Grafting of poly(glycidyl methacrylate) onto alginic acid

Eur. PO/WI. J. Vol. 21, No. 4, pp. 415-419, Printed in Great Britain. All rights reserved

GRAFTING

1985 Copyright

0014.3057185 $3.00 + 0.00 1”~ 1985 Pergamon Press Ltd

OF POLY(GLYCIDYL METHACRYLATE) ONTO ALGINIC ACID

M. T. VIJAYAKUMAR, C. RAMI REDDY and K. T. JOSEPH Polymer Division, Central Leather Research Institute, Adyar, Madras 600 020. India (Receiaed

18 June 1984, in rekedform

25 September

1984)

Abstract-Grafting of poly(glycidyl methacrylate) (PGMA) onto alginic acid was carried out using ceric ammonium nitrate as initiator, using various concentrations of monomer and initiator, and various temperatures and times. Percentage grafting, grafting efficiency and rates of polymerization, graft copolymerization and homopolymerization were evaluated in all cases. Infrared spectra for pure alginic acid, poly(glycidy1 methacrylate) and the alginic acid-poly(glycidy1 methacrylate) were taken to establish the occurrence of grafting. Differential scanning calorimetry was carried out for alginic acid, poly(glycidyl methacrylate). the graft copolymer and the physical mixture to establish evidence for grafting.

INTRODUCTION Controlled release technology has become very important and has emerged as an important area of research [l]. The main aspect of this technique is to incorporate the active ingredient in a solid polymer either by physical or chemical methods [2]. Polymeric carriers with pendant reactive functional groups are of interest in the chemical approach for binding the active ingredient. Natural polymers, although biodegradable, are prone to enzymatic degradation and suffer from limitations in fabrication [3]. To overcome these difficulties, the technique of graft copolymerization provides a promising approach to combine synthetic with natural polymers to produce tailor-made materials with the best properties of both types [41. Graft copolymerization of vinyl monomers onto water-insoluble polysaccharides is being exploited more effectively than on the water soluble polysaccharides [S]. The literature on the graft copolymerization of viny1 monomers onto alginic acid, a naturally occurring linear block copolymer of Dmannuronic acid and L-guluronic acid, is rather scanty [6, 71. For development of biomaterials, grafting of poly(g1ycidyl methacrylate) onto alginic acid has been carried out with ceric ammonium nitrate as initiator; the kinetics of the grafting are reported here. Percentage

of total conversion

Percentage

grafting

Grafting

efficiency

Rate of polymerization

Rate of graft copolymerization

(R,) (mall-’

=

weight ~-~

EXPERIMENTAL Materials Sodium alginate (Riedel) was converted to alginic acid [AA] by acid treatment as reported earlier [8]. Glycidyl methacrylate (GMA) (Fluka) was distilled under reduced pressure and the middle fraction was used. Fresh solutions of the initiator were used, prepared by dissolving the required amount of ceric ammonium nitrate (CAN) (Baker) in 5 ml of N-HNO,.

Preparation

C$ graft copolymers

Grafting reactions were carried out in a 250ml three necked flask fitted with condenser, stirrer and gas inlet. In a typical reaction, 2.5 g of alginic acid was dispersed in 60 ml of distilled water with constant stirring under N,. The required amount of initiator was added and stirred for 20 min. The required amount of monomer was added and the total volume was made up to 100 ml with distilled water. The grafting reactions were carried out for various periods and at various temperatures. After completion of the reaction, the contents were cooled and poured in methanol, filtered through weighed sintered crucible and dried in oacuo at 40”. The homopolymer was separated from the crude graft copolymer by exhaustive extraction with acetone by the tumble bottle method for 48 hr. The total monomer conversion, percent grafting (PG), grafting efficiency (GE), rates of polymerization (R,), graft copolymerization (R,) and homopolymerization (R,,) were evaluated as shown below: of polymer

grafted

weight

weight of polymer grafted (PG) = --: wetght of backbone (GE) = ~ Tweight set-‘)

weight of polymer

weight

of homopolymer -

x 100

grafted ~~-~ of homopolymer

X 100

charged

X 100

of polymer grafted

weight of polymer = ~-____-~_____

(R,) (mol 1-l set -‘) =

+ weight

of monomer

+ weight

grafted

+ weight

of polymer

_______~____--___-____~

of homopolymer

grafted

M . T . VIJAYAKUMAR et al.

