Soluble Polyimides Bearing (cis, trans)-Hydrogenated Bisphenol A and (trans, trans)-Hydrogenated Bisphenol A Moieties: Synthesis, Properties and the Conformational Effect

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.

Effects of thermal cycling on phenylethynyl-terminated PMDA-type asymmetric polyimide composites

The effects of thermal cycling on a polymerized monomeric reactant (PMR) type polyimide (TriA X) reinforced with carbon fibers were investigated. Composite specimens were subjected to 2000 thermal cycles between −54°C and 232°C. At 400-cycle intervals, laminates were inspected for microcracks, and glass transition temperature ( T g) and short-beam shear (SBS) strength were measured. The composites did not exhibit microcracks after thermal cycling, although after 2000 thermal cycles, mechanical properties of the matrix declined slightly. The matrix degradation decreased the resistance to microcracking upon further loading. No effects of thermal oxidative aging were observed from thermal cycling, and thermally driven fatigue and creep were identified as the primary and secondary factors inducing mechanical degradation of the matrix. T g of the composites exhibited no change after 2000 cycles, while the SBS strength decreased slightly (3–9%). The results highlight the potential for use of TriA X composites as long-term structural components in high-temperature service environments.

Thermal oxidation of PEPA-terminated polyimide

The thermal oxidative stability of a 4-phenylethynyl phthalic anhydride (PEPA) endcapped polyimide (PI) was investigated. A surface reaction layer formed due to oxidization during thermal aging and grew in thickness with increasing aging time. Analysis of the surface layer revealed a partial loss of aromatic ring, ether linkage, and imide linkage in the aged polymer. The partial loss of the imide linkage and ether linkage in the surface layer was corroborated by the observed release of carbon monoxide and carbon dioxide reaction products. The oxidized layer exhibited discoloration and an increase in glass transition temperature. The surface discoloration was attributed to the formation of the conjugated unsaturated or aromatic carbonyl groups and/or the charge transfer complex. Interior regions of the oxidized samples were largely unaffected, except for a more compact molecular configuration. Compared with aging at high temperatures (288°C, 316°C), aging at 204°C produced similar chemical changes in the cured polymer, albeit with a low degradation rate. The oxidative stability of the imide polymer surpassed that of conventional nadic-endcapped PIs due to the greater intrinsic thermal stability of the PEPA endcap. The unusual oxidative stability of the PI, combined with superior mechanical properties, warrant consideration as a composite matrix for future applications in high-temperature service conditions.