Biodegradable network polyesters from gluconolactone and citric acid

This paper presents biodegradable polyesters prepared from gluconolactone (G) and citric acid (C) with various molar ratios of G to C, 1/4, 1/3, 1/2, 1/1, 2/1, 3/1, and 4/1. Corresponding polyesters were named as GC14, GC13, GC12, GC11, GC21, GC31, and GC41, respectively. Prepolymers, prepared at 165 degreesC by a melt polycondensation, were cast from tetrahydrofuran solution and postpolymerized at 180 degreesC for various periods of time to form a network. The resultant films were transparent, flexible, and insoluble in organic solvents. The polyesters obtained were characterized by infrared spectroscopy, wide-angle X-ray diffraction analysis, density measurement, differential scanning calorimetry (DSC), dynamic mechanical analysis, thermomechanical analysis (TMA), and tensile test. The degree of reaction was estimated from the ratio between absorbance due to the stretching vibration of the hydroxyl group and that of the methylene group. Enzymatic degradability was performed at 37 degreesC in a buffer solution with Rhizopus delemar lipase. The degree of enzymatic degradation was decreased with decreasing C content in the polymers as follows: GC14 > GC13 > GC12 > GC11 > GC21 approximate to GC31 approximate to GC41 approximate to 0, which corresponded to the decrease of the average molecular weight between cross-linked sites. Two transition temperatures were measured on thermograms in DSC and TMA curves. The higher transition temperature was ascribed to the release of restricted motion in the molecule pinned by the cross-linked sites. The lower transition temperature was attributed to the transition due to the segmental molecular motion of the ester linkage. Large Young's modulus and tensile strength were obtained in the dried state of GC31 and GC21, respectively. Water absorbed in the polymer matrix was worked as a plasticizer, and largely depressed Young's modulus and tensile strength were measured for the sample in water.