416

Rate of homopolymerization ( R h ) ( m o l l - l s e c - l ) = ( m o l "

wt o q

weight of homopolymer ( time of the ~ ( volume of the

,] x 1000

\ m o n o m e r / × \reaction (see),] × \reaction mixture (ml),]

Infra-red spectra The i.r. spectra of poly(GMA) and alginic acid-gpoly(GMA) were taken in KBr pellets; that of alginic acid was taken in the form of a thin film using Perkin-Elmer Model 337 grating spectrophotometer.

Differential scanning calorimetric studies Differential scanning calorimetry (DSC) was performed on powdered samples of alginic acid, poly(GMA), a physical mixture of alginic acid and poly(GMA) (1 : 1, w/w) as well as the alginic acid-poly(GMA) graft copolymer. The sampies were weighed, packed into AI sample pans, caulked by a close fitting lid and heated at a rate of 5°/rain. The DSC curves were recorded against an indium standard, using D u Pont 990 Thermal Analyzer with 910 DSC module, with N 2 flow at 100 ml/min,

RESULTS AND DISCUSSION I n o r d e r to o p t i m i z e t h e c o n d i t i o n s f o r g r a f t i n g , t h e c o n c e n t r a t i o n s o f i n i t i a t o r a n d m o n o m e r , a n d also temperature and time were varied; the results were

u s e d to e v a l u a t e t o t a l c o n v e r s i o n , P G , G E , Rp, Rg a n d R h ( T a b l e s 1-4), respectively. W i t h a view to u n d e r s t a n d t h e i n f l u e n c e o f i n i t i a t o r in t h e g r a f t i n g o f p o l y ( G M A ) o n t o alginic acid, t h e a d d i t i o n o f m o n o m e r w a s d e l a y e d f o r 20 m i n a f t e r the addition of initiator, and the reaction was cond u c t e d f o r 5 h r at a fixed m o n o m e r c o n c e n t r a t i o n a n d t e m p e r a t u r e . It w a s o b s e r v e d t h a t t h e f o r m a t i o n o f h o m o p o l y m e r w a s c o n s i d e r a b l y less at l o w e r i n i t i a t o r c o n c e n t r a t i o n s while t h e r e w a s a s i g n i f i c a n t formation beyond a certain initiator concentration. H e b e i s h a n d M e h t a [9] also r e p o r t e d t h e effect o f delayed addition of monomer and found that homopolymer formation was practically eliminated when the monomer was added 3 hr after the addition of C A N i n i t i a t o r in t h e g r a f t i n g o f A N o n t o cellulose. T h i s effect m a y be d u e to t h e e l i m i n a t i o n o f direct c o n t a c t o f i n i t i a t o r w i t h t h e m o n o m e r . T h e active c e n t r e s c r e a t e d o n t h e b a c k b o n e a r e u s e d to p r o p a gate the grafted chain and hence formation of homo-

Table 1. Effect of initiator concentration in the graft copolymerization of GMA onto alginic acid (AA) Total [CAN] x 10 3

conversion

PG

GE

(mol l- l )

(~)

(%)

(~)

1.0 1.5 2.0 2.5 3.0 4.0 5.0 AA = 2.5 g; [GMA] = 1 0 0 ml.

Ro x 106

Rg x 106

R h × l0 6

(mol l- 1sec- i )

23.8 51.4 100.0 5.02 34.5 74.2 99.9 7.26 54.3 115.0 98.3 11.43 69.1 141.3 94.9 14.55 87.7 170.0 89.9 18.47 90.3 142.3 73.1 19.02 98.5 126.0 59.4 20.74 = 0.3792 mol l-~; temperature = 55°; time =

5.02 7.25 11.24 13.81 16.61 13.90 12.31 5 hr; total

-0.01 0.19 0.74 1.86 5.11 8.43 volume

Table 2. Effect of monomer concentration in the graft copolymerization of GMA onto alginic acid (AA) [GMA] (moll i)

Total conversion (~)

PG (~)

GE (%)

Rp × 106 Rg × 106 Rh × 106 (mol l-i sec-i)

0.071 99.8 29.1 72.2 3.94 2.84 0.142 97.6 63.1 80.1 7.70 6.17 0.213 94.8 96.7 84.2 11.22 9.45 0.284 91.3 129.1 87.6 14.40 12.61 0.379 87.7 170.0 89.9 18.47 16.61 0.568 60.4 158.3 81.1 19.07 15.47 0.710 48.4 142.8 73.0 19.10 13.95 AA = 2.5 g; [CAN] = 3 x 10-3 mol I i; temperature = 55°; time = 5 hr; total = 100 ml.

