Pulse Radiolysis Study on Aqueous Solutions of Polysaccharide

Chapter 15

Pulse Radiolysis Study on Aqueous Solutions of Polysaccharide Derivatives 1

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Seiichi Saiki , Yusa Muroya , Hisaaki Kudo , Yosuke Katsumura , Naotsugu Nagasawa , and Fumio Yoshii Downloaded by MONASH UNIV on April 9, 2016 | http://pubs.acs.org Publication Date: December 21, 2007 | doi: 10.1021/bk-2007-0978.ch015

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Department of Nuclear Engineering and Management, School of Engineering, The University of Tokyo, Tokyo, Japan NucIear Professional School, School of Engineering, The University of Tokyo, Tokyo, Japan Takasaki Advanced Radiation Research Institute, Japan Atomic Energy Agency, Takasaki, Japan 2

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Irradiated polysaccharides generally degrades, but some kinds of polysaccharide derivatives in an aqueous solution, for example carboxymethyl cellulose/chitin/chitosan etc, become crosslinked and form hydrogels in the case of concentrated solution. By irradiating polymer solutions, polymer radicals are produced by the reaction of polymer chains with water decompositions, OH radical and hydrated electron etc, and then the crosslinking takes place by the reaction of polymer radicals with each other. To study the early process of the gelation of polysaccharide derivatives by ionizing irradiation, we evaluated the reactivity of the reaction of polymer chains with OH radical, hydrated electron and with inorganic, sulphate or carbonate radicals by pulse radiolysis technique.

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© 2008 American Chemical Society

Celina and Assink; Polymer Durability and Radiation Effects ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

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Downloaded by MONASH UNIV on April 9, 2016 | http://pubs.acs.org Publication Date: December 21, 2007 | doi: 10.1021/bk-2007-0978.ch015

Introduction Recently pollution with unbiodegradable waste, for example a large amount of synthetic plastic consumed in a daily life, has become a problem. To use biodegradable materials is one means of reducing the environmental burden. One of biodegradable materials is polysaccharides, for example cellulose, chitin/chitosan, starch, alginate etc, and are expected as harmless materials for the environment. Especially, cellulose is famous as main component of plants and trees, and is the most abundant polysaccharide on earth. Structure of cellulose is described as a copolymer consisting of glucose units linked by β 1-4 glycosidic bond. As fiber, pulp and wood, cellulose is applied for building materials, paper, clothes and many tools used in a daily life etc, and has been indispensable in a human life. Chitin is a main component of shell of crab and shrimp and is the second abundant natural polymer after cellulose. Structure of chitin is described as a copolymer consisting of glycosamine and N acetylglucosamine units linked by β 1-4 glycosidic bond. Chitosan is generally obtained by deacetylation of chitin. These polysaccharides have characteristics of biocompatibility and biodegradability, and chitin and chitosan have antimicrobial and antifungal activities^. Nowadays, with progress for modification technique of these polymer, cellulose, chitin and chitosan can have been applied for more fields, agriculture and pharmacy, biotechnology, biomedical materials etc(2,3). For instance, cellulose and chitin have poor watersolubility and chitosan can be water-soluble only in an acidic solution, however carboxymethylated derivatives become water-soluble and have applied for cosmetics and food industry^. One of a modification method of polymers is a radiation technology. Not using a chemical, it is an advantage for the radiation technology that a harmful waste fluid doesn't come out from modifying process in the environment. The irradiation to polymers induces two effects for polymers. One is "scission", which lowers moleculer weight, and the other is "crosslinking" and "grafting" etc, which higher moleculer weight. Generally, it is known that the irradiation to polysaccharides of natural polymer induces chain scission reactionsf5,6,7J. It has been reported that the irradiated chitosan and alginate have a better anti-bacterial activity than the unirradiated and that can have been applied as a plant growth promoter and as a protectant of plants. (8,9,10,11) Recently, it has been reported that polysaccharide derivatives on a concentrated aqueous solution can be hydrogeled by ionizing irradiation. For instance, carboxymethyl cellulose(72,13), hydroxypropyl cellulose(74J, hydroxypropylmethyl cellulosef/J,), carboxymethyl chitin/chitosan(7$, and carboxymethyl starch^/ 7), and so on. These hydrogels consisiting of polysaccharides have also biodegradability as well as unirradiated, and some kinds of these hydrogels have various characteristics depending on a variety of



