Synthesis and self-assembly of biodegradable polylactide containing block copolymers and their stereocomplex blends

Date of Completion

January 2007

Keywords

Chemistry, Polymer

Degree

Ph.D.

Abstract

Polylactide (PLA), as an emerging biodegradable thermoplastic material derived from renewable resources, has attracted increasing research interest due to its biodegradability and biocompatibility. PLA containing block copolymers further diversify its application in the biomedical field, commodity materials, and even advanced nanoporous materials. ^ This thesis focuses on the fundamental aspects of morphology and phase behaviors of PLA-containing block copolymers, as well as their stereocomplex blends formed by blending incompatible enantiomeric PLA block copolymers. The work consists of two parts. First, using living ring-opening polymerization of L- and D-lactides (PLLA and PDLA), well-defined enantiomeric PLA block copolymers were synthesized from hydroxyl-terminated hydrophilic poly(ethylene oxide) (PEO) and hydrophobic poly(ethylene-co-1,2-butylene) (PEB) precursor polymers, respectively. Research effort was devoted to controlling the molecular weight of each block and narrow polydispersity. Quantitative stereocomplex formation was achieved by equimolar mixing of incompatible PEO- b-PLLA and PEB-b-PDLA. ^ Second, the morphology and phase behaviors of diblock copolymers and their stereocomplex blends were studied by small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) in both molten and crystalline states. In the stereocomplex systems, polymer brushes formed with PLLA/PDLA stereocomplex crystals as the substrate and PEO and PEB as the brushes. We were able to generate either balanced or unbalanced surface stresses on opposite sides of a stereocomplex crystal due to the crowded PEO and PEB brushes by changing the molecular weight of each block. When balanced surface stresses were generated, flat lamellar structure formed in both molten and crystalline states. Interestingly, if unbalanced surface stresses were generated, curved microstructures were found, and the detailed morphology depended on the molecular weights of PLLA/PDLA. A novel honeycomb structure with hexagonal cylinders being 2/3 of the volume fraction was achieved in the melt state when the PLLA/PDLA molecular weights were only 5,000 g/mol. Onion-like morphology was obtained in both the melt and crystalline states when the PLLA/PDLA molecular weights exceeded 10,000 g/mol. The detailed structures such as solid versus "hollow" onions were further controlled by the PLLA/PDLA molecular weights. Based on the SAXS and TEM results, tentative models were proposed to explain these novel structures. ^

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