Synthesis And Properties Of Polylactic Acid And Its Copolymers Of Microwave Radiation

Poly(L-lactide) (PLLA) has been found wide applications in biomedical fields as substitutes for tissue engineering owing to its favorable biocompatibility and biodegradation properties. With the gradual extension in the application range of PLLA copolymers and the continuous increase in materials properties demand, traditional PLLA polymer has failed to meet the demand for PLLA for its strong hydrophobicity, poor toughness, long degradation period of time as well as poor cytocompatibility, and which has become the most-highlighted and cutting-edge subject. The properties of the PLLA polymers can be mediated, and are even endowed with some special features by the method of copolymer formation, whereas the copolymers can be acquired via reforming PLLA compositions and specific architectures. Based on these descriptions, detailed work has been carried out in this thesis in the following aspects.1. The optimal technological factors of synthesizing PLLA were selected via experimental methods of orthogonal arrays design. Based on the above efforts, poly (L-lactide) (PLLA) polymers were synthesized via a microwave-assisted ring-opening and O=C-O groups exist in the structure of the resultant product, and that the as-synthesized PLLA is star-shaped in molecular structure. GPC measurements show that the resultant product assumes a single compositional polymer with narrow molecular weight distribution. The polymer has the glass transition temperature (Tg) of 45.3℃, melting point (Tm) of 124.7℃, and exhibits certain crystalline behavior. This crystalline property is corroborated by the appearance of the apparent diffraction peaks in XRD patterns. The dynamic contact angle measurements indicate that the PLLA powders have the contact angle of 83.0°.2. PLLA-g-MAAH polymers were synthesized by the graft reaction taking place between the terminated hydroxyl groups of s-PLLA and methacrylic anhydride using the as-synthesized s-PLLA as a raw material. The molecular structure was confirmed by NMR and FT-IR. GPC results show that the as-synthesized PLLA-g-MAAH polymer has narrow molecular weight distribution. The PLLA-g-MAAH polymers produce lower glass transition temperature in comparison with the s-PLLA, as demonstrated by DSC measurements. XRD analyses show that the PLLA-g-MAAH polymers are more conducive to be crystallized owing to the introduction of flexile methacrylate chains, compared with the s-PLLA. DCAT determinations indicate that the PLLA-g-MAAH polymers exhibit improved hydrophilicity, which is in favor of the applications in drug release carriers in the future.3. Poly(glycolic acid) was prepared by melt polycondensation ring-opening polymerization routes, using glycolic acid as a monomer and zinc lactate as a catalyst, companied by the traditional oil bath heating and microwave irradiation respectively; and their structural characterization was conducted by NMR and FT-IR. Their thermal properties and crystalline properties do not exhibit significant difference, as evaluated by DSC and XRD. SEM observations reveal that the surface morphologies have changed after the PGA synthesized by microwave irraradiation degrades in the buffer solution for 30 days, and with the pH value decreased, the degradation behavior becomes more obvious. AFM observations demonstrate that the oxygen plasma treatment makes the PGA surface morphologies more orderly than the untreated PGA. The contact angle measurements show that the contact angle becomes smaller and the hydrophilicity and hydrophobicity are enhanced after the oxygen plasma treatment. Since the increase in the surface free energy results mainly from the polar part, the surface hydrophilicity is enhanced. Therefore, the microwave radiation route is conducive to the synthesis of biodegradable PGA materials, while the plasma surface treatment is an effective method to further improve the surface biocompatibility of PGA. This work establishes a theoretical basis for the preparation of the biomedical materials in the future.Based on the above study, poly(lactic acid-g-glycolic acid) (PLGA) copolymers were prepared by means of the above microwave-assisted melt polycondensation ring-opening method in different ratios of lactic acid and glycolic acid monomers, and the structure of products was confirmed. GPC determination indicates that the molecular weight of PLGA copolymers is increased with increasing the monomer LA content, and the polydispersity Mw/Mn of the generated PLGA copolymers is less than 2.0 for several different monomer feed ratios. DSC analysis shows that the melting enthalpy and glass transition temperature corresponding to PLGA copolymers are reduced, and the crystalline properties deteriorate with the LA monomer content increased, which is consistent with XRD results. The contacts angle measurements show that the surface contact angles for PLGA copolymers become smaller, and the hydrophilicity is reinforced with increasing the LA monomer content. The PLGA copolymers synthesized by this method are hopeful to be used in biomedical fields.4. Thermosensitive PNIPAM-co-(PLLA-g-MAAH) copolymers and thermosensitive P(NIPAM-co-NMAM)-g-(PLLA-g-MAAH) graft copolymers were successfully synthesized via a free radical polymerization route using 2,2'-azobisisobutyronitrile (AIBN) as an initiator. The structure, molecular weight and polydispersity of the copolymer were characterized and confirmed by NMR, FT-IR and GPC. UV-vis transmittance analyses show that the obtained graft copolymers have the low critical solution temperature (LCST) of about 42℃close to the human body temperature. Surface tension measurements demonstrate that the graft copolymer aqueous solution bear the critical micelle concentration as low as ca.28 mg/L. The DLS and TEM results show that these graft copolymers form nearly sphere-like micelles in aqueous solution, particle size distribution ranges from 15 to 27 nm, and average size of approximate 20 nm. MTT assays indicate that the graft copolymers have slight toxicity depending on the copolymers and graft copolymer concentration. Therefore, the as-synthesized PNIPAM-co-(PLLA-g-MAAH) copolymers and P(NIPAM-co-NMAM)-g-(PLLA-g-MAAH) graft copolymers can be expected to be applied in the biomedical field as important drug delivery carrier materials.