Thermal gelation or gel melting: (Ethylene glycol)₁₁₃-(l -Alanine)₁₂ and (ethylene glycol)₁₁₃-(l -lactic acid) ₁₂

When PEG (M.W.∼5000 Daltons) is conjugated to poly(l-alanine), the polymer aqueous solutions (<10.0 wt.%) undergo sol-to-gel (thermal gelation), whereas it is conjugated to poly(l-lactic acid), the polymer aqueous solutions (>30.0 wt.%) undergo gel-to-sol (gel melting) as the temperature increases. In the search for molecular origins of such a quite different phase behavior, poly(ethylene glycol)-poly(l-alanine) (PEG-PA; EG₁₁₃-A₁₂) and poly(ethylene glycol)-poly(l-lactic acid) (PEG-PLA; EG₁₁₃-LA ₁₂) are synthesized and their aqueous solution behavior is investigated. PEG-PAs with an α-helical core assemble into micelles with a broad size distribution, and the dehydration of PEG drives the aggregation of the micelles, leading to thermal gelation, whereas increased molecular motion of the PLA core overwhelms the partial dehydration of PEG, thus gel melting of the PEG-PLA aqueous solutions occurs. The core-rigidity of micelles must be one of the key factors in determining whether a polymer aqueous solution undergoes sol-to-gel or gel-to-sol transition, as the temperature increases.