I Magnetic field effect on polymers and...
Transcript of I Magnetic field effect on polymers and...
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.
References1 Steiner U E & Ulrich T, Chem Rev, 89 (1989) 51.2 Krigbaum W R, in Polymer liquid crystals, edited by A.
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