SYNTHESIS, CHARACTERIZATION AND PHOTORESPONSIVE PROPERTY OF AZO POLYMERS

3. SYNTHESIS, CHARACTERIZATION AND PHOTORESPONSIVE PROPERTY OF AZO POLYMERS

Although many chromophores are known to undergo

changes in physical properties under photoirradiation, the

number of useful photoactive molecules for photoresponsive

polymers are limited. Polymers containing azobenzene

moieties rank among the best known photochromic system

in view of their potential use as photoresponsive

systems. 1 2 0 t 1 2 1 When an azo polymer is subjected to

irradiation, geometrical change of the chromophore occurs,

which is then transferred to the polymer chain causing

reversible conformational change and eventually this would

lead to changes in physical properties of the polymer.

Here the synthesis, characterization and photoresponsive

property change of the hitherto unreported photosensitive

polyesters with azobenzene residues in the backbone are

discussed.

Result and Discussion

3.1 Synthesis of polyesters with azobenzene residues in the backbone

For the synthesis of polyesters, the first s t e p was

the preparation of azobenzene dicarboxylic acid.

Azobenzene -4,4'-dicarboxylic acid (15) was prepared by

the reduction of para-nitro Senzoic acid in ethanol with

Zinc dust and sodium hydroxide. 122 The carboxylic acid

was converted to the corresponding dicarbonyl chloride

(16) by refluxing with excess of thionyl chloride in

presence of a few drops of N, N-dirnethyl formamide as

catalyst. 123,124 The excess thionyl chloride was

distilled off and the resultant solid residue was

recrystallized from cyclo hexane or petroleum ether. They

were characterized by different analytical and spectral

methods. In the IR spectrum of azobenzene dicarboxylic

acid, the peak around 1680 cm-l is due to hydroxyl group

of carboxylic acid. In the spectrum of dicarbonyl

chloride the peak at 1680 cm-I disappeared and a new peak

appeared at 1730cm-I showing the replacement of hydroxyl *

group by chlorine. The introduction of azo group is

confirmed by the peak at 1580 cm-I in the IR spectrum;

corresponding to the N = N stretching frequency. The

characterization data of azobenzene 4, 4'-dicarboxylic

acid and dicarbonyl chloride are shown in Table 3.1.

Table 3.1 Characterization data of azobenzene - dicarboxylic acid and azobenzene dicarbonyl chloride

Compound

AZO benzene-4,4' dicarboxylic acid

A20 benzene-4,4' dicarbonyl chloride

Polyesters having azobenzene residues in the backnone

were synthesized by interfacial poly condensation 125,126

of azobenzene dicarbonyl chloride with dihydroxy compounds

( d i o l s ) .

Scheme 3.1. Dihyciroxy comlmunds u~ied for the synthesis of polyesters

Figure 3.1

1500 1000

WAVENUMBER (CM-I)

IR Spechum of azobenzene 4,4'-Dic;irhoxylic acid (-) and azohenzene 4,4'-Dicrabonyl chloride (- - -)

Table 3.2 Synthesis and characterization of dihydroxy conpaunds used for the synthesis of polyesters (PE)

Dihydroxy Synthetic Separative Eluting Colour Yield ~p IR(IW)CE-' Compound reaction method solvent

20 Simple Column 20% rethanol Colourless 40% --- 3400(OH) 172O(C=O) condensat ion chro~atograpby cblorof on liquid 1080 (C-O-C)

rixture

21 Simple Colm 20% methanol Colourless 28% --- 3450(0H) 1720(C=O) condensation chroratography chloroform liquid 1050 (C-0-C)

mixture

22 Sirple Crystalli- --- White 78% 156'~ 3400jOH) 17%(C=O) condensation sation fro^ crystalline 1240 (C-0-C)

toluene solid

For the synthesis of polyester PEi (23), equimolar

amounts of the dihydroxy conpaund (17) in aqueous sodium

hydroxide solution and azobenzene 4,4'-dicarbonyl chloride

(16) in freshly distilled chloroform were stirred

well for 20 minute under nitrogen atmosphere in the

presence of an emulsifying agent Na2S04 at room

temperature. The reaction mixture was then added to

acetone to coagulate the product. It was filtered,

washed repeatedly with water and dried. An yellow powder

of the polymer PEi is obtained. The structure of the

polymer was confirmed by spectral methods. The IR

spectrum of the polymer showed characteristic ester

carbonyl (C=O) stretching frequency at 1690 cm-I and C-0

stretching frequency at 1070 cm-l. UV-visible spectrum

was recorded in N, N-dimethyl acetamide ( max 328, 419

nm) .

