Radiation induced crosslinking of poly(vinyl methylether)in aqueous solutions

S. Sabharwal a, *, H. Mohanb, Y.K. Bhardwaj a, A.B. Majali a

aIsotope Division, Bhabha Atomic Research Centre, Trombay, Bombay 400 085, IndiabChemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India

Received 23 June 1997; accepted 15 December 1997

Abstract

Radiation induced crosslinking of poly(vinyl methylether) (PVME) has been investigated in aqueous solutions.The spectral and kinetic features of the transients involved in the crosslinking reaction have been studied by pulse

radiolysis of dilute PVME solutions. H atoms reacts with PVME, like OH radicals, by abstracting an H atompredominantly from b-position with respect to 0OCH3 group, but the rate of reaction of H atom is an order ofmagnitude slower than that of OH reaction. The PVME radicals formed by H attack have been found to decay byusual 2nd-order kinetics unlike PVME radicals produced by OH attack that are reported to decay by a complex

time-dependent kinetics that deviates strongly from 2nd-order kinetics. The rate constant of eÿaq with PVME atpH 5.5 has been found to be 1.2 � 108 dm3 molÿ1 sÿ1. From the decay behaviour of the transient species formed byreaction of eÿaq with PVME, it has been shown that the transient initially reacts with solvent protons by a fast

reaction to yield radical species which subsequently recombine by a slow mode. The dependence of gelation doseand radiation yields of crosslinking (Gx) of PVME on various factors such as polymer concentration, dose rate, pH,presence of oxygen and crosslinking agent has also been studied by steady-state radiolysis using an electron-beam

accelerator. # 1999 Elsevier Science Ltd. All rights reserved.

1. Introduction

Radiation chemical studies of polymeric materials,

especially for water soluble polymers that can be cross-

linked by ionization radiation resulting in the for-

mation of hydrogels, are of current interest (Ulanski

and Rosiak, 1992; Nagaoka et al., 1993; Ulanski et al.,

1995a,b,c; Sonntag et al.,1995). Some of these cross-

linked polymer gels also display phase transition in re-

sponse to small environmental changes, viz. pH,

temperature, solvent composition and electric ®eld

(Tanaka, 1981; Ho�man, 1987; Gehrke and Cussler,

1989). Of these, temperature sensitive gels are pro-

duced by crosslinking linear polymers that display

lower critical solution temperature (LCST), e.g. poly-

(vinyl methylether) (PVME), poly(N-isopropyl acryl-

amide) (PNIPAAm) and poly(vinyl acetate-co-vinyl

alcohol) (Huang et al., 1989; Kabra et al., 1993).

Applications of these gels are being probed in variety

of areas such as recyclable absorbents, drug delivery

systems, robotics and as mechano-chemical actuators.

The rate of volume change (swelling/deswelling) is an

important characteristic of these gels specially for ap-

plications where a quick response is desired. The kin-

etic data reported in literature show that rate of

volume change for most conventionally polymerized

gels is very slow. A PVME gel, that is produced by

radiation crosslinking of linear water soluble polymer

PVME, swell and shrink orders of magnitude faster

than the previously reported responsive gels (Huang et

al., 1989; Kabra et al., 1992). PVME is a non ionic

polymer that is soluble in water at room temperature

but precipitates out when heated above 310 K due to

hydration and dehydration reaction respectively at low

and high temperatures as shown in Eq. (1)

Radiation Physics and Chemistry 54 (1999) 643±653

0969-806X/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved.

PII: S0969-806X(98 )00299-0

Radiation PhysicsandChemistry

PERGAMON

* Corresponding author.

ÿ�CH2ÿCH�nÿ 3ÿcool

heatÿ4 ÿ�CH2ÿCH�nÿ �H2O

j jOCH3 OCH3

:

