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Polymer Degradation and Stability 94 (2009) 584–590

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Polymer Degradation and Stability

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

Determination of scission, crosslinking and branching parameters of electronbeam irradiated methacrylate–acrylamide copolymer

Khelil Slimani a, Laurence Moine b,*, Caroline Aymes-Chodur a, Alexandre Laurent c, Denis Labarre b,Najet Yagoubi a

a Universite Paris-Sud11, EA 401, IFR 141, Faculte de Pharmacie, 92296 Chatenay-Malabry, Franceb Universite Paris-Sud11, UMR 8612 CNRS, IFR 141, Faculte de Pharmacie, 92296 Chatenay-Malabry, Francec Service de Neuroradiologie et Angiographie Therapeutique, AP-HP Hopital Lariboisiere, 75010 Paris, France

a r t i c l e i n f o

Article history:Received 20 November 2008Received in revised form8 January 2009Accepted 12 January 2009Available online 20 January 2009

Keywords:IrradiationScissionCrosslinkingBranchingPoly(methacrylate-co-acrylamide)

* Corresponding author. Tel.: þ33(0)146835994; faE-mail address: laurence.moine@u-psud.fr (L. Moi

0141-3910/$ – see front matter � 2009 Elsevier Ltd.doi:10.1016/j.polymdegradstab.2009.01.011

a b s t r a c t

The aim of this work was to investigate the impact of electron beam irradiation at different dose rates onthe molecular structure of linear methacrylate–acrylamide copolymer. In the first part, the radiationchemical yields of scission (Gs) and crosslinking (Gx) have been determined after irradiation for variousinitial molecular weights CL1 (40 000 g/mol), CL2 (90 000 g/mol) and CL3 (425 000 g/mol). Based on thiscalculation, solvent (ethanol) was found to increase the impact of irradiation especially at low concen-tration of copolymer. In the second part, the presence of branching in samples before and after e-beamirradiation was explored, and branching calculation was performed.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Ionizing radiation is often used in various applications and isone suitable method to obtain hydrogel for biomedical applicationswithout having recourse to any crosslinker or initiator [1,2]. Irra-diation is also used as an alternative method able to modify poly-mers’ surface properties and to enhance the surfacefunctionalisation of some biomaterials [2,3]. Grafting reactions onpolymers by irradiation techniques are also efficiently performed[4,5]. Furthermore, this technique is an effective way for steri-lisation of biodegradable medical devices [6] or drugs [7–9] that donot support traditional sterilisation such as steam sterilisation.However, this technique may alter polymer properties because ofcrosslinking and/or scission reactions that could occur on backbonechains [10,11]. The scission reaction leads to the degradation ofpolymers by rupture of covalent bond inducing a decrease ofmolecular weight and intrinsic viscosity. In contrast, crosslinkingreactions lead to the formation of inter- and intramolecular bonds,which increases molecular weight and intrinsic viscosity. For highcrosslinking yields, gelation may occur [12]. According to thechemical structure of polymers, crosslinking or scission will

x: þ33(0)146835312.ne).

All rights reserved.

predominate. The absence of tri or tetra substituted carbon atoms,as in polyethylene, polypropylene, polystyrene promotescrosslinking. Polymers with lateral side chains, like poly(methylmethacrylate) preferentially show scission reactions [13]. In addi-tion to the chemical structure, these two effects depend on otherparameters such as the physical state of the polymer (crystalline oramorphous solid, solution), the temperature and irradiationatmosphere [10]. Indeed, the presence of oxygen during irradiationtends to favour scission reactions [10,12,14] due to the oxidativeaction of formed peroxides [15]. Irradiation involves the formationof radicals that tend to react more rapidly with oxygen than withmolecular chains. The radical recombination or diffusion is stronglyaffected by molecular mobility. When temperature rises close tothe glass transition of the polymer, chain mobility and oxygendiffusion in the structure rise and may have different impact on thekinetics of radical decay [16,17].

In this study, we focused on the effect of irradiation by means ofelectron beam on a methacrylate–acrylamide copolymer. Thiscopolymer has shown suitable properties to be used like an effec-tive material in embolization treatment of hypervascularizedprocesses such as arterio-venous malformations and tumours orvein dilations such as varicoceles [18].

