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Radiation Physics and Chemistry 76 (2007) 1318–1323

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Crystallization of g-irradiated poly(ethylene oxide)

Irina Pucic�, Tanja Jurkin

RuXer Boskovic Institute, Zagreb, Croatia

Abstract

The shape of DSC thermograms and heats of phase transformations reflect the changes in crystallinity of poly(ethylene oxide) (PEO)

brought into it by 60Co g-radiation to selected doses, in air or in nitrogen. In samples irradiated in air, multiple peaks appeared

presumably due to degradation in more radiation-sensitive amorphous part producing new crystals of different macroconformations, the

changes increased during the post-irradiation period. Related morphological changes were detected. Additional crystallization at low

doses resulted in increase of crystallization heats, further degradation reduced crystallinity at higher doses. In the samples irradiated

under nitrogen crosslinking contributed to reduction of crystallinity, while crystallization temperatures decreased reflecting lower quality

of crystallites.

r 2007 Elsevier Ltd. All rights reserved.

Keywords: Multiple transitions; Degradation; Crosslinking; Crystallization; Post-irradiation; DSC; Morphology

1. Introduction

Poly(ethylene oxide) (PEO) is extensively studied semi-crystalline polymer that has numerous applications, forexample, in pharmaceutical industry for hydrogels, ex-cipients, etc. It can be a host to various metal salt systems,so PEO is frequently used to prepare polyelectrolytecomplexes of increasing importance (Dias et al., 2000;Schuster and Meyer, 2003). Since charge transport inamorphous phase is 2–3 orders of magnitude faster than incrystalline phase, intention is to prepare films with highercontent of amorphous phase while keeping or, possibly,improving its mechanical properties. Ionising radiation hasonly occasionally been used in an attempt to reduce thecrystallinity of PEO (MacCallum et al., 1984; Zainuddinet al., 2002; Ferloni and Magistris, 1991; Song et al., 1997,2002; Pucic and Turkovic, 2005) and there are relativelyfew recent studies on changes induced by high-energyradiation in pure PEO. In most cases, PEO was irradiatedfor a proposed application without an attempt to gainunderstanding of the impact of properties of PEO andirradiation conditions that can profoundly influence theoutcome of the irradiation of PEO. Because of its chemical

ee front matter r 2007 Elsevier Ltd. All rights reserved.

dphyschem.2007.02.023

ing author. Tel./fax: +385 1 45 61 166.

ess: pucic@rudjer.irb.hr (I. Pucic).

nature, both crosslinking and degradation have to be takeninto account. The changes of thermal properties of PEOmay provide some answers on what is going on inirradiated PEO samples. The crystallization is alreadystudied in great detail (Buckley and Kovacs, 1976; Chen etal., 1999; Cheng et al., 1993, 1988, and the referencestherein) particularly for low-molecular mass PEO, Mw

between 1000 and 10,000, with narrow distribution ofmolecular mass and the results of these studies can serve asmodel of PEO crystallization. The specificity of PEO isstepwise change of lamellar thickness causing polymorphictransitions (Buckley and Kovacs, 1976) unlike otherpolymers where the lamellar thickness changes graduallyand on melting or crystallization a single-DSC peakappears. Low molecular PEO preferably crystallizes inextended chains with helix conformation (Chen et al., 1999)that have the highest melting temperature of all macro-conformations (i.e. extended or variously folded chains),multiple folding of PEO was also encountered (Cheng andWunderlich, 1986). Folding of the PEO chain requiresbreaking of the hydrogen bonds that exist between –OHgroups of the PEO chain ends both in the amorphous andcrystalline part. Because of that the lamellae of folded PEOchain crystal are thinner and chain ends are being rejectedto the crystal surface. The cleavage of hydrogen bondsbetween chains ends along with formation of new ones on

ARTICLE IN PRESSI. Pucic, T. Jurkin / Radiation Physics and Chemistry 76 (2007) 1318–1323 1319

the crystal surface and influences the stability of theparticular macroconformation.

To our best knowledge, no study deals with the effect ofionising radiation on crystallization of pure PEO. Itschemical nature influences the way it crystallizes andvarious factors that may have great importance indetermining optimal irradiation conditions have yet tobe investigated. This work is the first step in this direc-tion with a goal to explore the influence of g-radiationon crystallization of pure PEO samples of differentmolecular masses, irradiated in various forms and condi-tions. The sensitivity of DSC enables fast determination ofchanges in crystallization on irradiation and during thepost-irradiation period while morphology provides someinformation on origin of effects encountered in thermograms.

