Poly(γ‐glutamic acid) hydrogel prepared from microbial poly(γ‐glutamic acid) and alkanediamine with water‐soluble carbodiimide

Abstract Poly(γ‐glutamic acid) (PGA) hydrogels have been prepared from microbial PGA produced by Bacillus subtilis F‐02‐1, water‐soluble carbodiimide (WSC), and alkanediamines such as 1,3‐propanediamine (1,3‐PD), 1,4‐butanediamine (1,4‐BD), and 1,6‐hexanediamine (1,6‐HD) in aqueous medium. The carboxyl groups of PGA were activated by the addition of WSC in deionized water, and the PGA‐WSC adduct was produced. PGA hydrogels could be produced after the mixing of PGA‐WSC and alkanediamine in deionized water. This alkanediamine to which both amino groups reacted with the carboxyl groups of PGA plays the role of a crosslinking point. When the amount of PGA was 100 mg, WSC was 50 mg or more, and 1,3‐PD was 25 μL or more in 2 mL of deionized water, PGA hydrogels could be produced. Specific water contents (weight of absorbed water/weight of dry gel) ranged from 300 to 1,993 g/g in the case of 1,3‐PD. If the PGA‐WSC adduct was freeze‐dried, the yield of the PGA hydrogel became higher than that when PGA‐WSC was not freeze‐dried. The highest yield of the PGA hydrogel from 100 mg of PGA, 100 mg of WSC, and 100 μL of 1,3‐PD in 2 mL of deionized water using the freeze‐dry method was 39.9 mg of dry PGA hydrogel with a 650 g/g specific water content. The order of yield was 1,6‐HD > 1,4‐BD > 1,3‐PD from 100 mg of PGA‐100 mg of WSC in 2 mL of deionized water. The order of the specific water content was 1,3‐PD (462 g/g) > 1,4‐BD (234 g/g) > 1,6‐HD (199 g/g). This order may be due to the higher reaction probability between the activated carboxyl groups in the PGA‐WSC and both amino groups in the alkanediamine with longer methylene chains, indicating that the crosslinking density of the PGA hydrogel is higher and the specific water content is lower. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65: 1889–1896, 1997