Indian Journal of ChemistryVol. 34A, September 1995, pp. 673-6R7

__ n_n_nn n __ mn_ n __ n IAdvances in Contemporary Research

Magnetic field effect on polymers and polymerization

Sukumar Maiti* & Dibyendu S Bag

Materials Science Centre, Indian Institute of Technology, Kharagpur 721302, India

Received 17 November 1994

The influence of magnetic field on polymerization reaction, polymer structure and properties hasbeen discussed in the light of the radical pair theory and the cage effect to explain the kffietics ofpolymerization and crosslinking reactions. The rate of polymerization and crosslinking increases dueto the singlet (- ) triplet intersystem crossing mechanism. While the molecular weight increases, themolecular weight distribution decreases under magnetic field. The polymer microstructure viz., tac­ticity, copolymer composition and monomer sequence in the copolymer chain are also affected bythe magnetic field. Polymerization of liquid crystal monomers under magnetic field results in highlyoriented liquid crystal polymers. As a result, the polymer properties like crystallinity, chain flexibil­ity, solubility, thermal behaviour are also changed. Conducting polyacetylene and magnetic polymersprepared under magnetic field also exhibit higher conductivity and magnetic properties respectively.Studies on biopolymers, solid state and plasma polymerization under magnetic field have also beenreported.

Dibyendu Sekluu Bag, after Obtaining M.Sc. in Chemistryfrom IlT, Kharagpur,joined Prof Maids researchgroup on aCSIR fellowship for his Ph.D. work on Magnetic Field Effecton Polymerization.

Dr Sukumar Mottl is Professor of Polymer Materials since1976 at the MaterialsScience Centre, lIT, Kharagpur. He hasextensive research and development experience in industry,both in USA and in India. He has authorisedlcoauthored3

books, published over 200 researchpapers and more than 50technical reports and popular science articles. He is alsoeditorof the Journal of PolymerMaterials.

Introduction

The term 'magnetokinetics' i.e., the magneticfield (MF) effects on the kinetics of chemicalreactions dates back to the discovery of nuclearand electronic spin polarization phenomena dur­ing chemical reactions (CIDNP, CIDEP). Thoughthe so-called radical pair mechanism is the heart ofthese phenomena, there are also other mechan­isms based on singlet-triplet, triplet pairs, or trip­let-doublet pairs. The field of magnetokineticchemical and related physical phenomena was re­viewed earlier. Recently Steiner and Ulrich 1 havemade a comprehensive review.

In early eighties a few polymerizations andcrosslinking reactions were studied under magne­tic field. The orientation of the liquid crystal (LC)polymer by external fields including magneticfield2 and to stabilize the alignment by freezing,crystallizing, drying, or by gel formation undermagnetic field have been studied recently. Theaim of this review is to report the results on thepolymerization and crosslinking reactions underthe influence of applied magnetic field, to throwsome light on the mechanism of such polymeriza­tion, and to examine the effect of magnetic fieldon the properties of such polymers such as solu­tion viscosity, molecular weight (MW), molecularweight distribution (MWD'), tacticity, change inmesomorphic state etc.

Dibyendu Sekhar BagDr Sukumar Maiti

674 [NDIAN J CHEM.. SEe. A, SEPTEMBER 1995

Fig. 1-Vec~or model of the three magnetic sublevels of atriplet radi~l pair and a singlet state of a radical pair. An"up" spin v~ctor is represented by a and a "down" spin V(lC-

tor is represented by j3c'.

Fig. 2-vec~r model of spin rephasing resulting from a dif­

ferent proce ional rate of the spin of one electron relative tothe other el tron about the z-axis (Hz). A magnetic compo­nent along x and y-axis (Hx or Hy) produces a torque which

i can flip an electron spin5•

... (1)

the precessional rates of the two spin vectors oftwo electrons are slightly different. The local mag­netic field arising from the electron spin influ­ences each electron's spin to the same extent andtherefore, does not affect the relative precessionalrates of two spin vectors of two electrons. So thiscannot be a cause of mixing of S and To by re­phasing the spin's precessional rate. But the localmagnetic fields due to spin-orbit coupling (equiva­lent to different 'g-factors') and hyperfine coupling(coupling of nuclear spin with the odd electron) atdifferent radical centres are different. So the L\g­mechanism and also the hyperfine interaction (hfi)mechanism allow evaluation of the difference in

precessional rate of the two spin vectors andcause the mixing of S and To states .

If, the local magnetic field acting in the z-direc­tion (Hz differs for two. electrons, rephasing ofthe spin vector occurs but the orientation (a or ~)of the spin vectors relative to z-axis remains thesame i.e., 'up' electron remains up and 'down'electron remains down. As a result S and Tostates are mixed and a pseudo-equilibrium is setup until a perturbation (e.g. chemical reaction ordiffusion out of the cage) removes S or To fromthe equilibrium. Local magnetic fields in the x and yplanes (Hx and Hy) may also cause rephasing butalso may provide a torque to twist one of thespins to flip (say, the a-spin of one electron into a~-spin) and also cause reorientation relative tothe z-axis. In other words, local magnetic fields,Hx and Hy, can flip the electron spin and causeconversion of S into either T + or T _.

So far the discussion has been dealt with theISC efficiency due to local magnetic field. Theremay be some effect of external (laboratory) mag­netic field on such ISC of a radical pair andhence on radical pair reactions. Actually there aretwo possible mechanisms by which the course of aradical reaction can be influenced by the externalmagnetic field. In the presence of external magne­tic field, different Larmor precessional rates ofthe radicals rephase S into To and vice versa. Thedifference in precessional rates of two electrons(L\w) is given by

L\w = (L\g (3Ho/1i) ± (a, - a2)1

g-factor effect Hyperfine effect

where, L\g= Igl - g21,gl and g2 are the g-values oftwD electrons, ~ is the Bohr ,magneton, 'Ii thePlanck's constant, Ho the external magnetic field,al and a) the hyperfine coupling constants of tworadiCalS and I the nuclear spin. So, the rephasingrate enhances as L\g and/or the tlux density (Ho)

THE SINGLETSTATE

TOT _

THE TRIPLET STATE ISIA

Hz! T.

"pUff" "mix~" "pum" pUffsinOI.t stat. tripl.t tr'pl.t

5.0 I 5·, 5.,

9r c;rR pF? R9. '''. (> " .. . U ; .' c=>

6\ Hz 6 ,,6Hq R7H Ph~:::'~1 .ind pllou in phall'':'' .. in phQseI /J a

I \ spin rephasing ----' L._ Spinflip _u __ •....J

I Hz H x • H y

! op.nat.s to ,..phoM op.rat. to flip

i

Radical p'r theory and the influence of magne-tic field on radical reactions .

In the dical pair theory3,4, the two radicalsgenerated y homolysis of a bond in a moleculemay remai as either singlet(S) or triplet (T +, To,T.) states. he spin states are represented pictori­ally as a v ctor in Fig. 1. The spin states (singletor triplet) nce prepared would remain in thesestates fore er, if magnetic torques operating onthem are i entical. But the mixing of singlet andtriplet stat s i.e., the intersystem crGssing (ISC)from a sin let to a triplet or vice versa, resultsfrom the 0 currence of different magnetic torquesthat either ephase or flip one of the spin vectorsto the othe . A magnetic field arising from withinthe molecu e (local magnetic field) provides themagnetic t rque required to rephase and/or toflip the ele~tron's spin vector. The local magneticfield and h~nce the magnetic torque is generated

by electronts orbital motion (spin-orbit indu.cedmagnetikc ~elds: spin-orbit coupling) or due to

other ma~tic spins (magnetic spin associated

with an el ctron's spin or· WIth a nuclear spin;spin-spin co pIing).

