Abstract
In this work, hydrogenated bisphenol A (HBPA) based dinitro mixed isomers (1a′ and 1a) were synthesized and separated via vacuum distillation under the monitor of DSC and 1H NMR. Corresponding diamines (2a′ and 2a) were separately polycondensed with five commercial dianhydrides via a two-step thermal imidization to obtain PI-(1′-5′) and PI-(1-5). All the polyimides could afford flexible, tough, and transparent films, and most of them were readily soluble not only in common polar solvents like DMAc, but also in low boiling point solvents such as chloroform. 1H NMR spectra of the polyimides demonstrated that HBPA moiety showed no conformation changes during the preparation of polymers. For a given dianhydride, PI-(1-5) exhibited better thermal stability than that of PI-(1′-5′), this can be attributed that the equatorial, equatorial C–O in PI-(1-5) promoted denser and more regular molecular chain stacking, as can be evidenced by the WAXD and geometric optimization results. Additionally, when the dianhydride was ODPA, BPADA or 6FDA, no apparent difference was found in either the transmittance or solubility between two series of polyimides, which could be attributed that twisted and flexible ether linkages, as well as bulky substituents, led to the “already weakened” inter- and intramolecular CT interaction and cohesive force. However, when it came to rigid and stiff dianhydride, e.g., BPDA, PI-3′ took an obvious advantage over PI-3 in transmittance and solubility, which was possibly owed to the larger molecular chain d-spacing imparted by equatorial, axial C–O. An overall investigation of PI-(1′-5′) and PI-(1-5) on aspects of thermal, mechanical, morphological, soluble and optical performance values was carried out, and the conformation effects of HBPA isomers on the properties of two series of polyimides were discussed in detail.
Highlights
Aromatic polyimides possess outstanding properties such as mechanical toughness, high-temperature resistance, dimensional stability, low dielectric value, radiation resistance, selectivity to gases, etc. [1,2,3,4]
Intra- and intermolecular charge transfer (CT) interactions between the electron-donating diamine and electron-accepting aromatic dianhydride are considered the main reason for the coloration of aromatic polyimides, and the stiff and ordered structure of the molecular chains led to the insolubility or infusibility of aromatic polyimides, which deteriorate their processing and applications [6,7,8,9]
As shown in Scheme 1, mixed dinitro monomers were gently synthesized via nucleophilic substitution reaction between hydrogenated bisphenol A (HBPA) and 4-fluoronitrobenzene
Summary
Aromatic polyimides possess outstanding properties such as mechanical toughness, high-temperature resistance, dimensional stability, low dielectric value, radiation resistance, selectivity to gases, etc. [1,2,3,4]. Ando et al [18] utilized a-BPDA/CHDA to developed colorless and processable semialiphatic polyimide films; Hasegawa et al [19] synthesized 1S,2S,4R,5R-cyclohexanetetracarboxylic dianhydride (H0 PMDA), and H0 PMDA based polyimides were approximately colorless with a transmittance of up to 90.3% at 400 nm; Fang et al [20] developed cis-1,2,3,4-cyclohexanetetracarboxylic dianhydride (cis-1,2,3,4-CHDA), and polyimides bearing cis-1,2,3,4-CHDA units presented good thermal, mechanical, and soluble properties, and the cut-off wavelength was around 270 nm These results suggested that the cyclohexane moiety in the backbone of polyimides promoted the ideal properties of the polymer. To the best of our knowledge, this is the first time that the conformation effects of HBPA isomers are explored in a set of different polyimides with varying dianhydride architecture
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