Graft copolymerization of 4-vinylpyridine onto cellulosics. effect of temperature

Eur. Po(vm. J. Vol, 25, No. 12, pp. 1193-1196, 1989 Printed in Great Britain. All rights reserved

Copyright t

0014-305789 $3.00 +0.00 1989 Pergamon Press plc

GRAFT COPOLYMERIZATION OF 4-VINYLPYRIDINE ONTO CELLULOSICS. EFFECT OF TEMPERATURE M. L. LEZA, I. CASINOS a n d G. M. GUZMAN Departamento de Ciencia y Tecnologia de Polimeros, Facultad de Ciencias Quimicas, Universidad del Pais Vasco, Apdo. 1072, 20080-San Sebastian, Espafia

(Received 7 April 1989) Abstract--Graft copolymerization of 4-vinylpyridine (4-VP) was carried out on unmodified cotton and partially carboxymethylated cotton (PCMC), using ceric ammonium nitrate as initiator. The grafting parameters were studied as a function of temperature and amount of cellulose. With unmodified cotton, the graft yield increased up to 40 ° and then decreased on increasing temperature. Graft yields for PCMC increased with increasing temperature and reached a constant value beyond 30c. Carboxymethyl groups attached to cellulose enhanced both graft yield and grafting efficiency as well as the ceric ion consumption. The molecular weight of grafted poly(4-VP) was obtained viscometricatly after hydrolysing away the cellulose backbone, and it was found to decrease while the graft frequency increased with temperature. The activation energies were found to be 32.6 and 8.7 k J/tool for PCMC and unmodified cotton respectively. The weight increase of cellulose due to grafting increased with increasing content of cellulose in the reaction mixture. The presence of occluded air in the grafting system caused a decrease in graft yield.

INTRODUCTION

O f the various m e t h o d s used for grafting, redox systems have attracted a t t e n t i o n in recent years [1]. A m o n g redox systems, the ceric ion m e t h o d has been the most widely employed [2]. It is generally believed that f o r m a t i o n of radicals on cellulose involves abstraction of h y d r o g e n from a c a r b o n a t o m carrying a hydroxyl g r o u p [3], oxidation o f glycol linkages leading to C - - C b o n d scission [4], oxidation of cellulose chain ends c o n t a i n i n g hemi-acetal linkages [5], or interaction o f ceric ions with c a r b o n y l and carboxyl groups on cellulose [6-8]. Thus, a l t h o u g h ceric ions have been extensively used to initiate graft copolymerization on cellulose, the m e c h a n i s m of the initiation process is not completely understood. The effect o f t e m p e r a t u r e on grafting has been examined by several investigators. The influence of t e m p e r a t u r e on grafting yields depends u p o n the type o f initiator, its c o n c e n t r a t i o n a n d the m o n o m e r employed [2]. In this p a p e r the effect of reaction t e m p e r a t u r e on the graft copolymerization of 4-vinylpyridine (4-VP) on unmodified c o t t o n and partially c a r b o x y m e t h y l a t e d c o t t o n using ceric a m m o n i u m nitrate ( C A N ) as initiator has been studied. The effects of occluded air and a m o u n t of cellulose were also examined.

EXPERIMENTAL PROCEDURES

The experimental work was performed as previously indicated [9]. Cotton (A,7~= 4.28 x I0 ~) was purified [10] before use. 4-VP was refluxed over KOH and distilled under N: prior to use. Ceric ammonium nitrate, reagent grade, was supplied by Fluka and was used without further purification. Partially carboxymethylated cotton (PCMC) was prepared by the Green method [11], using NaOH and chloroacetic acid. The degree of substitution (DS), i.e. the average number of carboxymethyl groups introduced per anhydroEPJ

