The Research on the Preparation of Copolymerized Polyimide Foams and Their Structure and Performance

Author:Li Jian Wei

Supervisor:zhang guang cheng


Degree Year:2018





Polyimide foams is made of polyimide resin or prepolymer through physical or chemical foaming method.It can be mainly divided into common foam materials and microcellular materials.Polyimide foams combines the properties of polyimide resin and porous materials,and has important application value in high technology fields owing to its excellent properties,such as high temperature resistance,light weight,heat insulation and flame retardancy.However,the current industrialized polyimide foams are mainly soft materials with single structure and high cost,and mainly used as shock absorption and sound insulation materials.It is difficult to meet the higher application requirements of the high technology fields,such as aerospace,aviation and microelectronics industries for its heat resistance,flame retardancy,low dielectric property,low water absorption,especially the polyimide foams with higher mechanical and thermal properties.Against the above background,the preparation mechanism of polyimide foam using PEAS(BTDA/ODA)precursor powders via thermal foaming method was explored in detail.Furthermore,the mechanical and thermal properties of PI foam were improved by introducing short cut carbon fiber(CF)and 6-amino-2-(4-amino phenyl)benzimidazole(BIA)containing benzimidazole structure unit.In addition,the crossinking rigid polyimide foam with high heat resistance and mechanical properties was explored using norbornene(NA)as reactive endcapping agent.The basic law of the preparation of polyimide microcellular films by thermal phase separation was also studied,and the heat resistance and mechanical properties of the microcellular films were improved by the introduction of BIA.The dielectric constant and water absorption of the microcellular films were reduced by introduction of fluorine two amine(FODA).The relationship between polyimide molecular structure,morphology and properties were discussed in detail.The main contents and conclusions of this paper are as follows:BTDA and ODA were used as polymeric monomers to explore the preparation process of polyimide foam via free volume thermal foaming method using polyester ammonium salts(PEAS)precursor powders.The main factors affecting the thermal foaming process are the melting viscosity of precursor powders,the volatile content in precursor powders and the foaming temperature.Through analysis of the DSC and the actual foaming process of the PEAS precursor powders,it can be found that the drying temperature of precursor powder should be below 90℃,the volatile content within the precursor powders should be controlled in the range of 1115 wt%,and the foaming temperature should be between 100150℃.The density of the polyimide foam can be regulated by changing the volatile content within the precursor powders and foaming temperature.The density of the obtained polyimide foams are mainly between 1050 Kg·m-3.Based on the previous exploration,a kind of chopped carbon fiber(CF)reinforced composite polyimide foams were explored.The results revealed that the chopped CF functioned as nucleation agent in the foaming process.It significantly influenced the density and morphology of foam.Through visual analysis,it can be seen that the chopped CF mainly and well dispersed in the cell arrises,as a result,the mechanical property of PI/CF foams was significantly reinforced.The compressive strength at 15%strain increased from0.18 MPa to 0.41 MPa as the CF content increased from 0 to 6 wt%,improved around 2.5times.The heat resistance and mechanical properties of polyimide materials can be improved by modifying the structure of poymer main chains.2-(4-aminophenyl)-5-amino benzimidazole(BIA)was synthesized and a series of novel PIBTDA/(ODA+BIA)foams containing benzimidazole units were prepared derived from PEASBTDA/(ODA+BIA)precursor powders.The experiment showed that the BIA content has a significant influence on foaming degree of PEAS precursor powders.With increasing the BIA content,the melt viscosity of PEAS precursor powders increased and the density of the obtained foams was increased gradually.Moreover,the thermal stability of the resultant co-polyimide foams presented a remarkable upward trend with incorporating more BIA units into the polymer chains.As the BIA loading up to 30mol%,the glass transition temperature(Tg)of co-polyimide foams increased around 50℃compared with the pristine PI-0.Furthermore,the mechanical property of the copolyimide foams gradually increased with increasing the BIA content.The compressive strength increased about 4 times at room temperature as the BIA content increased from 0 mol%to 30mol%.The formation of cross-linking network is helpful for the improvement of mechanical and thermal properties of polymer foams.A series of rigid polyimide foams with high mechanical properties and thermal stabilities were prepared using PEAS(BTDA+NA)/ODA powder via free volume foaming method.It is found that the rigid polyimide foam with uniform pore morphology can be obtained in temperature range of 280℃350℃.In addition,the density of rigid foams can be adjusted by changing the drying temperature of the PEAS precursor powders and foaming temperature.On the basis of the prophase process,a series of rigid polyimide foams containing BIA units were prepared derived from PEAS(BTDA+NA)/(ODA+BIA)precursor powders with various BIA/ODA molar ratios.The results showed that the BIA has a significant influence on the foaming process,especially on the foaming degree of PEAS precursor powders.The density of co-polyimide foams decreased with increasing the BIA content in the polymer chains.In addition,due to the introduction of BIA rigid structural units,the rigidity of polymer chains increased gradually,and the heat resistance of polyimide foam increased significantly.As the BIA loading up to 40 mol%,the Tg of co-polyimide foams increased around 50℃in comparison with the pristine PI-0.In addition,the compressive strength of the co-polyimide foams were in the range of 0.585.48 MPa.The tensile strength of rigid polyimide foams increased from 0.41 to 1.97 MPa,and the modulus increased from22.51 to 75.68 MPa as the BIA content increased from 0 to 30 mol%.The microcellular PI(BTDA/ODA)films were prepared through a phase separation process.The dibutyl phthalate(DBP)was selected as porogen.It was found that the main factors affecting the morphology of microcellular thin films were porogen content and phase separation time.Moreover,the microcellular polyimide films showed the best pore morphology uniformity as the DBP content was 20wt%,the thickness of the casting film was in the range of 200400μm.The pore size of the obtained microcellular material is about 10μm,and the porosity is about 60%.Furthermore,a series of porous polyimide films containing benzimidazole units were prepared using same method.The results showed that the morphology was mainly determined by DBP content.The structure variation has little impact on the morphology of resultant microcellular films.With increasing BIA content,the flexible ODA monomers was gradually replaced by high rigid BIA moieties,the thermal stability and mechanical property of the porous PI films were improved gradually.When the BIA was 30 mol%,the glass transition temperature of PI-3 increased about 20℃compared with the PI-0,and the initial decomposition temperature also increased.Compared with pure PI-0,the tensile strength of PI-3 increased nearly by 2 times,and the tensile modulus increased from 64.8 MPa to 258.1 MPa,which increased by nearly 4 times.The elongation at break of porous films also decreased with increasing BIA content.At the same time,because of the higher porosity,the dielectric constant of the microcellular films in the range of1.801.90,showed the excellent dielectric properties.Fluorine atoms have unique physical and chemical properties.A series of porous copolyimide(co-PI)films containing trifluoromethyl group(CF3)were prepared via phase separation process.It is found that the porosity of porous films were mainly determined by the content of porogen,the variation of PAA chemical structure has little effect on the morphology of co-PI microcellular films.The cell size of the co-PI films were mainly distributed between 711μm.The obtained porous co-PI films exhibit ultra-low dielectric constant in the range of 1.711.96at 1 MHz,and the specific values presented a decreased tendency with increasing the FODA mol%in the co-PI matrix.The water absorption of the microcellular film also decreased with increasing FODA content in the polymer matrix.The CF3 groups decreased the intermolecular forces and destroyed the regularity of the molecular chains,but the co-PI films also presented good mechanical and thermal properties.