Die Angewandte Makromolekulare Chemie 131 (1985) 169- 176 (Nr. 2104)

Polymer Division, Central Leather Research Institute, Adyar, Madras 600020, India

Modification of Gelatin by Grafting

Anne George, Ganga Radhakrishnan, and K. Thomas Joseph

(Received 12 November 1984)

SUMMARY: Graft copolymerization of butyl acrylate onto gelatin has been carried out using

potassium persulphate as initiator both in aqueous and in water-acetic acid medium. Kinetic parameters like rate of grafting, percent grafting, and grafting efficiency were determined for the two systems. The grafting was proved by IR spectra of the isolated graft chains which showed amide absorption bands characteristic for the amino acids.

ZUSAMMENFASSUNG: Die Pfropcopolymerisation von Butylacrylat auf Gelatine wurde mit Kaliumpersul-

fat als Initiator sowohl in Wasser als auch in Wasser-Essigsaure als Reaktionsmedium durchgefuhrt. Fur beide Systeme wurden kinetische Parameter wie Pfropfgeschwin- digkeit, Pfropfgrad und Pfropfausbeute bestimmt. Die erfolgte Pfropfung wurde durch IR-Spektren der isolierten Pfropfaste nachgewiesen, die fur Aminosauren cha- rakteristische Amidabsorptionsbanden zeigten.

Introduction

Studies on the modification of in general and gelatin6-8 in particular are scanty when compared with those of starch9-", cellulose12, and ~ 0 0 1 ~ ~ .

Modification of gelatin by various synthetic polymers has been the subject of our studies during the past with the hope of developing new and commer- cially important polymers. Envisaging graft copolymerization as a method for modification, it could be interesting to synthesize graft copolymers using gelatin as the backbone.

In the present investigation grafting of butyl acrylate onto gelatin was carried out in aqueous medium as well as in a water-acetic acid medium.

0 1985 Huthig & Wepf Verlag, Base1 0003-3146/85/$03.00 169

A. George, G . Radhakrishnan, and K. T. Joseph

Experimenta I

Materia Is

Gelatin (Riedel, Germany) was used as such. The monomer butyl acrylate (Rohm and Haas, USA) was purified in a manner reported earlierT4. The initiator used was potassium persulphate (KPS) (Riedel, Germany).

Graft Copolymerization Procedure

A 10% solution of gelatin in warm water was prepared and used in the graft co- polymerization reactions. A fresh solution of gelatin was prepared for each experi- ment to avoid bacterial growth. Requisite amounts of monomer, initiator, and gelatin solution were added. Deaeration was continued throughout the course of reaction. The tube was then thermostated at 60 f 0.1 "C. After a specific polymerization time (90 min) the reaction tube was immersed in a freezing mixture to stop the reaction. The contents were then poured into ice cold methanol. The polymer suspensions were filtered in weighed sintered glass crucibles (IG-3) and dried in vacuum at 40 - 50 "C overnight to a constant weight. In the second set of experiments water-acetic acid mixture (1 : 1 v/v) was used as a medium for grafting.

Isolation of the Graft Copolymers

In the synthesis of graft copolymers by any method, the reaction products are invariably contaminated with the homopolymer. The polymer obtained in the present case consisted of unreacted gelatin, graft copolymer, and unbound homopolymer. The poly(buty1 acrylate) homopolymer was removed from the graft copolymer by soxhlet extraction with acetone for 48 h. Residual solvent was removed by drying in a vacuum oven for 24 h. The amount of graft copolymer was obtained from the difference in weight of extracted and original specimens, each weighed in a moisture free state.

Calculation of Kinetic Parameters

The various kinetic parameters were calculated as follows: Grafting Efficiency was computed gravimetrically from the weight of homopolymer formed and the total weight of graft product and homopolymer:

x 100 Grafted Polymer (g)

Grafted Polymer (g) + Homopolymer (g) Grafting Efficiency =

Graft Copolymer (g) - Gelatin (9) Gelatin (8)

Percent Grafting = x 100

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The rate of graft polymerization ($) was computed from the weight of polymer bound as graft to the backbone. This was given by the amount of graft copolymer obtained after complete isolation of the free homopolymer:

Results and Discussion

Effect of Monomer Concentration

There is a regular increase in percent grafting, grafting efficiency, and rate of grafting with increase in monomer concentration in the aqueous medium as well as in water-acetic acid medium (Tab. 1). The grafting efficiencies in sythesis of gelatin graft copolymers in acetic acid medium were lower than in

Tab. 1 .

[BA] x 10 $ x 106 Grafting Percent (mole 1-' ) (mole 1-l SKI) efficiency grafting

A B A B A B

Effect of butyl acrylate (BA) concentration on grafting.

-~ -

0.702 4.43 1.20 72.68 30.16 3.53 10.12 1.404 8.25 2.42 80.10 42.24 16.24 19.10 2.106 15.38 5.21 83.34 53.46 25.46 23.32 2.808 20.79 10.24 85.56 59.38 30.19 40.41 3.610 26.97 16.67 88.42 63.17 38.21 60.36 4.212 42.09 25.82 91.29 68.15 46.30 72.19

[KPS] = t = 90 min; A = aqueous medium; B = water-acetic acid medium.

mol.1-I; [Gelatin] = 3.3 x mol.1-'; v = 50 ml; T = 60°C;

aqueous medium. This appears to be due to higher rates of termination of the growing graft chain radical and gelatin macro radical via chain transfer. This factor adversely affects grafting by lowering the molecular size of the graft in aqueous acetic acid medium. Similar results were obtained in the graft copolymerization reaction with cellulose Is.

