Melt processable homo- and copolyimides with high thermo-oxidative stability as derived from mixed thioetherdiphthalic anhydride isomers

Resin Transfer Moldable Fluorinated Phenylethynyl-Terminated Imide Oligomers with High Tg: Structure-Melt Stability Relationship

Phenylethynyl-terminated aromatic polyimides meet requirements of resin transfer molding (RTM) and exhibits high glass transition temperature (T_g) were prepared. Moreover, the relationship between the polyimide backbones structure and their melting stability was investigated. The phenylethynyl-terminated polyimides were based on 4,4′-(hexafluorosiopropylidene)-diphthalic anhydride (6FDA) and different diamines of 3,4′-oxydianiline (3,4′-ODA), m-phenylenediamine (m-PDA) and 2,2′-bis(trifluoromethyl)benzidine (TFDB) were prepared. These oligoimides exhibit excellent melting flowability with wide processing temperature window and low minimum melt viscosities (<1 Pa·s). Two of the oligoimides display good melting stability at 280–290 °C, which meet the requirements of resin transfer molding (RTM) process. After thermally cured, all resins show high glass transition temperatures (T_gs, 363–391 °C) and good tensile strength (51–66 MPa). The cure kinetics studied by the differential scanning calorimetry (DSC), ¹³C nuclear magnetic resonance (¹³C NMR) characterization and density functional theory (DFT) definitely confirmed that the electron-withdrawing ability of oligoimide backbone can tremendously affect the curing reactivity of terminated phenylethynyl groups. The replacement of 3,4′-ODA units by m-PDA or TFDB units increase the electron-withdrawing ability of the backbone, which increase the curing rate of terminated phenylethynyl groups at processing temperatures, hence results in the worse melting stability.

Rigid High Temperature Heat-Shrinkable Polyimide Tubes with Functionality as Reducer Couplings

Flexible and semi-rigid heat-shrinkable tubes (HSTs) have been used in thousands of applications, and here rigid high temperature HSTs are reported for the first time. These rigid HSTs are prepared with shape memory polyimides possessing glass transition temperatures (Tgs) from 182 to 295 °C, and the relationships between Tg and their molecular structures are studied. The polyimide HSTs (PIHSTs) can fix expanded diameters and shrink back to original diameters very well, and the mechanisms of their heat-shrinkage performance are discussed. Their differences from commercially available HSTs in heat-shrinkage are also analyzed. They can withstand low temperature of -196 °C, much lower than those of other HSTs. The PIHSTs can also connect subjects of different sizes by heat-shrinkage and then fix them upon cooling like reducer couplings, and the possible mechanisms of their reducer coupling effect are analyzed. With their unique characteristics, PIHSTs will expand the application areas of HSTs enormously.

Phenylethynyl-terminated imide oligomers derived from thioetherdiphthalic anhydride isomers with decreased melt viscosities

A series of oligomers based on the mixture of thioetherdiphthalic anhydride isomers and 4,4′-oxydianiline with 4-phenylethynylphthalic anhydride as reactive endcapping reagent were prepared. The calculated molecular weights were in the range of 1150–5070 g mol−1 with different degrees of polymerization ( n = 1, 3, 5, 7, and 9). The effect of molecular weight 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 typical oligomer (Oligo-1) could be melted at temperatures of 289–334°C to yield stable molten fluid with melt viscosity below 1.0 Pa s. The melt viscosity of the oligomers increased with the increasing molecular weight. After thermally curing at 370°C, the thermoset PIs exhibited good thermal properties. The glass transition temperatures of oligomers measured by differential scanning calorimetry were in the range of 286–326°C, and the temperature of 5% weight loss was 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.