Previous studies [19–22] have shown that ionizing radiationon polymethacrylates like poly(methyl methacrylate) or

O

CH3

O

OHH3C

CH2 C

H

C

H2C

ONHCH3C CH3CH3

n p

Fig. 1. Molecular structure of hydroxypropyl methacrylate-co-N-tert-butylacrylamidecopolymer.

K. Slimani et al. / Polymer Degradation and Stability 94 (2009) 584–590 585

poly(ethyl methacrylate) lead to scission. Shultz et al. [20] foundfor the poly-n-alkyl methacrylate series that there wasa competitive effect between scission and crosslinking reactions:an increase of the number of carbon atoms in pendent n-alkylgroups leads to an increase of crosslinking. Acrylamide polymersunlike methacrylate polymers show preference for crosslinkingunder gamma treatment. Using this property, hydrogels weresuccessfully synthesized after irradiation of acrylamide mono-mers [23–26]. Gel formation was also obtained from poly-acrylamide irradiated in solid state [27] and the initial moisturecontent revealed a high impact on the yield of crosslinking [28].

These two opposite behaviours between acrylamide andmethacrylate polymers under irradiation do not let us predictwhich reaction (scission and/or crosslinking) will predominateafter irradiation of our copolymer. We have tried to elucidate thebehaviour of the copolymer after irradiation by investigating themolecular weight distribution and determining the radiationchemical yields of scission (Gs) and crosslinking (Gx).

The presence of branching in polymers before and after e-beamirradiation was also explored.

2. Experimental

2.1. Materials

Monomers, reagents and solvents were used as receivedwithout any treatment. Hydroxypropyl methacrylate (HPMA 97%)was purchased from Aldrich (Saint Quentin Fallavier, France) and N-tert-butylacrylamide (TBA: 97%) and 2,20-azobisisobutyronitrile(AIBN 98%) from Acros Organics (Halluin, France).

2.2. Sample preparation

The copolymer hydroxypropyl methacrylate-co-N-tert-butyla-crylamide (Fig. 1) was obtained by bulk polymerization of HPMAand TBA in ethanol using AIBN as initiator, under nitrogenatmosphere at 85 �C during 4 h. Ratio of AIBN and volume ofsolvent was varied to obtain different molecular weights (Mw).Synthesis conditions and results are summarized Table 1. Threecopolymers having the same ratio of [HPMA]/[tBA] (70/30) butwith different Mw were obtained: CL1 (40 000 g/mol), CL2(90 000 g/mol), CL3 (425 000 g/mol). The polydispersity indexes(PI) were 1.9, 2.5 and 2.7 respectively.

Table 1Polymerization conditions of copolymers CL1, CL2 and CL3.

Sample Initial monomerscomposition (10�3 mol)

AIBN (10�3 mol) E

HPMA TBA

CL1 57 24 0.97 3CL2 226 97 0.97 3CL3 57 24 0.12

2.3. Analysis

2.3.1. SECMolecular weight distributions of the copolymers were

measured by Size Exclusion Chromatography (SEC) at 30 �C ona system equipped with a guard column and two GMH HRmcolumns (Viscotek, Irigny, France) with THF as eluent and flow rateof 1 ml min�1. A differential refractometer (Waters 410) anda double SEC detector (model 270 Dual, Viscotek) with RALS andviscometer in series were used to analyze the samples. The dataobtained were treated with OmniSEC 4.0 software (Viscotek).Branching index (g) of copolymers were also calculated from theOmniSEC software.

2.3.2. 1H NMR spectroscopyProducts were analyzed by 1H Nuclear Magnetic Resonance

(NMR) spectroscopy using a Bruker DPX 300 FT-NMR spectrometeremploying the solvent peak as reference. Analyses were performedon solutions of polymer in (CD3)2SO.

Copolymer composition was measured by 1H NMR and calcu-lated according to Eq. (1):

%HPMA ¼a3

a3þ

b� 83a

12

� 100 (1)

where a and b are the integral values of the peaks at d¼ 2.8–3.6 ppm and 0.2–1.8 ppm respectively. Results of monomer ratiomeasurement are summarized in Table 2.

2.3.3. IrradiationIrradiations were carried out under electron beam (e-beam)

(IONISOS – Orsay – France). Samples were irradiated at a constantdose rate of 300 kGy/h in air at room temperature to get total dosesof 25, 50, 75, 100, 125 and 150 kGy.

All samples CL1, CL2, CL3 were irradiated in solid form and CL2sample was also irradiated in ethanol solution at differentconcentrations: 10%, 20% and 30% (w/v).