2. Materials and methods

PEO powders produced by Aldrich of Mw 0.4� 106 and8� 106, were irradiated as received, in air, or pressedinto pellets. A part of the pellets was irradiated in airand the other part was irradiated in sealed flasks filledwith nitrogen. PEO powder of Mw 1� 106, produced byBDH, was irradiated as received and only in air. Allsamples were irradiated at room temperature in 60Co g-irradiation facility at RBI to various does at dose rates0.5 kGy/h (PEO powder of Mw 1� 106) and 25 kGy/h (allother samples). During the post-irradiation period thesamples were kept in sealed flasks in dessicator at roomtemperature.

DSC thermograms were recorded using Perkin-ElmerDSC 7, calibrated with In and Zn according to theprocedure proposed by its producer. The samples weresealed into Al sample pans, linear heating was performedunder nitrogen in the temperature range 0–90 1C at the rate10 1C/min, two heating and cooling cycles were performedfor each sample. For unirradiated PEO samples and PEOsamples irradiated to every dose and day at least twosamples were recorded, the peak temperatures and averagevalues of heats of fusion and crystalllization werecalculated.

Morphology was studied using Olympus BX51 polariza-tion microscope (magnifications: 10� and 50� ), equippedwith Linkam TH600 hot stage and Linkam TMS 93temperature controller. The heating rate was kept as closeto those in DSC experiments as possible.

3. Results and discussion

The thermograms recorded in the dynamic mode duringthe crystallization of unirradiated PEO and PEO irradiatedto 200 kGy in air or in nitrogen are shown in Fig. 1, plots ofderivative curves of thermograms are added below thecorresponding thermograms to make the changes morevisible and to ease detection of multiple transitions. Toavoid any thermal history and influence of samplepreparation that could contribute to artifacts, we presented

only the thermograms of crystallization from melt startingat about 30 1C above the melting point(s). All unirradiatedsamples of Mw 0.4� 106, 106 and 8� 106 show a singleexothermic peak, what is confirmed by the derivation of thecorresponding data. The thermograms and derivative plotsof PEO samples irradiated to 50 kGy (not shown) resemblethose of unirradiated PEO. Above that dose thermogramsof samples irradiated in air reveal multiple processes thatfirst appeared as the changes in the shape of the derivativecurves of samples irradiated to 100 kGy (not shown). In thethermograms of PEO samples irradiated in nitrogen at thefirst glimpse there seems to be no change even at 200 kGybut derivative curves are not so smooth any more andreveal some subtle changes.As previously stated, the appearance of multiple peaks in

PEO thermograms is common at melting (Buckley andKovacs, 1976; Cheng and Wunderlich, 1986; Chen et al.,1999) and at crystallization. Degradation that is, because ofchemical nature of PEO, highly likely especially on itsirradiation in air, causes formation of low-molecular weightchains and widens the molecular mass distribution. Whileunirradiated PEO samples have 2–3 orders of magnitudehigher molecular masses than the samples described inabove articles; the results of research on crystallization oflow-molecular PEO becomes even more relevant for theirradiated samples because of degradation. On heating totemperature high enough to erase all previous crystallinityand subsequent crystallization, each group of chains of aspecific molecular mass could crystallize in a correspondingmacroconformation(s). The increasing multiplicity in ther-mograms, particularly of PEO samples of molecular masses0.4� 106 and 8� 106 that were irradiated in air at higherdose rate is similar to those described in articles by theabove-mentioned authors and is consistent with significantdegradation at doses of 100kGy and higher. The wideningof molecular mass distribution increases the distribution ofthe crystallite size resulting in the widening of the DSCpeak(s), which can be seen in Figs. 1 and 3. Concurrentcrosslinking also widens the molecular mass distribution atthe same time posing some restrictions on crystallization.The length of the segments between crosslinks has to behigh enough for crystallization to occur and new peaks orshoulders in DSC thermogram to appear, Qiao et al. (2004)determined that the molecular mass of segments has to beover 1000. On increasing density of crosslinks, the segmentseventually become too short to crystallize. On crystal-lization from melt the crosslinks formed on irradiation arerejected to the crystal surface lowering the quality ofcrystallites, so the low-molecular mass fragments in sampleswith wide distribution of molecular mass can be treatedeven as a crystallizable or non-crystallizable impurities(Cheng and Wunderlich, 1986). Although in theory, a greatnumber of transitions and DSC peaks could be possible inirradiated high-molecular weight PEO, just several peaksappear in the thermograms because only stable andmetastable macroconformations that are present in highenough concentrations contribute.

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Fig.1.Thethermogramsrecorded

inthedynamicmodeduringthecrystallizationofPEO

samplesofdifferentmolecularmasses,unirradiatedandirradiatedto

200kGyin

airandin

nitrogen

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onthedayafter

theirradiationandthecorrespondingderivativeplots.