Preparation, characterization, and properties of poly(thioether imide)s from isomeric bis(chlorophthalimide)s and bisthiophenols

A series of isomeric poly(thioether imide)s (PTIs) containing thioether linkages were prepared by aromatic nucleophilic substitution reaction of isomeric bis(chlorophthalimide)s (BCPIs) and bisthiophenols. The glass transition temperatures ( Tgs) of the isomeric PTIs were 190–264°C, the 5% weight loss temperature ( T5%) reached up to 441–508°C under nitrogen and 472–520°C in air atmospheres, respectively. It was found that the Tg values of the PTIs from three isomeric BCPIs with the same bisthiophenol increased in the order of 4,4′-BCPI < 3,4′-BCPI < 3,3′-BCPI, while the T5% values gradually decreased in the order of 4,4′-BCPI > 3,4′-BCPI > 3,3′-BCPI. Flexible films that could be cast from the polymer solutions exhibited good mechanical properties with tensile strengths of 91–121 MPa, elongations at break of 8–12%, and tensile moduli of 2.2–2.6 GPa. The minimum melt viscosity of isomeric PTIs decreased with increasing the content of asymmetric 3,4′-substituted phthalimide unit, and the PTI (2c) showed the lowest melt viscosity about 760 Pa·s at 264°C among these isomeric polymers.

Comparative studies on melt processable polyimides derived from 2,3,3′,4′-oxydiphthalic anhydride and 2,3,3′,4′-thioetherdiphthalic anhydride

Two thermoplastic polyimides were controllably synthesized with similar molecular weight from 2,3,3′,4′-oxydiphthalic anhydride (3,4′-ODPA) or 2,3,3′,4′-thioetherdiphthalic anhydride (3,4′-TDPA) with 4,4′-diaminodiphenyl ether (ODA) by a one-step method of m-cresol. The solubility and mechanical properties of these polymers were investigated for comparison. The thermal and melt processable properties of these polymers were examined carefully by differential scanning calorimetry, thermogravimetric analysis (TGA), rheometer and melt index, and so on. It was found that the glass transition temperature ( Tg) of 3,4′-TDPA/ODA was about 15°C lower than that of 3,4′-ODPA/ODA, the melt indices of 3,4′-TDPA/ODA at different temperatures were higher than those of 3,4′-ODPA/ODA, while the minimum melt viscosity of 3,4′-TDPA/ODA was lower than those of 3,4′-ODPA/ODA. Furthermore, TGA results showed that 3,4′-TDPA/ODA had different degradation mechanism in air compared with 3,4′-ODPA/ODA. The long-term thermooxidative stability results showed that 3,4′-TDPA/ODA had better thermal oxidative stability than 3,4′-ODPA/ODA at 410°C for 6 h in air, which could be associated with the results of apparent activation energy ( Ea) of polymers calculated by TGA dynamics. The natural bond orbital charge of ethero atoms (S or O), and energy gap of the corresponding model compounds were also obtained by computational simulation to correlate with polymer properties.

Comparative study on polyimides derived from isomeric diphenylsulfonetetracarboxylic dianhydrides

In this study, three diphenylsulfonetetracarboxylic dianhydride (DSDA) isomers namely 2,2′,3,3′-DSDA (3,3′-DSDA), 2,3,3′,4′-DSDA (3,4′-DSDA) and 3,3′,4,4′-DSDA (4,4′-DSDA) were prepared from the corresponding diphenylthioethertetracarboxylic dianhydride isomers. Based on the three DSDA isomers, a series of isomeric polyimides were prepared. The properties, such as film-forming ability, solubility, wide-angle x-ray diffraction, thermal and mechanical properties as well as rheological behavior, were compared among the isomeric polyimides. It was found that 4,4′-DSDA-based polyimides had much better film-forming ability than 3,3′-DSDA- and 3,4′-DSDA-based polyimides. 3,4′-DSDA-based polyimides had better solubility than 3,3′-DSDA- and 4,4′-DSDA-based polyimides. Furthermore, for a given diamine, the glass transition temperatures of the polyimides from the three DSDA isomers decreased in the order of 3,3′-DSDA > 3,4′-DSDA > 4,4′-DSDA, while the 5% weight loss temperatures in both air and nitrogen atmospheres of the polyimides mainly increased in the order of 3,3′-DSDA < 3,4′-DSDA < 4,4′-DSDA. Moreover, 3,4′-DSDA- and 4,4′-DSDA-based polyimides showed much better melt stability than 3,3′-DSDA-based polyimide.