1.10 1.53 1.77 1.79 1.86 3.60 5.15 volume

Table 3. Effect of temperature in the graft copolymerization of GMA onto alginic acid (AA) Total Temperature

conversion

PG

GE

(°C) (~) (%) (~) 30 40.9 85.1 96.6 40 55.4 I 11.4 93.4 45 70.6 138.2 90.9 55 87.7 170.0 89.9 60 91.4 167.7 85.1 70 94.3 168.3 82.8 AA = 2.5 g; [GMA] = 0.3792 tool 1- J; [CAN] = 3 x volume = 100 ml.

Rp × 10 6

R~ × 106

R h × 106

(mol 1- t sec- 1) 8.61 8.32 0.29 11.66 10.89 0.77 14.86 13.51 1.36 18.47 16.61 1.86 19.25 16.39 2.86 19.86 16.45 3.42 10 3mol 1- t; time = 5 hr; total

Grafting of PGMA onto alginic acid

417

polymer is avoided. However, increase in initiator to the grafting site is small; the increase becomes less concentration beyond a certain value might have when the MMA concentration is higher. The same caused the initiator to promote homopolymerization, assumption may also be expected in the present case At a reasonably small initiator concentration, the as the concentration of GMA is several times higher PG is significantly high (Table 1). For grafting of than the concentration of CAN. vinyl monomers on carboxymethyl cellulose, it has It was observed that increase in the monomer been pointed out that ionization of carboxyl groups concentration up to 0.38 tool 1 Lincreased the rate of on the backbone resulted in increased attraction of polymerization and then it levelled off. This is indiceric ions to the backbone, which in turn leads to the cated by an increase in PG, GE, R~ and Rh (Table 2) creation of more active centres and hence the PG is whereas at higher monomer concentrations PG, GE, usually high [9]. Alginic acid behaves almost similarly Rg were found to decrease and Rh to increase to carboxymethyl cellulose as far as the carboxyl significantly. This is due to the formation of more group is concerned and is known to form salts with homopolymer which can be explained by supposing many metal ions. It is reasonable to assume the that the grafted chains act as diffusion barriers which ionization of the carboxyl groups on the uronic acid impede the diffusion of monomer into the backbone residues of alginic acid and hence a similar effect may [14].As a result, less monomer would be available for be operating, resulting in the formation of more grafting at the active centre and most of it would be grafted chains, utilized for homopolymer formation. A typical linear increase in the percentage conThe influence of temperature on the grafting is version was noticed with increase in initiator concen- shown in Table 3. The percentage conversion intration while there was a steady decrease in GE creases with temperature but levels off after 60'. The (Table 1). The fast dissociation of CAN may account rates of polymerization (R~,) and grafting ( R g ) and for its higher GE in the initial stages, since less Ce the percentage grafting become almost constant for (IV) would be available for initiation [10]. However, temperatures above 55°, after an initial increase at the percentage grafting reached a maximum of 170~o low temperatures. The grafting efficiency was found and decreased beyond 3 x 10-3 mol 1-1 initiator con- to decrease steadily with increase in temperature. centration. The R g values also followed the same From these results, increasing temperature seems to trend (Table 1). This may be attributed, as pointed cause a higher rate of dissociation of initiator, enout earlier, to the fact that a significant increase in the hanced ionization of the uronic acid carboxyl groups initiator concentration beyond 3 x 10 3tool I ~ not (which attract more ceric ions) as well as the diffusion only facilitates the formation of more active centres and mobility of the monomer from the aqueous phase on the backbone but also facilitates the homoto the backbone, resulting in considerable irapolymerization. Misra et al. pointed out that, for provement in the grafting yield. Similar results were ceric ion initiated graft copolymerization, the higher reported in the grafting of vinyl monomers on carthe concentration of Ce(IV) the greater will be the boxymethyl cellulose [9]. The decrease in GE with termination of growing grafted chains resulting in rising temperature may be attributed to the solubility reduction of PG [11]. It is reasonable to expect a of monomer in the aqueous phase at higher ternsimilar termination with increase in initiator concert- peratures and also to the acceleration of the termitration, the consequence of which leads to a steady nation process which leads to the formation of more decrease in GE. This type of observation was also homopolymer. Iwakura et al. [15] pointed out in the reported in the grafting of HEMA-co-MMA onto grafting of GMA onto cellulose that the conversion hide powder using CAN as initiator [12]. of the free radicals of the backbone to aldehyde At fixed [initiator], time and temperature, increase resulted in reduction of the number of grafting sites of [GMA] brought about a non-linear decrease in and hence GE. A similar reaction in the present case the percentage conversion rather than an increase would be expected to result in decrease of GE at (Table 2). Kojima et al. [13] attributed a similar trend higher temperatures. This is indicated by a sleady in the grafting of MMA onto cotton with tributyl increase in the rate of homopolymerization. borane to the idea that MMA itself is incapable of The relationship between the kinetic parameters of generating grafting sites, although it participates in the grafting reaction and the reaction time at fixed their generation. Thus, an increase in [MMA] is [initiator], [monomer] and temperature is shown in effective in the grafting as long as the ratio of MMA Table 4. It can be seen that the total conversion