168 substituents. For instance, carboxymethyl chitosan hydrogel has antibacterial activity(7$. Hydrogels are polymer networks crosslinked by hydrophilic polymer and can absorb water in their own structure. For their high water-absorbing property and good biocompatibility, applications of hydorgels have been studied on many fields, especially on biomedical f\e\d(l9,20,21,22). Radiation technology is advantageous for produce of hydrogels as biomedical material, because sterilization and crosslinking can take place simultaneously. Synthetic polymer hydrogel produced by radiation technology have been studied and applied for biomedical material(25J. The study concerning the radiation-induced crosslinking reaction of synthetic polymer such as poly (vinyl alcohol), poly (vinyl methyl ether), etc, have been teported(24,25,26). In the case of polysaccharides, though the hydrogel property has been reported (12,13,14,15,16,17), reports concerning the gelation mechanism of polysaccharides are a \\M\Q(27,28). In this report, focusing to carboxymethyl chitin/chitosan and carboxymethyl cellulose, we studied the reactivity of water radiolysis products with polymer chains using the pulse radiolysis method as the first step to clarify early gelation process of polymer radicals related to crosslinking.

Experimental Materials Carboxymethyl chitin/chitosan was obtained from the Kouyou Chemical Co., and carboxymethyl cellulose was obtained from the Daicel industry Co. Structures of two repeating units of these polysaccharide derivatives are shown in Figure 1 (carboxymethyl chitin/chitosan (left), carboxymethyl cellulose (right)). As the important indicator of polymer property, DS and DDA are showed on Table I. DS shows the average degree of substitution per one monomer unit. The upper bound of DS of carboxymethyl chitin/chitosan is 2, and of carboxymethyl cellulose is 3. DDA means the degree of deacetylation of carboxymethyl chitin/chitosan and shows average percentage amino group substituted for acetyl group of one polymer chain. ÇH OR

NH

2

Ç 2 H

2

OR

0 R

o-

ONHCOCH3

CH OR 2

OR

CH OR 2

Figure 1. Structure of carboxymethyl chitin/chitosan (left)and carboxymethyl cellulose (right)


169 Table I. Property of polysaccharide derivatives Polymer Carboxymethyl chitin Carboxymethyl chitosan Carboxymethyl cellulose

DS 0.64 0.54 2.2

DDA 26.8 61.8

Note : Units of DDA is percent (%)


Pulse Radiolysis Pulse radiolysis system is showed in Figure 2. Electron beam is from S Band LINAC (Energy 35MeV and pulse width 10 ns, Nuclear Professional School, School of Engineering, The University of Tokyo, Japan possesses) and Xe lamp is used as analysis light. The intensity of transmitted light was converted into an electric signal with the PIN-photodiode, and data processing of the absorbance was done by PC. The quartz cell of 2cm in the optical path length was used for the sample cell. The absorbance defined as equation (1). The value is determined from the ratio of the intensity of transmitted light to incident light, and is proportional to the concentration of solute absorbing light at measured wavelength. By following the decay of the absorbance, kinetics of intermediates can be measured. Absorbance ξ l o g ( / / 7 ) = sci

(1)

0

I : intensity of incident light I : intensity of transmitted light ε : absorption coefficient [M'V ] c : concentration of solute absorbing light [M] £ : optical path length [cm] 0

1

In this experiment, the rate constants of the reactions of hydrated electron and OH radical, generated by irradiation to water, with polymer chains were measured by the pulse radiolysis method. The rate constants could be related to the generation rate of polymer radicals which are involved in crosslinking reaction. In addition, the rate constants of the reactions of inorganic radical, sulphate radical and carbonate radical, with polymer chains were also measured. Sulphate radical is generated from K S 0 , and carbonate radical is from Na C0 or NaHC0 . It has been reported that hydrogels generated under the presence of inorganic radicals have different characterf2PMnd inorganic radicals attack a specific part of the sugar chain un\t(30), so it is interesting from physical properties viewpoint and fundamental science viewpoint to irradiate a polymer solution under inorganic solute existence. At the experiments about inorganic radical, the pH of the solutions was adjusted with HC10 and NaOH. The 2

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Figure 2. Pulse radiolysis system

radicals generated by irradiation and the ion dissociated from HC10 and NaOH don't influence measurement data and the absorbance. 4

Results and Discussion When polymer solution is irradiated, water chiefly absorbs the energy of the radiation, and water decomposition, which is hydrated electron, OH radical, and hydrogen atom, proton, etc, are generated in the solutionfreaction (2)]. These radicals having a high reactivity cause various chemical reactions in the solution. In a polymer solution, it is assumed that the radicals such as hydrated electron and OH radical produce some polymer radicals which cause crosslinking reactionsfreaction (3)(4)]. ionizing