HOH

�1�

Since PVME polymer is crosslinked by irradiating inpresence of water, reactions of the transients of water

radiolysis (i.e. eÿaq, H and.(OH) play an important

role in the crosslinking process. However, knowledgeabout the reactivity of various primary species of

water radiolysis with PVME as well as the mechanisticdetails about the crosslinking reaction occurring, thatcan help in better understanding of gel formation, is

lacking. We are interested in utilizing the extremelyhigh dose rates delivered by electron-beam EB accel-erator to crosslink such responsive polymers havinginhomogeneously crosslinked structure and also under-

stand the mechanistic details of the gel formation pro-cess under these conditions (Sabharwal et al., 1995;1996). In our earlier work, we have reported the spec-

tral and kinetic features, as well as the recombinationreactions of the transient species formed by reaction ofOH radical with PVME (Sabharwal et al., 1996). In

the present study, the characteristics of transientsinvolved in the reactions of H and eÿaq with PVMEhave been investigated by pulse radiolysis technique.Steady state radiolysis technique using EB has been

utilized to understand the macro e�ects of crosslinkingreaction by sol-gel analysis. The results of these studiesare presented in this paper.

2. Experimental

Solutions of PVME were made by diluting 50 w/w%solution of PVME (Aldrich, Mw090 000) with waterpuri®ed by the nanopure system (Barnstead). All other

chemicals were of Analar Grade and used as received.Sample solutions contained in 1 � 1 cm suprasil quartzcuvettes were irradiated with 50 ns/2 ms pulse of 7-

MeV energy electrons obtained from LINAC (RayTechnologies, UK). Details of pulse radiolysis set upare described elsewhere (Guha et al., 1987). The transi-ent species formed on irradiation were monitored using

a 450 W Xenon arc lamp as the light source and aHamamatsu R-955 photomultiplier was used as adetector. The photomultiplier output signal was digi-

tized using a 100 MHz storage oscilloscope (IwatsuModel 8123) and analysed for kinetic informationusing an IBM compatible microcomputer. Dosimetry

was carried out using aerated decimolar potassiumthiocyanate solution, with a value of Ge= 21 520 dm3

molÿ1 cmÿ1 at 500 nm (Fielden, 1982).

EB irradiation of samples was carried out using anindustrial ILU-6 accelerator (Budker Institute of

Nuclear Physics, Russia) under the following con-ditions: energy = 2 MeV, current = 10 mA and con-veyor speed = 13 mm sÿ1. The samples were irradiated

in 5 cm � 5 cm � 0.2 cm aluminium molds. The moldwas taken apart after irradiation to recover a ¯at sheetof gel. The radiation dose measurements were carried

out using FWT-60 nylon ®lm dosimeter and using aFaraday cup. PVME gels were also prepared by g radi-ation crosslinking using 60Co radiation source having a

dose rate of 1.5 kGy hrÿ1 as determined by Fricke do-simetry. For sol-gel studies, the crosslinked gels wereswollen in doubly distilled water at 298 K and thendeswelled at 318 K to leach out the sol-fraction. When

deswelling at 318 K, the water in which the gels wasimmersed became cloudy indicating that a sol fractionwas leaching out of the gel. The gels were again

swollen at 298 K and the process was repeated until nomore sol-fraction leached out, the gels were dried toconstant weight for gel determination.

3. Results and discussions

3.1. Characteristics of the linear PVME

The absorption spectrum of a dilute solution ofPVME shows that the polymer has two absorptionmaximum (lmax) at 226 and 278 nm with molar extinc-

tion coe�cient (e) = 55 and 53.4 Mÿ1 cmÿ1. The di�er-ential scaning calorimetric run of a 30 wt% PVMEshowed that the linear polymer has a lower critical sol-

ution temperature of 378C which agrees well with thereported values in literature (Huang et al., 1989).

3.2. Pulse radiolysis studies of PVME in dilute aqueous

solutions

3.2.1. Reaction of PVME with hydrated electrons

For studying the hydrated electron reactions, N2

saturated solutions of the polymer were irradiated inpresence of 0.2 mol dm3 t-butanol that was used as

OH radical scavenger. The t-butanol radicals formedwere found to be unreactive towards PVME. The reac-tion between eÿaq and PVME was monitored by follow-ing the optical absorption of eÿaq at 700 nm. In the

absence of polymer, the hydrated electron decay wasobserved with a half-life time of about 3 ms. However,under similar conditions, when the matrix containing

the monomer was radiolysed with the electron pulse,the hydrated electron decayed at a much faster ratedepending upon the concentration of the polymer. The

decay of absorption followed a pseudo ®rst order kin-etics and the ®rst order rate constant was determinedfrom the slope of log (O.D.) vs time plot. From the

