influence of gamma irradiation on chemical structure and thermal properties of polyethylene maleic...

J. Polym. Mater. Vol. 31, No. 4, 2014, 519-531© Prints Publications Pvt. Ltd.Correspondence author e-mail: [email protected]

Influence of Gamma Irradiation on Chemical Structureand Thermal Properties of Polyethylene Maleic

Anhydride

N. RAJESWARA RAOa, T. VENKATAPPA RAOa, S.V.S. RAMANA REDDYa ANDB. SANJEEVA RAOb

aDepartment of Physics, National Institute of Technology, Warangal-506004, India.bDepartment of Physics, Govt.Degree College, Mulugu,Warangal, India

ABSTRACT

The effects of gamma irradiation on polyethylene maleic anhydride (MANPE) are investigated byemploying various techniques. Radiation doses of 30, 60 and 90 kGy are selected to study theeffect on chemical structure and thermal properties on MANPE. Electron spin resonance (ESR)spectra of irradiated MANPE at radiation doses 30 and 60 kGy have resulted in the formation ofmacro-radicals such as (–CH

2-H-CH

2-) (I) and maleic anhydride (II). Recombination

of radicals was observed in temperature dependent ESR spectra of irradiated MANPE. Fouriertransform infrared analysis showed that absorption band at 929 cm-1 has shifted to 923 cm-1 onirradiation. The cleavage of side groups due to gamma irradiation on MANPE caused the meltingpoint to increase from 102°C to 112°C which is indicated by differential scanning calorimeter(DSC). The crystalinity of the MANPE is calculated from DSC results and are in good agreementwith X-ray diffraction data. The results indicate that on irradiation, MANPE preferably undergodegradation and crosslinking at low and high doses, respectively.

KEYWORDS: Gamma irradiation, Chemical structure, Melting behavior, Crystalinity, Microstructure.

1. INTRODUCTION

The role of polyethylene maleic anhydride(MANPE) as a compatibilizer is becomingprominent as it enhances compatibility betweenthe non-polar polyethylene (PE) and polarmaterials like chitin, PA6, cellulose, etc. [1-3]

which are promising new environmental

protection plastics. Moreover as acompatibilizer, it enhances the mechanicalstrength and reduces water uptake of LDPE/TPSS-kenaf fibre composites.[4] Degradationstudies on PE are widely reported in literature;however very few investigations have beencarried out on degradation of MANPE. Jiann-Wen et al. [5] have made an attempt to

520 N. Rajeswara Rao et al.

Journal of Polymer Materials December 2014

investigate thermal degradation of MANPE bythermo gravimetric analyzer (TGA) anddifferential scanning calorimeter (DSC). Theyreported that the thermal degradation ofMANPE proceeds by both chain scissions aswell as crosslinking. The presence of alkylradical has been predicted, which acceleratechain scission and reduce the thermal stabilityof MANPE, while R-OH groups formed duringthermal degradation interact with maleicanhydride (MAH) groups to form cross-linkedstructure leading to enhancement in thermalstability of MANPE.[5]

Silica nanoparticles have negligible effects onthermal degradation of LDPE andnanocomposites. But grafting of anhydridegroups improves thermal stability of LDPE.[6]

The improvement in thermal stability isassociated with the distributed degradationmechanism in the presence of anhydridefunctional group. Random chain scission is theprimary degradation pathway in polyethylene(PE) and it can also result in branching. In thepresence of anhydride groups, it is possiblethat there may be strong interaction betweenthem which may hinder evaporation ofmonomers. [7]

The effects of γ- irradiation on polyethylene filmswere studied by DSC and X-ray diffraction (XRD)techniques.[8] According to these studies PEdegrades initially and then undergoessubsequent crosslinking at low and high doses,respectively. The influence of montmorlinate onthe oxidative degradation of PE nanocompositeswas investigated by Fourier transform infrared(FTIR) and gas chromatography techniques.[9]

Recent studies have reported on productsformed during radiative degradation and thermalageing of PE cables used in nuclear power

stations.[10] They have deconvoluted the FTIRabsorption bands at 1700-1800 cm-1 into threecomponents which are attributed to carboxylicacid, carboxylic ester and carboxylic anhydridegroups. The intensity ratios of these groupsare reported to change with the oxidationtemperature.