3. Results and discussion

3.1. Analysis of scission/crosslinking effect

3.1.1. Irradiation in the solid stateWe studied the effect of the irradiation of the methacrylate–

acrylamide copolymers of different molecular weights (CL1, CL2and CL3) as a function of the absorbed dose (25, 50, 75, 100, 125 and150 kGy). The intrinsic viscosities of irradiated ([h]irr) and corre-sponding non-irradiated ([h]0) copolymers in THF at 30 �C weremeasured and the ratio ([h]irr/[h]0) as the function of the absorbeddose was plotted (Fig. 2). This ratio was found in all cases belowunity and decreased with the doses suggesting that scission ofmain-chain polymer occurred under e-beam treatment.

Impact of irradiation on copolymers having different molecularweights was done by analyzing the variation of radiation chemical

thanol (ml) Conversion (%) Mw (g mol�1) PI

60 81 40 000 1.960 85 90 000 2.530 88 425 000 2.7

Table 2Composition of copolymers.

Sample Initial monomer’scomposition

Copolymercompositiona (% mol)

HPMA TBA HPMA TBA

CL1 70 30 69 31CL2 70 30 72 28CL3 70 30 70 29

a Determined by 1H NMR spectroscopy.

0.8

1.3

1.8

2.3

2.8

0 25 50 75 100 125 150 175Dose (kGy)

Mw

0/M

w

CL1CL2CL3

Fig. 3. Graphical determination of s for irradiated copolymers CL1, CL2 and CL3 in solidform.

K. Slimani et al. / Polymer Degradation and Stability 94 (2009) 584–590586

yield of scission Gs and crosslinking Gx per 100 eV of irradiation. Thetheory of random scission and crosslinking in polymer chainsdeveloped by Charlesby [12] and Saito [29] using statisticaltreatment proposed the following Eqs. (2)–(5) to calculate Gs and Gx

[12,29–31].

Mnð0ÞMnðDÞ

¼ 1þ mðs� cÞD (2)

Mwð0ÞMwðDÞ

¼ 1þ mðs� 4cÞD (3)

Gs ¼ 9:65� 109s=M1 (4)

Gx ¼ 9:65� 109c=M1 (5)

with Mn(0): initial average number molecular weight, Mn(D):average number molecular weight after radiation dose D, Mw(0)

initial average molecular weight, Mw(D): average molecular weightafter irradiation dose D, m: average number of monomer units inpolymer chain (degree of polymerization), M1 is the molecularweight of the monomer unit, s and c are the respective probabilitiesof scission and crosslinking per Gray (Gy) per monomer unit withmolecular weight M1.

These mathematical quantitative expressions of irradiationmodifications involve the following assumptions:

� All polymers should be linear.� Initial polymers have sufficiently long-chain length to neglect

effects of end groups.� Gs and Gx are dose independent.� At high radiation doses, modifications of the chemical structure

and physical state of the polymer may happen andconsequently the measured chemical yields may differconsiderably from those of the initial polymer.

0.5

0.6

0.7

0.8

0.9

1.0

0 25 50 75 100 125 150 175Dose (kGy)

CL1CL2CL3

ηirr/ηo

Fig. 2. Variation of the ratio of intrinsic viscosity with irradiation dose for copolymersCL1, CL2 and CL3 irradiated in solid form.

� Determination of Gs and Gx is appropriate with samplessubjected to radiation doses below a critical gel dose Dg [30,31].

s and c values were obtained graphically from Eqs. (2) and (3).A value of Gx¼ 0 was obtained indicating that no crosslinkingoccurred during irradiation process (c¼ 0). Consequently, scissioneffect is the major effect of irradiation. So by replacing c in Eq. (3) by0, a simplified Eq. (6) [30] could be used to calculate s from theslope of Mw(0)/Mw(D)¼ f(D) (Fig. 3).

Mwð0ÞMwðDÞ

¼ 1þ msD (6)

After calculation of Gs using the graphical determination of s, wecould see in Table 3 that the chemical yield of scission is slightlyaffected by the molecular weight of the copolymers studied here.So, the intensity and probability to break a bond are quite similarwhatever the range of explored molecular weight. Nevertheless, itis important to note that random scission has a much greater effecton high molecular weight polymer with long chains than onpolymer of low molecular weight with short chains. Indeed, longchains show more fracture by electron beam radiation due to thepresence of more active sites (scissionable bonds).

