The mechanical relaxation behavior of polyimides based on a variety of 2,2'-disubstituted benzidines and rigid dianhydrides was investigated. Two transitions were observed in these polyimides. The glass relaxation process is relatively weak and occurs at high temperatures due to the linear and rigid nature of these polyimides. The subglass relaxation is very prominent in these polyimides and is due to main-chain rotational motion localized within the diamine (benzidine) segment. Changes in the dianhydride moiety have little effect on the temperature of the subglass transition and result in only minor changes in the magnitude of this relaxation. The presence of 2,2'-CF3 substituents on the benzidine moiety increases the magnitude and shifts the subglass relaxation approximately 150 °C to higher temperatures versus Cl or CHs in these positions. Incorporating a flexible ether linkage between the phenyl rings of the benzidine and the CF3 side group (e.g., OCF3) substantially reduces the temperature and to some extent the magnitude of the subglass relaxation. Replacement of the 2,2'-disubstituted benzidine unit (two phenyl rings) with one (benzene) or three (terphenyl) unsubstituted phenyl rings results in a substantial decline in both the temperature and magnitude of the subglass relaxation. Molecular modeling was used to clarify the nature of the subglass relaxation. Rotational energy barriers for the 2,2'-disubstituted benzidines, calculated from both semiempirical and density functional quantum mechanical calculations, are comparable in magnitude to the experimentally determined activation energies for the subglass relaxation.
Regioirregular poly(p-phenylene)s (PPPs) with OCF3, CF3 or OCH3 substituents (16a, 16b, or 16c, respectively) were synthesized via Ni(0)-catalyzed polymerization of 2-substituted-1,4-bis-[[(trifluoromethyl)sulfonyl]oxy]benzenes (5, 8, or 10, respectively). Regioregular head-to-head PPPs with OCF3 or CF3 substituents (17a or 17b) were prepared from 2,2′-disubstituted-4, 4′-bis[[(trifluoromethyl)sulfonyl]oxy]biphenyls 14a, 14b and 2,2′-disubstituted-4,4′-bis[methylsulfonyl)oxy]biphenyls 15a, 15b. Regioregular head-to-head statistical copolymers with combinations of OCF3 or CF3 substituents with OCF3, CF3, CO(p-FC6H5), CH3, or H substituents (17xy) were synthesized by the copolymerization of 15a or 15b with 2,2′-disubstituted-4,4′-bis[(methylsulfonyl)oxy]biphenyls 15a-15e. Regioirregular and regioregular PPPs with OCF3 or CF3 substituents were soluble (THF, CHCl3, dipolar aprotic solvents). The highest molecular weight PPPs were obtained from OCF3-substituted monomers. Regioirregular PPP (16a) with Mn ) 35 200 (corresponding to 220 phenylene groups relative to polystyrene standards) was obtained from 5. Regioregular PPP (17a) with Mn ) 54 500 (340 phenylene units) was obtained from 14a. Regioregular PPP copolymers (17ab) with Mn ) 55 200 (363 phenylene units) were obtained from copolymerization of 15a and 15b. The λmax values of OCF3-and CF3-substituted PPPs were 302-307 and 273-271, respectively. A 10% weight loss occurs in the range of 550-575 °C for PPPs with OCF3 substituents and 580-615 °C for those with CF3 substituents. Polymers 17a, 17b, and 17ab exhibit enantiotropic nematic and lyotropic mesophases. The thermotropic nematic phase of 17a, 17b, and 17ab is stable up to the decomposition temperature. Most probably, 17a and 17b also exhibit a second mesophase which is monotropic and becomes enantiotropic in 17ab. The rigid-rod character of these polymers was demonstrated by the Mark-Houwink coefficient a ) 1.58-1.60. Phosphorus was not detected (MALDI and 31 P NMR) in the structure of these homopolymers and copolymers.
The preparation of a range of 9,9-disubstituted xanthene dianhydrides is discussed, as is their use in the preparation of poly(amic acid)s and polyimides. Groups in the 9,9-positions include various combinations of phenyl, substituted phenyl, methyl, ethyl, butyl, trifluoromethyl, perfluoroethyl, perfluoropropyl, cyclotetramethylene, cyclopentamethylene, hydroxy, and acetoxy. Polyimide films prepared from some of these new dianhydrides exhibit good mechanical properties, lower dielectric constant, lower moisture absorption, and low coefficient of thermal expansion, demonstrating their potential use in electronic applications. In addition, these new materials may find use in other film, fiber, and part applications.
Two new fluorinated diamine monomers, 3,3′‐diamino‐5,5′‐bis(trifluoromethyl)biphenyl and 3,3′‐diamino‐6,6′‐bis(trifluoromethoxy)biphenyl, as well as a known nonfluorinated analog, 3,3′‐diaminobiphenyl, were synthesized. Reaction of these diamines with rigid, highly rod‐like dianhydrides produced poly(amic acid)s and polyimides, which were spin coated and thermally treated to produce polyimide films for evaluation in electronics applications. It was hoped that these polyimide films would exhibit an ideal combination of low thermal expansion, reduced water absorption, and low dielectric constant but with improved elongation due to the “crankshaft” nature of the 3,3′‐biphenyl unit. Unlike polyimide films from analogous 4,4′‐diaminobiphenyls, however, the 3,3′‐diaminobiphenyl‐based polyimides did not yield low in‐plane thermal expansion coefficient in spin‐coated films. In some cases high elongation was achieved, but with high thermal expansion. These new diamines may nevertheless find utility in polyimides and polyaramides for membrane, fiber, and other applications. Additionally, they may be useful in modifying the properties of polymer backbones via copolymerization. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 2441–2451, 1997
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