H0 2

+

-> e" ,*OH,*H,H ,OH",H 0 ,etc 2

2

radiation

e ,*OH,etc+ aq

polymer -» polymer*

polymer* + polymer* —> crosslinked

4-

products

polymer


(2)

(3)

(4)

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Rate Constants of the Reaction of the Radicals of Water Decomposition with Polymer Chains


Hydrated electron Evaluating the reactivity of hydrated electron with polymer chains[reaction (5) ], it is necessary to reduce the effect of the reaction of OH radical. So, tertbutanol was added to a polymer solution as scavenger of OH radical[reaction (6) ]. Tert-butmol radical produced by reaction (6) doesn't influence chemical reactions and absorbance data. Experiments were carried out under argon saturation for removal of oxygen, because oxygen reacts with hydrated electron and hydrated electron diminishes rapidly [reaction (7)]. Figure 3 is the result of pulse radiolysis experiment about the reaction of hydrated electron with polymer chains(0 or 30 mM carboxymethyl chitosan solution with 0.3 Mtert-butanolunder Ar saturation), and shows the decay of the absorbance as a function of time. This absorbance was measured at wavelength 720 nm, which is the absorption peak of hydrated electron. As seen in Figure J, the absorbance increases immediately after the irradiation, and attenuates afterwards. This means that hydrated electron is generated immediately after irradiation and diminishes gradually by some reactions of hydrated electron. Compared the absorbance decay of polymer solution with the decay of solution without polymer, the decay of polymer solution is faster than without polymer, so it is obvious that hydrated electron reacts with polymer chains. The decay curve can be fitted by pseudofirst-orderdecay. The pseudofirst-orderdecay is shown by equation (8). From estimating the slope of the pseudofirst-orderdecay rate of the absorbance at 720 nm against polymer concentration, the rate constant of the reaction of hydrated electron with polymer chains can be calculated (Figure 4). The rate constants of the reaction of hydrated electron with C M chitin and CM-chitosan was determined as l.lxlO and l ^ x l O ^ M ' V ] . These values are almost the same with the value of carboxymethyl cellulose(2 OH" + ter/BuOH*

e

I +0 -+0- + 0-,2q

[A) =

2

[A] e

-k[B]t

0


(5)

(6)

(7)

(8)


Figure 3. Time profile of the absorbance at 720 nm.

Figure 4. Plot of the pseudo first-order decay rate of hydrated electron against polymer concentration.


173 OH radical The experiments evaluating the reactivity of the reaction of OH radical with polymer chains were performed under N 0 saturation for reducing the influence of hydrated electron and oxygen. N 0 reacts with hydrated electron and produces OH radical[reaction (9)]. To measure the rate constants of OH radical with polymer chains, a competition method is suitable. An absorption peak of OH radical is at UV region and the absorption coefficient is very low, so it is difficult to observe OH radical directly. In this experiment, KSCN and KI were used as a competitor scavenger. OH radical reacts with SCN" or Γ and generates (SCN^" or (ΐ) ', which has a strong absorbtion peak at wavelength 472 nm and 380 nm [reaction (10)(11) and (12)(13)]. Irradiating the polymer solution under KSCN or KI existence, OH radical reacts with both polymer chains and a competitor scavenger. Reaction (15) shows the equation between the change of the absorbance of (SCN)*," or (l) ~ and the ratio of both reaction rates. A means 2

2


2

2

the absorbance of (SCN^' at 472 nm or (l) " at 380 nm of the polymer solution, and A means the absorbance of the solution without polymer. Concentration of scavenger was fixed at 2 mM, and the rate constant of the reaction of OH radical with SCN* and Γ are both \.\x\0 [M' s ](31). So, measuring the change of 2

0

ï0

l

]

the absorbance of (SCN^' or (ΐ) ', the rate constant of the reaction of OH radical with polymer chains can be calculatedfromequation (15). Figure 5 shows time profile of absorabance at 472 nm by pulse radiolysis (0 or 30mM carboxymethyl chitosan solution with 2 mM KSCN under N 0 saturation). As seen in Figure 5, the absorbance without polymer is obviously lower than a polymer solution. So, it was confirmed that OH radical reacts with polymer chains. Figure 6 shows the relation between the change of the absorbance of (SCN^" or (l) " and the ratio of both reaction rate. The slope of 2

2

2

the plot means the rate constants of the reaction of OH radical with polymer chains. The vertical axis is left-term of equation (15) and the horizontal axis is right-term of equation (15) except for koH+oiymer- From the slope, the rate constants of the reaction of OH radical with CM-chitin and CM-chitosan was determined as 1.4xl0 and l ^ x l O ^ M ' V ] using KSCN and as 9.3xl0 and l . l x l 0 [ M ' V ] using KI. Compared the rate constants of the reaction of hydrated electron and OH radical, the value of OH radical is almost 100 times as high as that of hydrated electron. The rate constants using KSCN is higher than using KI in both case of CM-chitin and CM-chitosan. This is assumed that SCN" accumulates around - N H , amino group. P

9

9

1

8

!