Figure 2 escribes the schematic representation

for singlet-t*plet mixing and intersystem crossing(ISC). A co~version between S and Tn occurs if

~ IIIi

MAITI et al.: MAGNETIC FIELD EFFECT ON POLYMERS 675

Fig. 3-Schematic representation of the Zeeman splitting ofthe triplet sublevels (T +, To, L). The effect of Zeeman inter­action g~Ho is to energetically split T ± and thereby inhibit

singlet - triplet ISC from or to these subIeveIs4.

increases, starting. with a radical pair in a singletstate, the triplet state is more frequently reachedand hence part of the cage products decreases inrelative to the escape products3•6• This concept isuseful for the radical polymerization and cross­linking reactions.

Homopolymerization under magnetic fi81dA few radical polymerizations have already

been studied under magnetic field (MF) with var­ious vinyl monomers and initiators under differentconditions. These polymerization systems alongwith respective percentage conversion, molecularweight (MW) and Mw/ Mn are listed in Table 1.The MF dependence of MWof a few polymers isshown in Fig. 4.

Mf. He

"M.T_~},E

S:{T+.T •.1-}

PhotopolymerizationTurro et aU utilized the radical pair theory for

the polymerization of vinyl monomers under MF.Zeeman splitting of triplet states in the presenceof an external MF increases the polymer yieldand the MW for the photoinduced emulsion po­lymerization of styrene. Emulsion acts as a cagefor the radical reactions including polymerization.

An important characteristic of emulsion polym­erization is to produce high MW polymer with arapid rate and for which water-soluble thermal in­itiators are best suited. Oil-soluble initiators arecommonly ineffective in emulsion polymerizationbecause such initiators produce pairs of radicalsin the micelle (polymerization loci), thereby fa­vouring termination before substantial polymergrowth can occur. But in the presence of MF highMW polymer can be achieved even by using anoil-soluble photoinitiator. The effect of MF is ob­served in the case of oil-soluble photoinitiator(e.g., dibenzylketone, DBK) and not in the case ofwater-soluble photoinitiator (e.g., f3-ketoglutaricacid) and water-soluble thermal initiator (e.g., so­dium persulfate), or oil-soluble initiator thatthermolyzes or photolyzes to produce micellizedsinglet radical pairs (e.g., 2,2'-azo-

increases (~g-mechanism). The singlet .•..•tripletconversion due to hyperfine interaction (hfi­mechanism) can also work without an externalmagnetic field. But in low magnetic field compar­able to local magnetic field, hyperfine interactioninduces ISC from the singlet to the three tripletstates (T+, To, T _) whereas in a sufficiently highexternal magnetic field only S - To transition oc­curs. However, there is also relaxation mechan­ism which is less effective than ~g- and hfi-me­chanism4• The relaxation transition rate also de­pends on external magnetic field. But being ofsmall importance in case of free radicals it is neg­lected here.

Another important influence of external magne­tic field is the Zeeman splitting of triplet states(T+, To, T _). In the absence of a magnetic field allthe three triplet sublevels themselves energeticallydegenerate as wen as with the singlet state. All thethree triplet sublevels interconvert with the singletstate, when the Zeeman interaction is small rela­tive to other interactions (such as hfi whosestrength is given by the hyperfine coupling con­stant). But when a magnetic field is applied, Zee­man splitting of the three magnetic sublevels oftriplet states, i.e., T +, To, T _ occurs. As a result,the energy of T + is raised, T _ is lowered and Toremains unchanged. The magnitude of this splitt­ing equals to gf3Ho. Since the singlet state has nonet magnetic moment (spin paired), its energy isunchanged by the application of magnetic field.Therefore, in the presence of external magneticfield, only To and S possess identical energies andhence only To state is accessible to switch into Sstate and vice versa (Fig. 3).

The triplet radical pair cannot recombine di­rectly until it passes into the singlet state. So, therecombination probability is proportional to thepopulation in the singlet state. Therefore, depend­ing upon the starting radical pair (singlet or trip­let) and the effect associated with external magne­tic field on different 'g-factors' (6.g-mechanism),hfi-mechanism or Zeeman splitting of triplet sub­levels, the corresponding cage or escape productsare determined as a consequence of singlet .•..•triplet ISC. For example, starting with a tripletradical pair the proportion of the cage productsrelative to the escape products decreases in thepresence of an external magnetic field4.6, becauseonly To is amenable to ISC due to Zeeman splitt­ing. Starting with a singlet radical pair, on theother hand, the cage product increases for thesame reason. Again since the rephasing timeshortens as ~g and/or the magnetic flux density

676 INDIAN J CHEM, SEe. A, SEPTEMBER 1995

Table I--Homopolymerization under magnetic field (MF)

t, a

Monolllpr Pol)'lllerl- In1tl~tl Polymerization conditionzation -----------techn1 queb Light Special d Tillie

or heat additives (h)

MF

(KG)l'.agnetie field e!feet"

"co::l'verslan Joft! x 1,,-5 FAReference

79999

108888888888

16

16

1614

14

14

1515

15

25

22

22

22

222129

29

29

29

1.54(2.27)'.53(2.31)'.54(2.31)

3.6(12.8)2.8(44.3)

3.1(11.1)5.2(3.9)2.66(3.37)

1.54(2.27)

54.,1(42.4)f57.1(34.6)f100.S(40.7)

4.0(0.75)

7.19(0.37)6,95(t':77)

5.oo(~.52)8.18(8.10)5.0(4.9)18(0.9)2(2)5(5)0.43(0.47)0.11(0.13)0.00(0.00)100(46)55(33)30(25)131(65)0.56(0.56)f2.7(0.56)f3.1 (0.89)0.72(0.72)2.4(0.74)

2.8(0.77)

91(8)81(19)94(8)91(92)79(72.4)

43(45)80(S5)

75(40)84(82)60(60)53(25)

69(47)3609

71h

17h30180.3(38.12) 4.3(3.2)11(6.6)8.2(6.2)19.3(8.8)9.7(0.9)

522

22155556.4

5555512

12

122

2

28111.11.11.11.11.11.•11.8

1.8

1.8

1.8

SDS

SOS+LaCl3 2(2.5)SOS+MgCl2 2SOS+hexane 2

SOS 2SDS 1.5

50sSOS

SDS

50S 1SOS 10

SUS 1

SDS 1.550s 1.550s 1

1

PVN.l 1PVN:2 1

1

PVBl 1

PVB..2 ,N-PEG-N

N-PEG-B

B-PEG-B

1

DENS (.50.5

0.5

0.5

0.5FfiD1A 10

4>m-300 10Pl'iEO/S.7 10

Poly(St1-b- ,0MAANa2)

llV

VV

UV

UV

UV

UV

UV

UV

65°C6SoC6SoCuv

UV

uv

UV

uv

uv

UV

UV

uv

UV

UV

UV

UV

UV

60°C

700C

70°C700C

70°C70°C600C

600C

60°C600C

:..