I. Pucic, T. Jurkin / Radiation Physics and Chemistry 76 (2007) 1318–13231320

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Fig. 2. Polarized optical micrographs of selected PEO samples: (a) PEO sample showing single peak in DSC on crystallization (magnification: 50� ), (b)

PEO sample that had multiple peaks in DSC on crystallization (magnification: 50� ), (c) unirradiated PEO at melting (magnification: 10� ), and (d) PEO

irradiated to 200kGy in air at melting (magnification: 10� ).

200 kGy, in air 200 kGy, in N2

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Fig. 3. The thermograms recorded in the dynamic mode during the crystallization of PEO samples (molecular mass: 0.4� 106) irradiated to 200kGy in air

and in nitrogen recorded on the first, 10th and 30th day after the irradiation and the corresponding derivative plots.

I. Pucic, T. Jurkin / Radiation Physics and Chemistry 76 (2007) 1318–1323 1321

ARTICLE IN PRESSI. Pucic, T. Jurkin / Radiation Physics and Chemistry 76 (2007) 1318–13231322

The reduction in inner order of the irradiated PEO canbe seen on comparison of morphology on polarized opticalmicrographs taken on crystallization of the sampleexhibiting a single transition (Fig. 2a) vs. the one exhibitingmultiple peaks (Fig. 2b). Clear, polygonal spherulites areseen in Fig. 2a, while the morphology of sample in Fig. 2bis typical for degraded polymers consisting of spheruliteswith recognizable fibrils and small branches. On heating,near the melting temperature, the unirradiated PEO(Fig. 2c) and PEO irradiated to 200 kGy in air (Fig. 2d)show different patterns. In the spherulites of unirradiatedsample Maltese crosses can still be seen although theboundaries between radical fibrillar elements are hardlyvisible. The decrease in order reduces the stability of theinner structure in the irradiated sample, so Maltese crossesdisappear simultaneously with spherulite boundaries. The

20

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Fig. 4. The variation of heats of crystallization (a) and temperatures of cryst

powder or pressed into the pellets in air and in nitrogen on dose.

occurrence of multiplicity and the number of peaksincreased not only with total dose, but the changes keptincreasing during post-irradiation period of 30 days(Fig. 3). If air is excluded during the irradiation socrosslinking is more likely than degradation, it seems thatthe network formed prevented changes in crystallization inthe post-irradiation period. That is why no further changestook place during the post-irradiation period in the PEOsamples irradiated in nitrogen, although the samples werelater kept in air.Because of the overlapping, the heats of fusion and

crystallization of multiple peaks were calculated as a singlepeak. At lower doses, the crystallization heats (Fig. 4a) ofall samples increased with the irradiation dose compared tothe corresponding unirradiated samples no matter whetherthey were irradiated in air or in nitrogen, although the

/ kGy

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powder Mw pelletsair

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r Mw pelletsN2air

allization (b) of PEO samples of different molecular masses irradiated as

ARTICLE IN PRESSI. Pucic, T. Jurkin / Radiation Physics and Chemistry 76 (2007) 1318–1323 1323

increase is more pronounced in the samples irradiated inair. The range of doses of increased heats of crystallizationis the highest for the sample of Mw 1� 106, most likelybecause it was irradiated at lower dose rate. The increase ofheats of fusion or crystallization has been encountered innumerous irradiated polymers: PE (Stojanovic et al., 2005),PVDF (Zhudi et al., 2002), etc. According to the literature,it seems that amorphous part is more sensitive to ionizingradiation than the crystalline (Minkova et al., 1989;Stojanovic et al., 2005). Short chains produced in theamorphous part can crystallize easier than the long ones; itis possible that they at least partly contribute to thickeningof the already existing lamellae. Fractionation, i.e. theexclusion of parts of polymer chain or chains of lowermasses during the nucleation, may also occur and it wouldcontribute to the decrease of the degree of crystallinity andcorresponding heat. The thermogram shape of samplesirradiated in nitrogen is seemingly unchanged but theincrease in the heat of crystallization is still present. Thatmay be the result of constraints caused by concurrentcrosslinking, that is expected to be more pronounced onthe surfaces of the lamellae (Stojanovic et al., 2005)improving the stability of the existing macroconformations.Although degradation under nitrogen is less pronounced, itcan still contribute to increase in crystalline content,especially at higher doses. The effect of concurrent cross-linking and degradation most likely caused the fastestdecrease of crystallization heats of PEO pellets irradiatedin nitrogen and at higher dose rate. Unlike the heats, thecrystallization temperatures decrease with increasing dosein the whole dose range under the consideration. For thesamples with multiple DSC peaks, the most prominentpeak temperature is shown in Fig. 4b. The defectsproduced on irradiation, including the crosslinks that arerejected to the crystal surface during crystallization,decrease the perfection of crystallites and reduce transfor-mation temperatures. Most of the samples have approxi-mately the same range and trend of crystallizationtemperatures that applies also for the PEO of Mw 1106that was irradiated at lower dose rate. The crystallizationtemperature of PEO of Mw 0.4� 106 irradiated as powderin air had the steepest decrease, consistent with easierdegradation in sample with wide surface in contact withair. Pressing of PEO into pellets seems to improve stabilityof its crystals, most likely by preventing the contactwith air. Otherwise the sample form in which PEOwas irradiated had no influence either on the shapeof thermograms or the heats and the temperatures ofcrystallization.