The studies on degradation effects on pureMANPE are very few in literature. MANPE isused as a compatabilizer to prepare polymerblends. During the process of melt mixing toprepare polymer blends, the MANPE may besubjected to degradation and can affect theblend properties. Further the formation ofdifferent types of radicals like macroradicals andperoxy radicals is reported in the literature.[5]

However experimental evidence is not providedfor the existence of these free radicals.Therefore we have investigated the effect ofgamma irradiation on MANPE using ESR,FTIR, DSC, XRD and scanning electronmicroscope (SEM) techniques. The informationobtained from DSC and SEM investigations willbe useful in ascertaining miscibility of MANPE,if it is used as a compatabilizer.

2. EXPERIMENTAL

2.1 Materials and Irradiation

MANPE in the form of powder is obtained from SigmaAldrich with viscosity 500 cP (140°C) and density 0.92g/mLat 25oC. Samples were subjected to thermal treatmentof about 50oC for 8 hours. Three MANPE sample packetswere prepared for gamma irradiation with differentradiation doses i.e. 30, 60 and 90 kGy. Gamma irradiationis performed in atmospheric air using a Gamma Chamber900 (GC-900) BRIT, India. The calibrated 60Co basedGC-900 with a central dose of 0.15 kGy/hr is used inpresent study. The uncertainty in doses delivered tosamples is ±5%, so the actual doses are 30±1.5 kGy,60±3 kGy and 90±4.5 kGy. The distribution of radiation

Influence of Gamma Irradiation on Chemical Structure andThermal Properties of Polyethylene Maleic Anhydride

521


dose is almost uniform as the samples are positioned atthe center of the chamber using a stand. The total dosegiven to the sample is controlled by time of irradiation.

2.2 Characterization Techniques

2.2.1 Electron Spin Resonance (ESR) ESRmeasurements are performed with Varian E linespectrometer at 9.1 GHz (X-band) and 100 kHzmodulation frequency at 25°C. The microwave powerused, 2.98 mW, were determined by considering thesaturation properties of MANPE ESR lines at 25°C. Thesamples were accommodated in quartz sample tubeand sample weight (5 mg) kept equal in order to getquantitative comparison of the ESR line intensitiesbetween the different samples. The spectrometerwas equipped with a variable temperature facility,so that samples irradiated at room temperature (RT)could be annealed to higher temperatures, thusallowing the reactivity of the radicals to beassessed.

2.2.2 Fourier Transform Infrared (FTIR) FTIR spectraof the sample pellet prepared by adding KBr arerecorded on Perkin Elmer 100S spectrometer with aresolution of 4 cm-1 and in the range 450-4000 cm-1 tocharacterize the chemical structure of the MANPE afterirradiation.

2.2.3 Differential Scanning Calorimeter (DSC)Differential scanning Calorimetry (DSC) was performedusing a DSC-TA Q10 calorimeter to study the thermalproperties of the unirradiated and irradiated MANPE.Samples consisted of approximately 5 mg of MANPEwere sealed into the aluminum crucibles and heatedfrom 50°C to 200°C at a heating rate of 10°C/min influshing Nitrogen gas. The crystalinity (Xc) of the MANPEwas calculated by ratioing the melting enthalpy of thesamples to 293 J/g, the melting enthalpy of 100%crystalline MANPE. [11]

2.2.4 X-ray Diffraction (XRD) The X-raydiffractograms were recorded on Bruker D8 AdvanceX-ray Diffractometer. The X-rays are produced using asealed tube and the wavelength of X-ray was 0.154nm (Cu Kα). The XRD pattern was recorded in the 2θrange from 10° to 50° at 0.2° steps and at a fixed

counting time of 10s. The x-rays were detected usinga fast counting detector based on Silicon striptechnology.

2.2.5 Scanning Electron Microscope (SEM) Thesamples were fixed on a cylindrical microscope stubcovered with carbon strip and surfaces of the samplewere gold coated using a sputter set at 10 mA for 60s.The morphology was then examined under Carl ZeissULTRA 55 at an accelerating voltage of 6 kV. A

magnification of 20,000× was used.

