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Radiation Physics and Chemistry 79 (2010) 745–750

Contents lists available at ScienceDirect

Radiation Physics and Chemistry

0969-80

doi:10.1

n Corr

E-m

pujari@

journal homepage: www.elsevier.com/locate/radphyschem

Free volume and microstructural investigation of poly(ethyleneterephthalate)-g-acrylic acid (PET-g-AA) copolymer films

S.K. Rath a, M. Patri a, S.K. Sharma b, K. Sudarshan b, P.K. Pujari b,n

a Naval Materials Research Laboratory, Shil-Badlapur Road, Addl. Ambernath, Ambernath 421506, Maharashtra, Indiab Radiochemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India

a r t i c l e i n f o

Article history:

Received 2 January 2010

Accepted 23 February 2010

Keywords:

Graft copolymers

Positron annihilation lifetime

Secondary relaxation

6X/$ - see front matter & 2010 Elsevier Ltd. A

016/j.radphyschem.2010.02.009

esponding author. Tel.: +91 22 25595326; fa

ail addresses: sudarshankathi@gmail.com (K.

barc.gov.in (P.K. Pujari).

a b s t r a c t

Acrylic acid (AA) was grafted onto PET films by preirradiation method using a 60Co g-radiation source.

Microstructural investigation of the PET-g-AA copolymers has been carried out by wide angle X-ray

diffraction (WAXD), dynamic mechanical analysis (DMA) and positron annihilation lifetime spectro-

scopy (PALS) techniques. The WAXD results showed only marginal variation in the crystallinity of the

graft copolymers. Dynamic mechanical analysis showed an increase in the temperature of the a-

relaxation and the tan d value of subroom temperature secondary relaxations. From the PALS results, a

complex variation of the free volume parameters, i.e. the o-positronium lifetime (t3), its intensity (I3),

fractional free volume (fv) and the intermediate lifetime component was observed for the graft

copolymers. Multiple phenomena of the effect of secondary relaxations, additive behavior of the

individual polymer free volume parameters and effects from interfacial defects have been invoked to

understand the fluctuational nature of the free volume properties in the graft copolymers.

& 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Modification of polymers by graft copolymerization of func-tional monomers is a well established technique to impartfavorable physical and chemical properties to the given polymers.Several techniques have been used to initiate the graft copoly-merization including ion radiation (He and Gu, 2003), ultravioletlight (Song et al., 2006; Chu et al., 2006), plasma treatment(Lazea et al., 2005; Gupta et al., 2002), etc. Among thesetechniques radiation induced graft copolymerization isadvantageous because of its extensive penetration into thepolymer matrix, and its ability to form radicals rapidly anduniformly for initiating the grafting in various kinds of polymers.

Poly(ethylene terephthalate) (PET) films are characterized bydetermining its properties like high crystallinity, thermal stabilityand hydrophobicity and presence of chemically inert functionalgroups (Karagiannidis et al., 2008). To get desirable properties,various functional monomers can be grafted onto PET (Avny et al.,1978). Radiation induced graft copolymerization of PET withvarious vinyl monomers such as styrene (Nasef, 2000, 2002),acrylic acid (Huglin and Zlatev, 1973), methacrylic acid(Uchida et al., 2000), 2-methyl 5-vinyl pyridine (Helbeish et al.,1978), etc. has been reported. Radiation induced graft

ll rights reserved.

x: +91 22 25505151.

Sudarshan),

copolymerization is well known to induce the structural changesin the host polymer matrix (Charlesby, 1960; Nasef, 2000). Suchchanges have a strong impact on the structural and hencethermo-mechanical properties of the polymers. Therefore, mon-itoring of these structural changes is important to evaluate theapplicability of the grafted PET films for various applications.

