Thermo-responsive Shape Memory Effect Of Propylene/α-Olefin Copolymers

Shape memory polymer(SMP)is widely used in packaging,biomedical and other fields due to advantages of light weight and low price,great processability and biocompatibility.Isotactic polypropylene(i PP),as the most widely used general-purpose polymer material in the world,has attracted widespread attention from researchers at home and abroad for its application in shape memory.This work is dedicated to exploring the shape memory principle of propylene copolymers.Using Ziegler-Natta catalyst to catalyze the copolymerization of propylene and a-olefin(1-butene,1-pentene),a series of propylene copolymers with different compositions were synthesized.Copolymers were used in the tensile deformation experiment.Through the analysis of crystalline phase before and after deformation,the mechanism of the shape memory effect of propylene copolymers with different compositions was proposed.The main research contents and conclusions are as follows:(1)By using Ziegler-Natta catalyst and slurry polymerization method,propylene/1-butene and propylene/1-pentene copolymers with different components were synthesized.By adjusting the ratio of reaction monomers,the content of 1-butene in the propylene/1-butene copolymer can be adjusted from 10 to 93 mol%,and the content of 1-pentene in the propylene/1-pentene copolymer can be controlled at 2 ~ 39mol% adjustable.The composition and sequence structure of the copolymer were analyzed by 13C-NMR,and the crystal form of the copolymer was studied by 1D WAXD.The shape memory properties of propylene/1-butene and propylene/1-pentene copolymers were measured by a single deformation-recovery cycle test,and it was found that the shape memory properties were significantly affected by the copolymer composition.(2)Through the 1D WAXD and 2D WAXD analysis of the crystalline phase of the sample before and after stretching,the mechanism of the shape memory phenomenon of the propylene/1-butene copolymer is proposed,that is,the phase transition of crystalline phase before and after stretching leads to shape memory phenomenon.In the copolymer with 1-butene content of 10-28 mol%,some alpha crystals of i PP undergo transition to mesophase during tensile deformation.In the copolymer with 1-butene content of 32-56 mol%,some alpha crystals of i PP undergo transition to trigonal crystals during the tensile deformation process.The newly generated mesophase or trigonal crystals act as a reversible phase with a fixed temporary shape,and the untransformed α crystals as a stationary phase.When the sample is heated to 60 ℃,the mesophase and trigonal crystals melt,and the sample returns to its original shape.After cooling,the mesophase and trigonal crystals have been converted back to α crystals,which can make the sample have significant and repeated shape memory behavior.The 1-butene content of 67-93 mol% copolymer has no phase transition during tensile deformation,and only a part of the molecular chain orientation causes a shape memory phenomenon with a low recovery rate.The mechanism of the shape memory phenomenon of the propylene/1-pentene copolymer is the same as that of the propylene/1-butene copolymer: when the 1-pentene content is 2-8 mol%,the phase transition of α crystal and mesophase is dominant,and the 1-pentene content At 8-39 mol%,the phase transition of α crystal and trigonal crystal is dominant.(3)The effect of the deformation rate on the trigonal crystal content,sample fixation rate and recovery rate in the propylene/1-butene copolymer was studied in detail.The results show that when the deformation rate is 600%,the sample has the highest trigonal crystal content(up to 89 %),The fixation rate(up to 80%)and recovery rate(up to 80%)are also the highest;the effect of recovery temperature on mesophase is studied,and it is found that the increase in temperature can effectively improve the transformation of mesophase to alpha crystal;The deformation process of copolymers with a content of 67-93 mol% is not very controllable.The surface of the sample after deformation and recovery was observed by SEM,and it was found that the sample surface was irreparably damaged during this process.