Studies On Enhancing Thermal And Hydrolytic Stability Of Poly(p-dioxanone)

Poly(p-dioxanone) is a linear aliphatic polyester which has good biocompatibility, biodegradability and mechanical properties. It has been used in medical field, such as monofilament sutures, bone and tissue fixation devices. Furthermore, PPDO also has a great potentiality for general uses such as films, molded products, laminates, foams, non-woven materials, adhesives and coatings. However, it has a poor thermal stability, the thermal degradation of PPDO is detectable above 140℃which is near to its melting point(110℃), so the temperature range of processing is very narrow. Due to the poor thermal stability, the molecular weight and melt viscosity of PPDO significantly decrease during processing which is a disadvantage for processing. In addition, we find that it could quickly degradation even in air. Therefore, how to enhance the thermal stability and hydrolytic stability of PPDO are very important to the processing and extensive applications of PPDO.According to the thermal degradation mechanism of PPDO, this investigation focused on enhancing thermal stability of PPDO in the melt by end capping and chain extending, studing the hydro-degradation of the end-capped PPDO and chain-extended PPDO at the same time.PPDO was end-capped by mono-isocyanate. The end-capped reagent of mono-isocyanate was prepared through the reaction of toluene-2,4-diisocyanate with equimolar n-hexyl alcohol, and was analysized and characterized by titration, IR and 1HNMR. The end-capped PPDO with low inherent viscosity of 0.25 dL/g was characterized by IR and 1HNMR. The reaction of mono-isocyanate with PPDO ([η]=1.20 dL/g) have been studied. The thermal degradation behavior of end-capped PPDO in air has been investigated by TG. Kissinger method and Friedman method have been used to determine the activation energies. The calculated activation energies were 91, 81 kJ mol-1 for PPDO, and 160, 149kJ mol-1 for end-capped PPDO, respectively. Differential scanning calorimetry (DSC) and wide angle X-ray diffractometry (WAXD) have been utilized to investigate the crystallization and morphology of PPDO and end-capped PPDO. The degrees of crystallinity of PPDO and end capped PPDO by DSC were 48% and 19%, respectively. The melting point of PPDO changed little through end capping. However, the glass transition temperature of end capped PPDO increased to some extent.PPDO was chain extended by hexamethylene diisocyanate(HDI). Usually, PPDO was synthesized through the ring-opening polymerization of p-dioxanone, the reaction time was 3-4 days, the inherent viscosity was around 1dL/g. In this paper, PPDO was prepared though ring-opening polymerization of PDO using SnOct2 as a catalyst at 125℃for 3 hours, followed by an increase in molecular weight through reaction with HDI (the product is PPDO-HDI). This was an easy and simple method for production of high molecular weight PPDO in large scale. The thermal degradation behavior of PPDO-HDI in air has been investigated by TG. The initial decomposition temperature of PPDO and PPDO-HDI were 223℃and 240℃, respectively. The degrees of crystallinity of PPDO and PPDO-HDI by DSC were 60% and 33%, respectively. The mechanical properties of PPDO and PPDO-HDI were studied by tensile testing machine. The results showed that the mechanical properties of PPDO and PPDO-HDI were almost at the same level.PPDO-HDI was end-capped by mono-isocyanate, N,N'-Dicyclohexylcarbodiimid -e(DCC), poly(carbodiimide)(PCD) , respectively. The thermal degradation behavior of PPDO and end-capped PPDO in air has been investigated by TG. The results showed that the thermal stability of end-capped PPDO were higher than that of pure PPDO. Differential scanning calorimetry (DSC) has been utilized to investigate the crystallization of PPDO and PPDO-HDI-PCD (PPDO-HDI end capped by PCD). The degrees of crystallinity of PPDO and PPDO-HDI-PCD by DSC were 60% and 38%, respectively. The mechanical properties of PPDO and PPDO-HDI-PCD were studied by tensile testing machine. The results showed that the tensile strength of PPDO-HDI-PCD was the same as PPDO, but the elongation was enhanced.The degradation tests of PPDO, end-capped PPDO and PPDO-HDI in air at ambient temperature have been performed. The results showed that the stability orders of various PPDO were as followed: PPDO < end-capped PPDO, PPDO < PPDO-HDI. The in-vitro degradation tests of PPDO, end-capped PPDO and PPDO-HDI were carried out in a buffer solution based on hydrogenated sodium phosphate and di-hydrogenated sodium phosphate with an initial pH of 7.4 at 37℃. The results showed that the hydro-stability of end-capped PPDO, PPDO-HDI weren't enhanced. We investigated the change of mechanical properties to study the stability of PPDO-HDI-PCD in air. The results indicated that the tensile strength and elongation at break of PPDO-HDI-PCD changed slower than that of PPDO.