In the same way interfacial polycondensation of

azobenzene dicarbonyl chloride and dihydroxy compounds 18,

19, 20, 21 and 22 leads to the formation of the polymer

PEii(24), PEiii(25), PEiv(26), PEv(27) and PEvi(28)

respectively. Details of polyesters synthesized are given

in Table 3 . 3 .

cIo c --Q+=N+-cocI

Scheme 3.2

COOH 0

PE vi CH3 HOO n 28

3.3 - POlyesters with abz monn in the harkl-mna

Table 3 . 3 Details of polyesters synthesized

Dihydroxy Azo benzene compound dicarbony 1 Poly ester Solvent Colour ~ ~ ( ~ e r ) n "

chloride

17 4 ,4 I-azobenzene PEi lone Pale yellow 2900(OH), 1120(C=0) dicarbonyl powder 1600(N=N), 1200(C+C) chloride

I PEi i i None Yellowish 2900(OH), l?lO(C=O)

powder 1600(N=N), 1270(C-0-C)

PEi v D#A Yellowish 3400(00), 1690(C=0) orange 1590(N=N), 1080(C-0-C) powder

PE" DMA Yellowish 3450(OH j , 1720(C=0) -i orange 1600(N=N), 1200(C-0-C)

powder

22 n PEv i DMA Redorange 3450(00), 1730(C=0)

powder 1590(N=N), 1060(C+C)

3.2 Photostimulated property changes of azo polymers

Nowadays photoresponsive polymers have attracted

considerable interest and become an active area of

research. The photoresponsive behaviour is t h e ultimate

result of the geometrical change of chromophore induced

by photoirradiation. In macromolecules containing azo

groups, cis-trans isomerisation is responsible for t h e

photoresponsive behaviour. The cis-trans isomerisation

of azobenzene residues incorporated in the polymer

backbone cause conformational change 127-129 ,f the

polymer chain affecting solution properties since

isomerisation involves appreciable change of polarity and

geometrical structure. The viscosity of a polymer system

directly reflect the extension of the polymer chain and

hence can conveniently used to assess the conformational

behaviour of polymer chain in solution. 130 If each

repeating unit of the polymer chain contains an

azobenzene group and if all are in the trans

configuration, the polymer chains a r e extended. Cn t h e

other hand if a major portion of the azo groups is *

converted to the cis form, the polymer chain form rather

compact coils. As the configuration of the attached

azobenzene group isomerises from trans to c i s form, the

extended conformation of polymer chain contracts rapidly

to a compact form. Photo irradiation easily carry out

this job without affecting the chemical properties of

polymer system. The conformational change of polymer

molecules in solution is well illustrated the Fig. 3.2.

trans

Q cis

Figure 3.2 Schematic illustration of photoinduced conformational changes of polymer chains in solution

3.2.1. Photoinduced viscosity changes of polyesters (PE) containing azo group in the backbone

The details of synthesis and characterization are

described in section 3.1. Polyesters PEi, PEii and PEiii

are insoluble in %all solvents and therefore not able to

study their solution properties. A dilute solution of

polymer PEi, (26) in N,N-dimethyl acetamide (0.5g/dl) was

irradiated with UV-visible light at 2 8 " ~ in a pyrex

immersion well reactor. Changes in viscosity were

measured at definite time intervals of irradiation.

Viscosity steadily decreases and af ter 5 hours of '

irradiation no further decrease in viscosity was observed

even though t h e solution was subjected t o irradiation

continuously for 10 hours. The solution regained the

original viscosity after keeping the solution under dark

for 12 hours at 2 8 ' ~ . ~rradiations followed by viscosity

measurements at definite time intervals were repeated

with solutions of different concentrations and similar

reversible photodecrease in viscosities were observed* A

graph is plotted with isp/C Vs. concentration and from

this plot, the [ 7 ] of the solution before and after

irradiations were computed. The intrinsic viscosity

before and after irradiation was 0.707 dl/g and .507 dl/g

respectively. The percentage decrease in intrinsic

viscosity as a result of irradiation was found to be

28.28% ( F i g . L 3 ) .

similar photoinduced viscosity studies were carried

out with other polyesters PE, ( 2 7 ) and PEVi (28) and of

solutions with different concentrations (Fig* 3 . 4 and

3 . 5 ) . The viscosity changes in these polyesters were also

found to be completely reversible in 12 to 14 hours, after

keeping the solution under dark.