Positron annihilation lifetime spectroscopy (PALS) has foundincreasing applications in probing the molecular arrangement ofpolymeric materials (Pethrick, 1997; Jean, 1996; Bamford et al.,2006). Both static as well as dynamic free volume holes can beprobed by using PALS as long as the time scale of molecularmotion, responsible for the dynamic free volume holes, is in theorder of nanoseconds, which is the typical order of ortho-Ps (o-Ps)pick-off lifetime in polymers. The pick-off annihilation of o-Ps inthe spherical free volume hole can be described by a simplequantum mechanical model of infinite potential well with anelectron layer thickness of DR. The semiempirical relationbetween the radius of the free volume hole, R, and o-Ps lifetime(t3), pick-off annihilation rate (l3) is given by (Tao, 1972; Eldrupet al., 1981; Liu et al., 1995)

1

t3¼ l3ðns�1Þ ¼ 2 1�

R

RþDRþ

1

2p sin2pR

RþDR

� �� �ð1Þ

where DR¼1.66 A is an empirical parameter. The product of thehole volume, calculated from the radius determined using Eq. (1),and o-Ps intensity is directly proportional to the free volumefraction for chemically similar polymeric samples (Jean et al.,2003).

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S.K. Rath et al. / Radiation Physics and Chemistry 79 (2010) 745–750746

In the present study PALS is used to characterize PET-g-AAsamples with the degree of grafting in the range of 1.8–12%. ThePALS results have been correlated with the tan d values forb relaxation of the samples obtained from the dynamic mechan-ical analysis (DMA) and the variation in the crystallinity evaluatedfrom wide angle X-ray diffraction (WAXD). Though, there havebeen reports of PALS studies of the base polymer, PET, (Chen et al.,1999; Olson et al., 2003) graft copolymers of PET and other graftcopolymer systems (Sudarshan et al., 2007; Sanchez et al., 1993),to our knowledge this is the first report on the free volumeanalysis of PET-g-AA graft copolymers by PALS.

2. Experimental

2.1. Preparation

2.1.1. Materials

PET films of 50 mm thickness (DU PONT) were washed withmethanol to remove any impurities adhering to the surface. Thefilms were dried under vacuum at 60 1C before using them forgrafting. Acrylic acid (Merck India Ltd.) was distilled undervacuum before using for the experiments. Mohr’s salt wassupplied by Qualigens, India. Distilled water was used for all theexperiments.

2.1.2. Irradiation

Irradiation of PET films was carried out in air by using a 60Cogamma radiation source at a dose rate (0.18 kGy/h) to give a totaldose of 40 kGy to the samples

2.1.3. Graft copolymerization

Graft copolymerization of acrylic acid onto PET was carried outby preirradiation method. The grafting was carried out in glassampoules of 2�10 cm2 size. A weighed amount of the irradiatedPET films (�600 mg) were placed into the ampoules containingaqueous monomer (acrylic acid) solution having monomerconcentration of 40% (vol%) along with Mohr’s salt of 0.1% (wt%)to prevent the homopolymerization of the monomer. Nitrogen gaswas purged into the ampoules to remove air trapped inside thereaction mixture. The ampoules were subsequently placed in awater bath maintained at 50, 60, 70 and 80 1C for a constantreaction time of 10 h. After each reaction, the ampoule wasremoved and the sample was washed with boiling water toremove any homopoylmer adhering to the sample surface. Thesamples were dried in an oven at 60 1C under vacuum and thedegree of grafting was determined using the followingexpression:

Degree of grafting ðDGÞ ¼Wg�Wi

Wi� 100 ð2Þ

where Wi and Wg are the weight of the ungrafted PET and graftPET copolymer films, respectively.

2.2. Characterization

2.2.1. FTIR spectroscopy

FTIR spectra of the neat PET and the graft copolymer sampleswere recorded with a Nicholet Omnic 3 Fourier transform infraredspectrometer with a resolution of 4 cm�1. Signals from 16 scansof the same sample were averaged before performing Fouriertransform.

2.2.2. Dynamic mechanical analysis

The measurements of dynamic mechanical properties of theneat PET and PET-g-AA samples were carried out at a frequency of1 Hz and a heating rate of 5 1C/min on a Rheometric ScientificDMTA. The temperature range of the measurements was from�130 to 200 1C.

2.2.3. Wide angle X-ray diffraction

The crystallinity of the neat PET and the graft copolymersamples were evaluated using a Cu Ka ray of wavelength 1.54 Abetween 2y range of 3–901 at 0.21/10 S with a Phillips X-raydiffractometer.

