Synthesis And Characterization Of Thermoplastic Polyimides From Isomeric Hydroquinone Diphthalic Dianhydrides(3,3′-HQDPA And 4,4′-HQDPA)

Polyimides(PIs) derived from 3,4′-dianhydrides attracts more and more attention of scientists from all over the world, due to their higher glass transition temperatures(Tg), better solubility, lower melt viscosity and wider processing window as compared with the traditional polyimides. The good performance of 3,4′-isomeric polyimides is owing to the presence of asymmetric structure as well as bent conformation in 3,4′-dianhydride moiety, which disrupt regularity of polymer chains and hinder denser chain packing. However, it is complicated and expensive to synthesize the 3,4′-isomeric dianhydrides. Therefore, the good performance of 3,4′-isomeric polyimides could be achieved through the copolymerization of 3,3′- and 4,4′-dianhydrides with diamines. Most researches of copolyimides focused on polyimides derived from isomeric biphenyltetracarboxylic dianhydrides(BPDA) 、 thioetherdiphthalic anhydrides(TDPA) and oxydiphthalic anhydrides(ODPA). It was found that the copolyimides drived from isomeric dianhydrides showed good thermal stability、excellent melt processability and solubility. Herein, a series of homo-and copolyimides were prepared from two hydroquinone diphthalic dianhydride(HQDPA) isomers, i.e., 3,3′,4,4′-HQDPA(4,4′-HQDPA) and 2,2′,3,3′-HQDPA(3,3′-HQDPA) with various diamines from point of view on both performance and cost efficiency. The dependence of Solubility, thermal properties, mechanical properties and melt processability of polyimides on the composition of HQDPA isomers and the effects of diamines structures were investigated in this work. The following works have been carried out:1. A series of homo- and copolyimides were prepared from isomeric dianhydrides(4,4′-HQDPA and 3,3′-HQDPA) with different ratios of the isomers and 4,4′-oxydianiline(4,4′-ODA) diamines by two-step polycondensation procedure. The inherent viscosities of the polyimides were about 0.48 d L/g. Solubility, thermal properties, mechanical properties and melt processability of PIs were investigated and compared with corresponding homopolyimides. It was found that all of polyimides were endowed with good mechanical properties with tensile strengths of 84-98 MPa, elongations at break of 5.3-8.1 %, and tensile moduli of 2.8 GPa. PIs showed excellent thermal stability with the 5% weight-loss temperatures higher than 500 °C in air and nitrogen. With the increasing of 3,3′-HQDPA, the polymers exhibited a regular increase in solubility and glass transition temperatures(Tgs), and Tgs increased from 223 °C to 257 °C. The melt processability was found to be dependent on the ratios of dianhydride units. Copolyimides showed better melt flowability than the homopolyimides. The copolyimides exhibited the best melt flowability, when the ratio of the isomers was 1:1. It was found that each isomeric polyimide showed obvious difference in crystallization behaviors. Thermoplastic polyimide based on 4,4′-HQDPA was semicrystalline resins which showed a melting peak at 350 °C. This melting/recrystallization phenomenon has been shown to give rise to the observed multiple melting endotherms in the DSC scans of samples annealing crystallized at different temperatures.3. A series of homo- and copolyimides were prepared from isomeric dianhydrides(4,4′-HQDPA and 3,3′-HQDPA) with different ratios of the isomers and 3,4′-oxydianiline(3,4′-ODA) diamines by two-step polycondensation procedure. The inherent viscosities of the polyimides were about 0.45 d L/g. Solubility, thermal properties, mechanical properties and melt processability of PIs were investigated and compared with corresponding homopolyimides. It was found that all of polyimides were endowed with good mechanical properties with tensile strengths of 102-116 MPa, elongations at break of 4.1-7.1 %, and tensile moduli of 3.1-3.5 GPa. With the increasing of 3,3′-HQDPA, the polymers exhibited a regular increase in solubility and glass transition temperatures(Tgs), and Tgs increased from 197 °C to 224 °C, which is about 23 °C lower than those from 4,4′-ODA. The 5% weight-loss temperatures of all polyimides were obtained near 500 °C in air and nitrogen which was comparable with polyimides based on 4,4′-ODA. In addition the melt processability was found to be dependent on the ratios of dianhydride units. Copolyimides showed better melt flowability than the homopolyimides and the copolyimide exhibited the best melt flowability, when the ratio of the isomers was 1:1. Compared with polyimides derived from 4,4′-ODA, polyimides derived from 3,4′-ODA exhibited much lower melt viscosity.