S. Sabharwal et al. / Radiation Physics and Chemistry 54 (1999) 643±653644

linear plot of observed ®rst order rate constant vs

[PVME], a bimolecular rate constant for the reactionwas determined to be 1.2 � 108 dm3 molÿ1 sÿ1.The time resolved spectrum of the the transient

species formed in the reaction of eÿaq with PVME isshown in Fig. 1. The transient can be characterized by

a broad absorption band having lmax at 320 nm with

molar extinction coe�cient (e320) = 3650 dm3 molÿ1

cmÿ1. The species was found to decay by two modes:

an initial very fast decay followed by a slower decay,when monitored at 320 nm (Fig. 1: Inset). The initialyield of the fast-decaying component increased with

Fig. 1. Time resolved spectra of the transient formed by hydrated electron reaction with PVME at pH 5.5. (a) 1.5 ms after the pulse

and (b) 5 ms after the pulse. Pulse duration = 50 ns; Dose = 14 Gy; inset: decay of the transient observed at 320 nm.

Fig. 2. Absorbance changes observed under di�erent conditions at 320 nm as a function of time on irradiating N2 saturated PVME

solution containing 0.5 mol dmÿ3 t-butanol under di�erent conditions: (a) [PVME] = 1 � 10ÿ2 mol dmÿ3, dose = 126 Gy; (b)

[PVME] = 1 � 10ÿ2 mol dmÿ3, dose = 56 Gy; (c) [PVME] = 5 � 10ÿ3 mol dmÿ3, dose = 56 Gy; (d) Time pro®le of decay of

PVME anion at pH 12. Other conditions same as in Fig. 1 (inset).

S. Sabharwal et al. / Radiation Physics and Chemistry 54 (1999) 643±653 645

increasing concentration of PVME in solution[Fig. 2(a)], as well as by increasing the dose delivered

per pulse [Fig. 2(b)]. However the apparent half-life ofthe fast-decaying component seemed to remain con-stant.

The appearance of an initial spike in the time pro®leof the transient species is interesting. Similar resultshave been reported by Tanaka et al. (1988) for the

decay of poly(vinyl biphenyl) anion in MTHF sol-utions. This has been attributed to the delayed gemi-nate-recombination occurring in the polymer

microdomains formed due to the entanglement of thepolymer chains. The reactions occurring inside thedomains will be dependent on the local motion ofpolymer chains while those occurring outside will be

relatively free from it. The radical anions produced in¯uid medium may disappear either by geminate-recom-bination, i.e. recombination of anion with parent cat-

ions in spur, or by a homogeneous randomneutralization reaction of free ions that escape thegeminate recombination reaction. Therefore, initially

we were also tempted to assign the fast-decaying com-ponent observed in the time-pro®le of PVME anion inaqueous solutions to the geminate recombination

within the polymeric microdomains. However, if this isthe case, the geminate recombination reaction shouldnot be a�ected by the pH of the solution, as geminaterecombination reactions are expected to be indepen-

dent of the pH of solution. Therefore, the e�ect of pHon the reaction of electron with PVME and the decaybehaviour of the resulting anion was investigated and

is described in the next section.

3.3. E�ect of pH and ionic salts on the reaction of eÿaqwith PVME

The bimolecular rate constant for the reaction of

eÿaq with PVME at pH 12 was also found to be1.2 � 108 dm3 molÿ1 sÿ1. This may be due to theabsence of any change in the structure as well as con-

formation of PVME molecules, in the pH range 6±12,particularly the site where hydrated electron getsattached. The time pro®le of decay of radical anions

formed was however considerably di�erent from thatobserved in the neutral solutions. At pH 12, no spikein the time pro®le of decay of transient species wasobserved. Instead, the transient decayed by a slow sec-

ond-order reaction [Fig. 2(d)]. In order to con®rm thatthe di�erence in the decay behaviour of anion is indeeddue to the change in pH, and not due to high concen-

tration of Na+ ions in solution, the decay behaviourof the transient was also investigated in presence of0.2 mol dm3 salts, viz. sodium chloride and sodium sul-

phate. The results clearly indicated that the presence ofthese ionic salts have no e�ect on the decay behaviourof the transient. The change in the decay behaviour

observed at pH 12, thus can be attributed only to thechange in pH of the solution. It was therefore con-

cluded that the initial fast-decay of the transientspecies in our studies does not seem be due to thedelayed geminate recombination reaction.

