponding to temperature change from T toT 1 DT, peak amplitude, melting peak temperature, andthe Gaussian radius, respectively; B is a constant. Integer ncan be 1, 2, or 3, and subscript i corresponds to the ith peak

of the endotherm.When n5 3, the endothermic peaks 1, 2,and 3 are designated as P1, P2, and P3 corresponding tothe crystalline forms b, a, and b9, respectively.15

Figure 1(b) provides an illustration of using Eq. (1) to fitthe experimental data for, as an example, an sPS samplegamma-irradiated at 800 kGy and isothermally crystal-lized at 240 °C for 15 min. Values of the parameters usedto fit the experimental data corresponding to this Tc aregiven in Table I, which also include values for othercrystallization periods. Note that at this Tc, three endo-thermic peaks corresponding to the three crystalline formsare present. All Ai and ci values increased with an increasein t, and each melting peak temperature, Ti, raised about1 °C when t was increased from 5 to 50 min. Tables IIand III provide values of parameters Ai, Ti, and ci for sPSsamples irradiated at the same dose but crystallized at250 and 260 °C, respectively. For these two high Tcvalues, the changes of both Ai and ci with t followed thesame trend with those for Tc at 240 °C but essentially hadno effect on Ti. In summary, sPS samples irradiated withgamma rays in air at doses from 0 to 1000 kGy composedthree crystalline forms, b, a, and b9 when they are crys-tallized in the 220–242 °C range. They contained twocrystalline forms, a and b, when Tc5 250 °C and had onlythe b form when Tc 5 260 °C.

Using the Gaussian function deconvolution proceduredescribed above, the effects of gamma radiation dose andcrystallization conditions (t and Tc) on melting enthalpy,amount of each crystalline form, and crystallization kineticsof air gamma-irradiated sPS were quantitatively investi-gated. Figure 2 depicts the total melting enthalpy (H) (i.e.,sum of all three peaks and is proportional to crystallinity ofthe sample) with crystallization time for four Tc values.Note that the 250 °C results are displayed in the insetbecause the crystallization time scale for this Tc is consid-erably greater than that for Tc at 220, 230, or 240 °C. If data

FIG. 1. (a) Typical DSC thermograms in the 245–280 °C range for different crystallization times of sPS irradiated in air with gamma rays at, as anexample, 400 kGy and isothermally crystallized at 230 °C, showing overlapping melting endothermic peaks, and (b) typical deconvoluted Gaussianfunctions for sPS irradiated in air with gamma rays at, as an example 800 kGy, and isothermally crystallized at 240 °C for 15 min.

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of all four crystallization temperatures were presented in thesame graph, the differences between the H versus t curvesfor the three lower Tc values cannot be readily seen.

Figure 2(a) shows a general S shape curve, similar tothat observed for nonirradiated sPS.20 However, theinduction time (defining here as the time between 0 andthe steep rise of the melting enthalpy), the rate ofH changewith crystallization time, and the time required to reacha constantH value changed greatly with Tc for air gamma-irradiated sPS. The induction period rose, the rate of Hchange with t decreased, and the time required to completethe crystallization markedly increased with increasing Tc.As revealed in Fig. 2(a), the crystallization started almostimmediately at Tc 5 220 °C, but it needed an inductionperiod of more than 10 min for this phase change to beginat 250 °C. Similarly, the crystallization time to attain asaturated amount of crystalline material increased withincreasing Tc. For example, it took approximately 5 min tocomplete the crystallization at Tc 5 220 or 230 °Cbut needed nearly 1 and 11 h for all crystalline materialsto form in the air gamma-irradiated sPS crystallized atTc5 250 and 260 °C, respectively. Obviously, the crystal-lization was faster, and the enthalpy change rate was higherwhen the difference between the melting temperature andthe crystallization temperature was greater.

The effect of gamma radiation dose on melting prop-erties is demonstrated in Fig. 2(b), where the dependenceofH on t at different doses is plotted for the highest Tc usedin this study (260 °C). The 0 kGy results represent thenonirradiated sPSmaterial. It is interesting to observe from

Fig. 2(b) that although the degree of influence is different,an increase in the gamma radiation dose had a similareffect on theH versus t behaviors as an increase in Tc. Thatis, the induction period and the time required to attain fullcrystallinity in the sample increased, whereas the rate of Hchange with t decreased with an increase of the radiationdose. Further, for a given t, the total melting enthalpyincreased with decreasing dose. Similar effects have alsobeen observed for other Tc values used in this study(220 °C # Tc # 260 °C).