Table 4. Effect of time in the graft copolymerization of G M A onto alginic acid

(AA) Total Time (hr)

conversion (?'o)

PG (%)

GE

1 2 3 4 5 6 7

48.5 57,4 77.9 86.4 87.7 89.1 91.0

82.66 100.5 139.8 160.9 170.0 169.1 169.3

79.1 81.3 83.3 86.4 89.9 88.0 86.3

AA-2.5g;[CAN]=3 x 10 ~moll 55 : total volume = 100 ml.

Rp × 106

(",~,) 51.04 30.22 27.34 22.74 18.47 15.64 13.69

Rg x 106 R h x l0 n (mol I i sec ~) 40.40 24.56

22.76 19.65 16.61 13.77 I 1.81

~:[GMA]~0.3792moll

10.7 5.7 4.6 3.1 1.9 1,9 1.9

t: t e m p e r a t u r e =

M.T. VIJAYAKUMARet al.

418

o

I

I

I

I

3500

3000

2.500

2000

[

I

I

[

I

[

I

1,500 1300 1200 1100 1000 900

800 700

Wove number (cm - 1 )

Fig. l. Infrared absorption spectra of (A) alginic acid, (B) poly(GMA) and (C) alginic acid-poly(OMA).

increases linearly with time up to 4 hr and then remains almost steady. A value of about 8 0 ~ was obtained for both P G and G E in the first hour. The P G increased linearly up to a maximum of 170~ and then levelled off while G E reached a maximum of 9 0 ~ at 5 hr and there was a slight reduction at longer times. This result may be attributed to the fact that the n u m b e r of grafting sites on the backbone increases with time and after a certain time their n u m b e r remains more or less constant. Also, as the time progresses, the grafted chains may act as diffusion barriers (due to decreased swelling of the backbone) which impede the diffusion of m o n o m e r into the backbone. As a result, the formation of homopolymer is favoured thereby reducing GE. The rate of polymerization decreased with increasing time for the grafting reaction. This effect can be attributed to the fact that the relative increment in the total yield is very much less when compared to that of time, and in the expression for Rp, the numerator becomes almost constant and when the time for the reaction is raised, the denominator becomes larger and the Rp will reduce accordingly. Since Rgand Rh are related to Rp, the relative decrease of Rg and Rh with time can be understood. Their decrease may also be due to depletion of initiator and m o n o m e r with time.

copolymer absorbs around 3000cm -~, characteristic of the assymmetric stretch of the methine r ] I | --C C--H [.| ~O j

groups and there is also t h e - - C - - O - - C symmetric stretch of the epoxide group at 1260cm -t which is not present in the alginic acid spectrum. The spectra of the alginic acid and the graft copolymer show peaks characteristic of carbohydrate systems in the

I

o "' - - " ~ ---.___ _~

\

A ,' !' ' ' ....

, ' ,, f

/1

,', ', ,

B ....

f

f , "i

f

Proof of grafting Figure 1 shows the i.r. absorption spectra of (A) alginic acid (B) poly(GMA) and (C) alginic acid-poly(GMA) graft copolymer. The graft copolymer has an absorption band at 1750 cm -~, characteristic of a carboxylic ester carbonyl and not present in the alginic acid spectrum. Also, the graft

t i i i i i i i i 356o 11o 16o 21o 260 31o 3eo 41o 460 Temperature (°c) Fig. 2. Differential thermograms of (A) alginic acid and (B) poly(glycidyl methacrylate).