DB!!:

DB!!:

DBK

DB!!:

DBK

BenzoinDPAE+MAP

DPAE

DPAE

AIEN

Na2S,20SDBK

DBK

PBK

DW-P

t-BDBK

.lIEN

AIEN

AIBN

AIEN

AIBN

AIBN

AIBN

AIaN

.lIEN

AISR

":252°8BPO

BPO

BPO

BPO

EP

EP

EP

EP

EP

EP

EP

EP

EP

EP

EP

SL

EP

EP

EP

EP

BP

BP

BP

BP

BP

BP

liPliP

BP

BP

EP

liP

SL

SL

BP

A

A

A

A

SS

S55S5555S5

MI'.A

MI'.A

MI1A

AA

55SSS

S~S5

1n the ab,ence of maqnet1c ~ield,fMF).. m rate increased by ,.qo I,;).

a. S=Styre e, MII.A=1'.ethylmethacrylate" AA=Acry11c ac1d, A/Il=Acrylon1trUe, BI.'A=Butylmethacryla teb. EP=Emul 10n pol}'l'ler1zation, SL=5oJ.ut10n, BP=Bulle, A=Aqueous

Co DBK••01b nzyl leetone .DPAE.'l ,t'-d1phenyl-l,l'-azoeth ane), IIAP",P-r:.thoxyacetoph enone, PPK=Phenylbenzylleetone. DP/.'P=1.2-d1phenyl-2-methyl propanone. t-Bn~K= p-t-butyl',d1benzyl leetone,• - -- 3 -

d. 5DS=50d urn dodecyl SUlfate. J'VNl",Polyv1nylnaphthalene (conc.=1.62xlO- mol/I), pI''''2=PVN (conc.9.12xlO-~lIlol /1) PVBl=Polyv1nyl benzophenone (conc.=o.98xlo-3mol /1), PVR:?=PVB(conc.=8.17xl0-3mol 11),D?NS=Do eeyl benzene natr1um sulflJnate, PEG=Polyethylene glycol IJI'/i=300), PI'EC>=Poly(olioo ethyleneoxide m thacrylate) (n",e.7) ,

The figlres with1n parentheses 1nd1cate corresoonding v.'

The nUIT,('1 average Illo1ecular weight. \I. The conv.The pFr entaqe of conversion 1ncreased by A~,The per entage of convprs10n decre'ased by l1f!

e.f•h.1.

I

bisisobutytonitrile, AIBN; 1,1' -diphenyl-1,1'-azoet~ane, DPAE), or even direct photoin­

duced i~iation8' The effect of MF is also

changed b the salt effects on micellar structure9.The ext mal MF effect on photoinduced emul­

sion poly erizationof styrene has been explainedon th~ bas s of photolysis of DBK producing trip-

let radical pairs in micellar solution. In the abs­ence of an applied MF, ISC from all the three tripletsublevels (T+, To, L) to singlet (S) occurs and afraction of the total geminate triplet populationundergoes recombination within micelle. But inthe presence of an applied MF only ISC from To toS occurs since the degeneracy of T ± with S is lift-

II Ilil ~ lit

MAITI et al: MAGNETIC FIElD EFFECT ON POLYMERS 677

The increase in rate of emulsion photopolymeri­zation of methyl methacrylate (MMA)lI initiatedby triethylaminebenzophenone and that of vinyla­cetate12 initiated by benzoin under MF (0.8 KG)were reported. The influence of MF on MW is al­so observed when conversion is low. But whenthe polymerization attains the limiting conversionthe effectofMF on MW is negligible.

Huang et aL13-15 utilized a second polymer s0­lution [viz.polyvinyl naphthalene (PVN) and poly­vinyl benzophenone (PVB), polyethylene glycol(PEG) with naphthyl end groups (N-PEG-N),PEG with benzoylbenzyl end grou~ (B-PEG-B)and PEG with a naphthyl group at one end and abenzoylbenzyl group at the other (B-PEG-N)] toprovide the cage in order to get the MF effect onphotopolymerization of vinyl monomers such asMMA and styrene in bulk or in aqueous solutionwithout adding an emulsifying agent. Besides pro­viding a cage, the dissolved polymer also acts asphotosensitizer. The polymers formed under MFpossessed higher MW, narrower molecular weightdistribution (MWD), higher stereoregularity andthermal stability13.The MW increases with theMF strength (Fig. 4, curves 3 to 6). By UV-irradia­tion, the triplet energy transfer from the excitedsensitizer moiety of the added macromolecules tosmall initiator molecules produces triplet radicalpair because of spin conservation, and these radi­cals initiate polymerization. The MF effect liesagain on the energy splitting of the triplet suble­vels (TH To.T_) and so only To remains degener­ate with S. Thus about two thirds of the radicalpairs remain in the triplet state. Or in other wordsthe triplet radical pairs have much longer life-timeand it is difficult to recombine them since thespins are parallel. As a result the triplet chain in­itiating species and propagating radicals havemore life-time to react with monomers ratherthan chain termination resulting in higher MWpolymer. Again the MWD of the polymer is nar­row because of similar life time of propagatingtriplet radical pair.

However, MW and concentration of theadaed polymers are important for the MFeffect16. The influence of MF is not observed ifnaphthalene or benzophenone and PVN or PVBwith a MW and concentration less than the criti­cal value are used. The polymer entanglement (si­milar to cage) provides efficient contact distancebetween macromolecules and small AIBN mole­cules for the benefit of triplet energy transfer (cri­tical distance: 10-15 A). It may also be noted herethat to get optimum MF effect, the reaction mustbe carried out at low conversion ( - 2ook) so that

6

5

2

3

4

14

1210

...'0)( 8~::l

6

420

0

6 8 10 12

Magnetic field (KG)

ed. SO,only 1/3 of the geminate triplet populationundergoes cage recombination and 2/3 escapefrom the micelle. So, the isolated radical in themicelle can grow uninterrupted until another radi­cal enters and terminates the radical. This is re­sponsible for both increased rate of polymeriza­tion and high MW of polymer in the micelles un­derMF.

Turro et aL showed mainly the variation of MWof the polymer with MF (Fig. 4, curve 2). It wasreported that MW increased by a factor of five bythe application of MF of 0.5 KG, and that thevariation of polymer yield with MF was the sameas that of MW. However, these authors did notstudy the effect of MF on the polymerization kin­etics. The effect of MF was observed at the earlystage of polymerization but it was not clear whythe MF effect was not observed in the latter stage.Mondal et aL10 studied the kinetics of the sameemulsion polymerization system with benzoin asphotoinitiator instead of DBK. Tl1ese authorsclaimed thijt the polymer particle nucleation peri­od is shoJlened in the presence of MF and hencethe rete of polymerization is increased.

Fig. 4- The variation of molecular weight (MW) with magne­tic field: (1) PAN prepared in bulk25, (2) PS produced byphotoinduced emulsion· polymerization7, (3) PS prepared inbulk in presence of PVN . (MW= 1.81 x 104,conc. = 1.62 x 10-3 moVI)!4, (4) PS prepared in bulk in pres­ence of PVB (MW= 1.15 x 104, conc.=2.76 x 103 moVl)!4, (5)PS prepared by aqueous polymeriza,tion in presence ofB-PEG-N (Mn=600, conc.= 1.45 x 10-4 moVl)15, (6) PS pre­pared by a~eous polymerization in presence of N-PEG-N

(Mn=600, conc.= 1.45 x 10-4 moVl)!5.

678 INDIAN J CHEM. SEe. A, SEPTEMBER 1995

Propagation and TI!rminotion

co

;.Y-MO 11(n-l) M ...-

R'V

Fig. S-Schernatic representation of photodecomposition ofketone initiators in cages and a radical polymerization of vi­nyl monomers (M) under magnetic field (circles represent

cages).

propagation starts in the cages as shown in I, II, IIIand IV. These are represented in a simplifiedmanner as in V, where M is a monomeric radical.The monomeric radical in the presence of R' canalso propagate to give polymeric radical (M;..)aslong as the triplet nature is preserved (step 16).As soon as ISC from triplet to singlet occurs, the

.growing polymeric radical terminates and givesthe polymer MmR (step 17). The polymer yieldand the degree of polymerization M are, there­fore, determined by the triplet to singlet ISC effi­ciency and are increased in the presence of MF.The escape of one radical, R from the cage ~step18) leads to the uninterrupted propagation of theisolated radical to form polymeric radical, M~(step 19). Thus propagation continues until a sec­ond radical, R'; enters and terminates the propa­gation (step 20). The indirect effect of MF which

the varia on of viscosity of bulk solution is not Generation of RlIdicals

&igenou to offset the influence of the variationoflocal . osity.

