Читать книгу Poly(lactic acid) - Группа авторов - Страница 92

Albertsson and coworkers [59, 60,62–64] reported extensive studies on copolymers of LA and 1,5‐dioxepan‐2‐one (DXO). Poly(1,5‐dioxepan‐2‐one) (PDXO) is a completely amorphous and hydrophilic wax‐like polymer with a T _g of −39°C. When DXO is used as a comonomer with LA, it increases the hydrophilicity and rate of degradation of the copolymers as compared with PLA. The copolymers show characteristics of thermoplastic elastomers that are suitable for biomedical applications such as slowly degrading sutures, temporary implants, and drug vehicles. Although the synthesis of DXO was reported as early as 1972, Mathisen et al. [59] in 1989 described an improved reaction scheme with a high yield. The DXO block forms a soft amorphous block, while LLA block forms a hard‐semicrystalline segment in the triblock copolymer of poly(LLA‐b‐DXO‐b‐LLA) [60, 62, 63]. Blocks of controlled lengths were synthesized using a tin oxide initiator. Degradable polyesters with strictly defined structure, unique mechanical properties, and tuned degradation profiles were prepared and characterized [64]. The morphology of spin‐coated films of triblock copolymers poly(LLA‐b‐DXOb‐LLA) was characterized by AFM. These studies revealed the absence of nanoscale morphology in copolymer films [64].