Based on the above results, the initial fast decay ofthe transient at pH 5.5 can be assigned to the reactionof PVME anion with the H+ present in the solution

leading to the formation of radical species [Eq. (2)]which subsequently decay by recombination reaction[Eq. (3)], as seen by the slow decay in the later time

scale.

PVMEÿ� �H� ! PVME� �2�

2 PVME� ! Products �3�

3.4. Decay of radical anions in presence of Thionine

In order to con®rm the above mentioned expla-nations, experiments were carried out to transfer theelectron from the PVME anion to the organic dyeThionine (TH+) in aqueous neutral solutions so that

thionine competes with H+ for the PVME anion andPVME radicals are not formed in the solution.Thionine readily undergoes one-electron reduction

yielding semi-thionine (lmax=400 nm, e400 = 6826dm3 molÿ1 cmÿ1) as it has a reduction potential valueof +0.05 V vs NHE (Guha et al., 1987). The matrix

and pulse dose conditions were chosen to ensure that(i) all the eÿaq predominantly react with PVME only toform anion and (ii) PVME anions formed completely

transfer the electron to thionine. The results of thesestudies are shown in Fig. 3. As is apparent from theseresults, in presence of Thionine, the transient decaysonly by a fast decay due to the transfer of electron

from anion to the thionine dye and the slower decaydue to recombination of PVME radicals is absent.These experiments also augment our explanation that

the decay of PVME anions at neutral pH can be ade-quately represented by Eqs. (4) and (5).

3.4.1. Reaction of H atoms with PVMEThe G-value of hydrogen atoms in the neutral sol-

utions is only (0.55) and their contribution towardsradiolytic reactions is generally neglected. However, in

acidic conditions, the hydrated electrons get convertedto H atoms and their yield is considerably enhanced.Therefore, the reactions of H atoms with PVME were

investigated in acidic deareated solutions at pH 1.

3.4.2. Reactivity of H atoms with PVME

The rate constant for the reaction of H atoms withPVME was determined by competition kinetics usingtetracycline as a reference solute (Fig. 4: inset). N2

S. Sabharwal et al. / Radiation Physics and Chemistry 54 (1999) 643±653646

saturated aqueous solutions at pH 1 containing

1 � 10ÿ4 mol dmÿ3 tetracycline (TC) and varying con-centrations of PVME in the range 0±6 � 10ÿ2 mol dmÿ3

and 0.5 mol dmÿ3 t-butanol as OH scavenger werepulsed and absorbence at 440 nm due to TCÿH

adduct was measured immediately after the electronpulse. Utilizing kTC + H of 2.5 � 109 dm3 molÿ1 sÿ1

(Sabharwal and Kishore, 1994), kPVME + H was evalu-

ated to be 4.5 � 106 dm3 molÿ1 sÿ1.

3.4.3. Spectral features of the transientFig. 4 shows the transient absorption spectrum

obtained by the reaction of PVME with H radicalsobtained by pulsing N2 saturated 10ÿ2 mol dmÿ3

PVME solution containing 0.5 mol dm3 t-butanol as.OH scavanger at pH 1. The spectrum shows anabsorption band at 330 nm. H radicals predominantly

react with organic compounds by addition or abstrac-tion reactions. H radicals generally undergo additionreactions only when some unsaturated site is available

in the molecule, and since PVME does not contain anyunsaturated group, the H radicals are expected to reactvia H abstraction just like OH radicals. This can leadto the formation of two kinds of radicals, radical for-

mation at a or b-position with respect to 0OCH3

group (Scheme 1).a-Radicals of well-de®ned model systems of similar

polymers have been characterized by an absorptionband around 290±330 nm whereas b-radicals have beenshown to absorb at wavelength E250 nm (Neta et al.,

1969; Simic et al., 1969). Therefore, the transientabsorption at 310 nm can be assigned to a-radicals ofPVME. The fraction of a-radicals formed can be deter-

mined by utilizing the di�erence in the reducing ability

of a- and b-radicals. The a-radicals of similar polymershave been reported to be easily reducible as comparedto b-radicals (Sonntag et al., 1995). The reaction ofthese radicals with thionine was used to quantitatively

determine the fraction of a-radicals formed in the reac-tion.