+

3

H 0 2

#

e + N 0 - » O H + OH" + N aq

2

2


(9)

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β

OH + SCN" -> OH" + SCN*

( 10)

SCN* + SCN" - » ( S C N £

(11)

•ΟΗ + Γ ->ΟΗ" + Γ

(12)

r + r->(i)r

(13)

9

*OH + polymer -» OH" + polymer

A

k

0

(14)

x[polymer]

OH+polymer

A

k

OH+scave»ger

^scavenger]

Rate Constants of the Reaction of Inorganic Radical with Polymer Chains Sulphate radical Sulphate radicals are produced by the reaction of sulphate acid ion with hydrated electron and hydrogen atom[reaction (16)(17)]. From the yields of water decomposition, most of sulphate radicals are produced by reaction (16). Sulphate radical have an absorption peak at 455 nm and the absorption coefficient is high(16000[M" cm" ] (32)), so we can measure an absorbance decay of sulphate radical directly. The absorbance at 455 nm decays following by pseudo first-order and the decay is faster under higher concentration. So, we confirmed the reaction of sulphate radical with polymer chainsfreaction (18)]. The rate constants of the reaction of sulphate radical with polymer chains were calculated from estimating the slope of the pseudo first-order decay rate of the absorbance at 455 nm against polymer concentration as well as the case of hydrated electron. As seen in Figure 7, the rate constants with CM-chitin and CM-chitosan, CM-cellulose at natural pH are 4.8xl0 (pH 8.8), 9.0xl0 (pH 9.3) and 1.7xl0 (pH 5.4) [M'V ]. These values are lower than that of OH radical. It shows that OH radical is more oxidative than sulphate radical. This experiment was done at various pH, but at lower pH than 6, CM-chitin and CM-chitosan solutions become white-clouded and were unable to be l

1

7

7

7

1



Figure 5. Time profile of the absorbance at 472 nm.

Figure 6. Competition kinetics ofpolymer and SCN, Γ by reaction with OH.


176 measured because light doesn't transmit through the sample cell. It can be assumed that - N H and -COO' are coexistent at less than pH 6 and polymer aggregates in the solution. The reactivity of sulphate radical with polymer chains have little pH dependency because the rate constants are almost constant at every pH solution measuring the data. At around pH 11, the rate constants are a little lower. It can be thought that this is because of the influence of the reaction of sulphate radical with OH". +

3

e ' +S 0 -^SOÎ"+S02-


a

q

2

8

• H + S Og -> SO" + HSO" 2

SO;

4

+ polymer -> HSO " + polymer* 4

(16)

( 17)

(18)

Carbonate radical Carbonate radical is generated by the reaction of OH radical with carbonate ion and bicarbonate ion [reaction (19)(20)], so this experiment was done under N 0 saturation[reaction (9)]. Carbonate radical has an absorption peak at 600 nm. As well as hydrated electron and sulphate radical, the rate constant of the reaction of carbonate radical with polymer chains[reaction (21)] can be calculated from estimating the slope of the pseudo first-order decay rate of the absorbance at 600 nm against polymer concentration. Then, the rate constants with CM-chitin and CM-chitosan, CM-cellulose were determined as (3.9-6A)x\0 [M~ s ](Figure 8). These values are lower than the value of OH radical and sulphate radical, and so this shows carbonate radical is less oxidative than OH radical and sulphate radical. Focusing the rate constants of CMchitosan, the value at around pH 9.5 is lower than over pH 10. This is because of pKa of amino group, protonation and unprotonation. For a weak reactivity of carbonate radicals, it can be assumed that carbonate radical have a selectivity attacking polymer chains. 2

5

l

]

•OH + HCOJ ->CO*T + H 0

(19)

•OH + CO|" ->C03~ + OH~

(20)

2


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1.2 10° • • • ^

CMchitosan CMchitin CMcellulose

Pulse Radiolysis Study on Aqueous Solutions of Polysaccharide

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