2.2.4. Positron annihilation measurements

PALS measurements were carried out by using a fast–fastcoincidence system with plastic scintillation detectors. The timeresolution of lifetime spectrometer was 230 ps (60Co) with timecalibration of 25 ps/channel. 22Na, (10 mCi) deposited and driedbetween two kaptons films of thickness 8 mm was used aspositron source. The positron source was sandwiched betweenstacks of the polymer films to stop positrons within the polymerfilms. For each sample, three spectra of 1�106 counts wererecorded. The computer program, PATFIT-88 (Kirkegaard et al.,1989) was used for the analysis of the spectra. Each spectrum wasanalysed into three lifetime components (ti) with their correspondingintensities (Ii), where i¼1, 2, 3. The three components correspond top-Ps, free positron and o-Ps annihilation, respectively. The results foreach sample reported are the average of three measurements.

3. Results and discussion

The grafting of acrylic acid onto PET films was accomplished intwo consecutive steps, i.e., radiation activation of the PETbackbone followed by the grafting of the monomer (acrylic acid)onto the activated surface. The irradiation of the base polymer,PET, in the presence of oxygen induces formation of hydroper-oxide groups along the polymer chains (Botelho et al., 2001). Thelatter is then subsequently used for initiation of polymerization ofacrylic acid onto it. The various parameters to control the degreeof grafting are the reaction temperature, monomer concentration,total dose to the base polymer and the additives. In the presentstudy, the degree of grafting was varied by carrying out thereaction for 10 h at different temperatures. The variation ofthe degree of grafting of AA with reaction temperature is shownin Fig. 1. The degree of grafting increases with the increase of thereaction temperature. It may be mentioned that Mohr’s saltconcentration for the graft copolymerization has been selected onthe basis of the reported values for effective suppression ofhomopolymer formation (Gupta et al., 2008) during graftcopolymerization. The graft copolymers were found to betranslucent compared to the neat PET films, which wereperfectly transparent.

The graft copolymers were identified by FTIR spectral analysis.Fig. 2 shows a typical FTIR spectrum of the PET-g-AA graftcopolymer with 12.1% degree of grafting. A broad band in therange of 2300–3400 cm�1 is observed in the PET-g-AA polymers.This broad absorption corresponds to the stretching vibrations of�OH of grafted acrylic acid.

The graft copolymers were then characterized for theirmicrostructural and free volume variation by DMA, WAXD andPALS. Fig. 3 shows the tan d values, from DMA measurement, vs.temperature plot of the neat PET and the graft copolymersamples. From the results, two different relaxation peaks arenoticed for the neat PET as well as the PET-g-AA graft copolymers.Thus, the grafting of AA onto PET does not introduce any new

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500

2

4

6

8

10

12

Deg

ree

of g

rafti

ng (%

)

Reaction temperature (°C)55 60 65 70 75 80

Fig. 1. Variation of the degree of grafting (%) of acrylic acid with reaction

temperature. Reaction conditions: monomer concentration —40%; time —10 h.

The line is eye guide only.

400

-C=O (ester) 1739 cm-1-C=O (carboxyl) 1709 cm-1

-OH

stre

tchi

ng

Abo

srba

nce

(a.u

)

Wave Number (cm-1)800 1200 1600 2000 2400 2800 3200 3600 4000

Fig. 2. Typical FTIR spectrum of PET-g-AA (12.1% degree of grafting).

-150

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

tan

δ

Temperature (°C)

PET 1.8 % 4 % 6 % 12.1 %

-100 -50 0 50 100 150 200

Fig. 3. tan d vs. temperature (1C ) plot of the neat PET and the PET-g-AA

copolymers evaluated from DMA.

S.K. Rath et al. / Radiation Physics and Chemistry 79 (2010) 745–750 747

relaxation behavior. However, the peak positions and thecorresponding magnitude of the relaxation peaks are affectedeven at low degree of grafting. The relaxation peak positions forPET observed in the present work are matching with thoseobserved by McGonigle et al. (2000) in their study of the influenceof physical ageing on the structural relaxation and molecularmotion in PET and related materials. The glass transitiontemperature or the a-relaxation of the samples are observed inthe temperature range of 100–130 1C, while the subroomtemperature secondary relaxations are observed in the range of�130 to 25 1C. The glass transition temperature (a relaxation)increases monotonically with the increase in the degree ofgrafting. This can be attributed to the incorporation of glassyacrylic acid graft chains onto PET matrix. The b relaxation(subroom temperature transition) position does not shift withthe increase in the degree of grafting. The magnitude of theb-relaxation peak decreases marginally in 1.8% grafting while itmarginally increases at 4% and 6% grafting. A discernible increasein the b relaxation is observed for 12% grafting. This suggests thatthe introduction of acrylic acid grafts increase the molecularmotion in the PET base polymer.