3.1.2. Irradiation in the liquid stateAs for the solid form, determination of intrinsic viscosity of

sample CL2 (90 000 g/mol) before and after irradiation lead to([h]irr/[h]0)< 1 showing that scission was also the major eventoccurring under e-beam treatment in liquid state whatever be theconcentration (Fig. 4).

Calculation of the scission yield from Eq. (6) always gave higherGs values in liquid form than in the solid state (Fig. 5 and Table 4).Moreover, this effect increased when the concentration of thepolymer solution decreased. The presence of solvent during theirradiation treatment led to the formation of radicals by radiolysisof this latter that favoured the scission reaction of polymer chains[12]. This effect was more pronounced in dilute solution due to thehigher mobility of polymer chains and to the higher diffusion offormed radicals. Moreover, in this study, the irradiation of CL2

Table 3Radiation chemical yield of scission (Gs) of copolymers CL1, CL2 and CL3.

Sample Mw (g/mol) Radiation chemical yieldof scission (Gs)

CL1 40 000 0.4CL2 90 000 0.5CL3 425 000 0.6

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

0 25 50 75 100 125 150 175 D (kGy)

CL2 10%CL2 20%CL2 30%

ηirr/ηo

Fig. 4. Variation of the ratio of intrinsic viscosity with irradiation dose for copolymerCL2 irradiated in liquid form.

Table 4Radiation chemical yield of scission (Gs) of copolymer CL2 in ethanol at differentconcentrations.

Sample concentration(w/v)

Radiation chemical yield ofscission (Gs)

10% 1.120% 0.830% 0.8

K. Slimani et al. / Polymer Degradation and Stability 94 (2009) 584–590 587

copolymer has been done in presence of oxygen. It is well knownthat an oxidative atmosphere could lead to polymer chain scissionby the action of radicals such as alkoxyl (RO�) or peroxyl (ROO�) [15].The presence of solvent may also promote this oxidative effect bydiffusion of oxygen in the vicinity of polymer [12]. The work ofAcharya [32] on the behaviour of hydroxymethyl acrylamide inaqueous solution under ionizing radiation have established thatformation of gel occurred only in N2 saturated solution confirmingthe effect of oxidative action by formation of peroxy species. So inour study, we could assume that chain scission reactions wereenhanced by the simultaneous presence of ethanol and oxygen. Anincrease of the copolymer concentration seems to involvea ‘‘protective’’ action from oxidative action by hindering the diffu-sion of radicals due to the viscosity of the medium.

Despite the presence of acrylamide units (30%) in the copolymerchain, no crosslinking reaction has been observed (Gx¼ 0) whileseveral studies [23,27,32,33] have found that acrylamide polymerstends to crosslink under irradiation. So, the behaviour of ourcopolymer under electron beam is mainly controlled by themethacrylate moieties.

-1,5

-1

-0,5

0

Lo

g [η]

A

3.2. Effect of irradiation dose on structure of the copolymer –analysis of branching

Long-chain branches can be introduced into linear polymerunder electron beam irradiation without crosslinking [34]. Asbranching is intrinsically related to the chemical and physicalproperties of the molecule, the presence of long-chain branching

0.9

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

0 25 50 75 100 125 150 175D (KGy)

Mw

0/M

w

CL2 10% (w/v)CL2 20% (w/v)CL2 30% (w/v)

Fig. 5. Graphical determination of s for irradiated copolymer CL2 in liquid form atdifferent concentrations.

(LCB) [35] affects the viscoelastic properties such as the intrinsicviscosity, sedimentation behaviour and angular distribution ofscattered radiation from dilute solutions, as well as the viscosityand elasticity of melts. Thus, to complete this study of the impact ofirradiation on polymer structure and to have a better under-standing of the recombination phenomenon, an analysis of thebranching has been done.

A variety of analytical techniques have been used to study thedifferent types of branching present on a polymer. But mainly sizeexclusion chromatography (SEC) and rheological measurementsare both sensitive methods to study LCB in the molecule havingbranch lengths of about 20 carbons or more [36].

Formation of LCB can be pointed out qualitatively on the Mark–Houwink (MH) plot, revealed by a deviation (breaking) of the slopefrom linearity. Moreover, the presence of a shoulder in the SECchromatograms can also be detected when LCB is present.

