X-ray investigation of polypropylene and irradiated i-PP_EVA blends

X-ray investigation of polypropylene and poly(ethylene-co-vinyl acetate)blends irradiated with fast electronsWAXS investigation of irradiated i-PP/EVA blends M.Mihailova a,*,M.Kresteva a,N.Aivazova a,V.Krestev a,E.Nedkov ba University of So®a,``St.Kliment Ohridski'',Faculty of Physics,Department of General Physics,5James Bourchier Blvd,1126,So®a,Bulgariab Central Laboratory of Polymers,Bulgarian Academy of Science,1040,So®a,BulgariaReceived26February1997;accepted18September1997AbstractBlends consisting of poly(propylene-ethylene)(PP)and poly(ethylene-co-vinyl acetate)(EVA)copolymers were investigated.Specimens were irradiated with fast electrons at di erent doses.Some of the samples show thermo-shrinkable properties.The interplanar spacing,paracrystalline factor,degree of crystallinity and crystallite sizes were determined by WAXS measurements.Results have been reported in respect to PP content and irradiation dose.A decrease of the crystallite's imperfections with the rise of the irradiation dose was observed.An interface built up of partially interpenetrated amorphous molecular chains of incompatible polymers and separate PP and EVA small crystallites is suggested.#1999Elsevier Science Ltd.All rights reserved.Keywords:Polymer blends;Irradiated polymers;Poly(propylene-ethylene);Poly(ethylene-co vinyl acetate);X-ray investigations1.IntroductionThe formation of thermo-shrinkable polymer®lms can be achieved by several technological processes. Each process causes changes in the material structure, in¯uencing the®nal properties of the®lm.The most important factors,determining the physical properties of polymer blends,are the degree of compatibility between the respective components and their mor-phology.Even in the case of completely incompatible polymers,such as PP and EVA,an interface layer can be created that a ects the mechanical properties of the blends.When the sizes of the dispersed particles are very small,the volume of interface regions can be sig-ni®cant.(Sperling,1984;Lipatov and Sergeeva,1979). To the best of our knowledge,there are only a few papers on the super-molecular structure of these blends.Thomas et al.(1987)have studied the mor-phology and mechanical properties of blends of i-PP and EVA with45wt%VA groups,irradiated with g-rays.They have found the maximum elongation and energy at break in the samples with30/70wt%PP/ EVA and irradiation dose of100kGy.The highest interpenetration between elastic and plastic phases has been proposed for the same composition and dose. The authors have suggested that irradiation cause cross-linking in the EVA phase and that cross-scission of polymer chains takes place predominantly in the PP phase.Minkova and Nikolova have investigated extruded®lms with di erent contents of PP and EVARadiation Physics and Chemistry56(1999)581±5890969-806X/99/$-see front matter#1999Elsevier Science Ltd.All rights reserved.PII:S0969-806X(97)00292-/locate/radphyschem*Corresponding author.E-mail address:krestev@phys.uni-so®a.bg(M.Mihailova)with 33wt%VA groups,irradiated with fast electrons,by DSC and thermomechanical tests (Minkova and Nikolova,1989a,b).They have found that the tempera-tures of melting were independent of the contents of the blends and decreased slowly with the rise of the ir-radiation dose and that irradiation from 53to 120kGy leads to the formation of 50±60%gel fraction.Samples with higher EVA concentration show a higher cross-link density at a given irradiation dose.The tem-perature interval of thermo-shrinkage spans from the beginning of EVA melting (80±908C)to the beginning of i-PP melting (1308C).The authors cited above have not considered the crystal structure and morphology of these blends.The aim of the present work is a structure investi-gation of extruded ®lms,composed of i-PP and EVA,and elucidation of the in¯uence of the composition and irradiation dose on the ®lm structure at di erent structural levels as well