Study On Synthesis And Properties Of Novel Soluble Polyimides Based On Unsymmetrical Aromatic Diamine Monomers

Aromatic polyimides possess excellent thermal and mechanical,electrical,and chemical properties.Therefore,they are being used in many applications such as electrics,coatings,composite material,and membranes.However,the commercial use of these materials is often limited because of their poor solubility,and high softening or melting temperatures.To overcome these problems,more and more attempts have been focused on the preparation of novel polyimides with improved properties.This paper summarizes the structural characteristics of polyimides with excellent properties,and analyzed the contributions of several structural modification methods to each aspect of polyimide properties.Four novel unsymmetrical aromatic diamines, i.e.1,4-(2′-trifluoromethy1-4′,4″-diaminodiphenoxy)benzene(PAPB),1,4-(2′,4″-diaminodiphenoxy) benzene(OAPB),1,4-(3′,4″-diaminodiphenoxy)benzene(MAPB)and 1,3-(2′-trifluoromethyl-4′,4″-diaminodiphenoxy)benzene(PMAPB)were designed and prepared based on the viewpoint of molecular design,which structures were confirmed by elementary analysis,FT-IR,NMR and MS.Derived from the resulted diamines and dianhydrides such as 4,4′-(hexafluoroisopropylidene)diphthalic anhydride(6FDA),4,4′-Oxydiphthalic anhydride(ODPA),3,3′,4,4′-benzophenonete-tracarboxylic dianhydride(BTDA)and Pyromellitic dianhydride(PMDA),four series of novel polyimides were successfully synthesized.The polymer constructions were confirmed by elementary analysis,FT-IR and ~1H-NMR.Their solubility,thermal property,mechanical property,aggregate structures,and dielectric property were investigated and discussed.A new aromatic unsymmetrical ether diamine with trifiuoromethyl pendent group, 1,4-(2′-trifluoromethyl-4′,4″-diaminodiphenoxy)benzene(PAPB),was successfully synthesized in three steps using hydroquinone as starting material and polymerized with various aromatic tetracarboxylic acid dianhydrides via the conventional two-step thermal or chemical imidization method to produce a series of the fluorinated polyimides.The polyimides were characterized by solubility tests,viscosity measurements,FT-IR,NMR spectroscopy,X-ray diffraction studies,and thermogravimetric analysis.The polyimides obtained had inherent viscosities ranged of 0.56-0.77 dL/g and were easily dissolved both in polar aprotic solvents and low boiling point common solvents.The resulting strong and flexible PI films exhibited excellent thermal stability with the decomposition temperature(at 5%weight loss)of above 522℃and the glass transition temperature in the range of 232-272℃. Moreover,the polymer films showed outstanding mechanical properties with the tensile strengths of 74.5-121.7 MPa,elongation at breaks of 6-13%and initial moduli of 1.46-1.95 GPa,and good dielectric properties with low dielectric constants of 1.82-2.53 at 10 MHz.The two novel unsymmetrial aromatic diamine monomers,i.e. 1,4-(2′,4″-diaminodiphenoxy)benzene(OAPB),1,4-(3′,4″-diaminodiphenoxy)benzene (MAPB),were successfully synthesized using hydroquinone as starting material and employed in the preparation of novel polyimides by polycondensation with several aromatic dianhydrides via the two-step method.The glass transition temperatures and the temperature at 5%weight loss in nitrogen of OAPB series and MAPB series were 230.2-299.0℃and 216.0-288.6℃,and 505-528℃and 515-530.6℃,respectively. The tensile strengths and modulus of OAPB series are 41.4-108.5 MPa and 1.15-1.68 GPa,respectively.The obtained OAPB films were also found fairly low dielectric constant of 2.24-2.80 at 10 MHz.And the tensile strengths and modulus of MAPB series are 39-116.0 MPa and 1.62-1.92 GPa,respectively.A new aromatic unsymmetrical ether diamine with trifluoromethyl pendent group, l,3-(2′-trifluoromethyl-4′,4″-diaminodiphenoxy)benzene(PMAPB),was successfully synthesized in three steps using resorcin as starting material and polymerized with various aromatic tetracarboxylic acid dianhydrides via the conventional two-step thermal or chemical imidization method to produce a series of the fluorinated polyimides.The resulting strong and flexible PI films exhibited excellent thermal stability with the decomposition temperature(at 5%weight loss)of above 540℃and the glass transition temperature in the range of 204.3-259.3℃.Moreover,the polymer films showed outstanding mechanical properties with the tensile strengths of 97.6-138.9 MPa,elongation at breaks of 7%-18%and initial moduli of 1.55-1.95 GPa,and good dielectric properties with low dielectric constants of 2.15-2.71 at 10 MHz.