Grafting of PGMA onto alginic acid o

--

,

, B

, ',

,~, ,' ',

,' , //k%ik,-, x A

~1

l ~ ,

~ff

/

~"

- ' - -"-":~"/

.-" ~[

I t

i'"

l

i

J

i

I

I

i

35 6o 110 160 210 z60 310 360 410 ,;60 Temperature (°C) Fig. 3. Differential thermograms of (A) physical mixture of alginic acid and poly(glycidyl methacrylate) and (B) alginic acid-poly(glycidyl methacrylate) graft copolymer,

419

group by the grafted chains which are linked through the hydroxyl groups of alginic acid. Also, the exothermic peak corresponding to the carbonization of alginic acid is shifted to higher temperature viz. 250' in the graft copolymer, whereas no shift of this exotherm was observed in the physical mixture. Further, the endotherm at 340 c~in the poly(GMA) as well as the physical mixture, appeared at 3 9 0 in the graft. All these results confirm grafting of poly(GMA) onto alginic acid by covalent bond formation between poly(GMA) and alginic acid. Further studies to understand the thermal stability by T G A as well as by DSC are in progress. Acknowledgements--One of the authors (M.T.V.K.) is

grateful to the Council of Scientific and Industrial Research, New Delhi, India, for financial assistance in the form of a fellowship to carry out this work. The authors also thank Mr T. S. Krishnan (Director, Central Leather Research Institute, Madras, India) for kind permission to publish this paper. REFERENCES

range 1000-1150 cm ~. Infrared spectral analysis has also been used to prove grafting by making use of variation in the intensity of the - - O H absorption band that may occur because of grafting [16]. It was found that the intensity of the OH band in the spectrum of alginic acid poly(GMA) graft copolymer is less than in the spectrum of pure alginic acid indicating that some of the grafted chains are linked through the hydroxyl groups. Differential scanning calorimetry has been very useful to establish the formation of graft copolymers [17]. Figures 2 and 3 show the DSC curves of alginic acid, poly(GMA), the graft copolymer and the physical mixture. Alginic acid showed an endothermic peak at 150: and an exothermic peak at 2 1 5 . The endotherm may be due to the loss of residual moisture and/or to dehydration of alginic acid by lactonization of the carboxyl group with the hydroxyl group and the exotherm is due to the carbonization of alginic acid [18]. Poly(GMA) exhibited endothermic peaks at 235, 300 and 340 ~' and an exotherm at 410'. The 1:1 (w/w) physical mixture of alginic acid and poly(GMA) exhibited peaks corresponding to the individual components. However, the endotherm due to poly(GMA) at 300 ° is not seen in the thermogram for the physical mixture. It is interesting to note that the magnitude of the peak at 12if' was considerably less in the graft copolymer than in the physical mixture. This difference may be attributed to the obstruction of lactonization of the carboxyl group with the hydroxyl

1. R. Langer and M. Karel, Polym. News 7, 250 (1981). 2. S. W. Kim, R. V. Peterson and J. Feijen, Drug Design (Edited byE. J. Ariens),Vol. 10, pp. 193 250. Academic Press, New York (1980). 3. B. Philipp, W. Bock and F. Schierbaum, J. Polym. Sci., Polym. Syrup. 66, 83 (1979). 4. R. J. Ceresa (Ed.), Block and Graft Copolymerization, Vol. 1. Wiley, London (1973). 5. T. Nishiuchi and M. Tani, Kogyo Kagaku Zasshi 73, 2699 (1970). 6. H. S. Blair and K. Moon Lai, Polymer 23, 1838 (1982). 7. V. Rusan, N. Asandei and Cr. Simionescu. Cell. chem. Tech. t, 655 (1967). 8. R. L. Whistler (Ed.), Methods in Carbohydrate Chemistry, Vol. 5, p. 71. Academic Press, New York (1965). 9. A. Hebeish and P. C. Mehta, J. appl. Polvm. Sei. 12, 1625 (1968). 10. A. Kantouch, A. Hebeish and M. H. EI-Rafie, Eur. Polym. J. 6, 1575 (1970). 11. B. N. Misra, 1. K. Mehta and R. Dogra. J. Macromol. Sci. Chem. AI2, 1513 (1978). 12. S. Amudeswari, C. Rami Reddy and K. Y. Joseph, Eur. Polym. J. 20, 91 (1984). 13. K. Kojima, S. lwabuchi, K. Murakami, K. Kojima and F. Ichikawa, J. appl. Polym. Sci. 16, 1130 (1972). 14. A. Hebeish, A. Kantouch and M. H. El-Retie, J. uppl. Polym. Sei. 15, I1 (1971). 15. Y. lwakura, T. Kurosaki, K. Uno and Y. Imai, J. Poh, m. Sci. C 4, 673 (1963). 16. D. Imrisova and S. Maryska, J. appl. Polvm. Sci. 11, 901 (1967). 17. C. I. Simionescu, I. A. Schneider and N. Hurduc, J. Poh,m. Sci. C 37, 325 (1972). 18. A. S. Perlin, Can. J. Chem. 30, 278 (1958).