as to analyse the changes of the interface layer thickness with irradiation dose.2.Materials and methodsThe polymer ®lms studied were blends of poly(pro-pylene-ethylene)(PP),Buplen 7623(Bulgaria)and poly(ethylene-co-vinyl acetate)(EVA),Lupolen V5510SX (BASF),Lupolen V5510SX is a copolymer of ethylene with 33wt%vinyl acetate (VA).Buplen 7623is an i-PP/E copolymer containing up to 10wt%ethylene groups.The sample compositions are pre-sented in Table 1.All the blends contain 0.15wt%of stabiliser,com-prising 0.05wt%2,6-diter-buthyl-4-methylphenol and 0.1%Irganox 1010(thermostabiliser of Ciba Geigy,Switzerland).Melt blending of the granules was carried out on a Brabender plasticoder at 2258C.Tubular ®lms extruded at 1908C and rapidly cooled to room tem-perature.All samples were irradiated in a linear elec-tron accelerator (E-250,Russia)in air,with doses of 53,83,100,113,166kGy.An X-ray study was carried out with Co K a radi-Table 1Composition of the blends Sample No.Buplen 7623(wt%)Lupolen V5510SX (wt%)1010021089.8533069.8545049.8556039.8520Scattering angle,I n t e n s i t y [i m p /s ]1816141210863002001000110(p p )040(p p )130(p p )200(p e )200(p p )220(p p )111+131+041(pp)+110(pe)Fig.1.X-ray di ractogram of an unirradiated sample containing 60wt%PP.Experimental data (thick line),®tted data (individual curves).The di use scattering has been subtracted.M.Mihailova et al./Radiation Physics and Chemistry 56(1999)581±589582ation(Fe®lter)under conditions of symmetrical trans-mission.Co radiation was used for the better separ-ation of the peaks.For the elimination of orientation e ects,the specimens were rotated rapidly around an axis perpendicular to the plane of the®lm. Corrections,such as Lorenz,polarisation and absorp-tion factors,were applied and the di racted intensities were calculated by using the following formula:I P s a P I L a ÈD exp ÀD a cos YÉ1where I L is the di racted intensity,corrected for Lorenz and polarisation factors;D is the X-ray optical density of the specimen,P s is the di racted intensity of a standard at®xed experimental conditions and P is the same intensity measured immediately before each scanning of the sample.The optical density D was measured for each specimen(Alexander,1969a).The di racted intensities thus corrected can be considered as absolute and their values can be compared quanti-tatively.The FIT software(Petrov and Bakaltchev,1990)was used for peak centre®nding,calculation of integral intensity and determination of di use scattering. Experimental peaks were®tted with the Gaussian func-tion.The base line was approximated with a second-order polynomial.The strongest peak in the X-ray diagrams(Fig.1)is a superposition of peaks of PP111,131,041and the most intensive re¯ex110of PE.The best separated peaks are110of PP and200of PE.The crystal sizes, interplanar spacings and integral intensities of the samples with di erent contents of PP and irradiation doses were determined.The values of crystal sizes of EVA were calculated using Scherrer's equation.The interplanar distances were calculated with a precisionof20.002nm.Two clearly expressed peaks in the same crystallographic direction were observed for PP:110 and220.This observation was used for the separation of the peaks broadening caused by the e ects of para-crystallinity and of the small crystal sizes(Alexander, 1969b):B cos Y 2 1a L2hkl p g II 4m4a d2hkl 2where B is the width at half peak height,L hkl is the crystal size in the hkl direction,g P is the paracrystal-line factor de®ned as g P=D d/h d hkl i,m the order of dif-fraction.The most probable distance between molecular seg-ments of d am in the amorphous regions was obtained from the maximum of the amorphous halo according to Bragg's law.The degree of crystallinity was calculated using the method of Matthews(Matthews et al.,1949):X c I cr a I cr I am X 3Taking into account the di use scattering,the degree of crystallinity was computed using the equation:X c I cr a I dif I cr I am 4where I cr is the