The formation of semi-thionine was monitored at400 nm by pulse radiolysing a 0.1 mol dmÿ3 PVMEnitrogen saturated solution containing di�erent con-centrations (0±4 � 10ÿ5 mol dmÿ3) of thionine and

0.5 mol dm3 t-butanol with a 50 ns pulse. The matrixand pulse dose conditions were chosen to ensure that(i) all the H radicals predominantly react with PVME

only and (ii) PVME radicals formed completely reactwith thionine. From the results of these experiments,the bimolecular rate constant for the reaction was

determined to be 2.1 � 109 dm3 molÿ1 sÿ1 and theyield of a-radicals has been estimated to be 26%. Thisvalue is the same as that obtained for OH radicals

Fig. 3. Time pro®le of decay of PVME anion at pH 5.5 (a) without thionine and (b) in presence of 1 � 10ÿ5 mol dmÿ3 thionine.

S. Sabharwal et al. / Radiation Physics and Chemistry 54 (1999) 643±653 647

Fig. 4. Absorption spectrum of the radicals formed by reaction of PVME with H atoms at pH 1 [PVME] = 1 � 10ÿ2 mol dmÿ3,Pulse duration = 2 ms, Dose = 225 Gy. Inset: Determination of rate constant of reaction of H atoms with PVME by competition

kinetics using tetracycline as reference solute. Absorbance measured at 440 nm in N2 saturated solution containing 0.5 mol dmÿ3 t-

butanol.

implying that H atoms react in a manner similar tothat of OH radicals, although the rate of reaction of H

atoms is an order to magnitude lower than that of OHradicals (Sabharwal et al., 1996). The absorbance ofthe transient band remains independent of solute con-

centration suggesting that the entire H radicals havereacted with PVME. Since only 26% of the H radicalsare used to form the transient band with

lmax=330 nm, the molar absorptivity of the transientwas determined to be 650 dm3 molÿ1 cmÿ1, which issimilar to that of the transient produced by

.OH rad-

ical attack on PVME (e310=730 dm3 molÿ1 cmÿ1)(Sabharwal et al., 1996).

3.4.4. Kinetic features of the radical recombination

The radicals formed due to the attack of H radicalson PVME via H abstraction can undergo two types ofreactions by recombination, viz. crosslinking and dis-

proportionation (Scheme 2). However, for both thereactions to occur, the essential step of polymer rad-icals di�using near each other to react, remains the

same. Recent studies of OH radical reaction with poly-mers in aqueous solutions have shown that decay kin-etics of the macroradical transient observed underdi�erent experimental conditions such as di�erent

polymer concentrations and at di�erent dose per pulse,

provide useful information about the inter and intra-molecular reactions that the macroradical can undergo

in solution (Sonntag, 1995; Ulanski et al., 1995c). Wehave earlier studied the reaction of

.OH with PVME

(Sabharwal et al., 1996). The macro-radicals of PVME

formed due to.OH attack decayed according to two

modes: an initial fast decay mode followed by a slowdecay, the ratio of the two modes also varied with the

concentration of the polymer as well as the radiationdose. These results showed that when the intra molecu-lar crosslinking reaction predominates, the decay kin-etics deviates from the classical second-order and

exhibit a time dependent behaviour.In order to study the recombination behaviour of

polymer radicals produced by H atom attack under

di�erent conditions, decay behaviour of macroradicalswas investigated at 330 nm for various concentrationsof PVME as well as radiation doses delivered to the

system. As shown in Fig. 5(a), a 0.1 mol dmÿ3 PVMEsolution containing 1.0 mol dm3 t-butanol at pH 1when pulsed with a 2 ms pulse (dose = 141 Gy), the

transient species formed decayed by a 2nd-order reac-tion. When the concentration of PVME was reducedto 1 � 10ÿ3 mol dmÿ3, keeping the dose constant, thespecies again decayed by a 2nd-order reaction, though

at a much slower rate [Fig. 5(b)].

