Hiroshi Mitomo

Radiation-induced degradation of sodium alginate

Abstract Alginates were irradiated as solids or in aqueous solution with Co 60 gamma rays in the dose range of 20 to 500 kGy to investigate the effect of radiation on alginates. Degradation was observed both in the solid state and solution. The degradation in solution was remarkably greater than that in the solid. For example, the molecular weight of alginate in 1% (w/v) solution decreased from 6×10 −5 for 0 kGy to 8×10 −3 for 20 kGy irradiation while the equivalent degradation by solid irradiation required 500 kGy. Degradation G-values were 1.9 for solid and 55 for solution, respectively. The free radicals from irradiated water must be responsible for the degradation in solution. The degradation was also accompanied by a color change to deep brown for highly degraded alginate. Little color change was observed on irradiation in the presence of oxygen. UV spectra showed a distinct absorption peak at 265 nm for colored alginates, increasing with dose. The fact that discoloration of colored alginate was caused on exposure to ozone suggests a formation of double bond in the pyranose-ring.

Hydrogel of biodegradable cellulose derivatives. I. Radiation-induced crosslinking of CMC

Radiation crosslinking of carboxymethylcellulose (CMC) with a degree of substitution (DS) from 0.7 to 2.2 was the subject of the current investigation. CMC was irradiated in solid-state and aqueous solutions at various irradiation doses. The DS and the concentration of the aqueous solution had a remarkable affect on the crosslinking of CMC. Irradiation of CMC, even with a high DS, 2.2 in solid state, and a low DS, 0.7 in 10% aqueous solution, resulted in degradation. However, it was found that irradiation of CMC with a relatively high DS, 1.32, led to crosslinking in a 5% aqueous solution, and 20% CMC gave the highest gel fraction. CMC with a DS of 2.2 induced higher crosslinking than that with a DS of 1.32 at lower doses with the same concentration. Hence, it was apparent that a high DS and a high concentration in an aqueous solution were favorable for high crosslinking of CMC. It is assumed that; high radiation crosslinking of CMC was induced by the increased mobility of its molecules in water and by the formation of CMC radicals from the abstraction of H atoms from macromolecules in the intermediate products of water radiolysis. A preliminary biodegradation study confirmed that crosslinked CMC hydrogel can be digested by a cellulase enzyme

Synthesis of antibacterial PVA/CM-chitosan blend hydrogels with electron beam irradiation

A series of excellent hydrogels were prepared from poly(vinyl alcohol) (PVA) and carboxymethylated chitosan (CM-chitosan) with electron beam irradiation (EB) at room temperature. Electron spectroscopy analysis of the blend hydrogels revealed that good miscibility was sustained between CM-chitosan and PVA. The properties of the prepared hydrogels, such as the mechanical properties, gel fraction and swelling behavior were investigated. The mechanical properties and equilibrium degree of swelling improved obviously after adding CM-chitosan into PVA hydrogels. The gel fraction determined gravimetrically showed that a part of CM-chitosan was immobilized onto PVA hydrogel. The further analyses of FTIR and DSC spectra of the prepared gels after extracting sol manifested that there was a grafting interaction between PVA and CM-chitosan molecules under irradiation. The antibacterial activity of the hydrogels against Escherichia coli was also measured via optical density method. The blend hydrogels exhibited satisfying antibacterial activity against E. coli, even when the CM-chitosan concentration was only 3 wt%.

Degradation of poly(l-lactic acid) by γ-irradiation

Abstract Poly( l -lactic acid) (PLA), was irradiated with γ-rays in air and in vacuum at 25°C. Melting point T m , glass transition temperature T g and number-average molecular weight M n decreased with increasing irradiation dose. At low doses, T m , T g and M n decreased sharply and then the rate became slower. Up to 200 kGy, PLA was predominantly degraded by random chain-scission. Above 200 kGy, recombination reactions or partial crosslinking may occur in addition to chain-scission. The degree of chain-scission G s of irradiated PLA was calculated to be 1.97 in air and 0.83 in vacuum. The decrease in M n , T m and T g of the sample in air was faster than in vacuum because of oxidative chain-scission. Biodegradation of PLA was retarded with increasing dose due to the introduction of crosslinking during irradiation. ESR spectroscopy showed five kinds of radicals assigned to three kinds of ESR spectra.

Hydrogels of polysaccharide derivatives crosslinked with irradiation at paste-like condition

Abstract Polysaccharides such as cellulose, starch, chitin/chitosan and their water-soluble derivatives have been known as degradable type polymers under action of ionizing radiation. Recently, we found that water-soluble polysaccharides derivatives such as carboxymethylcellulose (CMC), carboxymethylstarch (CMS) and carboxymethylchitin (CMCT), carboxymethylchitosan (CMCTS) lead to radiation crosslinking at high concentrated aqueous solution (more than 10%, paste-like state). It was proved that the crosslinking was remarkably affected by their concentration. It was assumed that radiation formation of hydrogels of these polysaccharides derivatives were mainly due to the mobility of side chains. Side-chains radicals were formed mostly via indirect effects, by the abstraction of H atoms by the intermediate products of water radiolysis. Some important characteristics of these novel hydrogels were also investigated. These hydrogels exhibited good swelling in water and possess satisfying biodegradability. In addition, the antibacterial activity against E.coli was also found in CMCTS hydrogel.