The effect on the rate of polymerization al­.th the viscosity of the medium for the

ylene blue) sensitized photopolymeriza­rylamide using triethanol amine as re­

ducing ent17• The rate of polymerization wasfound to increase in ethylene glycol or in water­glycerol . re, but not in aqueous medium on­ly. The otored'lction of methylene blue involv- Initiation

ing a tri et state forms triplet radical pairs con- --­taining 'cal ions. In the presence of MF thetwo-fold crease of the semiquinone free radicalsand esca of the radicals from the cage enhancethe rate f polymefization. The cage effect maybe due t the added ethylene glycol or glycerol assolvent. he viscosity uf the media has also animportan role in MF effect. The restricted move­ment of e radjcal in a viscous medium is, there­fore, also cause of MF effect.

~pMropoo/memaMnmffiem~let radical pair is commonly producedxcitation of ketone initiators like diben­, benzoin, benzophenone etc. Consider-R as an example, the generation ofical pair and polymerization of vinyl(M) in cages (micelles) under MF are

shown' Fig. 5. Initially the photolytic cleavageof the k one produces a triplet, 3(RCO"R), radi­cal pair step 3) followed by d(~boxylation toJ(R"R) ( tep 4) or coupling of the ReO' and R toproduce singlet radical pair, I(RCO"R) (step 5)and the (R-C0-R) (step 8). The triplet radicalpair, J(R 'R) produces singlet radical pair, I(R"R)(step 6) d finaliy the cage product, R-R (step 9)or one f the radicals, R' exits from the cage(step 7). Only the recombination back to singletfrom the corresponding triplet radical pairs (steps5 and 6 is lowered in the presence of MF. Butsteps 4 d 7 are directly independent of MF ef­fect. So' the competitiOftbetween steps 5 and 4,and step 6 and 7, steps 4 and 7 pn::dominate. Asa result 'gher extent of radical escape from thecage rs. The actual medumisti<; basis of theMF etf. t is the lowering of the triplet to singletISC in s ps 5 and 6 due to Zeeman splitting ofthe tripl t states, which enhances the radical es­cape fro the cage and consequt:ntly the quantlimyield fo the polymerization in the cages in­creases. i

The R~~O' or R radical formt:d can initiate po­

lynwriza,on an.d produce monomeric radicalOV1jor 1''''1:-) ~s shown in steps 11. 13 and 14. The

II "" " "'I111,1

I 'It

MAll et aL: MAGNETIC FIELD EFFECT ON POLYMERS 679

Transition state

increases the rate of radical escape from the cagealso leads to higher polymer yield and MW of thepolymer.

Thermal radical polymerizationSchmid, Muhr and Marek II! polymerized sty­

rene under MF (16 KG) at 80°C. The reducedrate of polymerization under MF was explainedto be due to the orientation of reacting molecules.H is likely that an external MF would orient notthe radicals but only the spin moments of the oddelectrons. The chain propagation step in the fieldfree space occurs by a preliminary uncoupling .ofthe n-electrons. The process can be representedas follows19.

! !!~Ph -CH-CH2-R

Radical

In the presence of a strong external MF,the uncoupled state is more probably

t t t[Ph- CH - CH2 - R], where all three electron spinmoments are oriented with respect to the externalfield. Such a configuration would not lead tobond formation in either direction and henceagrees with the observed deceleration by the ex­ternal ME Wojtczak20 found a strong inhibition ofacetaldehyde polymerization in aqueous solutionby steady MF but the rate increased under an al­ternating ME

The radical photopolymerization also followsthe above mentioned chain propagation. If themechanism of MF effect is the orientation of spinmoments, the rate and polymer yield of radicalpolymerization of vinyl monomer should alwaysdecrease. In photopolymerization, the predomi­nancy of triplet - singlet ISC should be consid­ered for the increased rate of polymerization. Butthe rate and yield of thermal radical polymeriza­tion of vinyl monomers should always decrease.However, recentIyZl- 29 experimental evidences areavailable where both the rate of polymerizationand polymer yield increase in thermal polymeriza­tion under ME Some of the properties of the p0­lymer synthesized under MF are also improvedthan those of the polymers prepared without MF.

Simionescu et aL polymerized MMA.21in bulk,and butyl methacrylate22 by various methods ofpolymerization (bulk, solution and emulsion) in­itiated by thermal decomposition of dibenzoyl

peroxide (BPO) in the presence and absence ofMF (1.1 KG). The increased rate in bulk, emul­sion and solution polymerization in polar solventsin the above cases was observed under ME Butin non-polar solvents the rate of polymerizationdecreases. The induction period and activationenergy of polymerization decrease while the MWand thermal stability of the polymers increase.But the electrical conductivity shows lower valuesas compared to those of the poly(butyl methacryl­ate) obtained in the absence of MF. These observ­ations were explained by the radical shifting fromthe singlet to the triplet state under the influenceof MF which increases initiation efficiencythrough the reduction of 'cage' recombinationreaction. But how the singlet to triplet conversionis influenced by MF and how the cage is made in,bulk and solution polymerization are not clear.Though the solvent may act as cage, it is morereasonable to assume that the cage may beformed by the polymer itself. The singlet to tripletconversion is also possible in the presence of MFif Ag# 0 (as discussed in Eq. 1).

The thermal decomposition of BPO producestwo identical radicals, viz., C6HsCOO' having thesame g-factor i.e., the Ag value for initially folllledradical pair is zero. So at the initial stage of p0­lymerization there should not be any possibility ofS-To conversion under the influence of MF.Thepossibility of one decarboxylation and formationof a radical pair (C6HsCOO"C6HS) cannot be ex­cluded. But more possible formation of theradi­cal pair (C~s'-C6H5) after both decarboxylationresults in zero Ag. The decrease in activation en­ergy of polymerization under MF is also contrad­ictory sioce the energy change even by the stron­gest external (laboratory) MP3 of 100 KG ismerely 0.03 kcaVmol even for paramagneticmolecules. However, in the course of the study onthe bulk copolymerization of styrene and acrylon­itrile initiated by BPO under UV-irradiation, it isalso observed that the yield of copolymer in­creases under MF. The decomposition of BPOthermally as well as by UV-irradiation is compli­cated in the presence of magnetic field24.

It has been reported that the polymer yield in­creases in the bulk polymerization of acrylonitrile(AN) at 60"C initiated by AlBN under MPs. TheMW of PAN also increases with the increasing MFstrength (Fig. 4, curve 1). The PAN polymer pre­pared under MF was thermally more stable andof higher degree of syndiotacticity than that pre­pared without MF. It may be noted that the ther­mal decomposition of AlBN produces .radicals

680 INDIAN J CHEM. SEe. A, SEPTEMBER 1995

with dg'= 0, and therefore the S -+ To ISC is notenhance by the MF, consequently the rate of po­lymeriz .on should not increa'ie. The bulk po­lymeriza .on of AN is heterogeneous in naturewith po er precipitating out of the monomerand is 'te different from the bulk polymeriza-tion Qf styrene, MMA and butyl methacrylate.The hi er rate and MW under MF may be dueto the i uence of MF on the termination bycombina .on of macroradicals which in turn is go­verned y the restricted translational motion ofmacrora icals as it precipitates out.