3. RESULTS AND DISCUSSIONS

3.1 Electron Spin Resonance studies

3.1.1 Effect of Radiation dose The ESRspectra of MANPE irradiated at variousradiation doses such as 30, 60 and 90 kGy isshown as curves 1, 2 and 3 in Figure 1,respectively. No ESR signal is found forunirradiated MANPE indicating no free radicalformation. The spectra observed for MANPEirradiated at low doses of irradiation (30 and60 kGy) possess an asymmetric doublet witha separation of 35G superposed on a hyperfinestructure. It is well established that irradiatedpolyolefins contain macroradicals [12-14] whichconvert to peroxy radical under oxidizingconditions.[15] Therefore efforts have been madeto resolve the hyperfine structure and peroxydoublet from the experimental spectrum.Initially peroxy doublet is subtracted from totalspectrum and resultant spectrum issimulated.[16, 17] Polyethylene on irradiationgives an octet (eight line) spectrum assignedto -CH

2-H-CH

3,[13] sixline spectrum assigned

to alkyl radical –CH=CH-H-CH3 or septet

(seven line) spectrum assigned to allyl radical–CH= CH-H-CH

2-.[12]



Figure 1. Dose dependent ESR spectra of MANPE irradiated to 30 kGy (1), 60 kGy (2) and 90 kGy (3) ofradiation dose

ESR spectrum of MANPE irradiated to 30 kGydose is simulated with the component spectrashown in Figure 2. Its component multipletassigned to free radical of the type ~CH

2-H-

CH2~ (alkyl radical). The values of theparameters used in the simulation are n

i=2,

mi=5 (n

i and m

i represents the number of lines

resulting from α and β-protons respectively),A

i=23.0G and B

i=13.0G (A

i and B

i represent the

hyperfine splitting of α and β-protonsrespectively; Ai ≠ Bi).

Figure 2. Resultant ESR spectrum (curve 1) after subtraction of peroxy radical (curve 2) from experimentalspectrum of MANPE irradiated to 30 kGy dose of radiation


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The magnetic parameters indicate thepresence of one α-proton and four β-protons.The structure of free radical is of the type ~CH

2-

Figure 3. Formation of free radicals in irradiated MANPE

H-CH2~ (I) whose formation is illustrated in

Figure 3.

Simultaneously when curve 1 of Figure 2 is

subtracted from the experimental spectrum(curve 1 of Figure 1), a difference peroxy doubletis obtained, shown as curve 2 in Figure 2. Thesuperposition of curve 1 and 2 of Figure 2 resultsin the experimentally observed spectrum curve1 Figure 1. Formation of macro radicals inMANPE is expected with the cleavage of

pendent groups on gamma irradiation. Thevalue of magnetic parameters employed tosimulate the component spectra are listed inTable 1. The cleavage of side groups is alsoconfirmed by DSC and FTIR results.

Considering the chemical structure of MANPE,

Table 1. Magnetic parameters of irradiated MANPE used in simulation

Radiation Relative Line Centre Hyperfineni m idose (R) Intensity width of spectra splitting

kGy Ymax ai Zoi Ai Bi

30 5.5 9.0 3224 23.5 13.5 2 5

60 4.0 7.5 3224 22.0 10.0 2 5



chain cleavage occurs at the tertiary carbonposition marked as ‘a’ in step-(1) shown inFigure 3, leading to the formation of radicals (I)& (II). Radical (I) is a chain (macro) radicalwhich gives an ESR multiplet spectrum. Macroradical transforms to peroxy radical underoxidized conditions as shown in step-(2) ofFigure 3. Radical (II) is stabilized to form aresonant structure having unsaturation is shownin step-(3) of Figure 3. Formation of unsaturatedstructure has been evidenced by FTIR studies.Radical (I) gives a component multiplespectrum, while doublet spectrum is expectedfrom radical (III). Since the irradiation is carriedout in oxygenated conditions both the radicalsmight abstract oxygen and convert into peroxyradical. The g-value of the radicals is found tobe around 2.0073 – 2.0098, which is close tothe reported value in case of polyethylene

peroxy radical.[15] The spectrum observed forMANPE irradiated to 60 kGy dose of radiationshown as curve 2 in Figure 1. The spectrumalso contains some hyperfine structure and canbe simulated with the same set of magneticparameters with minor changes, indicating thepresence of radicals (I) & (III).

