Study On The Controllable Polymerization Mechanism Of Polylactide And Its Effect On Zein-based Fiber Membrane

Corn is one of the main food crops in my country,with an output of 260 million tons in2020,of which about 36.5% is used in industrial production.The processing by-product zein has been used in food preservation coatings and food packaging and transportation materials because of its good film-forming properties.However,the pure zein film has poor mechanical properties and cannot be used alone as a packaging material,so it is usually necessary to add other polymer blending modifications to improve its mechanical properties.Polylactic acid,also known as "corn plastic",is an environmentally friendly green polymer that is fermented from corn starch and can be used for food packaging.It has been used in the food and medicine fields,but due to insufficient production efficiency and Its stereoselectivity in the polymerization process is not clear enough and other reasons,resulting in high production cost and unstable structure and performance,which has become the main factor hindering the promotion of polylactic acid in the market.In view of the above problems,this thesis first analyzes the control factors of the polymerization process of polylactic acid,and then wet-blends the two configurations of polylactic acid and the zein solution,and the blend is made into fibers under the action of a high-voltage electric field.Finally,the performance of the composite membrane material was characterized,aiming to improve the added value of corn by-product zein.The specific research results are as follows:(1)According to the mechanism of ring-opening polymerization,the polymerization reaction path of polylactic acid was explored,the key transition state and intermediate structure were found,and the energy calculation and decomposition were carried out.The results found that most of the polymerization reaction's rate-determining steps are nucleophilic attack processes.After considering temperature and solvation effects,the calculated activation energy can be in good agreement with the experimentally reported reaction rate,with a correlation coefficient of 0.93.Taking into account the energy difference caused by the copolymerization of lactide with different optical activity,and using Boltzmann's equation to calculate the probability of polymerization product formation,the experimental energy barrier of racemic lactide and the Pearson correlation coefficient of the calculated energy barrier reached 0.94.Finally,the energy is decomposed,and it is found that the main factor affecting the reaction rate is the correlation coefficient of the twisting energy of the molecular chain,0.81.If the twisting energy is caused by the interaction between the molecular chains,it is possible to control the reaction rate by appropriately changing the flexibility of the molecular chain.Selectivity is affected by both kinetics and thermodynamics.In this research system,chain end control caused by kinetic factors is the dominant factor in stereoselectivity.(2)The commercially available ±PLA and the laboratory polymerized PLLA are respectively mixed with zein in different proportions of polylactic acid in two configurations,and the blended fiber membrane is prepared by electrospinning under the same conditions.And test and characterize the microstructure,apparent morphology,mechanical and thermodynamic properties of these membrane materials.The results showed that the addition of polylactic acid significantly increased the spinnability of zein,the viscosity of the spinning solution increased from 0.2Pa/S to 2.1Pa/S,and the zein changed from a ribbon-like fiber structure to a uniform cylindrical shape.With the addition of polylactic acid,the thermal processing temperature of the zein-based fiber membrane can be increased by up to 20?,and the elongation at break can be increased from 6.8% to 110%,and due to the difference in the structure and molecular weight of the polymer itself,PLLA's The improvement effect is better than ±PLA.