Polym rization of AN in aqueous solution usingthe redo initiator system of H202 and thiourea atroom te perature under MF has been carried outin the a thors' laboratory26. The polymerizationkinetics ow sinusoidal shape with ME This maybe due t the sinusoidal change of viscosity anddensity f water under Mp27.However, detailed~ork is necessary to understand the processclearly. I may be possible that the sinusoidal na-'ture of ueous polymerization of AN (initiatorbeing wa er soluble) under MF is determined bythe diffu ion process of radicals and monomermolecule controlled by the sinusoidal change ofviscosity nd density of water.

Imoto t al.28 reported aqueous polymerizationof MMA initiated by the hydrophilic macromole­cule, pol 2-hydroxyethyl methacrylate) (PHEMA)at 20-30° under a MF of 141 KG. The initiationmechanis of polymerization takes place throughthe hydr gen atom transfer from the monomercomplexe at the OH groups of PHEMA to thefree mon mer. The polymer produced in threelayers, vi ., the MMA layer, the aqueous layerand the h drophobic area (HA) formed by swoll­en PHE were isolated separately by precipi­tating ou in methanol. Polymerization proceedsalmost en irely in the HA and even by heating to85°C for h it hardly proceeds in water phase inthe absen e of MF. But in the presence of MF,the poly erization of MMA increases markedlyin HA an takes place not only in the HA but al­so in wa r phase and the conversion in waterphase rea hes between 113 and 112 of the con­version in HA. Again, the conversion of monomerand MW f the polymer increase with increase inMF stren h in the range of 0-1 KG. But there isa tendenc 29 for the conversion to saturate be­yond 1 K . The tendency of grafting MMA ontoPHEMA i high, (> 90%) regardless of the intens­ityofMF. I

Though ~he initiation mechanism of PHEMA inthe HA i~different from the photoinduced po-

lymerization ·in cages, the effect of MF is quiteanalogous. At the first step of polymerization aradical pair in a singlet (S) state is generated in theHAformed bythe mother (added)polymer,PHEMA,which in the presence of MF is converted in­to the triplet state by ISC. This ISC may be dueto different g-factors of the two different radicalsand is pronounced by the ME Since the radicalpair in the triplet state cannot recombine, thesetwo radicals can initiate polymerization. As a re­sult the monomer conversion and the MW of thepolymer produced increase with increase in MF.Again, since the recombination rate of radicalpairs diminishes, the ease of radical escape fromthe HA to aqueous phase increases, thereby,causing polymerization in the aqueous phase inthe presence of MF. The overall activation energywith and without MF are 29.5 and 30.3 kJ/molrespectively.

There is also effect of MF on the radical po­lymerization of vinyl monomers initiated by sever­al other kinds of hydrophilic macromolecules. De­pending upon the hardness and the tightness ofhydrophobic areas (reaction areas) formed by dif­ferent kinds of mother polymers e.g.,graft or blockcopolymers or the mother polymers of differenttacticity, the extent of MF effect varies. The orderof the mother polymers for enhancement of po­lymerization is: PEG-300 < PHEMA < PMEO/4.0 <PMEO/8.7 < Poly(Stj-Co-ME02) and Po­ly(Stj-r-MAA-Na2) < Poly(Stj-b-MAA-Na2)·

Copolymerization under magnetic fieldSimionescu et al30 tried graft polymerization of

acrylonitrile (AN) onto the film of carboxymethylcellulose immersed in aqueous CeS04 and H2S04solution initiated by various forms of energy oneof which is the MF (1.44 KG) at 49-60°C. But nografting took place under ME However, in thepresence of MF (12 KG) the graft copolymeriza­tion of isoprene onto tetrafluoroethylene-propy­lene copolymer3! and that of MMA onto polyvi­nyl alcohol (PYA)32using UV-radiation and ben­zophenone as catalyst occurs. The graft ratio aswell as stereoregularity of grafts in both the casesincreased with MF reaching the maximum stere­oregularity at 85% graft ratio at the MF of 12KG.

Since PYA is a water soluble polymer, the graftcopolymerization of MMA onto PVA is very simi­lar to the emulsion polymerization. The cageformed by PYA macromolecules contains MMAand benzophenone. Under UV-irradiation, the ex­cited benzophenone triplet abstracts a hydrogenatom from the tertiary carbon of PYA and pro-

MAITI et al.: MAGNETIC FIELD EFFECT ON POLYMERS 681

~6

(4-/I-he xy loxyphen y I)-4-acry loy lox ybenzoate

Table 2-Liquid crystal (LC) monomers and their structurespolymerized under magnetic field (MF).

39

38

37

ReferenceMonomer

N-(fI-mcthacrylnylnx y hcnzilidcnl' )-fl-ct hnx yanilinc

N '(I'-methox y-o-hydrox ybenzy Iiden~ )-I'-alninost yrene

V. CH,ICHz'" C - COO--@- CH'" N -@-OCz",

N -(tHlcr.ylox ybcnz y Iidicnc )-/1-11 -hut y Ian iIinc

I CHz •••CH-COO-@-CH",N-@-CN ~sfI- anyloy lox yhcn7. y Iidene· tH'Y annan iIinc

II CHz'" CH-COO-@-CH=N-@-N"'CH-@-COO-CH = CHz35.39

IV MeO--<§(- CH= N-@- CH '" CHzOH

ViI N -p-acryloy lox ybenzy Iidene)- t,-d iaminnhcnzene

III CHz=CH-COO-@-COO-@- OC,H13

duces a triplet radical pair consisting of ketyl andPYA. In the presence of MF due to Zeeman ef­fect, the longer life of triplet pairs ensures forma­tion of higher graft ratio of the copolymer. Thegrafted copolymers prepared under MF showedimproved moisture and heat resistance properties.This is perhaps due to the increased graft ratioand greater stereoregularity of the copolymers ob­tained under ME

Thermal emulsion copolymerization of vinylacetate/MMNacrylamide was also reported un­der MFB. Decrease in reaction time and increasein rate of copolymerization under MF (1.3 KG)were reported. However, the effect of MF on var­ious aspects of copolymerization such as the cop­olymer composition, reactivity ratio of the comon­omers, comonomer sequence, steric arrangementsof the comonomers in the chain etc. has not beenreported. In the authors' laboratory, studies havebeen made in the copolymerization of styrene andacrylonitrile under MF and effects of MF onmonomer r'eactivity ratio, monomer sequence incopolymer formed, copolymer composition andtacticity of the copolymer chain have been exa­mined34•

Synthesis of liquid crystal polymers under mag­netic field

Polymers having rod-like conformation, orwith mesomorphic or potentially mesomorphicside groups may display liquid crystal (LC) behav­iour. To obtain mesomorphic phase in polymers,the monomers required need not necessarily bealways mesomorphic in nature. Again, the me­somorphic order of polymers obtained by polym­erization of monomers from LC phases is not ne­cessarily that of the monomers. A nematic mon­omer may also yield a polymer with smectic or­dering. Though examples of synthesis of LC po­lymers under MF where monomer is not LC areavailable, the main attention is to polymerize un­der MF the LC monomers which have mesomor­

phic side groups that produce well oriented po­lymers (Table 2). Here, it may be mentioned thatthe LC polymer itself can be oriented under MF;this feature is, however, beyond the scope of thepresent review.