Figure 3(a) depicts the dependence of the melting peaktemperatures, Ti, on Tc for sPS gamma irradiated in air at,

TABLE I. Effect of crystallization time t (min) on Ai (J/g, °C), Ti (°C), and ci (°C) for sPS irradiated in air at 800 kGy and crystallized at 240 °C; i5 1,2, and 3.

t A1 T1 c1 A2 T2 c2 A3 T3 c3

5 �0.18 6 0.01 260.58 6 0.22 1.88 6 0.04 �0.08 6 0.01 262.91 6 0.14 1.87 6 0.02 �0.23 6 0.03 269.51 6 0.43 1.90 6 0.036 �0.43 6 0.02 260.65 6 0.37 2.20 6 0.07 �0.15 6 0.02 262.95 6 0.32 1.99 6 0.01 �0.56 6 0.04 269.99 6 0.42 2.22 6 0.077 �1.22 6 0.08 260.92 6 0.41 2.25 6 0.06 �0.47 6 0.03 263.04 6 0.29 1.99 6 0.02 �1.58 60.08 270.10 6 0.23 2.45 6 0.0110 �2.47 6 0.05 261.31 6 0.35 2.44 6 0.03 �1.29 6 0.05 263.26 6 0.13 2.30 6 0.03 �3.15 6 0.07 270.13 6 0.25 2.45 6 0.0115 �2.95 6 0.02 261.53 6 0.56 2.50 6 0.02 �2.03 6 0.02 263.53 6 0.46 2.34 6 0.03 �3.88 6 0.01 270.20 6 0.21 2.60 6 0.0520 �2.99 6 0.02 261.77 6 0.44 2.51 6 0.01 �2.11 6 0.06 263.72 6 0.32 2.55 6 0.04 �3.94 6 0.02 270.27 6 0.19 2.63 6 0.0250 �3.05 6 0.03 261.86 6 0.62 2.53 6 0.02 �2.38 6 0.04 263.80 6 0.44 2.57 6 0.02 �3.97 6 0.05 270.55 6 0.40 2.65 6 0.03

TABLE II. Effect of crystallization time t (min) on Ai (J/g, °C), Ti (°C), and ci (°C) for sPS irradiated at 800 kGy and crystallized at 250 °C; i 5 1and 2.

t A2 T2 c2 A1 T1 c1

15 �0.03 6 0.00 264.06 6 0.66 0.39 6 0.03 �0.18 6 0.01 267.23 6 0.16 1.83 6 0.0320 �0.13 6 0.02 264.14 6 0.56 0.74 6 0.02 �0.27 6 0.02 267.26 6 0.29 2.33 6 0.0230 �0.46 6 0.02 264.23 6 0.43 1.20 6 0.04 �1.11 6 0.02 267.54 6 0.38 2.37 6 0.0340 �1.55 6 0.08 264.52 6 0.41 1.81 6 0.05 �2.36 6 0.05 267.65 6 0.31 2.49 6 0.0550 �2.41 6 0.06 264.64 6 0.45 2.35 6 0.06 �3.21 6 0.03 268.01 6 0.49 2.84 6 0.0780 �4.28 6 0.03 264.71 6 0.22 2.47 6 0.03 �3.24 6 0.02 268.13 6 0.42 3.01 6 0.02110 �5.25 6 0.02 264.78 6 0.39 2.65 6 0.02 �3.25 6 0.01 268.26 6 0.29 3.08 6 0.03140 �5.70 6 0.06 264.89 6 0.31 3.03 6 0.03 �3.44 6 0.05 268.52 6 0.22 3.12 6 0.04

TABLE III. Effect of crystallization time t (min) on Ai (J/g, °C) andci (°C) for sPS irradiated at 800 kGy and crystallized at 260 °C; i 5 1.

t A1 T1 c1

5 �0.24 6 0.01 270.44 6 0.36 0.48 6 0.017 �1.33 6 0.04 270.51 6 0.17 1.05 6 0.0410 �1.40 6 0.05 270.66 6 0.89 1.67 6 0.0513 �1.65 6 0.08 270.89 6 0.26 1.92 6 0.0616 �2.30 6 0.02 270.90 6 0.58 2.16 6 0.0319 �2.67 6 0.06 270.92 6 0.14 2.56 6 0.0223 �3.09 6 0.03 271.42 6 0.76 2.84 6 0.0126 �3.13 6 0.04 271.53 6 0.33 3.12 6 0.08

TABLE IV. Glass transition temperature of sPS, Tg, as a function of U.