Fig. 4 shows the storage modulus vs. temperature plot for theneat PET and the PET-g-AA samples. From the figure, it is seen thatthe graft copolymers have lower modulii compared to the basepolymer. However, no particular trend in the variation of themodulus with the degree of grafting is observed. The 12% graftedsample, contrary to our expectation, has the highest modulusvalue among the graft copolymers followed by the 6% graftcopolymer, while the 4% graft copolymer has the lowest modulus.

The variation in the degree of crystallinity induced by thestructural changes during radiation induced grafting of AA ontoPET was investigated by using wide angle X-ray diffraction(WAXD). Fig. 5 shows the diffractograms of the PET-g-AAsamples compared to the neat PET film. The relative crystallinityof all the samples was evaluated from the WAXD plots. The peakswere fitted to the Lorentzian distribution and the relativecrystallinity was calculated from the area under the crystallineand amorphous peaks. Only a marginal change (decrease) inthe relative crystallinity of the graft copolymers was observedcompared to neat PET which can be well within the error limits ofthe peak fitting procedure. However, with the increase in thedegree of grafting, a slight shift of the diffractograms to higherBragg angles is observed compared to the base polymer. Sincegrafting occurs only in the amorphous region of thesemicrystalline base polymers and the present degree of

-100100

1000

10000

Sto

rage

Mod

ulus

(MP

a)

Temperature (°C)

PET 1.8 % 4 % 6 % 12.1 %

-50 0 50 100 150

Fig. 4. Storage modulus (MPa) vs. temperature (1C) plot of the neat PET and the

PET-g-AA copolymers evaluated from DMA.

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212θ

Inte

nsity

(a.u

)

PET 1.6%

4%

6 % 12%

0

0

30000

60000 25.87608

B

A

22 23 24 25 26 27 28

20 40 60

Fig. 5. WAXD plot of the neat PET and the graft copolymers. Inset shows a typical

peak fitting of the WAXD plot to evaluate the relative crystallinity.

0

1.68

1.69

1.70

1.71

1.72

o-P

s lif

etim

e (τ

3 in

ns)

Degree of grafting (%)

011.5

12.0

12.5

13.0

13.5

14.0

14.5

15.0

15.5

o-P

s In

teni

ty (I

3 in

%)

Degree of grafting (%)

2 4 6 8 10 12

2 4 6 8 10 12

0

1.50

1.55

1.60

1.65

1.70

1.75

1.80

1.85

1.90

1.95

fract

iona

l fre

e vo

lum

e (f v

in %

)

Degree of grafting (%)

C

2 4 6 8 10 12

Fig. 6. (A) o-Ps lifetime, (B) o-Ps intensity and (C) fractional free volume in

PET-g-AA samples.

S.K. Rath et al. / Radiation Physics and Chemistry 79 (2010) 745–750748

grafting extends only up to 12.1%, the alteration of the crystalstructure of PET with the degree of grafting is not expected.

The o-Ps lifetime and intensity (t3 and I3), as a function of thedegree of grafting are shown in Fig. 6A and B, respectively. Theo-Ps lifetime, t3, is correlated to the free volume holes’ radius byusing Eq. (1) and the intensity, I3, is proportional tothe concentration of these free volume holes present in thepolymer. The t3 and I3 values of the base polymer, PET, were1.68 ns and 15.4%, respectively. The t3 and I3 values of the graftcopolymers were higher compared to the base polymer. However,the variation is not smooth. At 1.8% grafting the o-Ps lifetimeremains unchanged compared to the base polymer followed by anincrease up to 6% grafting, while 12.1% grafting registers a sharpdrop in the o-Ps lifetime. From the variation of the intensity withthe degree of grafting, we observe that the graft copolymers havelower I3 values compared to the base polymer, however, thedecreasing pattern is not continuous. A sharp drop of intensity at1.8% grafting is followed by an increase for 4% and 6% andsubsequently the lowest value is registered for the 12.1% graftcopolymer. The variation of the fractional free volume (fv) withthe degree of grafting is presented in Fig. 6C. The fractional freevolume (fv) has been calculated using the equation (Wang et al.,1990)