total integral intensity of the crystalline phase,I am the total integral intensity of the amorphous phase and I dif is an integral intensity of the di use scattering.3.Results and discussionX-ray di raction patterns obtained for®lms of in-itial unirradiated components are presented in Fig.2. Obviously,EVA is in the amorphous state and PP is in a``smectic''form(Stoyanov et al.,1989;O'Kane et al.,1994).At the same time,the X-ray patternsreveal Fig.2.X-ray di ractograms of pure components:(a)EVA, (b)PP.M.Mihailova et al./Radiation Physics and Chemistry56(1999)581±589583crystal peaks for all blends of EVA and i-PP (Fig.1).Generally,the PP is represented by a -monoclinic modi-®cation (Turner-Jones et al.,1964).All peaks of PP are shifted to smaller angles.The amount of g -phase is negligible,because its strongest peak 020is very weak even for samples with the highest content of PP.Re¯ections 110and 040of the a -phase are clearly recognised for all samples (Fig.1).Other peaks (130,131,200and 220)are also observed practically for all samples.b -Phase of i-PP is not detected.The most common orthorhombic crystals of poly-ethylene (Wunderlich,1973;Kavesh and Schultz,1970)are revealed by 110and 200re¯ections (Fig.1).The monoclinic phase (Wunderlich,1973)appears in some samples with very weak 200and 201re¯ections.Neither the interplanar spacings d 110of PP nor d 200of PE depend on the composition of the blends or on the irradiation dose.The calculated average interplanar spacing d 110of PP and d 200of PE are 0.632nm and 0.375nm respectively.The values are higher than those observed for the homopolymersÐ0.629nm for PP (Lotz et al.,1986;Turner-Jones et al.,1964)and 0.370nm for PE (Kavesh and Schultz,1970).Since both investigated polymers are copolymers,the pre-sence of a great number of crystal defects leads to an increase of the interplanar spacing.The lattice constant a of EVA,calculated from the 200orthorhombic re¯ection,is a =0.750nm and is higher than that for pure PE (a =0.741nm).Other authors have also found an increase of the a -axis with the VA-content (Kortleve et al.,1972;Bodily and Wunderlich,1966).It has been supposed that acetate groups form amorphous defects,involved in the crys-talline regions.Bodily and Wunderlich (1966)have suggested that these defects comprise seven CH 2-groups per acetate group,in addition to the branch point and the acetate group.According to Kortleve et al.(1972),at least 30%of the overall concentration of side groups are included in the crystalline regions.Crystallite sizes L 110and the paracrystalline factor g P of PP,calculated by Eq.(2),are given in Table 2.The crystallite sizes are of about 20nm.The average size of the crystallites of the irradiated samples is higher than that of the non-irradiated ones.The para-crystalline factor slightly increases with PP content at a constant dose.It is relatively low and varies from 0to 4%.The crystallite size of EVA crystals is calculated according to the Scherrer formula.The size varies by about 17nm and is not a ected signi®cantly by the concentration of PP and irradiation dose.The degree of crystallinity obtained from DSC measurements is about 30%,while the degree of crys-tallinity calculated from WAXS measurements,taking into account the di use scattering,is very high (50±60%).If the degree of crystallinity is computed accord-ing to Eq.(4),the value is almost the same as that cal-culated from DSC.As cited above (Kortleve et al.,1972;Bodily and Wunderlich,1966),the defects in the EVA crystals could be considered as micro-amorphous areas in the crystal lattice,resulting in an increase of the X-ray di use scattering (Cowley,1975).For this reason,the di use scattering should be taken into account,when the degree of crystallinity is calculated from WAXS measurements.Degree of crystallinity vs PP content is plotted in Fig.3.The degree of crystalli-nity decreases linearly with the rise of the PP content for all doses.The di use scattering can be used as a qualitative