Biodegradable Poly (1,4-dioxan-2-one)/Starch Blends: Preparation, Structures And Properties

Poly(1,4-dioxan-2-one) or poly (p-dioxanone), PPDO, is a well-known aliphatic polyester having good physical properties and bio-compatibility, and has been used to make monofilament sutures with good tenacity and knotting. Because the raw material that is used to produce PPDO, p-dioxanone (PDO), which can be obtained by a simple production process from diethylene glycol using some efficient catalysts, becomes inexpensive and available at present, PPDO is viewed as a candidate not only for medical use, but also for universal uses such as films, molded products, laminates, foams, non-woven materials, adhesives, and coatings.In order to lower its cost and expand its application, low-cost natural polymers such as starch can be blended with PPDO. However, the poor compatibility of PPDO with.starch made the blends behave unsatisfied performances. In order to improve their interfacial adhesion, a compatibilizer must be used in the blends. This motivated us to synthesize a novel compatibilizer, starch-g-PPDO, which is also biodegradable and can be used in starch/PPDO blends.Starch-g-poly(l,4-dioxan-2-one) (SGP) was synthesized by using the ring-opening graft polymerization of PDO monomer onto starch backbone via melting and solution, respectively. Due to poor solubility of starch in most organic solvents it is difficult to obtain graft copolymer in solution. So the trimethylsily-protected starch was produced by the reaction of starch and chlorotrimethylsilane in the presence of pyridine. PPDO can be grafted onto starch unit in tetrahydrofuran in the presence of potassium /err-butoxide (/-BuOK) Thereaction conditions of preparing trimethylsilylated starch have been studied systematically. SGP synthesized in solution was characterized by FT-IR and'H-NMR.Because the recovery and purification of solvents are very tedious in preparing SGP in solution, we synthesized SGP in bulk, in which PDO monomers were grafted on the hydroxyl groups of the starch in the presence of stannous octoate (SnOcti). The detailed analysis on the microstructure of the resulting SGP was conducted by using one- and two-dimensional NMR spectroscopy. Reactivity of graft polymerization has been studied. It was found that at the initial stage the grafting reaction occurs mainly on the €2- and Ca- branches of starch units due to lower PDO content and more hydrophilic medium. With the increase of PDO content, the medium becomes hydrophobic and therefore the grafting reaction occurs on the Ce-branch. When the molar ratio of PDO/starch unit increases from 4.76 to 7.94, the PPDO grafts on the starch cannot be further lengthened. The reason for this is that although the reactivity of PDO with starch is enhanced when the PDO content increases, the homopolymerization of PDO monomer is also promoted.The structures and properties of SGP were investigated by FT-IR, HNMR, HMQC, differential scanning calorimetry (DSC), thermogravimetry analysis (TGA), wide-angle-X-ray diffractometry (WAXD) and hot-stage polarizing optical microscope (PM). It was found that the graft structures of copolymers have great effect on their thermal and crystallization behaviors. Because there were many defect sites in the crystalline phase originated from the short grafted chains of PPDO, the crystal structure of the copolymers was much less perfect than that of PPDO. PM revealed that the spherulitic morphology of the graft copolymers depended on graft structures and crystallization temperatures. From the single polarized micrograph of the graft copolymers, it was observed clearly that the starch segments acted as nucleation sites. Both the PM observation and the DSC investigation indicated that the graft copolymers have a fast crystallization process compared with PPDO, especially at higher crystallization temperatures.The thermal properties, crystallization behaviors, spherulitic morphology,rheological properties, biodegradation and mechanical properties of PPDO/starch blends were investigated. Starch improved the thermal stability of PPDO and PPDO/St...