Processing-structure/morphology-property relationships in nanoscale fibers and their biomedical applications.

摘要

The objectives of this thesis are (1) to investigate the relationships between the structure, morphology, processing and materials properties; (2) to control molecular architecture and physical interactions including crystallization, molecular level mixing, and deformation; (3) to achieve desired degradation rates, mechanical properties, and shrinkage control of biodegradable nanofiber membranes via their microstructure and morphology; and (4) to explore potential applications of biodegradable nanofiber membranes in several areas such as for the prevention of post-surgery adhesions, and scaffolds for tissue regeneration.; The structural and morphological changes of poly(glycolide)- and poly(lactide)-based polymers (PLGA) during isothermal crystallization and during in vitro degradation have been investigated. The crystallization rate was found to increase, but the degree of crystallinity decrease with increasing GA/LA ratio. It was found that the lamellar morphology could best be described by the superstructure of PLGA polymers. During crystallization, both the average long period and the lamellar thickness exhibited decreasing values with time. Such decreases could be attributed to the mechanism of secondary crystallization in the form of lamellar-stacks insertion.; Based on the studies of in vitro degradation of poly(glycolide) homopolymer (PGA) and poly(lactide-co-glycolide) copolymer (LA/GA 10/90, PLA10GA90), a degradation mechanism model was proposed. The degradation of PGA-based materials having a large GA ratio proceeded through a combined processes of chain scission and cleavage-induced crystallization in the amorphous regions via two pathways: (1) the degradation occurred in the amorphous gaps between the crystal lamellar stacks, where the amorphous chains were broken down gaining greater mobility to form new crystal lamellae with thinner thicknesses. This process significantly reduced the average values of lamellar thickness and the long period; (2) the degradation process also occurred in the amorphous-layer domain between adjacent lamellae in the lamellar stacks, where chain scission caused the rapid decrease in polydispersity of PGA.; We have made significant efforts to understand the processing-structure/morphology-property relationships and to explore potential biomedical applications in PLGA membranes containing non-woven nanofibers via a novel electrospinning technique. We found that the fiber diameter and the membrane morphology largely depended on the processing parameters, such as solution viscosity, applied electric field strength, and ionic salt addition. The combination of different materials and processing parameters could be used to fabricate uniform nanofibers. Concentration and salt addition were found to have relatively larger effects on controlling the fiber diameter than the other parameters. The electrospinning process significantly retarded the crystallization of semi-crystalline polymers, such as L-PLA and PLA10GA90, and resulted in decreases in both the glass transition temperature and the crystallization temperature. For electrospun poly(lactide-co-glycolide) (LA/GA 25/75, PLA75GA25) membranes, which were completely amorphous, the materials lost most of the initial sizes in a one-day in vitro incubation period. If the membrane consisted of semi-crystalline polymers, such as L-PLA and PLA10GA90, a very small percentage of size shrinkage was observed. A model for structural and morphological changes during in vitro degradation of PLA10GA90 membranes has been proposed. In short, the PLA10GA90 membranes possess a fast solvent-induced crystallization process, followed by chain-cleavage induced crystallization during in vitro degradation. (Abstract shortened by UMI.)