Control of Degradation Properties of Polymer Gel
Authors: Akagi Yuki , Gosho Suguru , Anraku Yasutaka , Sakuma Ichiro 
Published: February 2020

Degradation behavior of hydrogel can be divided into two types, surface degradation that degrades from the surface of hydrogel and bulk degradation that degrades homogeneously. Most hydrogel show bulk degradation, but there are many scenes where surface degradation is more useful. This is because surface degradation does not press the surroundings more than bulk degradation. However, it is still unknown what conditions control bulk degradation and surface degradation. Whether the gel undergoes bulk degradation or surface degradation depends on the balance between the simultaneous phenomena of hydrolysis occurring at the hydrolysis site in the polymer constituting the gel and swelling of the gel. As for gel swelling, it is determined by the relationship between the elastic pressure of the gel and the osmotic pressure generated by the presence of polymer concentration difference inside and outside the gel. In this study, in order to change the elastic pressure, polymer concentration was changed. External solvent at the time of degradation was changed in order to control the hydrolysis rate and osmotic pressure. The effect of these parameters on degradation behavior was examined.

A polymer having a degradation site was synthesized as follows. Polyethylene glycol (3000 mg, 1 mol) was dried for 3 hours under reduced pressure, and then 40 mL of THF, 1.5 mL of potassium naphthalene / THF solution (0.15 mM) and 7.2 mL of succinic anhydride / THF solution (0.3 M) were added. The mixture was stirred for 24 h at room temperature. After the reaction, THF was removed with an evaporator. 100 mL of chloroform was added to the obtained residue, and the washing out was carried out using 160 mL of hydrochloric acid (0.5 N). Thereafter, sodium sulfate was added for dehydration, and chloroform was removed using an evaporator. Benzene was added to the resulting residue until dissolved, and polymer was recovered by a freeze-drying technique. Furthermore, 40 mL of methacrylic anhydride was added to the obtained polymer. The reaction temperature and time were 60 ℃and 24 hours. After cooling to room temperature, the macromers formed were precipitated in excess of ether and filtered. They were further purified by dissolution and reprecipitation with dichloromethane and ether, respectively. The polymer synthesis was confirmed by 1 H NMR spectrum and IR spectrum. The obtained polymer was dissolved in acetonitrile(30,40 and 50 %(w/v)). The solution was deoxygenated by purging with Ar gas for 20 min. Gel was prepared by photo-crosslinking. For the crosslinking agent, 2,2-dimethoxy-2-phenylacetophenone was used. In order to investigate the degradation of the gel, the gel was immersed in an external solvent temperature-controlled by thermostat, and the volume change was measured. For the external solvent, mixed solvent of phosphate buffer (pH 7.4) and acetonitrile (phosphate buffer: acetonitrile = 10:0, 8:2, 5:5, 2:8 and 0:10) was used to investigate the influence on gel degradation behavior.

Samples with different polymer concentrations during the process of gelation all showed surface degradation and it was found that it is difficult to control the type of degradation by changing the elastic pressure. In the evaluation of effect of external solvent, the results showed that the larger the ratio of acetonitrile, the longer time it took for the gel to degrade and the maximum volume of swelling was larger. Furthermore, when the ratio of phosphate buffer to acetonitrile was 2:8, the type of degradation switched from surface degradation to bulk degradation. This is because the rate of hydrolysis decreases as the amount of acetonitrile increases in the external solution, and as a result, the time to degrade becomes longer, the maximum volume of swelling becomes larger, and the whole gel is degrade to cause bulk degradation.