The Structure-Property Study Of Polyimides By Molecular Simulation And Molecular Design Approach

With the development of the hardware and software, thestructure-property relationships of polyimides with molecularsimulation approach provides a guideline for designing polyimides havingdesirable end-use properties. The importance of the computer simulationas a tool to predict and explain the properties of the newdesigning PIis being much in evidence and is upheld by more and more researchers.The studies in this dissertation have focused on the relationshipbetween the chemical structure and the chemical-physical property ofpolyimides and the establishment of molecular simulation approach forpolyimides with the biphenyl side chain and the homopolyimide with orderstructure aggregation.The first chapter is the backgroud of the this dissertation includingthe principle and forcefield of the molecular simulaiton.In the second chapter, with our early experimental research, 3copolyimide with different length methylene spacing group in biphenyl sidechains show that the polyimide PP6DA-PI with six methylene flexiblespacing group in biphenyl side chains has lower values in intrinsic viscosity, density and glass transition temperature but keep themechanical property close to that of polyimide PPODA-PI with no methylenespacing groups, and PP2DA-PI with two methylene spacing groups in theirside chains. In this chapter, the modeling of chain conformation showsthat the biphenyl side groups in PPODA-PI are almost vertical to thebackbone, whereas the biphenyl side groups in PP6DA-PI are almost parallelto the backbone, indicating that PP6DA-PI exhibits a strong intra-chaininteraction leading to in-situation reinforcement interaction between themain chain and the side chain, validated by the charge density, dipolemoment and force current property of the two phenyls in biphenyl sidechains. For this reinforcement in PP6DA-PI, the original modulus has beenenhanced to some extent. Meanwhile, though the PP6DA would be more reactivedue to higher charge density on N atom of the diamine monomer, the bulkstructure of the side chain hindering the polymerization approach of themonomers leads to lower intrinsic viscosity of PP6DA-PAA. The density ofPP6DA-PI does not decrease excessively with the increase in chain lengthbecause of the bended side chain in this system.In chapter 3, combined with XRD, POM, SEM measurements, therelationship of chemical structure and order aggregation of four kindsof polyimides based on BTDA monomer (3,3',4, 4'-benzophenonetetracarboxylic dianhydride) with various diamineswas investigated by molecular simulation. The diamine monomers used aremeta-phenylene diamine (m-PDA), para-phenylene diamine (p-PDA),4,4'-oxydianiline (ODA), benzidine (BZD), respectively.In procedure 1, due to the solvent assistance, the imidizationprocess and the order structure formation is ongoing at the same time,which is favorable to the sufficient movement of the polyimide chain.Therefore, from the thermal dynamics, the chain regularity is crucialto the perfection of the aggregation. The conformational analysis canoffer a complete image on the geometry of the chains and the stacking ordering of the polyimide models. The chains' geometries suggested thatthe order of chain regularity is: PI-B≈PI-P＞PI-M＞PI-O due to theconsideration of the whole chain: the carbonyl groups displacement andthe stacking of the backbone in bulk structure. This conclusion is inagreement with experimental results that in procedure 1 for these semicrystalline polyimide, the percent crystallinity is 40.0% for PI-O,52.2% for PI-M, 64.8% for PI-P, and 60.63% for PI-B. The differencetextures were obtained by SEM scanning as mentioned above. Set againstthe rotation barrier energy of four polyimides, the order of the chains'mobility is: PI-M＞PI-O＞PI-B＞PI-P. This result is consistent with theobservation in procedure 2, in which the banded spherulite was obtainedfor PI-M system at 180℃, underdeveloped spherulite was obtained for PI-Osystem at 250℃,irregular dotted crystals for PI-B at about 250℃, andno optical texture for PI-P during 2—300℃range.With the above established molecular simulation analysis for thestudy of the structure-property and the order aggregation of polyimides,we designed 6 different polyimides with different length side chain inbackbone. The used dianhydrid were PMDA, ODPA, and BTDA, respectively.Compared with different conformational geometry and chain flexibility,the modeling of chain conformation shows that the biphenyl side groupsin PMDA-6 are almost emanative to the backbone, whereas in ODPA-6 theside chains are almost recumbent to the backbone, and in BTDA-6, the sidechains are almost parellel to the backbone. Therefore, the predictedstacking showed that the BTDA-6 and PMDA-6 have the layered structureformation, while the ODPA-6 with strong intra-chain and inter-chainreaction has the amorphous bulk structure. Furthermore, for betterunderstanding and verifying the accuracy of molecular prediction, inthese 6 different polyimides, we chose 3 samples for the followingexperimental research.At last, 6 novel polyimides were prepared by polycondensation of diamine containing biphenyl mesogenic unit with BTDA, PMDA and ODPAdianhydrides. The predictive properties, such as solubility, density,glass transition temperature and the order structure were investigatedthrough DSC, X-ray diffraction, POM, and SEM measurements. Theexperimental results agree with the predicted properties well,indicating that the introducing of the long biphenyl side chain can leadto better solubility, lower density and glass transition temperature,and with different effects of the in-situation reinforcement withindifferent backbones and side chains of polyimides, the order structureis in large distinction from each other.