Synthesis And Properties Of The PLLA/PLAL Biodegradable Shape Memory Polymer Blends

In this paper, a kind of cyclic dimer consisting lactic acid and lysine monomer is polymerized by a series of organic reaction using D-alanine and lysine protected by Z-group as raw materials. The reaction conditions of intermediate reaction during the polymerization process are determined, and then the crude product is purified. After that, the poly(L-lactide-co-l-lysine) (PLAL) is copolymerized by ring-opening polymerization of L-lactide and cyclic dimer, and the chain structure of PLAL is characterized. The microstructure, mechanical properities, shape memory effect and degradation properties of PLAL/PLLA blends are investigated systematically by infrared spectrum (FTIR), polarizing microscope (POM), X-ray diffraction (XRD), scanning electronic microscope(SEM), tensile strength testing, differential scanning calorimeters(DSC), in-vitro degradation tests. The essence and relationship between the microstructure and properties are illuminated.Furhtermore, the effects of annealing treatment on the microstructure, mechanical properties and shape memory effect are studied and the mechanism is also revealed.The experimental results show the optimal reaction conditions for the synthesis of the cyclic dimer: diazo-reaction temperature is 2 ℃ and reaction time is 4h. Acyl chloride reaction temperature is 10 ℃ and reaction time is 2h. In the third step, the amout of Z-lysine is 16g, and the polymerization is carried out at 40 ℃ for 6h. Finally, the synthesis of cyclic dimer is polymerized at 50 ℃ for 2h. The FTIR results verified that the structure of the product for each step is in accordance with the theoretic results. The PLAL is successfully synthesized by ring-opening polymerization, and the chain structure of PLAL is also verified by FTIR and HNMR.PLAL/PLLA blends have good compatibility, and the PLLA crystal is the crystalline phase. The amount and size of spherlite decreases,as well as the crystallintity of PLLA/PLAL blends decreases with the increase of PLAL content. The PLAL content has apparently affect on the mechanical properties. The tensile modulus and tensile strength decrease gradually with the increase of PLAL content. However, the elongation at break increases with the PLAL content increasing. The storage modulus of PLLA/PLAL blends abruptly decreases and has a plateau above their glass transformation temperature, which indicates the PLLA/PLAL blends possess shape memory effects.The results show that the shape retention ratio of PLLA/PLAL blends decreases, while the shape recovery ratio and recovery stress of PLLA/PLAL blends initially increase and then decrease with the increase of the PLAL content. When the PLAL content is 20wt%, the PLLA/PLAL blend possesses the optimal shape memory properties. The shape memory process has obviously effects on the shape memory effect of the blends. The shape retention ratio of the blends slightly increases, where as the shape recovery ratio of the blends gradually decrases with the increase of deformation temperature. The higher recovery stress can be obtained at deformation temperature below Tg. However, the shape recovery ratio of the blends gradually decreases with the increase of deformation temperature. The effect of deformation on the shape memory rention of the blend is little. But, shape recovery ratio of the blends decreases with the increase of the deformation.The microstructure of the blends can not be changed by annealing treatment. But, the crystallinity of the blends increases after annealing treatment. The samples after annealing treatment possess enhanced tensile strength and lessened elongation at break.Vitro degradation tests showed that the degradation rate of PLLA/PLAL blends is higher than that of PLLA. With the increase of the PLAL content, the degradation rate of the blends increases. The in vitro pH value determination results indicate that the PLAL could neutralize the lactic acid resulting from the degradation of PLLA.