The intrinsic viscosity of polymer PEv before

irradiation was found to be 0.9066 dl/g and that after

irradiation was 0.6654 dl/g, The percentage decrease in

intrinsic viscosity is 26.60%.

CONCENTRATION ( g/dl )

.9

* BEFORE * AFTER

.8

Figure 3.3 Photo-induced viscosity changes in PEiv (26)

before and after irradiation

.7

.6

.5

-4

- - -

-

. . .... -- !-..-I !

0 0.1 0.2 0.3 0.4 0.5 0.

* BEFORE * AFTER ,. - .-

Figure 3.4 Photo-induced viscosity changes in PE,, (27)

before and after irradiation

In the case of polymer PEVi the intrinsic

viscosity [ 1 ] before irradiation was 1.3548 dl/g and

that after irradiation was 1,00155 dl/g. The percentage

decrease in intrinsic viscosity is 26.07%.

I t was reported t h a t t h e reversible photoinduced

viscosity changes of polyamides25~41 and po ly ~ r e a s ~ ~

having azobenzene residues in the main chain, arises from

t h e conformational change of the polymer chain. The

polyester chain having azobenzene groups acts like a well

known photochromic molecule, which undergoes

isomerisation from trans to cis form during irradiation.

In solution the trans form exist as extended chain

and during isomerisation from trans to cis form kinks T

t h e extended polymer c h a i n resulting in compact

conformation; Viscosity is a direct reflection of

molecular conformation. The photodecrease in viscosity

was explained as the shrinkage of the extended

conformation. After cutting off the light and keeping

the solution in dark, the compact conformation returns to

the original extended form and hence the polymer chain

regains their i n i t i a l viscosity.

Figure 3.5 Photo-induced viscosity changes in PEvi (28)

before and after irradiation

1.8. 1

* BEFORE -*- AFTER -

1 .6 - - -

1 .2 0 \

a U) r

1

0 .8

-

0.6

0 .4

--

.-- I L L - h I

0 0.1 0.2 0.3 0.4 0.5 0.6

CONCENTRATION ( g/dl )

A similar reversible photodecrease in viscosity was

reported4' in the case of a poly amide (Fig. 3.6) having a

photoisomerisable unsaturated linkage in the back bone of

the polymer. Here the intrinsic viscosity before

irradiation was 1.25 dl/g. and after irradiation was 0.5

dl/g. The intrinsic viscosity during irradiation is 60%

lower than the viscosity in the dark. The photodecrease

in viscosity here was explained as the shrinkage of the

polymer conformation, induced by the isomerisation of the

azobenzene residues from the trans to the cis form.

Figure 3.6 Viscosity of pcrlyanlide I in N ,N-dimrthylacetamide at 2 0 ' ~ (a) in the dark before irradiation and (o) under irradiation with ultraviolet light (410 > X > 350 nm)

3.2.2 Photo induced spectral change of polyesters

Photoisomerisation in an organic compound is very

well evidenced from its UV-visible spectrum. The UV-

visible spectrum of azobenzene and nearly all its

substituted derivatives 30#89 changes as the configuration

changes from trans to the c i s form. The spectrum contain

a principal absorption band ( n - r * ) in t h e W region and

a weak absorption band n - n* in the visible region. On

conversion to the c i s isomer by photoirradiation, a

decrease in intensity of r - x* band with a shift to

shorter wavelength region and an increase in the intensity

of the n - a* absorption is observed. lQ3 The reversible

photoinduced viscosity change as a result of

photoisomerisation in polyesters having azo group can

also be explained from its absorption spectrum. The

absorption spectrums of the polyesters PEiv (Fig 3 - 7 1 ,

PEv(Fig 3 . 8 ) , PEVi(Fig 3.9) were recorded in N,N-

dimethyl acetamide. A strong band was observed in the UV

region at 328 nm and very weak band in the visible region

at 419 nm.

Figure 3.7 Specma1 change of PEi, (-) before irradiation and (- - -) after irradiation

The position of the band is slightly shifted in

comparison with 'unsubstituted azobenzene , which can be

attributed to the substitution effect. On

photoirradiation of the polymer solution, the intensity

* of the n - s absorption band decreased accompanied by a

shift in absorption maximum, while the intensity of the

band at 419 nm increased. The change in intensities of

the bands is a clear evidence of the photoisomerisation of

azobenzene residues.