To quantify LCB, the branching index (g) could be calculated bySEC using the Zimm–Stockmayer [37] approach which defines g bythe ratio of mean-square radius of gyration of a branched polymerCR2

g DB to that of linear polymer CR2g DL (Eq. (7)).

g ¼ D

R2g

EBD

R2g

EL

!M

(7)

Values of intrinsic viscosities drop with increasing branching andbased on this rheological feature, Thurmond and Zimm [38]proposed Eq. (8) for branching factor calculation:

-2

-1,5

-1

-0,5

0

0,5

3 4 5 6 7Log [M]

Lo

g [η]

B

-23 4 5 6 7

Log [M]

Fig. 6. Mark–Houwink plot for non-irradiated copolymer CL1 (A) and irradiatedcopolymer CL1 in solid form at 150 kGy (B). The dashed line shows the Mark–Houwinkrelation for linear copolymer determined from the narrow weight distribution byOmniSEC (Viscotek).

-1,4

-1,2

-1

-0,8

-0,6

-0,4

3 3,5 4 4,5 5 5,5 6Log [M]

B

-1,9

-1,4

-0,9

-0,4

3 3,5 4 4,5 5 5,5 6Log [M]

A

-1,5

-1

-0,5

0

3 4 5 6Log[M]

C

Lo

g [η]

Lo

g [η]

Lo

g [η]

Fig. 7. Mark–Houwink plot for non-irradiated copolymer CL2 (A), irradiated copol-ymer CL2 in solid form at 150 kGy (B) and irradiated copolymer CL2 in liquid form (10%w/v) at 150 kGy (C). The dashed line shows the Mark–Houwink relation for linearcopolymer determined from the narrow weight distribution by OmniSEC (Viscotek).

-2

-1,5

-1

-0,5

0

0,5

3 4 5 6 7 8Log [M]

B

-2

-1

0

1

4 5 6 7 8Log [M]

A

Lo

g [η]

Lo

g [η]

Fig. 8. Mark–Houwink plot for non-irradiated copolymer CL3 (A) and irradiatedcopolymer CL3 in solid form at 150 kGy (B). The dashed line shows the Mark–Houwinkrelation for linear copolymer determined from the narrow weight distribution byOmniSEC (Viscotek).

10 12 14 16 18Retention volume (ml)

Fig. 9. Molecular weight distribution as a function of elution volume for copolymerCL3.

K. Slimani et al. / Polymer Degradation and Stability 94 (2009) 584–590588

g ¼ ðg0Þ1=3 (8)

with

g0 ¼�½h�B½h�L

�M

(9)

where [h]B is the intrinsic viscosity of branched polymer, [h]L theintrinsic viscosity of linear polymer and 3 the viscosity shieldingratio. This equation assumes that theta conditions (q) are satisfied.The 3 parameter depends on temperature, solvent and nature ofbranching [39]. Different values of 3 have been reported, 3¼ 0.5 forstar branching, 1.5 for long-chain comb and between 0.5 and 1.5 forintermediate branching structures [40] but the majority of 3 valuesfor randomly branched polymer were found between 0.7 and 0.8[35,41].

The branching index (g) defined in both Eqs. (7) and (8) requiresthe knowledge of the radius of gyration and/or the intrinsicviscosity of an appropriate linear standard which has the samechemical structure and covering the same molecular weight regionof the branched polymer.

Actually, the SEC triple detection (SEC3) coupled with differen-tial refractometer, right angle light scattering and viscometer isa powerful method to characterize and quantify polymer branching[42,43]. This method can also give important information about thebranching distribution across the molecular weight distribution(MWD) [44].

Results of SEC3 showed no deviation of the slope from linearityof the MH curves for CL1 (40 000 g/mol) and CL2 (90 000 g/mol)before and after e-beam treatment at 150 kGy neither in the solidnor in the liquid form (Figs. 6 and 7). Thus, the irradiation did notcreate long-chain branching for these two copolymers (CL1 andCL2) confirming our previous results that the major effect ofirradiation was scission of the main chain with no importantrecombination involving great modification of polymer architec-ture. In contrast, the irradiated and unirradiated copolymer CL3(425 000 g/mol) revealed the presence of LCB in the range of highmolecular weight (Fig. 8). This is confirmed by the presence ofa shoulder on the SEC chromatogram at low elution volumes(Fig. 9).