Growth-promotion of plants with depolymerized alginates by irradiation

Abstract Alginate has been degraded by gamma-ray irradiation from a Co-60 source in liquid state (aqueous solution) and in solid state (powder form). The irradiated alginate with a molecular weight less than 10 4 shows a strong effect on the growth-promotion of rice and peanut. Low concentration of degraded alginate from 4% solution irradiated at 100 kGy is effective for the growth-promotion of plants and the suitable concentrations are ca 50 ppm for rice and ca 100 ppm for peanut.

Radiation crosslinking of carboxymethylcellulose of various degree of substitution at high concentration in aqueous solutions of natural pH

AbstractCarboxymethylcellulose(CMC)hydrogelformedbyionizingradiationathighlyconcentratedaqueoussolutionswasfoundtoundergoswellingdependingonthepHoftheswellingmedia.SwellingincreasesatneutralandbasicpHduetoionizationofcarboxymethylgroupsonsidechains.Thepresenceofchargesdevelopsrepulsiveforcesbetweenpolymerchainsofthenetworkcausingitsexpansion.Hydrogelinrelaxedstateaswellasdriedgelrevealsgoodmechanicalproperties.Itwasconsideredthatintermolecularcrosslinkingreactionsoccurbyaradicalroute.RadicalsplacedonanhydroglucoserepeatingunitaswellasonsidechainsweredistinguishedfromESRspectraofCMC.Astabledoubletsignal with 2.0mT splitting constant belongs to a radical placed on the a-carbon atom of the substituent group,R–O– d CH–COO – .Itwasassumedthatthisspeciesparticipatesinintermolecularcrosslinking.r 2003ElsevierLtd.Allrightsreserved. Keywords: Carboxymethylcellulose;Hydrogels;Crosslinking;Ionizingradiation;Radicals 1. IntroductionBasicrequirementsforpolymericabsorbentsarelargefluid uptake and high mechanical strength. These twodemandsareclearlyconflicting,andasuitablecompro-mise needs to be found. A high degree of swelling inaqueous systems can be achieved by using polyelec-trolytesbecauseofmobilecounterions presentinsuchgels develop a large swelling pressure (Florry, 1953).Thus,carboxymethylcellulose(CMC),whichisthemostpopularandthecheapestcelluloseethercanbeusedforthis purpose. It is an anionic linear polymer in whichoriginal H atoms of cellulose hydroxyl groups arereplaced by carboxymethyl substituent, –CH

Radiation crosslinking of methylcellulose and hydroxyethylcellulose in concentrated aqueous solutions

Abstract The effects of ionizing radiation on aqueous solutions of cellulose ethers, methylcellulose (MC) and hydroxyethylcellulose (HEC) were investigated. The well-established knowledge states that cellulose and its derivatives belong to degrading type of polymers. However, in our study intermolecular crosslinking initiated by gamma rays or electron beam leaded to the formation of insoluble gel. This is an opposite effect of irradiation to the degradation. Paste-like form of the initial specimen, i.e. concentration 20–30%, when water plasticizes the bulk of polymer; and a high dose rate were favorable for hydrogel formation. Gel fraction up to 60% and 70% was obtained from solutions of HEC and MC, respectively. Produced hydrogels swell markedly in aqueous media by imbibing and holding the solvent. Radiation parameters of irradiation, such as yields of degradation and crosslinking and the gelation dose, were evaluated by sol–gel analysis on the basis of Charlesby–Rosiak equation. Despite of the crosslinked structure, obtained hydrogels can be included into the group of biodegradable materials. They undergo decomposition by the action of cellulase enzyme or microorganisms from compost.

Radiation crosslinked poly(butylene succinate) foam and its biodegradation

The gel fraction (%) for poly(butylene succinate) (Bionolle) blended with blowing agents was found to be higher than the pure Bionolle due to the presence of other component that also underwent crosslinking. It was also found that the cell diameter of the Bionolle foam produced from irradiated Bionolle blend decreases as the gel fraction increases, because the crosslink density increases and thus it retards the bubble growth. Lower gel fraction, less than 5% was effective enough to give sufficient melt strength for production of foams. It was found that the biodegradability of Bionolle foam in the enzymatic degradation test and soil burial test was higher than that of crosslinked Bionolle (before foaming) because it contained many bubbles.

Enzymatic degradation of radiation crosslinked poly(ε–caprolactone)

Abstract Poly( e –caprolactone) (PCL), crosslinked by gamma rays in a super cooled state, had high thermal stability, better transparency and heat shrink property. Biodegradation of such PCL was carried out using lipase AK enzyme in a phosphate buffer solution having a pH of 7.0. The enzymatic degradation was remarkably affected by reaction temperature. Preferable temperatures for enzymatic degradation were 50°C for uncrosslinked PCL and 55°C for crosslinked. The enzymatic degradation rate of crosslinked PCL is slower than that of the uncrosslinked due to network structure. However, it was confirmed that enzymatic degradation occurs even for high crosslink density of PCL such as 80% of gel content.