Liebert and Strzelecki35 copolymerized two LCmonomers: jracryloyloxybenzylidene- jrcyanoaniline(I) and di(N-jracryloyloxy benzyli­dene)-jrdiaminobenzene(ll) from their nematicphase under MF (8 KG). The polymer obtainedwas highly oriented and crystalline, having thesame mesomorphic phase as that of monomers.Similarly> the polymerization of LC vinyl monom-

VlI. CHz •• CH-COO-@-CH=N-N=CH-@-COO_CH=CHz

39

Di(N-tHlcryloyloxybcnzylidene) hydrazinc

VlIl CHz '" CH-@- N = CH-@- CH= N-@-CH=CHz

39

(I-phenylene his (N-Illethylenc-,..~tyrenc)

er, .(4- n-hexyloxyphenyl )-4-acryloyloxybenzo­ate(III) under MF results in a fully oriented po­lymer useful in integrated optics and optoelec­tronics36. The rate of polymerization, the viscosityand solubility of the polymerie product of the ne­matic LC monomer, N-(jrmethoxy-crhydroxybenzylidene)- jraminostyrene(IV), werenot affected by the application of MF (1.3 KG)during its polymerization37•

Perplies et al.38 studied the orientation as wellas rate of polymerization of 1£ monomer under astrong MF (70 KG). The 1£ vinyl monomer, jr(4-ethoxyphenyliminomethylidine)- jrethoxy­aniline(V), gave a LC polymer of the smectic Atype. The increase in rate U8der MF was ob­served during the polymerization of the monomerfrom its nematic phase and not from its isotropic.phase. The acceleration rate was explained by thetransformation of the nematic cluster structure tohomogenous nematic stmctuTe by the MF andthis homogeneous order has been ascribed by the

682 INDIAN J CHEM. SEe. A, SEPTEMBER 1995

authors ~o the decrease in the chain terminationstep. '

Clo'ug~ el aUY polymerized various LC mon-,omers apove the respective crystal melting tem­

perature I without addition of initiator under MF(4.5 KG. The polymerization of the monomer,di(N -p-a ryloyloxybenzylidene)- p-diamino enzene(II) in its nematic phase gave asmectic o\ymer having side chains aligned withthe field and the polymer backbone chains con­fined to erpendicular to the side groups. Copo­lymeriza 'on of nematic(II) with N-(p-acryloyloxy­benzylid ne)-p-n-butylaniline(VI) (1:1) also gave asmectic p Iymer. High degrees of side chain orien­tation (0 ientation parameter, f =, 0.7) have beenachieved i under ME Thermal e:xpansion of thispolymer ~howed large anisotropy. But the polym­erization Ii of both the monomers, di(N-p­

acryloylohbenzylidene) hydrazine(VII) and p­phenylenf bis (N-methylene-p-aminostyrene) (VIII)in their I nematic state resulted in polymerswith nemptic order. This phenomenon appears tobe depeddent on the ability of interacting sidegroups tq pack into an orderly layered arrange­ment witijout disturbing the regullar disposition ofthe poly ric backbone.

The m in objective of these polymerizationswas the 0 'entation of the polymers under MF al­though B rplies et al.38 studied the rate of polym­erization Iso. However, the polymerization of y­benzyI-L- utamate N-carboxy anhydride (BLG­NCA) un er MF (3.6 KG) provides an examplewhere th' monomer is not LC but the polymerobtained s LC40, The MW of the polymer, po­ly(y-benz l-L-glutamate) (PBLG) obtained underMF was Iso higher. The activation energies ofpolymeriz tion with and without application ofMF are t e same. So the rate of acceleration maybe explai ed by the suitable alignment of BLG­NCA mo omers and/or growing polymer chains

by the ME!

Crosslinki g reaction under magnetic fieldThermal c osslinking reaction

Very fe curinglcrosslinking reactions of po­lymers un er MF were available before 1980.Different 'lled and reinforced polymer systemsshow bett I' physico-mechanical properties whencured und I' ME The alignment of the magneticfiller parti les during curing under MF improvesthe proper ies of the crosslinked product. Polyes­ter compo 'tes containing Fe powder (particle size4-20 Il) d aligned under MF (0.4 KG) duringcuring, sh wed improved tensile and flexuralproperties4

The physico-mechanical properties of filled andreinforced epoxy resin increased 20-30% oncrosslinking under constant MP2. The physico­mechanical properties of epoxy resins crosslinkedunder MF showed a sinusoidal variation withthe pronounced maximum at 0.6-0.7 'KG whenED-5 was crosslinked in a MF with diethylenetriamine, triethylenc tctramine, hexamethylene di­amine and a reaction product of polyethylenepo­Iyamine with dimerized fatty acid esters of linseedoil43, The flex strength of polyamine crosslinkedepoxy resin ED- 5 increased to maximum withinthe first 15 min of treatment under the ME Long­er exposure to the MF produced no further im­provement of flexural strength44, Crosslinking ofED-20 epoxy resin with polyethylene polyamine at60°C for 6 h in a MF resulted in a highly cross­linked, more oriented polymer as compared tothe polymer obtained without MPs.

Crosslinking of phenol-furfural resin in a MFincreased the microhardness and hence mechani­

cal properties of the crosslinked polymer46. Thebending strength of the crosslinked polymer in­creased by 30-4()Oh,when crosslinked under ME

Like the orientation of certainliller particles ina polymer matrix, the orientation of matrix po­lymers, particularly liquid crystal (LC) polymers,is also possible under ME It is also possible tofirst orient the polymer matrix under MF andthen freeze the oriented structure by crosslinking.One advantage of magnetically orientednetworks over mechanically oriented net­work is that it does not loose its oreintation uponheating readily. Poly(y-benzyl-L-glutamate) (PBLG)was thermally crosslinked with 1,6-hexanediamineunder MF to get a permanent chain alignment ofthe polymer47, The permanent chain alignment ofnematic phase lasted over a year when cross­linked under ME

Barclay el al.48 treated some prepolymers underMF in order to obtain aligned and cured polym­ers. The epoxy terminated prepolymers based onoligoethers of 4,4'-dihydroxy-a-methylstilbene(DHMS) were crosslinked with stoichiometricamount of methylene dianiline (MDA) in the ne­matic state of the prepolymers under MF (135KG). The magnetically aligned LC epoxy networksshow different improved properties over the non­aligned or mechanically aligned networks. A signi­ficant degree of alignment was observed by thewide angle X-ray diffraction (WAXD) of the re­sulting crosslinked networks. For example, a parti­cular thermoset prepared from the mesogenicprepolymers under MF achieved an orientation

"

MAITI et al.: MAGNETIC FIELD EFFECT ON POLYMERS 683

Fig. 6-Schematic representation of the magnetic field effecton the network formation of LC prepolymers.

that in the absence of ME On the other hand, fordirect excitation of AZMS (MW= 10,300) underMF (0.89 KG) the gel fraction of AZMS dec­reases, and the efficiency of photocrosslinkingdecreases by 20%.

The degree of crosslinking measured by soV gelanalysis increased for the photocrosslinking ofbutadiene-styrene copolymer with ketonic initiator(eg., benzophenone or deoxybenzoin) by uv-radia­tion under the influence of MF (10 KG)56. Therate and mechanisms of crosslinking for sensi­tized, direct excitation and photolysis of initiatorsystems are different (Fig. 7). The discussion onthese systems follows.

Mechanism of photocrosslinkingThe photocrosslinking reaction follows a radical

mechanism and the effect of MF is explained onthe basis of radical pair model. The mechanismsof photocrosslinking with various initiating sys­tems (e.g., sensitized excitation52,53, direct excita­tion54,55'and photolysis of initiator56) are schema­tically represented in Fig. 7.

i(a) For the polymer (PH)/triplet sensitizer (SH),where the polymer has no photoactive group52,the. only process of crosslinking is through thesensitizer. The triplet sensitizer (3SH) by the ab­straction of H-atom from the polymer (PH) formsa triplet radical pair, 3P"SH2) (step 1, Fig. 7). Theinfluence of MF decreases the rate of conversionof triplet into singlet (step 2) as a result of theZeeman splitting· of the triplet state. Consequent­ly, the concentration of the triplet radical pairs in­creased. Thus the availability of the radicals forcrosslinking became enhanced which caused theincreased efficiency of crosslinking (step 3, Fig. 7).