S. Sabharwal et al. / Radiation Physics and Chemistry 54 (1999) 643±653648

Unlike OH radical reaction, the decay kinetics of the

transient species always followed the second-order

decay. In our earlier studies we have reported that.OH radical reacts with PVME with a rate constant

(kPVME + OH) 3.3 � 108 dm3 molÿ1 sÿ1 and the optical

absorbence signal of the macro-radicals of PVME pro-

duced by OH radicals decayed according to two

modes: an initial fast decay mode followed by a slow

decay, the ratio of the two modes also varied with the

concentration of the polymer as well as the radiation

dose, the fast decaying reaction has been ascribed to

the intramolecular crosslinking reaction and the slow

decaying mode to intermolecular crosslinking reaction

of the transient species (Sabharwal et al., 1996). The

present results show that the PVME macroradicals

produced by H atom attack predominantly undergo

intermolecular crosslinking reaction. The di�erence in

the behaviour of radicals produced by OH attack and

H attack can be attributed to the very large di�erence

in the reactivity of the two radicals. The rate constant

for OH radical attack is about two order of magnitude

higher than that of H radical. It has been suggested

Fig. 5. Absorbance changes as a function of time on pulse irradiating N2 saturated PVME solution containing 1.0 mol dmÿ3 t-buta-nol at pH 1: (a) [PVME] = 1 � 10ÿ1 mol dmÿ3, pulse dose = 141 Gy; (b) [PVME] = 1 � 10ÿ3 mol dmÿ3, pulse dose = 141 Gy; (c)

[PVME] = 1 � 10ÿ1 mol dmÿ3, pulse dose = 80 Gy.

S. Sabharwal et al. / Radiation Physics and Chemistry 54 (1999) 643±653 649

that classical chemical kinetics formulated for isolatedreactions in homogeneous systems fail to describe the

reactions occurring only at very short time scales forsystems which undergo rearrangement during the reac-tion (Plonka, 1991). This has been attributed to the

presence of matrix disorder at the microscopic levelthat leads to a distribution in the reactivity. In the caseof OH attack, the rate of internal rearrangement (the

time of PVME macroradical production and sub-sequent recombination) markedly exceeds the rate ofexternal rearrangement (the local motion of polymer

chains), the distribution in reactant reactivity is notpreserved during the course of reaction and the reac-tion rate constant depends on time. On the otherhand, the time taken for H atom reaction with PVME

to produce macroradicals exceeds the time required forlocal motion of polymer chains, thus for the recombi-nation the usual 2nd-order kinetics is observed. These

results also suggest that with H atom reaction, inter-molecular crosslinking reaction will predominate thatmay lead to the formation of a more homogeneously

crosslinked gel.

3.5. Gel formation of PVME in aqueous solutions:steady state radiolysis studies using EB accelerator

For technological applications, water soluble poly-mers are crosslinked in concentrated solutions by irra-diating them to high doses of radiation. Irradiation of

concentrated solutions of PVME in aqueous solutions(30 wt%) is known to lead to formation of crosslinkedhydrogels that swell/shrink about 1000 times faster

than the conventionally prepared hydrogels (Huang etal., 1989; Kabra et al., 1992; Sabharwal et al., 1996).Detailed investigations regarding the radiation chem-istry aspects of PVME gel formation have not been

studied so far. Therefore, radiation induced cross-linking of PVME in concentrated aqueous solutions,

that can result in the formation of gels, were investi-gated by irradiating the samples mainly using an elec-tron beam accelerator.

3.5.1. Dose rate e�ect on gel doseIn order to study the e�ect of dose rate on the gela-

tion dose, 30 wt% aqueous solution of PVME wereirradiated to varying doses up to 300 kGy using g radi-

ation a dose rate of 0.5 kGy hÿ1 and using an indus-trial electron beam accelerator. From the results shownin Fig. 6, it can be seen that the gel content obtained

by g irradiation is much higher at all doses as com-pared to EB irradiation. The gelation dose determinedby extrapolating Charlesby±Pinner plot to s + Zs= 2

was found to be 39 kGy for g radiation as comparedto 52 kGy required for EB irradiation. These e�ectscan be attributed to the large di�erence in the dose

rates between the two methods: 1.4 kGy hÿ1 for g radi-ation and 10 800 kGy hÿ1 for EB accelerator. Under girradiation conditions, the concentration of [PVME

.]

radicals generated on each polymer chain instan-

taneously is much lower than those generated underEB irradiation conditions. In the former case themacroradicals formed on di�erent chains will recom-

bine with the each other resulting in inter-molecularcrosslinks whereas in the latter case, since the densityof radicals formed on each PVME chain will be much

higher as compared to g radiation, the radicals willalso tend to undergo intra-molecular crosslinks orclosed loops that may not be e�ective in producing gel

phenomenon as it does not produce a linkage betweentwo separate chains. Thus, compared to g radiation,the dose required to produce gel is higher in the EB ir-radiation as well lower gel contents are obtained at the

Fig. 6. Comparison of gel content of (a) g crosslinked, and (b) EB crosslinked PVME at neutral pH.