U 1 kGy 0 200 400 600 800Tg (°C) 99.0 100.1 100.3 100.8 101.8

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as an example, 800 kGy. The results for nonirradiated sPSare presented in Fig. 3(b) for comparison. For both irra-diated and nonirradiated sPS materials, T1 and T2 in-creased linearly with Tc, but T3 was essentially unaffectedby Tc. Further, the rate of increase for T1 was greater thanthat for T2. For irradiated sPS [Fig. 3(a)], the T1 line

intercepted with the T2 line at approximately 242 °C. Theinterception between the two melting peak temperaturessuggests that the crystalline forms corresponding to T1 andT2, i.e., b and a, respectively, coexist in an air gamma-irradiated sPS material when it is crystallized at thistemperature. This result is different from that for the

TABLE V. Dependence of exponent n and crystallization rate k on Tc and radiation dose for crystalline a in air-irradiated sPS.

a form (kGy) Tc (°C) 220 230 240 250

0 n 3.29 6 0.03 3.41 6 0.04 3.46 6 0.02 3.49 6 0.14k (min�n) 0.9044 6 0.0575 0.2503 6 0.0024 0.0217 6 0.0002 2.19 � 10�4 6 0.06 � 10�4

200 n 1.42 6 0.03 1.53 6 0.02 1.92 6 0.03 2.13 6 0.04k (min�n) 0.6617 6 0.0023 0.2082 6 0.0417 0.0192 6 0.0065 2.14 � 10�4 6 0.02 � 10�4

400 n 1.36 6 0.01 1.41 6 0.05 1.87 6 0.06 2.11 6 0.12k (min�n) 0.3309 6 0.0026 0.1249 6 0.0032 0.0130 6 0.0054 2.10 � 10�4 6 0.09 � 10�4

600 n 1.09 6 0.04 1.34 6 0.07 1.86 6 0.05 2.06 6 0.01k (min�n) 0.0746 6 0.017 0.0361 6 0.0037 0.0044 6 0.0018 7.42 � 10�5 6 0.03 � 10�5

800 n 1.04 6 0.02 1.23 6 0.05 1.51 6 0.05 2.04 6 0.02k (min�n) 0.0196 6 0.0023 0.0106 6 0.0006 0.0021 6 0.0018 4.65 � 10�5 6 0.04 � 10�5

FIG. 2. Plots of total melting enthalpy versus t for sPS irradiated in air with gamma rays: (a) at 400 kGy and isothermally crystallized at different Tcvalues and (b) at different doses and isothermally crystallized at 260 °C.

FIG. 3. Relationship between melting temperatures, Ti, and Tc for (a) sPS irradiated in air with gamma rays and (b) nonirradiated sPS.

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nonirradiated sPS material, which showed that the T2 linewas always above the T1 line [Fig. 3(b)], similar to thatobserved by Woo and Wu.21 Experimental data obtainedat other Tc values also revealed that the highest meltingpeak (P3) of an air gamma-irradiated sPS was vanishedwhen Tc . 242 °C, and this result is indicated in Fig. 3(a).The disappearance of P3 suggests that the crystalline formcorresponding to this melting peak, i.e., b9, does not existin the air gamma-irradiated sPS material when it is cry-stallized above this temperature.

The relative amount of crystalline material, Xit, in %, ineach of the three melting peaks (P1, P2, and P3) ata crystallization time twas obtained from the area of the ithGaussian distribution as:

Xit %ð Þ ¼R ‘�‘

DHiðtÞDT dT

+n

i¼1

R ‘�‘

DHiðtÞDT dT

: ð2Þ

The relationship between Xit and t for sPS irradiatedwith gamma ray in air is representatively displayed inFig. 4 for two Tc values and at 200 kGy. For Tc at 220 °C[Fig. 4(a)], X1t values (i.e., crystalline fraction of P1) weresmall compared with those for X2t and X3t at all crystal-lization times. At this crystallization temperature, both X1t

and X3t values decreased, and X2t increased with t. Further,the rates of these changes were greater at shorter crystal-lization times (,25 min) than at longer ones. Figure 4(a)also showed that, among the three fractions, the relativedecrease for X1twas greatest: it lost nearly 75% of its valueafter 50 min of crystallization. Similar Xit versus t plotswere obtained for other Tc values between 230 and 242 °Cand at other gamma doses. The results showed that forall irradiation doses and Tc between 220 and 242 °C, X1t

and X3t decreased, and X2t increased, with increasing t.In summary, the amounts of all three crystalline forms in

an air gamma-irradiated sPS sample crystallized between220 and 242 °C varied greatly with t. Further, in this Tcrange and for all crystallization times, the fractions of X3t

and X1t were highest and lowest, respectively, with X2t

fraction was in between. Recalling that X1t, X2t, and X3t arethe crystalline contents of the b, a, and b9 forms, respec-tively, in an sPS material. Thus, an air gamma-irradiatedsPSmaterial crystallized for any duration in the 220–242 °Crange contains the largest amount of b9 and the smallestamount of b, with the quantity of a falls in between.

The variations of X1t and X2twith t up to 140 min for ansPS sample irradiated at 200 kGy and crystallized at250 °C is illustrated in Fig. 4(b). Note that at this Tc,which is closer to the 270 °C melting temperature, the X3t

curve disappears regardless of the radiation dose. At thisTc, X1t decreased and X2t increased rapidly between 15 and40 min but slowed down afterward. At low t values(15 and 20 min), X1t was the largest fraction in an air-gamma irradiated sPS (;78% after 15 min) but becamea minor component at high t values (;18% after 140 min).However, the quantity of X2t changed in the oppositedirection with crystallization time, from approximately22% after 15 min to almost 82% after 140 min. As alsoseen in Fig. 4(b), the X1t curve intercepted with the X2t

curve (at the 31 min, 50% point). Because the summationof X1t and X2t is unity, the X2t curve can be estimated fromthe X1t curve and vice versa. Further, the intercepted timeof the X2t and X1t curves increased with increasing dose.Other curves of X1t and X2t versus t of sPS samples irra-diated at other doses and then crystallized at Tc 5 250 °C(not shown) displayed similar trends to those of Fig. 4(b).

The effects of radiation dose on Xit at a fixed Tc or twere not well defined. In general, at a fixed t and in the Tcrange between 220 and 240 °C, X1t values at high doses(600 and 800 kGy) tended to be higher than X1t values fornonirradiated sPS or sPS irradiated at low dose (200 kGy).

FIG. 4. Variation of crystalline fractions of P1, P2, and P3 with t for sPS irradiated in air with gamma rays at, as an example, 200 kGy andisothermally crystallized at (a) 220 °C and (b) 250 °C (note that only the P1and P2 fractions are shown in Fig. 4(b) because P3 is vanished at 250 °C).

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The opposite was true for X2twhere its values at low doseswere higher than those at high doses. The effect of dose onX3t followed a similar trend to that of X1t for Tc 5 220 and230 °C. However, gamma radiation dose had a negligibleeffect on X3t at Tc 5240 °C. For more detailed results onthe effects of gamma radiation dose on Xit at various Tc andt values, see supporting Tables I–III.

The glass transition temperatures, Tg, of air gamma-irradiated sPS was also observed to increase slightly withan increase of radiation dose from 99 °C for nonirradiatedsPS to 101.8 °C for 800 kGy (supporting Table IV).Although the effect of radiation dose on Tg was small, butthe trend was obvious. A rise in Tg with increasing dosesuggests a decrease of chain mobility in the irradiated sPS,which was likely a result of an increase in the crosslinkingof the sample as the amount of radiation was increased.This relationship between crosslinking and radiation dosemay explain the decrease of H with increasing gammaradiation dose observed in Fig. 2(b). Higher crosslinkedpolymer chains tend to be more difficult to crystallizethan lower crosslinked ones. In our previous work,15 theTg of sPS irradiated with gamma rays in both vacuum andoxygen was found to decrease with increasing radiationdose, and this was explained as due to an increase of chainscission at higher doses. The contradiction on Tg betweenthis and our previous study15 probably arose from the doserate. The dose rate for the previous work was 9.57 kGy/hand that for this study was 30 kGy/h. Janik and Rosiak22

and Adem et al.23 also have reported similar observationfor sPS and suggested that, at high dose rates, polymerchains set in motion and move about before cleavage.But at low dose rates, they are essentially stationary untilbroken. Therefore, high dose rates tend to cause entangle-ment of the polymer chains and low dose rates result inchain scissions.

