Design,Synthesis And Properties Of Biodegradable Light-induced Shape Memory Polymers

Thermally induced biodegradable shape memory polymers (TBSMPs) have been paid much attention in recent years due to their excellent shape-memory function and biodegradability. They have found many application fields, especially in biomedical application such as sutures, bone-fixing materials to facilitate knotting and fixing in surgical operations and to avoid second operation to take them out of the body. With comparison to thermally induced SMPs, light-induced SMPs are characterized by temperature-independent shape memory effect and possible remote light triggering and may find biomedical and other applications. Nondegradable light-induced shape memory polymers were first reported by Lendlein Andreas in 2005. However, biodegradable and biocompatible LSMPs has not yet reported up to now though good biodegradability and biocompatibility are very desirded for biomedical applications.In this study, we aim to design and synthesize a kind of light-induced biodegradable shape memory polymers (LBSMPs)—biodegradable muti-block polyesterurethanes containing pendent photoresponsive cinnamamide group and possessing satisfactory shape memory effect and good biodegradability and biocompatibility.Firstly, N,N-Bis(2-hydroxyethyl)cinnamamide (BHECA), a diol monomer with a pendent photoresponsive cinnamamide group was successfully synthesized in high yield from methyl cinnamate (MC) and diethanolamine (DEA) under mild conditions. The product was characterized with element analysis, FTIR,1H NMR, MS, DSC and HPLC. The reaction was infuenced by ratio of raw materials, temperature and reaction time, etc. The reaction conversion increased with the increasing of the temperature, the dose of catalyst, the quantity of diethanolamine. It leveled off at 110℃,0.6wt% of MC and n(MC:DEA)=1:2, respectively. The reaction conversion can be as high as 99%. BHECA with purity of 96% was synthesized at a yield of 93% at the following conditions:reaction temperature 110℃, n(MC:DEA)=1:2,30min. This ammonolysis approach is characterized by readily available and safe raw material, high yield, easy separation and nonuse of organic solvent.Then biodegradable multiblock polyesterurethanes containing pendent photoresponsive cinnamamide groups were synthesized via a two-step polyaddition reaction using N,N-bis(2-hydroxyethyl) cinnamamide as molecular switch, aliphatic polyester diols as biodegradable soft and hard segments, and diisocyanate as coupling agent. They were characterized by IR,1H NMR, which furtherly confirmed its multi-block structure.The existence of pendent cinnamamide groups disturbed the molecular structure of soft segments and greatly destroyed the crystallinity of PCL. The amorphous nature of the soft segments facilitates to deform of the polymer and to be processed photomechnically at room temperature. However, the hard PLLA segments remained crystallized though the crystallization was depressed to cetain extent due to the existence of the soft segments. The melting temperature and enthalpy increased with PLLA molecular weight.The flexible PCL segments studded with BHECA moieties are amorphous and constitute the soft phase. The crystallized PLLA segments act as physical crosslinks and compose the hard phase. Therefore, the resultant polymers behaved as thermoplastic elastomers at room temperature. The tensile strength, modulus and elongation at break range 10-20 MPa,20-230 MPa and 230%-530%, respectively. The elongation increases with increasing the content of soft PCL segments.The pendent cinnamamide groups work as photoresponsive molecular switches and endow the polymer with light-induced shape memory behavior via reversible [2+2] cycloaddition crosslinking. The Rf, Rr range 32～55% and 73～98%, respectively, with different compositions. The strain fixity rate Rf increases with the content of BHECA moieties and the strain recovery rate Rr increases with PLLA content. The Rf reaches 50% at a BHECA content of 20 wt% and the Rτreaches>95% at PLLA content of 50 wt%.Besides, the shape memory effect was also dependent on the strain, the irradiation time and light density. The recovery rate increased with the light density, while too long irradiation time and large deformation were not favorable to the shape memory behavior. The best irradiation time was 60-90min. Temperature and cycles had no significant influence on shape memory effect, but the Rr will have a large decrease after repeated many times.