S. Sabharwal et al. / Radiation Physics and Chemistry 54 (1999) 643±653650

same radiation dose. The crosslinking yields (Gx) cal-culated using Eq. (4) (Wood and Pikeav, 1994)

Gx � 4:83� 103=DgMw;0 �4�

were determined to be 1.37 and 1.03 for g and EB ir-radiation respectively. Under these conditions, eÿaq and

H atoms mainly react with oxygen yielding relativelyunreactive Oÿ�2 and HO2 radicals and

.OH radicals are

the major reactive species. Under g irradiation con-

ditions, the G(crosslinking) value of 1.37 is very closeto 1/2 of G(OH) value of 2.8, and this suggests PVMEpredominantly undergoes intermolecular crosslinkingreaction under these conditions. On the other hand,

under EB irradiation conditions, the G(crosslinking)value of 1.03 suggests that a considerable amount ofPVME radicals produced by

.OH attack undergo intra-

molecular crosslinking reaction.

3.5.2. E�ect of oxygen

Ambient conditions are known to markedly e�ectthe crosslinking behaviour of many polymers in solidstate. In order to understand the e�ect of presence ofoxygen on gel formation in concentrated aqueous sol-

utions, deoxygenated as well as aerated 30 wt%PVME solutions were irradiated using EB acceleratorand the sol-gel studies of the PVME ®lm formed were

carried. The results of the study show that presence ofoxygen does has a marginal e�ect on the gelation doseas the Dgel=48 kGy in oxygen free solution whereas in

aerated solutions the Dgel was found to be 52 kGy.Since in neutral aqueous solutions, OH is the predomi-nant species that reacts with PVME leading to the for-

mation of macroradicals, the reactions of these withoxygen were investigated by following the decay ofthese macroradicals in (i) absence of oxygen, (ii) in aer-ated solutions and (iii) in oxygen saturated solutions,

at di�erent PVME concentrations. The rate of reactionwas found to be dependent on the concentration ofPVME in solution and decreased with increasing poly-

mer concentration. At 1 � 10ÿ3 mol dmÿ3 PVME con-centration, the rate constant kPVME + O2

was found tobe 1.3 � 108 dm3 molÿ1 sÿ1, and at 0.1 mol dmÿ3 it

was found to be E2.3 � 107 dm3 molÿ1 sÿ1. This alsocon®rmed that at high concentrations of PVME, pre-sence of oxygen should not have much e�ect on thecrosslinking behaviour of PVME. Therefore at very

high concentration, such as 30 wt% PVME concen-tration employed for gel formation, presence of oxygenshowed only a marginal e�ect on the gel formation.

The Gx was determined to be 1.1 in absence of oxygenas compared to 1.03 in presence of oxygen. However,it is evident from these results that crosslinking of

polymer in aqueous solutions is a�ected by the pre-sence of oxygen to a much lesser extent than when thepolymer is irradiated in solid state. This may be due to

the higher mobility of polymer chains in aqueous sol-utions as compared to solid-state, which facilitates the

crosslinking reaction.

3.5.3. E�ect of PVME concentration

PVME samples containing 10, 20 and 30 wt% poly-mer in aqueous solutions were subjected to di�erentdoses of radiation in the range from 40 to 300 kGy

using an EB accelerator. The Dgel values, obtainedusing sol-gel studies, increased with the increasingpolymer concentration and were found to be 48, 50

and 52 kGy for 10, 20 and 30 wt% solutions respect-ively. This is understandable as with increasing concen-tration the number of chains in solution increases,higher dose is required for gelation condition i.e. for-

mation of at least one crosslink between each polymerchain. From the Gx vs concentration plot, Gx value foran in®nitely dilute solution, which re¯ects the cross-

linking only due to the indirect e�ect of radiation, wasfound to be 1.15.