Considering the total area under the ESRspectrum which represents free radicalconcentration and these spectral areas arecalculated using the double integration method.A plot has been drawn between free radicalconcentration and dose of radiation isrepresented by curve 1of Figure 4. The graphindicates a gradual increase in free radicalconcentration implying that more number ofradicals might have formed with the increasein radiation dose as expected.

Figure 4. Variation of ESR spectral intensity of MANPE with radiation dose 30, 60 and 90 kGy (1)and temperatures 300K, 323K, 353K and 373K (2)


525


3.1.2 Effect of Temperature ESR spectraare recorded at different temperatures in orderto study the effect of temperature on freeradical formation in gamma irradiatedMANPE. Curves 1, 2, 3 and 4 in Figure 5represent the ESR spectra of irradiatedMANPE at 300K, 323K, 353K and 373Krespectively. The ESR signal intensitygradually decreases with the increase oftemperature and above 373K the signal totallyvanished. A similar radical decay associatedwith transitions in polymer is reported byvarious authors. When the temperature israised radicals gain thermal energy and

interact with themselves or with other radicalsand finally at the transition temperature theradical decay is completed.[18] Therefore basedon radical decay temperature, transitions inpolymer can be identified. This behaviorfurther confirms the formation of alkyl and alkylperoxy radical, which usually decay around373K. In the literature the decay temperatureof alkyl radicals in gamma irradiatedpolyethylene is in the range of 120oC to 140oC[19] and also the decay temperature of peroxyradicals is in the range of 50oC to 125oC.[15]

The DSC results confirm that the meltingtransition temperature of MANPE is around

Figure 5. ESR spectra of irradiated MANPE annealed from RT (300K) to temperatures 323K(2), 353K (3) and 373K (4)



380 K.

A plot of ESR intensity against temperatureis represented by curve 2 in Figure 4. The non-linearity of the curve indicates the complexform of the radical decay. These also suggestthat the free radicals have inhomogeneousdistribution in the polymer matrix. Presenceof macro-radicals and peroxy radicals inMANPE has been postulated.[5] However theauthors have not provided any experimentalevidence.

3.2 Fourier Transform Infrared Studies

FTIR spectra of unirradiated MANPE isrepresented by curve 1 in Figure 6, while curves2, 3 and 4 represent the FTIR spectra ofMANPE irradiated to 30, 60 and 90 kGy doseof radiation respectively. Various absorptionbands of MANPE are observed in Figure 6,which are assigned to chemical groups aslisted in Table 2. Among these absorptionbands, the 929 cm-1 absorption band hassuffered a low wavelength shift indicating thecleavage of maleic anhydride group.

Figure 6. FTIR spectra of MANPE: unirradiated (1) and irradiated to 30 kGy (2), 60 kGy (3) and90 kGy (4) of radiation dose


527


The absorption band at 1704 cm-1 that assignedto the carbonyl group (C=O) stretchingvibrations of maleic anhydride groups is shiftedto higher wavelength with a decrease in

Table 2. Assignment of various absorption bands in MANPE

S. No. Peak position cm-1 Assignment

1 3413 Hydrogen bonding between PE & MAH groups

2 2950 CH2 group of PE

3 1704 Symmetrical stretching of C=O group of MAH

4 1458 CH2 of MAH

5 1199 =C-O-C= of MAH

6 929 Ring stretching vibration of cyclic group

intensity.[20-22] This may means the decreasein concentration of MAH group. Furtherappearance of 1545 cm-1 absorption bandconfirms the presence of unsaturated groups.

3.3 Thermal Studies

DSC thermograms of unirradiated and irradiatedMANPE of doses 30, 60 and 90 kGy are asshown in Figure 7. Thermal parameters of both

irradiated and unirradiated MANPE are listedin Table 3. The endothermic peak observed inFigure 7 is associated with melting point (T

m)

of MANPE. The peak is shifted to higher

Figure 7. DSC thermogram of unirradiated and irradiated MANPE of doses 30, 60 and 90 kGy



temperature with increase of radiation dose andfurther suffered to a decrease. Badr et al. [8]

have observed a decrease in Tm

of polyethylene(PE) irradiated to low dose, which is to be dueto degradation.