25 12

A

glucose unit in the cellulose, was determined by the acidwash method reported by Eyler et al. [12]. A value of 0.24 was obtained. Graft polymerizations were carried out by mixing the cellulosic substrate (1 g) with a suitable amount of monomer (4-VP 18,6 mmol) and CAN (2 × 10 4 mmol) in a HNO~ aqueous medium to give a total volume of 42 mL under N, and continuous stirring (110 rpm) for 4 hr. Hydroquinone was added at the end of the reaction to stop polymerization. The homopolymer poly-4-VP, P(4-VP), precipitated by addition of aqueous NaOH, was filtered, washed with water, and dried over phosphorus pentoxide. Homopolymer was completely removed from the crude product by extraction with methanol in a Soxhlet extractor to constant weight. The concentration of the ceric ion in the reaction mixture was determined volumetrically [13] with ferrous ammonium sulphate using ferrous o-phenantroline as indicator. The graft copolymers were inmersed in 72% H,,SO4 (22 ml) and stirred for 6 hr at room temperature [14] to carry out hydrolysis of the cellulose of the graft copolymer backbone. The molecular weights of the separated P(4-VP) grafts were calculated from intrinsic viscosities in absolute ethanol at 2 5 [15] employing an Ubbelhode viscometer. The following grafting parameters were calculated as in a previous paper [9]. Graft yield, G, is the ratio of grafted polymer to cellulose; grafting efficiency, GE, is the ratio of grafted polymer to the total synthetic polymer; total conversion, C,, is the monomer fraction that polymerizes; grafted polymer conversion, Cg, is the monomer fraction that gives place to grafted polymer; homopolymer conversion, Ch, is the monomer fraction that gives place to homopolymer; graft frequency, GF, is the average-number of grafted polymer chains per cellulose chain.

RESULTS AND DISCUSSION

Effect o f temperature on grafting It is a p p a r e n t from Fig. 1 that, with an increase in temperature, the graft yield increases with b o t h unmodified cotton and P C M C to give a m a x i m u m

1193

1194

M.L. LEZAet al. 80

80

.//

70

A

°o

gso

•

0'

•

•

A 6O

o § 40

4O

o 2O ~O

I

1

I

I

20

40

60

80

Temperature

o I

(=C)

0

Fig. 1. Effect of temperature on graft yield: (©) unmodified cotton; ( 0 ) PCMC.

grafting of 37% for unmodified cotton and 57% for PCMC. Further increase in temperature leads to a considerable decrease in graft yield with cotton while it essentially remains constant with PCMC. A maximum in the curve of dependence of graft yield on the temperature has been reported also by Hebeish and Mehta [16, 17] with CAN and acrylonitrile as well as with methyl acrylate, and by Schwab et al. [18] for both ceric ammonium sulphate and CAN. Huque et aL [19] found a decrease in graft yield with ceric ammonium sulphate and methyl methacrylate onto jute as the temperature was increased from 30 ° to 50 °. The changes in grafting with temperature are due [2] to a variety of temperature dependent factors such as diffusion, absorption of monomer into substrate, and changes in the initiation, propagation and termination rates of graft copolymerization as well as homopolymerization. Increases in graft yield with temperature were accompanied by increases in rate of polymerization. Decreases in grafting were accompanied by decreased rates of polymerization, implying changes in the rates of initiation, propagation and termination with temperature [17]. Copolymer graft frequency and molecular weight of grafted P(4-VP) after cellulose hydrolysis of PCMC-g-P(4-VP) are shown in Table i. The graft frequency showed a tendency to increase with increasing temperature. This increase in graft frequency was compensated by a decrease in the molecular weight of the P(4-VP) grafts leading to constancy of graft yield beyond 30 °. The increase in graft frequency with temperature may be attributed to the faster decomposition of cellulose-ceric complex so that more active sites are generated on the cellulose chains. The decrease in molecular weight could be

I

I

I

I

20

40

60

80

Temperature

(*C)

Fig. 2. Effect of temperature on homopolymer conversion: (O) unmodified cotton; (0) PCMC. explained by an increase in the rate of termination of the growing polymer chains by ceric ion, as an increment in the ceric ion consumption is observed. P(4-VP) is simultaneously formed with the graft copolymer and the effect of reaction temperature on homopolymer conversion is shown in Fig. 2. With cotton, the homopolymer conversions are very low at 3° and increase with temperature up to a maximum at about 30 °. At higher temperatures the homopolymer conversion decreases. Fast termination by ceric ion at high enough temperature (i.e. 40 °) would account for this. Similar behaviour, characterized by a temperature range in which a maximum value of homopolymer conversion is achieved, is also noticed with PCMC. Increasing temperature causes a significant decrease in grafting efficiency in the case of unmodified cotton while it remains almost constant with PCMC (Fig. 3). That decrease in grafting efficiency arises from the fact that homopolymer formation is more favoured by increasing the temperature than grafting formation. The Ce(IV) consumption, (Ce~V)c, during grafting of 4-VP increases on raising the temperature for both PCMC and unmodified cotton (Fig. 4). However, the consumption is higher with the former than with the latter.

80--

A

~oL_ 60

40 I

Table I. Effect o f temperature on the graft frequency o f P C M C - g - P ( 4 - V P ) and on the molecular weight o f grafted P(4-VP) T('C)

G(%)

~ ' , x 10 -3

GF

3 22 30 40 50 60

20 53 69 67 66 68

217 197 159 150 142 ll0

0.40 1.15 1.84 1.91 1.98 2.62

~-'

30

20

40 Temperature

60

80

(eC)

Fig. 3. Effect of temperature on grafting effaciency: (©) unmodified cotton; (0) PCMC.

