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From Insulating to Conducting Polyimides
Published in Andreea Irina Barzic, Neha Kanwar Rawat, A. K. Haghi, Imidic Polymers and Green Polymer Chemistry, 2021
Göknur Dönmez, Ayça Ergün, Merve Okutan, Hüseyin Deligöz
PIs are polymers of imide monomers containing two acyl groups (C=O) bonded to nitrogen and they can exist in both thermosetting and thermoplastic forms. They are classically synthesized by the reaction of a dianhydride and a diamine. It can be said that the subject of PI chemistry is tremendous and the monomers used in the synthesis are abundant and diverse. Depending on the monomer type and synthesis routes, the form of the final product can be different. The two-step method is mostly used to obtain PIs over poly(amic acid) (PAA) intermediate.23 In the first step of this method, the pre-polymer (PAA) is obtained from a dianhydride and a diamine. Then, the PAA is converted to PI via imidization. The imidization can be carried out by chemical or thermal ways. The thermal imidization process is generally used to obtain cast products of PI such as films, fibers, and coatings. For example, in the preparation of a film, the PAA solution is cast with the aid of an applicator onto a glass plate and subsequently, it is heated to 250°C–400°C for a while. Initially, the volatile solvent is removed depending on the solvent type. Subsequently, the cyclization occurs at high temperatures and the product turns into PI structure. During thermal imidization, heating can be applied gradually to avoid obtaining products with dark colors. That’s why the following procedure, heating at 100°C for 1 h, at 200°C for 1 h, and finally at 300°C for 1 h, is mostly applied for thermal imidization.24 On the other hand, PAA can be treated with water-capturing agents in the presence of catalysts to convert PAA to PI structure by chemical imidization process.25 It is noted that the chemical imidization of PAA can be achieved by using aliphatic acid anhydrides and tertiary amines together. For instance, Francis et al.26 used acetic anhydride and β-picoline as dehydrating agent and catalyst, respectively, in the chemical imidization of PAA. Since the selected imidization method affects the morphology of the polymer,27 it is generally expected some differences in final product properties. For example, PIs obtained by chemical imidization are more soluble and have worse thermal properties compared to the ones prepared via thermal imidization.15 Classical PI formation reactions from PMDA and 4,4’ oxydianiline (ODA) over PAA intermediate are depicted in Figure 4.2.
Tailoring structure and properties of polymer-dispersed liquid crystal by quenching process
Published in Liquid Crystals, 2020
Gang Hu, Hongxiang Chen, Zhiqiang Liu, Song Zhang, Yu Zhou, Bailin Zhu, Huazhi Gu
Since the liquid crystal molecules usually contain aromatic structure, the aromatic epoxy resin is selected to ensure the solubility of liquid crystal molecules in the matrix [18]. The mixture of two diamines with different molecular weight is used as a cross-linking agent. The network cross-linked from diamine with lower molecular weight will hinder the mobility of liquid crystal molecules [19]. The network cross-linked from diamine with higher molecular weight will reduce the solubility of liquid crystal molecules, and meanwhile, it is beneficial to adjust the orientation of liquid crystal molecules under an electric field [20], thereby reducing the driving voltage and shortening the response time, thus showing better electro-optical properties. After formation of the polymer network, the liquid crystal molecules will dissolve in the matrix resin at a higher temperature, while the solubility of liquid crystal molecules in the matrix will decrease at a lower temperature, so they can precipitate out to provide heterogeneous nucleation sites [21]. During the growth of liquid crystal droplets, the diffusion of liquid crystal molecules will be affected by the polymer chains, while the mobility of chains will show the most significant variation near Tg, therefore, the quenching temperature and time are selected to regulate the morphology of liquid crystal droplets, thereby improving the electro-optical properties of PDLC film. To our best knowledgement, the study on PDLC properties regulated by the quenching process has not been reported yet.
Soluble, thermally stable, flame retardant quinoline-based poly(ester amide)s
Published in Soft Materials, 2018
Shahram Mehdipour-Ataei, Samal Babanzadeh, Ebrahim Abouzari-Lotf, Samaneh Khodami
Architecture of diamine monomer has remarkable effects on the final properties of related polyamides. In this research a two-step reaction was employed to prepare a novel diamine (AQAB) with special features including bulky and heterocyclic quinoline group and ester unit in a fully aromatic structure.