Therefore increase in peak position may beas a result of cleavage of side groups/chains.Due to irradiation of MANPE, formation ofradicals (I) and (IV) takes place as shown in

Figure 3. As a result of cleavage of MAHgroups, linearity of chain increases causingan increase in T

m. The radicals formed during

the initial stage, form cross-linked structuresresulting in branching causing a decrease inmelting point. A plot of melting temperature(T

m) and enthalpies (∆H) against radiation

dose is depicted in Figure 8. The studies areconsistent with the reported results inliterature.[23, 24]

Figure 8. Variation of melting point and enthalpy of MANPE calculated from DSC with radiation dose

Thermal stability of MANPE is higher than PEdue to the presence of MAH groups.[5] SincePE is a linear polymer in which thermal diffusionis easier, whereas in case of MANPE, the graftedMAH groups prevent the thermal diffusion. As aresult, MANPE is thermally more stable thanPE as reported in literature.[5] Regarding meltingbehavior, since PE is a linear polymer andgrafting of MAH groups on to PE results in an

increase of amorphous nature or decrease ofcrystalinity.[24] The calculated % crystalinity(X

c) of unirradiated and irradiated MANPE is

listed in Table 3. On irradiation the cleavageof MAH groups occurs resulting in theformation of linear chains which indicatesdecrease in amorphuosity or increase incrystalinity.


529


Table 3. Thermal parameters of MANPE

Radiation dose (kGy) TmoC ÄH J/g % Xc by DSC % Xc by XRD

Unirradiated (0) 102 100.7 34.1 36.2

30 109 109.8 36.3 38.8

60 112 115.4 38.4 41.1

90 98 136.5 46.7 42.4

3.4 X-ray Diffraction studies

The X-ray diffractogram of unirradiated andirradiated MANPE are shown in Figure 9. Curve1 represents XRD of unirradiated MANPE,while curves 2, 3 and 4 represent XRD ofMANPE irradiated to 30, 60 and 90 kGy doseof radiation respectively. Polyethylene is acrystalline polymer and shows an intense peakat 2θ=21.22° together with some additional

peaks. The peak at 2θ =21.22° is assigned toreflection of (110) plane as reported.[25] Graftingof PE with MAH groups has resulted a peak at2θ =21.5° and 24.0°.[26] The diffraction peakposition at 2θ =21.5° is found to shift withincrease in degree of grafting.[25] In the presentstudies the XRD peaks are observed forMANPE at 2θ= 15.67° and 18.51°. Onirradiation there is a minimal decrease in widthof the diffraction pattern.

Figure 9. XRD diffractograms of MANPE: unirradiated (1) and irradiated to 30 kGy (2), 60 kGy(3) and 90 kGy (4) of radiation dose



3.5 Microstructure studies

SEM micrographs of unirradiated and irradiatedMANPE are shown as (a), (b) in Figure 10respectively. The particle size has considerablyreduced and bears dark background indicating

the physical deformation of particles onirradiation. Before irradiation the particles areclosely packed while on irradiation the particlesare dispersed. When such irradiated MANPEis used as a compatabilizer, it can easily bedispersed in the polymer blend system.

Figure 10. SEM micrographs of (a) unirradiated and (b) irradiated MANPE

4. CONCLUSIONS

We have successfully investigated the effectof gamma irradiation of various doses onMANPE and found the cleavage of MAH groupsthat may lead to the formation of macroradicalstype (I). Under oxidizing conditions thesemacroradicals may convert into radicals type(III). The formation of radicals type (I) and (III)is proved by ESR and FTIR measurements.Likewise, experimental evidence was providedfor the above said radicals. On irradiation, thelinearity of polymer chains increases, by thecleavage of MAH macro-radicals, leading tothe enhancement of melting point as observedin DSC thermograms. This improves thestability of compatibilizer when processing athigher temperatures. XRD studies also indicatethe decrement in amorphuos nature of the

MANPE on irradiation, which is in goodagreement with the results obtained from DSC.

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Received: 13-11-2013

Accepted: 15-04-2014

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influence of gamma irradiation on chemical structure and thermal properties of polyethylene maleic...

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