A similar spectral change was reported4' in the case

of a poly amide (Fig. 3.10)

+ 2 * 5

Figure3.8 Spectral changes of PE, (-) before irradiation and (- - -) after irradiation

Figure 3.9 Spectral change of PE,, ( ) before irradiation and {- - -) after irradiation

Figure 3.10 Spectral changes of polyamide4' (-) before irradiation and (- - -) after hdiation

3 - 3 . Experimental *

General

T h e solvents used were purified according to

literature procedures. The melting points were recorded

in open capillaries on a hot-stage melting point

apparatus . Irradiations were carried out with a Philips

HPK 125W high pressure mercury vapour lamp in a pyrex

immersion well photochemical reactor. Dilute solution

viscosity measurements were performed at 2 8 " ~ with an AVS

4 0 0 automatic viscosity measuring unit with g las s

panelled thermostatic water bath.

Shimadzu UV-160A spectrophotometer was used for UV

spectral measurements. IR spectra were recorded on a

Shimadzu IR-470 spectrophotometer using K B r discs.

3.4 Preparation of azobenzene 4,4f-dicarboxylic acid (15 1

A mixture of sodium hydroxide (32 g, 0.8 mol) in

water (100 ml), P-nitrobenzoic acid (33.4 g, 0.2 mol) and

ethanol (250ml) was taken in a three necked round bottomed

flask fitted with a stirrer unit and a ref lux

condenser. Zinc powder (26 g, 0.4 mol) was added to t h e

above mixture and refluxed on a water bath for 15 hours

with vigorous stirring. The Sodium Zincate residue

filtered off while the mixture was hot. The Ethanol

was distilled off fram t h e mixture. The residual solution

was cooled and acidified with concentrated hydrochloric T

acid. The product obtained was filtered, washed

thoroughly with water and dried to yield reddish orange

powder of azobenzene-4.4' dicarboxylic acid yield 2 2 gm

(81%) m.p 3 2 3 ° C IR ( K B r ) (cm-l) : 2900 (OH), 1680 (C=O) ,

1580 ( N = N ) (Fig 3.1)

3-5 Preparation of azobenzene 4,4'-dicarbonyl chloride (16)

Thionyl chloride (180 ml, 2 . 5 mol) was added to

azobenzene 4,4'-dicarboxylic acid (l6g, 0.06 mol) taken in

a round bottomed flask. The mixture was refluxed for 5-6

hours in presence of a few drops of DMF as catalyst.

Excess of thionyl chloride was removed by distillation.

The residue obtained was recrystallized from petroleum

ether to get red needles of azobenzene-4,4'-dicarbonyl

chloride.yield 9.5g (52.28%). map. 163.~. IR (KBr) (cm-l) :

1730 ( C = O ) , 1590 ( N = N ) (Fig 3.1).

3.6 Synthesis of polyester PEi (23)

The polymer was synthesized by interfacial

polycondensation method. ~iethylene glycol (17) ( M - W =

106.12) (.24 mls, 0.0025 mol) was stirred with aqueous

sodium hydroxide (.2 g, 0.005 mol) in presence of sodium

su lphate under nitrogen a t room temperature. Azo

ber.zene-4,4'-dicarbonyl chloride (0.765 5, 0.0025 mi)

dissolved in freshly distilled chloroform (20 ml) was

then quickly added to the solution of diol and stirred

for 2 hours. The emulsified reac t ion mixture was poured

into acetone in order to coagulate the polymer. The

precipitated polymer was filtered, washed thoroughly with

water and dried to yield a pale yellow powder o f the

polymer PEI (23). y i e l d =0.810 g.

IR (KBr) (cmml) :2900(OH), 1720(C=O), 1600(N=N), 1200(C-o-C)

(Figure 3.11)

This polymer is insoluble and hence not ab le to study

th.e solution properties.

3.7 Synthesis of polyester PEiii ( 2 5 )

1,4-butanediol (19) (MW = 90.12, density = 1.0165)

(0.22 mls, 0.0025 mol) was stirred with aqueous sodium

hydroxide (0.2g, 0.005 mol) in presence of sodium

sulphate under nitrogen at room temperature. Azobenzene

4,4'-dicarbonyl chloride (0.765 g, 0.0025 mol) dissolved

in freshly distilled chloroform (20 ml) was then quickly

added to the solution of diol and stirred for t w o hours.

