Cellulose Diacetate-g-poly(p-dioxanone) Co-polymer: Synthesis, Properties and Microsphere Preparation
A novel biodegradable graft co-polymer based on cellulose diacetate (CDA) and poly(p-dioxanone) was synthesized by ring-opening polymerization of p-dioxanone (PDO). The molecular structure of co-polymers was characterized by one- and two-dimensional NMR. The graft co-polymers with different lengths of PPDO side-chains could be controllably synthesized by changing the in-feed ratio of CDA/PDO. It was found that the PPDO content had great effect on the thermal transition behavior, crystallization ability and thermal stability of the graft co-polymer. The in vitro degradation rates of CDA-g-PPDO were higher than that of linear PPDO due to their lower crystallinity. Moreover, porous microspheres of graft co-polymers with a diameter of about 5 μm, prepared through the solvent evaporation of water-in-oil emulsion (W/O), indicated it may have potential applications in drug-delivery systems.
Synthesis and Properties of Poly(d,l-lactide-co-p-dioxanone) Random and Segmented Copolymers
Two different polymerization routes, one-step and two-step bulk ring-opening polymerizations of d,l-lactide (LA) and p-dioxanone (PDO) monermers using stannous octoate [Sn(Oct)2]/n-dodecanol as the initiating system, were employed to synthesize poly(d,l-lactide-co-p-dioxanone) P(LA-co-PDO) random and segmented copolymers with different compositions and chain microstructure. For the two-step copolymers, the average sequence lengths of the lactidyl (LLA) and dioxanyl (LPDO) units calculated from the 1H-NMR spectra were much longer than those values for the one-step copolymers with the same LA/PDO feed ratio. Corresponding to this difference in microstructure, the two-step copolymers were semi-crystalline even when the PDO content was as low as 14.5 mol%, while the one-step copolymers were completely amorphous with PDO content below 60.6 mol%. However, irrespective of polymerization route, both types of copolymers displayed a single glass transition temperature that was in a linear relation with composition. The decrease of maximum decomposition temperature of the copolymers was in accordance with the decrease of LPDO value. The mechanical and degradation properties of the copolymers were significantly affected by both the polymerization route and the chemical composition as well. In conclusion, the properties of P(LA-co-PDO) copolymers could be adjusted conveniently to meet specific applications by changing the composition and microstructure of the copolymers via different polymerization routes.
Conception of a Language Learning Environment Based on the Communicative and Actional Approaches
This paper's aim is to give a conceptual framework about the conception of a Computer Assisted Language Learning environment based on the communicative and actional approaches. In this environment, the learner is implicated in a task that he/she has to complete by interacting with a partner using his/her language competencies. Given a task, different ways exist to accomplish it and the different competencies of the learner can then be solicited with different types of interaction. Works accomplished in human computer dialogue give models to manage this interaction.