(b) In the triplet sensitized photocrosslinking

parameter (f) of 0.57 in contrast to the maximumvalue of 0.16 achieved mechanically. Magneticallyoriented networks retain their orientation to tem­

peratures well above Tg and show reduction in thecoefficient of thermal expansion (CTE).

Similarly, the well-oriented and thermally stable(at least 100°C above the Tg) crosslinked LC tria­zine network with low values of CTE in the direc­

tion of the applied MF was obtained by thermalcyclotrimerization of dicyanate compounds ofring-substituted bis(4-hydroxyphenyl) terephtha­lates under MF (135 KG)49. The resulting 1,3,5­triazine rigid-rod networks formed a. mesophaseas the curing reaction proceeded and the meso­phase flowed less readily and was eventually 'froz­en' as the crosslinked network was formed andgrown to full potential. By the influence of MF,alignment of the LC phase during curing processproduced smectic ordered triazine networks. Thecuring of LC prepolymers in the presence andabsence of MF may be shown as in Fig. 6.

The application of MF may also influence thekinetics and the efficiency of crosslinking whereinitiators/sensitizers are used to produce radicalsfor crosslink formation. The radical can be gener­ated either by photodecomposition or by photo­excitation of added initiator/sensitizer. The influ­ence of MF on photocrosslinking reactions will bediscussed later. Martl et aL50,51observed the in­fluence of MF on the thermal crosslinking of (z)­

1,4-polybutadiene with bi~2,4-dichlorobenzoyl)peroxide (CBP) at different temperatures and withdifferent concentrations of CBP. With increase in

the MF strength the crosslink density decreased.However, the influence of MF on the kinetics ofcrosslinking reactions was not reported.

Photocrosslinking reactionMorita et aL52observed the enhancement of .the

photocrosslinking of bromo- and chloro-methylat­ed polystyrene (BCMS) using 2,4-diisopentyl thi­oxanthone (DITX) and UV-light (A> 330 nm) un­der ME With similar technique, the photocross­linking of poly( styrene- eo--vinyl benzylazide)(AZMS) of different MW and also with differenttriplet sensitizer system like DITX, 2-chlorothiox­anthone (CTX) and Michler's ketone (MK) hasbeen studied in the presence and absence ofMF53. Without the use of sensitizer, crosslinkingof AZMS by direct excitation has also been re­ported54.55. The efficiency (i.e., rate) of photo­crosslinking increases by 45% (at 1 KG) forAZMS/DITX system and by 20% (at 5 KG) forAZMS/CTX system (MW of AZMS 12,900) over

Diso•• ,.d Netvork

Curing

0,.,., Netvork

684 INDIAN] CHEM. SEe. A, SEPTEMBER 1995

II. Oir.ct excitation:

h~AN) -,---

I. s.nsiiiztd excitation:I SH~ISH~3SH

(a) Po~mers ho.int no photot'xcitabl. group. '5H 3 I I

PH -----... (p. SH2) - •. Crosslinking, ,J:

I( p' 'SHtl

I_I P.ltlMr. with ph.I ••• itobit groupI ,

! ItN, + 3ltN, + SH

I~-NI5H ( I, I.

IltN: -,- 3ltN: -.-- J (ltNH' • 5 I _ t1RNH•• 5)

'~PH ~II

IlRNHltPlL3(RNH!I P I ...2...-Cr,••• linking

IltN,

IJ-.,

tltN: -1¥-- 3ltl~:

'!'H 7~~'"RNH-P ~ l(RNH!!PI ~ 3(ltNH! !Pl

••C,. •• linki",

111. PhotaltSiS of Initiators:I h~ I 15C 3! R, - co -RI ---,--- R, -co- RI""- R, -co - RI

I 31: tt t"'IR,CO' 'R:l ••..• 3(It,co" RI)

5 l'"Cro •• linkirit

Fig. ~-~C!'ematic representation of the mechanism of photo­

crosslinkin under magnetic field. RN 3 represents polymermolecule ( ) (e.g. AZMS), SH represents sensitizer molecule(e.g., DTIX CTX, MK etc.) and R1-CO-Rz represents ketone

initi or (e.g., benzophenone, deoxybenzoin etc.)!

of the pqlymer (RN3 or PH) having photoactivegroups53 fe.g., azido group in AZMS), the triplet

senSitize~(3SH) produces triplet polymer (lR.N,)

(step 1). riplet nitrene (3RN:) originating from3RN3 (ste 2) and then abstracting H-atom fromthe poly er or the sensitizer, forms a triplet radi­cal pair, 3 RNHOOP(step 3) or 3(RNHooS) (step 4).

The decr~asing rate of ISC (steps 5 and 6) underthe influ~ce of MF increases the efficiency ofcrosslinki g (steps 7 and 8). The possibility ofISC of t plet nitrene to singlet nitrene (step 9)cannot be excluded, but again ISC rate decreasesby two f d under ME Besides, triplet nitrene ismuch m re reactive than singlet nitrene. So,crosslinki g occurs through the triplet nitrene.And the ecrease in the rate of ISC (T -> S) of3RN: -> ] : by the influence of MF is also ex-

pected t~' ,..ncrease the efficiency of photocross­

linking. other possibility of crosslinking isthrough t e H -abstraction by 3SH from the po­

lymer like (a) (Fig. 7) but it is a minor process.

ii. In the direct excitation of the polymer (RN3or PH) having photoactive groups 54 (e.g., azidogroup in AZMS), the singlet nitrene eRN:) is pro­9uced from the singlet excited polymer eRN3)(step 2). The singlet nitrene abstracts a H-atomfrom the polymer and forms the singlet radicalpair, '(RNH"P (step 3). The singlet radical paireasily produces recombination product (crosslink­ing of two polymers, step 4). The ISC rate of thesinglet radical pair to triplet (step 5) is decreasedunder ME So, photocrosslinking _ under MFdecreases (step 8). Although the possibility of ISCof the singlet nitrene to the triplet nitrene (step 6)to generate triplet radical pair, 3(RNHooP) (step 7)cannot be excluded completely, it is a minor pro­cess.

iii. In the crosslinking of polymer (PH) by thephotolysis of ketonic initiator56 (R]-CO-R2), thetriplet radical pair, 3(R;-CO "R2) is produced (steps1 to 3). Under the influence of MF, the rate oftriplet to singlet ISC (step 4) is reduced due tothe Zeeman splitting of the triplet states (T +, To,T _) and hence, with a longer life time, the tripletpair undergoes less cage recombination yieldingmore free radicals for crosslinking (step 5).

Miscellaneous polymerizations under magneticfieldSynthesis of magnetic polymers

Now-a-days, one of the frontier investigationsin the polymer field is magnetic polymers,which involves synthesis of conjugated stable 1'0­lyradicals with long range magnetic orderingamong the spins. The net moment of the spinningelectrons is responsible for the magnetic propert­ies. Now, the net magnetic moment will be en­hanced if all the radical centers i.e., spins in themolecules, are aligned in the same direction underME Ota et al.57 prepared a thermosetting resincomposed of triarylmethane structure (COPNA)by heating at 130-140°C for 1 h a mixture of ter­ephthalaldehyde (TPA) and pyrene (molar ra­tio = 1.25:1) blended with 5 wt % p-toluene sul­phonic acid in argon atmosphere under a MF of0.44 KG or 0.88 KG, and the product resin exhi­bited ferromagnetic characteristics. This resin pre­pared under ordinary conditions is diamagnetic.However, the ferromagnetism of the resin disap­peared after pulverization into a fine powder sug­gesting that stacking and orientation of the consti­tuent molecules in the resin is responsible for fer­romagnetism.