B. Morphological and chemical changes ofcrystallized air gamma-irradiated sPS

XRD technique was used to follow relative changes ofthe a and b forms as function of crystallization time forsPS irradiated under different environments and doses.Figure 5 displays the XRD patterns for two different tvalues for sPS irradiated in air followed by isothermalcrystallization at, as an example, 250 °C. The diffractionangles, diffraction planes, and the assignments of the peakshave been given previously.15 Clearly, under the crystal-lization conditions used in this experiment, a mixture ofa- and b-crystalline forms existed in the gamma-irradiatedsPS, consistent with the DSC enthalpy data shown above.For irradiation in air (Fig. 5), the a diffraction peaks at2h 5 6.7, 11.6, and 13.2° increased and those of the bdiffraction peaks at 2h 5 12.3 and 13.6° decreased withincreasing t. These results indicated that the crystallinityfractions of the a form increased and those of the b formdecreased with increasing t for sPS irradiated in air. These

results were consistent with those obtained by DSC as seenin Fig. 4(a).

We have shown previously that FTIR is useful for studiesof crystallization behaviors of gamma-irradiated sPS.15

It can not only characterize the chemical changes causedby the gamma radiation but also effectively follow theevolution of various materials formed during crystallizationof the irradiated samples, such as the amorphous materialand the a- and b-crystalline forms. Figure 6 presents theFTIR-ATR spectra in the 800–920 cm�1 region for several tvalues of sPS irradiated in air at, as an example, 800 kGyand then crystallized at 250 °C. This region was selected foranalysis because it is sensitive to polymer chain packingand phase changes in sPS.21,24 The bands at 840 and905 cm�1 have been assigned to the amorphous material,those at 857 and 911 cm�1 are associated with theb form, and the 901 cm�1 band is attributed to the aform.13,21 For irradiation in air, the band at 901 cm�1

was more pronounced when t was increased from 20 to80 min, but the band at 911 cm�1 had the opposite trend.

FIG. 5. XRD patterns of sPS irradiated in air at 600 kGy andcrystallized at 250 °C for two different times.

FIG. 6. FTIR spectra at different crystallization times of sPS irradiatedin air at 800 kGy.

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These results indicated that the crystallinity fraction of theb form decreased and that of the a increased with t for air-irradiated sPS. Qualitatively, FTIR results were consistentwith XRD and DSC data.

C. Crystallization kinetics of sPS irradiated withgamma rays in air

The isothermal crystallization kinetics of sPS irradiatedwith gamma rays under different environments anddoses have been investigated using the Avrami model[Eq. (3)]25–27:

1� Xit ¼ exp �kitnið Þ ; ð3Þ

where Xit is the crystalline fraction at time t, ki is thecrystallization rate, and ni is the exponent related to thegrowth mode and nucleation mode. Subscript i denotesthe melting peak corresponding to the crystalline form inthe polymers as stated previously. Although this modelwas developed for single-phase transformation from oneto another, it has successfully been applied for studyingthe crystallization kinetics of polymorphic nonirradiatedsPS.20,28–30 Equation (3) is valid only for crystal growth.As shown in Fig. 4(a), the amount of X2t (i.e., crystallinefraction of the a form) increased, but those of the X1t andX3t (b and b9 forms) in the 220 °C , Tc , 242 °C rangedecreased with t. Therefore, Eq. (3) is applied here to studycrystallization kinetics of the a form existed in sPSmaterials irradiated with gamma rays in air. Table V givesn and k values for the a form, as a function of radiationdose and Tc. The exponent n increased with increasing Tcbut decreased with increasing radiation dose. This isdifferent from that for nonirradiated sPS, where n wasreported to be constant in the Tc ranges of 122–132 °C and244–252 °C,31 and for n is equal to 3 for all Tc values.

28

However, the crystallization rate k decreased with anincrease of both Tc and radiation dose. Wang et al.31 alsoreported a decrease of crystallization rate with increasinggamma radiation dose. These results suggest that gammairradiation in air at high dose tends to retard the crystal-lization of the a form in sPS.

