Synthesis And Properties Of New Thermoset Polyimides Derived From Mixed Thioetherdiphthalic Anhydride Isomers

Thermoset polyimides display high heat resistance and great processablities,which are usually applied as matrix resins of carbon fiber-reinforced polymeric composites,high-temperature structural adhesives,coating resins and so on.For a long time,researchers have been always dedicated to improve the processablities,mechanical properties and heat resistance of thermoset polyimides to meet demands for large quantities of cost-effective and qualified polyimide materials in the high-tech fields.Phenylethynyl terminated oligomers not only possess good processability without the evolution of volatiles during the curing process,but also provide broader processing window and have drawn considerable attentions in these years.After curing,the polyimides showed appropriate chain crosslinkability and extendibility,leading to excellent mechanical properties with improved toughness and good thermal stability.In comparison with traditional dianhydrides,isomeric dianhydrides afforded corresponding resins with lower melt viscosities and higher Tgs(after curing),which could be attributed to their unique asymmetric architecture leading to decrease of intermolecular interactions and increase of rotation energy barrier.In recent years,most of the thermoset polyimides have been focused on isomeric biphenyltetracarboxylie dianhydride(BPDA)and oxydiphthalic dianhydride(ODPA).Very recently,our group has prepared a series of thermoplastic polyimides from the mixture of thioetherdiphthalic anhydride(m-TDPA),these kinds of isomeric copolyimides showed excellent melt processability similar to the homopolyimidefrom 3,4?-TDPA.In addition,m-TDPA was advantageous to prepare novel melt processable polyimides at relatively low cost.In this paper,m-TDPA and different diamine were used to synthesize thermoset polyimides with different calculated molecular weights.The effect of molecular weights and polymer chemical structures of the aromatic oligomers on their processability and solubility as well as the thermal and mechanical properties of the thermal-cured polyimides(PIs)was systematically investigated.The following works have been carried out:1.A series of oligomers based on m-TDPA and 4,4?-oxydianiline(4,4?-ODA)with 4-phenylethynylphthalic anhydride(4-PEPA)as reactive endcapping reagent were prepared.All of the oligomers were easily soluble in polar aprotic solvents such as N-methyl-2-pyrolidone(NMP)at room temperature.The typical oligomer could be melted at temperatures of 289-334 °C to yield stable molten fluid with melt viscosity below 1.0 Pa·s and the minimum melt viscosities was 0.63 Pa·s at 322 °C.The melt viscosity of the oligomers increased with the increasing molecular weight.After thermally curing at 371 °C,the thermoset polyimides exhibited good thermal properties.The glass transition temperatures of oligomers were in the range of 286-326 °C and the temperature of 5 % weight loss in N2 were higher than 524 °C.The cured films also showed good mechanical properties with tensile strength above 72 MPa and modulus more than 2.5 GPa.2.In order to improve solubility,novel imide oligomers based on m-TDPA with 2-phenyl-4,4?-diaminodiphenyl ether(p-ODA)in the presence of 4-PEPA as reactive endcapping agent were prepared.Then pyromellitic dianhydride(PMDA)was introduced to the polymerization of m-TDPA/p-ODA(molem-TDPA: molePMDA=1:1)to enhance the Tg of cured PIs.All the oligomers showed good solubility(33 wt%)in NMP and the gelation occurred after 15 days.The melt viscosity of the oligomers increased with the increase of molecular weight.The minimum melt viscosities was 1.28 Pa·s at 305 °C.After thermally curing at 371 °C for 1 h,the thermoset PIs exhibited good thermal properties.The glass transition temperatures of oligomers were in the range of 264-337 °C,and the temperature of 5% weight loss in N2 was higher than 505 °C.The cured films also demonstrated good mechanical properties with tensile strength and modulus greater than 52 MPa and 2.9 GPa,respectively.3.In order to obtain better solubility of oligomers and enhance the Tg of cured PIs,novel phenylethynyl terminated oligomers based on m-TDPA,2,2'-Bis(trifluoromethyl)benzidine(TFDB)were prepared.Then 3,4'-Oxydianiline(3,4'-ODA)was introduced to the polymerization of m-TDPA/TFDB to improve the elongation at breakage.All the oligomers showed good solubility(more than 33 wt%)in NMP and the solution were stable at room temperature for several month.The melt viscosity of the oligomers increased with the increase of molecular weight and the minimum melt viscosities was 0.87 Pa·s at 317 °C.After thermally curing at 371 °C for 1 h,the thermoset PIs exhibited good thermal properties.The glass transition temperatures of oligomers were in the range of 275-353 °C,and the temperature of 5% weight loss in N2 was higher than 536 °C.The cured films also demonstrated good mechanical properties with tensile strength and elongation at break as high as 116 MPa and 11.5 %,respectively.