in-dication of the amount of crystal defects.The integral di use scattering decreases with the irradiation dose (Fig.4).Therefore,we conclude that irradiationTable 2Crystallite sizes and paracrystalline factors for PP in directions perpendicular to the planes (110)PP content (wt%)Dose (kGy)10%30%50%60%0L hkl (nm)13171717g P (%)±0 1.7 2.153L hkl (nm)19202017g P (%)± 2.2 2.0 3.083L hkl (nm)17201717g P (%)± 1.6 1.7 2.3100L hkl (nm)25202017g P (%) 1.9 1.6 2.00113L hkl (nm)14212617g P (%)± 2.0 2.5 2.7166L hkl (nm)22172520g P (%)1.92.4M.Mihailova et al./Radiation Physics and Chemistry 56(1999)581±589584reduces the number of crystal defects.Such e ects have been observed for irradiated with g -rays poly(-ethylene oxide)(Nedkov and Tsvetkova,1994)and PP (Stoyanov et al.,1989,Krestev et al.,1986).Annealing decreases the number of crystal defects and the growth of the long period.This process has been called ``radi-ation annealing''by Nedkov et al.The di use scatter-ing decreases abruptly for samples with higher PP content (Fig.4).Hence,the ``radiation annealing''takes place predominantly in PP crystals.According to Keller et al.(1983)clustering of cross-links occurs,resulting in the coexistence of two discrete phases in irradiated alkanes:one crystalline,free of cross-links,and the other fully amorphous,containing most of the cross-links.These authors have assumed that the radi-ation energy results in the formation of active species,60Quantity of PP [ wt.% ]X c r , %4020302520150 kG y60Quantity of PP [ wt.% ]X c r , %40203025201553 kG y60Quantity of PP [ wt.% ]X c r , %4020030252015100 kG y60Quantity of PP [ wt.% ]X c r , %40203025201583 kG y60Quantity of PP [ wt.% ]X c r , %4020030252015166 kG y60Quantity of PP [ wt.% ]X c r , %402030252015113 kG yFig.3.Degree of crystallinity X cr vs PP content at di erent irradiation dose.M.Mihailova et al./Radiation Physics and Chemistry 56(1999)581±589585which migrate rapidly through the crystal lattice.Nedkov and Tsvetkova (1994)have associated these migrating active species with removal of crystal defects through the crystal lattice after irradiation at low doses.The nature of these active species has not been speci®ed in any report cited above.It is impossible to propose a suitable mechanism of this process only on the basis of WAXS measurement studies.There is a probability that the samples undergo the usual thermal annealing during irradiation.It is impossible to separate the amorphous halo into two components in the X-ray patterns for all samples (Fig.1).According to Lipatov et al.(1982a)the maxi-mum of the amorphous halo of PP is at y Cu =8.58and for Co radiation,y Co =9.98.The intensity of the X-ray scattering at y Co =9.98is very weak even for samples with maximum PP content of 60wt%(Fig.1).There is only one amorphous halo with peak position near y Co =11.88.The electron irradiation does not change the nature of the amorphous scattering.A single amor-phous phase has been observed by other authors for two incompatible polymers (Sung-Hee Ahn et al.,1993).The single T g transition has been assumed as an evidence for that.The observed single amorphous halo in our X-ray patterns allow us to suggest that the PP and EVA amorphous chains are partially interpene-trated in the interface regions.The interpenetration of the amorphous chains of the components is probably favoured by the molecular structure of the PP copoly-mer containing short PE blocks (De Gennes,1982).The most probable distance between molecular seg-ments in the amorphous regions d am vs PP content is shown in Fig.5.If there is no interaction between the two phases,d am should be a linearly additive function of PP content.The observed dependence is not linear except for the sample irradiated with 166kGy.Hence,this is more proof of interaction between the com-ponents.A well-de®ned minimum is seen at 30wt%PP.Thomas et al.