Investigation on the Dynamic Phase Evolution and Crystallization Behavior of Polymer/CO2 System

Author:Zhang Lei

Supervisor:zhao guo qun


Degree Year:2019





The accelerated consumption of resources and the severe environmental damage have let the whole society begin to pay unprecedented attention to the issues of energy conservation and emission reduction.For the polymer materials processing industry at a cost of oil consumption and white pollution,it is urgent to realize energy conservation and emission reduction.Because the high conformational entropy,strong molecular interactions effect and intermolecular topological entanglement of long-chain molecule,there are many questions such as narrow process window,complex flow behavior and product defects in the forming and functionalization process of polymer materials.Solution of the above problems can make the processing of polymer materials further reduce energy consumption,save raw materials and improve the service performance of products.CO2 is a non-toxic,cheap,bio-compatible and adjustable fluid.The incorporation of CO2 into the forming process of polymer materials can intervene the evolution of micro condensate state and regulate the formation of mesoscopic structure of polymer.As a result,the mechanical and service performances of polymer product can be enhanced.Such advantages cause the attention of the traditional polymer processing industry,and a variety of CO2-assisted polymer forming processes,including the polymer foaming injection molding process,have been developed.Furthermore,these advantages also attract many researchers to research the polymer/CO2 system,and investigate its application potentiality in the field of tissue engineering,drug transport,electromagnetic shielding,super heat insulation,sound absorption,oil absorption and so on.At present,in terms of the polymer/CO2 binary system,a lot of researches have been carried out in the aspects of morphology evolution of gas phase in polymer flow field and condensed state evolution of polymer in high-pressure CO2 environment.However,there are still several key issues need to be studied and solved.The whole morphological evolution process of bubbles in the polymer injection flow field has not been explored,the formation mechanism and elimination method of surface defects of polymer foam parts are not clear,and the influence rule of process parameters on polymer/CO2 system lacks theoretical explanation.The influence of pressurized CO2 on polymer crystallization has not been explored,and there is no theoretical guidance on how to control the morphology of polymer crystals by changing the CO2 pressure.Focus on the above issues,this paper investigates the dynamic phase evolution and crystallization behavior of the polymer/CO2 system.The main research work and achievements are as follows(1)An incompressible,non-isothermal,and unsteady three-dimensional mathematical model of multiphase flow was established.A setting method of boundary conditions with the exhaust function on the mold cavity walls was proposed,and the artificial loss of polymer melt was reduced to less than 1‰.The problem of temperature solution divergence on the interface between two phases with high viscosity ratio was solved through the coupling algorithm of energy equation and PIMPLE loop.The tracking accuracy of micron grade bubbles interface in macroscopic scale flow field was improved though adaptive meshing refining technique.Based on this model,the influence law of temperature field and velocity field on the bubble morphological evolution in the thickness cross-section of the injection flow field was obtained.The deformation,burst and vanishing process of bubbles with different initial sizes and locations in the shear and fountain flow field was predicted.Combined with the short shot experiment of polymer foaming injection molding(PFIM),the formation mechanism of pits,silver marks and collapses on the product surface manufactured by PFIM was revealed(2)A non-isothermal transient multiphase-VOF model based on finite volume method was established,and the implicit domain coupling algorithm was proposed to synchronously calculate the temperature field of mold and cavity regions.A in the Rapid Heat Cycle Molding(RHCM)assisted PFIM process(RHCM/PFIM)was also realized.Based on the simulation and experiment results,the transient flow behavior of supercritical CO2,polymer melt and air in mold cavity was analyzed.The effect of mold temperature on the deformation,bursting and collapsing process of bubbles in fountain flow field was revealed.The formation mechanism of the surface defects of RHCM/PFIM plastic part was also revealed(3)A non-isothermal fluid-solid coupling model based on the two-phase model was developed.The coupling heat transfer between the injection mold and the polymer melt is considered by using the implicit domain coupled algorithm.This model can predict the temperature field very accurately.The thermal response characteristics in RHCM/PFIM process were further analyzed.By combining the simulation results with the cellular morphology obtained by experiments,the formation mechanisms of the cellular morphology in RHCM/PFIM process was revealled.(4)An in-situ high-pressure microscopic system was developed with a sample cell whose temperature and pressure can be close-loop controlled.The crystal growth process of Poly(L-Lactic Acid)(PLLA)film was recorded by this system in pressurized CO2 environment with different film thicknesses,different molecular weights,different temperatures and different CO2 pressures.With a reasonable selection of experimental parameters,the preliminary stage of formation process of snowflake-shaped crystal was researched by using the self-established system.With the help of atomic force microscope,this research illuminated the growth mode of the snowflake-shaped PLLA crystal.(5)An in-situ high-pressure multiple-optical observing system composed of ordinary optics,polarization optics and small angle laser scattering was established to record the evolution process of polymer condensate state in pressurized CO2 environment with a scale range from 0.1 μm to 1 cm.The statistical results of Poly(L-lactic acid)(PLLA)crystal size and crystal number density obtained in CO2 were obtained by using the this system.Combining with atomic force microscopy,the growth behavior of PLLA dendritic crystal in high-prssure CO2 was recorded and analyzed.And a bamboo-shaped dendrite formed by rhythmic growth has been discovered.(6)The supercritical CO2 was introduced to intervene in the melting isothermal crystallization of PLLA film.A screw terrace crystal excluding the tie molecules and intercrystalline entanglements was prepared.Based on the results of Atomic Force Microscopy,Transmission Electron Microscopy,Nuclear Magnetic Resonance and X-ray Diffraction,intercrystalline phases on the lamella surface were determined,and the influence of amorphous chain state on the morphology of multilayer lamellae was researched.Finally,a theoretical explanation about the origination of the lamellar splaying was concluded.(7)By using an the self-established in-situ high-pressure microscopic system,a kind of crystallization experiment with online rising pressure was realized to research the polymer crystallization in CO2.Through this univariate experimental method,the effect of CO2 pressure on the PLLA crystal growth was investigated experimentally.A new model evaluating the polymer crystal nucleation rate was proposed based on the two-step mode.Combining with the experimental results,this research concluded the influencing mechanism of pressurized CO2 on the polymer crystal secondary nucleation through quantitatively calculating crystallization free energy,diffusion activation energy,mixing energy,adsorption energy and translational energy in polymer/CO2 system