Study On Synthesis And Properties Of Novel Polyimides Containing Pyridine Moiety In Main Chain

Aromatic polyimides are well known as functional materials with outstanding thermal resistance and thermal stability, low dielectric constant, and excellent mechanical strength, which are widely applied for adhesives, coating, and composites in advanced aerospace and electrical and electronic applications etc. With the fast development of high technology industries, ever-increasing requirements to polyimides have been raised, which attract more and more attentions on the preparation of novel polyimides with improved properties. Investigation of soluble polyimides aiming to improve their processability were adopted and have been one of the focuses.This paper discussed the developmental process of polyimides, displayed the structural characteristics of soluble polyimides, and analyzed the contributions to polyimide solubility for several modification methods. Based on the viewpoint of molecular design and combined the characteristics of heteroaromatic pyridine ring, novel pyridine-containing aromatic dianhydrides and diamines were designed and synthesized, which structures were confirmed by elementary analysis, FT-IR, NMR and MS. Derived from the resulted monomers, several 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 all-sided.Two novel pyridine-containing dianhydride monomers, i.e., linear monomer: 2,6-bis[4-(3,4-dicarboxy phenoxy)benzoyl]pyridine dianhydride(BPDA) and phenyl side-group monomer: 4-phenyl-2,6-bis[4-(3,4-dicarboxyphenoxy)phenyl]pyridine dianhydride (PPDA), were successfully synthesized, which respectively reacted with several aromatic diamines, such as p-phenylenediamine, 4,4-oxydianiline, 1,4-bis (4-aminophenoxy)benzene, 1,3-bis(4-aminophenoxy)benzene, and 1,4-bis(2-trifluoro-methyl-4-aminophenoxy) benzene, offering two series of novel polyimides containing pyridine moieties in main chain. The resulted polyimides exhibited good solubility in organic solvents. High quality polyimide films were obtained, which exhibited excellent thermal and mechanical properties. The glass transition temperatures and initial decomposition temperatures of BPDA series and PPDA series were 221-278℃and 223-256℃, and 523-529℃and 551-609℃, respectively. The tensile strengths and modulus of of BPDA series and PPDA series are 72.8-104.4 MPa and 70.7-97.6 MPa, 1.29-2.77 GPa and 1.34-2.82 GPa, respectively. The obtained films were also found fairly low dielectric constant. Comparing with each other, BPDA series polyimides exhibited slightly better solubility, mechanical property, and dielectric property, while PPDA series showed better thermal resistance and thermal stability.Taken p-hydroxyacetophenone and p-chloronitrobenzene as starting materials, two novel pyridine-containing aromatic diamine monomers, i.e. 4-phenyl-2,6-bis[4-(4-aminophenoxy)phenyl]pyridine (PAPP) and 4-(4-trifluoromethyl-phenyl)-2,6-bis[4-(4-aminophenoxy)phenyl]pyridine (3F-PAPP), were successfully synthesized and employed in the preparation of pyridine-containing polyimides by polycondensation with several aromatic dianhydrides via the conventional two-step method. Most of the resulted polyimides were soluble in common organic solvents. Meanwhile, strong and flexible polyimide films were obtained, which exhibited good thermal stability with the glass transition temperatures of 268-353℃, and the initial decomposition temperatures of 527-562℃in air atmosphere, as well as outstanding mechanical properties with the tensile strengths of 87.6-112.1 MPa, tensile modulus of 1.12-1.88 GPa and elongations at breakage of 9.5-15.4%. The polyimides were also found to possess low dielectric constants ranged from 2.49 to 3.11 at 1 MHz. The polyimides derived from trifluoromethyl-containing diamine exhibited better solubility and dielectric property than the non-fluorinated analogies.A series of novel polyimides containing pyridine moieties in main chain was prepared by the polycondensation of the two resulted pyridine-containing dianhydride monomers with the two resulted pyridine-containing diamine monomers via one-step and two-step method respectively. These polymers were subjected to solubility and thermal, optical, and dielectric property measurements. The resulted indicated that polyimides with excellent properties, such as fairly high molecule weight, good solubility, outstanding thermal stability and mechanical property etc., could be obtained by both one-step method and two-step method. The glass transition temperatures and initial decomposition temperatures in air atmosphere of the resulted polyimides were 239-278℃and 521-544℃, respectively, and their tensile strengths and modulus were 104.6-109.4 MPa and 1.92-2.58 GPa, respectively. The dielectric constants ranged from 2.80 to 2.92 at 1 MHz.