The emulsified reaction mixture was poured into acetone

in order to coagulate the polymer. The precipitated

polymer was filtered, washed with water and dried to yield

an orange powder of t h e polymer PEiii (25),

yield = 0.685 g.

IR(KBr) (cm-l) :2900(OH), 1710(C=0), 1600(N=N), 1270(C-0-Cj

(Figure 3.12)

This polymer is insoluble and hence not able to study

t h e solution properties.

3.8 Photoinduced dimensional change of polyester PEiv ( 2 6 1

3-8-1- Preparation of 1.4-phenylene carboxy ethylene glycol 2.5-dicarboxylic acid ( 2 0 )

Pyromelletic anhydride (4.36 gms, 0.02 mol) was taken

in a round bottomed flask fitted with a reflux

condenser. Ethylene glycol ( 5 . 5 7 7 mls, 0.1 mol) was

added to this and refluxed for 10 hours with stirring.

T h e product obtained was separated by column

chromatography using 20% methanol-chloroform mixture.

y i e l d 4 .23 g ( 4 0 % ) . IR (KBr) (an-') 3 4 0 0 (OH) 1720 ( C = O ) ,

1080 ( C - 0 - C ) (Figure 3-13).

3.8.2 Synthesis of PEiv ( 2 6 )

Interfacial polycondensation method was followed for

the synthesis of polymer P E ~ , * D ~ o ~ (20) (MW 342) (0.855 g

0 . 0 0 2 5 mol) was stirred with aqueous sodium hydroxide

(0.2 g, 0.005 mol) in presence of sodium s u l p h a t e under

Nitrogen a t room temperature. Azobenzene 4,4'-dicarbonyl

chloride (0.765 g , 0.0025 mol) dissolved in freshly

distilled chloroform (20 ml) was then quickly added to

the solution of diol and stirred for 2 hours . The

emulsified reaction mixture was poured into acetone in

order to esayuiate the polymer. The precipitated polymer

was filtered, washed thoroughly with water and dried to

yield yellowish orange coloured polymer PEiv (26).

Yield 1.2g ( 7 4 % ) 2 R (KBr) (cm-l) : 3 4 0 0 (OH), 1690 (C=O),

1590 (N=N), LO80 (C-0-C) (Figure 3.14).

3.8.3. Photoinduced viscosity change of PEiv (26)

A dilute solution of polymer PEiv (0.5 g/dl) in DMA

was prepared by stirring for about 30 hours . The intense

red coloured solution (130 ml) was irradiated. viscosity

measurements were done at definite intervals of time of

irradiation ( 2 , 4 , 6 , 8 , 10 hours) till it reaches a

constant value. The irradiated solution was kept under

dark for 14 hours to follow the reversibility.

Irradiations followed by viscosity measurements and its

reversibility were repeated with solutions of different

concentrations (0.4, 0.3, 0.2 and 0.1 g/dl). From the

time of flaw isp were calculated. qSp/c values were

plotted against concentrations (Fig.3.3).

The intrinsic viscosity of the solutions before and

after irradiation were computed and the [ tl IAI/[ 1 I B I (where A 1 represents after irradiation and BI represents

before irradiation) was calculated.

3.9. photoinduced dimensional change of polymer PE, (27 1

3.9-1 Preparation of l,4-phenylene carboxy diethylene glycol 2,5-dicarboxylic acid (21)

COOH

Pyromelletic anhydride ( 4 . 3 6 gms, 0.02 mols) was

taken in a round bottomed flask fitted with a feflux

condenser. Diethylene glycol (9.5 m l s , 0.1 mol) was added

to this and refluxed for 10 hrs. with stirring. T h e

product obtained was separated by column chromatography

using 20% methanol chloroform mixture.

Yield 28% IR(KBr) cm-I 3450 (OH), 1720 (C = O),

1050 (C-0-C) (Figure 3.15)

3.9.2 Synthesis of polyester PE, ( 2 7 )

Dihydroxy compound (21) (MW = 430) (2.15 g, 0 . 0 0 5

mol) was stirred with aqueous sodium hydroxide ( - 4 y,

0.01 mol) in presence of sodium sulphate under Nitrogen

at room temperature.Azobenzene-4,4'-dicarbonyl chloride

(1.53 g, 0.005 mol) dissolved in freshly distilled *

chloroform (40 ml) was then quickly added to the solution

of dihydroxy compound and stirred f o r 2 hours. The

emulsified reaction mixture was poured into acetone so

as to coagulate the polymer. The product obtained was

filtered, washed thoroughly with water and dried to yield

yellowish orange fibrous polymer PEv (27). Yield 1.8

g ( 4 8 % ) . IR ( K B r ) (cm-l) 3450 (OH), 1720 (C=O), 1600 (N=N),

1200 (C-0-C) (Figure 3.16).