Graft copolymerization of 4-VP onto cellulosics 100

Table 3. Effect o f t h e cellulose content on grafting parameters

•

Cotton

90

1195

~

.~ 80

(%)

(%)

(%)

(%)

GE

(%)

(Ce~V)c

(g)

0.50 0.75 1.00

11 14 17

64 61 67

75 75 84

42 37 34

14 23 20

93 99 80

Cs

Ch

Ct

G

(%)

~ ro .~_ 60 5O 4O

[ 20

0

f 40

f 60

f 80

Temperature (%)

Fig. 4. Effect of temperature on ceric ion consumption: (©) unmodified cotton; ( 0 ) PCMC.

Log (graft yield), after 4 hr, vs 1/Tis plotted for the initial portion of the curves (3 ° to 30-40 °) in Fig. 1. These values fall on a straight line (see Fig. 5). The calculated overall activation energy was 8.7 kJ/mol (2.1 kcal/mol) in the case of cotton and 32.6 k J/tool (7.8 kcal/mol) for PCMC. McDowall et al. [20] grafted acrylic acid onto rayon filaments using ceric ammonium sulphate as initiator and the activation energy calculated was 38 k J/tool (9.1 kcal/mol). Effect o f occluded air For a study of the effect of occluded air during grafting, a separate experiment was conducted with unmodified cotton at 30 °, in which N2 was not bubbled through the system. The results are presented in Table 2 and show that the graft yield and total conversion decreased when the reaction was carried out without a stream of N 2, but decreasing to a greater extent the conversion to homopolymer, so

4,5

4.0

3.5

5,0

2.5 3.1

I 3.2

I 3.3

I 3.4

I 3.5

L 3.6

that the graft efficiency increased by 12%. Thus, it can be stated that the occluded air partially inhibited the graft polymerization and homopolymerization, but the former to a greater extent. It can also be seen from Table 2 that the molecular weight of both homopolymer and grafted P(4-VP) was lower in air than that in N2, while the graft frequency was about the same order of magnitude, i.e. 1.09 in the presence of N 2 and !.13 under air. The similarity in these graft frequency values suggests that 02 does not interfere with the cellulosic radicals but with the growing chain radicals. The inclusion of air in a grafting system generally causes a decrease in graft yield [20-22] due to the inhibiting effect of 02. However, some investigators found that grafting with some monomers such as acrylonitrile under certain conditions was unaffected by the presence of 02 [16, 23]. Effect of the cellulose content Several experiments were carried out at 3 0 , varying the amount of unmodified cotton in the reaction mixture from 0.5 to 1 g. Amounts >1 g were not employed because the acidified solution (40 ml) was not sufficient to cover thoroughly such a quantity of substrate. It is seen from Table 3 that both homopolymer and grafted polymer conversions rise with increasing content of cellulose in the reaction mixture, but the ratio of grafted P(4-VP) to cellulose, or graft yield, decreases. Huang and Chandramouli [24] observed, for grafting methyl methacrylate onto wood cellulose by the ceric ion method, that both the graft yield and the molecular weight of grafted polymer as well as graft frequency decrease with increasing amount of cellulose used. The same behaviour was observed by Stepfi.n et al. [25] when vinyl chloride was grafted onto cellulose. They pointed out that the decrease in graft frequency could be due to a decreasing ratio of ceric ions per cellulose chain when the amount of cellulose in the grafting mixture was increased at fixed initiator concentration. Despite the decreasing graft frequency, the grafted polymer increased because the total number of grafted chains increased although their molecular weight diminished.

I 3.7

1 / T ( K "1) 10-~'

Fig. 5. The natural log of graft yield vs 1/T: (©) unmodified cotton; ( 0 ) PCMC.

Acknowledgements--The present work has been financed by Comision Asesora de Investigaci6n Cientifica y T~cnica. M. L. Leza thanks Ministerio de Educaci6n y Ciencia for a grant.

Table 2. Effect of occluded air on the graft copolymerization of 4-VP onto unmodified cotton

With N 2 Without N 2

Cs

Ch

C~

G

GE

( Celv )c

(%)

(%)

(%)

(%)

(%)

(%)

17 14

67 29

84 43

34 28

20 33

80 93

*h = homopolymer, r = grafted P(VP).

~'*h × 10

269 209

~

~Tt~r × 10

142 106

'

1196

M.L. LEZAet al.

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