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 (T_p) was increased with an increase in concentration of gelatin concentration (C_g). However, when C_g was lower than a critical value (C^∗_g), T_p was slightly increased with decreasing C_g. 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 (C_peg) and in the molecular weight of PEG (M_peg) raised the T_p 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 (T_m) increased with an increase in C_g and approached a saturation point. The addition of PEG raised the T_m curve closer to the saturation point, implying that T_m for the sample with a larger C_peg reached the saturation point at a lower C_g. Furthermore, melting enthalpy for the gelatin gel was increased with increases in C_peg and M_peg. These results indicate that the addition of PEG increases the strength of hydrogen bonding between gelatin chains. The appearance of minimum in the T_p curve might be originated from the interplay between self-aggregation via intra-chain concentration at a lower C_g and phase separation via inter-chain association at a higher C_g.