For the copolymer CL3, an estimation of the branching wasdone. Since the determination of the radius of gyration was verydifficult due to the low signal to noise ratio, the branching factorwas calculated from Eq. (8) using the intrinsic viscosity instead ofthe radius of gyration. However, as said previously, the knowledgeof the intrinsic viscosity of a corresponding linear reference isnecessary for the determination of g. In our case, it has not been

Table 5Influence of irradiation dose on the molecular weight, polydispersity index (PI) and branching index for copolymer CL3.

Dose (kGy) 0 25 50 75 100 125 150

Mw (g/mol) 425 000 376 600 277 700 243 900 210 400 188 000 160 200Branching (g) 0.74 0.84 0.90 0.93 0.95 0.94 0.85Polydispersity index (PI) 2.7 2.5 2.6 2.4 2.3 2.2 2.2

K. Slimani et al. / Polymer Degradation and Stability 94 (2009) 584–590 589

possible to synthesize the corresponding linear reference of CL3. So,an estimation of its intrinsic viscosity value was done from the MHparameters. Their determination was done from the MH plot by theextrapolation of the low molecular weight portion of the copol-ymer which is assumed to be non-branched. Moreover, for thecalculation of g, we have made the assumption that we were in thepresence of a randomly branched polymer which corresponds toa value of 3¼ 0.75 [42,43]. As seen from Table 5, the branchingindex increases with irradiation dose until 125 kGy indicating thatthe copolymer becomes less branched after irradiation. Theseresults confirm the predominance of scission reactions oncrosslinking reactions despite the presence of initial branching inthe sample.

It is also interesting to underline that the size distribution(PI) of the chains of CL3 decreases with the irradiation dose(Table 5) meaning that the number of long branches per poly-mer molecule decreases with the dose (chain-shorteningreactions).

At the specific dose rate of 150 kGy in the case of the copol-ymer CL3, the branching index value is found lower than theprevious ones at lower doses (Table 5). This change may indicatethat at higher dose rate the degradation mechanism graduallychanges from chain scission to chain branching probably due tothe higher amount of radicals formed at the same time. It resultsin more recombination of radicals and thus fewer chain scissions.So, the increase of branching at 150 kGy seems to reflect thebeginning of crosslinking reactions, whereas the calculation ofradiation chemical yield of crosslinking Gx was found equal to 0.So, branching calculation can be a more accurate method to detectthe dose rate at which the crosslinking starts to take place.Consequently at this dose rate, scission and branching reactionsoccur in same time and rise in the dose rate may enhance thecrosslinking reaction.

4. Conclusions

The present work reveals that scission is the major effect ofelectron beam irradiation of hydroxypropyl methacrylate-co-N-tert-butylacrylamide copolymer. No impact of the variation of molecularweight was found on the radiation chemical yield of scission.However, the presence of solvent (ethanol) during irradiation leads toan increase of the radiation chemical yield of scission and thisphenomenon is increased in dilute solutions. Calculation of branch-ing index g in the case of the copolymer CL3 was performed using theZimm and Thurmond approach with the help of SEC3. This method,which was much more sensitive to the structural modification thanthe determination of Gs and Gx, shows a decrease of long-chainbranching with irradiation dose. However, at the higher dose of150 kGy, LCB was introduced in the copolymer revealing thatrecombination reaction may start at this dose.

Acknowledgements

To Sebastien Rouzeau (Viscotek, Irigny, France) for his support inbranching calculation.

References

[1] Rosiak JM, Ulanski P, Pajewski LA, Yoshii F, Makuuch K. Radiation formation ofhydrogels for biomedical purposes. Some remarks and comments. RadiationPhysics and Chemistry – Second International Symposium on RadiationTechnology in Biomedical Materials 1995;46(2):161–8.

[2] Meinhold D, Schweiss R, Zschoche S, Janke A, Baier A, Simon F, et al. Hydrogelcharacteristics of electron-beam-immobilized poly(vinylpyrrolidone) films onpoly(ethylene terephthalate) supports. Langmuir 2004;20(2):396–401.

[3] Grondah L, Chandler-Temple A, Trau M. Polymeric grafting of acrylic acid ontopoly(3-hydroxybutyrate-co-3-hydroxyvalerate): surface functionalization fortissue engineering applications. Biomacromolecules 2005;6(4):2197–203.

