Spherulitic morphologies of the triblock Poly(GL)-b-poly(GL-co-TMC-co-CL)-b-poly(GL) copolymer: Isothermal and non-isothermal crystallization studies

Abstract

Crystallization of a biodegradable segmented copolymer constituted by polyglycolide hard segments and a middle soft segment constituted by a random disposition of glycolyl, e-caproyl and trimethylene carbonyl units has been studied by means of optical microscopy, atomic force microscopy and time resolved X-ray diffraction techniques. This Poly(GL)-b-poly(GL-co-TMC-co-CL)-b-poly(GL) copolymer is widely employed as surgical suture and has similar characteristics than previously studied copolymers having a middle soft segment constituted by only two monomers (i.e. glycolide and trimethylene carbonate).

FTIR and NMR spectroscopies demonstrated that the middle segment had an amorphous character and a random microstructure as consequence of transesterification reactions that took place during synthesis. Nevertheless, polyglycolide segments were able to crystallize giving rise to peculiar positive birefringent spherulites with a morphology, which depends on crystallization temperature (i.e. flat-on and edge-on crystals) as verified by AFM and electron diffraction patterns.

Complete bell shaped curves that defined the temperature dependence of the crystal growth rate could be experimentally obtained from both, isothermal and non-isothermal crystallizations. Data from both analyses were in close agreement and pointed out a secondary nucleation constant (2.42–2.88 × 105 K2) which was clearly higher than that determined for the related system with two components. Lamellar morphologic parameters were similar for samples crystallized from the melt state and after the reordering process that took place on heating. Comparing to the bicomponent system, significant differences were again observed highlighting the influence of the soft segment on the crystallization behavior