The crystallization half time s at each temperature wasdetermined using the relation:

sðTÞ ¼ ln 2=kðTÞ½ �1n ; ð4Þand the activation energy of isothermal crystallization ofair gamma-irradiated sPS was determined using a modi-fied Arrhenius equation:

1sðTÞ ¼

1s0exp � U

R T � T‘ð Þ� �

exp � A

T Tm � Tð Þ� �

;

ð5Þ

where s(T) is the crystallization half time at crystallizationtemperature T, s0 is the preexponent factor, U is the acti-vation energy, R is gas constant, A is the thermodynamicdriving force, Tm is the melting temperature, and T∞ is thetemperature in which polymeric chains stop segmentalmotion. Note that T∞ is usually taken as Tg � 30 K,32,33

where Tg is the glass transition temperature. Equation (5)can be rearranged as

Z ¼ �ln½sðTÞ� þ A=TðTm � TÞ ¼ � lns0� U=RðT � T‘Þ :

ð6ÞFigure 7 displays plots of Z versus 1= T � T‘ð Þ for the

a-crystalline form in sPS irradiated in air at differentradiation doses. U and s0 were obtained from the interceptand slope, respectively, of Fig. 7, and A was determinedusing the least square method described in Ref. 32. Valuesof U, s0, and A for the a-crystalline form is presented inTable VI. The preexponent factor k0, A, and the activationenergy U of crystallized air gamma-irradiated sPS in-creased with increasing dose. The results of U suggestedthat a higher energy is required to crystallize an sPSmaterialirradiated at a higher dose than a material irradiated ata lower dose. That is, both the rate of crystallization and thecrystalline fraction are lower for sPS irradiated with gamma

FIG. 7. Plots of Z versus 1/(T–T∞) for a-crystalline form of sPSirradiated in air with gamma rays at different doses.

TABLE VI. The values of crystallization half time s0 (min�1), ther-modynamic driving force A (104 � K2), and activation energy U (kJ) ofa form, samples of which are irradiated in air.

a phase 0 kGy 200 kGy 400 kGy 600 kGy 800 kGy

ln s0 2.434 4.530 6.388 10.043 12.041U 1.309 2.886 5.283 10.195 13.339A 3.660 6.122 6.540 7.340 7.421

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irradiation at higher dose than those exposed to the sameradiation at a lower dose, consistent with the results shownin Fig. 2(b). Furthermore, as seen from Table VI, thethermodynamic driving force increases with increasingdose. This is also implied that thermodynamic drivingforce is required to crystallize an sPS materials irradiatedwith a higher dose than with a lower dose. Note that if n isconstant, Eq. (5) has an alternative form,

kðTÞ ¼ k0 exp�U

RðT � T‘Þ �A

TðTm0 � TÞ� �

: ð7Þ

Equation (7) was used by Wesson28 and St. Lawrenceand Shinozaki32 for studying crystallization kinetics of sPS.

IV. CONCLUSIONS

The isothermal crystallization kinetics of gamma-irradiated sPS has been investigated. sPS samples wereirradiated with gamma ray in air at different doses from0 to 800 kGy at a rate of 30 kGy/h. They were meltedat 310 °C, quenched to crystallization temperatures (Tc)between 220 and 260 °C, and maintained for variousperiods of time. DSC was used to study the crystallizationkinetics. The melting envelop of crystallized irradiatedsamples recorded by the DSC method was decomposedinto Gaussian distributions corresponding to differentcrystalline forms. Significant results are given below:

(1) Crystallization time (t) and radiation dose have astrong effect on the melting enthalpies of air gamma-irradiated sPS. At the same Tc, the melting enthalpyincreases with increasing t, and the time to reach a saturatedamount of crystalline material increases with increasingradiation dose. At the same radiation dose and a given t,crystallinity of air gamma-irradiated sPS decreases withincreasing Tc.

(2) The a-crystalline fraction in air gamma-irradiatedsPS increases with increasing t, but those of the b and b9forms have an opposite trend.

(3) The isothermal crystallization rate of air gamma-irradiated sPS decreases with increasing radiation doseand Tc.

(4) The activation energy and thermodynamic drivingforce of crystallization increase with increasing gammaradiation dose.

(5) Glass transition temperature of air gamma-irradiatedsPS increases with radiation dose at 30 kGy/h dose rate,implying that this type of high energy radiation induceschain crosslink in this polymer.

ACKNOWLEDGMENT

This work was supported by the National ScienceCouncil, Taiwan.

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