fvð%Þ ¼ CVf I3 ð3Þ

where Vf is the volume of spherical free volume holes calculatedby using the radius (Eq. (1)), I3 (in %) is the o-Ps intensity and C istaken as 0.0018 nm�3 from the specific volume data (Wang et al.,1990). The fractional free volume variation follows the same trendas the intensity, with the increase in the degree of grafting.

Thus, there is a complex variation in the free volumeparameters with the degree of grafting. However, strictly onqualitative terms, an increase in o-Ps lifetime with a decrease inintensity is observed for the graft copolymers, compared to thebase polymer. It must be noted that the reported values of t3 andI3 for polyacrylic acid are 1.95 ns and 7.6%, respectively (Gomma,2007). Thus, the base polymer, PET, has more number of smallerholes, while the acrylic acid has lesser number of bigger holes.Purely on additive terms, graft copolymerization should result inexpansion of the average hole size and reduction in the number of

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0

390

400

410

420

430

Degree of grafting (%)

042

44

46

48

50

52

54

56

58

Degree of grafting (%)

τ 2 (p

s)I 2

(%)

2 4 6 8 10 12

2 4 6 8 10 12

Fig. 7. (A) intermediate lifetime, t2, (B) intensity, I2, in the PET-g-AA samples.

S.K. Rath et al. / Radiation Physics and Chemistry 79 (2010) 745–750 749

holes in the graft copolymers. Until 6% grafting the trend seems tofollow the simple additive rule, with an increase in the o-Pslifetime and a decrease in the intensity. This is also supported bythe subroom temperature secondary relaxation behavior of thegraft copolymers, observed from DMA, as a definitive relationbetween free volume properties and secondary relaxationsreported earlier (Chen et al., 1999; Sudarshan et al., 2007).However, at 12.1% grafting there is a simultaneous decrease in thet3 and I3 values, though it shows maximum increase in thesecondary tan d relaxation. It seems dynamic transition alonecannot account for the trend reversal.

The complicated variation of free volume parameters in thegraft copolymers can be due to the sensitivity of positron/Ps notonly to the preexisting free volume holes in the PET backbone andthe AA grafts but also to any interfacial spaces, such as thosecreated between boundaries of two phases, viz. PET backbone andAA grafts. This phenomenon has been reported especially forimmiscible blends (Liu et al., 1995). The graft copolymers ofhydrophobic PET base polymer and the hydrophilic acrylic acidare expected to have a complex phase separated morphology. The

effect of the newly formed interfacial spaces on the variation ofthe free volume parameters can be observed from the variation ofthe intermediate lifetime and intensity components, t2 and I2,with the degree of grafting, as shown in Fig. 7A and B,respectively. The intermediate lifetime component is attributedto the annihilation characteristics of free or trapped positrons ininterfacial spaces. The sharp increase in t2 at the higher degree ofgrafting substantiate our contention that there is trapping ofpositrons in the interfacial spaces which do not form positroniumand it is more evident with the increase in the degree of grafting.Thus, the complicated variation in PALS parameters can bereasonably likened to similar behavior in the immiscible blends.(Liu et al., 1995). However, the free volume hole propertiesobtained in these systems of more than one phases can be furtherinterpreted only when we study graft copolymers at higherdegree of grafting with different base and graft copolymers.

4. Conclusion

PALS was found to be a useful technique to study the freevolume hole properties and packing at the molecular level forPET-g-AA copolymers at low degree of grafting. The variation inthe free volume parameters had a direct correlation withstructural transition from DMA up to 6% grafting. The free volumeparameters followed simple mixture rule. At higher degree ofgrafting, a marked deviation in I2 was observed with a sharp dropin fractional free volume. Thus, we speculate that at higher degreeof grafting interfacial effects has a dominant role on positronbehavior. There is a need to investigate graft copolymers with awider range of grafting and on different base polymers by PALS togain more insight into their positron dynamics.

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