Study On Conformational Evolution And Mechanical Properties Of Polymers In Processing

Polymer processing is the last step of polymer from raw material to product.Its processing technology is various,such as blown film,extrusion molding,stretch film formation,injection molding and so on.However,there is a similar problem in these processes,that is,the processing flow field causes changes in the crystallinity,crystal form,crystal orientation,etc.of the polymer,thereby affecting the actual properties such as the optical,mechanical and electrical properties of the polymer product.Based on decades of experimental and theoretical work,researchers have also had some preliminary understanding of the phenomenon of flow-induced crystallization in polymer.One of the most important experimental observations is that the flow field can induce and accelerate the pre-ordering process of crystallization;the random twisted chain transitions to the straight-ordered chain.However,due to the limitations of the experimental methods such as infrared and Raman,the specific transformation process is still unclear.For example,the driving force of transformation and the transformation mode are still unsolved.At the same time,little is known about the mechanical properties of oriented semi-crystalline polymers after polymer flow field processing.The development of molecular dynamics simulations has been on the rise in recent decades.Based on its research scale at the atomic and molecular level,it is easy to reveal the microscopic mechanism and dynamic process of the flow induced chain conformation order.In this paper,the molecular dynamics simulation method is used to study the microscopic mechanism of stretch-induced coil-helix transition in isotactic polypropylene(iPP)molecular chain.The results show that the stretch can induce coil-helix transition(CHT)of the iPP molecular chain,and the content and average length of the helices increase during the process.The formation of long helices has a certain strain threshold,while the resulting dynamic path is mainly the combination of adjacent short helices.However,excessive stretching under large strains leads to the transition of the chain from helix conformation to the extended coil conformation.Based on statistical data and theoretical calculations,it is found that the driving force of stretch-induced CHT is that the stretching reduces the free energy difference of the transition,and the decrease of the free energy difference is mainly caused by the energy variation in the chain,rather than the entropy reduction described by the traditional theory.Unlike the generation of short helices,which is only based on intra-chain interaction,the formation of long helices requires interchain interaction,which are usually accompanied by the formation of helical clusters.For the oriented semi-crystalline polymer induced by the flow field,we studied the influence of the degree of orientation on the mechanical properties by molecular dynamics simulation.It is found that the mechanical strength of the oriented semi-crystalline polymer in the orientation direction is much larger than the vertical alignment direction,and the modulus increases as the degree of orientation increases.The crystal orientation in a semi-crystalline polymer mainly affects its early mechanical properties.