4. Conclusion

The additional peaks appearing in the crystallizationthermograms after the irradiation of PEO samples to dosesover 50 kGy, particularly those irradiated in air, ascompared to those irradiated under nitrogen, are causedby polymorphic transitions resulting of additional macro-

conformations that are caused by both the degradation andcrosslinking. The changes became more pronounced duringthe post-irradiation period and at higher dose rate, thesample form had less influence. The decrease of order isconfirmed by optical micrographs. Increase in the heats ofcrystallization of PEO at lower doses is consistent withdegradation since it is more pronounced in samplesirradiated in air. Under nitrogen, crosslinking of PEO ismore likely—additionally reducing the crystallization heat.The peak widths increased and the temperatures ofcrystallization decreased with dose as a result of lessperfect crystallization.This interesting results show the need for further

research to improve understanding of the influence ofionizing radiation on crystallization of PEO both tocontribute to the knowledge of the complex processes itscrystallization incorporates and to facilitate modificationof PEO for its various applications.

References

Buckley, C.P., Kovacs, A.J., 1976. Melting behaviour of low molecular

weight poly(ethylene oxide) fractions 2. Folded chain crystals. Colloid

Polym. Sci. 254, 695–715.

Chen, E.Q., Lee, S.W., Zhang, A.Q., Moon, B.S., Honigfort, B., Mann, I.,

Lin, H.M., Harris, F.W., Cheng, S.Z.D., Hsiao, B.S., Yeh, F.J., 1999.

Isothermal thickening and thinning processes in low-molecular-weight

poly(ethylene oxide) fractions crystallized from the melt. 6. Config-

urational defects in molecules. Polymer 40, 4543–4551.

Cheng, S.Z.D., Wunderlich, B., 1986. Molecular segregation and

nucleation of poly(ethylene oxide) crystallized from melt. I. Calori-

metric study. J. Polym. Sci., Part B: Polym. Phys. 24, 577–594.

Cheng, S.Z.D., Wu, S.S., Chen, J.H., Zhuo, Q., Qiurk, R.P., von

Meerwall, E.D., Hsiao, B.S., Habenschuss, A., Zschack, P.R., 1993.

Isothermal thickening and thinning processes in low molecular weight

poly(ethylene oxide) fractions crystallized from melt. 4. End-group

dependence. Macromolecules 26, 5105–5117.

Dias, F.B., Plomp, L., Veldhuis, J.B.J., 2000. Trends in polymer

electrolytes for secondary lithium batteries. J. Power Sources 88,

169–191.

Ferloni, P., Magistris, A., 1991. Effects of gamma-radiation on polymer

electrolytes: PEO and PEO20-LiClO4. Radiat. Phys. Chem. 37,

615–621.

MacCallum, J.R., Smith, M.J., Vincent, C.A., 1984. The effects of

radiation-induced crosslinking on the conductance of LiClO4 �xPEO

electrolytes. Solid State Ionics 11, 307–312.

Minkova, L., Stamenova, R., Tsvetanov, C., Nedkov, E., 1989. Structural

studies of radiation-crosslinked poly(ethylene oxide). Radiat. Phys.

Chem. 27, 621–642.

Pucic, I., Turkovic, A., 2005. Radiation modification of (PEO)8 ZnCl2polyelectrolyte and nanocomposite. Solid State Ionics 176, 1797–1800.

Schuster, M.E., Meyer, W.H., 2003. Anhydrous proton-conducting

polymers. Annu. Rev. Mater. Res. 33, 233–261.

Song, Y., Wu, S., Jing, X., Sun, J., Chen, D., 1997. Thermal, mechanical

and ionic conductive behaviour of gamma-radiation induced PEO/

PVDF(SIN)-LiClO4 polymer electrolyte system. Radiat. Phys. Chem.

49 (5), 541–546.

Stojanovic, Z., Kacarevic-Popovic, Z., Galovic, S., Milicevic, D.,

Suljovrujic, E., 2005. Crystallinity changes and melting behavior of

the uniaxially oriented iPP exposed to high doses of gamma-radiation.

Polym. Degrad. Stab. 87, 279–286.

Zainuddin, Albinska, J., Ulanski, P., Rosiak, J.M., 2002. Radiation-

induced degradation and crosslinking of poly(ethylene oxide) in solid

state. J. Radioanal. Nucl. Chem. 253, 339–344.