3.5.4. E�ect of pH on crosslinking reactionIn order to study the e�ect of pH on the cross-

linking reaction, 30 wt% solutions of PVME contain-

ing 0.4 M H2SO4 were exposed to various radiationdoses by EB accelerator in an oxygen free atmosphere.The ®lms formed after irradiation were analysed forgel content and results indicate that irradiation in

acidic conditions considerably reduces the gelationdose (Dg=16 kGy). This can be attributed to the reac-tion of H atoms that are produced in acidic condition

due to the reaction of hydrated electrons with H+

ions. Pulse radiolysis studies reported above have alsoshown that reactions of H atoms with PVME are simi-

lar to those of.OH radicals in bringing about the

crosslinking reaction. Under strongly acidic conditions,Gx was found to be 3.05 which is much higher than innear neutral solutions, and the crosslinking reaction

can be represented by the following reactions.

PVME� �OH �H� ! PVME� �H2O �H2� �5�

n PVME� ! Crosslinked gel �6�

3.5.5. E�ect of addition of crosslinking agentIn order to investigate the e�ect of crosslinking

agent on the gelation characteristics of PVME, 30wt% polymer solutions containing varying amounts(0±2%) of the crosslinking agent N,N-methylene bisa-

crylamide were subjected to electron beam irradiation.The results, shown in Table 1, indicate that the gela-tion dose reduces considerably by addition of even

small amounts of the crosslinking agent. In presence of2% crosslinking agent, the gel formation dose isbrought down to 13 kGy as compared to 52 kGy in

S. Sabharwal et al. / Radiation Physics and Chemistry 54 (1999) 643±653 651

absence of crosslinking agent. Further, at this concen-tration of crosslinking agent, 100% gel formation canbe achieved at 80 kGy whereas a dose of 200 kGy is

required to achieve the same in absence of crosslinkingagent. The extrapolation of straight line Gx=f([Mbaa])to [Mbaa] = 0 gives a radiation yield of Gx=2.2,which is considerably higher than Gx value of without

any crosslinking agent. These results indicate that theradiation dose for hydrogel formation can be ade-quately reduced by addition of small concentrations of

N,N 0-methylene-bis-acrylamide.A comparison of the gelation dose, Gx values

obtained for PVME with those of other water soluble

polymers is limited by the small number of detailed in-vestigation that have conducted in aqueous solutions,especially at such high dose rates. In recent studies,

Burczak et al. (1990) and Chapiro and Galant (1991)have reported crosslinking of poly vinyl alcohol(PVAL) and poly(N-vinylimidazole) respectively usingg radiation. The gelation dose for 10 wt% PVME sol-

ution (4.5 kGy) is lower than that of 8 wt% PVALsolution that has the Dg value of 5.54 kGy in argonsaturated solution. Both these polymers possess a simi-

lar basic 0[CH2 0CHR0] repeat unit characteristicof macromolecules that preferentially undergo cross-linking upon irradiation. The Gx yields of PVAL, 1.3

in deareated solutions and 0.8 under oxygenated con-ditions are also close to those of PVME. On the otherhand, aqueous solutions of PVI has extremely low Gx

value of 0.27. This may be due to the general increased

resistance of aromatic compounds towards radiation aswell as the bulkier imidazole may hinder the cross-linking reaction.

4. Conclusions

In summary, PVME in aqueous solution predomi-nantly undergoes crosslinking reaction. Both OH andH radicals react with PVME in a similar manner by H

abstraction reaction. However, the PVME radicalsformed by H atoms attack recombine by the conven-tional second-order kinetics at all polymer concen-

trations and dose rates unlike radicals formed by OHattack that have been earlier shown to recombine via acomplex time-dependent kinetics that deviates very

strongly from second-order kinetics resulting in theformation of more intramolecular crosslinks or a het-

erogeneous gel specially at high dose rates. The pulseradiolysis results clearly show that H atoms attack willlead to the formation of PVME radicals that recom-

bine only by intermolecular reaction and lead to theformation of a homogeneously crosslinked gel. Steadystate radiolysis results con®rm that Gx increases when

the polymer is irradiated in acidic solutions.

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Table 1

E�ect of addition of N,N 0-methylene-bis-acrylamide on gel

formation of 30 wt% PVME

[MBA], (wt%) Dgel (kGy) Gx

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