[4] Shimizu Takashi, Hayakawa Takashi, Sogabe Takahiro, Ichikawa T. Constructionof polymer skeletons with radiation-scissile groups at predetermined sites.Journal of Polymer Science Part A Polymer Chemistry 2008;46(6):1945–53.

[5] Iguerb O, Bertrand P. Graft photopolymerization of polyethylene glycol mon-oacrylate (PEGA) on poly(methyl methacrylate) (PMMA) films to prevent BSAadsorption. Surface and Interface Analysis 2008;40(3–4):386–90.

[6] Odeliusa Karin, Plikka Peter, Albertsson Ann-Christine. The influence ofcomposition of porous copolyester scaffolds on reactions induced by irradia-tion sterilization. Biomaterials 2008;29(2):129–40.

[7] Martınez-Sancho Concepcion,Herrero-Vanrell Rocıo,Sofıa Negro. Study ofgamma-irradiation effects on aciclovir poly(lactic-co-glycolic) acid microspheres for intra-vitreal administration. Journal of Controlled Release 2004;99(1):41–52.

[8] Kraskoa Michal Y, Jacob Golenserb, Abraham Nyskac, Meir Nyskad, BrindYaron S, Domb Abraham J. Gentamicin extended release from an injectablepolymeric implant. Journal of Controlled Release 2007;117(1):90–6.

[9] Bushell James A, Mike Claybourn, Williams Helen E, Murphy Damien M. AnEPR and ENDOR study of g- and b-radiation sterilization in poly(lactide-co-glycolide) polymers and microspheres. Journal of Controlled Release2005;110(1):49–57.

[10] Chapiro A. Radiation chemistry of polymeric systems. High polymers. NewYork: Wiley interscience; 1962. p. 728.

[11] Chapiro A. Chemical modifications in irradiated polymers. Nuclear Instru-ments and Methods in Physics Research Section B Beam Interactions withMaterials and Atoms 1988;32:111–4.

[12] Charlesby A. Atomic radiation and polymers. Oxford: Pergamon Press; 1960. p. 556.[13] Wilson JE. Radiation chemistry of monomers, polymers and plastics. New

York: Marcel Dekker Ltd; 1974. p. 633.[14] Yen-hsiung Feng Paul, Kennedy Joseph W. Electrical and chemical effects of

beta-radiation in polystyrene. Journal of the American Chemical Society1955;77(4):847–51.

[15] Tadao Seguchi, Shoji Hashimoto, Arakawa Kazuo, Hayakawa Naohiro,Kawakami Waichiro, Kuriyama Isamu. Radiation induced oxidative degrada-tion of polymers – I. Oxidation region in polymer films irradiated in oxygenunder pressure. Radiation Physics and Chemistry 1981;17(4):195–201.

[16] Deniz Veli, Guven Olgun, Ozbey Turan. Radiation chemistry of poly(ethyl-methacrylate) – II. Kinetics of free radical decay. Radiation Physics andChemistry 1985;26(1):39–41.

[17] Veli Deniz, Olgun Guven. Radiation chemistry of poly(ethyl methacrylate) – IV.Effect of irradiation temperature. International Journal of Radiation Applica-tions and Instrumentation Part C Radiation Physics and Chemistry1991;37(2):359–61.

[18] David G, Moine L, Dupuy A, Labarre D, Flaud P, Guiffant G, et al. Gellingsolution for vein embolisation: improvement of efficacy by microspheresaddition. Copenhagen, Denmark: CIRSE; 2008.

[19] Shultz AR, Roth PI, Rathmann GB. Light scattering and viscosity study ofelectron-irradiated polystyrene and polymethacrylates. Journal of PolymerScience 1956;22(102):495–507.

[20] Shultz AR, Roth PI, Berge JM. Radiation degradation of polymethacrylates.Dose rate and medium effects. Journal of Polymer Science Part A GeneralPapers 1963;1(5):1651–69.

[21] Veli Deniz, Olgun Guven. Radiation chemistry of poly(ethyl methacrylate) – III.Size exclusion chromatography results. International Journal of RadiationApplications and Instrumentation Part C Radiation Physics and Chemistry1991;37(2):353–7.

[22] Young-Shang Lin, Sanboh Lee, Chao Lin Bo, Pyeng Cheng Cheu. EPR studies ofhigh dose gamma-irradiated poly(methyl methacrylate). Materials Chemistryand Physics 2003;78(3):847–51.

