Date

2018-09-01

Author

Çulhacıoğlu, Yağmur
Hasırcı, Nesrin
Dilek Hacıhabiboğlu, Çerağ

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There is an increasing trend towards replacing synthetic polymers with their natural counterparts and applying environmentally friendly processing techniques [1]. For both industrial and biomedical applications, natural originated biodegradable polymers are intensely studied due to their environmentally benign and biocompatible nature. Supercritical carbon dioxide (scCO2)processing of polymers is a green approach since it reduces or eliminates the use of organic solvents and thus emissions of volatile organic compounds [2,3].ScCO2, which is non-toxic, non-flammable and inert with relativelylow critical temperature compared to other supercritical solvents, has a plasticizer effect on polymers having CO2 affinity by decreasing the glasstransition temperature of the polymer. Therefore, it can be used as a foaming agent for such polymers. In the first step of the foaming process, the polymeris saturated with scCO2 under high pressure [4]. Next,rapid depressurization and thus supersaturation of the polymer-scCO2 system is achieved, which leads to nucleation and growth of the pores in thepolymer matrix. Poly(L-lactic acid) (PLLA) is abiodegradable and biocompatible polymer that can be replaced with petroleum based polymers [4,5]. PLLA foams can be used inbiomedical applications such as drug delivery, scaffold for bone tissue or artificial vasculature systems. Due to its highly crystalline structure, high saturation temperatures close to the melting temperature of PLLA (about180°C) and pressures over 20 MPa are necessary for the foaming of the polymerwith scCO2, especially when the crystallinity of the polymers aregreater than 30% [4,6,7]. In this study, foaming of highlycrystalline PLLA composite films (>40%) was achieved only at 40oCand 21 MPa with the help of two novel CO2-philic components used ascell nucleators. These cell nucleators are liquid polyhedral oligomericsilsesquioxanes (POSS), which are monofunctional isooctyl POSS and bifunctionalmethacrylisooctyl POSS. These molecules have recently been identified to be CO2-philicwith different levels of CO2-philicity and physicochemicalproperties [8]. When PLLA film was processed with scCO2at the same conditions, pore formation was not observed within the polymermatrix. Therefore, POSS molecules act as not only cell nucleator but they also allow dissolution of CO2 in the matrix. The foam properties were analyzedwith various techniques. The EDX analyses show that the majority of the cell nucleatoris extracted by the scCO2 during the process. The mechanical tests show that the tensile moduli of the films increased by at least 80% after the CO2processing. Both films reached a tensile modulus of around 1100 MPa. Thenano-indentation analyses also revealed similar trend in the indentation hardness and moduli. The glass transition of the films increased by at least40% after the CO2 treatment and reached a value about 57 oCwhile the crystallinity of the CO2 processed films increased by 3%. The water contact angles of the films show that the process did not change the hydrophobicity of the polymer. The drug release studies are currently carried out and the results will be presented.

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https://hdl.handle.net/11511/83858

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