Biodegradable Shape Memory Material Preparation, Structure And Performance

A novel shape-memory polymer materials were synthesized from low unsaturated poly(propylene oxide) triols and L-lactide monomers.First, Poly(propylene oxide-lactide) diblock copolymer with triarms were prepared by ring-opening polymerization.Based on the characterization of these copolymers' chemical and segments structures,we investigated the physical properties of these copolymers.Then the crosslinked materials were synthesized using TDI as a crosslink agent,which showed shape-memory property.After investigated the crosslinked materials' chemical structure,thermal properties and mechanical properties of the materials were characterized.Also the shape changing and recovering process of materials were analyzed and evaluated by observing the transition process of microstructure.The thermal and alkali catalytic degradability were determined,and explained the degradation mechanism based on the structure of these crosslinked polymer.A series of PPO-PLLA pre-copolymer with various composition were synthesized by varying the PPO's molecular weight and the mole ratio of PO and LA.L-lactide occurred ring-opening polymerization on the terminal groups of PPO using Sn(oct)₂ as a catalytic,and formed triarms PPO-PLLA copolymer.The chemical structure of the PPO-PLLA copolymers was characterized by GPC,¹H NMR and FTIR.DSC results showed that,as the molecular weight of copolymer was constant,increasing the PPO segments' length,the cold crystalizaiton temperature of PLLA segments shifted to lower temperature,but improved the crystallinity of PLLA segments with slight higher melting temperature.When the molecular weight of PPO was constant, increasing the PLLA segments length would improve the cold crystallization temperature and crystallinity of the copolymer.It is interesting the cold crystallization peak would divide into two peaks from one as increasing the PPO's contents.The glass transition temperature raised with the PLLA contents increased,agreed with the prolepsis results by Fox-equation.The TGA results of copolymers indicated that PPO-PLLA copolymers' degradation temperature lied between PPO's and PLLA's, and the degradation temperature raised as the PLLA content increased.The crosslinked PPO-PLLA materials were synthesized using Sn(oct)₂ as a catalytic and TDI as a crosslink agent under various temperatures and times.The absorption of urethane bonds of samples by FTIR indicated that crosslink reaction occurred.It was also proved by swelling experiments.The thermal properties of the crosslinked polymers indicated that the crosslinked structure hided the PLLA segments crystallization from amorphous state.The thermal degradation temperature of the crosslinked materials shifted to low temperature as increasing the PLLA contents.On the mechanical properties,the crosslinked materials' break strength enhanced,elongation at break decreased,these materials' Young's modulus increased with the contents of PLLA increased,which meant that the crosslinked materials changed from ductile material to brittle material.As a result of containing soft and hard segment within the polymer chain,the crosslinked materials showed excellent shape-memory property.Research on the relationship between crosslinked materials' composition,the chain structures and shape-memory property was carried out.The results indicated that,these materials' temporary shape fixed ratio improved with the PLLA contents increasing,and all expressed ideally shape recovery ratio;the initial recovery temperature also increased with the PLLA contents increasing.Meanwhile,the higher crosslinkage density would accelerate the materials' recovery rate.In the other side,the deformation condition would also affect their recovery process.The lower the deformation temperature,the higher the recovery rate would be.Increasing the elongation ratio,the initial recovery temperature decreased,but recovery rate was slight faster.As the transforming cycle times increased,the recovery rate decreased to a constant value,and showed a stable recovery process.Based on the macromolecules and condensed states structure,and the views of polymer chains motion,we demonstrated the entropy elasticity shape-memory mechanism for Crosslinked PPO-PLLA polymers.Now,the biodegraded property of polymer materials is interested by researchers and industries.Because the chains contained biodegradable PLLA segments,the crosslinked triarms copolymers were a kind of the biodegradable materials.The ester group in PLLA chain could be decomposed using alkali as catalytic,and the weight loss of the materials sank in alkali solution at 80℃was calculated.The microstructures during the degradation process in alkali solution for various times were observed by electron microscope,and analyzed the effects of composition on degradability of crosslinked triarms copolymers.Compared with non-degradable PPO crosslinked polymer,the PPO-PLLA crosslinked polymers' microstructures were destroyed by the degradation process.