Synthesis of conducting polymersOne classical route to electrical conducting 1'0­

lymeris to bound the ferromagnetic and electri-

II ' I I ~'I ;I It, 1'1 I ill I IIIi

MAlTl et oL: MAGNETIC FIELD EFFECT ON POLYMERS 685

cally conducting filler with a hardenable organicadhesive. Salyer et 01.58 prepared conductingphenolic resins by curing them thermally underMF (4 KG) with 5-60% ferromagnetic and electri­cally conducting fillers such as Fe, Co, Ni, Gd andalloys that were coated with metals like Cu, AI,Zn or Cr. During curing, interface of the layerwas maintained normal to the lines of force of theMF so that the filler particles are oriented toform an electrically conducting bridge betweenthe units. Positive conductivity of the filled materi­als was observed as compared to the zero con­ductivity when cured in the absence of ME Mak­arov et 01.59 examined the change in the volumeresistivity of ED- 20 epoxy resin on crosslinkingunder an external constant ME Since the constantMF has a directional effect, an anisotropy in vo­lume resistivity of ED- 20 was observed with re­spect to the strength and direction of the ME Thedecrease in resistivity of ED-20 in the field direc­tion during the initial stage of crosslinking sug­gested an increased generation and mobility ofionic formations due to a rupture of a-oxidegroups in the polymer molecule.

For preparation of conducting polyacetylene(PA) various new methods have been used duringthe process of polymerization in order to producehighly oriented PA. One such method is polymeri­zation under high MF where the polymerizationcatalyst molecules are oriented along with theorientation of the liquid crystal (LC) medium in apreferred direction. Thus, PA with higher aniso­tropy is obtained. Aldissi60 polymerized acetyleneunder MF (4 KG) at 25°C with Ziegler-Natta ca­talyst, [Ti(OBu)4/ AlEt3] using (4-methoxybenzyli­dine )-4- n-butylaniline nematic LC as the mediuminstead of toluene. Similarly, Tsuji et 01.6,1 polym­erized acetylene under MF of 8 KG in the pres­ence of the above catalyst utilizing equimolar mix­ture of two different kinds of nematic LCs,4-( trans·-4-n-propylcyclohexyl)ethoxybenzene and4-(4- trans·-4-n-propylcyclohexyl )butoxybenzene asthe medium. PA obtained under MF exhibits a

high electrical anisotropy with a preferred orien­tation of the polymer chains and a more orderedfibrillar structure. But the c~trans compositionand its density are similar to those obtained whentoluene is used as solvent. Previously, the reactiontemperature was above 273 K to keep two com­ponent of LC mixture in the nematic phase. Butpolymerization at lower reaction temperaturedown to 233 K or even lower and using suitablefive- or six-component LC mixtures aligned underMF results in c~rich PA films (90% cis­

content)62,63. EPR measurement of pure PA pre-

pared under MF (3 KG), by the Shirakawa tech­nique has been reported64. The spin mobility inthe sample prepared under MF decreased.

BiopolymerizationTarbet et 01.65 investigated a biological polymer­

ization process under a strong MF. The slow rateof polymerization of fibrinogen under MF obtainsoriented fibrin fiber. The orientation during po­lymerization is explained due to the diamagneticanisotropy of aggregated fibrinogen, and is exa­mined by magnetically induced birefringence mea­surement65,66 and also by light transmissivity anddegree of polarization of transmitted light67.68as afunction of polymerization time. Neutron diffrac­tion and scanning electron microscopy were uti­lized to distinguish the surface morphology ofsample prepared with and without MF. At the be­ginning of polymerization because of a small di­amagnetic anisotropy of small fibrinogen mole­cules and fibrin monomers, the magnetic orientingenergy is much less than the thermal energy (kT)and consequently the orientation is poor. The up­surge in the birefringence commences with po­lymerization as many monomers corne together inan ordered way. This is because the diamagneticanisotropy of the aggregates is many times that ofa monomer and the magnetic orienting energy issufficiently large to allow a high degree of orien­tation. It may be mentioned here. that the orienta­tion is relatively poor even in the strongest MFand the production of oriented fibrin network un­der MF is different from the production of orient­ed polymer during polymerization of LC monom­ers which are oriented before polymerization.Other investigations on biomolecules includingthe growth of the living cell under the influence ofMF have been reviewed69.

Radiation induceasolid state and plasmapolymerization

Mori et apo, while studying radiation inducedsolid state polymerization of acrylonitrile (AN) at77 K, observed an increase in the initial rate withincreasing MF which become maximum at 4 KG.The MW was, however, unaffected by MF. But inthe case of radiation induced solid state polymeri­zation of acetaldehyde a decrease of yield was ob­served71 ifMF was applied.

Plasma polymerization is a general term to in­clude plasma deposition of organic and inorganicmaterials. However, the plasma deposition of vi­nyl monomers produces the deposited polymersusually in the form of thin films on the substrate.So, only the plasma polymerizations of vinyl mon-

686 INDIAN J CHEM. SEe. A, SEPTEMBER 1995

omers\ associated with the influence of MF are in­c1udeq here. The influence of MF on the plasmapolym rization of tetrafluoroethylene (1FE) wasobserv d in a capacitively coupled system withintern electrodes using radio-frequency (13.56MHzj7. audio-frequency (10 KHz), AC dis­charge 3, and in a magnetron glow discharge e1ec­trode74 In the absence of MF, the most activezone 0 the plasma is the centre of the interelec­trode g p whereas the influence of MF moves thisactive one closer to the electrodesn.73. The MFinfluen es the deposition rate. The chemical char­acterist' s of the deposited polymers as revealedby ele ron spectroscopy for chemical analysis(ESCA) are also changed by the influence ofMf"'72- 7 . The reduction of breakdown voltage ata partie lar pressure with increasing MF strengthwas als reported for the plasma polymerizationof ethyl e (C2H4) and fluoroethylene (C2H3F )75.

conclu~lon

The agnetic field (MF) can exert great influ-ences on chemical reactions including polymeriza­tion/cros linking reactions, as light, pressure andheat do. Though in some cases the influence ofMF on . etics of polymerization is contradicto­ry20--22, e influence of Zeeman splitting of thetriplet st tes (T+, To, T _) and the singlet +<0 tripletintersyste crossing due to Ag- and hfi-mechan­ism expl some of the observations reason­ably 7- 17. -Iowever, depending upon the systerns,the poly er yield and molecualr weight (MW)can be in reased under magnetic field, mainly, inradical p lymerization7,14 -16,21,22.25and crosslink­ing [eacti ns52,53,56.Wide molecular weight dis­tribution MWD) for radical polymerization in

general, m~y also be made narrower under the in­fluence of fnagnetic field8"o,'5.J6.The properties ofpolymers uch as crystallinity, chain flexibility,solubility d thermal behaviour can also be con­trolled by he application of magnetic field. Themicrostruc re of the polymer chain, mainly tac­ticity, mon mer sequence in copolymer etc. maybe altered nder magnetic field34. The extent ofgrafting31- 3 and the composition of copolymer34can also belmodified by the application of magne-tic field. i .

The de$d for orientated liquid crystal po­lymer may \be partially fulfilled by polymerizingrelevant mo omers under magnetic field where insitu oriente 35.36.38-40,45,47liquid crystal polymer isformed whi h is much more stable and orderedthan that 0 ained without magnetic field or eventhose orient d-by other external forces39,48. Theordered sf cture in the liquid crystal polymers

may also be locked by crosslinking such systemunder magnetic field47-49.

Synthesis of magnetic polymers57 and conduct­ing polracetylene60 under magnetic field is a re­markable achievement. This novel route appearsto be promising for such syntheses in near future.

AcknowledgementThe authors wish to thank the CSIR (India) for

award of a fellowship to DSB.

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