(1987)have shown that at this PP concentration the two phases form a continuous inter-penetrated network-like structure.From the thermo-mechanical studies of the same samples obtained by Minkova and Nikolova (1989b),it is evident that the thermal-shrinkable properties are most prominent for the specimens with 30/70wt%PP/EVA composition.All results mentioned above prove that the best expressed interaction between polymers is noticed at this composition.200Dose [ kGy ]D e f u s e s c a t t e r i n g1501005018001600140012001000800600Fig.4.Di use scattering vs irradiation dose for specimens with di erent PP content.M.Mihailova et al./Radiation Physics and Chemistry 56(1999)581±589586By increasing the irradiation dose,the d am values depend to a lesser extent on the PP content (Fig.5).The linear dependence of d am on PP content at a dose of 166kGy can be explained with predominating chain scissions over the cross-linkings in the PP phase (Dawes and Glover,1996).Lipatove et al.(1982b)have suggested a method for estimation of the interface layer thickness.According to this authors,polymer A is considered as sphericalparticles with radius r dispersed in the matrix of poly-mer B.Following this model,the integral intensity of re¯ection 110of PP should be a linear function of the PP content.The slope P of this function is:P Ktg a 5where K is K r 3a r t 36Quantity of PP [wt. %]d a m [ n m ]0.460.4460Quantity of PP [wt. %]d a m [ n m ]402000.460.4460Quantity of PP [wt. %]d a m [ n m ]402000.460.4460Quantity of PP [wt. %]d a m [ n m ]402000.460.4460Quantity of PP [wt. %]d a m [ n m ]402000.460.4460Quantity of PP [wt. %]d a m [ n m ]402000.460.44Fig.5.The most probable distance d am in the amorphous region vs PP content at di erent irradiation dose.M.Mihailova et al./Radiation Physics and Chemistry 56(1999)581±589587where t is the thickness of the interface layer,tg a depends only on the experimental conditions.In the case when t increases,the slope P decreases.As seen in Fig.6,the slope diminishes at higher irradiation doses.Obviously,irradiation has caused an increase of the interface layer thickness.Irradiation increases the con-centration of short chains and reduces the di erence inthe surface tensions between the two phases.The phase boundary becomes unstable and this causes the growth of the interface layer thickness.4.ConclusionsThe following explanations of our results are pro-posed.Interface layers between the components are formed during the mixing process.Highly dispersed EVA drops could serve as heterogeneous nuclei for PP crystallization.For this reason the ®lms prepared are amorphous (Fig.2),but all blends are semicrystalline (Fig.1).The crystallisation ®xes the created micro-het-erogeneous structure (Fig.7).We suppose that there are three di erent kinds of structure:relatively homo-geneous particles from PP and EVA copolymers,each of them consisting of crystalline and amorphous phases;interface regions with interpenetrating amor-phous chains of the two components and small crys-tals.PP and EVA crystallise separately.Each of the three regions could be considered as a network even before irradiation.The crystals serve as knots in this network.Irradiation leads to an increase of the thickness of the interface layer;up to 100kGy,it diminishes the imperfections in the crystal regions as can be seen by the decreasing di use scattering at higher irradiation doses (Fig.4).200Dose [ kGy ]S l o p e1501005001.11.00.90.80.7Fig.6.The slope P vs irradiation dose (explanation in text).EVA PPFig.7.Structural model of the interface between EVA and PP copolymers.M.Mihailova et al./Radiation Physics and Chemistry 56(1999)581±589588AcknowledgementsThe authors would like to thank the Foundation of Scienti®c Research,So®a University,``St.Kliment Ohridski'',for the®nancial support,and Dr Nikolova for the preparation of the samples.ReferencesAlexander,L.E.,1969a.X-Ray Di raction Methods in Polymer Science.John Wiley&Sons,Inc,New York,p.168.Alexander,L.E.,1969b.X-Ray Di raction in Polymer Science.John Wiley&Sons,Inc,New York,p.427. 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