Structural and thermodynamical investigation in triblock copolymers of polylactide and poly(ethylene glycol), PLA-b-PEG-b-PLA, envisaged for medical applications

A series of triblock copolymers based on poly(ethylene glycol) (PEG) and poly(lactic acid) (PLA), PLA-b-PEG-b-PLA, aimed for use in biomedicine, were synthesized and studied employing a combination of structure, thermal and dynamical techniques. The copolymers differ in the PLA/PEG molar ratios in the range from 640/1 down to 20/1 and, consequently, in the length of the built PLA side blocks. The copolymers exhibit high homogeneity, evidenced by their single glass transition, crystallization and melting events, which change systematically with the reduction of the PLA/PEG ratio. In the amorphous state, the glass transition temperatures decrease in the copolymers, from similar to 50 to about 40 degrees C, whereas the chains' cooperativity also drops. The recordings suggest the plasticizing role of PEG on PLA and the elevation of free volume within the copolymers. The crystal nucleation is hindered, whereas, in general, the degree of crystallinity is enhanced in the copolymers. The copolymers demonstrate a potential to tune the semicrystalline morphology to a wide extent. These properties are actually desirable, as they are strongly connected with the envisaged applications (drug carrier), the performance in general (mechanical, permeation, heat transport) as well as the facilitation of the renewability/compostability, aspects that are somewhat limited for bulk PLA.