[23] Rosiak JM, Burozak K, Pekala W. Polyacrylamide hydrogels as sustained release drugdelivery dressing materials. Radiation Physics and Chemistry 1977;22(3–5):907–15.

[24] Caulfield Marcus J, Xiaojuan Hao, Qiao Greg G, Solomon David H. Degradationon polyacrylamides. Part I. Linear polyacrylamide. Polymer 2003;44(5):1331–7.

K. Slimani et al. / Polymer Degradation and Stability 94 (2009) 584–590590

[25] Caulfield Marcus J, Xiaojuan Hao, Qiao Greg G, Solomon David H. Degradationon polyacrylamides. Part II. Polyacrylamide gels. Polymer 2003;44(14):3817–26.

[26] Rosiak JM, Ulanski P. Synthesis of hydrogels by irradiation of polymers inaqueous solution. Radiation Physics and Chemistry 1999;55(2):139–51.

[27] Guillermina Burillo, Takeshi Ogawa. The gamma ray-induced crosslinking ofpolyacrylamide in the solid state. Journal of Polymer Science Polymer LettersEdition 1983;21(8):615–20.

[28] Abd El-Rehim HA. Swelling of radiation crosslinked acrylamide-basedmicrogels and their potential applications. Radiation Physics and Chemistry2005;74(2):111–7.

[29] Osamu Saito. On the effect of high energy radiation to polymers. I: cross-linking and degradation. Journal of the Physical Society of Japan1958;13(2):198–206.

[30] Moad Catherine L, Winzor Donald J. Quantitative characterization of radiationdegradation in polymers by evaluation of scission and cross-linking yields.Progress in Polymer Science 1998;23(5):759–813.

[31] O’Donnell JH, Rahman NP, Smith CA, Winzor DJ. Chain scission and cross-linkingin the radiation degradation of polymers: limitations on the utilization of theo-retical expressions and experimental results in the Pregel region. Macromole-cules 1979;12(1):113–9.

[32] Anjali Acharya, Mohan H, Sabharwal S. Radiation induced polymerization andcrosslinking behavior of N-hydroxy methyl acrylamide in aqueous solutions.Radiation Physics and Chemistry 2002;65(3):225–32.

[33] Horia M, El-Din Nizam, El-Naggar Abdel Wahab M. Synthesis and character-ization of hydroxyethyl methacrylate/acrylamide responsive hydrogels. Jour-nal of Applied Polymer Science 2005;95(5):1105–15.

[34] Auhl D, Stange J, Munstedt H, Krause B, Voigt D, Lederer A, et al. Long-chainbranched polypropylenes by electron beam irradiation and their rheologicalproperties. Macromolecules 2004;37(25):9465–72.

[35] Striegel AM. Long-chain polymer branching: determination by GPC-SEC.Encyclopedia of chromatography. New York: Marcel Dekker; 2001. p. 497–500.

[36] Striegel AM, Krejsa MR. Complementarity of universal calibration SEC and 13CNMR in determining the branching state of polyethylene. Journal of PolymerScience Part B Polymer Physics 2000;38(23):3120–35.

[37] Zimm BH, Stockmayer WH. The dimensions of chain molecules containingbranches and rings. The Journal of Chemical Physics 1949;17(12):1301–14.

[38] Thurmond CD, Zimm BH. Size and shape of the molecules in artificiallybranched polystyrene. Journal of Polymer Science 1952;8(5):477–94.

[39] Flory PJ. Principles of polymer chemistry. Ithaca, New York: Cornell UniversityPress; 1953.

[40] Kulin LI, Meijerink NJ, Starck P. Long and short chain branching frequency in lowdensity polyethylene (LDPE). Pure and Applied Chemistry 1988;60(9):1403–15.

[41] Roovers J. Encyclopedia of polymer science and engineering. New York: JohnWiley & Sons; 1985. p. 478–99.

[42] Hamielec A. Characterization of complex polymer systems by size exclusionchromatography – homopolymers with long chain branching and copolymerswith composition drift. Pure and Applied Chemistry 1982;54(2):293–307.

[43] Hamielec A, Styring M. Recent advances in the characterization and analysis ofpolymers and polymer particles by size exclusion chromatography. Pure andApplied Chemistry 1985;57(7):955–70.

[44] Youlu Yu, DesLauriers Paul J, Rohlfing David C. SEC-MALS method for thedetermination of long-chain branching and long-chain branching distributionin polyethylene. Polymer 2005;46(14):5165–82.