Phase separation in binary polymer solution: Gelatin/Poly(ethylene glycol) system

We systematically investigated the phase behavior of gelatin and poly(ethylene glycol) (PEG) aqueous solution. With a decrease in temperature, the gelatin/PEG solution became opaque and underwent phase separation. The phase separation point (Tp) was increased with an increase in concentration of gelatin concentration (Cg). However, when Cg was lower than a critical value (C⁎g), Tp was slightly increased with decreasing Cg. In addition, the phase separation mechanism was changed from spinodal decomposition type to nucleation growth type at around C⁎g. The increases in PEG concentration (Cpeg) and in the molecular weight of PEG (Mpeg) raised the Tp curve. A further decrease in temperature resulted in a sol-gel transition for gelatin. In the case of a gelatin solution without PEG, the melting temperature of the gelatin gel (Tm) increased with an increase in Cg and approached a saturation point. The addition of PEG raised the Tm curve closer to the saturation point, implying that Tm for the sample with a larger Cpeg reached the saturation point at a lower Cg. Furthermore, melting enthalpy for the gelatin gel was increased with increases in Cpeg and Mpeg. These results indicate that the addition of PEG increases the strength of hydrogen bonding between gelatin chains. The appearance of minimum in the Tp curve might be originated from the interplay between self-aggregation via intra-chain concentration at a lower Cg and phase separation via inter-chain association at a higher Cg.