Figure 3.13 IR Spectrum of pyromelletic anhydride (-) and 1,Cphenylene carboxy ethylene glycol 2,5-dicarboxylic acid (20) (- - -)

WAVENUMBER (Ol-I)

: 3.14 IR Spectrum of polyester PIZi, (26)

WAVENUMBER ( CM-I)

Figure 3.15 IR Spectrum of pyromelietic anhydride (-) and 1,4-phenylene carhoxy diethylene glycol 2,5-dicarboxylic acid (21) (- - -)

Photoinduced viscosity change of polymer PE, ( 2 7 )

A dilute solution of polymer PE, (0.5 g/dl) in DMA

was prepared by stirring for about 32 hours. Viscosity

measurements were done at definite intervals of time

of irradiation till it reaches a constant value. The

solution after irradiation was kept under dark for 15

hours to follow the reversibility. Irradiations followed

by vis'cosity measurements and its reversibility were

repeated with solution of different concentration ( 0 . 4 ,

0.3, 0 . 2 and 0.1 g / d l ) . Intrinsic viscosity before and

after irradiation was calculated from the plot of Tsp/c

V s . C (Fig.3.4). The ratio [ 1 1 I B I was calculated.

3.10 2hotoinduced dimensional change of polymer PEvi ( 2 8 ) *

3.10.1 Preparation or 1,4-phenylene carboxy bisphenol A 2,5=dicarboxylic acid ( 2 2 )

Bisphenol A (4.56 g, 0.02 mol) is dissolved in

toluene (40 ml) and taken in a round bottomed flask

fitted with a ref lux condenser, ~yromelletic anhydride

(0,872 g, 0.004 mol) was added to this and refluxed for

about 10 hours. White crystalline product obtained was

filtered, washed with water, acetone and dried. Yield

4.2 g (78%). IR (KBr) (cm-I) : 3400 (OH), 1700 (C=O), 1240

(C-0-C) ( ~ i g u r e 3.17).

3.10.2 Synthesis of polyester PEVI ( 2 8 )

Dihydroxy compound (22) (MW 674) (3.37 g, 0.005 mol)

was stirred with aqueous sodium hydroxide (0.4 g,

0.01 mol) in presence of sodium sulphate under Nitrogen

at room temperature. Azobenzene-4,4'-dicarbonyl chloride

(1.53 g, 0.005 'mol) d i s s o l v e d i n freshly distilled

chloroform ( 40 ml) was then quickly added to the solution

of the above dihydorxy compound and stirred for two hours.

The emulsified r e a c t i o n mixture was poured into acetone

so as to coagulate the polymer. The precipitated polymer

was filtered, washed thoroughly with water and dried to

yield red orange coloured polymer PE,i (28). yield 2.3 g

( 4 7 % ) . IR ( K B r ) (cm-l) : 3450 (OH), 1730 (C=O), 1590

( N = N ) , 1060 ( C - 0 - C ) (Figure 3.18).

WAVENUMBER (CM-')

Fibwrr 3.17 1R Spectrum of pyromelletic anhydride (-1 and 1,Cphenylene carboxy hisphenol A 2,S-dicarboxylic acid (22) (- - -)

00 3000 2000 1500 1000 500 400

WAVENUMBER ( CM-I )

Figure 3.1 8 IR Spectrum of polyester PEVi (28)

3.10.3 Photoinduced viscosity change of polymer P q i (28 )

A dilute solution of polymer PEVi ( . 4 6 g/dl) in DMA

was prepared by stirring for about 36 hours. viscosity

measurements were done at definite intervals of time of

irradiation till it reaches a constant value. The

solution after irradiation was kept under dark for 15

hours to follow the reversibility. Irradiations followed

by viscosity measurements and its reversibility were

repeated with solutions of different concentrations.

Intrinsic viscosity before and after irradiation was

calculated from the plot of qsp/c Vs C (Fig. 3.5). Then

the r a t i o [ q ] A I / [ ~ l I B I was calculated.