In the aqueous system there is a steady increase in viscosity due to gel effect and due to this the termination rate of the growing chains was

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reduced. This is in conformity with the results obtained in grafting of poly(ethy1 acrylate) onto ~ 0 0 1 ' ~ .

Effect of Initiator Concentration

From Tab. 2 it is seen that when the initiator concentration is increased, the rate of grafting and the grafting efficiency increase up to 1.6 x mole 1-' and then decrease.

Tab. 2. Effect of initiator concentration on grafting.

[KPS] x Id & x 106 Grafting Percent (mol. 1-' ) ( m o l . l - ' ~ - ~ ) efficiency grafting

A B A B A B

6 3.95 3.81 75.27 77.61 18.21 5.20 10 7.91 6.62 78.31 70.10 20.32 13.88 16 12.80 10.50 83.46 55.12 21.45 21.55 20 8.03 9.78 70.10 48.10 15.01 23.90 24 5.60 7.37 61.43 34.20 12.30 20.12 30 2.46 6.00 52.66 30.15 10.24 17.92

[BA] = 0.1 x 4mol.1-'; [Gelatin] = 3.3 x t = 90 min; A = aqueous medium; B = water-acetic acid medium.

mol.l-', v = 50ml; T = 60°C;

As gelatin and potassium persulphate are soluble in water, the approach of initiating radicals to gelatin is facilitated. This results in an interaction with the functional groups of gelatin to produce backbone radicals and hence the rate of grafting increases. When the initiator concentration is increased side chain termination takes place even before the full growth of the side chain.

In the case of the water-acetic acid system the values for the rate of grafting efficiency are smaller. This may be due to the fact that a persulphate initiated polymerization is adversely affected by acidic pH" because the acid is believed to favour the decomposition of KPS by a mechanism'" that involves no free radicals. When the concentration of the initiator is increased beyond the optimum concentration, the homopolymerization rate increases

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Modification of Gelatin

which results in a decrease in graft yield. Similar results have been observed by other workers'*.

Effect of Gelatin Concentration

In both systems percent grafting and grafting efficiency increased initially with increase in gelatin concentration (Tab. 3). The initial increase may be due to the fact that the reactive sites increased with increase in the concentra-

Tab. 3. Effect of gelatin concentration on grafting. ~~~ ~

[Gelatin] x I @ % x 106 Grafting Percent (mol. 1-l) (mol.1-l s - l ) efficiency grafting

A B A B A B

2.2 2.5 3.2 3.8 4.8 6.3

10.01 4.90 70.21 27.53 12.24 18.62 12.06 6.10 74.62 31.19 16.81 21.49 14.52 7.40 78.31 35.52 20.00 27.52 17.13 8.80 80.24 43.21 22.32 31.17 16.14 6.00 76.31 39.15 18.46 29.14 12.12 2.70 70.12 33.56 15.31 25.53

[BA] = 0.1 x 4 mol.l-'; [KPS] = 10 x t = 90 min; A = aqueous medium; B = water-acetic acid medium.

mol.l- ' , v = 50 ml; T = 60°C;

tion of gelatin. The decrease is due to the destruction of radical activity on the backbone soon after it is formed. This is in agreement with the results obtained for the grafting of acrylonitrile onto ~ t a r c h ' ~ and grafting of methyl methacrylate onto gelatinz0.

Characterization of Graft Copolymers

i) Treatment of Isolated Graft with Ninhydrin

Proof of grafting can be ascertained by detection of amino acid end groups in the grafts produced by acidic hydrolysis of the graft copolymers.

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A. George, G . Radhakrishnan, and K. T. Joseph

The isolated grafts were treated with ninhydrin reagent which gave the characteristic blue colour normally associated with the presence of amino acids.

In the case of physical mixture obtained by thoroughly mixing gelatin with butyl acrylate no blue colour was detected. These results indicate that grafting of the polymer to the amino acid residues in gelatin has occurred.

ii) IR Spectra

Proof of grafting was further confirmed by IR spectroscopy. In the present investigation the infrared spectra of gelatin, gelatin graft copolymers, and poly(buty1 acrylate) showed that actual grafting took place (Fig. 1). The spectra showed the characteristic amide absorption at 1 550 and 1 660 cm-' in addition to carbonyl peaks at 1730 cm-'.

W 0 z a m

m

fY 0 cn Q

4000 3000 2000 1200 800 600 LOO

F RE CLUE NCY C M -'

Fig. 1 . IR spectra of pure gelatin (a), gelatin-g-poly(buty1 acrylate) (b), and poly(buty1 acrylate) (c).

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Modification of Gelatin

Conclusion

Various organic solvents have been used as a reaction medium for grafting. Favourable effects of organic solvents have been found for grafting onto substrates like cellulose2', nyl0n-6~~, and p~lyethylene~~. In the present investigation, grafting in aqueous medium was found to give better results for grafting of butyl acrylate onto gelatin. Acetic acid was found to depress the initial rate. Similar observations have been made in the aqueous polymerization of water soluble r n ~ n o m e r s * ~ - ~ ~ . The lowering of the rate is due to shrinkage of the hydration layer that protects the growing polymer chain. Water miscible organic solvents like acetic acid may tend to dehydrate the growing chains of hydrophilic macromolecules thus leaving their chain ends open to premature termination.

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